Penetration Laser Welding Research Articles

OCT monitoring data processing method of laser deep penetration welding based on HDBSCAN

To address the challenge of low accuracy in the measurement of laser deep penetration welding depth, this study introduces optical coherence tomography (OCT) technology for the direct measurement of this parameter, which is designed to enable real-time measurement and the accurate acquisition of depth information. This study has developed a laser welding depth inspection system that integrates Spectral-Domain OCT (SD-OCT), utilizing 304 stainless steel as the test material for inspecting laser deep penetration welding. A comparative analysis evaluated the impact of Hierarchical Density-Based Spatial Clustering of Applications with Noise (HDBSCAN), Density-Based Spatial Clustering of Applications with Noise (DBSCAN), and K-means clustering (K-mean) on the clustering of OCT raw data, in addition to the use of percentile filters combined with the exponential moving average method for extracting the depth-of-melt curve. The depth-of-melt curve, obtained via the HDBSCAN algorithm, more accurately reflects the actual depth curve, with an average error remaining below 5%. This error rate is 46% lower than that of DBSCAN and 42% lower than that of K-mean. The results of the experiments with continuously varying process parameters show that a high degree of accuracy can be demonstrated even when the welding process parameters are continuously varied. Experimental results indicate that the combination of the HDBSCAN algorithm with percentile filtering and the exponential moving average method significantly enhances the recognition accuracy of the deep penetration laser welding depth curve. This method provides more precise feature parameters for the real-time detection and identification of defects in deep penetration laser welding, thereby offering strong support for optimizing and controlling the laser welding process.

The investigation on the dynamic solidification behavior of the hybrid molten pool in layered bimetallic laser penetration welding

The dynamic solidification behavior of the hybrid molten pool in layered bimetallic DSS2205/X65 laser penetration welding has been investigated using high-speed photography and grayscale processing techniques. The results demonstrate that this welding process creates layered joints. Alongside the DSS2205 and X65 layers’ interconnection, an intermediate transition layer emerges proximate to their interface, exhibiting limited solute exchange. Understanding the dynamic solidification in the hybrid molten pool is crucial. Two Marangoni eddies form in the DSS2205 and X65 layers at the posterior end of the keyhole during welding. Despite encountering less external gas convection, the central transition zone of the hybrid pool undergoes rapid cooling, prompting preferential solidification. The grains then grow towards the sides due to the Marangoni eddies, resulting in a dumbbell morphology by wider ends and a narrower middle region. Thermodynamic calculations and microstructure observations reveal fine equiaxed grains in the transition zone influenced by the greater cooling rate and element distribution, while larger grains in the base layer, and columnar grains in the cladding layer. This confirms preferential solidification in the middle region. The study uncovers the intrinsic mechanism behind this phenomenon, offering valuable insights for controlling the microstructure, optimizing parameters, and enhancing mechanical properties in layered bimetallic composite joints using laser penetration welding.

A data-driven time-sequence feature-based composite network of time-distributed CNN-LSTM for detecting pore defects in laser penetration welding

A data-driven time-sequence feature-based composite network of time-distributed CNN-LSTM for detecting pore defects in laser penetration welding

Predicting weld pool metrics in laser welding of aluminum alloys using data-driven surrogate modeling: A FEA-DoE-GPRN hybrid approach

Multi-physics computational models based on finite element analysis, offer detailed insights into the dynamics and metrics in the weld pool formed by laser welding. Conversely, data-driven surrogate models provide a cost-effective means to predict desired responses. These models establish statistical or mathematical correlations with input–output data, eliminating the need for additional simulations during design optimization. This study proposes a data-driven surrogate model, employing the Gaussian process regression network (GPRN), to predict weld pool metrics, such as weld width and depth of penetration in laser welding of aluminum alloy. A 3D computational fluid dynamics-based numerical model is initially constructed and experimentally validated to predict weld pool metrics. Subsequent experimental runs, guided by the design of experiments, include various configurations of process parameter settings. The developed numerical model computes weld pool metrics for each experimental run, forming a dataset for training and testing the GPRN model. The GPRN model is evaluated against simulated data, showing adequacy with a mean square error of 1.7 µm and mean absolute percentage error of 10−7, with experimental validation further confirming its accuracy, revealing a minimum error of 1.7%, a maximum error of 8%, and an average error of 3%. The key contribution and novelty of this study lie in the development of the hybrid data-driven model, which accurately predicts weld pool metrics while minimizing experimental and computational efforts.

Investigation on the formation mechanism of collapse defects, microstructural evolution and mechanical properties in full penetration laser welding of thick-section steel

In this paper, we described full penetration laser welding (FPLW), a process capable of achieving single-pass welding of thick-section steel. This work summarized a welding process window encompassing various typical formations, including those well-formed and collapse defects. Furthermore, collapse defects were categorized as “collapse formation with backside humping” and “collapse formation with underfill”. High-speed camera (HSC) images reveal that the formation mechanisms of these two defects are attributed to the accumulation of liquid metal at the rear edge of the bottom molten pool and the ejection of a large amount of liquid metal from the weld in a spatter form, respectively. A computational fluid dynamics (CFD) numerical simulation model well-aligned with the experimental results has been established. The analysis of the molten pool flow behavior and keyhole evolution revealed that during well-formed welding, the keyhole status in the welding process exhibits a periodic closure-collapse mode. This periodic mode can instantaneously release the pressure at the bottom of the keyhole, inhibiting the excessive accumulation of liquid metal at the bottom of the weld and thus inhibiting the formation of collapse defects. The cooling rate in the upper part of the weld exceeds that in the middle and lower parts, primarily resulting in the formation of fine acicular ferrite and martensite. The increased tensile strength exhibited by the welded joint relative to the base material predominantly emanates from the mechanisms of grain boundary reinforcement and dislocation strengthening, precisely caused by the refined microstructure.

Monitoring laser weld penetration status from the optical signal

Spectrometers have demonstrated their value in laser welding by facilitating the comprehension of welding dynamics and the identification of defects. However, the complex interaction between the laser beam and the material being welded makes it difficult for spectrometers to accurately capture the depth and extent of weld penetration, predominantly because plasma formation during welding interferes. This study presents an innovative approach that integrates laser technology, spectrometers, and advanced data analysis methods to classify and characterize various penetration types in pulse laser welding procedures, with notable computational efficiency. The research entailed the execution of an experiment on a boron steel plate, wherein peak power (1000-1200 kW), pulse duration (2-4 ms), and pulse repetition rate (25-50 Hz) were systematically varied to achieve diverse penetration conditions. Two categories of joints were identified based on their depth of penetration through careful analysis of the collected data. The investigation demonstrated a positive correlation between the depth of weld penetration and the increment of laser energy, with peak power ranging from 1000 kW to 1200 kW. Consequently, an elevation in light intensity was observed related to deeper weld penetration. The information is essential for understanding the relationship between laser energy and weld penetration, highlighting the importance of controlling laser parameters to achieve desired welding results. The spectrums were analyzed using Principal Component Analysis (PCA) to distinguish between different welding conditions. Overlap was observed between data from different weld conditions due to limitations imposed by the restricted dataset. Expanding the sample size can rectify this limitation and improve the accuracy and dependability of analytical outcomes. This study’s results provide valuable insights into optimizing welding parameters and improving understanding of the welding process, specifically in Tailor Weld Blanks. The findings offer potential for improving welding quality and strengthening lightweight components in high-performance industries like aerospace and automotive engineering.

Inhomogeneous microstructure distribution and its formation mechanism in deep penetration laser welding of medium-thick aluminum-lithium alloy plates

Inhomogeneous microstructure distribution and its formation mechanism in deep penetration laser welding of medium-thick aluminum-lithium alloy plates

Frontal pyrometric snapshot measurements of the keyhole wall temperature in laser welding of pure aluminium

In deep penetration laser welding, the quality of the weld seam is strongly correlated with the process dynamics. Pronounced dynamics of the melt pool and the keyhole cause defects like pores and blow-outs which negatively affect the mechanical properties of the seam. The process dynamics are primarily governed by the fluctuations of the keyhole shape. Therefore, in order to avoid the occurrence of defects in the weld seam, it is necessary to gain control over the keyhole stability by understanding the underlying physical processes that contribute to its dynamics. Besides the intensity distribution of the laser beam on the keyhole walls, the keyhole wall temperature is a crucial influencing factor when it comes to the assessment of the local metal evaporation rate and the induced local recoil pressure acting on the keyhole wall. Using small high-melting tantalum probes as measuring channels, temperatures in the vicinity of the Knudsen layer at the keyhole front wall were measured at different depths below the sample surface by means of high-speed pyrometry. For this purpose, bead on plate welding was conducted at low process velocities in the Rosenthal regime using pure aluminium (EN AW-1050A) as substrate material. It is shown for different laser powers and process velocities that the melt temperature in the vicinity of the keyhole wall lies in the range of 3000 K ± 400 K, therefore intermittently exceeding the vaporisation temperature of the substrate material. Significant correlations between the keyhole wall temperature and the laser power as well as the measuring depth could not be identified.

Effects of solidification on flow dynamics: A novel comprehension of defects formation in laser penetration welding

The laser penetration welding for medium-thickness plates struggles with unstable molten pool behavior and a limited processing window, often leading to welding defects including spatters, depressions, and especially humps. In this study, forming processes of the defects were observed by using a high-speed camera, and forming mechanism of the defects were then analyzed in detail by simultaneously considering the influences of force and solidification. It was clear that the mass loss caused by spattering during the heating phase contributed to the depression, which was consistent with the existing knowledge. However, the formation mechanism of humps concluded in this study was different to the present understanding. Apart from the influences of force, the effects of solidification on molten pools and humps were analyzed. Under the specific conditions, middle part of molten pool solidified rapidly, causing localized necking, and disrupting the circular flow of the molten metal. As a result, molten metal once flowed downward due to the gravity and temperature can no longer refill to the upper part again, leading to the accumulation of molten metal at lower part and ultimately the formation of humps. These findings offered a fresh perspective on the formation of hump, provided valuable guidance for enhancing the quality and efficiency of thick-plate laser welding.

Integrated modeling of cracking during deep penetration laser welding of nickel alloys

During deep penetration laser welding of nickel alloys, interactions between composition and processing conditions can lead to the formation of defects. Inconel 740H, for example, has demonstrated a susceptibility to horizontal fusion zone cracking at locations between 70% and 80% of the weld depth during laser welding at powers above 5 kW. Coupling three-dimensional heat transfer, fluid flow, and stress modeling tools allowed that both the strain rate and stress normal to the solidification direction to be calculated. In the Inconel 740H welds, cracks formed at locations where the strain rate and stress simultaneously reached critical levels. No cracking was observed in the Inconel 690 welds, since the strain rate and stress did not simultaneously reach these critical levels.

Laser Welding of Fiber and Quartz Glass Ferrule.

Optical fiber sensors fabricated by bonding have several limitations. To address these limitations, a CO2 laser welding process for an optical fiber and quartz glass ferrule is proposed in this study. A deep penetration welding method with optimal penetration (penetrating the base material only) is presented to weld a workpiece according to the requirements of the optical fiber light transmission, size characteristics of the optical fiber, and the keyhole effect of the deep penetration laser welding. Moreover, the influence of laser action time on the keyhole penetration is studied. Finally, laser welding is performed with a frequency of 24 kHz, power of 60 W, and duty cycle of 80% for 0.9 s. Subsequently, the optical fiber is subjected to out-of-focus annealing (0.83 mm, 20% duty cycle). The results show that deep penetration welding produces a perfect welding spot and has good quality; the hole generated from deep penetration welding has a smooth surface; the fiber can bear a maximum tensile force of 1.766 N. The performance of the optical fiber sensor is stable, and the maximum pressure deviation corresponding to the cavity length fluctuation is about 7.2 Pa. Additionally, the linear correlation coefficient R of the sensor is 0.99998.

Comparative Study on the Behavior of Keyhole in Analogy Welding and Real Deep Penetration Laser Welding.

In deep penetration laser welding, the behavior of the keyhole has an important influence on the welding quality. As it is difficult to directly observe the keyhole and detect the pressure inside the keyhole during metal laser welding, theoretical analysis and numerical simulation methods are commonly used methods in studying keyhole behavior. However, these methods cannot provide direct real information on keyhole behavior. In this paper, a method of analogy welding is proposed, in which high speed gas is used to blow the liquid to generate the keyhole. Relevant process experiments were conducted to explore keyhole behavior in analogy welding and real deep penetration laser welding. The pressure balance of the keyhole, both in analogy welding and real deep penetration laser welding, were analyzed. The laws obtained in analogy welding and real deep penetration laser welding are similar, which indicates that studying keyhole formation and the maintenance principle using the analogy welding method proposed in this paper may be helpful for deep understanding of the keyhole formation and maintenance mechanisms in real deep penetration laser welding.

Predicting laser penetration welding states of high-speed railway Al butt-lap joint based on EEMD-SVM

The Al butt-lap joints are widely applied in China Railway High-speed (CRH) trains body joints. However, the gaps caused by the thermal deformation in Al butt-lap joints lead to the lack penetration or over penetration. To predict the penetration state of laser welds in Al butt-lap joints with different gaps, this work investigated the correlation between the plasma plume morphology and penetration state monitored by high-speed imaging system and the vertical gap and horizontal gap were utilized as intermediary. The ensemble experience mode decomposition (EEMD) was used to obtain the frequency features of plasma plume morphology. A support vector machine (SVM) model combined with EEMD was then established to classify the different penetration state, which adopted the original signals at time-domain and the mode decomposition signals at frequency-domain as input. The EEMD-SVM accuracy of 97.98% was the highest among the EMD-SVM of 86.44% and the SVM of 58.15%. The EEMD-SVM obtained both high Accuracy and Recall when the quantity of positive and negative samples were utmost different. The classification of the EEMD-SVM model was the most superior among the EMD-SVM model and SVM model. This proposed method provided a novel and accurate approach to perform process monitoring and penetration defects detection during laser welding of butt-lap joints.

Solid phase transformation effects on stress and strain in the thick plate EH40 welded butt joint

To accurately predict welding-induced residual stress and strain of high strength steel thick plate EH40 butt joint in single-pass full penetration laser welding, a complete solid phase transformation model according to the actual weld microstructure was proposed through material constitutive development. At the same time, the Case 1 considering the traditional thermo-elastoplastic were used as comparative study. The simulation calculation results and experimental results of each phase content were compared and verified. The stress and strain distribution of welded joints and the evolution mechanism of nodes in three zone including fusion zone, heat-affected zone and base metal were deeply studied and compared. The results indicated that the solid phase transformation had an obvious effect on the peak and distribution of longitudinal, transverse, normal and equivalent residual stress, as well as the stress gradient at the junction of heat-affected zone and BM. It increased the peak value of longitudinal residual compressive stress from –103 MPa to –768 MPa, with an increase of 645.6%. The stress gradient at the junction of heat-affected zone and base metal was up to 1299 MPa. The state distribution of longitudinal and transverse stress in the weld and its vicinity were completely opposite to the Case 1. The peak value of transverse plastic compressive strain increased from –0.059 to –0.111, with an increase of 88.1%. The fusion zone and heat-affected zone of the plastic strain in welded joints were more easily observed. It was found that martensitic transformation was the main reason for the obvious difference of stress and strain distribution and the evolution mechanism of stress and strain in the later stage of cooling stage.

Study on root hump formation mechanism in full penetration laser welding with backing gas for aluminum alloy

The dynamic behavior of molten pool in full penetration laser welding (FPLW) with backing gas or without backing gas for aluminum alloy was observed. It is found that the backing gas can greatly improve the root hump formation. A transient and multi-phase 3D numerical model considering the metallic vapor ejection process was developed to explain this phenomenon, in which an evaporation and condensation model was established to simulate the production of metallic vapor. Based on the proposed model, the metallic vapor ejection, periodic keyhole variation profiles and molten pool evolution were presented, which indicated that the flow pattern of melt in the molten pool was deeply affected by the vapor ejection pattern. The ejection velocity of vapor is 45–70 m s −1 . The high-speed vapor makes the melt flow (MF) velocity within 1 mm of the keyhole wall change from 5.5 m s −1 to 0.2 m s −1 . The high velocity gradient near the keyhole is the main reason to form an invert cone-shaped sagging under the keyhole and some globular spatters near the keyhole outlet. The surface tension plays an important role to shorten the invert cone-shaped sagging and improves root hump formation. Alumina film formed on the back surface of molten pool can worsen the root hump formation due to the lack of surface tension. • An innovative transient and multi-phase 3D numerical model considered the metallic vapor ejection process was developed to simulate the root hump formation process in full penetration laser welding of aluminum alloy. • The flow pattern of melt in the molten pool evolution was presented in the full penetration laser welding with backing gas. • Proposing that the high-speed metallic vapor results in a significant velocity gradient near the keyhole wall, which is the main reason to form an invert cone-shaped sagging under the keyhole. • Proposing that the surface tension plays an important role to shorten the invert cone-shaped sagging and backing gas is helpful for improving the formation of root hump.

Simulation of the Effect of Keyhole Instability on Porosity during the Deep Penetration Laser Welding Process

The quality of a laser deep penetration welding joint is closely related to porosity. However, the keyhole stability seriously affects the formation of porosity during the laser welding process. In this paper, a three-dimensional laser welding model with gas/liquid interface evolution characteristics is constructed based on the hydrodynamic interaction between the keyhole and molten pool during the laser welding process. The established model is used to simulate the flow and heat transfer process of molten. The Volume of Fluid (VOF) method is used to study the formation and collapse of the keyhole and the formation of bubbles. It is found that bubbles are easy to form when the keyhole depth abruptly changes. There are three main forms of bubbles formed by keyhole instability. The front wall of the keyhole collapses backward to form a bubble. The back wall of the keyhole inclines forward to form a bubble. The lower part of the keyhole produces a necking-down effect, and the lower part of the keyhole is isolated separately to form a bubble. In addition, when the keyhole does not penetrate the base metal, the stability of the keyhole is high and the percentage of porosity is low.

Solidification cracking of a nickel alloy during high-power keyhole mode laser welding

Nickel alloy Inconel 740H, a candidate material for use in ultra-supercritical power plants, is susceptible to solidification cracking during high power deep penetration laser welding. Here we examine how cracking is affected by welding variables and determine the locations where the cracks occur experimentally and theoretically. We use a solidification cracking model to calculate the effects of welding variables on cracking and the locations where the cracks form during high power laser keyhole mode welding of IN 740H. The parameters needed for the cracking model are obtained from a well-tested numerical heat transfer and fluid flow model for keyhole-mode welding. Model predictions of cracking and their locations for different welding conditions are verified by experiments. • Solidification cracking of nickel alloys during laser welding is poorly understood. • We propose a novel methodology to understand why, when, and where cracking occurs. • A heat and fluid flow and solidification model, and experimental data are used. • Model predictions of cracking locations and conditions are verified by experiments. • The work can be used to prevent weld failures in thick section laser keyhole welds.

Research on welding-induced inhomogeneous stress in thick plate EH40 butt joint considering phase transformation

In this work, the inhomogeneous residual stress distribution and evolution of thick plate in single-pass full penetration laser welding are systematically investigated using phase transformation model. The material constitutive model considering solid-state phase transformation according to the actual weld microstructure is proposed to predict the inhomogeneous stress distribution of the welded joint. The inhomogeneous stress distributions along the longitudinal, transverse and thickness direction surfaces of the welded joint are systematically analyzed and deeply discussed, respectively. The results show that there is an obvious inhomogeneous distribution of the temperature field between the start-welding, middle section and end-welding position of the weld. It is obviously observed that the top and bottom surfaces of welded joint produce remarkably high stress areas and high stress gradients. The maximum longitudinal and transverse stress gradient values in the top surface are up to 1240.1 MPa and 1175.7 MPa, respectively. It is found that martensitic transformation is the main reason for the inhomogeneous stress distribution including high stress areas and high stress gradients in thick plate welded joint. The current research is beneficial to reduce the detrimental effects of welding-induced inhomogeneous residual stress on thick plate welded joints.

Stress and distortion of the 10 mm thick plate EH40 and 316L different butt joints in 10 kW laser welding

Residual stress has a negative influence on the service life of different welded joints between high-strength steel and stainless steel, especially in corrosive environment, it is very easy to cause fracture failure of welded joints. In this study, three types of medium-thick plate butt joints in single-pass full penetration laser welding, including EH40 & 316L, EH40 & EH40 and 316L & 316L butt joints, were deeply studied and verified by thermo-elastoplastic finite element method and experiments. The peak exponentially increasing double cone heat source model was used to calculate the transient temperature field. The relevant data of longitudinal, transverse residual stresses and angular distortion in three cases were extracted for comparative analysis. At the same time, the influence of the same material and different materials on the evolution mechanism of nodes stress in the weld was also discussed. It had been found that the geometric profile average error between the simulation and experiment in three cases was less than 0.5%. The stress state changes of the weld and its vicinity nodes in EH40 & 316L butt joint were faster than EH40 & EH40 and 316L & 316L butt joints. Compared with the welded joints of the same material, the welded joints of EH40 and 316L in the weld and its vicinity were remarkable differences in transverse residual stress, equivalent residual stress and angular distortion. The transverse residual stress gradient value of the EH40 side in EH40 & 316L was almost twice that of EH40 & EH40. The mismatch material properties of EH40 & 316L aggravated the generation of transverse tensile residual stress on the EH40 side. The results obtained in the current study were beneficial to the assessment of the integrity in the butt joints and the control of potential risks.

Direct observation of keyhole with an innovative layer-by-layer imaging method during deep penetration laser welding

In order to solve the difficult problem of observing the keyhole in deep penetration laser welding of metal materials, an innovative keyhole direct observation method based on layer-by-layer imaging technology is first proposed in this paper. In this novel method, a specially-designed composite workpiece consisting of an upper metal wedge and a lower glass trapezoidal prism is used. During deep penetration laser welding, a keyhole will form in and penetrate the metal wedge, whose cross section image that appears on the inclined bottom surface of the wedge is directly observed by a high speed video camera through the transparent glass prism. Then, the deep penetration laser welding is carried out with a fiber laser unit, by using the above proposed keyhole direct observation method, the keyhole cross section images at different depths are captured by the high speed camera. Finally, the images of the keyhole cross sections are processed and the outlines of the keyhole cross sections are extracted, from which the variation of the keyhole cross section during deep penetration laser welding of metal material is discussed. The result shows that from the top to the bottom, on one hand, the size of the keyhole cross section decreases gradually in general, but fluctuates to some extent, which indicates that the laser beam energy may not be uniformly absorbed into the keyhole along the depth and the evaporation site may change its position depending on the energy equilibrium there, on the other hand, the position of the keyhole cross section moves backward in the opposite direction of the welding velocity, whose motion tend agrees with the bending keyhole in the real case of laser welding.