Neutron diffraction based non-destructive techniques for integrity studies on copper-stainless steel hybrid composites fabricated by friction stir forming

Abstract

A novel technique based on probeless tool aided friction stir forming (FSF) has enabled fabrication of bimetallic composites of copper (Cu) with immiscible metals such as tungsten (W) and stainless steel (SS). Thermomechanical bonding of Cu and the embedded SS-insert, with different interfacial geometries and with different intermediate metal powder layers, was investigated. A full 360 tomography of the Cu-SS composites was performed on the neutron imagining station DINGO, at ANSTO. Sequentially stacked neutron images were reconstructed into a 3D model of the sample to examine the existence of voids and tunnel defects. Complete encapsulation and continuous bonding of the SS-insert with thermo-mechanically conditioned Cu established the feasibility of FSF for fabricating such hybrid Cu-SS composites. Additionally, peak temperatures during the process were noted to reach up to 600C, bonding at such temperatures may lock in strain. Furthermore, on cooling significant thermal stresses might develop at the Cu-SS interface owing to the difference in the mechanical properties of two metals. Exposure to continuous heating and cooling cycle during the double pass technique might also result in residual stresses. These stresses occurring as a result of the thermomechanical bonding may affect the integrity of the composite. Owing to the architecturally intricate character of the fabricated Cu-SS hybrid composite, non-destructive neutron diffraction technique of KOWARI strain scanner at ANSTO was utilised. Through-thickness scans across the Cu-SS interfaces were performed using 2 x 2 x 2 mm3 gauge volume. The stress analysis performed in isolated spots of the Cu-SS composite showed minimal variation in the residual stress distribution in all three – longitudinal, traverse and normal directions.© 2017 Engineers Australia