Laser Welding Of Stainless Steel Research Articles

Experimental study of laser-welded stainless steel angle columns

Substantial research has been conducted in the last years into the structural responses of cold-formed and hot-rolled stainless steel equal-leg angle columns, whereas there is no experimental data on laser-welded stainless steel angle columns despite these sections being increasingly used in the modern construction industry. To fill this gap and provide benchmarks for design, a comprehensive experimental investigation into the structural strength and stability of laser-welded austenitic stainless steel equal-leg angle columns under pure axial compression was performed and described in this paper. A series of laboratory tests, performing on two test groups of specimens with nominal dimensions of 60 ×60× 6 mm and 100 ×100× 10 mm and different lengths, involved 6 stub column tests and 20 global buckling tests together with measurements on material properties, initial geometric imperfections and residual stresses. The experimental observations reveal that the column ultimate capacity and prevalent failure mode – major-axis flexural–torsional buckling and minor-axis flexural buckling – are highly dependent on the column slenderness and initial imperfections. The full load-lateral displacement, load-torsional rotation and load-axial strain responses of the specimens were fully reported and discussed. The results were employed to evaluate the accuracy of the current European and North American stainless steel design standards. The evaluation results revealed that the codified design procedures yield conservative resistance predictions of laser-welded stainless steel equal-leg angle columns, showing a prospective for their modification and improvement by providing more optimized and realistic results in design considering the actual residual stress distributions, material nonlinearity and strain hardening.

Pulsed laser welding and microstructure characterization of dissimilar brass alloy and stainless steel 308 joints

The laser welding of stainless steel and brass is difficult due to their different physical properties, including thermal conductivity and melting point. In this study, the laser welding of stainless steel 308 and brass was experimentally investigated. To systematically investigate the effects of different thermophysical properties, including the heat transfer rate, thermal conductivity, and laser beam interaction in relation to the beam absorption coefficient, the temperature of the melt pool zone was measured while changing welding conditions. Different temperature gradients were obtained by changing different parameters including the laser peak power, nozzle distance, and laser beam deviation. The results showed that an asymmetric melt pool formed, to which the melting of brass made a higher contribution. Due to the lower melting temperature and the higher thermal conductivity of brass, the measured temperature and the melt size of brass were higher. The microstructure of the melt pool consisted of intermetallic compounds. Variation of melt pool size (width and depth) and temperature could be affected by laser peak power and reductions in the nozzle distance than the other parameters. For instance, 1 mm increase in the nozzle distance reduced the melt pool zone temperature by approximately 20 °C for stainless steel. Furthermore, an increase in the laser peak power raised the maximum measured temperature to about 225 and 160 °C for brass and stainless steel sides, respectively. At the same time, the amount of temperature reduction at a period of 20 s for stainless steel is almost half that of brass.

On the interaction of microstructural morphology with residual stress in fiber laser welding of austenitic stainless steel

The development of compressive residual stress is governed by the metallurgical transformation and associated microstructure, which in turn influences the mechanical properties of a welded structure. In the present work, the microstructural interaction with the thermo-mechanical performance of Yb-Fiber laser-welded austenitic stainless steel (SS304) is evaluated at different heat inputs and varying defocus distances. The interrelation between microstructural morphology and the typical pattern of residual stress is systematically investigated. The development of the thermal-metallurgical-mechanical model particularly considers the effect of solid-state phase transformation (SSPT) such that the model predicts the phase fraction and its influence on residual stress distribution. The present study exclusively focuses on the mechanism of residual stress generation as a consequence of dual-phase microstructural evolution during solidification. Low heat input (45J/mm) leads to a high cooling rate, excellent tensile strength, and low tensile residual stress. The finite element-based thermo-mechanical model predicts the residual stress with a maximum deviation of ±50 MPa compared to the experimental value. The Ferritic-austenitic (FA) mode of solidification results in the combination of skeletal and lathy δ-ferrite morphology. Increasing peak intensity of δ {110} with a cooling rate confirms the enhancement of δ-phase within the austenite matrix and restricts the complete transformation to the austenitic phase. A relatively high amount of δ-ferrite due to enriched Cr and lean Ni content at the dendritic core with fine or acicular morphology at a high cooling rate reduces the longitudinal and transverse residual stress significantly. Besides, the increasing trend of primary dendritic arm-size of δ-ferrite yields a gradual increase in residual stress.

Investigation of dissimilar laser welding of stainless steel 304 and copper using the artificial neural network model

In this study, to accurately predict the temperature and melting ratio at low time and cost, the process of dissimilar laser welding of stainless steel 304 and copper was simulated based on artificial neural network (ANN). Among various ANN models, the Bayesian regulation backpropagation training method was utilized to model the current problem. This method was used considering the two temperatures of copper and steel and the two melting ratios of steel and copper as the four outputs, and the four parameters, pulse width, pulse frequency, welding speed, and focal length, as the inputs. According to the results, regression values had a good accuracy in all cases and the histogram diagrams indicated that the error distribution was mainly concentrated at the center; in other words, the major errors of the network were not very large. It was also observed that the error concerning the trained neural networks was acceptable in the experiment phase. Finally, this neural network could be used as a numerical model to estimate the four outputs of steel temperature, copper temperature, steel melting ratio, and copper melting ratio for all input values of pulse width, pulse frequency, welding speed, and focal length in the studied range, without any need to rerun the experiment.

Effect of welding speed, pulse frequency, and pulse width on the weld shape and temperature distribution in dissimilar laser welding of stainless steel 308 and brass alloy

Laser welding of brass and stainless steel alloys is of special importance due to its wide application in the industries related to energy production. In this study, laser welding of heterogeneous metals was performed in a laboratory. Measurements of the temperature around the molten pool showed changes in the welding conditions. By changing different parameters such as welding speed, frequency, and pulse width, different thermal gradients were obtained. The results showed that the formation of the molten pool was asymmetric, and it was mostly done by melting the brass alloy. Due to the lower melting temperature and the high heat transfer rate of the brass alloy, the measured temperature and the molten volume of this alloy were higher. The microstructure of the molten pool also included intermetallic compounds. Increasing the dimensions of the molten pool (width and depth) by raising the peak power and reducing the welding speed was more effective than other parameters. The microhardness results also indicated the higher weld strength of the brass alloy with stainless steel rather than pure copper. At the range of the investigated process parameters, the adjacent temperature near the molten pool was about 20% (30 °C) higher for the brass alloy, in comparison to stainless steel, when the pulse frequency and pulse width were changed.

Investigation of dissimilar fiber laser welding of low carbon steel and stainless steel by numerical simulation

A three-dimensional numerical simulation model is developed in this paper to analyze the symmetry of weld geometry and the homogeneity of alloy distribution in the molten pool during the dissimilar fiber laser welding of low carbon steel and stainless steel during different welding conditions. The simulated results are compared with the experimental results and good agreement has been obtained. The characteristics of weld geometry, flow field and alloy distribution are analyzed based on the numerical calculation. The obtained results indicate that the weld morphology is asymmetric and the weld width is increasing gradually with the laser power increasing. The convection formed in the molten pool is enhanced under high laser power welding condition, which results in that the alloy distribution in the molten pool is more homogeneous and the difference between the upper and lower widths of the weld is decreasing. The weld morphology and alloy distribution in the dissimilar fiber laser welding can be predicted. It is clearly seen that the proposed model is effective for optimal welding process parameters selection to obtain the desired welded joints with excellent performance, which demonstrates guiding significance for improving the quality of dissimilar laser welding.

Laser Welding of Stainless Steel 321 in Different Welding Positions

It was revealed that spatial position during laser welding of AISI 321 stainless steel influences the processes in the weld pool and the process of its crystallization. The geometry, structure, distribution of chemical elements depend on the cooling rate of the welded joint, which varies in different spatial positions during laser welding of stainless steel AISI 321. To achieve the lowest variance of results and the maximum values of mechanical characteristics of the welded joints of AISI 321 stainless steel it is recommended to produce laser welding in a vertical position.

Numerical and Metallurgical Analysis of Laser Welded, Sealed Lap Joints of S355J2 and 316L Steels under Different Configurations.

This paper presents the results of laser welding of dissimilar joints, where low-carbon and stainless steels were welded inthe lap joint configuration. Performed welding of austenitic and ferritic-pearlitic steels included a sealed joint, where only partial penetration of lower material was obtained.The authors presented acomparative study of the joints under different configurations. The welding parameters for the assumed penetration were estimated via anumericalsimulation. Moreover, a stress–strain analysis was performed based on theestablished model. Numerical analysis showed significant differences in joint properties, therefore, further study was conducted. Investigation of the fusion mechanism in the obtained joints wascarried out using electron dispersive spectroscopy (EDS) and metallurgical analysis. The study of the lap joint under different configurations showed considerable dissimilarities in stress–strain distribution and relevant differences in the fusion zone structure. The results showed advantages of using stainless steel as the upper material of a microstructure, and uniform chemical element distribution and stress analysis is considered.

Promoting austenite formation in laser welding of duplex stainless steel\u2014impact of shielding gas and laser reheating

Avoiding low austenite fractions and nitride formation are major challenges in laser welding of duplex stainless steels (DSS). The present research aims at investigating efficient means of promoting austenite formation during autogenous laser welding of DSS without sacrificing productivity. In this study, effects of shielding gas and laser reheating were investigated in welding of 1.5-mm-thick FDX 27 (UNS S82031) DSS. Four conditions were investigated: Ar-shielded welding, N2-shielded welding, Ar-shielded welding followed by Ar-shielded laser reheating, and N2-shielded welding followed by N2-shielded laser reheating. Optical microscopy, thermodynamic calculations, and Gleeble heat treatment were performed to study the evolution of microstructure and chemical composition. The austenite fraction was 22% for Ar-shielded and 39% for N2-shielded as-welded conditions. Interestingly, laser reheating did not significantly affect the austenite fraction for Ar shielding, while the austenite fraction increased to 57% for N2-shielding. The amount of nitrides was lower in N2-shielded samples compared to in Ar-shielded samples. The same trends were also observed in the heat-affected zone. The nitrogen content of weld metals, evaluated from calculated equilibrium phase diagrams and austenite fractions after Gleeble equilibrating heat treatments at 1100 °C, was 0.16% for N2-shielded and 0.11% for Ar-shielded welds, confirming the importance of nitrogen for promoting the austenite formation during welding and especially reheating. Finally, it is recommended that combining welding with pure nitrogen as shielding gas and a laser reheating pass can significantly improve austenite formation and reduce nitride formation in DSS laser welds.

Analysis of vapor plume and keyhole dynamics in laser welding stainless steel with beam oscillation

The influence of laser welding with beam oscillation on dynamic behavior of vapor plume, melt pool and keyhole was investigated. The stability of plume morphology is improved with a little fluctuation, and the height and area of plasma plume are decreased as the oscillation frequencies increased. The melt pool morphologies and flowing feature under beam oscillation conditions are also regulated due to the resultant energy deposition, which has an influence on weld solidification process. The re-melting effect of circular beam oscillation can eliminate pores formation and improve weld quality.

Free Vibrational Characteristics of Dual Side Nd: YAG Laser Welded Stainless Steel 304 Joints Reinforced with Al <sub>2</sub> O <sub>3</sub> Micro Powder Particles

<div class="section abstract"><div class="htmlview paragraph">Nd: YAG Laser Welding Process (LWP) is the most efficient method commonly used for the joining of different kinds of materials, whether it can be a sheet or plate. LWP in general, uses high power density, frequency and travel speed or feed rate as primary process parameters in order to perform a joining process across the metals. This paper investigates the effect of free vibrational characteristics for Stainless Steel 304 (SS 304) joints which are reinforced with Al<sub>2</sub>O<sub>3</sub> micro particles processed by dual side (welding performed on the top and bottom surface of sheets) Nd: YAG LWP. The inclusion of micro particles was inserted directly across the weld pool region, by fabricating drills with a constant gap between each drilled holes. Totally 12 samples were fabricated with different laser power, travel speed and by keeping the frequency level as constant for all the experiments. Vibration tests were performed on the fabricated samples made with and without incorporation of micro particles using accelerometer, data acquisition card, and the plots were obtained with the assist of DEWE software. The vibration equipment is used on the SS 304 joints in order to process the vibrational signals. Each sample has been tested with ten trials and the average values are drawn in order to reduce or minimize the measuring errors like (measuring error, fitting error, environmental impacts, etc.). The results revealed that vibrational characteristics are linked to factors like reinforcement of powders, selection of welding process and process parameters. It is also understood that the frequency and damping factor for a material can be varied with process parameters and inclusion of micro particles in the solidification process. Thus this paper will assist any designer to choose the process parameter ranges, reinforcement of powder particle methodology and their impact on the material behavior of fabricated joints.</div></div>

Adjustable Ring Mode (ARM) laser welding of stainless steels

As a combination of the annular laser beam and gaussian laser beam, adjustable ring mode (ARM) laser is a promising technology for stabilizing laser-based welding process. Process status and fusion efficiency of laser-materials processing was investigated experimentally under different shape of laser modes, including ring laser mode, center laser spot alone, and ring/center dual spots, with welding speeds ranging from 5 to 100 mm/s. The process characteristics of the keyhole entrance, including the area, length, and width were monitored and quantified by a high-speed CCD camera with the selected image processing method. The process intensity of metallic vapor and plasma was further monitored by a spectrometer, and the relationship between plasma intensity and the keyhole entrance geometry was analyzed correspondingly. Additionally, individual effects of shaped laser beam on the geometric characteristics of the fusion zone across the weld cross-sections such as effective depth of penetration, and the area and width of the fusion zone were analyzed accordingly. Experimental results showed that the process variation of the keyhole entrance differ with different welding energy modes, and the main difference came from the variation of keyhole length. The plasma intensity had a positive relationship with the keyhole entrance in different shape of laser beam welding. The fusion efficiency of cross-section was quite different with different laser beams. This work provided a new investigation for researchers and engineers to have a better understanding of shaped laser beam on materials welding.

Investigating the effect of process parameters on the mechanical properties and temperature distribution in fiber laser welding of AISI304 and AISI 420 sheet using response surface methodology

Fiber laser welding of austenitic and ferritic stainless steel was performed based on response surface methodology to systemically analyze the effect of process parameters on the weld characterizations. The laser welding input parameters were selected (laser power, welding speed and nozzle stand of distance) while the tensile strength of the joint and temperature near the molten pool were considered as the main responses. Variation of nozzle distance about 5 mm had less effect on the temperature domain near the molten pool and weld tensile strength. Increasing the nozzle distance and moving the focal point position to the upper surface of the sheet surface led to the notable improvement of the tensile strength about 20 percent. In other words, transition of the focal point thorough the material thickness clearly reduced the joint strength due to creating more volume of martensitic micro-structure and finally cracking during the tensile test from HAZ region of ferritic steel. The interaction of the laser power and welding speed had noticeable impact on the molten pool temperature. Evidently, the rate of temperature reduction due to an increased welding speed is greater at higher laser power. Among the investigated parameters, the welding speed had the highest effects on the tensile strength of the joint. Increasing the laser power from 300 to 360 W raised the tensile strength about 20%. Interaction of the laser power and welding speed resulted in about 300 °C of the temperature reduction due to an increased welding speed at higher laser power of 395 W variation of nozzle distance at different lase power levels less effect about 5 percent in comparison to the welding speed versus nozzle distance although the similar trend was observed for both of them.

Experimental and numerical studies of laser-welded stainless steel channel sections under combined compression and major axis bending moment

This paper presents an in-depth experimental and numerical investigation into the behaviour of laser-welded stainless steel channel sections under combined compression and bending moment about the major axis. Two laser-welded austenitic stainless steel plain channel sections were considered in the experimental investigation, and for each channel section, four eccentrically loaded stub column tests were conducted under various initial loading eccentricities. The experimental results were then adopted in a numerical investigation for the validation of finite element models, by means of which parametric studies were conducted to generate further structural performance data over a wider range of cross-section sizes and initial loading eccentricities. Both the obtained experimental and numerical results were carefully analysed and then used to evaluate the accuracy of the current codified design rules for welded stainless steel channel sections under combined compression and major axis bending. The evaluation results generally revealed that the codified design rules yield excessively conservative and scattered resistance predictions, owing to the neglect of the favourable material strain hardening of stainless steel and the beneficial stress redistribution within channel sections under combined loading. An improved design approach has been proposed through extension of the deformation-based continuous strength method (CSM) to the case of laser-welded stainless steel channel sections under combined compression and major axis bending. Quantitative evaluation of the new design approach was made through comparing the predicted resistances against the experimental and numerical failure loads, with the results revealing that the new design approach yields a much higher level of design accuracy and consistency than the current codified design rules. Finally, statistical analyses have been conducted to confirm the reliability of the new design approach according to EN 1990.

Fatigue Life Prediction of Dissimilar Metal Laser Weld Joints

Fatigue failure of dissimilar metal weld structures occurs when it is subjected to fluctuating loads. The structures made of dissimilar metal weld joints were found in many industrial applications. This paper is aimed to estimate the fatigue life of 0.76 mm 304 L stainless steel and mild steel laser welds using energy balance equation and infrared camera. The laser welding experiments are conducted as per the L4 Taguchi’s method, and optimal process parameters were estimated using Taguchi approach. The weld joint prepared at optimal process parameters is considered for prediction of fatigue life. The fatigue tests are conducted by varying pretension load and keeping amplitude constant. The rise in temperature of the specimen is captured during the fatigue life test using infrared camera. The results obtained from this approach are in good agreement with the experimental results. This study helps to predict the fatigue life of the welds quickly based on rise in temperature information acquired during the initial stages of fatigue test.

Characterization of laser beam offset welding TiNi alloy and 304 stainless steel with different joining modes

In this paper, laser welding of TiNi alloy and stainless steel (SS) dissimilar metal material with 38Zn-61Cu alloy as filler metal was carried. The results showed that the TiNi alloy and SS were joined by melting filler metal when the laser was concentrated in the SS side of the joint. One process was one pass welding involving creation of a joint with one fusion weld and one diffusion weld separated by remaining unmelted SS. When laser beam on the SS side was 1.2 mm, SS would not be completely melted in joint. During welding, the laser was focused near the TiNi alloy-SS interface, so that TiNi alloy and SS partially melted and diffusion welding process was realized at the TiNi alloy-SS interface, while element diffusion bonding occurred at the TiNi alloy-SS interface. No Ti-Fe intermetallic was detected in the joint. A diffusion weld was formed at the SS-TiNi alloy interface with the main microstructure of β-CuZn + Fe3Zn7, β-CuZn and Ti2Cu + TiNi. The joint fractured at the diffusion weld with the maximum tensile strength of 153 MPa.

Experimental investigation of temperature field and fusion zone microstructure in dissimilar pulsed laser welding of austenitic stainless steel and copper

Laser dissimilar welding of copper/ stainless steel was experimentally conducted to investigate the process parameters influence on the temperature field and formation mechanism of melt pool microstructures. The experimental approach is to estimate the influence of the Nd:YAG pulsed laser process parameters (e.g welding speed, laser power, focal length and laser beam deviation) on the temperature gradient and the resultant metallurgical aspects of the melt pool such as Solidification cracking, copper particle diffusion and microstructure of the fusion zone. Creating the appropriate temperature gradient during welding plays a crucial role to effectively control solidification cracking at the fusion zone in order to reduce heat conduction effect of the copper. Increasing welding speed from 2 to 6 mm/s not only reduces the temperature about 50 °C but also decreased the fusion zone cracking significantly. Evidently, increasing the laser power from 240 to 260 W transmitted the fusion zone cracks toward the stainless steel base due to creating higher temperature about 230 °C and 150 °C for steel and copper respectively. Therefore, higher discrepancy of the temperature field between copper and steel clearly changed the place and pattern of cracks propagation. The EDS analysis of the fusion zone confirms diffusion of copper particles inside the weld pool. The temperature measured near the molten pool region showed that clear discrepancy at the temperature gradient of the melt pool at the copper and stainless steel side has increased the possibility of cracking the stainless steel fusion zone. Also, high temperature gradient for compensating copper high rate of cooling could lead to excessive copper ablation or even diffusion of copper elements into the stainless steel fusion zone.

Flexural-torsional buckling of austenitic stainless steel I-section beam-columns: Testing, numerical modelling and design

Abstract The flexural-torsional buckling response and design of stainless steel I-section beam-columns are investigated in this paper. First, a series of laboratory tests on laser-welded stainless steel I-section beam-columns susceptible to flexural-torsional buckling is presented. The results obtained are supplemented by further data generated by means of numerical parametric studies on both conventionally arc-welded and laser-welded stainless steel members covering a wide range of member slenderness and combinations of loading. Existing provisions for the design of welded stainless steel I-section elements against flexural-torsional buckling are then assessed and found to require improvement. Finally, new formulae for the design of stainless steel I-section beam-columns susceptible to flexural-torsional buckling are proposed. The new proposals yield improved accuracy and consistency over existing provisions and their suitability for inclusion in the upcoming version of the European structural stainless steel design code EN 1993-1-4 is confirmed by reliability analysis in accordance with EN 1990.

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.

Understanding root humping in high-power laser welding of stainless steels: a combination approach

High-power laser welding is an efficient way for joining the thick plates in the industries. However, full penetration laser welding of thick plates is prone to form weld imperfections such as undercuts, root humps, and porosity. In this paper, numerical simulation and laser welding experiments are combinedly carried out to investigate the dynamics of the keyhole and molten pool and understand the formation mechanism of root humping during full penetration fiber laser welding of thick stainless steel plates. Besides, the microstructure of the welded joint is analyzed. Good agreements are obtained between the numerical simulation and experimental results. The results indicate that recoil pressure, surface tension, and gravity are the main driving forces for the formation of root hump. The melt flow driven by the recoil pressure near the bottom of the keyhole is an initial and direct formation cause of the root hump. Welding speed is an important factor for the growth of root hump. Local solidification of the rear weld pool path is a hinder for the continuous growth of root hump and can cause a periodic root humping weld.