Title of Invention

AN IMPROVED PROCESS OF PREPARING METAL ARTICLES, SUCH AS FROM COMMERCIALLY PURE TI AND STAINLESS STEEL WITH DIFFUSION BONDED JOINTS

Abstract Hereto fore difficulties were encountered in preparing bonded joints between dissimilar metals, e.g. Ti and stainless steel. The present invention provides a process for overcoming the above difficulties which comprises preparing and/or treating the mating surfaces to be joined to achieve maximum contact area by polishing the mating surfaces to a mirror-like finish, of the order of 0.2 - 21 Ra/m, followed by cleaning the said mating surfaces to make them free from adhering foreign particles, moisture, oil, grease or such other deleterious substances and bringing the mating surfaces together to intimate contact, the said process being characterised by - i) increasing the temperature of the interface to the desired level, in the rang(c) of between 750°C and 1000°C; preferably between 810°C and 850°C; ii) applying a controlled pressure of between 1 to 8 Mpa, preferably between 3 and 5 Hpa, to the components, thus pressurising the interface to achieve proper diffusion; iii) performing steps (i) and (ii) above under controlled atmosphere, usually under a vacuum or between 10-2 and 10-6 mbar, preferably between (2-6) x 10-4 mbar; iv) holding the interface at the prescribed temperature and pressure under controlled atmosphere for a specified period of the normally.
Full Text The present invention relates to an improved process for preparing metal articles, such as from commercially pure Ti and stainless steel, with diffusion bonded joints. In this specification, the terms "commercially pure Ti" and "diffusion bonded" have been designated as "CP-Ti" and "DB", respectively, for the sake of convenient.
More particularly, this invention pertains to a novel process for joining tubes, plates, bars, rods and like by diffusion bonding, e.g. between tube and tube, tube and tubesheet and plate to plate, between similar and / or dissimilar metals, which improves the characteristics and performance of the joints as compared to conventional joining techniques. The subject invention is capable of producing DB joints between CP-Ti and 304L stainless steel of 225-250 Mpa strength and 6-9% ductility, a feat not hitherto achieved.
In the known art, fabrication of engineering structures and articles by joining of components are usually achieved by mechanical fastening, welding or chemical bonding (using adhesives). Of these, the last named process finds its use limited to applications where both the load and temperature criteria of the joint are not critical, and ease and cost of joint preparation is of primary concern.
Of the former two, mechanical engineering fastening is limited to a number of factors, namely configuration and accessibility of the joint; load bearing capacity of the joint; temperature service of the joint; sealing of the joint under a given pressure differential existing, between the one and the other side of the joint, and most importantly, cyclic variations of one or more of the above factors, subjecting the joint to vibrating load, leading to fatigue. All the above limitations jointly or severally, may render mechanical fastening impossible.
These two processes may lead to changes in microstructures and mechanical properties of base material at the joining portion in such a fashion that the tendency of premature failure of the component may be enhanced in service condition. The problem is more severe for dissimilar metal -joints when they are physically and metallurgically
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incompatible.
The process of welding, which has replaced and is replacing mechanical fastening largely overcomes the above constraints counter indicative to mechanical fastening. Moreover, welding offers a wide variety of choices, ra'nging from gas welding, to shielded metal arc welding (SMAW), gas - metal arc welding (GMAW), submerged arc welding (SAW), electrical resistance welding, pressure welding, plasma welding and other techniques, from among which the process best suited to make the joint may be selected. However, welding suffers badly from one drawback-the problem of weldability of materials high metal content leads to changes in microstructure and mechanical properties at the joint and heat affected zone (HA2), enhancing the tendency of premature failure. The problem is also pronounced in welding of dissimilar metals. Generally, higher the content of carbon and other alloying elements in steel, greater is the difficulty in welding it, and also welding of steel to other metals like titanium etc. remains a difficult proposition even to this day.
Following difficulties are consequently encountered in making a joint between two materials of "low weldability", by conventional welding technique :
1. Bond strength and ductility is extremely poor,
2. Contamination in the welding zone occurs which makes the joint friable;
3. Distortion of the parent metal is enormous, which often defeats the very purpose of making the joint, or a higher production cost has to be incurred to overcome the problem through expensive after-machining;
4. Mechanical properties of the weld zone, and to a lesser extent the heat affected zone (HAZ) differ widely from that of the parent metals;
5. Corrosion resistance is not satisfactory;
6. Leak tight welds under the rated pressure service cannot be easily produced;
7. Even after lot of effort and cost involvement, a serviceable welded joint may be possible in limited cases, the life of such a joint is often substantially shorter than desired.
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Joining of dissimilar materials in most of the cases is a severe problem when both of them are metallurgically incompatible i.e, they have limited solid solubility at room temperature and form brittle intermetallic compound at the processing temperature. Mechanical fastening leads to cracking and distortion at the joint portion due to stress generation, whereas fusion welding of the two, develops segregation of brittle intermetallics, micro-cracks and distortion in shape and size of the material. All these may enhance the possibility of premature failure of the component in sen/ice.
In the light of above constraints, with the advent of advancement of science and technology, newer joining techniques have become the crying need for joining various materials of different shapes and configurations hitherto not possible or feasible using conventional processes. Diffusion bonding has emerged as a potential joining technique for a wide range of metals and alloys, and particularly dissimilar materials. In diffusion bonding (herein termed as DB) process, joining of similar or dissimilar materials is carried out in solid state or with the formation of a transient liquid phase where coalescence of faying surfaces is produced by diffusion of parent materials at elevated temperature and pressure for a definite time period with or without the use of interlayer under controlled atmosphere.
In this technique the mating surfaces are prepared to achieve maximum contact area (~85%). Subsequently they are brought together to intimate contact at high homologous temperature and moderate pressure under controlled atmosphere. Creep and diffusion processes help to transport atoms to the void surface from adjacent areas and reduces inrterfacial void volume. When sufficient time is allowed the voids will be removed and an atom bond across the original interface will result. DB has not yet found broad application areas due to economic reasons though it can be complementary to fusion welding in making complex joints and in some cases this technique is indispensable to achieve adequate strength and toughness in the bonded joints for their use in service condition.
The principal object of this invention is to provide a process to overcome the shortcomings of the traditional / conventional procedure in joining of similar and / or dissimilar metals and / or alloys.
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A further object of this invention is to provide a process for producing DB joints of better physical properties and better suitability for given service as compared to traditional / conventional procedures.
A still further object of this invention is to provide a process for producing DB joints, which involves transporting of atoms from one component to another by creep and diffusion process, resulting in an atom - to -atom bond at the interface.
Another object of the subject: invention is to provide a process for producing DB joints with longer service life as compared to traditional / conventional processes.
Yet another object of this invention is to provide a process for producing DB joints which are leak - tight at the required temperature and pressure.
To over come the aforesaid difficulties of the prior art, attempts have already been made to produce DB joints between CP-Ti and 304 Stainless Steel. As Ti and Fe are not completely soluble in each other in solid state and forms a number of brittle intermetallics, hence this prevents producing diffusion bonded joints of CP-Ti and Stainless Steel satisfactorily to use them as a structural component.
The present invention resides in an improved process for preparing metal articles with diffusion bonded joints from commercially pure Ti and stainless steel comprising -preparing and / or treating the mating surfaces to be joined to achieve maximum contact area by polishing the mating surfaces to a mirror-like finish,"of the order of 0.2-21 Raµm, followed by cleaning the said mating surfaces to make them free from adhering foreign particles, moisture, oil, grease or such other deleterious substances and bringing the mating surfaces together to intimate contact, the said process being characterised by -
i) Increasing the temperature of the interface to the desired level, in the range of between 750°C and 1000°C; preferably between 810°C and 850°C;
ii) applying a controlled pressure of between 1 to 8 Mpa, preferably between 3 and 5 Mpa, to the components, thus pressurising the interface to achieve proper diffusion;
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iii) performing steps (i) and (ii) above under controlled atmosphere, usually under a vacuum of between 10-2 and 10-6 mbar, preferably between (2-6) xl0-4 mbar;
iv) holding the interface at the prescribed temperature and pressure under controlled atmosphere for a specified period of time normally ranging from 60 to 150 minutes, preferably 90 to 120 minutes, to achieve diffusion bonding;
v) lowering the temperature of the samples to ambient under vacuum and then releasing the vacuum to bring pressure to ambient, resulting in the article with diffusion bonded joint of adequate strength and serviceability.
The following experiment describes in detail a process whereby commercially pure Titanium (CP-Ti) bars and tubes were joined with stainless steel (SS304L) by diffusion bonding process to yield a serviceable joint:
Experiment - 1
Two bars, one of CP-Ti and the other of SS304L material, each of a length of around 100 to 120 mm, and diameter 20 to 25 mm were taken for producing a diffusion bonded joint. One end of each bar, or rod was surface ground and polished to a mirror like finish, of the order of 0.4-10 Ra µm. The bars were then placed end to end, with the polished ends in contact, under a vacuum of 10-5 mbar. Next the temperature of the joint was raised to around 970°C, and subsequently an axial pressure of 6Mpa was applied to the bars, thus pressurising the joint. The arrangement was held under these conditions for a period of around 90 minutes, and cooled clown to a temperature of 150°C under vacuum
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before releasing the vacuum and cooling the joint to ambient temperature. The properties of
the resultant joint is detailed in the table below:
Table-1
Strength of a DB joint between CP-Ti and SS304Lbars:
Tensile strength 210-225 Mpa
Ductility 6 - 9%
It is to be noted that achieving properties anywhere near to those fisted above, using conventional / traditional methods would not have been possible to date.
The above experiment, with virtually the same parameters and sequence, has been successfully performed in making a DB joint between tubes of CP-Ti and SS304L, each having an outer diameter of 25mm and inner diameter of 21 mm, with substantially the same result.
A DB joint between CP-Ti and SS304L has been a particular case of study because this combination finds extensive uses inter alia e. g. in the fields of manufacturing cryogenic heat exchangers and drilling wells. Though presently CP-Ti and SS304L is presently welded using Tungsten Inert Gas (or TIG) method with a lot of precautions, the physical and chemical properties of such joints leave a lot to be desired. Joining by DB method, therefore, holds a key to this problem, hitherto not property solved. The method may be extended to other combinations of metals and alloys, which would enable joining combinations either impossible or unsatisfactory till date.
The scope of the above experiment can be further extended in achieving different joint configurations. For example the joint may be made a lap joint, instead of the 'butt joint described in the experiment. One of the members may have a straight or tapered internal bore or hole, into which the second element, having an outer surface finished to a 'press fit tolerance, or a matching taper as the case may be, is allowed to contact, The DB joint may then be produced between the components in the same manner described in the
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above experiment. Such arrangement may be adapted successfully for tube-to-tubesheet joining of heat exchanges.
A yet further extension of the process may be in producing butt or lap DB joints between two flat surfaces, e.g. plates or rolled sections. The process in each of the cases, however, should substantially be the same as described in the earlier experiment.
Advantages of DB joints as compared to traditional / conventional joints are listed as follows :
1. DB joints, as illustrated in the case of the joint between CP-Ti and SS304L illustrated in the experiment, have been found to have adequate strength and ductility for using them comfortably in service conditions;
2. Properties of DB joints are uniform and reproducible, with negligible scatter;
3. The joints have been produced without any micro-cracks, segregation and intermetallics formation;
4. More than one joint at a time, as in the case of tube-to-tubesheet welding can be possible using the DB process, increasingly the productivity; and making the process cost-effective.
5. Parent material distortion is eliminated;
6. Need of post bonding heat treatment is eliminated.
7. Leak-tight joints can be produced repeatedly and continuously;
8. The invented bonding technique can join 100 to 120 mm length with 20 to 25 mm dia. Solid rod. This technique can also be applied in case of tube having same length with external diameter 25mm and internal diameter 10mm. With the help of this technique plate of rectangular cross-section
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We claim:
1. Art improved process for preparing metal articles with diffusion bonded joints from
commercially pure Ti and stainless steel comprising - preparing and / or treating
the mating surfaces to be joined to achieve maximum contact area by polishing
the mating surfaces to a mirror-like finish, of the order of 0,2 - 21 Ra/m, followed
by cleaning the said mating surfaces to mate them free from adhering foreign
particles, moisture, oil. grease or such other deleterious substances and bringing
the mating surfaces together to intimate contact, the said process being
characterised by -
i) increasing the temperature of the interface to the desired level, in the range of between 750°C and 1000°C; preferably between 810°C and 850°C;
ii) applying a controlled pressure of between 1 to 8 Mpa, preferably between 3 and 5 Mpa, to the components, thus pressurising the interface to achieve proper diffusion;
iii) performing steps (i) and (ii) above under controlled atmosphere, usually under a vacuum of between 10-2 and 10-6 mbar, preferably between (2-6) x10-4 mbar; iv) holding the interface at the prescribed temperature and pressure under controlled atmosphere for a specified period of the normally ranging from 60 to 150 minutes, preferably 90 to 120 minutes, to achieve diffusion bonding; v) lowering the temperature of the samples to ambient under vacuum and then releasing the vacuum to bring pressure to ambient, resulting in the article with diffusion bonded joint of adequate strength and serviceability,
2. A process as claimed in Claim 1, wherein a DB joint is prepared between two
similar or dissimilar materials maintaining the sequence and parameters as in
Claim 1.
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(4 x10mm2 to 8 x 15mm2) can also be joined, when the total length of assembly becomes 60 to 100mm. This invention also rendered possible tube to tube and vessel to tube bonding , where the former is made of CP-Ti and the latter is Stainless Steel.
Application areas :-
(i) Cryogenic heat exchanger
(ii) Processing units of nuclear industries to treat the spent fluid for reclaiming
nuclear fuels like uranium and thorium, (iii) As helium vessel surrounding the accelerator for the accelerator production of
tritium super-conducting cavity, (iv) Accessories of drilling wells in chemical industries.
While the invention has been described in detail and with reference to the specific
embodiments thereof, it will be apparent to one skilled in the art that various changes and
modifications can be made therein without deviating or departing from the spirit and scope
of the invention. Thus the disclosure contained herein includes within its ambit the obvious
equivalents and substitutes as well.
Having described the invention in detail with example, it will now be more particularly defined by means of claims appended hereinafter.
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3. A process as claimed in Claim 1 and 2, wherein two similar or dissimilar materials
joined using DB process may be of any shape (bar, rod, tube, plate, cast, forged,
rolled), size or configuration.
4. A process as claimed in any of the above Claims, wherein multiple joints
between two similar or dissimilar materials, e.g. in a tube-to-tubesheet joining
configuration, may be prepared using DB process.
5. A process as claimed in any of the preceding Claims wherein the DB joints
between two similar or dissimilar materials may be lap, but, fillet, or of other such
configuration;
6. A process as claimed in Claims 1 and 3 above, wherein the DB joints between the male and female parts may be obtained by either a press-fit, or a matching taper, or a step-bore assembly, or combinations thereof.
7. A process as claimed in any of the preceding Claims, wherein a DB joint may be obtained between two materials optionally with a separate interlayer in between.
8. A process as claimed in Claims 1 and 2, wherein mating surfaces of the
constituent members to be attached by DB process are polished to a mirror like
finish of the order of 0.4 to 10 Raµm, the polished parts placed in end-to-end
contact under a vacuum of 10'5 mbar, then temperature of the joint is raised to
around 970°C and an axial pressure of around 6 Mpa is applied thereto, holding the arrangements for a period of around 90 minutes, followed by cooling to power temperature under vacuum, thereafter releasing the vacuum and An improved process for preparing metal articles from commercially pure Ti and Stainless Steel, with diffusion bonded joints, substantially as herein described
and exemplified.
Dated this 28th day of March, 2001
Dr. A. Basu Registered Patent Agent
&
Attorney for the Applicants
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Hereto fore difficulties were encountered in preparing bonded joints between dissimilar metals, e.g. Ti and stainless steel.
The present invention provides a process for overcoming the above difficulties which comprises preparing and/or treating the mating surfaces to be joined to achieve maximum contact area by polishing the mating surfaces to a mirror-like finish, of the order of 0.2 - 21 Ra/m, followed by cleaning the said mating surfaces to make them free from adhering foreign particles, moisture, oil, grease or such other deleterious substances and bringing the mating surfaces together to intimate contact, the said process being characterised by -
i) increasing the temperature of the interface to the desired level,
in the rang(c) of between 750°C and 1000°C; preferably between 810°C
and 850°C; ii) applying a controlled pressure of between 1 to 8 Mpa, preferably
between 3 and 5 Hpa, to the components, thus pressurising the
interface to achieve proper diffusion;
iii) performing steps (i) and (ii) above under controlled atmosphere,
usually under a vacuum or between 10-2 and 10-6 mbar, preferably
between (2-6) x 10-4 mbar;
iv) holding the interface at the prescribed temperature and pressure
under controlled atmosphere for a specified period of the normally.

Documents:

00183-cal-2001 abstract.pdf

00183-cal-2001 claims.pdf

00183-cal-2001 correspondence.pdf

00183-cal-2001 description(complete).pdf

00183-cal-2001 form-1.pdf

00183-cal-2001 form-18.pdf

00183-cal-2001 form-2.pdf

00183-cal-2001 form-3.pdf

00183-cal-2001 pa.pdf

183-cal-2001-granted-abstract.pdf

183-cal-2001-granted-claims.pdf

183-cal-2001-granted-correspondence.pdf

183-cal-2001-granted-description (complete).pdf

183-cal-2001-granted-examination report.pdf

183-cal-2001-granted-form 1.pdf

183-cal-2001-granted-form 18.pdf

183-cal-2001-granted-form 2.pdf

183-cal-2001-granted-form 3.pdf

183-cal-2001-granted-letter patent.pdf

183-cal-2001-granted-pa.pdf

183-cal-2001-granted-reply to examination report.pdf

183-cal-2001-granted-specification.pdf


Patent Number 194764
Indian Patent Application Number 183/CAL/2001
PG Journal Number 30/2009
Publication Date 24-Jul-2009
Grant Date 09-Sep-2005
Date of Filing 28-Mar-2001
Name of Patentee DR. SUBRATA CHATTERJEE
Applicant Address METALLURGY, B. E. COLLEGE (D.U) HOWRAH
Inventors:
# Inventor's Name Inventor's Address
1 MAINAK GHOSH METALLURGY B. E. COLLEGE (D. U.) HOWRAH-711 103
2 DR. SUBRATA CHATTERJEE METALLURGY, B. E. COLLEGE (D.U.) HOWRAH-711 103
PCT International Classification Number B23K 20/02
PCT International Application Number N/A
PCT International Filing date
PCT Conventions:
# PCT Application Number Date of Convention Priority Country
1 NA