| dc.description.abstract | Cyclic ageing of alloys, specifically heat-treatable aluminium alloys, is being explored as an alternative technique to strengthen them. Recent research has shown that this technique shows benefit in terms of reducing the time taken for strengthening when compared to the conventional technique of artificial ageing at elevated temperatures. It is seen that the application of small amplitude cyclic loads at room temperature, which impart small plastic strains to the material every cycle, result in the injection of vacancies, which are in excess of those naturally present in the material at room temperature. The vacancies are generated due to the non-conservative dragging of jogs on screw dislocations.
Solute elements present in solid solution, jump into these excess vacancies resulting in the precipitation of solute clusters that are of a few nano-meters in size . These clusters are responsible for the strengthening benefit. TEM and APT investigations have shown that these clusters are similar to the GP zones formed during conventional natural and artificial ageing of these alloys. It has been shown that uni-axial tensile strength and uniform strain, comparable to the peak aged (T6) condition can be obtained by this technique.
However, the existing studies are limited in terms of exploring the effect of different loading parameters such as the peak stress, frequency and number of cycles of processing for different alloys and the degree of strength and ductility that can be obtained for them through this technique. They are also limited relating to the fracture behaviour of these alloys processed by this technique. To this extent, this thesis, studied the effect of different cyclic ageing parameters. The effect on strength, as measured by tensile strength, yield strength and ductility, as measured by uniform strain and fracture strain for 3 different alloy systems was explored. These are, extruded flat plates of thickness 6 mm, of alloy compositions AA7003 - plate & AA6082 - plate, and, an axi-symmetric extruded round bar with diameter 50 mm, of alloy composition AA6082 - round.
It was found that for all three alloys, cyclic ageing under the conditions studied, resulted in strengthening. However, there exists a combination of peak stress, frequency and number of cycles to obtain the best compromise between strength and ductility. It has been seen that higher peak stresses, result in higher nominal yield and tensile strength but lower ductility in terms of nominal uniform strain, while higher frequencies result in lower nominal uniform strain but similar nominal yield and tensile strength. Moreover, AA6082 - round and AA6082 - plate can be processed at higher frequencies, without decreasing ductility and strength, as compared to AA7003 - plate. Interestingly, the optimal peak stress of cyclic ageing is of the order of 70 - 80% of the tensile strength of the peak aged (T6) condition for AA7003 - plate and AA6082 - round. More number of cycles during cyclic ageing can help in reducing the plastic strains imparted per cycle at the same or higher peak stress and frequency but lead to higher times of processing. The parameter set for
cyclic ageing can be chosen depending on the strength, ductility and processing time requirements for the concerned application.
The type of extrusion process affects the cumulative plastic strain accumulated during cyclic ageing. The axi-symmetric AA6082 - round accumulated lower total cumulative plastic strain compared to the flat plates of AA7003 - plate and AA6082 - plate at a similar magnitude of peak stress and number of cycles of processing.
Artificial ageing at high temperatures, such as at 140 degrees celcius and 120 degrees celcius, after cyclic ageing for AA7003 - plate, showed an initial decrease in hardness upon exposure. The hardness then gradually increases after this decrease. This indicates a low temperature stability at elevated temperatures initially. However, low temperature artificial ageing at 60 degrees celcius and natural ageing at room temperature showed a gradual increase in hardness upon exposure. These trends were also observed for a naturally aged sample, aged for 1 year after solutionizing and quenching (WQ + NA 1 year), upon ageing at
140 degrees celcius and 120 degrees celcius.
Subsequent artificial and natural ageing, after cyclic ageing, have shown to improve the tensile strength and uniform strain obtained from the CA condition. Room temperature storage for 10 days after cyclic ageing (CA + NA 10 days), increases the tensile strength by 20% for AA7003 - plate, while an 8% increase is seen for AA6082 - round after room temperature storage for 7 days after cyclic ageing (CA + NA 7 days). The tensile strength obtained in both these alloys is comparable to the T6 condition and naturally aged conditions, aged for several months after solutionizing and quenching (WQ + NA 1 year, WQ + NA 6 months for AA7003 - plate and WQ + NA 3 months for AA6082 - round). This implies that cyclic ageing followed by subsequent storage
resulted in accelerated natural ageing. Artificial ageing at 1400C after cyclic ageing (CA + AA 140) showed very similar yield & tensile strength as well as uniform and failure strains to the T6 condition for AA7003 - plate.
Despite good tensile strength and uniform strain from cyclic ageing, for AA7003 - plate, the CA conditions at all cyclic ageing parameters, CA + NA 10 days, WQ + NA 1 year, WQ + NA 6 months and CA + AA 60 conditions, all resulted in slant early ductile fractures with no necking after reaching maximum uniform strain, during the uni-axial tensile tests . The WQ, T6, CA + AA 120, CA + AA 140 and NA + AA 140 conditions, resulted in the regain of ordinary necking behaviour, with increased failure strains. From this, it was inferred that the damage imposed during cyclic ageing is not affecting the presence or absence of necking.
Uni-axial tensile tests of the naturally aged condition, aged for several months, (WQ + NA 6 months), conducted at -196 degrees celcius, also showed slant early ductile fracture without necking behaviour. From this, it can be reasoned that dynamic strain ageing (DSA), due to the difference in Mg and Zn in solid solution, is not a major cause of the occurrence of this behaviour for this alloy, even though it is present. However, Sun et al, reported ordinary necking, even for the CA condition, for axi-symmetric alloys of a similar composition. It suggests that the type of extrusion process and the precipitate type can affect the fracture behaviour. This hypothesis was not explored for this alloy in this thesis and is a recommendation for future work.
The type of extrusion process and DSA associated with the difference in precipitate state, has been shown to affect fracture behaviour for AA6082 - plate and AA6082 - round. On one hand, a slant early ductile fracture, without considerable necking, similar to AA7003 - plate, was observed in the WQ and cyclic aged to underaged (CA - UA) states. However, ordinary necking, with increased failure strain, was observed for the T6 and CA conditions for AA6082 - plate, a flat plate extrusion with orthotropic symmetry. On the other hand, ordinary necking behaviour was observed for the WQ, CA, T6 and WQ + NA 3 months conditions for AA6082 - round, an axi-symmetric round extrusion. Based on studies for AA6082 - round, it was also argued that the sample geometry used for cyclic ageing, flat square or cylindrical, was an unlikely factor to explain the occurrence of slant early fracture behaviour in AA7003 - plate.
From the above, it should be noted that for applications which require good deformation ability at high strengths, the alloy type, extrusion process and the strengthening technique, should be a point of consideration for applicability. | |
