Effect of Cooling Rate and Modification by Strontium on the Thermal Conductivity of Al-8Si Alloy

Cooling rate plays a critical role in determining the thermal conductivity of Al-Si alloys. Although the effect of morphology and size of Si (changed by heat treatment) on its thermal conductivity has been investigated, the effect of cooling rates on thermal conductivity has not been well studied. In this study, we investigated the microstructure of an Al-8Si (with and without modification by Strontium (Sr)) alloy with cooling rates from 46.2 °C/s to 234 °C/s. It was found that the effect of cooling rate on thermal conductivity of Sr modification and Sr-free samples are opposite from each other. As a result, while the cooling rate increased from 46.2 °C/s to 234 °C/s, the calculated thermal conductivity increased from 145.3 MS/m to 151.5 MS/m for Sr-free Al-8Si alloy, and the calculated thermal conductivity was reduced from 187.5 MS/m to 176.7 MS/m for the Sr-modified Al-8Si alloy. By discussing how thermal conductivity correlates with eutectic silicon morphology and secondary dendrite arm spacing, the relationship between cooling rate and thermal conductivity were explained. This work suggests a new design strategy for improving the thermal conductivity of Al-Si hypoeutectic alloys.

Effect of Alloying Elements on the Sr Modification of Al-Si Cast Alloys

Strontium-based modifier alloys are commonly adopted to modify the eutectic silicon in aluminum-silicon casting alloys by changing the silicon shape from an acicular to a spherical form. Usually, the modifier alloy necessary to properly change the silicon shape depends on the silicon content, but the alloying elements’ content may have an influence. The AlSr10 master alloy’s modifying effect was studied on four Al-Si alloys through the characterization of microstructural and mechanical properties (micro-hardness and impact tests). The experimental results obtained on gravity cast samples highlighted the interdependence in the modification of silicon between the Si content and the alloying elements. After modification, a higher microstructural homogeneity characterized by a reduction of up to 22.8% in the size of intermetallics was observed, with a generalized reduction in secondary dendritic arm spacing. The presence of iron-based polygonal-shaped intermetallics negatively affects Sr modification; coarser silicon particles tend to grow close to α-Fe. The presence of casting defects such as bifilm reduces Sr modification’s beneficial effects, and little increase in absorbed impact energy is observed in this work.

The Relationship between Residual Amount of Sr and Morphology of Eutectic Si Phase in A356 Alloy

This paper studied the relationship between the residual amount of Sr and the morphology of the eutectic Si phase in A356 obtained through different modification treatment processes; additionally, the cooling rates of molds were studied. The eutectic Si phase revealed a satisfactory modification effect at residual Sr amounts above 0.01 wt % in A356 alloys cast using an iron mould. Complete modification of the eutectic Si phase could be achieved at a Sr additive amount 0.03 wt % in an A356 melt. The addition of higher amounts of Sr (~0.04–0.06 wt %) did not improve the modification effect. With the addition of 0.06 wt % Sr into A356 alloy melt and holding at 750 °C, the anti-fading capacity of Sr modification effect could be sustained for 120 minutes. More Sr is needed to obtain a good modification of eutectic Si for an A356 alloy cast using a sand mold.

The effect of Sr on the microstructure and wear properties of AlSi5Cu1Mg alloy

The effect of different amounts (0 wt%, 0.02 wt%, 0.04 wt%, 0.06 wt%) of Sr modification on the microstructure of AlSi5Cu1Mg alloy was investigated. The wear resistances of the alloys were tested using an MMD-1 pin-on-disk wear-testing apparatus. Worn surfaces were examined using a scanning electron microscopy equipped with an energy-dispersive spectrometer. The relationship between the microstructure and wear properties of the alloy was discussed. The results show that the addition of Sr caused the grain refinement and AlSi5Cu1Mg with 0.04 wt% Sr has short rod-like Fe-rich intermetallic, minimal size of α-Al phase and secondary dendrite arm spacing, and granular or fibrous eutectic Si phase distributed uniformly at grain boundaries. Compared to the matrix alloy, the tensile strength, Brinell hardness, and elongation of the AlSi5Cu1Mg with 0.04 wt% Sr increased by 15%, 48%, and 73%, respectively. Both the lowest friction coefficient and the best wear resistance were achieved by the AlSi5Cu1Mg with 0.04 wt% Sr. Compared to the matrix alloy, the wear mass loss and friction coefficient of the AlSi5Cu1Mg with 0.04 wt% Sr increased by 42% and 18%, respectively. The adhesive wear and abrasive wear are the main wear mechanisms.

Effects of Holmium Additions on Microstructure and Properties of A356 Aluminum Alloys

Sr-modification of A356 alloys has distinct shortages due to the volatilization and oxidation during remelting and pouring, which often reduce the modification efficiency and mechanical properties of the alloys. To avoid the adverse effects and enhance the comprehensive properties, the effects of heavy rare earth element holmium (Ho) modification on the microstructure and properties of the alloy were investigated. Ho addition inhibited the intrinsic orientation growth of (111)Al planes and stimulated the growth of the (200)Al planes for α-Al crystals. The addition of 0.2 wt.% Ho produced the best refinement effect for α-Al grains; 0.3 wt.% of Ho addition yielded the most distinct modification effect for eutectic Si phases, which was further improved by a T6 treatment. The extra addition of 0.4 wt.% Ho resulted in the complete loss of the refinement and modification effects and in the abnormal growth of the α-Al crystals. Ho additions produced Al3Ho phases containing Fe elements, which were distributed on the boundaries of the α-Al dendrites. The corrosion-proof performance was enhanced by Ho addition and the T6 treatment; the tensile strength and elongation achieved the highest value upon 0.2 wt.% of Ho addition and the T6 treatment. Moreover, the hardness was also enhanced by Ho additions in both states.

Effects of Modification and Heat Treatment on Microstructure and Mechanical Property of Two-Step Foamed ZL111 Alloy Foam

ZL111 alloy foams with a porosity of 80% and an average pore diameter of 3.5 mm were fabricated using a two-step foaming process, and the effects of modification and heat treatment on their microstructure and mechanical property were studied. The results indicates that by Y&Sr modification, most of the eutectic silicon in the ZL111 alloy foam is transformed from plate-like into dot-like forms, and the average size of evenly distributed α-Al grain is reduced from 80~100μm to 30~40μm, which is more efficient than separate Y or Sr modifications. By combining Y and Sr modification and T6 heat treatment, the α-Al grain size of ZL111 alloy foam maintains its previous modified effect, eutectic silicon remains spherical and well-distributed, and CuAl2and Al9FeMg3Si5are dispersed homogeneously at the α-Al grain boundary. The Y&Sr modification and T6 heat treatment also significantly improved the compressive property of ZL111 alloy foam, when we compared them with the untreated ZL111 alloy foam. The compressive strength rises from 13.3 MPa to 22.6 MPa, the densification strain improves from 59.3% to 76.9%, and the energy absorption capacity increases from 4.87 MJ/m3to 13.77 MJ/m3.