Abstract: Leakage is a common quality problem of aluminum alloy castings. Leakage location usually exists in the position where distance between two machined surfaces is narrow and there are defects inside product or position where mold is severely sticky. Since development and production of right box of a motorcycle, qualified rate has been low, and leakage rate of water inspection after machining is as high as 86%. This paper analyzes and solves problem of leakage of this product from several aspects such as reducing wall thickness of product, optimizing screw hole core and optimizing extrusion pin. Finally, leakage rate of product was reduced from original 86% to 9.14%, which solved problem of leakage of product well.

Since development and production of right box of a motorcycle, qualified rate has been low, and leakage rate is as high as 86%. After impregnation, 8% of products will still leak, and due to processing technology requirements, left and right bodies need to be combined to process cylinder bore, which also causes qualified left body to be scrapped together with leaking right body, seriously affecting delivery of product and causing a lot of waste of manpower and financial resources. Through detailed leakage point inspection of waste leaking parts, it was found that leakage locations were mainly far-end M8 screw hole location and cylinder surface AB bolt screw hole location (Figure 1).

 

Product wall thickness is prone to heat nodes, and aluminum liquid solidifies late. If timely shrinkage compensation cannot be performed, it is easy to produce shrinkage holes and other defects in wall thickness, resulting in loose internal structure, which leads to leakage after machining.

(1) Far-end M8 screw hole location: Actual cutting inspection found that wall thickness at this location was about 9.4mm, and obvious shrinkage holes could be seen after cutting (Figure 2). This location was located at the far end of filling, and pressurization of press could not be effectively transmitted, resulting in shrinkage of internal structure of product.

 

(2) Cylinder surface AB bolt screw hole location: Because this location is located at pouring point and product parting is relatively complex, cylinder surface AB bolt screw hole and oil channel cannot be cored. Product wall thickness at circled location is about 20mm thick. Because wall thickness at this position is too thick, it is impossible to compensate for shrinkage at this position by simply relying on press to increase pressure. Therefore, an extrusion pin is added at thick position for local pressure increase to increase shrinkage compensation (Figure 3). However, in actual production process, extrusion pin often fails to squeeze. Failure of extrusion pin will cause structure at this position to be loose, cylinder surface screw hole and oil channel will leak each other after machining.

 

In product structure, there will inevitably be some small deep cavities or sharp structures. Such small structures on corresponding molds often cause heat to be unable to dissipate in time and concentrate at sharp positions during production process. When casting is demolded, these sharp positions will tear surface of casting due to excessive temperature.
Core of M8 screw hole at far end of product is 32.5mm long, 5.5mm in diameter, and 5.5 times length-to-diameter ratio. This slender core is very easy to accumulate heat, which will cause dense layer on the surface of casting to be damaged and leaked during demolding. Obvious traces of mold sticking and tearing can be seen from figure below (Figure 4).

 

(1) Far-end M8 screw hole position: Due to limitation of product characteristics, this position cannot be used for local shrinkage compensation by extrusion pins, etc. Therefore, a method of reducing wall thickness is adopted to solve problem. Wall thickness at this position is reduced from 9.4mm to 3.5mm, and mold is repaired by cutting (Figure 5).

 

After mold is repaired, flaw detection effect of product is good, there is no obvious shrinkage hole (Figure 6), and shrinkage hole requirement at flaw detection position is better than ASTME5052 grade standard.
(2) Cylinder surface AB bolt screw hole position: Regarding problem that extrusion pin cannot be squeezed in, through three-dimensional data analysis, it is preliminarily determined that gap between extrusion pin and extrusion pin sleeve is too small, which is only 1mm. Because aluminum liquid and metal contact position will solidify first, gap is too small, which will cause extrusion pin to squeeze on solidified metal surface when it is squeezed in, resulting in problem that extrusion pin cannot be squeezed. To address this problem, we optimized gap between extrusion pin and extrusion pin sleeve, increasing gap to 2mm (Figure 7) to prevent extrusion pin from squeezing above solidified aluminum liquid when squeezing in.

 

After extrusion pin was optimized, it was found through on-site tracking and actual measurement that extrusion pin could be smoothly squeezed into designed extrusion depth of 10mm.

To address this problem, distal M8 screw hole core was optimized. First, core diameter was increased from 5.5mm to 6.5mm to reduce machining allowance to avoid excessive machining allowance to expose internal defects, core surface was PVD coated and high-pressure fine core cooling water was added to increase core surface hardness and reduce core temperature, thereby avoiding occurrence of sticking mold pull (Figure 8).

 

After core was optimized, it was found that there was no obvious sign of sticking mold pull at this position (Figure 9), and leak detection after machining found that leakage problem at this position had been completely solved.

 

After solving wall thickness and mold sticking problems, leakage rate of product was reduced from 86% to 9.14%, leakage rate of leaking product after impregnation was reduced from 8% to 3.1%, and leakage problem was well solved. This not only provides a reference for development of similar products in the future, but also allows problems to be avoided during development, and also provides ideas for other products in solving leakage problems.