Experimental Analysis of Tensile and Metallurgical Properties in Similar and Dissimilar Metal Joints ^†

Abstract

This paper delves incto the tungsten inert gas (TIG) welding process, renowned for its efficacy in creating robust metal joints and widely employed in diverse industries for fusing similar or dissimilar materials. The focus of this study is the welding of mild steel with stainless steel, showcasing the method’s ability to amalgamate exceptionally sturdy metals and alloys. The resultant welded joints exhibit a meticulously refined microstructure and an impressive strength-to-weight ratio. The primary aim is to scrutinize TIG-welded joints, specifically those connecting mild steel with stainless steel, to elucidate their metallurgical and mechanical attributes. Notably, joints formed between distinct materials, such as mild steel and stainless steel, manifest commendable mechanical and metallurgical properties. This paper extensively investigates the metallurgical microstructures and tensile characteristics of both comparable and dissimilar metal junctions, contributing valuable insights to the field.

1. Introduction

Tungsten inert gas (TIG) welding is one of the most reliable techniques for metal welding. It is known and recommended for the production of quality, rugged joints. The TIG welding method finds application in aerospace and automobile industries among others, whereby various metals with varying compositions require fusion. TIG welding is very useful in cases when precision is crucial because the amount of heat input can be fine-tuned, hence producing a minimum distortion and a very good finish and weld strength. It is very important in welding dissimilar materials such as mild steel and stainless steel, where their mechanical properties and thermal behavior are quite different, which poses problems in fusion. This includes welding dissimilar metals such as the fusion of mild steel to stainless steel, which necessitates knowledge of the metallurgical transformation during the welding process as well as the end result of that process. The unique properties of mild steel, including high ductility and low cost, combined with stainless steel’s excellent corrosion resistance and high strength, make them a good combination for many industrial applications, but the welding process must be optimized to avoid potential issues such as cracking, poor fusion, or undesirable microstructures. This paper will discuss the process of TIG welding as it relates to the welding of mild steel with stainless steel to show how this method of welding can effectively combine different compositions and properties of material to achieve a better understanding of the process and optimize dissimilar metal joints [1]. Several contributions have been made over the years through various studies on TIG welding, especially with respect to dissimilar metal joints. It is in this regard that Pramod Kumar et al. (2021) [1] reported the optimization of the weld parameters of aluminum alloys in a critical study relevant for optimizing techniques for improving welding processes. It is particularly applicable for dissimilar materials, such as mild steel and stainless steel. Meanwhile, Mahmud Khan et al. (2021) [2] conducted research into the effects of post-weld heat treatment, filler metals, and welding procedures on dissimilar metal joints, specifically mild steel and stainless steel, highlighting the critical role of process control in achieving desirable mechanical properties, such as strength, toughness, and corrosion resistance. This study underscores the fact that TIG welding cannot be applied blindly, and careful thought needs to go into the process parameters if excellent results are to be ensured. K.R. Kavitha et al. (2021) [3] further explained the mechanical behavior of dissimilar metal weldments in detail while discussing the same point in the context of how the various parameters can actually influence tensile strength, hardness, and ductility in weldments. Shaik Mahaboob Subhani et al. (2019) [4] also studied the mechanical properties of TIG-welded joints between mild steel and SS 310 materials, further enriching the body of knowledge on dissimilar metal welding. This work highlighted the need to study the behavior of different materials during welding to avoid problems such as residual stress and cracking. Another study by Prashant Kumar Singh et al. (2015) [5] collated existing information on the TIG welding of dissimilar metals and identified areas requiring more research, particularly in terms of the optimization of welding parameters for the enhancement of joint performance. Such studies point out the difficulties in welding dissimilar metals and emphasize the carefulness required in the optimization of the process parameters so that the integrity of the welded joints is ensured [2,3,4,5].

Along with the optimization of welding parameters, many researchers focused on innovative techniques and inventions that improved the quality of TIG-welded joints, especially when welding dissimilar metals. In this regard, Akash Gupta et al. (2014) [6] presented a review of friction stir welding as an alternate technique for welding dissimilar materials. The researchers believed that friction stir welding could be employed together with TIG welding in order to improve the quality of joints. Sanjay G. Nayee et al. (2014) [7] explored the development of oxide-based fluxes to enhance the mechanical and metallurgical characteristics of dissimilar materials when welded by TIG, in particular. This study indicated that activating fluxes could improve weld quality through greater fusion between the base materials, with reduced defects regarding form porosity. In addition, Parthiv T. Trivedi et al. (2013) [8] reviewed methods for optimizing TIG welding parameters for aluminum, which gives useful insights into how similar principles could be applied to other materials, including mild steel and stainless steel. Vikesh et al. (2013) [9] assessed the effects of the welding parameters of A-TIG on penetration in mild steel; hence, there were many practical views about the extent to which process modification affects weld penetration and consequently, the joint strength. Taguchi and Grey Taguchi analysis to optimize the parameters in TIG welding. Therefore, there was a more systemic method of optimizing the process of improving welding efficiency and uniformity [10,11,12]. B.Y. Kang et al. (2009) [13] also studied the function of shielding gases in enhancing the integrity of dissimilar metal welds by analyzing different compositions of shielding gas to improve the quality of austenitic stainless-steel GTA welds. The findings from this research show how the right choice of shielding gas can reduce oxidation and other problems, leading to higher-quality joints. It further discusses how microstructure studies on dissimilar metal welds help researchers in understanding the mechanism, which affects the strength and toughness of the joint. For example, in their 2007 work, M. Ahmad et al. [14] studied the microstructure and phase characterization of TIG-welded joints of Zircaloy-4 with the stainless steel 304L and found some crucial metallurgical changes during the process. These could affect the weld joints’ performance. Peng Liu et al. (2006) [15] investigated the microstructural characteristics of TIG welds between magnesium and aluminum dissimilar materials, further deepening the knowledge of how different materials respond during welding. Such research contributes to the general improvement in the efficiency and quality of dissimilar metal TIG welds and emphasizes the current improvements in welding technology and methodology.

In addition, several works have undertaken profound investigations into advanced welding methods and fluxes, helping to enhance dissimilar metal welds. Cheng-Hsien Kuo et al. (2011) [16] investigated the use of activated TIG flux to enhancing dissimilar welds, mainly focusing on mild steel/stainless steel. Their studies illustrated how the incorporation of fluxes could improve weld integrity, minimize oxidation, and promote the overall mechanical characteristics of the weld. Along these lines, S.P. Gadewar et al. (2010) [17] carried out experimental work on the characteristics of single-pass TIG welding using SS304, increasing the depth of knowledge regarding process parameters and their influence on weld quality. The exploration of alternative welding techniques, such as laser–TIG hybrid welding, has also been a topic of interest and examining the microstructure of hybrid welds between dissimilar magnesium and aluminum alloys, highlighting the potential for improving the strength and quality of welded joints using this approach [18,19,20]. Then, the mechanical properties of the dissimilar metal joints are influenced by the flux composition, as shown in the research of Anand Baghel et al. (2021) [21] and Ramakrishnan A et al. (2021) [22]. Studies, such as those by Sairam Kotari et al. (2020) [23] and Anup Kulkarni et al. (2020) [24], on the use of dissimilar metals in TIG welding and brazing and the A-TIG welding of materials such as magnesium alloys and stainless steels show that the combination of different techniques can result in stronger and more durable joining. Lastly, the A-TIG welding of dissimilar P92 steel and 304H austenitic stainless steel was studied by Pratishtha Sharma et al. (2019) [25] in detail, exploring the mechanics, microstructure, and mechanical characteristics of the welds. These works refer to the continuous development of advanced TIG welding techniques for advanced quality and performance dissimilar metal weld, especially in critical applications.

2. Experimental Method

A metal plate-cutting machine was used to cut the workpiece materials in order to assure accuracy and reduce burrs. Plates made of mild steel and stainless steel were cut to the necessary measurements of 150 × 100 × 2.5 mm. Known for its flexibility, tungsten inert gas (TIG) welding was used to connect a range of thin and tiny materials. With a non-consumable tungsten electrode, filler metal can be used or not used when TIG welding is performed. In this investigation, different materials, such as mild steel and stainless steel, both with a thickness of 2.5 mm, were joined using TIG welding. The metal sheets underwent further processing and were machined into 150 mm × 100 mm rectangular welding samples. Using TIG welding equipment, these samples were longitudinally welded, with the mild steel sheet on the right and the stainless-steel alloy sheet on the left.

The filler metal that united the mild steel and stainless-steel sheets was made easier to melt by the argon gas released by the TIG welding flame during the welding process. The filler metal’s depth of penetration into the sheets was around 2 mm. The TIG welding parameters is shown in

Table 1. TIG welding parameters.

Exp No.	Material Used	Filler Metal	Voltage (v)	Current
1	SS-MS	SS 304	35	70
2	SS-SS	SS 304	35	70
3	MS-MS	ER70S2	35	70

. TIG welding uses the heat produced by an electric arc between the workpiece and the non-consumable tungsten electrode to fuse the metal in the joint region and form a pool of molten weld. The tensile strength of the welded material was then ascertained by performing a tensile test on a universal tensile testing apparatus. To start pulling the material, a load was supplied once the ends of the material were firmly secured between the grips. The material’s extension was measured using an extensometer, also known as a strain gauge, with the gauge length and applied force properly documented and recorded. A high-power microscope intended for examining opaque objects was also used for metallurgical microscopy. Unlike traditional biological microscopes, this kind of microscope works on the basis of reflected-light microscopy. This makes it possible to evaluate the welded joints’ microstructural properties and conduct a thorough analysis of their metallurgical structure.

3. Result and Discussion

3.1. Microstrucre Analysis

Understanding the metallurgical traits and mechanical qualities of the welded materials depends heavily on the microstructure study of TIG-welded joints. The welding of stainless steel 304 (SS) with mild steel A105 (MS) is the main topic of this study (Figure 1). To clarify phase changes, pore creation, layer development, and fractures in both the fusion zone and the base zone, SS-SS, MS-MS, and SS-MS combinations are examined. TIG welding, which is well known for its accuracy and adaptability, creates a pool of weld where the materials fuse together. Because of the high temperatures and quick cooling rates encountered during the welding process, the microstructure in the fusion zone experiences considerable changes. The basic form of stainless steel 304, which is distinguished by its austenitic structure, shows a mostly austenitic microstructure. The heat-affected zone (HAZ) forms coarse grains as a result of the thermal cycling that occurs in the austenitic structure during welding. These grains show that the heat input from the welding arc caused recrystallization to occur. On the other hand, the basic type of mild steel A105, a carbon steel, usually has a ferritic–pearlitic microstructure. Localized heating caused by the welding process results in the creation of a fusion zone that is distinguished by a combination of pearlite and ferrite. The heat-affected zone next to the fusion zone undergoes partial change, as the high temperatures cause the pearlite structure to coarsen.

Austenitic grains predominate in the fusion zone of SS-SS weld joints (Figure 2), maintaining the intrinsic microstructure of stainless steel 304. However, contingent upon the degree of thermal cycling and the velocity of cooling throughout the welding process, the heat-affected zone around the fusion zone may display a mix of austenite, ferrite, and carbides. A slow shift in microstructural phases occurs at the boundary between the fusion zone and the base material, creating a diffusion zone where intermetallic compounds can precipitate. Because of the nature of mild steel A105, the fusion zone microstructure in MS-MS weld joints is mostly ferritic (Figure 3). Within the fusion zone, the welding process encourages grain development and coarsening, which results in the production of a coarse ferritic structure with scattered pearlite colonies. Similar changes occur in the heat-affected zone next to the fusion zone, where the thermal input from the welding arc modifies the ferritic–pearlitic microstructure. The microstructure of SS-MS weld joints shows (Figure 1) a clear austenite-to-ferrite transition over the fusion zone. While the fusion zone next to the mild steel side acquires a ferritic structure, the fusion zone next to the stainless-steel side maintains its austenitic properties. Metallurgical bonding occurs at the interface between the two materials, where intermetallic phases are formed by atoms diffusing into one another.

Common defects in welded joints include pore formation and cracking, which greatly affect the mechanical properties and structural integrity of the weld. Pore formation usually occurs during the solidification process when gas is trapped in the weld pool, forming small voids or pores in the weld metal. The pores decrease the overall strength of the weld, which reduces its ductility and load-bearing capacity. Cracks are usually due to solidification shrinkage or residual stresses that occur during the welding process. These cracks either propagate along the fusion line or across grain boundaries and add to the weakness of the welded joint, increasing its susceptibility to failure under mechanical stress. Both of these defects—pores and cracks—detract from the general performance of the weld; thus, careful control over the welding parameters is a must to minimize their occurrence for reliable welds. Another crucial factor that affects the quality of welded joints is the generation of distinct layers with differing microstructures within the weld metal. These layers can occur due to differences in alloy compositions, cooling rates, or welding parameters, such as filler metal type, arc voltage, and welding speed. Each layer has unique microstructural characteristics that affect the mechanical properties of the welded joint. For example, differences in the thickness of these layers, as well as their specific microstructural morphology, can affect the strength, toughness, and resistance to the failure of the joint. Therefore, the welding process parameters should be understood and controlled to produce the optimal layer development. In other words, this will ensure that the weld produced exhibits the desired mechanical properties and meets the performance requirements of the application. The study of the formation process of layers in TIG welding joints is an important field of research because it ensures better weld quality, providing deep insights into the influence welded conditions have on the microstructure of the weld and hence on mechanical performance. Understanding the linkage between the mechanical properties and evolution of the weld’s microstructure is the key point in further improvement in the strength and reliability of welded structures. In both similar and dissimilar metal joints, the microstructure of the weldment is crucial for a comprehensive understanding of the phase transformations and metallurgical changes during welding, as well as in defect formation. This can help engineers to optimize welding parameters to achieve superior welds with minimum defects. Knowledgeable inputs enhance welded quality, the performance of welding components, and the weld structure’s reliability and lifecycle stability in different industrial applications. At each level, knowing more of the associated mechanical properties as well as the underlying microstructural developments in welded joints is one of the major keys through which welding processes may be maximally optimized and ensured to obtain structural integrity for welded products.

3.2. Tensile Analysis

Tensile strength values play a significant role in identifying the mechanical properties of welds made from combinations of various metals (Figure 4). From the results of the experiment, it is clear that the materials used have a huge effect on the tensile behavior of the welds. Among the combinations produced, SS-SS welds presented the highest tensile strength, followed by SS-MS welds and then MS-MS welds (

Table 2. Tensile strength readings.

Exp No.	Material Used	Filler Metal	Voltage (v)	Current
1	Tensile Strength	466.132 N/mm2	571.990 N/mm2	457.102 N/mm2

). This indicates that material selection is essentially crucial in determining welded joint strength. Microstructural examination of SS-SS welded joints shows that the weld zone mainly comprises austenitic grains. At the side of the weld zone is the HAZ, where mixtures of austenites, ferrites, and carbides are found, with a transition through microstructural phases via the interface. Key factors involved in improving the tensile strength of the SS-SS joint include the presence of austenitic grains and the gradual changes in phase within the HAZ. Thus, microstructure, or rather, the content and presence of the austenitic phase, plays an essential role in the enhancement of the mechanical properties of weldments.

On the contrary, a ferrite-based microstructure, where the heat-affected zone (HAZ) of the weld has a structure containing scattered pearlite colonies in a ferritic–pearlitic structure, forms the base of MS-MS weld joints. The tensile strength of MS-MS weld joints is mostly tied to the micro-constituent, wherein the ferritic phase contents strongly affect overall mechanical behavior. This indicates the presence of ferrite in weld joints, which contributes to the strength and ductility of the weld. SS-MS weld joints have an austenite-to-ferrite transition, which is clearly visible across the fusion zone. The mild steel side of the joint turns ferritic, while the stainless-steel side remains austenitic. The tensile behavior was found to be significantly influenced by the metallurgical bonding and intermetallic phases between two dissimilar materials at the interface, which are the main factors in determining the mechanical strength and integrity of the SS-MS joint under loads. In summary, tensile strength analysis highlights the need for optimized welding processes, both with respect to material composition and microstructural features. Controlling these factors is shown to improve the reliability and quality of welded structures, hence enhancing performance in various industrial applications.

4. Conclusions

• This study shows the direct correlation between the tensile strength of welded joints and material composition. SS-SS has the highest tensile strength, followed by SS-MS and MS-MS. This shows the substantial influence material combinations have on the mechanical performance of welded joints.
• Microstructural analysis is an important tool for understanding the mechanical behavior of welded joints. In MS-MS joints, the ferritic microstructure is predominant; it therefore, plays a significant role in determining tensile properties. In the case of SS-SS joints, the presence of austenitic grains contributes to a higher tensile strength value, indicating that microstructure also determines the overall mechanical performance.
• The distinct transition from austenite to ferrite, observed in SS-MS joints, indicates that mild steel and stainless steel are metallurgically bonded. The development of intermetallic phases at the interface between these materials further influences the tensile behavior of SS-MS joints, affecting their overall strength and integrity.
• The mechanical qualities and structural integrity of welded joints are greatly impacted by the presence of pores, fractures, and layer development. It is crucial to comprehend these mechanisms of defect generation in order to optimize welding processes and improve weld quality.
• The results emphasize how crucial it is to optimize welding processes in light of material composition and microstructural development. An improved knowledge of mechanical and metallurgical properties is essential to guaranteeing the dependability and caliber of welded structures in a range of industrial applications.