Analysis of Causes and Solutions for Porosity in Aluminium Alloy Castings
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- Release time: 2026-01-23
Key Point: In simple terms, porosity falls into two categories. The first is precipitation porosity, which occurs when gases precipitate out of the molten aluminium during solidification due to changes in gas solubility – the senior engineer has covered this in detail. The second is entrapment porosity, unrelated to the molten aluminium itself. This primarily arises from air entrained within the casting during filling due to turbulence, or from moisture present in the coating or mould cavity that has not fully dried. Entrapped porosity is closely linked to the rationality of the gating system; where it arises solely from coatings or water, it is purely due to improper operation. As for its occurrence after shot blasting, this is likely closely associated with the positioning of the high-speed transition point.
Precipitation and evolution were discussed in detail by the senior expert in this regard. The other type is entrainment porosity, which is unrelated to the aluminum liquid itself. It mainly refers to the air entrapped in the product due to turbulent flow during the aluminum filling process, as well as moisture from coatings or within the mold cavity that has not dried. Entrainment porosity is closely related to the rationality of the gating and venting system. Porosity caused solely by coating and water is purely due to improper operation. As for its appearance after shot blasting, it should be mainly closely related to the position of the high-speed switching point.
Question 1: We are troubled by a technical issue where ACD12 aluminum alloy die-casting parts exhibit excessive porosity after machining or sand blasting.
Reply:
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What kind of vacuum extraction equipment is being used?
It seems that porosity issues in die-casting parts cannot be completely resolved. They can only be controlled within a reasonable range by adjusting die-casting parameters, mold temperature, and modifying related mold temperatures.
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I. Human Factors:
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Is too much release agent being sprayed? Since release agents have high gas evolution, excessive usage that doesn't burn off completely before pouring will cause volatile gases to be trapped in the casting surface layer. This is one reason why some operators produce more porosity under the same conditions. Select release agents with low gas evolution, apply thinly and evenly, and close the mold after complete combustion.
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Are the overflow grooves and vent channels not cleaned regularly?
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Is mold opening too early? Is the mold preheated? Are all parts heated slowly and evenly to bring the cavity and core surface temperatures to 150°C～200°C?
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Are products produced when the mold temperature is low at the beginning isolated/segregated?
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If there is no preheating device, is aluminum alloy material slowly pushed into the cavity for preheating or heated by other methods?
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Is clean aluminum liquid being used? Is the oxide layer being injected into the shot sleeve?
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When pouring, is the ladle kept close to the shot sleeve inlet to avoid splashing, oxidation, or air entrainment and cooling?
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Is the injection performed immediately after the molten metal is poured into the shot sleeve? Has the temperature dropped?
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For cooling and mold opening, is the mold opening time selected according to different products?
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Is normal die-casting pressure not being used due to fear of molten aluminum splashing (water flashing)? Is there even hesitation to try appropriately increasing the specific pressure?
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Are operators strictly following die-casting processes?
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Is quantitative pouring being used? How is the pouring amount determined?
II. Machine (Equipment, Mold, Tooling) Factors: Mainly refers to mold quality and equipment performance.
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Is the die-casting mold design reasonable? Could it cause porosity? Reasons related to die-casting molds:
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Is the gate location selection and flow guide shape improper, causing frontal impact and vortex formation when molten metal enters the cavity? (Reduce injection speed to avoid vortex air entrainment)
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Is the runner shape poorly designed?
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Is the inner gate speed too high, causing turbulence?
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Is venting not smooth?
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Is the mold cavity position too deep?
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Is the machining allowance too large? Has it penetrated the dense surface layer, exposing subsurface porosity? The machining allowance for die-castings should be smaller, generally around 0.5mm. This reduces casting weight and machining costs while avoiding exposure of subsurface porosity. It's better not to exceed 0.5mm, as machined surfaces will basically show no porosity due to the protection of the hard layer.
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Are vent holes blocked, preventing air from escaping?
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Is there too much plunger lubricant, or is it burned? This is also a source of gas generation.
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Regarding gate location and flow guide shape, does the molten metal prematurely seal the overflow system on the parting surface?
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Is the inner gate position unreasonable, causing the metal to immediately impact the cavity wall after passing through, creating vortices and entraining gas into the metal flow?
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Is the vent channel position incorrect, causing poor venting conditions?
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Is the overflow vent area large enough? Is it blocked? Is it located at the last filling position? Are mold venting areas cleaned regularly to avoid losing venting function due to release agent blockage?
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Is the mold temperature too low?
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Are runner turns smooth? Is the inner gate appropriately enlarged?
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Are vent plugs installed in deep cavity areas, or is a split-insert form used to increase venting?
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Is there any difficult-to-vent area formed due to unreasonable die-casting design?
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Is the total overflow gate cross-sectional area less than 60% of the total inner gate cross-sectional area, resulting in poor slag removal?
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Under conditions of good mold filling, has the inner gate thickness been increased to reduce filling speed?
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Is the inner gate speed too high, with excessive turbulent motion causing severe gas entrainment in the metal flow?
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Is the inner gate cross-sectional area too small, causing severe jetting?
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Is sequential filling used to facilitate cavity gas evacuation? Are the sprue and runner of sufficient length?
III. Material Factors:
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Is supplier raw material composition control properly implemented? What is the iron content? (Required to be below 0.7%)
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Is aluminum purity guaranteed?
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Is secondary material (gate/runner material) used excessively without proper slag removal?
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Is excessive waste slag added to the molten aluminum during production and poured together with oxide skin?
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Does the company control the proportion of secondary use of waste materials? How is it implemented? Who checks?
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Can waste material be added to the aluminum liquid for important customer products?
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Try changing the ratio of new material to return material?
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Is the furnace charge clean?
IV. Method Factors: Mainly refers to die-casting parameters and operating processes.
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Are process parameters selected according to different products? (Die-casting aluminum liquid temperature 630-670°C) Reasonably select die-casting process parameters, especially injection speed. Adjust the high-speed switching starting point.
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Is the water content of the release agent reduced? Is a low gas evolution release agent used?
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Is the alloy melting temperature too high?
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How is the aluminum liquid temperature measured? Is the thermometer accurate?
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Is the injection speed and the switching point between slow and fast injection speeds adjusted in time according to the product?
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Are large machines being used to die-cast small parts, with insufficient shot sleeve fill ratio?
V. Environmental Factors: Is the die-casting environment humidity high?
Generally, the hydrogen content in surrounding air is not high. However, if the relative humidity is high, it will increase the solubility of gas in the aluminum liquid, forming seasonal porosity. For example, during the rainy season, due to high air humidity, the phenomenon of pinholes during aluminum alloy melting is more severe. Of course, when air humidity is high, aluminum alloy ingots, melting equipment, tools, etc., will also increase surface moisture adsorption due to damp air. Therefore, more attention should be paid to taking effective preheating and drying protection measures to reduce porosity generation.
