There are two main drivers of whether or not a metal alloy is printed today: printability and demand. Though there are a wide variety of metal 3D printing processes out there, nearly all rely on metal powders. These materials take two forms in printing: raw 3D printing metal powders, or bound powder 3d printing metal filament. As a result, the metal materials printable today are to a large degree constrained by powder availability and whether or not that powder can be effectively bound. Aluminum, as an example, is notoriously difficult to print well and is as a result relatively uncommon.
Metal 3D printing is useful for parts that are tricky to machine, either in complexity or material, because especially at low volumes it can be cheaper. Harder materials like stainless steels, tool steels, titanium, and others are more difficult to work with and require higher quality tooling, better machines, and more overhead costs. The added natural manufacturing costs adds to the relative value that 3D printing provides, allowing them to cross the "inflection point" at which 3DP becomes valuable. On the other side of the spectrum, materials that are easy and cheap to machine (low grade steel, aluminum), aren't as in demand because it's already easy to make them. This forms a grouping of "common" metal printing materials that are traditionally really hard to work with, made simple by additive instead of subtractive.
What are the most common metals for 3D Printing?
- Stainless Steels (17-4 PH, 316L, 304)
- Tool Steels (H13, A2, D2)
- Specialty Alloys (Inconel, Cobalt Chromium, and others)
- Titanium (Ti64)
- Aluminum (4047, 6061, 7075)
Steel 3D Printing
Steel is the most common metal 3D printing material. Its strength, ability to be turned into 3D printing metal powder, relatively low cost, and post-processability make it a material usable in many applications. Most varieties of steel can be printed: the two most common are stainless steels and tool steels. Not all steels are commonly printed -- Alloy steels, which are the most common to fabricate conventionally, are rarely printed. Due to their lower conventional fabrication costs and inferior material properties, they’re not as valuable of a material to print.
Stainless steels are strong, stiff steels that possess excellent corrosion resistance due to their significant Chromium content (at least 12%, often up to 18%). They come in two different varieties, austenitic and martensitic.
Austenitic stainless steels are the most common type of stainless steel. They’re corrosion resistant and can be both machined and welded, though they cannot be heat treated. 303 and 304 are the most common types of austenitic stainless steels, and 316L is a variant that maximizes corrosion resistance.
Martensitic stainless steels are much harder than austenitic steels, but more brittle and less corrosion resistant. As a group, they lack the general versatility of austenitic steels -- however, they can be heat treated and precipitation hardened. They’re best when you need a hard and stiff stainless steel. 17-4 PH is a particularly useful type of martensitic stainless steel that can be heat treated to fit a variety of material properties -- it’s also the most common martensitic steel, though others (like 420) are also printed.
Tool Steels are named for their central application -- tooling of all varieties. They contain carbide, an extremely hard compound that’s critical to their ability to cut, grind, stamp, mold, or form. Generally, they’re very hard, abrasion resistant, and usable at high temperatures. Tool steels are categorized by the AISI-SAE grading systems, divided into types by function. The three types most commonly metal 3D printed are A series, D series, and H series tool steels.
A Series tool steels are great general-use tool steels -- they balance wear resistance and toughness and are machinable. There are eight varieties of A Series tool steels, the most common of which is A2. It’s a versatile tool steel often used to make punches and dies, but usable in a wide variety of applications.
D Series tool steels are optimized for wear resistance and hardness. They’re not particularly tough, and only used for cold work applications. The most common variety of D Series tool steel is D2 -- it’s used for all kinds of cutting tools, from blades to industrial cutting tools and even knives.
H Series tool steels cut and shape material at high (or cycling) temperatures. They’re not as wear resistant as A or D Series tool steels at low temperatures, but hold their strength and stiffness well in increased heat. Though there are many types of H-Series tool steel, H13 the most common 3D printed variety. Its mix of excellent toughness, wear resistance, and heat resistance make it a good general use tool steel that’s optimized for use in high temperatures (hot working).
Titanium 3D Printing
While Titanium is by no means a common material to fabricate conventionally, its unique properties and high base cost make it a great candidate for 3D printing. It’s strong, incredibly lightweight, heat and chemical resistant, and can be biocompatible. Though there are a few different types of Titanium that can be printed, one is by far the most common: Titanium 64 (Ti-6Al-4V).
Ti64 is the most common type of Titanium in both 3D printing and conventional fabrication. It possesses an excellent strength to weight ratio and can be heat treated to further improve strength. The material also excels in adverse environments due to its corrosion and heat resistance. As a result, it’s used heavily in aircraft (missiles, rockets, airplanes) medicine (as orthopedic implants) and other places where high strength to weight ratio is beneficial.
Aluminum 3D Printing
Aluminum is notoriously difficult to print. As a result, it’s a relatively uncommon printing material despite being exceedingly common in conventional fabrication. The varieties that are printed are generally casting grade aluminum, not more common machinable types like 6061 or 7075. These casting grade aluminum alloys all contain significant (up to 12%) Silicon content, and are weaker and less stiff than 6061. It’s not immediately clear when Aluminum will become more readily available as a 3D printing material, but until then materials like steel and titanium achieve similar strength to weight ratios when printed with open cell infill.
In addition to more common metals, 3D printers can also fabricate parts out of alloys uniquely suited for hostile environments. These “superalloys” -- metals that exhibit high strength, heat resistance, good surface stability, and resistance to corrosion or oxidation -- can be used in a wide variety of high performance applications. Two subgroups are most common: Inconel and Cobalt Chrome.
Inconel is the most common and best known proprietary nickel alloy. It’s an extremely strong, stiff, and corrosion resistant material used in places like turbines, engine seals, and rockets. There are two main formulations that are 3D printed: Inconel 718 is stronger and tougher, and Inconel 625 is more heat resistant. Both materials are incredibly expensive to machine conventionally, making 3D printing a cost-effective alternative to fabricating high fidelity parts.
Cobalt Chromium is a superalloy known for its biocompatibility, high strength to weight ratio, and corrosion resistance -- it’s essentially a higher-grade, more expensive version of Titanium. Like Inconel, it’s used in turbines and other hostile environments. Unlike Inconel, it can be used in medical applications as orthopedic or dental implants.
The long term success of metal printers hinges to a large degree on their ability to support a wide variety of materials reliably. The current list is short and focused on higher grade materials that are financially beneficial to fabricate now. However, as metal 3D printing matures, expect to see more and cheaper metal 3D printing filaments and powders available across different metal printing platforms. These materials in turn will open up new applications for metal printing, furthering its adoption by the manufacturing masses.