Abstract
In this paper, Sr modifier (300 ppm) was added to 2L99 alloy sand castings to investigate its effect on bifilm defects in the castings. Two different sand molds were used in this study, with good and bad running system designs, to introduce different amounts of bifilm defects into the castings. The mechanical properties of the modified 2L99 castings were compared to the properties of unmodified castings and showed that with high bifilm defect contents (H) the Sr addition reduced the Weibull modulus of the UTS by 67 pct and the Position Parameter by 5 pct, and introduced a bimodal distribution into the Weibull plot of the pct Elongation. However, for castings with low bifilm defect content (L), the Weibull moduli of both the UTS and pct Elongation were significantly improved (by 78 and 73 pct, respectively) with the addition of Sr. The Position Parameter of the pct Elongation was improved by 135 pct. The results suggested that a desirable modification effect can only be achieved while the bifilm defect content in a casting was low.
Highlights
AL-SI alloys are widely used in the automotive and aerospace industries due to their high strength-to-weight ratio and castability
The tensile test results from the casting can only be accepted as following a Weibull distribution if R2 > R02.05
Tensile test results suggested that the addition of 300 ppm Sr in a high bifilm defect-containing 2L99 alloy tended to have a negative effect on mechanical properties of the castings, reducing the Weibull modulus of the UTS and increasing its variability
Summary
AL-SI alloys are widely used in the automotive and aerospace industries due to their high strength-to-weight ratio and castability. The plate-like coarse Si phase is usually accused of reducing mechanical properties of the alloy. To solve this problem, minor chemical modifications are used to modify the plate-like Si morphology into a fibrous or coral-like morphology so that mechanical properties, especially ductility, are improved. Various elements are reported to have a modification effect on Si, among which Sr and Na are the most commonly used.[1,2,3] Ba and Ca[4,5] are reported to have a similar but reduced effect on Si. Of the commonly used modifiers, Sr remains longer in the melt than Na but is well known for its association with enhanced gas porosity.[6] McDonald et al.[7] investigated porosity in a Sr-treated alloy and found that, at any Si content, porosity was increased when Sr was added. Tiedje et al.[8] suggested that Sr modification would increase the amount of porosity by 5 to 10 times in an Al-Si alloy
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