*3.2. FSW Temperature Field Distribution*

The simulation of the temperature field has a certain significance for the study of the friction stir welding process. The friction stir welding process is also the result of the joint action of the tool shoulder and the tool pin. The temperature field in the welding area is analyzed in three layers, in which the upper layer is the surface layer, the middle layer is 3.5 mm deep below the surface layer, and the bottom layer is the bottom surface of the workpiece. As shown in Figure 9, the temperature field in the surface layer presents the highest temperature, and one can observe that the temperature on the advancing side is significantly higher than that on the retreating side. The temperature on the advancing side is about 20 ◦C higher than that on the retreating side near the welding seam; furthermore, the experimental test can be referred to [26].

**Figure 9.** Temperature distribution of different thickness layers of the joint.

There is a thin layer of high temperature around the interface between the tool and the material that is to be welded. The material temperature in this area is about 500 ◦C. The rheological resistance of this area is extremely low, and the material is in a viscous state, which constitutes a viscous layer. The material temperature of the outer part of the viscous layer is about 450 ◦C, and the material is in a thermoplastic state, which constitutes a thermoplastic layer [37]. Since the tool pin is conical, the contact surface of the middle layer tool pin becomes smaller, and the temperature of the middle layer

material, in which the temperature radiation area is also relatively reduced, decreases; furthermore, the plastic softening layer area of the material decreases. Similarly, in the welding area, the contact surface of the bottom tool pin is the smallest, the welding temperature of the tool pin at the bottom is relatively low, and the material flow speed also decreases, which is one of the reasons for the welding defects at the bottom.

The temperature profile of the temperature distribution is obtained from the upper and lower temperature gradients. The temperature field simulation of the friction stir welding process can determine the temperature distribution of different thickness layers. The temperature field directly affects the material flow, and the shearing action of the tool pin drives the plastic softening layer material flow. Therefore, the effect of the temperature on the material flow in different areas of the weld can be further studied.
