Magnesium alloys are widely used in aerospace, automotive, electronics and other fields due to their light weight, high specific strength and specific stiffness, good shock absorption and easy recycling. Using magnesium alloys instead of steel or aluminum alloys is one of effective ways to achieve lightweight vehicles, which can reduce fuel consumption and gas emissions. Magnesium alloy liquid has low viscosity and good fluidity, is easy to fill complex cavities, solidifies quickly, and has good dimensional stability, making it especially suitable for die-casting processes. Therefore, die casting has become the most important forming process for magnesium alloy structural parts, is often used in production of automotive wheels, transmission bodies, laptop/mobile phone casings and other parts.
Currently, magnesium alloy die-casting parts have problems such as low absolute strength and poor high-temperature performance, which restrict their application. Research shows that shrinkage cavities, pressure chamber pre-crystallizations (ESCs), defect zones, etc. that appear in cold chamber die-casting magnesium alloys have a great impact on the mechanical properties of die-casting parts. There have been relatively in-depth studies on shrinkage cavities and ESCs, but related research on defect zones mainly focuses on structural characterization and phase analysis of defect zones. Research has found that morphology and distribution characteristics of defect zones have a certain correspondence with shrinkage holes, ESCs, etc. However, their formation mechanism has not been clear and unified.
Therefore, taking AM60B magnesium alloy tensile die-casting specimens as research object, microstructure, morphology and distribution characteristics of defective bands in die-cast magnesium alloys were systematically studied, corresponding relationship between defective bands, ESCs, shrinkage holes and die-casting process parameters was established. On this basis, formation and evolution mechanism of defective bands was discussed. It aims to provide a reference for optimizing die-casting process parameters, improving microstructure and mechanical properties of magnesium alloy die-casting parts.

Defect zones are unique structural features in cold chamber die casting structures, their morphology and distribution are relatively complex. Figure 1 shows defect bands with different morphologies and distribution characteristics in cross-section of magnesium alloy die-casting parts. It can be seen that some are distributed along cross-sectional contour of die-casting parts, and some are irregular; there are single defect bands, double defect bands and multiple defect bands. Previous studies have shown that defect zone is a band-like structure where holes are gathered, and there is a certain degree of solute segregation. Different structural observation methods are used to analyze microstructure of cross-section of magnesium alloy die castings, as shown in Figure 2. It can be seen that die-cast magnesium alloy structure can be divided into three parts from a macro perspective. The first part is structure from surface layer of die-casting part to outer surface of defect zone. It is characterized by fewer holes. ESCs structure is mostly broken dendrites without aggregation, so structure is relatively dense; second part is a defect zone at a certain distance from surface of die-casting part. There are a large number of holes with irregular shapes in interior, and ESCs structure is smaller than surrounding ESCs.

 

Figure 1 Defect zones with different morphologies and distribution characteristics in cross-section of magnesium alloy die-casting parts

 

Figure 2 Typical microstructure of cross-section of magnesium alloy die castings

 

Figure 3 Microstructure morphology of defect zones in die-cast magnesium alloy at different low speeds

Table 1 Die casting process parameters
Microstructure of die-cast magnesium alloys under different boosting pressures was studied. It was found that as boosting pressure increases, ESCs content in die-casting parts gradually decreases, its morphology tends to be spherical, distribution pattern changes from dispersion to segregation toward center of die-casting part. At this time, position and width of defective zone in die-casting part and morphology of internal holes have not changed significantly. Impact of pouring delay on defect zone is similar to that of low speed. Increase of pouring delay or decrease of low speed will increase ESCs content in die casting. ESCs appear in shape of more thick dendrites, and there is a large amount of shrinkage porosity at grain boundaries where ESCs gather. At this time, defect zone tends to surface layer of die casting and its width increases.

 

Figure 4 Microstructure morphology of defect zones in die-cast magnesium alloy at different high speeds

 

Figure 5 Schematic diagram of defect band formation mechanism based on pressure chamber pre-crystallization theory

 

Figure 6 Schematic diagram of defect band formation mechanism based on expansion shear theory
Some studies believe that defect band is an expansion shear band, which is formed due to rheological characteristics of granular materials during shear deformation exhibited by molten metal with a certain solid phase fraction, as shown in Figure 6. During solidification process, molten metal containing a certain solid phase fraction is equivalent to compacted granular materials. Under action of shear stress, they will push each other and expand, causing local deformation, and concentrate at shear zone to finally reach a critical state. Liquid metal is led into expansion shear zone due to pressure difference. As solidification proceeds, molten metal in band-shaped area compensates for solidification shrinkage of adjacent areas. However, in later stage of solidification, solidification shrinkage of strip area itself cannot be effectively fed, so concentrated shrinkage cavities and shrinkage porosity are formed in strip area. Applying expansion shear theory to magnesium alloy cold chamber die-casting process can explain formation mechanism of double defect bands to a certain extent. During die-casting filling process, due to chilling effect, a solidified shell layer will be produced on the surface of die-casting part. When mold cavity is filled at high speed, molten metal flow will generate large shear stress in area close to solidified shell, and this area is semi-solid. Under action of shear stress, grains will move and produce corresponding gaps. Finally, due to difficulty in feeding, a defective band structure near surface will be produced. During solidification process, pressurized pressure will lead to existence of shear stress, which will act on semi-solid grains. cause relative movement of grains and produce corresponding gaps. Finally, due to difficulty of feeding, a defective band structure near the center of die casting was produced.

 

Figure 7 Effect of metal flow morphology on the formation of defect zones in die-cast magnesium alloys

 

Figure 8 Analysis of chemical composition of tissue around hole in defect zone

Electron backscatter diffraction (EBSD) was used to analyze grain orientation of different parts of magnesium alloy die-casting. It was found that no twinning phenomenon was found in α-Mg grains on the surface of die casting and in intact ESCs dendrites in core. However, a large number of twins appeared in broken ESCs grains in core and ESCs grains inside defect zone. Analyzing reason, α-Mg grains on the surface of die casting nucleate and grow in mold cavity, and there is almost no external force acting on α-Mg grains; while ESCs particles are nucleated in pressure chamber and enter mold cavity through inner gate during injection process of punch. Due to high-speed and high-pressure characteristics of die-casting filling process, ESCs particles will be washed by liquid flow and subsequent boosting pressure when entering mold cavity, causing some ESCs grains to rotate and break. Residual stress will be generated inside eventually broken ESCs grains to form twins, while ESCs dendrites that have not been eroded by metal flow, pressurized are intact and do not have twins. A large number of twins appear in ESCs grains inside defect zone. It is found that there is a large stress in defect zone. It can be inferred that formation of defect zone is related to stress generated by erosion of metal liquid flow during filling process and subsequent boosting pressure.
Formation and distribution of defective bands are affected by shape of metal flow during die-casting filling process. Under action of violent erosion of high-speed molten metal and action of boosting pressure, crystal grains close to outer contour of molten metal flow are broken or rotated, forming gaps between crystal grains that are larger than volume of remaining molten metal. As solidification proceeds, a defect zone structure is formed in which holes are distributed along liquid flow contour.