The role of pore wall microstructure and micropores on the mechanical properties of Cu-Ni-Mo based steel foams

Mechanical behaviour of metal foams is not only affected by the macrostructure of the foam, i.e. porosity content, shape and size of the macropores, but also depends on the properties of pore wall. This study primarily concerns the role of pore wall microstructure and micropores formed in the pore wall which influences the mechanical behavior of steel foams. Steel foams having rather similar macropore structure but differences in pore wall microstructure and porosity content were produced by the space holder-water leaching technique in powder metallurgy. Pre-alloyed steel powders with compositions of 1.75 Ni-1.5 Cu-0.5 Mo-0.2 C, 4.0 Ni-1.5 Cu-0.5 Mo-0.2 C and 1.5 Mo-0.2 C as the parent material and carbamide as space holder material were used for the produced steel foams. The structural and mechanical property variations resulting from the use of spacer having different amounts and the various alloy elements in the chemical composition of powders were investigated. The formation mechanism of macro and micropores was discussed in terms of mechanical properties of the foams. The results showed that the size and amount of macropores increased with increasing spacer content, but the size and amount of micropores decreased. An increase in content of spacer improved the densification during compaction of powder mixtures by the plastic deformation of soft spacer particles. In addition, the copper in alloys reduced micropores in the pore walls. Compression tests were carried out to investigate the deformation mechanism and energy absorbing characteristic of the steel foams. The alloy that contained high nickel resulted in better compressive behavior. The strength and energy absorption capacity of the foams increased with a decrease in level of porosity.