Verification of error compensation for rapid investment casting

Through the actual observation of the casting, it is found that the appearance of the diffuser casting is complete, and the corresponding characteristic parts of the casting are measured. At the same time, the castings without dimensional error compensation and after dimensional error compensation are compared. Considering the actual radial shrinkage and Z-direction shrinkage effect of the diffuser casting, according to the structure of the casting, the specific test position of the diffuser part is selected as shown in Figure 1, including the lower inner diameter of the part, the lower outer diameter of the part, the maximum outer diameter of the part and the high characteristic size of the inner wall of the part. The vernier caliper is used to measure the corresponding feature position for five times, and the ideal size, the size before size compensation and the size after size compensation are compared. The size deviation of the corresponding feature test position is shown in Figure 2.

(a) Top view (b) Section

According to the measurement results of each feature position of the casting before the size correction in Fig. 2, the overall performance of the casting size relative to the ideal size is negative, and the deviation value is between – 1.73 and – 0.31. The size reduction degree of different feature points of the casting is not consistent, including the maximum outer diameter deviation > lower outer diameter deviation > inner wall height deviation > lower inner diameter deviation. It is not difficult to find that the maximum diameter deviation is the largest, which is mainly due to the lack of continuous heat input from the outside of the impeller during the SLS prototype preparation process, which leads to the larger contraction of the outside of the impeller during the SLS molding process, resulting in the larger size deviation than other characteristic positions The inner diameter deviation of the prototype is smaller than the outer diameter deviation of the lower part due to the input of the heat source in the sintered area; the inner wall height is affected by the SLS prototype sintering phenomenon, especially the secondary sintering and alloy shrinkage, so the overall shrinkage is consistent with the outer diameter shrinkage of the lower part.

According to the above results of casting dimension deviation, combined with the previous calculation, the X and Y dimension correction coefficient is 1.00985, and the Z dimension correction coefficient is 1.00339. The dimension error of SLS prototype is compensated. The dimension deviation of corresponding feature test position after dimension compensation is shown in Figure 3.

Through the analysis of the results of the feature point deviation diagram after the size compensation in Figure 3, it is found that the size deviation of each feature point has been significantly reduced after the size error compensation. In particular, the average size deviation of the lower diameter is -0.014mm, which is within the allowable error range. Among them, the maximum outer diameter deviation > lower outer diameter deviation > inner wall height deviation > lower inner diameter deviation, and the deviation sequence does not change. The smaller the dimension deviation is, the smaller the dimension deviation is. Therefore, the average size results are calculated according to different feature positions. The comparison results of the relative error of each feature position of the castings before and after size compensation.

According to the analysis of the above comparison results, it can be seen that the relative error of the overall size of the casting has been reduced after the dimensional error compensation, especially the size of the part has no shrinkage after the dimensional error compensation of the inner diameter of the lower part of the casting, while the relative error of the corresponding size of other parts gradually increases with the increase of the size of the test position. Especially in the XY plane, from the lower inner diameter to the lower outer diameter and then to the maximum outer diameter, the relative error of casting test feature position size changes from – 0.006% to – 0.382%, and the lower inner diameter size error is fully compensated, thanks to the SLS The inner sintering zone can provide the corresponding heat source in the molding process, so it has a small shrinkage rate. However, due to the lack of corresponding heat source compensation in other parts, the size error has been reduced to a certain extent after the size compensation, but it still does not meet the use requirements. Therefore, for the same type of parts, the size error calculation and compensation can be carried out directly in the place with heat source compensation, and the heat source can be added or the size error compensation amount can be increased appropriately in other places, which is generally 1.5 ~ 2 times of the actual calculation compensation amount. When the outer side is blade or other thin-walled structure, the larger size compensation rate is selected.

Through the gating system design of the qualified diffuser parts, the eight ingate gating scheme is finally determined, and ProCAST is applied The software was used to simulate the casting process. The mold filling, solidification time, casting defects and pouring process were studied. The actual defects were analyzed. Finally, the riser was added and the shell was roasted at 600 ℃ for 1 h, Qualified metal castings were successfully prepared under the casting process scheme (shell preheating temperature 1115 ℃, pouring temperature 1600 ℃, pouring speed 3kg / s), which verified the rationality of the rapid investment casting process. The results show that after the corresponding error compensation, there is a heat source compensation place in the SLS molding process, and the calculated size error calculation can be directly selected for compensation. For other positions, the heat source can be added appropriately or the size error compensation amount can be increased appropriately (the actual calculation is 1.5 ~ 2 When the outer side is a blade or other thin-walled structure, choose a larger size compensation ratio.