*3.5. Experimental Details*

For the purpose of validating the feasibility and accuracy of the presented model, the welding experiment was automatically carried out by an YC-400TX GTAW machine (Panasonic Welding Systems (Tangshan) Co., Ltd, Tangshan, China). The welding parameters used are shown in Table 3. It should be pointed that the welding efficiency in Table 3 was set as 0.8 [28]. Then, the specimens were mechanically ground with 120-grit SiC paper, were etched in a solution composed of 400 mL H2O + 40 g KOH + 40 mL H2O2, and purged with an ultrasonic cleaner in order to remove surface contaminants.

**Table 3.** Welding procedure parameters of Ti–6Al–4V alloy plate gas tungsten arc welding (GTAW).


An accurate simulation for the moving heat source of the weld process is a prerequisite to correctly predict the temperature distribution and dendritic growth. The parameters' calibration for the heat source, including the peak temperature, shape, and dimensions of the welding molten pool, is performed by optimizing parameters such as the welding voltage and the welding current. The parameter optimization is based on satisfying the macrograph experimentally observed for a welding cross section.

The calculated temperature field was validated by means of the K-type thermocouple measurement of the transient temperature captured at corresponding test points during the actual welding process, as shown in Figure 6a. In order to evaluate the accuracy for the simulated temperature distribution results, the test platform of the welding thermal cycle was built as shown in Figure 6b. It primarily consisted of a stored energy welding machine (self-developed), welding sample, K-type thermocouple, temperature measurement module (TR-W500, KEYENCE Corporation, Osaka, Japan), and computer. Since it is very hard to put the thermocouple very close to the welding sample surface without surpassing the temperature limitation for the thermocouple, the temperature distribution was measured at a smaller distance from the heat affected zone (HAZ). Hence, the K-type sheathed thermocouples were directly embedded in the welding sample surface by using the stored energy welding machine. The embedded thermocouples were located at four points on the workpiece surface; as shown in Figure 6c, these four points were evenly spaced along the welding direction.

**Figure 6.** Testing of the welding thermal cycle: (**a**) the welding sample geometry, (**b**) the diagram of the test platform, and (**c**) the picture of the test platform.
