Recent developments in metal additive manufacturing

Additive manufacturing (AM) or 3D printing has revolutionized the modern metal manufacturing industry. AM technology allows for fabrication of highly customized 3D objects where both shape and composition can be tailored. Compared to traditional methods, metal AM technology has advantages in saving time and cost. Recent developments in metal AM systems include upgrades in energy source and part resolution, which leads to better part quality and improved reliability. This brief review article summarizes recent developments in metal AM technologies as well as the current challenges and future trends.

Introduction

Additive manufacturing (AM) or 3D printing is a familiar term today across all ages in which a computer aided design (CAD) file is processed layer-by-layer to manufacture the 3D shape. This approach can be utilized to fabricate highly customized objects, which otherwise cannot be made using traditional manufacturing methods. Additionally, there are other advantages of AM including high material utilization, minimum fixed cost, not labor intensive, and generally environmentally friendly. Current AM technologies can be used with all types of materials; for example, metals, polymers, ceramics and composites. Among them, perhaps AM of metallic materials has shown the greatest impact in various industries including aerospace, automobile and biomedical [1,2^••,3,4^•,5, 6, 7, 8]. The advantages of using AM technologies to fabricate metallic materials are not only to produce complex geometries, but also to design and fabricate structures with customized properties using monolithic, bimetallic or multi-material compositions [9^••,10,11^••].

The AM technologies for metallic materials can be categorized based on the type of feedstock materials and energy source, which are shown in Figure 1. Powder and wire feedstock materials are commonly used in metal AM technologies. Among different metal AM technologies, powder bed fusion (PBF) (Figure 1a) and directed energy deposition (DED) (Figure 1b) are the most ones that use powder as feedstock material. Selective laser melting (SLM) and selective laser sintering (SLS) techniques are two types of PBF methods that uses laser as an energy source. Current laser based PBF systems equip optical fiber laser instead of CO₂ or Nd:YAG lasers, which improves the consistency and power of the laser. Another PBF technique is electron beam melting (EBM), which uses a high-power electron beam as the energy input instead of laser. Unlike the laser based PBF processing, which requires an inert gaseous printing environment, for EBM, the parts are fabricated in a vacuum chamber. In EBM processing, the electron beam preheats the entire powder bed before the printing of each layer is done. This could help to avoid the residual stresses in the fabricated object and the formation of martensitic phase due to rapid cooling. The latest PBF systems are able to achieve powder layer thickness as low as 20 μm, and minimum feature size between 100 and 150 μm [12, 13, 14]. The fine resolution could greatly improve the density and the quality of the as-fabricated parts along with surface finish. Quad-laser system is another advanced configuration of current SLM machines which substantially increase the print rate [15]. Directed energy deposition (DED) technique is a metal AM technology which directly feeds the powder(s) to the focal point of the laser by carrier gas. When the laser scans across the surface of the melted region, the previous molten pool experiences rapid solidification to form a bulk structure. Modern DED equipment involves optical fiber lasers as energy input to optimize the part’s quality and improve reliability. Another important feature of DED system is multiple powder feeders, where the powder feed rate of each powder feeder can be controlled individually. This feature is extremely useful for multi-material structure fabrication. Moreover, the latest DED systems utilize five-axis or free-axis CNC stage instead of three-axis. The deposition head modified with current co-axial powder deposition method shows better powder convergence at the focal point that has increased the efficiency of powder usage. Furthermore, current technology also offers various monitoring devices such as melt pool sensors, laser power monitor and layer control monitor adding to the metal AM system, which gives better in situ tracking of process and processing parameters control.

Using wire as feedstock materials for metal AM has also been found very promising [16]. The concept of wire-based deposition (Figure 1c) is very similar to powder-based DED but using metal wire. Arc-based, electron beam, and laser-based wire depositions are the three main energy sources. Besides using powder and wire form as feedstock materials, there are some other forms of feedstock materials as well. For example, ultrasonic consolidation (Figure 1d) uses thin metallic foils as feedstock. The metallic foil experiences normal load and high frequency ultrasonic vibrations which creates atomic diffusion across the metal-metal interfaces to achieve strong bonding between the layers. The concept of friction freeform fabrication (Figure 1e) is very similar to conventional friction welding, which uses a consumable rod as feedstock material. By rotating the rod at high speed against the substrate, the frictional heat is generated that consume the rod to achieve deposition. The HP® metal jet technology uses binding agent and a powder-bed to form green metallic structures. The as-fabricated parts need to be binder removed and sintered. Desktop Metal® and Markforged® are similar to conventional extrusion-based printers. The feedstock metal-polymer composite is made by high shear mixing of the metallic powders with polymeric binders. The parts made by this technology also require post processing – both binder removal and sintering. Although many manufacturing problems have been overcome by applying metal AM technologies, however there are still challenges that require further development.