Dimensional error of selective laser sintering rapid investment casting

The process of PS rapid investment casting based on selective laser sintering was studied. Starting from the process of rapid investment casting and the route of dimensional error transmission, the optimal parameters of wax impregnation process were obtained through the study of wax impregnation post-treatment process, and the dimensional error transmission law in SLS rapid investment casting process was studied. The reverse engineering technology is applied to verify the rule of dimension error transmission, and the dimension compensation of complex parts is completed by determining the path of dimension error compensation. Finally, the qualified metal castings were prepared by investment casting technology. The conclusions are as follows

(1) The SLS prototype was treated after wax soaking. The wax pattern has the best dimensional accuracy and surface quality when the first wax dipping temperature is 65 ℃ and the second wax dipping temperature is 75 ℃. The thickness of wax layer on one side after the second wax dipping is 0.0100 mm to 0.0683 mm.

(2) Study on the transfer law of dimensional error in SLS rapid investment casting process. When the Z dimension of CAD model is less than 22mm, the X and Y directions have the same change law, and the relative error is negative in the early stage. With the increase of Z dimension, the X and Y directions decrease first and then increase, but the overall relative error is negative. When the z-dimension is larger than 22mm, the SLS prototype has smaller relative error; after one wax immersion, the smaller the z-dimension is, the larger the volume increase is. Compared with the first wax immersion, the relative error of dimensions in all directions after the second wax immersion is more consistent; the relative error of castings relative to wax mold dimensions is negative, when the wax mold size is less than 20 mm, the shrinkage of metal castings is not obvious, within the allowable range, when the wax mold size is greater than 20 mm, the shrinkage of metal castings is obvious, and the average shrinkage rate is 0.75%. The X and Y transfer coefficients of the final CAD model are 0.99015, Z transfer coefficient is 1.01576, the corresponding X and Y dimension correction coefficient is 1.00985, Z dimension correction coefficient is 0.98424.

(3) Combined with the size error transfer law in the process of rapid investment casting, the size error compensation of typical complex parts is completed. The average positive dimension deviation of diffuser impeller is 0.4212mm, and the average negative dimension deviation is -0.7269mm. Combined with the existing size error transfer law, the theoretical X and Y direction size deviation should be -0.40789mm to -0.1236mm, but the actual deviation is greater than the theoretical value. Due to the larger impeller size of the diffuser, the temperature difference between the outer powder bed and the sintered part becomes larger, and the outer shrinkage increases, which makes the actual size deviation of the diffuser parts greater than the theoretical calculation. When the Z dimension of the model is larger than 22mm, the X and Y dimension correction coefficient is 1.00985, and the Z dimension correction coefficient is 1.00339. Finally, magics19 is used to complete the size correction and machining allowance compensation for the corresponding parts of the diffuser impeller.

(4) Experimental verification of error compensation in rapid investment casting after part size correction. Through the calculation of gating system, the scheme of 8 ingate is determined. After trial pouring, the specific casting defects are analyzed. The gating system was optimized by adding a blind riser to the upper blade and an open riser to the side blade, Under the pouring process scheme (shell preheating temperature 1115 ℃, pouring temperature 1600 ℃, pouring speed 3kg / s), the mold shell on the module was removed, the pouring riser was cut off, and then the residual mold shell and oxide scale were removed by sand blasting. Finally, the qualified casting was completed. Through the verification of casting size compensation, it is found that the heat source compensation can be directly used in the SLS molding process. For other positions, the heat source can be added in the rapid prototyping or the size error compensation can be increased appropriately (1.5 ~ 2 times of the actual calculation). When the outer side is blade or other thin-walled structure, the larger size compensation ratio can be selected.