Effects of Solidification Pressure and Heat Treatment on the Microstructure and Micro-Hardness of AlSi9CuMg Alloy.

Lijun Wei,Fan Ye,Baoshuai Han,Sujun Wu,Yanjin Xu

doi:10.3390/ma12142229

Lijun Wei, Fan Ye + Show 3 more

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https://doi.org/10.3390/ma12142229

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Abstract

The effect of high pressure (135 MPa) and the following heat treatment on the microstructure and micro-hardness of the squeezing cast AlSi9CuMg alloy is investigated, using optical microscopy (OM), Vickers tester, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The results indicate that the application of high pressure can increase under-cooling and the cooling rate during solidification and cause the refinement of the microstructure. The enhanced melt flow resulting from high pressure can also break the dendrite to form the spherical and elliptical primary α (Al) grains during the early stage of solidification. The winter-sweet flower-shaped primary α (Al) phases can also be formed through plastic deformation caused by the flow of the partially solidified melt. The ageing treatment results showed that a maximum (peak) micro-hardness value was obtained for each of the three ageing temperatures at different ageing times, and the highest peak value was achieved at 175 °C for 480 min. The micro-hardness change of the sample under different ageing processes was attributed to the variation of type, density, and size of the precipitates.

Highlights

Al–Si alloys are widely used in the automobile industry due to their excellent comprehensive mechanical properties and low density [1,2,3,4]
The fine expressed by Equation (1) [19], Ashiri et al [5] proposed a new version of the equation that can grains of the squeezing cast alloy shown in Figure 2b indicate that the application of solidification calculate the variation (∆Tf ) of the freezing point (Tf ) caused by solidification pressure (P), as shown in pressure can promote the microstructure refinement
The increment of the mechanical properties is associated with the type, size, and density of the precipitates distributed in the α (Al) matrix

Summary

Introduction

Al–Si alloys are widely used in the automobile industry due to their excellent comprehensive mechanical properties and low density [1,2,3,4]. In which molten metal solidifies under high pressure, is widely employed in the manufacture of components with the advantages of no porosity, high mechanical properties, good surface smoothness, and accurate dimensional precision [7,8,9]. In the conventional die casting process, porosities are caused by gas entrapment resulted from the high-speed filling of the liquid metal. Bubbles may be formed at the casting surface during heat treatment due to the expansion of the gas in the pores, which are detrimental to the surface quality and mechanical performance [10]. To avoid the formation of the bubbles at the surface, squeezing casting with a low filling speed, which can effectively expel the gas and eliminate the pores, has been proposed and adopted to fabricate some structural safety components

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