Showing papers by "Tarasankar Debroy published in 2021"

Metallurgy, mechanistic models and machine learning in metal printing

Abstract: Additive manufacturing enables the printing of metallic parts, such as customized implants for patients, durable single-crystal parts for use in harsh environments, and the printing of parts with site-specific chemical compositions and properties from 3D designs. However, the selection of alloys, printing processes and process variables results in an exceptional diversity of microstructures, properties and defects that affect the serviceability of the printed parts. Control of these attributes using the rich knowledge base of metallurgy remains a challenge because of the complexity of the printing process. Transforming 3D designs created in the virtual world into high-quality products in the physical world needs a new methodology not commonly used in traditional manufacturing. Rapidly developing powerful digital tools such as mechanistic models and machine learning, when combined with the knowledge base of metallurgy, have the potential to shape the future of metal printing. Starting from product design to process planning and process monitoring and control, these tools can help improve microstructure and properties, mitigate defects, automate part inspection and accelerate part qualification. Here, we examine advances in metal printing focusing on metallurgy, as well as the use of mechanistic models and machine learning and the role they play in the expansion of the additive manufacturing of metals. Several key industries routinely use metal printing to make complex parts that are difficult to produce by conventional manufacturing. Here, we show that a synergistic combination of metallurgy, mechanistic models and machine learning is driving the continued growth of metal printing.

Towards developing multiscale-multiphysics models and their surrogates for digital twins of metal additive manufacturing

Abstract: Artificial intelligence (AI) embedded within digital models of manufacturing processes can be used to improve process productivity and product quality significantly. The application of such advanced capabilities particularly to highly digitalized processes such as metal additive manufacturing (AM) is likely to make those processes commercially more attractive. AI capabilities will reside within Digital Twins (DTs) which are living virtual replicas of the physical processes. DTs will be empowered to operate autonomously in a diagnostic control capacity to supervise processes and can be interrogated by the practitioner to inform the optimal processing route for any given product. The utility of the information gained from the DTs would depend on the quality of the digital models and, more importantly, their faster-solving surrogates which dwell within DTs for consultation during rapid decision-making. In this article, we point out the exceptional value of DTs in AM and focus on the need to create high-fidelity multiscale-multiphysics models for AM processes to feed the AI capabilities. We identify technical hurdles for their development, including those arising from the multiscale and multiphysics characteristics of the models, the difficulties in linking models of the subprocesses across scales and physics, and the scarcity of experimental data. We discuss the need for creating surrogate models using machine learning approaches for real-time problem-solving. We further identify non-technical barriers, such as the need for standardization and difficulties in collaborating across different types of institutions. We offer potential solutions for all these challenges, after reflecting on and researching discussions held at an international symposium on the subject in 2019. We argue that a collaborative approach can not only help accelerate their development compared with disparate efforts, but also enhance the quality of the models by allowing modular development and linkages that account for interactions between the various sub-processes in AM. A high-level roadmap is suggested for starting such a collaboration.

Analytical estimation of fusion zone dimensions and cooling rates in part scale laser powder bed fusion

Abstract: The laser powder bed fusion process is increasingly used for the building of metallic parts by melting and solidification of alloy powders under a fast-moving finely focussed laser beam. A quick estimation of the resulting temperature field, fusion zone dimensions, and cooling rates is needed to ensure the manufacture of dimensionally accurate parts with minimum defects. A novel three-dimensional analytical heat transfer model with a volumetric heat source that can simulate the laser powder bed fusion process in part scale quickly and reliably is proposed here. The volumetric heat source term is constructed to analytically simulate the evolution of melt pools with a fair range of depth to width ratio. The proposed analytical model can simulate the building of multiple tracks and layers in part scale dimensions significantly faster than all the numerical models reported in the literature. The computed results of fusion zone shapes and sizes and cooling rates are found to be in good agreement with the experimentally reported results in builds of three commonly used alloys with diverse materials properties, SS316L, Ti6Al4V, and AlSi10Mg. Based on the analytically computed results, a set of easy-to-use process maps is presented to estimate multiple process conditions to obtain a set of target fusion zone dimensions without trial-and-error testing.

Stirring Solid Metals to Form Sound Welds

Abstract: Friction stir welding does not generate fumes and there is no loss of volatile alloying elements. This remarkable process maintains the solid state during welding and yet is able to fabricate structurally sound joints. A robust spinning-tool rubs against the abutting parts that require joining, generates heat by friction, thus softening the metals without melting it. The translation of the spinning-tool along the joint line forges the pieces together under great pressure. It is almost like locally and forcefully mixing the two sides of the joint together without changing the overall shape although the flow pattern during this mixing is asymmetrical. The lack of melting means that it is possible to join dissimilar metals or very difficult metals that cannot otherwise be joined. In spite of its relatively recent invention, the process is now ubiquitous in the welding of aluminium alloys in particular, but with many efforts to extend its horizon, for example, to the microelectronics field.

Welding: The Digital Experience

Abstract: Our understanding of welding has evolved mostly through experiments that have contributed so much to the success of the process. However, in some cases, experiments alone are not able to provide the insights needed to drive the most difficult of advanced materials-engineering. It is impossible, for example, to determine the entire flow field within a weld pool, the knowledge that can be critical in determining the ability of the pool to penetrate the joint. Well-tested mechanistic models of welding can do this quite effectively and inform on the best way forward. There are even occasions where the models can lead to entirely unexpected outcomes. A few of these concepts are illustrated here as an introduction to the power of digital intervention in welding.

Inventions that Enabled the Silicon Age

Abstract: Silicon-based electronic devices are all-pervasive, though this might have been unimaginable just a few decades ago. Their manufacture requires silicon with extreme purity, with less than one foreign atom per ten million of silicon. There are innovative processes that have enabled the mass production of such silicon. The base for discussion is the silicon that has been made since the nineteenth century for metallurgical applications, where the demands on purity are much less onerous. This is followed by the story of pure silicon crystals with minimal defects in their periodic structure, essential in the making of reliable semiconducting devices that are so remarkably affordable, so much so that there are now 56 billion transistors in existence per living person.

A Remarkable Innovation in Stainless Steel Making

Abstract: This story is about a young engineer, who when faced with conflicting data, refused to ignore the discrepancies, followed them up and as a result, created a disruptive technology that has changed the scenario for stainless steels. It shows corporate research at its best, with generic lessons for those yearning for novelty that changes established practice for the better. The experiments that led to the breakthrough, how they were scaled for manufacture, the ubiquitous adoption of the new process, and much more are described.

Dazzling Diamonds Grown from Gases

Abstract: Natural diamonds are difficult to mine. This, combined with marketing strategies, make them expensive and once polished to sparkle, they make the kind of gift that might seal a relationship between humans. However, their utility spans well beyond their dazzle. Diamonds are the hardest of natural materials, have a high thermal conductivity that if five times that of copper. There are therefore a myriad of engineering applications, such as in high-temperature electronics, cutting tool and windows that are transparent to X-rays, microwaves and infrared radiations. Many of these applications require diamonds to have large and non-planar surfaces. This limits the use of natural diamonds in some functional applications. Diamonds deposited from a vapour can overcome these difficulties through a process known as chemical vapour deposition. Industrial diamonds are now difficult to distinguish from those that are mined, without resorting to sophisticated tests. From a chemical point of view, diamond is simply another form of carbon, but a form that is not stable under ambient conditions and so can be induced to change into graphite.

Transition to Sustainable Steelmaking

Abstract: It would be hard to imagine a modern society without steel. Nevertheless, its production is a major contributor to carbon dioxide emissions that contribute to climate change. With the growing awareness of the damage that is caused by the accumulation of carbon dioxide in the atmosphere, the search for sustainable production technologies has intensified in recent decades. A few attractive concepts have emerged but are not yet ready for deployment. The development of innovative greenish technologies and their commercial viability will depend on the implementation of appropriate policies for environmental protection. Here we assess the reasons why steel production pollutes so much, the current status of sustainable technologies, and the policies that might make for a better future.

Metals That Do Not Forget

Abstract: A crystal is defined by the pattern in which the atoms within are arranged. This pattern can in the right circumstances transform into a new arrangement that has a different symmetry. One of the ways in which such a change can be achieved is by a homogeneous deformation which leaves all near-neighbour relationships intact. This represents a particular class of transformations that are known as martensitic transformations. There is no diffusion required and there is no composition change. As a result, the martensite retains a memory of the arrangement of atoms in the parent structure so a reversal of the transformation also reverses the deformation. We shall see in this chapter how this memory effect can be exploited to create devices that are life-changing and others that improve the ability to engineer improved mechanical devices.

Low-Density Steels

Abstract: Iron ordinarily has a density that in the context of aluminium, silicon, magnesium and lithium is large. It nevertheless has properties that are so superior that the quantity of iron used exceeds that of all other metals combined by a very large amount. However, there are specific engineering applications where it would be a tremendous advantage to have all the properties of iron but at a smaller density. The methods available for achieving this goal are described here. There has been substantial progress and some low-density iron alloys are now commercially available.

First Bulk Nanostructured Metal

Abstract: Sometimes, a combination of deep knowledge, serendipity and perseverance can lead to developments that set aside decades of attempts. The story of the world’s first bulk nanostructured metal is an example of this. The phase change that led to the material was, back in the 1970s, so controversial that textbooks of the time would not dare to venture opinions. The subject has calmed over a period of about half a century, so much so that the theory can be used to create materials that fire the imagination. The story is illustrated here.

Secrets of Ageless Iron Landmarks

Abstract: Metals and alloys exposed to the environment tend to react with oxygen, moisture and other gases. They therefore need to be protected to avoid degradation. However, there are several iconic structures that are over 1000 years old, have not been protected and yet, are quite intact. Recent research has revealed some of the secrets behind their unexpected endurance.

Picture to Parts, One Thin Metal Layer at a Time

Abstract: Additive manufacturing is a process of manufacturing that creates a three-dimensional object by progressively depositing thin layers of material guided by a digital drawing. The creation of metallic objects using this technology is one of the fastest growing implementations, although other materials such as concrete, ceramics and polymers are also amenable to this manufacturing process, enabling applications that might not otherwise have been possible. Stainless steels, aluminium, titanium and nickel alloys in the form of powders or wires are melted by heating with a high-energy source such as a laser beam, electron beam or an electric arc. Metal printing is now used in aerospace, consumer products, health care, energy, automotive, marine and other industries because in cases where it has advantages over conventional methods. Here we examine the key processes and principles for printing metallic parts, their unique features, and review how their microstructure and properties develop. There are major challenges that need to be addressed for its continued expansion, requiring imagination and ingenuity to drive the future of metal printing.