Fast X-ray Microscopy to Quantify the Nucleation of Hot Cracking

非技术概述：该项目将使用最先进的设备，使用高能同步辐射x射线进行超高速成像，以了解金属和合金凝固过程中的热裂问题。热裂意味着，在铸造或3D打印过程中，不会获得完全固态的金属，而是会留下裂纹，从而削弱材料。这也可能意味着必须丢弃和/或修剪破裂的部件，这是浪费。主要的重点将是研究激光3D打印过程中的裂纹，其中兆赫兹成像提供微秒级的分辨率，这种分辨率最为人所知的是子弹穿透装甲的定格动作电影。近年来，由于新相机的出现和X射线探测的改进，这种超高速成像技术取得了快速进展，并为我们理解熔化过程做出了实质性贡献。因此，它非常适合于成像裂纹的突然出现和扩展。将研究裂纹对凝固速度和化学成分变化的敏感性。计算机模拟将被用来检验关于材料微观结构如何发生裂缝的假设。在这项工作中获得的新认识对整个铸造行业产生了广泛的影响。它还可能激发对这个问题的新的理论分析，这通常发生在应用一种新的实验技术时。除了支持一名博士生外，还将招募本科生协助这项工作，这项工作涉及对图像序列的大量详细分析。这项工作还将传播到20多家公司，这些公司是CMU下一个制造中心的成员，对添加剂制造有强烈的直接兴趣。随着分析的进行，主要结果将被纳入PI的教学中，这将有助于确保CMU的MS和添加剂制造方面的工程次要程序保持最新。技术摘要：该建议将使用超快X射线显微镜，结合高能、高强度的同步辐射X射线，来测试凝固裂纹的形核是可变的，并取决于冻结过程结束时固体的形态的假设。鉴于在裂化理论中缺乏对成核点的直接测量和成核的任意方面，即使是对开始裂化的固体分数的测量也将是新颖的。测量裂纹的形核程度取决于柱状和等轴生长的程度，将进一步扩展我们对这个问题的基本知识。我们还将探索裂纹的异质形核，例如，在激光熔化铝基合金时经常观察到的小汽泡。预期的结果包括直接显示各种材料中的凝固裂纹作为温度梯度和冷却速度的函数，这些温度梯度和冷却速度由激光功率和扫描速度控制。我们将主要关注铝合金，部分原因是许多结构铝合金容易开裂，部分原因是为了便于成像，与不锈钢或镍合金进行比较。我们还将使用格子-玻尔兹曼模拟(来自之前的DMREF项目)来模拟凝固并量化糊状区，特别是在高固相分数时液态区的形状。最直接的影响将是激光粉末床印刷，但对铸造技术也存在更广泛影响的可能性。通过合作，我们将寻求获得旨在避免破裂的不同类型的改性粉末，例如，促进等轴微结构。我们将使用计算机断层扫描和切片来进行3D表征。超快X射线显微技术将主要在高级光子源进行，因为该设施具有高能量和高强度的最佳组合。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

NON-TECHNICAL SUMMARY: This project will use state-of-the-art facilities for ultra-high speed imaging with high energy synchrotron x-rays for the purpose of understanding the problem of hot cracking during the solidification of metals and alloys. Hot cracking means that, instead of obtaining fully solid metal during casting or 3D printing, cracks are left behind that weaken the material. It may also mean that the cracked component must be discarded and/or trimmed, which is wasteful. The main focus will be on studying cracking during 3D printing with laser light where mega-Hertz imaging provides micro-second resolution of the sort best known for stop-action movies of bullets penetrating through armor. Such ultra-high speed imaging has made rapid progress in recent years thanks to the advent of new cameras and improvements in x-ray detection and has already made substantial contributions to our understanding of the melting process. It is thus extremely well suited to imaging the sudden onset and growth of cracks. The sensitivity of cracking to variations in solidification speed and chemical composition will be investigated. Computer simulation will be used to test hypotheses about how the cracking happens with respect to the materials microstructure. The new understanding gained in this work has broad impacts in the casting industry in general. It is also likely to stimulate new theoretical analysis of the problem, which often happens when a new experimental technique is applied. In addition to supporting a doctoral student, undergraduates will be recruited to assist with the work, which involves a good deal of detailed analysis of sequences of images. The work will also be disseminated to the twenty-plus companies that are members of CMU's NextManufacturing Center and have a strong direct interest in additive manufacturing. As the analysis proceeds, the main results will be incorporated into the PI's teaching, which will help ensure that CMU's MS and engineering minor programs in additive manufacturing stay up to date.TECHNICAL SUMMARY:This proposal will use ultra-fast x-ray microscopy, with the high energy, high intensity synchrotron x-rays, to test the hypothesis that the nucleation of solidification cracking is variable and depends on the morphology of the solid near the end of the freezing process. Given the lack of direct measurement of the nucleation point and the arbitrary aspect of nucleation in cracking theories, even a measurement of the solid fraction at which cracking starts will be novel. Measuring the degree to which the nucleation of cracking depends on the extent of columnar versus equiaxed growth will further extend our fundamental knowledge of the problem. We will also probe for heterogeneous nucleation of the cracks from, e.g., the small vapor bubbles that are often observed in laser melting of Al-based alloys. The expected results include direct visualization of solidification cracking in a variety of materials as a function of temperature gradient and cooling rate, which are controlled by laser power and scan speed. We will mainly focus on aluminum alloys, partly because many of the structural Al alloys are prone to cracking and partly for ease of imaging, with stainless steel or nickel alloys for comparison purposes. We will also use lattice-Boltzmann simulations (from a previous DMREF project) to model the solidification and quantify the mushy zone, specifically the shape of the liquid zones at high solid fractions. The most direct impact will be on laser powder bed printing but the potential for broader impact on casting technologies also exists. Through collaboration, we will seek access to different types of modified powders that are intended to avoid cracking through, e.g., promoting equiaxed microstructures. We will use computed tomography and sectioning for 3D characterization. The ultra-fast x-ray microscopy will be mostly carried out at the Advanced Photon Source because this facility has the best combination of high energy and intensity.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.