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A recent increase in the use of electric vehicles demands an efficient and faster joining process for making electrical interconnects within the battery pack. The choice of tab and busbar materials to produce those electrical interconnects is mainly based on weldability, weight, electrical/ thermal conductivity and cost. To meet the high joining demand and low cycle time, laser welding is emerging as the main joining technology due to its ability to weld a variety of materials at a high speed. This paper investigates laser overlap welding for producing similar and dissimilar material tab-to-busbar interconnects for Li-ion battery assembly. In this research, 0.3 mm Al, Cu, Cu[Ni] and Ni tabs were welded with 1.5 mm Al and Cu busbars using a 150 W pulsed fibre laser system integrated with a wobble head. The weldability and joint suitability analyses were conducted by evaluating joint strength, joint intermetallic compound (IMC) formation, joint resistance and temperature rise with the aim of developing a better and safer battery system. It was observed that a maximum joint strength of 930 N was obtained from the Ni tab to Al busbar joints which was approximately 109%, 44% and 66% more than the strength obtained for Al, Cu[Ni] and Cu tab to Al busbar joints respectively. In the case of Cu busbar based tab connections, the maximum joint strength (1320N) was obtained from the Ni tab, which is 152%, 71% and 76% more than Al, Cu[Ni], and Cu tab to Cu busbar joint strength respectively. The strength obtained for the Cu tab to Al busbar (about 560 N) was slightly more than the Al tab to Cu busbar (about 520 N) due to the formation of CuAl2 IMCs at the weld interface of Al tab to Cu busbar joints. Weld microstructure studies provided insightful information on under-weld, good-weld and over-weld characterisation with respect to IMC formation and correlated with the joint strength. In addition, electrical resistance and temperature rise at the joint are equally important for electric vehicle battery applications. The change in contact resistance and joint temperature rise was measured simultaneously for 180 s at different amplitudes of current (i.e., 100 A, 150 A and 200 A) passed through the joints.

Aluminium alloys are increasingly used to fabricate cooling channels for the thermal management of Li-ion batteries. Cooling channel fabrication involves a number of manufacturing operations including material extrusion, forming and joining/welding. In general, welding of aluminium alloys is challenging as they are both highly reflective and thermally conductive. To address the joining challenges, this paper is focused on developing an optimised joining process to connect a thin, flanged cooling channel to the thick module manifold of the battery thermal management system to create a watertight joint with high mechanical strength. As continuous seam welding was required, laser welding was the preferred as it is a non-contact process combining high speed and precision. For this application, 0.4 mm Al cooling channel was welded with 1.5 mm Al endplate/module manifold using a wobble head integrated with 1 kW CW fibre laser system. The effect of process parameters including line energy, incident angle, laser power, welding speed and beam offset were investigated to optimise both the weld geometry and strength. Microstructure, micro-hardness and grain formation analyses were carried out to understand the metallurgical behaviour of the weld. Beam offset had the most significant effect on the responses such as weld strength, throat thickness and modified throat thickness, and laser power had a significant influence on two key geometric features of the fusion zone, i.e. penetration depth and weld width. Weld strength was optimised using a developed surrogate model and a maximum load of 646.89 N was achieved using 0.2 mm beam offset, 331.82 W laser power and 659.10 mm/min welding speed. Using this optimum combination, a leak-proof cooling channel and module manifold joint were produced for battery thermal management.

In many industrial applications, especially for automotive, aerospace and electronics, laser welding is the preferred joining process due to its high flexibility, short process time and high precision. In conventional single-mode laser welding, the laser beam is fine (typical spot diameter 10 – 40 µm) producing a very narrow weld profile. Wobbling or oscillating the laser beam ensures the weld width and depth of penetration can be achieved in a controlled manner. The wobble technique is most useful for highly conductive and reflective materials as it helps to reduce the spatter formation, back-reflection and instability with the laser. Joining with an overlap configuration created using the wobble technique have been most commonly investigated. But the fillet edge weld is considered more useful for sealing applications without the need for additional filler material. This paper investigates the feasibility of producing fillet edge welds, using the wobble technique, to join 0.3 mm thin aluminium to 1.5 mm thick aluminium sheet. A 1.0 kW CW IR laser was used to produce the fillet edge welds aiming for both joining and sealing applications. The effects of process parameters including laser power, welding speed, wobble amplitude and wobble frequency were studied to evaluate the weld quality. Joint strength and weld microstructural features including penetration depth and interface width were evaluated as indicators of weld quality. The relations between the input laser process parameters and output quality indicators were identified and the weld quality was classified into under-weld, good-weld and over-weld categories. In the case of the good-weld condition, average lap shear strength was around 418.64 N. The results suggested that preferred penetration depth and interface width can be obtained by controlling the laser power, welding speed and wobble amplitude. Overall, this paper demonstrates the feasibility of obtaining good fillet edge joint using laser wobble technique.

This paper investigates the microstructural and mechanical behaviour of overlap micro-joints produced by a blue laser system with a wavelength of ~450 nm. AISI 316L stainless steel was used to produce assemblies of 20 layers of 25 µm micro-foil welded to a single 200 µm foil. The welding speed was varied from 6.5 m/min to 9.5 m/min in incremental steps of 1.0 m/min whilst the power was kept at a maximum output level of 500 W. The joints were analysed for fusion zone microstructure, crystallographic grain structure, microhardness distribution and lap shear and T-peel strengths. Interface width and penetration depth were used as key quality indicators to characterise the fusion zone microstructure. The welding speed modulation indicated that the penetration depth can be easily controlled during blue laser welding. The mechanical strength was evaluated by performing lap shear and T-peel tests. The results indicated that the blue laser welding can have a considerable impact on weld geometry and joint strength as it can reduce discontinuities in welded regions and generate good mechanical properties. This study demonstrated for the first time that the blue laser can be successfully implemented for multiple stack-up joining of stainless steel.

This paper investigates the high rate tensile behaviour of fillet edge joints produced by ‘gap-bridged’ remote laser welding (RLW) using aluminium alloy AA5182. The RLW ‘gap-bridged’ test specimens were produced considering three levels of the part-to-part gap; 0.0, 0.2, and 0.4 mm. Lap shear tests were performed to evaluate the high rate sensitivity in the test speed range from moderate (0.1 m/s) to high-speed rate (10 m/s) at room temperature (~23 °C). This equates to a strain rate range from moderate (~ 10 s−1) to near 1000 s−1. Strain rate dependency was found to be low, however, an increase in tensile extension to failure was observed with increasing strain rate. Additionally, the effects of depressed (−50 °C) to elevated temperatures (up to 300 °C) on the joint tensile performance were evaluated. Fracture strain was computed at room temperature using the digital image correlation (DIC) method and the fracture strain across the weld area was in the range from 0.140 to 0.194 for all the gap and speed conditions. This paper compares the RLW experimental test results with finite element modelling for industrial use. To evaluate joint performance, the lap shear strength of RLW samples was also compared with self-piercing riveting and resistance spot welding.

This paper investigates the effects of process parameters on the joint strength and process robustness when multi-layered joints of dissimilar metals are produced by ultrasonic metal welding (UMW). Three layers of 0.3-mm aluminium sheet are welded with a single 1.0-mm copper sheet which is representative of electric vehicle battery interconnects. A process robustness study in which welding pressure, amplitude of vibration and welding time are varied to produce satisfactory welds is reported. The weld quality is evaluated by performing lap shear and T-peel tests where maximum loads are considered as the quality indicator. Response surfaces are developed to identify the relationship and sensitivity between the input process parameters and output quality indicators. A feasible weldability zone is defined for the first time by identifying the under-weld, good-weld and over-weld conditions based on load-displacement curves and corresponding failure modes. Relying on the weldability zone and response surfaces, multi-objective optimisation is performed to obtain maximum lap shear and T-peel strength which resulted in Pareto frontier or trade-off curve between both objectives. An optimal joint is selected from the Pareto front which is verified and validated by performing confirmation experiments, and further, used for T-peel strength analysis of different interfaces of the multi-layered joint. To conclude, this paper determines both the optimal weld parameters and the robust operating range.\ud \ud

This study reports an investigation on the effects of process parameters and part-to-part gaps on joint quality when producing fillet edge joint using the novel gap-bridged remote laser welding (RLW) technique. Joint geometries, microstructural characteristics, scanning electron micrographs (SEM), electron backscatter diffraction (EBSD) maps, microhardness and tensile strength behaviours are reported for the autogenous RLW fillet edge welds in an aluminium alloy, AA5182, used extensively for automotive structural applications. It has been found that laser process parameters and part-to-part gap directly influence key geometric features including penetration, leg length and throat thickness of the weld fusion zone, and subsequently, tensile performance. Lap shear test results demonstrate that both laser power and speed greatly influence the maximum mean load at failure and the corresponding tensile extension. Furthermore, both the weld macrostructure sections and lap shear strength confirm that the gap-bridging RLW technique can be successfully applied to close the part-to-part gaps. The microhardness profiles reveal that under zero gap and gap-bridged conditions the AA 5182 alloy base material has lower hardness (5–10%) compared to fusion zone. The EBSD maps exhibit the grain distribution from the base material to fusion zone under different gap conditions. As a result of directional cooling, a plane of weakness was formed at the junction of the horizontal and vertical grain developments when part-to-part gaps were employed. This study shows that the gap-bridging RLW technique is capable of creating good quality performance joints with automotive grade AA5182 aluminium alloy.

An automotive battery pack used in electric vehicle (EV) comprises several hundred to a few thousand of individual Lithium-ion (Li-ion) cells when cylindrical cells are used to build the battery pack. These cells are connected in series and/or parallel to deliver the required power and capacity to achieve the designed vehicle driving range. This triggers the need for suitable joining methods capable of providing mechanical strength together with the required electrical and thermal performances. A range of joining techniques are currently employed to connect large numbers of cells, and of these, laser welding is estimated to be the one of most efficient methods. Typically, the cylindrical cell casing is made of electrical grade steel which is electrically connected to copper tabs representing the cylindrical cell terminal to tab interconnect within the battery pack assembly. This study focuses on identifying the effect of laser welding process parameters on the mechanical, electrical and thermal responses of the laser welded joints produced using a 150 W Quasi-CW IR laser. Mechanical strength is assessed by evaluating the lap shear strength of the joint whereas the electrical and thermal responses are captured using voltage sensors and a thermal imaging camera respectively. It was observed that mechanical strength of the joint is highly correlated with electrical resistance and corresponding temperature raise at the joint. Furthermore, the optical micrographs reveal the microstructural characteristics of the joint.

Recent uptake in the use of lithium-ion battery packs within electric vehicles has drawn significant attention to the selection of busbar material and corresponding thickness, which are usually based on mechanical, electrical and thermal characteristics of the welded joints, material availability and cost. To determine joint behaviour corresponding to critical-to-quality criteria, this study uses one of the widely used joining technologies, ultrasonic metal welding (UMW), to produce tab-to-busbar joints using copper and aluminium busbars of varying thicknesses. Joints for electrical and thermal characterisation were selected based on the satisfactory mechanical strength determined from the T-peel tests. Electrical contact resistance and corresponding temperature rise at the joints were compared for different tab-to-busbar joints by passing current through the joints. The average resistance or temperature increase from the 0.3 mm Al tab was 0.6 times higher than the 0.3 mm Cu[Ni] tab, irrespective of busbar selection.

Use of lightweight materials to produce automotive body structures is one of the key trends adopted by automotive manufacturers to minimise emission of greenhouse gases, and subsequently, reduction of fuel consumption. Aluminium alloys are one of the promising lightweight materials which are increasingly used for automotive body-in-white structures. Such applications demand both efficient and effective joining/welding methods to produce repeatable, durable and strong joints without significant alteration of material properties. Remote laser welding (RLW) is an emerging joining technology and increasingly being used to produce lightweight joints as it satisfies the demand for high production throughput at low cost. This paper investigates the effects of process parameters when seam tracking remote laser welding is used to create an autogenous fillet edge weld of automotive grade aluminum alloy (AC-170PX) in lap configuration without shielding gas. The effects of laser power and welding speed on the key geometric features are reported together with details of the weld microstructure. Joint strength is evaluated by performing a lap shear test. It is found that the laser power and welding speed have dominant influence on key geometric features and subsequently on the lap shear strength. Relatively larger grain size in the fusion zone reduces the microhardness by up to 20% in comparison with the base material.

This paper investigates the microstructure, high-rate and temperature dependent tensile behaviour of fillet edge joints produced by novel ‘gap-bridged’ remote laser welding (RLW) using an automotive grade aluminium alloy AA6014, commercially known as AC-170PX, extensively used for automotive skin panel applications. Three part-to-part gap-bridged RLW fillet edge welds, produced with different gaps (0.2 mm, 0.4 mm and 0.6 mm) were examined for joint geometry and microstructure. Relatively larger columnar grains resulting from directional solidification were observed in the fusion zone and microhardness was reduced by ~15- 20% due to precipitates disappearance. Moderate (0.1 m/s) to high speed rate (10 m/s) tensile tests performed at room temperature (~23°C) were used to determine high-rate tensile performance. Although the strain rate dependency was found to be low, an increase in tensile extension was obtained. Additionally, the joint tensile performance was evaluated over a range of temperatures between -50°C and 300°C. Using digital image correlation (DIC), fracture strains were obtained in the range from 0.21 to 0.25 for all gap and speed conditions. Fusion zone based finite element simulations were performed using the Johnson-Cook material failure model to predict joint strength. Additionally, the suitability of gap-bridged RLW joints for automotive applications was determined by comparison with two industrial joining methods, self-piercing riveting (SPR) and resistance spot welding (RSW).

Maintaining dimensional tolerances during the manufacture of vehicle bodies is important to the automotive sector. With the industry's drive towards continuous improvement there is increasing focus on the effect of the joining process on the final assembly. A simulation tool that can predict the distortion resulting from assembly offers potential time- and cost-savings throughout design and manufacture. Although individual spot welds have been studied in detail, to-date methods to predict the distortion of a final assembly have concentrated on the variation of the constituent components. In the current paper simulation is used to predict the distortion of an assembly owing to spot welding using a local/global approach where the local distortion occurring around a single spot weld is projected onto a global assembly at each of the weld locations. The comparisons of the simulations of the assemblies with coordinate measuring machine (CMM) data under different clamping conditions suggest sufficient agreement with experimental data to be of value to engineers. A number of possible enhancements are suggested to improve the accuracy of final shape prediction.