Experimental materials Molenaar et al < 8 > believes that if the alloy | DFS/dT | small, solid phase fraction of it is easy to control, thus semi-solid processing performance is good. Scheil equation gives the solid fraction and temperature and the alloy distribution coefficient relationship: mlms1 KTf = 1 (1) type of fs as the solid phase fraction, Tm as pure metal melting, TL on behalf of the alloy melting point, T is the actual temperature, the partition coefficient K of alloy. To type available: (1) derivative KTf lmlmsk1d = 12 (2) from the type can be found on the smaller K, then | DFS/dT | is smaller, and more suitable for semi-solid processing. According to Sn - Pb binary alloy phase diagram, Sn, Pb rare metal materials and engineering vol. 36, alloy equilibrium distribution coefficient K0 = 0.0685 is very small, and the liquidus temperature is lower, Sn, Pb alloy for experiments. Therefore this study choose Sn - 15 pb alloy as the research object. By Sn Pb binary alloy phase diagram < 9 > also, you can see that the liquidus temperature of the alloy is 215 ℃, the crystallization temperature interval is 32 ℃. Accordingly, this study set up the experimental temperature area is 215 ℃ 183 ℃.

Using analytical pure grade and lead tin grain (purity > 99.8%) as raw materials, in a graphite crucible with argon gas protection in the mixture of Sn - 15 pb alloy. Then after heat preservation, slag casting in metal mold, made of phi is 14 mm x 120 mm test bar. Will test bar according to phi is 14 mm x 14 mm cutting, and remove the billet surface oxide skins experiment required cylinder billet.

Experiment considering the non-uniformity, electromagnetic stirring equipment complexity is higher, the preparation of semi-solid alloy of reason by mechanical agitation. Its device for dc torque tachometer generator drive, programmable PID digital regulator control temperature and heat resistance wire, high purity argon gas on the specimen surface protection, circulating cold water as a quenching medium.

Experimental scheme under argon protection will Sn - 15 pb alloy with H = 50 ℃ / min heated to 255 ℃ heating rate (above the liquidus temperature of 40 ℃) and thermal melt even 5 min, then cool to 235 ℃ and heat preservation after 5 min start stirring, shear rate of 280 s-1. Then with 2 ℃ / min cooling rate and cooling to 200 ℃. At this point according to the type (1) calculation, alloy solid fraction will be more than 0.45. Cut shear rate for 30 s - 1. Continue to reach below the solidus line temperature after stop stirring and quenching in order to obtain powder. As shown in the process temperature changes. So choose different quenching temperature, in the experimental process to determine the primary structure evolution of semi-solid behavior.

Tissue characterization of samples according to the longitudinal and transverse cutting after the bottom section, central section and surface profile for observation, and then, in turn, use 150 #, 600 #, 800 #, 1000 # SiC waterproof abrasive paper and particle size of 2.5 mu m, 1.5 m mu, 0.5 mu m synthetic diamond grinding paste grinding and polishing. Finally using glycerine 80% + 80% + 10% glacial acetic acid and nitrate solution for corrosion. Using quantitative metallographic microscope, AMARY LeitzLABORLUX12MES/ST - 1000 - b scanning electron microscope, the Finder - 1000 spectrometer analysis, measurement and Quantimet500 image processing software.

Results organization stirring moment in the continuous cooling process under the conditions of different temperature sample organization form as shown below. Experienced in accordance with the setting of control parameters, such as heating and cooling after two stages, alloy melts gradually into the liquid-solid two phase region. In the process, start nucleation in liquid phase, as shown in a. Can be observed by a, there's a very small chip in the melt; As further cooling and stirring, spherulite Numbers increased, its size grew up to a certain size, will no longer increase. As well as new small spherulites, as shown in b. Sample at this time the upper and lower solid-phase fraction were similar, organizing small and roundness is good; After the change of shear rate, still there will be a lot of new generation of small globular tissue, at the same time can see the ball group reunion and merger between growth, as shown in c. At this time on melt mixing torque force than under low solid fraction.

In sample upper solid-liquid separation phenomenon, namely a small number of scattered particles of aggregate on the sample surface, but the separation is not obvious; When the temperature has dropped below the solidus line temperature, according to the Scheil equation estimation, solid fraction has exceeded 0.6, stop stirring of sample liquid quenching, found the sample solid-liquid separation is very obvious, upper specimen appears a large amount of powder, powder with residual liquid phase bonding between and produce together. The AMARY - 1000 type b scanning electron microscopy (sem) observation of the flow pattern of particles as shown in d.

Powder obtained by d that is born, classes, spherical ball organization of aggregate, and the primary structure coated with a layer of thin film. These ball organization is from liquid phase nucleation and growth in the primary phase particles. Because it is passed by a core of solidification, which is formed by the nucleation and growth so it can be determined for the single crystal, and non-traditional shuangliu atomization, centrifugal atomization and mechanical atomizing powder preparation methods through rapid solidification of polycrystalline particles. Didn't find obvious inclusion between particles.

Component due to the alloy solidification process of solute segregation on the liquid-solid interface, so as the solidification process, liquid solute concentration will continue to improve. When the temperature reaches the solidus line temperature, according to the Scheil equation (1) calculation, the liquid fraction will be lower than 0.33, the enrichment of solute concentration in liquid phase will be close to Sn - Pb alloy eutectic composition (61.9% 61.9% Sn, Pb). The residual liquid part of the deposit at the bottom of the specimen in the solid phase particles surface part of package. Particles wrapped over the surface of the layer of liquid film is produced a large number of particles aggregate bond formation of an important reason. We have obtained as shown in d powder, energy spectrum analysis of different particle surface surface scan, the results such as shown below. The average chemical composition of 68.79% Sn, Pb 31.21%, very close to the eutectic composition of alloy (61.9% 61.9% Sn, Pb). This suggests that the primary particle surface is layer is low melting eutectic composition of package.

Many researchers on stirring under the condition of formation of the spherulite organization internal composition distribution were studied. Young people such as < > 10 in 304 stainless steel, such as Lehuy people < 11 > in the zinc - 27 Al alloy, Molenaar and Kool < > 8 in Al - 6 cu alloy, Zhang Shijiang < 12 > in the zinc - 22 Al alloy Jin Zhongdou primary spherulite was observed in composition distribution is more flat, it is also with conventional stirring solidification organization one of the important differences. Once again proved the results of particle internal line scan the results, as shown below. By that Pb concentration is uniform particle, relative to the particle surface, grain interior Pb concentration is higher; And the Sn is on the contrary, particle surface than the particle internal concentration is high. It once again proved that the particle surface is low melting eutectic composition. At the same time, this kind of uniform composition distribution is very useful for particle mechanics performance. Through this method in improving powder mixed preparation of single crystal metal powder particles, reduce segregation have obvious advantages.

Methods discussed in this study the preparation of metal powder particles mainly is to use the alloy within the low degree of supercooling stirring melt obtained primary nucleation is ball tissue and non-traditional solidification dendrite organization obtained the characteristic, and combining with the characteristics of mixing prompted solid liquid separation, the process for preparation of metal powder particles. Because each spherulite from low precipitation in supercooled melt liquid evenly and are within the melt of nucleation, so it is made up of a single grain. It has continuous photography technology is applied by the author and others through the real-time observation in butadiene nitrile - 5 at % water on the transparent model alloy were confirmed.