A new method to diffusion bond superalloys

European and USA Patents pending

 

A.A. Shirzadi and E.R. Wallach

Department of Materials Science and Metallurgy

University of Cambridge

Pembroke Street, Cambridge CB2 3QZ, UK

 

Background

Nickel and cobalt-base superalloys are especially suitable for the manufacture of components to be used in high temperature applications, e.g. turbine blades and rotary disks used in gas power generation plants and aircraft engines.  Due to the extremely high cost of manufacturing, maintaining and repairing such components, the joining of superalloys has been of a major interest to the power plant and aerospace industries, and a considerable amount of research is carried out in this field.  Despite recent developments in the fusion welding of superalloys using laser or TIG welding processes, the formation of hot cracks remains a major problem.  Other joining methods, such as brazing and transient liquid phase (TLP) diffusion bonding, normally require long bonding times and/or post-bond heat treatments.  Therefore, none of the existing methods for joining superalloys has proved entirely satisfactory to and viable for design engineers.  Hence, further improvements of the existing joining methods, as well as the development of new joining approaches, are necessary in order to meet some of the more demanding requirements when joining high performance materials.

A new diffusion bonding method

Diffusion bonding is a process by which faying surfaces are brought into sufficiently close contact using an applied pressure at elevated temperature to allow bond formation by atomic interdiffusion across the joint interface.  There are several hypotheses which have been proposed to suggest how a bond is formed in a diffusion bonding process.[]  One of these hypotheses emphasises the effect of surface oxide layers on the joining process.  It was proposed that the observed differences in the weldability of various metals can be attributed to the different properties of their surface films, and hence it is assumed that all metals will bond if thoroughly cleaned surfaces are brought together within the range of interatomic forces.

Although surface oxide films can easily be removed by grinding the faying surfaces of an alloy, a new oxide layer forms immediately due to the exposure of the ground metallic surface to ambient oxygen.  The reformation of the surface oxide is virtually instantaneous for many metallic alloys including most superalloys since they contain elements with a high affinity for oxygen, e.g. Ni, Cr, Al, Co, Ti, W.  Therefore, it would be beneficial to develop a method that can remove the surface oxide and then prevent its reformation on the cleaned faying surfaces prior to bonding.

A new method for diffusion bonding nickel and cobalt-base superalloys has been developed in this work, and is based on non-chemical oxide removal prior to the bonding process.  Using this method, most of the stable oxides on the faying surfaces of the superalloy are replaced with a very thin metallic layer.  The treated faying surfaces are believed to either be virtually oxide-free or contain far less stable and less detrimental surface oxides than the original surface oxide on an untreated surface. The details of this new method are to be published when patent protection has been completed.  However, it must be emphasised that the new oxide removal method is very rapid and also neither requires the use of any sophisticated equipment nor is a costly process.

Experimental procedure

Several nickel-base superalloys, including directionally solidified DSR142, single crystal SRR99, and a cobalt-base superalloy PWA647, were used in this work.  The compositions of the alloys bonded are shown in Table 1.

 

Superalloy

Cr

Co

Mo

W

Al

Ti

B

C

Si

Zr

Others

 

Inconel 718

18.3

0.1

2.85

 

0.5

0.92

0.003

0.02

0.08

0.01

0.08Mn, 0.0004S

Inconel 738

16

8.5

1.75

2.6

3.4

3.4

0.01

0.17

 

0.1

1.75Ta, 0.9Nb

C1023

15

10

8.5

 

4.2

3.6

0.006

 

 

 

 

DSR142

8.3

13

1.5

6

3.4

 

 

 

 

 

7Ta, 1.8Hf

PWA647

23

Bal.

0

 

0

0.2

0

0.6

0

0.5

10Ni, 7W, 3.5 Ta

 

Table 1:  Chemical compositions (wt%) of alloys used in this work (balance Ni except for cobalt-base superalloy PWA647.

 

Diffusion bonding of these various superalloys was carried out after treating the faying surfaces using the new oxide removal method.  The samples were bonded in vacuum (10-4 mbar) at a temperature between 900 to 1250C and under a pressure which ranged between 3 to 10 MPa.  The precise parameters were dependent on the particular alloy being joined.  The bonding time for all samples was 1 hour.  Some of the samples were post-bond heat-treated at 1150C for 24 hours.  A few joints between dissimilar alloys were also produced to assess the capability of the new method for producing more complex or multilayer components.

Examination of bond line microstructures

Optical microscopy, scanning electron microscopy (SEM) and X-ray energy dispersive spectroscopy (EDS) were used to study the microstructures and compositions of the bond lines.  Figure 1 shows the microstructures of the bonds made in two superalloys (C1023 and PWA647) with equiaxed grains in the as-bonded condition.  The microstructures of the bond lines are very similar to those of the corresponding bulk alloys, and hence locating the bond lines proved quite difficult.  Grains generally crossed the bond line and so left only a very small indication of the initial location of the joint interface.

Having heat-treated the already bonded C1023 sample at 1150C for 24 hours, back-scatter electron SEM imaging and EDS were used to investigate any compositional variations in the bond zone compared with that of the bulk.  Figure 2 shows SEM micrographs of this sample together with the results of EDS analysis on and away from the bond line.  It is clear that there is little compositional variation, and any possible difference between the compositions of the bond line and the bulk was below the detection limit of the EDS analyser used (<0.1 wt%).

In contrast, the joint interfaces in the bonded directionally-solidified superalloy DSR142 and the single crystal SRR99 could easily be located due to differences in grain alignment or phase distribution in the pieces bonded see Figure 3.

Further examination, using SEM and EDS, of the bond in DSR142 revealed that a discontinuous phase was formed on the bond line.  There was a high concentration of heavy elements such as Ta and Hf within this discontinuous phase see Figure 4.  However, despite the presence of this phase, the bond strength still was excellent, probably due to the discontinuity of these phases.  The effect of a post-bond heat treatment on the amount and distribution of this phase will be investigated in future work.

A dissimilar joint between the nickel-base superalloy Inconel 718 and the cobalt-base superalloy PWA647 was also produced.  Finger 5 shows the microstructure of the bond line and it is clear that a substantial amount of interdiffusion across the joint interface has occurred during the one hour bonding time.  No continuous interfacial phase was observed at the bond line and further investigation using SEM is in progress.

Evaluation of bond strength

In order to evaluate the bond strengths of the samples, thin slices with thicknesses between 300 mm and 1 mm were cut and bent across the bond line.  Although quantitative mechanical testing was not carried out in this work, the results after severe bending are extremely promising.  Most of the sliced samples showed no preferential failure or even lack of ductility at the joint interface.  In fact, the slices produced from bonded samples behaved like monolithic pieces of the alloy tested.

Despite the presence of a distinguishable bond line in the nickel-base DS alloy and single crystal, their bond strengths and ductility were also comparable to those of the parent alloys.  Similarly, the dissimilar joint between the nickel and cobalt-base superalloys, Inconel 718 and PWA647 respectively, had excellent bond strength.  Figure 6 shows some bonded samples, including dissimilar joints, which withstood severe mechanical deformation without showing any failure of or preferential fracture on the bond line.

Conclusion

A new diffusion bonding method, based on removing the surface oxide prior to bonding, has been developed.  Using this new approach, diffusion bonds in nickel and cobalt-base superalloys were produced with interfacial microstructures and compositions very similar to the bulk alloys.  The required bonding time is about one hour which is substantially lower than those used in previous diffusion bonding approaches, e.g. 10 to 48 hours.  The results of severe mechanical tests of the bonded samples, including DS, single crystal and dissimilar superalloys, are very promising.  The high temperature properties of bonded samples currently are being investigated.

 

 

 

 

 

 

 

 

 

 

 

 

 

 


Figure 1. Optical micrographs of two superalloys bonded using the new diffusion bonding method (etched).  Brackets [ ] show approximate location of the bond line.


 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 


Figure 2. SEM micrograph of heat-treated bond in C1023 superalloy and the results of EDS analysis show the microstructure and composition of the bond line are very similar to the bulk alloy.  Brackets [ ] show approximate location of the bond line.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 


Figure 3. Optical micrographs of a nickel-base directionally-solidified superalloy and a single crystal bonded using the new diffusion bonding method (etched).  The bond lines are clearly visible.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 


Figure 4. SEM micrograph of DSR142 superalloy and the results of EDS analysis show the formation of a discontinuous phase (bright) on the bond line containing heavy elements such as Ta and Hf.


 

 

 

 

 

 

 

 

 

 


Figure 5. Optical micrograph of a dissimilar joint between Inconel 718 and a cobalt-base superalloy PWA647 bonded using the new diffusion bonding method (etched).

 

 

 

 

 

 

 

 

 

 

 

 

 


Figure 6. Bonded samples of various nickel and cobalt-base superalloys, including dissimilar and multilayer joints, which have been subjected to severe mechanical deformation without showing any preferential failure of the bondline.

 

 



[] Kazakov N.F, Diffusion Bonding of Materials, Pergamon Press, 1985 (English version).