alsi42 alloys | Overview

Appropriate aging treatment temperature and time can significantly improve the uniformity of the structure and the morphology of the precipitates, thereby increasing the strength of the alloy, but too high temperature or too long aging time will reduce the strength of the alloy. Among the factors affecting the mechanical properties of A356 aluminum alloy, aging time has the greatest influence on tensile strength, yield strength, and elongation, and the magnitude of these properties increases first and then decreases with the increase of aging time. When the aging time is too long, the grains are obviously coarsened, and the coarsening and shape change of the grains directly reduce the hardness of the material. Secondly, the continuous and coarse brittle Mg2Si phase is formed when the aging time is too long, which also reduces the mechanical properties of the alloy. The precipitation phase Mg2Si is a hard and brittle intermetallic compound, which can effectively pin dislocations, stabilize the substructure, prevent the grain boundary from sliding, so that the strength, plasticity, toughness and hardness are well matched, and at the same time, the recrystallization temperature of the matrix is ​​increased. Thus, recrystallization is suppressed; in addition, the strength of the matrix is ​​improved. The stable precipitation hardening phase produced by the aged cast Al-Si alloy will not re-dissolve into the matrix, preventing the long-range movement of dislocations, thus improving the thermal fatigue resistance of the alloy. The fatigue properties of the alloys are mainly affected by the morphology and size of the Si particles, both of which are controlled by adjusting the heat treatment. The heat-treated alloy has excellent fatigue properties due to a large amount of fine Si spheroidization. Fine silicon particles exist in the cell structure, they can limit the expansion of fatigue cracks and delay fatigue fracture by changing the propagation direction. It is divided by small dimples, and no large dimples appear on the edge of the dimples, and its uniformity is better than that of the tensile fracture after T6 heat treatment. Therefore, the elongation of the alloy after double-stage aging is more excellent than that of the T6 process. The fracture surface of A356 alloy after T6 treatment is mixed with cleavage planes and some dimples, which is easy to form brittle cracks. For hypereutectic Al-Si alloys, the aging temperature affects the boundary dissolution and diffusion of alloying elements. With the increase of aging temperature, the boundary dissolution and diffusion of alloying elements accelerate, which is beneficial to improve the mechanical properties of the alloy. Appropriate aging treatment process will improve the wear resistance of the alloy. Sun Yu et al. studied the effect of heat treatment process on strontium-modified near-eutectic Al-Si casting alloys and found that aging treatment would reduce the plasticity of the material. Liu Tuanshen et al. found that aging treatment can improve the impact toughness of Al-20%Si alloy, which is related to the change of the shape of primary silicon and eutectic silicon and the strengthening of the matrix.

As-cast Al-Si alloys are mainly composed of α-Al dendrites and coarse eutectic silicon. For hyper-eutectic Al-Si alloys, there is primary silicon in addition to them, in which α-dendritic shapes are elliptical dendrites. For the bulk polygonal primary silicon, the larger the particle size and the more irregular the shape, the lower the strength, and it is easy to preferentially crack during the stretching process. Huang Caimin et al. found that when the high temperature aluminum liquid is cooled and solidified, due to the local temperature gradient and different cooling rates, the as-cast A356 alloy dendrites appear component segregation, and the matrix also has looseness, holes, inclusions, shrinkage holes and oxide films defect. The eutectic silicon of the unmodified A356 alloy is in the shape of coarse needles. Mg2Si is a precipitation strengthening phase, but the number of Mg2Si phases in the as-cast state is small and small, so it is not easy to be found. A large number of smooth quasi-cleavage planes appear in the tensile fracture morphology of the as-cast A356 alloy, and there are dimples of different sizes in the local area. Most of the dimples are small and shallow, and the number is relatively small. The reason for the characteristics of the cleavage plane is that cracks will occur at the junction of eutectic silicon and the substrate, which will expand and distribute in the eutectic region; Yifan Wang et al. found that the Al-7Si-0.6Mg interface forms covalent bonds between Al and Si atoms. , the covalent bond plays a key role in the interfacial bonding strength. According to the Griffith fracture theory, cracks first form and propagate inside the Al precipitation phase, and the interface can act as a protective layer to prevent crack propagation. Lou Huashan et al. found through the fracture of as-cast A356 aluminum alloy that when the crack propagation encounters the obstruction of eutectic silicon, the crack will cut off the eutectic silicon particles, and as the small crack grows and connects together to form a long crack, then The crack propagates and follows the principle of minimum energy consumption, and propagates through the weakest part of the grain boundary (lamellar structure), and finally manifests as brittle fracture. At the same time, S. Samat et al. found that the reduction of plasticity is related to the microstructural characteristics of harmful acicular β-AlFeSi intermetallic compounds and the existence of microscopic pores during solidification. For hypereutectic Al-Si alloys, the coarse primary silicon can improve the wear resistance of the alloy as a hard point, but because it is hard and brittle, the matrix is ​​severely split, so the mechanical properties of the alloy are reduced and the processing performance is deteriorated.

The addition of alloying elements is an important way to improve the microstructure and properties of Al-Si alloys. Commonly added elements in Al-Si alloys include Mg, Cu, Mn, Sr and RE. Mg element can be dissolved into α-Al to cause lattice distortion and play a role in solid solution strengthening. At the same time, Mg and Si form Mg2Si phase, which is a strengthening phase and improves the hardness of the alloy. The Cu content in the Al-Si alloy reaches 2.5%, and the number of Al2Cu phases increases, which is distributed at the interface of α-Al and eutectic silicon, and plays a strengthening role, but the coarse morphology and distribution of the strengthening phase make the alloy elongated rate decreased. Mn can reduce the number and size of primary silicon in the Al-Si alloy, and the eutectic silicon becomes a shorter needle-like structure. The Mn-containing hypereutectic Al-Si alloy will precipitate Mn-containing dispersed phase particles during the homogenization process, which has high density and high thermal stability, refines the recrystallized grains, and also becomes the nucleation core of the aging strengthening phase. The mechanical properties and processing properties of the alloy have a significant impact. Sr can make the morphology of the eutectic Si phase change from needle-like to fibrous; after the addition of Mn and Sr elements, the AlFeSi phase in the Al-Si alloy is uniformly distributed in the α-Al dendrite, and Mn improves the morphology of the needle-like Fe phase. The effect is larger than that of Sr. A certain amount of Ba has a good metamorphic effect on ZL109 eutectic silicon, and at the same time has good resistance to metamorphism and recession and remelting properties, and the alloy after metamorphism can obtain higher strength; but when the Ba content exceeds 0.125%, there will be appearance in the structure. A small amount of acicular phase is present, and the performance is correspondingly reduced. With the increase of Fe content, the size of the iron-rich phase in A356 aluminum alloy increases, the morphology changes from bone-like to needle-like, and the tensile strength of the alloy decreases. Large flake iron-rich intermetallic compound particles in high-iron aluminum alloy castings promote fatigue cracks The initiation of the alloy is one of the sources of crack sources, however, the increase of Fe content will increase the high temperature and short-term tensile strength of the alloy. After Sb is added to A356 for modification, the density of the alloy is increased, and the modification effect has a long-term effect; Zr can effectively refine grains and inhibit recrystallization. The addition of Zn element to a certain amount can form a eutectic group in the structure of the modified Al-Si alloy. As the amount of Zn increases, the hardness of the alloy increases and the elongation decreases. Phosphorus salt is added to the hypereutectic aluminum-silicon alloy to form an A1P hetero-core, the size of the primary silicon decreases, and the shape changes from a plate shape to a polygonal or agglomerate shape. The alloy has good mechanical properties, wear resistance and casting properties.