Elsevier

Progress in Materials Science

Volume 83, October 2016, Pages 630-663
Progress in Materials Science

Fabrication of NiTi through additive manufacturing: A review

https://doi.org/10.1016/j.pmatsci.2016.08.001Get rights and content

Abstract

Nickel-titanium (NiTi) is an attractive alloy due to its unique functional properties (i.e., shape memory effect and superelasticity behaviors), low stiffness, biocompatibility, damping characteristics, and corrosion behavior. It is however a hard task to fabricate NiTi parts because of the high reactivity and high ductility of the alloy which results in difficulties in the processing and machining. These challenges altogether have limited the starting form of NiTi devices to simple geometries including rod, wire, bar, tube, sheet, and strip. In recent years, additive manufacturing (AM) techniques have been implemented for the direct production of complex NiTi such as lattice-based and hollow structures with the potential use in aerospace and medical applications. It worth noting that due to the relatively higher cost, AM is considered a supplement technique for the existing. This paper provides a comprehensive review of the publications related to the AM techniques of NiTi while highlighting current challenges and methods of solving them. To this end, the properties of conventionally fabricated NiTi are compared with those of AM fabricated alloys. The critical steps toward a successful manufacturing such as powder preparation, optimum laser parameters, and fabrication chamber conditions are explained. The microstructural characteristics and structural defects, the influencing factors on the transformation temperatures, and functional properties of NiTi are highlighted to provide and overview of the influencing factors and possible controlling methods. The mechanical properties such as hardness and wear resistance, compressive behaviors, fatigue characteristics, damping and shock absorption properties are also reported. A case study in the form of using AM as a promising technique to fabricate engineered porous NiTi for the purpose of creating a building block for medical applications is introduced. The paper concludes with a section that summarizes the main findings from the literature and outlines the trend for future research in the AM processing of NiTi.

Introduction

In recent years, shape memory alloys (SMAs) have entered a wide range of engineering applications in fields such as aerospace and medical devices. Nickel-titanium (NiTi) is the most commonly used SMA due to its excellent functional features including shape memory (SM) and superelasticity (SE) effect. SE and SM behaviors consist of restoring large strains up to 8% by unloading and heating, respectively. These properties are based on a solid-solid phase transformation between martensite and austenite. Besides these two characteristics, low stiffness, biocompatibility, damping characteristics, and corrosion behavior of NiTi make this alloy an attractive candidate for biomedical applications (e.g., bone plates, bone screws, and stents) [1], [2], [3], [4], [5].

Unlike many of the conventional materials, there is no single fabrication recipe for realizing NiTi devices. Over the years, several common processing steps for manufacturing shape memory and superelastic NiTi devices are developed including casting and powder metallurgy processes. Fig. 1 summarizes various manufacturing methods for producing NiTi devices [1], [6].

Casting technique is one of the common conventional methods for producing NiTi. This technique is associated with high temperature melting procedures that result in an increase in the impurity level (e.g., carbon, oxygen), and, therefore, the formation of Ti-rich phases including TiC and Ti4Ni2OX. The functional properties of NiTi, are degraded as the result of these secondary phases formation. Since machining is used to form the final shapes from this alloys, another challenge associated with this technique is the difficulty of machining procedures for these super-ductile alloys which results in excessive tool wear [1].

Powder metallurgy (PM) is another conventional technique that is used for producing near-net-shape devices. Powder preparation is a required step prior to PM processing. One of the major disadvantages of this method is the high impurity pick-up that is resulted from the large surface area of the powder particles. Additionally, these techniques are limited in the complexity of the resulting parts and in controlling the size and shape of porosity, when desired [6].

In the past decade, Additive Manufacturing (AM) has gained significant attention for processing NiTi because they have circumvented many of the challenges associated with the conventional methods. These processes rely on the CAD data and entails adding material in consecutive layers made of powders and melted, in most cases by a laser [4]. The AM techniques for NiTi are either powder-bed based technologies such as Selective Laser Melting (SLM), or flow-based methods such as Laser Engineered Net Shaping (LENS). The powder-bed based techniques deal with deposition of the powder through a roller, blade, or knife, while the flow-based technologies deposit the powder through one or more nozzles that directly feed the powder into the laser focus. Powder-bed based technologies are more common for creating complex parts [1], [6], [7].

The first step in AM processing is preparing the NiTi powder. The ratio of Ni and Ti elements are important factors to guarantee the desired functional properties (i.e., shape memory or superelasticity) of the final part. Parts that are fabricated from a Ti-rich powder exhibit the shape memory effect [8], [9]. On the other hand, those fabricated from a Ni-rich powder demonstrate superelasticity behavior after subsequent solution annealing and aging [8], [9]. In addition to the Ni/Ti ratio, the procedure of preparing powder plays an important role in the features of the final products. The powders can be created either by pre-alloying or by elementally blending the Ni and Ti particles. It is notable that processing from elementally blended powders results in other intermetallic phases, pure nickel, and pure titanium in the fabricated parts [10]. Finally, the procedure of powder preparation (e.g., hydriding, mechanical attrition, water atomization, and gas atomization) is critical because it affects the final particle size, particle distribution, and the impurity of the resultant powders. EIGA (Electrode Inert Gas Atomization) procedure is shown to be more favorable due to producing more accurate particles as well as the acceptable level of impurity contents [11], [12].

The second important requirement for the AM processing is the processing parameters. Optimal parameters are methodically developed to make sure that the final product is not only fully dense, but also shows a low level of impurity contents. These two requirements are important as they affect the mechanical performance (e.g., high strength) and the success of the potential applications (e.g., medical applications) [13].

The third important requirement is to provide an inert atmosphere (e.g., argon) throughout the processing to minimize the oxidation and impurity pick-up (e.g., oxygen and carbon), increase the surface quality, enhance the density and, achieve similar functional behavior to the conventionally processed NiTi [14], [15], [16]. The level of impurity content is suggested to be less than 500 ppm in the produced parts based on ASTM F2063-05 [8]. In addition to providing inert atmosphere, it is suggested to preheat the build platform (i.e., substrate) prior to the fabrication in order to reduce the warping effect (i.e., separation of the sample from the substrate) (more details in Section 3.3) [17], [18], [19], [20].

One of the main potential areas of application of AM NiTi is in stiffness-tailored patient-specific implants. This method of processing allows for introducing interconnected porosity to match the stiffness of the host tissues as well as tailoring the shape to match the anatomical shape of the host tissues. The fabrication process is initiated based on the CT scan data of the patient [6]. It has been successfully demonstrated that AM NiTi has very low impurity contents for medical devices that satisfies the ASTM F2063-05 standard for medical devices [8], [15], [16]. Initial biocompatibility studies of AM NiTi have also shown promising results [3], [21], [22].

This paper is the second part of a recent review paper [6] on the methods of manufacturing of NiTi devices. To this end, the current paper highlights the main technologies and approaches that have been used to additively fabricate these alloys. In this context, the main advantages, as well as challenges of the additive manufacturing methods are studied. This review while summarizing the main lessons learned, also identifies the critical studies that should be conducted toward achieving repeatable functionality in additively manufactured NiTi shape memory and superelastic devices.

Section snippets

Additive manufacturing techniques to produce NiTi parts

The more commonly used AM techniques for producing NiTi products are powder-bed based technologies, such as selective laser sintering (SLS), direct metal sintering (DMLS), selective laser melting (SLM), and LaserCUSING [6]. Electron beam melting (EBM) is another AM technique that has potential to be used for making NiTi parts, however, no work has been conducted to realize NiTi via this technique so far [6]. For the powder-bed based techniques, the desired CAD model (The CAD file includes the

Powder preparation

The first step in the additive manufacturing of NiTi is to prepare the powder. Based on the desired functional properties (i.e., shape memory or superelasticity) of the final product, the Ni/Ti ratio needs to be selected. An ingot with a higher level of Ti (i.e., Ti-rich) results in higher transformation temperatures and shape memory properties in the final product. On the other hand, higher Ni contents (Ni-rich) produces parts with the superelastic behavior (more details in Section 5) [9], [21]

Microstructure of AM NiTi

In laser-based AM techniques, complex thermal history (i.e., high laser power and high cooling rate) contributes to a non-equilibrium solidification process which may result in the change of microstructure [70]. For instance, the high cooling rate (103–108 KS−1) associated with the AM techniques results in the formation of finer powders compare to conventional methods for producing metallic parts, and therefore, the mechanical properties as well as the density of the final product improves in

Transformation temperatures

Four transformation temperatures define the boundaries of the solid-state phase transformation in NiTi. At these temperatures the martensite and austenite phases start and finish (i.e., martensite start Ms, martensite finish Mf, austenite start As, and austenite finish Af). These four temperatures result in a hysteresis in the transformation process [79]. In addition to these boundary temperatures, it is also common to define the austenitic (Ap) and martensitic (Mp) peak transformation

Functional properties

The functional properties of NiTi are mainly related to the phase transition and are categorized into two distinct behaviors: (I) shape memory effects (thermomechanical memory) which are based on temperature-induced transformation and (II) superelasticity behavior (mechanical memory) which are associated with stress-induced transformation [13], [98]. Degradation of shape memory and superelasticity effects can be defined as the accumulation of irreversible strain inside the NiTi part during each

Hardness and corrosion resistance

Due to the rapid cooling associated with AM techniques, the resulting parts have increased residual stress, the formation of martensitic α′ grains, grain refinement and consequently the additional hardening of the NiTi matrix. If the optimum set of parameters is used to produce a crack and pore free structure, the high residual stresses can enhance the microhardness significantly [10], [72], [86]. As reported by Shishkovsky et al. [10], the microhardness of SLM processed NiTi is 1.5–2 times

AM enables the production of porous NiTi

Additive manufacturing (AM) has recently been of great interest for producing complicated porous NiTi structures. Porosity within the scaffold can be achieved either through decreasing the energy input (i.e., decreasing laser power, increasing scanning speed, and increasing hatch distance) or via imposing engineered porosity via CAD file. Table 8 summarizes the work of different groups on fabricating porous NiTi.

According to Meier et al. [27], [28], the introducing porosity via varying the

Perspectives on AM of NiTi

NiTi is a high-demanded engineering material which has not been extensively studied by AM technologies. So far, SLS, SLM, and LENS techniques have been used to produce NiTi product, however, no group has dedicated to fabricate NiTi through EBM method due to expected complications such as poor surface finish, high geometrical deviation, and costly process. Further studies are needed to be conducted to explore the possibilities of employing AM for producing repeatable, high quality, reliable, and

Conclusions

In this paper, the current state of research on the additive manufacturing of NiTi, associated challenges, trends, and opportunities are reviewed. A summary of the conclusions are mentioned as following:

  • 1.

    Powder preparation is the first step to additively manufacture NiTi products. Different techniques are implemented for preparing powder including hydriding, mechanical attrition, water atomization, and gas atomization, among which the gas atomization is the most common method. EIGA technique, a

Acknowledgements

The authors would like to acknowledge the financial support provided for the project “Nitinol Commercialization Accelerator” by the Ohio Department of Development through Grant WP 10-010. NSF support though award 0731087 Research to Aid Person with Disability is also appreciated.

References (101)

  • P.R. Halani

    Phase transformation characteristics and mechanical characterization of nitinol synthesized by laser direct deposition

    Mater Sci Eng: A

    (2013)
  • S. Shiva

    Investigations on the influence of composition in the development of Ni–Ti shape memory alloy using laser based additive manufacturing

    Opt Laser Technol

    (2015)
  • J.J. Marattukalam

    Microstructure and corrosion behavior of laser processed NiTi alloy

    Mater Sci Eng, C

    (2015)
  • X. Xu

    Microstructure evolution in laser solid forming of Ti–50wt% Ni alloy

    J Alloy Compd

    (2009)
  • S. Bernard

    Compression fatigue behavior of laser processed porous NiTi alloy

    J Mech Behav Biomed Mater

    (2012)
  • S. Bernard

    Rotating bending fatigue response of laser processed porous NiTi alloy

    Mater Sci Eng, C

    (2011)
  • M. Bidabadi et al.

    Thermophoresis effect on volatile particle concentration in micro-organic dust flame

    Powder Technol

    (2012)
  • N. Morgan

    Medical shape memory alloy applications—the market and its products

    Mater Sci Eng, A

    (2004)
  • K. Johansen et al.

    On the effect of TiC particles on the tensile properties and on the intrinsic two way effect of NiTi shape memory alloys produced by powder metallurgy

    Mater Sci Eng, A

    (1999)
  • E.O. Olakanmi et al.

    A review on selective laser sintering/melting (SLS/SLM) of aluminium alloy powders: Processing, microstructure, and properties

    Prog Mater Sci

    (2015)
  • E. Olakanmi et al.

    Densification mechanism and microstructural evolution in selective laser sintering of Al–12Si powders

    J Mater Process Technol

    (2011)
  • S. Li

    The development of TiNi-based negative Poisson’s ratio structure using selective laser melting

    Acta Mater

    (2016)
  • J. Frenzel

    Influence of carbon on martensitic phase transformations in NiTi shape memory alloys

    Acta Mater

    (2007)
  • L. Zhang

    Manufacture by selective laser melting and mechanical behavior of a biomedical Ti–24Nb–4Zr–8Sn alloy

    Scripta Mater

    (2011)
  • Y. Hedberg

    Correlation between surface physicochemical properties and the release of iron from stainless steel AISI 304 in biological media

    Colloids Surf, B: Biointerfaces

    (2014)
  • M. Mahtabi et al.

    Fatigue of Nitinol: the state-of-the-art and ongoing challenges

    J Mech Behav Biomed Mater

    (2015)
  • J. Van Humbeeck

    Non-medical applications of shape memory alloys

    Mater Sci Eng, A

    (1999)
  • A. Bansiddhi

    Porous NiTi for bone implants: a review

    Acta Biomater

    (2008)
  • C. Greiner et al.

    High strength, low stiffness, porous NiTi with superelastic properties

    Acta Biomater

    (2005)
  • J. Markwardt

    Experimental findings on customized mandibular implants in Gottingen minipigs – a pilot study

    Int J Surg

    (2014)
  • T. Bormann

    Combining micro computed tomography and three-dimensional registration to evaluate local strains in shape memory scaffolds

    Acta Biomater

    (2014)
  • M. Elahinia

    Shape memory alloy actuators: design, fabrication, and experimental evaluation

    (2015)
  • N. Shayesteh Moghaddam

    Toward patient specific long lasting metallic implants for mandibular segmental defects

    (2015)
  • I. Shishkovsky

    Porous biocompatible implants and tissue scaffolds synthesized by selective laser sintering from Ti and NiTi

    J Mater Chem

    (2008)
  • R. Rahmanian

    Load bearing and stiffness tailored niti implants produced by additive manufacturing: a simulation study

  • C. Haberland

    On the development of high quality NiTi shape memory and pseudoelastic parts by additive manufacturing

    Smart Mater Struct

    (2014)
  • C. Haberland et al.

    On the properties of Ni-rich NiTi shape memory parts produced by selective laser melting

  • M. Stobik

    Nanoval atomizing – capabilities, applications and related processes

  • Wielage B, Wilden J, Schnick T. Manufacture of SiC composite coatings by HVOF. In: ITSC 2001: international thermal...
  • C. Haberland

    Additive manufacturing of shape memory devices and pseudoelastic components

  • H. Meier et al.

    Structural and functional properties of NiTi shape memory alloys produced by selective laser melting

  • C. Haberland

    Additive manufacturing of complex NiTi shape memory devices and pseudoelastic components

  • C. Haberland

    Visions, concepts and strategies for smart Nitinol actuators and complex Nitinol structures produced by additive manufacturing

  • K. Kempen

    Producing crack-free, high density M2 HSS parts by selective laser melting: pre-heating the baseplate

  • Aggarangsi P, Beuth JL. Localized preheating approaches for reducing residual stress in additive manufacturing. In:...
  • Habijan T et al. Rapid manufacturing of porous nickel-titanium as a carrier for human mesenchymal stem cells. In:...
  • M.K. Ravari et al.

    A microplane constitutive model for shape memory alloys considering tension–compression asymmetry

    Smart Mater Struct

    (2015)
  • I. Gibson et al.

    Additive manufacturing technologies

    (2010)
  • ASTM International, F.-a.-S.T.f.A.M.T.;...
  • D. Hu et al.

    Modelling and measuring the thermal behaviour of the molten pool in closed-loop controlled laser-based additive manufacturing

    Proc Inst Mech Eng Part B: J Eng Manuf

    (2003)
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