The aluminum heat-treatment industry is an exciting place to work these days, as demonstrated by ongoing research activities into how mechanisms triggered by heat treatment can lead to shorter cycle times for solution heat-treating and aging processes. The hope is that this type of work will result in specification changes reflecting these advanced techniques in the not-too-distant future. Let’s learn more.
Heat-treatable aluminum alloys are ones that contain elements designed to increase their strength by thermal processing. These elements exhibit increasing solid solubility with increasing temperature. In addition, the alloying elements are present in concentrations that exceed their equilibrium solid solubility at room (and moderately higher) temperature.
A typical heat-treat sequence (Table 1) involves solution heat treating (solutionizing) followed by rapid cooling (quenching) and age hardening (aging, precipitation hardening) to obtain high strength and other important mechanical properties. The relative rates at which solution and precipitation reactions occur depends to a large extent on the respective diffusion rates of the different solutes in addition to their solubility and alloy contents. Bulk diffusion coefficients for a number of the commercially important alloying elements in aluminum have been determined by experimental methods.
Aging must take place below both the solvus temperature (the temperature at which the alloying elements will dissolve and disperse) and the Guinier-Preston (GP) zone solvus line (Fig. 1) in order for uniform precipitation to occur.
In most precipitation-hardenable systems, a complex sequence of time-dependent and temperature-dependent changes is involved. Three types of interfaces are common during nucleation and growth of the precipitates: coherent, semi-coherent and incoherent. The resultant precipitates can occur in a variety of shapes, generally determined by a minimal energy state, particle size, matrix misfit and interfacial energy. Examples include needles, spheres, cubes and disks. Precipitation normally occurs in the following sequence: solid solution, solute clustering, transition precipitates and equilibrium precipitates. Since strength is related to the volume fraction of the various precipitates (as well as their particle size), the number and type of alloying elements are important.
One of the concerns during precipitation hardening is the formation of precipitate-free zones (PFZ) that may form near grain boundaries (Fig. 2). PFZs can adversely affect ductility, fracture toughness and fatigue-crack-initiation resistance as well as increase susceptibility to stress corrosion cracking. Lowering the aging temperature, which effectively decreases diffusion rates and the solute super saturation, can minimize PFZs. Consequently, age-hardened alloys can be subjected to a double-aging treatment: a low-temperature age followed by a higher-temperature age. The low-temperature age increases the number of precipitates and minimizes the formation of PFZs, while the higher-temperature age accelerates the growth of precipitates.
Double aging can also result in shorter cycle times in certain alloy systems, offering potential savings in both time and energy. An example of research in this field is the study of 7075 aluminum, a major aerospace structural alloy. In particular, the effects of double aging (DA) and thermomechanical double aging (TMDA) on accelerating the kinetics of precipitation have been documented in literature, and the benefits have been briefly discussed. It has been reported that by using the appropriate heat-treatment parameters, the time to peak hardness can be reduced from 48 hours to two hours with higher ductility and only a 6% decrease in yield and tensile strength.
Perhaps the most important consideration for maximum hardness in the DA treatment is controlling the time of the first age (Fig. 3). Increasing the time in the first age at 121°C (250°F), the peak hardness is achieved after the second age at 177°C (350°F), provided the time is optimized (in this case 55 minutes).
With respect to the second age (Fig. 4), using the same conditions as in the first aging treatment, namely 121°C (250°F) and 55 minutes, the peak hardness is similar for several second-age temperatures, namely 149°C (300°F) and 177°C (350°F). However, the time to reach peak hardness decreases with an increase in temperature.
While space considerations prevent us from summarizing the work being done on other wrought and cast alloy systems, such as two-step and high-temperature solution treatments and rapid heat treatment of aluminum high-pressure die castings, progress continues in an effort to optimize cycle times.
Supporting research of this nature is critical to the heat-treatment industry as we strive to better serve our customers, increase productivity and save energy. The use of appropriate double aging and/or thermomechanical double aging is but one example that shows there can be a significant return on our investment from better understanding and then accelerating the kinetics of precipitation.
1. Illinois Institute of Technology/Thermal Processing Technology Center, Professor Philip Nash, Director (http:tptc.com)
2. Herring, Daniel H., “Understanding Aluminum Heat Treatment,” Industrial Heating, February 2005.
3. Concise Encyclopedia of Structure of Materials, J. W. Martin (Ed.), Elsevier, 2007.
4. Dr. D. Scott Mackenzie, private correspondence.
5. Emani. S. V., Benedyk, J. P. Nash and D. Chen, “Double aging and thermomechanical heat treatment of AA7075 aluminum alloy extrusions,”J. Mater Sci (2009) 44:6384-6391.
6. Herring, Daniel H., “Innovations in Aluminum Heat Treatment,” Industrial Heat-ing, February 2011.
7. Dr. Joseph Benedyk, Editor, Light Metal Age magazine, private correspondence as well as technical and editorial review.