The corrosion-resistant and strength-to-weight ratios are the primary factors in high-strength aluminum alloy. Hence, the AA2024 alloy is a possible candidate in the critical structural fabrication industry. The traditional joining method is ineffective for welding aluminum alloys. Higher melting point and temperature variations cause alloy isolation; porosity and hot cracking are caused by melting point variations. As a result, to fabricate joints, a light heat source laser beam was used. The weaker area of most fusion-welded joints was the heat-affected zone (HAZ). The post-weld heat treatment was used at HAZ to improve the properties. According to the experimental findings, the joint welded with solution treatment and artificial aging had a maximum tensile strength of 358 MPa. Re-precipitation of precipitates may accomplish in HAZ.
The high-strength aluminum alloy is the most prevalent material in structural fabrication industries, such as automobiles and aircraft due to its unique properties, high strength to weight ratio, and excellent corrosion resistance. The joining of this alloy by fusion welding is quite tricky. Alternate joining processes, solid-state welding, and low-energy beams welding have been used. Laser beam welding is one of the best-joining processes for welding aluminum alloy by the controlled heat input. In a comparative review, Zhan et al.  studied the laser beam welding (LBW) and electron beam welding (EBW) of 5A06 aluminum alloy. The assessment of microstructure and mechanical properties was carried out systematically. The effect of LBW on the mechanical properties of a 70/30 Cu–Ni alloy weld was investigated by Chakravarthy et al. . Welding speeds of 1.0–2.5 m/min were used to create the joints. Due to the formation of equiaxed grains and fine distribution of precipitates, the joint welded at 1.5 m/min had better mechanical properties. Sufizadeh et al.  examined the impact of Nd:YAG LBW of AISI 4340 and AISI 316L on weld shape and mechanical characteristics. The results revealed that with the pulse energy and frequency rise, the weld grain size and HAZ size increase. Wang et al.  examined the impact of the welding process on the microstructure’s characteristics of a LBW aluminum alloy joint. Hardness at the fusion line and in the fusion area, elongation and strength rise first, then drop with increasing heat input. Vyskoč et al.  examined the influence of joining factors on the characteristics of AA5083 aluminum LBW joints. The joint fractured due to the presence of Al2O3 particles. Zhan et al.  examined the impact of an externally supplied constant magnetic field on the morphology of a LBW joint composed of 4 mm thick 2024 aluminum alloy. At various laser energy and magnetic flux strengths, microstructures and part distributions were also investigated. Component aggregation and grain coarsening along the fusion line were helped by inhibiting Marangoni convection using Lorentz force.
Oliveira et al.  examined the influence of LBW on the strength of various 2024–7075 T-joints. Weld shape and porosity were affected by changes in welding parameters. However, they cause substantial changes in weld microstructures, but liquid cracking in the heat-affected zone of AA7075 and AA2024 was reported. Solidification cracking in 2024 aluminum alloy pulsed LBW was predicted by Sheikhi et al. . LBW of aluminum alloys was connected to solidification cracking. Several factors, including the starting temperature of the base metal and the temporal amplitude of the laser pulse, influenced the degree of cracking in these alloys. Srinivas et al.  investigated post heat treatment (PWHT) of different aluminum alloys using LBW. The PWHT method was utilized to simulate the aging process. The microstructural tests revealed that the grain size had changed significantly. The hardness profile alteration in AA6061 was more evident than in AA5083, according to microhardness findings. The tensile strength with the highest value was 246 MPa. Bal et al.  looked at how PWHT affected the tensile strength of LBW Hastelloy C-276 sheets at different heat inputs.
LBW is considered an essential welding process for the joining of aluminum alloy components due to its laser beam density property and focusability. It offers many advantages such as deep and narrow weld, less distortion, small heat-affected zone, and good mechanical properties . Nowadays, this technology can be used for welding 3D complex structures. By applying the conventional welding process, the heat input in the weld region is usually high. Due to slow cooling rates, it also promotes grain coarsening and wider heat-affected zones. Since the rate of heat dissipation is less in the conventional welding process, the weld strength of AA2024 aluminum alloy may not be satisfactory and might lead to weld defects. In order to avoid these weld strength-related issues, LBW is being used. Though LBW provides better strength compared to conventional welding, minimal welding defects such as low porosity and keyhole defects may arise due to their hydrogen entrapment in the weld region. As a result of extended cooling of various grades of aluminum alloy in critical sub regions, softening in HAZ of welded joints occurs. This affects the strength and other mechanical properties of the welded joints . In order to avoid such issues in HAZ, PWHT processes can be deployed on the weld for improved weld strength and other characteristics.
From the available literature study [1–12], a substantial amount of study has been done on the influence of process parameters on LBW joints of AA2024. Only few papers reported the effect of PWHT on tensile strength. Hence, an attempt has been made to compare the as welded joint with different heat treatment techniques such as aging, solution treatment, and solution treatment followed by artificial aging on tensile and microstructural behavior of LBW joints.
2 Experimental Work
AA2024 with 2 mm thick was used as a base material (BM) in this work. Tables 1 and 2 show the chemical and mechanical characteristics of BM, respectively. A do-all machine was used to cut the 75 mm × 75 mm test coupon. The buffing wheel was used to remove the burrs at first. The joints were made using laser beam welding (Table 3). The various regions in LBW are represented in Figs. 1(a) and 1(b). Figure 2(a) shows the micrograph of BM and photograph of joint represented in Fig. 2(b). The significance of post-weld heat treatment is used to enhance the lost properties in the weld region by re-precipitation of precipitates and density distribution of precipitates in the weld region. It was performed by solution treatment (ST), artificial aging (AA), and the combination of ST followed by AA. Both ST and AA processes were carried as per the ASME standard (volume 4) and literature review. AA, ST, and solution treatment followed by artificial ageing (STA) were the three types of PWHT. The aging procedure was done at 175 °C for 8 h (soaking time) and atmospheric air was used as the cooling medium. The solution treatment was done at 500 °C for a soaking period of 1 h. Then the samples were quenched in cold water immediately. Five samples were produced for each condition. Table 4 shows the tensile strength of as-welded (AW), AA, ST, and STA joints. The hardness characteristics of the joints were measured using a Vicker microhardness tester with a 0.5 N load and a 10 s dwell. A scanning electron microscope (SEM) was used to examine the fracture pattern of the tested materials. The elements present in the welded area were discovered via X-ray diffraction analysis. To determine the size and form of the precipitate, a transmission microscope was employed.
3 Result and Discussions
3.1 Effect of Post Weld Heat Treatment on Microstructure.
The micrograph of various regions in LBW joint treated with different heat treatment is shown in Figs. 3–6. The microstructure evaluation was taken at different locations: exemplary side interface, left side interface, topside, middle, and root side. In the age-hardening AA2024 aluminum alloy, the BM microstructure consists primarily of elongated grains in the rolling direction with a uniform distribution of CuAl2 precipitates. LBW changes the elongated grains of BM in the weld area. LBW is a low-heat-input, high-speed operation, so there is not enough heat available for grain growth over a long period. One of the explanations for fine-grained microstructure in the weld area may be this. Even though LBW involves low heat input and high speed, the interface region of weld metal and BM consists of a narrow heat-affected zone. Micrographs taken in the cross-section of the welds show a noticeable difference in grain size in the thickened section (Figs. 3 and 6(c)–6(e)). This may be because the LB diameter is reduced at the bottom of the plate during beam penetration to greater depths. As a result, less heat was produced at the bottom of the weld, i.e., there is a likelihood of a temperature gradient in the thickness direction. Therefore, the solidification rate varies from top to bottom. According to the optical, there is no discernible difference in grain size due to PWHT procedures on the top surface or in the cross-section of welded joints (Figs. 3 and 6(c)). The distribution of precipitates (CuAl2) and the scale of the precipitates show a significant difference. Since the precipitates were partly dissolved by the welding process, the weld region of an AW joint contains a much smaller number of precipitates (Fig. 7(a)). The precipitates were fully dissolved due to the solution temperature, the weld area of the ST joint is free of precipitates (Fig. 7(b)), followed by rapid quenching. Fine and coarse precipitates can be found in the AA joint’s weld area (Fig. 7(c)). Due to the agglomeration of precipitates, the left-over precipitates expand in size and become more prominent during welding. Due to the aging temperature, new precipitates are formed in the solid solution, smaller in size. STA joints have very fine precipitates in the weld zone (Fig. 7(d)). All precipitates were fully dissolved in the solid solution due to solution application, and no precipitates remain in the matrix. Since the precipitates emerge from solid solution immediately during aging, they are magnificent and evenly distributed in the matrix. The BM has a thick, closely spaced dislocation cell structure, which may be attributable to the material’s previous metalworking operations. The dislocation cell structure is not visible in the weld region of the AW and ST joints, which may be due to welds being exposed to the solutioning temperature during welding and PWHT. The dislocation cell structure in the weld region of the AA and STA joints is uniformly distributed and transparent, which may be attributable to dislocation movement during aging treatment.
3.2 Effect of PWHT on Strength.
The strength properties of LB welded AA2024 aluminum alloy are shown in Table 4. The yield and tensile strengths of BM are 325 MPa and 465 MPa, respectively. The yield and tensile strengths of AW joints are 210 MPa and 264 MPa, respectively. This means that LBW has resulted in a 40–45% drop in strength values. PWHT techniques improve the strength of welded joints in a variety of ways. The joints treated with STA had the highest strength values of the three PWHT procedures. STA joints have a yield strength of 289 MPa and tensile strength of 369 MPa, respectively, with a substantial increase of approximately 40% over AW joints. Similarly, AA joints’ yield strength and tensile strength were found to be 256 MPa and 336 MPa, respectively, with just a 20% increase in strength over AW joints. The ST joints’ yield strength and tensile strengths were 241 MPa and 303 MPa, respectively, with a 13% increase in strength values over the AW joints. The yield strength and tensile strength of LB welded AA2024 aluminum alloy joints have increased with PWHT procedures; among the three PWHT methods, STA substantially improves yield strength and tensile strength of LB welded AA2024 aluminum alloy joints.
3.3 Effect of PWHT on Ductility.
The ductile properties of BM and heat-treated joints are presented in Table 5. This indicates that LB welding reduces ductility by around 30%. STA joints deform to a greater extent than other joints due to plastic deformation. Compared to AW joints, STA joints have a 13% elongation and a 40% increase in ductility. Similarly, AA joints have an elongation of 11.6% and 8.5%, respectively, with a beneficial increase of just 30% as compared to AW joints. Compared to AW joints, the ST joints have demonstrated a 10.5% elongation and a 16% increase in ductility. It is evident from the observations that LBW has significantly decreased the tensile strength of AA2024 aluminum alloy. Of the three heat treatment methods employed, the combination of ST with AA gives the highest improvement in tensile characteristics. The tensile characteristics of LB welded aluminum alloy were improved on the application of simple aging treatment. Weld region failure was one of the prominent defects that was observed in most of the tensile specimens. This also proves that the weld region is weaker than the other sections. Based on the hardness test, it is also found that the properties that determine the harness of the weld are affected by the factors, namely, the distribution and size of the CuAl2 precipitates [13,14]. According to microstructural examination, weld metals are always made up of small, equiaxed grains.
These strengthening precipitates are formed as a result of the ST and subsequent AA. During welding, these precipitates should disintegrate, due to the rapid cooling rates required in welding. However, not all of them melt, and only a few survive as a fine spherical precipitate in the matrix. Because no additional precipitates can develop because no filler alloy was employed, the precipitates must be solely composed of Cu and Al. The form of these precipitates, on the other hand, is unlike any known Cu- Al precipitate, such as Guinier–Preston (GP) zones, these precipitates are the result of the precipitates disintegrating in BM prior to welding. Al dendrites and primary CuAl2 phases were found in the LBW region. PWHT precipitates and dislocation networks were obtained after PWHT. Precipitates shape as a result of age, increasing the micro-hardness (Fig. 6(f)). The formation of phase is favored by increasing the heating rate, and the alloy in the AW state has a large number of dislocations, which serve as nucleation sites for the phase. As a result, they discovered that microhardness increases as the number of passes increases. Rapid heating and cooling in the PWHT prevent plate-like formations from developing . The CuAl2 precipitates have completely disintegrated as a result of natural ageing.
Previously, Zhan et al.  revealed that the particles are rounded in the very early phases of precipitation, which was also seen in LBW. As a result, the precipitate particles in LBW metal may have developed after the weld metal had melted completely during the welding operation owing to natural aging. Due to the welding heat, the precipitates coarsen significantly in LBW. The rapid heating in LBW allows the weld and HAZ regions to reach solution temperature. Furthermore, since the LBW is a faster-joining process, the cooling rate is faster during the cooling process . As a result, the AW condition resembles the ST condition. The welded joints are allowed to cool at room temperature and are not quenched as the immediate quenching of welded material would cause surface brittleness. In order to retain the dissolved copper in the aluminum matrix, the welded material is made to undergo solution treatment where the material is rapidly quenched in a cold water. As a result of natural aging, the precipitate particles in LBW metal may have formed after the weld metal had completely melted during the welding procedure. The width of HAZ and the volume of phase precipitated across grain boundaries diminish as the heat input is reduced and the cooling rate is increased in LBW. Solid solution strengthening is the main strengthening process in the as-welded condition .
A nonuniform dispersion of solutes caused a heterogeneous spreading of particles in the post-weld state, resulting in strain localization. Due to a homogeneous dispersal of strengthening precipitates, the solution heat-treated and aged conditions produce the most significant overall weldment features. The ageing treatments increased the YS, UTS, and elongation of the weld metals. The intensity of the weldments increased by about 25% with age, but the elongation decreased from 2.5% to 1.1%. (Al3 Li) and (Al2CuLi) are the critical strengthening precipitates in the aluminum matrix. The (Al3 Zr) is a dispersoid particle that is used to refine grains. The strength and elongation of the weld metals are regulated by the distribution and morphology of these precipitates [16,17]. In the case of ST joints, the precipitate particles that form as a result of natural aging of the weld material after it has fully melted during the welding phase have dissolved in the matrix due to solutionizing (Fig. 7(b)), resulting in fewer precipitate particles than in AW joints. The precipitate particles in AA joints (Fig. 7(c)) have increased in size considerably and appear as bigger and incoherent particles due to prolonged heating. Related to the other PWHT joints, the STA joints have an exact and uniform distribution of precipitates (Fig. 7(d)). This may explain why STA joints have better tensile and hardness properties.
3.4 Effect of PWHT on Microhardness.
The hardness distribution in various region in LBW joint under different treatment is shown in Figs. 3 and 6(f). BM has a hardness of around 140 VHN when it is in the T4 state. On the other hand, the AW joint has a hardness of 105 VHN at the weld core. This indicates that LBW reduces the hardness of the weld center by 35 VHN. The PWHT procedures have enabled the hardness level at the weld center to be restored. Out of the three PWHT joints, the STA joints have a hardness of 140 VHN, which is 35 VHN higher than the AW joints. On the other hand, ST joints had a hardness of 110 VHN, which is 5 VHN higher than AW. The hardness of AA joints was 120 VHN, which is 15 VHN higher than that of AW. Fine grain heat affected zone (FGHAZ), coarse grain heat affected zone (CGHAZ), and BM regions have all shown a similar pattern. The FGHAZ region has a higher hardness than the WM region, which may be due to the formation of excellent recrystallized grains in that area. Not only does the size and distribution of precipitates affect the weld region’s hardness [20,21], but the composition of precipitates also affects the weld metal hardness. Copper-rich precipitates are more resistant to indentation, which may be one of the reasons for the higher stiffness of base metal and STA joints (140 VHN). Precipitates with a high aluminum content are less resistant to indentation, which may cause the AW and ST joints’ lower stiffness (110 VHN). Furthermore, the hardness of the weld region is influenced by the development of the dislocation cell structure in the weld region. Typically, a thick dislocation cell structure with very close spacing indicates the amount of strain hardening the material has experienced during previous metalworking operations. Strain hardening often increases the material’s stiffness, which may be one of the reasons for the higher hardness of base metal and STA joints [18–20]. Because of the first work hardening conducted on the material to attain the T6 state, the BM has a dense, meticulously spread-out dislocation cell structure.
The disarticulation cell structure in the weld region was disturbed by the welding heat. After treatment with a solution, the dislocation cell structure is completely removed. This is another reason for the reduced hardness of AW and ST joints. The aging method on AA and STA joints was helped by the restoration of the dislocation cell structure in the weld areas. The reformation of the dislocation cell structure increased the hardness of the weld zone of AA and STA joints, despite the fact that the cell structure is widely separated. The hardness of the weld top surface follows a similar pattern, but there is a 5–10 VHN reduction compared to the cross-sectional hardness. This may be due to the presence of coarse and elongated grains on the welds’ top surface. As compared to refined grains, coarse elongated grains have a smaller grain boundary region, resulting in less resistance to indentation and plastic deformation . This may be one of the factors for the lower hardness of the welds’ top surface. The results of hardness measurements of LBW of AA2024 aluminum alloy can be used to infer the following essential points: regardless of PWHT, the WM region has a lower hardness than the HAZ and BM regions. AW joints have a very low hardness (105 VHN), while STA joints have the highest (140 VHN). PWHT procedures are effective in increasing the hardness of the WM region of LBW of aluminum alloy.
3.5 Transmission Electron Microscope Micrograph.
The weld region is composed of coarse and fine precipitates, but formed in two morphologies such as coarse or more refined precipitates, depending on the heat input and dissipation during the LBW cycle. Figure 7 shows the TEM micrograph of AW, AA, ST, and STA joints. All the contained different sizes and shapes in the weld. The AW joint revealed coarse and fine needle-like precipitates, which are identified by energy dispersive X-ray spectroscopy (EDS) analysis. It may be the combination of Al, Cu, Mg, or Al, Fe, Mn. EDS analysis results are presented in Table 6. The fine precipitate in the range of 50–100 nm and coarse precipitate in the range of 100–500 nm were observed. Fine precipitates were dissolved in the AW joint with some coarse precipitate. Whereas ST joint showed complete dissolution of precipitates with no more significant in size precipitates. The AA treated joint revealed coarse precipitate, which is attributed due to aging. This precipitate has grown somewhat bigger due to artificial aging treatment. Because the treatment was carried out at 175 °C with a soaking period of 8 h, since it takes a prolonged time to cool, it causes coarse precipitate formation by consuming adjacent precipitates. Hence, it showed a bigger size. By the EDS analysis, this precipitate may be composed of Al–Cu–Fe. It may be Al2(FeMn6). On the other hand, the STA joint revealed uniform distribution of precipitates with fine particles. The density distribution of precipitate is due to the reprecipitation of precipitate during the aging process, which gives density distribution of precipitates in the weld [22–24]. Hence, the STA joint conceived superior mechanical properties than the other joints.
The fracture surface of LB welded joints in AW and heat-treated joints is shown in Figs. 8(a)–8(d). The formation of fractures pattern is based on the grain size. The fracture surface of the AW joint revealed coarse and elongated dimples, suggesting that a large stretch zone was presented at the tip of the crack. It could be by the recrystallized grains in the weld region. On the other hand, the heat-treated joint such as ST, AA, and STA joints revealed various fracture morphologies. The difference between the AA and ST joint fractograph showed a minor variation. It indicates that the influence of the recrystallization process and precipitate distribution at the weld region was less. In contrast, the surface morphology of the STA joint revealed a very fine and deep dimple. The large stretch zone was presented at the crack’s tip due to immense strain energy before fracture. Fine and deep dimples occurred due to the formation of fine and density distribution of strengthening precipitates. This precipitate (CuAl2) had given an anchoring effect during plastic deformation. Hence, the STA joint has higher strength than other joints.
From this investigation, the laser beam welded AA2024 aluminum alloy joints were post-weld heat treated successfully.
The joints treated with STA had the highest strength values of the three PWHT procedures. STA joints have YS and UTS of 295 MPa and 370 MPa, respectively, with a 40% increase in strength over AW joints.
With aging treatments, the weldments’ UTS, YS, and elongation increased. The intensity of the weldments improved by about 25% with age, but the elongation decreased from 2.5% to 1.1%.
The STA joints have a hardness of 140 VHN, which is 35 VHN greater than the welded joints of the three PWHT joints. ST joints, on the other hand, exhibited a hardness of 110 VHN and 5 VHN greater than welded joints. AA joints had a hardness of 120 VHN, which was 15 VHN greater than as welded joints.
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