For some engineers, 3D printing may appear out of nowhere in an unwelcome way. Company managers saw reports on how 3D printing could save the world, how to reduce the assembly of hundreds of parts to one, and how to process ultra-light lattice structure parts with surface skin. Engineers were forced to study 3D printing components carefully. Parts and workmanship. In some cases, they get the results they want.
background
The most common form of metal 3D printing is powder bed fusion. This type of process uses a heat source (the SLM process uses a laser, and the EBM process uses an electron beam) to fuse the powder particles point by point and process them layer by layer until the object is completed. Powder bed fusion systems have heat sources and powder distribution control mechanisms.
Direct energy deposition (DED: Directed Energy Deposition) and binder jetting methods can also be used to 3D print metal objects. The former sends powder or metal wire to the heat source, and the latter deposits liquid binder on the metal powder bed. After printing is completed, the latter heat-treats the object and sinters it in the furnace.
During the metal 3D printing process, a large number of problems may arise that equipment operators try to avoid, including porosity, residual stress, density, warpage, cracks, and surface finish.
surface finish
Before a metal 3D-printed part is placed in a showroom or used in an engine combustion chamber, it has undergone a large number of post-processing processes, such as CNC machining, shot blasting, or sandblasting, because the surface of 3D-printed metal parts is uneven.
Due to the nature of the process, direct energy deposition produces parts close to the final shape, which must be CNC processed to meet corresponding specifications. Powder bed fusion produces parts that are closer to their final shape but still have a rough surface. To improve the surface finish, finer powders, and smaller layer thicknesses can be used.
However, this method will increase the material cost, so a balance needs to be struck between surface finish and cost. Since all parts produced by the powder bed fusion process require post-processing to meet specifications, sometimes using coarser particle-size powder can reduce costs. Because parts can undergo varying levels of post-processing operations regardless of how rough their surfaces are, surface finish is less important than other issues that may arise with metal 3D printing.
pore
During the 3D printing process of parts, very small holes inside will form pores, which can be caused by the 3D printing process itself or the powder. These micropores reduce the overall density of the part, leading to cracks and fatigue issues.
Optical microscopy results comparing process-induced pores caused by incomplete melting to pores introduced by powdered feedstock are from a study titled “The Metallurgy and Processing Science of Metal Additive Manufacturing.”
During the atomization powdering process, air bubbles may form inside the powder, which will be transferred to the final part. For this reason, it is necessary to procure materials from good suppliers.
More commonly, the 3D printing process itself creates small holes. For example, when the laser power is too low, the metal powder will not be fully melted. When the power is too high, metal splashing will occur, and the molten metal will fly out of the molten pool and into the surrounding area.
When the size of the powder is larger than the layer thickness, or the laser overlap is too sparse, small holes will appear. Small holes can also occur if the molten metal does not flow completely to the corresponding area.
To address these issues, most equipment operators need to tune their equipment for specific materials and tasks. For specific materials and tasks, device parameters (e.g., laser power, spot size, and spot shape) need to be adjusted to minimize porosity.
In the powder bed fusion process, the laser zone scanning mode can also reduce the number of pores. This checkerboard-like filling pattern replaces the one-way scanning strategy and reduces the temperature gradient.
In the SLM process, powder spatter can be reduced by adjusting the spot shape. The well-known “pulse shaping” can achieve gradual melting of the area. For the EBM process, the electric current causes powder particles to splash from the powder bed, which can be improved by preheating the powder bed by rapidly scanning the electron beam.
Jim Gaffney, manager of the Forecast 3D metal 3D printing laboratory, gave the following suggestions for reducing pores: “For the SLM process, high-quality metal powder, appropriate processing parameters, and reasonable environmental control can ensure that the product density reaches more than 99%. Ultimately, parts can be hot isostatically pressed to remove residual porosity.”
Porosity can also be reduced by infiltrating other materials, such as copper infiltration. However, adding auxiliary materials will change the chemical composition of the part and may destroy the original design application scenario of the part.
density
The density of a part is inversely proportional to the amount of porosity. The more pores a part has, the lower its density is, and the more likely it is for fatigue or cracks to occur in a stressed environment. For critical applications, the density of parts needs to be above 99%.
In addition to the previously mentioned methods of controlling the number of pores, the particle size distribution of the powder may also affect the density of the part. Spherical particles not only improve the fluidity of the powder but also increase the density of the part. Additionally, a wider powder size distribution allows fine powders to fill the gaps between coarser powders, resulting in higher density. However, a wide powder size distribution reduces the flowability of the powder.
Good powder fluidity is necessary to ensure the smoothness and density of the powder spread. As you might imagine, this affects the porosity and density of the product. The greater the powder packing density, the lower the porosity of the part and the higher the density.
Jack Beuth, a professor at the School of Mechanical Engineering at Carnegie Mellon University and director of the NextManufacturing Center, can clearly explain the relationship between metal 3D printing parameter settings and the amount of porosity and density of the part.
“Maximizing the density of the part (minimizing the number of pores) is very important because the manufactured part will experience cyclic loading in practical applications.” Beuth explains: “In the research we carried out at CMU, by controlling the 3D printing process parameters, different sources of porosity can be controlled or effectively eliminated. No one process parameter is much better than all others in reducing porosity, but there is always an optimal combination of processing parameters for each process.”
Residual Stress
In metal 3D printing, residual stress is caused by hot and cold changes, expansion, and contraction processes. When the residual stress exceeds the tensile strength of the material or substrate, defects will occur, such as cracks in the part or warping of the substrate.
Residual stress is most concentrated at the connection between the part and the substrate, with greater compressive stress at the center of the part and greater tensile stress at the edge.
Residual stresses can be reduced by adding support structures because they are hotter than the substrate alone. Once the part is removed from the base plate, the residual stresses are relieved, but the part may deform in the process.
Researchers at Lawrence Livermore National Laboratory have proposed a method to reduce residual stress. To control temperature fluctuations, continuous laser scanning can be replaced by reducing the length of the scanning vector. Rotating the orientation of the scan vector based on the largest cross-section of the part may work.
Another way to reduce residual stress is to heat the substrate and material before printing. Preheating is more common in EBM processes than in SLM or DED processes due to lower operating temperatures.
Ingo Uckelmann is the Technical Manager of Metal 3D Printing Services and Materialize at the Enterprise Metal 3D Printing Technology Center in Bremen, Germany. Uckelmann explained that it is necessary to control residual stress in three stages, namely the data preparation stage, the printing process, and the post-processing stage.
“During the data processing phase, we use Materialize Magics to choose the appropriate placement orientation to prevent warping or deformation caused by later stresses,” Uckelmann said. “Magic can also use supports to firmly connect the parts to the platform and use volumetric supports to conduct heat quickly.”
Uckelmann pointed out that support structures play an important but “unreasonable” role in the metal 3D printing process. “On the one hand, a support structure is needed to offset the stress during the printing process and keep the part position unchanged. On the other hand, the support will dissipate the heat generated by printing, because excessive local temperatures may lead to the deterioration of surface quality or mechanical properties,” explains Uckelmann. “Magic uses hybrid bracing to play both roles.”
“During the printing process, we use the machine communication software Materialize Build Processor to cut the part into the shell and core area,” Uckelmann added: “Each part uses a different scanning strategy. The build processor can also support different structures. Specify different scanning strategies. For example, the support structure can be scanned every two layers to increase scanning efficiency and reduce stress. After printing is completed, we heat treat all parts to prevent stress deformation.”
crack
In addition to cracks occurring in the internal pores of a part, cracks can also occur when the molten metal solidifies or an area is further heated. If the heat source is too powerful, stress may occur during the cooling process.
Delamination may occur, causing breaks between layers. This may be caused by insufficient melting of the powder or the remelting of several layers below the molten pool. Some cracks can be repaired through post-processing, but delamination cannot be fixed through post-processing. Accordingly, the substrate can be heated to reduce this problem.
Beuth was also able to explain how cracks appear during the metal 3D printing process. He pointed out that cracking and its impact on past performance are not limited to additive manufacturing but are also concerns in traditional casting and other metal processing methods.
“Generally speaking, materials supported by the equipment manufacturer will not crack during the printing process,” Beuth said. “However, when users start trying to process materials that are not supported by the manufacturer, such as more brittle and harder alloys, this time you have to consider the issue of cracks. Similar to pore control, cracks can be reduced or eliminated by adjusting process parameters, which is a research hotspot in the field of additive manufacturing.”
Because cracks appear during the use of components, such as under fatigue loading. “Adjusting the 3D printing process parameters can control these defects to a large extent,” Beuth said. “One thing to note is that you don’t have to eliminate all pores or defects in the process of making the part. What’s important is that you know what pores or defects may be present. If you can anticipate these, engineers can consider these factors when designing and still make reliable and safe parts.”
Warp
To ensure a smooth start to the print job, the first layer of the print is fused to the substrate. When printing is complete, the parts are separated from the substrate through CNC machining. However, if the thermal stress on the substrate exceeds its strength, the substrate will warp, eventually causing the part to warp, risking the scraper hitting the part.
Carl Dekker, president of Met-l-flo and chairman of the ASTM F42 Committee on Additive Manufacturing, explained how this phenomenon occurs. “You’re dealing with multiple thermal factors during the printing process, and even if your product is very thick, it creates additional stress.” Dekker said: “The printing process has many rapid changes. Sometimes it will cause the part to detach from the support. Or there may be enough support that it will pull on the platform. It can cause the platform to deform, which does not happen while you are printing, but it will occur during the removal of the platform from the machine or during subsequent processing.”
Therefore, to prevent warping, you need to add the right amount of support in the right place. These settings are very difficult to determine without trial and error, with each part being printed. There are also some software solutions under development, such as 3DSIM’s printing prediction software.
When you have a sufficient understanding of the process of a device, you can also use Materialize’s Inspector software to control metal 3D printing quality. As Inspector Product Manager Vincent Wanhu Yang said: “At Materialize, we noticed the need for more refined quality control. Our Inspector software can process photos of the machining process to improve users’ understanding of the process and determine which areas are likely to be affected by warpage. By analyzing the root cause and detecting vectors, users can determine whether support is lacking and what is causing the deformation. Understanding the machining process is necessary for the next metal 3D print to go smoothly.”
other problems
Other deformations, such as swelling or spheroidization, may also occur during the metal 3D printing process. Expansion occurs when the molten metal exceeds the height of the powder. Similarly, spheroidization is when metal solidifies into spherical shapes rather than flat layers. This is related to the surface tension of the molten pool, which can be weakened by controlling the length-to-diameter ratio of the molten pool to be less than 1-2.
Exposure to oxygen or moisture may cause the composition of the alloy to change. For example, as the oxygen element in the Ti-6Al-4V titanium alloy increases, the aluminum element content may decrease. This phenomenon is particularly common when powders are reused. Repeated use will result in reduced powder sphericity and reduced fluidity.
The printing process may also cause changes in the composition of the alloy. Alloys are composed of multiple metallic elements, and low-melting-point elements may evaporate during printing. For Ti-6Al-4V, a commonly used aviation titanium alloy, Ti has a higher melting point than the Al element, and the composition of this material may change during the printing process.
