Heat transfer and solidification behavior of atomized droplets of superalloys

Heat transfer and solidification behavior of atomized droplets of superalloy SUN Jianfei, SHEN Jun, LI Zhenyu, JIA Jun, LI Qingchun National Key Laboratory of Precision Metal Processing, Harbin Institute of Technology. The variation of solid fraction and cooling rate with the droplet size and flight distance in the temperature distribution of Harbin j5oooi. The changing trend and the cause of the change are given. The effects of atomization pressure and alloy superheat on the above parameters are discussed.

1 Introduction Spray forming is a kind of new rejection of gold, rapid solidification technology uses this technology to prepare metal materials with a small group of micro-particles of push crystal technology. Weave and improved performance. It is well-known that the rapid solidification effect of the spray forming process is produced during the atomization process and relies heavily on the solid fraction controlled by the process conditions such as the atomization pressure superheat sink distance and so on. 14. Therefore, in order to properly understand the alloy The evolution of the microstructure determines the process parameters and it is necessary to reasonably describe and discuss the heat transfer of the atomized droplets.

The researcher has established the basic theory of the whisker field using the aluminum alloy iron-niobium alloy as the object of the study. Taking full account of the solidification characteristics of the high-temperature alloy, a mathematical model for calculating the thermodynamic melting and solidification process of the high-temperature alloy is established. The calculation results and related discussions are given.

2 Mathematical models 2.1 Velocity behavior of gas and droplets The deuterium oxide gas and metal droplet velocities are the analytical expressions of the Duncan's law in heat transfer and can be simplified as the velocity and density of the atomizing gas, respectively, and the drag coefficient. It can be the dynamic viscosity of 2 atomizing gas.

The gas velocity is greatest at the exit of the nozzle and decreases as the flight distance increases. According to the results of the large-scale experiments and theoretical analysis, 1 Jia et al. established the following relationship 2 Sun Jianfei, Ph.D. Associate Professor, currently mainly engaged in research on rapid solidification and new materials.

Flight distance.

2.2 Droplet heat transfer and solidification during atomization For a given atomized droplet, the superheat of the melt is determined by its initial temperature and heat is dissipated by heat transfer and radiation to the gas of the 1st order. In the above process, the release of crystallization latent heat will occur when the temperature of the droplet reaches the nucleation temperature. Based on the assumption that the internal temperature of the droplet is evenly distributed, the total up to Cs`A solid-liquid mixture in the temperature distribution during the flight can be given. The specific heat of the droplets is the specific heat of the solid and liquid droplets, respectively, and 5 is the nucleation temperature of the droplets.

The studied alloys have undergone a cooling phase, namely: liquid cooling, nucleation and re-fitting segregation solidification and solid-state cooling. The following formula 1 can be obtained in each phase of the following 1 liquid cooling 2 nucleation 4 retinoid dripping solid fraction, and the droplet and gas temperature, respectively. For the radiosity, also for 18, the constant, do for the convection heat transfer coefficient and can be described as the relation between the called solidification cooling 1 and the solid solid cooling rand number, for the gas specific heat to accurately predict the droplet temperature distribution. It is necessary to consider the supercooling and nucleation effects of dripping. The majority of researchers believe that the primary nucleation method in the atomization process of heterogeneous nucleation and nucleation, and the results of its calculation and discussion of the nuclear rate are known to Shan 124 Shan 1 . We do not have 1 size dripping nozzle and can twist in 7 people. For the latent heat of fusion, the scratching temperature is the temperature of the alloy liquidus, and the 9 is the self-coefficient that describes the nucleation ability. The start of the nucleation is the mark of the first critical nucleus due to the atomization gas dragging. The effect is accelerated to its maximum speed. At this time, the relative sliding speed between the droplet atomization gas is zero, and the droplet speed decelerates from the droplet speed exceeding the gas violation droplet. Can be found. Compared with the large-size droplets, the small-size droplet acceleration effect is more pronounced and its maximum speed is obtained within a short flight distance. If 10 drops are obtained when the distance between the feet is milk (1), the lowest point of obtaining the heart rate at 1 point is known from formula 6 as 2 to correspond to the maximum flight speed or Reynolds number equal to, at and beyond this point, the convective heat transfer The coefficient increases as the relative sliding speed gradually increases.

The solid phase fraction of the droplets from the 4 alloy can be clearly seen with the change in the flying distance. Corresponding to 3, the solid fraction curve is also divided into regions where the overheating phase is lost 6 is equal to 0. The rapid rise of the nucleation and re-growth phases of segregation increases slowly during the solidification phase and the solid-state cooling phase 18 equals to 1.

In addition. The solidification characteristics of the helium-casting process are as follows: 1. Different size droplets at a given flight distance have different solid-phase fractions, such as in-phase fractions of the siginificated droplets at the flight divergence.

The gas velocity of all the dripping drops has gone through the above mentioned aspects. In the aspect of the stomach, the size of the stomach increases with the size of the dripping droplets, since the convective heat transfer coefficient becomes smaller. Both the onset and end of solidification are delayed to a longer flight distance. For example, the initial and final points of the 80-diameter dripping condensate 1 river, in the course of 0.0 and 0.521, respectively, in the 0.51 and 2.451 flight distances, the flight distance m fraction with the change of the flight distance, the average diameter cooling speed with the flight distance .

When the liquid can be fully liquid, the cooling speed increases with the droplet size and the flying distance m speed varies with the flying distance. The variation coefficient of the row distance with the flying distance varies with the flying distance. The flying distance of the umbrella flying distance varies with the flying distance. The convective heat transfer coefficient varies with the flight distance for a sample of 80 internal diameters of a commercial temperature alloy droplet. When the melt overheated changes from 1 to 250 ft, the calculated droplet temperature and solid fraction fractional axial flight departure are respectively 10 and. With the atomization pressure on the solidified water as advised to the diameter of the pavilion, the initial cooling rate drops to 1.5,10. 31. During the re-growth phase, the cooling rate is changed to the heating rate or each cooling rate is negative. And in the segregation solidification stage after the thermal child's dissipating velocity approaching 0. along the heat release rate, the cooling velocity distribution is established and the minimum value of this stage is reached when the convection heat transfer coefficient is the lowest. The final solid-phase cooling rate gradually decreases from the initial cooling rate of 5.6 parent 10 in this stage until it is equal to the ambient gas temperature.

The tendency for the pressure to increase and decrease, but also for a given flight distance. The greater the atomization pressure, the higher the temperature. This is because the increase in flying speed of the droplets significantly reduces the flight time. Similarly, as the atomization pressure increases, the fraction of solid phase corresponding to a certain flight distance decreases. For example, the solid fraction of the flight enthalpy at 0.5.0 is 0.75 at 2.51. It is worth pointing out that the solid fraction corresponding to the re-growth, segregation, and solidification is not sensitive to the dry atomization pressure. 3, the atomization pressure has little effect on the cooling rate, and the flight distance, ra, varies with the distance of the flight. Atomizing gas Le Li is the most important process parameter of the jet forming art and has an influence on the dynamics and thermodynamic behavior of the droplet. 69, respectively, 80, the diameter of the droplet flying speed convection heat transfer coefficient temperature distribution and the solid fraction with the atomization pressure, then the atomization pressure will increase the flying speed of the droplet, such as the flight distance is 0. Such as fog The pressure is 1.0 heart and 25 he hits the scoop and the flying speeds are respectively for today and for the sake of knowledge.

3 o'clock. The large atomization force 1 causes the convective heat transfer coefficient curve to move, that is, its niobium position corresponds to the velocity of the droplet and the atomizing gas, or the highest droplet speed tends to the history with the increase of the atomization pressure. Flight distance. From 8 it can be seen that the greater the atomization pressure, the longer the solidification process, such as the completion of the segregation and solidification stage. The atomization pressure is 1.0 and this condition is at the nozzle 331, while the atomization pressure is 2.5 and it is at 0.9, 1 from the nozzle. In addition, the slope of the temperature distribution line in each phase corresponds to the solidification process that the solid phase fraction with the flight distance varies with the atomization flight distance m, and the temperature and solid fraction curves are shifted to the right. This change can be simply attributed to the fact that full liquid heat loss increases with the increase in superheat. On the other hand, in a certain flight, the melt temperature increases as the superheat increases. , The solid fraction becomes smaller. The superheat degree of the melt is directly related to the heat loss during the atomization process, so small changes in the fraction of the solid phase are sufficient to deposit the ingot structure. The performance has a significant effect on the change in superheat of the gold interface. There is no effect on the Reynolds number and convective heat transfer coefficient for the flying time of the droplet flying locust. Similar to the effect of the atomization 1 force, the cooling rate is also not sensitive to changes in superheat.

4 Conclusions The speed of the droplets in the process of atomization of superalloys firstly increases rapidly and then decreases slowly. The change coefficient of the solid-state fraction with flying distance of the heat transfer distance of the tank heat exchanger decreases first and the maximum height of the droplets rises. There is a minimum value at the speed.

The temperature distribution of the alloy droplets is a domain, and the onset and end of the solidification are delayed to a greater flying distance as the droplet size increases. Corresponding to the temperature distribution, the fraction of solid phase in the process of flying hall gradually increases. At a given distance from a flying hall. The size of the droplets is more people. The smaller the solid fraction.

The cooling rate of the alloy droplet in the full liquid state is 5 orders of magnitude, and rapid solidification characteristics emerge.

As the atomization pressure increases, the axial flight speed of the droplet increases. The convection heat transfer curve shifts to the right. At the same time, the solidification process is changed, and the phase fraction corresponding to a certain distance from a flycatcher increases. Alloy superheat, large corresponds to delayed solidification over 1; ghosts are at a certain flight distance. The higher the alloy superheat, the flight distance m temperature with the flight distance changes the flight distance ro temperature with the change of flight distance planted cage, the higher the temperature, the smaller the solid fraction.

8 Shen Jun. Atomization Deposition Process and Its Effect on Microstructure and Properties of Rapidly Solid High Strength Aluminum Alloy Harbin Institute of Technology doctoral dissertation. 1993.

9 Li Dianzhong. Numerical Simulation and Quality Control of the Formation Process of 417 Alloy Cast Steel Blades . Harbin Institute of Technology doctoral dissertation. 1998.

The National Powder Metallurgy 7 Technical Conference informed the China Qujin Institute of Metallurgical Professional Committee. Membrane and Metallurgical Academic Committee of China Society for Mechanical Engineering and Powder Metallurgy and Metal Ceramics of the China Society of Metals and Metals Co., Ltd. In May, 2001, the National Powder Metallurgy Academic Exchange Conference was held at Lake Wuhan, Hubei Province. Hey. Enthusiastically attend the meeting. Dissertation submission deadline, between June 2000 301. On the synonym of the manuscript sent to Beijing Haidian District, 30 Xueyuan Road, Beijing Science and Technology Institute of Powder Metallurgy Department 100083 Jiacheng Factory