Processing and characterization of high aluminum multicomponent (Co,Ni)-based superalloys for friction stir welding (FSW) tools

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Abstract

Despite the great potential of Friction Stir Welding (FSW) process, there is a lack of technical information on Co-based alloys designed for FSW tools. Therefore, this work evaluated two multicomponent (Co-Ni)-based superalloys tools of high aluminum contents for better wear and oxidation resistance. The two alloys differed on the presence of either Nb or Ta in their chemical composition. The alloys were produced via vacuum induction melting / investment casting and then heat treated at 1000 °C for 16 h. The as-cast and heat-treated microstructures of both alloys presented γ, γ′, Co7W6, CoAl, Laves and MC phases. Yield strength near 550 MPa at 900 °C and hardness values about 500 HV0.5 have been measured for both heat-treated alloys. Friction stir welding (FSW) tools of the two alloys were successfully used to weld (bead on plate) a plate of ASTM A1008 steel. However, superior results have been obtained from the Ta-alloy tool.

Introduction

Friction stir welding (FSW) is a solid-state joining process able to eliminate common welding defects from fusion welding processes [1,2]. The FSW technique uses a non-consumable rotating tool, which contains a shoulder and pin, that heats up and stirs the workpiece material through severe plastic deformation, resulting in reliable and improved joining interface [3,4]. Considering several features of the FSW tools and their constitutive materials, the metal flow around the pin can be quite complex [4] and tool wear becomes an important issue [2].

Usually, FSW tools are based on W-Re alloys, which are used to weld steel plates and titanium alloys [5,6]. These tools often show good fracture toughness; however, they tend to wear rapidly [5]. Polycrystalline cubic boron nitride (PCBN) tools show better wear resistance when compared to refractory W-Re tools [7], although they present high cost and low fracture toughness [5]. (Co-Ni)-based superalloys with γ/γ′ microstructure have been considered for these demanding applications, because they exhibit high mechanical strength and toughness, as well as high oxidation resistance. In addition, these tools can be obtained by low cost investment casting process, suitable for large scale production [8].

With respect to (Co-Ni)-based superalloys for FSW tools, the desired microstructure consists of embedded hard particles in a γ/γ′ matrix [9]. The γ′-Co3(Al,W) phase with L12 structure, coexisting with γ (Coss - FCC), plays the same role as in nickel-based superalloys [[10], [11], [12]]. Park et al. [8] evaluated a Co-based superalloy tool with γ/γ′ microstructure that successfully welded 45.7 m length of a mild steel, without major geometrical variation before 10 m of welding. However, significant wear of the tip and shoulder were reported at 45.7 m. Little deformation of the pin was observed after welding 200 mm length of Ti-6Al-4 V, with some wear at the shoulder. In addition, 1.4 m of Zircaloy 4 was welded without any important macroscopic changes of the tool geometry. Sugimoto et al. [13] tested several tool geometries of Co-based alloys produced by investment casting, aiming the weld of low carbon steel. Reasonable weld surfaces were obtained for travel speeds lower than 250 mm/min and pin shorter than 9 mm. According to the authors, the pin geometry is highly affected by the output power and promising results were obtained with a 6 mm pin and rotation speed of 250 revolutions per minute (RPM).

Despite the great potential of FSW process, there is a lack of technical information on Co-based alloys designed for FSW tools. Therefore, this work evaluated two multicomponent (Co-Ni) superalloys tools of much higher aluminum contents than those previously presented in the literature [6,[14], [15], [16], [17]] for better wear and oxidation resistance in terms of phase transformation temperatures, microstructural characterization of as-cast and heat-treated materials, Vickers hardness and hot compression tests. In addition, the produced tools were used for FSW experiments of an ASTM A1008 plate steel and both presented promising results.

Section snippets

Alloys production and heat treatments

Table 1 shows the chemical composition determined by X-ray fluorescence spectroscopy (XRF) of the two (Co-Ni)-based alloys investigated in this work. Boron and carbon contents are from the nominal compositions, based on the weighted amount of each material. The alloys are identified as Nb-alloy and Ta-alloy since the main difference between them corresponds to the presence or not of each of these elements in the alloy composition. Note that in both alloys the amount of either Nb or Ta was

Microstructural characterization of the as-cast alloys

Fig. 2 (a) presents the X-ray diffractogram of as-cast Nb-alloy, in which reflections of major phases γ, Co7W6 and CoAl were indexed. Peaks from γ′ have not been indicated in the diffractogram due to superposition with those from γ phase. SEM/BSE micrographs of as-cast Nb-alloy are shown in low and high magnification in Figs. 2 (b) and (c), respectively, depicting γ/γ′ dendrites, interdendritic γ′, Co7W6 and CoAl. Small solid state formed γ′ particles were observed throughout the γ dendrites.

Summary

In this work two alloys, one containing Nb and the other Ta, of composition close to Co (balance)- 42 Ni -15 Al - 9 Cr- 5 W - 3 (Nb/Ta) - 0.06B - 0.6C (at. %) were produced in the form of bars via vacuum induction melting / investment casting and heat-treated at 1000 °C for 16 h. Both alloys presented γ, γ′, Co7W6, CoAl, Laves and MC phases in the as-cast as well as in the heat-treated samples. The 0.2 % yield strength of the alloys is about 550 MPa at 900 °C and their hardness near 500 HV0.5

Author statement

Marcus Vinicius da Silva Salgado processed the alloys and wrote this paper; Bruno Xavier de Freitas performed XRD and FRX experiments and analysis; Alex Matos da Silva Costa performed the tests on mechanical simulator Gleeble 3800®; Victor Ferrinho Pereira performed de FSW experiments; Nabil Chaia performed the thermodynamics calculations and Vickers hardness measures; Maria Ismênia Sodero Toledo Faria performed the metallography and SEM/EDS experiments; Gilberto Carvalho Coelho revised the

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível superior – Brasil (CAPES) – Finance code 001. SALGADO, M.V.S. and FREITAS, B.X. acknowledges CNPQ for their scholarship grant numbers 140041/2018-4 and 142337/2019-6, respectively. Authors acknowledge Dr Julia Helena Bormio Nunes for English revision.

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