The Experts below are selected from a list of 360 Experts worldwide ranked by ideXlab platform
Michael Rethmeier - One of the best experts on this subject based on the ideXlab platform.
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equivalent heat source approach in a 3d transient heat transfer simulation of full penetration high power Laser Beam Welding of thick metal plates
International Journal of Heat and Mass Transfer, 2018Co-Authors: Antoni Artinov, Marcel Bachmann, Michael RethmeierAbstract:Abstract A three-dimensional multi-physics numerical model was developed for the calculation of an appropriate equivalent volumetric heat source and the prediction of the transient thermal cycle during and after fusion Welding. Thus the modelling process was separated into two studies. First, the stationary process simulation of full-penetration keyhole Laser Beam Welding of a 15 mm low-alloyed steel thick plate in flat position at a Welding speed of 2 m min - 1 and a Laser power of 18 kW was performed. A fixed keyhole with a right circular cone shape was used to consider the energy absorbed by the workpiece and to calibrate the model. In the calculation of the weld pool geometry and the local temperature field, the effects of phase transition, thermo-capillary convection, natural convection and temperature-dependent material properties up to evaporation temperature were taken into account. The obtained local temperature field was then used in a subsequent study as an equivalent heat source for the computation of the transient thermal field during the Laser Welding process and the cooling stage of the part. The system of partial differential equations, describing the stationary heat transfer and the fluid dynamics, were strongly coupled and solved with the commercial finite element software COMSOL Multiphysics 5.0. The energy input in the transient heat transfer simulation was realised by prescription of the nodes temperature. The prescribed nodes reproduced the calculated local temperature field defining the equivalent volumetric heat source. Their translational motion through the part was modelled by a moving mesh approach. An additional remeshing condition and helper lines were used to avoid highly distorted elements. The positions of the elements of the polygonal mesh were calculated with the Laplace’s smoothing approach. Good correlation between the numerically calculated and the experimentally observed weld bead shapes and transient temperature distributions was found.
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investigation of solidification cracking susceptibility during Laser Beam Welding using an in situ observation technique
Science and Technology of Welding and Joining, 2018Co-Authors: Nasim Bakir, Andrey Gumenyuk, Michael RethmeierAbstract:In recent years, Laser Beam Welding has found wide applications in many industrial fields. Solidification cracks are one of the most frequently encountered Welding defects that hinder obtaining a safe weld joint. Decades of research have shown that one of the main causes of such cracks are the strain and the strain rate. Obtaining meaningful measurements of these strains has always been a major challenge for scientists, because of the specific environment of the measurement range and the many obstacles, as well as the high temperature and the plasma plume. In this study, a special experimental setup with a high-speed camera was employed to measure the strain during the Welding process. The hot cracking susceptibility was investigated for 1.4301 stainless steel, and the critical strain required for solidification crack formation was locally and globally determined.
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Improved degassing in Laser Beam Welding of aluminum die casting by an electromagnetic field
Journal of Materials Processing Technology, 2018Co-Authors: André Fritzsche, Kai Hilgenberg, Fabian Teichmann, Helge Pries, Klaus Dilger, Michael RethmeierAbstract:Abstract An electromagnetic system was used to reduce the porosity in Laser Beam Welding of aluminum die casting. The difference of electrical conductivities between gases and molten aluminum is used by an electromagnetic system to displace the included gases to the top during a Laser Beam Welding process. It bases on generating Lorentz forces within the weld pool via an oscillating magnetic field. A partial penetration Laser Beam Welding of standard die-cast aluminum alloy AC-AlSi9MnMg in flat position is used. An optimized Laser Beam process was taken to investigate the porosity content and the surface smoothing by applying an electromagnetic field. The produced weld seams were analyzed in cross section views, by x-ray imaging and by computer tomography (CT). Depending on the magnetic flux density, a significantly reduction of the porosity down to 75% can be achieved. Especially big pores can be removed successfully. A surface smoothing of up to 75% can also be reached by using this system.
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Numerical simulation of solidification crack formation during Laser Beam Welding of austenitic stainless steels under external load
Welding in the World, 2016Co-Authors: N. Bakir, Andrey Gumenyuk, Michael RethmeierAbstract:Solidification cracking phenomena taking place under controlled tensile weldability (CTW) test conditions have already been investigated both experimentally and numerically via FEA in order to get a better understanding of the mechanisms of hot crack formation during Laser Beam Welding of austenitic steel grades. This paper develops a three-dimensional finite element model employing the contact element technique to simulate the formation and propagation of solidification cracks during Laser full penetration Welding of fully austenitic stainless steel 1.4376. During the experimental procedure, the resulting strain and displacement directed to the Laser Beam in the close vicinity of the weld pool was measured at the surface of the workpiece using a digital image correlation (DIC) technique with an external diode Laser as an illuminating source. Local strain fields, global loads and crack lengths predicted by the model are in good agreement with those observed in experiments.
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Numerical assessment and experimental verification of the influence of the Hartmann effect in Laser Beam Welding processes by steady magnetic fields
International Journal of Thermal Sciences, 2016Co-Authors: Marcel Bachmann, Vjaceslav Avilov, Andrey Gumenyuk, Michael RethmeierAbstract:Abstract Controlling the dynamics in the weld pool is a highly demanding challenge in deep-penetration Laser Beam Welding with modern high power Laser systems in the multi kilowatt range. An approach to insert braking forces in the melt which is successfully used in large-scaled industrial applications like casting is the so-called Hartmann effect due to externally applied magnetic fields. Therefore, this study deals with its adaptation to a Laser Beam Welding process of much smaller geometric and time scale. In this paper, the contactless mitigation of fluid dynamic processes in the melt by steady magnetic fields was investigated by numerical simulation for partial penetration Welding of aluminium. Three-dimensional heat transfer, fluid dynamics including phase transition and electromagnetic field partial differential equations were solved based on temperature-dependent material properties up to evaporation temperature for two different penetration depths of the Laser Beam. The Marangoni convection in the surface region of the weld pool and the natural convection due to the gravitational forces were identified as main driving forces in the weld pool. Furthermore, the latent heat of solid–liquid phase transition was taken into account and the solidification was modelled by the Carman–Kozeny equation for porous medium morphology. The results show that a characteristic change of the flow pattern in the melt can be achieved by the applied steady magnetic fields depending on the ratio of magnetic induced and viscous drag. Consequently, the weld bead geometry was significantly influenced by the developing Lorentz forces. Welding experiments with a 16 kW disc Laser with an applied magnetic flux density of around 500 mT support the numerical results by showing a dissipating effect on the weld pool dynamics.
Nikolai Kashaev - One of the best experts on this subject based on the ideXlab platform.
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prospects of Laser Beam Welding and friction stir Welding processes for aluminum airframe structural applications
Journal of Manufacturing Processes, 2018Co-Authors: Nikolai Kashaev, Volker Ventzke, G CamAbstract:Abstract The present study deals with Laser Beam Welding (LBW) and friction stir Welding (FSW) applied to high-strength aluminum alloys used in aircraft industry and displays their advantages compared with the riveting technique regarding structural integrity, weight and material savings. First of all, it is shown with respect to different applications and strength levels which high-strength aluminum alloys represent the state-of-the-art and which aluminum alloys are proposed as substitutes in the future. Furthermore, the respective joining process principles are described and demonstrated on different joint configurations, whereby mechanical and microstructural properties of Laser Beam- and friction-stir-welded joints are discussed and compared. The current study clearly demonstrates that these two joining techniques are not competing but complementary joining techniques in the aircraft industry. FSW, as a solid-state joining process, has the advantage that the joining is conducted at temperatures below the melting point of the materials to be joined. Therefore, improved mechanical performance of joints is expected compared to that of fusion joining processes such as LBW. Furthermore, better mechanical properties can be obtained when heat input during joining is reduced by employing stationary shoulder FSW and/or external cooling. On the other hand, LBW offers several advantages such as low distortion, high strength of the joint, and high Welding speeds due to its low localized-energy input. Thus, LBW - as a high-speed and easily controllable process - allows the Welding of optimized complex geometrical forms in terms of mechanical stiffness, strength, production velocity, and visual quality. Both joining processes have advantages and disadvantages, depending on joint geometries and materials. They both have the potential to reduce the total weight of the structure. The FSW process (particularly lower heat input stationary shoulder FSW process) is more advantageous in producing long-distance straight-line butt joints or overlapped joints of aircraft structures, whereas the high-speed and easily controllable LBW process allows the joining of complex geometrical forms due to its high flexibility, particularly in the new generation high strength Al-alloys (such as AA2198), the strengthening phases of which are more heat resistant.
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Laser Beam Welding of a cocrfenimn type high entropy alloy produced by self propagating high temperature synthesis
Intermetallics, 2018Co-Authors: Nikolai Kashaev, Volker Ventzke, N D Stepanov, D G Shaysultanov, V N Sanin, S V ZherebtsovAbstract:Abstract Fiber Laser Beam Welding of a CoCrFeNiMn-type high entropy alloy (HEA) produced by self-propagating high-temperature synthesis (SHS) was reported in this work. The SHS-fabricated alloy was characterized by both ∼2 times reduced Mn content in comparison with that of the other principal components and the presence of impurities including Al, C, S, and Si. The as-fabricated alloy was composed of columnar fcc grains with coarse precipitates of MnS and fine Cr-rich M23C6 carbides. Successful defect-free butt joint of the alloy was obtained using a Laser power of 2 kW and a Welding speed of 5 m/min. Welding resulted in changes in texture and structure of the fcc matrix. In addition, precipitation of nanoscale B2 phase particles in the weld zone was observed. A pronounced increase in microhardness from (153 ± 3) HV 0.5 (base material) to (208 ± 6) HV 0.5 (fusion zone) was observed. The B2 phase precipitation after Welding was found to be in a reasonable agreement with the ThermoCalc predictions. Quantitative analysis demonstrated that the increase in hardness can be associated with the B2 phase precipitation. Possibilities of the development of HEAs with intrinsic hardening ability after Laser processing are discussed.
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effect of nd yag Laser Beam Welding on weld morphology and mechanical properties of ti 6al 4v butt joints and t joints
Optics and Lasers in Engineering, 2016Co-Authors: Nikolai Kashaev, Volker Ventzke, Vadim Fomichev, Fedor Fomin, Stefan RiekehrAbstract:Abstract A Nd:YAG single-sided Laser Beam Welding process study for Ti–6Al–4V butt joints and T-joints was performed to investigate joining techniques with regard to the process-weld morphology relationship. An alloy compatible filler wire was used to avoid underfills and undercuts. The quality of the butt joints and T-joints was characterized in terms of weld morphology, microstructure and mechanical properties. Joints with regular shapes, without visible cracks, pores, and geometrical defects were achieved. Tensile tests revealed high joint integrity in terms of strength and ductility for both the butt joint and T-joint geometries. Both the butt joints and T-joints showed base material levels of strength. The mechanical performance of T-joints was also investigated using pull-out tests. The performance of the T-joints in such tests was sensitive to the shape and morphology of the welds. Fracture always occurred in the weld without any plastic deformation in the base material outside the weld.
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single sided Laser Beam Welding of a dissimilar aa2024 aa7050 t joint
Materials & Design, 2015Co-Authors: Josephin Enz, V Khomenko, Stefan Riekehr, Volker Ventzke, N Huber, Nikolai KashaevAbstract:Abstract In the aircraft industry double-sided Laser Beam Welding of skin–stringer joints is an approved method for producing defect-free welds. But due to limited accessibility – as for the Welding of skin–clip joints – the applicability of this method is limited. Therefore single-sided Laser Beam Welding of T-joints becomes necessary. This also implies a reduction of the manufacturing effort. However, the main obstacle for the use of single-sided Welding of T-joints is the occurrence of weld defects. An additional complexity represents the combination of dissimilar and hard-to-weld aluminium alloys – like Al–Cu and Al–Zn alloys. These alloys offer a high strength-to-density ratio, but are also associated with distinct weldability problems especially for fusion Welding techniques like Laser Beam Welding. The present study demonstrates how to overcome the weldability problems during single-sided Laser Beam Welding of a dissimilar T-joint made of AA2024 and AA7050. For this purpose a high-power fibre Laser with a large Beam diameter is used. Important Welding parameters are identified and adjusted for achieving defect-free welds. The obtained joints are compared to double-sided welded joints made of typical aircraft aluminium alloys. In this regard single-sided welded joints showed the expected differing weld seam appearance, but comparable mechanical properties.
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in situ study of phase transformations during Laser Beam Welding of a tial alloy for grain refinement and mechanical property optimization
Intermetallics, 2015Co-Authors: Peter Staron, Stefan Riekehr, N Huber, Andreas Stark, Norbert Schell, Andreas Schreyer, Martin Muller, Nikolai KashaevAbstract:Abstract In situ time-resolved X-ray diffraction by synchrotron radiation was used to monitor the phase transformations and grain-refining processes during Laser-Beam Welding of a γ-TiAl-based alloy. The heating rate plays an important role of grain refinement. A high heating rate suppresses solid–solid phase transformations. The superheated γ grains serve as heterogeneous nuclei for β grains on subsequent solidification and refine the lamellar colonies. At low heating rate, diffusion-based transformations are observed on heating and coarse lamellae are formed after Welding. The refined lamellar colonies improve the mechanical properties.
Stefan Riekehr - One of the best experts on this subject based on the ideXlab platform.
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effect of nd yag Laser Beam Welding on weld morphology and mechanical properties of ti 6al 4v butt joints and t joints
Optics and Lasers in Engineering, 2016Co-Authors: Nikolai Kashaev, Volker Ventzke, Vadim Fomichev, Fedor Fomin, Stefan RiekehrAbstract:Abstract A Nd:YAG single-sided Laser Beam Welding process study for Ti–6Al–4V butt joints and T-joints was performed to investigate joining techniques with regard to the process-weld morphology relationship. An alloy compatible filler wire was used to avoid underfills and undercuts. The quality of the butt joints and T-joints was characterized in terms of weld morphology, microstructure and mechanical properties. Joints with regular shapes, without visible cracks, pores, and geometrical defects were achieved. Tensile tests revealed high joint integrity in terms of strength and ductility for both the butt joint and T-joint geometries. Both the butt joints and T-joints showed base material levels of strength. The mechanical performance of T-joints was also investigated using pull-out tests. The performance of the T-joints in such tests was sensitive to the shape and morphology of the welds. Fracture always occurred in the weld without any plastic deformation in the base material outside the weld.
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single sided Laser Beam Welding of a dissimilar aa2024 aa7050 t joint
Materials & Design, 2015Co-Authors: Josephin Enz, V Khomenko, Stefan Riekehr, Volker Ventzke, N Huber, Nikolai KashaevAbstract:Abstract In the aircraft industry double-sided Laser Beam Welding of skin–stringer joints is an approved method for producing defect-free welds. But due to limited accessibility – as for the Welding of skin–clip joints – the applicability of this method is limited. Therefore single-sided Laser Beam Welding of T-joints becomes necessary. This also implies a reduction of the manufacturing effort. However, the main obstacle for the use of single-sided Welding of T-joints is the occurrence of weld defects. An additional complexity represents the combination of dissimilar and hard-to-weld aluminium alloys – like Al–Cu and Al–Zn alloys. These alloys offer a high strength-to-density ratio, but are also associated with distinct weldability problems especially for fusion Welding techniques like Laser Beam Welding. The present study demonstrates how to overcome the weldability problems during single-sided Laser Beam Welding of a dissimilar T-joint made of AA2024 and AA7050. For this purpose a high-power fibre Laser with a large Beam diameter is used. Important Welding parameters are identified and adjusted for achieving defect-free welds. The obtained joints are compared to double-sided welded joints made of typical aircraft aluminium alloys. In this regard single-sided welded joints showed the expected differing weld seam appearance, but comparable mechanical properties.
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in situ study of phase transformations during Laser Beam Welding of a tial alloy for grain refinement and mechanical property optimization
Intermetallics, 2015Co-Authors: Peter Staron, Stefan Riekehr, N Huber, Andreas Stark, Norbert Schell, Andreas Schreyer, Martin Muller, Nikolai KashaevAbstract:Abstract In situ time-resolved X-ray diffraction by synchrotron radiation was used to monitor the phase transformations and grain-refining processes during Laser-Beam Welding of a γ-TiAl-based alloy. The heating rate plays an important role of grain refinement. A high heating rate suppresses solid–solid phase transformations. The superheated γ grains serve as heterogeneous nuclei for β grains on subsequent solidification and refine the lamellar colonies. At low heating rate, diffusion-based transformations are observed on heating and coarse lamellae are formed after Welding. The refined lamellar colonies improve the mechanical properties.
Andrey Gumenyuk - One of the best experts on this subject based on the ideXlab platform.
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investigation of solidification cracking susceptibility during Laser Beam Welding using an in situ observation technique
Science and Technology of Welding and Joining, 2018Co-Authors: Nasim Bakir, Andrey Gumenyuk, Michael RethmeierAbstract:In recent years, Laser Beam Welding has found wide applications in many industrial fields. Solidification cracks are one of the most frequently encountered Welding defects that hinder obtaining a safe weld joint. Decades of research have shown that one of the main causes of such cracks are the strain and the strain rate. Obtaining meaningful measurements of these strains has always been a major challenge for scientists, because of the specific environment of the measurement range and the many obstacles, as well as the high temperature and the plasma plume. In this study, a special experimental setup with a high-speed camera was employed to measure the strain during the Welding process. The hot cracking susceptibility was investigated for 1.4301 stainless steel, and the critical strain required for solidification crack formation was locally and globally determined.
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weld pool shape observation in high power Laser Beam Welding
Procedia CIRP, 2018Co-Authors: Antoni Artinov, Andrey Gumenyuk, Nasim Aki, Marcel Achma, Michael RethmeieAbstract:Abstract The geometry of the melt pool in Laser Beam Welding plays a major role to understand the dynamics of the melt and its solidification behavior. In this study, a butt configuration of 15 mm thick structural steel and transparent quartz glass was used to observe the weld pool geometry by means of high-speed camera and an infrared camera recording. The observations show that the dimensions of the weld pool vary depending on the depth. The areas close to the weld pool surface take a teardrop-shape. A bulge-region and its temporal evolution were observed approximately in the middle of the depth of the weld pool. Additionally, a 3D transient thermal-fluid numerical simulation was performed to obtain the weld pool shape and to understand the formation mechanism of the observed bulging effect. The model takes into account the local temperature field, the effects of phase transition, thermo-capillary convection, natural convection and temperature-dependent material properties up to evaporation temperature. The numerical results showed good accordance and were furthermore used to improve the understanding of the experimentally observed bulging effect.
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Numerical simulation of solidification crack formation during Laser Beam Welding of austenitic stainless steels under external load
Welding in the World, 2016Co-Authors: N. Bakir, Andrey Gumenyuk, Michael RethmeierAbstract:Solidification cracking phenomena taking place under controlled tensile weldability (CTW) test conditions have already been investigated both experimentally and numerically via FEA in order to get a better understanding of the mechanisms of hot crack formation during Laser Beam Welding of austenitic steel grades. This paper develops a three-dimensional finite element model employing the contact element technique to simulate the formation and propagation of solidification cracks during Laser full penetration Welding of fully austenitic stainless steel 1.4376. During the experimental procedure, the resulting strain and displacement directed to the Laser Beam in the close vicinity of the weld pool was measured at the surface of the workpiece using a digital image correlation (DIC) technique with an external diode Laser as an illuminating source. Local strain fields, global loads and crack lengths predicted by the model are in good agreement with those observed in experiments.
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Numerical assessment and experimental verification of the influence of the Hartmann effect in Laser Beam Welding processes by steady magnetic fields
International Journal of Thermal Sciences, 2016Co-Authors: Marcel Bachmann, Vjaceslav Avilov, Andrey Gumenyuk, Michael RethmeierAbstract:Abstract Controlling the dynamics in the weld pool is a highly demanding challenge in deep-penetration Laser Beam Welding with modern high power Laser systems in the multi kilowatt range. An approach to insert braking forces in the melt which is successfully used in large-scaled industrial applications like casting is the so-called Hartmann effect due to externally applied magnetic fields. Therefore, this study deals with its adaptation to a Laser Beam Welding process of much smaller geometric and time scale. In this paper, the contactless mitigation of fluid dynamic processes in the melt by steady magnetic fields was investigated by numerical simulation for partial penetration Welding of aluminium. Three-dimensional heat transfer, fluid dynamics including phase transition and electromagnetic field partial differential equations were solved based on temperature-dependent material properties up to evaporation temperature for two different penetration depths of the Laser Beam. The Marangoni convection in the surface region of the weld pool and the natural convection due to the gravitational forces were identified as main driving forces in the weld pool. Furthermore, the latent heat of solid–liquid phase transition was taken into account and the solidification was modelled by the Carman–Kozeny equation for porous medium morphology. The results show that a characteristic change of the flow pattern in the melt can be achieved by the applied steady magnetic fields depending on the ratio of magnetic induced and viscous drag. Consequently, the weld bead geometry was significantly influenced by the developing Lorentz forces. Welding experiments with a 16 kW disc Laser with an applied magnetic flux density of around 500 mT support the numerical results by showing a dissipating effect on the weld pool dynamics.
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high power Laser Beam Welding of thick walled ferromagnetic steels with electromagnetic weld pool support
Physics Procedia, 2016Co-Authors: André Fritzsche, Vjaceslav Avilov, Andrey Gumenyuk, Kai Hilgenberg, Michael RethmeieAbstract:Abstract The development of modern high power Laser systems allows single pass Welding of thick-walled components with minimal distortion. Besides the high demands on the joint preparation, the hydrostatic pressure in the melt pool increases with higher plate thicknesses. Reaching or exceeding the Laplace pressure, drop-out or melt sagging are caused. A contactless electromagnetic weld support system was used for Laser Beam Welding of thick ferromagnetic steel plates compensating these effects. An oscillating magnetic field induces eddy currents in the weld pool which generate Lorentz forces counteracting the gravity forces. Hysteresis effects of ferromagnetic steels are considered as well as the loss of magnetization in zones exceeding the Curie temperature. These phenomena reduce the effective Lorentz forces within the weld pool. The successful compensation of the hydrostatic pressure was demonstrated on up to 20 mm thick plates of duplex and mild steel by a variation of the electromagnetic power level and the oscillation frequency.
Zeng Liying - One of the best experts on this subject based on the ideXlab platform.
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electron Beam Welding Laser Beam Welding and gas tungsten arc Welding of titanium sheet
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2000Co-Authors: Qi Yunlian, Hong Quan, Deng Ju, Zeng LiyingAbstract:Microstructures, properties and technical parameters of Welding specimen of 0.5 mm thick sheets of commercial purity titanium (C.P. Ti) have been studied via high vacuum electron Beam Welding (EBW-HV), CO2 Laser Beam Welding (LBW) and gas tungsten arc Welding (TIG), as well as optical microscope (OM) observation and microhardness measuring. The results indicate that the EBW is more suitable for C.P. Ti sheets Welding, and the Welding seam without defects can be obtained. The tensile strength and microhardness of joints are corresponding to matrix structure. The full-penetration butt welds are obtained by TIG, but they have many defects such as wide weld-seam, big deformation and coarse grains. The LBW has many advantages such as the narrowest weld-seam, the least deformation and the finest grains. The fine grains are good for properties of weld seam, but the LBW should be studied again for the reasons of unstable Welding technologies and strict condition.
