Additive manufacturing of complex metallic objects with overhanging features: slicing and path planning strategies

Panchagnula, Jayaprakash Sharma and S, Suryakumar (2017) Additive manufacturing of complex metallic objects with overhanging features: slicing and path planning strategies. PhD thesis, Indian institute of technology Hyderabad.

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Abstract

Additive Manufacturing is of great interest to various industries looking to create a part directly from the CAD model in a very short span. Amongst the laser, electron beam and arc based processes available today for the manufacture of fully dense metal parts, GMAW based arc wire deposition is of particular interest here owing to its advantages in terms of high material deposition rate, high power efficiency, lower investment costs, simpler setup and work environment requirements. The major challenge of weld-deposition method (also shared by processes based on other power sources) is the lack of support mechanism for realizing overhanging features. The aim of the proposed research is to investigate slicing and path planning techniques required for realizing such complex shapes. The associated investigations into the weld-deposition parameters and the thermal behavior of the part are also carried out. In the absence of a support mechanism, the complexity of the components fabricated would be limited to undercut free geometries that involve 3-axis kinematics. If the overhang is small, the features can be realized by exploiting the inherent overhanging capability of the weld-bead in 3-axis. However, if the overhang is large, the conventional 3-axis with uniform parallel slicing will not be applicable. In such geometries, it is possible to realize more complex shapes by adding extra degrees of freedom or by using higher order kinematics to the workpiece and/or to the deposition head by suitably aligning the overhanging feature in-line to the deposition direction. The kinematics analysis and coordinate frame transformations required for the same are presented in this thesis. The associated slicing and path planning required for higher axis kinematics have been divided into two categories viz., single-bead Multi-layer components and multi-bead Multi-layer components and analyzed separately. viii Single-bead Multi-layer components (or thin-walled components) refer to components with a single weld-deposition pass and no inner area filling. In the fabrication of such single-bead Multi-layer structures, while the deposition of the first layer is done on a flatwork plate, subsequent layers are deposited on the previous weld-beads which are curved. A geometrical model developed for predicting the weld-bead geometry for such bead-on-bead deposition. This mathematical model will be useful in the appropriate slicing of the CAD model, thus improving the dimensional accuracy of the structure. Multi-bead Multi-layer components, as the name reflects, involve multiple weld-deposition passes for a given layer. In the current study, objects with sudden or abrupt overhangs have been taken up for analysis. Some illustrative shapes, using the orthogonal weld-deposition were fabricated using this concept. Automated feature recognition (protrusion or depression and slots) from a tessellated CAD model and its deposition using higher order kinematics was also presented. An in-house MATLAB code was developed to realize the same. Owing to the sequential nature of the weld-deposition process, different parts of the component have different thermal histories. Maintaining consistent thermal conditions for every layer is hence important for ensuring uniform properties across the layers. A heat transfer conditions based thermal modelling was carried out to determine the dwell time required for the component to reach the required temperature.

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