Ultrasound Based In-line Assessment of Porosity for Laser-Sintered Parts

加法制造被视为促进轻量化和设计优化的关键技术，并在各种应用中实现降低能耗的承诺。然而，加法制造在用于生产规模运营方面一直进展缓慢。目前，孔隙率的可变性，即使是相同的构建参数，也是添加剂制造面临的重大技术挑战。该奖项支持选择性激光烧结(SLS)系统在制造过程中检测和量化孔洞的基础研究。这项研究的目的是发展超声散射理论，并将其与SLS试验台上的在线添加剂制造、质量控制和计量相结合。这一理论将通过计算机模拟来发展，并通过产品质量的离线和在线测量来验证。计算机模型将被用来开发简化的假设，使问题易于实时处理。一旦评估了假设的有效性和局限性，将使用标准超声换能器和定制激光系统进行在线质量评估，以实验验证该理论。在未来，这项研究可能会使加法制造更加可靠和可预测，从而产生更节能、更可靠的机械系统。在推进添加剂制造科学的同时，这项研究也将被用作催化剂，以激发K-12学生的兴趣。今年夏天，来自小型高中和农村学校的教师将加入研究团队，这些学校的毕业生历来没有学过工程学。在协助研究并接受无损评估和添加剂制造课程后，教师将在下一学年在各自的学校与学生一起开展一个项目。此外，教师将协助他们的学生在他们的社区为当地的小学生组织一个“工程博览会”来展示他们的项目。该项目的研究目标是1)发展散射理论以准确地从后向散射超声信号中定量孔隙度；2)开发基于激光的超声系统，用于基于散射理论的孔隙度评估的新方法。散射理论将通过对嵌入在钢、钛和钴铬微结构中的不同大小和密度的气孔的散射进行数值模拟来发展。这些模拟将被用来探索作为散射理论一部分的近似的局限性，并将包括来自压电换能器的聚焦超声波和由激光产生和检测的纵波的后向散射回波。基于激光的超声信号将被过滤，以允许在零件表面附近表征较小的缺陷。然后，将使用成品和正在加工的零件对散射理论进行实验验证。对正在加工的零件的验证将在定制的原型激光烧结系统上进行，该系统将作为该项目的一部分进行开发。该理论可应用于复合材料、压铸等其他制造技术以及其他添加剂制造技术。

Additive manufacturing is seen as a critical technology to promote light weighting and design optimization, and to achieve the promise of reduced energy consumption in a wide variety of applications. However, additive manufacturing has been slow to use for production scale operations. Currently, the variability in porosity, even for identical build parameters, is a significant technical challenge facing additive manufacturing. This award supports fundamental research into the detection and quantification of porosity in a selective laser sintering (SLS) system during fabrication. The goal of this research is to develop the theory of ultrasound scattering for pores and couple it with in-line additive manufacturing quality control and metrology in a SLS testbed. The theory will be developed by computer modelling and verified by both off-line and in-line measurements of production quality. The computer models will be used to develop simplifying assumptions to make the problem tractable in real-time. Once the validity and limitations of the assumptions have been evaluated, the theory will be validated experimentally using both standard ultrasound transducers as well as a custom laser-system for in-line quality assessment. In the future, this research could cause additive manufacturing to be more reliable and predictable, resulting in more energy efficient and reliable mechanical systems. While advancing the science of additive manufacturing, the research will also be used as a catalyst to excite K-12 students. In the summer, teachers from small high schools and rural schools, whose graduates have historically not studied engineering, will join the research team. After assisting with the research and receiving a course on nondestructive evaluation and additive manufacturing, the teachers will conduct a project at their respective schools during the following academic year with their students. In addition, the teachers will assist their students in organizing an "Engineering Fair" for local elementary students in their community to showcase their project. The research objectives of the project are to 1) Develop scattering theory to accurately quantify porosity from backscattered ultrasound signals and 2) Develop a laser-based ultrasound system for a new method of porosity assessment based on the scattering theory. The scattering theory will be developed using numerical simulations of scattering from pores of various sizes and densities imbedded in steel, titanium, and cobalt-chrome microstructures. The simulations will be utilized to explore the limitations of the approximations made as part of the scattering theory, and will include both focused ultrasound waves from a piezoelectric transducer and the backscattered echoes of longitudinal waves generated and detected by lasers. The laser-based ultrasound signals will be filtered to allow smaller defects to be characterized near the surface of the part. The scattering theory will then be experimentally validated using both finished and in-process parts. The validation on in-process parts will be conducted on a custom made prototype laser-sintering system that will be developed as part of the project. The developed theory may be applicable to other manufacturing technologies such as composites and pressure die casting as well as other additive manufacturing techniques.