تصنيع 2

    Cards (27)

    • Fabrication Techniques
      Various metal-forming methods, casting, powder metallurgy, welding, and machining
    • Factors affecting choice of fabrication technique
      • Properties of the metal
      • Size and shape of the finished piece
      • Cost
    • Casting
      A process in which molten metal flows by gravity or other force into a mold where it solidifies in the shape of the mold cavity
    • Casting process
      1. Melt the metal
      2. Pour it into a mold
      3. Cool and solidify
    • Ingot
      A large casting that is simple in shape and intended for subsequent reshaping by processes such as rolling or forging
    • Shape casting
      The production of more complex geometries that are much closer to the final desired shape of the part or product
    • Casting parts
      • Dental crowns
      • Jewelry
      • Engine blocks and heads for automotive vehicles
      • Machine frames
      • Railway wheels
      • Pipes
      • Pump housings
    • Materials that can be cast
      • Ferrous metals
      • Nonferrous metals
      • Polymers
      • Ceramics
    • Casting
      • Can be used to create complex part geometries, including both external and internal shapes
      • Some casting processes are capable of producing parts to net shape, requiring no further manufacturing operations
      • Some casting processes are near net shape, requiring some additional shape processing (usually machining)
      • Can be used to produce very large parts (more than 100 tons)
      • Can be performed on any metal that can be heated to the liquid state
      • Some casting methods are suited to mass production
    • Disadvantages of casting
      • Limitations on mechanical properties
      • Porosity
      • Poor dimensional accuracy and surface finish for some casting processes
      • Safety hazards to humans when processing hot molten metals
      • Environmental problems
    • Foundry
      A factory equipped for making molds, melting and handling metal in molten form, performing the casting process, and cleaning the finished casting
    • Mold
      A cavity whose geometry determines the shape of the cast part. The actual size and shape of the cavity must be slightly oversized to allow for shrinkage that occurs in the metal during solidification and cooling.
    • Mold materials
      • Sand
      • Plaster of Paris (gypsum CaSO4-2H2O)
      • Ceramic
      • Metal
    • Casting process
      1. Heat the metal to a temperature high enough to completely transform it into a liquid state
      2. Pour the molten metal into the mold cavity
    • Open mold
      The liquid metal is simply poured until it fills the open cavity
    • Closed mold
      A passageway, called the gating system, is provided to permit the molten metal to flow from outside the mold into the cavity
    • Solidification
      1. Occurs when the temperature of the molten metal in the mold drops sufficiently (e.g., to the freezing point for a pure metal)
      2. Involves a change of phase of the metal
      3. Time is required to complete the phase change, and considerable heat is given up in the process
    • Expendable-mold casting
      The mold must be destroyed in order to remove the casting. More intricate casting geometries are generally possible with these processes.
    • Permanent-mold casting

      The mold can be used over and over to produce many castings. It is made of metal (or, less commonly, a ceramic refractory material) that can withstand the high temperatures of the casting operation.
    • Specific heat and other thermal properties of a solid metal vary with temperature, especially if the metal undergoes a change of phase during heating
    • A metal's specific heat may be different in the solid and liquid states
    • Most casting metals are alloys, and most alloys melt over a temperature range between a solidus and liquidus rather than at a single melting point
    • There are significant heat losses to the environment during heating
    • Pouring
      The introduction of molten metal into the mold, including its flow through the gating system and into the cavity
    • Factors affecting fluidity of molten metal during pouring
      • Pouring temperature
      • Metal alloy composition
      • Viscosity of liquid metal
      • Heat transfer to the surroundings
    • Superheat
      The difference between the temperature at pouring and the temperature at which freezing begins
    • Problems caused by turbulent flow during pouring
      • Formation of metal oxides, degrading casting quality
      • Mold erosion, gradual wearing away of mold surfaces
      • Affected geometry of cast part due to erosion in main cavity
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