Experimental investigation on material flow and mechanical properties in friction stir welding of copper sheets

This paper focuses on the mechanism of onion rings emergence by analyzing optical macrographs and micrographs of friction stir welds on the copper alloy produced by a non-threaded tool pin. Also tensile tests, hardness measurements, and microstructural evaluations were performed on the welded specimens. The microstructural study shows that the weld has lamellar structure and the bands are sections of weld layers, which are produced by forward and rotational movement of the tool. The weld layers with same form, similar to the pin form, reside into each other with tool rotation and form a quasi-spherical shape. On the transverse cross section of the weld, the boundary of the layers is emerged like concentric quasi-circles, which are called onion rings. Each layer is emerged with one tool rotation, and its distance with the next consecutive layer is equal to the weld pitch. The formation of onion ring structures is due to the cutting of the weld layers with the same recesses and dimples. Hardness measurements show that the core of onion-ring-shaped structure at two points near the weld surface and weld root possesses maximum hardness. The average hardness of weld decreases from upper surface toward bottom until mid-depth, where then increases. HAZ has lower hardness on RS compared to that of AS. The scanning microhardness evaluation technique can be used as a complement method to the conventional hardness measurement procedures.