Effect of cooling speed on surface processing quality of alu

0 Introduction

In engineering practice, the material processing quality is closely related to the surface state of early casting. In order to obtain a better initial processing surface, it is particularly important to control the surface quality in the early casting process. In the process of electromagnetic die casting, the cooling rate and alloy melting temperature will directly affect the temperature field distribution in the alloy melt and the action time of the magnetic field, and have a serious impact on the surface quality of the ingot and the phenomenon of shrinkage and porosity. Therefore, different cooling rates will affect the effect of magnetic field. It is necessary to explore the influence of cooling conditions on electromagnetic die casting process [1]. It can be seen from references [2,3] that when a traveling wave magnetic field is applied during alloy solidification, the viscosity of the melt increases with the increase of solid phase rate, especially when the solid phase rate exceeds a certain critical value, the viscosity of the melt begins to increase rapidly. In the solidification process, the solid rate of melt is determined by the temperature of liquid-solid two-phase body. The temperature can be converted into a function of solid rate through Scheil equation. The functional relationship between solid fraction (FS) and temperature is:

Where: TM -- melting point of pure solvent; TL -- liquidus temperature of alloy; K -- equilibrium distribution coefficient.

In the process of solidification, the solid rate of the melt increases with the decrease of temperature. Generally, when the solid rate reaches 0.4, the viscosity of the melt will increase sharply. The following formula is the relationship between viscosity FS and solid rate:

η a=Aexp(Bfs) (2)

Where: A and B are coefficients.

The viscosity of the alloy melt depends on the solid phase ratio in the melt, and the solid phase ratio of the alloy melt is closely related to the internal temperature of the melt. It can be seen that the cooling rate and initial melting temperature of the alloy melt in the solidification process will seriously affect the control and improvement effect of the traveling wave magnetic field on the solidification process of the ingot, further affect the improvement effect of the traveling wave magnetic field on the surface quality of the ingot and the segregation of alloy elements, and further promote a positive impact on the subsequent machining of the product. Therefore, it is necessary to explore the influence of cooling conditions on electromagnetic die casting process and surface machining quality.

2 experimental method

The aluminum CNC block selected in this paper is Al-5% Cu alloy, which is a solid solution alloy α Phase and precipitated CuAl2. In this experiment, the influence of magnetic field (0.28 K / s) on ingot under air cooling condition, the influence of magnetic field (0.47 K / s) on ingot under water cooling condition and the influence of magnetic field (0.78 K / s) on ingot quality under rapid cooling condition were studied.

3 results and discussion

3.1 effect of cooling rate on composition segregation of aluminum CNC block

The axial content of Cu in aluminum CNC block under different cooling rates is shown in Table 1. Fig. 1 shows the axial content distribution curve of Cu element in ingot under different cooling rates.

Top of form

Bottom of form

It can be seen from table 1 and figure 1 that under the condition of no magnetic field water cooling, the content of Cu element gradually increases from bottom to top along the axial direction of the ingot, the maximum difference of Cu element content in the axial direction of the ingot is 1.03, and there is serious macro segregation of alloy elements in aluminum CNC block. Under the air cooling condition of traveling wave magnetic field, the variation law of Cu content is the same as that under the condition of no magnetic field from bottom to top along the axial direction of ingot, and gradually increases. The maximum difference of Cu content in the axial direction is 0.67. Under the condition of traveling wave magnetic field water cooling, the distribution law of Cu content has changed. The Cu content in the middle of the ingot is the lowest, the Cu content in the upper part is the highest, and the maximum difference of Cu content in the axial direction is 0.56. Under the condition of traveling wave magnetic field fast cooling, the change trend of Cu content is consistent with that under the condition of no magnetic field water cooling, showing a gradually increasing trend, and the maximum difference of Cu content in the axial direction is 0.78. It is found that the addition of traveling wave magnetic field changes the macro distribution of alloy elements in the axial direction of ingot and improves the macro segregation of alloy elements in the axial direction to a certain extent. The cooling speed has an impact on the traveling wave magnetic field to improve the macro segregation of alloy elements. In contrast, under the water cooling condition of traveling wave magnetic field, the distribution of Cu content in the axial direction is more uniform, but the maximum difference of Cu content in the axial direction is still as high as 0.56, which shows that the traveling wave magnetic field has a certain improvement effect on the macro segregation of alloy elements in the axial direction, but the effect is poor, There is still serious macro segregation of alloy elements.

3.2 effect of cooling rate on surface quality and shrinkage of aluminum CNC block

Figure 2 shows the surface quality of ingots under different cooling conditions. Figure 3 shows the internal interface of ingot under different water cooling conditions.

It can be seen from Fig. 2 (b) that under the condition of traveling wave magnetic field air cooling without approximate directional solidification, serious defects appear on the surface of the ingot, with a large number of continuous pits and holes, while in 2 (a), (c) and (d) of approximate directional solidification, there are no obvious pits and holes on the surface of the ingot, which shows that the cooling method of approximate directional solidification can significantly improve the surface quality of the ingot, A relatively smooth and flat ingot surface quality is obtained. It can be seen from Fig. 3 (b) that under the condition of traveling wave magnetic field air cooling without approximate directional solidification, it is found that there are a large number of holes in the ingot. The number and size of holes near the ingot surface are smaller than those in the shaft area of the ingot. In Fig. 2 (a), (c) and (d) with approximate directional solidification, it is found that there are basically no holes in the ingot, and the compactness of the ingot is significantly better than that in Fig. 2 (b) without approximate directional solidification. It can be seen that under the condition of mold casting, the cooling mode of approximate directional solidification can significantly improve the surface quality of the ingot, reduce the porosity in the ingot, and then improve the overall casting quality of the ingot. The speed of cooling and the presence or absence of magnetic field have no significant effect on the improvement of ingot surface quality and the reduction of porosity in ingot. Therefore, the appropriate cooling rate can significantly improve the surface quality of ingots and has a positive impact on the improvement of later machining quality.

4 Conclusion

In this paper, aluminum CNC block is used as the experimental material to carry out the application research of electromagnetic die casting, and the influence law of different cooling rate on the surface quality of ingot is studied. The results show that under the condition of mold casting, the cooling mode of approximate directional solidification can significantly improve the surface quality of the ingot, reduce the porosity in the ingot, and then improve the overall casting quality of the ingot.