The characterization of Fe-rich phases in a high-pressure die cast hypoeutectic aluminum-silicon alloy

doi:10.1016/j.jmst.2020.02.040

Abstract

Abstract:

Both microstructural characteristics and fracture behavior of Fe-rich phases in a high-pressure die-cast hypoeutectic Al-Si alloy were investigated. Attention was focused on the morphology and formation mechanism of Fe-rich phases, together with their influence on fracture. Results show that primary Fe-rich phases exhibited in blocky shape precipitated from liquid while secondary Fe-rich phase in a large net shape was distributed along eutectic boundary participating in a ternary eutectic reaction. Through synchrotron X-ray tomography characterization, three Fe-rich phases with different morphologies, i.e., polyhedral shape, fine compact shape and Chinese script-type shape were extracted along the radial direction. Lower slow-shot speed promoted the polyhedral Fe-rich phase to precipitate in slow-shot sleeve while decreased the formation of fine compact and Chinese script-type Fe-rich phases in die cavity. Because of a worse deformation compatibility, polyhedral Fe-rich phases fractured and became stress concentration sources before the failure of the casting.

Key words: Hypoeutectic Al-10Si, Fe-rich phases, Fracture morphology, High pressure die casting

X.Y. Jiao, C.F. Liu, Z.P. Guo, G.D. Tong, S.L. Ma, Y. Bi, Y.F. Zhang, S.M. Xiong. The characterization of Fe-rich phases in a high-pressure die cast hypoeutectic aluminum-silicon alloy[J]. J. Mater. Sci. Technol., 2020, 51: 54-62.

Figures/Tables 13

Table 1 Summary of Fe-rich phases in hypoeutectic Al-Si alloys.

Phase	Crystal structure	Space group	Lattice constant	Morphology	Nucleation position	Reference
β-Al5FeSi	Monoclinic	A12/a1, I4₁/acd	a = 0.616 b = 0.618 c = 2.081 β = 90.42	Plate-shaped	High Fe content and slow cooling speed	[18,21]
δ-Al4FeSi2	Orthorhombic	I4/mcm	a = 0.615 b = 0.615 c = 0.952	Needle-like shaped	Relatively high cooling speed	[18,22]
α-Al8Fe2Si	Hexagonal	P6₃/mmc	a = 1.24 c = 2.63	Polyhedral-shaped	High Fe content and slow cooling speed	[18,23]
α-Al15Fe3Si2	Cubic	Im3¯	a = 1.253	Polyhedral-shaped, Chinese script-type	High cooling speed and Mn addition	[14,18,24]

Table 2 The chemical composition of the AlSi10MnMg hypoeutectic Al-Si alloy (wt.%).

Si	Fe	Cu	Mn	Mg	Zn	V	Ti	Sr	Al
9.5-11.5	< 0.15	< 0.03	0.5-0.8	0.1-0.5	< 0.07	< 0.07	0.04-0.15	0.01-0.025	Bal.

Fig. 1. The configuration of (a) HPDC casting including two tensile test bars and four plates; (b) tensile test bar and sample extraction locations for OM, SEM, TEM, EPMA and synchrotron X-ray tomography; (c) tensile test plate and (d) in-situ tensile test specimen.

Table 3 The key processing parameters in HPDC.

Melting temperature (°C)	Initial mold temperature (°C)	Slow-shot speed (m/s)	Vacuum condition
680	120	0.05, 0.4	On

Fig. 2. Optical micrograph (a) of the cross section of the rod for the HPDC AlSi10MnMg alloy, (b) along the radial direction and corresponding local microstructure (b1) - (b5). (c) Area fraction of ESCs, eutectic and Al grains in five different regions. (d) EPMA liner-scan results of Mn and Fe elements, EPMA map-scan results of (e) Mn and (f) Fe in five different regions.

Fig. 3. SEM observation of (a) and (b) three kind of Fe-rich phases with different shape including coarse blocky shape, fine compact shape and net shape. TEM bright-field image of (c) fine compact Fe-rich phase and (d) Chinese script-type Fe-rich phase and corresponding selected area diffraction pattern of (e) [377]α and (f)[256]α, respectively.

Fig. 4. (a) shows the distribution of Fe-rich phases along the radial direction (from surface to center); (b), (c) and (d) show the 3-D morphology of Fe-rich phases in HPDC; (e), (f) and (g) show the SEM results of the morphology of Fe-rich phases corresponding to (b), (c) and (d), respectively.

Table 4 Chemical composition of Fe-rich phases in HPDC AlSi10MnMg alloy.

Phase shape	xAl[at.%]	xSi[at.%]	xMn[at.%]	xFe[at.%]	xMn/xFe
Polyhedral shape	69.24	11.40	16.91	2.45	6.9:1
Fine compact shape	86.69	5.88	6.28	1.15	5.5:1
Chinese script-type	85.64	4.74	7.20	2.42	3:1

Fig. 5. (a) 3D volume distribution of Fe-rich phases, (b) 3D volume distribution of a sub-volume of 1500 μm3 - 6000 μm3. The equivalent diameter of Fe-rich phases versus the shape factor at different slow-shot speeds of (c) Vl = 0.05 m/s and (d) Vl = 0.4 m/s.

Fig. 6. The Mn and Fe content in liquid versus solid fraction in nonequilibrium state simulated by JMatPro® software based on Scheil-Gulliver theory.

Fig. 7. EPMA analysis for (a) Al, (b) Si, (c) Fe and (d) Mn elements in polyhedral Fe-rich phase. Inner area of polyhedral Fe-rich phase (e) and the chemical composition of (f) P1 and (g) P2.

Fig. 8. The polyhedral Fe-rich (a) and the fracture morphology, (b)-(d) related to polyhedral Fe-rich phases. Polyhedral Fe-rich phase under (e) 0 N and (f) 410 N. Shrinkage under (g) 410 N.

Table 5 Stress intensity factor of shrinkage, planar Si and polyhedral Fe-rich phase.

Alloy	Tensile strength (σ)	Average crack length (a)	Geometry factor (Y)	Stress intensity factors (K_I)
Shrinkage	230 MPa	50 μm	1	2.88 MPa⋅m
Planar Si particle	230 MPa	2 μm	1	0.58 MPa⋅m
Polyhedral Fe-rich phase	230 MPa	10 μm	1	1.29 MPa·m

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