Influence of weld joint design on mechanical and corrosion properties of welded dissimilar metals

This research study makes fundamental observations about dissimilar metal-welded joints, which combine mild steel and aluminum alloy plates, both 6 mm thick. We joined this metal using the shielded metal arc welding (SMAW) process with E308L-16 as the filler metal. We conducted welding on prepared double-V and butt joints and assessed mechanical properties through tensile, impact, and hardness tests. We examined microstructural characteristics using an optical microscope. To investigate the corrosion behavior, we immersed the welded joint in a 3.5 wt% NaCl solution and analyzed it using the potentiodynamic polarization electrochemical method. The results showed that dissimilar weld joints in the double-V configuration outperformed butt joints in tensile and hardness properties. Moreover, the double-V joint displayed more excellent thermodynamic stability in the 3.5 wt% NaCl solution compared to the commonly used butt joint. Consequently, the double-V joint design is the most suitable choice for achieving optimal mechanical and corrosion characteristics in these dissimilar weld joints for engineering applications.


INTRODUCTION
Among various manufacturing technologies, joining has been recognized as a crucial enabling technology for innovative and sustainable manufacturing.techniques categorize themselves based on the welding process employed, encompassing methods such as SMAW (arc welding with wrapped electrodes), SAW (submerged arc welding), GMAW (gas metal arc welding), GTAW (tungsten arc welding), and more.
Moreover, the classification of welding further distinguishes between welding similar materials (similar welding) and welding dissimilar materials (dissimilar welding) (Arifin, 2020).This categorization and the versatility of welding techniques establish it as a foundational technology in the manufacturing industry, enabling the creation of a wide array of products across diverse materials and applications.In modern times, integrating different materials has become essential in various industries, including power plants, petrochemical and chemical sectors, and construction.This need for material integration is particularly prominent in fabricating items like boilers, rig towers, and automotive components.The primary goal of joining dissimilar materials is to create an optimal material combination for a specific design that offers the desired mechanical properties, corrosion resistance, and costeffectiveness (Musa et al., 2020).
Nevertheless, when it comes to welding dissimilar metals, achieving the same level of weld quality as with similar metals can be challenging.This challenge arises due to variations in the physical and chemical properties of the joined materials, even when welding conditions remain identical (Arifin,  Incorporating different Aluminum-Steel joints in a product offers several sustainable advantages.These include cost reduction by capitalizing on the price differences between the materials, weight reduction considering the differing densities, the combination of lightweight aluminum with high-strength steel, and the availability of hybrid properties (Mehta, 2019;Qin et al., 2022).However, welding dissimilar aluminum-steel materials is a complex process due to the intricate compatibility challenges during mixing.
Aluminum and steel differ significantly in their melting temperatures, with aluminum at around 850 °C.Their ability to form oxides at distinct temperatures also varies.Aluminum (Al) exhibits a thermal expansion coefficient and specific heat nearly twice that of steel.Furthermore, aluminum's thermal conductivity is six times higher than steel's.In terms of elasticity, aluminum's modulus is three times that of steel; besides, aluminum and steel exhibit low solubility in each other (Mehta, 2019).An electrochemical difference of 1.22 volts between aluminum and steel further complicates the welding process.
These disparities in thermophysical, chemical, and mechanical properties pose several challenges, including the formation of intermetallic compounds (IMCs), the presence of a heat-affected zone (HAZ), metallurgical precipitation, defects, distortion, and reduced mechanical joint properties (Mehta, 2019;Gill and Pradhan, 2022;Newishy et al., 2023).Hence, understanding how to use these essential materials for various engineering applications effectively has become a critical area of investigation.Numerous researchers have delved into various welding processes for joining dissimilar metals while examining their mechanical, corrosion, and other relevant properties.Additionally, they have considered the type of joint used, whether it is a lap or butt joint, accounting for the disparities between the two materials.For instance, Qin et al., (2022) studied the microstructure and mechanical properties of aluminum alloy/stainless steel dissimilar ring joints using inertia friction welding.This welding process exhibited a significant coupling effect between thermal and mechanical aspects.The steel experienced severe deformation and discontinuous dynamic recrystallization (DDRX).Increasing rotation speed during friction stir welding (FSW) enhanced the ultimate tensile strength (UTS).Irrespective of the specimen, tensile failure occurred at the SZ on the aluminum side.The collective findings of various researchers have observed that failures in aluminum and steel joints typically occur on the aluminum side due to their lower strength.Therefore, further investigation into parameters like weld joint design is crucial to understanding the strength and failure modes of such joints.There is a need to enhance the strength characteristics of these joints on the aluminum end.
The current study aims to examine the mechanical and corrosion characteristics of dissimilar metals, specifically aluminum and mild steel, using the shielded metal arc welding (SMAW) technique, considering both double-V and butt joints.SMAW, also known as Shielded Metal Arc Welding, is a widely used welding method in various industrial applications.This technique involves metal electrode wires coated with flux.The welder uses an arc to join a covered electrode with a weld pool.As the welder introduces the coated electrode into the welding pool, the coating decomposes, producing gases that shield the pool.This process does not require pressure and uses the metal filler from the electrode, contributing to metal deposits for joining metals and applying a metal surface to products (Arifin, 2020).

Materials
For this investigation, we utilized mild steel sheets compliant with ASME CODE SA-516, a standard specification for pressure vessel plates suitable for moderate-and lower-temperature services.Additionally, we used aluminum alloy sheets (Al5052) as the second base material.We procured these materials from a local supplier in Akure, Ondo State, Nigeria.We used a stainless-steel electrode with the code name E308L-16 as the filler metal in our welding process.This electrode features a rutile-basic coating explicitly designed for welding ASTM 304L base materials for low-temperature applications.We acquired this electrode from a local vendor in Akure, Ondo State, Nigeria, just like the base materials.The chemical composition of each material is in Tables 1 and   2, and the chemical composition of the stainless-steel electrode used in our welding work is in (Table 3).

Welding and Sample Preparation
The aluminum and mild steel sheets were cut into 60 x 80 mm pieces using a cutting machine equipped with a 4 1/2 -inch carbon cutting disc.In total, we prepared four plates, with two plates from each material.We then used a grinding machine with a four-1/2-inch carbon grinding disc to bevel one side of each plate to an angle of less than 3 mm.To create a double-V shape, we placed the prepared beveled pieces, one from aluminum and one from mild steel, adjacent.We repeated the same process for the remaining un-beveled pieces, forming butt joints.
The welding process was the Shielded Metal Arc Welding (SMAW) technique, following the specified requirements in (Table 4).
After completing the welding, we extracted dumbbell-shaped samples from both the welded joints and the base metals for tensile

ANALYSIS ARTICLE | OPEN ACCESS
Discovery 59, e122d1378 (2023) 4 of 12 strength testing.We also performed additional machining to facilitate hardness testing and acquire microstructural samples.These samples were separated into three sections: base metal (BM), heat-affected zone (HAZ), and weld metal (WM).

Tensile Test
Tensile test specimens are following the (ASTM A370-08A, 2008) standards.We conducted the tests using a Su Zhou Long Sheng universal testing machine.The machine applied a load of 300 KN to each specimen at a crosshead velocity of 5 mm/min, resulting in an initial strain rate of 0.98 s-1.We securely clamped the specimen at both ends using the machine's grips.The tensile testing equipment uniformly stretched the specimen while continuously measuring the applied load and elongation.Due to the applied load, each sample underwent permanent deformation and eventually fractured, resulting in the separation of the specimen into two distinct parts (Oladele et al., 2018b).

Hardness Test
Hardness assessment followed the Brinell hardness testing methodology, employing a load increment of Hardness Brinell Wolfram carbide (HBW) 5/250.We subsequently examined the hardness of the specimens, commencing at the fusion zone of the welded plates while also encompassing the corresponding section of the base metals to facilitate a comparative analysis of the findings.The indenter, in the form of a steel ball, was forcibly applied to the test material.The resulting indentation diameter (d) was duly measured, thereby enabling the calculation of the projected area and the subsequent determination of the material's hardness.

Impact Test
The impact tests were conducted on four samples, namely the base metals (aluminum and mild steel) and the welded samples (the butt and the double-V joint), in order to ascertain the impact strengths using the "V-notch method employing the Honfield Balance Impact Testing Machine".Prior to the installation of the machine, we notched a v-shaped edge on the test samples to a depth of 2mm using a hand file.Subsequently, we attached the notched test sample to the impact testing machine, and we activated the machine to apply a constant impact force to the test sample.We determined the impact strength by reading the calibrated scale on the impact testing machine, which quantifies the amount of impact energy absorbed by the specimen before yielding.

Metallographic Examination
The specimens were subjected to grinding using various grades of emery paper (220, 320, 500, 600, 800, and 1000), followed by polishing and etching for approximately 30 seconds using the corresponding etchants as outlined in (Table 5).Using a Nikon Optiphot metallurgical microscope and a Nikon D70 digital camera with magnifications of 50 and 100, we captured the micrographs of the samples.Optical microscopy was employed to examine the microstructures of the weld zone (WZ), heataffected zone (HAZ), and parent metal (PM).

Corrosion Test
We conducted potentiodynamic polarization and open circuit potential (OCP) measurements using the AUTOLAB PGSTAT 204N instrument.We performed the electrochemical investigations using a three-electrode cell configuration at ambient temperature.We used carbon steel as the working electrode, with a geometric area of 1 cm2 embedded in resins.We used platinum as the counter electrode and a silver/silver chloride reference electrode.The electrolyte was NaCl (sea water), with a weight percentage of 3.5%.

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We conducted the linear polarization analysis within a potential range of -250 mV to +250 mV, with a scan rate of 1.0 mVs-1; this allowed us to determine the current density (Icorr), corrosion rate, and anodic tafel slope (Ba).We performed the abovementioned process on the weld and heat-affected zones (HAZs) of equal sizes of (10 x 10) mm each.Oladele et al., (2018a) also carried out a similar procedure.

Tensile Properties
We assessed the tensile characteristics, including the ultimate tensile strength, tensile modulus, and maximum strain.The mean values in Figures 1-4 represent the data we collected from the measurements.The analysis presented in Figure 1 unveiled that, in terms of tensile properties, mild steel exhibited superiority over aluminum among the base materials employed, having an ultimate tensile strength of 464.24 MPa compared to 412.25 MPa of the aluminum alloy, as seen in (Figure 2).This discrepancy can be due to the inherent characteristics of mild steel, including its density, grain structure, atomic structure, and the alloying elements it encompasses.Moreover, we observed that the double-V joint of the welded component demonstrated superior tensile properties encompassing ultimate tensile strength, maximum strain, and tensile modulus in comparison to the butt joint of the welded component.
This enhancement can be due to the formation of intermetallic compounds between the two base metals and the proper filling strengthening mechanism made possible by the weld joint design.The intermetallic compounds result from the interaction between mild steel, aluminum, and the filler metal, leading to the creation of a phase that possesses greater strength than the parent metal (Oladele et al., 2018b).Also, using a double-V joint allows for the load distribution across two separate weld beads on either side of the joint.This distribution of load minimizes stress concentration at any specific point, thereby reducing the susceptibility of the joint to failure under tensile loads.Conversely, in a typical butt joint, the entire load is concentrated on a single weld bead at the center of the joint.
This load concentration can result in stress concentration and potentially give rise to welding defects or failures, particularly when subjected to high tensile loading.Likewise, from Figure 3, the double-V joint exhibited a higher maximum strain than the butt joint but slightly lower than that of the base metals.In contrast, in Figure 4, the double-V joint exhibited a higher tensile modulus than the other joint type employed and the base metal aluminum alloy 5052 but lower than the mild steel base metal; this can also be because of the presence of intermetallic phases formed during the welding process between the electrode used and the base metals.These results were in agreement with the work of (Brajesh et al., 2015).
Figure 1 Stress-strain curve of the base metals and weld joints.

Hardness Property
The hardness values of the base metals and welded components is presented in (Figure 5).From Figure 5, we observed that the base metal from mild steel exhibited the highest hardness of 142.89 HBR, followed by the double-V joint with a hardness of 109.4HBR and the butt joint with a hardness of 97.5 HBR.Conversely, the aluminum base metal demonstrated the lowest hardness value of 64.59 HBR.Thus, an enhancement of 69% was noted between the double-V joint and aluminum base metal, while we observed a 51% enhancement between the butt joint and aluminum base metal.This enhancement can be attributed to the diffusion of alloying elements in the weld pool, resulting in the formation of distinct phases from the base metals and ultimately affecting the mechanical properties.The dissimilar metal-welded samples exhibited intermediate strength and hardness values between mild steel and aluminum.Fusing these two dissimilar metals could be a viable fabrication method for applications requiring structural properties with specific hardness, considering environmental and cost factors.Figure 5 also shows that the double-V welded joint is superior to the butt welded joint, which is consistent with (Brajesh et al., 2015).

Impact Property
Figure 6 illustrates the measured impact energy in joules for both the base metal and the welded component.We observed that the base metals exhibited higher impact energy than the welded samples.Specifically, mild steel demonstrated the highest impact energy at 62.37 J, while the butt joint displayed the lowest impact energy at 13.23 J.This discrepancy can be because of the inherent challenges associated with welding dissimilar metals, namely their varying melting points, thermal conductivities, and expansion coefficients.Consequently, these differences can result in welding flaws such as cracking and porosity, ultimately diminishing the overall impact strength.Moreover, we noticed that the double-V joint exhibited tremendous impact energy when compared to the butt joint; this can be due to the double-V joint possessing a larger cross-sectional area.Consequently, the stress is distributed over a larger surface area, reducing the likelihood of failure under impact loading.

Corrosion Properties
The potentiodynamic polarization curves of the base metals and the welded joint, which consists of the weld zone and the base metals, were examined in a 3.5 wt% NaCl solution (saline environment), as depicted in (Figure 7).Meanwhile, Figure 8 provides the corrosion rate for each material.Notably, the samples displayed diverse polarization and passivity characteristics, as indicated by the distinct corrosion rate values in (Figures 7 and 8).The results revealed that aluminum exhibited a lower corrosion rate than mild steel among the base metals.This disparity can be due to the inherent corrosion resistance of aluminum relative to steel.
Regarding the welded samples, we also observed that the double-V joint displayed a lower corrosion rate compared to the butt joint.This discrepancy arises from the larger surface area exposed to the corrosive environment in the butt joint, rendering it more susceptible to cracking than the V-joint.Overall, both welded samples exhibited a higher corrosion rate than the base metals, as evident in Figure 8; this can be because of the formation of intermetallic compounds (IMCs) at the weldment interface.These IMCs Furthermore, the welding process may introduce defects in the weldment, such as porosity and cracking, which can serve as initiation sites for corrosion.

Metallographic Microstructural Analysis
The examination of the microstructure is evident in (Figures 9-11).It was noted in Figure 9a that the microstructural analysis of the mild steel displayed a heterogeneous composition consisting of both ferrite and pearlite phases.The dark spots represent the pearlite phase, while the bright spots signify the presence of the ferrite phase.Additionally, we observed that the pearlite phases, both globular and laminar, were dispersed throughout the ferrite phase.disperses some coagulated pearlite within the ferrite phase, but not to the same extent as in the base metal.A similar result was also

Functional
requirements and technological constraints drive the need for some form of joining in the majority of products.Products usually consist of multiple components, and joining processes are fundamental to ensuring product functionality and enhancing manufacturing process efficiency (Martinsen et al., ANALYSIS ARTICLE | OPEN ACCESS Discovery 59, e122d1378 (2023) 2 of 12 Consequently, the thermo-mechanically affected zone (TMAZ) on the aluminum side underwent inadequate recrystallization, resulting in zonal features.The TMAZ regions showed distinct differences in crystal orientation and grain size.The rapid cooling rate hindered the growth of intermetallic compounds (IMCs) on the joint faying surface.The metallurgical bonding characteristics ANALYSIS ARTICLE | OPEN ACCESS Discovery 59, e122d1378 (2023) 3 of 12 varied due to the discontinuous distribution of IMCs.The average tensile strength reached 161.3 MPa, equivalent to 92.2% of the strength of 2219-O.The fracture occurred in the base metal on the aluminum side, displaying ductile fracture characteristics.In a separate study, Newishy et al., (2023) investigated friction stir welding of dissimilar materials: Al 6061-T6 and AISI 316 stainless steel.Dissimilar butt joints were formed between these materials using different processing parameters.Electron backscatter diffraction (EBSD) analysis revealed significant continuous dynamic recrystallization (CDRX) in the stir zone (SZ) of the aluminum side, mainly comprising weaker aluminum along with steel fragments.

Figure 2
Figure 2 Ultimate tensile strength of the base metals and weld joints.

Figure 3
Figure 3Maximum strain of the base metals and weld joints.

Figure 4
Figure 4 Tensile Modulus of the base metals and weld joints.

Figure 5
Figure 5 Hardness (HBR) of the base metals and weld joints.

Figure 6
Figure 6 Impact Energy of the base metals and weld joints.

Figure 7
Figure 7 Potentiodynamic polarization curves of the base metals and the welded joints samples in the 3.5 wt% NaCl solution possess poor corrosion resistance, rendering the weldment more vulnerable to corrosion than the base metals.

Figure 8
Figure 8 Corrosion rate of base metals and the welded joints

Figure 9a
Figure 9aMicrostructure analysis of Mild steel (Base metal).

Table 1
Mild Steel Chemical composition

Table 3
Electrode Chemical composition

Table 5
Etchant for respective metal.