Preparation Technology and Properties of Graphene Nanoplatelets/AZ91D Magnesium Matrix Composites Using Thixomolding Process

Author:Chen Li Wen

Supervisor:zhao yu hong


Degree Year:2019





Magnesium(Mg)matrix composites(MMCs)have a series of advantages,such as high specific strength,high specific modulus,low density and good thermal stability.They have become the first choices for lightweight equipment in aerospace,defense,transportation,electronics and other fields.Graphene nanoplatelets(GNPs)with excellent mechanical,thermal and electrical properties are ideal reinforcements for MMCs.According to the mixing law,the MMCs reinforced with GNPs will have excellent performance.However,most of the current preparation methods,such as powder metallurgy and melt stirring,have restricted the performance improvement and application of GNPs/Mg matrix composites.The reasons are as follows:GNPs agglomeration in the matrix,adverse interface reactions,and the complex preparation processes.Researching on new casting preparation technology and exploring the homogenization integration technologies between GNPs and Mg alloy may solve the above problems.Furthermore,the developed technology may effectively shorten the cycle time and realize the molding of complex components,which is expected to realize mass practical application of GNPs/Mg matrix composites.In this paper,GNPs/AZ91D Mg matrix composites(AZ91D-GNPs MMCs)were prepared by the thixomolding process.The relationships between preparation technology,mechanical properties and microstructures of composites were studied,so as to reveal the GNPs uniform dispersion mechanism,fracture mechanism and strengthening mechanism.The main contents are as follows:(1)The microstructure evolution of composites during casting process was studied,and the influences of GNPs on microstructures and morphologies of composites were also investigated.After injection with high speed,the solidified microstructures of AZ91D-GNPs MMCs were fine and nearly spherical grain structures.The grains grew uniformly in all directions without obvious preferred grain orientation.After the addition of GNPs,the grain sizes of the composites were obviously refined,and the dislocation density was increased.The microstructure analysis shows that GNPs were evenly embedded in the Mg alloy matrix without inclusions,pores or holes observed around.XRD analysis showed that there were only the diffraction peaks of Mg and Mg17Al12 in the composites,and no other new intermetallic compounds and metal carbides were detected.The fracture of composites was diverted from the manner of brittle fracture to the manner of a mixed ductile-brittle fracture.(2)The effects of GNPs contents(C)and the barrel temperature(TB)on the microstructures and mechanical properties of AZ91D-GNPs MMCs were studied,and the thixomolding process was optimized.The influences of C and TB on the grain sizes and morphologies ofα-Mg grains were analyzed.The nonlinear function relations between the response variables(ultimate tensile strength(UTS),elongation(δ),Vickers hardness(HV)and porosity(P))and explanatory variables(C,TB)were established.The dimensional response surface plots have established an evaluation of the interaction effects between explanatory variables on mechanical properties.(3)The influences of screw rotate rate(RS)and injection velocity(VI)on the microstructures,dispersion of GNPs and mechanical properties of AZ91D-GNPs MMCs were studied,and the thixomolding process was optimized.The effects of RS and VI on the grain sizes,morphologies ofα-Mg grains and GNPs dispersibility were analyzed.The nonlinear function relations between the response variables(P,HV,UTS andδ)and explanatory variables(RS,VI)were established under different contents of GNPs(0,0.1,0.3,0.6,0.9 and1.2 wt%).The interaction effects between explanatory variables(RS and VI)on mechanical properties under different contents of GNPs were analyzed respectively utilizing dimensional response surface plots.(4)The GNPs dispersion mechanism of thixomolding process was studied.The homogeneous dispersion of GNPs in the matrix can be attributed to the following steps:(a)mixing AZ91D-chips with GNPs,(b)screw stirring process,and(c)injecting mixed slurry with fairly high velocity.After mixing,the GNPs were dispersed in the AZ91D-chips and there were three dispersion effects:covered,adsorbed and distributed.The AZ91D-chips-GNPs went through five stages(L0-L4)in the barrel:collided and rubbed(L0),compacted(L1),the outer melted firstly+small melting pools formed in the inner zone(L2),melting pools grow up and penetrate into the alloy melt(L3)and mixed slurry collected(L4).(5)The interface characteristics and reactions between GNPs and AZ91D Mg alloy matrix were studied.The fracture mechanism and strengthening mechanism were explored.A strong bonding interface between GNPs and alloy matrix had been formed,including GNPs/Mg interface,GNPs/Mg17Al12 interface,GNPs/MgO interface,Mg/MgONano and MgONano/GNPs interface.GNPs were maintained structural integrity and no metal carbides were found at the interface.The typical appearance of GNPs presented in the fracture surfaces:GNPs ruptured,GNPs pulled out and GNPs interlayer ruptured,respectively.The main fracture form was GNPs ruptured.The enhanced mechanical properties of the composites can be related to(i)the grain refinemen,(ii)strong bonding interface between GNPs and alloy matrix,(iii)the homogeneous dispersion of GNPs in the AZ91D matrix.