Basic Information
The raw materials of alloy are Titanium (Ti) and Nickel (Ni) powder. The properties depend on composition or impurity, and even only 0.1 present of impurity atoms will result in non-negligible change of the whole nitinol properties [1]. Commonly, the principle impurities in industry is carbides such as TiC, which has significant difference both in width of transition temperature range and fatigue property [1]. Therefore, it is important to control the process of alloying nitinol strictly, which takes aim at rising the purity of raw materials powder.
Take nitinol ingot product as example. The melting process must be selected to not only ensure the homogeneity of the melt, but to ensure as high purity of materials as possible [2]. There are 2 industrial melting process to melt the nitinol ingot, vacuum induction melting (VIM) and vacuum arc remelting (VAR) [2].
Vacuum Induction Melting (VIM)
For VIM (Figure 1 and 2), Ni and Ti are melted in graphite crucible and stirred by electromagnetic field. Simply, there are 2 series of processing routes [3] to wrought products.
Figure 1. sketch of VIM.
Figure 2. Basic elements of a vacuum induction.
Processing routes 1:
primary melting→subsequent treatment (extrude, forge, roll)→wrough products
Processing routes 2:
primary melting→secondary melting (atomize, VIM/VAR skull investment…)→final processing (near-net shaps, cast products, wrought products)
The purposes and functions of each step are different. Primary melting step mainly begins with refining, purifying, composition controling, virgin charge and scrap. [3] The secondary melting is important for the whole processing. It remelt for macrostructural control and microstructural control and it also need treatment including extruding, forging, hiping and rolling. For subsequent treament step, it can provide the final products, such as, billet, bar, sheet and wire, after treatment. Additionally, the final processing step aims at final products by deep treatment like laser-spray. [3] These processing route provide a good homogeneity, but the carbon in the graphic crucible may become the impurity.
Vacuum Arc Remelting (VAR)
For VAR (Figure 3), raw material are used as consumable electrode in water-cool copper crucible, as consumable electrode. A small amount of alloy to be remelted is placed at the bottom of the crucible. Feed current between raw material and crucible, electrical arc is struck between the raw material and the crucible to melt material at the bottom [2]. The metal melts continuously and the top electrode is gradually moved downwards by a mechanical ram in such a way that the arc is maintained steadily between the electrodes. The crucible is cooled with water jackets and solidification of the material is closely controlled. The quality of the resulting alloy is dependent on the cooling rate, gap between the electrodes and current. This refining process need repeated several times to promote its homogeneity. Nickel, titanium and special steels are often processed using VAR. [4]
Figure 3. sketch of VAR.
Figure 4. A visual depiction of VAR process.
A more advanced process is the VIM/VAR association. Ni and Ti are first melted in the VIM furnace and then subjected to the VAR or multiple VAR process. Low carbon level of copper crucible and high accuracy of pressure and raw material control both meet the requests of making high purity nitinol [1].
From the report published by the European Commission including critical materials for Europe, and the ‘Final List of Critical Minerals’ which is significant for the security and economic of the US, we can find that titanium is a kind of critical material, but nickel is not. Titanium is widely used as metal alloy and paint.[5]
It is important for the security and economic of every county, and it plays a vital role in the manufacture of products. As a common element and complete industrial chain, the use of nickel is extensive both in commercial and military industrial manufacturing field. [6]
References
[1] Elena villa (2015), Shape Memory Alloy Engineering ,4,80.
[2] Bethel, Menlo Park, New Hartford (2017) , Introduction to Nitinol [cited 5 June 2020]. Accessed from: https://www.memry.com/.
[3] Vacuum Induction Melting, ASM Handbook, 15(15): 1-8.
[4] F. J. Zanner (1981). "Vacuum consumable arc remelting electrode gap control strategies based on drop short properties." Metallurgical and Materials Transactions B, 12: 721-728.
[5] Interior Releases 2018’s Final List of Critical Minerals. Accessed on 9 June 2020 from: https://www.usgs.gov/news/interior-releases-2018-s-final-list-35-minerals-deemed-critical-us-national-security-and
[6] Nickel--U.S. Geological Survey(2014), Mineral Commodity Summaries[cited 5 June 2020]. Accessed from: http://metalpedia.asianmetal.com/metal/nickel/nickel.shtml