Synergistic Effects of Thermal Cycles and Residual Stress on Microstructural Evolution and Mechanical Properties in Monel 400 and AISI 316L Weld Joints

Abstract

The current study investigates the thermal, metallurgical, and mechanical results in similar and dissimilar weldments of Monel 400 and AISI 316L. Infrared thermography (IRT) was employed to record thermal cycles, while X-ray diffraction (XRD) was used to analyze the residual stresses post-welding. Mechanical properties were assessed through tensile and microhardness tests, and microstructural evolution was examined using energy-dispersive spectroscopy (EDS) and scanning electron microscopy (SEM). IRT results showed peak temperatures of 1788 °C for Monel 400 and 1750 °C for AISI 316L. Residual stress analysis revealed compressive stresses of 293 MPa in dissimilar welds, compared to 235 MPa in Monel 400 and tensile stresses of 57 MPa in AISI 316L. Ultimate tensile strength (UTS) values were 543 MPa for dissimilar welds, 533 MPa for Monel 400, and 556 MPa for AISI 316L, with corresponding microhardness values of 207 HV, 203 HV, and 168 HV, respectively. Microstructural analysis identified coarse Ni-Cu phases in the Monel 400 heat-affected zone (HAZ), austenitic structures in AISI 316L, and intermetallic compounds in dissimilar welds. The findings highlight the impact of thermal distribution, residual stress, and microstructural evolution on weld performance, providing insights into optimized welding parameters for improved joint integrity and mechanical properties.

1. Introduction

Welding is a common manufacturing process wherein two similar or dissimilar metals join to maintain homogeneity in the structure. Due to its process capability, this technique offers a wide range of applications in different sectors, including pressure vessels, boilers, ship bodies, chemical processing equipment, aerospace structures, and nuclear reactors [1,2]. One of the major classifications in welding is fusion welding, which is a process where high temperatures melt the base metals and allow them to solidify upon cooling. Gas tungsten arc welding (GTAW) is unique and popular for its accuracy and applicability in industrial applications, such as producing offshore structures, nuclear reactors, and petrochemical equipment [3,4,5,6]. However, temperature fields developed during welding are critical for generating residual stresses, which have been shown to affect weld integrity. Differential heating and cooling cycles cause significant thermal stress and microstructural changes. These changes may influence the strength, hardness, and ductility across the weld zone [7,8,9]. Dissimilar welding is more affected due to the different thermophysical properties of materials. Various types of welding techniques are used for joining dissimilar metals and alloys. Minze et al. discussed the dissimilar welding of AlCoCrFeNi2.1/316L, welded by the GTA welding process, commonly used in nuclear power plants [10]. Jiageng et al. discussed the corrosion behavior of dissimilar weld joints in lap-welded Al/Mg using the hybrid laser and arc welding process for vehicle structures [11]. Guo et al. discussed the dissimilar welding of joints between the 6005A alloy modified with Sc (designated as 6005A+Sc) and the 5083 alloy, welded using the MIG welding process [12]. Yuewei et al. discussed the keyhole and melt pool process in the laser welding of Q235 low-carbon steel and 316L austenitic stainless steel used for ships and bridges [13]. Explosive welding is popular for joining dissimilar aluminum and copper alloys subjected to shear loading for electrical applications [14,15].

Monel 400, a Ni-Cu-based alloy, is applied in marine or chemical environments primarily because of its high thermal conductivity and corrosion resistance characteristics [16]. AISI 316L austenitic stainless steel is popular for its resistance to oxidation and high mechanical strength [17,18]. Choosing a suitable filler wire, typically rich in nickel or stainless steel, is crucial for achieving compatible metallurgical bonding and minimizing defects in the fusion zone [19,20]. The microstructure of the weld critically affects the tensile and impact properties of naval HSLA steel weldments. The occurrence of acicular ferrite (AF) and bainite ferrite (BF) laths in the fusion zone enhances tensile strength (joint efficiency: 122%) and impact toughness [21]. Filler wire selection plays a crucial role in the microstructure and mechanical integrity of dissimilar AISI 321–AISI 347 stainless steel joints. Ferritic–austenitic ER2553 filler causes FZ fracture, while Ni-based fillers (ERNiCrMo-4, ERNiCrMo-10) result in a fracture location shift toward the AISI 321 base metal. Ni-based fillers improve joint efficiency and enhance impact resistance [22]. Researchers have attempted to improve weldments with available welding techniques, especially in temperature field determination and the residual stress effect on mechanical properties.

During any welding process, heat flow determines the characteristics of the welded joint. Based on these thermal cycles, different microstructures are developed in the HAZ and fusion zone, which largely determine the mechanical properties of weldments. Furthermore, such thermal cycles result in residual stresses, thermal distortions, and physical changes. Many authors have discussed and reported residual distortion and developments in the various fusion welding processes [23,24,25]. Yelamasetti et al. discussed the tungsten inert gas (TIG/GTAW), pulse TIG, and inter-pulse welding of dissimilar weld joints between 316L and Monel 400 steel [16]. Stress analysis of a 5 mm thick 316L steel plate, welded using the laser welding process and analyzed with ANSYS 19, was discussed [26]. Aarbogha et al. discussed the predicted displacement in the weld plate; they concluded that the results were not very accurate with the experiments and suggested the need for an accurate numerical model. The authors discussed the inaccuracy in the thermocouple measurements, which required further improvement [27]. Attarha et al. analyzed both dissimilar and similar joint welds through a numerical and experimental comparison. The research reported extensive temperature distribution in the HAZ of thin butt-welded joints with K-type thermocouples, presenting evident differences in peak temperature and cooling slopes between similar and dissimilar materials [28]. Zubairuddin et al. discussed the generation of distortion or displacement and residual stress in weld plates joined by laser and GTAW processes. The effects of phase transformation on stress and distortion were also discussed [29].

Due to the non-uniform thermal gradient, the thermal strains induced in the welded structures result in the formation of permanent stress within the weldments after the external load is removed. The performance of welded structures during service can be severely affected by residual stress [29,30]. Joining similar and dissimilar materials produces different lattice strains, further enhancing the strain. Dissimilar welding is more complicated and needs more research attention to study and fully understand. Various researchers have discussed the mechanical and metallurgical changes in the dissimilar welding of steels, which result from phase and structural transformations [31,32]. Balram et al. discussed the dissimilar welding of Inconel 718 and steel 430, developed using constant and pulse arc currents using the TIG welding process [33]. The research found that using a 4 Hz pulsed current for TIG welding of Inconel 718 and SS316L improves their mechanical properties, with a UTS of 524 MPa and a YS greater than other frequencies tested, particularly at 350 °C. Devedranath et al. discussed the dissimilar welding of marine grade alloys Monel 400 and AISI 904L, employing pulsed TIG welding and using two different batches of Monel 400 with ERNiCrMo-4 as the filler material. The authors also presented the effects of different filler wires, E309L and ENiCu-7, for dissimilar joining of AISI 304 and Monel 400 steel [34,35]. The research emphasized that using a pulsed current GTAW with ERNiCu-7 filler in joints between Monel 400 and AISI 904L provides a higher tensile strength of 607.5 MPa in comparison to 562.5 MPa with ERNiCrMo-4 due to lower partially melted zones. With the hardness values of the fusion zone being 209 HV (ERNiCu-7) and 225 HV (ERNiCrMo-4), the weldments exhibit greater mechanical integrity, making them applicable in marine and corrosion-resistant industrial applications. Lalit et al. also reported that the printed Monel 400 structure had an ultimate tensile strength of 718 MPa, a yield strength of 353 MPa, and 39% elongation; microhardness reduced from 286 HV₀.₁ at the bottom to 249 HV₀.₁ at the top. Furthermore, a very low corrosion rate of 0.03 mm/year was reported, affirming its potential use in marine and corrosion-resistant structural applications [36].

Shanthos et al. achieved a high tensile strength of 719 MPa for pulsed Nd:YAG laser-welded Hastelloy C-276 and Monel 400 sheets using optimized parameters, namely, a welding speed of 380 mm/min, pulse energy of 9.5 J, and pulse duration of 6 ms. The welds exhibited full penetration, finer dendritic grains, and ductile fracture, making them appropriate for high-strength, corrosion-resistant applications [37].

Shen et al. concluded that using Monel 400 filler in GMAW of the CoCrFeMnNi high-entropy alloy improved the mechanical performance via solid-solution strengthening, overcoming the generally low hardness of the fusion zone. New phases and carbides were formed in the fusion zone, and tensile testing ensured better micro- and macro-mechanical responses; these welds have great potential to be used in advanced structures and high-performance engineering applications [38].

Several researchers have studied the welding of high-strength alloys like Monel 400, Inconel, and other grades of stainless steel, with emphasis placed on their mechanical properties, microstructural development, temperature field, and the buildup of residual stresses during and subsequent to the welding process. Most joint strength and microstructural integrity-based welding techniques, such as the constant and pulsed current modes of GTAW, have been considered to optimize the weld joints. The studies have highlighted the roles of precipitates, grain refinement, and other phase transformations regarding mechanical performance. This is essential for measuring the temperature of the weld because transient heat transfer conditions create non-uniform contraction and expansion, leading to residual stresses in the fusion zone. This study aims to explore real-time temperature monitoring with IRT and residual stress measurement through XRD, along with the study of microstructures using EDS and SEM. Similar and dissimilar weldments of Monel 400 and AISI 316L are joined by using ERNiCrMo-3 filler wire through the GTAW welding technique. The effects of thermal cycles on residual stress formation, structural integrity, and mechanical properties of similar and dissimilar weldments are presented in this paper.

2. Materials and Methods

2.1. Welding Processes

Experiments were carried out on V-groove joints with two different alloy plates (i.e., Monel 400 and AISI 316L) with two different weld configurations (i.e., similar and dissimilar joints). The plates were cut into dimensions of 120 mm × 80 mm × 5 mm using a wire-electric discharge machining process; welding was performed along the 80 mm width, as shown in Figure 1. Monel 400, a Ni-Cu alloy, exhibited excellent corrosion resistance in marine and chemical environments. On the other hand, the excellent mechanical properties and resistance to pitting and crevice corrosion in AISI 316L are attributed to the presence of Mo as the primary alloying element. The ERNiCrMo-3 filler wire was used to join the 5 mm thick plates in multiple welds (3 passes). The chemical compositions of the base and filler materials are presented in

Table 1. Chemical composition of filler metals and base plates adopted from [16,17].

Base/Filler Metals	Fe	Cr	Ni	Mo	Mn	C	Cu	Si
ERNiCrMo-3	5.0	21.5	Bal	9.0	0.5	0.1	0.5	0.5
AISI 316L	Bal	17.6	10.6	2.5	2.0	0.03	Nil	0.9
Monel 400	2.5	Nil	Bal	Nil	2.0	0.3	31.65	0.5

. The physical properties of the base materials are given in

Table 2. Physical properties of Monel 400 and AISI 316L.

Base Metals	Coefficient of Linear Thermal Expansion (α), m/m/°C	Density (ρ), kg/m3	Thermal Conductivity (k), W/m°C	Poisson’s Ratio (ϑ)
AISI 316L	16.2	7980	16.4	0.28
Monel 400	13.6	8380	21.8	0.32

. The plates were prepared using a V-groove with a root gap of 1.5 mm for proper penetration and weld quality. The plates were properly clamped to prevent distortion caused by thermal stresses. Purging with argon gas at a flow rate of 10 lpm was used before and after welding to avoid oxidation and to maintain a clean welding environment. The experimental set-up with an IR camera (FLIR Thermacam T440, Stockholm, Sweden) (fixed 150 cm away from the workpiece) is shown in Figure 1a. The heat input rate (kJ/mm) is calculated using Equation (1). The welding efficiency (η) is 60% [39], and the welding process parameters are given in

Table 3. Welding parameters used in the GTAW process.

Weld Type Similar/ Dissimilar	Arc Voltage (V), V	Welding Current (I), A	Welding Speed (S), mm/min	Heat Input Rate (Q), kJ/mm
(a) AISI 316L-AISI 316L	14	130	108	0.708
(b) Monel400-AISI 316L	14	135	102	0.718
(c) Monel400-Monel400	14	135	102	0.718

. The welded samples obtained under these conditions are shown in Figure 1b.

$$HI={\displaystyle \frac{\left(V\times I\right)}{S}}\times \mathsf{ղ}$$

(1)

where V = voltage, V; I = current, A; and S = speed of welding (mm/min).

2.2. Infrared Thermography

IRT detects the infrared energy emitted from surfaces and converts it into visual representations of thermal distribution. The IRT camera uses high-pass band filters to record visual images of temperatures in place. The camera measures wide temperature ranges, from −20 °C to 2100 °C, with an accuracy of ±0.1 °C. It was mounted on a data logger to monitor and keep the recorded values in real-time, visually. The images were captured at 50 frames per second, ensuring high-quality thermal images with a resolution of 320 × 240 pixels. These thermal images, saved as digital files on a hard disk drive, provided comprehensive records for real-time monitoring of the thermal behavior when the weld was being developed. Thermal images were taken at three points, namely, the start, middle, and end of the weld line, to study overall thermal distribution, as shown in Figure 2. For accurate temperature measurements, the emissivity values for the base metals were added to the data logger before conducting experiments. Based on the literature data, emissivity values were set at ε = 0.72 for AISI 316L [40] and ε = 0.54 for Monel 400 [16]. The base metal was set to an initial temperature of 27 °C to minimize thermal variations.

2.3. Residual Stress Measurement

X-ray diffraction was used to measure and calculate the residual stresses at different distances from the weld centerline. Measurements were performed at −50, −15, −5, 0 (weld centerline), 5, 15, and 50 mm distances; negative values represent distances on the Monel 400 side. In XRD, X-ray beams interact with the crystal lattice, producing characteristic diffraction patterns that depend on the material’s stress state. Welding produces strains that change the inter-planar spacing of the {h k l} lattice planes due to contraction and expansion within the crystal lattice, reflecting changes in the reflection angle [41,42]. Bragg’s Equation (2) compares the stress-free and stressed states to determine the sum of principal stresses, as mentioned in Equation (3).

$$n\lambda =2d\times Sin\theta$$

(2)

$${\sigma }_{\varphi }={\displaystyle \frac{m}{d_0}}\times \left({\displaystyle \frac{E}{1+\upsilon }}\right)$$

(3)

where d is the interplanar lattice spacing and λ is the reflection order. To determine the stress, the inter-planar spacing d is used in the elastic stress–strain relation, m = the slope value derived from the lattice space D against ${sin}^2\phi$, E = elastic modulus, and v = Poisson’s ratio.

The back reflection technique near 2θ = 180 ° was used to measure the surface strains with vanadium-filtered Mn-Kα radiation with a wavelength of 1.07443 Å diffracted from the (3 1 1) planes at 2θ = 156.320. The $\phi$ angles ranged from −45 ° to +45 ° as shown in Figure 3. In this case, the residual stress value was determined to be −189 MPa at the weld zone. The precision of residual stress determination by XRD analysis can be observed.

2.4. Microstructural Characterization

The similar and dissimilar welded samples were prepared for metallurgical studies. The samples for this microstructural study were extracted from the welded joints transversely indicated in Figure 4. The composite area of the 50 mm × 5 mm × 5 mm welded samples was polished using silicon carbide grit sizes ranging from 220 to 2000 to visualize the microstructure evolution for different welding conditions. Furthermore, different etchants were used on different zones of the weldment. The reagent used for the HAZ and base metal of Monel 400 was Marble’s reagent, while on the fusion zone, a mixture of 10 mL HNO₃, 15 mL HCl, and 10 mL CH₃COOH was used on the base metal and HAZ. After etching, the welded samples were subjected to microstructure visualization and analysis using SEM and EDS.

2.5. Mechanical Testing

A schematic view of the dissimilar welded model and sample sections sliced for mechanical testing is shown in Figure 4. A universal testing machine (UTM) was used to perform tensile testing with a shear head speed of 2 mm/min as per ASTM E/8 standards [17,18]. Further, the Vickers microhardness tester was utilized to measure microhardness across the centerline of the weldment for both dissimilar and similar joints. The diamond indenter, with a load of 500 gf for about 10 s, and a 15 s dwell time, was displaced away from the center line at intervals of 0.2 mm. The results were analyzed to understand the interconnection between the process parameters, microstructure, and mechanical properties.

3. Results and Discussion

3.1. Infrared Thermal Cycles

IRT data from similar welds between Monel 400 plates were reported to understand the temperature distribution. The peak temperature values in the transverse and longitudinal directions were 1684 °C and 1788 °C, as shown in Figure 5. A steeper thermal gradient of the weld and a substantial temperature spread to the side of the base metal, Monel 400, are presented. The thermal behavior of welding in dissimilar plates between AISI 316L and Monel 400 shows significant variations. This is primarily due to differences in the thermal conductivity of the two materials. Research studies have shown that the thermal conductivity of Monel 400 (20 W/m·K) is more than AISI 316 (16 W/m·K); thereby, the heat distribution shows significant differences. As seen in Figure 6, the temperature profile oscillates along the longitudinal and transverse directions of the sample throughout each welding pass. During Pass-1, in a longitudinal direction, a temperature value of 1657 °C was observed, while in the transverse direction, the temperature ranged from 1641 °C to 1657 °C. The above thermal profiles illustrate more significant temperature gradients in the HAZ of Monel 400 compared to those found in AISI 316 because of the former’s excellent thermal conductivity, thus allowing it to dissipate heat more quickly and with a thinner HAZ. Continuous heat input throughout the welding process tends to increase the differences. Consequently, it produces non-uniform thermal profiles that will affect the dissimilar welds’ microstructural features and mechanical properties. The literature survey underlines that the thermal properties are crucial determinants of quality and performance in dissimilar welds, where higher thermal conductivity materials, like Monel 400, can contribute to enhanced heat transfer, as well as necessitate careful thermal management to prevent distortion and cracking risks [17,40,43].

For a similar joint of AISI 316L steel, the thermal distribution in the longitudinal direction at various time intervals is illustrated in Figure 7. The highest temperature was observed in the longitudinal direction at 1750 °C. The peak temperature values in longitudinal and transverse directions were 1750 °C and 1607 °C, respectively. High-temperature values were observed on either side of the weldment, increasing the HAZ width due to heat energy resistance at the base metals. This phenomenon highlights the influence of heat dissipation on the extent of HAZ growth, which is critical in determining the mechanical properties and microstructural stability of welded joints.

The thermal behavior of the similar (AISI 316L) and dissimilar (AISI 316L and Monel 4000) welds highlights the key differences in temperature distribution. In AISI 316L welds, a peak temperature of 1750 °C was observed, leading to an increased HAZ width. In dissimilar welds, temperatures of up to 1788 °C were recorded, with higher thermal gradients in the HAZ of Monel 400 due to its higher thermal conductivity. These findings highlight the significant impact of thermal properties on heat distribution and weld integrity, underscoring the need for effective thermal management to optimize performance and minimize distortion in the welded samples.

3.2. Residual Stresses

All the residual stress measurements of the welded samples under different conditions are tabulated in

Table 4. Residual stress (RS) measurements at different locations of the similar and dissimilar welded samples.

Weld Type	Distance (mm)	Location	Slope (m)	tan (dy/dx)	Young’s Modulus (MPa)	Poisson Ratio, (ϑ)	D-Spacing (A)	RS MPa
Dissimilar Monel AISI 316L	50	Monel Base	−0.0014	−0.0014	1.82 × 105	0.32	1.0744	−178
	15	Monel HAZ	−0.0008	−0.0008	1.82 × 105	0.32	1.0744	−109
	5	Monel Interface	0.0002	0.0002	1.82 × 105	0.32	1.0744	28
	0	Weld Center	−0.0023	−0.0023	1.82 × 105	0.3	1.0819	−293
	5	316L Interface	−0.0006	−0.0006	1.94 × 105	0.3	1.0819	−85
	15	316L HAZ	−0.0017	−0.0017	1.94 × 105	0.3	1.0819	−230
	50	316L Base	−0.0017	−0.0017	1.94 × 105	0.3	1.0819	−240
Similar Monel 400	15	Monel HAZ	0.0008	0.0008	1.82 × 105	0.32	1.0744	97
	5	Monel Interface	0.0003	0.0003	1.82 × 105	0.32	1.0744	41
	0	Monel weld	−0.0018	−0.0018	1.82 × 105	0.32	1.0744	−235
Similar AISI 316L	15	316L HAZ	−0.0021	−0.0021	1.94 × 105	0.3	1.0819	−285
	5	316L Interface	−0.0001	−0.0001	1.94 × 105	0.3	1.0819	−12
	0	316L weld	0.0004	0.0004	1.94 × 105	0.3	1.0819	57

. Residual stresses in similar welded samples, such as Monel 400 and AISI 316L, show different behaviors resulting from thermal and microstructural properties. For the Monel 400 similar welded samples, the residual stress is compressive. It reached a value of 235 MPa at the weld center (0 mm), which changed to lower compressive stresses at the base metals, as shown in Figure 8. In the Monel HAZ (15 mm), stresses were tensile, with a value of 97 MPa. The compressive residual stress at the weld center of the AISI 316L similar weld was about 285 MPa. In contrast, a tensile residual stress of 57 MPa was seen in the HAZ. This variation can be explained by differences in thermal conductivity and structural properties between Monel 400 and AISI 316L. Monel has a higher thermal conductivity and, thus, increases the cooling rates near the weld zone as compressive residual stresses, whereas, for AISI 316L, tensile residual stresses arise with low cooling rates. The residual stress distribution of the dissimilar welds between Monel and AISI 316L is based on the different thermal and microstructural properties of the two materials. Compressive residual stress of 293 MPa was recorded at the weld center, progressing toward tensile stresses at the interfaces and base metals. The tensile residual stress was measured at 28 MPa for the Monel interface at 5 mm, while compressive residual stress of 230 MPa was observed for the AISI 316L HAZ at 15 mm. This is because Monel has high thermal conductivity, which encourages rapid cooling and, thus, leads to the development of compressive residual stress. In contrast, AISI 316L has low thermal conductivity and maintains tensile stress because heat dissipation is slower (slow cooling).

In summary, the residual stresses in dissimilar and similar welds of AISI 316L and Monel 400 reveal the pronounced effects of thermal and microstructural properties. For similar Monel 400 welds, there is an evident trend of compressive stresses at the weld line and tensile stresses in the HAZ. Similarly, in the AISI 316L similar welded samples, a balance between tensile and compressive stresses has been observed, with increased compressive stresses of 285 MPa close to the weld center that decrease toward the base metals. In dissimilar welds, the thermal conductivity of Monel 400 is higher compared to the thermal conductivity of AISI 316L, resulting in a greater variation in residual stresses with significant compressive stresses near the interfaces of Monel 400 and tensile stresses at the AISI 316L HAZ. Proper thermal management is essential to reduce stress concentrations and ensure structural integrity in similar and dissimilar weldments.

3.3. Microstructural Evolution

Microstructural studies conducted on the weld joint showed tremendous variation throughout similar and dissimilar welded samples, as shown in Figure 9, Figure 10, Figure 11 and Figure 12, influenced by a thermal profile in material properties or the welding techniques applied. In a fusion zone, from the welded joints between AISI 316L substrates (as shown in Figure 9), elongated dendrite structures appeared distinctly with the prevailing austenite phase, mainly with an overall clear boundary due to efficient elements diffusion, like Ni, Cr, and Fe. Mo from the filler wire further stabilized and strengthened the austenitic phase. HAZ possessed grain structures refined through controlled thermal cycles, and balanced hardness and mechanical properties resulted. Enrichment of the fusion zone with Ni, Cr, Fe, and Mo, confirmed by EDS analysis, supported the stable formation of a face-centered cubic lattice, enhancing the strength of the weld joint.

In the microstructure of fusion zones in Monel 400 welds, as shown in Figure 10, coarse grains were dominated by intermetallic Ni-Cu phases. Ni, Cu, and Fe-based interdendritic networks could be observed, where grain boundary migration is prominently evidenced in the SEM images. This structural character is described due to its high thermal conductivity, which allows for very fast cooling, resulting in rapid solidification. Adding Nb and Mo from the filler wire also strengthened the Ni-Cu-rich phases, leading to higher hardness values in the fusion zone. Coarser grains were observed in the HAZ compared to the base metal because of the heat input during the welding process, more so in dual-pass welding, where the heat build-up also encouraged grain growth. Different microstructural features in the dissimilar joints were developed based on the different thermal properties of Monel 400 and AISI 316L, as shown in Figure 11. The microstructure of the fusion zone exhibited a complex dendritic structure consisting of elements such as Ni, Cr, Fe, Cu, and Mo that form intermetallic compounds, as shown in Figure 12. Long dendritic structures with enriched grain boundaries are primarily seen on the Monel 400 side due to the high thermal conductivity.

The HAZ of Monel 400 exhibited coarse grains of Ni-Cu phases, whereas the HAZ of AISI 316L revealed finer grains with austenitic structures. EDS analysis at the weld interface revealed a substantial compositional gradient, wherein peaks of Ni and Cu were significantly high on the Monel side, and peaks of Cr and Fe were high on the AISI 316L side by the thermal gradients established during welding, as shown in Figure 12. Similar findings were observed in other research works when the samples were joined with constant arc mode welding using TIG welding [35,37]. The microstructural characteristics of the weldments were significantly affected by the thermal properties of Monel 400 and AISI 316L, as dissimilar joints presented different microstructural characteristics. Since Monel 400 had a higher thermal conductivity of approximately 22 W/m·K, it allowed for rapid heat dissipation, resulting in a faster cooling rate that produced fine grains in the HAZ. On the other hand, AISI 316L—with lower thermal conductivity, approximately 16 W/m·K—exhibited a slower cooling rate and, consequently, coarse-grain structures. The dissimilar welds had uneven solidification patterns, leading to complex intermetallic phases and residual stress distributions [44].

3.4. Tensile Properties

The study of tensile tests provides a comprehensive understanding of the mechanical performance of welded samples. The engineering stress–strain curves, plotted for each weld sample, offer insights into the deformation behavior and failure modes. The fractured samples are shown in Figure 13. For the AISI 316L similar weld sample, the elongation was recorded at 53.58%, and the yield strength (YS) was 261 MPa with a UTS of 556 MPa, as shown in

Table 5. Tensile properties of the different welded samples.

Weld Type	Y.S (MPa)	UTS (MPa)	% Elongation	Fractured Location
AISI 316L similar	261	556	53.58	HAZ of AISI 316L
Monel 400-AISI 316L	253	543	26.48	Base metal Monel 400
Monel 400 similar	244	533	36.74	Base metal Monel 400

. This maximum percentage of elongation thus shows significant ductility, enabling such material to tolerate large deformation until failure. This fracture occurred extensively in the HAZ, where there was a microstructural change due to the welded thermal cycles. Due to these phenomena, the strength became lower compared to the base metal.

On the other hand, for dissimilar welding of Monel 400-AISI 316L welds, UTS and YS were observed at 543 MPa and 253 MPa, respectively, along with an elongation of 26.48%. These values show that there is reduced ductility compared to that of the AISI 316L welds. The fracture occurred on the Monel 400 side, illustrating the role of material properties in determining the failure area. The weld interface and HAZ at the Monel 400 side had higher stresses due to thermal expansion and conductivity differences. The Monel 400 similar welded samples exhibited a YS of 244 MPa, a UTS of 533 MPa, and an elongation of 36.74%. These welds showed relatively higher ductility than the Monel-AISI 316L welds but less than the AISI 316L similar welded samples. In this case, the fracture took place predominantly at HAZ with localized regions of higher tensile residual stresses. The tensile test results showed significant differences in the mechanical properties between similar and dissimilar weldments. The most significant increase in the tensile strength was found for the Monel 400 similar samples with UTS = 533 MPa, while for dissimilar joints (Monel 400-AISI 316L), strength decreased (UTS = 543 MPa). This is because the thermal expansion and contraction rates were different between Monel and AISI 316L, leading to increased residual stresses, especially at the Monel 400 side of the weldment. Faster cooling rates brought about by higher thermal conductivities in Monel 400 caused compressive residual stress to rise in HAZ, lowering weld strength [38].

After conducting the tensile tests, the fracture zones of the failed weld samples were visually examined; the fracture surfaces were analyzed at the macro level for both similar and dissimilar weld configurations. The failure locations for each sample are summarized in Table 5. Notably, the fracture surfaces exhibited typical cup-and-cone morphology, indicative of ductile failure, suggesting that significant plastic deformation occurred before fracture. This is further confirmed by the stress and strain curves (Figure 13).

In both the Monel 400 similar welds and the dissimilar Monel 400–AISI 316L joints, fractures consistently occurred within the Monel 400 base metal, reflecting its comparatively lower tensile strength. In contrast, for the AISI 316L similar welds, failure was observed in the HAZ of the AISI 316L. These results indicate that the Monel 400 side governs the failure behavior in both similar and dissimilar joints involving Monel, whereas for AISI 316L similar welds, the localized microstructural changes in the HAZ contribute to premature failure.

3.5. Microhardness

The average microhardness values for the various weld types are taken from the Vickers test. Microhardness values indicate the material’s deformation resistance and strength. The indentation values are plotted in Figure 14, giving a visual impression of the microhardness distribution across the different zones of the welds. For similar AISI 316L welds, the average microhardness (HV) values in these three regions are, respectively, the weld zone, 207; the HAZ, 192; and no significant difference in the base metal, as shown in Figure 14. This highlights the impact of thermal cycling on hardness in both zones, weld and HAZ. Similar Monel 400 welds yielded an average microhardness value of 155 HV in the HAZ and 203 HV in the weld zone. The base metal had a somewhat lower hardness, 155, showing a decrease in microhardness due to the thermal effects present at the fusion zone and the HAZ.

For dissimilar Monel 400-AISI 316L welds, the average microhardness values were 168 HV in the weld zone, 192 HV in the HAZ of AISI 316L, and 162 HV in the HAZ of Monel 400. The critical differences observed between the HAZs highlight how material properties and thermal effects influence microhardness distribution. A comparison of these values clearly shows that the highest hardness was obtained in similar AISI 316L welds, followed by similar Monel 400 welds, with dissimilar Monel 400-AISI 316L welds showing the lowest values. The difference in hardness values is attributed to the combined effects of thermal cycles and residual stresses, coupled with the type of tensile properties. Specifically, Monel 400 showed higher thermal conductivity, leading to lower microhardness in HAZ regions compared to AISI 316L [40].

4. Conclusions

The present paper carried out an experimental investigation of similar and dissimilar materials (i.e., AISI 316L and Monel 400) using the GTAW process. Further, the correlation between IRT, residual stress, microstructure, and mechanical properties was analyzed and presented. Based on the results and discussion, the following conclusions were observed.

• The maximum temperature in corresponding welds of Monel 400 was 1788 °C, and that for AISI 316L was 1750 °C. In the dissimilar joints between Monel 400 and AISI 316L, a non-uniform temperature field was noticed, with high thermal gradients showing variations in the thermal characteristics of the two materials.
• The weldments of Monel 400 and AISI 316L had compressive residual stresses of 293 MPa in the weld region as a result of contrasting thermal expansion and cooling rates. Similarly, welds of Monel 400 exhibited 235 MPa of compressive stress, while AISI 316L joints displayed 57 MPa of tensile stress based on their own thermal properties and shrinkage characteristics.
• Dissimilar weldments between Monel 400 and AISI 316L exhibited a UTS of 543 MPa, which is lower than that of a similar AISI 316L joint (556 MPa) but higher than that of a similar Monel 400 weld (533 MPa).
• Similar Monel 400 welded samples exhibited the highest average microhardness of 203 HV, with similar AISI 316L welds at 207 HV; the average microhardness found in dissimilar joints was 168 HV.

This study examined the roles of temperature fields, residual stresses, metallurgical studies, and mechanical properties in similar and dissimilar weldments. In similar welds, superior mechanical performance was exhibited through higher UTS, reduced residual stresses, and increased hardness, while contrasted thermal properties challenged dissimilar joints. The outcome of this work indicates that there is a need to balance strength and ductility in dissimilar weldments, requiring careful process control to optimize weld quality.