Fabrication of metallic parts with overhanging structures using the robotic wire arc additive manufacturing
Introduction
Additive manufacturing (AM), also called 3D printing, is used to fabricate parts by depositing materials layer upon layer using information from a 3D model. Ding, et al. [1] states that AM has become an alternative to conventional subtractive manufacturing and promising technology to produce metallic components, such as titanium and nickel alloys in the aerospace industry where extremely high buy-to-fly ratios are often suffered. Lockett, et al. [2] explains that Wire Arc Additive Manufacturing (WAAM), a direct energy deposition technology, is particularly suitable for fabricating medium to large scale metallic components due to its high deposition rates and relatively low cost. For conventional AM, a 3D CAD model is sliced into a set of 2.5D layers along the deposition direction, and the slicing direction is elaborately selected to enable each layer to be deposited in the downhand position. However, Jiang, et al. [3] points out that in order to fabricate complex components using the WAAM, extra support is usually unavoidable to deposit ‘overhang’ structures. As shown in Fig. 1, to produce a part with an ‘overhang’ structure (in red), a temporary support structure (in yellow) would be required if being deposited in the fixed vertical up direction, resulting in the deposition of extra material, and the additional cost of a post-machining process to remove it.
Multi-directional Additive Manufacturing, or Multi-orientational Additive Manufacturing, has the potential to build up overhangs without extra support to improve the efficiency of the AM process. Yang, et al. [4] describes in the multi-directional additive manufacturing system, a deposition nozzle is mounted on a multi-axis actuator, such as the Computer Numerical Control (CNC) machine or the industrial robot, to deposit materials in multiple directions. This positional variant of the process has been developed for various materials over the past 20 years. Polymers were initially studied for multi-orientational deposition due to their low density and large surface tension force. Yang, et al. [4] presented a new deposition method to minimise or avoid the use of support structure. Compared to the maximum inclined angle of 45˚ reported in previous studies, the proposed method was claimed to be able to deposit plastics in an arbitrary direction. It was also shown that the surface quality of the parts and the efficiency of the AM process were greatly improved. In a study by Singh and Dutta [5], the authors reported that polymers based AM has the ability to deposit part along multiple directions to improve the surface quality and reduce the support usage. An algorithm was developed first to determine how much of a part should be deposited in a particular direction. By employing the method, a few parts were successfully fabricated to demonstrate the effectiveness of the new multi-directional AM system. In addition, Allen and Dutta [6] proposed a wall thickness control strategy for producing the thin-walled polymer parts. The proposed method enables manufacturers to build up objects with less support material and overall manufacturing time.
For multi-directional WAAM, the downward flow of the molten metal, induced by the force of gravity, makes it difficult to properly control the deposition process. Shirali and Mills [7] addresses some several inherent defects formed during deposition, such as irregular bead shape, lack of penetration, porosity, undercut and formation of the humped bead, which are difficult to avoid. In the study of molten pool behaviour and mechanism of weld forming in positional GMAW welding, many researchers focused on the selection of the optimal welding parameters. In the studies of Randhawa, et al. [8] and Ghosh, et al. [9], the use of optimal welding parameters can significantly improve the weld bead geometry in vertical welding. Recently, three-dimensional numerical simulation is used to describe molten pool behaviours [10,11]. They investigated the molten pool flow patterns for various welding positions. In addition, some molten pool control methods, such as rotating arc [12,13], and tandem arc [14], were applied to the positional welding for a better weld bead geometry.
Kazanas, et al. [15] conducted a preliminary study of the GMAW based multi-directional WAAM. Through the conducted experiment, the feasibility of fabricating parts with overhangs using multi-directional WAAM was verified and the torch travel speed selection was claimed to be critical for the process. Recently, Radel, et al. [16] presented an approach to automatically manufacture complex truss structures without any support. To optimize the bead geometry, an image based closed-loop control strategy was developed to monitor the bead shape and computer-aided manufacturing (CAM) software was used to corrects the geometry of the bead for future deposition. Nevertheless, there are still several inherent technical challenges in the application of the GMAW-based multi-directional additive manufacturing. The forming mechanism of a weld bead in various deposition directions has not been explored for multi-directional WAAM. Therefore, it is necessary to establish a set of practical guidelines for multi-directional WAAM.
In this paper, a practical study of fabricating metallic parts with overhang features using multi-directional GMAW-based WAAM is presented. Initially, different GMAW transfer modes were evaluated based on the metal droplet kinetic and the weld bead geometry. Subsequently, the effect of process parameters on bead formation and process stability were quantitatively investigated. The remainder of this paper is organized as follows: Section 2 evaluates the different GMAW transfer modes. Section 3 describes the experimental details. Section 4 provides experimental results and discussion. Finally, three case studies are given in Section 5 and followed by a conclusion in Section 6.
Section snippets
Metal transfer in multi-directional GMAW-based WAAM
Compared to conventional GMAW-based WAAM, the multi-directional deposition process is more difficult to control mainly due to the influence of gravitational force. In this section, two major issues are addressed: (a) the transfer of metal droplets from filler wire tip to the molten pool in the proper position, and (b) weld pool geometry and formation of the desired weld bead profile.
Experimental setup
The plain carbon steel ER70S-6 filler wire, with a diameter of 0.9 mm, was selected as the feedstock material in the welding system. A Q235 steel base plate was used as the substrate to support metal deposition. The dimensions of this plate were 300 mm × 300 mm × 10 mm. The chemical composition (in wt. %) of the wire electrode ER70S-6 and base metal Q235 are given in Table 1. A shielding gas mixture of Argon (80 %) and CO2 (20 %) was used at a flow rate of 18 L/min.
The tests were implemented
Results and analysis
In order to produce weld beads with preferred geometry, the effects of each welding parameter on the metal transfer process are crucial to be identified. In this section, several major process parameters were studied, including WFS, TS, NTWD and welding torch angle. The results were recorded to investigate the effect of each process parameters and to give a guide for multi-directional WAAM technology.
For the horizontal deposition process, the length of the bead was set to 120 mm. To avoid the
Case studies
To further verify the performance of the proposed strategy, three scenarios were used to fabricate parts with more complicated features and compared to the experiments from previous sections. For comparative reasons, the filler material, shielding gas, base plate, and the robotic welding system utilized in previous sections were kept the same for these case studies.
Fig. 21 presents a 3D model of a part with an overhang structure, coloured in red. To examine the capability of depositing an
Conclusion
This study focused on a new strategy of fabricating parts with overhangs using the multi-directional WAAM. Due to the negative impact of gravitational force, it is difficult to transfer the molten droplet to the target position and form a regular bead geometry in the multi-directional deposition process. To achieve a stable deposition process, the forming mechanism of positional deposits is initially investigated. Based on the analysis, GMAW using the CMT technology was selected for the
Funding
No funding was received for this work.
Intellectual property
We confirm that we have given due consideration to the protection of intellectual property associated with this work and that there are no impediments to publication, including the timing of publication, with respect to intellectual property. In so doing we confirm that we have followed the regulations of our institutions concerning intellectual property.
Research ethics
We further confirm that any aspect of the work covered in this manuscript that has involved human patients has been conducted with the ethical approval of all relevant bodies and that such approvals are acknowledged within the manuscript.
IRB approval was obtained (required for studies and series of 3 or more cases)
Written consent to publish potentially identifying information, such as details or the case and photographs, was obtained from the patient(s) or their legal guardian(s).
Authorship
All listed authors meet the ICMJE criteria. We attest that all authors contributed significantly to the creation of this manuscript, each having fulfilled criteria as established by the ICMJE.
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Declaration of Competing Interest
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