Study On Stress And Deformation Of TC4 By Selective Laser Melting Based On Finite Element Simulation

Selective laser melting(SLM)is promising in advanced manufacture because of its ability to manufacture metal parts with high performance and accuracy.However,high temperature gradient induced in the process causes high residual stress and the deformation or even cracking,which limits its industrial application.Until now,there is still a lack of systematic research on the distribution and evolution of the thermal stress as well as the deformation of SLM.In this thesis,based on the finite element analysis(FEA)method,the distribution and evolution of thermal stress of TC4 are analyzed in detail.The influence of the laser scanning strategies and the geometrical profile of the sample on the magnitude and distribution of the residual stress is investigated.The effect of the part dimension and the geometrical profile on the deformation is also studied.The influence factors and disciplines of the thermal stress and deformation during SLM of TC4 are obtained.Furthermore,a deformation control method is proposed.The main research contents and the obtained results are as follows:(1)Based on the FEA method,three stress field models for SLM with different time-space-scale are developed by adopting the indirect coupled thermal-structural method and considering the additive manufacturing characteristics of point-line-face-body.The prediction results of the models agree well with the experimental results and each other.Among the three models,the block-by-block model,proposed by this thesis,can predict the residual stress of the parts with various scanning strategies and large dimensions simultaneously.However,the existing FEA models cannot.(2)The distribution and evolution of the thermal stress induced in SLM are revealed.It is found that for the single-track process,during the laser irradiating,the temperature near the laser spot is high,and the tensile stress is close to zero.But high compressive stress is generated near the surrounding area.When the laser moves away and the part is cooled down to the room temperature,a high residual tensile stress along the laser scanning direction generates,while the stresses along the other two directions are very small.For the single-layer and multi-track process,the thermal cycles caused by the laser heating induce the corresponding thermal stress cycles.Due to a high-temperature gradient along the laser scanning direction,the residual stress in this direction is the largest,followed by the stress perpendicular to the scanning direction,while the stress along the deposition height direction is the smallest due to small deposition height.For the multi-layer and single-track process,with the increase in the number of the layers,the stress along the scanning direction at the top of the part decreases,and a process that the stress changes from tension to compression occurs at the middle of the part.For the stress along the deposition direction,the tensile stress at both ends of the bottom of the part and the compressive stress at the bottom center of the part increase obviously with the deposition height,indicating that the stress along the deposition direction becomes the dominant stress.(3)Island scanning strategy is an effective method to reduce the residual stress in SLM.As the overlap rate increases,the X-component of stress and equivalent stress decrease first and then increase with the increase of the overlap rate,and an overlap rate of 25%-50%is recommended for the generally utilized island size(1×1 mm~2–10×10 mm~2).The laser vertical re-scanning the overlap region with a short scanning length is beneficial for reducing the residual stress,which is due to the small stress difference between directions parallel to and perpendicular to laser scanning directions and by the small temperature gradient.Therefore,the vertical overlap pattern can be used in the island scanning strategy.(4)The effect of the number of the laser beam on the residual stress is investigated.It is found that for the same part,as the laser beam number increases from 1 to 4,the residual stress at the top surface of the part decreases.Due to the change of laser rescanning times and laser scanning direction,the residual stress in the multi-laser overlap area is reduced.However,the internal and edge stresses of the part increase significantly,indicating that the crack susceptibility increases during the multi-laser SLM process.(5)The maximum stresses of all typical parts are found at the joint end of the part and the substrate.The second-largest stress occurs at the top center of the solid part,while occurs at the top center of the long side of the closed-loop and open-ended parts.The residual stress of closed-loop and open-ended parts is significantly lower than that of the solid parts.Compared with the thin-walled part,the residual stress of the inclined-wall part decreases,especially at the middle of the part.Adopting the rounded corner and the circular arc structure instead of the sharp angle and straight line structure,the stress is expected to be reduced,this can be considered as the structural design criteria for SLM.(6)For the thin-walled part,the deformation along the part length direction dominates,and the largest deformation is at the middle of the part,leading to a concave shape.This is the combined effect of the constraint of the substrate and the residual stress.The maximum deformation increases linearly first and then becomes stable with the increase of the length and the height.The maximum deformation decreases as the increase of the thickness.(7)All typical structures such as thin-walled part,incline-walled part,block,cylinders,and triangular prism have an internal concave deformation.Solid parts have a large shrinkage due to large volume,resulting in that the deformation is much larger than that of the closed-loop and open-ended parts.The maximum deformations of the closed-loop and the open-ended parts are similar.Based on the concave deformation characteristics of the typical parts,a control method of the pre-compensation design is proposed to reduce the deformation of the parts during the CAD design.The deformations along the part length direction of the thin-walled parts in the range of 60 mm-200 mm in length and 20 mm–105 mm in height are reduced from several hundred micrometers to±20?m by using this method.