**2. Materials and Methods**

The material used in this research is a stainless steel sheet in accordance with AISI 316 (equivalent to DIN 1.4401 and JIS SUS 316), which has a chemical composition measured from the machine brand, Thermo Scientific, model: ARL 3460 Metals Analyzer by optical emission spectrometry (OES) compared with the standard chemical composition [23] and the mechanical property values as in Tables 1 and 2, respectively.


**Table 1.** Chemical composition of AISI 316 by optical emission spectrometry.

**Table 2.** The mechanical property of AISI 316.


The workpiece used was 100 mm in width, 150 mm in length, and 3 mm in thickness using two sheets per experiment and using a closed square butt joint. The workpiece will be clamped by jigs to prepare for hot wire plasma arc welding by machine brand, Cebora, model: EVO 450T (Bologna, Italy), which is used as the current source. The plasma welding controller from the machine brand, Cebora, model: digital Console PW30 (Bologna, Italy), was employed, which is used for controlling various parameters for plasma welding. The hot wire machine used the brand, MAC, model: Power Assist IV-642 (Osaka, Japan), with a wire feed system to the welding torch, which is attached to the ABB robotic arm, model: IRB4400 (Västerås, Sweden) used for controlling the hot wire plasma arc welding path, as shown in Figure 1. The hot wire plasma arc welding parameters used in this research are shown in Table 3. The welding process starts from the right-hand side of the workpiece and ends at the left-hand side. Here, the plasma torch was at the front, while the hot wire was feed behind the plasma welding torch, with a total welding distance of 130 mm.

**Figure 1.** The hot wire plasma arc welding system.

For real-time temperature measurement during hot wire plasma arc welding, the high-speed infrared thermography camera brand, Infratec, model: ImageIR 8300 (Dresden, Germany), was used, with a temperature measurement accuracy of ±1%. The principle of temperature measurement was that all objects with temperatures above zero kelvin (−273.15 ◦C) emit infrared radiation that is invisible to the human eye. The camera with an Indium antimonide (InSb) sensor will measure the mediumwave infrared (MWIR) radiation, wavelength between 2 and 5 μm. The conversion of infrared radiation detected by sensors into temperature units, based on Planck's law and Stefan–Boltzmann's law according to the behavior of infrared pyrometer. Infrared emission of the object to be measured may be more or less depending on the wavelength. The parameters such as material composition, surface roughness, and measurement angle have some influence on wavelength [24]. Thus, the installation

of the camera must control the environment to be constant throughout the experiment to not affect the results. This camera is equipped with an ImageIR® standard lens with a focal length of 25 mm and a motorized filter wheel with spectral filters. This camera will be installed at a distance of 80 cm from the welding workpiece. The camera mounting point will be on the side of the workpiece so that the temperature can be seen throughout the welding length, as shown in Figure 2. This research sets the camera to record the temperature every 0.5 s (*f* = 2 Hz), using the recording time of 240 s. Using a high-speed infrared thermography camera will make it possible to be aware of the temperature occurring throughout the welding process, and to know the temperature in every position such as weldment position, HAZ, and base material, including the temperature that occurs on hot wire feed to the weld pool, from the beginning to the cooling rate after the welding is done. Although the plasma light is brighter, the infrared radiation is always emitted, which passes through the spectral filters into the camera's sensor. This camera uses an Indium antimonide (InSb) sensor that is narrow-gap sensitive at wavelengths between 2 and 5 μm. Therefore, other wavelengths will not affect the measurement of the temperature.



**Figure 2.** Installed high-speed infrared thermography camera.

The cooling rate at the temperature range 800–500 ◦C will be studied because the temperature range is the material phase transformation, which has a significant impact on the properties of the workpiece. The formula for calculating the cooling rate range 800–500 ◦C is calculated from a simple equation as shown in Equation (1):

$$\text{Cooking } rate\_{800-500^\circ \text{C}} = \frac{\Delta Temp\_{800-500^\circ \text{C}}}{\Delta time\_{800-500^\circ \text{C}}} \tag{1}$$

where Δ*Temp*800−500◦*<sup>C</sup>* = the temperature difference at 800 ◦C and 500 ◦C, Δ*time*800−500◦*<sup>C</sup>* = the time difference at 800 ◦C and 500 ◦C.

Each workpiece uses three positions to measure the resulting temperature, as shown in Figure 3. Here, position 1 (P1) is the center of the weldment; position 2 (P2) is at a distance of 5 mm away from the center of the weldment, which is the area of the HAZ; and position 3 (P3) is at a distance of 10 mm from the center of the weldment, which is the base material. Besides, the temperature of the hot wire at a height of 7.5 mm from the workpiece surface is measured.

**Figure 3.** Position for measuring temperature.

Once the installation of the hot wire plasma arc welding system is finished, the next step will be to design the experiment, in which this research will use the design of a full factorial experiment. The factors used in this study will be the factors of the hot wire process, which has two factors, wire feed rate and wire current, where each factor is assigned to have two levels caused by the trial and error of the preliminary study, whereby such parameters and ranges do not cause defect workpieces and obtain complete welds, such as no sputtering during welding, good penetration, no cracking of welds, uniform hot wire during welding, and the experimental design table, as shown in Tables 4 and 5, respectively.

**Table 4.** Determining the level of factors used in the experiment.



**Table 5.** Full factorial experimental design table.

When the hot wire plasma arc welding process is finished according to the experimental design. The workpieces in each experiment are divided into three parts, as shown in Figure 4. The parts number 1 and number 3 of each welded workpiece will be tested for ultimate tensile strength in accordance with ASTM-E8 to find the average ultimate tensile strength value and the dimension of the tensile test workpiece, as shown in Figure 5. Tensile testing uses the machine brand, Instron, model: 8801 (Norwood, MA, USA), equipped with a strain gauge to find an elongation at break.

**Figure 4.** Separation of welded workpieces.

**Figure 5.** The dimension of the test workpiece in accordance with ASTM-E8.

Part number 2 of each welded workpiece after going through the grinding and polishing processes will be tested for Vickers microhardness by machine brand, Anton-Paar, model: MHT-10 (Graz, Austria). The parameters used are 200 g of testing force or HV0.2; time of withstanding the load of 15 s; loading speed of 25 g/s, which will test the microhardness at the center of the cross-sectional area (measured at a distance below the surface about 1.5 mm); a total of 15 points; and two times per point to find the average, as shown in Figure 6. The results will be displayed in a graph that will show the microhardness value in each position.

**Figure 6.** Position for measuring Vickers microhardness.
