Effect of TIG welding parameters on 316 L stainless steel joints using taguchi L27 approach

The AISI 316 L stainless steel was welded using Tungsten Inert Gas (TIG) welding, utilizing ternary shielding gases Argon (Ar), Helium (He), and Nitrogen (N2). This study aimed to assess the effects of these ternary shielding gases on the microstructure, bead profile, and bead appearance. It provides a comprehensive grasp of welding parameters’ interplay with shielding gas compositions, enabling engineers to make informed choices that significantly influence the excellence, productivity, and lastingness of the welding process. The Taguchi L-27 approach was employed, incorporating different contents of N2 (2.5 vol. % to 7.5 vol. %) and He (10 vol. % to 30 vol. %) within the Ar shielding gas composition. Additionally, welding current intensities, ranging from 120 A to 180 A, were also used in the experiment. The results demonstrated that higher content of He and N2 resulted in elevated levels of austenite-forming elements. Therefore, for TIG welding at the arc current intensity of 150 A, it is recommended to utilize the shielding gas mixtures (2.5 vol. % N2 + 10 vol. % He + 87.5 vol. % Ar). Furthermore, by augmenting the content of both N2 and He within the Ar shielding gas mixture, in addition to adjusting the arc current, a notable expansion in both the width and depth of the weld profile was achieved. This achievement, in turn, played a pivotal role in securing comprehensive fusion throughout the welding process.


Introduction
TIG welding uses an electric arc between the non-consumable tungsten electrodes and the workpiece to create heat.It is utilized in a variety of applications, including power production, aviation, chemicals, petroleum pipelines, and furnace heat exchangers.In the welding process, the prevention of air contamination necessitates the utilization of shielding gases such as Argon (Ar), Helium (He), or Nitrogen (N 2 ).When selecting shielding gases, they may be used either in their pure form or mixture, such as pure Ar or Ar + He, Ar + N 2 mixtures, each gas has specific content and proportions.Furthermore, the type of material utilized, and the desired welding properties are considered, because the type of material is affected by the welding process due to variations in material properties such as melting point and thermal conductivity.Also, welding properties are influenced by changes in welding parameters like current, voltage, and travel speed [1][2][3].
Shielding gas shields the welding pool from oxidation and atmospheric contaminants, thereby stabilizing the welding process.This protects the material welded against flaws and maintains a uniform welding environment.The weld quality and durability depend on the gas selection and usage.Shielding gas protects the welding pool from outside contaminants based on its chemical and physical characteristics, according to Mvola and Kah [4].Thus, the heat transfer of shielding affects the welding pool's cooling rate, which affects the welding bead's quality and profile.Also, gas density determines how much gas is needed to cover the weld area.Shielding gas ionization potential affects arc stability and heat generation between the tungsten electrode and the workpiece.Welders may maximize welding performance and joint quality by carefully choosing the shielding gas and evaluating its characteristics, according to Kah and Martikainen [5].Table 1 provides shielding gas characteristics, which include density, ionization potential molecular weight, and gravity concerning air.
The use of an Ar and He shielding gas combination provides many benefits in welding.These benefits include enhanced arc stability, deeper penetration, and an increased welding speed, as noted by Srivastava and Garg [6].Sathiya et al [7] studied the effect of Ar and He as shielding gases on metrological and mechanical properties.They found that increasing the percentage of He in the shielding gas mixture can lead to a wider and deeper weld bead compared to using pure Ar.He has a lower density than Ar and can provide a higher heat transfer rate to the weld, resulting in a wider and deeper bead.Mastanaiah et al [8] studied welding bead profiles and mechanical properties.They found that bead profile and mechanical characteristics have a relationship, which may explain the welded joint's microstructure and features.They recommended that optimizing the mechanical performance of welded joints requires the balancing of the bead profile (top depression, width, penetration) and heat-affected zone (HAZ) softening.Also, conducted by Khrais et al [9] investigated the welding parameters including wire feed rate, welding current, and three shielding gas combination groups (G1, G2, G3) that affected bead height and width.They found the optimal bead height utilizing wire feed rate, welding current, and gas group (3.5 m min −1 , 160 A, G1).Furthermore, the optimal bead width was determined by wire feed, rate welding current, and shielding gas group (3 m min −1 , 120 A, G2).The selection of shielding gas mixture depends on factors such as the material being welded, welding parameters, and the desired welding joint properties.
Ar is commonly used as a shielding gas in TIG welding to provide an inert atmosphere and prevent defects.It is often used for welding aluminum and stainless steel.Furthermore, He can also be used as a shielding gas, especially for thicker metals, because it has a lower density than air, high thermal conductivity, and high ionization potential.As a result, it is utilized to weld aluminum and magnesium alloys because of their high heat input and limited tolerance for oxidizing elements.The He needs higher arc voltages than Ar.This leads to a hotter and wider arc which affects the weld bead's penetration depth and width [10].Therefore, He is preferred over Ar at higher welding current levels (Heat input).When welding thicker materials, particularly those with high thermal conductivity or melting temperatures, the He is utilized at a higher arc current level.Additionally, it can be used in high-speed automated projects while the Ar addition improves arc initiation and cleaning action education, according to the study of Jittavisuttiwong and Poopat [11].Adding He to Ar shielding gas mixtures improves melting and fusion strengths, resulting in a wider fusion width and better bead geometry.Using an Ar/ He mixture can improve the heat input efficacy of welding for nonferrous base metals and improve fusion characteristics.However, the exact effects of adding Helium depend on the specific welding application and parameters used according to the study by Saha et al [12].
In welding, careful consideration and adjustment are necessary for the He content (concentration) in the shielding gas mixture.Considering parameters such as the thickness of the workpiece, the welding process, and the desired bead geometry.To achieve sufficient fusion and enhance heat input, it is often necessary to increase the concentration of He when welding thicker workpieces.However, even a modest concentration of 20% He can affect the welding arc, making it imperative to carefully control the composition and flow rate of the shielding gas for optimal results, as highlighted by Cai et al [13].To achieve better outcomes, adjust both the composition and flow rate of the shielding gas mixture to optimize results.It's important to note that even a relatively small concentration of Helium (20% or below) can still impact the welding arc.Additionally, researchers Mvola and Kah [4] suggest that when working with thicker materials, it's advisable to raise the Helium percentage in the shielding gas.This can help enhance heat input and improve fusion during the welding process.Sathiya and Abdul Jaleel [14] examined bead geometry's influence on laser speed, beam power, focal distance, and shielding gas (Ar and He).Gas mixtures including Ar and He may decrease welding penetration.Because the shielding gas concentration of He contains more than pure Ar, arc pressure decreases, decreasing heat input and penetration depth.Traidia et al [15] found that adding 10 vol.% Ar to He may significantly reduce bead penetration.This is due to the higher electrical conductivity of the combination when Ar is added to the He shields gas.N 2 gas is utilized for the stability of arc welding, decreasing porosity, and strengthening welds.Moreover, N 2 increases the weld pool's cooling rate and narrows beads.N 2 in the shielding gas combination may also alter the weld's geometry profile, bead shape, and quality.However, this increase in N 2 concentration might contribute to a higher risk of cracking [16].Therefore, Vidyarthy and Dwivedi [17], and Deep et al [18] recommend careful management of N 2 and other gases and proper welding procedures to obtain ideal bead geometry and highquality welds.Several research studies have examined N 2 in shielding gas during welding.Zorc [19] investigated N 2 and N 2 -based mixtures for welding austenitic stainless steels.The research indicated that adding 20% N 2 to the shielding gas reduces the current needed for complete penetration by 40%.Yang et al [20] found that high N 2 causes weld porosity.To get the best welding results, the shielding gas N 2 volume must be carefully considered.Elmer et al [21] used N 2 instead of He to prevent plasma production.using the N 2 in shielding gas may brittle some steels.Ar gas generated porosity welds every time steel was welded, while N 2 gas created no weld porosity.Kshirsagar et al [22] examined the effects of shielding gas N 2 concentration on weld durability and quality.To determine the optimal N 2 concentration for achieving high-quality welds, welding experiments were conducted using various N 2 concentrations.Thus, they discovered that using a 10% volume N 2 concentration in the shielding gas not only increased penetration depth by 30% but also led to improvements in both bead geometry and mechanical properties.Liu et al [23] investigated N 2 's effects on welding high nitrogen stainless steel grades.They found the N 2 can enhance weld bead penetration, arc stability, and tensile strength.
Huang [24] discovered that the addition of N 2 to the shielding gas results in improved mechanical properties of 304 stainless steels.Enhancements were observed in properties such as tensile strength, heat input, hardness, penetration depth, and cross-sectional area of the welded metal.Additionally, there was a reduction in angular distortion.Furthermore, at the microstructure level, the N 2 presence decreased ferrite content, indicating its usage for enhancing weld quality.Vashishtha et al [25] also found that adding N 2 to shielding gas uniformly distributes chromium and nickel in the ferrite and austenite phases of duplex stainless steel.Also, adding N 2 to shielding gas may increase weld quality and microstructure.Weld penetration profile, speed, and quality are directly affected by welding current, according to Karadeniz et al [26].Recent studies show that increasing N 2 to the shielding gas can improve welding quality, but further study is required to fully explore the possibilities of alternative shielding gases in TIG welding.Ar, He, and N 2 shielding gas combination research for TIG welding is limited to researchers.Research is required to fully understand these gases' impact on welding properties and establish their appropriate utilization in diverse welding applications.To find the best gas mixtures for welding various materials and the best gas flow rates, welding currents, and other parameters for high-quality welds, further studies are needed.
This research investigates the effects of ternary shielding gas on AISI 316 L bead profile and microstructure by TIG welding.TIG welding parameters alter shielding gas concentrations and arc currents.He at (10 vol.%, 20 vol.%, and 30 vol. %), N 2 at (2.5 vol.%, 5 vol.%, and 7.5 vol.%), and Ar as a balanced gas.Arc currents of 120 A, 150 A, and 180 A were utilized.This investigation is one of the limited research studies to use a ternary-shielding gas in TIG welding parameters.This research reveals fresh insights into the geometric aspects of beads and the creation of microstructures, which impact mechanical capabilities.

Experimental setup 2.1. Welding machine
The Miller Syncrowave 350 LX machine was used to perform TIG welding in the conducted research.This welding machine is widely used in various applications, including precision metal fabrication, maintenance and repair, manufacturing, aerospace, shipbuilding, tube and pipe, automotive, and vocational purposes.The machine is designed to provide high-quality welds on a range of materials, such as aluminum, stainless steel, and mild steel.It has an amperage range of 3A-400A related output, with a maximum voltage of 80 V and a 40% duty cycle at 350 A and 34 VAC.

Workpiece (substrate preparation)
The samples used in this study were made of 316 L austenitic stainless steel.Table 2 displays the material's chemical composition and properties, consisting of chromium, nickel, and molybdenum with a lower carbon content (less than 0.008 wt%) than that of other steels and metal alloys.A sewing machine was used to cut specimens that were 100 × 100 × 5 mm in size.Subsequently, a V-edge was fashioned with an angle of 60°and a depth of 3 mm, as shown in figure 1.

Shielding gas mixture (Taguchi-L27)
The Taguchi L-27 Orthogonal Array is designed to address scenarios with 27 experimental factors, each at three levels.This arrangement allows us to investigate the main effects and interactions of these factors efficiently with a relatively small number of experimental runs.The L-27 design is a statistical approach employed to enhance product design, reduce variability, and elevate quality.This method involves conducting L-27 experiments, each with three levels for the nine control factors, and each factor is tested at three levels.This enables the determination of optimal levels that maximize performance and robustness [27].For this investigation, the primary factors examined were welding arc current (120 A, 150 A, 180 A), N 2 concentrations (2.5 vol.%, 5 vol.%, 7.5 vol.%), and He concentrations (10 vol.%, 20 vol.%, 30 vol. %), while maintaining Ar as the balancing gas.Table 3 displays the percentage composition of the various shielding gas mixtures used in the experiment.The L-27 orthogonal array is generated or created using MINITAB (statistical software).Furthermore, in this experiment, pure welding was used to investigate the effects of arc current on AISI 316 L, as shown in table 4.

Welding procedure
As shown in figure 2, the specific automatic TIG fixtures were used to create a constant welding feed.The plates in pairs were placed horizontally on an aluminum plate supported by wheels on a rotating shaft.The plates were moved only 10 mm at a constant speed of 1.68 mm s −1 , produced by a speed control motor that rotates the shaft.The welding torch was held constant at an angle of 45 0 by the clamp arm in the center of the mainframe.The distance between the contracted joint and the tip is constant at 5 mm.The total shielding gas flow rate was consistent throughout the experiments to be 10 l min −1 at constant pressure.

Material characterization procedure
This study performed a microstructure examination of the specimens to analyze the effects of the welding parameters.The specimens were first ground down using silicon carbide paper with progressively finer grits (120, 220, 600, 800, 1000, 1200, and 2000) until a fine surface finish was achieved.Then, they were polished using alumina diamonds with 1-micrometer-size particles to produce a mirror-like surface.Polishing samples before analysis is essential to create a smooth, contaminant-free, and mirror-like surface.After polishing, the samples were etched using an etching solution consisting of 5 gm of FeCl 3 , 10 ml of HCl, and 50 ml of H 2 O.The microstructure of the weld zone and base metal was examined using an optical microscope with a magnification of 50X.This preparation enhances the quality and resolution of images, reduces charging effects, and ensures consistency across multiple samples.

Results and discussion
3.1.Weld bead appearance Figures 3, 4, and 5 show the influence of the shielding gas composition on the appearance of the weld bead.These apparent were observed under different amounts of N 2 , and He added into the Ar shielding gas.He gas has the lowest ionization potential compared to Ar and N 2 gases.Therefore, increasing the concentration of He makes the welding arc more stable and the bead surface more uniform.Furthermore, increasing the percentage of He in the shielding gas mixture led to an increase in the bead width and bead height.This increase can be attributed to the high heat transfer of He, which was transferred to the workpiece and welding joint by the arc and resulted in high metal flow.
The oxidation on the bead surface increases as the N 2 gas content increases in the shielding gas mixture, because N 2 gas is a reactive element.This can be explained by the elevated temperatures promoting oxidation reactions with oxygen from the atmosphere, or other sources, combined with reactive N 2 to oxidize the molten metal.Hence, excessive oxidation on the bead surface can lead to issues such as porosity, reduced mechanical properties, and diminished weld quality.On the other hand, the N 2 enhances the fluidity of molten metal during welding.This improved fluidity enables better control over the weld pool, facilitating precise shaping of the weld and contributing to the overall control of bead geometry, it is similar to the study conducted by Mastanaiah et al [8].
The TIG welding at 120 A and 150 A arc current intensities generates a straight welding line without welding strikes, incomplete fusion, and an irregular bead profile.However, at 180 A arc welding current, the welding line becomes wider, which results in increased porosity, an irregular bead profile, and weld spatter.This causes an increase in angular distortion and transverse shrinkage as reported by Khrais et al [9].As a result, the best welding appearance was achieved at 120 A arc current for Samples No.  Figure 6 shows the welding appearance under pure Ar at three current values.The welding arc under Ar gas is stable due to the lower ionization energy of pure Ar (15.76 eV).Also, using pure argon as a shielding gas resulted in a shiny weld appearance with low surface oxidation, likely due to the higher density of argon compared to atmospheric air.Furthermore, an increase in welding current was observed to lead to a wider weld bead width due to the resulting increase in heat input.This conclusion was previously reported by Chuaiphan and Srijaroenpramong [28].

Bead profile (Side section view)
Figures 7, 8, and 9 show welded specimens' (Side section view) bead profiles under the three considered shielding gases at 120 A, 150 A, and 180 A arc currents, respectively.The Figures show that the welding area of the specimens increases as the N 2 and He percentages in shielding gas mixtures increase due to the contribution of both gases in increasing the heat input.
The addition of N 2 in the shielding gas mixture increased arc stability, decreased spatter, and increased heat input.This leads to a higher heat transfer rate from the arc current into the specimens, producing a deeper penetration weld [20].Furthermore, weld penetration increases slightly as the He content increases in the Ar-N 2 mixture [29].It was shown by Aguilar et al [30] that the arc acquired a greater voltage drop across the arc current due to He's higher ionization potential.Furthermore, the electromagnetic force (Lorenz) and the surface tension force (Marangoni) might have been increased on the pool surface due to the use of He gas more considerably than Ar, which led to deeper weld penetration, similar to Sathiya et al [7] and Vidyarthy and Dwivedi [31].It is clear that Samples No. 3, 6, 9, 12, 14, 15, 20, 21, 23, 24, 25, 26 and 27 exhibit the most favorable bead geometry.These samples have symmetrical bead geometry and demonstrate exceptional fusion quality.
The findings demonstrate that the penetration depth and cross-sections of the welded area increase as the welding current increases at a constant N 2 content, as reported by Adak et al [10].As a result, the increase in weld penetration depth is caused by an increase in the electromagnetic forces, which are proportional to the square of the current density.The higher the electromagnetic forces and arc pressure, the more inward flow is created in the metal pool, this results in a deeper weld penetration.According to Khrais et al [9] the weld pool profile is an equilibrium function of hydrostatic, capillary forces, and arc pressure.

Microstructure
The weld zone microstructure formation is affected by the metal alloy compositions, solidification factors, and weld pool geometry, as reported by Mvola and Kah [4].Hence, Gao et al [32] found that modifying the welding parameters has a positive impact on the welding microstructure.The weld zone microstructure is characterized by primary austenite (γ) grains, which appear white and are surrounded by a continuous network of ferrite (δ) grains, which exhibit a gray.δ-Ferrite is found around austenite grain boundaries and inside its phase.As reported by Yu et al [33].
Figure 10 (a)-(c) show the weld zone microstructure under various He content (10, 20, 30 vol.%) while maintaining N 2 constant content (2.5 vol.%).An increase in the concentration of He (10 to 30 vol. %) leads to a decrease in the size of grains inside the weld zone.Because He has a higher thermal conductivity compared to Ar or N 2 , which means He conducts heat more effectively.He and Ar can improve mechanical properties by  creating finer and more uniform grain structures in the weld zone.However, the addition of N 2 and He mixture in Ar shielding gas results in a decrease in the δ-Ferrite and an increase in the austenite formation phase.This is explained by Kshirsagar et al [22] and Kah and Martikainen [5].Furthermore, adding N 2 and He enhances nickel equivalent formation, leading to enhanced strength and corrosion resistance, as demonstrated by Başyiğit and  Kurt [34].Hence, Both N 2 and He gases impact the stability of the austenite phase in materials, with N 2 performing as a specific austenite stabilizer, notably in stainless steel.
Figure 11(A)-(C) show the microstructure of the weld zone formed by three different shielding gas combinations.Increasing the N 2 concentration from 2.5 to 7.5 vol.% results in a smaller grain microstructure because N 2 acts as a grain refiner, promoting the formation of more numerous and finer grains within the austenitic material (weld zone).The formation of welding joint grains is influenced by N 2 's high thermal conductivity.While higher thermal conductivity causes rapid cooling and solidifying.As a result, rapid welding solidification decreases grain sizes (fine grain).The Figures demonstrate that increasing the He concentration (10, 20 30 vol.%) decreased the grain size of the weld zone.This is supported by grain size decreases as the heat transfer rate increases, which can be achieved by increasing the cooling rate.Therefore, the rate of heat transfer for He is higher than Ar and N 2 [5].
According to the EDX analysis in table 4, increasing the concentration of He (10 vol.% to 30 vol.%) and N 2 (2.5 vol.% to 7.5 vol.%) when combined with a high arc current of 150 A can improve heat input and thermal conductivity.A rapid cooling rate causes the ferrite dendrites to be shorter and closer together.He has a higher thermal conductivity and a lower density than Ar and N 2 .As a result, He and N 2 are efficient shielding gases for a variety of welding applications requiring a more rapid cooling rate to achieve a finer grain structure.This reason is described by Chuaiphan and Srijaroenpramong [35] and Bansod et al [36].Furthermore, Increasing the N 2 content in the shielding gas can decrease the δ-ferrite formation in a welded joint.The slower cooling rate associated with N 2 decreases δ -ferrite to austenite transformation, resulting in a lower δ-ferrite formation in the final microstructure.This highlights the importance of the shielding gas composition in welding, in order to get the optimal microstructure formation of the welded joint as stated by Vidyarthy and Dwivedi [17].
The microstructure of the weld zone under different arc currents and pure argon shielding gas is shown in figures 12(A), 13(A), and 14(A).The formation of equiaxed dendrites in the microstructure may occur in the weld zone area when utilizing a low heat input (120 A) during welding and pure Ar as the shielding gas.Because of the rapid cooling rate, the grains are created in all directions and have a shorter length.Furthermore, low heat input could result in a higher proportion of δ-ferrite content than austenite, which is consistent with the findings reported by Chuaiphan and Srijaroenpramong [28].Thus, shielding gas selection influences ferrite-to-austenite formation, since different gases have different cooling rates even at a constant flow rate of 10 L min −1 .Furthermore, increased heat input content during welding results in the growth of longer dendrites, resulting in coarse grain formation in the microstructure, as reported by Bansod et al [36].As shown in figures 12(B), 13(B), and 14(B), welding at low-medium (120-150 A) arc currents leads to limited heat input and a different microstructure compared to welding at higher currents (180 A).However, using pure Ar as a shielding gas can contribute to producing a narrow HAZ, due to its low thermal conductivity and limited heat input.Additionally, the microstructure of the welded joint may also be affected by dendritic length.Thus, heat input and other cooling rates must be carefully considered to ensure the welding joint's intended properties, and overly high cooling rates in the HAZ may cause fine-grain structures.Thermal gradients, phase transformation, and residual strains from welding may cause HAZ twinning in welded joints.These variables cause HAZ grain crystallographic reorientations, producing twin grains as reported by Liu et al [37].Twinning may damage the material mechanical qualities; hence welding settings must be carefully controlled.
High-current welding (180 A) increases the amount of molten metal in the weld zone and the heat content, requiring more time to cool the weld pool.As a result, the welded joint produces coarse grain formation and longer dendrites.In contrast, welding at a low current has less heat input, and a smaller volume of molten metal in the weld zone, resulting in a different microstructure than welding at higher currents, this similar result was presented by Mortazavi et al [38].

Shielding gases mixtures' effect on the composition of the welded 316 L austenitic stainless steel (EDX)
The element composition of 316L stainless steel was investigated using Energy Dispersive x-ray (EDX) analysis, and table 5 summarized figures 15, 16, 17, 18, and 19.EDX results showed that the choice of shielding gas can significantly affect the element composition of the welded metal.Using pure Ar as a shielding gas results in increased δ-ferrite microstructure formation in the weld zone, decreasing the nickel equivalent (Ni eq ) while increasing the chromium equivalent (Cr eq ) In contrast, the use of a mixture of He, N 2 , and Ar during the welding process creates an austenitic microstructure with enhanced corrosion resistance.He and N 2 enhance austenite stabilizer elements.These results emphasize the need to carefully choose the shielding gas mixture to obtain desired features in welded materials.The findings are supported by the study of Sathiya et al [7] as well as Chuaiphan and Srijaroenpramong [39].
Sathiya and Abdul Jaleel [14] investigated the effects of He and Ar on welding stainless steel.They found that as the N 2 and He contents increase in the shielding gas mixture, the Cr Ni eq eq / ratio decreases.The Cr Ni eq eq / ratio is used to determine the solidification behavior of alloys and its impact on microstructure and properties.Furthermore, understanding the relative amounts of alloying elements that influence the phase transformation.When optimizing processing and performance, the Cr Ni eq eq / ratio should be evaluated with other data.A Cr Ni eq eq / ratio less than 1.35 indicates austenitic formation, while a ratio greater than 1.35 indicates ferrite formation.
The Cr eq and Ni eq values are calculated based on the weight percentages of the alloying elements in the weld metal, and their formulae are as follows: / ratio.Only welds made with pure Ar gas solidified in ferrite mode (20%).Hence, all the welded metals solidified in the austenitic mode.This is consistent with the findings of Sathiya and Abdul Jaleel [14].When the metals solidify in austinite mode, the welding integrity improves with smaller grain structures, corrosion resistance improves, and mechanical properties improve.Additionally, it decreases material distortion, and hot cracking risk, and facilitates welding dissimilar materials as reported by Sabzi and Castillo [40].It is recommended to set TIG welding parameters, which include an arc current intensity of 150 A, as well as a shielding gas mixture of 2.5% N 2 , 10% He, and 87.5% Ar.
Figure 20 shows the Schaeffler diagram, which is used in metallurgy to determine the phase balance, particularly the delta-ferrite content, in stainless steel welds by plotting the Cr eq and Ni eq values.Identifying their intersection on the diagram.All the data points showed the austenite formation mode, except for the point of  pure Ar shielding gas.The intersection of (Red, Green, Orange, Blue, and Yellow) corresponds to Samples No. (29,4,13,23,24) respectively.The process of solidifying in the ferrite-austenite mode starts with the formation of ferrite, which is then transformed into austenite via the peritectic reaction.During welding, the partial eutectic reaction leads to the formation of delta-ferrite in some areas, while the remaining liquid transforms   directly into austenite.While the content of N 2 and He increase in Ar shielding gas, the Cr eq value decreases.
Furthermore, the Ni eq value increases as the content of N 2 and He increases in Ar shielding gas.The lowest Cr eq value of 20.93 was determined for 5% N 2 and 30% He, indicating an enhanced austenitic microstructure and better corrosion resistance.In contrast, the highest Ni eq value of 21.69 was determined for 7.5% N 2 and 30% He, indicating a higher Ni eq and potential for increased mechanical properties in the resulting weld metal.As explained by Chuaiphan and Srijaroenpramong [35] as well as Bansod et al [36].

Conclusion
The following are the main points that can be withdrawn from the present study: 1.The addition of N 2 and He, or a mixture of the two in the shielding gas during welding can increase the penetration depth and cross-sectional area of the welded joint.This can lead to improving the molten weld pool and increased ionization of the shielding gas, resulting in a stronger and higher-quality welded joint.
2. Increasing the content of He in the shielding gas mixture results in a wider welding bead and a lower wetting angle.On the other hand, the stability of the arc improves when using pure Ar gas.Similarly, increasing the concentration of N 2 in the shielding gas mixture leads to a wider weld bead and more surface oxidation.
3. The addition of N 2 and He to the shielding gas can result in greater concentrations which promote austenite formation.Specifically, the N 2 gas stabilizes the austenitic phase, making it more thermodynamically useful  resulting in a decrease in the ferritic phase.This can improve the mechanical properties and corrosion resistance in the welded joint.
4. Increasing the content of N 2 in the shielding gas mixture during welding can lead to a decrease in the deltaferrite formation in the welding zone.N 2 acts as a diffusible element and can slow down the transformation of delta-ferrite to austenite during cooling, resulting in a lower amount of delta-ferrite in the final microstructure.
5. The addition of He and N 2 in the shielding gas enhances the formation of a fine-grained structure in the weld zone.On the other hand, an increase in welding current results in the formation of a coarser grain structure in the weld zone and the heat-affected zone (HAZ).

6.
It is recommended at least to use a shielding gas mixture (2.5 vol.% N 2 + 10 vol.% He + 87.5 vol.% Ar).All Samples have a Cr Ni eq eq / ratio less than 1.35, the weld metals will solidify in the austenitic solidification mode.

Figure 3 .
Figure 3.The effect of the He content on Ar-N 2 shielding gas on the surface weld joint (a) 10 vol.%, (b) 20 vol.%, and (c) 30 vol. % of He.At arc current 120 A.

Figure 4 .
Figure 4.The effect of the He content on Ar-N 2 shielding gas on the surface weld joint (a) 10 vol.%, (b) 20 vol.%, and (c) 30 vol. % of He.At arc current 150 A.

Figure 5 .
Figure 5.The effect of the He content on Ar-N 2 shielding gas on the surface weld joint (a) 10 vol.%, (b) 20 vol.%, and (c) 30 vol. % of He.At arc current 180 A.

Figure 6 .
Figure 6.Effect of pure Ar on surface welding joints under three values of welding current of Sample No. 28, 29, and 30.

Figure 7 .
Figure 7.The impact of the gases N 2 and He on the depth of penetration and the size of the welded area when using a welding arc current of 120 A.

Figure 8 .
Figure 8.The impact of the gases N 2 and He on the depth of penetration and the size of the welded area when using a welding arc current of 150 A.

Figure 9 .
Figure 9.The impact of the gases N 2 and He on the depth of penetration and the size of the welded area when using a welding arc current of 180 A.

Figure 13 .
Figure 13.Optical microstructure of (A) weld joint (B) HAZ welded under pure Ar at welding arc current 150 A (Sample No. 29).

Figure 12 .
Figure 12.Optical microstructure of (A) weld joint (B) HAZ welded under pure Ar at welding arc current 120 A (Sample No. 28).

Figure 15 .
Figure 15.EDX Analysis of Pure Ar welding current 150 A of Sample No. 29.

Figure 16 .
Figure 16.EDX Analysis of Welding at 150 A with 10% He − 2.5% N 2 Addition to Ar of Sample No. 4.

Figure 17 .
Figure 17.EDX analysis of welding at 150 A with 20% He-2.5% N 2 to Ar of Sample No. 13.

Figure 18 .
Figure 18.EDX analysis of welding at 150 A with 30% He −5% N 2 to Ar of Sample No. 23.

Figure 19 .
Figure 19.EDX analysis of welding at 150 A with 30% He-7.5 N 2 to Ar of Sample No. 24.

Table 2 .
Chemical composition of 316L austenitic stainless steel.

Table 3 .
Percentage of shielding gas mixtures.

Table 4 .
TIG welding parameters of Ar shielding gas.

Table 5 .
Compositions of welded metal at different shielding gas mixtures by EDX.