6063 aluminum alloy is mainly used for extruding architectural profiles. The metallurgical defects of its ingots include cracks, blowholes, slag inclusions, porosity, bright grains, feathery grains, coarse grains, etc. These metallurgical defects affect the yield of round ingot production and the finished product rate during extrusion to varying degrees, causing significant economic losses to the company. Therefore, it is imperative to prevent the occurrence of the aforementioned metallurgical defects.
Bright grains are found during the macrostructure inspection of 6063 aluminum alloy ingots. On the fracture surface of the sample, they appear as irregular light-colored bright spots, mainly concentrated at the root of the ingot. Characterized by coarse grain structure, few intracrystalline solutes and pale white color, they differ significantly from the normal crystalline structure and are generated during the casting stage.
1. Causes of bright grain formation
1.1 Chemical Composition
6063 is a low-alloyed Al-Mg-Si series high-plasticity alloy. During the charge batching process, 6061 alloy scrap containing 0.24%-0.32% copper (Cu) was added due to misoperation, which created the conditions for the formation of bright grain defects in 6063 aluminum alloy ingots, as shown in Table 1.
1.2 Low Casting Temperature
In order to improve the moldability and surface quality of the ingot, the casting temperature is often kept relatively low. This results in a lower temperature in the mold cavity during casting, causing the supercooled zone in the cavity to extend to the bottom area of the transition plate, where dendrites crystallize first. These dendrites grow over a long time in the cavity, forming bright grains. The growth rate of bright grains is very slow, and because the surrounding molten metal is constantly being refreshed, the composition of the liquid in that area does not change significantly during crystallization. The concentration difference between the bright grains and the liquid phase remains as it was at the beginning of crystallization, thus making the bright grains a solute-poor aluminum solid solution (1).
1.3 Low casting tray temperature
Due to the production characteristics of 6063 aluminum alloy round ingots, where there are long intervals between furnace batches and casting cycles, the casting trays are always in a cold state. This results in a lower melt temperature in the mold at the start of casting. The initially crystallized bright grains then 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 ingot's bottom.
1.4 Crystallizer
The crystallizer used when casting 6063 aluminum alloy is a water-eye type short crystallizer. The circulating water is dirty, the water temperature is high, and the concentrations of Ca²⁺ and Mg²⁺ ions are relatively high. During casting, this causes partial blockage of the water-eye, resulting in uneven temperatures of the melt around the ingot, which leads to fluctuations in metal flow and uneven distribution within the crystallizer. Under locally lower casting temperatures, the solidification rate of the melt varies, producing bright grains.
2 Preventive Measures
Based on the above analysis, we have taken the following measures:
(1) Thoroughly clean the furnace when changing the product type.
(2) Store alloy scrap of different types separately, and when preparing the melt, only scrap of the same type or similar composition is allowed to be added.
(3) Strengthen process operation management. Before casting, use a petroleum liquefied gas torch to bake the flow channels, distribution plate, flow pipes, and transition plates until they are red hot.
(4) Modify the circulating water filtration and cooling system, making full use of the softened water station to ensure that the quality and temperature of the circulating water and the concentrations of Ca, Mg, and other ions meet the requirements for casting operations, thus eliminating metallurgical defects caused by circulating water in the cast ingots. These defects are caused by high Cu content in the charge, low melt temperature at the time of entering the molds, poor preheating of the distribution plate, and inadequate cooling of the ingots by circulating water.
(5) By adjusting the Cu content in the charge, appropriately increasing the casting temperature and speed, preheating and baking the casting tools before casting, and strengthening the control of circulating water quality, the bright grain defects in 6063 aluminum alloy ingots can be completely eliminated.
