The effect of heat treatment on the structure and mechanical properties of cold-rolled sheets made of Al – Cu – Mn alloys with varying copper to manganese ratios

Abstract

This paper presents the results of analysis of the structure and properties of the commercial alloy 1201 (Cu:Mn=18,3) and the model alloy 2Mn2Cu (Cu:Mn =1,1) both in the cast state and after various modes of thermodeformation processing, including annealing at temperatures of 250–400 oC. It is shown that even after high-temperature homogenization at 540 oC, the ingot of alloy 1201 is not able to undergo high-quality cold deformation, and therefore the production of semi-finished products requires mandatory hot rolling. At the same time, cold-rolled sheets with a reduction ratio of 95% were obtained from the 2Mn2Cu alloy ingot without any intermediate annealing. The calculation method in the Thermo-Calc soft-ware shows that with an increase in temperature to 400 oC, more than 8 wt.% of the Al20Cu2Mn3 phase dispersoids are formed in the model alloy, which is more effective for containing the recrystallization process than the Al2Cu phase precipitates, which in thepredominant amount (~7 wt.% ) are formed in alloy 1201. The calculation results were confirmed by electron microscopy. After cold rolling, the structure of both alloys contains compact inclusions of intermetallides, and the grain structure shows a fibrous character. After processing according to the modes T, T1, T2 (T4, T6, T7) and annealing, the grade alloy underwent recrystallization at the stage of solution treatment, and after annealing at 400 oC, a coarsening of the secondary Al2Cu phase was observed. The grain structure of the model alloy after step annealing up to 400 oC remained unchanged, due to the presence of fine Al20Cu2Mn3 particles (with average size less than 100 nm). This difference especially affects the mechanical properties. In the state of maximum hardening, alloy 1201 has an advantage (UTS = 431 MPa, YS = 310 MPa, HV = 140). But after annealing at 400 oC strength properties degrade to the level of UTS =227 MPa, YS=86 MPa, HV=55. After step-by-step annealing up to 400 oC, the model alloy underwent a slight softening in terms of UTS value (371 MPa in the cold-worked state versus 277 MPa after annealing), but at the same the elongation increased from 2.7 to 11.5%. Based on the results of the work, it can be argued that the ternary model alloy 2Mn2Cu is a promising matrix for the development of heat-resistant aluminum alloys that do not require homogenization and quenching.This research was funded under Grant No. 20-19-00249 by the Russian Science Foundation.