March_2022_AMP_Digital

A D V A N C E D M A T E R I A L S & P R O C E S S E S | M A R C H 2 0 2 2 2 6 of contraction as the meltpool solidi- fies. But for mechanical properties, a high silicon content was less desirable, so the proportion of silicon in the al- loy was reduced from 12 to 10% and magnesium was added to increase the strength. The result was AlSi 10 Mg, but parts printed with it still didn’t meet many of the mechanical requirements of the fi- nal applications. One solution was to add more magnesium to A356, an alloy widely used in casting, which created A357, a stronger material that could be heat treated to better properties. But there was a catch: A357 also contains 0.04 to 0.07% beryllium. The alloy can be toxic to humans, especially if it is inhaled, which can happen during the powder-handling and post-pro- cessing that occur with additive man- ufacturing production. The final step? Eliminate beryllium from the alloy, with the result being the now “best” alumi- num for AM, F357 (think F for “free of beryllium”). Since then, F357 has become the L-PBF alloy of choice in aerospace (Fig. 3). Chief engineers like it because it’s been used for decades in the field. Swapping an A356 casting for an F357 L-PBF part is a relatively low technical risk for system material compatibility and corrosion. Additionally, the lower silicon content over AlSi 10 Mg makes it able to be anodized. PICKING THE OPTIMAL MATERIAL FOR AM APPLICATIONS Typically, an aerospace compa- ny that is already exploring AM and is thinking about expanding their inter- nal resources with new materials ca- pabilities—or, alternatively, one that outsources to a full-service contract manufacturer (CM) that provides start- to-finish AM services—has a new part design that can’t be made any other way besides AM. They have a specific alloy in mind to give them the desired heat, strength, ductility, or other char- acteristics they require, and they’ve developed their own internal specifica- tions that must be met. An existing material already qual- ified for use on an advanced AM sys- tem (like Alloy 718) may fit the bill for them. In that case, it may just be a mat- ter of accessing a dedicated machine; it’s considered best-practice to assign a particular material to a specific piece of equipment, to avoid cross-contamina- tion, as well as customize atmospheric conditions during printing. But if the need is for a complete- ly new alloy to be qualified for 3D print- ing, the AM-equipment maker will work closely with the customer, and/or a CM, to prove-out the material on the actual machine system. This involves identify- ing the customer’s application-specif- ic requirements, securing the material from a reputable vendor, and developing the pro- cess parameters and reci- pes required to print in that material. The printer must then achieve or exceed estab- lished criteria for porosity, ultimate tensile strength, surface roughness, and di- mensional accuracy. Skin and core samples are test- ed, along with sophis- ticated processes like zero-degree printing (which only the most advanced AM systems can achieve). It’s also common to perform a design of experiments on thermal treatments of the printed ma- terial to achieve the desiredmechanical properties. STRENGTH VERSUS TEMPERATURE REQUIREMENTS: A BALANCING ACT Some of the most recently qual- ified alloys for use in advanced metal AM systems include the aluminum alloy Scalmalloy, and the nickel-based su- peralloys Amperprint 0233 Haynes 282 (HS282), and Inconel 625 (IN625). Corrosion can be a challenge with aluminum alloys. Currently the high- est-strength aluminum alloy for AM is Scalmalloy, developed by APWorks at Airbus for, among other uses, critical structural aviation applications such as airframe parts or engine mounts. Achieving high strength in aluminum generally involves adding alloying ele- ments that may be detrimental to corro- sion behavior. In the case of Scalmalloy, APWorks used a strengthening mech- anism of precipitated ceramic phase Al 3 -Sc that enables retention of excel- lent corrosion resistance of 5000 series alloys and has been proven through nu- merous corrosion tests (Fig. 4). Nickel-based superalloys, on the other hand, are more corrosion resis- tant than aluminum alloys. The primary driver for advanced nickel-based mate- rials has been a demand for parts that can withstand higher temperatures that Fig. 3 — Heat exchanger 3D printed in Aluminum F357. Fig. 4 — Manifold 3D-printed using L-PBF with Scalmalloy.