Rainer J. Hebert

Viewpoint: metallurgical aspects of powder bed metal additive manufacturing

Metal additive manufacturing has emerged as a new manufacturing option for aerospace and biomedical applications. The many challenges that surround this new manufacturing technology fall into several different categories. The paper addresses one of these categories, the physical mechanisms that control the additive manufacturing process. Physical mechanisms control the effects of processing parameters on microstructures and properties of additively manufactured parts. Some mechanisms might not have been recognized, yet, and for those that are currently known, detailed quantitative predictions have to be established. The physical mechanisms of metal additive manufacturing are firmly grounded in metallurgy, branching into laser physics and the physics of granular materials. Powder bed additive manufacturing is described from the powder storage to post-processing and elements of metallurgy are highlighted that are relevant for the different aspects of the additive manufacturing process. These elements include the surface reactions on powder particles, the heating and melting behavior of the powder bed, solidification, and post-processing. This overview of the different metallurgical aspects to additive manufacturing is intended to help guide research efforts and it will also serve as a snapshot of the current understanding of powder bed additive manufacturing.

Nanocrystals in Metallic Glasses

Nanocrystals are considered as isolated, nanoscale particles in modern science or as grains of nanocrystalline material. The latter material is comprised of nanocrystals that form a 3dimensional polycrystal made of nanocrystals and grain-boundaries. Historically, however, nanocrystals are firmly rooted in colloid science and for a long time nanocrystals were used unknowingly as components of composite materials. For example, the coloring of glasses with colloidal gold nanocrystals dates back to the Romans (Freestone et al., 2007). In light of the historical use of nanocrystals as components of macroscopic composite materials it is not surprising that modern materials science continues and expands the use of nanocrystals for composite materials. The ever increasing array of nanoscale objects, for example, nanowires, nanofibers, nanobelts, nanopillars, or nanotubes, along with improving synthesis and characterization options offers a broad range of possible macroscopic composites with nanoparticle components. The range of applications is no longer limited to functional properties, but includes structural applications. While the nanocrystals are per definition crystalline and thus reveal a periodic arrangement of atoms, the surrounding matrix can be crystalline or amorphous. For metals-based nanocrystals, the modern era of composite research dawned with the discovery of Guinier-Preston zones and remained focused on nanocrystals embedded in crystalline matrices. For ceramic materials, an important application emerged with the dispersion of nanocrystals in amorphous or glassy ceramic matrices. For example, cook tops are widely available today that are made of ceramic glasses containing dispersions of oxide nanocrystals. On the metals side, bulk composite materials comprised of nanocrystals embedded in amorphous metallic matrices are still relatively novel materials by comparison with their ceramic counterparts. The interest in nanocrystals for metallic glasses has had two related motivations. From a viewpoint of fundamental material science, metallic glasses offer a very convenient approach for studying crystallization reactions. For conventional metals or alloys, solidification occurs almost instantaneously and it is usually only possible to study the completely crystallized phase experimentally but not the process of crystallization. Crystallization of a metallic glass, by comparison, can be induced as a “slow-motion” process that enables detailed experimental studies of the crystallization process. The formation of nanocrystals from metallic glasses thus represents an ideal vehicle to test and validate crystallization theories. The second motivation for studying nanocrystals in metallic glasses is much more practical and concerns improvements in properties. Iron-based metallic glasses were among the earliest metals-based glasses and it was soon discovered that the crystallization of transition-metal nanocrystals

Alloying reactions in nanostructured multilayers during intense deformation

Abstract Under intense deformation of metallic multilayer samples, a nanometer-scale layer thickness and grain size develops during repeated cold-rolling. Along with the evolution of the highly refined microstructure, a nanoscale interfacial alloying occurs that can result in an amorphization reaction. The deformation of multilayers exhibits driven system behavior during alloying. As the length scale of the layer thickness converges to the length scale of the mixing zone during rolling, amorphization develops in appreciable volumes. The results from selected experiments demonstrate that the relative specific interfacial area is the key microstructural metric to describe the deformation-driven alloying.

Deformation-Induced Nanocrystallization in Al-Rich Metallic Glasses

Deformation-induced nanocrystallization has been investigated in a marginally Al88Y7Fe5 glass forming alloy. Conventional calorimetry and microstructural analyses of materials that have been subjected to high pressure torsion straining (HPT) at room temperature indicate the development of an extremely high number density of small Al nanocrystals. The nanocrystals appear to be distributed homogeneously throughout the sample without any evidence of strong coarsening. Moreover, the comparison between nanocrystallization caused by the application of either HPT, cold-rolling or in-situ TEM tensile straining yielded the identification of the probable mechanisms underlying the formation of nanocrystals. These results form the basis for the development of advanced processing strategies for producing new nanostructures with high nanocrystal number densities which allow increased stability and improved performance.

Insight into point defects and impurities in titanium from first principles

Titanium alloys find extensive use in the aerospace and biomedical industries due to a unique combination of strength, density, and corrosion resistance. Decades of mostly experimental research has led to a large body of knowledge of the processing-microstructure-properties linkages. But much of the existing understanding of point defects that play a significant role in the mechanical properties of titanium is based on semi-empirical rules. In this work, we present the results of a detailed self-consistent first-principles study that was developed to determine formation energies of intrinsic point defects including vacancies, self-interstitials, and extrinsic point defects, such as, interstitial and substitutional impurities/dopants. We find that most elements, regardless of size, prefer substitutional positions, but highly electronegative elements, such as C, N, O, F, S, and Cl, some of which are common impurities in Ti, occupy interstitial positions.Defects: titanium impurities by first principlesFirst principles, not semi-empirical rules, can systematically predict how point defects form because of impurities in titanium. A team led by Sanjeev Nayak and Rainer Herbert at the University of Connecticut, USA, used density functional theory to model the effect of more than twenty impurities in titanium, including the first twenty chemical elements as well as transition metals commonly added to titanium. Electronegative elements such as carbon and oxygen were stable at octahedral interstitial positions in the titanium lattice, in an identical manner to interstitial titanium. In contrast, metallic impurities such as vanadium preferred substitutional sites. Finally, vacancies in the lattice only persisted if they were far enough from their interstitial titanium atom. Research into point defect formation and stability may help us control materials for advanced manufacturing processes such as 3D printing.

The effect of recycling on the oxygen distribution in Ti-6Al-4V powder for additive manufacturing

Abstract Ti-6Al-4V powder has been recycled 30 times in an electron beam melting system. A combination of electron microscopy techniques has been used to show that the recycled powder has a 35% higher oxygen content, and that the particles have a more irregular morphology, a narrower particle size distribution, and a much more variable microstructure than the virgin powder. The microstructures in the recycled powder particles vary from a martensitic α′ structure, which is identical to that in the virgin powder, to a two-phase α + β structure. This variability is related to the complex thermal history of the unmelted metal powder in the system. Despite these differences, all of the particles exhibit essentially the same surface oxide thickness; the excess oxygen in the recycled powders is instead located in the β phase. The possible consequences for the structure and properties of the resultant additively manufactured parts are discussed.

Electron Microscopy Analysis of 17-4 PH Powder for Additive Manufacturing

Metal additive manufacturing (MAM) has evolved into a production-ready manufacturing technology over a period of just a few years. The strong interest in MAM mainly derives from the ability to manufacture complex parts without the tooling and material waste typically associated with machining components from bulk alloys. Most of the attention for MAM has focused on Ti-6Al-4V and IN718 alloys, but several other alloy classes, such as stainless steels have gained traction as well. The stainless steels that have been considered for MAM include: 304, 316, and 17-4PH alloys, but to date none of these has been studied in great detail. 17-4PH is a heat-treatable martensitic stainless steel. Since MAM starts with the spreading or feeding of powders, a careful characterization of the starting powder is necessary to understand the way in which the microstructure and defects develop in the additively manufactured components, and how these lead to variations in component properties.

Deformation-driven catalysis of nanocrystallization in amorphous Al alloys.

Summary Nanocrystals develop in amorphous alloys usually during annealing treatments with growth- or nucleation-controlled mechanisms. An alternative processing route is intense deformation and nanocrystals have been shown to develop in shear bands during the deformation process. Some controversy surrounded the idea of adiabatic heating in shear bands during their genesis, but specific experiments have revealed that the formation of nanocrystals in shear bands has to be related to localized deformation rather than thermal effects. A much less debated issue has been the spatial distribution of deformation in the amorphous alloys during intense deformation. The current work examines the hypothesis that intense deformation affects the regions outside shear bands and even promotes nanocrystal formation in those regions upon annealing. Melt-spun amorphous Al88Y7Fe5 alloy was intensely cold rolled. Microcalorimeter measurements at 60 °C indicated a slight but observable growth of nanocrystals in shear bands over the annealing time of 10 days. When the cold-rolled samples were annealed at 210 °C for one hour, transmission electron images did not show any nanocrystals for as-spun ribbons, but nanocrystals developed outside shear bands for the cold rolled samples. X-ray analysis indicated an increase in intensity of the Al peaks following the 210 °C annealing while the as-spun sample remained “X-ray amorphous”. These experimental observations strongly suggest that cold rolling affects regions (i.e., spatial heterogeneities) outside shear bands and stimulates the formation of nanocrystals during annealing treatments at temperatures well below the crystallization temperature of undeformed ribbons.

SEM Technique Based Automatic Analysis for Metal Powders and Defects in Additive Manufactured Components

Metal additive manufacturing (AM) technologies such as powder-bed or directed energy deposition require the use of metal powders with carefully controlled characteristics. Characteristics such as the powder chemistry, size distribution, or sphericity play key roles in the beam-induced melting process and in the formation of various defects in the AM components. While established technologies exist to measure the size-distributions for either metal powders or porosity in AM components, these approaches have limited spatial resolution, and in some instances powder particles or pores significantly smaller than 1 μm are very difficult to assess with these techniques [1]. Scanning electron microscopy (SEM) is in general suited to determine the sizes, shapes, morphology, and microstructures of individual powder particles or of defects in AM components. However, to obtain a statistically significant sampling of the sizes and shapes, manual particle-by particle or defect-by-defect analysis would be very tedious. In this work, we show how the automated analysis mode for the FEI Aspex ExplorerTM can be used to obtain statistically significant powder particle size distributions and defect distributions efficiently.

Microstructural Study of the Heat-treated 17-4PH Stainless Steel Parts Prepared by Selective Laser Melting

Metal powder-bed additive manufacturing (AM) processes are very different from the conventional subtractive manufacturing methods, resulting in non-uniform microstructures and highly anisotropic material properties for the as-built parts. The microstructures of powder-bed AM processed samples can be affected significantly by various factors including beam energy density, beam speed, local heat transfer conditions, and scan strategy. In a study by Reafi et al. on AM parts prepared by Selective Laser Melting (SLM) using 17-4 precipitation hardenable (PH) stainless steel, it was reported that a nonequilibrium microstructure developed [1]. This microstructure exhibited a strong difference in texture components parallel and perpendicular to the build direction resulting from the very high cooling rates. The SLM-prepared as-built part also contained a substantial fraction of retained austenite, in contrast to wrought 17-4PH materials, which have a fully martensitic structure strengthened by fine Cu-rich precipitates. In recent work by Cheruvathur et al., post-build heat treatments were applied to AM 174PH materials to obtain a more uniform microstructure and promote the precipitation of the Cu-rich phase [2]. It is clearly important to understand the microstructural development during AM processing and during subsequent heat treatment. In this study, the effect of post-build heat treatments on SLM prepared 17-4PH as-built parts is studied in comparison to that of conventionally processed 17-4PH.