Abstract
In this investigation, an attempt is made to study the tensile and microstructural impact of different gas tungsten arc (GTA) welding processes of pulsed current, constant current, and magnetic arc oscillation welded AZ31B magnesium metal joints. These techniques were chosen because they have the potential to produce welds with high tensile strength and excellent microstructural integrity. Five joints were produced using each approach, each one employing a unique combination of parameters. According to the findings of this experiment, the joints that were manufactured utilising a welding process called magnetic arc oscillation yielded superior tensile qualities when compared to the properties of other joints. The improved tensile characteristics of these joints can be ascribed in a large part to the creation of smaller grains and surface hardness in the transition region, as well as uniformly dispersed precipitates.
1. Introduction
The realm of materials science is becoming increasingly interested in magnesium alloys as a result of their advantageous properties, which include low density, high damping qualities, machinability, dimensional stability, and inexpensive casting expenses [1, 2]. High stiffness-to-weight ratios make magnesium alloys useful in many applications. Automotive, industrial, materials handling, and aircraft all require improved composites [3, 4]. Magnesium alloys with additions of aluminium and zinc have high strength properties coupled with quite low density, which makes them very appealing as building components in applications where minimising weight is of utmost importance. These alloys can be made by adding aluminium and zinc to magnesium. Welding is an essential industrial process that is inextricably linked to the structural uses of magnesium alloys. Welding can be accomplished with relative ease on the vast majority of magnesium alloys by employing methods such as gas tungsten arc welding (GTAW), gas metal arc welding, electron beam, and laser beam welding [5]. The GTAW is a material joining procedure that is utilised extensively. Because of the reliability, clearance, and strength of the weld, GTA welds are considered to be of a higher quality than those produced by any of the arc-welding methods. However, the physical features of magnesium may readily produce several processing issues and welding flaws such as oxide coatings, fractures, and porosity. High thermal conductivity, melting and boiling temperatures, high solidification shrinkage, low viscosity, and good hydrogen solubility are just a few of magnesium’s unique qualities [6]. It also has a strong tendency to oxidise and has low melting and boiling temperatures.
Many weld flaws are less severe when the solidification pattern is improved [7]. In the fusion zone of welds, one method for fine-tuning grain structure is magnetic arc oscillation (MAO). The MAO method uses a two-pole magnetic probe to oscillate the arc column in the opposite direction of welding. Weld fusion zone dendritic columns are broken down mechanically by arc oscillation. The microstructure is refined as the fractured dendrites act as forming sites and boost the cooling rate [8, 9]. Inside the pulsed current method, the welding pulse is cycled amongst two levels. The arc is stable because the ambient current (Ib) is insufficient to melt the metal surface. In a small area, the peak current (Ip) melts the base plate, and the adjoining base material acts as a chill, causing the spot to cool more quickly. The weld pool is agitated and the cooling rate is increased by a series of pulses delivered at a precise frequency. This results in a continuous weld bead. This technique has been studied extensively, and the purported benefits include greater bead contour, high acceptance to heat sink changes, inferior heat input needs, minimum residual stresses, and distortions. Many studies have shown that pulsed current welding progresses the quality of the weld by reducing the grain size and microstructure. Grain structure in weld fusion regions and an increase in weld powered characteristics have been achieved using current pulsing.
The impression of magnetic arc oscillation on fusion zone grain growth and tensile behaviour in aluminium alloy welds were investigated by Janaki Ram et al. [10]. Sivaprasad and Raman investigated the microstructure and hot temperatures tensile stress of the 718 alloy TIG welded joints influenced by magnetic arc oscillations and current pulsing [11]. Mahajan et al. [12] investigated the impact of MAC of the grain boundary. An aluminium alloy GTA weld with magnetically agitated GTA welds was the subject of research by Rao et al. [13]. Magnetic arc oscillation has received a relatively small number of studies despite the fact that it has a number of advantages. Nevertheless, there is no evidence available on the effect of constant current, pulsed current, and magnetic arc oscillation on magnesium alloys. Keeping this in mind, a study was conducted to determine the inspiration of continuous, pulsed, and MAO welding procedures on the tensile and microstructural properties of AZ31B magnesium alloys joints; the findings are obtainable in this article.
2. Experimental Work
The thickness of the AZ31B magnesium alloy plates was trimmed to the specified measurements. Table1 presents the chemical composition as well as mechanical characteristics of the base metal. The joints were manufactured using a square butt joint arrangement. The axis of welding was parallel to the direction of rolling. Autogenous arc welding procedures (without filler metal) such as continuous current gas tungsten arc welding, pulse current gas tungsten arc welding, and magnetic arc oscillations were utilised to construct square butt joints. The MAO equipment is equipped and circumscribed with GTAW torch. In addition, it is interfaced with a microcontroller, which controls the arc oscillations frequency and amplitude. The experimental setup and MAO equipment is shown in Figure 1. Dimensions of tensile samples are exposed in Figure 2. The pictures of fabricated joints are revealed in Figure 3 steady flow rate of 20 l/min, argon gas has been used as a gasification agent.
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Input heat is an important factor that determines weld quality, Table 2 mechanical qualities, and metallurgical properties. As a result, heat input was calculated and included in this analysis. Heat input is proportional to voltage and amperage and inversely to welding speed for each unit length.
The welded joints were cut and then finished to the necessary dimensions in accordance with the ASTM E8M-04 standards for sheet-type materials (50 mm × 12.5 mm). For evaluating the transverse tensile characteristics of the welded joints, two distinct tensile samples were constructed. For the purpose of measuring yield point, tensile strength, and joint elongation, smooth tensile specimens were created. A 100 kN electromechanically driven universal testing machine was used to conduct the tensile test. The 0.2% offset yield strength was calculated using the load-displacement diagram. The proportion of elongation was also assessed, and the results are reported in Table 3. Using a light optical microscope equipped with image-analysing software, microstructural examination was performed (metal vision).
3. Results
3.1. Tensile Properties
At its maximum load, a notched specimen’s tensile strength (NTS) is compared to the UTS of an unnotched sample. Table 3 give the average of the three test findings for each condition, which were conducted on a total of three samples. Figure 4 depicts the tensile curves of the materials. In comparison to CCGTAW and PCGTAW, MAO welding produced better tensile characteristics. This type of joint has yield strength of approximately 192 MPa and tensile strength of approximately 153 MPa, 165 MPa, and 214 MPa, and these are the PCGTAW joints yield and tensile strengths. The MAO joints have yield and tensile strengths of 192 MPa and 248 MPa. MAO joints when compared to CCGTAW and PCGTAW, the MAO-fabricated joints have the highest values of the three types of welded junctions.
3.2. Macrostructure Study
The macrostructure image of the joints made with CCGTAW, PCGTAW, and MAO welding process at optimized conditions are presented in Figure 5. The joint fabricated using the different welding parameters from this various condition in every process only defect free joint with full penetration which are presented in Figure 5.
3.3. Microstructure
Autogenous arc weld optical micrographs are revealed in Figure 6. The grain size of the weld metal sections can be discerned from the micrographs. As a result, an effort was made to determine the normal weld metal grain diameter for all of the joints captured by the picture analysing software. CCGTAW joints have a mean grain diameter of 42 microns, whereas PCGTAW joints have a mean grain diameter of 30 microns. Pulsed current welding reduced the grain diameter by 12microns, according to this data. The mean grain diameter of MAO joints is 26 microns, which implies that the grain diameter is reduced by 16 microns as a result of the MAO process. The MAO process has the smallest grain in the weld metal associated to the CCGTAW and PCGTAW methods.
3.4. Microhardness
There was a microhardness study conducted from the welded zone to the base metal [14]. Based on differences in microstructure, the areas marked on Figure 7 are the areas of base metal, the heat impacted region, and the fusion region. In the weld metal region, the CCGTAW and PCGTAW joints have hardness values of HV 61 and HV 66. In contrast, the weld metal hardness of MAO joints is HV 68, which is higher than that of CCGTAW and PCGTAW joints.
3.5. XRD and EDS Analysis
Analysis of the chemical components of the weld area and the base metal was done using the XRD. Al12Mg17 precipitates are shown in Figure 8 XRD data, as well as traces of Mg2Zn11 and Al3Mg2. The matrix’s composition was also determined using energy dispersive spectroscopy (EDS). Figure 9 shows the EDS data, which reveal that magnesium and aluminium components dominate the matrix composition. Due of the increased peak temperature achieved by the weld area, zinc evaporation is also present in all the joints.
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4. Discussion
Compared to CCGTAW and PCGTAW weld joints, MAO joints are stronger in terms of yield and tensile strength. Weld thermal cycles, or more specifically, the heat input provided by the welding process, have an effect on this. The CCGTAW technique creates coarser grains in autogenous arc welds than the PCGTAW and MAO procedures (Table 3). The CCGTAW method has a larger heat input than the PCGTAW and MAO procedures (Table 3). The MAO method has a lower heat input than the PCGTAW and CCGTAW processes do. The microstructural features and toughness qualities of the weld are affected by these fluctuations in heat input during welding procedures. It is a version of the GTAW process that cycles the welding current between high and low levels at a predetermined frequency. The PCGTAW method peak current values are frequently selected to offer appropriate penetration and beading contour, while background current levels are controlled at a suitable level to keep a constant arc [11].“Because the base material is melted only when the maximum peak pulses for short durations, the heat dissipates into the raw product rather than being trapped in the weld pool” [10].
Grain refinement can be achieved through the usage of current pulsing and arc fluctuation in combination with their simultaneous application in the MAO process, which is an extension of the GTAW process. As can be perceived in Figure 9, the autogenous arc manipulation approaches produced fine grained microstructures, as well as the columnar grained structure that is typical of CCGTA weld metals. The satisfactory equiaxed grains in the welding process are the consequence of a variety of oscillation and pulse conditions. There is a strong correlation between oscillation/pulsing frequency and the equiaxed grain formation. When compared to CCGTAW and PCGTAW joints, MAO welds were shown to have a higher toughness (68VHN) (Figure 10). “Microhardness in fusion can be traced to a rapid cooling point due to steeper thermal gradients, which results in a microstructure with finer grain size” [15, 16]. The tensile characteristics of the fusion zone were improved as a result of grain refining. The junction created by welding MAO joints reached its maximum tensile strength. The higher tensile strength of the junction is also due to the weld metal’s spreading of higher hardness [17, 18]. “In the weld, very fine grains were produced as a result of enhanced cooling rates near the weld center in comparison to the fusion boundary” [19].
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The employment of arc manipulation techniques and arc oscillation provides motorized agitation in the weld transition region and breaks down the developing dendritic supports created during weld solidifying, according to the results of the tensile tests performed on the welded joints. Dendrites that have been shattered can serve as new nucleation points, resulting to a more refined grain structure. The tensile characteristics of welded joints should be affected by the development of grain structure in the fusion zone [20].
The SEM fractograph of base metal CCGTAW, PCGTAW, and MAO are revealed in Figures 10(a)–11(d), respectively. The fracture surface of the base metal shown in Figure 10(a) shows shear mode of failure with secondary cracking which is attributed to the high tensile strength. The fracture surface of CCGTAW joint shown in Figure 10(b) represents required mode failure with flat regions indicates the brittle mode of failure which is associated with the reduced ductility of the joints. The fracture surface of PCGTAW joint shown in Figure 10(c) reveals greater number of dimples than that of the CCGTAW joint which indicates the improvement in the ductility of the PCGATW in comparison to the CCGTAW joint. The fracture surface of the MAO weld joint (Figure 10(d)) consists of fine and large number of dimples with no flat featureless region indicating the ductile mode of failure in the joint in comparison to other weld joints [23, 24].
The TEM micrograph of the base metal is displayed in Figure 11(a) reveals Al12Mg17 precipitates dispersed in the Mg matrix. The TEM micrograph of CCGTAW shown in Figure 11(b) reveals the coarsening of precipitates during welding and even some clustering was observed. The TEM micrograph of the PCGTAW joint shown in Figure 11(c) reveals lesser and finer precipitates involved the PCGTAW joint than that of the CCGTAW joint. The precipitates tend to coarsen and aligned in the grain boundaries. The TEM micrograph of the MAO joint shown in Figure 11(d) reveals higher precipitates of finer size precipitates due to the disorder created in the weld pool of the MAO joint, by the stirring action of the arc [21, 22].
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5. Conclusions
The influence of GTAW variants on the tensile and mechanical characteristics of AZ31B magnesium alloy joints was considered. Important findings from this examination are as follows:(i)Associated to the CCGTAW and PCGTAW joints, the tensile strength of the MAO-fabricated joints was enhanced by 24 and 16 percent, respectively.(ii)MAO joints may have superior tensile properties compared to the CCGTAW as well as PCGTAW joints because of their lower heat input, thinner weld zone grain diameter, and greater fusion zone hardness.(iii)The strength difference of weld joints has mainly attributed to size and dispersal of precipitates in the MAO than the PCGTAW and CCGTAW joints.
Data Availability
There is no data availability statement.
Conflicts of Interest
The authors declare that they have no conflicts of interest.