Determination of the influence of a stress-relief heat treatment and additively manufactured surface on the fatigue behavior of selectively laser melted AISI 316L by using efficient short-time procedures

https://doi.org/10.1016/j.ijfatigue.2019.105301Get rights and content

Highlights

  • Effect of a stress-relief heat treatment (HT) on the fatigue behavior of 316L.

  • Improvement of fatigue strength because of increased defect tolerance after HT.

  • Locally restricted phase transformation at microstructural defects.

  • Significant reduction of fatigue strength due to as-built surface condition.

  • Double-staged S-Nf curves of SLM specimens with as-built surface condition.

Abstract

To use Additive Manufacturing (AM) technology for production of safety critical structural components, it is indispensable to investigate thoroughly the materials AM specific microstructure as well as the resulting mechanical and especially cyclic properties. Besides their special microstructure, AM materials show a highly inhomogeneous distribution of residual stresses as well as numerous process induced microstructural notches, e.g. pores in the materials volume and high roughness of additively manufactured surfaces. Therefore, in production of AM components stress-relief heat treatments as well as surface post-processing are commonly used. In the present work, the influence of stress-relief heat treatment as well as the additively manufactured surface condition on the cyclic properties was investigated at selectively laser melted (SLM) specimens made of AISI 316L stainless steel. Specimens were manufactured in horizontal as well as vertical building direction, leading to layer planes oriented parallel and perpendicular to the loading direction, respectively. To reduce the high material and time effort in fatigue investigations, the short-time methods PhyBaLCHT, which is based on cyclic indentation tests and enables a characterization of the materials defect tolerance, as well as load increase tests (LITs) were used for qualitative analyzes of the cyclic deformation behavior. Moreover, the physically based lifetime calculation approach (PhyBaLLIT) was applied for quantitative determination of the fatigue behavior of heat treated specimens, showing excellent correlation to additional constant amplitude tests (CATs), performed for validation. While the heat treatment does not significantly influence the fatigue behavior of vertically built specimens, an increase of fatigue strength could be observed for horizontal building direction, which is mainly caused by an improved defect tolerance. The higher defect tolerance was determined with PhyBaLCHT and could be proved by fracture mechanics considerations using the area-concept. Furthermore, the results show a significantly lower fatigue lifetime for additively manufactured surface condition compared with polished samples, which is more pronounced for vertically built specimens, because of the higher surface roughness of specimens manufactured in this building direction. Moreover, a double-staged S-Nf curve could be observed for specimens with additively manufactured surface, which correlates to the cyclic deformation behavior observed in LITs and CATs.

Introduction

The new possibilities of Additive Manufacturing (AM) technologies, especially in terms of design freedom of components, promise a high potential far beyond conventional manufacturing. Due to the extremely high complexity of possible geometries, AM is predestined for production of safety relevant and highly loaded structural components. However, to use AM technology also for such applications, a sound knowledge about the influence of the numerous manufacturing parameters on the microstructure as well as resulting mechanical and especially cyclic properties is indispensable.

The microstructure of AM materials has been thoroughly investigated in the recent years. The investigations of Casati et al. [1] and Riemer et al. [2] on AISI 316L, Mower et al. [3] on 316L and AlSi10Mg as well as Lewandowski et al. [4] on Ti6Al4V show a grain elongation of selectively laser melted (SLM) materials along the building direction, which corresponds to our own results on AISI 316L [5], [6], [7]. Furthermore, the results given in [1], [3], [5], [6], [7] show grain growth beyond the melt pool boundaries.

Beside this special grain structure, AM materials exhibit residual stresses because of the high thermal transients and gradients occurring in the manufacturing process [8]. The results of Mercelis et al. [9], Wu et al. [10] and Liu et al. [11] on 316L as well as Shiomi et al. [12] on chromium molybdenum steel JIS SCM440 show an increase of residual tensile stresses in the upper layer of the manufactured structures, whereas in the lower layers compressive residual stresses are observed. Furthermore, the results presented in [8], [10], [11], [13], [14] show a dependency of the residual stresses on the scanning direction as well as strategy. Due to the high level and gradient in residual stresses, a deformation of AM structures can be observed commonly during removal from the built plate [9]. To reduce the residual stresses, a stress relief-heat treatment can be performed before base plate removal, as shown by Riemer et al. [2] on AISI 316L. In this work also a dependency of the residual stresses on the building direction was observed [2].

Beside the dependency of the residual stresses on building direction, AM materials show significant, orientation induced differences of mechanical properties. This anisotropic behavior of AM structures in dependency with the building direction has been widely investigated for monotonic loading. Yadollahi et al. [15] on 17-4-PH, Brandl et al. [16] on AlSi10Mg, Casati et al. [1] and Spierings et al. [17] on 316L as well as Mower et al. [3] and Lewandowski et al. [4] on all these materials, reported a higher 0.2% yield and tensile strength for specimens with layer planes oriented parallel to the loading direction (horizontally built) compared to specimens with a layer plane orientation perpendicular to the loading axis (vertically built). Moreover, [2], [3], [4] show competitive or higher strength of AM specimens in relation to conventionally manufactured material.

Compared to the investigations of monotonic properties, the cyclic properties of AM materials have been investigated only in a limited scope. However, Yadollahi et al. [15] on 17-4-PH, Mower et al. [3] on 316L, Brandl et al. [16] on AlSi10Mg, as well as Rigon et al. [18] on maraging steel 18Ni300 show, in correlation to monotonic properties, a higher fatigue strength of horizontally built specimens, which correlates to our own results on 316L given in [6]. Note that Rigon et al. [18] also observed higher fatigue strength of vertically built specimens for a different batch of powder, which corresponds to our results reported in [5] and the results of Zhang et al. [19] on SLM 316L. However, compared to conventionally manufactured specimens, AM materials generally show a lower fatigue strength [3], [6], [15], [17]. This is mainly caused by the high defect density of AM structures as shown by Mower et al. on 316L, AlSi10Mg as well as 17-4-PH, Yadollahi et al. on 17-4-PH [15] and Günther et al. [20] as well as Leuders et al. [21] on Ti6Al4V. The investigations of Günther et al. [20] demonstrate similar fatigue properties as conventionally manufactured materials after hot isostatic pressing of the AM materials, indicating a high dependency of the fatigue properties on microstructural defects [3], [15], [16], [20], [21], [22], [23], [24].

For rating the influence of microstructural defects, i.e. pores and nonmetallic inclusions, Murakami et al. established the area-concept, which enables the determination of the fatigue strength in dependency with the size and location of the microstructural defect. This approach was successfully used for investigation of selectively laser melted AlSi10Mg [22] and 316L [5] fatigue specimens. Furthermore, this concept was applied to additively manufactured Ti6Al4V by Masuo et al. [23].

Besides microstructural defects in the materials’ volume, the additively manufactured surface lead to significant notch effects, due to comparably low surface quality. As shown by Riemer et al. [2] and Spierings et al. [17] on 316L as well as Greitemeier et al. [24], Vayssette et al. [25] and Lewandowski et al. [4] on Ti6Al4V, the additively manufactured surface lead to a significant reduction of the fatigue strength.

The limited scope of fatigue investigations is mainly caused by the high material and time efforts for fatigue investigations. Short-time procedures can reduce this effort and enable an efficient determination of the materials cyclic properties. The short-time procedures load increase tests (LITs), PhyBaLCHT as well as PhyBaLLIT are powerful means to determine cyclic properties of metallic materials [5], [6], [7], [26], [27], [28], [29]. As shown in former investigations, PhyBaLCHT can be used to efficiently rate the defect tolerance of different materials and heat treatment conditions [5], [6], [26], [29], [30]. For investigation on AM materials, the LIT has been used in previous investigations to characterize the anisotropy at cyclic loading in a comparably short time [5], [6], [7]. Based on the results of one LIT and two additional CATs, PhyBaLLIT enables a S-Nf curve estimation by means of only three fatigue tests, which has been successfully used for many material classes [26], [27], [28], including additively manufactured 316L [5], [7].

As obvious from the results in literature, AM structures show anisotropy as well as many and various microstructural notches, which significantly influence the fatigue properties. Furthermore, appropriate heat treatment of AM parts is necessary to reduce the residual stresses before removal from the built plate. Consequently, in the present work the influence of a stress-relief heat treatment on the cyclic properties of SLM specimens made of austenitic stainless steel AISI 316L was investigated. Therefore, heat treated specimens were compared to former results on non-heat-treated SLM specimens (published in [5]), manufactured with identical parameters and surface condition. Based on these results, the impact of the microstructural pores in the materials volume as well as the additively manufactured surface (as-built) on the fatigue behavior was determined in dependency of the building direction. For determination of the influence of additively manufactured surface condition, specimens with polished and as-built surfaces were compared in heat treated condition. To reduce the effort of this investigations, the materials cyclic deformation behavior was qualitatively analyzed by LITs in addition to conventional CATs. Moreover, the short-time procedure PhyBaLLIT was applied to utilize the information received from LITs for calculation of the S-Nf curves of polished specimens and verified by additional CATs. Furthermore, the PhyBaLCHT method was used for qualitative characterization of the defect tolerance of the differently heat treated conditions. Additionally, these results were compared to the results obtained by the area-concept.

Section snippets

Investigated materials

For specimen manufacturing by the SLM process a 3D Systems ProX DMP 320 device, equipped with a 500 W fiber laser (wavelength 1070 nm), was used. Bars with a diameter of 14 mm and a length of 122 mm were manufactured with a laser power of 250 W, a scan velocity of 900 mm/s and a layer thickness of 30 µm in a nitrogen inert gas atmosphere for oxidation prevention. The bars were manufactured in both, horizontal and vertical building direction, leading to layer planes oriented parallel,

Experimental methods

Light optical microscopy was carried out with a Leica 6000 DM device. The investigation of the fracture surfaces as well as the examination of the grain structure by EBSD-orientation mappings were conducted with a FEI Quanta 600 device. For EBSD-measurements samples with an electrolytically polished surface were used. Furthermore, the grain structure was visualized by a V2A etchant for LOMs. All microstructural images were taken in the center of the prepared samples. For advanced Focused Ion

Monotonic properties

To further investigate the influence of heat treatment on the monotonic properties, tensile tests were performed, which show nearly identical tensile strengths Rm and Young’s Moduli E of heat treated material in relation to reference specimens (see Fig. 3 and Table 3). In contrast to Rm, the 0.2% yield strength Rp0.2 is significantly decreased for heat treated specimens, leading to more pronounced strain hardening. While the horizontally built specimens show no difference in elongation at

Summary and conclusion

In the present work, the influence of a stress-relief heat treatment on the mechanical and especially cyclic properties of selectively laser melted (SLM) austenitic stainless steel AISI 316L was investigated. Furthermore, the influence of microstructural notches, i.e. process induced pores in the materials’ volume as well as the additively manufactured surface condition, was analyzed. Therefore, heat treated and differently oriented specimens with polished as well as additively manufactured

Acknowledgement

The support of this work by the Bundeswehr Research Institute for Materials, Fuels and Lubricants (WIWeB) is gratefully acknowledged. We further thank Prof. J. Seewig that we were allowed to perform the confocal microscopy investigations at the Institute for Measurement and Sensor-Technology.

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    On the occasion of his 70th birthday, this work is dedicated to our mentor, colleague and friend Prof. Dr.-Ing. habil. Dietmar Eifler.

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