The aerial aluminum profile is one of the high-end products in the aluminum industry. So, to manufacturing the aerial aluminum profile, it requires highly advanced processing technology. And a good treatment process matters the good performance of the aerial aluminum profile. In this article, we will tale about one of the optimal process method of heat treatment – the solid solution and aging heat treatment.
The solid solution and aging heat treatment is a new effective way to control the comprehensive performance of aluminum profile. And a new heat treatment technology often marks the creation of a new profile material. Therefore, the optimization of heat treatment process is of great significance for the development of new high-performance aluminum profile.
1. Solid solution process.
The purpose of solid solution treatment is to dissolve the alloy elements into the aluminum matrix in order to obtain the high super saturation solid solution after quenching, laying a foundation for the follow-up aging control performance. However, the alloy tends to recrystallization at the high temperature solid solution usually. The degree of solid solution of the alloy element, the recrystallization degree of the aluminum profile, and the size and orientation of the grain have a great influence on the final strength, fracture toughness, fatigue resistance and residual stress corrosion resistance.
The temperature and time of solid solution are two key factors that affect the performance of the aluminum profile. Compared with the solid solution time, the temperature affect more significantly. The increase of the solution temperature leads to the higher solute and vacancy concentration after the quenching of the aluminum profile, which is favorable for the precipitated phase precipitation during aging and improves the hardness and strength of the profile. As the profile often has some low melting eutectic phase, the solid solution temperature must be strictly controlled to prevent the occurrence of local overheating. For 7XXX profile, the graded strengthen solid solution can not only to achieve full solution of the alloy elements, but also control the organization of the plate. In order to reduce the difference between the surface and the center performance of the profile, the cross-section temperature of the thick plate should be as uniform as possible. The plate should be consistent with the furnace temperature in the direction of width and length. The advanced roller-type air heating spray quenching furnace can control the temperature within ±1.5 ℃. The solid solution time is also a parameter that must be controlled because the excessive time will lead to grain growing and coarsening, and also intensify oxidation and foaming of the profile, significantly reducing the performance of the aluminum profile.
2. Quenching process.
In the quenching process, the quenching transfer time and quenching rate are the process parameters that are necessary to control. The quenching transfer time refers to the interval when the aluminum plate is transferred to the quenching tank for quenching after solid solution, which is a difficult operation to control. The excessive transferring time will often lower the mechanical properties and corrosion performance of the profile. Roller-type spray quenching technology basically solved the problem of transferring quenching time of plate. In order to obtain a high aging effect, the faster the quenching rate, the better. However, the uneven cooling caused by rapid cooling will make aluminum sheet distortion, and thick cross-section products will produce large residual stress, resulting in product deforming, warping, or even cracking. In actual production, in order to control and reduce the residual stress, boiling water and other media can be used to properly reduce the surface quenching rate to easy the residual stress. The appropriate quenching medium and quenching temperature is important for a good quenching effect to control the cooling process. The polymer solution used in quenching can reduce the product warpage and deformation to get a better shape.
3. Aging process.
The aging is the last key step of heat treatment that determines the properties of high strengthaluminum profile. The appropriate regression temperature, heating rate, cooling rate and the regression time can achieve the good mechanical properties and residual stress corrosion resistance. According to the study, it shows that the profile has the best strength and residual stress corrosion resistance when the heating rate is 57 ℃/ min and after the regression, the cooling rate is 17 ℃/ min. The study also shows that the three-stage "aging-regression-re-aging" - RRA treatment system enables the 7XXX profile to have both high strength and high resistance to residual stress corrosion. By the current advanced level, it allows the RRA process continuously, using a lower regression temperature to prevent local aging.
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