Laser Spot Welding Research Articles

Toward Stabilizing the Keyhole in Laser Spot Welding of Aluminum: Numerical Analysis.

The inherent instability of laser welding, particularly keyhole instability, poses significant challenges in industrial applications, leading to defects such as porosities that compromise weld quality. Various forces act on the keyhole and molten pool during laser welding, influencing process stability. These forces are categorized into those promoting keyhole opening and penetration (e.g., recoil pressure) and those promoting keyhole collapse (e.g., surface tension, Darcy's damping forces), increasing instability and defect likelihood. This paper provides a comprehensive instability analysis to uncover key factors affecting keyhole and process instability, presenting future avenues for improving laser welding stability. Using a novel numerical method for simulating laser spot welding on aluminum with COMSOL Multiphysics 5.6, we investigated the effect of laser pulse shaping on keyhole and process instability. Our analysis focused on keyhole morphology, fluid flow behaviour, and force analysis. The results indicated that the curvature effect, Marangoni effect, and Darcy's damping force are primary contributors to instability, with the curvature effect and Darcy's damping force being the most dominant. Additionally, erratic and high-velocity magnitudes induce intense fluid flow behaviour, exacerbating keyhole instability. Moreover, single/quadruple peak triangular and variant rectangular ramp-down pulse shapes produced the least instability, while multi-pulse rectangular shapes exhibited intense instability. It was found that combining triangular/rectangular pulse shapes can reduce force and keyhole instability by smoothing spontaneous force spikes, resulting in a more stabilized welding process. Controlling fluid flow and abrupt force changes with appropriate pulse shaping is key to defect-free welded products.

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.

A Review on the Recent Trends in Forming Composite Joints Using Spot Welding Variants

Traditional resistance spot welding (RSW) has been unsuccessful in forming quality composite joints between steel– or aluminum–polymer-based composites. This has led to the development of spot welding variants such as friction stir spot welding (FFSW), ultrasonic spot welding (USW), and laser spot welding (LSW). The paper reviewed the differences in the bonding mechanisms, spot weld characteristics, and challenges involved in using these spot welding variants. Variants of RSW use series electrode arrangement, co-axial electrodes, metallic inserts, interlayers, or external energy to produce composite joints. FFSW and USW use nanoparticles, interlayers, or energy directors to create composite spot welds. Mechanical interlocking is the common composite joint mechanism for all variants. Each spot welding variant has different sets of weld parameters and distinct spot weld morphologies. FFSW is the most expensive variant but is commonly used for composite spot weld joints. USW has a shorter welding cycle compared to RSW and FFSW but can only be used for small components. LSW is faster than the other variants, but limited work was found on its use in composite spot weld joining. The use of interlayers in FFSW and USW to form composite joints is a potential research area recommended in this review.

Effect of pulse shaping on coupling efficiency of laser spot welding in AZ31 magnesium alloy

Effect of pulse shaping on coupling efficiency of laser spot welding in AZ31 magnesium alloy

Heat affected zone evolution in fine grained aluminum alloys during laser welding: Phase-field simulation and analytical investigation

One of the biggest challenges in the welding of strengthened alloys by grain refinement is the grain growth phenomenon in the heat-affected zone (HAZ), which would result in the degradation of mechanical properties. The goal of this study is to comprehend the HAZ evolution in the fine-grained aluminum alloy during laser welding as a low heat input process. Spot laser welding of fined grain 1050 aluminum alloy revealed that grain growth and HAZ formation occurred in samples with 4 µm grain size, but no grain growth occurred outside of the fusion line (FL) in larger grain samples. The microstructure of HAZ was simulated using the numerical phase-field technique. The numerical model's findings demonstrated that a critical grain size (Dcr) for the base metal can be established, above which grain growth would not occur outside FL. To predict grain size distribution outside FL, an analytical model was also used. According to both models, as grain size increases, the HAZ width decreases, and above a critical grain size, no HAZ will form outside of FL. In spot welding, Dcr was independent of power density but for continuous welding processes, Dcr has a direct relationship with heat input per unit length of weld. Moreover, HAZ grain growth mainly occurred in heating step for both types of processes.

Dynamics of Microbubbles Induced by Thermal Shock in Inconel 718 Pulsed Laser Spot Welding and Formation of Micropores after Solidification in Molten Pool

Dynamics of Microbubbles Induced by Thermal Shock in Inconel 718 Pulsed Laser Spot Welding and Formation of Micropores after Solidification in Molten Pool

Effect of initial/tail of the pulse waveform on surface morphology of AZ31 magnesium alloy laser spot welding

Effects of initial/tail of the pulse waveform on coupling efficiency and surface morphology of AZ31 magnesium alloy laser spot welding is investigated. Research shows that initial ramp-up waveform can improve the laser spot welding absorptance, but with limited effect (0.88 → 0.9), and melting time of ramp-up waveform is longer than rectangular for equal pulsed energies, making the welding energy coupling efficient and diameter larger. The ramp-down waveform can extend duration (9 ms → 12 ms) of molten pool above melting point temperature (630 °C), increase Marangoni number (R1: 29.29, R2: 32.11, R3: 23.70, R4: 26.62), slow down cooling rate (3 × 106 °C/s → 1 °C/s) and recoil pressure (R3: 52.90 P0 → 12.16 P0, R4: 95.95 P0 → 20.58 P0) of instantaneous closure of rectangular waveform, reduce the sensitivity of welding defects such as overfill, undercut and spatter, and improve the surface morphology of laser welding spots.

Outstanding ductility of dissimilar laser Al/steel spot joint using a high entropy alloy interlayer

Laser spot welding (LSW) technology was used for the joining of low carbon steel to AA5052 Al alloy using a FeCoNiCrMn equimolar high entropy alloys (HEAs) interlayer for the first time. The addition of the HEA interlayer promoted the formation of a Fe-rich Fe-Al IMC with less brittleness and hardness. It resulted in an increase of the joint fracture load from 963 N to 1473 N and a dramatic improvement of the displacement increased from 0.66 mm to 3.32 mm. Another reason for the outstanding ductility of the dissimilar laser joint was the formation of a conical fusion zone in the joint, which realized the dual coupling of metallurgical bonding and mechanical interlocking at the interface.

Pulsed Laser Spot Welding Thermal-Shock-Induced Microcracking of Inconel 718 Thin Sheet Alloy.

This paper investigates the change in solidification microcrack susceptibility under the influence of thermal-shock-induced effects for pulsed laser spot welding molten pools with different waveforms, powers, frequencies, and pulse widths. During the welding process, the temperature of the molten pool under the effect of thermal shock changes sharply, triggering pressure waves, creating cavities in the molten pool paste area, and forming crack sources during solidification. The microstructure near the cracks was analyzed using a SEM (scanning electron microscope) and EDS (electronic differential system), and it was found that bias precipitation occurred during the rapid solidification of the melt pool, and a large amount of Nb elements were enriched in the interdendritic and grain boundaries, which eventually formed a liquid film with a low melting point, known as a Laves phase. When cavities appear in the liquid film, the chance of crack source formation is further increased. Using a slow rise and slow fall waveform is good for reducing cracks; reducing the peak laser power to 1000 w is good for reducing cracks in the solder joint; increasing the pulse width to 20 ms reduces the degree of crack damage; reducing the pulse frequency to 10 hz reduces the degree of crack damage.

Correction to: Analytical and experimental investigation on the distribution of solidification crack initiation sites throughout a laser spot weld

Correction to: Analytical and experimental investigation on the distribution of solidification crack initiation sites throughout a laser spot weld

Analytical and experimental investigation on the distribution of solidification crack initiation sites throughout a laser spot weld

Analytical and experimental investigation on the distribution of solidification crack initiation sites throughout a laser spot weld

Development of free surface based Thermo-Fluidic model for conduction mode laser spot welding and comparison with the conventional flat surface based model

Development of free surface based Thermo-Fluidic model for conduction mode laser spot welding and comparison with the conventional flat surface based model

Optimization of Resistance Spot Welding Parameters in a Shop Floor Environment to Achieve Desired Spot Size in Low Carbon Steel Sheet

Resistance Spot Welding (RSW) is widely employed in automotive, electronics, furniture, Coach building and package industries. Although various spot weld methods, such as Friction Stir Spot Welding, MIG Spot welding, Laser Spot welding and Ultrasonic Spot welding are in use, Resistance spot welding is a versatile and easy method to adopt in simple fabrication shop. It is well known that various factors significantly influence the spot size and its quality. Designers, while selecting RSW process are particularly interested in fixing the appropriate spot size and pitch distance for a specific application. Against the above background, this study has been carried out to arrive at desired spot size by varying the parameters using a simple shop-floor spot welder which is readily available in any fabrication shop. Three levels of power input, three levels of welding current time and many levels of weld cycle time have been adopted for 2mm thick low carbon steel sheet of IS 513 grade. A novel technique has been developed to apply adequate torque to shear the spot weld joint without any damage or distortion. Spot size has been determined by using vernier and profile projector. The nugget quality has been examined using optical microscope at different magnifications. The bond strength of the spot weld has been determined by tensile shear test. By adopting this simple technique, it is possible to achieve desired spot size which is defect free and having excellent penetration. Keywords: Low carbon steel sheet, Resistance Spot welding, Nugget size, welding parameters, macro & micro examination.

Laser Beam Welding of CubeSat 1U Structure Parts Obtained by Powder Bed Fusion

This study contributes to a possible methodology for manufacturing CubeSats using additive manufacturing and laser beam welding. Titanium connectors were constructed by selective laser melting and electron beam melting and characterized from a topological point of view. The connectors can be joined to titanium tubes for the construction of CubeSats via laser spot welding. The fiber laser welds exhibited full penetration using pulses with 400 J of energy. The welds showed titanium acicular martensite grains with recesses and pores. The average hardness of the cast zone was 350 HV, which is close to the hardness of the connectors (400 HV) and more rigid than that of the tubes (100 HV). Spot welding has proven to be useful in resisting forces above 2000 N, which is sufficient for CubeSat frame space applications.

Combination of Eulerian and ray-tracing approaches for copper laser welding simulation

Laser welding of pure copper and its alloys is a challenging process with a growing industrial interest due to the latest development in the field of electric mobility. The difficulties are mainly related to the material's high thermal conductivity and a poor absorptivity of few percent at the classical IR laser (YAG). It is also well known that such a configuration can lead to the formation of undesirable defects, such as pores or spatters as a consequence of melt pool instabilities. It has been observed experimentally that the usage of a laser at both high speed and high power tends to limit those instabilities. Although this positive influence has already been observed for equivalent materials, a physical explanation is not yet available. In this perspective, a multiphysical simulation of the process at the melt pool scale is currently being developed by using comsol Multiphysics® software. The latter includes an Eulerian interface tracking method for the liquid-gas interface (phase field) and a ray-tracing description of the laser beam to take into account the well-known beam trapping effect under a keyhole regime. For the sake of time computation, the numerical model is first developed in an axisymmetric coordinate system (r,z) to be representative of a laser spot welding process and to validate the numerical coupling methodology. The model will then be extended to a 3D welding case and used as a predictive tool to make appropriate choices on welding parameters to obtain good quality welds (stable melt pool, low porosity rate, etc.).

Remote laser spot welding of AISI 430 sheets by fiber lasers—A phenomenal effect in refining weld microstructure with nanosecond pulses

In the present work, we have compared the results of microstructural and mechanical property characterization of AISI 430 ferritic stainless steel spot welds made remotely using a repetitive nanosecond pulsed fiber laser and a CW fiber laser. Optical/electron microscopy, microhardness test, and tensile shear tests were performed for evaluation of the welds. Welds made by the pulsed nanosecond laser were found to be superior due to the presence of the very narrow heat-affected zone along with finer grains in the fusion zone. The welds were also found to withstand more load before fracture and were more ductile. Use of short duration laser pulses with lower heat input were found to be responsible for the refinement of grains in the fusion zone and improvement of mechanical properties of nanosecond laser weld specimens.

In-situ x-ray phase contrast observation of the full penetration spot welding on limited aluminum material thickness

The laser-spot welding process of aluminum alloy 1050A with a limited thickness is observed with the x-ray phase contrast method to investigate the melt dynamic especially when the melt penetrates the material. The laser-spot welding is investigated with two different wavelengths of the laser beam source: 515 and 1030 nm to investigate the influence of the absorptivity. The melt progressively penetrates the material during the spot-welding process until reaching the bottom side of the material and when the melt penetrates the lower side of the material, the so-called “lens-like” melt appears at the lower side due to the surface tension. At a comparable beam intensity value, the oscillation of the “lens-like” melt at the lower side of the material is driven by the expansion of vapor capillary. This expansion occurs inside of the material and directly above the “lens-like” melt. The shape of the expanded vapor determines the volume as well as the geometry of the resulting melt volume. Furthermore, the transition from the heat conduction welding mode to the keyhole welding mode is investigated by defocusing the laser beam for the beam source with a 515 nm wavelength. At a given variation, a clear difference between either mode is observed with the x-ray phase contrast method.

Numerical Simulation and Experimental Verification of Laser Multi-Section Welding

To address the problems of large welding deformation and splashing in the resistance spot welding of the lubricating oil cooler plate, the laser spot welding was employed, instead of the resistance spot welding, and a novel laser spot welding was proposed, i.e., laser multi-section welding. The major processes involved in this study referred to a finite element model of pulsed laser lap welding built by adopting SYSWELD simulation software, as well as the laser welding of various welding methods. The effect of different welding methods on the welding quality was studied, the parameters of the average power and the duty cycle were optimized in line with the comparative analysis of the experimentally achieved results and the numerical simulation. As indicated from the experimentally achieved results, when the new 6-sections welding method was adopted, the resulting welded joint achieved the most uniform heat input and the largest welding fusion area, and the tensile properties exhibited by the welded joints were significantly enhanced, whereas some pores remained. By altering the duty cycle, pores could be eliminated to further improve the quality of the joint. The mentioned process method could tackle down the problems facing conventional resistance spot welding. Furthermore, it was capable of improving the uneven heat input of the laser spot welding.

Publisher Correction: Effect of ambient pressure change on pure tungsten laser spot welding

Publisher Correction: Effect of ambient pressure change on pure tungsten laser spot welding

Laser Spot Melting on Ti-6Al-4V Substrates: A Study on Thermal History and Keyhole Porosity

In situ monitoring techniques have been applied to laser spot welding experiments to study specific phenomena that also occur in metal additive manufacturing processes. Without the interference of powder particles and their interaction with the laser beam, melt pool signatures and pore evaluation can be monitored, which can help in understanding their effect on the end results in laser spot welding and in metal additive manufacturing. One in situ monitoring technique is thermography which uses infrared cameras to acquire surface temperatures in real time. However, one of the main challenges for infrared cameras is finding the unknown emissivity value of printed parts. Another challenge is that thermography cannot monitor the dynamics under the surface of the substrate. A new method was proposed by combining X-ray imaging and infrared imaging to estimate the emissivity and monitor subsurface phenomena. The method consisted of measuring the melt pool size in X-ray images and comparing it to infrared images. The proposed method was tested by analyzing X-ray and infrared images from laser spot melting experiments on Ti-6Al-4V substrates. Ti-6Al-4V is widely used in metal additive manufacturing processes because of its attractive mechanical properties consequently estimating its emissivity is important. The laser power and laser dwell time were varied for each experiment, and the process was observed in real-time using X-ray and infrared images. The objective was to achieve a confidence interval of the emissivity value for Ti-6Al-4V. In addition, an emissivity value was selected from the obtained confidence interval, which enabled the analysis of the thermal history of the surface of the substrate. Furthermore, the resulting emissivity was used to correct the reading of the temperatures and to estimate the melt pool size in the infrared images. The selected emissivity value of 0.5 from the confidence interval gave an accurate estimation of the melt pool size with an overall average error percentile with the melt pool size measured with the X-ray images of approximately 6.5%. After the temperature correction, solidification time, solidification rates and cooling rates were also calculated from the infrared images using the thermal history. Lastly, it has been observed that the size of the pore generated by the keyhole could affect the surface dynamic of the substrates, concluding that defects under the surface might be detected using thermal images from infrared cameras.