All fasteners undergo heat processing during their manufacture, and many undergo multiple heating processes. Aluminum fasteners, such as rivets used on aircraft, are solution treated and aged to provide the desired mechanical properties. 

Solution treatment involves heating the aluminum to a temperature of 430-540°C (800-1000°F), at which alloying constituents are taken into solution (i.e., brought near their melting point) prior to a rapid quench. This rapid quench retains the grain structure but leaves the material soft, requiring a subsequent aging operation. The heating time and temperature is dependent on the alloys contained in the aluminum and the cross-sectional thickness of the material.

The time required to extract the material from the furnace and quench it (i.e., quench delay) must be kept within a specified limit, typically 5 to 15 seconds, and varies depending of the alloy being processed. Thinner, cold-worked aluminum alloys require a faster quench than cast alloys. The quench medium is most commonly heated or ambient water, although glycol and air are used in certain cases.

After solution treatment and quenching, aluminum fasteners are age hardened at elevated temperature, which is also referred to as precipitation heat treatment. The aging process increases the strength and hardness of the material. Aging requires temperatures of 115-200°C (240-390°F) and heating times of 5 to 48 hours. The time-temperature parameters for aging are carefully selected based on the alloy used and the desired mechanical properties.

With some alloys, it is desired that aging be suppressed or delayed until a controlled artificial age can be performed, typically for maintaining production flow and desired batch sizes. Conventional practice allows for refrigeration, for example, of 2014-T4 alloy rivets at -18°C (0°F) for several days.

An Example

A leading aircraft manufacturer required a high-capacity solution-treatment system for aluminum rivets. The process required heating a 455-kg (1,000-pound) load of rivets carried in 12 254-mm-diameter x 152-mm-high (10-inch-diameter x 6-inch-high) baskets to 455°C (850° F) then water quenching them in 10 seconds.

A batch-style electrically heated solution-treatment furnace was provided (Fig. 1). To operate the system, the user places the baskets of rivets into the preheated furnace. Then after a predetermined soak time, the user quickly opens the door and manually pulls them from the furnace onto the quench elevator, which rapidly lowers them into the quench tank.         

The quenching is done in unheated water. Since the water temperature increased 14°C (25°F) after each quench, a chiller was provided to more quickly bring the water temperature back down prior to the next quench, which allows more frequent processing of the loads and higher throughput.

The furnace is rated for a 650°C (1200°F) maximum operating temperature and achieves a temperature uniformity of ±5.5°C (±10°F) at 430 and 590°C (800 and 1100°F) per AMS 2750 Class 2 furnaces and Type C Instrumentation. The heating system features Incoloy-sheathed heating elements and is rated for 36-kW heat input with SSR power control. The recirculation system utilizes a 4,760 m3/hour (2,800 cfm) fan powered by a 2.25-kW (3-HP) motor.