Z.L. Peng

Production of short-lived positron sources for spin-polarized positron beams using a 35 MeV α-beam

Abstract Various short-lived positron sources for polarized positron beams have been on-line produced by irradiating nickel, aluminum, and silicon targets for the first time using a 35 MeV alpha beam from the RIKEN AVF cyclotron. The β+ rays emitted from the targets with wide energy spread are converted to a monochromatic slow positron beam using a 10 μm tungsten polycrystalline moderator. In the current stage, ∼ 104 e+/(s μA) can be obtained from nickel and aluminum targets, which is in relatively good agreement with the calculated slow positron yield. For silicon, a relatively low slow-positron intensity, ∼ 103 e+/(s/μA) was obtained probably due to the charge up effect in the silicon surface during irradiation. For increasing the slow-positron yield loss of fast positrons in the material must be avoided, and as a new means for such improvement it is proposed to use gas targets.

Study on Micro Reversible Electrical Discharge Machining Method for the Fabrication of Micro Structures

A reversible machining method using micro electrical discharge machining (EDM) was developed. This new method can achieve depositing or selective removing of metal material for the fabrication of micro structures. It is easy to transform the machining process from deposition to removal in one EDM machining system. In micro EDM deposition process, brass, tungsten and steel material can be deposited successfully. The deposited material has compact fine texture and combines close to workpiece. Then, micro complex structures by series deposition strategy and sub-deposition strategy were deposited. In the selective removal process, the machining effects of different working mediums were researched. Results show that the machining effect in liquid medium is better than that of in air. Finally, using the micro reversible EDM process, a micro square column with 0.070mm in side length, 0.750mm in height and a micro cylinder with 0.140mm in diameter, 1.180mm in height were fabricated.

Electrochemical Micro Machining of Stainless Steel with Voltage Pulses

The purpose of this paper is to study electrochemical micro machining (ECM) technology on stainless steel. The micro machining of stainless steel is difficult by electrochemical machining, especially in machining deep micro holes, because of an oxide layer formed on the surface. To machining stainless steel, HF is usually used in electrolyte to destabilize the oxide layer. In this paper, ECM of stainless steel by applying short pulses in less toxic and corrosive electrolyte is research. The influence of electrochemical machining parameters such as voltage, current, electrolyte composition to machine stainless steel was investigated. The results showed that the oxide layer can be reduced by adding chloride and complex.

The Dissolution Mechanism of Copper Weld Brazing with Cu-Based Brazing Alloys

In this paper, the dissolution mechanism of copper weld brazing has been researched. The thickness losses of Cu foil in contact with molten Cu-P and Cu-Ag binary alloys at different temperatures have been studied. And the dissolution rate constants in both alloys is calculated and exists following relation: kCu-P (T)=10kCu-Ag (T), which explains the special phenomenon that the dissolving amount of copper in Cu-P liquid alloys is larger than that in Cu-Ag liquid alloys by using weld brazing technology. The dissolution rate of copper in filler metals is the main reason to realize weld brazing. It can be concluded that element P is indispensable in filler metals as the function of accelerating dissolution during weld brazing.

Study on the Characteristics of Discharge Waveform in Micro EDM Deposition Process

Based on the analysis of micro electrical discharge machining (micro EDM) principle, the process conditions of micro EDM deposition has been obtained. Micro EDM deposition is a new EDM method, in which the process conditions includes selecting air as working medium, short pulse duration, long pulse interval, low discharge current and setting the tool electrode as anode. In micro EDM deposition experiments, two types pulse generators of transistor type and RC type are applied respectively. The characteristics of discharge waveforms using each type pulse generator are researched. Results show that both two types pulse generators can be applied in the micro EDM deposition process. The transistor type is easy to obtain the same single discharge energy, but short circuit will damage the deposited material. While RC type can adjust discharge energy according to the discharge gap state, which is well used in micro EDM deposition process with the high deposition rate.

GTA Welding of Copper Thick Plates by Using ERCuTi Welding Materials

The new welding material – ERCuTi alloys filler metals were developed specially for gas tungsten arc welding (GTAW) of copper. The hot cracking in welding copper is inhibited completely as the addition of de-oxidant element Ti in copper welding material. The degree of addition of Ti (2-4wt%) is critical when the susceptibility of cracking is to be suppressed. If the level is allowed to exceed 4wt%, more low-melting point eutectics (β-TiCu4 and TiCu2) will be formed in the welds, and cracking susceptibility will be increased again. Results of mechanical properties tests show that although adding Ti increases the hardness and strength of the weld compared to the base metal, the impact ductility and the plastic properties are not decreased significantly.

Study on Micro Growth Process and Characteristics of Deposited Material in Micro EDM Deposition

The micro growth process and characteristics of deposited material in micro EDM deposition are researched in this paper. In the initial stage of the deposition the deposited material is not an even layer in the diameter direction of tool electrode. The emergence of deposited material will affect the next pulse discharge distribution between tool electrode and workpiece. A micro cylinder with the diameter of 85µm, height of 255µm formed successfully with the fine surface quality using the tool electrode diameter of 200µm. It has shown that micro EDM deposition process has the ability to fabrication more finer micro structures with thinner electrode. The characteristics of deposited material including bonding behavior between deposited material and workpiece, energy spectrum analysis, hardness test and resist compression test of the deposited material have been tested. Results show that the deposited material has compact microstructure and bonds close to the workpiece, which can be used in the occasion of withstanding compress.