Influence of Sr and Mn Additions on Intermetallic Compound Morphologies in Al-Si-Cu-Fe Cast Alloys

Abstract

The influence of Sr or Sr and Mn combined additions on the Fe-containing intermetallic compounds in Al-Si-Cu-Fe cast alloys has been investigated using Al-6.5%Si-3.5%Cu-1.0%Fe and Al-6.5%Si-3.5%Cu-1.0%Fe-0.3%Mn alloys (in mass%) with a similar composition to the 319 aluminum alloy. The results show that Sr successfully modifies the large, highly branched -needle-like phase ( :A l 5FeSi) into the individual, less-branched and finer one. The combined addition of Mn and Sr results in modification of the needle-like -phase as well as promotion of Chinese script and sludge morphology formation. The mechanism of the above morphological changes has been discussed in accordance with the mechanism of nucleation and growth of the -needle-like phase during solidification. Al-Si based cast alloys usually contain impurities or trace elements such as Fe, Mn, Cr that form preferentially high melting point and hard intermetallics with various morphol- ogies. The needle-like iron containing phase (-Al5FeSi) is the most harmful one for the mechanical properties of Al-Si based cast alloys. This phase, like most other intermetallic particles, is brittle. Its morphology is plate-like, appearing as needles in the optical micrographs. These characteristics, brittleness and plate-like morphology, greatly affect to decrease the strength and ductility of the cast alloys. It is, therefore, important to avoid or at least to control the formation of this phase. Mn is widely used as an alloying addition to neutralize the effect of iron and modify the needle-like -phase to less harmful morphologies. However, Mn is not always the best solution because it reacts with other elements existing in the melt and forms complex compounds. The major goal of this study is to find other effective elements to modify the -phase. Some limited work has been done on this subject. Sigworth 1) reported that the formation of Fe containing brittle phases were retarded in the Sr modified 319 alloy. Ouellet and Samuel 2) reported that the Sr modification of the Mg added 319 alloys caused the segregation of both the Al5Mg8Si6Cu2 and Al2Cu phases in the areas away from growing Al-Si eutectic regions. Samuel et al. 3) reported that the Sr addition led to the dissolution of more than two-thirds of the -needles and also to the modification of the Mg2Si particles in the Mg added 319 alloy. In this study, the influence of Sr or Sr and Mn combined additions on the intermetallic compound morphologies in an Al-Si-Cu-Fe cast alloy has been studied. The Fe concentration was as high as 1 mass% to make clear the morphological changes of the Fe- containing intermetallic compounds. 2. Experimental Procedure The alloys were melted in a graphite crucible by an electrical furnace with the composition of Al-6.5Si-3.5Cu- 1.0Fe (in mass%). Two categories of melt; 0.3%Mn-added and Mn-free melt were treated with 0.015% Sr addition. The alloys were cast into cast-iron and graphite molds to give different cooling rates. The effect of Sr on the microstructures was studied using an optical microscope and SEM. An image analysis method was utilized to measure the size and volume fraction of needle-like particles and number of intermetallic compounds per unit area. The chemical composition of the compounds was determined by an EDX method with a FE- SEM.