Microstructural evolution of Al-Si coating and its influence on high temperature tribological behavior of ultra-high strength steel against H13 steel

Phase Composition of Al-Si Coating from the Initial State to the Hot-Stamped Condition

“…According to the chemical composition (point 5, Figure 1 b), this sublayer was identified as η-Fe 2 Al 5 with an orthorhombic structure and the Cmcm space group, which has also been mentioned in Refs. [ 4 , 5 , 6 , 18 , 20 , 27 , 28 ]. The content of Si ( Table 2 ) in this phase corresponded to that observed by Windmann et al [ 6 ], who reported an even slightly higher content of Si in the η-Fe 2 Al 5 phase, reaching up to 6.1 at.%.…”

“…Contrary to this, the authors in Refs. [ 4 , 5 ] identified, in addition to the Fe 2 Al 5 and Fe 2 Al 8 Si phases, a FeAl 3 phase. The identification was based on the chemical composition determined by SEM-EDS.…”

“…In both variants, the input material is low-alloyed steel with a ferritic/pearlitic microstructure and a strength of approximately 600 MPa [ 2 ]. In direct hot stamping, the sheet blank is heated to temperatures of 850–950 °C and held for 3–10 min to obtain a fully austenitic microstructure with a low tensile strength and high ductility (~200 MPa, ~40% total elongation) [ 4 , 5 , 6 , 7 , 8 ]. After subsequent transferral into the water-cooled press and a temperature reduction to 600–800 °C, the blank is formed and hardened in a single step.…”

“…Therefore, the Al-Si-coated press-hardened steel is most widespread in the automotive industry. Its advantage is that the Al-Si coating can sustain high temperatures and protects the steel substrate from oxidation and decarburization by forming thermodynamically stable oxides rich in aluminum and silicon [ 2 , 4 , 5 , 9 ]. Moreover, in contrast to the Zn coating, the Al-Si coating does not suffer from liquid metal embrittlement during direct hot stamping, and further surface treatment, such as sandblasting or dry ice cleaning to remove the oxide layer, is not applied for the Al-Si coating compared to the Zn coating [ 11 ].…”

“…Furthermore, the continuous layer or island-like phase, denoted as FeAl in previous works, was described as Fe 2 SiAl 2 (τ 1 ) and Fe 2 Si 2 Al 5 (τ 2 ) in Refs. [ 5 , 22 , 23 ], with an interdiffusion layer formed of Fe 3 Al [ 5 , 23 ]. The most recent work from Cho et al [ 24 ] stated that the three-phase interdiffusion layer consisted of α-Fe(Al), Fe 3 Al, FeAl, the matrix of Fe 2 Al 5 , and a continuous layer or island-like phase distributed in the matrix as a mixture of FeAl and an unidentified Fe x Al x Si z ternary phase.…”