Junjie Wang

Effect of cryogenic treatment on wear resistance of Ti-6Al-4V alloy for biomedical applications.

The effect of cryogenic treatment on wear resistance of Ti-6Al-4V alloy for biomedical applications was experimentally investigated in this paper. Cryogenic treatments with the same soaking time of 24h at different temperatures of -80°C, -140°C and -196°C were conducted and the treatments at the same temperature of -196°C were then further given different soaking time of 3h, 48h and 72h to be investigated. After cryogenic treatment, the Vickers hardness of specimens was measured. Wear resistance of Ti-6Al-4V alloy was measured by pin-on-disk wear test under dry sliding condition. The results demonstrated that the Vickers hardness increased slightly with the reduction of temperature while it increased obviously with the elongation of soaking time at -196°C. The friction coefficients of specimens cryo-treated at -196°C were lower than those of untreated and of cryo-treated at -80°C and -140°C. And the longer the soaking time is during the cryogenic treatment, the higher the friction coefficient reduction can be achieved. The obvious reduction of mass loss can be obtained at -196°C with 72h soaking. The X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to detect the microstructure and worn surface of specimens. By cryogenic treatment, the plowing in the worn surface was smoothed and shallowed, and the degree of plastic deformation in the subsurface was decreased. There was no obvious phase transformation which can be detected in the microstructure after cryogenic treatment. However, the tendency of refinement in grain size can be detected by XRD which improved the wear resistance of Ti-6Al-4V alloy.

CFD study of heat transfer for oscillating flow in helically coiled tube heat-exchanger

Abstract The heat transfer and pressure drop for oscillating flow in helically coiled tube heat-exchanger were numerically investigated based on the Navier–Stokes equations. The correlation of the average Nussel number and average friction factor were proposed considering the frequency and the inlet velocity. The oscillating flow heat transfer problems are influenced by many factors. Hence we need an easy way to reduce the numbers of simulation or experiment. Therefore, the method of uniform design was adopted and the feasibility of this method was verified. The field synergy principle was used to explain the heat transfer enhancement of oscillating flow in helically coiled tube heat-exchanger. The result shows that the smaller the volume average field synergy angle in the helically coiled tube, the better the rate of heat transfer.

Jumping liquid metal droplet in electrolyte triggered by solid metal particles

We report the electron discharge effect due to point contact between liquid metal and solid metal particles in electrolyte. Adding nickel particles induces drastic hydrogen generating and intermittent jumping of a sub-millimeter EGaIn droplet in NaOH solution. Observations from different orientations disclose that such jumping behavior is triggered by pressurized bubbles under the assistance of interfacial interactions. Hydrogen evolution around particles provides clear evidence that such electric instability originates from the varied electric potential and morphology between the two metallic materials. The point-contact-induced charge concentration significantly enhances the near-surface electric field intensity at the particle tips and thus causes electric breakdown of the electrolyte.

A volatile fluid assisted thermo-pneumatic liquid metal energy harvester

A close-cycle self-driving thermal energy harvester using liquid metal as energy carrier fluid has been proposed. The driving force that pushes the liquid metal against flow resistance and gravity is provided by a resistively heated volatile fluid based on thermo-pneumatic principle. The tested harvester prototype demonstrated its capability to extract thermal energy between small temperature gradient, at a scale of 10u2009°C. During a 5-h operation, it further demonstrated robust liquid metal recirculating performance at a time-average volume flow rate of 14u2009ml/min with a 12.25u2009W heating load. The prototype also managed to self-adjust to variable working conditions which indicated the reliability of this method. Advantages of this method include simple-structural design, rigid-motion free operation, and low-temperature actuation. These advantages make it uniquely suited for solar energy and low-grade heat harvesting, high heat flux electronics cooling, as well as autonomous machines actuating.

Liquid metal actuated ejector vacuum system

An ejector vacuum system using nontoxic room-temperature liquid metal as actuating fluid was fabricated and experimentally demonstrated. With physical merits of high density, fluidity, and ultralow vapor pressure, the liquid metal could serve as a high momentum density carrier fluid. The main performance parameters of the liquid metal actuated ejector were thus found to be significantly improved by orders superior as compared with that of water actuating system. Under oxidation protection, an ultimate vacuum pressure of 170u2009Pa was achieved and capacity of evacuating a 500u2009ml nitrogen reservoir from atmospheric pressure to 480u2009Pa within 96 s was obtained.

The Effect of Cryogenic Treatment on the Microstructure and Properties of Ti-6Al-4V Titanium Alloy

The effect of cryogenic treatment on the microstructure and properties of Ti-6Al-4V has been studied in this paper. The program controlled SLX cryogenic box was used to conduct the cryogenic treatment and the subsequent low temperature temper. The scanning electron microscope was used to study the morphology of microstructure and fracture surface. As the results show that the cryogenic treatment increases the elongation of Ti-6Al-4V from 16.5 percent to 24.5 percent, at the same time, the strength increases slightly, this indicates that cryogenic treatment can improve the comprehensive mechanical properties. The microstructure measurement revealed that there is a tendency of reduction in the precipitated particles after cryogenic treatment. The cross section is flat and the size of dimples is more uniform. It is concluded that the change in the precipitation particle had a great influence in the mechanical properties.

Effect of Cryogenic Treatments on Mechanical Properties of 2A11 Aluminum Alloy

Effect of cryogenic treatments on mechanical properties of 2A11 aluminum alloy were studied. Cryogenic box with program control and electrical resistance furnace were employed to perform cryogenic treatment processes as well as heat treatment processes. Impact tester, tensile tester, three-coordinates measuring machine, high precision caliper and standard metallography were used to carry out tests. The influences of different process parameters on mechanical properties of 2A11 aluminum alloy were compared, and the results showed that cryogenic treatment could improve mechanical properties of 2A11 aluminum alloy. The dimensional stability increases after cryogenic treatment once, and increases further after cryogenic treatment again.

Effect of cryogenic treatment on the plastic property of Ti-6Al-4V titanium alloy

The effect of cryogenic treatment on the plastic property of Ti-6Al-4V plate was studied in the present work. After cryogenic treatment, the low temperature temper at 180 ▭ was conducted in one of the groups and the results were compared with that of the untreated and cryotreated ones. The SLX series program controlled cryogenic equipment was used for the cryogenic treatment. The tensile tests were conducted by universal tensile testing machine and parameters of elongation and area reduction were used to evaluate plastic property. The scanning electron microscope was used to study the morphology of microstructure and fracture surface. The results show that after cryogenic treatment alone the elongation increased 10.6% and the area reduction increased 13.5% while the strength reduced to a small extent. Cryogenic treatment followed with low temperature temper increased the elongation and area reduction just by the extent of 4.7% and 9.5%. It means that the additional low temperature temper after cryogenic is not beneficial to the tensile properties of Ti-6Al-4V alloy. The examination of microstructure by scanning electron microscopy revealed that cryogenic treatment reduced the content of β phase particles which is the main reason for the improvement in plasticity.

A novel Stirling type pulse tube cryocooler suppressing the third type of dc gas flow

This paper analyzes the zero and first order pressure drop along the pulse tube system and presents a simple but elegant way of eliminating the third type of DC gas flow which increases the efficiency of a Stirling type PTC. Since the optimization of a PTC depends sensitively on the adjustment of the phase angle between the pressure and mass flow because of its influence on the suppression of the third type of DC gas flow, we have done a series of experiments on a Stirling type multi-bypass PTC in order to attain the lowest refrigeration temperature. After the multi-bypass PTC reaches its lowest temperature, we open the metering valve to the best degree which connects the back pressure chamber of the linear compressor to the reservoir. For this novel type of PTC, the lowest refrigeration temperature drops from 38.8 K to 25.9 K after opening the metering valve. This approach offers a promising way for a compact single stage Stirling type PTC to reach liquid hydrogen temperatures.

Effect of cyclic cryogenic treatment on the properties and structure of 40CrNiMoA steel

In this work, the effect of different treatment processes on properties and structure of 40CrNiMoA steel after temperature cycling experiments were studied. Different treatment processes contained that heat treatment (HT), cryogenic treatment before tempering (CT-1) and cryogenic treatment after tempering (CT-2). After that, some samples of the different treatment processes were done by temperature cycling experiments, which was cooled from room temperature to −190°C for 4 hours, then heated from −190°C to 125°C for 4 hours, at last cooled to the room temperature. The temperature cycling experiments were repeated fifteen times. The rate of cooling and heating was 4°C/min. Both the quenching and tempering were done in a vacuum furnace. All the cryogenic treatments and temperature cycling experiments of fifteen times were done in a program-controlled cryogenic container. The results show that, the strength and plasticity achieve the optimal matching, when 40CrNiMoA steel was done by cryogenic treatment afte...