I. Introduction
- Although traditional machining methods of Titanium alloys can generate rather intricate contours and complex shaped parts, fabrication of three-dimensional (3D-) components can often be limited, and application of some additive technologies may be required.
- In the situation when no fusion welding or brazing is possible, diffusion bonding would be a precious ally.
- The present work is focused on solid-state techniques for joining two-phase phase (+) Ti-6Al-4V (ASTM Grade 5) alloy parts, which is an essential step in designing various “plate and shell”-type heat-exchangers.
II. Specification of the industrial problem
- A system of grooves is etched at the surface of the ASTM Grade 5 alloy plates, and then two such “surface patterned” parts are bonded (“face-to-face”) together. As a result, a system of channels (a “flow path” for coolant) inside (bulk) metal substrate is created (Fig. 1).
Fig. 1: Optical images (“top view”) of the initial Ti-6Al-4V alloy parts.
- Solid-state bonding of, for example, two-phase phase (+) Ti-6Al-4V (ASTM Grade 5) alloy components is generally performed at temperatures 900-950 C, at pressures ranging from 1.3-13.8 MPa and for times of about 1 to 6 hours [1, 2].
- Solid-state bonding of, for example, two-phase phase (+) Ti-6Al-4V (ASTM Grade 5) alloy components is generally performed at temperatures 900-950 C, at pressures ranging from 1.3-13.8 MPa and for times of about 1 to 6 hours [1, 2].
- Diffusion bonding pressure is important to control properties of the joints. However, high precision of the bonded structures cannot be kept if plastic deformation is too large. For this reason, there is a considerable interest in the extension of the solid-state joining to lower diffusion bonding pressures.
III. Direct low-pressure diffusion bonding of Ti-6Al-4V (ASTM Grade 5) alloys
- Diffusion bonding of titanium alloys can be carried out in vacuum or in an inert gas atmosphere with a very low dew point using standard furnaces.
- Given very high reactivity of the Titanium, a special attention is given to the material selection in designing suitable fixtures for the positioning alloy parts during joining.
- Joining of the Ti-6Al-4V alloy components across flat mating surfaces was accomplished by diffusion bonding in vacuum at 960 ºC (below the beta-transus) under external pressure 0.6 MPa [3].
- Examination of the microstructure in the vicinity of the contact surface reveals sound bonding at the interfaces: the original contact surface between the Ti-6Al-4V alloy parts is not discernible (Fig. 2).
Fig. 2: Cross-sectional view (Back-scattered Electron Image) of the Ti-6Al-4VTi-6Al-4V sample
diffusion bonded in vacuum at 960 C for 2 hours and external pressure 0.6 MPa. The (+) -Ti-
6Al-4V alloy microstructure consists of globular crystals of -phase (“grey” contrast) in an -
matrix (“white” contrast). Note that some pores (indicated by arrows) remain at the initial contact
surface of the joint [3].
- After diffusion bonding at 960 C (high in the (+) region) for 2 hours and subsequent (vacuum) furnace cooling, the two-phase structure of the parent alloy is preserved.
- Obviously, the associated production costs can be reduced by the lowering processing temperature. In principle, this can be achieved if diffusion bonding is conducted through a transient interlayer [1].
IV. Solid-state bonding of Titanium alloy components using transient interlayer
- There are some indications that joining temperature can be reduced significantly if diffusion bonding of the Titanium alloy parts is conducted through the transient Copper- interlayer.
- Joining of the Ti-6Al-4V alloy components can be accomplished by diffusion bonding in vacuum at 840 ºC through the electrochemically deposited Cu-interlayer under external pressures well below 1 MPa (Fig. 3).
Fig. 3: Secondary Electron Image of the (a+b) -Ti-6Al-4V (ASTM Grade 5) alloy parts after
joining through transient (electrolytically deposited) Cu-interlayer (840 °C; vacuum; 1 hour) and
subsequent annealing at these circumstances for 4 hours.
- The (electrochemical) deposition of Copper on Titanium alloys provides a basis for subsequent plating with other metal.
- One might expect that lowering processing temperature will not only reduce the production costs, but also improve overall dimensional stability of the bonding assembly.
V. Concluding remarks
- Joining of the Ti-6Al-4V (ASTM Grade 5) alloy components can be accomplished by direct (without any interlayer) diffusion bonding in vacuum at 960 C and external pressures well below 1 MPa.
- After diffusion bonding under these conditions and subsequent furnace cooling in vacuum the alloy microstructure consists of equiaxed grains of -phase and intergranular , and complete recrystallization was observed through the contact surface.
- Sound Ti-6Al-4V alloy joints can be fabricated by diffusion bonding in vacuum at 840 ºC through the electrochemically deposited Cu-interlayer, which reduces production costs and improves dimensional stability of the assembly.
- Clearly, the present project can be considered as a “first step” in solving much broader issue of 3D manufacturing of metallic structures.
VI. Outlook
- It is still not clear whether such a low-pressure diffusion bonding can be performed on mating surfaces of machined parts without any prior etching
- What quality of the Titanium alloy surface is required to allow direct (without any transient interlayer) low-pressure diffusion bonding?
- Another important issue that yet to be clarified is the possible influence of thermal treatment associated with the newly developed diffusion bonding procedures on mechanical properties of the base Titanium alloy.
- Issues of surface modification and development of cost-effective technology for metal (Copper or Nickel) electrochemical deposition on Titanium alloys (Grade 2 and Grade 5) are to be addressed.
- The newly developed diffusion bonding techniques (with and without transient interlayer) for Titanium alloys are to be optimized. Integration of newly developed joining technologies in the processes of 3D- manufacturing of Titanium-based components can now be initiated.
References
[1] N. F. Kazakov, “Diffusion Bonding of Materials”, Pergamon Press, 1985
[2] M.J. Donachie, “Titanium – A Technical Guide”, ASM International, 2000
[3] M.H. Biglari, L.C.P. Krassenburg, R.P.J. Denteneer, J.H.G. Brom, A.A. Kodentsov, “Low
pressure diffusion bonding of titanium alloys”. Proc. 10 th Int. Conf. LÖT 2013, Aaachen, Germany
