By invitation only: overview articleAdditive manufacturing of metals
Graphical abstract
Introduction
In contrast to conventional, subtractive manufacturing methods, additive manufacturing (AM) is based on an incremental layer-by layer manufacturing [1]. As such, most relevant AM technologies commonly use powder or wire as a feedstock which is selectively melted by a focused heat source and consolidated in subsequent cooling to form a part [2], [3]. AM has attracted much attention over the past ten years due to its immanent advantages, such as unrivalled design freedom and short lead times [4]. AM techniques have already been known for more than 20 years [5] but were at first limited to the rapid manufacturing of porous structures and prototypes. With the advancement of technology, part density and quality improved and first applications in tool inserts with conformal cooling evolved [6], as well as medical applications, e.g. in the form of dental prostheses [7]. Today, it has become possible to reliably manufacture dense parts with certain AM processes and for a number of materials, including steel, aluminium and titanium [8].
Thus, AM transforms more and more from rapid prototyping to rapid manufacturing applications [9], which require not only profound knowledge of the process itself, but also of the microstructure resulting from the process parameters and consequently of the properties of the manufactured parts. From the many technologies available, only a handful is able to produce metallic parts that fulfil the requirements of industrial applications. In this overview, the relationship between process, microstructure and properties is studied in detail for three AM technologies with the highest industrial relevance at the moment, Laser Beam Melting (LBM), Electron Beam Melting (EBM), and Laser Metal Deposition (LMD).
Section snippets
Additive manufacturing methods
AM methods can essentially be classified by the nature and the aggregate state of the feedstock as well as by the binding mechanism between the joined layers of material [10], [11]. In AM of metals a powder feedstock or more rarely a wire is fully melted by the energy input of a laser or electron beam and transformed layer by layer into a solid part of nearly any geometry.
The most popular processes for AM of metals are Laser Beam Melting (LBM), Electron Beam Melting (EBM) and Laser Metal
Feedstock for AM
Common materials for AM of metals are steel, Al alloys, Ti and its alloys as well as Ni-based superalloys, CoCr, and various other metallic materials. These metals are used in pulverized condition as feedstock in AM processes.
Microstructure and properties
This section at first describes the different microstructural features of AM parts, fabricated from steel, Al- and Ti-alloy powders, and then correlates these to the static and fatigue properties in section 5.
Influence of residual porosity of AM fabricated parts
With the advancements in AM technology over the past years, dense metallic parts with mechanical properties comparable to conventional manufacturing methods are achievable for a number of material and process combinations. As porosity is facilitating crack propagation and thus deteriorating mechanical properties [120], the manufacture of parts with a high density, typically > 99.5%, is regularly the first goal in AM process optimization. Besides other influences, part density is depending on
Applications
The recent extensive gains in knowledge on the influence of AM processing parameters on the microstructure and the related properties of AM metals, as discussed in the previous sections, enables AM to become not only a valuable method for rapid prototyping but more and more also for rapid manufacturing. Serial applications reach back some 10 years e.g. in the dental industry, where CoCr is used for dental prostheses [7]. Applications of tool steel, e.g. H13, in mould inserts and tools have also
Conclusions and outlook
An overview on the current state of AM of metals was presented with a focus on the interrelationship between process, microstructure and properties. The high temperature gradients involved in AM typically yield fine grained microstructures with outstanding strength according to the Hall-Petch law. Depending on material and process, non-equilibrium microstructures evolve in the as fabricated state, e.g. retained austenite in certain martensitic steel grades or the martensitic α′ phase in
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