The harm of shrinkage cavity and porosity in aluminum alloy low pressure casting

Shrinkage defects will reduce the elongation of the material, and have a great impact on the load-bearing performance, impact resistance and fatigue life of the product. For a single wheel, it is necessary to set up separate casting process parameters. The most frequent areas of this kind of defect are the spokes and wheel centers, which are tacitly allowed in the design of many wheels.

Research methods

At present, the most effective way to predict shrinkage cavity and shrinkage porosity is to simulate the heat transfer in the solidification process by computer, and then optimize the mold and process parameters based on the simulation results to avoid the key factors that lead to the formation of shrinkage cavity. For simulation, there are two key problems. First of all, the reasonable setting of boundary conditions. In the simulation process, reasonable boundary conditions are needed to define the heat conduction and heat exchange process, including the interface between the mold and the wheel, the interface between the mold and the mold, and the interface between the mold and the environment. However, there are few studies worthy of reference in this area. In addition, the definition of the boundary varies according to the specific situation, so it is difficult to get more reasonable data to define the boundary. Secondly, it is difficult to quantitatively measure the distribution of shrinkage cavity according to the temperature conditions. As a basic characteristic, the effect of hydrogen content and alloy modification on the formation of shrinkage cavity is a field to be developed. For example, it is well known that in the foundry industry, too low hydrogen content will lead to the increase of shrinkage cavity. Similarly, alloy modification can affect the mass transfer of liquid between dendrites, which can also change the tendency of shrinkage cavity formation. However, only a few quantitative studies have been reported.

Control method

From the production practice, the defect generally appears in the spokes, wheel centers and rims. According to the analysis of the formation mechanism and the process conditions, the formation of shrinkage defects is related to the temperature, filling speed, holding time and sequential solidification of the alloy liquid. Improvements can be made in the following ways:

1) To reduce the volume shrinkage caused by phase transformation can properly reduce the temperature of the liquid aluminum after filling;

2) At the end of filling, increase the pressure holding pressure, prolong the pressure holding time and enhance the feeding capacity;

3) Reasonable design of wheel mechanism, from the perspective of heat transfer, to prevent the generation of hot spots;

4) Seek reasonable cooling mode and cooling system to ensure that the sequence solidification is not disturbed.

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