Cladding thick Al plate onto strong steel substrate using a novel process of multilayer-friction stir brazing (ML-FSB)

Friction stir brazing (FSB) using a pinless tool together with a braze was developed to extend bond area over friction stir lap welding. When using a large pinless tool of Ø40 mm to clad 10 mm thick 5083Al plate to 16Mn steel by FSB with Zn braze, the thermomechanical effect was sufficient to achieve wetting on Al side by dissolving Al into molten Zn, but joint could not be formed due to poor wettability on steel side. To enhance mechanical disruption of oxide film on the steel, multilayer FSB (ML-FSB) process was proposed by adding a thin (3 mm thickness) and soft aluminum sheet as interlayer. The first FSB pass created bonding between the thin interlayer and steel substrate with 4.7 μm thick FeAl₃ without Zn residue, having 55 MPa shear strength. The second FSB pass created bonding between the thin interlayer and thick 5083Al clad with a diffusion zone with a little Zn, having 50 MPa shear strength. This work provided a new way to fabricate “thick clad” layered composite with “strong substrate”, and suggested that mechanical route for strong substrate and metallurgical route (eutectic reaction) for Al for removing oxide film should be preferential and available in FSB.