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Materials Processing and Design

Materials Processing and Design. Process Attributes. Process Selection. Classes of Processes. Process Selection Charts. Size-Shape chart Information Content-Size chart Size-Melting Point chart Hardness-Melting Point chart Tolerance and Surface Finish Process Cost. Size-Shape Chart.

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Materials Processing and Design

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  1. Materials Processing and Design

  2. Process Attributes

  3. Process Selection

  4. Classes of Processes

  5. Process Selection Charts • Size-Shape chart • Information Content-Size chart • Size-Melting Point chart • Hardness-Melting Point chart • Tolerance and Surface Finish • Process Cost

  6. Size-Shape Chart • Volume contours V = At • Aspect ratio  = t/l  t/A1/2 • There are inaccessible zones on the chart – it is not possible to create shape with smaller surface-to-volume ratio than that of a sphere

  7. Information Content-Size chart • Complexity of shape can be measured in terms of: • Number of independent dimensions • Precision with which these dimensions are specified • Symmetry, or lack of it. • The first two aspects are captured approximately by the quantity

  8. Size-Melting Point Chart • Low melting metals can be cast by any one of the casting techniques; as Tm rises, the range of primary-shaping techniques becomes more limited • The ‘surface-tension limit’ is a lower size limit for gravity-fed castings • The addition of a pressure, e.g. in pressure die casting or centrifugal casting, overcomes this limit

  9. Hardness-Melting Point Chart • Yield strength limits the ability to deform and machine • Forging and rolling pressure, tool loading and the heat generated during machining depends on the flow strength or UTS • Real materials occupy only the region between the two heavy lines because hardness (H) and Tm are inter-dependent.  Is the atomic or molecular volume

  10. Tolerance and Surface Finish Chart • Tolerance is the permitted slack in the dimension of a part, e.g. 100±0.1 mm • Surface finish is measured by the RMS amplitude of the irregularities on the surface, e.g R = 10 m. • Obviously, T > 2R. Real processes gives T which range from 10R to 1000R. • Processing cost increase almost exponentially as the requirement for T and R. • Polymer can easily attain high surface smoothness but T < 0.2 mm is seldom achievable.

  11. Tolerance and Surface Finish Chart

  12. Process Cost • Commonsense rules for minimizing cost • Keep things standard and simple • Do not specify more performance than is necessary • Breakdown of Cost • Cm: material cost • Cc: capital investment • CL: labour cost (per unit time) • n: batch size • : batch rate

  13. Case Studies – Forming a Fan • To make a fan of radius 60 mm with 20 blades of average thickness 3 mm • Must be cheap, quiet and efficient • Materials selection procedure identified aluminium alloys and nylon • Form in a single operation to minimize process costs, i.e. net-shape forming – leaving the hub to be machined

  14. Case Studies – Forming a Fan

  15. Case Studies – Forming a Fan Surface smoothness is the discriminating requirement

  16. Case Studies – Fabricating a Pressure Vessel • Tough steel was chosen as the material • Inside radius is 0.5 m and height is 2m, with removable end-caps; operating pressure is 100 MPa. • Outside radius is calculated as 0.7m, surface area  15 m2 and volume  1.5 m3; weight  12 tonnes • Precision and surface roughness are both not important

  17. Case Studies – Fabricating a Pressure Vessel Size is the discriminating requirement

  18. Case Studies – Fabricating a Pressure Vessel • Other consideration includes: • Casting is prone to including defects; elaborate ultrasonic testing needed • Welding is also defect-prone and requires elaborate inspection • Forging or machining from a forged billet are best because the large compressive deformation during forging heals defects and aligns oxides and inclusions in a less harmful way

  19. Case Studies – Forming a Silicon Nitride Microbeam • The ultimate in precision mechanical metrology is the atomic force microscope • Design requirements: • Minimum thermal distortion • High resonant frequency • Low damping • Silicon carbide and silicon nitride are suitable materials

  20. Case Studies – Forming a Silicon Nitride Microbeam

  21. Case Studies – Forming a Silicon Nitride Microbeam • Casting or deformation methods are impossible for the materials • Powder methods cannot achieve the size or precision required • CVD and evaporation methods of microfabrication are the best bet here

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