Characterization of Thin Laminates Present Inside the Aluminum Extruded Tubes

The commercial AA6061–T6 alloy was prepared from different grades of raw materials and casted in the form of billets. The 50-mm-thick tubes were extruded from this alloy, and the product quality was monitored through ultrasonic testing. The presence of internal defects within the bulk of material, their location, and size was also determined by 6 decibel drop method. To investigate the possible reasons for the inferior product quality and large rejection rate, the tubes manufactured from three different cast billets were produced and tested. The cross section of the polished specimens from the defect sites revealed the formation of oxide film laminates. The scanning electron microscope images showed the existence of entrapped hard phases which were aligned in the extrusion direction and formed internal fine cracks or fissures during extrusion process. The EDX, XRD, and XPS analyses of these oxide films also confirmed the presence of γ-Al₂O₃ and MgO along with some minor impurity elements (Fe, Si, Ca, Co, and Na).