Influence of the Chemical Composition of the Used Powder on the Fatigue Behavior of Additively Manufactured Materials

Abstract

To exploit the whole potential of Additive Manufacturing (AM), a sound knowledge about the mechanical and especially cyclic properties of AM materials as well as their dependency on the process parameters is indispensable. In the presented work, the influence of chemical composition of the used powder on the fatigue behavior of Selectively Laser Melted (SLM) and Laser Deposition Welded (LDW) specimens made of austenitic stainless steel AISI 316L was investigated. Therefore, in each manufacturing process two variations of chemical composition of the used powder were utilized. For qualitative characterization of the materials cyclic deformation behavior, load increase tests (LITs) were performed and further used for the physically based lifetime calculation method (PhyBaLLIT), enabling an efficient determination of stress (S)–number of cycles to failure (Nf) curves (S–Nf), which show excellent correlation to additionally performed constant amplitude tests (CATs). Moreover, instrumented cyclic indentation tests (PhyBaLCHT) were utilized to characterize the materials’ defect tolerance in a comparably short time. All material variants exhibit a high influence of microstructural defects on the fatigue properties. Consequently, for the SLM process a higher fatigue lifetime at lower stress amplitudes could be observed for the batch with a higher defect tolerance, resulting from a more pronounced deformation induced austenite–α’-martensite transformation. In correspondence to that, the batch of LDW material with an increased defect tolerance exhibit a higher fatigue strength. However, the differences in defect tolerance between the LDW batches is only slightly influenced by phase transformation and seems to be mainly governed by differences in hardening potential of the austenitic microstructure. Furthermore, a significantly higher fatigue strength could be observed for SLM material in relation to LDW specimens, because of a refined microstructure and smaller microstructural defects of SLM specimens.

Influence of microstructural defects and the surface topography on the fatigue behavior of “additively‐subtractively” manufactured specimens made of AISI 316L

“…The cyclic deformation curves obtained in fatigue tests at L‐PBF1 specimens reveal a significantly lower amount of plastic strain amplitude ϵ a,p compared to the curves from L‐PBF2 specimens, Figure 10a, b. Furthermore, the specimens from both parameter sets only show cyclic softening during the fatigue tests, which is in correspondence to preliminary results and is more pronounced for L‐PBF2 specimens [19–22]. The higher level of plastic deformation and stronger cyclic softening is caused by higher stress amplitudes σ a used in fatigue tests with L‐PBF2 specimens, Figure 10 a, b.…”

“…The continuously cast material shows globular grains, while elongated grains along the build‐up direction result from L‐PBF2, Figure 6c, d. The grains in materials produced with L‐PBF2 grow beyond the melt pool boundaries, which are visible in LOMs. The microstructure observed for additively manufactured material is characteristic and corresponds to [5–7, 19–22].…”

“…The investigated AISI 316L can be metastable at ambient temperature, resulting in deformation‐induced austenite‐α’‐martensite transformation at cyclic loading [19–21]. Consequently, the austenite stability, which mainly depends on the chemical composition, has to be considered.…”

“…The process‐induced microstructural defects, i. e., pores and lack of fusion, lead to a significant reduction of the fatigue lifetime of additively manufactured materials [17–22]. Besides these process‐induced defects, the relatively high surface roughness of additively manufactured materials results in notch effects and thus, in a reduction of the fatigue strength (>50 %) [7, 21, 23].…”

“…As additive manufacturing offers a high potential to produce safety relevant structural components, where a sound knowledge of the cyclic properties is indispensable, the presented work focuses on the fatigue behavior of specimens manufactured by laser‐based powder bed fusion. In accordance with preliminary own work, the commonly used austenitic stainless steel AISI 316L was utilized as investigated material [19–22, 26, 27, 35]. Because the “as‐built” surface of additively manufactured parts leads to a dramatic decrease of fatigue strength and additive manufacturing only offers a low level of dimensional accuracy, subtractive manufacturing of additively manufactured parts is of great interest.…”