Research Of Relationship Between Heat-treatment And Fatigue Property Of Aerial 2E12 Aluminum Alloy

The microstructure evolution and fatigue damage of alloys depend on the heat-treatment. Thus, it is very important to develop and optimize heat-treatment technology for better fatigue performance microstructure. By means of OM, SEM, TEM, XRD, EBSD, PAL, fatigue tester, DSC, etc. the microstructure evolution and mechanical properties of aerostatic aluminum alloy 2E12 after solution heat treatment, deformation, ageing and heat exposion were studied. The relationship between microtructure and fatigue behavior of 2E12 aluminum alloys at different status was analysied, main conclusions were presented as followed:(1) During fatigue process, residual particals were easily debonded from the matrix, which caused initial sites there. With the propagation of fatigue crack, those residual particles were bridged by the main crack. The fatigue crack propagation rate of the alloy was accerlerated by those particles. By introducing solution heat-treatment, fatigue properties of the alloy could be improved significantly. Propriate increasing the heating temperature and holding time could enhance the mechanical properties of the alloy. The solution heat temperature limit was 509℃. There was a indentical transformation peak from GPB zone to S" phase of room temperature and boil water quenched alloys. The coexistence of type I and type II S phase was observed in room temperature water and oil quenched alloys. Visible precipitates appeared in air and boil water quenched alloys. The precipitates in air quenched alloy were bigger in size and more in quantity than those in boil water quenched alloy. The reason caused difference of precipitates was cooling rate and cooling temperature.(2) There were two stages ageing process of 2E12 aluminum alloy at 180℃. Pre-deformation weakened the first ageing hardening process but accelerated the second. The precipitates became finer and more dispersed with the pre-deformation. The dislocations caused by pre-deformation provided beneficial sites for the nuleations of precipitates.(3) After the cold deformation, the grain size of the alloy increased along the rolling direction with the aspect ratio. There were three main texture of cold rolled alloy, S texture {123}<634>, Brass texture{011}<112> and Copper texture{112}<111>. Cold rolling and extension could increase surface residual stress and tensile stress respectively. The effect of extension on residual stress was stronger than cold rolling.(4) Fatigue crack propagation was obstructed by the grain boundaries. The crack propagation was transgranular. During the fatigue process, secondary cracks formed at the misorientation grain boundaries infront of the main crack. The spread plane of main fatigue crack was {111} plane, two or more{111}<110> slip systems started infront of the crack bifurcation. The cold rolled texture{110}<112>,{123}<634> and {112}<111> induced {111} plane deviated to the loading direction resulting lower fatigue crack propagation rate. The fatigue crack propagation rate could be affected by residual stress, orientation of the grains near main crack, size and the deformation torlerence of plastic zone. At the short crack propagation stage, the plastic zone size was at the grain size level, the residual stress and texture could suppress the crack propagation. At the steady propagation area, the crack propagation rate increased by the work hardening.(5) Applying 9kV/cm electric field during artifical ageing at 190℃could decrease the peak ageing time. The conductivity of artificial aged alloy with electric field showed same tendency with the traditional ageing process, however, the former one had a better performance the the latter one. The electric field dould decrease the formation energy of S phase during ageing. The formation energy of S phase were 7.9kJ/mol～12.7kJ/mol and 6.8 kJ/mol～22.6 kJ/mol lower calculated by Kissinger and General integration method. The electric field promoted transformation of typeⅠS phase to typeⅡS phase. The supersaturation of artificial aged alloy without electric field was higher than that with electric field. The precipitation rate of ones without electric field aged alloy was higher than that with. The volumen fraction of electric field aged alloy was higher than that without, and the precipitates were more dispersed of the former one. The electric field increased the quenched vacancies jumping rate, which promoted desolventizing of the solute atoms. The tensile flow stress of aged alloy with different ageing time was 10h>24h>5h. The refinement of precipitates under effection of electric field ageing caused higher yield strength and tensile flow stress of the alloy. The electric field had a positive effect on fatigue property of alloy. The 190℃/10h aged alloy with electric field had a 20% longer fatigue life and lower fatigue crack propagation rate.(6) During the heat exposion at 150℃, the microstructure evolution process was GPB zone→S"→S', the volume fraction increased with the exposion time, resulting hardness increased linerally. The fatigue life of alloy increased at first than decreased later with the heat exposed time. The 10h exposed alloy had the longest fatigue life, the fatigue life of 1000h exposed alloy decreased severesly. Precipitates in 100h and 1000h exposed alloy were S" and S' phases, which exhibited worse fatigue performance. It is because that those phases were less coherent than GPB zones, during fatigue process the amount of reversible dislocation decreased. Another reseaon is the deformatiuon coordination of precipitates free zone would caused microcracks near grain boundaries. Therefore, with the increase of exposed time, the fatigue life of alloy decreased. The size of GPB zone was affected by two factors, the cutting effect and temperature, and the latter was dominant. At higher temperature, the oxygen could penetrate through the surface of fatigue crack then caused AlxOy oxide film. Such oxide film accelerated fatigue crack propagation. The fatigue process could be treated as thermal activated, the activation energy decreased with cyclic loading stress. The thermal activated fatigue process of 2E12 aluminum alloy could be treated asβσ-Q0-RTln(N1/N0), where Q0 andβwere 8.54 and 1.52 respectively.