*3.1. Effect of FSW Parameters on the Heat Input*

Heat input is one of the important parameters associated with all welding processes and affects the weld quality and properties. Although, FSW is characterized by low heat input relative to the fusion welding processes, still heat input plays a significant role in controlling the joints properties and quality [33]. In this work, the control system in the FSW machine used allows the recording of the spindle torque *T* (N·m) that can be used with the other FSW parameters such as rotational speed ω (rpm) and the welding speed *v* (mm/min) to calculate the heat input. Heat input is defined as the heat energy applied to the workpiece per unit length in the unit of (J/mm). The source of heat generated during FSW is mainly from the friction between the tool and the stirred material and the heat input during FSW can be calculated using Equation (1) [33–35]:

$$\text{Heat Input (J/mm)} = \frac{power}{speed} = \eta \left(\frac{\omega T}{v}\right) \tag{1}$$

$$\text{Where } \omega = \left(\frac{2\pi r}{60}\right) \tag{2}$$

where *T* is the torque (N·m), ω is the rotational speed (rpm), *v* is the linear speed (mm/min) and η is the efficiency of heat transfer, (η = 0.9) [36,37]. The pseudo heat index is represented by the ratio of the square of the rotational speed to travel speed (ω2/*v*). As a function of FSW parameters, it can be considered a simple heat input metric and a well-known method to predict the heat generated during FSW. The maximum temperature highly depends on the rotation tool speed while the heating rate depends on the welding speed at a given tool geometry and plunge depth. The rotation tool speed term is squared because of its significant effect on the heat generated during the process [38]. The pseudo-steady-state welding parameters, calculated heat input and heat index are presented in Table 4.


**Table 4.** Key pseudo-steady-state welding parameters.

For the joints AA5083/AA5754, Figure 4a shows that the relatively high travel speed (60 mm/min) with low rotational speed (400 rpm) resulted in low ω/*v* value (6.66) and consequently low heat input value. Decreasing the welding speed to 40 mm/min for the same rotational speed of 400 rpm in Figure 4a increased the ω/*v* value to 10, leading to an increase in the heat input level. In Figure 4a, although the value of is the same (ω/*v* = 10) for a travel speed of 60 mm/min and rotational speed of 600 rpm as that in Figure 4a, the increased travel speed of 60 mm/min has showed a more dominant effect than the rotational speed (600 rpm) and resulted in decreasing the HI level. For the other system of joint (AA5083/AA7020; Figure 4b), the heat input can be interpreted in the same manner as explained in Figure 4a. The increased level of ω/*v* value (25) in Figure 4b (500 rpm and 20 mm/min) has resulted in obvious increase in the heat input level which reach the value of 261 J/mm. Changing the arrangement of the plates from AA5083/AA7020 to AA7020/AA5083 for the same ω/*v* value (6.25) has showed no difference in the power and heat input values, as shown in Figure 4b.
