Study On The Microstructure And Properties Of GNPs/AZ61 Magnesium Matrix Composites By Shift-speed Ball Milling

As energy shortage and environmental pollution become more and more serious,the requirements of carbon neutrality and carbon peaking are urgent.Therefore,the lightweight of transportation equipment has become more and more important in the engineering and technology community.Among the lightweight materials,magnesium(Mg)alloys have attracted much attention due to its low density,high specific strength/specific stiffness,high damping capacities,good electromagnetic shielding performance and abundant reserves.However,the low absolute strength and poor wear resistance limit the wide application of Mg alloys.Graphene nanoplatelets(GNPs)are expected to simultaneously improve the mechanical properties and wear performance of Mg alloys due to its extremely high strength and good lubricant nature.However,GNPs is difficult to uniformly disperse in the Mg matrix because of agglomeration and poor wettability.To handle this problem,the shift-speed ball milling(SSBM),composed of low-speed ball milling(LSBM)and high-speed ball milling(HSBM),was adpted to mix the powder.The dispersion and damage of GNPs as with as the morphologic evolution of the AZ61 alloy powders were systematically studied.Furthermore,the GNPs/AZ61 composites with excellent mechanical properties and good wear resistance were fabricated by a method coupling cold pressing,sintering,hot pressing and hot extrusion.The mechanical properties were evaluated by tension tests and microhardness tests,and the wear performance was tested by the tribometer.Besides,the strengthening and wear mechanisms of GNPs/AZ61 Mg matrix composites were also discussed.The main results as follows:(1)The morphology of the AZ61 powder and the dispersion of GNPs were controlled by the SSBM,which provided an effective way to fabricate high performance GNPs/AZ61 composites.With the increasing of LSBM time,the spherical AZ61 powders were flattened into flakes,while the agglomerated GNPs were gradually dispersed onto the AZ61 flakes with low structural damage.Besides,the grain refinement,relative density,microhardness and yield strength of the composites were gradually improved,while the elongation first decreased and then increased.Therefore,the optimal SSBM process was 150 rpm-16 h+300 rpm-1 h.(2)The GNPs enhanced the strength of the AZ61 matrix with accepted elongation.When the content of GNPs was 0.25 wt.%,the yield strength and ultimate tensile strength of the composite were 307.9 MPa and 400.0 MPa,respectively,which were 56.8 MPa and 41.1 MPa higher than that of the AZ61 matrix.And the fracture elongation was 13.9%,which was similar to the AZ61 matrix(14.1%),achieving the combination of high strength and good ductility of Mg matrix composites.The increased yield strength was mainly attributed to load transfer strengthening.(3)The wear behavior of GNPs/AZ61 composites was affected by the coupling of the content of GNPs and the applied loads.The wear resistance of the composites was gradually improved with the increasing of the content of GNPs.When the content of GNPs was 1 wt.%,the friction coefficient and wear volume were reduced by 16.7% and 31.8%,respectively,compared with the matrix.When the normal load was 1 N,the main wear mechanism of AZ61 matrix alloy was delamination wear,while the main wear mechanism of the 1.0GNPs/AZ61 composite was abrasive wear.Besides,the wear mechanism of the 0.25GNPs/AZ61 composite was abrasive wear and slight delamination wear at the low load,while the wear mechanism was severe adhesive wear at the high load.