The present paper focuses on the effect of alternating shielding gases and post weld heat treatment on tensile properties and porosity of GMA welded AA6061 joints. These properties of weld joints will be greatly influenced by microstructure and hardness of the welded joints. Hence, in this investigation tensile strength, porosity, microstructure and micro hardness measurements were carried out on the cross section of weldments. Investigations were conducted utilizing the Universal Testing Machine, Vickers Micro Hardness Tester, X-ray radiography, Optical Microscope, Scanning Electron Microscope (SEM), and X-ray Diffraction (XRD). Hence, in this research, a complete analysis has been carried out to understand the influence of shielding gases and post weld heat treatment on mechanical and metallurgical behavior of GMA welded joints of AA6061aluminium alloy. It was represented in this research; tensile strength of post heat treatment welded aluminum alloy was increased by 44.9% than non post treatment of welded joints. Also, the micro hardness has been greatly improved post weld heat treatment in AA6061 alloy specimens. It was evident from the micro structures and fractography analysis of the AA6061 alloy components.
Research Article
Effects of Alternating Shielding Gases and post weld heat treatment on microstructure and mechanical properties of the GMA welded Joints of AA6061 Aluminium Alloy
https://doi.org/10.21203/rs.3.rs-3303039/v1
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Shielding gas
gas metal arc welding
post weld heat treatment
aluminium alloy
Lightweight materials such as aluminum alloys are widely used in diverse industries like aerospace, automotive, etc. due to the excellent strength-to-weight ratio. In particular, the AA6061 aluminum alloy possesses better mechanical and welding characteristics. However, during the welding of AA6061 alloy promotes softening at heat affected zone [1]. In many applications, the Gas metal arc welding (GMAW) is a prominent welding process for joining of all Commercial Metals and aluminum alloys, which is the sole consumable electrode welding procedure. Welding speeds in GMAW can be increased due to greater metal filler deposition rates [2]. The electrode is MIG welded, resulting in high wire burn-off rates and efficient heat transmission into the weld pool via the filler wire. Also, it was noticed that argon provides a smooth, stable, and quiet arc compared to nitrogen and oxygen [3]. However, the arc interacts with the environment in the GMAW process, resulting in weld defects such as lack of penetration, burn through, and porosity. Therefore, in-process monitoring and control are required to ensure quality through defect-free welds [4]. Researchers have discussed the weld bead structure, hardness, and microstructures are influenced by welding process factors. Also, it was indicated that welded the lower heat input was attributed to decreasing the hardness values from the zone to the base metal, and when the welding parameters are changed, the grain size changes from smaller to larger in different passes [5–6].
Furthermore, researchers reported that the welding current, arc voltage, and welding speed were important parameters influencing micro structural, welding penetration, and hardness during the GMAW process [7]. Consequently, the higher ultimate tensile strength was obtained at high current; low preheat temperature and high welding speed [8]. Moreover, Wang et al. Investigated the cooling rates, solidification growth rates, and weld geometry of an aluminum alloy during single and double pulsed welding processes. It was indicated that the refinement of the microstructure during the welding process significantly depends on welding parameters [9]. The authors have revealed that the GMAW has identified porosity as the crucial influence influencing the mechanical characteristics of Al 5083 weldments to deteriorate [10]. On the other hand, any surface cracks were found in the copper cladding produced by GMAW, which were attributable to significant thermal stresses [11].
The Authors have concluded that the depth of penetration, ultimate tensile strength, and grain microstructure of the weld joint are all affected by variables such as current, gas flow rate, and voltage, which are attributed to determining the welded structure's life duration and quality. Also, it was indicated that dross and porosity might be caused by the GMAW process, which promotes less ductile welds and weaker welds [12]. Moreover, the increase of voltage and weld current deteriorated the stability of the welding process and led to large sputters [13].The shielding gas plays a significant role in controlling the weld bead geometry, porosity, weld microstructure, and mechanical properties of welded components. Also, it provides a suitable medium for the steady functioning of a sustained low-voltage arc as well as air contamination shielding. It was concluded that the weld performance was improved by the use of mixtures of two or more shielding gases [14].
Moreover, the shielding gas plays a predominant role in controlling the micro-structural and mechanical properties of aluminum and its alloys. It was pointed out that argon enhances oxide breakdown and weld quality by improving arc stability. Helium promotes an increase in welding rate, penetration, and weld puddle fluidity by providing higher heat input to the base metal [15]. Also, it was noticed that mixture of gas provides significant improvement in quality and structure of weld [16]. Izutani et al. have discussed the composition of shielding gas, current pulse wave pattern and size, and composition of welding materials for decreasing porosity effects in the weld of galvanized steel sheets. Also, it was concluded that 70% Ar and 30% CO2 were attributed to reducing the porosity defects [17]. Purwanin grum et al. investigated the influence of the mixture composition of shielding gas on the mechanical and physical properties during the welding of low carbon alloys. It was noticed that the welded metals have a better tensile strength than raw materials when the shielding gas is varied [18]. On the other hand, mixture of shielding gas provided greater dimensional stability, higher hardness, and lower porosity. However, the effects of non optimized composition of shielding gas might be attributed to reducing tensile strength with wide scatter, due to the microstructure's stable delta ferrite during the process [19–20]. Also, authors have pointed out that the optimal shielding gas promotes tensile strength and impact energy. However, the increase in shielding gas might affect the tensile strength properties during the high nitrogen stainless steel (HNSS) GMAW welding process [21].Also, it was indicated that, argon and helium mixture shielding gas promotes to suppressing the weld porosity defect was 50%., which lead to improve the keyhole stability [22]. The joints fabricated with tandem GMAW were found to have lower porosity, resulting in higher tensile shear strength over a wider range of wire feed rates [23].
Further, post weld heat treatment (PWHT) is predominately required for reducing tensile stress on the welded components. Consequently, fatigue life and tensile strength of welded components might be increased [24]. And, it was demonstrated that dislocation density and residual stresses were reduced after PWHT process [25]. The researchers have reported that residual stress might be developed while welding of thick components. It was indicated that reduced residual stress, promotes to improve the mechanical properties and changes in the micro structural at HAZ and weld metal region [26]. The researcher has evaluated mechanical properties of post weld heat treatment of titanium based welded joints. It was reported that the increase in temperature preheated conditions promotes to increase the heat affected zone width and fusion zones. Moreover, the micro hardness of welded joints might be reduced after preheating temperature up to 600ºC [27]. However, increasing grain size, lengthening and enhancing tensile strength will increase post-weld heat treatment [28]. The increase of holding time during post welds heat treatment promotes higher failure in the weld metal region compared with base metal region [29]. From the literature survey, it was observed that all researchers have investigated the influence of composition shielding gases on the GMAW process. Meanwhile, the effect of argon and helium composition on the welding of AA6061-T4 alloy using the GMAW process is required for investigation. The novelty of this research lies in the influence of alternating shielding gas on the micro structural behavior and mechanical properties on the welding of AA6061-T4 alloy using the GMAW process. Moreover, the post weld heat treatments (PWHT) are the predominant role to influence the mechanical properties of welded components. Hence the investigation is required for the joining of AA6061-T4 alloy components by using GMA welding. Consequently, PWHT and alternating shielding gases are required for investigation for decreasing welding defects and further improving the welding properties in aluminum alloy components.
The aim of this research is to study the influence of alternating protective gases from argon to helium and from helium to argon. The alternating shielding gases used are 0.25 s (He), 0.75 s (Ar), 0.5 s (He) and 0.5 s (Ar), and pure argon. Further, PWHT process is applied for the welding of AA6061-T4 alloy. The micro hardness, and tensile properties, porosity and micro structural influences were investigated on welded specimens. In this research, the welding currents 150A were considered in the GMAW process at a welding speed of 80–105 mm/min. The alternating time interval between 0.1 s and 1 s was used in this research. The microstructure analysis has been done in evaluating the influence of post weld heat treatment during the GMAW process.
2.1 Materials
Argon and helium were selected as alternating shielding gases in the GMAW process. Gas metal arc welding has been used in the fabrication of a 5 mm thick plate joint. An ER4043 type of filler material was used with the base metal. The mechanical properties of aluminum alloy are depicted in Table 1.
Table 1: Mechanical properties of aluminum alloy
2.2 Specimen welding and experimental characterization
The AA6061-T4 alloy components were fabricated using the GMA welding technique. For gas metal arc welding, the AA6061-T4 alloy plates were cut into 150 mm x 300 mm sections. GMA welding was used to produce the joints of the AA6061 aluminum alloy using the parameters listed in Table 2. The aluminum specimens were fabricated using GMAW and three different types of shielding gases, as illustrated in Fig. 1. Every aspect of the efficiency of arc welding is impacted by the alternate supply of shield gas technology. The technology's advantages are principally brought about by the dynamic action that the molten pool of welds undergoes. The alternative supply of shielding gas results in the occurrence of three separate phenomena: (a) fluctuation in arc pressure, (b) variation in weld pool fluidity, and (c) peaking in arc pressure. Each of these phenomena works in conjunction with the other two to stir the weld pool. Due to its ionisation properties, argon draws a larger current and creates a streaming metal transfer mode than helium and alternate shielding gases supplied to the welding torch [30]. The streaming transfer mode promotes to reduce droplet size, elongate the droplet formation and increase the metal transfer frequency, leads to increase the arc pressure and detachment of droplet from tip of electrode was delayed due to limited electromagnetic force. Consequently, when argon passes through the flame, the arc pressure might be raised. When helium passes the flame, the arc pressure drops. The pressure affects arc behavior linked to the gas flow regime around the arc. The arc voltage is increased due to the arc pressure. Due to above reason, the weld pool might be experienced a flow pattern when the shielding gases alternate between Ar to He and He to Ar. Meanwhile, the flow pattern with changes in arc pressure might not be significantly defined while welding with GMAW process. However, the typical diagram (Fig. 2) could be illustrated the changes in weld pool flow pattern and arc pressure.
The AA6061 aluminum is prone to developing porosity problems in the welding process. X-ray radiographic testing for welds were performed to evaluate weld porosity for weld joints. Figure 3a shows radiography testing typical image of welded specimen, in which porosity could be identified and then evaluated the amount of porosity in welded joints. Radiography is done in 1:1 scale, and the radiographs were compared with the ASTM standard reference radiographs (ASTM E 390 − 15) and confirmed the absence of the cracks. Regardless of shape or size, all porosities on the X-ray film for all the welded specimens were evaluated and then averaged in an X-ray radiographic test. In addition to this, the amount of porosity in the weld zone was evaluated by evaluating the optical micrograph pictures of the weld pool. Vickers micro hardness testing was performed on each sample using a 0.5 kg weight. The spacing between each indentation in the weld region, the heat impacted zone region, and the base metal region was set to 1mm to provide an understanding of the hardness without the effects of work hardening in the surrounding areas of each indentation. The tensile testing specimens were prepared according to ASTM E8M-04, which was shown in Fig. 4a. Tested specimen was represented in Fig. 4b. Optical microscopy and SEM have been used for further investigation of welded specimen.
Figure 5 depicts macroscopic images of the cross-section as well as variations in the width and depth of weld penetration recorded while using various types of shielding gases. From the macrographs it was concluded that, there is complete fusion between weld and parent metal is observed in the joints produced using all types of shielding gases and also found free from defects. Weld area arises from the adding helium to the shielding gas in the alternating supply, which results in a greater weld area than the constant argon gas, demonstrating that the shielding gas composition has a substantial influence on melting efficiency. As seen, the weld produced using all types of shielding gases resulted similar weld bead width. In addition to producing arc plasma with more uniformly distributed arc energy, pure helium gas has a better thermal conductivity than argon. The inner core of the argon arc plasma, which is extremely high-energy, and the outer mantle, which has less heat energy, are its characteristic qualities. The weld bead profile is significantly impacted by this variation in arc properties. A deep, broad weld bead is produced by the helium arc, whereas a papillary-like penetration profile is most frequently seen in the bead profile of the argon arc. In the current investigation, the penetration of the supply of alternating shielding gases 0.5 s (He) and 0.5 s (Ar) was greater and wider than that of Ar. This is typically caused by the combined impact of the properties of the arc concentration of Ar and the arc expansion of He, under the impulse action of the arc pressure with alternating supplies of shielding gases. [31].
Table 2: GMA Welding process parameters
The joints were treated for solution treatment followed by ageing in order to investigate the impact of post-weld heat treatment on mechanical properties. The solution was treated at 560°C for 60 minutes of soaking time. Joints were placed within the induction furnace as shown in Fig. 3b, and they were heated there at a rate of 100°C per hour from room temperature to 560°C. The joints were cooled in water that was 50°C in temperature following at the soaking time. A 8-hour soak in the artificial ageing treatment was conducted at 175°C. Once the soaking time was over, the joints were taken out and quenched in water that was 25°C warm.
3.1 Effect of shielding gases on tensile strength
Tensile investigation revealed that in all of the experiments, the fracture occurred at the weld zone of joints produced with various types of shielding gases. Figure 6 illustrates the experimentally assessed transverse tensile characteristics of welded joints made using three distinct types of alternating shielding gases (Pure argon, 0.25 s (He) and 0.75 s (Ar), and 0.5 s (He) and 0.5 s (Ar)). As can be observed, utilizing alternating shielding gases produced welds with strengths ranging from around 170 MPa in the basic case of continuous argon to about 182 MPa when using 0.25 s (He) and 0.75 s (Ar), and 196 MPa when using 0.5 s (He) and 0.5 s (Ar) alternating shielding gases. The tensile strength of the welded joints produced with alternating shielding gases 0.25 s (He) and 0.75 s (Ar) and 0.5 s (He) and 0.5 s (Ar) improved by 7% and 15.3%, respectively, in comparison to the welded joint made from pure argon as a shielding gas.
It has been demonstrated that the helium gas's alternating frequency affects the weld tensile strength, with a longer frequency period resulting in a stronger weld. The percentage porosity and the tensile strength of the weld are demonstrated to follow the same pattern in that decreasing porosity was produced with higher helium alternation frequencies, which in turn enhanced the tensile strength of the weld. Figure 6 displays the post-weld heat-treated joints' experimentally assessed transverse tensile characteristics. Tensile strengths of PWHT joints constructed with pure argon, 0.25 s (He) and 0.75 s (Ar), and 0.5 s (He) and 0.5 s (Ar) alternate shielding gases were 234, 267, and 284 MPa, respectively. And those in as non heat treated welded sample were 170, 182 and 196 MPa respectively. It was reported that the post-weld heat treatment increased tensile strength by 27%, 32%, and 31%, respectively, above the as-welded state. When solid solution treatment and ageing were applied, the microstructure of the PWHT 6061 aluminum alloy was significantly altered, increasing its tensile strength when it was welded using the GMAW technique. It was revealed that PWHT joints have higher tensile and hardness qualities because the weld metal region of these joints has a very fine and homogeneous distribution of precipitates when compared to other joints. This is further supported by other research [32], which demonstrated that tiny dimples enhance the material's hardness and tensile strength. This was attributable to precipitation hardening in the region of the weld metal.
3.2 Effect of shielding gases on porosity
Alternating shielding gases can be used to minimize the amount of porosity in the weld. The major cause of this is the weld pool's dynamic behavior brought on by the transition between various shielding gas conditions. The porosities on the X-ray film for each of the welded specimens were examined during the X-ray radiographic test. Additionally, the optical micrograph pictures of the weld pool were examined in order to estimate the number of porosity in the weld zone. These results led to the graph in Fig. 7 are being plotted comparing the type of gas supply and the percentage of porosity. From the graph, it can be inferred that the 0.25 s (He) and 0.75 s (Ar) alternating shielding gas created a decrease in percentage of porosity compared to the argon base case, which has been associated with an improvement in weld pool fluidity. Additionally, it should be highlighted that the application of a different class of alternating shielding gases—0.5 s (Ar)—reduces the percentage of porosity even more (0.5 s He). In contrast to 0.25 s (He) and 0.75 s (Ar) and pure argon, alternate shielding gases of 0.5 s (He) and 0.5 s (Ar) have a larger heat input because helium has a higher ionization potential. As a result of the increased heat input, the solidification rate was decreased, allowing the gas bubbles to escape from the weld pool region [33]. Additionally, it is shown that using alternate shielding gases can lessen the porosity that results from welding using aluminum alloy AA6061.
3.3 Effect of shielding gases on micro hardness
Figure 8 depicts the micro hardness profiles for both PWHT joints and as-welded joints. After post-weld heat treatment, all shielding gas combinations significantly enhanced the micro hardness of the welded joints. The post-weld heat treatment greatly increased the hardness value and significantly hardened the welded joints in all zones when compared to the as-welded samples. PWHT joints now have an enhanced average hardness value of 101Hv, which is 96% harder than base metal. The tensile strength of the joints is greatly increased after heat treatment because the micro hardness in the joints increases significantly. It was revealed that PWHT joints have higher tensile and hardness qualities because the weld metal region of these joints has a very fine and homogeneous distribution of precipitates when compared to other joints. This is further supported by additional studies [34], which discovered that fine dimples boost hardness and tensile strength. The region around the weld metal was thought to be where precipitation hardened.
3.4 Effect of shielding gases on microstructure
Figure 9 shows the microstructure in the fusion zone of samples of the AA6061 aluminum alloy that have been GMA welded with several types of alternating shielding gases (Pure Argon, 0.25 s (He) and 0.75 s (Ar) and 0.5 s (He) and 0.5 s (Ar)). Figure 9 depicts at the weld tiny grains of Al-SiMg eutectic particles in a matrix of the Al solid solution, demonstrating that all weld zones display an equiaxial crystal structure [38]. Grain nucleation and growth toward the weld's center occurred when the molten metal solidified in the weld. Meanwhile, the equiaxed grains developed at the weld's center. According to Fig. 9(a), (c), and (e), a welding joint will typically have a more uniform and finer microstructure for the alternating shielding gas which produced greater pressure differences, It is evident from the microstructure that the joint grains produced by alternating shielding gases 0.5 s (He) and 0.5 s (Ar) are significantly finer and more uniform than those produced by alternating shielding gases 0.25 s (He) and 0.75 s (Ar) and pure argon. The weld metal's susceptibility to solidification cracking during welding is significantly reduced by fine grains. The mechanical qualities of the weld could be improved by fine grains. Due to its ionization properties, argon draws a larger current and creates a higher arc pressure during the supply of alternate shielding gases than helium, whereas helium draws a lower current and produces a lower arc pressure. As a result, the change in welding current is directly proportional to the arc force [30].
In addition, the supply of alternate shielding gases can cause the arc force to fluctuate, which can modify the weld pool's surface and increase the stir effect. Additionally, the stirring effect of the alternating shielding gas supply on the weld pools causes the coarse columnar grains to break into smaller grains, which affects the uniformity of the alloy element distribution and promotes heterogeneous nucleation during the solidification process of the weld pools [35]. Both of these effects have an impact on the welding joint's micro morphology.
Furthermore, PWHT joint microstructure was shown in Fig. 9. It was indicated that equiaxial dentric crystal structure was observed in all PWHT welded structures. The precipitation of matrix-strengthening particles started when solution treatment and ageing combined to generate a supersaturated solid solution. With an increase in temperature and prolonged heating, PWHT progressively promotes the precipitated particles dissolved in the matrix. Hence, uniform distribution and fine particles of precipitates at the weld joints were attributed to enhance the hardness and tensile strength and elongation. It was observed that the grain boundary was obscured by precipitated particles that had adhered to the weld metal of the joint as it was being welded.
3.5 Effect of shielding gases on fracture morphology
SEM of the fractured surface of tensile specimens is used to evaluate the fracture behavior of joints. Figure 10(a-d) shows the fractographs of the tensile tested samples. It demonstrates fracture surfaces where dimples predominated. When compared to the parent metal, the fracture surface of a joint produced using pure argon as a shielding gas exhibits elongated dimples and a pattern that suggests ductile fracture processes caused the failure. The dimples on the fracture surfaces of the joints produced by alternating shielding gases 0.25 s (He) and 0.75 s (Ar) were greater than those of the joints made with alternating shielding gases 0.5 s (Ar): 0.5 s. (He). These tiny dimples could have been caused by the presence of numerous fine precipitates, and they might possibly be associated with greater precipitation dispersion.
Notably, joints produced by alternate shielding gases of 0.5 s (He) and 0.5 s (Ar) exhibits comparatively fine dimples that signify a ductile type failure. The whole dimples also display an almost equiaxed pattern in these joints. However, the specimen welded using 0.25 s (He) and 0.75 s (Ar) of alternate shielding gas resulted in a cleavage-type fracture mode due to the existence of shallow and coarse dimples on the fracture surface. Consequently, the elongated and coarse dimples reduce the tensile strength. Because argon and helium have different densities and ionization potentials, the pressure in the arc column varies in pulses. This periodic influence on the weld pool causes the metal to produce the welded joint with a fine-grained structure, high plasticity, and strength [36].
3.6 Effect of post weld heat treatment on fracture morphology
The fractographs of tensile tested joints subjected to post weld heat treatment are displayed in Fig. 11 (a-d).
The fracture surfaces of all PWHT samples show many cleavages similar to facets and ductile dimples. The mechanism of fracture in the joint produced using pure argon, alternating shielding gases 0.75 s (Ar):0.25 s (He) are brittle and are consistent with the joint's low elongation of 6.2% and 7.6%, respectively. In addition to cleavage lamellas, different sized dimples could be seen in the fracture surfaces of the joint produced by alternate shielding gases (He) 0.5 s (He) and 0.5 s (Ar). Figure 11(c) and (d) shows how dimples are dispersed in local fractures, at the points of acute cracks, and inside shearing fractures. Also, it could be reported that the relative fracture toughness is also rather high and the dimple depth is significant, both of which show that substantial plastic deformation occurs under loading. A common feature of ductile fractures is the appearance of dimples, which denotes increased ductility [1]. Therefore, these results are consistent with the joint's excellent elongation efficiency (10.84%).
3.7 The XRD Analysis
The XRD analysis could be used to investigate how shielding gases affect phase formation. In order to determine the phase formation for alternating shielding gas, XRD examination was performed at the weld site of the joint specimen. Figure 12 illustrates 0.5 s (He) and 0.5 s (Ar). SiO2 and Fe2O3 were found in the weld zone of an AA6061 joint that was produced utilizing alternate shielding gas pulses of 0.5 s (He) and 0.5 s. (Ar). A joint produced by alternating the shielding gases 0.5 s (He) and 0.5 s (Ar) attained less Fe2O3 and SiO2 oxide than the joint constructed without doing so. Improved mechanical characteristics were the result of less oxide forming [37].
In this present research, the influence of alternating shielding gas on mechanical and morphological properties during weld joining of AA6061 alloy. Further, the investigations have been carried out for post weld heat treatment of aluminum alloy. The tensile strength of PWHT specimen has increased by 44.9% than non heat treatment specimen for 0.5 s (Ar): 0.5 s (He) alternating shielding gas supply. This might be due to; it was evident in the micro structural analysis. Moreover, the micro hardness of PWHT specimen has significantly increased. It could be noted that 0.5 s (Ar) : 0.5 s (He) alternating shielding gas have prominent influences on mechanical and micro structural properties of aluminum alloy, which was evident in SEM and optical microscopy analysis. The alternating shielding gas effects also observed in the XRD analysis. The novelty of this research could be concluded that PWHT and alternating shielding gases are the prominent influences on the welding of aluminum alloy and improved mechanical properties compared with conventional shielding gas welding process.
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