Nitinol shape memory alloy is a novel functional material,which has entered the development stage of use or practicality in industrial developed countries. Due to the progress of science and technology and the increasing expansion of new uses,the demand of the market has also increased. Not only requires an increase in varieties and yields,but also expects to improve materials,increase productivity and reduce costs. Nitinol shape memory alloy is an intermetallic compound with a binary base.Its properties, especially its shape memory effect, are closely related to the melting method and processing technology. This article briefly introduces the smelting, processing and other processes of several typical nitinol shape memory alloys.
The premise of this smelting method is to first prepare a small amount of ni-ti mother alloy with known ingredients , place it in a graphite crucible in a vacuum induction furnace, energize and melt the ni-ti alloy bath, and then slowly add the ni and ti fragments according to the design composition ratio to the bath smelting under a protective atmosphere. the biggest advantage of this process is that the smelting energy consumption is less,the cost is low, and the alloy composition is more uniform. however , the problem is that smelting is carried out in a graphite crucible.graphite has considerable solubility with ni or ti at high temperatures. therefore , the contamination of impurity carbon is inevitably introduced. its controllable amount of carbon is about 0.2%. studies have shown that the tic formed by carbon in the alloy will hinder the movement of the twin boundary and the reorientation of martensite, which is unfavorable to the shape memory effect.
This smelting method is first refined from a vacuum self-consuming furnace into a basic alloyed ni-ti mother alloy , and then homogenized and remelted through a vacuum induction furnace. this process can effectively control the carbon content as long as the smelting temperature and smelting time are properly controlled.generally , the carbon content is not more than 0.1%. in the smelting process, we also found that with the increase of furnace age , even if there is contact between the liquid steel and the crucible for a long time , it will not cause a significant increase in the carbon content. the reason may be that the inner wall of the crucible forms a smooth and dense shell layer , which effectively blocks the diffusion of carbon atoms between the ni-ti liquid phase and the solid phase interface during smelting.
This is a new process of nickel-titanium alloy smelting that has been developing in recent years. the vacuum self-consuming furnace smelting method uses arc discharge between the poles to release heat energy to melt the self-consuming electrode to form alloy droplets into a water-cooled copper crucible to form ingots. the vacuum condensed shell furnace smelting uses the positive polarity connection method to cause high-speed electron flow to hit the bath to release heat energy , to ensure that the bath high temperature and electromagnetic stirring effect achieve full alloying and homogenization of nitinol alloys.
The biggest advantage of this process is that the smelting is not in the carbon crucible,so there is no introduction of impurity carbon , which significantly improves the processability of nitinol alloy , improves the material , and expands the furnace capacity of nitinol alloy. the ingot was expanded from dozens of kilograms to 140 kilograms, forming a batch and improving the yield, paving the way for the industrial production of nitinol alloys.