Effect of microstructural anisotropy on mechanical behavior of a high-strength Al-Mg-Si alloy

The mechanical behavior of an extruded aluminum alloy pipe has been investigated after repeated failures in an oil and gas industry. The pipe failures occurred by longitudinal cracking, and the mechanical properties of the pipe were blamed for the failure. The relevant critical properties of the pipe including basic tests of hardness, tensile, and impact behavior were measured, and extended fatigue testing of the material was conducted. Microstructural examination revealed a recrystallized grain structure and clusters of constituent particles aligned in the direction of extrusion. Tensile testing in both the longitudinal and circumferential directions showed virtually identical yield and tensile strengths. However, the material exhibited higher toughness in the longitudinal direction. Impact test showed that the energy absorbed during fracture was four times higher in the longitudinal direction. Fatigue testing displayed a shorter fatigue life in the transverse direction. The study showed that the microstructure after extrusion and the distribution of the constituent particles have a pronounced effect on the mechanical behavior of the extruded pipe and induced anisotropy in the material performance. Performance of the material can be improved by choosing the proper extrusion ratio to control the microstructure and by controlling the density and distribution of the constituent particles. © 2009 ASM International.