TRANSIENT TEMPERATURE AND HEAT FLUX MEASUREMENTS

Transient Temperature And Heat Flux Measurements In Ultrasonic Joining Of Battery Tabs Using Thin Film Micro Sensors



Abstract

Process physics understanding, real time monitoring, diagnosis and control of various manufacturing applications, such as battery manufacturing, are crucial for manufacturing quality assurance and cost reduction. While ultrasonic welding has been used in battery electric vehicles, its physics is still not well-understood, leading to time-consuming and expensive trial-and-error based weld process optimization. This study is to investigate thermal phenomena (i.e. measurement of temperature and heat flux) by placing micro thin film thermocouple (TFTC) and thin film thermopile (TFTP) arrays at the very vicinity of the ultrasonic welding spot during joining of 3-layer battery tabs and Ni-coated Cu buss bars (i.e., battery interconnect) as used in GM's Chevy Volt. The sensor arrays are directly fabricated on the surface of the buss bars using microfabrication techniques. The fabricated TFTCs and TFTPs exhibited good linearity and sensitivity. Experimental results showed TFTCs enabled the sensing of transient temperature with higher spatial and temporal resolutions than those of conventional thermocouples. Furthermore, it is found that TFTPs offered even higher sensitivity to the transient thermal processes in the welding than TFTCs. More significantly, heat flux change rate is discovered to provide valuable insight to the understanding of the fundamental physics of ultrasonic joining of battery tabs. It suggests that the ultrasonic bonding process involves three distinct stages, i.e., friction heating, plastic work and diffusion bonding stages. In addition, the heat flux change rate obtained from TFTPs has significant potentials for a reliable process monitoring and control of ultrasonic welding process for battery manufacturing.

Executive Summary

Thin-film micro sensors (TFTCs and TFTPs) are successfully used to measure transient temperatures and heat fluxes during ultrasonic joining of multilayer battery tabs with Ni-coated Cu buss bar.

Two different sensor layouts are designed to capture heat generation during the welding process. T

he weld itself has been attributed to phenomena such as plastic deformation and diffusion across the grain boundaries.

A prominent advantage of USMW is that it is a non-fusion joining, hence eliminating potential issues from a high temperature process

In order to fabricate sensor on top of buss bar coupon, a dielectric film (Polyimide) is applied to provide excellent dielectric insulation

and smoother surface for thin film sensor fabrication on the Cu buss bar.

It is discovered that the heat flux change rate provides fundamental insights in the ultrasonic joining mechanism for bonding multilayer battery tabs and Cu buss bar.

The bonding process involves three distinct stages, namely friction heating stage, plastic work stage and diffusion bonding stage.

The proximity sensitivity study suggests that TFTPs can still be effective for process monitoring when the distance is even 4.5 mm away from the welding zone.

This study suggest that thin film sensors can provide a powerful tool for process physics understanding while offering great potential for a robust process control of ultrasonic joining of battery tabs and bars.

Purpose of the Study

This purpose of this investigation is to measure the temperature and heat flux ...