FORM 2
THE PATENT ACT 1970
&
The Patents Rules, 2003
COMPLETE SPECIFICATION
(See section 10 and rule 13)
1. TITLE OF THE INVENTION:
"Deployment of Microtig Welding For EGR Cooler Application"
2. APPLICANT:
(a) NAME: TATA TOYO Radiator Ltd.
(b) NATIONALITY: Indian Company registered under
the provisions of the Companies Act, 1956
(c) ADDRESS: Survey No. 235/245, Hinjewadi,
Taluka - Mulshi, Pune-411027
3. PREAMBLE TO THE DESCRIPTION:
COMPLETE
The following specification particularly describes the invention and
the manner in which it is to be performed.

Deployment of Microtig Welding For EGR Cooler Application
Field of invention
Present invention relates to tube to tube sheet joint formation by using microtig welding method for shell and tube type Exhaust Gas Recirculation (EGR) cooler.
Background of the invention
Currently, core of Exhaust Gas Recirculation (EGR) cooler consist of welded flat tubes and formed inner fins. Specifically, material of tubes and fins is austenitic stainless steel of grade AISI 304 L. The cores are made by vacuum brazing at elevated temperature of about 1050° C. More specifically, Nickel brazing paste is used for brazing of the cores.
However, EGR core can be made with shell and tube construction by conventional TIG welding wherein the fine tuning of the heat input cannot be achieved and the probability of tube wall melting is high. Further, high input during tube to tube sheet welding widens the heat affected zone, which can results in early fatigue failure of joints during vehicle running.
Accordingly, there exists a need for autogenously welding for tubes of the core wherein there is no extra consumable used for joint formation and the welding joint is formed by pure fusion process.
Drawbacks in prior art:
1. Tube thickness is sufficiently less (0.25mm). Since fine tuning of heat input can not be achieved with conventional tig welding, probability of tube wall melting is high.
2. High input during tube to tube sheet welding widens the heat affected zone, which can results in early fatigue failure of joints during vehicle running.

Object of invention:
An object of the present invention is to deploy substitute welding source for tube to tube-sheet joint of EGR cooler which will render ease of fabrication and hence less internal process rejection.
Another object of the present invention of the present invention is to reduce early failure due to fatigue load.
Summary of the invention
Accordingly, there is provided a process of manufacturing a core of exhaust gas recirculation (EGR) cooler, the process comprising; welding a tube to tube sheet by a microtig welding process thereby forming a core of the EGR cooler having a plurality of passages, wherein welding current of the Microtig welding machine is low base current which rises steeply to a higher pulsing current depending on pulsing frequency, and drops back to the base, and wherein the thin tube section melts and solidifies quickly; and passing the welded core to a leak test, macro examination, micro-structural analysis and micro-hardness.
Brief description of drawings
Figure 1 shows a perspective view of the EGR cooler;
Figure 2 shows a cross sectional view of EGR cooler of figure 1;
Figure 3 shows a TIG pulsing operation mode;
Figure 4 & 4A shows a Microtig weld profile of a core, accordance with the
present invention;
Figure 5 shows a cross section of Microtig weld of figure 4; and
Figure 6 shows a micro-examination of the Microtig Weld.

Detailed description of invention:
The foregoing objects of the present invention are accomplished and the problems and shortcomings associated with the prior art, techniques and approaches are overcome by the present invention as described below in the preferred embodiment.
The present invention provides a process of manufacturing a core of exhaust gas recirculation by microtig welding process which renders ease of fabrication and hence less internal process rejection. Further, core made by microtig welding reduce early failure due to fatigue load.
This present invention is illustrated with reference to the accompanying drawings, throughout which reference numbers indicate corresponding parts in the various figures. These reference numbers are shown in bracket in the following description.
Referring now to figure 1-5, there shown a core of an Exhaust Gas Recirculation (EGR) cooler (100) made by Microtig welding. The Microtig welding machine is inverter based welding machine which offers welding with pulsing welding current. Welding current set at start at welding procedure is not necessarily ideal for the welding process. If the current is too low, the tube-sheet (10) of the EGR cooler (100) will not melt sufficiently and if the current is high, overheating occurs and tube of the core may melt.
In pulsing mode, low base current 1-G rises steeply to the sufficiently higher pulsing current I1 depending on pulsing frequency, drops back to the base current I-G. In this TIG pulsing (200), thin tube section melts quickly and then solidifies again quickly as shown in figure 4.

Following parameters are deployed during tube to tube-sheet welding.
1. Current Is: 12A.
2. Current I-G: 20 A
3. Current I1 :23.5 A
4. Pulse frequency: 3 Hz.
5. Current IE: 17.5 A
After welding, the welded core (20a) of the welded core assembly (20) a plurality of passages as shown in figure 4, 4A and 4 is subjected to leak test, macro examination, micro-structural analysis and micro-hardness.
Microstructure of weld metal shows interdendritic ferrites in austenitic matrix. Further, microstructure of heat affected zone (HAZ) shows absence of grain boundary carbides. Furthermore, microstructure of base metal (tube) shows no carbide along the grain boundary as shown in Figure 6.
Hardness survey of the weld metal, heat affected zone and base metal shows the following results.

Hardness HV-0.1 Kg

Location-1 Location-2 Location-3
Weld metal 226 222 221
Heat affected zone (HAZ) 217 219 217
Base metal (tube) 215 220 218

From above micro-structural analysis and micro hardness survey, it is confirmed that the heat input used during welding operation was in optimized range.
The present invention leads to deployment of Microtig welding source for core whrein Microtig welding heat input can be fine tuned since current can be varied in range of 0.5-80A.
Microtig Machine specifications are

Microtig welding machine.
Main Voltage 230V(+15%/-20%)
Current range 0.5-80 A
Open circuit voltage 85 V
Technique Pulsing mode.
The foregoing descriptions of specific embodiments of the present invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the present invention to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teaching. The embodiments were chosen and described in order to best explain the principles of the present invention and its practical application, to thereby enable others skilled in the art to best utilize the present invention and various embodiments with various modifications as are suited to the particular use contemplated. It is understood that various omission and substitutions of equivalents are contemplated as circumstance may suggest or render expedient, but such are intended to cover the application or implementation without departing from the spirit or scope of the present invention.

We Claim
1. A process of manufacturing a core of exhaust gas recirculation (EGR) cooler, the process comprising;
welding a tube to tube sheet by a microtig welding process thereby forming a core of the EGR having a plurality of passages, wherein welding current of the Microtig welding machine is low base current which rises steeply to a higher pulsing current depending on pulsing frequency, and drops back to the base, and wherein the thin tube section melts and solidifies quickly; and
passing the welded core to a leak test, macro examination, micro-structural analysis and micro-hardness.