I.Introduction

With the rapid development of the automobile industry and the requirements of automobile lightweight, aluminum, magnesium, and other alloy die-casting parts increased significantly, providing a broad prospect for the further development of the die-casting industry. Due to the lightweight demand of parts, the alloy material performance, product structure, and process design and control requirements are more stringent.

Each automobile factory on die casting requirements is more and more stringent on the die casting porosity requirements, generally 5% to 10% for some parts of the requirements and even 3%. For die-casting defects, detection methods and locations can be used in die-casting machine selection, mold design, and process design with the help of computer simulation analysis, experimental research, P-Q2 software, and other optimizations.

Die-casting porosity, shrinkage, and slag hole defects occur inside the casting, and the causes of defects are different. In order to eliminate defects, it is especially critical to identify the type of defects and analyze their causes. The tools and methods used to inspect the parts will affect the final judgment.

II.Porosity Inspection

For die casting porosity inspection, several locations must be emphasized:

1. Finite element analysis of the maximum stress location;

2. The location of the part simulation and analysis of the volume of air;

3. parts work critical parts (such as sealing surface, etc.).

General die casting can be used for X-ray inspection; defects can be found, and parts can be cut for further inspection. In the process control, according to ASTM E505 level 2 control, the critical parts should be according to ASTM E505 level 1 control.

Macroscopic involutional stomatal characterization

Pneumatic holes generally have a smooth surface and are round or oval, sometimes existing in isolation and sometimes clustered together.

While shrinkage holes and shrinkage pine are irregular in shape, with dark and non-smooth surfaces, dendritic structures can be found in the defect location under a microscope and an electron microscope. Sometimes, air holes and shrinkage holes exist simultaneously in the exact defect location, so they should be carefully observed.

Scanning electron microscopy of convoluted air pores

III.Die Casting Types of porosity formation

1.Hydrogen gas porosity

Hydrogen pores are tiny, shaped like needles, and uniformly distributed; parts’ surface machining can only be observed. Due to the thin wall of the die casting, the liquid metal solidification speed and sometimes hydrogen pores are difficult to observe with the naked eye. Water vapor is the most important source of hydrogen and may come from furnace gas, melting tools, ingots / recycled parts, oil contamination machining chips, and wet refining agents.

hydrogen orifice

Typically, aluminum alloy die casting uses a rotary degassing unit. Gas sources typically use argon, nitrogen, or chlorine. In the metal liquid into the gas, through the rotor cut into a large number of tiny bubbles, due to the concentration difference between the inside and outside of the bubble, the hydrogen gas will be sucked into the bubble, together with the discharge of the metal liquid outside.

Rotary degasser

The effect of degassing is affected by equipment, gas selection, degassing rotor speed, and degassing time. It is measured by detecting the density of the metal liquid after degassing. Collect a certain amount of aluminum liquid poured into a small crucible, into the decompression chamber, solidification under reduced pressure conditions, respectively, in air and water weighing, and then according to the following formula to find the relative density of the sample.

Rotary degassing principle

Where ρs is the relative density of the solidified specimen; ma is the mass of the specimen in air; mw is the mass of the specimen in water.

2.Rolled gas porosity

The air holes are round, clean inside, smooth, and glossy on the surface. They sometimes exist individually and sometimes cluster together. The following pictures show the characteristics of air pockets under microscope and SEM, respectively. Cylinder gas generally occurs in the punch system, sprue system, and cavity.

3.Punching system gassing

The flow of liquid metal from the pressure chamber or gooseneck to the inner gate involves a lot of air. Although changing the turbulent fluid flow pattern in the general die-casting process is impossible, improving the feeding system can reduce the amount of air entrained in the liquid metal reaching the inner gate.

For cold chamber die casting, the degree of filling, i.e., the ratio of liquid metal poured into the freezing chamber die casting machine to the chamber’s capacity, should be considered. When designing process parameters, the filling degree should be greater than 50%, with 70% to 80% being appropriate for a die casting fullness and volume of gas volume relationship graph.

A die casting fullness and oxygen quantity relationship graph

In the process of die casting machine selection and mold design, generally through the P-Q2 software calculations (P is the pressure, Q is the flow rate), select the appropriate chamber size and fullness. After the barrel size is determined, the pouring speed from the spoon to the barrel is considered. If the degree of filling is less than 50%, the upper space of the pressure chamber is ample, and the metal liquid will produce waves that reciprocate between the punch and the mold.

When the punch begins to move forward, creating a convergence of reflected waves in front of the punch and the middle of the shot cylinder, turbulence and air rolls occur. This increases the porosity of the casting and also causes the liquid metal in the press chamber to cool aggressively, which is detrimental to filling.

The best solution is to have the punch in motion before the metal wave is reflected, i.e., the punch is in the same direction as the initial wave, significantly reducing air rolls. In addition, the P-Q2 software was used to select more reasonable design parameters that satisfied at least 50% of the filling.

The following process factors should also be considered in the product development and design process:

● For cold chamber die casting, including pouring speed, injection delay time, low-pressure injection acceleration, gate speed, the gate to low-speed injection switching point, low-pressure injection speed, and fast injection starting point;

● For hot chamber die casting, including low-pressure injection acceleration and low-pressure injection speed to fast injection switching point, the above parameters are properly adjusted and monitored to minimize the degree of the gas roll.

4.Sprue system outgassing and venting

Under the speed of 64~160km/h, once the metal liquid encounters changes in the shape of the sprue, the impulse force will make the metal liquid produce a vortex, producing rolled gas porosity defects.

To solve this kind of rolled gas by rationally designing the shape of the sprue, it should be ensured that the metal liquid is smooth in the whole filling process, and it is necessary to choose the curve and size of the sprue reasonably.

5.Cavity gassing

Reduce the cavity volume air hole defects to ensure that the overflow system design is reasonable and the exhaust is smooth. The following figure shows a die-casting overflow system. The overflow system consists of an overflow tank, an exhaust tank, an overflow channel, and other parts.

The overflow system should ensure the discharge of liquid metal front gas. Usually, use Z-shaped or fan-shaped exhaust with a shallow depth in the mold’s edge to avoid spraying.

The overflow and exhaust channels are usually set at the last filling position of the liquid metal, which can be determined by mold flow analysis, and at the same time ensure sufficient exhaust size; the exhaust channel on the parting surface is usually set at the rear end of the overflow channel to enhance the effect of overflow and exhaust. A toothed exhaust channel has a good exhaust effect. In mold design, it is best to ensure that at least one toothed exhaust channel is used.

A die casting overflow system

Vacuum die casting will help solve such problems. The vacuum system is already in operation before the liquid metal arrives. In the operating standards, the time for the punch to reach the vacuum valve from the gate should be monitored and should generally be at least 1s. Sometimes, it is necessary to adjust the starting position of the low-speed die casting.

In the traditional die-casting, using the overflow channel and exhaust system, the beginning pressure at the inner gate reaches 180kPa, and the last filling place can reach 400kPa; Vacuum die-casting, using the vacuum channel and the vacuum valve, the beginning pressure at the inner gate reaches 20kPa, and the last filling place can reach 18kPa. Usually, under the condition of a vacuum, the pressure of the gas in the cavity reaches 2-7kPa; and under the condition of no vacuum, the pressure of the gas in the cavity reaches 2-7kPa; and under the condition of no vacuum, the pressure of the gas in the cavity reaches 2-7kPa. Usually, the gas pressure inside the cavity reaches 2~7kPa under vacuum conditions, while under no vacuum conditions, the gas pressure inside the cavity reaches more than 300kPa. Therefore, vacuum technology can effectively reduce the pressure inside the cavity.

6.Water vapor air holes

Externally, water vapor vents generally appear as round, gray, dull, uneven, dry, and scaly features, as shown in the figure below. The presence of this feature should be checked for mold release agent spray and mold cooling water line leakage.

Water vapor air holes

Water vapor is formed when liquid metal encounters water during the filling process. Expansion occurs during the conversion of water to water vapor. At the location of the water droplets, water vapor bubbles are formed. The bubbles occupy about 1500 times more space than the original water droplet. The gas is difficult to expel through the overflow system and exists somewhere in the metal in a location that is difficult to predict.

Approximately 98% of typical water vapor porosity comes from die-cast coatings. It occurs mainly in the following die casting processes:

1. Excessive spraying of water-based coatings on the mold, which is not completely dry in the cavity when the mold begins to close;

2. Water pipe leakage;

3. Leakage at the connection threads of water pipes;

4. Cracking of the mold with water infiltration;

5. When the mold is closed, water drops from the upper end of the mold flow into the cavity;

6. Water-based hydraulic fluid is left on the mold.

IV.Die Casting Solution:

1.Die casting mold process design considerations

In the process design, pay attention to the following points:

1. The sprue system should avoid square corners and ensure the surface of the sprue is smooth;

2. The overflow discharge system should be designed in the best position to ensure that it passes to the edge of the mold and that the exhaust area is sufficient and guaranteed to be adequate;

3. Vacuum systems are set on critical surfaces and connection parts to avoid leakage and interference from the surrounding environment; vacuum passages are correctly dimensioned, especially at the cavity inlet; pressure inside the cavity is measured and monitored, and alarms are raised, and parts are automatically scrapped if they are out of the monitoring range; vacuum valves are working correctly; and vacuum systems are cleaned out regularly.

2.Simulation analysis to predict the problem in advance

Die casting process simulation simulation technology, the casting filling process (flow field) simulation, can predict the barrel, sprue, and cavity roll gas situation.

Numerical simulation of the casting filling process can help technicians in the casting process stage with all kinds of volumes, gas pressure sizes, parts, and time to effectively predict the casting process design, ensure the quality of castings, shorten the trial period, and reduce production costs.

The following figure shows a die-casting volume gas simulation analysis, the actual location of air holes, and the simulation flow field analysis volume gas location in line.

Simulation analysis of air rolls                  actual occurrence of porosity castings

Simulation analysis of a die casting rolled gas

When the mold parameters and process parameters change in design, they should be re-simulated and carefully evaluated to ensure that the overflow system works effectively.

 

V.Conclusion

Effective process design, including optimizing the gating system, using vacuum die casting, and ensuring proper degassing, can significantly reduce porosity. These strategies produce high-quality, reliable automotive parts that meet stringent industry standards.