Aluminum alloy has characteristics of low density, good plasticity, high specific strength, good corrosion resistance and electrical conductivity, as well as good mechanical properties and process properties. Its industrial application is second only to steel. However, in production process of aluminum alloy casting, there are often casting defects such as pores, shrinkage porosity, cold shut and segregation, which are detrimental to performance of alloy.
Segregation is divided into micro-segregation and macro-segregation. Microsegregation is a short-range phenomenon of uneven chemical composition in grains, which can be eliminated by long-term high-temperature homogenization annealing. Macroscopic segregation is manifested as long-range changes in composition, and large-scale uneven composition, such as inconsistent composition inside and outside ingot. This kind of segregation cannot be eliminated by high temperature annealing and becomes a defect of material, which should be avoided during casting process. Using optical microscope, scanning electron microscope and other equipment, combined with examples to analyze segregation of Fe and Si elements in aluminum alloy castings, influence of segregation of Fe and Si elements on die castings and corresponding solutions are briefly described, providing reference for actual production.
Graphical results
Macro segregation is generally due to uneven chemical composition of melt, precipitation or floating of compound due to difference in density and matrix during melting process, or during solidification process, due to different cooling rates of various parts of casting, internal and external solidification shrinkage is not synchronized, gas in melt is precipitated, which causes pressure between dendrite channels, causes certain elements or low melting point substances to enter surface area of casting through dendrite channels, forming defects visible to naked eye. These segregation defects not only affect appearance of casting but also seriously affect performance of casting, should be avoided as much as possible during production process.

 

Fig.1 Morphology of defects in AlSi9Cu3 die castings

 

Figure 2 Analysis of EDS points at different defects in Figure 1

 

Fig.3 Tensile fracture morphology of AlSiMnFe phase aggregation
Formation of these larger AlSiMnFe phases is due to tendency of Fe and Mn elements in aluminum liquid to segregate to lower part of aluminum liquid. When content of Fe and Mn in alloy is high, or aluminum liquid stays for a long time, this segregation tendency will be stronger, AlSiMnFe phase will precipitate and grow at the bottom of aluminum liquid. Insoluble Al2O3 in aluminum liquid can be used as core of heterogeneous nucleation of AlSiMnFe phase, through motion aggregation, accelerate nucleation and growth of AlSiMnFe phase, finally form AlSiMnFe phase with a large number and large size. These Al2O3 heterogeneous particles may be obtained from transformation from softer γ-Al2O3 to hard α-Al2O3 after oxide film on the surface of aluminum liquid enters aluminum liquid and stays for a long time, and may also come from furnace wall or bottom that has not been cleaned.

 

Fig.4 Macroscopic appearance of wheel hub sample and cross-sectional schematic diagram of metallographic sample

 

Figure 5 Metallographic structure of longitudinal section at black spot

 

Figure 6 Si surface scanning of wheel hub

 

Fig.7 Surface SEM morphology after turning

 

Figure 8 EDS analysis of point 1 in Figure 7b
(1) Fe segregation in cast aluminum alloy die castings forms coarse Fe phases that appear as black spots in macroscopic view, which not only causes chipping during machining, but also seriously reduces mechanical properties of alloy. This defect can usually be solved by: ① fully stir alloy liquid during smelting and before pouring; ② shorten parking time of alloy liquid; ③ reduce Fe content under premise of satisfying performance; ④ accelerate cooling when alloy solidifies; ⑤ do not use a layer of aluminum liquid near bottom of furnace for prevention.
(2) Segregation of Si on the surface of cast aluminum alloy shows a large area of black spots, which not only affects appearance of wheel, but also shortens service life of wheel. This defect can usually be prevented by: ①increasing cooling rate; ②increasing local strong cooling at thick part of mold; ③applying an external magnetic field, electromagnetic stirring, ultrasonic and other methods.