Atomic level bonding mechanism in steel/aluminum joints produced by cold pressure welding
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Cold pressure welding of aluminum alloys and steels offers an attractive, cost-effective opportunity of joining the two most important structural materials. Aim of this work is to investigate the bonding mechanism between aluminum alloy EN AW6082 and carbon steel C15, joint by cold extrusion welding after targeted heat treatment. Mechanical testing under tensile loads revealed site specific interface strengths between 30 and 60% of the constituent materials. However, investigation of fracture surfaces indicates that interface strengths can exceed the AW6082 strength in areas where the bonding process substantially enlarged the Fe-Al interface region, due to the welding geometry. Near atomic scale investigations of the interface using aberration-corrected scanning transmission electron microscopy combined with electron energy loss spectroscopy disclose two different regions along the cold pressure welded interface. The majority of the interfacial area is oxygen free with Fe-based and Al-based crystals joining. No intermetallic phase was identified. Adhesion is attributed to a more covalent bond characteristic at the Fe-Al interface evidenced in the Al-L2,3 edge. A small fraction of the steel/aluminum interface was covered with oxidic regions of about 10 nm thickness. This oxide is predominantly an amorphous aluminum oxide, with only few Fe-O bonds. The observed metallurgical bond formation is explained by an extended Bay model through (i) cracking of the brittle, nanoscopic native oxide layer on the materials during forming, (ii) formation of metallic contact between Al and Fe, under reduction of iron oxides by aluminum, and (iii) formation of covalent-like Al-Fe bonds across the interface.