Full Penetration Laser Beam Welding Research Articles

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.

High-Speed X-Ray Investigation of Pore Formation during Full Penetration Laser Beam Welding of AA6016 Aluminum Sheets Contaminated with Lubricants

The presence of lubricants on the surface of sheets favors the formation of pores in laser welded seams. This formation process was investigated by means of high-speed X-ray imaging of the full penetration laser beam welding process of two AA6016 aluminum sheets in overlap configuration. The measurement of the growth velocity of the bubbles indicated their sudden growth once they started to form. Further analysis of the X-ray images identified the point of origin of the pores: a few millimeters behind the capillary between the two aluminum sheets. The study shows that the lubricant does not affect the stability of the capillary, which evidences that the formation of these pores is not caused by the fluctuations of the capillary. These results explain for the first time why pore formation cannot be avoided by process strategies, which stabilize the capillary, when welding uncleaned sheets.

Effect of Local Gas Flow in Full Penetration Laser Beam Welding with High Welding Speeds

Spatter formation is a major issue in deep penetration welding with solid-state lasers at high welding speeds above 8 m/min. In order to limit spatter formation, the use of local gas flows represents a technically feasible solution. By using the gas flow, the pressure balance inside the keyhole, and therefore the keyhole stability, is affected. Existing investigations demonstrate a reduction in spatter and pore formation for partial penetration welding up to a welding speed of 5 m/min. However, the effect of the gas flow is not yet clarified for full penetration welding at welding speeds above 8 m/min. By using a precisely adjustable shielding gas supply, the effect of a local gas flow of argon was characterized by welding stainless steel AISI304 (1.4301/X5CrNi18-10). The influence of the gas flow on the melt pool dynamics and spatter formation was recorded by means of high-speed videography and subsequently analyzed by image processing. Schlieren videography was used to visualize the forming flow flied. By the use of the gas, a change in melt pool dynamics and gas flow conditions was observed, correlating to a reduction in loss of mass up to 70%. Based on the investigations, a model of the acting effect mechanism was given.

On the search for the origin of the bulge effect in high power laser beam welding

The shape of the weld pool in laser beam welding plays a major role in understanding the dynamics of the melt and its solidification behavior. The aim of the present work was its experimental and numerical investigation. To visualize the geometry of the melt pool in the longitudinal section, a butt joint configuration of 15 mm thick structural steel and transparent quartz glass was used. The weld pool shape was recorded by means of a high-speed video camera and two thermal imaging cameras, a mid-wavelength infrared camera and a newly developed infrared camera working in the spectral range of 500 to 540 nm, making it perfectly suited for temperature measurements of molten materials. The observations show that the dimensions of the weld pool vary depending on the depth. The regions close to the surface form a teardrop-shaped weld pool. A bulge region and its temporal evolution were observed approximately in the middle of the depth of the weld pool. Additionally, a transient numerical simulation was performed until reaching a steady state to obtain the weld pool shape and to understand the formation mechanism of the observed bulging phenomena. A fixed keyhole with an experimentally obtained shape was used to represent the full-penetration laser beam welding process. The model considers the local temperature field, the effects of phase transition, thermocapillary convection, natural convection, and temperature-dependent material properties up to evaporation temperature. It was found that the Marangoni convection and the movement of the laser heat source are the dominant factors for the formation of the bulge region. A good correlation between the numerically calculated and the experimentally observed weld bead shapes and the time-temperature curves on the upper and bottom surface was found.

Thermoelectric currents and thermoelectric-magnetic effects in full-penetration laser beam welding of aluminum alloy with magnetic field support

Thermoelectric currents (TECs) caused by Seebeck effect were usually neglected in welding simulation. This paper characterizes the TECs and thermoelectric-magnetic phenomena during the magnetically supported laser beam welding (MSLBW) of thick aluminum (Al) alloy. The simulation is based on a computational fluid dynamic (CFD) model developed in three dimensions. The model considers multi-physical mechanisms including thermal transfer, solid–liquid (S/L) transition, molten metal convection, magnetohydrodynamics (MHD) and Seebeck effect. The computed TEC distribution in laser welding pool highly depends on welding time and temperature gradient. The large current densities occur near the weld pool edge and the vectors are opposite at both side of S/L interface. The maximum TEC density obtained in the stabilized weld pool is 2.14 × 106 A/m2, leading to a self-induced magnetic field of 2.27 × 10−3 T and a Lorentz force of 2.62 × 103 N/m3. The influences of TECs on weld pool dynamics are quite limited in LBW but notable in MSLBW. The inversion of MF direction leads to different Lorentz force distributions, weld pool dimensions and seam profiles. The simulation data agrees well with the experimental results.

Melt flow and thermal transfer during magnetically supported laser beam welding of thick aluminum alloy plates

A multi-field coupled numerical model was proposed to calculate the weld pool dynamics during the full-penetration laser beam welding of 12mm thick aluminum alloy plates under the function of a longitudinal magnetic field. The model dealt with the melt flow, heat transfer, solid-liquid transition and magnetic diffusion phenomena in welding process. The Lorentz force directing from the keyhole to the liquid-solid interface was induced in magnetic field supported cases. The Marangoni convection near the weld pool surfaces was obviously weakened due to the electromagnetic braking effect, which was featured with decreased velocity and compressed vortex. The heat transfer condition was largely changed, resulting in declined weld pool dimensions. When the magnetic flux density applied was larger than 0.5T, the convective heat exchange at the middle height of weld pool became notable again due to the reversed melt flow direction. The liquid-solid lines with lower curvature were achieved. The reinforcement of Hartmann effect became limited. The model and the simulated weld pool morphologies were verified in experiment.

Influence of steady magnetic field on dynamic behavior mechanism in full penetration laser beam welding

To study influence from external static magnetic field on dynamic behavior interaction between laser beam and EH36 steel, four cases with different magnetic strength (0mT, 15mT, 25mT and 30mT) are designed to capture high-speed image series of full penetration laser welding through quartz glass. It can be observed that: (1) Part of laser welding energy is compacted in bottom of weld pool to increase penetration and bottom width of bead geometry; (2) Eruption cycle and intensity (height 2.125mm, width 1.75mm) of vapor plume are both weakened; (3) Inhomogeneity and fluctuation of energy distribution are prominently dropped off; (4) Cooling period is delayed to reduce thermal gradient, depression area of the final profile is also reduced. Meanwhile, a polynomial model is developed to reveal relationship between thermal density and distance along the weld center. Therefore, external steady magnetic field has influence on dynamic behavior mechanism (weld pool, vapor plume, energy homogeneity, cool down stage) and even the final morphology, while 15mT is an optimal value of the magnetic strength in these four cases. It can be concluded that selecting an appropriate magnetic field strength is helpful to stabilize laser process and improve welding quality.

Finite element modeling of an alternating current electromagnetic weld pool support in full penetration laser beam welding of thick duplex stainless steel plates

An electromagnetic weld pool support system for 20 mm thick duplex stainless steel AISI 2205 was investigated numerically and compared to experiments. In our former publications, it was shown how an alternating current (AC) magnetic field below the process zone directed perpendicular to the welding direction can induce vertically directed Lorentz forces. These can counteract the gravitational forces and allow for a suppression of material drop-out for austenitic stainless steels and aluminum alloys. In this investigation, we additionally adopted a steady-state complex magnetic permeability model for the consideration of the magnetic hysteresis behavior due to the ferritic characteristics of the material. The model was calibrated against the Jiles–Atherton model. The material model was also successfully tested against an experimental configuration before welding with a 30 mm diameter cylinder of austenitic stainless steel surrounded by duplex stainless steel. Thereby, the effects of the Curie temperature on the magnetic characteristics in the vicinity of the later welding zone were simulated. The welding process was modeled with a three-dimensional turbulent steady-state model including heat transfer and fluid dynamics as well as the electromagnetic field equations. Main physical effects, the thermo-capillary (Marangoni) convection at the weld pool boundaries, the natural convection due to gravity as well as latent heat of solid–liquid phase transitions at the phase boundaries were accounted for in the model. The feedback of the electromagnetic forces on the weld pool was described in terms of the electromagnetic-induced pressure. The finite element software COMSOL Multiphysics 4.2 was used in this investigation. It is shown that the gravity drop-out associated with the welding of 20 mm thick duplex stainless steel plates due to the hydrostatic pressure can be prevented by the application of AC magnetic fields between around 70 and 90 mT. The corresponding oscillation frequencies were between 1 and 10 kHz and the electromagnetic AC powers were between 1 and 2.3 kW. In the experiments, values of the electromagnetic AC power between 1.6 and 2.4 kW at oscillation frequencies between 1.2 and 2.5 kHz were found to be optimal to avoid melt sagging or drop-out of melt in single pass full-penetration laser beam welding of 15 and 20 mm thick AISI 2205.

Full penetration laser beam welding of thick duplex steel plates with electromagnetic weld pool support

Full penetration high power bead-on-plate laser beam welding tests of up to 20 mm thick 2205 duplex steel plates were performed in PA position. A contactless inductive electromagnetic (EM) weld pool support system was used to prevent gravity drop-out of the melt. Welding experiments with 15 mm thick plates were carried out using IPG fiber laser YLR 20000 and Yb:YAG thin disk laser TruDisk 16002. The laser power needed to achieve a full penetration was found to be 10.9 and 8.56 kW for welding velocity of 1.0 and 0.5 m min−1, respectively. Reference welds without weld pool support demonstrate excessive root sag. The optimal value of the alternating current (AC) power needed to completely compensate the sagging on the root side was found to be ≈1.6 kW for both values of the welding velocity. The same EM weld pool support system was used in welding tests with 20 mm thick plates. The laser beam power (TRUMPF Yb:YAG thin disk laser TruDisk 16002) needed to reach a full penetration for 0.5 m min−1 was found to be 13.9 kW. Full penetration welding without EM weld pool support is not possible—the surface tension cannot stop the gravity drop-out of the melt. The AC power needed to completely compensate the gravity was found to be 2 kW.

Numerical simulation of full-penetration laser beam welding of thick aluminium plates with inductive support

A three-dimensional laminar steady-state numerical model was developed to investigate the influence of an alternating current (ac) magnetic field during high-power full-penetration laser welding on the weld pool dynamics and weld cross section of a 20 mm thick aluminium plate in flat position. Three-dimensional heat transfer, fluid dynamics including phase transition and electromagnetic field partial differential equations were solved iteratively with the commercial finite element software COMSOL Multiphysics using temperature-dependent material properties up to evaporation temperature. Thermocapillary convection at the weld pool surfaces, natural convection and latent heat of solid–liquid phase transition were taken into account in this model. Solidification was modelled by the Carman–Kozeny equation for porous media morphology. The ac magnet was mounted on the root side of the weld specimen. The magnetic field was aligned perpendicular to the welding direction. The flow pattern in the melt and thus also the temperature distribution were significantly changed by the application of oscillating magnetic fields. It was shown that the application of an ac magnetic field to laser beam welding allows for a prevention of the gravity drop-out. The simulation results are in good qualitative agreement with the experimental observations.

Effect of Laser Modification Welding on Formation of Aluminum Alloy

Effects of laser modification welding (LMW) on formation of full-penetration weld were studied, which were based on the laser beam welding (LBW) process of 5A02 aluminum alloy. The results indicated that the welding defects (such as undercut, surface depression and so on) could be improved obviously, and the microstructure of joint by LMW could be finer than that of joint only by LBW. Besides, the strength and toughness of joint would also be increased to some extent by LMW after full-penetration LBW. Therefore, the LMW is an effective method in order to strengthen joint of LBW.

Numerical and Experimental Analysis of Residual Stresses in Full-Penetration Laser Beam Welding of Ti6Al4V Alloy

A 3D thermal elastic-plastic finite element model to predict welding residual stresses induced by full-penetration laser welding of Ti6Al4V alloy was described in detail. The welding experiments and residual stresses measurements by through-thickness hole-drilling method were also performed to validate the simulated results. A uniform conical heat source model with parameters taken from the actual weld seam dimensions was developed to simulate the welding temperature fields with different welding heat inputs. The thermal elastic-plastic finite element simulation was employed to calculate the welding residual stresses. The boundary conditions and the size of finite element mesh were also discussed. The results show that the cross section profiles of the weld seam simulated with the conical heat source based on the configuration of weld seam agree well with the experimental results; the zone of residual stresses distribution in laser full-penetration welding of Ti6Al4V alloy is very narrow and the gradient of longitudinal residual stress is very steep; the residual stresses distribution on the surfaces are different from those in the interior of the welding seam; the measured residual stresses by through-thickness hole-drilling method are similar to the simulated ones in the interior of the welding seam.

A study on the metal flow in full penetration laser beam welding for titanium alloy

A mathematical model for flow simulation of full penetration laser beam welding of titanium alloy is presented. In this model, the heat source comprises a plane heat source on the top surface and a cylindrical heat source along the z-direction, which takes into account the plasma effect and the keyhole absorption. By solving the conservation equations of energy, momentum and mass, the temperature and flow fields are obtained. The momentum interpolation scheme with under-relaxation parameter is used to simplify the calculation algorithm and save the storage space of computer. The mushy region is introduced to provide a simple method to dispose of the pressure and velocity boundary conditions. Results calculated from the models are found to agree with the experimental results for the geometry profile of weld. The calculated results indicate the metal flow is the main reason for forming the typical “hourglass” cross-section profile.