Two unique fabrication technologies that have revolutionized the manufacturing industry are Additive Manufacturing (AM) and TIG welding. TIG welding is a conventional method of fusing metals, whereas AM is a state-of-the-art process enabling layer-by-layer creation of complicated shapes. Since TIG welding can yield precise, high-quality welds with superior mechanical qualities, it is a widely used technique for connecting titanium alloys. Moreover, high integrity joints are necessary for titanium alloys used in manufactured aircraft structures in order to satisfy design specifications [1], [2]. The techniques of deploying TIG, Electron Beam Welding (EBW), Plasma Arc Welding (PAW), and Laser Beam Welding (LBW) may all produce high-quality fusion-welded connections in titanium alloys. Deep penetration with little overall heat input is made possible by the creation of a keyhole in high energy beam welding procedures because to the highly concentrated energy input.

Relatively low residual stresses and limited fusion and Heat Affected Zone (HAZ) microstructural changes result in welds with suitable fatigue and mechanical qualities. At significantly reduced capital costs, TIG and PAW give the possibility to produce welds that are on par with EBW or LBW in quality [3-6]. It has been demonstrated that TIG welds can have mechanical qualities that are similar to EBWs in both cast and wrought base materials, despite the fact that the increased heat input in arc welding methods results in broader weld zones with coarser microstructure. The low density (equivalent to 4.43 g/cm3 for Ti and 2.80 g/cm3 for Al) and strong mechanical properties (the average ultimate tensile strength is equal to 300 MPa for Al alloys and 1205 MPa for Ti alloys) are the distinguishing features of Titanium (Ti) and Aluminium (Al) alloys [7], [8]. In aircraft and automotive applications, where weight reduction is necessary along with excellent mechanical strength and corrosion resistance, Al–Ti dissimilar welds are of great importance. However, due to their dissimilar physical characteristics and the development of Intermetallic Phases (IMC), which cause cracks to occur during cooling and solidification, connecting Al/Ti alloys remains a challenging technological task. Reducing the thickness of the IMC layer at the interface and the quantity of IMC particles scattered across the aluminium Fused Zone (FZ) are necessary for the successful connecting of aluminium to titanium [9-11]. Actually, despite the attractiveness of intermetallic compounds based on the Ti–Al system–which exhibit strong properties at elevated temperatures–the growth of IMC results in limitations on the deformation modes that are accessible. Therefore, the presence of such particles is linked to a decrease in fracture toughness and ductility but an increase in strength. The chemical makeup of the weld zone, the welding process, microstructural components, and other factors can all have an impact on the mechanical and corrosion properties of the weld. Zhang et al. [12] and Long et al. [13] observed that the β → α transformation's cooling rate-dependent process may alter the weld zone's shape. Increased cooling rate has the potential to alter weld cellular structure to a dendritic columnar shape. Furthermore, it has been said that epitaxial growth influences the grain size of Weld Metal (WM)via influencing the grain size of the HAZ [14], [15].

For instance, Ti-6Al-4V, a titanium alloy that is widely used, is characterized by its excellent reactivity to oxygen, low thermal conductivity, and relatively high strength at specific welding temperatures. Since titanium alloys maintain their increased strength and stability at comparatively high temperatures, they can be joined with other kinds of steels to achieve multifunctional applications. Ti-6Al-4V is an alpha-beta alloy; during the welding process, the microstructure's complexities are established in an effort to enhance the alloy's melting hardness and strength. As thermoplastic complex structures are used to withstand the static and fatigue pressures necessary for aerospace vehicles, their utilization is fast expanding and replacing that of their metallic and thermoset complex equivalents [16-19].

One of the least stressful metals to weld with TIG is titanium, which may be done both manually and automatically. Metals can be joined together in the presence of an inert gas, such as argon, helium, or a combination of the two, by creating an arc between the workpiece and the tungsten alloy electrode, which shields the welded area from air contamination [20], [21]. Additionally, the gasses facilitate faster welding and electrode penetration using TIG, titanium alloys can be welded independently (without filler) or with the aid of filler wire [22], [23]. Detailed procedure for setting up a TIG welding experiment are as follows:

I. Safety precautions: before starting the TIG welding experiment, it is essential to ensure the safety of the operator and the surrounding environment. This includes wearing appropriate personal protective equipment such as welding gloves, helmet, and apron. Additionally, make sure the work area is well-ventilated to prevent exposure to harmful fumes.

II. Equipment set-up: the next step is to set up the TIG welding equipment. This includes the TIG welding machine, gas cylinder, tungsten electrode, filler rod, and workpiece. Make sure all equipment is in good working condition and properly connected.

III. Tungsten electrode Preparation: the tungsten electrode is a crucial component of TIG welding as it generates the arc. To prepare the tungsten electrode, grind the tip to a point using a dedicated tungsten grinder. The electrode should be sharpened to a fine point to ensure a stable arc during welding.

IV. Gas selection: TIG welding requires the use of a shielding gas to protect the weld pool from atmospheric contamination. Common shielding gases used in TIG welding include argon and helium. Select the appropriate gas based on the material being welded and set the flow rate according to the manufacturer's recommendations.

V. Welding parameters: before starting the welding process, set the welding parameters on the TIG welding machine. This includes adjusting the amperage, voltage, and travel speed based on the material thickness and type. Refer to the Welding Procedure Specification (WPS) for guidance on the correct parameters.

VI. Welding technique: once the equipment is set up and the welding parameters are adjusted, it is time to start the welding process. Hold the torch at a 15-20 degree angle to the workpiece and initiate the arc by striking the tungsten electrode against the workpiece. Maintain a consistent travel speed and filler rod feed rate to create a smooth and uniform weld bead.

VII. Post-welding inspection: after completing the welding process, inspect the weld for any defects such as cracks, porosity, or lack of fusion. Use non-destructive testing methods such as visual inspection, dye penetrant testing, or ultrasonic testing to ensure the weld meets the required quality standards.

TIG welding is a versatile welding technique that requires careful preparation and attention to detail. By following this procedure, TIG welding experiment can be successfully set-up and high-quality welds achieved. Remember to prioritize safety, adhere to welding parameters, and inspect the weld for quality assurance. TIG welding experimental setup for the purpose of welding a butt joint is depicted in Fig. 1.

Because TIG welding is widely used in industries, it is important to understand how different parameters affect the mechanical behaviour of Ti6Al4V alloy in order to reduce welding costs and obtain high-quality welds. Researchers have used a variety of statistical and experimental techniques to identify the parameters that can be compromised for another in order to have a high-quality weld with good mechanical properties. Taguchi method, Response Surface Methodology (RSM), etc. are common techniques used in optimizing these parameters, and Box-Behnken and Artificial Neural Network (ANN) are commonly used in prediction analysis of these parameters [29-31]. Additionally, current pulsing has a significant impact on the parameter selection for TIG welding. The main factors influencing the quality of TIG welds are peak current, background current, pulse frequency, and pulse on time [32]. When compared to an unpulsed frequency process, an increase in pulse frequency enhances the preceding β grain size within the FZ and influences the stress created in TIG welded Ti6Al4V alloy. The metal's thickness affects the joint preparation. For butt joint welding of Ti6Al4V alloys, plates thicker than 3 mm must first undergo edge preparation, which involves cutting a V-groove to allow for appropriate penetration [33], [17]. Rough-edged plates often result in more porosity in the weld.

To lessen thermal stress within the welded zone in plates thicker than 6 mm, the J groove is utilized rather than the V groove. In order to have a constant and good penetration, the J groove angle should be between 45° and 65°. This will also prevent distortions. If it's necessary to weld from both sides, a double V joint preparation may be used [34-36]. The presence of α' martensitic microstructure in the weld zone of Ti6Al4V alloy welded by TIG is expected to increase the microhardness of the alloy. They further observed that the pulsed current method of TIG welding exhibits better micro-hardness towards the FZ than the unpulsed mode, albeit with a significantly lower average hardness value. A higher pulse frequency results in a decrease in micro-hardness in the Fatigue Zone (FZ) because the previous β grains are finer than in the unpulsed process. It can also be associated with a high amount of residual stress.

Cheepu et al. [37] presented microstructures of TC4 Titanium Alloy welds at Fusion Zone (FZ) produced with the welding current of 80, 120, 150 and 160 A as shown in Fig. 2a-2d. The increase in heat input in the welds leads in the formation of larger grains especially at the welding current of 120 A. the presence of single enlarged grain in the FZ may be one of the reason to decrease in strength of the joints. When the welding current increased from 150 A, the microstructures showed different from the lower welding currents. The microstructures at higher welding current and speed are showed finer grains and with the less number of enlarged grains. Due to this, the welds achieved highest strength at the higher welding currents even though the variation in the welding speeds.

Fig. 3a-3b presented by Szwajka et al. [38] illustrates the microstructure of pure grade 1 titanium weld, showing a fine acicular structure and coarse grains under the influence of heat input due to welding temperature. It was observed that as the flow of the shielding gas decreases, the hardness of the weld material increased and its brittleness also increased. A similar trend related to the amount of gas flow was also noticeable for the tensile strength of the joints. Due to the lack of proper protection of the weld root by shielding gas, the WM was contaminated by elements from the surrounding atmosphere, which resulted in porosity of the WM (see Fig. 3c). It was observed that the number of pores in the weld on the side of the root increased as the shielding-gas flow rate decreased. Fig. 3c shows a cluster of pores in the weld made with the lowest shielding-gas flow

As seen in Fig. 4, a welded metal is made up of three components: the Base Metal (BM), the HAZ, and the FZ. The area that receives the most heat during welding is known as the flux zone, or WZ. This zone reaches the liquidus temperature before solidifying as a result of cooling. The high hardness in the region is mostly caused by the α' martensitic microstructure seen in the FZ. Comparable to the liquidus temperature, the HAZ in Ti6Al4V is also composed of α' martensitic microstructure, but it reaches a lower temperature-below the liquidus temperature mostly above the β transus temperature of 995ºC. It can be observed that the α and β phases make up the BM [39-41].

The welding current may be computed using the Hall-Effect current sensor, and it is discovered that the current is encircled at the contact tip of the welding pistol and fixture, from which the arc voltage can also be computed. The process inputs (current and voltage) can be changed by comparing the variables (temperature, resistance, reflection, velocity) with the material responses. The arc voltage can protect the arc length and WM characteristics; as the arc voltage rises, the arc length rises as well, resulting in a broader bead thickness [32], [71]. Within a specific range, increasing the welding current during the TIG welding process results in increased heat and thermal force exerted on the workpiece. Consequently, as the current increases, the thermal heat also increases, leading to slight increase in voltage [72], [73].

The experiment on the arc and metal efficiency was carried out using the various processes, and it was determined that the arc efficiency has not varied much within a particular process for the choice of current explored. In the case of gas metal arc welding, droplet size measurement revealed that the droplet size falls and begins to deviate from the computed standards as the current increases. The changes in the weakening contact tube may be covered by the variations in the current for the steady range of voltage power source introduced by the Contact Tube-to-Work Distance (CTWD). Two areas arise in the TIG technique, which at first boost the tensile characteristics and then decline due to the grain size refinement, regardless of modifications in the base current and pulse-on-time [71-75].

The term "porosity" or "pores" usually refers to the cavities present in the welded metal. It is known that the WM porosity of a titanium alloy is formed from the base of the weld pool. The titanium welding procedure rounds up the pores, which is associated with the evolution of gas. The pore cracking process has properties on crack initiation and propagation according on the loading circumstances. The organization of the weld porosity happens as a result of imperfections caused by oil, grease, and dirt on the weld pieces outside [76- 79]. The weld bead geometry has the most control over the fatigue act of the joint in the microstructure in the WM area. Large levels of porosity are introduced into the FZ section as a result of improper laser welding conditions, and it is demonstrated that the porosity is close to the tapering FZ of the defective penetration welds. Pore cracking starts at the pores before leak pathways form, and leaks are predicted to occur from cracks with deeply buried large pores. Smaller weld pools at low heat input stages enable faster solidification, which in turn causes porosity arrangement because the bubbles that were previously unable to escape are trapped by the solidification process. Three phases-bubble formation, expansion, and escape define the closure of porosity development [79-82].

In the weld, the transverse residual stresses are tensile, while the exterior areas experience compressive residual stresses to balance the stresses. The maximum tensile residual stress (200 MPa) is located at the weld center and represents approximately 25% of the base substance's yield strength when the material in a friction stir welding process is exposed to enough air cooling roughly 760 degrees Celsius for 45 minutes in an argon atmosphere-residual tension can be reduced [83], [84]. High tensile residual stresses during the welding process encourage fracture in the weldment, while compressive residual stresses in the base shield connect with the deformation periodically and may reduce the buckling strength.

The fatigue strength of a material with high tensile residual stresses varies little, but it can be considerably developed by an appropriate compressive residual stress. Based on the morphology of titanium alloys, friction welds can lead to various problems with structural integrity. One of these is when the titanium has reduced thermal conductivity; in this case, the friction welds are inherently linked to high residual stress [85], [86]. The transverse and longitudinal residual stress standards decreased by the side of the welding direction, respectively, according to the unique effects of the Deep Cryogenic Treatment (DCT) on the residual stress of EBW of Ti-6Al-4V joints. In conclusion, the residual stresses are produced by two processes, according to the linear friction welds. The material's plastic deformation at high temperatures is the primary cause. The second reason is the difference in the substance's thermal extension and decrease during heating and cooling, which causes thermally induced strain [87-89]. Through wear and tear, environmental deterioration, and creep, residual pressures can exacerbate the damage. Because the weld material heats up and expands during the welding process, compressive yielding happens right around the molten material. On the other hand, tensile residual stress is created in the longitudinal direction of the weld during cooling and shrinkage of the metal. Due to the specimen's high temperature preheating during the application of the stress relief behaviour, in laser gas nitriding, the specimen reduces its residual stress value after the nitriding procedure. In most circumstances, it is possible to determine the precise criteria for a crack under combined load by selecting a fracture under tensile residual stress; nevertheless, the external load scenarios should have different requirements. It was discovered that the laser-welded joints had superior super plasticity at low strain rates. Peak stress of the material at the weld zone was shown to decrease with rising temperature and lowering strain rate, although the peak stress remained higher than that of the parent material [90-92].

In this research, the findings from conventional studies comparatively underscore the importance of understanding the complex interplay between TIG welding parameters, microstructure, and mechanical properties of titanium alloys. The review of existing literature has shown that the choice of welding parameters, such as welding current, welding speed, and shielding gas flow rate, can have a significant impact on the microstructure and mechanical properties of titanium alloys. For example, higher welding currents have been found to result in larger grain sizes and reduced mechanical properties, while higher welding speeds have been associated with finer grain sizes and improved mechanical properties. Furthermore, the review has highlighted the importance of optimizing welding parameters to achieve the desired balance between microstructure and mechanical properties. By carefully selecting the appropriate welding parameters, it is possible to tailor the microstructure of titanium alloys to meet specific performance requirements, such as improved strength, ductility, and fatigue resistance. By leveraging this knowledge, researchers and engineers can develop more efficient and effective welding processes for titanium alloys, ultimately leading to enhanced performance and reliability in a wide range of applications.

Based on the findings from conventional studies, the following recommendations are suggested in order to improve the overall performance of titanium alloy welds.

I. It is important to carefully control the welding parameters, such as welding current, welding speed, and shielding gas flow rate, to ensure the formation of a sound weld with minimal defects. Studies have shown that variations in these parameters can significantly affect the microstructure and mechanical properties of the weld, so it is crucial to establish optimal settings for each parameter based on the specific alloy being welded.

II. Pre-weld and post-weld heat treatments should be considered to refine the microstructure of the weld and enhance its mechanical properties. Heat treatments can help to eliminate residual stresses, improve the distribution of alloying elements, and enhance the overall strength and toughness of the weld. By carefully selecting the appropriate heat treatment process, the performance of titanium alloy welds can be significantly improved.

III. The use of filler materials with compatible composition and properties is essential for achieving high-quality welds in titanium alloys. Studies have shown that the selection of the right filler material can have a significant impact on the microstructure and mechanical properties of the weld, so it is important to carefully consider the compatibility of the filler material with the BM to ensure a strong and durable weld.

By carefully controlling welding parameters, utilizing appropriate heat treatments, and selecting compatible filler materials, the performance of titanium alloy welds can be optimized and their mechanical properties enhanced. Implementing these recommendations can help to improve the overall quality and reliability of titanium alloy welds in various industrial applications