Influence of cold-rolling deformation and annealing temperature on the grain growth of Al-4%Mg-Mn alloy

Abstract

The propensity of an alloy toward an abnormal grain growth (AGG) severely limits the annealing temperature range, capability for hot forming, and deteriorates mechanical properties. In this work, the results of a study of the effect of thermo-mechanical processing (TMP) of Al-Mg alloy AA5182 on the AGG are reported. TMP included cold rolling with a reduction range of 40-85 % followed by isochronal annealing (1 h) in a temperature range of 350-520°C or isothermal treatment at 480°C for different periods of time. The microstructure was characterized using optical microscopy in polarized light and an FEG SEM. The results showed that an increase in the degree of reduction lowers the annealing temperature for the onset of the AGG, while an increase in the annealing temperature increases the extent of the AGG for a given cold reduction. The abnormal grains tended to form in the region close to the surface of the plates, and with extended annealing bands, develope parallel to the surfaces in the rolling direction. However, no appreciable grain growth occurred in the plate center. The AGG and grain-boundary mobility showed a strong anisotropy with a much faster lateral growth than the motion of the growth front normal to the plate surface. Such anisotropy was attributed to the rod-like shape and alignment of Al6Mn dispersoids through Zener pinning. Keywords: abnormal grain growth, thermo-mechanical processing, Al-Mg-Mn alloy Nagnjenost zlitin na osnovi aluminija k abnormalni rasti kristalnih zrn (AGG) mo~no omejuje temperaturno obmo~je, v katerem se lahko izvaja njeno rekristalizacijsko`arjenje in sposobnost za njeno preoblikovanje v vro~em, ker vse skupaj poslab{uje njene mehanske lastnosti. V prispevku avtorji predstavljajo {tudijo vpliva termo-mehanske obdelave (TMP) na abnormalno rast kristalnih zrn Al-Mg zlitine vrste AA5182. TMP je bila sestavljena iz hladnega valjanja s stopnjo redukcije med 40 % in 85 % in eno-urnega izohronega`arjenja v temperaturnem obmo~ju med 350°C in 520°C ali razli~no dolge izotermne obdelave pri 480°C. Mikrostrukturo zlitine so preiskovali z opti~nim mikroskopom v polarizirani svetlobi in v FEG SEM. Rezultati ka`ejo, da pove~ana stopnja redukcije zni`uje temperaturo`arjenja za za~etek AGG medtem, ko zvi{anje temperature`arjenja povi{uje obseg AGG za dano stopnjo redukcije. Abnormalna kristalna zrna nastajajo pod povr{ino plo{~in s podalj{evanjem~asà arjenja nastajajo njihovi trakovi vzporedno s povr{ino v smeri valjanja. Vendar pa niso zaznali znatne rasti zrn v sredini plo{~e oz. vzorca. AGG in mobilnost mej kristalnih zrn sta mo~no anizotropni z mnogo hitrej{o bo~no rastjo kot je hitrost rasti~ela pravokotno na povr{ino plo{~e. Tak{no anizotropijo lahko pripi{emo Zenerjevemu pripenjanju usmerjenih pali~astih izlo~kov Al6Mn. Klju~ne besede: abnormalna rast zrn, termo-mehanska obdelava, zlitina na osnovi Al-Mg-Mn