Study On In-situ Synthesis Of TiN Nanophase Reinforced SLM Ti-based Composites And The Solid Solubility Extension

TiN nanophase reinforced Ti-based composites have a very attractive development prospect in aerospace and automotive manufacturing fields which combine the toughness of Timatrix with the high strength and hardness of ceramic reinforcement.In recent years,SLM technology has been developed rapidly and widely used in the field of devices with excellent performance,which can be used in the rapid production of complex and dense Ti-based composite parts.In order to solve the problems of poor interfacial bonding and wettability between ceramic phase and matrix and agglomeration behavior of reinforcement in Ti-based composites,in-situ synthesis is a very effective method in the introduction of reinforcement.Based on plasma ball milling has great advantages in realizing the in-situ gas-solid reaction on Tiat N₂ atmosphere,it is applied to the preparation of SLM formed Timatrix powder,in the hope of preparing nano TiN reinforced Ti-based composites.In addition,the study of plasma ball milling in expanding the solid solubility is also of great practical significance,such as further adding Al and V to Timatrix composites for solid solution strengthening and preparing industrial W-Cr powder for solid solution.Firstly,N₂ plasma ball milling process was used to mill Tipowder for 10 h with the high ratio of ball to material（60:1）,high speed（1350 rpm）,and synthesized Tipowder in situ which containing certain amount of nano TiN reinforcement.Then the radio frequency plasma spheroidizing was used to deal with it to get spherical powder,Rietveld finishing result showed that the content of TiN in the powder was about 30%,and the spherical powder was mixed with the original 3D printing Tipowder with the mass ratio of 1:9,then it was proceeded to SLM and obtained with the bulk part with the characteristic of double scale and double morphological organizational structure which had the distribution of coarse plate（coarse grain）and fine needle or lamellar（fine grain）interlaced with each other.Thanks to this double scale structure,the density of the part was 96.1%,which was close to 96.8%of the block part formed by original3D printing powder,but its hardness was up to 365 HV_0.5,and tensile strength was up to 1066MPa,which were far higher than the hardness(192 HV_0.5)and strength（532 MPa）of the part formed by original powder.On this basis,by further improving the ball milling process,the ratio of ball material was reduced to 30:1,the rotating speed was unchanged,and the ball milling time was 14 h.Then the Tipowder containing N precursor was prepared successfully,and obtained the sphere Tipowder of appropriate TiN content（Rietveld finishing result showed that the content was about 22%）through the radio frequency plasma spheroidizing,then it was mixed with the original powder.Finally,the double scale and double morphological structure of parts also obtained.Because the TiN content is slightly lower,the Vickers hardness of this part was slightly reduced to 344 HV_0.5,but its elongation has been greatly improved,from 21.9%to 33.4%,and its tensile strength was still up to 909 MPa,indicating that the elongation of this part was greatly improved while maintaining high strength and achieving high strength and high plasticity matching.Secondly,Ti,Al,V mixture was milled for 12 h by N₂ plasma milling and conventional milling,respectively.The results showed that N₂ plasma milling could also synthesize TiN in situ and promote the solid solution of Al and V at the same time.However,there was no TiN and it could not make the insoluble V enter to Tiin conventional milling.The powder after spheroidized showed regular of sphere,with reasonable particle size distribution,and only consisted of Ti（Al,V）solid solution and TiN（Rietveld finishing result showed that the content was about 48%）,then it was proceeding to SLM after mixed with the Original 3D printing Tipowder.The morphology of block parts also presented a fine lamellar or needle and plate structure interaction of double scale structure.The Vickers hardness of the component was further improved to 409HV_0.5 and its tensile strength reached 1039 MPa,which was mainly attributed to the combined effects of the strengthening of TiN reinforcement phase and the solution strengthening of Al and V.Finally,in order to further explore the solution mechanism of plasma ball milling,the W-Cr system in the electric contact material was milled by plasma ball milling.The results showed that the solid solubility of Cr in W was higher after plasma ball milling for 6h than that of conventional ball milling,and the supersaturated solid solution of W（Cr）can be obtained by complete solution of Cr in W after plasma ball milling for 10 h.Because the heating effect of discharge plasma generated in the plasma ball milling,the thermodynamic barrier of solid solution formation was greatly reduced.Therefore,the Gibbs free energy change of W-Cr alloy system based on Miedema model in plasma ball milling was only 5.60 kJ/mol,lower than that of conventional ball milling,which was 6.70 kJ/mol.Meanwhile,the higher grain boundary storage energy and dislocation strain energy as the total driving force（the result was 6.37 kJ/mol）generated in plasma ball milling made it easier to overcome the thermodynamic barrier of W-Cr system forming solid solution.