Surface hardening of MIM porous metals by ultrasonic vibration assisted pressing

Metal powder injection molding (MIM) has attracted attention as a cost-effective manufacturing method for serial producing complex designed micro metal parts. However, unavoidable micropores are observed in MIM parts that lead to poor bending strength, especially in small-sized products. This disadvantage is significant in the medical field. Ultrasonic vibration assisted surface hardening can be a solution. This surface strengthening method uses an ultrasonically vibrating punch to work-harden the surface, enabling localised hardening in a short time without using highly toxic chemicals or large-scale equipment. In addition, porous metals have the properties of absorbing sound and shock, which allows them to efficiently absorb the energy of ultrasonic vibrations and effectively harden the surface. In this study, we verified the effect of the ultrasonic vibration assisted surface hardening, the influence of the porous structure, and the mechanism of surface strengthening. As a specimen, MIM processed pure copper and stainless steel 316L specimens were used to compare with specimens without porous structure through investigations such as surface hardness test, electron backscatter diffraction analysis, and surface roughness measurement. After hardening, it was confirmed that the MIM specimens had an obviously increased surface hardness and a significantly reduced surface roughness compared to the specimens without porous structure. In addition, the hardening caused a increase in strain near the surface and around the pore walls inside the material. From these results, it is supposed that the pores inside the material absorbed the vibration energy and deformed slightly, and the strain on the surface increased due to the impact effect. This experiment demonstrated that ultrasonic vibration assisted surface hardening can effectively improve the surface strength and roughness of porous metals and can be used as a method that does not impair the original ductility by absorbing vibration energy through pores.