New research from the US suggests that a well-established method of reducing residual stresses in metal 3D printed parts may not be as effective as the additive manufacturing sector thinks it is.
Island scanning – a common laser scan strategy – is often used by manufacturers to destress metal parts 3D printed via laser powder bed fusion (PBF). The approach involves partitioning the layers of a build into smaller sub-sections, usually in the shape of squares, to reduce contractions in the part as it is 3D printed.
The team, comprising scientists from the National Institute of Standards and Technology (NIST), Lawrence Livermore National Laboratory, and other institutions, found that the island scanning method actually increased residual stresses in certain bridge-like geometries.
“This was very surprising and underscores the complexity of the problem,” said NIST materials research engineer Thien Phan, a co-author of the study. “It shows that, although island scanning may work in many cases, it did not work in ours, which really highlights the fact that we need to have accurate modeling.”
Residual stress: a pain point in metal 3D printing
When 3D printing industrial components made of metal, residual stresses are something to look out for. They are caused by the cyclic heating and cooling experienced by the feedstock as the laser scans over the powder bed. When the powder in the chamber melts, it expands, and when it cools back down, it contracts, pulling on the fused material adjacent to it and generating internal stress. In severe cases, these residual stresses can result in defects, cracks, and fractures in a cooling 3D printed part, ruining the build altogether.
“You end up with an incredible amount of residual stresses inside your piece,” explains Phan. “So it’s sitting there, tearing itself apart. The residual stress could crack the part and lift it up during the build, which could actually crash the machine.”
In the fight against residual stresses, modifying the scan strategy of the laser is one of the first lines of defense. Rather than employing a continuous scan path that melts the entire layer in one go, engineers can opt for island scanning, which melts small islands of metal in succession. The latter approach means there is less material contracting at any one time, reducing the overall stress in the part.
The effects of island scanning
Even though island scanning has been proven to work, comprehensive research into the actual effects of the method is limited, resulting in gaps in understanding. To analyze the approach in great detail, the US team 3D printed a number of titanium alloy bridges, all of which had lengths of around 2cm. Each of the test samples was fabricated using either a continuous or island scan strategy.
From the outside, all of the bridges looked the same, so the researchers used high-energy X-rays to calculate the stresses within. Interestingly, the highest levels of tension were found along some of the edges of the bridges 3D printed using island scanning.
“The island scan samples have these really large stresses on their sides and tops, which are missing or much less pronounced in the continuous scan samples,” said NIST physicist and co-author Lyle Levine. “If island scanning is a way that industry is trying to mitigate these stresses, I would say, for this particular case, it is far from successful.”
While the smaller size of the islands does reduce contraction, the team believes this also results in much faster cooling and greater temperature differences, which can increase stress. Ultimately, the results show that island scanning should be considered on a project-by-project basis, rather than being viewed as an all-encompassing ‘silver bullet’. To alleviate the effects of residual stress in metal 3D printing, manufacturers also need to consider other print parameters specific to the build.
Further details of the study can be found in the paper titled ‘Effect of the scanning strategy on the formation of residual stresses in additively manufactured Ti-6Al-4V’. It is co-authored by Thien Phan, Lyle Levine et al.
Defect elimination is an active area of research in the 3D printing community, with a whole host of novel techniques being developed. A team of researchers from Argonne National Laboratory and Texas A&M University have previously used real-time temperature data, together with machine learning algorithms, to predict defects in 3D printed parts. The method involved making correlative links between thermal history and the formation of subsurface defects during laser PBF.
Elsewhere, a group of Chinese and US-based researchers recently discovered a PBF 3D printing ‘speed limit’ at which part defects are less likely to occur. Through extensive X-ray imaging, the team was able to determine the parameters at which the J-shaped bubbles that form in a melt pool can be better controlled.
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Featured image shows stress maps for the 3D printed titanium bridges. Image via Lawrence Livermore National Lab.