Determination of Optimum Sr Level for Eutectic Si Modification in Al–Si Cast Alloys Using Thermal analysis and Tensile Properties

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

“…As the Sr amount increased to 0.03%, the fibrous eutectic Si phase became more refined, and the eutectic field became more compact (see Figure 2c). However, as shown in Figure 2d–f, increasing the amount of Sr (0.04–0.06%) in the aluminum alloys cast from the metal mold did not produce any further improvements, which agrees with the results observed by Zamani et al [13]. The results of the direct-reading spectrometer indicated that, in constitutional samples cast with an aluminum melt with initial Sr quantities of 0.02%, 0.03%, 0.04%, 0.05%, and 0.06%, the residual amounts of Sr were 0.011%, 0.019%, 0.029%, 0.039%, and 0.044%, respectively.…”

“…Five general features can be extracted from the cooling curves assistant by their first derivative curves, namely: The T G(α-Al) -Growth temperature of the primary α-phases, T NUC(Al-Si) -Nucleation temperature of the eutectic structure, T E,G -Eutectic growth temperature, T End(Al-Si) -Eutectic end temperature, and ΔT E,G = T E,G(unmodified) − T E,G(modified) . Among these parameters, T NUC(Al-Si) and T E,G are greatly affected by the cooling rate, while ΔT E,G , the most reliable indicator of the modification level, has been the object of more and more attention [13,19]. The addition of Sr leads to a depression the eutectic growth temperature which is reflected in the increased value of ΔT E,G .…”

“…This is consistent with the analysis of the microstructure, which mean that a further increase in Sr content does not achieve a better modification effect. Excess Sr will form a AlSiSr intermetallic compound which reduces the effect of the modification of Silicon and increases the eutectic temperature of the alloy [13]. Whether in the absence of Sr or not, the recalescence undercooling is hardly distinguished, which is inconsistent with the conclusion of Zamani [13], and therefore may be related to an inconsistency in the alloy’s composition.…”

“…Thus, to neutralize or offset the harmful effects of some trace elements, it is necessary to add larger amounts of Sr. However, an excessive amount of Sr forms a detrimental Al 2 Si 2 Sr phase [13,14,15], and frequently gives rise to increased hydrogen content in the melt [16,17,18]. Excess Sr can also initiate structural looseness and performance deterioration.…”

Effect of Cooling Rates on the Microstructure and Mechanical Property of La Modified Al7SiMg Alloys Processed by Gravity Die Casting and Semi-Solid Die Casting

“…The Al-Si-Mg alloys, which owe excellent casting characteristics including good fluidity and less solidification shrinkage [1,2], are broadly applied in casting industry [3][4][5]. However, in a normal casting process, the α-Al grains have coarse dendritic morphology, and the Si particles generally have acicular morphology and discontinuous distribution, which have detrimental influence on the strength and especially elongation of the aluminum alloys [6,7]. The chemical refinement and modification, which is achieved by adding small amount modifying elements (Ti, Sr, rare earth), is a useful method to refine the α-Al grains and improve the Si morphology, and therefore enhance the mechanical properties of alloys [8][9][10].…”