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Laser Additive Manufacturing using directed energy deposition of Inconel-718 wall structures with tailored characteristics
Vacuum 166 (2019) 270–278
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Laser Additive Manufacturing using directed energy deposition of Inconel718 wall structures with tailored characteristics
T
A.N. Jinoopa,b, C.P. Paula,b,∗, S.K. Mishrab, K.S. Bindraa,b a b
Homi Bhabha National Institute, Anushaktinagar, Mumbai, 400094, Maharashtra, India Laser Development and Industrial Applications Division, Raja Ramanna Centre for Advanced Technology, Indore, 452013, Madhya Pradesh, India
A R T I C LE I N FO
A B S T R A C T
Keywords: Laser additive manufacturing Directed energy deposition Process parameters Thin walls Post-processing Inconel 718
A process window is developed for fabricating thin walled Inconel 718 (IN718) structures for Laser Additive Manufacturing using Directed Energy Deposition (LAM-DED). In-house developed LAM-DED setup deployed to investigate the effect of process parameters on geometry and quality. The influence of post-heat treatment on microstructure, surface and mechanical properties is also studied. Full factorial experiments are carried out to derive the process window yielding requisite geometry, deposition rate and quality. The optimized process parameters are deployed for thin wall fabrication. The maximum and minimum limits for laser energy per unit length (E) and powder feed rate per unit length (F) are experimentally found to be 210 kJ/m, 105 kJ/m, 12.5 g/ m and 4 g/m, respectively for fabricating thin walls of requisite geometry. The fabricated thin walls are observed to be defect free with dendritic microstructure. The microstructural examination of heat-treated samples reveals recrystallized equiaxed grains with reduction in surface tensile residual stress up to 50%, and modified surface topography. The reduction of average hardness by 12%, significant improvement in ductility by 62.5% and improvement in energy storage capacity by 2.4 times are observed during the study.
1. Introduction Laser Additive Manufacturing (LAM) is one of the advanced metal additive manufacturing techniques which uses high power lasers for the fabrication of engineering and prosthetic components [1,2]. It is deployed for near net shaped fabrication of components directly from CAD model data using layer by layer methodology and the technology is attractive for “feature-based design and manufacturing”. LAM techniques can be broadly classified as powder bed fusion (LAM-PBF) and directed energy deposition (LAM-DED) based on material feeding. In LAM-PBF, a thin layer of powder (20–100 μm) is spread on the surface of a substrate or previously deposited layer. High power laser is used to melt the regions of a powder bed selectively as per the required geometry. Thus, a single layer conforming to component geometry is built, another layer of powder is laid and material is deposited using laser over the last layer. The process is repeated until whole number of layers yielding fabrication of full component is built. In LAM-DED, a high power laser is used as heat source to melt a thin layer of the substrate or previously deposited layer onto which the raw material (in form of wire or powder) is melted and deposited as per component geometry as a first layer. Subsequently, the successive layers are deposited one-overanother adopting same procedure resulting in three dimensional near
∗
net shaped components. LAM-PBF is used for fabricating complex shaped engineering components with intricate geometry and fine features, while LAM-DED finds application in fabricating multi-metal components, large sized parts and high performance coatings with higher build rate. LAM-DED is used widely for fabrication of automotive, aerospace, nuclear, medical components etc. with complexity and process control [1,2]. LAM-DED is effectively employed for processing wide variety of materials such as low alloy steels [3], HastelloyX [4], Titanium alloys [5], Inconel 718 [6] etc. LAM-DED is also successfully used for the fabrication of porous structures [2], graded materials [7,8] for various engineering applications. In addition to this, LAM-DED is investigated for the fabrication of thin walled structures [9]. Studies on the fabrication of thin walls [10] using pulsed Nd: YAG laser based wire deposition for austenitic stainless steel is reported. Another study on the stresses in thin walled structures [11] fabricated by wire arc additive manufacturing technique using Ti–6Al–4V reports that very high tensile deposition stress of 400 MPa is present in the asbuilt condition. The work emphasized the need for stress mitigation approaches to avoid the distortion of the components fabricated by Metal Additive Manufacturing (MAM). Yang et al. [12] studied the fabrication of thin walled structures using LAM-PBF and investigated the geometrical and microstructural characteristics of thin walled
Corresponding author. Homi Bhabha National Institute, Anushaktinagar, Mumbai, 400094, Maharashtra, India. E-mail address: [email protected] (C.P. Paul).
https://doi.org/10.1016/j.vacuum.2019.05.027 Received 29 March 2019; Received in revised form 16 May 2019; Accepted 17 May 2019 Available online 18 May 2019 0042-207X/ © 2019 Elsevier Ltd. All rights reserved.
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fibre laser based LAM-DED system [1,2] as shown in Fig. 1 (a) is deployed for the deposition of IN718. It consists of a 2 kW fibre laser system, coaxial nozzle, 5 axis manipulator in a glove box, computer numerical controller, twin powder feeder and gas analyzers. The pictorial image of co-axial nozzle used for LAM-DED is presented in Fig. 1 (b). The laser beam is fed through optical fibre and quartz lens and brought to the LAM-DED point on the table of 5-axis CNC laser workstation. The nozzle configuration is set to deliver a laser beam diameter of about 2.5 mm at the substrate surface. Sandblasted SS 304L substrate with thickness of 10 mm and diameter 75 mm is used for deposition. Argon gas with > 99.9993% purity (with O2 & H2O < 2 ppm, N2 < 3 ppm and total hydrocarbon < 0.2 ppm) is used as carrier gas for powder feeding and shielding gas for protecting melt pool from oxidation and subsequent contamination during the LAM-DED experiments. The flow rate of Argon gas was optimized experimentally to ensure regular-reproducible powder flow and oxidation prevention of molten metal during LAM-DED. The experimentally obtained values was found to be in the range of 6–8 L per minute (lpm) for various experiments. It is observed that the Argon gas flow rate plays a pivotal role in the quality of the material deposition. For value of flow rate lower than 6 lpm, the gas does not have enough force to carry the powder and provide a regular-reproducible powder flow, besides the shielding against the oxidation. On the other hand, the flow rate more than 8 lpm does not pose problem to oxidation, but results in a higher gas velocity for the same tube section, which results in powder ricocheting at the impingement point, reducing the powder catchment efficiency. In the present work, Argon flow rate of 6 lpm is deployed. Commercially available gas atomized IN718 powder with particle size ranging from 45 to 106 μm is used for the experiment. Fig. 2 (a) and (b)) presents the morphology of IN718 powders used for LAM-DED experiments. It can be observed that the powders are spherical in shape with fine satellite particle attached to it. As per literature, the most significant and common LAM-DED process parameters for parametric investigations are laser power (PL), scanning speed (v) and powder feed rate (mp) [1,4]. As it involves change in experimental setup, laser beam diameter is kept constant throughout the study to understand the influence of PL, v and mp on the track geometry. Thus, experiments are carried out at various combination of input parameters. Table 1 presents the control factors and their levels used in LAM-DED experiments. A number of tracks are laid at the various combination of process
structures. It is identified that process parameters are significant in controlling the geometry of structures, stability of melt pool and microstructure of thin walled components. Thus, it can be observed that a thorough investigation is necessary to develop the process window for fabricating thin walled structures using LAM-DED. Inconel 718 (IN718) is an important candidate for engineering applications of LAM parts. IN718 is a Nickel–Iron–Chromium based alloy [13] having high-temperature properties up to 700 °C with good corrosion resistance and fabricability. It is available in wrought, cast and powder metallurgy forms. IN718 is commonly used for various high end applications such as aircraft turbine engines, high-speed airframe parts, liquid fueled rockets, rings, castings and nuclear engineering. There are published reports on the fabrication of IN718 components by LAM-DED and LAM-PBF techniques and the effect of post-heat treatment on microstructure and mechanical properties. Wang et al. [14] investigated the microstructure and mechanical properties of IN718 fabricated by selective laser melting (SLM). The studies revealed variation in microstructure and mechanical properties before and after heat treatment. The tensile strength and ductility of heat treated IN718 are compared with the wrought material at room temperature. Zhao et al. [15] studied the microstructure and mechanical properties of Inconel 718 developed by LAM-DED. The mechanical properties of LAM-DED built IN718 samples were compared in as-built and heat-treated conditions. It was observed that gas atomized powders used for processing IN718 lead to low ductility and stress rupture. The ultimate tensile strength was improved through heat treatment by around 1.5 times as compared to that of as-built samples. However, these studies are limited to the bulk material, not on the development of thin walled structures. This paper reports LAM-DED of IN718 for developing a process window for fabricating uniform, continuous and defect free thin walled structures of IN718 with a wall thickness of around 2 mm for the fabrication of compact heat exchangers and thin walled honeycomb structures. Identified parameters are used for LAM-DED of thin walls and these walls are subjected to post-heat treatment to investigate its effect on microstructure, residual stress, surface topography and mechanical properties.
2. Material and methods In the present study, an in-house developed 2 kW continuous wave
Fig. 1. LAM-DED experimental setup (a) Schematic diagram (b) Co-axial Nozzle. 271
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Fig. 2. SEM image of IN718 powder.
at 5 different locations and average value of track dimensions is reported in Table 3. The maximum variation of 5% is observed during the measurement of track width and height. The tracks are analyzed for surface topography and uniformity. In addition, track geometry is measured to identify the combination of process parameters yielding track width (W) between 1.75 and 2 mm. It is observed that the track width (refer Fig. 3(a)) decreases with the increase in scan speed at constant powder feed rate (0.117 g/s). It is just because an increase in scan speed results in a reduction in laser energy per unit length leading to the generation of molten pool with smaller size. When the scanning speed increases, the track width is reduced and when the scan speed decreases, the track width increases and limits to laser beam diameter. In LAM-DED, the high power laser melts a thin layer of substrate or previous layer and powder material is added to the melt pool for track formation. For the parameters under investigation, the maximum size of the melt pool is limited to beam diameter. It is just because the chosen combination of processing parameters does not allow conduction of laser generated heat for creation of melt pool larger than the diameter of laser beam. These observations are in-line with earlier reported results [2]. The effect of process parameters on the track height (H) (refer Fig. 3(b)) shows that track height also decreases with an increase in scanning speed. It is primarily due to a decrease in powder feed rate per unit length yielding a lower amount of powder available for deposition and vice versa. These observations are in-line with earlier published research for other material [2]. Fig. 4 presents the top view of typical IN718 tracks laid on substrate using optical microscope with the digital imaging system. The geometry and quality of tracks are analyzed using two combined parameters: laser energy per unit length (E) and powder feed rate per unit length (F). E refers to the ratio between laser power and scan speed, while F refers to the ratio between powder feed rate and scan speed. Mathematically, E = PL/v (J/m) and F = mp/v (g/m). The limits of laser power, powder feed rate and scanning speed as presented in Table 1 is selected to avoid tracks/deposits with discontinuity and crack. Discontinuous tracks are observed below 76.92 kJ/m and above 16.71 g/m values of E and F, respectively. It is primarily because of unavailability of sufficient “E” for regular track formation/deposition. Cracked deposits are observed above 257.14 kJ/m and below 3.85 g/m. It is primarily because of larger thermal energy due to higher “E”. Thus, the range of parameters used for the present investigation are summarized in Table 1. As presented in Table 3, regular deposition is observed for all the processing parameters under investigation with spatters and blackening for the few. Blackening is primarily due to overheating of material at higher E value. Regular deposits without spatters and blackening is achieved when the value of E and F are below 142.8 kJ/m and 11.8 g/m, respectively. Fig. 5 presents the process map for track width 1.5 mm–2.5 mm for various E and F. Each processing condition is denoted by a pair of points representing the combination of parameters yielding thin wall of requisite thickness. In the present study, 1.75–2 mm thin walls are selected for one of our applications to develop
Table 1 Control factors and their levels used in LAM-DED experiments. Parameters
Unit
Level 1
Level 2
Level 3
Laser Power (PL) Scanning Speed (v) Powder feed rate (mp)
W m/s g/s
1000 0.007 0.050
1400 0.010 0.083
1800 0.013 0.117
Table 2 Post heat treatment parameters. Treatment
Solution treatment
HT950 HT1050
At 950 °C for 1 h soaking time followed by water quenching At 1050 °C for 1 h soaking time followed by water quenching
parameters as per full factorial experimental design. The identified combination of process parameters is selected by considering the track geometry and track characteristics. This selected combination of process parameters is used for laying multi-layered tracks for building the wall. Thus, built walls are subjected to post-heat treatment using an indigenously developed furnace (maximum temperature 1200 °C) as summarized in Table 2. The temperature for post-heat treatment is selected from the range of temperature provided in the literature for the solution treatment of IN718 [16]. LAM-DED fabricated samples are prepared as per standard metallographic procedure and subjected to various test in as-built and post-heat treated conditions. Track geometry and microstructure is measured using an optical microscope attached with digital camera (Make and Model: LEICA - DM 2700M). Surface Roughness studies are carried out in as-built condition using the optical profiler (Make and Model: Veeco-NT 9080). Residual stress and X-ray diffraction studies are carried out using X-ray diffraction technique (Make and Model: BRUKER - D8 Advance). Atomic force microscopy (AFM) (Make and Model: AGILENT 5420) is used to study the surface topography of the deposit. Vickers micro-hardness (Make and Model: INNOVATEST–Nexus Model 4305) is measured using a load of 1.96 N with a dwell period of 10 s. Compression testing is carried out using a 150 kN servo-hydraulic universal testing machine (Make and Model: BISSMedian). Ball indentation studies are performed using Automated Ball Indenter (Make and Model: BISS – LF–01-5020) with 1 mm diameter tungsten carbide indenter at maximum load of 80 N with a preload of 10 N. 3. Results and discussion 3.1. Parametric investigation For the detailed parametric investigation, a number of tracks are laid at the various combination of process parameters as per full factorial design (refer Table 3). The track dimensions are measured at least 272
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Table 3 Full factorial deposits with observations, measured dimensions and deposition rate. Specimen ID
01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27
PL
v
mp
E
F
Remarks
W
m/s
g/s
kJ/m
g/m
1000 1000 1000 1400 1400 1400 1800 1800 1800 1000 1000 1000 1400 1400 1400 1800 1800 1800 1000 1000 1000 1400 1400 1400 1800 1800 1800
0.007 0.01 0.013 0.007 0.01 0.013 0.007 0.01 0.013 0.007 0.01 0.013 0.007 0.01 0.013 0.007 0.01 0.013 0.007 0.01 0.013 0.007 0.01 0.013 0.007 0.01 0.013
0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.083 0.083 0.083 0.083 0.083 0.083 0.083 0.083 0.083 0.117 0.117 0.117 0.117 0.117 0.117 0.117 0.117 0.117
142.86 100 76.92 200 140 107.69 257.14 180 138.46 142.86 100 76.92 200 140 107.69 257.14 180 138.46 142.86 100 76.92 200 140 107.69 257.14 180 138.46
7.14 5.00 3.85 7.14 5.00 3.85 7.14 5.00 3.85 11.86 8.30 6.38 11.86 8.30 6.38 11.86 8.30 6.38 16.71 11.70 9 16.71 11.70 9 16.71 11.70 9
Regular Regular Regular Regular & Blackened Regular Regular Regular & Blackened Regular Regular Regular Regular Regular Regular & Blackened Regular Regular Regular & Blackened Regular Regular Blackened with more spatters Regular Regular Regular & Blackened Regular Regular Regular & Blackened Blackened with more spatters Regular
Height
Deposition Rate
mm
mm
mm3/s
1.52 1.25 1.12 1.86 1.84 1.82 2.33 2.22 2.16 1.47 1.41 1.27 1.88 1.85 1.72 2.38 2.03 1.98 1.58 1.42 1.21 1.95 1.85 1.70 2.20 2.01 2.00
0.26 0.22 0.20 0.27 0.23 0.12 0.39 0.26 0.22 0.26 0.24 0.22 0.49 0.32 0.25 0.58 0.35 0.25 0.48 0.35 0.25 0.62 0.40 0.32 0.77 0.53 0.36
0.66 0.69 0.75 0.84 1.06 0.73 1.51 1.44 1.58 0.64 0.85 0.93 1.54 1.48 1.43 2.30 1.78 1.65 1.26 1.24 1.01 2.02 1.98 1.81 2.82 2.66 2.27
beam diameter with E and F remains constant for other beam diameter also. Therefore, a ratio of requisite width to laser beam diameter can be deployed for selecting the parametric combination from Fig. 5. Subsequently, the optimum combination of process parameters is selected considering the requisite dimension (for the present study), maximum deposition rate and uniform surface topography. The deposition rate is defined as per equation (2), where n is geometry factor.
compact heat exchangers (CHEs) and thin walled honeycomb structures. Emax, Emin, Fmax and Fmin represents the maximum and minimum values of E and F, respectively for the development of thin walls with track width in the required range. These values are used to develop the process window and it is found that Emax and Fmax are 210 kJ/m and 12.5 g/m for development of IN718 thin walls of required dimensions. Similarly, Emin and Fmin are 105 kJ/m and 4 g/m respectively. A careful analysis of the process parameters and deposition process shows that Emax is limited by the required track dimensions, while Fmax is limited by the track dimensions and maximum energy available for melting the powder. Similarly, Emin is limited by the minimum energy required to melt the fed powder to build thin walls, while Fmin is limited by the minimum material required for the desired track dimension. Further, an empirical relation as presented in equation (1) is derived for track width. Statistical regression analysis of track width with respect to the process parameters showed a strong relationship with a linear regression coefficient of 0.965. Fig. 6 presents the dependence of the combined parameter with track width. It can be observed from equation (1) that PL and v are significant in determining the width. As mentioned before, PL and v govern the amount of energy available per unit length (E). Thus, E is significant in determining the width of deposition. As E increases, temperature of the melt pool increases resulting in the reduction of material viscosity, which leads to the spreading of material. It can also be observed from equation (1) that mp is less significant as compared to other factors which confirm that E is a major parameter as compared to F in controlling the width. The maximum deviation of 11% is observed between the experimental and predicted width.
W (m) = PL 0.8 (W ) v−0.2 (m / s ) mp0.009 (kg / s )
Width
Deposition rate = n × W × H
(2)
It is found that combination of process parameter complying to the above criteria is PL - 1400 W, v - 0.01 m/s and mp - 0.117 g/s (Specimen ID – 23). The selected process parameters are used for the development of multi-layered thin wall (10 layers) structures. Fig. 7 presents the macrostructure depicting transverse section of thin wall. It can be observed that the deposition is defect free without pores or irregularities. Further, the dimension of the wall is measured at different locations. It is observed that the maximum deviation of 15% is observed for the deposited wall as compared to the corresponding single track dimension. This may be due to the preheating effect during multilayer LAMDED. Preheating effect results in an increase of the average temperature during deposition and reduction in viscosity and density of the material. Thus, melt liquid will have a propensity to slide down and this results in the variation of dimension [17]. Surface roughness analysis of as-built transverse surface shows average roughness value (Ra) of 6.52 μm. This may be due to the stair stepping effect in LAM due to the layer by layer methodology and the presence of partially melted powders on the surface of the sample. Further, the fabricated structures are subjected to heat treatment and microstructure, surface topography and mechanical properties are studied.
(1)
The developed process window and empirical relation can be deployed for selecting the process parameter combination for the requisite dimension of a thin wall at a constant laser beam diameter. Track width is mainly influenced by beam diameter, E and F. In the present study, E and F are considered for parametric investigation and to analyze the track geometry. However, as beam diameter changes, the track width also changes. But, the relation between the ratio of track width and
3.2. Optical microscopy The microstructures of thin walls built by LAM-DED (in as-built and post heat-treated conditions) are studied using an optical microscope after preparing them as per ASTM E3. Fig. 8a presents the micrograph of the sample in the as-built condition. 273
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Fig. 3. Track Geometry (a) Typical cross-section of LAM-DED track (b) Variation of Track width with PL and v at a constant mp of 0.117 g/s (b) Variation of Track Height with PL and v at a constant mp of 0.117 g/s.
developed as a result of rapid cooling rate during LAM-DED. Fig. 8(b) and (c) presents the microstructure of the deposit after heat treatment at 950 °C (HT950) and 1050 °C (HT1050), respectively. After post-heat treatment, homogenized microstructure in all samples is observed with dissolved dendrites. The columnar grains present in the as-built samples are not visible in the HT950 and HT1050 samples. Recovery, recrystallization and grain growth takes place during post-heat treatment.
Few pores at isolated locations in the deposits are also observed. The pore formation may be attributed to rapid molten dynamics during LAM-DED yielding entrapped gas bubbles in the molten metal. The typical size of these pores are less than 15 μm. Microstructure examinations of LAM-DED deposits of IN718 reveal dendritic structure. The direction of dendrite growth is along the direction of deposition due to preferential cooling. The dendritic microstructures are
Fig. 4. Various IN718 deposits using LAM-DED (a) Regular deposition (b) Blackened deposition (c) Black deposition with more spatters. 274
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Fig. 5. Process window for nominal width of IN718 thin walls.
During recovery, the internal stresses are relieved and mainly takes place at lower temperature without affecting the mechanical properties of the material. During recrystallization, new grains are formed with same lattice structure with approximately same dimensions in all directions as appropriate conditions are provided. Thus, recrystallized equiaxed grains are formed during post-heat treatment. Further, coalescence of grains during grain growth increases the size of grains leading to reduction in strength and hardness with enhanced ductility.
3.3. X-ray diffraction studies X-ray diffraction studies are performed with scanning range of 30–100° at step size of 0.02° after calibration and characterization of the diffractometer with NIST standard reference material (Corundum). Fig. 9 depicts the XRD pattern of as-built, HT950 and HT1050 samples. The presence of face centered cubic γ matrix is noticed in the study as observed by other researchers [18]. Smaller crystallite size is revealed through the peak broadness. It is also observed that peak width decreased significantly which corresponds to the increase in the crystallite size of the material. Crystallite size is calculated based on the Scherrer formula as shown in equation (3). This provides a lower bound on the crystallite size as there are other factors which influence the width of the diffraction peak such as strain, dislocations etc.where d is the mean size of the crystal, k is a dimensionless shape factor, λ is the X-ray wavelength, β is the FWHM in radians and θ is the Bragg angle (in degrees). The estimated crystallite size is 9.71 nm, 21.21 nm and 23.17 nm, respectively for as-built, HT950 and HT1050 samples, respectively. As, mentioned before, the fine size of crystallites in the as-built condition can be due to the higher cooling rate during LAM-DED. It is also observed that there is a considerable change in average crystallite size between as-built sample and HT 950, but the size variation is less between HT950 and HT1050. The major reason for the increase in the crystallite size is due to the reduction of surface energy after heat treatment, which influences the particle size as a function of the temperature.
Fig. 6. Dependence of Track Width on the combined parameter.
Fig. 7. Macrostructure depicting transverse section of thin walled structure. 275
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Fig. 8. Microstructure of a) as-built b) HT950 c) HT1050 samples.
components are subjected to fluctuating mechanical load [19]. Thus, it is necessary to reduce this tensile residual stress, which can be achieved by appropriate post-processing of the material using solution treatment procedure adopted from the literature [16]. In our study, a significant reduction in residual stress is observed for the samples subjected to post-heat treatment at HT950 and HT1050. During the post-heat treatment, the material is heated and soaked for uniform temperature distribution within the material. The subsequent quenching induces the compressive stress in the material. This whole process causes reduction of tensile residual stress in the surface of LAM-DED material. 3.5. Atomic force microscopy AFM is used to understand the uniformity of the deposition in the nano-metric level. A similar approach is followed by earlier researchers [20–22]. Fig. 10(a) and (b) and (c) presents surface topography of the LAM-DED material in as-built, HT950 and HT1050 conditions, respectively using AFM with a magnification of 10 μm × 10 μm. The average value of surface roughness (Ra) at the nano-metric level is 1.87 nm, 2.39 nm and 2.46 nm for as-built, HT950 and HT1050, respectively. It is evident that the as-built samples have smoother surface, while sharp hills and valleys are clearly visible in the surface topography of HT950 and HT1050 samples. These observations are in line with earlier reported results for other deposition technique [23] and LAM-PBF samples [24]. The obtained value of Kurtosis (Rku) is 2.83, 3.03, and 3.80 in the as-built, HT950 and HT1050 samples, respectively. This reveals that as-built sample have fewer sharp peaks (Rku < 3) with uniform topography and post-heat treated samples have spikes (Rku > 3). During post-heat treatment, the grain aggregation occurs yielding larger clusters and it influences the topography. This was also confirmed by XRD using Scherrer equation which revealed increase in the crystallite size.
Fig. 9. X-ray diffraction results of the LAM-DED built samples under different conditions.
d=
k.λ β . cos θ
(3)
3.4. Residual stress measurement The surface residual stress measurement was carried out using XRay diffraction to investigate its variation in the LAM-DED material in as-built and post-heat treatment conditions. The samples are analyzed as per the standard procedure (ASTM E2860) without removing the deposit from the substrate and without any post-machining. Table 4 presents the variation of average surface residual stress in the material in as-built and post-heat treatment conditions. It may be seen that the average tensile residual stress induced on the surface of the LAM-DED sample is 512 MPa and reduction of 17.5% and 50% in the surface tensile residual stress is observed after HT950 and HT1050, respectively. The residual stress is induced in the material during LAM-DED is mainly due to the local heating, rapid solidification and self-quenching of the thin surface layer results in the accumulation of local thermal stress and thereby micro strain. The nature of micro strain is tensile because when the surface is heated during LAM-DED, it gives rise to local expansion and the surface is under compressive stress due to constraints offered by the material. This stress becomes zero, when the surface is melted. Subsequent solidification and cooling generates compression and thereby generates tensile stress at the surface. This tensile stress becomes critical causing an early failure when the
3.6. Micro-hardness measurement The micro-hardness of IN718 samples are examined in as-built and post-heat treated condition as per ASTM E384. Fig. 11 presents the variation of the hardness along the cross-section. It can be observed that as-built samples have higher hardness than post-heat treated samples. The average hardness of as-built, HT 950 and HT1050 are 234 ± 7 HV1.96N, 220 ± 6.1 HV1.96N and 206 ± 5.9 HV1.96N, respectively. Higher hardness of as-built sample can be due to the fine dendritic microstructure usually observed in LAM-DED samples owing to the high cooling rate [25]. A reduction of 6% and 12% in micro-hardness is observed for HT950 and HT1050 samples, respectively as compared to that of as-built samples. Grain agglomeration and coarsening due to the post-heat treatment as explained in the previous section may be the reason for the reduction in microhardness of post-heat treated samples as per Halls-petch relationship. Further, the result is compared with IN718 samples made by LAM-PBF and it was observed that the average micro-hardness is 319 HV1.96N, 307 HV1.96N and 260 HV1.96N in the asbuilt, HT950 and HT1050 condition, respectively [23]. It may be observed that the trend remains similar while the higher hardness is
Table 4 Average surface residual stress. Condition
Normal stress along X direction (MPa)
As-built HT950 HT1050
512 ± 15.46 422 ± 12.99 254 ± 11.80
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Fig. 10. Atomic force microscopy analysis (a) As-built (b) HT950 (c) HT1050.
stiffness remains almost the same up to certain values and thereafter it varies. The closer value of stiffness represents that the material is under an elastic regime in these loading conditions, while the region with difference corresponds to the plastic regime. It may be seen that the elastic regimes terminate early in post-heat treatment samples. The displacement at 20 kN is 0.668 mm, 0.8 mm and 1.086 for as-built, HT950 and HT1050 samples, respectively. It shows that an improvement in ductility by 19.7% and 62.5% for HT950 and HT1050, respectively due to post-heat treatment. 3.8. Ball indentation test Ball indentation tests (BI) is performed for comparing the energy storage capacity and maximum displacement of the material using a single cycle with maximum load of 80 N at a rate of 0.1 mm/min with unloading cycle till 10 N load as per ASTM STP1092. Fig. 13 presents the typical load displacement curve obtained during the BI of LAM-DED samples. The maximum displacement for as-built, HT950 and HT1050 samples are 0.050 mm, 0.070 mm, and 0.079 mm, respectively. The relatively lower depth of penetration in the as-built sample is primarily attributed to the higher hardness. These results are in line with the observation from micro-hardness and compression testing. The areas under the loading and unloading curve represents the entire work performed during loading and the reversible elastic contribution of the total work, respectively. The difference between the two areas provides the energy absorbed in plastic deformation, which can be correlated to the energy stored by the material. The area under the curves of as-built, HT950 and HT1050 samples are 0.49 N-mm, 1.09 N-mm, and 1.7 Nmm, respectively. BI studies reveal the improvement in energy storage capacity by 2.22 and 3.46 times for HT950 and HT1050, respectively to that of as-built. In earlier reported work, the maximum displacement for as-built, HT950 and HT1050 samples are 0.03 mm, 0.04 mm and 0.05 mm, respectively for LAM-PBF IN718 samples at maximum load of
Fig. 11. Variation of hardness along the cross-section.
observed in LAM-PBF samples. This can be due to the faster scanning rate as compared to LAM-DED which results in finer microstructures.
3.7. Compression testing Compression testing as per ASTM E−9 standard is carried out to understand the effect of heat treatment on the ductility of the material as ductility is important, when the material is subjected to compressive loading in real engineering applications, especially vacuum vessels, compact heat exchangers etc. Fig. 12 presents the load-displacement curve obtained from compression test, where the slope of the curve represents the stiffness of the material. It may be seen in Fig. 12 that the
Fig. 12. Load displacement curve during compression test.
Fig. 13. Load displacement curve during BI. 277
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50 N [23]. The area under the curves of as-built, HT950 and HT1050 LAM-PBF samples are 0.09 N-mm, 0.12 N-mm and 0.417 N-mm, respectively [23]. The displacement for LAM-PBF IN718 samples at 50 N is lower than the corresponding displacement for LAM-DED samples at 50 N as presented in Fig. 13. This is in agreement with the lower hardness of LAM-DED samples as compared to LAM-PBF samples.
[2]
[3]
[4]
4. Conclusions
[5]
One of the exciting applications of LAM-DED is the development of complex shaped thin walled engineering components. In the present work, a systematic study is performed for deploying LAM-DED for the development of IN718 thin walls and the process window is developed. The process window revealed maximum and minimum limits for both E and F are 210 kJ/m, 105 kJ/m, 12.5 g/m and 4 g/m, respectively for IN718 thin walls with requisite dimension. An empirical model is also derived for the track width using statistical regression analysis. Uniform deposition with requisite dimensions and higher deposition rate are found at laser power - 1400 W, scan speed - 0.01 m/s and powder feed rate - 0.117 g/s. Defect free thin walls fabricated by LAM-DED revealed fine dendritic microstructure. The post-heat treated samples shows recrystallized grains with dissolved dendrites. 50% reduction in surface residual stress is observed after post-heat treatment. Grain clustering during post-heat treatment changed the surface topography of the material. Micro-hardness studies revealed reduction up to 12%, compression testing shows ductility improvement up to 62.5% and BI studies revealed improvement in energy storage capacity up to 2.4 times. The obtained results of micro-hardness and BI studies using LAM-DED samples are compared with published literature for LAM-PBF samples. It is found that LAM-DED samples have lower hardness, higher ductility and higher energy storage ability as compared to that of LAM-PBF samples. This confirms its suitability for the intended applications and thus, this study opens avenues for thin wall fabrication to develop compact heat exchanger walls and thin walled honeycomb structures of requisite dimensions and properties.
[6]
[7]
[8] [9]
[10] [11] [12]
[13]
[14]
[15]
[16] [17]
Acknowledgment
[18]
A N Jinoop acknowledges the financial support by Raja Ramanna Centre for Advanced Technology (RRCAT), Department of Atomic Energy, Government of India and Homi Bhabha National Institute, Mumbai. The authors thank Mr. H. Kumar, Mr. U. Kumar and Mr. C.H. Prem Singh of LAM lab, RRCAT for their help during sample preparation. The authors thank the support from Dr. P. Ganesh, Dr. Archna Sagdeo, Mr. Sujan Kar, Ms. Preeti Pokhriyal, Ms. Sushmita Bhartiya of RRCAT and other members of Laser Additive Manufacturing Lab at RRCAT, Indore, India.
[19]
[20]
[21]
[22] [23]
Appendix A. Supplementary data [24]
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.vacuum.2019.05.027.
[25]
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Vacuum journal homepage: www.elsevier.com/locate/vacuum
Laser Additive Manufacturing using directed energy deposition of Inconel718 wall structures with tailored characteristics
T
A.N. Jinoopa,b, C.P. Paula,b,∗, S.K. Mishrab, K.S. Bindraa,b a b
Homi Bhabha National Institute, Anushaktinagar, Mumbai, 400094, Maharashtra, India Laser Development and Industrial Applications Division, Raja Ramanna Centre for Advanced Technology, Indore, 452013, Madhya Pradesh, India
A R T I C LE I N FO
A B S T R A C T
Keywords: Laser additive manufacturing Directed energy deposition Process parameters Thin walls Post-processing Inconel 718
A process window is developed for fabricating thin walled Inconel 718 (IN718) structures for Laser Additive Manufacturing using Directed Energy Deposition (LAM-DED). In-house developed LAM-DED setup deployed to investigate the effect of process parameters on geometry and quality. The influence of post-heat treatment on microstructure, surface and mechanical properties is also studied. Full factorial experiments are carried out to derive the process window yielding requisite geometry, deposition rate and quality. The optimized process parameters are deployed for thin wall fabrication. The maximum and minimum limits for laser energy per unit length (E) and powder feed rate per unit length (F) are experimentally found to be 210 kJ/m, 105 kJ/m, 12.5 g/ m and 4 g/m, respectively for fabricating thin walls of requisite geometry. The fabricated thin walls are observed to be defect free with dendritic microstructure. The microstructural examination of heat-treated samples reveals recrystallized equiaxed grains with reduction in surface tensile residual stress up to 50%, and modified surface topography. The reduction of average hardness by 12%, significant improvement in ductility by 62.5% and improvement in energy storage capacity by 2.4 times are observed during the study.
1. Introduction Laser Additive Manufacturing (LAM) is one of the advanced metal additive manufacturing techniques which uses high power lasers for the fabrication of engineering and prosthetic components [1,2]. It is deployed for near net shaped fabrication of components directly from CAD model data using layer by layer methodology and the technology is attractive for “feature-based design and manufacturing”. LAM techniques can be broadly classified as powder bed fusion (LAM-PBF) and directed energy deposition (LAM-DED) based on material feeding. In LAM-PBF, a thin layer of powder (20–100 μm) is spread on the surface of a substrate or previously deposited layer. High power laser is used to melt the regions of a powder bed selectively as per the required geometry. Thus, a single layer conforming to component geometry is built, another layer of powder is laid and material is deposited using laser over the last layer. The process is repeated until whole number of layers yielding fabrication of full component is built. In LAM-DED, a high power laser is used as heat source to melt a thin layer of the substrate or previously deposited layer onto which the raw material (in form of wire or powder) is melted and deposited as per component geometry as a first layer. Subsequently, the successive layers are deposited one-overanother adopting same procedure resulting in three dimensional near
∗
net shaped components. LAM-PBF is used for fabricating complex shaped engineering components with intricate geometry and fine features, while LAM-DED finds application in fabricating multi-metal components, large sized parts and high performance coatings with higher build rate. LAM-DED is used widely for fabrication of automotive, aerospace, nuclear, medical components etc. with complexity and process control [1,2]. LAM-DED is effectively employed for processing wide variety of materials such as low alloy steels [3], HastelloyX [4], Titanium alloys [5], Inconel 718 [6] etc. LAM-DED is also successfully used for the fabrication of porous structures [2], graded materials [7,8] for various engineering applications. In addition to this, LAM-DED is investigated for the fabrication of thin walled structures [9]. Studies on the fabrication of thin walls [10] using pulsed Nd: YAG laser based wire deposition for austenitic stainless steel is reported. Another study on the stresses in thin walled structures [11] fabricated by wire arc additive manufacturing technique using Ti–6Al–4V reports that very high tensile deposition stress of 400 MPa is present in the asbuilt condition. The work emphasized the need for stress mitigation approaches to avoid the distortion of the components fabricated by Metal Additive Manufacturing (MAM). Yang et al. [12] studied the fabrication of thin walled structures using LAM-PBF and investigated the geometrical and microstructural characteristics of thin walled
Corresponding author. Homi Bhabha National Institute, Anushaktinagar, Mumbai, 400094, Maharashtra, India. E-mail address: [email protected] (C.P. Paul).
https://doi.org/10.1016/j.vacuum.2019.05.027 Received 29 March 2019; Received in revised form 16 May 2019; Accepted 17 May 2019 Available online 18 May 2019 0042-207X/ © 2019 Elsevier Ltd. All rights reserved.
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fibre laser based LAM-DED system [1,2] as shown in Fig. 1 (a) is deployed for the deposition of IN718. It consists of a 2 kW fibre laser system, coaxial nozzle, 5 axis manipulator in a glove box, computer numerical controller, twin powder feeder and gas analyzers. The pictorial image of co-axial nozzle used for LAM-DED is presented in Fig. 1 (b). The laser beam is fed through optical fibre and quartz lens and brought to the LAM-DED point on the table of 5-axis CNC laser workstation. The nozzle configuration is set to deliver a laser beam diameter of about 2.5 mm at the substrate surface. Sandblasted SS 304L substrate with thickness of 10 mm and diameter 75 mm is used for deposition. Argon gas with > 99.9993% purity (with O2 & H2O < 2 ppm, N2 < 3 ppm and total hydrocarbon < 0.2 ppm) is used as carrier gas for powder feeding and shielding gas for protecting melt pool from oxidation and subsequent contamination during the LAM-DED experiments. The flow rate of Argon gas was optimized experimentally to ensure regular-reproducible powder flow and oxidation prevention of molten metal during LAM-DED. The experimentally obtained values was found to be in the range of 6–8 L per minute (lpm) for various experiments. It is observed that the Argon gas flow rate plays a pivotal role in the quality of the material deposition. For value of flow rate lower than 6 lpm, the gas does not have enough force to carry the powder and provide a regular-reproducible powder flow, besides the shielding against the oxidation. On the other hand, the flow rate more than 8 lpm does not pose problem to oxidation, but results in a higher gas velocity for the same tube section, which results in powder ricocheting at the impingement point, reducing the powder catchment efficiency. In the present work, Argon flow rate of 6 lpm is deployed. Commercially available gas atomized IN718 powder with particle size ranging from 45 to 106 μm is used for the experiment. Fig. 2 (a) and (b)) presents the morphology of IN718 powders used for LAM-DED experiments. It can be observed that the powders are spherical in shape with fine satellite particle attached to it. As per literature, the most significant and common LAM-DED process parameters for parametric investigations are laser power (PL), scanning speed (v) and powder feed rate (mp) [1,4]. As it involves change in experimental setup, laser beam diameter is kept constant throughout the study to understand the influence of PL, v and mp on the track geometry. Thus, experiments are carried out at various combination of input parameters. Table 1 presents the control factors and their levels used in LAM-DED experiments. A number of tracks are laid at the various combination of process
structures. It is identified that process parameters are significant in controlling the geometry of structures, stability of melt pool and microstructure of thin walled components. Thus, it can be observed that a thorough investigation is necessary to develop the process window for fabricating thin walled structures using LAM-DED. Inconel 718 (IN718) is an important candidate for engineering applications of LAM parts. IN718 is a Nickel–Iron–Chromium based alloy [13] having high-temperature properties up to 700 °C with good corrosion resistance and fabricability. It is available in wrought, cast and powder metallurgy forms. IN718 is commonly used for various high end applications such as aircraft turbine engines, high-speed airframe parts, liquid fueled rockets, rings, castings and nuclear engineering. There are published reports on the fabrication of IN718 components by LAM-DED and LAM-PBF techniques and the effect of post-heat treatment on microstructure and mechanical properties. Wang et al. [14] investigated the microstructure and mechanical properties of IN718 fabricated by selective laser melting (SLM). The studies revealed variation in microstructure and mechanical properties before and after heat treatment. The tensile strength and ductility of heat treated IN718 are compared with the wrought material at room temperature. Zhao et al. [15] studied the microstructure and mechanical properties of Inconel 718 developed by LAM-DED. The mechanical properties of LAM-DED built IN718 samples were compared in as-built and heat-treated conditions. It was observed that gas atomized powders used for processing IN718 lead to low ductility and stress rupture. The ultimate tensile strength was improved through heat treatment by around 1.5 times as compared to that of as-built samples. However, these studies are limited to the bulk material, not on the development of thin walled structures. This paper reports LAM-DED of IN718 for developing a process window for fabricating uniform, continuous and defect free thin walled structures of IN718 with a wall thickness of around 2 mm for the fabrication of compact heat exchangers and thin walled honeycomb structures. Identified parameters are used for LAM-DED of thin walls and these walls are subjected to post-heat treatment to investigate its effect on microstructure, residual stress, surface topography and mechanical properties.
2. Material and methods In the present study, an in-house developed 2 kW continuous wave
Fig. 1. LAM-DED experimental setup (a) Schematic diagram (b) Co-axial Nozzle. 271
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Fig. 2. SEM image of IN718 powder.
at 5 different locations and average value of track dimensions is reported in Table 3. The maximum variation of 5% is observed during the measurement of track width and height. The tracks are analyzed for surface topography and uniformity. In addition, track geometry is measured to identify the combination of process parameters yielding track width (W) between 1.75 and 2 mm. It is observed that the track width (refer Fig. 3(a)) decreases with the increase in scan speed at constant powder feed rate (0.117 g/s). It is just because an increase in scan speed results in a reduction in laser energy per unit length leading to the generation of molten pool with smaller size. When the scanning speed increases, the track width is reduced and when the scan speed decreases, the track width increases and limits to laser beam diameter. In LAM-DED, the high power laser melts a thin layer of substrate or previous layer and powder material is added to the melt pool for track formation. For the parameters under investigation, the maximum size of the melt pool is limited to beam diameter. It is just because the chosen combination of processing parameters does not allow conduction of laser generated heat for creation of melt pool larger than the diameter of laser beam. These observations are in-line with earlier reported results [2]. The effect of process parameters on the track height (H) (refer Fig. 3(b)) shows that track height also decreases with an increase in scanning speed. It is primarily due to a decrease in powder feed rate per unit length yielding a lower amount of powder available for deposition and vice versa. These observations are in-line with earlier published research for other material [2]. Fig. 4 presents the top view of typical IN718 tracks laid on substrate using optical microscope with the digital imaging system. The geometry and quality of tracks are analyzed using two combined parameters: laser energy per unit length (E) and powder feed rate per unit length (F). E refers to the ratio between laser power and scan speed, while F refers to the ratio between powder feed rate and scan speed. Mathematically, E = PL/v (J/m) and F = mp/v (g/m). The limits of laser power, powder feed rate and scanning speed as presented in Table 1 is selected to avoid tracks/deposits with discontinuity and crack. Discontinuous tracks are observed below 76.92 kJ/m and above 16.71 g/m values of E and F, respectively. It is primarily because of unavailability of sufficient “E” for regular track formation/deposition. Cracked deposits are observed above 257.14 kJ/m and below 3.85 g/m. It is primarily because of larger thermal energy due to higher “E”. Thus, the range of parameters used for the present investigation are summarized in Table 1. As presented in Table 3, regular deposition is observed for all the processing parameters under investigation with spatters and blackening for the few. Blackening is primarily due to overheating of material at higher E value. Regular deposits without spatters and blackening is achieved when the value of E and F are below 142.8 kJ/m and 11.8 g/m, respectively. Fig. 5 presents the process map for track width 1.5 mm–2.5 mm for various E and F. Each processing condition is denoted by a pair of points representing the combination of parameters yielding thin wall of requisite thickness. In the present study, 1.75–2 mm thin walls are selected for one of our applications to develop
Table 1 Control factors and their levels used in LAM-DED experiments. Parameters
Unit
Level 1
Level 2
Level 3
Laser Power (PL) Scanning Speed (v) Powder feed rate (mp)
W m/s g/s
1000 0.007 0.050
1400 0.010 0.083
1800 0.013 0.117
Table 2 Post heat treatment parameters. Treatment
Solution treatment
HT950 HT1050
At 950 °C for 1 h soaking time followed by water quenching At 1050 °C for 1 h soaking time followed by water quenching
parameters as per full factorial experimental design. The identified combination of process parameters is selected by considering the track geometry and track characteristics. This selected combination of process parameters is used for laying multi-layered tracks for building the wall. Thus, built walls are subjected to post-heat treatment using an indigenously developed furnace (maximum temperature 1200 °C) as summarized in Table 2. The temperature for post-heat treatment is selected from the range of temperature provided in the literature for the solution treatment of IN718 [16]. LAM-DED fabricated samples are prepared as per standard metallographic procedure and subjected to various test in as-built and post-heat treated conditions. Track geometry and microstructure is measured using an optical microscope attached with digital camera (Make and Model: LEICA - DM 2700M). Surface Roughness studies are carried out in as-built condition using the optical profiler (Make and Model: Veeco-NT 9080). Residual stress and X-ray diffraction studies are carried out using X-ray diffraction technique (Make and Model: BRUKER - D8 Advance). Atomic force microscopy (AFM) (Make and Model: AGILENT 5420) is used to study the surface topography of the deposit. Vickers micro-hardness (Make and Model: INNOVATEST–Nexus Model 4305) is measured using a load of 1.96 N with a dwell period of 10 s. Compression testing is carried out using a 150 kN servo-hydraulic universal testing machine (Make and Model: BISSMedian). Ball indentation studies are performed using Automated Ball Indenter (Make and Model: BISS – LF–01-5020) with 1 mm diameter tungsten carbide indenter at maximum load of 80 N with a preload of 10 N. 3. Results and discussion 3.1. Parametric investigation For the detailed parametric investigation, a number of tracks are laid at the various combination of process parameters as per full factorial design (refer Table 3). The track dimensions are measured at least 272
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Table 3 Full factorial deposits with observations, measured dimensions and deposition rate. Specimen ID
01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27
PL
v
mp
E
F
Remarks
W
m/s
g/s
kJ/m
g/m
1000 1000 1000 1400 1400 1400 1800 1800 1800 1000 1000 1000 1400 1400 1400 1800 1800 1800 1000 1000 1000 1400 1400 1400 1800 1800 1800
0.007 0.01 0.013 0.007 0.01 0.013 0.007 0.01 0.013 0.007 0.01 0.013 0.007 0.01 0.013 0.007 0.01 0.013 0.007 0.01 0.013 0.007 0.01 0.013 0.007 0.01 0.013
0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.083 0.083 0.083 0.083 0.083 0.083 0.083 0.083 0.083 0.117 0.117 0.117 0.117 0.117 0.117 0.117 0.117 0.117
142.86 100 76.92 200 140 107.69 257.14 180 138.46 142.86 100 76.92 200 140 107.69 257.14 180 138.46 142.86 100 76.92 200 140 107.69 257.14 180 138.46
7.14 5.00 3.85 7.14 5.00 3.85 7.14 5.00 3.85 11.86 8.30 6.38 11.86 8.30 6.38 11.86 8.30 6.38 16.71 11.70 9 16.71 11.70 9 16.71 11.70 9
Regular Regular Regular Regular & Blackened Regular Regular Regular & Blackened Regular Regular Regular Regular Regular Regular & Blackened Regular Regular Regular & Blackened Regular Regular Blackened with more spatters Regular Regular Regular & Blackened Regular Regular Regular & Blackened Blackened with more spatters Regular
Height
Deposition Rate
mm
mm
mm3/s
1.52 1.25 1.12 1.86 1.84 1.82 2.33 2.22 2.16 1.47 1.41 1.27 1.88 1.85 1.72 2.38 2.03 1.98 1.58 1.42 1.21 1.95 1.85 1.70 2.20 2.01 2.00
0.26 0.22 0.20 0.27 0.23 0.12 0.39 0.26 0.22 0.26 0.24 0.22 0.49 0.32 0.25 0.58 0.35 0.25 0.48 0.35 0.25 0.62 0.40 0.32 0.77 0.53 0.36
0.66 0.69 0.75 0.84 1.06 0.73 1.51 1.44 1.58 0.64 0.85 0.93 1.54 1.48 1.43 2.30 1.78 1.65 1.26 1.24 1.01 2.02 1.98 1.81 2.82 2.66 2.27
beam diameter with E and F remains constant for other beam diameter also. Therefore, a ratio of requisite width to laser beam diameter can be deployed for selecting the parametric combination from Fig. 5. Subsequently, the optimum combination of process parameters is selected considering the requisite dimension (for the present study), maximum deposition rate and uniform surface topography. The deposition rate is defined as per equation (2), where n is geometry factor.
compact heat exchangers (CHEs) and thin walled honeycomb structures. Emax, Emin, Fmax and Fmin represents the maximum and minimum values of E and F, respectively for the development of thin walls with track width in the required range. These values are used to develop the process window and it is found that Emax and Fmax are 210 kJ/m and 12.5 g/m for development of IN718 thin walls of required dimensions. Similarly, Emin and Fmin are 105 kJ/m and 4 g/m respectively. A careful analysis of the process parameters and deposition process shows that Emax is limited by the required track dimensions, while Fmax is limited by the track dimensions and maximum energy available for melting the powder. Similarly, Emin is limited by the minimum energy required to melt the fed powder to build thin walls, while Fmin is limited by the minimum material required for the desired track dimension. Further, an empirical relation as presented in equation (1) is derived for track width. Statistical regression analysis of track width with respect to the process parameters showed a strong relationship with a linear regression coefficient of 0.965. Fig. 6 presents the dependence of the combined parameter with track width. It can be observed from equation (1) that PL and v are significant in determining the width. As mentioned before, PL and v govern the amount of energy available per unit length (E). Thus, E is significant in determining the width of deposition. As E increases, temperature of the melt pool increases resulting in the reduction of material viscosity, which leads to the spreading of material. It can also be observed from equation (1) that mp is less significant as compared to other factors which confirm that E is a major parameter as compared to F in controlling the width. The maximum deviation of 11% is observed between the experimental and predicted width.
W (m) = PL 0.8 (W ) v−0.2 (m / s ) mp0.009 (kg / s )
Width
Deposition rate = n × W × H
(2)
It is found that combination of process parameter complying to the above criteria is PL - 1400 W, v - 0.01 m/s and mp - 0.117 g/s (Specimen ID – 23). The selected process parameters are used for the development of multi-layered thin wall (10 layers) structures. Fig. 7 presents the macrostructure depicting transverse section of thin wall. It can be observed that the deposition is defect free without pores or irregularities. Further, the dimension of the wall is measured at different locations. It is observed that the maximum deviation of 15% is observed for the deposited wall as compared to the corresponding single track dimension. This may be due to the preheating effect during multilayer LAMDED. Preheating effect results in an increase of the average temperature during deposition and reduction in viscosity and density of the material. Thus, melt liquid will have a propensity to slide down and this results in the variation of dimension [17]. Surface roughness analysis of as-built transverse surface shows average roughness value (Ra) of 6.52 μm. This may be due to the stair stepping effect in LAM due to the layer by layer methodology and the presence of partially melted powders on the surface of the sample. Further, the fabricated structures are subjected to heat treatment and microstructure, surface topography and mechanical properties are studied.
(1)
The developed process window and empirical relation can be deployed for selecting the process parameter combination for the requisite dimension of a thin wall at a constant laser beam diameter. Track width is mainly influenced by beam diameter, E and F. In the present study, E and F are considered for parametric investigation and to analyze the track geometry. However, as beam diameter changes, the track width also changes. But, the relation between the ratio of track width and
3.2. Optical microscopy The microstructures of thin walls built by LAM-DED (in as-built and post heat-treated conditions) are studied using an optical microscope after preparing them as per ASTM E3. Fig. 8a presents the micrograph of the sample in the as-built condition. 273
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Fig. 3. Track Geometry (a) Typical cross-section of LAM-DED track (b) Variation of Track width with PL and v at a constant mp of 0.117 g/s (b) Variation of Track Height with PL and v at a constant mp of 0.117 g/s.
developed as a result of rapid cooling rate during LAM-DED. Fig. 8(b) and (c) presents the microstructure of the deposit after heat treatment at 950 °C (HT950) and 1050 °C (HT1050), respectively. After post-heat treatment, homogenized microstructure in all samples is observed with dissolved dendrites. The columnar grains present in the as-built samples are not visible in the HT950 and HT1050 samples. Recovery, recrystallization and grain growth takes place during post-heat treatment.
Few pores at isolated locations in the deposits are also observed. The pore formation may be attributed to rapid molten dynamics during LAM-DED yielding entrapped gas bubbles in the molten metal. The typical size of these pores are less than 15 μm. Microstructure examinations of LAM-DED deposits of IN718 reveal dendritic structure. The direction of dendrite growth is along the direction of deposition due to preferential cooling. The dendritic microstructures are
Fig. 4. Various IN718 deposits using LAM-DED (a) Regular deposition (b) Blackened deposition (c) Black deposition with more spatters. 274
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Fig. 5. Process window for nominal width of IN718 thin walls.
During recovery, the internal stresses are relieved and mainly takes place at lower temperature without affecting the mechanical properties of the material. During recrystallization, new grains are formed with same lattice structure with approximately same dimensions in all directions as appropriate conditions are provided. Thus, recrystallized equiaxed grains are formed during post-heat treatment. Further, coalescence of grains during grain growth increases the size of grains leading to reduction in strength and hardness with enhanced ductility.
3.3. X-ray diffraction studies X-ray diffraction studies are performed with scanning range of 30–100° at step size of 0.02° after calibration and characterization of the diffractometer with NIST standard reference material (Corundum). Fig. 9 depicts the XRD pattern of as-built, HT950 and HT1050 samples. The presence of face centered cubic γ matrix is noticed in the study as observed by other researchers [18]. Smaller crystallite size is revealed through the peak broadness. It is also observed that peak width decreased significantly which corresponds to the increase in the crystallite size of the material. Crystallite size is calculated based on the Scherrer formula as shown in equation (3). This provides a lower bound on the crystallite size as there are other factors which influence the width of the diffraction peak such as strain, dislocations etc.where d is the mean size of the crystal, k is a dimensionless shape factor, λ is the X-ray wavelength, β is the FWHM in radians and θ is the Bragg angle (in degrees). The estimated crystallite size is 9.71 nm, 21.21 nm and 23.17 nm, respectively for as-built, HT950 and HT1050 samples, respectively. As, mentioned before, the fine size of crystallites in the as-built condition can be due to the higher cooling rate during LAM-DED. It is also observed that there is a considerable change in average crystallite size between as-built sample and HT 950, but the size variation is less between HT950 and HT1050. The major reason for the increase in the crystallite size is due to the reduction of surface energy after heat treatment, which influences the particle size as a function of the temperature.
Fig. 6. Dependence of Track Width on the combined parameter.
Fig. 7. Macrostructure depicting transverse section of thin walled structure. 275
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Fig. 8. Microstructure of a) as-built b) HT950 c) HT1050 samples.
components are subjected to fluctuating mechanical load [19]. Thus, it is necessary to reduce this tensile residual stress, which can be achieved by appropriate post-processing of the material using solution treatment procedure adopted from the literature [16]. In our study, a significant reduction in residual stress is observed for the samples subjected to post-heat treatment at HT950 and HT1050. During the post-heat treatment, the material is heated and soaked for uniform temperature distribution within the material. The subsequent quenching induces the compressive stress in the material. This whole process causes reduction of tensile residual stress in the surface of LAM-DED material. 3.5. Atomic force microscopy AFM is used to understand the uniformity of the deposition in the nano-metric level. A similar approach is followed by earlier researchers [20–22]. Fig. 10(a) and (b) and (c) presents surface topography of the LAM-DED material in as-built, HT950 and HT1050 conditions, respectively using AFM with a magnification of 10 μm × 10 μm. The average value of surface roughness (Ra) at the nano-metric level is 1.87 nm, 2.39 nm and 2.46 nm for as-built, HT950 and HT1050, respectively. It is evident that the as-built samples have smoother surface, while sharp hills and valleys are clearly visible in the surface topography of HT950 and HT1050 samples. These observations are in line with earlier reported results for other deposition technique [23] and LAM-PBF samples [24]. The obtained value of Kurtosis (Rku) is 2.83, 3.03, and 3.80 in the as-built, HT950 and HT1050 samples, respectively. This reveals that as-built sample have fewer sharp peaks (Rku < 3) with uniform topography and post-heat treated samples have spikes (Rku > 3). During post-heat treatment, the grain aggregation occurs yielding larger clusters and it influences the topography. This was also confirmed by XRD using Scherrer equation which revealed increase in the crystallite size.
Fig. 9. X-ray diffraction results of the LAM-DED built samples under different conditions.
d=
k.λ β . cos θ
(3)
3.4. Residual stress measurement The surface residual stress measurement was carried out using XRay diffraction to investigate its variation in the LAM-DED material in as-built and post-heat treatment conditions. The samples are analyzed as per the standard procedure (ASTM E2860) without removing the deposit from the substrate and without any post-machining. Table 4 presents the variation of average surface residual stress in the material in as-built and post-heat treatment conditions. It may be seen that the average tensile residual stress induced on the surface of the LAM-DED sample is 512 MPa and reduction of 17.5% and 50% in the surface tensile residual stress is observed after HT950 and HT1050, respectively. The residual stress is induced in the material during LAM-DED is mainly due to the local heating, rapid solidification and self-quenching of the thin surface layer results in the accumulation of local thermal stress and thereby micro strain. The nature of micro strain is tensile because when the surface is heated during LAM-DED, it gives rise to local expansion and the surface is under compressive stress due to constraints offered by the material. This stress becomes zero, when the surface is melted. Subsequent solidification and cooling generates compression and thereby generates tensile stress at the surface. This tensile stress becomes critical causing an early failure when the
3.6. Micro-hardness measurement The micro-hardness of IN718 samples are examined in as-built and post-heat treated condition as per ASTM E384. Fig. 11 presents the variation of the hardness along the cross-section. It can be observed that as-built samples have higher hardness than post-heat treated samples. The average hardness of as-built, HT 950 and HT1050 are 234 ± 7 HV1.96N, 220 ± 6.1 HV1.96N and 206 ± 5.9 HV1.96N, respectively. Higher hardness of as-built sample can be due to the fine dendritic microstructure usually observed in LAM-DED samples owing to the high cooling rate [25]. A reduction of 6% and 12% in micro-hardness is observed for HT950 and HT1050 samples, respectively as compared to that of as-built samples. Grain agglomeration and coarsening due to the post-heat treatment as explained in the previous section may be the reason for the reduction in microhardness of post-heat treated samples as per Halls-petch relationship. Further, the result is compared with IN718 samples made by LAM-PBF and it was observed that the average micro-hardness is 319 HV1.96N, 307 HV1.96N and 260 HV1.96N in the asbuilt, HT950 and HT1050 condition, respectively [23]. It may be observed that the trend remains similar while the higher hardness is
Table 4 Average surface residual stress. Condition
Normal stress along X direction (MPa)
As-built HT950 HT1050
512 ± 15.46 422 ± 12.99 254 ± 11.80
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Fig. 10. Atomic force microscopy analysis (a) As-built (b) HT950 (c) HT1050.
stiffness remains almost the same up to certain values and thereafter it varies. The closer value of stiffness represents that the material is under an elastic regime in these loading conditions, while the region with difference corresponds to the plastic regime. It may be seen that the elastic regimes terminate early in post-heat treatment samples. The displacement at 20 kN is 0.668 mm, 0.8 mm and 1.086 for as-built, HT950 and HT1050 samples, respectively. It shows that an improvement in ductility by 19.7% and 62.5% for HT950 and HT1050, respectively due to post-heat treatment. 3.8. Ball indentation test Ball indentation tests (BI) is performed for comparing the energy storage capacity and maximum displacement of the material using a single cycle with maximum load of 80 N at a rate of 0.1 mm/min with unloading cycle till 10 N load as per ASTM STP1092. Fig. 13 presents the typical load displacement curve obtained during the BI of LAM-DED samples. The maximum displacement for as-built, HT950 and HT1050 samples are 0.050 mm, 0.070 mm, and 0.079 mm, respectively. The relatively lower depth of penetration in the as-built sample is primarily attributed to the higher hardness. These results are in line with the observation from micro-hardness and compression testing. The areas under the loading and unloading curve represents the entire work performed during loading and the reversible elastic contribution of the total work, respectively. The difference between the two areas provides the energy absorbed in plastic deformation, which can be correlated to the energy stored by the material. The area under the curves of as-built, HT950 and HT1050 samples are 0.49 N-mm, 1.09 N-mm, and 1.7 Nmm, respectively. BI studies reveal the improvement in energy storage capacity by 2.22 and 3.46 times for HT950 and HT1050, respectively to that of as-built. In earlier reported work, the maximum displacement for as-built, HT950 and HT1050 samples are 0.03 mm, 0.04 mm and 0.05 mm, respectively for LAM-PBF IN718 samples at maximum load of
Fig. 11. Variation of hardness along the cross-section.
observed in LAM-PBF samples. This can be due to the faster scanning rate as compared to LAM-DED which results in finer microstructures.
3.7. Compression testing Compression testing as per ASTM E−9 standard is carried out to understand the effect of heat treatment on the ductility of the material as ductility is important, when the material is subjected to compressive loading in real engineering applications, especially vacuum vessels, compact heat exchangers etc. Fig. 12 presents the load-displacement curve obtained from compression test, where the slope of the curve represents the stiffness of the material. It may be seen in Fig. 12 that the
Fig. 12. Load displacement curve during compression test.
Fig. 13. Load displacement curve during BI. 277
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50 N [23]. The area under the curves of as-built, HT950 and HT1050 LAM-PBF samples are 0.09 N-mm, 0.12 N-mm and 0.417 N-mm, respectively [23]. The displacement for LAM-PBF IN718 samples at 50 N is lower than the corresponding displacement for LAM-DED samples at 50 N as presented in Fig. 13. This is in agreement with the lower hardness of LAM-DED samples as compared to LAM-PBF samples.
[2]
[3]
[4]
4. Conclusions
[5]
One of the exciting applications of LAM-DED is the development of complex shaped thin walled engineering components. In the present work, a systematic study is performed for deploying LAM-DED for the development of IN718 thin walls and the process window is developed. The process window revealed maximum and minimum limits for both E and F are 210 kJ/m, 105 kJ/m, 12.5 g/m and 4 g/m, respectively for IN718 thin walls with requisite dimension. An empirical model is also derived for the track width using statistical regression analysis. Uniform deposition with requisite dimensions and higher deposition rate are found at laser power - 1400 W, scan speed - 0.01 m/s and powder feed rate - 0.117 g/s. Defect free thin walls fabricated by LAM-DED revealed fine dendritic microstructure. The post-heat treated samples shows recrystallized grains with dissolved dendrites. 50% reduction in surface residual stress is observed after post-heat treatment. Grain clustering during post-heat treatment changed the surface topography of the material. Micro-hardness studies revealed reduction up to 12%, compression testing shows ductility improvement up to 62.5% and BI studies revealed improvement in energy storage capacity up to 2.4 times. The obtained results of micro-hardness and BI studies using LAM-DED samples are compared with published literature for LAM-PBF samples. It is found that LAM-DED samples have lower hardness, higher ductility and higher energy storage ability as compared to that of LAM-PBF samples. This confirms its suitability for the intended applications and thus, this study opens avenues for thin wall fabrication to develop compact heat exchanger walls and thin walled honeycomb structures of requisite dimensions and properties.
[6]
[7]
[8] [9]
[10] [11] [12]
[13]
[14]
[15]
[16] [17]
Acknowledgment
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A N Jinoop acknowledges the financial support by Raja Ramanna Centre for Advanced Technology (RRCAT), Department of Atomic Energy, Government of India and Homi Bhabha National Institute, Mumbai. The authors thank Mr. H. Kumar, Mr. U. Kumar and Mr. C.H. Prem Singh of LAM lab, RRCAT for their help during sample preparation. The authors thank the support from Dr. P. Ganesh, Dr. Archna Sagdeo, Mr. Sujan Kar, Ms. Preeti Pokhriyal, Ms. Sushmita Bhartiya of RRCAT and other members of Laser Additive Manufacturing Lab at RRCAT, Indore, India.
[19]
[20]
[21]
[22] [23]
Appendix A. Supplementary data [24]
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.vacuum.2019.05.027.
[25]
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