Tarasankar Debroy

Pennsylvania State University | Penn State · Department of Materials Science and Engineering

Geometrical conformity, microstructure and properties of additively manufactured (AM) components are affected by the desired geometry and many process variables within given machines. Building structurally sound parts with good mechanical properties by trial and error is time-consuming and expensive. Today's computationally-efficient, high-fidelity...

Structure and properties of components made by the powder bed fusion (PBF) additive manufacturing (AM) are often optimized by trial and error. This procedure is expensive, time consuming and does not provide any assurance of optimizing product quality. A recourse is to build, test and utilize a numerical model of the process that can estimate the m...

The rapidly evolving field of additive manufacturing requires a periodic assessment of the progress made in understanding the properties of metallic components. Although extensive research has been undertaken by many investigators, the data on properties such as hardness from individual publications are often fragmented. When these published data a...

The customized production of complex components by 3D printing has been hailed as a potentially trans-formative tool in manufacturing with important applications in health care, automotive and aerospaceindustries. However, after about a quarter of a century of research and development, only a handful ofcommercial alloys can be printed and the marke...

3D printing is now widely used in aerospace, healthcare, energy, automotive and other industries. Metal printing, in particular, is the fastest growing sector, yet its development presents scientific, technological and economic challenges that must be understood and addressed.

The manufacture of defect-free and dimensionally accurate parts in laser powder bed fusion (LPBF) is influenced by temperature field, deposited track geometry, and process-induced thermomechanical stress. The selection of an appropriate scanning strategy is key to achieving this goal. Well-tested numerical models of heat transfer and thermal stress...

During deep penetration laser welding of nickel alloys, interactions between composition and processing conditions can lead to the formation of defects. Inconel 740H, for example, has demonstrated a susceptibility to horizontal fusion zone cracking at locations between 70% and 80% of the weld depth during laser welding at powers above 5 kW. Couplin...

The grade 91 ferritic/martensitic steel is considered a promising structural or cladding material for various nuclear reactor applications. Here, grade 91 was fabricated via the Directed Energy Deposition Laser technique. This alternative manufacturing process potentially enables tailoring of the mechanical properties through increased control of t...

Lack-of-fusion porosity due to insufficient melting of the adjacent tracks and successive layers is a persistent problem in laser powder bed fusion (LPBF). A novel functional relation is proposed here following dimensional analysis for rapid estimation of the lack-of-fusion porosity fraction as a function of important process variables and alloy pr...

During additive manufacturing, the selection of alloys, process variants, and variables results in an exceptional diversity of microstructures, properties, and defects. Optimizing the structure and properties of parts by trial and error is not a viable option because of the large number of variables involved and the high cost of feedstocks and mach...

The vapor pressure values of common elements are available in the literature over a limited temperature range and the accuracy and reliability of the reported data are not generally available. We evaluate the reliability and uncertainty of the available vapor pressure versus temperature data of fifty common pure elements and recommend vapor pressur...

Grade 91 steel forms martensite during additive manufacturing and the extent of tempering of martensite significantly affects the mechanical properties of parts. Currently, there is a lack of quantitative understanding of the tempering kinetics for Grade 91 steel, and as a result, the effects of repeated thermal cycles on properties for different p...

Grade 91 steel forms martensite during additive manufacturing and the extent of tempering of martensite significantly affects the mechanical properties of parts. Currently, there is a lack of quantitative understanding of the tempering kinetics for this steel, and as a result, the effects of repeated thermal cycles on properties for different proce...

Numerical modelling of thermo-mechanical residual stresses for laser powder bed fusion is complex and computationally intensive. A novel analytical model is presented here that can compute the residual stress distributions through a printed part and the baseplate quickly and reliably using phenomenological modelling. The peak residual stress for ea...

Smooth surfaces of 3D printed parts are essential for long fatigue life and high dimensional accuracy. They are currently achieved by post-processing such as grinding of external surfaces which adds extra costs. We derive a dimensionless index to reduce roughness using experimental data and dimensional analysis of causative variables. The hierarchi...

A machine learning trick that everyone should be using - a video available at https://www.youtube.com/watch?v=mnUonRqOSqg In some cases, machine learning can be used with a small data set because the behavior of many complex engineering systems is often accurately described by a group of variables rather than individual variables. An example is th...

Smooth surfaces in the printed parts are essential for long fatigue life and high dimensional accuracy. They are currently achieved by post-process machining and grinding of external surfaces that add extra costs. Mitigating the surface roughness of internal channels remains a challenge. Here we use a high-throughput screening approach that analyze...

Nickel alloy Inconel 740H, a candidate material for use in ultra-supercritical power plants, is susceptible to solidification cracking during high power deep penetration laser welding. Here we examine how cracking is affected by welding variables and determine the locations where the cracks occur experimentally and theoretically. We use a solidific...

Process parameters and thermophysical and mechanical properties of alloys affect cracking which remains a major challenge in metal printing. Cracks occur because of multiple mechanisms and currently, there is no unified mitigation strategy. Here we evaluate the effects of variables related to the physics of cracking computed by a mechanistic model...

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 part...

In the past few decades, additive manufacturing has evolved for the one-step fabrication of various complex , customized metallic components that cannot be easily and economically produced by other means. However, widespread applications and market penetration of such components are often hindered by the formation of common defects that affect part...

The digital twin (DT) is a relatively new concept that is finding increased acceptance in industry. A DT is generally considered as comprising a physical entity, its virtual replica, and two-way digital data communications in-between. Its primary purpose is to leverage the process intelligence captured within digital models-or usually their faster-...

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...

Wire-arc additive manufacturing provides the fastest metal printing rate among all printing processes. Heat transfer and fluid flow models offer a usable connection between process variables and the parameters that affect the structure and properties of parts. Here we develop a computationally efficient, three-dimensional, transient, heat transfer...

Several key industries routinely make complex parts using metal printing, but its continued growth will require the ability to control the microstructure and properties of parts. Many process variables affect the spatially variable thermal cycles that affect the microstructure and properties of parts. Here we show that the evolution of hardness of...

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 intensi...

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...

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...

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...

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 f...

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 p...

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 particu...

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...

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 unexpe...

Most metallic alloys contain small concentrations of solutes because they are sufficient to achieve amazing properties on a grand scale of production and utilisation. In the early 1980s, a different concept was introduced that involved mixing large concentrations of solutes in roughly equal amounts so that there is no predominant solvent. When this...

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...

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...

Functionally graded materials (FGMs) with site specific chemical composition are commonly manufactured by directed energy deposition (DED). Although previous work fabricated an FGM with a compositional variation between a ferritic and austenitic alloy, difficulties arose due to variations in deposit shape with composition change. This problem also...

This book provides an invaluable reference of materials engineering written for a broad audience in an engaging, effective way. Several stories explain how perseverance and organized research helps to discover new processes for making important materials and how new materials with unmatched properties are theoretically conceived, tested in the labo...

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...

In powder bed fusion, distorted temperature fields and deposit dimensions, undesirable surface features, and defects have been attributed to asymmetry in track geometry. It is also thought to affect sensing and control. While data on the asymmetry have been widely reported in the literature for many commonly used alloys, the origin of the asymmetry...

Additive manufacturing (AM), also known as 3D printing, is gaining wide acceptance in diverse industries for the manufacture of metallic components. The microstructure and properties of the components vary widely depending on printing process and process parameters, and prediction of causative variables that affect structure, properties and defects...

Residual stresses and distortion are common serious defects in wire-arc additive manufacturing. Commercial thermomechanical models are often used to understand how these defects form. However, no clear mitigation strategy has evolved from previous research. Identification of the hierarchy of variables that influence residual stresses will help to u...

Residual stresses and distortion are common serious defects in wire-arc additive manufacturing. Commercial thermomechanical models are often used to understand how these defects form. However, no clear mitigation strategy has evolved from previous research. Identification of the hierarchy of variables that influence residual stresses will help to u...

Keyhole mode laser welding under reduced ambient pressure is known to provide improved weld penetration, narrower width, and reduced incidences of defects, but the underlying mechanism for these benefits is not known. We sought to elucidate the mechanism by an experimental and theoretical program of investigation. Potential causative factors, such...

Since friction stir welding tools fail in service under various mechanisms, it is difficult to mitigate tool failure based on mechanistic understanding alone. Here we use multiple machine learning algorithms and a mechanistic model to identify the causative variables responsible for tool failure. We analyze one hundred and fourteen sets of experime...

Although laser welding is now widely used in ship building, automobile, and other industries, high-quality deep penetration (> 10 mm) welds frequently suffer from defects such as porosity and root defects. Since reduced pressure keyhole welding was found to provide deeper penetration and reduced incidences of defects, an experimental and theoretica...

Although laser welding is now widely used for the welding of thick plates in keyhole mode, defects such as porosity frequently occur because of instability of the keyhole. It has been shown recently that a reduction of ambient pressure results in defect free welds of thick plates. However, direct observation of the keyhole behavior and melt flow du...

Since the deposition patterns affect the stresses and distortions, we examined their effects on multi-layer wire arc additive manufacturing (WAAM) of Ti-6Al-4V and Inconel 718 components experimentally and theoretically. We measured residual stresses by hole drilling method in three identical components printed using different deposition patterns....

Friction stir welded joints often contain voids that are detrimental to their mechanical properties. Here we investigate the conditions for void formation using a decision tree and a Bayesian neural network. Three types of input data sets including unprocessed welding parameters and computed variables using an analytical and a numerical model of fr...

Additive manufacturing (AM) of metallic materials and welding share many similarities in their physical processes and the way the microstructure and properties of the products evolve. This synergy can be a powerful driver for the scientific and technological advancements of both these important technologies for the benefit of all people. Science an...

A printability database can help in the selection of a printing process-alloy combination to reduce, and in some cases avoid, common defects in printed parts. The extensive testing of parts is not a viable option for determining printability because printing processes are inherently slow and expensive. Here we evaluate printability of stainless ste...

Laser welding was performed on a U6wt%Nb uranium alloy using a 400 W solid state laser with welding speeds from 20 to 2500 mm/s. This speed range spanned melt pool sizes from traditional welding to surface modification and additive manufacturing. With increasing scan speed, the ratio of weld length relative to depth and width increased, with melt p...

Previous research on the powder bed fusion electron beam additive manufacturing of Inconel 718 has established a definite correlation between the processing conditions and the solidification microstructure of components. However, the direct role of physical phenomena such as fluid flow and vaporization on determining the solidification morphology h...

Heterogeneous grain structure is a source of the inhomogeneity in structure and properties of the metallic components made by multi-layer additive manufacturing (AM). During AM, repeated heating and cooling during multi-layer deposition, local temperature gradient and solidification growth rate, deposit geometry, and molten pool shape and size gove...

Components manufactured by additive manufacturing often exhibit improper fusion among layers and hatches. Currently, there is no practical way to select process parameters and alloy systems based on scientific principles to mitigate these defects. Here, we develop, test and demonstrate a methodology to predict and prevent these defects based on a n...

The rapidly evolving field of additive manufacturing requires a periodic assessment of the progress made in understanding the properties of metallic components. Although extensive research has been undertaken by many investigators, the data on properties such as hardness from individual publications are often fragmented. When these published data a...

Structure, properties and serviceability of components made by wire arc additive manufacturing (WAAM) depend on the process parameters such as arc power, travel speed, wire diameter and wire feed rate. However, the selection of appropriate processing conditions to fabricate defect free and structurally sound components by trial and error is expensi...

Dissimilar metal joints between ferritic and austenitic alloys are susceptible to premature failure due to diffusive carbon loss from the ferritic alloy driven by abrupt changes in carbon chemical potential. Compositional grading of transition joints fabricated using laser-based directed energy deposition additive manufacturing offers a means for l...

The most important metallurgical variables that affect the structure and properties of components produced by powder bed fusion (PBF) are examined using a model, proposed and validated in part-I of this paper. These variables include the temperature and velocity fields, build shape and size, cooling rates, solidification parameters, dendrite arm sp...

Since its inception, significant progress has been made in understanding additive manufacturing (AM) processes and the structure and properties of the fabricated metallic components. Because the field is rapidly evolving, a periodic critical assessment of our understanding is useful and this paper seeks to address this need. It covers the emerging...

Additively manufactured compositionally graded joints are potentially attractive to minimize abrupt changes in residual stresses and distortion of dissimilar alloy joints. Performance of these graded joints depends on the residual stresses and distortion governed by the transient temperature field during additive manufacturing and local mechanical...

Since the pulsation of heat input provides a flexible and effective way to control temporal variation of weld pool geometry, cooling rate and solidification parameters, double pulsed (DP) gas metal arc welding (GMAW) serves as an unique tool for controlling the structure and properties of welds. A comprehensive model of DP-GMAW, when adequately tes...

Vaporization of alloying elements during laser beam welding adversely affects weld metal composition and properties. Alloying element vaporization and weld metal composition change during partial and complete joint penetration keyhole mode Nd:YAG laser welding of two stainless steels containing different concentrations of manganese were examined ex...

In fusion welding, variations of cooling rate and microstructure are commonly achieved by changing heat input. Here we show that the cooling rates can be adjusted and microstructural features could be refined at constant heat input while changing the pulsing parameters. The temporal variations of cooling rates, solidification growth rate and weld g...

Properties and serviceability of additively manufactured components are affected by their geometry, microstructure and defects. These important attributes are now optimized by trial and error because the essential process variables cannot currently be selected from scientific principles. A recourse is to build and rigorously validate a digital twin...

Evolution of microstructure during keyhole mode welding involves several special features such as multiple inflections of weld pool boundary curvatures, strong spatially variable thermal cycles and negligible undercooling. These systems are difficult to characterize rigorously, because depending on the sections selected, significantly different gra...

The effects of many process variables and alloy properties on the structure and properties of additively manufactured parts are examined using four dimensionless numbers. The structure and properties of components made from 316 Stainless steel, Ti-6Al-4V, and Inconel 718 powders for various dimensionless heat inputs, Peclet numbers, Marangoni numbe...

In laser assisted additive manufacturing (AM) an accurate estimation of residual stresses and distortion is necessary to achieve dimensional accuracy and prevent premature fatigue failure, delamination and buckling of components. Since many process variables affect AM, experimental measurements of residual stresses and distortion are time consuming...

Additively manufactured parts are often distorted because of spatially variable heating and cooling. Currently there is no practical way to select process variables based on scientific principles to alleviate distortion. Here we develop a roadmap to mitigate distortion during additive manufacturing using a strain parameter and a well-tested, three-...

The structure and properties of welded and additively manufactured alloys are affected by the microstructural evolution in the fusion zone (FZ) and heat affected zone (HAZ). The motion of the liquid pool and the interdependence of grain growth in both the solid and liquid regions are important in the evolution of the final grain structure. Previous...

The evolution of grain morphology during solidification of a moving aluminum alloy pool is simulated by considering heat transfer, flow of liquid metal in the molten pool and solidification parameters. The computationally efficient model consists of a 3D coupled heat transfer and fluid flow simulation to predict the molten pool shape and temperatur...

In a laser based additive manufacturing process, the alloy powders undergo a rapid heating, melting, solidification and cooling process. The morphology and the scale of the solidification structure depend on the temperature gradient and the growth rate during the additive manufacturing process. A comprehensive three dimensional transient heat trans...

For successful commercial applications of additive manufacturing, metallic parts need to be defect free, structurally sound and reliable. However, additively manufactured metallic parts are susceptible to distortion, lack of fusion defects and compositional changes. Methods to measure susceptibilities of various alloys to these defects are needed b...

For successful commercial applications of additive manufacturing, metallic parts need to be defect free, structurally sound and reliable. However, additively manufactured metallic parts are susceptible to distortion, lack of fusion defects and compositional changes. Methods to measure susceptibilities of various alloys to these defects are needed b...

Although additive manufacturing (AM), or three dimensional (3D) printing, provides significant advantages over existing manufacturing techniques, metallic parts produced by AM are susceptible to distortion, lack of fusion defects and compositional changes. Here we show that the printability, or the ability of an alloy to avoid these defects, can be...

Laser and hybrid laser-arc welding present significant opportunities for thick section welding of nickel-based alloys during the construction and repair of nuclear power plant components. However, the impact of these welding processes on the fusion zone geometry and defect levels in Alloy 690 are not well understood. A series of laser and hybrid la...

In a laser based additive manufacturing process, the alloy powders undergo a rapid heating, melting, solidification and cooling process. The morphology and the scale of the solidification structure depend on the temperature gradient and the growth rate during the additive manufacturing process. A comprehensive three dimensional transient heat trans...

For successful commercial applications of additive manufacturing, metallic parts need to be defect free, structurally sound and reliable. However, additively manufactured metallic parts are susceptible to distortion, lack of fusion defects and compositional changes. Methods to measure susceptibilities of various alloys to these defects are needed b...

In this work, two series of specimens with Hammar and Svensson's Cr- and Ni-equivalents (Creq+Nieq) = 35 and 45 wt% were used to cover a wide range of austenitic grades. These were laser welded with different energy inputs achieving cooling rates in the range of 103°C/s to 104°C/s. As high cooling rates and rapid solidification conditions could fav...

Striking differences in the solidification textures of a nickel based alloy owing to changes in laser scanning pattern during additive manufacturing are examined based on theory and experimental data. Understanding and controlling texture are important because it affects mechanical and chemical properties. Solidification texture depends on the loca...