Laser-beam Bilateral Synchronous Welding Research Articles

Bridging microstructure and crystallography with the crack propagation behavior in Ti-6Al-4V alloy fabricated via dual laser beam bilateral synchronous welding

The microstructure evolution and crack propagation behavior were systematically and thoroughly investigated in α/β Ti-6Al-4V alloy obtained under the inhomogeneous heating effect of a coupled dual laser beam via a combination of postmortem electron backscattering diffraction analyses and numerical simulations. The effect of microstructural attributes (grain size, grain boundary, dislocation) on the fracture property and crack propagation behavior was investigated based on thermodynamic and crystallographic analyses, as well as the examinations of dislocation density and the Burgers vector. By quantifying the degree of variant selection using the degree of variant selection (DVS) equation, it was found that the higher the energy input, the weaker the variant selection, as the secondary α phase suppresses further dislocation-induced variant selection via autocatalytic nucleation and growth. The proportion of high angle grain boundaries (HAGBs) concentrated around 60° gradually increases with improving cooling rate, mainly due to the change in the tendency of martensite transformation to be induced. The grain refinement was the primary mechanism for the enhancement of Ti-6Al-4V alloy T-joints, while the suppression of crack propagation by HAGBs also played a crucial role in improving strength and ductility. The crack propagation direction frequently changes when the order is approximately parallel to the short axis of α′ martensite grains, forming a zigzag propagation path. The above findings should shed light on optimizing dual laser beam bilateral synchronous welding (DLBSW) technology to tailor the microstructure and improve the mechanical properties of Ti-6Al-4V alloy T-joints.

Research on keyhole-induced porosity and melt flow kinetic behavior of 2219 alloy T-joint manufactured by dual laser-beam bilateral synchronous welding

Research on keyhole-induced porosity and melt flow kinetic behavior of 2219 alloy T-joint manufactured by dual laser-beam bilateral synchronous welding

Research on the cryogenic axial tensile fracture mechanism for 2219 aluminum alloy T-joint by dual laser-beam bilateral synchronous welding

In this paper, dual laser-beam bilateral synchronous welding (DLBSW) technology is performed to fabricate 2219 aluminum alloy T-joint. The cryogenic axial tensile strength of the DLBSW T-joint in the direction parallel to the weld at −182℃ is tested. The microstructure of T-joint, the cryogenic axial tensile strength, fracture location and tensile fracture morphology are comprehensively investigated to explore the cryogenic axial tensile mechanism. The average value of cryogenic axial tensile strength of T-joint is 415 MPa, 76.9% of base metal (BM). Under the axial tensile force, the grain deformation and intergranular cracks are respectively generated in weld seam (WS) and heat affected zone (HAZ) near the fracture location. It is found that the cryogenic axial tensile fracture mechanism of the T-joint is intergranular ductile fracture, contains cleavage fracture and microvoid coalescence fracture. The degradation of cryogenic axial tensile property is attributed to multiple factors, such as impurity phases, pores, eutectics.

Effect of laser power on the grain morphology and microhardness of dual laser-beam bilateral synchronous welded 2219 aluminium alloy T-joint

Dual laser-beam bilateral synchronous welding (DLBSW) technology has been widely used in the connection of skin-stringer T-joint. In this study, DLBSW technology was performed to fabricate T-joint of 2219 aluminium alloy. Furthermore, the effect of laser power on grain morphology and microhardness of the T-joint was investigated. The microstructure of different parts was observed and the grain size was quantitatively analysed. The results show that the grain quantity and microhardness of the welded joint are increased significantly as the welding heat input increases. It is found that the influence of laser power on the grain size at the edge of the weld seam is greater than that in the centre of weld. Besides, the microhardness of the non-dendritic equiaxed zone decreases with the increase of laser power, which is caused by the decrease of Al element.

Research on microstructure evolution and tensile characteristics of Ti6Al4V double lap-fillet joints fabricated by dual laser-beam bilateral synchronous welding

Employing prefabricated “boss” on the substrate to replace wire feeding, an attempt is made to explore the non-filler welding process of Ti6Al4V double lap-fillet welded joints with the technology of dual laser-beam bilateral synchronous welding (DLBSW). The weld quality is evaluated in terms of bead geometry, microstructural morphology and mechanical property. An optimal range of welding speed (1.3 m/min) with constant unilateral laser power of 950 W is identified to achieve satisfactory weldments with no underfill defect and acceptable porosity. Phase transformation and microstructural evolution occurring through thermal history in different regions of the welded joint are clarified. The average length of α'-martensite in the weld seam (WS) decreases significantly as welding speed increases. A connection between dimensional variation of α'-martensite and tensile strength is developed, and the fracture mechanism of the lap-fillet joint is discussed. The existence of fine α'-martensites helps to attain high-performance joints with strength comparable to the base metal (BM). The fracture position of tensile specimens is located in the region near the weld toe where stress concentration occurs. Fractography analysis indicates that the failure of weldments is in ductile mode with the feature of a great number of dimples.

Microstructure and texture evolution of Ti–6Al–4V alloy T-joint fabricated by dual laser beam bilateral synchronous welding

The microstructure and microtexture evolution of Ti–6Al–4V T-joints produced by dual laser beam bilateral synchronous welding (DLBSW) were investigated using the optical microscopy (OM) and electron back-scattered diffraction (EBSD) and correlated with the mechanical properties of the joint. We demonstrate important differences in temperature field, grain morphology, misorientation angles and texture intensity between the base material (BM), the skin-side heat-affected zone (SK-HAZ), the stringer-side heat-affected zone (ST-HAZ) and the weld seam (WS). Through in-depth crystallographic analysis, we demonstrate that the kernel average misorientation curve of the WS is steeper and the peak misorientation angle is larger, revealing that the WS has a lower geometrically necessary dislocations density and a smaller residual strain. Due to the different orientation of each a-phase colony at the interface of the two phases, it is not conducive to the deformation mode of dislocation slip, so a large number of twins are generated in the WS. Furthermore, the WS has a strong crystal preferential orientation and the maximum polar density of the pole figure is 17.05, which is significantly higher than the polar density of the base material. It is found that the gradient of microstructure variation in the narrow HAZ is higher than that in the WS, which leads to more heterogeneous properties in the HAZ compared to the weld. This research result is consistent with the phenomenon that all the fractures of the resultant T-joints occurred in the HAZ.

Study on the grain morphology and fracture performance of T-joints for Ti6Al4V alloy manufactured by dual laser beam bilateral synchronous welding

Titanium alloys have become attractive candidates for structural materials in aviation industry, benefited from their reliable mechanical properties. In the present investigation, dual laser beam bilateral synchronous welding experiments of Ti6Al4V alloy under various welding speeds were performed using optic fiber laser. Experimental results indicate that the heat affect zone has become the weakest region of the joints theoretically. Inappropriate welding speed lead to a severe coarsening of the equiaxed grains below the lower fusion zone, which is the main reason why the tensile strength of the construction decreased significantly. Additionally, the ultimate tensile strength of the T-joint under the welding speed of 22 mm/s has reached 808 MPa, exceeding 90% of the strength of the base metal. According to the fractographs, the T-joints adopted higher welding speeds have failed in the form of microporous aggregation fracture. In contrast to the others, it can be concluded that the fracture mode of the T-joint under the lowest welding speed is a mixed fracture composed of cleavage fracture and ductile fracture.

Effect of porosity morphology and elements characteristics on mechanical property in T-joints during dual laser-beam bilateral synchronous welding of 2060/2099 Al-Li alloys

The 2 mm thick 2060-T8/2099-T83 Al-Li alloy T-joints were welded by dual laser-beam bilateral synchronous welding (DLBSW) with ER4047 filler wire. The metallurgical porosity defects under different welding parameters were investigated to study the influence of laser power and weld speed on the porosity distribution, formation mechanism and mechanical properties during DLBSW process. In order to explore the special effect of welding heat input on the difference of alloying elements content around porosity, a comparative analysis of elements content between the porosity boundary and matrix around porosity was studied. It was found that the weld porosity defects were increased significantly as the laser power increased. Besides, the heat affected zone (HAZ) and branches of dendritic crystals were the key positions of porosity nucleation and formation. The results of tensile test and SEMoffailuresurface demonstrated that porosity was one of the important factors leading to weld bead failure. In particular, the porosity in the bottom fusion zone and EQZ significantly weakened the performance of the T-joint.

The cryogenic shear property and fracture mechanism for 2219 aluminum alloy T-joint fabricated by dual laser-beam bilateral synchronous welding

The dual laser-beam bilateral synchronous welding (DLBSW) experiment with self-melting process of prefabricated bosses in the 2219 aluminum alloy T-structure, consisted of skin and stringer components, was performed. The analyses of cryogenic shear strength at −182 °C, fracture morphology and element distribution were carried out to investigate the influence of different weld structure with prefabricated bosses on cryogenic shear performance. For the T-joint with prefabricated bosses on skin, the shear crack was extended from the bottom fusion line (FL) to the skin in the bottom of weld seam (WS). For the T-joint with prefabricated bosses on stringer, the shear fracture was located at equiaxed dendrites zone near the upper FL. It was indicated that the T-joint with prefabricated bosses on skin obtained higher cryogenic shear strength, 48.4% of base metal.

Effect of Grain Size on Mechanical Properties of Double Laser-Beam Bilateral Synchronous Welding Joint

Effect of Grain Size on Mechanical Properties of Double Laser-Beam Bilateral Synchronous Welding Joint

Study on keyhole coupling and melt flow dynamic behaviors simulation of 2219 aluminum alloy T-joint during the dual laser beam bilateral synchronous welding

In this research, the keyhole coupling and melt flow behaviors of 2219 aluminum alloy T-joint during dual laser beam bilateral synchronous welding (DLBSW) process are investigated by simulation, which involves fluid heat transfer and fluid flow. The calculated mesh model of the T-joint is established by simplifying the equal boss of the skin. The combined Gaussian laser heat source is adopted to simulate the laser energy loaded on the base metal during the welding process. The obtained simulation results are reliably matched with the experimental results. Subsequently, not only the coupling process of two keyholes and the evolution characteristics of the coupled keyhole are analyzed, but also the characteristics of molten pool and the flow behaviors of the liquid metal are studied in detail. The results demonstrate that the keyhole profile’s oscillations continuously exist, whether before or after the keyhole coupling. The flow direction of the liquid metal near the coupled keyhole wall includes the flow parallel to the keyhole wall and that around the keyhole wall. Besides, the maximum depth of the molten pool slightly lags behind the rear wall of the keyhole. On the coupled keyhole wall and molten pool surface, the melt flow velocities are significantly greater than that inside the molten pool. The vortexes are produced near the upper and lower regions of the coupled keyhole wall.

Effect of temperature alternating load on stripping performance of Al-Li alloy T-joint manufactured by dual laser beam bilateral synchronous welding

In this work, the temperature alternating load (TAL) with four cycles is performed to the 5A90 Al-Li alloy T-joint produced by dual laser beam bilateral synchronous welding (DLBSW). The grain details, stripping strength, fracture micro-morphology and elemental composition of the samples are analyzed by optical microscope, self-designed specific fixture, scanning electron microscopy (SEM) and energy disperse spectroscopy (EDS), respectively. The experiment results indicate that the TAL treatment has a slight decrease on the width of non-dendritic equiaxed zone (EQZ) near the fusion line. The average striping strength and post-break elongation of the TAL treated samples are separately 126.01 MPa and 3.60%, which are both reduced compared with the values 145.61 MPa and 5.54% of the untreated samples. Meanwhile, the results of fracture micro-morphology and elemental composition are utilized to investigate the decline of the tensile performance of the T-joint with the TAL treatment. It is found that the fracture features such as tear layers, shear dimples and torn pores are relatively insignificant compared to the T-joint without the TAL treatment. The non-uniformity of element distribution exists in the TAL treated and untreated sample.

Study on 2219 Al-Cu alloy T-joint used dual laser beam bilateral synchronous welding: Parameters optimization based on the simulation of temperature field and residual stress

In the current research, the dual laser beam bilateral synchronous welding (DLBSW) of 2219 Al-Cu alloy T-joint is investigated by finite element (FE) simulation of temperature field and residual stress, which aims at the optimization of the welding process parameters. The FE mesh model of the T-joint is established by simplifying the equal boss of the skin. The combined Gaussian laser heat source is adopted to simulate the laser energy loaded on the Al-Cu alloy during welding process. Furthermore, a test table of process parameters with the laser power and welding speed is designed to investigate the parameters via the simulation results. The simulation with 16 groups of parameters are performed based on the heat source model which is checked by the experimental result. Two cases of parameters are filtered from the simulation results for the hoop tensile test experiment. It is employed to estimate the tensile and yield strength of the T-joints with the filtered parameters. Welding process parameters in case 6, which are the laser power of 4300 W and welding speed of 2.5 m/min, are the optimized parameters in the test table based on the tensile strength (337.28 MPa) and yield strength (243.51 MPa) obtained by the tensile test results.

Study on the shear performance and fracture mechanism of T-joints for 2219 aluminum alloy by dual laser-beam bilateral synchronous welding

Double laser-beam bilateral synchronous welding (DLBSW) of T-joints for aluminum alloy is an advancing technology, which is significant for the aerospace panel structures in the lightweight and efficient manufacturing. In this paper, the DLBSW technology is employed in the 2219 aluminum alloy T-joint with skin and stringer. It is notable that the skin component accompanied with two bosses is designed to replace the welding wire. The influences of the microstructure, defects, and stresses distribution on the shear performance of the T-joints are investigated. It is found that the T-joint owns fewer defect and higher shear performance at a weld speed of 1.3 m/min than at the weld speed of 4.0 m/min. Although the tensile strength of the T-pull tests is superior, the shear strength of the shear tests is only about 40% of base metal (BM). Besides, the dynamic shear process of welded joint based on finite element (FE) model of laser welding process for 2219 aluminum alloy is simulated. It is indicated that the weld toe is the stress concentration zone under the shear load. The shear failure propagates direction is from the highest stress node to the lowest stress node. According to the analyzation of both experiment and simulation results, it is concluded that the formation of EQZ and high stress together contribute to the origin of shear fracture.

Effect of welding direction on deformation of Ti6Al4V alloy coplanar double lap-joint produced by dual laser beam bilateral synchronous welding

The coplanar double lap-joint is the weld type mainly used for the connection of skin and stringer. The welding deformation in the component will reduce fabrication accuracy and bearing capacity and the excessive welding deformation can even render the product unusable. In the present work, a thermal-elastic-plastic finite element model is developed to investigate the characteristics of welding deformation in the coplanar double lap-joint. The experiment is carried out to verify the accuracy of simulated results. The results of the simulation shows a good agreement with the experimental measurements. The influence of welding direction on welding deformation is studied by finite element analysis. Moreover the mechanism of deformation mode in the coplanar double lap-joint is discussed. The results show that the change of welding direction has different effects on the distribution of residual deformation. Through the analysis of simulation results, the deformation mode of the joint is similar to a concave-convex mode.

The effect of cryogenic applications on tensile strength of aluminum 2219-T87 T-joint welded by dual laser-beam bilateral synchronous welding

2219 aluminum alloy has been widely used in aerospace industry due to its high specific strength, excellent weldability as well as outstanding ductility. The dual laser-beam bilateral synchronous welding (DLBSW) technology is employed in the joining of 2219-T87 aluminum alloy skin-stringer structure in T-joint. In order to establish the tensile strength of T-joints at room and cryogenic temperatures, the tensile testing is carried out. The microstructure of weld seam is observed for exploring the relationship between the grain size and tensile strength. In addition, the fracture morphology is observed by scanning electron microscope (SEM). The elemental analysis of fractured surface is done by energy dispersive spectrometer (EDS) and is used to study the effect of chemical element composition on tensile strength. The results show that the highest tensile strength of 360.01 MPa was achieved when the tensile test obtained at room temperature while the joint welded by same parameter exhibit a much higher tensile strength of 422 MPa when tested at cryogenic temperature. According to the results of microstructure analysis, grain size reduction was observed when the joint was exposed to cryogenic temperature. From the fracture morphology of joints, it was found that dimples of the sample tested at cryogenic temperature are bigger and deeper than that at room temperature. The results of EDS show that the amount of the Al2Cu increases at 77 K which causes an increase in tensile strength.

The effect of temperature alternating load on residual stresses for Al-Li alloy T-joints welded by dual laser beam bilateral synchronous welding

The finite element (FE) simulation is employed to predict the residual stresses, which is induced by the welding and the temperature alternating load. The simulation of 5A90 Al-Li alloy T-joint welded by the dual laser beam bilateral synchronous welding (DLBSW) is carried out. Then the evolution of the residual stresses is obtained under the temperature alternating load which is achieved by changing the ambient temperature on T-joint. In addition, the accuracy of simulation model has been verified by comparing the experimental results with the simulation results. According to the comparison, the simulation results are consistent with the experimental results, which shows that the residual stresses of 5A90 T-joints decrease under the temperature alternating load. Besides, the FE simulation is a relatively efficient and accurate method to predict the change of residual stresses under the temperature alternating load.

Influence of Laser Power on Grain Size and Tensile Strength of 5A90 Al–Li Alloy T-joint Fabricated by Dual Laser-Beam Bilateral Synchronous Welding

Dual laser-beam bilateral synchronous welding is introduced to produce the 2.5 mm thick 5A90 Al–Li alloy T-joint. The grain morphology and grain size of weld metal (WM) in the T-joint are analyzed and calculated. The tensile experiment, scanning electron microscopy and energy disperse spectroscopy are respectively employed to study the tensile strength, fracture morphology and chemical composition of the T-joint. The results reveal that when the laser power is increased from 2500 to 3000 W, the grain sizes of fine-grained layers and columnar dendrites near the fusion line are significantly reduced. Conversely, that of equiaxed dendrites at the WM center is not sensitive to the variety of laser power. Moreover, the degree of elemental segregation in WM near the fusion line is also aggravated with the increasing of the laser power. The tensile strength of the T-joint with the laser power of 2500 W is significantly higher than that with the laser power of 3000 W. The tensile fracture locations are occurred in the weld toe with obvious pores, shear dimples and tear ridges, which are the typical characteristics of ductile fracture. Besides, the chemical compositions of the second phase particles in the WM are more sensitive to than the variation of laser power compared with that of the matrix.

Effect of welding parameters on porosity distribution of dual laser beam bilateral synchronous welding in 2219 aluminum alloy T-joint

The dual laser beam bilateral synchronous welding (DLBSW) for T-joint is limited to develop for the porosity defect in joint. In order to investigate the effect of porosity in T-joint, the DLBSW experiment of 2219 Aluminum Alloy T-joint is carried out. While, the distribution of porosity is scanned by X-ray non-destructive testing (NDT) and the size of porosity is measured by particle size analysis software Nano measurer 1.2. The morphology of pore in fracture and the elemental concentration around the pore are investigated by scanning electron microscopy (SEM) and energy spectrometer (EDS). Furthermore, the analysis results of porosity distribution show that the optimal parameter is 3700 W of laser power and 2.5 m/min of welding rate, under which the number of porosities reaches the minimum. The main factor, caused the fracture of weld seam, is the concentrated porosities in T-joint. In addition, technical porosity is mainly found in the center of the weld seam or at the bottom of the weld seam, and it can be eliminated by improving welding parameters. Metallurgical porosity mainly exists at the edge and the top of the weld seam, including hydrogen porosity and metal vapor porosity which formed by the burning of metal elements.

FE analysis of the residual stresses for the laser welded T-joint of Al-Li alloy under service loads

This paper attempts to reveal the evolution of welding residual stresses (WRS) in skin-stringer structure of Al-Li alloy exposed to service loads. 3D thermal-structural finite element (FE) formulation of the dual laser-beam bilateral synchronous welding process is first derived, and its accuracy is verified by experimental tests to acquire accurate WRS. FE numerical analyses incorporated with the elastic-plastic constitutive model, taking the WRS, strains distribution and total equivalent plastic strains as an initial condition, are next carried out to explore the residual stresses redistribution of the T-joint under service loads. The results unveil that longitudinal stress relaxation is pronounced (especially at the weld) in the skin after the end of the loads applying. However, the longitudinal stress in the middle of the stringer is not sensitive to the service loads. The extended high tensile stress concentration zone inside the joint in case 2 and case 4 may be the potential failure location after the progressive loading of the skin-stringer welding structure. Additionally, the weld toe, which is the obvious stress sudden change zone in the service process, is a non-negligible position for the improved failure estimation of the aerospace skin-stringer structure.