Development of Advanced Nanocomposite for Micro/Nanosystems Packaging

Ph.DDOCTOR OF PHILOSOPH

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

Effect of Interface Wettability on Additively Manufactured Metal Matrix Composites: A Case Study of 316L-Y₂O₃ Oxide Dispersion-Strengthened Steel

Laser powder bed fusion (LPBF) is a fusion-based additive manufacturing process. It has the advantage of allowing the manufacturing of metal matrix composites. This advantage arises from its small melting zone and rapid cooling rate, which minimize the risk of reinforcement segregation. In this work, 0.3 wt% and 1.0 wt% Y2O3 nanoparticles were added to 316L to fabricate oxide dispersion-strengthened (ODS) steels using the LPBF process. Notably, Y2O3 agglomerates were identified in the LPBF-fabricated 316L ODS steels, without inducing grain refinement, while the impact on tensile strength of Y2O3 addition proved negligible. Tensile elongation was decreased due to the poor bonding of the Y2O3 agglomerations to the matrix. The crucial role of the wettability of the reinforcement and the matrix in facilitating grain refinement and strength enhancement is discussed. The poor wettability of the Y2O3 particles and 316L emerged as the primary cause for Y2O3 agglomeration. This finding highlights the importance of addressing wettability issues to optimize the manufacturing process and enhance the overall performance of LPBF-fabricated metal matrix composites

Mitigation of liquation cracking in selective laser melted Inconel 718 through optimization of layer thickness and laser energy density

Conventionally, the main goal of process optimization in selective laser melting is to achieve the highest relative density. However, for Inconel 718, this study has demonstrated that the highest relative density does not correspond to the best mechanical properties. Moreover, similar relative densities can result in significant differences in mechanical properties. This phenomenon arises from the presence of cracks in the microstructures. The research was carried out to study the problem systematically using combinations of 2 layer thicknesses (40 and 50 μm) and 2 laser energy densities (3.17 and 3.47 J/mm2). Microcracks were observed near the melt pool boundaries and within the heat-affected zones of the newly deposited layer, occurring along the grain boundaries and interdendritic regions. Evidence was obtained to show that the cracking was associated with remelting of Laves phase; therefore, it was identified as liquation cracking. It is interesting to observe that layer thickness has a much greater influence on crack formation than laser energy density, owing to the significant change in the melt pool shape and grain boundary morphology when the layer thickness was changed. The influence of laser energy density was only observed at the larger layer thickness as the severity of cracking was amplified when laser energy density was increased due to microstructural coarsening. Thus, this presents a unique problem in additive manufacturing (AM) regarding liquation cracking in Inconel 718 as one of the key differences from conventional manufacturing is the successive heating and reheating of multiple layers of material in AM.Agency for Science, Technology and Research (A*STAR)Nanyang Technological UniversityThe work was supported by the A*STAR Additive Manufacturing Centre (AMC) Initiative: Work Package I (High Temperature Materials Development for 3D Additive Manufacturing) with project No. 1426800088, and supported by the Nanyang Technological University

Preliminary Study on Nano Particle/Photopolymer Hybrid for 3D Inkjet Printing

3D inkjet printing is one of the new generation Additive Manufacturing technologies for the production of multi-material and multifunctional 3D products. Due to the critical condition for ink deposition and the stringent requirements for ink solidification, only limited ranges of materials are suitable for 3D inkjet printing. These include low viscosity photo curable polymers, and low melting temperature materials like wax. In this study, a hybrid printing ink prepared by mixing nano silica particles into photopolymer resin was developed for 3D printing, and the influence of silica particle concentrations on the viscosity, surface tension, printability and mechanical properties of cured polymer was studied. Results show that untreated nano silica particles would influence the rheological properties which is critical to the printability, but did not improve the mechanical properties due to the aggregation of nanoparticles during photo curing.ASTAR (Agency for Sci., Tech. and Research, S’pore)Published versio

Characterization of carbide particle-reinforced 316L stainless steel fabricated by selective laser melting

In this study, we have fabricated a TiC particle strengthened 316L stainless steel metal matrix composite using selective laser melting and characterized the microstructure with a particular focus on the TiC carbides in terms of their crystallography and orientation relationship with the austenitic matrix. Two families of TiC carbides are found to form in the SLM fabricated microstructure – firstly, the carbides that form along the high angle grain boundaries and secondly, those that form in the grain interior. The latter is primarily nano-crystalline TiC that are believed to precipitate out from the melt pool following a cube-on-cube orientation relationship with the f.c.c. austenitic matrix. Under this crystal registry, due to the differences in lattice parameters, a lattice mismatch between two phases occurs. The formation of TiC carbides and their morphology and distribution have been explained on the basis of melting of the powder bed under laser beam and strong melt pool dynamics during SLM process. Finally, the contribution of coherent nano-sized TiC precipitates on the overall strength of SLMed 316L-TiC metal matrix composite has been discussed wherein the TiC precipitates were found to contribute to ~50% of the strength increment in comparison to SLMed pure 316L

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

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 defects and specimen size with rectangular cross-section on the tensile properties of additively manufactured components

To evaluate specimen size dependence on the tensile properties of additively manufactured (AM) components, various rectangular specimens, ranging from miniaturised to ASTM standard specimens, are machined from electron beam melted Ti-6Al-4V and used for the tensile testing. It is found that the elongation is strongly related to the sample size while the yield and ultimate tensile strengths exhibit an independent feature. Three major aspects, (i) presented location of lack of fusion, (ii) size and segregation of pores, and (iii) slimness ratio, have a synergic influence on the elongation of different specimen sizes with various cross-section area. Our findings suggest that microscale tests arise uncertainties in measurement, which must be considered in order to provide quantifiable levels of confidence when applying such tests to discriminate a material’s behaviour. The experimental results and analyses provide a guideline for the design and testing of non-standard specimens for AM components