【China Aluminum Industry Network】 In recent years, energy saving and emission reduction has become a trend of the times, and vehicle weight reduction is also a general trend. Under these two backgrounds, the use of aluminum alloy materials in automobiles has become more widespread, with the help of die-casting automotive components. more and more. As the core components of the car, most of the engine blocks are made of aluminum alloy and cast iron. Among them, die-cast aluminum alloy blocks have received more and more recognition. Japanese and South Korean and European and American automobile companies mostly use die-cast aluminum alloy blocks.

In the field of cylinder production, ordinary sand-type cast iron cylinders have the advantages of simple process, low cost, good rigidity and heat resistance, but they also have a disadvantage that they are too heavy. If the crankshaft below the cylinder block and the upper cylinder sleeve are divided into two, the use of aluminum alloy and the use of cast iron above it will serve two purposes. This will not only reduce the mass of the cylinder but also maintain the advantages of the cast iron cylinder.

The lower cylinder refers to the crankshaft portion of the lower engine after such a division into two. Since the lower cylinder is a thick-walled part and the wall thickness varies greatly, die-casting molding is very difficult. We learn from the relevant experience at home and abroad and developed a set of lower die casting technology for a 1.5T engine design. The test was very successful.

1 Aluminum alloy lower cylinder die casting difficulties

The aluminum alloy lower cylinder casting quality is 8.4kg, the outline size is 382mm×258mm×67mm, the die casting quality is 11.1kg, the material is A380, and the average wall thickness is 7.2mm. Because the lower cylinder is connected to the crankshaft, cast iron inserts are also required at the bottom.

The die casting process of the lower cylinder casting is complex, and the difficulties are mainly as follows:

At the front, castings need to be placed in five pieces of cast iron inlays. The cast iron inlays must be perfectly set on the aluminum alloy castings, and separation cannot occur.

Second, the thickness of the lower part of the casting of the lower part is as thin as 2mm, and that of the thick part is as much as 24mm. The distribution is severely uneven.

Thirdly, due to the large difference in the thickness of the two side walls of the insert, it is very difficult to fill the flow of the aluminum alloy liquid, and it also tests its ability to feed.

Fourth, aluminum alloy castings are prone to defects such as porosity, shrinkage holes, cracks, shrinkage, and the like, and quality control is difficult.

2 Aluminum alloy lower die casting technical points According to the test analysis, we believe that the following die casting production technology mainly has the following points:

At the front, scientifically design the casting system of the cylinder die castings. The position of the inserts in the middle of the lower cylinder is a thin wall, and the upper and lower parts are thick parts. Therefore, we choose one-side pouring. In this way, the liquid aluminum can be fed from the bottom side. After reaching the top through the middle insert.

Second, we used a toothed chilled exhaust block for vacuum die casting. The use of a toothed chilled exhaust block combined with a vacuum machine can improve the lack of flow caused by thin sidewalls and ensure good casting quality.

Thirdly, in order to improve the degree of wetting of aluminum alloy liquids and cast iron inserts, we performed preheating of the inserts. This not only ensures that the cast iron parts do not separate from the aluminum alloy after forming, but also improves the fluidity of the aluminum liquid.

After testing, the internal structure of the lower cylinder we obtained was dense and the appearance was well formed. In the die-casting process of the lower cylinder, the scientific and reasonable process parameters are the guarantee for obtaining the high-quality lower cylinder. We believe that the following process parameters are the key influencing factors for casting molding:

Front, die casting temperature. In the die casting process, the aluminum liquid temperature should be controlled well, because the temperature is too high or too low can not achieve a good casting effect, too high is easy to lead to shrinkage and shrinkage, and too low is easy to cause poor filling. Generally speaking, the reasonable temperature of aluminum liquid should be between 650-665°C, and the temperature after mold spraying should be between 150-200°C.

Second, the temperature of the insert. When the insert reaches 120-140°C, aluminum liquid overflows the side of the tank, which improves internal quality.

Third, die casting fast, slow shot speed and pressure. The fast shot and slow shot speed 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 liquid aluminum. As the base material of the casting, the quality of the aluminum liquid determines the quality of the casting. Therefore, to ensure the quality of the aluminum alloy liquid, each package of aluminum liquid must be refined and degassed to avoid contamination.

3 Aluminum Alloy Die Casting Defects and Countermeasures

After castings were formed, we conducted X-ray inspection of the castings and found that the castings had some internal defects such as shrinkage holes, porosity, shrinkage, and the like. In order to improve defects and improve quality, we have put forward corresponding countermeasures. The main directions are as follows:

At the front, improve the structure of the overflow tank. The overflow tank has the function of excluding the gas in the cavity, storing the mixed gas, and transferring the shrinkage cavity/shrinkage portion. After repeated trials and studies, we have found that we can use methods such as prolonging and adding overflow tanks to improve shrinkage, porosity, and other defects. Due to the phenomenon of shrinkage in the middle of the casting, the overflow port is set on a large surface and the filling pressure will be affected. Therefore, a vertical overflow port is usually selected.

Second, optimize the mold cooling system. Casting shrinkage generally occurs at locations where the local temperature is too high or the wall thickness is too large. Through the study, we found that the temperature is high at the places where the thickness of the two side walls is large, and it is easy to cause shrinkage. Because the diameter of the cold pipe, which was chosen earlier, is 12mm, the above position cannot be effectively cooled. Therefore, we have improved the structure of the cooling water pipe, using high-pressure cooling equipment and stainless steel spot cooling pipes with an internal 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 castings.

Third, improve the separation of the inserts. For the separation phenomenon between the inserts and the aluminum alloy castings, we have adopted the following means: First, the use of thinners to clean the inserts to improve the wettability; Secondly, to perform positioning holes inspection and appearance inspection of the inserts, using steel wire Some of the rust plaques were removed for spotting. Third, the preheating test was performed on the inserts. It was found that when the temperature reached 120° C. or above, the problem of separation of the inserts could be effectively solved.

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