PNNL

Abstract

A combination of increased strength and lightweight materials design offers significant importance in automobile industries. Friction-stir Assisted Scribe Technique (FAST) is a viable method to join dissimilar materials. In this study, FAST was employed to join AZ31 magnesium alloy to Zn-coated steels including hot dipped galvanized 590 and electroplated 270 steels with and without coating. The mechanical properties and performance of a FAST joint are governed by its underlying interfacial and microstructural characteristics, which in turn are determined by the FAST tool dimensions and process parameters. The FAST tool was utilized to create a different stirring depth that is responsible for the level of chemical intermixing and size of mechanical hooks. Microstructure and chemistry investigations on the weld interface were assessed using scanning electron microscopy and electron dispersive spectroscopy. Mechanical properties including lap shear test of welded joints and nanoindentation of joint interfacial regions at different locations were correlated to the local microstructures. Meanwhile, a finite element-based modeling approach was developed to identify the optimal interfacial characteristics that lead to desirable mechanical performance of a FAST joint. The results of the computational model were compared to the experimental observations presented in this paper. A sensitivity analysis was conducted to study the variation in the mechanical response and failure modes of the joint with respect to the interfacial characteristics. Results indicate that interfacial characteristics including hook shape can have a significant impact on the joint strength and failure modes.