In aluminum casting, molten aluminum has the characteristics of high temperature (700°C), strong chemical activity, easy oxidation, and good wettability to refractory materials (easy adhesion and penetration). This places extremely strict requirements on the material of the riser tube:
Core performance requirements:
1: Excellent high-temperature strength: When working for a long time above 700°C, it must maintain sufficient structural strength to resist the static pressure of aluminum liquid and filling pressure, without deformation or fracture.
2: Excellent thermal shock resistance: The riser tube needs to repeatedly undergo drastic temperature changes from room temperature to high temperature (aluminum liquid immersion/filling) and then to room temperature (replacement/maintenance). The material must be able to withstand such thermal shock without cracking or peeling.
3: Excellent resistance to aluminum liquid erosion/wettability: The material must be able to resist the chemical erosion and physical dissolution of high-temperature aluminum liquid, and be not easily wetted by the aluminum liquid (with a large contact Angle), preventing the aluminum liquid from seeping into the material's pores or climbing along the surface, which could lead to "fire leakage", blockage, thickening of the pipe wall, or even fracture.
4: Good resistance to alumina adhesion: An alumina film is constantly formed on the surface of the molten aluminum. These films tend to adhere to the inner wall of the riser pipe, gradually accumulate and thicken, eventually blocking the pipe or changing its inner diameter, affecting the filling stability and flow rate. The material needs to be not prone to adhering to aluminium oxide.
5: Low porosity/high density: Reduces the channels for molten aluminum to penetrate, enhancing erosion resistance and mechanical strength.
6: Good dimensional stability and wear resistance: Ensures stable inner diameter and long service life.
7: Certain chemical inertness: Does not undergo harmful reactions with molten aluminum or protective atmospheres.
Common material types and analysis:
Al₂TiO₅ Aliumina Titanate Riser Tube:
Characteristics and Advantages:
Ultra-low coefficient of thermal expansion, close to zero expansion.
It offers excellent thermal shock resistance and can withstand severe temperature cycles (room temperature →700 ° C molten aluminum → room temperature).
Low thermal conductivity:reduce heat loss and maintain the temperature stability of molten aluminum.
Chemical inertness:it has poor wettability to molten aluminum (with a large contact Angle) and is not easy to adhere to the aluminum oxide (Al₂O₃) film.
Defects and Challenges:
Defects and Challenges
Insufficient high-temperature strength
Decomposition sensitivity problem
Low fracture toughness
The manufacturing difficulty is high.
Applicable scenarios
Only for low-temperature or short-cycle operating conditions:
It is suitable for small castings with a filling temperature of no more than 700℃ and a slow production rhythm (such as low-pressure casting small shells).
Si₃N₄ silicone nitride riser tube
Characteristics and Advantages:
High strength and high toughness
It maintains excellent flexural strength (>500 MPa) from room temperature to 1200℃ and has strong resistance to mechanical impact.
Outstanding thermal shock resistance
A medium coefficient of thermal expansion (3.2×10⁻⁶/℃) combined with high thermal conductivity (30 W/m·K) alleviates thermal stress.
Outstanding erosion resistance
A SiO₂ passivation film forms on the surface, preventing the penetration of molten aluminum and chemical erosion.
Low wettability
The contact Angle with molten aluminum is greater than 90°, significantly reducing the risk of alumina adhesion.
Defects and Challenges
The cost is extremely high
Restrictions on large-sized components
The resilience is still insufficient
Applicable scenarios
High-performance special demand fields
Small precision castings with strict strength requirements for aerospace, military industry and other fields.
Si₃N₄-bonded SiC riser tube
Characteristics and Advantages:
Optimal balance of strength, toughness and corrosion resistance:
Silicon carbide skeletons offer ultra-high hardness, wear resistance and thermal conductivity (120 W/m·K).
Silicon nitride bonded phase fills the pores, enhances toughness and blocks the penetration of molten aluminum.
Ultimate thermal shock resistance
The high thermal conductivity of SiC rapidly disperses heat, and Si₃N₄ buffers thermal stress, withstanding over a thousand thermal cycles.
Top-notch resistance to aluminum liquid erosion
A dense Si₂ON₂/SiO₂ layer forms on the surface, with a wetting Angle greater than 110°, and it hardly adheres to alumina.
Optimal lifespan and cost-effectiveness
The service life reaches over 3,000 furnace cycles (aluminum titanate < 500 cycles), and the comprehensive cost is lower than that of pure Si₃N₄.
Defects and Challenges:
The manufacturing cost remains relatively high
The reaction sintering process requires precise control of the silicon infiltration process and has a high technical threshold.
Low-temperature oxidation sensitive
Slight oxidation may occur below 600℃ (without affecting the main performance).
Applicable scenarios:
The absolute mainstream materials for aluminum casting:
It covers over 90% of automotive parts (hubs, cylinder blocks, etc.) and civilian industrial fields.
First choice for mass production
It is especially suitable for automated production lines, ensuring high stability and low downtime maintenance costs.
