An Investigation of the Microstructure and Fatigue Behavior of Additively Manufactured AISI 316L Stainless Steel with Regard to the Influence of Heat Treatment

Abstract

Abstract:To exploit the whole potential of Additive Manufacturing, it is essential to investigate the complex relationships between Additive Manufacturing processes, the resulting microstructure, and mechanical properties of the materials and components. In the present work, Selective Laser Melted (SLM) (process category: powder bed fusion), Laser Deposition Welded (LDW) (process category: direct energy deposition) and, for comparison, Continuous Casted and then hot and cold drawn (CC) austenitic stainless steel AISI 316L blanks were investigated with regard to their microstructure and mechanical properties. To exclude the influence of surface topography and focus the investigation on the volume microstructure, the blanks were turned into final geometry of specimens. The additively manufactured (AM-) blanks were manufactured in both the horizontal and vertical building directions. In the horizontally built specimens, the layer planes are perpendicular and in vertical building direction, they are parallel to the load axis of the specimens. The materials from different manufacturing processes exhibit different chemical composition and hence, austenite stability. Additionally, all types of blanks were heat treated (2 h, 1070 • C, H 2 O) and the influence of the heat treatment on the properties of differently manufactured materials were investigated. From the cyclic deformation curves obtained in the load increase tests, the anisotropic fatigue behavior of the AM-specimens could be detected with only one specimen in each building direction for the different Additive Manufacturing processes, which could be confirmed by constant amplitude tests. The results showed higher fatigue strength for horizontally built specimens compared to the vertical building direction. Furthermore, the constant amplitude tests show that the austenite stability influences the fatigue behavior of differently manufactured 316L. Using load increase tests as an efficient rating method of the anisotropic fatigue behavior, the influence of the heat treatment on anisotropy could be determined with a small number of specimens. These investigations showed no significant influence of the heat treatment on the anisotropic behavior of the AM-specimens.

Determination of the anisotropic fatigue behaviour of additively manufactured structures with short-time procedure PhyBaLLIT

“…9, are in good accordance with the data of additional CATs in the HCF-regime. Similar to the S-N f -curves shown in [10], the S-N fcurves for vertically built specimens show a slightly steeper slope compared to horizontal building direction (see Fig. 9 and Table 5).…”

“…These differences in chemical composition significantly influence austenite stability [11], which can be rated with the M d30 -Temperatures determined from equation (1) according to Angel [12]. As shown in [10], austenite stability has to be taken in account for comparison of the mechanical properties of the differently manufactured specimens made of 316L, because lower austenite stability can result in higher deformation or stress induced transformation of the austenite into α'-martensite, which strongly influences the mechanical properties. .5Mo [12] (1) In the Additive Manufacturing processes blanks with a simple geometry were manufactured using the facilities and powders given in Table 2 [10,11].…”

“…As shown in [10], austenite stability has to be taken in account for comparison of the mechanical properties of the differently manufactured specimens made of 316L, because lower austenite stability can result in higher deformation or stress induced transformation of the austenite into α'-martensite, which strongly influences the mechanical properties. .5Mo [12] (1) In the Additive Manufacturing processes blanks with a simple geometry were manufactured using the facilities and powders given in Table 2 [10,11]. To focus the investigations on properties of the volume microstructures, and to exclude the influence of surface topography, resulting from the Additive Manufacturing process, blanks were by subsequently turned to the final specimen geometry given in Fig.…”

“…However, horizontally built specimens show lower lifetime scatter (comp. [10]). In correlation to SLM-material, horizontally built specimens show lower scatter compared to vertical building direction in LDWmaterial.…”

“…Additional to the results shown in Blinn et al [10], LITs at a frequency of 2 Hz and lifetime calculations by the short-time procedure PhyBaL LIT [3] were conducted. Therefore additionally to the LITs, CATs in the HCF-regime were performed.…”