Thermal cycling of Fe3Al based iron aluminide during the wire-arc additive manufacturing process: An in-situ neutron diffraction study
Introduction
Fe3Al based iron aluminide has been considered as a promising replacement of regular stainless steel in piping and tubing for fossil energy systems, due to a combination of advantages such as excellent oxidation and sulfidation resistance, considerable high temperature strength and creep resistance, low density and low cost [1]. Extensive efforts have been made to improve the room-temperature ductility and high-temperature strength of iron aluminide, by means of adding alloying elements and heat treatments [2]. To date, the room-temperature tensile elongation of Fe3Al based iron aluminide has been improved up to 11% with the selection of suitable alloying elements and appropriate thermo-mechanical processing combined with subsequent annealing [3]. On the other hand, these combined processing techniques have on the other hand increased the manufacturing cost of this alloy and counterbalance their advantages.
For the successful introduction to the market, a cost-effective manufacturing method for iron aluminide is necessary. In recent years, the wire-arc additive manufacturing (WAAM) process has obtained considerable progress in both manufacturing accuracy (through improved robotic path planning) [4], [5], and application scope of materials, such as aluminum alloys, steel and titanium alloys [6], [7], [8]. Moreover, the WAAM process has already proved its capability of in-situ fabricating intermetallics of titanium aluminide [9], [10] and iron aluminide [11], [12] with controllable chemical compositions. Compared to traditional methods of producing iron aluminide, such as furnace casting/melting [13] and mechanical hot pressing [14], the WAAM process is capable of directly producing structures with full density which eliminates the need for expensive post-fabrication processing. Also, the cost of filler wires in the WAAM process is much lower than the high-purity metal powder which is necessary for powder metallurgical methods as used to prevent casting defects [15].
In order to further understand and develop the WAAM process, profound knowledge of the materials behavior during the multi-deposition process is required. During the process of additive manufacturing, the pre-deposited layer will be partially remelted by the next deposition process, and substantially reheated several times during the buildup. Specific to the buildup process of Fe3Al based iron aluminide, the as-deposited material will experience phase transformation between Fe3Al and FeAl every time the subsequent layer is deposited [16] as related to the phase diagram shown in Fig. 1, which will induce stress and influence the mechanical properties of the buildup structures. Therefore, an in-situ observation and quantitative analysis of the phase transformation processes and grain structure variation are desired.
In the present research, the in-situ diffraction experiment was performed using the high-flux neutron diffractometer WOMBAT located on the TG1 thermal neutron guide at the Open Pool Australian Lightwater (OPAL) reactor. It is equipped with a 120° position sensitive detector, for which the major sensitive area is set up for high-speed recording and capable of measuring real-time phase transformations in seconds [17], [18], [19], [20], [21], [22]. The neutron diffraction data were accumulated in 35 s time slices while the specimen was heated in a vacuum furnace to simulate the heat-up process during WAAM. Similar methods have been widely applied to investigate specific material properties in recent years [23], [24], [25]. Since both D03 structured Fe3Al and B2 structured FeAl phases are ordered structures, their existence can be observed in superstructure diffraction peaks. Accordingly, the phase transformations can be quantitatively detected by the variation and disappearance of the corresponding peaks. Also, the grain structure variation occurring during the heat-up process can be analyzed using the obtained diffraction patterns.
Section snippets
Sample preparation
The setup of the WAAM process is shown in Fig. 2. The process was powered using a gas tungsten arc welding (GTAW) arc which was generated by a commercial inverter power source and a matching (2.4 mm diameter) tungsten welding torch [11], [12]. Two wire feeders with independent speed controls, one for 1080 grade aluminum wire (695 mm/min) and one for annealed high purity (99.99%) iron wire (1000 mm/min), were applied to feed the wires into a single molten pool and fabricate in-situ the designed
X-ray and neutron diffractograms
The X-ray and neutron diffractograms of the as-fabricated sample are shown in Fig. 5. The solid X-ray specimen has been cut adjacently from the neutron sample, while also been ground to powder. The dominant bcc structure, based on pure α-Fe, is recognized by the strong 110, 200, 211 and 220 peaks in the sc indexing. The ordered FeAl structure of B2 is expressed by the 100 superstructure peak, seen weakly for X-rays and strong for neutrons. The neutron diffractograms show additional, weak peaks
Conclusion
According to the XRD results, EDS results and the morphologic characterizations, a Fe3Al based buildup of iron aluminide with consistent 30 at% Al content has been successfully fabricated by the WAAM process. However, compared to the XRD results in which no FeAl phase was detected, the neutron diffraction data has revealed the existence of FeAl with about 3.36 mass% at room temperature. It is due to the accuracy of the XRD equipment that phase fractions under 5 mass% are hardly detected.
Acknowledgement
The authors gratefully acknowledge financial support from China Scholarship Council, University of Wollongong, Welding Technology Institute of Australia (WTIA), and use of the facilities within the UOW Electron Microscopy Center. The authors also would like to acknowledge the support of the Bragg Institute, Australian Nuclear Science and Technology Organization (ANSTO), in providing the high-intensity neutron powder diffractometer WOMBAT used in this work, and financial assistance (Proposal ID
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