The Best 3D Printer Materials: Metals Edition

As powerful as plastics are for 3D printing, nothing says “critical parts” like metal additive manufacturing (AM). As humans learned long ago, metals can provide strength like no other material can. As we’ve continued our journey on this Spaceship Earth, humans have harnessed metals for ever greater specialized applications, from radiation shielding on spacecraft to conductive components on printed circuit boards (PCB).

ENGINEERING.com hopes to break down the wide variety of metals used in 3D printing as best we can, but, before we can do that, what does metal 3D printing look like?

Powder Bed Fusion Processes

There are two broad categories of direct metal 3D printing, with some indirect forms and several emerging technologies that may have a profound impact on the industry. Regardless of the form, the metals themselves only change with regard to the delivery method necessary for a given process.

Selective laser melting (SLM) and direct metal laser sintering (DMLS) are terms used to describe powder bed metal AM technologies in which an energy source is directed at a bed of metal powder. In the case of SLM, the particles are fully melted together, whereas in DMLS, the particles are only sintered. While both technologies feature a high powered laser, electron beam melting (EBM) is a specific form of SLM invented by Arcam, in which powder is melted with an electron beam.

Parts made with powder bed fusion can have a high amount of geometric complexity, despite the fact that metal support materials are required to make them. This means that internal cavities may be difficult to produce because support structures within hollow parts need to be machined after printing.

Directed Energy Deposition

The other dominant form of metal 3D printing is directed energy deposition (DED), whereby a metal wire or powder is fed directly to an energy source as it is deposited. With DED, more than one material can be printed at a time and the use of a multi-axis system makes it possible to 3D print materials onto an existing part (to add features or for repair purposes).

Some forms of DED may require specially refined powders, but others can use materials already on the market for more traditional forms of manufacturing. For instance, Sciaky’s Electron Beam Additive Manufacturing (EBAM) technology relies on metal wire made for the welding industry, fusing the material with an electron beam at a rapid rate. “Our process uses welding wire as a feedstock,” stated John O’Hara, Global Sales Manager for Sciaky. “The wire feedstock is typical welding wire, a supply chain that has existed for decades.”

The reliance on welding wire opens the EBAM process to the wide range of materials already on the market. “The most common alloys we work with, Titanium and Nickel alloys, exhibit excellent properties relative to the wrought and forged properties.  Unique to our process, refractory alloys such as any versions of Molybdenum, Tantalum, Tungsten and Niobium, exhibit excellent properties and geometries with our process,” O’Hara said.

Though powder bed parts can be treated to reach densities of close to 100 percent, DED processes are even closer to the properties of forged metals. As O’Hare explained, “Sciaky’s metal is typically going to be nearly completely dense, and will have properties that will approach or meet the requirements of forged or wrought metals.  As with any AM process, the results will heavily depend on the material and the post-deposition heat treatment. By completely dense, we mean what little porosity would be found (nothing is perfect) would be considered micro porosity, and will occur at a very low frequency:  in general, will meet the inspection requirements of Forgings.”

The geometric complexity of DED parts is somewhat limited, with most processes producing a near-net-shape part that is further machined to achieve the final product. That said, what DED lacks in geometric complexity, it makes up for in speed and size. Of note, Sciaky makes the largest metal 3D printing machines currently available.

Other Metal 3D Printing Processes

There are also binder jetting methods for 3D printing metal parts. Companies like ExOne manufacture systems that deposit a binder material onto a bed of metal powder. Once printed, this “green” object is placed in a furnace, where the binder is burned out and the part is combined with bronze to create the final part.

Fabrisonic uses an interesting hybrid technique called Ultrasonic Additive Manufacturing to fuse metal foils. The technique requires ultrasonic welding before a CNC tool cuts the part from the excess foil material. This method makes it possible to combine different types of metals, and because no melting occurs, electronic components can be incorporated into the part without fear of destroying them.

Emerging technologies from Markforged, Desktop Metal and Admatec also use an indirect form of metal AM. In the case of Markforged and Desktop Metal, metal powder in a thermoplastic matrix is deposited on a printbed similar to a fused deposition modeling (FDM) process. The part is then sintered in an oven, which burns out the thermoplastic binder. In contrast, Admatec exposes a photopolymer loaded with metal particles to UV light, hardening the object layer by layer. The green part is then sintered in an oven.

Finally, XJet has invented an inkjet approach to metal 3D printing. Their method deposits metal nanoparticle inks from a printhead within a heated chamber. Desktop Metal appears to have developed a similar technology which will be released in 2018.

Making Metal Powders

In powder bed fusion processes, highly refined and often expensive metal powders are used. These powders are usually produced through a gas or plasma atomization process, where induction heating or plasma torches are used to melt metal. Molten metal is then poured into an atomization chamber and high velocity gas is used to break up the metal stream into small droplets which fall through a vertical chamber as they solidify.

LPW Technology is a UK-based firm that is focused entirely on the production and supply of metal powders and control and monitoring technologies for AM. The firm uses a variety of processes to produce a very wide range of materials for different AM technologies. John Hunter, General Manager of LPW Inc., explained to ENGINEERING.com that gas atomization is used for over 90 percent of metal powders for AM, but that plasma-based methods are used for higher purity metals such as titanium based alloys or nickel-based superalloys.

“Plasma will give you a better morphology, a more spherical particle than gas atomization. Gas atomization also produces spherical particles, but not quite as perfect,” Hunter said. Both of these processes differ from water atomization (utilized to produce the majority of metal powders outside of AM) for metal injection molding, hot isostatic pressing and other applications. However, the particles produced by water atomization are more irregular, making them difficult to use in 3D printing, partially due to the smooth flow needed in AM.

Whereas Directed Energy Deposition (DED) processes that use powder work with coarser particles – which can be in excess of 100 microns – and Electron Beam Melting (EBM) operates with metal powder particles between 45 and 100 microns, other powder bed systems use powder particles between 10 and 45 microns in size.

Due to the various patents related to powder bed processes, manufacturers have often adopted proprietary methods for spreading powder across the bed during printing, as well as refilling the build platform. While one firm’s machines may use flat metal bars to spread powder and gravity to refill the build area, others use a cylindrical roller and elastomeric material to spread the powder and a piston feed to refill the chamber.

“When you’re talking about Particle Size Distribution (PSD), one of the things that LPW does is that we sell to a number of machine OEMs. We know which powder characteristics work with various machines. They’re all a little bit different,” Hunter continued. “Some are more forgiving about a powder that flows less well than another.”

For that reason, when LPW sells material to a customer (whether it’s an end user or research lab), the company will ask what type of machines are being used and will provide the range of acceptable particle sizes for those machines.

Other factors to be considered when producing powder include the actual chemistry of the alloy itself, as well as the density and porosity of the powder. Those last two variables are particularly important for DED because the larger sizes of the particles leave more room for gas used in the manufacturing process to create bubbles within the material. This can ultimately end up in the 3D-printed part, resulting in porosity which can ultimately lead to cracks or changes in performance.

For that reason, LPW offers a variety of services and products in addition to the powders themselves. This includes software and sensors for recording and monitoring powder quality, tools for verifying and maintaining powders, labs for analyzing the material and interpreting a company’s powder-related data, consultation services and powder lifecycle management.

An Alternative Form of Metal Powder Production

Outside of the more traditional processes for producing metal powder for AM, there is a method for producing metal powder known as electrolysis, which is characterized by being more energy efficient and generally more controllable in terms of the particle output.

Electrolytic powder manufacturing is an electrochemical process in which a metal oxide is introduced to an electrified salt bath, usually made up of molten calcium chloride. As the current passes between the metal oxide (acting as the cathode) and a carbon anode, oxygen is removed from the metal oxide. The result is a purified powder form of the metal, which is then cleaned and dried for use.

A leading manufacturer in the AM realm that uses electrolysis is the UK company Metalysis. Dion Vaughan, CEO of Metalysis, explained to ENGINEERING.com that the company’s process offers a number of benefits when compared to other forms of powder production.

“With processes like plasma atomization, you get a normal distribution of particle sizes,” Vaughan said. “Essentially, if you are trying to produce powders for AM, the range of particle sizes may be a very narrow slice of what you actually convert and what you’re actually producing. If you’re producing 100 tons of material per year, the particle range for a given AM system, such as SLM, may be just 10 tons, for instance.”

With electrolysis, it’s possible to control the process in such a way that almost all of the output can be used for a specific AM system. Therefore, if a company is producing powder for an SLM machine from SLM Solutions, the process can be tuned to manufacture particles only in that necessary range, and the same would be true for an EOS machine or a DED system.

Because electrolysis operates at temperatures between 800°C and 1000°C, the energy required is much less than what would be needed to melt the same amount using heat. “If you compare our process to conventional titanium powder production, plasma atomization, you’re using roughly 50 percent less energy, we estimate,” Vaughan said.

This energy reduction has environmental consequences, but also factors into a reduced cost for the customer. Additionally, electrolysis can be used to produce powder from a much broader specrum of metals, regardless of the high melting temperature.

Metalysis is now on its fifth generation of the technology, which began as an R&D development, and is about to undergo feasibility studies to expand its fully-fledged powder production capabilities. Vaughan stated that the Generation 5 powder production system will expand on the Generation 4 system Metalysis is scheduled to complete this year. Generation 4 will be capable of producing 20 tons of light powder and 60 tons of heavier metals, and Generation 5 will provide manufacturing options for hundreds to thousands of tonnes per annum of high value metal and alloy powders. The technology will be available through a unique licensing model which is flexible depending on the need of the customer.

Titanium

Though Metalysis’ process is capable of producing a variety of metal powders, the company’s primary focus is on titanium. “In general terms, titanium is a wonder metal,” Vaughan asserted. “It is light, but strong and very corrosion resistant.”

The primary forms of titanium available for 3D printing are titanium alloys 6Al-4V (known as grade 5 or Ti64) and 6Al-4V ELI, referred to as grade 23 or Ti64ELI. Grade 5 titanium is the most widely used form of titanium on the planet due to its utility. The material can be welded, heat treated for increased strength, can withstand temperatures of up to 600° F, has a high strength-to-weight ratio and offers high corrosion resistance. For these reasons, grade 5 titanium is often found in high performance industries like aerospace, medical, marine and chemicals.

Grade 23 is a higher purity and biocompatible form of this alloy that can be shaped into coils and wires while maintaining a high strength-to-weight ratio, corrosion resistance and toughness. The material is often found in biomedical applications, including surgical devices and implants.

Aluminum

The two most common forms of Aluminum alloys available for 3D printing are AlSi12 and AlSi10Mg. Though both are made up of a combination of aluminum and some silicon, AlSI10Mg also includes magnesium. Both are casting alloys that are particularly useful for creating parts with thin walls and complex geometries.

These materials feature impressive strength and hardness and can be used for high loads. Their light weight and temperature resistance make them ideal for applications such as motorsports or the interior parts of aircraft. They can also be easily post-processed via machining, welding, shot-peening and polishing.

Steel

There are a variety of steel materials that fall into the three broad categories of stainless steels, tool steels and maraging steels. Maraging steels are created through an extended heat treatment process that enable high strength and toughness, without losing malleability. This means that they can be machined easily after printing and further hardening. As a result, maraging steel may be used for series production parts and tooling.

Stainless steels are known for their high abrasion, wear and corrosion resistance. Therefore, these materials are often used in applications such as cutlery and surgical instruments, and in the construction of acid or corrosion resistant parts.

Tool steels are used to produce tools and molds for manufacturing due to their hardness, abrasion and deformation resistance, and the ability to maintain a cutting edge, unlike maraging steels. Tool steels can then stand up to the wear and tear required to form other materials. Once 3D-printed, unique cooling channels can be integrated into tool steel parts, optimizing the injection molding process.

Cobalt

A variety of cobalt-chrome alloys are available for 3D printing and often employed for their high strength, hardness, and resistance to corrosion and high temperatures. Cobalt is usually combined with materials like chromium and tungsten to create heavy duty cutting tools or dies, as well as with magnetic and stainless steels for jet and gas turbine components.

Nickel

Nickel-based superalloys Inconel 625, 718 and HX (all made from nickel and chromium) are the most predominant nickel alloys in 3D printing. These materials are heat, oxidization and corrosion resistant and exhibit high strengths in very elevated temperatures up to 1200 °C. Parts made from nickel alloys are very weldable and can be post-processed with heat treatment for further strength. Altogether, these materials are used in the aerospace and motorsport industries in cases where high temperatures and oxidation are a significant risk, such as combustion chambers and fans.

Though Inconel 625 has higher corrosion resistance and long-term stability under high temperatures when compared to 718, the latter is about twice as strong and much more conductive. Inconel HX may be the most weldable of the three varieties.

A representative from powder manufacturer Additive Metal Alloys (AMA) told ENGINEERING.com at the AMUG event in Chicago this year that while AMA manufactures a variety of materials, nickel alloys are a big focus for the company. Located near the GE Aviation plant in Cincinnati, Ohio, AMA sees aerospace as a big market for nickel-alloys due to their strength, resistance, and hardness when heat treated.

“Titanium, is not as heat resistant, but it’s very lightweight and strong, giving it a very high strength-to-weight ratio,” the representative explained. “Nickel-alloys are heavier, but because of the heat resistance the material, it can operate inside of an engine.”

Copper

Copper is not widely available in 3D printing, but a couple of companies do manufacture copper alloy powder for powder bed fusion processes. Additionally, DED systems may use copper materials already available to the welding industry. The material can be used for jewelry and the arts because of the greater aesthetic and hardness values when compared to silver alloys.Copper is also being used in aerospace.

NASA’s Materials and Processing Laboratory at Marshall Space Flight Center and Aerojet Rocketdyne have used copper alloys in powder bed fusion systems to 3D print rocket engine components with unique cooling channels. The high conductivity of the material makes it possible to quickly heat rocket fuel.

Precious Metals

Precious metals available for 3D printing include silver, gold and platinum. These materials are often ductile, shiny and less reactive. In many cases, they may also be highly conductive. Along with Concept Laser, Cooksongold is one of the few to offer 3D printing with gold (yellow, rose and white) and platinum. The main applications for these materials are for jewelry and artistic objects.

Silver nanoparticle inks have been utilized by several companies like Voxel8 and Nano Dimension to print conductive traces on parts. Nano Dimension, in particular, is working to develop nickel-based and copper-based inks, which have higher conductivity. Silver inks make it possible to 3D print circuits, either for PCB prototyping or to integrate electronics directly into a 3D-printed object.

Refractory Metals

Refractory metals are a small class of metals (niobium, molybdenum, tantalum, tungsten, and rhenium) known for their extremely high heat and wear resistance. They all maintain melting points above 2000 °C, are chemically inert, have high densities and are very hard.

Tantalum has a high corrosion resistance and is very conductive, which makes it useful for electronics. According to Los Alamos National Laboratory, 60 percent of the material’s use is for vacuum furnace parts and electrolytic capacitors. It has been theorized that Tantalum could be used to increase the radioactivity of nuclear fallout, a terrifying process known as “salting”.

In its pure form, Tungsten has the highest melting point of any element at 6,192 °F (3,422 °C). Highly dense, the material is also difficult to work with, but its rigid composition makes it desirable for wear resistance in knives, drills, mills and saws. Tungsten is also very resistant to oxygen, acids and alkalis and can be used for radiation shielding.

Global Tungsten & Powders (GTP) is one of the few companies that produces tungsten, tungsten carbide and molybdenum powders, marketing only powders that have been successfully printed. Rick Morgan, R&D Manager of W and WC Powders at GTP, explained the process the company uses to make its tungsten materials: “GTP is vertically integrated and has the ability to take tungsten ore and chemically refine it to APT, oxidize and reduce it to W metal powder, take the W metal powder and carburize it to WC, spray dry it with cobalt, and then spheroidize it for use in Additive Manufacturing,” Morgan said.

ExOne offers a bonded tungsten material for its binder jetting process. The company markets the material as a replacement for lead, which is more toxic, in medical devices and aerospace parts. GTP’s tungsten carbide cobalt material was successfully binderjet printed by ExOne, and the company has developed a debinding/sintering schedule to achieve acceptable density.

The Future of Metals

SmarTech Markets Publishing has estimated that the market of metal powders for AM would grow to $930 million by 2023, and recently suggested that this growth would be driven in large part by the aerospace industry. Davide Sher, Senior Analyst for the European division of SmarTech and founder of 3D Printing Business Media, was able to provide some insights by relying on the company’s recent report on metal powders, including which materials will be the most popular.

“For the foreseeable future the most used metal AM powders are going to be Steel, Titanium, Nickel Alloys (Inconel) and Cobalt Chrome,” Sher said. “Titanium is the most used in aerospace because cost is not an issue and the slightly improved performances compare to other less costly metals justify the expense. Inconel—also costly—is also used primarily in aerospace and defense manufacturing. Titanium is also very much used in medical applications (implants) - once again costs are justified by performance.”

Dion Vaughan, CEO of Metalysis, also believes that titanium use will increase and that his company’s process will aid in a greater adoption of AM technology: “Historically, titanium production has been challenged by the fact that conventional processing routes were very energy inefficient and expensive, even today with advanced plasma processing,” Vaughan said. Electrolysis, however, is more efficient and less expensive, which should result in a greater use of metal AM and further drive down the cost of the powder.

Vaughan envisions the possibility of powder production co-located with the manufacturing process, which would enable greater efficiency. This would also be important for the emerging trend of distributed manufacturing, where part production is closer to the point of end use.

Davide Sher has stated that due to its primary use in the large AM segment of dentistry, cobalt chrome will be even more important for part production. Steel, the first metal powder for 3D printing, is usually the most common choice. Aluminum is much lower in cost, which makes it feasible for producing parts for the automotive industry, resulting in higher volumes of parts. Precious metals (particularly platinum) may have interesting applications, but their use will be limited, according to Sher.

“[T]he recent expansion of powder fed (DED, high deposition rate technologies) are going to drive up demand significantly,” Sher mentioned. “Now these technologies are no longer used just for part repair but also for large part production. Also the size and speed of powder bed fusion technologies is going up very quickly, almost doubling every two years.”

According to Sher, metal may not be king in industrial 3D printing, as high performance plastics like PEEK and PEKK, as well as carbon fiber reinforced materials can be used to replace metals for a lower cost. “[These] may take away some of the market for metal AM in aerospace and medical applications especially,” concluded Sher.

GE’s formation of GE Additive, after acquiring Arcam and Concept Laser, certainly speaks to the growth estimated by SmarTech. The purchase of Arcam gave the corporate behemoth simultaneous access to a 3D printer manufacturer as well as a titanium powder maker.

John Hunter, of LPW, used this example to suggest that more vertical integration would be coming. He added that powder manufacturers, including LPW, had been adding capacity for powder production. His branch in the United States is moving to a larger facility where it will focus on conditioning the materials. LPW is also increasing its activities related to recycling materials, a trend that Hunter said is increasing in popularity.

This growth, he pointed out, is being driven by the use of AM for production, as opposed to the prototyping it was initially used for. “The powder market is growing so quickly as more and more of these machines are installed,” Hunter said. “Instead of just making prototypes and then testing them for a couple of months, a lot of these prototypes are now in production. So, these machines are running daily or weekly building parts. They’re using a lot more powder now than they were just a year ago.” Hunter did stress that his optimistic comments about the future of AM and the power market are only his personal opinions and observations.