1 Introduction

Functional materials, especially shape memory alloys (SMA), such as Nitinol, which is a binary intermetallic alloy with extraordinary mechanical and functional properties [17, 39], are gaining ever greater attention in scientific and engineering communities. Nitinol is a near-equiatomic alloy produced out of nickel and titanium. Due to its biocompatibility and certain mechanical properties, Nitinol is used in medical engineering for producing bone implants, stents, medical tools and other devices [18, 23, 27, 37]. It is also used in a wide range of engineering fields from aerospace to civil engineering [15] and energy engineering [1, 12, 31, 40]. Naturally, for different applications different Nitinol properties are of high importance. For example, for medical applications shape memory effect (SME) of Nitinol is exploited, which means that at the application temperature Nitinol is in a fully martensitic phase [15] and when it is mechanically deformed it returns to its initial shape once it reaches its phase transformation temperature (PTT). On the other hand, for energy engineering, namely elastocaloric cooling, superelasticity of Nitinol is utilised, that is the material is in a fully austenitic phase at the application temperature [15], so that when the mechanical load is removed it instantly comes back to the initial state and releases a certain amount of energy related to its phase transformation [40].

To increase the geometrical complexity of the Nitinol devices used in various applications, attempts to produced Nitinol goods employing additive manufacturing (AM) techniques have been carried out [27, 36, 38, 42]. However, AM processes prove not only to affect the PTT range [2, 9, 38, 42] of produced Nitinol parts, but as it is shown in Fig. 1 and is reported by Stepputat et al. [38], to have a tremendous effect on the enthalpy of the final parts. This is indeed a significant drawback if additively manufactured parts are to be considered for energy engineering applications, such as elastocaloric cooling. It must be mentioned that the data presented in Fig. 1 were obtained using different differential scanning calorimetry (DSC) apparatuses using different measuring principles and following different sample preparation protocols as well as on samples having initially different PTTs. Nevertheless, since a similar trend was also reported by Stepputat et al. [38], where DSC results were obtained on conventionally prepared and additively manufactured samples intended to have the same PTT, and the same DSC apparatus and sample preparation protocol were used, the data presented in Fig. 1 could be relied on for obtaining a general understanding about the implications of AM processes on Nitinol properties.

Fig. 1
figure 1

DSC results obtained on cold drawn Nitinol wire and on additively manufactured porous Nitinol cubes [25, 27]

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A further big disadvantage of additive manufacturing is the poor surface quality of manufactured parts [6]. It has been concluded in a number of studies that the functionality, mechanical stability as well as likelihood to develop a bone-implant interface of elastocaloric regenerators, medical instruments and implants is influenced the most by the surface quality of these parts [3, 7, 18, 19, 28]. Thus, a post-processing step is inevitable in order to improve the surface integrity of additively manufactured parts.

Naturally, due to the geometrical complexity of AM parts many of conventional surface treatment techniques have a very low or no efficacy at all. For example, mechanical polishing like grinding or particle blasting could not be effectively used on porous structures such as reported by Navickaitė et al. [27]. Therefore, a type of (electro)chemical treatment like etching and/or electrochemical polishing is used for improving the surface integrity of complex parts [13, 16, 34, 35]. Unfortunately, these methods are known to use hazardous and/or toxic acids [5, 18, 34].

However, recent developments in electrochemical processes have led to the establishment of an environmentally friendlier surface treatment technology. Namely, a plasma electrolytic polishing (PEP). In contrast to the conventional electrochemical polishing (EP) process, PEP uses material-specific low concentration water-based non-hazardous salt or acid, e.g. ammonium sulphate, citric acid, acetic acid, solutions as electrolytes [5, 24, 30, 41]. However, to achieve a stable PEP process, certain conditions must be meet. Firstly, the temperature of the electrolyte must be increased compared to the EP process [5]. Further, a higher applied voltage is needed, so that the current density of the PEP process increased by the order of a magnitude compared to the EP. A detail comparison between the conditions required for a stable PEP and EP processes is given by Navickaitė et al. [25]. However, it must be emphasised here, that the PEP process would require significantly less time to achieve same results compared to the EP process. Of course, duration of an individual PEP process depends on the initial surface quality and the desired end result.

The working mechanism and schematics of the PEP process were exhaustively presented and discussed in a number of scientific articles [4, 11, 30, 32, 43]. Nevertheless, it must be admitted that the working mechanism of PEP is still an object of heated debates in the scientific community. However, the authors of this study adopt the theory of PEP being a predominantly electrochemical process during which a more energy intensive anodic water electrolysis reaction is replaced by an anodic metal dissolution that provides the surface smoothening [4, 29].

As mentioned above and also provided in several scientific references [4, 22], a temperature of an electrolyte plays an important role in the stability of the PEP process. The thermal energy, among other sources, is generated by chemical reactions taking place on electrodes and due to the Joule heating [10]. The latter is of particular interest because it is caused by the electrical resistance of the electrolyte [10], and thus it is believed that a workpiece, i.e. a part that is polished, cannot be exposed to temperatures higher than the boiling temperature of the electrolyte, which is similar to that of boiling temperature of water. However, Kusmanov et al. [22] reported that if the electrolyte temperature is not exceeding te = 50 °C, the temperature of a work piece made out of carbon steel exceeds tw ≥ 400 °C. Furthermore, an experimental study carried out on additively manufactured stainless steel concluded that due to Ohmic resistance samples reached the recrystallization temperature of up to 1050 °C during the PEP process [14]. In addition, due to the cooling effect of the plasma-vapour envelope, the outer layers of the workpiece are rapidly annealed [14]. These findings clearly demonstrate the clear need to completely comprehend the mechanism behind the PEP process and how it affects not only conventional but also functional materials like Nitinol, especially, owing to its high sensibility to temperature.

Taking into consideration that the conventional procedure of Nitinol shape setting in a furnace takes place at around from 400 to 520 °C for 10 min followed by a rapid quenching [8], one could assume that a PEP process carried out at a relatively low electrolyte temperature could enable simultaneous shape setting and polishing of Nitinol. This hypothesis is strengthen even more by the recent experimental results obtained on attempts to achieve shape setting of superelastic Nitinol wires and tubes by means of the resistance heating [8]. There satisfactory results of shape setting to Nitinol tubes and wires were achieved in no more than 15 s, which even proved to be superior to the results obtained using the conventional methods [8].

Therefore, the applicability of the plasma electrolytic polishing to be used for simultaneous shape setting and polishing of Nitinol is investigated in this study. A successful application of PEP for shape setting and polishing of Nitinol goods at the same time would allow to make the manufacturing chain of Nitinol products more resource- and time-efficient. However, a detail analysis of the process applicability as well as its effect on the functional and mechanical properties of Nitinol is needed. Thus, to gain a basic understanding of the correlation between the phase change temperature of Nitinol and the temperature of the PEP process, samples cut out of Nitinol wire with three different PTTs Af = 4.5 °C, Af = 31 °C and Af = 61 °C were plasma electrolytic polished at the electrolyte temperatures of te = 50 °C, te = 65 °C and te = 80 °C for τPEP = 60 s, τPEP = 120 s and τPEP = 300 s. Note that the Nitinol wire was purchased with desired Af temperatures and was not additionally heat treated priori the PEP experiments. In this preliminary study, the response of the Nitinol wire to the shape setting was analysed taking into consideration the change in its surface roughness. Furthermore, the PEP process stability was taken into consideration as well.

2 Materials and methods

2.1 Nitinol wire

In total 27 Nitinol wire samples with PTTs Af = 4.5 °C, Af = 31 °C and Af = 61 °C were plasma electrolytic polished at the electrolyte temperatures of te = 50 °C, te = 65 °C and te = 80 °C. All samples, except those with PTT Af = 4.5 °C, had a dark oxide layer in as-received conditions. Note that no additional surface treatment for removing this existing dark oxide layer was performed. The samples with the phase transformation temperature Af = 4.5 °C had a shiny metallic appearance in as-received conditions. Figure 2 shows the segments of the characteristic wire samples in as-received conditions.

Fig. 2
figure 2

Photographs of the characteristic segments of the analysed Nitinol wire samples with the transformation temperature of a Af = 4.5 °C, b Af = 31 °C and c Af = 61 °C

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The characteristic dimensions and mass of the Nitinol wires were measured before and after the PEP process. The obtained results are summarised in Table 1. Note that wire samples Af31-6 and Af31-9 were not polished since samples polished before them, i.e. Af31-5 and Af31-8, broke during the PEP process. The exact process details are explained in Sect. 2.2. It is also noticeable that the mass difference Δm before and after the PEP process for sample with Af = 4.5 °C is significantly lower compared to the rest of the samples. The reasons for this phenomenon are explained in Sect. 2.2 as well.

Table 1 Characteristic dimensions of the Nitinol wire samples before and after the PEP
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2.2 Plasma electrolytic polishing

From Fig. 3, one can see that during the so-called bath-PEP process, a part 7 is connected as an anode and is immersed into an electrolyte 6. The electrolyte tank 9 is then connected as a cathode. The electrolyte 6 is heated to the required temperature by a heater 10. Note that the heater is turned off during the process. During the PEP process the part 7 is enclosed by a highly conductive plasma envelope 8, which is surrounded by the water vapour 5. The part is handled, i.e. moved in and out of the electrolyte by the part handling system 3. The energy for the PEP process is supplied by process energy source 1 that supplies direct current.

Fig. 3
figure 3

A principal scheme of a PEP process used in this study [26]

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It must be mentioned that the size of the part that can be treated using the bath-PEP technology depends on the available power of the energy source, since the part surface area and the required process current are directly proportional. However, this issue could be solved by applying a jet-PEP technology [21, 33, 44]. During the jet-PEP process, the electrolyte is applied on a selected surface area. This allows to lower the peak energy necessary to initiate the process as well as post-process larger parts and simultaneously achieve a selective/functional surface finish.

2.3 Experimental procedure

As mentioned above, the sample wires were polished at different electrolyte temperatures for three different times. Namely τPEP = 60 s, τPEP = 180 s and τPEP = 300 s. The process times were selected based on the previous experience obtained on post-processing additively and conventionally manufactured Nitinol samples [25, 27] as well as taking into consideration the time required for the shape setting step using conventional and heat resistance methods [8]. It must be emphasised that during the whole experimental campaign, the values of electrolyte electric conductivity and pH were maintained constant at around κ ≈ 107 mS·cm−1 and 4.5, respectively. Thus, the obtained changes of the surface quality in terms of a root mean square average of profile height, Rq, are caused only due to the polishing duration and electrolyte temperature.

The experimental procedure consisted of the following steps:

  1. 1.

    Mounting the sample wire into the form shown in Fig. 4 for shape setting;

  2. 2.

    Plasma electrolytic polishing at the respective conditions provided in Table 2;

  3. 3.

    Demounting the polished sample;

  4. 4.

    Testing the achieved shape memory effect in a water bath at tw = 70 °C and subsequently using a hot air stream at ta = 110 °C. The water and hot air temperatures were selected to be above the PTT of the tested Nitinol wires.

Fig. 4
figure 4

A stainless steel form for shape setting of Nitinol wires fitted with a test Nitinol sample

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Table 2 The PEP process conditions
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Each sample was polished individually and at once, i.e. there were no intentional interruptions in the PEP process, unless samples broke and the process was stopped abruptly. These samples are listed in Table 2 and could be recognised from shorter PEP time. The reason of samples breaking under the PEP process, especially after some PEP process time is not yet fully understood.

The samples with PTT Af = 4.5 °C could not be polished while fitted into the shape setting form. They broke immediately after being immersed into the electrolyte. Therefore they were polished in as-received shape, i.e., in a long form as shown in Fig. 2, and only the change of their surface quality was evaluated. It must be emphasised, that the PEP process starts with closing the electric circuit and then submerging workpieces into electrolyte. In other words, the direct current was applied to samples before they were submerged into the electrolyte. In order to determine whether the wire failures were caused due to purely thermal overload or a combination of the electrochemical processes coupled with the thermal loading of the samples, a test wire with the PTT Af = 4.5 °C was mounted into the shape setting form and immersed into a water bath which was then heated from t = 18.6 °C to t = 97.7 °C in τ = 30.2 min. The test wire neither broke nor showed any observable loss of its properties. i.e., it remained superelastic and returned to the initial form after demounting it from the shape setting form. This simple experiment confirmed that Nitinol that is superelastic at room temperature is highly susceptible to electrochemical part of the PEP process while undergoing the shape setting step. Navickaitė et al. [25], on the other hand, demonstrated that superelastic Nitinol plates could be successfully polished using the PEP process. Nevertheless, no shape setting was imposed on these plates during the PEP.

Rest of the samples were polished according to the experimental procedure under the conditions presented in Table 2. Note that samples Af31-6 and Af31-9 were not polished at all, because samples Af31-5 and Af31-8 broke during the PEP process causing short circuiting and abrupt stop of the respected experiments. Thus, anticipating similar behaviour from Af31-6 and Af31-9, provided their longer foreseen PEP time, it was decided not to conduct these experiments on them at all. Sample Af31-3 broke twice during the process, the first time at τ = 217 s and the second time at τ = 251 s causing a short-circuit and abrupt stop of the process. However, during the first wire breaking only audible wire breaking sound was noticed and no PEP process interruption was caused. Samples Af31-2 and Af61-6 also broke during the PEP process. Nevertheless, this did not cause any noticeable disturbance in the process which was carried out to the planned end. It is also noticeable that the polished length of the samples with Af = 4.5 °C was up to l = 100 mm, while other samples were polished up to their full length of l = 400 mm. Therefore, the previous samples have lower mass removal rate. Sample Af61-1 partially slipped out from the shape setting form during the PEP process, thus the ends of the remaining samples were firmly clamped using screws. This resulted in a reduction of the polished sample length.

3 Results and discussion

The volumetric mass removal rate, MRRV, and mass removal rate MRRM, reported in Table 2 were calculated as denoted in Eqs. 1 and 2, respectively.

$${\mathrm{MRR}}_{v}\text{=}\frac{\left(\mathrm{\pi l }\frac{{\left(\overline{\mathrm{d} }\right)}^{2}}{4}\right)}{{\uptau }_{\mathrm{PEP}}}$$
(1)
$${\mathrm{MRR}}_{M}\text{=}\frac{\Delta \mathrm{m}}{{\uptau }_{\mathrm{PEP}}}$$
(2)

where MRR is the mass removal rate and subscripts V and M denote the volumetric and mass, respectively, Δm is the mass difference before and after the PEP, l is the polished sample length, \(\overline{d }\) is the average sample diameter calculated from the initial and final values of the sample diameter, τPEP is the PEP process time.

From Fig. 5 one can see that with increasing the PEP time, the mass removal rate MRRV and MRRM decreases. This is due to the fact that the electrolyte temperature te is increasing during the PEP process, thus the mass as well as volumetric ablation is slowing down. And since the PEP setup used in this study does not permit the maintenance of a constant electrolyte temperature te, MRRV and MRRM per unit time is decreasing with the process time τPEP.

Fig. 5
figure 5

Mass removal rate MRR as a function of the polishing time τPEP

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3.1 Surface roughness

The surface roughness was measured using the confocal microscope Mahr Surf and following the ISO 21920 Standard. Before evaluating a root mean square average of profile height, Rq, a form removal, namely a 5th degree polynomial, and surface levelling filters were used. The used “cut-off” λs and λc were 2.5 μm and 8 μm, respectively.

It is interesting to observe from Fig. 6 that the surface roughness after the PEP was reduced up to 7.2 times only for samples with the PTT of Af = 4.5 °C, i.e., those samples that could not be polished in the shape setting form. However, the surface roughness for most of other investigated samples increased tremendously with the maximum increase being more than 20 times for sample Af31-8. Nevertheless, for sample Af61-2 that was polished in the shape setting form at the electrolyte temperature of te = 50 °C for τPEP = 180 s, a surface roughness reduction up to 1.3 times of the initial value was observed.

Fig. 6
figure 6

Surface roughness of the analysed samples before and after PEP. Note that the change in surface roughness plotted below the blue highlight denotes that the surface roughness of these samples increased after PEP

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To avoid tedious enumeration, Fig. 7 presents only the samples with the largest reduction/increase in surface roughness after the PEP as well as those whose surface roughness decreased while being polished in a shape setting form. Note that the previously mentioned samples were plasma electrolytic polished at different electrolyte temperatures. However, one can see that comparatively smooth surface without any visible defects has been achieved on sample Af4.5–5 after polishing it for τPEP = 180 s at te = 65 °C. On the other hand, the surface of sample Af31-8, which was polished for the same amount of time but at te = 80 °C, became rougher and some surface defects were uncovered. One can see from Fig. 7, that the surface of Af4.5-5, with the reduced surfaces roughness, is smooth and damage or crack-free as it is for the sample Af61-9 (Fig. 7j) even though the surface roughness parameter Rq increased after the PEP process. It should also be noted, that based on previous experimental observations made by the authors, the surface roughness of Nitinol samples containing an oxide layer tend to increase after a short, i.e., τ ≤ 60 s, PEP procedure. It is assumed that during this short period, only the oxide layer of Nitinol samples is removed, exposing the material surface that is rougher compared to the initial oxide layer. Therefore, it is not surprising that the surface roughness of samples Af31-1 and Af61-2 increased after the PEP process that lasted only 60 s. On the other hand, it is surprising that surface roughness was reduced for samples Af4.5-1, Af4.5-2 and Af4.5-3, that were polished for 60 s only. That clearly demonstrates that Nitinol responds to the surface treatment differently being in different phase conditions. However, the underlying mechanism of this phenomenon is not yet investigated and thus no informed assumptions of specific target-oriented PEP conditions could be made.

Fig. 7
figure 7

A micrograph of the selected samples before and after PEP. a Af4.5-5 in as-received condition, b Af4.5-5 after PEP, c Af31-1 in as-received condition, d Af31-1 after PEP, e Af31-8 in as-received condition, f Af31-8 after PEP g Af61-2 in as-received condition, h Af61-2 after PEP, i Af61-9 in as-received condition and j Af61-9 after PEP

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Korolyov et al. [20] pointed out that using the PEP technology different electrolytes for polishing Nitinol in austenitic and martensitic phases are used. In this study, on the other hand, a single electrolyte was used for post-processing Nitinol in both phases. Moreover, as reported in a number of previous studies on Nitinol workpieces polished using the same electrolyte conducted by Stepputat et al. [38] and Navickaitė et al. [25, 27] , satisfactory results on the reduction of the surface roughness have been achieved. Thus, the poorer surface quality obtained after the PEP process is most likely caused by not yet fully understood microstructural changes undergoing in the material that are caused by the simultaneous shape setting attempt. After all, it was experimentally proven in this study that the success of simultaneous shape setting and polishing of Nitinol samples depends on the combination of the PTT, electrolyte temperature and the process time. Thus, to fully comprehend the ongoing changes in the material structure, a much more complex experiments and measurements are required. It is worth mentioning, that the chemical composition of Nitinol plates before and after PEP was investigated using EDX and reported by Navickaitė et al. [25]. The study reported no changes in chemical composition of the Nitinol plates in terms of the main alloy elements or adsorbents from the electrolyte. These results also proved that the selected electrolyte does not cause a selective material ablation, i.e. none of the substances in the alloy is removed more intensely than others. However, the main disadvantage of the EDX measurements is their inaccuracy in detecting light materials such as oxygen and hydrogen, that might be affecting the Nitinol workpieces. Therefore, the generation/diffusion of oxygen and hydrogen in/into the alloy under the PEP conditions is going to be considered in follow up studies.

3.2 Shape setting

To investigate the possibility to set a desired shape and simultaneously polish the Nitinol samples, a complex shoe-lace-like form was chosen that could be achieved using a simple shape setting tool. The shape setting tool had to be electrically conductive to enable the PEP of the tested samples, meaning that it would require higher applied current I. Thus, the shape setting tool had to have the smallest possible surface area. Therefore, it was constructed out of stainless-steel wire as shown in Fig. 4. Another important criterion for choosing the shape setting tool and the shape itself was that by no means any plastic deformation could be inflicted on the samples resulting in a falls assumption of a successful shape setting after the PEP procedure.

However, the selected shape setting tool prevented the possibility to qualitatively evaluate the quality of the shape setting step. That is, that due to the mechanical flexibility of the tool no concrete geometrical parameters, like distance between the two loops, bending angle, etc., could be obtained. Furthermore, as shown in Fig. 8, presenting the characteristic sample wire shape obtained after the samples were demounted from the shape setting tool and subsequent training, it is obvious that the recovered shape is insufficient for qualitative measurements.

Fig. 8
figure 8

Sample Af61-5 in a a characteristic form obtained after demounting a sample from the shape setting from shown in Fig. 4 and b the final sample shape obtained after the training procedure

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All the samples, that did not break during the PEP process, formed a loop-like shape with regions of straight wires after demounting them from the shape setting tool. All samples one by one were immersed in hot water bath at tw = 70 °C. All samples with the PTT at Af = 31 °C assumed their long shape, i.e. returned into pre-polishing shape, demonstrating the failure of the shape setting attempt. All samples with the PTT at Af = 61 °C assumed Ω-like form. For the second testing step, samples with the PTT at Af = 61 °C were stretched out to the pre-polishing shape. Then all samples, with Af = 31 °C and Af = 61 °C, were exposed to a stream of hot air at ta = 110 °C. The samples with Af = 31 °C remained in their pre-polishing shape, while the samples with Af = 61 °C formed a bit wider Ω-like form as shown in Fig. 8b. After repeating the last step of the shape memory testing for several more times, no changes in final form were observed.

It must be emphasised that with the exception of sample Af61-1, all samples with Af = 61 °C demonstrated the same shape memory behaviour and produced the same final shape regardless of the polishing duration and the electrolyte temperature. Sample Af61-1, on the other hand, produced the final Ω-like form with somewhat larger radius compared to the rest of the investigated samples. Nevertheless, the shape memory effect exhibited by all samples stabilized after two training tests.

4 Conclusions

In this study, a preliminary experimental campaign investigating the possibility to perform shape setting and simultaneous polishing of Nitinol samples is presented. The obtained results are somehow intriguing since there is an obvious link between the material response to the PEP process coupled to the shape setting procedure. And this link depends on the PEP process conditions and material PTT. To be able to fully understand and characterise the phenomenon observed in this preliminary study, a much deeper and more thorough analysis of the changes happening in the material needs to be conducted including an investigation of mechanical and functional macroscale properties. It is also immensely important to fully understand whether any microstructural changes that might be happening while the Nitinol samples are exposed to the simultaneous polishing and the shape setting step are taking place due to the physical restriction for the samples to move, or this is a general effect of the PEP process. Proving the former being the case, it would call for more comprehensive studies on how PEP affects mechanical and functional properties of Nitinol and other functional materials. Nevertheless, conclusions drawn from the current experimental results could be summarised as follows:

  • All samples with Af = 61 °C, with the exception of one sample Af61-6, which broke during the PEP process, could be successfully polished in the combination to the shape setting step. Nevertheless, it also could transform to the Ω-like shape. Samples with lower PTT that were investigated in this study could not be treated following the full experimental protocol.

  • The highest surface roughness reduction was observed for samples polished without the shape setting step. These samples were post-process for at least τPEP = 180 s at the electrolyte temperature te ≥ 50 °C.

  • Only sample Af61-2, that was polished for τPEP ≥ 60 s, showed reduced surface roughness after being exposed to PEP and shape setting step.

Finally, a more thorough investigation is needed to fully understand the relation among the material PTT, the temperature of the PEP process and the process time in order to find out whether simultaneous shape setting and polishing of Nitinol parts could be applicable in any engineering applications. However, it is safe to say, that every application where Nitinol with a PTT at or below Af = 31 °C is considered, could be eliminated from that list.