During natural aging, the solid solution obtained immediately after quenching starts to form precipitates at room temperature. This process is called natural aging, and hardening during natural aging is almost entirely due to the homogeneous precipitation of solute-rich GP regions and the accumulation of vacancies. Precipitation occurs naturally at room temperature, but in supersaturated solid solution after quenching, the influence of precipitation on mechanical properties can be greatly accelerated and improved through high temperature aging after quenching. This is done at temperatures typically in the range of about 200°F to 400°F (95°-205°C). Aging at elevated temperatures is known as precipitation heat treatment or artificial aging .
Precipitation hardening is a mechanism by which hardness, yield strength and ultimate strength increase dramatically over time at a constant temperature (aging temperature) after rapid cooling from a much higher temperature (solution heat treatment temperature). This rapid cooling or quenching results in a supersaturated solid solution and provides the driving force for precipitation.
During the artificial aging process, the supersaturated solid solution produced by quenching at the solution heat treatment temperature begins to decompose. Solute atoms gather near the vacancies. Once enough atoms diffuse into these initial vacancy clusters, coherent precipitation forms. The strain field surrounds the solute clusters due to the mismatch between the solute atomic clusters and the aluminum matrix. As more solute diffuses into the cluster, the matrix is no longer able to accommodate the matrix mismatch. A semi-coherent precipitate formed.
When the semi-coherent precipitate grows to a large enough size, the matrix no longer supports the crystal mismatch and an equilibrium precipitate is formed. Heating quenched material in the range 95°-205°C accelerates precipitation in heat treatable alloys. This acceleration is not entirely due to a change in the reaction rate.
Structural changes occur depending on time and temperature . Generally, the increase in yield strength that occurs during artificial aging occurs faster than the increase in ultimate tensile strength. This means that the alloy loses its ductility and toughness. T6 performance is higher than T4 performance, but the ductility is reduced. Overaging reduces tensile strength and improves stress corrosion cracking resistance. It also increases resistance to fatigue crack growth. It also imparts dimensional stability to the part. In artificial aging, the degree of precipitation and the morphology of precipitation are controlled by aging time and temperature. Within certain limits, approximately equal effects can be obtained for shorter times at higher temperatures or longer times at lower temperatures. When aging is performed at elevated temperatures, a series of different transition precipitations can occur.
Commercial aging practices are a compromise in order to provide the desired mechanical and corrosion properties. The suggested soak times assume that the characteristics of the furnace and the load are such that the load is heated to temperature reasonably quickly. Overaging can result if the rate at which soaking temperature is approached is unusually slow, as a result of heavy compact loads, overloading of the furnace, or use of a furnace with insufficient heating capacity
Temperature control and furnace response need to be considered to avoid overaging or overaging. During soaking, the furnace shall be capable of maintaining the metal temperature within ±5°C (±10°F) of the recommended temperature. With the thermocouple properly placed within the load, the immersion time shall be counted from the time the lowest temperature in the load reaches within 5°C of the specified temperature. The suggested soak times assume that the characteristics of the furnace and the load are such that the load is heated to temperature reasonably quickly. Overaging can result if the rate at which soaking temperature is approached is unusually slow, resulting from heavy, tight loads, overloading of the furnace, or use of a furnace with insufficient heating capacity.
Not using a load thermocouple and estimating the soak time based on the total time in the oven will result in insufficient aging. During artificial aging, the mechanical properties are improved. The yield strength will increase, as will the ultimate tensile. Yield strength increases faster than ultimate tensile strength. Therefore, the ductility decreases as the aging sequence progresses. Once peak aged conditions are reached, the yield and ultimate strengths decrease while the ductility increases.
However, other factors may greatly support an overused temper. For example, in some applications, the strength factor as a criterion for tempering selection is replaced by resistance to SCC (the SCC resistance of some alloys improves significantly with aging) or the greater high temperature service dimensional stability provided by aging. Some paint/bake operations are in the temperature range typically used to age aluminum. As a result, body panels can be formed in T4 conditions with high formability and then aged to higher strength in a paint/bake cycle. Alloy 6010 has been developed to provide the best possible response to aging in the temperature range typically used for baking paints. The stresses generated during solution heat treatment quenching are reduced during artificial aging. The amount of stress relief is related to artificial aging time and temperature. Oldest tempers (T6) can reduce stress by 10-35%, while older tempers (T7X) can greatly reduce residual stress
