Aluminum alloy die-casting molds operate under extreme conditions of high temperature, high pressure, and intense erosion by molten metal. Therefore, material selection requires a systematic consideration of high-temperature resistance and thermal fatigue resistance. The material must possess excellent high-temperature stability, meaning it must maintain its microstructure without significant changes during repeated heating-cooling cycles. The distribution density of heat-resistant alloy phases such as molybdenum carbide is a key indicator. These hard phases maintain hardness at high temperatures, preventing cavity deformation or collapse due to softening. For example, high-molybdenum mold steels, by forming a stable carbide network, can effectively resist grain boundary softening at high temperatures.
Improving thermal fatigue resistance depends on the material's toughness reserves and stress dispersion capabilities. During aluminum alloy die casting, the mold cavity surface experiences alternating thermal stress due to rapid heating and cooling. If the material's toughness is insufficient, microcracks are prone to initiation and propagation at grain boundaries or inclusions. When selecting alloy steels containing chromium and molybdenum, a balance must be struck between red hardness and toughness—while excessively high tungsten content can improve heat resistance, it significantly reduces impact resistance and increases the risk of hot cracking. Modern mold steels, through optimized alloy ratios, such as high-chromium, high-molybdenum composite designs, enhance toughness while maintaining heat resistance, thus extending the incubation period for thermal fatigue cracks.
The service conditions of molds directly influence material selection strategies. For pure aluminum or alumina die-casting molds without water cooling, the operating temperature is higher than that of conventional aluminum alloys, requiring the selection of low-chromium heat-resistant steel. These materials, by adjusting the type and distribution of carbides, form a dense oxide film at high temperatures, reducing the erosion effect of molten metal on the cavity. For thin-walled, multi-cavity, or high-ribbed radiator molds, materials need to possess both high hardness and erosion resistance. In this case, high-hardness, high-toughness mold steel becomes the preferred choice, improving surface wear resistance and inhibiting the formation of erosion pits through grain refinement and homogenization.
The size and structural complexity of molds impose differentiated requirements on material properties. Large automotive aluminum alloy die-casting molds, due to their large size, have a higher risk of stress concentration due to thermal expansion, necessitating the preferential selection of low-alloy chromium-based hot-work mold steels. These materials reduce heat treatment deformation by lowering the content of alloying elements, while maintaining sufficient toughness to resist cracking. Conversely, small and medium-sized molds prioritize a balance between heat resistance and crack resistance. High-chromium, high-molybdenum mold steel has become the mainstream choice due to its excellent comprehensive performance. Optimized carbide morphology and distribution can significantly slow the propagation rate of thermal fatigue cracks.
The quality of smelting and the heat treatment process have a decisive impact on the material's performance. High-purity mold steel has low levels of harmful impurities such as sulfur and phosphorus, reducing stress concentration sources at grain boundaries and lowering susceptibility to hot cracking. Vacuum heat treatment technology, through precise control of the heating-cooling rate, avoids decarburization or carburization on the cavity surface, while optimizing microstructure uniformity and improving the mold's thermal fatigue life. For complex mold structures, multi-stage preheating and graded quenching processes can effectively control deformation and ensure that the cavity dimensional accuracy meets requirements.
Surface strengthening technology provides a supplementary means to improve the high-temperature resistance and thermal fatigue resistance of molds. Shot peening introduces a residual compressive stress layer on the cavity surface, which can offset some thermal stress and delay crack initiation. Nitriding treatment forms a hard compound layer that significantly improves surface hardness and wear resistance, reducing erosion damage to the cavity from molten metal. Steam treatment enhances corrosion resistance by generating a dense oxide film on the surface. These technologies can be applied individually or in combination, with reinforcement solutions customized based on the specific failure mode of the mold.
The selection of materials for aluminum alloy die-casting molds requires comprehensive consideration of factors such as operating temperature, stress state, structural dimensions, and cost. Through optimized alloy design, controlled smelting quality, precise heat treatment, and surface strengthening, a synergistic improvement in high-temperature resistance and thermal fatigue resistance can be achieved. The development trend of modern mold steels is towards higher purity, higher uniformity, and functional versatility to meet the continuous demands of the aluminum alloy die-casting industry for long mold life, high precision, and low maintenance costs.
