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There has been no investigation of additive manufacturing (AM) of semi-finished sheets for subsequent forming. By combining laser powder bed fusion of metals with forming technology, we demonstrate the potential for resource efficiency and effective use of build chambers. This process chain is designed to save up to 50% on processing time and strengthen materials by hardening hastelloy x sheet, x-sheets. 

An essential aspect of this paper is the understanding and characterization of the flow behavior of additively manufactured semi-finished parts for forming the nickel-based superalloy Hastelloy X. Monolithic additively manufactured tensile, compression, and in-plane torsion specimens are used to characterize sheet metal forming. The resulting characterization and yield criteria can be used to predict the forming behavior of additively manufactured semi finished parts with integrated functions like cooling channels that are formed in their final geometry. 

Stress-strain diagrams are corrected for surface roughness using a correction function. The material exhibits a high anisotropic yield stress with a near isotropic hardening behavior. The heat treatment reveals a homogenization of the material accompanied by an isotropic initial yield stress but anisotropic yield behavior. Different yield surfaces are considered to numerically model those effects in light of the material characteristics presented previously. The additively manufactured Hastelloy X is found to have high tensile strength as well as excellent formability.

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Introduction

AM provides a resource-efficient way to manufacture geometrically complex parts, as opposed to milling. However, it is important to reduce the drawbacks such as an ineffective use of build chamber volume and a three-dimensional packing problem. Until now, additive manufacturing has mostly been used in toolmaking for creating complex shaped dies that are integrated with cooling channels or sensors. The method can also be used to repair damaged tools at a reasonable cost. 

An integrated cooling channel was developed by Hölker and Tekkaya in a hot extrusion die. A conventionally shaped die bridge is manufactured by drilling and milling, and Selective Laser Melting (SLM) is used to additively manufacture the mandrel with cooling channels. A laser beam melting process was used to manufacture a hot forming-tool insert with internal cooling, according to Muller et al. An incremental sheet metal forming and laser powder deposition (LPD) process was invented by Hölker et al.. An incremental forming operation is applied to a flat sheet and then a complex shaped element is additively manufactured on top of the formed sheet. 

It was presented and analysed how to combine deep drawing with Laser Beam Melting to produce a functional element on top of a thin preformed sheet as a hybrid process. It was shown by Silva et al. that wire arc welded aluminum blocks could be bent to reduce unwanted support structures, and compression tests were conducted on the aluminum blocks to determine if they could be formed. With the combined process chain of bending and laser powder deposition, Babach showed that a forging operation could be replaced. The part was produced with time- cost- and energy savings of ≈30−40%, which count for small lot sizes and for the specific process route of combining bending and welding onto the preformed substrate. 

An advantage of laser powder deposition in this case is the high build-rates. A forming operation has not been performed on the additively manufactured part. There has been no direct research that focuses on cold forming additively manufactured sheet metal produced by laser powder bed fusion and AM-parts are still being regarded as unsuitable for cold forming. Combining additive manufacturing with a forming operation is a new approach in additive manufacturing. This study compares two possible routes to produce a U-shaped part.

Process route A, shows the part that is additively manufactured to its final shape. It requires a support structure and extensive post-processing. Route B, on the other hand, involves AM combined with forming. For efficient use of the build chamber volume, sheets are manufactured with a core structure and integrated functional elements (e.g. cooling channels) and stacked parallel to each other. The final shape and contour are determined by the subsequent forming operation (bending, deep-drawing, etc.). 

The flexibility and free complexity of AM-processes combined with the high productivity of metal forming operations allows time savings of up to 50%. In addition to strengthening the part itself, this combination of processes can also be used intentionally in areas where the part is heavily loaded by strain hardening caused by the forming process. As the AM-process allows an optimized core structure to be designed, sandwich sheets for a subsequent forming operation bring many advantages and challenges. 

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A subsequent forming operation, on the other hand, presents a challenge. The material is seldom characterized for the purpose of a forming operation and is highly dependent on the process parameters. In this study, bending is performed on sandwich sheets created by laser powder bed fusion of metals at the Institute of Forming Technology and Lightweight Components (IUL) and formability is strongly influenced by core structure characteristics.

Simulations are therefore necessary to develop formable core structures. In order to perform a successful simulation, the material must be characterized. For additive manufacturing, tensile test specimens are usually round turned according to DIN ISO 6891-1. A topologically optimized structural bracket for aerospace applications was numerically analysed by Brusa et al. Through linear static analysis, he derived Young's modulus, yield strength, and Poisson's ratio from round tensile tests and reduced the mass to 80% of its original value. 

Neither anisotropy, strain hardening, nor ultimate tensile strength were taken into account. A layerwise build up of AM-materials leads to anisotropy, which is important to take into consideration during forming. Etter et al. The effect of post heat-treatment (HT) and different scanning strategies on the anisotropy of Young's modulus of Hastelloy X was studied. By using HT and rotating the scanning direction by 67° between the layers, the anisotropy could be reduced. Anisotropy in the plastic state was not taken into account, but it is crucial to understanding deformation behavior at high strains. In hot compression tests, Bambach et al. found that Laser-Metal-Deposited (LMD) specimens can be forwarded in the same way as wrought materials at high temperatures based on their results.

A study by Strößner et al. examined how different heat treatment routes affect Inconel 718's microstructural and mechanical properties. He demonstrates that the material exhibits highly anisotropic behavior in vertical and horizontal directions. As a result of the HT route, the material hardens to high ultimate tensile stresses. The ductility is measured by strain to failure, which is not an appropriate measure of formability because it includes necking strain. Sufiiarov et al. Provides a shallow overview of the dependency of parts quality on process parameters. The mechanical properties show a large deviation, which shows the dependency of the material properties on process parameters. According to Babu et al.

Inconel 718 has been extensively studied for its qualification challenges, process parameters, and material characteristics. Using different hot isostatic pressing conditions, he investigated stress-strain behavior. Despite the above cited literature, a numerical model of a forming operation is lacking due to the lack of investigation and understanding of flow behavior. Even at temperatures of approximately 600 °C, Inconel 718 still exhibits low ductility at high strengths. However, Hastelloy X has good formability and corrosion resistance, even at elevated temperatures, but has never been studied for forming applications with additive manufacturing.

There has been no investigation of material behavior under shearing. Suh et al. suggest that uniform elongation is influenced by surface roughness. The uniform elongation and strength are found to decrease when the ratio of specimen thickness to surface roughness is reduced. This can be attributed to a change in surface to volume ratio, and thus the topography of the surface roughness cannot be neglected anymore. Surface roughness of AM parts is ten times higher than that of rolled sheet metals. This effect should be investigated.

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As a conclusion to the current state of the art, additive manufacturing has not yet been explored for semi-finished parts that will be used for subsequent forming operations. Institute of Forming Technology and Lightweight Components has been researching new combinations and routes for processing lightweight components. In addition to increased productivity and cost-efficiency, additive manufacturing and subsequent forming have a number of other advantages. Strain hardening provides a higher grade of lightweight construction by strengthening the formed part. Subsequent forming processes like ball burnishing of AM-surfaces are already used. 

Due to the complex material properties of AM parts, establishing more complex and global forming operations, such as bending or deep drawing, is currently hindered by the lack of numerical simulation capability for such processes. In order to build a predictive simulation model, it is imperative to describe the plastic flow behavior under various stress states of the AM material. As semi-finished sandwich sheets have a complex structure, Hastelloy X is experimentally investigated with monolithic specimens because of its complex structure. As a result of examining plastic flow behavior and its physical relationship to additive manufacturing, a valuable contribution can be made to the state of the art of combining additive manufacturing and forming technologies.

In this paper, we examine the elastic-plastic characterization at high strains of monolithic additively manufactured Hastelloy X under different stress states in order to develop core structures and simulations for the future. Different surface finishes and heat treatments are investigated for tension, compression, and shearing. A variety of metal forming methods are used to characterize the material, including flat tensile tests, round compression tests, and in-plane torsion tests. Tests are performed on hot rolled sheet metal as a reference material and in a direction perpendicular to the build platform with Laser Powder Bed Fusion of Metals (L-PBF-M). 

A set of possible yield surfaces is derived from the mechanical characterization, which serves as a foundation for simulating additive manufacturing operations.

Experiments

Material stress-strain responses are determined by testing specimens under different stress states, after heat treatment and different surface finishing operations.

These are the dimensions of the flat tensile specimens according to DIN ISO 6891-1 and DIN 50125. The additively manufactured specimens were made in the xz-building plane and the specimens' longitudinal axis was x. 

Yield surface

The yield surface includes the yield stresses under all three dimensional stress-states in the principal axes system (σI,σII,σIII). A possible set of yield surfaces for additively manufactured Hastelloy X is discussed using the previously achieved mechanical characterization. Because of this, the principal axes directions are the same as those of the AM build.

The mechanical properties in the y-direction are assumed to be equal to those in x-direction. This is due

Conclusion

As part of this study, a fundamental material characterization was performed in order to determine a yield criterion based on three dimensions. In comparison with rolled sheet metal, additively produced Hastelloy X cannot be accurately modeled using von Mises yield criteria. An anisotropic Drucker-Prager model of AM can be used, with four input parameters that also account for tension-compression symmetry.

As a result of additive manufacturing, the surface roughness is high

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