Evaluation of diffusion bonds formed between superplastic sheet materials

Abstract

Diffusion bonds produced in microduplex titanium and stainless steel sheet materials for various bonding conditions have been evaluated using a range of techniques. These include light and scanning electron microscopy (SEM), scanning acoustic microscopy (SAM) and compressive lap shear testing. The potential of other procedures such as ultrasonic inspection and resistivity measurement are also discussed. For imperfect bonds, the bond line in titanium alloys consists of clearly defined interfacial voids separated by metallurgically sound bonded regions, while the unbonded regions in stainless steel often consist of long flat voids in which the opposing surfaces have contacted but not bonded. It was observed that light microscopy and SEM observations provide a convenient and reliable method for the assessment of the bond quality, and in the case of titanium alloys it is possible to obtain quantitative data on the extent of bonding. High frequency SAM also proved to be an effective procedure for qualitative assessment. A linear relationship between the fraction of parent metal strength achieved and bonded area fraction as determined by metallography was observed for titanium alloys.