B. Vamsi Krishna

Studies on cold solid state joining of dissimilar powder metallurgical preforms

Abstract 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 e=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.

Inferences on plastic properties and coefficient of friction during simultaneous compression deformation of dissimilar sintered powder metallurgical preforms

Abstract Plastic working of powder metallurgical (PM) material necessitates the development of fundamental data such as flow stress, densification behaviour, coefficient of friction, apparent strength coefficient, apparent strain hardening exponent, plastic Poissons ratio, etc. In the present work compression and standard ring compression tests have been carried out to generate the fundamental data for simultaneous deformation of sintered steel and copper powder metallurgical preforms. The results reveal that the behaviour of individual materials during simultaneous deformation is strongly influenced by local micromechanical interactions at the metal - metal interface. In addition to this, the test conditions (iso-stress and iso-strain) strongly influence the severity of interaction. The interfacial friction coefficients are less than that of the same material when tested between hard tools. The optimal process parameters with higher interfacial friction, which can enhance the solid state joining of dissimilar materials, have been identified. The flow stress of the composite (steel - copper combination) during simultaneous deformation can be estimated if the flow stress of the individual materials comprising the combination/composite are known. With these studies, it should be possible to extend the inferences to the major deformation processes.

Use of powder metallurgy preforms as alternative to produce bimetallic tubes

Abstract Coextrusion is one of the major processes used to produce bimetallic rods, tubes and wires. Previous investigations into the coextrusion of wrought metals have shown that, for the production of a sound coextrusion, the available processing zone is very narrow, with little flexibility in terms of tool and process parameter selection. These limits have many practical implications in the selection of suitable material and process parameter combinations. The present investigation envisages the production of sound bimetallic tubes by controlling the initial materials characteristics through the powder metallurgy (PM) and cold extrusion route. It is shown that the consolidation of PM preforms during extrusion reduces the differential velocity between the core and the sleeve, and high friction at the interface aids sound flow over a wide range of processing conditions. In addition, the non-uniformities of deformation (especially on the hard metal side) are accommodated by the softer preform near the interface, through micromechanical interaction. The present approach implies manufacturing flexibility and reduction of tool/amortisation costs for the production of bimetals.