Research On Solid Carburizing And Boronizing Process Of Ti6Al4V And Its Microstructure And Properties

In this thesis,the process experiments of carburizing and boronizing on Ti6Al4V titanium alloy were studied by using solid penetrating agent.Different carburizing and boronizing process parameters had effected on layer thickness,microstructure,phase composition and formation mechanism which were investigated.Besides,influence factors of carburizing and boronizing layer’s friction wear behavior and corrosion resistance were explored.Research results show that:（1）When carburizing temperature was 950℃or above,compound layer could be formed on the surface of Ti6Al4V titanium alloy.Compound layer appeared a large number of small TiC particle phase,which grown up and gave birth to secondary phase particles,then accumulated continuously,exhibited the characteristics of cluster growth.However,due to the uneven growth of the TiC particles resulted in numerous small groove and nanoscale pores.The microstructure was composed of homogeneous,sub micron granular TiC phases,furthermore the grains average diameter was about 0.70μm.With carburizing temperature rising and carburizing time prolonging,TiC compound layer’s thickness and density were all increased.Meanwhile carburization samples’ microhardness were all increased evidently,the maximum was 1195 HV which about 3.5 times higher than Ti6Al4V titanium alloy substrate.（2）NaCl solution lubrication wear experiment results indicated that the friction coefficient and volume wear rate of carburization samples were all decreased,which proved that Ti6Al4V titanium alloy’s wear resistance properties improved greatly after carburizing.Thick and dense TiC compounds was a decisive factor to improve the wear resistance of Ti6Al4V titanium alloy.Moreover,NaCl solution corrosion experiment results displayed that the corrosion potential and corrosion current density of carburization samples were all increased,which meant that Ti6Al4V titanium alloy corrosion resistance decreased slightly after carburizing.Nevertheless,which still belonged to corrosion resistant material.Pore and micropore structure of carburizing compound layer had an adverse effect on the corrosion resistance of Ti6Al4V titanium alloy.（3）Cellular structure TiB and TiB₂ compound layer could be formed on the surface of Ti6Al4V titanium alloy after different solid boronizing process,which thickness ranged from 2to 7μm.With boronizing temperature rising,TiB and TiB₂ compound layer became more thicker.What’s more,boronization samples’ surface microhardness were all higher than 1200HV0.1,the maximum was 1924HV0.1 which about 5.6 times higher than Ti6Al4V titanium alloy matrix.The results of friction and wear experiment announced that the friction coefficient and volume wear rate of boronization samples all decreased significantly which implied that Ti6Al4V titanium alloy wear resistance properties increased visibly after boronizing.The results of electrochemical corrosion test disclosed that boronization samples’ corrosion potential increased and corrosion current density all decreased when compared with Ti6Al4V titanium alloy matrix.Compact and corrosion resistant TiB and TiB₂ surface compound layer was a fatal factor to improve corrosion resistance of Ti6Al4V titanium alloy.