Die casting porosity analysis and solution

R-C (2)

With the rapid development of automobile industry and the requirement of automobile lightweight, aluminum, magnesium and other alloy die casting parts increase significantly, which provides a broad prospect for the further development of die casting industry. Due to the lightweight requirements of parts, the requirements of alloy material properties, product structure and process design and control are more stringent. The requirements of auto factories on die casting are more and more strict. The porosity of die casting is generally 5%-10%, and the requirements of some parts are even 3%. In view of the detection method and detection location of die casting defects, the selection of die casting machine, mold design and process design, with the help of computer simulation analysis, experimental research, using P-Q2 software for optimization. The porosity, shrinkage hole and slag hole defects of die casting occur in the casting, and the causes of the defects are different. In order to eliminate defects, it is critical to identify the types of defects and analyze their causes, and the tools and methods of inspecting parts will affect the final judgment.

一.Stomatal check

For the porosity inspection of die casting, several positions should be considered emphatically: ① the maximum stress position of finite element analysis; ② Part simulation analysis of entrainment position; ③ Key parts (such as sealing surface, etc.).

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

The stomata are generally smooth, round or oval, sometimes isolated, sometimes clustered together.

Die casting porosity analysis and solution

The shape of shrinkage pores and shrinkage porosity is irregular, and the surface color is dark but not smooth. Under the microscope and electron microscope, dendrite structure can be found at the defect location. Sometimes pores and shrinkage pores exist at the same defect location, which should be carefully observed.

二. Porosity formation

  1. Hydrogen pores

Hydrogen pores are small, needle-like, and evenly distributed, and can only be observed after the surface of the part is processed. Due to the thin wall of die casting and the fast solidification speed of liquid metal, sometimes hydrogen pores are difficult to be observed by naked eye. Water vapor is the main source of hydrogen, which may come from furnace gas, smelting tools, aluminum ingots/recycled parts, oil-contaminated machine cuttings and wet refiners.

Die casting porosity analysis and solution

Usually aluminum alloy die casting using rotary degassing device, gas sources generally use argon, nitrogen or chlorine. In the liquid metal through the gas, through the rotor cut into a large number of tiny bubbles, due to the concentration of the bubble inside and outside the hydrogen inhalation bubble, together with the discharge of liquid metal.

Die casting porosity analysis and solution

Degassing effect is affected by equipment, gas selection, degassing rotor speed and degassing time, and measured by detecting the metal liquid density after degassing. A certain amount of liquid aluminum was collected and poured into a small crucible, then solidified in a reduced pressure chamber, weighed in air and water respectively, and then the relative density of the sample was obtained by pressing the following equation.

Die casting porosity analysis and solution

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

2. Coil air hole

Air enrolling pores are round, clean inside, smooth and shiny surface, air enrolling sometimes exists alone, sometimes clustered together.

Die casting porosity analysis and solution

(1) Punch system air entrainment

During the flow of liquid metal from the pressure chamber or gooseneck to the inner gate, a lot of air is involved. It is not possible to change the turbulent liquid flow pattern in the general die casting process, but the volume of liquid metal coiling to the inner gate can be reduced by improving the feeding system.

For cold chamber die casting, the degree of fill should be considered, which is the ratio of the amount of liquid metal poured into the cold chamber die casting machine to the capacity of the occupying chamber. In the design of process parameters, the degree of filling should be greater than 50%, 70%-80% is appropriate. FIG. 8 shows the relationship between filling degree and volume of a die casting.

In the process of die casting machine selection and die design, generally through P-Q2 software calculation (P is the pressure, Q is the flow rate), select the appropriate pressure chamber size and degree of filling. After the barrel size is determined, the pouring speed from the ladle to the barrel should be considered. If the filling degree is less than 50% and the upper part of the chamber is large, the liquid metal will create waves that will move back and forth between the punch and the die. Turbulence and air entrainment occur when the punch begins to move forward, creating a confluence of reflected waves in front of the punch and in the middle of the barrel. In this way, the porosity of the casting is increased, and the liquid metal in the pressure chamber is chilled, which is not good for filling.

Die casting porosity analysis and solution

The best solution is for the punch to start moving before the metal wave is reflected, that is, the punch is in the same direction as the initial wave, which can greatly reduce the enrolling. In addition, P-Q2 software was used to select more reasonable design parameters to meet at least 50% of the full degree.

In the process of product development and design, the following process factors should also be considered: (1) for cold chamber die casting, including pouring speed, injection delay time, low pressure injection acceleration, gate speed, gate to low speed injection switching point, low pressure injection speed and fast injection starting point; (2) For hot chamber die casting, including low pressure injection acceleration, low pressure injection speed to fast injection switch point. Adjust and monitor the above parameters appropriately to minimize the degree of air entrainment.

(2) Coil and exhaust of the runner system

At the speed of 64-160km /h, once the shape of the runner changes, the impulse force will cause the liquid metal to generate whirlpool, resulting in the air coil hole defect.

In order to solve this problem by designing the shape of runner reasonably, it is necessary to ensure that the metal liquid is stable in the whole filling process, and it is necessary to choose the curve and size of runner reasonably.

(3) cavity air entrainment

To reduce the defects of the cavity coil air hole, it is necessary to ensure the reasonable design of the discharge system and smooth exhaust. Figure 9 shows the overflow system of a die casting. The discharge system consists of an overflow tank, an exhaust tank and an overflow channel.

The discharge system shall ensure that the liquid metal front-end gas is discharged. Z-shaped or fan-shaped exhaust is usually used, which is shallow and located at the edge of the mold to avoid injection.

The overflow tank and exhaust tank are generally set at the last filling position of the liquid metal, which can be determined by mold flow analysis, while ensuring adequate exhaust size; The exhaust groove on the parting surface is usually arranged at the rear end of the overflow groove to enhance the effect of overflow and exhaust. Tooth shaped exhaust has good exhaust effect, mold design, it is best to ensure that there is at least one tooth shaped exhaust.

Die casting porosity analysis and solution

Vacuum die casting will help solve such problems. The vacuum system is up and running before the liquid metal arrives. In the operating standard, the time for the punch to reach the vacuum valve from the gate should be monitored, which should generally be at least 1s, and sometimes the low-speed injection starting position should be adjusted.

In the traditional die casting, using the overflow tank and exhaust system, the pressure at the inner gate can reach 180kPa at the beginning and the filling can reach 400kPa at the end. When vacuum die casting, vacuum channel and vacuum valve are used. The pressure at the inner gate starts to reach 20kPa, and the final filling can reach 18kPa. Usually, under vacuum condition, the gas pressure in the cavity reaches 2 — 7kPa; In the absence of vacuum, the gas pressure in the cavity reaches more than 300kPa. Therefore, vacuum technology can effectively reduce the pressure in the cavity.

In process design, pay attention to the following points: ① the gate system to avoid square Angle, and ensure that the surface of the gate is smooth; ②The discharge system should be designed in the best position to ensure that it reaches the edge of the mold, the exhaust area is sufficient and the exhaust is sufficient; ③ The vacuum system is arranged on the key surface and connection part to avoid leakage and ambient interference; The vacuum channel size is correct, especially at the cavity inlet; Measure and monitor the pressure in the cavity, if it is beyond the monitoring range, alarm and scrap the parts automatically; The vacuum valve works normally; Clean the vacuum system regularly.

(4) Simulation analysis

The simulation technology of die casting process can simulate the mold filling process (flow field) and predict the air enrolling situation in the cylinder, runner and mold cavity. The NUMERICAL SIMULATION OF CASTING MOLD FILLING PROCESS CAN HELP TECHNICIANS TO EFFECTIVELY PREDICT THE SIZE, LOCATION AND OCCURRENCE TIME OF ALL KINDS OF AIR ENTRAPMENT PRESSURE IN CASTING PROCESS stage, SO AS to OPTIMIZE casting process design, ENSURE casting quality, shorten trial production cycle and reduce production cost. FIG. 10 shows the simulation analysis of enrolling gas of a die casting, and the actual location of pores is consistent with the location of enrolling gas analyzed by the simulated flow field.


Die casting porosity analysis and solution

  1. Steam pores

The appearance of water vapor pores is generally round, gray, dull, uneven and dry scaly. This feature should be checked for release agent spray and mold cooling water pipe leakage.

Die casting porosity analysis and solution

When the liquid metal meets water during the filling process, it forms water vapor. During the transformation of water into steam, expansion occurs. In the position of water droplets, bubbles of water vapor form. The bubble takes up about 1,500 times as much space as the original water drop. The gas is hard to get out through the discharge system, and it’s somewhere in the metal where it’s hard to predict.

About 98% of the typical water vapor pores come from die casting paint. It mainly appears in the following die casting process: ① Too much water-based coating is sprayed on the mold, and when the mold begins to close, the cavity is not completely dry; ② water pipe leakage; ③Water pipe connection thread leakage; ④Mold cracking, water infiltration; ⑤ When the mold is closed, the water droplets at the upper end of the mold flow into the mold cavity; ⑥ Water-based hydraulic liquid remains on the mold.