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.
岛扫描 - 一种常见的激光扫描策略 - 制造商经常使用通过激光粉床融合（PBF）打印的金属零件3D。该方法涉及将构建的层划分为较小的子段，通常是正方形的形状，以减少零件打印的零件的收缩。
该团队包括来自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
当3D打印工业组件由金属制成时，要注意残余压力。它们是由原料在粉末床上扫描的循环加热和冷却引起的。当腔室中的粉末融化时，它会膨胀，当它冷却后，它会收缩，将融合的材料与它相邻的融合材料拉开并产生内部压力。在严重的情况下，这些残留应力会导致冷却3D印刷部分中的缺陷，裂缝和骨折，从而完全破坏了构建。
“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
即使已被证明岛屿扫描能够起作用，但对该方法的实际影响的全面研究受到限制，从而导致理解差距。为了详细分析该方法，美国队3D打印了许多钛合金桥，所有桥梁的长度约为2厘米。每个测试样品都是使用连续或岛屿扫描策略制造的。
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’。它由Thien Phan，Lyle Levine等人合着。
缺陷消除是3D打印界的一个积极研究领域，并开发了许多新型技术。一组研究人员阿贡国家实验室and德克萨斯农工大学have previously used实时温度数据，以及机器学习算法，预测3D打印零件中的缺陷。该方法涉及在激光PBF期间的热历史与地下缺陷的形成之间建立相关联系。
Elsewhere, a group of Chinese and US-based researchers recently discovered aPBF 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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特色图显示了3D打印钛桥的应力图。图片通过劳伦斯·利弗莫尔国家实验室（Lawrence Livermore）。
