Study On The Fabrication Properties And Process Of Stainless Steel Parts By Selective Laser Melting

Selective Laser Melting (SLM) is the important development direction in rapid manufacturing field. Compared with conventional manufacturing methods, SLM has the advantages to directly fabricate any complex shape of dense metal parts. But now the study of SLM is still in the initial stage, including the powder melting and fabrication propertied, process controlling and optimizing, overhanging structure fabrication and the technical process of controllable porous structure, above issues need further discussion.This paper gives a deep and detailed theoretical analysis of the properties and process during SLM fabrication of 316L stainless steel, the main research contents as follow:The SLM equipment was improved. In order to optimize the powder bed preparing effect, different powder preparation devices were used according to various geometrical features, and preloading powder preparation device and elastic device with flexible racks were used respectively. Preloading powder device was mainly used to fabricate precision parts with no supports, or parts with small cross section; elastic device with flexible racks was mainly used to fabricate fine-structure parts with less supports, or parts with large cross section. In order to blow the black smoke and the vaporization substance away, and to improve the air flow in the chamber, "side blowing jet" is installed on one side of the penetrative mirror. The improvement of the SLM equipment increases its stability and safety.Research on fabrication performance of 316L stainless steel during SLM process. The performance includes relative density, surface appearance, dimensional accuracy, microstructure, and fabrication capability of different geometrical features. In order to analysis the relationship between energy input and relative density, energy input formula was deduced according to effects of different parameters on energy input, and the energy requirements needed to fabricate dense metal parts was discussed. Then, through a 6 factors and 5 levels orthogonal experiment and measurement of the samples’density, the relationship between the energy input and relative density was got, and the profile was divided into straight up stage, yield stage and uncertain stage. Six types of surface appearance were obtained at different line energy input. The dimensional accuracy of SLM parts was studied, relationship formula among practical layer thickness, theoretical layer thickness, powder’s apparent density and part’s relative density was deduced, and the formula indicated that the layer thickness gradually become thicker as the layers increase, which would affect z dimension accuracy. The microstructure analysis showed that the microstructure of the SLM part is mainly composed of columnar crystal and equiaxed grains, and the grain size can reached as small as 0.3-1μm. The SLM capability for producing typical geometrical features was validated. CAD model with thin wall, sharp angle, sophisticated characters, and so on was designed, then real part was manufactured by the above optimized process parameters. All the geometrical features can be well fabricated except the overhanging structures.Research on single track, multi-track, multi-layer fabrication during SLM process, and the related heat accumulation effect was discussed too. Four different track types were obtained under different energy inputs, then regional distribution diagram of track pattern was depicted, the widths of single track fabricated with different parameters were summarized.The effects of different overlapping rates for different track types were studied, and the optimized overlapping rate for manufacturing dense metal parts were got. The effect of scanning strategy on multi-track fabrication was discussed. The experiment showed that inter-layer stagger scanning strategy, which contributes to reduce the overlapping track and improve the inter-layer strength, was used in multi-layer fabrication. The part’s surface would gradually deteriorate. As a result of the heat accumulation, the distribution diagram of track patterns would slowly shift to the lower right in multi-track and multi-layer fabrication. By simplifying the heat accumulation model, the amount of heat accumulated during multi-track overlap and multi-layer fabrication was calculated. Based on above process experiments and optimization, dense metal parts with tensile strength and hardness 20-25% more than casting part were obtained.Research on the SLM fabrication process of overhanging structure. The warping and hanging dross defects occur frequently during overhanging structure fabrication, which deteriorate the shape accuracy and dimensional accuracy. In order to improve the fabrication quality of overhanging structure, the factors that affect the fabrication of overhanging structure, and the defects that caused were analyzed theoretically. The effects of tilting angle, scanning speed, laser power, stress accumulation and vector length on SLM fabrication of overhanging structure were studied. It is found that overhanging structure will warp easier as the tilting angle, the scanning speed become small, and the laser power, scan line length, the amount of fabricating layers get larger. The relationships among laser power, scanning speed and the critical tilting angles were constructed, and the critical tilting angles include reliable tilting angle and minimum building angle. According to above analysis, space placement adjusting and local parameters controlling were adopted to improve the fabrication quality of overhanging structure, several parts with typical overhanging structure were manufactured successfully.Research on the SLM fabrication process of controllable porous structure. Porous structure includes controllable and uncontrollable porous structures, and the first type and its fabrication process was a key discuss here. The blocking defect in the unit hole, caused by hanging dross during SLM fabrication process, was avoided by designing space symmetric unit hole, and the relation formula among main geometrical parameters was calculated. By adjusting the energy input and the unit hole’s geometrical parameters, the sticky powder and splashing defects during SLM process were strictly controlled. Controllable porous structure part with 0.1-0.15 mm wire-frame diameter, 0.2-0.25mm pore size and 45o wire-frame tilting angle was got. The fabrication effects satisfy the requirements of high porosity, small holes and thin line width.