The Study Of The Microstructure And Mechanical Properties Of Ultrafine Grained AA6063-SiC Nanocomposite Synthesised By Powder Metallurgy

As one of the Al-Mg-Si alloy,AA6063 is widely used in industry because of their resistance to corrosion,good workability and heat treatable properties.However,AA6063 alloy is still not strong enough even if treated by T6 treatment.In this study,ultrafine grained AA6063-SiC nanocomposite is synthesized by the combination of grain boundary strengthening and dispersion strengthening.The study presented in this thesis covers:(1)the microstructure of AA6063 chips and the microstructural evolution of AA6063-SiC nanocomposite powders during high energy ball milling;(2)the microstructure evolution and the mechanical properties of ultrafine grained AA6063-SiC nanocomposite fabricated by 3 consolidaiton methods;(3)the effect of SiC nanoparticle content on the microstructure and mechanical properties of AA6063-SiC nanocomposite;(4)grain growth and recrystallization behavior of AA6063-5vol.%SiC nanocomposite during ball milling,SPS process,heat treatment and extrusion process;(5)precipitation behavior of AA6063-5vol.%SiC nanocomposite.With the increasing milling time,the grain size of AA6063 chips/ AA6063-SiC nanocomposite powders decreased until the critical milling time.The shape of chips transformed to flat chips,subtle chips and near spherical powders by the both influence of the cold wielding and fracture.By increasing the volume fraction of SiC nanoparticles,the milling process to equilibrium state is accelerated and powder particle size is decreased.The hardness of AA6063 chips increased because of the grain boundary strengthening caused by refined grains and solution hardening caused by impurity atoms for longer milling process.Ultrafine grained AA6063-(1,5,10)vol.%SiC nanocomposite samples have been fabricated by a powder metallurgy route which combines high energy ball milling of a mixture of 6063 alloy granules made from machining chips and SiC nanoparticles and powder consolidation through spark plasma sintering and hot extrusion.The microstructures and tensile mechanical properties of the samples were investigated in details.With increasing the SiC nanoparticle content from 1 to 10 vol.%,the yield strength and ultimate tensile strength increased from 295.5 MPa and 342.5 MPa to 544.7 MPa and 603.3 MPa,respectively,and the elongation to fracture decreased from 10.0%,to 2.3%,respectively.As expected,a higher SiC nanoparticle content renders a stronger inhibiting effect to the grain growth during the thermomechanical powder consolidation process.Analysis of the contributions of various strengthening mechanisms shows that a higher SiC nanoparticle content leads to a higher level of contribution by nanoparticle strengthening,but grain boundary strengthening still contributes most to the strength of the nanocomposite.Higher SiC content causes higher sensitivity to the defects of the ultrafine grained nacocomposite which leads to the fracture.The yield strength,ultimate tensile strength and elongation to fracture of the sample produced by a combination of SPS and hot extrusion at 500℃ are 445 MPa,496MPa and 4.4%,respectively,while those of the sample produced by die pressing and hot extrusion are 432 MPa,524MPa and 2.1%,respectively.Pre-sintering of the powder compact using SPS prior to extrusion improves the ductility of the consolidated material without causing significant decrease of strength,suggesting that SPS has a beneficial effect on improving the consolidation level of the material.The UFG microstructure of the AA6063 matrix is the result of recrystallization of the as-milled nanocrystalline AA6063-5vol.%SiC nanocomposite powder particles during extrusion.With increasing the extrusion temperature from 500 to 550℃,the average grain size of the consolidated sample increases from 276 to 627 nm,leading to a significant decrease of the yield strength from 449 to 233 MPa,but a drastic increase of the elongation to fracture from 2.1 to 8.3%.By comparing the microstructure of as-milled,as-SPSed,as-heat treated and as-extruded samples,grain growth is observed.Microstructural characterization of the as-SPSed,heat treated and extruded samples revealed rapid growth of Al nanograins during SPS;slight Al grain growth and recovery during heat treatment;and dynamic recrystallization,rapid growth and grain boundary dragging of SiC nanoparticles leading to their aggregation during extrusion.The aggregation of SiC nanoparticles during extrusion further leads to formation of regions with finer and coarser Al grains respectively.The microstructural changes after each of the process steps lead to changes of hardness of the material.Unlike solution treated and quenched CG AA6063 alloy,the solution treated and quenched UFG 6063-5vol.%SiC nanocomposite does not exhibit any clear age hardening effect during aging at 170℃.The reasons for this lack of age hardening effect of the UFGNC sample are formation of platelet shaped GP zones parallel to Al{111} planes which are not very effective in acting as barriers for dislocation movement along {111}/<011> slip systems and the dominance of grain boundaries and SiC nanoparitcles working as barriers for dislocation movement.It is likely that the concentration of vacancies inside the Al(Si,Mg)grains in the as-quenched UFGNC sample is very low due to the high effectiveness of the high area of grain boundaries and SiC nanoparticles/Al(Mg,Si)matrix interfaces acting as vacancy sinks.The lack of vacancies make it impossible to form needle shaped β?? precipitates during ageing at the low homologous temperature and only allow the limited growth of the platelet shaped GP zones.