This paper presents the application of multi-layer artificial neural networks (ANNs) and backward elimination regression for the prediction of values of the coefficient of friction (COF) of Ti-6Al-4V titanium alloy sheets. The results of the strip drawing test were used as data for the training networks. The strip drawing test was carried out under conditions of variable load and variable friction. Selected types of synthetic oils and environmentally friendly bio-degradable lubricants were used in the tests. ANN models were conducted for different network architectures and training methods: the quasi-Newton, Levenberg-Marquardt and back propagation. The values of root mean square (RMS) error and determination coefficient were adopted as evaluation criteria for ANNs. The minimum value of the RMS error for the training set (RMS = 0.0982) and the validation set (RMS = 0.1493) with the highest value of correlation coefficient (R2 = 0.91) was observed for a multi-layer network with eight neurons in the hidden layer trained using the quasi-Newton algorithm. As a result of the non-linear relationship between clamping and friction force, the value of the COF decreased with increasing load. The regression model F-value of 22.13 implies that the model with R2 = 0.6975 is significant. There is only a 0.01% chance that an F-value this large could occur due to noise.

Modeling of Friction Phenomena of Ti-6Al-4V Sheets Based on Backward Elimination Regression and Multi-Layer Artificial Neural Networks.

This article investigated the mechanical behavior of Ti-6Al-4V alloy (VT6, an analog to Ti Grade 5) in the range of strain rates from 0.1 to 103 s−1. Tensile tests with various notch geometries were performed using the Instron VHS 40/50-20 servo hydraulic testing machine. The Digital Image Correlation (DIC) analysis was employed to investigate the local strain fields in the gauge section of the specimen. The Keyence VHX-600D digital microscope was used to characterize full-scale fracture surfaces in terms of fractal dimension. At high strain rates, the analysis of the local strain fields revealed the presence of stationary localized shear bands at the initial stages of strain hardening. The magnitude of plastic strain within the localization bands was significantly higher than those averaged over the gauge section. It was found that the ultimate strain to fracture in the zone of strain localization tended to increase with the strain rate. At the same time, the Ti-6Al-4V alloy demonstrated a tendency to embrittlement at high stress triaxialities.

Localization of Plastic Deformation in Ti-6Al-4V Alloy

Milling and selective laser melting (SLM) technology have become new options for removable partial denture (RPD) processing. However, whether milling and SLM technology has an impact on the properties of RPD remains unclear, which is also the aim of our study. To investigate the effects of milling and SLM technology on pure titanium, mechanical property, corrosion resistance, and anti-adherence of specimens were evaluated, and specimens processed by lost-wax casting were used as control. Compared with casting and milling groups, the SLM group showed enhanced Vickers hardness (402.1 ± 13.0 HV), tensile stress (694.4 ± 4.5 MPa), and larger electrochemical capacitance arc radius compared with casting and milling groups. A series of adhesion-related genes (Als1, Als3, and HWP1) of Candida albicans cultured on SLM specimens were upregulated for more than two times that of casting and milling groups. However, images from scanning electron microscopy and confocal laser scanning microscopy exhibited similar biofilm morphology and biomass of C. albicans on a titanium disk processed by casting, milling, and SLM. Dwindled water contact angle (64.7 ± 0.6°) and higher TiO2 constituents (40.82%) in the SLM group might lead to the incompatibility of genetic expression and biofilm generation. Our findings indicated that SLM is an ideal process to produce titanium dentures, providing a reference on the selection of processing technology for dentists.

Mechanical properties, corrosion resistance, and anti-adherence characterization of pure titanium fabricated by casting, milling, and selective laser melting.

Based on the modified Hopkinson torsion bar, a high-temperature dynamic shear test method was proposed for the Ti-1023 alloy, and the microstructural evolution of the tested material at different temperatures was studied. By using the modified high-temperature Hopkinson torsion bar, high-temperature testing within 1000 °C can be achieved. As the specimen-heating rate was fast, and the temperature gradient of the experimental environment was small, valid experimental data can be ensured during the experiment. The experimental results show that stress-induced martensite can significantly enhance the strength of the Ti-1023 alloy. Dynamically recrystallized grains similar to those in adiabatic shear bands appear in the microstructure of the Ti-1023 alloy after severe plastic deformation. Therefore, it is possible to regulate the content of stress-induced martensite in the microstructure to improve the mechanical properties of other alloys that are similar to β titanium alloys.

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Experimental Study on the High Temperature Impact Torsional Behavior of Ti-1023 Alloy

Ti–6Al–4V alloy manufactured by selective electron beam melting (SEBM) was heat treated with solution treatment at 1020°C for 2 h and aging at 500°C for 8 h, and the as-printed and heat-treated samples were shock loaded with split Hopkinson pressure bar (SHPB) at different strain rate (1700 s-1and 2700 s-1) and loading directions. The effects of microstructure, phase composition and orientation on adiabatic shear behavior were discussed. The results showed that the adiabatic shear behavior of as-printed Ti–6Al–4V was anisotropic, the differences of critical strain for adiabatic shearing between as-printed longitudinal and radial simples are 0.025 at 1700 s-1 and 0.004 at 2700 s-1 respectively; however, the differences of critical strain decrease to 0.002 at 1700 s-1 and 0.001 at 2700 s-1 after heat treatment, which indicate the heat treatment significantly decrease the anisotropy. The as-printed longitudinal samples with the were more prone to adiabatic shearing than the radial samples, due to the great influence of the orientation of α phase cells on adiabatic shear susceptibility. For example, when the c-axis of α phase cells was parallel to the loading direction, i.e., the as-printed longitudinal sample, it was difficult to active the basal{0001} < 11 2‾ 0 > and prismatic{101‾ 0} < 112‾ 0 > slip systems during shearing and only the pyramidal{101‾ 1} < 112‾ 0 > slip system with higher critical resolved shear stress (CRSS) could be activated, which increased the flow stress and enhances the thermal softening effect, and resulting in a higher adiabatic shear susceptibility of the as-printed longitudinal samples. The adiabatic shear susceptibility of the heat-treated samples in the longitudinal and radial directions was similar, because the orientation of the microstructure became weaker after recrystallization. The heat-treated samples were more susceptible to adiabatic shearing, because the lamellar spacing was reduced and the aspect ratio of the lamellar α phase became larger, which resulted in a larger α-phase interface area compared to that of the as-printed sample, causing greater resistance for dislocation moving during deformation, so the flow stress was elevated and the corresponding thermal softening effect was enhanced, which eventually led to an increase in adiabatic shear susceptibility. 

Effect of heat treatment on adiabatic shear susceptibility of Ti–6Al–4V titanium alloy manufactured by selective electron beam melting

Titanium Ti6Al4V alloy is a superior material that has extremely high strength, hardness and good anti-corrosion resistance. Dynamic shear-compression experiments were carried out on the alloy to investigate the micro-mechanisms of adiabatic shear banding (ASB) formation. The split Hopkinson pressure bar (SHPB) setup were used for the tests at high strain rates. It was found that the shear deformation localization (SDL) was considerably affected by the complex loading conditions. The micro-mechanisms for the ASB formation relied on different shear compressive proportion of loadings (SCLPs). Scanning electron microscope (SEM) observations showed that the ASB width was related with the SCLP and the fracture failure of alloy was induced by the nucleation and growth of microvoids. In transmission electron microscope (TEM) analysis, the microstructural changes of material within the ASB were characterized by dynamic recrystallization (DRX) and twining grain formation, dislocation migration, and stacking and grain refining processes. The results in this article demonstrates a complex image of microstructural evolution of alloy in the shear localization process.

Tao Jin

Papers

Micro-Mechanisms of Shear Deformation Localization of Ti6Al4V Alloy under Shear-Compressive Loading Conditions.