Forming and Heat Treatment: Which Temper, and Ice-Box Rivets

Which temper to form

Match the original. Replacement aluminium sheet is identical to the original, or properly altered. If another alloy is considered, MMPDS (the successor to MIL-HDBK-5, which AC 43.13-1B cites) has the comparative strengths (AC par. 4-53).

The forming decides the temper. Single curvature with straight bend lines and a large radius can be formed from heat-treated sheet. A part such as a fuselage frame is formed from soft, annealed sheet and heat-treated after forming (AC par. 4-53a).

Cold forming in the tempered condition, at room temperature, saves the warp and twist that heat treatment brings. Expect springback. If the metal cracks over a small radius, form it annealed. Small radii and compound curves are formed annealed, then heat-treated to temper before use (Handbook p. 4-21).

Do not leave a structural part annealed. An annealed repair of equal strength weighs more than a heat-treated one (AC par. 4-53b). Unpainted parts are made of clad sheet (par. 4-53a).

Bend radius. The minimum radius depends on composition, temper and thickness: thinner sheet takes a tighter radius and a harder temper needs a larger one (AC par. 4-55a). Look it up in the bend radius chart. Bend lines should lie across the grain, preferably at 90 degrees, and relief holes go at the ends of bend lines (par. 4-55b).

Work hardening

Too much hammering work-hardens the metal: it stops responding to bending and feels springy. In some cases the part must be annealed during the operation, and then heat-treated again before it goes on the aircraft (Handbook p. 4-75).

A patch plate that must be formed, such as to fit the contour of a lightening hole, is made from sheet in the O condition and heat-treated after forming (Handbook p. 4-106).

Heat treatment, and its limits

Structural aluminium parts are heat-treated to the manufacturer's instructions, in the sequence the specification gives for the temper. The temperatures and times are in that specification and are not repeated here (AC par. 4-54).

Quench from the solution heat-treating temperature as fast as possible after the furnace. Cold water is preferred; hot water or air blast is used for bulk sections such as forgings, to lessen quenching stress. If the part warps, straighten it immediately after the quench. Heat-treat riveted parts before riveting, to avoid warping and corrosion (AC par. 4-54, 4-54a).

The 212 degree rule. Reheating 2017 and 2024 above 212 degrees Fahrenheit impairs the original heat treatment. That includes baking primers. It is not acceptable without a full, correct heat treatment afterwards (AC par. 4-54b). Mind this before putting a formed part in a paint oven.

Temper marks. T4 is solution heat-treated; T6 is solution heat-treated and artificially aged; T3 adds cold work. The full list is in the identification guide.

Ice-box rivets

Some rivet alloys are too hard to drive as received. 2117 (AD) is driven as received. 2017 rivets above 3/16 in diameter and all 2024 (DD) rivets are kept packed in dry ice or refrigerated in the quenched condition until driven, or are reheat-treated just before driving (AC par. 4-57d).

The Handbook adds that DD rivets are annealed and stored in a freezer to slow hardening, hence the name. Most have been replaced by E-type rivets (7050), which are driven as received (Handbook p. 4-32). The E head mark differs by document: MS20470 Rev. H and MS20426 Rev. L require an indented ring, and the Handbook's strength chart says raised ring (Fig. 4-77, p. 4-33). The strength chart says DD must be driven in the W condition; D rivets give 38 KSI as driven and 34 KSI when reheat-treated (p. 4-33).

The AC gives no time limit out of the freezer. The Handbook glossary says only that they harden within a few hours at room temperature and that a subfreezing icebox delays this for several days (p. G-19). A US Navy training manual prints 20 minutes and 24 hours: see the rivet identification chart. Take the limit from the rivet specification and the shop's procedure. The rivet alloy calculator checks the head mark and condition.