Particle-reinforced aluminum matrix composites (PAMCs) have been mainly used in aerospace and military fields since their creation, but there are few reports on their application in civilian field. In addition to price factors, it is mainly due to high strength and high hardness of added reinforced particles that subsequent cutting processing of PAMCs is difficult and production cost is high. Die-casting technology has characteristics of high production efficiency, good product quality and less subsequent processing. It is widely used in mass production of aluminum and magnesium alloy castings. Stir casting method is used to prepare PAMCs melt in batches, and die-casting technology is used to achieve near-net shape, which can reduce subsequent cutting processing and reduce costs, has positive significance for promotion and application of PAMCs.
Die-casting process is relatively complex, resulting in a large difference between die-casting structure and ordinary casting structure. During die-casting filling process, after metal melt enters mold cavity from pressure chamber, it is quenched in contact with mold wall, forming a skin layer (also called a quench layer) composed of fine grains. It is generally believed that skin layer has characteristics of fine-grained materials and can improve mechanical properties of castings, but some studies have pointed out that it has a small impact on properties of castings. Regarding formation of skin layer, research shows that it is mainly affected by collision method between melt and mold wall during mold filling process and solidification sequence of casting. Skin layer is not easy to form during direct collision, but is easily formed in edge area of indirect collision, backflow and preferential solidification. In addition, during die-casting process, melt fills mold cavity at high speed under high pressure, which is prone to air entrainment and formation of hole defects. A large number of studies have shown that hole defects reduce effective bearing area of material, and large-sized holes can easily induce fracture failure of castings, leading to a decrease in mechanical properties. Vacuum-assisted die-casting technology can effectively reduce hole defects and improve mechanical properties of castings.
Compared with base aluminum alloy, density, viscosity and heat transfer coefficient of PAMCs have greatly changed, so impact of die-casting process on structure and properties of PAMCs castings is greatly different. However, there are currently few research reports on die-casting forming of PAMCs, and no research on skin layer, hole defects and their impact on mechanical properties of die-cast PAMCs has been reported. Therefore, this project carried out vacuum-assisted die-casting experiments on PAMCs to understand formation mechanism of skin layer and hole defects in die-cast SiC/6061Al composite materials, to explore impact of skin layer and hole defects on mechanical properties of castings.

SiC particles with an average particle size of 50 μm were selected as reinforcing phase, and commercial 6061 aluminum alloy was selected as matrix material. Its chemical composition is shown in Table 1. Figure 1 shows SiC/6061Al composite die-casting test device. Using self-developed PAMCs stirring device (see Figure 1a), SiC/6061 composite melt was prepared using semi-solid stirring casting method. For detailed steps, see reference literature. 65kg of SiC/6061Al composite melt with a SiC volume fraction of 10% was prepared.

Table 1 Chemical composition of 6061 aluminum alloy (%)

 

(a). PAMCs stirring preparation device

 

(b). SiC/6061 composite die castings
Figure 1 SiC/6061 composite material die-casting test device
 
Figure 2 Microstructure of die-cast SiC/6061Al composite samples with different wall thicknesses

 

(a). Epidermal layer tissue

 

(b) Mechanism of action
Figure 3 Epidermis layer organization and its mechanism of action on SiC particle distribution
It can be seen that formation of the skin layer is related to type of matrix alloy. Analysis found that liquidus temperature of 6061 aluminum alloy is 653℃, while liquidus temperature of A356 aluminum alloy is 617℃, with a difference of 36℃. Therefore, under same mold temperature and pouring temperature conditions, 6061 aluminum alloy melt is more likely to obtain a greater degree of supercooling, forming a large number of crystal nuclei at mold wall. At the same time, due to rapid cooling rate of sample, formed crystal nuclei did not have time to grow, resulting in formation of a chilled crystal structure, that is, a skin layer, on the surface of sample. Formation of skin layer has an important impact onthe distribution of SiC particles, and there are fewer SiC particles in skin layer. This is because formation of skin layer can push particles close to mold wall into interior of sample, causing reinforced particles to move into casting. Figure 3b shows mechanism of skin layer on particle distribution. In addition, skin layer has an important influence on pressure resistance, corrosion resistance, air tightness, machining and mechanical properties of die castings.

 

(a)2mm

 

(b)4mm

 

(c)6mm

 

(d)8mm
Figure 4 3D morphology and distribution characteristics of holes in SiC/6061Al composite die-cast specimens with different wall thicknesses

 

Figure 5 Number and roundness of holes in SiC/6061Al composites with different wall thicknesses
Studies have shown that shear stress generated by boosting pressure can cause relative movement of semi-solid grains to create gaps, and ultimately form defect bands. In this study, shear stress generated by boosting pressure will cause residual liquid phase in core area to flow in filling direction between semi-solid grains. Obstructed by α-Al and SiC particles, flow direction is deflected, and part of residual liquid phase forms a long strip-shaped small molten pool. Finally, due to difficulty in feeding, long strip-shaped shrinkage cavities were formed after solidification shrinkage, as shown in Figure 6b.

 

Figure 6 6061 matrix alloy solid phase ratio and temperature relationship curve, strip shrinkage cavity formation mechanism

 

Figure 7 Relationship between mechanical properties, porosity and number of large-sized holes of die-cast SiC/6061Al composite samples

 

(a)2mm

 

(b). 2mm, heart enlarged

 

(c)4mm

 

(d). 4mm, heart enlarged

 

(e)6mm

 

(f) 6mm, enlarged heart

 

(g)8mm

 

(h)8mm, heart enlarged
Figure 8 Tensile fracture morphology of die-cast SiC/6061 composite materials with different wall thicknesses

(1) For SiC/6061Al composite materials, due to high liquidus temperature of base 6061 aluminum alloy, die-cast sample is easy to form a thicker skin layer (about 0.5mm); during formation of skin layer, SiC particles can be pushing inward, resulting in fewer SiC particles in epidermal layer.
(2) Wall thickness has an important influence on type, number and distribution of holes in die-cast SiC/6061Al composites. When wall thickness is 2mm, solidification time of sample is short, a large number of pores are not compressed by boosting pressure and are distributed throughout casting; when wall thickness increases to 4mm, boosting pressure action time is extended, most of pores are compressed, hole type changes to mainly shrinkage holes and number of shrinkage holes increases significantly, is concentrated in the center of specimen axis area; after wall thickness increased to 6mm, core area of sample tended to solidify like paste, and a certain number of strip-shaped shrinkage holes were formed under influence of boosting pressure.
(3) Large-sized holes in the center of sample can easily become source of fracture, and scattered holes promote expansion of cracks, resulting in low brittle fracture mechanical properties of die-cast SiC/6061Al composite sample. Change in tensile strength has an opposite relationship with porosity and number of large-sized holes.
(4) A small amount of particles in skin layer destroys continuity of fine-grained structure, and stress concentration is easily generated during stress process, leading to brittle damage of skin layer and reducing mechanical properties of sample. In summary, in order to improve mechanical properties of die-cast SiC/6061Al composite materials, die-casting process should be optimized and an appropriate aluminum alloy matrix should be selected to reduce porosity and number of large-sized holes, control thickness of skin layer.