FSW differs greatly from other welding processes in that there is no external heat source and all of the heat involved in the joining process is generated through the tool-material interaction. The tool action on the workpiece generates considerable stress and strain at the tool-workpiece interface. The heat is primarily generated by high shear stresses and strain rates in the material at the tool/workpiece interface [2]. For a given alloy and plate thickness with a particular tool, the primary operating process variables which affect the heat generation phenomena are the pin geometry and then the downward force, tool plunge depth, rotation speed, and traverse speed. The downward force is a preset variable (if welding is not accomplished under position control), while the tool plunge depth needed is defined by sample thickness. This leaves rotation speed and traverse speed as the undefined process variables. Increased rotation speed leads to higher levels of heating and hence temperatures induced in the material (Figure 3). Tang et al. (1998) performed a test in which the tool rotation speed was varied and the resulting temperature of the weld in a 6061-T6 aluminum plate was measured for various speeds; all other parameters were kept constant. At a tool rotation speed of 300 rpm, a temperature of 425 °C was recorded. An increase to 650 rpm resulted in a 40 °C increased to 465 °C, and a further increase in speed to 1000 rpm resulted in a further increase of 20 °C to 485 °C [35]. Variation in the second key parameter, the transverse speed, is found to directly impact weld quality. There is an optimal set of speed conditions with degraded welds occurring when the speed is either too slow or too fast. When the transverse speed is too fast, occurrence of defects is common because the pin moves too fast to give the time needed to properly mix the material and to generate the needed level of heat input. A slow traverse speed will create too much heat within the weld and lead to different forms of defects, knows as flash or voids [36].

Tool properties such as geometry play a critical role in determining material flow and it controls what is the optimal transverse speed for good quality weld formation [38,39]. The tool is designed with a shoulder and a pin, as shown in Figure 1 and it has two main functions: to control the material flow and generate heat. The pin and shoulder can be any one of multiple designs which have been developed to encourage the formation of the desired microstructure and resulting mechanical properties. More modern tools are designed so as to reduce displaced volume. For example the MX Triflute pin reduces displaced volume by 60% when compared to earlier conventional pins [40]. The advances in design of this tool reduces welding force, enables easier flow of plasticized material, creates downward auguring, and decreases the interface area between the pin and the plasticized material [40]. Additional tool designs have been investigated so as to be able to perform various functions during welding [41,42]. In producing a tool design a combination of life and performance parameters also need to be considered for a FSW processes so as to minimize the equipment costs associated with the process [43].

Anvil back plates have a significate effect of properties of FSW due to the interaction with the plunge force as well as influence on heat dissipation of the process. The influence on heat dissipation will affect the microstructure evolution in the weld nugget and thermomechanically affected zone. For example, the conventional steel anvil back plates have a high thermal conductivity, which increases the dissipation of the generated heat [44].