Effects of hot isostatic pressing on the elastic modulus and tensile properties of 316L parts made by powder bed laser fusion

The microstructure and mechanical properties of 316 L steel have been examined for parts built by a powder bed laser fusion process, which uses a laser to melt and build parts additively on a layer by layer basis.Relative density and porosity determined using various experimental techniques were correlated against laser energy density. Based on porosity sizes, morphology and distributions, the porosity was seen to transition between an irregular, highly directional porosity at the low laser energy density and a smaller, more rounded and randomly distributed porosity at higher laser energy density, thought to be caused by keyhole melting. In both cases, the porosity was reduced by hot isostatic pressing (HIP).High throughput ultrasound based measurements were used to calculate elasticity properties and show that the lower porosities from builds with higher energy densities have higher elasticity moduli in accordance with empirical relationships, and hot isostatic pressing improves the elasticity properties to levels associated with wrought/rolled 316 L. However, even with hot isostatic pressing the best properties were obtained from samples with the lowest porosity in the as-built condition.A finite element stress analysis based on the porosity microstructures was undertaken, to understand the effect of pore size distributions and morphology on the Young's modulus. Over 1–5% porosity range angular porosity was found to reduce the Young's modulus by 5% more than rounded porosity. Experimentally measured Young's moduli for samples treated by HIP were closer to the rounded trends than the as-built samples, which were closer to angular trends.Tensile tests on specimens produced at optimised machine parameters displayed a high degree of anisotropy in the build direction and test variability for as-built parts, especially between vertical and horizontal build directions. The as-built properties were generally found to have a higher yield stress, but lower upper tensile strength and elongation than published data for wrought/hot-rolled plate 316 L. The hot isostatically pressed parts showed a homogenisation of the properties across build directions and properties much more akin to those of wrought/hot-rolled 316 L, with an increase in elongation and upper tensile strength, and a reduction in yield over the as-built samples

A comparison of grazed and conserved grass and concentrate diets in terms of the performance and carcass composition of beef cattle and lambs

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Validated Computational Modelling Techniques for Simulating Melt Pool Ejecta In Laser Powder Bed Fusion Processing

Industry currently require faster build rates from laser powder bed fusion processes. As such, higher power lasers and multi-laser systems are being explored. Due to instabilities in the melting process, material is ejected from the melt pool in the form of spatter and vapour. Previous work has shown that these ‘ejecta’ can result in attenuation of the laser and redeposition of lager particles onto the powder bed; which can lead to poor mechanical properties. ANSYS Fluent was used to create a CFD model which was validated against hot wire anemometry results from Renishaw’s RenAM 500Q. This was then coupled with a Discrete Phase Model (DPM) to track the ejection of spatter and vapour from the melt pool through the chamber. This has led to a better understanding of the removal of ‘ejecta’, leading to increased mechanical properties and lower rates of build failure.Mechanical Engineerin

Controlling Thermal Expansion with Lattice Structures Using Laser Powder Bed Fusion

Tuning the Co-efficient of Thermal Expansion (CTE) of a component is traditionally limited by material choice. Laser Powder Bed Fusion (LPBF) enables the designer to create complex geometries including lattice structures. When combined with a secondary material, these metallic lattice structures can be designed to exhibit different CTE’s whilst retaining stiffness. This allows the designer the freedom to adjust the CTE by changing CAD variables such as lattice angle, and member thicknesses. This paper aims to develop an arrangement for CTE matched components for high precision optical systems. Development pursued using a Static Thermo-Structural Finite Element Analysis model to determine the best arrangements for the required CTE change. The results are incorporated into a new design prototype of a full cylindrical lens system in metal on a Laser Powder Bed Fusion machine.Mechanical Engineerin

Computational Modelling and Experimental Validation of Single In625 Line Tracks in Laser Powder Bed Fusion

Laser track experiments are performed using INCONEL® nickel-based powder alloy, IN625, in a Powder Bed Fusion (PBF) system. Optical microscopy is used to obtain key track dimensions and morphology for various machine parameters, allowing direct validation of ESI Group’s ICME suite of tools for modelling AM. The high-fidelity powder bed model simulates the melt pool formation based on solution of the Navier-Stokes equations and heat transfer, radiative powder-laser interaction, phase change, surface tension, Marangoni forces and recoil pressure. Models are enhanced by measured thermophysical material properties. Validation of the solidified melt geometry showing that conductive mode melting and instabilities such as balling can be captured with existing models and pave the way for models which capture the onset of keyholing. Examination of the melt track microstructures can also be used to determine local cooling rates, granting insight into the phase evolution differences between the alloys.Mechanical Engineerin

Controlling Grain Evolution of IN625 Parts Produced by LPBF-AM

Laser Powder Bed Fusion (LPBF) Additive Manufacturing (AM) is rapidly being adopted globally due to its capability of producing complex net-shape parts in a range of alloys with mechanical properties as good as, or better than conventional processes. The alluring possibility is in controlling microstructural features during processing such as grain size, solidification morphology and texture, giving mechanical properties tailored for intended applications. LPBF-AM microstructures are dominated by sizeable columnar growth, which along with even small levels porosity contribute to lower fatigue and creep strength in comparison to wrought. This would limit high temperature applications where IN625 is used in aeroengine exhaust sections. In contrast, the less dominating equiaxed grains lead to a reduction in crack propagation and improve fatigue performance at the surface. In this work a combination of physical experiments and modelling is used to study the controllability of grain growth and orientation of IN625 made by LPBF-AM.Mechanical Engineerin

A Study into the Effects of Gas Flow Inlet Design of the Renishaw AM250 Laser Powder Bed Fusion Machine Using Computational Modeling

Previous work has highlighted the importance of the gas flow system in laser powder bed fusion (L-PBF) processes. Inhomogeneous gas flow experienced at the surface of the powder bed can cause variations in mechanical properties over a build platform, where insufficient removal of by-products which cause laser attenuation and redisposition of byproducts are believed to contribute to these variations. The current study analyses the gas flow experienced over a build platform in a Renishaw AM250 metal powder bed fusion machine via Hot Wire Anemometer (HWA) testing. Velocity profiles of the flow directly above the powderbed and through the centre plane normal to the inlets have been categorized. These HWA results illustrate the inhomogeneity of the gas flow experienced over the build platform and from literature imply that there will be insufficient removal of by-products and hence variable build quality in specific areas of the build platform. A Computational Fluid Dynamics (CFD) model was created in ANSYS Fluent and validated against HWA results coupled with a Discrete Phase Model (DPM) representing the expulsion of spatter. Velocity contours of simulated against experimental are compared, where the results appear in good agreement. The multiphase CFD model was then used to explore the effects of changing inlet design parameters using a Design of Experiments (DOE) study based on an Optimal Space Filling (OSF) method. This was to understand the effect of design parameters on flow uniformity, local gas velocity over the processing area and spatter particulate accumulation within the build chamber. The initial design study found that flow uniformity could potentially be increased by 21.05% and spatter accumulation on the processing area could be reduced by 26.64%. In addition, this has given insight into important design considerations for future generation of LPBF machines.Mechanical Engineerin

In-situ Modification of a High Entropy Alloy with 2.4% Molybdenum Using LPBF, and Its Effect on Microstructure and Corrosion Resistance

Laser powder bed fusion (LPBF) components display higher porosity compared to parts made by conventional processes and these pores act as preferential initiation sites for pitting corrosion to occur. In stainless steels such as 316L, molybdenum is 3.5X more effective at enhancing the pitting resistance than chromium, without adding unwanted nitrides to the alloy. In this work, the effect on corrosion resistance is reported for an Al-Cr-Mn-Ni-Fe high entropy alloy (HEA) gas atomised specifically for LPBF, as well as the effects of modifying the alloy by blending the HEA with molybdenum. In-situ LPBF processing, even for low levels of additions has made the comparison difficult, as the pitting resistance is so strongly linked to the porosity, which is higher in the in-situ process. Pitting resistance for both the original HEA and the doped HEA will be compared between samples processed by casting and by LPBF.Mechanical Engineerin

Structural investigation of the stability in temperature of some high entropy / multi major components alloys as a function of their electronic structure

High Entropy Alloys (HEA) can be classified in three domains according to their e/a and r values, with e/a, the number of itinerant valence electrons and r the average radius for a 12 nearest atoms neighborhood. The phase composition, thermal stability and possible phase transformations of a series of HEA alloys, CoCrzFeNi-XY (with X and Y = Al, Cu, Pd, Ru, Ti and z = 0 or 1), selected according to their e/a ratio were investigated in cast conditions (T0), after 3 h homogenization at 1100 °C (T1) and after 3 h annealing at 700 °C (T3). When observing the behavior of the different Domains of HEAs as classified by electronic structure it is observed that for the alloys from Domain I which contain fcc structures, the microstructure transforms from multi-to almost single-phase under homogenization (T1). In Domain III alloys containing cubic (bcc and/or B2) structures, very small multi-structural changes are observed. Alloys in Domain II have a mixed structure, i.e. several different structures in the diffraction pattern, which changes during heat treatments