Introduction to Manufacturing

1
Introduction to Manufacturing

• Structure of Metals (l.u. 2/1/10)

2
Properties of Metals

• Behavior (properties) depends on structure
• Some materials are hard, ductile, brittle, soft,
  tough
• Crystal/Grain structure affect properties
• Processing affects grains (material properties)

(www.townwest.com)
(www.gw-svr-a.org.uk)
3
Material Selection

• Mechanical, physical, and chemical properties
  must be taken into account
• Atomic structure of metals can significantly
  affect
• Designs (form, fit, function)
• Service requirements (durability, life, wear,
  fatigue, etc.)
• Compatibility with other materials
• Environmental and economic factors

4
Deformation/Strength of Crystals

• When a crystal is subjected to an external force
  it undergoes
• 1. Elastic (resilient) deformation.
• or
• 2. Plastic (permanent) deformation.
• Basic mechanisms by which deformation occurs
• slipping (shearing force)
• twinning (bending, hcp)

5
Structure of Metals

• When metals solidify from a molten state, the
  atoms arrange themselves into various
  configurations, called crystals (or grains)
• Typical Unit Cell Patterns for Metals
• bcc (body-centered cubic)
• fcc (face-centered cubic)
• hcp (hexagonal close-packed)

6
Body-Centered Cubic (bcc)

• Good Strength/Moderate Ductility (48 slip
  systems, high shear stress)
• Molybdenum, tungsten, alpha iron, chromium,
  tantalum

7
Face-Centered Cubic (fcc)

• Moderate Strength/Good Ductility (12 slip
  systems)
• Aluminum, Copper, Nickel, Silver, Lead, Gamma
  Iron, Gold, Platinum

Steel Making Rolling Steel (Hot working) Steel
Origins
8
Hexagonal Close-Packed (hcp)

• Brittle at room temperature (3 slip systems)
• Magnesium, Zinc, Beryllium, Cobalt, Zirconium,
  Alpha Titanium, Cadmium

9
(Black Kohser, 2008, p. 60)
10
Allotropism

• (Polymorphism) Structure (atomic) change with
  change in temperature
• Different crystal structures, within the same
  metal, may form at different temperatures.
• E.g. Hotworking a steel may change from BCC to
  FCC as temperature rises (becomes more ductile,
  soft easier to process)

11
Deformation/Strength of Crystals

• Slip systems
• combination of slip plane and slip direction
• The higher the slip systems, the higher the
  probability that applied shear stress will cause
  slip along one of the systems
• Slip systems of 5 and above are considered ductile

12
Deformation/Strength of Crystals

• Crystal Structure Imperfections
• Line Defects dislocations.
• Point Defects atom missing (vacancy), extra
  (interstitial), or foreign (impurity).
• Inclusions non-metallic elements (oxides,
  sulfides, and silicates).
• Planar Imperfections grain boundaries.

13
Grains and Grain Boundaries

• Metals are polycrystal structures, composed of
  many individual, randomly oriented grains
  (crystals).
• Several factors affect the number, size and
  distribution of the grains (nucleation).

14
Zinc Grains
(www.doitpoms.ac.uk)
15
Grain Structure of Eutectoid Steel

• (www.matsci.ucdavis.edu)

16
Grains and Grain Boundaries

• Generally, rapid cooling will produce small
  grains while slow cooling will produce larger
  grains.
• Grain size affect properties (large grain size is
  generally associated with low strength, hardness,
  and ductility).
• Grain boundaries also affect properties since the
  atoms in the boundaries are packed less
  efficiently.

17
(www.westcoastcorrosion.com)
(www.substech.com)
18
Deformation of Polycrystalline Metals

• During plastic deformation grain boundaries
  remain and mass continuity is maintained,
  although the metal will exhibit greater strength
  because of the entanglement of the dislocations
  with the grain boundaries.
• Strain hardening
• Can be through compression, rolling, etc.

19
Deformation of Polycrystalline Metals

• Rolling threads (compression) results in stronger
  structure as opposed to cutting threads

(www.5bears.com)
(www.precisionscrewthread.com)
20
Deformation of Polycrystalline Metals

• Anisotropy (different properties for each
  direction), plywood example.
• Preferred Orientation when crystals are
  subjected to tension, they align themselves
  toward the direction of pulling.
• Under Compression slip directions are aligned
  perpendicular to direction of compression
• Mechanical Fibering result of alignment of
  impurities and voids in metal during deformation.

21

• At the left, an isotropic grain structure The
  grains are in random orientations and have equal
  properties.
• Above-right, the grain structure exhibits
  elongation as a result of an applied force.
  Alignment of the grains (from pulling) in the
  right image is known as Preferred Orientation.

22
Grain Flow - Evident through Processing

• Compressive force results in anisotropic
  (unequal) properties throughout part.

• (Black Kohser, 2008, p. 372)

23
Recovery, Recrystallization, Grain Growth
(through heating)

• 1. Recovery
• at certain temperatures (below
  recrystallization), stresses of highly deformed
  regions are relieved (subgrain boundaries begin
  to form).
• 2. Recrystallization
• at particular temperatures, new strain-free
  grains form to replace older grains (strength
  goes down, ductility goes up).
• 3. Grain Growth
• by raising the temperature of a metal, the grains
  grow (eventually exceeding the original size),
  affecting mechanical properties.

24
(Black Kohser, 2008, p. 67)
25
Cold, Warm, and Hot Working

• Cold Working
• plastic deformation is carried out at room
  temperature.
• Hot Working
• plastic deformation is carried out above
  recrystallization temperature.
• Warm Working
• plastic deformation is carried out above room
  temperature but below recrystallization levels.

26
Reversing Cold Working Effects
Making Cases
27
Annealing

• Restoring original properties by heating within a
  certain temperature range for a period of time
• May facilitate easier processing (further
  processing)

Annealed Brass
28
Superplastic Flow

• A materials large (300-2000) uniform
  elongation (prior to necking/fracture)
• Bubble gum behavior
• Plastics, glass, titanium alloys, zinc-al alloys

29
Superplastic Flow of Grains in Pb-Sn

• (www.mse.mtu.edu)

30
Topic Support

• Platinum Grains http//www.platinummetalsreview.c
  om/dynamic/article/view/50-3-120-129
• Cold-Rolled Niobium Grains http//www.mpie.de/137
  0/?type1
• Squeeze Casting http//www.key-to-nonferrous.com/
  default.aspx?IDCheckArticleNM172