Industrial Lubrication｜Aluminum Strip Surface Darkening Reasons and Lubrication Measures
February 21, 2022
At present, aluminum sheet and strip users have higher and higher requirements for the surface quality of aluminum sheet and strip. The improvement of surface quality and improvement of surface defects is one of the most important means to enhance the competitiveness of users. Generally, the finished surface of a good aluminum hot-rolled strip should be relatively uniform, fine, with a certain metallic luster and no obvious defects. Many factors will cause problems on the surface of hot-rolled strip, such as surface sticking to aluminum, black wire, black line, color difference, residue and so on. Strip surface roughness, lubrication and cooling uniformity, equipment, process and process lubrication adaptability and matching degree, etc. may be the reasons. Reason 1: Metal to Metal Contact Case 1 Comparison of two hot rolled surfaces (pictured), normal and darkened surfaces. A "dark" surface, viewed from an angle, has "dark" and "bright" lines. SEM/EDX test: darkened surface with deep grooves and transverse tears with particle intrusion. The two surfaces have very similar chemical compositions at the macroscopic level. Al, O, C are not significantly different.
Cause Analysis Through investigation and analysis of all aspects of details (process, equipment, emulsion, etc.), it is initially judged that the possible cause is the direct contact between metal and metal in the last pass of the bite zone, that is, at the exit of the rolling bite zone, the roll coating The protruding rolls scrape the rolling surface. One of the reasons may be due to insufficient lubrication between the rolls. Possible Measures Consider comprehensive process variables, especially those affecting oil film thickness, such as emulsion parameters (concentration, active ingredients, etc.), process parameters (rolling speed, reduction, roll roughness, etc.). Case 2 Surface defects cause darkening of the board surface. During rolling, the surface of the aluminum plate is partially forcibly separated by the roller, and the continuous surface oxide is transferred between the roller and the plate, and the metal oxide is pressed into the plate. As shown, anodizing (AQ) can be observed more clearly.
Cause Analysis High magnesium alloys are prone to appear in hot rolling. SEM-EDX characterization and identification revealed the presence of both high oxygen/high magnesium elements. In addition, a work roll coating analysis was performed. Possible Measures The most important influencing factors include emulsion, roller brush, and work roll temperature. ●Is emulsion lubrication and cooling suitable? ●Is the lubrication condition, active ingredients, management of emulsion, etc. reasonable? ●Is the cooling and spraying of work roll temperature appropriate? ●Type of brush roller, speed, brush marks ●Ingot and milling related issues
Reason 2: Aluminum powder and metal oxide (smut) Aluminum powder and metal oxide fragments (replaced by Smut below) are inorganic fragments produced during rolling, mainly composed of fine Al2O3 particles. Smut adheres to the surface of the strip and can be wiped off.
Produces the result of Smut Might make the board darker/gray (full surface) or black/gray - dots/flakes/strips, etc. Smut does not affect the mechanical properties, but does affect the visual effect, the surface looks "dark/dirty". Not good for high surface requirements. Reason for Smut The surface oxide layer falls off during rolling. Due to the relative movement of the roll and the strip during rolling, when the abrasive particles are worn, the softest material particles are easily formed by falling off. The Smut generated on the rolling surface accumulates under certain conditions and becomes visible to a certain extent. Variables affecting Smut strength: roll roughness, roll gap oil film thickness, Smut size, strip temperature, etc. Possible Measures A comprehensive discussion of process, equipment and emulsions is required to reduce the effects of Smut accumulation and adsorption onto the board. Among them, the surface state of the work roll, the thickness of the oil film, the degree of contamination of the emulsion, and the temperature can be given priority.
Reason 3: Organic Residues The organic residues mentioned here include emulsions, oils, etc., which are excessively left on the surface of the aluminum plate, causing the surface to be dark and flowery. During the rolling process, if the purging is not good or other factors, the excess emulsion or oil will remain, adsorbed and involved in the strip coil.
Cause Analysis There are various reasons, which may be that the temperature of the strip is too low, the purging is not good, the emulsion or oil is polluted, and other processes are improperly coordinated. Among them, the temperature of aluminum hot-rolled strip, especially the too low final rolling temperature of strip is an important factor, which needs special attention, because at low temperature, the emulsion vapor film is destroyed, and the emulsion is easy to adhere. possible measures Several aspects can be focused on, such as ●Process equipment (purging, emulsion injection system, material temperature control, etc.) ●Emulsion technology (optimization of emulsion components, new technology development and improvement - Quaker Houghton has been working on this) ●Emulsion control and use (emulsion testing and management, pollution control, cleaner production, etc.)
