Low-pressure casting is a metal casting process, mainly used to produce high-quality aluminum alloy, magnesium alloy and copper alloy castings. Compared with traditional gravity casting and high-pressure die casting, low-pressure casting has the advantages of dense casting structure, high yield, excellent mechanical properties, easy mechanization and automation, etc., and is widely used in industrial fields such as automobiles, aviation, machinery, and weapons.

1. The basic principle of low-pressure casting

Low-pressure casting uses the air pressure in a sealed crucible to slowly press the liquid metal from the bottom into the metal mold above to complete the filling and solidification. Its working principle is as follows:

Add a sealing cover to the top of the crucible to maintain a certain airtightness;

A certain low-pressure gas (such as air, nitrogen) is passed into the upper part of the crucible through a pressure pipe to form pressure above the metal liquid surface;

Liquid metal passes through the riser immersed in the crucible, rises along the riser under the action of air pressure, and enters the mold cavity;

Maintain air pressure until the metal is completely solidified;

Decompress or release the pressure, and the remaining unsolidified metal liquid flows back into the crucible, and then the casting is taken out.

The air pressure is generally controlled between 0.02 and 0.06MPa, and the filling process is stable and controllable, thereby reducing defects such as slag inclusions and oxide interlayers.

2. Process flow of low-pressure casting

The typical process flow of low-pressure casting is as follows:

1. Mold preparation

The mold usually uses a metal mold (such as a steel mold) with good thermal conductivity and strength. The mold needs to be preheated before each pouring, and the temperature is generally 150℃～350℃ to avoid the cold metal liquid from solidifying rapidly after contacting the mold, resulting in incomplete filling.

2. Alloy smelting and processing

Common alloy materials include aluminum alloy (such as ADC12, A356), copper alloy, magnesium alloy, etc. The temperature must be strictly controlled during smelting, and aluminum alloy is generally controlled at 700℃～750℃. At the same time, degassing and slag removal are carried out to improve the purity of the metal.

3. Filling process

Under the set air pressure, the molten metal slowly enters the mold cavity through the riser. Due to the constant pressure and controllable filling speed, it can effectively avoid defects such as flash, burrs, shrinkage, etc., and improve the quality of castings.

4. Solidification and cooling

The solidification process of the molten metal is completed under continuous air pressure. The mold is usually equipped with cooling water channels to control the cooling rate, achieve directional solidification, and further improve the structural density and mechanical properties of the casting.

5. Demolding and cleaning

After the casting solidifies, the pressure is released to allow the molten metal to flow back, and then the mold is opened to remove the casting. Then, post-processing steps such as grinding, riser removal, and sand removal are carried out.

6. Inspection and processing

After the casting undergoes quality inspections such as X-ray flaw detection, metallographic analysis, and dimensional inspection, it is mechanically processed if necessary to meet the final use requirements.

3. Advantages of low-pressure casting

Smooth filling: The filling process is from bottom to top, and the molten metal fills the cavity at a stable flow rate, effectively reducing defects such as eddy currents, slag inclusions, and scale;

Good density: Under the action of air pressure, the molten metal is fully filled, the organization is dense, and the mechanical properties are better than gravity casting;

High yield rate: Due to the use of metal molds, automatic control and other means, the scrap rate is low and the repetitive production capacity is strong;

Strong adaptability: Suitable for the production of small and medium batches of complex structural parts, especially for thick-walled parts, wheels, cylinders, brackets and other products;

Strong controllability: It can realize automated and intelligent production with high precision and efficiency;

High material utilization rate: The molten metal can be refluxed, the riser is small, and the material loss is low.

4. Disadvantages of low-pressure casting

Although low-pressure casting has outstanding advantages, it also has certain limitations:

High investment cost: high prices of equipment and molds, suitable for mass production;

High process control requirements: more precise pressure control, mold temperature control and filling parameters are required;

Not suitable for oversized castings: the length of the riser is limited, and the size of the casting is subject to certain restrictions.

5. Typical application examples

Low-pressure casting is widely used in the following fields:

Automobile industry: production of aluminum alloy wheels, cylinder blocks, cylinder heads, brackets, gearbox housings, etc.;

Aerospace: used to manufacture lightweight and high-strength structural parts;

General machinery: such as motor housings, pump bodies, valve bodies, etc.;

Home appliances and electronics industry: used for the production of precision aluminum alloy housings and structural parts.

For example, the use of low-pressure casting in automobile wheels can significantly reduce weight while ensuring mechanical strength, improving fuel economy and safety performance.

VI. Future Development Trends

With the development of intelligent manufacturing and green manufacturing, low-pressure casting is developing in the following directions:

Intelligent control system: integrating multi-parameter control such as temperature, pressure, and solidification process to realize intelligent production;

Quick mold replacement and maintenance system: improving equipment versatility and maintenance efficiency;

Use of environmentally friendly and energy-saving materials: such as low-melting-point environmentally friendly alloys, lead-free alloys, etc.;

Combination of composite processes: such as low-pressure casting and extrusion, precision machining, heat treatment and other process combinations to further improve product quality.