1. Different coefficients of thermal expansion, potentially resulting in excessive distortion and residual stresses.

2. Limited solubility of the joining partners (in some important cases, such as aluminium and iron), potentially resulting in brittle intermetallic phases in the joining zone and inadequate joint properties.

9.3.1 General considerations

With respect to laser-based joining processes for lightweight structures, the stage of the joining process in the process chain has to be considered in order to select the appropriate process for joining. Basically, the joining of dissimilar metals can be performed during the production of semi-finished products such as tailored blanks (Engelbrecht et al., 2006) or during the production of the structure itself. The first option has the advantage of a mostly two-dimensional joining task and excellent fit-up tolerances. However, the tailored blank in general and the joint in particular will have to show a good formability for the subsequent stamping process, which is usually required to obtain three-dimensional structural elements. Conversely, joining of three-dimensional structural elements will not face the challenge of formability, but rather of poor fit-up tolerances (Thomy et al., 2007a).

Table 9.3 gives a concise, yet not comprehensive overview of some recent research work in the field of laser joining of dissimilar materials, grouped by material combinations investigated. Although these combinations are among the most challenging, most research work was carried out on joining of aluminium to steel and aluminium to titanium, two combinations with significant relevance for the automotive industry (Al-Fe) and the aircraft industry (Al-Ti), respectively.

9.3.2 Laser brazing and welding

For material combinations with low solubility, most laser-based joining processes can generally be grouped into those designed to melt only the aluminium side of the joint with or without filler wire and laser brazing processes, trying to avoid the melting of both base materials.

In the latter processing approach for joining of aluminium to steel, mostly high-power diode lasers (Saida et al., 2008) or Nd:YAG lasers are used for standard seam geometries such as raised edge welds and fillet welds in overlap configuration (Mathieu et al., 2005). As filler materials, a wide range of alloy systems based on zinc, aluminium or silver were considered. Partly depending on the selection of filler materials as well as on the surface layer of the steel, the use of flux is suggested.

The other processing approach, i.e. melting only the lower melting material with or without filler wire, is called ‘braze welding’ by some authors (e.g. Mathieu et al., 2007). The bond is then obtained by brazing with the lower-melting melt onto the higher melting substrate material (e.g., steel).

A typical example for joining aluminium to steel in overlap configuration by this approach is provided by Mathieu et al. (2007), where heterogeneous steel-aluminium assemblies were joined by welding in the aluminium and brazing on the steel sheet. The filler material was a Zn-based alloy chosen for its compatibility with the aluminium alloys and its low melting temperature. Similar results are described in Vrenken et al. (2009), however at elevated brazing speed of up to 4 m/min at a laser power of 4.5 kW.

For joining of aluminium to steel (zinc-coated) in butt joint configuration, a laser welding process using either CO₂ or Nd:YAG laser for sheet thicknesses in the range of 1 mm was suggested (Fig. 9.2) (see, e.g., Thomy et al, 2007a). The laser beam was positioned completely on the aluminium side of the joint to obtain a deep-penetration weld. The molten aluminium then wetted the steel sheet (to which flux was applied). Mainly for process stabilisation (and not predominantly for metallurgical reasons), filler wire was used.

Crucial factors of influence were the exact positioning of the beam relative to the interfacial region between aluminium and steel and the use of specially designed clamping devices to avoid the opening of a gap during the process. Using an Nd:YAG laser with 4 kW power, a joining speed of up to approx. 8 m/min was demonstrated, with the joints mostly failing in the aluminium.

A comparable approach was pursued for joining of aluminium to titanium for aircraft applications (see, e.g., Möller et al., 2010a). In this process, again the aluminium is molten in order to wet the higher-melting joining partner, the titanium. However, contrary to the above example for joining of aluminium to steel, a heat conduction and not a deep-penetration laser welding process is carried out. The sheets are arranged in vertical position with aluminium on top to achieve a good wetting of the titanium sheet under the effect of gravitation (Fig. 9.3).

A typical joint appearance from Möller et al. (2010a) and a cross section are given in Fig. 9.4. By the simultaneous joining process, a reproducible, constant wetting of the titanium for joint lengths of 1 m was achieved. For the same material combination, Chen et al. (2010) and Chen et al. (2011) investigated a single-sided joining process using filler for good wetting.

Whereas the two combinations steel/aluminium and aluminium/titanium (which are the focus of this chapter) are mainly considered for structural applications, joints between aluminium and copper or stainless steel and copper are of importance in electronics and precision mechanics.

Welding of (stainless) steel to copper (or brass) is a task encountered, for example, in precision mechanics for joining heat conduction elements (Mai and Spowage, 2004). These materials, such as aluminium and iron, typically also show limited solubility. However, as part dimensions and the melt pools are typically much smaller, heat input is smaller and the cooling rate is much higher than for macro applications considered for aluminium-steel and aluminium-titanium joints. Consequently, using pulsed laser spot welding techniques (melting both materials) can also be feasible, resulting in at least sufficient mechanical properties of the joints.

Such pulsed laser spot welding techniques are also used for joining of aluminium to copper especially in the micro range (Jokiel et al., 2005). For thicker sections, either direct laser welding in overlap configuration or laser joining using oscillating beam techniques (Kraetzsch et al., 2011) was considered to join sheet thicknesses of up to 2 mm, for example for application in electrical conductors.

9.3.3 Combined and special processes involving lasers

In order to further improve joint properties, a variety of combined processes have been suggested:

In Radscheit (1997), a process for joining of aluminium to steel in overlap configuration with additional, superimposed clamping/forming force enacted by a specially designed welding head is presented. The joining zone is irradiated by either Nd:YAG or high-power diode laser, thus heating both materials, melting the aluminium and wetting the steel with the aluminium (Fig. 9.5). By optimising heat input, the formation of intermetallic phases was restricted significantly. The basic advantage of this process is that joints with one flat side (optimised for material flow in forming tools) can be obtained (Fig. 9.6), however at the limitation that only two-dimensional welds with limited curvature can be produced (Ozaki et al., 2010). This seems to limit the process to tailored blank applications or aluminium.

More recently, a laser MIG hybrid welding process for dissimilar metals was suggested by Kreimeyer and Vollertsen (2006) and developed further by Thomy and Vollertsen (2009). In this process, a laser beam (CO₂, Nd:YAG, YB fibre or Yb:YAG) and a MIG-arc operated in the pulsed mode are used.

The process principle is depicted in Fig. 9.7. The aluminium and steel sheet are arranged preferably in butt joint configuration, also allowing a gap. The laser beam is positioned on the aluminium side. During joining, the edge of the aluminium sheet is molten, and together with the molten wire, the gap between the aluminium and the steel is bridged and the steel is wetted by the aluminium melt. Figure 9.8 gives the top view and the cross section of a typical aluminium-steel joint obtained by laser MIG hybrid welding using optimised parameter settings. It is obvious that the weld seam is smooth and its width and the wetting length are fairly constant over the total seam length. In particular, no weld sagging, undercut or excessive spatters were observed. There was no melting of the steel sheet, and the weld showed only few small pores close to the face side of the steel sheet. This was attributed to the use of flux on the face side, which can contain humidity and, as a consequence, be a source of hydrogen to the melt pool. Moreover, the authors have suggested a parameter envelope giving basic rules for adjusting the parameters to obtain appropriate joint properties (i.e. with respect to seam appearance, tensile strength and phase layer thickness), which equal or surpass those reported in the literature, however at a significantly elevated speed of 6 mm/min or higher (Fig. 9.9).

Although this process yielded state-of-the-art properties at high productivity, it was still required to apply flux to the uncoated edge of the zinc-coated steel sheet to support wetting. In order to avoid the use of potentially corrosive flux, a laser-plasma hybrid welding was developed by Möller et al. (2009) as a coaxial combination of laser beam and plasma arc. The principle of the laser plasma hybrid joining process is depicted in Fig. 9.10. A ring-shaped plasma beam is used to heat the steel and the aluminium and activate and clean the surfaces, whereas an Nd:YAG laser beam increases heat input and stabilises the plasma arc, thus allowing a higher joining speed and the joining of thicker aluminium sheets. The combined heat input led to a melting of the aluminium and a wetting of the steel by the aluminium, which was further promoted by the heating of the steel by the plasma arc. Overlap joints (ENAW 6016, 2mm + DC 05 + Z, 0.8 mm) were carried out using AlSi12 filler wire, and the authors found joining aluminium-steel tailored hybrid blanks without flux at an elevated speed of 2.6 m/min feasible.

9.3.4 Application potentials

As discussed above (and in detail in Thomy, 2009), one of the important application fields for laser joining of dissimilar materials is the transportation industry. The first significant area of application is the production of tailored hybrid blanks preferably from aluminium and steel, the focus of many application studies (see, e.g., Thomy and Vollertsen, 2009). An example of such a tailored hybrid blank (produced by Nd:YAG laser MIG hybrid welding) is shown in Fig. 9.11. The aluminium and steel sheet with a length of 400 mm were joined in butt joint configuration at a competitive speed of 66 m/min. Consequently, and taking into account the limits in formability in design of the parts as well as forming tools, such hybrid blanks may be used in lightweight structures in a way comparable to conventional tailored blanks.

The second field of application is the welding of tailored hybrid tubes or other three-dimensional structural elements (see, e.g., Thomy et al., 2007a). As an example, the tube displayed in Fig. 9.12(a) was produced by Nd:YAG laser MIG hybrid welding (welding speed 4 m/min) from aluminium and zinc-coated steel tubes with a diameter of approx. 10 cm. Joint configuration was fillet welding in overlap configuration, with the aluminium tube on top. Comparable elements and structures may be used for structural elements (e.g. axles) in the automotive industry.

However, not only the automotive industry, but also shipbuilding may be a potential field of application of dissimilar joints in more complex structures. In modern shipbuilding (especially of yachts and military vessels), material combinations of aluminium and steel are already used in order to improve structural properties predominantly in view of stiffness, weight and position of the centre of gravity. Whereas the hull is normally manufactured from steel, the upper decks or the deckhouse are made from aluminium alloys. A structural part of a yacht deckhouse (steel on the lower part of the demonstrator and aluminium on the upper part, thickness 3 mm each, length 1 m) joined in butt joint configuration by laser MIG hybrid welding using a CO₂ laser is shown in Fig. 9.12(b) (see, e.g., Thomy et al., 2007c). With this demonstrator produced in an industrial environment, a static strength of up to 140 MPa was achieved. This is a significant improvement compared to the so-called ‘Triclad®’ profiles (explosion welded transition profiles between aluminium and steel) used in shipbuilding today.

Comparably large structures from dissimilar materials are also found in aircraft. A potential application for laser joining of dissimilar materials is found in the so-called seat track, to which seats and other cabin interior are mounted. Currently, in most cases, aluminium is used for these seat tracks. However, to avoid defects from the presence of liquids in the passenger cabin, it is desired to use titanium instead of aluminium in the top area of the seat track. Since the remaining seat track part should still be made of aluminium (especially for cost reasons), laser joining for aluminium titanium seat tracks was developed (Möller et al., 2010a). Figure 9.13 shows a demonstrator seat track with a length of 1 mm produced by the process presented in Section 9.3.3, fulfilling all relevant industrial requirements with respect to process and properties.

9.4.1 Joint formation and the intermetallic phase layer

In most laser processes discussed, the bonding between the different joints is obtained by brazing, which makes two aspects especially crucial for the properties of the joints:

For the case of joining of aluminium to steel, several authors have identified the relevance of a zinc coating on the steel substrate (Kreimeyer et al., 2004; Güngör and Gerritsen, 2008) for the wetting process in general and the resulting wetting length in particular. In case wetting occurs, typically a marked zinc-rich region is observed in the interfacial zone, mostly close to the tip of the wetted zone (Fig. 9.14) (see, e.g., Mathieu et al., 2007). Studies by Vollertsen and Thomy (2009, 2011) seem to suggest that the wetting length is essentially correlated to the melting isotherm of the zinc coating. However, the behaviour of the zinc coating during wetting is not completely elucidated yet.

Provided wetting has occurred, typically an intermetallic phase layer is formed. For the case of joining aluminium to steel, Wirth et al. (2007) has taken a closer look at a typical intermetallic phase layer (Fig. 9.15 right) and subjected it to EDX (Fig. 9.15 left) and EBSSD (Fig. 9.16) (see also Thomy, 2009). It was established that the intermetallic phase layer consisted of Al₅Fe₂ and Al₃Fe. The Al₅Fe₂ phase forms a continuous layer between the aluminium and the steel, whereas the Al₃Fe-phase forms needle-shaped structures protruding into the aluminium matrix. Comparable results were found by Mathieu et al. (2007).

Regarding crystal orientation, there seems to be a preferred growth direction for Al₅Fe₂ perpendicular to the steel sheet surface, whereas the Al₃Fe needles show a more or less random orientation. This was interpreted by Wirth et al. (2007) as follows: initially, only the Al₃Fe-phase is created during heating by diffusion of Fe atoms into the aluminium as a layer between the two metal sheets. The continuous phase of Al₃Fe₂ is created by diffusion of Al atoms into the iron at temperatures above the melting temperature of aluminium (Radscheit, 1997). The formation of the needle-shaped Al₃Fe phase should then be connected to the cooling at temperatures between 50 and 350°C.

As is to be expected by the nature of phase formation, heat input and, consequently, process parameter variations can have a significant effect on phase layer thickness. For example, for the case of Nd:YAG laser MIG hybrid welding of aluminium to steel, Thomy and Vollertsen (2009) have found a slight trend towards an increase in intermetallic phase layer thickness with increasing laser power (Fig. 9.17), staying below approximately 4 mm, however, for most laser powers. For a laser power of 3400 W, a marked increase in phase layer thickness to 12 mm on average at significant scatter was observed, and cracking occurred without external load. This substantiates the generally held belief that a phase layer thickness exceeding 10 mm should be avoided in all cases (see, e.g., Radscheit, 1997).

For the case of joining of aluminium to titanium, Möller et al. (2010a) have investigated phase layer thickness depending on heat input (Fig. 9.18). Again, it was found that with increasing heat input, phase seam thickness is also increased.

9.4.2 Mechanical properties and formability

The mechanical properties in general, and tensile as well as fatigue strength in particular, of dissimilar joints are closely related to the two crucial aspects of bonding:

Generally, a strong correlation between wetting length and tensile strength is found (Möller et al, 2009). Figure 9.19 gives the correlation between wetting length and tensile strength for aluminium-steel overlap joints. For the same case, Mathieu et al. (2007) have found a roughly linear correlation between wetting length and tensile strength, despite significant scatter.

However, not only wetting length, but also the shape of the weld seam (which is, of course, often associated with wetting length; Möller et al, 2009), have an influence on static strength. In general, it is found that a smoother transition between the braze metal and the substrate (i.e., a smaller wetting angle, indicating better wetting) should improve tensile strength even under static conditions (Mathieu et al., 2007).

In fatigue testing of laser MIG hybrid welded samples, an influence of several geometrical parameters of the seam on fatigue strength was identified (Thomy et al., 2007b). In particular, with increasing gap and, consequently, decreased wetting length at otherwise unchanged parameters, the upper tension was reduced (Fig. 9.20).

Provided seam quality as such is adequate, the location of fracture not only depends on wetting length, but also on the cross section of the aluminium and the steel sheet. However, the interface zone with its steep property gradient is often associated with crack initiation or crack propagation understatic load as well as in fatigue testing. Thomy and Vollertsen (2009) found that in those cases where the interface zone was involved in fracture, either a phase layer thickness in the range of 10 mm or above or a defect close to the interface zone was identified. In the latter case, fracture typically started at a small pore close to the interface zone and then deviated along the intermetallic phase layer (Fig. 9.21). A closer look at the fracture surface typically reveals remainders of the intermetallic phase layer on both the steel and the aluminium side (see, e.g., Mathieu et al, 2007).

Comparable results were found for aluminium-titanium joints (Chen et al., 2010). Whereas for lower heat input, fracture was in the fusion zone, higher heat input resulted in a steep decrease in tensile strength due to fracture in the interface zone, which was often associated with porosity occurring at higher heat input.

Of significant relevance, especially for tailored hybrid blanks, is formability. For aluminium-steel tailored hybrid blanks produced at adequate parameters, a bend test is typically passed (Thomy and Vollertsen, 2011; Fig. 9.22). In particular, transverse cracking normally does not occur, and delaminations or cracks between the aluminium weld metal and the steel sheet surface are not observed. Moreover, typical limit drawing ratios of at least 1.6 are achieved.

In hydromechanical deep-drawing (with a multi-axial stress state), the shape given in Fig. 9.12 (length approx. 250 mm, width approx. 2500 mm, height approx. 45 mm) was achieved applying a pressure of 65 bar. Aside from some slight transverse cracks in the weld metal in the zone of the edge radius of the die plate, no defects were observed. Increasing the pressure much further resulted in longitudinal cracks in the weld metal. Aside from the difference in stress condition compared to static tensile testing and bend testing, another reason is assumed to lie in the fact that both tool and part design were not yet fully optimised. In order to further improve formability, both the forming process and the joint properties of the tailored hybrid blanks before forming should be dealt with. One means to address the latter aspect should be connected to the selection of filler wire with the aim of obtaining a more ductile weld metal.

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