Direct Metal Laser-sintered Stainless Steel: Comparison Of Microstructure And Hardness Between Different Planes

Microstructural analysis and micro-hardness measurements were performed on different planes of 316L stainless steel fabricated by direct metal laser sintering (DMLS) technique. A fine cellular network was observed within the steel microstructure, where morphology of most cells changed from columnar on XZ-plane (vertical section) to equiaxed on XY-plane (horizontal section). Correspondingly, morphology of most grains was found to alter from columnar for the XZ-plane to equiaxed in the case of the XY-plane. Moreover, X-ray diffraction (XRD) analysis revealed a fully austenitic structure for both the planes. The average micro-hardness value for the XZ-plane and XY-plane was insignificantly (≈ 3%) different, which was attributed to the random grain orientation observed on both the planes. However, the average micro-hardness of the DMLS-fabricated 316L stainless steel in this contribution was approximately 25% higher than that of the as-cast one

Experimental Measurement Of Residual Stress And Distortion In Additively Manufactured Stainless Steel Components With Various Dimensions

Disk-shaped 316L stainless steel parts with various diameters and heights were additively manufactured using a direct metal laser sintering (DMLS) technique. Neutron diffraction was used to profile the residual stresses in the samples before and after removal of the build plate and support structures. Moreover, distortion level of the parts before and after the removal was quantified using a coordinate measuring machine (CMM). Large tensile in-plane stresses (up to ≈ 400 MPa) were measured near the as-built disk top surfaces, where the stress magnitude decreased from the disk center to the edges. The stress gradient was steeper for the disks with smaller diameters and heights. Following the removal of the build plate and support structures, the magnitude of the in-plane residual stresses decreased dramatically (up to 330 MPa) whereas the axial stress magnitude did not change significantly. The stress relaxation caused the disks to distort, where the distortion metric was higher for the disks with smaller diameters and heights. The distribution of the residual stresses revealed a marked breakdown of self-similarity in their distribution even comparing disk-shaped samples that were fabricated under identical printing parameters; the stress field profiles were not linearly scaled as a function of height and diameter

Fast Prediction Of Thermal Distortion In Metal Powder Bed Fusion Additive Manufacturing: Part 1, A Thermal Circuit Network Model

The additive manufacturing (AM) process metal powder bed fusion (PBF) can quickly produce complex parts with mechanical properties comparable to wrought materials. However, thermal stress accumulated during PBF induces part distortion, potentially yielding parts out of specification and frequently process failure. This manuscript is the first of two companion manuscripts that introduce a computationally efficient distortion and stress prediction algorithm that is designed to drastically reduce compute time when integrated in to a process design optimization routine. In this first manuscript, we introduce a thermal circuit network (TCN) model to estimate the part temperature history during PBF, a major computational bottleneck in PBF simulation. In the TCN model, we are modeling conductive heat transfer through both the part and support structure by dividing the part into thermal circuit elements (TCEs), which consists of thermal nodes represented by thermal capacitances that are connected by resistors, and then building the TCN in a layer-by-layer manner to replicate the PBF process. In comparison to conventional finite element method (FEM) thermal modeling, the TCN model predicts the temperature history of metal PBF AM parts with more than two orders of magnitude faster computational speed, while sacrificing less than 15% accuracy. The companion manuscript illustrates how the temperature history is integrated into a thermomechanical model to predict thermal stress and distortion

Fast Prediction Of Thermal Distortion In Metal Powder Bed Fusion Additive Manufacturing: Part 2, A Quasi-static Thermo-mechanical Model

The additive manufacturing (AM) process metal powder bed fusion (PBF) can quickly produce complex parts with mechanical properties comparable to that of wrought materials. However, thermal stress accumulated during Metal PBF may induce part distortion and even cause failure of the entire process. This manuscript is the second part of two companion manuscripts that collectively present a part-scale simulation method for fast prediction of thermal distortion in Metal PBF. The first part provides a fast prediction of the temperature history in the part via a thermal circuit network (TCN) model. This second part uses the temperature history from the TCN to inform a model of thermal distortion using a quasi-static thermo-mechanical model (QTM). The QTM model distinguished two periods of Metal PBF, the thermal loading period and the stress relaxation period. In the thermal loading period, the layer-by-layer build cycles of Metal PBF are simulated, and the thermal stress accumulated in the build process is predicted. In the stress relaxation period, the removal of parts from the substrate is simulated, and the off-substrate part distortion and residual stress are predicted. Validation of part distortion predicted by the QTM model against both experiment and data in literature showed a relative error less than 20%. This QTM, together with the TCN, offers a framework for rapid, part-scale simulations of Metal PBF that can be used to optimize the build process and parameters

Comparing Microstructure And Hardness Of Direct Metal Laser Sintered AlSi10Mg Alloy Between Different Planes

Microstructural analysis and micro-hardness measurements were performed on different planes of AlSi10Mg alloy fabricated by direct metal laser sintering (DMLS) technique. A fine cellular dendritic network was observed within the alloy microstructure, where morphology of most cells changed from columnar on XZ-plane (vertical section) to equiaxed on XY-plane (horizontal section). Correspondingly, morphology of most grains was found to alter from columnar for the XZ-plane to equiaxed in the case of the XY-plane. Thus, the morphology of the solidification cells was determined to be strongly dependent on the morphology of the grains containing them. Moreover, X-ray diffraction (XRD) analysis revealed a similar phase assemblage for both the planes. The average micro-hardness value for the XZ-plane and XY-plane was insignificantly (≈ 3%) different, which was attributed to the random grain orientation observed on both the planes