A Study on the mechanical and electrical performance of Al/Cu laser multi-seam welded joints

Aluminum alloy and copper used in lithium-ion batteries have low absorption and high reflectivity, so it is difficult to obtain stable joint strength when laser welding is applied. In order to overcome problems, this study has applied multi-seam weld to increase joint area of welded joints. An AA1050-H18 and C11020P sheets with a thickness of 0.5 mm were welded by 2 kW fiber laser welding. Effect of the welding parameters of pitch (0, 1, 4, 8 mm) on weldability in welded joints has been investigated. As the pitch of multi-seam increased, the tensile-shear load tended to increase. The maximum tensile-shear load in the multi-seam weld of pitch 8 mm was 1.25 kN, which was about 108% compared to the single-weld seam 0.6 kN.


Introduction
As global carbon emissions regulations become stricter, the automotive market is transitioning to electric vehicle alternatives with the goal of achieving carbon neutrality by 2050.
Therefore, the welding process used to supply the battery pack requires high productivity and reliability [1,2].Welding processes applied in the battery manufacturing process include ultrasonic welding and laser welding, among others.The laser welding process offers control over heat input between dissimilar materials, allowing for high-speed welding and automation, making it well-suited for mass production [3][4][5][6].BRAND et al. revealed that Al/Cu laser welding resulted in high strength and low electrical resistance [7].Therefore, laser welding is considered the most efficient method in the battery pack welding process.
However, when laser welding different materials such as Al/Cu, intermetallic compounds (IMCs) are formed, which reduce strength and increase electrical resistance.IMCs are irregularly distributed in the welded joints during the solidification process, and their brittle characteristics make them the primary cause of fracture.Al/Cu IMCs formed include Al2Cu (θ), AlCu (η2), Al3Cu4 (ξ2), Al2Cu3 (δ), and Al4Cu9 (γ1) [8][9][10].Zuo et al. reported that Al2Cu (Θ) reduces strength and is the main cause of brittle fracture, while Al4Cu9 has a positive effect on strength [11].
Furthermore, it has been reported that an increase in IMC thickness is responsible for the decrease in strength and increase in electrical resistance [12].ABBASI et al. reported that when the IMC thickness of Al/Cu obtained through the cold rolling process increases more than 2.5 μm, the strength and electrical resistance of the welded joints are significantly reduced [13].Therefore, minimizing the formation of IMCs is crucial for improving the strength of the welded joints, reducing electrical resistance, and ensuring reliability.
Moreover, various technologies are being developed to overcome the limitations of strength and electrical resistance due to insufficient joint area.Kim et al. reported that by using laser beam oscillation, the molten pool can be stirred to effectively control the increase in the joint area, microstructure, and temperature gradient [14].KRAETZSCH et al. found that the joint area was increased by weaving, cracks were suppressed, and the strength of welded joints was achieved by about 80% compared to the aluminum base material [15].Furthermore, it has been reported that multi-beam laser welding, such as ARM (Adjustable Ring Mode) laser, can achieve the same effect as beam oscillation [14].Therefore, researchers are studying various welding processes to improve the welding quality issues of Al/Cu dissimilar materials in laser welding.
In this study, the objective was to enhance the welding quality of Al/Cu dissimilar materials by securing the joint area through multiweld seam (MWS) laser welding, leading to improved strength and reduced electrical resistance in welded joints.Therefore, we aimed to evaluate mechanical and metallurgical characteristics and electrical resistance with respect to process parameters by applying laser multiwelding in joining aluminum to copper.

Materials and experimental method 2.1 Materials and laser welding process
The base materials used in this study were 1000 series aluminum (AA1050-H18) and oxygen-free copper (C11020P) with dimensions of 100 mm (L) × 25 mm (W) × 0.5 mm (t), as shown in Fig 1 .The lap joints of aluminum and copper were welded, and the chemical compositions and mechanical properties of the two base metals are given in Table 2 and 3. A laser welding machine used a single-mode fiber laser with a maximum output power of 2 kW.The laser beam had a focal length of 255 mm, a spot diameter of 50 μm, and a beam parameter product (BPP) of 4 mm rad.As shown in Table 4, the process parameters were selected as follows: welding speed of 600 mm/s, focal location of 0 mm, 2 EA weld seams, and pitch values of 0, 1, 4, and 8 mm.

Mechanical and metallurgical test
In this study, the weldability of the Al/Cu dissimilar metal welded joints was evaluated by investigating defects in the welded joints and bead width.Additionally, mechanical properties were assessed through a tensile-shear load test and micro Vickers hardness measurement.The hardness was measured using the Vickers scale (HV 0.5) in both the horizontal and vertical directions, as shown in Fig 2 .To evaluate the metallurgical characteristics, optical microscopy (OM), scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), and fracture surface analysis were performed.

Electrical resistance test
For the measurement of electrical resistance in welded joints, the four-point probe method is applied to determine the contact resistance.As shown in Fig 3, the specimens were fixed between four electrodes with a 40 mm separation.Electrical resistance measurements were performed between two electrodes using a multimeter instrument while applying a current of 1 A.

Bead profiles according to pitches
The bead cross-sections of the aluminum and copper at various pitches are shown in Table 5.
Upon observing the cross-sections of the weld joints, no cracks were observed, but porosity was detected at the interface on the Al side.It is assumed that the fast welding speed and solidification speed, characteristic of laser welding, prevented hydrogen emission, resulting in porosity at the interface [18].Additionally, it was confirmed that the higher amount of upper sheet Al melting was due to the different melting points of the two base materials [16,17].

Tensile-shear load
The characteristics of the tensile-shear load and the fracture location were investigated based on changes in pitch.As shown in Fig. 4, the strength increased with an increasing pitch, and the maximum tensile-shear load was 1.25 kN at a pitch of 8 mm.This represents an approximately 108% increase compared to a load of 0.6 kN at 0 mm.This can be attributed to the larger joint area achieved in MWS compared to single-weld seam (SWS), resulting in an improvement in the tensile-shear load.It is assumed that the heat influence between the first and second beads led to a concentration of heat in the local area, resulting in a decrease in strength [19].Additionally, it is inferred that the heat influence between the beads did not significantly affect the strength at a pitch of 4 mm.
The fracture locations following the tensile-shear load test are shown in Table 6.As a result, interface fracture occurred in the case of SWS, while fracture occurred in the heat-affected zone (HAZ) of the first bead on the Al side in the case of MWS.

Hardness distribution
The measured distribution of hardness in the horizontal direction according to pitch is shown in Table 7.As a result, it was observed that hardness tends to increase as it approaches welded joints.Compared to the aluminum base material, a rapid reduction in hardness of approximately 20 HV was observed within ±0.2 mm of the centerline of Al side welded joints.This is presumed to be the decrease in hardness at the HAZ due to material softening caused by heat input during welding.Therefore, it is considered that fracture location after the tensile-shear test occurred in HAZ.
Table 8 shows the hardness distribution in the vertical direction according to pitch, and hardness increases as it gets closer to the joint interface.It is presumed due to the formation of brittle IMCs at the joint interface.In the case of a 1 mm pitch, it was observed that the hardness of the second bead increased due to heat influence between beads.On the other hand, it was observed that the heat influence between beads decreased at pitches of 4 and 8 mm, resulting in a similar hardness distribution.The growth of IMCs depends on temperature and time [13].Therefore, it is presumed that as pitch decreases, the increased heat input from the first bead to the second bead leads to the increased formation of IMCs.9.
In the case of SWS, brittle AlCu, and Al3Cu4 phases were formed through a eutectic reaction and a peritectic reaction.Therefore, it is assumed that the main cause of SWS is interface fracture caused by IMCs.In the 1 mm and 4 mm pitch conditions, primarily Al2Cu, AlCu, and Al/Cu eutectic phases were formed, while at an 8 mm pitch, Al/Cu eutectic, Al2Cu, and Al4Cu9 phases were formed.As the pitch decreased, the formation of IMCs increased due to the higher heat input.

Fracture mode
The fracture locations following the tensile-shear load test are shown in Fig. 6 and summarized in Table 10, presenting the results obtained from the composition analysis of EDS on IMCs in the fractured surface, indicating the interface failure mode.As a result, fractures were observed due to the formation of phases such as AlCu, Al2Cu3, and Al3Cu4 in the Al/Cu welded joints [11].Fig. 7 shows the fractured surface morphologies for the 1 mm and 8 mm conditions, where fractures occurred in the HAZ.At a pitch of 1 mm, cleavage fractures with brittle characteristics were found.Furthermore, under the condition of the maximum tensile-shear load at an 8 mm pitch, large and deep dimples with micropores were observed on the fracture surfaces, contributing to improved tensile strength.

Electrical resistance
Fig. 8 shows the electrical resistance of welded joints with pitches of 0 and 8 mm, and the electrical resistance ranges of the base metals used in this study were 117 µΩ for Al and 70 µΩ for Cu, respectively.The results of the electrical resistance test indicated that the electrical resistance decreased as the pitch increased from 0 mm to 8 mm.The minimum electrical resistance obtained was 77 µΩ at a pitch of 8 mm, which was approximately 49% lower than that at a pitch of 0 mm.Overall, these results suggest that a sufficient joint area was secured, and the increased pitch affected the smooth flow of current due to decreased IMCs in the welded joints.

Conclusions
In the present study, the dissimilar welding of 0.5 mm thick aluminum and copper sheets was investigated to examine the effect of process parameters on mechanical and metallurgical properties, as well as electrical resistance, using fiber laser welding.The results are summarized as follows: 1.With an increase in pitch, the tensile-shear load also increased due to the reduced heat input effect between the beads.The maximum tensile-shear load was 1.25 kN at a pitch of 8 mm, representing an approximately 108% improvement compared to a pitch of 0 mm.

2.
The hardness exhibited a rapid decrease within ±0.2 mm of the welded joints on the Al side.As the pitch decreased, the hardness increased due to the formation of IMCs at the joint interface.

3.
It was observed that the formation of IMCs decreased as the pitch increased.At a pitch of 1 mm, cleavage fractures with brittle characteristics were found.Furthermore, under the maximum tensile-shear load condition at an 8 mm pitch, large and deep dimples were observed on the fracture surfaces, contributing to improved tensile strength.

4.
As the pitch increased, the electrical resistance decreased.The minimum electrical resistance obtained was 77 µΩ at a pitch of 8 mm, which was approximately 49% lower compared to that at a pitch of 0 mm.

Fig. 1
Fig. 1 Schematic experimental set-up for lap joint of dissimilar Al and Cu sheets

Fig. 2
Fig. 2 Measurement position for hardness distribution

Fig. 4
Fig. 4 Comparison of tensile-share load with pitchesTable 6 Fractured location of welded joints with pitches Fractured

Table 7 Fig 5 .
Fig 5.  shows the SEM-EDS analysis of the welded joint interface in Al and Cu produced at a pitch of 0-8 mm.The EDS analysis results are summarized in Table9.In the case of SWS, brittle AlCu, and Al3Cu4 phases were formed through a eutectic reaction and a peritectic reaction.Therefore, it is assumed that the main cause of SWS is interface fracture caused by IMCs.In the 1 mm and 4 mm pitch conditions, primarily Al2Cu, AlCu, and Al/Cu eutectic phases were formed, while at an 8 mm pitch, Al/Cu eutectic, Al2Cu, and Al4Cu9 phases were formed.As the pitch decreased, the formation of IMCs increased due to the higher heat input.

Fig. 5
Fig.5SEM-EDS analysis of the welded joint interface for various pitches Table9Summarized of EDS analysis for various pitches

Fig. 6
Fig. 6 Fractured surface of welded joints with SWS

(a) pitch 1 Fig. 7
Fig. 7 Al side fractured surface of weld specimen with MWS of pitch 1 and 8 mm after tensile-shear test

Fig. 8
Fig. 8 Comparison of electrical resistance with pitches

Table 1
Physical and chemical properties of Al/Cu intermetallic phases

Table 2
Mechanical properties of base materials

Table 9
Summarized of EDS analysis for various pitches

Table 10 .
Summarized of EDS analysis for various pitches