Laser Spot Welding Research Articles

Using pulse shaping to control temporal strain development and solidification cracking in pulsed laser welding of 6082 aluminum alloys

High-speed observation of visible and infrared radiation was performed to measure the molten pool geometry, velocity of the solid–liquid interface and temperature profile during laser spot welding of aluminum. Hot cracking occurred at a late stage of solidification for the investigated laser pulse shapes. Hot cracking could be minimized by using a pulse shape with two distinct power levels and a final cooling slope to shut down the laser power. The drop of the laser power from the first to the second power level led to a high cooling rate and high interface velocity at the beginning of solidification. This drop in temperature and molten pool diameter released strains originating from thermal contraction and solidification shrinkage at the beginning of solidification, where spot welding is expected to have a higher ductility. After this initial high solidification rate, low interface velocities were observed during solidification within the second power level. The final solidification rate increased again as a function of the cooling time of the last laser pulse section. This type of solidification process was also found in metallographic microstructures with greater dendritic structures during solidification within the second power level. The strain release at the beginning of solidification minimized residual strains for the remaining solidification, so that crack-free and full penetration bead-on-plate seam welding with overlapping spot welds was possible.

Comparative study on CO2 laser overlap welding and resistance spot welding for automotive body in white

Abstract Our previous study included a comparative analysis of the characteristics of CO 2 laser overlap welding and resistance spot welding in terms of the weld fundamentals, tensile–shear performance, microstructure, hardness, and corrosion resistance of welding joints. In this study, the processing properties of the two types of welding methods were further investigated. First, experiments were conducted on dissimilar auto body sheets using resistance spot welding and laser welding with different weld lengths. Next, the layout of the laser welding joints was analyzed. Finally, experiments were performed on an auto door sub-assembly using resistance spot welding and laser welding respectively. Experimental results showed that both the sheet thickness and weld pool width were correlated with the laser weld length (LWL). When the LWL was increased to 20 mm, the tensile–shear performance of the specimen was better than that of one resistance-spot welding point under various conditions. When performing weld joint layout using the “short-length and multi-segment” scheme, it was possible to produce laser welding specimens with a preferable joint morphology and mechanical properties. The laser welded auto door with welding joints having an optimized weld length and layout had better quality and performance, and the laser welding process had higher flexibility.

Laser Spot Welding of Thermoplastic and Ceramic: An Experimental Investigation

Laser welding of dissimilar materials classes has vast potential in a variety of applications due to the non-contact nature of the process. This study investigates the effects of CO2 laser process parameters on the characteristics of spot welded joint between ceramic and thermoplastic. Using a full-factorial design an experimental investigation is carried out studying the effect of welding parameters (laser exposure, number of spots, stand-off distance) on the tensile strength and the diameter of spot weld. The study reveals an increase of joint tensile strength with the increase in the number of spots and the duration of exposure. However at low laser power, the effect of stand-off distance on the spot weld diameter is found to be inconclusive. Optimal process parameters are estimated using the multi-objective optimization on the basis of ratio analysis. The analysis shows that the higher number of spots and the longest duration of laser exposure result in optimal weld characteristics. Alongside these conditions, a larger stand-off distance is seen to have beneficial impact on the weld characteristics.

Thermomechanical Modeling of Laser Spot Welded Solar Absorbers

A finite element (FE) approach is developed to investigate the laser spot welding (LSW) of flat-plate solar absorbers and the stress and distortion fields that develop after fabrication and during operation. Numerical calculations at two different levels are carried out. At a microscopic scale, the details of a spot weld are analyzed. At a macroscopic level, a global approach is used to simulate the joining of the pipeline to the absorber plate and the “restoration” (flattening) process of the absorber. The simulated welding-induced distortion is compared with experimental measurements. The thermomechanical behavior of a solar absorber under working conditions is also studied and operational stresses and the critical locations for structural failure are reported.

Modeling of keyhole dynamics and analysis of energy absorption efficiency based on Fresnel law during deep-penetration laser spot welding

Deep-penetration laser spot welding can effectively improve the utilization rate of the laser beam through forming the keyhole. In this paper, a model based on Fresnel law was developed with the computational fluid dynamics (CFD) software FLUENT. By considering the recoil pressure, surface tension, buoyancy and evaporation, a ray tracing technique was proposed to describe the absorption efficiency of the laser beam during the process of keyhole forming. It was found that there was large temperature gradient in the keyhole, recoil pressure and Marangoni flow forced the molten metal liquid to flow from bottom to top. At last it formed a nail head at the bottom and crowned at the top. The energy absorption efficiency rose from 42% to 59% during the keyhole forming, which was approximately linear with the depth of the keyhole.

Weld bead formation and corrosion behavior of pulsed laser welded zirconium alloy

Pulsed laser spot welding was developed for the fabrication of Zircaloy spacer grid assembly in pressurized light water reactors. The effects of laser welding parameters on the size of the weld beads were discussed. The influence of oxygen and hydrogen uptakes during the welding process on the corrosion resistance was investigated after the corrosion tests. The results indicate that the optimum weld bead size could be obtained by adjusting the pulsed laser parameters, including laser peak power, number of shots, and pulse width. Pulse width should be taken into consideration to control the weld width. When the moisture and oxygen contents were maintained at a low level, i.e., 200 parts per million (ppm) and 30 ppm, respectively, the oxide thickness of specimens corroded for 120 h was about 0.7 μm compared to about 1.5 μm at high moisture and oxygen contents (1,000 and 280 ppm). The combination of the oxidation and hydriding induced cracking was responsible for the crack of the specimens welded under high moisture and oxygen contents, thereby decreasing the corrosion resistance of the welding specimens.

Study on Factors of Axial Crushing of Thin-Walled High-Strength Steel Sections Used in Passenger Car

This paper firstly studied the structure of front longitudinal, then reviewing comprehensive literature draw the similarity of collision of the thin-walled tube and front longitudinal,finally ensure the study of collision of the thin-walled tube. And establishing three kinds of thin-walled welded rectangular beam finite element model of spot welding, seam welding and laser welding, arrive a more superior technology through researching and analyzing the model.

Mechanical and Corrosion Properties of Welded Joints in New Generation Ferritic and Duplex Stainless Steels

This study covers the very preliminary results of welding and characterisation of one novel duplex and two ferritic stainless steel grades. The so-called ”lean duplex” grade EN 1.4162 (UNS S32101) has 21.5% Cr-5% Mn-1.5% Ni-N, the ferritic EN 1.4509 (UNS S43940, AISI 441) 17% Cr, and the other, improved 21% Cr ferritic grade that fulfills also UNS S44330 standard requirements, but has no standard EN designation yet. Both ferritic stainless steels have low (< 0.02%) carbon content and double (Nb+Ti) stabilisation. The materials were used as 1.5 and 2 mm sheets, hence laser and resistance spot welding were selected for welding experiments. The joints were subjected to mechanical testing and critical pitting temperature (CPT) corrosion tests, which were performed on both the base materials and welds. The mechanical tests of the welds did not reveal any significant softening effect due to welding operations. Comprehensive CPT data were achieved for base materials and their welds using two different polarisation potentials, and the new 21% Cr ferritic grade shows a great promise in both as-received and welded conditions. This paper was written as part of the Finnish Metals and Engineering Competence Cluster (FIMECC)’s Demanding Applications (DEMAPP) program.

The influence of adhesive viscosity and elastic modulus on laser spot weld bonding process

Laser spot weld bonding (LSWB) is a novel joining technology, which combines laser spot welding with a layer of structural adhesive in a single joint. The purpose of this paper is to investigate the effect of the adhesive properties on the joining process, the peel and the shear strength of the LSWB joints. The present work demonstrates that the adhesive viscosity has great influence on the vaporized adhesive gas exhaust process, and the low viscosity is good for the exhaust process. The mechanical test result shows that the tension–shear load of LSWB joint isn׳t always higher than that of the adhesive bonded joint, and LSWB joint with high elastic modulus of adhesive may get the same tension–shear load as the adhesive bonded joint gets. The reaction zone produced by the carbon diffusion between the adhesive and the metal sheet will influence the mechanism of LSWB joint.

Laser Spot Welding of Copper-aluminum Joints Using a Pulsed Dual Wavelength Laser at 532 and 1064 nm

Abstract A modulated pulsed laser source emitting green and infrared laser light is used to join the dissimilar metals copper and aluminum. The resultant dynamic welding process is analyzed using the back reflected laser light and high speed video observations of the interaction zone. Different pulse shapes are applied to influence the melt pool dynamics and thereby the forming grain structure and intermetallic phases. The results of high-speed images and back-reflections prove that a modulation of the pulse shape is transferred to oscillations of the melt pool at the applied frequency. The outcome of the melt pool oscillation is shown by the metallurgically prepared cross-section, which indicates different solidification lines and grain shapes. An energy-dispersivex-ray analysis shows the mixture and the resultant distribution of the two metals, copper and aluminum, within the spot weld. It can be seen that the mixture is homogenized the observed melt pool oscillations.

Laser spot welding of AISI 304L: metallurgical and mechanical properties

The present study aims at investigating the effect of laser mean power on mechanical and metallurgical characterisation of AISI 304L austenitic stainless steel laser spot welds. Macro- and microstructural examinations, microhardness test and the quasi-static tensile shear test were performed. Peak load, energy absorption and failure mode were used to describe the mechanical behaviour of welds. It was found that changing laser transfer mode from conduction to keyhole mode occurs with increasing the laser mean power. Moreover, increasing the laser mean power increases the penetration depth and the area of the fusion zone, changing the failure mode from interfacial to pullout. It was found that heat affected zone softening and fusion zone size are key controlling factors for mechanical properties of laser spot welded AISI 304L.

Analysis of Acoustic Emission Signals During Laser Spot Welding of SS304 Stainless Steel

In this article, an in-process monitoring scheme for a pulsed Nd:YAG laser spot welding (LSW) is presented. Acoustic emission (AE) was selected for the feedback signal, and the AE data during LSW were sampled and analyzed for varying process conditions such as laser power and pulse duration. In the analysis, possible AE generation sources such as melting and solidification mechanism during welding were investigated using both the time- and frequency-domain signal processings. The results, which show close relationships between LSW and AE signals, were adopted in the feature (input) selection of a back-propagation artificial neural network, to predict the weldability of stainless steel sheets. Processed outputs agree well with LSW experimental data, which confirms the usefulness of the proposed scheme.

Interfacial analysis of the ex-situ reinforced phase of a laser spot welded Zr-based bulk metallic glass composite

To study the interfacial reaction of the ex-situ reinforced phase (Ta) of a Zr-based ((Zr48Cu36Al8Ag8)Si0.75+Ta5) bulk metallic glass composite after laser spot welding, the interfacial regions of the reinforced phases located at specific zones in the welds including the parent material, weld fusion zone and heat affected zone were investigated. Specimen preparation from the specific zones for transmission electron microscopy analysis was performed using the focused ion beam technique. The test results showed that the reinforced phases in the parent material, weld fusion zone and heat affected zone were all covered by an interfacial layer. From microstructure analysis, and referring to the phase diagram, it was clear that the thin layers are an intermetallic compound ZrCu phase. However, due to their different formation processes, those layers show the different morphologies or thicknesses.

Effect of enhanced heat and mass transport and flow reversal during cool down on weld pool shapes in laser spot welding of steel

In the literature on numerical simulations of Marangoni driven hydrodynamics in laser spot weld pools, it has been proven impossible to obtain good agreement between simulated and experimentally observed weld shapes without artificially enhancing the thermal conductivity and the dynamic viscosity of the liquid steel by one to two orders of magnitude. This has mostly been ascribed to flow instabilities that are not properly accounted for in the simulations. However, whereas experimental weld shapes are obtained post solidification, the cooling and solidification stage is generally neglected in reported simulations. In the present work, we report a detailed study on the role of the artificial diffusivity enhancement factors in weld pool simulations, and we extend the simulations into the cooling and solidification stage. We show that during the cooling stage, flow reversal may occur in the weld pool, which enhances the downward heat and momentum transfer. This leads to a deeper weld pool that agrees better with experimental results. By including the cooling and solidification stage into the simulations of weld pool hydrodynamics, an improved agreement with experimentally observed weld shapes can be obtained with a reduced necessity to artificially enhance the thermal conductivity and the dynamic viscosity.

Effect of gap on plasma and molten pool dynamics during laser lap welding for T-joints

The aim of the present research is to discuss the effect of gap on plasma plume, keyhole, and molten pool dynamics during laser lap welding for T-joints. The authors observe plasma plume, keyhole opening, and molten pool images by high-speed camera in different gaps during CO2 laser overlap welding of T-joints. The results show that gap causes beam energy fluctuations in the keyhole and leads to the instability of welding process. In laser spot welding, zero-gap and small gap greatly affect the stability of plasma and keyhole, which causes the formation of cavities in the weld metal, while a proper gap can help prevent porosity formation. In laser continuous welding, the disruption and closure of front keyhole wall at the gap periodically changes with the gap, which causes the formation of plenty of porosities at the gap. The instability of keyhole is closely related to dynamics of plume and molten pool, which gives an insight into the mechanism of porosity formation during laser overlap welding.

Pulsed laser spot welding of intersection points for Zircaloy-4 spacer grid assembly

A pulsed laser spot welding technology has been developed for joining intersection points of 0.457mm-thick Zircaloy-4 straps. Weld beads size, mechanical properties and microstructure characterization of the weld beads were investigated. The results indicate that peak power of the pulsed laser has significant influence on the penetration depth of weld beads. Pulse width and number of shots should be taken into consideration mainly to control the weld width. The average value of ultimate tensile loads of the intersection points are continuously increasing as the penetration depth and weld width of weld beads increase. Fracture was located at the fusion line and the fracture surface could be characterized as a mixture of ductile and cleavage feature. Column grains and equiaxed grains were observed in fusion zone and heat affected zone, respectively. The fusion zone consists of a mixture of α-Zr and β-Zr phases among which lamellar Widmanstatten structure α-Zr+Zr3Fe was distributed. The Zr(Fe, Cr)2 second phase particles were precipitated inter and intragranular of α-Zr grains.

Characterization of Joint Quality in Ultrasonic Welding of Battery Tabs

Manufacturing of lithium-ion battery packs for electric or hybrid electric vehicles requires a significant amount of joining, such as welding, to meet the desired power and capacity needs. However, conventional fusion welding processes, such as resistance spot welding and laser welding, face difficulties in joining multiple sheets of highly conductive, dissimilar materials to create large weld areas. Ultrasonic metal welding overcomes these difficulties by using its inherent advantages derived from its solid-state process characteristics. Although ultrasonic metal welding is well-qualified for battery manufacturing, there is a lack of scientific quality guidelines for implementing ultrasonic welding in volume production. In order to establish such quality guidelines, this paper first identifies a number of critical weld attributes that determine the quality of welds by experimentally characterizing the weld formation over time using copper-to-copper welding as an example. Samples of different weld quality were cross-sectioned and characterized with optical microscopy, scanning electronic microscopy (SEM), and hardness measurements in order to identify the relationship between physical weld attributes and weld performance. A novel microstructural classification method for the weld region of an ultrasonic metal weld is introduced to complete the weld quality characterization. The methodology provided in this paper links process parameters to weld performance through physical weld attributes.

Welding of shape memory alloy to stainless steel for medical occluder

Dissimilar metal joining between NiTi shape memory alloy (SMA) and stainless steel was conducted. A cluster of NiTi SMA wires were first joined with tungsten inert gas (TIG) welding process, then the NiTi SMA TIG weld was welded to a stainless steel pipe with laser spot welding process. The microstructure of the welds was examined with an optical microscope and the elemental distribution in the welds was measured by electron probe microanalysis (EPMA). The results show that TiC compounds dispersively distribute in the NiTi SMA TIG weld. However, the amount of TiC compounds greatly decreases around the fusion boundary of the laser spot weld between the NiTi SMA and stainless steel. Mutual diffusion between NiTi shape memory alloy and stainless steel happen within a short distance near the fusion boundary, and intermetallic compounds such as Ni3Ti+(Fe,Ni)Ti appear around the fusion boundary.

A two-dimensional axially-symmetric model of keyhole and melt pool dynamics during spot laser welding

For a better understanding of the physical phenomena associated with the appearance of defects in laser welding, a heat and fluid flow model is developed using Comsol Multiphysics®. This first step of the project is focused on the modeling of a static laser shot on a sample of steel. This 2D axially-symmetric configuration is used to study the main physical phenomena related to the creation of the keyhole. This model takes into account the three phases of the matter: the vaporized metal, the liquid phase and the solid base. To track the evolution of these three phases, coupled equations of energy and momentum are solved. The liquid/vapor interface is tracked using the Level-Set method. The calculated velocity and free surface deformation are analyzed. Melt pool shapes are compared with experimental macrographs and the influence of some parameters such as laser power is discussed.

Predicting Transient Softening in the Sub-Critical Heat-Affected Zone of Dual-Phase and Martensitic Steel Welds

To improve vehicle fuel economy and crash worthiness the automotive industry has been redesigning parts from advanced high strength steels such as dual-phase and martensitic steels. These steels have high strengths with the higher formability characteristics when compared to lower strength conventional steels of similar ductility. These steels derive their unique properties from their complex microstructures containing ferrite and martensite. During assembly welding, the martensite within the sub-critical region of the heat-affected zone tempers, which locally reduces mechanical properties. Although this phenomenon is well studied, it has yet to be quantified. The present work proposes a technique to measure the softening kinetics of dual-phase and martensite steels using rapid isothermal tempering. The resulting model was then validated by predicting the heat-affect zone softening that occurs in laser and resistance spot welds as well as by comparing the microstructures of the rapid tempered samples to the microstructures found in the sub-critical heat-affected zone of welded samples.