What causes porosity defects in bronze castings?

Pores in copper castings (including brass, bronze, purple copper, etc.) are common casting defects, usually caused by gas evolution in the molten metal, poor exhaust of molding sand or molds, improper melting processes, and other factors. The following are specific reasons and solutions:

1、 Types and characteristics of stomata 1 Characteristics of precipitating pores: small, dispersed, circular or elliptical, mostly located in thick parts of castings or at the final solidification point. Reason: Gases dissolved in copper liquid (such as H ₂ CO、 Water vapor precipitates and forms bubbles during solidification.  

2. Characteristics of reactive pores: Smooth or oxidized pore walls, often appearing on or near the surface of castings. Reason: Copper liquid reacts chemically with molding sand, coating or slag to generate gases (such as CO ₂, SO ₂).  

3. Characteristics of rolled in pores: irregular shape, often accompanied by slag inclusions, distributed along the direction of metal flow. Reason: During the pouring process, gas is drawn into the molten metal (such as turbulent pouring and poor exhaust).  

2、 Main cause analysis

1. Hydrogen absorption during the smelting process (key factor): Copper liquid is highly prone to absorbing hydrogen gas at high temperatures (especially copper and tin bronze), and the solubility of hydrogen drops sharply during solidification, forming pores. Source: The furnace material is damp, oily, or contains organic matter (such as recycled copper containing oil and grease). The melting environment has high humidity (such as not dehumidifying during the rainy season). Insufficient fuel combustion (gas furnace, coke furnace produce water vapor).  

2. Insufficient deoxidation results in the oxidation of copper liquid to form Cu ₂ O, which reacts with hydrogen: Cu ₂ O+H ₂ → 2Cu+H ₂ O ↑ * *, and water vapor forms pores. Commonly seen in: phosphor bronze (requiring phosphorus deoxidation), brass (insufficient zinc boiling deoxidation).  

3. Improper design of the pouring system can result in excessive pouring speed, high gate height, or insufficient cross-sectional area of the sprue, leading to turbulent flow of the molten metal and entrainment of air. Insufficient riser or exhaust channels prevent gas from escaping.  

4. Sand/mold problems: Poor air permeability of sand molds (such as high compactness and poor collapsibility of sodium silicate sand). When resin sand or oil sand is cast, the binder emits a large amount of gas (such as H ₂ and CH ₄ produced by the high-temperature decomposition of furan resin). When casting metal molds, the mold does not have exhaust grooves or the coating is too thick.  

5. Improper process operation: The pouring temperature is too high (exacerbating hydrogen absorption) or too low (the gas cannot float up in time). Not fully allowed to settle (poured without degassing the copper liquid). 3、 Solution

1. Smelting control degassing refining: Purple copper/bronze: Deoxidize with phosphorus copper (P-Cu) or refine with nitrogen/argon gas. Brass: Utilize the "self boiling" effect of zinc to remove hydrogen and control the melting temperature (brass ≤ 1100 ℃). Drying furnace materials: Waste copper needs to be roasted to remove oil stains, and the furnace lining and tools need to be preheated before melting. Cover protection: Cover the copper liquid with charcoal or glass slag during smelting to isolate water vapor.  

2. The optimization of the pouring system adopts bottom injection or stepped pouring system to reduce turbulence. Increase the cross-sectional area ratio of the transverse and inner runners (e.g. 1:2:1.5) and reduce the flow velocity. Set up slag collection bags and exhaust risers (especially in thick and large areas).  

3. Sand casting/mold improvement: Control the moisture content of the sand (≤ 4.5%) and add breathable materials (such as coal powder and sawdust). Metal mold casting: The mold is equipped with an exhaust groove (depth 0.1~0.3mm) and coated with zinc oxide paint. Resin sand: Reduce the amount of resin added or switch to low nitrogen resin.  

4. Process parameter adjustment pouring temperature: 1200~1250 ℃ for copper, 980~1050 ℃ for brass, 1100~1180 ℃ for bronze. Slowly cool after pouring (such as covering with insulation sand) to extend the gas release time.  

5. Auxiliary measures for melt detection: Use vacuum solidification test method to check the gas content of copper liquid. Post processing: Hot isostatic pressing (HIP) is performed on key castings to eliminate internal porosity. 4、 Typical case of brass (Cu Zn) porosity: Zinc volatilization leads to insufficient "zinc boiling" and residual hydrogen gas → Zn content needs to be controlled (≤ 40%), and stirring should be strengthened during melting. Tin bronze (Cu-Sn-P) porosity: Insufficient phosphorus deoxidation or Sn oxidation → 0.03%~0.05% phosphorus copper needs to be added for rapid casting to reduce oxidation.  

By systematically investigating processes such as melting, shaping, and pouring, it is possible to significantly reduce porosity defects in cast copper. If the problem persists, it is recommended to further locate the gas source through metallographic analysis of pore composition (such as energy dispersive spectroscopy).