Study On The Pulse Current Preparation Mechanism And Processing Properties Of B₄C/6061Al Composites

The rapid development of industry,national defense and other high-tech fields has put forward more stringent requirements for service materials,the traditional single metal structure materials cannot meet the demand.Aluminum matrix composites prepared by adding hard B₄C ceramic particles into Al matrix can obtain more excellently comprehensive mechanical properties,thus expanding the application range of Al alloy in automotive and aerospace industries.However,the improvement in mechanical properties is limited by the large differences in physical and chemical properties between Al matrix and B₄C particles,the serious aggregation of B₄C particles and the generation of harmful reaction products at the interface and the grain coarsening.In this study,B₄C/6061Al composites with three volume(3%,5%,7%)were fabricated by pulse current sintering.The temperature field and current density were calculated based on ANSYS finite element simulation.The microscopic mechanism of high-energy pulse current on powder particle connection during sintering densification was studied.Combine with the microstructure characteristics in the contact area of particles,the dynamic connection process between powder particles were restored,which can provide theoretical guidance in the preparation of aluminum matrix composite with good interface and excellent mechanical prosperity.The particle distribution,microstructure and mechanical properties of B₄C/6061Al composites prepared by pulse current sintering were characterized and tested.The hot deformation mechanism of composite was studied by Gleeble hot compression test,and the rheological behavior of composite under different deformation parameters was investigated.The constitutive equation was obtained and the processing map was established,which provides theoretical guidance for the subsequent extrusion and rolling deformation.The recovery,recrystallization and grain refining behavior of composite during extrusion and rolling were analyzed,and the strengthening mechanism and fracture failure mechanism were also discussed.Finally,as-rolled composites plate was treated by T6 heat treatment and electropulsing treatment,and the similarities and differences of precipitation,recovery and recrystallization behavior under two technologies were compared.Below are key research findings:The temperature field and current density of pulsed current sintering were calculated by ANSYS finite element simulation.The results showed that the high energy pulsed current flows preferentially through particle contact surfaces,and the current density can be up to 3.48×10⁵ A/m² at the initial stage of sintering.At this time,the conditions for producing tip discharge were met,and the temperature between particles rose to 1278°C.However,the current density inside the particles was only 8187 A/m²,and the temperature inside the particles was only 79°C.The sharp increase of temperature between particles leads to the change of contact mode from point contact to surface contact.Based on the finite element calculation results and the microstructure characteristics of particle connect interface,the connection mechanism of powder particles under the action of pulse current was investigated.The results showed that the microstructure at the interface of Al particles was mainly fine recrystallized grains,without obvious preferential orientation.The main connection mechanism at the initial sintering stage was particle surface melting(even evaporation).With the change of particle contact to surface contact,joule heat became the main bonding mechanism.At the final stage of sintering,the loading pressure can rapidly reduce the internal pores of the composites,where joule heat and plastic deformation jointly promote the dentification between particles.In above,the pulse current sintering process can be regarded as a dynamic process of melting(evaporation),joule heat and plastic deformation.The microstructure and mechanical properties of B₄C/6061Al composite prepared by pulsed current sintering were characterized and tested.The results showed that micro/nano B₄C particles have been evenly embedded into the surface of soft Al particles after high energy ball milling and low energy ball milling.After sintering,micro/nano B₄C particles distributed in a network structure at the contact interface of Al particles.The increase of B₄C volume fraction inevitably brings about the phenomenon of B₄C aggregation.The particle tip spark was inhibited and the bonding interface quality was decreased.The average grain size of 3 vol.%B₄C/6061Al composites was only 3.12?m under short-time sintering.The tensile strength,yield strength and elongation are178 MPa,90 MPa and 13%,respectively.The tensile fracture morphology demonstrated the main fracture mechanisms was ductile fracture.The rheological behavior of the B₄C/6061Al composite during hot compression deformation was investigated in detail.The results showed that the optimum hot working conditions were determined to be deformation temperature of 773-823 K and strain rate of 0.001-0.003 s^-1 with a maximum power dissipation efficiency of 44.3%.Under low deformation temperature and high strain rate,the softening mechanism of the composite was mainly dynamic recovery.With the increase of temperature and the decrease of strain rate,the main softening mechanism was transformed into dynamic recrystallization.The optimal deformation parameters were verified successfully by hot extrusion test.The microstructure,interaction mechanism between micro/nano B₄C particles and dislocation of as-extruded and as-rolled B₄C/6061Al composites were investigated.The results showed that pores were closed effectively after extrusion rolling deformation.The network structure disappeared and micro/nano B₄C distributed more homogeneously.Pin effect of nano-size B₄C particles stimulated the formation of low-angle grains.The average grain size was refined from 3.12?m to 1.56?m after deformation.The nanoindentation properties and mechanical properties of as-extruded and as-rolled B₄C/6061Al composites were tested and analyzed.The results showed that micro/nano B₄C particles were uniformly distributed in the Al matrix,the microhardness from 0.86 GPa to 1.08 GPa after extrusion and rolling.Meanwhile,higher microhardness values were found around micro-size B₄C particles under the action of particle stimulation nucleation(PSN).The tensile strength,yield strength and elongation of 3 vol.%as-rolled composites were increased to 305 MPa,168 MPa and 12%,respectively.The main strengthening mechanisms are dislocation strengthening,Orowan strengthening and fine grain strengthening.The similarities and differences of the interaction mechanism under the action of T6 heat treatment and electropulsing treatment were investigated.The results showed that the dispersive Mg₂Si phase was precipitated inside the Al matrix after T6 treatment.Obvious recrystallization occurred in the deformation area around the micro-size B₄C particle,while the region with uniform distribution of nano-size B₄C particle was still dominated by the substructured grains.The average grain size grew from 1.56?m to 1.87?m under the high temperature of solution aging treatment.After electropulsing at the same temperature,the dislocation mobility was greatly promoted,thus getting rid of the pinning effect of nano-size B₄C particles and promoting recrystallization.Nano-indentation test results show that electropulsing is a better treatment technology,which can be used to improve the plasticity of composites and eliminate the anisotropy.