NUMERICAL STUDY ON FREE-CUTTING PHASE PRECIPITATION BEHAVIOR IN Fe-Bi-Mn TERNARY ALLOY MULTIPHASE TRANSFORMATION- DIFFUSION SYSTEM
WANG Zhe1, WANG Fazhan1,2(), HE Yinhua1, WANG Xin1, MA Shan2, WANG Huimian3
1 College of Materials and Mineral Resources, Xi′an University of Architecture and Technology, Xi′an 710055 2 School of Mechanical and Electrical Engineering, Xi′an University of Architecture and Technology, Xi′an 710055 3 Technology Center, Shanxi Taigang Stainless Steel Co. Ltd., Taiyuan 030003
Cite this article:
WANG Zhe, WANG Fazhan, HE Yinhua, WANG Xin, MA Shan, WANG Huimian. NUMERICAL STUDY ON FREE-CUTTING PHASE PRECIPITATION BEHAVIOR IN Fe-Bi-Mn TERNARY ALLOY MULTIPHASE TRANSFORMATION- DIFFUSION SYSTEM. Acta Metall Sin, 2014, 50(11): 1393-1402.
The solidification process of alloys are not just liquid to solid phase transformation, in fact in some alloys liquid to gas and gas to liquid phase transformation processes happen. A method incorporating the full diffusion-governed phase transformation kinetics into a multiphase volume average solidification model is presented. The motivation to develop such a model is to predict the multiple effect of inclusions precipitation behavior in castings. A key feature of this model, different from most previous ones which usually assume an infinite solute mixing in liquid lead to erroneous estimation of the multiphase diffusion path, is that diffusions in solid, liquid and gas phases are considered. Here solidification of Fe-Bi-Mn ternary alloy is examined. As MnS and Bi have large differences in the solute partition coefficient, diffusion coefficient and liquidus slope, the multiphase diffusion path shows differently from those predicted by infinite liquid mixing models. In this work, a three-dimensional mathematical model for a three-phase flow during its horizontai solidification was studied based on diffusion-governed phase transformation kinetics. Effects of Fe-Bi-Mn ternary alloy solidification on solid-liquid-gas phase transformation were considered. The free-cutting phase precipitation behavior was studied and multiphase transformation and multiphase diffusion path of free-cutting phase precipitation behavior were analyzed. Results show that the multiphase transformation-diffusion is strongly influenced by free-cutting phases precipitation behavior: MnS has a relatively large partition coefficient and small diffusion coefficient with larger Mls,MnS (solid-liquid mass transfer rate of MnS). During solidification, C*s,MnS (solid interface concentration of MnS) may become even larger than Cl,MnS (liquid concentration of MnS), MnS in liquid is assumed to be fully ‘trapped’ in solid and there is no longer any enrichment of MnS; however Bi has a relatively small partition coefficient and large diffusion coefficient with smaller Mls,Bi (solid-liquid mass transfer rate of Bi) and negative Mgl,Bi (liquid-gas mass transfer rate of Bi), during solidification, Cl,Bi (liquid concentration of Bi) always greater than C*s,Bi (solid interface concentration of Bi). In addition, due to the existence of Bi-gas phase, Bi continuous to flow, enriched in the solidified around MnS. Calculated results show good agreement with experimental data.
Fund: National Science and Technology Pillar Program During the Twelfth Five-Year Plan Period (No.2011BAE31B02) and Preparation and Processing of Non-ferrous Materials of High Performance for Innovative Research of Xi′an University of Architecture and Technology of China
Table 1 Conservative equations and transfer rate equations[10,14,16]
Fig.1 Schematic of solidification process of the Fe-Bi-Mn ternary alloy (l, m, s represent liquid zone, solid zone, mushy zone, respectively)
Fig.2 Schematic of multiphase transformation-diffusion system (The interfacial transfer of different species are indicated by Z1~Z12)
Parameter
Fe
MnS
Bi
Atomic fraction / %
55.8
87.0
209.0
Melting point / K
1808
1630
544
Density / (kg·m-3)
7850
3990
9780
Specific heat / (J·kg-1·K-1)
460
303
130
Viscosity / (kg·m-1·s-1)
0.0059
0.0073
0.0021
Heat conductivity / (W·m-1·K-1)
80.4
34.0
7.9
Latent heat / (J·kg-1)
246400
148390
11300
Liquidus slope / K
-55.0
-4.8
-2.7
Partition coefficient
0.36
0.84
0.25
Solid diffusion coefficient / (m2·s-1)
1.0×10-9
1.2×10-13
2.2×10-9
Liquid diffusion coefficient / (m2·s-1)
2.0×10-8
4.0×10-9
1.6×10-8
Gas diffusion coefficient / (m2·s-1)
-
-
5.7×10-7
Thermal expansion coefficient / K-1
1.43×10-4
1.07×10-4
2.83×10-4
Solutal expansion coefficient / (%-1)
1.1×10-2
0.2×10-2
1.9×10-2
Initial temperature / K
1873
1873
1873
External temperature / K
298
298
298
Table 2 Physical parameters used in simulation
Fig.3 Schematic of 3D model with boundary and initial conditions
Fig.4 3D distributions of Bi and MnS isosurfaces of solidified Fe-0.3%Bi-0.9%Mn ternary alloy
Fig.5 Section zone IV shown in Fig.4 at 900 s of alloy solidification
Fig.6 Isolines in section zone I shown in Fig.4 at 900 s of alloy solidification
Fig.7 Section zone IV shown in Fig.4 at 1500 s of alloy solidification
(a) isolines of liquid fraction and nephograms of mass fraction of MnS
(b) isolines of liquid fraction and nephograms of mass fraction of Bi
Fig.8 (Cl,MnS, Cl,Bi) , (C*l,MnS, C*l,Bi) and (C*s,MnS, C*s,Bi) multiphase diffusion paths of path I
Fig.9 Curves of solid, liquid and gas-phase solute concentration and their interfacial concentration with volume fraction of solid caused by multiphase diffusion
(a) mass fraction of Bi (b) mass fraction of MnS
Fig.10 Experimental (a, c) and simulated (b, d) microstructures in transversal (a, b) and vertical (c, d) directions of Fe-0.3%Bi-0.9%Mn free-cuting stainless steel ingot
Fig.11 Experimental (a) and simulated (b) COVd (coefficient-of-variance of the mean near-neighbor distance) of Bi and MnS on ingot section
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