# Geometric Limitation and Tensile Properties of Wire and Arc Additive Manufacturing 5A06 Aluminum Alloy Parts

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## Abstract

Wire and arc additive manufacture (WAAM), as an emerging and promising technology of metal additive manufacturing, it lacks of experimental works to clarify the feature of geometrical configuration, microstructure and tensile properties, which can be used for further evaluating whether the as-deposited part can be used directly, and providing design reference for structure optimization. Taking 5A06 aluminum alloy additive manufacturing for example, in this paper, the geometric limitation and tensile property criteria are characterized using experimental method. The minimum angle and curvature radius that can be made by WAAM are 20° and 10 mm when the layer width is 7.2 mm. It shows isotropy when loading in build direction and perpendicular one. When loading in the direction of parallel and perpendicular to texture orientation, the tensile properties are anisotropic. The difference between them is 22 MPa.

## Keywords

geometric limitation mechanical anisotropy microstructure tensile property wire and arc additive manufacture## Introduction

As an important engineering structure material for aircraft and space vehicles, 5A06 aluminum alloy is widely used as fuselage skin, aerofoil, fuselage frame, fuel tank and so on. To ensure longevous service and the high reliability, large-scale, integrated and functional 5A06 aluminum alloy components are preferable for aeronautical manufacturing. Wire and arc additive manufacture (WAAM) characterized with high efficiency and low cost is thought to be preferable for rapid prototyping large-scale aluminum alloy part. However, it lacks basic data to clarify the geometric limitation of WAAM and performance criteria, which are the reference data to evaluate whether the geometric configuration can be achieved using WAAM and whether the mechanical properties of as-deposited part can meet the requirement of service conditions.

Cranfield University together with Rolls-Royce started to center on developing wire and arc additive manufacture as a means of reducing the wastage levels of expensive and high-performance alloys that can occur in conventional processing, such as nickel-based alloy and titanium-based alloy at the beginning of 1990s (Ref 1). Until a major three and a half year €2.7 million European research project entitled rapid production of large aerospace components (RAPOLAC) was approved to develop WAAM based on cold wire feed gas tungsten arc welding (GTAW) in Cranfield University, WAAM technology has got more and more attention for its distinct advantages in forming efficiency and cost. Research work about WAAM continues at the University of Nottingham (Ref 2), the University of Wollongong (Ref 3) and Southern Methodist University. Kazanas investigated the production of geometric features using wire and arc additive manufacturing with positional welding. It is also useful for building features with limited accessibility without manipulating the workpiece. Inclined, horizontal wall and enclosed features could be built using an inclined torch (Ref 4). They clarified the forming ability of spatial geometric shape using WAAM. How about the forming ability of plane geometric shape? It determines whether the final part can be deposited with layer-up-layer fashion using WAAM. In AM path planning, a stack of 2D closed contours is obtained when a 3D model is sliced. Each slice may have a set of closed contours or polygons (Ref 5). Many types of tool-path patterns have been developed for AM, such as raster scanning path technique (Ref 6), which is simple implementation and suitability for almost any arbitrary; zigzag tool-path generation (Ref 7), which is the most popular one in commercial AM machine; and spiral tool-path generation (Ref 8), which has been widely used in numerically controlled (NC) machining. They all concentrate on the importance of filling the outline of the image with vector motions. However, WAAM is different from laser additive manufacturing, and because of the large molten pool and surface tension, sharp angle and curve shape with large curvature usually cannot be formed accurately. Ding et al. (Ref 9) provided a method of decomposing 2D geometries into a set of convex polygons basing on a divide and conquer strategy, so as to simplify the complex shape. However, it also encountered the above-mentioned problem. Plane geometric limit is a constrain condition for 2D path planning.

The feature of geometric configuration, microstructure and property of aluminum alloy deposited using WAAM technology is still needed to be further studied, which can be used for further evaluating whether the as-deposited aluminum part can be directly put into service. Similar to casting and forging, WAAM should also establish related standard to facilitate the industrial application. Now it lacks industrial standard to evaluate the additive manufactured parts from geometric configuration and mechanical property perspective. What is the minimum limit of geometry, such as the minimum radius of curvature, the minimum angle? The as-deposited Ti-6Al-4V part has distinct microstructure and mechanical anisotropy (Ref 10), which will confine the structure layout comparing the isotropic materials. Then, is the as-deposited 5A06 aluminum alloy anisotropic or isotropic? This paper aims to clarify them and provide basic data about additive manufactured 5A06 aluminum alloy for path planning and structure designer. So plane geometric configuration and tensile properties (ultimate tensile strength and elongation) are characterized.

## Experimental Procedure

Major chemical composition and mechanical properties of the raw 5A06 wire

Major chemical composition, wt.% | Mechanical properties | |||
---|---|---|---|---|

Mg | Al | Tensile strength, MPa | Elongation, % | |

5A06 | 6.4 | 93.6 | 346 | 12 |

*I*

_{p}) was set to 120, 140, 160 and 180 A, the background current (

*I*

_{b}) was set to

*I*

_{b}=

*I*

_{p}– 50 A, pulse frequency was 50 Hz, and duty cycle was 0.5. Travel speed (

*v*

_{T}) was 0.15, 0.20, 0.25 and 0.30 m/min. Wire feed speed (

*v*

_{W}) was 1.2, 2.0, 2.8 and 3.6 m/min.

Deposition parameters

Parameters, unit | Notation | Value |
---|---|---|

Peak current, A | | 160 |

Base current, A | | 110 |

Average current, A | | 122 |

Peak time, s | | 0.1 |

Base time, s | | 0.1 |

Travel speed, m/min | | 0.25 |

Wire feed speed, m/min | | 2.0 |

Average volt, V | | 16.4 |

Layer height, mm | | 1.3 |

## Results and Discussion

### Geometric Limitation for WAAM Process

Increasing the centerline spacing from half of the layer width to nine-tenths of the layer width, the two weld beads are detached gradually. Nine-tenths of the layer width is the minimum centerline spacing to obtain separate weld beads. If the spacing is less than this value, the final obtained shape is determined by the surface tension and the two weld beads are overlapped. So in Fig. 3(c), when the vertical distance increases to 7.5 mm what is nearly equal to the layer width, the initial and terminal sides of the angle begin to separate from each other. In Fig. 4(c), when the curvature radius is set to 5 mm, molten metal accumulates rapidly at the curve segment where the span is equal to nine-tenths of layer width, so inner outline of which is distorted and the designed plane curve shape cannot be deposited accurately. Plane angle of 20° and curvature radius 10 mm are the geometric limitation for WAAM when layer width is 7.2 mm.

### Tensile Properties in the Direction of Parallel and Perpendicular to Build Direction

The samples extracted from the single wall in x and y directions are isotropic when varying the process parameters. There is a little change in tensile strength when deposited with varying parameters, so is the yield strength. The average value of tensile and yield strength is 273 MPa and 124 MPa, respectively. Elongation in two directions is in the range of 31–36%, and the average value is 34%. The mean square error of the three tested mechanical parameters is 3.032, 2.216 and 1.374, respectively. So build direction and process parameters are not constraint conditions during formation of path planning, and loading in x and y directions is identical in service.

### Tensile Properties in the Direction of Parallel and Perpendicular to Texture Orientation

In Fig. 11(a), the average tensile strength of specimens in the direction of perpendicular to texture orientation is 251 MPa, and the mean square error is 10.504. The average yield strength of specimens in the direction of perpendicular to texture orientation is 101 MPa, and the mean square error is 8.920. The average tensile strength of specimens in the direction of parallel to texture orientation is 239 MPa, and the mean square error is 7.918. The average yield strength of specimens in the direction of parallel to texture orientation is 90 MPa, and the mean square error is 7.653. In Fig. 11(b), the average elongation of specimens in the direction of perpendicular to texture orientation is 34%, and the mean square error is 1.856. The average elongation of specimens in the direction of parallel to texture orientation is 37%, and the mean square error is 5.066.

Grain boundary sliding is a main mechanism to dominate the plastic deformation of aluminum. In Fig. 12(a), when loading in the direction of parallel to texture orientation, grain boundary slides along the texture orientation (i.e., loading direction) in the layers. In the bounding region, grain boundary slides along x direction. So the plastic deformation will lead to dislocation generation and piling up in the bounding region. With the strain increase, dislocation density increases rapidly. A main dislocation band with high dislocation density in bounding region will be generated during the loading process. However, the region of high dislocation density is unstable. It is prone to crack nucleation and propagation. The cracks are torn or forming pores after the sample break. The pores can be found by observing SEM fractograph. When loading in the direction of parallel to texture orientation, as shown in Fig. 12(b), grain boundary sliding happens in bounding region firstly. In layers, tangential force along texture orientation is small, and only high-angle boundary can start to slide, which is accompanied by intragranular dislocation glide and climb. So the tensile strength is higher than tensile in the direction of parallel to texture orientation. The tensile sample fractures in the bounding region. Laminated tearing can be observed by observing SEM fractograph.

The tensile strength in y (or x) direction is 22 MPa higher than that in the direction of perpendicular to texture orientation and 34 MPa higher than that in the direction of parallel to texture orientation. So wire and arc additive manufactured 5A06 aluminum alloy also shows moderate anisotropy among y (or x) direction, directions of parallel to texture orientation and perpendicular one. In wire and arc additive manufacturing 5A06 aluminum alloy process, slicing along the plane perpendicular to primary loading direction is advisable because of the anisotropy, or else taking the minimum mechanical performance index as reference value to optimize geometric structure and size.

## Conclusion

- (1)
Under the deposition parameters in the paper, angle exceeding 20° is preferable for WAAM. The initial and terminal sides of the angle will overlap if the angle is smaller than 20°. The minimum curvature radius that can be made by WAAM is 10 mm when the layer width is 7.2 mm. If the curvature radius is set to smaller than this value, the inner outline will distort, which goes against the following layer deposition.

- (2)
In x (horizontal) and y (vertical) direction, the tensile test results show isotropy. The average value of the tensile strength, yield strength and elongation is 273 MPa, 124 MPa and 34%, respectively.

- (3)
In the direction of parallel and perpendicular to texture orientation, the tensile properties are anisotropic. The average tensile strength and yield strength of specimens in the direction of perpendicular to texture orientation are 251 and 101 MPa. The average tensile strength and yield strength of specimens in the direction of parallel to texture orientation are 239 and 90 MPa. The average elongation in the direction of parallel and perpendicular to texture orientation is 37 and 34%.

## Notes

### Acknowledgments

This work was supported by the National Natural Science Foundation of China (Grant Nos. 51475376 and 51575451) and the Research Fund of the State Key Laboratory of Solidification Processing (NWPU), China (Grant No. 109-QP-2014).

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