POWER SPEED REDUCTION UNITS FOR GENERAL AVIATION PART 8: CASING DESIGN FOR METAL DIRECT ADDITIVE MANUFACTURING

This paper introduces a practical design procedure for Direct Metal Addictive Manufacturing (AM) by Hybrid Selective Laser Sintering or Melting (SLS/SLM). Theoretically AM frees the designer from many design constraints, practically it introduces new problems and it requires a brand new design of the part. Hybrid manufacturing reduces the problems without solving them totally. Supports can be introduced but represent a waste of time during manufacturing and during the post-mill removal. Before a 3D-printed metal part becomes usable, it has already undergone significant postprocessing in the form of CNC milling, hot-hyping, shot-peening or sand-blasting. Powder-bed fusion parts shows a quite rough surface finish. Considering that post-milling will be necessary in any case, surface finish is less important than other issues metal 3D printing. For example, very small cavities form within the part as it is being built. These defects lead to cracks during the part life cycle. Too few power applied by the wiper, too low laser intensity, excessive or inefficient cooling are the most common causes. Geometry of the part, optimized printing pattern and machine parameters are used to address these problems. AM machine operators have to tune their machines for a given material and print job. In the SLS/SLM process, density is achieved with quality powders, optimized build parameters and controlled machine environment. Hot isostatic pressing treatment as a post-process removes the porosity and reveals excessive defect by deforming the part. Residual stress is a result of the printing process. As the powder is melted and cooled, expansion and contraction occur. The residual stress is compressive at the center of the part and tensile at the boundary surfaces. Therefore, thin walled continuous parts are to be preferred. In fact, the classical ribbed structure has several thickness discontinuity requiring continuous adjustment in the build pattern and in laser modulation. To ensure the quality of the part material, the initial layers of the print are removed via CNC milling after the build is complete. Unfortunately, the thermal stress of the substrate will cause the part itself to warp modifying its geometry. Support structures are positioned in the right locations to prevent distortion or warpage. In addition, overhanging surfaces or down-skins faces have poorer surface finish ad are subjected to warping. For this reason, they also need additional supports. For these reasons, the Direct Metal AM part may have a completely different geometry than a cast component. An example of redesigning of a PSRU (Power Speed Reduction Unit) is provided in this paper.