Mechanical Property Improvement in Dissimilar Friction Stir Welded Al5083/Al6061 Joints: Effects of Post-Weld Heat Treatment and Abnormal Grain Growth

The 5083 and 6061(T6) aluminum (Al) alloys are widely used in transportation industries and the development of structural designs because of their high toughness and high corrosion resistance. Friction stir welding (FSW) was performed to produce the dissimilar welded joint of Al5083-Al 6061(T6) under different welding parameters. However, softening behavior occurred in the friction stir welded (FSWed) samples because of grain coarsening or the dissolution of precipitation-hardening phases in the welding zone. Consequently, this research intended to investigate the effect of the post-weld heat treatment (PWHT) method on the mechanical property improvement of the dissimilar FSWed Al5083-Al6061(T6) and governing abnormal grain growth (AGG) through different welding parameters. The results showed PWHT enhanced the mechanical properties of dissimilar joints of Al5083-Al6061(T6). AGG was obtained in the microstructure of PWHTed joints, but appropriate PWHT could recover the dissolved precipitation-hardening particle in the heat-affected zone of the as-welded joint. Further, the tensile strength of the dissimilar joint increased from 181 MPa in the as-welded joint to 270 MPa in the PWHTed joint, showing 93% welding efficacy.

Some advanced welding technologies applied for repair welding in power plants

Steels are subjected to many time-dependent degradation mechanisms when they are applied in electric power plants. They are exposed to high temperatures, multi-axial stresses, creep, fatigue, corrosion, and abrasion during such services. Used under these threatening conditions, those materials could develop various damages or failures or even form cracks. Therefore, it is desirable to prevent in-service failures, improve reliability, and extend the plant's operational life. The efficiency of the electric power plant, among other processes, depends on effective maintenance. The paper presents the evaluation of advanced procedures and knowledge in the field of steel repair welding in the maintenance of the power plants. Most repair welding of low alloy steels requires high-temperature post-weld heat treatment (PWHT), but in certain repairs, however, this is not always possible. Application of the nickel-based filler metal could also be an alternative to performing post-weld heat treatment (PWHT). The repair work expenses could be reduced if the repair is performed on-site. The novel developed repair welding procedures presented in this paper were applied for emergency weld repairing of the steel pipelines in thermal power plant, repairing without disassembling the working wheel of the coal mill in thermal power plant and "on-site" repairing turbine shaft of the hydropower plant. For all the presented repair welding procedures, weldability analysis based on the analytical equations and technological ''CTS'' and ''Y'' tests to determine the sensitivity to cold and hot crack forming were applied. Tensile tests, absorbed energies tests, banding tests, and hardness measurements were performed on trial joints, which were used to develop and verify the applied methodologies. Presented advanced weld repair technologies enable repairs for a shorter time and at lower costs compared to conventional procedures.