In the production of large aluminum bronze castings, induction furnaces are the preferred choice due to their strong electromagnetic stirring, rapid heating, and ability to ensure chemical homogeneity. Namun, the tendency of aluminum bronze (seperti ZCuAl10Fe3) to oxidize and absorb gas easily presents significant challenges.
Technical analysis of porosity control and process optimization in aluminum bronze smelting
SAYA. Porosity Mechanisms and Refining/Degassing Processes
Porosity in aluminum bronze is primarily caused by hydrogen (H₂) and entrapped alumina (Al₂O₃) films. At temperatures near the melting point, aluminum reacts readily with water vapor:
2Al + 3H₂O → Al₂O₃ + 6[H]
1. Dynamic Refining and Degassing
While electromagnetic stirring ensures a uniform melt, it can also pull surface oxide films deep into the melt.
- Inert Gas Purging: Toward the end of the melting process, use a graphite tube to inject high-purity argon (Ar) or nitrogen (N₂) into the bottom of the furnace. Rising bubbles carry away dissolved hydrogen via partial pressure differences.
- Flux Refining: Use a covering flux composed of chlorides and fluorides (MISALNYA.,NaCl, KCl, Na₃AlF₆). This isolates the melt from the air and uses physicochemical action to wet and adsorb floating alumina dross.
2. Static Holding
Before pouring, the power should be cut or held at low power for a short duration. This allows tiny bubbles and inclusions to float to the surface due to density differences.
Ii. Furnace Lining Selection and Slag Buildup (Hanging Slag) Pencegahan
Alumina (Al₂O₃) buildup is a notorious issue in aluminum bronze melting. Slag “pengerasan kulit” adheres to the furnace walls, reducing furnace volume, lowering electrical efficiency, and potentially causing severe inclusion-related porosity if the buildup peels off into the melt.
1. Recommended Materials
- Neutral or Basic Linings: Avoid acidic (silica-based) linings because aluminum will reduce Sio₂. Recommended materials include high-alumina (Al₂O₃) atau magnesium-aluminum spinel (MgO · Al₂O₃) dry-ramming mixes.
- Density Requirements: The particle size distribution of the lining material must be precisely graded to minimize physical penetration by the melt.
2. Anti-Buildup Techniques
- Surface Smoothness: Ensure the inner wall of the furnace is as smooth as possible during the lining process.
- Anti-Sticking Flux: Use specialized fluxes that alter the surface tension of the oxide slag, keeping it in a liquid or loose state for easier removal.
- Thermal Stability: Minimize drastic temperature fluctuations, which can cause micro-cracks in the lining where oxide slag can “take root.”
AKU AKU AKU. Temperature Control Strategy
Temperature is the most direct lever for controlling porosity.
| Fase | Control Point | Logic |
| Meleleh | Rapid melting; strictly avoid overheating | Reduces exposure time at high temperatures and lowers gas absorption rates. |
| Pengilangan | 1150°C – 1200°C | Ensures flux activity and facilitates slag-metal separation. |
| Penuangan | 1100°C – 1150°C | Adjusted based on casting wall thickness; avoid high temperatures to prevent secondary gas absorption. |
Prinsip Inti: “Melt fast, Control temp, Pour quickly.” Leverage the high power density of tungku induksi to minimize the time the metal spends above 1200° C..
Iv. Comprehensive Guidelines for Porosity Prevention
- Charge Pre-treatment: Biaya tungku (including returns) must be clean, dry, and free of oil. Preheating the charge to above 200° C. effectively removes surface moisture.
- Aluminum Addition Timing: Since aluminum oxidizes easily, it is typically added late in the melting process and immediately covered with flux.
- Gating System: For large aluminum bronze parts, gunakan a bottom-pour gating system to maintain a smooth filling process and prevent turbulence that traps air.
- Mold Preparation: Sand molds must be thoroughly dried. If using metal molds, apply an even coating and preheat them.
Expert Note: Oxidation and gas absorption in aluminum bronze are inextricably linked. If the flux cover is insufficient, broken oxide films will carry moisture into the melt, generating localized hydrogen. Karena itu, maintaining a complete flux cover on the melt surface is the top priority.
