Peeling of aluminum casting is a common defect in sand casting. The reason of peeling should be observed first, then the area and position of peeling should be checked, and then the solution should be found according to the causes of peeling. Therefore, in order to prevent casting aluminum parts from peeling, we should take some corresponding measures.
After the metal is poured into the sand mold, a very thin dry layer is formed on the surface of the mold, and then a high water layer is formed in the dry layer. The hot and wet tensile strength of the mold is much lower than that of the wet sand mold. The surface of the mold expands when heated, and the stress concentration and cracks appear in the high water layer with low strength, which further develops from rat tail to sand inclusion scar. Because of the low strength of the high water layer of the mold and the separation from the mold body due to the expansion of the dry layer on the surface of the mold, it can slide freely. Therefore, it is concluded that the conditions for sand inclusion formation are too long pouring time, flat surface and wall thickness of the casting.
The degree of scab is related to the time of crack on the surface of molding sand. If the hot and wet tensile strength of high water layer molding sand is high, the cracking start time of surface dry sand can be prolonged. If the mold sand is completely filled with molten metal, no sand inclusion and scab defects will be formed. The longer the cracking time of all molding sand is, the less likely it is to produce sand inclusion scab. The main preventive measures of casting aluminum parts peeling are: using sodium bentonite or increasing clay content. The content of SiO2 in sand should not be excessively pursued in iron castings. The sand inclusion tendency of coarse sand is smaller than that of fine sand, and that of dispersed particle size is smaller than that of concentrated particle size.
To control the moisture content of molding sand, the moisture content of molding sand should be strictly controlled in green molding, and surface drying can be used. The addition of sawdust (0.2 ~ 2%), yam powder or dextrin can increase the deformation of the mold. The addition of pulverized coal, residual oil and other additives can reduce the expansion of molding sand. Master the molding operation, the compactness should not be too high in the wet mold (controlled at 65 ~ 85), tie the air hole, brush less water as far as possible, the repair part should be connected with the body, and the sand mold can be reinforced by inserting nails. To improve the pouring process, low temperature and fast pouring process should be adopted in the possible range, and the internal runner should be set up dispersedly, so as to avoid the excessive concentration of metal heat effect. In a word, in order to prevent sand inclusion defects, in addition to the main control of sand mold quality, it is also necessary to cooperate with the correct molding and pouring process operation to obtain better results.
Metallic hard spots of cast aluminum parts:
1、 It's mixed with undissolved silicon
1. When melting silicon alloy, do not use silicon powder.
2. When adjusting alloy composition, do not add silicon element directly, use master alloy.
3. The melting temperature should be high and the time should be long, so that the silicon can be fully melted.
2、 The raw materials for promoting primary silicon crystal growth are mixed
1. The range of casting temperature fluctuation is reduced to keep the molten state.
2. It is necessary to prevent alloy ingot from solidifying when adding cold material.
3. The composition that promotes the growth of primary silicon should be reduced as much as possible.
3、 It is mixed with crystalline materials that form intermetallic compounds
1. The range of temperature fluctuation should be reduced, and the temperature of alloy should not be too high or too low.
2. While controlling the impurity content of alloy composition, pay attention not to increase impurities.
3. The materials that can produce intermetallic compounds should be melted at high temperature. In order to prevent the increase of impurities, a small amount should be added.
Cleaning method of cast aluminum parts:
1. Casting shell cleaning: Generally speaking, further cleaning can be carried out after the aluminum casting is completely cooled. The cooling time of the casting in the mold is directly related to the material of the cast aluminum part and the shape and size of the aluminum casting and the production conditions. Generally, if the cleaning work of aluminum alloy casting is earlier, the deformation and crack of aluminum casting will be easily produced. The cooling time of castings with large size and complicated process should be longer than that of castings with small size and thin wall process.
2. Cutting gate and Riser: the main methods of cutting gating and riser are: gas cutting, grinding wheel cutting, sawing machine, hydraulic cutting, anode cutting, etc.
3. Surface cleaning of cast aluminum parts: after the casting aluminum parts are shelled, the total residual oxide scale on the surface of the casting, especially for the castings with complex inner cavity, deep groove and blind hole, can not be completely cleaned, so the surface cleaning is required.
4. Shot blasting treatment: the casting with special requirements can be shot blasted. Shot blasting cleaning is to use the centrifugal force generated by the impeller of high-speed rotating shot blasting device to cast iron shot to the surface of casting, so as to make the surface of aluminum casting residual sand, sticky sand or iron oxide scale. Generally speaking, the surface of casting after shot blasting is related to the size of iron shot.
5. Repair welding: usually refers to the casting which does not meet the acceptance technical conditions or the provisions of the order agreement and has casting defects but can be repaired. First of all, the repair welding parts should be cleaned to make the surface of the aluminum casting completely clean and exposed metal luster. The size of defects should be welded from small to large. If the repair welding area is large and the defects are in the important parts of the casting, annealing treatment should be carried out.