47
Stress
Stress
C
f
f
B
S
B
2
y
y'
Sy
y
E
elastic
( a)
elastic
( b)
energy
E
1
energy
recovered
1
recovered
O
A
O
A
D
permanent set
Strain
Strain
permanent set
F I G U R E 2 - 13
Strain Hardening a Ductile Material by Cold Working ( a) First Working ( b) Second Working If significant plastic deformation is required for manufacture, such as in making deep-drawn metal pots or cylinders, it is necessary to cold form the material in stages and anneal the part between successive stages to avoid fracture. The annealing resets the material to more nearly the original ductile stress-strain curve, allowing further yielding without fracture.
HOT WORKING All metals have a recrystallization temperature below which the effects of mechanical working will be as described above, e.g., cold worked. If the material is mechanically worked above its recrystallization temperature (hot working), it will tend to at least partially anneal itself as it cools. Thus hot working reduces the strain-hardening problem but introduces its own problems of rapid oxidation of the surface due to the high temperatures. Hot-rolled metals tend to have higher ductility, lower strength and poorer surface finish than cold-worked metals of the same alloy. Hot working does not increase the hardness of the material appreciably, though it can increase the strength by improving grain structure and aligning the “grain” of the metal with the final contours of the part. This is particularly true of forged parts.
FORGING is an automation of the ancient art of blacksmithing. The blacksmith heats the part red-hot in the forge, then beats it into shape with a hammer. When it cools too much for forming, it is reheated and the process is repeated. Forging uses a series of hammer dies shaped to gradually form the hot metal into the final shape. Each stage’s die shape represents an achievable change in shape from the original ingot form to the final desired part shape. The part is reheated between blows from the hammer dies which are mounted in a forging press. The large forces required to plastically deform the hot metal require massive presses for parts of medium to large size. Machining operations are required to remove the large “flash” belt at the die parting line and to machine holes, mounting surfaces, etc. The surface finish of a forging is as rough as any hot-rolled part due to oxidation and decarburization of the heated metal.
Virtually any wrought, ductile metal can be forged. Steel, aluminum, and titanium are commonly used. Forging has the advantage of creating stronger parts than casting or machining can. Casting alloys are inherently weaker in tension than wrought alloys.
The hot forming of a wrought material into the final forged shape causes the material’s
48
MACHINE DESIGN -
An Integrated Approach
2
F I G U R E 2 - 14
Forged Steel Crankshaft for a Diesel-Truck Engine - Courtesy of Wyman-Gordon Corp., Grafton, MA
internal “flow lines” or “grain” to approximate the contours of the part, which can re-
sult in greater strength than if a stock shape’s flow lines were severed by machining to
the final contour. Forgings are used in highly stressed parts, such as aircraft wing and
fuselage structures, engine crankshafts and connecting rods, and vehicle suspension
links. Figure 2-14 shows a forged truck crankshaft. In the cross section, the grain lines
can be seen to follow the crankshaft’s contours. The high cost of the multiple dies
needed for forged shapes makes it an impractical choice unless production quantities
are large enough to amortize the tooling cost.
EXTRUSION is used principally for nonferrous metals (especially aluminum) as it
typically uses steel dies. The usual die is a thick, hardened-tool-steel disk with a tapered
“hole”
or orifice ending in the cross-sectional shape of the finished part. A billet of the
extrudate is heated to a soft state and then rammed at fairly high speed through the die,
which is clamped in the machine. The billet flows, or extrudes, into the die’s shape. The
process is similar to the making of macaroni. A long strand of the material in the desired cross section is extruded from the billet. The extrusion then passes through a water-
spray cooling station. Extrusion is an economical way to obtain custom shapes of constant cross section since the dies are not very expensive to make. Dimensional control and surface finish are good. Extrusion is used to make aluminum mill shapes such as angles, channels, I-beams, and custom shapes for storm-door and -window frames, sliding-door frames, etc. The extrusions are cut and machined as necessary to assemble them into the finished product. Some extruded shapes are shown in Figure 2-15.
2.5
COATINGS AND SURFACE TREATMENTS
F I G U R E 2 - 15
Many coatings and surface treatments are available for metals. Some have the prime purpose of inhibiting corrosion while others are intended to improve surface hardness Extrusions - Courtesy of
and wear resistance. Coatings are also used to change dimensions (slightly) and to al-The Aluminum Extruders
Council
ter physical properties such as reflectance, color, and resistivity. For example, piston