Materials Processing and Design

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Materials Processing and Design
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Process Attributes
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Process Selection
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Classes of Processes
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Process Selection Charts

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

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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

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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

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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

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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
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Tolerance and Surface Finish Chart

• Tolerance is the permitted slack in the dimension
  of a part, e.g. 1000.1 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

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Tolerance and Surface Finish Chart
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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

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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

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Case Studies Forming a Fan
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Case Studies Forming a Fan
Surface smoothness is the discriminating
requirement
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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

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Case Studies Fabricating a Pressure Vessel
Size is the discriminating requirement
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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

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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

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Case Studies Forming a Silicon Nitride Microbeam
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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