The Analysis of Automobile Aluminum Alloy Lower Cylinder Blocks
In recent years, energy-saving and emission reduction have become the trend of the times, and the lightweight of automobiles is also the general trend. Under these two major backgrounds, aluminum alloys are used more and more widely in automobiles, and more and more auto parts are manufactured by die casting. As the core components of cars, most of the engine cylinder blocks are made from aluminum alloys and cast iron. Among them, die-cast aluminum alloy cylinder blocks have gained more and more recognition. Japanese, Korean, European, and American automobile companies mostly use die-cast aluminum alloy cylinder blocks.In the production field of cylinder blocks, ordinary sand cast iron cylinder blocks have the advantages of simple processes, low costs, good rigidity, and heat resistance, but they also have a disadvantage, that is, the excessive weight. If the crankshaft below the cylinder block is made from aluminum alloys and the cylinder liner above cast iron, we can serve a double purpose, which not only reduces the weight of the cylinder block, but also maintains the advantages of the cast iron cylinder block. The lower cylinder block refers to the crankshaft of the lower part of the engine. Because the lower cylinder block is a part with great thickness and the wall thickness varies greatly, it is very difficult to die-cast. We learned from relevant experience at home and abroad, and then designed and developed a die casting technology for the lower cylinder block. The experiment was very successful.
1. Difficulties in the die casting of aluminum alloy lower cylinder blocks
The aluminum alloy lower cylinder casting has a weight of 8.4kg, an overall dimension of 382 ×258×67 mm, a die casting weight of 11.1kg and an average wall thickness of 7.2 mm, and is made from A380. Because the lower cylinder block is connected to the crankshaft, a cast iron insert needs to be placed at the bottom. The die casting process of the lower cylinder block casting is complicated, and the main difficulties are as follows:
- 5 pieces of cast iron inserts need to be placed for the casting, and the cast iron inserts must be perfectly embedded on the aluminum alloy die casting without any separation.
- The lower cylinder block casting has the thinnest thickness of 2 mm and greatest thickness of 24 mm. The thickness is severely uneven.
- The big difference in the thickness of the two sides of the insert brings great difficulty to the flow filling of the aluminum alloy liquid, and also tests the feeding capacity.
- Aluminum alloy castings are prone to defects such as gas holes, shrinkage cavity, cracks, and shrinkage porosity, and quality control is difficult.
2. Key points of the die casting technology of the aluminum alloy lower cylinder block
According to the experimental analysis, we believe that the main technical points of the lower cylinder block die casting are as follows:
- Scientifically design the gating system of the cylinder block die casting. The insert placed at the position that is in the middle lower cylinder block should have thin walls, and the upper and lower parts are thick and large. Therefore, we choose to pour from a single side so that the molten aluminum can be filled from the bottom side. It will reach the top after it flows through the middle insert.
- The chilled exhaust block vacuum die casting with tooth form is adopted. The combination of the chilled exhaust block and the vacuum machine can improve the problem of insufficient fluidity caused by thin walls at two sides and ensure good casting quality.
- In order to improve the wettability of the aluminum alloy liquid and the cast iron insert, we preheat the inserts, which not only ensures that the cast iron and aluminum alloy do not separate after forming, but also improves the fluidity of the aluminum liquid. After testing, the internal structure of the lower cylinder block we obtained is dense and the appearance is well-formed.
In the die casting process of lower cylinder blocks, scientific and reasonable process parameters are the guarantee of obtaining lower cylinder blocks with good quality. We believe that the following process parameters are the key influencing factors for molding:
- Die-cast temperatures: in the die casting process, the temperature of the molten aluminum should be well controlled, because if the temperature is too high or too low, a good die casting effect cannot be obtained. The too high temperature will easily lead to shrinkage cavity and shrinkage porosity. Too low temperature will easily result in the poor filling. Generally speaking, the reasonable temperature of molten aluminum should be between 650 and 665℃, and the temperature after spraying of the mold should be from 150 to 200℃.
- The temperature of the insert: when the temperature of the insert reaches from 120 to 140℃, the molten aluminum will flow at one side of the overflow tank, which can improve the internal quality.
- Fast and slow injection speed and pressure of die-cast: the fast injection and slow injection speeds should be controlled at about 4m/s and 0.22m/s respectively, and the pressure should be controlled at about 70MPa.
- The quality of molten aluminum: As the basic material of die castings, the quality of molten aluminum determines the quality of die castings. Therefore, the quality of molten aluminum should be ensured. Each bag of molten aluminum must be refined and degassed to avoid pollution.
3. Defects and measures of die-cast aluminum alloy lower cylinder blocks
After the die casting part was formed, we ran an X-ray inspection on the die casting part and found that there were some internal defects such as shrinkage holes, gas holes, and shrinkage porosity. To reduce defects and improve quality, we proposed the following corresponding measures:
- Improve the structure of the overflow tank. The overflow tank has the functions of expelling gas in the cavity, storing mixed gas, and transferring shrinkage cavity parts. After repeated trials and research, we found that we could use methods such as extending and creating overflow tanks to improve shrinkage cavity, gas holes, and other defects. Because there is more shrinkage porosity in the middle of the casting, the filling pressure will be affected if the overflow port is set on a large plane. The vertical overflow port is usually selected.
- Optimize the mold's cooling system. Shrinkage cavity generally appears at locations where the partial temperature is too high or the wall thickness is too great. Through research, we found that the temperature for walls with great thickness at two sides is high, which can easily cause a shrinkage cavity. Because the minimum diameter of the spot cooling pipe initially selected is 12 mm, the above-mentioned parts cannot be effectively cooled. Therefore, we improved the structure of the cooling water pipe by adopting high-pressure cooling equipment and a stainless steel spot cooling pipe with an inner diameter of 4 mm. We cooled the mold cores on both sides of the die casting to about 180°C, which greatly reduced the shrinkage cavity and improved the quality of the die casting very much.
- Improve the separation of inserts. In view of the separation between inserts and aluminum alloy die castings, we have taken the following measures: firstly, use thinner to clean the inserts to improve wettability; secondly, perform positioning hole inspection and appearance inspection for the inserts and use steel wire to remove spots on the inserts with rust; thirdly, the preheating test was carried out for the inserts. The study found that when the temperature reaches over 120℃, the problem of the separation of inserts can be effectively solved.
