1. Inoculation and Modification Treatment
Inoculation treatment refers to the process of adding a small amount of other substances to molten metal during solidification to promote nucleation and inhibit growth, thereby refining grains and improving the structure. Conventionally, adding additives to cast iron is called inoculation treatment; adding additives to cast steel and aluminum alloys is termed modification treatment. In essence, inoculation treatment primarily affects nucleation, while modification treatment changes the crystal growth mechanism (inhibiting growth), thus affecting crystal morphology.
Currently, there is a wide variety of inoculants for cast iron. Inoculants containing barium, bismuth, and rare earth elements can slow down the fading of the inoculation effect. Inoculation methods (i.e., the method of adding the inoculant) mainly include ladle inoculation, stream inoculation, instantaneous inoculation (pouring cup inoculation, cored wire inoculation, stream inoculation), and in-mold inoculation. Instantaneous inoculation and in-mold inoculation can prevent inoculation fading, require a small amount of inoculant, and achieve good inoculation effects, leading to their increasingly widespread application in cast iron production.
The development trend for modification treatment is towards low-cost, high-efficiency, non-polluting, and multi-functional composite modification. Cast steel undergoes modification treatment using alloying modifiers containing rare earth elements, which refine grains while also purifying the molten steel. The ZL109 alloy, modified with a composite of Cu-P and Al-Ti-B, results in well-refined primary silicon and eutectic silicon in its structure. Sr salts can change the silicon phase in aluminum-silicon alloys from needle-like to short rod-like (with a length below 100 μm) or curved fibrous forms.
2. Dynamic Crystallization
Besides inoculation and modification treatments for grain refinement and microstructure improvement, subjecting molten metal to ultrasonic vibration treatment or electromagnetic stirring can also achieve the goal of refining grains and improving casting quality. Ultrasonic treatment can refine the as-cast structure of AS41 magnesium alloy, making the MgSi phase finer and spheroidized, with grain sizes only 30% to 50% of those without ultrasonic treatment. Currently, grain refinement technology is moving towards combination methods, forming combined processes such as inoculation treatment coupled with electromagnetic stirring. For example, when pure aluminum is subjected to a direct current pulsed magnetic field combined with inoculation treatment using 0.05% Al-5Ti-B, the resulting grain size is even finer and more uniformly distributed compared to applying the direct current pulsed magnetic field alone.
3. Directional Solidification
Directional solidification, also known as directional crystallization, is a process method that enables metals or alloys to grow crystals directionally within the melt. The main methods for directional solidification include the exothermic mold method, the power reduction method, the rapid solidification method, and the liquid metal cooling method. Directional solidification is primarily used to prepare columnar grains and single crystals and has been successfully applied in the production of nickel-based columnar grain and single crystal engine blades. Columnar grain structures eliminate transverse grain boundaries, while single crystals have no grain boundaries at all, significantly enhancing the high-temperature strength, creep resistance, stress-rupture properties, and thermal fatigue performance of the blades.
Single crystal blades produced by directional solidification are formed through the spiral selection growth from polycrystals. Whether for single crystal growth or columnar grain growth structures, the <100> direction is the preferred growth orientation. Through years of intensive research and development, the industry has mastered the core technology for single crystal engine blades, breaking monopolies. Extensive data and experience have been accumulated in areas such as ceramic core technology, corundum shell mold technology, single crystal selection and directional solidification processes, recrystallization control for single crystal blades, firing temperature control, pouring process parameters, pulling rate, process route adjustments, and mold improvements. The precision casting technology for hollow single crystal blades is becoming increasingly sophisticated.
