In a previous blog post we discussed, in general, issues related to residual stress in welding. In this post, we’ll follow up with a more detailed look at a specific welding application – linear friction welding of titanium alloys.
Titanium alloys are widely used in aerospace applications for their high strength to weight ratio, good corrosion resistance, and metallurgical stability. New joining methods are being implemented that allow for more efficient manufacture of titanium components. Linear friction welding (LFW) is a solid phase bonding process, which is particularly appropriate for titanium alloys. Due to the titanium’s great affinity for oxygen, nitrogen, and hydrogen, protective atmospheres must be used to prevent contamination of the welded material. LFW avoids the formation of a liquid phase during the welding process, and can therefore be carried out in air. Likewise, the typical defects caused by melting and solidification during traditional welding process such as pores, pinholes, shrinkage cracks and grain coarsening are avoided. However, as with all welding and deformation processes, understanding the weld residual stress is important.
Hill Engineering has performed many residual stress measurements on linear friction welded parts and test specimens. The contour method is a very useful technique for evaluating weld residual stress because it provides a 2D map of the residual stress distribution. For example, the result from a contour method residual stress measurement on a test specimen containing a linear friction weld is shown below. The test specimen consisted of two 0.5 inch thick plates of Ti-6Al-4V that were joined together using linear friction welding. In general, the weld residual stresses from the LFW process are shown to be high in magnitude and localized near the weld. For titanium alloys it is possible to perform a post-weld thermal stress relief to significantly reduce the magnitude of the weld residual stress, which has also been evaluated using the contour method.
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Weld residual stress measured in a linear friction weld specimen using the contour method.