Characterization of high-pressure die-cast hypereutectic Al-Si alloys based on microstructural distribution and fracture morphology

doi:10.1016/j.jmst.2018.12.005

Abstract

Abstract:

The fracture behavior of high-pressure die-cast hypereutectic (HPDC) Al-Si alloys was investigated using a high-resolution laboratory CT and synchrotron X-ray tomography with a particular focus on the influence of HPDC microstructure. Results showed that microstructure of the alloy was mainly comprised of primary silicon particles (PSPs), Al dendrites, Cu-rich phases and pores. Most of the coarse PSPs, Cu-rich phases and pores were located in the center of the specimen. The rapid solidification of HPDC led to a heterogeneous microstructural feature. Elemental Cu was enriched in the frontiers of solid-liquid interface, causing the formation of large size dendritic arms. The pores were formed in the interdendrites which endured high stress intensity under high applied stress. Microcracks were originated from pores and further connected Cu-rich phases causing intergranular fracture. PSPs worked as obstacles causing piling-up dislocations in the phase interface. In the regions where large size of PSPs enriched in, PSPs ruptured rather than debonded from matrix, indicating transgranular fractures of PSPs. Microcracks originated around pores and PSPs tended to converge on the main cracks to decrease the energy required for crack propagation.

Key words: Hypereutectic Al-Si alloy, Primary silicon particles, Porosity, Fracture morphology, High pressure die casting

X.Y. Jiao, J. Wang, C.F. Liu, Z.P. Guo, G.D. Tong, S.L. Ma, Y. Bi, Y.F. Zhang, S.M. Xiong. Characterization of high-pressure die-cast hypereutectic Al-Si alloys based on microstructural distribution and fracture morphology[J]. J. Mater. Sci. Technol., 2019, 35(6): 1099-1107.

Figures/Tables 10

Fig. 1. Configuration of (a) casting produced by HPDC, (b) tensile test bar and sampling locations, (c) key processing parameters in HPDC, (d) Vickers hardness test, (e) chemical composition of three hypereutectic Al-Si alloys, and (f) stress-strain curve of alloys A, X and S.

Fig. 2. Optical micrographs of the cross section of the rods of HPDC Al-Si alloys (a) A, (b) X, (c) S. (d)-(f) show optical micrographs of alloys A, X and S along the radial direction, respectively.

Fig. 3. SEM observation of the microstructure of alloys (a) A, (b) X and (c) S.

Fig. 4. Vickers hardness along the radial direction from surface to center. From top to bottom correspond to alloys (a) A, (b) X and (c) S, respectively. Fig. 1(d) shows the position of Vickers indentation. The black line represents the evolution of the average Vickers hardness.

Fig. 5. Fracture morphology and contour map of the undulation height of the fracture surface for alloys (a) A, (b) X and (c) S. Green region is used as baseline.

Fig. 6. Distribution and volume percent for the pores, PSPs and Cu-rich phases of alloys (a) A, (b) X and (c) S along the radial direction (from surface to center) buried under fracture surface.

Fig. 7. SEM observation of (a) microstructure near pores and (b) enlarged Cu-rich phase region and corresponding EDS result in (c) and (d).

Fig. 8. Thermodynamic diagram of the equivalent diameter of PSPs along relative distance away from surface and crack surface fluctuation (corresponding to the arrow in black dotted line frame in Fig. 5) of alloys (a) A, (b) X and (c) S.

Fig. 9. SEM observation of ruptured PSPs in center of alloys (a) A, (b) X and (c) S.

Table 1 Geometry factor and stress intensity factor of the hypereutectic Al-Si alloys.

Alloy	Tensile strength (σ)	Geometry factor (Y)	Stress intensity factors (K_I)
A	320 MPa	1.50	34.02 MPa·$sqrt{m}$
X	300 MPa	1.50	31.89 MPa·$sqrt{m}$
S	300 MPa	1.28	21.51 MPa·$sqrt{m}$

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