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Evaluation of the mechanical properties of Inconel 718 components built by laser cladding
International Journal of Machine Tools & Manufacture 51 (2011) 465–470
Contents lists available at ScienceDirect
International Journal of Machine Tools & Manufacture journal homepage: www.elsevier.com/locate/ijmactool
Evaluation of the mechanical properties of Inconel 718 components built by laser cladding I. Tabernero a, A. Lamikiz a,n, S. Martı´nez a, E. Ukar a, J. Figueras b a b
Department of Mechanical Engineering, University of the Basque Country ETSI, ETSII-UPV, c/Alameda de Urquijo s/n, 48013 Bilbao, Spain IDEKO-IK4 Research Center, Pg. Arriage 2, 20870-Elgoibar, Spain
a r t i c l e i n f o
abstract
Article history: Received 16 December 2010 Received in revised form 31 January 2011 Accepted 10 February 2011 Available online 19 February 2011
Laser-cladding process is one of the most relevant new processes in the industry due to the particular properties of the processed parts. The main users of this process are aeronautical turbine parts manufacturers and engineering maintenance services. The main advantage of laser-cladding process is the possibility of obtaining high quality material deposition on complex parts. Thus, laser cladding can be applied in the repair of high added-value and safety critical parts. This ability is especially useful for high-cost parts that present wears or local damage due to operating conditions. Different types of parts can be processed, such as housings, blades or even complete turbine rotors. Once the parts are repaired by laser cladding, they can be reassembled on the engine, reducing lead times. Laser-cladding process can permit buildup of complex geometries on previously forged or machined parts, such as stubs or flanges. However, one of the main drawbacks of the laser-cladding process currently is lack of knowledge on the properties of the deposited material. Most of the available data relate to the microstructure and the final hardness values. Nevertheless, there are few data of the mechanical properties of the parts. Moreover, it is difficult to gather data related to the influence of the laser-cladding parameters and strategies on the mechanical behaviour of a part. This paper presents the mechanical properties of a series of samples builtup by laser cladding. Two different types of specimens are tested: first, hybrid parts, in which laser cladding deposits materials built up layer-by-layer onto a substrate and the resulting part is a combination of deposited material and the substrate and second, complete rapid manufactured test samples. The results of tensile tests on various parts show that the laser-cladding strategy has a significant influence on their stress–strain curves. In addition, the laser-cladding process can result in a high directionality of their mechanical properties. The direction depends on the particular strategy in use. The study demonstrates that these properties present high anisotropy, a factor that should be carefully considered when selecting the most appropriate laser-cladding strategy. & 2011 Elsevier Ltd. All rights reserved.
Keywords: Laser cladding Mechanical properties Inconel 718 Deposition strategies
1. Introduction Material deposition techniques are being used for decades in the industry for repair, maintenance or to build up, layer by layer, fully functional near-net shape parts. The main advantages of these techniques are lead times and waste material reduction. Within this group of processes, arc welding or plasma deposition techniques are commonly used. Recently, laser-cladding process has become an alternative process to the traditional techniques. Laser-cladding process presents lower deposition rates than plasma or arc welding processes, but on the other hand it can n
Corresponding author. Tel.: +34 946014221; fax: +34 946014215. E-mail addresses: [email protected] (A. Lamikiz), jfi[email protected] (J. Figueras). 0890-6955/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijmachtools.2011.02.003
produce a much better coating, with minimal distortion and better surface quality [1]. Therefore, laser cladding is commonly used for high-cost materials and high added-value manufacturing of parts [2]. Furthermore, laser cladding can be used in the hybrid manufacture of details or structures attached to previously machined substrates. This application is particularly interesting for large parts in turbines, where the forgings can be reduced in thickness and material may be added as protruding geometries, such as flanges or stubs [3]. This is one of the most interesting applications for the aeronautical industry because of the reduction of wasted material. This application includes also the repair of high added-value and highly complex parts such as rotor blades, discs and turbine housings. With regard to this application, Yilmaz et al. [4] presented a repair methodology based on digital scanning of added material on turbine components for
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later machining. Alternatively, laser cladding may also be applied for coatings. This application consists of the deposition of a different material onto the substrate to improve wear resistance, friction or other tribological property. Laser cladding for coating application has been examined in various works focused on aeronautical parts coated with ceramics and superalloys [5,6]. In all of its applications, the laser-cladding process uses a highenergy laser beam to create a melt pool on a substrate. Once the melt pool is created, a filling material is injected into the melted material. The heat-affected zone is limited to the melted material and the surrounding areas and minimum heat distortions are present in the part, due to high-energy density of the laser beam, but a relatively low global input energy is used to melt the substrate. The filling material can be injected in powder or wire form but the most common configuration for laser-cladding operations is powder material injection through a coaxial nozzle. In this case, the powder is injected coaxially with respect to the laser beam and cladding can be carried out in any direction [1]. The laser cladding steps start with a 3D design and generation of tracking paths. The conventional method of generating lasercladding paths is by programming the CNC of a machine or robot that guides the laser and the nozzle. In a way similar to machining operations, if the part presents complex geometry it requires the use of a CAD/CAM system in order to obtain complex laser paths and build the geometry. The result of the laser-cladding operation is a near-net shape geometry of the part, which usually presents a 0.3–1 mm stock. Abrasive techniques or machining of the component gives it the required surface finish and tolerances. One of the most relevant laser-cladding applications focuses on turbine components for aircraft industry. These parts are usually manufactured from thermally resistant superalloys, mainly nickel and titanium-based alloys. These alloys are expensive and present low machinability, so the reduction of wasted material increases the productivity and reduce significantly the manufacturing costs [7–9]. Aircraft engine components can be divided into two groups: parts made of titanium-based alloys that are unable to withstand high temperatures, and parts made of nickel-based alloys that have to withstand high temperatures. Many of the components that are repaired or manufactured by laser cladding are from the last group, and more specifically are made of Inconel 718. This material is a precipitation-hardenable nickel alloy containing significant amounts of elements that include iron, chromium, molybdenum and niobium, among others. Its density can be compared with the density of steel, but it ensures the mechanical properties at high temperatures (above 650 1C). There are some works that evaluate the optimum process parameters for different substrate–powder combinations, including Inconel 718. These works study window process parameters and the final quality of the deposited material [10,11], including the influence of laser power, feed rate, powder size, etc. in order to achieve optimum process parameters. Peng et al. [12] studied the optimum strategies for direct manufacturing of nickel alloy parts, presenting different combinations of rastering and offsetting paths, although their study was limited to surface roughness and tolerances, without considering the influence on mechanical properties. Moreover, Nickel et al. [13] mentioned the relevance of deposition strategies on the mechanical properties of the final part, presenting deflection values due to residual stresses but no data on final ultimate tensile strength values (UTS) or the ductility of the parts. Relatively few works have focused on the study of laser-cladding mechanical properties, and most of them present only hardness values. Paul et al. [14] evaluated the mechanical properties of test specimens made of Inconel 625 manufactured by Laser Rapid Manufacturing. The main conclusion of their work was that the process could improve
mechanical properties. The study also discussed different rastering strategies. Zhao et al. [15] studied the microstructures and mechanical properties of Inconel 718. They also focused on the Rapid Manufacturing process and obtained UTS values comparable to those of wrought Inconel 718. In this case, different scenarios were studied, including mechanical properties as deposited, after heat treatment, and wrought Inconel 718. In any case, only complete parts built by rapid manufacturing were studied, but no data were provided on the mechanical properties of hybrid parts manufactured in part by laser cladding onto a substrate. As previously mentioned, there are further works that focus on the final hardness or the microstructure quality of deposited material, but mechanical data such as UTS, ductility, and stress–strain curves, among others are limited. The present work evaluates the mechanical properties of different laser-cladding tests on Inconel 718. First, full rapid manufactured parts have been tested. Second, hybrid test specimens, half-deposited material and half-substrate material have been also tested. In both cases, mechanical properties have been directly measured as deposited, and after a precipitation-hardening treatment. This heat treatment is commonly applied to aeronautical parts made of the alloy. The results show that the laser-cladding strategy can have a significant influence on the mechanical properties of the part and that there is a high risk in obtaining lower mechanical properties than those of wrought Inconel 718.
2. Experimental procedure The tests were carried out using a laser-cladding system based on a High Power Diode Laser Rofin-Sinar DL015-S with a 1.5 kW maximum peak power. The powder material was injected through a COAX-8 continuous coaxial nozzle, developed by the Fraunhofer IWS Institute. The complete description of the nozzle and the resulting powder flow distribution is presented in [16]. Argon was used as both the protective and the carrier gas. Finally, both the laser and the nozzle were fixed to a conventional 3-axis CNC machine centre. Therefore, the entire programming of the laser-cladding tracks could be performed with conventional CNC programming using a CAM system. The complete laser-cladding system is shown in Fig. 1. As previously made clear, the tested material (both substrate and powder) is Inconel 718. The composition (wt%) of this alloy is basically 50–55% nickel, 19% iron and 17–21% chromium along with smaller amounts of other elements, including niobium, molybdenum, titanium and cobalt. Inconel 718 powder is manufactured by gas atomisation and supplied in different powder sizes. A typical powder size, ranging from 70 to 180 mm, was selected in order to develop the optimum test parts. Finally, the powder feeder is a conventional rotary disc feeder where powder mass flow can be controlled with the rotation speed of a metering disc. Two different types of test specimens were designed, in order to study the maximum number of cases. The first type combines substrate and deposited material, while the second type is completely built by laser cladding. Each type of test part was built using two different strategies, resulting in four different test specimens. Fig. 2 shows the designs of the four different test parts. The laser-cladding conditions used in the test were obtained from previous studies. In these tests the optimum laser power was set to 1100 W and the feed rate to 700 mm/min. The mass flow was set to 5.2 g/min, measured using the procedure described in [16]. Once the test parts had been built up by laser cladding, the final shape of the specimens was obtained by wire electrodischarge machining (WEDM) cutting. Although the WEDM process can damage the surface of the test parts, the
I. Tabernero et al. / International Journal of Machine Tools & Manufacture 51 (2011) 465–470
Power Supply/Water Z Axis
Gas + Powder
467
Laser
Powder feeder Nozzle
Test specimen
Y Axis
X Axis
Chiller
Fig. 1. (a) Laser-cladding system scheme. (b) Details of the laser-cladding nozzle.
1
2
Deposited material by Laser Cladding
3
4
Type No. 1 2 3 4
Substrate
Specimen type Substrate + Cladding Substrate + Cladding Only cladding Only cladding
Strategy Zigzag Spiral Transversal Zigzag Longitudinal Zigzag
Substrate
Fig. 2. Test part designs and laser-cladding strategies.
a
b
Tensile test specimens
Laser cladding Substrate
Fig. 3. (a) Tensile test of a specimen. (b) Test parts made by laser cladding and WEDM.
damaged layer thickness is below 20 mm, so there is no appreciable change in the structural properties of the specimens. Finally, all the specimens were tested following the EN 10002-1: 2002 standard on a universal testing machine. Fig. 3 shows type 1 tensile specimens (substrate+cladding with zigzag strategy) and the tensile testing of one of the parts.
3. Test results and discussion The test results revealed a series of stress–strain curves. A minimum of 3 tests were performed on each specimen type, in order to consider the mechanical properties of dispersion. The stress–strain curves of tensile tests are shown in Figs. 4 and 5.
A first evaluation of the test results showed that the UTS values obtained in all the tests were lower than the wrought Inconel 718 values, which ranged between 1150 and 1340 MPa, depending on the type of test and heat treatment [15,17]. A further important conclusion is that all tests on type 1 and 2 specimens, which presented a hybrid structure of deposited Inconel 718 on a forged Inconel 718 substrate, showed signs of failure in the deposited material zone. In other words, no failure was detected on the substrate, even in the heat-affected zone, which was caused by the laser-cladding process. Finally, the results show that the mechanical properties vary significantly with laser-cladding strategies. This effect can be clearly observed in different results between type 3 and 4 specimens, which have been manufactured by similar processes.
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The only difference is the direction of the laser-cladding tracks. While the type 3 specimens present a transverse direction for the laser tracks with respect to the tensile test force, the type 4 specimen laser tracks are parallel to tensile test force. Thus, in type 3 specimens the UTS values are 55–60% lower than those obtained in the type 4 specimens. These results show that
the mechanical properties of parts manufactured by laser cladding present high anisotropy due to the laser tracks. These tracks have a similar effect as those of the fibres of a composite material. The results also show that there are relatively high deviations for UTS and ductility. This dispersion may be caused by the presence of porosity in the deposited material. Furthermore, the best results were obtained for the samples from the central zone, while the tests on samples from the outer zones show lower values for both UTS and ductility. This is attributed to edge effects that occur in the laser-cladding process, because the temperature gradients on the edges are higher than those on the internal areas of the parts. A complete analysis with scanning electron microscopy (SEM) was performed, in order to study the deposited material structure. It showed that the substrate material presented two types of precipitates: one for grain boundaries and the other with an acicular shape inside the grains. The grains are dendritic in the deposited material, present columnar growth and are larger than the substrate material. Scattered porosity was also detected throughout the deposited material less than 5 mm in size. The structure of one of the test parts is shown in Fig. 6. In order to quantify the porosity, the microscopic images were processed to detect the number, size and the relative area of porosity. Fig. 7 shows the results of one analysis. The larger pore size did not exceed 5 mm and percentage porosity in areas of deposited material was less than 1%. However, the presence of pores is a sign of poor mechanical properties and high dispersion.
Fig. 4. Stress–strain curves for type 1 and 2 test specimens.
4. Influence of the precipitation-hardening treatment on mechanical properties
Fig. 5. Stress–strain curves for type 3 and 4 test specimens.
a
Finally, once repaired, most of the Inconel 718 aircraft engine parts are heat-treated. Heat treatment is a precipitationhardening technique that is based on solid solubility with temperature to produce disc shape nanoparticles of Ni3Nb, g00 phase, which prevents the movement of dislocations and hardening of material. This heat treatment consists of a solution treatment for 1 h at 980 1C followed by double aging for 8 h at 720 1C with furnace cooling plus other 8 h at 620 1C with air cooling. The substrate for non-treated specimens is in solution condition, 1 h at 980 1C. The type 1 specimen was taken as the reference to evaluate the influence of the precipitation-hardening treatment on mechanical properties. Then, new specimens of this type were manufactured, but with a precipitation-hardening treatment. As shown in Fig. 8, these specimens exhibited more homogeneous properties between the deposited material and the substrate. It may be observed that the hardness values between the substrate and the
b
Deposited material
Zone A
Zone A Dendrite direction
Porosity
Substrate
Fig. 6. (a) Substrate and deposited material structure. (b) Details of the deposited material structure.
I. Tabernero et al. / International Journal of Machine Tools & Manufacture 51 (2011) 465–470
469
0 100
Image 200 Processing 300 400 500 600 700 800 900 200
400
600
800
1000
1200
Fig. 7. Porosity analysis by image processing technique.
reductions in the mechanical properties should be taken into account if the laser-cladding process is used. The stress–strain curves are presented in Fig. 9.
5. Conclusions
Fig. 8. Hardness evolution from substrate to deposited material. (a) Untreated specimens; (b) treated specimens (1 h–980 1C/air cooled+ 8 h–720 1C/furnace cooled+ 8 h–620 1C/air cooled).
The results of the study have shown that laser-cladding strategies have a significant influence on the mechanical properties of Inconel 718 parts. Two types of tests were performed. First, hybrid specimens with substrate and laser-deposited material were tested. The results show reduced mechanical properties in comparison with the values for wrought Inconel 718. Moreover, all specimens present failures in the zone of the deposited material. The second type of specimen was a 100% deposited material. In this case, the mechanical behaviour of the parts presented high anisotropy and was relatively similar to that of a composite material, in which the fibres are comparable to the laser-cladding tracks. Therefore, the optimum laser-cladding strategy is achieved by aligning the laser-cladding tracks in the direction of the main stress lines. Furthermore, precipitation hardening improves the mechanical properties because of homogenisation of the microstructure of the deposited material. The tests on treated specimens showed higher tensile strength and ductility increments as much as 100%. However, it can be also concluded that there is a reduction in the specimens’ mechanical properties, which is about 75% of the strength of the wrought Inconel 718 in the best of cases.
Acknowledgements The authors wish to thank the ITP S.A. team for their collaboration and support with the characterisation and treatment of Inconel 718. Special thanks also go to the Spanish Ministry of Science and Innovation for financial support provided through the SURFACER Project (Ref.: DPI2010-20317-C02-01).
Fig. 9. Stress–strain curves for treated and untreated type 1 specimens.
deposited material present similar values, while the non-treated structures show differences of up to 200 HV. A significant increase in mechanical properties was noted in the test results: UTS was 20% higher in all cases and 100% higher ductility was obtained. However, maximum UTS was slightly over 1000 MPa, while the values of the base material were between 1200–1300 MPa. Thus, despite the improvement in the mechanical properties of precipitation hardening, the properties of these specimens do not match those of wrought Inconel 718. These
References [1] E. Toyserkani, A. Khajepour, S. Corbin, Laser Cladding, CRC Press LLC, 2005. [2] G.N. Levy, R. Schindel, J.P. Kruth, Rapid manufacturing and rapid tooling with layer manufacturing (LM) technologies, state of the art and future perspectives, CIRP Annals—Manufacturing Technology 52 (2003) 589–609. [3] L. Sexton, S. Lavin, G. Byrne, A. Kennedy, Laser cladding of aerospace materials, Journal of Materials Processing Technology 122 (2002) 63–68. [4] O. Yilmaz, N. Gindy, J. Gao, A repair and overhaul methodology for aeroengine components, Robotics and Computer-Integrated Manufacturing 26 (2010) 190–201. [5] R.L. Sun, Y.W. Lei, W. Niu, Laser clad TiC reinforced NiCrBSi composite coatings on Ti–6Al–4V alloy using a CW CO2 laser, Surface and Coatings Technology 203 (2009) 1395–1399.
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[6] J. Dutta Majumdar, I. Manna, Ajeet Kumar, P. Bhargava, A.K. Nath, Direct laser cladding of Co on Ti–6Al–4V with a compositionally graded interface, Journal of Materials Processing Technology 209 (2009) 2237–2243. [7] D. Dudzinski, A. Devillez, A. Moufki, D. Larrouquere, V. Zerrouki, J. Vigneau, A review of developments towards dry and high speed machining of Inconel 718 alloy, International Journal of Machine Tools and Manufacture 44 (2004) 439–456. [8] L.N. Lo´pez de Lacalle, J. Pe´rez, J.I. Llorente, J.A. Sa´nchez, Advanced cutting conditions for the milling of aeronautical alloys, Journal of Materials Processing Technology 100 (2000) 1–11. [9] S. Nowotny, S. Scharek, E. Beyer, K. Richter, Laser beam build-up welding: precision in repair, surface cladding, and direct 3D metal deposition, Journal of Thermal Spray Technology 16 (2007) 344–348. [10] F. Liu, X. Lin, G. Yang, M. Song, J. Chen, W. Huang, Microstructure and residual stress of laser rapid formed Inconel 718 nickel-base superalloy, Optics and Laser Technology 43 (2011) 208–213. [11] D. Clarka, M.R. Bacheb, M.T. Whittakerb, Shaped metal deposition of a nickel alloy for aero engine applications, Journal of Materials Processing Technology 203 (2008) 439–448.
[12] L. Peng, et al., Direct laser fabrication of nickel alloy samples, International Journal of Machine Tools and Manufacture 45 (2005) 1288–1294. [13] A.H. Nickel, D.M. Barnett, F.B. Prinz, Thermal stresses and deposition patterns in layered manufacturing, Materials Science and Engineering A317 (2001) 59–64. [14] C.P. Paul, P. Ganesh, S.K. Mishra, P. Bhargava, J. Negi, A.K. Nath, Investigating laser rapid manufacturing for Inconel-625 components, Optics and Laser Technology 39 (2007) 800–805. [15] X. Zhao, J. Chen, X. Lin, W. Huang, Study on microstructure and mechanical properties of laser rapid forming Inconel 718, Materials Science and Engineering A 478 (2008) 119–124. [16] I. Tabernero, A. Lamikiz, E. Ukar, L.N. Lopez de Lacalle, C. Angulo, G. Urbikain, Numerical simulation and experimental validation of powder flux distribution in coaxial laser cladding, Journal of Materials Processing Technology 210 (2010) 2125–2134. [17] /www.matweb.comS, (accessed 24.11.2010).
Contents lists available at ScienceDirect
International Journal of Machine Tools & Manufacture journal homepage: www.elsevier.com/locate/ijmactool
Evaluation of the mechanical properties of Inconel 718 components built by laser cladding I. Tabernero a, A. Lamikiz a,n, S. Martı´nez a, E. Ukar a, J. Figueras b a b
Department of Mechanical Engineering, University of the Basque Country ETSI, ETSII-UPV, c/Alameda de Urquijo s/n, 48013 Bilbao, Spain IDEKO-IK4 Research Center, Pg. Arriage 2, 20870-Elgoibar, Spain
a r t i c l e i n f o
abstract
Article history: Received 16 December 2010 Received in revised form 31 January 2011 Accepted 10 February 2011 Available online 19 February 2011
Laser-cladding process is one of the most relevant new processes in the industry due to the particular properties of the processed parts. The main users of this process are aeronautical turbine parts manufacturers and engineering maintenance services. The main advantage of laser-cladding process is the possibility of obtaining high quality material deposition on complex parts. Thus, laser cladding can be applied in the repair of high added-value and safety critical parts. This ability is especially useful for high-cost parts that present wears or local damage due to operating conditions. Different types of parts can be processed, such as housings, blades or even complete turbine rotors. Once the parts are repaired by laser cladding, they can be reassembled on the engine, reducing lead times. Laser-cladding process can permit buildup of complex geometries on previously forged or machined parts, such as stubs or flanges. However, one of the main drawbacks of the laser-cladding process currently is lack of knowledge on the properties of the deposited material. Most of the available data relate to the microstructure and the final hardness values. Nevertheless, there are few data of the mechanical properties of the parts. Moreover, it is difficult to gather data related to the influence of the laser-cladding parameters and strategies on the mechanical behaviour of a part. This paper presents the mechanical properties of a series of samples builtup by laser cladding. Two different types of specimens are tested: first, hybrid parts, in which laser cladding deposits materials built up layer-by-layer onto a substrate and the resulting part is a combination of deposited material and the substrate and second, complete rapid manufactured test samples. The results of tensile tests on various parts show that the laser-cladding strategy has a significant influence on their stress–strain curves. In addition, the laser-cladding process can result in a high directionality of their mechanical properties. The direction depends on the particular strategy in use. The study demonstrates that these properties present high anisotropy, a factor that should be carefully considered when selecting the most appropriate laser-cladding strategy. & 2011 Elsevier Ltd. All rights reserved.
Keywords: Laser cladding Mechanical properties Inconel 718 Deposition strategies
1. Introduction Material deposition techniques are being used for decades in the industry for repair, maintenance or to build up, layer by layer, fully functional near-net shape parts. The main advantages of these techniques are lead times and waste material reduction. Within this group of processes, arc welding or plasma deposition techniques are commonly used. Recently, laser-cladding process has become an alternative process to the traditional techniques. Laser-cladding process presents lower deposition rates than plasma or arc welding processes, but on the other hand it can n
Corresponding author. Tel.: +34 946014221; fax: +34 946014215. E-mail addresses: [email protected] (A. Lamikiz), jfi[email protected] (J. Figueras). 0890-6955/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijmachtools.2011.02.003
produce a much better coating, with minimal distortion and better surface quality [1]. Therefore, laser cladding is commonly used for high-cost materials and high added-value manufacturing of parts [2]. Furthermore, laser cladding can be used in the hybrid manufacture of details or structures attached to previously machined substrates. This application is particularly interesting for large parts in turbines, where the forgings can be reduced in thickness and material may be added as protruding geometries, such as flanges or stubs [3]. This is one of the most interesting applications for the aeronautical industry because of the reduction of wasted material. This application includes also the repair of high added-value and highly complex parts such as rotor blades, discs and turbine housings. With regard to this application, Yilmaz et al. [4] presented a repair methodology based on digital scanning of added material on turbine components for
466
I. Tabernero et al. / International Journal of Machine Tools & Manufacture 51 (2011) 465–470
later machining. Alternatively, laser cladding may also be applied for coatings. This application consists of the deposition of a different material onto the substrate to improve wear resistance, friction or other tribological property. Laser cladding for coating application has been examined in various works focused on aeronautical parts coated with ceramics and superalloys [5,6]. In all of its applications, the laser-cladding process uses a highenergy laser beam to create a melt pool on a substrate. Once the melt pool is created, a filling material is injected into the melted material. The heat-affected zone is limited to the melted material and the surrounding areas and minimum heat distortions are present in the part, due to high-energy density of the laser beam, but a relatively low global input energy is used to melt the substrate. The filling material can be injected in powder or wire form but the most common configuration for laser-cladding operations is powder material injection through a coaxial nozzle. In this case, the powder is injected coaxially with respect to the laser beam and cladding can be carried out in any direction [1]. The laser cladding steps start with a 3D design and generation of tracking paths. The conventional method of generating lasercladding paths is by programming the CNC of a machine or robot that guides the laser and the nozzle. In a way similar to machining operations, if the part presents complex geometry it requires the use of a CAD/CAM system in order to obtain complex laser paths and build the geometry. The result of the laser-cladding operation is a near-net shape geometry of the part, which usually presents a 0.3–1 mm stock. Abrasive techniques or machining of the component gives it the required surface finish and tolerances. One of the most relevant laser-cladding applications focuses on turbine components for aircraft industry. These parts are usually manufactured from thermally resistant superalloys, mainly nickel and titanium-based alloys. These alloys are expensive and present low machinability, so the reduction of wasted material increases the productivity and reduce significantly the manufacturing costs [7–9]. Aircraft engine components can be divided into two groups: parts made of titanium-based alloys that are unable to withstand high temperatures, and parts made of nickel-based alloys that have to withstand high temperatures. Many of the components that are repaired or manufactured by laser cladding are from the last group, and more specifically are made of Inconel 718. This material is a precipitation-hardenable nickel alloy containing significant amounts of elements that include iron, chromium, molybdenum and niobium, among others. Its density can be compared with the density of steel, but it ensures the mechanical properties at high temperatures (above 650 1C). There are some works that evaluate the optimum process parameters for different substrate–powder combinations, including Inconel 718. These works study window process parameters and the final quality of the deposited material [10,11], including the influence of laser power, feed rate, powder size, etc. in order to achieve optimum process parameters. Peng et al. [12] studied the optimum strategies for direct manufacturing of nickel alloy parts, presenting different combinations of rastering and offsetting paths, although their study was limited to surface roughness and tolerances, without considering the influence on mechanical properties. Moreover, Nickel et al. [13] mentioned the relevance of deposition strategies on the mechanical properties of the final part, presenting deflection values due to residual stresses but no data on final ultimate tensile strength values (UTS) or the ductility of the parts. Relatively few works have focused on the study of laser-cladding mechanical properties, and most of them present only hardness values. Paul et al. [14] evaluated the mechanical properties of test specimens made of Inconel 625 manufactured by Laser Rapid Manufacturing. The main conclusion of their work was that the process could improve
mechanical properties. The study also discussed different rastering strategies. Zhao et al. [15] studied the microstructures and mechanical properties of Inconel 718. They also focused on the Rapid Manufacturing process and obtained UTS values comparable to those of wrought Inconel 718. In this case, different scenarios were studied, including mechanical properties as deposited, after heat treatment, and wrought Inconel 718. In any case, only complete parts built by rapid manufacturing were studied, but no data were provided on the mechanical properties of hybrid parts manufactured in part by laser cladding onto a substrate. As previously mentioned, there are further works that focus on the final hardness or the microstructure quality of deposited material, but mechanical data such as UTS, ductility, and stress–strain curves, among others are limited. The present work evaluates the mechanical properties of different laser-cladding tests on Inconel 718. First, full rapid manufactured parts have been tested. Second, hybrid test specimens, half-deposited material and half-substrate material have been also tested. In both cases, mechanical properties have been directly measured as deposited, and after a precipitation-hardening treatment. This heat treatment is commonly applied to aeronautical parts made of the alloy. The results show that the laser-cladding strategy can have a significant influence on the mechanical properties of the part and that there is a high risk in obtaining lower mechanical properties than those of wrought Inconel 718.
2. Experimental procedure The tests were carried out using a laser-cladding system based on a High Power Diode Laser Rofin-Sinar DL015-S with a 1.5 kW maximum peak power. The powder material was injected through a COAX-8 continuous coaxial nozzle, developed by the Fraunhofer IWS Institute. The complete description of the nozzle and the resulting powder flow distribution is presented in [16]. Argon was used as both the protective and the carrier gas. Finally, both the laser and the nozzle were fixed to a conventional 3-axis CNC machine centre. Therefore, the entire programming of the laser-cladding tracks could be performed with conventional CNC programming using a CAM system. The complete laser-cladding system is shown in Fig. 1. As previously made clear, the tested material (both substrate and powder) is Inconel 718. The composition (wt%) of this alloy is basically 50–55% nickel, 19% iron and 17–21% chromium along with smaller amounts of other elements, including niobium, molybdenum, titanium and cobalt. Inconel 718 powder is manufactured by gas atomisation and supplied in different powder sizes. A typical powder size, ranging from 70 to 180 mm, was selected in order to develop the optimum test parts. Finally, the powder feeder is a conventional rotary disc feeder where powder mass flow can be controlled with the rotation speed of a metering disc. Two different types of test specimens were designed, in order to study the maximum number of cases. The first type combines substrate and deposited material, while the second type is completely built by laser cladding. Each type of test part was built using two different strategies, resulting in four different test specimens. Fig. 2 shows the designs of the four different test parts. The laser-cladding conditions used in the test were obtained from previous studies. In these tests the optimum laser power was set to 1100 W and the feed rate to 700 mm/min. The mass flow was set to 5.2 g/min, measured using the procedure described in [16]. Once the test parts had been built up by laser cladding, the final shape of the specimens was obtained by wire electrodischarge machining (WEDM) cutting. Although the WEDM process can damage the surface of the test parts, the
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Fig. 1. (a) Laser-cladding system scheme. (b) Details of the laser-cladding nozzle.
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Fig. 3. (a) Tensile test of a specimen. (b) Test parts made by laser cladding and WEDM.
damaged layer thickness is below 20 mm, so there is no appreciable change in the structural properties of the specimens. Finally, all the specimens were tested following the EN 10002-1: 2002 standard on a universal testing machine. Fig. 3 shows type 1 tensile specimens (substrate+cladding with zigzag strategy) and the tensile testing of one of the parts.
3. Test results and discussion The test results revealed a series of stress–strain curves. A minimum of 3 tests were performed on each specimen type, in order to consider the mechanical properties of dispersion. The stress–strain curves of tensile tests are shown in Figs. 4 and 5.
A first evaluation of the test results showed that the UTS values obtained in all the tests were lower than the wrought Inconel 718 values, which ranged between 1150 and 1340 MPa, depending on the type of test and heat treatment [15,17]. A further important conclusion is that all tests on type 1 and 2 specimens, which presented a hybrid structure of deposited Inconel 718 on a forged Inconel 718 substrate, showed signs of failure in the deposited material zone. In other words, no failure was detected on the substrate, even in the heat-affected zone, which was caused by the laser-cladding process. Finally, the results show that the mechanical properties vary significantly with laser-cladding strategies. This effect can be clearly observed in different results between type 3 and 4 specimens, which have been manufactured by similar processes.
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The only difference is the direction of the laser-cladding tracks. While the type 3 specimens present a transverse direction for the laser tracks with respect to the tensile test force, the type 4 specimen laser tracks are parallel to tensile test force. Thus, in type 3 specimens the UTS values are 55–60% lower than those obtained in the type 4 specimens. These results show that
the mechanical properties of parts manufactured by laser cladding present high anisotropy due to the laser tracks. These tracks have a similar effect as those of the fibres of a composite material. The results also show that there are relatively high deviations for UTS and ductility. This dispersion may be caused by the presence of porosity in the deposited material. Furthermore, the best results were obtained for the samples from the central zone, while the tests on samples from the outer zones show lower values for both UTS and ductility. This is attributed to edge effects that occur in the laser-cladding process, because the temperature gradients on the edges are higher than those on the internal areas of the parts. A complete analysis with scanning electron microscopy (SEM) was performed, in order to study the deposited material structure. It showed that the substrate material presented two types of precipitates: one for grain boundaries and the other with an acicular shape inside the grains. The grains are dendritic in the deposited material, present columnar growth and are larger than the substrate material. Scattered porosity was also detected throughout the deposited material less than 5 mm in size. The structure of one of the test parts is shown in Fig. 6. In order to quantify the porosity, the microscopic images were processed to detect the number, size and the relative area of porosity. Fig. 7 shows the results of one analysis. The larger pore size did not exceed 5 mm and percentage porosity in areas of deposited material was less than 1%. However, the presence of pores is a sign of poor mechanical properties and high dispersion.
Fig. 4. Stress–strain curves for type 1 and 2 test specimens.
4. Influence of the precipitation-hardening treatment on mechanical properties
Fig. 5. Stress–strain curves for type 3 and 4 test specimens.
a
Finally, once repaired, most of the Inconel 718 aircraft engine parts are heat-treated. Heat treatment is a precipitationhardening technique that is based on solid solubility with temperature to produce disc shape nanoparticles of Ni3Nb, g00 phase, which prevents the movement of dislocations and hardening of material. This heat treatment consists of a solution treatment for 1 h at 980 1C followed by double aging for 8 h at 720 1C with furnace cooling plus other 8 h at 620 1C with air cooling. The substrate for non-treated specimens is in solution condition, 1 h at 980 1C. The type 1 specimen was taken as the reference to evaluate the influence of the precipitation-hardening treatment on mechanical properties. Then, new specimens of this type were manufactured, but with a precipitation-hardening treatment. As shown in Fig. 8, these specimens exhibited more homogeneous properties between the deposited material and the substrate. It may be observed that the hardness values between the substrate and the
b
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Fig. 6. (a) Substrate and deposited material structure. (b) Details of the deposited material structure.
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Fig. 7. Porosity analysis by image processing technique.
reductions in the mechanical properties should be taken into account if the laser-cladding process is used. The stress–strain curves are presented in Fig. 9.
5. Conclusions
Fig. 8. Hardness evolution from substrate to deposited material. (a) Untreated specimens; (b) treated specimens (1 h–980 1C/air cooled+ 8 h–720 1C/furnace cooled+ 8 h–620 1C/air cooled).
The results of the study have shown that laser-cladding strategies have a significant influence on the mechanical properties of Inconel 718 parts. Two types of tests were performed. First, hybrid specimens with substrate and laser-deposited material were tested. The results show reduced mechanical properties in comparison with the values for wrought Inconel 718. Moreover, all specimens present failures in the zone of the deposited material. The second type of specimen was a 100% deposited material. In this case, the mechanical behaviour of the parts presented high anisotropy and was relatively similar to that of a composite material, in which the fibres are comparable to the laser-cladding tracks. Therefore, the optimum laser-cladding strategy is achieved by aligning the laser-cladding tracks in the direction of the main stress lines. Furthermore, precipitation hardening improves the mechanical properties because of homogenisation of the microstructure of the deposited material. The tests on treated specimens showed higher tensile strength and ductility increments as much as 100%. However, it can be also concluded that there is a reduction in the specimens’ mechanical properties, which is about 75% of the strength of the wrought Inconel 718 in the best of cases.
Acknowledgements The authors wish to thank the ITP S.A. team for their collaboration and support with the characterisation and treatment of Inconel 718. Special thanks also go to the Spanish Ministry of Science and Innovation for financial support provided through the SURFACER Project (Ref.: DPI2010-20317-C02-01).
Fig. 9. Stress–strain curves for treated and untreated type 1 specimens.
deposited material present similar values, while the non-treated structures show differences of up to 200 HV. A significant increase in mechanical properties was noted in the test results: UTS was 20% higher in all cases and 100% higher ductility was obtained. However, maximum UTS was slightly over 1000 MPa, while the values of the base material were between 1200–1300 MPa. Thus, despite the improvement in the mechanical properties of precipitation hardening, the properties of these specimens do not match those of wrought Inconel 718. These
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