**1. Introduction**

Tungsten Inert Gas welding, also known as TIG or GTAW, is a clean and widely used welding technology for metals. Nevertheless, a main disadvantage of this welding process is the limited penetration ability. It may require multipasses to complete the weld for medium thickness materials, which results in very low productivity. In order to overcome this drawback, K-TIG was developed as a new TIG variant to increase the penetration capability and production efficiency. In K-TIG welding, a free burning arc is applied as the heat source to melt the weldment by forming a cylindrical keyhole [1], which provides a nice alternative to high power density welding. This type of keyhole welding is characterized by its dependence on arc force, as opposed to ablation pressure in laser-based and electron beam-based welding, and a combination of stagnation and recoil pressure in plasma arc welding (PAW), as reported by Jarvis and Ahmed [2]. Compared with standard melt-in mode TIG welding, K-TIG process possesses higher energy density, process efficiency and greater penetration ability. Although heat density of K-TIG is far lower compared to high power density welding, it is easier to operate and more cost-effective, as stated by Liu et al. [3]. In addition, Liu et al. [4] reported that K-TIG was able to achieve continuous open keyhole and was more stable than PAW. It was also demonstrated by Liu et al. [5] that K-TIG had much wider operating window compared with PAW owing to the fact that it has much smaller arc pressure/arc current ratio.

To date, K-TIG has found applications in industries for joining medium thickness materials, say titanium [6], zirconium [7], stainless steel [8], low carbon steel [9] and dissimilar metal welding [10]. It was shown in these studies that K-TIG can complete the medium thickness weld in a single pass with a V-shaped morphology. It was also shown that K-TIG produced a welded joint with smaller fusion zone and larger heat-affected zone compared with conventional TIG welding process. In addition, it has been demonstrated that K-TIG has the potential to increase the productivity while maintaining the mechanical properties at a level similar to the joints produced by conventional TIG welding. Recently, it was reported by Cui et al. [11] that K-TIG can even be used in underwater condition and can produce a duplex stainless-steel weld that meets underwater welding standards. In addition, because of the fact that the material properties of some welded joints produced by K-TIG are not satisfactory, several optimisation technologies have been introduced into the K-TIG welding process. For example, Fei et al. [12] introduced filler materials into K-TIG to improve the weld microstructure and mechanical properties of armour steel weld. Fei et al. [13] also applied specially designed interlayer to tailor the microstructure and solidification sequence of K-TIG-welded armour steel joint. Post-weld heat treatment and ultrahigh frequency were utilised by Xie et al. [14] to improve both the mechanical properties and corrosion resistance of 430 ferritic stainless steel welded joint. On the other hand, since K-TIG has a narrower operating window for carbon steel because of its higher thermal conductivity, several studies have focused on the keyhole behaviour and stability improvement, such as the application of back purging unit [15], one pulse one keyhole technique [16] and high frequency pulse current waveform [17]. Furthermore, in order to develop on-line control strategy for the K-TIG welding process, several penetration monitoring systems have been developed, such as arc voltage-based oscillation frequency sensing [18] and combination of acoustics and vision-based sensing [19].

Overall, the current research directions of the K-TIG welding technology can be divided into four parts, namely metallurgical qualification of K-TIG welded joints, optimisation technology for joint microstructure and properties, process dynamics and stability improvement as well as development of on-line monitoring system. Up to date, there has been little research on the relationship among process parameters, weld formation and mechanical properties in the K-TIG welding process. More importantly, unlike conventional TIG welding for which the weld formation can be partially controlled or compensated by the operators, the K-TIG operates in a completely automated mode, with the weld formation being entirely dependent on the welding parameter combination. Although in some studies, small amount of welding trials were conducted to test the influence of welding parameters in the K-TIG welding, the main purpose was to find a parameter combination to achieve full penetration, as did by Cui et al. [8] and Feng et al. [20]. The mechanism for defect formation and the correlation between weld formation and mechanical properties in the K-TIG welding process have not been addressed. To a practical view, the mechanical properties, especially tensile properties, are easily affected by weld formation which is a function of process parameters used during welding. Figuring out the relationship among welding parameters, weld formation, and mechanical properties is helpful to the process optimisation and is crucial for the avoidance of defect formation and premature failure during service.

In this study, K-TIG welding was performed on 6.2 mm thickness high hardness armour (HHA) plates using different welding parameter combination, aiming at evaluating the effect of heat input on weld formation and tensile properties. The mechanism by which various behaviours occurred has been discussed. The results would provide a practical guidance of parameter optimisation and quality assurance for the K-TIG welding process.

#### **2. Materials and Methods**

The material used for investigation is 500 grade armour steel, also known as HHA. Detailed chemical composition and mechanical properties of the base plate are listed in Tables 1 and 2, respectively. The chemical composition was measured by atomic emission spectroscopy. The base

metal shows tempered martensite microstructure, as shown in Figure 1. It is worth noting here that as this paper is intended to analyse the relationship among welding parameters, weld formation and tensile properties, the material-dependent aspect will not be discussed in detail.


**Table 1.** Chemical composition of HHA (wt%).

### **Table 2.** Mechanical properties of HHA.


**Figure 1.** Microstructure of the HHA base metal.

As depicted in Figure 2, the experimental equipment designed to conduct the welding trials was composed of a K-TIG power supply, a data acquisition device and a high-speed camera. The welding output was provided by a special power source with capability up to 1000 A. A high-speed camera in combination with a 10% transparency neutral filter was placed beside the worktable in order to record the arc behaviour. An inert gas back purge unit was designed to provide shielding from the atmospheric contamination on the root side of the weld pool. The argon shielding gas was transferred into the back purging unit from the left-hand side hose hole and allowed to be ejected through the switch on the right-hand side to avoid accumulation of pressure inside the unit. The image capturing was conducted after the process reached stable. During welding, the torch and high-speed camera were kept stationary while the worktable moved at the pre-set speed.

To test the influence of process parameters on weld formation and tensile properties, seven butt-joint welding experiments were performed on 6.2 mm thickness HHA plates by using control variable method. Process window identification tests were conducted to find the appropriate parameter combination leading to full penetration with the absence of both incomplete penetration because of insufficient heat input and weld pool collapse because of excessive heat input. Detailed parameter combination is depicted in Table 3. The heat input was calculated by the product of welding current and voltage divided by the travel speed plus a thermal coefficient of 0.6. The arc length was measured by the stacked thickness measurement device before welding. The dimension of the plates used in this study is 250 mm × 75 mm × 6.2 mm. The welding current and travel speed were chosen as variables. The fixed parameters for all experimental groups are depicted in Table 4. After welding, both transverse and longitudinal cross sections of the welds were cut from the weldments. The longitudinal cross section was cut in the arc extinguishment area in a bid to capture the in-situ profile during welding. The surface morphology and cross section macrographs were captured by Nikon digital camera and

Leica M205A stereomicroscope, respectively. The mounted samples were etched with 2 wt% nital for both macro and microstructure observation. Microhardness test was undertaken using Struers DuraScan-70 automatic hardness tester 2 mm below the surface with 0.5 mm interval under a load of 1 kg for 10 s dwell time. The preparation of tensile samples and the execution of tensile tests were in accordance with ASTM E8/E8M-16a guideline. The tensile samples were cut perpendicular to the welding direction using Wire Electrical Discharge Machining. Tensile tests were performed in as-cut condition without any finishing on the surface of tensile samples with 1 mm/min crosshead speed and 25 mm gauge length (Figure 3). The thickness of all tensile test specimens is 6.2 mm. A CCD camera was used to measure the total elongation by capturing the movement of two white spots with 25-mm interval on the tensile specimens. In order to ensure the reproducibility of tensile results, three samples were tested and the average value was considered.

**Figure 2.** Schematic of experimental system.


**Table 3.** Welding parameter combination.

**Table 4.** Fixed welding parameters (all experiments).


**Figure 3.** Dimension of tensile specimen.
