Characteristics Of Casting Structure Formed During Solidification

The crystallization process of titanium and titanium alloys consists of two stages: nucleation and crystal growth. This process determines the grain structure after casting, the shape and size of the crystal, the uniformity of the composition within and between the grains, and each The formation of defects such as shrinkage and cracks. Therefore, it has a great influence on the quality and performance of castings and the subsequent heat treatment process.
The solidification crystallization of titanium and titanium alloy castings belongs to primary crystallization, also called primary crystallization. The main parameters of the process are the rate of nucleation and the rate of crystal growth. Changes in these two parameters directly affect the shape and size of the crystal after crystallization and others. For this reason, some people compared the crystallization process of titanium with the crystallization of aluminum and iron. At the same crystallization rate, the nucleation rate of titanium is about 1 order of magnitude smaller than that of aluminum and 2 orders of magnitude smaller than that of iron. It can be seen that the growth rate of crystals during the crystallization process of titanium and its alloys is faster than that of crystal nuclei. As a result, titanium and titanium alloys tend to form coarse grain structures after cooling and solidification.
The main thermophysical properties that affect the crystallization process of titanium alloy are: crystallization heat, crystallization temperature interval, heat capacity, thermal conductivity and density, especially thermal conductivity and crystallization heat. The thermal conductivity of titanium itself is relatively poor. With the addition of alloying elements, the thermal conductivity of titanium alloys will become worse and solidify more buns, eventually resulting in coarse crystal structure. The crystallization heat of the alloy has a great influence on the solidification and crystallization process of the drill alloy.
In addition, the size of the crystallization temperature interval of the titanium alloy also has a great influence on the crystallization characteristics after casting and solidification. It is generally believed that the wider the crystallization temperature interval, the larger the absolute size of the primary crystals. Under the same conditions of other conditions, the crystallization rate should be greater, and the final grains formed will also be coarser.