Abstract:The microstructural evolution of the intermetallic compound (IMC) layer formed on and in boron steel that was hot dipped in an Al-7Ni (wt %) bath at 690 • C for 30-180 s was investigated, and the growth mechanism of the IMC was identified. Except for the solidification structure of Al, the reaction layer consisted of one layer on the steel surface and two layers in the steel interior. The reaction phase formed on the original surface of the steel was the Al 9 FeNi (T) phase, which has a monoclinic (space group: P21/c) crystal structure. The reaction phase formed from the T phase was the Fe 2 Al 5 (η) phase, which is orthorhombic (space group: Cmcm). The variation in thickness of the η phase increased linearly with increasing dipping time, which is in accordance with the diffusion growth, a growth mechanism of the η phase in Al-coated pure Fe and low-carbon steels. The Fe 3 AlC (κ) phase (which had a band shape with a width of 100 nm) and a cubic system were formed along the interface between the Fe2Al5 (η) phase and the steel.

Abstract:In laser welding and hot stamping Al-Si-coated boron steel, there is a problem that the strength of the joint is lowered due to ferrite formation in the fusion zone. The purpose of this study is to develop an Al-7 wt.% Mn hot-dip coating in which Mn, an austenite stabilizing element, replaces the ferrite stabilizing element Si. The nucleation and formation mechanism of the reaction layer was studied in detail by varying the dipping time between 0 and 120 s at 773 • C. The microstructure and phase constitution of the reaction layer were investigated by various observational methods. Phase formation is discussed using a phase diagram calculated by Thermo-Calc TM . Under a 30 s hot-dipping process, no reaction occurred due to the formation of a Fe 3 O 4 layer on the steel surface. The Fe 3 O 4 layer decomposed by a reduction reaction with Al-Mn molten alloy, constituent elements of steel dissolved into a liquid, and the reaction-layer nucleus was formed toward the liquid phase. A coated layer consists of a solidified layer of Al and Al 6 Mn and a reactive layer formed beneath it. The reaction layer is formed mainly by inter-diffusion of Al and Fe in the solid state, which is arranged on the steel in the order of Al 11 Mn 4 → FeAl 3 (θ) → Fe 2 Al 5 (η) phases, and the Fe 3 AlC (κ) in several nm bands formed at the interface between the η-phase and steel.

Abstract:The formation mechanism of intermetallic compounds formed in boron steel hot-dipped in Al-7Ni (wt %) at 690 • C for 10-120 s was studied by identifying the intermetallic phases and investigating the growth process. Initially, a Fe 3 O 4 oxide layer formed on the steel. The oxide layer separated into multiple layers sporadically; following this, the Al-Ni molten alloy permeated into the region of the oxide layer breakdown and formed the Al 9 FeNi (T, monoclinic, space group: P21/c) phase on the steel surfaces. The Al 9 FeNi (T) phase formed from the reaction between the Al-Ni molten alloy and Fe eluted from the steel; this phase not only acts as an Al interdiffusion channel, but also as a barrier for Fe; and facilitates only grain growth without a significant change in thickness. Inside the steel, the Fe 2 Al 5 (η, orthorhombic, space group: Cmcm) phase grows along the c-axis in the [001] direction; and has a long columnar structure. The Fe 3 AlC (κ, Cubic, space group: Pm3m) phase is formed owing to a reduction in the Al concentration and the simultaneous diffusion and discharge of C toward the steel interface, as C cannot dissolve in the Fe 2 Al 5 (η) phase.