How to Prevent Bright Grain in 6063 Aluminum Alloy Round Cast Ingots

6063 aluminum alloy is mainly used for extruded architectural profiles. Its cast ingot metallurgical defects include cracks, porosity, inclusions, looseness, bright grains, feather-like grains, and coarse grains. These metallurgical defects affect the finished product yield of round ingot production and the yield during extrusion to varying degrees, causing significant economic losses for the company. Therefore, it is essential to prevent the occurrence of the above metallurgical defects.

Bright grains are observed when examining the low-magnification structure of 6063 aluminum alloy cast ingots. On the cross-section of the specimen, they appear as irregular, light-colored bright spots, mainly concentrated at the root of the ingot. The grain structure is coarse, with fewer solutes within the grains, and the color is light white, showing a significant difference from normal crystalline structure. This occurs 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 within the mold cavity, forming bright grains. Since the growth rate of bright grains is very slow, and the surrounding molten metal continuously renews, the composition of the liquid phase in this area does not change significantly during crystallization. The concentration difference present at the beginning of crystallization is maintained between the bright grains and the liquid phase, making the bright grains a solute-poor aluminum solid solution.

1.3 Low Temperature of the Casting Plate
Due to the production characteristics of 6063 aluminum alloy round ingots, where there are long intervals between furnace batches and casting cycles, the casting plate is always in a cold state. This results in the molten metal in the mold being relatively low in temperature at the start of casting. The initially crystallized bright grains adhere to the transition plate and gradually grow. When they grow to a certain extent, they fall into the ingot, forming bright grain structural defects at the base of the ingot.

1.4 Crystallizer
The crystallizer used for casting 6063 aluminum alloy is a short water-pipe type. The circulating water is contaminated, the water temperature is high, and the concentration of Ca²⁺ and Mg²⁺ ions is high. During casting, this causes partial blockage of the water pipes, leading to uneven temperatures around the periphery of the ingot. This results in fluctuations in the molten metal flow within the crystallizer and uneven distribution of the flow. At locally lower casting temperatures, the molten metal solidifies at different rates, producing bright grains.

2 Preventive Measures

Based on the above analysis, we have implemented the following 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.

(3) 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.

(4) 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.

(5) 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.