A Review: Interlayer Joining of Nickel Base Alloys

This article provides a comprehensive review of the improvements and results in 9 interlayer bonding of nickel and its alloys to other metal alloys. The development of the interlayer 10 bonding process in joining nickel-based alloys is of high interest to the aerospace and power 11 generation industries, with the idea of on-site repair of turbine components of focus. History of the 12 diffusion bonding process has been summarized, and bonding parameters and methods for various 13 alloy combinations have been outlined. The relationship between hardness and strength, to the 14 intermetallic compounds and porosity present in the bond region has been illustrated. The literature 15 shows the methods for manipulating the volume of these compounds, and subsequent strength. The 16 paper also shows the microstructural changes that occur during interlayer bonding and how these 17 may be manipulated by changing bonding parameters and interlayer composition. Recent and 18 influential papers have been summarized, with the key findings outlined, the type of interlayer and 19 alloy/s being joined have been headlined for ease of navigation, when available, bond strengths and 20 mechanical property values have been highlighted to illustrate bond soundness. This review does 21 not concern traditional fusion welding methods. 22


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8% of operating costs for airline operators are for engine maintenance and repair, and so 26 techniques that have the potential to join high-performance alloys using relatively low energies are  primarily on metals with lower melting points, such as titanium alloys [18]. The process developed 150 at Swansea university, using powder-based interlayers to join Ti-6Al-4V, is outlined below.

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The two parts to be bonded are held in the opposing sites of a servo-hydraulic rig via collets, 152 with an interlayer attached to one of the titanium parts. The parts are brought into contact, so that 153 the faying surfaces sandwich the interlayer. To avoid oxidation, the entire bonding procedure is 154 performed in an argon chamber, which is contained in a quartz glass tube that surrounds the parts 155 to be bonded, figure 2. The initial heating is provided by a water-cooled induction coil, where 156 900˚C is reached at a heating rate of approximately 6˚C/s. However, it is important to not exceed the

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Electroplating is also used to create interlayers and is ideal when a very thin interlayer is 198 required. A disadvantage to electroplating is the restriction to only plating a single metal onto the 199 surface of one of the bonding metals [9,19]. Typical thicknesses of an electroplated interlayer sit at 200 2-10 m.

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Powder-based interlayers are still relatively new. An advantage of these interlayers is that they 202 can be made on-site with just the powder base and a few ingredients, however, they currently need 203 to be made one at a time, directly onto one of the faying surfaces immediately prior to bonding. The

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For ease of access,

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Increasing the brazing temperature to 1190-1225 ˚C led to an increased amount of liquid and 259 subsequent eutectic at the joint than that present after an equivalent holding time at lower 260 temperatures (1120-1175 ˚C); this was a deviation from predictions by TLP diffusion models.

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with thickness 40 -60 m, they were prepared using a single roller rapid solidification apparatus.

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The microstructures of the prepared interlayers were fine and homogeneous. The TLP welding

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The microstructure of the bonded joint was found to be consistent with the base metal, and the   It was concluded that further study is needed to achieve a joint with similar microstructure to

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The Ni-rich boride and binary Ni-Si eutectic were observed in the centreline of the sample bonded 306 at 1070 ˚C for 5 min. Precipitate density at the DAZ increased with increasing temperature.

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No pores and cracks were observed at the interface of the TLP joint with or without a PWHT.

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Few white precipitates were found at the centreline of the joint of the no PWHT sample, EDS 341 analysis showed these to be nickel-rich boride phases due to the low solubility of boron in the which was seen in the shear strength and hardness data, where the hardness was 50 Hv higher than the base metal, but shear strength was at 66.8% of the base metal. After the PWHT, relatively can be attributed to the increase of volume fraction of ' phase in the bonding region.

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The amorphous interlayer exhibited excellent wettability on the single crystal nickel superalloy

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Foil interlayer, SX superalloy: An investigation into the TLP bonding of a single-crystal nickel 369 superalloy using a custom interlayer was completed by Zheng et al (1993). It is notoriously difficult 370 to weld single crystals whilst retaining the majority of the material strength, so an effort was made 371 to find an interlayer material which could allow successful high integrity joining. After much 372 analysis and experimentation, the composition for the interlayer was chosen as: Ni-10Co-8Cr-4W-      between 1000-1100 ˚C, and contact forces of 5-10 kN were applied. Most bonds were performed The TLP bonding process was achieved at 1050˚C held for one hour, the bonded specimens 506 were subsequently furnace cooled to room temperature. The bond region had three distinct zones:

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The bonding process utilized a bonding pressure of 5.58 x 10 -3 MPa at a temperature of 1523K.

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These parameters meant the process was transient liquid phase, meaning the cloth interlayer 519 melted during the process. The bonding times were varied, and post-bond heat treatments were 520 used. Figure 7 shows the difference in microstructure caused by bonding time.

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The interlayer had similar composition to the base metal being joined, with a couple of         TiAl alloy to NI-based superalloy has high crack sensitiviy, thus poor weldability. This is due to the 679 formation of brittle intermetallic compounds with large differences in linear expansion coefficients.

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The formation of these compounds was greatly reduced when using the V/Cu interlayer. The  treatment was required to restore most of IN718's strength after a typical Nioro brazing cycle. The stainless steel alloy, not affected from secondary phase precipitation or dissolution, maintained its ultimate tensile strength after a typical Nioro brazing cycle. It was concluded that Nioro is the leading using a pure titanium interlayer was investigated by Cai et al (2018). A YAG pulse laser with pure 740 argon gas blown on the front and back of the specimens during welding to prevent oxidation.

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The role of the thin film multilayer was primarily to increase reactivity between the alloys, thus 756 improving diffusivity at the lower than standard bonding temperature of 800 ˚C. It was found that

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The formation mechanism of the TLP joint was summarized as follows: the interlayer metls first and the base metal adjacent to the liquid frontier dissovles subsequently. Due to microstructural

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In joining of dissimilar nickel alloys, a challenge that arises often is the difference in diffusivity of certain 819 atoms in the two alloys. This can lead to bonding dissymmetry and hardness variation across the bondline 820 [31,35]. This is typically overcome by modifying the interlayer composition, though changing the heating 821 method to a more localized induction coil setup could allow for more controlled, focused heating that may allow 822 for a temperature gradient across the two materials; meaning one could be at a slightly lower bonding temperature than the other. The formation of brittle intermetallics plagues almost every dissimilar joint, though bond performance is still improved over raw diffusion bonding via the use of an interlayer [19,33]. Due to the added complexity of joining dissimilar nickel alloys, many studies did not include mechanical testing and rely 826 only on microstructural analysis to judge the soundness of the bonds. This is understandable, as if compositional 827 analysis shows a myriad of intermetallic-forming elements across the bond interface, one can assume the bond 828 will not perform as well as its parent materials [30,34]. However, future studies should include mechanical 829 testing to help understand the degree of strength gained over raw diffusion bonding or lost compared to the 830 base alloys. The majority of dissimilar interlayer bonds utilize foil interlayers, other types of interlayer, such as 831 powder-based, should be trialed more in future studies.

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Post-bond heat treatments are common in interlayer joining of nickel-based alloys, proving to be necessary 833 in many of the studies noted here, in both similar and dissimilar joining. Although not particularly an issue in 834 laboratories, PBHTs would make on-site repairs of nickel-components much more challenging, due to the long 835 treatment times. As this is one of the most promising factors of interlayer bonding, the hurdle that is PBHTs may 836 need to be overcome for the process to achieve its potential. Clever manipulation of the interlayer material and 837 bonding parameters can optimize the joining process to remove as many of the brittle intermetallics that form 838 to disrupt the mechanical properties [21,25]. Commercially available interlayers are being used in many 839 investigations, showing an increase in demand for the benefit of interlayers for joining nickel alloys; these 840 interlayers also act as great bases for one to build/ customize a more specific interlayer material.

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Processes utilizing more specialized methods such as nano-sputtering interlayers have shown good joint 842 quality [34], but the development of more accessible and stable interlayers is perhaps more beneficial for the