Keyhole Mode Laser Welding Research Articles

Review and Analysis of Modern Laser Beam Welding Processes.

Laser beam welding is the most modern and promising process for the automatic or robotized welding of structures of the highest Execution Class, EXC3-4, which are made of a variety of weldable structural materials, mainly steel, titanium, and nickel alloys, but also a limited range of aluminum, magnesium, and copper alloys, reactive materials, and even thermoplastics. This paper presents a systematic review and analysis of the author's research results, research articles, industrial catalogs, technical notes, etc., regarding laser beam welding (LBW) and laser hybrid welding (LHW) processes. Examples of industrial applications of the melt-in-mode and keyhole-mode laser welding techniques for low-alloy and high-alloy steel joints are analyzed. The influence of basic LBW and LHW parameters on the quality of welded joints proves that the laser beam power, welding speed, and Gas Metal Arc (GMA) welding current firmly decide the quality of welded joints. A brief review of the artificial intelligence (AI)-supported online quality-monitoring systems for LBW and LHW processes indicates the decisive influence on the quality control of welded joints.

Comparative Numerical Analysis of Keyhole Shape and Penetration Depth in Laser Spot Welding of Aluminum with Power Wave Modulation

Keyhole mode laser welding is a valuable technique for welding thick materials in industrial applications. However, its susceptibility to fluctuations and instabilities poses challenges, leading to defects that compromise weld quality. Observing the keyhole during laser welding is challenging due to bright process radiation, and existing observation methods are complex and expensive. This paper alternatively presents a novel numerical modeling approach for laser spot welding of aluminum through a modified mixture theory, a modified level-set (LS) method, and a thermal enthalpy porosity technique. The effects of laser parameters on keyhole penetration depth are investigated, with a focus on laser power, spot radius, frequency, and pulse wave modulation in pulsed wave (PW) versus continuous wave (CW) laser welding. PW laser welding involves the careful modulation of power waves, specifically adjusting the pulse width, pulse number, and pulse shapes. Results indicate a greater than 80 percent increase in the keyhole penetration depth with higher laser power, pulse width, and pulse number, as well as decreased spot radius. Keyhole instabilities are also more pronounced with higher pulse width/numbers and frequencies. Notably, the rectangular pulse shape demonstrates substantially deeper penetration compared to CW welding and other pulse shapes. This study enhances understanding of weld pool dynamics and provides insights into optimizing laser welding parameters to mitigate defects and improve weld quality.

Mitigating spatters in keyhole-mode laser welding by superimposing additional ring-shaped beam

Mitigating spatters in keyhole-mode laser welding by superimposing additional ring-shaped beam

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.

Effect of non-equilibrium solid state phase transformation on welding temperature field during keyhole mode laser welding of Ti6Al4V alloy

The effect of non-equilibrium solid state phase transformation on numerical simulation of welding temperature field is researched during keyhole mode laser welding of Ti6Al4V alloy. The simulation of the welding temperature field considering equilibrium phase transformation is obtained by means of the model of the heat transfer and fluid flow after the keyhole presetting. Based on the simulation considering equilibrium phase transformation, the simulation considering non-equilibrium phase transformation is modified, in terms of partitioning the regions and endowing the physical parameters considering non-equilibrium phase transformation. Compared with those considering equilibrium phase transformation in the horizontal planes, the temperature contours greater than or equal to 1300 K considering non-equilibrium phase transformation are elongated, while the temperature contours between 1100 K and 1300 K considering non-equilibrium phase transformation are shortened. The difference of the welding temperature fields considering equilibrium and non-equilibrium phase transformations basically emerges in the cooling stage. The temperature fields considering non-equilibrium phase transformation after modifying once and twice are almost the same, signifying that the simulation considering non-equilibrium phase transformation after modifying once is sufficient. The computational results considering non-equilibrium phase transformation agree well with the experimental ones in contrast with those considering equilibrium phase transformation, showing that the simulation considering non-equilibrium phase transformation after modifying once is reliable and satisfactory.

Penetration Depth Modeling and Process Parameter Maps for Laser Welds Using Machine Learning

Penetration control is an important factor in determining the weld quality in keyhole mode laser welding, which enables deep penetration. In this study, machine learning models and neural network models were developed by using 380 published welding data which were constructed for steel base metals under the following welding conditions: a laser power of 0.3-16.7 kW, a welding speed of 0.3-20.0 m/min, and a bead diameter of 0.05-0.78 mm. A machine learning model SVM (supported vector machine) could accurately predict the penetration depth with a coefficient of determination, R2 of 0.95. A shallow neural network model with five nodes in only one hidden layer was developed with a slightly improved accuracy with R2 of 0.98. It was confirmed that neither model was overfitted, and process parameters (welding speed and beam diameter) maps with penetration depth contours were provided for a laser power of 2-8 kW.

A New Heat Source Model for Keyhole Mode Laser Welding

Abstract Laser welding is a common technique for joining metals in many manufacturing industries. During welding, a weld gun traverses the interface of the parts to be joined causing them to melt, fuse, and solidify when the temperature decreases, thus joining the parts. Due to the heat input and the resulting melting and solidification, the parts deform causing residual distortion and residual stresses. To assure the geometrical and functional quality of the product, computational welding mechanics (CWM) is often employed in the design phase to predict the outcome of different design proposals. Furthermore, CWM can be used to design the welding process with the objective of assuring the quality of the weld. However, welding is a complex multiphysical process including the weld pool flow, microstructure dynamics, and structural mechanics. In a design process, it is typically not feasible, for example, to employ fluid simulation of the weld pool in order to predict deformation of a welded assembly, especially if a set of design proposals is under investigation. This is because of the high resolution needed for these fluid simulations in combination with challenges to couple fluid simulation with structural simulation. Instead, what is used is a heat source that emulates the heat input from the melt pool. An example of a heat source is the standard doubled ellipsoid. This heat source has been efficiently used for a large number of welding simulation. However, standard heat sources are typically not flexible enough to capture the fusion zone for deep keyhole mode laser welding. In this study, we presented a new heat source model for keyhole mode laser welding. In an industrial case study, a number of bead-on-plate welds have been employed to compare standard weld heat sources and develop the new heat source model. The proposed heat source is based on a combination of standard heat sources. From this study, it was concluded that the standard heat sources could not predict the observed melted zone for certain industrial application while the new heat source was able to do so. Therefore, the proposed heat source model can be employed to model keyhole mode laser welding, which enables welding simulation of a set of design proposals during the design process in a larger number of industrial cases.

Material ejection attempts during laser keyhole welding

Abstract Material loss during keyhole mode laser welding often leads to surface imperfections that can negatively impact component performance. High-speed imaging identified four types of material ejection attempts: classical spatter, re-captured spatter, protuberances, and scalloping. The momentum attributed to the melt body, which is influenced by the keyhole properties, dictates whether the ejection attempt is successful or if the material is re-captured. The relationship between the dynamics of the keyhole and melt pool was elaborated in an extended systematic description of melt ejection attempts, which were then classified. Ejection attempts were often observed to be proceeded by a co-current swelling of the melt, adjacent to the keyhole, followed by a melt depression. The melt swell would elongate into a melt column with a concentration of momentum, where excessive momentum causes the melt to neck and separate into spatter. Trajectory determines if the spatter becomes a permanent fixture or re-incorporates into the melt body, with the latter having the possibility to cause further melt body disturbances leading to more ejection attempts. If the melt column fails to neck and separate, or an additional force acts upon the column, a protuberance or a scallop could then form. Keyhole and melt pool fluctuations were sometimes observed to be accommodated, avoiding material ejection. In these cases, a stable weld could be obtained with large variations in the dimensions of the melt pool and the keyhole.

Enhanced Penetration Depth during Reduced Pressure Keyhole-Mode Laser Welding

Keyhole-mode laser welding under reduced ambient pressure is known to provide improved weld penetration, narrower width, and reduced incidences of defects, but the underlying mechanism for these benefits is not known. We sought to elucidate the mechanism by an experimental and theoretical program of investigation. Potential causative factors, such as the depression of the boiling point of al-loys at reduced pressures and the changes in laser beam attenuation by metal vapors/plasma, were investigated using a well-tested heat transfer and fluid flow model of keyhole-mode laser welding for various ambient pressures. The model was tested with experimental data for the weld-ing of four alloys — Structural Steel Q690, Aluminum Alloy A5083, commercially pure titanium, and Nickel 201 — that have very different thermophysical properties. The results showed the changes in the boiling point alone were unable to explain the enhanced depth of penetration at low ambi-ent pressures. The experimental and calculated fusion zone geometries showed excellent agreement when both the boiling point depression and the beam attenuation by metal vapor were considered. The reduction of ambient pressure also affected the heat transfer pattern near the keyhole, owing to a decrease in the keyhole wall temperature and changes in the temperature gradient near the keyhole wall.

Effect of Welding Speed on the Prediction Accuracy of Residual Stress in Laser Welded 1.2 mm Thick Dual Phase Steel

Post-weld residual stress and distortion are significant issues in laser welding as they reduce fatigue strength and toughness. In the present work, cold rolled dual phase steel sheets of 1.2 mm thickness were laser welded in both bead on plate and butt configurations. The optimum heat input was observed to be approximately 25 kJ/m for visually defect free weld with narrow weld bead and through-thickness penetration. A novel attempt was made to increase the welding speed whilst maintaining the optimum heat input. It was observed that, high welding speed led to high residual stress, resulting in longitudinal cracks, whereas the speed at a limiting value of 0.08 m/s was found to be beneficial for welding without longitudinal cracks. Literature suggests researchers have already proposed different mathematical models on keyhole mode laser welding that can predict the residual stress and deformation. However, most of them failed to predict it repetitively, especially for thin sheets. This work tried to find out the rationality of this argument. For low-speed condition, the range of residual stress values was substantially wide, making it difficult to predict actual residual stress with the aid of mathematical models. The range of residual stress values was found to be much lower for the optimum heat input and high-speed condition, resulting into better prediction accuracy. Residual stress values were also validated using kernel average misorientation (KAM) study.

A three-dimensional model of coupling dynamics of keyhole and weld pool during electron beam welding

In electron beam welding (EBW), the coupling dynamics between the keyhole and the weld pool play significant roles in joint quality, yet the mechanisms of these dynamics have not been well understood, since the rise of EBW in the 1960s. The present study reported a novel three-dimensional mathematical model of EBW and used it to theoretically understand the coupling behaviors between the keyhole and the weld pool during EBW of a Ti-6-Al-4V alloy. A new phenomenological heat source model, dependent on the dynamic keyhole profile and temperature, was developed. The physical effects, including the beam reflection and absorption of the keyhole wall, scattering of secondary electrons, keyhole evaporation heat loss and beam defocus, were rigorously treated in the heat source model. The transient free surface keyhole dynamics were tracked with a high-resolution level set method (LSM). On the free surface boundary, the majority of the key physical effects, such as the recoil pressure, Marangoni shear stress, the surface tension and hydrodynamic pressure of the molten liquid, were incorporated using a recently developed sharp interface model. The coupling behaviors of the three-dimensional keyhole evolutions, heat transfer, and fluid flow in the weld pool asa function of the process parameters in the EBW of Ti-6-Al-4-V alloys were simulated, of which the results reasonably agreed with independent experimental data. The dynamical keyhole violently exhibited fluctuations within the sub-millisecond characteristic time in the radial direction. The oscillations of the keyhole, which were characterized by its surface area and volume, were theoretically studied asa function of the welding speed and surface tension coefficient. The welding speed has stabilization effects on the keyhole and fully stabilized the keyhole under certain high welding speed conditions, an effect similar to that observed in keyhole mode laser welding. The surface tension governed the keyhole oscillation periods and was proposed asa major source of oscillations. In addition, the complex and violent flow patterns of weld pool were predicated and consistent with the well-accepted X-ray imaging experimental results. The EBW transient keyhole and weld pool dynamics were self-consistent. The present study is to present the successful development of EBW, thereby allowing the establishment of the self-consistent keyhole and weld pool dynamics asa function of the process parameters and material properties, to provide a promising avenue for process optimizations in industrial applications.

An octree-based adaptive mesh refinement method for three-dimensional modeling of keyhole mode laser welding

Numerical simulations of laser welding could provide unprecedented insight into the physical details of the welding process; however, it remains a central challenge to understand laser welding with a high-efficiency and high-resolution method. This paper reports an octree-based adaptive mesh refinement method for efficient process modeling of laser welding in three dimensions. Based on multiresolution analysis algorithm, we also propose a hybrid adaptive mesh refinement strategy depending on local temperature, fluid velocity, and volume of fluid (VOF) value of weld pool of laser welding. Using the proposed method, the predicted heat transfer, and fluid flow behaviors of weld pool are consistent with experimental and previous theoretical results. Spatter formation at the beginning stage of laser welding is observed with a grid resolution in tens of microns. Under certain circumstances, the liquid jet can be produced when the fluid speed near the top surface of the weld pool is 1m/s. The time for breakup of a liquid jet into spatters is about 0.065ms. Furthermore, using the method, we just need 7hours to complete a simulation of a 40-ms real laser welding process on a small workstation (2.60GHz CPU, 16 cores), comparing with 120hours needed using uniform grids. In a word, this method shows great potential for efficient computer simulation-based process optimization of laser material processing.

Crystal growth during keyhole mode laser welding

Evolution of microstructure during keyhole mode welding involves several special features such as multiple inflections of weld pool boundary curvatures, strong spatially variable thermal cycles and negligible undercooling. These systems are difficult to characterize rigorously, because depending on the sections selected, significantly different grain structures and topological features are observed. Here we uncover the special features of crystal growth during keyhole mode laser welding considering the motion of the melt pool and the interdependence of the grain growth in both the fusion zone and the heat affected zone. The temperature distribution and the transient thermal history of welds were combined with the grain growth simulation using a Monte Carlo approach in a computationally efficient manner. The computed results were tested against independent experimental data for keyhole mode laser welding of copper where the grain structure can be easily resolved. The results showed that the curved columnar grains growing from the fusion zone boundary coexisted with axial columnar grains near the centerline of welds. The effects of welding speed on the dimension, distribution, orientation and morphology of the columnar and equiaxed grains were studied.

Factors controlling keyhole-induced porosity in cold wire laser welded aluminum

Production of lightweight laser-welded assemblies to meet the requirements for higher performance and reduction of carbon emission requires substantial effort in mitigating the effects of processing on welding defects. The factors controlling keyhole-induced porosity during cold wire laser welding of AA6061-T6 were studied. The laser beam intensity distribution parameters and the laser energy deposited per unit length of the weld determine the keyhole behavior, the shape and size of the surrounding molten material and, consequently, the percentage of porosity and average size of pores that resulted from keyhole collapse during welding. Changes in keyhole size and porosity are not noticeably significant with respect to the rate of feeding the cold wire into the molten weld pool. The vapor plume that forms above the molten pool due to the ejection of evaporated metal particles from the keyhole effectively attenuates the laser beam and reduces the depth of penetration. However, the erratic behavior of the plume results in an inconsistent porosity response. The use of a gas jet for deflecting the plume is beneficial for more consistent porosity response. Results from this work provide guiding principles for the selection of process variables for keyhole-mode laser welding of aluminum alloys, depending on the particular application and the criteria for acceptable level of defects.

Solidification Map of a Nickel-Base Alloy

The solidification behavior of the advanced nickel-base alloys, such as Inconel® Alloy 690, is important for understanding their microstructure, properties, and eventual service behavior in nuclear power plant components. Here, an experimental and theoretical program of research is undertaken with the aim of developing a quantitative understanding of the solidification behavior under a wide range of temperature gradients and solidification growth rates. The temperature gradient and solidification rates vary spatially by several orders of magnitude during keyhole mode laser welding. Therefore, the solidification structure is experimentally characterized from microscopic examinations of the resulting fusion zones and correlated with fundamental solidification parameters to provide a widely applicable solidification map that can be employed for a broad range of solidification processes. The cell and secondary dendrite arm spacings are quantitatively correlated with cooling rates. An Alloy 690 solidification map, which illustrates the effect of temperature gradient and solidification rate on the morphology and scale of the solidification structures, is also presented.

Microstructure characterization of laser welded Ti-6Al-4V fusion zones

The as-welded microstructure of laser-welded Ti-6Al-4V is characterized as a function of CO2 key-hole mode laser welding speed. Martensitic α′ is the predominant phase, with some α and retained β. Phase transformation is affected by the cooling rate through laser welding speed. A higher welding speed of 1.6 to 2.0m/min produced more martensite α′ and less retained β in the welds. 1.4m/min welding speed produced small amounts of α, besides the martensite α′. A trace of δ titanium hydride phase seems to have formed in the weld fusion zone. Moiré fringes are a common feature in the TEM microstructure, due to abundance of multi-phase interfaces. Tensile twins and clusters of dislocations indicate that plastic deformation has happened in the as-welded microstructure, indicating the local stress levels to be approaching the yield stress on-cooling during laser welding.

Porosity Formation after the Irradiation Termination of Laser

Porosity is formed because of the keyhole collapse. The porosity formation is associated with the melt pool dynamics, the keyhole collapse and solidification processes. The objective of the paper is t to investigate porosity formation mechanisms and fluid flow in the melt pool using the volume of fluid method. The results indicate that the formation of porosity in continuous wave keyhole mode laser welding is associated to keyhole collapse, backfilling of liquid metal close the gas exit of the laser welding keyhole, surface tension of the gas/liquid interface play an important role in the backfilling downward to the keyhole right after laser beams left.Keywords: porosity; keyhole; collapse; welding; model

A New Model for Keyhole Mode Laser Welding Using FLUENT

Evolution of the free surface at gas–liquid interface during keyhole mode welding is complex and its calculation is computationally expensive. Similarly, models based on only heat conduction without considering vapour cavity and liquid convection around it, are computationally efficient but are not effective in defining the weld pool shape especially for low conducting material like steel. In the present study a useful yet computationally efficient model has been presented for keyhole mode laser welding using commercial software FLUENT. Here instead of evolving the free surface of keyhole in a rigorous way, various possible steady keyhole shapes are assumed partially based on literature evidence and subsequently their dimensions are calculated by an overall heat balance. The estimated keyhole profile is then mapped into the thermo-fluid framework of FLUENT and steady computational fluid dynamics calculations is carried out around the keyhole that is considered rigid wall at boiling temperature. Next, an optimized keyhole shape is identified by comparing the predicted fusion lines with the experimental weld fusion lines reported in literature. Finally, using this optimized keyhole shape independent predictions are made for two materials of widely different thermal conductivities, like steel and aluminum, under different operating conditions. In all cases the results of the present simulation is found to in close agreement with experimental data and even better than the model predictions reported in literature. The present model emerges as a simple yet effective model for predicting the weld bead profile encompassing wide range of materials under different operating conditions.

Role of surface-active elements during keyhole-mode laser welding

During high power density laser welding of mild steel, the keyhole depth, liquid metal flow, weld geometry and weld integrity are affected by base-metal sulfur content and oxygen (O2) present in the atmosphere or shielding gas. The role of these surface-active elements during keyhole-mode laser welding of steels is not well understood. In order to better understand their effects, welding of mild steel specimens containing various concentrations of oxygen and sulfur are examined. In addition, a numerical model is used to evaluate the influence of the surface-active elements on heat transfer and fluid flow in keyhole-mode laser welding. Increase in base-metal sulfur concentration or O2 content of shielding gas results in decreased weld widths. Sulfur results in a negligible increase in penetration depth whereas the presence of O2 in shielding gas significantly affects the weld penetration. It has earlier been proposed that oxygen, if present in the shielding gas, can get introduced into the weld pool resulting in formation of carbon monoxide (CO) at the keyhole surface and additional pressure from CO can result in increased penetration. Numerical modelling has been used in this work to understand the effects of formation of CO on the keyhole and weld geometries.

Alloying elemental change of SS-316 and Al-5754 during laser welding using real time laser induced breakdown spectroscopy (LIBS) accompanied by EDX and PIXE microanalysis

Experimental studies of pulsed laser welding of stainless steel 316 in keyhole mode was done to examine a vaporization model based on the kinetic theory of gases and the thermodynamic laws. A long pulsed Nd:YAG laser with variable duration of 1–12 ms and 9–17 Gw/cm2 was employed. The undesirable loss of volatile elements affects on the weld metal compositions and the alloy properties. The model predicts that the loss of alloying elements strongly takes place at higher peak powers and longer pulse durations. On the other hand, the model shows the rapid migration of Mn and Cr based on the pressure and concentration gradients from the molten pool. Accordingly, the concentrations of iron, chromium, nickel and manganese were determined in the weld pool by means of the energy dispersive x-ray analysis (EDX) and proton induced X ray characteristics (PIXE) microanalysis. The change of weld metal composition of aluminium alloy 5754 in keyhole mode laser welding, was investigated using the model and was supported by the successive measurements. The model predicts that the concentration of magnesium in the weld metal decreases, while the aluminium concentration increases. Moreover, the real time concentrations of aluminium and magnesium elements in the weld metal were determined by laser induced breakdown spectroscopy (LIBS) at different conditions. We conclude that variation of the Al to Mg concentration ratio is negligible with various laser power densities while it is strongly correlated to the pulse duration.