Ref/IM/048R/fs.12 26.10.12 1 DR3.5 QCOALA PROJECT TECHNICAL REPORT Grant Agreement Number: 260153 Project Acronym: QCOALA Project Title: Quality Control of Aluminium Laser-welded Assemblies Funding Scheme: FoF.ICT.2010.10-1 Seventh Framework Programme Name, title and organisation of the scientific representative of the project's coordinator 1 : Paola De Bono Senior Project Leader Electron Beam, Friction and Laser Processes Group TWI Ltd Granta Park, Great Abington, Cambridge CB21 6AL United Kingdom F: +44 (0)1223 892588 W: www.twi.co.uk Tel: +44 (0)1223 899530 Direct E-mail: [email protected]Project website 2 address: www.qcoala.eu QCOALA Project Document Reference: IM/048R/fs/12 Author(s): Ioannis Metsios 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: http://europa.eu/abc/symbols/emblem/index_en.htm ; logo of the 7th FP: http://ec.europa.eu/research/fp7/index_en.cfm?pg=logos). The area of activity of the project should also be mentioned.
Transcript
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QCOALA PROJECT TECHNICAL REPORT
Grant Agreement Number: 260153
Project Acronym: QCOALA
Project Title: Quality Control of Aluminium Laser-welded
Name, title and organisation of the scientific representative of the project's coordinator
1:
Paola De Bono Senior Project Leader Electron Beam, Friction and Laser Processes Group TWI Ltd Granta Park, Great Abington, Cambridge CB21 6AL United Kingdom F: +44 (0)1223 892588
Deliverable D3.1 ‘performance of 1064nm Wavelength Lasers when Welding Aluminium and Copper Joints’ detailed the outcome of the work performed from month 1 to month 12 in Task 3.1. The work reported in this deliverable report was split by the envisaged end application of the laser welding process; ie thin-film photovoltaic cells and electric car batteries. As detailed in D3.1, five different laser sources were evaluated for welding copper 101 (Cu101) and/or aluminium alloy AA3003 (AA3003) in the butt, lap and lap-filler joint configurations for the battery application, specifically:
GSI JK125; a 125W maximum average power pulsed Nd:YAG rod laser.
LASAG SLS 200 CL60; a 220W maximum average power pulsed Nd:YAG rod laser.
LASAG FLS 1042; a 800W maximum average power pulsed Nd:YAG rod laser.
IPG YLR-1000; a 1kW continuous-wave Yb-fibre laser.
Trumpf TruDisk 8002; a 8kW continuous-wave Yb:YAG disk laser. It should be noted that within the scope of work reported in D3.1, it was not possible to perform trials with every permutation of laser source, material and joint configuration. Nevertheless, the results gave a good indication of what was achievable with the range of commercially available laser sources evaluated. In addition, electron beam welding of Cu101 and AA3003 samples was also performed, with a maximum power up to ~1.3kW. For laser welding of Cu101, the results highlighted the difficulty of welding copper with a pulsed laser of moderate peak and average powers (the GSI JK125 was used, having a 3kW maximum peak power). In order to achieve an appreciable penetration depth in Cu101 (>0.1mm in this instance), surface pre-treatment and pre-heating was necessary. Nevertheless, even with surface pre-treatment and pre-heating the reproducibility of the process was low in comparison to other metallic materials. In comparison, a higher power continuous-wave laser (Trumpf TruDisk 8002) was capable of producing homogenous welding seams, free of cracks and porosity at penetration depths up to 2.5mm. For laser welding of AA3003, initial results achieved with a moderate power pulsed Nd:YAG laser highlighted that although a 0.5mm penetration depth was achievable, cracks in the weld metal were visible in both the lap and lap-fillet welds. For laser welding of Cu101 to AA3003, the results achieved with three different laser sources (LASAG SLS 200, LASAG FLS 1042 and IPG YLR-1000) highlighted the difficulty of producing crack-free welds in this material combination. Based on the recommendations in D3.1 and the Task 3.1 description in the Description of Work for the QCOALA project, further work has been performed with a pulsed laser source of 1064nm wavelength (ie an Nd:YAG laser source) on the materials and joint configurations of interest to the battery application. All of the work reported in this deliverable report has been performed with one laser source, a LASAG FLS 542 pulsed Nd:YAG rod laser. This laser source, along with the LASAG FLS 1042 used in month 1 – month 12, is considered to be a state-of-the-art near infra-red laser for pulsed welding. The primary difference between the LASAG FL 542 and the LASAG FLS 1042, is the maximum pulse energy and the maximum average power. A comparison of the laser and optic combinations used in the work reported here and the range of laser and optic combinations used in D3.1 is detailed in Table 1.
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Table 1 Comparison of laser sources used in Task 3.1
Laser sources used in Task 3.1
Parameter GSI
JK125
LASAG SLS 200
CL60
LASAG FLS 1042
LASAG FLS 542
IPG YLR-1000
Trumpf TruDisk
8002
Wavelength (nm)
1064 1064 1064 1064 1070±10 1070±10
Operation mode
Pulsed Pulsed Pulsed Pulsed Continuous
-wave Continuous
-wave
Pulse length (ms)
0.2-20 0.1 – 200 0.3-100 0.3-100 n/a n/a
Peak power (kW)
2.3 8.0 20 15 n/a n/a
Pulse energy max. (J)
17 110 150 70 n/a n/a
Pulse repetition rate (Hz)
0.1-1000 0.1-500 0.1-500 0.1-500 n/a but
modulation to 5kHz
n/a but modulation
to 5kHz
Average power max. (W)
125 220 800 500 1000 8000
Delivery fibre diameter (µm)
150 400 600 600 15 200
Magnification ratio of the delivery fibre
1 0.5 and 1 0.5 0.33 and
0.83 2 1
Nominal spot size, diameter (µm)
150 200 and
400 300
200 and 500
30 200
Max power density (kW/mm
2)
129 254 and 64 282 478 and
76 1415 254
The LASAG FL 542 and the specific optic combinations were chosen ahead of the other laser sources for the following reasons:
The results from D3.1 on laser welding of Cu101 indicated that particularly high laser powers were required in order to initiate melting, and consequently vaporisation, in materials having high room temperature reflectivity at near infra-red wavelengths.
The results from D3.1 on laser welding of AA3003 indicated that the prevention of cracking in the weld metal was critical when pulsed laser welding AA3003. It was thought the parameters of the laser source gave sufficiently high average power, pulse energy and pulse length, in order to tailor the pulse length such that cracking was avoided.
The high pulse repetition range and a relatively high average power (500W) will allow investigation of speed performance for the demanding battery contacts welding application. It is useful to keep in mind that typical performance of a commercial pulse welding laser is at 20 to 200W average power, 3kW peak power and 100Hz maximum pulse repetition rate.
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2 Target Battery Welding Specification
As detailed in D2.2, TWI are responsible for the process development work related to the battery application. The materials and thicknesses for this application were chosen by VW and are specified below:
Pure Cu (henceforth referred to as Cu101).Thickness 0.5 and 1.0mm.
Aluminium alloy AA3003 (henceforth referred to as AA3003). Thickness 0.5 and 1.0mm.
Lap welds: Cu101-to-Cu101 (thicknesses combinations: 0.5mm-to-1mm, 0.5mm-to-0.5mm). AA3003-to-AA3003 (thicknesses combinations: 0.5mm-to-1mm, 0.5mm-to-0.5mm). AA3003-to-Cu101 (thicknesses combinations: 0.5mm-to-1mm, 0.5mm-to-0.5mm). Cu101-to-AA3003 (thicknesses combinations: 0.5mm-to-1mm, 0.5mm-to-0.5mm). Target penetrations are in the range 0.5-1mm. Butt welds: Cu101-to-Cu101 (thicknesses combinations: 1mm-to-1mm, 0.5mm-to-0.5mm). AA3003-to-AA3003 (thicknesses combinations: 1mm-to-1mm, 0.5mm-to-0.5mm). AA3003-to-Cu101 (thicknesses combinations: 1mm-to-1mm, 0.5mm-to-0.5mm). Cu101-to-AA3003 (thicknesses combinations: 1mm-to-1mm, 0.5mm-to-0.5mm). Target penetrations are in the range 0.5-1mm. Table 2 details acceptable limits for imperfections for quality levels. Table 2 Acceptable limits for imperfections for quality levels, from BS EN ISO 13919-2
Limits for imperfections for quality levels
Imperfection/designation Moderate D
Intermediate C
Stringent B
Cracks
Lap welds Not permitted Not permitted Not permitted
Butt welds Not permitted Not permitted Not permitted
Porosity h*≤0.5t** h*≤0.4t** h*≤0.3t**
For lap and butt welds h≤0.25mm (for 0.5mm penetration depth). h≤0.5mm (for 1mm penetration depth).
*h=diameter of pore, **t=penetration of the weld; ***h1=lack of fusion
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3 Experimental Approach
3.1 Materials and material preparation
The welding trials were conducted on AA3003 and Cu101 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 3 and 4. Table 3 Main constituents of Cu101 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)
Table 4 Main constituents of AA3003 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 AA3003 and Cu101 samples was chemically etched. Specifically, hydrofluoric acid-based 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. AA3003 and Cu101 specimens used for butt weld trials were edge milled.
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Figure 1 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
Lap-Stake Weld
Cu Cu
Butt Weld
Cu Al
Figure 1 Laser welding joint configurations
3.2 Equipment
A LASAG FL 542 laser source was used for all the work reported in this deliverable. The laser was located at LASAG’s application laboratory facilities in Thun, Switzerland. As detailed above in Section 1, this laser source is considered state-of-the-art for a pulsed 1µm wavelength laser source. The two main advantages of the LASAG FL 542 compared with other commercially available pulsed laser sources are its peak power and pulse energy. The specific details of the laser source are given in Table 1. The laser beam was delivered to the process head through a 0.6mm core diameter fibre. The process consisted of a 180mm focal length collimating lens, and a focussing lens of 150mm focal length; giving a focused spot diameter of 500µm. In order to avoid back reflection, a 75° angle of incidence between the process head and the workpiece was used. The samples were clamped between two aluminium plates, with a 5mm gap between the plate edges to allow access by the laser beam. The clamping fixture was mounted on an XY stage, which provided motion of the workpiece relative to a stationary laser beam.
A
Laser Beam
B
A
Laser Beam
B
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Argon gas of 99.98% purity (gas type I1 to BS EN ISO 14175) was used as the shielding gas in most experiments. The gas was supplied at a flow rate of 10ℓ/min through a copper pipe of 10mm in diameter.
3.3 Scope of work
3.3.1 Development of pulse parameters for AA3003
Initially, bead-on-plate (BOP) spot welds were produced on AA3003 plates in order to determine the power density thresholds for melting and keyhole generation. Only single sector square pulses were used for the power density threshold experiments. The parameters and their ranges detailed in Table 5 were varied at this stage. Table 5 Parameters and their ranges varied when determining the power density thresholds required for melting and vaporising AA3003.
Parameter Range
Pulse energy (J) 2.5 - 40
Pulse width (ms) 3-5
Pulse repetition rate (Hz) 2-5
Peak power (kW) 0.5 - 10
Based on the results achieved on the power density threshold experiments above, pulses with a leading high peak power sector were applied to meet the melting threshold. The high peak power sector duration is 1ms. The purpose of the high peak power sector is to create a molten pool in the AA3003, ie to simulate the ‘green pulse’ of the GreenMix system being developed in this project. Following the creation of the melt pool, different trailing sectors were tested in order to determine their effect on penetration depth and bead width. The parameters and their ranges detailed in Table 6 were varied at this stage. Table 6 Parameters and their ranges varied when determining the effect of different trailing sectors of the laser pulse on penetration depth and bead width in AA3003.
Parameter Range
Pulse tail width (ms) 1.5 - 39
Pulse energy (J) 4.08 - 47.8
Power (kW) 0.5 - 2.5
3.3.2 Development of pulse parameters for Cu101
A similar scope of work to that detailed above was performed for developing the welding process parameters for Cu101. Initially, bead-on-plate (BOP) spot welds were produced on Cu101 plates in order to determine the power density thresholds for melting and keyhole generation. The parameters and their ranges detailed in Table 7 were varied at this stage. Table 7 parameters and their ranges varied when determining the power density thresholds required for melting and vaporising AA3003.
Parameter Range
Pulse energy (J) 5 - 40
Pulse width (ms) 0.48 - 5
Pulse repetition rate (Hz) 2 - 5
Peak power (kW) 6 - 5
Based on the results achieved on the power density threshold experiments above, pulses with a leading high peak power sector were applied to meet the melting threshold. The high peak power sector duration was either 1 or 2ms. The purpose of the high peak power sector is to create a molten pool in the Cu101, ie to simulate the ‘green pulse’ of the GreenMix system
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being developed in this project. Following the creation of the melt pool, different trailing sectors were tested in order to determine their effect on penetration depth and bead width. The parameters and their ranges detailed in Table 8 were varied at this stage. Table 8 Parameters and their ranges varied when determining the effect of different trailing sectors of the laser pulse on penetration depth and bead width in AA3003.
Parameter Range
Pulse tail width (ms) 1.5 – 9
Pulse energy (J) 14.27 – 42
Power (kW) 2.5 – 6.25
3.3.3 Production of welds in Cu101
The VW requirements for the battery contacts are detailed in deliverable D1.2. To summarise, a joint area of 25mm
2 is required per weld, and 24 welds must be produced in 21 seconds.
Parameter combinations were developed in order to determine the maximum productivity possible with the laser system when welding Cu101 in the lap-joint configuration, hence obtaining a reference for performance comparison of a mixed green-NIR laser. The results of the trials described above were used as a starting point for defining the parameter combinations.
3.3.4 Development of process parameters for welding AA3003 to Cu101
Based on the parameters developed in the above experiments, experimental trials were performed to develop the laser welding parameters for AA3003 to Cu101 in the butt joint configuration (0.5mm thickness coupons). Table 9 details the parameters, and their ranges, varied. Table 9 Parameters and their ranges varied when developing process parameters for welding AA3003 to Cu101.
Parameter Range
Pulse energy (J) 30
Peak power (kW) 3 – 12
Pulse width (ms) 2.5 – 10
Offset of the focused spot 100µm towards AA3003 coupon - 400µm
towards Cu101 coupon
3.4 Weld Quality Evaluation
Weld quality (specifically cracks, lack of fusion and porosity) was assessed in accordance to the criteria detailed in BS EN ISO 13919-2. Selected welds were sectioned, transverse and/or longitudinal to the welding direction, mounted in an epoxy resin, ground, polished and etched, in order to determine their weld profile. Selected welds were also radiographed to determine the presence of volumetric defects in the weld metal. Table 2 details the acceptable limits for imperfections for quality levels B, C and D of BS EN ISO 13919-2.
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4 Results and Discussion
4.1 Development of process parameters for welding AA3003
For the power density threshold experiments on AA3003, Table 10 details the process parameters for selected welds. Also detailed in Table 10 are the measured penetration depths for these combinations of process parameters. As detailed in Section 4, only single sector square pulses were used for the power density threshold experiments. Images of selected weld top beads and cross-sections (transverse and longitudinal) are detailed in Figures 2 and 3 respectively. It can be seen from the results that with a peak power up to 1.2kW (power density of 6.1kW/mm
2) and pulse energy up to 5J, there is only sufficient power density to cause surface
effects on the AA3003 workpiece (ie oxidation is present). With a small transition in energy to 7.5J, and a peak power transition to 1.5kW (power density of 7.6kW/mm
2), the material melting threshold was surpassed.
With a further peak power transition from 3 to 4kW (power densities of 15.3kW/mm
2 and
20.4kW/mm2) the formation of a keyhole in the workpiece occurred.
Figure 2 details the result of applying pulses at selected parameter sets, from the surface of the samples, demonstrating these effects. Figure 3 details selected cross-sections of the spot welds shown in the aforementioned figure. The samples in the two figures were chosen in order to indicate the transition points between surface effects, melting and keyhole generation. Table 10. Results of selected power density threshold trials performed on 1mm thick AA3003 coupons with a nominal spot size diameter of 500µm.
Sample number
Energy (J)
Pulse Width (ms)
Rep Rate (Hz)
Peak power (kW)
Travel Speed
(mm/min)
Penetration depth (µm)
3c 2.5 5 5 0.5 400 1/2 (oxide)
3b 5 5 5 1.0 400 17
12c (see Figures 1a, 2a)
5 3 5 1.2 400 46
1a (see Figures 1b, 2b)
7.5 5 5 1.5 150 102
2a 10 5 5 2.0 400 146
2b 15 5 5 3.0 400 222
2c (see Figures 1c, 2c)
20 5 5 4.0 400 1070
4a 30 5 2 6.0 400
Full penetration
4b 40 5 2 8.0 400
Full penetration
4c 30 3 2 10.0 400
Full penetration
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a)
b)
c) Figure 2 Microscopy images showing the effects of transition from surface effects to molten pool formation and to keyhole formation on selected samples from those listed in Table 1. a) Sample number 12c, indicating surface oxidation effects. b) Sample number 1a, indicating melt pool initiation. c) Sample number 2c, indicating keyhole initiation.
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a)
b)
c) Figure 3 Cross-section images laser welds shown on Figure 2. a) Sample number 12c, indicating surface oxidation effects (50µm penetration depth). b) Sample number 1a, indicating melt pool initiation (100µm penetration depth). c) Sample number 2c, indicating keyhole initiation (860µm penetration depth).
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FigurFigure 4 details the effect of changing the power density on weld depth (ie penetration depth) when welding AA3003 with a laser pulse of 5J pulse energy. The curve demonstrates the onset of the keyhole formation at power densities of 20 kW/mm². At the onset of keyhole formation, it can be seen that the rate at which penetration depth increases as a function of the power density changes significantly. The penetration depth increase is normally expected to continue with the same rate for bulk material. However, lack of effective heat dissipation eventually dominates and energy being deposited to the material achieves ablation and thus limits further conduction and growth. This effect is more evident in thin sheet material as heat sinking is further restricted in comparison to the bulk. A line has been drawn on the graph to demonstrate the anticipated behaviour if this work was performed in material thicknesses above 1mm.
Figure 4 The effect of changing the power density on weld depth (ie penetration depth) when welding AA3003 with a laser pulse of 5J pulse energy. Table 11 lists the power density thresholds (kW/mm
2) that were found in this work programme
to result in melting or keyhole generation of 1mm thickness AA3003. The melting transition was evident by consistent generation of round deformations on the surface of the material. The keyhole transition was evident by the intense localised flash generated by the process (observations were made live on the coaxial view camera) and the cross-sections taken. Theoretically the thresholds are also a function of pulse energy. Practically however, for a pulsed welding system, the thresholds apply for pulse lengths of 500µs minimum duration. Longer pulses up to 20ms may heat up the sample or increase the affected depth, but are not anticipated to shift the thresholds substantially.
Table 11 Power density thresholds, kW/mm2, for AA3003.
Melting threshold Keyhole threshold
7.6 20.4
Following the determination of power density thresholds required to initiate melting and keyholing, single spot welds were produced using an initial pulse sector of 1ms and sufficient power density to initiate keyholing. The trailing sector of the pulse was varied in order to determine its effect on the resultant spot weld. As above, the laser nominal spot size was 500µm in diameter for all trials. Table 12 details selected results achieved with an initial sector of the pulse of 2.5kW peak power (1ms initial pulse sector), whilst varying the pulse width of the trailing sector between 1.5 and 39ms. The power of the trailing sector was 20% that of the initial sector of the pulse (ie 500W). As detailed in Table 12, the diameters of the weld bead (measured at the weld face) did not change as a result of the varying pulse width of the trailing sector. Similarly, the weld penetration depth varied randomly between 100 and 130µm, without showing an ascending or descending pattern. Table 12 Gradual increase of pulse energy below the melting or keyhole threshold with a molten pool initiation sector of 1ms and tail at 20% of the initiator’s peak power.
Sample number
Energy (J)
Pulse Tail Width (ms)
RepRate (Hz)
Peak power (kW)
Bead diameter (µm)
Penetration depth (µm)
15c 4.08 1.5 4 2.5 500±25 117
15b 5.5 4 4 2.5 500±25 103
15a 8.7 9 4 2.5 500±25 122
16a 14.3 19 4 2.5 500±20 127
16b 19.7 29 4 2.5 490±10 117
16c 25.1 39 4 2.5 490±10 121
Table 13 details selected results achieved with an initial sector of the pulse of 5kW peak power (1ms initial pulse sector), whilst varying the pulse width of the trailing sector between 1.5 and 39ms. The power of the trailing sector was 20% that of the initial sector of the pulse (ie 1kW). As previously, the weld width and depth appeared to be governed by the interaction of the initial sector of the pulse with the material. Table 13 Gradual increase of pulse energy below the melting or keyhole threshold with a molten pool initiation sector of 1ms and tail at 20% of the initiator’s peak power.
Sample number
Energy (J)
Pulse Tail Width (ms)
RepRate (Hz)
Peak power (kW)
Bead diameter (µm)
Penetration depth (µm)
17a 7.48 1.5 4 5.0 1055 511
17b 10.3 4 4 5.0 927 458
17c 15.5 9 4 5.0 1054 473
18a 26 19 4 5.0 1108 492
18b 37 29 4 5.0 948 405
18c 47.8 39 4 5.0 1020 468
Table 14 details selected results achieved with an initial sector of the pulse of 5kW peak power (1ms initial pulse sector), whilst varying the pulse width of the trailing sector between 1.5 and 9ms. The power of the trailing sector was 50% that of the initial sector of the pulse (ie 2.5kW). As previously, the weld width and depth appeared to be governed by the interaction of the initial sector of the pulse with the material.
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Table 14 Gradual increase of pulse energy above the melting threshold and beneath the keyhole initiation threshold, with a molten pool initiation sector of 1ms and tail at 50% of the initiator’s peak power.
Sample number
Energy (J)
Pulse Tail Width (ms)
RepRate (Hz)
Peak power (kW)
Bead diameter (µm)
Penetration depth (µm)
19a 9.75 1.5 4 5.0 1069 657
19b 16 4 4 5.0 1058 663
19c 28.5 9 4 5.0 1068 605
The small variations in penetration depth and weld bead diameter observed in Tables 12, 13 and 14 are likely related to surface reflectance of the material during the interaction with the initial sector of the pulse.
4.2 Development of process parameters for welding Cu101
For the power density threshold experiments on Cu101, Table 15 details the process parameters for selected welds. Also detailed in Table 15 are the measured penetration depths for these combinations of process parameters. As detailed in Section 4, only single sector square pulses were used for the power density threshold experiments. Images of selected weld top beads and are detailed in Figures 5, 6 and 7. For welds performed under argon shielding gas (samples 5a, 5b, 5c, 7a, 7b, and 7c in Table 15), it can be seen that a peak power of 8kW was sufficient to start melting of Cu101. The power density threshold for keyholing the copper was found to present with a peak power of 13kW. In previous work, reported in deliverable D3.1, surface preparation and pre-heating of the workpiece were required in order achieve an appreciable penetration depth. A similar result was found in this work programme, if the Cu101 sample was processed in air rather than shielded with argon. In air, a peak power of only 8kW was required to initiate keyholing of the workpiece. Normally, copper oxide forms on the surface of copper in atmosphere at 200 to 300ºC, however the reaction rate is very slow. It is anticipated that in ambient atmosphere as the laser heats the material surface it enhances the rate of oxidation. Since copper oxide is very absorptive in the near infra-red wavelength and hence the rest of the pulse is being absorbed much more, resulting in melting and keyhole generation depending on peak power. Table 15 Heat penetration trials on 1mm thickness Cu101 coupons, as bead on plate, with a 500µm diameter laser focal spot.
Sample number
Energy (J) Pulse Width
(ms)
Rep Rate (Hz)
Peak power (kW)
Travel Speed
(mm/min)
Penetration depth (µm)
5c 30 5 3 6.0 400 2
5b 40 5 3 8.0 400 26
5a 30 3 2 10.0 400 124
7c 13.5 1 3 15.0 400 253
7b 11.8 1 3 13.0 400 202
7a 10.5 1 3 11.5 400 120
6a† 30 3 2 10.0 400 653
6b† 40 5 3 8.0 400 201
6c† 30 5 3 6.0 400 9
†Performed in air
The effect of material heating is also evident in sample 13c (Figure 7) where pulses of 9.8kW peak power and 5J energy in argon are slowly heating the material and achieving a melt only after four successive pulse applications in the sequence presented.
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Figure 5 Sample 5, indicating initiation of molten pool on Cu101 in argon, as bead on plate spot welds with a 500µm laser focal spot diameter. The welding direction was from the right to the left.
Figure 6 Sample 7, indicating initiation of keyhole on Cu101 in argon, as bead on plate spot welds with a 500µm laser focal spot diameter. The welding direction was from the right to the left.
Figure 7 Sample 6, indicating initiation of keyhole on Cu101 in atmosphere, as bead on plate spot welds with a 500µm laser focal spot diameter. The welding direction was from the right to the left.
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Table 16 lists the power density thresholds (kW/mm2) that were found in this work programme
to result in melting or keyhole generation of 1mm thickness Cu101. The melting transition was evident by consistent generation of round deformations on the surface of the material. The keyhole transition was evident by the intense localised flash generated by the process (observations were made live on the coaxial view camera) and the cross-sections taken. Theoretically the thresholds are also a function of pulse energy. Practically however, for a pulsed welding system, the thresholds apply for pulse lengths of 500µs minimum duration. Longer pulses up to 20ms may heat up the sample or increase the affected depth, but are not anticipated to shift the thresholds.
Table 16 Process thresholds in kW/mm
2 peak intensity for Cu101.
Melting threshold Keyhole threshold
Copper in air 30.6 50.9
Copper in Argon 58.6 76.4
Following the determination of power density thresholds required to initiate melting and keyholing, single spot welds were produced using an initial pulse sector of either 0.5 or 1ms duration and sufficient power density to initiate keyholing. The trailing sector of the pulse was varied in order to determine its effect on the resultant spot weld. As above, the laser nominal spot size was 500µm in diameter for all trials. Table 17 details selected results achieved with an initial sector of the pulse of 12.5kW peak power (1ms initial pulse sector), whilst varying the pulse width of the trailing sector between 1.5 and 9ms. The power of the trailing sector was 20% that of the initial sector of the pulse (ie 500W). As detailed in Table 17, the diameters of the weld bead (measured at the weld face) did not change as a result of the varying pulse width of the trailing sector and corresponding increase in pulse width. Similarly, the weld penetration depth varied randomly between 340 and 415µm, without showing an ascending or descending pattern. Table 17 Gradual increase of pulse energy below the melting or keyhole threshold with a molten pool initiation sector of 1ms at 12.5kW and tail at 20% of the initiator’s peak power.
Sample number
Energy (J)
Pulse Tail Width (ms)
Rep Rate (Hz)
Peak power (kW)
Bead diameter (µm)
Penetration depth (µm)
20a 18.8 1.5 4 12.5 727 396
20b 25.05 4 4 12.5 814 413
20c 37.5 9 4 12.5 775 341
Table 18 details selected results achieved with an initial sector of the pulse of 15kW peak power (0.5ms initial pulse sector), whilst varying the pulse width of the trailing sector between 1.5 and 9ms. The power of the trailing sector was 20% that of the initial sector of the pulse (ie 3kW). As previously, the weld width and depth appeared to be governed by the interaction of the initial sector of the pulse with the material. Table 18 Gradual increase of pulse energy below the melting or keyhole threshold with a molten pool initiation sector of 0.5ms at 15kW and tail at 20% of the initiator’s peak power.
Sample number
Energy (J)
Pulse Tail Width (ms)
Rep Rate (Hz)
Peak power (kW)
Bead diameter (µm)
Penetration depth (µm)
21d 14.27 1.5 4 15.0 710 196
21b 21.2 4 4 15.0 633 190
21c 37.1 9 4 15.0 626 105
Table 19 details selected results achieved with an initial sector of the pulse of 12.5kW peak power (1ms initial pulse sector), whilst varying the pulse width of the trailing sector between 1.5 and 9ms. The power of the trailing sector was 50% that of the initial sector of the pulse (ie
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2.5kW). As previously, the weld width and depth appeared to be governed by the interaction of the initial sector of the pulse with the material. Table 19 Gradual increase of pulse energy above the melting threshold and beneath the keyhole initiation threshold, with a molten pool initiation sector of 1ms at 12.5 and tail at 50% of the initiator’s peak power.
Sample number
Energy (J)
Pulse Tail Width (ms)
Rep Rate (Hz)
Peak power (kW)
Bead diameter (µm)
Penetration depth (µm)
22a 24.2 1.5 4 12.5 699 310
22b 39.8 4 4 12.5 735 241
22c 42 9 4 12.5 750 353
The small variations in penetration depth and weld bead diameter observed in Tables 17, 18 and 19 are likely related to surface reflectance of the material during the interaction with the initial sector of the pulse. Despite the wide range of trailing pulses tested in AA3003, crack-free welds could not be produced.
4.3 Production of welds in Cu101
Based on the results presented above in Section 5.2, experimental trials were performed to produce lap welds between 1mm thickness Cu101 coupons. This is the most demanding Cu101-Cu101 joint required for the electric battery application, given the necessary depth of penetration. The following parameters were found to be the most suitable for producing lap welds of suitable penetration between 1mm thickness Cu101 coupons:
Nominal spot size of 500µm diameter.
Repetition rate of 13Hz.
Pulse energy of 31J.
Average power of ~400W.
Pulse width of 3ms. The shape of the laser pulse used is detailed in Figure 8. The power ramp down sectors (from 2.0 to 2.7ms, and from 2.7 to 3.0ms) were considered necessary as a single square sector of 2ms at 13kW generated a weld bead with significant underfill and inconsistency. Such a pulsing regime (in terms of power density and pulse duration) is very similar to the drilling regimes utilised with these lasers.
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Figure 8 The optimised pulse shape found in this programme of work for welding Cu101 with a LASAG FLS 542 laser source. The process parameters described above were used to produce welds of 25mm
2 on the top
surface, in an elapsed time of five seconds. It should be noted that the surface area of the joint at the weld interface was 11mm
2. An image of a typical weld face produced with these
process parameters is detailed in Figure 9. Radiographic examination of the welds indicated that none of them met the BS EN ISO 13919-2 Class B, C or D criteria in terms of porosity. The welds also fail in terms of the moderate limit of continuous undercut (as defined in EN ISO 6520-1) presenting undercuts larger than 30% of the layer thickness and repeatability of undercut features in intervals less than five times the width of the weld.
Figure 9 Dual line joint on copper to copper lap joints through 1mm thickness sheet at a speed of 240 mm/min. It should be noted that numerous attempts were made to produce lap welds between the Cu101 coupons using different pulse shapes to improve the weld bead quality:
Attempts were made at pulse energies of 31J but with a lower peak power (6 to 9kW) than that used to produce the weld in Figure 9. No weld joining the two Cu101 components was achieved under these conditions.
In order to increase the welding speed further, the energy per pulse was lowered to 20J, with the peak power subsequently being lowered to 10kW. However, the penetration depth produced with such a pulse shape was insufficient to weld the Cu101 coupons.
Even if the pulse could be shaped such that it produced a weld bead of acceptable quality, the productivity of such a pulsed system would still be below VW’s requirements. As stated in deliverable D1.2, the time available to perform each 24 welds (with an area of 25mm
2 at the
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weld face) is 21 seconds, whereas the time taken to produce the weld in Figure 9 was 5 seconds. In an effort to increase productivity, the pulse width of the successful pulse regime (ie as depicted in Figure 9) was then compressed to half its duration, i.e. 1.5ms in total, with a 1ms sector at 15kW peak power. A total pulse energy of 18J was delivered at pulse repetition rate of 20Hz. This enabled a process speed of 480mm/m to be used at the average power limits of the system. However, this regime has only produced welds with very small occasional penetration in the second copper layer, and a very weak joint. Hence the pulse energy of 30J or over is considered necessary in the near infra-red wavelength in order to create lap welds between 1mm thickness Cu101 coupons. It should be remembered that the LASAG FLS 542 system has a relatively high average power combined with very high peak powers in comparison with other commercial pulse lasers, thus enabling copper welding at relatively high speeds. To the author’s awareness, there is only one other pulsed laser source of higher average power than the FLS 542. This is the LASAG FLS 1042, which has a maximum average power of 800W; ie twice that of the FLS 542. A linear extrapolation of the results in this work programme, indicate that such a laser would be capable of doubling the productivity achieved with the LASAG FLS 542, although such a productivity would still be considerably below VW’s productivity requirements.
4.4 Production of butt welds between AA3003 and Cu101
Laser welding of dissimilar metallic materials presents several potential problems, specifically:
The materials are likely to have different thermo-physical properties (ie melting temperature, vaporisation temperature and thermal conductivity, specific heat capacity etc).
The combination of the different metallic materials may lead to the formation of intermetallic phases, with mechanical properties significantly lower than the parent materials being welded.
The difference in the incident laser beam absorption coefficients of the two materials. Figure 10 shows the Cu-Al phase diagram for which Cu-rich phases are highly susceptible to intermetallic/brittle phase formation.
Figure 10 Cu-Al phase diagram (Hanser and Anderko, 1958).
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In this section of work, butt welds were produced between 0.5mm thickness AA3003 and 0.5mm thickness Cu101. Numerous welds were produced with the parameters varied within the ranges specified in Table 9. Figure 11 shows images of weld beads produced with two different sets of process parameters, both with the centre of the focused laser beam positioned 300µm towards the Cu101 coupon:
The weld shown in Figure 11a was produced with a pulse energy of 30J, a peak power of 12kW, a pulse duration of 2.5ms, and a repetition rate of 2Hz.
The weld shown in Figure 12b was produced with a pulse energy of 30J, a peak power of 5kW, a pulse duration of 5ms, and a repetition rate of2Hz.
As a first approach, optical microscopy of the weld beads was used to examine the degree of material mixing. It can be seen in Figure 11, that the molten pool appears to have dragged material from either side and has achieved a non-homogeneous mix as a result of convection forces in the weld pool and the dynamics of the keyhole. The chemical composition of the bead, as initially interpreted from its colour, in each of the overlapping pulses seems to be varying, moving from the Cu101 to the AA3003 side. A yellowish tint is present in both images, attached to the Cu101 side. At a certain distance from the centre of the weld towards the edges of the bead, the material colour acquires the characteristic copper colour, while on the other side, the silver-like aluminium colour is dominant from more or less the centre of the joint to the unaffected material.
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a)
b) Figure 11 Butt welds between AA3003 and Cu101 with overlapping pulses, displaced by 300µm from the joint towards the copper coupon. a) Produced with a pulse energy of 30J, a peak power of 12kW, a pulse duration of 2.5ms,
and a repetition rate of 2Hz. b) Produced with a pulse energy of 30J, a peak power of 5kW, a pulse duration of 5ms, and a
repetition rate of2Hz. Figure 12 is a cross-section of a joint between AA3003 and Cu101 produced with the laser beam positioned 147µm towards the Cu101 coupon, and with a pulse energy of 30J, a peak power of 6kW, and a pulse duration of 5ms. Figure 12 demonstrates how the materials intermix through thickness, with the darker material being Al rich and the lighter Cu rich; due to the interaction of the etchant with the two materials. It is evident that convection flow interweaves the two materials, but the microstructure of the weld is far from homogeneous.
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Figure 12 Cross-section of a joint between AA3003 and Cu101 produced with the laser beam positioned 147µm towards the Cu101 coupon, and with a pulse energy of 30J, a peak power of 6kW, and a pulse duration of 5ms. Longitudinal cracks were evident in the overlapping spot welds produced between the AA3003 and the Cu101 coupons, as is evident in Figures 11, 12, 13 and 14.
Figure 13 Butt weld between AA3003 and Cu101 produced with a pulse energy of 30J, a peak power of 12kW, a pulse duration of 2.5ms, a repetition rate of 1Hz and a travel speed of 60mm/min.
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a)
b)
c) Figure 14 Cross-sections of butt welds between AA3003 and Cu101demonstrating through cracks on the copper side, produced with: a) A pulse energy of 30J, a peak power of 6kW, a pulse duration of 5ms, and with the spot positioned 64µm towards the Cu101 coupon. b) A pulse energy of 30J, a peak power of 6kW, a pulse duration of 5ms, and with the spot positioned 102µm towards the AA3003 coupon. c) A pulse energy of 30J, a peak power of 6kW, a pulse duration of 5ms, and with the spot positioned 19µm towards the AA3003 coupon.
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The longitudinal cracks were also with non-overlapping pulses as displayed in Figure 15 below; hence, the cracks do not form because of stresses created in successive pulses that generate a continuous weld bead, but are effects equally present and created within single spot welds.
a)
b) Figure 15 Spot weld between AA3003 and Cu101 produced with a 30J pulse energy, 10ms pulse duration, 3kW peak power, 1 Hz repetition rate and sample travelling at 60mm/min. a) overview, b) transverse cross-section.
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Considering the difference in reflectivity and melting point of copper and aluminium, shifting the beam from the copper side to the aluminium side of a copper to aluminium butt weld is expected to affect the effectiveness of the process by concentrating the power input more to one or the other material. Laser pulses were thus been applied in sequence on copper to aluminium interfaces at different displacement from the joint line. A selection of these results were detailed above, and some further selected results are detailed below. As the spot was moved over to the copper side, at around 200µm displacement, cracks appear in the centre of the weld as highlighted in Figure 16.
Figure 16 Butt weld between AA3003 and Cu101 produced with 12kW peak power, 2.5ms pulse duration, 30J pulse energy and the laser beam focused 230µm on to the Cu101 side of the joint. By moving the beam further, towards the copper at around 300µm materials starts mixing with greater homogeneity as it is evident by the colour of the surface image and the cross-section in Figures 17a and b respectively. The welds are also reproduced consistently. However, the cracks now form on the aluminium rich side as shown in both images. Considering that a displacement like this may offer optimum conditions for copper to aluminium pulsed laser welding, this effect is likely to occur from the susceptibility of AA3003 to cracking when welded with a pulsed laser. Experiments with pure aluminium and copper should clarify this observation. It was found that the best results for welding 0.5mm thickness coupons of AA3003 and Cu101 with a near infra-red laser beam, were achieved by displacing the beam towards the copper at distances between 150 to 300µm. The welds were achieved using a peak power of 6kW to 12kW, linearly varying in proportion to the displacement distance between the two mentioned limits. Even in this regime, cracks stretching to at least 40% of the sheet thickness were observed in all cases. The cracks appear either on the Cu101 side when the laser beam is closer to the joint-line and towards the AA3003 side when further into the Cu101 side. As a result, the welds produced between AA3003 and Cu101 in this investigation did not meet the weld quality criteria of Class B, C or D of BS EN ISO 13919-2.
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a)
b) Figure 17 Butt weld between AA3003 and Cu101 produced with 12kW peak power, 2.5ms duration, 30J pulse energy and the laser beam focussed 313µm on the side of the Cu101 coupon. a) Surface view microscopy image b) Microscopy image of section of the same weld; the dark material is aluminium rich, the crack is a thin line next to it extending down by 60% of the material width. In contrast to the Cu101 – Cu101 welds made in this work, the Cu101 to AA3003 butt welds made with the centre of the laser beam focus positioned ~300µm towards the Cu101 coupon showed relatively low levels of subsurface porosity. The 10mm weld depicted in Figure 18 shows no signs of pores large enough for the resolution of the imaging device.
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Figure 18 Radiography image of a butt weld between AA3003 and Cu101 produced with 12kW peak power, 2.5ms duration, 30J pulse energy and the laser beam focussed 313µm on the side of the Cu101 coupon.
5 Conclusions
Work using a LASAGL FLS 542 pulsed laser source has allowed the following conclusions to be drawn within the scope of work performed: Experiments carried out on AA3003:
Power densities of 7.6 and 20.4 kW/mm2 were required to achieve melting and keyholing in
1mm thickness AA3003 coupons respectively.
Penetration depths up to ~650µm were achieved, and this was found to be dependent upon the initial peak power of the laser pulse.
Solidification crack-free butt welds could not be produced in AA3003 with the pulsed laser source used in this investigation, ie the welds did not meet BS EN ISO 13919-2 Class B, C or D.
Experiments carried out on Cu101:
Power densities of 58.6 and 76.4 kW/mm2 were required to achieve melting and keyholing
in 1mm thickness Cu101coupons respectively.
Penetration depths enabling joining of 1mm thickness Cu101 coupons in the lap-joint configuration were achieved, by tailoring the pulse.
The quality of welds produced with a quasi-optimised laser pulse did not meet BS EN ISO 13919-2 Class B, C or D.
The productivity of the laser when pulsed welding 1mm thickness Cu101 was found to be significantly below VW’s requirements.
Experiments carried out on AA3003-Cu101 dissimilar joints:
Full penetration butt welds could be produced with a wide range of laser parameters.
The mixing of the weld metal was found to be significantly influenced by the offset of the laser beam with respect to the joint line.
Crack-free welds could not be produced with any of the laser parameter combinations used, ie the welds did not meet BS EN ISO 13919-2 Class B, C or D.
6 Recommendations and way forward
Based on the results presented here, and those detailed in D3.1, it is evident that laser welding of Cu101 and AA3003 of thicknesses suitable for VW’s battery application is not achievable with a pulsed Nd:YAG laser source; in terms of both the resultant weld quality and the productivity of the welding process.
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For welding of Cu101, it is likely that the quality of the welds produced could be increased by utilising a mixed wavelength laser welding system. Such a system would enable lower initial peak powers to be used, thereby enabling reduced peak powers to be initially used. It is thought that these reduced peak powers will allow more control of the welding process, as the initial sector of the pulse would not be as deleterious on the resulting weld quality. Nevertheless, the average power of the GreenMix system being developed in this project is 50W (ten times less than the FLS 542 used in this project). Therefore, even if an acceptable weld quality may be reached for 0.5 to 1.0mm thickness Cu101, the productivity would be well below VW’s requirements (as per deliverable D1.2). For welding of AA3003, it was not possible to overcome solidification cracking in the weld metal by tailoring the pulse. Given the limited heat input of the GreenMix system being developed in this project, it is strongly thought that cracking in this material will not be avoided as the mechanism is inherent to pulsed welding and the chemical composition of the workpiece. For welding AA3003 to Cu101 dissimilar joints, it was also not possible to overcome cracking in the weld metal when welding with the FLS 542 system. From other work performed by TWI (not within the scope of the QCOALA project), TWI is confident that the problems detailed above could be addressed using a suitably focussed and manipulated continuous-wave 1µm wavelength laser beam. It is therefore recommended, that the remainder of the project concerning >0.5mm thickness AA3003 or Cu101 is centred around the use of commercially available 1µm wavelength laser source. However, the use of a GreenMix laser system for penetration depths between 0.1 and 0.5mm may still hold some advantages, and experimental work should continue to establish the resultant weld quality and productivity possible in this thickness range.
7 References
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 1-85233-698-6.
