Refined Spherulites of PP Induced by Supercritical N2 and Graphite Nanosheet and Foaming Performance.

The isothermal crystallization properties of polypropylene/graphite nanosheet (PP/GN) nanocomposites under supercritical N2 were systematically studied by a self-made in situ high-pressure microscope system. The results showed that the GN caused irregular lamellar crystals to form within the spherulites due to its effect on heterogeneous nucleation. It was found that the grain growth rate exhibits a decreasing and then increasing trend with the enhancement of N2 pressure. Using the secondary nucleation model, the secondary nucleation rate for spherulites of PP/GN nanocomposites was investigated from an energy perspective. The increase in free energy introduced by the desorbed N2 is the essential reason for the increase in the secondary nucleation rate. The results from the secondary nucleation model were consistent with those acquired through isothermal crystallization experiments, suggesting that the model can accurately predict the grain growth rate of PP/GN nanocomposites under supercritical N2 conditions. Furthermore, these nanocomposites demonstrated good foam behavior under supercritical N2.

Constitutive model of 6063 aluminum alloy under the ultrasonic vibration upsetting based on Johnson-Cook model.

Establishing an accurate constitutive relation in ultrasonic vibration assisted metal forming, can provides a reliable theoretical basis for analyzing the mechanism of the ultrasonic vibration on materials. A constitutive model of 6063 aluminum alloy under the ultrasonic vibration upsetting at room temperature was constructed based on Johnson-Cook Model and experimental results. The influence of amplitude and frequency on the yield strength, hardening coefficient and exponent were analyzed quantitatively. Results showed that the yield strength was reduced due to the softening effect induced by the ultrasonic vibration. The maximum decreasing amount was 68.8% when imposing the maximum ultrasonic energy in this study. The hardening coefficient and exponent increased by 10.9% and 16.6% in maximum, respectively. However, the ultrasonic vibration has little impact on the strain rate hardening. The modified Johnson-Cook constitutive model under the ultrasonic vibration upsetting was established and was in good agreement with the experimental results.

A nanocrystalline-amorphous mixed layer obtained by ultrasonic shot peening on pure titanium at room temperature.

A nanocrystalline-amorphous (NC-A) mixed layer was obtained by ultrasonic shot peening (USP) on pure titanium at room temperature and observed by X-ray diffraction pattern (XRD), scanning electron microscope (SEM) and high-resolution transmission electron microscope (HRTEM). The results showed that the amorphization percentage in the NC-A mixed layer increased continuously with the increase of the peening duration, shot diameter and sonotrode amplitude or the decrease of the peening distance. The maximum amorphization percentage achieved in this study was 44.09%. Moreover, with the amorphization percentage in the NC-A mixed layer increasing, the surface hardness increased constantly. Base on the experimental results, the amorphization mechanism during USP treatment was also analyzed.

Effects of ultrasonic vibration on the compression of pure titanium.

The technology of ultrasonic vibration assisted plastic forming possesses a great many merits, such as reducing the deformation resistance and friction, as well as improving the surface quality of parts. In this study, the ultrasonic vibration assisted compression tests were carried out on pure titanium in order to improve its formability. The results indicating that the ultrasonic vibration had no effort on elastic deformation, and the temperature of material only increased by 6 °C after compression with applying the ultrasonic vibration. Therefore the influence of temperature increase on reduction of flow stress could be ignored. After excluding interface friction and temperature effects, ultrasonic vibration can still decline the flow stress, the mechanism of deformation includes ultrasonic softening, stress superposition and strain hardening. In the intermittent vibration tests, the material shows the residual softening effect after stopping vibration. By observing the microstructure of material with SEM, it shows that the ultrasonic vibration can promote the generation of deformation twins, causing the grain refinement and the reduction of the twins, which is the major factor of affecting the residual softening effect.