International Scientific Journals
of Scientific Technical Union of Mechanical Engineering "Industry 4.0"

The need to protect the environment by reducing the energy intensity of production, the amount of waste, and reducing greenhouse gas emissions, as well as shrinking bauxite resources, means that secondary raw materials are playing an increasingly important role in aluminum alloy smelting. However, the increase in the proportion of scrap results in a higher content of various impurities, the worst of which is iron in the group of metallic ones. It is well known that manganese is one of the most commonly used additives to neutralize morphologically unfavorable β-Al5FeSi phases. However, there is little information about the effects of other transition elements, such as chromium. Accordingly, the study’s results concern the effect of chromium on the microstructure of AlSi7Mg alloy with increased iron content (from 0.4wt.% to 1.4wt.%, in 0.2% increments). Based on SEM/EDX and XRD studies, it was found that the addition of chromium causes the transformation of the lamellar-needle phase β-Al5FeSi into the α-Al13(Fe,Cr)4Si4 phase with a dendritic morphology (so-called “Chinese script”). However, care should be taken not to exceed the permissible value of the chromium/iron quotient (Cr/Fe ~ about 1/3). Otherwise, the morphology of the α-Al13(Fe,Cr)4Si4 phase changes from more “fluffy” to massive polygons. This is accompanied by an unfavorable increase in the slime ratio from about 1.6 to more than 3.0%.

Defects in die castings can include underfilling, blistering, sticking to the mold, and cracking. However, the factor that most affects the quality of castings is porosity. The primary causes of porosity are gaseous impurities and improper mold venting, which lead to gaseous porosity. Additionally, a poorly selected gating system, low casting speed, excessively short piston paths, low post-pressure, and low casting temperatures all contribute to shrinkage porosity. An additional factor contributing to the occurrence of pores is the increasing proportion of scrap (from production and post-production), which contains a wide variety of impurities. Gaseous inclusions (e.g., hydrides) can be removed from the liquid alloy by refining it, but metallic impurities are worse. In Al-Si alloys, one of the most detrimental elements is iron, which enters solution due to its low solubility in the solid state, at levels exceeding 0.6 wt%. At high percentages, it crystallizes in morphologically unfavorable phases, which deteriorate service properties, increase brittleness and porosity of castings, and limit their use.
This paper presents the results of a study of the effect of increased iron content (from 0.8wt.% to 1.5wt.%, in 0.2wt.% increments) on the porosity of AlSi7Mg alloy die castings. Porosity evaluation, conducted using microscopic metallography methods, was performed both qualitatively and quantitatively. It was found that the unfavorable morphology and dimensions of the Al5FeSi phase hinder the free flow of liquid alloy at the crystallization front. The lamellar-ligneous separations “close” the space between the dendrites of the α(Al) solid solution, causing the formation of shrinkage porosity. Increasing the iron content of die-cast Al-Si alloys forces the use of higher doping pressures, but not enough to cause “ejaculations” of the alloy in the dividing plane of the casting mold.