An experimental investigation on work hardening behaviour of elliptical shaped billets of aluminium during cold upsetting

The present investigation has been undertaken to evaluate the work hardening phenomenon experienced in elliptical shaped billets of commercially pure aluminium during axial compression tests using graphite mixed with oil as lubricant on both sides. It is very essential to study the plastic deformation of any material and also is an important parameter used to describe the work hardening characteristics of materials. The parameters namely the instantaneous strain hardening exponent (ni) and the instantaneous strength coefficient (ki) were evaluated during the cold upsetting of commercially pure aluminium under various stress state conditions namely the uniaxial, the plane and the triaxial. Samples with three different aspect ratios (ratio of height to diameter), namely, 0.5, 0.75 and 1.0 and five different b/a ratios (ratio of minor diameter to major diameter) of 0.6, 0.7, 0.8, 0.9 and 1.0 were prepared and cold upset forged. The strain hardening exponent (ni) and the strength coefficient (ki) were obtained for each aspect ratios. It has been observed that the value of strain hardening exponent (ni) varies very much and found to be different for different aspect ratios of the billets tested. Further the value of the strength coefficient (ki) is found to be increasing for different aspect ratios under different stress state conditions.