Metrology and microscopy analysis of multisheet packs manufactured via superplastic forming to study possible diffusion bonding

A number of titanium alloys multisheet packs with predefined complex features were manufactured via superplastic forming (SPF) and investigated via metrology and microscopy analysis to determine the possible occurrence of diffusion bonding. Four sheets of titanium alloys were welded using resistance seam welding based on a defined pattern to manufacture a composite sheet of four layers. Each composite sheet structure was composed of four sheets: two core sheets of the same titanium alloy material - Ti64 or Ti54M, and two external sheets of similar titanium alloy material - Ti64 or Ti6242. The composite sheet structures were inflated via SPF process in pockets where the sheets were not welded to each other to form a complex component. A pressure cycle was determined via the analysis of the numerical data from finite element simulations and a laboratory optimization method to form each multisheet pack. The maximum elongation due to stretching of sheets by SPF could reach 134% of initial part pre-forming.

The wall thickness of each inflated packs was measured via GOM scanning all features of the formed structures. The thickness reduction imposed by SPF to the component surfaces was found to be up to 59% at some regions of the packs. Several samples from selected regions of each inflated pack were investigated via scanning electron microscopy (SEM) to study whether diffusion bonding occurred between the sheets. The GOM scanning and image analysis demonstrated that during SPF, the multisheet packs underwent a degree of diffusion bonding where the adjacent sheets exhibited thickness reduction under compression forces.