The microstructure evolution and tensile properties of Inconel 718 fabricated by high-deposition-rate laser directed energy deposition (original) (raw)

Highlights

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The IN718 sample with deposition rate of 2.2 kg/h and height 75 mm was prepared.
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δ, γ" and γ' phase are precipitated in bottom and middle region due to thermal cycle.
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Abstract

In order to meet the requirements for rapid manufacturing of large-scale high-performance metal components, the unique advantages of high-deposition-rate laser directed energy deposition (HDR-LDED, deposition rate ≥ 1 kg/h) technology have been attracted great attention. HDR-LDED technology significantly improves the efficiency by simultaneously increasing the mass and energy input on basis of conventional laser directed energy deposition (C-LDED, deposition rate ≤ 0.3 kg/h), which dramatically changes the solidification condition and thermal cycling effect compared to C-LDED processes. Based on this, Inconel 718 bulk samples were fabricated with a deposition rate of 2.2 kg/h and a height of 75 mm. Through experimental observation combined with finite element simulation, the precipitation morphology, thermal cycling effect and tensile properties at room temperature of the block samples at heights of 6 mm (bottom region), 37 mm (middle region) and 69 mm (top region) from the substrate were investigated. The results show that both temperature interval and incubation time satisfy the precipitation conditions of the second phases because of the intense thermal cycling effect so that δ, γ" and γ' phase are precipitated in the bottom and middle region of the as-deposited sample during the HDR-LDED process. As a result, the micro-hardness and the yield strength of the bottom region (385 HV; 745.1 ± 5.2 MPa) are similar to those of the middle region (381 HV; 752.2 ± 12.1 MPa), respectively. And they are both higher than those of the top region (298 HV; 464.7 ± 44.2 MPa). The tensile fracture mechanism is shown in both fracture and debonding of the Laves phase. The inhomogeneous microstructures and corresponding mechanical property differences of Inconel 718 fabricated by HDR-LDED along the deposition direction suggest the necessity to conduct further research of the post heat treatment in the future.

Introduction

As one of the most representative nickel-base superalloys, Inconel 718 is widely used in large-scale integrated equipment such as blades (Blade-Integrated-Disks) and turbine engines due to its excellent mechanical properties and oxidation resistance. To realize the manufacturing of large-scale complex Inconel 718 structural components, laser directed energy deposition (LDED) technology has gradually become widely accepted. However, for the manufacturing of integral structural parts, the production efficiency of conventional laser directed energy deposition (C-LDED), with a deposition rate of approximately 0.5 kg/h, has been not meet the rapid production requirements [[1], [2], [3]]. As of today, the deposition rates of HDR-LDED technology is at least ten times as high as of C-LDED [[4], [5], [6]], which greatly reduces manufacturing time and costs. Therefore, HDR-LDED technology, which provides an effective way for rapid production of large-scale metal structural parts, is expected to become one of the most important technologies in this field.

Compared with C-LDED, HDR-LDED technology is achieved by significantly increases both energy and mass input at the same time, which changes the process conditions during HDR-LDED compared to C-LDED process. Firstly, the change in process conditions focused on the solidification process in HDR-LDED technology. Ma proposed that a lower energy input will lead to a refined microstructure and less Laves phase in the as-deposited Inconel 718 sample since the cooling rate is increased [7]. Zhong investigated the microstructure of Inconel 718 fabricated by HDR-LDED and found that Laves phase is coarser in HDR-LDED compared to C-LDED, which is the result of the decrease in cooling rate [8]. Additionally, there are differences in Nb concentrations between C-LDED (4.92 wt. %) and HDR-LDED (5.2 wt. %) Inconel 718 powder, which was used by Zhong. He found that the concentration of Nb (15.82 wt. %) in the Laves phase of high-deposition-rate laser direct energy deposited (HDR-LDEDed) Inconel 718 was significantly lower than that of conventional laser direct energy deposited (C-LDEDed) Inconel 718 (25.44 wt. %) [4,9]. This research indicates that the changes of solidification conditions will affect micro-segregation regardless of alloy composition. Secondly, the change in the process conditions focused on the thermal cycling effect. Currently, many scholars have studied the effects of thermal cycling on the microstructure [[10], [11], [12], [13]]. Sames et al. found that during electron beam melting (EBM) process, the intense thermal cycling effect causes a large amount of the strengthening phases γ" and γ' to precipitate in the as-deposited Inconel 718 sample and the corresponding micro-hardness is up to 400 HV [13]. Tian et al. also observed this phenomenon in C-LDED technology, which is considered to be the result of thermal cycle as well [11]. According to the above research results, it can be said that the change of thermal cycling effect significantly affects the precipitation phase of the as-deposited sample. However, F.C. Liu et al. also chose the C-LDED technology to prepare the as-deposited Inconel 718, and no precipitation of the strengthening phase was observed in the samples, although there is the thermal cycling effect in C-LDED technology. Wang hasn't observed the precipitation of strengthening phase in selective laser melting (SLM) technology when investigating the effects of the thermal cycling on the microstructures [10]. In HDR-LDED, the increase in energy input will intensify the thermal cycling effect, and conversely, the increase in mass input will weaken this effect. It remains unclear how the combined factors of the two will affect the thermal cycle. Based on literature, the question whether the change in thermal cycling effect influences the microstructures of the as-deposited Inconel 718 remains unanswered. This needs to be researched especially in HDR-LDED.

In terms of the microstructure of HDR-LDEDed Inconel 718, C.L. Zhong initially deposited the single-track cladding layer, and it was found that there was no significant difference between the microstructure of HDR-LDEDed samples and that of C-LDED [14], although the mass and energy input were increased during the forming process. However, it is worth mentioning that the dilution area of the single-track cladding layer is very large according to the results of the research. This means that the re-melting depth of the forming layer to the solidified layer will be significantly increased when the bulk sample is deposited. Correspondingly, the average temperature inside the bulk sample and the duration time of the high temperature range will be improved. As a result, the thermal cycling effect inside the deposited sample will be particularly significant. The intense thermal cycling effects may cause the temperature to rise to the phase transition point or even induce solid phase transition during HDR-LDED technology since Inconel 718 is a precipitation-strengthening alloy. Therefore it is estimated that when the sample transitions from a single-track to bulk one, the thermal cycling effect may play an important role in the microstructure and further affect the type of precipitated phases. Obviously, the changes of the precipitation phases will eventually affect the tensile properties and fracture mechanism of the as-deposited sample. Based on the above analysis, whether the thermal cycling effect will affect the microstructure and tensile properties of the bulk samples in HDR-LDED technology still needs to be further explored.

In this paper, the influences of the thermal cycling effect on the precipitation phase, micro-hardness and tensile properties of HDR-LDEDed Inconel 718 alloy have been investigated. In addition, the tensile fracture mechanism of HDR-LDEDed Inconel 718 also has been analyzed.

Section snippets

Experimental materials and procedures

The experimental equipment is established by State Key Laboratory of Solidification Processing. This system consists of a 6 kW semiconductor laser, a three-dimensional numerical controlled working table, an inert atmosphere processing chamber (oxygen content ≤50 ppm), a powder feeder with a coaxial nozzle and an adjustable automatic feeding device with high precision (type DPSF-2).

A set of HDR-LDED parameters (diode laser power, 4000 W; scanning speed, 1600 mm/min; beam diameter, 5 mm; overlap,

Microstructure

Fig. 2 shows the microstructures of HDR-LDEDed Inconel 718 from the bottom region to the top region. Columnar grains growing epitaxially along the deposition direction in the bottom, middle and top region can be seen in the optical micrographs (Fig. 2(a), (b) and (c)). Therefore the heat flow direction during HDR-LDED process is approximately perpendicular to the surface of the substrate or the pre-deposited layers [18]. The columnar grains in these three regions are large and some even extend

Conclusion

In summary, the thermal cycle during HDR-LDED process influences the precipitation of δ, γ" and γ' phase in the as-deposited Inconel 718. In the bottom and middle region, δ phases precipitate around the inter-dendritic Laves phases. Furthermore, a certain amount of γ" and γ' phases also form in these two regions. However, there is no δ, γ" and γ' phase existent in the top region. As a result, the micro-hardness of the bottom region (385 HV) is similar to that of the middle region (381 HV), and