Magnesium and its alloys have good application prospects in 3C products, transportation, aerospace and other fields due to their high specific strength, good vibration damping and heat dissipation, excellent electromagnetic shielding, good casting process performance and easy recycling. Currently, 90% of magnesium alloys used in engineering applications are formed by die casting. During die-casting production, for thin-walled and complex-shaped parts, it is difficult to not only meet integrity of casting filling, but also ensure that casting has good performance. At present, numerical simulation software is often used in production to predict filling, solidification processes and defects of die casting in order to optimize process design and parameter settings.
For purpose of engineering application, in view of special solidification characteristics of magnesium alloy, structural analysis of motorcycle crankcase right cover was carried out. With the help of orthogonal tests and use of Anycasting2.4 software, die-casting filling and solidification process of parts was simulated. By observing filling situation of molten metal, predict and analyze general rules of occurrence of internal defects in die-casting parts, find factors that have the greatest impact on quality of die-casting parts, and based on this, optimized process parameters were obtained to reduce occurrence of internal defects to a certain extent, in order to provide a reference for die-casting production of similar structural parts.

Normal service temperature of motorcycle crankcase right cover exceeds 120℃, which requires high density and strength. Outline size of die-casting part is 265.06mm*471.41mm*110.00mm, see Figure 1. Shape of casting is relatively complex, and wall thickness varies greatly. The thinnest is 3.8mm and the thickest is 18mm, which puts forward higher requirements for process design and parameter setting.

 

Figure 1 Three-dimensional model diagram of die casting

Table 1 Physical property parameters of castings and molds

Table 2 Orthogonal test plan

Table 3 Orthogonal test results

 

Figure 2 Filling process into inner gate

 

Figure 3 Filling process of castings filled to 50%

 

Figure 4 Solidification process of group 1 die castings

 

Figure 5 Casting defect distribution

 

Figure 6 Internal shrinkage cavities and shrinkage porosity in Group 1

Table 4 Visual analysis table of test results
As can be seen from Figure 6, shrinkage cavities and shrinkage porosity defects appear in parts with larger wall thickness of casting. This is mainly because uneven wall thickness in these parts creates hot spots where molten metal gathers. Since shrinkage cavities and porosity produced when magnesium alloy die castings solidify under high pressure are related to shape and structure of castings, shrinkage cavities and shrinkage porosity generally occur on thick wall side of connecting area between thin and thick walls. When thickness of casting is uneven, solidification speed is different. Shell is thinner and solidifies before thick-walled area. At this time, thick-walled area is difficult to be fed by external alloy liquid, so shrinkage holes in casting will appear concentrated in hot spots where molten metal finally solidifies and accumulates.

 

Figure 7 Optimized solution casting filling process

 

Figure 8 Optimized solution casting solidification process

 

Figure 9 Internal shrinkage cavities and shrinkage porosity

 

Figure 10 Sample drawing

(1) During process of filling mold cavity with molten metal, slow injection speed has the most significant impact on filling time. The greater slow injection speed, the shorter filling time, and the more unstable molten metal, which can easily cause defects such as inclusions, shrinkage cavities, and shrinkage porosity.
(2) During solidification process, mold temperature has the most significant impact on solidification time. The higher mold temperature, the smaller temperature difference between molten metal and mold, and the longer solidification time.
(3) Slow injection speed has the greatest impact on maximum solidification rate. The greater slow injection speed, the smaller temperature drop in pressure chamber. The higher temperature reaching inner gate, the greater temperature difference with mold, and thus the greater solidification rate. This also affects setting time to some extent.
(4) Distribution of shrinkage cavities and shrinkage porosity defects in castings shows that four factors have similar effects on die-casting defects, and defects appear in thick walls near thin-walled shells.