Streaking on aluminium alloy extrusions
Extrusion is a process in which a billet of most commonly aluminium is pushed through a die aperture of a desired cross sectional shape to produce lengths of extrudate. The type of streaking being investigated is due to differing thermomechanical conditions created due to changing cross sectional thicknesses of an extrusion profile. These differing thermomechanical conditions then may cause possible issues of differing distributions of intermetallics or grain orientations. At the start of this research many possible reasons for streaking were presented in literature but no single mechanism identified as being the primary cause. Therefore it needed to be identified what the mechanism for causing streaking on the product being investigated was due to. In addition the evolution of streaking after different etch durations needed to be examined as this type of finding has not been reported in literature. The streaking observed therefore might not be the same for all levels of material removal.
To conduct this research on streaking, samples were submitted to differing chemical etch durations which produced different levels of material removal. The chemical etch differs in and out of the streak region and the different durations thus allow possible different types of streaking to be observed at different levels of material removal. In addition to chemical etching an electro-etching experiment was conducted so that primarily the grain boundaries were etched out to examine if these features play an insignificant, minor or major role in the streaking phenomenon.
In summary there are two features that are etched on the surface of an extrudate, these are grain boundary grooves and etch pits. It was established through a thorough investigation that etch pits are the primary factor for streaking to occur with the grain boundary grooves being an insignificant factor for the extrusion product examined. For the top surface layers a glossy streak appeared and as more material was removed two dull parallel bands and then a single dull band streak appeared. The dull streaking was due to more pitting in the streak region and the glossy streaking due to less pitting and less densely distributed die lines. Overall the streaking observed most commonly in the manufacturing environment is the dull gloss streaks which occur after a longer etch duration and larger volume of surface material removal. Therefore the streaking phenomenon is mainly a subsurface issue but only to a small depth. Therefore if abrasive blasting in conjunction with a short etch duration is used the issue of streaking created by etching can be resolved.