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The traditional welding processes are challenging to obtain an excellent jointed structure with a thickness of over 80 mm. Meanwhile, cyclic heating for large-scale manufacturing components worsens microstructure and mechanical properties after welding. Laser Melting Deposition (LMD) technology is put forward to break through the limit of large-scale jointing components widely used for manufacturing aerospace products. Therefore, the LMD experiment with coaxial powder feeding for joining a pair of X-groove Ti-6Al-4V thick plates (80 mm) with different thermal behaviors (including laser scanning speeds and cooling rates) is carried out in this paper. The morphology evolution of its microstructure at the equiaxed grain zone (EQZ) and the deposition area under different thermal behaviors are systematically investigated. Meanwhile, the relationship between microstructure evolution mechanism and tensile properties is further explored. The volume fraction and size of acicular α/α' phases at the EQZ or the deposition area keep a stable rising when the laser scanning speed increase from 10mm/s to 20mm/s. The α-Ti cluster phases are usually precipitate around the β grain boundary with a low scanning speed and cooling rate. By contrast, the orthogonal acicular martensitic α/α' phase can precipitate inside the β grain, forming a Basketweave structure with a high scanning speed and cooling rate. The Basketweave structure forming in β grains can coordinate the deformation and reduce the stacking effect of dislocations, which is beneficial for improving ductility. However, the α-Ti cluster phases precipitating around the β grain boundary usually cause local stress concentration at these locations, forming microcracks at this location and deteriorating the tensile properties.

The laser additive manufacturing technology with laser melting deposition as the core technology presents superior efficient connectivity of titanium alloy components with large thickness and complex structure in the aerospace field. The content of nanoparticles affects the microstructure and properties of TC4 large thickness titanium alloy joint when it is manufactured with laser additive manufacturing. A pair of TC4 titanium alloy test plates with a thickness of 80 mm were connected by laser additive manufacturing technology with the mixed powder as filler material which was produced by adding different content Y2O3 nanoparticles into TC4 powder. Optical microscopy, scanning electron microscopy, micro-hardness test and tensile test were conducted to study the effect of Y2O3 content on the microstructure and mechanical properties of the TC4 titanium alloy joint. The strengthening mechanism and agglomeration weakening mechanism of Y2O3 nanoparticles on microstructure and mechanical properties was investigated based on the experimental results. It was found that the addition of more Y2O3 nanoparticles resulted in partial powder agglomeration and degradation of tensile strength. The results indicated that the columnar grains in the middle of the deposition zone with Y2O3 nanoparticles were finer than those without Y2O3 nanoparticles. The joint fracture mode gradually changed from plastic fracture to quasi-cleavage fracture with the increase of Y2O3 content. The average hardness values of the joint with various Y2O3 contents are all higher than that of the TC4 titanium alloy substrate.

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 ...

5A06 aluminum alloy plates with a thickness of 6.9 mm are welded by laser-metal inert gas (MIG) hybrid welding. To investigate the influence of laser heat input on grain size and microhardness, opt...

The 5A06 aluminum alloy butt joint is fabricated by laser-metal inert gas (MIG) hybrid welding in this investigation. The macro morphology and porosity of welded joints under different heat input are observed by optical microscope. The effect of the laser, MIG, and hybrid heat input on macro morphology and porosity of welded joint is investigated. Furthermore, the force condition inside the laser-MIG hybrid welding pool is analyzed to explain the influence mechanism of heat input on macro morphology and porosity in theory. It is found that the cross-section area of weld seam (WS) is increased with the welding heat input. The porosity of the full-penetration WS is smaller than the partial-penetration WS owing to the different flow characteristic of molten pool. The higher MIG heat input is beneficial to reduce the porosity. It is illustrated that the welded joint with a lower porosity is expected to be obtained under the energy ratio coefficient of 0.5.

With the purpose of improving the modeling efficiency, the model database is established targeting that intelligent modeling of laser welding for aerospace aluminum alloy. The model database consists of material database, heat source model database and weld joint structure database. The aerospace aluminum alloys are classified reasonably in order to establish the material model database which can manage the thermo-physical properties of aluminum alloys. The heat source models of laser welding are summarized to establish model database on the basis of the different energy distribution characteristics. The joint structure database includes butt joint, T-joint and lap joint. Modeling of arbitrary structures can be implemented quickly and efficiently when modelers invoke and assemble this model database. The finite element model is efficiently established and accurately solved by taking the 5A90 and 2219 aluminum alloy laser welding as an example. Finally, it is found that the model database established in this paper can greatly improve the modeling efficiency of laser welding numerical simulation.

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.

Porosity is one of the significant defects existing in the weld seam of aluminum alloy by laser. In the current research, the evolution of porosity and grain coupling growth in the weld seam is rea...

Laser direct joining of carbon fiber reinforced thermoplastic (CFRTP) composite plate and titanium alloy plate with a thickness of 2 mm was performed with swing laser. Numerous air bubble of submillimeter size were observed inside the fusion zone of CFRTP and titanium alloy at the cross section of the joints. The air bubble characteristics were analyzed based on the morphology and size, while the formation mechanism of air bubble was further elucidated according to the nucleation mode, nucleation site and nucleation position. The results demonstrated that the nucleation modes of air bubble are substantially divided into homogeneous nucleation and heterogeneous nucleation, which is related to the nucleation sites. The nucleation mode presents a crucial factor influencing the position and morphology of air bubble. In addition, the air bubble characteristics are also determined by the clamp pressure and resin flow. The final morphology of air bubble is primarily represented by four typical types.

Laser-MIG hybrid welding is an advancing technology for the joining of aluminum alloy with the medium-thickness owing to the high efficiency and excellent gap bridging ability. In this paper, the laser-MIG hybrid welding experiment for 5A06 aluminum alloy with the thickness of 6.9 mm is carried out. The process porosities and metallurgical porosities in the weld cross-section and tensile fracture are investigated. Furthermore, it is novelty that the formation mechanism of process porosities and metallurgical porosities are comprehensively analyzed based on the porosity morphology after welding, as well as characteristics of molten pool and keyhole during laser-MIG hybrid welding. It is found that the formation of process porosity is owing to the fact that the tip of the keyhole is easily closed by the molten metal stream and separated to form a process bubble. The formation of the metallurgical porosity is attributed to the atomic hydrogen. The inner wall of the metallurgical porosity with the second phase is observed. It is inferred that the formation of second phase is accompanied by the atomic hydrogen. Besides, the area with dense metallurgical porosities and the aggregate metallurgical porosity are formed under the driven force of the liquid metal flow behavior. The formation process of aggregate metallurgical bubble goes through three states, separation state, contact state and aggregate state.

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.

Taking TA15 titanium alloy as the research object, laser welding experiments of are performed on the lock-bottom joints, and the microstructure and tensile performance of the joints are analyzed. The laser welding speed is constant and the laser power is 1000 w, 1300 kw, 1600 kw respectively. The bottom weld width and the width of high temperature heat-affected zone (HT-HAZ) increase significantly as laser power increase. The microstructure of the heat-affected zone (HAZ) is relatively complicated, mainly composed of secondary α, acicular α structure and β. The microstructure at the weld metal is comprised of the martensite α′ and acicular α. The tensile strength, fracture morphology and chemical composition of the welded joint are studied by tensile test, scanning electron microscope (SEM) and energy dispersive spectrometer (EDS) analysis techniques. The results show that all the specimens fractured near the heat-affected zone, and the fracture is a ductile fracture in the shape of a dimple. The tensile strength of welded joint decreases with the increase of laser power. When the laser power is 1.0 kw, 1.3 kw and 1.6 kw, the tensile strength is 723 MPa, 705 MPa and 695 MPa respectively, which is 64.55%, 62.95% and 62.05% of the base material strength. EDS results show that Mo and V elements are the strengthening elements, while the burning loss of Mo and V reduce the strength of the joint.

Temperature distribution has an important influence on grain morphology during laser welding process. In this investigation, a three-dimensional finite element model considering the effect of diffe...

Hybrid laser arc welding (HLAW) features advantages such as higher welding speed and gap tolerance as well as smaller welding deformation and heat-affected zone than arc welding. Porosity in hybrid laser arc weld due to keyhole fluctuation tends to be the initial source of crack propagation, which will significantly diminish the weld performance. A high-speed imaging technique was adopted to record and analyze the droplet transfer and keyhole fluctuation behavior during hybrid laser arc welding of aluminum alloys. A heat transfer and fluid flow model of HLAW was established and validated for a perspective of the evolution process of droplet transfer and keyhole fluctuation. The relationship between keyhole fluctuation and weld porosity was also revealed. During the droplet transfer stage, liquid metal on the top surface of the weld pool flows toward the keyhole originated by globular transfer, and the keyhole fluctuates and decreases significantly, which has a higher tendency to form a bubble in the weld pool. The bubble evolves into porosity once trapped in the mush-zone near the trailing edge of the weld pool. Therefore, globular transfer during HLAW is the principal origin of keyhole fluctuation and weld porosity. Welding current has a significant influence on keyhole fluctuation and weld porosity rate. Droplet transfer frequency, keyhole fluctuation, and porosity rate increase with higher welding current under the globular transfer mode. The porosity rate shows a nearly positive correlation with the standard deviation of keyhole fluctuation.

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.

Al-Li alloys have been applied in modern aerospace manufacturing fields due to their excellent properties. In this investigation, 2-mm-thick 2060 Al-Li alloy laser beam welding was carried out under different heat input by KUKA Robot. The macro profile and micro morphology characteristics of weld seam cross section were observed. For further investigating the effect of micro morphology in different zones on mechanical properties of welded joints, micro-hardness test and tensile test were carried out and the experimental results were analyzed. It was found that the micro-hardness distribution was corresponding to the grain morphology distribution. The average value of micro hardness in the weld seam center was the smallest because of the biggest grain size. During the tensile test process, there was no obvious necking phenomenon observed in the fracture section and the displacement of tensile specimen was extremely small. According to the analysis of fracture morphology, the bigger and deeper dimple in fracture contributed to better tensile performance. Besides, the strengthening phases were rich in Cu element that was considered as the result of elements redistribution during laser beam welding process.

Laser joining technology with the advantages of high efficiency and good processing flexibility has the good application prospects in the connection of the Carbon Fibre Reinforced Thermoplastics Plastics (CFRTP) and aluminum alloy. Laser joining of CFRTP and 6061 aluminum alloy is investigated with various laser processing parameters. The microstructure characteristics and energy dispersive spectroscopy (EDS) results are analyzed. Besides, the fracture morphologies of the laser connection joints are studied. The results indicate that there is a partial unconnected area between the resin and the 6061 aluminum alloy due to the existence of the air bubbles, while most of the interface area is tightly connected where the mechanical anchoring effect exists.

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.

Ultraviolet(UV) laser treatment on the surface of the carbon fiber reinforced polymer (CFRP) laminate becomes an effective method to benefit the bonding strength of adhesively bonded joint in aerospace industries. In the present research, homomorphic CFRP laminates with different resin distribution on the surface are bonded into single-lap joints. Their shear strengths are tested to evaluate the effect of surface resin distribution on bonding mechanical performance. The different resin distributions on the surface of CFRP laminate are obtained by UV pulse laser with different laser scanning speeds. The scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) are conducted to analyze the laser treated surfaces and fracture surfaces of tested joints. The experimental results indicate that the residual resin of CFRP surface increases with the increase of scanning speed. Compared with both the reference surface without laser pre-treatment and that with no-residual resin for bonding, the surface with partial residual resin results in an enhancement of the shear strength of bonded joint. Moreover, the shear strength of the reference sample is higher than that bonded by the surface with no-residual resin. The research lays foundation for understanding the relationship between surface resin distribution and bonding strength.

The promising dual laser-beam bilateral synchronous welding (DLBSW) technology was employed in the 2219 aluminum alloy T-joint which consisted of the skin and stringer components. The coherent weld seam (WS) is achieved under the heat affect of dual laser beam. The stripping experiment was carried out to test the stripping strength of DLBSW joint. With the purpose of discussing the relationship between the welding shape and stripping strength, H 1 was measured to evaluate the shape of welded T-joint. The microstructure characteristics of T-joint were observed to discuss the crack location during striping experiment. Besides, the fracture morphology was observed by scanning electron microscope (SEM). The chemical element composition on the impure phase in the fracture was detected by energy dispersive spectrometer (EDS). The highest stripping strength is 317.46 MPa with the laser power of 5500 W and welding speed of 4.2 m/min. The stripping crack locations are mainly divided into three categories. The highest stripping strength is achieved under the second category of crack location which is taken place near the upper fusion line. It is indirectly found that the strength of WS in case 6 with the laser power of 5500 W is highest than the others. The degrading of stripping strength is mainly owing to the reducing of plasticity as well as the formation of internal defects such as pores, bonded structure and nubby impure phases. The nubby impure phase is mainly metal oxides (TiO 2 , Al 2 O 3 ) which generally degrade the weld quality. It is notable that a new skin component accompanied with two bosses is firstly designed in the present study to replace the wire filler.

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 ...

Laser-metal inter gas (MIG) hybrid welding technique provides higher overall productivity over the traditional fusion welding processes for joining medium-thick aluminum alloy. In this paper, laser-MIG hybrid welding experiment was performed by the TruDisk 12003 Laser and KUKA robot. The second fusion line, the laser-dominated region and laser-MIG hybrid-dominated region were introduced to this study to discuss the microstructure distribution. A three-dimensional finite element model was established by MSC. Marc to study the thermal field distribution of laser-MIG hybrid welding process. It found that the temperature in the laser-MIG hybrid-dominated region was much higher than the laser-dominated region. The formation of the second fusion line in the welded joint is mainly owing to the different heat effect of laser-arc hybrid-dominated region and laser-dominated region. The crystalline size of equiaxed dendrite grains in the laser-MIG hybrid-dominated region is finer than the laser-dominated region. In addition, the tensile property was measured to dissect the failure mechanism of the laser-MIG hybrid welding joints. The element content in fractured surface at the position of dimple and pore wall were comprehensively analyzed. The element content of Mg and O in pore wall are higher than dimple. In the solidification process, the growth of equiaxed dendrite grains is in part hindered by the existence of coarse pores in the laser-MIG hybrid-dominated region.

The surfaces of the carbon fiber resin matrix reinforced polymer (CFRP) are completely cleaned and partially cleaned by controlling the ultraviolet laser (UV) scanning speed. Residual resin and surface energy of laser cleaning samples are analyzed and correlated to the adhesive bonding tensile properties. The results show that the main damage form of CFRP bonding joints obtained by UV laser complete cleaning and partial cleaning are mixed failure, in which the interface failure and cohesion damage play an important role in the tensile properties of the two kinds of joints, respectively. The surface of the samples after UV laser partial cleaning has been proved to have obvious infrared absorption peaks. Residual resin on the surface of partial cleaning samples is beneficial to improve the tensile properties of bonded samples. The CFRP surface energy by UV laser partial cleaning is 83.35 mJ/m2, which is far higher than that of the original surface. Nevertheless, the CFRP surface energy after UV laser complete cleaning is 56.67 mJ/m2, which is lower than that of UV laser partial cleaning. To a certain extent, the magnitude of the surface energy reflects the bonding property of CFRP. Therefore, the strength of the CFRP adhesive joint after partial cleaning is stronger than that of CFRP joints after complete cleaning.

The Invar alloy coatings were prepared using laser cladding technology. The effects of heat input on the coatings size, microstructure and porosity of laser cladding layers were investigated by optical microscope (OM), scanning electron microscope (SEM) and electronic differential system (EDS). In this analysis, the microstructure of coatings consist of elongate cellular crystals and equiaxed cellular crystals. The results showed that the size, HAZ and crystals size of the samples clearly show a noteworthy increase by improving the heat input from 81.4 to 230 J/mm2. Meanwhile, an interesting phenomenon was discussed that the subgrain size near the layer line were extraordinarily distinct from the normal. The pores in the coatings are considered to be divided into two categories. The one is fusion lack porosity which is caused by the unfinished powders and rapid solidification. The other one, bubble porosity, is composed of trapped shielding gas and metal vapor. It is demonstrated that the porosity factors increase until a threshold heat input is reached and then decrease as the heat input increases. In this paper, the heat input of 126 J/mm2 is more suitable to be applied in laser cladded Invar alloy.

Adhesive bonding technique for carbon fiber reinforced plastics (CFRP) composites is believed to be a promising connection method in terms of aircraft composite structural components. The laser surface cleaning processing is an effective way to improve the mechanical properties of CFRP adhesive bonding joints. The comparison between infrared laser surface treatment and ultraviolet (UV) laser surface treatment of CFRP laminates is investigated with various laser processing parameters. The microstructure characteristics and energy dispersive spectroscopy (EDS) results are analyzed. In addition, the detections of bonding strength and contact angles are also performed. The results indicate that the infrared laser thermal effect leads to the burning of carbon fibers, while UV laser surface processing with no thermal effect guarantees the integrity of fibers. Compared with infrared laser processing, the adhesive bonding joints obtained by UV laser treatment have a higher average shear strength.

The simulation method in laser-MIG hybrid welding, which involves two heat sources and multiple welding parameters, is beneficial to reveal the complex physical phenomena and dynamic behavior of molten pool keyhole during welding process. In this investigation, laser-MIG hybrid welding for 6-mm-thick 6061 aluminum alloy was performed under different heat input by the high-power disc laser, MIG welding system and KUKA Robot. The high-speed camera system was used to observe the droplet transition phenomenon in the welding process. Besides, a thermal-fluid coupling model was established to simulate the temperature field and flow field, which were changed by the droplet transfer during laser-MIG hybrid welding. The experimental and simulated results showed that the droplet transition behavior affected the formation of the keyhole. The keyhole was the smallest when the droplet contacted the molten pool. In addition, the droplet transition brought external momentum and energy to the molten pool, which was conducive to the increase of the flow rate of the molten pool.

Invar alloys are widely used in the aerospace, but their manufacturability and mechanical properties are not well understood when they are employed in additive manufacturing. The laser melting deposition (LMD) technology is utilized to the surface repair of Invar alloys. In this work, the microstructure evolution, quantitive subgrain size, as well as the subgrain orientation are investigated using optical microscopy. Moreover, the effect of partially remelting of successive layers is exhibited. It is explicit that the cellular crystal orientation is strongly influenced by the heat flow which is subject to the scanning direction. Considering to the basic laws of Invar alloy microstructure evolution during LMD process, the relations between heat input and produced subgrain characteristics are further elucidated. The results show that a slight increase of λ from 4.4 μm to 5.2 μm at the function of heat input varied from 98.6 J/mm2 to 114 J/mm2. Moreover, it was notable that a reverse change in the dimension discrepancy of remelted and non-remelted subgrains occurred with two kinds of heat input.

Dual laser-beam bilateral synchronous welding technique is considered to replace riveting in terms of skin–stringer joints. The 2.0-mm thick Al-Li alloy T-joint, which is welded by dual laser-beam bilateral synchronous welding between 2099-T83 stringer and 2060-T8 skin, is investigated in the current study. Under the tailored process conditions, the examinations and analyses of microstructures, microhardness distribution, hoop tensile mechanical properties, and fractographies of the T-joint are conducted. Experimental results show that the maximum tensile strength of the weld seam is up to 85% of the tensile strength of the skin material. In order to explore the special fracture characteristics of dual laser-beam bilateral synchronous welding joints after tensile test, a three-dimensional finite element model is established and the residual stress of the weldment is analyzed. Considering the scanning electron microscopy results and simulation results comprehensively, stress concentration is the main reason for the weldment failure in tensile test.

Thermoelectric currents (TECs) caused by Seebeck effect were usually neglected in welding simulation. This paper characterizes the TECs and thermoelectric-magnetic phenomena during the magnetically supported laser beam welding (MSLBW) of thick aluminum (Al) alloy. The simulation is based on a computational fluid dynamic (CFD) model developed in three dimensions. The model considers multi-physical mechanisms including thermal transfer, solid–liquid (S/L) transition, molten metal convection, magnetohydrodynamics (MHD) and Seebeck effect. The computed TEC distribution in laser welding pool highly depends on welding time and temperature gradient. The large current densities occur near the weld pool edge and the vectors are opposite at both side of S/L interface. The maximum TEC density obtained in the stabilized weld pool is 2.14 × 106 A/m2, leading to a self-induced magnetic field of 2.27 × 10−3 T and a Lorentz force of 2.62 × 103 N/m3. The influences of TECs on weld pool dynamics are quite limited in LBW but notable in MSLBW. The inversion of MF direction leads to different Lorentz force distributions, weld pool dimensions and seam profiles. The simulation data agrees well with the experimental results.

Laser-MIG hybrid welding (LMHW) has not been extensively applied in welding of plate-pipe dissimilar steel with thick-walled because of the complex welding process requirements. In this study, numerical simulation and experimental studies of LMHW with different parameters of 25CrMo4 steel and 33MnCrB5-2 steel are presented. A 3D finite element model with coupled thermal-structural is established for analyzing the effects of welding parameters on the posted-weld thermal field, residual stress, and deformation distribution by using combined ellipsoid-Gaussian heat source model. Comparison of the calculated and experimental results suggested that the LMHW process is characterized well by the combined heat source model. Additionally, the inter-layer cooling time is determined by the amount of heat input, which also affects the magnitude and distribution of residual stress. Residual stress and deformation are controlled within the adaptable range. But, the component cannot be penetrated when laser power of backing weld P is 6000 W and welding current I is 282 A.

Purpose This paper aims to describe a three-dimensional mathematical and numerical model based on finite volume method to simulate the fluid dynamics in weld pool, droplet transfer and keyhole behaviors in the laser-MIG hybrid welding process of Fe36Ni Invar alloy. Design/methodology/approach Double-ellipsoidal heat source model and adaptive Gauss rotary body heat source model were used to describe electric arc and laser beam heat source, respectively. Besides, recoil pressure, electromagnetic force, Marangoni force, buoyancy as well as liquid material flow through a porous medium and the heat, mass, momentum transfer because of droplets were taken into consideration in the computational model. Findings The results of computer simulation, including temperature field in welded plate and velocity field in the fusion zone were presented in this article on the basis of the solution of mass, momentum and energy conservation equations. The correctness of elaborated models was validated by experimental results and this proposed model exhibited close correspondence with the experimental results with respect to weld geometry. Originality/value It lays foundation for understanding the physical phenomena accompanying hybrid welding and optimizing the process parameters for laser-MIG hybrid welding of Invar alloy.

5A06 aluminum alloy plates with thickness of 6.9mm are welded by KUKA Robot with a fiber laser of IPG YLS-6000. The laser-metal inert gas (MIG) hybrid welding method is adopted to provide high overall productivity in this paper. To investigate the joints microstructure and porosity distribution, optical microscope observation, scanning electron microscope (SEM) and energy dispersive spectrometer (EDS) detection are conducted. The pores of laser-MIG hybrid welding joint are mainly metallurgical pores which are caused by the dissolved gases and evaporation of Mg element. This metallurgical pores are densely populated in the upper area of weld joint. Besides, the micro-hardness along the weld center line and perpendicular to the weld center line are separately analyzed. The joint softening can be evidently observed near heat affected zone (HAZ) and in weld bead (WB), where the grain enlargement or pores problem exist. It claims that the formation of pores has a much greater influence on the joint softening than grain enlargement. In order to get a low-porosity weld joint, as well as owning a high level of micro-hardness, the porosity and micro-hardness in different laser-MIG hybrid weld joints are analyzed comprehensively. The welding speed of 3m/min is more suitable to be applied in laser-MIG hybrid welding for the 6.9mm 5A06 aluminum alloy.

Adhesive bonding is considered to offer advantages over the connections of the aircraft composite structures. The infrared laser surface processing of various laser process parameters and peel ply treatment are performed in this study. To investigate the microstructure and properties of adhesively bonded composite single-lap joints obtained by infrared laser surface treatment and peel ply processing, the examinations of microstructures, surface free energy and shear strength of the joints are conducted. The results indicate that the adhesively bonded joints acquired by infrared laser treatment have lower average shear strength of 20.158 MPa, comparing with the joints of 32.574 MPa in average shear strength treated by peel ply processing. In addition, the surface roughness test is conducted and the surface roughness of 4.926 μm of the specimen after peel ply treatment is higher than that of the specimen after infrared laser treatment.

An externally applied magnetic field has been proved effective in controlling the molten flow behavior during the laser beam welding. This paper reveals how a magnetic field applied in different orientations influences the weld pool dynamics during the full-penetration laser butt welding of thick aluminum alloy plates. A multi-physics coupling model was developed and numerically solved by considering the electromagnetic interaction between static magnetic field and magnetofluid. The dynamic and electromagnetic information within the weld pool were predicted and comparatively analyzed. The results show that Lorentz force and Hartmann effect induced in magnet-assisted weld pool are quite sensitive to magnetic field direction. The horizontally directed magnetic field leads to limited Hartmann effect by inducing additional molten vortexes, while the field aligned vertically to aluminum surface can obviously reduce the molten flow velocity and weaken the Marangoni convection. The work can provide a reference in optimizing the operating parameters when an electromagnetic system is employed to improve the welding quality.

The morphology of microstructure affects the quality of Invar alloy mould when it is repaired with laser melting deposition (LMD). However, it is quite hard to observe the evolution of microstructure in real time. The current study adopted a CA-FE model, i.e, cellular automaton coupled with finite element, to simulate the microstructure evolution of Invar alloy in the LMD process. The mechanism of columnar-to-equiaxed transition in the molten pool was investigated based on the simulation results. In addition, an analysis was completed by comparing the simulation with the experimental results, which verified the feasibility of simulating the microstructure evolution with the CA-FE model in the LMD process. The results indicated that the bottom of the molten pool is filled by the epitaxial growth of columnar grains and the orientation of the columnar grains is consistent with the direction of the maximum temperature gradient in the molten pool. The top of the molten pool is occupied by equiaxed grains. This study provides a referenced value on the quality improvement of the mould repaired by LMD.

Purpose The purpose of this paper is to establish a three-dimensional flow field model of the Invar alloy laser–metal inert gas (laser–MIG) hybrid welding process to investigate the influence of different heat sources between different layers and to analyze the flow field based on the two different heat source models for the multilayer welding. Design/methodology/approach The Invar steel plates with 19.5 mm thickness are welded into three layers’ seam using the hybrid laser–MIG welding technology. The flow field based on different heat source models is studied and then used to investigate the influence of different heat sources in different layers during the laser–MIG hybrid welding process. The simulation results of flow field using two different heat source models are compared with experiments. Findings The flow field simulations results show that using the Gaussian rotating body heat source model to simulate the temperature field is more consistent with the experiment of the hybrid laser–MIG welding where its flow field between different layers better reflects the characteristics of the hybrid laser–MIG welding. Originality/value The findings will be useful in the study of a variety of thick-plate laser–MIG hybrid welding process fluid flows.

A multi-field coupled numerical model was proposed to calculate the weld pool dynamics during the full-penetration laser beam welding of 12 mm thick aluminum alloy plates under the function of a longitudinal magnetic field. The model dealt with the melt flow, heat transfer, solid-liquid transition and magnetic diffusion phenomena in welding process. The Lorentz force directing from the keyhole to the liquid-solid interface was induced in magnetic field supported cases. The Marangoni convection near the weld pool surfaces was obviously weakened due to the electromagnetic braking effect, which was featured with decreased velocity and compressed vortex. The heat transfer condition was largely changed, resulting in declined weld pool dimensions. When the magnetic flux density applied was larger than 0.5 T, the convective heat exchange at the middle height of weld pool became notable again due to the reversed melt flow direction. The liquid-solid lines with lower curvature were achieved. The reinforcement of Hartmann effect became limited. The model and the simulated weld pool morphologies were verified in experiment.

To regulate the inhomogeneous interfacial reaction during laser welding-brazing of Ti/Al dissimilar alloys, an advanced method of dual laser-beam bilateral synchronous welding-brazing was proposed. The formation mechanism of the intermetallic compounds (IMCs) with various dual laser-beam input was analyzed. The experimental results revealed that sound brazing joints with homogeneous serrated-shaped IMCs along the Ti/Al interface were obtained when the laser power on both sides is the same. An interesting phenomenon can be found that the thickness of IMCs layer on both sides is thicker than the middle. The highest bonding strength of Ti/Al butt joints could reach up to 71% of aluminum alloy base metal due to homogeneous serrated-shaped IMCs with a thickness of 3–4 μm.

Welding of dissimilar aluminum alloys is gaining an attention in academic and industrial communities because of wide spread industrial application. In this study, the dissimilar laser welding of AA2060-T8 and AA6061-T6 thin slabs is conducted using circular mode oscillation welding with amplitude of 0.5 mm and different oscillation frequencies. With the aid of numerical simulation, optical microscopy (OM) and electron backscatter diffraction (EBSD), the microstructure evolution mechanism of the obtained welded joints and their correlations with mechanical properties and failure modes are systematically studied. In comparison to rectilinear weldment, the mechanical properties of welded joint could be significantly improved via beam oscillation welding, with the maximum of tensile strength reaching to 277 MPa at oscillation frequency of 30 HZ, (about 89% of the AA6061-T6 sheets) and the elongation improving by about 65% compared to the sample with non-oscillation welding. Higher frequency dramatically decreases the EQZ area at AA2060 side, and sharply expenses the equiaxed grain zone at central area of welded joints. In EBSD analyses, the orientations of refined columnar grains present randomization tendency, and the coarse ones show solidification texture component with parallel to growth direction. The relationships of these microstructural variations and fracture modes are analyzed. In addition, to fully understand the formation mechanisms of welded microstructures, thermal simulation is conducted in this paper. From results of the thermal simulations, it can be observed that the 30 HZ oscillating weldment has a similar cooling rate with the one of non-oscillation mode, coupled with the stirring effect of beam oscillation, resulting in a refinement of the solidified microstructure and relatively higher mechanical properties.

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.

More and more applications of location-based services lead to the development of indoor positioning technology. As a part of the Internet of Things ecosystem, low-power Bluetooth technology provides a new direction for indoor positioning. Most existing indoor positioning algorithms are applied to specific situations. Thus, they are difficult to adapt to actually complex environments and different users. To solve this problem, this paper proposes a precise dead reckoning algorithm based on Bluetooth and multiple sensors (DRBMs). To address positioning accuracy, this paper improves the traditional Bluetooth propagation model and calculate the steps and step lengths for different users in the process of multisensor track calculation. In addition, this paper fuses the localization results of Bluetooth propagation model and multiple sensors through the Kalman filter. The experiment results show that the proposed DRBM algorithm can obtain accurate positions. The localization accuracy is within 1 m, and the best can be controlled within 0.5 m. Compared with the traditional Bluetooth positioning methods and the traditional dead reckoning methods, the proposed algorithm greatly improves positioning accuracy and universality.

Considering the laser power, scanning speed and powder feeding rate, the experimental investigation of the laser melting deposition (LMD) with coaxial powder feeding for Fe-36Ni Invar alloy was presented in this paper. The influence of different parameters on morphology of deposition layer (DL) was analyzed in detail. Besides, the microstructure in HAZ of LMD for Fe-36Ni Invar alloy and the width of HAZ were quantitatively studied. Meanwhile, a three-dimensional finite element (FE) model considering the effect of laser power and scanning speed on heat transfer process was established to investigate the thermal field of LMD process for Fe-36Ni Invar alloy. In addition, the linear heat input was introduced into this paper to analyze the comprehensive influence of process parameters on the thermal field during LMD process. Moreover, combining the simulation results of thermal field, the thermal physical mechanism of the microstructure formation in HAZ was discussed.

A thermal-electrical-mechanical coupled model was established to simulate the Parallel-gap resistance welding (PGRW) process between the Germanium-based solar cell and the silver interconnector. The simulated results showed that the peak temperature during PGRW is lower than the melting temperature of the base material. It is indicated that the connection mechanism of PGRW was mainly the interdiffusion and recrystallization due to pressure of electrode and the resistance heat. A detailed calculation method of current was proposed using semi-layered resistance model and layered resistance model. By comparing these models, it was found that the layered resistance model was more accurate to calculate the current value. The maximum residual stress was generated within the region under the welding electrode, while the maximum deformation was generated on the edge of the interconnector. The current variation trend predicated by the simulation results is in good agreement with the results obtained by the experiments.

Disk laser beam welding technique is used to fabricate 2-mm 2A14-T6 aluminum alloy plates under different welding parameters. Effects of welding parameters on microstructures and mechanical properties of the weld are investigated. Grain size and porosity ratio in the disk laser welds firstly decrease, then increase with the increase of the heat input. Hardness at the fusion line and in fusion zone, tensile strength and elongation firstly increases, then decreases with the increasing heat input. When laser power is 2500 W, welding velocity is 2.0 m/min and heat input is 75 kJ/m, the finest microstructures, the minimum porosity ratio, the highest hardness and the maximum tensile strength are obtained. The fracture type in all samples is defined as a ductile fracture, and the maximum tensile strength reaches up to 261.7 MPa, about 61.2% of that of the base metal.

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.

In this investigation, the 2219 aluminum alloy sheets with a thickness of 1.5 mm are welded by different types of lasers. By analyzing the macro-morphology and microstructure of the weld obtained by two different laser welding methods, the characteristics of welding aluminum alloy by using these different lasers are studied. The results indicate that under the gravity of molten pool and different laser power densities, the weld seams (WS) welded by fiber laser and disc laser are in X-shape and Y-shape respectively. Besides, under the influence of Marangoni convection, the equiaxed fine grains of the weld are brought into the middle part, forming a small and regular equiaxed crystal zone (EQZ). It is found that the widths of the WS and the heat-affected zone (HAZ) obtained by fiber laser are both wider than those welded by disc laser. The grain size of columnar dendrites and equiaxed dendrites in the WS welded by fiber laser is coarser than that welded by disc laser. The tensile strengths of disc-laser-welded joints are also higher. In short, disc lasers are more suitable for high-power laser welding aluminum alloys.

The pulsed metal inert gas welding and continuous metal inert gas welding with various parameters are implemented on 2-mm-thick Invar36 alloy plates. The pulsed metal inert gas welding and continuo...

In order to solve the problem of large deformation, low production efficiency and high tendency of hot cracking in welding 19.05 mm thick plates of Fe36Ni Invar alloy, laser-MIG hybrid multi-layer welding technique (LMHMW) has been developed. To investigate the influence of different welding parameters on the joint properties, optical microscope observation, SEM, EDS and microhardness measurement were conducted. Experimental results illustrated that different matching of welding parameters significantly affected the depth-to-width ratio, formation of defects and HAZ width. Besides, weld zone were consisted of two regions according to the different grain shape. The region near center of weld seam (region 1) was columnar dendrite induced by laser, while the region far away from weld seam center (region 2) was cellular dendrite which was mainly caused by MIG arc. The peak value of microhardness appeared at the center of weld seam since the grains in region 1 were relatively fine, and the lowest hardness value was obtained in HAZ. In addition, results showed that the sheets can be welded at optimum process parameters, with few defects such as, surface oxidation, porosity, cracks and lack of penetration in the welding seam: laser power of backing weld P = 5500 W, welding current I = 240 A, welding speed v = 1 m/min. laser power of filling weld P = 2000 W, welding current I = 220 A, welding speed v = 0.35 m/min. laser power of cosmetic weld P = 2000 W, welding current I = 300 A, welding speed v = 0.35 m/min.

Laser beam welding technique offers obvious advantages over other fusion welding processes in terms of joining titanium alloy. The microstructure of welded seam and heat affected zone resulted from diverse welding speeds and laser powers were investigated after simulating welding heat treatment. The analysis of the thermal transport properties successfully explained the morphology. Optimal process parameters were obtained. The simulation results were consistent with the corresponding experimental observations.

In this study, a three-dimensional (3D) finite element model was established to predict the shape of the molten zone and the temperature fields of multi-layer laser–MIG hybrid welding for Fe36Ni Invar alloy. The multi-layer laser–MIG hybrid welding experiments of 19.05-mm-thick Invar plates were conducted to obtain the well-welded joints. The molten zone shapes were compared to the simulated results to figure out the relation between the energy distribution factor and the welding temperature field. A combined heat source model was proposed to describe the interaction between laser heat source and arc heat source. Besides, the empirical formula of energy distribution factor was given. The metallography detection results showed that the increasing ratio of laser power in hybrid welding could increase the weld penetration depth and the increasing ratio of arc power in hybrid welding could increase the weld width. In addition, it was also found that the laser heat source in hybrid welding heat source contributed equiaxed grains, and weld centerlines had rotation, due to different ratios of energy distribution during the multi-layer laser–MIG hybrid welding.