Effect of secondary yielding on nett-shape forming

Nett-shape forming requires better understanding of the phenomena affecting component accuracy. Some of these are subtle or not fully recognised and are, therefore, being disregarded. This paper reveals secondary yielding in some cold bulk metal forming operations. Secondary yielding of a component results from unloading by punch removal and die contraction. The process of closed-die upsetting of a cylindrical workpiece is examined. The theoretical analysis of this process explains the notion of secondary yielding. The main factors affecting secondary yielding are the flow stress of the workpiece material and the elastic properties of both the workpiece and the die. Secondary yielding refers to the plastic flow of material during the unloading cycle and, therefore, affects the final dimensions of the workpiece. The present analysis, which has been carried out in terms of the elastic strains of both the workpiece and the die, enables formulation of the die compensation procedure for this case. It appears that the die compensation procedure used for processes which involve secondary yielding is different from that for processes characterised by purely elastic unloading.