Dear Colleagues,

Research in additive manufacturing (AM) of metals has witnessed a dramatic rise in global attention during the past decade. Some AM processes have evolved from conventional welding processes, while others, such as powder bed fusion processes, have been developed with the specific intent of enabling the manufacture of complex 3D geometrical objects. One key feature of all AM processes is that material is only added where it is really needed, thereby permitting near net shape manufacture utilizing starting feedstock in powder or wire form, with virtually no residual material waste if all the unmelted material can be fully recycled.

The distinct heating–cooling cycles associated with various AM processes result in different as-built microstructures and varying types of defects, which are additionally governed not only by the process parameters used but also by the geometry of object(s) being built as well as by the local environmental conditions prevailing during processing. Consequently, in-process monitoring of different parameters is important to understand the process parameter–microstructure relationships during layer-on-layer manufacturing. For low-stressed, statically loaded components, the microstructure determines the average mechanical properties. However, for cyclically loaded critical parts, such as aeroengine or turbine components, defects limit the lower bound of the mechanical properties and are, therefore, a major concern as they restrict the loading conditions during operation. In view of the above, post-build treatments such as hot isostatic pressing (HIP) that can minimize certain types of defects such as porosity can become relevant, depending on the material and AM process in question. Other in situ/post-build treatment/hybrid solutions have also been considered, such as inducing residual compressive stresses in built material to reduce the influence of surface topography/defects/residual stresses on properties. From an implementation standpoint, the final quality of finished parts also needs to be ascertained using appropriate non-destructive evaluation (NDE) methods, which represent an area under active development.

It is thus apparent that AM involves a complex manufacturing chain spanning a number of different expert competences that have to be coordinated appropriately to enable successful economical serial production of AM components. Since such a complex knowledge chain is challenging for any single organization to internally complete, establishing collaborative networks that stitch together complementary competences is often a key enabler.

This Special Issue intends to address the latest progress in various facets of metal AM that constitutes the entire value chain. Topics include but are not limited to the following:

• Directed energy deposition processes (DED);
• Powder bed fusion processes (PBF, EBM, SLM, and more);
• Hybrid–AM techniques;
• Process parameter–microstructure/defects–mechanical property relationships;
• Advanced characterization of AM utilizing SEM, TEM, synchrotron radiation diffraction, neutron scattering, and more;
• Post-build/in situ treatments (HIP, HT, machining, shot peening, hybrid manufacturing, and more) and their influence on material properties and quality;
• In-line monitoring techniques for process-build evaluation and control;
• AM process modeling, including areas such as temperature history, phase transformation, precipitation kinetics, microstructure, defects, cracks, and residual stress/distortion;
• Development of alloys customized for AM;
• Digitalization of AM.

I hope the comprehensive AM-related focus of this Special Issue will encourage submissions of manuscripts incorporating recent research findings in any of the above topics, as well as manuscripts focused on the complex chain of activities required for maturing AM into a serial manufacturing technique.

Prof. Dr. Robert Pederson
Guest Editor

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• additive manufacturing
• metals
• titanium alloys
• Ni-base superalloys
• steels
• aluminum alloys
• directed energy deposition processes
• powder bed fusion processes
• hybrid processes
• wire
• powder
• recycle
• process parameters
• defects
• porosity
• lack of fusion
• microstructure
• texture
• characterization techniques
• microscopy
• synchrotron-/neutron-scattering measurements
• residual stress
• post-build treatments
• heat treatment
• hot isostatic pressing
• shot peening
• mechanical properties
• on-line/in-line monitoring
• process control
• process regulation system
• micro-/macro scale material/process modeling
• simulation
• validation
• qualification
• digitalization
• sustainability