The thermomechanical, time-dependent, elastic-plastic creep behaviour of a representative tool in a high-temperature superplastic forming press has been simulated using a sequential finite element-based heat transfer and mechanical stress analysis approach. The heating and cooling aspects of the heat transfer model have been previously validated against laboratory measurements. This paper focuses on the prediction of die life based on this model. The determination and modelling of the high-temperature mechanical properties of the materials and die are described for both so-called (a) major cycles, relating to die heat-up and cool-down from and to ambient temperature, and (b) minor cycles, relating to die heat-up and cool-down associated with part removal and blank insertion. A creep-fatigue life prediction method using non-linear damage summation is developed and applied to the die-life problem. Isothermal fatigue and creep-fatigue interaction tests at 700 and 900°C on 40%Ni-20%Cr die material XN40F are carried out to estimate the required strain-life relationships.

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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 8C and short-term tensile creep tests at 900 8C 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.

Shot peen forming different from shot peening is a metal forming process widely used in aeronautical industry. Different types of tension-tension fatigue specimens after peen forming were tested. The results show that the fatigue life of specimens peen formed with large balls except edges is 35.4% higher than that of unpeened specimens, while the life of the specimens with whole surface peen formed is 23.2% lower than that of the unpeened specimens. In addition, shot peening specimens which have been peen formed as a surface strengthening way can significantly improve the fatigue resistance. Moreover, when the specimen is peen formed except edges, the position of crack source is from the indentation region to subsurface.

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