Fabrication of metallic parts with overhanging structures using the robotic wire arc additive manufacturing

https://doi.org/10.1016/j.jmapro.2020.03.018Get rights and content

Highlights

  • A theoretical analysis of the metal transfer behaviour and bead shape formation using positional GMAW are provided.

  • The effects of various process parameters on the stability of positional deposition are investigated.

  • The effectiveness of the proposed strategy is verified by three complex samples using a positional GMAW-WAAM process.

Abstract

Robotic wire arc additive manufacturing (WAAM) technology has been widely employed to fabricate medium to large scale metallic components. It has the advantages of high deposition rates and low cost. Ideally, the deposition process is carried out in a flat position. The build direction is vertically upward and perpendicular to a horizontal worktable. However, it would be difficult to directly deposit complex parts with near horizontal ‘overhangs’, and temporary supports may be required. Thus, it is necessary to find an alternative approach for the deposition of ‘overhangs’ without extra support in order to simplify the deposition set-up. This paper proposed a fabrication method of producing metallic parts with overhanging structures using the multi-directional wire arc additive manufacturing. Firstly, based on the metal droplet kinetics and weld bead geometry, two different Gas Metal Arc Welding (GMAW) metal transfer modes, namely short circuit transfer and free flight transfer, were evaluated for the multi-directional wire arc additive manufacturing. Subsequently, the effects of process parameters, including wire feed speed (WFS), torch travel speed (TS), nozzle to work distance (NTWD) and torch angle, on the stability of positional deposition were investigated. Finally, the effectiveness of the proposed strategy was verified by fabricating three complex samples with overhangs.

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.

We confirm that the manuscript has been read and approved by all named authors.

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Contact with the editorial office

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Declaration of Competing Interest

No conflict of interest exists.

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