Comprehensive Evaluation of Modification Level Assessment in Sr-Modified Aluminium Alloys

Abstract

One way to improve the mechanical properties of Al-Si alloys is to add chemical modifiers that affect the microstructure of the cast components. Small amounts of Sr cause the modification of the eutectic Si particles from a coarse plate-like morphology into a fine fibrous one. This study sets out to analyse the effects of chemical modification on an Al-7%Si-0.3%Mg foundry alloy treated with Sr, ranging from 62 to 820 ppm. Assessment of modification level was evaluated by both thermal analysis technique and quantitative microstructural investigations. Up to now, little attention has been paid to the role played by microstructural characteristics in evaluating the modification level, which is commonly performed by comparison with standard images. In the present investigation, numerical methods proposed in the literature have been considered in an attempt to understand their effectiveness. The correlation between thermal analysis results and geometrical parameters of eutectic Si particles highlighted the impact of microstructural homogeneity on modification-level evaluation. In addition, the presence of Ca in Srbased intermetallics has been further investigated by means of scanning electron microscopy combined with energy-dispersive X-ray spectroscopy.

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

“…Thus, it can be inferred that the concentration of Sr in the melt had a critical value of ~0.012% (to have an effect on modification of the eutectic Si phase), which is very close to the value (equal to 0.01%) given by Fortini et al [16]. Zarif et al [15] also pointed out that only 50 ppm is good enough to alter the morphology of Si.…”

“…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.…”

Sr addition and its effect on the melt cleanliness of A356

“…It was found that porosity was increased with increased Sr and pores were associated with intermetallics. Barrirero [11] and Fortini [12] also found similar results. Nampoothiri [13] claimed that Sr was beneficial for eutectic modification, however, porosity was increased and localised particularly in the interdendritic region.…”

Study of the Influence of Zirconium, Titanium and Strontium on the Properties and Microstructure of AlSi7Mg0.3Cu0.5 Alloy

“…However, research has shown that the presence of small-size oxides helps to form nucleation site, especially those based on Al 3 Zr. Unlike the combination of Zr and Sc or Zr and Ti, the combination of Zr and Sr in aluminum alloys does not change the structure to non-dendritic and, at the same time, the given phases can bind Fe to them [ 28 , 29 , 30 , 31 , 32 , 33 , 34 ].…”