Finite-element-based methodology for predicting the thermo-mechanical behaviour of superplastic forming tools

The time-dependent, thermo-mechanical behaviour of superplastic forming (SPF) dies is studied using finite-element (FE) analysis and experimental testing. The objective of the work is to predict the cyclic, non-linear stress-strain response of the dies under the severe cyclic thermal conditions that lead to short lives in service. Experiments are carried out on a simple rectangular-shaped die within a laboratory SPF press for a 20%Ni-24%Cr high-temperature tool steel to validate the FE modelling methodology. Tensile tests at four different temperatures up to 900°C and short-term tensile creep tests at 900°C are carried out to measure the high-temperature mechanical behaviour of the 40%Ni-20%Cr die material XN40F. Finally, time-dependent, non-linear, sequentially coupled, FE analyses are employed to study the behaviour of a representative XN40F SPF die. Attention is focused on the practical combined thermal cycles consisting of (a) major cycles, which correspond to controlled heat-up and free cool-down, and (b) minor cycles, which correspond to smaller temperature changes associated with the opening and closing of the press and die for part removal and blank insertion. It is shown that significant cyclic inelastic strains develop at die locations that correspond to locations at which major cracking has been observed during service. Elastic-plastic-creep, stress-strain loops are presented corresponding to these locations as a basis for die life prediction.