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　　Analysis of Defects in Aluminum Extrusion

　　The aluminum extrusion process is widely used to produce various aluminum profiles, but it is not without its challenges, as several defects can occur during this manufacturing process. These defects can compromise the quality, functionality, and appearance of the final aluminum products.

　　One common type of defect is related to the extrusion press. Pickups, for example, are small particles of aluminum and aluminum oxides that are torn from the surface of the aluminum profile during extrusion and then redeposit on its surface. They typically appear in the form of commas or comets, oriented in the direction of extrusion. This defect is often associated with die lines, which are grooves running in the extrusion direction and often end at points where aluminum material particles adhere. Die lines can be caused by minor irregularities or the build - up of aluminum or non - metallic inclusions on the bearing surfaces of the die. Even with highly polished die bearings and optimal extrusion temperatures, micro die lines, which are finer and shallower, can still occur.

　　Bearing streaks are another extrusion - press - related defect. These are light or dark streaks running in the extrusion direction, usually resulting from a too - abrupt change in the length of the die bearing. Uneven cooling, especially in areas with large changes in wall thickness, can also lead to bearing streaks. Chamber and web markings are streaks that appear in the area of changes in the profile cross - section, such as in webs or hollow chambers. These markings are caused by differences in structure, which result in convex/concave bulges or different reflections on the strand surface.

　　Longitudinal extrusion seam streaks, also known as weld lines or charge welds, are narrow streaks running in the extrusion direction. They are process - related and, although unavoidable, their extent can be influenced. Front - end defects, or transverse - weld defects, are parabolically shaped and are caused by oxides and lubricants from the billet end surface being trapped when two billets are welded during billet - to - billet extrusion, resulting in poor welds. Back - end defects, on the other hand, are cone - shaped and are a result of surface contaminations and oxides building up in front of the advancing extrusion pad due to container friction, leading to annular separation in the rear of the extrusion.

　　Speed cracks, or speed tears, are cracks that appear on the surface of the strand, mostly running transversely to the pressing direction or at an angle (45º) to the extrusion direction, often at the edges of the strand. In extreme cases, they can resemble a fir tree. These cracks are typically caused by localized high temperatures. Blisters are another defect, appearing as bubbles of different sizes arranged in lines or irregularly in the extrusion direction. They are formed when air penetrates a subsurface zone, and if a blister ruptures, it leaves a void often termed a “blow hole.” Understanding these defects and their root causes is crucial for manufacturers to implement appropriate preventive measures and improve the quality of aluminum extrusion products.

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