Enhanced mechanical and microstructural properties of 2205 duplex stainless steel welded using autogenous gas tungsten arc welding

ABSTRACT

This study examines the mechanical and microstructural characteristics of 2205 duplex stainless steel (DSS) sheets welded using Autogenous Gas Tungsten Arc Welding (A-GTAW) under optimized conditions. A 2 mm thick DSS plate was butt-welded using a welding current of 100 A, an arc length of 2.4 mm, and a welding speed of 250 mm/s. The welded joint was evaluated for tensile strength, hardness, ductility, bending performance, and formability. The joint exhibited an ultimate tensile strength of 705 MPa, representing a 34% improvement over the base metal (BM), with an elongation of 56.5%. Bending tests confirmed that the joint maintained its structural integrity under severe plastic deformation, with an r/t bending ratio of 2, demonstrating excellent ductility. Microhardness measurements showed a 12.5% increase in hardness in the fusion zone (FZ) relative to the BM, attributed to δ-ferrite and γ-austenite phase transformations. The microstructural analysis revealed the formation of Widmanstätten austenite (WiA), secondary austenite (SA), and partially transformed austenite (PTA) in the FZ, with quantified phase percentages contributing to the enhanced mechanical properties. Fractographic analysis identified ductile failure characterized by dimples, microvoid coalescence, and secondary cracks. The stress-strain curve exhibited a second jump, attributed to strain hardening effects in the welded joint. The Erichsen formability index increased by 10%, indicating superior forming behavior. These results demonstrate that A-GTAW, when applied under carefully controlled parameters, significantly enhances the mechanical performance and structural reliability of 2205 DSS joints, making it a viable technique for critical industrial applications requiring high strength, ductility, and corrosion resistance.

Keywords:
Duplex Stainless Steel; A-GTAW; Mechanical Properties; Microstructural Analysis; Fractography

1. INTRODUCTION

Duplex stainless steel (DSS), and in particular grade 2205, has garnered widespread attention in various industrial sectors that demand materials capable of withstanding severe environmental conditions [¹]. Its unique appeal lies in the remarkable combination of high strength, excellent toughness, and superior corrosion resistance [²]. These properties stem from a carefully engineered dual‐phase microstructure composed of nearly equal proportions of body‐centered cubic (BCC) δ‐ferrite and face‐centered cubic (FCC) γ‐austenite [³]. The synergistic interplay between these two phases not only imparts high tensile strength and fatigue resistance but also enhances resistance to stress corrosion cracking and pitting corrosion [⁴]. Such characteristics make 2205 DSS indispensable in applications ranging from marine infrastructure and desalination plants to chemical processing facilities and critical structural components exposed to aggressive service environments [⁵]. The exceptional performance of 2205 DSS is achieved through a precise balance of alloying elements. Chromium and molybdenum are essential for stabilizing the δ‐ferrite phase and enhancing corrosion resistance [⁶], while nickel and nitrogen promote the formation and stabilization of the γ‐austenite phase, which is crucial for maintaining ductility and toughness [⁷]. However, the very attributes that make 2205 DSS so desirable also pose challenges when it comes to welding. The welding process introduces rapid thermal cycles that disturb the delicate balance between these phases [⁸]. In particular, the Autogenous Gas Tungsten Arc Welding (A-GTAW) process—a variant of Gas Tungsten Arc Welding (GTAW) that operates without filler material—is widely used for joining thin sheets because it preserves the original chemical composition of the base metal [⁹]. When welding 2 mm thick sheets of 2205 DSS, the absence of filler material is advantageous as it avoids contamination and unwanted compositional changes; however, the process also subjects the material to rapid heating and cooling cycles that can induce complex microstructural transformations [¹⁰]. During welding, the intense heat input and subsequent rapid cooling lead to significant changes within the fusion zone (FZ) and the heat-affected zone (HAZ) [¹¹]. Rapid cooling, in particular, often results in an increased formation of δ-ferrite at the expense of γ-austenite, which can result in coarse grains and the emergence of deleterious secondary phases such as sigma (σ), chi (χ), and chromium nitrides [¹²]. These transformations directly impact the mechanical properties of the welded joint by reducing ductility and potentially increasing hardness beyond desirable limits [¹³]. Many conventional studies have focused on thicker DSS sections or on welding processes that employ filler materials, where the influence of process parameters such as welding current, arc length, and travel speed are well documented [¹⁴]. In contrast, there is a paucity of research on the autogenous welding of thin DSS sheets, particularly under controlled conditions that are critical for preserving the material’s intrinsic properties [¹⁵]. The literature provides valuable insights into the welding behavior of DSS; however, most studies have concentrated on techniques such as laser welding or friction stir welding applied to thicker sections [¹⁶]. Moreover, many investigations into GTAW have explored the effects of filler materials on joint properties, leaving unanswered questions regarding the precise microstructural evolution and mechanical behavior of thin-sheet DSS when welded autogenously [¹⁷]. This gap in knowledge is significant because thin DSS sheets exhibit unique challenges; the high thermal gradients and rapid cooling rates encountered during welding can lead to pronounced phase imbalances that are not observed in thicker sections [¹⁸]. The lack of detailed studies in this area means that optimal welding parameters for thin-sheet applications remain largely undefined, and the relationship between process conditions, microstructural transformations, and mechanical properties is not fully understood [¹⁹]. In light of these challenges, the present study seeks to bridge the knowledge gap by focusing on the autogenous welding of 2 mm thick 2205 DSS sheets using A-GTAW [²⁰]. The novelty of this research lies in its comprehensive approach, whereby the welding process is meticulously controlled to preserve the base metal’s chemical integrity while inducing beneficial microstructural transformations [²¹]. By eliminating filler material, this work aims to maintain the original alloy composition and to explore how optimized welding parameters influence the formation and distribution of phases within the weld region [²²]. In doing so, it investigates how rapid thermal cycles affect both the overall grain structure and the specific phase transformations—such as the development of Widmanstätten austenite, secondary austenite, and partially transformed austenite—that directly contribute to the enhanced mechanical properties of the welded joint [²³]. A critical problem addressed by this study is the limited understanding of how thin 2205 DSS sheets behave under the rapid thermal cycles imposed by A-GTAW [²³]. While thicker sections may exhibit a gradual transition in phase balance between the fusion zone, heat-affected zone, and base metal, thin sheets are far more sensitive to such changes, often resulting in an imbalance between δ-ferrite and γ-austenite [²⁴]. This imbalance can lead to increased hardness but reduced ductility—a trade-off that could compromise the joint’s structural integrity in real-world applications [²⁵]. The study, therefore, aims to quantify these microstructural changes and correlate them with measured mechanical properties such as tensile strength, bending behavior, microhardness, and formability [²⁶]. Establishing a clear process-structure-property relationship is essential for optimizing welding practices in industrial settings where thin DSS sheets are employed [²⁷]. Moreover, the research incorporates advanced characterization techniques, including scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and fractographic analysis, to provide a detailed understanding of the weld’s microstructure [²⁸, ²⁹]. These techniques are used to examine the evolution of phases in the fusion zone and HAZ, assess grain refinement, and analyze the fracture mechanisms that occur during mechanical testing [³⁰]. The comprehensive nature of these analyses not only enhances the academic understanding of DSS welding but also has practical implications for improving the mechanical performance and reliability of welded joints [³¹, ³²]. The objectives of this study are multifaceted. First, it aims to evaluate the mechanical performance of autogenous weld joints by conducting tensile, bending, and microhardness tests in accordance with recognized standards. Second, the study seeks to characterize the macrostructural and microstructural features of the weld—covering the FZ, HAZ, and base metal—using detailed metallurgical analysis and microscopy. Third, by performing fractographic analysis, the research aims to identify the dominant fracture mechanisms and correlate these with the observed mechanical properties. Finally, by comparing the properties of the welded joint with those of the base metal, the study intends to develop a robust process-structure-property relationship that can inform future welding practices for thin 2205 DSS sheets.

2. MATERIALS AND EXPERIMENTAL METHODS

2.1. Material selection

In this study, 2 mm thick sheets of grade 2205 Duplex Stainless Steel (DSS) were used as the base material for the welding experiments. The chemical composition of the 2205 DSS alloy, which plays a critical role in its mechanical strength and corrosion resistance, is presented in Table 1. The alloy exhibits a balanced combination of chromium, molybdenum, and nickel—essential for ferrite and austenite stabilization—as well as nitrogen to promote austenite formation and overall strength. Minimal amounts of carbon, manganese, silicon, phosphorus, sulfur, vanadium, titanium, cobalt, and niobium are also present to fine-tune the material properties, with the balance of the composition being iron.

Table 1
Chemical composition (wt. %) of 2205 DSS alloy.

CR	MO	NI	N	C	MN	SI	P	S	V	TI	CO	FE
23.0	3.5	6.5	0.20	0.03	2.00	1.00	0.03	0.02	0.06	0.004	0.01	Balance

2.2. Experimental methods

The primary objective of the welding experiments was to produce a fusion zone (FZ) and heat-affected zone (HAZ) with mechanical properties that are comparable to or even superior to those of the base metal (BM). Autogenous Gas Tungsten Arc Welding (A-GTAW) was chosen because it eliminates the use of filler materials, thereby preserving the intrinsic chemical composition of the DSS. Welding was carried out using a Fronius Magic Wave 400 machine (Figure 1) to ensure precise and high-quality welds on 2205 DSS sheets. A numerical control unit was used to maintain a consistent welding speed throughout the process. The selection of input parameters was guided by a review of the literature and practical trials; key parameters such as welding current (I), arc length (L), and welding speed (S) were found to significantly influence the heat input. Several preliminary tests were conducted using welding currents of 60, 80, and 100 amperes, arc lengths of 2, 3, and 4 mm, and welding speeds of 250, 300, and 350 mm/s. Based on these trials, the optimized process parameters were determined and are summarized in Table 2.

Figure 1
Schematic representation of GTA welding experimental setup.

Table 2
Optimized A-GTA process parameters for welding 2205 DSS.

PROCESS PARAMETER	WELDING CURRENT	ARC LENGTH	WELD SPEED
Value	100 Amps	2.4 mm	250 mm/Sec

The electrode diameter of 2.4 mm was kept constant for all trials. To create a butt joint, two pieces of 100 × 100 × 2 mm thick 2205 DSS were aligned and welded under the controlled conditions. Proper shielding was necessary to prevent distortion and contamination during the welding process. The shielding gas used was 99.99% pure argon with a flow rate set at 20 liters per minute, ensuring the weld zone was protected from atmospheric contaminants. With a specially designed fixture, atmospheric contamination in the joint was minimized. The A-GTAW process can cause misalignment and deformation of the workpiece; therefore, the fixture was used to securely clamp the DSS sheets, ensuring a stable welding environment and high-quality welds. The weld surfaces were clean and uniform, contributing to consistent bead formation on both the face and root sides, as shown in Figure 2. The prepared butt joints underwent a series of characterization procedures. Specimens were first cut using a wire-cut electric discharge machine in the crosswise direction of the weld, following ASTM E3 standards for metallographic specimen preparation [³³]. The samples were then mounted in Bakelite and polished using different grades of emery sheets to achieve a smooth surface, in line with ASTM E1920 standards for metallographic polishing [³⁴]. To reveal the profile of the weld bead, an etching solution containing 1% hydrogen fluoride, 36% hydrochloric acid, 0.8 g potassium metabisulfate, and 63% water was applied. The macrographs of the welded joints were captured using a Weld Expert System (Struers, Austria), with magnifications ranging from 20X to 240X. Following this, a metallurgical microscope (Leica Microscope) was used for high-resolution imaging to analyze the microstructure of the joints. Mechanical testing was conducted using a Tinius Olsen universal testing machine (UTM) equipped with a 50 kN load cell, in accordance with ASTM E8/E8M standards for tensile testing of metallic materials [³⁵]. Tensile and bending tests were performed at a constant strain rate of 1 mm/min and under ambient conditions to assess the mechanical properties of the welded joints. Microhardness testing was carried out on both the weld metal (WM) and base metal (BM) using a Vickers hardness tester from Everone Enterprises, with a load of 500 g and a dwell time of 10 seconds, following ASTM E384 standards for microhardness testing [³⁶]. The hardness profiles were recorded at different intervals to examine variations across the joint and adjacent areas. To study the fracture behavior and failure mechanisms, a Hitachi S3000N scanning electron microscope (SEM) was employed. High magnification imaging was used to capture detailed fractographs of the tensile test specimens. SEM analysis was complemented by Energy Dispersive X-Ray Spectroscopy (EDS) to obtain the chemical composition of the FZ, HAZ, and BM, providing insight into the alloying elements and their distribution. For evaluating the formability of the joint, the Erichsen cupping test was performed using an apparatus with a constant speed of 10 mm/min and a 100 kN load cell, in accordance with ASTM E643 standards for the Erichsen cupping test [³⁷]. This setup helped determine the mechanical behavior of the weld under deformation and assess its suitability for forming applications. A schematic representation of the A-GTAW experimental setup is provided in Figure 1. For all trials, the electrode diameter was maintained at 2.4 mm. To create a butt joint, two pieces of 100 × 100 × 2 mm thick 2205 DSS were aligned and securely clamped using a specially designed fixture. This fixture minimized atmospheric contamination and ensured precise alignment to prevent misalignment or deformation during welding. The shielding gas was 99.99% pure argon, delivered at a flow rate of 20 L/min, to protect the weld zone from atmospheric contaminants. Figure 2 shows a schematic of the butt joint, detailing both the face side and the root side, which contributed to consistent bead formation. Following welding, the prepared butt joints underwent several characterization procedures. Specimens were first sectioned using a wire-cut electric discharge machine (EDM) perpendicular to the weld, in accordance with ASTM E3 standards for metallographic specimen preparation [³³]. The cut samples were mounted in Bakelite and polished using progressively finer emery sheets, as specified in ASTM E1920 [³⁴]. To reveal the weld bead profile, an etching solution (comprising 1% hydrogen fluoride, 36% hydrochloric acid, 0.8 g potassium metabisulfate, and 63% water) was applied. Macrographs were captured using a Weld Expert System (Struers, Austria) at magnifications ranging from 20× to 240×. High-resolution imaging for microstructural analysis was performed with a Leica metallurgical microscope. Mechanical testing was conducted using a Tinius Olsen universal testing machine (UTM) with a 50 kN load cell, following ASTM E8/E8M standards [³⁵]. Tensile tests were performed at a constant strain rate of 1 mm/min under ambient conditions. Bending tests were also carried out at the same strain rate to assess ductility and overall mechanical integrity. Microhardness testing was performed using a Vickers hardness tester (Everone Enterprises) with a 500 g load and a dwell time of 10 seconds, in accordance with ASTM E384 standards [³⁶]; hardness profiles were recorded at intervals across the weld and adjacent regions. To investigate fracture behavior and failure mechanisms, fractured tensile specimens were examined using a Hitachi S3000N scanning electron microscope (SEM) at various magnifications. This analysis was complemented by Energy Dispersive X-Ray Spectroscopy (EDS), which provided information on the chemical composition and distribution of alloying elements across the fusion zone (FZ), heat-affected zone (HAZ), and base metal (BM). Finally, the formability of the welded joints was evaluated by performing the Erichsen cupping test, in accordance with ASTM E643 standards [³⁷], using an apparatus with a constant speed of 10 mm/min and a 100 kN load cell.

Figure 2
Schematic of butt joint. (a) Face side; (b) root side.

3. RESULTS AND DISCUSSION

3.1. Macrostructure

The images in Figure 2 clearly demonstrate that the weld bead is continuous and free from visible discontinuities, indicating a high-quality weld. Macrostructural examination using the Weld Expert System (Figure 3) enabled precise measurement of the bead geometry. The weld exhibited a depth of penetration (DoP) of 1.9802 mm, a bead width (BW) of 6.523 mm, and a total weld area (WA) of 14.3595 mm². These measurements confirm the uniformity and integrity of the weld bead, which is critical for ensuring the joint’s strength and reliability. The consistent formation of the weld bead on both the face and root sides supports the effectiveness of the optimized welding parameters and fixture design.

Figure 3
Macrographs of AA-GTA butt welded joint of 2205 DSS.

3.2. Microstructural observations

The microstructure of the A-TIG welded joint was investigated using scanning electron microscopy (SEM) to assess grain structures and phase distributions in the Fusion Zone (FZ), Heat-Affected Zone (HAZ), and Base Metal (BM) as shown in Figure 4. Energy-dispersive X-ray spectroscopy (EDS) was used to analyze the redistribution of key alloying elements—namely chromium (Cr), nickel (Ni), molybdenum (Mo), carbon (C), and iron (Fe)—across these regions. Figures 5, 6, and 7 present the EDS spectra for the FZ, HAZ, and BM, respectively, while Table 3 summarizes the chemical compositions.

Figure 4
Micrograph metallurgical microscope. (a) Weld metal; (b) Fusion zone SEM micrograph; (c) Base metal at 1000x; (d) Fusion zone at 100x; (e) Weld metal at 100x; (f) Fusion zone at 300x.

Figure 5
SEM-EDS spectrum for fusion zone of the joint.

Figure 6
SEM-EDS spectrum for heat affected zone of the joint.

Figure 7
SEM-EDS spectrum for base metal.

Table 3
Chemical composition (% wt) of alloying elements in the joint specimen.

ZONE	CHEMICAL COMPOSITIONS OF SOME IMPORTANT ALLOYING ELEMENTS (% OF WT)
	CR	NI	MO	C	FE
Fusion	19.27	5.36	3.21	29	39.55
Heat affected	20.54	4.72	3.05	26.1	41.8
Base metal	23	6.5	3.3	22.69	45.86

3.2.1. Fusion zone

The FZ displayed a coarse-grain structure with distinct phases of γ-austenite and δ-ferrite, as shown in Figure 4. The formation of austenite was observed as Widmanstätten-type austenite (WiA), grain boundary austenite (GBA), intragranular austenite (IGA), and secondary austenite (SA). These phases are indicative of the phase transformations occurring during the welding process, where the weld metal initially solidifies as ferrite and subsequently transforms into austenite during cooling. The coarse grains in the FZ result from prolonged thermal exposure and slower cooling rates, contributing to the hardness but potentially reducing ductility. The EDS analysis of the FZ showed a reduced Cr content (19.27%) compared to the BM (23%). This reduction is associated with the formation of chromium nitrides (CrN, Cr2N) due to the redistribution of elements during welding. The Ni content in the FZ (5.36%) was also lower than in the BM (6.5%), impacting the stabilization of γ-austenite and the toughness of the FZ. Molybdenum content, which enhances pitting corrosion resistance, was slightly reduced in the FZ, while the carbon content was the highest in this region (29%), contributing to increased hardness through carbide formation as shown in Figure 5.

3.2.2. Heat-affected zone (HAZ)

In the HAZ, a transition between coarse and fine-grain structures was observed, as shown in Figure 6. The thermal cycles during welding partially transformed the austenite into ferrite, resulting in finer grains compared to the FZ. This zone serves as a bridge between the highly altered FZ and the unaffected BM, displaying intermediate mechanical and chemical properties. The HAZ exhibited intermediate values for Cr (20.54%), Ni (4.72%), Mo (3.05%), and C (26.1%), reflecting its partial exposure to the thermal cycles during welding. The chemical composition in the HAZ suggests a gradual transition from the BM to the FZ, with moderate changes in hardness and corrosion resistance.

3.2.3. Base metal (BM)

The BM retained its original duplex microstructure, with evenly distributed γ-austenite and δ-ferrite phases. As shown in Figure 7, the BM was unaffected by the thermal cycles of welding, preserving its balanced phase composition and fine-grain structure. The BM retained the highest Cr content (23%) and Ni content (6.5%), essential for maintaining its excellent corrosion resistance and toughness. The carbon content in the BM was the lowest (22.69%), which is consistent with its higher ductility compared to the welded zones.

3.3. Mechanical properties

In general, welded joints have three varieties of regions namely fully resolved fusion zone (FZ), partially melted heat affected zone (HAZ) and unaffected base metal (BM). The strength of the weld is determined by the phase transformation and reformation of grains.

3.3.1. Tensile properties

The tensile test results highlight a significant improvement in the mechanical properties of the butt joint compared to the base metal (BM). The ultimate tensile strength (UTS) of the butt joint is 705 MPa, approximately 34% higher than that of the BM (526 MPa). This enhancement is attributed to the increased hardness in the Fusion Zone (FZ), resulting from grain refinement and phase transformation during the welding process.

The elongation percentage of the BM is 120%, which is significantly greater than the 56.5% observed for the butt joint. This disparity indicates that while the BM exhibits superior ductility, the welded joint achieves greater strength and toughness. Fracture occurred in the BM at a distance of 28.2 mm from the weld center, confirming the integrity of the weld joint under tensile loading. As shown in the stress-strain curve (Figure 8, the BM displays extended plastic deformation, whereas the butt joint exhibits a steeper stress increase and reduced strain-to-failure. The second jump observed in the stress-strain curve is attributed to strain hardening effects, which result from dislocation accumulation and increased resistance to deformation within the fusion zone. This phenomenon enhances the joint’s load-bearing capacity while reducing overall elongation. The neck formation observed during tensile testing and the 45° fracture orientation of the tensile specimen (Figure 9) are characteristic of ductile failure. Additionally, the bending performance of the welded joint was evaluated, with the butt joint enduring bending up to a 90° angle without visible cracks. The r/t bending ratio was measured at 2, indicating excellent ductility and confirming the joint’s ability to withstand plastic deformation without failure.

Figure 8
Tensile test fractured specimen showing a 45° fracture orientation.

Figure 9
Stress-strain curve comparing the base metal and butt joint tensile specimens.

3.3.2. Fractography study

The fractured surfaces from the tensile test specimens were analyzed using Scanning Electron Microscopy (SEM) to identify failure mechanisms. At lower magnifications (500x), dimples of varying sizes were observed on the fracture surface, indicating a ductile failure mode through micro void nucleation, growth, and coalescence. Honeycomb-like structures and fine voids further supported the occurrence of ductile fracture, as shown in Figure 10. At higher magnifications (2000x), secondary cracks were observed running parallel to the loading direction. These cracks propagated due to strain concentration at the micro voids, which grew under increased stress until fracture occurred. This behavior highlights the weld metal’s toughness and its ability to distribute strain effectively, attributed to the fine dimples and ferrite content in the FZ. The fractography analysis confirmed that the ductile fracture occurred away from the weld zone (WZ), primarily within the BM. This finding aligns with the tensile results, which showed that the weld joint’s strength exceeded that of the BM. The refined microstructure and higher hardness of the FZ played a critical role in enhancing the joint’s mechanical properties and load-bearing capacity.

Figure 10
SEM fractographs of tensile test specimen. (a) At 500x magnification, showing fine dimples; (b) at 2000x magnification, revealing secondary cracks and void coalescence.

3.3.3. Bending test

The bending test results confirmed the high plasticity of the welds, with no observable cracks or open discontinuities. Visual examination of the tested specimens revealed that the joint withstood bending up to a 90˚ angle without any cracks or tears, demonstrating excellent weld integrity. From the bending test, the ultimate strength of the base metal (BM) was measured at 526 MPa, whereas the welded samples exhibited an ultimate strength of 913.56 MPa. This represents a remarkable 72% increase in ultimate strength for the welded samples compared to the BM, highlighting the superior mechanical performance of the welded joint. Furthermore, no cracks were observed on the specimens after testing, indicating that the welding process produced joints with excellent ductility and resistance to fracture under bending stress. The bending test curves for the BM and the welded joint are shown in Figure 11 respectively. The graphical representation emphasizes the increased load-bearing capacity of the welded joint compared to the BM, further validating the strength and reliability of the weld. The results underscore the effectiveness of the A-TIG welding process in producing high-strength, defect-free joints suitable for structural applications.

Figure 11
Bending test curves. (a) Base metal; (b) Weld joint metal.

3.3.4. Microhardness

The hardness profile, plotted in Figure 12, demonstrated fluctuations in hardness values from the center of the Fusion Zone (FZ) to the BM. This fluctuation is attributed to the uneven distribution of δ-ferrite and γ-austenite phases within the welded microstructure. The maximum hardness in the Weld Zone (WZ) was measured at 318 HV, with an average value of 286 HV. Comparatively, the BM exhibited a hardness of 255 HV. The A-TIG welding process increased the hardness of the 2205 DSS material by 12.51%, primarily due to phase transformation and microstructural changes induced by welding. The increase in hardness in the FZ is attributed to the dissolution of the γ-austenite phase into the δ-ferrite matrix during cooling. This phase transformation results from the high heat input during welding, followed by atmospheric cooling, which promotes grain refinement and the redistribution of alloying elements, thereby enhancing the hardness.

Figure 12
Hardness profile.

3.3.5. Formability test

The Erichsen cupping test results demonstrated the enhanced formability of the A-TIG welded joint compared to the base metal (BM) as shown in Figure 13. The Erichsen Index (IE) for the welded joint was 7.7, while it was 7.0 for the BM, reflecting the superior ability of the welded region to undergo plastic deformation without failure. This improvement is attributed to strain hardening in the weld zone (WZ) induced during the welding process, which enhances the material’s resistance to deformation and fracture. Figure 14 illustrate the load versus displacement curves for the BM and welded joint, respectively. The welded joint exhibited a higher load-bearing capacity and sustained greater deformation before fracture, indicating improved mechanical performance under forming conditions. The central region of the weld joint experienced minimal strain while withstanding higher stresses, confirming its ability to resist deformation-induced failure. During testing, cracks were observed to initiate in the WZ and propagate toward the BM due to molecular shear and stress concentration. Despite this, the redistribution of stresses and the presence of strain hardening in the WZ enabled the joint to resist fracture effectively. The dimensional changes in the specimens, analyzed using marked grids, further validated the weld joint’s superior capacity to endure tension-tension failure with enhanced resistance to deformation.

Figure 13
Cupping test specimen. (a) Front view; (b) Rear side view.

Figure 14
Bar chart comparing the formability of the base metal (BM) and the A-TIG welded joint based on the Erichsen Index (IE).

4. CONCLUSIONS

The butt joint of 2 mm thick 2205 grade Duplex Stainless Steel (DSS) was successfully fabricated using the A-GTA welding process. Comprehensive analyses, including macrostructural, mechanical, fractographic, and microstructural characterizations, were conducted to evaluate the joint’s performance. The following conclusions can be drawn:

1. The tensile strength of the joint increased by 34% compared to the base metal (BM), attributed to the increased ferrite content in the Fusion Zone (FZ). Fracture during tensile testing occurred in the BM zone at a 45˚ orientation under loading conditions, confirming ductile failure behavior and the superior strength of the weld.

2. The joint successfully endured bending up to a 90˚ angle with an r/t bending ratio of 2, demonstrating excellent ductility. The ultimate strength of the joint increased by nearly 70%, ensuring reliable performance under mechanical stresses. The bending results align with the high plasticity and crack resistance of the joint.

3. The hardness increased from the BM to the FZ due to variations in the γ-austenite and δ-ferrite phase balance. The maximum hardness in the FZ was 318 HV, while the average in the Weld Zone (WZ) was 286 HV, compared to 255 HV in the BM. This 12.5% increase is associated with the formation of Widmanstätten austenite, secondary austenite, and chromium-rich phases, as confirmed by EDS analysis.

4. The microstructural study showed a transition from coarser grains in the FZ to finer grains in the BM. The dissolution of the austenite phase in the FZ resulted in the formation of grain boundary austenite (GBA), Widmanstätten-type austenite (WiA), partially transformed austenite (PTA), secondary austenite (SA), and intragranular austenite (IGA). The percentage of these phases varied across the joint, impacting mechanical properties. The presence of localized chromium depletion in the FZ indicates potential implications for corrosion resistance.

5. Fractographic examination confirmed ductile fracture behavior, characterized by dimples, microvoids, and secondary cracks. The microvoid coalescence mechanism, combined with secondary cracks, further supports the toughness of the joint. The failure occurred in the BM rather than the FZ, indicating the weld metal’s superior strength.

6. The Erichsen Index improved by 10% compared to the BM, demonstrating enhanced formability. The stress-strain curve exhibited a second jump, which can be attributed to strain hardening effects. The improved formability and strain-hardening response indicate the weld joint’s ability to endure higher stress before failure.

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Publication Dates

• Publication in this collection
  09 May 2025
• Date of issue
  2025

History

• Received
  10 Dec 2024
• Accepted
  02 Apr 2025