Solidification is a phase change, its essence is process of material transforming from liquid phase to solid phase. During die-casting process, molten metal fills mold cavity at high speed under action of punch and completes solidification under action of high pressure. Rapid solidification is a major feature of die casting. In actual product production process, cooling rate of metal can often reach hundreds or even thousands of degrees Celsius per second. At such a fast cooling rate, microstructure inside die casting is often smaller, and the overall performance of die casting is also higher compared to traditional castings.

 

Figure 1. Pure water at a certain degree of supercooling rapidly solidifies and crystallizes after encountering external nucleation core

Similar to various forming processes, defects can also occur in die castings during solidification process. "Defects" here refer to heterogeneous structures that are different from matrix structure of casting, and the most common solidification defects are well-known "pores", "shrinkage holes" and "shrinkage porosity".
Stomata are gases that are forced into interior of casting under high pressure. They are smooth spherical and randomly distributed. Its formation is related to vacuum degree of mold cavity, release agent, and vaporization of moisture condensed on the surface.
Shrinkage cavities and porosity are caused by solidification shrinkage and insufficient supply of molten metal. They are irregular in shape and are mainly distributed in the area where casting is finally solidified, such as overheated area at the corner of mold cavity, center area of thick castings, etc.
Shrinkage cavities and shrinkage porosity are difficult to define in essence. Generally speaking, we believe that shrinkage cavities are relatively large holes, while shrinkage porosity is relatively small, continuously distributed holes. Shrinkage is a more common solidification defect during die casting process. In general, shrinkage defects are caused by insufficient local fluid supply and differences in thermal shrinkage between heterogeneous component and surrounding tissue.
Feeding is a behavior in which liquid flows toward holes under pressure (induced by gravity or forced by external forces). If feeding channel is clear and external force is sufficient, liquid will spontaneously flow to and fill holes, thereby avoiding formation of shrinkage porosity. On the contrary, if feeding channel is not smooth or external force is insufficient, shrinkage defects will form.
Shrinkage holes caused by difference in thermal shrinkage between heterogeneous components and surrounding tissues are another main cause of shrinkage defects. So-called heterogeneity mainly refers to difference from surrounding matrix structure. From perspective of actual die-casting products, heterogeneity mainly refers to oxidation inclusions. This kind of oxidation inclusion is mainly a defect caused by oxide layer on the surface of metal liquid being drawn into liquid during filling process.
For aluminum alloys, this oxide layer is mainly aluminum oxide, while matrix structure is mainly a microstructure formed by aluminum dendrites and eutectics. Since thermal expansion and thermal shrinkage of alumina are very different from surrounding matrix structure, thermal shrinkage voids will be formed around alumina during subsequent solidification process, thereby forming shrinkage defects.

 

Figure 2. Supercooling degree △T=3.6℃, simulation of Al-Cu dendrite coarsening process
In cold chamber die casting process, alloy liquid is first poured into pressure chamber. Since temperature of pressure chamber is relatively low, liquid alloy in it will partially solidify. These solidified structures are mainly primary phase dendrites or grains, also known as pre-crystallization. These formed pre-crystallized grains will be pushed into mold cavity by punch along with remaining molten metal during subsequent filling process, will complete final solidification.
Since environment (pressure chamber) in which pre-crystallized structure itself is formed is different from solidification environment in mold cavity, to a certain extent, these pre-crystallized structures are also heterogeneous components. In subsequent solidification process, holes or shrinkage defects will also be caused due to differences in thermal shrinkage with matrix. In die-casting process, the lower low speed, the more pre-crystallized structure in pressure chamber, the less supply of molten metal in late solidification period, the more difficult feeding is, and the easier it is to form shrinkage defects.

Shrinkage defects formed during die-casting solidification process can be completely restored using CT detection technology. In most cases, shrinkage porosity and pores (filling air) in die castings cannot be completely separated, but are mixed together, ultimately affecting performance of casting. Through in-situ tensile and fatigue tests, it can be seen that shrinkage porosity often becomes source of cracks, and under action of external forces, cracks often expand by connecting different shrinkage porosity holes, eventually leading to material damage.
Tendency of crack origin is often directly proportional to size of holes. Effectively controlling size of holes inside casting will greatly promote improvement of performance of casting. Therefore, during filling process of molten metal, if tendency of material to be oxidized can be effectively controlled, number of oxidized inclusions can be reduced to a great extent, and formation of subsequent shrinkage porosity can be effectively controlled. This is also main reason why vacuum die casting is widely used at this stage.
Using vacuum die-casting, air in cavity is extracted during filling process of molten metal, which can effectively ensure isolation of molten metal and oxygen, thereby greatly controlling formation of oxide layers and oxidized inclusions. At the same time, we can use a special diverter cone to collect and block pre-crystallized structure before entering mold cavity, thereby avoiding holes and shrinkage defects that may occur during subsequent solidification process.
In die castings, especially magnesium alloy castings, we can often observe another type of defect: this defect is not a single, isolated hole, but is distributed in a band, annular or layered manner inside casting. We call this defect a "defect band".

 

Figure 3. SEM metallographic structure morphology of AZ91D die-casting solidification shrinkage defect
In existing research, it is generally believed that defect zone is formed due to shearing of hotter molten metal on colder, solidified metal surface, which is related to characteristics of die-casting filling and solidification itself. In die casting, hotter molten metal fills mold cavity at high speed. Molten metal that enters mold cavity early comes into contact with colder cavity surface and solidifies rapidly, forming a chill layer on cavity surface. Subsequent inflow of molten metal contacts this chill layer and causes shearing, resulting in defective bands.
Due to high filling speed, probability of this kind of intertwined hot and cold shear occurring during die-casting filling process is extremely high. At the same time, subsequent die-casting pressurization will often continue to increase layer migration of remaining liquid phase and intensify shear intensity, thereby promoting formation of defective zones. Using CT to detect defective bands in die-casting parts, it can be seen that three-dimensional geometric shape of defective bands is not symmetrical, but has obvious flow genetic characteristics, which also proves that formation of defective bands is more affected by flow.
Another key factor affecting formation of shrinkage defects in die castings is heat transfer, that is, heat exchange state of casting-mold interface.
When temperature of mold reaches a good state, a relatively efficient heat transfer state will be achieved between casting metal and casting mold, solidification state of internal structure of casting and feeding state of metal liquid will also be optimal, which will have a positive effect on reducing formation of shrinkage porosity within casting. There are many factors that affect heat exchange state of casting-mold interface, but after research, it was found that the most important factor is temperature state of casting mold itself. Therefore, using a mold temperature controller combined with actual casting rhythm to maintain mold temperature in a relatively optimal range can effectively reduce formation of shrinkage and porosity inside casting.

 

Figure 4. Mold surface temperature and possible hot spots in casting
For different alloys, due to differences in properties of materials themselves, optimal casting temperatures are also different. Experiments have shown that for most aluminum alloys, optimal casting surface temperature is about 230℃-270℃; while for magnesium alloys, this temperature is slightly higher, about 260℃-300℃. When die casting is performed in this temperature range, quality of castings obtained is often better than in other temperature ranges.
In addition to actual experience, in order to avoid formation of shrinkage porosity, we can adopt a more effective method, namely computer simulation technology.
In simulation technology, calculation of heat transfer is relatively mature, it is not difficult to solve temperature field change itself. Key factor that determines calculation accuracy is casting-mold interface heat transfer coefficient or interface thermal resistance.
In numerical calculations, only by accurately setting interface heat transfer coefficient can an accurate temperature field be obtained, so that in subsequent analysis, solidification hot spots of casting can be obtained and location and size of potential shrinkage porosity can be determined.