Common defects and prevention of low pressure casting
(1) Pore: hole-like defects formed by gas inside the casting. Its surface is generally smooth, mainly pear-shaped, round or oval. Generally not exposed in the casting surface, often exist in large holes, holes are in groups.
(2) Subcutaneous air holes: scattered pores located beneath the skin of the casting. Reactive gas holes generated by the chemical reaction between the molten metal and the sand (mold, wet core, paint, chilled iron surface). Shape acicular, tadpole-like, spherical, pear and so on. Different sizes and depths. Usually found after machining or heat treatment.
(3) gas nest (gas pit surface pores): the casting surface concave to a smoother pores.
(4) Shrinkage: Dispersed porosity and shrinkage and shrinkage combined hole casting defects.
(5) Pinhole: The pinhole size is generally distributed in the casting section of the precipitated pores. Aluminum alloy castings often appear in such pores, a great harm to the casting performance.
① Point-like pinholes: Such pinholes in the low magnification microscopic dot-like, clear outline and are not connected to each other, can count out the number of pinholes per square centimeter area and measure the diameter of the pinhole. Such pinholes and shrinkage cavity shrinkage easy to distinguish. Dot-like pinhole formed by the precipitation of the casting bubble, and occurred in the crystallization temperature range, good casting ability of casting, such as ZL102 alloy castings. When the solidification rate is faster, from the eutectic composition of ZL105 alloy castings will appear punctate pinhole.
② Mesh pinhole: This type of pinhole dense microstructure at low magnification associated with a mesh, accompanied by a small number of larger holes, not easy to count the number of pinholes, difficult to measure the diameter of the pinholes, often with the tip , Commonly known as "flies feet." Crystallization temperature of the alloy, the casting slowly solidified precipitation of gas distribution in the grain boundary and developed dendrite gap, when the crystallization of the share price has been formed, the shrinkage channel is blocked, they formed in the grain boundary and dendrite gap Mesh pinhole.
③ mixed-type pinhole: This type of pinhole pinhole pinhole and mesh pinhole mixed together, common in the structure of complex, uneven thickness of the casting.
Pinholes can be graded according to national standards, the worse grades, the lower the mechanical properties of castings, the worse the corrosion resistance and surface quality. Casting will be scrapped when the pinhole level allowed by the casting technology is not reached, in which the mesh pinhole splits the alloy base and is more harmful than the pinhole.
(6) Pinholes on the surface: scattered pores grouped in the casting surface. Its characteristics and formation of the same reasons and subcutaneous air holes, usually exposed on the casting surface, after machining 1 ~ 2mm can be removed.
(7) choke fire (choke hole): a large number of gas generated during the casting process can not be successfully discharged, boiling occurs in the liquid metal, resulting in a large number of pores in the casting, casting defects or even incomplete.
(1) Precipitated pores: These pores are evenly distributed in the interior near the gate, riser, hot section and other areas with high temperature, the pores are small and scattered, often with shrinkage coexistence.
Precipitation: that is, the gas containing aluminum, not completely remove the net precipitation during solidification.
(2) Reactive porosity: These pores are uniformly distributed on the contact surface between the wall and the casting. Stomatal surface is smooth, silver white (steel castings), metallic bright color or dark color.
Reactions: Mold, core, chill iron, paint, etc. Contains substances that react with aluminum water and produce gas.
(3) Invasive stomata: These stomata are distributed in the upper part of the casting, with large and smooth holes.
Invade: the gas in the cavity, did not discharge in time, but invaded into the casting.
3. Stomatal formation mechanism
Low-pressure casting mold is basically sealed, liquid metal filling faster, too late to discharge gas, the package formed in the casting pores or pinholes.
(1) Gas dissolved in the molten metal Precipitation - Precipitated pores (pinholes), gases contained in the molten metal When the liquid metal is cooled and solidified, gas is precipitated due to the decrease of the solubility of the gas, .
Liquid aluminum gas, high content of inclusions, the refining effect is poor, the casting solidification rate is low.
(2) wet core, paint, chilled iron with unclean surface, gas-reactive pores (hypodermic pores) generated after pouring heat, pores produced by chemical reaction between the wall material and the liquid metal or inside the liquid metal .
(3) The gas in the cavity is not expelled in time due to the type of outside-invasion stomata (single atmosphere). Due to the unreasonable design of the casting process, such as the poor exhaust of the mold or the core or the inadvertent operation, Such as blocking the injection when the eye (pouring too fast), the cavity of the gas was suppressed in the casting caused.
4, prevention and control measures
(1) The strict implementation of smelting operating procedures, to avoid liquid metal suction, and serious out of gas. Prevent precipitation porosity
① metal raw materials and charge should be dry, no rust, no oil, etc., before use to preheat.
② melting temperature should not be too high. The higher the liquid metal melting temperature, the more dissolved gas (mainly hydrogen) is. Therefore, should be strictly controlled melting temperature, especially for nonferrous alloys.
③ any kind of metal melting time should be as short as possible to prevent over-long smelting liquid metal suction increased, a factory producing aluminum-iron-manganese brass castings, 2.5h melt out of the furnace, pouring castings Air tightness are qualified; but after 6h molten furnace casting pouring out, under the premise of the same process castings rejected due to gas-tight failure. After recovering the melting time, the airtightness of the castings is all qualified, which fully shows the influence of the melting time on the airtightness of the castings.
④ aluminum alloy should be as far as possible without frequency furnace smelting, because of this strong stirring capacity of the furnace, aluminum and air contact easily oxidized into Al2O3, and into the liquid metal into a slag, but also for the precipitation of gases opportunity. At the same time also easy to react with H2O, the liquid metal inhalation of hydrogen H2. Reflectance furnace if used, far infrared heating furnace, and even the use of oil or gas furnace reflex furnace can be. Practice has proved that: with these furnace smelting aluminum alloy gas content, the amount of impurities are less.
⑤ feeding should be put into the low melting point of the material, followed by the introduction of high melting point of the material. This will make the metal intake less, the reason is that the charge and air contact area and time are reduced.
⑥ liquid metal degassing should immediately slag, and then pouring, can not stay too long, to prevent breathing again.
⑦ with hexachloroethane or argon gas refining or vacuum to the gas.
(2) minimize the paint, sand core, metal type (core) and other gas production. Choose a good quality gas less of the paint, mold and core paint to fully dry. Prevent reactive stoma
① types of paint should choose the right, paint the amount of gas can not be high. Paint also has a certain exhaust.
② mold and core should be preheated, and then spray paint, after the end must be roasted before use.
③ paint spray can not be tedious. Where the paint off, should immediately make up spray.
④ sand core must be thoroughly dried to use.
⑤ metal and cold iron surface should be smooth and clean, and after drying use.
(3) to improve the mold and core exhaust conditions. According to the characteristics of the casting, considering the casting filling situation, choose a reasonable location of the exhaust and different exhaust measures: exhaust groove, exhaust, exhaust pin, exhaust plug, exhaust vent exhaust .
(4) Select the appropriate filling speed, and strive to smooth liquid metal-filled type, to prevent the involvement of gas. Metal liquid rising speed is generally controlled at 50mm / s. Gravity casting that is reasonable casting process: pouring temperature, mold temperature, pouring speed, pouring time and so on.