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Process robustness of laser braze-welded Al/Cu connectors.

ABSTRACT

Laser welding of dissimilar metals such as Aluminum and Copper, which is required for Li-ion battery joining, is challenging due to the inevitable formation of the brittle and high electrical-resistant intermetallic compounds. Recent research has shown that by using a novel technology, called laser braze-welding, the Al-Cu intermetallics can be minimized to achieve superior mechanical and electrical joint performance. This paper investigates the robustness of the laser braze-welding process. Three product and process categories, i.e. choice of materials, joint configurations, and process conditions, are studied. It is found that in-process effects such as sample cleanness and shielding gas fluctuations have a minor influence on the process robustness. Furthermore, many pre-process effects, e.g. design changes such as multiple layers or anodized base material can be successfully welded by process adaption. The minimization of the interface gap is identified as the most significant influence of process stability. The specimen were validated by mechanical lap shear tests and metallographic analysis.

CITATION: Schmalen, P., Plapper, P., and Cai, W., "Process Robustness of Laser Braze-Welded Al/Cu Connectors," SAE Int. J. Alt. Power. 5(1):2016.

INTRODUCTION

The development of sustainable and energy-efficient electric drive vehicles is considered the key challenge for future automotive industry. The performance of new electric vehicles depends on the power and capacity of the energy storage. Hence, the manufacturing of battery assemblies is one competitive technology for the deployment of electric cars.

Today, battery pack assemblies consist of hundreds to thousands of battery cells which need to be joined robustly and economically. For electrochemical reasons one cell consist of electrolyte, separator and two electrodes with unequal electronegativity. For Lithium-ion cells, the cells with highest energy density, the current collectors are usually aluminum and copper (see figure 1). The connectors between the electrodes are manufactured using various materials, e.g. Al or Cu. During assembly, various thickness and number of electrodes need to be joined for performance requirements and manufacturing efficiency [1].

Dissimilar materials are difficult to join due to their different physicochemical characteristics, e.g. Aluminum and Copper are considered as not weldable by conventional welding techniques. The fusion joining of Aluminum and Copper causes a seam between both materials, where binary alloys are formed. These Intermetallic Compounds (IMC) are usually brittle and have less shear strength properties and a low conductivity factor [2], e.g. the electric resistance of Al-Cu IMCs is about 8 times greater than the electric resistance of the base materials. There is a critical width of the IMCs at which the connection still shows a ductile behavior and the losses in strength and conductivity are strongly reduced. According to [3], this width must be less than 2.5 microns.

Existing joining techniques enable the production of those dissimilar connectors, e.g. ultrasonic welding [4] or frictions stir welding [5]. Current reviews of different joining techniques can be found in [6] and [1]. However, laser welding techniques are showing high potential for joining Aluminum and Copper [7 and 8]. Lasers are frequently used in high volume applications and offer high automation opportunity through its low inertia positioning system and contact less power delivery, as described by the authors in [9] and [10]. Novel laser processing techniques control the energy input to reduce the interaction of the joining materials, whereby the IMC seam thickness is minimized [11, 12, 13 and 24].

Recent publications with the focus on laser welding of dissimilar Al-Cu Connectors were concentrated on optimizing the joining process on specific purposes, e.g. strength [7]. The authors in [14] enhance the ductility of the connections by using filler materials. In [15], the authors analyzed the effects of a tin foil as a filler material on laser joining of aluminum and copper. Other publications are focused on characterization of Al-Cu connectors [11, 12, 16 and 17] and the joint fracture behavior is analyzed [18].

This paper investigates the influence of external effects on the joining process performance. The resistance of the process to external disturbances is interpreted as process robustness. Different effects will be identified and analyzed to the laser welding process to prove the competitive manufacturing readiness of the process.

ORGANIZATION

During the investigations, dissimilar Al/Cu connectors were joined with defined pretreatments to analyze and evaluate the influence on the process robustness. The tests were chosen on expert experience based on preliminary investigations and on requirements from the industry.

The following structure was defined (see figure 2):

A. Material effects

1. Hardness

2. Surface

B. Design effects

3. Multiple layers

4. Coating

5. Anodization

C. Process effects

6. Shielding gas

7. Gap

8. Cleanness

The material effects were linked to the base material properties and the design effects are usually requirements due to the product or manufacturing requirements. These effects were taken into account before processing and cannot be influenced by the process. The direct process effects were grouped separately. The analysis of the weld performance was performed by post-process methods.

All tests will be performed by using the braze-welding process [10]. which will be explained hereafter. The tests will deal with laser joining of Aluminum to Copper. A summary will point out the main advantages and issues on laser welding of Al-Cu connectors. The outlook will state opportunities to enhance the braze-welding process.

THE BRAZE-WELDING PRINCIPLE

The braze-welding process is based on the laser keyhole welding principle. The beam is focused on the material surface with an energy density of at least 1 MW/[cm.sup.2] [31], which melts and vaporizes the material. The vaporized metal forms a hole wherein the laser is reflected multiple times, thus the energy absorption of the metal is increased. This high energy input enables a fast process and a selective melting of the upper aluminum can be achieved, while the lower copper remains in solid state (see figure 3). Thus, the process is called a braze-welding process [24]. The realized fusion layer thickness between Al and Cu can be reduced to under 5 [micro]m. The formation of IMC is minimized, whereby a ductile metallic joint with reduced electric resistance is realized.

The mechanical properties of the joint depend on the surface area of the welded interface. The width of the interface is controlled by using a spatial power modulation [25] (figure 4), which furthermore can be used to define the weld pool geometry [12 and 26]. Current investigations have shown that high-frequency power oscillated laser-beam stabilize the welding process [27, 28 and 29] by reducing the penetration depth fluctuation and achieving a homogenous heat input. The author in [30] developed a guide for choosing process parameters for pulsed lasers for similar materials and analyzed their effect on heat flow, weld dimension and weldability.

METHODOLOGY

For the laser welding experiments, bare AA 1050/1100 and anodized AA 1145 were joined to SF/E-Cu. The coating and thickness changed depending on the test. The materials were always joined in overlap configuration. Each sample had a size of 40 x 40 mm with an overlap of 10 to 15 mm. The weld seam was welded with a length of 30 mm in the center of the overlap. The materials were joined without cleaning, except for tests in section C.8. The surface of the base material was dust and grease free, as well as uncrumpled. No shielding gas was used, except for tests in section C.6.

The laser machine applied in all the experiments was a Trumpf TruFiber 400 fiber laser machine with a wavelength of 1070 nm and a maximum power of 400 W. Taking into account the 31 [micro]m spot diameter, a maximum power density of approximately 50 MW/[cm.sup.2] can be reached depending on the defocus setting. The spatial laser modulation was adapted to have a weld seam width of 0.5 mm and the temporal modulation was performed with a pulse frequency of 18 kHz. The pulse time was varied from 27 to 36 [micro]s to control the energy input into the melt pool. The used parameters were developed during previous investigations [24]. They were optimized to enhance the mechanical durability and to minimize the IMC layer thickness. The laser path was controlled by a Scanlab HS20 2D scanner head.

The weld seams were evaluated by performing mechanical and metallurgical tests. Cross-sectional micrographs were used to investigate the intermetallic interface, i.e. the layer thickness (see figure 5). Furthermore, the weld porosity and cracks could be evaluated. The samples were grinded with SiC-Paper up to roughness 1000. The samples were then polished with 6, 3 and 1 [micro]m diamond suspension. After cleaning with de-ionized water, the samples were etched with a Keller solution for 3 to 5 seconds. The cross sections were analyzed with an optical microscope using unpolarized light.

The mechanical performance was evaluated by a shear pull test by measuring the maximum shear force and the toughness of the joint. The tests were performed on a Zwick/Roell Z010 material testing machine with a pulling velocity of 3 mm/min and a sampling rate of 10 kHz. The toughness T can be calculated by:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (1)

With:

I: pulling distance upon failure

F: pulling force

s: strain

For a good joint quality, the maximum pulling force and toughness should be increased. Since the base material influences both factors, see chapter A. 1, material control tests should be performed at the beginning of an investigation.

TESTS TO EVALUATE PROCESS ROBUSTNESS

A.1. Hardness

During the investigations it was found that the material hardness has an effect on the mechanical performance, particularly on the maximum pulling force. The influence of material properties on the process has also been reported by [14]. During the manufacturing process of the sheets, the metals were strain hardened, the strength is increased, but the total elongation is decreased [33]. Depending on the strain hardening history and heat-treatment, the metals can be divided in soft to hard in intermediate grades: annealed=soft; quarter hard; half hard; hard; etc. Figure 6 shows three Aluminum base material samples with different hardness.

The aluminum (0.2 mm thick) was joined to soft, annealed Copper ([sigma]= 220 MPa, 0.5 mm thick). Figure 6 shows the impact of the base materials hardness to the mechanical properties of the joint. The results of figures 6 and 7 were directly linked, the softer base material caused a reduced maximum pulling force with a higher strain, hard base materials showed an inverse behavior.

By using a harder base material, the maximum pulling force can be increased, but the joint will become more brittle (see figure 7) The softer material gives the advantages of more elongation before break. A ductile base material can compensate material stresses induced by thermal distortion and therefore prevents the formation of cracks. The soft joint is more likely to withstand alternating mechanical loads compared to the joints with hard Aluminum temper. The joints were broken next to the weld seam in the heat affected zone in the aluminum base material. Thus, the joints cannot be validated only by comparing the maximum pulling force, the maximum strain also needs to be accounted. It was found that the calculated toughness can be used to estimate the mechanical performance of the joint.

A. 2. Surface

Aluminum is a highly reflective material for infrared lasers and has a high thermal conductivity. Thus, a high power density is needed to heat and melt the metal. The surface of the workpiece could affect the process performance during pulsed laser welding by increasing or decreasing the absorptivity which results in a deficient weld quality [32].

The effect of the material surface will be investigated by preparing three different samples with varying surface treatment. The first one is the natural aluminum, untreated, as used in most applications. The second sample has been ablated with a marking laser before joining. (Parameters: Defocus: 1,5 mm; Width: 0,05 mm; Power: 30 W; v: 1600 mm/s; Frequency: 40 kHz). The third sample were sanded with corundum. All samples were welded with the same process parameters and validated by a pull test. The mechanical results are shown in figure 8.

The natural and the ablated samples delivered similar toughness and maximum pulling force. Hence, the effect of ablation of the surface on the process is insignificant. The sanded samples reached a higher pulling force, put the toughness has been reduced. This result can be explained by the knowledge gained in the chapter A. 1. The sanding process changed trough the impact energy of the corundum the hardness of the surface of the material. Increasing the hardness of a material goes along with an increased strength and a decreased toughness. The sanded surface did not affect the welding process, the cross sections for all samples were equivalent (see figure 9).

The main factor for absorption, especially during the whole process is the material temperature. During the starting phase, the surface affects the heating up phase of the material, until a saturation value for absorption is reached [34]. During the keyhole welding process, a plasma is formed, the absorption is not depending on the surface anymore. The two dominating effects for absorption are inverse Bremsstrahlung of the plasma and Fresnel absorption at the cavity walls [35], Thus, the relation between surface state and absorptivity can only be made at ambient temperature.

For keyhole welding, an effect of the surface can only be seen during the starting phase. With the formation of the keyhole, an effect of the state of surface was not detected. The starting phase can be reduced by using high peak power pulse at the beginning [25]. The process stabilizes due to the formation of the keyhole capillary, which was also found by [28].

B.3. Multiple Layers

The manufacturing of battery modules requires the joining of multiple layers of aluminum to copper [36]. In order to reduce the experimental effort for a new material combination, a comparison of one single layer joint to a multiple layers with the same thickness in total has been performed (see figure 10)

The tests have been performed with one single layer of 0.50 mm thickness and four layers of 0.125 mm of thickness. The parameters were identical for both setups. Pull tests were not performed because of the lack of a defined and uniform mechanical test for multiple layers. The cross sections showed for the welding of 4 Al-layers a thin IMC layer, however the penetration for the single layer was strongly increased (see figure 11). The multiple layer configuration showed a weld seam with flat-angle flanks compared to the single layer joint. The weld seam width of the multiple layer configuration was smaller compared to the single layer configuration.

During the welding process, the laser liquefies and vaporizes aluminum, the keyhole is formed for both configurations similarly. The zone next to the keyhole is affected by heat conduction, whereat the single layer can be heated uniformly. The multiple layers were thermal delimited by each geometrical transition. Thus, the layer next to the copper stays colder than the upper layers, which results in a more flat-angle weld flank. Small steps in the liquefied weld can be observed (figure 12).

To conclude, it was shown that the braze-welding process is able to weld multiple layers of Al to Cu. Each layer transition causes a thermal insulation, causing the layer next to the Cu to be "colder", with a reduced melt pool. The welding of multiple layers therefore needs a higher energy input compared to single layer connectors to achieve an equal weld seam width.

B.4. Coating

Despite Aluminum and Copper are considered not weldable to each other, both metals are able to form an alloy with other metals, e.g. nickel or tin. These materials could act as intermediary, aluminum and copper were only joined to the so called filler material [14 and 15], The filler material can be applied during the process or coated base materials could be used. The thickness of the coating influences the joint properties, a thin coating can lead to the formation of a ternary alloy, and a thick coating could cause a heat barrier, which resulted in a difference in weld depth [15].

The following experiment was performed to identify possible benefits of a nickel coated copper to bare copper in mechanical strength. Nickel can be welded to copper and aluminum, thus can be used as filler material. Bare aluminum was welded to bare copper and to Nicoated copper with 3-5 [micro]m coating thickness. The bare copper had a thickness of 0.3 mm, the Ni-coated copper a thickness of 0.25 mm and the aluminum 0.2 mm. Both materials were joined with equal parameters.

By comparing the mechanical results shown on figure 13, no benefit of Ni-coated copper can be seen compared to the bar copper. The maximum pull forces are similar, same for the strain. The cross section (see figure 14) show an equal intermixture in the weld seam.

Thus, a coating to enhance the compatibility of the dissimilar metals is not needed, the materials can be joined directly by laser braze-welding with similar mechanical properties of the joint. A beneficial metallurgic effect of the Ni to the mechanical properties of the joint was not found.

B.5. Anodization

The anodizing procedure is commonly used in industry to enhance the robustness of Aluminum. The artificial thickened oxide layer makes the material more resistant to mechanical impacts and corrosion. Regarding the weldability, the anodized aluminum is unwanted because of the high melting point and the porosity of the oxide layer, wherein atmospheric moisture could condense. The welding of an anodized base material will cause a weak and porous joint (see figure 15).

Thus, the anodized surface has to be removed, common techniques here for are laser ablation or mechanical polishing with sand paper [17]. Laser ablation can be selectively applied where material will be welded and therefore will be used for the experiments (see figure 16).

The anodized aluminum will be joined in a 2-step process. First, the bottom surface of the Al, which will be contacted to the Cu directly, is ablated. Only the oxide layer next to the welded area will be removed, the residual anodization on the workpiece can be preserved. The second step consist of laser-braze welding of ablated Al to Cu. The anodized base materials had two different surfaces with unequal anodization. As delivered, one side of the base metal was matt, the other side had a grounded appearance, possibly generated by a rolling process (figure 17). The ablation process changed the thickness of the material (see table 1). All samples were welded with same process parameters.

The anodized Al, welded in the 2-step process was compared to the bare Al received by ablating both sides. The Al materials were welded to 0.5 mm bare copper. The mechanical results (see figure 18), show a dependence on which side has been ablated. The toughness and maximum pull force has been the highest for the joint with the mate side ablated. If the other surface, the grounded one is ablated, the mechanical properties decrease. The ablation of both sides increases the shear strength compared to the grounded surface ablation, but the toughness decreases again.

The mechanical results for the one side matt ablated Al were best. By ablating the grounded side, a drop in maximum pulling force and toughness of approximately 5% was found compared to the matt side. By ablating both sides, the toughness decreased by 20 %.

The results depend on which surface was ablated, respectively which surface remained at the sample. The cross sections show that the energy absorption for both sided ablated aluminum was reduced, which resulted in the reduced toughness (figure 19).

To conclude, anodized aluminum can be joined in a two-step process, but the mechanical properties of untreated aluminum-copper connections were superior. The anodizing process change properties of the surfaces unequally, thus a two-step process cannot be applied on any surface, it has to be adapted to the surface of the workpiece. The anodizing of both surfaces before joining will reduce the thickness of the base material (see table 1), thus results in reduced mechanical properties.

C.6. Shielding Gas

Aluminum reacts strongly with ambient oxygen. Similar to the IMC, the generated [Al.sub.x][O.sub.y] were brittle and unwanted. A shielding gas is used to cover the weld seam from the reaction with the air. Mostly, argon, a chemical inert gas, is used as shielding gas. The joining of dissimilar metals is feasible with [14] or without shielding gas [29]. Pull tests were performed to retrieve the effect of welding with/without shielding gas on the mechanical properties.

It was found that the shielding gas had a negligible influence on the mechanical strength of the joint (see figure 20) All mechanical results were equally distributed within the process variation. The optical analysis of the weld seam welded with shielding gas revealed more geometric details, the spatial modulation can clearly be identified. The weld seam without shielding gas appeared rougher and burned (see figure 21) The cross sections were comparable for both setups.

The braze-welding principle is based on a laser keyhole welding process, which includes the evaporation of metal gases and the formation of a plasma on top of the weld seam [35]. Thus, the surrounding air is displaced and the metal is prevented from oxidation. Shielding gas should be used, if closed and clean weld seams need to be achieved, for example for Cr/Ni steels [9]. The shielding gas can even more influence the process dynamics by influencing the vaporized metal. According to [37], the shielding gas can have a positive or a negative effect on the process, e.g. increasing the penetration depth by influencing the plasma plume.

C.7. Gap

The braze-welding process is a laser process which does not add material to the process. Thus, air gaps in the joint can lead to not desired joint properties, like porosity. Most authors claim to minimize the air gap and to obtain "a nearly zero gap" [4], [14]. The following experiment is conduced to quantify the importance of a "nearly zero gap". The author in [35] recommended that a gap between two blanks should be less than 10% of the thickness of the thinner blank.

A gap test has been performed (figure 22). The gap h was variated from 0 mm to 105 [micro]m by using stacked 15 [micro]m thick aluminum foil. Cross sections were analyzed. Pull tests were not performed. 200 [micro]m thick Aluminum was welded to 300 [micro]m thick copper.

The cross sections reveal the formation of gas pores and cracks (see figure 23) for gaps of 15-30 [micro]m. The amount of gas pores as well as the total surface covered by them was counted, but a direct correlation between gap offset and amount of gas pores was not found. The cracks were mostly formed with the presence of an air gap next to the interface.

With a larger gap less gas pores and cracks were observed. A gap of 45 [micro]m or more allowed the liquid metal to flow into the gap at the interface of both metals, thus preventing the formation of cracks on the edge (figure 24, left). With the increasing gap, gas pores were reduced again. A gap of 105 urn was successfully bridged and a mechanical joint was formed (figure 24, right). The width of the interface is reduced with increasing gap and the weld seam becomes increasingly unstable.

To summarize, a small air gap between Al and Cu of 15-30 [micro]m reduces the overall joint performance, but an increased air gap up to 50% of the base material thickness, 105 [micro]m, was still successfully joined. An evident link between gap and joint performance was not founded. The formation of pores and cracks at the interface will be furtherly investigated. The best joint performance, as shown for the other tests, was found for a nearly zero gap width, which was realized by an appropriate clamping device.

C.8. Cleanness

In order to achieve a grease- and dust-free surface, workpieces were commonly cleaned before joining. Unfortunately, detailed information about the cleaning process are sparely delivered. Common cleaning techniques for metals are alcoholic solutions, acetone, and ultrasonic bath [32].

An experiment has been setup to investigate the effect of the cleaning technique to the weld quality. Hence, the two common techniques, acetone and ultrasonic bath were compared with an untreated, natural sample. All samples were joint with the same process parameters. The samples were handled with gloves to avoid fingerprints. After cleaning the samples for two minutes in the ultrasonic bath, the samples were cleaned with demineralized water. All samples with their specific treatment dried for 30 minutes on the atmosphere.

The pull tests (see figure 26) generated similar results for all samples. The variation was highest for the natural samples and the toughness was lightly increased for samples cleaned with acetone. The cross sections correlate with the mechanical results, regardless the cleaning medium, a homogenous and uniform weld seam was formed (figure 25).

Summarizing, the sample cleaning has a minor, regarding the process variance, negligible effect in the mechanical properties of the joint. In most cases, the samples can be joined without a cleaning routine, a dust and fingerprint-free surface fulfills the requirements for the braze-welding process.

SUMMARY

In this paper, the robustness of the braze-welding process has been analyzed. From the material side it was found that the base material hardness had an effect on to the Al-Cu joints. High strength base material will lead to high strength joints, compared to tough or ductile materials. An effect of the state of the surface was not found due to the keyhole process.

Multiple layers, which were considered as a design factor, could be joined by braze-welding, but it was found that more layers require an increased energy input and the process parameter need to be adapted. The Ni-coating on the other hand had no distinguished changed compared to bare copper and no effect on the process robustness. The anodized material were joined by a two-step process to remove the oxide layer before welding to enhance the process quality. The joint performance was affected by which side of Al was ablated.

The process was found to be mostly insensitive to the material preparation, the sample cleanness was of less importance. Shielding gas is not needed for the process, but the surface appearance can be enhanced. The gap finally had an increased impact to the joint performance, but a direct link between gap width and porosity/cracks was not found.

To conclude, the braze-welding process is able to join aluminumcopper connections under variable configurations and conditions. Most effects were identified in the pre-process phase and can therefore be minimized, e.g. by parameter optimization. The most critical in-process effect is a possible air gap between both base materials. In order to achieve the highest joint quality, base materials with high weldability, e.g. without anodization or other treatment, were preferred. A nearly zero gap has to be pursued and base material tests should be performed. For joint evaluation, the use of multiple results, e.g. maximum pull force, toughness and cross sections is recommended.

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CONTACT INFORMATION

Corresponding author:

P. Schmalen

Tel:+352 466644 5850

pascal.schmalen@uni. lu

peter.plapper@uni. lu

(P. Plapper)

wayne.cai@gm.com

(W. Cai)

ACKNOWLEDGMENTS

This work is supported by the Fonds National de la Recherche (FNR) in Luxembourg under grant no. AFR 10155468.

DEFINITIONS/ABBREVIATIONS

IMC - Intermetallic Compounds

Al - Aluminum

Cu - Copper

Ni - Nickel

Pascal Schmalen and Peter Plapper

University of Luxembourg

Wayne Cai

General Motors Co.

Table 1. Thicknesses of ablated anodized aluminum.

                  Thickness [[micro]m]

No ablation           215-218
One side ablated      207-209
Two side ablated      200-203
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Author:Schmalen, Pascal; Plapper, Peter; Cai, Wayne
Publication:SAE International Journal of Alternative Powertrains
Article Type:Report
Date:May 1, 2016
Words:5573
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