Additive manufacturing of metallic alloys is an emerging technology that prints 3D components. The main advantages of this technology are the manufacture of complex geometry, the gain of raw material, and the rapid production of any geometry, which reduces the supplying time in case of maintenance. However, as-built additive manufactured components generally contain unfavorable porosities and a high surface roughness. These geometric flaws involve that fatigue properties of as-built components are inferior to that of their conventional counterparts. Fatigue properties represent the capability of a material to resist crack initiation under repeated loading. The fatigue phenomenon is a major source of fracture in industry, and is usually the prime property in mechanical design. As a result, enhancement of fatigue properties of metallic additive manufacturing remains a paramount research topic for industrial applications. In order to enhance fatigue properties of as-built components, post-treatments such as hot isostatic pressing, machining, and/or polishing can be performed. While the hot isostatic pressing is costly, polishing or machining a component is time consuming. 

Shot peening has been used for decades to increase fatigue properties of material. In regard to additive manufacturing, studies have shown that shot peening introduced beneficial compressive residual stress as well as cold work, and decreased as-built surface roughness (Ardi et al., 2020; Bagherifard et al., 2018; Eric et al., 2013; Wood et al., 2019). Therefore, shot peening appears to be the perfect candidate to increase the fatigue properties of metallic additive manufacturing at low cost and time. The main objective of this study is to demonstrate that shot peening can enhance fatigue life of as-built additive manufacturing material. A secondary objective is to compare fatigue properties of as-machined and as-built fatigue samples to conclude whether post machining as-built structures is mandatory to increase fatigue properties.

Material

The laser-power bed fusion was the additive technology used to manufacture the fatigue samples. The feedstock powders were Inconel 718 (IN718) nickel superalloy. The scanning strategy was a combination of contour and hatch strategies, as schematically described in Figure 1. The corresponding microstructure was a combination of small and large grains resulting from the different cooling rates experienced during manufacturing. 

Microstructure of additive manufacturing material differs from conventional route manufactured material. Herein, a duplex grain microstructure was observed, as shown in Figure 2a. Finer grains resulting from the contour strategy at the surface of the sample was also observed, as shown on the bottom of Figure 2a. Figure 2b shows a material optical micrograph, in which a high density of circular and acicular porosities were observed. 

Methodology

Since IN718 material was intensively used in aerospace industry for high temperature applications, fatigue tests were performed at 550°C. Samples were fatigue tested at a stress ratio (R) of -1 and at a maximum stress (σmax) of 650 MPa.  In regard to the present objectives, four surface finishes were compared, namely, as-built (AB), as-built and shot peened (AB-SP), as-machined (AM), as well as as-machined and shot peened (AM-SP). Three fatigue tests were performed for each surface finish.

Results and discussion

Figure 3 presents a histogram of the fatigue results, from which two points are drawn. 

Firstly, the average fatigue life of the AB samples was about 5 times higher than that of the AM samples, although the average surface roughness of the AB samples was about 4 times higher. Studies on IN718 (Balachandramurthi et al., 2018; Kelley et al., 2015) have shown at room temperature that fatigue lives of AM samples were drastically higher than those of AB samples.  The authors attributed the superior fatigue properties of AM samples by the lower surface roughness. However, the observed trend herein invalided this rationalization for the studied material. In fact, the contour strategy technique used here was most likely the cause of the superior fatigue properties of AB samples. Indeed, this technique introduced finer grains at the surface yielding to a surface layer having enhanced fatigue properties. In addition, fractographic observations after fatigue tests showed surface crack initiation on lack of fusion porosities for the AM samples, as shown in Figure 4a, while cracks initiated at the surface on roughness irregularities for the AB samples, as shown in Figure 4b. In summary, the contour strategy had beneficial effects on fatigue life by introducing finer grains in the near surface layer without porosities.

Secondly, shot peening AM and AB samples increased average fatigue lives and decreased fatigue dispersion of AB surface condition. The fractographic analyses of the shot peened samples showed that cracks systematically initiated on lack of fusion porosities at the subsurface, as shown in Figure 4c. The baseline conditions, namely, AM and AB had either a short fatigue lifetime when crack initiated at the surface, or a longer fatigue lifetime when crack initiated at the subsurface. Shot peening introduced beneficial in-depth compressive residual stress over 200 µm with a value at the surface of about 740 MPa, and decreased surface roughness of AB samples by four. These beneficial effects of shot peening protected the surface layer from crack initiation and thus increased fatigue lifetime. In conclusion, shot peening enhanced fatigue lifetime, and can be used as a surface treatment on additive manufactured structures in an as-built state.

Conclusion

Additive manufacturing is an emerging technology that has the drawback of producing materials with lower fatigue properties than those produced using traditional manufacturing processes. 

The present study showed that shot peening enhanced fatigue lifetime at 550°C of IN718 additive manufactured. This study indicated that shot peening can be a beneficial surface treatment to improve the fatigue properties of additive manufactured structures in an as-built state.

Charles Bianchetti 

Ph.D, Research Scientist

ONERA (Office Nationale d'études et de recherches aérospatiales)

DMAS - Department of materials 

and structures

M3S - Metallic material mechanics group

E-mail: charles.bianchetti@onera.fr

Co-authors:

Louise Toualbi 

Ph.D, Research Scientist

Pascale Kanouté

Ph.D,Research Scientist

E-mail: louise.toualbi@onera.fr

pascale.kanoute@onera.fr