6063 aluminum alloy is mainly used for extrusion of architectural profiles. Its ingots have metallurgical defects such as cracks, porosity, slag inclusions, looseness, bright grains, feather-like grains, and coarse grains. These metallurgical defects affect the yield of round ingot production and the material yield during extrusion to varying degrees, causing significant economic losses to the company. Therefore, it is necessary to prevent the occurrence of the above metallurgical defects.
Bright grains were discovered during the low-magnification inspection of 6063 aluminum alloy ingots. They appear as irregular light-colored bright spots on the cross-section of the specimen, mainly concentrated at the root of the ingot. The grain structure is coarse, with relatively few intragranular solutes, and the color is light white, showing a significant difference from the normal crystalline structure. These grains are formed during the casting stage.
1 Causes of Bright Grain Formation
1.1 Chemical Composition
6063 is a low-alloy A1-Mg-Si high-plasticity alloy. During the charge process, due to misoperation, 0.24%-0.32% of 6061 alloy scrap containing Cu was added, which created conditions for bright grain defects in 6063 aluminum alloy ingots, as shown in Table 1.
1.2 Low Casting Temperature
To improve the formability and surface quality of the ingot, the casting temperature is often kept relatively low. At such temperatures, the temperature in the mold cavity is lower, and the supercooled zone extends to the bottom area of the transition plate. As a result, dendrites crystallize first at the bottom of the transition plate; these dendrites grow for a long time in the mold cavity, forming bright grains. Since the growth rate of bright grains is very slow, and the surrounding molten metal is constantly refreshed, the composition of the liquid phase in this area does not change significantly during crystallization. The concentration difference between the bright grains and the liquid phase remains as it was at the start of crystallization, resulting in the bright grains becoming an aluminum solid solution poor in solute (1).
1.3 Low temperature of the casting tray
Due to the production characteristics of 6063 aluminum alloy round ingot casting, where there are long intervals between furnace and casting runs, the casting tray is always in a cold state. This causes the molten metal in the mold to be relatively low in temperature at the beginning of casting. The initially crystallized bright grains adhere to the transfer plate and gradually grow. When they reach a certain size, they fall into the ingot, forming bright grain defects at the base of the ingot.
1.4 Crystallizer
The crystallizer used for casting 6063 aluminum alloy is a water-eye type short crystallizer. The circulating water is dirty and the water temperature is high, with elevated concentrations of Ca²⁺ and Mg²⁺ ions. During casting, this causes partial blockage of the water-eye, resulting in uneven temperatures of the molten metal around the ingot, leading to fluctuations in the metal flow and uneven flow distribution within the crystallizer. Under locally lower casting temperatures, the solidification rate of the molten metal varies, producing bright grains.
2 Preventive Measures
(1) Thoroughly clean the furnace when changing production alloys.
(2) Store different types of alloy scrap separately, and only allow scrap of the same type or similar composition to be added during alloy preparation.
(4) Strengthen process operation management, and preheat the runners, distribution plates, flow channels, and transition plates to a red-hot state using a petroleum liquefied gas torch before casting.
(5) Modify the circulating water filtration system and cooling system, make full use of the softened water station, and ensure the quality and temperature of circulating water, as well as the concentrations of Ca, Mg, and other ions, meet the requirements for casting operations. This eliminates metallurgical defects in ingots caused by circulating water. This issue was caused by high Cu content in the charge, low temperature of the melt entering the mold, insufficient preheating of the distribution plate, and poor cooling of the ingot by circulating water.
(6) By adjusting the Cu content in the charge, appropriately increasing the casting temperature and casting speed, preheating and baking the casting tools before casting, and strengthening control over circulating water quality, it is possible to completely eliminate the bright grain defects in 6063 aluminum alloy ingots.
