Elsevier

Materials Characterization

Volume 110, December 2015, Pages 126-135
Materials Characterization

Microstructural characterisation of friction stir welding joints of mild steel to Ni-based alloy 625

https://doi.org/10.1016/j.matchar.2015.10.023Get rights and content

Highlights

  • Comprehensive microstructural characterisation of dissimilar joints of mild steel to Ni-based alloy

  • Friction stir welding of joints of mild steel to Ni-based alloy 625 produces sound welds.

  • The interface region showed deformed and recrystallised fcc grains with NbC carbides and a length of 2.0 μm.

Abstract

In this study, 6-mm-thick mild steel and Ni-based alloy 625 plates were friction stir welded using a tool rotational speed of 300 rpm and a travel speed of 100 mm·min 1. A microstructural characterisation of the dissimilar butt joint was performed using optical microscopy, scanning and transmission electron microscopy, and energy dispersive X-ray spectroscopy (XEDS). Six different weld zones were found. In the steel, the heat-affected zone (HAZ) was divided into three zones and was composed of ferrite, pearlite colonies with different morphologies, degenerated regions of pearlite and allotriomorphic and Widmanstätten ferrite. The stir zone (SZ) of the steel showed a coarse microstructure consisting of allotriomorphic and Widmanstätten ferrite, degenerate pearlite and MA constituents. In the Ni-based alloy 625, the thermo-mechanically affected zone (TMAZ) showed deformed grains and redistribution of precipitates. In the SZ, the high deformation and temperature produced a recrystallised microstructure, as well as fracture and redistribution of MC precipitates. The M23C6 precipitates, present in the base material, were also redistributed in the stir zone of the Ni-based alloy. TMAZ in the steel and HAZ in the Ni-based alloy could not be identified. The main restorative mechanisms were discontinuous dynamic recrystallisation in the steel, and discontinuous and continuous dynamic recrystallisation in the Ni-based alloy. The interface region between the steel and the Ni-based alloy showed a fcc microstructure with NbC carbides and an average length of 2.0 μm.

Introduction

Dissimilar welds between Ni-based alloys and ferritic steels have been used in engineering in a wide range of applications, including pressure vessels and oil exploration [1]. In general, the applications take advantage of the excellent corrosion and oxidation properties of Ni-based alloys in combination with the low cost of steels. Ni based alloy 625 is used in the construction of a large number of power plant components that require creep, thermal-fatigue and corrosion resistance. In contrast, the steel ASTM A516 is used mainly for the construction of equipment components for the oil industry, and it has the highest tensile strength and ductility of conventional pressure vessel steels. In corrosive environments, this steel has the longest time to fracture [2]. Fusion welding is usually employed to join steel to Ni-based alloys, most often used in cladding. However, there are process and metallurgical challenges associated to this dissimilar joint. On the process side, there are large differences in melting temperatures and fluidity that may lead to lack of fusion defects. Regarding the metallurgical weldability, there are solidification cracking, hard interfacial regions, materials intermixing and C diffusion issues that may compromise the joint performance [1], [3]. In addition, the mechanical and corrosion properties may be compromised due to the as cast structure [4], [5]. Therefore, performing solid state joining of these alloys overcomes or minimises some of the aforementioned challenges, making it a promising alternative [1].

Friction stir welding (FSW) is a solid state joining technique developed at TWI in 1991 [6], [7], [8], [9]. A non-consumable tool rotates and travels along the joint or material being processed [10]. Heat generated by friction and deformation softens the material, which flows under severe plastic deformation condition [7]. FSW has become a viable and important manufacturing alternative in several industries. Although, it was originally developed for joining light Al and Mg alloys, this technique has evolved to become useful with higher-melting-temperature alloys, such as steel, titanium and Ni-based alloys [11].

The use of FSW for joining Ni-based alloys, particularly alloy 625, has resulted in significant grain size refinement in the stir zone compared to the base material [4]. In addition, there have been enhancements to the material's mechanical properties due to the refined grain size. Similar improvements to mechanical properties have been reported for alloys 600 and 718 welded using FSW [12], [13], [14]. In the case of dissimilar FSW joints involving steel and Ni-based alloys, there are few reports in the literature [15], [16]. Song et al. [15] studied the microstructure and mechanical properties of a lap joint of alloy 600 to low carbon steel. The results showed friction stir lap joints without defects, notable grain refinement in the stir zone and improvement of mechanical properties. In addition, the formation of a steel hook in the alloy 600 was observed. This hook, formed at the interface, enhanced the joint peel strength. In previous publications [16], grain size refinement in the heat-affected zone and allotriomorphic and Widmanstätten ferrite formation in the stir zone of the low carbon steel were found. For alloy 625 the dynamic recrystallisation led to austenite grain refinement. However, the microstructural character of FSW dissimilar butt joints of steel to a Ni-based alloy has not been reported. In the current study, the microstructure character of butt joints steel to Ni-based alloy is described.

Section snippets

Experimental procedure

ASTM A516 Gr 60 (A516) carbon steel and Ni-based alloy 625 (A625) plates (500 × 90 × 6.6 mm) were friction stir welded using a W-Re PCBN (polycrystalline cubic born nitride — PCBN) composite tool in force control mode. The alloy compositions are presented in Table 1. The tool's rotational speed and travel speed were 300 rpm and 100 mm·min 1, respectively. The development and selection of welding parameters are shown elsewhere [17]. An axial offset of 1.63 mm (Fig. 1) and an axial force of 30 kN were

Results and discussion

The FSW parameters used produced sound welds without cracks or cavities. The solid-state nature of and lack of melting in FSW eliminated the problems of solidification and cracking. Fig. 2 shows a low magnification optical image of the weld cross-section. The material flow caused by the tool rotation and displacement during FSW caused the formation of a vortex-like shape in the weld, which has been also seen in dissimilar FSW of other alloys [18]. The plastic deformation caused some portions of

Conclusions

The FSW of mild steel and Ni-based alloy 625 produces sound welds, without volumetric defects. Different microstructures were recognised in the welds. In the steel, the HAZ showed three regions, the ICHAZA516 and FGHAZA516 consisted of ferrite and pearlite colonies with different morphologies, and the CGHAZA516 showed allotriomorphic and Widmanstätten ferrite. The thermo-mechanically affected zone (TMAZ) was not observed in the steel because the allotropic transformation hid the deformation

Acknowledgements

The authors gratefully acknowledge Petrobras (Contract Number 0050.0050438.09.9) for financial support, the Brazilian Nanotechnology National Laboratory (LNNano-CNPEM) and the Electron Microscopy Laboratory (LME-LNNano) for technical support.

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