Superplastic forming of friction stir welds in Titanium alloy 6Al‐4V: preliminary results

Abstract

AbstractThe trend in design and fabrication of aerospace structure is moving rapidly towards the use of composite materials and the consolidation of many pieces into large monolithic assemblies. Titanium alloy 6Al‐4V is more compatible with composite materials than aluminum alloys because of its superior corrosion resistance and closer match to the coefficient of thermal expansion. In addition, many components that are used for the newer composite based aircraft, and are subjected to high service temperatures, are fabricated from titanium using Superplastic Forming (SPF) and Diffusion Bonding (SPF/DB). However, the use of SPF titanium parts has been limited up until now due the size restriction of standard sheets from the titanium mills, which is generally available at a maximum size of 1.2 m x 3.6 m. The purpose of this study was to develop the Friction Stir Welding (FSW) process for both standard and fine grain titanium alloy 6Al‐4V in a bid to find a process that would allow the joining of multiple pieces to fabricate much larger components. Further, the FSW process was refined such that the welds were made to have superplastic properties equal to those of the parent sheet. A secondary goal of this effort was to build full size SPF prototype parts of a generic jet engine nacelle Lipskin using one FSW titanium blank. SPF of 7475 aluminum had been reported previously in the literature by Mahoney, Barnes, Mishra and others. During this study, the FSW process for 5083 Superplastic grade aluminum was developed simultaneously along with titanium 6Al‐4V. The aluminum material was used to reduce the cost of developing the SPF manufacturing process to fabricate full scale engine inlet test components. FSW blanks of both materials were used for the initial forming trials.