3.2.1 Microstructure of joint surface at different overlapping lengths
Figure 10 shows microstructure along the longitudinal section at overlapping lengths of 10 mm, 9 mm, 6 mm, and 4 mm, respectively. The distribution of shear strength changed with variation of overlapping length by comparing mechanical properties and peel morphology at different overlapping lengths, indicating that the joining process changed in the process of MPWed Al-Ti.
At the overlapping length of 10 mm, the joint interface closed to the free end is shown in Fig. 10(a1). It has been tightly bonded with a uniformly distributed transition layer, showing an effective joining area. The shear strength is 50MPa, and the effective joining length of about 2-3mm, as observed in Fig. 6(a). Fig. 10 (a2) shows that the transition layer was not observed between BM of Al and Ti but with a crack gap, indicating an invalid joining at an overlapping length of 5mm. Fig. 10(a3) shows the microstructure of joint surface near the end of BM, indicating an effective joining with a shear strength of 65MPa, which is higher than that of the wavy surface with a transition layer, and effective joining area has a length of about 2-3mm with waves being observed. Yu et al.  pointed out that wave interface was caused by Helmholtz instability, and starting position of waves is regarded as an efficient impact starting point. As seen in Fig. 10 (b1), as the overlapping length decreased to 9 mm, there is a noticeable gap between Al and Ti from the free end to end of BM, because the outer tube impacted the inner rod at an angle parallel to the inner rod, which could not form an effective joining. However, the BM's end was constrained with an effective impact angle, resulting in a small area of tight joining (Fig. 10(b2). Fig. 10 (c) shows microstructure along the longitudinal section at an overlapping length of 6 mm, where Al and Ti were closely joined. Microstructure distribution is relatively regular in the whole joining range. The joint surface is divided into two types. The one is the joint surface with a discontinuous transition layer and a shear strength of 45MPa (Fig. 10 (c1)), which is similar to that of the joint with a transition layer of large thickness at an overlapping length of 10mm. And the other one is the wavy surface without transition layer (Fig. 10 (c2)) with a shear strength of 63Mpa.
The outer aluminum alloy tube has undergone severe deformation at the left side of point D, mainly caused by material flow when the outer tube impacted the inner rod at a higher impact speed. At the right side of point D, there are regular waves on the joint surface with its amplitude increasing gradually from point D to the right, thus point D is the initial point of the wave, namely, the initial point of impact. When overlapping length was reduced to 4mm, a thin transition layer was observed at point D1 with a shear strength of 48MPa, similar to the thick transition layer interface (Fig. 10(a2)). Therefore, the thickness of the transition layer has little influence on shear strength and there is a wavy feature with a small wavelength in the D2 zone with a maximum joining strength of 55MPa.
In conclusion, the strength of interface without a transition layer is greater than that with a transition layer in MPW joint of Al-Ti alloy, and the IMC Al3Ti was found in the transition layer in session 3.3, reducing mechanical properties. However, noticeable waves were observed in the effective joining area according to microstructure (Fig. 10) and joining strength (Fig. 6). Meanwhile, the boundary point between wave and invalid joining area is deemed as the initial point of the wave, namely, the starting point of efficient welding. Therefore, at an overlapping length of 10 mm, the central outer tube of the overlapping part first impacted the inner rod, then the impact extended gradually from center to both sides and the effective joining began at points A and B. At an overlapping length of 9mm reaching the equilibrium length, effective joining is hard to achieve without adequate impact angle until the point reached D. As the overlapping length was reduced to 6 mm and 4 mm, the impact position started from the free end, thus useful impact and effective joining started from point D to the end of joint.
3.2.2 Impact modes of the outer tube
The MPWed Al-Ti joints can be divided into three impact modes, bidirectional impact mode, overall impact mode and single orientation impact mode according to mechanical properties and cross-section characteristics of the joint surface at different welding parameters (Fig. 11). In bidirectional impact mode (Fig. 11(a1-a3)), the central overlapping area of the flyer impacted the inner rod firstly due to the maximum magnetic force in this area. And the impact angle is too small to achieve an effective joining area at this stage, which resulted in the invalid joining area at the central zone. As the impact progresses, the impact point shifts along the Ti bar surface horizontally on two opposite sides from the center. The impact angle is suitable for the welding conditions, forming the effective joining area at both ends as shown in Fig. 11(a3). In welded joints produced by overall impact mode at an overlapping length of 9 mm, the flyer impacted the rod at an angle nearly parallel to the inner rod (Fig. 11(b2)). It is hard to form an effective joining owing to the minor impact angle, only a tiny area of effective joining formed at the right side of the magnetic field zone (Fig. 11(b3)). In single-orientation impact mode, the initial impact point is close to the free end of the outer tube as shown in Fig. 11(c2). And then, the impact extended from the free end to the non-free end, and an effective joining formed in the whole overlapping area (Fig. 11(c3)) due to proper impact angle and impact speed.