FIELD OF INVENTION
The present invention generally pertains to fusion welding of reactive materials. More particularly the invention relates to laser based fusion welding of reactive materials by holding in a fixture while performing the butt welding of reactive materials without contamination.
BACK GROUND OF THE INVENTION
Refractory materials like Titanium, Niobium, Zirconium, Tantalum, and Vanadium and their alloysfind applications in aerospace, defence, nuclear, etc. Most of these materials and their alloys have good weldability and can be welded by both fusion and non-fusion welding processes. However, these materials are highly reactive and have very high affinity for the atmospheric contaminants like Oxygen, Hydrogen and Nitrogen at temperatures much less than the melting point.Titanium and its alloys are highly reactive beyond 450 °C temperature, whereas, materials like Niobium& its alloys are reactive beyond a temperature as low as 250 °C. Hence, during welding, a proper shielding of the weld and heat affected zone (HAZ) will be required until the materials cool to a temperature below the reactive temperature. If not, the contamination of the weld can lead to excessive strengthening and embrittlement.
These reactive materials in the form of sheets and plates are used for many applications and can be welded by all fusion welding processes like TIG, Plasma, etc.

Laser welding, coming under a power beam fusion welding process utilizes the focused laser beam to melt and effect the coalescence of the abutting surfaces. Due to high power densities coupled with smaller spot sizes, can weld deeper with narrow fusion and at a very low heat input. Since, the fusion zone is narrow, the shielding can be simple and easy. This makes laser welding a good candidate process for welding such exotic materials. However, even though the fusion zone and HAZ is narrow, a proper and effective shielding will be required for welding reactive materials. Hence, the laser welding process can be used for such applications if suitable fixtures are used. Accordingly, a suitable fixture has been designed and fabricated for laser butt welding of sheets of such reactive materials and the details are explained.
The US patent US 2014/0305910 A1 and international patent WO 2012/044285 A1 relate to the use of fiber laser in narrow band width with Argon shielding gas in cross flow or in controlled environment around the welding region for reducing the contamination in the resulting plasma. It describes a method to shift the spectral lines of fiber laser more towards Nd: YAG laser which alters the plasma plume-Argon shield gas interaction phenomena thereby reducing the soot formation and contamination of welds.
The international patent WO 2014/114353 A1 relates to the use of a fixture with electromagnet, a nonmagnetic support and one or more clamp shoes for clamping two or more sheets for welding with electron beam, laser or plasma arc welding.

The US patent US 2013/0156493 A1 and the international patent WO 2013/090499 A1 relate to method of joining Titanium and its alloys to ferrous materials by using a Tantalum interlayer to prevent the formation of brittle Fe-Ti intermetallic phases. All the prior arts mentioned are discussing techniques like defect free welding of reactive materials by altering the bandwidth of the laser and dissimilar welding involving Titanium and its alloys or using a magnetic fixture for holding the specimens for laser welding. None of the prior arts have discussed about a fixture which is suitable for defect free and contamination free laser welding of reactive materials.
The various novel features that characterizes the invention are pointed out in the detailed description of the invention.
OBJECTS OF THE INVENTION
An object of the present invention is to develop a fixture for laser butt welding. Another object of the invention is to develop a method for providing proper inert gas shielding to protect weld and heat affected zone from atmospheric contamination.
A further object of the invention is to use the fixture for butt welding of Titanium, Niobium, Zirconium, Vanadium and Tantalum.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig. 1 - The assembled laser butt welding fixture for proper shielding of weld and
HAZ along with details of copper tubes. Fig. 2 - The details of the bottom part 1. Fig. 3 - The details of the top part 2.

DETAILED DESCRIPTION OF THE INVENTION
The sheets of reactive materials like Titanium, Niobium, Zirconium, Vanadium and Tantalum are frequently used in defence, aerospace and nuclear industries. These materials are highly reactive to the atmospheric contaminants like Oxygen, Nitrogen and Hydrogen and form brittle oxides, nitrides and hydrides respectively, the welding is carried out with proper shielding of the areas and to prevent from being exposed to temperatures above 250 °C. Laser welding characterized by very deep penetration is capable of achieving the required weld penetration in single pass at speeds much higher than conventional welding process. This produces welds with narrow fusion and heat affected zone.
Also, the fine spot sizes achievable in laser welding is making it suitable for precise welding in the desired location. Hence, laser welding is one of the good choice for welding of reactive materials with effective shielding as the area to be shielded will be less. However, laser butt welding of these reactive materials will be critical as the shielding has to be given in such a way to avoid weld contamination by either reduced shielding gas or because of turbulence.
In the present invention, a suitable fixture has been developed for laser butt welding of reactive materials without any contamination. The fixture is made in two parts, a bottom part (1) and a top part (2) with a groove/slot (7). A plate (6) with a cut out (8) at the central position is placed over the groove (7) of top part (2). A stopper plate (5) disposed over bottom part (1) abut the top part (2) from the front side such that an enclosed chamber is created in between the top part (2), bottom part (1), plate (6) and stopper plate (5).

The sheets for welding are butted together and placed over the bottom part (1). The top part (2) is placed over the sheets and fixed with the bottom part (1) by means of allen screws through the matching holes (9) and (10) by taking measures to avoid any misalignment during laser welding.
The sheets after proper placing divides the enclosed space into two halves, the top region covering the top side of the job and the bottom region covering the root side.
Copper tubes (3,4) are used for supplying shielding gas to protect the root and top side of the weld respectively.
The copper tube (4) has perforations to provide supply of inert shielding gas without any turbulence to the top side of the weld including HAZ. The copper tube 3 supplies inert shielding gas to protect the root side of the weld and HAZ at the bottom. The laser beam can be passed through the groove provided in the top part to carry out the welding of the sheets without any obstruction.
During welding, to avoid contamination of weld and HAZ by atmospheric contaminants, Argon or Helium shield gas shall be first passed through the copper tubes to purge the entire area at both top and bottom sides. Subsequently, the laser beam shall be focused on the joint location through the slot (8) provided on the top members (6) and the welding shall be accomplished.

WE CLAIM:
1. A fixture for laser butt welding of reactive materials with shield gas purging
comprising:
- a bottom part (1);
- a top part (2) disposed over the bottom part (1);
- the top part (2) having a central slot (7) for accommodating a plate (6) having a central cut out (8) for passing laser beam to job work for welding;
- a front stopper plate (5) disposed over the bottom part (1) which abut the top part central slot (7) from front side;

- pipes (3,4) are placed for providing inert/shielding gas to the enclosure;
- the top part (2) provided with 4 holes (9) at corners which matches with the threaded holes (10) of the bottom part (1) wherein the job work to be laser butt welded are placed over the bottom part (1) in between the bottom part (1) and top part (2) and then tightened with the help of alien screws through matching holes (9) and (10) of top part (2) and bottom part (1) respectively in a manner such that the job plate divides the enclosed space in two halves, the bottom space is filled with inert gas coming through pipe (3) and the top space is filled with inert gas coming through pipe (4), wherein the laser beam is sent through the cut out (8) of the plate (6) for laser welding of the job, the operation being performed in an enclosed space filled with shielding gas that prevents contamination during welding.
2. The fixture as claimed in claim 1, wherein pipe (4) is provided with perforations
for passage of shielding gas in the enclosed zone.