CHAPTER 2
PART 2
FUNDAMENTALS OF
METAL CASTING
Ass.Pro.Dr. Nguyen Ngoc Ha
6. CONTRACTION
6.1. Shrinkage during Solidification and Cooling
• (0) starting level of molten metal immediately
after pouring; (1) reduction in level caused by
liquid contraction during cooling
6.1. Shrinkage during Solidification and Cooling
• (2) reduction in height and formation of
shrinkage cavity caused by solidification; (3)
further reduction in volume due to thermal
contraction during cooling o
74 trang |
Chia sẻ: Tài Huệ | Ngày: 19/02/2024 | Lượt xem: 64 | Lượt tải: 0
Tóm tắt tài liệu Fundamentals of metal casting - Nguyen Ngoc Ha (Part 2), để xem tài liệu hoàn chỉnh bạn click vào nút DOWNLOAD ở trên
f solid metal
6.2. Solidification Shrinkage
• Occurs in nearly all metals because the solid
phase has a higher density than the liquid
phase
• Thus, solidification causes a reduction in
volume per unit weight of metal
• Exception: cast iron with high C content
– Graphitization during final stages of freezing causes
expansion that counteracts volumetric decrease
associated with phase change
Solidification Contraction or Expansion
6.3. Hot Spots
Hot Spots and Shrinkage Cavities
Hot Spots and Shrinkage Cavities
6.4. Risers and Hot Spots
Why Riser?
• The shrinkage occurs in three stages,
1. When temperature of liquid metal drops from pouring to
zero temperature
2. When the metal changes from liquid to solid state, and
3. When the temperature of solid phase drops from freezing
to room temperature
• The shrinkage for stage 3 is compensated by providing
shrinkage allowance on pattern, while the shrinkage during
stages 1 and 2 are compensated by providing risers.
• The riser should solidify in the last otherwise liquid metal will
start flowing from casting to riser. It should promote
directional solidification. The shape, size and location of the
risers are important considerations in casting design
Progressive Solidification in Riser
6.5. Shrinkage Allowance
• Patternmakers correct for solidification
shrinkage and thermal contraction by making
the mold cavity oversized
• Amount by which mold is made larger relative
to final casting size is called pattern shrinkage
allowance
• Casting dimensions are expressed linearly, so
allowances are applied accordingly
6.6. Directional Solidification
• To minimize effects of shrinkage, it is desirable
for regions of the casting most distant from
the liquid metal supply to freeze first and for
solidification to progress from these regions
toward the riser(s)
– Thus, molten metal is continually available from risers to
prevent shrinkage voids
– The term directional solidification describes this aspect
of freezing and methods by which it is controlled
Achieving Directional Solidification
• Directional solidification is achieved using
Chvorinov's Rule to design the casting, its
orientation in the mold, and the riser system
that feeds it
– Locate sections of the casting with lower V/A
ratios away from riser, so freezing occurs first in
these regions, and the liquid metal supply for the
rest of the casting remains open
– Chills - internal or external heat sinks that cause
rapid freezing in certain regions of the casting
Achieving Directional Solidification
6.7. Chills
• Chills
Pieces of material placed in the mold to speed
up heat transfer in thicker areas of the part to
prevent shrinkage porosity
• Internal chills are left within the cast part;
external chills are removed
External Chills
• (a) External chill to encourage rapid freezing of
the molten metal in a thin section of the
casting; and (b) the likely result if the external
chill were not used
External Chills
Internal Chills
Types of Internal and External Chills used
in Casting
Various types of (a) internal and (b) external chills (dark areas
at corners) used in castings to eliminate porosity caused by
shrinkage. Chills are placed in regions where there is a larger
volume of metal, as shown in (c).
6.8. Riser Design
• Riser is waste metal that is separated from the
casting and re-melted to make more castings
• To minimize waste in the unit operation, it is
desirable for the volume of metal in the riser to be a
minimum
• Since the shape of the riser is normally designed to
maximize the V/A ratio, this allows riser volume to be
reduced to the minimum possible value
Riser
• Provide additional material to fill in as
shrinkage occurs
• Live risers receive hot metal directly entering the mold
• Dead risers are filled by hot metal that has already
passed through the mold
• Blind risers are closed to the atmosphere
• Open risers penetrate through the mold cavity
• Side risers feed the part through the runner / gate
system
• Top risers are attached directly to the part
Location of Risers and Open and Closed Risers
• Top riser has the
advantage of
additional pressure
head and smaller
feeding distance
over the side riser.
• Blind risers are
generally bigger in
size because of
additional area of
heat conduction
Location of Risers and Open and Closed
Risers
7. STRESSES IN CASTINGS
Hot Tears in Castings
Examples of hot tears in castings. These defects occur because the
casting cannot shrink freely during cooling, owing to constraints in
various portions of the molds and cores. Exothermic (heat-
producing) compounds may be used (as exothermic padding) to
control cooling at critical sections to avoid hot tearing
Hot Tears in Castings
8. CASTING DEFECTS
•Defects may occur due to one or more of the
following reasons:
– Fault in design of casting pattern
– Fault in design on mold and core
– Fault in design of gating system and riser
– Improper choice of moulding sand
– Improper metal composition
– Inadequate melting temperature and rate of
pouring
8.1. Classification of casting defects
Casting defects
Surface
Defect
Internal Defect Visible defects
Blow
Scar
Blister
Drop
Scab
Penetration
Buckle
Blow holes
Porosity
Pin holes
Inclusions
Dross
Wash
Rat tail
Swell
Misrun
Cold shut
Hot tear
Shrinkage/Shift
8.2. Surface Defects
These are due to poor design and quality of sand molds and
general cause is poor ramming
Blow is relatively large cavity produced by gases which displace
molten metal from convex surface. Scar is shallow blow generally
occurring on a flat surface. A scar covered with a thin layer of
metal is called blister. These are due to improper permeability or
venting. Sometimes excessive gas forming constituents in
moulding sand
8.2. Surface Defects
• A scab when an up heaved sand gets
separated from the mould surface and the
molten metal flows between the displaced
sand and the mold.
• Penetration occurs when the molten metal
flows between the sand particles in the mould.
These defects are due to inadequate strength
of the mold and high temperature of the
molten metal adds on it.
8.2. Surface Defects
• Drop is an irregularly-shaped projection on
the cope surface caused by dropping of sand.
• Buckle is a vee-shaped depression on the
surface of a flat casting caused by expansion
of a thin layer of sand at the mould face. A
proper amount of volatile additives in
moulding material could eliminate this defect
by providing room for expansion.
8.3. Internal Defects
The internal defects found in the castings are mainly due to
trapped gases and dirty metal. Gases get trapped due to hard
ramming or improper venting. These defects also occur when
excessive moisture or excessive gas forming materials are used for
mould making.
Blow holes are large spherical shaped gas bubbles, while porosity
indicates a large number of uniformly distributed tiny holes. Pin
holes are tiny blow holes appearing just below the casting surface.
Inclusions are the non-metallic particles in the metal matrix,
Lighter impurities appearing the casting surface are dross.
8.4. Visible Defects
8.4. Visible Defects
• Insufficient mould strength, insufficient metal,
low pouring temperature, and bad design of
casting are some of the common causes.
• Wash is a low projection near the gate caused
by erosion of sand by the flowing metal. Rat tail
is a long, shallow, angular depression caused by
expansion of the sand. Swell is the deformation
of vertical mould surface due to hydrostatic
pressure caused by moisture in the sand.
8.4. Visible Defects
• Misrun and cold shut are caused by
insufficient superheat provided to the liquid
metal.
• Hot tear is the crack in the casting caused by
high residual stresses.
• Shrinkage is essentially solidification
contraction and occurs due to improper use of
Riser.
• Shift is due to misalignment of two parts of
the mould or incorrect core location.
9. DESIGN FOR CASTS AND MOLDED
PARTS
9.1. Visualize the Casting
• It is difficult to follow section changes and shapes
from blueprint.
• Create a model to scale or full size to help designer
to:
– See how cores must be designed, placed or
omitted
– Determine how to mold the casting
– Detect casting weaknesses (shrinks / cracks)
– Determine where to place gates and risers
– Answer questions affecting soundness, cost and
delivery
9.2. Design for Soundness
Most metals, alloys, and plastics shrink when
they solidify
Design components so that all parts increase in
dimension progressively to areas where feeder
heads (risers) can be placed to offset shrinkage
• Disguise areas of shrinkage when unavoidable
- The mold and pattern should be made larger
than the casting by the amount of shrinkage
9.2. Design for Soundness
- Shrinkage of casting varies not only with
material but also with shape, thickness,
casting temperature, mold temperature and
mold strength
- Thicker areas will cool slower than thinner
areas. Areas of transition between thick and
thin (ribs, walls, embosses, etc) will be prone
to sink marks. Different tool shops and
different materials will require a certain rib-to-
wall thickness ratio.
9.2. Design for Soundness
• The table below shows an average amount of
shrinkage for important cast metals
Design Rules: Disguising Sink Marks
9.3. Avoid Sharp Angles & Corners
• When two or more sections conjoin, mechanical
weakness is induced at the junction interrupting free
cooling (most common defect in casting design).
– Replace sharp angles with radii and minimize heat and
stress concentration
– In cored parts avoid designs without cooling surfaces
– A rounded junction offers a more uniform distribution of
strength
Design Rules:Corners & Radii
Good Corner Design Incorrect Corner Design
Incorrect Corner Design Incorrect Corner Design
• Generous radius
• Uniform wall thickness
• Smooth flow transition
• Very sharp radii
• High stress concentration
• Sharp flow transition
• Inside / outside radius mismatch
• Non-uniform wall thickness
• Non-uniform flow transition
• Outside corner and inside radius
• Non-uniform wall thickness
• Non-uniform flow transition
• Shrinkage stress / voids / sinks
Sink
9.4. Minimize the Number of Sections
A well designed casting brings the minimum number
of sections together at one point.
• Staggering sections (where possible)
• Minimizes hot spot effects
• Eliminates weakness
• Reduces distortion
• Where staggering sections is not possible use a cored
hole through the center of the junction.
• Helps to speed solidification
• Helps to avoid hot spots
9.5. Employ Uniform Sections
• Thicker walls will solidify more slowly.
– This means they will feed solidifying thinner walls.
– Results in shrinkage voids in the thicker walls
• Goal is to design uniform sections that solidify
evenly.
– If this is not possible, all heavy sections should be
accessible to feeding from risers.
Design Rules: Wall Uniformity
Original Part Design
• Very thick wall sections
• Non-uniform wall thickness
• Sharp inside and outside radii
Improved Part Design
• Thinner wall sections
• More uniform wall thickness
• Inside and outside radii (when possible)
9.6. Correctly Proportion Inner Walls
• Inner sections of castings cool much slower than
outer sections.
– Causes variations in strength properties
• A good rule of thumb is to reduce inner sections to
90% of outer wall thickness.
• Avoid rapid section changes
– Results in porosity problems similar to what is
seen with sharp angles.
Design Rules: Wall Uniformity
Part gated from “thin to thick” hinders packing of thicker sections
and can create flow problems.
Gating from “thick to thin” when possible to improve flow
and allow thicker sections to be packed.
Internal runner to assist / improve the ability to pack the
thick section when gating from “thin to thick” is
necessary.
Correctly Proportion Inner Walls
• Whenever complex cores must be used, design for
uniformity of section to avoid local heavy masses of
metal.
• The inside diameter of cylinders and bushings should
exceed the wall thickness of castings.
– When the I.D. is less than the wall it is better to cast the
section as a solid.
– Holes can be produced by cheaper and safer methods than
with extremely thin cores
9.7. Fillet All Sharp Angles
• Fillets (rounded corners) have three functional purposes:
– To reduce the stress concentration in a casting in service
– To eliminate cracks, tears and draws at re-entry angles
– To make corners more moldable by eliminating hot spots
– Improves flow of material
• The number of different size fillet radii used in a pattern
should be the minimized
• Large fillets may be used with radii equaling or exceeding the
casting section.
– Commonly used to fulfill engineering stress requirements
– Reduces stress concentration
• Note: Fillets that are too large are undesirable – the radius of
the fillet should not exceed half the thickness of the section
joined.
9.7. Fillet All Sharp Angles
• Large fillets may be used with radii equaling or exceeding
the casting section.
– Commonly used to fulfill engineering stress requirements
– Reduces stress concentration
• Note: Fillets that are too large are undesirable – the radius
of the fillet should not exceed half the thickness of the
section joined.
• Tips to avoid a section size that is too large at an “L”, “V”
or “Y” junction.
• For an “L” junction :
– Round an outside corner to match the fillet on the inside wall. (If
this is not possible the designer must make a decision as to
which is more important: Engineering design or possible casting
defect)
• For a “V” or “Y” junction:
– Always design so that a generous radius eliminates localization
of heat.
Design Rules: Fillets & Corners
Design Rules: Fillets & Corners
Design Rules: Fillets & Corners
9.8.Avoid Abrupt Section Changes
• The difference in relative thickness of adjoining
sections should not exceed a ratio of 2:1.
• With a ratio less than 2:1 the change in thickness
may take on the form of a fillet.
• Where this is not possible consider a design with
detachable parts.
• With a ratio greater than 2:1 the recommended shift for
the change in thickness should take on the form of a
wedge.
– Note: wedge-shaped changes in wall thickness should not taper
more than 1 in 4.
• Where a combination of light and heavy sections is
unavoidable, use fillets and tapered sections to temper the
shifts.
Design Rules: Section Changes
Wall Thickness Transitions
Tapered Transition
Gradual TransitionStepped Transition
Core out thicker
areas where possible
Poor
Design
Better
Best
9.9. Maximize Design of Ribs & Brackets
• Ribs are only preferable when the casting wall cannot
be made strong or stiff enough on its own.
• Ribs have two functions:
– They increase stiffness
– They help to reduce weight
• Common mistakes that make ribs ineffective:
– Too shallow
– Too widely spaced
Design Rules:Rib Dimensions
• The thickness of the ribs should be
approximately 80% of the adjoining thickness
and should be rounded at the edge.
• The design preference is for ribs to be deeper
than they are thick.
• Ribs should solidify before the casting section
they adjoin.
• The space between ribs should be designed
such that localized accumulation of metal is
prevented.
• Preferably the ribs connect the attachment to
the loading point.
Design Rules:Rib Dimensions
Design Rules:Rib Dimensions
Correct Proportions
Radius
(fillet)
Sink
Mark
Shrinkage
VoidsExcessive
Radius
Part Wall
Rib
Excessive
Rib Wall
Thickness
9.9. Maximize Design of Ribs & Brackets
• Generally, ribs in compression offer a greater
safety factor than ribs in tension.
• Exception: Castings with thin ribs in
compression may require design changes to
provide necessary stiffening and avoid
buckling.
• Thin ribs should be avoided when joined to a
heavy section or they may lead to high stresses
and cracking
9.9. Maximize Design of Ribs & Brackets
• Avoid cross ribs & ribbing on both sides of a casting
– Cross ribbing creates hot spots and makes feeding difficult
– Alternative is to design cross-coupled ribs in a staggered
“T” form.
• Avoid complex ribbing
– Complicates molding, hinders uniform solidification and
creates hot spots
• Ribs meeting at acute angles may cause molding
difficulties, increase costs and aggravate the risk of
casting defects
• “Honeycombing” often will provide increased
strength and stiffness without creating hot spots
Design Rules: Rib Manufacturability
Design Rules: Rib Design
Maximize Design of Ribs & Brackets
• Brackets carrying offset loads introduce bending
moments, localized and in the body of the casting.
• Tips to avoid this problem:
– Taper “L” shaped brackets and make the length of contact
with the main casting as ample as possible.
– Brackets may frequently be cast separately and then
attached, simplifying the molding.
Maximize Design of Ribs & Brackets
• A ribbed bracket will offer a stiffness advantage, but
avoid heat concentration by providing cored openings
in webs and ribs.
– The openings should be as large as possible
– The openings should be consistent with strength and
stiffness
• Avoid rectangular-shaped cored holes in ribs or webs.
– Use oval-shaped holes with the longest dimension in the
direction of the stresses
Recommended Configurations
May complicate die
construction
External ribs may cause poor
distribution of stresses
May complicate die
construction
Sharp corners,
small radii
H ≤ T H > T core out
underside
Good distribution of
stresses
Sharp corners, small radii,
little draft
Generous draft and fillets, angular
transitions
Ribs inside, good distribution of metals
for all purposes.
Avoid Using Bosses, Lugs & Pads
• Bosses and pads can have adverse effects on castings:
– They increase metal thickness
– They create hot spots
– They can cause open grain or draws
• If they must be incorporated into a design you should
blend them into the casting by tapering or flattening
the fillets.
Reducing Heavy Masses & Die
Simplification
A
a c
B
b
d
Reducing Heavy Masses & Die
Simplification
A B C
a
dc
b
Reducing Heavy Masses & Die
Simplification
A B
9.10. Avoid Using Bosses, Lugs & Pads
• The thickness of bosses and pads should be
less than the thickness of the casting section
they adjoin but thick enough to permit
machining without touching the casting wall.
• Exception: Where a casting section is light the
following should be used as a guide:
9.10. Avoid Using Bosses, Lugs & Pads
• Bosses should not be used in casting design when the
surface to support bolts may be obtained by milling
or countersinking.
• A continuous rib instead of a series of bosses will
permit shifting hole location.
• Where there are several lugs and bosses on one
surface, they should be joined to facilitate machining.
– A panel of uniform thickness will simplify machining
– Make the walls of a boss at uniform thickness to the casting
walls
Design Rules: Boss Design
• Poor Boss Designs:
– Result in the potential for sink marks and voids.
– Sinks / Voids / Cooling stresses
• Improved Boss Designs:
– Bosses attached to the walls using ribs
– Thick sections cored out
– Gussets reinforce free standing bosses
Design Rules: Boss Design
Các file đính kèm theo tài liệu này:
chapter_2_fundamentals_of_metal_casting_nguyen_ngoc_ha_part.pdf