Deposited Powder Layer Research Articles

Shape effects in binary mixtures of PA12 powder in additive manufacturing

The quality of powder spread in additive manufacturing devices depends sensitively on particle shapes. Here, we study powder spreading for mixtures of spherical and irregularly shaped particles in Polyamide 12 powders. Using DEM simulations, including heat transfer, we find that spherical particles exhibit better flowability. Thus, the particles are deposited far ahead of the spreading blade. In contrast, a large fraction of non-spherical particles hinders the flow. Therefore, the cold particles are deposited near the front of the spreading blade. This results in a temperature drop of the deposited particles near the substrate, which cannot be seen with spherical particles. The particles of both shapes are homogeneously distributed in the deposited powder layer.

Modelling of microstructures development in laser powder bed fusion process – Application to IN718 superalloy

In laser powder bed fusion (L-PBF) process, a deposited powder layer is melted by a laser and solidifies when the laser moves away. During solidification, the microstructure of the part is formed due to both epitaxial grain growth and nucleation. This structure has a strong influence on the final mechanical properties of parts and is influenced by the choice of process parameters such as the laser power and velocity or the scanning strategy. As a consequence, the prediction and characterization of this microstructure is of prime interest considering size, crystallographic orientations and shapes of grains. Among the approaches reported in the literature to model microstructure development, the Cellular Automaton (CA) method is a relevant choice to describe grain structure evolution. This model has been adapted to investigate microstructure formation during L-PBF process applied on an IN718 nickel-base superalloy. The steady state thermal behaviour at the scale of the melt pool is analysed and used to compute the development of the structure. This structure is computed at the scale of the part also considering realistic scanning strategies.

Factors to control the alignment of surface-treated titanium dioxide powders to maximize performance of sunscreens.

Titanium dioxide powders are contained in a large class of colour cosmetics and sunscreen formulas. When they are used, the formation of a uniform functional powder layer on the skin is an important factor to show their functionality, such as aesthetic and UV protection. Attempts were made to extract the factors that affect the UV shielding ability of the deposited powder layer. Seven kinds of surface treatments were conducted on nano-sized titanium dioxide powder to modify the surface characteristics. Dispersion samples were prepared by mixing these powders with liquids, such as mixed solutions of cyclopentasiloxane, isododecane, coconuts alkane and dimethicone using a disperser and a bead mill. The dispersions were applied using an applicator on cellulose triacetate film, polycarbonate film and polymethyl pentene film. Laser microscope observation and micro-gloss glossmeter analysis were carried out to assess the flatness of the deposited powder layers, and the UV shielding ability was evaluated using SPF analyser. Factors whose influences on the structure and UV shielding ability of the deposited powder layer being analysed were pseudo-HLB of the powders, liquids for preparing the dispersions and material of the substrates. Higher UV shielding ability was attained when powders having pseudo-HLB at around 6 were employed independent from the kinds of liquids and substrates. Flatness of the deposited layer was found to enhance the UV shielding ability of the UV-B region, while that of the UV-A region was scarcely influenced by the flatness. Employing lower surface tension liquids for preparing the dispersions and materials exhibiting lower polar components of surface free energy as substrates tended to enhance the UV shielding ability. Surface treatments conducted on the powders in this study were found to change UV shielding ability, especially UV-B shielding ability, and the relation between pseudo-HLB and UV shielding ability was scarcely influenced by the kinds of liquids. Both surface tension of liquids and the polar component of surface free energy of substrates affected the UV shielding ability. It was suggested that pseudo-HLB calculated based on chemical structure becomes useful information to choose optimum surface treatment to make uniform powder alignment independent from the surrounding environment.

A discrete element framework for the numerical analysis of particle bed-based additive manufacturing processes

This paper investigates the potential of the discrete element method to simulate the physics of particle bed-based additive manufacturing. This method naturally captures the discrete aspects of additive manufacturing processes, such as material addition. The proposed discrete element framework uses constitutive relations for loose powder, bonding kinematics and the thermo-mechanical behaviour of bonded particles. The mechanical bond interactions consist of beams that interconnect the particles. These beams are able to transfer forces as well as moments. The thermal conductive bond interactions assume an effective conductive area and density to account for the voids in the system. Simulated compression tests reveal that the macroscopic Young’s modulus and Poisson’s ratio of the bonded material are controlled by only two micro-scale parameters. Furthermore, a heat conducting rod of both powder and bonded material is simulated and compared to a continuum finite element simulation. The proposed discrete model is able to simulate a complete printing process, capturing the solid material behaviour accurately. A simulation of a printed sample shows various additive manufacturing aspects such as: the deposited powder layer, G-code input, heat source interaction, contact, bonding, thermal conduction and the accumulation of residual stresses and deformations.

Part scale estimation of residual stress development in laser powder bed fusion additive manufacturing of Inconel 718

Residual stress has been among the primary problems in the laser powder bed fusion (LPBF) additive manufacturing. Nevertheless, complex physics and multi-scale nature of the problem make an accurate prediction of residual stress at the part level a great challenge. Thus, the present study developed the finite element framework to predict the part scale residual stress development in the LPBF of Inconel 718. Two-scale models were used coherently. A mesoscale model calculated the stress formation in a deposited powder layer by considering the influence of process conditions and scan strategies. Subsequently, the inherent strain approach was used for the part level prediction. The inherent strain values were estimated according to numerical results from the mesoscale model. Numerical validation was made by comparing the transient development of residual stress with reported measurements from the neutron diffraction of over 200 data points. The proposed framework provided the prediction which matched excellently with experimental results. Ultimately, the present study established the proven framework which could be used for further investigation on part scale-related issues in the LPBF process.

A DEM study of powder spreading in additive layer manufacturing

In this paper, discrete element method simulations were used to study the spreading of an idealised, blade based, powder coating system representative of the spreading of spherical, mono-sized, non-cohesive titanium alloy (Ti6AlV4) particles in additive layer manufacturing applications. A vertical spreader blade was used to accelerate a powder heap across a horizontal surface, with a thin gap between the blade and the surface, resulting in the deposition of a thin powder layer. The results showed that it is inevitable to deposit a powder layer with a lower packing fraction than the initial powder heap due to three mechanisms: shear-induced dilation during the initiation of powder motion by the spreader; dilation and rearrangement due to powder motion through the gap; and the inertia of the particles in the deposited powder layer. It was shown that the process conditions control the contribution of these three mechanisms, and that the velocity profile in the shear layer in front of the gap is critical to the final deposited layer packing fraction. The higher the mean normalised velocity in the shear layer the lower the deposited layer packing fraction. The gap thickness and the spreader blade velocity affect the properties of the deposited layer; with the former increasing its packing fraction and the latter decreasing it. The analysis presented in this study could be adapted to powders of different materials, morphologies and surface properties.

Ultrasonic material dispensing-based selective laser melting for 3D printing of metallic components and the effect of powder compression

Selective laser melting (SLM) is one of the most commonly used metallic component 3D printing techniques. In a previous investigation of multiple materials SLM reported by The University of Manchester, high porosities and cracks were found in the regions where the powder was deposited via an ultrasonic powder dispenser. The low powder packing density was identified as a critical reason for this. In this paper, we report a new method to compress the ultrasonically deposited powder layer in order to increase the powder packing density. The effects of powder deposition velocity, powder track overlap distance and powder compression force on the deposited powder characteristics were investigated. The microstructure, tensile strengths, and porosity of the laser-fused samples were analyzed. The results indicated that powder compression could reduce porosity and component distortion and increase the mechanical strength of the printed parts.

Fundamental analysis of the influence of powder characteristics in Selective Laser Melting of molybdenum based on a multi-physical simulation model

Selective Laser Melting (SLM) offers great potential for the processing of molybdenum as it allows the production of parts that can hardly be fabricated in a classical way. The workpiece is built up by melting up powder layer by layer. Besides the applied laser parameters, the characteristics of the powder and the deposited powder layer have a crucial influence on the dynamics of the process and the final processing result. In order to get a fundamental understanding of these influences, numerical simulations are a versatile tool as they allow a detailed look on isolated parameters. In this contribution a coupled thermo-fluid dynamical simulation model for SLM is applied for a fundamental analysis of the influence of powder characteristics in SLM of molybdenum. The results allow a deeper insight into the influences of powder particle size and powder particle arrangement on the SLM process and the formation of the molten track.

EVALUATION OF SINGLE TRACKS OF 17-4PH STEEL MANUFACTURED AT DIFFERENT POWER DENSITIES AND SCANNING SPEEDS BY SELECTIVE LASER MELTING

In Selective Laser Melting, the initial units produced are single tracks that overlap to create a single layer; from the sequence of layers, a 3D object is manufactured. The properties of the parts produced by SLM depend heavily on the properties of each single track and each layer formed by these tracks. This study evaluates the effect of processing parameters on the geometrical characteristics of single tracks manufactured from 17-4PH stainless steel powder. A single-mode continuous-wave ytterbium fibre laser was used to manufacture single tracks at laser powers in the range of 100-300 W with a constant spot size of ∼ 80μm. The single tracks produced were subjected to standard metallographic preparation techniques for further analysis with an optical microscope. Deep molten pool shapes were observed at low scan speeds, while shallow molten pool shapes were observed at high scan speeds. At higher laser power densities, under-cutting and humping effects were also observed. The dimensions of single tracks processed without powder generally decrease with increasing scan speed at constant laser power. However, the geometrical features of the single tracks processed with powder revealed pronounced irregularities believed to be caused by non-homogeneity in the deposited powder layer.

On the Possibility of Selective Laser Melting of Quartz Glass

Quartz glass is considered as model ceramic material for selective laser melting (SLM). Single beads of fused powder bonded with a continuous substrate of the same material are obtained. Neither the beads nor the substrates near them are cracked. Low closed porosity in the beads and detachment of the bead borders from the substrate are observed. The quality of the obtained beads is estimated to be sufficient for fabrication of three-dimensional parts by SLM. Comparison with calculations of heat transfer and evaporation reveals that the mass and the energy losses by evaporation are considerable and that the substrate surface should be locally heated up to 1600 K for strong bonding with the bead. Decreasing the thickness of the deposited powder layer is proposed to improve the adhesion between the bead and the substrate with the minimum loss in the productivity of the process. Finer powder is expected to decrease the residual closed porosity in the bead and to make the pores finer too.

Measurement of the electrostatic powder coating properties for corona and triboelectric coating guns

Measurements have been made of the deposited powder layer for conventional electrostatic corona powder guns and triboelectric guns. By selectively removing the powder layer under computer control conditions, measurements of the powder thickness, and the adhesive properties of the powder layer have been obtained for variation in the coating process and detailed comparisons have been obtained for both coating systems.

Three-dimensional finite element modeling of laser cladding by powder injection: Effects of powder feedrate and travel speed on the process

This article addresses a novel three-dimensional transient finite element model of the laser cladding by powder injection process. The proposed model can predict clad geometry as a function of time and process parameters including beam velocity, laser power, powder jet geometry, laser pulse shaping, and material properties. In the proposed method, the interaction between powder and melt pool are assumed to be decoupled and as a result, the melt pool boundary is first obtained in the absence of powder spray. Once the melt pool boundary is calculated, it is assumed that a layer of coating material based on powder feedrate and elapsed time is deposited on the intersection of the melt pool and powder stream in the absence of laser beam. The new melt pool boundary is then calculated by thermal analysis of the deposited powder layer, substrate and laser heat flux. The results of numerical modeling for different process velocities and different powder feedrates are presented and compared with experimental results. The comparisons show a good agreement between the modeling and experimental geometry of the clads.

3-D finite element modeling of laser cladding by powder injection: effects of laser pulse shaping on the process

This paper introduces a 3-D transient finite element model of laser cladding by powder injection to investigate the effects of laser pulse shaping on the process. The proposed model can predict the clad geometry as a function of time and process parameters including laser pulse shaping, travel velocity, laser pulse energy, powder jet geometry, and material properties. In the proposed strategy, the interaction between powder and melt pool is assumed to be decoupled and as a result, the melt pool boundary is first obtained in the absence of powder spray. Once the melt pool boundary is obtained, it is assumed that a layer of coating material is deposited on the intersection of the melt pool and powder stream in the absence of the laser beam in which its thickness is calculated based on the powder feedrate and elapsed time. The new melt pool boundary is then calculated by thermal analysis of the deposited powder layer, substrate and laser heat flux. The process is simulated for different laser pulse frequencies and energies. The results are presented and compared with experimental data. The quality of clad bead for different parameter sets is experimentally evaluated and shown as a function of effective powder deposition density and effective energy density. The comparisons show excellent agreement between the modeling and experimental results for cases in which a high quality clad bead is expected.

Gloss and Texture Control of Powder Coated Films

The desired appearance of powder coated film depends upon the specific industrial application. While a glossy, mirror-like finish with a high distinctiveness of image (DOI) is required for automotive clear coat applications, much outdoor equipment and many household appliances require smoothly textured finishes that hide surface irregularities, fingerprints, and surface markings. This textured finish of powder coated film, often termed orange peel appearance, can be controlled by adjusting the chemical formulation and the physical properties of the powder and by optimizing the electrostatic application process. These adjustments allow a wide-ranging control of gloss and orange peel texture. The chemical formulation of the powder is often determined by the functional needs of the coatings: providing protection against corrosion, heat, UV radiation, and physical impact. In many cases where the formulation cannot be changed, the electrostatic spraying process for the powder can be adjusted to achieve the desired gloss and texture. In electrostatic powder coating, back corona on the deposited powder layer often causes defects in the film. However, if the electrical field intensity inside the powder layer is below the breakdown level, the adverse effect of back corona can be avoided, thus improving the uniformity of the deposited powder layer. In the absence of back corona, increasing the high voltage applied to the corona gun improves both the smoothness and the glossiness of cured films. Back corona can be reduced by increasing the surface conductivity of the powder. Increasing the relative humidity during spraying also decreases the roughness, since the augmented surface conductivity allows the powder layer charge to decay faster. The optimal relative humidity value for applying powder for uniform film thickness was found to be 60%; increasing humidity beyond that point led to surface degradation, possibly due to excess moisture trapped in the powder layer. While the surface smoothness increased, the glossiness did not vary significantly with the change in relative humidity. Another factor for film surface properties is the "hiding power" of the unevenness of the substrate surface. A rough substrate can be responsible for a rough film surface when film thickness is less than 25 m. It was found that films 40 to 50 m thick can successfully cover scratches of about 20 m depth on the substrate. Typical values of film waviness varied from 1.5 to 3 m, and film roughness varied from 0.2 to 0.4 m for epoxy film with 60 to 65 m thickness.

Measurement of Surface Topography, Packing Density, and Surface Charge Distribution of an Electrostatically Deposited Powder Layer by Image Analysis of Fluorescent Latex Spheres

A novel method of nonintrusive measurement of surface profile, packing density, and surface charge distributions of a powder layer deposited on a substrate is reported. The method employs the deposition of electrostatically charged monodispersed fluorescent latex spheres (FLS), approximately 2 m in diameter, on the surface of: (1) the substrate before deposition, (2) the powder layer after deposition, and (3) the film formed by curing the powder layer. The surface topography in all cases was mapped using an epi-fluorescent microscope with a vertical resolution of - 2 m in the z axis and - 10 m in the x and y axes. An area of 1 cm 2 1 cm is scanned in 1 mm segments, providing approximately 100 data points per cm 2 for the surface topography. For each measurement of surface topography, the substrate was positioned on the microscope stage in a manner such that the reference points (x, y, and z) remained the same for all measurements of the substrate. The surface profiles, with respect to the same reference points, were plotted using Origin 6.0 software for 3D presentation of the topography. The method was also applied to map the surface charge density distribution of electrostatically charged surfaces. The FLS imaging method provides a new tool for examination of surface profiles, packing density, and charge distribution of powder layers on a microscopic scale not provided by optical or atomic force/electrostatic force microscopy (AFM/EFM). While AFM and EFM are very effective in providing similar information with nanometer resolution, they cannot be directly applied on a larger macroscopic scale to study powder layers and for a larger surface area (up to 1 cm 2 or greater) involving deposited particles in the range of 1-50 m in diameter. For AFM, the range in the z-axis is limited to - 3 m and the x-y scan area is limited to 100 m 2 100 m. The FLS method has a much wider range but it is operated manually; an automated scanning process is required for rapid measurement. A comparison of the FLS and EFM techniques as they apply to analyzing charge distribution on coal surfaces is presented.

Adhesion Measurements for Electrostatic Powder Coatings

The total force holding a layer of electrostatically charged powder onto a work piece results from a combination of effects; an interfacial electrochemical phenomenon at the surface, inter particle/particle cohesion and a bulk electrostatic attractive force. Attempts have been made to obtain the total integrated effect by assessing the force that is required to selectively remove the powder layer from a metal substrate. This is a destructive measurement that disrupts the powder layer. All the measurements are undertaken on the powder layer before fusing. Powder adhesion measurements have been obtained from three different techniques consisting of (i) the centrifugal spinning of a powder coated cylinder, (ii) measuring the mass of powder removed when a powder coated sample is subject to an impulse from a drop test and (iii) selectively removing powder under computer control with a series of micro air pulses. The results tend to be in agreement and from these different test procedures they assist in modelling the structure of a deposited powder layer. Spray tests using different spraying systems indicate that the outer layer of the coating tends to have higher adhesive characteristics when the specific charge (Q ∗) is < 2 μC/g. When the specific charge is > 3.5 μC/g the adhesion of the outer layer is reduced.

POWDER COATING PROCESS PARAMETERS FOR A TRANSFER EFFICIENCY MODEL

ABSTRACT The trajectories of charged powder particles in a powder coating system are governed by the electrostatic, gravitational and aerodynamic forces acting on the particles. A mathematical model of particle trajectories inside a powder coating booth must consider (1) the aerodynamic flow field, (2) particle size and charge distributions, (3) the electrostatic field distribution, and (4)the geometry of the target. Our approach is to employ a grid generation and flow solver to examine the air flow pattern and an iterative technique where the Charge Simulation Method can be used to compute the electric field strength and the Method of Characteristics can be used to compute the charge density in the gun-to-target region. The electrostatic forces due to the deposited powder layer and image charge are to be taken into account to determine if the particle will deposit on the substrate or not. The model is applied to the geometry of a high-voltage electrode consisting of a long thin rod with a hemispherical e...