Characteristic control, processing, applied technology
5.1 Characteristic control
The mixing ratio of Ni-Ti alloy has a great influence on the transformation temperature, and change of 0.1at% (or 0.1wt%) can cause a difference of about 10℃ in the transformation temperature. Since the variation of the transformation temperature is caused by the variation of the blending value and the influence of impurities such as carbon and oxygen, it is important to control the melting and casting conditions such as the level of vacuum. Therefore, the melting and casting process of Ni-Ti alloys is usually carried out in a vacuum induction melting furnace (VIM) or vacuum arc remelting (VAR).
5.2 Heat treatment to obtain Shape Memory and Super-elastic effect
In order to obtain the Shape memory or Super-elastic effects, Ni-Ti alloys are constrained into the shape of a straight or coiled spring, and heat treated at medium temperature (Shape memory treatment). The most common heat treatment temperature is 400~550℃. Depending on the size of the jig, it is often kept for 30 to 60 minutes. Water-cooling or air-cooling by a fan is desirable because of the precipitation behavior around 400°C and the increase in transformation temperature with slow cooling.
Ni-Ti wire can be formed into springs using a coil forming machine. Since it has a large spring-back and is easy to be seized, the tool wear is large and the forming process is not easy, but various measures have been taken for practical use. For press work and other processes that require large deformation, annealing (heating at around 700℃) is recommended in advance.
5.4 Jointing and plating technology
Jointing and plating of Ni-Ti alloys were considered difficult because of the large amount of deformation during use, but brazing after plating or direct bonding has been adopted for eyeglass frames production. The key point of ingenuity is to design a joint portion that does not cause large deformation. As for plating, physical surface treatment methods (ion plating, sputtering, etc.) have been successful where deformation is large.
5.5 Rolling, Sheet processing
The maximum cold working rate during annealing is much lower than that of iron, copper, aluminum, etc. Ni-Ti alloy is a difficult alloy to work. It is also easy to crack when subjected to high rolling work. Hot rolling and cold rolling are possible, but the working rate must be controlled and the processing distortion must be recovered by annealing after cold working. Sheet, strip, and tape processing are possible, but they are very costly. Currently, Furukawa Techno Material has basically withdrawn from sheet and strip manufacturing.
5.6 Cautions for use
When heated, the material may recover the shape rapidly, which requires caution from a safety standpoint. From the property point of view, if a large strain (>8%) is considered, slip will occur and the shape will not fully recover. In addition, if the material is deformed at high temperature (>Af+50°C) or constrained at high strain, some of deformation will remain. To prevent this, it is necessary to consider a design structure, such as installing a stopper to prevent excessive deformation from being concentrated.