Laser Welding Of Stainless Steel Research Articles

A NOVEL DOUBLE-SIDE LASER WELDED THICK PLATE: MICROSTRUCTURE AND NUMERICAL PREDICTION OF TENSILE TEST

The study aims to provide firsthand knowledge on finite element simulation (FES) of double-side laser welded (LW) stainless steel 304 (SS-304) thick plates used for automobile application. Double-side welding technique is an idea of achieving complete penetration with minimal heat input. The microstructural evaluation revealed the presence of dendritic growth that results in the enhancement of tensile strength (TS) for the welded sample. There is a slight increase in the delta ferrite content in weldment. The change in microstructure is due to thermal history during solidification. The TS of base metal (BM) was [Formula: see text][Formula: see text]MPa whereas the welded sample exhibited strength of [Formula: see text][Formula: see text]MPa. FES was successfully employed to simulate TS with error percentage less than 1.

Study on molten pool flow and hot cracking of narrow gap laser welding of 316LN stainless steel for fusion reactor

In this paper, narrow-gap laser welding (NGLW) of 316LN stainless steel was studied. The results indicated that continuous laser welding with filler wire (CLWFW) and pulsed laser welding with filler wire (PLWFW) can obtain good weld formation under the transition of the wire bridge, and when the pulsed laser was a trapezoidal wave, the hot cracks of the filling layer can be greatly inhibited. The intermittent output of the pulsed laser reduced the temperature gradient of the molten pool and enhanced the morphology of the caudal region of the molten pool. Meanwhile, timely liquid backfilling effectively relieved the stress concentration of the weld, thus inhibiting the generation of hot cracks. Besides, the intermittent pulse energy input had a stirring effect on the molten pool, which disturbed the growth direction of the dendrite grain and refined the grain; consequently, the impurity elements were fully diffused, and the distribution of elements in the weld center area was more uniform.

Real-time tracking method for motion spatter in high-power laser welding of stainless steel plate based on a lightweight deep learning model

The high-power laser welding of stainless steel is prone to spatter defects. The dynamic behavior of motion spatter is closely related to the welding state, but the relevant features are difficult to obtain. This paper extensively describes a motion spatter tracking method based on a lightweight deep learning model and a multi-target tracking algorithm. Firstly, an in-situ observation platform with a high-speed camera-based high spatiotemporal resolution for motion spatters is established. This platform can record the generation and motion process of different spatters during welding. To reduce the interferences and highlight the spatter characteristics, a linear point operation-based image low-brightness metal vapor plume removal method is also proposed. Then, a deep learning-based lightweight spatter detection model is established, and an automatic spatter label generation method is proposed to quickly obtain a sufficient amount of highly discriminative training data to train the established model comprehensively. Compared with the basic model, the validation results show that the lightweight spatter detection model runs 5.11 times faster, with a spatter detection accuracy of 96.71%. Finally, a motion spatter tracking method based on the DeepSORT algorithm is proposed to distinguish different spatters and associate the same spatters. The spatter size, velocity, quantity, and other features are also able to be accurately extracted via this method, achieving spatter defect monitoring. In summary, this study describes a novel and robust method for motion spatter real-time tracking during laser welding and this method can provide reliable data support for accurate and fast process condition monitoring as well as weld quality assessment.

Influence of Wire Feeding Speed on the Morphology and Properties of Stainless Steel Laser Welding Joints

Abstract Using laser filler wire welding technology, welded joints were produced at varying wire feeding speeds. The microstructure and mechanical attributes of these joints were then examined through optical metallography, scanning electron microscopy, universal tensile testing, and microhardness testing. It demonstrates that superior surface formation and defect-free welded joints can be achieved through the filling wire welding process. The main components are austenite and ferrite, with slightly varying morphology and content. Through the results of tensile tests and fracture microstructure scanning, when the rate of wire supply is 2.0 meters per minute, the tensile strength reaches 650 MPa. The second phase particles were discovered in the fracture scan. According to microhardness measurements, the overall hardness of the welded joints at different wire feeding speeds is higher than that of the base material.

Laser welding of molybdenum and stainless steel with Cu[sbnd]Ti filler metal

Brittle MoFe intermetallic compounds (IMCs) are usually fatal for the joining of Mo and steel. In the present work, Ni-free joining of Mo and stainless steel was successfully achieved by laser welding with CuTi filler metal. The entry of Mo element into the weld seam and the formation of MoFe IMCs was significantly suppressed. TiFe, TiFe2, Ti5Cr8Fe16, and Cu-Ti-Fe phases were found inside the weld and near the interface. The IMCs were separated by Cu matrix and no continuous layered IMCs were found. The brittle fracture characteristics of TiFe IMCs and plastic deformation of the Cu matrix was found on the fracture surface in the weld.

Numerical modeling of the temperature distribution and melt flow in dissimilar fiber laser welding of duplex stainless steel 2205 and low alloy steel

The research outlined in this paper pertains to the computational modeling and experimental investigation of laser keyhole welding between duplex stainless steel 2205 and low carbon steel (A516). A crucial aspect of keyhole welding entails predicting the phase transition behavior of the materials involved. The vaporized metal generates a recoil pressure and distorts the interface between the liquid and vapor phases, thus forming a vapor capillary. A multiphysics model employing the finite volume method was proposed to achieve a highly accurate simulation of the temperature and velocity fields. The findings demonstrate that the resulting weld bead relies heavily on the keyhole's structural characteristics. Additionally, analysis of the temperature distribution at a specific temporal and spatial point reveals that duplex stainless steel exhibits elevated temperatures owing to low thermal penetration at a distance of 5 mm from the juncture of the two components compared to A516. The numerical simulation results were in good agreement with experimental results for prediction of temperature filed and melt flow that have impact on weld bead and melt pool geometry. The weld bead width changes according to variation of welding speed and laser power has been in good agreement with the temperature distribution at upper surface of the workpiece extracted from simulation results. The melt pool geometry predicted from the simulation results (melt flow and temperature field) has been in good agreement with the melt pool geometry of the experiment. The temperature distribution from simulation results has a good correlation with the microstructural changes of the melt pool region according to the melting, solidification and temperature gradient at the melt pool region and adjacent HAZ of base metals.

Microstructure and mechanical properties of Flexible Ring Mode laser welded 304 stainless steel

Flexible Ring Mode (FRM) laser welding is a relatively new composite laser welding technology. The present study focused on the FRM laser welded joints produced from 4.5 mm-thick 304 stainless steel. The effects of various laser modes on the macroscopic morphology, microstructure, microhardness, and tensile properties of the produced welded joints were systematically investigated. The results showed that, when the ring laser power was set at 1000 W and the center laser power at 3000 W, a maximum weld area of 6.23 mm2 was obtained. Employing a composite laser with a center laser power greater than the ring laser power resulted in the fewest number of welding spatter particles on both sides of the weld bead, indicating an effective inhibition of welding spatter by the ring laser. Conversely, using a composite laser with a center laser power equal to the ring laser power still led to the presence of small welding spatter particles, indicating differences in the laser combination modes in suppressing welding spatter for the welding of 304 stainless steel. Increased cooling and a large presence of small turbulence contributed to the grain refinement in the fusion zone of the welded joints produced using a composite laser with a central laser power greater than the ring laser power. Consequently, this welded joint exhibited the highest maximum tensile strength among all tensile test specimens reaching 95.2 % of the base metal’s maximum tensile strength. However, the fractured surfaces of the tensile test specimens produced using a composite laser with a center laser power equal to the ring laser power exhibited the highest number density and largest size of oxide particles, resulting in a joint elongation of only 77.3 % of the base metal’s elongation. This study emphasizes the potential application of this FRM laser and provides theoretical guidance for selecting the optimal laser combination mode for FRM laser welding of stainless steel.

Response of the melt pool and vapour capillary on dynamic beam shaping in the kHz regime during laser welding

Beam shaping technologies provide the possibility to distribute the optical energy of the laser beam on the workpiece in multiple ways. One of the latest technologies is coherent beam combining (CBC) of 32 fiber lasers, which allows quasi-instantaneous switching of beam shapes during laser material processing with frequencies up to 10 MHz. A study was performed for laser welding of stainless steel 1.4301, to investigate the influence of the switching frequency between two beam shapes on the capillary shape. Therefore, the beam shape was switched between a single spot and four spots, which leads to a maximum intensity variation. Frequencies between 10 Hz and 10 kHz were investigated. The results show, that the melt pool shape and the capillary shape follow the switching frequency until a limit frequency.

Reducing capillary depth fluctuations in high-speed laser welding of stainless steel using multi-core laser technology

Laser welding was carried out in AISI304 (X5CrNi18-10) stainless steel using a multi-core fiber laser. High-speed X-ray imaging was used to monitor the process. When employing core-power only (400W) at high welding speeds (12m/min), the capillary depth was found to be inconsistent. The capillary penetration into the material varied from approximately 0.8 mm to approximately 0.2 mm, with fluctuation periods of between single milliseconds and tens of milliseconds. Employing a 0.5:0.5 ring-core power distribution (390W:390W) to achieve the same penetration (0.8mm) was found to reduce fluctuations in the depth of the welding process and the capillary penetration. Possible reasons for this reduction in depth fluctuations are discussed together with a detailed description of the resulting changes in weld cross-section.

Underwater wet laser welding of duplex stainless steel under various water depths

Reports on experiments of underwater wet laser welding (UWLW) are scarce because of the poor forming quality of weldments. To address the key manufacturing challenges such as failure of welding with water depth exceeding 7 mm and the generation of pores and cracks which caused by the rapid water cooling, a specific self-designed flux assisted welding at different water depths (0–30 m) was innovatively proposed in this study. The microstructure, phase distribution, and properties of underwater wet laser welded duplex stainless steel joints were examined. The welding process successfully produced joints free from pores or cracks, even when performed within a 15 m water depth. In conjunction with thermodynamic calculation, the evolution process of welds and the effect of nickel from the flux on phases structure were analyzed. As the water depth increases and the welding speed decreases, the number of pores inside welds increase significantly, and no cracks are found. The penetration of all welds made in deep water is obviously smaller than that of shallow water and increased heat input does not seem to contribute to the increase of weld penetration. With the increase of water depth, a large amount of coarsened austenite consisted of columnar grains and cellular grains appear in weld zone. Texture and dislocation of the weld made in shallow water were investigated. Many dislocations including edge dislocations and dislocation entanglements arranged in austenite phase, while seldom dislocations appear in ferrite phase.

A coaxial nozzle assisted underwater laser welding of 316L stainless steel

As a promising maintenance technique, underwater laser welding (ULW) technology aids in advancing oceanographic engineering. However, water has an attenuating effect on laser making direct ULW challenging, and water intrusion may deteriorate weld formation. This paper qualitatively analyzed the visualization of ULW welding process dynamics under varying water depths, based on image sequences recorded by a high-speed camera, and then focused attention on macro-forming characteristics of ULW joints. The results demonstrate that wet ULW fails when the water depth exceeds 6 mm due to the inability to establish a “beam channel”, which is required for ULW to proceed. Thus, a self-designed coaxial nozzle was employed to minimize water interference in underwater environment, as demonstrated by numerical simulations. Porosity is sensitive to the welding parameters and can be minimized by lowering laser power, increasing velocity, and choosing a modest shielding gas flow rate. Pore formation is linked to the dynamic behavior of the local gas phase space, which is distinctive to the ULW processing. This work offered a comprehensive understanding of the ULW that was highly feasible.

I-section stainless steel portal frames: Loading tests and numerical modelling

Austenitic stainless steel is an attractive structural material due to its high ductility, appropriate toughness, significant strain hardening and good fire resistance. Also the laser welding technology for fabrication of structural stainless steel profiles has recently become increasingly popular. However, there is still only a limited amount of experimental data on laser-welded stainless steel members. And loading tests of whole structures made of these sections have still not been published. Therefore, loading tests were performed on two austenitic stainless steel frames made of laser-welded I-sections. The frames had pinned columns and were loaded both vertically and horizontally. The objective of the loading tests was to capture the effects of material nonlinearity on the global sway behaviour of the stainless steel frames. In fact, material nonlinearity leads to a gradual loss of stiffness, which results in higher deformations and thus a higher second-order effect. The resulting load-displacement diagrams of the experiment are tabulated, allowing their further use. A numerical model of the studied frames was developed and validated on the basis of the performed experiments. Based on the elastic buckling load factor and the modified elastic buckling load factor, a large influence of material nonlinearity on second-order effect has been demonstrated.

Monitoring of thermo-cycles in fibre laser welding of duplex stainless steel 2205 sheets and its correlation with microstructures and mechanical properties

The study reports the influence of change in the heat supplied (43 J mm−1 to 18.5 J mm−1) on the microstructures as well as mechanical properties of weld joints obtained by welding of Duplex stainless steel 2205 using fibre laser. In-process thermal monitoring of the molten weld pool was carried out using IR pyrometer. Cooling rates (i.e. solidification and solid) were calculated from the thermo-profiles of weld pool, and it increases with the decrease in heat input. From the optical images, it is observed that columnar grains originated from the fusion zone walls and merged at the center. Since, solidification front velocity is comparable on both sides’ leads to a central edge. Ferrite phase content observed in fusion zone microstructure, increases with the increase in solid cooling rate. The result suggests that the joints fabricated at lowest heat input displayed highest tensile strength. The maximum tensile strength been reported to be 872.5 ± 10.8 MPa, and failure occurred at parent metal. Tensile strength of weld joints of DSS 2205 was found to have improved with increasing cooling rate. Higher cooling rate results in the formation of fine dendritic grains as well as higher ferrite content in the weld metal.

Study on the Step by Step Process and Performance of Laser Welding for the Spent Fuel Pool Floor

Abstract In order to realize the steel liner underwater repairing of the spent fuel pool of the third generation nuclear power plant, the laser welding process tests were carried out step by step in three environments: air, shallow water, and simulating-repairing of the spent fuel pool floor(high-pressure condition). Through the process optimization, the high-quality forming of the underwater laser welding of duplex stainless steel was realized, and the underwater local dry laser welding process suitable for the spent fuel pool floor of nuclear power plant was developed. The results of nondestructive testing (including visual testing, liquid penetrant testing, ultrasonic testing, and radiographic testing) of welding test pieces under three environments were qualified, and the test results of properties (including tensile, impact, bending, intergranular corrosion, and ferrite content) meet the standard requirements. The underwater weld performance is similar to that in the air environment, and the weld quality meets the requirements of the spent fuel pool construction standard, laying a technical foundation for the application of the spent fuel pool underwater repairing.

Using the numerical simulation and artificial neural network (ANN) to evaluate temperature distribution in pulsed laser welding of different alloys

The temperature field during laser welding process plays an important role on determining the quality and quantity of the weld bead size, microstructure characterizations and mechanical properties of the welding interface in the thermal engineering applications. In this study, using the numerical simulation, the influence of pulse duration and frequency on the temperature distribution and velocity field in distinctive laser welding of stainless steel 420 (S.S 420)/stainless steel 304 (S.S 304), and Bohler 303 (B 303)/stainless steel 304 (S.S 304) was examined. The results of numerical modeling illustrated that shear stress of Marangoni and buoyancy force are the most curtail aspects in the formation of the flow of liquid metal. A novel artificial intelligence method is proposed to optimally predict the melting ratio, and maximum temperature of the materials. To this end, a combination of ANN and Particle Swarm Optimization (PSO) algorithms are employed. The PSO algorithm is used to optimize the architecture and training algorithm of the ANN, while the ANN is employed for the regression problem. Based on the results, a three-layer feed-forward architecture with sigmoid transfer functions having 17 and 8 neurons in the hidden layers combined with the scaled conjugate gradient backpropagation training scheme is recognized by the PSO as the optimal configuration. Application of optimal ANN to the regression problem results in an acceptable level of error for the training, validation, and test datasets. Finally, the optimized ANN can be utilized to anticipate the melting ratio and thereby the resultant temperature.

Flexural buckling of laser-welded stainless steel T-section columns

The present paper reports experimental and numerical studies on the flexural buckling behaviour and resistances of laser-welded stainless steel T-section columns. An experimental programme, including initial global and local geometric imperfection measurements and ten fixed-ended column tests, was firstly performed. The test results, including failure loads, load–mid-height lateral deflection curves and two types of failure modes (‘C’-shaped and ‘reverse C’-shaped flexural buckling), were reported in detail. Following the experimental programme, a numerical modelling programme was carried out, where finite element models were developed and validated against the test results and then used to perform parametric studies to generate further numerical data over a wide range of cross-section dimensions and member lengths. On the basis of the test and numerical results, the relevant design buckling curves, as given in the European code and American specification, were evaluated. The evaluation results revealed that both codified design buckling curves generally resulted in accurate and consistent flexural buckling resistance predictions for laser-welded stainless steel T-section columns.

Effect of post-weld heat treatment on the microstructure and properties of titanium/304 stainless steel laser welding dissimilar metal joints with copper as the transition layer

Low elongation has been an important factor limiting the application of titanium/304 stainless steel (304SS) laser welding of dissimilar metal joint products. By adopting the post-weld heat treatment (PWHT) process of holding the welded joint at 400 ℃, 550 ℃ and 700 ℃ for 30 min, this study increases the elongation of the welded joint from 0.3 % to 2.21 % in the welded state and further analyses and investigates the structural changes in the organization and the formation process of intermetallic compounds (IMC) within the weld seam. The results showed that the acicular structure in the weld could not be eliminated by holding at 400 ℃ for 30 min, but only part of the residual stress was released, and the elongation was increased to 0.85 %. After holding at 550 ℃ for 30 min, a large number of equiaxed crystals appeared to replace the acicular structure in the weld and released the residual stress. With the enhancement of the IMC layer thickness, grain gap and deformation coordination ability, the elongation increased to 2.21 %. After holding at 700 ℃ for half an hour, the IMC layer becomes thinner, the grain gap decreases, the deformation coordination ability deteriorates, and the tensile strength and elongation decrease. In the tensile test, the fracture position was all at the weld. After being held at 400 ℃ for 30 min, the average tensile strength of the tensile sample increased from 330 MPa in the welded state to 352 MPa; after being held at 550 ℃ for 30 min, the average tensile strength of the tensile sample was 340 MPa, and the fracture elongation was 7.4 times that of the welded state, reaching 331 µm.

Membrane residual stresses and compressive behaviour of laser-welded stainless steel T-sections

This paper reports experimental and numerical investigations into the membrane residual stresses and compressive behaviour of laser-welded stainless steel T-sections. An experimental programme was firstly conducted on eight T-sections and included tensile coupon tests, membrane residual stress measurements, initial local geometric imperfection measurements and sixteen stub column tests. On the basis of the measured data, a new membrane residual stress predictive model was proposed for laser-welded stainless steel T-sections. Following the experimental programme, a numerical modelling programme was performed, with finite element models developed and validated against the test results and then employed to perform parametric studies to generate additional numerical data over a wide range of cross-section dimensions. The obtained test and numerical data were used to evaluate the relevant local buckling design rules specified in the European code and American specification. The evaluation results revealed that (i) the codified slenderness limits were generally accurate when used for cross-section classification of laser-welded stainless steel T-sections in compression and (ii) both design codes resulted in slightly conservative and scattered cross-section compression resistance predictions.

Dissimilar laser welding of CrMnFeCoNi high entropy alloy and 316LN stainless steel for cryogenic application

The microstructure and mechanical properties of dissimilar laser beam welded joint between CrMnFeCoNi alloy and 316LN stainless steel was investigated. The results showed that the defect-free dissimilar joint was obtained by laser beam welding. The ultimate strength of the welded joints can reach ∼90% of the base materials at both room and cryogenic temperatures. The deformation substructure mainly consisted of planar dislocation, the stacking faults and the dissociation of stacking faults into nanotwins. The volume fraction of the nanotwins was increasing at cryogenic temperature. The hardness fluctuates greatly in welded joint and the lowest hardness was located at fusion zone near the fusion line. The fracture of the welded joint was located at the fusion zone in consistence with the lowest hardness area. It is mainly attributed to the coarse grain and stress concentration at this area.

Toward accurate prediction of partial-penetration laser weld performance informed by three-dimensional characterization – Part II: μCT based finite element simulations

The mechanical behavior of partial-penetration laser welds exhibits significant variability in engineering quantities such as strength and apparent ductility. Understanding the root cause of this variability is important when using such welds in engineering designs. In Part II of this work, we develop finite element simulations with geometry derived from micro-computed tomography (μCT) scans of partial-penetration 304L stainless steel laser welds that were analyzed in Part I. We use these models to study the effects of the welds’ small-scale geometry, including porosity and weld depth variability, on the structural performance metrics of weld ductility and strength under quasi-static tensile loading. We show that this small-scale geometry is the primary cause of the observed variability for these mechanical response quantities. Additionally, we explore the sensitivity of model results to the conversion of the μCT data to discretized model geometry using different segmentation algorithms, and to the effect of small-scale geometry simplifications for pore shape and weld root texture. The modeling approach outlined and results of this work may be applicable to other material systems with small-scale geometric features and defects, such as additively manufactured materials.