Home Medicine Effects of heat treatment and welding process on superelastic behaviour and microstructure of micro electron beam welded NiTi
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Effects of heat treatment and welding process on superelastic behaviour and microstructure of micro electron beam welded NiTi

  • Isabel Balz EMAIL logo , Uwe Reisgen , Julia Schoft and Christian Otten
Published/Copyright: September 30, 2016
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Current Directions in Biomedical Engineering
From the journal Current Directions in Biomedical Engineering Volume 2 Issue 1

Abstract

Medical devices with small dimensions made of superelastic NiTi become more popular, but joining these parts remains challenging. Since laser welding was found to be an option, electron beam welding seems to be an interesting alternative as it provides additional advantages due to the precise beam positioning and the high vacuum. Superelasticity is influenced by microstructure and surface layer composition that are mainly affected by welding process and by heat treatment and therefore will be investigated in the present paper.

Keywords: DSC; micro electron beam welding (MEBW); microstructure; NiTi; phase transformation behaviour; XPS

1 Introduction

Since the discovery of NiTi and its extraordinary properties in 1963 [1], especially superelastic (SE) alloys containing about 56 wt.% Ni became the material of choice for medical devices such as stents and orthodontic devices [2]. Next to the rubber-like deformation behaviour, SE NiTi is characterised by good biocompatibility and high corrosion resistance [2], [3], [4]. The disadvantage in using NiTi as construction material is its difficult processing. Above all joining by welding is still challenging, as SE properties are impacted by heat input. In the literature are different microstructural modifications discussed that may affect the stoichiometric ratio of NiTi matrix. From this a shift of phase transformation temperatures up to the loss of SE characteristics may occurs, if no austenitic structure at ambient temperature is present.

Heat treatment is necessary for adjustment of superelasticity and shape setting of NiTi components. High temperatures above 550°C lead to Ni3Ti2-precipitations, which bind high amounts of Ni from the matrix determining a sharply increased austenite finish temperature (Af). On the contrary, heat treatment of <10 min at 500–550°C reduces the Af due to the dissociation of Ni-rich Ni14Ti11-precipitations [5].

Longer heat treatment causes the formation of a titanium oxide layer (TiO2) and a second nickel-rich layer (Ni3Ti) beneath [6], [7]. Another possibility to reduce the Af is to generate fine grains as no precipitations appear in a structure with grain sizes <150 nm, accompanied by higher tensile strengths and decreased residual elongation [8].

During the last 20 years, studies were focused on laser welding for joining NiTi which was found to be suited because of a minimal heat affected zone and high cooling rates [9], [10], [11]. Additionally to these advantages micro welding with electron beam (MEBW) features material independent energy absorption and contactless processing under high vacuum, which is necessary particularly with regard to very reactive elements, like titanium. Also due to nearly inertia free and precise beam deflection in combination with in-situ quality control using the high-resolution SEM mode MEBW is well suited for welding medical applications in micro-range. However, scientific results about EBW of NiTi are rare. Yang et al. [12] worked out that nickel evaporates earlier in the weld pool of an electron beam process due to the lower saturation vapour pressure compared to titanium causing a substantially higher Af. The majority of scientists examining laser welding of NiTi observed also an increase of transformation temperature in the weld metal and explained that with nickel-rich precipitations [10], [13]. Heat treatment and welding is generally a combined process of NiTi applications, but the outcome of both is still insufficiently investigated. Therefore, the aim of this study is to analyse the changing of microstructural and SE properties depending on pre heat treatment at different temperatures and various energy input on thin NiTi foils using MEBW particularly because this method of micro joining had not been considered until now.

2 Experimental details

Ti-55.8 wt.% Ni foils with 200 μm thickness were used throughout this investigation. The as-received flat annealed and oxide free material was heat-treated under atmosphere for 10 min at 350°C, 450°C or 550°C (sample abbreviations: HT – heat treated, NHT – not heat treated) to determine the influence of the heat treatment and oxide layer on SE behaviour of the welding joint. All experiments were done with a micro electron beam welding machine “MEBW-60” of the FOCUS company. The used cathode type for investigations was a LaB6 single crystal, which allows a beam diameter of <25 μm and thus a constant beam characteristic and an increased power density compared to a tungsten hairpin filament. The maximum beam power is 2 kW with an accelerating voltage of 60 kV. Bead-on-plate welds with different heat input per unit length “Q” (welding speed: 20, 40, 60, 80 mm/s) were produced on various heat-treated foil samples.

Micrographs were made along cross direction of the welded joints and further treated with a colour etching process with Beraha III stock solution. The microstructure of the base material (BM), heat-affected zone (HAZ) and welding zone (WZ) were studied in polarized light with metallographic microscope Zeiss “Axio Imager M2m“. According to the ASTM F2004 standards, the thermal transformation behaviour was determined by differential scanning analysis (DSC), which was operated using a “DSC 204 Phoenix” of the Netzsch company within a temperature range from 193 K to 373 K with an controlled cooling/heating rate of 10 K/min. Samples of 13—22 mg were cut from base material and from a field of parallel welded seams (Figure 1).

Figure 1: Field of parallel electron beam welded seams for DSC-samples of the weld.
Figure 1:

Field of parallel electron beam welded seams for DSC-samples of the weld.

X-ray Photoelectron Spectroscopy (XPS) was used to obtain the elemental composition and binding states (oxide state) with an information depth of about 6 nm on selected samples. In this study the XPS measurements were performed with an “Axis Ultra” system (Kratos) of the employing monochromatised Al Kα X-Ray radiation at pass energy of 20 eV (high resolution scan) and an off angle of 45°. Additionally, energy dispersive X-ray spectroscopy (EDX) with “Leo 1530” (Zeiss) at an accelerating voltage of 15 kV was used to examine chemical compositions of sample surface with an information depth of a few microns.

3 Results and analysis

3.1 Microstructure and hardness

The samples welded by 60 mm/s showed the best compromise between a equiaxed grain structure in the weld seam and a low transformation temperature, so this parameter was chosen for further investigations. Considering the microstructure as it is shown in Figure 2, dendritic growth of large grains perpendicular to the weld centreline can be seen in the micrograph of the WZ. As Q decreases, those dendrites become less oriented and more compact. In contrast, fine recrystallised equiaxed grains are located in the HAZ. Grains in the BM are smaller than in the WZ and additional the existence of dislocations in the BM may cause the highest hardness is measured. These dislocations annihilate through the heat input in WZ and HAZ, which implies a decrease in hardness. Furthermore, it must be considered that the SE of austenitic microstructure in BM leads to lightly biased micro hardness measurements with increased values.

Figure 2: Colour etched micrograph of a cross-section of the welded joint (60 mm/s) and corresponding microhardness profile.
Figure 2:

Colour etched micrograph of a cross-section of the welded joint (60 mm/s) and corresponding microhardness profile.

3.2 Composition of surface layer

Surface composition (presented in at.%) and the corresponding Ti/Ni measured by XPS surface scan to a depth of 6 nm for BM and WZ with a welding speed of 60 mm/s of NHT samples are summarised in Table 1. The presence of a high C content and small amounts of N, Ca and Si is due to environmental contaminations.

Table 1:

Surface composition (at.%) of BM NHT and WZ NHT (60 mm/s).

SampleNiTiTi/Ni ratioOCCa, N, Si
BM NHT1.7111.476.7141.0941.684.05 (Ca, N)
WZ NHT 601.847.834.2626.8560.443.04 (Si)

The surface Ti/Ni ratio of the WZ sample without HT is 4.26 and smaller than measured in the BM sample, which is 6.71. That results mainly from an increase of Ti which is caused by chemical affinity for oxygen. The high power density of the electron beam leads to destruction of oxide layer and the vacuum prohibits fast regeneration, which can be seen at the low O content on the surface of the weld seam.

As depicted in Figure 3, three oxidation peaks in accordance with [14] were measured in the WZ sample: TiO (454.6 eV), Ti2O3 (456.7 eV) and TiO2 (458.7 eV), in which the majority of titanium is in the form of TiO2 (65.7 at.% to 100 at.%).

Figure 3: High resolution scan of Ti2p for BM NHT and WZ NHT (60 mm/s).
Figure 3:

High resolution scan of Ti2p for BM NHT and WZ NHT (60 mm/s).

For both samples, the high resolution scan of Ni shows the existence of metal-bonded Ni (852.6 eV). In addition, EDX measurements in BM show an increase of oxygen content on surface with increasing heat treatment temperature (Figure 4). The formation of Ti oxides on the surface of BM leads to a lower Ni content on the surface and might cause the rise of Ni content in NiTi matrix.

Figure 4: Plots of oxygen and nickel content measured by EDX against heat treatment.
Figure 4:

Plots of oxygen and nickel content measured by EDX against heat treatment.

3.3 Superelastic properties

As shown in Table 2, there is a strong correlation between Af and Q. With higher energy input the transformation temperature rises.

Table 2:

Dependence of austenite finish temperature (Af) on heat input per unit length (Q).

Q [Ws/mm] v [mm/s]1.05 80 mm/s1.15 60 mm/s1.35 40 mm/s1.8 20 mm/s
Af (WZ NHT)30.0°C30.5°C47.4°C51.5°C

However, the Af is compared to the as-received material (Af = 10.1°C) substantially increased and reaches values above room temperature, resulting from the MEBW process. So, no matter which weld speed and beam current is chosen, mainly martensite is present in the melted area.

If the SE NiTi-alloy is heat treated under atmosphere before the welding process, the Af results as seen in Figure 5. While the SE properties of the BM seem not to be influenced by HT at 350°C, temperatures at 450°C cause a significant increase of Af. Ni-rich precipitations [5] mainly affect the stoichiometric ratio of NiTi matrix and cause a rise of Af at 450°C and dissociate above 500°C.

Figure 5: Af of BM and WZ as a function of heat treatment.
Figure 5:

Af of BM and WZ as a function of heat treatment.

Next to that the formation of oxide layer must have a positive influence on SE behaviour. Interesting is the effect by heat treatment at 550°C upon the BM. The large drop of Af at −5.5°C, which is below Af of the as-received material, probably results as well from the formation of Ti oxides on the surface and therefore causes the rise of Ni content in NiTi matrix. This thesis is confirmed by further DSC measurements on equal heat treated BM (but under vacuum), which show in contrast an approximately constant curve linearity of Af (∼45°C ± 5°C).

The Af of the melted material in the WZ decreases slightly with higher previous heat input, but is without exception clearly above room temperature. The loss of SE properties in the WZ corresponds with foregoing studies [10], [13].

4 Conclusion

The effects of micro electron beam welding and preheating on the changes of microstructural and superelastic properties were studied. The key findings are:

  1. The microstructure consists of columnar dendritic structure in weld zone and a recrystallised structure in the HAZ. As the heat input decreases, also the Af of the WZ decreases, but still remains above room temperature (martensitic microstructure).

  2. Pre-heating at temperatures of 550°C (10 min) favoures SE of the base material by a large reduction of Af, but getting lost through welding process.

  3. Next to Ni-rich precipitation the formation of Ti-rich oxide layer affects the stoichiometric ratio of the NiTi matrix and therefore may be responsible for lower austenite finish temperatures.

Author’s Statement

Research funding: The work is funded by German Federal Ministry of Economics and Technology (BMWi) under grant number KF2683605CJ4. Conflict of interest: Authors state no conflict of interest. Material and Methods: Informed consent has been obtained from all individuals included in this study. Ethical approval: The research related to human use complies with all the relevant national regulations, institutional policies and was performed in accordance with the tenets of the Helsinki Declaration, and has been approved by the authors’ institutional review board or equivalent committee.

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Published Online: 2016-9-30
Published in Print: 2016-9-1

©2016 Isabel Balz et al., licensee De Gruyter.

This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 License.

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https://doi.org/10.1515/cdbme-2016-0007
Keywords for this article
DSC; micro electron beam welding (MEBW); microstructure; NiTi; phase transformation behaviour; XPS
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BY-NC-ND 4.0

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  61. A method to determine the kink resistance of stents and stent delivery systems according to international standards
  62. Comparison of stented bifurcation and straight vessel 3D-simulation with a prior simulated velocity profile inlet
  63. Transient Euler-Lagrange/DEM simulation of stent thrombosis
  64. Automated control of the laser welding process of heart valve scaffolds
  65. Automation of a test bench for accessing the bendability of electrospun vascular grafts
  66. Influence of storage conditions on the release of growth factors in platelet-rich blood derivatives
  67. Cryopreservation of cells using defined serum-free cryoprotective agents
  68. New bioreactor vessel for tissue engineering of human nasal septal chondrocytes
  69. Determination of the membrane hydraulic permeability of MSCs
  70. Climate retainment in carbon dioxide incubators
  71. Multiple factors influencing OR ventilation system effectiveness
  72. Evaluation of an app-based stress protocol
  73. Medication process in Styrian hospitals
  74. Control tower to surgical theater
  75. Development of a skull phantom for the assessment of implant X-ray visibility
  76. Surgical navigation with QR codes
  77. Investigation of the pressure gradient of embolic protection devices
  78. Computer assistance in femoral derotation osteotomy: a bottom-up approach
  79. Automatic depth scanning system for 3D infrared thermography
  80. A service for monitoring the quality of intraoperative cone beam CT images
  81. Resectoscope with an easy to use twist mechanism for improved handling
  82. In vitro simulation of distribution processes following intramuscular injection
  83. Adjusting inkjet printhead parameters to deposit drugs into micro-sized reservoirs
  84. A flexible standalone system with integrated sensor feedback for multi-pad electrode FES of the hand
  85. Smart control for functional electrical stimulation with optimal pulse intensity
  86. Tactile display on the remaining hand for unilateral hand amputees
  87. Effects of sustained electrical stimulation on spasticity assessed by the pendulum test
  88. An improved tracking framework for ultrasound probe localization in image-guided radiosurgery
  89. Improvement of a subviral particle tracker by the use of a LAP-Kalman-algorithm
  90. Learning discriminative classification models for grading anal intraepithelial neoplasia
  91. Regularization of EIT reconstruction based on multi-scales wavelet transforms
  92. Assessing MRI susceptibility artefact through an indicator of image distortion
  93. EyeGuidance – a computer controlled system to guide eye movements
  94. A framework for feedback-based segmentation of 3D image stacks
  95. Doppler optical coherence tomography as a promising tool for detecting fluid in the human middle ear
  96. 3D Local in vivo Environment (LivE) imaging for single cell protein analysis of bone tissue
  97. Inside-Out access strategy using new trans-vascular catheter approach
  98. US/MRI fusion with new optical tracking and marker approach for interventional procedures inside the MRI suite
  99. Impact of different registration methods in MEG source analysis
  100. 3D segmentation of thyroid ultrasound images using active contours
  101. Designing a compact MRI motion phantom
  102. Cerebral cortex classification by conditional random fields applied to intraoperative thermal imaging
  103. Classification of indirect immunofluorescence images using thresholded local binary count features
  104. Analysis of muscle fatigue conditions using time-frequency images and GLCM features
  105. Numerical evaluation of image parameters of ETR-1
  106. Fabrication of a compliant phantom of the human aortic arch for use in Particle Image Velocimetry (PIV) experimentation
  107. Effect of the number of electrodes on the reconstructed lung shape in electrical impedance tomography
  108. Hardware dependencies of GPU-accelerated beamformer performances for microwave breast cancer detection
  109. Computer assisted assessment of progressing osteoradionecrosis of the jaw for clinical diagnosis and treatment
  110. Evaluation of reconstruction parameters of electrical impedance tomography on aorta detection during saline bolus injection
  111. Evaluation of open-source software for the lung segmentation
  112. Automatic determination of lung features of CF patients in CT scans
  113. Image analysis of self-organized multicellular patterns
  114. Effect of key parameters on synthesis of superparamagnetic nanoparticles (SPIONs)
  115. Radiopacity assessment of neurovascular implants
  116. Development of a desiccant based dielectric for monitoring humidity conditions in miniaturized hermetic implantable packages
  117. Development of an artifact-free aneurysm clip
  118. Enhancing the regeneration of bone defects by alkalizing the peri-implant zone – an in vitro approach
  119. Rapid prototyping of replica knee implants for in vitro testing
  120. Protecting ultra- and hyperhydrophilic implant surfaces in dry state from loss of wettability
  121. Advanced wettability analysis of implant surfaces
  122. Patient-specific hip prostheses designed by surgeons
  123. Plasma treatment on novel carbon fiber reinforced PEEK cages to enhance bioactivity
  124. Wear of a total intervertebral disc prosthesis
  125. Digital health and digital biomarkers – enabling value chains on health data
  126. Usability in the lifecycle of medical software development
  127. Influence of different test gases in a non-destructive 100% quality control system for medical devices
  128. Device development guided by user satisfaction survey on auricular vagus nerve stimulation
  129. Empirical assessment of the time course of innovation in biomedical engineering: first results of a comparative approach
  130. Effect of left atrial hypertrophy on P-wave morphology in a computational model
  131. Simulation of intracardiac electrograms around acute ablation lesions
  132. Parametrization of activation based cardiac electrophysiology models using bidomain model simulations
  133. Assessment of nasal resistance using computational fluid dynamics
  134. Resistance in a non-linear autoregressive model of pulmonary mechanics
  135. Inspiratory and expiratory elastance in a non-linear autoregressive model of pulmonary mechanics
  136. Determination of regional lung function in cystic fibrosis using electrical impedance tomography
  137. Development of parietal bone surrogates for parietal graft lift training
  138. Numerical simulation of mechanically stimulated bone remodelling
  139. Conversion of engineering stresses to Cauchy stresses in tensile and compression tests of thermoplastic polymers
  140. Numerical examinations of simplified spondylodesis models concerning energy absorption in magnetic resonance imaging
  141. Principle study on the signal connection at transabdominal fetal pulse oximetry
  142. Influence of Siluron® insertion on model drug distribution in the simulated vitreous body
  143. Evaluating different approaches to identify a three parameter gas exchange model
  144. Effects of fibrosis on the extracellular potential based on 3D reconstructions from histological sections of heart tissue
  145. From imaging to hemodynamics – how reconstruction kernels influence the blood flow predictions in intracranial aneurysms
  146. Flow optimised design of a novel point-of-care diagnostic device for the detection of disease specific biomarkers
  147. Improved FPGA controlled artificial vascular system for plethysmographic measurements
  148. Minimally spaced electrode positions for multi-functional chest sensors: ECG and respiratory signal estimation
  149. Automated detection of alveolar arches for nasoalveolar molding in cleft lip and palate treatment
  150. Control scheme selection in human-machine- interfaces by analysis of activity signals
  151. Event-based sampling for reducing communication load in realtime human motion analysis by wireless inertial sensor networks
  152. Automatic pairing of inertial sensors to lower limb segments – a plug-and-play approach
  153. Contactless respiratory monitoring system for magnetic resonance imaging applications using a laser range sensor
  154. Interactive monitoring system for visual respiratory biofeedback
  155. Development of a low-cost senor based aid for visually impaired people
  156. Patient assistive system for the shoulder joint
  157. A passive beating heart setup for interventional cardiology training
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