Due to its low density, high strength and corrosion resistance, aluminum alloy is widely used in production of auto parts. Aluminum alloy is strengthened by alloy, its strength is obviously improved, which meets requirements of clutch housing, oil pan and differential housing working in harsh environments. At the same time, under demand of energy saving and emission reduction, expanding application of aluminum alloy can significantly reduce weight of automobiles. As an important part of automobile rotation system, differential case has more and more demands for automobile manufacturing and higher quality requirements. Therefore, under condition of ensuring high product quality, how to reduce reject rate is a problem that needs to be solved.
Aluminum alloy differential case is one of key components of automobile transmission system, its structure is relatively complex, and its internal quality requirements are high. In production process, large-scale processing is required, so exposed internal defects are greatly increased, especially slag inclusions and pores. In order to solve problem of high scrap rate caused by defects, numerical simulation is used to compare aluminum alloy liquid filling state with actual distribution of defects in castings, then dissect and analyze casting structure to optimize gating system. By designing a reasonable gating system to reduce defects and reduce scrap rate.

Die-casting parts of automobile differential housing are shown in Figure 1. Outline size of part is 236.91mm*191.23mm*187.41mm, weight of die casting is 3.35kg, average wall thickness is 6.2mm, projected area is 97532.21mm2, and casting material is ADC12 alloy. The overall wall thickness of casting is relatively thick, and wall thickness is not uniform, so position of hot joint is easy to cause shrinkage cavity during die-casting process, and processing area is large. Gating system design is shown in Figure 2, all parts are required to be free of burrs, scars, and flash defects; take tensile samples at position of bearing bush, and tensile strength obtained from test is greater than 177MPa, no defects such as pores and shrinkage can occur, air-tightness inspection is required , leak test pressure is 200kPa, and allowable leakage is <3mL/min.

 

Figure 1 Die-cast parts of differential case

 

Figure 2 gating overflow system

 

Figure 3 Differential case filling simulation

 

Figure 4 Air pressure simulation of differential housing cavity

 

Fig.5 Simulation of solidification process of differential case
According to simulation results, it can be seen that gating system can basically realize smooth filling of aluminum materials, entrained gas is mainly concentrated in slag ladle and some local non-processing dead corner areas of product. According to simulation analysis, it is found that there are two problems: ①Flow rate of molten aluminum on fixed mold side is faster, but there are two thicker bosses on fixed mold side that require more aluminum material; ②Amount of gating on right side is obviously stronger than that on left side, but processing area on left side is larger, and internal quality requirements are higher, so quality of processing area needs to be guaranteed. Comprehensive consideration, program is feasible. Due to uneven wall thickness of shell, local cooling rate is not uniform, so a more reasonable cooling system can be designed.

 

Figure 6 X-ray detection results when high-speed switching point is 370mm

 

Figure 7 X-ray detection results when high-speed switching point is 390mm

 

Figure 8 X-ray results when high-speed switching point is 420mm

Table 1 Statistics of production data

 

Fig.9 Location and appearance of slag inclusions

 

Figure 10 Shrinkage hole next to left core-pulling needle and right and upper core-pulling
Bottom stomata

 

Figure 11 Leakage location and pore morphology
According to distribution of defects, there are 4 problems that need to be improved: ①Core-pulling bearing bush on left side shrinks and loosens, and after processing, it communicates with threaded hole on the side to leak air; ②Pinhole shrinkage process on both sides of bearing bush is exposed; ③Interlayer leaks after processing in deep cavity of core-pulling on the right; ④Air hole at the bottom of pinhole on the right-side core-pulling.

 

Figure 12 Comparison before and after gate improvement

 

Figure 13 Position and structure of needle shrinkage hole

 

Figure 14 Initial rough needle and processed pinhole

 

Figure 15 Internal wrapping and gate changes
Through analysis of trial production of differential case, it can be concluded that numerical simulation can provide a reference for quality control and defect analysis of aluminum alloy die-casting parts in the early stage, so as to design a more reasonable gating system and shorten development cycle; then compare problems in actual production process with simulated state, so as to further optimize gating system of product, improve forming process conditions, and improve quality of casting. In addition, modifying casting structure also has positive significance for improving quality of die casting.