For manufacturers, 3D printing or additive manufacturing provides a way to build complex-shaped parts that are more durable, lighter and more environmentally friendly than parts made by traditional methods. The industry is booming. Some people predict that its size will double every three years, but it also has a series of problems with its rapid development.
Residual stress is a by-product of repeated heating and cooling in the metal printing process. It can introduce defects into the parts and, in some cases, can damage the printer. In order to better understand how residual stress is formed and how to suppress it, researchers from the National Institute of Standards and Technology (NIST), Lawrence Livermore National Laboratory, Los Alamos National Laboratory (Los Alamos) Alamos National Laboratory) and other research institutions have carefully studied the effects of different printing patterns on titanium alloy parts made by ordinary laser methods.
The researchers tested four different printing patterns, in which the laser either continuously melted back and forth in the metal powder, or melted in different square islands, and extended parallel to the long side of the part or diagonal to the part.
The results of their research published in the journal Additive Manufacturing showed that a printed pattern called “island scanning” performed the worst of all research methods, which did not match the team’s expectations. . The data they produce can help manufacturers test and improve predictive models for 3D printing. If the models are accurate, they can avoid destructive levels of residual stress.
This is very surprising and highlights the complexity of the problem. The co-author of the study and NIST materials research engineer Thien Phan said that this shows that although island scanning may be effective in many situations, it is It doesn’t work at work, which really emphasizes the fact that we need accurate modeling.
Researchers at NIST and other institutions used laser-based methods to 3D print four miniature metal bridges, each of which was made with a different printing pattern (line advertising). The research team measured the tension (red) and compression (blue) levels inside the bridge in three different directions, and determined the vertical stretch levels along the edges of some samples.
The team’s research focused on a popular additive manufacturing method called laser powder bed fusion (LPBF). In this method, a laser scans a layer of metal powder, melting and fusing particles together on the surface in a predetermined pattern. . When the molten metal cools into a solid, the worktable supporting the material is lowered, and the printer adds a new layer of powder on it, allowing the laser to continue manufacturing parts layer by layer.
Once the second floor of the building begins, the residual stresses begin to rise. The metal used in LPBF cools quickly, which means that when the printer’s laser starts to heat the new layer, the metal in the previous layer is already solid. The molten metal layer shrinks inward as it cools, pulling the solid metal below and creating pressure. The greater the temperature difference, the greater the pulling force of the melting layer. This process is repeated for each layer until the part is complete, locking the stress in the solid metal.
You end up creating incredible residual stresses inside your workpieces, Phan said. Residual stresses can crack the parts and rise during the manufacturing process, which can actually crash the machine.
The simplest printing mode in LPBF is continuous scanning, where the laser scans back and forth from one end to the other. But an alternative called island scanning has emerged as a way to relieve stress. The idea behind this method is that melting a small part of metal, or islands, one at a time, instead of an entire layer, can reduce the shrinkage of the metal while reducing the overall pressure.
Island scanning has gained the favor of manufacturers, but past research on this technology has been inconsistent. More broadly, the relationship between scanning strategy and residual stress remains largely a mystery. To fill these gaps, the multi-agency team began to analyze in detail the impact of island scanning on stress.
The authors of this new study printed four titanium alloy bridges over 2 cm (0.8 inches) in length. These samples are created by continuous or island scanning, with the laser running along their length and width or at an angle of 45 degrees.
At first glance, these bridges appear to be printed from a printer, but the researchers did not just look at the surface value, but carefully studied their details.
They fired high-energy X-rays into the depths of the sample from a powerful tool called a synchrotron. By measuring the wavelength of the X-rays reflected by the metal, the team accurately extracted the distance between the metal atoms. From there, the researchers calculated the pressure. The greater the distance, the greater the pressure on the metal. After mastering this key information, they generated a map showing the location and degree of stress in the entire sample.
The stress of all samples is close to the yield strength of the titanium alloy-the strength point at which the material undergoes permanent deformation. But these maps also revealed something that surprised researchers.
NIST physicist and co-author Lyle Levine said that there is a lot of stress on the sides and top of the island scan sample, which is not, or not so obvious, in the continuous scan sample. If island scanning is a way for the industry to try to relieve these pressures, I would say that for this particular case, it is far from successful.
In another test, they removed one leg of each bridge from the metal substrate. The authors of the study measured the distance the bridge legs bounced upwards, thereby obtaining another measure of the residual stress stored in the vault of each bridge. Similarly, the island-shaped scan samples performed poorly, and their legs deflected more than twice as much as the other samples.
The author suggests that island scanning may be a double-edged sword. Although small islands may reduce shrinkage, islands may also cool down much faster than larger melting pools, creating a greater temperature difference and thus greater pressure.
Pan said that although the island scan is not suitable for the specific parts, materials and equipment used in the research, it is still a good choice under different circumstances. The results do show that it is not a panacea for residual stress, however. In order to avoid pressure, manufacturers may need to adjust scanning strategies and other parameters according to their specific models-computer models will greatly help this work.
If the prediction is accurate, manufacturers can use the model to quickly and cheaply determine the best parameters, rather than optimizing printing through trial and error. Levine said that modelers can test their tools through rigorous benchmark measurements, just like data obtained in new research, thereby increasing their confidence in the tools.
This work provides a new perspective on a popular printing strategy, adds a key part to the puzzle of residual stress formation, and ultimately brings 3D printing closer to its full potential.
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