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Research Papers

# Transient Temperature and Heat Flux Measurement in Ultrasonic Joining of Battery Tabs Using Thin-Film Microsensors

[+] Author and Article Information
Hongrui Jiang

Department of Mechanical Engineering,
1513 University Avenue,

Jeffrey A. Abell

Manufacturing Systems Research Lab,
GM Global R&D,
Warren, MI 48090-9055

Xiaochun Li

Department of Mechanical Engineering,
1513 University Avenue,
e-mail: xcli@engr.wisc.edu

Contributed by the Manufacturing Engineering Division of ASME for publication in the JOURNAL OF MANUFACTURING SCIENCE AND ENGINEERING. Manuscript received July 5, 2012; final manuscript received April 19, 2013; published online September 16, 2013. Assoc. Editor: Robert Gao.

J. Manuf. Sci. Eng 135(5), 051015 (Sep 16, 2013) (8 pages) Paper No: MANU-12-1200; doi: 10.1115/1.4024816 History: Received July 05, 2012; Revised April 19, 2013

## Abstract

Process physics understanding, real time monitoring, and control of various manufacturing processes, such as battery manufacturing, are crucial for product quality assurance. While ultrasonic welding has been used for joining batteries in electric vehicles (EVs), the welding physics, and process attributes, such as the heat generation and heat flow during the joining process, is still not well understood leading to time-consuming trial-and-error based process optimization. This study is to investigate thermal phenomena (i.e., transient temperature and heat flux) by using micro thin-film thermocouples (TFTC) and thin-film thermopile (TFTP) arrays (referred to as microsensors in this paper) at the very vicinity of the ultrasonic welding spot during joining of three-layered battery tabs and Cu buss bars (i.e., battery interconnect) as in General Motors's (GM) Chevy Volt. Microsensors were first fabricated on the buss bars. A series of experiments were then conducted to investigate the dynamic heat generation during the welding process. Experimental results showed that TFTCs enabled the sensing of transient temperatures with much higher spatial and temporal resolutions than conventional thermocouples. It was further found that the TFTPs were more sensitive to the transient heat generation process during welding than TFTCs. More significantly, the heat flux change rate was found to be able to provide better insight for the process. It provided evidence indicating that the ultrasonic welding process involves three distinct stages, i.e., friction heating, plastic work, and diffusion bonding stages. The heat flux change rate thus has significant potential to identify the in-situ welding quality, in the context of welding process monitoring, and control of ultrasonic welding process. The weld samples were examined using scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) to study the material interactions at the bonding interface as a function of weld time and have successfully validated the proposed three-stage welding theory.

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## Figures

Fig. 1

Schematic of USMW machine and weld area and the actual machine

Fig. 2

USMW workpieces

Fig. 3

Two different thin-film microsensor design layouts

Fig. 4

Surface finish of as-received Cu buss bar

Fig. 5

Surface finish of Cu buss bar after PI coating

Fig. 6

Thin-film sensor fabrication process

Fig. 7

TFTP unit for calibration (each sensor junction area: 30 μm by 30 μm)

Fig. 8

TFTP calibration set up

Fig. 9

Characterization of TFTP

Fig. 10

Experimental setup for in-situ temperature and/or heat flux measurement

Fig. 11

In-situ temperature and heat flux measurement results during USMW

Fig. 12

Comparison between measured heat flux from TFTPs and the calculated one from two TFTCs

Fig. 13

Heat flux and heat flux change rate

Fig. 14

Heat flux change rates for different TFTPs with 4.5 mm to welding zone

Fig. 15

SEM pictures of welding interfaces

Fig. 16

SEM pictures of sample welded with a duration of 0.8 s

Fig. 17

SEM pictures of sample welded with duration (a) 1 s (b) 1.2 s, and (c) 1.5 s

Fig. 18

Oxygen wt. % determined by EDS spot analysis across the welding center line

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