Starting from the inspection of die casting defects, the types of pores are identified according to the characteristics of pores. Various pore formation mechanisms are discussed, and the causes of pore formation are identified with the help of computer simulation analysis and corresponding field process investigation and analysis. The corresponding solutions to various porosity defects are proposed to mitigate or prevent the occurrence of porosity defects.
With the rapid development of the automobile industry and the requirements for automobile lightweight, aluminum, magnesium, and other alloy die-casting parts significantly increased, providing a broad prospect for the further development of the die-casting industry. Due to the lightweight demand of the parts, the alloy material performance, product structure, and process design and control requirements are more stringent.
Each automobile factory on the requirements of die casting more and more stringent, the requirements of die casting porosity, generally 5% to 10%, and the requirements of certain parts even to 3%.
Die casting porosity, shrinkage, and slag hole defects occur within the casting, and the causes of defects vary. To eliminate defects, it is critical to identify the type of defect and analyze its cause; the tools and methods used to inspect the part will affect the final determination. The following article will discuss how to solve the problem of aluminum and magnesium alloy die-casting porosity.
Table of Contents
For die-casting porosity inspection, several locations:
① finite element analysis of the maximum stress location;
② parts simulation analysis of the location of the volume of gas;
③ parts work critical parts (such as sealing surface, etc.).
General die castings can be used for X-ray inspection; cut open the parts for further inspection after discovering defects. According to ASTM E505 grade 2 control, the process control should control critical parts according to ASTM E505 grade 1.
Pores are generally smooth, round, or oval, sometimes isolated, and sometimes clustered. Figure 1 shows the surface of die-casting pores.
And shrinkage and shrinkage lose an irregular shape, the surface color is dark and not smooth, under the microscope and electron microscope, can be found in the defect location exists dendritic structure, see Figure 2. sometimes pores. Shrinkages exist simultaneously in the same defect location, to be carefully observed.
Hydrogen gas pores
Figure 3 shows hydrogen gas pores. Hydrogen pores are tiny, needle-like, and evenly distributed and can be observed only after the surface processing of the parts. Due to the thin wall of the die casting and the fast solidification speed of the metal liquid, sometimes it is difficult to observe the hydrogen gas pores with the naked eye. Water vapor is the most important source of hydrogen gas, which may come from furnace gas, melting tools, aluminum ingots/recycled parts, oil-contaminated machining chips, wet refining agents, etc.
Usually, aluminum alloy die casting uses a rotary de-gassing device (see Figure 4). The gas source usually uses argon, nitrogen, or chlorine gas. The gas is introduced into the metal liquid and cut into a large number of tiny bubbles by the rotor. Due to the concentration difference between inside and outside the bubbles, hydrogen is drawn into the bubbles and discharged out of the metal liquid together (see Figure 5).
The degassing effect is affected by equipment, gas selection, degassing rotor speed, and degassing time and is measured by detecting the density of the metal liquid after degassing. A certain amount of aluminum liquid is collected and poured into a small crucible, put into a reduced pressure chamber, solidified under reduced pressure conditions, weighed in air and water, respectively, and then the relative density of the specimen is obtained according to the following formula.
Where ρs is the relative density of the solidified specimen; ma is the mass of the specimen in air, g; MW is the mass of the specimen in water, g.
The coiled gas pores are round, clean inside, with smooth and shiny surfaces, sometimes alone and clustered together. Fig. 6 and Fig. 7 show the characteristics of the air-rolling pores under the macroscopic and scanning electron microscope, respectively. The air rolls generally occur in the punch, sprue, and cavity.
Air entrainment in the punch system
A lot of air gets involved during the flow of the metal fluid from the press chamber or gooseneck to the inner gate. The general die-casting process cannot change the turbulent liquid flow pattern, but it is possible to reduce the amount of air rolled into the inner gate by improving the feeding system.
For cold chamber die casting, should consider the degree of filling, i.e., the ratio of the amount of liquid metal poured into the cold chamber die casting machine to the chamber’s capacity. In the design process parameters, the degree of filling should be greater than 50%, to 70% to 80% is appropriate. Figure 8 shows a die-casting filled degree and the relationship volume between the air volume.
In the die casting machine selection and mold design process, generally through the P-Q2 software calculation (P is the pressure, Q is the flow rate), select the appropriate pressure chamber size and the degree of filling. After determining the injection barrel size, the pouring speed from the pouring ladle to the injection barrel is considered. If the filling degree is less than 50%, the upper space of the pressure chamber is large, and the metal liquid will create waves that move back and forth between the punch and the mold. When the punch starts to move forward, forming a confluence of reflected waves in front of the punch and the middle of the injection barrel, turbulence and rolled air will occur. It will increase the air holes in the casting and cause the liquid metal in the press chamber to cool down, which is not good for filling.
The best solution is to have the punch already moving before the metal wave is reflected, i.e., the punch and the initial wave are in the same direction, which can significantly reduce the air roll. In addition, use the P-Q2 software to select more reasonable design parameters that meet at least 50% of the fullness.
During the product development and design process, the following process factors should also be considered: (i) for cold chamber die casting, these include pouring speed, the delay time of injection, low-pressure injection acceleration, gate speed, the gate to low-speed injection switching point, low-pressure injection speed and fast injection start point; (ii) for hot chamber die casting, these include low-pressure injection acceleration, low-pressure injection speed to fast injection switching point. Properly adjust and monitor the above parameters to minimize the degree of air roll.
Sprue system roll air and exhaust
Once the metal liquid encounters the change of the sprue shape at the speed of 64~160km/h, the impulse will cause the metal liquid to swirl, resulting in the generation of rolled air and air hole defects.
This rolled air is solved by the good design of the sprue shape, which should ensure the metal liquid is smooth throughout the filling process and requires a proper selection of the curve and size of the sprue.
Cavity roll air
To reduce the cavity roll air hole defects, ensure that the overflow system is properly designed and venting smoothly. Figure 9 shows a die-casting overflow system. The overflow system consists of an overflow tank, exhaust tank, overflow channel, and other parts.
The overflow system should ensure gas discharge from the front of the metal liquid. Usually, using Z-shaped or fan-shaped exhaust, shallow depth and located at the edge of the mold can avoid the production of jets.
The overflow channel and exhaust channel are usually set at the last filling position of the liquid metal, which can be determined by mold flow analysis while ensuring sufficient exhaust size; the exhaust channel on the parting surface is usually set at the back end of the overflow channel to enhance the effect of overflow and exhaust. Tooth-shaped exhaust channel has a good exhaust effect. It is better to ensure at least one tooth-shaped exhaust channel when designing the mold.
Vacuum die casting will help solve such problems. The vacuum system is already running before the metal fluid arrives. In the operating standard, the time for the punch to reach the vacuum valve from the gate should be monitored and should normally be at least 1s, and sometimes it is necessary to adjust the low-speed die casting start position.
In conventional die casting, using an overflow tank and exhaust system, the pressure starts at 180kPa at the inner gate and can reach 400kPa at the final filling; in vacuum die casting, using a vacuum channel and vacuum valve, the pressure starts at 20kPa at the inner gate and can reach 18kPa at the final filling. Usually, under vacuum conditions, the gas pressure inside the cavity reaches 2-7kPa, while in no vacuum, the gas pressure inside the cavity reaches 300kPa or more. Therefore, vacuum technology can effectively reduce the pressure in the cavity.
In process design, pay attention to the following points:
① Avoid square corners in the sprue system and ensure the smooth surface of the sprue;
② Drainage system should be designed in the best position to ensure access to the edge of the mold, sufficient venting area, and adequate venting;
③ Vacuum system is set on critical surfaces and connection parts to avoid leakage and surrounding environment interference; vacuum channels are correctly sized, especially at the cavity inlet; measure and monitor The pressure in the cavity is measured and monitored, and if the monitoring range is exceeded, an alarm is given and the part is automatically scrapped; the vacuum valve works properly, and the vacuum system is cleaned regularly.
The die-casting process simulation technology, the casting filling process (flow field), can predict the rolled air in the injection barrel, sprue, and cavity. Numerical simulation of the casting filling process can help technicians effectively predict the magnitude, location, and time of occurrence of various possible rolled gas pressure in the casting process stage to optimize the casting process design, ensure the quality of the casting, shorten the trial period and reduce the production cost. Figure 10 shows the simulation analysis of a die casting roll gas, the actual location of the air hole, and the simulation of the flow field analysis of the location of the roll gas consistent.
When the mold parameters and process parameters design changes, the simulation analysis should be re-run and carefully evaluated to ensure that the overflow discharge system works effectively.
Water vapor porosity
Water vapor pores generally appear round, gray, dull, uneven, and dry, with scaly features, see Figure 11. This feature should be checked for mold release agent spraying and cooling water pipe leakage condition.
Water vapor is formed when the metal fluid encounters water during the filling process. During the conversion of water to water vapor, expansion occurs. At the location of the water droplet, water vapor bubbles are formed. The bubbles take up approximately 1,500 times more space than the original water droplets. 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 the typical water vapor gas bubbles come from die-casting coatings.
It mainly occurs in the following die casting processes:
① too much water-based paint is sprayed on the mold, and the cavity is not completely dry when the mold starts to close;
② leaking water pipes;
③ leaking water pipes at the connection threads;
④ cracking of the mold with water infiltration;
⑤ water droplets from the top end of the mold flowing into the cavity when the mold is closed;
⑥ water-based hydraulic fluid remaining on the mold.
One of the most important things is to ensure that the mold is dry when it is closed. Sometimes the mold is visually dry, but the part will still have air holes caused by water vapor. In this case, it is necessary to close the mold, lock it and wait for some time, and then open the mold for inspection after confirming that there are no die-cast parts. It is also important to prevent the slider from squeezing the release agent into the mold cavity during its movement.
Hot chamber die casting machine leaks tend to occur at the sprue. This area often cracks and causes leaks due to thermal stresses and the formation of water vapor vents. It should be checked and eliminated promptly at each start-up. Especially for magnesium alloy castings, a leak at the sprue may produce a very dangerous explosion. Since the distance between the mold surface at the sprue and the water pipe is short, it should inspect carefully and often.
Sometimes the water vapor porosity only occurs on the hot mold parts. This way, the spraying amount should be reduced during the mold start-up phase when the mold is warmed up and stabilized. The temperature condition of the mold should be measured by borrowing temperature measuring instruments to determine the amount of spraying during the start-up process.
The amount, time, and spraying of the mold according to the temperature of the mold and the initial spraying parameters of each part of the mold can be determined by simulation analysis. At the same time, the actual conditions of the trial production are combined with appropriate adjustments to finalize the spraying process.
Lube oil air hole
Figure 12 shows the punch lube oil air hole. Lubricating oil causes air holes, bronze, brown or black surface, and a relatively smooth surface. The lubricating oil mainly comes from the die and punch.
When the metal fluid encounters the die and punch lubricant, evaporation, boiling, and combustion occur. In this case, the gas produced will be involved in the metal fluid and cannot be discharged through the overflow discharge system, but the degree of porosity can be reduced by vacuum die casting.
In-room die-casting machine, the main source of the gas hole is the punch lubricant, which should be added to the upper part of the punch, not the front end of the punch, during use. Otherwise, the front-end lubricant enters the injection barrel, encounters the metal fluid to produce evaporation, gets involved in the cavity, and can cause parts to be porous.
The best solution is to reduce the amount of lubricant used and ensure the amount is consistent each time. If necessary, replace the pressure injection barrel and the end of the punch.
When the die is pulled during production, air holes will appear at the pulling die. In addition, if the punch has worn, a large number of air holes will appear on the die casting surface. Therefore, if the punch has wear characteristics, it should be replaced in time. The die casting factory determines to replace the punch according to the pressure and die times of die casting and takes measures before the wear so that can prevent some suspicious boundary parts.
Pores caused by inserts
The surface of the insert is often with antirust oil, cutting fluid, or other foreign matter. When the metal fluid encounters the insert, these substances will evaporate. If the amount of gas and mold exhaust is insufficient, it will form pores. The general pore is located in the insert and metal bonding parts, Figure 13 for die-casting cylinder block, cylinder sleeve in the pore.
In the die-casting before, the insert eliminates gas by preheating. The process should verify the preheating temperature and time, monitor the production process in real-time, and set up an error-proof system; meanwhile, the inserts should be guaranteed to be clean before use. In addition, the amount of gas generated by the insert is checked to prevent the scrapping of the die casting due to gas holes.
In conclusion, there are 3 steps to solve the porosity defects. First, through macroscopic or with the help of other testing equipment, determine the porosity, shrinkage, or other defects; then, according to the characteristics of the porosity, determine whether the porosity is caused by hydrogen, coiled gas, water vapor, lubricating oil, or inserts; finally, according to the mechanism of porosity formation, with the help of simulation technology analysis or on-site corresponding process investigation and analysis, determine the cause of porosity formation, and verify and take corresponding Measures.
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