ABSTRACT
Additive manufacturing/three-dimensional printing (AM/3DP) processes promise a flexible production modality to fabricate a complex build directly from its digital design file with minimal postprocessing. However, some critical shortcomings of AM/3DP processes related to the build quality and process repeatability are frequently experienced and reported in the literature. In this study, an in situ real-time nondestructive monitoring framework based on the dispersive properties of phononic crystal artifacts (PCAs) to address such quality challenges is described. Similar to a witness coupon, a PCA is printed alongside a build while it is interrogated and monitored with ultrasound. A PCA is substantially smaller than the actual build. Due to its periodic internal structures, a PCA creates pass and stop bands in its spectral response, which are sensitive to the variations in its process and material parameters. These periodic structures, representing the geometric complexities of an actual build, are designed for a specific monitoring objective(s) in AM/3DP. As a model application, in this demonstration study, the effect of the laser scanning speed of a slective laser melting (SLM) printer on the spectral properties of metal PCAs (mPCAs) is ultrasonically evaluated offline. The dependency of the pressure and shear wave speeds, the apparent Young's and shear moduli, and Poisson's ratio on the scanning speed are quantified, and it is found that they are highly sensitive to the laser scanning speed of an SLM printer. The sensitivity of the peaks of the pressure and shear spectral waveforms acquired for the identical mPCA designs printed on the same build plate with the same process parameters is also quantified. For powder-based AM/3DP technologies, where scanning speed is among the crucial process parameters such as laser power and bed temperature, the reported correlations between scanning speeds and the mechanical and spectral features of the mPCAs are expected to be instrumental in developing in situ real-time monitoring systems.
ABSTRACT
The brittle phase (Laves) of Inconel 718 parts formed by laser metal deposition (LMD) represents a bottleneck of the engineering applications. In order to investigate effectiveness of laser remelting (LR) technology on suppressing the formation of Laves phase, different laser scanning speeds of the LR process were adopted to build and remelt the single-pass cladding layers. The evolution of phase composition, microstructural morphology, and hardness of the LMD and LMD + LR specimens were analyzed. The experimental results show that different laser scanning speeds can obviously change the microstructural evolutions, Laves phase, and hardness. A low laser scanning speed (360 mm/min) made columnar dendrite uninterruptedly grow from the bottom to the top of the cladding layer. A high laser scanning speed (1320 mm/min) has a significant effect on refining Laves phase and reducing Nb segregation. When the laser scanning speed of LR process is equal to that of LMD, the cladding layers can be completely remelted and the content of Laves phase of the LMD + LR layer is 22.4% lower than that of the LMD layer. As the laser scanning speed increases from 360 to 1320 mm/min, the mean primary dendrite arm spacing (PDAS) values of the remelting area decrease from 6.35 to 3.28 µm gradually. In addition, the low content of Laves phase and porosity contribute to the growth of average hardness. However, the laser scanning speed has a little effect on the average hardness and the maximum average hardness difference of the LMD and LMD + LR layers is only 12.4 HV.
ABSTRACT
A porous ß-tricalcium phosphate (ß-TCP) bioceramic scaffold was successfully prepared with our homemade selective laser sintering system. Microstructure observation by a scanning electron microscope showed that the grains grew from 0.21 to 1.32 µm with the decrease of laser scanning speed from 250 to 50 mm min-1. The mechanical properties increased mainly due to the improved apparent density when the laser scanning speed decreased to 150 mm min-1. When the scanning speed was further decreased, the grain size became larger and the mechanical properties severely decreased. The highest Vickers hardness and fracture toughness of the scaffold were 3.59 GPa and 1.16 MPa m1/2, respectively, when laser power was 11 W, spot size was 1 mm in diameter, layer thickness was 0.1-0.2 mm and laser scanning speed was 150 mm min-1. The biocompatibility of these scaffolds was assessed in vitro with MG63 osteoblast-like cells and human bone marrow mesenchymal stem cells. The results showed that all the prepared scaffolds are suitable for cell attachment and differentiation. Moreover, the smaller the grain size, the better the cell biocompatibility. The porous scaffold with a grain size of 0.71 µm was immersed in a simulated body fluid for different days to assess the bioactivity. The surface of the scaffold was covered by a bone-like apatite layer, which indicated that the ß-TCP scaffold possesses good bioactivity. These discoveries demonstrated the evolution rule between grain microstructure and the properties that give a useful reference for the fabrication of ß-TCP bone scaffolds.