The present study deals with the numerical analysis of the solidification process of NiTi binary alloy in a lab-scale continuous casting device. The physical medium is taken as an incompressible fluid where the heat is transferred by conduction and convection, including the thermal phase change phenomenon. The energy equation which includes both convection-diffusion heat transfer and a mushy region for the phase-change (solidification) is modelled by using an enthalpy-based formulation. The numerical approach is based on the finite volume method in body fitted coordinates with a SIMPLE scheme to couple the pressure and velocity fields. For varying casting speed, the results are presented for the temperature field, as well as for the NiTi mass fraction during the solidification process. Results show that the casting speed has a siginificant effect on the temperature distribution and, consequently on the solidification as well as metallurgical length. High casting speed may cause inadequate thickness of the solidified material at the mold exit to withstand the hydrostatic pressure of the molet NiTi below the mold and may also laed to breakout due to sticking of solidified shell and mould because of lack of slag film for lubrication between the two.
COBISS.SI-ID: 19771158
Superelastic (SE) and thermo-activated (TA) archwires, produced from equimolar nickel-titanium (NiTi) are used in everyday orthodontic practice. The aim of our study was to investigate and compare the cytocompatibility between two SE (Rematitan and TruFlex) and two TA archwires (Equire and TruFlex), based on the hypothesis, which has not been tested so far, that the differences in manufacture and microstructure of the archwires may influence their biocompatibility. The archwires were studied as received and after conditioning for 35 days in cell culture medium under static conditions. We found that all the archwires were non-cytotoxic for L929 cells in a direct contact assay (6cm2 surface area/ml of culture medium), but inhibited proliferation (metabolic activity) of the cells up to 30%. SE archwires had stronger inhibitory effects. The conditioning medium (CM) of the archwires exerted lesser degree of inhibition. All of the archwires could be considered as non-cytotoxic, according to the current ISO standards. However Rematitan SE, both as received and conditioned, induced apoptosis of rat thymocytes in direct contact. In contrast, TruFlex SE and Equire TA increased proliferation of thymocytes. The cytotoxic effect of Rematitan SE on thymocytes correlated with higher release of Ni iones in CM, higher concentration of surface Ni and with increased oxide layer thickness after the conditioning. In conclusion, the direct contact is more important for the assessment of cytocompatibility than the use of corrosion products of NiTi archwires. A new sensitive test on thymocytes may be relevant for screening the cytotoxicity, and in this context Rematitan SE was less cytocompatible than other archwires.
COBISS.SI-ID: 19654422
In this paper we present the research which is connected to the performance of the series of experiments combined with vacuum induction melting and continuous vertical casting of NiTi alloy in order to produce the strand. The theoretical chosen parameters enabled obtaining a continuously cast strand with a diameter of 11 mm. The strand microstructures were investigated with a light and scanning electron microscope, while the chemical composition of the single phase was identified with the semi-quantitative micro-analysis Energy-dispersive X-ray Spectroscopy and Inductively Coupled Plasma - Optical Emission Spectrometry. The research showed that the microstructure is dendritic where, in the inter-dendritic region the eutectic is composed of a dark NiTi phase and bright TiNi3-x phase. In some areas we found Ti–carbides and phases rich with Fe. Micro-chemical analysis of the NiTi strand showed that the composition changed over the cross and longitudinal section, which is a proof that the alloys are inhomogeneous. In the final part, the electrochemical behaviour of NiTi strand samples were compared to a commercially available NiTi cast alloy with the same composition.
COBISS.SI-ID: 20059926
Patients with higher body mass index treated with fully modular Ti-alloy THA may be at increased risk to experience catastrophic failure of the device. Orthopaedic surgeons should avoid using long necks whenever possible, as these are especially prone to develop a vicious circle starting with the fretting process and crevice corrosion at the taper-cone connection, leading to crack initiation and crack propagation, accelerated by the increased vulnerability of the Ti-alloy in biologic media, ultimately ending as fracture at the typical site. Serum Ti concentration may represent a rough estimation of taper degradation and patients with elevated levels should be warned and followed accordingly.
COBISS.SI-ID: 20099094
The objective of the present work is to analyse the solidification process of NiTi binary alloy. The physical medium is taken as an incompressible fluid with variable (i.e. temperature dependent) physical properties where the heat is transferred by conduction and convection, including the thermal phase change phenomenon. The last one is modelled by the improvement procedure called enthalpy-porosity formulation, where the liquid-solid mushy zone is treated as a porous zone with porosity equal to the liquid fraction. The numerical model is based on the finite volume method in body fitted coordinates with a SIMPLER scheme to couple the pressure and velocity fields. The results are presented for the velocity and temperature field, as well as for the NiTi mass fraction during the solidification process.
COBISS.SI-ID: 18864150