Lost foam casting technology is known as “casting technology in the 21st century” and “green revolution of casting industry”, which is naturally a great praise, but nothing is single-sided and a double-edged sword. Although there are many advantages of lost foam casting, there are many defects in the production process, resulting in more defective products. In terms of lost foam casting of aluminum alloy, in addition to the defects easily produced by general lost foam casting, combined with the characteristics of aluminum alloy, there will be some typical characteristics different from other alloy lost foam casting. Here, the typical defects of aluminum alloy lost foam casting will be stated, and other defects common in the process of lost foam casting will not be repeated.
1. Insufficient pouring and cold insulation
Insufficient pouring and cold shut are common defects in lost foam casting, especially for aluminum alloy thin-walled complex parts. The main reason for this kind of defect is that the fluidity of liquid metal in the mold filling process is not good. There are two main reasons for the poor fluidity of liquid metal: one is the back pressure. During the filling process, the foam model absorbs heat and decomposes a large amount of gas, which hinders the filling of molten metal and reduces the fluidity of molten metal. Second, the front temperature of molten metal decreases. Compared with cast iron and other casting methods, aluminum alloy lost foam casting has a prominent feature of low temperature. Moreover, in the pouring process, the gasification of the model further reduces the heat and the temperature is lower, which is more likely to produce defects such as insufficient pouring and cold insulation.
2. Pinhole
The generation of pinholes is also one of the technical defects that are very tenacious and difficult to avoid in the process of lost foam casting of aluminum alloy in China. It is mainly caused by the gas entrapment of liquid metal. There are two specific reasons: first, the pouring temperature of aluminum alloy is as high as 750 ~ 780 ℃, which is much higher than that of ordinary cavity pouring, and the temperature is so high. In addition, lost foam casting adopts dry sand molding, the cooling speed is slow, the solidification time of liquid metal becomes longer, tends to paste solidification, and hydrogen is easy to precipitate during cooling to form dispersed pinholes (often accompanied by slag). In addition, if the operation is not carried out in strict accordance with the basic requirements of the production process, such as too high or too low pouring temperature; The returned materials with organic matter, aluminum rust and other sundries are not fully preheated; Problems such as insufficient refining of liquid aluminum will also easily produce pinholes.
3. Shrinkage and porosity
In the process of lost foam casting of aluminum alloy, another point that needs to be paid attention to is the design of riser. If the temperature and pressure of molten metal at the riser of lost foam casting of aluminum alloy are low, it is easier to produce shrinkage cavity and porosity defects than that of cast steel and cast iron.
4. Surface carbon defect (wrinkle)
The carbon defect on the surface of aluminum alloy lost foam casting, also known as wrinkle, is mostly due to the decomposition product of polystyrene in the casting process. Polystyrene presents different states under different temperature conditions. Once the temperature is lower than 750 ℃, it will become a liquid state similar to asphalt. These liquids are easy to absorb. Usually, after being absorbed, a film will be generated, which will be adsorbed between the liquid and the coating to further generate crystalline carbon, which leads to the generation of carbon defects. If you want to reduce the defect rate, you should be more strict in the control link, and take pre-treatment and post remedial measures. At the same time, it is necessary to strictly control the quality of raw materials, including the coating and foam template used, strictly according to the scientific and reasonable process in the pouring process, do well in every key link of production, and strictly control the pouring temperature, the quality of coating metal liquid and the production speed.
5. Surface irregularities
The surface concave convex phenomenon is also one of the common defects in the process of lost foam casting. Its causes are mostly the unevenness of the exhaust plug on the template and the defects on the mold surface. This defect can be found before production and belongs to congenital defect. For such defects, our preventive measures are simpler. We can conduct regular inspection, management and maintenance of mold templates before production, screen out unqualified mold templates, and actively deal with small problems once they occur. In the procurement of molds, try to select manufacturers with quality assurance to ensure that the quality of mold templates is excellent, so as to produce good lost foam casting parts.
6. Others
The defects of aluminum alloy lost foam casting are: sand sticking, nodulation, sand collapse, surface holes, bead surface, etc. their causes are different, and the solutions are cumbersome. I won’t elaborate here.