Influence of introducing Zr,Ti,Nb and Ce elements on externally solidified crystals and mechanical properties of high-pressure die-casting Al–Si alloy

High pressure die casting(HPDC)AlSi10Mn Mg alloy castings are widely used in the automobile industry.Mg can optimize the mechanical properties of castings through heat treatment,while the release of thermal stress arouses the deformation of large integrated die-castings.Herein,the development of non-heat treatment Al alloys is becoming the hot topic.In addition,HPDC contains externally solidified crystals(ESCs),which are detrimental to the mechanical properties of castings.To achieve high strength and toughness of non-heat treatment die-casting Al-Si alloy,we used AlSi9Mn alloy as matrix with the introduction of Zr,Ti,Nb,and Ce.Their influences on ESCs and mechanical properties were systematically investigated through three-dimensional reconstruction and thermodynamic simulation.Our results reveal that the addition of Ti increased ESCs'size and porosity,while the introduction of Nb refined ESCs and decreased porosity.Meanwhile,large-sized Al_3(Zr,Ti)phases formed and degraded the mechanical properties.Subsequent introduction of Ce resulted in the poisoning effect and reduced mechanical properties.

Explosion Event of Cavitation in Intact Xylem Conduit

Cavitation in plant conduits only involves two processes of air bubbles: the gradual expansion and elongation, and the explosion event. An explosion event of cavitation, which can only occur in intact conduit at water tension, trigs acoustic (or ultrasound) emission and induces air to diffuse with high speed, simultaneously. Synchrotron X-ray phase contrast microscopy (XPCM) was used to capture cavitation event in intact conduits of leaves of corn and rice. Cavitation events occur in certain areas of leaves and have a certain time frame. Before XPCM experiment, several preliminary experiments were done as follows: 1) Paraffin sections of leaves of different species were observed to select samples and to determine the occurrence area of cavitation event of leaves. 2) The time frame of cavitation occurrence was determined by ultrasonic emission. 3) The water potentials of leaves were determined, to know the water state of the leaves during cavitation. Locked the area and time frame of cavitation event in the leaves, consecutive XPCM images of cavitation process were more easily acquired. The images show that the phenomenon of gas bubble fully filling conduits for an instant took place in intact conduits of detached leaves of corn and rice more easily. It is that the gas diffusing in a moment was caused by the explosion of the air seeds which had entered in the intact conduits of the leaves. For living plants, it is suggested that the explosion event of cavitation is the most important for embolism formation.

In situ study on the oscillation of mobile droplets and force analysis during the directional solidification of Al-Bi alloy

During solidifications of immiscible alloys,the motion of droplets at the solid/liquid(S/L)interface is gen-erally driven by dragging force,gravity force,repulsion force of interface,and thermal-solutal Marangoni force,However,there is few in situ study investigating kinetics behavior to analyze the forces on droplets.The mechanism of droplet motion remains unclear due to the unavailability or uncertainty of the effect of convection and solutal Marangoni force on droplet behavior.In this study,directional solidification of im-miscible Al-Bi alloy was observed via synchrotron radiography,and the horizontal oscillation of droplets at S/L interface was detected for the first time.Forces,especially solutal Marangoni force,were calcu-lated based on the in situ measured radius of droplets and thermal-solutal gradients.The experimental results cannot be reasonably explained by the previous analysis model which neglects melt convection.The non-negligible effect of flow on droplet motion was demonstrated,and the force balance of droplet both vertically and horizontally can be obtained considering a lift force of 6.39 × 10^(-9) N and a modified solute-related parameter dσ/dc of 0.45-0.65 J m^(-2),respectively.