Aluminum is one of the most commonly used metals in investment casting. In investment casting (lost wax casting), aluminum and its alloys are widely used in aerospace, automotive, electronics and consumer products due to their light weight, high thermal conductivity, corrosion resistance and good castability.
The following is a detailed analysis of suijin on the key characteristics of aluminum metal in investment casting, common alloys, process points and application cases:
I. Advantages of aluminum metal in investment casting
Lightweight: Aluminum metal has a low density (about 2.7 g/cm³), which is suitable for manufacturing lightweight parts (such as aviation structural parts, automobile engine parts).
High thermal conductivity and electrical conductivity are used in scenarios such as radiators and electronic housings that require rapid heat conduction.
Aluminum surface is easy to form a dense oxide film (Al₂O₃), which has good resistance to atmospheric and chemical corrosion.
Good fluidity, moderate shrinkage (about 6-8%), and good casting performance are suitable for complex thin-walled parts molding.
Scrap aluminum can be 100% recycled and reused, in line with sustainable development needs.
II. Common aluminum alloys for investment casting
Aluminum-silicon system (Al-Si): best fluidity, low shrinkage (such as A356, A357).
Aluminum-copper system (Al-Cu): high strength, but poor castability (such as 201.0, 204.0).
Aluminum-magnesium system (Al-Mg): strong corrosion resistance (such as 514.0, 520.0).
Aluminum-zinc system (Al-Zn): natural aging hardening (such as 713.0).
III. Key points of aluminum investment casting process
Melting and pouring
Temperature control: The temperature of aluminum liquid is usually 680-750°C to avoid overheating and oxidation inclusions.
Degassing treatment: introduce argon or use a rotary degasser to reduce hydrogen pores (aluminum liquid is easy to absorb hydrogen).
Pouring speed: fast filling is required to reduce the risk of oxide film rupture.
Mold and shell design
Shell material: silica sol or aluminosilicate ceramic slurry, good high temperature stability (sintering temperature 900-1100°C).
Shell thickness: complex parts require multi-layer coating (5-8 layers) to ensure a balance between strength and permeability.
Post-processing
Heat treatment: T6 treatment (solid solution + artificial aging) significantly improves strength (such as A357-T6 strength up to 345 MPa).
Surface treatment: anodizing, sandblasting or electroplating to enhance wear resistance and aesthetics.
IV. Challenges and solutions for aluminum investment casting
1. Oxidation inclusions (Al₂O₃ defects)
During the smelting and pouring process, aluminum liquid easily reacts with oxygen to form an aluminum oxide film (Al₂O₃), resulting in inclusions inside the casting, reducing mechanical properties and surface quality. Inert gas protection is its solution. During smelting, argon or nitrogen is introduced to cover the surface of the aluminum liquid (such as using a rotary degasser + Ar mixed gas). Use vacuum pouring technology (vacuum degree <10⁻² mbar) to completely isolate oxygen (such as aerospace precision castings). Add a NaCl-KCl composite flux layer to absorb oxides and form a protective barrier. Use a bottom pouring gate or a serpentine runner to reduce aluminum liquid turbulence (the probability of oxide film rupture is reduced by 50%).
2. Shrinkage and shrinkage (solidification shrinkage defects)
The solidification shrinkage rate of aluminum alloy is relatively high (6-8%), and thick and large cross-sectional areas are prone to internal voids due to insufficient shrinkage compensation. The riser position can be optimized through simulation software (ProCAST/MAGMASOFT) to ensure that the thick wall area solidifies last. Place graphite chilled iron in the hot zone to accelerate local cooling (shrinkage volume is reduced by 40%). Spray zirconium oxide chilled coating inside the shell (cooling rate is increased by 2-3 times). Add trace strontium (Sr) or titanium (Ti) to refine the grains (such as A356+0.02% Sr, shrinkage rate is reduced by 30%).
3. Thermal cracks (solidification stress cracking)
Complex castings cool unevenly due to differences in wall thickness, and internal stress exceeds the tensile strength of the material. Suijin's solution is to select low-stress alloys and use Al-Si alloys (such as A357). A silicon content of 7% can improve crack resistance. The shell preheating temperature is increased from 200°C to 450°C to reduce the cooling gradient (crack rate is reduced by 60%). Optimize the structure, round corner design (R≥3mm) to avoid stress concentration, and the slope of the thin-thick transition zone is ≤15°. After the casting is shelled, apply 20-50Hz mechanical vibration to release residual stress (crack risk is reduced by 70%).
4. Surface roughness and dimensional accuracy
The inner surface defects of the ceramic shell (such as slurry bubbles) are transmitted to the casting, affecting the accuracy (Ra>6.3μm). Suijin recommends high-precision shell preparation, using nano-scale silica sol (particle size <50nm) slurry, and the shell surface finish Ra<1.6μm. Using 3D printed ceramic shells (such as ExOne S-Max Flex), the resolution reaches 140μm and the dimensional error is ±0.1mm. Electrolytic polishing of castings (voltage 12V, time 5min), Ra can be reduced from 6.3μm to 0.8μm.
5. Complex thin-walled structures are difficult to form
Although aluminum alloys have good fluidity, they are prone to incomplete filling or cold shut when the wall thickness is <1mm. Low-pressure/vacuum assisted pouring is required, and the pressure is controlled at 0.5-1.2 bar, and the filling speed is increased by 30% (suitable for thin-walled parts of drone frames). Optimize the permeability of the shell, add 30% mullite fiber to the ceramic slurry, and the permeability is increased from 15 cm³/(min·cm²) to 45 cm³/(min·cm²). Ultra-fine treatment of alloys, so that electromagnetic stirring + ultrasonic vibration can jointly refine the grains (grain size is reduced from 200μm to 50μm), and the fluidity is increased by 25%.
V. Future Trends
Development of high-strength lightweight alloys
Nano-enhanced aluminum-based composite materials (such as Al-SiC) to improve wear resistance and high-temperature performance.
Digital process optimization
AI-based casting simulation predicts shrinkage and automatically optimizes the casting system.
Green casting technology
Ceramic shell mold with bio-based binder reduces carbon emissions from roasting.
Aluminum metal has become a core material for high-end manufacturing in investment casting due to its lightweight, high thermal conductivity and excellent casting performance. Through alloy optimization (such as AlSi10Mg), process innovation (vacuum casting) and post-processing strengthening (T6 heat treatment), aluminum castings can meet the stringent requirements of aerospace, automotive and other fields. In the future, with the integration of new materials and digital technologies, aluminum investment casting will further improve efficiency and sustainability.