Study On The Dimensional Accuracy Of Ti6Al4V Typical Structure Fabricated By Selective Laser Melting

Selective laser melting（SLM）is a new near net forming technology developed in recent years because of its abilities of fabricating complex metal components with high accuracy and performances.It is widely used in the industry of aerospace and medical devices,et al.Although it has so many advantages,the dimensional accuracy of forming parts is still unable to meet the requirements of industrial application,in most cases it still needs post-machining.Till now,there are a few studies on SLM dimension accuracy,and the existing studies are almost at the experimental level and lacking theoretical analysis,which can not be popularized and applied.The forming dimensions of SLMed parts have been systematically studied in this thesis according to the additive manufacture characteristics of point by point,line by line and layer by layer in SLM.The mechanism and regularity of dimensional deviation of SLMed Ti6Al4V are obtained.Furthermore,some methods to improve the dimensional accuracy of SLM are proposed.The main contents and conclusions of this thesis are as follows:（1）The experimental studies on the width and surface quality of single-track,thin wall,single-layer and cuboid samples in SLM forming were performed.It is found that when the layer thickness and the hatch space are fixed,the laser power is 200W⁴00W and the scanning speed is 500mm/s²500mm/s,with the decrease of the scanning speed and the increase of the laser power,the three surface morphologies of single-track are presented in turn:discontinuous and irregular,continuous but irregular,continuous and regular;the surface linear roughness decreases and the track width increases.Under the action of the vertical thermal cycles of SLM,the track width and the surface linear roughness increase.Under the action of the horizontal thermal cycles of SLM,the track width increases,the surface morphology of the track improves and the surface linear roughness increases first and then stabilizes.Under the action of the complex thermal cycles of SLM,the track width increases;due to the comprehensive effect of the thermal accumulation and the changed layer thickness,the surface roughness increases first and then decreases,and then tends to stabilize.（2）The effects of the equipment and data pre-treatment,processing parameters and post-treatments on the forming dimensions of SLMed parts were systematically studied.For the data pre-treatment,0.001mm or less small geometric errors of STL files should be used for parts with arc structure;for the non-arc structures,the selection of geometric error will not change its dimensional accuracy and surface quality,so that large geometric errors can be used to improve the efficiency of the software.The processing parameters of scanning speed,hatch space and laser power are the significant factors for the forming dimension in X/Y directions,but not the significant factors for the forming dimension in Z direction.The forming dimension in X/Y directions increases with the decrease of the scanning speed and the hatch space,and the increase of the laser power.When the scanning speed is 500mm/s,the hatch space is 0.14mm and the laser power is 400W,the maximum dimensional errors in X/Y direction are 417μm and 420μm respectively.When the scanning speed is 1500mm/s,the hatch space is 0.08mm and the laser power is 200W,the minimum dimensional errors in X/Y direction are 109μm and 139μm respectively.（3）The theoretical model of Z-direction dimension deviation was established.It is found that the dimensional deviation of the Z direction mainly caused by three causes:the difference between the solidification height and the layer thickness of the powder of the last layer,the solidification shrinkage when the temperature of the whole sample is cooled to room temperature after processing,and the surface roughness Rz of the top surface.Among the three cause,the the surface roughness Rz of the top surface increases the Z-direction dimension,the other two causes decrease the Z-direction dimension.When the height of the part is larger than 20 mm,the dimensional deviation of Z-direction decreases linearly with the increase of the height.The experimental results are in good agreement with the predicted results by the theoretical model,and the difference between them is within 15μm.（4）The theoretical models of the horizontal dimension deviation of straight wall structure and inclined wall structure,the angle deviation of the angular structure were developed.It is found that the dimension deviation of X/Y direction is caused by two causes:the track width and the cooling shrinkage.The track width enlarges the forming dimension,and the cooling shrinkage reduces the forming dimension.When the dimension of X/Y direction is very small,such as the thickness of the thin wall structure,the dimension deviation is mainly caused by the track width.When the dimension of X/Y direction is large,such as the length of the thin wall structure,besides the track width,the cooling shrinkage can not be neglected,even it is the main cause of the dimension deviation.The dimension deviation in the thickness direction of the inclined wall structure include not only the track width and the cooling shrinkage,but also the dimension deviation caused by the overhanging effect.The predicted results of X/Y dimension deviation model of straight wall structure and inclined wall structure are in good agreement with the experimental results,and the difference between them is within 20μm.The value of the SLMed angular structure without a hanging surface is equal to the design value.（5）The cooling shrinkage of the horizontal dimension includes three parts:unilateral shrinkage of track in the processing(W_s1),the shrinkage of the whole layer after single layer processing(W_s2)and whole shrinkage of the parts after processing(W_s3).Theoretically,as the increase of the X/Y dimension,W_s1 and W_s2 increase first and then towards stability,W_s3 increases linearly.However,due to the restraint of substrate,the cooling shrinkage of SLM in X/Y direction increases linearly first and then towards stability.Therefore,in order to meet the requirements of engineering application,an empirical formula for calculating the solidification shrinkage of the straight wall structure in X/Y direction was developed based on the large number of experimental data.The experimental verification was carried out and found that it agreed well with the theoretical analysis.The difference between them is less than 20μm.（6）For the four different typical structures of straight wall structure,inclined wall strucure,angular structure and arc structure,the methods of pre-compensation,combination of the contour scanning and the pre-compensation,the variable hatch space and layer thickness,and combination of the contour and the solid scanning strategy are proposed respectively to improve the dimensional accuracy.In this thesis,by using these methods,the dimension deviation of the parts after forming is less than±20μm,the roughness Ra of the bottom surface is reduced from 20.31μm to 15.84μm,and the shape accuracy of the angular structure and the arc structure improves.The experimental results show that these methods can improve SLM dimension accuracy significantly.The dimension accuracy of the SLMed typical structures is up to the IT7 standard of standard tolerance value GB/T1800.3-1998.