Concise Summary on Cast Metal Forming Technologies and Powder Metallurgy

  • Casting Overview:

    • Involves pouring molten metal into molds to create shapes.
    • Articles produced via casting are often near final shape, unlike ingots, which are subsequently processed.
    • Economical compared to forging or fabrication, and can offer non-directional properties.

  • Basic Casting Considerations:

    1. Metal Melting: Must melt cleanly and economically using various fuels (coke, oil, gas).
    2. Mould Cavity: Should account for cooling shrinkage, allow gas escape, and not restrain cooling casting to avoid internal stress or failure.
    3. Removal of Casting: Solidified casting must be removable; issues arise with permanent molds (like die-casting).
    4. Pattern Making: Patterns are made by patternmakers and can be used directly or to create pattern plates or dies, with appropriate allowances for shrinkage.

  • Casting Techniques:

    • Sand Casting: Uses reclaimable sand mixtures; good for complex shapes but single-use per mold.
    • Mould Strength: Proper sand ramming is critical for strength; improperly ramming can cause defects (porosity).
    • Permanent Mould Casting: Uses durable metallic molds for high-volume production.
    • Die Casting: Metall is injected into molds at high pressure; limited shapes due to die complexity.
    • Investment Casting: Utilizes expendable patterns (often wax) for high-precision parts; results in smooth surface finishes.

  • Casting Defects and Challenges:

    • Common defects include porosity, surface imperfections, and troubles related to chemical interactions with the surroundings.

  • Powder Metallurgy:

    • Process includes powdering metals through various methods (mechanical disintegration, atomisation, etc.).
    • Involves pressing, sintering, and often results in components that are difficult to machine or of complex shapes.
    • Key advantages: reduced scrap, high production rates, ability to form complex shapes.
    • Limitations: generally lower strength than traditionally made components, higher costs for powders and dies.

  • Metal Injection Moulding (MIM):

    • Combines features of powder metallurgy with injection moulding; produces complex shapes effectively but involves extensive debinding processes.