The phenomenon of significant improvement in strength and hardness of aluminum alloy over time after quenching is called aging, also known as age hardening of aluminum alloy. This is one of the important methods for strengthening aluminum alloys.
According to the definition, the prerequisite for aging strengthening of aluminum alloys is first to stop quenching and obtain a saturated single-phase structure. In the solid solution obtained by rapid quenching, not only are the solute atoms supersaturated, but the vacancies (crystal point defects) are also supersaturated, that is, in a double supersaturation state. Taking Al-4% Cu alloy as an example, after solid solution treatment, the chemical composition of the supersaturated alpha solid solution is the chemical composition of the alloy, that is, the steel content in the solid solution is 4%. According to the Al Cu phase diagram, at room temperature equilibrium, the copper content of the α solid solution is only 0.5%, indicating that 3.5% Cu is supersaturated in the α phase. When the temperature approaches the melting point of pure aluminum, the vacancy concentration is close to the order of 10-3, while at room temperature, the vacancy concentration is on the order of 10-11, with a difference of 10-8 orders between the two. After discussion, it is known that:; The higher the solid solution treatment temperature of aluminum alloy, the greater the supersaturation level after treatment, and the greater the aging strengthening effect produced after aging. Therefore, the criterion for selecting the solid solution treatment temperature is to increase the solid solution treatment temperature as much as possible while ensuring that the alloy does not burn.
The aluminum copper alloy after solid solution treatment undergoes aging process when left at room temperature or a certain temperature. This process is essentially the precipitation of the second phase Al2Cu from a supersaturated solid solution. This process is interrupted by molding and growth, and is a diffusion type solid-state phase transition. It terminates in the following order: a over → G P zone → θ '' phase → θ 'phase → θ' phase → θ phase
G. The P region refers to the solute rich atomic region, and for Al Cu alloys, it is the copper rich region. G. of aluminum steel alloy The P region is formed by the segregation or coalescence of copper atoms on the (100) crystal plane, presenting a circular shape. It does not have a complete crystal structure and is coherent with the parent phase. No more G. is generated at 200 ℃ Zone P. Room temperature aging G The P zone is very small, with a diameter of about 50A and a density of 1014-1015/mm3. The spacing between the G. P zones is 20-40 °?. After aging at 130 ℃ for 15 hours, the diameter of the G.P zone grew to 90?, Is it 4-6 thick?. No matter how high the temperature is, the number of G.P zones begins to decrease. It can induce elastic strain at the crystal surface. The θ '' phase is formed when the effective temperature increases or the aging time prolongs, the diameter of the G.P region rapidly increases, and copper and aluminum atoms gradually form a regular arrangement, that is, a square ordered structure. The elastic coherent stress field or lattice distortion region formed to the left of the θ '' transition phase is greater than G The stress field generated in the P zone results in a greater time-dependent strengthening effect of the θ '' phase compared to G The strengthening effect of P zone. The θ 'phase refers to the transformation of the θ' phase into the θ 'phase when the aging time is further increased or the aging temperature is advanced. The θ 'belongs to a square structure, and it is aligned with the base aluminum on the (001) plane. However, due to significant mismatch in the z-axis direction, the aligned relationship on the (001) and (100) planes is locally destroyed. The θ phase is an equilibrium phase, and its composition is Al2Cu, which is a square ordered structure. Due to the intact detachment of the θ phase from the parent phase and the loss of its coherent relationship with the matrix, the stress field is significantly weakened. This also means that the hardness and strength of the alloy have significantly decreased.
What are the factors that affect the effectiveness of time hardening?
The time limit is terminated in a certain number of times, and the strengthening effect is affected by the following factors.
(1) Aging temperature. Fixed aging time, for alloys with the same composition, the relationship between aging temperature and aging strengthening effect (hardness). At a certain aging temperature, a Z-large hardening effect can be achieved, and this temperature is called the Z-optimal aging temperature. The aging temperature at which alloys with different compositions achieve Z-large aging strengthening effect is different. Statistics indicate that there is a relationship between the Z-best aging temperature and the melting point of the alloy as follows:
T0 = (0.5 – 0.6)T
(2) Timeframe. The peak values of hardness and strength are in the late stage of the usual θ '' phase and the early stage of the θ 'transition phase. The late stage of θ' has passed the aging period and begins to soften. When a large amount of θ phase is present, softening is already very severe. Therefore, within a certain aging temperature, in order to achieve a Z-large aging strengthening effect, there should be a Z-optimal aging time, which is the time required for the generation and transformation of θ '' to θ '.
(3) Quenching temperature, quenching cooling speed, and quenching transfer time. Theoretical proof shows that the higher the quenching temperature, the faster the quenching cooling speed, the shorter the intermediate transfer time during quenching, the greater the level of solid solution supersaturation obtained, and the greater the strengthening effect after aging is stopped.
(4) Timeliness process. The time limit can be selected as single level or graded time limit. Single stage aging refers to the aging process that is terminated at room temperature or below 100 ℃. It has a simple process, but the average organization is poor, and it is difficult to achieve a good combination of tensile strength, yield strength, conditional yield strength, fracture toughness, and stress corrosion resistance performance. Graded aging refers to stopping aging twice or repeatedly at different temperatures. Suspending pre aging at lower temperatures aims to achieve high-density G. in the alloy Zone P, due to G The P region is usually nucleated on average, and when it reaches a certain size, it can become the center of subsequent precipitation phases, thereby improving the uniformity of the tissue. Suspend Z final aging at slightly higher temperatures for a certain period of time. Due to the slightly higher temperature, the possibility of the alloy entering the over aging zone increases, resulting in a slightly lower strength of the obtained alloy compared to single-stage aging. However, the alloy treated by such graded aging has a higher fracture toughness value and improved corrosion resistance, enhancing stress corrosion resistance.
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