The filling process of liquid metal is the first stage of casting formation. Many defects of castings are formed in this process. In order to obtain high-quality and sound castings, this process must be mastered and controlled. To this end, the ability of the liquid metal to fill the mold is studied in order to obtain a well-shaped, well-defined casting that prevents defects during the filling phase.
First, the filling concept Liquid alloy full cavity, the formation of a clear outline, the integrity of the shape of the ability of high-quality castings, known as the fluidity of liquid alloys, also known as filling capacity. The better the fluidity of the liquid alloy, the easier it is to cast a clear, thin, and complex-shaped casting, and it helps the liquid alloy to be replenished when it shrinks in the mold, facilitating the floating of gas and non-metallic inclusions in the liquid alloy. With exclusion. If the fluidity is not good, the castings can be easily deficient in casting, cold septa, porosity, slagging and shrinkage.
Liquid metal filling molds are a complex physical, chemical, and hydromechanical issue involving various properties of the metal fluid, such as density, viscosity, surface tension, oxidizability, oxide properties, and wettability. The size of filling capacity affects the forming of castings. It is difficult for alloys with poor filling ability to obtain large-scale, thin-walled, and complicated structurally sound castings.
The good flow properties make the shrinkage pores produced during the solidification of the castings be complemented by the liquid metal, and the thermal cracking of the castings that is hindered at the end of solidification can be filled and sealed by the liquid metal, which is beneficial to prevent the defects from being produced.
The fluidity of a liquid alloy is usually measured as the length of a spiral flow specimen. As shown in Figure 2-3,
The liquid alloy was injected into the spiral sample mold and the length of the spiral was measured after condensation. For easy measurement, bump marks are made every 50mm on the standard sample. The longer the length of the spiral measured under the same pouring process conditions, the better the fluidity of the alloy. The fluidity of commonly used alloys is shown in Table 2-1. Among them, gray cast iron and silicon brass have the best fluidity, followed by aluminum alloys, and the worst in cast steel.
In general, alloys with good fluidity have strong filling ability; alloys with poor fluidity have poor filling ability
In the actual casting production, it is possible to improve the filling ability by improving the external conditions. According to the requirements of the casting and the filling ability of the alloy, corresponding technological measures are taken to obtain a sound high-quality casting.
Second, the factors affecting the filling ability The factors affecting filling are two ways: first, affect the heat exchange conditions between the metal and the mold, thus changing the flow time of the metal liquid; second, affecting the liquid metal The hydraulic conditions in the mold, thus changing the flow rate of the molten metal. There are many factors affecting liquid metal filling, which can be grouped into four categories: 1 The first type of factors, which are metal properties, mainly include metal density, specific heat, thermal conductivity, latent heat of crystallization, dynamic viscosity, surface tension and crystallization characteristics. Wait
Different alloys have very different fluidities. For the same kind of alloy, the chemical composition is different and its fluidity is also different. When melted to the same temperature above the liquidus, pure metals, eutectic components and compounds have the greatest cavitation capacity, while the alloys at the largest crystallization temperature interval have the smallest filling ability.
The influence of the alloy composition on the flowability is mainly caused by the difference in the crystallization characteristics of the alloy when the composition is different. The pure metal, eutectic composition and compound are solidified at a fixed temperature. The solidified solid layer pushes from the surface of the casting layer to the center, and the interface between the solid and the not yet solidified liquid is clear, and the inner surface of the solid layer is relatively smooth. The flow resistance is small, that is, the flow speed is large. In addition, when these alloys precipitate more solid phases, they stop flowing and the flow time is longer, so their fluidity is good.
When alloys with a wide crystallization temperature range flow in the cavity, due to the presence of well-developed dendrites on the casting cross-section, there are also two-phase regions where the unsolidified liquid and the solid phase are intermingled, and the dendrites are closer to the front of the flow. The more the number is, so when the number of dendrites at the front of the flow reaches a critical value, the metal liquid stops flowing; the wider the crystallization temperature interval of the alloy, the wider the two-phase region, and the more developed the dendrites, the sooner the molten metal will be. The flow stopped, so the liquidity was poor. This is mainly due to the fact that dendrites roughen the inner surface of the solid layer and increase the resistance to liquid alloy flow. The wider the crystallization temperature range of the alloy is, the wider the area where the liquid-solid two phases coexist, and the greater the flow resistance of the liquid alloy, the worse the fluidity. Obviously, the closer the alloy composition is to the eutectic composition, the better the flowability. Figure 2 - 4 shows the relationship between the fluidity and the C content of the Fe-C alloy. It can be seen from Figure 2-4.
With the increase of C content in hypoeutectic cast iron, the crystallization temperature range decreases, and the fluidity increases.
(2) The second type of factors are those of the nature of the mold, such as the heat storage coefficient, density, specific heat, thermal conductivity, temperature, coating layer, and gas-generating property, and air permeability. The resistance of the mold affects the filling rate of the molten metal, and the heat exchange strength between the mold and the liquid metal influences the flow time. Therefore, improving the filling ability of the metal by adjusting the thermophysical properties of the mold can often receive good results. For example, preheating the mold can reduce the temperature difference between the molten metal and the mold, reducing the heat exchange between the two and increasing the mold filling capacity.
The greater the heat-conducting speed of the mold material, the faster the cooling rate of the liquid alloy, thereby making the fluidity worse. For example, the fluidity of the liquid alloy in the metal mold is worse than that in the sand mold; the wall thickness of the casting is too small and the shape is complex, which increases the flow resistance of the liquid alloy and also reduces the fluidity of the alloy. Therefore, when designing a casting, the wall thickness of the casting must be greater than the specified minimum allowable wall thickness, and the shape should be simple.
The high moisture content of cast sand or the poor air permeability of the cast produces a large amount of gas during pouring and cannot be discharged in time, resulting in an increase in the gas pressure in the cavity, which increases the resistance of the flow of the liquid alloy, thereby reducing the fluidity of the alloy. Therefore, improving the air permeability of the mold, reducing the moisture content of the molding sand, and setting up the air outlet, etc., can improve the fluidity of the liquid alloy.
When the mold has a certain gas generating capacity, a gas film is formed between the liquid metal and the mold to reduce the flow friction and facilitate filling. According to the experimental study, when less than 6% of water and less than 7% of pulverized coal are added to the wet sand type, the filling ability of the liquid metal increases, but when the water and coal powder content is too high, the filling ability decreases. When the content of water, pulverized coal, and other organic matter is too high, the cooling rate of liquid metal increases. Under the effect of the heat of molten metal, the gas in the cavity expands, the water in the mold evaporates, and the combustion of pulverized coal and organic matter generates a large amount. Gas, if not discharged in time, will hinder the flow of molten metal
3 The third type of factors, which are the pouring conditions, mainly include the casting temperature of liquid metal, static pressure head, the loss of the head in the pouring system and the influence of external force field force, vacuum, centrifugal, vibration survey, etc.
The pouring temperature has a decisive influence on the filling ability of the liquid metal. In a certain temperature range, the pouring temperature is increased, the overheating heat of the alloy is increased, the heat content per unit volume of the alloy is increased, and the filling capacity increases linearly with the increase of the pouring temperature; after a certain temperature is exceeded, oxidation is serious due to the increase of metal suction. , filling capacity decreases
The larger the filling head, the greater the pressure on the liquid metal in the direction of flow, and the greater the flow rate of the liquid metal, the better the filling ability. The method of increasing the hydrostatic head of the liquid metal is commonly used in production to increase the mold filling capacity. However, when the filling speed of the liquid metal is too high, spraying and splashing may occur, the oxidation of the metal liquid may increase, and the “iron bean†defect may occur, and the cavity The medium gas is not exhausted, the back pressure increases, resulting in insufficient or cold insulation defects
The more complex the structure of the gating system, the greater the flow resistance, and the worse the filling ability when the static head is the same. In the casting of aluminum alloys and magnesium alloys, serpentine and flat runners are often used to make the molten metal flow smoothly. The greater the flow resistance, the significantly decreased filling capacity.
In the thin-wall castings to reduce the lack of pouring, cold septa and other defects important measures. However, the casting temperature is too high, castings easily produce shrinkage, shrinkage, sticky sand, pores, coarse crystals and other defects, in the premise of ensuring that the thin-walled part of the casting can be filled, the pouring temperature should not be too high. The casting temperature range of various alloys is: cast iron is 1230-.14500C; cast steel is 1520-16200C; aluminum alloy is 680--780gC. Thin-walled complex parts to take the upper limit, the thickness of large parts to take the lower limit
(4) The fourth factor, which belongs to the casting structure, mainly includes the thickness of the casting, and the head loss caused by the complexity of the cavity specified by the casting structure.
When the castings have the same volume and the casting conditions are the same, the casting with a large thickness will have a small surface area in contact with the mold. The thinner the casting wall is, the smaller the thickness of the casting is, and it is not easy to be filled. The structure of the casting is complex, and there are many transitional surfaces in the thin wall, the structural complexity of the cavity increases, the flow resistance is large, and the filling ability decreases.
Third, the commonly used measures to improve the replenishment capacity of the factors affecting the filling ability to improve the filling ability measures, can still start from the above four types of factors
1In terms of alloy design, the composition of the alloy can be adjusted to the eutectic composition in accordance with factors such as the size of the casting, the thickness, and the nature of the mold, etc.; certain process measures are taken to refine the alloy grains without affecting the performance of the casting. , also help improve filling ability
Since inclusions affect filling ability, raw materials should be cleaned during smelting and measures should be taken to reduce gas and non-metallic inclusions in liquid metal.
2In the aspect of casting mold, the temperature of the mold is increased for the metal mold and the mold shell, the thermal resistance of the mold is increased by the coating, the exhaust capacity of the mold is increased, and the gas generation rate of the mold during metal filling is reduced. Helps increase filling capacity
3 Pouring conditions, appropriately increasing the pouring temperature, increasing the filling pressure head, and simplifying the pouring system are all conducive to improving the filling ability.
4 casting structure can provide limited measures
It should be pointed out that when adopting the above measures, it often brings other problems. At this time, we must grasp the main contradiction and solve the main problems. Other problems arising therefrom should be secondary, and can be solved by other measures.
In production, especially for castings with high requirements, it is unrealistic to take measures in both alloy composition and casting structure design. For large thin-walled castings, the following three measures are generally used to improve the molding problem:
1 increase the pouring temperature
2 increase the filling rate. The speed here is not the linear speed of the flow, but the volume velocity of the filling. Increase the area of ​​the gate and fill it quickly when the line speed is low
3Use coating to increase the thermal resistance of the mold
C steel purlin
C Steel Purlin is composed of automatic processing C steel purlin molding machine molding. The C steel purlin molding machine can automatically complete the molding process of the C steel purlin according to the given size of the C steel. C steel purlin is formed by bending coil thin wall, light weight, high strength, excellent section properties, compared with traditional channel, the same strength can save 30% of the material.
Description of C steel purlin:
Material: Q235, Q345
Size: C120-320
Thickness: 0.8-3mm
Length: 1-12m according to your requirement
Standard: AISI ASTM BS DIN GB JIS ETC
Surface: Painted or hot dip galvanized
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Package:Standard seaworthy bulk package by 20ft/40ft container or as customer request
Delivery time: Within 15 days after we get your deposit
Payment:T/T, L/C, Western Union
FAQ:
1. Q: Are you a manufacturer?
Yes, we are a manufacture with our own steel structures factory.
2. Can you do the OEM producing?
Yes, we can.
3. What`s the MOQ?
Usually,it`s 25tons.
C Steel Purlin
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Shijiazhuang Zhouming Steel Building Materials Co., Ltd. , http://www.zmsteels.com