THE REASON ANALYSIS AND RESOLUTIONS OF THE "WHITE SPOT" IN THE PROCESS OF ALUMINUM ALLOY PROFILE OXIDATION

1.Statement of Problem

　　In actual production, the processing large (ε>95%), with thin wall thickness (δ≤1.5mm) of T5 state of 6063 aluminum alloy extrusion profiles in after sulfuric acid anodized treatment, the surface will appear regular (and sometimes irregular) distribution of white spots (or no light spots); When severe, the dark spot - “white spot”. The distribution regularities and features of “white spot”is:it is a substantially equidistant on the plane parallel to the extrusion direction, presenting linear or flat quadrilateral shape or irregular star points (pieces),which relative to the substrate surface with small depth is a surface defect with concave shape[5,6].A white spot is usually distributed on one or several surfaces of the profile, and is sometimes distributed on all surfaces of the profile (for thin-walled hollow sections, which are distributed on the inside and outside of a plane or surface).

２.Reason Analysis

　　In the scene, "white spot" formed in the "alkali erosion" process,did not disappear after the dilute nitric acid(or sulfuric acid) “neutralization”,and was more clearly presented by sulfuric acid anodizing.

2.1 the influence of ingot quality

　　The main phase composition of the 6063 aluminum alloy is: α (Al) solid solution, free Si (anode phase) and FeAl3 (anode phase).When the iron content is greater, there are β(FeSiAl) (anode phase).When the iron content is less, there are α(FeSiAl) (cathode phase).Other possible impurity phases are: MgZn2, CuAl2 etc.

　　In the production,6063 aluminum alloy ingots obtained from the non-equilibrium crystallization process often have macroscopic segregation or transgranular segregation.Therefore, Si, Mg, Zn,Cu and other elements in ingots are not evenly distributed.And some aluminum processing enterprises due to economic factors, hardly ever do a homogenizing annealing treatment for ingot of a small specifications (such asφ100 mm below) to eliminate the segregation phenomenon, so as to create conditions for the generation of the “white spot”.

2.2 Impact of Extrusion - Heat Treatment Process

　　In order to improve the production efficiency,in production operations,often using high speed extrusion at low temperature,the “heat effect”caused by the extrusion make the quenching temperature of products greatly increased in the mould exit, and the fixed on the discharge contacts with surface temperature of 80 ~ 110 ℃ (or lower) of the graphite plate (or round) , material surface will be caused by rapid cooling heat transfer effect and make that part of the concentration of alloy elements such as Mg and Si a little higher than the normal parts.n the subsequent artificial aging process, the site will precipitate a large β'(Mg2Si) phase.Without homogenization annealing treatment and heating temperature on the low side of 6063 aluminum alloy ingot caused by extrusion of the “heat effect” is not enough to make the profile of the quenching temperature to rise above 500℃, which will not only cause one of the ingots a small part of the β(Mg2Si) phase remains in the profile, and the Mg and Si elements in the α(Al) matrix phase at high temperature are changed as described above.These factors set the organizational conditions for the production of "white spots".

2.3 Effects of Surface Alkali Erosion Treatment

　　When the content of Si is larger than that of Fe, excess Si is segregated in the α (Al) crystal or near the grain boundary to form a free single crystal Si phase.Cathodic phase Si and segregated anode phase Mg2Si, or anode phase α(Al) matrix and coarse cathode phase Mg2Si in the alkaline solution occurred in the “original battery effect”.As a result, the α(Al) solid solution around the free Si is rapidly dissolved or the coarse Mg2Si phase is preferentially dissolved in the α(Al) solid solution, leaving a shallow and flat "corrosion pits" on the surface of the profile.

　　In addition, some scholars have suggested that white spots are related to the hydrolysis of NaA1O2.When the ratio of Al3 + concentration to total NaOH concentration is more than 0.35, the stability of NaAlO2 decreases and Al (0H) 3 separated out of water precipitates on the surface of aluminum. Due to the incomplete washing, it is also prone to spots or massive “white spots”.However, the author believes that this is mainly related to the role of scale inhibitors (such as hydroxycarboxylates, sodium tartrate, etc.) contained in alkaline etch additives.In particular, the hydroxycarboxylate reacts with Al (0H) 3 in a stable alkaline etching process to produce a soluble complex anion.

2.4 Influencing Factors of Sulfuric Acid Anodizing Process

　　In general, when the concentration of sulfuric acid is too high, or the electrolysis temperature is too high, or the Al3 + content in the oxidation bath sulfuric acid solution is greater than 20 g / L, With the increase of Al3 +, the following ionization equilibrium state at the normal temperature (about 20℃) is destroyed, The Al (0H) 3 in sulfuric acid oxidation tank was deposited in a flocculation and attached to the groove of the profile surface or Al2O3 film pinhole, and the water was not clean and the sealing hole was not easy. After drying, white spots appear on the surface.

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