Relation between Process Parameters and Pressure during Friction Stir Forming in the Development of a Superplastic Composite Steel Sheets

Abstract

In previous research conducted by the authors, a new vibration-damping steel sheet was developed using a new method employing friction stir forming (FSF) to create a laminated composite sheet in which steel sheets and superplastic alloys are laminated in layers. However, the details of the bonding mechanism have not yet been clarified. The present study investigates the relationship between the pressure during the process and the weld interface in the creation of the mentioned superplastic composite sheet. More specifically, a 0.5mm thick perforated steel plate is inserted between two Zn-22Al superplastic alloys and the FSF is applied to the top layer of Zn-22Al. The probe of the FSF tool passes directly above the perforated steel plate, the material stirred by the probe plastically flows into the hole and is joined to the underlying Zn-22Al interface by superplastic diffusion bonding (SPF/DB). It was revealed that the process parameters (rotational speed and tool feed rate) must be perfectly adequate to produce adequate heat input and pressure leading to a proper plastic flow of the material and the occurrence of superplasticity in Zn-22Al. In this study, as a first step to clarify the detailed joining mechanism, the amount of pressure applied to the specimen during the process is measured. While changing the process parameters, the pressure was measured at three points, under the probe of the friction stir process tool and, on the advancing, and retreating sides, to investigate the relationship between the parameters and the pressure at the joint interface.