Ferrous metals refer to darker-colored metals, primarily including iron, steel, and cast iron. These metals are mostly silvery-white or dark gray, possessing good electrical and thermal conductivity and ductility, and are widely used in manufacturing and construction. Non-ferrous metals refer to metals other than ferrous metals, such as copper, aluminum, zinc, lead, nickel, and tin. These metals typically exhibit a variety of colors, possessing high corrosion resistance, electrical and thermal conductivity, and are widely used in industry, electronics, and chemicals.
1. Iron is Key to Ferrous Metals
Ferrous metals contain iron, while non-ferrous metals do not. The presence of iron gives the metal higher electrical conductivity. The metal also possesses good tensile strength, allowing it to withstand high stress without breaking. With their superior mechanical properties, strength, and durability, ferrous metals can be used to manufacture parts for electrical and high-stress applications.
It is important to note that many ferrous metals are susceptible to corrosion due to their iron content. In applications involving large amounts of water, moisture, solvents, oils, and other corrosive substances, parts may wear down and eventually fail. To address this issue, additives such as chromium can be added during the smelting process of ferrous metals to enhance their corrosion resistance. By enhancing the properties of these alloys, manufacturers can develop precision castings suitable for such harsh environments.
Types of ferrous metals include steel, cast iron, stainless steel, and carbon steel. Generally, ferrous metals are less expensive than non-ferrous metals because they are widely used in a variety of fields, including aerospace components.
2. Common Corrosion Resistance of Non-ferrous Metals
Many non-ferrous metals are not easily corroded or rusted due to their lack of iron content. These metals are highly ductile and lightweight, but have lower tensile strength. They may also be magnetic or non-magnetic, offering more options depending on the application.
The main advantage of non-ferrous metals lies in their thermal and electrical conductivity. When metals possess these properties, current and heat can easily pass through with minimal resistance. For castings used in electrical or electronic systems, conductive metals allow current to pass through while absorbing and dissipating the heat generated by the current. This property prevents the system from overheating, thus avoiding performance degradation and malfunctions.
3. Key Differences in Investment Casting Processes
The core process of Feinguss is “wax pattern making → shell coating → dewaxing → baking → pouring → cleaning.” The parameter selection and operational requirements for each stage differ significantly between ferrous and non-ferrous metals, as follows:
3.1. Wax Pattern and Module Making: Different Strength Requirements
The wax pattern is the “prototype” in Feinguss, and its strength must match the subsequent shell coating and handling requirements. Due to the different weight and dimensions of the castings after pouring, the requirements for wax patterns differ between the two types of metals:
- Ferrous Metals: Investment Castings are typically thicker (≥3mm) and heavier (from several kilograms to tens of kilograms). Therefore, the wax pattern needs higher room temperature strength and high temperature stability (to avoid deformation during shell coating). A “paraffin-stearic acid” composite wax (with a higher proportion of stearic acid, resulting in greater hardness) is often used, or reinforcing components such as polyethylene are added.
- Non-ferrous metals: Investment Castings are mostly thin-walled parts (less than 1mm, such as aluminum alloy components for aerospace) and lightweight. Wax models prioritize fluidity and molding precision (replicating complex details, such as precision gears and decorative patterns). Pure paraffin wax or composite waxes with a low stearic acid ratio are often used. Some precision parts even use “low-temperature wax” (melting point 40-60℃, easy to dewax and produces a fine finish).
3.2. Shell Preparation: Differences in Refractory Materials and Coating Layers
The shell is the “mold” for Feinguss, and it must withstand the high-temperature erosion and chemical corrosion of molten metal. Therefore, the selection of refractory materials and the number of coating layers must match the melting point of the metal:
| Process Steps:
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Ferrous Metals (taking stainless steel as an example):
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Non-ferrous metals (taking aluminum alloy as an example):
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| Refractory Materials
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Inner Layer (in direct contact with molten metal): High-melting-point materials, such as corundum (Al₂O₃) and mullite (3Al₂O₃・2SiO₂) (resistant to temperatures above 1600℃ and corrosion by molten steel); Outer Layer: Low-cost silica sand + silica sol (providing structural strength).
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Inner layer: Low to medium melting point materials, such as quartz powder and zircon powder (resistant to temperatures below 1000℃, low cost); Outer layer: Quartz sand + water glass (good fluidity, easy to coat). Titanium alloy is special: high-purity corundum is required (to avoid reaction between titanium and silicon). |
| Number of Coating Layers
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6-10 layers (Due to the high temperature and impact of the molten metal, a thick shell is required to prevent cracking)
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3-6 layers (Low molten metal temperature, thin-walled parts require a thin shell to ensure heat dissipation and avoid shrinkage cavities in the Feinguss)
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| Mold Drying
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Long drying time (8-12 hours per layer), requiring strict humidity control (silica sol shells are sensitive to humidity to avoid strength reduction)
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Short drying time (4-8 hours per layer), water glass shells can dry naturally, resulting in higher efficiency.
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3.3. Dewaxing and Firing: Differences in temperature control
Dewaxing removes the wax pattern to form the cavity, while firing further removes residual wax and improves shell strength. The temperatures for both must be matched to the melting point of the wax pattern and the characteristics of the metal.
Dewaxing methods:
- Ferrous metals: Mostly use “steam dewaxing” (100-120℃ saturated steam, quickly melts the wax pattern, suitable for high-strength wax patterns); some large parts use “hot water dewaxing” (80-90℃, low cost).
- Non-ferrous metals: Low-temperature wax is dewaxed using hot water (60-80℃ to avoid softening and deformation of the wax model); high-temperature wax (such as wax for copper alloys) is still dewaxed using steam.
Firing temperature:
- Ferrous metals: 800-1100℃ (to thoroughly burn off residual wax, ensuring a dense sintered shell and resistance to molten steel erosion), firing time 2-4 hours.
- Non-ferrous metals: Aluminum alloys/Magnesium alloys: 400-600℃ (to avoid over-sintering of the shell, which could reduce permeability and cause porosity in the casting); Copper alloys: 600-800℃; Titanium alloys: 1000-1200℃ (requires high-vacuum firing to prevent oxidation).
3.4. Casting Process: Temperature, Atmosphere, and Flowability Control
Feinguss is the core step of injecting molten metal into the mold. Due to differences in melting point and oxidizing properties, the investment casting parameters differ significantly between the two types of metals:
Casting Temperature:
- Ferrous Metals: Significantly higher than their melting point (e.g., carbon steel melting point 1450℃, casting temperature 1550-1650℃). Increased flowability is needed to fill thick-walled castings and avoid cold shuts.
- Non-ferrous Metals: Close to their melting point (e.g., aluminum alloy melting point 660℃, casting temperature 700-750℃). Excessive temperatures can easily lead to oxidation and shrinkage cavities in the casting. Titanium alloys are special (melting point 1668℃), requiring high-temperature casting at 1800-1900℃, and necessitating vacuum or inert gas protection (titanium readily reacts with O and N at high temperatures).
Casting Atmosphere:
- Ferrous Metals: Can be cast in the atmosphere (steel and cast iron oxidation can be removed through subsequent cleaning, and oxidation has minimal impact on thick-walled parts).
- Non-ferrous metals: Aluminum and magnesium alloys require degassing treatment (argon or hexachloroethane is used to remove hydrogen from the molten metal before casting to prevent porosity); copper alloys can be cast in the atmosphere, but must be covered with flux to prevent oxidation; titanium alloys must be vacuum cast (absolute vacuum < 10⁻³ Pa).
Casting methods:
- Ferrous metals: Large parts use “bottom casting” (the molten metal fills the mold smoothly, avoiding erosion of the mold shell); small parts use “top casting” (to increase the filling speed).
- Non-ferrous metals: Thin-walled parts use “pressure casting” (such as low-pressure investment casting, which forces the molten metal to fill the mold cavity, reducing defects); precision parts use “vacuum casting” (to increase density).
3.5. Post-treatment: Differences between Cleaning and Heat Treatment
Post-treatment determines the final performance and appearance of the investment casting. Due to differences in surface characteristics and performance requirements, the processes for the two types of metals differ significantly:
Surface Cleaning:
- Ferrous Metals: Due to the high hardness of the mold shell, residual shells need to be removed by sandblasting (corundum sand) or shot blasting (steel shot); oxide scale is removed by pickling (hydrochloric acid + nitric acid).
- Non-ferrous Metals: The mold shell is easily detached; it can be removed by sandblasting (quartz sand, avoiding damage to the investment casting surface) or chemical descaling (e.g., dissolving quartz in NaOH solution for aluminum alloys); copper alloys can be pickled (sulfuric acid + hydrogen peroxide).
Heat Treatment:
- Ferrous Metals: Require normalizing + tempering (to adjust hardness and eliminate internal stress); stainless steel requires solution treatment (to improve corrosion resistance).
- Non-ferrous Metals: Aluminum alloys require aging treatment (to improve strength); magnesium alloys require annealing (to eliminate stress and prevent deformation); copper alloys require quenching + tempering (to improve toughness).
4. Development Trends
Ferrous Metals: Development of low-shrinkage alloys (e.g., high-nickel steel) combined with 3D printing wax investment casting technology.
Non-ferrous Metals: The widespread adoption of vacuum melting processes for magnesium/titanium alloys is driving the development of lightweight aerospace.
Through appropriate material selection and process optimization, investment casting can meet the needs of various fields, from micro-jewelry to large turbine blades.
