The advent of additive manufacturing has made a significant change to traditional manufacturing processes. It offers a new model for the manufacturing industry and has many applications in medicine and aviation as well as in construction, defense, consumer and consumer applications. The rapid advancement of additive manufacturing technology since its inception in the 1980s has opened new avenues for metal production. Recent years have seen increased attention to additive manufacturing at the nano- and microscale.
"The current state and future prospects of microscopic selective laser melting technology," published by the Chinese Academy of Engineering Journal "Engineering", 4th edition 2019. It examines the use of selective melting technology to produce micro-sized features in metallic materials. The paper also includes a comprehensive evaluation of different research papers and commercial systems that allow for the fabrication of small parts using selective laser sintering and selective laser melt.1. Introduction
In recent years, there has been a growing demand for micromachining techniques to meet the development needs of various industries, including electronics, medicine, automotive, biotechnology, energy, communications, and optics. Many products and components, including microdrives, micromechanical devices, sensors and probes, microfluidic components, medical implants, microtransducers, optical devices, memory chips, micromotors, magnetic hard disk heads, computer processors, ink print heads, pins, electronic connectors , tiny fuel. cells and, most importantly, microelectromechanical systems (MEMS) devices are fabricated using micromachining techniques.
Microscale fabrication processes can generally be divided into MEMS-based (or lithography-based) and non-MEMS (or non-lithography-based) processes. Significant progress has been made in the application of metallic materials in microcomponents, largely due to their applicability in terms of mechanical and electrical properties (i.e. strength, ductility, electrical conductivity, etc.). Micromachining of metals is typically achieved by non-lithographic methods such as machining, molding, and bonding. Conventional micromachining techniques have one or more of the following limitations: difficulty in fabricating complex shaped components, material limitations, tooling issues, inability to perform true three-dimensional (3D) fabrication, etc.
The development of additive manufacturing (AM) technology over the past 20 years has opened up new areas for the production of metals, as additive manufacturing allows the manufacture of components of any complex shape. AM integrates powder or wire raw materials step by step into the final product. The AM process begins with a 3D modeling of the desired part, which is then cut into different 2D (2D) layers. The raw material is then deposited and each layer is selectively added using an energy source.
AM technologies can generally be divided into seven categories: material extrusion, photopolymerization, material spraying, binder spraying, laminating, directed energy deposition (DED), and powder bed melting (PBF).
Material extrusion, photopolymerization and material spraying are generally applied to non-metallic materials; lamination can process metals based on precision cutting of metal sheets and then stacking by gluing, welding or ultrasonic reinforcement;< /p>
However, binder blasting, DED and PBF are considered the most suitable processes for metal processing. Binder jet spraying consists of applying the binder to a metal powder, which then solidifies into a "green" part. The last part is achieved by sintering the wet part with an optional infilterntom from another material or nanoparticles of the same metal. Forced heat treatment and high porosity are common limitations of the binder blasting process as they prevent its application on a microscopic scale.
DED, also known as laser cladding, laser metal deposition (LMD) and laser engineered grid shape (LENS), is another important additive manufacturing process used to make metal parts. In DED, raw materials are placed directly into a melt bath, which is produced using a concentrated energy source. The feedstock can be powder or wire, where powder-fed DEDs typically have higher resolution than wire-fed DEDs. Because DED only produces nearly pure forms, further processing is required.
PBF is often used for small parts that require good surface quality because PBF provides better resolution than DED. PBF typically has a smaller melt pool and layer thickness, resulting in better resolution and surface finish. The PBF process involves the selective melting or sintering of a layer of powder using energy. An electron beam and a laser beam are the two main energy sources used for the PBF process, namely electron beam melting (EBM) and selective laser melting (SLM)/selective laser sintering (SLS), in that order. In addition, SLM makes it possible to produce components with mechanical properties similar to traditional manufacturing processes.
Although metal additive technology has been applied in various areas of biomedicine and aerospace (including the manufacture and repair of aerospace components), its application is limited to large-scale and mesoscale production. Recently, additive modeling techniques have been developed for micron-scale manufacturing to create 3D microelements on a variety of materials, including ceramics, polymers, and metals. The next section will focus on previous additive manufacturing methods for fabricating metal microcomponents.2. Micrometal AM
In recent years, micro- and nanoscale AM have attracted attention, as evidenced by the appearance of review articles on relevant techniques.
Engström et al. published a white paper on additive nanomanufacturing (ANM) technology, which uses a variety of materials, including metals, polymers and organic molecules, to produce final parts with sub-100 nm resolution. Research by Hirt et al. is focused on micro-AM metals technology, which is divided into metal transfer technology and in-situ synthesis technology. They defined the reference element size for AM microtechnology as 10 µm. Vaesi et al. divided 3D micro-AM technology into two main categories, namely direct 3D writingand scalable AM. As shown in Figure 1, direct 3D writing includes nozzle-based ink spray and aerosol jet technology, laser delivery technology, and beam methods. such as laser chemical vapor deposition (LCVD), focused ion beam (FIB) recording, and electron beam (EB) recording. Although the direct recording process usually has a high resolution suitable for nanoscale fabrication, the processing is extremely complex and slow. In the category of scalable AM techniques, microstereolithography (MSL) is considered the most successful micro AM technique due to its high resolution and reproducibility, albeit limited by the choice of materials. Fused deposition modeling (FDM) and layer-by-layer manufacturing (LOM) methods have difficulties in processing metals, and, in addition, they have limitations in obtaining high resolution features. Although metallic inks have been used in inkjet printing, this approach is still strictly limited to non-metals. 3D printing (3DP)/jet inkjet printing (BJP) is promising for multi-material printing and cold working, but the porosity of printed parts is usually high.
Fig. 1. Main categories of additive manufacturing methods for microscale production. MSL: microstereolithography; FDM: deposition modeling; LOM: Layered Object Manufacturing. Adapted from reference with permission from Springer-Verlag London, ©2012
SLM and SLS (i.e. laser layer-by-layer melting or sintering of powder layers) have shown potential. Extensive existing knowledge on the application of SLM and SLS in macroscale processing can be used to scale the technology to the microscale. This article focuses on SLM and SLS for creating microscale features. The difference between SLM and SLS lies in the degree of melting. SLM can achieve complete melting of the powder, while SLS can only achieve a sintered state or partial melting of the powder. There are no differences between SLM and SLS in terms of process settings and mechanisms other than full or partial melting of the powder particles. Therefore, in order to compare process components and process parameters, this article treats SLM and SLS as consistent. The discussion of re-powder coating systems and hybrid processes later in the article can also be applied to the miniaturization of other PBF technologies.3. Selective laser melting
In fig. 2 shows a diagram of the installation of the SLM process. With SLM and SLS, a layer of powder is first applied to the building substrate. The laser beam melts or sinters the powder according to the desired geometry. The hardened part is then coated with the next layer of powder, followed by laser melting/sintering. The heating and cooling rates in the SLM process are high due to the short interaction time of the laser source with the powder. Since the geometry of the formed melt pool significantly affects the microstructure, the mechanical properties of the machined parts differ from those of traditional processes. Detailed reports on the mechanism of the SLM process are given in references [6,7,21]. Due to the involvement of complex systems and mechanisms, a large number of process parameters affect the final quality of SLM components.
Fig. 2. SLM Process Diagram
The SLM process parameters can be roughly divided into powder, laser, and powder bed variables based on their properties, as shown in Figure 3. Most powder-related process parameters, such as chemical composition, particle size, and shape , as well as surface morphology, are invariable in a real production environment. Parameters related to laser systems that affect the SLM process include the type of laser [i.e. e. continuous (CW) or pulsed], laser power and spot size. Scan parameters such as scan strategy, pattern pitch, and scan speed have a significant effect on the part characteristics set by SLM. The third category of SLM process parameters are powder bed properties. In most powder bed processes, powder is added to the build platform using a rake mechanism, also known as recoating. The efficiency of a powder supply system is affected by several parameters, including the type of recoater, the number of recoating powder loads, the amount of powder recovered per load, and most importantly, the properties of the powder. The recoating thickness is one of the important process parameters that determine the characteristics of the part. Layer thickness, particle size distribution (PSD) and laser parameters affect the interaction of the laser with the material and hence the properties of the melt pool.
Fig. 3. Summary of SLM process parameters
The performance of AM components made using SLM is often evaluated over several processes, depending on the application. On fig. 4 shows some important features of the SLM parts. As with any conventional process, feature resolution, surface quality, mechanical properties, and microstructure were characterized to evaluate the quality of the final manufactured part, and hence the SLM process. On fig. 5 shows the defects that can occur in SLM. Defect formation is inherently dependent on process variables that need to be optimized to produce defect-free parts. A detailed account of the shortcomings in the AM process is available elsewhere.
Fig. 4. SLM process output summary
Fig. 5. Typical defects of the SLM process4. microscopic selective laser melting
Commercial SLM systems typically use powder particles with a particle size of 20-50 µm and a coating thickness of 20-100 µm. In order to make the application of traditional SLM more accurate and improve feature resolution, the author mainly conducts research on three aspects: laser beam diameter, coating thickness, and particle size (as shown in Figure 6). Fisher et al. defined the micro SLM range as follows: laser beam diameter less than 40 µm, coating thickness less than 10 µm, particle size less than 10 µm.
Fig. 6. Characteristic requirements of microSLM
(1) Existing technical level
More than a decade ago, a laser research institute called "Mittelsachsen" used a Q-switched yttrium aluminum garnet (Nd:YAG) laser (0.5~2kW) to produce the first SLS microscopic system, a microsintered laser. The system includes a special raking step in which a thick layer of powder is first applied, followed by continuous shearing in the opposite direction to obtain a thin layer. To ensure submicron accuracy of coating thickness, the rag and model platform have a resolution of 0.1 µm. Microcomponents made by this method have a structural resolution of less than 30 µm, an aspect ratio of more than 10, and a surface roughness of 5 µm. As shown in fig. 7, various metals were tested, such as tungsten (W), aluminum (Al), copper (Cu), and silver (Ag). On fig. 7(a) shows one of the original features of this device made using 300 nm tungsten powder. Although the powder is raked better in a vacuum of 10–3 Pa, the powder bed density (PBD) after raking is still around 15%. A mixture of tungsten-copper powder can be sintered to achieve a maximum part density of 90%.
Figure 7. Microscopic features obtained by laser microsintering. (a) Sintered experimental structure from tungsten powder (300 nm), (b) three nested hollow spheres, (c) concentric rings, (d) laser sintering of several materials (Cu and Ag). (a) and (b) reproduced by reference with permission of Emerald Group Publishing Limited, © 2007 (b) reproduced by reference with permission of WILEY-VCH Verlag GmbH & Co. KGaA, © 2007 (c) reproduced by reference with permission from Emerald Group Publishing Limited, © 2005. Licensed here
The research team has also developed an advanced system with two round tine arms for powder distribution. On fig. 7(b)~(d) show various shapes of elements made with the improved device. The difference between the two machines is the powder re-applying mechanism, the new rake moves in a circular motion between the powder tank and the build platform. Metal cylinders with sharp edges servingt rake. The dual leading edge design allows parts to be made from multiple materials, or the grain size of the part varies with the thickness gradient of the part, as shown in Fig. 7(d). In addition to raking, the recoating system can also compact the powder manually under pressure. This unique facility is capable of producing micro-components from various metals including tungsten, aluminium, copper, silver, 316L, molybdenum (Mo), titanium (Ti) and 80Ni20Cr by laser microsintering. After continuously improving the process characteristics, laser microsintering of metals has a minimum resolution of 15 µm and a surface roughness of 1.5 µm. The maximum detail density of 98% and 95% is registered for oxide ceramics and alloys.
In 2013, Giesecke et al. developed the SLM microsystem for the production of American Iron and Steel Institute (AISI) 316L hollow microneedles with a minimum wall thickness of 50 µm. To visualize fine details, the laser spot diameter is reduced to 19.4 µm. To make needles with an inner diameter of 160 µm and a layer thickness of 20 µm, the researchers used powders with a particle size of 5 to 25 µm. Despite the small size of the spot and powder, the obtained parts had a low surface roughness (Ra ≈ 8 μm). Agglomeration of fine powders can cause uneven distribution of the powder, which may explain poor finishing results. Due to the high energy input, the sticking of the powder to the wall is noticeable. Although some spacers failed, a more complex helical shape was also made, with a minimum spacer diameter of 60 µm. Subsequently, Gieseke's research team used a shape memory alloy (Ni-Ti) to make parts, as shown in Fig. 8(a), with a resolution of 50 μm at lower laser power and higher scanning speed. Yadroitsev and Bertrand used a commercial PM 100 system to fabricate a 904L stainless steel (SS) microfluidic system as shown in Fig. 8(b), with a spot diameter and a layer thickness of 70 µm and 5 µm, respectively. They also made normally functioning parts with a size of 100-150 microns, and structural elements with a size of 20 microns. It should be noted that the spot diameter here is still large, and the surface roughness leaves much to be desired.
Fig. 8 Parts made using microscopic SLM. (a) Ni-Ti microactuator, (b) top view of the SS 904L microfluidic system, the small figure shows its internal structure. (a) Extracted from link with permission from Elsevier B.V., © 2010 (b) Extracted from link with permission from DAAAM International, © 2010
In 2014, Fisher et al. used the EOSINT µ60 system to study microscopic SLM process parameters. The minimum roughness and maximum resolution of the elements reached 7.3 µm and 57 µm, respectively. The maximum relative density of the SLM cubic structure can reach 99.32%. Although the powder used is relatively finem, with a particle size of 3.5 µm, the resolution achieved cannot meet the microcomponent size specifications. Abele and Knipkamp further improved the surface quality of microscopic SLM parts using a profile scanning strategy, achieving a minimum surface roughness of 1.69 µm along the wall construction direction. Knipkamp et al. also used parameter optimization to fabricate microscopic SLM parts with top surface roughness of less than 1 µm. Recently, Robert and Thien used microscopic SLS to fabricate SS microelectrode arrays with vertical and lateral resolutions of 5 µm and 30 µm, respectively.
The latest results of microscopic additive modeling research have come from the University of Texas at Austin, whose microscopic SLS system consists of an ultrafast laser, a micromirror-based optical system, substrate heating, and an accurate recoating system with feature resolution down to 1 µm. They made three important modifications to a typical SLS system:
• Using a new coater design combined with precision blades and rollers. The rollers are equipped with linear voice coil drives providing high frequency vibrations with very low amplitude. This new machine uses vibration to compact the powder, resulting in thin layers of a few microns thick.
• The galvanic mirror commonly used in SLM machines has been replaced in this setup by a digital micromirror device (DMD) to increase system throughput.
• An additional focusing optics has been added to the setup to obtain a 1 µm spot. In addition, the device uses a linear actuation system to increase the resolution of the powder layer to tens of nanometers.
Even though researchers have added a vibrating roller to the SLS system as a powder coating machine, powder particle agglomeration still exists. The researchers made two modifications to the SLS microscopic system: ① replaced the dry powder with nanoink; ② Changed the particle distribution mechanism from traditional blade/roller to slot die or spin coating technology. In a modified setup, the SLS microscopic system adds a flexible slot die coating mechanism. Thanks to precise dosing and controlled distribution, the slot die coating can be applied in thicknesses from 20 to 150 µm. In addition, the system is equipped with a precise nano-positioning platform using voice coil drives to ensure accuracy. However, this system is only suitable for slurries or inks, since fine dry powders are agglomerated by van der Waals forces.
Table 1 summarizes research work on the processing of metallic materials using microscopic SLM/SLS. It should be noted that in microSLM systems use both CW and pulsed lasers, while conventional SLM systems are dominated by CW lasers. Regenfuss et al. originally used a Q-switched pulsed laser in laser microsintering device Effective reasons are as follows: ① Improve component resolution ② Reduce residual voltage ③ Reduce oxidation effects that can cause shielding effects due to gas or plasma expansion; Problems such as poor adhesion of the main parts and sublimation of the material, which usually occur when using continuous laser sintering of submicron powders; ⑤ suitable for processing dielectrics. Compared with continuous laser, pulsed laser has higher laser intensity and can produce narrow and deep incision, freeze jets and level pits. However, the pulsed laser melt pool is unstable, resulting in poor surface finish, irregular trajectory, and spheroidization.
Ke et al. compared cw and pulsed lasers in laser microsintering of nickel (Ni) powder with an average particle size of 4 µm. Experiments have shown that the spheroidization phenomenon of a cw laser is more obvious than that of a pulsed laser; the leveling effect and the fast cooling rate of the plasma reduce the phenomenon of spheroidization of the latter. In addition, the wettability of the pulsed laser is better. However, a single track created by a pulsed laser produces ripples and grooves, and the surface quality leaves much to be desired. Similarly, Knipkamp et al. reported poor surface quality with discontinuous tracks using a 50W pulsed fiber laser. Fisher et al. sampled a large amount of laser power and pulse repetition rate data for testing, but found that pulsed lasers could not produce uniform single tracks without defects. In addition to metals, the pulsed wave laser in microscopic SLS has also been tested with ceramics and proved to be effective. For ceramics, the resolution obtained with Q-switched pulsed lasers is higher than the resolution obtained with CW lasers because pulsed lasers do not accumulate heat. Although laser microsintering devices can successfully sinter some metal and ceramic materials with Q-switched pulsed lasers, there are still limitations in using pulsed lasers in microscopic SLM, such as surface finish, melt pool stability, and defects. These limitations and the widespread use of CW lasers in conventional SLMs may explain why recent research in this area has been performed using CW lasers.
Table 1. Literature review of SLM/SLS technology for micromachining
NS: not specified. D90: particle diameter, 90% of particle distribution of which is less than thisvalue.
It should be noted that research work in the field of micro-SLM is quite limited, which is not in line with the enthusiasm for the traditional field of macro-SLM. For traditional SLMs, the effect of various process parameters (as shown in Figure 3) on process performance has been extensively described in the literature. Although the microscopic parameters of the SLM process are expected to have a significant impact on the results of the process, including feature resolution, defects, surface finish, and microstructure, not many studies of the microscopic parameters of SLM are mentioned in the literature. Kniepkamp et al. reported that during microscopic SLM of 316L powder, the dimensional accuracy of some parts of the part increased with decreasing laser power. studied the formation of single-track and bulk features using 316L microscopic SLM powder over a range of scan speeds and laser powers, and determined the process window for uniform tracks and dense cubes. Abele and Knipkamp investigated the effect of profile scan strategy, laser power, and scan speed on the roughness and surface morphology of a vertical wall during 316L microscopic SLM powder. With optimized exposure settings, profile scanning reduces the vertical surface roughness of the part. Despite these efforts, the mechanical properties, microstructure, or residual stress distribution of fabricated elements have not been reported in previous work on microscopic SLM/SLS. Since the main focus of these works was to obtain small and dense features with smooth surfaces, only characteristics such as feature resolution, detail density, and surface finish were reported. Most parts made with conventional SLM have structural applications where mechanical properties and microstructural factors such as grain morphology and crystal structure are important. Because components fabricated with microscopic SLM can also have requirements for mechanical properties, residual stresses, and microstructure, it is necessary to understand the fundamental behavior of the process.
The formation of microstructures in SLM is influenced by many mechanisms, including heat transfer, thermophysical properties of materials, and phase transitions. The solidification pattern and the resulting microstructure are controlled by the melt pool temperature gradient (G) and the liquid-solid interface velocity (i.e., solidification rate, R), which is represented by a solidification map (image G vs. P). Solidification forms include equiaxed dendrite, columnar dendrite, honeycomb crystal, and flat crystal. The microstructure often seen in SLM has been found to be columnar, as AM processes typically undergo rapid heating, solidification, and reheating during melting of adjacent layers. The formation of columnar grains in SLM can be mainly associated with large hailstones.temperature factor along the assembly direction. The microstructure obtained in SLM is mainly controlled by process variables such as laser power, scan speed, and scan strategy, although factors such as elemental composition, assembly direction, and part geometry also play a role.
While there is a large body of literature on microstructures obtained with conventional SLM, there is no similar report on microscopic SLM. Recently, attempts have been made to study the effect of laser spot diameter (see Section 4.2) by beam defocusing in PBF processes such as EBM and SLM. Al-Bermani reported that defocusing the electron beam by changing the focus shift significantly affects the molten pool morphology during EBM in SS. A similar approach by Phan et al. used a narrowly focused beam in the EBM of a cobalt-based (Co) alloy, whereby horizontal dendrites limited the growth of typical columnar dendrites. McLauth et al. studied laser beam focus shift during IN718 SLM and found that due to the higher power density, the smaller the spot size, the finer the microstructure and the better the equiaxed structure. In our recent microscopic study of the unidirectional formation of 316L powder using microscopic SLM, due to the small laser spot size we studied, the observed melt pool morphology on the “double peak” surface was compared with that formed in macroscopic SLM. The above studies of the effect of defocusing show that the size of the laser spot diameter can play an important role in the characteristics of the microscopic SLM process. Due to the smaller spot size, thinner layer thickness, and finer powder in microscopic SLM, it is expected that its microstructure formation will differ from conventional SLM. In addition, due to the small spot size of microscopic SLMs, the temperature gradient and solidification rate are expected to be larger, which can lead to a higher cooling rate and hence thinner dendrites. However, the microstructure of microscopic PMS is difficult to predict because it depends on many factors involving complex mechanisms. Through many studies, we have been able to know the mechanical properties of parts made by traditional SLM methods, including material hardness, tensile strength and fatigue properties. However, the mechanical properties of the microscopic components of SLS have hardly been studied in the literature. Mechanical properties are often affected by defects, microstructure, residual stresses and subsequent heat treatment.
According to published reviews regarding SLM and PBF, the following post-thermal treatments are commonly used: stress relief, aging, solution treatment, and hot isostatic pressing (HIP). The purpose of heat treatment is to reduce or eliminate defects, control microstructure, improve performanceand removal of residual stresses. HIP is typically used to seal internal pores and cracks, recrystallization improves the microstructure to equiaxed fine grains, and aging controls sludge formation. Since the microstructure obtained by SLM is different from that formed by traditional processes, the heat treatment method is also different. As previously mentioned, the ultra-small spot size can cause microscopic SLMs to form microstructures that are different from conventional SLMs. It is expected that with proper heat treatment, the microstructure will be controlled while improving the mechanical properties. Predicting the appropriate heat treatment for microscopic SLMs is challenging because the post-heat treatment of SLM components depends on many factors, including initial microstructure, defects, residual stresses, elemental composition, and desired output properties. Thus, future research on microscopic heat treatment of SLM will be very valuable, as it will open up significant opportunities for expanding related applications. However, microstructural features generated by microscopic SLM of various materials, such as grain morphology and phase formation, must first be understood in order to determine the optimal post-thermal treatment.
Table 2 compares various characteristics of commercially available AM systems for micromachining techniques in terms of build volume, achievable layer thickness, laser specification, laser spot size, recoating system, processing materials, etc. The first commercial microscopic SLS system is based on a patented technology based on laser microsintering. The micro-SLS process has been commercialized as "EOSINT μ60" by 3D MicroPrint GmbH, founded by 3D-Micromac AG and EOS GmbH, which specializes in the development of micro-SLS systems for precision metal processing. Table 2 shows that the laser spot diameter of current commercial systems is greater than or equal to 20 µm. It should be noted that in the future we will have to reduce the size of the laser spot as much as possible in order to produce accurate components. Since the part is built in layers in the SLM/SLS process, it is necessary to keep the thickness of the layers as small as possible to reduce the resolution of the features. With the exception of the EOSINT μ60, other existing microscopic SLS systems typically produce 10–50 µm thick layers and cannot be used to achieve microscopic performance at the submicron scale. Although attempts have been made to use various recoating systems, commercial systems still, however, all use a blade or roller system that is similar to the SLM macro system. The ability to reduce the layer thickness is related to the particle size of the powder used. Conventional SLM/SLS typically uses powders with a diameter of 20-50 µm, while the micro-SLS process requires particles with a diameter much smaller thanless than 10 µm.
Table 2. Comparative performance of commercial AM systems for micromanufacturing technologies
Recently, researchers at the Singapore Institute of Manufacturing Technology (SIMTech) have developed their own microscopic SLM system (Fig. 9(a)) with a small laser spot and a new powder recoating method that can handle fine powders. Initial experimental results using SS 316L powder (D50 ≈ 10 µm, where D50 is the particle diameter, with 50% particle diameter distribution below this value) showed that the developed SLM microscopic system can produce microscopic features with good surface finish. Various experimental validations of this system have been performed by varying the laser power, scanning strategy, scanning speed and incubation density. On fig. Figure 9(b) shows various elements fabricated using the SLM microscopic system, the process parameters of which are as follows: layer thickness 10 µm, spot diameter 15 µm, laser power 50 W, scanning speed 800–1400 mm s. − 1 with a hatching step of 10 µm. Currently, the smallest achievable element size is 60 µm with a minimum surface roughness (Ra) of 1.3 µm, while the system is capable of processing sub-micron and nano-sized powders to achieve 1 µm thick layers. By further reducing the layer thickness and powder particle size with this development system, it is possible to achieve higher fine detail resolution (< 15 µm) and surface roughness of less than 1 µm.
Figure 9 (a) Microscopic SLM system developed by SIMTech; (b) Various elements made using microscopic SLM; (c) Scanning electron microscope (SEM) image of the top surface of the element
Scaling down from traditional SLM to microscopic SLM requires certain considerations, which can be divided into the following categories: ① hardware-related, ② process-related, and ③ post-processing factors. Most process mechanisms and the effect of process parameters can be obtained with various SLM sizes. Fine spot size and particle size naturally reduce layer thickness and incubation time, resulting in longer cycle times. An article by Regenfuss et al. mentions that by reducing the layer thickness and grain size by an order of magnitude, the processing time of the same component printed by laser microsintering increased by a factor of 12. The power density will be greatly improved when applying an accurate spot on a microscopic scale. Thus, process throughput can be increased by using less laser power and/or faster scanning. The design of the support structure is another challenge for micro SLM, as the removal of the structure is difficult and can affect part dimensions. Similarpreheating can be a problem in high elongation thin wall situations, especially when supporting structures are difficult to build.
Scaling factors associated with equipment include construction platforms, optics, powder recoating, powder handling, and powder recycling. For microscopic SLM systems, the size of the building platform and the footprint of the entire device are smaller. To fulfill one of the basic requirements for achieving accurate spot sizes, it is necessary to modify the optical assembly, which will be described in Section 4.2. Another important requirement for micro-SLM is to achieve a thinner layer thickness, which can be achieved with precision drives for powder distribution and building platforms. The main downscaling equipment problem is the need to use fine powders on the submicron or even nanometer scale. Due to the safety and health hazards associated with environmental exposure to small nanoparticles, it is recommended that manual handling of these powders be kept to a minimum. It is very important for any SLM machine to provide a sealed enclosure for the assembly room. The effect of powder particle size and recoating system will be discussed in Sections 4.3 and 4.4 respectively. Differences in post-processing include surface and heat treatments performed on AM parts. Heat treatment of thin micro-components can lead to deformation of the part. Powder sticking to walls is a common occurrence in SLM and requires additional handling after printing. At the microscale, processing of thin walls is not possible. Non-contact finishes such as electropolishing can also be ineffective, as noted by Gieseke et al. Therefore, it is necessary to produce parts with good surface finish and along the wall, rather than relying on secondary machining. Section 5 discusses the effects of surface treatment in detail.
(2) Laser spot
The diameter of the laser beam is one of the most important parameters that affects the resolution of elements. The spot size at the intersection of the lasers is the smallest and is often used for AM processes because the power density can be maximum at this focal point. The PBF process uses a laser beam with a diameter of 50 to 100 µm, while the DED process uses spots on the order of millimeters. Ma et al. studied the difference in metal melting shown by SS 316L fabricated with laser cladding (LCD) and SLM processes where the spot size of the LCD process (>1 mm) is much larger than that of the SLM process (0.12 µm). 0.15 mm). In the SLM process, the size ratio and cooling rate of the melt bath are higher, the distance between the main honeycomb sleeves is smaller, the particle size ratio is lower, and the microhardness and strength are higher. Although it is difficult to attribute the performance of the SLM process to the beam diameter in this study, this study provides some clues.phenomena for future studies, showing that changing the spot size will lead to different energy input and solidification rates. difference in the microstructure of the melt pool. Liu et al. used SS 316L powder to study the effect of laser beam diameter in the SLM process. When the beam diameter was reduced from 48 µm to 26 µm, the process improved in terms of part density, surface finish, and mechanical properties. used two systems using beams of different diameters (80 µm and 240 µm) to investigate the effect of increasing spot size in PBF laser processes. To study the influence of the beam diameter, the power density was kept constant. It has been found that smaller beam diameters and lower laser powers result in narrower and shallower melt pools and consequently smaller fill intervals and layer thicknesses.
Helmer et al. studied the effect of laser spot size on the EBM process by changing the laser focus. The results show significant differences in melt bath geometry and microstructure for spots of different sizes corresponding to focused (400 µm) and defocused beams (500 µm). A recent paper by MacLout et al. extends the laser focus change analysis to the SLM process. IN718 samples fabricated at the focus of the laser had a finer microstructure compared to samples fabricated using a defocused beam. This behavior is explained by the higher power density as a result of the smaller spot size. Simultaneous studies of the effect of shifting the laser focus on porosity, surface roughness, and tensile strength revealed significant changes in the properties of prefabricated parts upon shifting the focus. Various melt behaviors were observed ranging from insufficient melting at negative displacement (–2 mm) to keyhole formation due to excess energy at positive displacement (+3 mm). Changes in the input energy, as well as shifts in focus and spot size, correspond to beam divergence in a Gaussian distribution. However, the study also notes that the optimal focus shift and spot size depend on scan speed and laser power. Studies on a similar process (i.e. laser welding) have shown the effect of smaller laser spot sizes on improving welding performance by increasing welding speed or deeper penetration by increasing power density.
Although the SLM process has been extensively studied, it is worth noting that studies on the impact of spot size on process performance, especially on feature resolution, are very scarce. Table 1 shows that the spot size of the micro-SLM system is in the range of 20–30 µm, and the corresponding minimum element resolution is similar to or slightly larger than the spot size. Similarly, commercial micro-SLM systems have laser spot sizes greater than 20 µm (Table 2). To achieve fine microscopic characteristics, it is necessary to achieve a more accurate spot size.on the laser beam. DebRoy et al. emphasized the need for finer detail resolution at the expense of small spot size and low power. The spot size can usually be expressed as a function of the diameter of the fiber core, focusing and collimating lenses. Reducing the size of the laser spot is not difficult with proper optics design. The optical system in the SLM process usually consists of a collimator, a beamformer, a scanner, and an F-theta lens as an objective lens. Scanning systems in conventional and microscopic SLM machines typically use a two-mirror galvanometer to guide the laser beam along at least two axes. The initial set of SLS systems developed by Regenfuss et al. used a 25 mm × 25 mm SCANLAB beam scanner with a Q-switched Nd:YAG laser at 0.1–10 W in TEM00 mode. . To achieve finer spot sizes, the optical design may also include other mechanisms such as digital mirror devices. However, the details of the optical system evaluation are beyond the scope of this study.
Several properties of the powder (Figure 3) affect the throughput of the SLM process and therefore the quality of the manufactured part. Powder shape, size and surface roughness are the most important parameters influencing powder flow and hence powder bed properties, melt bath characteristics and part properties.
Olakanmi studied the effect of powder properties on the efficiency of SLM/SLS processes for pure aluminum and aluminum alloys. The results show that the shape of the powder particles has a significant effect on the processing structure and the compaction process, and irregularly shaped powder particles in the powder exacerbate the formation of agglomerates and pores. Analysis of the shape of the original Ti-TiB powder in the SLM showed that irregularly shaped powder particles adversely affect the compaction process and hence the tensile strength. In studying powder properties, Cordova et al. used a variety of metal powders and found the highest packing density of the powder with the most uniform morphology (i.e., the largest spherical shape). Liu et al. observed that 11 µm powders sprayed with water had a lower PBD compared to apparent and shaken density due to their irregular angular morphology and fine particle size. These studies show that it is common to use spherical powder particles in SLM and AM processes.
As described by Sutton et al., the effect of particle diameter on SLM has been extensively studied. Smaller particle size usually means better powder packing (higher apparent density) and worse flowability. In contrast, the use of the finer IN718 powder showed lower apparent gravity, tapping density and PBD. After the SLM process, the finer powder results in better surface roughness of the final part, but with increased porosity.Simchi reported that smaller powder particle size or higher surface area during SLM results in better part compaction without agglomeration. The optimal powder particle size depends on other process parameters, since the use of powders larger than the laser spot size and layer thickness often leads to uneven energy distribution, which further affects the behavior of the melt pool.
In addition to particle size, PSD can also significantly affect the SLM process. Liu et al. found that the wider the PSD, the better the surface roughness and part density, and the narrower the PSD, the better the hardness and tensile strength. Determining the optimal particle size of powder and PSD is challenging because fine powders with narrow PSDs cause aggregation, while coarse powders with wider PSDs cause segregation. In addition, many studies have shown that bimodal or multimodal powder distribution increases powder bulk density and detail density. Based on this advantage, Vaesi et al. proposed a bimodal microscale binder spray process to improve the surface quality of a part.
Traditional SLM/SLS typically uses powders with a particle size of 25-50 µm, while the micro-SLS process requires particles much smaller than 10 µm in diameter. Micron and submicron powders have been tested in microSLS systems but have shown limitations in terms of part quality. Regenfus et al. used 0.3 µm powders for a laser microsintering process to obtain the features shown in Figure 7. Fisher et al. used 3.5 µm powders, but the finest detail resolution was about 57 µm. Nanopowders are needed to fabricate submicron elements. However, nanopowders suffer from excessive aggregation and oxidation due to the high surface area to volume ratio. On fig. 10 shows an accumulation of irregularly shaped and finely spherical powder particles. On the nanoscale, van der Waals forces are greater than gravity. Agglomeration increases friction between particles and reduces the fluidity of the powder, resulting in uneven powder layering. Additional effects include spheroidization effects and increased porosity.
In addition to agglomeration, in order to improve the microscopic SLM system, other problems related to fine powder particles must be solved. These questions are as follows:
• The reflectivity of fine powder particles is high, which reduces the absorption rate of laser radiation during the SLM process.
• Nguyen et al. observed that during SLM IN718, an inert gas flow carried away fine powder particles smaller than a few microns in size.
• As can be seen from SLM, fine powder particles can evaporate at very high energy densities, resulting in reduced detail density.
• Another disadvantage is the reactivity of fine powder particles, which requiresadditional safety measures during handling and transportation.
Fig.10 (a) Agglomeration of submicron granular tungsten powder, (b) irregularly shaped copper nanoparticles (average particle size 100 nm), (c) 40 nm spherical copper nanoparticles. (a) By reference, with permission from Emerald Group Publishing Limited, © 2007; (b) and (c) Linked, with permission from Elsevier B.V., © 2018
(4) Re-powder coating system
It has been reported that the main problem with the metal micro SLM/SLS process is that the traditional recoating system cannot effectively deposit the powder on the powder bed. The academic community has always believed that it is necessary to develop a new powder recoating mechanism to uniformly disperse submicron or nanoscale powder. However, as mentioned earlier, nanopowders are prone to excessive aggregation due to the high surface area to volume ratio and the resulting high surface energy. At the nanoscale, van der Waals forces exceed gravity, resulting in inhomogeneous layers of powder during the recoating step of the additive manufacturing process. To achieve effective layering with good powder packing density, micro-SLM requires one or more of the following methods:
• Efficient powder distribution strategy to avoid clogging;
• mechanical separation of agglomerated powders;
• Thermal energy increases packing density (preheat/presinter);
• Efficient dosing (based on suspension) with an additional binder.
In order to develop new powder dispensing strategies with applications not limited to micro-SLMs, it is necessary to understand the existing technologies currently used in conventional SLMs.
1. Current ranking method
Recoating in the powder bed depends on the fluidity of the powder, which is affected by the properties of both the powder itself and the equipment. The flowability must first be increased for better distribution of the powder, and the powder must be distributed intact. Most commercial SLM/SLS systems use a squeegee or roller to reapply the powder coat (Figure 11) as described in Table 2.
Fig. 11 Schematic diagram of the existing AM powder bed raking system. a - scraper; b - roll of direct rotation (VP); c - reverse rotation roll (VR); d - combined VR-VR; e - combined scraper and vibration VR; e - three-roll system; g) Cylindrical raking system with compaction mechanism
The most widely used mechanism is scraper leveling, as shown in fig. 11(a). A scraper is a small piece of metal or ceramic used to scrape the powder off the surface of the powder bed. Since the powder is not fluidized by the paddle spreader, high forces are applied to the previously applied layer.shift. It is expected that the application of ultrasonic vibrations to the blade will reduce these shear stresses.
Rollers are the second most commonly used powder raking equipment. The drum creates a forward rotational movement by translational movement or clockwise rotation on the powder bed, called the forward rotation (FR) drum, as shown in fig. 11(b). This method tends to compact the powder because there is more powder in front of the drum during the movement. But while moving forward, the powder sticks to the drum and forms pits in the powder layer. Rotating the drum in the opposite direction, called counter-rotation (CR), provides better flowability because it pushes the powder up while it is fluidized (Fig. 11(c)). However, powders cannot be compressed using the CR method. Niino and Sato proposed a combined FR and CR tuning, as shown in Fig. 11(d). CR first removes excess powder from the layer, which contributes to better compaction of the powder at FR. Budding and Vanekar replaced the CR with a spatula to get the same scraping effect and reduce processing time. However, these methods still result in pitting in the powder bed. Roy and Cullinan used a scraper and a CR, respectively, to level and compact the powder bed, respectively. In the setup shown in Fig. 11(e), CR vibration was added to compact the powder initially dispersed by the scraper. Haferkamp et al. used a combination of three rolls to provide forward and counter-rotation (Fig. 11(f)), where the layer thickness was controlled by the distance between the rolls. In addition to scrapers, Regenfuss et al. have also used compacting cylinders to disperse and compact fine powders in micropowder bed processes. A schematic diagram of the powder raking system is shown in fig. 11(g). In this setup, the build substrate, the melted area and the remaining powder under the new powder layer are lifted up to the manual lid to compact the powder. Table 3 compares literature descriptions of various powder raking systems.
Table 3. Comparison of powder raking systems
Existing raking systems are effective for traditional SLM processes where small inaccuracies in powder dispersion are negligible. However, on a microscopic scale, such problems can lead to large deviations in the dimensions of manufactured parts. This situation is exacerbated by the use of micropowders for microscopic SLM. While efforts have been made to improve raking methods, these methods lack the accuracy needed for micro-SLM. Existing recoating methods cannot provide a uniform dense layer of fine powder on a powder bed. The interaction between the fine powder particles and the rake components greatly affects the spreading efficiency of the powder.
A review of the literature showed that the systemThe raking bins not only spread the powder across the powder bed, but also provide better packing density. Therefore, efficient powder re-coating systems are needed to control the layer thickness with sub-micron or nanometer precision while ensuring that the powder is evenly distributed along the powder layer.
2. Dry Powder Distribution
To overcome the problems existing in the existing powder distribution system, Vaesi et al. proposed the use of dry powder distribution technology, especially for the microscopic PBF process. Mechanical dry powder distribution methods include pneumatic, volumetric, and auger/screw methods, which have low feed rates and cannot handle fine powders. The spatial resolution of these methods is at least two orders of magnitude lower than that required for micro-SPM.
More and more attention is being paid to the vibration method in the field of fine powder feeding. These methods use vibrational behavior to increase free volume and thus improve particle displacement. The shaking method also destroys particle agglomerates. Matsusaka was the first to use the vibration of a vertical capillary [as shown in Fig. 12(a)] to control the flow of fine alumina powder with a particle size of 20 μm and an irregular shape. Due to cohesion, fine powders cannot pass completely through the capillary due to gravity. When vibration is induced on the capillary by an alternating direct current (DC) motor, it propagates into the powder, resulting in reduced friction between the tube wall and the powder. Both amplitude and frequency of vibration are key parameters that affect the flow rate. The powder flow rate is directly proportional to the vibration frequency but inversely proportional to the amplitude. The research team used an ultrasonic transducer to vibrate the capillary. Jan and Evans [shown in fig. 12(b)] developed a similar setup for printing polygonal tungsten carbide powder particles using 12 µm particles on a substrate. used ultrasonic vibrations generated by piezoelectric transducers to feed copper and stainless steel powders with a size of 3 μm. A thin layer of powder near the inner wall behaves like a lubricant due to microvibrations at ultrasonic frequencies. Because ultrasonic waves propagate through capillary tubes, the advantages of ultrasonic powder feeding lie in its ability to prevent powder aggregation and ensure continuous and uniform powder feeding. Young and Evans developed the system shown in Fig. 12(c) for mixing and depositing multiple materials using separate powder bins and mixing bins where flow rates are controlled by acoustic vibrations. These research efforts have demonstrated the capabilities of ultrasonic-based microflows.sound that can be integrated with lasers and used in conventional AM systems.
Fig. 12 Schematic diagram of a vibrating dry powder dosing system. (a) Vibration using a DC motor, (b) Vibration using an ultrasonic source, (c) Multiple powder distribution system using AM acoustic powder bed. (a) From link, with permission of the Japan Society of Powder Technology, © 1996 (b) From link, with permission of Elsevier B.V., © 2004 (c) From link, with permission of the Japan Society of Powder Technology, © 2007 < /p>
Another promising powder feed mechanism for AM is electrostatic separation. Electrostatic coating and spraying are widely used in industrial coatings and construction. It has been used for dry coating of pharmaceutical tablets, as detailed in a recent review by Yang et al. The method works on the principle of electrostatic attraction between opposite charges. As shown in fig. 13(a), powder particles are charged when subjected to a strong electric field. Negatively charged particles are attracted to a substrate that is positively charged or grounded. In electrostatic spraying, as the powder particles pass through the atomizer, the powder is charged and deposited on a grounded substrate. Compared to other dry coating methods, electrostatic coating greatly improves coating efficiency and adhesion due to electrical attraction.
Fig. 13 Schematic diagram of an electrostatic dosing system for dry powder. (a) electrostatic spraying; (b) SLM powder distribution based on electrophotography; (c) electrostatic compaction of the powder; (d) electrostatic distribution of the powder by the AM powder bed. (a) By reference with permission from the China Particle Society and the Institute of Technological Design of the Chinese Academy of Sciences, © 2016; (b) From link, © 2018 with permission from Liberty Fabrication Lab and the University of Texas at Austin; (c) From reference. link; (d) from links
Electrophotography is another common application using electrostatic techniques, in which photo paper is printed with toner particles. In electrophotography, a light-sensitive photoconductor is first charged with a high-voltage corona, and then its surface is selectively discharged by a light source to produce a latent image on the photoconductor. Charged toner particles are deposited on a photoconductor, which is then transferred to paper. Based on electrophotography, Liu et al. developed a secondary powder deposition system for the fabrication of several materials using SLS. In a simple experimental setup, a Teflon scraper is used to separate the negatively charged toner, which is then applied to the positively charged paper. Kumar and Zhang developed electrophotography-based powder deposition methods such as SLM/SLS for powder bed technology.i, which can also be used to spray the binder. The schematic diagram of their installation is similar to the diagram of the electrophotographic process, as shown in fig. 13(b). Polystyrene powder with a particle size of 5 µm was applied to an aluminum building platform and fused with heated rolls to obtain parts with a thickness of 1 mm. In this technology, the layer thickness is controlled by parameters such as the speed of the photoconductive tape, the charge per unit mass, and the speed of the developer roller. Thomas et al. also developed an electrophotographic-based powder deposition method for the SLM process. Their setup demonstrated good transfer of polymer powder from the loading plate to the substrate. Both studies presented multi-material powder deposition using electrophotography and found that the deposition efficiency depends on the potential and distance between the charging plate and the substrate. Despite the initial formation of a homogeneous single-layer powder on the substrate, it is difficult to control the stacking of other layers required for SLM in electrophotographic-based deposition. In order to achieve powder deposition in a typical SLM process, they proposed two methods for maintaining a constant potential between the photoconductor and the substrate or surface of the cured part: ① removing the residual charge from the molten layer; ② using a corona device Additional charge to increase the charge density.
Melvin and Beeman developed a screen feed system for use in SLS. Unlike electrophotography, screen-feed systems work by discharging static charge. In sieve systems, the powder is pressed against the powder bed by charged or crushed sieves and leveling is done by scrapers or rollers. After sintering polycarbonate powder using a sieve feeding system, the strength of the components is increased by 3-4 times, and the density of the parts is increased by 10-15% compared to feeding by rollers. It was observed that the experimental results were associated with a corresponding increase in PBD, which was caused by the removal of the electrostatic charge from the powder passing through the sieve. However, it is difficult to achieve precise layering and uniform coating thickness with this system. The same researchers developed a method for recoating SLS powders based on electrostatic coatings. Although electrostatic powder application provides better dispersion than rolls, the sintered body still has more porosity.
In a recent Applied Materials Inc. patent. an electrostatic charge is used to compact the diffusion powder layer with the substrate or pre-sintered part, as shown in fig. 13(c). Electrostatic sealing is used when the potential drop across the gap between the electrode and the fresh starting powder layer is greater than the potential drop across the sintered and fresh starting material layers. Plasma generated by gas flowohm, can also be used to increase the compaction force. In this case, most of the potential drop is due to any previously deposited layers and the layer of fresh starting material. Paasche et al. developed the concept of an AM powder bed system using electrostatic powder deposition as shown in Fig. 13(d). In their setup, a positively charged substrate collects powder from a negatively charged powder container with an applied voltage. After applying the powder, the substrate is moved towards the laser beam for subsequent melting. This process is repeated until the entire part is made.
The implementation of this system may have the following problems: ① positioning the substrate at the focus of laser radiation and the specific location of powder application for each layer is time-consuming and may lead to errors; ② passing the substrate between each layer may lead to inaccurate positioning and misalignment of parts; ③ It may be difficult to dispose of the captured powder before displacement. In addition, the system may still lack the ability to achieve further tiering.
While vibratory and electrostatic powder distribution has proven to be useful for precise and selective layering in powder bed processes, these methods have certain limitations:
(1) Powder distribution through nozzle-based systems is highly dependent on the process fluid, and clogged nozzles reduce powder delivery reliability.
(2) Dry powder dosing systems have longer settling times than traditional powder recoating methods. The cycle time of the powder bed process will be increased when AM has already solved the problem of higher cycle times compared to the conventional manufacturing process.
(5) Powder layer characteristic
For micron-scale SLM, the application of a thin layer of powder is an important step, as it greatly affects part resolution, surface finish, porosity, microstructure, and mechanical properties. According to Liu et al., PBD has a significant effect on the density of manufactured parts in SLS. It is worth noting that there are currently no process variables that can be used to compare different powder dosing methods. If present, then by the density of sintered or fused parts. SLM is affected by many process parameters, so it is difficult to isolate the effect of powder bed properties when comparing final results. This section details PBB as it is an important factor influencing microscale powder bed systems.
Powder filling during powder bed processing affects the density of the part. However, there is no standard method for determining the density of powder layers. developed a method for determining the density of the powder layer for inkjet printing binder. First, the powder is deposited on the powder bedusing CR. Next, a jet of glue is applied along the contour of the cup, leaving loose powder in the cavity. After printing, remove the cup and measure the loose powder weight. Since the weight and volume of the cup are known, the PBD can be calculated. A similar approach was taken by Liu et al. For SLM, PBD is measured by melting the walls of a square vessel. In two studies, PBD was found to fall between the apparent and shaken density of the powders. Gu et al. developed a method for calculating PBD without binder or sintering along the disc. A stainless steel disk with a diameter of 60 mm was placed on the building platform of the sintering machine, and three layers of powder 0.03 mm thick, with a total height of 0.09 mm were applied on it, so that the powder volume could be determined. The disc was then removed from the reference plate and weighed with and without powder, the difference being the weight of three layers of powder. Calculate PBD using mass and volume. From the results, no correlation was observed between powder flow (angle of repose) and PBD. In the experiments of Zocca et al. The density of the powder layer was determined by weighing the powder after applying 50 layers of powder (each 100 µm thick) to the build platform of the printer and dividing the mass by the resulting geometric volume. .5. Surface finishing and mixing
The surface roughness of parts made with SLM typically exceeds 10 µm, so post-processing is still required. Although many efforts have been made to obtain a smooth surface with a roughness of less than 1 µm, secondary processing of microscopic additive parts is still unavoidable. This section first looks at typical surface treatment methods for additive manufacturing components and the possibilities of these methods. The following discussion briefly discusses whether these methods can be applied to SLM microscopic parts, i.e. whether SLM parts can be post-processed separately or integrated with SLM microscopic parts to form a hybrid system.
Table 4 compares some common surface treatment methods for additive manufacturing components. Traditional subtractive machining is often used to improve the surface finish of near-clean shaped parts made by additive manufacturing. Simple mechanical grinding and/or polishing, which is usually not up to the standards required for high quality parts, may be sufficient for some applications.
Table 4 Comparison of surface treatment technologies for AM parts
CNC: computer numerical control; CHE: chemical etching; ECP: electrochemical polishing.
Chemical and electrochemical polishing (ECP) is more suitable for complex elements than traditional processing methods. Pika et al. used chemical etching (CHE) and ECE to study the open pore structure of a titanium alloy substrate. It was found that CE mainly removes attached pore particles.scale, and ECP additionally reduces roughness. Alrbey et al. used ECP to reduce the roughness of SS 316L made by SLM from 10~17.5 µm to 0.5 µm. Electrolytic polishing of the Ti6Al4V samples prepared by the ELP method made it possible to reduce the surface roughness from 23 µm to 6 µm. The study observed an inconsistent loss of shape and polish accuracy across regions and over time. In addition to the associated environmental concerns, ECP is susceptible to material corrosion, which can also lead to deviations in dimensional accuracy.
Laser polishing or laser remelting has become a potentially cost-effective surface treatment process for SLM and can use the same laser source as AM. After laser remelting SLM-made steel SS 316L with an initial roughness of 12 µm, Yasa et al. obtained a final surface roughness of 1.5 µm and no cracks or pores in the heat-affected zone. Laser polishing AISI 420 additively fabricated stainless steel with bronze infiltration reduces the surface roughness (Ra) from 7.5~7.8um to less than 1.49um. Ma et al. observed that the surface roughness of titanium-based alloys decreased from 5 µm to less than 1 µm. Marimutu and others. reduced the roughness of the fabricated SLM Ti6Al4V from 10.2 µm to 2.4 µm without the formation of α-shells or thermal cracks. Although laser polishing of AM parts is possible, this method is limited to flat surfaces and external features.
In addition, surface remelting also affects the surface chemistry and residual thermal stress. Abrasive blasting, commonly referred to as sandblasting, is widely used in industry for surface cleaning, engraving and deburring. Sand, abrasives and nutshells are used as the blasting medium, driven by compressed air or liquid. De Wild et al. used sandblasting to finish porous orthopedic titanium implants made by SLM. The surface roughness (Sa) of the implants after diamond blasting decreased from 3.33 µm to 0.94 µm. used polycrystalline particles of tetragonal yttrium zirconium (Y-TZP) for sandblasting the surface of Y-TZP to a roughness of 1.7 μm. Klotz et al. used sandblasting with corundum and glass beads to polish gold alloys made by the SLM method with an initial roughness of 12.9–4.2 µm. Sandblasting is also used to improve the aesthetic appearance of maraging steel produced by the SLM process. Qu et al. reported that the surface roughness of EDM-machined WC-Co rough parts was significantly improved by sandblasting, with the average surface roughness (Ra) reduced from 1.3 µm to 0.7 µm.
Table 5 summarizes the effect of various sandblasting treatments on the final surface quality of various materials. Can be done inThe conclusion is that sandblasting can effectively reduce surface roughness by 50-70% with a minimum Ra value of less than 1 µm. Although the reproducibility of the abrasive blasting process is limited, it is often used for micro-components. Because it has advantages in terms of process simplicity, flexibility, cycle time and cost.
Table 5. Comparison of the effect of different sandblasting conditions on surface finish
HSS: high speed steel; CVD: chemical vapor deposition; NA: No data available.
In order to meet the demanding surface treatment requirements, some new and different techniques have been applied to complex AM components. Tang and Yeo have developed a new technology for additive components - ultrasonic cavitation-abrasive processing. In this method, cavitation bubbles created by ultrasonic pressure waves in a liquid medium remove some of the molten powder. The collapse of the cavitation bubbles causes a shock wave that spreads the abrasive particles over the surface of the sample, removing the material. The surface roughness of the IN625 receiving substrate has been reduced from 6.5~7.5µm to 3.7µm. used abrasive blasting (AFM) to process SLM components. AFM is a well-known finishing method in which a semi-hard abrasive is moved across a surface. After AFM, the surface quality of the aluminum alloy produced by the SLM method improved significantly, and the surface roughness decreased from 14 µm to 0.94 µm. Magnetic Abrasive Polishing (MAF) can reduce the surface roughness of 316L stainless steel internal channels from 0.6 µm to 0.01 µm by generating an abrasive force from the magnetic force acting on the magnetic abrasive. Guo et al. have developed a modified version of MAF Vibratory Magnetic Abrasive Polishing (VAMAP) to modify microchannels and grooves. This method provides a surface finish of the microgroove expansion from 2.2 µm to 0.3 µm.
Complex grinding methods such as vibratory grinding and barrel polishing are detail-based. AM components use the principle of sliding between surfaces and abrasive particles. Vibratory grinding was applied to the fabricated SLM Ti6Al4V with an average roughness of 17.9 µm, resulting in a final roughness of 0.9 µm. However, vibration grinding results in a large number of rough grooves on the surface. Boschetto et al. used barrel polishing (removal of material by tilting the rotating barrel) to complete production of SLM-made Ti6Al4V. This method significantly reduces the surface roughness of the SLM samples (from 13.3 µm to 0.2 µm within 48 hours of treatment). Although this technology has good surface finishing performance and process simplicity, it has the disadvantage of being labor intensive.
To aboutTo determine an appropriate surface finish for microscopic SLM components from the set of available methods discussed earlier, many factors must be considered, including the initial roughness of fabricated features, part size, geometry, minimum feature size resolution, process complexity, cycle. time, etc. The dimensions of SLM microscopic components are typically on the millimeter scale, and the resolution of the smallest element is in the range of a few micrometers (Table 1). The acceptability of the methods used for microscopic SLM components is shown in Table 4. While bulk grinding methods can provide good surface finish, microscale characteristics can be compromised in the process. It is possible to machine microscopic SLM parts with numerical control (CNC), but micromachining and toolpath control in complex geometries is difficult. In particular, precision machining of thin walls and precision machining of internal parts and features with a large aspect ratio are difficult and time consuming. CHE and ECP typically require flat surfaces and material erosion at the edges, which can lead to large dimensional errors in small parts. Abrasive blasting is often used on small parts in many industries such as dentistry and jewelry making, so it can be an ideal choice. Micro-abrasive blasting is one of the most commonly used surface treatments in a number of medical applications, for example to obtain the surface needed to support osseointegrated dental implants. Kennedy et al. used ceramic balls for micromachining high speed steel (HSS) and coated carbides, which reduced surface roughness by 60%, with Ra of the smallest surface being 0.4 µm. Laser polishing is another suitable option, although the thermal stresses caused by remelting can cause the part to deform, especially residual thermal stresses are very strong for weak parts.
Hybrid manufacturing systems integrate AM with subtractive or other auxiliary systems to increase the productivity and customization capabilities of machine systems. In hybrid additive machining systems, the laser deposition head (in the case of LMD) is mounted on the z-axis of the milling machine, and then the laser system and the CNC milling machine are integrated. Overall, the design of the system should improve the performance of the structure, the accuracy and surface finish of the structures with minimal post-processing. In the case of Powder Melt Additive Manufacturing (PBF-AM), few hybrid systems are available other than Sodick OPM250E and Matsuura LUMEX Avance-25, although component surface quality after PBF-AM has been problematic. Although machining accuracy is greatly improved with powder bed additive machining, a hybrid system includinging additive and subtractive processing, has not been developed for the fabrication of metal materials at the microscale. Compared to the precision machining processes listed in Table 4, laser remelting or laser polishing combined with micro-SLM to develop a hybrid system seems to be the most viable option. The same laser source or a different laser source can be used in existing SLM systems. However, it should be recognized that each precision machining technology has its advantages and limitations, and the choice of the ideal surface treatment process depends on the initial conditions and finishing requirements of the SLM parts. Therefore, the ability of SLM technology to produce elements with fine surface finishes must be improved to eliminate the need for any secondary processing.6. Possible applications
Microscopic AM (especially microscopic SLM) is used in the production of precision devices and components in various fields. Microfluidic devices find applications in cell biology, biomedicine, and clinical diagnostics. This article attempted direct type AM microfluidic devices, but it was found that the performance of this method is much lower than typical injection molding processes. The most common methods for fabricating microfluidic devices are injection molding and thermal compression. These methods require a master mold or tool insert to reproduce the elements on the substrate. Microfluidics master molds are typically made using lithography, coating and shaping (LIGA) and LIGA-like processes. However, these methods are limited by materials and design. It is also a method of using electroforming nickel to make a metal master mold, but the hardness of the resulting mold is insufficient, and the strength of the micro mold needs to be improved. Sophisticated additive manufacturing techniques for making metal microforms can increase tool life and thus increase productivity. The same technique can be used to produce high aspect ratio microstructures, which are increasingly being used in MEMS. Roy et al. used the micro-SLS process to fabricate electrical connectors and dielectrics to assemble integrated circuit (IC) components. Two flexible substrates are connected by printing silver electrodes and silver connectors on prefabricated tracks.
Microscopic AM can also be used in dentistry. Currently, in addition to the most common stereolithography and digital light projection (DLP), SLM and SLS are also used in dentistry. Dental bridges and crowns, dental implants, partial dentures and model castings are some of the potential applications of micro-AM in the dentalindustry.
Over the past decade, the jewelry industry has been experimenting with the use of additive machining to process jewelry. This area is constantly evolving, as almost all major additive manufacturing manufacturers are increasingly using additive manufacturing for the processing of precious metals: alloys such as gold, platinum and palladium. In addition to some of the usual advantages of additive manufacturing, such as near-net shape production, reduced material waste, and higher overall process cycle speed for small batches, the ability of microscopic additive manufacturing to produce thin walls, filaments, meshes, and lightweight parts can improve designs, degree of freedom, and beauty is the specific attraction of the jewelry industry. Numerous studies by jewelry manufacturers have shown that despite existing limitations, SLM will co-exist with traditional casting for cost savings and design versatility.
Hirth and others envision the possibility of directly printing devices and sensors on existing technologies in the aerospace, automotive, medical and optical industries. Components with micron or nanometer resolution facilitate the creation of controlled microstructures. Precise microstructure control is used to improve the mechanical strength and tribological properties of additive manufacturing components.7. Conclusion
This article systematically discusses the application of SLM technology to implement microscale functions on metallic materials. Micro SLM differs from traditional SLM in three factors: laser spot size, powder particle size, and layer thickness. Existing research on microscopic SLM has successfully demonstrated the ability to fabricate microscopically resolved elements on a variety of materials, including polymers, ceramics, and metals. Current microscopic SLM systems have a minimum feature resolution of 15 µm, a minimum surface roughness of 1 µm, and a maximum detail density of 99.3%. Given the limited academic research in this area, it is surprising that there are already some commercial SLM microscopic systems on the market.
Commercial systems can achieve a minimum spot size of 20 um and a maximum layer thickness of 1 um. The major drawback of the literature is the inability to examine the physical properties and microstructures of fabricated parts. This makes it difficult to compare SLM processes at different scales.
The SLM system needs to be modified in order to create microscopic SLM technology. This includes optical system adjustment, powder distribution and powder re-coating, as well as molding step control. The limiting factor is currently the powder properties and powder recoating system. These are key factors to obtaining uniform and thin powder layers. The literature has shown that most current powder coating recoating techniques are done using blades or rollers. These are not suitable for handling fine powders. This document discusses several dry powder dosing options for powder bed AM systems. Among the IM methods that have been tested and implemented, electrostatic and vibratory powder distribution methods were used. The fastest coating cycle times are reported to be achieved by electrostatic technology. A micro-SLM strategy that is effective involves integrating all subsystems, such as powder collection, powder distribution and sifting and creating a closed loop feedback mechanism.
This article will also discuss the surface treatment technology for SLM components. Although most micro-SLM processes can achieve a surface finish of less than one um, there are many factors that influence the choice of the right micro-SLM process. These include part geometry, feature resolution and finishing requirements. Sandblasting is a popular method of finishing micro-parts, according to the literature. Laser polishing is a more common secondary finishing method for microscopic SLM than any other mixed processing methods.
SLM/SLS are not the only factors that limit micro-AM's use. It is also difficult to model. These issues prompted the development of new systems that use new powder materials. Configuration and post-processing.
Two aspects will be key to the future of micro-SLM: process-related and equipment-related. The system should be able to handle metal powders of nano size that are prone to agglomeration. It is important to develop a powder recoating system that produces a uniform sub-micron layer of powder without slowing down recoating speed. More research is required to better understand the interactions between nanosized powder particles, laser beams, and other process information. Given the lack of research on micro-SLM components, it is necessary to further understand their microstructures and mechanical properties. SLM's potential for further development will be expanded as a result of the increasing use of metal particles in precision engineering, biomedmedicine and dentistry, and possibly even jewelry.