The interest in the joining of Cu and Al alloys for some industrial sectors, such as the electrical mobility one, has been growing in recent years, despite their difficulties in laser based processing. Welding by means of lasers operating in the near-infrared (n-IR) field (typically 1060 nm) suffers from the low absorption coefficient of highly reflective materials, making joining these alloys difficult and inefficient. Recently, new laser sources have appeared on the market, with emission at 450 nm (indicated as blue radiation). Absorptivity coefficient of blue laser is significantly higher than n-IR laser in the case of Cu. The present work explores the use of a high-power blue laser source, lasers having enough power to be adopted in real industrial environments, for dissimilar welding of thin Cu and Al sheets. The evolution of the shape and size of the welded beads obtained by employing different combinations of laser power (2–3 KW) and welding speed (10–100 mm/s) values was evaluated. Thereafter, compositional and microstructural investigation, as well as mechanical tests, were performed to evaluate the quality of the joints. The selected process conditions promoted the generation of satisfactory welded beads, exhibiting good strength thanks to the efficient mixing of Cu and Al.
I. INTRODUCTION
Laser welding is one of the most widespread joining techniques and allows to obtain high quality beads in several industrial sectors, such as automotive,1 aerospace,2 and biomedical ones.3 In the last few years, novel applications of laser technology in welding and cutting high-reflective (HR) elements and alloys, such as Cu and Al ones, are emerging, thanks to the rising attention to electrical mobility.4,5 The adoption of common emission wavelengths in the infrared field, and mainly in the n-IR field with the characteristic values of 1060–1080 nm, is associated with very low absorption coefficients for HR materials, making their laser processing very inefficient and pretty far from large volume industrial applications.6 Moreover, welding needs to be conducted in a keyhole mode for achieving sufficient absorption coefficients, while conduction welding cannot be approached. Additionally, recent works investigated the pulse shaping in the n-IR field during welding. Ma et al. studied the effect of different pulse shapes, such as rectangular, trapezoidal, and triangular, in the welding of Al plates placed on top of Cu plates. Limited penetration of the welded beads from the Al into the Cu plates was achieved, due to the limited absorption coefficient.7
The absorption coefficient of HR materials increases at shorter emission wavelengths;8,9 therefore, green lasers were tested for welding pure Cu and Cu alloys with promising results, but the limited available powers, up to 1 kW, prevented large interest in such solutions.10,11
On the other hand, new laser sources, with low emission wavelengths down to 450 nm (called blue lasers), are emerging in the last few years: their main advantage lies in the huge increase in the absorption coefficient associated with HR alloys. For instance, the absorption coefficient of Cu at 450 nm can be up to ten times higher than in the n-IR field.9 Recently, literature reports some works exploring the impact of blue lasers in welding of Cu and Al in bead on plate configurations in low-power (200–500 W) and high-power (up to 2 kW) regimes: a widened processability window with respect to IR-laser welding and the formation of defect-less beads were observed.12 The combination of a blue and IR laser into a hybrid welding configuration was successfully explored, as well.13
Due to the limited irradiance values allowed by blue diode lasers, conduction mode welding is typically implemented for obtaining a stable process. Only high-power blue lasers can promote surface vaporization and hence the generation of deep welded beads.14 Tang et al. showed that Cu and Al plates can be successfully welded in the bead-on-plate configuration, achieving stable joining without spatter generation. Cu showed an absorption coefficient significantly higher than Al, promoting an increase in its melting efficiency; on the contrary, the higher thermal conductivity of Cu played an opposite role, decreasing its melting efficiency with respect to Al.15
Some also dealt with the welding in the overlapping configuration of low-thickness Cu to mild steel. Iqbal and Sadeghian found that no cracks or a substantial number of pores affected the beads quality using 1–1.5 kW power. An increase in hardness was detected in the heat affected zone (HAZ), justifying the fracture taking place in the HAZ.16
Currently, no studies on the dissimilar welding of HR alloys, such as Cu and Al plates, using high-power blue lasers can be found in the literature. Apart from their high reflectivity, the key issues in joining these materials are related to their different thermophysical properties, and the possible formation of brittle intermetallics during solidification. However, since in the field of electrical mobility the opportunity of joining Cu and Al is very promising, the present work aims to explore the use of a high-power blue laser (i.e., 3 kW) for dissimilar welding in overlapped Al/Cu and Cu/Al configurations. Welded bead shapes and sizes were correlated to the process conditions at varying laser power and welding speed; compositional analysis and mechanical testing of the beads, performed in selected process conditions, were carried out as well.
II. EXPERIMENT
A. Materials, laser equipment, and design of experiments
Thin sheets in pure Al (1.3 mm thick) and pure Cu (0.9 mm thick) were used in the present work. The main thermophysical properties of both elements are listed in Table I. The welding test was conducted by placing the sheets in overlapping (lap-joint configuration): Cu sheet overlapped to Al sheet and vice versa.
Principal thermophysical properties of pure Cu and Al elements (Ref. 17).
Parameter . | Cu . | Al . |
---|---|---|
Specific heat capacity (J/g °C) | 0.44 | 0.88 |
Latent heat of fusion (J/g) | 188 | 398 |
Density (g/mm3) | 0.008 94 | 0.002 70 |
Thermal conductivity (W/m °C) | 401 | 237 |
Melting point (°C) | 1083 | 660 |
Absorption coefficient at 450 nm | 0.6 | 0.1 |
Parameter . | Cu . | Al . |
---|---|---|
Specific heat capacity (J/g °C) | 0.44 | 0.88 |
Latent heat of fusion (J/g) | 188 | 398 |
Density (g/mm3) | 0.008 94 | 0.002 70 |
Thermal conductivity (W/m °C) | 401 | 237 |
Melting point (°C) | 1083 | 660 |
Absorption coefficient at 450 nm | 0.6 | 0.1 |
The welding system consists of a continuous wave high-power blue laser source (mod. LDF Blue 3CC from LaserLine) and a scanner head (mod. Intellscan 330 from ScanLab). The main characteristics of the laser equipment, whose schematic is depicted in Fig. 1, are listed in Table II.
Main characteristics of the laser equipment used for the dissimilar welding.
Max. laser power (kW) . | Emission wavelength (nm) . | Beam product parameter (mm mrad) . | Fibre core diameter, dfo (mm) . | Focal length, ffoc (mm) . | Collimation length, fcol (mm) . | Working distance, wD (mm) . |
---|---|---|---|---|---|---|
3 | 450 | 30 | 0.6 | 133 | 112 | 182.8 |
Max. laser power (kW) . | Emission wavelength (nm) . | Beam product parameter (mm mrad) . | Fibre core diameter, dfo (mm) . | Focal length, ffoc (mm) . | Collimation length, fcol (mm) . | Working distance, wD (mm) . |
---|---|---|---|---|---|---|
3 | 450 | 30 | 0.6 | 133 | 112 | 182.8 |
Before welding, all the sheets were cleaned with acetone to remove any surface contamination. After preliminary testing aimed at obtaining a large feasibility area, a design of experiments was conducted, according to the variation of laser power and welding speed (see Table III), for welding in lap joint the Cu sheet on Al and vice versa. Linear welding lines, having a length of 30 mm for reaching a stable behavior, were performed in each investigated process condition. Three replications for each process condition were done.
Design of experiments used for the investigation on Cu/Al and Al/Cu dissimilar welding. The welded beads obtained from the process conditions highlighted with * were analyzed in terms of microstructure and mechanical behavior.
Laser power (kW) . | Welding speed (mm/s) . |
---|---|
3 | 60–80*–100 |
2 | 10–20*–40–60 |
Laser power (kW) . | Welding speed (mm/s) . |
---|---|
3 | 60–80*–100 |
2 | 10–20*–40–60 |
B. Sample characterizations
The analysis of the results was conducted to explore the feasibility map at varying laser power and welding speed. At first, the picture of the upper and the lower surfaces of the beads was acquired by optical microscopy (OM, mod. Leitz Aristomet). Then, the cross sections of the beads were prepared according to the standard metallographic procedure: grinding the surfaces with SiC sand papers with mesh from 180 to 2500, then tissues with lubricants with mesh from 6 μm down to 1 μm; no final chemical etching was adopted. All cross sections of the beads were analyzed via OM with unpolarized light for highlighting the shape of the welded beads as well as for measuring the corresponding dimensions, such as width (W) at the top and the maximum penetration (P), as depicted in Fig. 2(a).
Schematic representation of the welding configuration (a) and the samples designed for mechanical testing in the shear configuration (not to scale) (b). The reversed configuration was used for Al–Cu joining.
Schematic representation of the welding configuration (a) and the samples designed for mechanical testing in the shear configuration (not to scale) (b). The reversed configuration was used for Al–Cu joining.
After the analysis of the beads shape and depth, one condition for each laser power value (see the conditions highlighted with * in Table III) was selected for further microstructural and mechanical characterizations. In detail, these selected cross sections were characterized with OM with polarized light for highlighting details about the dispersion of the two elements in the liquid pool and with scanning electron microscopy (SEM, mod. Leo 1413) equipped with EDS for compositional analysis.
Moreover, mechanical testing was conducted in the single-lap shear configuration, according to ASTM D1002 standard, on two selected process conditions, highlighted with * in Table III. An electro-mechanic MTS 2/M system was employed at a constant cross head speed of 0.5 mm/min, at room temperature. The specimens used for mechanical qualification, having a length of 100 mm and a width of 10 mm, are shown in Fig. 2(b).18,19
III. RESULTS
A. Exploration of the principal process parameters in the dissimilar laser welding of Cu/Al and Al/Cu configurations
Figures 3(a) and 3(b) show the upper and bottom views of the welded beads, obtained in the Cu/Al configuration in all the investigated process conditions. Upper sides of the beads presented a constant width in all the investigated conditions, while the bottom side exhibited full penetration in the most energetic condition only (3 kW at 60 mm/s). No spatter generation can be observed in any of the welding beads.
Top and bottom views of the welded beads, obtained in the Cu/Al configuration.
Figures 4(a) and 4(b) show the morphology of the beads produced in the Al/Cu overlapped configuration. The presence of Al on top prevents a successful joining regardless of the investigated process parameters. It is worth mentioning that Al exhibits an absorption coefficient and thermophysical properties, which are quite different from the ones of Cu.
Upper (a) and bottom (b) views of the beads obtained in the Al/Cu configuration.
Trend of the input energy E values, at varying welding speed and laser power, absorbed by both the Cu (a) and Al (b) sheets. Different vertical scales were used for better visualization of the calculated values.
Trend of the input energy E values, at varying welding speed and laser power, absorbed by both the Cu (a) and Al (b) sheets. Different vertical scales were used for better visualization of the calculated values.
Cu is also characterized by higher thermal conductivity and melting point than Al: this suggests that the heat accumulation during the Cu melting is less intense than for Al. In fact, the results depicted in Fig. 4(a) may be justified by the susceptivity of Al to cracking and oxidation reaction, promoting an excessive and unstable melting on the upper part of the sheet in the most energetic conditions. A fine, stable but not penetrating bead was produced in the other process conditions. Considering these preliminary results, the Cu/Al overlapping configuration was selected for further analysis.
B. Morphological characterization of the beads obtained in the overlapped Cu/Al configuration
The present paragraph focuses on the most promising configuration (Cu/Al), which allowed us to achieve satisfactory performance in terms of weldability. Figure 6 shows the cross sections of the welded beads, obtained at 3 and 2 kW at varying welding speed. The adoption of high power (3 kW) produced a complete penetration through the upper Cu sheet for all the investigated values of welding speed. Limited drop out on the Cu sheet can be seen at 60 and 80 mm/s. The lowest value of welding speed (60 mm/s) promoted a full penetration through both the overlapped sheets thanks to the highest E (120 J/mm). At higher welding speeds (3 kW–80 mm/s), a partial fusion of Al to Cu was obtained: no evident defects, such as cracks or pores, can be detected in this condition. A further increase in the welding speed up to 100 mm/s caused a narrowing of the joint region, which, despite the absence of defects, is likely too thin to grant sufficient mechanical resistance.
Micrographs showing the shapes of the welded beads, carried out at 3 and 2 kW and varying the welding speed from 10 to 100 mm/s. Yellow dotted lines highlight the contours of the welded beads.
Micrographs showing the shapes of the welded beads, carried out at 3 and 2 kW and varying the welding speed from 10 to 100 mm/s. Yellow dotted lines highlight the contours of the welded beads.
When lower laser power (2 kW) was adopted, the feasibility window was slightly reduced: only at 10 mm/s, the welded bead was deep enough to join the Al sheet to the Cu one, while the increase in the welding speed in the 20–60 mm/s range prevented full penetration in the upper Cu sheet. Differently from the high-power condition, the bead obtained at 2 kW and 10 mm/s presented large pores but no cracks.
The shape of all the welded beads, carried out at 3 kW, regardless of the welding speed, and at 2 kW and 10 mm/s, suggests that a deep-penetration condition was achieved. On the contrary, the use of 2 kW in the 20–60 mm/s speed interval promoted the typical, shallow beads characterizing conduction mode.
The evolution of the characteristic upper width (W) and penetration (P) of the beads is plotted in Fig. 7. W decreased with the increase in welding speed, while it increased at higher laser power. In detail, W decreased from 1.5 mm down to 1 mm at 3 kW by varying the welding speed from 60 to 100 mm/s. At 2 kW, W decreased from 1.3 to 0.7 mm as welding speed increased from 10 to 60 mm/s.
Geometrical features of the welded beads, W (a) and P (b), at varying welding speed and laser power.
Geometrical features of the welded beads, W (a) and P (b), at varying welding speed and laser power.
The evolution of the bead penetration, P, needs to be carefully analyzed because of its significant impact on the overlapping joining performance: Fig. 7(b) highlights the correspondence between P and the relative positioning in the joint between the two sheets (see the schematic on the right). At 3 kW, P increased from 1.2 to 2.4 mm by decreasing the welding speed from 100 to 60 mm/s: full penetration of the Cu sheet was always obtained, but full penetration of the Al plate was achieved only at the slowest welding speed (3 kW at 60 mm/s).
At lower laser power (2 kW), the overlapping joint was guaranteed only at the lowest welding speed (10 mm/s), resulting in a liquid pool depth of 1.4 mm. By increasing the welding speed, the values of W ranged in the 0.3–0.6 mm interval, preventing any possibility of joining the Cu and Al sheets.
The main output of this analysis was the identification of two process conditions, able to offer two overlapping joints, at both laser power values. Therefore, the selected process conditions are the following: (i) 2 kW at 10 mm/s and (ii) 3 kW at 80 mm/s. The evaluation of the mechanical performances of the Cu/Al joints, carried out in both the selected conditions and supported by microstructural and compositional investigation, is addressed in Secs. III C and III D.
C. Mechanical characterization of the beads obtained in the overlapped Cu/Al configuration
Figure 8 shows the results of the single-lap shear tests performed on samples welded with the chosen parameters at both low and high power. It may be immediately appreciated that samples welded at 2 kW were much more brittle than the ones welded at 3 kW. Indeed, 2 kW samples presented a maximum shear stress of 47.2 MPa, whereas an almost doubled value of 82.6 MPa was attained by 3 kW samples. This difference shall be ascribed to the much higher ductility of the latter group rather than to an intrinsic higher mechanical strength. As shown in Fig. 8(b), 2 kW samples failed inside the welded bead, on the Al plate side, due to the presence of large pores at the bottom of the melted pool (see Fig. 7) and the reduced penetration inside the Al plate itself. On the other hand, the use of the higher power allowed a more considerable penetration, as well as the absence of defects and a satisfactory mixing of the two elements, producing sound and strong beads, which resulted in high strength and satisfactory ductility. This resulted in an overall absolute toughness of 1.5 J, which, if normalized over the beads length, corresponds to 0.15 J/mm. Indeed, samples welded at 3 kW failed at the interface between the weld and the Cu plate, as visible in Fig. 8(b): it may be inferred that the heat diffusing from the bead toward the base material gave rise to a heat affected zone, likely causing local recrystallization. The reduction in local strength associated with the increase in grain size (Hall–Petch relation) may be responsible for the premature failure of the Cu–bead interface. Nevertheless, this point is a further testimony of the excellent quality and strength of the beads obtained in optimized processing conditions.
Mechanical characterization of the joints produced at 2 kW and 10 mm/s and at 3 kW and 80 mm/s in the single-lap shear configuration (a); photograph of the fractured samples (b).
Mechanical characterization of the joints produced at 2 kW and 10 mm/s and at 3 kW and 80 mm/s in the single-lap shear configuration (a); photograph of the fractured samples (b).
D. Metallurgical characterization of the beads obtained in the overlapped Cu/Al configuration
The present section focuses on the microstructural characterization of the welded bead produced at 3 kW and 80 mm/s, which resulted to offer the best mechanical performances. Figure 9 shows its cross section, where only a few rounded pores of micrometric size and no cracks can be detected.
Microstructural characterization of the welded bead obtained at 3 kW and 80 mm/s: optical micrographs acquired with bright-field (a) and cross-polarized (b) illumination; EDS map (c) and back-scattered electron SEM micrograph (d).
Microstructural characterization of the welded bead obtained at 3 kW and 80 mm/s: optical micrographs acquired with bright-field (a) and cross-polarized (b) illumination; EDS map (c) and back-scattered electron SEM micrograph (d).
A complex mixing of Cu and Al takes place within the welded bead, as evidenced by both the alternation of gray, yellow, and red colors in Fig. 9(a) and by the EDS map in Fig. 9(c): the likely formation of turbulent vortices during laser melting causes a strong intermixing of the two metals and an evident outcrop of molten Al in the Cu plate. Indeed, during deep penetration welding, the high temperature at the bottom of the keyhole generates a recoil pressure, which pushes away the locally melted material (Al in the present configuration). The Al flow, deflected by melt pool borders and aided by its lower density, will be directed upwards, generating the grayish stripes elongating along the section of the melt pool in Fig. 9(a).20 Concurrently, a back and forth flow of materials parallel to the welding direction, driven by Marangoni force, is expected to take place and further aid in the mixing of different elements.21 Moreover, a lateral enlargement of the molten pool can be observed in the Al plate: as Al is characterized by a lower melting temperature than Cu, the heat propagating from the Cu molten pool is likely able to melt a larger amount of Al, thus causing the lateral expansion below the Cu–Al transition line. The observation of Figs. 9(b) and 9(d) allows to recognize several phases, which are typical of the Cu–Al alloys and reflect the progressive shift in composition in the related phase diagram.22 Pure Al can be recognized as the darkest (i.e., the lightest) phase in the back-scattered electrons (BSE)-SEM micrograph (point 1); above the melt pool border, a transition area exists, which is about 160 μm thick and encompasses several regions: a fine dendritic area originating from the solidification of a quasi-pure Al melt (region 2), a seamless, middle-gray eutectic area (region 3), the ϑ-Al2Cu phase solidifying as relatively elongated dendritic branches (region 4) and, finally, the brighter η-AlCu phase (region 5) forming a continuous layer on the Cu side and a cellular structure next to the ϑ area. On top of this transition layer, which encompasses regions 2–5, the previously described alternation of Cu-poor (region 6) and Cu-rich (region 7) bands, produced by vortices, can be observed. The use of cross-polarized illumination clearly shows that region 7 [appearing as a very bright, light-blue area in Fig. 9(b)] contains acicular structures (round inset), which can be attributed to β′ martensite, probably originating from a parent cubic β phase because of the high cooling rate experienced by the welded bead after solidification.23 On the contrary, region 6 does not present any acicular structure, as its higher Al content prevented the activation of the previously mentioned martensitic transformation: it may be related to the γ-Al4Cu9 phase.
IV. CONCLUSIONS
The overlapping welding between thin Cu and Al sheets, conducted by a high-power blue laser, was investigated in terms of feasibility, microstructure, and mechanical response.
The following conclusions can be drawn:
- Successful joining of overlapped Cu and Al was achieved, creating a deep and stable bead, while the reversed Al/Cu configuration did not result in a satisfactory joining. This behavior can be explained by the large variation of the absorption coefficients between Cu and Al, but also by their different thermophysical properties.
- In the overlapping Cu/Al configuration, the 3 kW laser power granted the full penetration of the Cu sheet at all the investigated welding speeds. An intermediate welding speed (80 mm/s) was selected for further microstructural and mechanical characterization. On the contrary, an extremely low welding speed of 10 mm/s was required to obtain joining at 2 kW.
- Single-lap shear testing demonstrated that a relevant shear strength, up to 82.6 MPa, and a satisfactory ductility could be obtained by the bead welded at high power. Failure occurred outside from the welded bead, further confirming its good quality.
The use of blue lasers opens new opportunities in the electrical mobility field, where highly reflective materials need to be processed with high productivity and quality requirements. Further investigations will focus on deeper analysis of the microstructure in optimized process conditions.
AUTHOR DECLARATIONS
Conflict of Interest
The authors have no conflicts to disclose.
Author Contributions
Jacopo Fiocchi: Data curation (equal); Formal analysis (equal); Investigation (equal); Writing – original draft (equal). Stefano Zarini: Investigation (equal). Tugay Kurtay: Investigation (equal). Ausonio Tuissi: Data curation (equal); Funding acquisition (equal); Project administration (equal). Carlo Alberto Biffi: Conceptualization (equal); Data curation (equal); Formal analysis (equal); Investigation (equal); Methodology (equal); Project administration (equal); Writing – original draft (equal).