M Aerospace RTC
Metals in 3D printing
In this article, metal powders and filaments used in additive manufacturing will be discussed in a non-exhaustive manner. Powders are the predominant material used in most additive manufacturing systems. On a small scale, filaments are used in DED (Directed Energy Deposition), foils in SL (Stereolithography), and metal particles in suspensions are used in ME (Material Extrusion) and MJ (Material Jetting).
Only a small fraction of the alloys available for conventional manufacturing methods have been optimized for additive manufacturing applications. The high sensitivity of additive manufacturing technologies to process parameters and the unique microstructure and properties of parts manufactured by additive manufacturing require the development of optimized process recipes and process property databases for many new alloys that could be suitable for additive manufacturing processes.
Most of the development in materials for metal additive manufacturing has been focused on ferrous and titanium alloys, followed by aluminum alloys, nickel and cobalt-chromium superalloys. Additive manufacturing of refractory metals (e.g. molybdenum and its alloys), precious metals (e.g. gold, silver, platinum, tantalum, etc.), tungsten, magnesium, copper, and intermetallic materials have been investigated by both academia and industry. The development of metal powders (and other forms) oriented to the various additive manufacturing processes is an active field of research. Growth is expected as additive manufacturing applications increase.
Arc Bicycle made of stainless steel by MX3D and TU Delft. Source: TU Delft
Given the numerous ferrous alloys such as stainless steels, tool steels, maraging steels, etc., employing them in additive manufacturing processes has always been at the forefront of R&D efforts. In a review of ferrous alloys used in additive manufacturing, the majority of paper publications are attributed to the PBF (Powder Bed Fusion) process (68% for LPBF (Laser Powder Bed Fusion) and 1% EB-PBF (Electron Beam Powder Bed Fusion) ), followed by LDED (Laser Directed Energy Deposition) (28%) and BJ (Binder Jetting) (3%), indicating the high popularity of PBF technology for additive manufacturing of metals. Among various ferrous alloys, 316L stainless steel, 300 martensitic stainless steel, H13 tool steel, 17-4 PH stainless steel, M2 tool steel, P20 tool steel, and 304L stainless steel stand out as the most studied alloys. The EB-PBF (Electron Beam Powder Bed Fusion) process has worked ferrous alloys in a reduced way, limited to H13 tool steel and 316L stainless steel. Applications such as the fabrication of porous biomedical implants, shaped cooling channels, tooling and lattice structures have been reported for ferrous alloys in additive manufacturing.
Titanium intervertebral disc implants. Sources: Wikimedia Commons
Titanium alloy Ti-6Al-4V is the most studied alloy in metal additive manufacturing processes. Titanium alloys that feature unique properties such as biocompatibility, high strength while being lightweight and high corrosion resistance have extensive applications in the biomedical and aerospace industries. Being available in powder and filament form, all three main categories of additive manufacturing (PBF, DED, BJ) can use titanium alloys as raw material. Titanium is well suited for additive manufacturing because of its difficult and expensive machining. In addition to Ti-6Al-4V alloy and pure titanium, other alloys of interest for additive manufacturing include Ti-24Nb-4Zr-8Sn, Ti-6Al-7Nb, and Ti-6.5Al-3.5Mo-1.5Zr-0.3Si. Even Ti-6Al-4V, Ti-48Al-2Nb-0.7Cr-0.3Si, γ-TiAl, and Ti-24Nb-4Zr-8Sn alloys have been used in EB-PBF.
Ignus-II engine for the Vulcan 2 rocket made of Inconel 718. Source: SEDS en UC San Diego.
Nickel superalloys are another category of materials that have gained the attention of the additive manufacturing community for their excellent resistance to extreme conditions and high temperatures. These properties and the freedom of design and optimization that additive manufacturing offers provide a great combination for creative innovations in the aerospace, aerospace, aerospace, and tooling industries. Inconel alloys make up the majority of the nickel alloy R&D effort in additive manufacturing, with Inconel 718 and Inconel 625 being the most studied materials in this category followed by Hastelloy X as another popular alloy in this category for additive manufacturing.
Aluminum manifold for a 1977 Ford F-150. Source: Ford Performance.
Aluminum is a lightweight material with high thermal and electrical conductivity, high ductility and high corrosion resistance with applications in multiple industries such as aerospace, construction, and automotive. The weldability of an alloy is a good indicator of its compatibility with additive manufacturing processes. Aluminum alloys are not very weldable but are easy to machine. In addition, they have demonstrated low processability with lasers due to their high reflectivity and low viscosity in the melt phase. As a result, aluminum has not been a material of interest in additive manufacturing research compared to other alloy groups such as titanium, nickel and iron.
However, given the benefits that additive manufacturing of aluminum offers, such as lightweight structures for aviation, there have been increasing efforts to optimize the processing parameters in the PBF and DED processes. So far the main aluminum alloy used in manufacturing has been AlSi10Mg; however, the possibility of printing AlSi12, AlSi9Mg, AlSi7Mg0.3 and AlSiMg0.75 has been demonstrated. Al-Cu and Al-Sc filaments have been used with the EB-DED process.
Gold and Platinum Rings by VENTURY. Source: Wikimedia Commons
In the case of precious metal 3D printing there are 2 approaches, direct manufacturing and indirect manufacturing. Indirect manufacturing refers to the fabrication of a pattern that is then used to make a function mold, this roughly speaking. While in the case of direct manufacturing we refer to the fabrication of the final part from a reference model, we are going to focus on the latter for this section.
Precious metals are used in additive manufacturing due to their special properties. In addition to the jewelry industry, they are also important in the biomedical industry for the manufacture of implants due to their good biocompatibility. Their outstanding physical properties, such as high strength even at high temperatures and stable behavior at fluctuating temperatures, give them a predestined place in the aerospace industry. The precious metals used are the so-called noble metals among which platinum (Pt), Rhodium (Rh), Iridium (Ir), Palladium (Pd), Silver (Ag) and Gold (Au) as well as some customized alloys such as PtRh20, PtIr50, Sterling Silver (or .925 Sterling Silver) and Red Gold. They are usually processed by additive manufacturing printers using EBM (Electron Beam Melting) and SLM (Selective Laser Melting) processes.