Deep Penetration Welding Process Research Articles

Inline detection of process anomalies during laser deep penetration welding of hidden T-joints

Laser deep penetration welding processes are sensitive towards contaminations inside of the process zone. Automized production facilities report the risk of residual packaging material within the process zone to result in seam defects. These can impair the mechanical properties of the seam and raise the demand for post-process quality control. In-line process monitoring is a promising tool to detect process anomalies correlated to such defects and potentially reduce the demand for time-consuming post-process control. This study investigates the influence of polystyrene particles, a common packaging material, enclosed in the gap of hidden T-joints onto the process dynamics. For process monitoring optical coherence tomography and airborne sound measurement were employed. The provoked process anomalies resulted in increased porosities and significant signal changes of the OCT and airborne sound measurements. This proves the in-line detection capability of process anomalies which occur due to polystyrene particles in joint gaps of hidden T-joints.

Influence of superimposed intensity distributions on weld seam quality and spatter behavior during laser beam welding of copper with green laser radiation

With the increasing demand for copper connections in the field of renewable energies, e.g., for electric vehicle applications, various approaches were pursued to reduce the challenging spatter and melt ejection susceptibility in laser beam welding of copper materials. One is the use of adjustable intensity profiles of multi-mode beam sources with a combination of a core and ring beam in order to affect a modification of the flow field in the melt pool surrounding the highly dynamic keyhole in the deep penetration welding process. This should favor a reduction of spattering and melt pool ejections, especially at low weld speeds and high penetration depths in pure copper, therefore enabling a more stable deep penetration welding process. In this work, the influence of different superimposed intensity distribution ratios for green laser radiation with summarized power up to 3 kW on the generation of process imperfections was investigated conducting welding experiments on Cu-ETP using high-speed imaging for enhanced process understanding. In addition, the effects of different power distribution conditions and welding speeds on the seam dimensions were analyzed. It was found that a significant amount of laser power in the ring beam leads to a widening of the melt pool in the area near the sample top-surface, which effectively reduces spatter behavior. The associated change in process zone morphology in laser beam direction was furthermore observed via sandwich analysis, allowing a detailed view into the laser–matter interaction area through a borosilicate glass sheet clamped in front of a processed sample. This setup was found to be a cost-effective method for obtaining further information about the keyhole formation mechanism and melt dynamics under comparative boundary conditions.

Inline Weld Depth Evaluation and Control Based on OCT Keyhole Depth Measurement and Fuzzy Control

In an industrial joining process, exemplified by deep penetration laser beam welding, ensuring a high quality of welds requires a great effort. The quality cannot be fully established by testing, but can only be produced. The fundamental requirements for a high weld seam quality in laser beam welding are therefore already laid in the process, which makes the use of control systems essential in fully automated production. With the aid of process monitoring systems that can supply data inline to a production process, the foundation is laid for the efficient and cycle-time-neutral control of welding processes. In particular, if novel, direct measurement methods, such as Optical Coherence Tomography, are used for the acquisition of direct geometric quantities, e.g., the weld penetration depth, a significant control potential can be exploited. In this work, an inline weld depth control system based on an OCT keyhole depth measurement is presented. The system is capable of automatically executing an inline control of the deep penetration welding process based only on a specified target weld depth. The performance of the control system was demonstrated on various aluminum alloys and for different penetration depths. In addition, the ability of the control to respond to unforeseen external disturbances was tested. Within the scope of this work, it was thus possible to provide an outlook on future developments in the field of laser welding technology, which could develop in the direction of an intuitive manufacturing process. This objective should be accomplished through the use of intelligent algorithms and innovative measurement technology—following the example of laser beam cutting, where the processing systems themselves have been provided with the ability to select suitable process parameters for several years now.

OCT Capillary Depth Measurement in Copper Micro Welding Using Green Lasers

The transition of the powertrain from combustion to electric systems increases the demand for reliable copper connections. For such applications, laser welding has become a key technology. Due to the complexity of laser welding, especially at micro welding with small weld seam dimensions and short process times, reliable in-line process monitoring has proven to be difficult. By using a green laser with a wavelength of λ=515 nm, the welding process of copper benefits from an increased absorption, resulting in a shallow and stable deep penetration welding process. This opens up new possibilities for the process monitoring. In this contribution, the monitoring of the capillary depth in micro copper welding, with welding depth of up to 1 mm, was performed coaxially using an optical coherence tomography (OCT) system. By comparing the measured capillary depth and the actual welding depth, a good correlation between two measured values could be shown independently of the investigated process parameters and stability. Measuring the capillary depth allows a direct determination of the present aspect ratio in the welding process. For deep penetration welding, aspect ratios as low as 0.35 could be shown. By using an additional scanning system to superimpose the welding motion with a spacial oscillating of the OCT beam perpendicular to the welding motion, multiple information about the process could be determined. Using this method, several process information can be measured simultaneously and is shown for the weld seam width exemplarily.

Deep-Penetration Welding as a Method for Saving Materials and Increasing the Load-Carrying Capacity of Welded Structures

The study presents possible savings resulting from the use of deep-penetration welding processes in the fabrication of welded structures. The study-related tests revealed 80% savings of filler metals and the 50% reduction of welding distortions without compromising the maximum load-carrying capacity of welded joints. The tests involved steel grades S355J2, S460NL, S700MC, S690QL and 450HBW (Hardox) having thicknesses restricted within the range of 8 mm to 20 mm as well as filler metal grades G4Si1 and G69.

Adaption of a topographical measurement system for beam welding processes with high temporal and spatial resolution

Both typical beam welding processes laser beam welding LBW and electron beam welding EBW feature the deep penetration welding process. Theses processes rely on a vapor capillary with a delicate, highly dynamic equilibrium of various opposing forces. The direct observation of the vapor capillary's inner surface is hindered with growing capillary depth by the thermal optical emissions with high intensities due to the high temperatures inside the capillary. Even with the use of high intensity laser based illumination and band pass filters adjusted for the laser wavelength, it is merely possible to observe only the upper most part of the capillaries opening. The lack of observability, especially with high temporal resolution to assess the highly dynamic effects contributes to the state of the capillary models that actually are describing quasi-static modes or implement the less dynamic effects that can be observed with state of the art methods.The topographical measurement system, developed by the Institut für Strahlwerkzeuge Universität Stuttgart IFSW, uses the process inherent thermal emission of the capillary governed by the complex refractive index of the base material to reconstruct the inner geometry of the visible capillary surface. This is achieved by using the dependence of the emissivity between emission angle and polarization that is described by the Fresnel equations (Sawannia et al., 2018). The measurement system uses the fact that the quotient of two or more polarization components of the emission from a surface increment is depending on the angle of surface inclination but are almost independent of the surface temperature (Weberpals et al., 2011). Based on the results of the application of the measurement principle with laser beam welding, an adaption to electron beam welding is developed to assess the observability of capillaries with large depth and to contribute in the improvement of capillary models in the wake of the development. To allow the use of the improved measurement system to capture the EBW process capillary with unmatched spatial and temporal resolution, a complete new optical observation beam path was developed and fitted insight a custom-made pressurized chamber inside an electron beam welding machine.

Microstructure and Mechanical Properties of Aluminum Alloy/High Strength Steel Double Beam Laser Deep Penetration Welded Joint

A new dual beam laser deep penetration welding technology for lap joint of 1.5 mm thick aluminum alloy and high strength steel was explored in this paper, and the effects of three different beam energy ratios (RS=0.25,0.33,0.5) on weld formation, interface microstructure and mechanical properties were studied. The result shows that under certain conditions of other parameters, double beam laser deep penetration welding process can be applied to lap joint of aluminum alloy / high strength steel with good weld shape when RS=0.25,0.33,0.5. As RS increases from 0.25 to 0.5, the penetration of the weld reduces from 575 μm to 424.2μm, the thickness of intermetallic compound (IMC) layer at the interface between aluminum alloy and weld metal reduces from 3.4 μm to 2.5 μm, the average microhardness of the IMC layer decreases from 771.1 HV to 571.9 HV, the mechanical resistance of the joint raises from 95.7N/mm to 115.2N/mm. When RS=0.5, double beam laser deep penetration welding of aluminum alloy / high-strength steel joints has the highest mechanical resistance of joints, because of the relatively good plastic ductility of the joint.

Deriving spectral information upon the laser welding process employing a hyperspectral imaging technique

In this paper we present results from process observations of deep penetration laser welding using a hyperspectral imaging (HSI) technique. For monitoring different types of laser-based material processing, we developed an appropriate high-speed camera-based HSI system. This system enables us to derive spectra of the deep penetration welding process with high time resolution. These spectra can be used for temperature determination or to extract spectral characteristics, both of which can help to further investigate process dynamics and properties of the vapor. The presented results contain particularly HSI-derived spectral information of the evolving vapor plume that is related to corresponding high-speed images and spectra acquired with a conventional VIS-spectrometer.

Formation mechanisms of pores and spatters during laser deep penetration welding

During laser deep penetration welding process, defects cannot be completely avoided yet. Due to the dynamic process, especially process pores and spatters occur. Keyhole dynamics are assumed to be responsible for the initiation of both pores and spatters. However, it is not completely clear yet how spatters and pores are formed. A semianalytical model of a keyhole is used to simulate dynamic keyhole properties of laser beam welding of aluminum. These are related to characteristics of spatters and pores occurring during laser deep penetration welding recorded using high-speed-imaging technique and x-ray analysis, respectively. Correlations of keyhole shapes and pore formation show that the volume underneath a keyhole collapse can be sufficient to capture the gas that is necessary for the formation of a pore and a keyhole expansion is not necessarily needed to produce the pores found in the weld seam. Calculated keyhole wall fluctuations are found to be not sufficient to detach spatters from the keyhole wall without additional forces from the melt pool.

Numerical simulation of laser beam welding using an adapted intensity distribution

The automotive industry has a high demand for lightweight solutions for car body components to reduce the carbon dioxide emissions and to increase the range of electric cars. In this context, the joining methods play a significant role in enabling the lightweight construction. Specifically, the use of aluminum alloys for structural components or body panels poses a major challenge for joining technologies. These parts are often made from aluminum alloys AA6xxx, which are very susceptible to hot cracks during fusion welding. As laser beam welding is increasingly used for welding car body components, special techniques are required to avoid hot cracks in weld seams. Besides the use of filler wire, laser welding using an adapted intensity distribution is an innovative approach to get a defect-free weld seam coupled with a high surface quality. Due to the lack of flexible beam shaping optics for investigations on high power material processing using an adapted intensity distribution, a simulation method for this technique is presented. The impact of the adapted intensity on the process characteristics, e.g., the temperature field, the temperature gradients, or the molten pool geometry, can be determined by using this numerical model. The heat input by the adapted intensity distribution is composed of a stationary capillary geometry for the deep penetration welding process and an additional surface heat source. An experimental analysis was carried out to calibrate the simulation model. Using design of experiments, the weld seam geometry depending on the laser parameters can be predicted. Finally, the impact of an adapted intensity distribution on the geometry of the molten pool and the temperature field is shown.

Laser Deep Penetration Welding of an Aluminum Alloy with Simultaneously Applied Vibrations

In aluminum welding, the grain structure of produced seams is an essential factor with respect to the seam properties. In the casting technology the effect of mechanical vibrations on the grain growth during the solidification of liquid metals is known as a refinement method. In this paper, the transferability of this approach from comparatively long-time processes in the field of casting to the short-time process of laser deep penetration welding is investigated. Therefore, specimens were sinusoidal vibrated with frequencies up to 4 kHz during bead-on-plate full-penetration welding experiments. The resulting grain size was determined by applying the circular intercept procedure on the center of a cross-section micrograph of each weld. The results show that grain refinement is in general achievable by mechanical vibrations in the audible frequency range during laser full penetration keyhole welding of the aluminum alloy EN AW-5083.

Keyhole stability during laser welding—part I: modeling and evaluation

The keyhole is a requirement in order to establish the energy efficient process of laser deep penetration welding. However, the process is highly unstable which results in unwanted pore and spatter ...

Spatters during Laser Deep Penetration Welding with a Bifocal Optic

The energy efficient, high-speed laser deep penetration welding process is a technology which is increasingly used for industrial applications. In order to guarantee weld seams of high quality a stable process needs to be established. Especially when welding aluminium alloys the weld quality is reduced due to occurring spatters which entails a loss of material. Solidified spatters remain on the surface of the specimen after welding and need to be cleaned for further processing steps. One method to change the process behaviour is beam shaping. In this work, a bifocal optic is used to produce two foci along the beam axis in order to manipulate the energy input into the keyhole. Bead-on-plate welds are produced in aluminium alloy EN AW-6082 and mild steel S235. For comparison, welding is conducted using standard optics. The spatter occurrence is compared when using these different beam shapes. While a reduced number of spatters per time are observed the spatter size increases when using the bifocal optic in this study.

Development of a new combined heat source model for welding based on a polynomial curve fit of the experimental fusion line

To improve the accuracy of finite element (FE) models used to simulate electric arc and beam-based fusion welding processes, a combined heat source model is proposed. The combined model, termed the polynomial heat source, is based on a Gaussian heat density distribution and employs a disc source to account for surface heating effects while a polynomial equation is used to define the volumetric heat power source. This provides an improved capability for matching the heat source power distribution to fusion zones having variable curvature and also results in a simplified process for defining heat source parameters as only three numerical values need to be determined. To validate the heat source model, fusion zone cross sections, thermal cycles, angular distortion, and residual stresses obtained using SYSWELD were compared to measurements taken from gas metal arc (GMA) welds made on 10-mm-thick high-strength low-alloy (HSLA) plates. Predictions obtained from SYSWELD using double ellipsoid and conical heat source models were also compared. An analysis of the FE predictions obtained from each of the three heat source models showed that the best agreement between simulated and experimental values was achieved using the polynomial model. This is attributed to the fact that heat power distribution can be adjusted at the top and bottom of the fusion zone and results in an improved level of bead cross section matching. It is expected that the proposed heat source model will be applicable for simulating both shallow and deep penetration welding processes where the fusion zone is symmetric about its centerline.

Distinct morphology of keyhole wall during high power fibre laser deep penetration welding

The keyhole wall is the interaction interface between the laser and the material during the laser deep penetration welding. Measuring the morphology of the keyhole wall is thus of significance for understanding the high power fibre laser deep penetration welding process. In this paper, the clear keyhole wall was reserved by suddenly closing laser during high power fibre laser welding of copper alloy. The results indicate that the keyhole can be divided into laser action region and metallic vapour pressure maintenance region. The laser action region is on the front wall of keyhole, and a series of concentric elliptical rings are observed in this region. In another region, its diameter is significantly larger than the spot diameter and the keyhole wall is basically smooth. The results are different from those generally accepted viewpoints, which clarify the flowing law of molten fluid in the keyhole and are thus of great guiding significance for optimising the welding technology.

Effect of welding parameters and the heat input on weld bead profile of laser welded T-joint in structural steel

The high power fiber laser has become one of the most efficient energy sources for deep penetration welding processes used in heavy manufacturing and marine industries. Combinations of cost-efficient, easily automatable process together with fairly mobile and flexible welding equipment have raised high expectations for improved quality and economic feasibility. In this study, the fillet welding of a low alloyed structural steel was studied using a 10 kW fiber laser. Plates of 8 mm thick AH36 were welded as a T-joint configuration in flat (1F) and horizontal (2F) positions using either an autogenous laser welding or a hybrid laser arc welding process. The effect of heat input on the weld bead geometry was investigated using one variable at a time approach. The impact of single process parameter such as laser power of 4.5–6 kW, welding speed of 0.5–2.5 m/min, beam inclination angle of 6°–15°, focal point position of −2 to +2 mm, and welding positions of 1F and 2F were studied. All welds were visually evaluated for weld imperfections described in EN ISO 13919-1 standard. Penetration depth, geometries of the fusion and heat affected zones, and hardness profiles were measured. Produced joints have a high depth to width ratio and a small heat affected zone; full penetration welds with acceptable weld quality on both sides of the joint were produced. The parameter configurations for optimizing the welding processes are proposed.

Characteristic line emissions of the metal vapour during laser beam welding

This article provides an insight into the origin of the radiation emissions during laser beam welding of aluminium and steel alloys. It is the fundamental basis for the development of a system monitoring the welding process of challenging alloys with respect to their chemical composition in the melt pool. To reduce welding faults, e.g. cracks, brittle phases or other metallurgical based defects, a high degree of process knowledge is necessary. This manuscript presents novel spectroscopic investigations used to analyse laser beam welding setups to gain a better understanding of the emission formation. Therefore, also the spatial origin of the emitted radiation of the deep penetration welding process was examined. These empirical results are accompanied by theoretical considerations about the excitation degree of different alloying elements. Together they are a progressive step to develop a system to control the chemical composition of the melt pool during the active welding process. Knowing the composition of the melt allows to control e.g. a filler unit to change the melt composition. The empirical results were conducted by processing common alloys of today’s manufacturing applications.

Bifocal Hybrid Laser Welding -More than a Superposition of two Processes

The shortage of resources demands a consequent lightweight design with aluminium alloys. Process stability and a high seam quality in welding processes of this material are difficult to achieve. Bifocal hybrid laser welding (BHLW) demonstrates how this material can be welded reliably without defects. The used hybrid laser system consists of a Nd:YAG laser and a high power diode laser (HPDL). This leads to a high welding penetration in combination with a high seam quality. This publication demonstrates that the BHLW process is more than a coupled deep penetration welding (DPW) process with a heat conduction welding (HCW) process. The synergetic effect is a better quality in the entire welding seam.

Process Studies on Laser Welding of Copper with Brilliant Green and Infrared Lasers

Copper materials are classified as difficult to weld with state-of-the-art lasers. High thermal conductivity in combination with low absorption at room temperature require high intensities for reaching a deep penetration welding process. The low absorption also causes high sensitivity to variations in surface conditions. Green laser radiation shows a considerable higher absorption at room temperature. This reduces the threshold intensity for deep penetration welding significantly. The influence of the green wavelength on energy coupling during heat conduction welding and deep penetration welding as well as the influence on the weld shape has been investigated.

STUDY ON THE THERMAL LAG EFFECT OF KEYHOLE IN CONTROLLED PULSE KEY--HOLING PLASMA ARC WELDING

The behaviors of keyhole formation determine to a great degree the deep penetration welding process and the weld depth-width ratio in welding of medium and large thickness plates. Obtaining a deep insight into the dynamic keyholing process in the weld pool in plasma arc welding is of great significance for widening the process parameter-widow and improving the process robustness as well as weld quality stability. Based on the Level-Set theory, a numerical model is developed to describe the keyhole behaviors in the weld pool in stationary plasma arc welding (PAW), and employed to track the evolution process of keyhole boundary. In this paper, the fluid flow fields in both the plasma and the keyhole regions are constructed according to the experimental and simulation data in the previous literatures. The combined volumetric heat source model is used to numerically analyze the transient temperature field and then to determine the weld pool geometry. The algorithm of Level-Set theory combined with the transient thermal conduction model is used to determine the evolution of both keyhole and weld pool geometry at different time steps. The dynamic information on the weld pool and keyhole geometry and sizes under a few process conditions is obtained by the numerical simulation of keyholing phenomena in welding of stainless steel plates. It is found that a complete keyhole is established at 2.7 and 2.5 s for the current levels of 170 and 180 A, respectively. During the stationary PAW process, the cross-section geometry of keyhole transforms from U-shape at initial stage to V- shape later and shows finally a hyperbola shape after a complete keyhole is formed. The numerical analysis of keyhole formation is verified by measuring the efflux plasma voltage signals at the moment of keyholing.