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, and 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 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 and distribution characteristics of defective bands in die-cast magnesium alloy 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 defect bands were 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; the second part is a defect zone at a certain distance from surface of die-casting part. A large number of holes with irregular shapes are concentrated inside it, and ESCs structure is smaller than surrounding ESCs.
 

(a) Defect-free tape

 

(b) Defect zone near surface

 

(c) Defect zone near center

 
(d) Double defective strip
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, and distribution pattern changes from dispersion to segregation toward the center of die casting. However, at this time, position and width of defect zone in die casting and morphology of internal holes do not change significantly. Influence 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 the 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

 

(a) Changes in grains under action of shear stress

 

(b) Microstructure of expansion shear band
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 feeds solidification shrinkage of adjacent areas. However, in the later stage of solidification, solidification shrinkage of band-shaped 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, metal flow will generate large shear stress in the area close to solidified shell layer, and this area is 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 and cause relative movement of grains and produce corresponding gaps. Finally, due to difficulty in feeding, a defective band structure near the center of die casting will be produced.

 

(a) Two forms of internal gates

 

(b) Defect zones in die castings corresponding to two different inner gates

 

(c) Hole scanning and three-dimensional reconstruction of die-casting parts corresponding to two different inner gates

 

(d) Top view corresponding to figure (c)
Figure 7 Effect of metal flow morphology on formation of defect zones in die-cast magnesium alloys

 

(a) Scanning area of pore morphology and composition of defect zone

 

(b) Mg element distribution

 

(c) Al element distribution
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 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. Eventually, residual stress will be generated inside eventually broken ESCs grains to form twins, while ESCs dendrites that have not been eroded by metal flow and pressurized are intact and do not have twins. From large number of twins 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 grains that are larger than volume of remaining molten metal. As solidification proceeds, a defect band structure is formed in which holes are distributed along contour of liquid flow.