PROJECT DELIVERABLE REPORT - QCoala Project Performance of 1064nm.pdf · WP3/Deliverable Number 3.1...

PDB/gdp/087DR.11 September 2011

PDB/gdp/087DR.11 WP3/Deliverable Number 3.1 10 November 2011

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PROJECT DELIVERABLE 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

Date of latest version of Annex I against which the assessment will be made:

18 May 2010

Deliverable Number and Title: D 3.1: Performance of 1064 nm Wavelength Lasers when Welding Aluminium and Copper Joints

Name, title and organisation of the scientific representative of the project's coordinator

1:

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)1223 892588 W: www.twi.co.uk

Tel: +44 (0)1223 899530 Direct

E-mail: [email protected]

Project website2 address: www.qcoala.eu

QCOALA Project Document Reference: PDB/gdp/080DR.11

Author(s): Thierry Gachon / SAFEL Paola de Bono / TWI Uwe Hildebrandt / TWI Kerstin Kowalick / RUB

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.

http://www.twi.co.uk/

mailto:[email protected]

http://www.qcoala.eu/

http://europa.eu/abc/symbols/emblem/index_en.htm

http://ec.europa.eu/research/fp7/index_en.cfm?pg=logos



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Performance of 1064nm Wavelength Lasers when Welding Aluminium (Al) and

Copper (Cu) Joints (D 3.1)

1 Introduction

The objectives of the QCOALA project comprise the development of a new laser processing system for the welding of thin-gauge Al and Cu, 0.1-1.0mm in thickness, to provide a reliable, high-speed, low-cost and high-quality joining solution for electric car battery and thin-film photovoltaic (PV) cell interconnections. As shown in the Laser Technology Assessment Report (Deliverable D2.1), there is significant level of general knowledge about laser welding of Al and Cu in the public domain. However, the interaction of the laser beam and its parameters on Cu and Al surfaces and its consequential influence on the welding results is not fully understood, particularly on thin materials. This information is required to establish the primary laser welding parameters and tolerance levels for subsequent dual beam trials and to assist in the generation of a tailored energy strategy to allow the development of the temporal pulse capability of the QCOALA laser platform (WP2). To develop this information, a detailed welding performance study for 1064nm wavelength infra-red lasers was carried out for the welding of Cu and Al alloy. Work Package 3 “Intelligent laser welding” includes two different laser applications: thin film PV cell interconnections and electric car battery welding. The former requires the welding of Al or Cu ribbon (interconnects) to an electrically conductive thin film layer on the solar cell. In the latter application, the joining of Cu to Cu, Al to Al and Cu to Al with material thicknesses up to 1mm is in the focus of the work. These two applications are being developed in parallel, therefore, this report on Task 3.1 “Performance of 1064 nm Wavelength Lasers when Welding Al and Cu Joints” is composed of two parts:

Thin film PV cell laser welding

Electric car battery laser welding

2 Thin Film PV Cell Laser Welding

Work conducted by RUB Author: Kerstin Kowalick,

2.1 Introduction

Main interest of both the consortium solar end-users centers the welding of Al or Cu ribbon (interconnects) to an electrically conductive thin film layer on the solar cell or to a second ribbon. Electrically conductive thin film layer as well as ribbons can differ in their raw material but also in alloying. Since physical processes and process limits for the different materials and layer thicknesses involved have not been examined sufficiently in the past, it was considered essential for the first period of the project to investigate the weldability of mainly Al foils of approximately 100µm in thickness. 2.2 Materials

Al material used for electronic industry (e.g. EN AW1350) has commonly a purity of 99.5% or slightly above, with well adapted additions. Because this Al alloy is difficult to purchase at a thickness of 100µm, trials were started on EN AW1050 Al alloy, which has the same purity but a slightly different chemical composition.



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2.3 Laser system

The following laser system was used on these initial trials: Laser Lasag SLS200CL16

Wavelength 1064nm

Fiber 400µm

Beam Profile Top hat

Spotsize 400µm

Processing gas Argon

Pulse Length 0,3-100ms with Pulse shaping

Mean Power 50W

Angle of Incidence 10°-15° For Al the absorption behavior is similar for the visible and near infrared range. Its highest absorption is at about 830nm. In contrast to Cu the reflectivity decreases by approximately 4% using a frequency doubled Nd:YAG (532nm) instead of a normal Nd:YAG (1064) Laser. Whether or not this difference in absorptivity will have an observable impact on the process will be the subject of upcoming investigations. 2.4 Welding trials

The main reason for difficulties with Al spot-welding result from the high reflectivity of Al in the visible and infrared range and its high heat conductance. Figure 1 shows the instability of the laser process using rectangular shaped pulses.

Figure 1 Lap-welding of 100µm foils with rectangular shaped pulses Practically identical process parameters lead to different results, for example:

Metallurgically-bonded connections

Form closer

Holes An overview of potential influence factors for this process instability on thin film welding is outlined in Figure 2.



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Figure 2 Potential factors reducing process stability The following parameters have been evaluated in terms of their influence on the process itself as well as the process stability:

Gas atmosphere

Cleanliness

Focus position

Groove-direction

Heat Congestion

Gap between foils It can be summarized that none of the factors seem to have an explicit influence on the process stability. Consequently, the influence of the individual factors on the process itself cannot definitely be determined. However, cleanliness and groove direction show tendencies toward different absorption behavior. For comparison, only the starting period (coupling in) of a welding process was addressed. High peak power (up to 2kW) and pulse length (about 600µs) were used. Al increases its absorption properties with raising temperature. It is already known from the macro welding that pulse shaping can have a positive influence on the welding behavior and the controllability of the process. The welding process is subdivided into the following: 1. Pre-heating – to rise absorptivity 2. Power boost – breaking oxide layer 3. Welding – determines pool size 4. Cool down – determines grain structure

It can be seen from Figure 3 that pulse shaping can influence the melting zone and melt behavior on thicker 1mm sheet material.

Figure 3 Bead on plate weld of 1mm sheets: left, without pulse-shaping, right, with pulse-shaping For welding thin Al foils, no significant changes in welding behavior could be determined to-date, but investigations on pulse shaping will be a key activity for the upcoming period. At this point it can be



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seen from Figure 4, which shows the top view of three welds produced using the same parameters, that even with pulse shaping, no stable processing behavior were obtained.

Figure 4 Overlap welding of 100µm foils with pulse shaping and same parameters All the trials conducted showed that due to high conductivity and reflectivity of Al, the processing window becomes too small to be reached constantly. Since for Al there are very limited possibilities to increase the absorptivity, especially in an automated process, the main focus of upcoming work will be on dealing with the high conductivity. All the trials have been done with a tilted sample holder and a processing head perpendicular to the optical table. Most samples with a smooth weld puddle to edge transition show an almost constant angle between weld puddle and base material (Figure 5). Since this angle is in size similar to the tilted sample holder, a correlation can be assumed. For that reason the set up will be rearranged for upcoming trials.

Figure 5 Surface geometry of an overlap weld measured with white-light interferometer

As described previously, thermal conductivity of the base material is one of the main factors influencing the process. The dissipated heat diffuses quickly into the surrounding material and it is unclear how differences in heat conduction arise and impact the process result. The large differences which the conductivity observations revealed, may indicate differences in thermal influence. Observation with high spatial and temporal resolution can be conducted with specialized equipment. A test set up demonstrating a spatial resolution of <25µm has been realized and includes a custom-designed lens system to magnify the image and increase the resolution and a thermal camera (Figure 6).



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Figure 6 Lateral process monitoring system with infrared camera: top, schematic of setup, bottom, real setup The camera records wavelenghts in the range from 3-5µm. Figure 7 shows an image of a heated wire taken with the monitoring system. In order to increase temporal resolution a suitable high-speed thermal camera is needed. Such cameras are commercially available at high prices but not currently available to the project partner.

Figure 7 Image of a heated wire taken with infrared monitoring system

3 Electric Car Battery Laser Welding

Work conducted by: TWI (Laser) and SAFEL (Electron Beam) Authors: Uwe Hildebrandt / TWI

300µm



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Paola De Bono/ TWI Thierry Gachon / SAFEL

3.1 Introduction

The laser welding trials for the battery terminations were focussed on 0.1-1mm thick Cu and Al sheets. Key aspects of this work were:

Establishing the feasibility of laser welding 0.1-1.0mm thick Al 3003 and Cu 101 in terms of mono-metallic and dissimilar material joints.

Establishing laser parameters required to produce welds in 0.1-1.0mm thick Al 3003 and Cu 101, including the influence of continuous and pulsed laser systems.

To assess quality of welds when produced without the reflectivity and oxidisation issues of current infra-red laser systems. These trials were conducted using electron beam (EB) technology.

The performance of Al-Cu joints is influenced by their respective compositions. The resultant crystal structures and often complex intermetallic phases in the weld will have a significant impact on the hardness and toughness of the joint. The influence of the laser parameters and welding geometries on the formation of these structures was also studied in these trials. To achieve the above information, welding trials were conducted on a range of laser systems from low power (125W average power) pulsed to high power (8kW average power) continuous wave. Additional work was conducted on a 3kW electron beam machine. 3.2 Materials

Al alloy and Cu materials were employed for car battery application trials. Specifications of the materials used are reported in Table1: Table1 Materials used on the battery termination welding trials

Material Cu Al

Type/Alloy Cu 101 Al 3003

Composition 99.99 % Cu min Al Balance

5ppm oxygen max Cu 0.05 - 0.2

Iron 0.7 max

Manganese 1 - 1.5

Silicon 0.6 max

Zinc 0.1 max

Physical Specification:

Density 8.94 x 103

Kg/m3

2.73 x 103

Kg/m3

Melting point: 1084C 655C

Boiling point: 2562C 2467C

Specific heat: 385 J/Kg K 896 J/Kg K

Thermal conductivity (RT): 401 W/Km 198 W/Km

3.3 Weld joint configuration

The weld geometries (Figure 8) and the material thicknesses (0.5-1.0mm) were selected following discussion with the automotive end user (VW).

Joint Configuration

Material Combinations

A B



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Lap-Stake Weld

Cu Al

Al Cu

Cu Cu

Al Al

Lap-Fillet Weld

Cu

Al

Butt Weld

Cu

Al

Figure 8 Laser welding joint configurations

The laser welding trials were conducted on three material combinations:

Laser welding Cu to Cu

Laser welding Al to Al

Laser welding Cu to Al

The electron beam welding trials were conducted on Cu to Al joints.

A

Laser Beam

B

A

Laser Beam

B

A

Laser Beam

B



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The equipment, joint configurations and results obtained are detailed below.

3.4.1 Laser welding Cu 101 to Cu 101

The Cu welding trials were conducted on an infra-red Nd:YAG pulsed laser and continuous wave disk laser:

Pulsed: GSI JK125, pulsed 125W Nd:YAG Crystal Rod laser

Continuous Wave: Trumpf TruDisk 8002, 8kW disk-laser. The pulsed crystal rod laser was chosen as a typical commercial Nd:YAG laser system that is used for micro-welding. The second (disk laser) is a less widely used laser that has high power and higher beam quality. 3.4.1.1 Welding Cu to Cu with a 125W Nd:YAG pulsed laser

The welding tests were performed on a GSI JK125 pulsed laser system (Figure 9). Specifications of the laser system are: Laser GSI JK125

Wavelength 1064nm

Average power 125W

Delivery fibre diameter 0.15mm

Collimating lens focal length 160mm

Focusing lens focal length 160mm

Beam diameter at focus 0.2mm

Figure 9 125W Nd:YAG pulsed laser system A compressed air cross-jet was used to protect the cover slide and lens from any contamination and damage from welding fume and spatter. The work piece was placed in a clamping jig and manipulated by an integrated 2-axis stage. This Anorad X-Y table system was composed of WRL series linear motor stages with a stated positional accuracy of ±9μm and a repeatability accuracy of ±1.5μm.



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Materials Trials were conducted on 0.1mm and 1.0mm Cu foils. Sample pre-treatment To establish the influence of surface reflectivity and thermal control factors, surface pre-treatment and pre-heating approaches were investigated, as follows:

Samples were processed as purchased without any modification of the top surface.

The top surface of the samples was subjected to abrasion and then coated with a black ink

The top surface of the samples was subjected to abrasion prior to coated with a black ink. In addition, the samples were pre-heated to approximately 100ºC prior to welding.

Welding Trials Welding trials were carried out using 1mm thick plate and 0.1mm foil Cu work pieces. The welding runs were performed over a range of laser parameter combinations and processing conditions. Foil-to-plate and foil-to-foil lap and spot welding trials were carried out using the treated and untreated work pieces. An overview of processing conditions used is summarised in Table 2. Table 2 Range of processing conditions used on the welding trials

Laser parameters Range value

Average output power (J) 50-120

Repetition rate (Hz) 10-15

Pulse energy (J) 2-12

Pulse duration (msec) 2-6

Travel speed (mm/sec) 2-4

Focus position At focus

The results of the trials identified the following key points: (1) Melt runs carried out on the untreated Cu plates showed that the laser beam at 1064nm was, in

the majority of cases, reflected off the surface of the Cu work piece and in most cases no melting of the surface occurred.

(2) Abrading the surface and coating it with black ink improved the optical absorption and enabled

weld penetration, with the depth increasing the average power of the beam (Figure 10).

Figure 10 Weld penetration depths vs. average power in 1mm thick plate. (3) The combined effect of a surface pre-treatment with pre-heating at approximately 100°C gave the

best quality welds among the range of processing conditions investigated (Figure 11).

1mm copper plate

0

20

40

60

80

100

120

140

40 60 80 100 120

Average power (W)

Pen

etr

ati

on

dep

th (

mic

ron

)



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Figure 11 Spot weld between two 0.1mm thick Cu foils, specimen subjected to surface and pre-heating treatment (65W average power, 5msec pulse duration, 10Hz repetition rate). In this case, the presence of the black ink on the top surface acted as coupling agent and increased the absorption of the work piece, while the pre-heating treatment improved the control of the laser energy introduced into the specimen. Figure 12 shows a comparison of two samples from batches 2 and 3, clear improvements in results can be observed.

(a) Weld from Batch 2 (b) Weld from batch 3 Figure 12 Comparison between two welds from Batch 2 and Batch 3. (4) Cu is very sensitive to small adjustments of processing conditions and work piece/beam

interactions

3.4.1.2 Welding Cu to Cu with continuous wave Nd:YAG disk laser

An opportunity arose to obtain time on a continuous wave Trumpf TruDisk 8002 continuous wave Disk-laser. These high average power (multi-kW range) lasers have relatively high beam quality. A limited range of trials were conducted with the aim of establishing the impact of using a substantially higher power laser on weld performance and potential speed capabilities, when used on Cu. These data could then be used in combination with the information generated with low power lasers to establish an envelope of potential process speed. Specifications of the laser system were: Laser: Average power: 8kW Wavelength: 1064 nm Beam quality: 8 mm x mrad



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Optics: Beam delivery: 200μm diameter single step fibre Collimating lens: 200mm Focussing lens: 200mm

Ratio Focus/LLK: 1

Shielding gas: N

2, trailing orientation

Surfaces of all samples were milled before the welding trials to ensure reproducible surface conditions. The material thickness was 4mm. Figure 13 shows a cross-section of this type of weld, penetration ~0.65mm. The seam is very homogeneous without cracks or cavities, but with occasional spatter on the surface.

Figure 13 Cross section of a CW seam weld in Cu. Laser power 6 kW, v = 9m/min Figure 14 shows the relationship between weld penetration and speed. It can be seen that a weld penetration of less than 1mm can be achieved with feed rates more than 6m/min when using a laser power of 8 kW and spot size 200μm.

Figure 14 Penetration vs. feed rate, material thickness 4mm laser power 8 kW, spot size 200μm

3.4.2 Welding Al 3003 to Al 3003 with a 220W pulsed Nd:YAG laser

The work conducted on Al to Al joints was conducted on a LASAG SLS 200 CL60 pulsed laser system. The laser set up of experimental trials is shown in Figure 15.

0

0.5

1

1.5

2

2.5

3

0 2 4 6 8 10

welding speed [m/min]

weld

ing

pen

etr

ati

on

[m

m]



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Figure 15 220W Nd:YAG pulsed laser system The welding geometries were Butt, and Lap joints (stake and fillet). Specifications of the laser are: Laser parameters: Average power: 220 W Wavelength: 1064nm Beam quality: 8mm x mrad Peak power Pp: 8 kW

Pulse duration tp: 0.1 - 200ms

Max. pulse energy Ep: 110J

Optics: Beam delivery: 400μm diameter fibre Collimating lens: 200mm Focussing lens: 100mm

Ratio Focus/LLK: 0.5

Shielding gas: N2, trailing orientation

The samples/surfaces were not pre-treated prior to welding. Using the above laser system, the results of the initial trials on Al showed a variation in performance between the three joint types. Butt welds had no visible major cracks when welding 0.5mm thick plate (Figure16), however, with the investigated parameters cracking occurred in the cases of lap-stake (Figure 17) and lap-fillet joints (Figure 18). These cracks are probably caused by the thermal boundary conditions during cooling and solidification of the molten pool. Further work is required to avoid these cracks by the use of tailored temporal pulse shaping to get slower cooling and solidification without stress induced by high thermal gradients. Additionally, reduction in restraint through the design of the joint will be considered.



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Figure 16 Butt weld in 0.5mm thick Al 3003 Pulse power: 1.5kW Pulse duration: 5msec Pulse energy: 5.27J

Figure 17 Lap-stake weld in 0.3mm thick Al 3003 Pulse power: 3kW Pulse duration: 5msec Pulse energy: 10.27J

Figure 18 Lap-fillet weld in 1.2mm thick Al3003 Pulse power: 2.5kW Pulse duration: 10msec Pulse energy: 11.92J

3.4.3 Laser welding of Cu 101 to Al 3003



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For the initial welding trials on Cu to Al joints, two geometries were selected: lap-stake and butt welding. The material thicknesses were between 0.3 and 1.2mm. Trials were conducted on both pulsed and continuous wave laser systems. The laser systems were: Pulsed systems: Laser Spot size Peak power LASAG SLS 200 200, 400µm 3.0 kW LASAG FLS 1042 300µm 4.0 kW Continuous wave system: IPG YLR 1000 30µm 1000W

3.4.3.1 Laser lap-stake welding of Al 3003 on top of Cu 101

Welding with pulsed laser systems One self-evident fact of lap-stake welding is that the upper material is melted first. This means, that the heat transfer to the lower material is done either by heat conduction (at low laser intensity) from the top surface or directly by the keyhole (higher laser intensity). The laser beam was pointed on the top surface of the Al specimen to create the joint with the underlying Cu plate. Initial trials highlighted some interesting issues/problems when welding with the Al as the top sheet. It was possible to produce joints with a lack of welding to the Cu-surface even though the Al was fully melted down to the interface. 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. When the energy density was further increased there was sufficient heat in the Cu to just start melting the underlying Cu plate and form a small crack-free weld (Figure 19).

Figure 19 Lap-stake weld of Al (0.3mm) on Cu (0.5mm) Spot size: 300μm Pulse power = 3.0 kW Pulse duration: 5 msec Higher energy densities produced full penetration keyhole welds (Figure 20). Cracking was observed on the majority of these full penetration welds. These phenomena appear to be related to the degree of mixing of the materials and the intensity of the laser beam at the surface of joint.



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When the laser beam was pointed on the top surface of the Al specimen (to create the joint with the underlying Cu sample) the mixing of the materials was limited, a Cu-rich phase remained near the Cu surface, the Cu-content in the Al-rich phase was small and at the seam near the Cu boundary a significant level of intermetallic phases were formed, which can result in cracks and small cavities.

Figure 20 Lap-stake weld of Al (0.5mm) on Cu (0.5mm) Spot size 400μm Pulse power = 3.5 kW Pulse duration = 20 msec

3.4.3.2 Laser butt welding of Al 3003 to Cu 101

When butt welding, both materials are molten at the same time at the start of the weld. Furthermore the volume of one or other of the materials in the weld pool can be adjusted by moving the position of the beam to be preferentially over either the Al or Cu base materials. Therefore, it is possible to change the volume of either the Al or Cu in the melt pool and potentially produce a better mix of the materials. The aim being to reduce or avoid intermetallic phases in the weld. Material: Cu 101: 0.9mm Al3003 1.2mm Laser: IPG YLR 1000 Laser parameter: CW, 1 kW Fibre: single mode Optic: collimation 100mm, focussing 200mm Spot size: 30μm Focus position: At focus on the top surface of the work piece It was found that when butt welding and using a continuous wave laser there was a significant level of material mixing in the joint. The crystallographic sections were larger than found in the lap joints with pulsed mode, but Cu-rich intermetallic phases were still present (Figure 21).



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Figure 21 Butt welding using the CW-laser and 0.1mm beam off-set into the Cu. Travel speed= 12 m/min (laser from below) 3.4.3.3 Summary of laser welding trial on Cu101 to Al3003

Cu and Al-samples with different thickness from 0.3 - 1.2mm were butt and lap welded. The welding tests were performed with CW and pulsed laser systems. None of them produced completely satisfactory results when welding this particular combination of Al and Cu alloys. The primary problem is the formation of intermetallic phases in the Cu-rich sections in the weld pool.

3.4.4 Electron beam welding of Al 3003 and Cu 101

Trials were conducted using an electron beam system to enable joints to be studied that had not been influenced by the reflectivity of surfaces (as occurs with infra-red lasers) and had been conducted in a non-oxidizing atmosphere. This enables a closer examination of the material interactions which can then be utilized in subsequent laser developments. The following joint types were investigated:

Lap-stake welding of Cu on Al

Lap-stake welding of Al on Cu

Butt welding of Al and Cu For the butt welds, in order to change the proportion of Al and Cu in the weld pool, the beam was initially positioned at the joint interface, then for subsequent samples it was moved away from the joint line (offset) increasing the offset on the Cu side or the Al side. The offset was increased by 0.1mm for each run. Because Al and Cu do not mix well together, butt welding was conducted with both a fixed beam and with an oscillating beam; in the former case using an increasing offset on the Cu and on the Al and in the latter case on the Al only. 3.4.4.1 Samples

The dimensions of the samples were: Cu 101: 10x20x0.8mm Al 3003: 10x20x1.2mm The edges were milled before welding, so that there were no bevels.

Al 3003 1.1mm thick

Cu 101 0.9mm thick



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Chemical cleaning of the samples was conducted prior to welding to remove the surface oxide, as below:

Chemical cleaning for Al: MAGNUS 841X, contains phosphoric acid.

Chemical cleaning for Cu: Water, CrO3, H2SO4 3.4.4.2 Electron beam machine

The following small electron beam machine was used for the trials (Figure 22): Vacuum chamber with a capacity of 30 l. Electron gun: FE3000 (SAFMATIC), max power 3kW. The power used for the welding is: Power = HV x Current HV is the high voltage used to accelerate the beam. Unit: kV Current is the measure of the electrons bombarding the samples. Unit: mA The power is given in W. The spot diameter of the fixed beam on Al was ~1.5mm. The spot diameter of the rotating beam on Al was ~1.75mm.

Figure 22 Electron beam system used for the welding trials



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Sample clamping tool A tool made of Al was used to clamp the samples in the vacuum chamber (Figure 23).

Figure 23 Sample clamping table 3.4.4.4 Welding trials

For each joint configuration/material the power was set up to obtain full penetration. The same power was then applied on the samples made with a beam offset.

Cross sections (macrographs) of each welding case were produced to enable an initial inspection of the welds profiles and composition (Figures 24 -27).

Electron Beam Lap Welding of Cu 101 on top of Al 3003

Parameters Power: 1290W HV: 30kV Current: 43mA Foc. Position: Surface Welding speed: 1m/mn Full penetration delay: 5mm Good Weld: large seam, good surface, no visible cracks

Figure 24 Lap-stake joint (Cu on top surface) using electron beam welding - mid feed rate It was noticed that due to the difference of melting temperatures between Al and Cu there was a delay of 5mm before the full penetration.

Electron Beam Lap Welding of Al 3003 on top of Cu 101

Tool

Sample



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Cross section at mid feed rate:

Parameters Power : 1290W HV : 30kV Current : 43mA Foc. Position : Surface Welding speed : 1m/mn No penetration delay Good Weld Large seam in Al

Figure 25 Lap-Stake joint (Al on top surface) using electron beam welding - mid feed rate As observed in the first case (Cu on Al), the melted zone is larger in Al than in Cu. There was no penetration delay. It was noticed that there was a relatively high sensitivity to welding power: only 60W between the two following samples, the backsides are completely different (Figure 26).

Back side at 1290 W Back side at 1350 W

Figure 26 Back surfaces of test samples when electron beam welding

Thin seam

Large seam



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Electron Beam Butt Welding of Al 3003 to Cu 101 The first sample was welded with the beam positioned at the joint interface Cross section at:

Parameters Power : 560 W HV : 30kV Current : 18.7mA Foc. Position : Surface Welding speed : 1m/mn No penetration delay Good Weld Large seam Smooth surface No visible cracks

Figure 27 Electron beam welded Al to Cu butt weld - mid feed rate A good weld was obtained. Electron Beam Butt Welding of Al 3003 to Cu 101 – Using an Offset Beam The following samples were welded with an increasing offset (each 0.1mm) in the Cu, until the melted zone appears to be tacked to Cu (Figure 28 and Figure 29).

Figure 28 Sketch of weld offset

Cu Al

First seam

n x 0.1mm



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Parameters : Power : 585 W HV : 30kV Current : 19.5mA Foc. Position : Surface Welding speed : 1m/mn No penetration delay Max. shift in Cu: 0.3mm With the 0.4mm offset, the power is insufficient.

Figure 29 Electron beam welding with offset The maximum shift is smaller in Cu than in Al.

Electron Beam Butt Welding of Al 3003 to Cu 101 – Using a Rotating Beam and Offset A circular magnetic field was introduced in order to rotate the beam and increase the mix the materials (Figure 30). Figure 30 Offset with rotating beam.

Cu Al

First seam

n x 0.1 mm



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The first sample is welded with the beam positioned at the joint interface (Figure 31). Cross section at mid feed rate:

Parameters : Power : 585 W HV : 30kV Current : 19.5mA Foc. Position : Surface Welding speed : 1m/mn Rotating amplitude : 0.25mm No penetration delay Good weld

Figure 31 Electron beam welding with rotation. Rotating and offset beam butt welding For the following samples (Figure 32), we have shifted the beam on Al every each 0.1mm.

Parameters : Power : 660W HV : 30kV Current : 22 mA Foc. Position : Surface Welding speed : 1m/mn Rotating amplitude : 0.25mm Max. shift in Al: 0.6mm With the 0.7mm shift, the melted zone seems to be glued on Cu.

Figure 32 Electron beam welding with offset and rotation. Because the diameter spot is larger, the maximum shift is more important in Al using a rotating beam.



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3.4.4.5 Conclusion

EB welding allows good welds.

3.5 Discussion: Electric car battery welding

These initial trials on the welding of Cu 101 and Al 3003 were set up to develop a better understanding of the welding process when using lasers with 1064nm wavelengths. To achieve this, a range of different laser types and energies were employed to cover the full range of material sizes and joint configurations potentially required by the battery application. To establish additional data on high energy beam welding, without the impact of surface reflectivity and oxidation, electron beam welds were also investigated. These trials identified a number of key factors that need to be to be taken into accounted when planning the future development of the battery welding application. These include the following: 3.5.1 Cu 101 to Cu 101 welding using 1064nm wavelength lasers

Good Cu to Cu welds can be produced on both thin (eg 0.1mm) and thick (eg1.0mm) materials. In the former case, using a pre-treatment and conduction limited pulsed technique and in the latter, continuous wave and keyhole welding. With respect to these limited initial trials, it was found that the surface reflectivity of Cu and its high thermal conductivity restricted both the tolerance range of the process for each laser type and the quality (eg porosity, spatter, undercut) of the joints produced. Using pulse shaping techniques and enhancing the surface absorption where found to be beneficial in improving the welding process. This is the direction being pursued by the QCOALA project through the development of a dual wavelength pulsed system, to overcome the reflectivity issues and gain greater control over the primary welding pulse. 3.5.2 Al 3003 to Al 3003 welding using 1064nm wavelength lasers

The trials on pulsed laser welding of Al 3003 material resulted in a problem of solidification cracking when welding in the lap stake and fillet joint configurations. It was interesting to note that this cracking was not evident on this scale in the butt joints. This suggests the failures are related to the additional constraint on the latter joint when cooling, increasing the potential of solidification cracking. This cracking is of interest because in conventional arc welding processes, Al 3003 alloy is generally considered to be a weldable material. This suggests that for these more difficult joints, the route forward is to tailor the laser parameters such that they melt and mix the interfaces in a similar way to the arc process, for example: increasing the weld size and cool down times. It was noted in Section 2.6 of deliverable D2.1 that low-alloyed Al alloys of the 3003 type are crack sensitive when pulsed laser welding {See also reference: L.A. Weeter, Proc. ICALEO 1985, p.81}. However, it is considered that using modern pulsed laser technology systems with their capabilities of higher pulse energies, longer pulse durations (50ms) and laser pulse shaping it may be possible to overcome this problem by configuring their output to retain heating on the weld, and protect against fast solidification. Larger spot sizes or a laser beam with a top-hat or an annular beam profile will also help in increasing the molten pool diameter and volume, and generating a thermal profile on the weld with a smoother temperature gradient. Both approaches are within the scope of future work in the QCOALA project. Oscillating the beam to increase the weld pool volume could also be considered. Further work is required to investigate this issue. 3.5.3 Al 3003 to Cu 101 welding using 1064nm wavelength lasers and electron beams

The laser and electron beam trials on Al 3003 to Cu 101 identified a potential problem when welding this combination of materials, namely the generation of an intermetallic phase in the weld pool. In metallurgical terms this is not fully unexpected as shown in the Cu-Al phase diagram of below (Figure 33). These materials can form numerous intermetallic phases.



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. Figure 33 Phase diagram Cu-Al-alloy M. Hauser, K.Anderko, Constitution of binary alloys, McGraw-Hill, 1958 The typical properties of these intermetallic phases are:

complex crystal structure

high hardness combined with high brittleness

low toughness These properties can cause cracks and cavities in the welds, induced by high internal stress during the cooling and solidification phase. It was interesting to see that there was no evidence of this cracking and the cavities in the electron beam welded samples. To enable a direct comparison of the two processes, similar samples butt and lap stake were welded and the resultant joints metallographically sectioned and examined. The welding for each process was set up to achieve full penetration joints. It is important to note that these conditions were not optimised. Welding parameters for the butt joins are shown below and the results in Figures 35-38.

Parameter EB Welding Laser Welding

Spot size 1.5mm 0.03mm

Power (cw) 560 - 1290W 1000W

Feed rate 1m/min 12 - 15m/min

Energy input 35 - 77 J/mm 4 - 5 J/mm

Pre-treatment Chemical cleaning none

Geometry Butt welding Butt welding

It can be seen from the above parameters that the electron beam had a larger spot diameter and a significantly slower feed rate which results in a 10-15 times higher energy input per unit length. This resulted in a broader weld seam and larger weld volume. This can be seen from figures 34 and 37 which show the top of the seam welds.



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Figure 34 Electron Beam Seam Weld (top) Figure 35 Laser Seam Weld (top) It can be seen from Figures 37 and 38 that the laser weld has a lower melted volume and there is a noticeable difference in the material mixing in the weld. As the electron beam weld is a significantly larger, it means that the temperature in the seam weld and surrounding area is higher and the molten pool has had more time to mix, diffuse and cool down.

Figure 36 Electron beam butt weld – Figure 37 Laser 3003 butt weld – Al 3003 (1.1mm) to Cu (0.9mm) Al 3003 (1.1mm) to Cu (0.9mm) Figures 38 and 39 show lap-stake welds using electron beam and laser welding. In this case the electron beam weld has produced slightly larger welds but the shape and appearance are similar. In both cases materials have not mixed homogeneously and Cu-rich phases are concentrated in the melt located in the Cu sheet. In these sections the electron beam welds appear to have slightly fewer solidification cracks but it is not thought to be significant.

Figure 38 Electron beam lap-stake weld Figure 39 Laser beam lap-stake weld Al (1.2 mm on Cu 0.9mm) Al (0.5 mm on Cu 0.55mm)



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Power: 1490 W, v = 1m/min Pp = 3.5 kW, tp = 20ms,

Ep = 50 J

The electron beam welding has identified some interesting points that may assist in the next phase of the laser development, namely:

Can the laser weld be improved on these materials/joint configurations by increasing the molten pool size and slowing the weld heating/cooling times?

Would benefit be gained from rotating the beam to increase the weld size?

How much impact/influence (if any) does the pre-cleaning of the materials have on the weld micro-structure and will the green wavelength overcome this impact.

3.6. Summary - electric car battery welding

In Deliverable 2.1, Laser Technology Assessment Report, an overview was given of laser beam welding of Cu and Al. Starting from this review, a detailed welding performance assessment study was carried out to establish the key influences on laser welding Al alloy and Cu for the battery interconnections (thickness 0.1 - 1mm) when using a wavelength of 1064nm. The aim being to develop information that would aid the development of the QCOALA system This study was carried out using disk and pulsed Nd:YAG laser systems. Significant attention was given to gaining a better understanding of the metallographic behaviour of laser welded Cu and Al welds Good welds were produced on both thin (0.1mm) and thick (>0.1mm) Cu to Cu joints using in the former case conduction limited and in the latter keyhole welding. With respect to these limited initial trials, it was found that the surface reflectivity of Cu and its high thermal conductivity restricted both the tolerance range of the process for each laser type and the quality (eg porosity, spatter, undercut) of the joints produced. The trials on pulsed laser welding of Al 3003 material resulted in a problem of solidification cracking when welding in the lap joint configuration. It was interesting to note that this cracking was not evident on this scale in the butt joints. This requires further investigation as this alloy is considered as weldable with conventional Arc processes. The laser welding of Al 3003 to Cu was found to be difficult due to the generation of intermetallic phases and subsequent solidification cracking. This was not so evident in similar electron beam welds. In terms of the joint metallurgy, this combination of material is likely to require precise control of the material mixing in the weld pool if successful joints are to be repeatable produced. Further work is required to optimise the approach to developing a laser welding solution for this material combination. 3.7 Proposed approach for next 12 months

The work over the next 12 months will focus on applying the above knowledge on the specific joint profile and tolerances required for the battery application. This will include: 1. Conducting trials on the new dual wavelength higher power laser currently under development at

LASAG and benchmark against trials on comparable commercial Nd:YAG lasers. 2. Establishing conditions for welding Al to Cu without solidification cracking (e.g. pulse shaping,

duration and joint design). 3. Optimising weld conditions for welding Cu to Cu and Cu to Al joints. 4. Develop welded test samples for the non-destructive testing programme. 5. Establish data to assist the development of the process optimization integration and closed

feedback inspection systems.



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PROJECT DELIVERABLE REPORT - QCoala Project Performance of 1064nm.pdf · WP3/Deliverable Number 3.1...

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