6063 aluminum alloy is mainly used for extrusion of building profiles. The metallurgical defects of ingots are cracks, pores, slag inclusions, loose, bright grains, feather-like crystals, coarse grains and so on. These metallurgical defects affect the yield of round ingot production and the yield rate during extrusion to varying degrees, which brings great economic losses to the company. Therefore, it is necessary to prevent the above metallurgical defects.
The bright grain is found in the inspection of the low-fold structure of the 6063 aluminum alloy ingot. It shows some irregular light-colored bright spots on the sample section, mainly concentrated in the root of the ingot, the grain structure is coarse, and the crystal is inside. The solid solution is less, the color is light white, and it is quite different from the normal crystal structure, which is produced in the casting stage.
1 Causes of bright grain formation
1.1 chemical composition
6063 is a low-alloyed A1-Mg-Si high-plastic alloy, which was added by mistake ("Cu): 0.24%-0.32% of 6061 alloy scrap in the furnace charge process, which is 6063 aluminum alloy cast The conditions for the ingot to produce bright grain defects create conditions, see Table 1.
1.2 low casting temperature
In order to improve the formability and appearance quality of the ingot, the casting temperature is often lowered, so that the temperature of the liquid cavity is low during casting, and the subcooling zone in the liquid cavity extends to the bottom area of the adapter plate, and the bottom of the adapter plate is advanced. The dendrites crystallize and grow in the liquid cavity for a long time to form bright crystal grains. Since the growth rate of the bright crystal grains is very slow, and the metal liquid flow around it is continuously renewed, the liquid phase composition at the place has no large change in the crystallization process, and the crystal crystallization is always maintained between the bright crystal grains and the liquid phase. The difference in concentration makes the bright crystal grains a poorly soluble aluminum solid solution (1).
1.3 casting tray temperature is low
Since the 6063 aluminum alloy round ingot is characterized by a long production interval between the heat and the casting, the casting tray is always in a cold state, causing the liquid melt temperature to be lower when the casting starts, and the first crystallized bright crystal grains are It will be attached to the adapter plate and gradually grow up. When it grows to a certain extent, it will fall into the ingot and form a bright grain structure defect at the root of the ingot.
The crystallizer used in 6063 aluminum alloy casting is a water-eye type low crystallizer. The circulating water has dirty water, high water temperature, high concentration of Ca2+ and Ms2' ions, and some water eyes are blocked during casting, resulting in partial melt around the ingot. The temperature is different, which causes the metal liquid flow in the crystallizer to fluctuate and the liquid flow distribution is uneven. At a locally low casting temperature, the melt crystallization rate is different, and bright crystal grains are produced.
2 Preventive measures
Through the above analysis, we have taken the following measures:
(1) Wash the stove thoroughly when changing production varieties.
(2) Separate storage of alloy scraps of different varieties, and only allow scraps of the same variety or composition to be added during preparation.
(4) Strengthen the process operation management, and use the petroleum liquefied gas torch to bake the flow cell, the distribution plate, the flow tube and the adapter plate to a red hot state before casting.
(5) Renovate the circulating water filtration system and cooling system, make full use of the softened water station, so that the circulating water quality, water temperature and Cab, M and wide ion concentration meet the requirements of the casting operation, and eliminate the metallurgical defects caused by the circulating water to the ingot. The content of Cu in the charge is too high, the temperature of the melt flows into the liquid cavity is low, the preheating of the distribution plate is not good, and the circulating water is not good for cooling the ingot.
(6) By adjusting the Cu content in the charge, appropriately increasing the casting temperature and casting speed, preheating and baking the casting tool before casting, and strengthening the control of the circulating water quality, completely eliminating the bright grain defects of the 6063 aluminum alloy ingot .