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

Iowa State University, Ames IA

Investigators

Abstract

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.