state. The purpose of heat treating is to make a metal more useful by changing or restoring its mechanical properties. Through heat treating, metal can become harder, stronger, and more resistant to impact. On the other words, the purpose of the heat treatment is to cause desire changes in the metallurgical structure and thus in the properties of metal parts. Lopes (1996) stated that heat treatment is a method used to alter the physical and sometimes chemical properties of a material. Differences in type, volume fraction, size, and distribution of the precipitated particles govern properties as well as the changes observed with time and temperature, and these are all affected by the initial state of the structure. The initial structure may vary in wrought products from unrecrystallized to recrystallized and may exhibit only modest strain from quenching or additional strain from cold working after solution heat treatment. These conditions, as well as the time and temperature of precipitation heat treatment, affect the final structure and the resulting mechanical properties (Totten, 1997).
2.2.1 Heat Treatment of Aluminum Alloy
Heat treating is a critical step in the aluminum manufacturing process to achieve required end-use properties which frequently restricted to the specific operations employed to increase strength and hardness of the precipitation-hardenable wrought and cast alloys. Basically, not all Aluminum Alloys are heat treatable. Only the precipitation hardenable wrought and cast alloys are heat treated (to increase strength and hardness) (ASM, Aluminum Alloys) whereas Wrought aluminum alloys can be divided into two categories which are non-heat treatable and heat treatable. Non-heat-treatable alloys, which include the 1xxx, 3xxx, 4xxx and 5xxx series alloys derive their strength from solid solution and are further strengthened by strain hardening or, in limited cases, aging. Heat-treatable alloys include the 2xxx, 6xxx and 7xxx series alloys and are strengthened by solution heat treatment followed by precipitation hardening (aging)(Peeler, 2002).
According to Hatch (1984), the heat treatable alloys contain of soluble alloying elements that exceed the equilibrium solid solubility limit at room and moderately higher temperatures. The amount present may be less or more than the maximum that is soluble at the eutectic temperature. Figure 2.1 below show a portion of the aluminum-copper equilibrium diagram which illustrates these two conditions and the fundamental solution-precipitation relationships involved.
Figure 2.1: Partial equilibrium diagram for aluminum-copper alloys with temperature ranges from heat treating operation. (Source: adapted from Hatch, J. E. 1984, Aluminum: Physical properties and metallurgy)
As shown in Figure 2.1 above, two alloys containing 4.5 and 6.3% copper are represented by the vertical dashed line "(a) and (b)". The solubility relationships are heat treating behavior of these compositions approxiamate those of commercial alloys 2025 and 2219, and the priciples apply to the other heat treatable alloys (Hatch, 1984). Regardless of the initial structure, by holding the 4.5% copper alloy at 515 to 550°C (960 to 1020°F) until equilibrium is attained causes the copper to go completely into soild solution where this operation is generally known as Solution Heat Treating (SHT). If the temperature is then reduced to below 515°C (960°F), the solid solution become superssaturated, and there is a tendency for the excess solute over the amount actually soluble at the lower temperature to precipitate. According to Hatch (1984), by referring again to figure above, the alloy with 6.3% copper which exceeds the maximum content soluble at the eutectic temperature, consists of a solid solution plus additional undissolved CuAl2 when heated to slightly below the eutectic temperature. The solid solution has a higher copper concentration than that of the 4.5% copper alloy if the temperature exceeds 515°C (960°F). The increased copper in solid solution provides greater driving force for precipitation at lower temperature and increases the magnitude of property changes that may occur.
The solid solution formed at high temperature may be retained in a supersaturated state by cooling with sufficient rapidity to minimize precipitation of the solute atoms as coarse, incoherent particles. Controlled precipitation of fine particles at room or elevated temperatures after the quenching operation is used to develop the mechanical properties of heat treated alloys (Hatch, 1984). Most alloys exhibit property changes at room temperature after quenching which called "natural aging" and may start immediately after quenching or after incubation period. The rates vary from one alloy to another over a wide range, so that the approach to a stable condition may require only a few days or several years (Hatch, 1984). Hatch (1984) also stated that precipitation can be accelerated by heating above room temperature which referred as "artifical aging" or "precipitation heat treating". As is apparent from the descriptions of the Aluminum Association temper designations applicable to heat treatable alloys with combination and sequences of cold working and precipitation heat treatments are used (Hatch, 1984).
2.2.2 Basic Aluminum Heat Treatment Designation
According to Hatch (1984), the tempers of age hardenable alloys are all designated by the letter T, followed by one to five digits, while H temper designation are designated for strain hardenable alloys as shown in Table 2.4 below.
Table 2.4: Basic Temper Designation.(Source: adapted from Hatch, J. E. 1984, Aluminum: Physical properties and metallurgy)
F
As Fabricated: Applies to the products of shaping processes in which no special control over thermal conditions or strain-hardening is used. For wrought products, there are no mechanical property limits.
O
Annealed. Applies to wrought products that are annealed to obtain the lowest strength temper and to cast products that are annealed to improve ductility and dimensional stability. The O may be followed by a digit other than zero.
W
Solution heat-treated: An unstable temper applicable only to allys which spontaneously age at room temperature after solution heat treatment. This designation is specific only when the period when the period of natural aging is indicated.
H
Strain-hardened: Applies to those products which have had an increase in strength by reduction through strain-hardening or cold working operations.
T
Thermally treated to produce tempers other than F, O, or H: Applies to products which are thermally treated, with or without supplimentary strain hardening to prodyce stable tempers.
A period of natural aging at room temperature may occur between or after the operations listed for the T temper. Numerals 1 through 10 indicate specific sequences of the heat treatment process as shown in Table 2.5 below
Table 2.5: Subdivisions of "T" Temper Heat Treatable Alloys. (Source: adapted from Hatch, J. E. 1984, Aluminum: Physical properties and metallurgy)
T1
Cooled from an elevated temperature shaping process and naturally aged to a
Substantially stable condition.
T2
Cooled from an elevated temperature shaping process, cold worked, and naturally
aged to a substantially stable condition. Usually associated with cast products.
T3
Solution heat-treated, cold worked, and naturally aged to a substantially stable
condition. (T4+cold work)
T4
Solution heat-treated, and naturally aged to a substantially stable condition.
T5
Cooled from an elevated temperature shaping process and artificially aged. Usually
associated with extruded products in the 6XXX series alloys. (T1+artificial age)
T6
Solution heat-treated, and artificially aged. (T4+artificial age)
T7
Solution heat-treated, and over-aged/stabilized. Applies to alloy products which are
thermally over-aged after solution heat-treatment to carry them beyond the point of
maximum strength to provide control of some special characteristic.
T8
Solution heat-treated, cold worked, and artificially aged. (T3+artificial age)
T9
Solution heat-treated, artificially aged and cold worked. (T6+artificial age)
T10
Cooled from an elevated temperature shaping process, cold worked, and artificially
aged. Usually associated with cast products.(T2+artificial age)
2.2.3 Heat Treatment of Aluminum Alloys 7075
As mentioned before, heat treatment is a process to achieve the desired properties of the material. Hatch (1984) stated that, in aluminum alloys heat treatment process, it consists of three steps which are:-
Solution heat treatment; dissolution of soluble phases
Quenching; development of supersaturation
Age hardening; precipitation of solute atoms either at room temperature (natural aging) or elevated temperature (artificial aging or precipitation heat treatment)
2.2.3.1 Solution Heat Treatment
Solution heat treatment are purposed to redistributed the alloying constituent that segregate from the aluminum during cooling from the molten state or in the other words is the dissolution of the maximum amount of soluble elements from 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 the quenching rapidly to retain the homogeneous condition to achieving increased hardness and strength (Capman, 2004). According to Hatch (1984), solution heat treatment is achieved by heating a cast or wrought products to a suitable temperature, holding at that temperature long enough to allow constituents to enter into solid solution, and cooling rapidly enough to hold the constituents in solution.
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. While in the case of under heating, solution treatment is incomplete and strength values lower than normal can be expected. In certain cases extreme property loss can occur. The solution soak times for castings can be reduced significantly by placing the casting directly into the solution furnace immediately following solidification. The casting is maintained at a temperature above a process critical temperature, and the alloy solute is still in solution. (Andreatta et al., 2002). .
2.2.3.2 Quenching
After the product discharged from the solution furnace, quenching process must take place immediately (ASM Handbook, The Metallurgy of Aluminum). According to Rolan and Robinson (2004), to achieve optimal mechanical properties many heattreatable aluminium alloys require a rapid quench from the solution heat treatment (SHT) temperature by either immersion in, or spraying using cold water. Hatch (1984) stated that, to avoid precipitation detrimental to mechanical properties or to corrosion resistance, the solid solution formed during solution heat treatment must be quenched rapidly enough (and without interruption) to produce a supersaturated solution at room temperaturethe optimum condition for precipitation hardening. However, the resistance to stresscorrosion cracking of certain copper-free aluminum-zinc-magnesium alloys is improved by slow quenching. The quenching rate must be sufficiently high where it depends on the quenching technique:
Immersion in water,
Spray quenching, and
Forced-air quenching.
Most frequently, parts are quenched by immersion in cold water (Hatch, 1984). With increasing product thickness, quenching becomes slower and hence less efficient in the centre of the product. Hatch (1984) also stated that, the highest strengths attainable and the best combinations of strength and toughness are those associated with the most rapid quenching rates as well as resistance to corrosion and stress-corrosion cracking are other characteristics that are generally improved by maximum rapidity of quenching where some of the alloys used in artificially aged tempers, and in particular the copper-free 7xxx alloys, are exceptions to this rule.
