Knowledge of aluminum die casting process

Die casting process is the organic combination of die casting machine, die casting mold and die casting alloy three elements to be used comprehensively, these process factors complement each other and restrict each other. Only by choosing and adjusting these factors correctly and reasonably, can we get the expected results.

1. Pressure

The existence of pressure is the main feature of the die casting process that is different from other casting methods. Pressure is an important factor to make castings obtain dense tissue and clear outline.
In die casting production, the expression of pressure formation has two types of injection force and specific pressure:

(1) Injection force

The injection force is the force of the injection mechanism of the die casting Aluminum die casting machine to promote the movement of the movable plug. The size of the injection force is determined by the cross-sectional area of the injection cylinder and the pressure of the working fluid.
P beam =P2.g
P2. Pressure of working fluid in injection cylinder.

d. Injection cylinder diameter.

g. 9.8 kg/cm2

2. Specific pressure

The pressure of molten metal in the unit area of the press chamber is called the specific pressure, the specific pressure is the ratio of the injection force to the area of the press chamber P ratio =P injection /F chamber
The specific pressure of F is the expression of the actual force of the molten metal in each stage of the filling process, reflecting the root of the force of the molten metal in each stage of the filling and the metal flowing through different cross-sectional areas. The specific pressure of the filling stage is called the filling specific pressure (also known as the injection specific pressure), and the specific pressure of the booster stage is called the booster specific pressure.
The filling pressure is used to overcome the flow resistance in the pouring system and cavity, especially the resistance at the inner gate, so that the metal flow can ensure that the required inner gate speed is reached. The boost specific pressure determines the pressure on the solidifying metal and the size of the swelling force formed at this time.

3.The influence of specific pressure on the mechanical properties of castings
1) The specific pressure increases, the crystallization is finer, and the fine crystal layer is thickened. Due to the improvement of filling energy, the surface quality is improved, and the influence of porosity is reduced, so the tensile strength is increased, but the elongation is reduced.
2) Influence on filling conditions.
When the alloy melts, the cavity is filled under the action of high specific pressure, the alloy temperature increases and the fluidity improves, which is conducive to the improvement of casting quality.

4.Injection speed

In the process of die casting, the injection speed is directly affected by the pressure, and the pressure plays an important role in the internal quality of the casting, the surface and the clarity of the profile.

(1) Injection speed
During the injection process, the injection punch moves the molten metal in the press chamber at a speed called the injection speed, and the injection speed is divided into two stages in the process, that is, the slow injection speed and the fast injection speed. Slow injection speed refers to the movement speed before the punch sends the indoor metal liquid into the inner gate, and the main function of fast injection speed is to fill the cavity.

1. The effect and influence of fast injection speed

① Improve the injection speed, the kinetic energy is converted into heat energy, and the flow of the alloy liquid is improved, which is conducive to eliminating the flow mark, cold insulation and other defects, but when the speed is too fast, the alloy molten liquid is mixed with the gas in a fog, resulting in serious eddy current entraining.
②The effect of fast injection speed on the filling characteristics
The increase of injection speed makes the alloy melt temperature rise when filling cavity, but the injection speed is too high, and the filling condition deteriorates, especially in thick wall castings.
2. Fast injection speed and selection and consideration
① Die-casting alloy and characteristics, melting latent heat and alloy and specific heat and thermal conductivity, solidification temperature range.
②When the mold temperature is high, the pressure rate can be appropriately reduced.
③Casting quality requirements, high surface quality requirements and thin-walled complex castings, the use of high pressure injection speed.

5.The speed of the inner gate
1. The speed at which the molten metal is imported into the cavity through the inner gate under the action of the punch, so the inner gate speed is usually 15-70m/s.Molten metal under the action of the punch, through the cross runner to the inner gate, and then fill into the cavity, when the machine injection jet, molten metal through the inner gate can be considered unchanged (or change very little).
2. After the molten metal passes through the inner gate, it flows into each part of the cavity (that is, when it is filled). Due to the influence of various factors such as the thickness of the cavity (the wall thickness of the casting), the thermal state of the mold (the temperature field and distribution), the flow speed changes at any time, which is called the filling speed.
Usually, only the constant speed is selected to measure the process parameters, so the inner gate speed is one of the important process parameters.
The speed of the inner gate has a great influence on the mechanical properties of the casting. The speed of the inner gate is too low, the strength of the casting decreases, the speed increases, and the strong teeth decrease.

6.The choice of speed

In die casting production, speed and pressure together play an important role in the internal quality of the casting, surface requirements and profile clarity.To sum up, if the mechanical properties of the die casting, such as tensile strength and density put forward high requirements, do not choose too large inner gate speed, which can reduce the eddy current due to turbulence, the eddy gas is easy to make the internal structure of the casting porosity, shrinkage and pinholes and other defects, thereby reducing the mechanical properties of the casting.
If the casting surface requirements are high, once the thin wall casting, appropriate consideration can be given to increasing the speed of the inner gate.
1.About the starting point of pressurization selection
The starting point of pressurization should not be selected too early or too late, too early, the cavity has not yet filled, the pressurization start, the casting can not reach the required density, too late, the casting has solidified, the pressurization pressure can not play the role of feeding.
2.Injection chamber and punch wear cause analysis
The injection punch bites for a long time, which seriously affects the stability of pressure transmission and injection speed, as well as the quality of castings and normal production. The main reason is that due to temperature fluctuations, the gap between the pressure chamber and the punch is also in a constantly changing state, and this gap change is particularly significant in the large-diameter pressure chamber, so the larger the diameter of the pressure chamber, the more serious the wear of the punch.

7. Temperature

In the die casting process, temperature plays an important role in the thermal state of the filling process, as well as the efficiency of the operation. The temperature referred to in die casting refers to the pouring temperature and the mold temperature, and temperature control is an important factor in obtaining excellent castings.
(1) Pouring temperature
The casting temperature of molten metal refers to the average temperature when it enters the cavity from the pressure chamber. Because the temperature measurement of the metal liquid in the pressure chamber is inconvenient, the temperature of the general insulation furnace is expressed.
Effect and influence of pouring temperature
The effect of alloy temperature on the mechanical properties of castings: with the increase of pouring temperature, the mechanical properties are improved, but after exceeding a certain limit, the performance deteriorates.

① The gas melts in the alloy, and increases with the temperature, although the gas melts in the alloy, it is difficult to precipitate during the die casting process, which has an impact on the plasticity.

② The iron content increases with the increase of alloy temperature, which reduces the fluidity, makes the crystallization coarse and deteriorates the performance.
③ The oxidation of aluminum alloy and magnesium alloy increases with the increase of temperature, and the oxidation of inclusions deteriorates the alloy properties. Therefore, the alloy overheating, easy to produce shrinkage holes, cracks, pores oxidized inclusions, so the mechanical properties are reduced. The alloy temperature is too high, and under the action of high speed, easy to produce turbulence, eddy current aeration, in the solidification range of a wide range of alloys, the use of high pressure, low temperature (semi-solidified state), low speed pouring, is conducive to the formation of sequence filling, improve the quality of casting, deni cause sticky mold and molten iron, is not conducive to the mold heat balance, but also reduce the life of the mold.

(2) Mold temperature

In the die casting process, the mold needs a certain temperature. Die temperature is an important factor in die casting process, which plays an important role in improving production efficiency and obtaining high-quality castings.
1. The role and influence of mold temperature
In the filling process, the mold temperature has a great influence on the metal flow temperature, viscosity, fluidity, filling time and filling flow state. When the mold temperature is too low, the surface layer is broken by high-speed liquid flow after condensation, resulting in surface defects, and even cannot be “formed”. When the mold temperature is too high, although it is conducive to obtaining a smooth casting surface, it is easy to shrink and doing down.
(2) Die temperature has obvious influence on cooling rate, crystallization state and shrinkage stress of molten alloy. The mold temperature is too low, the shrinkage stress increases, and the casting is prone to crack.
(3) Die temperature has a great influence on die life, and intense temperature changes form complex stress alternating leading to early cracking.
(4) The influence of mold temperature on the size tolerance level of the casting, the mold temperature is stable, the size shrinkage rate of the casting is also stable.