On the apparent strength-coefficient, strain-hardening exponent and boundary-friction coefficient of sintered P/M copper compacts

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.