Effect Of Laser Melting Deposition Parameters Om Microstructure And Properties Of Ti6Al4V Alloy

Titanium alloy has the advantages of high specific strength and excellent comprehensive properties,however,its processing performance is challenging.Laser melting deposition(LMD)technology can achieve near-net-forming,which can make up for the deficiency of titanium alloy and maximize the potential of titanium alloy.LMD technology requires higher powder,and titanium alloy has higher activity,so the preparation of titanium alloy powder is a major test.Ti6Al4 V is the most mature(α+β)Two phase titanium alloy.In this paper,Ti6Al4 V alloy bars and powder were prepared by vacuum consumable arc melting(VAR)and electrode induction gas atomization(EIGA),and then different process parameters were selected for LMD experiment.The phase composition and microstructure of titanium alloy bars were analyzed.The characteristics,microstructure and the formation reason of powder defects of titanium alloy powder prepared by atomization were studied.The effects of LMD process parameters on the microstructure and properties of titanium alloy were systematically investigated.Ti6Al4V alloy bar is mainly composed of α phase and β phase,and its microstructure is Widmannian structure.Most Ti6Al4 V alloy powders are spherical and have high sphericity.The particle size of Ti6Al4 V alloy powders is unimodal normal distribution,mainly distributed in the range of 27.5～175.7μm,and the average particle size D50 is73.5μm.By further analysis of the powders,it was found that the phases of the powders were composed of α phase and α’ phase.The surface morphology of powders with different particle sizes had great differences,but the microstructure of the powders were all needle-like.The influence of LMD process parameters on the formability of Ti6Al4 V titanium alloy was investigated.It is found that increasing the laser input power will increase the density of titanium alloy sample and the width of the deposition layer,while the height of the deposition layer first increases and then decreases.With the increase of the scanning speed,the density of titanium alloy sample,the height and width of the deposition layer decrease.With the increase of the powder feed rate,the density of titanium alloy sample decreases and the height of deposition layer increases,while the width of deposition layer increases first and then tends to be stable.Increasing the laser power and reducing the scanning speed have similar effects.Both of them will increase the energy input,increase the temperature of the molten pool and enlarge the area,thus increasing the height and width of the deposition layer.When the powder feeding rate is high,the energy input is limited,so it can’t melt all the powder.Therefore,the utilization rate of the powder is not high,and it is easy to cause metallurgical defects.The samples formed under various technological parameters are composed of αphase and β phase.The change of technological parameters will not affect the phase composition of the samples,but will obviously affect the phase content.Increasing the laser input power will increase the energy absorbed by the molten pool,leading to the heat accumulation in the titanium alloy samples.The decrease of temperature gradient and cooling rate leads to the coarsening of microstructure and the enhancement of texture.However,increasing the deposition head movement speed will reduce the energy absorbed by the molten pool,resulting in an increase in temperature gradient and cooling rate.Therefore,the microstructure of the alloy is gradually refined and the texture is weakened.In addition,increasing the powder transport rate will increase the amount of melted or undissolved powder in the molten pool.This kind of powders can be used as heterogeneous nucleated particles to refine the microstructure.The mechanical properties of alloy samples are related to the internal metallurgical defects and microstructure.When there are metallurgical defects in the alloy sample,the mechanical properties of the material will be seriously deteriorated.Under the parameters of 1500 W laser input power,300mm/min scanning speed and 1.0R/min powder feeding rate,the alloy samples have better comprehensive properties,including tensile strength of 1191 MPa,yield strength of 1129 MPa and elongation of 8.3%.