Aluminum alloys are finding wider application in the transport industry as manufacturers seek to reduce vehicle weight, minimize fuel consumption and reduce exhaust emissions. A 10% reduction in motor vehicle weight is understood to deliver a 5.5% decrease in fuel consumption. Each 1kg (2.2 lb) of weight saved is estimated to lower CO2 emissions by 20 kg (44 lb) for a vehicle covering 170,000 km (105,000 mi). Considerable global research and development activity is focused on improving the properties of aluminum alloys and reducing processing costs to further facilitate adoption of aluminum into vehicles.
Aluminum alloy heat treatment
Heat-treatable aluminum alloys are those alloys that typically experience substantial increases in strength due to the precipitation of a fine distribution of second phases throughout the grains. To achieve such precipitation, the alloy is first solution treated at a temperature that dissolves the alloying elements (corresponding to the a-phase region in Fig. 1); quenched to ambient temperatures to "freeze" the solute into the matrix and produce a supersaturated solid solution; and then held either at room temperature (for natural aging below the GP zone solvus), or more commonly, at an artificial aging temperature (typically in the range 150 to 200C, or 300 to 390F) for a T6 temper [1]. For an alloy to be age hardenable, it must display solid solubility of the alloying element in the base element (e.g., Cu in Al), and the solid solubility must decrease with decreasing temperature.
The aging treatments enable solute atoms to diffuse together to form discrete strengthening precipitates, and several metastable phases may form during the process. For example, the complete precipitation sequence for the model alloy Al-4Cu is:
SSSS°'GP zones°'θ double
Prime °'θprime°'θ
where SSSS is supersaturated solid solution, GP zones is Gunier-Preston zones (crystallographically coherent precipitations), and the precipitate phases are based around the chemistry Al2Cu.
Materials that have been processed to the T6 condition (solution treated, then artificially aged) generally exhibit a peak in tensile properties together with minimum values of ductility and toughness (resistance to fracture). These properties depend largely on the precipitate phase present, as well as its size and distribution within the alloy. Typically, as an alloy gets stronger, its ductility and toughness decreases, largely due to exhaustion of the local work-hardening capability leading to strain localization and micro-void coalescence.
The nature of the precipitates formed and their influence on alloy properties is largely dependent on the quantities and ratios of the alloying elements. Age hardenable aluminum alloys are mostly based around the 2000, 6000 and 7000 series alloys, where quantities of Cu plus Mg, Si plus Mg, and Zn plus Mg(plus Cu) are added respectively. Some alloys also contain additions of Li, but these have found limited applicability in service.
If an alloy is heat treated for a shorter duration than that required to reach peak strength in a T6 temper, it can be encouraged to undergo the remainder of the strengthening process at lower temperatures through a process termed secondary precipitation, which is the focus of this article.
CSIRO's T6i4 aging schedule
By exploiting secondary precipitation so that it becomes an integral part of the heat treatment cycle, CSIRO has developed two patented heat-treatment processes that apply to all age hardenable aluminum alloys [2,3]. There are also multiple variants of each of these tempers, designed specifically for targeted property enhancement [4]. The first of these tailored tempers is designated T6i4, and has the scheme of aging represented schematically in Fig. 2. The changes that occur to the microstructure of the model alloy Al-4Cu during T6i4 processing are shown in Fig. 3.
During this heat-treatment, the conventional solution treatment and quenching procedure is followed by a short period of artificial aging to effect a prescribed degree of underaging at the same artificial aging temperature usually used to produce the T6 temper. The alloy is then held at temperatures in the range 25 to 65C (80 to 150F) for durations sufficient for the hardness and strength to increase to levels close to those normally recorded for the T6 temper. This new schedule can be viewed as an interrupted (i) T6 temper in which the aging cycle is halted prematurely and followed by a period of low temperature or natural aging (4); that is, T6i4.
The T6i4 treatment is demonstrated using a (Vickers) hardness-time plot for the aluminum casting alloy 357 (Al-7Si-0.5Mg) in Fig. 4. In some cases, such as for the wrought alloy 7075 (Al-5.5Zn-2.5Mg-1.5Cu-0.2Cr), the hardness developed by the T6i4 treatment can significantly exceed that of the T6 temper (Fig. 5) over extended periods. Additionally, because this process requires shorter periods of exposure to the artificial aging temperature, furnace costs and, therefore, component costs should be reduced.
Alloy Properties
Table 1 shows comparative mechanical properties of the casting alloy 357 resulting from the T6 and T6i4 heat treatments. The tensile properties resulting from the T6i4 treatment are close to those for the T6 temper, whereas the fracture toughness has increased by 40%.
Comparative mechanical properties for selected wrought alloys are shown in Table 2 for T6 and T6i4 treatments. The T6i4 results typically show tensile properties close to those for the T6 tempers, whereas for some alloys such as 6061, increases were observed. Fracture toughness values were either little changed (alloys 2214, 2001) or increased (38% for 7050, and 17% for 6061). The basis of the improved fracture properties of alloys given tempers including secondary aging (i.e., T6i4 and related tempers) suggest that enhanced homogeneity of deformation arises from the novel microstructure, and microvoid coalescence, which leads to fracture, is significantly impeded [5].
Examples of the mechanical changes in fracture toughness afforded by the T6i4 heat treatment are shown in Figs. 6 and 7 for the casting alloy 357. Figure 6 is an example of the load displacement fracture curves recorded during fracture, and show that there are increases in both the load required to initiate fracture and the displacement of the sample to effect crack opening, leading to a 500% increase in energy absorbed by the T6i4 specimen during fracture. This difference in behavior is also visible from the post-mortem fracture surfaces of the alloy, as shown in Fig. 7.
Incorporation into the paint-bake cycle
The automotive industry is making increased use of aluminum alloys for car bodies and other components such as alloy wheels. These components are often painted and dried for a period of time in an oven at a temperature in the range 150 to 180C (300 to 355F). This "paint-bake" temperature is similar to that used for artificial aging, and is often used to achieve some of the heat treatment strengthening required. However, the short period of time required to bake the paint (typically less than one hour) is not sufficient to achieve alloy strengths close to the T6 values.
A means of incorporating the paint-bake cycle into processing occurs when all heat treatment is carried out during paint-bake processing. The T6i4 temper is highly effective in this regard. Provided that the material is handled and processed appropriately, the tensile properties are very close to that for the T6 treatments. Among the data presented in the tables are the mechanical properties of an automotive body sheet alloy (6111) and the automotive casting alloy 357 having undergone conventional T6 and T6i4 heat treatments. For example, for the 6111 alloy, the time of T6 heat treatment is approximately 15 hours at 177C (350F), whereas the T6i4 treatment can achieve similar properties at 177C for only 45 minutes.
One of the advantages of the T6i4 process is the reduction in processing costs afforded by the reduced duration of heat-treatment required. In addition, the potential to reduce the footprint of the artificial aging furnace in a plant, the reduction in handling times following artificial aging, the improved material properties (such as hardness and toughness) and potential to modify alloy composition to optimize the response to secondary aging, all offer opportunities to further reduce component costs and increase the use of aluminum in transport vehicles.
Conclusions
A new aging process has been developed at CSIRO that offers the potential to improve the mechanical properties of aluminum alloys and reduce heat-treatment costs. Designated T6i4, this process involves aging for a short time at elevated temperature, after which alloys are quenched and allowed to undergo secondary aging at ambient or slightly higher temperatures. The technology can be applied to all aluminum alloys that respond to age hardening, and treatments can be tailored to improve specific properties. One example of cost savings is the opportunity to incorporate the paint-bake cycle as part of the T6i4 treatment when age hardening cast and wrought automotive components. IH Acknowledgement: The authors thank Barrie Finnin (barrie.finnin@csiro.au) for assistance with business development. Discussion regarding licensing opportunities for the CSIRO technologies is welcome.