Friction stir welding joint microstructure analysis

When the welding preparation and welding process specifications, welding process parameters are appropriate, friction stir welding can obtain welded joints with excellent performance. The welded joint formed after welding can be divided into four different areas from the microstructure, namely: Shoulder-Affected Zone (SAZ); Thermo-Mechanically Affected Zone (TMAZ); Welding Nugget; Heat-Affects Zone (HAZ). Figure 1 shows the micro-morphology of the LD10 aluminum alloy friction stir welded joint, which is also the typical microstructure of the friction stir welded joint. Among them, AS represents the advancing side of the agitating tool, and RS represents the retreating side of the agitating tool.

The shape of the friction stir welded joint of aluminum alloy will change with the different process parameters and welding conditions, which is manifested by the different microstructure and area of ​​the weld nugget.

Generally, the deformed aluminum alloy is in a rolled state and is distributed in a slat. However, due to the strong agitation, the weld nugget area undergoes a high temperature thermal cycle, and the tissue undergoes dynamic recrystallization, and the microstructure changes from the original lath to the fine equiaxed recrystallized structure. Figure 3 shows the microstructure of the weld nugget area of ​​the 2219 aluminum alloy friction stir welded joint. The size and number of grains in the weld nugget zone are determined by the weld heat input and the upset force during the welding process.

Figure 3 The microstructure of the 2219 aluminum alloy weld nugget area

 

The heat affected zone is affected by mechanical agitation and welding thermal cycle during the welding process, but its influence is smaller than that of the weld nugget zone, so the region forms a recovery grain structure in the lath structure.

 

The heat-affected zone is only subjected to thermal cycling during the welding process, no deformation occurs, and only the recovery reaction occurs. Compared with the base metal, the grain in this area is slightly coarse.