PROJECT DELIVERABLE REPORT

Transcription

1 PROJECT DELIVERABLE REPORT Grant Agreement Number: Project Acronym: Project Title: Funding Scheme: Date of latest version of Annex I against which the assessment will be made: Deliverable Number and Title: QCOALA Quality Control of Aluminium Laser-welded Assemblies FoF.ICT Seventh Framework Programme 18 May 2010 D3.2: Performance of 532nm Name, title and organisation of the scientific representative of the project's coordinator 1 : Tel: Paola De Bono Senior Project Leader Specialist Materials and Joining Sector Advanced Materials and Processes Group TWI Ltd Granta Park, Great Abington, Cambridge CB21 6AL United Kingdom F: +44 (0) W: (0) Direct Project website 2 address: QCOALA Project Document Reference: Paola.debono@twi.co.uk Ref/PDB/fs/117/ Author(s): Paola De Bono and Paul Hilton (TWI) Kerstin Kowalick and Andreas Ostendorf (Bochum) 1 Usually the contact person of the coordinator as specified in Art.1 of the grant agreement. 2 The home page of the website should contain the generic European flag and the FP7 logo which are available in electronic format at the Europa website (logo of the European flag: ; logo of the 7th FP: The area of activity of the project should also be mentioned. 1

2 1 Introduction As made clear in Deliverable 2.2, Lasag did not provide a green laser platform, instead developing from the outset of the project, the dual wavelength greenmix pulsed laser source. This meant that in order to fulfill the requirements of the DOW (specifically Task 3.2), the partners engaged in laser welding trials had to either use existing green laser sources or utilise green lasers from alternative sources. Deliverable 2.2 also showed that the supply of pulsed laser sources operating at an output wavelength of 532nm, either lab based or commercially available, is significantly limited. For the work reported here, TWI sourced and used two different types of green laser: A lamp pumped frequency doubled Nd:YAG laser manufactured by Miyachi. A Q-switched diode pumped frequency doubled Nd:YAG laser manufactured by Lee Laser. The Miyachi laser offered pulse durations in the msec range whilst the Q-switched unit, from Lee Laser, offered pulse durations in the nsec range. In addition, Task 3.2 of the DoW mentions that a wavelength of around 840nm may be advantageous to weld Al. Therefore, some trials were carried out using a 150W Laserline CW diode laser, operating at 808nm. Using these lasers TWI have investigated the welding of copper (Cu 101) and aluminium (Al 3003). As mentioned above, Lasag delivered the first prototype GreenMix pulsed laser and made this available to Bochum University, early in the project. As a result Bochum used the GreenMix laser for the work reported in this deliverable. For the work reported here, Bochum University used: The Lasag GreenMix laser. With this laser, Bochum University investigated the welding of Cu (SE-CU) and Al 1050A. 2

3 2 Laser Welding for ThinFilm PV Cell Applications 2.2 Introduction For low power applications, a prototype of the GreenMix laser platform could be used at Lasag at an early stage of the project. Due to the large amount of investigations that needed to be carried out in terms of process influencing factors, resulting from the challenges in layer thickness, no welding performance assessment at 532nm was carried out for the solar applications part. Trials rather focused on tailored energy concepts, i.e. to temporally shaping the pulse using the dual wavelength setup. By temporal pulse shaping the thermal response of the material can be controlled very precisely. For example, smaller peak power in the beginning can be used to heat up the material followed by a short high power welding time and a pigtailed remaining part with long times and small power in order to control the cooling, which avoids hot cracking in some aluminium alloys. A typical pulse shape to tailor the absorbed energy and the temperature in the welding zone is depicted in Figure 1. 0,15kW 1kW 0,75kW 3,5ms 1ms 2,5ms 2.3 Materials Figure 1 Sketch of typical pulse shape to tailor the absorbed energy The material used for the performance assessment were aluminium and copper material with a purity of 99,5% and above (Aluminium: 1050A, Copper: SE-CU), supplied as foils in the thicknesses range of 30µm-100µm. 2.4 Laser system Laser Lasag SLS200-GX Lasag SLS200-CL16 Wavelength Greenmix (532nm+1064nm) 1064 nm Peakpower 1.3kW 6kW Fiber 100µm 400µm Beam Profile Top hat Top hat Spotsize 100µm 50µm 400µm Pulse Length: 0,3-3ms (Pulse shaping) Length: 0,3-7ms (Pulse Shaping) Angle of Incedence All trials with the GreenMix-laser were carried out with an A-Scale factor of 200% (Figure 2). The A-Scale factor describes an excessive increase in peak power during the first 300µs (FWHM) of the pulse when the green wavelength is efficiently combined. 3

4 2.5 Trials Figure 2 A-Scale factor of 200% for the GreenMix-laser. The first trials using the GreenMix laser (performed at Lasag) were concerned with the spot welding of aluminum to aluminum at 100µm foil thickness. The main difficulties with aluminium spot welding arise by reason of the high reflectivity of aluminium in the infrared range. Unfortunately, the behavior is also similar in the visible range. The highest absorption is at about 830nm. In contrast to copper, the reflectivity of aluminium decreases by only 4% using a frequency doubled Nd:YAG (532nm), instead of a normal Nd:YAG laser. To clarify whether the slight change in absorptivity would have an observable impact on the process, some initial trials were made at 1064nm. These showed that there was an observable change in absorptivity, which could be recognized by a decrease in energy needed for complete penetration. However, a significant increase in process stability could not be recognized, showing that an improvement in absorptivity of 4% for aluminium at least does not have a dominant impact on process quality. Figure 3 depicts in the left part the influence of Argon atmosphere. It is obvious that the resulting gap between the foils is smaller when using Ar. Also the formation of hot cracks is less pronounced when using Ar. It should be noted that for achieving a stable welding process the laser pulse power must a slightly higher when using Ar instead of air. The process window in general is quite small. In the right part of Figure 3 a cross section is shown with a slightly higher laser pulse power applied. The accumulated heat does not form a stable melt pool. Rather the material coiled up leading to a much worse result. Figure 3: Overlap-welding of 100µm aluminum foils using the 1064nm laser wavelength and a pulse length of 10ms (left top: without Argon atmosphere, laser power 0,9kW, left bottom: Argon atmosphere, laser power 1,0kW, right: without Argon atmosphere, laser power 1,2kW). Using the single wavelength of 1064nm for overlap welding of copper foils (30µm) showed that it was almost impossible to couple in any energy. There were very few samples that showed any welding penetration. However, good results could be achieved by switching to the 4

5 GreenMix laser system. The highest possible energy at the highest possible focusing was needed for coupling into the copper. Once energy was coupled in, a very low energy level was needed to maintain the welding process and the process was much more stable compared to aluminum welding. This was two wavelengths and the according improvement in energy coupling. As Figure 4 shows, the process also seems to be much more tolerant to gaps between foils. As this has not fully been proven, further trials will be performed. 1,4kW 0,8ms Figure 4: Overlap-welding of 30µm copper foils with the GreenMix laser (right: pulse parameters). Although some progress has been made, the process currently shows a high tendency towards pores and crack formation (Figure 5), compared to the welding of pure aluminium. A further series of experiments will deal with these difficulties. 1,4kW 1,4kW 0,75kW 1ms 0,5ms 1ms Figure 5: Defects - Overlap welding of 50µm copper foils with the GreenMix laser, pores (left), crack (right) and pulse parametes (lower part) More trials were performed welding copper to aluminium using the single wavelength laser system (1064nm) with aluminium on top and the GreenMix for the inverse arrangement. The outcome clearly shows (Figure 6) that the possibility of having the copper foil on top, available with the GreenMix laser system is advantageous compared to the inverse arrangement. Aluminium on top leads to poor mixing of materials in the joint due to its low melting temperature. Just some very fragile bonding can be seen in some samples. This particularly becomes crucial at layer thicknesses in the micrometer range. 5

6 0,15kW 3,5ms 1ms 1kW 2,5ms 0,75kW Figure 6: Overlap welding of aluminum and copper; (left) aluminum on topside welded with single wavelength 1064nm, (right) copper on topside welded with the GreenMix laser (foil thickness 50µm, pulse parameters: lower part). A tendency toward defects was again found in many samples (Figure 7). There is a high demand to continue with experiments trying to overcome these difficulties, by a continuation of varying the process parameters (e.g. pulse shaping). 0,7ms 1,4kW 1,4kW Figure 7: Overlap welding of aluminum to copper welded with the GreenMix laser showing a large surface pore. (Foil thickness 50µm, pulse parameters: right). 2.6 Discussion of results The use of SE-CU material has been chosen due to its superior electrical conductivity. This material has been deoxidized by a small amount of phosphorus atoms. Usually, oxygen is used in the copper production process in order to oxidize impurities which decrease the conductivity. However, oxygen negatively influences the processing properties, e.g. weldability. Therefore, phosphorous is added to deoxidize the material. It has been reported in a few published papers that a small amount of phosphorus atoms can induce pores during welding. The pores in Fig. 3 can be attributed to this effect. Regarding welding of dissimilar materials (aluminum to copper), the process with aluminum on top results basically in a soldering process due to the strong mismatch in the physical and chemical parameters of the material, especially the melting temperature. It seems more interesting to focus on the copper-on-top configuration, where a good mixing of the two phases could be observed. However, there should be intermetallic alloys such as CuAl, CuAl2, and Cu9Al4 in the welding zone, which will reduce the stiffness. From the crosssections it seems that there are regions which are rich in copper and others which are rich in aluminum while the brittle intermetallic phases are hardly visible. 2.7 Conclusions While the use of the GreenMix laser provides only a slightly better process quality in thin foil aluminum overlap welding compared to a single-wavelength laser, it is superior in welding of copper/copper or copper/aluminum. Improvements have to be carried out in order to further enhance the process stability and to reduce pores, cracks, and material ejections, in order to 6 0,7ms

7 fulfill the solar application specifications. The welding of dissimilar combinations seems to be possible with copper-on-top and only very small amounts of intermetallic phases have been observed. 2.8 Recommendations and way forward The welding of thin aluminum foils in an overlap configuration will be further improved. Although acceptable welds could be achieved, the reproducibility is still lacking. The GreenMix laser offers far more parameter variations than a single wavelength laser, and showed a better adaption to the process. In copper/copper welding, pores have been observed, believed due to phosphorous atoms in the material. From welding of austenitic steel it is known that an optimized temporal pulse shaping can significantly reduce pores. First, an optimum pulse shape has to be generated and if the pores can be detected by non-destructive testing (eddy-current sensing), it will be possible to accelerate the development process. For welding of dissimilar materials, a better joining strength could be expected for the aluminum-on-top configuration when using a fluxing agent in order to remove the oxygen layer. However, this process configuration will always end up in a soldering process. For the more promising, copper-on-top configurations, metallographic analysis has to prove the element mixing in the welding zone and the formation of copper rich and aluminum rich areas, without the formation of intermetallics at the boundaries. 7

8 3 Laser welding for car battery applications 3.2 Introduction Laser welding trials related to the battery application were focused on bead-on-plate melt runs and welds in 0.1-1mm thickness Al 3003 and Cu C101 sheets and foils. Hereafter, these will be referred to simply as Al and Cu. This work focused on the following activities: Experiments with green and infrared wavelength Miyachi lasers: o o o o Bead-on-plate melt runs on 0.5mm thickness Cu and 1mm thickness Al using the green wavelength. The reproducibility of spot and seam melt runs on Cu and Al was assessed. Cu-to-Cu, Al-to-Cu and Cu-to-Al lap joints using 0.1mm thickness foils. Dissimilar joining of Al-to-Cu butt welds, using 1mm thickness plates. Bead-on-plate melt runs on 1mm thickness Al plates. Comparison between results achieved using 1064nm and 532nm light under similar processing conditions. Experiments with a Q-switched diode pumped frequency doubled Nd:YAG laser: o Bead-on-plate melt runs on 0.5mm thickness Cu specimens. Penetration as a function of the pulse power was investigated. The threshold for generation of a keyhole was also established. Experiments with a 150W Laserline diode laser operating at 808nm: o Bead-on-plate melt runs on 0.5mm and 1.0 mm thickness Al The effect of laser parameters (pulse duration, pulse repetition rate, average and peak powers) on the welding of Al and Cu was investigated in terms of weld quality and process stability. Weld quality was evaluated by transverse cross-sections allowing evaluation of weld profiles. 3.3 Materials and Joint Configurations The welding trials were conducted on Al 3003 and Cu C101 thin sheets and foils, which are commonly used for electrical connections in automotive and solar cell applications. Chemical composition analyses of the two materials used are reported in Tables 1 and 2. Table 1 Main constituents of Cu C101 samples (TWI test ref. S/12/74) Constituent Element, % Manganese (Mn) <0.01 Nickel (Ni) <0.01 Iron (Fe) 0.01 Lead (Pb) <0.01 Tin (Sn) <0.01 Aluminium (Al) <0.01 Zinc (Zn) <0.01 Silicon (Si) <0.01 Phosphorus (P) <0.01 Copper (Cu) Reminder (99.95) 8

9 Table 2 Main constituents of Al 3003 samples. (TWI test ref. S/12/78) Constituent Element, % Silicon (Si) 0.11 Iron (Fe) 0.48 Copper (Cu) 0.14 Manganese (Mn) 1.05 Magnesium (Mg) <0.01 Nickel (Ni) <0.01 Chromium (Cr) <0.01 Titanium (Ti) <0.01 Zinc (Zn) 0.02 Lead (Pb) <0.01 Tin (Sn) <0.01 Aluminium (Al) Remainder (~98.4) The material was supplied as 1mm, 0.5 mm and 0.1mm thickness sheets and foils and was prepared by cutting into coupons measuring 10x30mm. The specimens were cleaned with acetone prior being processed, in order to remove any grease from the metal top surface. A limited batch of Al and Cu samples was chemically etched. Specifically, hydrofluoric acidbased etch (composition 190ml de-ionised water, 3ml hydrochloric acid, 5ml nitric acid and 1ml hydrofluoric acid) was used for etching Al, while 40% acetic acid solution was used for etching Cu. After chemical etching, samples were sealed in a desiccator to prevent oxide formation prior to being laser processed. Al and Cu specimens used for butt weld trials were edge milled. Figure 8 shows an overview of bead-on-plate and joint configurations selected for experimental trials. Bead-on-Plate and joint configurations Material combinations A B Al or Cu Laser Beam Cu Al Al Cu A B Cu Cu Lap-Stake Weld 9

10 Laser Beam A B Cu Al Butt Weld Figure 8 Laser welding joint configurations. 3.4 Equipment Frequency Doubled Miyachi ML 8150 A Green Laser A Miyachi ML 8150 A frequency doubled Nd:YAG-Laser (532nm) was used. An image of the laser system used is shown in Figure 9. Figure 9 Images of Miyachi laser welder ML 8150 A system. Trials with this system were performed at the laser application centre of Miyachi Europe in Puchheim, DE. The laser beam is fibre-guided and the beam focusing optic was installed inside the Miyachi laser station Nova 6 with 3 NC-axis. This pulsed laser system has a maximum peak pulse power of 1.5kW, maximum pulse length of 5msec, pulse energy of 4J and an average power of 5W. Some significant specifications of the laser are reported in Table 3. 10

11 Table 3 Specifications of the Miyachi Laser ML 8150 A Laser Miyachi ML 8150 A Optical System Wavelength 532nm Step index fibre (mm) 0.2 Operation mode Lamp Collimating lens (mm) f=60 pulsed Pulse width (ms) Focusing lens (mm) f=60 Peak power (kw) 1.5 Max Image scale 1:1 Pulse Energy (J) 4 Max Minimum spot size (μm) 200 Pulse shape Rectang ular Repetition rate (Hz) 1-30 The sample clamping jig was the standard one of the laser station. Shielding gas was argon. The shield gas supply was a single nozzle, trailing the laser beam. Melt and welding runs were performed over a range of parameter combinations and processing conditions. An overview of laser parameters used for the experimental trials is reported in Table 4. Table 4 Overview of laser parameters used at 532nm wavelength. Miyachi ML8150 A. Laser parameters Range value Peak power (kw) Pulse energy (J) 4 Pulse duration (msec) Focus position at work-piece surface At focus Due to the limited repetition rate, processing speed was limited to 25 mm/min. Pulse energy was kept constant to 4J, which was the maximum value of energy that could be associated with the pulse while varying pulse duration and peak power, to potentially observe the transition from conduction limited to keyhole mechanisms, should this occur Lee Laser LDP 200 MQS Because it was recognised that the power density of the Miyachi laser was not enough to create a keyhole in Cu, tests with a Q-switched green laser were performed to determine the behaviour of copper at higher power densities. For these trials, a Lee Laser LDP 200 MQS was used. This is a diode pumped Nd-YAG laser. The frequency conversion happens inside the resonator. The laser was located at SET, Burgwedel (DE), where experiments were performed. The experimental set-up is shown in Figures

12 Figure 10 Experimental setup LEE-laser, welding station. The laser beam was mirror-guided to a workstation with one NC-axis. The focussing optic was fixed on to micro-slides, to adjust the focal spot to the surface of the target. The specifications of this laser are listed in table 5. Table 5 Specification of the Lee Laser LDP 200 MQS Laser LEE LDP 200 MQS Optical System Wavelength 532nm Mirror guided beam Operation mode Q-switched Collimating lens Beam expanding optic Pulse width (ns) 150 Focusing lens (mm) f=100 Peak power (kw) 26 Max Minimum spot size (μm) 80 Pulse Energy (mj) 4 Max Repetition rate (khz) Melt runs were performed over a range of parameter combinations and processing conditions. An overview of laser parameters used for experimental trials is reported in Table 6. Table 6 Overview of laser parameters used at 532nm wavelength. Lee LDP 200 MQS. Laser parameters Range value Average power (W) Current driving the flashlamp (A) 8 18 Pulse duration (nanosec) 150 Frequency (khz) 10 Focus position at work-piece surface At focus Lamp pulsed Nd:YAG Miyachi ML 2050 A Infrared Laser To compare the welding behaviour of Al at 532nm and 1064nm, additional welding trials were performed using an Nd:YAG-laser, Miyachi ML 2050 A, with 1064nm emission wavelength. The specifications of this laser are listed in table 7. 12

13 Table 7 Specifications of the Miyachi Laser ML 2050 A Laser Miyachi ML 2050 A Optical System Wavelength 1064nm Step index fibre (mm) 0.2 Operation mode Lamp pulsed Collimating lens (mm) f=120 Pulse width (ms) Focusing lens (mm) f=120 Peak power (kw) 5 Max Image scale 1:1 Pulse Energy (J) 70 Max Minimum spot size (μm) 200 Pulse shape Rectangular Repetition rate 1-30 (Hz) The clamping jig, fixtures and gas shielding were the same as the ones used in the case of the green wavelength ML 8150 system LDL Laserline Diode Laser, 808nm Wavelength A Laserline LDL diode laser, operating at 808nm wavelength, was used to perform some trials on 1mm thickness Al coupons (Figure 11). Figure 11 Images of LDL Laserline diode laser. The maximum output power achievable when operating at 808nm was 150W average power. Some significant specifications of the laser are reported in Table 8. Table 8 Specifications of the diode laser LDL Laser diode LDL Wavelength 808nm Operation mode Continuous wave Average power (W) 150 Max Optical System Focusing lens (mm) f=100 13

14 The laser was mounted above a 700x700mm x/y table capable of reading 12m/min. The clamping jig was composed of two strong magnets and a steel base-plate which maintained the specimen in position during the laser melt runs. 3.5 Results with Green and Infrared Wavelength Miyachi Lasers Bead-on-plate melt runs on 0.5mm thickness Cu using a 532nm beam. A series of experiments, producing single spot welds, were carried out to establish reproducibility of results. During experiments, the pulse energy was kept at approximately 4J, while peak power was varied from 1kW to 1.5kW (therefore pulse duration varied from 4msec to 2.8msec respectively). Examples of improved reproducibility can be seen in Figure 12, which shows spot melt runs performed on 0.5mm thickness Cu plates. Laser parameters used were pulse duration of 2.8msec, pulse energy of 4J and peak power of 1.5kW. Figure 12 Image of single spot melts on 0.5mm thickness Cu, showing reproducibility of the process (Sample 1a; pulse duration: 2.8msec, pulse energy: 4J, peak power: 1.5kW). Bead-on-plate spot melts, using single pulses of variable duration, on 0.5mm thickness Cu plates, were performed in order to observe how the penetration depth varied as function of the applied peak power intensity (peak power divided by the area of the focused laser beam). The Cu plates were sectioned and polished to determine the melt penetration. The results of plotting penetration depth as a function of the peak power intensity in the spot, for fixed pulse energy, are presented in Figure

15 Figure 13 Penetration depth vs peak power intensity for bead-on-plate spot melts performed on 0.5mm thickness Cu plates. It was observed that penetration depth varied from a minimum of 11µm (intensity 3.2x106 W/cm2) to a maximum of 52µm (intensity 4.6x106 W/cm2). Figures 14 and Figure 15 show micrographs of the top surfaces and cross sections of the spot melts, for the cases of maximum and minimum penetration depths achieved on the 0.5mm thickness Cu plates. Maximum penetration depth was found at the maximum performance of the laser, in terms of peak power and pulse energy (Figure 14). At the maximum pulse energy it was observed that at least 1kW peak power was needed to observe the start of melting on the surface of the workpiece (Figure 12). 52μm penetration Figure 14 Micrographs of top surface and cross section in the case of maximum penetration depth for 0.5mm thickness Cu thin sheet (Sample 1a; pulse duration: 2.8msec, pulse energy: 4J, peak power: 1.5kW). 15

16 11μm penetration depth Figure 15 Micrographs of top surface and cross section in the case of minimum penetration depth for 0.5mm thickness Cu thin sheet (Sample 1b; pulse duration: 4msec, pulse energy: 4J, peak power: 1kW). Similar results to those shown in Figure 14, in terms of penetration depth and reproducibility, were observed in the case of bead-on-plate seam melt runs produced using a series of overlapping spots (Figure 16). a) b) Figure 16 Micrographs of top surface a) and cross section b) of bead-on-plate overlapping spot melt runs on 0.5mm thickness Cu thin sheet (Sample 4b; pulse duration: 2.8msec, pulse energy: 4J, peak power: 1.5kW, repetition rate: 1Hz, travel speed:25mm/min) Bead-on-plate melt runs on 1.0 mm thickness Al 3003 using a 532nm beam. Bead-on-plate single spot melts performed on Al plates using a 532nm beam showed good reproducibility, at least observing the surface of samples. Figure 17 shows an example of a series of spot melt runs performed on 1mm thickness Al 3003 plate using a pulse duration of 3.1msec, a pulse energy of 4J and a peak power of 1.3kW. 16

17 Figure 17 Image of spot melt run on 1mm thickness Al, showing reproducibility of the process (Sample 2a ; pulse duration: 2.1msec, pulse energy: 4J, peak power: 1.3kW). The variation of penetration depth as a function of the peak power intensity was observed by changing pulse duration from 2.8msec to 4msec and peak power from 1kW to 1.5kW (for a fixed pulse energy of 4J). The results of plotting penetration depth as function of the peak power intensity in the spot, are shown in Figure 18. Figure 18 Penetration depth vs peak power density for bead-on-plate melt runs performed on 1mm thickness Al plates. For the processing conditions investigated, it was observed that penetration depth varied from a minimum of 115μm (intensity 3.2x106 W/cm2) to a maximum of 162μm (intensity 4.8x106 W/cm2). Figure 19 and Figure 20 show micrographs of the top surfaces and cross sections, for the cases of the minimum and maximum penetration depths achieved on 1mm thickness Al plates. 17

18 Figure 19 Micrographs of top surface and cross section in the case of minimum penetration depth for 1mm thickness Al thin sheet (Sample 7b; pulse duration: 4msec, pulse energy: 4J, peak power: 1kW). Figure 20 Micrographs of top surface and cross section in the case of minimum penetration depth for 1mm thickness Al thin sheet (Sample 5a; pulse duration: 2.8msec, pulse energy: 4J, peak power: 1.5kW) Performance differences for Cu and Al To investigate the relative effect of the Miyachi green laser on Cu and Al, bead-on-plate melt runs were performed on the two materials under the same processing conditions. A series of overlapping spot melt runs were performed at fixed energy of 4J and repetition rates to 1Hz, while pulse duration was varied from from 2.8msec to 4msec and peak power from 1kW to 1.5kW. Figure 21 shows sections of Cu and Al overlapping melt runs using pulse duration of 2.8msec, pulse energy of 4J, peak power of 1.5kW and a frequency of 1Hz. It was observed that the resulting penetration depth was greater in the Al, by a factor of nearly three. 18

19 a) Copper b) Aluminum Figure 21 Bead-on-plate overlapping spots performed on 0.5mm thickness Cu and 1mm thickness Al under the same laser processing conditions: a) Copper (sample 4b): pulse duration: 2.8msec, pulse energy: 4J, peak power: 1.5kW, repetition rate:1hz; b) Aluminium (sample 4b ): pulse duration: 2.8msec, pulse energy: 4J, peak power: 1.5kW, repetition rate:1hz. Al and its alloys oxidise readily in both the solid and molten states and the rate of oxidation increases with temperature (Davis et al, 1993). Bead-on plate melt runs were also performed on Al thin sheets chemically etched prior to being laser processed, to observe how this would affect the laser processed area when no oxide was present on the surface of the work-piece. The results were compared with melt runs performed on un-etched Al samples. The results can be seen in Figure 22. a) No chemical etching. b) Chemically etched. Figure 22 Bead-on-plate overlapping spots performed on Al chemically etched and un-etched, under the same laser processing conditions: a) No chemical etching (sample 4b ): pulse duration: 2.8msec, pulse energy: 4.2J, peak power: 1.5kW; b) Chemical etching (sample 8b): pulse duration: 2.8msec, pulse energy: 4.2J, peak power: 1.5kW). From Figure 22 it is possible to observe that no major differences occurred in terms of penetration depth and metallographic structure. 19

20 3.5.4 Cu-to-Cu, Al-to-Cu and Cu-to-Al lap joints using 0.1mm thickness foils. Experiments on bead-on-plate single spot melt runs, reported in section 2.4.1, showed that the maximum achievable penetration depth for spot melt runs, using the Miyachi 532nm beam, was ~50µm on 0.5mm thickness Cu (see Figure 14). When the maximum achievable peak power (using the same Miyachi system at 532nm wavelength) was applied on five 0.1mm thickness Cu foils, overlap-clamped together to achieve an equivalent thickness of 0.5mm, it was observed that a a penetration of the order of 250μm was achieved, fully penetrating two foils and partially penetrating into the third. This value of penetration depth is much greater than the 52µm penetration depth achieved in the case of using a 0.5mm Cu sheet, and the same laser processing conditions. Figure 23 shows a macro-section of the joint achieved between the three foils. Figure 23 Spot lap weld of three Cu foils using a 532nm wavelength laser system. (Sample 2a ; pulse duration: 2.8msec, pulse energy: 4J, peak power: 1.5kW). Dissimilar lap seam welds between 0.1mm thickness Cu and Al foils were also produced, with both Cu and Al facing the laser beam. For these experiments, peak power was varied from 1.2kW to 1.5kW, pulse duration from 2.8msec to 3.4msec, while pulse energy was kept fixed at 4J. The repetition rate was 1Hz. When Cu was the top layer, results of top surface and cross section analysis often showed the formation of cracks within the welds. Figure 24 and Figure 25 present examples of micrographs made using a peak power of 1.2kW, a pulse duration of 3.4msec, a pulse energy of 4J, a repetition rate of 1Hz and a feed rate of 6 mm/min. These clearly show the difficulties in clamping thin foils for overlap welding. 20

21 Figure 24 Cu-to-Al lap weld of 0.1mm thickness foils using a 532nm wavelength laser system. (Sample 9; pulse duration: 3.4msec, pulse energy: 4J, peak power: 1.2kW, feed rate: 6 mm/min). Figure 25 Cu-to-Al lap weld of 0.1mm thickness foils using a 532nm wavelength laser system. (Sample 11; pulse duration: 3.4msec, pulse energy: 4J, peak power: 1.2kW, feed rate: 6 mm/min). 21

22 When the joint configuration used Al on the top no joints could be made. Examples of achieved top surface and resulting cross sections are presented in Figure 26 and to Figure 27. In this case laser parameters used were peak power of 1.2kW, pulse duration of 3.4msec, pulse energy of 4J, repetition rate of 1Hz and feed rate of 0.1mm/sec). Figure 26 Al-to-Cu attempted lap weld of 0.1mm thickness foils using a 532nm wavelength laser system. (Sample 12; pulse duration: 3.4msec, pulse energy: 4J, peak power: 1.2kW, feed rate: 6 mm/min). Figure 27 Al-to-Cu attempted lap weld of 0.1mm thickness foils using a 532nm wavelength laser system. (Sample 8; pulse duration: 3.4msec, pulse energy: 4J, peak power: 1.2kW, feed rate: 6 mm/min). 22

23 3.5.5 Dissimilar joining of Al-to-Cu butt welds, 1mm thickness plates. Al and Cu plates, with and without chemical etching prior to being laser processed, were used to produce Al-to-Cu butt welds. The aim of this experiment was to compare the effect of removal of the oxide layer from the specimens to be welded together. One set of processing conditions was used in this case, which comprised a pulse duration of 2.8msec, a pulse energy of 4J, a peak power of 1.5kW, a repetition rate of 1Hz and a feed rate of 6mm/min. Figure 28 shows the top surface of four Al-to-Cu butt welds, produced using non chemically etched a) and chemically etched b) samples, with the edges milled. The laser spot was positioned at the joint interface. Trials showed that in all cases the results were more repeatable in the case of chemically etched samples. A planar discontinuity through the full thickness of parent materials at the joint interface was observed when using unetched Cu and Al plates as well as formation of brittle butt welds. Figure 29 presents micrographs of the top surface a) and cross section b) of welds using chemically etched samples. a) No chemical etching b) Chemically etched Figure 28 Al-to-Cu butt weld of 1mm thickness plates using a 532nm wavelength laser system: a) No chemical etching (Sample 16): pulse duration: 2.8msec, pulse energy: 4.2J, peak power: 1.5kW, repetition rate: 1Hz, feed rate: 0.1mm/s. b) Chemical etched (Sample 18): pulse duration: 2.8msec, pulse energy: 4.2J, peak power: 1.5kW, repetition rate: 1Hz, feed rate: 0.1mm/s. 23

24 a) b) Figure 29 Al-to-Cu butt welds using a 532nm beam on chemically etched samples, 1mm thickness (Sample 18; pulse duration: 2.8msec, pulse energy: 4.2J, peak power: 1.5kW, feed rate: 0.1mm/s): a) Top surface; b) Cross-section. Figure 29 shows micrographs of welds with either brittle Cu-rich phases or wettability issues in the Al-rich phases. Cross sections also show that clamping should be improved Comparison between results achieved with 1064nm versus results achieved with 532nm under similar processing conditions. To examine the influence of the laser wavelength on the welding behaviour of aluminium, welding trials with two different laser systems with 532nm and 1064nm emission wavelengths were performed, as described in section 3.3. Both laser systems provided the same spot size on the surface of the target. Bead-on-plate single spot melt runs were performed on Al 1mm thickness plates. Parameters varied during experiments were peak power (from 1kW to 1.5kW) and pulse duration (from 2.8mse to 4msec), while pulse energy was kept constant at 4J. In all cases, results of cross sections showed that penetration depth was less when using the 1064nm beam, compared to the 532nm wavelength, under the laser processing conditions. Figure 30 shows micrographs of cross sections comparing penetration depths achieved using 532nm and 1064nm beams. Figure 31 presents penetration depth plotted as function of the power intensity, where a progressive decrease of penetration depth is observed when decreasing the peak power, while keeping pulse duration fixed to 4J. 24

25 a) b) c) Figure 30 Micrographs of cross sections of single spot melt runs, to compare effects of green (left) and IR (right) wavelengths using 1mm thickness Al plates: a) Samples 5a and 9a: pulse duration: 2.8msec, pulse energy: 4.0J, peak power: 1.5kW. b) Samples 6b and 10b: pulse duration: 3.1msec, pulse energy: 4.0J, peak power: 1.3kW. c) Samples 7b and 11b: pulse duration: 4.0msec, pulse energy: 4.0J, peak power: 1.0kW. 25

26 Figure 31 Penetration vs. power intensity of green (blue line) and IR-laser (red line) beams on Al. 3.6 Experiments with the Q-switched diode pumped frequency doubled Nd:YAG laser: Bead-on-plate melt runs on 0.5mm thickness Cu C101 The goal of these trials was to examine the behaviour of Cu under a green laser beam at high power densities. During experiments the average power was increased stepwise from 2.6W to 32.5W, by increasing the current of the pumping diodes (from 9Ampere to 14Ampere). A series of spot overlapping melt runs were performed at a feed rate of 18mm/min and a fixed pulse duration of 150nsec. Images of the top surfaces and cross sections produced show the occurrence of an ablation process rather than melting of the materials. In Figures 32 and 33 respectively, it is possible to observe a clear transition from a just ablated surface (a) and (b), to the start of the cutting/deep penetration (c), and finally deep keyhole/cutting through the thickness of the material (d) and (e). 26

27 e) d) c) b) a) Figure 32 Micrographs of top surfaces. Bead-on-plate melt runs on 0.5mm thickness Cu. Repetition rate: 10kHz and feed rate: 18mm/min: a) Samples : pulse duration: 150nsec, average power: 2.6W. b) Samples : pulse duration: 150nsec, average power: 7.4W. c) Samples : pulse duration: 150nsec, average power: 10.9W. d) Samples : pulse duration: 150nsec, average power: 22W. e) Samples : pulse duration: 150nsec, average power: 32.5W. 27

28 a b c d e Figure 33 Micrographs of cross sections. Bead-on-plate melt runs on 0.5mm thickness Cu. Repetition rate: 10kHz and feed rate: 18mm/min: a) Samples : pulse duration: 150nsec, average power: 2.6W. b) Samples : pulse duration: 150nsec, average power: 7.4W. c) Samples : pulse duration: 150nsec, average power: 10.9W. d) Samples : pulse duration: 150nsec, average power: 22W. e) Samples : pulse duration: 150nsec, average power: 32.5W. 3.7 Experiments with the 150W Laserline diode laser operating at 808nm All bead-on-plate melt runs were performed on 0.5mm and 1mm Al plates. The power was progressively increased up to the maximum value of 150W, while maintaining a spot size of 0.4x0.4mm (focus position at work-piece surface). Under the processing conditions investigated, the surface of the Al specimens remained unaffected by the laser beam, therefore no micrographs of cross sections were produced for this piece of work at the 808nm wavelength. 4 Discussion 4.2 Bead-on-plate melt runs on 0.5mm thickness Cu C101 and 1.0 mm thickness Al 3003 using a 532nm beam. All results achieved with the lamp pumped Miyachi laser showed a reproducible performance with good weld quality. The use of a frequency doubled Nd:YAG laser enabled some of the difficulties of Cu laser welding to be overcome. The absorptivity of Cu at 532nm and at room temperature is approximately 30%, which is ten times higher than at 1064nm at the same temperature (Figure 34, Steen 2003). 28

29 Figure 34 Spectral absorptivity of different materials (Steen, 2003) From the experimental trials carried out in this work, it was observed that 30% absorptivity of Cu at 532nm, at room temperature is already a sufficient condition to achieve enhanced reproducibility of the spot welds in 0.5mm Cu plates, at least from the surface of samples, when compared to earlier results using pulsed 1064nm light (D3.1 deliverable report). Penetration depths achieved were up to ~60μm on 0.5mm thickness plates and up to ~250μm when processing up to five overlap-clamped 0.1mm Cu foils. At higher power intensities and pulse energies that are currently available in this project, the green wavelength may play a significant role in achieving the VW battery welding specifications, in terms of higher weld penetration and processing speeds. The absorptivity of Al when using a frequency doubled Nd:YAG (532nm) laser is only 4% higher than the absorptivity of Al using an Nd:YAG (1064nm) laser. Regarding bead-on-plate laser melt runs on Al, the increased absorptivity of Al by only 4% using 532nm, is already a sufficient condition to produce reproducible spot welds. However, in contrast to Cu, there are no major differences in Al, in terms of reproducibility and stability of the process, when comparing results achieved with both 532nm and 1064nm light (Figure 30). The differences when using the two wavelengths, under the same processing conditions, are just in terms of achievable penetration depth. In addition to the absorptivity aspects, the lower melting temperature of Al ( C) compared to Cu (melting temperature 1084 C) plays a major role in the achievement of higher penetration depth in Al, when using the same processing conditions. The penetration depth achieved in experiments carried out on 1mm thickness Al, showed differences in penetration depth of a factor of three (e.g. 61μm in 0.5mm thickness Cu, versus 168μm in 1mm thickness Al). Chemical etching of Al specimens, to remove the oxide layer on the top surface of the material, did not have any remarkable impact on penetration depth and metallographic structure of the weld. 4.3 Cu-to-Cu, Al-to-Cu and Cu-to-Al lap joints on 0.1mm thickness foil using a 532nm beam. Due to the limited output power of the Miyachi system, lap joint configurations were investigated using 0.1mm thickness Cu foils. During trials on five Cu overlap clamped foils (total thickness of 0.5mm), it was observed that up to three foils could be joined together (single spot welding) with a resulting weld penetration depth of ~250μm (Figure 23). Greater penetration depths in the sandwiched foils were achievable than in the case of single spot 29

30 melt runs on 0.5mm thickness Cu plates (penetration depth was ~50μm), using the same laser and under the same parameters. This might be related to the higher heat sinking capability combined to heat conduction present in the thicker material, while the presence of air/discontinuities at the interface of the thin foils clamped together might contribute to better control of heat dispersion during the welding process. This again highlights the importance of thermal mass and clamping in this type of experiment. In the case of dissimilar material Al-Cu and Cu-Al lap joints (Figures 24 to 27), weld quality was influenced by the respective compositions of Cu and Al in the weld and also by the material on the top of the combination (Al or Cu). Figure 35 shows the Cu-Al phase diagram for which Cu-rich phases are highly susceptible to intermetallic/brittle phase formation. Figure 35 Cu-Al phase diagram (Hanser and Anderko, 1958). When the laser beam was focused on the top surface of the Cu specimen (to create the joint with the underlying Al sample), cracks were observed on both the top surface and in the cross sections of the welds. This is attributed to the formation of Cu-rich intermetallic phases, which are known to lead to formation of cracks in welds. When welding Al-Cu with the Al as the top sheet, samples with a lack of any kind of joint to the Cu-surface were produced, even though the Al was fully melted down to the interface. In addition, wettability and vaporisations issues of the Al top-layer occurred. This is probably due to the significantly lower melting temperature of Al and insufficient heat transfer to the Cu, combined with a lack of surface wetting. 4.4 Dissimilar joining of Al-to-Cu butt welds, 1mm thickness plates, using a 532nm beam. Similarly to results achieved in Al-Cu and Cu-Al lap joints, in the butt welds, quality was influenced by the respective compositions of Cu and Al in the joint area. Specifically, cracks observed in cross sections of welds produced can be attributed to the formation of intermetallic phases in the weld. Poor quality butt welds were observed in both the cases of chemically etched and nonchemically etched samples. However, experiments showed that results were more repeatable 30

31 in the case of chemically etched samples. Butt welds performed on chemically etched samples (to remove the oxide layer from top surface of coupons prior to welding) all showed either crack formation or poor surface wettability at the Cu-Al interface. 4.5 Comparison between results on Al using 1064nm and 532nm wavelengths under similar processing conditions. The absorptivity of Al at 1064nm is approximately 4% less than that at 532nm. Cross sections of bead-on-plate melt runs on 1mm thickness Al plates, using both wavelengths, showed that there are no major differences, in terms of reproducibility and stability, of the process (Figure 27). In terms of penetration, the green laser outperformed the 1064nm laser and the better performance was stronger at lower applied power density. A green wavelength might be considered therefore for the welding of thin Al, however, the higher cost of the green laser, would have to be taken into account in any industrial application. 4.6 Experiments with the Q-switched diode pumped frequency doubled Nd:YAG laser: bead-on-plate melt runs on 0.5mm thickness Cu C101 Results with the green Miyachi system showed that the pulsed msec green laser system was not suited to perform welds with penetrations greater than approximately 0.3mm in Cu, which are required for the battery application. Therefore, trials with a nsec Q-switched green system were also tried. The laser beam parameters of the Q-switched laser available are in the range where ablation is the dominating process. On the heat penetration experiments on 0.5mm Cu coupons, it was observed that up to an average power of 2.6W (intensity 0.3 x 10 8 W/cm 2 ) there is no effect of the ns 532nm beam on the Cu surface. With a transition in average power to 10.9W (intensity 0.3 x 10 8 W/cm 2 ) the material melting threshold was surpassed. In the transition from 22W to 32.5W (intensity 3.8 x 10 8 W/cm 2 ) keyholing becomes evident and cutting occurs nearly all the way through the thickness of the material. Formation of a keyhole by using a q-switched green system might be able to induce deep penetration welding in Cu when 532nm light is simultaneously combined with an infrared msec pulsed system. 4.7 Experiments with the 150W Laserline diode laser operating at 808nm No effects of the laser beam were seen on both 0.5mm and 1mm thickness Al plates, up to an average applied power of 150W. This is due to the power density of laser beam which was not sufficient to induce any ablation or any presence of melting on the surface of the Al specimen. 5 Conclusions Work using the pulsed laser sources available to the project has allowed the following conclusions to be drawn. Experiments carried out on Cu: o o o The use of 532nm light gives enhanced reproducibility of spot and overlapping spot welds on thin Cu plates, when compared to results using pulsed 1064nm light. Up to three Cu foils, 0.1mm thickness, were successfully spot lap joined achieving a maximum penetration depth of 250μm. By using a q-switched green system, capable of achieving a keyhole, it was estimated that the threshold intensity required for keyholing in Cu was 3.8x10 8 W/cm 2. 31

32 Experiments carried out on Al: o o The use of 532nm light on Al did not show major differences in terms of reproducibility and stability of the process when comparing results achieved using 1064nm light. The main differences when using the two wavelengths, under the same processing conditions, were just in terms of penetration depth. Chemical etching of Al specimens, to remove the oxide layer on the top surface of the material, did not have any remarkable impact on penetration depth and metallographic structure of the weld. Experiments carried out on Cu-Al dissimilar joints: o Trials performed on dissimilar materials (butt and lap welds) all showed the formation of intermetallic phases which led to the presence of cracks in the welds. The quality of welds produced to date cannot be considered acceptable. 6 Recommendations and way forward This work has focused the investigation on green pulsed laser systems since they offer advantages for welding Cu within the scope of the QCOALA project. Results with the green Miyachi system showed good reproducibility and process stability on both thin Cu and Al samples. Specifically, major benefits were observed when processing Cu, when comparing to results achieved using a 1064nm light. The use of a pulsed green msec laser is suitable for processing Cu thin foils when a conduction limited mechanism was sufficient to produce a joint (up to ~0.3mm penetration depth). However, currently available pulsed green laser systems are not able to perform welds with penetrations greater than approximately 0.3mm (which are required for the Volkswagen battery application). Consistent sample clamping is believed critical for processing foils (in the case of lap-joints) and thin sheets (in the case of butt-joints) and needed to improve process control. At the beginning of Year 3, it is planned to schedule a brainstorming session with the relevant QCOALA partners to define approaches for designing suitable clamping fixtures. Formation of a keyhole using a q-switched green system, when combined with a 1064nm source, might prove beneficial for further process development work on Cu and investigation with combined wavelengths (ns pulsed green and msec pulsed infrared) is currently being carried out as part of Task 3.3 Development of tailored energy strategies and Task 3.5 Welding of electric battery interconnections, which are currently scheduled to be completed at M31 of the QCOALA project. Trials performed on dissimilar materials (butt and lap welds) showed that all welds made are highly susceptible to intermetallic/brittle phase formation, which led to crack formation in the welds. The quality of welds in Cu-Al dissimilar materials, achieved using a pulsed system, cannot be considered currently acceptable based on the quality requirements requested in the ISO standard BS EN ISO References Davis J R & Associates, 1993: Aluminum and Aluminum Alloys, ASM International. Handbook, The Material Information Society, p51. Hanser M and Anderko K, 1958: Constitution of binary alloys, McGraw-Hill Inc. Steen W M, 2003: Laser Materials Processing, Published by Springer-Verlag London Ltd. ISBN