The United States, Europe and Japan are traditional die casting
powers worldwidely, especially in the pillar industries of represented by automobiles. They have a long history of development, large market coverage and rich experience, forming an unshakable status due to the market competitiveness. Although China has been a global leader in passenger vehicle sales, foreign cars with intellectual property rights still have great advantages and shares, which is not only a pressure for China but also an impetus.
Choosing AI-Si series alloy as the main body of the aluminum alloy die casting
is determined by the die casting process
and characteristics of forming solidification; AI-Si series alloy is are not only the most widely used alloy in aluminum alloy die casting, but also has the largest yield. Because of this, the exposed problems are concentrated and we should pay attention to the common problems.
The analysis of die casting hole defects
Liquid aluminum alloys react with the water in the air and produce Al2O3 and H2; this kind of Al2O3 has a loose tissue and its specific gravity is similar to aluminum's, so Al2O3 is easy to suspend in the aluminum liquid and has a strong adsorption with H2, forming an adsorption core. Secondary aluminums are widely used in O melting of aluminum alloy die casting, if the pretreatment is not proper, it will cause hidden risks. Si element is the main element in Al-Si alloy. Its melting point is twice higher than that of aluminum and it is lighter than aluminum; it is necessary to stir violently at high temperature in order to make it uniform in the melting, and it is a certain thing that the gas will get in and it will cause oxidation. Plus, the adding of Si element has reduced the solubility of H2 in the aluminum solution and the viscosity and surface tension has also been increased, which hinders the formation of bubbles in aluminum liquid. These are reasons for die casting hole defects of AI-Si series. That's why we pay particular attention to alloy refining process in aluminum alloy melting. We just remove the gas in the past, but many failure analysises of the defects indicate that we should have purification treatment for the refining process. In this process, we not only need to degas, but also need to deslag. Or you will get half the results with twice the effort. We also optimize the device of alloy purification treatment nowadays. It reaches the purposes of degassing and deslagging through the distribution hole which is at the end of the graphite rotor introduces the pure argon to the rotating vortex of aluminum alloy liquid by rotation, using argon oxygen as the carrier of the refining flux.
Interpretation of chemical compounds and hard spots of aluminum alloy metals
Hard spots are often found in mechanical machining process of Al-Si-based binary alloy die casting, especially in Al-Si-Cu alloy. Microscopic analysis shows that most of them are intermetallic compounds, which are mainly ternary. We can also find primary crystal silicon particles.
The formation of these defects occasionally occurred in the aluminum alloy die casting. One reason is that the pretreatment of die casting is not standardized, and the refining and purification processes are not complete. These hidden dangers in die casting process are very obvious, while the risks of the similar kind of alloy in the gravity casting are difficult to be seen see. Another reason is the size, proportion and flow rate in the die casting process, unreasonable designs of slagging and exhaust systems will lead to defects. It should be emphasized that all Al-Si series aluminum alloys are sensitive to the gas absorbed in before, so we need to pay attention to melting of die casting alloy ingots, pretreatment of raw materials and receiving examination of die casting alloy ingots.
Interpretation of air tightness problem of die castings under pressure
The pressure part which bears air pressure, hydraulic pressure and oil pressure in die castings occasionally leak due to the lack of air tightness, especially those with complicated shapes and uneven wall thickness. Our advice is to make sure the trial-manufacturing of the die casting is done well. Whether mold design and manufacturing can meet the quality of die casting or not, it all depends on the metallurgical quality of the test-piece, the surface quality and the inner quality of the die casting, and there should be 3 to 5 batches of stable production trial results. At last, they need to pass the processing examination of die castings. Among all those steps, the acceptance of the mold is the most important. It involves the runner design in the mold debugging process, the analysis on the heat balance of cavity, designs of the slagging and exhaust duct, as well as the recycling, water cooling, paint spraying and measurements for partial cooling. If air tightness of the die casting is critical to the technical requirements, all the measurements we have taken are not entirely certain. And finally we need to use soakage treatment to solve the pressure leakage in order to ensure the quality of die castings.