Study On Grain Size Control And PLC Effect In Al-4.5Zn-1.8Mg-0.5Cu Alloy

Al-cladded Al-4.5Zn-1.8Mg-0.5Cu alloy an aluminum alloy developed for aerospace applications.The difference in the grain size between the clad and the matrix,as well as the Portevin-Le Chatelier(PLC)effect can influence the surface quality of the final product.The current research investigated the influence of heat treatment parameters on the grain size and the mechanism of PLC effect in Al-cladded Al-4.5Zn-1.8Mg-0.5Cu alloy.Optical microscopy(OM),backscattered electron(BSE),electron backscatter diffraction(EBSD),transmission electron microscopy(TEM),three-dimensional atomic probe(3DAP)were used to characterize the microstructure;and tensile tests,hardness and electrical conductivity tests were used to characterize the mechanical property.Following results were obtained.(1)The influence of the annealing temperature and heating rate during single step annealing on the grain size were studied.It was found that the grain size of the Al-clad is significant lower than that of the Al-4.5Zn-1.8Mg-0.5Cu matrix when the annealing temperature was between 330°Cand 450°C.The grain size of the Al clad doesn't change much with the annealing temperature but grow when the annealing temperature is 450°C.The grain size of the Al-Zn-Mg-Cu matrix showed a bimodal distribution.The finest grain size was obtained at annealing temperature between 380°Cand 400°C,and the grain start to grow when the annealing temperature is higher than 410°C.Increase of heating rate can lower down the grain size.(2)In comparison with single step annealing,much finer grains can be obtained by double step annealing,because many fine grains were obtained after the first annealing due to very fast heating rate.The mechanical property of the Al-cladded AlZnMgCu alloy can be improved by two-step annealing due to finer grain size(3)In cold rolled Al-4.5Zn-1.8Mg-0.5C alloy,the concentrations of the solute atoms is low.When it is annealed the low temperature,the concentrations of the solute atoms don't increase much,therefore,no PLC was observed.While it is annealed at temperature higher than 300°C,PLC effect was observed due to high concentration of solute atoms and vacancies.(4)Strong PLC effect can be observed in solution-treated Al-4.5Zn-1.8Mg-0.5Cu alloy.Artificial aging can strongly reduce the PLC effect.At 60°Cand 120°C this was mainly due to the precipitations;while at 180°C,this was mainly due to the low concentration of solute atoms in matrix after the precipitation of ?.