Friction Stir Welding Of Ultrafine-grained Materials

Ultrafine-grained(UFG) materials have a series of advantages compared with conventional materials and will be widely used as structural materials.However,to the best of our knowledge,seldom studies on the jointing of UFG materials have been reported so far,especially using friction stir welding(FSW).Hence we thought it would be necessary to study in this field.In this paper,5A05 UFG aluminum alloy prepared by Equal Channel Angular Pressing(ECAP) was subjected to FSW.The microstructure of 5A05 UFG aluminum alloy samples was observed using optical microscopy,transmission electron microscopy, scanning electron microscopy and electronic tensile equipment.The study leads to several conclusions:The average grain size was refined to around 340nm after 8-pass ECAP by Bc route.The elongation of 6.2%and the hardness of 120.7 HV were achieved after 4-pass ECAP.The tensile strength of 390MPa was achieved after 6-pass ECAP.The UFG materials prepared by 8-pass ECAP were joined using FSW,and the optimal joint could be obtained with a rotation speed of 1500rpm and a welding speed of 37.5mm/min.The tensile strength was 270MPa,which was 98.2%of that of the original base metal,and the hardness was higher than that of the original base metal.Tensile fracture is characterized by ductile rupture.The original base metal subjected to FSW possessed nugget zone grains of around 1μm in size.The average grain size after 8-pass ECAP was refined to around 0.34μm, while the grain size of the 8-pass ECAP samples subjected to FSW was around 0.4μm in nugget zone.The tensile strength of that FSW samples was 270MPa,which was 70% of the base metal_without FSW(385MPa).However,it was as 1.125 times as that of the original base metal joint(240MPa).The hardness of UFG material joint was higher than that of the original base metal joint,but it was lower than that of the base metal without FSW.Tensile fracture is characterized by ductile rupture.