Prevention of nine common defects in low pressure casting
時間：2021.09.07 瀏覽量：22
1、 Stomata
1. Features
(1) Porosity is a kind of hole defect formed by gas in the casting. Its surface is generally smooth, mainly pear shaped, round or oval. Generally, it is not exposed on the casting surface. Large holes often exist in isolation, and small holes appear in groups.
(2) Subcutaneous pores are dispersed pores located under the skin of the casting. It is a reactive pore produced by the chemical reaction between molten metal and sand mold (mold, wet core, coating, cold iron with unclean surface). The shapes are needle like, tadpole like, spherical, pear like, etc. Different sizes and depths. It is usually found after machining or heat treatment.
(3) Air pocket (air pit type surface air hole) a smooth air hole is recessed in the surface of the casting.
(4) Gas shrinkage cavity is a kind of casting defect formed by the combination of dispersed pores, shrinkage cavity and shrinkage porosity.
(5) Pinholes are generally precipitated pores with the size of the needle distributed on the section of the casting. Such pores often appear in aluminum alloy castings, which do great harm to the properties of castings.
① Punctate pinholes: these pinholes are round dots in the low magnification microstructure, with clear outline and are not connected to each other. The number of pinholes per square centimeter can be counted and the diameter of pinholes can be measured. This kind of pinhole is easy to distinguish from shrinkage and porosity. Punctate pinholes are formed by bubbles precipitated during solidification of castings. They often occur in castings with small crystallization temperature range and good feeding capacity, such as ZL102 alloy castings. When the solidification rate is fast, punctate pinholes will also appear in ZL105 alloy castings far away from eutectic composition.
② Reticular pinholes: these pinholes are densely connected into a network in the low-power microstructure, accompanied by a small number of large holes. It is difficult to count the number of pinholes, and it is difficult to measure the diameter of pinholes. They often have terminals, commonly known as "fly feet". For the alloy with wide crystallization temperature, the gas precipitated during slow solidification of the casting is distributed on the grain boundary and in the developed dendrite gap. At this time, the crystallization stock price has been formed and the feeding channel is blocked, so a network pinhole is formed on the grain boundary and in the dendrite gap.
③ Mixed pinholes: such pinholes are mixed with dot pinholes and mesh pinholes, which are common in castings with complex structure and uneven wall thickness.
Pinholes can be graded according to national standards. The worse the grade, the lower the mechanical properties of the casting, and the worse the corrosion resistance and surface quality. When the pinhole grade allowed by the technical conditions of the casting is not reached, the casting will be scrapped, in which the mesh pinhole cuts the alloy matrix, which is more harmful than the dot pinhole.
(6) Surface pinholes are dispersed pores distributed in groups on the surface of the casting. Its characteristics and causes are the same as those of subcutaneous pores. It is usually exposed to the casting surface and can be removed after machining for 1 ~ 2mm.
(7) Choking (choking hole) a large amount of gas produced in the pouring process can not be discharged smoothly, boiling occurs in the liquid metal, resulting in a large number of pores in the casting, and even incomplete defects of the casting.
2. Pore classification
(1) Precipitated pores are evenly distributed in areas with high temperature such as gate, riser and hot spot. The pores are small and dispersed, and often coexist with shrinkage pores.
Precipitation: that is, the aluminum water contains gas, which is not completely removed, and precipitates during solidification.
(2) Reactive pores are evenly distributed on the contact surface between the mold wall and the casting. The surface of pores is smooth, silvery white (steel castings) and metallic light or dark.
Reaction: mold, core, cold iron, coating, etc. contain substances that react with molten aluminum to produce gas.
(3) Intrusive pores, which are large and smooth, are distributed in the upper part of the casting. Intrusion: the gas in the mold cavity intrudes into the casting without being discharged out of the mold in time.
3. Pore formation mechanism
The mold of low-pressure casting is basically sealed. The liquid metal filling mold is relatively fast, and the gas has no time to be discharged. It is wrapped in the casting to form pores or pinholes.
(1) Precipitation of dissolved gas in liquid metal - precipitated pores (pinholes), the gas contained in molten metal. When the liquid metal is cooled and solidified, the gas precipitates due to the decrease of gas solubility, which is too late to be eliminated, resulting in pores in castings.
The gas in molten aluminum has high inclusion content, poor refining effect and low solidification rate of castings.
(2) Wet core, coating, cold iron with unclean surface, gas generated after pouring and heating - reactive pores (subcutaneous pores), pores generated by chemical reaction between mold wall material and liquid metal or inside liquid metal.
(3) The gas in the mold cavity is not discharged out of the mold in time - invasive air hole (single large air hole), which is caused by unreasonable casting process design, such as poor exhaust of mold or core, or careless operation, such as blocking the air hole during pouring (pouring speed is too fast), and the gas in the mold cavity is confined in the casting.
4. Prevention and control measures
(1) Strictly implement the smelting operation procedures, avoid liquid metal suction, and carefully degassing. Prevent precipitation porosity
① Metal raw materials and returned materials shall be dry, free of rust, oil stain, etc., and preheated before use.
② The melting temperature should not be too high. The higher the melting temperature of molten metal, the more gas (mainly hydrogen) dissolved in it. Therefore, the melting temperature should be strictly controlled, especially for non-ferrous alloys.
③ The melting time of any kind of metal should be shortened as far as possible to prevent the long melting time from increasing the suction capacity of liquid metal. A factory produces aluminum iron manganese brass castings, which are melted out in 2.5h, and the air tightness of the cast castings is qualified; However, the castings poured after 6h melting out of the furnace are scrapped due to unqualified air tightness on the premise of unchanged process. When the melting time is restored, the air tightness of the castings is all qualified, which fully shows the influence of the melting time on the air tightness of the castings.
④ Aluminum containing alloys should not be melted in power frequency furnace as far as possible, because the stirring capacity of this furnace is very strong, and aluminum is easy to oxidize into Al2O3 in contact with air and enter into liquid metal to become slag, which also provides opportunities for gas precipitation. At the same time, it is also easy to react with H2O to make the liquid metal inhale hydrogen H2. If resistance reverberatory furnace, far infrared heating furnace, or even reverberatory furnace with oil or gas can be used for smelting. Practice has proved that the aluminum alloy melted in these furnaces has less gas content and impurity quality.
⑤ When feeding, the materials with low melting point shall be input first, and the materials with high melting point shall be input in turn. This will reduce the metal suction, which is due to the reduction of the contact area and time between charge and air.
⑥ After degassing the liquid metal, the slag shall be removed immediately and then poured. It shall not stay too long to prevent re inhalation.
⑦ Degassing by refining with hexachloroethane or argon or vacuum degassing.
(2) Minimize the gas generation of coating, sand core, metal mold (core), etc. Select the coating with good quality and low air generation, and fully dry the mold and core coating. Prevent reactive pores
① The type of coating shall be selected appropriately, and the air emission of coating shall not be high. The coating also has certain exhaust property.
② The mold and core shall be preheated first, and then the coating shall be sprayed. After that, it must be dried thoroughly before use.
③ The paint shall not be troweled after spraying. Where the paint falls off, it shall be sprayed again immediately.
④ The sand core must be thoroughly dried before use.
⑤ The surface of metal mould and cold iron shall be flat and smooth, and shall be used after drying.
(3) Improve the exhaust conditions of mold and core. According to the characteristics of the casting and comprehensively considering the mold filling of the casting, a reasonable exhaust position and different exhaust measures can be selected: exhaust groove, exhaust sheet, exhaust needle, exhaust plug, exhaust hole, etc.
(4) Select the appropriate filling speed to ensure the smooth filling of molten metal and prevent gas from being involved. The rising speed of molten metal is generally controlled at 50mm / s. That is, the reasonable pouring process of gravity casting: pouring temperature, mold temperature, pouring speed, pouring time, etc.
2、 Shrinkage cavity and porosity
Shrinkage defect: the defect caused by the failure of liquid metal to effectively feed the casting during metal solidification and shrinkage. Including shrinkage cavity, shrinkage porosity, shrinkage depression, shrinkage subsidence, etc.
1. Features
① Shrinkage cavity: there is a very irregular hole in the casting, the hole wall is rough and has dendrite, which is called shrinkage cavity defect. It mostly appears in the last solidification part of the casting.
② Shrinkage porosity: there are scattered and small shrinkage cavities on the casting section, sometimes called shrinkage porosity defects with the help of a magnifying glass. For example, when aluminum piston is produced by low-pressure casting, shrinkage porosity sometimes occurs at the top of the piston.
③ Porosity: a very small hole in the slow solidification zone of a casting. Distributed in and between dendrites, it is a mixed defect with dispersive pores, micro shrinkage porosity and coarse structure, which reduces the compactness of castings and is easy to cause leakage.
④ Shrinkage: the collapse of the upper plane at the junction of the thick end face or section of the casting. Sometimes there is a shrinkage cavity below the shrinkage cavity, and sometimes the shrinkage cavity also appears near the inner shrinkage cavity.
⑤ Shrinkage sink: a casting defect produced in the production of castings using water glass limestone sand mold, which is characterized by the expansion of casting section size.
⑥ Shrinkage crack: crack caused by improper feeding, blocked shrinkage or uneven shrinkage of castings. It may occur just after solidification or at a lower temperature.
2. Causes
The reason for the formation of shrinkage cavity and shrinkage porosity is that during the solidification process of liquid metal, due to the liquid shrinkage and solidification shrinkage of the alloy, that is, the volume loss caused by volume shrinkage can not be compensated, that is, feeding can not be obtained, and holes often appear at the last solidification part of the casting.
Different from general gravity casting, low pressure casting is filled from bottom to top, and the gate is at the lower part. In order to make the casting get enough feeding, it must form a sequential solidification from top to bottom, that is, solidification at the place far away from the sprue first and solidification at the sprue last, otherwise shrinkage cavity and porosity defects will occur.
3. Preventive measures (simultaneous solidification or sequential solidification)
Since both low-pressure casting and differential pressure casting are counter gravity casting, and gravity is hindering feeding at all times, the quality of liquid level pressurization control system is the key link to determine the compactness of castings, whether for sand mold casting, metal mold casting, simultaneous solidification or sequential solidification. Especially for thin-walled metal mold casting, the solidification time is not long. When the mold is filled to the top, the solid fraction in the liquid metal has occupied a considerable proportion. At this time, the pressure should be increased rapidly immediately in order to overcome the negative effect of gravity and feed. At this time, the compactness of the casting is an extremely critical moment. At present, some liquid level pressurization control systems still pressurize slowly according to the mold filling speed at the critical moment, while others are worse. They can pressurize normally when the pressure is low, but the higher the pressure is, the slower the pressure rise speed is. The so-called parabolic filling with opening downward.
When the solidification of liquid metal has basically ended, the control system will raise the pressure of pressurization and feeding. Obviously, it is too late, which will not play a good role in the density of castings. In production, sometimes the feeding pressure is already very high (up to 0.2MPa), but the castings still have shrinkage porosity defects, resulting in too high pressurization leakage rate. When the feeding channel is reasonable, this is mainly because the timing of pressurization of the control system is not well controlled, rather than the wrong saying that "the feeding pressure has little impact on the compactness of castings". For example, a factory has tried to produce a large thin-walled part, and failed to cast qualified castings for more than two years. The problem is that the castings have more shrinkage porosity, poor compactness and serious pressurization leakage.
When the old liquid level pressurization control system was replaced by the "clp-3" low pressure casting liquid level pressurization control system with closed-loop feedback, the situation changed greatly, and the qualified castings were produced without major changes in the original process.
A factory in Shenyang used the manual control system to produce thin-walled shell castings on the differential pressure casting machine, and the scrap rate was almost as high as 80% ~ 90%. After replacing the "CLP" differential pressure casting liquid level pressurization control system designed by Harbin University of technology, the scrap rate immediately decreased significantly, and cast qualified castings with clear external edges and corners and plump printing. It can be seen that the liquid level pressurization control system plays an extremely important role in differential pressure and low pressure casting.
Specific preventive measures: for the shrinkage cavity caused by sequential solidification of metal mold, the elimination methods are as follows.
(1) Make the mold temperature distribution reasonable, that is, the upper temperature is low and the lower temperature is high. It is best to use clp-5 liquid level suspension pressurization control system, which can improve the lower temperature and increase the feeding capacity.
(2) The heat capacity distribution of the mold itself is reasonable, that is, the lower heat capacity is small and the upper heat capacity is large (that is, the lower mold wall is thin and the upper mold wall is thick).
(3) Forced cooling shall be adopted for local hot joints to adjust a temperature field distribution consistent with feeding.
(4) For the "cold joint" that locally affects feeding, drill holes and mill grooves around the back, and then fill with thermal insulation materials to increase thermal resistance and give a reasonable temperature field.
(5) Reduce the mold filling speed and mold temperature, but be appropriate to prevent cold shut and insufficient pouring
(6) Properly reducing the pouring temperature has a significant effect on reducing shrinkage porosity. For shrinkage porosity solidified at the same time, the elimination methods are as follows: make the mold temperature distribution reasonable, the upper temperature is high and the lower temperature is low; The heat capacity distribution of the mold is reasonable, that is, the heat capacity of the upper part is small and the heat capacity of the lower part is large, that is, the upper part of the mold wall is thin and the lower part is thick; The treatment methods of local hot section and "cold section" are the same as above.
(4) The treatment methods of mold temperature, mold filling speed and pouring temperature are opposite to sequential solidification.
It is convenient to change the casting process of sand mold, so there are many methods to eliminate shrinkage cavity whether it is simultaneous solidification or sequential solidification. Specific preventive measures are as follows:
(1) For large and medium-sized non-ferrous alloy and ferrous metal castings, the wall thickness varies greatly. A riser is set and pressurized from the riser to strengthen feeding and prevent shrinkage cavity and porosity.
(2) Properly reduce the pouring temperature or pouring speed.
(3) Reasonably design the casting process and establish sequential (simultaneous) solidification conditions.
3、 Inclusion
1. Features
(1) Inclusion defect is the general name of various metallic and non-metallic inclusions in castings. Usually, impurity particles such as oxide, sulfide and silicate are mechanically retained in the solid metal, or formed in the metal during solidification, or formed in the metal in the reaction after solidification. Including inclusions, cold beans, introgressive beans, slag inclusion, sand holes, etc.
(2) Inclusions are particles different from the composition of the base metal in or on the surface of the casting. Including: slag, sand, coating layer, oxide, sulfide, silicate, etc.
(3) Endogenous inclusions in the process of melting, pouring and solidification, inclusions generated due to the chemical reaction between liquid metal and furnace gas (including mold), and inclusions precipitated due to the decrease of liquid metal temperature and solubility.
(4) Exogenous inclusions are inclusions caused by slag and foreign impurities.
(5) Slag inclusion is a kind of inclusion defect in castings caused by molten slag, low melting point compounds and oxides wrapped in molten metal due to impure pouring liquid metal or improper design of pouring method and pouring system. Because its melting point and density are usually lower than that of liquid metal, it is generally distributed on the top or upper part of the casting, the lower surface of the core and the dead corner of the casting. The fracture is dull and dark gray.
(6) The coating slag hole is caused by the coating layer remaining on the casting surface after pulverization and falling off, and contains the irregular pit of residual coating accumulation material. The coating on pouring tools, mold, riser and sand core falls off, especially after the sand core is coated
