Fabrication of bilayer tube consisting of outer aluminum foam tube and inner dense aluminum tube by friction stir back extrusion

Bilayer tubes consisting of an outer Al-Si-Cu ADC12 foam tube and an inner commercial-purity Al A1070 tube (ADC12 foam/A1070 bilayer tube) with metal bonding between them were fabricated by applying friction stir back extrusion (FSBE). By applying FSBE, it was shown that a tubular ADC12 precursor can be satisfactorily obtained and that an A1070 tube can be successfully formed and coated inside the tubular ADC12 precursor with no gap between them. By heat treatment of the ADC12 precursor/A1070 bilayer tube, the ADC12 precursor can be sufficiently foamed outside of the A1070 tube. During static compression tests, the uniform ADC12 foam tube exhibited brittle deformation owing to the brittle Al-Si eutectic nature of ADC12, resulting in a nonmonotonic stress–strain curve. In contrast, brittle deformation was only observed at the surface of the ADC12 foam tube of the ADC12 foam/A1070 bilayer tube, resulting in a monotonic stress–strain curve. This is because the inner ductile A1070 tube can prevent the brittle deformation of the ADC12 foam tube to a certain extent. The combination of the ADC12 foam tube and inner A1070 tube with metal bonding between them improves the compression properties of the tube.