Improving GMAW weld metal and HAZ properties through friction stir processing

Steel is one of the most widely used engineering materials and it is popularly welded in fabrication industries using Gas metal arc welding (GMAW) process. The microstructure obtained in the heat affected zone is often characterized with large grain size. Depending on the GMAW process parameters, the weld metal may consist of Allotrimorphic ferrite if the heat input is high. Therefore, the weld metal and the heat affected zone may have poor weld metal toughness. Efforts have been made to modify the microstructure of the weld metal by performing friction stir processing. Initially bead on plate welding was performed on mild steel plate using GMAW process using standard 1.2 mm consumable wire and CO2 as the shielding gas. The top surface of the weld was processed using a tungsten carbide tool. The weld reinforcement was removed using milling process and the area to be processed was made smooth before performing FSP. The plate was secured in an FSW machine and friction stir processing was carried out with a FSW tool having pin length of 2 mm. The GMAW weld and the weld that has been subsequently modified using FSP were characterized using standard techniques. The microstructure of the top face showed an improvement from Widmanstätten to fine equiaxed structure after being friction stir processed. The microstructure in the HAZ also got refined. It is expected that this structure would improve the mechanical properties of the weld particularly on the surface.