Study of of Additively Manufactured 316L Stainless Steel

Laser powder bed fusion (LPBF) additive manufacturing (AM) is a relatively new manufacturing method in which metal parts are manufactured layer-by-layer through rapid heating and cooling of the powder bed. Consequently, complex geometries can be produced to simplify assemblies and reduce material wastage. These benefits make adopting additive manufacturing enticing, however mechanical and electrochemical properties of AM metals are significantly influenced by their unique microstructural and defect features that are highly dependent on the build parameters. In this experiment, we will investigate the interplay between mechanical and electrochemical effects on additively manufactured stainless steel 316L (SS316L). Tests are designed to perform corrosion characterization on AM coupons in stressed and stress-free conditions. Corrosion properties such as pitting potential, corrosion potential, and corrosion current density will be characterized under both stress conditions. Results of the AM samples will be compared with the results of the wrought SS316L to understand the effects of microstructure and defects on corrosion properties. Conclusions will be made about how significant the impact of additive manufacturing is on key properties of this material and if there are any relevancies to its microstructural characteristics. This increase in insight will reveal how to implement AM SS316L safely so its previously mentioned benefits can be realized when appropriate.

Hello everyone, my name is Ben Fuentes and I work with my mentor, Dr. Amiri, in the Reliability and Mechanics of Failure Lab under GMU’s Mechanical Engineering Department. Traditionally, metals parts are manufactured subtractively to their desired shapes with excess material being removed. However, a relatively new technique, called additive manufacturing, operates differently as metal powders are rapidly heated and cooled layer-by-layer to build the part. The effect of this difference in process is a great concern as it could influence important properties of metals. As a result, this project investigates these changes by comparing the corrosion and fatigue, which are two very important factors in aerospace applications, of 316L stainless steel manufactured traditionally and additively. The foundation of this experiment is a fatigue test which examines how many times a material can be loaded and unloaded before failing and is essential for creating safe designs. An everyday example of fatigue is the consistent bending of a paper clip or metal wire which causes it to break clean. With respect to this project, fatigue is tested using the micro-fatigue tester at the lab. A metal sample is attached rigidly through the use of bolts and a load is supplied through an electric motor and gears. On the other side, a load cell is present which measures the amount of force being applied to the sample. The other essential part of any test involving load is measuring the strain. Strain, which is the measure of elongation, is typically measured 1-Dimensionally through the use of an extensometer. However, there is a process called Digital Image Correlation and allows for strain to be traced in 2 and 3-Dimensions. This process consists of imprinting a pattern onto the surface which was accomplished by lightly corroding the sample. Next, a series of photos of the sample are captured throughout its elongation. Finally, the images are passed through a DIC program to track displacements over time. The second portion of this experiment involves corrosion which is the loss of material due to its interactions with the environment. Fundamentally, corrosion is a redox reaction which means that electrons are transferred and positively charged ions are ejected from the material which ultimately results in a loss in mass. This exchange in electrons can be captured as electrical current which ultimately allows for corrosion to be a quantifiable property so it can be compared with other materials tested in a similar environment. This experiment was designed to compare the fatigue strength, or the number of cycles until failure, of traditionally and additively manufactured samples of the same geometries at various loadings and corrosive environments. While this project experienced several difficulties which delayed the collecting of results, predictions can be made through the analysis of other experiments that are similar in nature. For example, researchers at the University of Toledo and University of Memphis found that additive manufactured titanium had worse fatigue strengths which could be explained by the buildup in residual stresses due to the iterative cooling and heating or through other induced defects experienced through manufacturing. Additionally, a research group in Belgium found that resistance to corrosion in additive manufactured aluminum alloys were generally equivalent to or better than their counterparts despite the large number of defects present. Regardless of how additive manufactured metal performs against its traditional counterparts, the primary value in this experiment is understanding the limitations of additive manufactured materials so they can be implemented safely and effectively so that their benefits, such as reduced material wastage and ability to produce complex parts, can be utilized fully. The following slides cite sources where information was obtained from, and I’d also like to acknowledge OSCAR URSP for providing funding and supplemental learning for this project. Thanks for watching!