MANUFACTURING TECHNOLOGY FOR AEROSPACE MATERIALS Forging Forgings 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 open and 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
Blocker
Extensive
Low
Low
Conventional
More
High die cost
500 or more
High definition
Very less or nil (near net shape)
Less machining cost
Typical shape
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 5 –10% 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 to eight 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 draw forming, stretch wrapping, and radial draw forming. Forming
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 100 –300%. 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 requirement for 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 sheet is heated to its superplastic temperature range, gas pressure is injected through inlets in the upper die.
Cavitation can be minimized, or eliminated, by applying a hydrostatic back pressure to the sheet during forming, as shown schematically in Fig. Back pressures of 100 –500 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 in this process. The advantages are very smooth
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 precision matched 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 to metal 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 0.060 –0.080 in. or higher without excessive warpage, while with high speed machining, wall thicknesses as thin as 0.020 –0.030 in. without distortion are readily achievable.
High speed machining of aluminum was originally implemented on the F/A- 18E/F fighter to save weight. It soon became apparent 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 weight reduction. 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 195±5_ F with small amounts of triethanolamine to improve the surface finish. The etchant rate is in the range of 0.0008 –0.0012 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
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
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. No electrode 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 µm, while that of Nd:YAG laser is 1.06 µm. Since the absorption of the beam energy by the material being welded increases 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
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 of down a drain. The garnet abrasive is a non-toxic natural substance that can be recycled for repeated
Methods: Unfortunately,
most
internal threads cannot be made by thread rolling.
Nontraditional Machining Processes – A Summary Summary of Chemical NTM Processes
Specific HP 3 (hp/in /min)
Penetration Rate (ipm) or Cutting Speed (sfpm)
30 in /min
Chemical energy
0.001-0.002 ipm
0.001-0.006; material and process dependent
4-32, but can go as low as 2 or 1 or better
Very slow
50-200 amperes per square foot
0.00050.0015 ipm
NA ; process used to obtain finish
High quality, no stress surface; removes residual stresses; make corrosion resistant surfaces; may be considered to be an electrochemical process
63-250, but can go as low as 8
Same as chemical milling
DC power
0.00040.0020 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
Burr-free
Minute with rapid cycle time
NA
NA
NA
For burrs and fins on cast or machined parts; deburr steel gears automatically
Process
Surface Finish AA (µ/in)
Chemical machining
63-250, but can go as low as 8
Electro polishing Photochemic al machining (blanking) Thermoche mical machining (combustion machining)
Metal Removal Rate 3 (in /min)
3
Accuracy (in)
a
Comments
Most all materials possible; depth of cut limited to ½ inch; no burrs; no surface stressed; tooling low cost
Summary of Electrochemical NTM Processes
Process
Surface Finish AA (µ/in)
Metal Removal Rate 3 (in /min) 0.06 in W, Mo 0.16 in CI 0.13 in steel, Al 0.60 in Cu
Specific HP 3 (hp/in /mi n)
Electrochemical machining (ECM)
16-63
160
Electrochemical grinding (ECG)
8-32
0.010
High
Electrolytic hole machining (Electrostream)
16-63
NA
NA
Penetration Rate (ipm) or Cutting Speed (sfpm) 0.1 to 0.5 ipm
Accuracy (in)
Comments
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
Cutting rates about same as grinding; wheel speeds, 40006000
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
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; multiplehole drilling; typical holes 0.004 to 0.03 inch in diameter with depth- todiameter ratio of 50:1
Summary of Thermal NTM Processes
Process
Surface Finish AA (µ/in)
Electron beam machining (EBM)
32-250
Laser beam machining (LBM)
Metal Removal Rate 3 (in /min)
Specific HP 3 (hp/in /m in)
Penetration Rate (ipm) or Cutting Speed (sfpm) 200 sfpm
Accuracy (in)
Comments
0.0005 max.; Extremely low
10000
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.
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
0.0020.00015
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 discharge Wire 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 inches thick; 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 0 to 8 inches thick with 5 to 10 taper
6 ipm
possible
Summary of Mechanical NTM Processes
Process
Typical Surface Finish AA (µ in)
Typical Metal Removal Rate
Typical Specific Horsepower 3 (hp/in /min)
Typical Penetration Rate (ipm) or Cutting Speed (sfpm)
Typical Accuracy (in.)
Low
0.0010.002
Typically used to finish inaccessible integral passages; often used to remove recast layer produced by EDM; used for burr removal; (cannot do blind holes)
Used in heat-sensitive or brittle materials; produces tapered walls in deep cuts
Comments
30-300; can go as low as 2
Low
Abrasive jet machining
10-50
Very low; fine finishing process, 0.001
NA
Very low
0.005 typical, 0.002 possible
Hydrodynamic machining
Generally 30-100
Depends on material
NA
Depends on material
0.001 possible
Used for soft non metallic slitting; no heataffected zone; produces narrow kerfs (0.0010.020 inch); high noise levels
Ultrasonic machining
16-63; as low as 8
Slow, 0.05 typical
200
0.02-0.150 ipm
0.0010.005
Most effective in hard materials, Rc > 40; tool wear and taper limit hole depth to width at 2.5 to 1; tool also wears
Abrasive flow machining
NA
