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Processing Aluminum Stampings

By: Peter Ulintz

Sunday, June 1, 2014
 

Aluminum alloys can successfully be formed into complex shapes—automotive body panels for example—using existing pressroom production equipment. Production examples include inner and outer door panels, hoods, deck lids, lift gates, cross members, and structural and reinforcement components. Aluminum properties differ greatly than those for steel, which means that aluminum deforms differently during stamping. This requires a die- and process-design strategy that accounts for these differences.

From ASM Specialty Handbook: Aluminum and Aluminum Alloys
Table 1—From ASM Specialty Handbook: Aluminum and Aluminum Alloys
The first step: Understand the alloy and temper designations for the specific stamping being processed. Aluminum alloys are designated by a four-digit code that describes their primary alloying elements (Table 1).

The 1xxx series alloys essentially are commercially pure aluminum—very soft and formable—and are not used where strength is a prime consideration. They typically find use in electrical and chemical industries.

The 2xxx series alloys exhibit high strength, toughness and, in some cases, weldability. They do not resist atmospheric corrosion as well as other aluminum alloys do, so they typically are painted or clad for added protection.

The 3xxx series alloys are the most widely used—stronger than 1100 but still readily formable. Typical applications include radiators, heat exchangers and beverage-can bodies.

The 4xxx series alloys exhibit a relatively low melting point and find use as welding wire.

The 5xxx series alloys find use in consumer electronic cases (strength, appearance and anodizing) and automotive structural components. Since this series of alloys is not heattreatable, any beneficial strengthening from cold working may be lost if the final part is subjected to a paint-bake cycle.

The 6xxx series alloys provide relatively good formability and will strengthen during the paint-bake cycle, making them useful for automotive body panels and closures.

The 7xxx series alloys are high-strength heattreatable grades that are not easily joined by commercial welding processes, so they are routinely joined by riveting.

Temper Designations

Metalformers select from four basic temper designations when specifying aluminum sheetmetal:

Minimum mechanical properties for 0.060-in.-thick sheet
Table 2—Minimum mechanical properties for 0.060-in.-thick sheet (Ref. ASTM B209)
*ASM Specialty Handbook: Aluminum and Aluminum Alloys
• As-fabricated (F)

• Strain hardened by cold work (H)

• Fully soft (O)

• Heattreated (T).

The temper designation follows the alloy designation number with a dash and a letter, 5052-O for example.

H and T tempers often are accompanied by additional numeric designations that further describe the tempering method.

H—Strain hardened (cold worked) with or without thermal treatment

H1x—Strain hardened without thermal treatment

H2x—Strain hardened and partially annealed

H3x—Strain hardened and stabilized by low temperature heating

The second digit indicates the degree of hardness: 2 = ¼ hard; 4 = ½ hard; 6 = ¾ hard; 8 = full hard; and 9 = extra hard. An example would be 5052-H32—strain hardened without thermal treatment to a ¼-hard condition

Minimum bend radii for select alloys and material thickness
Table 3—Minimum Bend Radii for Select Alloys and Material Thickness
T—Heat reated to produce stable tempers

T1—Cooled from hot working and naturally aged (at room temperature)

T2—Cooled from hot working, cold worked, and naturally aged

T3—Solution heattreated and cold worked

T4—Solution heattreated and naturally aged

T5—Cooled from hot working and artificially aged (at elevated temperature)

T6—Solution heattreated and artificially aged

T7—Solution heattreated and stabilized

T8—Solution heattreated, cold worked and artificially aged

T9—Solution heattreated, artificially aged and cold worked

T10—Cooled from hot working, cold worked and artificially aged

Processing Planning

Aluminum-alloy type and temper are a prime concern for the process engineer and die designer. Formability—in terms of total elongation—varies with alloy type and temper (Table 2).

Determining an appropriate cutting clearance between punch and die depends on material type and thickness. Suggested punch-to-die clearances, in terms of percent of sheet thickness (t), also are provided in Table 2 for cutting, blanking and hole punching.

When punching and cutting aluminum, especially dead-soft alloys (O-temper), metalformers must closely follow the appropriate maintenance routines, and use sharp tooling. Dull edges on punches and dies can produce burrs similar to those caused by excessive clearance, with burr height being particularly problematic. A lubricant suitable for aluminum stampings will help reduce tool wear and produce quality shear edges.

Bending aluminum requires special attention of the die designer. While for most steels the minimum bending radius relative to sheet thickness is approximately constant, primarily because ductility (total elongation) tends to be the limiting factor, this is not the case with aluminum. In general, the ratio of bend radius to sheet thickness will increase with sheet thickness (Table 3). MF

Note: Look for a special, in-depth article on bending, forming, deep drawing, cutting and punching of aluminum alloys, authored by Pete Ulintz and Stuart Keeler, coming in the July 2014 issue of MetalForming.

 

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