Abstract:
Crown bubbles are the main casting defects of pistons with casted crowns, which are also one of the primary causes of engine piston failure. Through reproduction tests of crown bubbles, this study investigates the influence of aluminum liquid temperature, pouring speed, filter mesh structure, and filter position on these defects. The results indicate that the missing or misplaced filter is the main cause of crown bubbles, which is further verified by simulation analysis. By optimizing the structure of the fixation groove in the filter and using an infrared sensor to detect missing filters, the deviation of the filter position is effectively solved, and the crown bubbles are eliminated.
Keywords: Aluminum Piston; Casting; Casting Defects; Crown Bubble
1. Introduction
With the increasing demand for low fuel consumption in automobiles, enhancing thermal efficiency and developing efficient, energy-saving internal combustion engines have become increasingly important. Optimizing the shape of the piston crown can achieve full fuel combustion, resulting in better engine performance and meeting the requirements for improved thermal efficiency. However, this also complicates the shape of the piston head [1-2]. Complex piston head shapes can be machined to remove defects such as pores and inclusions, resulting in low scrap rates. However, the complex surface shape and significant machining quantities require a longer processing cycle, significantly increasing equipment and labor costs. Alternatively, the top of the piston can be formed in one casting process, reducing the machining takt time by 30%, lowering manufacturing costs, and providing better economic benefits. Since there is no machining allowance on the piston head, casting defects cannot be removed by machining, reducing the head’s strength and increasing the risk of cracking during high-speed reciprocating operation, directly affecting the engine’s normal function. Therefore, pistons with casted crowns place higher demands on the casting process.
2. Basic Information on Defects
A certain piston project undertaken by our company underwent product audits after engine dynamometer testing at the client’s end. Defects were found on the piston crowns through endoscopy. Defective parts were inspected using CT (GE XS 240) tomography upon return from the client, revealing a single defect near the top of the piston head. The location distribution of casting defect from different perspectives. To further determine the defect type, SEM (MA10) was used to observe the defect morphology. Combining CT and SEM analysis results, it can be seen that casting defect surface is smooth without significant irregularities, indicating it is a porosity defect. This porosity is located in a moderately stressed area of the piston. According to the piston appearance defect standards, the allowed defect size in this area is 1 mm, with a maximum of 3 defects, and each must be more than 5 mm apart. The optical microscope (VHX-500F) was used to measure casting defect, resulting in dimensions of 3.423 mm × 1.063 mm, far exceeding casting defect standard requirements, necessitating analysis and improvement.
3. Defect Analysis and Reproduction
3.1 Analysis of Crown Bubble Defects
KS aluminum piston casting employs a one-mold, two-piece, central injection, gravity casting method with a metal mold featuring a lower core-pulling structure. The area from the pouring gate to the filter closure is highly enclosed, favorable for blocking slag and generating a high static pressure head. Aluminum liquid passing through the filter enters the wide cross-gate, where the flow rate slows, aiding in the floating of slag. This casting method facilitates smoother filling of the aluminum alloy liquid, suitable for mass production of small and medium-sized castings. The piston adopts the Lite KS2 design, made of AlSi12Cu5Ni2 material. A glass fiber filter with a thickness of 0.6 mm and mesh specifications of 1.4 mm × 1.4 mm is used. Through sorting of client-side, finished, and in-process products, a total of five similar defects were found, concentrated in a specific shift, with no defects found in other shifts. The distribution of porosity defects on the piston crown, mainly concentrated in areas A2 and A3, both located above the inner gate side.
Porosity originates from gas not escaping from the metal liquid. If the solidification speed is greater than the escape speed of porosity, small porosity will remain in the casting, forming porosity defects. Crown bubble defects differ from shrinkage porosity and porosity caused during solidification, generally being larger in size. Their formation is due to turbulence in the aluminum liquid during filling, causing gas entering the mold cavity to be encapsulated by the aluminum liquid without timely expulsion. During the filling process, the pressures of the liquid phase, gas phase, and free surface are dynamically changing. If macroscopic porosity does not rupture under continuous flushing and compression of the liquid phase, it will float to the casting surface, forming crown bubble defects [5]. For cast crowns, the defective area has no machining allowance, and defects cannot be removed by machining, thus remaining in the finished product, causing piston scrap.
3.2 Reproduction Tests
During the aluminum liquid filling process, possible causes of crown bubbles include out-of-tolerance aluminum liquid temperature, excessive filling speed, unreasonable gating structure, unsuitable filter specifications, missing filters, and filter misalignment. Combining the sorting results of the associated batches of defective parts, defects were concentrated in one shift, suggesting a process失控. Therefore, the reproduction tests did not verify the gating structure but only investigated pouring process parameters and filters, as shown in Table 1.
Table 1: List and Results of Defect Reproduction Tests
| Number | Pouring Temperature (°C) | Pouring Time (s) | Gating Structure | Filter Specifications (mm) | Filter Used | Filter Misalignment | Verification Quantity | Defect Quantity |
|---|---|---|---|---|---|---|---|---|
| A1 | 790 | 4 | T-type | 1.4 × 1.4 | Yes | No | 20 | 5 |
| A2 | 770 | 4 | T-type | 1.4 × 1.4 | Yes | No | 20 | 3 |
| A3 | 780 | 4 | T-type | 1.4 × 1.4 | Yes | No | 20 | – |
| A4 | 800 | 4 | T-type | 1.4 × 1.4 | Yes | No | 20 | – |
| B1 | 790 | 3 | T-type | 1.4 × 1.4 | Yes | No | 20 | – |
| B2 | 790 | 5 | T-type | 1.4 × 1.4 | Yes | No | 20 | – |
| C1 | 790 | 4 | T-type | 1.8 × 1.8 | Yes | No | 20 | – |
| D1 | 790 | 4 | T-type | 1.4 × 1.4 | No | No | 20 | 5 |
| E1 | 790 | 4 | T-type | 1.4 × 1.4 | Yes | Yes | 20 | 3 |
Each set of tests poured 20 castings to assess the occurrence of casting defects. After pouring, the top near the gate side of the castings was pressed with a ballpoint pen, and subsequently inspected visually for any defects at a 100% rate. Number A1 represented the key pouring process parameters used in mass production.
By comparing A1 with A4, it was confirmed through testing that aluminum liquid temperatures ranging from 770 to 800 °C did not result in the formation of casting defects. Additionally, Tests B1, A1, and B2 employed the same filter specifications while varying the pouring time between 3 and 5 seconds. This series of tests aimed to investigate the impact of pouring speed on casting defect formation, ultimately confirming that changes in pouring speed within this range did not produce casting defects.
The tests were designed to identify potential causes of top porosity defects in aluminum piston castings, which might include factors such as excessive aluminum liquid temperature, excessive pouring speed, unreasonable gating structure, improper filter specification, missing filters, or misplaced filters. Given that casting defects were concentrated in one specific shift, it was suspected that a process might have occurred. Therefore, casting defect reproduction tests focused on examining pouring process parameters and the filters, rather than the gating structure.
These tests were crucial in isolating the factors that could contribute to the formation of top porosity defects. By systematically varying and testing the key parameters, researchers were able to determine that aluminum liquid temperature and pouring speed, within the tested ranges, were not the causes of casting defects. This narrowed down the potential causes to issues related to the filters, which led to further investigations and ultimately to the identification of the main causes of casting defects.