Analysis of micro error sources in rapid investment casting

In SLS rapid investment casting process, there are SLS prototype preparation, SLS prototype post-processing and alloy casting solidification process, and there are some dimensional errors in the above processes. In the process of SLS, there are many kinds of sintering phenomena, including powder spheroidization, secondary sintering, z-axis surplus in microcosmic, sintering shrinkage and surface step effect in macroscopical. The existence of sintering phenomenon will affect the size error of SLS prototype, as follows.

Powder spheroidization occurs in the sintering process. Because the powder particles are often irregular in shape, they are approximately equivalent to ellipsoid. Nelson et al. Believe that ellipsoid powder has higher surface energy due to its larger surface area. When the powder is heated to its glass transition temperature, the powder particles will spontaneously transform from high surface energy to spherical shape with lower surface energy, as shown in Figure 1 As shown in. Specifically, after laser sintering, the long axis a shortens, the short axis B becomes longer, and finally becomes a ball with radius R. The occurrence of this phenomenon will make the size of SLS prototype increase in z-axis direction.

Secondary sintering is an additional sintering phenomenon caused by laser main sintering. When the laser acts on the powder bed to complete the current sintering, due to the energy radiation and heat transfer, the powder in the adjacent unscanned area will also be sintered. The scanning mode is XY alternate scanning mode. In theory, the X and Y directions have the same size deviation without considering the force of powder laying roller on the sintered parts. Therefore, SLS prototype in the role of secondary sintering, will make the X, Y direction presents the trend of size increase. Z-axis surplus sintering phenomenon refers to that in order to ensure that the two sintering layers can be fully sintered, the laser energy must be transferred to the previous sintered layer. Therefore, when the first layer of powder is sintered, the surplus powder on the powder bed is sintered due to the transfer of laser energy, which causes the Z-axis dimension to increase. The details are shown in Figure 2.

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