Metal Injection Molding of Multi-Functional Materials

Abstract

Metal injection molding (MIM) enables processing of multi-functional components with combinations of properties including, magnetic and non-magnetic; magnetic response and corrosion resistance; controlled porosity and high thermal conductivity; high inertial weight and high strength; high thermal conductivity and low thermal expansion coefficient; wear resistance and high toughness; high thermal conductivity and good glass-to-metal sealing; high elastic modulus and high damping capacity; and magnetic response and electrical resistance. Such materials can be processed by MIM by co-injection molding and co-sintering, but compositions and sintering cycles must be optimized to minimize stresses arising from shrinkage mismatch while providing the desired properties To determine compatible combinations, individual materials, including various stainless steels and tool steels are mixed with a thermoplastic binder and injection molded. Debound components are subjected to dilatometry to determine dimensional change during sintering. The compatibility of these materials is predicted based on calculations of the thermal stress during co-sintering of concentric rings. For this geometry, shrinkage mismatches result in both radial stresses, which are the highest at the interface and lead to interfacial separation, as well as hoop stresses, which lead to radial cracking. These stresses are dependent on the thicknesses of the inner and outer rings. Defect-free components can be produced when the tensile hoop stresses do not exceed the intrinsic strengths of the component materials. Based on the dilatometry results, low stresses are predicted between a combination of magnetic and non-magnetic stainless steel and between a combination of austenitic stainless steel and tool steel. Example MIM bi-material components are processed from these combinations. The magnetic properties of the tough/wear-resistant bi-material are measured. General predictions and processing guidelines for other multi-functional materials are given.