Effect of Mn and Ti Addition on Microstructure and Hardness of 2024 Al Alloy

2024 Alloy is a medium-strength alloy that retains mechanical properties up to approximately 150°C and develops these properties principally through the precipitation-strengthening mechanism. The main alloying elements are Cu and Mg, with small additions of Mn, Cr and Zr. The primary purpose of Cu and Mg is to improve strength, mainly by precipitation of second phases but also by solid solution strengthening mechanism [1]. On the other hand, mechanical alloying produces alloyed powders with a controlled fine microstructure. It occurs from the repeated welding and fracture of a mixture of metallic and non-metallic powders. Raw materials used in mechanical alloying must include at least one ductile metal to act as a host or binder to hold the other ingredients together. Ductile metals include elements such as Ni, Cu, Fe, Al, and even normally brittle metals [2]. Precipitation hardening is achieved by producing a dispersion of particles that impede the motion of dislocations. The degree of strengthening obtained depends mainly on the metal system involved, the volume fraction and size of the particles, and the nature of the interaction of the particles with the dislocations [3]. Intermetallics have been used as particle reinforcements with promising results [4]. Generally, intermetallics offer an increase in behavior against wear and corrosion and an improvement in mechanical properties. Through mechanical alloying, both Al-Mn and Al-Ti master alloys were produced in a Spex 8000 M with elemental powders of 99.99 % purity. The material obtained was then added to the 2024 alloy previously melted in an electric furnace; after mixing and casting, the alloy was hot extruded with an extrusion ratio of 16 at 500 °C. The samples obtained were then put through solubilization for 4h at 493 °C, quenching and artificial aging process for 30, 60, 300, 600, 3000, and 6000 min at 180 °C. The microstructural characterization was carried out by scanning electron microscope HITACHI SU3500. The hardness was evaluated by Vickers microhardness and Rockwell hardness tests. The first was measured following the ASTM E92 standard using a LECO LM300 AT tester, and the second was measured following the ASTM E18 standard using a Wilson C503 R tester in the HRB scale. Figure 1 shows most of the Cu in a solid solution, and most of the Mn and Ti added remain dispersed particles across the aluminum matrix. Figure 2 shows that these dispersed particles have the effect of strengthening the alloy in aging conditions. Throughout the aging process, these reinforced alloys present the highest hardness values overall due to the combination of precipitation hardening and the strengthening generated by the dispersed particles in the aluminum matrix. Micrographs and EDS of the samples 2024 (a), 2024-1 wt. % Mn (b) and 2024-1 wt. % Ti (c) in As-Cast, Extruded and Solubilized condition. a) Vickers and Rockwell hardness in As-cast, Extruded & Solubilized condition. b) Vickers and Rockwell hardness during aging treatment.