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. © 2002 Elsevier Science B.V. All rights reserved.

D. Durgalakshmi

P. Venugopal

D. R.Gopalakrishna Achar

Annealing

Compression testing

COPPER POWDER

Joints (structural components)

Preforming

Shear strength

Sintering

Welding

SOLID STATE JOINING

Powder metallurgy

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IIT Madras

Studies on cold solid state joining of dissimilar powder metallurgical preforms

Journal of Materials Processing Technology

The present article reports the results of preliminary studies, carried out for the first time, related to the production of bimetallic tubes by tailoring the initial materials characteristics through powder metallurgy (P/M) and cold extrusion route. It is shown that the domination of consolidation (volume change) during simultaneous deformation of P/M preforms reduces the differential velocity between the core and the sleeve as indicated by lower failure propensity of extrudes. High interfacial friction at the dissimilar P/M preform interface and in turn the high interfacial bond aids the sound flow over a wide range of processing conditions. In addition, the non-uniformities of deformation would be accommodated by the softer preform near the interface through micro-mechanical interactions. It is believed that the current approach of producing the bimetals greatly enhances the manufacturing flexibility and reduces the tool cost. © 2005 Elsevier B.V. All rights reserved.

Materials Science and Engineering A

Co-extrusion of dissimilar sintered P/M preforms - An explored route to produce bimetallic tubes

Solid state joints made between Cu and Al powder metallurgical preforms by cold extrusion were heat treated at various temperatures and for various times with and without an interlayer (Ni) to evaluate the interfacial microstructure and weld strength. The heat treatment conditions were optimised for highest weld strength without deleterious effects In terms of intermetallic compounds, Kirkendall voids, etc. Heat treatment of these joints made without an interlayer resulted in a 40% increase In the strength. However, under some processing conditions different intermetallic compounds and phases were formed during heat treatment without an Interlayer. Application of an interlayer in the form of powder allowed partial interdiffusion of metal at the interface and this minor interdiffusion had no influence on the weld strength and microstructure. Finer interlayer particle size increased the weld strength by eliminating physical discontinuities and residual particles at the Interface. The heat treatment of Cu-Al joints made using 1 μm Ni powder as the interlayer increased the strength by 82% In comparison with that in the as extruded condition. © 2005 Institute of Materials, Minerals and Mining.

Science and Technology of Welding and Joining

Effect of heat treatment and interlayer on weld strength and microstructure of solid state joints between Cu and Al powder metallurgical preform tubes

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. © 2005 Institute of Materials, Minerals and Mining.

Materials Science and Technology

Use of powder metallurgy preforms as alternative to produce bimetallic tubes

The present investigation describes about producing a sound transition joint between two dissimilar metal tubes by tailoring the initial materials characteristics through powder metallurgy (P/M) and cold extrusion route. An attempt has been made to analyse the influence of process parameters such as density ratio, volume ratio, interfacial angle and strain on successful solid state joining of steel-Cu and Cu-Al P/M preform tubes. The correlation between the process parameters and failure propensity, weld strength, extrusion force, change in interfacial angle, strain in each material and microstructural features of the joints is also presented. © 2004 Elsevier B.V. All rights reserved.

Solid state joining of dissimilar sintered P/M preform tubes by simultaneous cold extrusion

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 Poisson's 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.

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

Mathematical expressions have been derived for relating the values of apparent strength coefficient Ka (in N/mm2) and strain hardening exponent na to preform density ρ{variant}0, based on axial ring compression tests. These equations are consulted for prediction of cold hooker extrusion forces for P/M preforms of various starting preform densities, ρ{variant}0. Experimental values of extrusion forces are compared with those predicted. Such analysis demonstrates the usefulness of the generated Ka, na against ρ{variant}0 equations to avoid elaborate experimentation on the extrusion. Besides, the analysis suggests that the void closure is persistent till the theoretical density and if corrections are applied for force, theory and experiment agree well. © 1988.

Journal of Mechanical Working Technology

Some uses of na and Ka in the prediction of cold extrusion forces of sintered powder metallurgical preforms

In forming calculations (such as in the estimation of force) in the selection of the machine, the capacity of the motor, the tool and the tool material, and in the estimation of the manufacturing tolerances of formed parts, basic data describing the flow properties of the work metal in the form Y = K · ε{lunate}n are of great value. The values of K and n are different (from those of wrought parts), when extended to sintered P/M parts on account of matrix hardening and densification and are commonly known as the apparent strength-coefficient, Ka, and the apparent strain-hardening exponent, na. Compacts with wide variation in Ka and na with respect to equivalent wrought parts would give rise to difficulties while forming. It is to be expected that compacts prepared at an appropriate green compacting load and pressing rate followed by optimal controlled sintering would give values of Ka and na nearer to those comparable with equivalent wrought parts: such values could then be regarded as an index of good sound flow properties of the compacts. Cold forming of P/M parts rivals the expensive hot isostatic pressing and hydrostatic pressing processes from the conservation of energy and easy adaptability points of view. However, cold forming is limited by tool stresses and forces. The friction associated with cold forming is responsible, to a large extent, for the increased tool working stresses. Further, friction influences the densitification of the compacts, as a result of which the values of Ka and na are affected. Higher strain rate and optimal lubrication will reduce the role played by friction. The present paper is concerned with the estimation of Ka, na and μ for sintered copper preforms by means of the standard ring-compression test. © 1987.

Journal	Journal of Materials Processing Technology
ISSN	09240136
Open Access	No