ISSUE 115 MARCH 2025 LASER WELDING
THE LASER USER
PRE- AND POST-PROCESSING OF Cu-
TO-STEEL IN EV BATTERY JOINING PRATIK SHUKLA With the transportation sector transitioning
towards e-mobility, sustainability, and net-zero carbon emissions, the demand for electric vehicle (EV) batteries is surging. This has created a need for innovative manufacturing solutions, with lasers emerging as a pivotal technology. Lasers play a crucial role throughout the battery production cycle, from electrode manufacturing to cell assembly and battery pack production. The expanding EV market demands precision in welding copper- to-steel joints for battery applications. This article presents work on a three-step process combining green laser cleaning, blue laser welding, and green laser shock peening to enhance joint quality and longevity.
Introduction
In EV batteries, steel contacts in each cell must be precisely joined to a copper bar to connect all cells in series, generating total output power. This joining requires minimal heat-affected zones (HAZ) and high precision. Effective surface cleaning is essential before joining because poor weld quality can lead to early battery failure. Welded joints are also susceptible to tensile stresses, microstructural defects and fatigue, which are exacerbated by thermal and mechanical cycles during operation. Hence, innovative methods to produce robust and durable joints are crucial.
Copper has exceptional malleability and high thermal and electrical conductivity, making it ideal for various applications [1]. However, copper's high reflectivity to infrared lasers poses challenges in welding, requiring significant power. Increased absorptivity at higher temperatures can lead to blowouts and spattering, while its thermal conductivity causes deformation under excessive heat. Shorter wavelength blue (450 nm) and
green (515 - 532 nm) lasers, improve the absorption efficiency, enabling effective welding with reduced power [2, 3].
Using a 450 nm blue laser, previous studies achieved penetration depths of 312 μm (Cu- ETP) and 648 μm (CuSn6), with protective gas enhancing results. Blue lasers provide faster processing and deeper penetration with less heat. Similarly, green lasers have demonstrated stable conduction welding at speeds up to 3 m/s, facilitating smooth transitions to deep penetration welding [2,3].
Research on Ni-coated copper and mild steel found that increasing the laser power to 1.5 kW improved weld depth and hardness but introduced thermal damage. Optimised parameters — spot size, pulse duration, shielding gas, and airflow — enhanced fracture force, penetration, and electrical resistance in pulsed green laser welding [4]. These findings highlight the effectiveness of advanced laser technologies in overcoming copper welding challenges.
This study demonstrates the potential and feasibility of blue lasers for welding copper to tinned steel and its pre-processing using green laser cleaning and post processing also using a green laser with a view to enhance the weld absorption and its strength.
Materials and Methods
Copper alloy (C106) and tinned steel were used, measuring 0.9 mm and 0.3 mm thick respectively, welded as a lap-weld. Samples were laser-cut to 90 mm × 50 mm dimensions. The material pairing is representative of EV battery applications where efficient electrical connectivity and mechanical stability are critical.
Welding was performed with a 1.5 kW blue laser (445 nm, CW mode). Spot welding configurations varied laser active time (0.8–1.2 seconds) and spot overlap. The beam was delivered through a
galvanometer system mounted on a robotic arm. Microstructural evaluation revealed increased hardness but also porosity and cracking in some samples.
Pre-weld laser cleaning and post-weld laser shock peening were both conducted using a q-switched green pulsed Nd:YAG laser (532 nm, 100 – 525 mJ/pulse) with an ultra-portable laser shock peening system by Bright-Beams Laser Technology Ltd. Laser shock peening was conducted without the use of the typical confinement of water and instead, a 3 mm thick polymer confinement layer was used. This was important because if it is used for post-processing live battery welding, then dry confinement will be a practical and safer solution. Samples were characterised using optical microscopy, universal tensile tester, hardness testing and residual stress analysis using X-ray diffraction (XRD).
Results and Discussion Laser Cleaning
The material removal mechanism, ablation, removed the top surface, which contained impurities and oxidation (Figure 1), at the same time increasing surface roughness by around 50%. The as-received roughness was measured to be 0.548 µm whilst the roughness after cleaning was measured to be 0.825 µm and 0.967 µm respectively, after blue laser welding and laser peening.
Blue Laser Welding
In the cross-section of the welds, porosity, sagging and occasional cracking were observed, particularly with longer laser active times (Figure 2). These defects were attributed to trapped air in the keyhole and uneven heat dissipation. The first spot shows just over 50% penetration with some welding defects on the top whilst the second spot penetrated to the bottom
Figure 1: Cleaned copper surfaces using a green laser source as pre-weld preparation. 20
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