Investigation On Microstructure And Properties Of Multi-walled Carbon Nanotubes Reinforced Magnesium Alloy Composites By Friction Stir Processing

In this paper, multi-walled carbon nanotubes reinforced AZ80magnesium matrixcomposites(MWCNTs/AZ80) were fabricated by multi-pass friction stir processing(FSP). Under the premise of assuring the composite good performances, the compositedensity was minimized through adding as much carbon nanotubes as possible. Theeffect of different processing parameters on composite forming was mainly explored,and the changes of composites microstructure and mechanical properties were mainlystudied, the complex mechanism of CNTs and the substrate was analyzed finally. Toachieve mass production of composites materials by FSP early, the practical foundationand theoretical basis are provided. The main results are as follows:FSP method was used to prepare the high volume fraction MWCNTs/AZ80. Aftertesting on multiple sets of different processing parameters, the processing parameterswhich are suited to prepare high volume fraction composite by "grooving method"adding CNTs is that, on the premise of1~3passes, the rotational speed of tool is300r/min, the processing speed is23.5mm/min; on the premise of4~7passes, the rotationalspeed of tool is950r/min, the processing speed is23.5mm/min; the depression amountis maintained in the range of0.5~1mm, and the angle of inclination is2.5°.The composite is prepared by three, five and seven passes of FSP. With theprocessing passes increasing, the compound zone area is enlarged, the CNTs volumefraction is decreased, and the stirred organization is uniform gradually. The study showsthat the composite hardness is gradually increased with the increase of the volumefraction of CNTs. When the volume fraction of CNTs is15.0%, the average hardness ofthe composite is1.31times than the based metal; When it is19.5%, the averagehardness is1.67times than the base material; When it is between15.0%and19.5%, thetensile strength of the composites decreased after the first rising, and which is lowerthan that of the based metal. The trends of the above show that the volume fraction ofthe highest tensile strength(340.6MPa) of the composite is16.5%, and the tensilestrength is98%of the based metal.The SEM observation shows that, bridging of CNTs and the matrix are debondedand pulled out on the tensile fracture surface of the composite in the composite of three, five and seven passes. In addition, there are some CNTs agglomeration clusters on thefracture surface of different passes; especially, the agglomeration phenomenon of3passes composite is most serious, and there is large crack on the bonding surfacebetween CNTs and matrix. However, CNTs agglomeration clusters area is largest. Withthe increase of processing passes, CNTs cluster area and the gap are significantlyreduced, and the length of the pulled out CNTs is shorten on the fracture surface, therole of CNTs bridging is enhanced in the matrix, and the adhesion strength with thematrix is increased gradually.The TEM observation reveals that, in the composite of5passes, CNTs dispersed inthe matrix, the multi-wall structure which is not damaged by mechanical stir is stable,and the interface with the matrix is good, but fractured by mechanical stir in thelongitudinal direction. In the composite of7passes, there are some superfinenanocrystals, the average size of which is only5nm. At the same time, a small amountof fractured MWCNTs reacted with the substance Al, finally, the Al4C3is generated,which bonds with the magnesium matrix in the form of completely coherency crystalface, and the interface is compact highly.