Quantitative multiscale correlative microstructure analysis of additive manufacturing of stainless steel 316L processed by selective laser melting

Highlights

• Hierarchical growth model development based on structural and compositional analyses

• Understanding the melt-pool boundary structure and formation

• KAM and ECCI calculations of the dislocation density

• The impact of the structure on the yield-strength calculations

Abstract

In this study we have assessed the microstructure details of 316L stainless steel produced by the additive-manufacturing selective-laser-melting technique under industrial conditions and correlated them with the mechanical properties. The employed micro- and nano-scale imaging electron microscopy techniques revealed the formation of a rigid hierarchical microstructure, which was driven by the rapidly changing solidification rates. The latter also affected the alloying atoms' distribution in the melt-pool boundary area as well as in the dislocation-dense regions. The melt-pool boundaries in themselves did not produce structural irregularities, but were shown to have a slightly different chemical composition. The arrangement of the complex dislocation cells that developed in the whole material volume led to an increase in the yield strength. The calculated twenty-times-higher dislocation density compared to that of the forged material was linked to a very low strength hardening. The similarity of the calculated yield strength, which came from the experimentally determined structure parameters, with the experimental value additionally supported the structure/mechanical properties correlation, derived in this work.

Introduction

Stainless steel is an essential material for modern society as it is used for various applications in the automotive and transportation sector, construction, medicine, energy and heavy industries and food preparation and handling. Among the different types of steel, austenitic AISI 316L stainless steel (316L SS) is one of the most common materials for engineering applications due to its good corrosion resistance and good mechanical properties [1,2]. Most of the 316L SS final products are produced by cold rolling or forging in order to strengthen the final engineering parts. However, the demand for increasingly complex steel structures of smaller dimensions that can be manufactured with fewer technological steps led to the development of various additive-manufacturing (AM) techniques [[3], [4], [5], [6]]. The advantages of these AM approaches are the high degree of design freedom as well as the resource and production process efficiency compared to conventional processes. This enables the tool-free manufacturing of highly complex components [7].

The two general approaches to the AM of metals according to the ISO/ASTM 529000:2017 standard are direct-energy deposition (DED) and powder-bed fusion. While the former allows the direct reconstruction and repair of deteriorated parts and is a faster method, only the latter permits the manufacturing of complex shapes and yields a finer and less porous microstructure. In the powder-bed fusion manufacturing, a 3D object is formed layer-by-layer from a metal powder feedstock that is selectively melted by a high-energy laser source or electron beam. The produced part is then consolidated in subsequent cooling. There are several commercial names for this process. The one where a laser source is used to selectively melt the powder is called selective laser melting (SLM) [8,9].

In contrast to traditional metallurgical routes, 316L SS produced by SLM can achieve better mechanical properties, particularly tensile strength and hardness [[10], [11], [12], [13], [14], [15], [16]]. The SLM process can also promote unique microstructures where the trade-off between tensile strength and ductility can be overcome [17]. The reported extraordinary mechanical properties of SLM-manufactured 316L SS are said to be related to the fine-grained microstructure, additional small-scale compositional variations and the formation of sub-grain structures with nano-inclusions [11,[17], [18], [19]]. All of these phenomena are linked to the initial laser-powder interaction process. Rapid melting and subsequent solidification causes the formation of non-equilibrium conditions in the melt-pool regions. This promotes the development of a fine microstructure and the appearance of a large amount of dislocations, particularly in the form of elongated cells [13,17,19]. Due to the different temperature gradients and heat-flux directions during the solidification process, different regions of the melt pool crystallize differently [20]. Consequently, dislocation cell structures have not been observed in the whole sample [17,18]. Y.M Wang et al. [17] proposed that the main contribution to the yield strength is made by this cellular dislocation structure, according to Hall-Petch-type strengthening behaviour. In some studies, the observed nano-inclusions were considered to act according to the Orowan strengthening model. Wang and co-authors estimated by calculations, however, that the pinning of dislocations by these inclusions delivers only a minor contribution to the overall material's strengthening. Saeidi et al. [19] and Springer et al. [21], on the other hand, connected the improved strengthening of the SLM-manufactured material with the observed nano-inclusion formation in the presence of a partial oxygen pressure.

The aim of our study is to contribute to the further understanding of the microstructures vs. property relationship of SLM 316L SS manufactured without strengthening additives [[22], [23], [24], [25]] under industrial conditions. To this end, along with mechanical testing, the produced material's microstructure was studied on different length scales from the macroscopic observation of the melt pools to the state-of-the-art scanning and transmission electron microscopy observations of the material's structure on the nanoscale. The material's texture, the melt-pool boundary structure and composition, the formation of the dislocation network, the calculated number of dislocations and the influence of the nano-inclusions are discussed in connection with measurements of the tensile strength.