50 Years of Image Analysis

Welding Metallurgy of

Additive manufacturing, often known as three-dimensional (3D) printing, is a process in which a part is built layer-by-layer and is a promising approach for creating components close to their final (net) shape. This process is challenging the dominance of conventional manufacturing processes for products with high complexity and low material waste1. Titanium alloys made by additive manufacturing have been used in applications in various industries. However, the intrinsic high cooling rates and high thermal gradient of the fusion-based metal additive manufacturing process often leads to a very fine microstructure and a tendency towards almost exclusively columnar grains, particularly in titanium-based alloys1. (Columnar grains in additively manufactured titanium components can result in anisotropic mechanical properties and are therefore undesirable2.) Attempts to optimize the processing parameters of additive manufacturing have shown that it is difficult to alter the conditions to promote equiaxed growth of titanium grains3. In contrast with other common engineering alloys such as aluminium, there is no commercial grain refiner for titanium that is able to effectively refine the microstructure. To address this challenge, here we report on the development of titanium-copper alloys that have a high constitutional supercooling capacity as a result of partitioning of the alloying element during solidification, which can override the negative effect of a high thermal gradient in the laser-melted region during additive manufacturing. Without any special process control or additional treatment, our as-printed titanium-copper alloy specimens have a fully equiaxed fine-grained microstructure. They also display promising mechanical properties, such as high yield strength and uniform elongation, compared to conventional alloys under similar processing conditions, owing to the formation of an ultrafine eutectoid microstructure that appears as a result of exploiting the high cooling rates and multiple thermal cycles of the manufacturing process. We anticipate that this approach will be applicable to other eutectoid-forming alloy systems, and that it will have applications in the aerospace and biomedical industries.

/pdf/additive-manufacturing-of-ultrafine-grained-high-strength-5boc0pisnn.pdf

Additive manufacturing of ultrafine-grained high-strength titanium alloys

Additive manufacturing (AM), also known as 3D printing or rapid prototyping, is gaining increasing attention due to its ability to produce parts with added functionality and increased complexities in geometrical design, on top of the fact that it is theoretically possible to produce any shape without limitations. However, most of the research on additive manufacturing techniques are focused on the development of materials/process parameters/products design with different additive manufacturing processes such as selective laser melting, electron beam melting, or binder jetting. However, we do not have any guidelines that discuss the selection of the most suitable additive manufacturing process, depending on the material to be processed, the complexity of the parts to be produced, or the design considerations. Considering the very fact that no reports deal with this process selection, the present manuscript aims to discuss the different selection criteria that are to be considered, in order to select the best AM process (binder jetting/selective laser melting/electron beam melting) for fabricating a specific component with a defined set of material properties.

Additive Manufacturing Processes: Selective Laser Melting, Electron Beam Melting and Binder Jetting-Selection Guidelines.

Wire Arc Additive Manufacturing (WAAM) is attracting significant attention in industry and academia due to its ability to capture the benefits of additive manufacturing for production of large components of medium geometric complexity. Uniquely, WAAM combines the use of wire and electric arc as a fusion source to build components in a layer-by-layer approach, both of which can offer significant cost savings compared to powder and alternative fusion sources, such as laser and electron beam, respectively. Meanwhile, a high deposition rate, key for producing such components, is provided, whilst also allowing significant material savings compared to conventional manufacturing processes. However, high quality production in a wide range of materials is limited by the elevated levels of heat input which causes a number of materials processing challenges in WAAM. The materials processing challenges are fully identified in this paper to include the development of high residual stresses, undesirable microstructures, and solute segregation and phase transformations at solidification. The thermal profile during the build poses another challenge leading to heterogeneous and anisotropic material properties. This paper outlines how the materials processing challenges may be addressed in WAAM by implementation of quality improving ancillary processes. The primary WAAM process selections and ancillary processes are classified by the authors and a comprehensive review of their application conducted. Strategies by which the ancillary processes can enhance the quality of WAAM parts are presented. The efficacy and suitability of these strategies for versatile and cost effective WAAM production are discussed and a future vision of WAAM process developments provided.

https://purehost.bath.ac.uk/ws/files/183236429/StrategiesAndProcessesForHighQualityWireArcAdditiveManufacturing_Manuscript.pdf

Invited review article: Strategies and processes for high quality wire arc additive manufacturing

In situ tailoring microstructure in additively manufactured Ti-6Al-4V for superior mechanical performance

Additive manufacturing (AM) of metals, also known as metal 3D printing, typically leads to the formation of columnar grain structures along the build direction in most as-built metals and alloys. These long columnar grains can cause property anisotropy, which is usually detrimental to component qualification or targeted applications. Here, without changing alloy chemistry, we demonstrate an AM solidification-control solution to printing metallic alloys with an equiaxed grain structure and improved mechanical properties. Using the titanium alloy Ti-6Al-4V as a model alloy, we employ high-intensity ultrasound to achieve full transition from columnar grains to fine (~100 µm) equiaxed grains in AM Ti-6Al-4V samples by laser powder deposition. This results in a 12% improvement in both the yield stress and tensile strength compared with the conventional AM columnar Ti-6Al-4V. We further demonstrate the generality of our technique by achieving similar grain structure control results in the nickel-based superalloy Inconel 625, and expect that this method may be applicable to other metallic materials that exhibit columnar grain structures during AM. 3D printing of metals produces elongated columnar grains which are usually detrimental to component performance. Here, the authors combine ultrasound and 3D printing to promote equiaxed and refined microstructures in a titanium alloy and a nickel-based superalloy resulting in improved mechanical properties.

/pdf/grain-structure-control-during-metal-3d-printing-by-high-23r8jaxm4i.pdf

Grain structure control during metal 3D printing by high-intensity ultrasound.

Recent progress has shown that Ti–6Al–4V fabricated by selective laser melting (SLM) can achieve a fully lamellar α + β microstructure using 60 µm layer thickness in the as-built state via in situ martensite decomposition by manipulating the processing parameters. The potential to broaden the processing window was explored in this study by increasing the layer thickness to the less commonly used 90 µm. Fully lamellar α + β microstructures were produced in the as-built state using inter-layer times in the range of 1–12 s. Microstructural features such as the α-lath thickness and morphology were sensitive to both build height and inter-layer time. The α-laths produced using the inter-layer time of 1 s were much coarser than those produced with the inter-layer time of 12 s. The fine fully lamellar α + β structure resulted in tensile ductility of 11% and yield strength of 980 MPa. The tensile properties can be further improved by minimizing the presence of process-induced defects.

/pdf/new-development-in-selective-laser-melting-of-ti-6al-4v-a-46geffkgfl.pdf

New Development in Selective Laser Melting of Ti–6Al–4V: A Wider Processing Window for the Achievement of Fully Lamellar α + β Microstructures

https://espace.library.uq.edu.au/view/UQ:408229/UQ408229_OA.pdf

Effects of chip conditions on the solid state recycling of Ti-6Al-4V machining chips

Ti-6Al-4V machining chips were recycled using equal channel angular pressing. The as-recycled material was fully dense and well bonded, but contained chip boundaries decorated by entrapped surface oxide, giving rise to brittleness in tensile loading. Annealing at high temperatures was effective in removing the oxide. The times required for completely dissolving the oxide layers were calculated using models based on oxygen diffusion in α- and β-Ti, respectively. It is shown that the oxide dissolution is rapid, taking from several minutes to less than one second at temperatures between 973 K and 1323 K (700 °C and 1050 °C) for thicknesses of up to 1 μm. In addition, bands of grains finer than those in the matrix occurred in the vicinity of the prior chip boundaries, caused by the enhanced level of oxygen diffusing away from the dissolving oxide which hindered local grain growth. It would take hours of annealing to homogenize the grain size and composition. The as-recycled material was subjected to conventional mill-annealing, leading to a finer microstructure with superior yield strength (~1150 MPa) and equivalent tensile ductility (~25 pct), compared to a commercial mill-annealed Ti-6Al-4V.

E. W. Lui

Papers

In situ tailoring microstructure in additively manufactured Ti-6Al-4V for superior mechanical performance

Grain structure control during metal 3D printing by high-intensity ultrasound.

New Development in Selective Laser Melting of Ti–6Al–4V: A Wider Processing Window for the Achievement of Fully Lamellar α + β Microstructures

Effects of chip conditions on the solid state recycling of Ti-6Al-4V machining chips

Achieving Superior Strength and Ductility in Ti-6Al-4V Recycled from Machining Chips by Equal Channel Angular Pressing