Determination of the load acting on the probe by separating force and torque during FSW of AA 6060 T66

Abstract

The development of suitable welding processes is required to meet the ever-increasing demands on joining processes, particularly for lightweight construction and increasing environmental awareness. Friction stir welding (FSW) represents a promising alternative to conventional fusion welding processes, particularly for the joining of low-melting-point materials such as aluminium and magnesium alloys, which present a number of challenges, including the formation of pores and the occurrence of hot cracks. The central element of the process is the friction stir welding tool, which consists of a shoulder and a probe. The rotation and the simultaneous application of pressure during the joining process create a friction-based heat input through the tool. The excellent mechanical properties resulting from dynamic recrystallisation during the welding process are a major advantage of the process. As a result, strengths comparable to those of the base material can be achieved. However, FSW is subject to process-specific challenges, including high process forces, which result in the fabrication of complex and robust devices. Additionally, high dynamic loads on the friction stir welding tools must be considered. In many cases, the design of friction stir welding tools is based on empirical data. However, these empirical values are machine-, component- and material-specific, which often results in under- or overmatching of friction stir welding tools. Sudden probe failure, component scrap, and low process reliability are the direct consequences of undermatching. Overmatching results in enlarged tools with limited accessibility, high heat input, and high process forces, leading to component deformation. The aim of this study is to determine the load on the probe by separating the forces and torque of the shoulder and the probe in order to be able to make statements about the load acting on the probe and the resulting stress state. The knowledge of the stress state can be employed to design friction stir welding tools, both statically and dynamically, for a specific welding task. A strategy was devised to distribute the load exerted on the shoulder and probe. To this end, the length of the probe was gradually reduced between the welding tests. The investigations were carried out with a force-controlled robotized welding setup in which AA 6060 T66 sheets with a thickness of 5 mm were welded. A Kistler multicomponent dynamometer type 9139AA allows to measure the Cartesian forces to be recorded in the x-, y-, and z-directions with a sampling rate of 80 kHz. The weld seam properties were determined by visual and metallographic inspections as well as tensile and bending tests in accordance with DIN EN ISO 25239–5.