Analysis and application of thermal field in the h

Thermal field analysis and its application in the casting process

[Abstract] through the thermal field analysis of the specific internal energy distribution in the casting pouring and solidification process, determine the benign thermal field, bad thermal field and the homogenization degree of the process thermal element, bad thermal element and thermal field under various process conditions, analyze the main causes of casting defects, take corresponding process measures to eliminate the impact on the thermal field distribution, and obtain a sound casting. Through the analysis of three castings, the practical application of the thermal field analysis method is illustrated. However, the thermal field analysis method of casting process depends on fluid mechanics knowledge and rich casting experience. At present, only qualitative analysis can be carried out, and quantitative analysis or computer CAD can not be carried out

keywords Process analysis of specific internal energy of thermal field during the period from the time when the molten metal is completely poured into the mold cavity to the time when the molten metal solidifies, the distribution of specific internal energy (energy contained in unit particle j/kg) of molten metal in the mold cavity (including casting and pouring system) with temperature as the symptom is the thermal field

in the traditional casting process theory, the material enrichment part in the geometric structure of the casting, that is, the part with large geometric section, is determined as the hot spot, and the hot spot is brought into the focus of process control. These are based on the fact that the temperature of the molten metal in the mold cavity is the same at the moment when the molten metal is poured, which is contrary to the national policy. Then, according to the conditions of uniform heat dissipation, the part with the smallest unit specific surface area, that is, the part with the slowest decline in specific internal energy, is the last solidification part, that is, the part where the general hot spot is located. However, using the distribution of casting hot spots to arrange the casting process, process defects often appear in production practice, that is, the defects caused by local overheating and overheating of castings do not necessarily appear at the geometric hot spots. Therefore, the concept of thermal field is introduced. Through the study of the overall temperature distribution of molten metal and the change of temperature distribution with the difference of cooling conditions after the pouring of the mold, the distribution of the thermal field is determined, and the concept of specific internal energy is used to replace the hot spot, so as to determine the layout of the casting process, so as to make up for the deficiency of simply relying on the hot spot to arrange the casting process

-, classification of thermal field

according to the different casting processes, whether the formed thermal field is beneficial to the sequential solidification of castings, the thermal field can be divided into benign thermal field and poor thermal field

take the simplest plate casting as an example to illustrate the influence of casting process layout on thermal field and casting defects. Figure 1 shows the bad thermal field, and Figure 2 shows the benign thermal field. The casting process layout in Figure 1 cannot find any improper place according to the geometric hot spot method, but the castings produced according to this process will have shrinkage defects at position a. This is because the temperature distribution of molten metal in Figure 1 decreases with the increase of the sequence number in the figure. The reason for this phenomenon is the heat absorption of the mold and the radiation and heat dissipation of the open riser. Once the metal liquid near the dead corner (No. 6) in the figure is filled, it will no longer flow, and it will begin to cool and crystallize before the pouring is completed. The middle area is the filling channel, and the temperature is always close to the pouring temperature. The molding sand temperature near this area is also relatively high, So that the sequential solidification phenomenon of decreasing temperature cannot be formed in the feeding channel, while the upstream and downstream of the feeding channel solidify first, resulting in shrinkage defects at a. Therefore, it can be seen that for castings with uniform geometric section, when the heat dissipation conditions of all parts are basically the same, the specific internal energy of the prepared parts of the casting is uneven due to the uneven temperature distribution, so an uneven thermal field, that is, a poor thermal field, is formed in the uniform quality field, which is unfavorable to the sequential solidification of the castings and prone to casting defects. The formation and degree of bad thermal field are related to many physical conditions, such as: ① the greater the difference between the temperature of molten metal and the temperature of the mold, the more serious this phenomenon is, and vice versa, the less it is. It can be said that cast steel is far more sensitive to this phenomenon than cast aluminum; ② The greater the difference between pouring temperature and solidification temperature of molten metal, the more serious this phenomenon is. Therefore, reducing pouring temperature can reduce casting defects; ⑧ The slower the pouring speed is, the more serious this phenomenon is; ④ Under the condition of producing this phenomenon, this phenomenon is more serious by increasing the riser volume; ⑤ The greater the tendency of heat absorption or heat transfer of the mold, the more serious this phenomenon is; ⑥ The smaller the solidification range of the alloy, the more serious this phenomenon is. Changing the above factors can reduce or eliminate the bad thermal field, change the solidification process of castings, and reduce or eliminate casting defects. (a) Casting process and defects

(b) solidification process (poor thermal field) if the process scheme shown in Figure 1 is changed, the original pouring direction is reversed, the riser is changed into the gate, and the original gate is the exhaust and slag channel, the thermal field changes, and the solidification process becomes sequential solidification. Although the thermal field is also uneven, it is conducive to the sequential solidification, so it is called a benign thermal field. The process and solidification process are shown in Figure 2. A casting process that will directly integrate the mature research and development achievements of carbon fiber composites in Europe, once the benign thermal field is determined, the dependence on other physical conditions will become smaller, and some can even be ignored,

for example: ① at present, the temperature difference between mold and molten metal and the endothermic tendency of mold are not considered

② the influence of pouring temperature on thermal field can also be ignored, but its influence on other defects should be considered

⑧ the control range of pouring speed can be extended for a long time, and the volume of riser can be greatly reduced

④ has nothing to do with the solidification temperature range of the alloy. The establishment of benign thermal field is a prerequisite for improving the quality of castings. 2、 Process thermal element and thermal field uniformity

in Figure 1a, area a is not a material enrichment area, that is, it is not a geometric hot spot, but it is a pouring filling channel, which has a higher specific internal energy as the geometric hot spot, but it is caused by the process layout, so it is called process hot element. For the process heat element, it must be the same as the geometric heat node to provide the necessary feeding to ensure the casting quality

Figure 3 shows a