Study On The Stress Evolution And Control In Selective Laser Melting Of Ti6Al4V

Selective laser melting(SLM)has attracted wide attention in the industry in recent ten years because of its ability to fabricate complex metal parts with high performance and accuracy.However,due to the high stress,the parts fabricated by SLM are easy to deform and crack.Due to the lack of systematic research on the mechanism of stress generation and evolution,the influence factors,variation tendency and control methods of the stress during SLM processing are also limited,which greatly limits the development and application of this technology.In this thesis,the generation and evolution mechanism,influence factors and control methods of the stress are studied by the experiments and the calculation of the finite element numerical model(FENM)and indirect analytical model(IAM)calculation.The main research contents and results of this thesis are as follows:(1)Two models,FENM and IAM of temperature and stress field of SLMed Ti6Al4V,were established,respectively.The FENM of temperature and stress field of SLM was established by using the thermal-structural coupling method and considering the material thermal physical parameters changing with material phase and temperature.The IAM of SLMed Ti6Al4V temperature field was established based on the Rosenthal equation.The IAM of SLMed Ti6Al4V stress field was established based on the thermoelastic function and the material properties of Ti6Al4V with only elastic deformation below the mechanical melting point.The two models and the experimental results agreed well.The theoretical and experimental results all proved that the change rate of the temperature gradient at the last mechanical melting point(850°C)in SLMed Ti6Al4V during the thermal cycle is positively related to the residual stress at this point.(2)The single-layer and multi-layer processes of SLMed Ti6Al4V were simulated by the FENM,and the stress evolution and distribution process were obtained.For the current layer,the stress evolution process can be divided into four stages:the stress generation caused by melting,the stress release caused by annealing effect,the stress accumulation caused by the reduction of thermal cycle temperature peak value,and the stress accumulation caused by the temperature reduction as the laser scanning away.For the formed layer,the stress evolution process can also be divided into four stages:stress reduction caused by temperature increase,new stress caused by material remelting,stress release caused by annealing effect and stress accumulation caused by temperature decrease.The stress distribution is not uniform,the maximum stress is located at the combination of the specimen and the substrate.With the number of building layers increasing,the surface and inner stresses of the part both decrease,but the maximum stress increases,its position does not change which will increase the crack susceptibility.(3)The influence and mechanism of the process parameters on stress were revealed by the FENM and IAM.In different process parameter ranges,the relationship between the residual stress and the process parameters is different.The change rate of the temperature gradient increases with the increase of the input laser energy and decreases with the increase of the distance from the laser center.Because of the competition of these two mechanisms,the relationship between the residual stress and the process parameters is very complex.The significance of the influence in the residual stress is scanning speed>laser power>hatch spacing.(4)Three kinds of stress control methods that can be used in actual SLM were systematically studied,and their influence and mechanism on the stress and forming quality were obtained.The effective stress control methods which can be used in SLM process were proposed.The average stress in the sample increases first and then decreases as the scan line length increasing,so the medium scan line length should be avoided.The scan line length of 5～7 mm should be avoided in this thesis.The addition of the support can effectively reduce the stress due to the decrease of the temperature gradient.The stress reduction increases with the decrease of supporting area.In this thesis,the maximum average stress reduction is 28%.The influence of the rescanning process parameters on residual stress is similar to the process parameters,and the influence is very complex.The residual stress and surface roughness can be reduced and the relative density can be improved by using a suitable rescanning process parameter.In this thesis,the stress reduction is 22%,the surface roughness decreased 12%and the relative improved from 99.96%to 99.98%when an optimized rescanning process parameter was used.(5)The stress-relieving annealing process after SLM forming and its effect on microstructure and microhardness were studied.The annealing efficiency of SLMed Ti6Al4V alloy is much higher than that of traditional forging.The annealing efficiency increases with the annealing temperature increasing.In this experiment,the stress-relieving annealing process of 100%stress relief is 650 ~oC for 60min,500 ~oC for 300 min or 350 ~oC for 600 min.When the stress relief annealing temperature is higher than 500 ~oC,the precipitating time of the whiteβphase decreases as the annealing temperature increasing,and the precipitation amount increases as the holding time increasing.The microhardness hardness values of the annealed samples are higher than that of the deposited sample,and the maximum value is achieved at the annealing temperature of 500 ℃.