点阴极电场辅助阳极连接中的紧密接触面积和电流

用等效电路的方法研究了点阴极情况下的电场辅助阳极连接效应，计算了连接电流，连接孕育期和紧密接触面积，本文结果表明，连接初期电流迅速衰减，随后缓慢变小，直至某一稳定值，孕育期随温度上升指数下降，连接温度高时，孕育期将难以观察到；待连接表面之间紧密接触面积随时间延长而非线性扩展，紧密接触区域增大时，其边界向外推进速度渐小，而接触面积增加渐快。     

Semi-solid rapid compression testing of spray-formed hypereutectic Al-Si alloys

The rheology of Al-30Si-5Cu and Al-30Si-5Cu-2Mg spray-formed alloys has been characterized after holding at various times and temperatures, using semisolid rapid compression to obtain load-displacement curves. Microstructures were examined after partial remelting and compression in an attempt to understand the flow behavior. The initial resistance to flow generally increased with holding time at temperature. This increase was attributed to the increased strength of a three-dimensional (3-D) solid silicon network present in these alloys in the semisolid state, with the initial resistance to flow smaller in the magnesium-containing alloys. With increased temperature, the initial resistance decreased due to increased liquid content and grain spheroidization. The viscosity vs shear rate response was studied using the approach of Laxmanan and Flemings. The materials shear thinned rapidly during the test conditions, in which the shear rate rapidly increased from approximately 2.5 to 92 s⁻¹. The results were fitted to a power-law model, and the viscosities showed a drop of approximately two to four orders of magnitude when extrapolated to a shear rate of 1000 s⁻¹ (a typical value in actual component manufacture). The rapid compression test, which is close to conditions present in component manufacture, was shown to have more in common with a shear rate jump experiment, rather than steady-state viscometry.     

World’s largest dry gas seal marks five-year milestone

This case study covers what is believed to be the world’s largest dry-running, non-contacting gas seal, manufactured by John Crane. Recently returned for routine maintenance, it has reached its five-year anniversary while remaining in excellent condition.     

A process model for active brazing of ceramics: Part II Optimization of brazing conditions and joint properties

In the present investigation, the process model developed in Part I has been applied to evaluate the microstructure and strength evolution during active brazing of ceramics. As a starting point, reaction-layer growth is assumed to occur isothermally with no restrictions in the supply of reactive element. Different kinds of diagrams are then constructed to show how specific process variables (e.g. the heating and cooling period, the limiting layer thickness, and the diffusion mechanism) affect the growth kinetics. It is concluded that the key to improved joint properties lies in control of the reaction-layer thickness through optimization of the brazing conditions, and an illustration of this is given. This revised version was published online in November 2006 with corrections to the Cover Date.