Investigation Of Microstructure And Mechanical Properties Of Al-7Si-0.3Mg Alloy Fabricated By Thermomechanical Consolidation

Aluminum alloys have become an important metal material and has a great support to the development of the national economy due to its low density,corrosion resistance,oxidation resistance,In recent years,materials with lightweight and high specific modulus are urgent needed to reduce weight and improve performance of the components used in automobile,aviation and aerospace fields,which is a challenge to the traditional aluminum alloys.Thermomechanical consolidation of metallic powders attracts more and more attention due to its advantage in producing high-performance aluminum alloys with fine grains and uniform microstructure.Consolidation processing,temperature and the amount of plastic deformation affect the microstructure and mechanical properties of the consolidated materials.The apparent difference in grain size between coarse grained and ultrafine grained aluminum alloy makes them demonstrate different microstructural evolution behavior during consolidation,which affects their mechanical properties.Howerver,the published results focusing on the camparison of microstructural evolution during consolidation between coarse grained and ultrafine grained aluminum alloy are very limited.High energy ball milling is an effective tool to produce nanocrystalline powders which can be consolidated by thermomechanical consolidation of metallic powders to produce ultrafine structured aluminum alloys and their composite with improved mechanical properties.The microstructure of ball milled nanocrystalline powder has an important influence on the mechanical properties of the consolidated bulk ultrafine grained materials.During high energy ball milling,grain refinement,the fracture of second phase particles and the dissolution of solute atoms occur,which make the microstructural evolution quite complex.In addition,defects such as grain boundaries,dislocations and vacancies are usually introduced during the preparation of ultrafine grained Al alloys,which make ultrafine grained Al alloys exhibit different precipitation behavior such as accelerated precipitation dynamic,the change of morphology and size of precipitates,as well as the change of precipitation sequence comparing with the coarse grained counterparts.However,the reported work is very limited and the influence of grain size on the precipitation mechanism of thermal mechanaical consolidated Al alloys need to be clarified.In this study,we investigated the effect of extrusion on the microstructural evolution and intergranule bonding of the coarse grained Al-7Si-0.3Mg alloy samples made by granule compact extrusion of Al-7Si-0.3Mg granules produced by crushing recycled machining chips of same composition.Fracture and redistribution of Si and Al₃FeSi particles along the extrusion direction occurred during the extrusion of Al-7Si-0.3Mg alloy granule compacts,leading to excellent ductility of the consolidated samples with an elongation to fracture ranging from 16.6 to 20.5%,being much higher than that of the Al-7Si-0.3Mg casting alloy.The Al-7Si-0.3Mg alloy rods showed a bimodal distribution of?-Al grains as a result of recrystallization and growth of new grains.With increasing extrusion temperature from 400 to 500?,the mean grain size of the?-Al matrix increased from 5.6 to 7.8?m.Meanwhile,rapid establishment of good intergranule bonding was achieved in the Al-7Si-0.3Mg alloy rods during extrusion due to effective breaking of oxide layers on the granules and fast atomic diffusion bonding between the atomically fresh surfaces of the granules.With same extrusion temperature of 500?,increasing the extrusion ratio from 9:1 to 25:1 causes the coarsening of precipitates and the deterioration of mechanical strengthes.We investigated the microstructural evolution and microhardness change of the powder particles during high energy mechanical milling of Al-7Si-0.3Mg alloy granules produced by mechanically crushing machining chips of this alloy.Ball milling significantly reduced the sizes of eutectic Si particles from micrometer level to submicrometer level,changed their morphology from being faceted into being rounded,and caused formation of stacking faults in them.Ball milling also resulted in a drastic change of the Al?Si,Mg?matrix by increasing its Si content from 0.16at%to 1.81at%,and reducing its average grain size to 45nm.It can be envisaged that sharp corners of the faceted Si particles are favorable sites for dissolving Si into the Al?Si,Mg?solid solution due to the Gibbs-Thomson effect associated with the curvature of the Al?Si,Mg?/Si interfaces.The fragmentation of the micrometer sized Si particles into submicrometer and nanometer sized Si particles cause drastic increase of the total Al?Si,Mg?/Si interfacial energy.In the meantime,the increase of dislocation density and the total area of subgrain and grain boundaries caused by grain refinement also caused the increase of the free energy of the system,providing an additional driving force for the dissolution of Si particles.We discussed the strengthening mechanism of nanocrystalline Al-7Si-0.3Mg alloy powders.The microstructural changes of the powder particles lead to a dramatic increase of the microhardness of the powder particles to 187 HV,which is more than twice that of the original granules.Analysis of the contributions of various strengthening mechanisms shows that the major contribution to the microhardness increase comes from grain refinement.We investigated the microstructures,mechanical properties and fracture behavior of ultrafine grained Al-7Si-0.3Mg alloy samples fabricated by two routes of thermomechanical consolidation of a nanocrystalline Al-7Si-0.3Mg alloy powder prepared by HEMM.One route combines vacuum hot pressing?VHP?and hot extrusion?HE?,and the other route combines SPS and HE.Dynamic recrystallization,Al grain growth,Si particles growth and precipitation of GP zones occurred during consolidation.Meanwhile,with increasing the amount of plastic deformation during extrusion the uniformity of Si particles distribution increases due to the flow of Si particles.With the VHP-HE route,increasing the extrusion ratio from 9 to 25 improved the tensile strength from 320MPa to 345MPa and elongation to fracture from 3.9%to 5.9%due to decrease of average grain size and enhancement of inter-particle bonding.Similarly,with the same extrusion ratio of 9,the use of SPS instead of VHP for the first consolidation step improved the tensile strength by 2.2%and elongation to fracture by 90%for the same reasons.Analysis of the various contribution mechanisms to the yield strength of the ultrafine grained Al-7Si-0.3Mg alloy samples shows that grain boundary strengthening and precipitation strengthening due to GP zones make the major contributions.We investigated the influence of grain size on precipitation behavior of Al-7Si-0.3Mg alloy.The grain size has no obvious effect on the precipitation rate,but coarse grained Al-7Si-0.3Mg alloy showed larger precipitates???with size of 20-50nm than that of ultrafine grained one showing needle shaped precipitate???with size of 5-10nm and rounded GP zones with size of 5nm following the same T6 heat treatment process.The orientation relationship between Al matrix and???is as follow:Al[110]????[010],Al?002??????002?,Al?2^-20??????200?,and???precipitates along Al[^-111]direction.Significant precipitation strengthening behavior is observed in ultrafine grained Al-7Si-0.3Mg alloy during T5 heat treatment.At last,we discussed the microstructure stability and the influence of microstructure on mechanical properties.Following T6 heat treatment,the ultrafine grained Al-7Si-0.3Mg alloy shows improved ductility as reflected by the elongation to fracture of10.2%.This may attribute to the decrease of dislocation density and the increase of grain size caused by T6 heat treatment.With the same T6 heat treatment process,ultrafine grain Al-7Si-0.3Mg alloy exhibit better YS and UTS than coarse grained counterpart due to its fine grain size which produces more grain boundary strengthening.The grain coarsening is not serious in ultrafine grained Al-7Si-0.3Mg alloy treated by T6,showing the good microstructure stability which is mainly attributed to the pinning of silicon particles and Al₃FeSi particles to grain boundaries.