WO1999012694A1

WO1999012694A1 - Friction assisted diffusion welding

Info

Publication number: WO1999012694A1
Application number: PCT/CA1998/000849
Authority: WO
Inventors: Hang Li
Original assignee: Liburdi Engineering Limited
Priority date: 1997-09-09
Filing date: 1998-09-09
Publication date: 1999-03-18
Also published as: CA2212250A1; AU9057598A

Abstract

A process and an apparatus for diffusion bonding two elements together with the assistance of instant mechanical friction is disclosed in which the areas to be joined or the whole welding assembly are heated either by localized heating methods or furnace. A constant or a variable axial pressure is applied to bring the members into contact. The bonding stress produced by the applied axial pressure, constant or variable, is maintained below the compressive yield strength of the elements throughout the entire diffusion bonding temperature range. At different stages of the bonding process, instant in-situ friction treatments at the joint are conducted by means of introducing small scale relative movement between the contact surfaces of the elements. The instant in-situ friction treatment is only aimed to reduce oxidation and to overcome the oxide barrier at the joint interface to stimulate inter-diffusion and coalesence. Microscopic mixing and localized grain growth are also achieved at the joint during friction treatment. The combination of temperature, pressure, time and instant friction applied at the contact surfaces does not produce any significant macroscopic deformation of the elements. Addition of insert material at the joint interface is optional.

Description

Friction Assisted Diffusion Welding

BACKGROUND OF THE INVENTION

The present invention relates to a new diffusion welding process and an apparatus for carrying out this process. This process relies on the assistance of instant mechanical friction treatments to reduce oxidation and to overcome any oxide barrier that exists at the joint interface which in turn assists inter- diffusion and coalescence. Microscopic mixing at the bonding interface also occurs during friction treatment at holding temperature of the process.

Conventional diffusion bonding techniques include two forms of process — solid state and liquid phase. In the former, all the reactions are taking place in the solid state while in the latter, a liquid phase is formed by using an insert material at the joint interface. Diffusion bonding involved in liquid phase is also called transient liquid phase bonding. Comprehensive reviews of these processes can be found in references

[1,2,3,4,5]. Diffusion bonding processes have been generally successful in joining of aerospace materials such as Ni or Co base superalloys, Al alloys as well as Ti alloys. However, these processes highly rely on the application of controlled atmosphere, complicated joint surface treatments and flux application to overcome the oxide barrier at the joint interface that prevents the inter-diffusion, wetting and coalescence at the joint.

While the known friction welding techniques consist of rotary motion or linear reciprocating motion at the interface of tne two elements to be joined. Both techniques rely on the friction action to generate heat required to form metallurgical bond. The rotary type of friction welding is limited by its annular configuration requirement of the elements to be joined, while the linear friction welding needs the application of very high compressive pressure. Each technique produces significant amounts of plastic deformation at the joint interface. Typical examples of the know linear friction welding and rotary type of friction welding techniques are illustrated in U.S. Patents 52480'⁷7, 5551623 and French Patent 2,641,222 respectively.

Combination of the rotary motion type of friction welding method with induction heating has been disclosed in U.S. Patent 5,240,167. In this disclosure, additional induction heating is applied aiming to heat treat the elements to be joined before and/or after the friction welding operation. The process basically bears the same limitations as for any conventional rotary type of friction welding techniques which require annular configuration of elements to be joined and produces significant plastic deformation at the joint

SUMMARY OF THE INVENTION

A process and an apparatus for diffusion bonding two elements together with the assistance of instant mechanical friction at the joint interface of the two elements to be bonded is disclosed, in which the areas to be bonded or the whole welding assembly are heated either by localized heating methods or a furnace. In the stages of heating and holding operations of this process, an axial compressive force is applied and maintained to bring the members into contact and produce only localized microscopic plastic deformation at the contact interfaces . The axial force can either be a constant or a variable. If the force is a constant, then simply maintain it at a level which produces a compressive bonding stress at the joint that is slightly lower than the compressive yield strength of the elements at the holding temperature during the entire heating and holding operations. If the force is a variable, then the compressive bonding stress product must tailor the yield strength variation of the elements as the yield strength varies with temperature during heating. The variable compressive bonding stress is always maintained at levels slightly less than the compressive yield strength of the elements during heating and holding operations. At certain stages of the process, instant m-situ mechanical friction treatments at the joint are conducted by means of introducing small scale relative movement between the contact surfaces. The friction motion can either be rotary type or linear reciprocating type depending upon individual application. The instant in-situ friction treatment in this process is by no means aimed to produce friction heat at the contact interfaces to forge bonding. The friction action only serves to break the oxide layer at the interfaces to be bonded and help to bring the fresh bulk material into direct contact. The instant friction action also produces localized material mixing at the joint interface of the elements. The combination of temperature, pressure, instant friction and time applied at the joining interfaces does not produce any significant macroscopic plastic deformation of the elements. The assistance of insert materials at the joint is optional in this invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a schematic representation of a first embodiment of the friction assisted diffusion bonding apparatus according to the present invention.

Fig 2 is a partial schematic representation of a second embodiment of the friction assisted diffusion bonding apparatus according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

As illustrated in Fig. 1, the first embodiment of the friction assisted diffusion bonding apparatus utilized to bond elements 1 and 2 to each other involves placing the elements in jaws or chunks 3 and 4, respectively. One of the elements, in this case element 1, is held stationary by jaws or chunk 3, while the other element, element 2, is made movable by jaws or chunk 4 about axis 13. Axis 13 passes through the centroid of the surface S2, while axis 14 through the centroid of the surface SI. The jaws or chunk 4 is attached to a mechanical device 8 (a transmission-motor system or a mechanical vibrator also represented) through a clutch 12 and to a hydraulic pump 9.

The hydraulic pump 9 moves element 2 back and forth along axis 13 and exerts and maintains a controllable axial force F between surfaces SI and S2 during bonding operation. The mechanical device 8 is capable to provide rotation movement or linear reciprocating movement of element 2 about the axis 13 against the stationary element 1 under the action of the axial force F. Instant mechanical friction treatments to the contact area between element 1 and element 2 can be conducted by means of engaging and disengaging clutch 12. Heating is applied to the bonding assembly either by localized heating at the joint surfaces SI and S2 or furnace heating depending upon particular application. If localized heating is required, then induction heating coil 5 or optionally direct electrical resistant heating by the power supply unit 10 can be used. Induction coil 5 (or furnace 5) is connected to power supply unit 7. Power supply unit 10 with pulse capability provides better heating control. If furnace heating is adopted, then heating at the surfaces SI and S2 is no longer very localized. During heating, sensor 6 measures the temperature at the joint and sends signal to control unit 11. Sensor 6 may either be a remote type or thermal couples. Together with the sensor 6, the power supply units 7 ad 10, the hydraulic pump 9, the clutch 12, and the mechanical device 8 are all connected to the control unit 11. The core element of the control unit 11 is a desk top computer with a pre-installed database containing information of the temperature-compressive yield strength relationships of the elements to be joined and an appropriate software to control all the process variables and to coordinate the actions of each individual device involved.

Conventional diffusion bonding processes have four major process variables including temperature, time, axial pressure (usually a constant) and oxide removal. Oxide removal can be done through means of atmospheric control, flux application and other bonding surface treatment methods. The process of this invention also consists of four variables comprising axial pressure, temperature, instant friction treatment and time. The introduction of the instant friction at the contact surfaces to be bonded in this invention is capable to make the process much less dependent on the conventional methods of oxide removal. The controls of these variables together with the process sequences of this invention are described as the following:

Bonding Axial Pressure Selection - To start the process, element 2 is moved toward element 1 through the jaws or chunk 4 by the hydraulic pump 9 such that surfaces SI and S2 come into contact with each other. Contact is maintained by an axial force F through axis 13. The axial force F should produce a compressive bonding stress σ. between the surfaces SI and S2 which is slightly lower than the peak temperature compressive yield strength of the elements σ_y in the process. In case that dissimilar materials are involved, σ„ is selected from the element with the lower σ, at the holding temperature of the process. The axial pressure is maintained throughout the heating and holding periods of the process.

Bonding Temperature Control - After application of axial pressure at the contact surfaces SI and S2, heating then can be applied to the bonding assembly either by localized heating at the contact area of SI and S2 or furnace. If localized heating is required, then induction heating coil 5 or direct electrical resistant heating power supply 10 or combination of these two methods is used. Temperature at the joint is monitored by sensor β and signal sent to the control unit 11. When the temperature at the joint interface SI and SI reaches the predetermined holding temperature of the diffusion bonding process, control unit 11 then maintains the temperature in the remaining time of the process.

Instant Friction Assistance and Control - In addition to bonding axial pressure selection and temperature control, this invention incorporates a function of instant mechanical friction treatment aimed to produce a tight gap and to break any oxide barrier between the contact surfaces SI and S2. At certain stages of the diffusion bonding process, instant rotation (if device 8 is a transmission-motor system) or linear reciprocating motion (if device 8 is a mechanical vibrator) of element 2 about the axis 13 is introduced against the stationary element 1 under the action of the axial force F producing instant friction action at the contact area between surface SI of the element 1 and surface S2 of element 2. The above instant friction treatments are conducted by means of engaging and disengaging clutch 12 between the jaws or chunk 4 and mechanical device 8. A one step friction approach is to give an instant friction treatment to the contact area between SI and S2 when the joint interface reaches the holding temperature. A two step friction approach includes starting the first instant friction treatment before the commencement of the heating operation and then giving the second instant friction treatment immediately upon reaching the process' holding temperature at the joint. The first friction action is aimed to increase the microscopic plastic deformation at the joint surfaces SI and S2 such that the contact area is increased before heating. This will reduce the tendency of joint interface oxidation at SI and S2 during the subsequent heating operation. The second friction treatment is aimed to break any oxide barrier at the joint interface SI and S2 and to bring fresh bulk materials from element 1 and element 2 into direct contact so that inter-diffusion between the elements can proceed. Limited microscopic mixing between the bulk materials from element 1 and element 2 at the joint interface S1/S2 is achieved through the instant friction treatment as well as at the holding temperature. Unlike the known friction welding techniques, friction treatment in this invention is always maintained in a very limited scale and only applied instantaneously. Friction action in this process is not designed to generate heat for welding. Limited Scale of rotation or linear reciprocating movements is defined as few rounds of rotation or few cycles of vibration, respectively. Due to the nature of the instant friction treatment described in this invention, unlike the known friction welding techniques, there are no requirements that element 1 and element 2 being annular in configuration and axis 14 and axis 13 in alignment. In addition, at the process holding temperature, the required axial pressure is greatly reduced comparing with that required for linear friction welding process.

Holding Time - After the friction treatment for the contact area of surfaces SI and S2 at the holding temperature, the holding time becomes the only remaining variable left in the process. The holding time in this invention depends on the holding temperature, the axial pressure selected, and the metallurgical/physical/chemical properties of the elements involved. Sufficient inter-diffusion at holding temperature between element 1 and element 2 is essential for the formation of strong joint. Holding time can be optimized to achieve desirable mechanical properties of the joint. Meanwhile diffusion between the joint interfaces S1/S2 and the bulk materials in element 1 and element 2 also dissolves certain types of residual oxide remained in the interface region.

Due to its unique oxide barrier breaking capability, this invention has made diffusion bonding process much less dependent on atmospheric control, flux application and other bonding surface treatment methods. Al alloy 6061 and INCONEL alloy X- 750 with bonding area 3-4 cm have been successfully bonded in open atmosphere utilizing this invention. Furnace heating with a holding temperature of 450 °C and induction heating with a holding temperature of 1000-1060 °C are adopted for Al alloy addition. The instant friction treatment not only effectively breaks the oxide barriers for both alloys at the joint interface but also promotes localized grain growth in the area immediately adjacent to the bonding interface of the elements such that grains at each side of SI and S2 readily advance into one another. This is an achievement that even vacuum diffusion bonding may not be easy to accomplish. Tensile test samples of the open air diffusion bonded Al alloy 6061 frequently failed in the parent material. Under the examination of microscope, the joining interface of open air diffusion bonded INCONEL alloy X- 750 almost disappeared in just 20 minutes bonding time. The applications of the invention are particularly suitable to be extended to join certain kinds of metal matrix composites and oxide dispersion strengthened superalloys .

Depending upon the particular applications, the invention may assume two other variations.

One variation of the first embodiment, as shown in Fig. 1, comprises the possibility of variation of the axial force F applied through axis 13 such that the bonding compressive stress σ. produced at the contact area of surfaces SI and S2 always tailors the variation of compressive yield strength, σ" of the elements 1 and 2 and produces a constant σ._ ratio slightly less σ, than unity. This constant σ,_ ratio is maintained throughout the σ, heating and holding operations of the process. In case of joining dissimilar materials, σ_r is selected from the element that has a lower σ_y at the peak temperature. For a process utilizing variable axial pressure, during heating, sensor 6 continuously monitors the temperature increase at the joint and sends signal to the control unit 11 where they are compared with the database. The control unit then makes adjustment accordingly to reduce the axial force F at the joint surfaces SI and S2 through the hydraulic pump 9 such that σ»_ σ, ratio remains constant. Immediately after the temperature at the joint surface SI and SI reaches the pre-determined diffusion bonding holding temperature, the control unit 11 acts to maintain the temperature and the axial Force F through the power supply units

7 and/or 10 and the hydraulic pump 9 respectively in the remaining holding time of the process. Maintaining a constant

σ, ratio can establish a tight gap between the surfaces SI and S2 during the entire bonding operation, which in turn decreases the potential of high temperature interfacial oxidation at the joint .

The other variation of this invention utilizing the second embodiment of this invention is illustrated in Fig. 2. In this figure, elements having the same or similar function as those in Fig. 1 are denoted by these same reference numerals. As can be seen, elements 1 and 2 are to be joined together and, as in the previously described embodiment, are clamped in jaws or chunks 3 and 4, respectively. An insert material represented by element 15 is placed in between the surfaces of SI and S2. Element 15 can either be filler materials for transient liquid phase (TLP) bonding process or simply any conventional brazing filler materials in the form of foil or past. Element 15 can also be a specialized coating applied to surfaces SI and/or S2. The bonding procedures utilizing this embodiment is similar to those applied for the first embodiment illustrated in Fig. 1. Comparing with conventional brazing and transient liquid phase bonding techniques, the axial pressure and the instant friction treatment delivered at the contact area of surfaces SI and S2 described in this invention, utilizing the second embodiment, not only can squeeze out any excessive liquid insert materials, which otherwise may retain at the joint, but also be able to eliminate micro-voids formed at the joint in many solid state diffusion bonding processes. The involvement of liquid phase through the introduction of insert materials also helps to overcome any potential problem associated with lack of bonding near the edge of the joint interfaces SI and S2 in some of the solid diffusion bonding processes.

The foregoing description is provided for illustrative purposes only and should not be construed as any way limiting this invention, the scope of which being defined solely by the appended claims.

Claims

I claim:

1. In a method of friction assisted diffusion bonding of two similar or dissimilar elements of metallic materials or metal matrix composites together having the steps of:

orienting the elements such that surfaces to be diffusion bonded face each other; holding one of the elements stationary; moving the other element and bringing the elements into contact with each other; applying and maintaining an axial pressure to the surfaces to be joined such that the bonding stress produced is slightly lower than the holding temperature yield strength of the elements; applying heat to the joint region; monitoring the temperature changes at the joint; providing small scale instant mechanical friction treatments at the contact area of the elements by means of introducing rotation or linear reciprocating motion of the moveable element against the stationary element under the action of the axial pressure before heating and/or during heating and upon reaching the predetermined holding temperature; keeping the elements at the holding temperature under the action of the axial pressure after the last instant friction treatment for a desirable period of time to complete the bonding process, the invention comprising the steps of providing means of introducing small scale mechanical friction treatments, rotary type or linear reciprocating type, to the area between the contact surfaces of the elements to be diffusion bonded at different stages of the process .

2. The process of claim 1 comprising the first additional variation of possibility of applying variable axial pressure at the contact surfaces of the elements to be diffusion bonded by tailoring the variation of compressive yield strength of the elements involved during heating operation such that the bonding compressive stress applied being maintained always slightly lower than the yield strength of the elements throughout the heating and holding periods of the process.

3. The process of claim 1 comprising the second additional variation of the possibility of introducing insert material, diffusion type such as transient liquid phase bonding filler materials or non-diffusion type such as brazing filler materials, to the contact area between the surfaces of the elements to be bonded.