Microstructures, Properties And Fatigue Fracture Characteristics Of Aluminum Alloy After Age Forming

Stress free aging and age forming of 2324 and 7075 aluminum alloy was studied through mechanics experiment and microstructure observation with OM,SEM,TEM and EPMA.The microstructure evolution under the two different states was observed and the variation was explained;the main mechanical performance including strength,elongation,toughness and fatigue crack propagation rate was compared and the difference was analyzed.The fracture morphology was observed and the fracture mechanism was studied.The observation of the precipitates indicates that:due to the difference of the diffusion coefficients between in the parallel stress direction and vertical stress direction,the precipitates grow preferentially parallel to(1(?)1) during age-forming while grow uniformly along {201} under stress free ageing,which is the typical "stress orientation effect".The preferential orientation is decided by the form(compressive stress or tensile stress) and the orientation of the external stress.Because of the high symmetric sphere morphology of 7075 GP zone,and "stress orientation effect" occurs at the beginning of aging,"stress orientation effect" wasn't observed for 7075 A1 alloy after age forming.There are three kinds of particles in the matrix of 2324 and 7075:1) impurity phase rich of Fe and Si,the size of which is often between 0.1-10μm.It often makes source of crack due to its brittleness and the low binding force with the matrix.2) middle sized particles mainly composed of Mn,Cr and Zr,it already exists when homogenizing,the size is often 0.05-0.5μm, can inhibit the growth of the grain and recrystallization.3) small precipitates,the size of which is 0.01-0.5μm,can obviously enhance the mechanical performance of the material.With the stress field and temperature filed combined,the grains will be further lengthened and bruised after age forming.This will cause bad compatibility of the grains and reduce the plasticity of the material compared with stress free aging.The strength,elongation and fracture toughness will all be decreased for some extent.After rolling,the undissolved and hard dissolved phases will be arranged orderly along the rolling direction.This will lead to better fracture toughness and slower fatigue crack propagation rate in the vertical rolling direction than the rolling direction. The fatigue crack propagation rate did not change obviously after age forming compared with stress free aging no matter it is 2324 or 7075,however,it's slower in the vertical rolling direction than in the rolling direction under the same heat treatment condition.It indicates that the key factor deciding the fatigue crack propagation rate is the arrangement of the unsolved phase.The morphology variation of the grain and the preferred arrangement of precipitates have little effect on the fatigue crack propagation rate.