0102030405
Die Casting Defect Series - Porosity
2025-09-17
Porosity, a common internal defect in die-castings, is a regularly shaped, smooth hole formed by trapped gas. Porosity, or pinholes, are currently one of the most significant defects in die-castings. Controlling porosity is particularly critical in areas that require sealing.
Defect feature description:
Pores are holes in die castings formed by gases released from the alloy liquid, gases drawn in during the alloy hydraulic injection filling process, and gases generated by the decomposition of release agents and punch oils that infiltrate the alloy liquid. The inner surface of the pores in die castings is relatively smooth and round, with a relatively full shape.
Due to the different causes and locations of pores, they can be large or small, concentrated or scattered. Pores inside castings often appear in shapes such as round, plump or irregularly round, pear-shaped, and needle-shaped. There are also cases where a large amount of gas, shrinkage cavities, and oxidation inclusions coexist, forming irregular pores.
Possible causes:
(1) Unreasonable product or mold structure design
①. The runner shape is poorly designed, the alloy liquid flows unevenly in the runner, causing turbulence and air suction.
② The direction of the alloy liquid introduced into the mold cavity from the inner gate is unreasonable, and the jet splashes violently.
③. The designed ingate cross-sectional area is too small, resulting in too high a metal liquid filling speed, severe jetting, and excessive gas entrained in the liquid flow.
④. After passing through the ingate, the molten metal directly impacts the mold wall to form a vortex, entraining air.
⑤. The jetted liquid flows block the exhaust groove too early, resulting in air holes in poor exhaust or deep cavities.
⑥. The pressure chamber is too small to be filled, and the required low speed is high, which will entrain the gas.
⑦. The mold cavity is too deep, and exhaust is impossible or poor in the deep cavity, dead corner, or middle of the cavity.
⑧. The exhaust groove is incorrectly positioned and the cross-sectional area is not large enough, resulting in poor exhaust capacity and a large back pressure in the cavity.
(2) Unreasonable casting design
① There are parts of the casting that are difficult to vent;
② The wall thickness of the casting is too thick;
(3) Alloy Melting:
The gas in aluminum alloy solution is mainly hydrogen, accounting for more than 95% of the total gas. During the aluminum alloy melting process, hydrogen is very easy to dissolve in liquid aluminum and aluminum alloy. The higher the temperature, the greater the solubility of hydrogen. In particular, when the melting point temperature changes from solid to liquid, the solubility of hydrogen increases sharply. At the melting point temperature of aluminum, the solubility of hydrogen in liquid aluminum is almost 15 to 30 times that of solid aluminum. The higher the temperature, the more hydrogen is absorbed by the aluminum liquid, and the longer the aluminum liquid is kept at high temperature, the greater the amount of hydrogen absorbed.
①. Due to the impurity of molten metal or the high melting temperature, the gas content adsorbed by the molten metal increases, and when solidifying, it precipitates and remains in the casting. Generally, it is a small pore below 0.5mm, which converges with the shrinkage cavity in the center of the casting to form a larger hole.
② The charge is not clean and the degassing and refining is poor. If there are dense small pinholes on the machined surface, it is often more related to the alloy melting factors.
(4) Spraying of release agent and punch oil
Too much release agent and punch oil are sprayed, the release agent generates a large amount of gas, and the release agent is not burned out before pouring, so that the gas generated by the release agent is drawn into the surface of the casting. The inner surface of this pore is mostly dark gray.
①. The die-casting process is unreasonable, such as low specific pressure and high speed.
② The high-speed injection starts too early, causing the alloy liquid to contain more gas in the runner and cavity.
③. The slow injection speed is fast, the fast injection is early, the boost pressure is late or not established, the accumulator lacks nitrogen, etc.
④. The Hot Chamber Die Casting machine's punch returns too early, and the alloy liquid that has not yet condensed is drawn back to form pores, which are likely to form pores and shrinkage cavities at the gate of the casting.
⑤. There are pores on the inner gate fracture. The pores with bright inner surface are caused by too fast speed at low speed. The dark surface is caused by water leakage from the punch or mold release agent sprayed into the injection chamber. The black or yellow surface is caused by too much oil in the punch.
Countermeasures and suggestions:
The formation of stomata is often caused by the combined action of multiple factors. It is recommended to analyze and solve it from the following aspects:
(1) Alloy smelting
① When melting aluminum alloy, we should strive to achieve rapid melting and shorten the time of staying at high temperature. The melting temperature should not be too high and should be ≤760℃. The long-term holding temperature should be 620~640℃.
② The smelting process should be strictly followed. The alloy liquid should be fully and thoroughly degassed and deslagging, and the refining should be carried out at a sufficient temperature and time.
③ Use dry and clean charge and strictly follow the smelting process. It is strictly forbidden to add charge with dust, oil, paint, or moisture into the furnace. Return charge and aluminum ingots should be dried before loading into the furnace.
④. It is prohibited to use undried furnaces, crucibles, tools, refining agents, slag removers, chloride salts, etc. Undried helium, chlorine and other gases must not be used directly for refining. It is prohibited to vigorously stir the alloy liquid during refining and slag removal.
⑤. The calcium content must be strictly controlled when smelting aluminum alloy.
① The filling of alloy liquid should be conducive to exhaust. Select the gate position, guide shape and direction that are conducive to the exhaust of gas in the cavity to guide the metal liquid to flow smoothly and fill the cavity in an orderly manner, which is conducive to the exhaust of gas. Make the alloy liquid enter the middle part, deep and high part or the wide part of the bottom layer of the cavity first, and push the air inside the cavity outward into the exhaust groove to prevent the metal liquid from blocking the overflow system on the parting surface first.
②. Make sure to open a venting groove with sufficient cross-section size at the last filling part of the cavity.
③. Improve the location and size of the overflow and vent grooves, and avoid the location where the overflow and vent grooves are set to be blocked by the molten metal first. Change the location of the parting surface to facilitate the installation of overflow and vent grooves for exhaust.
④. The gas in the cavity is not discharged and the exhaust is not smooth. You can set an overflow groove and a push rod at the air hole, and set a core at the small hole to cast a bottom hole.
⑤. Be careful not to make the overflow port too thin, otherwise it will be blocked prematurely and the casting will have air holes around the overflow groove. Clean the oil and waste on the exhaust groove in time.
⑥ To solve the problem of difficult exhaust in deep cavities and hot spots, exhaust plugs are opened in deep cavities, the matching clearance of the mosaic block structure is used to design the mold surface into a tortuous parting surface, and exhaust push rods and movable cores are added to exhaust.
⑦. If there are pores, cold shut, paint accumulation and loose compaction on the surface at the end of filling, the most effective way is to add an inner gate and directly fill the defective parts.
⑧. When filling at high speed, the alloy liquid will entrain gas, and there will be a large number of small pores inside the die casting.
⑨. Select a smaller machining allowance while meeting the requirements of fine machining, and set cores or push rod exhaust in blind holes of castings and places where pores are prone to occur, so as to reduce the pores that appear during fine machining.
(3) Die Casting Process parameters
① If the injection and filling speeds are too high, the metal flow will be entrained with more gas. Adjusting the fast injection speed to reduce or extend the filling speed and time can change the size of the bubbles. Prolonging the injection time, reducing the first-stage injection speed, and delaying the high-speed injection start switching point can reduce entrained gas. Increasing the injection pressure and boost pressure to ensure the thickness of the material cake can reduce the size of the pores.
② Controlling the mold temperature, lowering the pouring temperature, and increasing the specific pressure are all beneficial to eliminating porosity defects.
③. Check that the gap between the punch and the pressure chamber is 0.03~0.15mm to prevent the punch from getting stuck.
④. Ingate air holes: Factors such as too high low speed, too much punch oil, punch leakage, and release agent water sprayed into the injection chamber will affect the ingate air holes.
⑤. Use mold cooling water with adjustable switch to control the temperature of the mold at a higher temperature: only cool the mold with water during the period from the start of injection filling to the mold opening, and turn off the cooling water at other times.
(4) Release agent spraying
① If the release agent produces excessive gas or is used in excessive amounts, it may not completely burn out before pouring, trapping volatile gases on the casting surface. Therefore, use minimal release agent. During spraying, check that the spray is white. Before closing the mold, check the cavity for any residual gas evaporating. After spraying the release agent, use compressed air to blow out any remaining moisture in the mold cavity. Alternatively, replace the coating or increase the coating-to-water ratio. Therefore, the choice of release agent is crucial.
② After spraying the release agent, make sure that the air pipe does not leak, no water condensed from the compressed air is sprayed out, and the punch and gate sleeve do not leak.
Many people may also encounter the difficulty of distinguishing between pores and shrinkage cavities, which often occur simultaneously. I will also analyze shrinkage cavities in detail later. Based on the causes and corrective measures mentioned above, the key to solving the pore problem is to address all the associated links and possible factors that may involve gas.