Microstructure and Surface Characterization of Incrementally Formed AA 7075

Abstract

Lightweighting in the automotive and aerospace industries is critical for improving fuel economy without sacrificing safety and performance. Replacing heavier steel components with lightweight materials, such as aluminum alloys, reduces the overall vehicle’s weight; a 10% weight reduction can result in a 6-8% improvement in fuel economy. However, the decision to commercialize new lightweight metal components is dependent on the economics of production volume and manufacturing costs. Conventional forming processes such as stamping and deep drawing are economical for high volume production, while additive manufacturing processes are economical for low volume production.

Incremental Sheet Forming (ISF) is a die-less or low-cost die manufacturing process where components are formed from metal sheet through a series of localized plastic deformations. ISF uses much lower forming forces than conventional processes; it also allows for the forming of detailed, customizable parts and requires shorter forming times than additive processes. These advantages allow for it to be competitive for mass customization and low volume production. It is of high interest for prototyping, after-market service and creating complex products with high strength at low costs.

For ISF to be widely used, current challenges predicting the microstructure and mechanical properties of the as-formed parts must be addressed, as well as concerns about tribological and mechanically induced surface features that may impact fatigue life. An improved sample preparation technique for Electron Backscatter Diffraction (EBSD) analysis was developed, which allowed a majority of the grain structure to be resolved at the highly deformed areas. Grains elongated proportionally to the stretch of the sheet and no evidence of recrystallization was found in the formed parts. This suggested that the starting grain structure can be used to predict the microstructure evolution due to a known strain path and Integrated Computational Materials Engineering (ICME) tools can be used to predict texture.

Additionally, inconsistencies in the amount of contact between the forming tool and sheet induced variations in surface finish on the as-formed components, even within a single tool striation. The majority of the deformation occurred under the tool with limited deflection at the microscale and material that was pushed forward by the forming tool formed a macroscopic bulge after the final tool pass, which is a defect part manufacturers want to avoid. These surface metrology studies enabled characterizing the surface features that developed on the formed parts, which may have an impact on the fatigue properties. Tribological grooves formed on the tool side surface and the native oxide layer cracked on the non-tool side surface. The density of the grooves was affected by process variant, wall angle and squeeze factor and their orientation depended on the rolling direction of the initial sheet material. While cracking in the oxide layer is unlikely to be sites of fatigue crack initiation, cracks in the second phase particles (Al-Mg-Zn) may serve as areas of high stress concentration, leading to potential fatigue crack growth.