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Quality Control of Aluminium and Copper Laser-welded Assemblies

QCOALA is an innovative new collaborative project funded through the European FP7 programme with the objective of improving the laser welding of highly reflective materials through the development of a fully integrated dual wavelength manufacturing work cell. It will develop a new fully integrated manufacturing system for automated high yield, laser welding of copper and aluminium (0.1-1.0 mm thick) for car battery (Fig 1) and photovoltaic cell interconnect (Fig 2). Welding strategies are being developed to suit these types of applications and materials, and are integrated with process monitoring and non-destructive inspection sensors.

The project focuses on energy-efficient, environmental-friendly and agile manufacturing, through the feed-back of in-line-monitoring and inspection information into the production line, allowing process control and continuous quality improvement and waste reduction. Whereas the concept of the project is aimed at smarter and more energy-efficient manufacturing, the applications that are addressed in the project are categorised in the 'green' alternative energy market.

QCOALA fig 1
Fig 1 Image of super-capacitor battery type (Image courtesy of Volkswagen).

The super-capacitor battery comprises a 'bank' or 'stack' of batteries, which are interconnected using aluminium (and sometimes copper) tabs. The cumulative aluminium weld area determines the electrical conductivity of the connection, and as such, the performance of the device. Laser welding offers the advantage of high-speed, low heat input and low-distortion compared with more the conventional resistance spot or TIG welding process.

Flexible organic and inorganic solar cells are increasingly becoming important as an alternative source of energy (Figure 3). Although photovoltaic or solar cells are a proven concept / product, many challenges remain in the production process of these flexible thin-film solar cells. For instance, an outstanding question is how to interconnect the individual cells? Flexible metal tabs or foils, generally made of aluminium or copper, are welded to the cells to form modules. An accurate, low heat input process is required, since PV cells on <100?m thick foils are extremely fragile and sensitive to mechanical, chemical and thermal stress. Laser welding offers the advantage of a low chemical, thermal and mechanical impact (due to its temporally and spatially selective energy input) compared with ultrasonic welding and conductive adhesives.

QCOALA fig 2
Fig 2 Image of flexible solar cell (Image courtesy of Flisom).
  • A dual wavelength (combined 532nm and 1064nm wavelengths) laser test platform prototype for thin sheet welding (Figure 3) and is being tested on thin copper and aluminium connections
QCOALA fig 3
Fig 3 Dual wavelength test platform (Courtesy of LASAG)
  • A prototype weld Monitoring System (WMS) for 1064nm laser wavelengths, has been developed for the in-line quality monitoring of high quality laser welds in thin-gauge aluminium and copper.
  • An on-line temporal pulse control system is being developed with respect to the closed-loop control of the laser welding process (Figure 4)
QCOALA fig 4
Fig 4 Prototype Weld Processing/Monitoring Head
  • The initial database to contain welding monitoring data has been initiated.
  • The initial assessment of eddy current and digital radiography technology to detect small defects (up to sizes of a few tens of nm) in thin-gauge aluminium and copper interconnections has been conducted, using modelling and experimental work.
  • Laser welding processing parameters have been investigated at 1064nm and 532 nm wavelengths, for the joining of: aluminium to aluminium, copper to copper and aluminium to copper. Processing parameters investigated include wavelength, spot size, beam quality, pulse length, average and peak power, and repetition rate )Figures 5 & 6)

QCOALA fig 5 QCOALA fig 6
Figure 5 Spot welding of 3 copper foils,
Thickness 0.1mm at 532nm (Photo: TWI)
Figure 6 Reproducible spot welds with a 532nm laser.
(Photo: TWI)

Benefits and Impact of QCOALA technology

The above, if successful, will facilitate the introduction of advanced laser automation into mainstream manufacturing of products requiring high yield, small scale joining and interconnect. This has the potential of increasing productivity by 50-100% and process yields by 2-10%:

The technologies being developed for thin sheet aluminium and copper welding have the potential of being scaled to larger joint sizes and being applied to other applications requiring automation, in-process control and monitoring.

The research leading to these results has received funding from the European Community's Seventh Framework Programme (FP7/2007-2013) under grant agreement no 260153

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