Interface microstructure and tensile properties of a third generation aluminium-steel butt weld produced using the Hybrid Metal Extrusion & Bonding (HYB) process

The Hybrid Metal Extrusion & Bonding (HYB) process is a patented solid state joining method for metals which utilizes filler material additions to consolidate the weld. In the present investigation the interface microstructure and tensile properties of a 4 mm thick joint, belonging to the third generation Al-steel HYB butt welds, are characterized. The mechanical testing shows that the HYB weld exhibits excellent tensile properties, displaying ultimate tensile strength values in the range from 238 to 266 MPa. Moreover, supplementary digital image correlation analysis of the strain evolution occurring during tensile testing reveals that all plastic deformation is localized to the soft heat-affected zone on the aluminium side of the joint, leading to necking and final fracture in the aluminium. Scanning and transmission electron microscopy examinations of the Al-steel interface show that bonding occurs by a combination of microscale mechanical interlocking and intermetallic compound formation. The intermetallic layer has a thickness varying between 0.1 and 1 μm and is composed of Al–Fe–Si crystals. This makes the butt joint highly resistant against interfacial cracking. The subsequent benchmarking against commercial fusion and solid state welding methods reveals that the tensile properties of the Al-steel HYB weld even surpass those reported for comparable friction stir welds. At the same time, the HYB process allows butt welding to be performed at much higher speeds compared to friction stir welding, without compromising the mechanical integrity of the weldment. •Tensile strengths up to 265 MPa can be achieved in Al-steel welds made by HYB.•The tensile properties exceed those reported for comparable friction stir welds.•Final fracture always occurs in the HAZ on the Al-side of the joint.•Bonding occurs by a combination of microscale interlocking and IMC formation.•The 0.1–1 μm thick IMC layer is composed of Al–Fe–Si crystals.