All DAE

DAE

Laser related activities at CAT

 
 
Dr. A. K. Nath
Head, Industrial Lasers,
Centre for Advanced Technology

 


20 kW Laser System developed at CAT

 

The importance of lasers was recognized early in India and many Indian laboratories started working on lasers in the early sixties. BARC was one of the first laboratories in India to develop a semiconductor laser, which was used for setting up an optical communication link over a distance of 20 km. Since then, the laser programme at BARC was expanded to include development of other important lasers and to take up research in areas such as non-linear optics, laser-plasma interaction etc. With the establishment of the Centre for Advanced Technology (CAT), most of the laser activities at BARC have been shifted to the new centre. CAT has now been identified by the Government of India as a premier institution for the development of lasers. It is also playing a pivotal role in co-ordination and implementation of the National Laser Programme, providing major thrust to laser technology in India.

 

CAT has been developing various types of lasers, laser related components and laser based instruments and applications of lasers to industry, medicine, and basic sciences.

 

CO2 Laser Systems:

 

Ever-since the development of carbon dioxide lasers in 1964, long strides have made towards achieving more power, better beam quality, safety of the operation and the cost effectiveness to use its immense potentials in wide variety of the applications. The availability of cheaper indigenous laser system will promote the utilization of laser potential in India. A comprehensive programme on the development of the CO2 lasers has been initiated since 1986 at the DAE’s Centre for Advanced Technology. The CO2 laser systems developed at CAT are given in the Table below.

 

20 kW transverse flow CW CO2 laser system. Beam size: 47 mm x 45 mm (Multi-mode)
Beam divergence ˜ 5 mrad
Electrical power supply: IGBT based SMPS
Electro-optic efficiency: 15 %
Deep penetration welding of thick metal plates, laser cladding of engg. components, thick concrete blocks processing for decontamination and decommissioning applications.
10 kW transverse flow CW CO2 laser system with 3 axes workstation. Beam size: 38 mm x 35 mm (Multi-mode)
Beam divergence ˜ 5 mrad
Electro-optic efficiency: 15 %
Welding of metal plates, laser cladding of engg. components, cutting of concrete blocks, laser surface modifications.
3.5 kW transverse flow CW CO2 laser system with 2 axes workstation. Beam size: 30 mm x 28 mm (Multi-mode)
Beam divergence ˜ 3 mrad
Electro-optic efficiency: 12 %
Laser cutting, laser welding, laser cladding and laser rapid prototyping applications.
High Repetition rate TEA CO2 Laser System Energy per pulse: 2 J
Maximum rep.rate: 500 Hz.
Average power at high repetition rate: 500 W
Pre-ionization scheme: Ballast free inductive.
Selective photochemical reaction studies, laser paint stripping.
Fast axial flow CO2 laser system with 5 axis workstation Laser power: 1 kW
Power supply: Radio frequency (6.5 MHz)
Various LMP, like, laser cutting, laser welding etc.

 

Laser material processing carried out at CAT:

 

End plug welding of PFBR fuel clad tube Laser welding of end plug weld of the fuel clad tube of the Prototype Fast Breeder Reactor was carried out with 3.5 kilowatt continuous wave carbon dioxide laser. This involved welding of alloy D9 (15 Cr-15 Ni-2 Mo stainless steel) fuel clad tube with end plug made of AIS 316 M stainless steel.

 

Being a non-contact process, laser welding does not generate active wastes in the form of used electrodes, and defects like tungsten inclusions, are completely eliminated. The austenitic mode of solidification associated with alloy D9 makes this alloy particularly susceptible to solidification cracking. Primary mode of solidification of the laser weld was substantially modified by effecting relatively greater degree of fusion of the end plug material with respect to that of clad tube. Ramping the laser power during welding successfully eliminated crater and associated defects appearing at the weld termination site.

 


Photomicrograph showing three layers of stellite6 on AISI 304 stainless steel specimen

 

Development of graded overlay of stellite 6 on AISI 304 stainless steel:

 

In nuclear reactors, applications of stellite cladding on austenitic Stainless Steel are employed to obtain high wear resistance at elevated temperatures in the range of 550oC. Large difference in thermal behaviour of austenitic Stainless Steel (SS) and stellite often results in build up of severe thermal stresses at substrate clad interface leading to cracking. A solution to this problem was seen in the development of stellite overlay with graded composition.

 

At CAT, graded overlay of stellite 6 on AISI 304 stainless steel was successfully developed. This graded overlaying brought about significant reduction in micro hardness gradient across substrate/clad interface with respect to that of directly cladded stellite 6 on AISI 304 SS specimen. This signified enhanced cracking resistance of graded overlaid specimen. Thermal cycling test conducted on graded overlaid as well directly stellite 6 clad stainless steel specimen demonstrated significantly superior cracking resistance of graded overlaid specimen.

 


Details of the weld of PFBR fuel pin (a), Cross-section of the laser weld (b)

 

Laser-induced synthesis of aluminides on mild steel and Inconel 600 substrates.

 

Protective coatings play an important role in extending the performance of super alloys operating at elevated temperatures. Most common type of coatings for environmental protection of super alloys is the aluminum diffusion coating, which is based on the formation of intermetallic compounds like NiAl and CoAl via a diffusion process. Their usefulness is derived from protective nature of Al2O3 scale that forms on the surface at the operating temperature.

 

The objective of the present study at CAT was to develop aluminides on the surface of mild steel and nickel based super alloy. The process involved irradiating the surface of mild steel and Inconel 600 substrates carrying composite coatings of Al/Fe/Al and Al/Ni/Al, respectively. Surface alloyed layer on mild steel substrate was found to be composed of Fe3Al and FeAl whereas in the case of Inconel 600 surface alloyed layer exhibited presence of NiAl, Ni3Al besides small amounts of Al3Ni and Al3Ni2.

 

Laser applications in decontamination and decommissioning of nuclear facilities:

 

High power lasers are emerging as potential tool for decontamination and decommissioning of nuclear facilities. Major advantages associated with process includes:-

  1. Ability to perform operation remotely.
  2. Reduced amount of active waste generation and
  3. No involvement of reaction forces thus eliminating engineering complexity.

Laser surface Cleaning: Paint stripping

 

In nuclear processing plants, paints are used to form a sacrificial layer on the surfaces of buildings and equipment in low-level radioactive environment. During decommissioning, these paints have to be removed and disposed of in a controlled manner. Current technologies for paint stripping include abrasive blasting and chemical decontamination. These methodologies generate a substantial amount of secondary waste volume. Also these methods are typically slow and labour intensive. An alternate method of paint stripping by laser ablation process has been successfully demonstrated on helicopters and military aircrafts to facilitate necessary periodic metallurgical inspections. Decontamination of radioactive surface via paint removal with lasers is now being experimented. Lasers are capable of higher decontamination rates and surface pore cleaning. Contaminated surfaces can be surgically removed without causing any melting of the underneath (which may otherwise cause further contamination due to mixing of radioactive species). The contaminants so removed are captured in a filtering system. Pulsed CO2 laser, Nd:YAG laser, excimer lasers and diode laser have been used in for this application. The overall efficiency of paint removal is the highest for pulsed CO2 laser. To remove lead based paint from different types of substrates like aluminum, steel, plywood and concrete, studies have been carried out which have shown that the short duration (~100ns) CO2 laser pulses were more efficient in removing paint than the longer (~microsecond) duration pulses.

 

Laser Scabbling, Glazing and Drilling of concrete block:

 

In many instances, contamination in concrete is confined to a few mm thick surface layers. The use of lasers to effect the removal of a concrete surface layer has been demonstrated as a useful tool in the area of decontamination. Removal of the contaminated layer would greatly reduce the volume of active concrete waste to be disposed.

 

We carried out experiments on the large-scale ablation (scabbling) and drilling holes in concrete blocks with high power CW CO2 laser. The study demonstrated that different mechanisms of material removal are operative under different regimes of laser irradiation parameters.

 


Laser glazed & mechanically scraped concrete surface

 

Laser irradiation induces spalling if the power density is about 150 W/sq cm and scans speed is in the range of 10-40 cm/min. Up to 5 mm thick surface layers of concrete were ablated in single pass. If the laser power density is above 300 W/sq cm and the scan speed is below 30 cm/min, there is a formation of the glassy surface, which can be simultaneously removed by a suitable mechanical scraper.

 

Drilling of 6 thick concrete blocks and 3 refractory bricks was carried out at an average power density of 1.2-3 kW/sq cm. Drilling efficiency was found to be the maximum when the block surface was tilted by ~ 20º with respect to the vertical direction, and the laser beam was incident normal to the surface. A 6 concrete block was cut by the hybrid process of glazing and mechanical scraping.