Quality Improvement of Aluminum Alloy Cylinder Block Die Casts

Quality Improvement of Aluminum Alloy Cylinder Block Die Casts

At present, people's requirement of automobiles tend to be high performance, low energy consumption and low pollution based on energy and environmental protection. There are two ways to realized that. One is to improve the power system; the other is to reduce the weight of the car, that is, the lightweight car. Aluminum alloy is the most widely used light metal in automobile engines, because aluminum alloy has excellent properties such as light weight and good corrosion resistance, which make aluminum alloy meet the requirement of engine pistons, cylinder blocks and cylinder heads working in the harsh environment. Let's take the example of a new engine. The new engine is 14.7kg lighter than the old engine. The fuel consumption is reduced by 1L per 100km and the emission of CO2 is reduced by about 10g per kilometer.

Automotive engine die casting parts such as cylinder blocks, cylinder heads, sprocket shells and oil pans are die casts with complex shapes, changeable structures, precision sizes and good compactness. The requirements of high standards and high rates of finished products have challenged techniques and quality assurance of aluminum alloy die casts.

In order to obtain die casting parts with high quality, the key process parameters such as temperature, speed, pressure and time must meet the requirements of die casting production.

Temperature control
Pouring temperature should be controlled at 640 to 680 ℃. When pouring temperature is too high, shrinkage is great, which will make the crack and big grain easily appear on the die casting part and the die cast stick to the mold; when the pouring temperature is too low, defects such as cold shuts and metal shortages will occur. Make sure that oxide layers on surfaces of aluminum alloy liquid in the holding furnace are cleaned timely; otherwise, the oxide layer may lead to oxide inclusions.

The die casting mold should be preheated to a certain temperature before it is used. In continuous production, temperature of the die tends to increase. Too high temperature can lead to slow cooling of the die cast besides the molten metal being stick to the mold, making the grain big and deform.

Speed and pressure control
Changes of the injection process parameters can greatly influence quality of the cylinder block die cast. High speed easily leads to increased gas in the die casting part, while too low speed will easily cause poor filling. When injection pressure is too low, defects such as blow holes and shrinkage cavities will increase; when the injection pressure is too high, defects like the joint flash will increase, which will cause damage to the mold. Adopting appropriate injection speed or pressure, determining the location of a reasonable speed conversion and rapidly increasing pressure on the die casting part before the solidification process can reduce defects on the die cast.

A die casting machine with 28000kN should be adopted due to large sizes, complex structures and great difference in wall thickness of cylinder blocks. Set the slow injection speed at about 0.2m/s, the slow injection stroke 400mm, the fast injection speed about 5.5m/s and pressure at about 45MPa.