In high temperature and high humidity environment, the aging failure of plastic cast aluminum parts is the result of the synergistic effect of multiple factors, and its mechanism involves multiple dimensions such as material properties, interface bonding, and environmental erosion. This kind of complex environment poses a severe challenge to the stability of parts, which requires in-depth analysis from the physical, chemical, and mechanical levels.
High temperature environment will significantly accelerate the thermal oxidation aging process of plastic materials. Under the continuous high temperature, the intermolecular force of the polymer chain in the plastic is weakened, the chain segment movement is intensified, and it is easy to break or cross-link. For example, engineering plastics such as polyamide (nylon) will cause molecular chain degradation due to oxidation at high temperature, which is manifested as brittle surface, yellowing color, and a significant decrease in mechanical strength (such as tensile strength and impact toughness). At the same time, high temperature will cause the plasticizer, stabilizer and other additives in the plastic to precipitate or volatilize, destroying the original structure of the material and further reducing its mechanical properties and weather resistance. This thermal aging process is irreversible. The plastic part that has been in a high temperature environment for a long time will gradually lose its bearing capacity, and even cracking, breaking and other failure phenomena will occur.
High humidity environment will have a double erosion effect on plastic cast aluminum parts. On the one hand, water molecules will penetrate into the material through the micropores or molecular gaps on the plastic surface, triggering a hydrolysis reaction. For polar plastics such as polyesters and polyethers, water molecules are easy to combine with polar groups on the molecular chain, destroying chemical bonds and causing a decrease in material strength. For example, ABS plastic may experience "stress cracking" in a high-humidity environment, that is, after water molecules penetrate into the interior, they accelerate crack propagation in the stress concentration area. On the other hand, a humid environment provides conditions for electrochemical corrosion. After the metal surface of the cast aluminum part comes into contact with moisture and oxygen, it is easy to form micro-batteries, causing oxidative corrosion. After the aluminum matrix is corroded, loose aluminum oxide hydrates will be generated. The volume expansion may cause the surface of the part to bulge and peel off. At the same time, the corrosion products will diffuse to the plastic interface, weakening the bonding force between the two.
The interaction between temperature and humidity will further aggravate the aging of parts. High temperature will increase the activity of water molecules and accelerate their diffusion rate inside the plastic, while high humidity will reduce the glass transition temperature of the plastic, causing the material to soften and deform at a lower temperature. This synergistic effect may cause the interface between the plastic and the cast aluminum to fail first. For example, under the action of high temperature and high humidity cycles, the stress generated by the thermal expansion and contraction of plastics and the swelling stress caused by the penetration of water molecules are superimposed on each other, forming a complex stress field in the interface area, causing the adhesive layer or transition layer to gradually peel off, and eventually causing the overall structural instability of the part.
Interface bonding defects are the key cause of aging and failure of parts under high temperature and high humidity environments. The thermal expansion coefficients of plastics and cast aluminum are significantly different (plastics are usually (100-200)×10⁻⁶/℃, and aluminum is 23×10⁻⁶/℃), which will produce large thermal stress when the temperature changes. If the bonding process at the interface is poor (such as incomplete surface treatment and improper adhesive selection), the repeated action of thermal stress at high temperature will cause microcracks to initiate at the interface, and water molecules in a high humidity environment will penetrate into the interior through the cracks, forming a "stress corrosion" cycle. This microcrack will gradually expand into a macro crack, separating the plastic from the cast aluminum, and eventually causing the part to lose its function. For example, the plastic cast aluminum connectors in the engine compartment of a car often first debond and break at the interface due to long-term exposure to high temperature, high humidity and vibration.
Material compatibility issues will also be highlighted in complex environments. Different plastics have different chemical affinities with cast aluminum. The surface energy of some plastics (such as polypropylene) and aluminum is quite different, and it is difficult to form a strong bond only through physical bonding. In a high temperature and high humidity environment, additives in plastics (such as flame retardants and lubricants) may migrate to the interface and react chemically with the oxide film on the aluminum surface, weakening the interfacial bonding force. In addition, alloy elements (such as silicon and magnesium) in the aluminum matrix may undergo electrochemical reactions in a humid environment, and the generated ions will diffuse into the plastic, causing material degradation. This compatibility defect will cause the parts to gradually lose their integrity during long-term service, manifesting as failure forms such as surface delamination and peeling.
Environmental erosion will also affect the functionality of parts. For example, plastic cast aluminum parts used for heat dissipation of electronic equipment may experience reduced insulation performance after aging in a high temperature and high humidity environment, and may even cause a short circuit risk; while the corrosion products of the cast aluminum parts will block the heat dissipation channel, resulting in reduced heat conduction efficiency and affecting the normal operation of the equipment. For plastic cast aluminum gears in mechanical transmission, the increased wear of the tooth surface plastic and the corrosion of the aluminum matrix will occur simultaneously, resulting in reduced transmission accuracy, increased noise, and ultimately mechanical failure due to bite failure. This type of functional failure is often hidden and difficult to detect at the beginning, but it will significantly shorten the actual service life of the parts.
To deal with aging failures in high temperature and high humidity environments, it is necessary to start from multiple dimensions such as material selection, structural design, and process optimization. For example, select engineering plastics that are resistant to high temperature hydrolysis (such as polyetheretherketone PEEK), passivate or plate the surface of cast aluminum to enhance corrosion resistance, and optimize the interface bonding process to reduce thermal stress. At the same time, by simulating the actual service environment through accelerated aging tests (such as high temperature and high humidity chamber tests), the reliability of parts can be evaluated in advance, providing data support for improving design and extending service life. Understanding the nature of the aging failure mechanism is a key prerequisite for improving the environmental adaptability of plastic cast aluminum parts.
