Inspection of laser welded electrical connections for car batteries using eddy currents

Transcription

1 11th European Conference on Non-Destructive Testing (ECNDT 2014), October 6-10, 2014, Prague, Czech Republic More Info at Open Access Database Inspection of laser welded electrical connections for car batteries using eddy currents Abstract John RUDLIN, Paola De BONO and Shiva MAJIDNIA TWI Cambridge CB1 6AL, UK Telephone Fax Laser welding techniques using mixed red and green lasers have been developed for welding copper-copper and copper-aluminium lap joints for car battery electrodes. Laser welding assists in the long term stability of the joint whilst maintaining good electrical contact. To monitor the quality of the welds in production, radiography and eddy current methods were adopted. The challenges to be overcome are small size, difficult materials and geometry and difficult materials, particularly the copper to aluminium joint. This paper describes the results obtained with the prototype eddy current technique. Keywords: battery terminals, laser welds, copper, aluminium, eddy currents 1 Introduction The market for Electric Vehicles and Hybrid Electric Vehicles is growing rapidly, and, with this, the demand for electric batteries is growing too. Lithium-ion batteries require several stages of construction, with one particular stage requiring the joining of highly-conductive aluminium (or copper) busbars to the electrode cell ends of the battery. Laser welding has been considered for replacing the current mechanical screwing joining technique, which is considered too slow and produces too many imperfections. 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 lowdistortion compared with more conventional resistance spot or TIG welding processes. However the laser welding of copper to copper and copper to aluminium is not straightforward, as a copper surface reflects much of the laser energy from the high power red lasers most often used for welding. This makes the process inefficient and variable. The QCOALA (Quality Control of Aluminium Laser-welded Assemblies) project investigated the use of a green and red mixed laser to overcome this problem. To enable the quality assurance, a prototype weld monitoring system, digital radiography system and eddy current systems were developed to inspect the weld. This paper describes the eddy current development. The geometry of the battery is shown in Figure 1. There are a series of welds 13mm long which connect the copper busbar to the electrodes below, which are aluminium or copper depending on the polarity. Figure 2 gives a cross section of the weld geometry. The inspection system has to inspect each one in turn.

2 71, 15 71, Laser-weld seam Figure 1 Battery showing weld locations 1mm Copper/copper weld Copper/aluminium weld Figure 2 Weld Geometry The difficulty for the eddy current tests can be seen, the welds are quite small (13mm long, around 1mm wide and up to 2mm deep). They are close to the edge and have different flaw types (see below) and an unhelpful surface profile. 2 Flaws in welds The types of flaws found in these welds are as follows: in the copper to copper welds the most common flaw is porosity, which usually appears at the surface (Figure 3). Lack of penetration (no weld) is also a possibility. For the copper to aluminium welds the flaw types are very different. There are often porosity flaws at the interface, but the main problem occurs when the aluminium migrates to the surface. This causes a brittle intermetallic material to form which often cracks (Figure 4).

3 Figure 3 Porosity in weld (top good weld, bottom-porosity at surface) Cross section Length section Surface appearance Figure 4 Aluminium to copper welds (top-slight mix with intermetallic formation (G324); middle-complete mix in weld bead (G325); bottom-aluminium migrated to surface with intermetallic formation and cracking (G326) 3 Approach Initially a theoretical and experimental study was carried out using copper and aluminium samples with slots and holes. This was carried out together with a study of probe geometry. It

4 was shown in these studies [1] that depth of penetration of the eddy currents depended on probe size as well as frequency. It was concluded that a probe size of at least 1mm diameter, and a frequency of 5-10KHz was needed to penetrate to the weld interface. It should perhaps be noted that the eddy current method cannot detect the lack of penetration flaws, because the interface is parallel to the flow of the eddy currents. Using the probe type above it was shown that a 0.2mm slot on the opposite side from the probe could be detected at 5kHz. It was expected that the surface porosity would be detected by a higher frequency, although potentially the 5KHz frequency could also detect it. For the copper to aluminium welds it was required that a reading be taken of the weld material, to establish the aluminium content (if possible). This requires that a reference reading be obtained prior to a scan of the weld. It was decided that the best reference in this case would be to balance the probe in air, then take a reading of the metal, balance again and then scan the weld. Automation of the process was required, as the inspection would need to be carried out in a laser cell. 4 Description of Equipment The chosen eddy current system for the QCOALA inspection was the EtherNDE Veritor instrument. This was chosen because it works directly with a laptop and could be integrated easily with an inspection robot. The electronic system for integration with the robot is shown in Figure 4, and the sequence of operations in Table 8. The system was demonstrated on the battery application. Figure 4 Components of eddy current inspection system.

5 Table 1 Simplified sequence of operations (after welding pass, instrument values preset). File named Robot goes to scan start Acquisition started on Veritor Signal (e.g TTL level) sent from PLC to DIO to NULL EC software Signal sent from PLC to DIO to start data collection Probe is moved to surface Signal sent from PLC-DIO to stop data collection.txt file of data transferred from EC software to output Analysis carried out Eddy current parameters reset for surface scan Signal (e.g TTL level) sent from PLC to DIO to NULL EC software Robot starts scan Robot finishes scan Signal sent from PLC-DIO to stop data collection.txt file of data transferred from EC software to output Analysis carried out and sentence made(prior to start of next pass) The software was a Labview programme. 5 Results Basic data showing how the metal sorting is carried out is shown in Figure 5, which shows the indications obtained when the probe is balanced in air and then put on to the two metals in turn. Figure 6 shows the case where the probe alights on a copper weld with a pore immediately below the probe. In this case the apparent change in the metal is due to the edge of the pore, and the reduced signal due to lift off. Different mixes of the copper and aluminium appear between the two indications shown in Figure 5, as shown in Figure 7. Figures 8 and 9 show a comparison of the indication from a good Cu weld and one with surface porosity. The movement of the spot in the impedance plane display can be clearly seen to be much greater for the porosity sample. A similar effect is shown in Figures 10 and 11 for a Cu/Al weld. Figure 12 shows cracks in the surface due to the formation of intermetallics and these could be observed with the eddy current system, however the rough surface of the weld made this difficult to identify individual indications.

6 Aluminium Above sample Copper Figure 5 Basic Set-up showing indications when probe alights on Cu and Al. Indications between the two show the mix. Figure 6 Indication when the probe alights on a pore. The deflection is smaller because of the pore and changes direction because of the edge.

7 G326 G325 G324 Figure 7 Indications from the copper to aluminium welds Surface of sample G339 (good weld) Screen display on scan Figure 8 Example of indication of a good Cu/Cu weld. Surface of Sample G331 Indication on scanning the surface Figure 9 Example of scan of a Cu/Cu weld with porosity

8 Surface of sample G325 Indication on scanning the surface Figure 10 Example of scan of good Cu/Al weld Surface of sample Indication Figure 11 Indication from a poor Cu/Al weld Figure 12 Crack on surface 6 Implementation The eddy current system deployed on a simulated battery is shown in Figure 12. The production line is simulated by an X-Y table. A stepper motor drive is used to produce the vertical movement required. For the eddy current system the speed of the inspection would be able to match the welding speed (100mm/sec) but the time to change the instrument settings and re-balance the

9 instrument before each scan slows down the eddy current system. This could easily be reduced by making the software more efficient. Figure 12 System deployed on simulated battery 7 Conclusions 1 A semi-automated eddy current system has been developed and constructed to inspect Cu/Cu and Cu/Al lap welds 2 The eddy current system was able to detect lack of fusion through the cross section, less than 0.2mm in cross-section length at any point in the cross-section in copper welds of 0.6mm thickness. Inspection of mixed aluminium and copper joints was more difficult, but still possible. Using a phase discrimination method enabled the proportions of the copper/aluminium mix in the weld to be estimated and surface flaws detected. 8 Acknowledgement This work was funded by the EC in contract in funding scheme FoF.ICT Seventh Framework programme. 9 References 1 S. Majidnia and J. Rudlin Depth of Penetration Effects in Eddy Current Testing, BINDT Conference Daventry UK, September 2012.