Investigation into Freeform Fabrication of Multi-Material Parts by 3D Welding and Milling Process
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Mat. Res. Soc. Symp. Proc. Vol. 625 © 2000 Materials Research Society
(a)Welding
(b)Face milling
(d) Product
(c) Finishing
Fig. 1: Process principle of 3D Welding and Milling EXPERIMENT Starting from the single bead experiment, the influence exerted by the main process parameters, voltage, current and welding speed on the formation of single beads was investigated. Then, multiple layers were built to examine the microstructure and the hardness of the layers. The focus was also on the feasibility study of direct fabrication of multi-material parts. Principally, there are two approaches available when it comes to deposit multi-materials, Fig. 2. The first approach is to adapt the number of welding guns to the number of materials to be used for the deposition. However, the different thermal expansion coefficients have to be considered when selecting the material combination. The second approach consists in building only the shell without filling the inner area. When the build-up process is completed, these areas are then filled with low-melting alloys. This allows a short deposition time at lower warpage. 1. Use of two welding guns
2. Building shell structure and subsequent filling
Fig. 2: Two different approaches for depositing multi-material parts
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To realize these approaches, the "HybridRP" machine was developed in cooperation with Korea Institute of Machinery and Materials KIMM. It consists of a 3-axis vertical milling machine and two straight MIG welding guns attached to the machine spindle. These two welding guns allow depositing either two different sorts of wire material or two different sizes of the same wire sort in a layer. The single beads are deposited on a substrate plate which is preheated up to 200 0 C. This preheating is necessary to reduce the thermal stress build-up during welding. This kind of machine configuration has a considerable advantage over other SFF machines in that only a simple retrofitting of an existing 3-axis milling machine is required in order to carry out the process, thus eliminating any need to purchase special equipment. RESULSTS Optimizing the parameters in the ranges of 16V-36V and 60A-140A at a welding speed between 200 mm/min and 1400 mm/min, permitted the deposition of single, connected beads instead of loose spheres. The suitable welding parameters for the deposition of mild steel AWS 5.18 E70S-6 with a wire diameter of 0.9 mm were found to be 19 V and 120 A at a welding speed of 1.2 m/min. Fig. 3 shows a thin wall made of 8 layers with and without finishing. In case of the part without machining, the instability of the arc welding process caused a defect in the middle of the bead deposited. Initially small in size, the defect in the layer influenced the height of the next bead deposited, thereby increasing the size of the defect, until no further deposition was possible. This result shows that face milling is necessary not only to increase geometrical accuracy in z direction, but also to stabilize the deposition process. In case of thin walls, the minimum
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