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

增材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，增材制造在大规模生产中应用的速度很慢。目前，即使在相同的构造参数下，孔隙率的变化也是增材制造面临的重大技术挑战。该奖项支持在选择性激光烧结（SLS）系统制造过程中孔隙度检测和量化的基础研究。本研究的目的是在SLS测试平台上发展超声散射理论，并将其与在线增材制造质量控制和计量相结合。该理论将通过计算机建模来发展，并通过离线和在线生产质量测量来验证。计算机模型将用于简化假设，使问题易于实时处理。一旦评估了假设的有效性和局限性，该理论将使用标准超声换能器以及用于在线质量评估的定制激光系统进行实验验证。在未来，这项研究可能会使增材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。在推进增材制造科学的同时，这项研究也将被用作激发K-12学生的催化剂。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，这些学校的毕业生以前没有学过工程学。在协助研究并接受无损评估和增材制造课程后，教师将在下一学年与学生一起在各自的学校开展一个项目。此外，教师将协助学生组织“工程展”，让当地的小学生在他们的社区展示他们的项目。本课题的研究目标是：1)发展散射理论，从后向散射超声信号中准确量化孔隙度；2)开发基于激光的超声系统，为基于散射理论的孔隙度评估提供一种新的方法。散射理论将利用数值模拟从不同大小和密度的孔中散射嵌入钢、钛和钴铬微结构。模拟将用于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。基于激光的超声波信号将被过滤，以允许在零件表面附近表征较小的缺陷。然后，散射理论将使用成品和在制品进行实验验证。在加工零件的验证将在一个定制的原型激光烧结系统上进行，该系统将作为项目的一部分开发。所开发的理论可能适用于其他制造技术，如复合材料和压力压铸以及其他增材制造技术。

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.