Cause and Analysis of Aluminum Extrusion Surface Cracking
Sometimes found in the Aluminum Extrusion surface with varying degrees of irregular arrangement of point-like dark gray corrosion point, this corrosion point and zinc particles caused by the corrosion of its shape is completely different, and, in the Aluminum Extrusion process Is intermittent. Some people think that the reason for the operator did not perform the correct surface treatment process; tank there are some harmful impurities ions; material is not good, mixed with too much. In this regard, we analyze the following.
Analysis of Corrosion Point
According to our years of production experience and the Aluminum Extrusion production process parameters of the inspection, as well as the implementation of the operation of the operator to follow the survey, that the type of dark gray corrosion of the main reasons for the following aspects:
(Mg) / ω (Si) is in the range of 1.0 to 1.3, Aluminum Extrusion which is much smaller than the optimum ratio of 1.73 (generally controlled at (1)) for some reason. 1.3 to 1.5). In this way, although the content of magnesium and silicon is within the range of (ω (Mg) = 0.45% to 0.9% and ω (0.2) to 0.6%). But there are some surplus silicon exists, this part of the surplus silicon in addition to a small amount of silicon in the presence of free, in the aluminum alloy will also form a ternary compound. When α (Si)> ω (Fe), the formation of more α (Al12Fe3Si) phase, it is a brittle compound, when ω (Si)> ω (Fe), the formation of more β (Al9Fe2Si12) Phase, which is a more brittle acicular compound, its harmful effect is larger than α, and tends to make the alloy easy to break along it. These insoluble impurity phases or free impurity phases formed in the alloy tend to accumulate on the grain boundary, while weakening the grain boundary strength and toughness, making the weakest part of the corrosion resistance, where corrosion is first generated.
(2) In the smelting process, although the addition ratio of magnesium and silicon is within the range specified in the standard, the uneven distribution and insufficientness of the silicon are sometimes caused by the uneven distribution of silicon in the melt. Area. Because the solubility of silicon in aluminum is very small, eutectic temperature 577 ℃ when 1.65%, while the room temperature is only 0.05%, after casting rods also produced a non-uniform phenomenon, it is directly reflected in the industrial Aluminum Extrusion products , The aluminum matrix in the presence of a small amount of free silicon, not only reduce the corrosion resistance of the alloy, and coarse alloy grain.
(3) the control of the process parameters of Aluminum Extrusion, such as billet preheating temperature is too high, the metal extrusion flow rate, squeeze when the air-cooled strength, aging temperature and holding time and other improper control are easy to produce silicon segregation and free, So that magnesium and silicon do not completely become Mg2Si phase, and some free silicon exists.
Corrosion in surface treatment
Excess and free silicon 6003 Aluminum Extrusion in the surface treatment of the following phenomenon: When the profile into the acid tank (sulfuric acid 15% to 20%), can be clearly observed in the profile surface has a lot of small bubbles, with the Time extension and bath temperature rise, the reaction rate is faster and faster, indicating that the original battery electrochemical corrosion has been generated. At this point the profiles from the tank to observe, it will be found in the profile surface a lot of normal and the surface color is not the same point. Continued subsequent treatment, such as alkali corrosion, acid neutralization and sulfuric acid anodic oxidation, this dark gray corrosion point will be exposed more obvious and intuitive.
Zinc caused by corrosion and silicon-induced corrosion in the appearance of the appearance of some differences. Zinc caused corrosion points like snowflakes, spread out along the grain boundary, Aluminum Extrusion there is a certain depth of the pit [6,7]. And the silicon element caused by the corrosion point of the dark gray point, along the crystal interface is not outward diffusion, but also do not feel the depth and with the processing time, the number of more and more until the complete reaction after the termination. This dark gray point can be substantially eliminated or reduced by prolonging the etch time or unwrapping treatment.
Silicon caused by 6063 Aluminum Extrusion corrosion behavior is completely preventable and control, as long as the purchase of raw materials, alloy composition for effective control to ensure that the proportion of magnesium, silicon in the range of 1.3 to 1.7, and the parameters of the process (such as smelting , Stirring, casting cooling water temperature, billet preheating temperature, extrusion hardening air cooling strength, aging temperature and time, etc.) to avoid segregation and dissipation of silicon, silicon and magnesium as much as possible to form a favorable Mg2Si enhanced phase.