Technical Feasibility of Laser Dissimilar Welding of Superalloys on Casted Nozzle Guide Vanes

Technical Feasibility of Laser Dissimilar Welding of Superalloys on Casted Nozzle Guide Vanes

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Available online at www.sciencedirect.com

ScienceDirect Procedia CIRP 41 (2016) 963 – 968

48th CIRP Conference on MANUFACTURING SYSTEMS - CIRP CMS 2015

Technical feasibility of laser dissimilar welding of superalloys on casted nozzle guide vanes Fabrizia Caiazzoa,*, Vittorio Alfieria, Vincenzo Sergia, Andrea Tartagliab, Michele Di Foggiab, Antonio Niolab a

Department of Industrial Engineering, University of Salerno, Via Giovanni Paolo II 132, Fisciano (SA), 84084, Italy b EMA Europea Microfusioni Aerospaziali, Zona Industriale ASI, Morra De Sanctis (AV), 83040, Italy

* Corresponding author. Tel.: +39-89-964323. E-mail address: [email protected]

Abstract An increasing degree of automation is required both in the automotive and aircraft industry in order to allow scrap reduction and flexibility. In this frame, the shift from arc welding to laser beam welding is being investigated for a number of applications on metals in aerospace engineering, where strict standards apply. In comparison with conventional welding methods, a number of advantages are benefited; nevertheless, when moving to a new technology, some issues must be addressed. Hence this study is aimed to investigate laser dissimilar welding of real metal components, in order to assess the technical feasibility as well as to discuss set-up and operating issues in view of the implementation of the process for actual industrial application. A second-stage stator of low-pressure turbine is considered: lightening of the airfoils of the nozzle guide vane is achieved thanks to inner hollows which are drained from wax upon casting; afterward, the core exits on the outer side of the nozzle must be conveniently closed off by means of metal plates. Joining of the plates to the nozzle is performed by fusion welding along the edge of each plate and a condition of dissimilar welding is in place, being the nozzle and the plates made of C1023 and Nimonic 75, respectively. A mixed factorial plan has been arranged, laser power, welding speed and focus position being the leading processing parameters; a convenient welding set-up is proposed. Reasons are given for the implementation of laser beam welding as an alternative to conventional arc welding. © 2015 2015 The The Authors. Authors. Published Published by by Elsevier © Elsevier B.V. B.V. This is an open access article under the CC BY-NC-ND license Peer-review under responsibility of the Scientific Committee of 48th CIRP Conference on MANUFACTURING SYSTEMS - CIRP CMS (http://creativecommons.org/licenses/by-nc-nd/4.0/). 2015. Peer-review under responsibility of the scientific committee of 48th CIRP Conference on MANUFACTURING SYSTEMS - CIRP CMS 2015 Keywords: Welding; Laser; Superalloys

1. Introduction A major focus both in automotive and aircraft is the development and the integration of sustainable technologies toward the optimization of process-related assets [1, 2]. Furthermore, reliability and innovation are key factors to be addressed and are currently being investigated among a number of priorities in advanced manufacturing environments which are commonly referred to as factories of the future. In particular, high-tech processes must be performed by means of adaptive and smart equipment. Hence, an increasing degree of automation is required [3] in order to allow scrap reduction and flexibility in solving any issue arising on-line.

In the field of laser welding, the need for automation has been remarkable for the sake of competitiveness due to different reasons: the optimized welding condition must be maintained during the process and must be reproducible over a series of welds; seam-tracking sensors are capable of being included for automatic teaching of the seam path as well as for correcting small errors from a pre-defined trajectory [4]; furthermore, a number of on-line signals can be collected to extract critical features relating to overall bead quality [5]. In addition, automation is the answer to safety issues, since laser sources are recognized as being potentially dangerous when improperly used. Given these reasons, laser welding is currently one of the major application areas for industrial robots [6] which have undergone severe evolution [7]. In this frame, the shift from

2212-8271 © 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of 48th CIRP Conference on MANUFACTURING SYSTEMS - CIRP CMS 2015 doi:10.1016/j.procir.2015.12.133

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arc welding to laser beam welding is being investigated for a number of applications on metals in aerospace engineering [8, 9, 10] where strict standards apply. In comparison with conventional welding methods, a number of advantages are benefited when considering laser beams [11]: the extension of both the fusion zone and the heat affected zone is restrained to few millimeters, deep penetrative beads are produced and excessive heating up of the base metal is prevented, thus reducing thermal distortion and degradation of metallurgical properties. Further advantages are achieved with a new generation of high brightness lasers, such as fibre and thin disk lasers [12], since high output power, high efficiency and good beam quality are simultaneously delivered. Moreover, low maintenance costs as well as small floor space requirements are benefited [13]. Nevertheless, some issues must be addressed when moving to any new technology. Laser dissimilar welding of real metal components with no need for wire metal is explored in this study. The assessment of the technical feasibility, with relating set-up and operating issue in view of the implementation of the process for actual industrial application is first aimed. Furthermore, investigations on chemical composition and mechanical properties are conducted to discuss the response in the base metals, so to suggest a proper set-up of the governing parameters. The study is based on the current design of the base component; possible changes are suggested afterward. 2. The component A second-stage stator of low-pressure turbine is considered. Lightening of the airfoils of the nozzle guide vane (NGV) is achieved thanks to inner hollows which are produced by means of core items and are drained from wax upon casting. Afterward, any core exit on the outer side of the nozzle must be conveniently closed off by means of 0.8 mm thick, approximately 5 mm wide, metal plates (Fig. 1) whose shape is required to match the corresponding exit, so to prevent possible inclusions of dust and debris under normal operating conditions. Core exits on the inner side of the NGV are mechanically concealed in final assembling of the engine, instead. Joining of the plates to the NGV is performed by fusion welding along the edge of each plate. In particular, a condition of dissimilar welding is in place, being the NGV and the plates made of C1023 and Nimonic 75, respectively.

Table 1. C1023, nominal chemical composition (wt.%), NGV side. Al

Ti

C

14.5 ÷ 16.5 9 ÷ 10.5 7.6 ÷ 9.0 4.2

Cr

Co

Mo

3.6

0.16

Fe

Nb

Mn

Ni

<0.5 <0.25 <0.2 Bal.

Table 2. Nimonic 75, nominal chemical composition (wt.%), plate side. Cr

Co

Ti

Nb

Mn

18.9 ÷ 21.0 0.15 ÷ 0.08 0.6 ÷ 0.2 <0.25 <1.0

Si

Cu

<1.0

<0.5

Fe

Ni

<0.5 Bal.

Both are nickel-based superalloys (Table 1 and Table 2) which are specifically developed to combine increased mechanical strength and creep rupture properties in order to cope with demanding stress and temperature [14]. Both can be joined by means of conventional methods; nevertheless, wire metal is required when performing arc welding and is neglected in this study so to benefit from simplified tooling in the mechanical set-up, in view of possible actual implementation of the process. 3. The welding set-up Each plate is manufactured to match the shape of the corresponding core exit; nevertheless, positioning of the plates is critical as manually accomplished, irrespective of the welding method. As a consequence, lack of fusion may typically occur at the edge and proper tooling to clamp the plates on the NGV is required. In addition, as nickel-based superalloys are prone to oxidation when in fused state, shielding with inert gases is crucial in order to obtain silver sound joints, although no specific requirements of weld strength are stated for the application in exam. Given the difference between arc and laser welding, the overall set-up must be redesigned to convert the process; namely, new clamping and shielding must be engineered (Fig. 2). Laser head

Shielding duct

Shielding duct

Outer core exit

Suction / shielding

Fig. 1. Metal plate (light gray) to be welded at core exits on the base component (dark gray).

Inner core exit

Fig. 2. Welding set-up; laser head with argon shielding ducts at outer core exit on the NGV.

Fabrizia Caiazzo et al. / Procedia CIRP 41 (2016) 963 – 968 Table 3. Main technical data of the welding system.

Laser light wavelength [nm] Beam Parameter Product [mm × mrad] Fibre core diameter [mm] Focus diameter [mm] Rayleigh range [mm]

1030 8.1 0.300 0.300 2.8

Table 4. Testing levels for each governing factor of the experimental plan.

Power [W] Welding speed [mm s-1] Focus position [mm]

-1 600 20 -2

Levels 0 800 30

1 1000 40 0

A suction pipe entering each draining channel from the corresponding core exit on the inner side of the NGV is considered to prevent unwanted slippage of the plate from its nominal position; four welding spots are performed preliminarily and are thought to be overlaid by the final continuous bead to be performed. Indeed, upon spotwelding, the draining channel is filled with inert gas for back-side shielding, so to prevent discoloration despite the weld root not being in view. To the purpose of top-side shielding instead, considering the welding path to track and given the shape of the NGV preventing easy access to the outer core exits, a specific device consisting in twin shielding ducts has been developed as an upgrade of the laser welding head. Focused shielding on the plate is hence allowed, being the twin ducts in synchronous move with the laser head. In agreement with common industrial practice, 99.995% pure argon has been supplied at both sides of the welding bead; a flow rate of 30 l/min has been set. A proper time delay has been considered before welding to prepare inert atmosphere with stable shielding flux; additional delay has been required to effectively shield the bead upon laser switch-off. All of the analyses in terms of type of gas, flow rate, time delay and positioning of the components of the shielding device are a carryover from a prior patent [15]. Welding has been eventually performed along the boundary of each metal plate in continuous wave emission by a welding robot receiving the path from the controller, based on a CAD model of the NGV and the shape of the plates. A thin disk laser based system has been used (Table 3).

consequence of variation of irradiance; to the additional purpose of seam tracking of complex parts in case of possible robot inaccuracy, the effect of a defocused beam is investigated in order to benefit from larger spots. When individually considered, each one of the base alloys has its own processing range. Nevertheless, a common processing window can be investigated. A mixed factorial plan has been arranged: three levels have been chosen for both the laser power and the welding speed, whereas two levels have been chosen for the focus position as categorical factor (Table 4); namely, a diameter of 0.370 mm is achieved in the selected defocused condition. In particular, a condition of focused beam is chosen to provide the highest irradiance on the upper surface of the plate to be welded; on the other hand, the effect of negative defocusing is investigated, with the focal spot locating beneath the surface when key-hole conditions are in place. All of the possible combinations of the factors have been considered; a full factorial plan with 18 processing conditions resulted. 5. Results and discussion Visual inspections have been conducted upon welding to check the effectiveness of gas shielding; uniform, smooth and shiny beads have been produced with no spattering. Although the demand on quality is not as tight as for other components of the engine, non destructive tests in the form of fluid penetrant inspections have been conducted; the beads are found to be sound with no indications. 5.1. Bead aspect Bead aspects and macrographs of the transverse crosssections are properly descriptive to compare the results in arc welding (Fig. 3) and laser beam welding (Fig. 4). Uniform width along the welding path, smooth edges and reduced extent of the fusion zone are produced by laser welding. The area of the fusion zone has been measured for each processing condition in laser beam welding to discuss the dependence on the governing factors.

4. Design of the experiments Based on the literature [13, 16] and past experience on dissimilar welding of superalloys [8, 17], preliminary trials in the form of bead-on-plate tests have been conducted on the base metals so to find a suitable window for the processing parameters. Laser power and welding speed are primary governing factors to be taken into account as they determine the energy input to the work-piece. In addition, although wide depth of focus and restrained beam spread are usually accomplished in laser systems for industrial applications, the focus position is worth investigating since it has been proven that even small shifts may significantly affect the responses [18], depending on the material being welded as a

Fig. 3. Bead aspect and corresponding macrograph of the transverse crosssection as resulting from arc welding.

Fig. 4. Bead aspect and corresponding macrograph of the transverse crosssection as resulting from laser beam welding.

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Area of the fusion zone [mm ]

2

Area of the fusion zone [mm ]

defocused beam focused beam

1.8

1.2

0.6 600

800

Table 5. Average weight percentages of the main alloying elements in the corresponding sites of interest.

1.8

Al

Co

Cr

Fe

Mo

Ni

Ti

Fusion zone 1.665 4.473 17.099 1.869 3.684 67.060 1.885 C1023 2.645 9.786 15.652 9.104 58.436 3.341 Nimonic 75 20.231 3.980 73.277 0.263

1.2

0.6

1000

Power [W]

20

30

40

Welding speed [mm/s]

Fig. 5. Area of the fusion zone as a function of power and welding speed for given focus position.

At this stage of the analysis, it is worth noting that the expected trend for the response as a function of power and speed is followed; namely, the area of the fusion zone increases for increasing power, whereas a decrease is yielded for increasing welding speed (Fig. 5). Furthermore, the effect of focus position is far less significant, with no interactions with either power and speed, thus allowing improved reproducibility to the purpose of industrial application in case of possible imperfect control of focusing due to robot inaccuracy on complex paths to be tracked.

Fig. 6. Microstructure in base C1023, NGV side: bright nickel-based matrix with fine dark precipitates.

5.2. Microstructure and energy dispersive spectrometry Three sites of interest are clearly defined in the crosssection: the base metal at the NGV side, the base metal at the plate side and the fusion zone where a mixture of new phases is expected to be produced upon solidification [19].

30

C1023

10 8 6 4 2 0 0

0.5 1 Distance from weld surface [mm]

1.5

fusion zone

90

C1023 Nickel weight percentage [%]

fusion zone

Molybdenum weight percentage [%]

Cobalt weight percentage [%]

12

Fig. 7. Microstructure in base Nimonic 75, plate side: chromium segregating at grain boundary.

20

10

0

fusion zone

C1023

75 60 45 30 15 0

0

0.5 1 Distance from weld surface [mm]

1.5

0

0.5 1 Distance from weld surface [mm]

Fig. 8. Cobalt, molybdenum and nickel percentage in the fusion zone at the NGV (C1023) side.

Fig. 9. Areal maps for distribution of cobalt, molybdenum and nickel in the cross-section (800 W, 20 mm/s, -2 mm).

1.5

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5.3. Micro-hardness Considering the outcome of EDS inspections and based on the assumptions that precipitates at the NGV side as well as carbides at the plate side dissolve upon heating, a different microstructure must be expected in the fusion zone with respect to the base metals. Hence, Vickers microhardness testing have been conducted [20] to assess the response. Two indentation patterns, mutually orthogonal, have been scanned for each condition of the experimental plan in order to draw the trend of micro-hardness at each side of the joint; an indenting load of 0.200 kg has been used for a dwell period of 10 s. Depending on the welding condition, as a consequence of different thermal inputs and different cooling rates, average micro-hardness in the fusion zone ranges from 250 to 390 HV, being it intermediate between the reference values resulting for base C1023 and Nimonic 75 (Fig. 10). NGV side

fusion zone

plate side

450 425 400

HV0.2

375 350 325 300 275 Pattern 1

250 -1500

-1000

-500

Pattern 2

0

500

1000

1500

indent position [µm]

Fig. 10. Micro-hardness trend in the cross-section at both sides of the joint (800 W, 20 mm/s, -2 mm); patterns are mutually orthogonal.

defocused beam focused beam

defocused beam focused beam

Vickers micro-hardness

Vickers micro-hardness

With respect to these, inspections have been conducted by means of energy dispersive spectrometry to investigate the chemical composition for each site. Base C1023 at the NGV side has been found to consist of a bright nickel-based matrix with dispersed fine dark chromium-cobalt precipitates, whereas molybdenum is the main element in coarse grains (Fig. 6). Carbides are found in base Nimonic 75 instead; type is usually M23C6 and chromium is reported to segregate at grain boundary (Fig. 7). An amorphous structure is found in the fusion zone, whose content in terms of the main alloying elements has been compared with the measured composition in the base alloys (Table 5). To further discuss the issue, linear scanning patterns have been considered. As an example, the trend for cobalt, molybdenum and nickel percentage at the NGV side at midthickness of the cross-section is shown (Fig. 8): it is worth noting that the concentration for each element on a macroscopic scale is uniform in the fusion zone, due to vigorous whirling convective mixing of metal in fused state. The distribution of the alloying elements is even clearer when matching the average percentages with the areal maps (Fig. 9). There is no point in comparing these conclusions with the chemical composition of the fusion zone in case of arc welding where wire metal is used, thus dramatically affecting the response.

330

300

270

600

800 Power [W]

1000

330

300

270

15

20

25

30

35

40

45

Welding speed [mm/s]

Fig. 11. Average micro-hardness in the fusion zone as a function of power and welding speed for given focus positions.

Namely, average micro-hardness in the fusion zone is found to increase for increasing power and decrease for increasing welding speed (Fig. 11). Even in this case, the effect of focus position is far less significant, with no interactions with either power and speed, thus allowing improved reproducibility of the welding condition. Once more, there is no point in comparing these conclusions with the trends of micro-hardness in case of arc welding where wire metal is used and different dynamics are in place; nevertheless, it is worth noting that tighter heat affected zones are benefited in laser welding. Based on the outcome of the experimental plan, an optimal condition of welding can be suggested within the investigating domain. In particular, a condition with minimal extension of the fusion zone and intermediate micro-hardness must be preferred to the purposes of restrained heat affected zones and mechanical continuity with the base metals. Both the processing conditions with a power of 800 W and a welding speed of 20 mm/s can be chosen, irrespective of the focus position; nevertheless, the condition with defocused beam is suggested so to benefit from larger spots. Given the application of the joint, tensile tests which are required in usual practice to assess the optimum can be neglected. 6. Current and future improvements Once proven that an upgrade of the welding process on the base metals is viable on actual current components both in terms of operating set-up and weldability, further remarks can be drawn. It has been widely reported in the literature that shorter processing times are benefited from laser beam welding in comparison with traditional methods. For this application, although additional time for spot-welding and delay for proper shielding have been required in the suggested welding set-up, a reduction in the order of 80% has been estimated with respect to conventional arc welding. Moreover, a suitable housing slot at the outer core exit would allow to better position each metal plate, with no need for preliminary spot-welding. This would further reduce the processing time. As a consequence, from an economic perspective, the move from arc to laser welding seems to be affordable provided that increase of production and sharing of the same laser system among different processes are set.

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Grounds are then given to the development of a further improved advanced manufacturing environment where several departments are capable of benefiting from laser material processing as an alternative to traditional technologies. 7. Conclusions The shift from arc welding to laser beam welding with no need for wire has been investigated to explore possible improvements to join real metal component for aerospace application: namely, the core exits on the side of a C1023 nozzle guide vane have been conveniently closed off by means of Nimonic 75 metal plates. Hence, a condition of dissimilar welding is in place. A disk laser has been used, so to benefit from better beam quality with respect to conventional laser sources. Given the difference between arc and laser welding, new clamping and shielding have been engineered to convert the process. Uniform, smooth and shiny beads have been produced with no spattering; no indications resulted from non destructive tests. A mixture of new phases is produced in the fusion zone upon solidification; the concentration of the main alloying elements is found to be uniform on a macroscopic scale. Both the area of the fusion zone and its average micro-hardness depend on power and welding speed, whereas the effect of focus position is far less significant, thus allowing improved reproducibility to the purpose of this application in case of imperfect control of focusing due to robot inaccuracy and complex paths to be tracked. A processing condition has been suggested. Given the application of the joint, tensile tests have been neglected. Shorter processing times are benefited in comparison with traditional methods; a reduction of overall processing time in the order of 80% has been estimated. Valuable grounds are given for actual industrial application, in view of advanced manufacturing environment with increased production and shared resources. Acknowledgements The authors gratefully acknowledge Eng. Gaetano Corrado for his valuable support in designing and implementing the welding set-up; Eng. Federica Palma for her worthy commitment in the experimental work. References [1] Radziwon A, Bilberg A, Bogers M, Madsen E. The smart factory: exploring adaptive and flexible manufacturing solutions. Procedia Engineering 2014;69:1184-90. [2] Haapala K, Zhao F, Camelio J, Sutherland J, Skerlos S, Dornfeld D, Jawahir I, Clarens A, Rickli J. A review of engineering research in sustainable manufacturing. ASME Journal of Manufacturing Science Engineering 2013;135,4:599-619. [3] Carpanzano E, Jovane F. Advanced Automation Solutions for Future Adaptive Factories. CIRP Annals - Manufacturing Technology 2007; 66,1:435-8.

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