Development Of Nano TiC/Ti Grain Refiner And Its Effect On Microstructure And Mechanical Properties Of Al-Cu-Mg Alloy

Al-Cu-Mg alloy is an important structural material in aerospace,weapon equipment and automobile industry.It is usually made into casting billet and then toughened by solid solution ageing after plastic processing.Due to the high content of alloying elements and large solidification temperature interval,the microstructure of the alloy is prone to problems such as large grain size and dendritic segregation,which seriously affect its plastic processing and mechanical properties.Refining grain is an important means to improve the formability and mechanical properties.At present,the commonly used Al-Ti-B refiner has good refining effect on Al-Cu-Mg alloy,but it still has some problems such as the aggregate and deposit of TiB₂ particles and the"poisoned"tendency in the melt.Therefore,the development of new type of aluminum alloy refiner is of great significance for refining the as-cast microstructure of Al-Cu-Mg alloy,improving its machining performance and improving its formability and mechanical properties.Adding ceramic nanoparticles into liquid aluminum can provide large number of dispersed substrate to promote the heterogeneous nucleation and refine the structure of aluminum alloy.The process has a wide application prospect because of its good controllability and the ability to overcome the uncertainty of the number and size of particles in endogenous particle method.However,because of the poor wettability,the nanoparticles are difficult to evenly disperse into the melt of aluminum alloy.Therefore,improving the wettability of ceramic particles and molten is one of the key point on applications of nano ceramic particle refiners.In addition,the effects of nano-ceramic particles on the solidification process,microstructure formation and heat treatment of aluminum alloys have not been systematically studied yet.Therefore,in this paper,high-energy ball milling method was used to prepare Ti powder supported TiC nanoparticles composite grain refiner?TiC/Ti refiner?.The influence of ball milling time and powder ratio on the structure and refining effect of the refiner was studied,and the refining mechanism was discussed.On this basis,the effects of TiC/Ti refiner on the microstructure and properties of as-cast and heat treatment Al-Cu-Mg alloy were studied,and the mechanism of the influence of TiC nanoparticles on the aging behavior was revealed.The following research results were obtained.The effect of milling time and powder ratio on the structure and refining effect of the refiner were studied.The results show that the increase of milling time within 0-20 h was beneficial to the refinement of Ti powder and the dispersion of TiC nanoparticles.However,long time milling would make the surface of Ti powder smooth,which was not conducive to the load of TiC nanoparticles.Increasing the content of TiC appropriately?from TiC:Ti=1:20 to 1:10?was beneficial to the crushing of Ti powder and the loading of nanoparticles.However,when the TiC/Ti ratio was increased to 1:5,the TiC nanoparticles might coat on the surface of Ti powder and hinder the deformation and breakage of Ti powders..With the powder ratio TiC:Ti=1:10 and 5 h ball milling,the nanoparticles were of good dispersion and appropriate loading capacity,which were chosen as process parameters for refiner preparation.With the addition of 0.1 wt.%TiC/Ti refiner,the as-cast microstructure of pure aluminum transformed from millimeter thick columnar crystal to equiaxial crystal with average size of 132?m,which was better than that refined with 0.1wt.%Ti powder.The grain refinement mechanism of TiC/Ti refiner for pure aluminum was studied.It was found that,in the solidification process of pure Al with TiC/Ti refiner added,the?-11-1?lattice plane of?-Al was attached to the?11-1?planes of TiC crystal,and the carrier Ti aggregated to TiC particles to form a"Ti-rich transition zone"in the interface layer of?-Al crystal,which caused lattice distortion of the?-Al crystal,improved the interface relationship between?-Al and TiC,and promoted the heterogeneous nucleation of?-Al.With the assistance of the Ti and the nucleation of TiC nanoparticles,the addition of 0.5 wt.%of TiC/Ti refiner increased the solidification initial temperature of pure aluminum from 656.7?to 664.4?,and the microstructure of pure aluminum transformed from millimeter columnar grains to equiaxial grains with average size of 114?m.The effect of the refiner on the as-cast microstructure and properties of Al-4.5Cu-1.5Mg alloy was studied.The results show that the average grain size of Al-Cu-Mg alloy decreased first and then increased with the increase of the TiC/Ti refiner.When the addition content was 0.2 wt.%,the grain size of Al-Cu-Mg alloy decreased from415?m?unrefined?to 88?m,indicating a decrease of 78.7%.In addition,the refinement of?-Al phases improved the distribution of intercrystalline second phases?net-like?-Al+?-Al₂Cu+S-Al₂CuMg eutectic and block-like Al₂Cu?,and increased the solid solubility of Cu and Mg atoms in?-Al phase.The Vickers hardness,yield strength and ultimate tensile strength of Al-Cu-Mg alloy reached to 102 HV,125 MPa and 249 MPa,respectively,which increased by 13.4%,68.9%and 39.8%in comparison with unrefined alloy.The effect of TiC/Ti refiner on the microstructure and properties of Al-Cu-Mg alloy during heat treatment was studied.The results show that,after 495?/2 h of solid solution treatment,the recrystallization grain size of the thermal deformed Al-Cu-Mg alloy was 76?m.Compared to samples without and with Ti refining,the grain size decreased by 66.7%and 37.7%respectively.TiC nanoparticles could act as the substrate for recrystallization nucleation to promote the recrystallization process.After 180?/6 h of ageing treatment,high resolution transmission and interface structure analysis showed that there was of good interface matching relationship between S'?021?//Al?001?//TiC?001?,which promoted the precipitation of GPB?Guinier-Preston-Bagaryatsky?region and S'-Al₂CuMg phase to improve the strengthening effect.The Vickers hardness,yield strength,tensile strength and elongation were increased to 156 HV,287 MPa,421 MPa and 25.5%,respectively.Compared with the samples without refiners,the properties increased by11.4%,22.6%,14.7%and 109.1%,respectively.The fracture mode of the alloy changed from ductile intergranular fracture to ductile transgranular fracture.