Research on Microstructure and Growth Mechanism of Different Primary Silicon in Hypereutectic Aluminum Alloy

The as-cast microstructure of a typical hypereutectic Al-25Si alloy was studied, and the growth mechanism of different primary silicon phases was analyzed. The results show that the as-cast microstructure phase composition of the alloy is mainly primary silicon and eutectic silicon. Primary silicon is mainly petal-like, massive and other complex polyhedrons, and there are a lot of cavities, cracks and other defects in the interior and boundary; Eutectic silicon is coarse and long needle-like, and the distribution is relatively messy, which seriously deteriorates the mechanical properties and cutting performance, and hinders the further application of the alloy in the field of lightweight pistons. Petal-shaped primary silicon is grown by combining five tetrahedral crystal nuclei in the melt into a decahedron, while bulk primary silicon is mainly caused by the unbalanced aggregation of impurity elements. And these two types of silicon phase growth methods are related to the twin groove growth mechanism, which is the result of a combination of multiple mechanisms.

Influence of MHD - plasma melt processing on the structure and properties of cast aluminum alloy A390

Growth of production of cast products and the desire of enterprises to reduce the cost of manufacturing metal products led to a significant increase in requirements for the structure and properties of aluminum alloys. Increasing of physical and mechanical properties of alloys is most effectively at the stages of their preparation in liquid state. At that, it is possible to affect effectively on the quality of cast metal by external actions on alloys, deep refining from gases and harmful impurities, active modifying of alloy, reducing or eliminating the negative impact of heredity of charge materials. The main disadvantage of the processes of structure refinement of alloys by using modifiers is instability of their results, which depends on various reasons. One of the most important reasons is providing conditions for the formation and preservation of active modifier particles in the melt volume. They are assimilating by liquid alloy and acting on crystal nucleus at crystallization. It is known that only ~10% particles are active of the total number of particles added with the ligature into the melt. Other particles dissolve in the melt, take away by the crystallization front, or push back on to intergranular boundaries. The considered methods of electromagnetic, MHD and plasma actions on liquid metal allow to refine and modify alloys without use of special reagents. The paper presents studying of the structure and properties of supereutectic silumin A390 after treatment in casting magnetodynamic installation (MDI) by submerged into melt the plasma argon jet and alternating electromagnetic field & magnetohydrodynamic (MHD) effects, including simultaneous combination. There are developed the scientific and technological bases of MHD-plasma processing of liquid hypereutectic silumin A390 and original equipment for their realization. It provides dispersed structure of solidified alloy. Thus, there is a significant decreasing of sizes both particles of primary silicon and dendrites of α-solid solution of aluminium. Also, strength characteristics of alloys increased to 10%, and elongation rises up in 1.5-2 times. Keywords: plasma jet, magnetodynamic installation (MDI), aluminum alloy, mechanical properties.

Spheroidization Behavior of Nano-Primary Silicon Induced by Neodymium under High-Current Pulsed Electron Beam Irradiation

The spheroidization behavior of the nano-primary silicon phase induced by Nd under high-current pulsed electron beam (HCPEB) irradiation was investigated in this study. The study results revealed that, compared to the Al–17.5Si alloy, spheroidized nano-primary silicon phase emerged in the alloy’s HCPEB-irradiated surface layer due to the presence of Nd. Because Nd was abundantly enriched on the fast-growing silicon crystal plane, its surface tension was reduced under the extreme undercooling caused by HCPEB irradiation, causing the growth velocity of each crystal plane to be the same and spherical nanometers of silicon to appear. The spheroidization of nano-primary silicon phases occurred in the remelted layer. The microhardness test revealed that Nd could depress the microhardness of the Al matrix at the same number of pulses, but conversely increase the microhardness of the primary silicon phase, compared to the Al–17.5Si alloy. The tribological test showed that the presence of spherical nano-primary silicon could significantly improve the alloy’s tribological property.

Effect of Si Content on the Properties of A356 Aluminum Alloy

The effects of different Si contents on the microstructure and mechanical properties of A356 aluminum alloy were studied by metallographic microscope analysis and tensile property test. The results show that when the silicon content is between 7% and 11 %, with the increase of silicon content, the eutectic silicon in the matrix increases, and the tensile strength and elongation decrease. When the silicon content increased to 13%, the primary silicon structure appeared in A356 aluminum alloy, and its mechanical properties increased.

Microstructural Studies and Material Characterization of Alumina Nanoparticulate Reinforced Functionally Graded Al-12Si (wt.%) alloy, produced using Centrifuge Casting Technique

The current research work discusses the concept of reinforcing alumina nanoparticulate to Al-12Si (wt.%) alloy using a unique casting technique known as centrifuge casting system established in-house at our advanced manufacturing centre. The resulting cast solid cylindrical specimen will be functionally graded along the length of the specimen containing higher percentage of primary silicon and alumina nanoparticulate at the top region than the bottom region, with a gradual transition in between the two regions. The microstructure of the functionally graded Al-12Si (wt.%) alloy, reinforced with 0.5, 1 and 1.5wt.% of alumina nanoparticulate was analysed using “Scanning Electron Microscope (SEM)” and the elemental composition of the cast specimens were obtained using “Energy-dispersive X-ray analysis (EDX)”. The outcomes of the EDX analysis confirmed the presence of alumina nanoparticulate in the cast specimens. Further, the mechanical properties of the cast Al-12Si (wt.%) alloy, reinforced with alumina nanoparticulate were studied, and it was found that the specimen with the addition of alumina nanoparticulate showed enhanced mechanical properties when compared with the Al-12Si (wt.%) alloy, without alumina nanoparticulate addition, cast under identical conditions.

INVESTIGATION OF THE INFLUENCE OF THE MODIFIERS P, SR, TI AND COMBINATIONS OF THEM ON THE STRUCTURE AND MECHANICAL PROPERTIES OF ALSI25 ALLOY

The most commonly used elements to modify primary silicon crystals in the structure of hypereutectic aluminum-silicon alloys are phosphorus and sulfur. Phosphorus has been shown to have the highest coefficient of modification with respect to the primary silicon and is therefore a preferred modifier. There are also data on the positive effect of the modifiers Sb, Sr, Ti, and B on the silicon crystals in the structure of this type of alloys. The influence of the modifiers phosphorus, strontium, titanium and combinations of them on the size and shape of both the primary silicon crystals and the silicon crystals in the composition of the eutectic of the AlSi25 alloy has been studied in this work. Mechanical tests have been performed to determine both the strength and the plastic parameters of the investigated alloy (in unmodified and modified state). The classic for this type of alloys modifier - phosphorus - has been introduced into the melt by the ligature CuP10. Strontium has been introduced by the ligature AlSr10, and titanium - by the ligature AlTi5B1, the two ligatures in the form of rods. The investigated alloy has also been modified by combinations of the used modifiers: phosphorus and strontium, phosphorus and titanium.The influence of the used modifiers on the structure and mechanical properties of AlSi25 alloy has been discussed.

INVESTIGATION OF THE INFLUENCE OF DIFFERENT MODIFIERS ON THE EUTECTIC SI IN THE COMPOSITION OF ALSI18 ALLOY

The structure of hypereutectic aluminum-silicon alloys consists of primary silicon crystals arranged in a eutectic matrix. In the present work the influence of different types of modifiers on the size and shape of the silicon crystals in the composition of the eutectic of the AlSi18 alloy has been studied. The classic for this type of alloys modifier (phosphorus), as well as the nanomodifiers SiC and nanodiamonds (ND) have been used. The results of the microstructural analysis show that the three modifiers used affect differently the shape and size of the eutectic silicon of the investigated alloy.

Effect of primary silicon crystals on the fluidity of eutectic and hypereutectic Al-Si alloys in lost foam casting

The hypereutectic Aluminum-Silicon (Al-Si) Alloys are gaining impetus in the automotive components, mainly due to their wear resistance. This property in these alloys is derived from the primary silicon crystals. However, there are unique metal flow and mold filling problems associated with hypereutectic Al-Si alloys in Lost Foam Casting (LFC). This investigation is a pioneering work undertaken to gain a better understanding of the role of primary silicon crystals and other phases in the LFC of hypereutectic Al-Si alloys. Time-temperature and first derivative curves were used to determine velocity of metal flow and to calculate solid fractions. Process parameters such as superheat, gating design and alloy composition were manipulated to change the morphology of primary silicon crystals. Microscopy and image analysis of castings enabled study of precipitated particles. Solidification, interlocking and melt sluggishness of precipitated particles significantly influenced fluidity, and the routine process parameters played a minor role.

Effect of primary silicon crystals on the fluidity of eutectic and hypereutectic Al-Si alloys in lost foam casting

The hypereutectic Aluminum-Silicon (Al-Si) Alloys are gaining impetus in the automotive components, mainly due to their wear resistance. This property in these alloys is derived from the primary silicon crystals. However, there are unique metal flow and mold filling problems associated with hypereutectic Al-Si alloys in Lost Foam Casting (LFC). This investigation is a pioneering work undertaken to gain a better understanding of the role of primary silicon crystals and other phases in the LFC of hypereutectic Al-Si alloys. Time-temperature and first derivative curves were used to determine velocity of metal flow and to calculate solid fractions. Process parameters such as superheat, gating design and alloy composition were manipulated to change the morphology of primary silicon crystals. Microscopy and image analysis of castings enabled study of precipitated particles. Solidification, interlocking and melt sluggishness of precipitated particles significantly influenced fluidity, and the routine process parameters played a minor role.