Microstructure refinement of the hottest high sili

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Microstructure refinement of high silicon aluminum alloy suspension casting

abstract under certain overheating conditions, adding different particle sizes and different amounts of rapid solidification powder with the same composition to Al-30Si metal liquid can refine the coarse primary silicon in the alloy, so as to explore a simple and effective method to refine the primary silicon of high silicon aluminum alloy

key words: high silicon aluminum alloy suspension casting microstructure refinement

high silicon aluminum alloy obtains abnormally coarse primary silicon according to the ordinary casting method. In order to refine the primary silicon, modifier can be added to the liquid metal [1]. However, the amount of modifier is strictly controlled, and the cost is increased. The process is more complex, and sometimes due to the addition of foreign particles, the number of foreign particles and inclusions in the metal increases, reducing its performance. The suspension casting method is used in the test. A quantitative amount of metal powder with the same composition is added to the metal liquid, which is evenly mixed with the melt and suspended in it. It plays the role of endothermic, nucleation, promoting solidification, preventing the continuous growth of primary silicon, and thus obtaining the obviously refined structure of primary silicon. Therefore, this method is a method to directly control the solidification of liquid metal. In this paper, the effects of the particle size and amount of solid particles added on the precipitation of primary silicon in Al-30Si alloy under certain overheating conditions (150 ℃) were studied

1 test method and device

the test uses ZL102 (Al-12Si) and industrial pure silicon to produce Al-30Si alloy. The multi-stage atomization device manufactured by our institute is used to produce 20 ~ 80 mesh, 80 ~ 150 mesh and 150 mesh powder. The massive particles (3 ~ 5 mm) used are mechanically crushed, and then the device shown in Figure 1 is used, While pouring into the crucible with the same temperature as the metal liquid, add metal powders with different mass fractions (10%, 16.7%, 25%, 33, respectively. Reduce the investment of human resources by 3%), and get the required samples after cooling in the crucible after stirring

Figure 1 Schematic diagram of suspension casting

1 Alloy powder 2 Crucible 3 Metal flow 4 Suspension casting fluid

2 test results and analysis

2.1 test results

through the metallographic analysis of the sample (Fig. 2), it can be found that with the reduction of solid particle size, the precipitated primary silicon becomes finer, and with the reduction of the addition amount, the primary silicon tends to be refined, achieving a good refining effect when the mass fraction is 10%. In the samples with massive solid particles, it is found that almost all primary silicon has a local small region with low silicon content, and its boundary is very irregular

a. conventional casting structure B. add 3 ~ 5 mm massive particles (10%); C. add -20 ~ 80 mesh powder (20%); D. add -80 ~ 150 mesh powder (10%); e. add -150 mesh powder (10%)

Figure 2 the effect of the addition of various solid particles on the microstructure

2.2 discussion

on the refinement of primary silicon of high silicon aluminum alloy, The general method is to add modifiers (such as sodium salt, Al-Ti-B, rare earth, etc.) [1]. In suspension casting, the Al Si alloy powder used is prepared by the rapid solidification atomization device of our institute. The cooling rate reaches 105 ~ 107 K/s. The primary silicon is fully refined and dispersed in the powder particles [2]. Metals, like living things, have genetic phenomena [3]. The former Soviet Union has made in-depth and detailed research on metal genetics [4]. After adding rapid solidification powder into high silicon aluminum alloy liquid, a large number of fine primary silicon in rapid solidification powder formed a relatively stable group. These groups still maintain the effectiveness of crystallization under certain overheating conditions of molten metal, and they preserve the microstructure in the powder like genetic factors under subsequent crystallization conditions. To sum up, the reasons are as follows

(1) because the added aluminum silicon alloy powder absorbs the heat of the liquid metal very quickly, it produces a degree of supercooling. Under this supercooling, the composition fluctuation area (high segregation concentration) in the liquid metal forms a primary silicon atom aggregate, that is, a group. This group just forms a crystalline core and further grows, and the liquid metal can produce a new and further growing crystal nucleus under the supercooling condition

(2) when aluminum silicon alloy powder is added to the metal liquid, the metal around the metal powder solidifies to form a solid metal shell of primary silicon, and then the shell melts into the atomic aggregate. Because the metal liquid cools quickly and reaches a certain degree of supercooling, these groups can grow into crystalline cores

(3) due to the addition of fine powder, the number of nucleation cores is huge, which can make the primary silicon in the metal liquid be analyzed. When the amount of solid phase added exceeds a certain limit value, some groups will fuse with each other through growth, resulting in the coarsening of primary silicon. Therefore, under the condition of certain overheating, there is an optimal amount of solid phase added

with the precipitation of primary silicon, the composition of the liquid is gradually close to the eutectic composition. With the continuous decline of temperature passing through the adoption of aluminum matrix composites in the rotor, the remaining metal liquid will solidify in the eutectic structure. Because the alloy powder with the same composition is added, there is good wettability between the solid and the liquid. By stirring, the powder can quickly melt into the metal liquid, evenly distribute and nucleate, and will not float on the surface of the metal liquid and oxidize

this method was first used in powder injection metallurgy in steelmaking. [5] it is used for the modification of ladle, which can significantly improve the microstructure and properties of ingot, especially the phenomenon of coarse precipitation of the second phase. There are also reports on how to refine the primary silicon of high silicon aluminum alloy in China [6 ~ 10]. Suspension casting is a simple and effective method with low cost

3 conclusion

(1) the addition of solid particles can accelerate cooling and promote solidification

(2) the powder particles have obvious nucleation effect, and the smaller the particles are, the more nucleation cytoplasmic points are formed, and the more obvious the effect of refining the particles is

(3) there is the best amount of solid phase added under certain overheating

about the author: Li Xiaoping, male, born in 1967, engineer, Institute of nonequilibrium materials, Central South University of technology, Changsha, Hunan (410083)

author unit: Li Xiaoping, Chen Zhenhua, Cao Biao, Fu Dingfa, Central South University of technology


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