Investigation of Quality and Integrity in QCW Fiber Laser Welding of Tab to Busbar for EV Battery Interconnects

Abstract

Quasi-continuous wave (QCW) fiber laser welding has emerged as a promising technique for battery tab-to-busbar joining due to its ability to deliver high peak power in pulsed mode with flexible control over energy distribution. In spite of these benefits, there is a limited understanding of how QCW laser process parameters, particularly pulse frequency and scan speed, influence weld quality, bond failure load, and electrical performance in tab-to-busbar interconnects. Therefore, this paper is focused on the effects of laser frequency (25-100 Hz) and scan speed (1-7 mm/sec) on the weld quality and reliability in terms of bond failure load, contact resistance, and weld morphology of Ni-plated Cu (Ni-plated [Cu]) busbar to stainless steel–copper–stainless steel (SS-Cu-SS) sandwich tab joint. A parametric study revealed that joint quality is primarily governed by pulse overlap and linear energy density (LED). The study reports that a minimum overlap of 10% is required for bonding, while an optimal 20–60% overlap and an LED of 20–40 J/mm produce the highest bond failure loads ($>500$N) and the lowest contact resistance ($<160$ $\mu \Omega$). A 43.6% reduction in contact resistance was achieved by increasing the frequency from 25 Hz to 75 Hz at a scan speed of 7 mm/s, corresponding to an improvement in overlap. Stable weld widths are obtained (210–240 $\mu$m) within the optimal overlap range; however, a jump in width to 260–275 $\mu$m occurs under extreme parameter combinations. Higher pulse frequencies and slower scan speeds improve heat input and material transport, increasing microhardness; the maximum value (258 ${\mathrm{HV}}_{0.05}$) was attained at 50 Hz and 3 mm/s. The findings demonstrate that obtaining low-resistance and high-strength welds in battery interconnects requires ideal overlap and linear energy density.