Influence of inconel interlayer on microstructural, mechanical and electrochemical characteristics in single-pass ATIG welding of dissimilar austenitic and duplex stainless steel

This study investigates the impact of Inconel 625 interlayer on dissimilar welded low nickel austenitic stainless steel (LNiASS) and super duplex stainless steel (S32760) using activated tungsten inert gas (ATIG) welding. Two weldments were prepared: with and without (autogenous) interlayer. Geometrical investigation of the weld cross sections revealed that interlayer-based welding significantly increased the depth of penetration and decreased weld width as compared to autogenous welding at the same welding current. The dual microstructure was observed in the weld zone (WZ) of autogenous weldment while fully austenitic structure with few intermetallics was observed in the WZ of interlayer-based weldment. Mechanical properties, particularly impact strength observed to be improved in the case of interlayer-based weldment (91 ± 2 J) compared to autogenous weldment (68 ± 2 J). Lower microhardness was noticed for the WZ of interlayer-based weldment (258 ± 3 HV0.2) than WZ of autogenous (279 ± 2 HV0.2) weldment due to the presence of higher content of Ni. However, UTS of interlayer-based weldment (654 MPa), falls short in comparison to the autogenous weldment (693 MPa), indicating a compromised joint efficiency of 5.96%. The corrosion resistance was observed to be higher for the WZ of interlayer-based weldment attributed to the higher content of Ni and Mo. The sensitization study revealed 47.33% degree of sensitization in the WZ of autogenous weldments due to dual microstructure, while interlayer-based weldments showed no sensitization.


Introduction
One well-known and frequently utilized welding technique for stainless steel is tungsten inert gas (TIG) welding.It is well known for both the extraordinary quality of the welds it produces and the versatility it offers in the process.TIG welding does, however, have some drawbacks, one of which being its shallow penetration in a single pass.A method of welding called Activated TIG (A-TIG) has been proposed to get around this restriction.Activated flux is applied to the plate in a thin layer prior to welding in this method.The inclusion of this activated flux is thought to improve the weld's penetration, raising the TIG welding process's overall quality and efficiency [1,2].With the aid of active metallic powders, an arc is formed between a non-consumable tungsten electrode and a workpiece during the gas-shielded arc welding method known as ATIG.This procedure transfers heat to cause the coalescence of metals.ATIG welding provides improved arc stability, and reduced heat-affected zone (HAZ) dimensions compared to traditional TIG [3,4].During Ni-based super alloy welding trials, Sivakumar et al [4] has reported a significant reduction in the width of the HAZ through the implementation of ATIG welding.ATIG welding has found applications in various industries, including structural steel welding, automotive manufacturing, aerospace, pressure vessel fabrication, and large-diameter pipe welding [5].ATIG can be used to weld thicker materials, such as stainless-steel plates up to 12 mm thick, in a single pass [6].The ATIG process can be operated with or without a filler wire addition.ATIG welding offers several advantages and can help mitigate some of the challenges associated with traditional fusion welding processes.The formation of sigma phase and grain coarsening are examples of unfavorable metallurgical changes that can occur in the HAZ during typical fusion welding methods such as arc and beam welding.Pre-and post-heat treatments, rapid welding speeds, and other procedures are usually necessary to overcome these problems.However, ATIG can reduce grain coarsening in the HAZ and the weld metal due to the unique characteristics of the ATIG process [7].The activated flux used in the TIG helps to refine the weld grain structure, resulting in improved toughness and ductility compared to conventional fusion welding methods [8].According to Kuo et al [9], activating flux can increase the weld penetration during the SiO 2 aided ATIG process for mild steel and stainless steel.The authors further claimed that the excellent surface appearance was caused by the alteration in the surface tension gradient that the activating flow brought about.According to Shyu et al [10], using oxide fluxes when TIG welding stainless steel improves weld penetration and decreases angular distortion.Accordingly, the chemical composition has a significant impact on ATIG welding; as a result, new steels with diverse chemical compositions require the development of new techniques.
Austenitic stainless steel is widely used material in petrochemical, chemical, marine and nuclear industry and is also known as Cr-Ni stainless steel.Currently, there is a growing demand among stainless steel manufacturers, and due to the price of Ni, the producers are looking to develop alternatives to Cr-Ni grades that contain low nickel.Given the similar composition of the Cr-Ni steel the Ni is replaced by Mn (austenitic stabilizer).The LNiASS (low Ni ASS) stainless steel is a type of Cr-Mn stainless steel and could be considered as a potential substitute for type Cr-Ni stainless steel either for partial replacement in cases of equipment failure or complete replacement in new product manufacturing [11].However, welding of ASSs possess several challenges, such as, coarse grains formation in HAZ, liquation cracking in the partially melted zone (PMZ), and weld zone (WZ) [12].On the other hand, the duplex stainless steels (DSS) are another type of low Ni stainless steel that possess a two-phase microstructure consisting of both austenitic and ferritic phases in roughly equal proportions.DSSs are suitable replacements for Cr-Ni stainless steel and offer excellent resistance to corrosion, high strength, and good weldability.However, these steels when welded, it possess several challenges, especially decrease in corrosion resistance and/or toughness attributed to formation of intermetallic phases, such as, sigma phase, chi phase, secondary austenite [13][14][15].The dissimilar joint involving duplex structure like super-DSS S32760 and ASS such as LNiASS is widely used for the fabrication of various industrial components.The dissimilar welding is always challenging due to difference in the chemical compositions and properties of the materials.The choice of an appropriate welding method and filler metal are the two main concerns in the dissimilar welding.Inadequate filler metal selection and welding technique lead to poor mechanical and metallurgical qualities and joint failure.Rahmani et al [16] studied the dissimilar welding of 304 L ASS with S32750 super-DSS and reported that ferrite dominated in the HAZ of super-DSS, whereas in the HAZ of ASS, coarse austenite grains were present along with small amounts of vermicular grain boundary ferrite precipitates.Moteshakker et al [17] studied the impact of filler electrodes on dissimilar welding of 2205 DSS with 316 L ASS using gas tungsten arc welding (GTAW) technique and reported that ER 309 L filler electrode exhibited the most favourable mechanical and corrosion properties.
The interlayer employed has various advantages over filler wire, such as, (i) lowering of fluctuation of metal transfer rates from the base metals (BM) to WZ, (ii) preventing improper filler deposition rates, (iii) minimizing C migration from low Cr to high Cr steel, (iv) eliminating the positional angle concern associated with filler wire, (v) avoiding reinforcement on the weld bead results decline in the filler consumption and (vi) eliminating the necessity of groove preparation.Kulkarni et al [18] used Inconel 800 and Inconel 600 as interlayers to weld P91 and 316 L ASS using ATIG welding technique and observed that the modification in microstructure was observed on using the interlayers.The WZ of weldment without interlayer displayed a fully martensitic structure, however, the weldment Inconel 600 exhibited with a completely austenitic structure while the Incoloy 800 interlayer exhibited partial austenitic structure.Fande et al [7] studied the impact of Inconel 625 interlayer on dissimilar welding of super-DSS S32760 and 202 ASS using TIG and ATIG welding technique and concluded that the ATIG is efficient as compared to TIG welding.
In this study, the dissimilar welding was performed between to super-DSS S32760 and LNiASS using ATIG welding technique.A comparative analysis of mechanical and corrosion properties is performed between the weldments with and without interlayer incorporation.The BMs, i.e., super-DSS S32760 DSS and LNiASS used in the present study contains less Ni content, and it is well known fact Ni enhances the corrosion properties of stainless steels [19].Therefore, to improve the corrosion properties of the WZ, the Inconel 625 is used as an interlayer between the BMs.

Sample preparation and welding process parameters
In this study, super-DSS S32760 plates and LNiASS of mm 6 thick were welded using the ATIG welding technique using Inconel 625 as interlayer.The materials were purchased from Pratham Steel Industries, Mumbai, India.The BMs for welding were prepared in dimensions of 150 × 70 × 6 mm 3 (S32760 and LNiASS).The chemical compositions of the BMs and interlayer are listed in table 1.
The automated weld torch set-up was developed in-house to maintain the arc length and torch speed constant throughout welding.Pure argon gas with 99.99% purity was used as shielding gas during ATIG welding.The ATIG operating principle, autogenous weld arrangement and interlayer placement are schematically shown in figure 1.Two weldments were prepared, (i) autogenous, i.e., without interlayer and (ii) with Inconel 625 interlayer of 2 mm width.TiO 2 and SiO 2 (50:50) activated flux powder was used during ATIG welding.The utilization of SiO 2 flux resulted in the narrow weld bead primarily because of the prevalence of arc  convergence, while TiO 2 flux contributed to the highest penetration depth by favouring the reversal of Marangoni convection [1,[20][21][22].Consequently, in the previous work, enhanced weld bead cross-section by augmenting penetration depth and reducing the width of weld bead was noticed [7,23,24].Table 2 lists the ATIG welding process parameters.

Microstructural characterization
For metallography study, wire cut electric discharge machine was used to prepare the samples.The cutting of the samples was done perpendicular to the weld interfaces.The cross-sections of these joints were then polished using a grit size of 400 up to 2000 SiC polishing paper followed by cloth polishing with 75 μm alumina suspension.The samples were then ultrasonicated in distilled water for 5 min to remove the dirt and alumina particles.To examine the microstructure, the samples underwent an electrolytic etching process in 10% chromic acid.The microstructural investigation was done through an optical microscope (Leica DMi8C) and a scanning electron microscope (SEM, Model: JEOL 6380 A).The SEM coupled with energy dispersive spectroscopy (EDS) was employed to study the chemical composition of secondary phases in the weld zone.To identify the phases and compounds present in the WZ of the weldments, X-ray diffraction (XRD, Model: XRD-6000 equipped with a Cu Kα tube) analysis was conducted, operating at a voltage of 40 kV, and an amperage of 40 mA by keeping a step size of 0.2°.The XRD test samples were polished using emery paper up to grade 1500 followed by cloth polishing with 0.75 μm alumina powder.To remove the surface impurities after polishing the samples were ultrasonicated in di-ionized water for 15 min at room temperature.The XRD was performed for the welded portion including WZ, HAZ and UMZ of the weldments and the XRD patterns were a recorded range from a 20°i ncidence angle to 95°using a silicon photodiode point detector [25].

Mechanical and corrosion properties
For microhardness testing, the samples were cut at the dimensions of 30X10X6 mm 3 , the transverse section of the sample was polished using emery papers followed by alumina-smeared cloth polishing.The samples were then cleaned ultrasonically in distilled water for 5 min.The hardness of the welded sample was tested using a Shimadzu microhardness tester (Shimadzu Corp. in Kyoto, Japan).The test applied a 200-gram force for 10 s.
Measurements were taken at 0.5 mm intervals on the transverse section of the weld joint.Tensile tests were performed in accordance with ASTM-E8-M-04 standard utilizing an Instron universal testing machine (Model-5582).SEM was used to examine the tensile specimens' fractures.The samples were made with dimensions of 55 ×10 x 6 mm 3 (ASTM-E-23) in order to determine the toughness of the welded connection.A 300 J capacity hammer was used to test the samples using a Charpy V-notch testing machine (Pendulum Impact Tester Model- IT 30).The susceptibility to intergranular corrosion (IGC) was assessed via non-destructive double-loop electrochemical potentiokinetic reactivation (DL-EPR) testing, using aggressive electrolytes (0.01 M KSCN and 0.5 M H 2 SO 4 in 300 ml deionized water).The 1.67 mV/s scan rate was kept with a potential range of −0.5 V SCE to +0.3 V SCE during the forward scan and +0.3 V SCE to −0.5 V SCE to during reverse scan of the test.Potentiodynamic polarization (PDP) tests and electrochemical impedance spectroscopy (EIS) were performed in 3.5% NaCl (sea water environment) at room temperature to examine the electrochemical properties of the weldments.The PDP tests were performed in the potential range of −0.5 V SCE to +1.2 V SCE at a scan rate of 0.167 mV s −1 .The EIS test was performed in the range of 10 6 to 10 −2 using 10 mV sinusoidal AC signal.Utilizing a three-electrode cell configuration with platinum serving as the contour electrode, saturated calomel electrode (SCE) serving as the reference electrode, and sample (WZ of the weldments) as the working electrode, the DL-EPR, EIS, and PDP tests were carried out.All the tests were in Biologic VMP-300 potentiostat.To achieve data reproducibility, an average of three samples testing were analyzed.A fresh electrolytic solution (maintained at ambient temperature) was used after each test.3(a)).In contrast, the microstructure of super-DSS S32760 (figure 3(b)) exhibits a balanced microstructure austenite phase (white areas) and the ferrite phase (dark areas).The reduced grain size of the ferrite in super-DSS S32760 BM can be attributed to the precipitation of the sigma phase, which results in lower levels of Cr and Mo within the ferrite phase [3,4].The primary aim of examining the metallography of the BM was to understand the alterations in the microstructure and solidification process of the fused metal that occur during welding procedures.

Results and discussions
The SEM macrographs of weldments are illustrated in figure 4. It can be observed that the widths of the WZ in both the weldments are almost the same.Further, full penetration depth can be observed in both the weldments, suggesting the sound weld joints, however, it can be observed that the penetration depth of interlayer-based weldment was slightly higher than the autogenous weldment.
The optical micrographs of interfaces of the autogenous and interlayer-based welded samples are shown in figures 5 and 6, respectively.It can be observed that in both the weldments, the HAZ has formed in the super-DSS S32760 side while un-mixed zone (UMZ) has formed in LNiASS side.The HAZ of both weldments showed the formation of continuous network of intergranular austenite (IGA), Widmanstatten austenite (WA) and grain boundary austenite (GBA), as can be seen in figures 5(c) and 6(c).The cooling transformation of the grain boundary of the ferrite phase leads to GBA [4][5][6].It has been reported that the formation of GBA usually takes place in the temperature range of 1350 °C-800 °C [26].The WA needles are the result of the displaced and diffused mechanism of the alloying elements from GBA.The WA enriched in Ni as compared to the ferrite matrix, however, WA typically contains lower amounts of Cr, Mo, and N than the GBA [26,27].Further, IGA forms due to the cooling rate effect, it requires a greater driving force and precipitates later at a lower   temperature [28].These IGA, WA and GBA collectively called as secondary austenite [29,30].Further, the width of HAZ of interface-layer based weldment (∼0.25 mm) was slightly higher than the autogenous based weldment (∼0.22 mm), however, the difference is not significant see figures 5(a) and 6(a).Due to marginally higher heat  input because of slower welding speed, the interlayer-based weldment resulted in slightly wider HAZ.The higher heat input resulted in the slower cooling rate which led to longer heat retention, thus the width of HAZ increased [31,32].Towards the LNiASS side of both weldments, a significant band of dendritic microstructure can be observed in figures 5(b) and 6(b).This band of microstructure is also shows the unmixing of the weld region to the BM and referred to be an unmixed zone (UMZ) [33].Austenitic grades usually have lower thermal conductivity, which may contribute to the formation of UMZ by facilitating rapid and effective cooling in a prominent direction of heat flow [34,35].Further, no significant difference in the width of UMZ was observed for the interlayer-based weldment (∼0.19 mm) and the autogenous weldment (∼0.21 mm).The HAZ was not visible towards the LNiASS side of the autogenous based weldment (figure 5(b)), however, very minor grain coarsening, say HAZ, was noticed on the interlayer-based weldment (figures 6(b), (d)).
Further, the SEM microstructures of the WZ are illustrated in figure 7.In the autogenous based weldment, the formation of various types of austenite (IGA and WA) as well as ferrite near the interface of both the BMs and WZ can be observed (figure 7(a)).On the other hand, a rich austenitic microstructure was observed in the WZ of the interlayer-based weldment.This rich microstructure is the result of the presence of a higher austenitic stabilizer element in Inconel 625 interlayer.The higher percentage of Ni was resulted in the formation of cellular along with columnar dendritic patterns in the WZ of the interlayer-based weldment, whereas an equiaxed grains, were observed near the weld center (figure 7(b)).Further, the lower temperature gradient to solidification rate at the weld center results in the microstructure transitioning from columnar dendritic to equiaxed grains [36].
The solidification from the weld interface to the weld center caused the microstructural changes from columnar (∼300 μm) to fine equiaxed dendritic (∼16 μm) in the case of interlayer-based weldment.Further, the formation of precipitates can be observed in the interlayer-based weldment.EDS analysis was performed to quantify the precipitations, as shown in figure 7(c), and it was observed that the phases manifesting as white blotches in the WZ of the interlayer-based weldment were enriched with Nb and Mo.The formation of these precipitates can be observed in both the grain boundary and dendrites.
From the XRD analysis (figure 8), the findings revealed that the BM LNiASS exhibited distinct peaks associated with the austenitic γ phase, specifically the (111), ( 200), ( 220), (311), (222) planes.However, the BM super-DSS S32760 shows the dual microstructure, i.e., γ [(111), (200), (220), (311)] and α [(110), (220), (211)] peaks.Furthermore, the spectrum shows the evolution of the higher intensity of δ ferrite in the case of autogenous weldment at the plane (110) corresponding to 2θ values of 29.54 as compared to interlayer-based weldment, which is in agreement with optical and SEM microstructures.After welding, the microstructural changes at the interface and WZ of the weldments would depend on the specific conditions of the welding process, such as the welding parameters and the composition of the interlayer and BMs [37].However, it is important to note that the presence of dual microstructure in the WZ of autogenous weldment, however, after employing Inconel 625 interlayer, the WZ exhibited purely austenitic structure, which can be attributed to higher content of Ni in the Inconel 625 which is austenite stabilizer.The tiny ferrite peaks observed in the interlayer-based weldment could come from the UMZ portion of the weldment (figures 6(b), (d)).These variations of δ ferrite in both the weldments can impact their mechanical and electrochemical properties [38].

Mechanical characterization 3.2.1. Microhardness test
The microhardness distribution profile of both autogenous and interlayer-based weldment is shown in figure 9 [39].Interestingly, the microhardness increased from LNiASS to S32760 in the case of autogenous weldment, while in the interlayer-based weldment, the microhardness firstly decreased from LNiASS to WZ and then increased from WZ to S32760.However, the microhardness profile indicates some variations in microhardness from the WZ and its interface.The average measured values at the WZ were 279 ± 2 HV 0.2 for autogenous weldment, and 258 ± 3 HV 0.2 for interlayer-based weldment.This difference in microhardness can be attributed to the difference in microstructures.As observed, the microstructure of autogenous weldment exhibited dual microstructure while the microstructure of interlayer-based weldment exhibited fully austenitic microstructure (figure 7).The WZ of autogenous weldment exhibited higher microhardness attributed to the presence of ferrite.It has been reported that the presence of ferrite restricts dislocation motion, leading to enhanced hardness [40].Also, Verma et al [40] noted that the presence of alloying elements such as Ni reduces microhardness.In the case of autogenous weldment towards the S32760 HAZ side, the average hardness value was 285 ± 5 HV 0.2 .However, in the case of interlayer-based weldment towards the S32760 HAZ side, the average hardness value was 262 ± 2 HV 0.2 .In this case, the difference in hardness can be attributed solely to variations in temperature, solidification time and ferrite content.The average microhardness at the WZ obtained for the autogenous and interlayerbased weldment are listed in table 3.

Tensile test
The tensile test findings for the BMs and welded samples, showing the ultimate tensile strength (UTS) and percentage elongation are summarized in figure 10.The results of the tensile test showed good agreement with the hardness data.With an elongation of 26%, the average UTS of the Interlayer based weldment was 654 MPa.Nevertheless, this UTS of the Interlayer-based weldment was significantly less than the UTS of 693 MPa of the autogenous weldment (30% elongation), leading to a decreased joint efficiency of 5.96%.Base metal S32760 shows the highest UTS of 820 MPa with percentage elongation of 35% among all the base metal and welded samples.Followed base metal S32760 base metal LNiASS shows the UTS of 778 MPa with percentage elongation of 31%.Lower hardness in the case of interlayer-based weldment was identified as the cause of the face-centered cubic (FCC) austenitic microstructure in the welded zone.As FCC crystal structure ductile in nature results reduced hardness in the welded zone [41].Also, an evolution of macro-segregation (intermetallics) in the interdendritic arms was found by microstructure analysis, which led to an uneven element distribution in the matrix and weakened the weld zone of Interlayer-based weldment.Using a SEM, this study examined the fracture surfaces of autogenous and interlayer based weldments to obtain insight into the aspects of failure.The surface micrographs of the fracture tensile samples are shown in figure 11.The surface fractographs of tensile fractured samples revealed the existence of many bigger voids with ductile tearing edges in both autogenous and Interlayer based weldment.

Impact test
The samples for the impact testing were cut as per the guideline of ASTM E23.An average of three trials of impact tests were carried out at room temperature to ensure results reproducibility.The fractographs of the impact tested weldments are shown in figure 12.The interlayer-based weldment was observed to exhibit higher impact value 91 ± 2 J compared to the autogenous weldment 68 ± 2 J which could be attributed to its fully  austenite microstructure (Face Centered Cubic (FCC)).The FCC structure is nearly ductile in nature which could lead to have higher impact resistance [42].Furthermore, FCC structure provides more flexibility which absorbs more energy during deformation leads to higher impact value [43].
The surface morphology of autogenous weldment and interlayer-based weldment mainly consisted of tiny and large dimple-like structure which indicates that the material has undergone plastic deformation under highimpact loads.When autogenous welding was used, the quasi-cleavage pattern on the fractured surface increased while the dimple morphology on the impact fractured surface decreased as a result of the ferritic austenitic dual microstructure evolving in the WZ [44].The quasi-cleavage pattern also shows the reduction in the toughness properties of the welded joint [45,46].Overall, the fractography analysis revealed that only a ductile mode was responsible for both weldment fractures.

Electrochemical behaviour of weldments 3.3.1. Electrochemical impedance spectroscopy (EIS) test
The evaluation of passive film resistance in WZ of weldments was conducted through EIS test.Figure 13 displays the EIS Nyquist plots of the WZ.The Nyquist experimental data were fitted using the Randles equivalent circuit [Rs (CPE || Rct)}], where R s stands for solution resistance, R ct for charge transfer resistance, and CPE for the constant phase element.The following equation (1) is used to calculate the CPE's impedance: In equation (1), Z 0 is a constant, n is an exponent, and ω is the angular frequency.The CPE can act as an ideal resistor n = 0 or capacitor n = 1, depending on the value of n [47].The partial semi-circular arcs with variable widths shown in nyquist plots are linked to the film properties and are a result of the charge transfer process that takes place at the electrode/electrolyte interface.The diameter of the plots corresponds to the corrosion resistance, indicating that larger diameters signify greater resistance and, consequently, enhanced corrosion resistance [48].The Nyquist plot of the interlayer-based WZ exhibits the largest diameter compared to the autogenous weldment WZ.Moreover, figure 14 illustrates the EIS Bode plots of the interlayer-based and autogenous weldments.Greater impedance values in the high-frequency region of the Bode plots indicate the passive film's compactness, while the low-frequency region's impedance modulus indicates corrosion resistance [49].Impedance values for the interlayer-based WZ are higher than those for the autogenous, in line with the  Nyquist plots.The impedance modulus in the low-frequency range has this same pattern.These results indicate that the corrosion resistance of the interlayer-based WZ is higher than that of the autogenous WZ.This discrepancy can be explained by the fact that the interlayer-based weldment has larger concentrations of alloying elements (Mo and Ni) than the autogenous weldment.

Potentiodynamic polarization (PDP) test
To determine the resistance of the metal samples against pitting, the PDP test is popularly used [50].The PDP curves and SEM micrographs after PDP test of the WZ of the weldments are shown in figure 15.The obtained polarization data after the potentiodynamic test is given in table 4. The pitting potential (E pit ), corrosion current density (i corr ) and corrosion potential (E corr ) values were compared between the autogenous weldment and interlayer-based weldments.It is reported that steels with greater E corr values are said to be thermodynamically more stable than those with lower E corr values [51].The WZ of autogenous weldment has shown higher corrosion potential (E corr = −0.40mV SCE ) compared to the interlayer-based weldment (E corr = −0.52 mV SCE ).However, WZ of interlayer-based weldment shows a slightly higher pitting potential (E pit = 0.55 mV SCE ) as compared to the autogenous weldment (E pit = 0.48 mV SCE ).Also, the i corr for WZ of interlayer-based weldment is lower as compared to its counterpart.From EIS Nyquist plots (figure 13), it was observed that the unfinished semi-circular diameter of WZ of autogenous weldment was significantly lower as compared to interlayer-based  weldment.This shows the higher charge transfer between the electrolyte and passive film, thus resulting in lower E pit and i corr values.This difference could be attributed to the difference in microstructure and use of interlayer in interlayer-based weldment.
Furthermore, it is well known that the pitting resistance is assessed using the pitting resistance equivalent number (PREN) as described in equation (2) [47] From the equation (2), it can be seen that the PREN is dependent on Cr, N and Mo.Table 1 shows the difference in Cr and Mo content in the BMs and Inconel 625.The Inconel 625 has a significantly higher content of Mo as compared to the BMs.Hence, upon using the Inconel 625 as an interlayer in the interlayer-based weldment, the presence of Mo in the WZ may have resulted in the higher pitting potential.Also, the presence of Ni results in the formation of NiO within the passive film, contributing to higher pitting potential [52].Also, the presence of dual phase in the WZ of autogenous weldment may form galvanic cell between the phases resulting in lower pitting potential.

Intergranular corrosion (IGC) test
The need for non-destructive and highly sensitive tests to detect IGC in stainless steel is crucial, as conventional methods can inadequate in detecting low degrees of sensitization and consume a large amount of material [53,54].KSCN in the electrolyte solution was used as a depassivator for in situ observation.Dissimilar welds commonly experience sensitization in various industries application.The sensitization curves and micrographs obtained after DL-EPR test of WZ is presented in figure 16.The DLEPR test findings of autogenous weldment yielded a precise of the anodic activated peak current (I a = 0.0037 A cm −2 ) and reactivated peak current (I r = 0.000045 A cm −2 ) showed that the degree of sensitization = DOS % 100 47.33%.This can be attributed to the dual microstructure, and it has been reported that the ferrite phase is typically more susceptible to IGC.According to Moon et al [55], in corrosive conditions, galvanic cells may form between the Cr-rich phase (ferrite) and Cr-depleted phase (austenite), which could result in IGC.According to reports, the body-centered cubic (BCC) ferrite structure with loose packing shows a higher tendency to react than the more  thermodynamically stable FCC austenite structure with firmly packed structure.The dual phase structure also shows a higher activation energy [40].Also, the microstructure after DLEPR test revels the sensitized area in the ferrite phase (figure 16(b)).However, in interlayer-based case no sensitization was noticed.These zero sensitization of interlayer-based weldment could be due to the fully austenitic microstructure in the WZ [7,56], also, FCC phase is more thermodynamically stable.

Conclusions
Based on the detailed experiments conducted on autogenous and interlayer-based dissimilar ATIG welding of LNiASS and super-DSS S32760, the following conclusions can be drawn.
1.A significant increase in depth of penetration (D) and reduction in weld width (W) is achieved in the case of ATIG welded interlayer-based weldment (D = 8.57mm and W = 9.11 mm) at the same welding current as compared to autogenous weldment (D = 6.06 mm and W = 9.52 mm).
2. The dual microstructure (austenite + ferrite) was formed in WZ of autogenous weldment while fully austenite with intermetallics was formed in WZ of interlayer-based weldment.
3. The formation of a significant band of dendritic microstructure [unmixed zone (UMZ)], in the austenitic (LNiASS) side of both autogenous and interlayer-based weldments attributed to the lower thermal conductivity of austenitic grades.
4. Microhardness measurement shows that a significant reduction in the hardness value of the WZ of the interlayer-based weldment (258 ± 3 HV 0.2 ) compared to the autogenous weldment (279 ± 2 HV 0.2 ) due to the higher content of Ni.
5. The interlayer-based weldment exhibits a reduced UTS and joint efficiency compared to the autogenous weldment, attributed to macro-segregation (Intermetallics), as revealed by SEM/EDS microstructure analysis.
6.The impact study result showed that the improvement in the impact strength in case of interlayer-base weldment (91 ± 2 J) as compared to the autogenous weldment (68 ± 2 J).
7. The polarization study revealed the higher pitting potential for WZ of interlayer-based weldment (E pit = 0.55 mV SCE ) as compared to the autogenous weldment (E pit = 0.48 mV SCE ) attributed to higher content of Ni and Mo.

Figure 1 .
Figure 1.(a) The operating principle of the ATIG Welding (b) schematic of autogenous based weldment (c) schematic of interlayerbased weldment.

3. 1 .
Macro and microstructure characteristics The photograph of the welded sample is shown in figure 2 revealing no visible defects.The microstructures of BMs are shown in figure 3. The microstructure of the LNiASS BM, composed of austenite with a twin boundary

Figure 5 .
Figure 5. Optical microstructures of the autogenous based weldments (a) super-DSS S32760 side, (b) LNiASS side, (c) magnified region of HAZ of super-DSS S32760 side and (d) magnified region of UMZ of LNiASS side.

Figure 6 .
Figure 6.Optical microstructure of the interlayer-based weldments (a) super-DSS S32760 side, (b) LNiASS side, (c) magnified region of HAZ of super-DSS S32760 side and (d) magnified region of UMZ of LNiASS side.

Figure 8 .
Figure 8. X-ray diffractometry analysis of the BMs and WZ of weldments.

Figure 10 .
Figure 10.Tensile test results of the BMs and dissimilar weldments.

Figure 11 .
Figure 11.SEM tensile fractographs showing the fractured surface of the (a) autogenous welded joint (b) interlayer-based welded joint.

Figure 15 .
Figure 15.PDP curves and SEM micrographs after PDP test of WZ of autogenous and interlayer-based weldments.

Figure 16 .
Figure 16.DLEPR curves and optical micrographs after DLEPR test of the WZ of autogenous and interlayer-based weldments.

Table 2 .
Process parameters used for ATIG welding.

Table 3 .
Mechanical properties of the autogenous and interlayer-based weldment.