[China Aluminum Industry Network] In the production of aluminum alloy building decorative profiles, it is common to see hollow, semi-hollow, and even some solid extruded materials with large section curvatures. After the sulfuric acid anodizing production process, the surface Partially, there will appear a continuous, longitudinally-smoothed, coarse-grained (pear-like), apparently visible, scintillation-grained surface defect—a “flickering pattern†(or “bright patternâ€). . The law of its distribution is: 1 in the direction of extrusion, the tail is more visible than the head, in severe cases, both ends are obvious; 2 in the direction perpendicular to the extrusion axis, "pattern" generally only appears in the local, especially in the profile The locations where the curvature is larger, or the weld zone of hollow or semi-hollow profiles, or the 6063 aluminum alloy is subjected to frictional resistance during the formation of the profile.
Second, the cause of the analysis 2.1 The effect of pre-oxidation treatment process After squeezing and washing with sulfuric acid in some extrusion materials, the surface has no abnormal changes, and when it is in the wZn2+≥4×10-6 alkaline solution, it is eroded normally. Immediately after the effective washing immediately, you will see the presence of "flickering patterns." The author analyzes the extruded structure of the extruded material and the results show that the "flickering pattern" corresponds to the structure of the recrystallized structure of coarse equiaxed crystals with a much larger grain size than the normal part, ie, coarse-grained rings. The coarser the grain, the more pronounced the “flickering patternâ€; this phenomenon is also more and more obvious as the erosion progresses.
The formation of the "flickering pattern" is not only related to the microstructure of the alloy composition (especially Zn) and extrusion material (RCS state), but also related to [Zn2+] in the alkaline solution. Experiments show that in the alloy, wZn≥0.033%, and the presence of coarse crystal rings on the surface of the profile, as long as the wZn2+ in the alkaline solution is ≥4×10-6, a “flickering pattern†will be produced.
The root cause of the "flickering pattern" is the selective intergranular corrosion caused by Zn contamination in the alkaline solution. The mechanism of intergranular corrosion is electrochemical and is the result of local galvanic action within the grain boundary. Due to the difference in corrosion potential between the precipitated second phase Mg2Si precipitated along the grain edge and the lean solid solution, the primary cell α-Al-Mg2Si was formed in the alkaline electrolyte solution. In actual production, the Si content is generally required to be excessive, and the intergranular corrosion susceptibility is increased because the free silicon located in the grain boundary and its vicinity has a strong anode property.
The results of the study showed that the amount of Zn contained in the grain boundary of the "scintillation grain" and its vicinity is relatively high, that is, Zn participates in the corrosion process. Reference [4] speculates that Zn promotes grain boundary corrosion in the form of "dissolution-redeposition". In caustic washing, Zn dissolved in α-Al dissolves with dissolution of α-Al; when wZn2+ in the bath ≥ 4×10-6, reaction occurs: Al+Zn2+→Zn+Al3+, elemental Zn is selectively in the cathodic Area deposition further exacerbates localized corrosion.
2.2 The influence of ingot quality We know that the main phase composition of 6063 aluminum alloy is: free Si (anode phase) and FeAl3 (anode phase), when wFe ≥ wSi, there is α-(Al-Fe-Si) (anodic phase ); when wFe ≤ wSi, there is β-(Al-Fe-Si) (cathode phase). In actual production, the composition of 6063 alloy is required to comply with the provisions of international GB3190-82, and it is required that the excess amount of Si element with relative excess of wFe:wSi=1.73:1 is not more than 0.20%. In the alkali etching treatment of 6063 aluminum alloy profile (RCS state), when other conditions are met, as long as wZn in the alloy is ≥ 0.03%, “flickering pattern†defects may be generated; and the degree of clarity of this defect is in the alloy. The Zn content increases. In particular, it should be pointed out that under the same conditions, when the "flickering pattern" defect occurs, the influence of Zn content in the alloy on the hollow profile is more significant than that on the solid profile.
2.3 Effect of extrusion-heat treatment process factors The literature points out [5] that when the low-level fault energy metal (such as α-Al) is squeezed (ε=90%), only dynamic recovery occurs, and then static recovery and static reactivation occur. crystallization. The recrystallization temperature of pure aluminum is about 280°C, while the recrystallization temperature of 6063 aluminum alloy is about 320°C. We know that whether "recovery" or "recrystallization" is the diffusion and migration of atoms in the solid phase. This is related to the chemical potential required for the diffusion of atoms and it needs to be carried out at higher temperatures. When the temperature is too low, the diffusion should not be carried out, so that the "recovery" and "recrystallization" processes are suppressed.
From the above theory, it can be seen that 6063 aluminum alloy extruded profiles in the RCS state are produced under normal process conditions. After extrusion-quenching, the microstructure is: the atoms of elements such as Mg and Si are dissolved in α-Al. The formation of supersaturated aluminum-based solid solution and free Si simple substance, etc., and the crystal grains are fine and evenly distributed, and become a processing structure in which only dynamic recovery or static recovery has occurred. After artificial aging treatment, the main phase composition of 6063 aluminum alloy profile is: α-Al, free Si, main strengthening phase Mg2Si, and so on. The microstructure state is that the fine Mg2Si grains are uniformly dispersed in the α-Al matrix, while the free Si is distributed in the grain boundary and its vicinity. However, when the production conditions are not properly controlled (for example, the quench cooling strength is insufficient), "static recrystallization" and "recrystallized grain growth" may occur to form coarse crystal rings.
The distribution rules of the coarse-grained rings are: thicker coarse-grained rings appear near the part of the extruded tube wall; thicker coarse-grained rings are present in the part where the frictional resistance of the mold works is greater; and the crystal grains at the thicker coarse-grained rings are coarser. The thickness distribution of coarse-grained rings in the extrusion direction is: the thin head and thick tail; when severe, coarse-grained microstructures appear on the entire section. The larger depth of the coarse-grained ring is: 2.0-2.5 mm.
The formation mechanism of the coarse-grained ring is [6]: The coarse-grained ring is often the site where the metal material undergoes severe additional shear deformation. In the extrusion process of hollow and semi-hollow profiles of soft aluminum alloys (such as 6063 alloy), the outer grains of the aluminum alloy are subjected to more severe shear deformation than the inner ones, and the tail is more severe than the head in the extrusion direction. Far more, the additional shear deformation of the metal subjected to the strong external friction and the long friction time is larger. The degree of fragmentation and lattice distortion of the crystal grains is also relatively severe. Therefore, the metal in the site is in a thermodynamically unstable state with high interfacial energy, which reduces the recrystallization temperature at this site (this temperature difference is about 35° C.), and increases the driving force for grain growth and growth. At the same time, due to the sharp increase in the temperature of the metal in the severe friction part, when the air cooling is not timely or the cooling strength is insufficient, some of the disperse particles (such as MnAl6CrAl7) precipitated during the extrusion re-dissolve in the solid solution α-Al and hinder the recrystallization conditions. Disappearance also lowers the recrystallization temperature, and increases the driving force for recrystallization nucleation and growth.
This shows that: When the 6063 aluminum alloy extrusion temperature is too high, the extrusion speed is too fast, the local work of the mold with too long, so that the alloy profile out of the mold hole temperature is high and not timely air-cooled to below 250 °C, it is easy Local static recrystallization and aggregation of the recrystallized grains occur, which results in coarse-grained rings.
The presence of a coarse-grained ring creates an organizationally objective condition for the formation of a "flickering pattern."
Third, preventive measures Due to the presence of "flicker pattern" defects, a large number of process waste products have caused significant losses to the company's production and operations. Therefore, specific prevention measures must be taken to prevent the occurrence of such surface defects.
1 According to the size of the coarse-grained ring and coarse crystal on the surface of the extruded material, the production process control parameters are adjusted within the scope of the production process specification to minimize the influence of the coarse-grained ring on the surface quality of the oxide material.
2 Select excellent alkaline etching additives. When the [Zn2+] in the alkaline solution is too high, excessive NaS or polysulfide should be added to the bath in time.
3 According to the following ionization equilibrium equation at operating temperature: Zn2++2OH-Zn(OH)22H++Zn, in order to suppress the adverse effects of Zn2+, the concentration of free NaOH can be increased, thereby reducing the concentration of Zn2+, so that wZn2+3×10-6. Or use a part of the old tank liquid to be discharged first, and then add a considerable amount of new tank fluid, which can also make wZn2+3×10-6.
4 According to the provisions of international GB3190-82, starting from the actual production of enterprises, develop a corporate standard 6063 aluminum alloy components suitable for the situation of the enterprise. Strict control of Si, Fe, Mg, and Zn content requires that the content of Si be excessive with respect to the content of Si required by wMg:wSi=1.73:1, but the excess amount is not more than 0.20%; wZn2+ ≤ 0.050%; The homogenization annealing treatment eliminates segregation.
5Adjust the mold structure, reduce the local severe friction and shorten the length of the working part of the corresponding part, increase the inclination of the empty knife or ensure that the height difference between the work belt plane and the joint of the empty knife slope is not less than 0.5mm, adjust the arrangement of the shunt hole or rely on The size of the former splitter ratio K1 and the shape of the lower end of the mold bridge section reduce the relative friction between the two metals when the weld and the upper chamber meet.
6Adjust the control parameters of the heating temperature, extrusion temperature and extrusion speed of the casting and ingot extrusion cylinders, strictly control the quenching cooling process, and avoid the occurrence of static recrystallization.
IV. Concluding remarks In summary, the formation of the “flickering pattern†is regular and its influence factors are not unique. In production, different companies should proceed from reality, analyze specific issues, take appropriate measures to solve specific problems, and increase economic efficiency.
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