Mechanical Properties Of Welded Joints Research Articles

Enhancing tensile properties of MIG welded AA6061 joints: Effect of pulse mode and post-weld heat treatment

Metal-Inert Gas (MIG) welding is one of the most important welding technologies commonly used in the manufacturing industry due to its high degree of automation and welding adaptability. However, softening is prone to occur in fusion zone (FZ) and heat affected zone (HAZ) of AA6061 MIG welded joint, which limits its application. In this work, AA6061 was welded by MIG welding with different pulse modes, and the post-welding heat treatment was carried out. The results show that the dual pulse MIG (DP-MIG) and PWHT can significantly improve the mechanical properties of AA6061 butt joint, and its ultimate tensile strength (UTS) is 331.3 MPa, even more than 90% of the base material (BM). In this paper, the microstructure of welded joints under different pulse conditions before and after PWHT was compared, and the relationship between the welding process, microstructure, and mechanical properties was established Furthermore, the influence mechanisms of pulse mode and PWHT on the mechanical properties of welded joints were explained in detail.

Improvement of microstructure and mechanical properties of laser welded Al-Si coated 22MnB5 steel joints by austenitic element copper

In this experiment, the effects of different copper foil thicknesses on the morphology, structure, and mechanical properties of laser lap welded 22MnB5 steel joints were studied. The aim was to reduce the damage of Al-Si coating and increase the mechanical properties of welded joints. As the thickness of copper foil increases from 0 to 100μm, there was a gradual decrease in the presence of δ-ferrite in the fusion zone (FZ) and fusion boundary (FB), while the copper-rich content of lath martensite (LM) gradually increased. The reason was that Cu is an austenite element which can expand the γ phase area and play a stabilizing and strengthening role. The result of JmatPro software can support it. It found peritectic reaction (L+δ→γ) range expanded from 1414 to 1422°C to 1436–1453°C which led to more γ phases formed during the solid-state phase transition (δ→γ) at high temperature. Based on this phenomenon, as the copper foil increased (0–100μm), the joint tensile strength increased from 361 MPa to 706 MPa. However, after adding too much copper foil (200μm), the microstructure of FZ varied to a copper-rich phase (ε-Cu) with low hardness and brittleness which adhered to the martensite grain boundaries and reduced the bonding force of the crystal boundaries, so the the mechanical properties of the weld joint seriously reduced (232 MPa). When the copper foil was thin or non-existent (0–30μm), there was still more delta ferrite in FB (low strength and brittle), and the fracture type of the joint mainly appeared as a cleavage fracture. When adding 100μm copper foil, the fracture expressed ductile fracture due to the interfacial fracture due to the lack of δ-ferrite in FB. If the thickness of Cu increased to 200μm, the fracture changed back to FB which expressed a quasi-cleavage fracture.

Softening effect in the heat-affected zone of laser-welded joints of high-strength low-alloyed steels

AbstractThe softening effect, which occurs as an undesirable consequence of microstructural changes in the heat-affected zone as a result of the welding process, is an inherent aspect of welding high-strength low-alloyed steels. One of the recommended ways to minimize these changes is the application of laser beam welding as a lower heat input technology. Hence, this work compares and investigates the effects of laser beam welding on the weld joint properties of S690QL, S960QL, S1100QL, S700MC, S960MC, and S1100MC steels. This research operates on the assumption that the mechanical properties of the zones surrounding the soft zone—base metal and weld metal—affect the mechanical properties of weld joints as well. The work shows that the total value of yield strength, tensile strength, and elongation of welded joints increases when the value of the strength of the weld metal and the soft zone increases and when the width of the soft zone narrows, and vice versa. Furthermore, the study demonstrates that the amount of C, Cr, Mn, Mo, Cu, and Ni in steel as well as the thermal cycle is directly associated to strength in these zones. The findings indicate that although the welded joints’ yield strength and tensile strength values remained over 96% of the base metal’s value, in certain cases the elongation values decreased to a mere 21% of the base metal’s value.

Effect of heat treatments on microstructure and mechanical behavior of laser welded LPBF AlSi10Mg alloys

In this work, heat treatments were performed on autogenously laser welding of laser powder bed fusion (LPBF) AlSi10Mg alloys to improve mechanical properties of welded joints. After welding, the welded joint was heat treated by direct aging (DA) and solution heat treatment and artificial aging (T6) treatment. The porosity, microstructure and mechanical performance of the welded joints after DA and T6 treatments were investigated and compared with the as-welded joint. Results show that the microhardness of weld metal (WM) has been increased to over 93.5 HV0.1 after DA treatment, while the microhardness in the T6 welded joint is reduced to 77.0 HV0.1, from 82.0 HV0.1 in the as-welded joint. The ultimate tensile strength (UTS) of the DA treated joint reaches 276.5 MPa, showing an increase of 43.0% compared with as-welded joint. The DA heat treatment of the LPBF AlSi10Mg welded joints effectively improves the hardness and the UTS, which can be mainly attributable to the reduction of the hydrogen pores, the enhancing effect of solid solution strengthening coming from the Si element in the Al matrix and the dispersion strengthening of Si precipitation.

A comparative study on welding characteristics and mechanical properties of Ti–6Al–4V laser welded joints under the sub-atmospheric pressure and beam oscillation

Sub-atmospheric pressure environment and oscillation laser are both excellent ways to improve welded joint quality. However, the differences in their effects and mechanisms are unclear. Therefore, the differences between sub-atmospheric pressure and oscillating laser in weld formation, porosity suppression and grain size refinement were analyzed in this study. The results revealed that penetration depth of welded joint was increased to 6.09 mm under sub-atmospheric pressure environment while it was decreased to 3.3 mm under oscillation laser. Furthermore, reducing fluctuating vapor pressure and suppressing vortex formation were the main means to improve the keyhole stability and promote the escape of bubbles in a sub-atmospheric pressure environment. The improvement of the oscillating laser in these two aspects was attributed to enlarging keyhole area, shortening escape path and oscillation keyhole absorption. Additionally, the average grain size in the upper weld was reduced from 134.6 μm to 118.6 μm and 114.1 μm under the sub-atmospheric pressure environment and laser oscillation, respectively. The reduction of the temperature gradient led to former grain refinement while the increase of heterogeneous nucleation rate was responsible for latter grain refinement. Correspondingly, the mechanical properties of welded joints were improved. The study offered valuable insights for the application of laser welding with oscillation laser and sub-atmospheric pressure in industrial production.

Role of material properties on metal transfer dynamics in gas metal arc welding

In the gas metal arc welding (GMAW) process, metal transfer dynamics, influenced by wire material properties such as electrical conductivity and surface tension, serve as key sources of mass and heat for metal deposition on substrates. To elucidate the role of these material properties on metal transfer dynamics from a driving force perspective, this study examined molten droplet elongation across five distinct wires: Al, Cu, Fe, Ni, and Ti. Additionally, a unique non-transferred arc welding setup equipped with a consumable wire anode and two tungsten cathodes was introduced, allowing for observations at minimized arc lengths and preventing current concentration in molten droplets. For Al and Cu wires, while only projected-spray transfer was observed under sufficient arc length conditions, a shift to minimized arc length resulted in significant droplet elongation and a transition to streaming-spray transfer. This contrast emphasized the impact of high electrical conductivity in reducing the internal gradient of electromagnetic pressure and inhibiting the occurrence of streaming-spray transfer. Further substantiating this mechanism, Fe, Ni, and Ti wires revealed that higher electrical conductivity corresponded to an increased transition current to streaming-spray transfer. Additionally, surface tension played a crucial role in the formation of molten droplets and ensured transfer stability, with its influence varying at different current levels. These insights contribute to our comprehensive understanding of arc plasma and molten metal interactions. Our study proposes material-specific strategies to optimize metal transfer dynamics, leading to enhanced control over weld bead formation and overall mechanical properties of welded joints in the GMAW.

The effect of thermomechanical welding on the microstructure and mechanical properties of S700MC steel welds

Grain refinement by plastic deformation during conventional TIG welding can help to compensate for the loss of mechanical properties of welded joints. The thermomechanical welding (TMW) tests were performed on S700MC steel with different combinations of TIG arc energy and high frequency hammering over three target cooling times (t8/5 = 5s, 15s, and 25s). Additionally, the effect of initial microstructures on the weld joint quality was analysed by testing three materials conditions: hot-rolled (as-received) and cold-rolled with 10% and 30% thickness reductions, respectively. The effects of plastic deformation and the mechanical vibration on the grain refinement were studied separately. Optical microscopy, electron backscattered diffraction, and Vickers hardness were used to characterise the weld microstructure heterogeneity. The weld width and depth and the mean grain size were correlated as the function of cooling time t8/5. The results show that the weld dimensions increase with increasing the t8/5. The weld microstructures transformed from the mixed martensite and bainite into mixed ferrite and bainite with increasing the t8/5 time, and the related mean grain size increased gradually. The TMW welds exhibit smaller grains compared to TIG welds due to the coupled effects of mechanical vibration and plastic deformation. The mechanical vibration contributes to weld metal homogenisation, accelerating TiN precipitation in the fusion zone. The proposed TMW process can refine the weld microstructure of S700MC steel, enhancing its mechanical properties.

Effect of Microstructure on Mechanical Properties of X65MS Welded Pipe Joint

The mechanical properties of welded joints have a crucial impact on the safety of pipelines. This article focuses on examining the correlation between the microstructure and mechanical properties of welded joints of X65MS pipeline steel for sour service. The results indicate that the welding thermal cycle forms the microstructure consisting of acicular ferrite, granular bainite, and polygonal ferrite. The hardness across the joint ranges from 210 and 270 HV, peaking within the welded zone and decreasing towards the base metal (BM). At −40 °C, the welded zone shows the lowest impact energy of 53 J, while heat‐affected zones exhibit superior impact toughness exceeding 270 J with the highest impact energy of 326 J. This can be attributed to the exceptionally fine‐grained microstructure, a small quantity of small‐sized martensite‐austenite (M/A) constituents in the heat affected zone. As for the low toughness of the weld, the main factors can be attributed to the presence of large‐sized elongated M/A constituents and inclusions.

Microstructure and Mechanical Properties of IN690 Ni-Based Alloy/316LN Stainless-Steel Dissimilar Ring Joint Welded by Inertia Friction Welding.

Inertia friction welding (IFW) was used to join large-diameter hollow bars made of Inconel 690 and 316LN successfully. The interfacial characteristics, microstructure, mechanical properties and fracture mechanism of welded joints under different process parameters were investigated. The results indicated that a joining mechanism with mechanical interlocking and metallurgical bonding was found in IFW joints. There was a significant mechanical mixing zone at the welding interface. The elemental diffusion layer was found in the "wrinkles" of the mechanical mixing zone. A tiny quantity of C elements accumulated on the friction and secondary friction surfaces. The tensile strength and impact toughness of the joints increased with the total welding energy input. Increasing the friction pressure could make the grain in all parts of the joint uniformly refined, thus enhancing the mechanical properties of welded joints. The maximum tensile strength and impact toughness of the welded joint were 639 MPa and 146 J/cm2, reaching 94% and 68% of that for Inconel 690, respectively, when the flywheel was initially set at 760 rpm, 200 MPa for friction pressure, and 388 kg/m2 for rotary inertia. Due to the Kirkendall effect in the welded joint, superior metallurgical bonding was at the welding interface close to the Inconel 690 side compared to the 316LN side.

An innovative pulsed current arc welding technology for armor steel: Processes, microstructure, and mechanical properties

This study presents a novel pulsed current arc welding technology, specifically pulsed current gas tungsten arc welding (PC-GTAW), single and double pulse gas metal arc welding (SP/DP-GMAW), and dual process welding (D-process). This study aims to investigate the effectiveness of these welding techniques for armor steel weldments. This research focused on investigating the welding processes, resulting microstructures, and mechanical properties of welded joints. Based on the macrography analysis, it was determined that the weldments exhibit no signs of porosity, inclusions, or inadequate penetration. This finding substantiates the optimum settings for the weldment production process. The weldment optical microstructure, fusion, and heat-affected zone were examined in the weld interface. The microstructural properties of the FZ include δ-ferrite, austenite, bainite, and acicular martensite. The HAZ has a bainite-untempered martensite microstructure. The chemical composition and variations across the weldment (base, weld and HAZ) were identified. The grain borders contained greater quantities of Cr, Ni, Mn, Si, and Mo than those of the grain cores. EBSD revealed average weld zone grain sizes of 17.7 µm, 32.6 µm, 30.8 µm, and 24.6 µm for PC-GTAW, SP-GMAW, DP-GMAW, and D-Process, respectively.Furthermore, the hardness measurements showed a progressive transition across different regions. Additionally, the D-Process-fabricated material exhibited a maximum ultimate tensile strength of 872 MPa and an elongation of 4%. The mechanical properties exhibited by this new process variant exceed those of other weldments, indicating its potential for fabricating armor steel. The impact toughness demonstrated superior performance in all the weldments. Within this context, the new use of pulsed current advanced welding methods has shown the ability to generate welded parts of exceptional quality.

Comparison of electron-beam-welded joints manufactured by rolled and selective laser melted reduced activation ferritic/martensitic steel

As the core component of China Fusion Engineering Test Reactor (CFETR), the forming and manufacturing of the first wall faces severe challenges due to the constraints of materials, service environment and geometric structure. The microstructure and mechanical properties of welded joints obtained by rolled and laser selective melted (SLMed) reduced activation ferritic/martensitic (RAFM) steel were investigated in depth. The microstructure analysis results showed that the weld metal (WM) was composed of coarse lath martensite and δ-ferrite, while the microstructure were mainly coarse grains in heat affected zone (HAZ) which was easy to break. The average grains size of SLMed RAFM steel (S-RAFMs) was smaller than that of rolled RAFM steel (R-RAFMs), since the rapid melting and solidification of laser in the additive manufacturing process prevented grains to grow. The microstructure of R-RAFMs joint was relatively more uniform and there was no obvious texture and preferred orientation, while the WM of S-RAFMs joint had a weak cubic texture. The results of mechanical properties showed that the S-RAFMs joints exhibited higher strength (UTS = 697.46 MPa) and the R-RAFMs joints exhibited better plasticity (EL = 13.49 %) under the same tensile conditions. The tensile and impact fracture morphologies showed that the fracture mechanism of two joints was ductile-brittle mixed fracture, and the dimples of R-RAFMs joints were finer and denser. It was clearly observed that the hardness of WM (∼390–430 HV0.3) was higher than that of the base metal (BM) (∼190–230 HV0.3) from the hardness distributions along the width directions. The average hardness of BM in R-RAFMs joint (227 HV0.3) was lower than S-RAFMs joint (197.3 HV0.3), which was mainly related to the fine grain strengthening effect during SLM forming. The above results may provide theoretical guidance for the high-performance manufacturing of the first wall and promote the application of SLM and EBW technology in the field of nuclear fusion.

Effect of High-Temperature Tempering on Microstructure and Mechanical Strength of Laser-Welded Joints between Medium-Mn Stainless Steel and High-Strength Carbon Steel

The strengthening effect due to high-temperature tempering (HTT) at 700 °C on the microstructure and mechanical properties of welded joints between medium-Mn stainless steel (MMnSS) and high-strength carbon steel (CS) was studied. The microstructure of the weldments was investigated using Laser and scanning electron microscopes. An Electron probe microanalyzer (EPMA) was used to assess quantitatively the elemental distribution profiles of alloying elements within the weld zone. The strengthening precipitates induced during welding and HTT were characterized by transmission electron microscopy (TEM). Uniaxial tensile tests and microindentation hardness (HIT) measurements of the weld joints were conducted to evaluate the strengthening effect. Fully fresh-martensite and fine-tempered martensitic structures were promoted in the as-weld and HTT processes, respectively. The HTT structure exhibited a remarkable improvement in mechanical properties (a better combination of yield and tensile strength together with moderate ductility) compared to its weld counterpart. TEM investigation revealed that various types of precipitates have been promoted in the structures of the weld and HTT, e.g., nanosized vanadium and chromium carbides. It is apparent that the proposed HTT of the joints is an effective treatment for improving the mechanical properties due to inducing the formation of fine interphase precipitates, resulting in enhanced mechanical strength of the joints.

MODELLING THE INFLUENCE OF WELDING MODES ON THE MECHANICAL PROPERTIES OF WELDED JOINTS

Introduction. Welding is one of the most common and effective methods of joining metal structures, which is used in many industries, from the automotive to the oil industry. However, in the process of welding, the joints are subjected to significant thermal and mechanical loads, which can lead to defects and a decrease in the mechanical properties of welded joints. The main part. Modelling the influence of welding modes can be performed using various methods, such as mathematical modelling based on the laws of physics and mechanics, experimental studies of welded joints and their analysis, or a combination of these methods. Basically, finite element methods, boundary element method, finite difference method, and hybrid modelling method can be applied, where, for each individual element of influence on welding, the most effective method of mathematical modelling is used. It is considered what software can be used for modelling a mathematical model. The methods of experimental research used to determine the mechanical properties of a welded joint are analyzed. The methods of comparing experimental data to the mathematical model are also analyzed. Conclusions. The basic methods of mathematical modelling of welding, methods of experimental research, and comparison of the obtained data of the mathematical model with the conducted research have been considered. Based on the comparison, it is possible to find out how a particular welding mode will affect the quality of the welded joint.

EFFECT OF A MAGNETIC FIELD ON THE MICROSTRUCTURE AND PROPERTIES OF A 22MnB5-CP780 WELDED JOINT

A steady-state magnetic field-assisted laser welding test of 1-mm-thick automotive 22MnB5 and CP780 high-strength steel plates was carried out using a low-power fiber laser. The effects of different magnetic induction intensities on the microstructure and mechanical properties of welded joints were investigated under a heat input of 150 J/mm. When no magnetic field was applied, there was splashing on the welded joint surface, the width was wide and the main solid-state phase transitions in the weld center included ferrite transformation and bainite transformation. The application of a magnetic field resulted in an acceleration of the cooling rate of the molten pool due to the thermoelectric magnetic force. This, in turn, led to the predominance of the bainite transformation and martensite transformation as the primary solid-state phase transitions in the weld center. In a range of 5–25 mT, the welded joints were narrow, symmetrical, well-formed and free of defects such as splashing. The experiment proved that as the magnetic induction intensity increased, the laser energy became more concentrated. This led to an increase in the average hardness at the center of the weld, a gradual reduction of the softening in the heat-affected zone and an increase in both the yield strength and elongation of the welded joint. The optimal comprehensive mechanical properties of the welded joints were observed when B = 25 mT.

Monitoring of thermo-cycles in fibre laser welding of duplex stainless steel 2205 sheets and its correlation with microstructures and mechanical properties

The study reports the influence of change in the heat supplied (43 J mm−1 to 18.5 J mm−1) on the microstructures as well as mechanical properties of weld joints obtained by welding of Duplex stainless steel 2205 using fibre laser. In-process thermal monitoring of the molten weld pool was carried out using IR pyrometer. Cooling rates (i.e. solidification and solid) were calculated from the thermo-profiles of weld pool, and it increases with the decrease in heat input. From the optical images, it is observed that columnar grains originated from the fusion zone walls and merged at the center. Since, solidification front velocity is comparable on both sides’ leads to a central edge. Ferrite phase content observed in fusion zone microstructure, increases with the increase in solid cooling rate. The result suggests that the joints fabricated at lowest heat input displayed highest tensile strength. The maximum tensile strength been reported to be 872.5 ± 10.8 MPa, and failure occurred at parent metal. Tensile strength of weld joints of DSS 2205 was found to have improved with increasing cooling rate. Higher cooling rate results in the formation of fine dendritic grains as well as higher ferrite content in the weld metal.

SELECTION OF TECHNOLOGICAL PARAMETERS OF MODES FOR MANUAL ARC WELDING OF SHUT-OFF VALVES MADE OF STEEL ALLOYED WITH 3–4 % NICKEL

Body of ball valves can be manufactured from A350-LF3 steel according to the US standard ASTM A350, alloyed with nickel up to 4 %. This steel has a tendency to harden in the heat affected zone during welding. The results of computer simulation showed that cooling rates during shielded metal arc welding exceed the recommended range. This can lead to the formation of hardening structures, the development of cold cracks and a decrease in the performance properties of welded joints of ball valves made of A350-LF3 steel.
 To prevent the formation of cold cracks, the preheating and concurrent heating temperatures were calculated. According to the results of computer simulation, it was found that pre-heating and concurrent heating provide the cooling rate of heat affected zone in the recommended range.
 The technology has been developed for welding ball valves made of A350-LF3 steel with mounting nozzles. This technology prevents the formation of cold cracks and provides a high level of strength and toughness of welded joints. The high level of mechanical properties of welded joints is confirmed by results of mechanical tests and metallographic studies.

Strength calculation and microstructure characterization of HAZ softening area in 6082-T6 aluminum alloy CMT welded joints

Degradation of mechanical properties in the heat-affected zone (HAZ) is a common issue when welding 6082-T6 aluminum alloy. Even for cold metal transfer (CMT) welded joints with little heat input, it is always challenging to avoid this problem. In this study, we examined the effects of various heat inputs on the softening of CMT welded 6082-T6 aluminum alloy joints as well as the mechanism underlying the softening of HAZ. The hardness of the welded joint at the HAZ softening area directly decreases as the heat input increases. The softening of HAZ is the most severe under the impact of thermal cycling, with a softening zone width of up to 5.5 mm and a minimum hardness of about 61 HV, reaching 58.65% of the base material (BM). According to the strength model established in the study, the most significant factors affecting the strength of welded joints were the weakening of precipitation strengthening and solid solution strengthening due to welding thermal cycling. These factors accounted for 51.51% and 30.26% of the total strength, respectively. Compared with BM, the contribution of precipitation strengthening and solid solution strengthening in HAZ softening area decreased by 85 MPa and 23 MPa, respectively. Where the weakening of precipitation strengthening is mainly due to the β" and β' phase transitions induced by welding thermal cycling. The decrease in solid solution strengthening is then mainly influenced by the change in the content of Mg and Si elements in the matrix. The revelation of softening mechanism provides theoretical support for how to control the loss of mechanical properties of welded joints of Al-Mg-Si aluminum alloy.

Prediction of the tensile strength of friction stir welded joints based on one-dimensional convolutional neural network

Friction stir welding (FSW) is a complex thermo-mechanical coupling process. Tensile strength is an important evaluation index of the mechanical properties of welded joints. How to realize the real-time prediction of tensile strength of the friction stir welded joints to reflect the dynamic change of welding state is a problem in the field. To solve this problem, this paper presents a multi-scale one-dimensional convolutional neural network (Multi-scale 1D CNN) prediction model using time series data of temperature and axial force as inputs to realize the online prediction of tensile strength of welded joints. Firstly, FSW experiments are carried out to obtain time series data of temperature and axial force. Tensile strength values of the welded joints is obtained by tensile tests. The time series data and tensile strength values are fused as a dataset. Then Multi-scale 1D CNN, traditional 1D CNN and Multi-channel 1D CNN prediction models are established and trained with the dataset, respectively. Finally, by comparing the prediction performance of the three models, Multi-scale 1D CNN is proved to be more suitable for analyzing time series data to feedback the dynamic change of tensile strength of the joints during welding.

Effect of post-welding coating process on the organization and properties of MIG-welded T-joints of 6063-T6 aluminum alloy

Abstract With the rapid development of the precision industry, aluminum alloy structural parts have been widely used. However, in the welding process of aluminum alloy, there are problems such as welding deformation and weld tumor defects due to the special physical characteristics of aluminum alloy. Therefore, this paper mainly focuses on 6063-T6 aluminum alloy as the research object to analyze the effect of the coating process on the organizational properties of MIG welded T-joints of 6063-T6 aluminum alloy. First, based on the Simufact welding finite element analysis platform, the establishment of a three-dimensional welding simulation model, the simulation calculations and test results for comparison, the peak temperature and temperature change trend of each measurement point between the two. Secondly, according to the T-joint welding process, welding numerical simulation of 6063-T6 aluminum alloy rear compartment structural parts to obtain the rear compartment assembly welding residual stress is mainly concentrated in the vicinity of the weld. Finally, the impact of welding current on the organization and properties of T-shaped welded joints of 6063-T6 aluminum alloy was discussed. The results show that the coating process has a good strengthening effect on the mechanical properties of welded joints. Compared with the pre-coating process, the welded joint tensile strength increased by 14.93%, and the welding coefficient of 73% of the parent material heat-affected zone microhardness increased by 21.82% compared to the pre-coating process. In this paper, the finite element theory of the coating process and MIG welding, T-joint organization between the found welding heat-affected zone after the coating process can make Mg, and Si atoms continue to be enriched, and mechanical properties strength is enhanced.

Investigation of microstructure evolution on different planes in laser welding of aluminum alloy

The solidification behavior of a molten pool is a critical factor affecting the mechanical properties of welded joints. This paper develops a multi-scale model combining the macroscale heat transfer and fluid flow model with the microscale phase field model for calculating the microstructure evolution on two different planes that are perpendicular to the thickness direction in the laser welding of the aluminum alloy. To obtain the time-varying temperature gradient (G) and solidification velocity (R) used in the simulation, a transient solidification conditions model is proposed. These models are validated by comparing the simulation results with the experimental results. The results indicate that G decreases, while R increases during solidification process. G/R decreases on both two planes, which results in the transformation of the microstructure from planar to cellular and then to the columnar grain. Additionally, it is found that the primary dendrite arm spacing of columnar grains on the lower plane is smaller, which is related to lower G−1/2R−1/4.