| Summary: | 碩士 === 國立臺北科技大學 === 材料科學與工程研究所 === 107 === In recent years, selective laser melting (SLM) technology has been widely used to manufacture porous metallic materials. However, there are few literatures on the fracture mechanism of the SLM porous structure. This study produced porous SLM Ti-6Al-4V alloys with the unit cells of Cubic, G7, Rhombic dodecahedron (Rd), COH and COH-z structures. The porosities of Cubic, G7 and Rd structures were fixed at 80%. By changing the strut size, the porosities of COH structures were 45% and 59%, and the porosities of COH-z structures were 33% and 50%. The aim of this study was to investigate the fracture mechanisms of previous five porous structures during loading using digital image correlation (DIC) technique.
The uniaxial compression results show that the first stress peaks of COH and COH-z structures were much higher than those of Cubic, G7 and Rd structures due to the lower porosity and coarser strut size. During uniaxial compression, the strain distribution shows that the five porous structures were collapsed layer-by-layer. Only the COH-z structure with a porosity of 33% produced shear band when the compressive strain was close to 50%. Based on the fracture surface, Cubic, G7, and COH (porosity 59%) structures were ductile failures. The fracture surfaces of Rd, COH (porosity 45%) and COH-z structures could not be identified due to the compactness of the specimens after compressive test.
Better strain mapping results can be obtained by adjusting the DIC subset and step. To observe small strains before breaking, the number of subset should be reduced to show a smaller range of strain distribution. Conversely, a large number of subset could be used when the situation is close to fracture due to a larger range of average images. Even if the material is broken, it can still be operated until the end of the test. However, the spatial resolution was reduced accordingly. After the metallographic treatment, the feature points can improve the calculation of the DIC. However, if a structure has too many oblique struts, only a few struts could be observed on the grinding plane. The improvement effect was thus limited, such as G7 and Rd structures.
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