The process parameters of SLS were predicted as follows: temperature 95 ℃, laser power 30W, powder layer thickness 0.14mm, scanning speed 3000mm / s. The test position is shown in Figure.
The dimension accuracy of SLS rapid investment casting prototype is the relative dimension error of SLS prototype relative to CAD model, which is calculated by the following formula:
Where,
ε 0 – relative dimension error of SLS prototype relative to CAD model;
L1-sls prototype size, mm;
L0 – dimension of CAD model, mm.
It can be seen from Figure 2 that the relative error of Z dimension changes greatly with the change of test position, and the relative error of Z dimension decreases with the increase of Z dimension; while the relative error of Y dimension keeps a small change at different test positions; with the increase of Z dimension, the relative error of X dimension increases to a certain extent. The relative error of Z-direction dimension is changed from 5.857% to – 0.444%, and the decrease is 108%. This shows that the Z dimension has a great influence on the relative error of SLS prototype. This is due to the warping deformation of the first sintering layer caused by the temperature difference between the powder bed and the first sintering layer when the z-direction dimension is small. At the same time, due to the effect of the secondary sintering phenomenon, the size expansion of the specimen is much larger than the sintering shrinkage, which is shown as the size increase in the macro. With the further increase of z-dimension, the relative error of SLS prototype changes to a negative value. This is because when the Z dimension of SLS prototype is large, the influence of warpage and secondary sintering phenomenon is not obvious, and the specimen is mainly affected by sintering shrinkage, which further reduces the Z deviation. Especially when the z-dimension is less than 12mm, the relative error of z-dimension decreases from 5.857% to 2.667%. When the z-dimension is larger than 12mm, the relative error of z-dimension decreases. This shows that when the z-dimension of CAD model is less than 12mm, the influence of z-dimension on the relative error increases, while when the z-dimension of CAD model is greater than 12mm, the influence of z-dimension on the relative error decreases. When the z-dimension is larger than 22mm, the relative error of specimen size changes from positive value to negative value, and the influence of secondary sintering on CAD prototype gradually decreases.
Compared with the change of Z-direction overall dimension relative error, the x-direction overall dimension relative error shows a trend from negative value to positive value, in which the x-direction dimension relative error changes from -1.000% to 1.100%, especially at test point ① and ②, the x-direction dimension relative error basically remains unchanged, while the test point ⑤ shows a positive value. This is due to the warping deformation of the specimen during the initial sintering process, which leads to the negative value of X-direction of SLS prototype, while the x-direction of test point (1) and (2) is mainly affected by the initial warping deformation, so the relative error of X-direction dimensions of the two is basically the same. In the later stage, with the increase of z-dimension, the influence of warpage on the upper layer during the initial sintering process gradually decreases, and the relative error of X-dimension gradually changes from negative to positive. At test point 5, the relative error of X-direction dimension is positive due to the residual PS powder in XY plane and secondary sintering. The results show that the relative error of y-dimension decreases and then increases from a small range, and the relative error of y-dimension changes from – 1.000% to – 1.556%. The reason is that the Y direction is not affected by the additional force of the powder laying roller, only by the comprehensive effect of the incomplete sintered PS powder remaining in the X and Z planes and the sintering shrinkage of the specimen. At this time, the specimen is mainly affected by the sintering shrinkage, and the relative error of the Y direction dimension is negative.
The mean value of relative error of y-dimension is less than 0, which indicates that the two-dimensional dimension is shrinking and z-dimension is increasing. Therefore, the transfer rule from CAD model to SLS rapid investment casting prototype is as follows.
The transmission law of X direction is as follows
The transfer law of Y direction is as follows
The transfer law of Z direction is as follows
Where, ε x1, ε Y1 and ε Z1 are the relative errors of X, y and Z dimensions of SLS rapid investment casting prototype relative to CAD model.