A tool-path generation strategy for wire and arc additive manufacturing

Introduction

Additive manufacturing (AM) machines have evolved over the last three decades from a limited number of expensive prototypes to widely available small-scale commodity production tools . These machines can automatically fabricate arbitrarily shaped parts layer-by-layer from almost any material. The AM process has drawn significant research interest for a variety of industrial applications in the manufacturing, medical, architecture, aerospace, and automotive sectors. A particularly interesting application is the production of large sized aerospace components that are currently machined from costly wrought material, such as Ti-6Al-4V. AM offers the possibility of producing these parts at very low fly-to-buy ratios in comparison to current production practices. Many techniques have been developed for manufacturing metal structures in AM, such as Selective Laser Sintering , Direct Metal Deposition , Electron Beam Freeform Fabrication , Shape Deposition Manufacturing , and WAAM [6-8], etc. In terms of power sources, AM can be classified into three groups, namely, laser, electron, and arc. WAAM is by definition an arc-based process that uses either the Gas Tungsten Arc Welding (GTAW) or the Gas Metal Arc Welding (GMAW) process, and is considered to be a promising technology for fabricating functional metal parts. With the advantages of higher deposition rate, lower costs, and safer operations, WAAM is considered to be a more realistic

method for manufacturing aerospace components with median to large size. Generally, the deposition rate of laser or electron beam deposition is in the order of 2-10 g/min, compared to 50-130 g/min for WAAM . However, a mature WAAM system is still not commercially available to industry at present due to a number of inherent technical challenges, such as distortion and residual stress from excessive heat input, and uneven weld bead geometry distribution within weld paths. The issue of distortion and residual stress could be minimized through using thermal tensioning technology or adopting a low heat input process such as Cold Metal Transfer (CMT), but these issues are beyond the scope of this paper. This paper focuses on improving the weld bead geometry and increasing the surface accuracy of the built parts through improved tool-path planning. Uneven weld bead geometry may lead to the accumulation of errors in the vertical direction after the deposition of several layers. Figure 1 shows an example of thin walls built by weld deposition where there are significant differences in bead geometry at the start and end of the weld paths . As can be seen, the errors introduced in lower layers are compounded as further layers are added. To overcome the issue of uneven surface induced by arc start and arc end procedures, Zhang et al.  adjusted the deposition parameters at the start and end portions of weld paths to flexibly control the weld bead geometry. However, the control procedures are empirical and time-consuming. Hybrid layer manufacturing processes, which combine the means of both additive and subtractive manufacturing, have recently been developed . The hybrid processes employ intermediate machining of the upper surface between successive layer depositions to overcome the layer surface roughness and to avoid the cumulative deviations in build height. Nevertheless, such cleaning steps increase the complexity of the system, and reduce the productivity of AM technology.

Fig.1 Thin walls built by weld deposition showing the changing bead geometry at the start and end of weld paths

This paper develops a path planning algorithm to improve surface accuracy of the WAAM process. The proposed path planning algorithm is able to generate a continuous toolpath to fill a large class of geometries without starting-stopping sequences. The current state of research on path generation strategy is reviewed in Section 2. Section 3 introduces the detailed path planning algorithms.