Study On Strengthening Mechanism Of CNTs/Al Composites By Rotational Friction Extrusion

Carbon nanotubes reinforced 7075 aluminum matrix(CNTs/7075Al)composites were prepared by Rotational Friction Extrusion(RFE). The effect of RFE processing and heat treatment on composite materials grain and the second phases of the composites were studied. The average grains size of the composites was calculated. The influence of CNTs, RFE processing and heat treatment on the second phase of the composites was research. The dispersed state of CNTs in Al matrix and interface reaction between CNTs and aluminium matrix were investigated.The main results show that as following:The 7075 aluminum alloy is composed of fine equiaxed grains after RFE. there are un-completed recrystallization grains in the extruded organization. the composites could get a good shape with the parameter of n=315rpm and v=0.28mm/s, Migration of grain boundaries was hampered by putting in CNTs. It results to the composites to be finer equiaxed grain after RFE and heat treatment. The recrystallization grains are completed thoroughly by heat treatment. CNTs in composites is uniform distribution and have no tangle. The product of interface reaction between CNTs and aluminium matrix is Al4C3 phase which were formed a thin layer coated on the outer wall of CNTs.The phase could increase the wettability between CNTs and Al matrix. It is beneficial to improve of the composite properties.The parameters of solution treatment of the composites are 480 ℃ solid treatment temperature and 2h solid treatment time. With the increasing of the CNTs content, the peak-age hardness of samples is correspondingly improved, and the time of peak-aged is shortened. The hardness of the composites increased at first and then decreased when the aging temperature is 120 ℃.With the increasing of the CNTs content, the hardness is increased. The hardness of 5%CNTs/7075 Al composites is 191 HV which increased nearly 18% comparing with that of the aluminium matrix and nearly 7.8% comparing with that of the 7075 aluminum alloy prepared by RFE after heat treatment.The second phase in 7075 aluminum alloy distributes more uniform with smaller size by heat treatment. The composition of the second phase is η phase, which does not change after RFE and heat treatment. The dispersion of the second phase get worse and the size of the second phase get larger, while the rod and elliptical shape of the second phase that it used to be totally turns into elliptical caused by the addition of CNTs. thesecond phase is η phase, The addition of CNTs also result in the decline the effect of second phase strengthing.The strengthing of the CNTs/7075 Al composites rely mainly on the interaction of fine grain strengthening, second phase strengthening, CNTs strengthening. The fine grain strengthening and CNTs strengthening can effectively improve the hardness of the composites, and the second phase strengthening effect is weakened.