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Aluminum castings are widely used in aerospace, automobile manufacturing, machining and many other fields due to their advantages of light weight, high strength and corrosion resistance. However, in the production process of aluminum castings, due to the influence of various factors, some defects often occur, which not only affect the appearance quality of aluminum castings, but also may reduce their performance and service life, causing huge economic losses to the production enterprises. This article will analyze in detail the most common defect types of aluminum castings, provide root causes and solutions. And help you optimize the production process, reduce costs, and improve product qualification rate.
Round or oval holes appear inside or on the surface of the casting, with varying sizes and numbers, reducing the mechanical properties and sealing.
● Melting Process: During the melting process, if the melting temperature is too high, the melting time is too long or the stirring is uneven, the aluminum liquid will absorb a large amount of gas, such as hydrogen, oxygen, etc. In addition, if the surface of the charge is not clean and contains impurities such as oil, water, etc., it will also produce gas during the melting process and enter the aluminum liquid.
● Pouring Process: During pouring, if the pouring speed is too fast or the pouring system is not designed properly, the aluminum liquid will generate turbulence during the filling process, and a large amount of air will be drawn in. At the same time, if the mold is not vented smoothly, the gas in the cavity cannot be discharged in time, which will also form pores in the casting.
● Mold Factors: Rough mold surface, uneven coating or excessive gas emission of coating will cause the aluminum liquid to react with the gas on the mold surface during the filling process, resulting in pores. In addition, mold temperature that is too high or too low will also affect the filling and solidification process of the aluminum liquid, increasing the possibility of pores.
● Optimize the smelting process: Strictly control the smelting temperature and time to avoid overheating of the aluminum liquid and long smelting time. During the smelting process, use refining agents to refine the aluminum liquid to remove the gas and impurities. At the same time, ensure the quality of the charge, strictly inspect and clean the charge, and remove impurities such as oil and moisture on the surface.
● Improve the pouring system: Rationally design the pouring system so that the aluminum liquid can flow smoothly during the filling process to avoid turbulence and air entrainment. Increase the cross-sectional area of the pouring system, reduce the flow rate of the aluminum liquid, and reduce the possibility of air entrainment. At the same time, set filters in the pouring system, such as ceramic filters, foam ceramic filters, etc., to filter out impurities and gases in the aluminum liquid.
● Improve mold exhaust: Optimize the mold structure, increase the number and size of exhaust grooves and exhaust holes, and ensure that the gas in the cavity can be discharged in time. Use mold materials with good air permeability, such as graphite molds, ceramic molds, etc., to improve the exhaust performance of the mold. In addition, apply an appropriate amount of mold release agent on the mold surface to reduce the friction between the aluminum liquid and the mold surface and reduce the probability of gas generation.
Irregular holes (shrinkage cavities) or small loose parts (shrinkage porosity) appear in the thick walls of castings, affecting the structural strength.
● Solidification shrinkage: Aluminum will shrink in volume during the solidification process. If the structural design of the casting is unreasonable, such as uneven wall thickness, the presence of hot spots, etc., it will lead to inconsistent solidification sequence and form shrinkage cavities and shrinkage in thick wall parts or hot spots.
● Insufficient shrinkage compensation: Improper setting of risers and chillers cannot provide sufficient shrinkage compensation for castings, so that castings cannot get sufficient liquid metal during solidification, resulting in shrinkage cavities and shrinkage. In addition, pouring temperature that is too high or too low will also affect the fluidity and shrinkage compensation capacity of aluminum liquid, increasing the probability of shrinkage cavities and shrinkage.
● Alloy composition: If the content of certain elements in aluminum alloy is too high or too low, it will affect the solidification characteristics of the alloy, such as solidification temperature range, shrinkage rate, etc., thus leading to shrinkage cavities and shrinkage. For example, if the silicon content in aluminum silicon alloy is too low, the solidification temperature range of the alloy will increase, the shrinkage rate will increase, and shrinkage cavities and shrinkage will easily occur.
● Optimize casting structure: When designing castings, try to make the wall thickness uniform to avoid hot spots. For unavoidable hot spots, you can use methods such as setting reinforcing ribs and inserts to improve the solidification sequence of the casting and reduce the occurrence of shrinkage cavities and shrinkage.
● Reasonable setting of risers and chills: According to the structure and solidification characteristics of the casting, the position, size and shape of the risers should be reasonably designed to ensure that the risers can provide sufficient shrinkage compensation for the casting. At the same time, chills should be set at the thick-walled parts and hot spots of the casting to speed up the cooling of these parts, promote sequential solidification, and reduce the formation of shrinkage cavities and shrinkage.
● Control the pouring process parameters: Strictly control the pouring temperature and pouring speed to ensure that the aluminum liquid has good fluidity and shrinkage compensation ability. The pouring temperature should not be too high to avoid excessive shrinkage and air absorption of the aluminum liquid; the pouring speed should be moderate to avoid being too fast or too slow. In addition, during the pouring process, multiple pouring and supplementary pouring can be used to improve the shrinkage compensation effect of the casting.
Cracks appear on the surface or inside the casting, which may lead to breakage in severe cases.
● Thermal stress: Thermal stress will be generated during the solidification and cooling process of the casting due to uneven temperatures in different parts. If the thermal stress exceeds the yield strength of the aluminum casting, it will cause cracks in the casting. The magnitude of thermal stress is related to factors such as the structure of the casting, wall thickness, cooling rate, and the thermophysical properties of the alloy.
● Mechanical stress: During the demoulding, cleaning and processing of castings, if they are subjected to excessive external forces, such as unbalanced ejection, collision during transportation, etc., mechanical stress will be generated in the castings. When mechanical stress is superimposed on thermal stress, cracks are more likely to occur.
● Alloy characteristics: Some aluminum alloys have a greater tendency to hot cracking, such as aluminum-copper alloys, aluminum-magnesium alloys, etc. During the solidification process of these alloys, due to the segregation of solute elements, low-melting-point eutectic structures will be formed at the grain boundaries, reducing the strength of the grain boundaries, thereby increasing the possibility of hot cracking. In addition, excessive content of impurity elements in the alloy will also reduce the plasticity and toughness of the alloy, increasing the risk of cracking.
● Reduce thermal stress: Optimize the structural design of castings, reduce wall thickness mutations and sharp corners, so that the castings can dissipate heat evenly during solidification and cooling, and reduce the generation of thermal stress. Reasonably control the mold temperature and cooling rate to avoid excessive thermal stress caused by rapid temperature changes during the cooling process. For large or complex castings, segmented cooling, slow cooling and other methods can be used to reduce thermal stress.
● Avoid mechanical stress: During the demoulding, cleaning and processing of castings, be careful to avoid applying excessive external force to the castings. Ensure the balance of the ejection system to prevent the castings from being subjected to uneven forces and cracks during the ejection process. When transporting and storing castings, take appropriate protective measures to avoid collision and extrusion of the castings.
● Adjust alloy composition: For aluminum alloys with a high tendency to hot cracking, the hot cracking sensitivity of the alloy can be reduced by adjusting the alloy composition. For example, the copper content can be appropriately increased in aluminum-copper alloys, and the magnesium content can be controlled within a suitable range in aluminum-magnesium alloys. At the same time, appropriate trace elements such as titanium and boron can be added to refine the grains and improve the plasticity and toughness of the alloy.
The casting contains non-metallic impurities (such as oxides and sand particles), which reduce the mechanical properties.
● Smelting process: During smelting, if the charge is not clean and contains a lot of oxides, slag and other impurities, or if the smelting tools are not thoroughly cleaned, these impurities will be mixed into the aluminum liquid. In addition, during the smelting process, the aluminum liquid reacts with the furnace gas to generate oxide inclusions such as aluminum oxide.
● Pouring process: Improper design of the pouring system, such as improper gate position, too small runner cross-sectional area, etc., will result in the aluminum liquid being unable to effectively block impurities from the mold cavity during the filling process, causing the impurities to enter the casting with the aluminum liquid. At the same time, improper operation during pouring, such as excessive stirring of the aluminum liquid, unstable pouring, etc., will also cause impurities such as slag to be drawn into the aluminum liquid.
● Mold factors: Impurities such as sand and paint particles on the mold surface may be washed into the casting during the aluminum liquid filling process, forming inclusions. In addition, mold wear will also produce metal debris, which will mix into the aluminum liquid and also cause inclusion defects.
● Strengthen smelting management: Strictly control the quality of the charge, conduct strict inspection and screening of the charge, and remove impurities and oxides. During the smelting process, use effective slag removers to remove slag from the aluminum liquid to ensure the purity of the aluminum liquid. At the same time, regularly clean the smelting tools and furnace body to prevent impurities from mixing into the aluminum liquid.
● Optimize the pouring system: Rationally design the pouring system so that the aluminum liquid can effectively block impurities from the mold cavity during the filling process. Set up slag blocking devices in the pouring system, such as slag collection bags and filters, to filter out slag and impurities in the aluminum liquid. In addition, control the pouring speed and pouring method to avoid excessive stirring of the aluminum liquid and reduce the possibility of impurities being involved.
● Strengthen mold maintenance: Clean and maintain the mold regularly to remove impurities such as sand and paint particles on the mold surface. Repair the worn parts of the mold in time to prevent metal debris from mixing into the aluminum liquid. Before using the mold, preheat the mold to ensure that the mold surface is dry and clean to reduce the occurrence of inclusion defects.
Incompletely fused seams appear on the surface of the casting, affecting strength and sealing.
● Liquid aluminum fluidity: Insufficient liquid aluminum fluidity is one of the main reasons for cold shut and insufficient pouring. Liquid aluminum fluidity is related to factors such as pouring temperature, alloy composition, mold temperature, and the structure of the pouring system. If the pouring temperature is too low, the content of impurity elements in the alloy is too high. The mold temperature is too low, or the resistance of the pouring system is too large, the liquid aluminum fluidity will deteriorate and it will be difficult to fill the entire cavity.
● Pouring speed: If the pouring speed is too slow, the aluminum liquid will dissipate heat too quickly during the filling process, the temperature will drop, and the fluidity will further decrease, thus leading to cold shut and insufficient pouring. In addition, if there is a pause during the pouring process, an oxide film will form on the surface of the aluminum liquid that enters the cavity first, hindering the fusion of the subsequent aluminum liquid and causing cold shut defects.
● Mold structure: Unreasonable structural design of the mold, such as complex cavity, uneven wall thickness, poor venting, etc., will increase the difficulty of filling the mold with molten aluminum, making it difficult for the molten aluminum to be smoothly filled into various parts, thus leading to cold shut and insufficient pouring.
● Improve the fluidity of aluminum liquid: Increase the pouring temperature appropriately to ensure that the aluminum liquid has good fluidity. However, pay attention to controlling the pouring temperature to avoid excessively high temperatures that may cause the aluminum liquid to absorb air and oxidize. Optimize the alloy composition and select aluminum alloy materials with good fluidity. At the same time, increase the mold temperature to reduce the heat loss of the aluminum liquid during the filling process and improve the fluidity of the aluminum liquid.
● Adjust the pouring speed: According to the structure and size of the casting, reasonably adjust the pouring speed to ensure that the aluminum liquid can quickly and smoothly fill the mold cavity. During the pouring process, the continuity of pouring should be maintained to avoid pauses. For thin-walled or complex castings, special pouring processes such as high-pressure pouring and vacuum pouring can be used to improve the filling capacity of aluminum liquid.
● Optimize mold structure: Optimize the mold design, simplify the cavity structure, and reduce the resistance of aluminum liquid filling. Reasonably set the gate and exhaust system to ensure that the aluminum liquid can enter the cavity smoothly and the gas in the cavity can be discharged in time. For some parts that are difficult to fill, auxiliary gates, overflow troughs, etc. can be used to improve the flow of aluminum liquid.
The common defects of aluminum castings are the result of the combined effect of multiple factors. Which seriously affect the quality and performance of aluminum castings. Through in-depth analysis of the characteristics and causes of common defects such as pores, shrinkage and shrinkage, cracks, inclusions, cold shuts and insufficient pouring, and taking corresponding solutions, the defects of aluminum castings can be effectively reduced and the quality and production efficiency of aluminum castings can be improved.
