Microstructure and fatigue properties of double-sided friction stir welded Ti-4.5Al-2.5Cr-1.2Fe-0.1C alloy plate for aerospace applications

Abstract

Welding is an important technology for using titanium alloy plates in aerospace applications. However, pores (welding defects) are introduced into the welded zone during welding, including during the melting process, leading to a decrease in the fatigue strength of the welded titanium alloy joint. Friction stir welding (FSW) seems to be effective for avoiding the deterioration of fatigue strength because it is difficult for pores to form in the welded zone during FSW, which does not include a melting process. Therefore, the microstructure and mechanical properties, including the fatigue properties, of an (α+β)-type titanium alloy, Ti-4.5Al-2.5Cr-1.2Fe-0.1C alloy (Ti-531C), subjected to FSW were investigated. In this study, FSW was conducted from both sides of the plates (double-sided FSW) to ensure that the samples were subjected to the same experimental conditions (plate thickness, specimen geometry, etc.) used in our previous study on laser-welded Ti-531C plates for comparison. No pores are observed on the fractured surfaces of Ti-531C subjected to double-sided FSW. Consequently, higher fatigue strength is obtained for Ti-531C subjected to double-sided FSW compared with the samples subjected to double-sided laser welding. However, an acicular secondary α phase and a grain-boundary α phase in the first welded zone are coarsened by heating in the second welded zone, leading to a difference in hardness in the direction of the loading axis for the fatigue test. As a result, the boundary between these two regions became a stress-concentration site that deteriorates the fatigue strength of Ti-531C subjected to double-sided FSW.