Manufacturing Process And Structural Performance Of TC4 And TC18 Titanium Alloy Fabricated By The Selective Laser Melting (SLM)

Titanium and titanium alloys are becoming more and more popular for aerospace applications due to their benefits of low density,high specific strength,high-temperature resistance,and corrosion resistance.Due to its outstanding overall performance,titanium alloy TC4,a typicalα+βduplex metal,is frequently used in aerospace applications.In addition,due to its great strength and toughness,the near-beta titanium alloy TC18 is frequently employed in aircraft landing gear and other load-bearing components.But as aviation technology advances and the demand for cost and quality control increases,more and more complicated parts are used in the aerospace industry,which unquestionably creates insurmountable challenges for the processing of conventional titanium alloy parts.Selective laser melting（SLM）technology,which has recently experienced rapid development,offers a fresh approach to overcoming this challenge.When compared to conventional parts processing and forming technology,SLM has the distinct advantages of rapid manufacturing,near-net forming,and manufacturing intelligence.Thus,SLM is appropriate for producing intricate titanium alloy parts like TC4 and TC18.While there are still many crucial issues that need to be researched in the process of creating titanium alloy by SLM,such as spheroidization of the melt,porosity defects of the melt pool,the microstructure of non-equilibrium solidification,and so forth.These issues not only have an impact on the shape,function,and quality of the titanium alloy parts but also on their safety.Most of the time,optimizing the SLM process settings can solve the spheroidization phenomenon and melt porosity flaws,and subsequent heat treatment can control the non-equilibrium microstructure.To enhance the forming quality,control the microstructure,and enhance the performance of TC4 and TC18 titanium alloys,it is therefore of considerable value and significance to research the SLM forming process parameters and heat treatment.Accordingly,a thorough investigation into the issues is conducted in this dissertation,with key findings as follows.Firstly,the effect of process parameters and energy density on the density of SLM forming of TC4 and TC18 titanium alloys was studied experimentally.Reasons for the variation of density caused by different process parameters and energy density were analyzed.Regarding the SLM-formed TC4,the optimum density can be obtained when the laser power,scanning speed,scanning spacing,and powder layer thickness are 250 w,1150 mm/s,100μm,and 50μm,respectively.For the SLM-formed TC18,the density was more than 98%when the energy density ranged from 30 J/mm³ to 120 J/mm³,and more than 98.8%when the energy density ranged from 30 J/mm³ to 60 J/mm³.Secondly,In addition,a numerical model of the SLM procedure was built.When the energy density is less than 100 J/mm³,the model can be applied to numerical calculations.The model was employed to determine the impact of various laser intensities on the size of the melt pool for the SLM-formed TC4 titanium alloy.It was determined that the laser power should not be too low in order to prevent defects brought on by unmelted powder and to make sure that the depth of the melt pool does not fall below the thickness of the powder layer.At a certain energy density,the melt pool width remains constant regardless of the processing parameter composition.In the direction of the melt pool depth,similar findings are made.Additionally,the volume of the melt pool varies roughly linearly with the energy density.Thirdly,the effects of different heat treatment regimes on the microstructure regulation and properties of SLM and forming of TC4 and TC18 titanium alloys were investigated.It was found that when the heat treatment temperature was higher than Tβ,the obtainedβgrains were equiaxed,and the cyclic annealing treatment successfully induced a bimorphic organization in the SLM-formed TC4 titanium alloy.Regarding TC18 alloy,the bimorphic and lamellar tissues were obtained afterαβ-STA and BASCA heat treatments,respectively.The lamellar tissues showed high strength（yield strength of 1295±8.7 MPa）and fracture toughness(70.0±2.2 MPa.m^1/2).TC4 and TC18 alloys with bimorphic tissues had good ductility（elongation of 9.8±1.8%）and strength（yield strength of 1054±9.6 MPa）.The tensile properties depend on the size and morphology of the theα-phase.The fracture toughness of TC4 and TC18 was controlled by the curvature of the crack path and the plastic deformation of the material.Finally,the dissertation probed into the corrosion resistance of TC4 and TC18 alloys formed by SLM after the heat treatment.By analyzing the microstructure of samples that suffered the circuit potential,the potentiodynamic polarization,and the electrochemical impedance,the heat treatment was verified to control and match micro-phases of samples and to strengthen the corrosion resistance of prepared alloys.