The effect of process parameters and tool geometry on mechanical properties of friction stir welded aluminum butt joints

In Friction Stir Welding (FSW), process parameters and tool geometry play a fundamental role in obtaining desirable mechanical properties and microstructures in the welded zone. The results of experimental activity on friction stir welding of aluminum alloys, focused on the above mentioned aspects, are reported in the present paper. The experimental campaign was performed on sheets 5 mm thick of AA6060 alloy butt joined by means of a CNC machine tool. Several tests were carried out by varying process parameters (namely tool rotational speed and feed rate) and tool geometry (two different tools were adopted for this purpose). The first tool was a very simple device fabricated using AISI 1040 steel, with a flat shoulder and a cylindrical pin. The second one was a more complex tool based on two main components: a tool holder, with a flat shoulder, and a threaded probe obtained using a commercial thread forming tap. A similar solution for tools fabrication was already presented by other Authors. Because of the assembling solution adopted the probe height can be adjusted so resulting in a very adaptable tool. Tensile properties of the friction stir welded joints were evaluated as a function of the different process parameters for both the adopted tools. With respect to the joints obtained using the simplest tool, good results were found in terms of Ultimate Tensile Strength (UTS) and corresponding strain; in some experimental conditions the tensile strength of the joints resulted to be close to the properties of basic material. Nevertheless, the experimental results showed how a good weld can be obtained across a very wide range of conditions even though feed resulted to be more critical than rotation speed. Interesting results were also obtained using the tools with the thread forming tap. A comparison between macrostructures of specimens obtained in different welding conditions was finally carried out.