MAKING THE MOST OF FORGING BENEFITS:
SHAPE + SIZE CAPABILITY BROADENS DESIGN ALTERNATIVES
To the well-seasoned designer, it's no secret that forgings are regularly selected for their high strength, structural integrity, extra-long service life, unmatched impact toughness, and other desirable characteristics. Not so well known is the tremendous design versatility of forgings, attributable to the wide ranges of part sizes, shapes and complexities that can be forged with current technology.
Processing refinements, implementation of computerized equipment, and newly developed alloys are just some of the evolutionary forging developments that now permit larger-size components, more complex configurations, and closer tolerances to be achieved in most forging processes. In forging near-net shapes, significant gains in part size are accompanied by an ever-increasing degree of part complexity.
With few limitations, virtually any shape imaginable can be forged today, or can be economically put together with forged components. Shapes range from bars with basic cross-sections (round, square, hexagonal) and simple shafts to complex, contoured profiles that integrate a thin rib-and-web structure and built-in attachment features. Many unique shape possibilities are presented in the accompanying photos, supplied by FIA member companies.
Similarly, size capability for all forging processes covers an enormous range: from ounces to more than 150 tons. However, each of the many forging processes has its own special design niche in terms of size, shape, part complexity and material options.
Shape, size options for impression-die forgings
Commonly referred to as closed-die forging, impression-die forging of steel, aluminum, titanium and other alloys can produce an almost limitless variety of 3-D shapes that range in weight from mere ounces up to more than 25 tons. Impression-die forgings are routinely produced on hydraulic presses, mechanical presses and hammers, with capacities up to 50,000 tons, 20,000 tons and 50,000 lb, respectively.
As the name implies, two or more dies containing "impressions" of the part shape are brought together as forging stock undergoes plastic deformation. Because metal flow is restricted by the die contours, this process can yield more complex shapes and closer tolerances than open-die-forging processes. Additional flexibility in forming both symmetrical and nonsymmetrical shapes comes from various preforming operations (sometimes bending) prior to forging in finisher dies.
Part geometries range from some of the easiest to forge-simple spherical shapes, block-like rectangular solids, and disc-like configurations-to the most intricate components with thin and long sections that incorporate thin webs and relatively high vertical projections like ribs and bosses. Although many parts are generally symmetrical, others incorporate all sorts of design elements (flanges, protrusions, holes, cavities, pockets, etc.) that combine to make the forging very nonsymmetrical. In addition, parts can be bent or curved in one or several planes, whether they are basically longitudinal, equidimensional or flat.
Most engineering metals and alloys can be forged via conventional impression-die processes, among them: carbon and alloy steels, tool steels and stainlesses, aluminum and copper alloys, and certain titanium alloys. Strain rate- and temperature-sensitive materials (magnesium, highly alloyed nickel-based superalloys, refractory alloys and some titanium alloys) may require more sophisticated forging processes and/or special equipment for forging in impression dies.
Accounting for a large majority of custom shapes, impression-die forgings span the industrial spectrum, ranging from structural parts for autos, trucks, aircraft/aerospace and heavy-duty manufacturing equipment to hardware, fixtures and hand tools.
Wide shape selection via open die
Open-die forging can produce forgings from a few pounds up to more than 150 tons. Called open-die because the metal is not confined laterally by impression dies during forging, this process progressively works the starting stock into the desired shape, most commonly between flat-faced dies. In practice, open-die forging comprises many process variations, permitting an extremely broad range of shapes and sizes to be produced. In fact, when design criteria dictate optimum structural integrity for a huge metal component, the sheer size capability of open-die forging makes it the clear process choice over non-forging alternatives. At the high end of the size range, open-die forgings are limited only by the size of the starting stock, namely, the largest ingot that can be cast.
Practically all forgeable ferrous and nonferrous alloys can be open-die forged, including some exotic materials like age-hardening superalloys and corrosion-resistant refractory alloys.
Open-die shape capability is indeed wide in latitude. In addition to round, square, rectangular, hexagonal bars and other basic shapes, open-die processes can produce:
- Stepped cylindersor "step shafts"--basically shafts or cylindrical solids whose diameter increases or decreases (steps down) at multiple locations along the longitudinal axis. Common applications include all kinds of shaft-like parts from small diameter camshafts to gigantic prop shafts for ocean-going vessels. (It should be noted that stepped configurations or "step-downs" can be produced either by increasing the nominal diameter by upsetting, or decreasing the nominal diameter by basic open-die forging techniques.)
- Hollows--cylindrical in shape, usually with length much greater than the diameter of the part. Length, wall thickness, ID and OD can be varied, as needed.
- Ring-like partscan resemble washers or approach hollow cylinders in shape, depending on the height/wall thickness ratio. All are seamless for improved performance over cast, welded and fabricated rings.
- Contour-formed metal shellslike pressure vessels, which may incorporate extruded nozzles and other design features.
Not unlike successive forging operations in a sequence of dies, multiple open-die forging operations can be combined to produce the required shape. At the same time, these forging methods can be tailored to attain the proper amount of total deformation and optimum grain-flow structure, thereby maximizing property enhancement and ultimate performance for a particular application. Forging an integral gear blank and hub, for example, may entail multiple drawing or solid forging operations, then upsetting. Similarly, blanks for rings may be prepared by upsetting an ingot, then piercing the center, prior to forging the ring.
Because of all the shape options available, certain open-die processes--hollows, rings and upsets--deserve further consideration.
Forged hollows deliver unique shapes
Formed by open-die forging techniques--either forging over a mandrel or forging between an enlarging bar and the top die of a press--forged hollows offer unique part geometries. While identical shapes are produced, the two processes work the metal differently. With a mandrel, the ID of the hollow takes on the dimension of the mandrel; reducing the wall thickness increases the length. Here, a bottom die is required. In contrast, using an enlarging bar, which is smaller than the ID of the hollow, involves working the metal between the bar and the top die of a press. In this case, the length is held constant and the diameter enlarges as wall thickness decreases. No bottom die is necessary.
Basically cylindrical hollows, these carbon-, alloy- and stainless-steel forgings typically weigh a few hundred up to thousands of pounds. Although less common, hollows weighing as much as several hundred thousand pounds have been produced.
Most hollows range in diameter from about 24 to 40 in., with the smallest diameter forgings coming in just under 12 in. However, with certain restrictions on other dimensions, forged hollows can attain 200-in. diameters or 300-in. lengths. In general, as diameter decreases, length can increase.
Wall thickness, too, depends on other dimensional constraints. For the largest diameters, a 2 to 3-in. minimum wall thickness is required so that the part can support its own weight. According to forgers who specialize in large hollows, maximum thickness is limited only by press "daylight" and mandrel/bar size. Although not intended as an upper limit, 24-in. wall thicknesses have been produced.
Currently, hollows can be forged with steps on the outside, as well as steps and tapers on the inside, presenting in many smaller diameter components a higher-performance alternative to tubing. Even large hollows can be contoured, forged with different diameters along the length, and incorporate forged-in ports or extruded nozzles by use of special forging techniques that sometimes approximate closed-die forging on an open-die press.
Innumerable options in rings
Made by ring rolling or hammer forged over a saddle/mandrel set- up, forged rings weigh < 1 lb up to 350,000 lb, while ODs range from just a few inches up to 30-ft in diameter. Performance-wise, there is no equal for forged, circular-cross-section components in energy generation, mining, aerospace, off-highway equipment and other critical components.
Seamless ring configurations can be flat (like a washer), or feature higher vertical walls (approximating a hollow cylindrical section). Heights of rolled rings range from less than an inch up to more than 9 ft. Depending on the equipment utilized, wall- thickness/height ratios of rings typically range from 1:16 up to 16:1, although greater proportions have been achieved with special processing. In fact, seamless tubes up to 48-in. diameter and over 20-ft long are extruded on 20 to 30,000-ton forging presses.
Even though basic shapes with rectangular cross-sections are common, rings featuring complex, functional cross-sections can be forged to meet virtually any design requirements. Aptly named, these "contoured" rolled rings can be produced in thousands of different shapes with contours on the inside and/or outside diameters. A key advantage to contoured rings is a significant reduction in machining operations. Not surprisingly, custom-contoured rings can result in cost-saving part consolidations.
The two primary processes for forging rings differ not only in equipment, but also in quantities produced. Also called ring forging, saddle-mandrel forging on a press is particularly applicable to heavy cross-sections and small quantities. Essentially, an upset and punched ring blank is positioned over a mandrel, supported at its ends by saddles. As the ring is rotated between each stroke, the press ram or upper die deforms the metal ring against the expanding mandrel, reducing the wall thickness and increasing the ring diameter.
In continuous ring rolling, seamless rings are produced by reducing the thickness of a pierced blank between a driven roll and an idling roll in specially designed equipment. Additional rolls (radial and axial) control the height and impart special contours to the cross-section. Ring rollers are well suited for, but not limited to, production of larger quantities, as well as contoured rings. In practice, ring rollers produce seamless rolled rings to closer tolerances or closer to finish dimensions.
Ring rolling mills can produce continuous-rolled rings with face heights approaching 10 ft. Diameter-wise, they range from 2-in. IDs up to 360-in. ODs. Compared to flat-faced seamless rolled rings, maximum dimensions (face heights and ODs) of contoured rolled rings are somewhat lower, but are still very impressive in size.
High tangential strength and ductility make forged rings well- suited for torque- and pressure-resistant components, such as gears, engine bearings for aircraft, wheel bearings, couplings, rotor spacers, sealed discs and cases, flanges, pressure vessels, and valve bodies. Materials include not only carbon and alloy steels, but also nonferrous alloys of aluminum, copper and titanium, as well as nickel-base alloys.
Upsets for multidiameter parts
Upsets are ideal for cylindrical parts that incorporate a larger diameter or "upset" at one or more locations along the longitudinal axis. Usually forged from bar, upsets typically weigh < 1 lb up to about 400 lb for 12-ft-long components, although heavier parts up to 30-ft in length have been forged. Depending on upsetter size, upset diameters range from just under 3 in. up to 17 in.
Once limited to simple parts "headed" on one or both ends, upset forgings currently include components with larger diameter sections upset in the center, more complex shapes via multiple dies, internal and offset upsets, and even double-ended upset hollows formed from tubular stock.
Targeted for such applications as gears, stub shafts, axles, roller shafts, shell bodies, as well as blanks for further processing in other forging equipment (e.g., pinion-gear blanks), upsets are typically made of carbon or alloy steel, although any forgeable alloy can be used.
Precision forging for 'net' shapes
Component designs achieved by precision forging feature most functional surfaces forged to net dimensions with virtually no contour machining required. In contrast to conventional forging, part designs do not require generous draft. In fact, components can accommodate significant undercuts, thanks to the refinement of segmented-die techniques by forgers specializing in this process.
Typical structural part shapes in aluminum include: channel or "C" sections with flat backs; spar-type parts (long narrow parts with an "H" section or a combination of an "H" and channel with cross ribs) that can be 6 to 8 in. wide by 60 to 80 in. long; and large parts in a variety of shapes.
Along with design complexity, the size of precision aluminum forgings has also increased. High-definition parts (web thicknesses in the 0.070 to 0.080 in. range, ribs at about 0.100 in. thick, and contours on both sides) have just recently been produced with plan-view areas (PVAs) up to 600 sq. in. Previously, parts with thin rib/web structures were limited to about 300 sq. in., many of which were less than 150 sq. in.
In general, the achievable PVA of a part depends on how restrictive the rib/web structure is, as well as on rib and web thicknesses, rib heights, etc. Larger, heavier parts (thicker ribs and webs) with 800 sq. in. PVAs are just now entering production development. Similarly, precision-forged titanium components have increased in size and complexity. Current designs now surpass 300 sq. in. and feature wall thicknesses down to 0.100 in., depending on the part configuration.
Because of the above advances in size and shape, the types of parts that can be precision forged have also changed. Once relegated to less critical brackets, clips and other secondary components, precision forgings today are focused on main structural parts of aircraft. Wing ribs, for instance, now incorporate the airframe contour on the outside of the forging.
Lots of shapes through cold forging
Cold forging encompasses many processes--bending, cold drawing, cold heading, coining, extrusion, punching, thread rolling and more-to yield a diverse range of part shapes. These include various shaft-like components, cup-shaped geometries, hollow parts with stems and shafts, all kinds of upset (headed) and bent configurations, as well as combinations.
Most recently, parts with radial flow like round configurations with center flanges, rectangular parts, and non-axisymmetric parts with 3- and 6-fold symmetry have been produced by warm extrusion. With cold forging of steel rod, wire, or bar, shaft-like parts with 3-plane bends and "headed" design features are not uncommon.
Typical parts are most cost-effective in the range of 10 lb or less; symmetrical parts up to 7-lb readily lend themselves to automated processing. Materials options range from lower-alloy and carbon steels to 300 and 400 series stainlesses, selected aluminum alloys, brass and bronze.
Often chosen for integral design features such as built-in flanges and bosses, cold forgings are frequently used in automotive steering and suspension parts, antilock-braking systems, hardware, defense components, and other applications where high strength, close tolerances and volume production make them an economical choice.
Special shapes via hot-die, isothermal forging
Hot-die forging and isothermal forging make it possible to forge superalloys, vacuum-melted and other exotic alloys to net and near-net shape. Such materials are far more difficult to forge by conventional techniques. Isothermal forging is suitable for superalloys, such as Waspaloy and Astroloy, and P/M alloy IN100; hot-die forging, for titanium alloys and some heat-resistant nickel-based superalloys.
In hot-die forging, dies are heated close to the forging temperature of the alloy being forged. With isothermal forging at about 2000F, a controlled atmosphere or vacuum is required to protect tooling and workpiece from oxidation. These processes dramatically increase the formability of titanium alloys and superalloys, resulting in shapes that are forged much closer to the finish part profile than can be achieved on conventional presses.
Most forgings are disc-type shapes with diameters from about 6 to 36 in. and weights from about 70 to 1,000 lb. In general, thicknesses can range from approximately 1/2 up to 9 in., while a cross-section in a typical part may have thicknesses that vary from a maximum of 3 to 7 in. down to 1/2 in. in a thinner web section. Part shapes are not as complex as those that can be achieved by precision forging of aluminum, but rib/web titanium components can incorporate rib thicknesses below 1/4 in., approaching 1/8 in. for small parts.
Although most current applications are critical components for gas turbine engines and similar high-temperature environments, these forging processes also exhibit potential for structural airframe components in difficult-to-forge titanium alloys.
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