Fabrication of the τ5c Intermetallic Compound Monoliths by a Novel Powder Metallurgy and Hot-Dipping Approach
2020, The Minerals, Metals & Materials Society and ASM International. The precipitation of discrete iron-based intermetallic compounds (IMC) is an inevitable phenomenon in a steel strip alloy coating pot due to the continuous supply of iron from the steel strip and temperature perturbations in the pot. These IMC [τ5c: Al20Fe5Si2(Zn) or Al8Fe2Si(Zn)] particles grow by coarsening and agglomeration and are the primary source of coating inclusions for coated steel products. They are further responsible for excessive bottom dross build-up in the alloy coating pot. It is hypothesized that they will re-dissolve into the pot during the heating cycle of the coating bath, but their dissolution kinetics remains unclear. The underlying hold-up is the lack of large IMC samples for quantitative studies of their dissolution kinetics in the molten coating alloy. To quantify and understand the dissolution of the τ5c IMC phase, this study reports the fabrication of monolith samples of the τ5c phase by a novel powder metallurgy and hot-dipping approach. Firstly, τ5c IMC particles were extracted by acid dissolution of the bottom dross collected from a commercial alloy coating pot. A powder metallurgy route was then used to fabricate an intermediate form of the monolithic τ5c IMC disks of 14.80 mm in diameter and 17 mm in height. X-ray diffraction (XRD) and electron transmission microscopy (TEM) analyses revealed that the initial τ5c IMC phase transformed into α-Al8Fe2Si (τ5H) and Al3Fe phases during isothermal sintering at 900 °C due to devolatilization of Zn from the τ5c phase. However, when these intermediate IMC monoliths were immersed in the Zn-concentrated and iron-saturated 55 pct Al-43.4 pct Zn-1.6 pct Si (AZ) alloy coating bath at 595 °C, both the α-Al8Fe2Si (τ5H) and the Al3Fe phases transformed back into the τ5c IMC phase through diffusion of Zn into the as-sintered monoliths. The fabrication of the τ5c monolith samples, which can be shaped in any geometry, provides an essential basis for quantifying and understanding the dissolution of the τ5c intermetallic phase in a commercial steel strip coating pot. Dissolution analysis of the IMC monoliths revealed a mass transfer coefficient (k) of 5.3 x 10−8 m/s in an unsaturated AZ alloy coating bath at 595 ± 2 °C. It is suggested that dissolution kinetic of IMC monoliths is sluggish; therefore, IMC(τ5c) particles precipitated in the AZ alloy bath will unlikely dissolve back at industrial steel strip coating processing conditions.