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.

The study concerns cooling curves of AlSi9Cu3(Fe) alloy, into which iron (in the form of Al-Fe master alloy – as a substitute for scrap) was introduced with a content of 0.5 to 1.5wt.%. Crystallization analysis by ATD and microstructure studies of Al-Si-Mg(Fe) alloys revealed that up to approximately 0.4wt.% Fe, the formation of iron phases does not significantly affect the microstructure. They are part of multicomponent eutectics such as α(Al)+(Al2Cu,Fe)+β(Si) or α(Al)+(AlXFe;CuYSiZ)+β(Si), which crystallize after the formation of the α(Al)+β(Si) eutectic. In the range of about 0.5wt.%Fe to 0.9wt.%Fe, there is a pre-eutectic crystallization of iron phases, mainly the lamellar-neutectic phase β-Al5FeSi. At more than 1.0wt.%Fe, the morphology of this phase becomes even more unfavorable (due to primary crystallization) and is accompanied by numerous clusters of shrinkage porosity. Such a microstructure has an adverse effect on mechanical properties, primarily affecting plastic properties (yield strength and elongation). To prevent this, the positive impact of manganese additive (in the form of AlMn20 master alloy) was found to transform the morphologically unfavorable β-Al5FeSi phase into α-Al15(Fe,Mn)3Si2 phase with dendritic structure.

Laser processing is often used in many technological processes for welding, cutting, marking, etc. However, it has been gaining popularity for several years as a surface modification process, including cleaning, polishing, surfacing, and hardening. These methods are used to modify the physicochemical properties to enhance hardness, wear resistance, corrosion resistance, and the propagation of microcracks on the metal surface. It has been found that the effectiveness of laser radiation (short pulses of laser light) depends on the thermal properties of the alloy being treated and the process parameters. Also significant is optical absorption, which determines the coupling of laser radiation energy to the workpiece during the melting of its surface. The study, considered preliminary, was conducted on flat sparrows of AlSi11 alloy under argon shielding. It was possible to fragment the microstructure components of the studied alloy at depths of several micrometers, depending on the laser power density, spot size, and travel speed. The fragmented α(Al) dendrites in the melted laser zone are oriented toward the temperature gradient. At the very surface, they gradually lose their secondary arms, assuming the morphology of compact cells. This method can harden large or local areas that are inaccessible to other surface treatment methods, such as piston ring gaps in internal combustion engines, specifically the part of the piston shell that operates under extreme thermo-mechanical and tribological stresses.

Using an increasing share of aluminum scrap (production and post-production) requires paying special attention to its impurities. Gaseous inclusions (e.g., hydrides) can be removed from the liquid alloy by refining, but the situation is worse with metallic impurities. In Al-Si alloys, one of the worst is iron, which goes into solution due to its low solubility in the solid state, at a content of over 0.4wt.%, it crystallizes in morphologically unfavorable phases, which worsen the functional properties and increase the porosity of aluminum alloys, limiting their use. The crystallization of these phases causes thermal effects that various methods can record. The paper presents studies of phase transformation, especially iron phases, using thermal-derivative analysis occurring in the AlSi7Mg alloy with different iron content. The studies were performed on Crystaldigraph NT3-8K coupled with the MLab program. It was found that the most unfavorable phase is β- Al5FeSi, which crystallizes preeutectic (and mainly primary crystallize) dimensions of up to 1000 μm, causing the formation of shrinkage porosities.