A 125 motorcycle engine aluminum alloy case, with a processed mass of 1.84kg, was cast using a DC800T cold chamber die-casting machine. The ingate was opened on the non-processed surface. After the biscuit was knocked off, slag inclusion defects appeared at the gate. Since this is the appearance surface and the oil storage part of the crankcase, it was basically scrapped and could not be repaired.
In 2013, the scrap rate due to slag inclusion was 1.91%. After a series of studies, the scrap rate was successfully reduced to 0.1%, and it was promoted and applied in other parts with good results. This product uses YDC11 die-casting aluminum alloy, and its chemical composition is shown in Table 1.
Table 1 Chemical composition of YDC11 aluminum alloy (%)
This topic mainly analyzes the causes and improvement measures of slag inclusion in the gate of aluminum alloy die castings from the perspective of the occurrence mechanism and actual casting conditions, in order to provide a reference for the production of aluminum alloy box die castings.
Types and Causes of Slag Inclusion
The aluminum alloy crankcase of motorcycle engines often has slag inclusion in the gate during die casting, and the slag inclusion in the gate is mostly in the form of shrinkage, oil stain and hard layer. The main causes of slag inclusion are excessive mold temperature, excessive punch oil and high slag content in Al liquid.
To solve the problem of shrinkage-type slag inclusion, the main approach is to lower the mold temperature. To solve the problem of oil-type slag inclusion, the amount of punch lubricating oil, the filling position and the supply method can be adjusted. To deal with the slag inclusion in the cold hard layer, the main measures include adjusting the injection delay and the casting soup ladle return standby time, the introduction of the filtering casting soup ladle, and standardizing the requirements for Al liquid slag removal and degassing operations.
Statistical Analysis of Slag Inclusions
Through the analysis of scrap aluminum alloy boxes, there are three main types of slag inclusions in the gate, shrinkage, oil stains and chilled layers. Among them, the number of oil stains is the largest, accounting for more than 60% of the total number of slag inclusions. The chilled layer accounts for about 35% of the total number of slag inclusions. And the shrinkage type accounts for 5% of the total number of slag inclusions.
Shrinkage-Type Slag Inclusion
When the gate of the aluminum alloy box is knocked off, a hole similar to a shrinkage cavity with irregular shape, relatively clean inside and rough surface appears at the gate, as shown in Figure 1.
The surface of the casting gate with shrinkage cavity slag inclusion is concave, and the appearance is still not smooth after grinding and painting. The reason is that the temperature of the ingate is relatively high (the highest mold temperature measured is 380℃), which is the last part to solidify, causing the Al liquid there to have no time to compensate for shrinkage, resulting in shrinkage defects.
Oil-Type Slag Inclusion
Lubricating oil needs to be added to the punch for each casting cycle. The punch oil is not completely burned after the Al liquid is poured, and is wrapped in the Al liquid to form impurities and fill the cavity together. Since the Al liquid wrapped with punch oil has a lower temperature, it will solidify earlier and flow slowly. So it stays at the end of the product at the end of filling. In addition, the cross-sectional area of the ingate is small and the flow resistance is large. This part of the Al liquid impurities wrapped with punch oil and solidified in advance is more difficult to pass through and stagnates near the ingate. After knocking off the gate, a black hole appears at the ingate, the inner wall is rough, and there is obvious oil on the surface, as shown in Figure 2.
Check the slag inclusions in the gate and biscuit, and such impurities are also found.
Chilled Layer Slag Inclusion
The chilled layer refers to the shell formed by the rapid condensation of the Al liquid injected into the cylinder and the low-temperature surface of the cylinder (the chemical composition of the chilled layer was checked by spectrometer and no abnormalities were found, all within the standard allowable range). It is filled into the cavity together with the Al liquid during high-speed filling, as shown in Figure 3.
The density of the solid chilled layer (2.7g/cm2) is greater than that of the liquid (2.45g/cm2) Al liquid. During the low-speed operation of the punch, the chilled layer slowly gathers at the end of the Al liquid. When the punch runs at high speed, part of the chilled layer will be drawn into the cavity, forming a defect. The morphology of the chilled layer that finally remains in the gate is shown in Figure 4.
The chilled layer is mostly in the form of thin sheets, with a shiny and hard surface, and there are many gaps with the surrounding base materials, which not only affects the appearance, but also reduces the strength of the parts.
FAQ
Q1: What causes slag inclusion in the gate of aluminum alloy die-cast crankcases?
A1: The main causes are excessive mold temperature, excessive punch oil, and high slag content in the aluminum liquid. Slag inclusions typically appear as shrinkage, oil stains, or chilled layers.
Q2: How can shrinkage-type slag inclusion be prevented?
A2: The primary method is to lower the mold temperature, ensuring the last-solidifying part of the ingate has enough time for aluminum liquid to compensate for shrinkage.
Q3: What causes oil-type slag inclusion, and how can it be solved?
A3: Oil-type slag forms because punch lubricating oil is not completely burned and gets wrapped in the aluminum liquid, solidifying prematurely. Prevention measures include:
- Adjusting the amount of punch lubricating oil
- Changing the filling position and supply method
Q4: What is a chilled layer and how does it cause defects?
A4: The chilled layer is a shell formed by rapid solidification of aluminum against cold surfaces of the die or cylinder. Its density (2.7 g/cm³) is higher than the liquid aluminum (2.45 g/cm³), causing it to gather at the end of the flow and form defects during high-speed injection.
Q5: How are chilled layer defects prevented?
A5: Measures include:
- Adjusting injection delay and ladle standby time
- Using a filtered casting soup ladle
- Standardizing aluminum liquid slag removal and degassing operations
Q6: What were the results of implementing these improvements?
A6: After optimization, the scrap rate due to slag inclusion dropped from 1.91% to 0.1%, and the method was successfully promoted to other parts with good results.
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