Manufacturing Technology for Aerospace Materials

7/30/2019 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

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Manufacturing Technology for Aerospace Materials