Skin layer of A380 aluminium alloy die castings and its blistering during solution treatment

doi:10.1016/j.jmst.2019.05.011

Abstract

Abstract:

Skin layer is a characteristic microstructure of aluminium die castings, which would effect the surface blistering during solution treatment. In this study, the microstructures of skin layer were investigated by the methods of optical microscope (OM), scanning electron microscope (SEM) and electron probe micro-analyzer (EPMA). High resolution X-ray CT was used to compare the skin layer with normal surface before and after solution treatment. With the aid of computational fluid dynamics (CFD), the formation mechanism of the skin layer was discussed based on microstructure distribution, solute segregation, porosity distribution and surface blistering. The results suggested that the skin layer is related to a succession of complex processes before the filling process finished. Pore clusters or laminar defects would be formed in skin layers during solution treatment and cause severe surface blistering.

Key words: Skin layer, Die castings, Solution treatment, Blistering

Zihao Yuan, Zhipeng Guo, S.M. Xiong. Skin layer of A380 aluminium alloy die castings and its blistering during solution treatment[J]. J. Mater. Sci. Technol., 2019, 35(9): 1906-1916.

Figures/Tables 19

Fig. 1. An expanded laminar defect leading to blistering in an A380 alloy die casting solution treated 20 min at 490 °C [25]. The arrows point to an unexpanded laminar defect.

Table 1 The composition of the A380 alloy used in this study.

Elements	Si	Cu	Fe	Mn	Zn	Ni	Mg	Al
wt%	8.67	3.27	0.82	0.42	2.57	0.03	0.08	balance

Table 2 Key processing parameters during HPDC.

Parameters	Value
Melt temperature	680 °C
Slow shot speed	0.15 m/s
Gate speed	around 60 m/s
Intensification pressure	79 MPa

Fig. 2. (a) The specific die casting employed in the high pressure die casting experiment. (b) Configuration of the tensile test bar used in the present study.

Fig. 3. Density distribution of the bar within the gauge length. (a) A reconstructed cross-section with a resolution of 7.5 μm. (b) A reconstructed skin layer with a resolution of 0.667 μm. (c) Distribution of skin layer within the gauge length of a tensile bar.

Fig. 4. Microstructure contrast between (a) skin layer and (b) normal surface.

Fig. 5. Structure of the skin layer. (a) Overview of the transitional region between the skin layer and the sub-skin region. (b) Sub-skin region (SSR). (c) Outer sub-layer of the skin layer (OSL). (d) Inner sub-layer of the skin layer (ISL). (e) Normal surface. (f) Central region of the bar.

Fig. 6. Composition distribution from the surface to the center of the bar.

Fig. 7. Distribution of element concentration obtained by the method of EPMA. (a) SEM image. (b) Si distribution. (c) Cu distribution.

Fig. 8. Surface blistering during solution treatment of 30 min at 490 °C. (a-b) for the normal surface, (c-d) for the skin layer. (a) and (c) for the die-cast bar, (b) and (d) for the same locations after solution treatment. The CT resolution was 2.5 μm.

Fig. 9. Laminar gas pores after solution treatment, which was very similar to Fig. 1 [25]. The CT resolution was 7.5 μm.

Table 3 Porosity in different regions.

Region	Before solution treatment	After solution treatment	Expansion time
OSL	1.34%	4.05%	3.02
ISL	2.50%	14.6%	5.84
SSR	0.74%	5.54%	7.49

Fig. 10. Blistering after solution treatment of 15 min at 490 °C. (a) Normal surface, (b) skin layer. The CT resolution was 7.5 μm.

Fig. 11. Contour of the entrained air volume fraction at a series of moments during filling the rightmost bar in Fig. 2a.

Fig. 12. Contour of the entrained air volume fraction of a cross-section of the bar (see Fig. 11e).

Fig. 13. Density of α-Al and liquid in semi-solid temperature interval.

Fig. 14. Mechanisms of the solid-liquid segregation. (a) Impingement and depositing. (b) Centrifugation.

Fig. 15. Variation of solidification rate. (a) Rapid solidification in OSL. (b) Slow solidification in ISL. (c) Rapid Solidification in SSR.

Fig. 16. The effect of intensification pressure on the gas pressure inside gas pores.

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