Laser-welded Joints Research Articles

Improving the Quality of Welded Joints in Laser Welding of Silver Products

The article deals with promising areas of application of pulsed laser welding for products made of silver-based alloys. The results of experimental studies on the choice of modes of pumping, defocusing to improve the quality of the weld and the efficiency of the welding process.

Microstructure and properties of underwater laser welding of TC4 titanium alloy

Underwater local dry laser welding of TC4 alloy was carried out through a double-side gas-shielding nozzle, and the heat input and defocus distance were optimized by analysing the weld quality. With increasing heat input, the fusion zone (FZ) and heat-affected zone (HAZ) expanded, while the amount of vapour increased, resulting in an increase in the degree of oxidization of the underwater joint; the mechanical properties improved initially and subsequently deteriorated, influenced by bead oxidization and welding defects. With increasing defocus distance, the width of the FZ increased gradually, whilst the width of the HAZ remained largely unchanged. Excellent tensile strength and impact toughness were obtained when the laser beam was focused on the upper surface. In the FZ of the underwater laser welding (ULW) joint, the microstructure comprised full martensite α′ with an acicular morphology, whereas a small amount of grain boundary α-phase appeared in the FZ of the in-air joint, and the thickness of the α′ lamellae decreased compared to that of the in-air joint due to the higher cooling rate of the ULW joint. The impact toughness and tensile strength of the underwater joint reached ˜90% of the values for the in-air joint, and a mixed fracture mode was operative in the fracture surfaces.

Evaluation of Distortion and Residual Stresses in Dissimilar Metal Laser Welds

The distortion and residual stresses in laser weld joints play an important role in performing the intended functions. The objective of this research work is to evaluate distortion and residual stresses induced in dissimilar metal butt weld joints. The 304 L stainless steel and mild steel sheets of 0.98 mm thick are used in automobile industries. The laser welding experiments are conducted on dissimilar metals as per the L9 orthogonal array design matrix and optimal process parameters obtained using Artificial Neural Network. A mathematical model is proposed to estimate the angular distortion and longitudinal residual stresses in dissimilar weld joints. The longitudinal, transverse and twisting type distortions are evaluated using the Coordinate Measuring Machine, whereas residual stresses using an X-ray Diffractometer. The standard correction techniques are applied for background error correction and peak position was obtained using the Gaussian curve fitting technique. The outcome of the research work is useful for the evaluation of distortion and residual stress in dissimilar metal weld joints.

Effect of Preheating Temperature on Microstructure and Properties of 42CrMo4/38MnVS6 Heterogeneous Laser Welded Joint

Laser-welded forged steel pistons can meet the needs of the new era of heavy truck engines. 42CrMo4 and 38MnVS6 are widely used as piston materials due to the good mechanical properties. This study investigates the influence of preheating on microstructure and mechanical properties of 42CrMo4/38MnVS6 laser welding joint. The experimental results show preheating increases the laser absorption capacity of the metal, which can lead to an increase in weld width. The microstructure of weld is the high-hardness and poor toughness twin martensite without preheating. As the temperature of preheating increases, the twin martensite in the weld begins to transform into lath martensite and regenerates ferrite and bainite. As the preheating temperature increases, the plane fracture toughness (K1C) of the weld increases and then decreases, reaching the highest value of 2322.94 MPa·mm−1/2 at 150 °C. Compared with no preheating conditions, the tensile strength of the welded joint after preheating is improved. The fracture mode of welded joints changes from brittle fracture to ductile fracture. When the preheating temperature is 100–200 °C, the tensile strength of the welded joint reaches 1018.1–1032.5 MPa; when the preheating temperature is 250 °C–300 °C, the tensile strength decreases.

Laser Heat Treatment of Low-Alloy Ultrahigh-Strength Steel Laser-Welded Joints

Laser Heat Treatment of Low-Alloy Ultrahigh-Strength Steel Laser-Welded Joints

Microstructure of a Laser-Welded Joint between a Chromium–Nickel Steel and a Titanium Alloy with a Copper Insert

Microstructure of a Laser-Welded Joint between a Chromium–Nickel Steel and a Titanium Alloy with a Copper Insert

Effect of laser heat treatment on bending property of laser welded joints of low-alloy ultra-high strength steel

This paper investigates the effect of laser heat treatment on the bending property of low-alloy ultrahigh strength steel laser-welded joints. A laser rectangular spot is used for local heat treatment. The results indicate that the back bending angle of joints can be improved from 30° to over 90° with a joint efficiency of 76%. With the increase of laser power and scanning times and the decrease of scanning speed, the heat treatment temperature at the back of the joint increases gradually. The peak temperature under different parameters changes from 450 to 700 °C, at which tempering takes place. This is consistent with the microstructural evolution from original lath martensite of weld metal and heat-affected zone to tempered martensite after laser heat treatment. The hardness between weld and softened zone is similar under the same parameters. Analysis of variance reveals that the hardness is mainly influenced by the peak temperature instead of heating time. The weld hardness decreases gradually with the increase of the peak temperature, and hardness less than 35.0 HRC is obtained when that temperature is over 600 °C. The weld hardness is closely related with the bending property of joints. When the hardness is less than 35.0 HRC, a bending angle over 90° can be achieved. This is because laser tempering makes the hardness of joints uniform, consequently improving the bending performance of the joints.

Effect of shielding gas on the properties of CP titanium Grade 2 laser weld joints

Effect of shielding gas on the properties of CP titanium Grade 2 laser weld joints

Microstructural evolution and mechanical properties of laser-welded joints of medium manganese steel

The cold-rolled 5% medium Mn steel was butt-welded using a fiber laser. The microstructure, distribution of microhardness, and tensile properties of the base metal (BM) and welded joint were investigated. The results showed that the fusion zone of the welded joint had the highest microhardness due to the formation of 100% martensite. A finely mixed microstructure of martensite, ferrite, and austenite was formed in the heat-affected zone, and there was no softened zone in this area. The tensile test results indicated that the ultimate tensile strength and yield strength were higher for the joint than for BM. The joint efficiency was approximately 100%. All samples of the welded joint failed at the location of BM during tensile deformation. The fracture surfaces of the BM and welded joint were mainly ductile fractures. The BM and welded joint exhibited strain rate independence of the tensile strength and yield strength at strain rates of 0.01–1 s−1, while the yield strength of the BM and welded joint increased rapidly when the strain rate reached 5 s−1 due to changes in the dislocation movement mechanisms. The uniform elongation of the BM and welded joint decreased with increasing strain rate.

Structural and Mechanical Features of Laser-Welded Joints of Zinc-Coated Advanced Steel Sheets

We analyze the quality of laser-welded bodywork steel sheets. The welding was done by a CO2 laser. We studied the welds made on steel sheets of advanced grades: high-strength low-alloy steel, dual phase steel, and multiphase transformed steel with induced plasticity. Both sides of steel sheets were gal- vanized. The quality of welded joints was assessed by visual inspection. We performed the tensile tests of the specimens, including fracture tests. The Vickers microhardness was evaluated in the individual re- gions of joints. The macro- and microstructural analyses were performed by microscopy. The parame- ters recommended by the International Institute of Welding were used for welding. According to the da- ta of nondestructive and destructive tests, the analyzed welds were classified as satisfactory.

Enhancement of weld strength of laser-welded joints of AA6061-T6 and TZM alloys via novel dual-laser warm laser shock peening

In this paper, an experimental study is presented on an investigation to improve the weld strength of laser-welded joints via post-processing by warm laser shock peening (wLSP). A dual-laser setup was utilized to simultaneously heat the sample to a prescribed temperature and to perform the wLSP process on the laser-welded joints of AA6061-T6 and TZM alloys. Joints in overlap and bead-on-plate configurations were created by laser welding by a high-power fiber laser and post-processed with wLSP. The tensile tests carried out on wLSP-processed AA6061-T6 samples demonstrate an enhancement in the strength by about 20% over as-welded samples and the ductility of samples processed by wLSP improved by 30% over as-welded samples. The bead-on-plate (BOP) welds of TZM alloy processed with wLSP demonstrated an enhancement in strength by about 30% and the lap welds processed with wLSP demonstrated an increase in the joint strength by 22%. Finite element analysis revealed that the depth and magnitude of compressive stresses imparted by wLSP were greater than room temperature laser shock peening (rtLSP), which contributed to the enhancement of the joint strength for processed samples.

Effects of Sn-foil addition on the microstructure and mechanical properties of laser welding joint for dual phase steel and magnesium alloy

The aim of present research was investigating the effects of Sn-foil addition on the microstructure and mechanical properties of laser welding joint for DP600 dual phase steel and AZ31 magnesium alloy in an overlap steel-on-magnesium configuration. Results showed that when Sn-foil was added as interlayer, some defects, such as surface pit, spatter and softening in heat affected zone, can be avoided,oxide and porosity are also no obvious in the weld. A good weld surface with continuous and uniform fish scale welding line is obtained. Fe–Sn phases, such as FeSn, Fe1.3Sn, and Fe3Sn, are formed in steel side, while Mg2Sn is found in magnesium side. These new phases are favorable for realizing the steel/magnesium metallurgical connection, and adding Sn-foil laser welding is an effective way in joining steel to magnesium alloy. However, mechanical property for steel/magnesium joint is closely connected with the brittleness of Mg2Sn phase, how to regulate its brittleness is the key in order to improve the joint properties.

The Evolution and Distribution of Microstructures in High-Energy Laser-Welded X100 Pipeline Steel.

High-energy beam welding was introduced for pipeline steel welding to reduce pipeline construction costs and improve the efficiency and safety of oil and gas transportation. Microstructures and their distribution in X100 laser-welded joints, which determine the joints’ strength and toughness, are discussed in this paper. Welded joints were prepared by an automatic 10,000-watt robot-based disc laser-welding platform for 12.8 mm thick X100 pipeline steel. Then, the grain, grain boundary, orientation, and distribution pattern of each zone of the welded joints were studied by optical microscopy (OM), scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), and electron backscattered diffraction (EBSD) analysis techniques. The results showed that the grain boundary density, contents of the high-angle and low-angle grain boundaries, distribution states, and evolution trends of coincident site lattice (CSL) grain boundaries were essentially the same in each zone from the base metal (BM) to the weld of the X100 pipeline steel laser-welded joint. The relative content of grain boundaries above 55°, which were composed of the Σ3 type CSL grain boundary, showed a considerable impact on the mechanical properties of the joint. The content of twin grain boundaries was closely related to the thermal cycles of laser welding, and the effect of the cooling rate was greater than that of the process of austenization.

Finite element simulation of the residual stress in Ti6Al4V titanium alloy laser welded joint

AbstractIn this study, in order to investigate the mechanical heterogeneity of a Ti6Al4V titanium alloy laser-welded joint, microstructural examinations, microhardness tests and shear punch tests were carried out. Meanwhile, hole drilling was deployed to measure the residual stress in the joints. In residual stress simulation studies, the complex phenomenon of welding was numerically modelled by indirectly coupled transient, non-linear thermo-mechanical analysis. An elongated combined heat source was used here for modelling the temperature field. The effects of the residual stress after welding, both for considering and neglecting mechanical heterogeneity, were investigated systematically. The obtained results indicate that the residual stress of the simulation, when mechanical heterogeneity is considered closely, matches the experimental data.

Semiconductor properties of passive film and corrosion resistance of 2205 duplex stainless steel joint welded by laser hybrid welding

Purpose There are obvious differences in corrosion resistance of different 2205 welding joints with different ratios of austenite and ferrite, from the top to the bottom, the austenite content decreased gradually while the ferrite increased. In each region of welded joint, the pitting resistance number of ferrite is higher than that of austenite; pitting corrosion is more likely to occur in austenite phase first on the top region of the weld and in the secondary phase precipitates on the other regions of the weld. The fluctuation of the ratio of austenite and ferrite has a great influence on performance of passive film in 3.5 per cent NaCl solution. Design/methodology/approach To study the corrosion behavior of welded joint, the samples were obtained by laser hybrid welding. Pitting corrosion was studied in different area of welded joint. The Mott–Schottky curves of welded joints were measured to study the passive film on the different welded joint area. Findings Due to the difference of heat input and the limit of filler depth of the wire, the microstructure of duplex stainless steel laser welding joint has obvious difference in the thickness direction. In addition, there will be harmful secondary phase (such as chromium nitride and σphase) precipitates in the lower part of the joint. For the welded joint, the corrosion resistance decreases with the increase in the difference of the microstructure. Pitting corrosion usually takes the two phases as the nucleation point and grows up. The surface of 2205 duplex stainless steel laser hybrid welding joint cannot form a complete passive film in 3.5 per cent NaCl solution, and the more the ratios of austenite and ferrite deviate from equilibrium position (50:50), the worse the performance of passive film is. Originality/value In this paper, the authors attempt to establish the correlation between the semiconductor electronic properties of passive film and the difference of microstructures and the component in a joint welded by laser hybrid welding. The effect of passive film on the corrosion resistance of the weld was further investigated.

Applying Digital Image Correlation Technique for Studying the Growth of a Fatigue Crack in VT23 Titanium Alloy Welded Joints

We present results of applying a combined approach to studying fatigue fracture processes in VT23 titanium alloy with a laser-welded joint. The approach involves fractographic examination using scanning electron microscopy and analysis of strain fields at the sample surface (using digital image correlation technique). Samples with permanent connections are presented in two forms: in the initial state and subjected to post-welding treatment by ultrasonic forging and pulsed high-energy impact. It is shown that applying the proposed integrated approach to the analysis of fatigue fracture has allowed identifying features in the development of processes of nucleation and propagation of cracks and locating them in the bulk of the material, as well as determining running times after which transition from one stage to another takes place: crack initiation, stable growth, and rupture. The results have made it possible to explain the mechanisms of how post-welding treatment of permanent joints affects their fatigue properties.

Laser welding of copper‐niobium microcomposite wires for pulsed power applications

AbstractThe laser welding of copper‐niobium microcomposite wires was investigated. It was determined that the joint structure does not have welding defects, while microscopic examination of the joint cross‐section showed that the microstructure of the autogenous weld consists mainly of a copper‐based solid solution strengthened by niobium‐rich precipitations. The weld obtained with use of filler material consists of two distinct zones, which are formed due to melting of filler wire and microcomposite wire. This structure of the joint provides an insignificant increase in electrical resistance and sufficient ultimate strength and plasticity of the joint. The tensile strength of the sample welded without filler material reaches 335 MPa, but such welded joints are very brittle due to very low ductility. However, an autogenous laser welding joint has about 1.6 times better ductility, and the tensile strength of the joint depends on the applied filler material and is equal to the tensile strength of this material.

Investigations on in-process control of penetration depth for high-power laser welding of thick steel-aluminum joints

Over the last few decades, the demand for lightweight constructions has been increased continuously for several industrial applications, like automotive and ship building, to reduce the weight of vessels in order to minimize the CO2 emissions as a result of a lower fuel consumption. Lightweight construction is almost applied for ship applications, especially for yachts, which are designed by using aluminum for the deck constructions and steel for the ship hull. For joining these parts, a high-power laser welding process shall be developed. However, the welding of these dissimilar materials is associated with great challenges, due to the different physical properties and the formation of hard and brittle intermetallic phases, which may influence negatively the properties of the weld seam. The quality of dissimilar joints depends strongly on the mixture ratio between the molten amount of steel and aluminum. However, the mixture ratio varies over the weld seam length due to a high dynamic of the keyhole resulted by welding of this material combination. Furthermore, different batches of materials and varied sheet thicknesses t may influence the mixture ratio. In this study, a high-power laser welding process is developed with in-process control of the penetration depth tP by analyzing the spectral process emissions for dissimilar lap joints of aluminum alloy EN AW-6082 (t = 8 mm) and steel S355 (t = 5–7 mm). In the context of these investigations, an increase of occurring cracks within the weld seam and ejections of molten material with increasing penetration depth tP can be observed. To achieve a relative high joint strength, the penetration depth tP must be kept constant at a value of 1.4 mm. In case of varied batch of material, thickness t of the used sheets, welding speed vS, and leap of the steel sheet thickness t, the penetration depth tP requested cannot be achieved. Using the in-process control of the penetration depth tP, the weld seam quality remains almost constantly over the weld seam length, as shown in visual inspections, metallographic analyses, profiles of the penetration depth tP, and tensile shear testing. Among other things, the appearance of ejections of molten material can be avoided by using the in-process control of the penetration depth tP.

Weldability, microstructure and mechanical properties of laser-welded selective laser melted 304 stainless steel joints

Laser welding is a promising process for joining small components produced by selective laser melting (SLM) to fabricate the large-scale and complex-shaped parts. In the work, the morphology, microstructure, microhardness, tensile properties and corrosion resistance of the laser welded stress-relieved SLMed 304 stainless steel joints are investigated, as the different sections of stress-relieved SLMed 304 stainless steel are joined. Results show that the SLMed 304 stainless steel plates have a good laser weldability. The microstructure of laser-welded joints consists of the cellular dendrites in austenite matrix within columnar grains, exhibiting a coarser dendrite structure, lower microhardness (∼220 HV) and tensile properties (tensile strength of ∼750 MPa, and area reduction of ∼27.6%), but superior corrosion resistance to those of SLMed plates. The dendrite arm spacing of the joints varies from ∼3.7 μm in center zone, to ∼5.0 μm in fusion zone, to ∼2.5 μm in epitaxial zone. The SLMed anisotropy shows a negligible effect on the microstructure and performance of the laser-welded joints. The laser welding along the building directions of the SLMed base plates can induce a slightly finer dendritic structure and higher tensile properties.

Power and welding speed influence on bead quality for overlapped joint laser welding

Laser welding has gained more and more industry acceptance due to narrow heat affected zone, low distortion levels, possibility of remote processing, and outstanding welding speeds when compared to traditional electric arc processes. Automotive industry requires a significant amount of overlapped joint welding, which usually is applied to the chassis assembling. Thus, as a mean to support laser users to optimize the welding process, this paper presents an approach through an experimental methodology followed by a phenomenological discussion about the quality of overlapped welded joints. Arguments are based on power and welding speed parameters variation and their related outcomes. This approach allows mapping of main defects such as porosity, undercut, and insufficient penetration, as well as identifying eventual preventive actions. A 10 kW IPG Photonics YLS 10000 fiber laser was used to weld 3 mm SAE 1008 steel sheets. The power ranged from 5 to 7 kW while the welding speed ranged from 2 to 5 m/min. High power levels at low welding speeds led to high temperature, resulting in an undercut issue, and low power levels at high welding speed led to insufficient penetration. Porosity was the most common defect. The best results, which did not show any type of discontinuity, were obtained at a laser power of 5 kW and a welding speed of 3 and 4 m/min.