A 125 motorcycle engine aluminum alloy case produced by a certain enterprise has a processed mass of 1.84kg and is cast using a DC800T cold chamber die casting machine. Inner gate is opened on non-processed surface. After knocking off material cake, slag inclusion defects appear at gate. Since this is appearance surface and oil storage part of crankcase, it is basically irreparable and scrapped. In 2013, scrap rate due to slag inclusion was 1.91%. After a series of studies, scrap rate was successfully reduced to 0.1%, it was promoted and applied in other parts, achieving good results. This product uses YDC11 die-casting aluminum alloy, and its chemical composition is shown in Table 1. This topic mainly starts from occurrence mechanism and actual casting conditions of aluminum alloy die castings, analyzes causes and improvement measures of gate slag inclusion, in order to provide reference for production of aluminum alloy housing die castings.
Aluminum alloy crankshaft housing of motorcycle engines often has gate slag inclusion when die casting blanks, and gate slag inclusion is mostly in three forms such as shrinkage cavity, oil stain and chilled layer. Main causes of slag inclusion are high mold temperature, excessive punch oil and high slag content in Al liquid. To solve shrinkage slag inclusion, the main approach is to lower the mold temperature; to solve oil stain slag inclusion, amount of punch lubricating oil, filling position and supply method can be adjusted; to deal with chilled layer slag inclusion, main measures are to adjust injection delay and spoon return standby time, introduction of filtering spoon style, standardize Al liquid slag removal and degassing requirements.
Graphical results

 

Table 1 Chemical composition of YDC11 aluminum alloy (%)
Through analysis of waste aluminum alloy boxes, there are three main forms of gate slag inclusions: shrinkage, oil and cold hard layer. Among them, number of oil inclusions is the largest, accounting for more than 60% of the total number of slag inclusions; cold hard layer accounts for about 35% of the total number of slag inclusions; and shrinkage type accounts for 5% of the total number of slag inclusions.
After knocking off aluminum alloy box runner material cake, irregularly shaped, relatively clean inside, and rough surface-like shrinkage holes appeared at gate, as shown in Figure 1. Surface of casting gate with shrinkage slag inclusions is concave, appearance is still not smooth after grinding and painting. Reason is that temperature of inner gate is relatively high (the highest mold temperature measured is 380℃), which is last solidified part, causing Al liquid there to have no time to compensate for shrinkage, resulting in shrinkage defects.

 

Figure 1 Slag inclusions in gate similar to shrinkage cavities
Lubricating oil needs to be added to punch in each casting cycle. Punch oil is not completely burned after Al liquid is poured, is wrapped in Al liquid to form impurities and fill cavity together. Since Al liquid wrapped with punch oil has a lower temperature, it will solidify earlier and flow slowly, so it stays at the end of product at the end of filling. In addition, cross-sectional area of inner gate is small and flow resistance is large. This part of Al liquid impurities wrapped with punch oil and solidified in advance are more difficult to pass through and stagnate near inner gate. After knocking off biscuit, a black hole appears at inner gate, inner wall is rough, and there is obvious oil on the surface, see Figure 2. Check slag inclusions in gate and biscuit, and such impurities are also found.

 

Figure 2 Oily gate inclusions
Cold hard layer refers to shell formed by Al liquid injected into barrel and rapid condensation of low-temperature surface of barrel (chemical composition of cold hard layer was checked by spectrometer and no abnormalities were found, all within standard allowable range). It is filled into cavity with Al liquid during high-speed filling, as shown in Figure 3. Density of solid cold hard layer (2.7g/cm2) is greater than that of liquid (2.45g/cm2) Al liquid. During low-speed operation of punch, cold hard layer slowly gathers at the end of Al liquid. When punch runs at high speed, part of cold hard layer will be rolled into cavity, forming a defect. Morphology of cold hard layer that finally remains in inner gate is shown in Figure 4. Cold hard layer is mostly in the form of thin sheets, with a shiny and hard surface, and there are many gaps with surrounding parent materials, which not only affects appearance but also reduces strength of parts.

 

Figure 3 Schematic diagram of formation position of cold hard layer

 

Figure 4 Slag inclusion in cold hard layer

 

Figure 5 Existing oil supply method

 

Figure 6 Changing bottom filling of punch oil to top filling

 

Figure 7 New and old soup spoons
Conclusion
For slag inclusion in gate of die-cast motorcycle crankcase, starting from clarifying mechanism of slag inclusion, summarizing shortcomings of existing production conditions and correlation with slag inclusion, so as to formulate corresponding improvement measures, ensure appropriate mold temperature, correct wrong punch lubricating oil supply method, realize standardization of aluminum alloy smelting, and verify effect, so that problem of slag inclusion in gate of crankcase can be effectively controlled.