A Review on Present Status of Friction Stir Welding of NiTinol, a Functionally Advanced, Versatile and Widely Used Shape Memory Alloy

Abstract

Ni-Ti alloys are extensively utilized in different fields of manufacturing because of their typical pseudoelastic effect and the shape memory properties. Welding of NiTinol is always essential to manufacture diverse geometrical structures with sufficient design flexibility following the application necessities. NiTinol is susceptible to compositional variations and microstructural changes because of the welding process. As a result, the mechanical and microstructural properties along with other functional properties get deteriorated with time. Welding of NiTinol without melting is extremely substantial because of the avoidance of the volatilization of the compositional constituents. Friction stir welding, a solid-state welding method, satisfies all the vital necessities of NiTinol alloy welding. This chapter will describe the friction stir welding of NiTinol in both similar and dissimilar material combinations in detail. The effect of different welding process variables on mechanical and metallurgical properties will be described along with the description of the smart functionality of the welded structures and the corrosion resistance performance.

1. Introduction

In today’s modern engineering world, material science has progressed to develop unique and functional materials with advanced and strong structural performance and functionalities. The shape memory materials with smart functionalities fall beneath the wide variety of innovative engineering materials with superior and unique properties to identify and react to some certain stimulation by altering their chemical or physical properties. Shape memory alloys (SMA) can remember their original shape and can come back to it when thermal stimulation is applied to the deformed shape. SMA shows higher activation energy density and can recuperate to its original shape under high amounts of applied loads in comparison with other smart materials [1, 2]. Depending on the constituent components, SMAs can be characterized as NiTi-based (NiTi, NiTiZr, NiTiHf, NiTiCu), iron-based (FeNiCoTi, FeMnSi) and copper-based alloys (CuZnAl, CuAlBe, CuAlMn) [3, 4, 5, 6].

SMAs have different crystal structures of different phases and the properties of different phases vary considerably. NiTinol has two phases. A steady phase at high temperature with a cubic structure is identified as austenite phase. The product phase at low temperature with monoclinic structure is known as martensite phase. The solid-state reversible phase transformation because of the shear lattice deformation in place of diffusion of atoms is the cause of the distinctive functional properties of SMA, such as pseudoelasticity (PE) and shape memory effect (SME).

The recovery of the shape by application of thermal stimulation is acknowledged as SME. NiTi is in the twinned martensite phase at room temperature (point 1). After solicitation of load, the detwinning of NiTinol happens. The maximum and minimum stress needed to distort the SMA is known as detwinning finish stress (σf) and detwinning start stress (σs), correspondingly [1]. The detwinning phenomena prompt a recoverable distortion where the allied stress will be smaller than the plastic yield stress value of low-temperature unstable martensite phase. After the elimination of the load, the elastic recovery occurs (point 4 to point 5) by holding the detwinned martensite phase. The shape retrieval happens because of the conversion of detwinned martensite to austenite by heating and it is known as reverse conversion. The start and end of this conversion sequence are represented as point 6 and point 7 equivalents to austenite start (As) and austenite finish temperature (Af), correspondingly. At austenite finish temperature (point 7), NiTi is existing in the austenite phase. This transformation sequence is known as forward conversion. The transformation starts and finish temperature of twinned martensite from austenite is characterized by Ms. and Mf. This whole process occurred in a cycle and is known as one-way shape memory effect (OWSME) as the shape recovery was obtained by heating and detwinning mechanism of the twinned martensite by application of outside mechanical load. OWSME is extensively applied for a widespread use. On contrary to this, the SMA may also be accomplished to show cyclic mechanical properties by the utilization of cyclic heat load devoid of any necessities of outside mechanical load. In plain words, simply, the cyclic phase conversion between austenite and martensite phase only by application of thermal load is recognized as two-way shape memory effect (TWSME). Because of the lower amount of strain recovery and the requirements of training, the TWSME is less employed for different practical applications [7].

The strain reclamation characteristics determined by stress-generated martensite conversion at temperatures over the austenite finish temperature (Af) but below Md is known as the pseudoelastic effect. The SMA will recuperate to the initial shape by removing the load at temperatures over Af [8, 9]. Over Af temperature, the pseudoelastic transformation can be initiated and can be continued to develop by application of outside load which helps in the generation of the detwinned martensite phase. Preferably, after the elimination of load, the route converses and high-temperature steady austenite phase generates at no load state. Conversely, the pseudoelastic transformation cycle of SMA helps in stabilizing the stress or stress-generated martensite and lowers the amount of strain recovery. The amount of irrecoverable strain escalates after each pseudoelastic cycle because of generated lattice defects and dislocations [10, 11, 12]. By solicitation of load, the austenite state goes through elastic loading (1–2). The beginning of the conversion from the austenite phase to detwinned martensite commences and the SMA faces a high value of inelastic strains [2, 3]. σ^Ms and σ^Mf are the values of required stress for starting and finishing the forward conversion correspondingly. After the accomplishment of forward conversion, the enhancement of stress level helps in elastic piling (3–4) which is basically the elastic transformation of the detwinned martensite. After removal of the force, martensite undergoes elastic unloading [4, 5]. After reaching the start stress (σ^As) of the austenite phase, the converse conversion from martensite phase to austenite starts and the recovery of the ends at austenite finish stress (σ^Af) (5–6). The route from point 6 to point 1 is described as the elastic repossession of the parent austenite phase. The whole loading-unloading curve of an SMA forms a hysteresis loop, and the area inside the loop depicts the amount of heat dissolute during a cycle.

2. About NiTinol alloy

NiTinol is the widely utilized and most preferred SMA among design engineers because of its synergistic amalgamation of superior functional and mechanical properties, pseudoelasticity and shape memory effect. This alloy has very good density and high deformation recovery properties [13]. The thermal responsive shape change has helped the utilization of NiTinol as actuators and sensors in different fields of robotic and industrial applications. Biocompatibility is the major requisite of any materials to be used in biomedical applications. Biocompatibility means that the material should not show any harmful effects, including any inflammatory, toxic or allergic reaction) during the working period inside the human body. Any biomaterial has to face a very critical environment inside the human body. NiTinol, a functionally advanced biomaterial, has very good corrosion protection characteristics and biocompatibility than any other alloy used in biomedical fields [14, 15]. A skinny and long-lasting protective layer of titanium oxide (TiO, Ti₂O₃ and TiO₂) on the surface of NiTinol helps to ensure the good corrosion resistance property of NiTinol. A good combination of superior characteristics, such as biocompatibility, shape memory effect, pseudoelasticity and kink resistance, helps in the use of NiTinol in the biomedical industry. A few of the extensively utilized biomedical NiTinol components are cardiovascular stents, orthopedic implants (spacers, connectors, staples and plates), orthodontic braces, minimally invasive surgery devices, forceps and aortic pumps.

The stress-strain relationship of NiTinol is greatly comparable with human bones and tissues [16]. As a result of this, the biomechanical characteristics of NiTinol match with the human body. A resemblance in loading and unloading between NiTinol and human bone was observed. The recoverability of strain of steel is below 0.5% but it shows a value of 8% for NiTinol. Typically, NiTinol having pseudoelasticity and the austenite finish temperature lower than the body temperature is favored for biomedical implants.

Even the shape recovery property of NiTinol was also used for surgical devices and implants. The biomedical devices could be activated in the body by utilizing body heat or any other exterior heat source to advance the bone joining and or nominal invasive surgery, correspondingly.

The requirement of close compositional constitute variance [9] of NiTinol demands a high degree of care during the manufacturing of the alloy. The steps involved in the production of NiTinol are very similar to the steps utilized for the production of traditional metallic materials. Those include melting and different hot and cold working methods. Along with these, an additional stage of shape memory treatment is carried out. Maintenance of appropriate and uniform constitutional composition and purity must be done during the melting phase to ensure the appropriate properties of the alloy. Carbon and oxygen contamination must be barred to control the purity. Alumina and magnesia crucibles are not generally favored because of oxide contamination issues. Generally, graphite crucibles are reused multiple times as the repetition in use helps the development of NiTinol coating on the inner surface of the crucible which basically decreases the carbon exposure in subsequent stages. The vacuum induction furnace (VIM) along with vacuum arc remelting (VAR) and electromagnetic excitation is used in sequential melting and remelting stages to confirm the uniformity in the molten material during the melting phase.

Because of the low formability and machinability of NiTinol, the processing is done in between the temperature range of 700 and 950°C. If cold working is performed for wire drawing and in any other applications, the intermediary annealing process is used to counteract the work hardening.

Shape memory treatment shows a vital role in the production of NiTinol. Basically, two aims are fulfilled by shape memory treatment. They are: (i) regulation of phase conversion temperature and (ii) memorizing the preferred shape. The shape memory treatment is basically performed in the temperature range of 300°C and 500°C. The processing and manufacturing steps and conditions considerably disturb the conversion temperatures and the mechanical characteristics. The alloy can show anyone of pseudoelasticity or shape memory effect at room temperature depending on the manufacturing route and constitutional composition. Maintaining the shape memory behavior is also an acute need in the processing of NiTinol. The occurrence of any unwanted and brittle intermetallic phases (Ti₂Ni, Ti₂Ni₃, and Ti₃Ni₄) or inclusion of nitrogen, hydrogen and oxygen elements should be eliminated. The occurrence of these elements changes the superior characteristics severely and may be the reason for the embrittlement of the NiTinol devices. In that scenario, the pseudoelastic property or the actuation mechanism by stress will not be shown.

The edge of the NiTi is nearly perpendicular to the Ti-rich side, but the Ni-rich side reduces with the reduction in temperature, and at about 500°C the solubility drips to insignificant. The diffusional conversion can happen and different phases, like Ti₂Ni₃, TiNi₃ and Ti₃Ni₄ can be generated depending on the amount of constituent components, the aging time and the temperature. The degree of diffusional conversion in comparison with the rise in aging time and temperature is as follows: Ti₃Ni₄ → Ti₂Ni₃ → TiNi₃. The NiTi phase exhibits the B2 structure at room temperature. The compositional constituent’s amount must be in a specific range for this structure. B2 structure transforms to BCC at a temperature of 1090°C. This phase is retained during furnace cooling and quenching up to room temperature. The phase conversion temperature plays a vital part in the determination of the shape memory alloys’ applicability. A negligible change (even 0.1 at %) in the constitutional composition has a significant effect on phase conversion (more than 10°C) [5, 6]. The phase conversion temperature may be custom-made depending on the solicitation by varying the Ni amount or by precipitate formation. The martensite start temperature rests almost constant till 49.7 at % of Ni. Above this percentage of Ni, the Ms. temperature exhibited a reducing trend. In this region, the austenite finish temperature (Af) is more or less 30 K above Ms. temperature.

The equiatomic conformation can show the highest value of austenite finish temperature (Af) of 120°C. The springs, made of NiTinol and utilized for hot water regulators, have Ni amount of 50.5 at %. With 51% Ni content, NiTinol can show pseudoelasticity having an austenite finish temperature of 40°C [1]. The elemental constituent range for the B2 state is very slender beneath 700°C. Depending on the heat treatments, the alloy having Ni more than 50% helps the development of Ni-rich intermetallic phases such as Ti₂Ni₃ and Ti₃Ni₄. The generation of precipitate alters the phase conversion temperature. To regulate the phase change temperature, different elements such as vanadium, copper and chromium can be supplemented.

3. Inevitability and obligations for NiTinol welding

Fabrication of NiTinol complex products are challenging and costly affair because of the poor machinability and low formability of the alloy. As a result of this, welding is a vital method for the fabrication of NiTinol devices with adequate freedom in design. The welding of NiTinol is a fascinating work because good welding needs sufficient mechanical properties and preservation of superior properties (pseudoelasticity and shape memory effect) of the alloy. If the shape memory property is not retained in the welded structure, then the activation by application of either stress or temperature becomes difficult and prevents the desired application.

Microstructure and the constitutional composition were varied during welding. These variations may affect the phase conversion temperature of NiTinol and must have an unwanted effect on the thermomechanical stability of the fabricated structure [11, 12]. If there is a considerable mismatch in the phase conversion temperature of the joint and the parent material, then the control and actuation of the joined structure turn out to be very problematic. Still, a considerable variation in phase conversion temperature is fascinating and can impart a functionally graded characteristic in the fabricated structure. Welded NiTinol joints have a wide range of applications depending on the necessity of pseudoelasticity or shape memory effect. Dissimilar material combinations with steel have well-known applications in petrochemical, nuclear and aerospace applications [17]. NiTinol was fabricated to a titanium structure for a notched and adaptive nozzle used in Boeing B-777 for reducing noise levels during landing and launch [18]. While launching, the temperature at the outlet will be higher to activate the NiTinol actuator. The NiTinol activating device helps to reduce the engine noise by acting as a protrusion which basically tugs the whole structure downwards. In the cruising state at high elevation, the outlet temperature reduces because of variations in the environment and the engine condition. At this point, the base structure made of titanium will perform as the bias and helps to boost the assembly back to its original form [19, 20].

In multi-way activation of NiTinol actuators, two SMA sheets with dissimilar phase conversion temperatures need to be fabricated. Here, welding is the best method to fabricate this type of structure. The fabricated structure can be at three distinct locations depending on the temperature variation. At normal temperature i.e., at room temperature (RT), the plates should be in a straight-line situation. With the increase in temperature, the plate, with austenite temperature ranging from 40 to 50°C, activates. By additional increase in temperature, the second SMA sheet will also trigger. The whole activation of the plate generates the third position. This system is extensively used for morphing the rotor blades in aircraft. In this technology, each blade is accomplished at a particular position separately and can be fabricated to a principal hub. Depending on the fluid flow and the thermal criteria, the blades will twist consequently and the subsequent mechanical yield from the rotor can be changed.

In concrete structures reinforced with NiTinol for seismic isolation, the NiTinol bars having a diameter of 3 mm and length of 446 mm were stressed till 7% strain was reached. Then those stressed bars were twisted into circles and the boundaries were welded utilizing the TIG welding method. The same type of rings was coupled with some constant gap among them to generate the ribbed cylinder structure. After that concrete cylinders with a diameter of 15 cm and height of 30 cm were built around the NiTinol reinforcement. This type of concrete structure has improved strength properties and a higher value of failure strain than the normal concrete structures without NiTinol reinforcement [21].

Resistance spot welding was successfully used to join cylindrical NiTinol wire with rectangular one for orthodontic applications [22]. The kind of orthodontic wires, made of NiTinol and used for tooth adjustment, improve the design flexibility by fabricating a small piece of NiTinol wire with a leveling wire. The small pieces of wires would perform as hooks to twist the elastics, stops or omega loops to help the dentist to make the adjustment in teeth. Welding is basically an easy, faster and cost-effective method to fabricate patient-specific hooks, loops to wire and rings of different dimensions.

Because of the significance of welding as a method of fabrication, there are numerous published literature and articles on welding of NiTinol [23, 24, 25, 26, 27]. The possibility of solid-state welding of NiTinol was studied in a few literatures. Most of the work was concentrated on different fusion welding techniques of NiTinol, mostly laser welding of NiTinol. In view of the unexceptional benefits of the solid-state joining process for the fabrication of composition dependent alloys and dissimilar material combinations, this review work has been structured to talk about the friction stir joining of NiTinol alloy in detail.

4. Advantages of solid-state welding

Fusion joining of NiTinol was vastly reported by many researchers. Different fusion welding methods such as laser welding, tungsten inert gas welding, resistance welding and electron beam welding have been applied to join NiTinol [23, 24, 26]. The fusion welding of NiTinol undergoes different disadvantages in the generation of different intermetallic phases, favored vaporization of some elements. The solid-state fabrication of NiTinol was not studied comprehensively in comparison with different fusion welding methods.

Fusion welding of Nitinol has been comprehensively studied. Various synergistic arrangements of temperatures and pressures are used to join materials together. Here, no melting of materials takes place. The defects related to solidification, such as cracks and porosities, can be prevented significantly as melting of the material is not occurring. The atomic level bonding at the contact surface of the two materials is formed by heavy plastic deformation and high temperature during the solid-state welding method. Generally, filler materials are not used. In many cases, the interlayers of the lesser melting point material are utilized to help in the welding process. As the molten state of the material is prevented by solid-state welding, the generation of different brittle intermetallic compounds, which basically deteriorates the mechanical characteristics of the joint, can be avoided. Solid-state welding produces a joint with good dimensional control. The generated residual stress is less than any fusion welding method. The conservation of shape memory effect in the welded samples is far better for solid-state welding than soldering, brazing and different fusion welding methods. There are different categories of solid-state welding depending on the principle of plastic deformation and heat generation. They are impact-based processes, diffusion-based processes and friction-based processes. Here we will discuss about the friction stir welding of NiTinol.

Now a day, friction stir welding (FSW) is deliberated as a powerful method of joining materials in solid-state after its invention in the year 1991 by the Welding Institute (TWI) [28, 29, 30] for resolving the issues related to welding of aerospace grade aluminum alloys which were considered as non-weldable at that time because of the development of porosity in the weld-bead, reduced mechanical properties and differences in microstructure across different zones of the welding [31, 32, 33, 34, 35, 36, 37, 38, 39]. From that point of time, FSW was moving faster as a feasible welding method for a wide variety of alloys and metals to be used for different applications starting from space shuttles to microelectromechanical systems (MEMS) [40].

5. Friction stir welding

In FSW, a non-consumable and external rotational tool is utilized to do the welding in a solid-state. The tool mainly contains a pin (probe) and a shoulder. The diameter ratio between the pin (smaller) and the shoulder (larger) basically depends on the type and the thickness of the material to be welded [41] and in a few cases on the tool material as well [42]. During the FSW process, the tool revolves at a predetermined rate and also plunges into the workpiece material till the full depth of penetration is achieved by the application of pressure from the shoulder of the tool to the top surface of the workpiece. This will generate heat and cause the material softening around the tool by severe plastic deformation. At this point, the tool starts moving forward with a pre-set value (mm/min) along the line of the joint in solid-state. During processing, the plastically deformed softened material about the tool is relocated from the advancing side (where tool rotation and direction of travel are the same) to the retreating side (where tool rotation and direction of the travel are opposite to each other). The ratio between tool rotational speed and the welding speed has a considerable effect on the formation of joint area. After completion of the welding, the tool leaves the workpiece with a keyhole which is one of the distinguishing characteristics of FSW. In the initial phase, the revolving tool starts plunging on the meeting surfaces. In the second stage, the plunging gets completed before starting of the tool traverse along the joint interface. In the third stage, welding gets completed and extraction of the tool happens.

5.1 Friction stir welding of NiTinol

In the year 2017, the feasibility of FSW of NiTinol was demonstrated by Mani Prabu et al. [43]. Consequently, in-depth experimentation on the effect of tool rotating speed on different aspects of FSW-ed NiTinol was described by them [44]. The tool made of Densimet with a normal cylindrical probe was utilized for the experimentation [45]. The marks on the welding top surface and root face undoubtedly portrayed different phases of FSW like dwelling/ plunging phase, welding with traverse movement and retraction of the tool [46]. Severe plastic deformation and high temperature helped to refine grains through dynamic recrystallization phenomena. No detrimental intermetallic phases were observed in the joint nugget. The variation of phase conversion temperature after FSW was marginal in comparison with fusion welding methods. The welding done at 800 rpm had the least effect on phase transformation temperature in comparison with all other welds made at upper tool rotational speed. Moreover, a minor deviation in phase conversion temperature was observed across the different zones of the welding like stir zone, retreating side and advancing side [44]. This change in phase conversion temperature was caused by the variances in residual stress, grain size and dislocation density. The welding done at 1000 rpm revealed 17% higher yield stress in contrast with the base samples and the tensile strength value was about 66% of the base material. The tensile characteristics of the welding carried out at 1200 rpm were reduced because of the presence of the tool fragments inside the weld. The temperature dissemination during FSW of NiTinol was simulated using Comsol Multiphysics software on the finite element analysis platform. The boundary on the probe has lodged the extreme temperature on the advancing side. The forward velocity component and the tangential component of the same acts in the same way on the advancing side. As a result of this, the advancing side has displayed comparatively more temperature in comparison with the retreating side. The temperature was augmented linearly with the escalation in tool rotating speed because of the increase of heat produced by friction and excitation of the weld zone.

Deng et al. performed the welding of Nitinol with the help of the W–Re tool having a cylindrical probe. The overall cross-section illustrating diverse zones of the friction stir welded NiTinol was revealed by them. The stir zone consisted of refined and recrystallized grain. However, the HAZ and TMAZ consisted of refined and elongated grains because of the difference in plastic deformation and temperature across different areas. The overall tensile property of the joint after welding was reduced by the creation of different defects like kissing bond, lamellar structure, tunnel defects, tool inclusions (W₁₃Re₇), intermetallic compounds (Ti₂Ni) and tunnel defects. The post-weld pickling (PWP) treatments were done to eliminate the brittleness of the joint. The PWP carried out at 600 rpm has significantly improved the tensile property and the ultimate tensile strength value reached is 751 MPa, which is 79.1% of that of the parent material.

Abdollah Bahador et al. used a tungsten carbide tool with a plain cylindrical pin for welding of NiTinol [47]. The weld texture was examined with the help of EBSD analysis. It was found that refined and equiaxed grains form the entire welded region. The grain size was decreased from 49 μm of base material to 6.6 μm of the welded samples. Because of the inborn asymmetry of temperature and strain rate at diverse zones of FSW, the diverse areas of the joint had dissimilar and inhomogeneous microstructure initiated by diverse degrees of recovery and recrystallization through the weld. Likewise, the variation in tool rotating speeds helps in the creation of different textures. The welded specimen shows an improved yield strength value than the parent samples because of texture formation, spreading of tool wear elements in the joint and grain refinement. The welding done at 350 rpm showed the maximum yield stress and maximum fracture stress of value 765 MPa and 870 MPa consequently.

5.2 Corrosion resistance performance of friction stir welded NiTinol samples

The corrosion protection performance of friction stir welded samples was measured in 3.5% NaCl solution by potentiodynamic polarization (PDP) test, an electrochemical method of testing corrosion [45]. The overall breakdown potential was decreased to a lower value after welding. However, the current density increased after welding which considerably improved the deterioration of the corrosion resistance property. The amount of corrosion of the parent material and the welding at 1000 rpm were 4.97 × 10⁴ mm/year and 1.62 × 10³ mm/year consequently. It is well known that the parent material had a homogeneous grain structure but the FSW-ed samples had certain gradients in the grain distribution because of strain rate and temperature variation across diverse zones of the weld. Generally, the pin-affected area has the finest grains than other adjacent areas. Because of this, the generation of homogeneous and inactive oxide layer was prevented and caused in lowering the corrosion resistance performance of the welded samples. Even the variation in residual stress across different zones along with the inclusions of wear particles from the tool were the reasons behind the degradation of corrosion protection performance value. Parker et al. [48] also reported that the overall corrosion protection performance of the friction stir welded samples was depreciated in contrast with the parent material.

5.3 Functional properties of FSW-ed NiTinol samples

The damping characteristics of the FSW-ed samples were studied by using a dynamic mechanical analysis (DMA) study. Though the damping capability of the welded samples are bit lower than the parent samples, all welded samples exhibited good damping capabilities in the verified frequency value of (1, 10 and 20 Hz.) [49]. Even the cyclic load-deformation behavior for FSW-ed NiTinol was tested using a tensile cycle loading test. The maximum value of tensile stress was 320 MPa, 495 MPa and 590 MPa at strain levels of 2.5%, 3.5% and 4.5% [45].

Likewise, Abdollah Bahador et al. examined the cyclic loading characteristics under tensile load for FSW-ed NiTinol samples [47]. They reported deterioration of the cyclic behavior because of the occurrence of the twisted and increased number of dislocations, tool fragment particles in the weld material, high Schmid factor and high texturing. Even they reported the breakage of a few samples after a few number of tensile cycles.

The actuation characteristics of the FSW-ed NiTinol samples under diverse actuating methods such as electrical heating, laser heating and hot plate heating were studied by Mani Prabu et al. [44]. The recovery of the bending mechanism was utilized for the hot plate method. The specimen was set to a predefined U-shape putting it in an ice immersion and then placed it in a hot plate at a temperature of 65°C. After putting the sample on the hot plate, it completely recovered the generated stress and went back to the parent shape. The specimen has regained the parent shape by recovering the induced strain within 27 seconds.

The cyclic load-deformation behavior under electrical actuation was also studied by Mani Prabhu et al. [45]. The welded sample was kept in a cantilever arrangement with some predefined load on the free side and then the specimen was stimulated for up to 300 cycles devoiding of any noteworthy deterioration of the displacement behavior. This electrical activation is economic and gives improved regulation of the actuation of SMA. Still, there is some possibility of destruction of the electrical contact during processing because of the halted actuation by contact mode.

Even the laser actuation method was also studied for achieving better displacements. The laser activation techniques offered noncontact heating mode and as a result, no connectors and wiring were required. Improved actuator movement was achieved by laser actuation rather than electrical actuation. The FSW-ed strips were actuated in a cantilever arrangement with the help of the laser heating method [49]. Laser actuation offers better actuation capability and a higher value of displacement than electrical heating because of the produced high temperature and impulsive nature of heating.

5.4 Dissimilar FSW of NiTinol

Deng et al. made the welding between NiTinol sheets and Ti6Al4V sheets with the help of the FSW technique [50]. They have used W-Re tool having a cylindrical probe with a tapered cross-section. In the initial stage, they have found the crack generation and tunnel defect formation in the nugget area because of the lower fluidity of the material and internal residual generation. To avoid this defect formation, they have preheated the back plate along with the base material up to 200°C. The preheating helped to form good joining by increasing the fluidity of the material together with the reduction in cooling rate and temperature gradient. The macrostructural cross-section of the welding at a rotating speed of 475 rpm and traverse speed of 23.5 mm/min, 30 mm/min and 60 mm/min consequently were analyzed. It was observed that onion rings were formed by material interweaving technique at 30 mm/min welding speed. The micrograph images showed the occurrence of lamellar construction in all the welded samples. The tunnel formation and kissing bond defect formation were reported at the interface of the weld at traverse speeds of 60 mm/min and 30 mm/min because of a reduction in metallic fluidity. While welding, the NiTinol moved towards the Ti6Al4V plate and formed the landmasses of Ti₂Ni in the matrix of Ti6Al4V. The joint got at a welding speed of 23.5 mm/min and showed an extreme value of tensile strength of 269 MPa.

Parker et al. made a joint between stainless steel and NiTinol with the help of W-Re tool having a tapered cylindrical probe [51]. The welding cross-sectional microstructure has shown a good mingling of the material in the stir zone. Along with this, various welding zones were noticeably observable. A broader TMAZ and HAZ were observed in the steel part. Contrarily, a narrower TMAZ with no HAZ was observed on NiTinol side. Because of grain size modification, the grains in stir zone get reduced and enhancement of microhardness was observed at the stir zone. The FSW-ed samples revealed the extreme tensile strength value of 705 MPa at normal temperature and 438 MPa at raised up temperature of 121°C. The welded samples showed a considerably low value of impedance in the electrochemical corrosion test during the corrosion behavior determination because of the non-uniformity of the oxide layer. The Tafel plot graphs for FSW-ed samples were studied. The corrosion current density and corrosion potential of the weld were moderately higher and lower in comparison with the base material consequently.

The composites use bulk NiTi ribbon for strengthening purposes as in the Al matrix was made by FSW techniques. The hybrid method as well as the conventional method supported by Joule heating was utilized in the fabrication of the composites. The high value of temperature because of the hybrid method of heating helped in enhanced material flow and caused high interfacial and tensile strength properties [52].

6. Summary of the study

In FSW, frictional force, generated by tool rotation is used for making the joint. Usually, this force produces heavy plastic deformation and heat at the interface of the welding. The dynamic loading is being facilitated in the FSW process and as a consequence of this, a high amount of strain was generated during welding. The properties and microstructure of the hot deformation process have similarities with the FSW process.

There are not many studies on the FSW of NiTinol. More researches are needed for a better understanding of the FSW of NiTinol. Works related to the effect of different tool profiles, tool material, welding speed, axial force, tool rotational speed and tool tilt angles should be tried to solve the issues related to the welding of NiTinol. This review work, shows that good quality joints of NiTinol can be made using FSW. The generated heat and cooling rate primarily control the microstructure in welding and generates grain size difference across different region of welding. Because of the grain size variation and inhomogeneity in microstructure across different regions in welding, the mechanical properties of welding structure get deteriorated. In FSW of NiTinol, a fine-grained microstructure across different areas of welding was obtained. As a result, the overall mechanical properties and the toughness of the joint were enhanced.

FSW is basically a pollution-less process as no toxic metal vapor and gas are generated. Porosity and hydrogen embrittlement were not formed during FSW. The heat-affected zone is very low and the overall mechanical and metallurgical properties are far better than any type of fusion welding process. The primary aim of this review work was to detect the advantages and disadvantages of FSW while joining the NiTinol. In comparison with any fusion welding method, FSW has many advantages like lesser heat input, improved mechanical and metallurgical properties, higher corrosion resistance property, lesser heat affected zone and no requirement of any special protective environment and filler material. FSW needed considerably less amount of power than any fusion welding method. The microhardness of FSW-ed NiTinol sample was very good. The microstructural, mechanical and metallurgical properties along with phase conversion temperature have a significant influence on different process parameters because of induced stresses and chemical composition variation during welding. The induced changes in FSW are minimal in comparison with any fusion welding method. As a result, FSW can be utilized for different applications where the phase conversion temperature of the welding should lie close to the parent sample. FSW helps to obtain enhanced weld strength because of substantial recrystallization phenomena achieved through severe plastic deformation.

This review on FSW of NiTinol shows that more work in similar and dissimilar material combinations is needed using traditional and hybrid FSW processes to acquire more knowledge in the field so that FSW could be a practical fabrication route for NiTinol components in aerospace, automotive, biomedical, hydrospace and civil structural application. Such research work would reduce the joining costs and help in the commercialization of the process for NiTinol fabrication.

The above work shows that most of the research work deals with the strength enhancement of the joint and metallurgical and mechanical aspects. In most of the research work the tool is made of expensive material. Such tools enhanced the overall cost of machining. It was observed that the generated microstructure in the biomaterial region was acceptable but the strength of the joint is not sufficient for industrial use for dissimilar NiTinol joints. At present different secondary heat sources were used for FSW of high-strength alloys to enhance the material flow around the tool and to get good quality joints at low heat input. So far, such studies with secondary heating have not reported for FSW of NiTinol. The research related to hybrid FSW of NiTinol with a secondary heat source needs more attention in the near future to commercialize the process for industrial adaptation of FSW of NiTinol at comparatively lower heat input.

7. Future scope of study

As FSW has exceptional proficiencies in joining NiTinol, a wide scope of future study is prominent. A few major research fields that should be explored in forthcoming years are listed here.

1. The FSW of NiTinol with Cu and Fe-based shape memory alloys should be studied as a dissimilar material combination with functional application because of the considerable difference in their phase conversion temperatures.

2. Simulation and modeling study of FSW is required for a better understanding of the effect of process parameters on weld qualities.

3. The consequence of FSW on fatigue property and the cyclic load-deformation behavior of NiTinol should be studied to know about the service life of the fabricated component.

4. Though a few corrosion studies of FSW-ed NiTinol in 3.5% NaCl solution was performed by previous researchers, the corrosion study in different physiological solution has not been explored yet.

5. The limited tool life because of the very high wear rate and the occurrence of tool rubbles in the weld zone is a serious problem that needs more research and attention to enhance tool life by reducing toll wear and preventing the tool debris particles to include in the welded region.

6. Hybrid FSW of the tool with some secondary heating source should be studied for industrial adaptation of FSW of NiTinol. These hybrid FSW processes reduce the requirement of overall heat input and enhance the material flow around the tool pin and helps in the formation of good-quality joint.

Acknowledgments

This work was supported by SERB, India. File Number: PDF/2021/001330.