As a technology, metal 3D printing has seen remarkable growth in recent years, with new tools and new materials being added all the time. The majority of metal 3D printing processes use metal powders as feedstock, although there are some exceptions. Hence, as metal additive manufacturing (AM) becomes more common, the metal powder market will grow along with it.
In 2021, the global metal powder market was valued at $6.75 billion and projected to grow to $10.79 billion by 2029. While the COVID-19 pandemic had a negative impact on overall demand—as it did for so many industries—it’s not unreasonable to expect the market to rebound if 3D printing can relieve issues in the global supply chain.
In any case, given how crucial metal powders are to metal AM, it’s worth taking a closer look at what makes them different from other bulk materials and what to keep in mind when selecting, storing and using them for additive manufacturing.
Let’s start with the basics.
A Brief Primer on Powder Metallurgy
The process of converting raw metals into fine powders and then working those powders into solid objects dates all the way back to the Incas and ancient Egyptians, who both used powdered metals to produce jewellery and other valuable artifacts. In the 19th century, Russian rubles were made from platinum powder that was sintered to create coins, but the mass production of metal powders didn’t really take off until the introduction of incandescent lightbulbs and the tungsten wires therein.
While there are a variety of ways to produce metal powders, the most common are the so-called sponge iron process and atomization via water or gas. The sponge iron process involves mixing magnetite ore with coke and lime and then reducing it via a retort inside a kiln, which creates an iron “cake” and slag. Once separated from the slag, the cake is then crushed and annealed into powder. The resulting particles are highly porous, hence the term ‘sponge’ iron.
Atomization, on the other hand, involves forcing a molten metal stream through a nozzle and introducing turbulence into the resulting flow via inert gas supplied just before the stream leaves the nozzle (gas atomization) or via high-speed streams of atomized water, that both breaks the stream into drops and cools them (water atomization).
Gas Atomized Powders Versus Water Atomized Powders for Additive Manufacturing
Gas atomized powders tend to be spherical, which improves their flowability and therefore makes them desirable for metal additive manufacturing. Water atomized powders tend to be more irregular in shape, but there have been recent efforts to improve their morphology to make them more amendable to AM processes.
Gas atomized powders also have higher purity levels compared to water atomized powders, as the use of inert gases in processing minimizes the risk of contamination. It should be noted, however, that impurities in water atomized powders can be removed in post processing, e.g., through sieving and washing. Nevertheless, in applications where ensuring low oxygen content is critical, gas atomized powders are generally the better option.
The one caveat to add is that gas atomized powders are generally more expensive to produce, due to the complexity of processing involved. According to Koushik Viswanathan, assistant professor of mechanical engineering at the Indian Institute of Science (IISc), “Gas atomized powders can be from 2x to 30x more expensive for common materials.” As water atomized powders are most cost-effective, they can be an attractive option for less demanding applications or more cost-sensitive projects.
Metal Powders in Additive Manufacturing Versus Other Manufacturing Processes
Historically, metal powders were processed into solid objects via powder compaction, where the powder is pressed into a die to form a desired shape and then sintered to make it into one solid piece. If you’ve worked with processes of this kind—including die pressing, powder forging, hot isostatic pressing (HIP) or metal injection molding (MIM)—you’re already well acquainted with many of the important properties of the metal powders used in metal AM.
However, as Ryan Dehoff, director of the Manufacturing Demonstration Facility at Oakridge National Laboratories (ORNL) explains, there are some important differences between the powders used for these processes: “AM technologies typically use a narrow range of powder size distributions that meet flowability requirements for the process, size ranges for proper melting and specific chemistries based on process.”
Ankit Saharan, head of metals technology at EOS agrees, adding: “Metal powders for AM, specifically L-PBF [laser powder bed fusion] or DMLS [direct metal laser sintering] are different from MIM powders in the sense that MIM powder tends to be much finer and not necessarily spherical in shape. Metal AM powders have historically been preferred to have spherical morphology, although some advances in this process suggest that we might be able to process non-spherical powder like flakes (chips) or water atomized powder through AM processes as well.”
Viswanathan notes that when powder is being “rolled on” by a spreading blade for powder bed fusion (PBF), flowability is particularly important. “Perfect spherical shapes are strongly preferred to prevent powder sticking,” he explains. “Think of walking on a layer of ball bearings versus sand. On the other hand, a process like directed energy deposition (DED), where powder is delivered via a carrier gas, is more tolerant of randomly shaped particles. The same is true of processes such as MIM, where flowability is often not a primary concern because the material is being squeezed anyway.”
Important Factors to Consider When Selecting Metal Powders for Additive Manufacturing
The most crucial consideration when it comes to powder selection for metal AM is, not surprisingly, chemical composition. “Chemistry is likely the dominant factor, but this is usually derived from the end application target,” says Dehoff.
“Conformance to chemical composition of the metal powder is crucial,” adds Saharan. “Especially in applications where material properties and mechanical performance is critical, like aerospace and medical. This would include the oxygen content in the material, which would affect the aging of the powder in this process.” Viswanathan also cites the presence of nitrogen being a concern in some cases, depending on AM process.
Beyond chemistry, Dehoff and Saharan both noted the importance of powder cleanliness (i.e., free of contaminates and surface oxides), powder uniformity (consistent particle size distribution results in uniform melting and minimized porosity in the final product) and powder consistency. With regard to this last attribute, “Batch-to-batch variation during production can lead to variations in the end performance of the printed component,” explains Dehoff. “These variations are traditionally very difficult to track down and eliminate.”
Other factors to consider are powder flowability and spreadability. The former is important if you’re using a top-fed or gravity-fed machine, while the latter matters for uniform powder layers. Both derive from the other factors cited above but, as Saharan points out, one must nevertheless be careful to distinguish spreadability from flowability, as these are very distinct features.
Viswanathan points out that although flowability is often cited as a primary consideration because of how common PBF is, “mileage may vary depending on which AM process you’re considering.”
Finally, there’s the ever-present question of material cost. As with more traditional manufacturing processes, striking a balance between material performance and cost is not easy, especially for cost-sensitive applications. “A metal powder that can be atomized is not enough,” says Saharan. “It also needs to be processed at high enough speeds inside the machine to make a profitable business case.”
To sum up, there are seven important factors that need to be considered when selecting metal powders for additive manufacturing applications:
- Chemistry
- Cleanliness
- Uniformity
- Consistency
- Flowability
- Spreadability
- Cost
Refractory Metals & Additive Manufacturing
One of the often-touted advantages metal AM has over subtractive processes such as milling and turning is its ability to work with metals that are difficult to machine (DTM). These include higher grades of steels, such as stainless steels and hardened steels, as well as non-ferrous metals such as titanium, tungsten and various nickel-based alloys. While it’s true that DTM metals are easier to work with when producing workpieces additively rather than subtractively, refractory metals with high melting points, such as molybdenum and tungsten, present their own challenges.
“The refractories, due to their high melting point, have a higher transition temperature than what most machines can accommodate today, which makes these materials very brittle under the current processing conditions,” explains Saharan. “As a result, these material processes tend to suffer from microcracking and lack-of-fusion defects, especially in bulky parts.”
Dehoff notes that ORNL has dedicated significant resources to improving the printing process for tungsten, molybdenum and other refractories. “It is difficult to put enough energy into the material in order to melt it,” he says. “If you can get the material melted, cracking during solidification or upon cooling can be a major issue.”
Because refractory metals require very high energy densities—even compared to DTM metals like titanium and certain steels—the powder ages more aggressively. This is due to the larger heat affected zones (HAZ): non-melted areas that undergo changes in material properties due to their exposure to high temperatures. Viswanathan notes that, for this reason, tungsten powders—specifically tungsten carbide—are more common in powder metallurgy applications where the cooling rate can be more accurately controlled in a furnace, such as making cutting tool inserts.
“The other challenge is the speed of the process is very slow, owing to their high melting points,” says Saharan. “This tends to make the parts from these processes non-commercially viable in many cases.”
Metal Powders & Different AM Processes
Metal 3D printing encompasses a multitude of technologies, including PBF, DED and metal binder jetting (MBJ). Moreover, these three broad categories can be further broken down by process.
For example, PBF includes electron beam melting (EBM) and laser beam melting, sometimes called laser powder bed fusion (L-PBF) or selective laser melting (SLM). Direct energy deposition includes wire arc additive manufacturing (WAAM) and electron beam additive manufacturing (EBAM).
While it can be difficult to keep track of all these processes and their accompanying acronyms, one basic distinction that may be helpful in the context of metal powder is whether the process involves spreading layers of metal powder in a bed (PBF and MBJ) versus flowing metal powder through a nozzle (DED). Each process was developed with specific powder requirements in mind.
“Laser powder bed fusion, for example, typically uses powders between 15-45 microns, while DED powder is typically larger, 15-150 microns,” says Dehoff. According to Saharan, coarser powders can be advantageous in DED because they allow for higher deposition rates, which is important for larger-scale applications. “However, particle size distribution should be controlled to avoid clogging or inconsistencies during the deposition process,” he adds.
In addition to powder particle size, Viswanathan highlights the importance of the spot size of the laser. “Ideally, you’d want a large number of particles within the spot to ensure pore-free fusion,” he says.
Advice for Storing and Handling Metal Powders
Storing and handling metal powders properly is important not only for maintaining their purity by preventing contamination, but also for the sake of safety. Due to their high ratio of surface area to volume, metal powders can be highly reactive and present health hazards from inhalation. Even metals that are normally considered relatively benign can pose toxicological risks in a finely powdered form. Metal powders can also be explosive. As Viswanathan points out, aluminum powder is used as fuel in solid rocket boosters for this very reason.
In short, from the perspective of health and safety, a pound of aluminum powder is considerably more hazardous than a one-pound aluminum billet. For this reason, metal AM experts usually recommend special training and expertise to ensure safe handling and storage of metal powders. “ORNL tests all of the powders we use during processing to understand the flammability, combustibility and any other dangers that may result from the use of powders,” says Dehoff.
Saharan recommends storing metal powders in a dry and controlled environment, away from moisture, humidity and heat sources. Airtight containers made of non-reactive materials that are compatible with the powder are best for preventing contamination. Personnel should wear gloves, respirators, and other personal protective equipment (PPE) to avoid any direct contact with the powders, and they should be trained in safe handling practices as well as emergency procedures.
“Familiarize yourself with the MSDS [Material Safety Data Sheets] provided by the powder supplier and incorporate them into the risk mitigation plan for machine operators,” advises Saharan. “Careful attention also needs to be paid to their disposal according to local safety guidelines. The MSDS contains essential safety and handling information specific to each powder.”
Recycling Metal Powders for Additive Manufacturing
Metal AM is often cited as being more cost-effective—and even more sustainable—than traditional machining because the former uses less material than the latter. Producing parts additively rather than subtractively means that by the end of a build, all you’re left with is the part, rather than the part and a pile of metal chips. This raises questions of whether and to what extent metal powders are reusable in additive processes.
“Over time, AM powders can degrade in quality,” says Dehoff, “and there are many research papers looking into how often powders can be used in an AM process before they recommend being discarded.” He notes that because powders have considerably more surface area than bulk materials, this can lead to the accumulation of moisture and impurities from the atmosphere, such as oxygen. As a result, recycling powders can be difficult without resubjecting them to the full refining process.
“If suitable chemical protocols can be established for minimizing chemical contamination (oxides are the usual culprit, at least for steels) then there is a strong case to be made for using recycled powders,” says Viswanathan. “However, a suitable AM process must be chosen for this (DED works well, PBF not so much) so that the resulting powder morphology can be handled without hassle.”
In addition to the potential for contamination, Saharan adds several other points that must be kept in mind. These include:
- Changes in particle size distribution
- Safety concerns from the added risk of exposure or explosion
- Difficulties meeting regulatory and quality standards, especially in the aerospace and medical industries
“Despite these limitations and risks, recycling metal powders is always a viable option and pursued in almost all industries using metal AM,” adds Saharan. “Proper powder handling, storage and recycling practices, along with rigorous material testing and quality control, can help mitigate some of these challenges and improve the sustainability of AM processes.”
Metal Powders for AM: What You Need to Know
Although metal powders have been around for centuries, there’s still considerable room for improvement when it comes to their use in additive manufacturing. If you’re just getting started with metal 3D printing, these are the questions you should ask yourself to help ensure success:
- Is my application better suited to gas atomized powders or water atomized powders?
- What are my requirements in terms of chemistry, cleanliness, consistency, etc.?
- Does my application involve refractory metals that may not work well with AM?
- Do I have everything I need to safely store and handle metal powders?
- Are there any constraints that would make it difficult to recycle my metal powders?
If you know the answers to these questions, you should be well prepared to begin your journey into metal additive manufacturing.