Microstructure Construction And Strengthening Mechanisms Of Al-Zn-Mg-Cu Alloy Reinforced By Trace TiB₂+TiC Nanoparticles

Aluminum alloys reinforced by ceramic particles have higher specific strength and more extensive applications,which occupy a pivotal position in lightweight researches on aluminum alloys.Compared with microparticles,nanoparticles have greater advantages of particle reinforcement effect.However,researches on the enhancement of the high-strength Al-Zn-Mg-Cu alloy by nanoparticles are not systematic.Changes and mechanisms of the as-cast microstructure and mechanical properties of Al-Zn-Mg-Cu alloys under reinforcement of multi-phase nanoparticles are required to be further discussed.In addition,Al-Zn-Mg-Cu alloys are usually required to be hot deformed and friction stir welded to satisfy the industrial requirements.However,studies on hot defomed and friction stir weled Al-Zn-Mg-Cu alloys enhanced by nanoparticles are not mature to be further investigation.Therefore,in this paper,Al-Zn-Mg-Cu alloys reinforced by TiB₂+TiC nanoparticles were prepared by the direct melt reaction method.The solidification behavior,hot extrusion and friction stir welding of Al-Zn-Mg-Cu alloys reinforced by TiB₂+TiC nanoparticles were investigated.Laws and mechanisms behind the microstructures manipulation and mechanical properties enhancement of Al-Zn-Mg-Cu alloys with TiB₂+TiC nanoparticles were discussed and revealed in this paper.The innovations of this article are as follows:1)Effect mechanisms of trace TiB₂+TiC nanoparticles on the as cast microstructures and mechanical properties of Al-Zn-Mg-Cu alloys were revealed:i)It has been calculated that TiB₂+TiC nanoparticles have a low degree of mismatch with the?-Al phase,which means these nanoparticles can act as the heterogeneous nucleation site of?-Al during solidification and obstruct the growth of?-Al dentrites at the front of the solid-liquid interface.And finally, grain sizes of Al-Zn-Mg-Cu alloys with different Zn-Mg content are stably refined.In addition,the diffusion of solute atoms were also hindered by nanoparticles,which is beneficial to the distribution of elements in as cast microstructures of Al-Zn-Mg-Cu alloy.And Al-Zn-Mg-Cu alloy can achieve better heat treatment effect under conventional heat treatment process.ii)The addition of trace TiB₂+TiC nanoparticles greatly increases the grain boundary areas which increases the channels for solute atoms to diffuse into crystals.Thus,the element can diffused more uniformly duringr solid solution treatment.Moreover,the thermal mismatch between nanoparticles and Al-Zn-Mg-Cu alloy forms a large number of lattice defects,which can performed as the nucleation sites and nucleation energy for the nucleation of the precipitated phase.And finally,the size of precipitated phase of the Al-Zn-Mg-Cu alloy after T6 heat treatment reduced from 97.13 nm to 69.88 nm.iii)After the addition of trace TiB₂+TiC nanoparticles,the mechanical properties of as cast Al-Zn-Mg-Cu alloys after T6 heat treatment were significantly improved:, the strength and plasticity of Al-Zn-Mg-Cu alloys with different Zn-Mg contents are simultaneously improved.The strengthening mechanisms of Al-Zn-Mg-Cu alloy reinforced by nanoparticles were summarized to fine grain strengthening effect,thermal mismatch strengthening effect and Orowan strengthening effect and Orowan strengthening effect contributes a major strengthening effect.2)The microstructure evolution and mechanical properties improvement mechanisms of hot extruded Al-Zn-Mg-Cu alloys reinforced by trace TiB₂+TiC nanoparticles can be concluded as followed:i)During hot extrusion process,TiB₂+TiC nanoparticles impeded the migration of grain boundaries and the movement of dislocations through the pinning effect,so that the Al-Zn-Mg-Cu alloy can maintain the refined state when deformed.In addition,the pinning effect also help grains accumulating higher deformation energy which maked more recrystallized grains sprouted in deformed grains when deformation and T6 heat treatment.ii)As a result of finer grains and the suppress of element segregation generated by TiB₂+TiC nanoparticles,precipitates in hot extruded Al-Zn-Mg-Cu alloy after T6 heat treatment achieved uniform distribution.And the width of the PFZ was reduced from 26 nm to 16 nm.iii)After the addition of trace TiB₂+TiC nanoparticles,the yield strength of hot-extruded Al-Zn-Mg-Cu alloy sheet increased from 653 MPa to 681 MPa,the tensile strength increased from 708 MPa to 738 MPa and the fracture strain was reduced from 12.16%to 10.21%,which means alloys got better mechanical properties.The main strengthening mechanisms of hot extruded Al-Zn-Mg-Cu alloy sheets improved by trace nanoparticles were concluded to fine grain strengthening effect,dislocation strengthening effect and precipitation strengthening effect.And precipitation strengthening played a major role in strengthening mechaniasms.3)It was found that the welding heat input had a significant effect on the macroscopic morphology of the friction stir welded joint.And microstructure evolution and mechanical improvement mechanisms of friction stir welded joint of hot extruded Al-Zn-Mg-Cu alloy sheets reinforced by trace TiB₂+TiC nanoparticles before and after heat treatment were realed as followed:i)It was found that Al-Zn-Mg-Cu alloy can achieve a good macroscopic quality and beautiful friction stir weld at a 0.3 mm shoulder reduction,a rotation speed of 600 rpm/min and a travel speed of 100 mm/min.ii)After improved by trace TiB₂+TiC nanoparticles,microstructures of different areas of the friction stir welded joint of the Al-Zn-Mg-Cu alloy achieved different degrees of grain refinement.And due to pinning effect of nanoparticles which obstructed the expansion of grain boundaries in weld nugget zone and TMAZ on the advancing side during the heat treatment,microstructures of the welded joint were also effectively refined after T6 heat treatment.iii)The microhardness of the friction stir welded joints of the Al-Zn-Mg-Cu alloy reinforced by trace TiB₂+TiC nanoparticles was higher than that in alloy without reinforced by nanoparticles.Morever,after T6 heat treatment,the the tensile strength of welded joints of alloy improved by nanoparticles increased by 15.6%.The mechanical properties strengthening mechanisms of friction stir welding joints of Al-Zn-Mg-Cu alloy reinforced by nanoparticles were attributed to fine-grain strengthening effect,dislocation strengthening effect and precipitation strengthening effect and the precipitation strengthening effect was the main strengthening effect.