In the field of metal 3D printing, due to the different melting points of various rare metals, many material manufacturers are trying to mix more metals to produce materials that can be processed and applied. Scientists at Oak Ridge National Laboratory (ORNL) have developed a new molybdenum (Mo) compound that has been specifically optimized for use in electron beam melting (EBM) 3D printers. The results show that the new material can withstand extreme temperatures, making it ideal for aerospace applications.
3D printed SEM image of "Mighty Mo" material by the ORNL team
As a refractory metal, molybdenum has a number of properties that make it an attractive option for use in ultra-high temperature sensitive applications. The alloy is characterized by a high melting point up to 2622 o C, low coefficient of thermal expansion, thermal conductivity and corrosion resistance, but at certain temperatures has poor impact strength.
In addition, molybdenum is very sensitive to nitrogen and oxygen contamination during processing, which can lead to grain boundary segregation and cracking of parts. In the limited research done in this area, scientists have mixed metals with other materials in an attempt to better control their recrystallization and grain size, with little success.
In 2017, researchers at the Austrian powder manufacturer Plansee Group were able to use simulation data to quantify how molybdenum particle size affects SLM printability, but failed to completely solve the problem. In contrast, the ORNL team has now discovered that by adding TiC particles to an alloy and converting to EBM, microstructures with higher levels of strength and stiffness can be created.
The ORNL team uses a special Arcam 3D printer for research
To create the material, the scientists mixed Mo and TiC powders in a ratio of 60:40 in a graduated cylinder filled with argon to prevent oxidation. The resulting metal matrix composite was then subjected to mechanical alloying using a planetary ball mill for 8 hours until it was ready for 3D printing.
To work with the new powder, the ORNL team has developed a special Arcam S12 EBM 3D printer with an improved print chamber, consisting of a piston, feeder, comb and powder bed feed system. The machine upgrade has effectively streamlined small batch production with enhanced process monitoring and secondary feed.
Using their machine, the researchers decided to 3D print six 12mm (D) by 13mm (H) parts that have a sandwich structure consisting of a layer of molybdenum wrapped between two layers of pure molybdenum. strong molybdenum. Interestingly, the SEM image showed that none of the clean samples had cracks, but they did have some surface irregularities due to powder spreading.
The team later ran thermodynamic simulations, which also showed that the process remained extremely sensitive to changes in composition and temperature. As a result, ORNL scientists suggest that tight control of process inputs will be key to fabricating future microstructures using molybdenum without causing changes in part layer consistency or temperature gradients during processing.
Finally, the researchers also concluded that they have demonstrated the feasibility of 3D printing pure molybdenum without cracks, and with perfect parameter tuning, the alloy can be found in fields such as aerospace or energy conversion, such as in heat transfer components, a new application.