Microstructure and Non-destructive Evaluation of Aluminum 6009 Alloy Sheets Joined by Friction Stir Welding

Friction Stir Welding (FSW) is one of the advanced welding techniques in which the material joining occurs purely in the solid state, i.e., without any material melting. FSW has material joining through the combined effects of frictional heat and the plastic deformation of the materials. Aluminum 6009 alloy is important from automobile body panel point of view. In this present work, Aluminum 6009 alloy plates were friction stir welded at four different sets of process parameters using alloy steel FSW tool. After the welds were successfully obtained, they were subjected to Radiographic Testing (RT), a Non-Destructive Testing (NDT) technique, for analysis of weld defects formed during FSW of the plates. Furthermore, to analyze the microstructural changes at Stir Zone (SZ), Thermomechanically Affected Zone (TMAZ), Heat-Affected Zone (HAZ), and Base Metal (BM), Optical Microscopy (OM) was carried out on these welds. Also, mechanical properties such as impact strength and microhardness were evaluated in order to analyze the variations in them due to different sets of weld parameters. The RT-NDT results indicated the exit hole defects as well as ribbon flash defects in all the welds obtained from different process parameters. The microstructural analysis indicated the presence of finer grains in the Stir Zone (SZ) of the welds. Enhancement of tool rotational speed led to reduction of impact strength, whereas increase of weld traverse speed resulted in improvement of impact strength. On the other hand, combination of lower tool rotation speed and higher weld traverse speed provided better hardness, with maximum hardness obtained in the weld’s advancing side.