Nickel-titanium Shape Memory Alloy Research Articles

Microstructure and phase transformation of nickel-titanium shape memory alloy fabricated by directed energy deposition with in-situ heat treatment

Additive manufacturing has been vastly applied to fabricate various structures of nickel-titanium (NiTi) shape memory alloys due to its flexibility to create complex structures with minimal defects. However, the microstructure heterogeneity and secondary phase formation are two main problems that impede the further application of NiTi alloys. Although post-heat treatment is usually adopted to improve or manipulate NiTi alloy properties, it cannot realize the spatial control of thermal and/or mechanical properties of NiTi alloys. To overcome the limitations of uniform post-heat treatment, this study proposes an in-situ heat treatment strategy that is integrated into the directed energy deposition of NiTi alloys. The proposed method will potentially lead to new manufacturing capabilities to achieve location-dependent performance or property manipulation. The influences of in-situ heat treatment on the thermal and mechanical properties of printed NiTi structures were investigated. The investigations were carried out in terms of thermal cycling, microstructure evolution, and mechanical properties by 3D finite element simulations and experimental characterizations. A low-power laser beam was adopted to localize the in-situ heat treatment only to the current printed layer, facilitating a reverse peritectic reaction and a transient high solution treatment successively. The proposed in-situ heat treatment on the specimen results in a more obvious phase transformation peak in the differential scanning calorimetry curves, about 50~70% volume reduction for the Ti2Ni phase, and approximately 35 HV reduction on microhardness.

Continuous Heating Dissolution and Continuous Cooling Precipitation Diagrams of a Nickel-Titanium Shape Memory Alloy

Binary NiTi alloys are the most common shape memory alloys in medical applications, combining good mechanical properties and high biocompatibility. In NiTi alloys, the shape memory effect is caused by the transformation of an austenite phase to a martensite phase and the reverse process. Transformation temperatures are strongly influenced by the exact chemical composition of the NiTi phase and the presence of precipitates in the microstructure induced by thermo-mechanical treatment, especially solution annealing and ageing. Isothermal time–temperature precipitation diagrams can be found in the literature. Cooling is frequently not considered, as water quenching is typically assumed to be sufficient. To the best of our knowledge, continuous heating dissolution (CHD) and continuous cooling precipitation (CCP) diagrams do not exist. Differential scanning calorimetry (DSC) is a common method to analyse the austenite/martensite transformation in shape memory alloys, but it has not yet been used to analyse precipitation processes during continuous temperature changes. We have enabled DSC to analyse dissolution and precipitation processes in situ during heating as well as during cooling from the solution annealing temperature. Results are presented as CHD and CCP diagrams, including information from microstructure analysis and the associated changes in the austenite/martensite transformation temperatures.

Molecular dynamics study of phase transformations in NiTi shape memory alloy embedded with precipitates

The present study utilizes molecular dynamics simulations to study the athermal and stress-induced martensitic transformation of B2 to B19′ phase in a Ni-Ti alloy and the effect of precipitates on the phase transformation. The simulations demonstrate the existence of an intermediate B19 phase between martensite and austenite phases. The Nickel-Titanium shape memory alloy with precipitates is studied by introducing Ni3Ti, NiTi2 and Ni4Ti3 precipitates individually using atomistic simulations. The results show that the phase transition temperature decreases in the presence of a large volume fraction of precipitates. A blended precipitates model with Ni3Ti and NiTi2 is simulated to study the phase transformation in equiatomic NiTi alloy. The results indicate that the precipitates would initiate the emergence of the B19 phase and reduce the transition temperature. In addition, the variation of Nickel content by embedded precipitates would lead to a change in the microstructural phenomena.

Blast Performance Evaluation of Steel Moment-Resisting Frame Equipped with Smart Bolted Connection

This paper deterministically evaluates the blast performance of steel moment-resisting frame (MRF) smart structures equipped with nickel titanium shape-memory alloy (NiTi SMA) bolted connections. The recentering capacity of the NiTi SMA bolts is exploited to limit the structural damage and reduce the residual drift after a near-field explosion event. A simplified blast over-pressure calculation approach is used to draw the blast overpressure profile. Numerically, the NiTi SMA–based connection is designed conforming to current European standards and is modeled with a solid finite element. The effect of high-strain-rate actions is considered. The response of the NiTi SMA–based connection is obtained by the nonlinear transient analysis and compared with the steel bolted connection. The NiTi SMA–based connection provided excellent recentering capacity. Later, the moment-rotation response of the NiTi SMA–based connection is utilized in the MRF structure. A simplified self-centering connection is proposed to produce smart behavior in the MRF structures. The results reveal that using NiTi SMA connections keep interstory drift relatively low and substantially provide reasonable structural lateral resistance under near-field blast loading.

Spatial characteristics of nickel-titanium shape memory alloy fabricated by continuous directed energy deposition

Additive manufacturing has been adopted to process nickel‑titanium shape memory alloys due to its advantages of flexibility and minimal defects. The current layer-by-layer method is accompanied by a complex temperature history, which is not beneficial to the final characteristics of shape memory alloys. In this study, a continuous directed energy deposition method has been proposed to improve microstructure uniformity. The spatial characterization of nickel‑titanium shape memory alloy fabricated by continuous directed energy deposition is investigated to study the temperature history, phase constituent, microstructure, and mechanical properties. The results indicate that the fabricated specimen has a monotonic temperature history, relatively uniform phase distribution and microstructure morphology, as well as high compressive strength (2982 MPa–3105 MPa) and strain (37.7%–41.1%). The reported method is expected to lay the foundation for spatial control during the printing of functional structures.

Seismic Performance of Ni-Ti SMA Wires Equipped in the Spatial Skeletal Structure

Nickel Titanium (Ni-Ti) Shape Memory Alloy (SMA) can be used to limit response of structure during external disturbances such as large seismic events. This paper presents a seismic performance study of Ni-Ti SMA wires equipped in the spatial skeletal structure. First, an improved Graesser-Cozzarelli (G-C) numerical constitutive model of the Austenitic phase of NiTi SMA wire is established. By contrast, the model based on uniaxial cyclic loading experimental tests is demonstrated as feasibility and validity. Next, a method consisting of a three-layer steel spatial skeletal structure model equipped with SMA wires is employed for simulation and experimental tests. According to the obtained constitutive numerical model, the simulation program of vibration control is written to simulate the effect of vibration control of seismic EL-centro wave. Furthermore, a shaking table experimental test was designed to verify the vibration control effect under the same action of seismic EL-centro wave. By comparison of the results of the numerical simulation and shaking table test, dynamic responses of the displacement and acceleration for different floors with control and without control was concluded. The superior superelastic properties of SMA wires used in passive control are investigated and the correctness of the constitutive numerical model are verified as well. The results show that such a comprehensive analysis integrates seismic-resistant behavior of Ni-Ti SMA wires in this type of structure. Besides, proposed method has broad application prospects to address the issues in passive control field of building structures.

Part II.: Dissimilar friction stir welding of nickel titanium shape memory alloy to stainless steel – microstructure, mechanical and corrosion behavior

In the present work, nickel-titanium (NiTi) shape memory alloys (SMAs) is joined to 304 stainless steel (SS) using friction stir welding (FSW) using process parameters of 400 rpm and a constant tool translation speed of 75 mm/min. The macrostructure of the dissimilar weld showed some surface tunneling defects but there were regions of the weld that were defect free and without evidence of intermetallics at the interface. The weld stir zone (SZ) showed grain refinement compared to both base materials (BM). NiTi side of the joint showed very fine thermomechanical affected zone (TMAZ) and absence of a distinct heat affected (HAZ) zone, resulting in very little variation in microhardness. SS side of the weld showed typical FSW microstructural zones with higher microhardness in the SZ (270 HV) compared to BM (185 HV). Tensile testing of the weld showed a small initial plateau region, characteristic of shape memory alloys (SMAs) arising from NiTi part of the weld. Importantly, the transformation temperatures of NiTi, measured by differential scanning calorimetry (DSC) from different weld zones, varied by no more than 4 °C. Corrosion properties of the weld zone investigated in 3.5 wt. % NaCl solution showed moderately inferior corrosion resistance of dissimilar weld, compared to individual base metals, as characterized by the surface film impedance and corrosion current density. The decreased corrosion resistance was attributed to inhomogeneous passive film formed across the weld due to zonal heterogeneities. The predicted and measured galvanic parameters of the galvanically coupled base metals suggested that there appeared to be negligible galvanic effects. The results from the study show that with further development, FSW can be successfully utilized to create dissimilar joints between NiTi and stainless steel. • Nickel titanium shape memory alloy joined to stainless steel using friction stir welding process. • Refined grained along with absence of intermetallics was observed in the weld zone. • The transformation temperature on NiTi side across the different weld zones shows very little variation. • Under tensile loading, dissimilar joint showed small plateau region, characteristic of SMAs in martensitic phase. • Weld zone showed slightly inferior corrosion property compared to individual base material, while negligible galvanic effect was observed.

An In-Silico Corrosion Model for Biomedical Applications for Coupling With In-Vitro Biocompatibility Tests for Estimation of Long-Term Effects.

The release of metal particles and ions due to wear and corrosion is one of the main underlying reasons for the long-term complications of implantable metallic implants. The rather short-term focus of the established in-vitro biocompatibility tests cannot take into account such effects. Corrosion behavior of metallic implants mostly investigated in in-vitro body-like environments for long time periods and their coupling with long-term in-vitro experiments are not practical. Mathematical modeling and modeling the corrosion mechanisms of metals and alloys is receiving a considerable attention to make predictions in particular for long term applications by decreasing the required experimental duration. By using such in-silico approaches, the corrosion conditions for later stages can be mimicked immediately in in-vitro experiments. For this end, we have developed a mathematical model for multi-pit corrosion based on Cellular Automata (CA). The model consists of two sub-models, corrosion initialization and corrosion progression, each driven by a set of rules. The model takes into account several environmental factors (pH, temperature, potential difference, etc.), as well as stochastic component, present in phenomena such as corrosion. The selection of NiTi was based on the risk of Ni release from the implant surface as it leads to immune reactions. We have also performed experiments with Nickel Titanium (NiTi) shape memory alloys. The images both from simulation and experiments can be analyzed using a set of statistical methods, also investigated in this paper (mean corrosion, standard deviation, entropy etc.). For more widespread implementation, both simulation model, as well as analysis of output images are implemented as a web tool. Described methodology could be applied to any metal provided that the parameters for the model are available. Such tool can help biomedical researchers to test their new metallic implant systems at different time points with respect to ion release and corrosion and couple the obtained information directly with in-vitro tests.

Position Control and Force Estimation Method for Surgical Forceps Using SMA Actuators and Sensors.

Minimally invasive surgery is increasingly used in many medical operations because of the benefits for the patients. However, for the surgeons, accessing the situs through a small incision or natural orifice comes with a reduction of the degrees of freedom of the instrument. Due to friction of the mechanical coupling, the haptic feedback lacks sensitivity that could lead to damage of the tissue. The approach of this work to overcome these problems is to develop a control concept for position control and force estimation with shape memory alloys (SMA) which could offer haptic feedback in a novel handheld instrument. The concept aims to bridge the gap between manually actuated laparoscopic instruments and surgical robots. Nickel-titanium shape memory alloys are used for actuation because of their high specific energy density. The work includes the manufacturing of a functional model as a proof of concept comprising the development of a suitable forceps mechanism and electronic circuit for position control and gripping force measurement, as well as designing an ergonomic user interface with haptic force feedback.

Part I.: Friction stir welding of equiatomic nickel titanium shape memory alloy – microstructure, mechanical and corrosion behavior

In this study, friction stir welding (FSW) process is utilized to join equiatomic nickel-titanium (NiTi) shape memory alloys as a solid-state welding alternative to fusion welding. In the studied range of process parameters (spindle speeds – 350 – 450 rpm and constant translation speed 75 mm/min), combination of 400 rpm spindle speed and 75 mm/min tool translation speed produced a defect-free joint without any discernable voids. The weld microstructure consisted of a typical stir zone with fine grains; a rather small thermo-mechanically affected zone and heat affected zone. Vickers microhardness measurements after appropriate thermal cycling, showed uniform hardness across the weld zone, indicating a joint free of detrimental precipitates. The transformation temperatures measured by differential scanning calorimetry, showed little to no change across the weld zones. Under tensile loading, at ambient temperature a maximum joint efficiency of 93% was obtained in the martensitic phase while at elevated temperature a joint efficiency of 84% was obtained in the austenitic phase. The welded specimen showed martensitic detwinning plateau, characteristic of SMAs tested in martensitic phase, indicating that martensitic detwinning was the dominant mode of deformation at low strains, even within the weld zone. The weld zone showed comparatively lower corrosion resistance than the base material, owing to less dense TiO2 passive layer formation. Galvanic effect between the weld zone and base materials was negligible, indicating good compatibility between the different weld zones. Our results show that friction stir welding can be successfully applied for joining NiTi SMAs sheets.

Electrochemical corrosion response of ZrN coated Nickel-Titanium alloy in 0.9% NaCl solution

Abstract: The biomedical application as metallic body implants of Nickel-Titanium shape memory alloys are of high interest due to its unique property of pseudo elasticity. However, high content of nickel and its potential to release from the surface due to corrosion raises safety concerns about its allergic reactions, toxicity and carcinogenicity. Hence, it is necessary to produce proper passivity to prevent surface layer degradation. In this study, ZrN coating on Nickel-Titanium shape memory alloys was obtained by vacuum cathodic arc physical vapor deposition technique to improve electrochemical corrosion resistance. The electrochemical corrosion response was studied in 0.9% NaCl solution at 370C using Tafel extrapolation method. Surface morphology was studied using SEM/EDAX and XRD. ZrN coating increases corrosion stability and micro hardness. Keywords: Shape memory alloy; pseudoelasticity; vacuum cathodic arc PVD; electrochemical corrosion; micro hardness.

Research Status and Prospect of Additive Manufactured Nickel-Titanium Shape Memory Alloys.

Nickel-titanium alloys have been widely used in biomedical, aerospace and other fields due to their shape memory effect, superelastic effect, as well as biocompatible and elasto-thermal properties. Additive manufacturing (AM) technology can form complex and fine structures, which greatly expands the application range of Ni-Ti alloy. In this study, the development trend of additive manufactured Ni-Ti alloy was analyzed. Subsequently, the most widely used selective laser melting (SLM) process for forming Ni-Ti alloy was summarized. Especially, the relationship between Ni-Ti alloy materials, SLM processing parameters, microstructure and properties of Ni-Ti alloy formed by SLM was revealed. The research status of Ni-Ti alloy formed by wire arc additive manufacturing (WAAM), electron beam melting (EBM), directional energy dedication (DED), selective laser sintering (SLS) and other AM processes was briefly described, and its mechanical properties were emphatically expounded. Finally, several suggestions concerning Ni-Ti alloy material preparation, structure design, forming technology and forming equipment in the future were put forward in order to accelerate the engineering application process of additive manufactured Ni-Ti alloy. This study provides a useful reference for scientific research and engineering application of additive manufactured Ni-Ti alloys.

Molecular Dynamics Investigation of the Influence of Voids on the Impact Mechanical Behavior of NiTi Shape-Memory Alloy.

To date, research on the physical and mechanical behavior of nickel-titanium shape-memory alloy (NiTi SMA) has focused on the macroscopic physical properties, equation of state, strength constitution, phase transition induced by temperature and stress under static load, etc. The behavior of a NiTi SMA under high-strain-rate impact and the influence of voids have not been reported. In this present work, the behavior evolution of (100) single-crystal NiTi SMA and the influencing characteristics of voids under a shock wave of 1.2 km/s are studied by large-scale molecular dynamics calculation. The results show that only a small amount of B2 austenite is transformed into B19’ martensite when the NiTi sample does not pass through the void during impact compression, whereas when the shock wave passes through the hole, a large amount of martensite phase transformation and plastic deformation is induced around the hole; the existence of phase transformation and phase-transformation-induced plastic deformation greatly consumes the energy of the shock wave, thus making the width of the wave front in the subsequent propagation process wider and the peak of the foremost wave peak reduced. In addition, the existence of holes disrupts the orderly propagation of shock waves, changes the shock wave front from a plane to a concave surface, and reduces the propagation speed of shock waves. The calculation results show that the presence of pores in a porous NiTi SMA leads to significant martensitic phase transformation and plastic deformation induced by phase transformation, which has a significant buffering effect on shock waves. The results of this study provide great guidance for expanding the application of NiTi SMA in the field of shock.

Cyclic testing of an innovative self-centering X-braced ductile shear panel

This paper presents an innovative seismic load-resisting component of braced ductile shear panel (BDSP) with self-centering ability. Tension-only nickel-titanium (NiTi) shape memory alloy (SMA) wires installed along the inclined segments are employed to facilitate the self-centering ability. Thin shear panel with pinching hysteretic feature is adopted to decrease the amount of SMA wires, and it provides sufficient lateral bearing capacity and lateral stiffness. A total of eight specimens at one-third scale tests were performed to investigate the hysteretic behavior and re-centering capacity under cyclic loading. The experimental results show that the adoption of SMA wires inside traditional BDSP component realized the desirable self-centering ability and retained moderate energy dissipation capacity with the equivalent viscous damping ratio in the range of 0.124–0.158. Compared to solid shear panel, the employment of slotted shear panel obtained a considerably pinching hysteretic loop and reduced the amount of SMA wires for providing the re-centering ability. The local buckling and damage were primarily focused on the shear panel. The self-centering BDSP specimens were used as replaceable fuse elements for realizing the repairability after an earthquake. The tension field inclination in the solid shear panel ranged from 18° to 33.5°, which was approximately consistent with the main diagonal direction according to observed experimental phenomenon. Furthermore, the surrounding flanges of shear panel did not play an important role on improving lateral strength. The average overstrength factor of the self-centering BDSP specimens was 1.49.

Investigations on the actuation behaviour of friction stir–welded nickel titanium shape memory alloy using continuous fibre laser

The friction stir welding (FSW) is found to be an effective solid-state process to join Nickel Titanium (NiTi) shape memory alloy. The retention of shape memory effect has enabled the welded NiTi alloy to be exploited in various functional applications. In this article, the NiTi sheets of 1.2 mm thickness are welded using FSW. The tool selection, geometry design and process parameters required to weld NiTi sheets are explored. Interestingly, an attempt is made to actuate the welded NiTi alloy, using laser actuation technique. The laser beam is scanned over the sample at a particular speed, enabling the increase in temperature suitable for physical actuation. A minimum and maximum displacement of 10 mm and 28 mm are recorded for the laser powers of 10 W and 50 W, respectively. Apart from laser actuation, the dynamic mechanical analysis of the welded NiTi alloy is investigated.

Molten pool characteristics of a nickel-titanium shape memory alloy for directed energy deposition

Fabrication of nickel-titanium shape memory alloy through additive manufacturing has attracted increasing interest due to its advantages of flexible manufacturing capability, low-cost customization, and minimal defects. The process parameters in directed energy deposition (DED) have a crucial impact on its molten pool characteristics (geometry, microstructure, etc.), thus influencing the final properties of shape memory effect and pseudoelasticity. In this paper, a three-dimensional numerical model considering heat transfer, phase change, and fluid flow has been developed to simulate the cladding geometry, melt pool depth, and deposition rate. The experimental and simulated results indicated that laser power plays a critical role in determining the melt pool width and deposition rate while scan speed and powder feed rate have less effect on cladding geometry and deposition rate. The fluid velocity has a huge influence on the distribution of elements in the molten pool. The temperature gradient G, solidification rate R, as well as shape factor G/R were calculated to illustrate the underlying mechanisms of grain structure evolution. The grain morphology distribution of cross-section from the experimental samples agreed well with the simulation results. The model reported in this paper is expected to shed light on the optimization of the deposition process and grain structure prediction.

PVA/gelatin-based hydrogel coating of nickel-titanium alloy for improved tissue-implant interface

Nickel-titanium shape memory alloys have attracted notable interest for biomedical applications due to their unique properties. However, there are still some concerns on the potential release of nickel ions causing adverse reactions in-vivo at high concentrations. In this work, samples were coated with a polyvinyl alcohol/gelatin hydrogel. Coating stability, corrosion resistance and biocompatibility were investigated. Coated samples exhibited similar shape memory response to that of the uncoated substrates in wire form. Additionally, the hydrogel coating led to a decrease in the surface temperature and significantly improved the corrosion resistance of the shape memory substrate. The presence of the hydrogel layer does not affect the biocompatibility of the alloy while providing a semi-degradable, hydrophilic surface that forms a cushion at the tissue-implant interfaces.

A Differential Evaporation Model to Predict Chemistry Change of Additively Manufactured Metals

The desire for increased performance and functionality has introduced additional complexities to the design and fabrication of additively manufactured (AM) parts. However, addressing these needs would require improved control over local properties during the fabrication process. In this regard, differential evaporation is an inherent characteristic in metal AM processes, directly influencing local chemistry, material properties, functionality, and performance. In the present work, a differential evaporation model (DEM) is presented for laser powder bed fusion (LPBF) AM to predict and control the effect of evaporation on chemistry and properties on local and part-wide scales. The DEM model is coupled with an analytical thermal model that is calibrated against 51.2 Ni [at.%] nickel titanium SMA single-track experiments and a multi-layer model that accounts for the AM part’s multi-layer design and the inherent melt pool overlap and chemistry propagation. The combined hierarchical model, consisting of the thermal, evaporation, and multi-layer components, is used to predict location-specific chemistry for LBPF AM fabrication of 50.8 Ni [at.%] nickel titanium shape memory alloys(NiTi SMAs). Model predictions are validated with values obtained from multi-layer experiments on a commercial LPBF system, resulting in a root mean square error (RMSE) of 0.25 Ni [at.%] for predicted Ni content. Additionally, martensitic transformation temperature, Ms, is calculated and compared with empirical data, resulting in an RMSE of 18.6 K. A practical account of the cumulative and propagative thermal-induced evaporation effect on location-specific chemistry is made through this linkage of models. Fundamentally, this model chain has also provided a solution to the forward modeling problem, enabling steps to be taken towards resolving the inverse design problem of deter-mining processing parameters based on desired location-specific properties.

Evaluation of the mechanical properties and clinical application of nickel-titanium shape memory alloy scaphoid arc nail.

To investigate the mechanical and biomechanical properties of nickel–titanium (Ni–Ti) shape memory alloy scaphoid arc nail (NT‐SAN) fixator as well as study the surgical method of treating carpal scaphoid fractures and evaluate its clinical efficacy. (1) Static and dynamic bending tests with embedded axial bending fixture were conducted to study the mechanical properties. (2) To evaluate biomechanical strength and fatigue, 32 scaphoid samples were classified into four groups to perform the fixation rigidity test: intramedullary Kirschner fixation (group A), Kirschner straddle nail fixation (group B), screw nail fixation (group C), and NT‐SAN fixation (group D). Next, 24 scaphoid waist fracture models were classified to conduct fatigue experiments as follows: Kirschner straddle nail fixation (group E), screw nail fixation (group F), and NT‐SAN fixation (group G). (3) The Krimmer score chart was used for clinical evaluations. (1) NT‐SAN showed excellent mechanical performance and a long lifespan. (2) NT‐SAN was fixated with a strong intensity and an anti‐fatigue outcome. (3) Ninety‐eight interviewed patients were satisfied with the therapeutic effects of the arc nail (satisfaction rate: 95.92%). The designed strength and hardness of NT‐SAN corresponded with the anatomical characteristics of the scaphoid, and the designed mechanical properties met the biomechanical requirements of a scaphoid fracture. The fatigue strength can meet the requirements of bone healing after the scaphoid fracture. Clinical trials on NT‐SAN scaphoid fracture treatment have shown that the surgery is simple and the clinical results are satisfactory. The therapeutic level of NT‐SAN is III; thus, it is worth promoting.

Prediction of white layer formation in μ-WEDM process of NiTi shape memory superalloy: FEM with experimental verification

Microscopic changes in the surface of nickel-titanium (nitinol) shape memory alloys (SMAs) in micro-wire electro-discharge machining (μ-WEDM) due to the formation of a resolidified layer on the machined surface, called white layer, are one of the main drawbacks in the processing of such alloys. Since these changes significantly affect the shape memory and elastic recovery characteristics of these alloys, reduction of the white layer thickness (WLT) based on the selection of optimum process parameters is essential to raise the quality of the machined parts. In this regard, a finite element model (FEM) has been developed to simulate the effects of μ-WEDM process parameters, including discharge current, pulse on-time, pulse off-time, and servo voltage, on the heat distributing in Ni55.8Ti SMA to predict the WLT. The flushing efficiency of electric discharges and the effect of flow regime of the dielectric fluid on the heat distribution in the workpiece and the formation of the WLT are analyzed. Experimental data are used to verify the accuracy of the FEM. The results show that the developed model can predict the WLT in μ-WEDM process of Ni55.8Ti SMA with an average error of 14%. The effects of discharge parameters on the formation of the WLT are discussed in details based on the results of the FEM.