Performance and processing of NiTinols in medical device applications
志联
2024-01-31
Binary NiTinol titanium is now a preferred material for many medical devices, but challenges remain in producing NiTinol titanium alloys into the required shapes and surface smoothness. In addition, the performance of binary NiTinol titanium needs improvement, especially in terms of radiopacity, superelasticity, and fatigue limit. This will explore the development of NiTinol titanium alloys related to various devices and new approaches to improving performance and advanced manufacturing techniques.
I. Environment
However, as the miniaturization of implantable devices increases, the need for further improvements in the radiopacity, strength, fatigue, and biocompatibility of this NiTinol titanium alloy family also increases. Since NiTinol titanium alloys are primarily used to produce rods, wires, amorphous ribbons, and tubes for medical devices, starting with the melting and forging of ingots, it is a topic that should be addressed.
II. Ingot Melting
The melting of NiTinol titanium alloys and the forging of ingots for primary and secondary processing are related to the challenges of the following characteristics:
Sensitive to oxygen and carbon contamination
Strict compositional control requirements
Solidification criteria to minimize micro and macro porosity
Prevention of non-metallic inclusions
Many of the metallurgical processes already successfully used in titanium alloy manufacturing are also useful for NiTi. Currently, the common processing method for NiTi shape memory alloys is to use vacuum induction melting (VIM) for primary alloy production, followed by vacuum arc remelting (VAR) to improve microstructure homogeneity. Porosity characteristics are a function of phase diagram characteristics and solidification rate; faster cooling rates favor smaller twin spacing, which equates to less porosity. It should be noted that for alloys with NiTinol content exceeding 55.0 wt%, a 1% error in the relative content of NiTinol or titanium can cause a change in transformation temperature of approximately 100 °C, and analytical techniques cannot accurately predict the transformation temperature.
While VIM-VAR is currently the preferred metallurgical process for NiTinol titanium alloys, vacuum induction skull melting may produce higher purity ingots. In this process, the crucible has a unique geometry, shaped like a crown, with a water-cooled base and side walls composed of square water-cooled bars spaced approximately half the bar diameter apart. The crucible is surrounded by an induction coil, and the entire assembly can be tilted to pour into a mold placed in a vacuum chamber. When the NiTinol titanium alloy melts, a thin layer of solidified metal forms at the bottom and sides of the crucible, confining the melt within the already molten alloy in the crucible or skull, avoiding crucible-melt interaction and minimizing contamination of the highly reactive melt.
NiTinol titanium compositions that are less sensitive to compositional changes in transformation temperature, such as NiTiCu and NiTiNb ternary systems, can be used. However, the addition of Cu can lead to embrittlement, which is a problem during heat treatment, while Nb broadens the transformation hysteresis. There are many NiTinol titanium alloying possibilities that may reduce compositional sensitivity and improve superelastic properties, leading to better ingot processing.
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