Al/CFRP joining with interlayers

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157 structures, as they both have a cubic centred face (cfc) crystallographic structure thus, it can also act as a dumper to reduce the impact force on the CFRP tube.

The coatings were produced with electrolytic deposition over the CFRP tube to improve the adhesion between them. If welding is established between the aluminium tube and the metallic coating it will have good adhesion to both parts, the CFRP tube and the aluminium fitting, thus, the joint strength will increase. Welding can also help to homogenize the forces distribution to avoid cracks on the aluminium tube.

Cracks on the aluminium tube were avoided using a thicker aluminium wall (1.5 mm), however, with the metallic coating cracks have been avoided even using 1 mm thick aluminium tubes due establishing a weld with the coating which has good adhesion with the CFRP tubes. By having continuity along the joint, the stress can be more evenly distributed between both parts thus, the CFRP tube does not transfers all the force to the aluminium, being some transferred to the coating, which is more ductile, thus, more deformable.

As achieved for the 1 mm thick flyer, a good fit between the aluminium tube and the CFRP tube was produced with the 1.5 mm thick counterparts.

The aluminium was tight to the CFRP tubes both with the 6 (Figure 6.27 detail a) and the 8-turn coil (Figure 6.27 detail b) for 2.5 kJ of discharge energy. However, with 3 kJ the joint height in contact with the target is larger which could result in a higher joint resistance, as shown in section 6.3.

Figure 6.27 – Interface details from samples with 1.5 mm thick wall flyer:

a) G32 parameters: DE=2.5 kJ; SD=1 mm; 6-turn coil and aluminium mandrel);

b) G33 parameters: DE=2.5 kJ; SD=1 mm; 8-turn coil and aluminium mandrel).

Good adhesion was achieved for both electrolytic coatings of copper and nickel with the CFRP substrate.

Welding between the coating and the tube was observed for both discharge energies tested (Figure 6.28) but it had smaller welded length and was more intermittent

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for the lower energy resulting in failure and delayed aluminium cracking. Nevertheless, with 3 kJ, a larger welded length was achieved which helps to avoid cracks on the aluminium side (Figure 6.29 and Figure 6.30).

Figure 6.28 – Details from samples with 50 μm thick copper coating on the CFRP tube:

a) G38 parameters: DE=2.5 kJ; SD=1 mm; 6-turn coil and aluminium mandrel;

b) G39 parameters: DE=3 kJ; SD=1 mm; 6-turn coil and aluminium mandrel.

Figure 6.29 – Cross section of sample G38 welding parameters: DE=2.5 kJ; SD=1 mm; 6-turn coil, aluminium mandrel and 50 μm thick copper coating on the CFRP tube)

Figure 6.30 – Cross section of sample G39 welding parameters: DE=3 kJ; SD=1 mm; 6-turn coil, aluminium mandrel and 50 μm thick copper coating on the CFRP tube

159 The larger continuous weld observed was between the Aluminium tube and the Nickel coating, due to the better affinity between both materials and a good adhesion to the substrate (Figure 6.31).

Figure 6.31 – Details from sample G40 welding parameters: DE=3 kJ; SD=1 mm; 6-turn coil, aluminium mandrel and a 80 μm thick nickel coating on the CFRP tube

No significant damage was observed, showing that some of the energy is now used to weld and to deform the coating thus, less energy will be transferred to the CFRP tube.

No compound at the nickel aluminium interface was observed showing that no liquid phase was present due to the low energy transferred contrary to other studies where Ni-Al binary phases in the form of an amorphous alloy were observed at the interface due to melting and rapid solidification of both materials. This intermediate layer was visible for discharge energies of 2 kJ, in sheet to sheet overlap configuration. Ni particles were observed encrusted to the Al matrix near the interface and no intermetallics were found[102].

Welding between the aluminium tube and the nickel coating was confirmed with the SEM and EDS inspections. As depicted in Figure 6.32 the nickel has good adhesion to the CFRP tube. Between the aluminium and the coating, a tight interface was achieved with almost a full perimeter.

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Figure 6.32 – SEM image of the interface of specimen G40 Welding parameters: DE=3 kJ; SD=1 mm; 6-turn coil and aluminium mandrel

The black line of Figure 6.32 is a shadow effect between both materials since, as shown in Figure 6.33 there is material in that region, mostly of the coating but also of aluminium (around 16%).

Figure 6.33 – EDS profiles from the nickel aluminium interface of specimen G40 Welding parameters: DE=3 kJ; SD=1 mm; 6-turn coil and aluminium mandrel

Concluding, the materials selected proved to be efficient to absorb some impact energy, avoiding damages to the CFRP tubes, with good adhesion with the substrate.

Welding was observed for both the copper and the nickel coatings however, the welded length was larger for the Ni coating due to the better affinity between both materials. Also, no interlayer was observed due to the presence of a liquid phase during the MPW process, at the interface, since the energy used was low for tubular overlapped configurations.

The coatings also help avoiding the delayed cracking phenomena reported due to both absorbing part of the energy which would be stored as elastic deformation on the CFRP tube and increasing the resistant section by welding with the aluminium tube. Due to the good adhesion with the substrate, the expanding force is more evenly distributed however, it should be combined with a larger wall thickness on the aluminium tube (1.5 mm) to ensure the aluminium cracks are completely avoided.

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