Effect of interfacial characteristics on magnesium to steel joint obtained using FAST

Interfacial reaction during friction stir assisted scribe welding of immiscible Fe and Mg alloy system

“…An iron-rich particle of approximately 100 nm size was observed ~400 nm away from the interface on the Mg side. This is likely a result of the scribing, similar to what has been routinely observed in macro cross sections in previous FaST work 12 . Nevertheless, this is a first observation of nano sized stray steel particles in FaST welding.…”

“…These regions were mechanically transferred to the Mg side by the scribe action during the FaST process. While micron sized stray steel particles in the Mg side has been reported previously 12 , this is the first observation of nano-sized steel particles on the Mg side in FaST process. Additionally, the overall region on the Mg alloy side is enriched to ~ 10 at.% Fe.…”

“…1 (b)) shows the interface bonded area (inside solid rectangular outline). In comparison to typical FaST joints 12 , smaller plunging depth resulting in ~ 40 μm of scribe engagement was used during welding to ensure interfacial fracture. Thus, the characteristic steel hook feature is significantly reduced.…”

“…It is likely that Al diffused from the Mg side towards the interface in response to high temperature, pressure and to a negative heat of mixing with Fe. Owing to high welding speed, short diffusion time, and weak diffusion coupling of Al towards Fe, a nanosized Al rich layer (40 ± 10 nm) formed at the interface 12 , 18 , 19 . Al has excellent coherence with both Mg and steel facilitating its intrusion into the interface as a coupling layer.…”

“…We used two specific combinations: (a) AZ31 Mg with uncoated DP 590 steel and (b) pure Mg with uncoated DP 590 steel. The Zn layer was chemically removed from the as-received DP 590 to avoid Mg/Zn interaction and eutectic formation at the interface 12 . In case (b), commercially pure elemental Mg 13 was used to avoid the effects of alloying elements.…”