How are shape memory alloys produced?
This section will help you to learn more about the production of SMA and to deepen your knowledge from the first parts of this course.
Based on some metallographic basics, you will get to know SMA with more than two alloy components and their typical application areas. Thereafter, you will gain a more in-depth understanding of martensite and austenite with regard to SMA and learn the underlying reasons for the geometric precision of your SMA. In addition you will understand the effects of heat treatments during production on the properties. Also, you will find more details on heat treatment for defining desired shapes, referred to as shape setting. Next you will learn about common and alternative processing methods of the various NiTi SMA provided by us. In the last section, you will find more details on surfaces and the use of SMA in bonded / composite materials.
Alloys and material design
In the section semi-finished products and components you have learned about binary NiTi SMA, that is to say SMA with two components. The SMA currently provided by Ingpuls almost exclusively based on the components nickel (Ni) and titanium (Ti). This is where we exhibit an exceptional world class expertise. Already small changes (e.g., 0.01 At %) in the composition ratio already will result in your SMA behaving very differently.
This applies in particular to mechanical behaviour and transformation temperatures. In the guise of this, we avail of our expertise in material design to design your functional SMA specifically for your application and your industry. In order to be able to produce according to your requirements, therefore NiTi based SMA with more than two alloying components are available to you.
Thereby we can customise your product by adding other elements. In the full course, you will find out which properties and areas of application can be obtained by which additional alloying elements. Download the full SMA course as a PDF HERE. It includes a concise summary for later reference.
Martensite und Austenite
The properties of SMA are determined by the microstructure. By now, you should already have become acquainted with more details in other sections such as Application or Characterisation.The shape memory effect results from a phase transformation between martensite and austenite which is responsible for the different behaviour of the shape memory effect. As you will remember: With martensite as their microstructure, NiTi SMA are apparently plastically deformable, the microstructure of the element converting into austenite during heating. During the heating process, the element will remember its previously programmed shape.
For example, a bent wire will straighten up or a compressed spiral spring will expand back to its original shape. The conversion taking place is geometrically unambiguous because austenite exhibits a highly symmetrical crystal structure and therefore will not permit other shapes. In the free course, you will find out the reasons behind the geometric precision and the background of the phase transformation with superelastic material. Click here to receive immediate and free access to your SMA-course.
Cold forming and heat treatment
In order to refine the micro structure of SMA, it is required that the SMA undergoes several process steps. For this you will alternately resort to cold forming and heat treatment (intermediate annealing) and in most cases to an ultimate heat treatment for setting, finally for setting the parameters: The latter is referred to as the shape setting heat treatment. In the SMA course you will get to know some parameters and their background as well as finding out why cold-formed SMA do not exhibit shape memory properties, or at least to a very limited extent.
Shape setting heat treatment
The heat treatment for shape setting is the stage during which a specific shape is programmed into an SMA component. This process is commonly referred to as “shape setting”. You have already learned the principles of shape setting by heat treatment in the Characterisation section. For this purpose, usually a shaping tool with a target shape is chosen. Your component will be fixed into the tool before the shape setting
heat treatment takes place. In the course, you will also get to know some of the types of tools and processes which you may use at this stage. Further, you will learn why overheating your component – also important during the service life of your component – will permanently impair the performance of your SMA component.
From your conventional material, you may be quite familiar with several machining technologies for removing material, such as drilling, turning or milling. Generally speaking, these methods can also be used for NiTi SMA. However, some material properties of SMA make conventional processes more challenging.
Machining creates a high, local heat input into the workpiece. Without extensive additional measures this will lead to the fact that you will create local changes in the material structure. The consequences this will have for you are also outlined in your free course Download it right here.
Alternative production and processing methods
There are also some alternative production and processing methods at hand. With SMA, no process can be described as the best fit means of handling SMA: You ought to weigh up the advantages and disadvantages of the respective processes for your specific application.
In your SMA course we have listed the most common alternative production and processing methods for SMA. In addition, you will find out more about available process routes such as powder metallurgy or the option to produce SMA within a thin-film process using so-called sputter targets.
Especially the titanium contained in NiTi SMA prove highly reactive. If you refrain from taking special measures, you will therefore find a thin oxide layer (of type TiO) forming during the production process. This is the reason when wishing to avoid this phenomenon, you should perform casting processes and heat treatments under vacuum. However, a defined oxide layer may also prove desirable since it exhibits an auto-passivating effect. This way, it protects against corrosion and contributes to NiTi SMA’s good biocompatibility. For the efficiency of the overall production, each case must therefore individually be examined in order to weigh
possible process steps; i.e. in which process an oxide layer creates more advantages and from which point it turns into a disadvantage.In some cases, the processing of surfaces may be undertaken purely for aesthetic reasons. For this, usually you will refer to established processing methods that are common for surface treatment (for example grinding, vibratory polishing, polishing, pickling or electropolishing). But besides creating your desired surface ex post, already during the manufacturing stage are you able to positively impact the desired surface by choosing the right processing steps and production parameters.
Now, in the course, you will learn the reason for choosing particular types of surfaces and find out which types of application typically go in tandem with which surface type. Further, the tools required for achieving these surfaces will be outlined. Download your PDF for your free use here and start your first step with SMA today.
Coatings and composite materials
In addition, it is possible to use NiTi-SMA as coatings, portraying several advantages. While SMA is not favourable for achieve good bonds with polymers, inversely, tools for injection moulding coated with SMA prove advantageous. What SMA is to the injection moulding process, Teflon is to a pan. You will be able to achieve longer lasting moulds and injection moulded products that are more precise, saving you big time on costs in maintenance and servicing. SMA coatings are however also perfectly suited for the compensation of thermal play or for vibration damping. In both cases, alloys which portray superelastic properties at the
operating point (operating temperature) are used. You will remember that the metal then reacts like rubber. As example, when other elements expand, SMA can contract. When other elements contract due to cooling, SMA can expand. The mechanical hysteresis of the superelastic SMA is excellent for damping vibration. In a loading and relief cycle, the mechanical energy of the vibration is converted into heat via phase transformation within the NiTi. This heat is then expelled into the environment. This leads to the fact that the service life of bearings, transmissions, wind power installations or machines can be significantly
increased. If the coating is heated by an additional system, further, the mechanical properties can be influenced. So of your SMA coating is coupled to machine parameters, SMA coatings help you achieve the perfect degree of damping for each operating condition. The possibilities are unlimited!
Recap on what you should have understood in this section:
- There are binary NiTi SMA (with two alloying components) and higher grade NiTi SMA (with more than two alloying components)
- Conversion to austenite is geometrically unambiguous; therefore the shape can be precisely determined in beforehand
- To achieve the desired shape memory behaviour, various targeted heat treatments are essential
- during manufacture/production (especially annealing)
- in the stage of shape setting (aka shape-setting heat treatment)
- For the production and processing of NiTi, conventional and alternative processes can be used
- These affect both the geometric shape and the surfaces. However they are subject to some significant restrictions (in particular cutting processes)
- SMA can be combined with one another and with other materials using suitable methods
- By using coatings in SMA, vibrations can be damped, play compensation created, and injection tools can be optimised