Zhan Lihua

Effect of Stress-Aging Treatments on Precipitates of Pre-retrogressed Al-Zn-Mg-Cu Alloy

Abstract According to retrogression and re-aging treatment (RRA), a novel retrogression and stress-aging treatment (RSA) was put forward for Al-Zn-Mg-Cu alloys. The effect of stress-aging time and external stress on precipitates of pre-retrogressed Al-Zn-Mg-Cu alloy was investigated by transmission electron microscope (TEM). The observation shows that there is a large number of matrix precipitates (MPts) and slightly discontinuous grain boundary precipitates (GBPs) in the pre-retrogressed alloy. The effects of stress-aging time and external stress on precipitates of the retrogressed alloy are significant. With the increase of stress-aging time and external stress, the size of MPts increases and the density of MPts decreases. Meanwhile, the size of GBPs, the distance between GBPs and the width of precipitate free zone (PFZ) increase. Compared with the traditional retrogression and re-aging treatment, the retrogression and stress-aging treatments increase the size of MPts, narrows the width of PFZ and make the GBPs more discontinuous for the studied Al-Zn-Mg-Cu alloy.

Formation, Influence Mechanism and Experimental Characterization of Composite Porosity

Abstract In view of the common issue of composite porosity, the formation and influence mechanism of composite porosity were elucidated from the aspects of mechanical mixed nucleation theory and classic nucleation theory, and the characterization of porosity morphology, distribution and size were conducted through specific experiments. Results show that poor wettability of fibers, retention of vapor molecules and molecular volatilization generated during crosslinking reaction are the main reasons of porosity formation. The lower cure pressures have little effect on eliminating the porosity. With the increase of cure pressure, the porosity factor and porosity size decrease significantly. There is a power function relation of three times among porosity factor, porosity diameter and cure pressure.

On synchronization control strategy of large scale water press’s unloading procedure

Moving beam of the 300 MN large scale water press is properly damage the poles by bigger deflection angle which is brought by changing eccentric load during the time that the driving system is discharging. Eccentric load can be indirectly measured by pressure sensor because center of forging blank deforming drag change little, consequently moving beam position holding system can be discharged simultaneously when the driving system is discharged. The results of simulation and experiment both showed that this method is very useful to decrease the maximum deflection angle when driving system is in the state of being discharged.

Rheological behavior of continuous roll casting process of aluminum alloy

The rheological behavior of aluminum alloy and its influencing factors in physical simulation of continuous roll casting process were studied by using a Gleeble-1500 thermal-mechanical simulation tester with a set of special clamp system. The relationships between the flow stress and the strain rate in the deformation process of simulating roll casting experiment were obtained. The results show that four different characteristic stages exist in the temperature range of the whole rheological process. The first occurs when the temperature is higher than 600 °C, which belongs to the creep deformation stage; the second occurs when the temperature lies in the range of 500–600 °C, and it can be regarded as the high temperature and low stress level deformation stage; the third occurs when the temperature decreases to the range of 300–500 °C, it is considered to be the middle stress level deformation stage; the last occurs when the temperature is less than 300 °C and the strain rate is less than 1.00 s−1, it belongs to middle stress level deformation stage. But when the strain rate is larger than 1.00 s−1, it belongs to the high stress level deformation stage. And the relative constitutive models suitable for the four different stages of continuous roll casting process were established through multivariate linear regression analysis of the experimental data.

Stress Concentration Numerical Analysis of a Panel with Big Grooves

To improve the fatigue life of a panel with grooves, the stress relief groove is added after machining the tangent round transition structure. The purpose of this method is to reduce the stress concentration at the edges of rounded transition structure in the present study. To confirm the effectiveness of the method, analyses were performed by using software for two-dimensional elastic problems based on the body-force method. The calculation results show that the existence of stress relief grooves further reduced the stress concentration at the edges of rounded transition structure. A close distance between two stress relief grooves results in little influence on stress concentration at the edge of rounded transition structure and much stress reduction at the edge of stress relief grooves. Also, a lower stress concentration was obtained by increasing the diameter and numbers of stress relief grooves. Results prove the effectiveness and certain engineering practicability of this method. The method is helpful for structure design.