The effect of nickel, cobalt and tin on the evolution of microstructural and mechanical properties of Al-Zn-Mg-Cu alloys (7000 series) via casting, powder metallurgy and friction stir processing techniques

Abstract

Casting and alloying, powder metallurgy (PM) and Friction Stir processing (FSP) techniques were used to produce Al-Zn-Mg-Cu alloy of superior properties. The main objective of this research is to improve the microstructure, mechanical properties, corrosion and solid particle erosion resistance of Al-Zn-Mg-Cu alloys. Conventional casting route includes the production of Al-Zn-Mg-Cu alloys with and without Ni, Sn and Co additives using a semi direct chill casting technique, thereafter homogenization treatment at different conditions to the as cast Al-Zn-Mg-Cu alloys had carried out. Additionally, hot extrusion process at 450°C on some homogenized Al-alloys was applied to induce working stress in the surface to improve its erosion resistance. Aging at T6 (120°C/24h) and retrogression and reaging (RRA) at (120°C/24h+180°C/30min+120°C/24h) were conducted on non-extruded and extruded Al-alloyed samples. Comparison the strength and hardness values of Al-Zn-Mg-Cu alloy underwent RRA, was about of 380 MPa and 211 Hv with similar Al-alloy of Ni and Co additives underwent similar heat treatment after extrusion showed that the maximum tensile strength acquired were on the order of 665 MPa and hardness of about 265 HV. On the other hand, powder metallurgy routes comprised both mixing and ball milling (BM) processes. Al-Zn-Mg-Cu compacted from their as received powder mixtures were produced. Other Al-Zn-Mg-Cu with Ni or/and Co or Al2O3 samples were produced for comparison purpose. All the mixed and ball milled Al-PM alloys were cold compacted and sintered under argon atmosphere at 650°C/2hr then and underwent homogenization at 470°C/1.5hr, and followed by aged at T6 and RRA treatment. Findings showed that the saturation limits of the alloying elements were changed. The solubility of nickel in the PM Al-alloy underwent ball milling was extended. In fact, this was the primary target of the mechanical alloying (MA) brought about by BM process application. Additionally, there was a notable increase in hardness and the sintered density for the BM Al-PM alloy of Ni and Co additives after the RRA process. Microhardness of 107 Hv and an increase in the sintered density of 11.5% g/cm3 were acquired as compared to the similar Al-alloy which underwent the similar sintering conditions without additives.