A practical example of an optimization shrinkage porosity problem
“Shrinkage porosity”. It is a common internal defect in aluminum alloy die-casting parts and often occurs in locations where the product wall thickness is large or hot spots are easily formed. Generally speaking, as long as shrinkage porosity does not affect the use performance of the product, it will be judged as qualified. However, for some important parts such as cooling water channel holes or lubricating oil channel holes of automobile engine cylinder blocks, shrinkage porosity is not allowed to be judged as qualified.
In this case, an aluminum alloy engine crankcase is cast using a Buhler 28 000kN cold chamber die casting machine with ADC12 alloy, and the composition is shown in Table 1. The casting blank weighs 6.3 kg. When the X-ray inspection is performed in the subsequent process, shrinkage porosity is found in the second crankshaft bearing hole oil passage, about 8 mm away from the oil passage, which poses a significant oil leakage risk. According to statistics, the scrap rate of shrinkage porosity at this location was 5% in 2017. After a series of explorations, the scrap rate was successfully reduced to 0.2%.
So how does shrinkage porosity form in die-casting parts?
Shrinkage porosity is a common defect in aluminum alloy die-casting parts. The main reason for this is that the aluminum alloy does not compensate for shrinkage during the solidification process. The reasons for shrinkage porosity are varied and include the following:
- The improper temperature gradient of the mold, which causes uneven local shrinkage of the aluminum liquid;
- insufficient amount of aluminum liquid poured, resulting in a thin cake and insufficient pressure during the pressurization stage;
- the mold has a hot spot or sharp area;
- the width of the inner gate of the mold is not enough and the area is small, which causes premature solidification of the casting and blockage of pressure transmission during the pressurization stage;
- the casting pressure setting is too low, resulting in a poor compensation effect.
Figure 1 shows a schematic diagram of the formation of shrinkage porosity in aluminum alloy castings.
Figure 2 shows the shrinkage porosity of a certain engine crankcase. It is nearly elliptical and is about 10 mm away from the bearing oil hole.
The inner wall is rough and dull. The wall thickness of the casting in the shrinkage porosity area is relatively large, about 22 mm, and there is no cooling water in front of the oil hole plug, so the mold temperature in this area is relatively high.
Due to the large working load and severe wear of the two main journals (main journal and connecting rod journal) of the automobile engine crankshaft, pressure lubrication must be performed during operation.
Therefore, if there is shrinkage porosity near the oil hole of the journal, it will seriously affect the lubrication effect.
How can we avoid the shrinkage porosity occurring in the crucial spot?
The main reasons for the casting defects in aluminum alloy die-casting parts are the structural characteristics of the product itself, improper design of the pouring system and cooling system of the mold, or inappropriate design of process parameters. Based on the common causes of casting defects and the defect handling process for aluminum alloy castings, corresponding countermeasures are explored to solve the problem of shrinkage porosity in thick parts of aluminum alloy die-casting parts:
Early Analysis and Countermeasures
Analysis and countermeasures in the early stage Starting from the easy-to-operate process parameters, through on-site measurement and observation, it was found that the thickness of the pouring port in the mold was 4 mm, and the internal pouring speed was calculated to be 40 m/s. The thinnest part of the product wall thickness was 4.6 mm; the thickness of the cake was 25 mm; and the casting pressure was 60 MPa. According to experience, it can be known that the mold design conforms to the structural characteristics of the product, and there should be no problem of insufficient pressure compensation during the pressurization stage of the mold pouring system. However, the shrinkage compensation of aluminum liquid during the pressurization stage is directly related to the thickness of the cake and the pressurization pressure. Only appropriate cake thickness and casting pressure can form a casting with a dense internal structure. Therefore, it can be reasonably suspected that shrinkage porosity is caused by low casting pressure and thin cake.
Two countermeasures were used to eliminate shrinkage porosity in the early stage:
① The casting pressure was increased from the previous 65 MPa to 90 MPa;
② The thickness of the cake was adjusted from the original 25 mm to 30 mm.
After using the above measures and verifying them in small batches, the shrinkage rate was reduced from 5% to 4.8%, but the effect was not significant, indicating that process parameters are not the main cause of shrinkage porosity in castings.
Mid-term analysis and countermeasures
Since the root cause of shrinkage porosity in castings is insufficient shrinkage compensation during aluminum liquid solidification, and uneven mold temperature distribution can easily cause improper aluminum liquid solidification sequence and insufficient shrinkage compensation.
Therefore, the mid-term countermeasure analysis mainly starts with ensuring a reasonable mold temperature. From the 3D model of the product, it can be seen that the wall thickness of the casting shrinkage porosity is 22.6mm, which is relatively large and easy to causes a higher mold temperature.
When the aluminum liquid solidifies, the aluminum liquid inside the casting with a larger wall thickness may still be in the liquid phase or solid-liquid mixed phase due to its higher temperature, while at this time, the channel for shrinkage compensation through the internal pouring port may have already solidified. In this way, during the pressurization stage, the casting cannot perform aluminum liquid shrinkage compensation, which may lead to shrinkage porosity.
To ensure a suitable mold temperature, a thermal imager was used to measure the highest temperature of the mold after spraying the release agent was 272 ℃ (see Figure 3), which is higher than the normal temperature after spraying the release agent. The overall mold temperature and its distribution in other areas are normal. Therefore, it is necessary to reduce the mold temperature at the shrinkage porosity location. In addition, it was found that the distance between the bottom of this cooling water hole and the surface of the mold cavity is relatively large at 20 mm. Because a larger heat transfer distance will reduce the cooling effect of the mold, it is necessary to improve the cooling water hole.
To reduce the mold temperature at the shrinkage hole location, three methods were mainly adopted:
① Improve the mold cooling system. Deepen the cooling water hole of the shrinkage hole attachment from 20 mm away from the mold surface to 12 mm, so as to quickly take away the heat near the mold and reduce the mold temperature; unify and number all mold cooling water pipes and water pipes one by one to prevent mistakes in mold preservation and affect cooling effect.
② Reduce the pouring temperature from 675 ℃ to 645 ℃.
③ Extend the spraying time of the mold at the shrinkage hole from 2 s to 3 s.
After implementing these rectification measures, the temperature of the mold sprayed in the shrinkage area was greatly reduced, which was about 200℃ and belonged to a normal range. The shrinkage rate was reduced from 4.8% to 4%, indicating that such measures have a certain effect on shrinkage problems, but cannot completely solve shrinkage problems in this area.
Post-analysis and countermeasures
Through the previous two improvements, it is basically ensured that the die-casting mold is in the theoretically best state, that is, the reasonable design of the pouring system, appropriate arrangement of the cooling system, and optimal design of process parameters; however, the rate of shrinkage defects in castings is still as high as 4%. Due to the wall thickness of the shrinkage defect area of the casting being 22.6 mm, which is much larger than other parts of the wall thickness, a larger wall thickness may cause insufficient compensation for shrinkage when the center of the casting solidifies, and this area has not completely solidified after the end of pressurization, and continues to shrink (see Figure 4 for mold flow analysis).
Therefore, how to solve the insufficient compensation for shrinkage at the shrinkage defect area of the casting may be the key to the problem. Usually, the compensation for shrinkage of castings is carried out through a path of cake → sprue → inner gate → casting. In this case, after the thick part of the casting solidified behind the inner gate, it cut off the compensation channel for late-stage shrinkage after pressurization and caused it unable to compensate for shrinkage.
During the conventional pressurization stage, the injection head applies cast pressure to the cake to achieve the compensation effect. The measure taken is to add a structure similar to a slag bag near the shrinkage defect area of the casting to act as a cake. A pair of oil cylinder core-pulling mechanisms are used as injection heads to perform secondary pressurization compensation on areas that are prone to shrinkage defects in the late stage of casting solidification to eliminate shrinkage defects.
Usually, such a secondary pressurization mechanism is called an extrusion pin. Its pressurization principle is to apply appropriate pressure after pouring metal liquid or alloy liquid until complete solidification to enhance the effect of casting solidification compensation and achieve the purpose of improving casting density and reducing or eliminating shrinkage defects. Pressurized solidification can change the physical parameters and crystallization process of metals and their alloys, change the distribution and size of loose cavities, improve the density of castings, and improve performance such as tensile strength and hardness of castings.
According to the compensation and pressurization rules of castings, the extrusion pin action signal adopts the pressurization signal of the casting process and starts with a delay on this basis. Therefore, the extrusion pin mainly controls two parameters: extrusion depth and extrusion delay time. The extrusion depth depends on the casting structure and shrinkage distribution and size, generally 10-20 mm; the extrusion delay time mainly refers to the pressurization time setting, generally 2-5 s. In actual engineering, the determination of extrusion parameters is optimized based on empirical values according to casting conditions. To facilitate the adjustment of extrusion parameters, a separate oil cylinder is usually used to control the action of the extrusion pin.
In this case, the later improvement measure is to symmetrically arrange two extrusion pins near the mold-bearing hole (see Figure 5). By adjusting the two main parameters of extrusion depth and extrusion delay time and optimizing the compensation effect of secondary pressurization of extrusion pins, the shrinkage rate of castings can be reduced. Based on the above measures, after adding two extrusion pins to the mold, the shrinkage rate decreased significantly, and the defect rate dropped from 4% to 0.2%. At the same time, in the 0.2% shrinkage defect products, the size of shrinkage defects has also decreased significantly. Therefore, the extrusion pin scheme has played a good role in controlling the shrinkage rate of castings with increased wall thickness. However, during this improvement process, there was also a fluctuation in the casting shrinkage defect rate. By optimizing extrusion parameters such as extrusion depth of 15 mm, extrusion delay time of 2.5 s, and specifying relevant specifications such as extrusion pin service life (8000 molds), the defect rate of castings was stabilized at around 0.2%.
Figure 6 shows the X-ray inspection comparison before and after the improvement of the shrinkage area of the casting. It can be seen that before the improvement, the shrinkage of the casting appeared near the bearing hole, with a wide and scattered distribution and a relatively loose structure. Due to the need for pressure lubricating oil to pass through the cylinder body bearing hole, there is a risk of oil leakage during use; after improvement, there is no longer a loose shrinkage distribution on the X-ray inspection photo, and the internal structure of the casting appears denser.
Conclusion:
1. Shrinkage is a common internal defect of castings, which is prone to occur in areas with larger wall thickness and higher mold temperature. It is usually approached from several aspects such as mold design (casting system, cooling system), process parameter setting, and casting condition guarantee. For castings with larger wall thickness involved, traditional improvement measures can only play a relieving role, but cannot completely solve the problem.
2. Two extrusion pins were designed based on the compensation effect of secondary pressurization of the punch during the pressurization stage, which had a significant effect on the shrinkage area.
