TAGRA 2024

Microalloyed steels are widely used in many industries due to their high strength, light weight and weldability. These steels, which are preferred for components such as chassis and body structures in automotive manufacturing, require welding joining methods to adapt to complex designs. For these steels, which are generally joined by methods such as fusion, laser and resistance welding, fusion and laser welding in particular can cause problems such as grain growth, cracking and weakening of mechanical properties in the weld zone due to high heat input. This situation results in loss of performance after welding, especially in thin sections and high strength materials. Friction stir welding (FSW), one of the solid-state welding methods, can largely avoid such problems as it is a process performed without melting. In this study, the weldebility of microalloyed steels by FSW, the microstructure and the mechanical properties of the welded zone were investigated. The microalloyed steel sheets with 1.5 mm thick and 200x50 mm were butt welded by FSW method using a tungsten carbide (WC) tool. The shoulder diameter of the tool was 14 mm, the pin diameter was 4 mm, the pin length was 1.3 mm and the conical angle was 30°. During the FSW process, the tool speed was set at 800 rpm, the trverse rate was set at 65 mm/min and the tool down force was set at 5 kN. Microstructural analysis of the weld zone was carried out by optical microscope and scanning electron microscope. Mechanical properties were evaluated by Vickers microhardness test and tensile test. The results showed that the initial ferrite-carbide microstructure of the microalloyed steel was significantly refined and the grain size was reduced after the FSW process. This refined microstructure resulted in a significant increase in both hardness and strength values in the weld zone. While the initial microhardness of the microalloyed steel was 180 HV0.3, this value increased to avarege 240 HV0.3 in the FSW zone. Furthermore, no loss of hardness was observed the heat affected zone. The strength of the weld zone increased compared to the base material, while the total elongation value decreased. These results show that microalloyed steels can be successfully welded by the FSW method and that the weld performance is acceptable.