Full Penetration Laser Welding Research Articles

The change of the absorptance at the transition from partial- to full-penetration laser welding

AbstractFull-penetration laser welding processes are necessarily associated with significant changes of the geometrical properties of the keyhole at the beginning of the process when the keyhole expands all the way through the workpiece and finally pierces the bottom of the sheet. The impact that this transition has on the absorptance was investigated by means of X-ray imaging to determine the geometry of the keyhole and subsequent raytracing to calculate the distribution of the absorbed irradiance. The results show a significant drop of the overall absorptance when the bottom of the capillary opens through the rear side of the workpiece which in practice is noticed by an unstable behavior of the keyhole. Since the drop of the absorptance is less pronounced for smaller diameters of the keyhole, one may recommend the application of laser beams with small diameters at least during the initial phase until the keyhole is fully developed and reliably reaches through the bottom surface of the welded sheet.

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.

Study on full-penetration laser welding of aluminum alloy under electromagnetic field support and subatmospheric pressure

Study on full-penetration laser welding of aluminum alloy under electromagnetic field support and subatmospheric pressure

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.

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.

Investigation of Weld Root Defects in High-Power Full-Penetration Laser Welding of High-Strength Steel

The currently available high-power laser shows promising opportunities for the welding of thick plates in a single pass. However, weld-root defect frequently occurs when a high-power laser is used to join thick plates in a full-penetration mode, which has a significantly adverse effect on the serviceability of the weld joint. The purpose of this work is to understand the defect formation mechanism and reduce these defects through controlling welding parameters. In this study, the characteristics of weld root defects were investigated using a 10 kW fiber laser using a program of experiment and theoretical analysis. The corresponding defect formation mechanisms were discussed based on the bottom molten pool behaviors observed by the high-speed camera. The results showed that there were four types of weld-root appearances as follows with an increase of linear heat input from 300 J/mm to 1000 J/mm: weld-root humping (30 mm/s), sound weld (25 mm/s), weld sagging (20 mm/s) and excessive weld sagging. The remedies for reducing weld-root defects were also presented to obtain sound weld bead by optimizing welding parameters. Weld-root humping was formed due to the quasi-full-penetration keyhole. Weld sagging resulted from the imbalance of the hydrostatic pressure and surface tension in the condition of a through keyhole. It was also found that the sound weld was formed when a through keyhole and a proper molten pool size were obtained. Thus, the state of the keyhole and molten pool geometry were the major factors that affect weld-root defects.

Dynamic behavior of molten pool backside during full-penetration laser welding of Ni-based superalloys

Dynamic behavior of molten pool backside during full-penetration laser welding of Ni-based superalloys

Effect of welding parameters on spatter formation in full-penetration laser welding of titanium alloys

The spatter formation process was observed by high-speed imaging during the full-penetration laser welding (FPLW) process of 4 mm-thick TC4 alloy plates. The effects of process parameters on spatter formation were studied. The results showed that the process parameters have obvious effects on the fluid flow characteristics in the melt pool and the dynamic behaviors of metal vapor in the keyhole, which and then affects the spatter behaviors. When using low-power and low-speed parameters, the fluid flow in the melt pool is unstable. When the melt flow impacts the keyhole, the keyhole opening diameter shrinks instantly, resulting in an immediate increase in the metal vapor eruption velocity at the keyhole opening and leading to the formation of weld spatter with a small size and high flying speed. When using high-power and high-speed parameters, a large amount of metal vapor erupts at high velocity from the keyhole opening transiently, resulting in the formation of a higher liquid column on the keyhole wall. The liquid column fractures to form weld spatter with a large size and low flying speed. When the pressure of the metal vapor in the keyhole is too large, the surface tension of the liquid metal under the keyhole cannot withstand it, causing an explosion on the backside of the melt pool. During the explosion, a large amount of spatter and plasma is ejected from the keyhole exit, and the plasma plume above the melt pool is weakened.

Synchrotron X-ray Analysis of the Influence of the Magnesium Content on the Absorptance during Full-Penetration Laser Welding of Aluminum

Full-penetration laser beam welding is characterized by a weld seam whose depth equals the material thickness. It is associated with a stable capillary and is therefore widely used for welding of sheet metal components. The realization of lightweight concepts in car body production requires the application of high-strength aluminum alloys that contain magnesium as an alloying element, which significantly influences the evaporation temperature and pressure. This change of the evaporation processes influences the geometry of the capillary and therefore its absorptance. In order to quantify the influence of magnesium on the capillary, their geometries were captured by means of high-speed synchrotron X-ray imaging during the welding process of the aluminum alloys AA1050A (Al99.5), AA5754 (AlMg3) and AA6016 (AlSi1.2Mg0.4). The 3D-geometries of the capillaries were reconstructed from the intensity distribution in the recorded X-ray images and their absorptance of the incident laser beam was determined by the analysis of the reconstructed 3D-geometry with a raytracing algorithm. The results presented in this paper capture for the first time the influence of the magnesium content in high-strength aluminum alloys on the aspect ratio of the capillary, which explains the reduced absorptance in case of full-penetration laser beam welding of aluminum alloys with a high content of volatile elements. In order to improve the absorptance in full-penetration welding, these findings provide the information required for the deduction of new optimization approaches.

Determination of heat source model for simulating full penetration laser welding of 316 LN stainless steel by computational fluid dynamics

The selection of appropriate heat source model is vital in predicting the temperature distribution and visualizing the dynamic fluid flow behaviour during Laser keyhole welding of Type 316 LN stainless steel. Three-dimensional (3D) numerical models were designed utilizing ANSYS Fluent by employing different heat source models. Different heat source models considered include volumetric heat sources namely Goldak double ellipsoidal, 3D conical, a coalition of 3D conical and cylindrical heat source, 3D volumetric Gaussian and Rotary Gaussian. The simulation was carried out by solving the fundamental conservation equations of mass, momentum and energy. The predicted weld bead profile by different heat sources were compared with the actual weld bead obtained from the laser welding experiment on 0.0055 m thick 316 LN stainless steel (SS) plates. It was found that simulation results of the conical-cylindrical combined (CCC) model exhibited good agreement with actual weld bead dimensions and thermal data obtained during the experiment. Therefore, the conical-cylindrical heat source model is recommended for CFD simulation of full penetration laser welding of 316 LN stainless steel for studying the effect of variation of laser power and surface active element on the weld pool development.

Mechanism study of thermal fluid flow and weld root hump suppression in full penetration laser welding of Al alloy with alternating magnetic field support

In this work, the influence mechanism of alternating magnetic field on the thermal fluid flow and weld root hump suppression in full penetration laser welding of aluminum alloy was studied. A novel three dimensional transient numerical model was developed, in which a multiple reflection laser beam model, a varied metallic vapor force model and an electromagnetic model including Lorentz force and Joule heat were considered. The experimental results indicated that when the effective magnetic flux density was 57 mT and its alternating frequency was 400 Hz, a sound weld bead for 10 mm plate could be obtained. The numerical analysis showed that the periodically varied electromagnetic force exerted an upward support to the weld pool, making its geometry and flow pattern stable. The keyhole collapse at upper and middle region was enhanced and the higher laser absorption was attributed to the decreased occurrence frequency of through-keyhole. The weld root sagging formation was instantaneous, while its suppression was a persistent process. The effect of weld root hump suppression depended on the dynamic competition among the electromagnetic force and the forces that promote the weld root sagging generation in the welding process.

Weld bead characteristics for full-penetration laser welding of aluminum alloy under electromagnetic field support

An oscillating electromagnetic field weld pool support system combined with a welding sample travel mechanism was designed for improving the weld bead formation in full-penetration laser welding of A5083 aluminum alloy. The experimental results show that a proper oscillating electromagnetic field can effectively reduce the weld root sagging. When the root mean square of magnetic induction intensity on the back surface Brms0 = 80 m T and its electromagnetic frequency f = 400 Hz, the weld root sagging for the joint of aluminum alloy plate with thickness of 10 mm can be almost eliminated. The electromagnetic frequency and the laser power have a great impact on the weld root sagging height and the weld back surface roughness, while the influence of magnetic induction intensity is relatively small within its variable range in this study. The analysis of welding joint microstructure shows that the electromagnetic field can also refine the grains to some extent and decrease the width of columnar grain zone, decreasing the cracking susceptibility and improving the mechanical properties of the welding joint. However, the undercut defects are inevitable in full-penetration laser welding of aluminum alloy unless filler wire is used, because the metallic evaporation and splash are unavoidable during the high-power laser welding process.

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

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

Heat transfer and fluid flow and their effects on the solidification microstructure in full-penetration laser welding of aluminum sheet

Understanding the behaviors of heat transfer and fluid flow in weld pool and their effects on the solidification microstructure are significant for performance improvement of laser welds. This paper develops a three-dimensional numerical model to understand the multi-physical processes such as heat transfer, melt convection and solidification behavior in full-penetration laser welding of thin 5083 aluminum sheet. Solidification parameters including temperature gradient G and solidification rate R, and their combined forms are evaluated to interpret solidification microstructure. The predicted weld dimensions and the microstructure morphology and scale agree well with experiments. Results indicate that heat conduction is the dominant mechanism of heat transfer in weld pool, and melt convection plays a critical role in microstructure scale. The mushy zone shape/size and solidification parameters can be modulated by changing process parameters. Dendritic structures form because of the low G/R value. The scale of dendritic structures can be reduced by increasing GR via decreasing heat input. The columnar to equiaxed transition is predicted quantitatively via the process related G3/R. These findings illustrate how heat transfer and fluid flow affect the solidification parameters and hence the microstructure, and show how to improve microstructure by optimizing the process.

Study of molten pool dynamics and porosity formation mechanism in full penetration fiber laser welding of Al-alloy

To study molten pool dynamics, root hump formation, periodic keyhole behavior in full penetration laser welding (FPLW) of aluminum alloy, a numerical simulation was carried out, in which volume of fluid (VOF) method and ray-tracing algorithm were adopted. A varied metallic vapor shear stress model was considered. Meanwhile a series of welding experiments on specimens being composed of aluminum alloy and quartz glass were conducted to analyze the porosity formation process. It was found that the over-heated sagging with low surface tension stagnated at the bottom side and solidified to form root hump. The dominant backward heat convection at the lower part can well explain the extreme spreading of molten pool at bottom surface. Periodic behavior of the keyhole was identified during quasi-steady stage. The porosity can be effectively suppressed in FPLW. Both the porosity ratio and average porosity number were reduced obviously. The main mechanism of porosity formation in FPLW is the collapse of the rear keyhole wall, mainly caused by bulges on the front keyhole wall. Bubble coalescence is responsible for large porosity size and coalescence efficiency depends on bubble size difference.

A numerical model for full and partial penetration hybrid laser welding of thick-section steels

Full and partial penetration welding using high power lasers exhibit different melt pool geometries. The main difference is that full penetration welds tend to widen at the root side. In recent years, full penetration laser welding has been modeled using sophisticated multiphase numerical models. However, less computationally-complex thermo-mechanical models that are capable of accounting for the root widening phenomenon in full penetration laser welding are lacking. In this study, hybrid laser welding was performed for full and partial penetration modes on butt joints of structural steel. A numerical model was presented that used a three-dimensional transient Finite Element (FE) analysis and thermal conduction heat transfer for calculating the temperature fields in both full and partial penetration welding modes. For this purpose, a double-conical volumetric heat source was developed based on the three-dimensional conical (TDC) heat source in the literature. The model was validated and calibrated with different experiments. The results show that the model is capable of calculating the transient temperatures for the common melt pool geometries obtainable by the full and partial penetration hybrid laser welding of thick-section steels. The model can potentially be employed as the basis for predicting e.g. microstructural properties or residual stresses for a given welding procedure.

Comparison of processing window in full penetration laser welding of thick high-strength steel under atmosphere and sub-atmosphere

In order to show the advantages of laser welding under vacuum or sub-atmosphere in fabrication of thick section structures, fiber laser welding of thick high-strength steel was performed under atmosphere and sub-atmosphere, respectively. The effect of ambient pressure on penetration depth and cross section was investigated, and the processing window in full penetration welding of 10 mm thick high-strength steel under atmosphere and sub-atmosphere was compared. The results showed that the processing window of full penetration laser welding under atmosphere (101 kPa) was very narrow because of the appearances of various weld defects. Full penetration sound welds could only be achieved in a small processing window near the parameter of 10 kW laser power and 1.5 m/min welding speed. It was found that there was a sharp increase in penetration depth and a significant change of cross section from “nail-head” shape to parallel shape with a decrease in ambient pressure. Full penetration welds joint under sub-atmosphere (0.1 kPa) displayed high weld quality with extreme smooth surface appearances. The processing window under sub-atmosphere with laser power ranging from 4 kW to 6 kW and welding speed ranging from 0.5 m/min to 1.5 m/min was much wider than that under atmosphere.

Root defects in full penetration laser welding of thick plates using steady electromagnetic force

This article proposes an approach to stabilize the process of full-penetration laser welding of 16 mm 316L stainless steel by introducing steady electromagnetic force. Transient formation of root defects was captured by high-speed camera (CCD) and using the “sandwich” method. The stable and sustained welding process benefited from electromagnetic force, which weakened hydrostatic pressure at the bottom surface and altered the liquidity of the metal pool. Moreover, the electromagnetic force compensated for the insufficient surface tension and successfully prevented root defects such as sagging and drop-out.