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Welding problems have restricted the applications of lightweight SiCp/Al composites in aerospace. In this work, the controllability of the keyhole mode, nucleation mechanism, and mechanical properties of the laser-welding process are studied in detail. The extrusion effect formed by the open keyhole pushes SiC particles to the bottom of the molten pool. To observe the nucleation and growth mechanism of the Al4C3 of keyhole laser-welded SiC/2A14Al composites, the dissolution–precipitation behavior of the SiC particles and growth behavior of the Al4C3 tip were observed. Owing to its high chemical properties, Al4C3 mainly begins nucleating from the open edges of the SiC particles and its formation is mainly distributed along the direction of Al grains. Al4C3 was the main crack source and propagation path while the small-sized SiC particles hindered the crack propagation. This work innovatively regulates the keyhole mode to achieve ‘small damage breakdown’ laser welding, greatly reducing the volume fraction of Al4C3. It also provides new insight into fusion welding of SiC particle-reinforced Al matrix composites.

A computational fluid dynamics (CFD) simulation of the molten pool in laser keyhole welding was utilized to acquire temperature data for further metallurgical and mechanical calculations. For the CFD simulation, the governing equations were solved, and the scattering and absorption of the laser beam in the plume were modeled at both the standard atmospheric condition (101,325 Pa) and a vacuum condition (3,000 Pa). A stochastic ray-tracing algorithm was adopted to effectively implement the transmission and scattering of laser bundles of rays. The temperature data from the CFD simulation were then imported to a finite element method (FEM)-based heat conduction analysis to simulate the thermal-metallurgical-mechanical behavior during the cooling phase of the weldment. The strain, residual stress, and distortion were calculated using an elastoplastic model based on the phase transformation-dependent material properties. An element deactivation scheme was used to take care of the zero-strength condition of the elements in the molten pool and keyhole region. The Vickers hardness and the residual stress were measured to verify the simulation model, and the experimental and simulation results had a similar tendency.

Laser keyhole initiation and termination-related defects, such as cracking and keyhole cavities due to keyhole collapse, are a well-known issue in laser keyhole welding of thick section steels. In longitudinal welding, run-on and run-off plates are used to avoid this problem. However, such an approach is not applicable in circumferential welding where start/stop defects remain within the workpiece. These issues can hinder industry from applying laser keyhole welding for circumferential welding applications. In this paper, the effect of inter-pass temperature on laser keyhole initiation and termination at the weld overlap start-stop region was investigated. This study has identified that defects occurring within this region were due to laser termination rather than laser initiation because of keyhole instabilities regardless of the thermal cycle. The laser termination defects were mitigated by applying a laser defocusing termination regime to reduce the keyhole depth gradually and control the closure of the keyhole. [ABSTRACT FROM AUTHOR]

A vacuum environment results in a deep penetration depth in laser keyhole welding. In this study, the numerical simulations of laser keyhole welding in various vacuum conditions were performed with consideration for multiphysics phenomena. To consider the vacuum environment in numerical simulations, a modified recoil pressure model with the change in pressure and vaporization temperature according to ambient pressure and a laser power attenuation model by scattering and absorption due to the nanoparticles in the laser-induced plume was considered. For the scattering and absorption calculations, artificial coefficients obtained for a 1 atm condition were used, and then the coefficients in the other pressure conditions were calculated under assumptions suggested in this study. The simulation results using the models based on the assumptions could depict a deeper penetration in vacuum conditions considered in this study, and they were in good agreement with the experimental results in terms of weld cross section. [ABSTRACT FROM AUTHOR]

A laser keyhole welding process was performed to join of 6mm thick aluminum (Al) alloys to steel plates in a butt configuration without any filler materials. High quality metallurgical bonding joints were produced by controlling welding parameters.The effect of different laser beam effsets and different locations of joints on the morphology, phase components and thickness of the intermetallic compound (IMC) layers was investigated by using optical microscope (OM) and electron backscatter diffraction (EBSD) analysis technology. The results show that the main phases of IMC layers are composed of η-Al5Fe2 and θ-Al13Fe4 phases. The laser beam offsets have an important influence not only on the average thickness of IMC layers but also on the IMC grain size:increasing laser beam offsets significantly reduces the amount of needle-like θ-Al13Fe4 phases, refines the grain sizes of η-Al5Fe2 layers, and decreases the average thickness of η-Al5Fe2 layers at the interfaces. In considering the influence of the different locations through the steel/Al alloy joint, under the conditions of two laser beam offsets, the minimum grain size and thickness of η-Al5Fe2 layers have been obtained at the middle part of the steel/Al alloy joints. In addition, it shows clearly that the θ-Al13Fe4 phases are generated between the η-Al5Fe2 layers and the steel substrates are confirmed by the EBSD phase analysis.

In the field of automotive manufacturing, it is pivotal to obtain a moderate penetration status when joining automotive parts by laser keyhole welding. As a typical characteristic of laser keyhole welding, keyhole behavior can directly reflect the penetration status of the weld bead. In this paper, a coaxial vision monitoring system with a laser auxiliary illuminant is established to collect the keyhole images from the top-face of the weldment during laser welding. The interferences of the bright spot caused by mirror reflection and metal vapors in the collected images are fully considered. Then, a hybrid adaptive keyhole detection algorithm is proposed to accurately segment the keyhole region. Five static image features are extracted from the perspective of the average gray value, area, and perimeter in the keyhole region. The feature selection method of Wrapper, which combines a sequential forward searching algorithm with a random forest classifier, is used to select ten penetration status features (PSFs). The constructed prediction model of weld penetration based on the selected PSFs has excellent performance. The proposed monitoring method in laser keyhole welding irradiated by a laser auxiliary illuminant is contributed to the closed-loop control of weld penetration to obtain a moderate penetration status. [ABSTRACT FROM AUTHOR]

The interfacial reaction 3SiC + 4Al → Al 4 C 3 + 3Si has bottlenecked the fusion welding of aluminum matrix composites for decades, hindering further improvement of the mechanical properties of high-strength lightweight SiC p /Al joints. Using unweldable SiC p /2xxxAl as an experimental model, we designed a targeted reinforcing phase that can be alloyed in situ with Si and C elements. L1 2 –(Al,Si) 3 Ti and TiC were generated in situ from Si and C precipitated from SiC particles. In addition, the in situ-formed Al 3 Ti adhered to the surface of the SiC particles, enhancing the load transfer of the alloy film during the tensile process. The mechanism of titanium on the joint grain nucleation and refinement and the contribution of each strengthening mechanism to mechanical properties were discussed in detail. Quantitative calculations showed that the combined effects of the nanoscale TiC phase (Orowan strengthening) and high density of the geometrically necessary dislocations (dislocation strengthening) increase the yield strength of the joints. The ultimate tensile strength of the joint was 393 MPa (with a strength coefficient of 83%), which was highly comparable to that of SiC p /Al joints prepared by friction stir welding. This work provides fundamental guidance for the welding of high-strength SiC p /2xxxAl alloy. • The L1 2 –(Al,Si) 3 Ti and TiC reinforced SiC/2A14 welding joint was prepared. • The mechanism of joint strengthening was quantitatively calculated. • The tensile strength of the SiC/2A14 joint is 393 MPa. [ABSTRACT FROM AUTHOR]

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Laser welding of thick plates in production environments is one of the main applications of high power lasers; however, the process has certain limitations. The small spot size of the focused beam produces welds with high depth-to-width aspect ratio but at times fails to provide sufficient reinforcement in certain applications because of poor gap bridging ability. The results of welding shipbuilding steel AH36 with thickness of 8 mm as a single-sided T-joint using a 10 kW fiber laser are presented and discussed in this research paper. Three optical setups with process fibers of 200 μm, 300 μm and 600 μm core diameters were used to study the possibilities and limitations set by the beam delivery system. The main parameters studied were beam inclination angle, beam offset from the joint plane and focal point position. Full penetration joints were produced and the geometry of the welds was examined. It was found that process fibers with smaller core diameter produce deeper penetration but suffer from sensitivity to beam positioning deviation. Larger fibers are less sensitive and produce wider welds but have, in turn, lower penetration at equivalent power levels. [ABSTRACT FROM AUTHOR]

Dissimilar Al/steel butt joints of 6.0 mm thick plates have been achieved using fiber laser keyhole welding autogenously. The cross sections, interface microstructures, hardness and tensile properties of Al/steel butt joints obtained under different travel speeds and laser beam offsets were investigated. The phase morphology and thickness of the intermetallic compound (IMC) layers at the interface were analyzed by scanning electronic microscopes (SEM) using the energy-dispersive spectrometry (EDS) and electron back-scattered diffraction (EBSD) techniques. The results show that travel speeds and laser beam offsets are of considerable importance for the weld shape, morphology and thickness of IMC layers, and ultimate tensile strength (UTS) of Al/steel butt joints. This proves that the IMC layers consist of Fe2Al5 phases and Fe4Al13 phases by EBSD phase mapping. Increasing laser beam offsets from 0.3 mm to 0.7 mm significantly decreases the quantity of Fe4Al13 phases and the thickness of Fe2Al5 layers at the interface. During tensile processing, the Fe2Al5 layer with the weakest bonding strength is the most brittle region at the interface. However, an intergranular fracture that occurred at Fe2Al5 layers leads to a relatively high UTS of Al/steel butt joints. [ABSTRACT FROM AUTHOR]

The purpose of this paper is to present and implement a novel approach to data-driven modeling and robust control of a laser keyhole welding process. The objective is to maintain the penetration depth of the laser welding process at a desired value under system uncertainties. A framework was proposed to estimate the keyhole diameter and the keyhole penetration depth in real time by type-1 and type-2 fuzzy basis function networks, and an adaptive divided difference filter. A robust Takagi-Sugeno fuzzy controller was implemented to adjust the laser power of the welding process. Experimental results demonstrated that the fuzzy models provided an accurate estimation of both the welding geometry and its variations due to uncertainties, and the robust Takagi-Sugeno fuzzy controller successfully reduced the penetration depth variation and improved the quality of the welding process. [ABSTRACT FROM AUTHOR]

Laser welding of thick plates in production environments is one of the main applications of high power lasers; however, the process has certain limitations. The small spot size of the focused beam produces welds with high depth-to-width aspect ratio but at times fails to provide sufficient reinforcement in certain applications because of poor gap bridging ability. The results of welding shipbuilding steel AH36 with thickness of 8 mm as a single-sided T-joint using a 10 kW fiber laser are presented and discussed in this research paper. Three optical setups with process fibers of 200 µm, 300 µm and 600 µm core diameters were used to study the possibilities and limitations set by the beam delivery system. The main parameters studied were beam inclination angle, beam offset from the joint plane and focal point position. Full penetration joints were produced and the geometry of the welds was examined. It was found that process fibers with smaller core diameter produce deeper penetration but suffer from sensitivity to beam positioning deviation. Larger fibers are less sensitive and produce wider welds but have, in turn, lower penetration at equivalent power levels.

• A vision monitoring system for weld geometry of laser keyhole welding is proposed. • A high-accuracy and robust Multi-task CNN model is proposed and validated. • The proposed monitoring system can obtain the weld depth and width in real time. This paper presents a low-cost, robust, in-situ monitoring system for weld geometry that can achieve multi-task prediction. First, the system uses a low-cost CCD camera to monitor the melt pool in the laser keyhole welding process. Then, the proposed novel multi-task convolutional neural network (Multi-task CNN) model is used to simultaneously complete the two prediction tasks of weld depth and width. Furthermore, the learning process of the Multi-task CNN model is explored using a visual feature map approach and the robustness of the model is demonstrated. Compared with Support Vector Machine, K-Nearest Neighbor, Bayesian Ridge, Decision Tree, the proposed Multi-task CNN model has the highest prediction accuracy. The model predicts a mean absolute percentage error (MAPE, relative to ground truth) of 3.0% and 1.9% for weld depth and width. The in-situ monitoring results show that the system can achieve accurate predictions, and the average time-consuming of the system is 23.35 ms. [ABSTRACT FROM AUTHOR]

Both zinc-coated and uncoated DP 590 steel sheets were used for laser welding experiment with a 2 kW multi-mode fiber laser under the same experimental conditions. Using systematically obtained keyhole data, how the keyhole dynamically responds to an incident laser beam was studied by defining several key factors, such as keyhole expansion factor, keyhole motion range factor, average aperture diameters, mass loss fraction and melt pool volume size. The relative configuration of the keyhole and the laser beam was found to be the most influential factor for obtaining good welds, and when the beam was located away from the front keyhole wall a good weld was obtained. When the beam irradiates on the front keyhole wall, the zinc evaporation region between the two sheets can be directly heated by the beam, and the zinc vapor pressure can become extremely large, and therefore the melt pool is severely disturbed. The keyhole bottom aperture diameter and the keyhole expansion factor for the bottom aperture were both largely proportional to the mass loss for both zinc-coated and uncoated steels. Zero-gap laser welding of zinc-coated steels can be indeed successful and a theoretical basis is presented. [ABSTRACT FROM AUTHOR]

Laser beam absorption phenomena during laser welding of zinc-coated and uncoated DP 590 steel sheets were simulated using the finite-difference time-domain (FDTD) method for the Maxwell equations. For accurate simulations, the keyhole shapes that were measured using a coaxial observation method during multi-mode fiber laser welding of the above steels were used, and the beam divergence and focusing characteristics of the laser were accurately matched. Also, the wavelength enlargement method was used to deal with a large computational domain required for laser welding simulation. Absorption mechanisms, multiple reflection patterns, and heating patterns were found to be significantly different for the two steels because of the different keyhole shapes. Zinc-coated steel has a much lower keyhole absorptance than uncoated steel at the same experimental condition, which was experimentally validated by melt pool size measurement. Ray tracing simulations were also performed for comparison purposes. [ABSTRACT FROM AUTHOR]

Experiments were conducted over a large process parameter space using a 2 kW multi-mode fiber laser, and the effect of zinc-coating on the keyhole geometry was investigated using a high-speed coaxial video camera. From the top and bottom coaxial surface images of erratic keyhole motions that were obtained from separately conducted experiments, time-averaged keyhole structures were calculated using a physics-based semi-statistical frame analysis. For uncoated steel, the keyhole bottom aperture is located mostly within the laser beam area. In this case, because there is no zinc evaporation, the bottom aperture tends to be closed to absorb enough laser energy for sustaining a keyhole. For zinc-coated steel, the keyhole is mostly open at the bottom, and the front keyhole wall is tilted so that the laser beam is located on the front keyhole wall. The keyhole tilting angle is more important for zinc-coated steel and the beam interaction area is more relevant to uncoated steel. [ABSTRACT FROM AUTHOR]

In keyhole welding, welding quality is closely related to the stability of the keyhole, which is primarily determined by keyhole geometry during the welding process. Three essential attributes to describe the simplified three-dimensional keyhole shape include keyhole size, penetration depth, and keyhole inclination angle. However, when using traditional measurement techniques, it is very challenging to take in-process measurements of penetration depth and inclination angle, even if the keyhole size can be detected by using a visual monitoring system. To realize the on-line estimation of keyhole dynamics and welding defects, a data-based radial basis function neural network state observer is adopted for estimating penetration depth and inclination angle in the transient state when welding parameters change suddenly. First, a static neural network is trained in advance to establish a correlation between the welding parameters and unobservable keyhole geometry. The dynamic state observer is trained based on the transient welding conditions predicted by a numerical model and then used to estimate the time-varying keyhole geometry. Meanwhile, a coaxial monitoring system is used to observe the keyhole shape from the top side in real time, which not only provides input to the neural network but also indicates the potential welding porosities. The predicted results are validated by experimental data obtained by welding of stainless steel 304 and magnesium alloy AZ31B. [ABSTRACT FROM AUTHOR]

Partial penetration welding with a fiber laser at a 9-kW laser power was carried out on 20-mm-thick plates at different positions and analyzed by both experimental and numerical methods. Experiments were carried out for four different angular positions at 1.5 m/min welding speed. All four cases showed a tail-like structure of molten pool on the top surface. The in-depth mechanisms of the energy input characteristics in fiber laser position welding for eight different positions were studied by numerical simulation using the volume-of-fluid (VOF) method. Experimental and numerical results were compared for four cases and showed fair agreement. Observation of flow pattern and bead shape revealed that gravity had little influence on bead shape and flow structure, but changed the pore structure considerably. Flow structure showed a periodic behavior which probably nullified the effect of gravity in position welding. Most of the laser rays reached the bottom of the keyhole without interruption and then multiple reflections started within the keyhole. The first five reflections inside the keyhole delivered around 70 % of the total energy. [ABSTRACT FROM AUTHOR]

Laser welding of wrought magnesium alloy has been investigated through experimentation and simulation. Laser butt welds and laser lap welds were performed on 2.0mm thick magnesium alloy AZ31 plates using a 1 kW fiber laser and shielded with argon gas. The effects of laser power and welding speed on weld geometry and microstructure were investigated. Tensile tests were performed to verify weld quality. Through experimentation. a novel processing map was created, which gives the ranges of operating parameters of laser power and welding speed that resulted in viable, defect-free welds. Numerical simulations were performed to predict the weld pool geometiy and keyhole stability, and resultant microstructures are shown to be in good agreement with experimental results. [ABSTRACT FROM AUTHOR]

A multi-scale model is developed in this paper to investigate the growth of grains and sub-grain dendrites during the molten pool solidification of austenitic stainless steel AISI304 in laser keyhole welding processes. A macro-scale dynamic model is used to predict the fluid flow and heat transfer in the keyhole as well as the molten pool. The generated thermal history is fed to a three-dimensional Cellular Automata model to predict the meso-scale grain growth and to a two-dimensional Cellular Automata–Phase Field model to predict the micro-scale dendrite morphology. The multi-scale model is applied to simulate multiple cases of different welding parameters. The model predictions are validated against experiments and studied further for detailed information. The effects of the welding parameters are discussed based on the numerical and experimental results. [ABSTRACT FROM AUTHOR]

• Laser welding needs high-speed in situ monitoring for quality assurance and control. • Coherent imaging measures depth but with speckle and autocorrelation artifacts. • We introduce a second imaging channel with no loss of resolution or imaging speed. • Uncorrelated noise and artifacts are now easily filtered out. • Laser keyhole weld tracking rate improves by as much as 67%. In situ coherent imaging provides on-the-fly measurement of morphology changes during high-power laser processing (such as welding and additive manufacturing) at high speeds (exceeding 300 kHz line rates) but suffers from speckle and image artifacts particular to interferometric imaging. We present a self-witnessing inline coherent imaging system that incorporates a second imaging channel to allow complete removal of autocorrelation artifacts, and an increase in signal to noise by a factor of two, with no loss of imaging rate or spatial resolution and no need for balanced detection or specialized light sources. When applied to laser welding of 316L stainless steel, the keyhole tracking rate is increased by as much as 67%. [ABSTRACT FROM AUTHOR]

Abstract: The flow of molten metal on the front wall of a laser generated welding keyhole has been observed by high speed photography, optically measured by mapping the flow of ripples on the liquid surface and theoretically calculated. A clear downward flow can be observed and measured by a Particle Image Velocimetry algorithm. A theoretical calculation of the melt thickness on the keyhole front is also presented. Results indicate that the thickness of the liquid on the keyhole front is similar to that of the resolidified layer found in micrographs of the front if the laser is suddenly turned off. The measured surface ripple flow speeds are between two and four times as high as the theoretical average fluid flow rate. [Copyright &y& Elsevier]

For keyhole welding regime there is a particular of the weld cross-section shape aspect given by the heat affected zone area. It analyzes the laser welds made with Nd:YAG laser on low alloy steel plates using irradiation in continuously regime. The power, welding speed and defocusing are varied. Experiments were conducted after the full factorial experimental design 2. This showed the effect of laser power for setting keyhole welding regime. [ABSTRACT FROM AUTHOR]

Abstract: Welding of zinc coated sheets in zero gap configuration is of eminent interest for the automotive industry. This Laser welding process would enable the automotive industry to build auto bodies with a high durability in a plain manufacturing process. Today good welding results can only be achieved by expensive constructive procedures such as clamping devices to ensure a defined gad. The welding in zero gap configuration is a big challenge because of the vaporised zinc expelled from the interface between the two sheets. To find appropriate welding parameters for influencing the keyhole and melt pool dynamics, a three dimensional simulation and a high speed imaging system for laser keyhole welding have been developed. The obtained results help to understand the process of the melt pool perturbation caused by vaporised zinc. [Copyright &y& Elsevier]

[Display omitted] • CFD-FEM coupled simulation method is adapted for laser keyhole welding of AH36 steel. • CFD simulation is necessary for acquiring temperature history and molten pool shape. • Atmospheric pressure conditions can be compared using modified recoil pressure and laser power attenuation models. • Using the CCT diagram, the hardness and residual stress can be predicted in laser welding. • The lower pressure generates a deeper penetration depth and a slower cooling rate. A computational fluid dynamics (CFD) simulation of the molten pool in laser keyhole welding was utilized to acquire temperature data for further metallurgical and mechanical calculations. For the CFD simulation, the governing equations were solved, and the scattering and absorption of the laser beam in the plume were modeled at both the standard atmospheric condition (101,325 Pa) and a vacuum condition (3,000 Pa). A stochastic ray-tracing algorithm was adopted to effectively implement the transmission and scattering of laser bundles of rays. The temperature data from the CFD simulation were then imported to a finite element method (FEM)-based heat conduction analysis to simulate the thermal-metallurgical-mechanical behavior during the cooling phase of the weldment. The strain, residual stress, and distortion were calculated using an elastoplastic model based on the phase transformation-dependent material properties. An element deactivation scheme was used to take care of the zero-strength condition of the elements in the molten pool and keyhole region. The Vickers hardness and the residual stress were measured to verify the simulation model, and the experimental and simulation results had a similar tendency. [ABSTRACT FROM AUTHOR]

• Laser welding of copper is difficult because of its high reflectivity. • Surface reflectance of copper can be lowered using femtosecond laser patterning. • Conductive heat loss of copper causes the keyhole to weaken and eventually collapse. • Regularly placing low-reflectance patterns can maintain the keyhole over weld length. To improve laser beam coupling to the copper surface for the formation of a keyhole, a method of changing the surface reflectance of copper using femtosecond laser patterning is proposed in this study. For 2 kW fiber laser welding of copper, a 2 mm × 5 mm pattern was fabricated on the copper surface at the welding start point, taking a maximum of 27 s to produce a suitable pattern. The surface reflectance of copper was subsequently reduced from 97% to as low as 3% (for a 1 μm wavelength) by femtosecond laser patterning based on laser-induced periodic surface structures and their variants. Moreover, when a low reflectance pattern was employed, it was possible to increase welding speed by up to 300%, and once the melt pool was fully established, its size remained relatively stable for the length of the weld line. However, owing to melt pool fluctuations and the large conductive heat loss of copper, the depth and width of the fusion zone decreased and the laser coupling to the copper surface was lost in several cases. This problem was overcome by placing the patterns at regular intervals along the welding direction, indicating that this method has long weld-length applications. [ABSTRACT FROM AUTHOR]

• Metal evaporation flux across Knudsen layer is discussed in laser keyhole welding; • A high-gas pressure state is indicated at the keyhole bottom; • Evaporation flux decreases with the keyhole depth due to the high-gas pressure; • The decrease of evaporation flux suppresses the element loss in the fusion zone. This study aims to calculate the metal evaporation flux across the Knudsen layer inside the keyhole in laser welding of Al–Mg alloys. The pressure balance at the keyhole wall was analysed and used as the boundary condition in the calculation. Based on the extracted keyhole shape parameters from high-speed images, the surface temperature and gas pressure inside the keyhole were calculated, and the evaporation fluxes for both Al and Mg were acquired. The results show that the front wall of the keyhole absorbs more laser energy than the rear wall. The gas pressure inside the keyhole increases and the evaporation flux decreases with increasing keyhole depth. In addition, the evaporation flux of Mg is much higher than that of the base element Al, which can result in the loss of Mg inside the melt pool. The distribution of the Mg evaporation flux inside the keyhole agrees with the measured Mg loss in the welded section of the Al–Mg alloy. Thus, the calculation method of the metal evaporation flux inside the keyhole can be applied in future simulations of element loss in laser keyhole welding. [ABSTRACT FROM AUTHOR]