Ultrasonic characterization of microstructures in inertia friction welds on SiC-reinforced 8009 aluminum (1998)

The microstructures of inertia-friction welds (IFRWs) on an aluminum metal matrix composite have been investigated using several ultrasonic examination (UE) techniques as well as standard characterization methods. This study represents one of the first efforts to utilize UE methods for microstructural characterization of welds. The flow line patterns and dimensions of the different weld zones in IFR welds on a SiC-reinforced 8009 aluminum composite (8009/SiC/11p) have been determined using optical microscopy (OM) and scanning-electron microscopy (SEM) as well as the following UE methods: 1) acoustic microscopy, 2) C-scan imaging, 3) axial backscattering, and 4) radial backscattering. The UE techniques exploited the variations in ultrasonic scattering produced by changes in the distribution of the SiC particles. The ultrasonic procedures allowed rapid determination of microstructural features in the IFR welds. The acoustic microscopy technique provided detailed pictures of the flow lines in longitudinal sections of the welds, as well as the dimensions of the different weld zones as functions of radial position. Ultrasonic C-scan imaging determined the flow line patterns in thin disks centered at different axial distances from the weld centerline. The C-scan imaging technique also revealed the presence of purposely induced defects that resulted from lack of bonding near the axial centerline of one of the welds. Axial ultrasonic backscattering measurements were used to determine the widths of the various weld zones as functions of radial position. Finally, radial ultrasonic backscattering measurements provided the overall widths of the entire weld zone at a fixed radial position. The results of this study were self-consistent in that weld microstructures and dimensions determined with one UE technique correlated closely with those found with other UE techniques, as well as with OM and SEM. The ultrasonic methods complement OM and SEM by providing similar microstructural information over the size scales examined. The UE techniques described in this study may also be employed to study IFR welds on similar materials and can likely be adapted to other processes involving metal matrix composites (MMCs) and comparable materials. The information determined from this study also contributes to an improved understanding of process/structure/property relations for IFR welds on 8009/sic/11p as well as plastic flow resulting from the IFRW process. The SiC particles in the as-received extrusion were distributed into alternate SiC-rich and SiC-depleted bands oriented parallel to the extrusion direction. The flow lines of SiC-rich and SiC-depleted bands followed spiral-shaped paths in the IFR welds. Lower values for strain to failure and elastic modulus (relative to the base material) in subscale tensile samples from these welds tested in previous studies are attributed to the realignment of the bands during IFRW. Results confirm that flow in the IFR welds took place with components in the axial, radial and tangential directions. Moreover, the strength or dominance of the components of flow varied with axial and radial position for a constant weld axial force. The strength of the various flow components also changed with the axial force used for welding. The information on the final patterns of flow lines and weld dimensions may be useful in corroborating the final results of computational process models for IFRW.