Superplastic Sheet Metal Forming with Focus on the Warm Bulge Test and its In-Process Monitoring

Material characterisation is of prime importance in understanding its response to forming loads, which in turn influence the product design. The flow stress in plastic loading defines the evolution of the yield surface and depends on a variety of factors such as strain, strain rate and temperature. The degree of influence of the strain rate increases at higher temperatures. This effect can be well described with superplastic forming, in which the material is loaded very slowly at superplastic temperatures. This paper deals with monitoring of process parameters during superplastic sheet metal forming of magnesium alloys, with special emphasis on in-process measurements for bulge test purposes at elevated temperatures. The strain evolution near the part‟s pole region has been recorded in-process by using the ViALUX photogrammetric strain analysis system AutoGrid. The recorded data provides a lot of information about the forming process such as evolution of strains, strain rate, flow stress and the limit strains, which allows determining all relevant material and process parameters for superplastic forming.