Microstructure and fatigue properties of extruded aluminum alloys 7046 and 7108 for automotive applications

Abstract

This work aimed to obtain by extrusion two aluminum alloys, 7108 and 7046. The microstructural characterized at each stage of the manufacturing process and the mechanical fatigue behavior of the extruded profile were evaluated. The aluminum alloys in the as-casted condition exhibited an interdendritic microsegregation inside the grains, with intermetallic phases precipitated on the grain boundaries. In the homogenization of the billets of the AA7108 and AA7046, the precipitated phases dissolved. The AA 7046, in the as-casted condition, displayed shrinkage microvoids throughout the length of the billet and a microstructure of heterogeneous grains, with the presence of a remelted layer at a maximum depth of 10 mm. The AA7046 displayed better tensile and fatigue properties than the AA7108. A higher surface roughness facilitated the initiation and propagation of cracks during the fatigue cycles. This effect was more pronounced with low levels of maximum stress and high number of cycles to failure. Deep secondary cracks perpendicular to the growth direction of the main crack were visible on all fracture surfaces. In the medium and high cycle fatigue tests of the AA7108 and AA7046, the cracks advanced in a perpendicular direction to the elongated grains resulting from the extrusion process. The fracture surfaces of the tested samples with a high number of cycles exhibited a region with a smaller crack propagation area and a larger region of rupture due to mechanical overload. The region of crack propagation is characterized by plastic deformation and the presence of striations perpendicular to the growth direction.