A Study on the Forging of AM Parts

Additive manufacturing (AM) has become a specific topic of interest in the general manufacturing community due to its novelty and projected abilities for ground-up automation. In metalworking, AM processes have been studied in various depths over the past 20 years, just recently making significant strides as the popularity of metal-additive processes grows. Despite this growth, understanding the effects of AM artifacts on mechanical behavior is comparatively basic. Most metal-based AM processes have required significant post-processing to account for deficiencies in mechanical performance. Most mainstream material treatments are incapable of working the material and thus cannot take advantage of the superior performance imparted by deformation-based processes.

The Role of Additive Manufacturing

From a process perspective, it seems likely that AM will be most attractive in conjunction or in combination with the benefits of other traditional processes. This may include forging, if the mechanical behavior of metals with AM artifacts can be understood sufficiently to appropriately design such a process. The Forging Industry Educational and Research Foundation (FIERF) funded a project to begin to understand how these new additive processes could be combined with forging to create parts with better performance. This study sought to characterize how AM metal behaves during compressive plastic deformation and to compare strength, anisotropy and fatigue performance in traditionally manufactured and AM metal.

The Pros and Cons of AM

In short, AM has two key advantages: highly specific geometries can be made with generally the same ease as simple geometries, and internal voids and passageways can be easily built into the part to allow for easy lightweighting and geometric optimization. From a design perspective, however, AM has significant drawbacks in cost and mechanical performance.

Mechanical characteristics such as strength, isotropy and fatigue performance usually suffer because of porosity in the layup, layup directionality, surface finish and microstructure. For example, Figure 1 shows a fatigue sample from this study produced by direct metal laser sintering (DMLS). The print lines are easily visible on the part as-manufactured.

Internally, artifacts sometimes referred to as “scan tracks” are easily visible even after polishing and have significant effects on overall porosity, pore geometry and microstructure. Figure 2 shows a cross section of DMLS material etched to show scan tracks and related pores from the welding process. Also, the various methods to improve mechanical performance generally slow down the process, adding to cost.

Forging as a Post-Process

The use of forging as a post-process to AM could provide several significant benefits. Compressive plastic deformation on the outer layer of an AM preform could reduce porosity (thus improving mechanical performance) while simultaneously providing better microstructure and surface finish.

Hot isostatic pressing (HIP) is commonly used to improve AM part performance by collapsing microscopic internal voids. While the heat in the process can remove scan tracks, it generally does not improve microstructure beyond that of typical annealed material.

The core benefit AM can give to forged part design is its ability to produce optimized internal voids. While microscopic pores generally decrease mechanical performance, macroscopic internal voids such as those found in trabecular bone (also known as cancellous, or spongy, bone tissue) could not only mitigate fatigue crack growth but provide for selective stiffness, density and even heat transfer.

Open-cell internal structures could be supported during compressive deformation in a variety of ways, maintaining the functional structures in the preform while allowing the forging process to improve the mechanical characteristics of both the outer and internal surfaces. In short, this process could provide an avenue to produce forged parts that benefit from the advantages of AM while improving material performance to a level AM alone cannot otherwise achieve.

Mechanical Properties

This study characterized the effects of compressive plastic deformation on thin AM samples to examine their strain-hardening behavior and the resulting effects on porosity and fatigue life. Rolled sheet metal of the same bulk material was used as a control; 316L stainless steel was used due to its high ductility, ease of AM fabrication and comparatively low cost to other additive metals. Deformation was done cold in order to avoid heating as a confounding variable.

As the samples were deformed, their level of strain hardening measured by comparing true stress-strain curves from tensile tests of successive rolling steps (called total true strain here) was compared to the actual deformation introduced (apparent true strain). The results are shown in Figure 3.

The AM samples appeared to experience some deformation without bulk hardening during the first step and then hardened at a faster rate than was predicted with a basic friction model. The directionality of the laser scan had a significant effect. Samples that were rolled transverse to the laser-scan direction hardened at a slightly lower rate than those rolled longitudinally, and measurable bulk density changes were detected in the first rolling step of the longitudinal samples but not in the transverse samples. The rolling process clearly collapsed pores, but initial pore geometry, size and distribution were not measured or modeled in this work, so the localized effects could not be directly modeled.

While the AM samples appear to have experienced more internal redundant work, the mechanism by which they did was not immediately apparent. Microhardness tests were conducted across the cross sections of as-received and rolled AM and sheet samples. Results showed more absolute hardening in the sheet samples than the AM samples.

Fatigue properties appeared unaffected by strain hardening, but no endurance limit was detected in these particular samples. Scan tracks dominated the fatigue-failure surfaces, likely because the deformation process was done cold and didn’t remove the scan tracks. Figure 4 shows the sample fatigue-failure surfaces of sheet and AM samples. The sheet specimens exhibit a clear neutral axis where cracks met at fracture. Unrolled AM samples had an extremely irregular fracture surface that seemed to be dominated not by sharp crack growth but instead by rounded paths that left powder artifacts from the printing processes intact. Rolled DMLS samples did not have the same fracture-surface appearance as the unrolled DMLS samples, but they still did not exhibit surfaces commensurate with normal crack growth in continuous material.

AM metals are often characterized by modulus, yield strength, ultimate tensile strength and porosity/density in comparison to traditionally manufactured metals of the same rough chemistry. Indeed, increased porosity and reduced modulus have been shown to be significant indicators of poorer print quality and poorer resulting mechanical properties in additive materials, but there is a caveat: Most methods for determining porosity characterize the relative volume of empty pores in the material but not the shape or orientation. Pores that change significantly in shape as they are deformed could cause unexpected deformation characteristics, including localized hardening at much lower or higher levels than conventional modeling of bulk shape change would reflect.

This weakness could be manipulated into something useful. The introduction of purposefully shaped internal voids could allow more strain hardening than was previously possible with the same deformation process. By compressing a very small void with a high aspect ratio, the material could be worked more around the quadrants of the void than the bulk dimensional change of the elements around it would reflect.

AM preforms, or even modified conventional preforms with selected areas of additive manufacture, could be used to produce parts with exceptionally higher performance than normal by introducing additional deformation in key areas on the part. Work is currently being done to create simple models and corresponding specimens to explore this phenomenon. If successful, such a tool could be used to create highly optimized parts for lightweighting or fatigue performance.

Conclusion

Plastic deformation of AM material stands to improve mechanical performance. While metal additive manufacturers strive to achieve material properties through chemistry and process parameters, the ability to work the material still provides microstructural controls that are otherwise very difficult to achieve. While the market for such a process is likely niche, there is potential for some significantly advanced work in combining additive and deformation processes to produce high-performance parts.