Influence of tool shoulder geometry on properties of friction stir welds in thin copper sheets

Abstract

The aim of this work is to study the influence of the shoulder geometry on friction stir welding of 1 mm-thick copper-DHP plates. The welds were produced using three different shoulder geometries, flat, conical and scrolled, and varying the rotation and traverse speeds of the tool. The flat shoulder tool proved to be inadequate for performing welds, because many defects were produced for all welding conditions. In turn, the scrolled shoulder tool is more effective than the conical one in the production of defect free welds. However, both geometries required a minimum rotational speed to avoid internal defects. For the same welding parameters, greater grain refinement, higher hardness and improved strength are also achieved in the nugget of the welds, using the scrolled tool.

Introduction

Although many researchers recognize that tool geometry is a fundamental parameter in the friction stir welding (FSW) process, the knowledge of its influence on heat generation, material flow and the properties of the welds is still limited (Nandan et al., 2008). In fact, as the geometry of the tools is complex and difficult to characterize, most of the studies consider the effect of the tool shoulder and pin separately, as in Zao et al. (2006) and Leal et al. (2008). As it is well-known, the basic functions of the tool are to generate heat, in order to plasticize the material, and to direct the flow of plasticized material, preventing the formation of defects. In particular, the tool pin plays an important role in the flow of material in the thickness direction, especially in thick plates. This way, in recent years, instead of the traditional cylindrical or conical threaded pins, tools with pins of complex geometry have been developed, as showed by Thomas et al. (2003). Fuller (2007) and Rai et al. (2011) summarize very well the tool geometries developed in recent years as well as their effects on the welding process. The virtues attributed to these complex geometries are related to the increased flow of material in the thickness direction as well as to higher heat generation due to the increased interface area between the tool and the workpiece. However, Schmidt et al. (2004) developed an analytical model for a tool with a conical shoulder and cylindrical pin, which showed that most of the heat is generated by the shoulder and only about 14% is generated by the pin. Moreover, Mehta et al. (2011), on FSW in 7075-T6 alloy, pointed out that the heat generated increases with increasing shoulder diameter. These authors also stated that the optimum diameter is a function of the tool's rotation speed.

Dawes and Thomas (1999), in a further shoulder geometry study, showed that the use of a scrolled-shoulder tool allows welds in aluminum alloys to be produced without tool tilting, which besides improving the surface finishing and the mechanical properties of the welds, allows higher welding speeds. Although this study was published in 1999, only few recent studies, such as Scialpi et al. (2008), Gratecap et al. (2008) and Leal et al. (2011), have addressed the influence of the shoulder geometry on the quality of friction stir welds, particularly in reduced thickness welds, where material flow is more constrained due to the cooling effect of the backing plate.

Copper and alloys, although expensive, have great potential to be used in industry due to their high electrical and thermal conductivities and excellent corrosion resistance. As mentioned by McNelley et al. (2007) and Cederqvist et al. (2009), they are used for components in a wide range of marine systems and, for example, for the encapsulation of nuclear waste material. However, copper alloys are difficult to weld by fusion because adherent oxides inhibit welding, or volatile and toxic elements, such as zinc, may be present in the alloys, requiring adequate ventilation. Therefore, FSW is a good option for joining parts made of these materials. The optimization of FSW tool geometry and process parameters is still required. To this end, the current study addresses the effect of tool shoulder geometry on the microstructure and mechanical properties of friction stir welds in thin copper sheets. The effect of shoulder geometry on the heat generated in the process is also discussed.