Investigation on Microstructure and Tensile Properties of High-Strength AA2014 Aluminium Alloy Welds Joined by Pulsed CMT Welding Process

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Introduction
AA 2xxx alloy is precipitation hardening aluminium alloy preferentially employed in aerospace and automotive application owing to its good mechanical properties, resistance to general corrosion, high strength-to-weight ratio, and better formability. e light combat aircraft interstage skin and the automotive wheel are made from this alloy. e main problems encountered in welding high-strength agehardened Al-alloys, including AA2014, are cracking in weld metal and partially melted zone, porosity, and alloy segregation which constrain the fusion welding of AA2014 aluminium alloy (Babu et al, [1][2][3][4][5][6][7][8][9]). Hence, novel methods have been developed and commercialized over few decades. e TIG-welded aluminium alloy joints disclosed inferior strength properties which is almost half of the base material strength. It refers to high heat input requirement in TIG welding of aluminium alloy.
is made researchers to practice low heat input solid state welding process popularly known as friction stir welding (FSW) process [10,11] [ [12][13][14][15][16]. Several grades of aluminium alloys and dissimilar materials were fabricated using FSW. Moreover, the strength of friction stir welded joints is 75%-85% of the base material strength [17]. Even though, this process has limitations such as the exit hole at the end of the weld, CMT was invented in 2004 and is foreseen to be a breakthrough method in aluminium and its alloy welding. CMT is an advanced variant of GMAW in which the digital control detects the short circuit timing and detaches the drop by the retracting motion of welding wire during welding; the wire is moved forward and again pulled back as the short circuit happens. us, CMT arc introduces heat input only during the burning phase of arc. is lowers the heat input than traditional dip transfer welds [18]. Shang et al. investigated the mechanical and metallurgical characteristics of CMT-welded dissimilar materials. e intermetallic compounds have considerably affected strength. Pickin and Young [19] reviewed the role of CMT welding process in thick materials and pulsed GMAW. Finally, the CMT welding process was compared with pulsed GMAW. CMT welding process deposited a small amount of filler material in the weld by pulse frequency. Gungor et al. [20] used AA5083 and AA6082 for CMT welding. e materials were joined using two welding processes such as pulsed robotic CMT welding and MIG welding.
e test results showed that the CMT welded joints had good tensile and fatigue strength. Azar [21] proposed a model to predict the heat input and geometry of CMT weld.
is model explained the behavior of weld pool and formation of weld pool with respect to heat input. [22] and analyzed the formation of weld defects in CMT welded 5083 plates and joint performance. In the fusion zone, a dendritic structure with a significant percentage of big pores was found. Inadequate surface cleaning before welding was the cause of the creation of big pores. e failure mechanism was also influenced by the existence of large pores, which dictated the fracture growth path. e crack propagated within the fusion zone by jumping from one pore to the next, resulting in brittle failure. e research work on CMT welding of aluminium alloy is mainly reported on 5xxx, 6xxx, and 7xxx series grade. e information available on P-CMT welding of AA2014 aluminium alloy is scanty. It is necessary to investigate the strength properties and microstructural features of AA2014-T6 aluminium alloy and compare the joint performance with base material for its viability in the concerned applications. Hence, an attempt has been made to weld AA2014-T6 aluminum alloy using pulsed CMT welding.

Work Methodology
Cold rolled sheets (AA2014-T6) with a thickness of 2 mm were used in this work. e chemical properties and mechanical properties of the base material (BM) were presented in Tables 1 and 2, respectively. e mechanical properties of BM were evaluated using a tensile test. e grain size of BM was 50 µm (Figure 1(a)). A test coupon size of 150 mm × 150 mm was prepared for the weld. CMT-2000x version machine (Figure 1(b)) was used to make a joint with a constant current of 110 A, a voltage of 12 V, welding speed of 45 cm/min, gas flow rate of 5 lpm, and wire travel speed of 555 cm/min (Table 3). e photograph of P-CMT welded joint is shown in Figure 1(c). Composite tensile specimens were extracted from the weld in accordance with the ASTM E8-M04 guidelines (Figure 1(d)). e process parameters were illustrated in Figure 1(e). e machined samples were used for tensile test. Tensile test was conducted using 100 kN electromechanically controlled UTM with a slide velocity of 0.15 cm/min. For metallography examination, a rectangular piece of the sample was cut from the middle portion of the weld line. Different grades of sand paper were used to polish the weld sample with alumina powder. e heat developed during welding was calculated using the following equation: Welding current(A)xThermal efficiency factor(η)xVoltage(V) 1000xWelding speed(mm/min) . (1) To reveal the structure of weld and other regions, the polished sample was etched with Keller's reagent. e hardness distribution across the weld was performed using Vickers microhardness test and was carried out as per the ASTM-E384, and the applied load of 0.5 N and dwell time of 15 sec were used. A scanning electron microscope was used to measure the type of failure pattern in the tensile tested sample. Moreover, EDS analysis was performed in the fractured region using energy dispersive spectroscopy.

Tensile Test.
e strength properties of AA 2014 P-CMT joints are presented in Table 4. e strength properties were taken from an average of three samples in tensile test. From the results, the welded joint yields inferior strength than the parent material in as received conditions because of the differences in microstructural features, orientation of grains, and sizes in different regions. It affects the plastic deformation behavior during tensile load. However, the temperature rises above 200 degrees, and it causes the reversion of precipitate (θ") at WZ and HAZ on cooling. is may be the reason for lower hardness and strength. P-CMT weld conceived a maximum tensile strength of 303 MPa, and it is 33% lower than the base material strength. e yield strength and percentage of elongation of CMT weld were 258 MPa and 6.5%, respectively ( Figure 2). e reason for the material in as received condition possesses maximum strength is the presence of strengthening precipitates in its matrix and tempering condition. e P-CMT joint showed an efficiency of 67%.

Hardness Survey.
e hardness measurement across the transverse direction of weld is shown in Figure 3. e hardness distribution in the materials depends on two criteria such as size and distribution of precipitates and size of the grains. It is an inverse proportion to the grain size and precipitate size. e formation of recrystallized grains and distribution of strengthening precipitates in CMT weld is carried out by the heating and cooling process during thermal cycle. Hence, the hardness distribution has a direct relationship with weld strength. e weld region conceived a peak hardness of 88 HV and HAZ was 72 HV. Moreover, the HAZ is the weaker region irrespective of the welding process. Due to formation of the soft region [24], HAZ is adjacent to the fusion zone, where this zone experienced severe heat input followed by a slow cooling rate. As results, the grains were coarsened. e precipitate in HAZ may be coarsened or partially destructed. Moreover, the dispersion of strengthening precipitate was lower. Hence, the soft region was formed. is region will act as a crack initiation   point. Strengthening precipitate in as received condition is sensitive to the heat input during the welding cycle. It reduces the strength properties of joint, particularly, in the HAZ region. During artificial ageing of precipitation hardening aluminum alloy (AA2014-T6), the primary precipitates are needle-like (θ"), which is the main strengthening precipitate in α-aluminium [25]. e spread of precipitate is uniform in the matrix. Due to thermal driving forces, the needle-like precipitate was transferred into a metastable (θ') precipitate whose presence along with aluminum becomes predominant in the WZ.
is phenomenon is the main factor to reduce the hardness and strength. Moreover, the precipitate changes from θ" to θ', causing a soft zone in the WZ [26].

Microstructure.
e macrograph of P-CMT joint is revealed in Figure 4(a). No visible flaws were observed in the welded joints. e fusion zone of the weld showed brighter appearance than the other region due to recrystallization of grains and the geometry of the weld exhibits in elliptical shape. e mean value of grains in the weld (Figure 4(b)),      Advances in Materials Science and Engineering partially melted zone (Figure 4(c)), and heat affected (Figure 4(d)) zone was 35 μm, 38 μm, and 45 μm, respectively. e fusion zone of P-CMT weld is framed with columnar grains with the dendritic structure. e grain morphology varies in the order of equiaxed dendrites and coarse dendrite. e equiaxed dendrites begin from the fusion line to the center of the weld and are perpendicular to the fusion line and parallel to the weld line. As the rate of solidification slows down, the amount of solute in the solution increases. Furthermore, the field of constitutional super cooling significantly expands. When the size of equiaxed dendrites and its respective volume present in liquid weld metal exceed a certain stage, columnar dendrites stop growing and turn into equiaxed dendrite.

Fractography.
e SEM (JEOL India Pvt. Ltd. Type: 6610LV JSM Oh, Japan) analysis was conducted primarily to clarify the failure mode (i.e., both brittle or ductile). e fracture morphology of tested specimens is shown in Figure 5.
e fractography does provide useful information about the role of microvoids on strength properties of joint. e fracture surface of high-strength aluminium alloy in as received condition showed fine and deep dimples ( Figure 5(a)), whereas wider and shallow dimples were found in the joint welded with P-CMT. It suggests the larger fracture energy stored during tensile load. Although, the fracture was propagated along the grain boundary, it suggests the fracture mode was intergranular.

Discussion
e P-CMT welded AA2014-T6 joint revealed maximum tensile strength of 303 MPa compared to 455 MPa of parent metal. is extends joint efficient up to 67%. By short circuiting and current pulsing, the highest weld strength is due to wire retraction, resulting in the development of refined dendritic grains in weld metal, enhancing the tensile properties of CMT joint in the pulsing mode. Coarse grain formation with partial destruction or size variation in harder precipitates in the HAZ area and is the cause for the breaking point in the HAZ zone [24]. As compared to arcing and short circuiting, the welded metal microstructure of P-CMT joints yields refined dendritic grains that are columnar in shape since the process is connected to a pulse stage with adequate heat input [19]. is pulsing effect, along with the faster cooling rate, helps the grains refine thoroughly. e microstructure of the welding metal is not as refined as it is for CMT joints [27]. e hardness properties in different regions of the joint were based on its microstructural behavior due to the unavailability of pulsation compared to the P-CMT joint [28]. Its high hardness was given by various Advances in Materials Science and Engineering intermetallic compounds of Mg, Cu, and Al or by two of them produced near the fusion region [29,30]. e weld metal revealed somewhat higher hardness associated to the HAZ zone; although dissolution of precipitate in the weld zone was completely. e region adjacent to the fusion zone is a soft region. Due to the microsegregation of brittle eutectic (α-Al) + θ"), when the grain boundary is heated above the eutectic temperature, the liquation will partially form [31]. is eutectic microsegregation weakens the PMZ, and in the tensile strains, it can result in intergranular cracking. e PMZ is differentiated in the fusion welding process by the formation of equiaxed to columnar grains caused by high heat input. And, the volume of microsegregated eutectic rises as the peak temperature increases, while the concentration of solutionized Cu in the alpha aluminium decreases. In addition, the high concentration electromagnetic force produces a constitutional super-cooling effect to the P-CMT process in the fusion region, which helps regulate phase division tremendously. e hardness of the fusion zone was, therefore, superior to the other zones. e mechanism behind the formation of weld in highstrength aluminum alloy is the enhancement of the solid solution with Cu and Mn. In the softened area, the minimum hardness was by coarsening or complete destruction of harder phases (precipitates) [24,[32][33][34]. e greater electromagnetic force contributes the P-CMT process to a constitutional fastercooling impact that helps to regulate the division of stages immensely. In P-CMT, the pulsing effect is similar to the CMT process, which helps to dramatically monitor the degree of phase segregation and distribution in the weld region. By the low heat input (0.164 KJ/mm) during the welding cycle, brittle phase and variation in precipitate intensity were reduced considerably. With the formation of weld metal, a common soluble zone was created between the α-aluminium and the welded metal. To disclose the cause behind the fracture point in the welded joint, EDS analysis was performed on the fractured sample. Most researchers claimed that HAZ was the fracture region of the fusion welding process. is area has since lost its precipitate reinforcement due to heat sensitivity during the welding cycle. EDS analysis (Figure 6)

Conclusions
e microstructural features and tensile properties of AA2014-T6 joints developed under optimized condition of P-CMT welding were evaluated, and conclusions were made as follows: (i) e AA2014-T6 aluminium alloy can be welded successfully using pulsed mode of cold metal transfer-welding (CMT) process without defects of cracking and porosity.  Advances in Materials Science and Engineering (ii) e P-CMT AA2014 joints developed using a welding current of 110 A, a welding speed of 450 mm/min, a voltage of 12 v, and a wire feed rate of 5500 mm/min disclosed a maximum tensile strength of 303 MPa compared to 455 MPa of parent metal. is extends joints efficiency up to 67%. (iii) e enhancement in strength of P-CMT AA2014 joint is attributed to the combination of the wire's withdrawal motion and pulsing effect in the P-CMT process. e related pulsing mechanism, as well as the arcing and short-circuiting phases, help in refining the dendritic grains in weld metal which aids in controlling the eutectic phase segregation. is enhances the hardness of the weld metal region. (iv) e failure occurred at the HAZ of P-CMT AA2014 joint due to the formation of lower eutectic elements in the HAZ and grain growth which lowers the hardness of HAZ compared to weld metal.

Data Availability
e data used to support the findings of this study are included within the article.

Conflicts of Interest
e authors declare that they have no conflicts of interest.