Direct metal laser sintered (DMLS) process to develop Inconel 718 alloy for turbine engine components

Direct metal laser sintered (DMLS) process to develop Inconel 718 alloy for turbine engine components

Journal Pre-proof Direct Metal Laser Sintered (DMLS) process to develop Inconel 718 alloy for turbine engine components B. Anush Raj, J.T. Winowlin Ja...

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Journal Pre-proof Direct Metal Laser Sintered (DMLS) process to develop Inconel 718 alloy for turbine engine components B. Anush Raj, J.T. Winowlin Jappes, M. Adam Khan, V. Dillibabu, N.C. Brintha

PII:

S0030-4026(19)31633-X

DOI:

https://doi.org/10.1016/j.ijleo.2019.163735

Reference:

IJLEO 163735

To appear in:

Optik

Received Date:

17 August 2019

Accepted Date:

6 November 2019

Please cite this article as: Anush Raj B, Winowlin Jappes JT, Adam Khan M, Dillibabu V, Brintha NC, Direct Metal Laser Sintered (DMLS) process to develop Inconel 718 alloy for turbine engine components, Optik (2019), doi: https://doi.org/10.1016/j.ijleo.2019.163735

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Direct Metal Laser Sintered (DMLS) process to develop Inconel 718 alloy for turbine engine components B. Anush Raj1, J.T. Winowlin Jappes1, M. Adam Khan1*, V. Dillibabu2 and N.C. Brintha3 1

– Department of Mechanical Engineering & Centre for Surface Engineering, Kalasalingam Academy of Research & Education, Virudhunagar, Tamilnadu, India

– Scientist, Small Turbo Fan Section, Gas Turbine Research Establishment (GTRE), DRDO, Bangalore, India.

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– Department of Computer Science and Engineering, Kalasalingam Academy of Research &

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Abstract

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* Corresponding Author: [email protected]

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Education, Virudhunagar, Tamilnadu, India

Nickel based superalloys are difficult to manufacture in conventional production process.

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Laser based fusion bed method is adopted to develop metal. Direct metal laser sintering (DMLS) is one of the best to produce difficult to produce metal component. The developed alloy was planned to perform heat treatment process and the metallurgical properties are enriched. To

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investigate initial effects of microstructure, the material as deposit and treated material under are studied in detail. To compare the result of DMLS and commercial alloy were also heat treated at same condition. In this research, corrosion behavior of DMLS alloy and commercially available Inconel 718 are tested with 1.0M H2SO4 solution at room temperature. The electrochemical results of DMLS and commercial alloy are compared for corrosion rate. It was found that the

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DMLS sample give good corrosion resistance than the commercial IN718 alloy due to the precipitation γ” is high. The segregation of iron element is more in commercial alloy and nickel element is high in DMLS alloy was found in the EDS analysis.

Keywords: laser; metal; superalloy; heat treatment

INTRODUCTION The utilization of additive manufacturing is various industries have increased during the past decades. The fabrication of complex parts can be easily manufactured by this additive manufacturing technique. The additive manufacturing is similar to the CNC machining process. Both takes the input data from the computer but in additive machining process the metal is added to built parts and in CNC machining subtracting the material to build parts. When compared to conventional casting the additive manufacturing gives good material efficiency, resource efficiency, production flexibility and dimensional accuracy. It is also a layer-by-layer

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manufacturing process [1-4]. The AM produce fully dense material when compared to other conventional process [5]. Direct Metal Laser Sintering (DMLS) is one of the processes used to

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manufacture product in additive manufacturing technique. DMLS produce models directly from 3D CAD data. It uses the gas atomized metal powder and ytterbium fiber laser power to sinter

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the cross section of the product [6].

Ali Keshavarzkermani et al. [7] reported that the IN718 has high strength, low thermal

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conductivity and high hardness tools will wear out quickly when it is machined so additive manufacturing is preferable. DMLS additive manufacturing is best suitable for manufacturing

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alloy metals. Since each element have different melting point the DMLS is recommended. Persenot et.al [8] reported that build orientation of additive manufacturing has impact on surface finish of the as-built samples. The build orientation of 45˚ and 90˚ (vertical) has low

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surface roughness when compared to 0˚ (horizontal) orientation samples. Nicoletto et.al [9] reveals that the surface roughness of DMLS Sample has low surface roughness when compared to EBM in same orientation. When compared EBM samples and DMLS samples the DMLS sample gives high bending strength. Overall the horizontal orientation build sample gives high fatigue strength. Jian Xing et.al [10] surveyed that transient temperature of building chamber

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also the factor to produce quality products. IN 718 is a nickel based superalloy founds wide application in aerospace and power

generation industries. Aerospace industries and gas turbine application requires a complex shapes which can be easily manufactured by additive manufacturing process. IN718 has high strength and good corrosion resistance in high temperature application [11]. The uniform pericipation of γ’ and γ’’ in IN718 nickel alloy improve the strength of the alloy [12]. As deposited the laves phase contains rich amount of niobium element and γ’’ is the formation of

Ni3Nb which is the major strength to the alloy [13]. Due to the solutionizing and ageing heat treatment gives good strength which makes difficult to machine the material this is because of high hardness and very low thermal conductivity [14]. Finding an option to subtractively fabricate of high-temperature materials a significant point of research wherein additive manufacturing is now turning into a handy choice. The mechanical properties of additive manufacturing created parts are reliant on various parameters on building direction and post-processing heat treatment [15]. The residual stress is caused due to the uneven heat distribution and rapid cooling. In

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Selective Laser Melting (SLM) Process the residual stress is observed at the lower portion of the build surface. Since the molten layer formed on the top of the solidify material and shrink to

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thermal contraction. The residual stress is found to be larger in parallel to the scan direction and lower in perpendicular direction [16]. Post-processing heat treatment is required to relieve the

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residual stress in the material [17]. The residual stress act on the material affects the corrosion resistance of the material [18].

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Nurbanasari [19] reported that the turbine blade failure is caused by corrosion. The chromium present in the alloy prevents the dissolution of protective oxide layer and gives good

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hot corrosion resistance. The chromium present above 16% weight is recommended for nickel alloys. The cracks formed on the surface due to the formation of corrosion pits. Through the cracks the corrosion products infiltrate and cause damage [20].

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The corrosion behavior of DMLS heat treated superalloy in H2SO4 is not found in any literature. Thus the paper deals with the electrochemical corrosion resistance of IN 718 superalloy obtained by direct metal laser sintering (DMLS) and conventional casting are studied. The process with and without heat treatments of the samples are designed to increase the metallurgical properties. The results were compared using optical metallography (OM), electron

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microscopy and energy-dispersive X-ray spectroscopy (EDX). EXPERIMENTAL 2.1 Materials

DMLS sample are fabricated with EOS M280 DMLS machine at INTECH DMLS, Pvt

Ltd, Bangalore with orientation of building platform (xy-plane) and building direction (z-plane). The parameter used for fabricating the samples are as follows: laser power 285 W; scan speed 970 mm/s; hatch distance 0.15mm; layer thickness 40 µm; and beam diameter of 0.08mm The

sample obtained from DMLS process with chemical composition in weight percent: 52.44 Ni, 19.78 Fe, 17.13 Cr, 4.77 Nb, 3.38 Mo, 1.11 Ti, 0.59 Al, 0.05 C, 0.16 Co, 0.23 Cu and 0.23W.

2.2 Heat Treatment The commercial and DMLS alloy are subjected to Solutionizing at 1100˚C for two hours at a heating rate of 15˚C/min and followed by ageing at a temperature of 845˚C for 24 hours [21]. The heat treatment was performed in tubular furnace controlled by PID controller having

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temperature tolerance of ±2˚C.

2.3 Electrochemical Test

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The electrochemical test is performed as per ASTM G3-14 standard. The electrochemical measurement is carried out using a computer controlled potential analyzer (ACM instruments

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Gill AC, UK) with a conventional three-electrode cell. It consists of 1cm2 specimen holder cell, standard calomel reference electrode, Platinum counter electrode and the sample as the working

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electrode [22]. Corrosion test is carried out in room temperature in aerated 1.0M H2SO4 solution. The potential ranges from -250mV to 250mV at a sweep rate of 1mV/sec is used.

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2.4 Analysis of Corroded Surface

To evaluate the microstructure the samples are etched as per ASTM 407 standard of 87 Glyceregia consisting of 15ml HCl, 10ml Glycerol and 5ml HNO3. The micro structural changes

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occurring in the DMLS sample with and without heat treatment surfaces have been investigated by optical Microscope. The scanning electron microscope (Make: Zeiss–FE SEM) equipped with energy-dispersive spectroscopy (Make: Brukers EDS) was used to observe corrosion surface morphology and chemical composition. Result and Discussion

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3.1 Micro-structural features

Figure 1 is the optical micrograph showing the microstructure in the vertical plane of

DMLS as-built specimen. The individual deposited tracks can be found on the low magnification of optical image. The image clearly shows the laser scanning line by line in horizontal direction and the melt pool with arc-shaped features formed on the layer by layer in the vertical direction. Figure 1b shows the melt pools at higher magnification. It clearly shows that the metal is fully

dendritic structure formed on the metal pool boundary. The heat input and dissipation mode is constant when each track was deposited, encourage the continuity of direct solidification. (b)

50μm

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(a)

100μm

(b) lower magnification

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Figure 1: Optical image of DMLS sample before heat treatment: (a) higher magnification;

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Figure 2 shows the recrystallization structure were formed on the alloy after heat treatment. It also observe that melt pool is fully disappeared and homengenized grain structure is

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formed on the alloy.

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Figure 2: Optical image of DMLS sample after heat treatment

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The comparison of XRD diffractogram of bare commercial and DMLS IN718 alloy with heat treated alloy is shown in figure in 3. Cao et.al [23] investigated on the precipation of γ’, γ” and δ in heat treated IN718 alloy produced by selective laser melting process. In the commercial IN718 alloy γ, γ’ and γ” the intensity at 51.3˚ is very low when compared to DMLS alloy. Also in heat treated DMLS alloy the δ phase is also found. This δ phase is not found in the bare DMLS and Commercial alloy. The precipation of γ” is high in case of DMLS alloy which improves the mechanical strength and corrosion resistance.

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Figure 3: Comparison of XRD diffractogram of heat treated commercial and DMLS IN718 Alloy

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3.2 Electrochemical Test

The influence of heat treatment on the kinetics of the cathodic and anodic processes of IN

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718 alloy fabricated by DMLS process in 1.0 M H2SO4 solution was established on the basis of TAFEL polarization experiment was shown in Figure 4. The corrosion data deduced from the

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polarization curve are listed in Table 1. The current density is high and corrosion potential of commercial alloy is low when compared to DMLS fabricated sample. It was found that lower current density (0.5360mA/cm2) is found on the heat treated DMLS sample and higher current

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density (1.3142mA/cm2) was found on the commercial sample. The more positive corrosion potential is found in the DMLS alloy. The corrosion rate is depends on the corrosion potential and current density. The corrosion potential is more positive and current density is low the corrosion rate is reduced. The current density and corrosion rate are the measure of the corrosion and describes how much amount of material loss due to

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corrosion. Hence, lower the current density and corrosion rate is reduced. The corrosion rate increase in the order of heat treated DMLS, bare DMLS, heat treated commercial and bare commercial bare alloy.

DMLS Bare

DMLS Heat Treated Commercial Bare

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Commercial Heat Treated

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Figure 4: The potentiodynamic polarization graph obtained for the DMLS bare and Heat treated & Commercial bare and heat treated

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3.3 Corrosion morphology through SEM and EDS

To observe the corrosion morphology, the corroded samples are investigated by SEM

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equipped with EDS. Figure 5 shows the samples exposed under electrochemical investigation with 1.0M H2SO4 solution. Rubben et.al [24] reported that the corrosion attack penetrating is initiated at the metal pool boundary of the sample is confirmed. Similarly, the uniform corrosion

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takes place in the DMLS as-built samples and corrosion starts in the overlap region of the sample. The figure 5b shows the presence of large pit on the surface of DMLS heat treated sample. The uniform small pits is formed on the bare commercial alloy is shown in figure 5c. It is also noted that small pits are formed on the interior and exterior of the grain boundary. After heat treatment of commercial alloy the intergranular corrosion takes place on the surface is

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shown in figure 5d.

(a)

(b)

Corrosion starts in the over-lap region

Pitting corrosion

DMLS HT

(c)

(d)

Intergranular Attack

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DMLS as-built

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Pitting

Commercial as-cast

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Commercial HT

Figure 5: SEM image after corrosion: (a) DMLS as-built; (b) DMLS heat treated;

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(c) Commercial as-cast; (d) Commercial heat treated The segregation of element was formed on the EDS investigation. The Figure 6 shows the EDS analysis of DMLS alloy. In spectroscopy analysis it is found that precipitation of nickel

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element found in the sample. The segregation of iron content is very low and nickel and molybdenum is very high in case of DMLS alloy when compared to commercial alloy. Also, the

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oxides are formed on the DMLS alloy is high. Due to the high segregation of nickel element and oxides present in the DMLS alloy the corrosion rate is reduced. The segregation of chromium is

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same in both the commercial and DMLS alloy.

Element wt.% Fe = 19 Ni = 37 Cr = 27 Mo = 8 O =7

FeK 19%

CrK 27%

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NiK 37%

MoL 8%

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Figure 6: SEM image with EDS mapping of element distribution on the exposed surface of the DMLS heat treated alloy

The segregation of element of commercial alloy is shown in figure 7. It was found that

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the iron (Fe) was high in commercial alloy. The presence of nickel is very low in commercial

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alloy and in DMLS alloy it is drastically increased.

Element wt.% Fe = 66 Ni = 6 Cr = 28

CrK 28%

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NiK 6%

FeK 66%

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Figure 7: SEM image with EDS mapping of element distribution on the exposed surface of the commercial heat treated alloy

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Conclusions

The DMLS process used to produce IN718 parts offers the complex shape of an additive manufacturing process and ensures corrosive behavior is good as compared to conventional material. The corrosion resistance of heat treated DMLS IN718 is due to the segregation of high nickel, molybdenum and oxygen. The precipitation of γ” in the DMLS alloy improves the

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corrosion resistance. The protective film of oxides is present in the DMLS alloy reduce corrosion.

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Table 1: Potentiodynamic polarization data Material

Rest Potential Corrosion Potential Current Density Corrosion Rate RP (mV) Ecorr (mV) Icorr (mA/cm2) CR (mm/yr) 3.19

-202.46

1.3142

23.116

Com 24

-330.97

-343.85

1.3002

22.871

3D Bare

-0.73

-9.9925

0.8074054

14.202

3D 24

-49.04

-13.284

0.5360531

9.4291

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Com Bare