Main reason for shrinkage of die castings is the phase change shrinkage that is inevitable when metal melt changes from liquid phase to solid phase after filling mold. Since solidification characteristics of die castings are cooling from outside to inside, when wall thickness of casting is large, shrinkage will inevitably occur inside. Therefore, for die castings, especially for thick die castings, existence of shrinkage is inevitable. This topic starts from causes of shrinkage of aluminum alloy die castings, combined with the entire process of actual casting production, step by step to find causes of shrinkage holes in castings, so as to formulate corresponding countermeasures and successfully reduce shrinkage defect rate of castings.

 

In view of problem of casting shrinkage cavity (Figure 1) in a hole of a fuel vehicle oil pump body casting, hole and oil channel hole leaked after processing, and defect rate was 20%, far exceeding company's target value for defect rate of single defect of castings. Material of die casting is ADC12 aluminum alloy, and main components are shown in (Table 1).

 

Chemical composition and physical properties of aluminum liquid and casting were tested, and test results were qualified. At the same time, slag content of aluminum liquid was tested using K mold, which was also within qualified range. Therefore, two influencing factors of excessive aluminum liquid composition and abnormal slag content can be basically ruled out.

 

Through CAE analysis (Figure 2), this position is too thick, uneven wall thickness, and long duration of hot node. During transition of aluminum alloy from liquid phase to solid phase, aluminum liquid that contacts mold surface will cool down in advance during filling process, resulting in insufficient shrinkage after pressurization, leading to occurrence of shrinkage cavity (Figure 3).

 

In view of this problem, initial countermeasures were formulated:
① When mold temperature is uneven, temperature of some areas on mold will be too high, while temperature of other areas will be too low. Shrinkage of aluminum liquid is inconsistent. By spraying hot spot position of mold, checking cooling water, and using a thermal imager to check mold temperature, mold temperature is balanced.
② Increase thickness of mold gate, and thicken gate from 2.5mm to 3mm to avoid premature solidification of casting, obstruction of pressure transmission in pressurization stage, and inability of aluminum liquid to compensate for shrinkage.
③ Optimize die-casting process and adjust thickness of material cake. If thickness is too thin, it will affect transmission of supercharging pressure. Adjust injection stroke, speed, pressure, etc.
After above measures are formulated, improvement effect of shrinkage position is not obvious, and company's target value for defect rate of single defects in castings cannot be achieved. If it is only solved by adjusting die-casting process, it is necessary to analyze and optimize from other angles.

Since main cause of shrinkage is excessive wall thickness, uneven wall thickness, and long duration of hot spots, aluminum liquid that contacts mold surface during transition from liquid phase to solid phase of aluminum alloy will cool down in advance during filling process, resulting in insufficient compensation after supercharging.

 

From CAE analysis, it can be seen that solidification time here is long, and it cannot be effectively improved by die casting pressurization. It can only be solved by locally increasing extrusion compensation. An extrusion pin is added on the surface of shrinkage hole of casting, and an extrusion cylinder is added separately to mold (Figure 4). According to shrinkage compensation and pressurization rules of casting, extrusion pin action signal adopts pressurization signal of casting process, and is delayed as start signal on this basis. Therefore, extrusion pin mainly controls two parameters: extrusion depth and extrusion delay time.
Extrusion depth depends on structure of casting and distribution and size of shrinkage holes. Since area of pore position of casting is small, a Ø9 extrusion pin is used; extrusion delay is mainly set with reference to pressurization time setting, which is set to 0.5s, and extrusion pin service life (times/6000 molds) is specified. In actual engineering, determination of extrusion parameters is based on empirical values and then optimized according to casting situation.

 

After adding extrusion structure, X-ray flaw detection of shrinkage hole area of casting can be seen to be significantly improved (Figure 5). It can be seen that shrinkage holes of castings appear near bearing holes, with wide and scattered distribution, loose structure. Since holes need to be passed with pressurized lubricating oil, there is a risk of oil leakage during use of castings. After improvement, loose shrinkage hole distribution can no longer be seen from X-ray flaw detection photos, and internal structure of castings appears to be more dense, achieving expected goal.

From process principle, solution to shrinkage defects of castings can only be carried out according to process idea of shrinkage compensation. Phase change shrinkage during solidification process of castings is a natural physical phenomenon. We cannot violate laws of this natural phenomenon, but can only follow its laws. When this problem cannot be solved, it is necessary to consider physical forced shrinkage compensation. The thicker wall thickness of casting or the larger heat node, the greater volume shrinkage. Conventional improvement measures can only play a mitigating role, and cannot fundamentally solve problem.