The effect of Mg on the microstructure and mechanical behavior of Al-Si-Mg casting alloys

Abstract

The microstructure and tensile behavior of two Al-7 pct Si-Mg casting alloys, with magnesium contents of 0.4 and 0.7 pct, have been studied. Different microstructures were produced by varying the solidification rate and by modification with strontium. An extraction technique was used to determine the maximum size of the eutectic silicon flakes and particles. The eutectic Si particles in the unmodified alloys and, to a lesser extent, in the Sr-modified alloys are larger in the alloys with higher Mg content. Large Fe-rich -phase (Al 9 FeMg 3 Si 5 ) particles are formed in the 0.7 pct Mg alloys together with some smaller ␤-phase (Al 5 FeSi) plates; in contrast, only ␤-phase plates are observed in the 0.4 pct Mg alloys. The yield stress increases with the Mg content, although, at 0.7 pct Mg, it is less than expected, possibly because some of the Mg is lost to -phase intermetallics. The tensile ductility is less in the higher Mg alloys, especially in the Sr-modified alloys, compared with the lower Mg alloys. The loss of ductility of the unmodified alloy seems to be caused by the larger Si particles, while the presence of large -phase intermetallic particles accounts for the loss in ductility of the Sr-modified alloy.

Effects of metallurgical parameters on the decomposition of л-AlFeMgSi phase in AL-SI-MG alloys and its influence on the mechanical properties /

“…casting. 5 In their study of the effects of Mg, Fe, and solidification rate on the fracture toughness of Al-7Si-xMg-yFe, Murali et al 44 observed that, at 0.2 wt% Fe for all the solidification rates studied, increasing the Mg content from 0.32 to 0.65 wt% causes a reduction in the total impact energy by about 50%. This reduction in the impact energy was attributed to the presence of the coarse ^-phase in the microstructure which acts as a stress raiser and facilitates crack nucleation and propagation.…”

“…Moreover, the volume fraction of the iron-rich intermetallics in such alloys is larger than it is in 356 alloys containing 0.45wt% Mg as a result of the formation of larger amounts of the 7r-AlFeMgSi iron-intermetallic phase. 4 ' 5 The stability of the ;r-phase in 357 alloys after applying the standard heat treatment causes a reduction in the amount of free Mg, found in solid solution, which is available for forming Mg2Si precipitates during the aging step. The strength of this kind of alloy is thus lower than expected, leading to the saturation of the UTS and YS values.…”

“…23 ' 24 Lu et al? 5 reported that increasing the iron level from 0.3 to 0.7 wt% causes a major change in the precipitation sequence of the /?-phase. At low iron levels, the /?-phase forms at lower temperatures through the ternary eutectic reaction displaying a small needle-like morphology.…”

“…70 Peyman et al phase of iron mold cast Al-6Si-3.5Cu-l.0Fe alloys. 71 Samuel et al 72 studied the effects of strontium on formation of the /?-Al 5 FeSi phase in 319 alloys. According to these researchers, the addition of strontium acts as a poison on the nucleation site of the /5-needles, thereby reducing the number of sites ultimately available for nucleation.…”

Residual Stress in Metal-Matrix Composite Cylinder Measured by Neutron Diffraction and Contour Method

“…The microstructure is shown in Fig.1 and has been described previously [7]. It is highly nonuniform, comprising coarse dendrite colonies (grains) some 0.8 mm in diameter, dendrites with a secondary dendrite arm spacing of ~60 µm, and inter-dendritic eutectic Si inclusions with a volumeequivalent diameter ~3 µm and an aspect ratio of ~1.6 [8] together with age-hardening nanoprecipitates of MgSi.…”