The related literature was searched on FSW assisted by various types of auxiliary energy and donor materials. Wu et al provided a comprehensive review  on the methods that provides auxiliary energy. These methods majorly are using electricity, induction, laser, and ultrasonic vibration etc., which partially heat and/or soften the alloys before or during the FSW, reduce the load requirement on the tool. Thus, it improves the tool performance and life, optimizes process window, and enhances welding efficiency and quality, etc.
Electrically assisted FSW (EAFSW) softens materials by Joule heating effect and electroplastic effect [14, 15]. Due to both effects, the EAFSW process produced significantly higher temperatures in the plunge stage to soften the material, which reduced the plunge force significantly [14, 15]. This would facilitate welding of thick part, improve the wear resistance of the tool, and increase the tool life and performance . Luo et al applied EAFSW for joining light alloys of AZ31B and Al 7075 [16, 17]. Morphologies of the weld line in  confirmed smoother surfaces with finer arc shaped features due to more uniform and denser corrugation of the material. It was also reported that the influence of electric current would reduce the average grain size in the SZ . Although the average grain size was almost unchanged in the TMAZ, the grain distribution was more uniform with more elongated grains due to severe plastic deformation. Also, it formed less narrow and more equiaxed grains in the TMAZ due to more severe dynamic recrystallization caused by electric current . The difference in grain refinement in the SZ and TMAZ of the Al and Mg alloys is induced by the different responses of the alloys to nucleation rate of dynamic recrystallization under similar conditions of heat input and plastic deformation . The higher is the nucleation rate of the dynamic recrystallization, the finer are the grains formed. The grains in the HAZ of EAFSW joints were coarsen with increasing electric current because of the long exposure of the HAZ to high temperature and secondary recrystallization. The hardness profiles of the EAFSW welds were consistent with the grain refinements and microstructure developments. The hardness values varied directly with the grain refinements. The higher heat inputs improved material flow. The viscoplasticity at the root of the weld improved the weld quality by reducing the risk of various defects in welds.
Induction assisted FSW (IAFSW) uses electrical inductance to generate heat to soften the workpiece materials. IAFSW has been reported for joining medium strength Aluminium alloy  and high strength steel [19, 20]. Both researchers in [18 and 19] reported that the downward force was reduced, but mechanical properties in the joints still maintained. Further, IAFSW would reduce the residual stress distortion and HAZ softening and increases the plastic deformation due to its low heat input flux and well defined heating. This would eliminate the heavy clamping systems thus reducing the size of the FSW machine . When joining steel, the IAFSW, comparing to the FSW, process induced more intense grain refinement , which induced an increase in the hardness and tensile strength of the SZ. However, the ferrite/austenite ratio of this steel remained unchanged for both processes.
Laser assisted FSW (LAFSW) is the most widely used external energy assisted FSW. The application of laser preheating lowered the resistance of the material to tool penetration and forward motion . Therefore, the need to apply large forces on both the tool and the workpieces is lowered. When applying EAFSW to high strength steel with ferrite pearlite structure, significant amount brittle Martensite and Bainite phases were found in the joints, but the LAFSW could prevent the brittleness  and retain the original phases. LAFSW was even applied for joining of super alloys such as Inconel . Laser preheat induced intense grain refinement in the SZ, so it significantly improved the overall strength and mechanical properties of Inconel alloy. An improvement was reported in the fatigue behavior of Aluminium lap joints produced by LAFSW . Alvarez et al. also reported the life of a PCBN tool used in the LAFSW of marine grade steel was increased up to several feet .
Ultrasonic energy assisted FSW (UAFSW) uses high frequency vibration to soften material without significant heating it. UAFSW reduces the friction heat by complementing the softening. However, at the same time, it would increase the heat generation from the additional deformation. The overall result of these two effects may increase the heat input slightly. UAFSW expanded the plastically deformed region  and improved the material flow , thus it improved lap shear force and hardness of the joints . UAFSW also suppressed the formation of voids and tunnel defects [24, 26].
Numerical [10–12] and experimental [11–12] studies of donor materials that assisted FSW suggest that local preheating at the plunge stage is generated by friction between the tool pin and the donor material and also by the plastic deformation in the material. This preheating would generate heat in the copper donor material due to high frictional forces and this preheating is expected to be transferred to the work piece by conduction, which will result in softening the workpiece material and reducing the plunge force. This in turn is expected to reduce tool wear. For the selection of a donor material, it is desired for the donor material to possess high thermal conductivity to allow rapid heat transfer into the workpiece. Additionally, it is desired that the donor material will not advance along the welding path, because this will influence the weld quality by producing a nonhomogeneous weld line. It has been demonstrated by previous studies that material moves around the tool pin and becomes deposited behind it without being transported forward with the tool’s pin [27–29]. Hence, donor material from the plunge phase is restricted to the initial weld area and can be sacrificed.
Comparing to the auxiliary energy assisted FSW, the donor material assisted FSW does not require the welding tool to be electrically conductive, and it needs no installation of complex insulation system between the tool and the FSW machine. It also avoids the difficulty in IAFSW on controlling the current flow, generating the spark, and heating of conductive materials involved in the current path. It also avoids the complex equipment for laser and ultrasonic energy. This donor material assisted FSW idea is unique. Except the paper [10–12] on examining the plunge stage, there is no reported work examining the effect of donor material on the welding stage and the joint properties of FSW. To fill this gap, the proposed methodology is presented in Sect. 3.