Finding the Perfect Aluminum for Additive Manufacturing
When weight reduction is the primary goal, 3D-printed aluminum alloys are a frequent choice for aerospace and high-performance motorsports applications. Aluminum is much lighter than nickel alloys and has been particularly popular for laser powder-bed fusion (LPBF) because it’s good for prototyping and easy to post-process.
There are indeed lightweight metal alloys with higher specific strengths (a better ratio of strength to density) than aluminum, such as the titanium grades. In the case of thermal management, there are certainly materials with better heat-transfer coefficients, such as copper alloys. For the lowest density or a higher galvanic potential, magnesium alloys are also a great choice. But aluminum alloys have persisted because, when it comes to the trade-off between cost, performance, and manufacturability, they remain among of the best materials to optimize all three.
These same trade-offs drove much of the early development in aluminum when it came to metal 3D printing, especially LPBF. Aluminum alloys are generally grouped as either casting alloys or wrought alloys, and much of the original success in printed aluminum was with casting alloys. Wrought alloys can be desirable for demanding applications, particularly in aerospace where alloys like 2024, 6061, or 7075 see a lot of use, but these higher strength alloys suffer from poor weldability. Even 6061, which is considered to be a weldable aerospace-grade alloy, isn’t well-suited for laser powder bed fusion. Since LPBF is at its most basic level a welding process, how can this be the case? It turns out that “weldability” is not the primary criterion here, but rather “autogenous weldability”.
Autogenous weldability means that an alloy is weldable without a filler material. This isn’t as much of a problem in a normal welding application, but the powder bed in an LPBF printer is a single material, meaning that there isn’t a great way to get a “filler” into the process. Because of this, alloys that aren’t autogenously weldable can present a problem, which in many of these cases presents itself as a tendency to crack while printing.
So, casting alloys took lead in development of aluminum for 3D printing. Some of the original success in printing aluminum came with AlSi12, an alloy that is 12% silicon. This is a fairly significant amount of Si for an aluminum alloy, but the Si serves to increase the flowability of the meltpool, and also to decrease the amount of contraction as the meltpool solidifies. In this sense, the more silicon the better! But in the sense of mechanical properties, a high silicon content is not a good thing.
The next step was a logical one: reduce the proportion of silicon in the alloy from 12 to 10% and add magnesium to increase the strength. The resulting was AlSi10Mg, affectionately known by insiders as “alsitenmag.”
Even with the additional magnesium, however, AlSi10Mg wasn’t an ideal stopping point; parts printed with it still didn’t meet many of the mechanical requirements of the final applications. This printed alloy tends to have low elongation, which is pretty significant: the higher the elongation, the tougher the material. Even with this deficiency, many people stopped here and settled for “alsitenmag” as acceptable, even though it wasn’t what they were really looking for.
The Safety Factor
A large proportion of the applications that were good candidates for printing were originally castings in A356, one of the most widely used cast-aluminum alloys. It is lightweight and extremely corrosion resistant. To push the mechanical properties a bit further, one can add more magnesium, which leads to A357, a stronger alloy that can be heat treated to better properties but is a bit harder to cast. This could be an ideal candidate for LPBF, but there’s a catch: A357 also contains 0.04 to 0.07% beryllium—and beryllium is one of the most toxic metals to humans there is. Especially if it is inhaled, which can happen during powder-handling and post-processing. Not an ideal fit for AM.
Fortunately, the beryllium can be eliminated from the alloy, with the result being F357 (think F for Free of beryllium). F357 is lightweight, offers great weldability, can be anodized, has high corrosion-resistance and is tolerant of a wide range of temperatures. It’s an excellent candidate for those AM parts with thin walled, complex structures that you see in a number of aerospace and high-end motorsports applications.
While a number of leading AM equipment makers are exploring F357 now, if you have a need for this aluminum alloy there are several things to consider as you decide on which LPBF 3D printing system to use to deliver your parts at the highest levels of quality.
Making Aluminum F357 Work for You
Surface finish is important, especially at those increasingly low angles that AM machines can print these days, sometimes completely support-free. Not to mention those complex interior geometries where post-processing of a surface is limited, if not impossible. Many of the applications where you see F357 being used are in high-stress environments—and a superior surface finish right out of the printer provides better corrosion resistance and fatigue life.
Hot tearing defects can occur with F357 because its reduced silicon content can lead to cracks as it cools. The most advanced AM systems have defined parameter sets and high-fidelity laser controls that are specifically tuned to address this potential problem.
Humidity can be another issue when you’re printing with aluminum. The powder is often “stickier” and tends to clump, especially with “snowplow” recoater systems. Aluminum powder is an excellent desiccant, and moisture will adsorb onto the surface of the particles immediately. When the humidity inside the build chamber creeps up, aluminum powder can be difficult to spread, and can often lead to build-killing powder-bed defects. A non-contact recoater completely avoids such problems. Also look for the tightest build chamber that can maintain extremely low oxygen and humidity levels, with active control of both at all times throughout the build.
The combination of F357 with the advanced metal AM capabilities now available provides significant advantages over AlSi10Mg and legacy metal AM systems—eliminating engineering compromises that in the past could not be avoided and freeing the design engineer to achieve that desirable mix of geometric complexity and high performance at the lightest weights possible.
This article was written by Zach Murphree, Vice President Technical Partnerships, VELO3D (Campbell, CA). For more information, visit here .
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