Studies on cold solid state joining of dissimilar powder metallurgical preforms

The present work examines the feasibility of ensuring solid state joining of two dissimilar powder metallurgical (P/M) parts, viz. electrolytically annealed copper powder and steel powder preforms. As the present work was a feasibility study, a simple compression test was chosen to document the inferences, related to the influence of: (a) volume ratio (copper to steel), (b) density ratio (steel to copper), and (c) strain, in achieving a good joint. The copper P/M preform was press-fitted into a steel P/M preform and compressed between two parallel platens to various strains. The joints were then subjected to the tear test evolved exclusively in the present investigation for inferring the weld strength, both at pre-sintering and post-sintering conditions of these joints. Supporting evidences in terms of metallography (optical, SEM) were obtained to verify the joint. Results revealed that a volume ratio (copper:steel) of 1:4 subjected to a cold plastic deformation ratio of ε=0.6 (with flow stress ratio≈1.0) and with a steel to copper density ratio of 0.98 ensured a very good mechanical bonding substantiated by large interfacial area, reflecting a weld strength ratio of 0.89 (weld strength ratio=weld strength/shear strength of copper P/M). From the fundamental studies carried out it could be concluded that lower the volume ratio of copper to steel, lower the density ratio of steel to copper and optimal the plastic deformation, it should be possible to get a sound joint of sintered P/M parts of electrolytically annealed steel and copper powder after post-sintering.