For some engineers, 3D printing may be coming on in an unwelcome rush. Pressured by management forces who have read how additive manufacturing (AM) will save the world, reducing a hundred-part assembly into one part, or making super-lightweight parts with only skins over lattices, the engineers are subjecting the 3D-printed components and process to the microscope and, in some cases, have found them wanting.

Background

The dominant form of metal 3D printing is powder bed fusion, in which an energy source—a laser in the case of selective laser melting (SLM) or an electron beam in electron beam manufacturing (EBM)—fuses particles of metal powder together point by point, layer by layer until an object is complete. Powder bed fusion systems have mechanisms for controlling the energy source and the distribution of powder.

A diagram of the powder bed fusion process. (Image courtesy of Wikipedia.)

Directed energy deposition (DED) and binder jetting are also used to 3D print metal objects. In the case of the former, powder or a metal wire feedstock is introduced to an energy source. In the case of the latter, a liquid binder is deposited onto a bed of metal powder. After the print is complete, the object is heat treated and sintered in a furnace.

Various configurations of DED processes. (Image courtesy of Wikipedia.)

In the metal 3D printing process, a number of issues can occur that machine operators attempt to avoid. These include porosity, residual stress, density, warping, cracking and surface finish.

Surface Finish

Before a 3D-printed metal part hits the showroom floor or the fuel chamber of an engine, it has already undergone significant post-processing in the form of CNC machining, shot peening or sandblasting. That’s because the metal 3D printing process is a bumpy one when it comes to surface finish.

Two Ti-6Al-4V titanium alloy brackets 3D printed via EBM, before and after machining. (Image courtesy of NASA.)

Due to the nature of the process, DED typically produces near-net-shape parts that must be CNC machined to meet specifications upon printing; however, powder bed fusion parts are somewhat closer to their final shape when produced. Nevertheless, the surface finish is still rough. To reduce this, finer particles of powder and finer layer thicknesses can be printed.

This, however, drives up the cost of the material. Therefore, there may be a tradeoff between surface finish and cost. Because all powder bed fusion parts will be refined to specification during post-processing, it sometimes makes more economic sense to use slightly larger particle sizes anyway. This also means that surface finish is less important than some of the other issues that can occur in metal 3D printing, since the part will undergo some level of post-processing regardless of how smooth or rough the end part is.

Porosity

Porosity occurs when very small cavities form within the body of a part as it is being printed. This can be caused by the 3D printing process itself or even from the powder used in the process. These microscopic holes reduce the overall density of the part and can lead to cracks and fatigue.

Light optical microscopy showing comparison of process-induced, lack of fusion porosity to entrapped, gas porosity transferred from the powder feedstock. From a study titled "The Metallurgy and Processing Science of Metal Additive Manufacturing". (Image courtesy of International Materials Reviews.)

During the powder atomization process, gas pockets can form within the powder feedstock, which will then be transferred to the final part. For this reason, it’s necessary to purchase materials from a reputable supplier.

More often, pores can occur within a part due to the 3D printing process itself. When not enough power has been applied, the laser intensity is too low for example, the metal particles may not fuse adequately. When too much power is used, on the other hand, a phenomenon known as spatter ejection occurs, in which melted metal flies out of the melt pool and into adjacent areas.

Pores may also be formed when powder particles are larger than the layer thickness of a part or if the powder is too loosely packed. It can also occur if melted metal does not flow adequately into the desired melt region.

To address these problems, most machine operators basically have to tune their machines for a given material and print job. The machine parameters, such as the power, spot size and spot shape of a laser, will be modified for a given material and print job until porosity is minimized.

Porosity may also be reduced in power bed fusion processes through the use of an “island scanning” pattern employed by the laser. This checkerboard pattern features alternating unidirectional fills to reduce temperature gradients by distributing the heat.

In SLM, the shape of the laser can be modified to reduce spatter ejection. Known as “pulse shaping,” it’s possible to heat the melt area gradually. For EBM, powder particles can be ejected from the powder bed by the electrical charge. This can be reduced by quickly scanning the print bed and the diffuse beam to perform pre-melting of the powder.

Jim Gaffney, additive metal/DMLS lab manager at Forecast 3D, gave this advice for reducing porosity: “With the SLM process, density of over 99 percent can be achieved with quality raw materials, the proper build parameters and an environmentally controlled machine lab. Hot isostatic pressing treatment as a post-process can aid in the removal of any remaining porosity in final parts.”

It may also be possible to infiltrate powder bed fusion parts with another material, such as bronze. However, the use of auxiliary materials will naturally affect the chemistry of the primary metals, potentially disrupting the desired application of the part in the first place.

Density

The porosity of a part is inversely related to its density. The more pores a part has, the less dense it is and the more likely it is to experience fatigue or to crack under pressure. For critical applications, a density of above 99 percent is required.

In addition to controlling for porosity through the methods mentioned above, particle size distribution may aid in increasing the density of a printed part. Spherical particles will not only improve the flowability of the powder, but can also improve density. Additionally, a wider particle size distribution allows fine particles to fill spaces between larger particles, resulting in higher density. Unfortunately, this may decrease the flowability of the powder.

Flowability is necessary to ensure an even, densely packed layer of powder, which, as you might guess, affects both the porosity and density of a 3D-printed part. The more tightly packed a layer is, the denser and less porous the final object will be.

Jack Beuth, a professor in the department of mechanical engineering and the director of the CMU NextManufacturing Center at Carnegie Mellon University, was able to shed light on the advances being made in defining the parameters associated with porosity and density in metal 3D-printed parts.

“Maximum density (which translates to minimum porosity) is important as manufacturers move toward fabricating parts that will be subjected to cyclic (fatigue) loading in their applications,” Beuth explained. “In research carried out within our NextManufacturing Center, we have demonstrated that porosity from a variety of sources can be controlled and effectively eliminated by manipulating AM process variables. There is no one process that is particularly better at avoiding porosity than others, but for each process, there are combinations of process variables (we define them as a ‘processing window’) that do so.”

Residual Stress

Residual stress is a result of heating and cooling, expansion and contraction, that occurs during the metal 3D printing process. When residual stress exceeds the tensile strength of the printing material or substrate, defects, such as cracking in the part or warpage of the substrate, can occur.

A titanium part is ripped from the build plate during a powder bed process due to residual stress buildup. (Image courtesy of Penn State CIMP-3D.)

Residual stress is in its highest concentration at the interface between the printed part and the substrate onto which it adheres. The stress is more compressive at the center of the build and tensile at the edge of the build.

Support structures are implemented to reduce some of the residual stress because they will have a higher temperature than the substrate alone. Once the part is removed, the residual stress is relieved, but the part may become deformed in the process.

Lawrence Livermore National Laboratory researchers have shown that one method for reducing residual stress is by decreasing the laser scan vector length, instead of relying on a continuous laser scan, in order to regulate fluctuations in temperature. Rotating the laser scan vector in relation to the largest section of the part may also help.

Another method for reducing residual stress is through the heat treatment of the substrate and heating the material before it is hit by the energy source. Due to a lower operating temperature, preheating the substrate is more often possible with EBM than it is with SLM or DED.

Based at the company’s Metal 3D Printing Competence Center in Bremen, Germany, Ingo Uckelmann is the technical manager of metal 3D printing for 3D printing service and software company Materialise. Uckelmann explained that it’s necessary to control for residual stress in metal 3D-printed parts at three different stages, “during the data preparation, during the actual printing process and then after the parts are built.”

“We use Materialise Magics during our data prep to choose an optimal build orientation and thereby prevent warpage or later stress-related deformities,” Uckelmann said. “Magics also helps with firmly anchoring the parts to the build platform and uses volumetric support elements to conduct heat effectively.”

Uckelmann pointed out that support structures play an important yet “somewhat counterintuitive” role in the metal 3D printing process. “On the one hand, they need to counteract the stresses generated in the metal AM process and hold the part in place, while, on the other hand, they remove the heat generated by the process, as excess local temperatures may lead to poor surface quality and/or poor mechanical properties,” said Uckelmann. “Magics uses patented hybrid support structures to reconcile these two roles.”

“Then, during the printing process, we use machine communication software—the Materialise Build Processor—to divide parts into a hull and a core,” Uckelmann continued. “Each are built with a different scanning strategy. Build processors can also assign different scanning strategies to different types of support structures. For instance, support structures can be scanned every two layers, speeding up the scanning process and reducing stress. Following the printing process, as a third layer of protection against stress-related deformity, we use heat treatment on all 3D-printed metal parts.”

Cracking

In addition to springing from pores within a part, cracking may occur when the melted metal solidifies or during further heating of an area. If the energy source is too strong, stress may occur during the solidification process.

Stress has caused these metal bars to crack during the powder bed process. (Image courtesy of Albert To and University of Pittsburgh’s Swanson School of Engineering.)

Delamination may also occur, leading to cracks between layers. This may happen as a result of the powder not melting sufficiently or layers below the melt pool remelting. Some cracks may be repaired through post-processing, but delamination is not one of them. Instead, substrate heating may be implemented to reduce this issue.

Beuth was also able to speak to how cracking may occur in metal 3D printing. He pointed out that cracking and its effect on part performance are not limited to AM, but are concerns in traditional casting and other metals processing methods.

“As a rule, we do not see cracking occurring during AM processing for alloy systems that are supported by machine manufacturers,” Beuth said. “However, as users begin to experiment with fabricating components out of unsupported, more brittle, hard-to-weld alloys, cracking can be a concern. Similar to the control of porosity, cracking during processing can be reduced or eliminated by manipulating process variables. This is an active area of research in the AM community.”

As cracking occurs during component use, such as under fatigue loading, Beuth said, “[M]anipulation of AM process variables can largely control such flaws. An important item to note is that it is not necessary to eliminate all pores or flaws in a fabricated part. What is important is to know what pores or flaws might exist. If that is known with confidence, engineers can take it into account in their component designs and still fabricate reliable, safe parts.”

Warpage

To ensure that a print job begins properly, the initial layers of a print are fused with a substrate that must be removed via CNC machining after the print is complete. However, if the thermal stress of the substrate exceeds the strength of the substrate material, the substrate will begin to warp, ultimately causing the part itself to warp and potentially causing the powder recoater to collide with the part.

(Image courtesy of the Center for Additive Manufacturing and Logistics at North Carolina State University.)

Carl Dekker, President at Met-l-flo Inc and Chair of the ASTM F42 committee on standards for AM, explained how this phenomenon occurs. “You’re dealing with a number of thermal factors going in the process, even more when you have varying thicknesses of material. So you’re going to have additional stresses.” Dekker said. “You’ve got a lot of state changes happening rapidly. Sometimes, it could be that the part’s going to rip away from the supports or also called anchors, causing it to separate undesirably. It could be that you’ve got enough anchors and the pulling is on the platform. It literally can start twisting the platform, not as you’re building it, but when you release that platform from the machine or go thru secondary processing.”

Therefore, in order to prevent warpage, it’s necessary to place the ideal number of support structures in the right locations. This can be difficult to determine without performing trial and error with each new geometry that gets printed. There are software solutions in development, such as print preview–style software created by 3DSIM.

Materialise’s Inspector software can also be used to perform quality control operations in metal 3D printing by building up process knowledge associated with a machine. As Vincent Wanhu Yang, product manager of Inspector at Materialise, explained, “At Materialise, we noticed the need for more detailed quality control, so our Inspector software processes piles of build images to increase the process knowledge of the user and indicate areas that could have been affected by warpage. By performing a root cause analysis and checking the vectors, the user can, for instance, detect that support structures were lacking, which caused deformation. Understanding the build process is essential to achieve a successful metal build next time.”

Other Issues

Other distortions, such as swelling or melt balling, can occur during the metal 3D printing process as well. Swelling takes place when the solidified metal rises over the powder. Similarly, melt balls are created when the material solidifies into a spherical shape instead of solid layers. This is caused by issues related to surface tension connected to the melt pool and may be mitigated by controlling the length-to-diameter ratio of the melt pool to less than 2 to 1.

Melt ball formation and delamination in EBM stainless steel. From a study titled "The Metallurgy and Processing Science of Metal Additive Manufacturing". (Image courtesy of International Materials Reviews.)

Exposure to oxygen and humidity can cause metal alloys to change composition. For instance, as oxygen increases in Ti-6Al-4V titanium, the aluminum content may decrease. This is particularly true when metal powder is recycled. Recycling also causes powder particles to become less spherical and to flow less well.

The printing process may also see changes in the composition of metal alloys as well. In a metal alloy, in which multiple metals are combined (i.e., alloyed), the metal with the lower melting point could potentially evaporate in the process. In the case of Ti-6Al-4V, a popular metal for 3D printing in aerospace, titanium has a much higher melting temperature than aluminum, and it’s possible to alter the composition of the material during the 3D printing process.

As Bill Bihlman, the founder of boutique aerospace management consultancy Aerolytics, explained, “If you put too much energy in, you can vaporize the aluminum. You can also burn through the additional layer below. Every time you reheat and cool that area, it affects the residual stress, and eventually it degrades the material itself.”

Making Metal 3D Printing Better

As one might guess from reading this, avoiding issues in metal AM still requires a lot of process knowledge and trial and error. Every geometry alters the variables of the machine, often forcing machine operators to print the same part several times in order to finally overcome issues such as warpage, cracking and porosity. Once successfully complete, the components have to be tested to ensure they meet the proper standards.

As may also be clear from reading this, the industry is building up knowledge regarding each metal 3D printing process. Businesses who have nailed down their systems’ processing parameters may not be the first to explain exactly what they did to achieve successful outcomes.

“There is fight to primacy so first movers for any of these companies that go into that space are able to differentiate themselves,” Bihlman explained. “They’re using the fact that there’s a very steep learning curve and this is a frontier, and they’re not disseminating anything more than what they have to, which isn’t very much.”

One option to access some of this expertise is to join a trade organization, such as America Makes, in which industrial members share data with one another as they research new technologies and define processes. Outside of such organizations are research institutions that are publishing data publicly. Also, among the first movers in the industrial AM space are software companies, such as Materialise and 3DSIM, who are developing software solutions to address metal 3D printing issues.

For now, the wild frontier of metal 3D printing remains untamed. The landscape will be very different in the next five years. As Beuth said, “One of the predictions made by our NextManufacturing Center is that within five years, the ability to define these processing windows to eliminate porosity effectively will be a widespread industry practice.” When that happens, not only will individual industry players be able to capitalize on the knowledge gained from metal 3D printing, but the industry itself will as a whole.