Preparation And Surface Micro-arc Oxidation Of Graphene Reinforced Magnesium Matrix Composites

As a lightweight structural material,magnesium and its alloys have a widely applied prospect due to their low density,high specific strength and specific stiffness.Since the magnesium is an active metal and its crystal structure is closely packed hexagonal structure,its poor mechanical properties and corrosion resistance can be well explained.Graphene can be added into the magnesium alloy as a good reinforcement due to its excellent mechanical properties.Combined with surface micro-arc oxidation technology,it's expected to prepare a lightweight and high strength and good corrosion resistance of graphene reinforced magnesium matrix composites,which can broaden its application in aerospace,automotive manufacturing and other fields.In this paper,graphene / AZ91 D magnesium matrix composites were prepared by microwave sintering.The effect of sintering temperatures,pressing pressures and holding times on the microstructure and microhardness of AZ91 magnesium alloys,which can further affirm the sintering process of the graphene / AZ91 D magnesium matrix composites.Electroless plating copper on the surface of graphene was studied to improve the compatibility of the interface between graphene and metal matrix.Moreover,the effect of graphene contents on the structure,mechanical properties,wear resistance and corrosion resistance of the graphene / AZ91 D magnesium composites were investigated.The effects of electrolyte systems,micro-arc oxidation times and graphene contents on microstructure,phase composition and corrosion resistance of micro-arc oxidation coatings were also investigated.The results show that the relative density and microhardness of AZ91 D magnesium alloy increase first then decrease with increasing the pressing pressures,sintering temperatures and holding times,reaching the maxium values at the molding pressure of 350 MPa,the microwave sintering temperature of 550 ? and the holding time of 20 min.The nano-copper particles are evenly distributed on the surface of the graphene after copper plating.At the same time,the graphene is agglomerated while its structural integrity is not destroyed.The effect of the graphene addition on the grain size of the matrix grain refinement is not obvious.With increasing the copper coated graphene contents,the relative density of the composites decreases,the microhardness increases,and both of the bending strength and compressive strength increase first,and then decrease,reaching the maximum values at the copper coated graphene content of 0.1wt%,respectively.The wear resistance of the composites enhances,while the corrosion resistance reduces with increasing the copper coated grapheme contents.Compared with the non-copper-coated graphene reinforced magnesium matrix composites,both of the mechanical properties and wear resistance are improved,while the corrosion resistance is reduced.Three kinds of electrolyte systems have been employed for micro-arc oxidation of graphene / AZ91 D magnesium matrix composites.The coating prepared in the silicate system is smooth and the pores are uniformly distributed over the surface.Many cracks appear on the surface of the coating prepared in the phosphate system,and the pore distribution is inhomogeneous.The coating prepared in the aluminate system exhibits loose and possesses the smallest pore size.Compared with the main phase of MgO in the phosphate system and aluminate system,the coating prepared in the silicate system is mainly composed of Si O2 and MgO phases.Furthermore,all the three kinds of electrolyte system show the better corrosion resistance compared with that of the substrate,and the corrosion resistance of the silicate film is best among the three kinds of the electrolytes.In the silicate solution,with increasing the oxidation times,the cracks begins to appear at the composite ceramic coating surface.When the oxidation time runs up to 5 min,it possesses the best corrosion resistance owing to its well-distributed,smooth and flawless structure.As increasing the contents of graphene,the pore size of the composite microarc oxidation coating decreases,while the corrosion resistance decreases.