| Ceramic particle reinforced aluminum alloys are important lightweight materials because they have the advantages of light weight and high specific strength.It has broad application prospects in the fields of automobile,aircraft,aerospace and other fields.However,the basic theory of ceramic particles reinforced aluminum alloys is still not mature enough,especially the manipulated mechanisms of ceramic particles on the solidification behavior of aluminum alloys and the effect on recrystallization.And most of the applications of aluminum alloys require welding,but there are many problems in welding aluminum alloys,such as the coarse microstructure of the melted zone after welding,and the coarsening of the heat-affected zone,which causes the mechanical properties of the welded joint to decline.The current problems in ceramic particles reinforced aluminum alloys need to be solved urgently to adapt the vast demand.In this paper,6061 aluminum alloy was used as the matrix,and micro-nano dual-scale TiC-TiB2 ceramic particles were added to prepare 6061 aluminum alloy reinforced with ceramic particles.The effect and mechanism of dual-scale TiC-TiB2 ceramic particles on the microstructure and solidification behavior of as-cast 6061 aluminum alloy were revealed.The 6061 sheet was prepared by two-way vertical controlled rolling.The effect and mechanism of TiC-TiB2 ceramic particles on the deformation and recrystallization of aluminum alloy were studied.The effect of TiC-TiB2 ceramic particles on the tensile properties and strengthening mechanism of 6061 sheet was analyzed.The influence of TiC-TiB2 ceramic particles on the micro structure and tensile properties of TIG welding joint was studied,and the micro structure and strengthening mechanism of ceramic particles reinforced joint were analyzed.The friction stir processing of 6061 aluminum alloy and the effect rules and refinement mechanism of ceramic particles on the friction stir structure,the effect rules on the mechanical properties and the strengthening mechanism were studied.The main innovations of this paper are as follows:1)The effects of as-cast micro structure and manipulated mechansim about dual phase TiC-TiB2 ceramics particles were revealed.With the increase of TiC-TiB2 contents,the refiment degree was remarkably improved.In the presence of 0.5 wt.%TiC-TiB2,the mean grain size decreased from 187 ?m to 90 ?m,decreased by 51.9%.The grain topography became more regular and uniform.The manipulated mechanisms are the following.Firstly,ceramic particles served as heterogeneous nucleation cores of a-AI grains,increasing the nucleation rate of a-AI.Secondly,Ceramic particles are enriched at the front of the solidification interface,which strongly inhibits the solidification interface and inhibits the growth of dendrites.2)The recrystallization bahavior and refinement mechanism,the effects of tensile properties and strengthening mechanisms about dual-scale TiC-TiB2 ceramic particles were revealed.a)The refinement behavior and mechanisms of dual-scale TiC-TiB2 ceramic particles were revealed.With the addition of 0.5 wt.%TiC-TiB2 creamics particles,the mean sizes of recrystallization micro structure were drecreased from 19.6 ?m to 16.4?m,reduced by 16.3%.The refinement meachanisms were studied.Firstly,the original as-cast microstructure was refined,leading to the increase of the boundary area,which makes the dislocation aggravated.Secondly,the ceramiac particles hinder the dislocation movement,leading to the dislocation tangle around the particles and promoting the recrystallization nucleation.Thirdly,the ceramic particles can hinder the development of grian interface,so that the grain growth is restrained.b)The tensile properties were remarkably improved with the addtion of dual-scale TiC-TiB2 creamic particles.When the 0.5 wt.%TiC-TiB2 creamic particles was added,the yield strength and ultimate tensile strength was increased from 230 MPa to 290 MPa,from 302 MPa to 350 MPa,which increased by 17.3%and 29.1%,respectively.The fracture strain was not reduced.The strengrhening mechanisms were mainly grain refinemrnt strengthening,Orowan strengthening of precipitation phase and ceramic particles,and thermal mismatch strengthening.And the Orowan strengthening of precipitation phase and ceramic particles were dominating.3)The effects of dual-scale TiC-TiB2 creamic particles in welding micro structure and tensile properties were systematically studied.a)The effect and manipulated mechanism of dual-scale TiC-TiB2 ceramic particles on the microstructure of weld fusion zone and heat-affected zone were revealed.The size of the dendrite arm decreases obviously.The average grain size in the heat affected zone decreased from 28 ?m to 23 ?m,with a decrease of 17.9%.After adding ceramic particles,the grain size becomes more uniform.The manipulating mechanisms are as follows.Firstly,the ceramic particles in the melting zone improve a-Al nucleation efficiency,thus the grains are refined.In thermal affected zone,ceramic particles inhibit grain boundaries migration and reduce grain coarsening.b)The effect of dual-scale TiC-TiB2 ceramic particles on the tensile properties of welded joints and the strengthening mechanisms were revealed.In the as-welded state,the yield strength and ultimate tensile strength were increased from 96 MPa to 125 MPa,and from 167 MPa to 182.MPa,increased by 30%and 9.0%,respectively.However,the fracture strain is not improved.The strengthening mechanisms are:grain refinement strengthening,Orowan strengthening of ceramic particles and thermal mismatch strengthening.The reduction in fracture strain is mainly due to the coarsening of the precipitation phase caused by welding thermal cycling and the agglomeration of ceramic particles.4)The effect of dual-scale TiC-TiB2 ceramic particles on microstructure and mechanical properties of friction stir processing were revealed.And the refinement and strengthening mechanism were systematically studied.a)The grain size was reduced obviously with the addition of ceramic particles.At 800 rpm,the mean grain size decreased from 4.12 ?m to 3.37 ?m,decreasing by 18.2%,while the 0.5 wt.%TiC-TiB2 was added.At 1200 rpm,the grain size decreased from 3.36 ?m to 2.83?m,reduced by 15.8%.The refinement mechanism is that a large number of dispersed ceramic particles hinder the migration of grain boundaries during friction stirring,thereby inhibiting the growth of the grains.Meanwhile,the induced ceramic particles as the nucleation sites accelerates the dynamic recrystallization nucleation.b)The effect of dual-scale TiC-TiB2 ceramic particles on the mechanical properties of friction stir joints and its strengthening mechanisms were revealed.At 800 rpm and 1200 rpm,the hardness of the nugget zone increased from 61 HV to 76 HV,from 70 HV to 80 HV,which increased by 24.6%and 14.3%,respectively.At 800 rpm,the yield strength,ultimate tensile strength,and fracture strain were increased from 145 MPa to 150 MPa,from 190.9 MPa to 200.3 MPa,from 22.5%to 26.6%,which increased by 3.5%,5.6%,and 16.4%,respectively.At 1200 rpm,the yield strength and ultimate tensile strength increased from 165 MPa to 170 MPa,from 207.9 MPa to 215.9 MPa,respectively,which increased by 9.1%and 5.6%.However,the fracture strain decreased slightly.The strengthening mechanisms are as following.Firstly,the fine grain strengthening,that is the addition of ceramic particles makes the structure of the weld nuclei significantly refined.Secondly,the thermal mismatch strengthening,dislocations generated around the ceramic particles,increasing the dislocation density.Thirdly,the dispersed phase strengthening.During the friction stirring processing,the coarse precipitated phase is broken,and the distribution is more uniform with plastic deformation.The dispersed phase strengthening plays a major role. |
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