Aluminium-copper hybrid parts, as a substitution to copper parts, result in weight and cost reduction, and are relevant in applications related to the electronic, heating and cooling sector. However, aluminium to copper joined by thermal welding processes presents challenges in terms of achieving good joint quality. This is attributed to their dissimilar mechanical and thermal properties which result in large stress gradients during heating. This study investigated joining of aluminium to copper sheets by electromagnetic pulse welding, which is a solid-state process that uses electromagnetic forces for joining of dissimilar materials. Hybrid sheet welds were obtained for all parameters conditions, selected according to a Taguchi L18 design. The structural and mechanical characteristics were examined and related to the welding parameters by means of a Pareto analysis and response graphs. The welded zone started with a wavy interface with interfacial layers and defects and evolved to a flat interface without interfacial layers. The maximum transferable force depended on the minimum specimen thickness and the strength of the hybrid sheet weld. In case of aluminium sheet thickness reduction, the maximum transferable force was linearly correlated with the aluminium sheet thickness. High quality joints were obtained for no aluminium sheet thickness reduction and for a sheet weld strength which was at least as high as that of the base material. The most effective way to increase the transferable force was to lower the initial gap and to increase the free length, which resulted in no aluminium sheet thickness reduction. Alternatively, the use of a rounded spacer decreased the effect of the aluminium sheet thickness on the transferable force. An increase in weld width was achieved for an increase in capacitor charging energy and gap, whereas an increase in weld length was obtained for a decrease in gap. An increase in weld width did not necessarily result in an increase in the transferable force. In the regarded cases, a hybrid sheet with narrow weld width could therefore have higher quality.
Global trends force industry to manufacture lighter, safer, more environmental friendly, more performant and cheaper products. A multi-material design exploits materials with desired properties for each part of the component or product. However, design of products is hindered by challenges in the field of joining technology. Conventional thermal welding technologies are mostly suitable for assemblies of similar metals with comparable melting temperatures, and hence restrict the materials that can be joined, the type and the quality of the joint. In contrast, solid-state welding technologies allow both dissimilar and similar metals to be joined, since no significant melting of the base metals occurs.
Electromagnetic pulse welding is an innovative solid-state welding technology that belongs to the group of pressure welding processes, since it uses electromagnetic forces for the deformation and joining of materials. It was originally suggested in [
Compared to thermal welding processes, important advantages of electromagnetic pulse welding have been documented in literature. One important issue is that pressure instead of heat is employed to realise the metallic bond, as stated by [
Dissimilar aluminium-copper joints are of interest for wiring and cable systems, where they are commonly employed in transition pieces for high direct current bus systems. The partial substitution of copper with aluminium allows for both mass and cost reduction, with a minimum loss of conductivity. In the field of e-mobility, aluminium-copper joints are used to connect additional cells to batteries consisting of aluminium and copper electrodes [
However, the hybrid structure of aluminium to copper presents a number of challenges in terms of achieving a high strength joint. Both copper and aluminium have some similar properties that make it difficult to weld by fusion welding. These include high thermal conductivity, high thermal expansion coefficient, relatively low melting point, brittleness at elevated temperatures and less viscosity of molten metals. More- over, aluminium to copper joined by fusion welding can contain large stress gradients, due to their highly dissimilar mechanical properties and different thermal expansion coefficients [
Electromagnetic pulse welding can be used for tubular and sheet applications, placed in the overlap configuration. Mainly aluminium-copper tubular components have been documented in literature for their interfacial morphology and mechanical properties [
The objective of the present work was to investigate the electromagnetic pulse welding of aluminium to copper sheets achieved at different parameter combinations. These were selected according to a Taguchi L18 design. First, the weld shape, interfacial morphology, transferable force, failure modes and dimensions of the weld seam were analysed in detail. Subsequently, the effect of the welding parameters on these structural and mechanical characteristics was investigated using a statistical approach. Finally, a modification of the experimental set-up is examined to improve the maximum transferable force of the hybrid sheet weld.
ENAW-1050 H14 and Cu-DHP R240 (Phosphorus-Deoxidised Copper) sheet materials were used for the present study. Both sheets have a width of 240 mm and a thickness of 1 mm. Their mechanical characteristics are shown in
The electromagnetic pulse welding installation comprises 4 main parts, as shown in
Sheet material | Hardness [HV] | Yield strength (Rp0.2) [MPa] | Tensile strength (Rm) [MPa] | Elongation (A50 mm) [%] |
---|---|---|---|---|
EN AW-1050 H14 | 30 | ≥85 | 105 - 145 | ≥2 |
Cu-DHP R240 | 65 - 95 | ≥180 | 240 - 300 | ≥8 |
1) High-voltage cabinet containing the grid power supply, which is used to charge the capacitor bank,
2) Energy storage bank containing the capacitors, which stores the energy that is used to accelerate the sheet,
3) Work table containing the coil, which generates the magnetic field necessary for accelerating the sheet and producing the weld,
4) Control panel, for setting the required voltage.
The main characteristics of the pulsed power generator (Pulsar MPW 50/25) are summarized in
The tool coil used for all tests is illustrated in
Characteristic | Value |
---|---|
Maximal charging voltage | 25 kV |
Maximal storage energy | 50 kJ |
Maximal pulse energy | 40 kJ (22.36 kV) |
Maximal pulse current | 500 kA |
Voltage-energy ratio |
which the aluminium flyer sheet is separated from the copper target sheet prior to discharge. The overlap is the overlap distance between the tool coil and the aluminium flyer sheet. The free length is the part of the aluminium flyer sheet that is being accelerated towards the copper target sheet.
The electromagnetic welding principle is as follows: in first instance, an AC current in the power supply is rectified which charges the capacitor bank to the selected energy level. The electrical energy stored in the capacitors is the discharge energy. Once the bank is charged, the switch is closed and the energy is instantaneously discharged through the coil. A damped sinusoidal current induces a strong transient magnetic field in the coil. The magnetic flux lines intersect with the aluminium flyer sheet, which results in an induced electromagnetic force and corresponding Eddy currents and magnetic field. This magnetic field caused by the Eddy currents in the aluminium flyer sheet opposes the original transient magnetic field caused by the coil current. The difference in magnitude of the magnetic fields generates Lorentz’ forces that repel the aluminium flyer sheet away from the coil. Hence, the aluminium sheet impacts with the copper target sheet at high velocity in a few microseconds. If collision angle and velocity are in the range of the weldability window, a jetting effect occurs which causes the expulsion of oxides and contaminations on both sheet surfaces. After collision, the resulting Lorentz forces press the atomically clean surface of the aluminium flyer and copper target sheet together to form a weld. Bonding between the two materials occurs when the distance between their atoms becomes smaller than the range of their mutual attractive forces [
Based on exploratory welding experiments, a set of parameters was identified that produced a sound weld. However, 54 parameter combinations would have to be considered
in order to perform a full factorial research. Other statistical methods were therefore employed for the design of experiments and the interpretation of the results. This allowed reducing the number of experiments without losing a significant amount of information and accuracy. For this purpose, the Taguchi design L18 was applied, for which only 18 out of the 54 parameter combinations were selected. The according parameter combinations are listed in
The following welding parameters were varied, namely:
Capacitor charging energy E,
Overlap between flyer sheet and actuator o,
Initial gap between flyer and target ginitial,
Free length l.
The fixed parameters were chosen as:
Capacitance C = 160 μF,
Target thickness ttarget = Flyer thickness tflyer = 1 mm,
Target width wtarget = Flyer width wflyer = 240 mm,
Active length and width of tool coil = 150 and 10 mm.
Specimen no. | Varied parameters | Fixed parameters | ||||||
---|---|---|---|---|---|---|---|---|
Capacitor charging energy E [kJ] | Overlap between flyer and actuator o [mm] | Initial gap between flyer and target ginitial [mm] | Free length l [mm] | Capacitance C [µF] | Target thickness ttarget = Flyer thickness tflyer [mm] | Flyer width wflyer = Target width wtarget [mm] | Active length and width of tool coil [mm] | |
1 | 5.51 | 8 | 1 | 10 | 160 | 1 | 240 | 150 and 10 |
2 | 5.51 | 6 | 1 | 15 | ||||
3 | 5.51 | 8 | 2 | 10 | ||||
4 | 5.51 | 6 | 2 | 20 | ||||
5 | 5.51 | 6 | 3 | 15 | ||||
6 | 5.51 | 8 | 3 | 20 | ||||
7 | 6.48 | 8 | 1 | 15 | ||||
8 | 6.48 | 6 | 1 | 20 | ||||
9 | 6.48 | 6 | 2 | 10 | ||||
10 | 6.48 | 8 | 2 | 15 | ||||
11 | 6.48 | 8 | 3 | 10 | ||||
12 | 6.48 | 6 | 3 | 20 | ||||
13 | 7.50 | 6 | 1 | 10 | ||||
14 | 7.50 | 8 | 1 | 20 | ||||
15 | 7.50 | 6 | 2 | 15 | ||||
16 | 7.50 | 8 | 2 | 20 | ||||
17 | 7.50 | 6 | 3 | 10 | ||||
18 | 7.50 | 8 | 3 | 15 |
All parameter combinations were performed according to the principle set-up configuration shown in
Prior to conducting a welding experiment, the flyer sheet and target sheet dimensions were measured to ensure the correct geometrical parameters. Subsequently, both parts were cleaned thoroughly using steel wool and acetone. This removed all contaminations from the surfaces, such as dust particles or lubricating oil from the machining process. Otherwise, these contaminations could possibly interfere with the creation of a welded joint. In [
The weld quality was first determined by visual inspection of the weld shape and quantification of the length of the weld seam. Furthermore, metallographic examination was conducted to quantify the width of the weld seam and to investigate the interfacial morphology. Lap shear testing was performed to obtain the maximum transferable force. To that end, the hybrid sheet weld was cross sectioned perpendicular to the weld seam. In this way, 1 specimen (dimensions 5 × 30 mm) for metallographic examination and 3 specimens for lap shear testing (dimensions 45 × 270 mm) was obtained (see
Furthermore, the load course during magnetic pulse welding was characterised via measuring the inductor current. This was done with the aid of a Rogowski coil during all experiments. The following significant characteristics were determined from the current courses (see also
Significant frequency f, according to [
Maximum inductor current Imax
It turned out that the significant frequency varied between 42.7 and 48.5 kHz for all considered parameter combinations. Thus, none of the regarded varied parameters had a noteworthy effect on the frequency. The maximum inductor current varied between 229 and 301 kA.
Aluminium/copper sheet welds were successfully obtained for all parameters conditions considered. A typical example of such a hybrid sheet weld is shown in
all parameter combinations varied from 165 to 182 mm, which corresponded to approximately 69% - 76% welded length of the total sheet length. A summary of the weld lengths obtained is listed in
Visual examination showed that an indent shape located at the extremities of the weld was present, as indicated in
A typical metallographic cross-section obtained at the centre of the hybrid sheet weld is illustrated in
A detailed view of a typical interfacial morphology evolution of a welded zone is presented in
The width of the weld seam was quantified, based on measurements on the metallographic cross-sectional views as shown in
Lap shear testing was conducted in order to obtain the maximum transferable force of
Specimen no. | Visual inspection | Metallographic analysis | Lap shear test | |||
---|---|---|---|---|---|---|
Length of the weld seam [mm] | Width of the weld seam [mm] | Aluminium sheet thickness after welding [mm] (original thickness: 1 mm) | Failure position | Absolute value of the maximum transferable force in the lap shear test [kN] | Maximum transferable force/specimen width [N/mm] | |
Average of the 3 lap shear specimens | Average of the 3 lap shear specimens | |||||
1 | 173.4 | 1.1 | 0.9 | In the joint | 4.6 | 102.2 |
2 | 174.8 | 3.3 | 1.0 | In the aluminium base material | 5.3 | 117.8 |
3 | 168.5 | 4.8 | 0.8 | In the aluminium base material | 4.4 | 97.8 |
4 | 168.8 | 3.8 | 1.0 | In the aluminium base material | 5.2 | 115.6 |
5 | 165.2 | 2.3 | 0.7 | In the aluminium base material | 4.1 | 91.1 |
6 | 176.4 | 4.4 | 1.0 | In the aluminium base material | 5.1 | 113.3 |
7 | 175.9 | 1.4 | 1.0 | In the joint | 5.1 | 113.3 |
8 | 176.6 | 4.0 | 1.0 | In the aluminium base material | 5.3 | 117.8 |
9 | 172.6 | 3.8 | 0.8 | In the aluminium base material | 4.7 | 104.4 |
10 | 171.2 | 5.9 | 0.7 | In the aluminium base material | 4.2 | 93.3 |
11 | 167.9 | 4.9 | 0.5 | In the aluminium base material | 2.6 | 57.8 |
12 | 168.9 | 3.2 | 1.0 | In the aluminium base material | 5.2 | 115.6 |
13 | 177.8 | 4.2 | 0.9 | In the aluminium base material | 5.2 | 115.6 |
14 | 182.0 | 4.4 | 1.0 | In the aluminium base material | 5.3 | 117.8 |
15 | 173.9 | 4.3 | 1.0 | In the aluminium base material | 5.2 | 115.6 |
16 | 174.8 | 6.1 | 1.0 | In the aluminium base material | 5.2 | 115.6 |
17 | 174.4 | 5.2 | 0.6 | In the aluminium base material | 3.8 | 84.4 |
18 | 171.8 | 5.0 | 1.0 | In the aluminium base material | 5.1 | 113.3 |
the hybrid sheet weld. The dimensions of the lap shear test specimen were 45 × 270 mm (see
aluminium base material due to necking, either close to the joint or further away from the joint. In contrast, failure in the joint occurred in very few cases only.
For hybrid sheet welds with no aluminium thickness sheet reduction after welding,
Also, the maximum transferable force obtained was linearly correlated to the aluminium sheet thickness after the welding operation (see
wise, for failure in the joint, a transferable force of 5.1 and 4.6 kN was achieved at no aluminium thickness reduction and at a 10% aluminium thickness reduction, respectively. The highest % reduction of the aluminium sheet thickness was therefore observed for failure in the aluminium base material. This correlation between the maximum transferable force and the aluminium sheet thickness reduction will be discussed in more detail in Section 4.4. In this section, the welding parameters are related to the maximum transferable force.
Moreover, the occurrence of the 2 types of failure modes is related to the measured weld width (see
The relation between the various welding parameters and the aluminium thickness sheet reduction after welding, the maximum transferable force and the dimensions of the weld seam was thoroughly investigated. To this end, the individual results of the 18 different parameter combinations (orange dots in
examined. Based on that analysis, the main parameters affecting the weld characteristics and their general trends were identified (blue line in
Main parameters affecting the aluminium sheet thickness reduction near the welded zone
A strong interaction was suspected between these 2 parameters, which can be explained geometrically. For the lowest value of the free length of 10 mm, only an aluminium sheet length of 2 or 4 mm (depending on the overlap distance) remains that was not covered by the coil conductor. This small aluminium flyer sheet length should bend towards the copper target sheet over an initial gap distance of 3 mm in a time period of μs. Therefore, this parameter combination resulted in the largest reduction of the aluminium sheet thickness. The original aluminium sheet thickness of 1 mm was most likely retained for the lowest initial gap of 1 mm and the highest free length of 20 mm.
Main parameters affecting the maximum transferable force
A similar trend as observed for the aluminium sheet thickness has been identified for the effect of the welding parameters on the transferable force.
Therefore, the transferable force was investigated as a function of the aluminium sheet thickness (see
failure mode during lap shear testing. This is because welds with a large aluminium sheet thickness reduction of 20% or more, tended to fail in the aluminium base material due to necking (see
No effect of the capacitor charging energy or the overlap distance on the transferable force was identified for the regarded experiments. A Pareto analysis confirmed that the gap and the free length were the 2 dominating parameters affecting the transferable force (see
Main parameters affecting the weld length
An increasing weld length was mainly noticed at a smaller gap of 1 mm (see
Main parameters affecting the weld width
The capacitor charging energy and gap were the main parameters causing variation of the total weld width (see
A Pareto analysis confirmed that the capacitor charging energy and the gap were the dominating parameters affecting the weld length (see
More detailed insight in the dominating effects of the welding parameters was given via the predicted response graphs. These graphs show for each of the influencing parameters how it affects the weld characteristics, assuming that all other parameters are fixed to the pre-selected values. In particular, the predicted response graphs in
The predicted response graphs confirm the same trend of the effect of the capacitor charging energy, overlap, initial gap and free length on the weld characteristics, as observed for the individual results plotted in
Furthermore, an increase in weld width did not necessarily result in an increase in the transferable force. A hybrid sheet with narrow weld width, but without remarkable thinning of the aluminium, could therefore have a higher quality compared to one with long weld width but severe pre-damage in the aluminium. However, no unambiguous relation between the weld width and the transferable force was identified in these test series. This can be attributed to the aluminium sheet thickness reduction, which is the dominant parameter affecting the transferable force attained (see
operation, in order to eliminate the effect of thickness reduction on the transferable force.
Based on the results obtained by the Taguchi L18 design, 3 specimens showed a large reduction (from 30% up to 50%) of the aluminium sheet thickness near the welded zone after welding. It was proven that this was attributed to the sharp edge of the right spacer in combination with the specific parameters considered. This aluminium sheet thickness reduction in the welded zone significantly lowered the maximum transferable force attained. Hence, these particular parameter combinations were repeated using a rounded spacer. The modified experimental set-up showing a rounded spacer with a radius of 1.5 mm is illustrated in
All specimens with a rounded spacer displayed no visual reduction of the sheet thickness, which was beneficial for the joint quality and in particular the transferable force. However, when the sheet thickness was measured by optical microscopy, a small thickness reduction of 10 % was still present compared to the original sheet thickness of 1 mm. This small reduction was probably caused by elongation of the sheet material during the welding operation.
The transferable force for hybrid sheet welds produced with a straight, sharp-edged spacer and with a rounded spacer is compared in
Specimen no. | Hybrid sheet welds obtained with a straight, sharp-edged spacer | Hybrid sheet welds obtained with a rounded spacer | ||
---|---|---|---|---|
Aluminium sheet thickness after welding [mm] | Average maximum transferable force of 3 lap shear specimens [kN] | Aluminium sheet thickness after welding [mm] | Average maximum transferable force of 3 lap shear specimens [kN] | |
11 | 0.5 | 2.6 | 0.9 | 4.6 |
17 | 0.6 | 3.8 | 0.9 | 4.3 |
5 | 0.7 | 3.9 | 0.9 | 4.4 |
maximum transferable force is noticeable, the highest transferable force (up to 5.3 kN) which was obtained for welds without a sheet thickness reduction (see
The use of a rounded spacer in producing the hybrid sheet weld therefore had a beneficial effect. An increase of 28% - 80% in the aluminium sheet thickness and an increase of 13% - 77% in the transferable force was achieved, compared to the use of the straight spacer (see
Aluminium-copper sheets were joined by means of the electromagnetic pulse technology using different parameter combinations, selected according to a Taguchi L18 design. The weld shape, interfacial morphology, transferable force, failure modes and dimensions of the weld seam were examined in detail. Subsequently, these structural and mechanical characteristics were related to the welding parameters, namely the capacitor charging energy, the initial gap, the overlap distance and the free length. A Pareto analysis allowed identifying the most influential welding parameters. The trends between these dominating parameters and the weld characteristics were determined using predicted response graphs. A modification to the experimental set-up configuration consisted of using a rounded spacer instead of a straight, sharp-edged spacer. This was done to avoid aluminium sheet thickness reduction near the welded zone. The following conclusions can be drawn from the present experimental study.
Hybrid sheet welds were successfully obtained for all parameters conditions considered. An indent shape located at the extremities of the weld was attributed to the change of the current direction at the end of the coil conductor, resulting in a current concentration at the sheet extremities. The centre of the hybrid sheet welds evolved from a non-welded zone to a welded zone. Based on measurements and modelling studies found in literature, this can be explained by the evolution in the direction and magnitude of the Lorentz forces, which determine the impact angle and velocity during the welding process. Under the correct welding conditions, a jet occurred that removed the oxide layers and hence established a metallurgical bond. The welded zone started with a wavy interface with interfacial layers and evolved to a relatively flat interface without interfacial layers.
The absolute values of the maximum transferable force during lap shear testing ranged from 2.6 to 5.3 kN. Accordingly, the transferable force/specimen width during lap shear testing ranged from 57.8 to 115.6 N/mm for a lap shear specimen width of 45 mm. The maximum transferable force depended on the minimum specimen thickness and the strength of the hybrid sheet weld. For no aluminium sheet thickness reduction, the maximum transferable force was determined by the hybrid sheet weld strength and failure occurred in the base material. Hence, high quality joints were obtained for no aluminium sheet thickness reduction and for a sheet weld strength which was at least as high as that of the base material. For small aluminium sheet thickness reductions (approximately 10%), and for a sheet strength slightly lower compared to that of the base material, failure occurred in the joint. At large aluminium sheet thickness reduction of 20% or more, the maximum transferable force was linearly correlated to the aluminium sheet thickness and hence failure occurred in the base material. In this case, a reduced aluminium sheet thickness resulted in a lower maximum transferable force.
The most effective way to increase the transferable force was to lower the initial gap to 1 mm and to increase the free length to 20 mm, since both parameters resulted in no aluminium sheet thickness reduction near the welded zone. Alternatively, the use of a rounded spacer to produce the hybrid sheet welds resulted in a higher transferable force and a smaller aluminium sheet thickness reduction, compared to hybrid sheet welds produced with a straight, sharp-edged spacer. Therefore, this modified experimental set-up allowed to decrease the effect of the aluminium sheet thickness reduction on the transferable force achieved. The weld width obtained ranged from 1 to 6 mm and an increase was mainly achieved for an increase in capacitor charging energy and initial gap. However, an increase in weld width did not necessarily result in an increase in the transferable force. A narrow weld width could therefore have higher joint quality. The weld lengths obtained ranged from 165 to 182 mm and an increase was noticed for a decrease in the initial gap.
This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No. H2020-FoF-2014-677660― JOIN-EM.
Kwee, I., Psyk, V. and Faes, K. (2016) Effect of the Welding Parameters on the Structural and Mechanical Properties of Aluminium and Copper Sheet Joints by Electromagnetic Pulse Welding. World Journal of Engineering and Tech- nology, 4, 538-561. http://dx.doi.org/10.4236/wjet.2016.44053