| Due to series of advantages such as high specific strength,excellent abrasion resistance and good fracture toughness,7075 aluminum alloy shows potential application in the fields of aerospace and aircraft industry,as well as automobile industry.However,it is prone to cause types of detects such as blow hole,slag inclusion,liquefaction cracks and so on by traditional fusion welding technology,which limits the application of 7075 aluminum alloy.Friction stir welding(FSW)as well as friction stir processing(FSP)technology,which is a kind of solid state processing technology,can solve those problems partly.Studies about the FSW/FSP of 7075 are mainly focusing on Stir Zone(SZ),few studies are about Swirl Zone(SWZ)and Shoulder affect zone(SFZ).Besides,there are few reports concerning the effect of heat treatment on the friction stir processing of 7075 aluminum alloy.In this work,7075 aluminum were friction stir processed(FSPed)under different parameters(rotating speed n:400-1200rpm;advancing speed ?: 60-100mm/min),and part of FSPed samples were followed by heat treatments(470°C?3h+120°C?24h,FSPed-T6).The influence of heat treatment and processing parameters on microstructures and mechanical properties of FSPed 7075 aluminum were investigated.The main results are as follows:(1)With the increase of rotating speed,the “corner” phenomenon becomes more and more obvious in the common border of heat affected zone in advancing side(TMAZ-AS)and nugget zone(NZ).For each FSPed sample,the grain size comparison of nugget zone is as follows: stir zone(SZ)>swirl zone(SWZ)> shoulder affect zone(SFZ).(2)After solution treatment + peak aging treatment,all samples appear the abnormal grain growth(AGG)phenomenon in NZ,but not in TMAZ or heat affect zone(HAZ).and The area of AGG become larger with the increase of heat input(HI).The areas of AGG in SWZs are extended to SZs in all samples,except for 1200-80-FSPed-T6 and 800-60-FSPed-T6 samples.The contents of low angle grain boundaries(LAGBs)before heat treatment in the AGG area are between 13.3%-34.9%.Compared with non-AGG area,AGG areas show high content of LAGBs.Besides,there are many grains in the orientation of <111>//ND in SZ of FSPed samples,and this kind of grains almost do not appear AGG in heat treatment.(3)The hardness distribution curves of FSPed specimens with different process parameters show that the hardness distributions of different layer in cross section face show a pattern of "W" shape.Micro hardness maps of cross section surface indicate that as the heat input increases,the high micro hardness area(>150Hv)of samples becomes larger and then smaller,and the low micro hardness area(<120Hv)shows an increasing and then remained constant trend.After T6 treatment,"W" shape hardness distribution disappeared and the hardness values of different zones are basically equal.Hardness distribution of FSPed7075 aluminum alloy is related to second phase distribution.For FSPed samples,the content of grain boundary precipitates in SFZ?SZ and SWZ is obvirous and there are much coarse second phase in SWZ in all FSPed samples.In TMAZ,the second phase is distributed along the direction of mechanical streamline,and the size of second phase is fine.In heat affect zone(HAZ),the second phase are coarsened.Processing parameter have little influence on the distribution of second phase in FSPed areas.After T6 treatment,the difference of second phase distribution in different areas of each sample disappeared.Tensile results show that the heat treatment does not completely eliminate the weak mechanical properties areas formed during the friction stir processing.However,compared with FSPed samples,after solid solution + peak aging treatment,yield strength,tensile strength and elongation of FSPed-T6 samples increase,respectively. |
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