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Workability domain without the onset of flow instability was developed by numerical simulation and response surface method (RSM) for complex-shaped straight spur gear forging.The processing map of AZ31B alloys was calculated from flow stress curves and then integrated with the finite element model to simulate the distribution of flow instability in the straight spur gear undergoing isothermal forging process.Occurrence of flow instability depends on forging temperature,punch velocity and billet reduction.Taking forging temperature and punch velocity as design variables,while billet reduction as response variable,RSM of workability domain was established.Analysis of variance indicates that forging temperature is the most significant factor determining the appearance of flow instability in the forged gear.Flow instability is easier to take place at lower temperatures of 250 and 300 ℃C in the early stage of forging but at higher temperatures of 350 and 400 ℃C in the later stage of forging,which is attributed to different deformation mechanisms and dynamic recrystallization behaviors at different temperatures or deformation levels.Meanwhile,increasing punch velocity further reduces the workability of the forged gear.Four different processing paths were chosen to carry out the gear forging trials.Visual observations and metallographic examinations demonstrate that the developed workability domain contributes to optimization of forging parameters.