Laser Welding Of Steel Research Articles

Microstructure and mechanical properties of fiber laser welded QP980/press-hardened 22MnB5 steel joint

Fiber laser welding of QP980 steel and press-hardened 22MnB5 steel in butt configuration was performed in this study. The effect of heat input on microstructure evolution and mechanical properties of dissimilar joint was investigated. Laser power employed was varied from 1.6 to 2.4 kW while welding speed was kept constant at 2 m/min. The microstructure of fusion zone (FZ) was changed with the increasing laser power. Martensite and few bainite were observed when laser power ranged from 1.8 to 2.2 kW. Out of this range, the microstructure in FZ consisted of mixed structure of martensite and ferrite. Transformation from liquid phase to ferrite was promoted by Al-Si coating on the surface of press-hardened 22MnB5 steel. Heat affected zones (HAZs) of dissimilar joint were composed of martensite and bainite. Softened zone (SZ) appeared at the HAZ of 22MnB5 steel due to tempering behavior. The hardness of SZ and QP980 steel base metal declined with microstructure evolution. The fracture occurred at QP980 base metal when dissimilar joints were produced at laser power of 1.8 and 2.0 kW. When laser power reached 2.2 kW, the tensile samples failed at SZ owing to the increased softening degree. With further increasing laser power, the weakest region was FZ since ferrite declined the whole joint strength. A better Erichsen value was obtained at 2.0 kW (4.04 mm), which was balanced by more crash energy absorption and better deformation resistance.

Impact of Wavelengths of CO<sub>2</sub>, Disk, and Green Lasers on Fusion Zone Shape in Laser Welding of Steel

Impact of Wavelengths of CO<sub>2</sub>, Disk, and Green Lasers on Fusion Zone Shape in Laser Welding of Steel

On the feasibility of AISI 304 stainless steel laser welding with metal powder

Laser welding is a manufacturing process that allows the joining of metallic parts even if accurate positioning and preparation of the welding edges are mandatory for successful welding. The use of a filler material in the form either of wire or powder can mitigate the drawback entangled with that preparation. In this work, bead-on-plates were performed on of AISI 304 stainless steel plate with AISI 316 L powder as filler material. Tests were carried out using different combinations of process parameters. The influence of process parameters like laser power, translation speed, powder feed rate, carrier gas flow rate and laser spot diameter on geometry, on microstructure and on porosity of the weld seams were investigated. The transition between conduction to keyhole regime was detected, both with and without powder addition during welding. It was found that the variation of laser spot diameter is fundamental in generating the different welding regimes. Moreover, porosity was detected only in the keyhole regime with a maximum value of 5.5%. Finally, “V-shaped” grooves were machined on AISI 304 substrate and filled by two overlapped tracks. The results of the study demonstrated the feasibility of the laser welding of AISI 304 stainless steel with AISI 316 L powder as filler material.

Effect of Welding Speed on Texture in Laser-Welded Dual-Phase Steel

The present study makes an attempt to understand the influence of welding speed on the formability of a welded microalloyed steel. An optimum heat input for laser welding was maintained in this study, under bead-on-plate and butt-welding conditions, with varying welding speed. Initially, the heat-affected zone (HAZ) had a moderate γ-fiber texture, which was later distorted with the increased speed of welding. There was an enhancement of rotated cube {001}〈110〉 and cube {001}〈010〉 orientations. The fusion zone (FZ) exhibited mainly random texture, without any presence of γ-fiber. However, faster speeds resulted in strengthening of intensities close to the rotated cube {001}〈110〉 and cube {001}〈010〉 orientations in the FZ. The increased welding speed resulted in enlargement of dimples of fracture surface for the HAZ, finally leading to a transition into a mixed mode of fracture. With increased welding speed, the FZ exhibited a drop in the equiaxiality of the austenite grains. The enhancement of the columnar nature of the austenite grains at the FZ could be correlated with the cube texture formation and deterioration in formability of the welded material.

Remote Fibre Laser Welding of Advanced High Strength Martensitic Steel

The study presents the results of remote robotic laser welding of advanced high strength Docol® 1200 M martensitic steel. One mm thick samples were welded in a lap joint configuration using a special clamping system. Welding was done using a continuous-wave (CW) fibre laser with a constant welding power of 300 W and constant focus diameter Ø 1.8 mm. Welding was done using 12 different welding speeds in the range from 0.15 to 1 m/min, whereas the inclination angle was kept constant at 0°. The influence of various welding speeds and linear heat inputs during welding on microstructural changes were examined by the occurrence of acicular and allotriomorphic ferrite or martensite. Results revealed big influence of the clamping system on the accumulation of the laser beam energy, heat sink and consequently weld size and geometry, as well as its microstructure and joint strength. Tensile-shear strength, microstructure and hardness results confirmed laser power of 300 W and 0.6 m/min welding speed as the optimal parameters, at which a martensitic structure was obtained in the weld. The width of the heat affected zone (HAZ) in this case is 1100 μm.

Weldability and mechanical behavior of laser-welded TRIP 750 steel sheets

Transformation-induced plasticity steels have been developed and widely applied in the automotive and aerospace industries. They exhibit ductility and mechanical strength associated with a high formability due to their complex microstructure of bainite-associated pro-eutectoid ferrite and significant retained austenite fractions. The weldability of these steels is limited by the high content of alloying elements in the composition, causing the thermal cycle to modify the carefully designed microstructure, which in turn generates unsatisfactory weld mechanical properties. Laser welding has a relatively low thermal input, and, therefore, a narrow heat-affected zone is obtained. As known, the literature had not been definitively reported the microstructural features of the fusion and the heat-affected zones after laser welding of TRIP steels in conjunction with their mechanical behavior. The aim of the present work is to characterize the microstructure and mechanical behavior of laser-welded TRIP steel after uniaxial tensile and Erichsen formability tests. The coupons of TRIP 750 steel sheets were subjected to different laser welding conditions in order to analyze their impact on the microstructure, hardness, and mechanical strength of the material. After some preliminary tests, the laser power was fixed at 900 W and the weld speed fixed at 50 mm/s as the best choice of operating parameters. Under these conditions, the fusion zone was almost completely martensitic, while the heat-affected zone had a mixture of ferrite and martensite. The martensite transformation is corroborated by finite elements analyses as the cooling rate was 4200 °C/s for material at martensite start temperature. The average hardness of the fusion zone was 530 HV and the heat-affected zone was 550 HV, compared with 270 HV of the base material. In terms of mechanical behavior, the tensile strength of the welded coupons was found to reach 740 MPa and the ductility reached 22% in uniform deformation. The Erichsen index for the welded sheets attained 15 mm for a load of 48.5 kN, similar with the non-welded base material. Both in the case of the uniaxial tensile testing and in the Erichsen testing, the fracture occurred in the base material away from the weld, showing a good toughness of the welded component.

An Overview of Laser Welding of High Strength Steels for Automotive Application

High strength steel and their variations are found in abundance and are an excellent material selected for automotive applications. However, their usage in automotive applications requires the joining of components via conventional joining and welding techniques. Recently, there has been advancement in photons-based manufacturing technology where laser as a localized and robust heat source is employed to join similar or dissimilar material combinations. A brief overview of lasers employed in welding technology is outlined. Secondly, various types of steel important for automotive applications are introduced. Thereafter, laser welding of steel and the corresponding effect of parameters and their effect on weld geometry are discussed. It can be concluded that the variation of laser welding parameters can cause high thermal distortion and greatly affects the mechanical performance of the joint. Hence, an optimum parameter value should be investigated to obtain a strong joint. It is iterated that additional literature review should be carried out investigating various ways of improving weld quality, techniques, jigs development to avoid welding defects formation. Additionally, better observation and characterization through the state of art available materials testing could be employed to further gain insights and to have a better understanding of microstructural behaviour

Prediction of Preheating Temperatures for S690QL High Strength Steel Using FEM-Simulation for High Power Laser Welding

This study investigates a method for predicting the effect of preheating temperatures on the resulting hardness for high power laser welding of high strength steel. An FEM model is introduced containing a hardness calculation based on an existing model. Moreover, the hardness values of experimental results have been measured in order to show the performance of the model. The hardness calculation requires the chemical composition and the t8/5-time at the point of measurement. It is claimed that a calibration of the melt pool width and depth at room temperature only is enough to get reasonable results from the FEM-model for higher preheating temperatures. From the experimental result of a single experiment the width of a weld seam and the depth was deducted. In this study experiments have been done at various preheating temperatures in order to show the correlation between the model and the experimental results at various temperatures. The hardness equation provides suitable results in the verification with the measurements. The prediction of preheating temperature can be done with the resulting t8/5-time of the FEM-model. This method can decrease the amount of time and costs within a production according to testing and analyzing a matrix of process parameters. Moreover it is concluded that this methodology might be used for single item production.

Numerical and experimental analysis of a laser welded joint made of dissimilar materials S355 and 304 steel

One of the latest trends in modern welding industry is joining materials with different physical properties. Performing such welded joints results from the needs of the modern industry (like aviation or automotive). It is required that welded joints have appropriate strength, anti-corrosion and heat properties. The welded joints of this type are quite troublesome due to the large differences between the materials being joined. This work presents numerical and experimental research of laser butt welding of dissimilar materials. Experimental tests were made for laser welding of S355 steel with 304 austenitic steel (X5CrNi18-10). Macroscopic research is performed. On the basis of experimental research numerical investigations of the thermomechanical phenomena of the welding process are carried out using commercial software. Numerical analysis takes into account temperature dependent thermomechanical properties of welded materials. The temperature distribution in the welded joint is determined on the basis of results of calculations. The shape of melted zone and heat affected zone are numerically estimated. Stress distributions and displacement field in welded joint are numerically determined. Obtained results of the numerical analysis are compared to the experimental results.

Optimization of laser welding of DP780 to Al5052 joints for weld width and lap-shear force using response surface methodology

A laser welding of DP780 steel to Al5052 joint has been investigated to optimize the weld width and lap-shear force. Four types of steel-Al welded joints and fracture model have been proposed. Response surface methodology and central composite design were applied to develop mathematical relationships between the control parameters and the responses. Then, the validation and optimization experiments were carried out to check the models adequacy. The results show that there exists good consistency between the predicted and actual value. This indicates the developed models could predict the weld width and lap-shear force within the limits of the process parameters.

Experimental research on formation mechanism of porosity in magnetic field assisted laser welding of steel

The dynamic keyhole behaviors are directly observed in laser welding of AH36 steel/glass, and Zirconia particle is used to observe the convective pattern. The weld bead formation, keyhole instability and porosity formation are analyzed. Moreover, the differences in keyhole instability and porosity formation between no magnetic field assisted and magnetic field assisted laser welding are analyzed. With the magnetic field is added, the weld width, small concave, the formation of the keyhole collapse and the convective pattern are similar in laser welding of steel, and the generated Lorentz force hinders the flow of liquid metal. Widening of bottom weld width without undercut defect can be obtained with magnetic field assisted. The lower fluctuation of the weld depth, the longer time of plasma plume above the weld pool, fewer undercuts and spatter defects contributes to the increase of the keyhole stability, and thus restrains the porosity formation. Besides, an easier keyhole open, the larger bubble escape velocity and the lower solidification rate also promote the reduction of the porosity formation.

Application of Laser Welding in Car Bodies Manufacturing

The paper deals with a problem of laser welding of zinc-coated steel (DX53D+Z and DX54D+Z) plates. Experiments were carried out with the use of steel plates 0.8; 1.0; 1.3; and 1.75 mm thick. High power 2.5-2.9 kW CO2 laser was employed. CO2 was used as the shielded gas. The optimal parameters of welding for material under study were determined. The mechanical properties of welded joints were measured through the tensile loading and microhardness measurement. Tensile performances of the specimens fabricated at different welding rate and laser beam power were compared with that of the base material.

The fatigue stress on laser welded joints of DC06EK steel

The paper analyses the influence of the fatigue characteristics of laser welding steels sheets. The combination of laser welded steel sheets DC06EK has been subjected to fatigue stress testing. To assess the condition of the material under the cyclic stress and to determine the material characteristics such as fatigue or resp. fatigue time limit was performed according to STN 42 0363. Fatigue tests were performed on the test piece. The experimental material had a laser weld with a rough surface. The test specimens were subjected to a pulsating positive disappearing thrust at a constant value of the coefficient of unbalance of 0‹R‹1 and a frequency of 25 Hz. The fatigue limit was determined at 260 MPa. The analysis of fracture surfaces showed on the sides of the weld joint the formation of a de-cohesive fracture. The resulting test values graphically presents stress-number curve.

Laser welding with deep alloying of the elements of the truck chassis

A method of laser welding with simultaneous alloying of 2.4060 over the entire depth of the melt bath for connecting the crankcase and the steering knuckle of a truck chassis is described. The results of experimental studies of the macro and microstructure of various zones of laser welding of 35HGSA steel with steel 1.0570 are presented. A comparative analysis of the hardness of the melt zone and the heat affected zones for samples with and without alloying is carried out.

Cold Cracking Resistance of Butt Joints in High-Strength Steels with Different Welding Techniques

Investigation results are presented to improve the structural strength (cold crocking resistance) of weld joints in high-strength steels with the yield limit over 600 MPa. The effect of arc, laser, and hybrid laser-arc welding conditions on the weld metal structure and diffusion hydrogen saturation of build-up and fused base metals was experimentally studied. The diffusion hydrogen saturation of build-up (arc and hybrid welding) and fused (laser welding) metals was chromatographically examined. In gas-shielded arc welding, the diffusion hydrogen content in the fused base metal was shown to be limited to the concentration that does not exceed 0.4 ml/100 g due to an increase in the welding speed from 18 to 50 m/h. In laser and hybrid laser-arc welding of high-strength steels with the yield limit over 600 MPa, the diffusion hydrogen content in the fused metal makes up 0.07 and 0.2–0.3 ml/100 g, respectively, regardless of the welding speed. The cold cracking resistance was evaluated by a commonly accepted procedure of special reference butt samples. Optical and transmission microscopic studies permitted of revealing the effect of arc, laser, and hybrid laser-arc welding conditions on the weld metal structure and gaining detailed information on the dislocation density distribution. The relation between the level of local internal stresses and the structural factors of dislocation density distribution in the weld metal was established. In arc and laser welding, local internal stresses were shown to be reduced to the values that do not exceed 0.22 of the theoretical metal strength if the welding speed would make up 50 m/h. In hybrid laser-arc welding at 72–110 m/h, maximum local internal stresses are also less than 0.22 of the theoretical metal strength. An increase in the cold cracking resistance in butt weld joints of high-strength 14KhGN2MD and N-A-XTRA-70 steels was established to be reached due to a low concentration of diffusion hydrogen in fused metal and formation of the fine-grained structure of lower bainite with the uniform dislocation density distribution.

Microstructure of intercritical heat affected zone and toughness of microalloyed steel laser welds

Microstructure of laser welds of the X70 low-carbon pipe steel was studied. High cooling rates after laser welding and non-uniform distribution of carbon in the ferrite-pearlite base metal caused formation of regions with increased microhardness (up to 650 НV) in inter-critical heat affected zone (ICHAZ). These regions consisted of finely dispersed degenerate upper bainite and martensite-austenite constituents of a slender shape and small fraction of a massive shape along the boundaries of bainite laths, as well as twinned martensite. High concentration of martensite-austenite constituents (10–16%) and residual stresses in ICHAZ, as well as a dendritic martensitic structure with carbide interlayers along the boundaries of martensite laths in fusion zone were the main reasons of sharp decrease in charpy impact energy of the welded samples. High microhardness of the laser welds was decreased down to 320 HV and their brittleness was improved by annealing. Also, in ICHAZ, degenerate upper bainite and the regions of martensite-austenite constituents decayed forming tempered sorbite and Fe2C and Fe3C carbides, respectively. Charpy impact energy of the welds doubled after annealing compared to the welds without annealing, and ductile-brittle transition temperature decreased down to –60°С.

Optimizing pulsed Nd: YAG laser welding of high-Mn TWIP steel using response surface methodology technique

In the present study, the microstructural and mechanical properties of laser beam-welded thin sheet twinning-induced plasticity (TWIP) steel were investigated. The pulsed neodymium: yttrium-aluminum-garnet (Nd: YAG) laser beam welding process parameters were modeled and optimized based on experimental data and statistical analysis using response surface methodology (RSM) technique. Process parameters range, i.e. the power input (2000–3000 W), welding speed (0.2–1 mm/min), and spot size (0.3–0.7 mm) were selected properly in order to obtain the desired mechanical properties. Main effects of each factor along with interaction effect with other factors were determined quantitatively. The predicted and actual values of the mechanical properties compared using analysis of variance (ANOVA) in order to verify the adequacy of the developed model. Optimal laser beam welding parameters were identified as the power input, welding speed and spot size of 2586 W, 0.53 mm/min, and 0.48 mm, respectively. Using parameters in the optimal conditions, a welding joint with tensile load of 2001 N (% 94 strength of the base metal) was obtained. In addition, the welding zone with an average grain size coarser than the one for the base metal and a random texture was identified.

Fiber laser welding of HSLA steel by autogenous laser welding and autogenous laser welding with cold wire methods

6 mm thickness high strength low alloy (HSLA) steel butt joints were fabricated by fiber laser welding. The influence of welding parameters on morphologies, weld process behavior, microstructure and mechanical properties were investigated. Defects (undercut, spatters and humping) were observed with the increase of heat input (HI) during autogenous laser welding (ALW), and weld morphologies changed from partial penetration to root humping first, then fine result, and finally over penetration (cutting) via high-speed images (HSI). Three typical transition modes were observed during autogenous laser welding with cold wire (LWACW), including spreading-transition, liquid-bridge transition and explosion transition. Sound process stability was achieved in liquid-bridge transition with the optimized parameters of 6 kW laser power, 0.8 m/min welding speed and 3.5 m/min feeding wire speed. The microstructure in the fusion zone was composed of acicular ferrite with some equiaxed grains, while that of autogenous laser welding was mainly made up of the acicular ferrite. The microhardness of base metal was relatively lower than that of fusion zone in both types, and both joints fractured at the side of base metal during tensile test.

Effect of galvanneal-coating evolution during press-hardening on laser welding of 22MnB5 steel

ABSTRACTUnderstanding phase evolution and growth kinetics of Zn-coating during press-hardening is essential in predicting welding behaviour of press-hardened 22MnB5 steel. The results showed that during press-hardening, the initial δ and Γ-Γ1 phases in the coating transform into α-Fe(Zn), Γ, and ZnO phases. As heat-treating continued at a constant temperature, the α-Fe(Zn) thickness grew and the overall Zn-content decreased. This resulted in the full-penetration weld window shifting towards higher heat-inputs. However, when heat-treating at higher temperatures, severe oxidation and Fe-Zn inter-diffusion led to the reduction of Zn-content in the evolved coating. It was found that higher heat-treatment times resulted in wider weld beads, thereby increasing joint strength.

Research of a laser welding of thin-shit construction steels by continuous laser radiation

The results of the laser welding by continuous radiation of thin-sheet low-carbon steels on the example of steel 08U presented in this paper. Metallographic studies of the laser weld were conducted. Tensile tests were carried out as a result of which it was determined that the nature of the fracture is adhesive, and after cohesive failure of the specimens, it is show the high quality of the welded joints at the beginning of the weld.