What Is Directional Solidification?

Directional solidification, also known as directional crystallization, refers to a process that allows a metal or alloy to grow crystals in the melt. Directional solidification technology is a process of establishing a specific temperature gradient in the mold to make the molten alloy solidify and cast along the opposite direction of the heat flow and according to the required crystal orientation. It can greatly improve the comprehensive performance of superalloys. ^[1]

Directional solidification is the use of forced means during the solidification process to establish a temperature gradient in a specific direction in a solidified metal-like unsolidified melt, so that the melt nucleates on the gas wall in a direction opposite to the heat flow, as required The technique of solidification of crystal orientation. This technology was originally established and refined in the development of superalloys. The adoption and development of this technology was originally used to eliminate the lateral grain boundaries generated during the crystallization process, thereby improving the unidirectional mechanical properties of the material. This technology is applied to the production of gas turbine engine blades. The obtained material with columnar or even single crystal structure has excellent thermal shock resistance, longer fatigue life, higher creep resistance and moderate temperature plasticity, thus improving the blade The service life and operating temperature became one of the major events that shocked the metallurgical and industrial circles at that time.

Directional solidification technology provides an extremely effective means for the theoretical study of metal solidification and the development of new superalloys. However, the length of the casting obtained by the traditional directional solidification method is limited, and equiaxed grains are easy to appear at the end of solidification, and the grains are easy to coarse. For this reason, continuous directional solidification technology has appeared, which combines the advantages of continuous casting and directional solidification, and makes up for each other's shortcomings and deficiencies, so that ingots or castings with ideal directional solidification structure, arbitrary length and sectional shape can be obtained. Its appearance indicates that the directional solidification technology has entered a new stage.

The biggest advantage of directional solidification technology is that the alloy material prepared by it eliminates the influence between the matrix phase and the enhanced phase interface, and effectively improves the overall performance of the alloy. At the same time, this technique is also an important method for scholars to study the theory of solidification and the law of metal solidification. ^[1]

Two conditions are required to achieve directional solidification: first, heat flow flows in a single direction and is perpendicular to the solid-liquid interface during growth; second, there is no stable crystal core in the melt in front of crystal growth. For this reason, measures must be taken to avoid lateral heat dissipation in the process, and at the same time, a large temperature gradient should be caused in the melt near the solid-liquid interface. This is the basic principle to ensure that the growth of non-oriented columnar and single crystals stops and is oriented correctly Elements.

The realization of directional solidification should meet the requirements of stable directional growth at the solidification interface, and suppress the large component undercooling zone that may appear in front of the solid-liquid interface, which leads to the generation of free grains. according to

Heating agent method

The so-called heating agent method is to pour molten metal into a side wall.

A large number of equiaxed grains are obtained by ordinary casting. The length and width of the equiaxed grains are approximately the same, and the number of longitudinal grain boundaries and transverse grain boundaries are also approximately the same. Accidental analysis of superalloy turbine blades found that the horizontal grain boundaries are more likely to crack than the vertical grain boundaries due to the centrifugal force experienced by the blades when the turbine rotates at high speed. By applying the directional solidification method, columnar crystals grown in one direction were obtained without lateral grain boundaries, which greatly improved the unidirectional mechanical properties of the material. Single crystal blades obtained by single crystal casting can significantly improve modern aviation. For magnetic materials, the application of directional solidification technology can make the direction of columnar crystals aligned with the direction of magnetization, which greatly improves the magnetic properties of the material. Directional solidification technology is also widely used in the production of in-situ composites. In-situ composites obtained by directional solidification eliminate the influence of the interface between the reinforcing phase and the matrix during the preparation of other composites, and greatly improve the performance of the composite. ^[3]