The aluminum alloy melted by adding some metal or non-metal elements to pure aluminum is a new type of alloy material. Because of its small specific gravity, high specific strength and good comprehensive performance, it is widely used in aviation industry and automobile manufacturing. Industry, power meters, tools and civilian appliances manufacturing. With the development of the national economy and the advancement of the economic integration process, its production volume and consumption have greatly exceeded the trend of steel. Strengthening the research on the properties of aluminum alloy materials and ensuring the quality of aluminum alloy castings is an unshirkable responsibility of every one of our scientific and technological workers. It is also a top priority related to our daily lives. This article combines the author's aluminum alloy casting production practice experience to talk about aluminum alloy castings stomata and prevention problems.
1. Stoma Category
Due to the severe oxidation and inhalation tendency of aluminum alloys, the smelting process directly contacts the furnace gas or the external atmosphere. Therefore, if the control during the smelting process is slightly improper, the aluminum alloy can easily absorb the gas and form pores, which are more common. It is a pinhole. Pinholes (gas porosity/pin-hole) usually refer to precipitated pores less than 1mm in castings, which are mostly round and unevenly distributed throughout the entire section of the casting, especially in the thick section of the casting and the cooling rate is small The location. According to the distribution and shape characteristics of the aluminum alloy's precipitated pores, the pinholes can be further divided into three categories1, namely:
(1) Dot-shaped pinholes: In the low-powered tissues, the pinholes are in the shape of circles, the pinholes are clearly defined and discontinuous, and the number of pinholes per square centimeter can be counted and their diameters can be measured. Such pinholes are easily distinguished from shrinkage holes, shrinkage, and the like.
(2) Mesh pinholes: In the low-powered tissues, the pinholes are densely connected and meshed. There are a few large holes, so it is difficult to check the number of pinholes per unit area, and it is difficult to measure the diameter of the pinholes.
(3) Synthetic pores: It is an intermediate type of punctiform pinholes and reticular pinholes. From the viewpoint of low magnification, there are many large pinholes, but they are not punctate but polygonal.
The practice of aluminum alloy production has proved that the main gas component of the aluminum alloy due to inhalation is hydrogen gas, and its appearance does not have a certain rule to follow. It is often the case that all or most of the castings have a pinhole phenomenon; materials are also Without exception, aluminum alloys of various compositions are prone to pinholes.
2. Pinhole formation
When smelting and casting, aluminum alloy can absorb a large amount of hydrogen, and when cooled, it continuously precipitates due to the decrease of solubility. Some materials introduce 2, the more dissolved hydrogen in the aluminum alloy, its solubility increases with the temperature of the alloy liquid, decreases with the decrease of the temperature, and the solubility of hydrogen in the aluminum alloy changes from the liquid state to the solid state. Decrease 19 times. (The relationship between hydrogen solubility in pure aluminum and temperature is shown in Figure 13). Therefore, during the cooling solidification process of the aluminum alloy liquid, at a certain time of hydrogen, the hydrogen content exceeds its solubility, that is, it precipitates in the form of bubbles. Hydrogen bubbles formed by the supersaturation of hydrogen are less likely to float out, and form fine, dispersed pores during the solidification process, which is what we commonly call gas porosity. The degree of supersaturation reached prior to the formation of hydrogen bubbles is a function of the number of hydrogen bubble nucleations, while oxides and other inclusions act as bubble cores.
It is difficult to avoid pinholes under normal production conditions, especially in thick sand castings. The smelting and pouring of aluminum alloys in a relatively high-humidity atmosphere makes the pinholes in the castings particularly severe. This is why we often wonder in the production that there are fewer pinhole defects in aluminum alloy castings during the dry season than in wet and humid seasons.
In general, for aluminum alloys, if the crystallization temperature range is large, the probability of mesh pinholes is much greater3. This is because in the general casting production conditions, castings have a wide range of solidification temperatures, making it easier for aluminum alloys to form well-developed dendrites. In the later stage of solidification, the residual aluminum liquid in the dendrite gap portion may be isolated from each other and exist in an approximately closed small space, because they are subjected to external atmospheric pressure and the static pressure of the alloy liquid is less, when residual aluminum liquid further When the cooling shrinks, a certain degree of vacuum can be formed (ie, the feeding channel is blocked), so that supersaturated hydrogen in the alloy is precipitated to form pinholes.
3. Source and Precipitation of Hydrogen Forming Pores
The generation of pores in aluminum alloys is caused by the inhalation of aluminum alloys. However, gases in the form of gas molecules generally cannot be dissolved in alloy liquids. Only when the gas molecules are decomposed into active atoms can they be dissolved. The amount of dissolved gas in the alloy fluid is not only related to whether the molecule is easily decomposed into an active atom, but also directly related to the type of gas atom. In the melting process of aluminum alloys, the furnace gases that are usually in contact with are: hydrogen, oxygen, water vapor, carbon dioxide, sulfur dioxide, etc. These gases are mainly generated by combustion of the fuel, and the refractory materials, metal charge and flux, and the gas are in contact. Tools and so on can also bring in a certain amount of gas, such as new linings, refractories of furnaces, crucibles, etc., which usually takes days or weeks, and the chemically bound hydrogen can be fully released from the binder. . In general, the furnace gas composition is determined by the type of fuel and the amount of air. Common coke oven oven, furnace gas components mainly for carbon dioxide, sulfur dioxide and nitrogen; gas, heavy oil crucible furnace mainly for water vapor, nitrogen; and most of the smelting furnace used by the current electric furnace smelting, the furnace gas composition is mainly hydrogen. Therefore, when smelting using different smelting furnaces, the amount of suction of aluminum alloy and the degree of generation of pores are different.
The practice of aluminum alloy production shows that hydrogen is a gas that can be dissolved in aluminum or aluminum alloys in large quantities, and is the main cause of the formation of pores in aluminum alloys. It is a relatively harmful gas in aluminum alloys and a gas with a relatively high solubility in aluminum alloys. The porosity generated during the solidification process of the casting due to the precipitation of hydrogen not only reduces the actual cross-sectional area of ​​the casting but also is the source of the crack. Inert gases are insoluble in aluminum or aluminum alloys, and other gases generally react with aluminum or aluminum alloys to form aluminum compounds such as Al2O3, AlCl3, AlN, Al4C3, and the like. As can be seen from Fig. 1, the solution of hydrogen in liquid aluminum or aluminum alloy has a large degree of solution, and is almost insoluble in solid aluminum (having a solubility of about 0.003 or less at room temperature).
When aluminum alloy is smelted, the content of hydrogen in the surrounding air is not large, the more common source of hydrogen is the reaction of aluminum and water vapor, and the water vapor mainly comes from the moisture in the furnace gas, the moisture absorbed by equipment and tools, and some The crystal water of the material is decomposed with aluminum rust Al(OH)2. The reaction formula is as follows:
3H2O (steam)+2Al=Al2O3+6[H](1)
Magnesium-containing aluminum alloys absorb hydrogen more easily because of the following reactions:
H2O (steam) + Mg=MgO+2[H](2)
In addition, oil, organic matter, salt flux, etc. brought into the metal charge or return charge react with the aluminum liquid to generate hydrogen:
4mAl+3CmHn=mAl4C3+3n[H] (3)
Magnesium, sodium, and lithium can change the oxide film on the surface of aluminum to allow active hydrogen atoms to enter; metal fluoride and tantalum can form a denser oxide film on the surface of aluminum, reducing the diffusion rate of hydrogen into aluminum or aluminum alloys. , To protect the aluminum alloy. Elements that form hydrides, such as calcium, titanium, lithium, and ruthenium, can strongly expand the solubility of hydrogen in aluminum liquids. The solubility of active hydrogen atoms in aluminum liquids or aluminum alloys at different temperatures is shown in Table 1.
4. Influence of Stoma on Properties of Aluminum Alloy Castings
The effect of pinholes on the properties of aluminum alloys is mainly manifested in the decrease in density and mechanical properties of the castings. Therefore, in the production practice of aluminum alloy castings, it is important to strengthen the research on the effect of the stomatal grade on the mechanical properties, and to ensure the quality of aluminum alloy castings by controlling the pinhole grade. Pinhole rating, low magnification test according to GB10851-89, when in dispute, according to the provisions of Table 2; X-ray photography according to GB11346-89 aluminum alloy casting pinhole grading standards, the standard selection of two commonly used in industrial production The test results of alloys ZL101 (Al-Si-Mg) and ZL201 (Al-Cu-Mn) and determination of бb and σ5 in T4 state indicate that the mechanical properties of various pinhole specimens of ZL101T4 and ZL201ST4 are shown in the table 3, Table 4): The mechanical properties of the casting and the pinhole grade are linearly related. As the pinhole grade level increases, the mechanical properties gradually decrease; every increase in the pinhole grade, the mechanical бb drops about 3%, σ5 About 5% drop. For performance test requirements for aluminum alloy castings, see page 9
It should be pointed out here that, due to the wall thickness effect of the casting, even if the severity of the pinhole is the same, the mechanical properties of the wall thickness are reduced, and the wall thickness is small. Because the mechanical properties of castings depend on a variety of factors, not only related to the pinhole grade, but also related to the fluctuation of the chemical composition of the alloy, the solidification rate of the casting, the effect of heat treatment, and the existence of other defects, so the same level of pinhole samples Mechanical properties will fluctuate within a fairly large range.
5. The main factors of pinhole formation in aluminum alloy castings
In summary, the pinhole is a kind of casting defect that is easily occurred in the aluminum alloy casting and has a certain impact on the quality of the casting. Hydrogen is the main cause of the pinhole (introduction of some materials, the dissolved gas in the aluminum liquid 80%-90% is hydrogen, and the main source of hydrogen is the decomposition of water vapor. Therefore, the cause of water vapor in the aluminum alloy during the smelting process is the main factor directly affecting the formation of pinholes. The main factors affecting pinhole formation are:
5.1 Influence of Raw Materials and Auxiliary Materials
In the smelting and casting of aluminum alloys, the use of raw materials, auxiliary materials, crystal water in some materials, and the decomposition of aluminum rust AL(OH)2 will generate moisture. There are many types of organic and inorganic excipients in the molding materials. In the casting materials, accessories, paints, etc. due to poor preheating water, etc., in the smelting of aluminum alloy casting, due to the decomposition of water vapor to generate a lot of gas, these gases may cause the casting to produce pores. Although the binder in the paint can increase the thickness of the coating, it also increases the amount of gas generated.
5.2 Influence of Smelting Equipment and Tools
When different smelting equipment melts the aluminum alloy, the amount of suction of the aluminum alloy and the degree of formation of pores are different. The new crucible and the old crucible with rust and dirt should be blown with sand or cleaned by other methods before use, heated to 700°C-800°C, and kept warm for 2h-4 h to remove the water and other chemicals absorbed by the crucible. , Otherwise, it will cause water vapor to generate pores during the smelting and casting. Newly built furnaces usually need to use a few days or weeks for oven drying treatment, otherwise the moisture and chemically combined hydrogen contained in the refractory materials cannot be released, resulting in the formation of pores during smelting.
Melting tools such as ladle, bell jar for degassing, etc., must be cleaned of residual metal, scale, and other contaminants before use; tools used in aluminum-magnesium alloys should be placed on carnallite before use. The flux is washed clean. Then apply a protective coating and preheat and dry. If the preheating is poor, the water sucked by the surface will generate a large amount of gas during the smelting and casting process due to the formation of steam by heating, resulting in the formation of pinholes in the casting.
5.3 Impact of Climate
Under normal circumstances, the amount of hydrogen in the surrounding air is not much, but if the relative humidity in the air is large, the solubility of the gas in the alloy liquid will increase, forming seasonal air holes, such as in the rainy season, due to high humidity, when the aluminum alloy is smelted The phenomenon of pinholes is more serious. Of course, when the air humidity is high, aluminum alloy ingots, smelting equipment, tools, etc. will also increase the amount of surface water adsorption due to air humidity, so it should pay more attention to take strong preheating and drying protective measures to reduce the generation of air holes.
5.4 Effect of Melting Operation
When the aluminum alloy is smelted, since hydrogen needs to be dissolved in the aluminum liquid, it is necessary to strengthen the control of the smelting process and control the amount of inhalation of the aluminum alloy. The production practice shows that the hydrogen absorption of aluminum liquid is performed on the surface. It is not only related to the partial pressure of the aluminum liquid surface, but also has a great relationship with the alloy melting temperature and melting time. The higher the melting temperature of the alloy, the longer the melting time and the holding time of the molten aluminum after melting, the more fully hydrogen is diffused in the aluminum liquid, and the greater the amount of hydrogen absorbed by the aluminum liquid, the greater the probability of occurrence of pinholes. Some people have done experiments, the longer the aluminum liquid storage time, the aluminum alloy internal gas volume increases approximately proportionally. Therefore, under mass production conditions, in order to reduce the absorption of hydrogen when smelting aluminum alloys, it is necessary to strictly enforce the aluminum alloy smelting process regulations. Generally, the holding time of aluminum alloys after melting cannot exceed 3h-5h, and the melting temperature of aluminum alloys cannot be too high. Generally controlled below 760 °C, higher initial smelting temperature should not exceed 920 °C.
5.5 Effect of sand casting molds
The mold has a high moisture content and the higher the amount of hydrogen in the aluminum alloy. Some people use the same furnace alloy cast into different water content of the mold, the hydrogen content of the alloy is determined by a very big difference 3: the mold water content of 5%, the mold hydrogen content of 1.5ml/100g; mold containing When the amount of water is 6%, the hydrogen content in the mold is 2.5 ml/100 g; when the water content of the mold is 8%, the hydrogen content in the mold is 3.0 ml/100 g. Therefore, sand casting aluminum alloy, it is better to use dry or surface drying type, such as the use of wet type, water content should be controlled at 6% or less. This is because during wet casting, due to the low vaporization temperature of moisture, when heated to the melting temperature of molten aluminum, a large amount of gas will be generated in the sand type. As the pressure increases, the volume expands and the gas with a large pressure enters the type. The liquid in the cavity or cavity causes the formation of invasive air holes.
5.6 Effect of Mold Casting Cavity
Since the metal mold has no concession and no air permeability, during the filling and pouring of the metal mold, the gas inside the mold cavity is compressed on the one hand with the filling of the aluminum liquid metal; on the other hand, it is rapidly and strongly heated. As the pressure rises, the resulting backfill pressure will impede the filling of the aluminum liquid metal cavity. When the pressure exceeds a certain limit, the gas may break through the surface layer of the metal fluid stream and escape through the internal gate, destroying the metal fluid continuously. It flows and causes strong oxidation. When the gas passes through the molten metal, if it is blocked by the primary crystal or solidified layer, it will remain in the molten metal to form pores. When casting with a sand-type metal mold, during the filling process of the liquid metal, the sand mold is decomposed by the binder, and the effect of insufficient drying of the coating or preheating of the metal mold increases the amount of gas in the cavity. When the gas inside cannot be fully discharged, the gas will stay in the castings to form pores, and part of the residual gas is enriched between the wall of the mold and the molten metal to form an “air resistanceâ€. These gas resistances cause the castings to be under-slurred or cold. Interval defects.
6. The main measures to prevent the formation of pinholes in aluminum alloy castings
From the above analysis, it can be seen that aluminum alloy castings are prone to pinhole defects. It is related to the nature of the aluminum alloy itself, and it is also related to a series of external factors. In order to avoid or reduce the generation of pinholes in the smelting of aluminum alloys and ensure that the aluminum alloy castings have excellent quality, appropriate preventive measures can be taken to prevent them.
6.1 Preparations for smelting and pouring
6.1.1 Strictly handle the charge according to the requirements of the technical regulations. Before the charge is used, blow sand or other methods to remove the rust, sediment, and other dirt on the surface of the charge, and preheat the burden. Preheat temperature: 350°C-450°C, keep it for more than 3 hours, and prevent moisture and oil pollution. Grade I castings required by QJ169-75 are allowed to use only one-stage return charge. Type II and III castings are allowed to use secondary return charge. However, the total amount of returned material for type II castings is not allowed to exceed 70%. Allowed for the production of basic products.
6.1.2 Before using, ingot molds, smelting tools should be used to clean the surface of oil, dirt and other clean. Preheated to 120 °C -250 °C, coated with protective coating.
6.1.3 New mortars, new furnaces, rusted old mortars, and other methods of blowing sand before use to clean the surface and oven treatment. Generally should be heated to 700 °C -800 °C, heat 2h-4h, in order to remove the water and other chemical substances absorbed by the radon.
6.1.4 Before the enamel, ingot moulds and smelting tools that have been applied must be preheated, the enamel should be preheated to dark red (500°C-600°C); the smelting tools should be preheated to 200°C-400°C for 2h Above (except when using an induction furnace to smelt the alloy, helium may not be preheated.)
6.2 Strict implementation of process rules, and strive to achieve rapid smelting
When the aluminum alloy is smelting, it must strive to achieve rapid smelting and shorten the time spent at high temperatures. When the Al-Mg alloy and other aluminum alloys remain molten for a long time after being melted, the surface of the aluminum alloy needs to be covered with a flux to prevent the aluminum alloy from inhaling. Once an abnormality occurs in the production process, it is necessary to contact the on-site technical personnel in time and take Take decisive measures to deal with it. According to the QJ1182-87 standard, the time required for the melting of each alloy from the beginning of melting to the completion of casting is not more than 4 h for sand casting, 6 h for metal casting, 8 h for die casting, and 760 °C for higher alloy temperatures. Thickness must not be less than 60mm.
6.3 Strengthen Preventive Measures During the Moist Season
In the wet season or when the air is cast in the wet season, we must pay more attention to the precautionary measures to degas, and we must preheat the smelting tools, ingot molds, crucibles, and furnace burdens in strict accordance with the regulations to prevent overheating. Water, oil, etc., cause various types of pinholes.
6.4 Refining degassing to remove gas from aluminum alloy
Under normal circumstances, the so-called "degassing" (also known as "degassing") is to remove the gas in the alloy, "refining" refers to the removal of inclusions in the alloy. Due to the smelting of aluminum alloys, the degassing and refining processes are often combined and carried out. Therefore, these two processes are customarily referred to as refining in production practice. Since the gas in the aluminum alloy is mainly hydrogen, degassing is mainly hydrogen removal. At present, the main method of degassing is to produce a large amount of gas in the aluminum alloy by refining the degassing agent (the gas in the bubbles may be generated by the chemical reaction inside the aluminum liquid, and may also be directly brought in via the partial refining degassing agent. The use of partial pressure principle allows the hydrogen atoms dissolved in the aluminum liquid to diffuse into the air bubbles (at this time, the partial pressure of the air bubbles is zero). Since the specific gravity of the air bubbles is light, when the air bubbles float to the surface of the aluminum liquid, the air bubbles break and the hydrogen gas escapes. Into the atmosphere, the purpose of removing hydrogen is finally achieved.
At present, in order to eliminate the pinholes of aluminum alloy castings, the more commonly used methods are dechlorination with chlorine salt and chloride in the melting process, degassing with chlorine gas and nitrogen, degassing with vacuum, degassing with ultrasonic waves, filtration and degassing, etc. . The use of commonly used refined degasifiers is shown in Table 5. When using chlorine salt and chloride degassing agent to degas, use a bell jar to press the degassing agent into the bottom of the crucible 100mm and move along the circumference of the crucible diameter 1/3 (distant from the inner wall of the crucible) at a uniform speed. In order not to spray a large amount of molten aluminum, the degassing agent may be added in batches, the degassing may be finished and the slag may be removed and allowed to stand for the time specified in Table 6.
6.5 increase the solubility of gas in the alloy
Rapid or high-pressure solidification methods are used to increase the solubility of gases in aluminum alloys, and to promote the elimination of pinholes. The specific method is limited in length, and it does not elaborate too much here.
6.6 Degassing Using Process Methods
Under normal circumstances, sand casting can also be used to degas, such as standing, multi-barrier vents and enlarged feeders. Here is a simple introduction to the precautionary measures of metal mold casting degassing. Due to the non-breathable nature of metal mold casting, precautions must be taken when designing metal molds. The commonly used exhaust methods in production are:
(1) Exhaust by using the clearance of the parting surface or the combination surface of the cavity parts: Because the metal parts are combined, there will always be gaps, the general parting gap is between 0.08mm and 0.15mm, and the movable part gap is Between 0.1mm-0.2mm, these gaps can be used for exhausting, but it is not allowed to expand the gap too much for the exhaust, resulting in blocking of the molten metal, thereby increasing burrs on the casting and reducing the dimensional accuracy of the casting.
(2) Open and exhaust slots: that is, on the parting surface or the combination of cavity parts, the core block and the top rod surface do exhaust slots, so that it can not only exhaust, but also gas, prevent liquid metal inflow, Therefore, it is widely used in metal mold casting and metal mold low pressure casting.
(3) Vent hole: The vent hole is generally located in the upper part of the metal type, or in the metal type, where “gas resistance†may occur.
(4) Design of exhaust plugs: Exhaust plugs are commonly used metal type exhaust facilities. When several exhaust plugs need to be set up on a plane, an exhaust ring can be used instead, and it can be designed at the "air block" of the cavity or on the large plane of the cavity so that the exhaust can be unblocked. If the exhaust plug is designed for the thick part of the casting, the exhaust plug can be made of good thermal conductivity copper, and it can also play a role in strengthening the cooling of the casting. The location and amount of exhaust plug installation is often determined when the metal type is corrected. In the production of metal type small batches, in order to simplify the production of the vent plug, often in the place where the vent plug needs to be set, the drill hole of 5-10 mm drill hole, the plug of water glass sand, can also play the role of exhaust plug The role.
7. Process principles to prevent the formation of blowholes in aluminum alloy castings
The above analysis of the main factors of the formation of air holes in aluminum alloy castings, and a series of appropriate preventive measures are discussed in a targeted manner, the purpose is to prevent the generation of air holes and inclusions in the castings, and obtain good quality castings. Comprehensive analysis from the perspective of the casting process to prevent the formation of stomata and eliminate pores and oxide inclusions, we can use the "anti-", "row", "dissolved" three-word process principles to summarize.
"Defense": It is to prevent moisture and various contaminants from entering the crucible or furnace.
"Row": It is to exclude the inclusion of oxygen and hydrogen in the aluminum liquid, because only effective removal of dispersed inclusions (mainly Al2O3) suspended in the aluminum liquid can prevent the hydrogenation of the aluminum liquid and eliminate the dehydrogenation barrier. Obtain pure aluminum fluid and cast qualified castings. This is what "slag is done and gas is indispensable."
"Solubility": It means that the hydrogen in the aluminum liquid can be partially or completely dissolved in the alloy structure during solidification so as not to form pores in the casting.
Therefore, in the smelting arrangement of aluminum alloys and the selection of “anti-â€, “dischargeâ€, and “dissolve†three sets of technological measures, we must follow the “principle of prevention, priority to row,†but better smelting or Remelting method, focus should still be placed on the word "defense."
Of course, when the aluminum alloy is smelted or remelted, it implements the principle of “mainly prevention and queuing,†and correctly implements three sets of technological measures of “prevention,†“discharge,†and “dissolve,†and must also have excellent smelting operations. Basic skills, basic operations of smelting operations include: preparation and processing of refining equipment, smelting furnace tools, pre-preparation of solvents, modificators, refining, deteriorating slag removal techniques, mixing operation techniques and reasonable pouring, etc. We only have excellent operations. Basic skills can effectively prevent the formation of blowholes in aluminum castings.