Fig. 1. Localized overheating of an aluminum-copper alloy (rosette pattern)
When we deal with applications where strength-to-weight ratio is a critical consideration, we often turn to solutions involving the so-called “light metals.” Aluminum, magnesium, titanium and in some cases beryllium enhance engineering performance while minimizing the weight of components and structures. Most of us involved in heat treating these materials know how we do it, but it is equally important to understand why we do it. Let’s learn more.
First, it is important to remember that light metals possess other physical properties that may be of importance in selection or service. These include the good electrical and thermal conductivity of aluminum, the machinability and noise dampening of magnesium or the extreme corrosion resistance of titanium. Our heat-treatment processes must retain and possibly enhance these properties.
Diffusion
As with all nonferrous alloys, the role of diffusion-related mechanisms in light metals must be understood to select the optimum heat treatment. The time/temperature relationship (Fig. 1) is critical in determining how changes will occur in the microstructures, which affect the resultant properties.Diffusion can simply be thought of as the rearrangement of the atoms inside the crystal (lattice) structure of the metal. Diffusion is controlled by the rate at which atoms change position and increases exponentially when temperature is applied. A few simple mechanisms are involved:
- Vacancy diffusion – the predominant diffusion mechanism in metals due to the (relatively) small amount of energy required for atom movement.
- Chemical diffusion – the exchange of atoms when two metals or alloys are placed in contact. Migration across the contact boundary occurs by atom exchange.
- Interstitial diffusion – occurs if a sufficiently small solute atom moves to a position between larger solvent atoms in an energy-favorable configuration.
- Grain-boundary diffusion – diffusion occurs along these defects in the crystal structure due to high interfacial energy and relatively weak bonding.
Fig. 2. Annealed AZ31 magnesium - 3% aluminum, 1% zinc alloy[2]
(a) Uniform microstructure after the 10th pass with intermediate 2-5 minute infrared heating to 750°F (400°C) with average rolling reductions of 16.5% per pass
(b) As-rolled sheet with good surface finish and minimal edge cracking
(a) Uniform microstructure after the 10th pass with intermediate 2-5 minute infrared heating to 750°F (400°C) with average rolling reductions of 16.5% per pass
(b) As-rolled sheet with good surface finish and minimal edge cracking
Heat Treatment
Light metals are usually heat treated either to improve mechanical properties or as a means of conditioning for specific fabricating operations. The type of heat treatment selected – annealing, homogenizing, solution treating, aging, stress relief – depends on alloy composition, the form (cast or wrought), manufacturing methods and ultimately on the anticipated service conditions.Annealing
In shaping of light metals by deformation – cold, warm or hot working – there is a limit to the amount of plastic deformation possible without failure by fracture. Annealing the material prior to reaching this limit places the part microstructure in a condition that can allow additional deformation to take place if necessary.
The annealing temperature for many of these metals is near the recrystallization temperature. For example, pure aluminum recrystallizes at a temperature of 300°F, and recovery/annealing temperatures normally vary from 300-650°F (150-340°C). By contrast, extensively cold-worked commercial aluminum alloys subject to recrystallization require heating for several hours into the 650-775°F (340-410°C) range.
Wrought magnesium alloys (Fig. 2) in various conditions of cold work, strain hardening or temper must be annealed for one or more hours by being heated at 550-850°F (290-455°C) depending on alloy.
By contrast, many titanium alloys are placed in service in the annealed state. The annealing of titanium and titanium alloys serves primarily to increase fracture toughness, ductility at room temperature, dimensional and thermal stability and creep resistance.
Fig. 3. Machined component ready for stress relief (Photograph courtesy of Solar Atmospheres, Inc.)
Most often a mill process, homogenization uses high temperatures (near the solidus) usually for prolonged periods to eliminate or reduce microsegregation – for improved workability – in castings that are to be hot or cold worked.
Solution Heat Treatment and Aging
Solution heat treating improves the strength of many light metals by causing the alloying elements to go into a solid solution and controlling the rate and extent of their return. Aging or precipitation hardening promotes full transformation of the material from its metastable state.
For example, a wide range of strength levels can be obtained in titanium alpha-beta (a-b) or beta (b) alloys by solution treating and aging. The origin of heat-treating responses of titanium alloys lies in the instability of the high-temperature beta phase at lower temperatures. Heating an alpha-beta alloy to the solution-treating temperature produces a higher ratio of beta phase. This partitioning of phases is maintained by quenching. On subsequent aging, decomposition of the unstable beta phase occurs, providing high strength.
Stress Relief
Stress relief of wrought components is performed to reduce or remove residual stresses induced by cold working (shaping, forming) and hot working (forging), straightening, welding or even cooling. The removal of such stresses helps maintain shape stability and eliminates unfavorable conditions such as the loss of compressive yield strength – the Bauschinger effect.
Stress relief of castings is particularly important to avoid residual stresses and associated warpage caused by differential cooling rates or from aggressive machining practices. Preventing stress-corrosion cracking is another reason.
For example, when magnesium extrusions are welded to hard-rolled sheet, a lower stress-relief temperature and longer time should be used to minimize distortion. It is better to use 300°F (150°C) for 60 minutes rather than 500°F (260°C) for 15 minutes. Other materials such as titanium and most titanium alloys can have a stress-relief operation performed (Fig. 3) without adversely affecting strength or ductility.
