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A technique to achieve an excellent strength-ductility balance in AA2024 alloy

Roodgari Mohammad Reza, Jamaati Roohollah

Archives of Civil and Mechanical Engineering

2023

Vol. 23, no. 1

art. no. e12, 2023

In the current study, the microstructure and mechanical properties of AA2024 alloy produced by a new technique consisting of solution treatment, instantly followed by asymmetric cold rolling with a reduction of 65%, and subsequent artificial aging (heat treatment) with four temperatures of 190, 300, 400, and 500 °C was investigated. The additional shear strain in asymmetric rolling led to the formation of a high amount of shear bands in the microstructure of AA2024 alloy. During aging treatment at the temperature of 300 °C, recrystallization was locally started in the shear bands. The grain morphology of the rolled sample was not much changed after aging at the temperature of 190 and 300 °C. However, the samples aged at 400 and heat-treated at 500 °C had different microstructures. In addition, with increasing the temperature to 500 °C, numerous dispersoids were formed in the microstructure of the AA2024 alloy. The sample after aging treatment at 190 °C had the maximum hardness, yield strength, and ultimate tensile strength of 207.4 HV, 481.7 MPa, and 605.1 MPa, respectively, along with a desirable elongation (7.9%). By increasing the aging temperature, the hardness and strength of the alloy considerably decreased. The aging treatment at 400 and heat treatment at 500 °C led to the complete elimination of the strain hardening effect and recurrence of Portevin-Le Chatelier (PLC) in the stress-strain curves. The fracture mode was often a ductile mode for all samples. By increasing the aging temperature, the number and size of dimples increased. As a consequence, the processing technique used in the present study resulted in an excellent strength-ductility balance due to an appropriate combination of strain hardening and precipitation hardening.

Precipitation Processes during Non-Isothermal Ageing of Fine-Grained 2024 Alloy

Kozieł J., Błaż L., Włoch G., Sobota J., Lobry P.

Archives of Metallurgy and Materials

2016

Vol. 61, iss. 1

169--176

Mechanical alloying and powder metallurgy procedures were used to manufacture very fine-grained bulk material made from chips of the 2024 aluminum alloy. Studies of solution treatment and precipitation hardening of as-received material were based on differential scanning calorimetry (DSC) tests and TEM/STEM/EDX structural observations. Structural observations complemented by literature data lead to the conclusion that in the case of highly refined structure of commercial 2024 alloys prepared by severe plastic deformation, typical multi-step G-P-B →θ” →θ’ →θ precipitation mechanism accompanied with G-P-B →S” →S’ →S precipitation sequences result in skipping the formation of metastable phases and direct growth of the stable phases. Exothermic effects on DSC characteristics, which are reported for precipitation sequences in commercial materials, were found to be reduced with increased milling time. Moreover, prolonged milling of 2024 chips was found to shift the exothermic peak to lower temperature with respect to the material produced by means of common metallurgy methods. This effect was concluded to result from preferred heterogeneous nucleation of particles at subboundaries and grain boundaries, enhanced by the boundary diffusion in highly refined structures. Transmission electron microscopy and diffraction pattern analysis revealed the development of very fine Al4C3 particles that grow due to the chemical reaction between the Al matrix and graphite flakes introduced as a process control agent during the preliminary milling of chips. Al4C3 nano-particles are formed at high temperatures, i.e. during hot extrusion and the subsequent solution treatment of the samples. Highly refined insoluble particles such as aluminum carbide particles and aluminum oxides were found to retard recrystallization and reduce recovery processes during solution treatment of preliminarily milled materials. Therefore, the as-extruded material composed of a milled part and chip residuals retained its initial bimodal structure in spite of solution heat treatment procedures. This points to a high structural stability of the investigated materials, which is commonly required for new technologies of high-strength Al-based materials production.