Titanium materials are ideal targets for selective laser melting (SLM), because they are expensive and difficult to machinery using traditional technologies. After briefly introducing the SLM process and processing factors involved, this paper reviews the recent progresses in SLM of titanium alloys and their composites for biomedical applications, especially developing new titanium powder for SLM. Although the current feedstock titanium powder for SLM is limited to CP-Ti, Ti–6Al–4V, and Ti–6Al–7Nb, this review extends attractive progresses in the SLM of all types of titanium, composites, and porous structures including Ti–24Nb–4Zr–8Sn and Ti–TiB/TiC composites with focus on the manufacture by SLM and resulting unique microstructure and properties (mechanical, wear/corrosion resistance properties).

Selective Laser Melting of Titanium Alloys and Titanium Matrix Composites for Biomedical Applications: A Review†

Today, a large number of different steels are being processed by Additive Manufacturing (AM) methods. The different matrix microstructure components and phases (austenite, ferrite, martensite) and the various precipitation phases (intermetallic precipitates, carbides) lend a huge variability in microstructure and properties to this class of alloys. This is true for AM-produced steels just as it is for conventionally-produced steels. However, steels are subjected during AM processing to time-temperature profiles which are very different from the ones encountered in conventional process routes, and hence the resulting microstructures differ strongly as well. This includes a very fine and highly morphologically and crystallographically textured microstructure as a result of high solidification rates as well as non-equilibrium phases in the as-processed state. Such a microstructure, in turn, necessitates additional or adapted post-AM heat treatments and alloy design adjustments. In this review, we give an overview over the different kinds of steels in use in fusion-based AM processes and present their microstructures, their mechanical and corrosion properties, their heat treatments and their intended applications. This includes austenitic, duplex, martensitic and precipitation-hardening stainless steels, TRIP/TWIP steels, maraging and carbon-bearing tool steels and ODS steels. We identify areas with missing information in the literature and assess which properties of AM steels exceed those of conventionally-produced ones, or, conversely, which properties fall behind. We close our review with a short summary of iron-base alloys with functional properties and their application perspectives in Additive Manufacturing.

Steels in additive manufacturing: A review of their microstructure and properties

The double-edge effect of second-phase particles on the recrystallization behaviour and associated mechanical properties of metallic materials

Key in powder-bed-based additive manufac- turing is the use of appropriate powder materials that fit to the process conditions. There are many parameters affect- ing the build process and the corresponding quality of the parts being built. Therefore, an accurate assessment of the powders becomes important. Such an assessment involves, besides others, the powder flowability, which should be sufficient in order to create good-quality powder layers. The current study aims at the development of suitable parameters and values for the qualification of metal pow- ders for selective laser melting (SLM) with regard to their flowability. The powder flowability is assessed by the statistical analysis of the measured powder avalanche angles and the powder surface fractal, which give valuable information about the significance of inter-particle forces. A set of 21 different Fe- and Ni-based powders has been analysed and good correlations between the powder ava- lanche angles, the surface fractal and the particle shape with the optically evaluated flowability could be derived. The method allows a quantitative powder flowability assessment, which correlates with the experiences for powders for powder-bed-based additive manufacturing, especially for SLM.

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Powder flowability characterisation methodology for powder-bed-based metal additive manufacturing

Metal additive manufacturing (AM), also known as 3D printing, is a disruptive manufacturing technology in which complex engineering parts are produced in a layer-by-layer manner, using a high-energy heating source and powder, wire or sheet as feeding material. The current paper aims to review the achievements in AM of steels in its ability to obtain superior properties that cannot be achieved through conventional manufacturing routes, thanks to the unique microstructural evolution in AM. The challenges that AM encounters are also reviewed, and suggestions for overcoming these challenges are provided if applicable. We focus on laser powder bed fusion and directed energy deposition as these two methods are currently the most common AM methods to process steels. The main foci are on austenitic stainless steels and maraging/precipitation-hardened (PH) steels, the two so far most widely used classes of steels in AM, before summarising the state-of-the-art of AM of other classes of steels. Our comprehensive review highlights that a wide range of steels can be processed by AM. The unique microstructural features including hierarchical (sub)grains and fine precipitates induced by AM result in enhancements of strength, wear resistance and corrosion resistance of AM steels when compared to their conventional counterparts. Achieving an acceptable ductility and fatigue performance remains a challenge in AM steels. AM also acts as an intrinsic heat treatment, triggering ‘in situ’ phase transformations including tempering and other precipitation phenomena in different grades of steels such as PH steels and tool steels. A thorough discussion of the performance of AM steels as a function of these unique microstructural features is presented in this review.

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Additive manufacturing of steels: a review of achievements and challenges

Mechanical response and deformation mechanisms of ferritic oxide dispersion strengthened steel structures produced by selective laser melting

Characterisation of a complex thin walled structure fabricated by selective laser melting using a ferritic oxide dispersion strengthened steel

High resolution microstructural studies of the evolution of nano-scale, yttrium-rich oxides in ODS steels subjected to ball milling, selective laser melting or friction stir welding

Oxide Dispersion Strengthened (ODS) alloys are a long established class of materials manufactured using powder metallurgy techniques. These alloys can offer exceptional high temperature strength and resistance to radiation damage, thus are envisioned to be used in a number of future nuclear and fossil energy power applications. However, due to the manufacturing steps involved, the overall cost to build components with these materials can be high. This paper presents work conducted to assess the feasibility of applying Selective Laser Melting (SLM) techniques to either coat or direct build on substrates with Fe-based Oxide Dispersion Strengthened (ODS) alloys. SLM is a rapid prototyping technique which can be used to manufacture near net-shape solid components from layered metallic powder beds. Two different geometries were of interest in this study — a simple button configuration with a nickel-base superalloy (IN939) substrate and a more complex hexagonal shaped wall with a mild steel substrate. Powders of PM2000 (a FeCrAl based ODS alloy) were deposited in both cases. Heat treatments were subsequently conducted on these structures to investigate effects of temperature on the bond characteristics and secondary recrystallisation. Electron microscopy examination revealed significant amounts of diffusion between the nickel and the ODS powders which enhances the bond strength. The studies have revealed the existence of a strong bond between the substrate and the interface even after prolonged exposure at elevated temperatures.Copyright © 2011 by ASME

Thomas Boegelein

Papers

Mechanical response and deformation mechanisms of ferritic oxide dispersion strengthened steel structures produced by selective laser melting

Characterisation of a complex thin walled structure fabricated by selective laser melting using a ferritic oxide dispersion strengthened steel

High resolution microstructural studies of the evolution of nano-scale, yttrium-rich oxides in ODS steels subjected to ball milling, selective laser melting or friction stir welding

Selective Laser Melting of Oxide Dispersion Strengthened Steels