powder layer
Recently Published Documents

Selective laser melting (SLM) additive manufacturing (AM) exhibits uncertainties, where variations in build quality are present despite utilizing the same optimized processing parameters. In this work, we identify the sources of uncertainty in SLM process by in-situ characterization of SLM dynamics induced by small variations in processing parameters. We show that variations in the laser beam size, laser power, laser scan speed, and powder layer thickness result in significant variations in the depression zone, melt pool, and spatter behavior. On average, a small deviation of only ~5% from the optimized/reference laser processing parameter resulted in a ~10% or greater change in the depression zone and melt pool geometries. For spatter dynamics, small variation (10 μm, 11%) of the laser beam size could lead to over 40% change in the overall volume of the spatter generated. The responses of the SLM dynamics to small variations of processing parameters revealed in this work are useful for understanding the process uncertainties in the SLM process.

Selective laser sintering technology has broad application prospects in the manufacture of small batch parts with complex structure. In the sintering process, the preheating efficiency and temperature of powder layer determine the processing quality. A method of preheating powder by lamp radiation and tropical heat conduction is proposed in this paper. The thermal radiation model is established, and the angle coefficient is introduced to describe the proportion of radiation energy on the surface of powder layer. Based on the geometric characteristics of the powder cylinder, the heat conduction process is simplified to one-dimensional heat conduction along the radial direction, and the heat conduction model is established. The coupled temperature field under two actions is obtained by combining the heat radiation model with the heat conduction model. The uniformity coefficient [Formula: see text]/[Formula: see text] of the temperature field is defined to represent the uniformity of the preheating temperature field of the powder layer. By comparing the uniformity coefficient [Formula: see text], a more uniform temperature field can be obtained when the height coefficient is 1.8 under combined action. The validity of the model is verified by a comparative experiment with processed water atomized iron powder. Constructing uniform temperature field can effectively reduce the deformation of parts and improve the forming quality.

With the development of the powder bed 3D printing process, sand casting can be performed with methods that are more advanced than the traditional ones, thus enabling new research on applied materials. When sand is 3D-printed with cement as a binder, its casting performance is improved and sufficient thermal stability of conventional organic and inorganic binders is ensured. In this study, to ensure high resolution and strength in a physical and simple mixture of cement and sand, the compatibility for casting was confirmed using submicron-level cement with ingredients and sizes similar to commercial sand, which is uniformly controlled at 4 µm, instead of conventional sand. To enable quick 3D printing, calcium aluminate cement, which has quick binding properties, was used for high-temperature casting. The strength up to 6 h after hydration was compared to determine the curing rate of silica, mullite, and alumina sand containing cement components. By investigating the change in strength due to heat treatment and comparing the adhesion drop test results after powder bed formation, the material containing silica sand was determined as the most suitable for powder layer 3D printing for application to the mold.

Powder spattering and splashing in the melt pool are common phenomena during Laser-based Powder Bed Fusion (LPBF) of metallic materials having high fluidity. For this purpose, analytical and computational fluid dynamics (CFD) models have been deduced for the LPBF of AlSi10Mg alloy. The single printed layer’s dimensions were estimated using primary operating conditions for the analytical model. In CFD modelling, the volume of fluid and discrete element modelling techniques were applied to illustrate the splashing and spatter phenomena, providing a novel hydrodynamics CFD model for LPBF of AlSi10Mg alloy. The computational results were compared with the experimental analyses. A trial-and-error method was used to propose an optimized set of parameters for the LPBF of AlSi10Mg alloy. Laser scanning speed, laser spot diameter and laser power were changed. On the other hand, the powder layer thickness and hatch distance were kept constant. Following on, 20 samples were fabricated using the LPBF process. The printed samples’ microstructures were used to select optimized parameters for achieving defect-free parts. It was found that the recoil pressure, vaporization, high-speed vapor cloud, Marangoni flow, hydraulic pressure and buoyancy are all controlled by the laser-material interaction time. As the laser-AlSi10Mg material interaction period progresses, the forces presented above become dominant. Splashing occurs due to a combination of increased recoil pressure, laser-material interaction time, higher material’s fluidity, vaporization, dominancy of Marangoni flow, high-speed vapor cloud, hydraulic pressure, buoyancy, and transformation of keyhole from J-shape to reverse triangle-shape that is a tongue-like protrusion in the keyhole. In the LPBF of AlSi10Mg alloy, only the conduction mode melt flow has been determined. For multi-layers printing of AlSi10Mg alloy, the optimum operating conditions are laser power = 140 W, laser spot diameter = 180 µm, laser scanning speed = 0.6 m/s, powder layer thickness = 50 µm and hatch distance = 112 µm. These conditions have been identified using sample microstructures.

In the laser powder bed fusion (LPBF) process, the operating conditions are essential in determining laser-induced keyhole regimes based on the thermal distribution. These regimes, classified into shallow and deep keyholes, control the probability and defects formation intensity in the LPBF process. To study and control the keyhole in the LPBF process, mathematical and computational fluid dynamics (CFD) models are presented. For CFD, the volume of fluid method with the discrete element modeling technique was used, while a mathematical model was developed by including the laser beam absorption by the powder bed voids and surface. The dynamic melt pool behavior is explored in detail. Quantitative comparisons are made among experimental, CFD simulation and analytical computing results leading to a good correspondence. In LPBF, the temperature around the laser irradiation zone rises rapidly compared to the surroundings in the powder layer due to the high thermal resistance and the air between the powder particles, resulting in a slow travel of laser transverse heat waves. In LPBF, the keyhole can be classified into shallow and deep keyhole mode, controlled by the energy density. Increasing the energy density, the shallow keyhole mode transforms into the deep keyhole mode. The energy density in a deep keyhole is higher due to the multiple reflections and concentrations of secondary reflected beams within the keyhole, causing the material to vaporize quickly. Due to an elevated temperature distribution in deep keyhole mode, the probability of pores forming is much higher than in a shallow keyhole as the liquid material is close to the vaporization temperature. When the temperature increases rapidly, the material density drops quickly, thus, raising the fluid volume due to the specific heat and fusion latent heat. In return, this lowers the surface tension and affects the melt pool uniformity.

Numerical methods for simulating selective laser melting (SLM) have been widely carried out to understand the physical behaviors behind the process. Numerical simulation at the macroscale allows the relationship between input parameters (laser power, scanning speed, powder layer thickness, etc.) and output results (distortion, residual stress, etc.) to be investigated. However, the macroscale thermal models solved by the finite element method cannot predict the melt pool depth correctly as they ignore the effect of fluid flow in the melting pool, especially in the case of the presence of deep penetration. To remedy this limitation, an easy-implemented temperature-dependent heat source is proposed. This heat source can adjust its parameters during the simulation to compensate for these neglected thermal effects related to the fluid flow and keyhole, and the heat source’s parameters become fixed once the temperatures of the points of interest become stable. Contrary to the conventional heat source model, parameters of the proposed heat source do not require a calibration with experiments for each process parameter. The proposed model is validated by comparing its results with those of the anisotropic thermal conductivity method and experimental measurements.

In the present work, the flow field in a slab continuous casting mold with thicknesses of 180 and 250 mm are compared using high temperature quantitative measurement and numerical simulation. The results of the numerical simulation are in agreement with those of the high temperature quantitative measurement, which verifies the accuracy and reliability of the numerical simulation. Under the same working conditions, the velocities near the mold surface with the thickness of 180 mm were slightly higher than those of the mold with the thickness of 250 mm. The flow pattern in the 180 mm thick mold maintains DRF more easily than that in 250 mm thick mold. The kinetic energy of the jet dissipates faster in the 250 mm thick mold than in the 180 mm mold. For double-roll flow (DRF), as the argon gas bubbles can be flushed into the deeper region under the influence of strong jets on both sides, the argon bubbles distribute widely in the mold. For single-roll flow (SRF), as the argon bubbles float up quickly after leaving the side holes, the bubble distribution is more concentrated in the width direction, which may cause violent interface fluctuation and slag entrainment. The fluctuation at the steel-slag interface in the mold with 180 mm thickness is greater than that in the mold with 250 mm thickness but less than 5 mm. The increase of mold thickness may lead to a decrease of the symmetry of the flow field in the thickness direction and uniformity of mold powder layer thickness. In summary, the steel throughput should be increased in the 250 mm thick mold compared with that in the 180 mm thick mold.