Microstructure evolution of diffusion welded 304L/Zircaloy4 with copper interlayer
Graphical Abstract
Introduction
Zirconium alloys Zircaloy4 and Zircaloy2 are key structural materials in nuclear reactors due to their very low thermal neutron absorption, their satisfactory mechanical properties and stability under neutron flux, and to their good corrosion resistance to high-temperature pressurized water [1], [2]. These advantages are counterbalanced by its high cost and thus the will to replace Zirconium alloys by stainless steels in component where the interaction with neutrons is less critical [3], [4]. These two materials need however to be joined together at one point, which can become an issue due to the metallurgical incompatibility between Zirconium alloys and steels.
Up to now many conventional and non-conventional welding processes were applied to join these two materials. Brutto et al. recommended the swaging process to weld tubes of great length of Zircaloy2 and steel AISI 304L [5]. D.Mukherjee et al. used TIG method for joining Zircaloy2 with stainless steel 302 [6]. The weld contained intermetallic compounds and cracks. M. Ahmad et al. used the same process for joining Zircaloy4 and steel AISI 304L and reported a high concentration of eutectic phases Zr(Cr, Fe)2, Zr2Fe and Zr2Ni in the melting zone [7]. Hairong and Bangxin examined the explosive welding of Zircaloy4 to AISI 304L steel and indicated the formation of the Zr(Fe,Cr)2 compound [8]. M. Ahmad et al. studied the connection between the Zircaloy4 and steel AISI 304L achieved by electron beam welding and showed the presence of rod-shaped Zr(Fe,Cr)2 compound and ZrCr2-liq(Zr,Fe) and Zr2Fe - ZrNi2 eutectic phases in the fusion zone [4].
The use of diffusion welding in the solid state is performed below the melting temperature of the base materials [9], [10]. The studies on direct diffusion bonding between Zircaloy4 and steel AISI 304L can lead to variable reaction layer thicknesses [9], [10], [11], [12] and to reaction layers containing several microstructurally different areas [13], [14], depending on the processing parameters. The complex microstructure observed in diffusion bonded Zircaloy4 (Zy4) - stainless steel AISI 304L (SS 304L) joints is again inherited from the coexistence of numerous phases, from solid solutions to intermetallics and Laves phases. The Laves phases in particular are expected to be the main source of crack initiation during mechanical testings due to their very high hardness. Although many studies have been undertaken, identification and kinetics formation of some phases remain controversial [11], [14]. A recent study performed on the diffusion bonding at 1323 K for 45 min holding time, indicates a ∼700 μm thick reaction layer containing many phases and revealing the presence of a transient liquid phase [15]. Based on EDX measurements, the phases were identified as the α-(Fe,Cr) solid solution, the intermetallic compound Zr2(Fe, Ni) and the Laves phase Zr(Cr,Fe)2. An eutectic Zr(Fe,Cr)2 - α-Zr microstructure was also shown. Impact tests performed on these samples revealed the embrittlement of the welds by the Zr2(Fe, Ni) compound. In all the cases presented above, the nucleation of brittle phases (either Laves-type or intermetallics) has the strongest impact on the joints mechanical properties and corrosion resistance [14]. Improving these properties requires decreasing the occurence of these compounds at the Zy4 - SS 304L interface, as well as the decrease of the diffusion zone thickness.
A common solution is to use a metallic layer to avoid the formation of embrittling phase, that is commonly copper, nickel or niobium [16], [17], [18]. This paper focuses on the use of a 50 μm copper layer to diffusion weld SS 304L to Zy4. Different samples were held from 1173 to 1323 K for 45 min. The emphasis is held on the phase identification, the mechanisms of microstructure and phases formation and the consequences on the hardness of the reaction zone of the welded specimens depending on the welding temperature.
Section snippets
Materials and methods
The nominal composition of the Zy4 and SS 304L used in this study is shown in the Table 1 . The interlayer material is pure copper. The samples to be welded are machined in the form of cylinders dimension 8 × 7 mm. The Copper layer is a 50 μm thick foil and is inserted between both base materials. To ensure the optimum planarity and an intimate contact between the base materials and the interlayer, the materials are ground and polished mechanically until grit 1200 SiC paper. Subsequently the
Influence of the holding temperature on the extent of the reaction layer, the diffusion affected zone and their hardness
The diffusion welded microstructure overviews at the SS 304L/Cu/Zy4 interfaces are shown in the Fig. 1 (a–d) on low magnification SEM BSE images and correspond to the investigated temperatures of 1173, 1223, 1293 and 1323 K during a holding time of 45 min. In all the figures, the SS 304L appears in dark grey at the bottom of the image, the zircaloy in light gray on top of the image, with the copper and the reaction zone at the center of the image. In the following, what will be called (i) the
Influence of the holding temperature on the microstructure
Table 4 summarizes the identified phases and their measured compositions depending on the holding temperature. In the diffusion-affected zones a eutectoid microstructure composed of α-Zr and C14-Fe2Zr was found in addition to the α-Zr phase for all the investigated temperatures. Table 4 shows the composition of the eutectoid mixture that is constant in Zr but is richer in Cu at low temperature and richer in Fe at higher temperature. A detailed description of the microstructure for each
Influence of copper on phase stability
Regarding only the major elements of the two base metals considered i.e. Fe and Zr, the expected phases in the investigated temperature range would be, in addition to the terminal solid solutions of Fe and Zr, the C15- Fe2Zr Laves phase, the FeZr2 phase and the liquid. The high chromium content of the stainless steel can stabilize the C36 phase at the expense of the C15 phase as shown by Yang et al. in their reassessment of the Fe-Cr-Zr system [28], while the previous studies on the diffusion
Conclusions
Diffusion-welding of 304L stainless steel with zircaloy-4 was performed with a 50 μm thick copper interlayer, in the temperature range of 1173 to 1323 K, for a constant holding time of 45 min with application of a dynamic load. The different samples were characterised in terms of microstructure and microhardnesses. The set of temperatures chosen for the study allowed the establishment of the reaction scheme between the interlayer and the base materials and has lead to the following conclusions:
- 1.
The
Acknowledgements
D. Aboudi would like to thank the Algerian Ministry of Higher Education and Research (267/PNE/ENS/2015-2016) for funding his work. The department of metallurgy of the Center for Nuclear Research of Draria, Algiers, is also acknowledged for having produced the diffusion welded samples.
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