Abstract: Air leakage is a common quality problem in aluminum alloy castings, and the location of air leakage usually exists in the position where the distance between the two machining surfaces is narrow and there are defects inside the product or in the position where the mold is severely scratched. Since the development and production of the right chassis of a certain motorcycle, the pass rate has been consistently low, with a water leakage rate of up to 86% after machining. This article analyzes and solves the problem of air leakage in the product from several aspects, such as reducing the product wall thickness, optimizing the screw hole core, and optimizing the extrusion pin. Finally, the air leakage ratio of the product was reduced from the original 86% to 9.14%, which effectively solved the problem of air leakage in the product.

1. Background
Since the development and production of a certain motorcycle's right body, the qualification rate has been low, with a leakage rate of up to 86%. Even after infiltration, 8% of the products still leak. Due to processing requirements, the left and right bodies need to be combined to process cylinder holes, which also leads to the qualified left body being scrapped together with the leaking right body, seriously affecting product delivery and causing a lot of waste of manpower and financial resources. Through a detailed investigation of the leakage points of the waste gas components, it was found that the main leakage locations were the remote M8 screw hole position and the AB bolt screw hole position on the cylinder surface (Figure 1).

2. Cause analysis
2.1 Excessive wall thickness of the product
The thick parts of the product wall are prone to producing hot spots, and the aluminum liquid solidifies later. If timely shrinkage repair cannot be carried out, defects such as shrinkage holes may occur at the wall thickness, resulting in an internal structure that is not dense, leading to air leakage after machining.
(1) Remote M8 screw hole position: Physical sectioning inspection found that the wall thickness at this position is about 9.4mm, and obvious shrinkage can be seen after sectioning (Figure 2). This position is also located at the far end of the filling, and the pressure increase of the press cannot be effectively transmitted, resulting in internal tissue shrinkage and looseness of the product.

(2) Position of AB bolt hole on cylinder surface: Due to its location at the pouring point and the complexity of product classification, the AB bolt hole and oil passage on the cylinder surface cannot be extracted. The wall thickness of the product at the circled position is about 20mm thick. Due to the excessive thickness of the wall at this location, it is not possible to compensate for this by simply increasing the pressure with a press. Therefore, an extrusion pin was added to the thick meat position for local pressure increase and compensation (Figure 3). However, in the actual production process, it is common for the extrusion pin to not move, and its failure can lead to loose tissue at this location, resulting in leakage between the cylinder surface screw hole and the oil passage after machining.

2.2 Severe local heat accumulation and strain
In the product structure, it is inevitable to have some small deep cavities or sharp structures. These small structures on the corresponding mold often cause heat to be unable to dissipate in a timely manner during the production process and concentrate in sharp positions. When the casting is taken out of the mold, these sharp positions may scratch the surface of the casting due to high temperature.
The remote M8 screw hole core of this product has a length of 32.5mm, a diameter of 5.5mm, and a length to diameter ratio of 5.5 times. This slender structure core is prone to heat accumulation, which can cause damage to the dense layer on the surface of the casting during demolding, resulting in air leakage. The obvious adhesive mold strain marks can be seen from the following figure (Figure 4).

3. Solution and Effect Verification
3.1 Problem of excessive wall thickness
(1) Remote M8 screw hole position: Due to limitations in product features, it is not possible to use compression pins or other methods for local shrinkage. Therefore, a method of reducing wall thickness was adopted to solve this problem. The wall thickness at this position was reduced from 9.4mm to 3.5mm, and the mold was repaired by cutting (Figure 5).

After mold repair, the product has good flaw detection effect, with no obvious shrinkage holes (Figure 6), and the requirement for shrinkage holes in the flaw detection position is better than the ASTME5052 level standard.
(2) Position of AB bolt hole on cylinder surface: In response to the problem of the extrusion pin not being able to be squeezed in, through three-dimensional data analysis, it is preliminarily determined that the gap between the extrusion pin and the extrusion pin sleeve is too small, only 1mm. Due to the fact that the contact area between aluminum liquid and metal will solidify first, a small gap can cause the extrusion pin to squeeze onto the already solidified metal surface, resulting in the problem of the extrusion pin being unable to move. In response to this issue, we optimized the gap between the extrusion pin and the extrusion pin sleeve, increasing it to 2mm (Figure 7) to avoid the extrusion pin being squeezed above the solidified aluminum liquid during extrusion.

After optimizing the extrusion pin, it was found through on-site tracking and testing that the extrusion pin can smoothly squeeze into the designed extrusion depth of 10mm.
3.2 Adhesive mold strain problem
In response to this issue, the remote M8 screw hole core has been optimized. Firstly, the diameter of the core was increased from 5.5mm to 6.5mm to reduce the machining allowance and avoid exposing internal defects due to excessive machining allowance. PVD coating process was applied to the surface of the core and high-pressure fine core cooling water was added to improve the surface hardness and reduce the core temperature, thereby avoiding mold sticking and pulling (Figure 8).

After optimizing the core, it was found that there were no obvious signs of mold sticking or pulling at this position (Figure 9), and after machine testing, it was discovered that the leakage problem at this position had been completely resolved.

4. Conclusion
After solving the problems of wall thickness and mold sticking, the leakage rate of the product decreased from 86% to 9.14%, and the leakage rate after infiltration of the leaking product decreased from 8% to 3.1%, indicating a good solution to the leakage problem. This not only provides a reference for developing similar products in the future, but also helps to avoid problems during development, and provides ideas for solving leakage problems in other products.

Article: Yang Huai, Shuai Haijiang, Zhang Qingfeng, Ren Jinshan from Chongqing Zongshen Power Machinery Co., Ltd