Effect Of Cr, Zr And Er Composite Microalloying On The Microstructure And Properties Of Al-Mg-Mn Alloys

The Al-Mg alloy is called a rust-proof aluminum,with medium strength,excellent corrosion resistance,good workability and weldability.Al-Mg alloy is a kind of non-heat-treatable alloy,of which the comprehensive properties were mainly improved by means of the work-hardening and micro-alloying.In this paper,the microstructure and properties of Al-Mg-Mn alloy with different additions of Cr,Zr and Er were studied.The base alloy selected for the experiment is Al-6.5Mg-0.5Mn,with Cr in 0.1wt%,0.15wt%and 0.2wt%;Zr in 0.08wt%,0.13wt%and0.18wt%;Er in 0.15wt%,0.2wt%and 0.25wt%,respectively.The alloys with 9 compositions were prepared by the orthogonal experiment design.The alloys were treated by a series of processings,homogenization annealing,hot-rolling,intermediate annealing,cold-rolling and stabilizing annealing.The microstructure of the alloys was characterized by OM,XRD,SEM,and the mechanical and corrosion resistant performances were tested using the brinell hardness test,tensile test and intergranular corrosion experiment.The effect of different combinations of Cr,Zr and Er on the microstructure and properties of Al-6.5Mg-0.5Mn alloy was different.In the alloy with 0.2Cr-0.08Zr-0.25Er,the as-cast microstructure and the grain size of the alloy were relatively small,and the hardness of the alloy was the largest.After homogeneous annealing,the microstructure of the alloy was uniform,the dendrites were almost eliminated,the hardness of the alloy was obviously improved,and the Al3Mg2 phase was precipitated along the grain boundary,in the alloy with0.15Cr-0.18Zr-0.15Er,the precipitates of Al7Cr,Al3Zr,Al3Er,Al3?Zr,Er?were precipitated in the alloy,and the hardness of the alloy was the largest.After a series of processing,a large amount of precipitates such as Al₇Cr,Al₃Zr,Al₃Er,Al₃?Zr,Er?be separated out from the matrix at the same time.In the process of deformation and heat treatment,the particles could pin dislocations and grain boundaries strongly,which hindered dislocation slip and grain boundary migration.At this time,the strength of the alloy was higher and the elongation was lower.The tensile strength,yield strength and elongation of the processed alloy with 0.15Cr-0.18Zr-0.15Er were 539MPa,444MPa,and 5.80%,respectively.Adding different combinations of Cr,Zr and Er into Al-6.5Mg-0.5Mn alloy,the amount and distribution of Al3Mg2 phase precipitated along grain boundaries were different,which made the potential difference between the alloy grain and grain boundary different,and the corrosion resistance of the alloy be very different.The alloy with 0.2Cr-0.08Zr-0.25Er possessed the best intergranular corrosion resistance,and the mass loss was only15.58mg/cm²;while the intergranular corrosion resistance of the alloy with0.15Cr-0.18Zr-0.15Er was the most poor,and the mass loss reached 24.40mg/cm².After stabilizing annealing,the internal recovery process occured in the alloy,and the internal dislocation density of the alloy decreased.During the annealing process,the Mg precipitated from the matrix and the solid solution strengthening of Mg decreased,which made the strength lower than that of the cold-rolled alloy and the elongation increase.The Al₇Cr,Al₃Zr,Al₃Er and Al₃?Zr,Er?particles continued to precipitate at the annealing temperature of 250?²70?,which enhanced the migration of dislocation and the migration of subgrain boundaries.In this temperature range,the strength of the alloy increased,but the elongation decreased with the increase in the annealing temperature.When annealing at270?³10?,the particles precipitated in the matrix would grow up and coarsen,and the effect of particle pinning and obstruction would be weakened,leading the strength decreasing and the elongation increasing.The alloy with 0.1Cr-0.18Zr-0.25Er had the highest strength after stabilization annealing at 270?/2h.The tensile strength of the alloy was 447MPa,the yield strength was 295MPa and the elongation was 11.44%.With the increase of the stabilization annealing temperature,the Al₃Mg₂ phase,which is uniformly dispersed in the grains and grain boundary,became more uniform,and the potential difference between the grains and the grain boundary was reduced,making the corrosion resistance of the alloy more excellent.The alloys with 0.1Cr-0.08Zr-0.15Er,0.1Cr-0.18Zr-0.25Er,0.2Cr-0.18Zr-0.2Er,possessed relatively good corrosion resistance with the mass loss values of 7.59mg/cm²,8.12mg/cm² and 8.13mg/cm²,respectively.