Precipitation hardening in 7075 Aluminum alloy.

Age hardened aluminum alloys are desirable for applications that require lightweight materials. In this experiment, the precipitation hardening behavior of the Al-6%Zn-2%Mg-2%Cu (7075) alloy was studied by solution treatment, quenching and aging. As a function of aging time, changes in hardness and electrical conductivity of the specimens were noted, and the effect of time (3, 10, 30, 60, 90 minutes) and temperature variations (160°C and RT) were compared. Also, the relationship between the hardness and aging time was found to be linear and explains the increase in the generation of GP zones.

Introduction

In age hardening, also known as precipitation hardening, the strength of a multiphase material is increased by reducing the movement of dislocations.  This is achieved by increasing the grain size and allowing grains of different phases to collide and intersect. This process takes place under high heat (above the solvus line) over a sustained period so that one phase becomes thermodynamically stable while the other phase dissolves. To achieve this result known as solid-solution heat-treatment, the 7075 Al specimen requires 890°C for 30 minutes. This is because the precipitation process is not instantaneous – precipitates need time to form, or age. Hardness and strength are inversely proportional to precipitate size. As the precipitate size becomes coarser, therefore, hardness and yield strength will decrease. At some point of aging 7075 Al, the precipitates tend to coarsen and the spacing between them increases, therefore leading to reduction in hardness. Since coarsening is dependent on the motion of atoms, the maximum point is typically achieved at a higher temperature.

Figure 1. A comparison of the hardness-aging time relationship.

GP zones of spherical shape accompany the aging process of aluminum alloys. GP zones increase in size as the aging cycle progresses, therefore leading to increased strength of the alloy. Aging times and temperatures that allow for the highest strengths to be achieved typically involve GP zones with average diameters of 20 to 35 angstroms and have high concentrations of zinc atoms. There is a temperature known as the GP zone solvus temperature, which when achieved, causes heterogeneous nucleation of GP zones, therefore inhibiting thermodynamic equilibrium and consequently decreasing strength of the alloy through reduction of solute that catalyzes homogeneous nucleation (higher strength). GP zone growth is dependent on its size, but more importantly on exposure temperature. A two-step aging process is often required for above-mentioned aluminum-Zinc-Magnesium alloys since most of the large GP zones transform to transition precipitates below and above the solvus temperature.

Procedure

Pre-preparation:  Six specimens of 7075 aluminum, of roughly 1 x 1 x 0.5 in. were stamped with identifying letters;  E, M, K, R, T and V.  The specimens were then measured for Rockwell B hardness (HRB) and electrical conductivity (IACS%).  The results of the initial measurements before heat exposure are shown in Table 1.  The six specimens were then placed in a ceramic crucible, which was placed in the furnace at 480°C.  The specimens were placed in the furnace at 19:40 and removed at 20:10, for a 30 minute total exposure at temperature. Upon removal, the specimens were quenched by pouring them from their ceramic crucible into a pail of room temperature (RT) water.  Note that in the interest of time, the marking, initial measurements and placement of the specimens in the furnace were performed by the professor.  The student group removed the specimens and quenched them.   The results of the initial measurements of the specimens provided to the work group are found in Table 2.

From the above, five of the aluminum specimens were then placed back in the crucible, and placed in a furnace at 160°C.  One specimen (letter V) was held back for use as a RT control.  This control specimen was not subjected to heating at 160°C.  The specimens were removed from the crucible one at a time, at intervals of 3 minutes, 10 m, 30 m, 60 m and 90 m.   As they were removed, the individual samples were immediately quenched in the pail of water at RT.  After about 1 m, the specimens were dried, the letter recorded, and that specimen re-measured for hardness and conductivity after the heating period.  As each specimen was removed from the furnace and re-measured, the control specimen was also re-measured at that same time interval.  This was to compare the effect of heat aging with natural, room temperature aging. The results from the age hardening are shown in Table 3.

Results and Discussion

Initial measurements of the specimens before any heat exposure:

Table 1:  Initial Measurements of the Test Specimens Before Heat Exposure

Measurements of the specimens after exposure to 480°C for 30m.

Table 2:  Measurement of Test Specimens After Heat Exposure (480°C)

Measurements of the test specimens after exposures to 160°C at the time intervals shown are found in Table 3.  Note also that when each of the heat-aged samples was measured, a control sample (marked as “V”) that was held at room temperature was also measured, and the results recorded.

Table 3:  Measurements of the Test Specimens after Heat Exposure (Aged @ 160°C)

Figure 2: Hardness (HRB) VS Log Time of aging (seconds)

In Figure 2, the hardness of the Al alloy is plotted against its time of aging. With the same amount of aging time, there is a tremendous change in hardness of the Al alloy at 160°C compared to the reference Al alloy at room temperature. At each aging temperature, the hardness of the Al alloy continually increases with the aging time. This increase in the hardness can be explained with the generation of GP zones increasing in size. When the zone size is large enough, the zones transform to transition precipitates above the GP zone solvus temperature. On the other hand, there is a 2% copper content, the Al alloy can get more precipitation hardening with some contribution of copper atoms to zone formation.

Figure 3: Conductivity VS Time of aging (s)

In Figure 3, conductivity of the Al alloy is plotted against its time of aging. With the same amount of aging time, the conductivity of Al alloy at 160°C increases while the conductivity of the reference Al alloy at room temperature decreases. The measurement of the conductivity is an indirect way to understand the nature and distribution of the transition precipitates. As the aging begins, the conductivity drops significantly as seen in Figure 2, it is due to the formation of closely spaced GP zones. This results in a formation of precipitates involved in the rearrangement of Al and Cu multilayers (reference…). As the aging progresses, the precipitates lose the coherence and increase in size. As a result, the conductivity starts to increase as demonstrated in Figure 3. Thus, this phenomenon shows the effects of over-aging and regular aging.

Conclusion

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