In casting production, the porosity and shrinkage of castings are sometimes associated or independent. When porosity and shrinkage occur, it is necessary for us to quickly determine whether the defect is porosity or shrinkage to solve the problem. To quickly identify these two types of defects, one must first have a thorough understanding of their characteristics. Today, we will teach you how to quickly identify porosity and shrinkage defects by recognizing their characteristics.
Stomatal defects and prevention methods
In casting production, porosity defects are common and also one of the defects that cause significant losses to foundries. Pore defects are divided into porosity and shrinkage. Porosity is often caused by the intrusion, wrapping, and involvement of gases in the molten metal. The location of casting porosity on the casting is different, and the reasons for its occurrence are also different. This requires our casting technicians to master the principles behind the occurrence of various types of pores and their unique abilities when determining the cause of porosity defects. Only in this way can the remedy be tailored to the situation and the occurrence of stomatal defects be resolved. Next, let's take a look at the characteristics of air holes generated by different reasons: (1) Entangled air holes: During the filling process, metal liquid forms air holes in the casting due to gas entrapment, which are mostly isolated circular or elliptical large air holes, and their positions are not fixed, generally leaning towards the middle and upper parts of the casting. (2) Invasive pores: pores generated by mold, core, coating, core support, and cold iron invade the surface of the casting, forming pores that are mostly pear shaped or elliptical in size, with smooth pore walls and oxidized surfaces. (3) Reaction pore: A group of pores formed by chemical reactions between certain components inside the metal liquid or between the metal liquid and the mold or core at the interface. The needle shaped or waist shaped reaction pores located on the surface of the casting are called surface needle voids and subcutaneous pores, which are caused by the interface reaction between the metal liquid and the mold and core coatings; Dispersed or clustered needle reaction pores distributed throughout the entire cross-section or in a local area of the casting. Reason for formation (1) Due to the dampness, rust, oil stains, and humid climate of the furnace material, the melting tools and ladle have not been dried, the composition of the metal liquid is improper, and the alloy liquid is refined and insufficient, resulting in a large amount of gas or gaseous substances in the metal liquid, leading to the precipitation of pores or reaction pores in the casting. (2) The mold and core are not fully dried, with poor breathability, poor ventilation, excessive moisture and gas content, undeveloped coatings or excessive gas content, rusty spots, oil stains or undeveloped cold iron and core supports, poor exhaust of the metal mold, and the formation of invading pores in the casting. (3) The pouring system is unreasonable, the pouring and filling speed is too fast, and the metal mold has poor exhaust, which causes turbulence, eddy current, or interruption of the metal liquid during the pouring and filling process, resulting in gas entrapment and the formation of entrapment pores in the casting. (4) The alloy liquid is easily breathable, and effective refining, protection, and purification measures are not taken during the melting and pouring process, resulting in a large amount of gas, slag, and gas inclusions in the metal liquid. During the filling and solidification process, precipitation pores and reaction pores are formed. (5) Improper preparation of molding sand, cores, and coatings results in interface reactions with the molten metal, forming surface pinholes and subcutaneous pores. (6) The pouring temperature is too low, the metal mold temperature is too low, the slag removal of the metal liquid is poor, and the viscosity is too high, which makes it difficult for the gas involved in the pouring and filling process and the gas precipitated from the metal liquid to exhaust the mold or float up to the riser or outlet in time. (7) When melting alloys that are easily aspirated during humid weather seasons, the alloy liquid absorbs a large amount of gas, resulting in batches of castings being scrapped. (8) Excessive addition of resin and curing agent in resin sand, high fluorine content in resin, high angular coefficient and particle size of raw sand and recycled sand, high ignition loss and micro powder content, resulting in high gas generation and low permeability of molding sand. Prevention method (1) During non ferrous alloy melting, furnace materials, solvents, tools, and pouring bags should be fully preheated and dried to remove rust and oil stains. The addition amount of furnace materials for multiple remelting should be appropriately limited. (2) To prevent excessive oxidation and suction of molten metal during the melting process, deoxidation, degassing, and slag removal are carried out. A solvent is added to the surface of the metal melt pool inside the ladle to prevent secondary oxidation, suction, and harmful impurities from returning to the melt pool. When using aluminum to deoxygenate cast steel and cast iron, the residual aluminum content should be strictly controlled. For steel liquid with a severe tendency to absorb air, aluminum deoxygenation should be avoided as much as possible. AVD, VOD, multi hole plug blowing inert gas, powder spraying method, etc. can be used to refine the steel liquid outside the furnace to remove gases and harmful impurities in the steel liquid; For ductile iron, desulfurization should be strengthened to reduce the flow rate of the original soup. On the premise of ensuring spheroidization, the amount of spheroidizing agent added should be minimized as much as possible to reduce the residual magnesium content of the cast iron, and inoculation treatment should be strengthened. (3) During pouring, the metal liquid should not be cut off, and the filling speed should not be too high. The casting pouring position and pouring system should be set to ensure that the metal liquid smoothly fills the mold cavity and facilitates the smooth discharge of gas in the opening cavity. (4) During casting, it is necessary to ensure smooth exhaust of the mold and core. An exhaust channel should be opened inside the sand core, and the gap between the core heads should be filled during mold closing to avoid drilling into the metal liquid blocking the exhaust channel. (5) Increase the height of the sprue to increase the hydrostatic pressure of the filled metal. (6) Reduce the amount of resin and curing agent added to resin sand, use low nitrogen or nitrogen-free resin and raw sand and recycled sand with rounded shape, moderate particle size, low ignition loss, and low micro powder content to reduce the gas generation of resin and improve the permeability of resin sand
Shrinkage defects and prevention methods
There are many reasons for shrinkage and porosity in castings, including casting and pattern design, improper sand box design, gating and riser design, sand mold displacement avoidance, as well as manufacturing reasons, improper mixing of metal chemical components, improper operation in the melting process, and pouring reasons. Due to the numerous causes of shrinkage in castings, it is often time-consuming to identify the root cause. How to quickly determine the internal causes of shrinkage defects in castings requires us to compare multiple cases of shrinkage defects in castings, memorize theoretical features, strengthen learning, and thus improve our understanding and problem-solving abilities for such defects.