Additive manufacturing enables the printing of metallic parts, such as customized implants for patients, durable single-crystal parts for use in harsh environments, and the printing of parts with site-specific chemical compositions and properties from 3D designs. However, the selection of alloys, printing processes and process variables results in an exceptional diversity of microstructures, properties and defects that affect the serviceability of the printed parts. Control of these attributes using the rich knowledge base of metallurgy remains a challenge because of the complexity of the printing process. Transforming 3D designs created in the virtual world into high-quality products in the physical world needs a new methodology not commonly used in traditional manufacturing. Rapidly developing powerful digital tools such as mechanistic models and machine learning, when combined with the knowledge base of metallurgy, have the potential to shape the future of metal printing. Starting from product design to process planning and process monitoring and control, these tools can help improve microstructure and properties, mitigate defects, automate part inspection and accelerate part qualification. Here, we examine advances in metal printing focusing on metallurgy, as well as the use of mechanistic models and machine learning and the role they play in the expansion of the additive manufacturing of metals. Several key industries routinely use metal printing to make complex parts that are difficult to produce by conventional manufacturing. Here, we show that a synergistic combination of metallurgy, mechanistic models and machine learning is driving the continued growth of metal printing.

Metallurgy, mechanistic models and machine learning in metal printing

7xxx Al alloys such as Al-Zn-Mg-Cu are typical lightweight materials of excellent mechanical performance. Their near-net-shape manufacturing by selective laser melting (SLM) additive manufacturing, however, remains challenging due to hot-cracking prone nature of these alloys, when subjected to rapid solidification during the SLM process. In this study, we propose that co-incorporation of submicron Si and TiB2 to an Al-Zn-Mg-Cu alloy is capable to solve the long-standing problem by reducing solidification shrinkage and, simultaneously, enhancing its fracture toughness. Results show that solidification cracks indeed have been eliminated by the co-incorporation, along with much-refined microstructure. The resultant mechanical properties are high in ultimate tensile (556 ± 12 MPa) and yield strengths (455 ± 4.3 MPa). For disclosing the underlying mechanism, analytical means including high-resolution computer tomography, transmission electron microscopy and electron backscatter diffraction, as well as finite element simulation have been employed. It is aspired that the current approach can enable SLM to process critical engineering materials such as the hard-to-weld Al-Zn-Mg-Cu alloys.

Selective laser melting additive manufacturing of 7xxx series Al-Zn-Mg-Cu alloy: Cracking elimination by co-incorporation of Si and TiB2

Metal additive manufacturing offers a tool to bring formerly unmanufacturable, geometrically complex, engineered structures into existence. However, considerable challenges remain in controlling the unique microstructures, defects and properties that are created through this process. For the first time this work demonstrates how LaB6 nanoparticles can be used to control such features in Al alloys produced by Selective Laser Melting (SLM). A novel and efficient mechanical agitation process is used to inoculate AlSi10Mg powder with LaB6 nanoparticles which resulted in a homogenous, crack-free, equiaxed, very fine-grained as built microstructures. The substantial grain refinement is attributed to the good crystallographic atomic matching across the Al/LaB6 interfaces which facilitated Al nucleation on the LaB6 nanoparticles. The LaB6-inoculated AlSi10Mg exhibited near-isotropic mechanical properties with an improved plasticity compared with un modified AlSi10Mg.

A novel method to 3D-print fine-grained AlSi10Mg alloy with isotropic properties via inoculation with LaB6 nanoparticles

Aviation and automobile industries demand high strength, fatigue resistant, and wear-resistant materials in combination with lightweight, especially for structural applications. On the other hand, biomedical applications demand materials with low modulus and stiffness for optimized implants better matching the modulus of human bone combined with enhanced strength and wear resistance. For all the aforementioned applications in various fields, the fabrication of parts with desired size and shape without the need for joining or welding operations is desired while simultaneously reaching improved mechanical properties and more resistance to environmental attack, which are stringent requirements for almost all the applications. To achieve all these demands, both material developments, as well as modification of process conditions and parameters, are essential. Along these lines, a lot of research work focusses on advanced or even disruptive manufacturing routes and proper alloy development (e.g. Ti, Al, steels, and so on) applying additive manufacturing (AM) techniques for various applications. Among different AM methods, selective laser melting (SLM) is in high demand and preferred for achieving fully dense products in the required dimensions. Titanium alloys designed for AM have replaced a variety of other alloys due to their superior properties such as lightweight or good fatigue and corrosion resistance, achievable through modified microstructures gained by the faster heating and cooling rates realized upon laser printing. Ti alloys with a single (α) or dual (α+β) microstructure are mostly implemented in the aviation and automobile industries, whereas β alloys with exceptionally low modulus close to that of human bone are intensively studied for bio and dental implants but have not been commercialized yet. The modification of microstructure and properties in Ti-based materials with the addition of suitable reinforcement is also a reliable method. In this article, critical aspects for the optimization of processing parameters affecting the properties of SLM manufactured Ti alloys and titanium matrix composites (TMCs) will be presented, and future prospects of such materials will be critically assessed. This work is expected to be helpful for future studies on Ti alloys and composites with enhanced properties processed by laser manufacturing.

Selective laser manufacturing of Ti-based alloys and composites: impact of process parameters, application trends, and future prospects

The application of conventional Al–Zn–Mg–Cu alloys for selective laser melting (SLM) of additive manufacturing is hindered by hot cracking during the SLM process. In the present work, a novel Si- and Zr-modified Al–Zn–Mg–Cu (Al7075) prealloyed powder was developed for SLM, and the printability, microstructure, and mechanical properties of the as-built samples were evaluated at various laser powers. The as-built samples are completely crack-free at various laser powers, showing that Si and Zr effectively prevent hot cracking for SLM printed Al–Zn–Mg–Cu. The mechanisms of tensile property enhancement are as follows: first, the low melting point Si-rich eutectics backfill the cracks; second, the Al3Zr significantly decreases the grain sizes. The microstructure has a bipolar grain distribution with fine grains at the bottom of the melt pool (2–5 μm) and coarse columnar grains (10–30 μm) at the top of the melt pool. The synergistic effect of the Si and Zr contribute to outstanding tensile properties (446 MPa tensile strength and 6.5 % elongation), which are higher than those for SLM printed Al–Zn–Mg–Cu alloys without the addition of Si and Zr.

Microstructures and tensile properties of a selective laser melted Al–Zn–Mg–Cu (Al7075) alloy by Si and Zr microalloying

Selective laser melting enabled additive manufacturing of Ti–22Al–25Nb intermetallic: Excellent combination of strength and ductility, and unique microstructural features associated

T. Ebel

Papers

Selective laser melting enabled additive manufacturing of Ti–22Al–25Nb intermetallic: Excellent combination of strength and ductility, and unique microstructural features associated