Pulsed Laser Welding Process Research Articles

Optimization and GRA prediction of Al–Cu pulsed laser welding process based on RSM

In this paper, the process parameters of pulsed laser welding of Al–Cu are studied using response surface methodology (RSM) and grey relation analysis (GRA) to propose optimal directions for lap-shear strength of the joints. A single-factor experiment was conducted to find the suitable process parameter windows. The RSM models were established and analyzed with laser power, welding speed, pulse width, and frequency as inputs and joint lap shear, interfacial weld width, and weld-penetration-depth as outputs. With the use of GRA of interfacial weld width and weld penetration depth with the lap-shear force of joints, the improved direction for the lap-shear force of joints can be proposed.

The role of asymmetric metal flow on weld formation and solidification characteristics during pulsed laser butt welding with assembly tolerance

In comparison to other laser-assisted welding techniques, the pulsed laser welding process (PLBW) provides cost-effectiveness and ease of operation while effectively connecting butt-assembled thin plates with manufacturing tolerances (both air gap and edge misalignment). A combination of numerical simulations and experimental studies was conducted to analyze the forming mechanism and solidification behavior of the weld. In this paper, a transient three-dimensional heat-flow-metallurgical model has been established for a PLBW process of mild steel plates with a thickness of 1.8 mm. The predicted weld pool profiles were consistent with the experimental findings. The finding suggested the molten metal, driven by surface tension and recoil pressure, successively generated liquid bridges on the rear and front sides of the pool. The two sheets were joined as the molten metal gradually solidified. The energy absorptivity was highly correlated with the weld pool and keyhole dynamics, and the laser energy absorbed by the metal was mainly transferred in the form of thermal convection. The kinematic features of the melt were observed by the liquid phase mass transfer rate and surface fluctuations. According to the Fourier analysis results, the molten pool oscillated at the characteristic frequency of the laser pulse frequency. The spatial and temporal distributions of solidification parameters aided in analyzing the solidification features within the pool. The asymmetric flow had a greater influence on the temperature gradient. These findings addressed the roles of PLBW on fluid flow, heat behaviors, and solidification parameters of welds with assembly tolerances, as well as the reasons for improved weld formation.

Impact of microstructure evolution on the corrosion behaviour of the Ti–6Al–4V alloy welded joint using high-frequency pulse wave laser

In this work, a high-frequency pulsed laser-welding process was successfully implemented to fabricate a Ti–6Al–4V alloy butt joint. The microstructure of each zone of the welded joint was systematically characterized by carrying out scanning electron microscopy and electron backscatter diffraction measurements. In addition, the corrosion resistance of the welded joint was tested using traditional electrochemical and local electrochemical methods, while the main factors affecting corrosion resistance and the mechanism of the galvanic corrosion were also thoroughly investigated. From the acquired results, it was demonstrated that the α′ martensitic phases constituted the microstructure in the fusion zone (FZ), where the grain size, grain orientation, and misorientation distribution of the different zones varied, and the α′ martensitic and bulk α covered the heat-affected zone (HAZ). The corrosion resistance in the HAZ was slightly better than that in the base material, and both were worse than that of the FZ, which was related to the phase type. In addition, the corrosion current density values of FZupper were about 1/4.6 and 1/2.6 those of FZmiddle and FZdown, respectively, which was attributed to differences in the grain size and grain orientation. The FZ was protected as a cathode during galvanic corrosion, which is of great importance for ensuring the service life of the component during practical applications.

Laser Welding Penetration Monitoring Based on Time-Frequency Characterization of Acoustic Emission and CNN-LSTM Hybrid Network

In-process penetration monitoring of the pulsed laser welding process remains a great challenge for achieving uniform and reproducible products due to the highly complex nature of the keyhole dynamics within the intense laser-metal interactions. The main purpose of this study is to investigate the feasibility of acoustic emission (AE) measurement for penetration monitoring based on acoustic wave characteristics and deep learning. Firstly, a series of laser welding experiments on aluminum alloys were conducted using high-speed photography and AE techniques. This allowed us to in-situ visualize the complete keyhole dynamics and elucidate the generation mechanism of acoustic waves originating from pressure fluctuations at the keyhole wall. Then, an adaptive time-frequency technique namely VMD (Variational Mode Decomposition) was proposed to characterize the acoustic energy distribution among the nine subsignals with low-frequency and high-frequency components under different welding penetrations. Lastly, a novel hybrid model combing CNN (Convolutional Neural Network) and LSTM (Long Short Term Memory) was designed to deeply mine the spatial and temporal acoustic features from the extracted frequency components. Extensive experiments demonstrate that our proposed approach yields a remarkable classification performance with a test accuracy of 99.8% and a standard deviation of 0.21, which obtains a high recognition rate. This work is a new paradigm in the digitization and intelligence of the laser welding process and contributes to an alternative way of developing an efficient end-to-end penetration monitoring system.

A novel macroscopic computational methodology to predict the locations and orientation of solidification-cracks: Application to pulsed laser welding

The prevention of solidification cracks in critical engineering applications demands generalised predictive approaches. The first part of the manuscript reports the computation of transient temperature and flow fields using high-fidelity CFD simulations of the mixed -mode pulsed laser welding process, using iterative calculations of the volumetric heat source. The temperature field is verified by comparing the experimental and numerical weld profiles with different power-deposited-per-unit-length ϕ,which varied from 76J/mm to 177J/mm. The second part of this manuscript presents a novel and generalised macro-scale computational methodology which allows in situ numerical estimation of the Crack Vulnerable Index (CVI). Unlike the previous attempts, here, only the thermal characteristics, i.e., the temperature, melt flow velocity, and fluid fraction, are used to predict the locations and orientation of the cracks dynamically. The predictions agree well with the experimentally obtained findings of pulsed laser welding. The study shows that the heat-flow direction correlates closely with the crack-orientations obtained from experiments. It is experimentally observed that the cracking tendency is substantially reduced as the power-deposited-per-unit-length is increased. The bulk weld-pool solidification velocity is inversely proportional to the number-of-cracks (a measure of cracking tendency) observed in the post-solidified weldments. Further, for the case with the lower cracking tendency, the peak values of the temperature gradient and the normalized bulk weld-pool solidification velocity are ∼33% and 11% higher than the respective values for the case with the higher cracking tendency.

Optimizing the mechanical properties of weld joint in laser welding of GTD-111 superalloy and AISI 4340 steel

The present research pertains to the optimization of the mechanical properties of weld joint between GTD-111 superalloy and 4340 steel through Nd:YAG pulsed laser welding process. For this purpose, the response surface methodology in combination with desirability function technique were employed to optimize the laser welding parameters. The parameters of laser power (1500, 2000, 2500 W), welding speed (0.5, 1, 1.5 mm/s) and pulse frequency (5, 10, 15 Hz) were considered to improve the responses of tensile strength, elongation, hardness and weld penetration, simultaneously. The microstructure characteristics of the weld joint were examined using optical and scanning electron microscopy. The results indicated that an increase in the laser power up to 2500 W decreased the tensile strength (from 1017 to 705 MPa) and hardness (from 460 to 375 kgf) due to formation of the Laves phase and nucleation of solidification cracks, arising from segregation of Nb, Ti, Mo, W and Ta elements. The increase of welding speed decreased the amount of Laves particles due to lower heat input, which resulted in an improvement in the tensile strength and hardness up to 911 MPa and 417 kgf, respectively. However, an increase of welding speed reduced the elongation and weld penetration to 12.7 % and 1147 μm, respectively. Concurrent increase of the laser power and pulse frequency associated with the formation of shrinkage voids in HAZ of 4340 steel side and the nucleation of liquation cracks in HAZ of GTD-111 superalloy side. The optimal values of parameters were found to be a laser power of 1812 W, pulse frequency of 15 Hz and welding speed of 1.3 mm/s. Under the optimal conditions, the following values of responses have been obtained: tensile properties = 1013 MPa, elongation = 16.9 %, hardness = 416 kgf and weld penetration = 1165 μm.

Measurement of pulsed laser welding penetration based on keyhole dynamics and deep learning approach

The keyhole instability is a key concern in laser deep-penetration welding of high reflectivity materials, potentially impacting the penetration status and weld quality. Monitoring and control the keyhole behavior still remain a great challenge for obtaining a desired welded joint. For the pulsed laser welding of thin-sheet aluminum alloy, an active visual monitoring system was established to systematically probe the dynamic keyhole behavior from multi-view sensing. Combining with the image processing method and process analysis, the keyhole surface area and depth were extracted to quantify the keyhole formation dynamics under different welding conditions. Furthermore, a data-driven deep learning model with hyperparameter optimization was constructed to identify different penetration states and it has a high accuracy and good reliability. The experiment results show that our proposed measurement scheme based on multi-view monitoring and deep learning approach could guide the development of real-time control of the pulsed laser welding process.

A mesoscale approach to simulate residual deformations in complex laser welding processes

Laser welding can be characterized by very small radii of beam, in the order of tenths of a millimeter, and very short high power inputs (more kW in few ms), and thus, it can be certainly classified as a microscale process with a high level of physical complexity. This is clearly incompatible, due to the high computational costs, with the analysis of macroscale processes related to large geometries and non-uniform welding patterns. In order to overcome this issue, a simplified finite element method (FEM)–based thermo-elastoplastic model is presented to simulate heat transfer and residual deformations due to thermal expansion and material plasticity. The idea is to substitute the microscale analysis with a mesoscale approach that renounces to describe in detail all the physical phenomena occurring in the heated zone and focuses attention on the correct prediction of the keyhole depth and weld pool size, that are the most important parameters to describe the mechanical characteristics of the welded joint. The concept of passive element, based on the numerical adjustment of the material properties in order to take into account the orthotropic behavior during the keyhole formation, is introduced. In particular, the new approach has been tested on the pulsed laser welding process of two overlapping DC04 steel plates with thickness of 0.5 mm (so-called sandwich) and validated through experimental tests involving different input parameters, such as power, pulse duration and frequency, speed, and geometrical pattern.

Effect of Nd:YAG pulsed laser welding process on the liquation and strain-age cracking in GTD-111 superalloy

The weldability of GTD-111 nickel-based superalloy by Nd:YAG pulse laser with an average power of 250 W was studied using several pre- and post-weld heat treatment cycle, and the characteristics of liquation, solidification and strain-age cracks were also investigated. The results revealed that liquation cracks in the GTD-111 Nickel-Based Cast Superalloy were associated with the constitutional liquation of γ′ particle, MC carbides, inter-dendritic γ-γ׳ eutectic and melting of Cr-rich boride. Also, cracking during welding occurred at the places that the concentration of Al and Ti is high. Crack-free laser welds were observed in the MT1 conditions (1200 ℃ for 2 h) owing to dissolution of deleterious phases before welding. γ′ phase had little effect on the incidence of cracking in the MT1 condition. An analysis of the microstructures indicated that the cracking was caused primarily by liquation in the as-welded condition and was exacerbated by post-weld heat treatment cracking during the subsequent heat treatment. aging heat treatment of samples which were undergone aging heat treatment before welding, resulted in the formation of strain-age cracking due to γʹ and γ-γ׳ eutectic phases precipitation. The results of micro-hardness indicated that the hardness of fusion zone (FZ) was higher than that of HAZ and base metal zone (BMZ). The liquation cracking in the heat affected zone (HAZ) was observed to be affected by the hardness of base-alloy.

Welding deformation of ultra-thin 316 stainless steel plate using pulsed laser welding process

The ultra-thin plate (thickness <0.1 mm) is widely used as the raster plate in the high-precision collimator, but the welding induced distortion is the main problem restricting its application. In this study, the effect of laser power on the welding deformation of an ultra-thin steel plate during pulsed laser welding was studied by means of both numerical simulation and experiment. A three dimensional thermal-elastic-plastic FE analysis was conducted to predict the temperature field and distortion of the ultra-thin steel plate. The thermal and mechanical material properties used in this simulation all followed the non-linear relations as a function of temperatures. Meanwhile, a pulsed laser welding experiment was carried out, the temperature field was measured by a pyrometer, and the distortion was measured by a laser displacement sensor. The results showed that the model prediction was consistent with the experimental measurements results.

Optimizing pulsed Nd:YAG laser beam welding process parameters to attain maximum ultimate tensile strength for thin AISI316L sheet using response surface methodology and simulated annealing algorithm

Pulsed laser welding is a powerful technique especially suitable for joining thin sheet metals. In this study, based on experimental data, pulsed laser welding of thin AISI316L austenitic stainless steel sheet has been modeled and optimized. The experimental data required for modeling are gathered as per Central Composite Design matrix in Response Surface Methodology (RSM) with full replication of 31 runs. Ultimate Tensile Strength (UTS) is considered as the main quality measure in laser welding. Furthermore, the important process parameters including peak power, pulse duration, pulse frequency and welding speed are selected as input process parameters. The relation between input parameters and the output response is established via full quadratic response surface regression with confidence level of 95%. The adequacy of the regression model was verified using Analysis of Variance technique results. The main effects of each factor and the interactions effects with other factors were analyzed graphically in contour and surface plot. Next, to maximum joint UTS, the best combinations of parameters levels were specified using RSM. Moreover, the mathematical model is implanted into a Simulated Annealing (SA) optimization algorithm to determine the optimal values of process parameters. The results obtained by both SA and RSM optimization techniques are in good agreement. The optimal parameters settings for peak power of 1800 W, pulse duration of 4.5 ms, frequency of 4.2 Hz and welding speed of 0.5 mm/s would result in a welded joint with 96% of the base metal UTS. Computational results clearly demonstrate that the proposed modeling and optimization procedures perform quite well for pulsed laser welding process.

Spike laser welding for the electrical connection of cylindrical lithium-ion batteries

The transition toward renewable energies implicates decentralized and time-dependent ways of energy generation. To compensate for the resulting fluctuation in energy supply, local storage systems are necessary. Larger systems may consist of thousands of battery cells. Therefore, the reliable interconnection between the individual battery cells is a prerequisite for the economic production of these systems. Laser beam welding is a suitable process to contact batteries. Due to the high requirements regarding the heat input and the reproducibility of the joining process, thorough investigations are necessary. Experiments on pulsed laser beam welding of cylindrical lithium-ion cells were conducted by applying a strategy named spike welding. Suitable process parameters were identified and tested with regard to their impact on the reproducibility of the welded joint. For the measurement of the heat input during the welding process, the temperature rise inside the battery case was determined. Results indicate that a pulsed welding strategy with discrete weld seams is preferable in order to minimize the heat input and to maximize the flexibility during the welding process. Finally, battery cells were contacted and electrical properties of the cell compound were determined. The pulsed laser welding process proved to be superior compared to conventional resistance spot welding since no negative influence on the electrical properties could be observed and the heat input was kept at a minimum.

Optimization of pulsed laser welding process parameters in order to attain minimum underfill and undercut defects in thin 316L stainless steel foils

In this study, the optimization of pulsed Nd:YAG laser welding parameters was done on the lap-joint of a 316L stainless steel foil with the aim of reducing weld defects through response surface methodology. For this purpose, the effects of peak power, pulse-duration, and frequency were investigated. The most important weld defects seen in this method include underfill and undercut. By presenting a second-order polynomial, the above-mentioned statistical method was managed to be well employed to balance the welding parameters. The results showed that underfill increased with the increased power and reduced frequency, it first increased and then decreased with the increased pulse-duration; and the most important parameter affecting it was the power, whose effect was 65%. The undercut increased with the increased power, pulse-duration, and frequency; and the most important parameter affecting it was the power, whose effect was 64%. Finally, by superimposing different responses, improved conditions were presented to attain a weld with no defects.

Feasibility of using acoustic method in monitoring the penetration status during the Pulse Mode Laser Welding process

In this paper, the feasibility of using acoustic method to monitor the depth of penetration was investigated by determine the characteristic of the acquired sound throughout the pulse mode laser welding process. To achieve the aim, the sound signal was acquired during the pulsed laser welding process on the 2 mm structural carbon steel plate. During the experiment, the laser peak power and pulse width was set to be varied while welding speed was constantly at 2 mm/s. Result from the experiment revealed that the sound pressure level of the acquired sound was linearly related to the pulse energy as well as the depth of penetration for welding process using 2ms pulse width. However, as the pulse width increase, the sound pressure level show insignificant change with respect to the change in the depth of penetration when the pulse energy reaches certain values. The reported result shows that this was happen due to the occurrence of spatter which suppressed the information associated with the generation of plasma plume as the product of high pulse energy. In this work, it was demonstrated that in some condition, the acoustic method was found to be potentially suitable to be used as a medium to monitor the depth of weld on online basis. To increase the robustness of this method to be used in wider range of parameter, it was believed that some other post processing method is needed in order to extract the specific information associated with the depth of penetration from the acquired sound.

Laser welding of low friction nanostructured sintered composites: technical and environmental aspects

Low friction materials in nanostructured sintered composites are used for different pairs of moving parts of MEMS configuration. The paper aims to present some elements regarding the behaviour of such type of materials in pulsed laser welding process. The influences that are developed during the interaction between the laser beam and the nanostructured composite have been analysed. Major instabilities have been observed and that involved a very accurate choosing of the laser power. The particles have been measured and visualised from the emitted fume by using special extractors that analysed the emitted gases as well. The particles were put in suspension and visualised by using nanosight microscope and they were measured by using a nanosizer. The analysis showed that they were in nanometric domain that meaning higher impact on the human health comparing to the welding of the classic materials, according to the OSHA.