Process Optimization,microstructure And Properties Of TC4 Alloy By Laser Hot Wire Additive Manufacture

Titanium and its alloys are widely used in the aerospace field owing to its excellent mechanical properties,like high strength-to-density ratio,high corrosion resistance and excellent mechanical performance under high temperature.However,in large-scale and complicated titanium components manufacturing process,traditional subtractive manufacturing process faces obstacles due to the difficulty of fabricating,as well as the massive waste of material,thus,a new method of titanium components fabricating is urgently looking forward to.Additive manufacturing,as a near-net-shape fabricating method with the ability to shape the production of complex parts at a low cost with minimal material waste,has broad prospects for development.However,a significant problem in additive manufacturing of titanium alloys is the anisotropy of microstructure and mechanical properties.Due to the strong temperature gradient caused by directional cooling during the additive process,the epitaxial growth of the additive structure makes the common microstructure of the additive titanium alloy appear as coarse columnar grains,which leads to the significant anisotropy of mechanical properties of the parts.In order to solve the common problem of anisotropy of microstructure and mechanical properties in as-additive-manufactured titanium alloy,this work takes TC4 titanium alloy as material,and controls the distribution of the heat input so as to optimize the microstructure properties with the method of laser hot wire additive manufacturing.The forming process research results showed that the forming process of laser hot wire additive manufacturing was more stable and the forming quality was improved after adding hot wire.Compared with wire laser additive manufacturing,laser hot wire additive manufacturing reduced the laser power required to achieve stable deposition due to the introduction of additional heat input,and the larger the hot wire current was,the lower the laser power required to match it,which could achieve stable deposition process at low power.Under the condition of 30 A hot wire current,the laser power was reduced by 25% to achieve stable forming.Due to the preheating of the wire before entering the melting pool,the adaptability of the process was expanded,the process window for stable forming was wider,and the stable forming could be formed at faster deposition speed,and the upper limit of deposition speed for stable deposition was increased by 23%.In the process of single-pass multilayer thin wall forming,the stability of deposition process of laser hot wire additive manufacturing was improved,and the forming quality of thin-wall samples was improved after the addition of hot wire,specifically,the surface roughness was decreased by 62.8%,and the effective width proportion was increased by 33.7%.The effect of different hot wire current on the microstructure and properties of TC4 titanium alloy thin wall specimens fabricated by laser hot wire additive was studied.With the increase of the hot wire current,the heat input into the molten pool decreased accordingly.According to the analysis of the temperature field in the additive process combined with the solidification theory,the temperature gradient and solidification velocity at the solidification front inside the molten pool decreased,and the decrease of the ratio of the two promoted the equiaxial growth of β grains rather than the epitaxial columnar growth in the titanium alloy augmentation.The area fraction of equiaxed grains increased from 3.1% to 37.47%,and the distribution of grain orientation was more uniform.The crack growth path tended to be consistent when different directions were applied.Finally,the anisotropy of mechanical properties was significantly improved,and the anisotropy coefficient of tensile strength changed from 1.036 to 1.002.The elongation changed from 0.772 to 0.975.In addition,the lower heat input of the molten pool also reduced the heat accumulation,and the temperature of the in-situ heat treatment in the additive process decreased,further refining the α phase and improving the comprehensive mechanical properties.Finally,the effect of interlayer cooling time on the microstructure and properties of laser hot wire additive manufacturing TC4 titanium alloy was studied.With the increase of interlayer cooling time,the temperature gradient of the molten pool and the solidification rate both increased,and the increase of the temperature gradient of the molten pool was the dominant factor.The equiaxed crystal content decreased significantly with the increase of cooling time,the area fraction decreased from 19.9% to 10.9%,and the anisotropy increased,and the anisotropy coefficient of tensile strength increased from 1.005 to 1.019.The elongation changed from 0.997 to 0.855.