| Selective Laser Melting(SLM) is an important technique in the 3D printing field. The process principle of this technique breaks through the restriction of the traditional subtractive manufacturing process. It is capable of directly fabricating three-dimensional, individuation and customization parts with complex structure in a short time. Although it has so many advantages, the key issues on SLM technique, such as spatter behavior, thermal stress concentration, warping and cracking, low precision of parts were less well studied, let alone the researches in manufacturing ability of typical geometry features. Accordingly, this thesis focuses on the manufacturing ability of geometry features by SLM based on the key issues on process. The main contents and conclusions are as follows.(1) Thermal physics process during SLM process. Combining the fractal theory with finite element method to predict the effective thermal conductivity of metal powder. In contrast with the experimental data of reference, it is found that the predictive values are agreement with experimental values, illustrating the high precision of finite element model.The spatter behavior of SLM process was also studied, we divide the spatter behavior into two types, one is the droplet spatter, and the other is powder spatter. Both of them are induced by the impact of metallic vapor. We setup a high speed photography system to observe the spatter behavior and to analyze the influence of energy input on spatter intensity. The EDS and XRD analysis results showed that there are a large amount of oxides in spatter particles, it explains that SLM process is sensitive to oxygen content. Finally, we used fresh powder and powder polluted by spatter to manufacture tensile test specimens respectively, and found that spatter reduce the mechanical property of SLM parts seriously.(2) The thermal residual stress and the warping & cracking during the SLM process.High temperature gradient, small grain size and phase transformation are the main reasons for generation of residual stress during SLM process. We adapted the XRD to measure the residual stresses along the height direction and horizontal direction, what’s more, we also studied the impact the Substrate Thermal Cycling(STC) and process parameters on the stress distribution. According to the study, higher energy input and longer track length induce larger residual stress in SLM parts. Therefore, sector scanning strategy can reduce residual stressefficiently. The peak values of residual stress always occur at the onset of scanning track,which indicates the both ends of part are easy to warping and cracking. At last, we studied the influencing factors on the warping & cracking and proposed several measures to avoid it.(3) Accuracy of geometry features manufactured by SLM process. Manufacturing errors of SLM mainly come from the systematic error and the densification of powder materials(Here it is named as shrinkage of materials). In this thesis, the influence of process parameters(such as laser power, scanning speed, scanning hatching, layer thickness) on the shrinkage ratio of materials were studied by using Response Surface Methodology(RSM), it was found that the shrinkage ratio in the horizontal direction is smaller than that in height direction, and dimensional compensation can improve the accuracy in X/Y/Z directions. The relative error in X/Y directions can be limited in 0.4%, while relative error in Z direction can be limited in0.6%.(4) Manufacturing ability of typical geometry features. We drew typical geometric features of mechanism, such as thin-walled part, hole, column, angle, sphere, high aspect ratio part. The quality of SLM manufactured parts were evaluated, the evaluation criteria includes the maximum/ minimum size, dimensional error and the surface topography. At last,design rules of geometric features based on SLM direct manufacturing were extracted.(5) Key issues of non-assembly mechanism manufactured by SLM. The minimum clearance size manufactured by SLM was determined by a series of experiment, and the influence of process parameters on the size accuracy and inner surface roughness of clearance features. Under the premise of reducing the overhanging within the clearance feature of mechanism, we proposed an optimized design method for the revolution pair, prismatic pair and spherical pair manufactured by SLM. The needed support structures can be reduced sharply by changing the assembly configuration of mechanism, thus, the difficulty that SLM direct manufacturing of non-assembly mechanism can decrease. This above method was verified on spatial mechanisms. |
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