The heat treatment of aluminum and aluminum alloys are precision processes that must be carried out in furnaces and ovens properly designed to provide very precise thermal conditions and equipped with adequate control instruments to ensure required uniformity and repeatability of temperature-time cycles. Heat-treat process details must be carefully established, controlled and documented for each type of product to ensure that required properties are achieved. Let's learn more.
Types of Heat Treatment
Heat treatments applied to aluminum and its alloys are:
- Preheating or homogenizing to reduce chemical segregation of cast structures and to improve material workability
- Annealing to soften strain-hardened (work-hardened) and heat-treated alloy components, to relieve stresses and to stabilize properties and dimensions
- Solution heat treatment to improve mechanical properties by putting alloying elements into solution
- Precipitation (age-hardening) heat treatment to provide hardening by precipitation of constituents from solid solution
Homogenization (Ingot Preheating Treatment)
The initial thermal operation applied to castings and ingots (prior to hot working) is referred to as homogenization, which has one or more purposes depending on 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.
Annealing can be used for both heat treatable and non-heat treatable alloys to increase ductility with a slight reduction in strength. There are several types of annealing treatments dependent to a large part on the alloy type, initial structure and temper condition. In annealing, it is important to ensure that the proper temperature is reached in all locations of the load. The maximum annealing temperature is also important in achieving good results.
Full annealing (temper designation “O”) produces the softest, most ductile and most versatile condition. It is performed for both heat treatable and non-heat treatable aluminum alloys. The rate of softening is strongly temperature dependent; that is, the time required can vary from a few hours at low temperature to a few seconds at high temperature.
Other forms of annealing include stress relief annealing 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.
Heat-treatable aluminum alloys contain amounts of soluble alloying elements that exceed the equilibrium solid solubility limit at room (and moderately higher) temperature. The amount present may be less or more than the maximum that is soluble at the eutectic temperature.
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.
Solution Heat Treating
The purpose of solution heat treatment is the dissolution of the maximum amount of soluble elements in 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.
Care must be taken to avoid overheating or under-heating. In the case of overheating, eutectic melting can occur with a corresponding degradation of properties such as tensile strength, ductility and fracture toughness. If under-heated, solution is incomplete, and strength values lower than normal can be expected. In certain cases, extreme property loss can occur.
In general, a temperature variation of +/-10°F (+/-6°C) from control set point is allowable, but certain alloys require even tighter tolerances. The time at temperature is a function of the section thickness of the material and may vary from several minutes to many hours. The time required to heat a load to the treatment temperature also increases with section thickness and loading arrangement and, thus, the total cycle time must take these factors into consideration.
Rapid, uninterrupted quenching in water or polymer 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, which provides the optimal condition for subsequent age (precipitation) hardening.
Quenching is in many ways the most critical step in the sequence of heat treating operations. The objective of quenching is to preserve as nearly intact as possible the solid solution formed at the solution heat treating temperature by rapidly cooling to some lower temperature, usually near room temperature.
Water and polymer quenchants are the most widely used quenching media. In immersion quenching, cooling rates can be reduced by increasing temperature. Conditions that increase the stability of a vapor film around the part decrease the cooling rate.
Aging (Age Hardening)
Age hardening is achieved either at room temperature (natural aging) or with a precipitation heat treatment (artificial aging) cycle. The same general rules used in solution heat treatment (temperature uniformity, time at temperature) apply for precipitation hardening.
Aging at room temperature (natural aging). Most of the heat-treatable alloys exhibit age hardening at room temperature after quenching, the rate and extent varying from one alloy to another. Since the alloys are softer and more ductile immediately after quenching than after aging, straightening and forming operations are normally performed in the as-quenched condition. The process window for forming after quenching can be enlarged by keeping the alloy refrigerated prior to forming.
Aging at elevated temperature (artificial aging). The effects of precipitation on mechanical properties are greatly accelerated (and usually accentuated) by reheating the quenched material to a temperature range of about 212 to 424°F (100 to 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, decreases. 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, because 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 general, the principles and procedures for heat treating wrought and cast aluminum alloys are similar. However, for cast alloys, soak times and quenching media are often different due to the section sizes involved. Typically, soak times are longer and quenchants such as boiling water are used to reduce quenching stresses in complex shapes. IH