Analysis of key points of die-casting technology of automobile aluminum alloy lower cylinder and countermeasures for defects

2021-06-04 by No Comments

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 backgrounds, aluminum alloy materials are used more and more widely in automobiles, and more and more auto parts are formed by die-casting. As the core components of cars, most of the engine cylinder blocks are made of aluminum alloy 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 field of cylinder block production, ordinary sand cast iron cylinder blocks have the advantages of simple process, low cost, good rigidity and heat resistance, but there is also a disadvantage, that is, the weight is too large. If you divide the crankshaft below the cylinder block and the cylinder liner above the cylinder block in two, use aluminum alloy below and cast iron on the top, you can do two things with one stone, which not only reduces the quality of the cylinder block, but also maintains the advantages of the cast iron cylinder block.
Lower cylinder refers to the lower part of the crankshaft of the engine after such a division into two. Because the lower cylinder body is a thick-walled part, and the wall thickness varies greatly, it is very difficult to die-cast molding. We learned from relevant domestic and foreign experience, designed and developed a set of lower cylinder die-casting technology for a 1.5T engine, and the test was very successful.
1 Difficulties in die casting of aluminum alloy lower cylinder block
The aluminum alloy lower cylinder casting has a mass of 8.4kg, an outline size of 382mm×258mm×67mm, a die-casting mass of 11.1kg, a material of A380, and an average wall thickness of 7.2mm. Since 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 body casting is complicated, and the main difficulties are as follows:
First, the casting needs to be placed in 5 pieces of cast iron inserts, and the cast iron inserts must be perfectly embedded on the aluminum alloy castings without separation.
Second, the thickness of the lower cylinder body castings is as thin as 2mm at the thinnest part and 24mm at the thickest part, and the distribution is severely uneven.
Third, due to the large difference in the thickness of the two side walls of the insert, it brings great difficulty to the flow filling of the aluminum alloy liquid, and also tests its feeding ability.
Fourth, aluminum alloy castings are prone to defects such as porosity, shrinkage, cracks, and shrinkage, and quality control is difficult.
2 Key Points of Die Casting Technology of Aluminum Alloy Lower Cylinder Block According to experimental analysis, we believe that the key points of die casting production technology of lower cylinder block are as follows:
First, scientifically design the gating system of the cylinder block die casting. The insert position in the middle of the lower cylinder block is thin-walled, and the upper and lower parts are thick and large parts. Therefore, we choose single-side pouring, so that the molten aluminum can be fed from the bottom side. , Flow through the middle insert and reach the top.
Secondly, we use tooth-shaped chilled exhaust block vacuum die-casting. The combination of tooth-shaped chilled exhaust block and vacuum machine can improve the problem of insufficient fluidity caused by thin side walls and ensure good casting quality.
Third, in order to improve the wettability of the aluminum alloy liquid and the cast iron inserts, we have carried out the insert preheating, which not only ensures that the cast iron and the aluminum alloy do not separate after forming, but also improves the fluidity of the aluminum liquid.
After experimentation, the internal structure of the lower cylinder body we obtained is dense and the appearance is well formed. In the process of die casting of the lower cylinder, scientific and reasonable process parameters are the guarantee for obtaining high-quality lower cylinder. We believe that the following process parameters are the key influencing factors for casting molding:
First, die casting temperature. During the die-casting process, the temperature of the molten aluminum should be well controlled, because the temperature is too high or too low to achieve good casting results, too high will easily lead to shrinkage and shrinkage, and too low will easily lead to poor filling. Generally speaking, the reasonable temperature of molten aluminum should be between 650-665℃, and the temperature after spraying of the mold should be between 150-200℃.
Second, the temperature of the insert. When the insert reaches 120-140°C, the molten aluminum overflows one side of the tank, which can improve the internal quality.
Third, die casting fast and slow injection speed and pressure. The speed of fast injection and slow injection should be controlled at about 4m/s and 0.22m/s respectively, and the pressure should be controlled at about 70MPa.
Fourth, the quality of molten aluminum. As the basic material of castings, the quality of molten aluminum determines the quality of castings. Therefore, to ensure the quality of molten aluminum, each package of molten aluminum must be refined and degassed to avoid pollution.
3 Aluminum Alloy Lower Cylinder Block Die Casting Defects and Countermeasures
After the casting was formed, we carried out X-ray inspection on the casting and found that the casting had some internal defects, such as shrinkage cavity, porosity, shrinkage porosity, etc. In order to improve defects and improve quality, we have proposed corresponding countermeasures, the main directions of which are as follows:
First, improve the overflow tank structure. The overflow groove has the functions of expelling gas in the cavity, storing mixed gas, and transferring shrinkage/shrinkage parts. After repeated trials and research, we found that we can use methods such as extending and adding overflow grooves to improve shrinkage, pores and other defects. Since there are more shrinkage and porosity in the middle of the casting, the filling pressure will be affected if the overflow port is set on a large plane, so vertical overflow ports are usually selected.
Second, optimize the mold cooling system. Shrinkage holes in castings generally appear at locations where the local temperature is too high or the wall thickness is too large. Through research, we found that the higher temperature at the thicker side walls can easily cause shrinkage cavities. Since the minimum diameter of the spot cooling pipe initially selected is 12mm, which cannot effectively cool the above-mentioned locations, we have improved the structure of the cooling water pipe, using high-pressure cooling equipment and a stainless steel spot cooling pipe with an inner diameter of 4mm. We cooled the mold cores on both sides of the casting to about 180°C, which greatly reduced the shrinkage phenomenon and greatly improved the quality of the casting.
Third, improve the separation of inserts. In view of the separation phenomenon between inserts and aluminum alloy castings, we have adopted the following measures: first, use thinner to clean the inserts to improve wettability; second, perform positioning hole inspection and appearance inspection on the inserts, using steel wire Part of the rust-spotted inserts was removed; thirdly, the preheating test was carried out on the inserts. The study found that when the temperature reaches 120℃ or higher, the problem of insert separation can be effectively solved.

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