Effects of thermal and mechanical treatments on the fatigue performance of friction welded joints

Abstract

Friction welding (FRW) is presented as one of the most economical, simple and productive welding methods. FRW is a solid state joining process providing a unique approach for joining alloys that are usually unsuitable to weld using fusion welding techniques. Residual stresses (RS) in FRW may be considerably less than those induced in fusion welds since FRW occurs at lower temperature. However, the rigid clamping system used in FRW can still produce high tensile RS in the welding joints. These RS can remarkably affect the service performance of the welded materials by reducing their fatigue life and promoting the fatigue crack growth process. This has instigated the demand for techniques and methods that can relieve the tensile RS in welded parts. Therefore, the main objective of this study is to improve the surface layer properties and impact toughness as well as to enhance the fatigue performance of rotary and orbital friction-welded metallic joints. In this study, the continuous drive rotary friction welding (RFW) has been used to weld similar joints from commercially pure titanium (CP-Ti) and dissimilar joints of stainless steel AISI 316L to carbon steel AISI 1012. The process parameters were varied in a wide extent to determine the optimum operating parameters using the design of experiment (DoE) technique. In addition, the orbital friction welding (OFW) was used to weld joints from the high temperature titanium alloy Ti-6Al-2Sn-4Zr-2Mo-0.1Si (Ti-6242). Combination of mechanical surface treatments such as shot peening (SP) and roller burnishing (RB) and thermal ones namely post-weld heat treatment (PWHT) have been applied on the weld joints to introduce compressive RS that compensate the tensile ones at the weld, which results in significant improvements of the fatigue life of these joints up to 87%. The process parameters of the SP and RB have been varied until reaching the optimum parameters that led to improved surface layer properties and enhanced fatigue performance. The residual stress depth profiles and surface residual stress distribution across the weld line were measured using the incremental hole-drilling method (IHD) and laboratory X-Ray diffraction (LXRD), respectively. The phase transformations during welding processes were investigated using X-ray diffraction technique. This investigation demonstrated that both SP and RB markedly increased in the fatigue performance and hardness levels of the joints welded by rotary and orbital friction welding. A significant enhancement in the ductility of the joints has been reported after PWHT process. Moreover, applying SP and RB after PWHT led to a remarkable improvement of the fatigue life of the welded joints. According to these results, it is recommended to apply a combination of surface treatments and PWHT after FRW to improve the integrity of the weld. Furthermore, it could be concluded that the mechanical surface treatment (SP and RB) can control the fracture location either in the weld zone or the lower strength base metal of the welded joints by distinguishing the reinforcement zone.