A 125 motorcycle engine aluminum alloy casing produced by a certain enterprise has a processed weight of 1.84kg and is cast using a DC800T cold chamber die-casting machine. The internal gate is located on the non machined surface, and after knocking off the material cake, there is a slag inclusion defect at the gate, which is the appearance surface and the oil storage area of the crankcase. It is basically irreparable and scrapped. 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 promoted in other components, achieving good results. This product is made of YDC11 die cast aluminum alloy, and its chemical composition is shown in Table 1. This project mainly analyzes the causes and improvement measures of gate slag inclusion in aluminum alloy die-casting parts from the perspectives of the occurrence mechanism and actual casting conditions, in order to provide reference for the production of aluminum alloy box die-casting parts.
The aluminum alloy crankcase of motorcycle engines often experiences slag inclusion at the sprue during die-casting, which is often in three forms: shrinkage, oil stains, and cold hardening layers. The main reasons for slag inclusion are high mold temperature, excessive punch oil, and high slag content in Al liquid. To solve the problem of slag inclusion in shrinkage cavities, the main approach is to reduce the mold temperature; Oil contaminated slag can be treated by adjusting the amount, position, and supply method of lubricating oil for the punch; To deal with slag inclusion in the cold hard layer, measures mainly rely on adjusting the injection delay, adjusting the standby time of the spoon, introducing filter spoon styles, and standardizing the requirements for Al liquid slag and gas removal operations.
Graphic and textual results
Through analysis of the waste aluminum alloy box, there are three main forms of slag inclusion in the gate: shrinkage cavity, oil contamination, and cold hardening layer. Among them, the majority of slag inclusions are oil contaminated, accounting for over 60% of the total slag inclusions; The cold hard layer accounts for about 35% of the total slag inclusion; And shrinkage cavities account for 5% of the total slag inclusions.
After knocking off the sprue cake of the aluminum alloy box, irregular shaped, clean inside, and rough surface shrinkage like holes appeared at the sprue, as shown in Figure 1. The surface of the sprue of castings with shrinkage cavities and slag inclusions is concave, and the appearance is still not smooth after polishing and coating. The reason for the occurrence is that the temperature of the sprue is relatively high (the highest measured mold temperature is 380 ℃), which is the final solidification part, causing the Al liquid in this area to not have enough time to shrink, resulting in shrinkage defects.
Each casting cycle requires lubricating oil to be added to the punch. 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 mold cavity together. Due to the lower temperature of the Al liquid wrapped with punch oil, it will solidify earlier and flow slower, thus staying at the end of the product at the end of filling. In addition, the small cross-sectional area and high flow resistance of the sprue make it difficult for Al liquid impurities, which are wrapped in punch oil and have solidified in advance, to pass through and stagnate near the sprue. After knocking off the cake, black holes appeared at the inner gate, with rough inner walls and obvious oil stains on the surface, as shown in Figure 2. Upon inspection of the sprue and material cake for slag inclusion, similar impurities were also found.
The cold hard layer refers to the shell formed by the rapid condensation of Al liquid injected into the material cylinder and the low-temperature surface of the material cylinder (no abnormalities were found in the chemical composition of the cold hard layer by the spectrometer, all within the standard allowable range). It is filled into the mold cavity together with the Al liquid during high-speed filling, as shown in Figure 3. The density of the solid cold hard layer (2.7g/cm2) is higher than that of the liquid Al liquid (2.45g/cm2). During the low-speed operation of the punch, the cold hard layer slowly accumulates at the end of the Al liquid. When the punch runs at high speed, some of the cold hard layer will be rolled into the mold cavity, forming defects. The final morphology of the cold hard layer remaining in the sprue is shown in Figure 4. The cold hard layer is mostly thin and has a glossy and hard surface, with gaps between it and the surrounding base material, which not only affects the appearance but also reduces the strength of the parts.
conclusion
Starting from understanding the mechanism of slag inclusion in the gate of the die-casting motorcycle crankcase, summarizing the shortcomings of existing production conditions and their correlation with slag inclusion, corresponding improvement measures are formulated to ensure appropriate mold temperature, correct incorrect punch lubricant supply methods, standardize aluminum alloy melting, and verify the effect, so as to effectively control the problem of slag inclusion in the crankcase gate.
Author of this article:
Yang Cheng, Yang Xingguo
College of Automotive Engineering, Chongqing University
Tang and Yong, Yang Zhengping
Chongqing Construction Yamaha Motorcycle Co., Ltd
This article is from the magazine "Special Casting and Nonferrous Alloys",
Strategic Partner of Die Casting Weekly
