**3. Results and Discussion**

The program used for high-speed infrared thermography camera for temperature recording and data retrieval was the IRBIS® 3.1 Professional program. Figure 7 shows an example of an image captured from a high-speed infrared thermography camera. Each pixel of the image can show the resulting temperature and, therefore, allows knowing the exact location of the temperature. Real-time temperature measurement results using a high-speed infrared thermography camera according to the design of the experimental design could be graphed as shown in Figures 8–11, which are shown, at position 1 (position of weldment), position 2 (position of HAZ), and position 3 (position of base material), to have a graph in the shape like an "M". Position 1 has the highest measured temperature of around 1300 ◦C and a minimum of about 900 ◦C. It was shown that the weld pool will have a temperature between 900 and 1300 ◦C and then gradually cool down. The occurrence of the graph in the shape like an "M" owing to the area in front of the weld pool has the area of plasma charge that will cause the workpiece to form a keyhole and this area has the highest temperature, as shown in the graph. Behind the plasma area, caused by the hot wire feeding, causing the temperature to decrease and the tail of the weld pool, the temperature will drop slightly close to the temperature of the plasma area, as shown in Figure 12. Therefore, using a high-speed infrared thermography camera to record the real-time temperature will result in the graph in the shape of an "M". Whereas, if the recording frequency is not high or using a thermocouple or if the hot wire is not fed, the "M" graph will not be produced. The weldment position will have a maximum temperature of about 1300 ◦C higher than the HAZ position, which has a maximum temperature of about 1000 ◦C, higher than the base material position, which has a maximum temperature of about 700 ◦C, respectively. The temperature decreases in this order owing to the increasing distance from the center of the weld, in which the highest temperature corresponds to Figure 13. For peak temperature at different distances from the welding center line, as shown in Figure 13, we found that the resulting graph has very little temperature differences owing to the same type and size of material used, even when using different parameters of welding. The maximum temperature at the weldment position of run order 1 is 1324.64 ◦C, run order 2 is 1324.67 ◦C, run order 3 is 1324.69 ◦C, and run order 4 is 1324.62 ◦C.

**Figure 7.** The captured image from a high-speed infrared thermography camera.

**Figure 8.** The temperature profile of run order 1.

**Figure 9.** The temperature profile of run order 2.

**Figure 10.** The temperature profile of run order 3.

**Figure 11.** The temperature profile of run order 4.

**Figure 12.** The characteristics of the weld pool results in an M-shaped graph.

**Figure 13.** Peak temperature by the distance of the workpiece.

In the graph temperature of the hot wire from Figures 9 and 11, which use hot wire current at 30 A, the hot wire has an average temperature of about 800 ◦C throughout the welding process, and the final position when the hot wire is pulled back, as shown in Figure 14, at the tip of the hot wire, will have a temperature of around 1200 ◦C. It was found that the hot wire during the welding process is still lower than the weld pool, resulting in the hot wire possibly not melting homogeneously with the workpiece, resulting in the workpiece having lower strength according to the tensile strength results obtained from Table 7. In Figures 8 and 10, which use a hot wire current at 40 A, the hot wire has an average temperature of about 1000 ◦C throughout the welding process, and the final position when the hot wire is pulled back at the tip of the hot wire will have the temperature around 1200 ◦C. During the welding process, the hot wire has the temperature close to the weld pool, causing the hot wire to melt homogeneously with the workpiece, resulting in the workpiece being stronger, such as in the results obtained from Table 7. Therefore, if the workpiece needs to be strong as per the standards, it is necessary to control the hot wire current so that the hot wire has an average temperature during welding around 900–1300 ◦C to make the workpiece as strong as possible.

**Figure 14.** The captured image while the hot wire pulls back.

From the recorded temperature, the cooling rate can be calculated according to Equation (1); the values used to calculate the cooling rate are shown in Table 6, which found that the cooling rate of 800–500 ◦C is between 13.42 and 17.31 ◦C/s. Here, position 1 (position of weldment) has a slightly lower cooling rate than position 2 (position of HAZ) in the same workpiece. This is because position 1 has a higher temperature than position 2. Moreover, it was found that the parameters of the hot wire process affect the cooling rate. If the wire current increased, the cooling rate tends to be slower owing to the hot wire temperature.


**Table 6.** Cooling rate calculation at position 1 and position 2.

The tensile test results found that run order 3 gave the best results, followed by run order 1, 2, and 4 respectively. Run order 3 and 1 gave the ultimate tensile strength, yield strength, and elongation close to the reference workpiece, which shows that the parameters of run order 3 and 1 are suitable for use. Run order 4 gave the least value, by visual inspection of a broken workpiece, it was found that the fracture was in the position of the workpiece's joint (center of the welds). The breakage characteristics are brittle, resulting in lower ultimate tensile strength, yield strength, and elongation compared with other workpieces, where other workpieces fracture at the base material near the HAZ. The results of the tensile strength given in Table 7 show the consistency of the hot wire parameters, the temperature of the hot wire, and the cooling rate, as shown in Figure 8 to Figure 11. If the hot wire current was 30 A, the temperature of hot wire was about 800 ◦C during welding, which was lower than the temperature of the weld pool (around 900–1300 ◦C). On the other hand, if the hot wire current was 40 A, the hot wire will have a temperature of around 1000 ◦C, which was in the temperature range of the weld pool, causing the hot wire to melt homogeneously, making the workpiece stronger. Furthermore, it was found that the relationship between the cooling rate and tensile strength was inversely proportional; the workpiece with a lower cooling rate will be stronger than the fast cooling rate workpiece, as shown in Figure 15.

**Table 7.** Tensile strength results according to the experimental design.


\* As-received material.

**Figure 15.** The relationship between cooling rate and tensile strength.

For Vickers microhardness values measured at each specified position shown in Figure 16, the microhardness characteristics were shaped like a "W", with all the smallest microhardness values at the HAZ area that was away from the welding center line about 5 mm, the measured microhardness was in the range of 180–210 HV, which was more than the standard microhardness of AISI 316 specified at 146 HV. It can be concluded that this parameter was suitable for use. The relationship between cooling rate and microhardness is shown in Figure 17. It was found that, if the cooling rate of 800–500 ◦C was between 13 and 16 ◦C/s, the microhardness will increase as the cooling rate increases. Although the cooling rate of 800–500 ◦C greater than 16 ◦C/s will result in a significant decrease in microhardness, the microhardness value is still higher than the standard value. However, the cooling rate was very important, thus the cooling rate of 800–500 ◦C should be controlled between 13 and 16 ◦C/s for the suitable results. Therefore, to control the cooling rate appropriately for this research, the hot wire current should be used at 40 A and the hot wire feed rate of 1.5 m/min or 1.7 m/min will give the most appropriate tensile strength and microhardness results.

**Figure 16.** Vickers microhardness results according to the experimental design.

**Figure 17.** The relationship between cooling rate and Vickers microhardness.
