Microstructure and mechanical properties of friction stir welded Haynes 282
Introduction
Advanced ultra-supercritical (A-USC) next generation power plants are designed to operate at steam temperature in the range of 700–760 °C and pressure up to 350 bar (35 MPa) to obtain the target efficiency of 45–52% [1]. The increase in operational efficiency in turn reduces the coal consumption, lowers flue gas emissions, and lowers the carbon tax. Owing to the extreme steam parameters, material exhibiting superior high temperature properties are required, such as precipitation strengthened nickel-based superalloys. However, a number of nickel-based alloys such as CM247LC and IN738 with higher γ′ fraction and higher temperature strength generally exhibit poor weldability. Therefore, the proposed A-USC alloys should exhibit a good fabricability in addition to high temperature strength and corrosion resistance.
HAYNES 282 is a γ′-strengthened nickel-based superalloy developed by Haynes international for high temperature structural applications in aerospace- and land-based gas turbine engines [2,3]. The alloy was designed with lower Al and Ti content to have increased fabricability as compared to R-41 and Udimet 720 [2,3]. Mo content was increased to 8.5 wt% to exhibit creep strength comparable to high Al + Ti containing alloys. Due to reduced γ′ fraction, strain-age cracking resistance of Haynes 282 is higher than high γ′ containing alloys such as Udimet 720 and R-41. Haynes 282 could be a candidate material for high temperature structural components in A-USC powerplant, however further research and development is needed to access its adoption as a high temperature structural alloy in A-USC plants. A few potential applications include hot-gas-path components in land-based turbines, and exhaust and nozzle components in aircraft gas turbines.
Fusion based welding processes are commonly employed manufacturing techniques during fabrication of components and are also utilized in the repair and refurbishment of in-service parts. Therefore, as a part of the fabricability, weldability investigations using various fusion welding are generally conducted to develop production capable processes and understand the type, location, cause, and effect of potential defect modes, such as porosity, cracking, etc. Commonly occurring defects during fusion welding of precipitation strengthened Ni-base superalloys are solidification cracking, liquation cracking, and strain-age cracking [4,5]. Various investigations predominantly reported the absence of solidification cracking in the fusion zone of Haynes 282, and the presence of grain boundary phase liquation and consequent grain boundary cracking in the heat affected zone (HAZ) [[6], [7], [8], [9], [10], [11], [12]]. Osoba and Ojo [7] observed HAZ cracking in laser welded Haynes 282 and attributed the cracking to grain boundary liquation. Furthermore, increasing the heat input reduced the extent of HAZ liquation cracking [7]. Based on the observations, HAZ cracking was due to the presence and liquation of M5B3 boride particles [8]. Additionally, Gleeble hot ductility tests showed that the reduction in grain size from 90 μm to 40 μm improved the hot ductility from 10% to 40%, thereby displaying the positive effect of grain size reduction on the extent of HAZ cracking [8]. However, grain size control might not be feasible in the fusion welding Haynes 282 components. Moreover, the impact of liquated grain boundaries on the long-term mechanical properties such as creep and high cycle fatigue are yet to be understood. A recent investigation by Hanning and Anderson reported the presence of solidification cracking in the fusion zone of wrought Haynes 282 alloy welded using manual gas tungsten arc welding [9].
To potentially overcome the shortcomings of fusion welding of γ′ strengthened Ni-base superalloy, solid-state welding techniques such as friction welding and friction stir welding (FSW) have been considered and investigated for feasibility [13,14]. Details on friction stir welding/processing (FSW/P) can be found in [15]. FSW of low melting point structural materials such as Al and Mg alloys have successfully been fabricated since the invention of the process in 1991. However, FSW of high temperature materials required the deployment of FS tools that could withstand large processing forces at high temperature. In the recent years, FSW of high temperature materials have been made possible due to friction stir (FS) tool materials such as polycrystalline cubic boron nitride (PCBN), WC-Co, W-Re, and W-Re-HfC that could withstand the processing forces. These materials retain significant strength and toughness above 700–800 °C; temperatures associated FSW/P [14,[16], [17], [18], [19], [20], [21], [22], [23]]. Considerable amount of research exists on the FSW/P of solid solution strengthened nickel alloys such as Inconel 600 [17,21,24,25], Inconel 625 [19,22], and Inconel 718 [20,23,26] while research on FSW/P of γ′ and γ″ strengthened nickel alloys are scarce [16]. This is due mainly to the high temperature strength of precipitation strengthened nickel alloys and its adverse effect on the FS tool wear. In the case of FSW of nickel alloy 625 using PCBN tool with a W-Re binder phase, wear of the FS tool and the presence of FS tool material in the processed region was noted [22]. Higher tool rotational speed resulted in poor weld quality and tool wear while reduction in tool rotational speed improved the weld quality and reduced the tool wear. Another method to reduce/eliminate the tool wear is to employ laser or induction heating ahead of the tool to minimize the processing forces thereby reducing/removing the mechanical wear of the tool [27]. In addition to the tool wear, defects such as internal void and lack of consolidation on the advancing side are common in friction stir welds if inappropriate weld parameters or FS tool designs are used [28]. Varying the processing parameters such as tool rotational speed, traverse rate, the axial force, and/or FS tool design usually results in defect-free welds. As expected, the FS processed zone of various nickel alloys typically exhibits recrystallized, fine grains resulting in improved hardness and tensile properties. To the best of authors' knowledge, only a few investigations exist on friction stir processing of Haynes 282 to modify the cast microstructure [16,23]. The processed cross-section was free of defects and exhibited mixture of nickel alloy 282 and 600. The regions containing Haynes 282 exhibited lamellar microstructure with fine particles and resulted hardness of above 400 HV in the post heat-treated condition. Therefore, there is a strong need to investigate the feasibility to friction stir weld Haynes 282, investigate the microstructure, and evaluate the mechanical performance.
In the current investigation, friction stir welding of Haynes 282 was carried out at low tool rotational speed and slow traverse rate in an effort to minimize the tool wear and produce defect free welds. Then, optical and scanning electron microscopy investigations were carried out to investigate the presence of defects and the evolved microstructure. Mechanical property evaluations were carried out using microhardness measurements and cross-weld tensile tests.
Section snippets
Experimental Methods
Haynes® 282® alloy plates of the following dimensions 508 × 203 in a nominal thickness of 9.5 mm was procured from Haynes International, Inc. in solution treated condition (AMS5951). A Q60 friction stir welding tool (60% pcBN and 40% W-Re) purchased from Megastir was used. The welding was conducted in a temperature control mode and the tool temperature was maintained at ~825 °C by varying the weld power via tool rotational speed (revolutions per minute, RPM). Temperature was measured using a
Friction Stir Welding
The measured temperature from the thermocouple located in the FS tool shoulder and tool rotational speed variation that is needed to maintain the tool temperature of 825 °C is presented in Fig. 1. Tool rotation speed varied between 50 and 80 RPM to maintain the set temperature while the tool traverse rate was kept constant (0.42 mm/s). Consequently, the tool temperature was maintained with a standard deviation of 1 °C. A constant weld temperature was maintained to eliminate the temperature
Concluding Remarks
Friction stir welding of precipitation strengthened Haynes 282 nickel alloy was successfully carried out followed by microstructure and mechanical property analysis. Following are the concluding thoughts.
- 1.
It is possible to create defect-free welds with the solid-state friction stir welding. Uniform weld temperature was maintained throughout the weld. Lower rotation speeds appear to be important for creating defect-free welds.
- 2.
A fine-grained wrought microstructure was noted in the friction stir
Data Availability Statement
The raw/processed data required to reproduce these findings can be obtained upon a direct request to the corresponding author.
Funding
The work related to this publication was funded by the Department of Energy, Office of Fossil Energy (FE).
Declaration of Competing Interest
The authors declare that they have no conflict of interest.
Acknowledgements
The authors are grateful for the efforts of Anthony Guzman for preparation of specimens for microstructural characterization and Timothy Roosendaal, Robert Seffens, and Ethan Nickerson for mechanical testing. The authors are grateful for the discussions with Anand Kulkarni and Kyle Stoodt from Siemens Corporation. The authors also appreciate and are grateful for the guidance of Vito Cedro at US DOE Fossil Energy Office. Pacific Northwest National Laboratory is a multiprogram national laboratory
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