CHALLENGES AND SOLUTIONS IN SIMULATING SHS-SYNTHESIZED POROUS TITANIUM NICKELIDE USING ANSYS

ABSTRACT

Nickel-titanium alloy has a wide range of applications as a biomaterial due to its high ductility, low corrosion rate, and favorable biocompatibility. Although the Young’s modulus of NiTi is relatively low, it still needs to be reduced; one promising method is to introduce a porous structure. Traditional manufacturing processes, such as casting, can hardly produce complex porous structures. Accordingly, this paper aims to describe the design of the porous structure by ANSYS software. It points out the current challenges. Given the increasing application of NiTi implants, this review paper may open up new horizons for advanced and novel designs.

Keywords: NiTi; Porous structure; Ansys.

Introduction

Lastly, SHS-synthesized porous Ti2Ni structures include many flaky/Ti particles/TiN impurities that actually are isolation bodies and significantly influence the mechanical responses of Ti2Ni. However, due to the weak coupling of thermal elasto-plastic body-conductivity algorithm between commercial software, the possible rapid quenching effects of Ti/TiN particles are unsupportive to implement. With significant drawbacks, it becomes necessary to use a nonconducting porous network and masking methodology.

Moreover, SHS may result in great inhomogeneity in the microstructures and properties of the products or several wrapped bodies. Unlike the well-prepared ones, real SHS-synthesized bodies are often provided with irregular shapes, which makes the fast and automatic precise modeling of SHS-synthesized Ti2Ni challenging, especially for a wrapped one with many holes. Accordingly, a universal method is necessary to conduct this modeling, especially for commercial software. [1]

The first challenge comes from the extremely low porosity and the high melting point/ignition temperature relative to the normal porous ones. That is to say, the porosities are usually above 70% and the relatively low melting points are around 1100 °C. In comparison, the porosity of SHS-synthesized porous Ti2Ni is only 50–75%, and the high melting point/ignition temperature is around 2200–2500 °C. [2]

Self-propagating high-temperature synthesis is an advanced method suitable for producing advanced porous materials, which are essential components in industrial applications such as rocket engines, aerospace, and bone defect therapeutics. Accordingly, it is necessary to simulate their macroscopic behaviors for further understanding and applications using finite element software such as ANSYS. Simulating this special porous material, however, poses great challenges.

Overview of Simulation Techniques in Material Science

Accuracy is a primary condition for any physical model and stimulus for its implementation. However, approximation is a necessary condition for reducing computational costs within acceptable limits through the use of simplified models. In arithmetic, accuracy is measured as a discrepancy of a model and experimental or known data. The simulation of SHS represents a mathematical, physical, and computational challenge because the burning front motion is perpendicular to the temperature gradient and non-isothermal processes take place in short times. Therefore, there is no simple stratification of the properties field, and the computational domain mentioned for direct numerical solutions may have any form.

Simulating processes in material science is very complex as it involves many physical phenomena simultaneously. When applying traditional methods, this problem becomes even more difficult because the resulting models are very large and hence may have unaffordable computational cost. In contrast, the various high-level and simplified models provide a feasible solution to the reduced accuracy in return for much better convergence. In certain conditions, the necessary computational cost cannot be reduced — and this is the case in SHS where the reaction media moves with the flame front and changes thermo-physical and chemical properties and phase transitions of the reacting materials occur. Performance of the computational techniques can be described by well-known characteristics that should be achieved for designed simulating: accuracy, robustness, speed, constructedness, maintainability, and flexibility.

Challenges in Simulating SHS-Synthesized Porous using ANSYS

The aim is to improve the simulation model of titanium nickelide combustion in the ANSYS software package to reduce the loss on porosity and to obtain material with dense and adjustable porosity not only during "bridge" thickness formation but also at the stage of heating incoming parts of reactants. The authors consider the solution to the question of how to take into account the change in the properties of the titanium nickelide reactive system. Some other issues that have been resolved are also summarized, in particular, how to eliminate the error of using large physical constants in the calculation of the adiabatic combustion temperature during the transition to the highly porous structure and its experimental comparison with different EH calculations. [3]

The present authors' previous study has shown that in the process of SHS synthesis of titanium nickelide, it is possible to adjust the porosity and the material density due to the use of phase-stabilizing additives. Functionally graded cellular structures can form due to combustion wave stratification by heat release. The indicated properties and features are key parameters for the practical use of such material in applications such as filters, mufflers, and shock absorbers. To understand the laws of cellular material formation and to predict the SHS synthesis process of porous titanium nickelide, numerical simulation in the ANSYS software package is often performed. When conducting a simulation, it is important to take into account the non-stationarity of a burning jet, as well as the influence and change in the physical and mechanical properties of the material. However, the software ANSYS is used quite rarely to simulate and explain such processes. Unfortunately, its capabilities in modeling the burning process of porous substances obtained using the high-temperature SHS synthesis method are not disclosed at all. [4]

Solutions and Strategies for Overcoming Simulation Challenges

The following creative strategies have been sought and implemented in order to answer the modeling concerns described in the preceding section: Minimizing the excessive usage of computer system memory and processing time in solving problems with certain difficulty problems in ANSYS. The main concerns of slow performance of the simulation were eliminating program memory overflow, reducing time consumption, and decreasing exit code 345 errors, which all directly related to the capability of CPU physical memory and the amount of virtual memory. For advanced material modeling problems, there are some much more significant issues related to rigid timelines such as meeting the graduate-to-marked date for the master's thesis or providing the student research team research deliverables on time. Hence, time is of the essence. The need to frequently access the graph of the behavior of model parameters can be therefore addressed by using activation or deactivation of the graph generation feature, using the contents of direct text output of the ANSYS program without 3D graphical representation, at the expense of obtaining image content of the graphs. 3D graphical images are obtained after post, sometimes even after deleting the graphical window of the FE model. [5]

The 3D spatial distribution of temperature would greatly depend on the amount of heat transfer between porous Ti2Ni and the region very close to the symmetry boundary. Here, there is the possibility of long small thermal conduction times of the porous components through their sharply deposited profiles. To suppress such sensitivity, a symmetry boundary condition that actually represents some kind of a heuristic model boundary, an enlarged perimeter was added to the original geometry by approximating a sheet of new nodes across the free surface cut. The boundary condition switches the usual Dirichlet fixed profile temperature boundary condition model into Fourier model. Outside of this enlarged region, the temperature profile would typically not have such a strong variation normal to the symmetry boundary as the porosity is much less in comparison. However, it is well approximated by the usual symmetrical temperature boundary condition with an initial kernel, usually a fixed Gaussian kernel. Since this does not affect simulations involving porous alumina, and since porous alumina is outside the symmetry boundary, during the inclusion of porous Ti2Ni in the porous Ti2Ni/porous alumina biphasic model only porous Ti2Ni bounded by porous alumina is imported, unfortunately with the enlarged boundary condition. However, only the porosity in the vicinity of the boundary is of interest. Such a workaround supplementary model allowed simulating thermal transmission without increasing render time cost, favored by the presence of double time-steps allowing the use of a lower CFL coefficient. [6]

The predicted geometry of interfaces cannot preserve the geometrical structure between particle agglomerates inside the embedded porous model, or from which part of the elementary porous representation of the particle agglomerate lies in the model fragment, so careful, layer-by-layer verification should be done in search of voids inside the structure. In general, since such heuristic models can be subjected to bugs or other lacks when operated over the allotted time window of the modeling experiment, i.e., using the key design philosophy of acknowledging the absurdity of a premise for the purposes of liberating an architect design decision, always additional model-checking and post-processing were needed. If the high value of overheating is a problem when directly assembled to the fully periodic geometry from nano tomography images, in creativity, the feature of simplicity is overactive because basics are ignored or are simplified and over-ignored. The periodicity is only present through symmetrical boundary conditions, so the simplicity aspect reigns supreme. As paraorthogonalization is more of a qualitative rather than quantitative nature, simple settings, like time-step initialization, can give unpredictably extreme results.

Conclusion

This paper reviews recent advances in the design of porous structures and mechanical properties, however, there are still many challenges to be addressed. The mechanism of the SLM process that affects the microstructure of the parts has not been clarified. Moreover, the design of functionally graded porous structures still needs further research. [7] have conducted a series of researches on unit cell design, but they are still insufficient, and more novel unit cell designs are needed. Considering the combination of different pore sizes and pore shapes to form a biomimetic structure is a promising direction. Designing new 3D models and establishing standardized frameworks for sharing datasets and machine learning models among the research community.

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