New Way of Process Parameters Optimization in Sff Based on Deposition by Welding

To successfully control the welding process and to make it appropriate for solid freeform fabrication (SFF), it is necessary to fully understand the influence of the welding parameters and the geometry of the substrate on the resulting weld bead dimensions. Extensive experiments with different welding parameters and complex geometrical features such as edges and corners have been designed and completed. The experimental data show a clear correlation between the heat input, the weld bead dimensions, and the two dimensional (2D) geometrical features. This correlation may be used for on-line welding process control. It is found that the geometry of the molten pool is directly related to the heat transfer conditions determined by surrounding mass of material. A machine vision system based on a high-resolution CCD camera coaxially integrated with a gas tungsten arc welding (GTAW) torch is used to acquire the images of the molten pool. The results demonstrate the capability to adjust GTAW process parameters according to complex external and internal geometrical features of the substrate. The heat loss affected by the surrounding mass of material will be used to determine the optimal energy input. Introduction In the last decade, it has been shown that solid freeform fabrication (SFF) through layered material deposition is an attractive method for 3D-object generation. Common to all layered forming techniques is the incremental nature of the material build up. The quality of each deposited layer as well as the quality of the bond between the layers determines the quality of a built part [1]. The goal of any SFF technique is the fast construction of precise parts directly from CAD drawings, or as it is often called “from computer to component” process. While most of the SFF systems still make parts from non-metallic materials, not specified by the designer of the parts, SFF based on deposition by welding overcomes this disadvantage. Several researchteams [1 – 8] have shown that this technique is capable of making diverse part shapes. Spencer et al. made a vertical wall and a hollow box. Ribeiro et al. made a ‘chimney’ shape, a ‘bow tie’ shape, and a ‘pint glass’ shape. Kovacevic et al. also made a thin vertical wall, cylinders, cubical 3D parts with straight channels, a 3D part with complex geometry and a 3D network of the conformal channels. Relatively small changes to the welding parameters dramatically influence welding quality during the building of 3D parts by welding. So, an intelligent control system that controls the process parameters has to be developed. There are a number of parameters in GTAW which have a significant impact on the weld (i.e. bead) quality, such as: filler metal feeding speed, welding current, arc voltage, welding speed, shielding gas flow rate, electrode to substrate angle, type of shielding gas, shape of the tungsten electrode etc. Also, it should be noted that some of these parameters are non-linear and have transient-dependent and temperature-dependent