Shrinkage cavities are common internal defects in aluminum alloy die-casting parts. A small amount of shrinkage cavities are generally allowed in die-casting parts. However, when they appear in important parts of product (such as finishing surface, near high-pressure oil passage, sealing surface, etc.), it will affect its performance, and in severe cases, it will directly lead to scrapping of casting. According to statistics, in 2016, defective rate of LD68 hole on A side of left crankcase of a certain 125 motorcycle model caused by casting shrinkage was 0.12%, far exceeding company's target value for defective rate of single defects in castings. Moreover, this hole is located near lubricating oil passage, which can easily cause a series of problems such as poor pressure inspection and oil leakage, so it must be strictly controlled. This die-casting part is made of ADC11 aluminum alloy, and its main components are shown in Table 1. Components of aluminum liquid and castings were tested with a spectrometer, and it was found that contents of alloy elements were within standard range. In addition, K-mold was used to detect slag content of aluminum liquid, and it was found that it was also within qualified range. Therefore, the two influencing factors of excessive aluminum liquid composition and abnormal slag content can basically be eliminated.
This topic starts from mechanism of shrinkage cavities and shrinkage porosity in aluminum alloy die castings, combines actual casting production conditions (such as process parameters, mold temperature, injection mechanism and ancillary equipment) and manifestations of shrinkage cavities in die castings to find out causes of shrinkage in castings, so as to formulate corresponding countermeasures and successfully reduce shrinkage cavity defective rate of castings.

Table 1 Chemical composition of aluminum alloy ADC11 (%)

 

Figure 1 Characteristics of shrinkage holes in LD68 crankcase die-casting parts
For type A shrinkage cavities, feeding characteristics of liquid aluminum during solidification are mainly analyzed, starting from two aspects: process parameters and mold design. Process parameters: pressure is 67MPa; material thickness is 25mm; pouring volume is 3.65kg, all are within range of process requirements and remain stable. Possibility of shrinkage cavities in castings caused by inappropriate process parameters is basically eliminated. Then, gate size of mold with Class A shrinkage cavities was measured. Average thickness was 2.85mm, which was basically consistent with design value. A thermal imager was used to measure temperature of LD68 hole pin of mold and nearby mold opening temperature. It was found that average temperature in this area was about 300℃, and maximum temperature of pin head (corresponding to bottom of shrinkage cavity) reached 320℃. Average mold temperature after spraying release agent is 230℃. Analysis found that LD68 hole pin is long and head diameter is small, so cooling water pipes cannot be arranged. Moreover, shape of casting is complex and it is easy to form hot spots, resulting in a high mold temperature and final solidification area of aluminum liquid. Therefore, type A shrinkage cavities are determined to be caused by excessive local mold temperature, and later countermeasures mainly consider how to reduce mold pin temperature.

 

Figure 2 Effects before and after improvement of release agent spraying device

 

Figure 3 Mold setting cavity

 

Figure 4 Fixed model core cooling water layout
Compared with type A shrinkage cavities, situation of type B shrinkage cavities is relatively complicated. Type B shrinkage cavities have impurities similar to punch oil sintered matter around the hole, and shapes are basically same. It is initially believed that it is due to incomplete combustion products formed by punch oil droplets being wrapped in aluminum liquid. This kind of sintered material often has a large surface area, causing large regional defects, and has a large hardness, which can easily damage tool during processing. After investigation, it was found that punch oil used for lubrication during casting was added to barrel by dripping. Since punch oil has high viscosity and poor fluidity, it can only form a lubricating oil film in a small area. Most of punch oil is ignited by poured aluminum liquid to form incomplete combustion products, which are filled into mold cavity together with aluminum liquid. Due to random position of punch oil combustion products in aluminum liquid, location of type B shrinkage cavities is uncertain. Therefore, for type B shrinkage cavities, main consideration is to control punch oil from being wrapped by aluminum liquid.
Starting from formation mechanism of shrinkage cavities in aluminum alloy die castings, it was found that main factors leading to shrinkage cavities in LD68 are excessive local mold temperature, insufficient feeding of aluminum liquid, and encapsulation of burnt oil in punch oil. For two types of shrinkage cavities, corresponding improvement measures were formulated, namely forming a reasonable mold temperature gradient, reducing amount of punch oil and changing its supply method, and achieved good improvement results.