Fatigue Failure

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Primary forming process (casting)
A casting is produced by pouring molten
metal into a mould cavity and allowing it to
solidify.
The mould cavity is the shape of the required
component.
Vital factors in determining the outcome are :
Fluidity (easy to flow)
Fusibility (low melting point)
Casting
A fabrication process whereby molten metal is poured
into a mold cavity having the desired shape; upon
solidification, the metal assumes the shape of the mold
but experiences some shrinkage.
Casting techniques are used when:
1.
The finished shape is so large or complicated that
any other method would be impractical.
2.
A particular alloy is so low in ductility that forming
by either hot or cold working would be difficult.
3.
In comparison to other fabrication processes,
casting is the most economical.
Classification of casting process
Sand Casting
Investment Casting
Permanent Mould Casting
Die Casting
Sand casting
The traditional method of casting metals is in
sand moulds and has been used for many
years.
A two-piece mold is formed by packing sand
around a pattern that has the shape of the
intended casting.
The major features of sand moulds
The flask (cope and drag)
A pouring cup
A sprue
Risers
Cores
Vents
Sequence of operations for sand casting
Investment casting
Also called lost-wax process
First used 4000 – 3000 BC
The pattern is made of wax or of a plastic by molding or
rapid prototyping techniques
Term investment derives from the fact that the pattern is
invested with the refractory material
Need careful handling because they are not strong enough
to withstand the forces involved in mold making
Wax can be recovered and reused
Investment casting
The pattern is made from a wax or plastic that
has a low Tm. Around the pattern is poured a
fluid slurry, which sets up to form a solid mold or
investment.
The mold is then heated, such that the pattern melts and is
burned out, leaving behind a mold cavity having the desired
shape.
This technique is employed when high dimensional accuracy,
reproduction of fine detail, and an excellent finish are
required (in jewelry and dental crowns and inlays, and blades
for gas turbine and jet engine impellers)
Sequences involve in investment casting
1. WAX INJECTION : Wax replicas of the
desired castings are produced by injection
molding. These replicas are called
patterns.
2. ASSEMBLY : The patterns are attached to
a central wax stick, called a sprue, to form
a casting cluster or assembly.
3. SHELL BUILDING : The shell is built by immersing
the assembly in a liquid ceramic slurry and then into a
bed of extremely fine sand. Up to eight layers may be
applied in this manner.
4. DEWAX : Once the ceramic is dry, the wax is melted
out, creating a negative impression of the assembly
within the shell.
Die casting
Further example of permanent-mold casting
Molten metal is forced into the die cavity at pressures
ranging from .7MPa – 700MPa
Parts made from here range from:
Hand tools
Toys
Appliance components
There are two basic types of die casting machines
Hot-chamber - involves the use of a piston to push
molten metal in to the die cavity
Cold-chamber – molten metal is poured in to the
injection chamber & the shot chamber is not heated
Die casting
The liquid metal is forced into a mold (die) under
pressure and at a relatively high velocity, and
allowed to solidify with the pressure maintained.
A two-piece permanent steel mold is employed; when
clamped together, the two pieces form the desired shape.
When complete solidification has been achieved, the mold
pieces are opened and the cast piece is ejected.
Rapid casting rates are possible, making this an
inexpensive method; a single set of molds may be used for
thousands of castings.
This technique lends itself only to relatively small pieces and to alloys of low melting
points such as Zn, Al, and Mg
Hot chamber die casting
1. The die is closed and the piston
rises, opening the port and
allowing molten metal to fill the
cylinder. Pressure range up to 35
MPa
2. The plunger moves down and seals
the port pushing the molten metal
through the gooseneck and nozzle
into the die cavity, where it is held
under pressure until it solidifies.
Hot chamber die casting
3. The die opens and the cores, if any, retract.
The casting remains in only one die, the
ejector side. The plunger returns, allowing
residual molten metal to flow back through
the nozzle and gooseneck.
4. Ejector pins push the casting out of
the ejector die. As the plunger
uncovers the filling hole, molten
metal flows through the inlet to refill
the gooseneck, as in step (1).
Cold chamber die casting
1. The die is closed and the
molten metal is ladled into the
cold-chamber shot sleeve.
2. The plunger pushes the molten metal
into the die cavity where it is held
under pressure until solidification.
Pressures ranges from 20 to 70 MPa.
Cold chamber die casting
3. The die opens and the plunger
advances, to ensure that the casting
remains in the ejector die. Cores, if
any, retract.
4. Ejector pins push the casting
out of the ejector die and the
plunger returns to its original
position.
Adv/disadv of different casting process
Process
Advantage
Disadvantage
Sand casting
Almost any metal, no size limit
Wide tolerances, finishing
required
Die casting
Excellent dimensional accuracy,
high production rate
Die cost high, size
limited, usually limited to
nonferrous
Investment casting Intricate shapes, excellent
surface finish, almost any metal
cast
Part size limited,
expensive mould
Casting defects
Various defects can develop in manufacturing processes
depending on factors such as materials, part design, and
processing techniques.
While some defects affects only the appearance of the
parts made, others can have major adverse effects on the
structural integrity of the parts.
Casting defects - fins
Metallic Projections: fins (flash), swells, and scabs
Fins are excessive amounts of metal created by
solidification into the parting line of the mold
Fins are removed by grinding
Casting defects - swells
Swells are excessive amounts of metal in the vicinity of
gates or beneath the sprue
Casting defects - scabs
Scabs are surface slivers caused by splashing and rapid
solidification of the metal when it is first poured and
strikes the mold wall
Casting defects- blowholes/pinholes
Blowholes, pinholes, shrinkage cavities, & porosity
Blowholes and pinholes are holes formed by gas entrapped
during solidification
Casting defects- shrinkage
Shrinkage, which causes dimensional changes, is the result of the
following three sequential events:
Contraction of the molten metal as it cools prior to solidification.
Contraction of the metal during the phase change from liquid to
solid.
Contraction of the solidified metal (the casting) as its temperature
drops to ambient temperature.
Casting defects- shrinkage cavities
Shrinkage cavities are cavities that have a rougher shape
and sometimes penetrate deep into the casting
Shrinkage cavities are caused by lack of proper feeding or
non-progressive solidification
Casting defects- porosity
Porosity is pockets of gas inside the metal caused by microshrinkage during solidification.
Casting defects- lack of fusion
Lack of fusion is a discontinuity caused when two streams of liquid
in the solidifying casting meet but fail to unite
Rounded edges indicate poor contact between various metal
streams during filling of the mold
Casting defects- hot tear, hot crack
Cracks in casting and are caused by hot tearing, hot cracking, and lack
of fusion (cold shut)
A hot tear is a fracture formed during solidification because of
hindered contraction
A hot crack is a crack formed during cooling after solidification
because of internal stresses developed in the casting
Lack of fusion is a discontinuity caused when two streams of liquid
in the solidifying casting meet but fail to unite
Rounded edges indicate poor contact between various metal
streams during filling of the mold
Casting defects- discontinuities
Cracks in casting and are caused by hot tearing, hot cracking, and lack
of fusion (cold shut)
A hot tear is a fracture formed during solidification because of
hindered contraction
Casting defects- hot crack
A hot crack is a crack formed during cooling after solidification
because of internal stresses developed in the casting
Casting defects- defective surfaces
Casting surface irregularities
Casting defects- inclusions
Particles of foreign materials in the metal matrix
Casting design guidelines
Account for shrinkage
- geometry
- shrinkage cavities
Casting design guidelines
(a) avoid sharp corners
(b) use fillets to blend section changes smoothly
(c1) avoid rapid changes in cross-section areas
Casting design guidelines
Avoid large, flat areas
- warpage due to residual stresses (why?)
Casting design guidelines
Avoid rapid changes in cross-section areas
If unavoidable, design mold to ensure
- easy metal flow
- uniform, rapid cooling (use chills, fluid-cooled tubes)
Casting design guidelines
Provide drafts and tapers
easy removal, avoid damage
along what direction should we taper ?
References
1. S. Kalpakjian, S.R. Schmid: Manufacturing Engineering &
Technology, 5th edition, Prentice-Hall International, 2006.
2. E. Paul Degarmo, J. R. Black, R. A. Kohser; Materials and
Processes in Manufacturing, 9th edition, John Wiley & Sons,
Inc, 2003.
3. R. L. Timings, S. P. Wilkinson: Manufacturing Technology,
2nd edition, Pearson Education Limited, London, 2000.
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