**1. Introduction**

Hot wire plasma arc welding is relatively a new welding process, a hybrid process between the plasma arc welding (PAW) process and the hot wire process. The hot wire process can be used as a hybrid with other welding processes such as gas tungsten arc welding (GTAW) process or laser welding process [1–3]. The basic principle of the hot wire process is to heat the consumable filler wire by a separate power source until close to the melting point and feed to the weld pool [4]. There are many advantages over the cold wire process, mainly high deposition rates [5,6], which will assist to increase welding speed and productivity. The work of [7] has been conducted to study mechanical properties to compare the results between the use of hot wire and cold wire of GTAW. The results showed that the mechanical properties obtained were not significantly different between the use of hot wire and cold wire. In general, hot wire process parameters (such as wire current, wire feed rate, wire feed angle, wire size, wire contact length, gas shield) have a significant effect on the quality of the obtained workpiece [8–10]. The selection of inappropriate parameters can result in weld defects [11]. Therefore, choosing the appropriate parameters will increase the mechanical properties and achieve quality welding workpieces. The optimization of hot wire laser welding parameters has been studied [12] based on ensemble metamodels and non-dominated sorting genetic algorithm. This research studied

the three parameters, that is, laser power, welding speed, and hot wire current. The results showed that the optimal process parameters gave results consistent with the experiment. Although research articles related to the hot wire process are available [13–15], research on hot wire plasma arc welding is limited. Therefore, the study of the basic parameters of the hot wire process is interesting to obtain good mechanical properties of the welding workpiece. Recently, cameras have been widely used to study the real-time welding process [16–18], which are non-contact measure and give results with high accuracy and precision. The information obtained from the camera gives insight into various forms of weld results such as temperature, geometric shape, and defects. The temperature data recorded from the camera can be used to calculate the cooling rates that occur in the range of 800–500 ◦C, which is a material phase transformation that affects the microstructure and mechanical properties. The effects of cooling rate in a super duplex stainless steel on pulsed GTAW welding have been studied [19] using three process parameters (heat input, wire feed rate, wire feed technique). The results found that the heat input plays an important role in the cooling rate, which affects microstructure in the heat-affected zone (HAZ) and weld zone. From the literature review [20–22], we found that the main factors affecting the cooling rate were heat input, type of material, and the thickness of the material.

This paper presents the hot wire plasma arc welding process using a high-speed infrared thermography camera to measure the temperature in real time. The resulting temperature profile allows to calculate the cooling rate of the workpiece, which will obtain the relationship between the cooling rate and the mechanical properties (tensile strength, microhardness). Furthermore, the appropriate parameters for the hot wire plasma arc welding process are obtained that allow the welding workpiece to have tensile strength and microhardness within the specified standards.
