Manufacturing Technology for Aerospace Materials
Transcript of Manufacturing Technology for Aerospace Materials
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MANUFACTURING TECHNOLOGY FOR AEROSPACE MATERIALS
ForgingForgings are often preferred for aircraft bulkheads and other highly loaded parts because the forging
process allows for thinner cross-section product forms prior to heat treat and quenching, enabling
superior properties. It can also create a favorable grain flow pattern which increases both fatigue life
and fracture toughness when not removed by machining. Also, forgings generally have less porosity than
thick plate and less machining is required.
Alloys can be forged using hammers, mechanical presses, or hydraulic presses. Hammer forging
operations can be conducted with either gravity or power drop hammers and are used for both openand closed die forgings. Hammers deform the metal with high deformation speed; therefore, it is
necessary to control the length of the stroke, the speed of the blows, and the force being exerted.
Hammer operations are frequently used to conduct preliminary shaping prior to closed die forging. Both
mechanical and screw presses are used for forging moderate size parts of modest shapes and are often
used for high volume production runs. Mechanical and screw presses combine impact with a squeezing
action that is more compatible with the flow characteristics of aluminum than hammers. Hydraulic
presses are the best method for producing large and thick forgings, because the deformation rate is
slower and more controlled than with hammers or mechanical/screw presses.
Die forgings can be subdivided into four categories
Type Machining reqd Cost Prod volume Typical shape
Blocker Extensive Low Low
Conventional More High die cost 500 or more
High definition Very less or nil (nearnet shape)
Less machiningcost
Precision Mostly nil Most Expensive High
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Precision Mostly nil Most Expensive High
Brake Forming
In brake forming, the sheet is placed over a die and pressed down by a punch that is actuated by the
hydraulic ram of a press brake.
Deep Drawing
Punch presses are used for most deep drawing
operations. In a
typical deep
drawing
operation,
shown in Fig.2.17, a punch or
male die pushes the sheet into the die cavity while it is
supported around the periphery by a blankholder.
Clearances between the punch and die are usually equal to
the sheet thickness plus an additional 10% per side for the intermediate strength alloys, while an
additional 510% clearance may be needed for the high strength alloys. Excessive clearance can result in
wrinkling of the sidewalls of the drawn shell, while insufficient clearance increases the force required for
drawing and tends to burnish the part surfaces. The draw radius on tools is normally equal to four toeight times the stock thickness.
Stretch Forming
In stretch forming (Fig. 2.18), the material is stretched over a tool beyond its yield strength to produce
the desired shape. Large compound shapes can be formed by
stretching the sheet both longitudinally and transversely. In
addition, extrusions are frequently stretch formed to mouldline
curvature. Variants of stretch forming include stretch drawforming, stretch wrapping, and radial draw forming. Forming
lubricants are recommended.
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Superplastic Forming
Superplasticity is a property that allows sheet to elongate to quite large strains without localized necking
and rupture. In uniaxial tensile testing, elongations to failure in excess of 200% are usually indicative of
superplasticity. Although superplastic behavior can produce strains in excess of 1000%, superplastic
forming (SPF) processes are generally limited to about 100300%. The advantages of SPF include the
ability to make part shapes not possible with conventional forming, reduced forming stresses, improved
formability with essentially no springback and reduced machining costs. The disadvantages are that the
process is rather slow and the equipment and tooling can be relatively expensive. The main requirementfor superplasticity is a high strain rate sensitivity (m). The strain rate sensitivity describes the ability of a
material to resist plastic instability or necking. For superplasticity, m is usually greater than 0.5 with the
majority of superplastic materials having an m value in the range of 0.4 0.8, where a value of 1.0 would
indicate a perfectly superplastic material. In the single sheet SPF process, illustrated in Fig. 2.21, a single
sheet of metal is sealed around its periphery between an upper and lower die. The lower die is either
machined to the desired shape of the final part or a die inset is placed in the lower die box. The dies and
sheet are maintained at the SPF temperature, and gas pressure is used to form the sheet down over the
tool. The lower cavity is maintained under vacuum or can be vented to the atmosphere. After the sheetis heated to its superplastic temperature range, gas pressure is injected through inlets in the upper die.
This pressurizes the cavity above the metal sheet forcing it to superplastically form to the shape of the
lower die Gas pressurization is applied slowly so that the strains in the sheet are maintained in the
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Cavitation can be minimized, or eliminated, by applying a
hydrostatic back pressure to the sheet during forming, asshown schematically in Fig. Back pressures of 100500 psi
are normally sufficient.
Casting
Plaster and Shell Molding
Plaster mold casting is basically the same as sand
casting except gypsum plasters replace the sand inthis process. The advantages are very smooth
surfaces, good dimensional tolerances, and
uniformity due to slow uniform cooling. However, as
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Die Casting
Die casting is a permanent mold casting process in which the liquid metal is injected into a metal die
under high pressure. It is a very high rate production process using expensive equipment and precisionmatched metal dies. Since the solidification rate is high, this process is amendable to high volume
production. It is used to produce very intricate shapes in the small to intermediate part size range.
Characteristics of the process include extremely good surface finishes and the ability to hold tight
dimensions; however, die castings should not be specified where high mechanical properties are
important because of the inherently high porosity level. The high pressure injection creates a lot of
turbulence that traps air resulting in high porosity levels. In fact, die cast parts are not usually heat
treated because the high porosity levels can cause surface blistering. To reduce the porosity level, the
process can be done in vacuum (vacuum die casting) or the die can be purged with oxygen just prior tometal injection.
Investment Casting
Investment casting is used where tighter tolerances, better surface finishes, and thinner walls are
required than can be obtained with sand casting. A brief description of the process is that investment
castings are made by surrounding, or investing, an expendable pattern, usually wax, with a refractory
slurry that sets at room temperature. The wax pattern is then melted out and the refractory mold is
fired at high temperatures. The molten metal is cast into the mold and the mold is broken away after
solidification and cooling. Suited well for Titanium.
Machining
High Speed Machining: HSM is somewhat an arbitrary term. It can be defined for aluminum as
machining conducted at spindle speeds greater than 10000 rpm. It should be emphasized that while
higher metal removal rates are good, another driver for developing high speed machining of aluminum
is the ability to machine extremely thin walls and webs. For example, the minimum machining gage for
conventional machining might be 0.0600.080 in. or higher without excessive warpage, while with high
speed machining, wall thicknesses as thin as 0.0200.030 in. without distortion are readily achievable.
This allows the design of weight competitive high speed machined assemblies in which sheet metal
parts that were formally assembled with mechanical fasteners can now be machined from a single or
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High speed machining of aluminum was
originally implemented on the F/A- 18E/F
fighter to save weight. It soon becameapparent that the higher metal removal
rates could also save costs by eliminating
multiple parts and assembly costs.
Chemical Milling: Shallow pockets are sometimes chemically milled into aluminum skins for weightreduction. The process is used mainly for parts having large surface areas requiring small amounts of
metal removal. Rubber maskant is applied to the areas where no metal removal is desired. In practice,
the maskant is placed over the entire skin and allowed to dry. The maskant is then scribed according to a
pattern and the maskant removed from the areas to be milled. The part is then placed in a tank
containing sodium hydroxide heated to 1955_ F with small amounts of triethanolamine to improve the
surface finish. The etchant rate is in the range of 0.00080.0012 in./min. Depths greater than 0.125 in.
are generally not cost competitive with conventional machining, and the surface finish starts to degrade.
After etching, the part is washed in fresh water and the maskant is stripped.
Joining
Welding
Gas Metal Arc Welding (GMAW): Gas metal arc welding, as shown in Figure is an arc welding process
that creates the heat for welding by an electric
arc that is established between a consumable
electrode wire and the workpiece. The
consumable electrode wire is fed through a
welding gun that forms an arc between the
electrode and the workpiece The gun controls
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and argon shielding gas are used. In general, material less than 0.125 in. thick can be welded without
filler wire addition if solidification cracking is not a concern.
Plasma Arc Welding
Automated variable polarity plasma arc (VPPA) welding is often used to weld large fuel tank structures.
Plasma arc welding, shown in Figure, is a shielded arc
welding process in which heat is created between a
tungsten electrode and the workpiece. The arc is
constricted by an orifice in the nozzle to form a highly
collimated arc column with the plasma formed through
the ionization of a portion of the argon shielding gas. The
electrode positive component of the VPPA process
promotes cathodic etching of the surface oxide allowing
good flow characteristics and consistent bead shape.
Pulsing times are in the range of 20 ms for the electrode
negative component and 3 ms for the electrode positive
polarity. A keyhole welding mode is used in which the arc fully penetrates the workpiece, forming a
concentric hole through the thickness. The molten metal then flows around the arc and resolidifies
behind the keyhole as the torch traverses through the workpiece. The keyhole process allows deep
penetration and high welding speeds while minimizing the number of weld passes required. VPPA
welding can be used for thicknesses up to 0.50 in. with square grooved butt joints and even thicker
material with edge beveling. While VPPA welding produces high integrity joints, the automated
equipment used for this process is expensive and maintenance intense.
Resistance Welding
Resistance welding can produce excellent joint
strengths in the high strength heat treatable
aluminum alloys. Resistance welding is normally used
for fairly thin sheets where joints are produced with
no loss of strength in the base metal and without the
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Laser Welding
There is considerable interest in laser beam welding (LBW) of high strength aluminum alloys. The
process is attractive because it can be conducted at high speeds with excellent weld properties. Noelectrode or filler metal is required and narrow welds with small HAZs are produced. Although the
intensity of the energy source is not quite as high as that in electron beam (EB) welding, EB welding
must be conducted in a vacuum chamber. The coherent nature of the laser beam allows it to be focused
on a small area leading to high energy densities. Since the typical focal point of the laser beam ranges
from 0.004 to 0.040 in., part fit-up and alignment are more critical than conventional welding methods.
Both high power continuous wave carbon dioxide (CO2) and neodymium-doped yttrium aluminum
garnet (Nd:YAG) lasers are being used. The wavelength of light from a CO2 laser is 10.6 m, while that of
Nd:YAG laser is 1.06 m. Since the absorption of the beam energy by the material being weldedincreases with decreasing wavelength, Nd:YAG lasers are better suited for welding aluminum. In
addition, the solid state Nd:YAG lasers use fiber optics for beam delivery, making it more amenable to
automated robotic welding.
Friction Stir Welding
A new welding process which has the potential to revolutionize aluminum joining has been developed
by The Welding Institute in Cambridge, UK. Friction
stir welding is a solid state process that operates by
generating frictional heat between a rotating tool
and the workpiece, as shown schematically in
Figure. The welds are created by the combined
action of frictional heating and plastic deformation
due to the rotating tool. A tool with a knurled probe
of hardened steel or carbide is plunged into the
workpiece creating frictional heating that heats a
cylindrical column of metal around the probe, as
well as a small region of metal underneath the
probe. As shown in Figure, a number of different
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Water jet machining
A water jet cutter is a tool capable of slicing into metal or other materials using a jet of water at high
velocity and pressure, or a mixture of water and an abrasive substance. The process is essentially the
same as water erosion found in nature but greatly accelerated and concentrated. It is often used during
fabrication or manufacture of parts for machinery and other devices. It has found applications in a
diverse number of industries from mining to aerospace where it is used for operations such as cutting,
shaping, carving, and reaming.
The cutter is commonly connected to a high-pressure water pump where the water is then ejected from
the nozzle, cutting through the material by spraying it with the jet of high-speed water. Additives in the
form of suspended grit or other abrasives, such as garnet and aluminum oxide, can assist in this process.
Water jet cuts are not typically limited by the thickness of the material, and are capable of cutting
materials over 45 cm thick.
An important benefit of the water jet cutter is the ability to cut material without interfering with the
material's inherent structure as there is no "heat-affected zone" or HAZ. Minimizing the effects of heat
allows metals to be cut without harming or changing intrinsic properties.
Water jet cutters are also capable of producing rather intricate cuts in material. The kerf, or width, of
the cut can be changed by changing parts in the nozzle, as well as the type and size of abrasive.
Waterjet is considered a "green" technology. Waterjets produce no hazardous waste, reducing waste
disposal costs. They can cut off large pieces of reusable scrap material that might have been lost using
traditional cutting methods. Parts can be closely nested to maximize material use, and the waterjet
saves material by creating very little kerf. Waterjets use very little water, and the water that is used can
be recycled using a closed-looped system. Waste water usually is clean enough to filter and dispose ofdown a drain. The garnet abrasive is a non-toxic natural substance that can be recycled for repeated
use. Garnet usually can be disposed of in a landfill. Waterjets also eliminate airborne dust particles,
smoke, fumes, and contaminates from cutting materials such as asbestos and fiberglass. This greatly
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Methods:
Unfortunately, mostinternal threads cannot be
made by thread rolling.
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Nontraditional Machining Processes A Summary
Summary of Chemical NTM Processes
ProcessSurface
Finish AA
(/in)
Metal
Removal
Rate
(in3/min)
Specific HP
(hp/in3/min)
Penetration
Rate (ipm)or Cutting
Speed
(sfpm)
Accuracy
(in)Comments
Chemical
machining
63-250, but
can go as
low as 8
30 in3/min
Chemical
energy
0.001-0.002
ipm
0.001-0.006;
material and
process
dependent
Most all materials possible; depth of cut
limited to inch; no burrs; no surface
stressed; tooling low cost
Electro
polishing
4-32, but can
go as low as
2 or 1 orbetter
Very slow
50-200
amperes per
square foot
0.0005-
0.0015 ipm
NAa; process
used to obtain
finish
High quality, no stress surface; removes
residual stresses; make corrosion resistant
surfaces; may be considered to be anelectrochemical process
Photochemic
al machining
(blanking)
63-250, but
can go as
low as 8
Same as
chemical
milling
DC power0.0004-
0.0020 ipm
10% of sheet
thickness or
0.001-0.002
inch
Limited to thin material; burr- free blanking of
brittle material; tooling low cost; used
microelectronic
Thermoche
mical
machining
(combustion
machining)
Burr-free
Minute with
rapid cycle
time
NA NA NAFor burrs and fins on cast or machined parts;
deburr steel gears automatically
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Summary of Electrochemical NTM Processes
Process
Surface
Finish AA
(/in)
Metal
Removal
Rate
(in3/min)
Specific HP
(hp/in3/mi
n)
Penetration
Rate (ipm) or
Cutting Speed
(sfpm)
Accuracy
(in)Comments
Electrochemical
machining
(ECM)
16-63 0.06 in W,
Mo 0.16 in CI
0.13 in steel,
Al 0.60 in Cu
160 0.1 to 0.5 ipm 0.0005-0.005 =
0.002 in cavities
Stress free metal removal in hard to
machine metals; tool design expensive;
disposal of chemicals a problem; MRR
independent of hardness; deep cuts will
have tapered walls
Electrochemical
grinding (ECG)
8-32 0.010 High Cutting rates
about same as
grinding; wheel
speeds, 4000-6000
0.001-0.005 Special form of ECM; grinding with ECM
assist; good for grinding hard
conductive materials like tungsten
carbide tool bits; no heat damage,burrs, or residual stresses
Electrolytic hole
machining
(Electrostream)
16-63 NA NA 0.060-0.120 ipm =0.001 or 5% of
dia. Of hole
Special version of ECM for hole drilling
small round or shaped holes; multiple-
hole drilling; typical holes 0.004 to 0.03
inch in diameter with depth- to-
diameter ratio of 50:1
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Summary of Thermal NTM Processes
Process
Surface
Finish AA
(/in)
Metal
Removal
Rate(in
3/min)
Specific
HP
(hp/in3
/min)
Penetration
Rate (ipm)
or Cutting
Speed
(sfpm)
Accuracy
(in)
Comments
Electron beam
machining
(EBM)
32-250 0.0005 max.;
Extremely
low
10000 200 sfpm
6 ipm
0.001-.0002 Micromachining of thin materials and hole drilling
minutes holes 100:1 depth to diameter ratios; work
must be placed in vacuum but suitable for automatic
control; beam can be used for processing and
inspection; used widely in microelectrons.
Laser beam
machining
(LBM)
32-250 0.0003;
Extremely
low
60000 4 ipm 0.005-0.0005 Can drill 0.005 to 0.050 inch dia . holes in materials
0.100 inch thick in seconds;same equipment can
weld, surface heat treat, engrave, trim, blank, etc,;has heat affected zone and recast layers which may
need to be removed.
Electrical
discharge
machining
(EDM)
32-105 0.3 40 0.5 ipm 0.002-
0.00015
possible
Oldest of NTM processes; widely used and
automated; tools and dies expensive; cuts any
conductive material regardless of hardness ;
delicate, burr free parts possible; always for recast
layer.
Electrical
dischargeWire cutting
32-64 0.10-0.3 40 4 ipm 0.0002 Special form of EDM using traveling wire cuts
straight narrow kerfs in metals 0.001 to 3 inchesthick; wire diams. of 0.002 to 0.010 used; N/C
machines allow for complex shapes
Plasma beam
machining
(PBM)
25-500 10 20 50 sfpm; 10
ipm; 120 ipm
in steel
0.1-0.02 Clean, rapid cuts and profiles in almost all plates up
to 8 inches thick with 5 to 100
taper
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Summary of Mechanical NTM Processes
Process
Typical
Surface
Finish AA( in)
Typical
Metal
RemovalRate
Typical
Specific
Horsepower(hp/in3/min)
Typical
Penetration
Rate (ipm) or
Cutting Speed(sfpm)
Typical
Accuracy
(in.)
Comments
Abrasive flow
machining
30-300;
can go as
low as 2
Low NA Low0.001-
0.002
Typically used to finish inaccessible integral
passages; often used to remove recast layer
produced by EDM; used for burr removal;
(cannot do blind holes)
Abrasive jet
machining10-50
Very low;
fine
finishing
process,0.001
NA Very low
0.005
typical,
0.002
possible
Used in heat-sensitive or brittle materials;
produces tapered walls in deep cuts
Hydrodynamic
machining
Generally
30-100
Depends on
materialNA
Depends on
material
0.001
possible
Used for soft non metallic slitting; no heat-
affected zone; produces narrow kerfs (0.001-
0.020 inch); high noise levels
Ultrasonic
machining
16-63; as
low as 8
Slow, 0.05
typical200
0.02-0.150
ipm
0.001-
0.005
Most effective in hard materials, Rc > 40; tool
wear and taper limit hole depth to width at 2.5
to 1; tool also wears