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Forging of Aluminum Alloys

Forging of Aluminum Alloys - Process and Operation

Forging Process

Aluminum can be forged with virtually any type of forging process. The types of aluminum alloy forgings are:

  • Open-die forgings
  • Closed-die forgings (blocker, conventional, precision, and cored), which are most frequently used.
  • Other forging types, such as rolled ring, roll forging, rotary (orbital) forging, etc.
  • Various forging processes have been discussed in separate papers.

    Most frequently used forging process is close-die forging. Near-net-shape production practices are extensively employed. Hydraulic presses are very widely used in forging of aluminum alloys because heating of the dies to a temperature near the workpiece temperature-that is, to about 425 C (800 F)-eliminates die chilling. As a result, the time of contact between the forging and the dies, which usually is extensive in hydraulic presses, does not affect the forging process. In addition, the easy control of the hydraulic press allows forging to proceed at a moderate speed and/or permits the forging operation to be stopped at any time during the stroke. Thus, a part can be forged in the same die, using several forging strokes.

    Selection of forging process follows the common rule for forging of all metals. Where quantities are large, tool costs per unit are negligible, and close-die forging may be preferred. For low quantities, it may be uneconomical to produce a part other than as an open-die forging, unless there are some special requirements such as close tolerance. Parts requiring several thousand pounds of metal can be produced only as hand forgings.

    Forging Operation

    Common forging operations for all metals are discussed in a separate paper. This section only discusses selected aspects of aluminum specific operations.

    Stock Preparation and Heating: The two methods most used for cutting stock into lengths for forging are sawing and shearing. Abrasive cutoff can be used, but it is slower than sawing for cutting aluminum, and, like sawing, produces burrs. Gas-fired semimuffle furnaces are the most widely used for heating aluminum alloys for forging, mainly because gas is widely available and is usually the least expensive source of heat. Oil-fired furnaces can be used if gas is not available. Electric furnaces are entirely satisfactory for heating aluminum alloys, but in most areas they cost more to operate than fuel-fired types. Time at temperature is not critical for aluminum alloys. Long soaking times provide no advantage but, except for the high-magnesium alloys such as 5083, are seldom harmful. Forging reheat temperatures must be maintained below that at which the alloy is hot short and fractures or develops internal cracks during forging. Excessive moisture in the furnace atmosphere should be avoided, to minimize the formation of internal voids resulting from diffusion of atomic hydrogen. Correct, uniform stock temperatures are essential for close dimensional control. Too high a forging temperature, even within the allowable range, sometimes can produce roughened surfaces and too rapid metal movement around the corner radii of the die. The latter condition can cause undesirable laps or cold shuts, which must be removed. Too low a temperature also can affect the final grain size of the forging, causing coarse grains. This may be undesirable because of its effect on mechanical properties, appearance and weldability.

    Die Heating. During forging of aluminum alloys the dies are normally heated. Die temperature is another critical element in the forgeability and forging process optimization. The die temperature ranges typically used for several aluminum forging processes are listed in the Table 1 [173]. For slower deformation processes, such as hydraulic press forging, die temperature should be kept in the same as that for metal deformation. Heated dies employing automatic temperature control are the best means to maintain die temperatures. For close-tolerance forgings, reliable die temperature control is essential to obtain the required dimensional accuracy.

    Table 1: Die temperature ranges for the forging of aluminum alloys [173]

    Forging process/equipment

    Die temperature

    C

    F

    Open-die forging

    Ring rolling

    95-205

    200-400

    Mandrel forging

    95-205

    200-400

    Closed-die forging

    Hammers

    95-150

    200-300

    Upsetters

    150-260

    300-500

    Mechanical presses

    150-260

    300-500

    Screw presses

    150-260

    300-500

    Orbital (rotary) forging

    150-260

    300-500

    Roll forging

    95-205

    200-400

    Hydraulic presses

    315-430

    600-800

    Use of Die. Forging of aluminum alloys requires the use of dies specially designed for these alloys, for at least three reasons [173]:

  • Aluminum alloys are seldom fullered or bent the forging sequence; two forging stages, preforming and finishing, are most commonly used.
  • Allowances for shrinkage are greater than for steel.
  • Temperature control of dies for forging aluminum is critical; thus, facilities for heating dies and controlling die temperature during forging must be considered in die design.
  • Finish on dies used for forging of aluminum alloys is more critical than that on dies used for steel. Cavities must be highly polished to obtain acceptable surface finish on the forgings.

    Lubricants: Aluminum alloys have a tendency to adhere to steel at elevated temperatures. A lubricant is necessary in forging aluminum alloys, to prevent metal sticking to the dies, insure smooth surfaces, reduce die wear, and facilitate metal flow. Unsatisfactory lubricants or improper application may result in undesirable metal flow, especially around corner radii of the die and in web areas enclosed by ribs, causing nonfill, laps and cold shuts. Many proprietary lubricants are used for forging aluminum. These usually are water or oil base, including compounded medium viscosity oils, and often contain soaps, other fatty compounds, and graphite. As an example, if metal flow is a problem, as in forging metal into narrow rib sections, soap is added to the graphite mixture.

    Deduction of Residual Stresses. Heat treating the finished forging is usually required, and undesirable residual stresses may be introduced. If the residual stresses are too high, subsequent machining of forgings can disturb the balance of these stresses and result in undesirable warping. Residual stresses of most concern in forgings are tensile stresses at the surface. They can result in stress-corrosion cracking in some aluminum alloys in a corrosive environment. So during the forging and subsequent heating treating, it is intended to reduce residual stresses:

  • The use of special racks and racking procedures during heat treatment helps to minimize such distortion.
  • Excessive warping of forgings on quenching should be avoided.
  • To minimize warping resulted from heat treatment and subsequent machining, it is helpful to perform rough machining before heat treatment, and to provide relief holes to allow better circulation of the quench water.
  • If straightening is required, it should be done promptly after quenching, to minimize creating more localized residual stresses or cracking.
  • For maximum assurance against stress-corrosion cracking, both alloy and temper must be considered in the design of forged aluminum products. Aluminum forging alloys 6061 and 6151 are virtually immune to stress-corrosion cracking in all tempers. Some alloys have high resistance to stress-corrosion cracking only in certain tempers, such as 7075 in T73 and 2219 in T6 and T8.
  •   
    References

    [156] James F. Young & Robert S. Shane: Materials and Processes. Part B: Processes. Marcel Dekker, Inc. 1985. ISBN 0-8247-7198-2
    [173] ASM: Metals Handbooks, Desk Edition. ASM 1998. ISBN 0-87170-654-7
    [177] ASM: Aluminum. Vol. 3. Fabrication and Finishing. Ed. By Kent R. Van Horn.

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