Powder bed fusion processes: main classes of alloys, current status, and technological trends

Metal powder-based additive manufacturing (AM) has increasingly gained importance in the last decades. Powder-based techniques are well-known because of their versatility, allowing high degrees of geometric complexity and a wide range of chemical compositions. It enables a myriad of alloys to be printed and, e.g., the production of parts with functional gradient given the possibility of tailoring the alloy composition by changing in situ the process parameters. Moreover, the processes have been constantly developed, leading to fast, reproducible, and ready-to-use net shape parts becoming powder-based AM economically attractive. The factors mentioned above attracted the attention of aerospace, biomedical, energy and others, being the focus of intense investigations in processing and related material processing phenomena; these led to the understanding of key phenomena and consequently to the improvement of AM as a production tool. This chapter aims to explore recent developments in the powder-based AM of elementary classes of alloys for different applications. The first section is devoted to aluminium alloys, where one discusses the general challenges of printing these alloys by laser powder bed fusion (LPBF) technique, such as crack formation, porosity and the influence of building direction on the tensile properties. Further, a general overview of essential aluminium alloys produced by laser-based techniques is given. AlSi10Mg, the most common one, is the first discussed with a focus on the heat treatment of as-produced parts and its impact on tensile behaviour. Attention is also given to AlSi12, the promising Sc, Zr-based Al alloys, the hardly processable Al-Cu, the strong AA7075, and AA6061. The following two sections clarify how LPBF and laser metal deposition (LMD) are suitable for printing tool steels. The former explores hot work tool steels, giving a complete overview of the effect of processing on microstructure, phase transformation/precipitation, and the formation of defects such as cracks; high speed and cold work tool steels are briefly explored at the end of this section. The latter section deals with printing several tool steels (hot and cold work, and high speed) by LMD, correlating, e.g., printing strategies and cooling effect on the mechanical properties (such as hardness), microstructure, phase transformation and precipitation. Ahead, a section is dedicated to the powder-based AM of shape memory alloys (SMA). A general overview of the processes currently employed for printing SMAs is given. Moreover, some highlightable results of the effects of processing parameters on the transformation temperatures and functional properties are explored. Also, the microstructure evolution based on different process parameters of directed energy deposition samples is clarified. The fabrication of NiTi-based high-temperature SMA and in situ alloying of NiTi SMA are briefly explored. Some examples of application in the biomedical and micro-electromechanical are illustrated, followed by the last section, where the AM of alloys other than NiTi (e.g., iron and copper-based SMAs) is discussed. High entropy alloys (HEA) are in the subsequent section. An explanation about this novel class of alloys comes first, followed by a short technological overview and a concise sub-section regarding the powder development of HEAs. Successfully printed HEAs may be found in two separate tables where it is possible to find the technique and related process parameters. Lastly, in this section, one compares the mechanical properties of several printed HEAs. In sequence, it is possible to find the AM of magnetic materials. An introduction about Nd-Fe-B magnets is presented, and some techniques used to print magnets are explored within this sub-section; the same applies to the sub-section on Fe-Co alloys. Lastly, the AM of soft magnetic materials is explained using some examples of the effect of process parameters on the magnetic properties and the role of in situ alloying in overcoming the difficulties of printing magnets. Still, on the topic of in situ alloying, the last section of this block on specific classes of alloys is dedicated to exploring this method, focusing on the powder quality and mixing, the melting temperature, energy input and homogeneity, i.e., feedstock properties and process features. The last section is preceded by one focused on the recyclability of Ti-64 powder, where it is possible to keep up the influence of the reuse on the powder itself, the built parts and respective mechanical properties. Then, this chapter finishes with an outlook of new powder-based AM processes based on sintering-debinding, binder jetting, metal AM based on extrusion of highly filled polymer filaments, lithography and cold spray-based AM. The basics of each process are enlightened, and some examples are given to illustrate the capability of each process.