Mechanical performance of 316 L stainless steel by hybrid directed energy deposition and thermal milling process

https://doi.org/10.1016/j.jmatprotec.2020.117023Get rights and content

Abstract

Due to its excellent temperature resistance, 316 L stainless steel (SS) is widely used in integral impellers; however, as the engine performance requirements have increased, the shape of integral impellers has become increasingly complex. Directed energy deposition (DED), with a high production rate and low cost, is used to directly fabricate complex structural geometries. Nevertheless, the surface quality is lower than that achieved by conventional methods, i.e., milling. To overcome this problem, a novel hybrid approach with DED followed by a subtractive milling process within a single workstation is developed. This method can directly produce internal and highly complex structural parts with ideal dimensional accuracy. However, the process parameters and mechanical properties of DED and subtractive thermal milling (starting milling temperature of 200−300℃) after each deposition of a set of layers have rarely been evaluated. The purpose of this study is to address these research limitations. The densification, phase composition, microstructure and mechanical behaviour are studied, and a correlation between process parameters and performance is newly established. The results indicate that the nearly fully dense 316 L SS specimens exhibit high microhardness and tensile strength under the optimum process parameters, which is attributed to the high density and fine microstructure. Moreover, the highest tensile strength (683.3 MPa) among all tensile samples is obtained with v = 8 mm/s. The tensile strength values for wrought (hot work-annealed), wrought (cold-worked), cast samples, and for the industry requirement for 316 L SS are 480, 574, 552 and 450 MPa, which are 42.97 %, 19.56 %, 24.33 %, and 52.51 % lower, respectively, than that for the hybrid DED and thermal milling process. The test results and a comparison analysis show that the components from the hybrid DED and thermal milling process can satisfy the industry requirements for 316 L SS.

Introduction

Aeroengine blades are a vital functional component of aeroengines. As engine performance requirements increase, the shape of the integral impeller becomes increasingly complex, with the characteristics of small blade spacing and complex freeform surfaces. These characteristics present formidable challenges to the manufacturing of integral impellers through traditional subtractive manufacturing (SM) due to the low utilization rate and high production costs. Therefore, additive manufacturing (AM) technologies have been widely studied and applied in important areas, such as the aeronautic, astronautic and medical fields, with minimum waste and excellent processing flexibility (Mozaffar et al., 2019). In contrast to SM, the obvious advantage of AM is that it can directly fabricate complex and internal characteristic components through line-by-line and then layer-by-layer processes from raw materials without the need for secondary machining, while achieving optimized mechanical performance, which cannot be accomplished by traditional processing methods. Laser directed energy deposition (DED) is an increasingly applied 3D AM technology that employs a laser beam with a high laser energy density to quickly melt each powder layer before deposition of the next layer; this step is followed by solidification and cooling with an intricate heat history to improve the mechanical and chemical properties (Wolff et al., 2017). Simson et al. (2017) reported the surface roughness values Rz and Ra from 30−69 μm and 7−14 μm at 100-fold magnification, respectively, for components produced with selective laser melting (SLM). Wust et al. (2020) compared the statistical optimum of the surface quality for specimens fabricated by a single AM and milling process. The results showed that the optimized surface roughness values Sa were 6.5 μm and 0.327 μm for parts fabricated by the single AM and milling processes, respectively. Generally, the surface quality of AM components is not ideal due to the high surface roughness and micro-cracking as well as residual stresses, which restrict the use of these parts in applications with high tolerance and high fatigue strength requirements. Indeed, it is difficult to directly manufacture components that satisfy the needs of industry via the DED process independently due to spheroidizing, unmelted powder particles, the stair-step effect and overheating. Therefore, it is important to decrease the surface roughness of components fabricated with DED, so that they can be used in critical aerospace applications. Therefore, a subsequent SM milling process is needed to obtain a low surface roughness and high dimensional precision.

Here, a novel hybrid DED and SM milling process with a single workstation is developed. This hybrid technology uses DED to fabricate metal parts with near net shapes as well as milling to remove the rough surface of the part for finish machining. The distinctive advantage of this hybrid technology is that it can rapidly switch between DED and milling process. Thus, the design can be altered, and adjustments can easily be completed in real time, which is a substantial benefit over DED, as DED simply cannot alter or modify components that have been produced through the addition of new materials. Moreover, a hybrid process can also reduce the adverse impact on the environment and improve manufacturing efficiency as well as cause synergistic influences on high-geometrical-complexity components. Li et al., 2018a, 2018b proposed a 6-axis robotic arm with AM and SM heads to achieve additive and subtractive hybrid manufacturing (ASHM) process. It was reported that the ASHM process has the advantages of shortening the production time, improving the surface quality and not requiring a supporting structure. Moreover, Liu and To (2017) demonstrated a topology optimization method of continuum structures for the ASHM process. Sun et al. (2018) adapted the theoretical modelling and prediction of surface roughness as well as a tri-dexel model to simulate the ASHM process. Additionally, Yan et al. (2018) demonstrated a hybrid manufacturing (HM) process to produce Ti-6Al-4 V thin-wall parts with a good build strategy to ensure the forming quality. However, a considerable amount of time was spent setting up and positioning the work on the DED and computer numerical control (CNC) machines in their research because the near net shape creation and finishing processes were carried out in different positions. Bai et al. (2020) reported a hybrid process that combined SLM and milling processes to fabricate 6511 martensitic stainless steel (SS) parts. The effects of processing parameters on the machining features and the residual stress were analysed, but the mechanical properties of the parts fabricated by the ASHM process were not analysed. Soshi et al. (2017) studied an injection mould manufacturing and cooling method using DED and CNC machines. The hybrid technology rapidly fabricated a mould with conformal cooling channels, thus substantially improving the cooling performance of the mould. However, the effect of the build strategy on the geometrical precision and mechanical performance was ignored. Zhao et al. (2018) reported a hybrid process by combining laser additive manufacturing (LAM) and a subtractive milling process to fabricate an FeCr alloy. This research mainly reported the influence of milling on residual stress for LAM-fabricated parts and pointed out that milling altered the near surface stress distribution. Overall, the combination of subtractive machining and AM provides a hybrid manufacturing route that can overcome the inherent problems of a poor surface finish and a low dimensional accuracy of AM-fabricated components. Previous research on the ASHM process focused mainly on the surface quality, stock utilization ratio and productivity efficiency, and very few studies have dealt with a property analysis of the hybrid method to fabricate components, which is also essential for ASHM approaches. Woo et al. (2019) identified a useful scanning strategy to ensure a favourable finish quality from the DED process. However, the hybrid DED and thermal milling approaches described in that paper still require additional research; in particular, to our knowledge, few studies have investigated the relationship between the mechanical performance and scanning strategy, which is also important for a hybrid DED and thermal milling process after each deposition of a set of layers.

316 L SS has good corrosion resistance and temperature resistance and is widely used in marine engineering and aerospace applications. In our recent work (Yang et al., 2020), four sets of samples were fabricated from 316 L SS using four different scanning strategies by the ASHM method. That work determined the optimal scanning strategy (the chessboard scanning strategy) through various mechanical tests and then analysed the machining characteristics during the milling process. In the present work, 316 L SS powder was prepared as a raw material, and cubic specimens were fabricated by hybrid DED with a chessboard scanning strategy and thermal milling (starting milling temperature of 200−300℃) after each deposition of a set of layers. Then, the densification behaviour, phases, microstructural evolution and mechanical properties were investigated. Moreover, the mechanical properties consisting of Vickers hardness and tensile behaviours were analysed. This work lays a solid foundation for the wide application of hybrid DED incorporating layer-by-layer deposition and thermal milling processes to achieve complex structural parts, such as closed impellers.

Section snippets

Experimental methods

A hybrid DED and subtractive milling process (SVW80C-3D, Dalian Sunlight Technology Co., Ltd., China) was used in this study, as shown in Figs. 1(a)-(c). The DED subsystem consisted mainly of an IPG 2000 fibre laser, a coaxial feeding nozzle, cooling equipment and a laser head. Before the deposition process began, a 40# steel (U20402, according to GB/T699–1999) substrate was attached to the building platform and milled to eliminate a very thin oxidation layer (Li et al., 2018a, 2018b).

Densification level

Fig. 5 presents the influence of v on the densification level of 316 L SS parts fabricated by the hybrid DED and thermal milling process. The density initially increased and then decreased with increasing v. At a low v of 6 mm/s, numerous pores appeared in the cross-sectional microstructure. As a result, there was a relatively low densification level (ρrel = 93.52 %). When v increased to 8 mm/s, the cross-sections of the specimens had an improved microstructure with comparatively few pores, and

Conclusions

The mechanical properties of 316 L SS parts fabricated with the hybrid DED method with a chessboard scanning strategy and thermal milling (starting milling temperature of 200−300℃) after each deposition of a set of layers were experimentally investigated in this study, and the conclusions are as follows:

  • (a)

    A novel method was employed to fabricate 316 L SS specimens. Moreover, the thermal milling process was performed after each deposition period, which not only maintained the relatively high

CRediT authorship contribution statement

Yuying Yang: Conceptualization, Investigation, Methodology, Writing - original draft. Yadong Gong: Project administration, Funding acquisition, Writing - review & editing, Supervision. Changhe Li: Project administration, Funding acquisition, Writing - review & editing, Supervision. Xuelong Wen: Conceptualization. Jingyu Sun: Methodology.

Declaration of Competing Interest

The author declare no conflict of interest.

Acknowledgement

This research was financially supported by the following organizations: National Natural Science Foundation of China (51775100, 51975305, 51905289), Innovation Talent Supporting Program for Postdoctoral Fellows of Shandong Province (SDBX2020012), Major Research Project of Shandong Province (2019GGX104040, 2019GSF108236), Shandong Provincial Natural Science Foundation of China (ZR2019PEE008), Major Science and technology innovation engineering projects of Shandong Province (2019JZZY020111),

References (38)

Cited by (0)

View full text