Materials Science Forum Vols. 628-629

Abstract: During plasma spray forming, the forming quality is directly influenced by the flattening behavior of the droplet impacted onto the substrate, which is determined by the flattening configuration and solidification state of the droplet. In this study, according to the rule of Reynolds number and Peclet number, the flattening behavior of big-size molten droplet impacted onto the substrate with low-speed were experimentally simulated during flattening. The droplets of Sn30Pb70, Zn and ZA12 impacting onto the stainless steel substrate with different velocity were investigated. Splashing degree was introduced to evaluate the flattening profiles of the splats. The relationships between the splashing degree and the impact velocity, Reynolds number and Weber number of the droplet were established. Experimental results show that the flattening and diffusion are impeded by the dynamic viscosity and surface tension of the droplets, and the impeding effects are more remarkable when the impact velocity is lower. The research has a significant ap

Abstract: Zinc-alloy with low melting point can be easily sprayed directly on the production surface to fabricate a mould with high precision by arc spraying. But the zinc-alloy mould has the disadvantage of easy invalidation due to its relatively low hardness and poor anti-wear property. In this paper, the surface modification of zinc-alloy tooling by composite electro-brush plating was researched. The joining mechanism between the plating film and sprayed coating was investigated, and the law of quality control on the unification of rapid tooling and surface modification was revealed. A new modification technology possessing the advantage of economical and high quality surface relating to moulding was developed with independent intellectual property right.

Abstract: The wear resistance, corrosion resistance and hardness can be greatly increased by using low temperature QPQ salt-bath nitriding treatment. And it is a new strengthening method without distortion in the treating process. In this paper, 3Cr2W8V steel is dealt with QPQ salt-bath nitriding at 520°C, 540°C, 560°C and 2h, 4h, 6h, respectively. The treated surface microstructure was analyzed by using SEM. The depth of nitriding layer, scratch hardness and wear-resistance were tested for both QPQ salt-bath treated and untreated specimen. The corrosion resistance was tested in the 5%NaCl water by using spraying method. The experimental results indicate that a certain depth of white layer and diffusion layer of the steel can be obtained by using low temperature QPQ salt-bath nitriding treatment. The nitriding compound layer with high hardness, superior wear-resistance and stable microstructure can also be obtained on the surface of the parts. With increasing the temperature and the nitriding time, the depth of nitriding layer, scratch hardness and wear-resistance of 3Cr2W8V steel were greatly increased. Comparing with the untreated specimen, its hardness enhances 95.0%, wear resistance enhances 212.3%, anti-corrosion enhances 1288.9%. The experimental results show that the low temperature QPQ salt-bath nitriding treatment has many advantages, such as fast nitriding speed, uniform heating, short process time, low treating temperature, small distortion, high production rate, low cost, stable nitriding quality without pollution and so on.

Abstract: The present work proposes a novel plasma cutting system using the ultrasonic frequency pulsed arc, and then experimentally investigates the effect of pulse process parameters on cut quality in the frequencies of 20-60 kHz. It is shown that kerf widths, bevel angle, straightness and dross attached level decrease obviously with the appropriate increases in the frequency and amplitude of pulse current. Furthermore, cutting heat affected zone narrows and cut hardness drops clearly in the pulsed cutting. This current pulsation can thus improve plasma cut quality remarkably, and finally leads to a narrower, flatter and more perpendicular cut of better remachinability. Experimental results simultaneously demonstrate that the effectiveness of the ultrasonic-frequency pulsed cutting process developed at the low current of arc.

Abstract: Compare to atmosphere die casting or moderate vacuum die casting process, use of the high vacuum die casting can enhance excluding gas volumes in the mold, improve the casting mechanical properties and density, and increase the fabrication feasibility for casting small feature size, shape, and higher quality of product. The process of high vacuum die casting for dumbbell specimens is established and implemented for evaluation of this innovative manufacturing technology. A transparent acrylic mold is fabricated to investigate the vacuum injection for vacuum die casting process and a metal mold is fabricated to test the real manufacturing of high vacuum die casting. To visualize the flowing pattern inside the mold under the consequence of evacuation processing, flow visualization experiments are conducted. Then, the specimen is molded by the high vacuum die casting process and the specimen castings are test by a universal testing machine for its loading-elongation relationship. The porosities of specimen casting are examined by scale weighting and direct visual inspection ways. Experimental results show that the high vacuum die casting process is effective to reduce the porosity (5.47% increment in weight) and to increase the strength (4.83% increment in maximum stress) of the investigated specimen. An obviously vortex roll is established along the centerline position of dumbbell specimen which may be became the source of porosity in real processing. Meanwhile, an excellent vacuum sealing is critical to the success of high vacuum die casting.