Provided by Timothy Cyders, assistant professor, mechanical engineering, Ohio University; Athens, Ohio

This FIERF-sponsored project experimentally investigated some of the fundamental aspects of using forging or other deformation techniques as post-processes to additively manufactured (AM) metals. Most metal AM processes use powder metallurgy to deposit, sinter or weld material as a part is built up. As such, additive metals generally lack the main benefit of wrought metals. Work hardening and the resulting grain refinement are absent from the deposition processes, generally resulting in relatively deficient material properties.

Therein lie some interesting opportunities for forging processes to become critical post-processes for specialized AM parts. It may be possible to produce parts that get the weight reduction, internal structures and topological optimization available through AM while enjoying the superior tensile and fatigue properties imparted by working the material. This study explored the effects of plastic deformation on properties of AM parts.

Experimental Approach

The main goal of this experiment was to characterize the effects of plastic deformation alone on the metallurgy and performance of AM parts. Since thermal processing can significantly change the nature of these materials, a cold process was used to obviate these effects. 316L stainless steel was selected as the material of interest, such that the grain structure would be relatively simple and consistent, and the same material chemistry could be produced by both AM and traditional processes. 

Tensile and fatigue samples were produced via direct metal laser sintering and from standard off-the-shelf sheet material. These samples were then put through a sequential cold-rolling process to introduce compressive strain. The apparent strain hardening in the samples was then characterized and compared to the actual deformation introduced to the materials to compare the behavior of the two processes in plasticity.

Results

Strain hardening considerably improved the mechanical properties of the AM material. Stress-strain behavior followed traditional strain-hardening models quite well. Fracture surfaces of the AM material appeared brittle compared to the typical ductile appearance of those in the sheet samples, even with high strain before failure (>30%). Metallography revealed that “scan tracks” (artifacts of the additive process) dominated the grain structure and anisotropy present in the AM samples. 

The directionality of the build process had a significant effect on both the tensile properties in the material and the effects of the cold work. Material rolled perpendicular to the build direction had opposite changes in anisotropy due to plastic deformation and exhibited different levels of strain hardening in response to deformation than those rolled along the build direction. In both cases, AM samples exhibited more strain hardening than was indicated by the induced thickness strain, suggesting that tensile performance of preforms made through this process may benefit significantly more from a given amount of deformation than would normally be expected. This difference was significantly higher than could be explained by traditional modeling of redundant work.

As expected, the high-cycle fatigue performance of AM samples was far below that of standard sheet samples. While the increase in ultimate tensile strength appeared to improve the fatigue performance of the standard sheet at high cycle life near the endurance limit, the AM samples experienced no increase in fatigue performance – even with an increase in ultimate tensile strength of nearly 50% – and exhibited no endurance limit. Failures occurred well into the 106-107 cycle regime. Fatigue fracture surfaces of the AM samples suggested, as did the tensile results, that the AM material did not completely behave as a material continuum and that fracture in fatigue of AM material is dominated by a variable other than the tensile properties of the material alone. Crack initiation appeared to be driven by the presence of scan tracks at the outer bending surface.

Conclusion

These results show that it appears to be possible to use work hardening to improve the tensile performance of AM parts, perhaps even more than would normally be possible through the same deformation of a solid material. More needs to be done to characterize the factors affecting high-cycle fatigue of these materials and the plastic behavior of the material, such that optimum design parameters for additive preforms can be developed. While this capability appears to be niche, there currently exists no other way to get the advantages of wrought processes in additive parts. The right modeling and implementation may put forging at the top of desired processes for AM parts demanding the utmost in mechanical performance.