Effect on microstructure and mechanical properties of friction stir welded 5A06 aluminum alloy joints by deep cryogenic treatment

In order to improve the comprehensive mechanical properties of the welded joints of the 5A06 aluminum alloy, friction stir welded (FSW) joints were subjected to deep cryogenic treatment (DCT). The microstructure and mechanical properties were characterised using metallographic microscopy, x-ray diffractometer (XRD), energy spectrometer, microhardness tests, and tensile tests. The experimental results show that DCT refines the structure significantly due to the large temperature difference. This refinement results from an increase in Mg atoms within the α-Al solid solution through the precipitation of Al atoms, forming the Al3Mg2 phase. This enhancement in plasticity is achieved through dispersion distribution. Moreover, as the treatment time of DCT increases, the mechanical properties of the welded joint also improve significantly. The microhardness of the welding joint peaked at DCT3h, rising from 78.8 HV at DCT0h to 87.2 HV at DCT3h. Meanwhile, the tensile strength of the joint reached its maximum at DCT12h, rising from 358.7 MPa at DC0h to 385.3 MPa at DCT12h, representing a 7.4% increase. These experimental results underscore the significant impact of DCT on improving the welded joints.


Introduction
Aluminum alloys, being a novel type of metal, exhibit exceptional performance in comparison to carbon steel.They are characterized by their high strength, superior formability, and machinability, as well as excellent corrosion resistance.Consequently, they have gradually emerged as a viable alternative material [1].Among the high-strength 5xxx Al-Mg alloys, the 5A06 aluminum alloy has found extensive applications in pressure vessels, ships, automobiles, aerospace, and airlifts.Due to its elevated strength-to-weight ratio and hardness, the 5A06 aluminum alloy offers more favourable weldability and improved corrosion resistance as compared to the nonheat-treatable Al-Mg alloys [2].The 5A06 aluminum alloy possesses remarkable potential for diverse industrial applications owing to its exceptional properties.Prospective investigations can concentrate on enhancing the mechanical properties and optimising the manufacturing process of this alloy, thus rendering it an exceedingly competitive material in the industry.
The traditional technique of arc welding poses challenges when it comes to joining aluminum alloy due to the occurrence of welding defects such as hot cracks, significant distortion, and high residual stress [3,4].Consequently, the widespread application of aluminum alloy is hindered.Regarding the welding of 5A06 alloy, numerous scholars have employed various high-efficiency welding methods to enhance the performance of the joint, such as laser welding [5], variable polarity plasma welding [6], and laser-arc hybrid welding [7].However, all the joints obtained through these welding methods still exhibit welding defects such as pores and cracks, and it is impossible to achieve a perfect and defect-free welded joint.However, friction stir welding (FSW) presents itself as an innovative solid-state welding approach that was invented by The Welding Institute (TWI) in 1991.FSW has demonstrated its exceptional aptitude for joining aluminum alloys [5,8].FSW is highly suitable for welding 5A06 aluminum alloy, as it can avoid welding defects caused by traditional fusion welding.However, the softening issue caused by FSW seriously affects the strength of the joint, limiting the application of 5A06 aluminum alloy.
Experiments on friction stir welding (FSW) joints were conducted by a cohort of researchers.Nevertheless, the investigations pertaining to FSW revealed that a notable defect associated with this welding technique is the influence of thermal-mechanical coupling on the heat affected zone [9].Numerous endeavours have been undertaken to rectify the mechanical attributes of FSW joints, with a study being the application of a post-weld aging heat treatment.
Previous investigations have demonstrated that the application of heat treatment after welding can effectively enhance the tensile strength.Chen et al [9]found that post-weld heat treatment significantly improves the mechanical properties of 2219-O FSW joints.Surveys conducted by Elangovan [10] indicated that the simple artificial aging treatment is more advantageous in enhancing the tensile properties of FSW AA6061 aluminum alloy joints compared to other treatment methods.V Varghese et al [11] presented the impact of post-welded heat treatment on the mechanical properties of butt joints using FSW in aluminum alloys 2024-T6 and 7075-T6.It has been observed that the tensile properties of the butt FSW joints, when subjected to post-welding heat treatment at a low temperature (200 °C), are comparable to those of the as-FSW joints.Liu et al [12] studied on investigating the influence of post-weld heat treatment on the tensile properties of FSW joints made from 2219-T6 aluminum alloy.The tensile strength of the joints was found to increase, while the elongation at fracture decreased because of post-weld heat treatment.
Although the use of post-weld heat treatment in welding has been extensive, its application in heat-treatable aluminum alloys has been limited.Extensive investigation has been conducted in assessing the effectiveness of post-weld heat treatment, as mentioned above [13].However, an alternative technique known as deep cryogenic treatment (DCT), which involves treating materials at liquid nitrogen temperature (−196 °C) for a specific duration, has been explored as an extension of conventional heat treatment [14].Moreover, DCT can improve the joint strength without compromising other performance characteristics.Only a few studies have focused on investigating the DCT of FSW welds.For instance, Vinay Dwived et al [15] examined the impact of DCT on the micro-hardness of FSW Al 7050-T7451 and observed that cryogenically treated joints exhibited higher microhardness compared to those without cryogenic treatment.Similarly, Wang et al [16] studied the effect of DCT on the FSW joints of 2024-T351 aluminum alloy and discovered that pre-DCT facilitated the redissolution or dispersion of unstable phases in the as-welded joints.
Jincheng Guo et al [17] studied the microstructure and mechanical properties of Al-Cu-Mg-Ag alloy treated with DCT.The results showed that deep cryogenic treatment can promote the nucleation and growth of precipitate phases and introduce dislocations.The interaction of these two factors increased the yield strength of the alloy from 391.4 MPa to 472.7 MPa after DCT1h, significantly improving the mechanical properties of the alloy.Marwan Abbas Madhloom et al [18] studied the effect of cryogenic treatment on the microstructure and mechanical properties of 6061 aluminum alloy.The results showed that the hardness of the alloy increased from 64 HV to 74 HV after cryogenic treatment, and both the tensile strength and yield strength increased to different degrees.A large number of fine precipitate phases were observed within the structure, and many fine particles were also found in the fracture observation.
While previous studies have explored the topic of deep cryogenic treatment (DCT), there have been limited empirical examinations conducted specifically on DCT applied to Friction Stir Welded (FSW) joints.The primary objective of this investigation is to establish a theoretical foundation for understanding the impact of deep cryogenic treatment on the microstructure and mechanical properties of 5A06 aluminum alloy joints that have undergone FSW.Furthermore, this research also addresses potential mechanisms associated with changes in the microstructure.

Material and method
2.1.Material All the work in the experiments was carried out using aluminum alloy 5A06(Al-6Mg-Mn-Si) with the size of 300 mm × 150 mm × 5 mm, which were subjected to friction stir welding.The chemical compositions of 5A06 aluminum alloy are given in table 1.
To mitigate the occurrence of welding defects and to enhance the overall surface welding quality, it was imperative to eliminate both the oxidation film and oil soil that were present on the workpiece surface prior to the commencement of the welding process.The various welding parameters that exerted an influence on the stability of the operation encompassed the rotating speed, travel speed, tool shoulder, tool pin length, and tool tilt angle all of which were set at 1200r•min −1 , 150 mm•min −1 , 15 mm, 4.7 mm, and 2.5°respectively.
The welded joints underwent deep cryogenic treatment (DCT) by immersing them in liquid nitrogen.The parameters for the DCT experiments are detailed in table 2, following an analysis of the experiments.These experiments were categorized into five groups, with one group serving as the control, receiving no DCT.The remaining four groups were immersed in liquid nitrogen for 3 h, 6 h, 9 h, and 12 h, respectively.The DCT process curves are shown in figure 1.

Method
The microstructure of the samples was characterized using two different microscopy techniques: a LEICA DMi8 optical microscope (OM) and a Zeiss scanning electron microscope EVO 10 (SEM).To prepare these samples for examination, one specimen from each group in the specimen set underwent a polishing process using abrasive papers.Subsequently, all samples underwent electrolytic etching with Barker's reagent for 180 s to reveal the microstructure characteristics.
To perform tensile testing, standard specimens were prepared according to GB/T228-2018 guidelines, each having a length of 110 mm.The ultimate tensile strength (UTS) and percentage elongation (%) were calculated by conducting the tensile tests in the rolling direction.These tensile tests were carried out using an MTS testing machine at a constant speed of 1 mm min −1 .To ensure accuracy, three specimens were selected from each group in the specimen set, and the average values were recorded.The sampling location of the specimen is shown in figure 2. For the analysis of fracture characteristics, one fractured tensile specimen from each group in the specimen set was subjected to further examination using a scanning electron microscope (SEM).This analysis helped determine the nature of the fracture.
To assess the micro-hardness distribution perpendicular to the weld direction, the TMVS-1 Vickers hardness instrument has been used with a 500 g load and a 10-s loading time.Vickers hardness measurements at 1 2 h 0.5 mm intervals were performed, starting from both sides and moving towards the weld centre.In addition to hardness measurements, the phase composition of the welded joint was analysed using an x-ray diffractometer (XRD).

Welding joint profile
The morphology of the FSW welded joint in cross-section is shown in figure 3. Upon overall observation, owing to variations in material flow behaviour and thermal cycling, the FSW joint is consequently segregated into four distinct regions: the weld nugget zone (NZ), the thermo-mechanically affected zone (TMAZ), the heat-affected zone (HAZ), and the base metal (BM).The microstructure in these distinct regions displayed variations in size and morphology.As a result of the heat effect, the microstructure in the HAZ is coarsened in comparison to the other regions, and its properties are inferior.Therefore, the changes in microstructure and properties in this region due to low-temperature treatment were examined as the primary focus.

Effect of DCT on grain structure
The metallographic microstructure of the Heat-Affected Zone (HAZ) in the welded joints subjected to varying DCT durations is presented in figure 4. The microstructure predominantly comprises elongated striated grains and punctate precipitates.It is evident from the illustration below that DCT contributes to grain refinement, resulting in a reduction in grain size with increased cryogenic exposure.As demonstrated in figure 4(a), the grains in the absence of DCT are coarse and unevenly distributed.Utilising the intercept method, the grain size in the joint without DCT measures between 36 μm and 43 μm parallel to the rolling direction, and between 22 μm and 26 μm perpendicular to the rolling direction.Figure 4(b) displays the welded joint structure after 3 h of Deep Cryogenic Treatment (DCT).While the grain distribution remains uneven, some grain sizes have undergone noticeable changes.Upon measurement, it was determined that the grain sizes parallel to the rolling direction fall within the range of 37 μm to 41 μm, while those perpendicular to the rolling direction range from 18 μm to 21 μm.Moving on to figure 4(c), it illustrates the welded joint structure after 6 h of DCT.Here, the number of smaller grains increases with the extended cryogenic treatment time.The measured grain sizes parallel to the rolling direction are within the range of 26 μm to 33 μm, and perpendicular to the rolling direction, the grain sizes range from 14 μm to 19 μm.
In figure 4(d), the welded joint structure following 9 h of DCT is presented.In this case, we observe a growing number of smaller grains and a relatively uniform distribution of grain structures.The grain size parallel to the rolling direction ranges from 26 μm to 30 μm, while perpendicular to the rolling direction, the grain sizes are between 12 μm and 14 μm.
Finally, figure 4(e) exhibits the welded joint structure after 12 h of DCT.In this instance, the microstructure becomes finer and more uniform.After conducting measurements, it was determined that the grain size parallel to the rolling direction ranges from 24 μm to 26 μm, and perpendicular to the rolling direction, the grain sizes are between 12 μm and 14 μm.
According to Debby's heat capacity theory and the stress equation [17]: As for the aluminum alloy 5A06, the linear expansion coefficient a is 20.8 × 10 -6 K −1 .The initial temperature T 0 is initial (20 °C) and the liquid nitrogen temperature T 1 is −196 °C, therefore, the temperature drop ΔT is 216 K, and the calculated compressive stress is about 211 MPa.The volume of alloy will shrink under compressive stress during DCT in the form of α (∂V/ ∂T) ρ according to the general relationship between temperature and volume [19], where a stands for the linear expansion coefficient.The initial temperature and volume are assumed as T 0 and V .
0 When the temperature is changed to T , 1 the final volume V T can be deduced from E : Therefore, the volume shrinkage rate f can be calculated by E : The volume shrinkage rate was determined to be 0.45% through calculations.It is important to note that under the influence of compressive stress, the presence of point defects in the crystal structure is temperature dependent.As the temperature of the crystal structure decreases, the number of point defects within the crystal structure increases, as expressed by equation [20]: N d represents the number of point defects, N denotes the total number of atomic sites, E d stands for the activation energy required for point defect formation, K is the Boltzmann constant, and T represents the absolute temperature [21].As a result, the sub-zero treatment induces the formation of high-density crystal defects and dislocations within the matrix, as depicted in figures 5 and 6.
In figure 5, the diagram illustrates the process of dislocation formation during DCT.As per the laws of thermodynamics, entropy becomes zero at absolute zero temperature [22].As the welded joints are immersed in liquid nitrogen, the atomic spacing decreases due to lattice contraction.The shrinkage disrupts the internal stress balance within the material, leading to the generation of strong internal stress.Such stress, in turn, results in significant lattice distortion and the creation of a multitude of dislocations.With an increase in immersion time, the stored internal strain energy continues to grow, providing the impetus for dislocation movement [23].
In scenarios of low strain energy, when the material returns to its initial temperature, strain energy can be released through the formation of new dislocations and the short-range movement of existing dislocations.The process serves to prevent grain fragmentation.Furthermore, due to the different stress states within grains caused by short-term cryogenic treatment, variations in free energy may occur, potentially leading to grain coarsening [24,25].After the welded joints have been immersed for an extended period, the strain energy gradually accumulates.Once this energy surpasses a certain threshold, dislocations arrange themselves in a manner that mostly aligns the grains along sub-boundaries when returning to the initial temperature.Consequently, the microstructure tends to form numerous fine sub-grains within the matrix, contributing to the refinement of the grains [26].This deep cryogenic treatment can refine the structure of the weld due to significant temperature variations during the cryogenic process.
Figure 6(a) presents the TEM image of the FSW joint prior to Deep Cryogenic Treatment.This initial joint displays a considerable internal grain size with minimal dislocation line distribution.In contrast, figure 6(b) illustrates the TEM image of the FSW joint after undergoing DCT.During the DCT process, the joint undergoes compressive stress, leading to a reduction in its volume, microplastic deformation, and the formation of numerous dislocation tangles and dislocation blocks within the joint.Consequently, the joint's interior is in a high-stress metastable condition.Under the combined effects of compressive stress and volume reduction,  strain energy is generated within the joint.This energy encourages dislocation movement through the continuous accumulation of strain energy.However, due to grain boundaries, dislocation motion within the grain is obstructed, causing dislocations to migrate toward the grain boundary via a combination of dislocation glide and dislocation climb.These dislocations accumulate at the boundary.Once a sufficient accumulation of dislocations is reached, dislocation rearrangement takes place.This results in the formation of new orientations with significant angular differences, promoting dynamic recrystallization and the creation of subgrain boundaries that divide large grains into smaller grains, effectively refining the grain size.Furthermore, highdensity dislocations offer additional pathways for atomic diffusion.Internal atomic diffusion in the pre-DCT joint is primarily dictated by grain boundary diffusion.In the post-DCT joint, a substantial number of dislocations are generated, effectively 'capturing' adjacent atoms and providing shortcuts for atomic diffusion.This reduces the activation energy of atomic diffusion and increases the diffusion coefficient, consequently accelerating atomic diffusion rates.This mechanism is particularly prominent in solid-state atomic diffusion processes at lower temperatures.Dislocations not only serve as paths for atomic diffusion but also offer additional nucleation sites for atoms.This facilitates the precipitation of the second phase and enhances its dispersion.

Effect of DCT on precipitates
Deep Cryogenic Treatment (DCT) exerts a significant influence on both the grain morphology of welded joints and the precipitation of phases [27,28].Given that these precipitates exert a strong impact on the properties of aluminum alloy, a variety of characterization methods, including SEM, XRD, EDS, and TEM, are employed to identify and distinguish these precipitates.In figure 7, we observe the microstructure of untreated welded joints and those subjected to DCT.The SEM images of the Heat-Affected Zone (HAZ) at different DCT durations predominantly consist of an α-Al matrix structure and white, granular β-Al 3 Mg 2 precipitated phases.It is evident from the figures that the quantity of precipitated phase increases progressively with longer DCT times.As the temperature decreases, the Mg and Si atoms within the α-Al matrix become unstable, leading them to accumulate around edge dislocations.This results in an observable phenomenon where the concentrations of Mg and Si increase around the dislocations.Additionally, metal compounds diffuse along dislocation lines and grain boundaries, forming precipitated phases within the welded joint.As the temperature decreases, the solubility of the base metal decreases, further contributing to the increase in precipitated phases.Deep cryogenic treatment has a profound impact on the FSW joint, affecting not only its grain size but also promoting the precipitation of the second phase.As shown in the figure 7, the microstructure of the FSW joint primarily consists of a grey Al matrix and white granular second phases.Figure 7(a) illustrates the HAZ structure of the joint in the absence of DCT.In this scenario, the grain structure is influenced by the welding thermal cycle, resulting in an expansive phenomenon.Only a few white granular second phases are randomly dispersed within the matrix.Moving on to figure 7(b), we observe the HAZ structure after 3 h of DCT.Although there is no significant change in grain size, there is a notable increase in the number of second phases, with most of them congregating along grain boundaries.Figure 7(c) presents the HAZ structure after 6 h of DCT, where the number of secondary phases remains steady.However, smaller second phases and a refined grain size are evident.In contrast, figure 7(d) displays the HAZ structure after 9 h of DCT.At this stage, the number of second phases has substantially increased, and some second phases exhibit aggregation distribution.Figure 7(e) reveals the HAZ structure of the joint subjected to 12 h of DCT.While the number of second phases remains relatively unchanged, the size of the second phase is considerably smaller and more dispersed.Additionally, the grain size is further refined.
As the FSW joint submerges in liquid nitrogen, the temperature of the joint decreases from room temperature (25 °C) to a frigid −196 °C.This drastic temperature change disrupts the joint's original stress state, resulting in the emergence of internal compressive stress and ongoing microplastic deformation.The interplay of compressive stress, microplastic deformation, and low temperatures places the material in an unstable, metastable state.As the temperature continues to drop, solute elements like Mg, Si, Fe, and Mn experience reduced solid solubility within the Al matrix of the joint.This reduced solubility becomes a significant factor that triggers the precipitation of the second phase [29].Throughout the process of Deep Cryogenic Treatment (DCT), numerous point defects and dislocations emerge within the joint.Supersaturated point defects and dislocations react with solute atoms at low temperatures, accelerating the precipitation of solute atoms and providing additional nucleation sites to enhance the precipitation of the second phase.However, with insufficient processing time for DCT, there is not enough driving force for the precipitation of the second phase, and an inadequate number of dislocations are available to serve as nucleation sites.Consequently, only a limited number of second phases form, clustering together.Upon reaching a DCT duration of twelve hours, the driving force for the precipitation of the second phase becomes substantial, with a sufficient number of dislocations promoting diffusion and providing the necessary nucleation sites.The gradual diffusion and distribution of the second phase contribute to the enhancement of the mechanical properties of the joint.In figure 8, the outcomes of the EDS elemental composition analysis of the precipitates are depicted.These precipitates are primarily located at grain boundaries following DCT.This location can be attributed to the higher dislocation density at grain boundaries, capturing more solute atoms.The EDS results of the precipitates at specific points consist of Al-Mg phases.
X-ray Diffraction (XRD) was employed to test and analyse the nature and content of the phase with and without Deep Cryogenic Treatment (DCT), as illustrated in figure 9.A comparison with the experimental XRD results reveals that cryogenic treatment significantly influences the precipitation of the β-phase.The figure clearly shows that the position of the x-ray diffraction peak remains unchanged after DCT.The stability indicates that no new phase forms as a result of DCT, and the microstructure still primarily comprises the matrix α-Al and the compound β-Al 3 Mg 2 .
Submerging welded joints in liquid nitrogen disrupts the internal stress balance within the material and leads to lattice contraction due to the influence of compressive stress.As the material enters a metastable state, the solid solubility of magnesium in aluminum decreases [29].This change in solubility is pivotal.At low temperatures, oversaturated point defects and dislocations in the aluminum alloy proliferate.They interact with solute atoms, resulting in the precipitation of sediment, particularly the β-phase, which forms along dislocation lines and grain boundaries after cryogenic treatment [30]. Figure 10 shows the interaction between solute atoms and dislocations following DCT.During the DCT process, lattice strain occurs under compressive stress, and leads to compression of the matrix lattice atoms around the aluminum atoms.Notably, Mg atoms are concentrated around edge dislocations, significantly exceeding the average concentration.In some instances, this concentration is even high enough to form metal compounds around the dislocations.The presence of solute atoms around the dislocations reduces the strain energy of the dislocation, enhancing its stability.The increased stability reduces the likelihood of dislocation movement and, consequently, improves the crystal's resistance to plastic deformation.As a result, the volume contraction and the reduction in lattice constant caused by ordered solid solution induce the precipitation of the second phase.The volume contraction fosters the generation of substantial compressive stresses, which store deformation energy.This deformation energy, accumulated during the cooling process in liquid nitrogen during cryogenic treatment, serves as the driving force for the precipitation of the second phase.The energy accumulation is a product of cryogenic treatment and manifests as residual stresses in the matrix.The intensity of these residual stresses varies with the duration of exposure to cryogenic temperatures.In the end, cryogenic treatment gives rise to a substantial number of fine second phase precipitates [31].reduction in lattice constant and solubility of Mg atoms, triggered by plastic deformation during the deep cryogenic process, serves as the primary driving force behind the precipitation of these phases.Additionally, dislocations generated during the deep cryogenic process act as pathways for the diffusion of Mg atoms, thereby facilitating diffusion under the challenging conditions of low-temperature deep cryogenic treatment.The validity of these observations and phenomena has been substantiated through various experiments, including microstructure observations, SEM images, and EDS spectra.

Mechanical properties of the joints
The microhardness distribution of the four weld zones in the welded joint increases with prolonged cryogenic treatment, as demonstrated in figure 12.This microhardness distribution forms a characteristic 'W' shape, with the microhardness in the Nugget Zone (NZ) significantly higher than that in the Thermo-Mechanical Affected Zone (TMAZ) and Heat-Affected Zone (HAZ).The average microhardness of the base metal measures 89.5 HV. Figure 12 further illustrates that the microhardness in the Base Metal (BM) is higher than in the HAZ, and the microhardness in the NZ surpasses that in the HAZ.The microhardness is closely related to the presence of precipitation phases and grain size.The HAZ experiences grain coarsening due to the influence of welding thermal cycles, causing precipitated phases to accumulate at grain boundaries.The low temperatures in the HAZ lead to an overaging effect, resulting in a minimum hardness of 82.5 HV.On the other hand, the NZ demonstrates a microhardness of 90.1 HV due to its fine microstructure brought about by the stirring needle action, which promotes grain refinement and the strengthening of fine grains.After cryogenic treatment, there is a significant increase in the microhardness of the welded joint.Prior to cryogenic treatment, the minimum hardness of the joint is 78.8 HV.Following 3 h of cryogenic treatment, the hardness reaches a peak at 87.2 HV, marking an increase of 8.4%.The average microhardness of the entire weld area increases by 6.95% after  cryogenic treatment compared to the untreated state.This notable increase in microhardness in welded joints after cryogenic treatment is primarily attributed to the precipitation of the second phase during the cryogenic process.In contrast to the aluminum matrix, Al 3 Mg 2 is a hard and brittle particle with a much higher hardness value than that of the matrix grain.As cryogenic time increases, the microhardness of the welded joint reaches its maximum.However, with prolonged DCT, the increase in alloy hardness becomes less pronounced as the precipitation of the strengthening phase reaches a limit.
Figure 13 presents the tensile properties of joints treated with varying durations of DCT.The joints without DCT exhibit a Ultimate Tensile Strength (UTS) of 358.7 MPa and an elongation of 13.9%.It can be found that DCT significantly enhances the strength of FSW 5A06 alloy.It's evident that the UTS progressively increases with extended DCT time, reaching a maximum tensile strength of 385.3 MPa after 12 h of DCT, with a substantial 7.4% increase compared to joints without DCT.However, the elongation of the welded joint initially decreases and then increases with DCT time.It experiences a noticeable decrease at 3 h and 6 h of DCT, but significantly increases, surpassing that of the non-DCT joint, with a remarkable increase of 23.7%.These results demonstrate that applying DCT can lead to a considerable enhancement in both the strength and plasticity of FSW 5A06 alloy [32].
The alterations in tensile properties can be attributed to microstructure modifications.Submerging the welded joint in liquid nitrogen generates substantial internal stress within the joint, resulting in a proliferation of dislocations, which in turn significantly improves joint strength [16].After 6 h of DCT, both the strength and plasticity of the welded joints show marked improvement.This may be attributed to the accumulation and movement of dislocations after the conclusion of DCT, leading to the formation of new grain boundaries, causing coarse grains to fragment into finer ones.This grain fragmentation phenomenon ultimately contributes to the enhanced strength and plasticity of the welded joints.Moreover, the hindrance caused by grain boundaries to crystal deformation is greater when grain size is smaller, as it increases the surface area of grain boundaries.Consequently, this refined structure leads to a strength increase in the material.Additionally, cryogenic treatment promotes the precipitation of the second phase, which plays a role in immobilising dislocations and effectively obstructing their movement [33] The precipitation of the second phase is instrumental in increasing the tensile strength of the welded joint.
Based on tensile tests, SEM was employed to gain a deeper understanding of the fracture mechanism.Figure 14 presents the tensile fracture morphology of 5A06 joints with different DCT durations, comparing those with and without DCT.By observing the dimple sizes on the fractured surfaces in the presence and absence of DCT, it becomes evident that DCT leads to significantly larger dimples.In general, larger dimples correspond to an improved material capacity to withstand applied loads, showcasing a higher degree of plastic deformation before fracture.The results in increased mechanical properties of the material.It is obvious that a larger dimple forms due to extensive plastic deformation [34].Conversely, smaller dimples, as observed in the fracture morphology of tensile testing without cryogenic treatment, indicate a reduced material capacity to withstand applied loads, demonstrating lower plastic deformation and, consequently, relatively inferior mechanical properties.

Figure 3 .
Figure 3.The macro cross-section of the whole weld.

Figure 5 .
Figure 5. Mechanism diagram of dislocation formation and grain refinement during DCT.

Figure 6 .
Figure 6.TEM images of the dislocation before and after DCT.

Figure 11 (
a) displays the TEM image of the precipitated phase before DCT.In this figure, one can observe large grain sizes and a limited number of precipitated phases.In contrast, figure 11(b) reveals the TEM image of the precipitated phase after DCT.Notably, the number of precipitated phases within the joints has significantly increased following DCT treatment.Furthermore, the grain size has undergone a noticeable refinement, and a substantial proportion of the precipitated phases are concentrated at the intersection of dislocation lines.The

Figure 10 .
Figure 10.Interaction of solute atoms with dislocations after DCT.

Figure 11 .
Figure 11.TEM images of the precipitated phase before and after DCT.

Figure 12 .
Figure 12.Microhardness distribution of the joints under different DCT conditions.

Figure 13 .
Figure 13.The tensile properties curves at different DCT times.