Dependence of Microstructure and Mechanical Properties on Heat Treat Cycles of Electron Beam Melted Ti-6Al-4V

The EBM Ti-6Al-4V alloy has generally superior mechanical properties, owing to finely spaced α−β laths which give a good combination of strength and ductility. The grain structures in the as-printed structures are long columnar which can give rise to anisotropic mechanical properties. Moreover the non-uniformity in microstructure can also arise from part geometry where the thin features have propensity to form martensite phase. Heat treatment provides a viable solution to modify the microstructure and to tailor to the properties as desired. A wide range of heat treatment experiments were performed, followed by microstructure and tensile property analyses. It was observed that the microstructure and the tensile properties significantly changed depending on the heat treat cycle performed. Tensile properties of solution treated air-cool plus aged samples yielded globular equiaxed grains with fine α−β lath structure, which were found to be the best among the different heat treated samples and better than ASTM F1472 specifications.Mechanical Engineerin

Creative destruction in science

Drawing on the concept of a gale of creative destruction in a capitalistic economy, we argue that initiatives to assess the robustness of findings in the organizational literature should aim to simultaneously test competing ideas operating in the same theoretical space. In other words, replication efforts should seek not just to support or question the original findings, but also to replace them with revised, stronger theories with greater explanatory power. Achieving this will typically require adding new measures, conditions, and subject populations to research designs, in order to carry out conceptual tests of multiple theories in addition to directly replicating the original findings. To illustrate the value of the creative destruction approach for theory pruning in organizational scholarship, we describe recent replication initiatives re-examining culture and work morality, working parents\u2019 reasoning about day care options, and gender discrimination in hiring decisions. Significance statement It is becoming increasingly clear that many, if not most, published research findings across scientific fields are not readily replicable when the same method is repeated. Although extremely valuable, failed replications risk leaving a theoretical void\u2014 reducing confidence the original theoretical prediction is true, but not replacing it with positive evidence in favor of an alternative theory. We introduce the creative destruction approach to replication, which combines theory pruning methods from the field of management with emerging best practices from the open science movement, with the aim of making replications as generative as possible. In effect, we advocate for a Replication 2.0 movement in which the goal shifts from checking on the reliability of past findings to actively engaging in competitive theory testing and theory building. Scientific transparency statement The materials, code, and data for this article are posted publicly on the Open Science Framework, with links provided in the article

Adsorption refrigeration system based on the silica gel-water pair

Adsorption refrigeration system is a system that researchers and scientists are looking into to find a sustainable solution to refrigeration. The system uses environmentally friendly refrigerants and requires lesser electrical energy than the conventional systems. In this project, the theory and methods of refrigeration are discussed in order to further understand the experimental system. A literature review was conducted to study what scientists and researchers had found out through their experiments and understand the advantages and disadvantages of the system. The project will also look into the previous final year project reports to study the modifications and performance observations of the experimental system. Minor modifications were done to the existing experimental system while the main scope of the project was to conduct experimental investigations on the existing set-up to study its drawbacks in order to find solutions to improve the system. Various sets of experiments using a variety of parameters were conducted in order to understand the performance of the system. The performance indicators obtained and calculated from the experimental results had shown that the system is working though minimal cooling was achieved. Finally, a set of modifications had been listed in order to improve the system and rectify the drawbacks present in the system in the future. In addition, recommendations were made to conclude the project and give a general direction for future continuation of the project.Bachelor of Engineering (Mechanical Engineering

Effect of Building Height on Microstructure and Mechanical Properties of Big-Sized Ti-6Al-4V Plate Fabricated by Electron Beam Melting

Electron beam melting (EBM) is a layer by layer additive manufacturing technology, which has the capability of producing near-net shaped parts with complex geometries. It is also suitable for handling high melting point and reactive metallic materials, such as Ti alloy, which is widely used in the aerospace and biomedical applications. The present study focused on the relationship between the microstructure and mechanical properties of big-sized Ti-6Al-4V parts. A plate (6mm×180mm×372mm) was additively manufactured by EBM. The microstructure evolution and variation of mechanical properties were investigated by using the x-ray diffraction, optical microscope, scanning electron microscope and tensile test. The results revealed that with an increasing in the build height, there was a variation in the microstructure and the mechanical properties of the build plate. Although only α phase and a relatively small fraction of β phase were detected in both the bottom and top specimens of the build plate, yield strength and ultimate tensile strength decreased with an increase of build height. This was attributed to the increase of α lath width which was caused by the different thermal histories along the build height of the plate

Effects of Processing Parameters on Surface Roughness of Additive Manufactured Ti-6Al-4V via Electron Beam Melting

As one of the powder bed fusion additive manufacturing technologies, electron beam melting (EBM) is gaining more and more attention due to its near-net-shape production capacity with low residual stress and good mechanical properties. These characteristics also allow EBM built parts to be used as produced without post-processing. However, the as-built rough surface introduces a detrimental influence on the mechanical properties of metallic alloys. Thereafter, understanding the effects of processing parameters on the part’s surface roughness, in turn, becomes critical. This paper has focused on varying the processing parameters of two types of contouring scanning strategies namely, multispot and non-multispot, in EBM. The results suggest that the beam current and speed function are the most significant processing parameters for non-multispot contouring scanning strategy. While for multispot contouring scanning strategy, the number of spots, spot time, and spot overlap have greater effects than focus offset and beam current. The improved surface roughness has been obtained in both contouring scanning strategies. Furthermore, non-multispot contouring scanning strategy gives a lower surface roughness value and poorer geometrical accuracy than the multispot counterpart under the optimized conditions. These findings could be used as a guideline for selecting the contouring type used for specific industrial parts that are built using EBM

Effect of Building Height on Microstructure and Mechanical Properties of Big-Sized Ti-6Al-4V Plate Fabricated by Electron Beam Melting

Electron beam melting (EBM) is a layer by layer additive manufacturing technology, which has the capability of producing near-net shaped parts with complex geometries. It is also suitable for handling high melting point and reactive metallic materials, such as Ti alloy, which is widely used in the aerospace and biomedical applications. The present study focused on the relationship between the microstructure and mechanical properties of big-sized Ti-6Al-4V parts. A plate (6mm×180mm×372mm) was additively manufactured by EBM. The microstructure evolution and variation of mechanical properties were investigated by using the x-ray diffraction, optical microscope, scanning electron microscope and tensile test. The results revealed that with an increasing in the build height, there was a variation in the microstructure and the mechanical properties of the build plate. Although only α phase and a relatively small fraction of β phase were detected in both the bottom and top specimens of the build plate, yield strength and ultimate tensile strength decreased with an increase of build height. This was attributed to the increase of α lath width which was caused by the different thermal histories along the build height of the plate

Compressive properties of electron beam melted lattice structures with density gradient

Lattice structures are used in many applications such as lightweight design, energy absorbers and medical implants. Incorporating a density gradient in the design of lattice structures provides distinctive properties compared to designs with uniform density. In this study, density graded lattice structures of four different architectures were fabricated by electron beam melting technique with Ti-6Al-4V as building material. The samples were tested for compressive properties in comparison to their counterparts with uniform density. Under quasi-static uniaxial loading conditions, density graded samples exhibited more predictable deformation behavior and higher energy absorption than samples with uniform density. Observation with scanning electron microscopy showed that the fracture surfaces of the compressed density graded samples changed across the structure according to strut diameter. Finite element simulation was also conducted to compare the structural stiffness and to identify locations of highest stresses of the different lattice designs during deformation, and the results were compared with the deformation behavior observed from experiments. The distinctive properties of density graded lattice designs demonstrated in this study encourage further research to achieve advanced and tailored functionality.ASTAR (Agency for Sci., Tech. and Research, S’pore)Published versio

Superior energy absorption of continuously graded microlattices by electron beam additive manufacturing

Electron beam melted (EBM) Ti-6Al-4V functionally graded materials (FGM) with continuously graded densities are investigated for dimensional accuracy, compressive properties, fractography and build direction effect in comparison to uniform density counterparts of the same volume. It is found that FGMs exhibit progressive layer-by-layer deformation mode regardless of unit cell designs and build direction. This deformation behavior is highly favourable for uni-directional impact absorption applications compared to uniform density counterparts with random or diagonal failure. Overall, the EBM-built FGM exhibits superior energy absorption than counterparts of uniform density. Significant improvement in the quasi-elastic gradient and energy absorption is obtained by changing the build direction for specific designs. Compared with other FGM or uniform density lattice structures from the literature, the energy absorption of lattice structures with lower relative density could outperform those with higher relative density by changing the unit cell design or density profile

Mechanical Properties and Microstructures of As Printed and Heat Treated Samples of Selective Laser Melted IN625 Alloy Powder

The current study focusses in evaluating the mechanical properties (yield strength, ultimate tensile strength and elongation %) and microstructures of as printed and heat treated tensile samples produced by Selective Laser Melting (SLM) which is an Additive Manufacturing (AM) technique. The as printed IN625 alloy exhibited good mechanical properties as compared to cast and wrought alloy, owing to fast cooling rates that give very fine dendritic/cellular structures during the SLM process. Standard solutionizing and precipitation hardening treatment was given to the SLM as printed samples. The as printed sample’s ductility increased from 30% elongation to 47% elongation with small drop in tensile strength from 878MPa to 836MPa. The increase of ductility after heat treatment was evident in the change of fracture morphology from long straight striations with dimple shape ductile like features in the as printed sample to mainly dimple shaped ductile-like fracture surface in the heat treated sample as observed in the FESEM

Additively manufactured CoCrFeNiMn high-entropy alloy via pre-alloyed powder

An equiatomic CoCrFeNiMn high-entropy alloy (HEA) powder is produced by vacuum induction melting gas-atomization with argon gas. A detailed characterization reveals that gas atomized HEA powder is suitable for powder-bed-fusion additive manufacturing (PBFAM). Therefore, the powder with size ranging from 45 to 106 μm is applied in one of PBFAM, electron beam melting (EBM). The final EBM part is strongly affected by the feedstock. The entrapped gas pores result in a high porosity of EBM-built parts, requesting extra efforts to eliminate the pores for processing parameters optimization. After the optimization, the highly dense EBM-built parts with a hierarchical microstructure and a strong 〈100〉 texture are obtained. The hierarchical microstructure displays long columnar grains containing intragranular cellular structure (dendrites) along build direction. Deformation mechanism of the EBM-built CoCrFeNiMn HEA is dominated by dislocation with limited contribution from mechanical twinning. Experimental results reveal that the cellular structure (dendrites) does not contribute the strength without the pre-existing surrounding dislocation network. As a result, EBM-built CoCrFeNiMn HEA parts exhibit comparable tensile properties to their conventional cast counterparts. It is suggested that the EBM process and gas atomized HEA powder can be employed for fabricating single-phase HEA parts with complex geometry and good mechanical properties.ASTAR (Agency for Sci., Tech. and Research, S’pore)Published versio