The present invention relates to a method of diffusion bonding of joints of two
dissimilar metallic materials like titanium and stainless steel without the use of any
interlayer. The bonding was achieved in two step processes namely 1) stage of liquid
phase formation at the interface for short duration at high temperature and 2) solid
state diffusion stage where the joint is kept at lower temperature for longer duration.
BACKGROUND OF THE INVENTION
Dissimilar material combination between commercial pure titanium and 304L stainless
steel are widely applied in chemical processing, aerospace and nuclear industries due to
their attractive properties. Generally, diffusion bonding process is employed for the
joining this dissimilar combination. Diffusion bonding is an extremely valuable technique
in joining components, in many industry, and involves the pressing together of heated
components so that the atoms in the components inter-diffuse to form a metal to metal
bond. When the titanium material is directly bonded to stainless steel during the
welding process, many TiFe and TiFe2 metallic compounds are formed in the weld joint
because the solubility of Ti and Fe is very small. TiC which was hard and brittle is also
formed because Ti is a strong carbide forming element. On the one hand, the formation
of TiFe, TiFe2 , and TiC results in brittleness of the weld joint; on the other hand, a

large internal stress is formed because of a large difference of linear expansion and
heat transmission coefficient between titanium and steel, which leads to a bonding
crack. With use of thin interlayer (thickness 10-30 microns) such as copper, nickel or
silver, indirect diffusion bonding can be achieved without the formation of harmful
phases. Generally, mirror like surface finish prepared through strenuous metallography
method and perfect flat surface preparation is necessary in order to achieve intimate
interfacial contact. In order to avoid these complications, this invention discloses the
direct bonding between the dissimilar materials without any interlayer. Moreover,
medium rough polishing using 80 grit emery paper is sufficient. In this direct diffusion
bonding process, titanium, which have strong tendency for the formation of surface
oxide layer tends to dissolve in the metal during rapid heating and can be readily
diffusion bonded to stainless steel in two stage process.
PRIOR ART
US patent US5221039 discusses a method of a liquid phase diffusion bonding using an
insert material such as B, C, Si and Hf having a high diffusivity and a melting point
higher than that of the base metal Iron base alloys and nonferrous alloys.
US patent US4331286A relates to methods for pressure diffusion bonding of dissimilar
metals or the same metal by utilizing eutectic reaction.

China patent CN105200269A discloses the use of tailor made interlayer alloy in
instantaneous liquid phase diffusion bonding of nickel based high temperature alloy
without any harmful precipitation phase.
JPH06234082A shows a liquid phase diffusion bonding method using an insert material
having a melting point higher than that of a base metal, and the insert material is not
melted but the insert material and the base metal are reacted with each other at an
appropriate bonding temperature in such a manner that the diffusion bonding can be
carried out.
US patent US4700881A discusses the transient liquid phase bonding process through
the use of multiple boronized interlayer foils to eliminate the porosity problem in nickel
base super alloy material.
US6427904B1 discloses the application of thin pure aluminium/Cr on the surface of
dissimilar material by electro plating or PVD technique to prevent oxidation at the
interface during press or roll bonding for use in material which are difficult for bonding
process.
OBJECTS OF THE INVENTION
Therefore, it is an object of the invention to propose a method of diffusion bonding of
joints of two metallic dissimilar materials like titanium and stainless steel which is
capable of bonding the said dissimilar materials without the use of any interlayer.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING
Figure 1 - Specimen prepared from stainless steel and commercial pure titanium
Figure 2 - Stitched macrograph of the joint at the interface
Figure 3 - SEM–EDS spot scan analysis at four location of the diffusion interface
Figure 4 - Plot of load (kgf) versus extension (mm) for tension test conducted
Figure 5 - SEM-EDS line scan analysis at the diffusion interface
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE
INVENTION
This invention relates to a method of bonding the dissimilar material combination of
commercial pure titanium and 304L stainless steel. Joining of particular material
combination by conventional fusion welding creates number of brittle intermetallic
phase on the both side of the fusion zone and has high tendency to crack. Alternately,
diffusion bonding widely applied for the various critical joints is employed for this
combination. Cylindrical specimen with 10 mm diameter and 45 mm length extracted
from both materials and contact surface is made flat and rough polished by
metallographic method by 80 grit paper. Then the surface is cleaned using acetone
solution. Using the percussion welder, 0.15 mm thick Type K thermocouple was welded
to the edge of the stainless steel specimen for measuring and recording the

temperature. Then, both the specimens are kept in contact without any interlayer in
thermo mechanical simulator with contact pressure of 1.5 MPa and required bonding
was achieved in two stage processes. In stage 1 portion, the specimen is heated at the
rapid heating rate of 200°C/s to the peak temperature of 1150°C using copper grips
with minimum free span in an inert argon atmosphere to achieve the required fusion
showing mild liquid metal squeeze out at the interface. In the stage 2 part, the same
specimen is heated at lower temperature of 850°C with a heating rate of 1oC/S for the
soaking period of 90 minutes to activate the diffusion of atoms and gain the strength of
the joint. Tensile test conducted on the joint shows the ultimate tensile strength of 301
Mpa, yield strength of 159 Mpa and elongation of 1.5 - 2%. The SEM image shows the
four distinct regions in the micrograph namely 1) austenitic grain structure portion 2)
interface region having width of 7-8 microns 3) bcc beta titanium phase and 4) alpha -
beta titanium phase.
The SEM image shows the four distinct regions in the micrograph (Figure 3). Region
001, belongs to stainless steel part and showing austenitic grain structure and region
002 corresponds to the interface region having width of 7-8 microns. Below 882°C,
commercial pure Titanium has a hexagonal close packed (alpha phase) structure and
undergoes allotropic transformation at 882°C to a bcc structure (beta phase). On the
contrary, 304 stainless steel does not show any phase transformation up to its melting
point. Region 002 close SS fusion side shows 67Fe-0.9 Ti-19.7Cr-1.9Mn-9.3Ni beta

titanium phase. It has been observed that region 003 contains substantial amount of
strong beta stabilizers like iron and chromium. So, high temperature body centered
cubic (bcc) structure of titanium has been retained even at room temperature. The
width of region 003 is nearly 30 microns and has chemical content of 10Fe-86Ti-
3.2Cr.During cooling period after bonding process, beta phase transforms to aggregate
of alpha-beta phase of acicular morphology in area in the region 004.
In a distance of 190 microns (0.19 mm), chemical composition was line scanned at 25
locations at an equal interval of 7.6 microns (Refer Figure 5), and considerable
concentration of Fe, Cr and Ni are seen in the titanium side, whereas the small amount
of titanium was seen in stainless steel portion.

WE CLAIM
1. A method of diffusion bonding of joints of two dissimilar metallic materials like
titanium and stainless steel without the use of any interlayer, the said method
comprising the steps of;
arranging cylindrical specimen of pre-determined diameter and length of two
dissimilar materials titanium (T) and stainless steel (S);
making contact surface flat and rough polished by metallographic method with
80 grit abrasive paper;
cleaning the contact surfaces with acetone solution;
welding 0.15 mm thick type K thermocouple (K) to the edge of the stainless steel
specimen (S) for measuring and recording the temperature;
keeping both the specimens (T, S) in contact without any interlayer in thermo
mechanical simulator to achieve required bonding;
wherein,
the required bonding is achieved in two stages, wherein in first stage, the
specimen is heated at the rapid heating rate of 200oC/s to a peak temperature of
1150oC with contact pressure of 1.5 MPa with copper grips with minimum free span
in an inert argon atmosphere to achieve the required fusion when mild liquid metal

squeeze out at the interface, wherein in second stage, the specimens are heated at
lower temperature of 850oC with a heating rate of 1oC/s and soaking for a period of
90 mns to activate the diffusion of atoms and gain the strength of the joint.
2. The method as claimed in claim 1, wherein the joint of two dissimilar materials
achieves a tensile strength of 301 MPa, yield strength of 159 MPa and elongation of
1.5-2%.
3. The method as claimed in claim 1, wherein pre-determined diameter and length of
specimens of two dissimilar materials (S, T) are 10 mm and 45 mm.