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.
Aluminum 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. The deformation or strain rate can be very fast (> 10 s-1) for processes such as hammer forging or very slow (< 0.1s-1) for hydraulic presses. Since higher strain rates increase the flow stress (decrease forgeability) and the 2XXX and 7XXX alloys are even more sensitive than other aluminum alloys, hydraulic presses are usually preferred for forging these alloys. Hydraulic presses are available in the range of 500-75 000 tons (biggest press is in Russia) and can produce forgings up to around 3000 lb.
For the 7XXX alloys, the forging pressure is actually higher than that for low carbon steels. As shown in Fig. 2.12, the flow stress of 7075 is considerably
higher than that of 1025 plain carbon steel at the same strain rate, while the flow stress of 2219 is about the same as 1025. Although flow stress represents the lower bound required for forging, it does illustrate that the 7XXX series, such as 7010, 7049, 7050, and 7075, require substantial pressure. As the temperature increases toward the maximum in the range, the flow stress decreases and the forgeability increases.
Dies for forging aluminum alloys are usually made from die steels such as the hot work steels H11, H12, and H13 heat treated to 44-50Rc. The surfaces may be hardened for additional durability by carburizing, nitriding, or carbonitriding. The die cavities are normally polished to produce a good surface finish on the forging. The dies are usually heated for forging of aluminum alloys, normally from as low as 200° F to as high as 800° F. Since the higher temperatures are used for hydraulic press forging, this operation is essentially conducted in the isothermal range in which the dies and part are at or near the same temperature.
Aluminum alloys are heated to the forging temperature with a wide variety of heating equipment, including electric furnaces, muffle furnaces, oil furnaces, induction heating units, fluidized beds, and resistance heating units. Regardless of the heating method, it is very important to minimize the absorption of hydrogen, which can result in surface blistering or internal porosity, referred to as bright flakes, in the finished forging. The 2XXX alloys are normally forged at 800-860° F, while the 7XXX alloys are forged at 750-825° F. Since the temperature range for forging is generally less than 100° F, and always less than 150° F, temperature control during forging is very important. Soak times of 1-2 h are usually sufficient with temperatures controlled to ±10° F. Before forging, graphite based lubricants are sprayed on the dies.
Open die forgings, also know as hand forgings, are produced in dies that do not provide lateral restraint during the forging operation. In this process, the metal is forged between either flat or simply shaped dies. This process is used to produce small quantities where the small quantities do not justify the expense of matched dies. Although open die forgings somewhat improve the grain flow of the material, they offer minimal economic benefit in reduced machining costs. Open die forging is often used to produce preforms for closed die forging.
Most aluminum forgings are produced in closed dies. Closed die forgings are produced by forging ingots, plates, or extrusions between a matched set of dies. Closed die forging uses progressive sets of dies to gradually shape the part to near net dimensions. Die forgings can be subdivided into four categories from the lowest cost, least intricate to the highest cost, most intricate. A comparison of the relative amount of part definition for these different forging processes12 is shown in Fig. 2.13.
• Blocker forgings may be chosen if the total quantities are small (e.g., 200). Since they have large fillet and corner radii, they require extensive machining to produce a finished part. The fillets are about 2 times the radius and the corner radii about 1.5 times the radii of conventional forg-ings. Therefore, a blocker forging costs less than a conventional forging but requires more machining. Finish only forgings are similar to blocker forgings in that only one set of dies are used; however, since they have one more squeeze applied, they have somewhat better part definition. Fillets
Fig. 2.13. Part Definition Produced by Different Forging Methods
Fig. 2.13. Part Definition Produced by Different Forging Methods are about 1.5 times the radius of conventional forgings with corner radii about the same as conventional forgings. A quantity of about 500 might justify the use of finish only forgings.
• Conventional forgings require two to four sets of dies with the first set producing a blocker type forging that is subsequently finished in the other sets. This is the most common type of aluminum forging and is usually specified for quantities of 500 or more. Conventional forgings have more definition and require less machining than blocker forgings but the die cost is higher.
• High definition forgings contain even better definition and tolerance control than conventional forgings with less machining costs. These forgings are near net shape forgings produced on multiple die sets. In some applications, some of the forged surfaces may not require machining.
• Precision forgings produce the best part definition and highest quality but are, of course, the most expensive. These forgings have tighter tolerances than those produced by even high definition forgings with better grain flow. Minimal or no machining is required to finish these forgings.
Other common forging methods for aluminum alloys include upset forging, roll forging, orbital or rotary forging, spin forging, ring rolling, and mandrel forging. The choice of a particular forging method depends on the shape required and the economics of the number of pieces required traded off against higher quality and lower machining costs.
All 2XXX and 7XXX forged alloys are heat treated after forging. To minimize distortion during quenching, the racking procedures are important in obtaining uniform cooling rates. Aluminum forgings are often straightened between solution treatment and aging. Compressive stress relieving with a permanent set of 1-5% is used to reduce internal stresses and minimize distortion during subsequent machining operations. When this is conducted in the finishing dies, it is designated as TXX54 temper. When it is conducted in a separate set of cold dies, it is designated as the TXX52 temper.
Residual stresses generated during forging and subsequent heat treatment can cause significant problems when the part is machined. The 7050-T7452 forging shown in Fig. 2.14 bowed almost a foot after machining. A study13 of the residual stresses in this type of part revealed that the sequence used to conduct compression stress relief can be important. If a large number of small "bites" are taken along the length during the incremental compression stress relief, the residual stress pattern is not as uniform than if a smaller number of larger bites is taken.
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