SOME INVESTIGATIONS IN FRICTION STIR WELDING OF CARBON STEEL

Abstract

Friction stir welding (FSW) is the most promising welding technique that represents a significant advancement in the field of metal joining. While extensive research has been conducted in this area, initial investigations primarily focused on low-temperature softening materials such as aluminum alloys. However, as industries involved in steel welding seek more efficient and environment-friendly alternatives, there is a growing interest in exploring the application of FSW for steel joining. Welding steel poses unique challenges due to its high temperature softening and high-strength properties. Although aluminum alloys were initially preferred for research purposes due to their inherent difficulty in conventional welding processes, the numerous advantages offered by FSW have prompted the need to develop this innovative technique for welding steels as well. Considering the increasing concerns over environmental pollution and health hazards associated with conventional welding processes, the development of a greener welding process holds great significance for various welding applications. In this investigation, FSW was performed on AISI 1018 carbon steel plates, 3 mm thick. The welding process resulted in the successful creation of a butt joint extending along a length of 200 mm. FSW machine equipped with a high-powered spindle motor capable of generating the necessary torque was carefully selected for the study. Through experimentation, a specific grade of tungsten carbide was chosen, considering its ability to provide a longer tool life. A series of extensive trial runs were conducted to establish an optimal welding procedure that would yield defect-free joints. Through these trials, the process parameters that have an impact on the weld quality were identified. The range of variation for these parameters was also determined. To examine the impact of input variables on the mechanical properties of the welds, experimentations were done using Taguchi's L9 orthogonal array (OA), with three repetitions for each experiment. All 27 welds underwent thorough mechanical and metallurgical testing, requiring the extraction of samples from the welded plates. The samples underwent the transverse tensile test, reduced section tensile test, and impact test. Additionally, the fatigue test was conducted to evaluate the response of the welds and the base metal to cyclic loading. vi To obtain the best possible machine settings for the welding process, both the single-response and multi-response optimization techniques were employed. Through the combined use of single-response and multi-response optimization techniques, the study aimed to find the optimal machine settings that would yield welds with improved mechanical properties and superior performance compared to the base metal. Metallurgical testing was carried out using microhardness testing, microstructural examination, and spectroscopic analysis of the welds. The strength, impact energy absorbed, and fatigue properties of the weld were all found to have improved over the parent material properties. The process parameter, particularly welding speed, was found to have a significant effect on response parameters. Based on the experimental values, statistical models have been developed to find the effect of input variables on output variables. Microhardness values in the weld nugget and the HAZ were found to be more than the base metal.