Aluminum is supplied in almost an infinite variety of shapes and sizes for use in industries as diverse as transportation (automobiles, aircraft, railcars), packaging (foils, food, beverage cans), building products (siding, structural) and sports equipment (bats, shot-put circles, bleachers) to name a few [1]. Aluminum has an excellent strength-to-weight ratio and is easily cast, fabricated, formed and machined.

Table 1. Wrought Alloy Designation System

Aluminum Heat Treatment

Heat treating is a critical step in the aluminum manufacturing process to achieve required end-use properties. The heat treatment of aluminum alloys requires precise control of the time-temperature profile, tight temperature uniformity and compliance with industry-wide specifications so as to achieve repeatable results and produce a high-quality, functional product. The most widely used specifications are AMS2770 (Heat Treatment of Wrought Aluminum Alloy Parts) and AMS2771 (Heat Treatment of Aluminum Alloy Castings)[2], which detail heat-treatment processes such as aging, annealing and solution heat treating in addition to parameters such as times, temperatures and quenchants. These specifications also provide information on necessary documentation for lot traceability and the quality-assurance provisions needed to ensure that a dependable product is produced.

Wrought aluminum alloys (Table 1) can be divided into two categories: non-heat treatable and heat treatable. Non-heat-treatable alloys, which include the 1xxx, 3xxx, 4xxx and 5xxx series alloys, derive their strength from solid solutioning and are further strengthened by strain hardening or, in limited cases, aging. Heat-treatable alloys include the 2xxx, 6xxx and 7xxx series alloys and are strengthened by solution heat treatment followed by precipitation hardening (aging).

Table 2. Cast Alloy Designation System

Cast aluminum alloys (Table 2) cannot be work hardened, so they are used in either the as-cast or heat-treated conditions. Common heat treatments include homogenization, annealing, solution treatment, aging and stress relief. Typical mechanical properties for commonly used casting alloys range from 20-50 Ksi (138-345 MPa) ultimate tensile strength and 15-40 Ksi (103-276 MPa) yield strength with up to 20% elongation [3].

Heat Treatment Processes

In general, the principles and procedures for heat treating wrought and cast alloys are similar. For cast alloys, however, soak times tend to be longer if the casting is allowed to cool below a process-critical temperature for the particular alloy. Solution soak times for castings can be significantly reduced to durations similar to that for wrought alloys if the castings are placed into the solution furnace while still hot (above the process-critical temperature) immediately following mold filling and solidification. The reduction of stress in complex cast shapes is achieved in large part by the control of quenching parameters such as agitation rate, quenchant temperature, rate of entry and part orientation in the quench.

Fig. 1. Typical age hardening equipment

Aging

The goal of aging is to cause precipitation dispersion of the alloy solute to occur (Fig. 1). The degree of stable equilibrium achieved for a given grade is a function of both time and temperature. In order to achieve this, the microstructure must recover from an unstable or “metastable” condition produced by solution treating and quenching or by cold working.

The effects of age hardening or precipitation hardening on mechanical properties are greatly accelerated, and usually accentuated, by reheating the quenched material to about 212°F-424°F (100°C-200°C). A characteristic feature of elevated-temperature aging effects on tensile properties is that the increase in yield strength is more pronounced than the increase in tensile strength. Also ductility – as measured by percentage elongation – may decrease. Thus an alloy in the T6 temper has higher strength but lower ductility than the same alloy in the T4 temper.

In certain alloys, precipitation heat treating can occur without prior solution heat treatment since some alloys are relatively insensitive to cooling rate during quenching. Thus they can be either air cooled or water quenched. In either condition, these alloys will respond strongly to precipitation heat treatment.

In most precipitation-hardenable systems, a complex sequence of time-dependent and temperature-dependent changes is involved. The relative rates at which solution and precipitation reactions occur with different solutes depend upon the respective diffusion rates, in addition to solubility and alloy contents.

Fig. 2. Coil batch annealing furnace with nitrogen atmosphere and controlled cooling

Annealing

Annealing is used for both heat-treatable and non-heat-treatable alloys to increase part ductility with a slight reduction in strength. There are several types of annealing treatments dependent to a large extent on the alloy type, initial and final microstructure and temper condition. In annealing it is important to ensure that the proper temperature is reached in all portions of the load (Fig. 2). The maximum annealing temperature needs to be carefully controlled.

During annealing, the rate of softening is strongly temperature dependent – the time required can vary from a few hours at low temperature to a few seconds at high temperature. Full annealing (temper designation “O”) produces the softest, most ductile and most versatile condition. Other forms of annealing include: stress-relief annealing, used to remove the effects of strain hardening in cold-worked alloys; partial annealing (or recovery annealing) done on non-heat-treatable wrought alloys to obtain intermediate mechanical properties; and recrystallization characterized by the gradual formation and appearance of a microscopically resolvable grain structure.

Fig. 3. Homogenization of aluminum loop

Homogenization (Ingot Preheating Treatments)

The initial thermal operation applied to castings or ingots (prior to hot working) is homogenization (Fig. 3), which has one or more purposes depending upon the alloy, product and fabricating process involved. One of the principal objectives is improved workability since the microstructure of most alloys in the as-cast condition is quite heterogeneous. This is true for alloys that form solid solutions under equilibrium conditions and even for relatively dilute alloys.

Fig. 4. Three-chamber ingot, t-bar or saw preheater

Preheating

Preheating of aluminum ingots prior to rolling, extruding, forming, forging or melting (Fig. 4) is used to reduce energy consumption by improved process efficiency, reducing cycle time and increasing safety.

Fig. 5. Roller hearth furnace for solution heat treatment of cast aluminum brake components

Solution Heat Treatment

The purpose of solution heat treatment is the dissolution of the maximum amount of soluble elements from the alloy into solid solution. The process consists of heating and holding the alloy at a temperature sufficiently high and for a long enough period of time to achieve a nearly homogenous solid solution in which all phases have dissolved (Fig. 5).

Care must be taken to avoid overheating or underheating. In the case of overheating, eutectic melting can occur with a corresponding degradation of properties such as tensile strength, ductility and fracture toughness. If underheated, solution treatment is incomplete and strength values lower than normal can be expected. In certain cases extreme property loss can occur. The solution soak times for castings can be reduced significantly by placing the casting directly into the solution furnace immediately following solidification. The casting is maintained at a temperature above a process-critical temperature (PCT), and the alloy solute is still in solution.

In general, a temperature variation of ±10°F (±5.5°C) from control set point is allowable, but certain alloys require even tighter tolerances. Tighter thermal variation (±5°F) allows the set point to be controlled closer to the eutectic, thus improving proportion and reducing required soak time. The time at temperature is a function of the solubility of the alloy solute and the temperature at which the aluminum casting or wrought alloy is removed from the mold and placed into the solution furnace. This time may vary from several minutes to many hours. The time required to heat a load to the treatment temperature increases with section thickness, air space around the casting for hot air to flow and the loading arrangement.

Fig. 6a. Vertical tower drop-bottom furnace for 32-foot aluminum aerospace extrusions (includes polymer quench system)

Quenching

Rapid and uninterrupted quenching in water or poly (alkylene) glycol in water is, in most instances, required to avoid precipitation detrimental to mechanical properties and corrosion resistance. The solid solution formed by solution heat treatment must be cooled rapidly enough to produce a supersaturated solution at room temperature that provides the optimal condition for subsequent age (precipitation) hardening. Quench types include hot water immersion, ambient water immersion, water spray, forced air, forced air with mist and poly (alkylene) glycol in water.

Quenching is, in many ways, the most critical step in the sequence of heat treating. In immersion quenching, cooling rates can be reduced by increasing the quenchant temperature. Conditions that increase the stability of a vapor film around the part decrease the cooling rate. Four factors that minimize distortion in the aluminum include:
  • Temperature of the quenchant
  • Agitation rate of the quenchant
  • Speed of entry of casting into the quenchant
  • Orientation of the aluminum part as it enters the quenchant


Fig. 6b. Continuous roller hearth system with age-hardening oven for aluminum forgings to the T6 condition

Stress Relief

Stress-relief annealing can be used to remove the effects of strain hardening in cold-worked alloys. No appreciable holding time is required after the parts have reached temperature. Stress-relief annealing of castings provides maximum stability for service applications where elevated temperatures are involved.

Table 3. "T" Temper Designations

Tempering

Tempering can be performed on heat-treatable aluminum alloys to provide the best combination of strength, ductility and toughness. These may be classified by the following designations:
  • “F” - as fabricated
  • “H” - strain hardened
  • “O” - annealed
  • “T” - thermally treated
  • “W” - solution treated


The temper designation (Table 3) follows the alloy designation and consists of letters. Subdivisions, where required, are indicated by one or more digits following the letters.

Fig. 7. Three gas-fired drop-bottom batch furnaces with common mobile quench tank for solution treatment of aluminum castings

Batch Installations

Batch designs (Fig. 7) are typically used for aluminum applications where throughput is not predictable or consistent, where the process varies from load to load or where a ramp-up in production favors sequential implementation. Batch units often run a variety of load configurations, so it is critical to have versatile airflow and tight temperature control. Typically, systems operate in temperature ranges of 350°F, 500°F, 650°F and 850°F. Heating systems can be provided in electric, direct gas or indirect gas heating. Loading can be accomplished by truck, rack/shelf, car, overhead trolley or monorail. Single and multiple chamber units provide maximum process flexibility. Common applications include aging, annealing, homogenizing and stress relief.

Fig. 8. U-configuration continuous roller hearth solution heat-treatment furnaces with age-hardening ovens producing cast alumi-num wheels

Continuous Installations

Continuous designs (Fig. 8) are typically used for automated production and include mesh-belt conveyors, roller hearths, roller rails, rotary hearth, pusher and walking-beam units. Capacities ranging from several hundred to several thousand pounds per hour are typical. Parts are loaded consistently and uniformly for continuous process flow, so minimum labor is required. Continuous furnace systems significantly reduce operating costs. Typically, systems operate in temperature ranges up to 1100°F. Heating systems can be provided in electric, direct gas or indirect gas heating. If desired, loading can be automated in a number of different ways. Common heat treatments include aging, annealing and solutioning for products as diverse as castings, forgings, plate, bar and tube products.

Integration with Lean and Agile Manufacturing

In order to integrate aluminum heat-treating systems into lean and green manufacturing strategies, system automation and control for loading, unloading and transferring workloads is required. Robotics, roller conveyor systems, manipulators and charging cars are typical examples of equipment supplied to increase production efficiency while reducing manpower requirements.

The processing of aluminum requires a combination of rapid heating and close temperature uniformity throughout the entire load. Many components are safety-critical and must be heat treated with high precision and repeatability. Often plant floor space is important and compact designs are highly desirable. Integrating with the SCADA systems for real-time data acquisition and integration with upstream and downstream processes is essential. The heat treatment of aluminum demands that all aspects of the process are monitored and controlled. IH

For more information: Robert D. Howard is Manager-Ovens & Furnaces for Consolidated Engineering Company, 1971 McCollum Pkwy., N.W., Kennesaw, Ga. 30144; tel.: 770-422-5100; fax: 770-422-6968; web: www.cec-intl.com

Additional related information may be found by searching for these (and other) key words/terms via BNP Media SEARCH at www.industrialheating.com: solution heat treatment, wrought, aging, precipitation harden, strain harden, yield strength, elongation, homogenization, anneal