Processing Parameter Effects on Residual Stress and Mechanical Properties of Selective Laser Melted Ti6Al4V

Selective laser melting (SLM) process is characterized by large temperature gradients resulting in high levels of residual stress within the additively manufactured metallic structure. SLM-processed Ti6Al4V yields a martensitic microstructure due to the rapid solidification and results in a ductility generally lower than a hot working equivalent. Post-process heat treatments can be applied to SLM components to remove in-built residual stress and improve ductility. Residual stress buildup and the mechanical properties of SLM parts can be controlled by varying the SLM process parameters. This investigation studies the effect of layer thickness on residual stress and mechanical properties of SLM Ti6Al4V parts. This is the first-of-its kind study on the effect of varying power and exposure in conjunction with keeping the energy density constant on residual stress and mechanical properties of SLM Ti6Al4V components. It was found that decreasing power and increasing exposure for the same energy density lowered the residual stress and improved the % elongation of SLM Ti6Al4V parts. Increasing layer thickness resulted in lowering the residual stress at the detriment of mechanical properties. The study is based on detailed experimental analysis along with finite element simulation of the process using ABAQUS to understand the underlying physics of the process

A pragmatic continuum level model for the prediction of the onset of keyholing in laser powder bed fusion

Laser powder bed fusion (L-PBF) is a complex process involving a range of multi-scale and multi-physical phenomena. There has been much research involved in creating numerical models of this process using both high and low fidelity modelling approaches where various approximations are made. Generally, to model single lines within the process to predict melt pool geometry and mode, high fidelity computationally intensive models are used which, for industrial purposes, may not be suitable. The model proposed in this work uses a pragmatic continuum level methodology with an ablation limiting approach at the mesoscale coupled with measured thermophysical properties. This model is compared with single line experiments over a range of input parameters using a modulated yttrium fibre laser with varying power and line speeds for a fixed powder layer thickness. A good trend is found between the predicted and measured width and depth of the tracks for 316L stainless steel where the transition into keyhole mode welds was predicted within 13% of experiments. The work presented highlights that pragmatic reduced physics-based modelling can accurately capture weld geometry which could be applied to more practical based uses in the L-PBF process

Ionization degree for strong evaporation of metals

Kinetic equations for ions and neutrals are numerically solved in the plasma sheath formed at a condensed phase when strong evaporation is taking place. The Boltzmann distribution is assumed for electrons. A weakly ionized vapor with the Debye length much shorter than the mean free path is considered. This is typical for laser evaporation of metals. Under these conditions, the sheath consists of a Knudsen layer and a thin charge separation layer between the Knudsen layer and the condensed phase. The self-consistent electrostatic field in the Knudsen layer is obtained from the quasineutrality condition. The potential barrier in the charge separation layer is determined by the charge balance. Kinetic boundary conditions for neutrals and charges are estimated by the detailed balance principle from the parameters of the saturated vapor. The transport of charges in the sheath is controlled by ions and depends on ion-neutral collisions and the self-consistent electrostatic field. Ionization degree in the vapor formed by strong evaporation increases with the Mach number and can attain values about 30% higher than the ionization degree in the saturated vapor. Two factors contribute to this increase. The first is the drop of the potential barrier in the charge separation layer and the second is the strengthening of the field in the Knudsen layer. The ionization equilibrium may be disturbed by a considerable excess of charges

Contact thermal conductivity of a powder bed in selective laser sintering

Estimation of the temperature field in the powder bed in selective laser sintering process is a key issue for understanding the sintering/binding mechanisms and for optimising the technique. Heat transfer may be strongly affected by formation and growth of necks between particles due to sintering When the contact conductivity becomes predominant in the powder bed effective thermal conductivity. The necks often remain small as compared to the particle size. To calculate the effective contact conductivity of such structures a model of independent small thermal contacts is proposed. The conductivity of the considered cubic-symmetry lattices and the random, packing of equal spheres depends on the three structural parameters: the relative density, the coordination number, and the contact size. The present model agrees with the known numerical calculations in the range of contact radius to particle radius ratio below 0.3. The strong dependence on the contact size is qualitatively confirmed by experimental data. (C) 2002 Elsevier Science Ltd. All rights reserved.status: publishe

Pulsed laser deposition of antifriction thin-film MoSeₓ coatings at the different vacuum conditions

MoSeₓ coatings were obtained by pulsed laser deposition in vacuum at the pressure of background Ar gas up to 10 Pa. The deposition temperature was 200 °С. The films were studied by means of X-ray diffraction, scanning and transmission electron spectroscopy, X-ray photoelectron spectroscopy, and Rutherford backscattering spectroscopy of helium ions. The tribological properties of thin-film coatings were investigated by pin-on-disk testing in air with 50% relative humidity. In addition, wear tracks were studied by micro-Raman spectroscopy. Chemical composition, structure, and tribological properties of the coatings were found to be sensitive to the presence of the inert gas. Thus, increasing the gas pressure from 10¯⁴ to 10 Pa changes the chemical composition, so that the ratio of the atomic concentrations of Se and Mo (x= Se/Mo) increases from 1.5 to 2.4 in the principal deposition zone. The changing of the structure concerns the accumulation of distortions in the lattice of MoSeₓ nano-crystals as increasing the distance between the basal planes and intensive formation of nano-sized inclusions of the amorphous phase and Mo nano-crystals in the volume of the coatings. At the optimal gas pressure (∼2 Pa), the composition of the coating was close to the stoichiometric one, and the layer adjacent to the substrate consisted of MoSeₓ nano-crystals with the basal planes parallel to the substrate surface or oriented at small angles to the surface. The thickness of the oriented layer in such coatings was greater than the thickness of the similar layer in the coatings deposited in vacuum (10¯⁴ Pa). The tribological properties of MoSeₓ coatings deposited on substrates of stainless steel type 95 × 18 (18 at.% Cr) depend on the gas pressure. The friction coefficient in air decreases from 0.08 for deposition at the background pressure of 10¯⁴ Pa to 0.04 for deposition at the optimal pressure. This change in the deposition conditions has only a marginal effect on the coating durability. Means to increase the durability are also considered.status: publishe

Photopyroelectric measurement of thermal conductivity of metallic powders

The effective thermal diffusivity of metal powders in air at room temperature is measured by the photopyroelectric technique. The thermal conductivity is calculated from the diffusivity, the relative density, and the specific heat obtained from literature. Maxwell's model is a good prediction but underestimates the measured effective thermal conductivity, especially for irregular particles. Due to the large difference between the thermal conductivity of metals and air, the effective conductivity is mainly determined by the relative density of the powder bed but not by the properties of the powder material. A theoretical model showing the influence of grain size and gas pressure is presented. The dependence on the particles' size and pressure is explained by the gradual transition from the free molecular to conductive mechanism of heat transfer in gaps between particles. The theory gives a precise estimation of effective thermal conductivity for metallic powders with a narrow size distribution of spherical particles and it underestimates this value in the case of a wide size distribution or in the case of irregular particles. (C) 2005 American Institute of Physics.status: publishe