Mechanical Engineering: The Selective Laser Melting of Metals and In-situ Aluminium Matrix Composites

Selective laser melting (SLM) is an additive manufacturing technique to produce complex three-dimensional parts through solidifying successive layers of powder materials on top of each other, from the bottom to top. The powder base nature allows the SLM to process a wide variety of materials and their mixtures and fabricate advanced and complicated composite parts. However, the SLM is a newly established process and seeks detailed scientific studies to develop new materials systems for the consumption of industry. These scientific studies are particularly important because of many issues associated with the SLM process, such as porosity, balling, delamination, thermal stress, etc, which can be varied from one material system to another. This PhD project aims to elucidate the fundamental mechanisms governing the microstructure and mechanical properties of the metallic and in-situ Al matrix composite parts made by SLM. The research starts with a preliminary study on SLM of stainless steel in order to explore the usage of SLM machine and related parameters. It illustrates the effect of part layout on the quality of products. The main research focuses on the in-situ formation of particulate reinforced Al matrix by using SLM of Al/Fe2O3 powder mixture. It is a pioneering research to integrate in-situ interaction with laser melting to produce advanced Al composites. It investigates the mechanisms governing SLM assisted in-situ reaction and also the effects of various parameters such as SLM layer thickness, laser power and scanning speed as well as the proportion of Fe2O3. It examines the influence of Al alloy powder and it describes the effect of hot isostatic pressing (HIP) post-treatment. The physical, mechanical, and metallurgical properties of the products are extensively assessed using various techniques. The processing windows of the process are sketched. The findings demonstrate unique microstructural features due to combined in-situ reaction and laser rapid consolidation, and contribute to provision of an in-depth scientific understanding of novel Al matrix composites by using SLM assisted in-situ processes. As part of this PhD project, industrial collaborative research has also been conducted to characterise the surface finish, metallurgical quality, process accuracy and mechanical properties of various SLM made metallic parts using Al, Ti, stainless steel, and super alloys. This part of research has generated scientific data and results for industrial applications of metallic fabrication using SLM

Effect of Layer Thickness in Selective Laser Melting on Microstructure of Al/5 wt.%Fe 2

In situ reaction was activated in the powder mixture of Al/5 wt.%Fe2O3 by using selective laser melting (SLM) to directly fabricate aluminium metal matrix composite parts. The microstructural characteristics of these in situ consolidated parts through SLM were investigated under the influence of thick powder bed, 75 μm layer thickness, and 50 μm layer thickness in various laser powers and scanning speeds. It was found that the layer thickness has a strong influence on microstructural outcome, mainly attributed to its impact on oxygen content of the matrix. Various microstructural features (such as granular, coralline-like, and particulate appearance) were observed depending on the layer thickness, laser power, and scanning speed. This was associated with various material combinations such as pure Al, Al-Fe intermetallics, and Al(-Fe) oxide phases formed after in situ reaction and laser rapid solidification. Uniformly distributed very fine particles could be consolidated in net-shape Al composite parts by using lower layer thickness, higher laser power, and lower scanning speed. The findings contribute to the new development of advanced net-shape manufacture of Al composites by combining SLM and in situ reaction process

In Situ Formation of Particle Reinforced Al Matrix Composite by Selective Laser Melting of Al/Fe2O3 Powder Mixture

This work presents a novel in situ reaction approach to produce Al matrix composites from a powder mixture of Al/5a wt% Fe 2O 3 by using selective laser melting (SLM). It is found that the SLM process not only is able to produce three-dimensional parts, but also is capable of activating an in situ reaction in the powder mixture, producing particles mainly from alumina (Al 2O 3) and iron combinations (such as Fe 2+Al 2O 4) in Al matrix. These particles (as reinforcements) can be distributed uniformly with a good particle/matrix interface under controlled conditions. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.status: publishe

Mechanical engineering : the selective laser melting of metals and in-situ aluminium matrix composites

Selective laser melting (SLM) is an additive manufacturing technique to produce complex three-dimensional parts through solidifying successive layers of powder materials on top of each other, from the bottom to top. The powder base nature allows the SLM to process a wide variety of materials and their mixtures and fabricate advanced and complicated composite parts. However, the SLM is a newly established process and seeks detailed scientific studies to develop new materials systems for the consumption of industry. These scientific studies are particularly important because of many issues associated with the SLM process, such as porosity, balling, delamination, thermal stress, etc, which can be varied from one material system to another. This PhD project aims to elucidate the fundamental mechanisms governing the microstructure and mechanical properties of the metallic and in-situ Al matrix composite parts made by SLM. The research starts with a preliminary study on SLM of stainless steel in order to explore the usage of SLM machine and related parameters. It illustrates the effect of part layout on the quality of products. The main research focuses on the in-situ formation of particulate reinforced Al matrix by using SLM of Al/Fe2O3 powder mixture. It is a pioneering research to integrate in-situ interaction with laser melting to produce advanced Al composites. It investigates the mechanisms governing SLM assisted in-situ reaction and also the effects of various parameters such as SLM layer thickness, laser power and scanning speed as well as the proportion of Fe2O3. It examines the influence of Al alloy powder and it describes the effect of hot isostatic pressing (HIP) post-treatment. The physical, mechanical, and metallurgical properties of the products are extensively assessed using various techniques. The processing windows of the process are sketched. The findings demonstrate unique microstructural features due to combined in-situ reaction and laser rapid consolidation, and contribute to provision of an in-depth scientific understanding of novel Al matrix composites by using SLM assisted in-situ processes. As part of this PhD project, industrial collaborative research has also been conducted to characterise the surface finish, metallurgical quality, process accuracy and mechanical properties of various SLM made metallic parts using Al, Ti, stainless steel, and super alloys. This part of research has generated scientific data and results for industrial applications of metallic fabrication using SLM.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Contouring strategies to improve the tensile properties and quality of EBM printed Inconel 625 parts

This work systematically analyzes the influence of rough surfaces and porous subsurfaces in electron beam melting (EBM) printed components. Consequently, it applies various contouring strategies to improve the tensile properties of EBM printed Inconel 625 alloy parts. It is shown that no contouring (i.e., only hatching) creates a rough surface with numerous surface voids (as translated to surface notches). Although the commercially used multi-spot contouring can smoothen the surface to some extent (∼34 %), it fails to create a defect-free superficial region by leaving ∼25 % surface voids (translated to large surface notches) and ∼4 % subsurface porosity. These superficial defects form due to an interrupted shrinkage, occurring on the surface and in the contouring region. In contrast, optimal post-hatching high energy continuous contouring creates a thick and consistent post-hatching track that can successfully reconsolidate surface voids remaining from the hatching step. In comparison with the multi-spot contouring, this reduces the surface and subsurface porosity down to ∼10 % and ∼0.4 %, respectively, and hence increases the apparent stiffness by ∼140 %, tensile strength by ∼105 % and elongation by ∼260 %. This nearly reaches the mechanical properties of the conventionally machined parts (UTS ∼635 ± 20 MPa and elongation ∼50 ± 2 %).QC 20210119</p

Influence of Contouring and depth of machining on tensile properties of Inconel 625 made by Electron Beam Melting

This research is to evaluate the manufacturability and to characterize the performance of Inconel 625 (IN625) by electron beam melting (EBM). After further modifying the commercial EBM parameters for Inconel 718, such as speed function for hatching and the line offset for both hatching and multi-beam contouring, nearly full dense samples (over 99.7% of the wrought material) are produced. It is shown that no contouring strategy generates a relatively rougher surface (approximately 29% in average) compared to the samples printed with contour, requiring even a further post-process machining. Furthermore, the microhardness after EBM is comparable to as-rolled and annealed IN625 material. The samples machined from bulk specimens exhibit good tensile properties regardless of the contouring strategy due to the high depth of machining (5.5mm). However, for the near-net shaped specimens with only 2.1 mm machining of the surfaces, elongation is significantly affected by the contouring strategy. This is in such a manner that the contoured near-net shape parts show relatively 32% less elongation compared to the samples without any contour. Accordingly, multi-beam contouring is better to be avoided to reach tensile properties comparable to the wrought material after a shallow machining, despite the fact that it can lead to a relative smoother surface finish after EBM.QC 20200204</p

Contouring strategies to improve the tensile properties and quality of EBM printed Inconel 625 parts

This work systematically analyzes the influence of rough surfaces and porous subsurfaces in electron beam melting (EBM) printed components. Consequently, it applies various contouring strategies to improve the tensile properties of EBM printed Inconel 625 alloy parts. It is shown that no contouring (i.e., only hatching) creates a rough surface with numerous surface voids (as translated to surface notches). Although the commercially used multi-spot contouring can smoothen the surface to some extent (∼34 %), it fails to create a defect-free superficial region by leaving ∼25 % surface voids (translated to large surface notches) and ∼4 % subsurface porosity. These superficial defects form due to an interrupted shrinkage, occurring on the surface and in the contouring region. In contrast, optimal post-hatching high energy continuous contouring creates a thick and consistent post-hatching track that can successfully reconsolidate surface voids remaining from the hatching step. In comparison with the multi-spot contouring, this reduces the surface and subsurface porosity down to ∼10 % and ∼0.4 %, respectively, and hence increases the apparent stiffness by ∼140 %, tensile strength by ∼105 % and elongation by ∼260 %. This nearly reaches the mechanical properties of the conventionally machined parts (UTS ∼635 ± 20 MPa and elongation ∼50 ± 2 %).QC 20210119</p

Developing processing windows for powder pre-heating in electron beam melting

Powder pre-heating is a critical step in electron beam melting (EBM), while there has been no systematic work tostudy the corresponding processing windows so far. Accordingly, this work investigates the relation between thesintering and the issues appearing during pre-heating (e.g., smoking or excessive sintering) in EBM of highlysusceptible-to-smoke Nickel-Titanium (NiTi) powder. First, the EB spot size was assessed depending on differentfocus offsets and beam currents from beam tracking experiments on a ceramic-coated stainless steel plate. Af-terwards, the smoke tests were carried out at different focus offsets and beam currents in terms of beam speeds. Itis shown that a smaller EB spot can effectively prevents smoking by enhancing the sintering degree. However,since this high sintering degree can cause strong powder bonding preventing the powder recycling, less focusedbeam (or larger EB spot) was selected to reach medium but efficient sintering in the level of around 30 %.Moreover, due to the influence of the diverging angle on the EB-material interaction, it is found that the negativedefocused EB mitigates the smoke phenomenon compared to the positive defocused EB with a similar spot size.Based on the smoke test results, linked to the sintering degree, the processing windows for pre-heating NiTipowder are developed demonstrating three different modes: smoke-heating, melting-heating and healthy-heating. QC 20220930</p

Influence of Contouring and depth of machining on tensile properties of Inconel 625 made by Electron Beam Melting

This research is to evaluate the manufacturability and to characterize the performance of Inconel 625 (IN625) by electron beam melting (EBM). After further modifying the commercial EBM parameters for Inconel 718, such as speed function for hatching and the line offset for both hatching and multi-beam contouring, nearly full dense samples (over 99.7% of the wrought material) are produced. It is shown that no contouring strategy generates a relatively rougher surface (approximately 29% in average) compared to the samples printed with contour, requiring even a further post-process machining. Furthermore, the microhardness after EBM is comparable to as-rolled and annealed IN625 material. The samples machined from bulk specimens exhibit good tensile properties regardless of the contouring strategy due to the high depth of machining (5.5mm). However, for the near-net shaped specimens with only 2.1 mm machining of the surfaces, elongation is significantly affected by the contouring strategy. This is in such a manner that the contoured near-net shape parts show relatively 32% less elongation compared to the samples without any contour. Accordingly, multi-beam contouring is better to be avoided to reach tensile properties comparable to the wrought material after a shallow machining, despite the fact that it can lead to a relative smoother surface finish after EBM.QC 20200204</p

Influence of Contouring and depth of machining on tensile properties of Inconel 625 made by Electron Beam Melting

This research is to evaluate the manufacturability and to characterize the performance of Inconel 625 (IN625) by electron beam melting (EBM). After further modifying the commercial EBM parameters for Inconel 718, such as speed function for hatching and the line offset for both hatching and multi-beam contouring, nearly full dense samples (over 99.7% of the wrought material) are produced. It is shown that no contouring strategy generates a relatively rougher surface (approximately 29% in average) compared to the samples printed with contour, requiring even a further post-process machining. Furthermore, the microhardness after EBM is comparable to as-rolled and annealed IN625 material. The samples machined from bulk specimens exhibit good tensile properties regardless of the contouring strategy due to the high depth of machining (5.5mm). However, for the near-net shaped specimens with only 2.1 mm machining of the surfaces, elongation is significantly affected by the contouring strategy. This is in such a manner that the contoured near-net shape parts show relatively 32% less elongation compared to the samples without any contour. Accordingly, multi-beam contouring is better to be avoided to reach tensile properties comparable to the wrought material after a shallow machining, despite the fact that it can lead to a relative smoother surface finish after EBM.QC 20200204</p