*2.2. TIG Welding of Similar and Dissimilar Alloys and Metals*

One of the most critical factor to consider for TIG welding is the filler metal, which mainly depends on the alloys to be welded. Ishak et al. [28] investigated the welding of dissimilar AA6061 and AA7075 using different filler metals, i.e., ER4043 (Si-reach) and ER5356 (Mg–reach). The depth analysis revealed that ER5356 penetrated deeper compared to the ER4043 and the depth of penetration plays an important role towards the strength of the weld or joint. The microstructural analysis revealed the existence of fusion zones that are normally identified on dissimilar TIG joints. The fusion zone (FZ) filled with ER5356 had finer grain sizes compared to the FZ filled with ER4043. Average hardness values for ER5356 filler specimens were higher compared to ER4043 filler specimens. TIG welding using the ER5356 filler yielded better joint compared to ER4043.

Borrisutthekul et al. [29] have evaluated the feasibility of using TIG welding technique in joining the dissimilar materials, i.e., steel plate and aluminium alloy plate. The microstructural analysis reveal that there was an existence of fusion zone (FZ) and heat affected zone (HAZ) which are the characteristics of the TIG welding. All the specimens were fractured on the same location during the tensile analysis, i.e., HAZ of aluminium alloy side. This type of behaviour was due to the growth of grain sizes that were observed through microstructural analysis. The existence of intermetallic reaction layers was also observed during the microstructural analysis.

Most studies that are studying different aspects of welding seem to be dominated by mostly two dissimilar aluminium alloys, i.e., AA5083 and AA6061 [30–35]. Waleed and Subbaiah [30] have evaluated the effect of using ER4047 filler rod in welding aluminium alloys AA5083-H111 and AA6061-T6. Similar analysis was also performed by other researchers with the focus on different aspects and different welding parameters on different grades of AA5083 [31–35]. Waleed and Subbaiah focused on analyzing the mechanical behaviour of the joint formed through the use of ER4047 filler rod. The tensile strength of the joint was lower than that of the base metals. The hardness value of the joint was varying in each side of the joint. This variation was caused by the formation of the magnesium-silicon (Mg2Si) precipitates on the AA6061 side. The microstructural analysis showed the elongation of grains towards the rolling direction. There was also an existence of cavities and micro-pores at the intersection point of the weld. There was a notable decrease in ductility and this decrease was caused by the presence of columnar grain.

In another study, the AA2195 was joined using the ultrasonic assisted TIG welding with the aim of analyzing the weld characteristics in terms of size and porosity [36]. It was found that the pores existed at the weld adjacent to the surface. The size of the porosity was found to increase with the decrease in welding speed. Additionally, the increase in ultrasonic power resulted to the decrease in weld porosity. Wang et al. [37] used the TIG welding technique to fabricate the AA7A05-T6/AA5A06-O dissimilar joint in order to study the mechanical properties of the said joint. The results revealed that the dissimilar joint had a tensile strength which was 78.8% and yield strength of 97.24% of the base metal (AA5A06-O). The elongation was about 84.29%to that of the base metal AA7A05-T6. The microstructural analysis results showed a coarse grain sizes due to high heat input which resulted in hardness and strength drop.

Narayanan et al. [38] have evaluated the impact of TIG welding parameter variation on the AA5083 joints. The welding current and the shielding gas flow rate were the two parameters that were being varied for the duration of the study. The tensile results and the hardness value for the joint was lower than that of the commercial base metal. The microstructural analysis showed that the grains in the HAZ region were coarser compared to the base metal hence the brittle failure. The welding quality improvement of AA6031 plates using an automated TIG welding system was performed by Mohan [39]. The mechanical analysis showed that the joint performance was found to be way lower than that of the base metal. There was an inverse proportionality observed between the welding speed and the tensile strength of the joint. There was also a variation of hardness value across the weld.

Automated pulse TIG welding using AA5083 and AA6061 dissimilar plates was performed by Baghel & Nagesh, [40]. The main purpose was to evaluate the mechanical behaviour of the joint formed through this technique. The radiographical analysis revealed the presence of porosities which were caused by the lack of proper penetration. The tensile results for the joint were lower than the base metal. There was a variation of hardness value which was caused by non-uniformity of the grain sizes across the weld. The surface fracture exhibited the ductile failure mode. The impact of the welding speed variation towards the quality of the AA5083 TIG welded joints was analyzed by KumarSingh et al. [41]. All the other parameters were kept constant but only the welding speed that was varying. The tensile results showed a linear relationship with the welding speed until 100 mm/min. The notable decrease in

tensile results was observed at the welding speed beyond 100 mm/min. The microstructure of the weld pool showed a refined grain size in comparison to the base metal.

TIG welding of dissimilar AA2014 and AA5083 was investigated by Sayer et al. [42]. One-sided TIG welding was applied with two passes. The microstructural analysis results in the weld region showed nonhomogeneous less equiaxed grain distribution with bigger diameters when compared to AA2014 and AA5083-O base metals. The grain size increase was said to be due to severe heat input. The tensile test results were lower than those of base metal. The tensile test specimens fractured in the welded region revealing brittle mode of failure. There was a variation in hardness across the weld with a sharp decrease at the center. This sharp decrease at the center was caused by the high silicon content in the filler material which dominated the center of the weld. Singh et al. [43] reported the mechanical properties of TIG welding at different parameters with and without the use of flux. The welding parameters used were all varied with the purpose of determining the optimal welding parameter combination. The variation of current effected the decrease in hardness value of the joint. The hardness values for joint formed with flux were higher compared to those formed without flux.

Dissimilar AA7075-T651 and AA6061-T6 plates were TIG welded with the aim of investigating the hardness of the center of the weld joint [44]. The Al-Si alloy filler wire type was used in performing all the welding. The maximum hardness value for the joint was found to be lower than that of the base metals. There were voids that were observed through microstructural analysis. Kumar et al. [45] also discovered that the use of pulsed current during TIG welding improves the mechanical properties of the welded joint in comparison to continuous current welded ones. This was found to be caused by the microstructural grain refinement which occurs in the fusion zone. Table 2 summarizes the mostly studied welding parameters during welding similar and dissimilar alloys/materials. It also shows the typical plates profile and filer wire used for different analysis.


**Table 2.** TIG welding of dissimilar materials/alloys. (WC—welding current, WS—welding speed, *Q*—gas flow rate, FZ—fusion zone, YS—yield strength, *V*—voltage, *F*—frequency, FW—filler wire, BM—base metal, UP—ultrasonic power, NS—not specified).
