Ring-compression tests on sintered iron preforms

doi:10.1016/0378-3804(88)90139-8

Journal of Mechanical Working Technology

Volume 16, Issue 1, February 1988, Pages 51-64

https://doi.org/10.1016/0378-3804(88)90139-8 Get rights and content

Abstract

Standard ring-compression tests have been carried out on sintered iron preforms of different initial densities. The values of the apparent strength coefficient and the strain hardening exponent have been estimated to identify which conditions of the perform result in mechanical properties closest to those of equivalent wrought part. The coefficient of friction has also been estimated for compacts as well as for wrought parts, so that near identical frictional conditions could be employed for comparison purposes, in computations of their respective force requirements.

Compacts of different length-to-diameter ratios have been prepared for different compacting pressures and the optimal conditions of the billet geometry for the least non-uniform densification along the length of the compact have been established.

The main aim of this fundamental work was to prepare, and explore the properties of, sintered preforms of different initial densities and of greater length-to-diameter ratio so that future cold Hooker studies on these compacts would be possible.

References (4)

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H.A. Kuhn et al.
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(1971)

There are more references available in the full text version of this article.

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In addition to the matrix work hardening, the P/M preforms undergo geometric work hardening or densification hardening which further enhances the flow stress of the material [18]. P/M processed preforms undergo extremely high friction coefficients during deformation due to the influence of inherent porosity [19,20]. In addition, during deformation of sintered powder materials, the non uniformity of density distribution increases with decrease in initial relative density and increase in the coefficient of friction [21].
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This is due to decrease in the na and increase in the Ka values which were obtained by the deformation test of cylindrical parts earlier in this work, and in consequence the dead metal zone at the tool-metal interface was decreased that resulted in low friction values. Venugopal et al. [24] was observed extremely high friction conditions on PM iron preforms due to the influence of porosity, and reported decrease in na and increase in the Ka values as the relative density of preforms increase which yields lower friction values. In addition, the friction factor increases with increase in the content of copper in the composites.
Formability is concerned with the extent of deformation that the materials undergo before failure; thus its investigation is critical for successful processing of materials during bulk deformation. The present investigation has been undertaken to generate the forming limit diagrams for powder metallurgical aluminium–copper composites for different initial relative densities and copper contents. Sintered aluminium–copper composite compacts of 2%, 4% and 6% copper content with different initial relative densities have been prepared by applying recommended powder compaction pressures. The material properties such as apparent strain hardening exponent and strength coefficient were determined using stage wise compression test to generate the formability limit diagram. Densification curves were plotted to investigate the effect of initial relative density and copper content on the pore closure phenomena during deformation. Theoretical and experimental investigations using standard ring compression test were carried out to determine friction factor between tool and work piece interfaces for different initial relative density and copper content. The critical transition densities vide the forming limit diagram were found to be 84%, 85.3%, 86% and 87.5% for pure sintered aluminium, Al–2%Cu, Al–4%Cu and Al–6%Cu composites respectively. The friction factor between tool and work piece interfaces has showed increasing pattern for all the cases with decrease in the initial relative density and increase in the copper content of the composite.
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The current paper deals with the generation of formability limit diagrams (FLD) for sintered preforms of steel from room to warm working temperatures. Steel compacts of different resident densities have been prepared by applying recommended pressures. Stage wise upsetting has been carried out to obtain the densification curves from room temperature (27 °C) to 600 °C in steps of 100 °C to explore the possibility of in situ sintering. Compression tests were performed and the critical transition densities vide formability limit diagrams were found to be 6.28 g cm⁻³ at room temperature, 6.21 g cm⁻³ at 100 °C, 6.08 g cm⁻³ at 200 °C, 5.95 g cm⁻³ at 300 °C, 5.72 g cm⁻³at 400 °C and 5.5 g cm⁻³ at 500 °C. The decreasing trend in critical transition density addresses the energy conservation and cost effectiveness during secondary processing of sintered preforms of steel.
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Hence die angle and aspect ratio have been kept as 45° and 1.5, respectively, while calculating extrusion stress for the development of nomograms. The required formability properties for the determination of extrusion stress viz., strength coefficient (k), strain hardening exponent (n) and coefficient of friction for P/M copper and iron preforms were obtained from the earlier experimental works [7,8]. The required stress for cold solid extrusion of P/M copper preforms can be ascertained from Fig. 1.
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Powder metallurgy (P/M) manufacturing route has wide industrial applications due to a host of techno-economic advantages. However, defects free and superior products can be obtained only if the process parameters are carefully controlled during forming. Moreover, in metal forming processes conducting trial experiments is costlier and time consuming as well. Hence modelling technique provides effective control in scaling down costs. Mathematical models were developed for P/M working process, using regression analysis and analysis of variance (ANOVA) in order to study the main and interaction effects of process parameters, viz., strain rate, preform density and temperature. The adequacies of the developed models were verified by calculating correlation coefficient. These models can be effectively used to predict the strength coefficient (K) and strain-hardening exponent (n) and subsequently to estimate the flow stress of P/M copper preforms.

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Ring-compression tests on sintered iron preforms

Abstract

Equipment for the economic manufacture of special shapes by pressing and sintering, Sprechsaal Keram

Glas Email Silik.

Deformation characteristics and plasticity theory of sintered powder materials

Int. J. Powder Metall.