Progress in the application of cold gas dynamic spraying to repairing continuous casting molds

A new continuous casting mold repairing method--cold gas dynamic spraying （CGDS） is introduced. The study investigates the advantages of the CGDS process regarding repairing operation, such as convenient, in-situ repairation,little heat delivery, microstructural and dimensional stability and other special applications. Microstructure and mechanical properties of the copper alloy coating, nickel coating, ceramic composite coating, and their interface to the substrates ,which are usually used in repairing operation have been researched by means of optical microscopy （ OM）, scanning electron microscopy （SEM）, transmission electron microscopy （TEM） and micro-hardness tests. Experimental results have demonstrated the relative density of the copper alloy coating is as high as 98.7%, and that no obvious difference can be observed between the CrZrCu substrate and the Cu alloy coating in terms of microstructures; thus the interface is quite difficult to be identified. The bonding strength and micro-hardness of the Cu alloy coating reach up to 37 MPa and 310 HV0.2 ,respectively. The interface between the copper alloy coating and the nickel coating is either zigzag or wave shaped, and the cohesion is relatively good. As-sprayed nickel coating is dominated by severely deformed particles,and the relative density is up to 98.5%. Complete recrystallization occurred after annealing at 900℃ for one hour,while its micro-hardness remains as high as 124.1 HV02. All these results have indicated that CGDS is a promising technology for repairing the continuous casting mold and that its future development is prosperous as well.

Numerical simulation of the effects of the impact velocity on the particle deposition characteristics in cold gas dynamic spraying

In this study, the effects of the impact velocity on the particle deposition characteristics in cold gas dynamic spraying （CGDS） of 304 stainless steel （SS） on an interstitial free （IF） steel substrate are numerical simulated by means of a finite element analysis （FEA）. The results have illustrated that when the particle impact velocity exceeds a critical value at which adiabatic shear instability of the particle starts to occur. Meanwhile, the fatten ratio and impact crater depth （or the effective contacting area ） increase rapidly. The particle-substrate bonding and deposition mechanism can be attributed to such an adiabatic shear deformation induced by both the compressive force and the slide friction force of particle. The critical velocity can be predicted by numerical simulation, which is useful to optimize the CGDS processing parameters for various materials.

Electromagnetically Induced Transparency in a Cold Gas with Strong Atomic Interactions

Electromagnetically induced transparency （EIT） is investigated in a system of cold, interacting cesium Rydberg atoms. The utilized cesium levels 6S1/2, 6P3/2 and nD5/2 constitute a cascade three-level system, in which a coupling laser drives the Rydberg transition, and a probe laser detects the EIT signal on the 6S1/2 to 6/23/2 transition. Rydberg EIT spectra are found to depend on the strong interaction between the Rydberg atoms. Diminished EIT transparency is obtained when the Rabi frequency of the probe laser is increased, whereas the corresponding linewidth remains unchanged. To model the system with a three-level Linclblad equation, we introduce a Rydberg-level dephasing rate γ3 = κ×（P33/Ωp）^2, with a value κ that depends on the ground-state atom density and the Rydberg level, The simulation results are largely consistent with the measurements. The experiments, in which the principal quantum number is varied between 30 and 43, demonstrate that the EIT reduction observed at large Ωp is due to the strong interactions between the Rydberg atoms.

Microstructure and micro-hardness of the Inconel 718 superalloy coating prepared by cold gas dynamic spraying

Inconel 718 superalloy coating was prepared on a martensitic steel substrate by cold gas dynamic spraying （ CGDS ）. Microstructure and micro-hardness of the coating were characterized by means of scanning electronic microscope （SEM） ,energy dispersive spectrometer （EDS）, and micro-hardness and nano-indentation tests. The results have indicated that the coating＇ s thickness can reach more than 300 um, and there exists a good interfacial cohesion between the coating and the substrate. The quantity and size of the defects at the bottom, middle and top areas increase gradually. There is no significant change to the original microstructure of the Inconel 718 superalloy particle after the process of CGDS ;and an obvious torsional deformation on the particle＇ s profile is produced while little deformation at the center part occurs. The micro-hardness of the coating at the bottom, middle and top areas decreases in turn. Compared with the bottom area, the micro-hardness of the middle and top areas decreases by about 10% and 21%, respectively. The nano-hardness of the coating is much lower than the original particle,which decreases by about 13.5% at the bottom area and 28% at the top area,respectively. The distribution of micro-defects is an important factor to the micro-hardness of the coating.

Influence of the preheated substrate temperature on the microstructure deposition behavior of the 304 stainless steel coatings deposited by cold gas dynamic spraying

The effects of the substrate temperature on the deposition and microstructure of the 304 stainless steel （SS） cold gas dynamic spraying （CGDS） coatings were investigated. It was found that the higher substrate temperature could increase the deposition rate of the 304 SS particles,even the oxide films existed at the interface. There was a critical oxide film with a thickness of 3 -4 um which could be destroyed by the impacted particles. The micro-hardness and microstructure of the 304 SS coatings under different substrate preheating temperatures were almost the same.

Study on the 316L stainless steel coatings and their properties by cold gas dynamic spraying

316L stainless steel coatings were prepared by cold gas dynamic spraying （CGDS） on a carbon steel. A new heater of sprayed powders was added to the traditional facility. The results show that the coating quality is obviously improved, due to better interface bonding and reduced porosity.

Numerical simulation of particle tracks in the cold gas dynamic spraying process

In this study,the distribution behavior of the particle flow field in cold gas dynamic spraying （CGDS） was simulated through the Computational fluid dynamics （CFD） method. Traces of the particles with different diameters in the gas flow field were analyzed, and effects of fiat and sphere substrates on the particle tracks were also compared .Simulation results indicate that different escaping directions of particles flow with the two substrates. These investigations. gave instructions on how to design the powder recovery and dusting machines in a CGDS system.

Effects of standoff distance on the supersonic flow fields in cold gas dynamic spraying

In this study,the effects of standoff distance （SOD） on the supersonic flow fields （including gas and particle flow fields） during cold gas dynamic spraying （ CGDS ） are investigated by means of computational fluid dynamics （CFD）. The variation of velocity,temperature,pressure and density with different SoD is elucidated through the analysis of the distribution properties of the flow fields. It is found that the shock waves in front of the substrate remarkably influence the gas and particle flow fields. The wave system of expansion waves and pressure waves come into being continuously. The velocity of gas reaches the super.,;onic speed at the position of the Mach disc, while it decreases sharply when the gas flow crosses the Mach disc. The optirnal SoDs are 40 mm for 1 um particles and 50 mm for both 5 um and 22 um particles.

INFLUENCE OF COLD NITROGEN GAS AND OIL MIST IN MACHINING NICKEL-BASE K424 ALLOY WITH CERAMIC CUTTING TOOLS

The role of cold nitrogen gas and oil mist on tool wear and surface roughness is investigated in turning the K424 nickel-base super alloy with Sialon and SiC whisker-reinforced alumina ceramic tools. A new cooling system is developed and used to lower the temperature of the compressed nitrogen gas. Experiments are performed in three different cooling/lubrication modes, i.e. the dry cutting, the cold nitrogen gas （CNG）, and the cold nitrogen gas and oil mist （CNGOM）. Experimental results show that the depth-of-cut notching severely limits the tool life in all the cooling/lubrication modes. Compared with the dry cutting, the use of CNG and CNGOMcan yield higher wear rate of depth-of-cut notching and worse surface finish.

Elastic Modulus of 304 Stainless Steel Coating by Cold Gas Dynamic Spraying

304 stainless steel coating was deposited on the IF steel substrate by cold gas dynamic spraying （CGDS）, and the elastic modulus of the 304 stainless steel coating was studied. The elastic modulus of cold sprayed 304 stain- less steel coating was measured using the three-point bend testing and the compound beam theory, and the other me- chanic parameters （such as the equivalent flexural rigidity and the moment of inertia of area） of the coatings were also calculated using this compound beam theory. It is found that the calculated results using the above methods are accu- rate and reliable. The elastic modulus value of the cold sprayed 304 stainless steel coating is 1. 179 X 105 MPa, and it is slightly lower than the 304 stainless steel plate （about 2 X 105 MPa）. It indicates that the elastic modulus of the cold sprayed coatings was quite different from the comparable bulk materials. The main reason is that the pores and other defects are existed in the coatings, and the elastic modulus of the coatings also depends on varies parameters such as the feed stock particle size, porosity, and processing parameters.

Fracture behavior of 304 stainless steel coatings by cold gas dynamic spray

304 stainless steel coating was deposited on the Interstitial-Free steel substrate by cold gas dynamic spraying （CGDS）. Three-point bending test of the cold sprayed 304 stainless steel coating was tested by SHIMADZU electro-hydraulic servo-controlled fatigue testing machine and the fracture morphology was examined by scanning electron microscopy. The results showed that the fracture behavior of the cold sprayed 304 stainless steel coating was brittleness fracture. The crack in the coating occurred in the interfaces between particles and the crack extended to the internal of the coating with the increase of the load. When the crack has extended to the combination interface between coating and substrate, the crack extended to the two sides. The microstructure and mechanical property of the cold sprayed 304 stainless steel coating have been optimized after heat treatment.

Resonant Energy Transfer, with Creation of Hyper-Excited Atoms, and Molecular Auto-Ionization in a Cold Rydberg Gas

A cold Rydberg gas, with its atoms prepared initially all in the excited state |n0> , with n0 »1, contains an excessive amount of energy, and presumably is to relax by the Penning-type molecular auto-ionization (MAI), in which a portion of excess energy of one atom is given to another near-by atom and ionizing it. Its complementary process, the resonant energy transfer (RET), is discussed, in which the excess energy of one atom is used on another to form a hyper-excited atomic state |na> with na»n0. This process is always present, provided certain resonance energy conditions are satisfied. In this report, the n0 and density dependences of the RET rates are studied in detail, employing a simple model: 1) at low densities, the RET is mediated by the dipole-dipole coupling Vdd and its rates are generally much smaller than that of MAI, especially for small n0. But 2) as the density increases, our model shows that the rates become of comparable magnitude or even larger than the MAI rates. The Vdd is no longer adequate. We, then construct a semi-empirical potential to describe the RET process. 3) At high densities, we show that the atomic orbital of |na> overlaps with that of neighboring atoms, and the electron-electron potential becomes prominent, resulting in much higher rates.

Topology optimization of reactive material structures for penetrative projectiles

Recently,reactive materials have been developed for penetrative projectiles to improve impact resistance and energy capacity.However,the design of a reactive material structure,involving shape and size,is challenging because of difficulties such as high non-linearity of impact resistance,manufacturing limitations of reactive materials and high expenses of penetration experiments.In this study,a design optimization methodology for the reactive material structure is developed based on the finite element analysis.A finite element model for penetration analysis is introduced to save the expenses of the experiments.Impact resistance is assessed through the analysis,and result is calibrated by comparing with experimental results.Based on the model,topology optimization is introduced to determine shape of the structure.The design variables and constraints of the optimization are proposed considering the manufacturing limitations,and the optimal shape that can be manufactured by cold spraying is determined.Based on the optimal shape,size optimization is introduced to determine the geometric dimensions of the structure.As a result,optimal design of the reactive material structure and steel case of the penetrative projectile,which maximizes the impact resistance,is determined.Using the design process proposed in this study,reactive material structures can be designed considering not only mechanical performances but also manufacturing limitations,with reasonable time and cost.

A review of cryogenic power generation cycles with liquefied natural gas cold energy utilization

Liquefied natural gas （LNG）, an increasingly widely applied clean fuel, releases a large number of cold energy in its regasification process. In the present paper, the existing power generation cycles utilizing LNG cold energy are introduced and summarized. The direction of cycle improvement can be divided into the key factors affecting basic power generation cycles and the structural enhancement of cycles utilizing LNG cold energy. The former includes the effects of LNG-side parameters, working fluids, and inlet and outlet thermodynamic parameters of equipment, while the latter is based on Rankine cycle, Brayton cycle, Kalina cycle and their compound cycles. In the present paper, the diversities of cryogenic power generation cycles utilizing LNG cold energy are discussed and analyzed. It is pointed out that further researches should focus on the selection and component matching of organic mixed working fluids and the combination of process simulation and experi- mental investigation, etc.

Experimental and numerical study on pressure oscillation in a combustor with rotary valve

A rotary valve coupled in a combustor assembly can generate periodic pressure oscillations inside the combustor and can be used to study the combustion instability in the combustion chamber of a rocket engine.This paper proposes a cold gas flow experimental system based on a rotary valve and a corresponding rotation model.A 3 D numerical model is proposed to obtain the transient flow inside the rotary valve,and the dynamic mesh technique and User-Defined Functions(UDFs)are adopted to implement a swing motion instead of a rotary motion.Several cold gas flow experiments are carried out at rotating speeds of 75225,and 375 rpm to verify the validity of the numerical model.The effects of rotating speed,stroke length ratio and radius of the RED(Rotor Exhaust Duct)on the pressure oscillations are studied using this numerical model.The results show that the maximum and peak-to-peak values of the pressure oscillations gradually decrease with increasing rotating speed.The ratio of the corresponding peak-to-peak value to the maximum pressure(pressure amplitude ratio)is reduced from 1.81 to 0.6%.The stroke length ratio affects the pressure waveform because it leads to a change in the time spent in the non-exhaust stage.When the stroke length ratio is 0,the waveform closely resembles a sine wave.With the increase in the stroke length ratio,the pressure waveform exhibits a more square or a triangular wave shape.Finally,a high-frequency and high-amplitude pressure oscillation can be obtained by appropriately increasing the radius of the RED.