Numerical Simulation On Thermo-mechanical Coupling Behaviors Of Laser Solid Forming Ti-6Al-4V

Titanium alloy can meet the urgent needs of advanced aircraft for high reliability,high maneuverability and long life due to its light weight,high specific stiffness,high specific strength and good corrosion resistance.Therefore,the application level of titanium alloy often represents the advance of the aircraft.Laser Solid Forming(LSF),one of typical metal Additive Manufacturing(AM)technologies,can achieve the accurate forming of high-performance complex parts,and has been gradually applied to the fabrication of the large and complex titanium alloys in aerospace high-end equipment such as advanced aircraft.However,LSF process is a non-uniform,fast and multi-scale thermal-microstructure-stress coupled process.During LSF process,the materials undergo repeated heating,melting,solidification and cooling.The dynamic non-uniform temperature field in the component results in large residual stresses and distortion,which affects the geometric accuracy and mechanical properties of the fabricated compnents.The thermo-mechnical coupling model based on finite element method is expected to effectively predict the mechanical behavior of components during LSF,control the geometry and properties of LSF parts in real-time and reduce residual stress and distortion.Hence,this paper focuses on the prediction of thermo-mechanical behavior of LSF Ti-6Al-4V alloy and combines numerical simulation with benchmark evaluation experiments to reveal the influence of the process parameters on the evolution of the thermo-mechnical fields during LSF.The main conclusions obtained are as follows:1)Different mechanical properties coming from different literatures for the same Ti-6Al-4V alloy have been tested.The results obtained showed large discrepancies.The most accurate response to correctly characterize the mechanical behavior of the metal deposition in LSF is obtained.The sensitivity analysis of the mechanical properties of Ti-6Al-4V alloy shows that the distortion and residual stresses are strongly influenced by the thermal expansion coefficient while are slightly affected by the Young's modulus.The influence of the elastic limit is also very significant,because it changes the formation and evolution of the plastic strains.2)The in-situ measurement platform of full-field thermal and mechanical responses in LSF process were further improved.Thermocouples and infrared thermal imaging were combined to achieve accurate measurement of the surface temperature of the fabricated part during LSF.The displacement sensor was used to monitor the distortion of the substrate in manufacturing process.Meantime,digital image correlation technology was used to monitor the strain field of the single-walls in real-time.3)The LSF thermo-mechanical coupled model was experimentally calibrated and validated by in-situ measurements.The numerical results are in agreement with the experimental measurements.In the LSF process,the generation and development of distortion and stress are mainly resulted from the large temperature gradient and high cooling rate.Note that in the initial stage the extremely high temperature gradient occured,and the distortion increment in final cooling process accounted for 60% of the residual distortion.The maximum temperature gradient and the maximum tensile stress occured in the deposition of the first layer.With the increase of the number of the deposited layers,the temperature gradient and the maximum tensile stress first decreased rapidly and then remained stable.The heat accumulation,residual stress and distortion of the built were strongly affected by the laser power but slighjtly affected by the scanning speed.4)The effects of preheating methods,cooling rates and deposition locations on the residual stress and distortion of the parts were investigated.The results show that the laser preheating substrate can reduce residual stress,but may lead to the larger distortion.The entile preheating of the substrate is an effective method to reduce distortion and residual stress.Increasing the pre-heating temperature,the mitigation is more marked.In addition,it is advantageous to reduce residual stress and distortion by optimizing the deposition position and designing specific substrate structures.5)The effect of different scanning paths on the residual stress and distortion of the built was investigated.The results show that when the 90o alternating scanning method is adopted,the residual stress and distortion of the part can be effectively alleviated.Additionally,increasing the heat accumulation of the substrate and reducing the length of the scanning line are beneficial to reduce residual stress and distortion.6)During the LSF process,the physical properties of Ti-6Al-4V thermo-mechanical model will obviously affected by many factors such as laser,component size and external environment,among others.Therefore,for different AM processes,specific material types and forming environments as well as other factors mentioned are used to determine the boundary conditions and physical properties used in the AM model.