Solidification Microstructures And High Temperature Mechanical Properties Of Heat-resistant Al-Si Alloy Manipulated By Micro-and Nano-sized Particles

Due to the excellent wear resistance and thermal stability of Si phase and Al-CuNi phases at high temperature,Al-Si-Cu-Ni alloy is widely used to manufacture automobile parts such as engine cylinders and pistons.A novel strategy to improve the high temperature performance of aluminum alloys is proposed by introducing nanoparticles into the aluminum matrix to pin the grain boundaries.However,as for conventional Al-5Ti-1B master alloy,the corresponding TiB₂ particles are large in size.More importantly,the residual Al3 Ti intermediate phase in this master alloy can react with Si and damage its manipulating effects in Al-Si alloys,i.e.Si poisoning.Therefore,we used the combustion synthesis method to prepare a novel TiB₂/Al master alloy,which contained micro-and nano-sized TiB₂ particles,with sizes between 50 and 450 nm,and avoided the formation of Al3 Ti.This novel TiB₂/Al master alloy was used to achieve the import and dispersion of nanoparticles in the aluminum melt.Considering the different role of micro-and nano-sized ceramic particles in aluminum alloy,the heat-resistant eutectic Al-Si alloy was taken as the research object in this study,and the in-situ TiB₂/ Al master alloy was used as the nano-particle carriers.The mechanical stirring and ultrasonic-assisted dispersion were added into the Al-Si alloy melt to achieve the import and uniform dispersion of in-suit nanoparticles in the Al alloy melt.The effects of micro-and nano-sized TiB₂ particles on the solidification behavior and microstructure of eutectic Al alloy were studied.The modified mechanism of micro-and nano-sized TiB₂ particles on the solidification behavior and microstructure of Al-13 Si alloy was revealed.The effects of micro-and nano-sized TiB₂ particles on the microstructure evolution and mechanical properties of Al-13 Si alloy after heat treatment were studied.The effects of single modification and complex modification on the microstructure and mechanical properties of Al-13 Si alloy were investigated.The manipulating mechanisms of micro-and nano-sized TiB₂ particles,as well as complex modification of Al-13 Si alloy at room temperature and high temperature strengthening were revealed.Here,three innovations of this study are as follows:1)The refinement mechanisms of micro-and nano-sized TiB₂ particles on ?-Al phase and eutectic Si phase were revealed.Micro-and nano-sized TiB₂ particles refined ?-Al phase mechanism: micron and submicron TiB₂ particles can act as effective heterogeneous nucleation substrates/sites;nano TiB₂ particles were easier to be pushed by the solid/liquid interface front during solidification and finally distribute around the grain boundaries,hindering the solute atoms diffusion.Micro-and nano-sized TiB₂ particles can refine ?-Al grain growth and reduce solute microsegregation in promoting nucleation and hindering solute diffusion.However,the refinement efficiency of ?-Al phase did not increase with the increase of micro-and nano-sized TiB₂ particle content.When the particle content exceeded 0.5 wt.%,?-Al phase did not refine further.Mechanism of micro-and nano-sized TiB₂ particles refined eutectic Si phase: micro-and nano-sized TiB₂ particles could promote the Al-Si eutectic transformation.Also,after the addition of micro-and nano-sized TiB₂ particles,?-Al dendrites transformed from coarse columnar to fine equiaxed shape,and the liquid phase region between dendrites decreased.However,the nucleation and growth of the eutectic Si phase can only initiate in these liquid phase regions,so the formation of needle-like eutectic Si phase was restrained,and the eutectic Si phase was refined.2)The optimal heat treatment process of Al-13 Si alloy strengthened by micro-and nano-sized TiB₂ particles was optimized.It was found that the addition of nanosized TiB₂ particles can reduce the aging treatment time significantly,and the precipitates were more fine and dispersed.The strengthening and toughening mechanisms of Al-13 Si alloys at room temperature and high temperature manipulated by micro-and nano-sized TiB₂ particles were revealed: the precipitation strengthening caused by the reduction of precipitate size;the thermal mismatch between micro-and nano-sized TiB₂ particles and the alloy;Orowan strengthening by TiB₂ partcles hindering the dislocation slip;grain refinement strengthening by ?-Al phase refinement.The plasticity enhancement mechanism of Al-13 Si alloy strengthened by micro-and nano-sized TiB₂ particles: the main reason was the significant refinement of ?-Al phase.The small-sized dendrites were more conducive to the coordinated deformation of each other during the plastic deformation process;the refinement of the eutectic Si phase and the Al-CuNi phases was beneficial to reduce stress concentration and improve toughness and plasticity.It was found that the addition of nanoparticles can significantly reduce the aging treatment time,and the precipitates were more fine and dispersed.The optimal heat treatment process parameters for the Al-13 Si alloy strengthened by micro-and nano-sized TiB₂ particles was: solution at 510 ? for 8 h + water quenching + aging at 165 ? for 8 h.3)The effect and mechanism of single modification and complex modification on microstructure and mechanical properties of Al-13 Si alloys were revealed.The refinement mechanisms of the primary Si phase by Al-3P modification and Al-3P+TiB₂ were revealed.Al-3P and Al-3P+TiB₂ can make the eutectic point shift to the right and decrease the nucleation temperature of the alloy.At the same time,the primary Si phase growed inadequately,the primary Si phase was refined,and the primary Si content also reduced.Al-3P and Al-3P+TiB₂ can serve as heterogeneous nucleation sites,reduce the nucleation undercooling for the primary Si phase,improve the nucleation efficiency of the primary Si phase and finally refined the primary Si.The strengthening mechanism of Al-13 Si alloy modified by Al-3P+TiB₂ was revealed: precipitation strengthening caused by finer precipitates;effect of nanoparticles on pinning grain boundaries;Orowan strengthening of submicron particles and nanoparticles;the pinning effect of refined primary Si on grain boundaries and the tendency of crack initiation were reduced.The toughening mechanism of Al-13 Si alloy modified by Al-3P+TiB₂: ?-Al phases were refined and the refined grains can reduce the stress concentration at the grain boundaries by coordinating deformation between grains.The increased grain boundary can also increase the crack propagation path.The refinement of primary Si and eutectic Si: Si refinement reduced the stress concentration and reduced the probability of crack initiation.