A. K. Nath

Laser boronising of Ti–6Al–4V as a result of laser alloying with pre-placed BN

Abstract This paper discusses the effect of CO 2 laser alloying of pre-placed BN coating with Ti–6Al–4V alloy. The formation of titanium boride and titanium nitride investigated using energy dispersive X-ray diffraction (EDXRD) result were related to the microhardness and microstructure. The nitrogen and boron diffusion during the laser boronising process identified using secondary ion mass spectrometry (SIMS) and X-ray photoelectron spectrometry (XPS) analysis was compared with the EDXRD results. The surface hardness HV1500–1700 observed at the boronised layer was five to six times higher than that of untreated Ti–6Al–4V alloy. This was compared with needle platelet and dendrite type microstructures. Theoretically estimated surface temperature values were used to interpret the compound formation in the laser alloyed layer.

Laser Cladding of Austenitic Stainless Steel with Hardfacing Alloy Nickel Base

Abstract A nickel base alloy Colmonoy 6 was chosen as a candidate material for hard facing many components in the prototype fast breeder reactor. This alloy offers outstanding wear resistance and high hardness at elevated temperatures. Previous research using the gas tungsten arc welding process revealed that the hardness of the deposit is influenced by the dilution from the base metal up to a deposit thickness as high as 2.5 mm. The present study aims to control the level of dilution in the hard face deposits by carrying out deposition using a low heat input laser cladding process. The samples were characterised by macroscopic and metallographic examination, and microhardness measurements, SEM - EDAX and XRD analysis were used to determine their properties. The study demonstrated that by controlling laser cladding parameters, thin deposits with a low level of dilution can be laid on austenitic stainless steel substrate. Cracking of the deposit was minimised by controlling the heating and cooling rates.

Three-Dimensional Conduction Heat Transfer Model for Laser Cladding Process

Cladding is the process of depositing a superior built-up layer on a substrate by fusion. In the present study a three-dimensional conduction heat transfer model is developed and solved using the finite-volume method in a nonorthogonal grid system for a blown-powder laser cladding process. Comparisons with experimental data for deposition of copper powder on SS316 stainless steel show that the developed model can predict the geometry of the buildup layer above the substrate within an acceptable range of tolerance. The overall absorption of the CO2 laser radiation is in the range of 14–17%.

Laser power coupling efficiency in conduction and keyhole welding of austenitic stainless steel

Laser welding of thin sheets of AISI 304 stainless steel was carried out with high power CW CO2 laser. The laser power utilized in the welding process was estimated using the experimental results and the dimensionless parameter model for laser welding; and also the energy balance equation model. Variation of laser welding efficiency with welding speed and mode of welding was studied. Welding efficiency was high for high-speed conduction welding of thin sheets and also in keyhole welding process at high laser powers. Effect of pre-oxidization of the surface and powder as filler material on laser power coupling is also reported. The paper also discusses effect of microstructure on the cracking susceptibility of laser welds.

Analysis and synthesis of laser forming process using neural networks and neuro-fuzzy inference system

To apply laser forming process in reality, it is required to know the relationships between the deformed shape and scanning paths along with heating conditions. The deformation due to laser scanning depends on various factors, namely laser power, scan speed, spot diameter, scan position, number of scans, and many others. This article presents soft computing-based methods to predict deformations for a set of heating conditions, and also to determine the heating lines and heat conditions, in order to get a desired shape (i.e., inverse analysis). A novel attempt has been made in this paper to carry out analysis and synthesis (inverse analysis) of laser forming process using both genetic-neural network (GA-NN) and genetic adaptive neuro-fuzzy inference system (GA-ANFIS). During the analysis, laser power, scan speed, spot diameter, scan position and number of scans are taken as inputs and bending angle is considered as the output. A batch mode of training has been used for both the approaches with the help of some experimental data. The performances of the developed approaches have been tested on some real experimental data. Both the approaches are found to be effective to predict the bending angles and carry out the process synthesis successfully. GA-NN approach is found to perform better than the GA-ANFIS approach in predicting the bending angles, and both the approaches are able to provide comparable predictions in inverse analysis.

Operational characteristics and power scaling of a transverse flow transversely excited CW CO2 laser

Transverse flow transversely excited (TFTE) CO2 lasers are easily scalable to multikilowatt level. The laser power can be scaled up by increasing the volumetric gas flow and discharge volume. It was observed in a TFTE CW CO2 laser having single row of pins as an anode and tubular cathode that the laser power was not increasing when the discharge volume and the gas volumetric flow were increased by increasing the electrode separation keeping the gas flow velocity constant. The discharge voltage too remained almost constant with the change of electrode separation at the same gas flow velocity. This necessitated revision of the scaling laws for designing this type of high power CO2 laser. Experimental results of laser performance for different electrode separations are discussed and the modifications in the scaling laws are presented.

Effect of active flux addition on laser welding of austenitic stainless steel

Abstract The use of active flux in tungsten inert gas (TIG) welding is known to increase its weld depth. The present paper involves study of active flux laser beam welding (ALBW) of austenitic stainless steel sheets with respect to its effect on plasma plume, microstructure and mechanical properties of the resultant weldments. ALBW performed with SiO2 as the flux significantly modified shape of the fusion zone (FZ) to produce narrower and deeper welds. Plasma plume associated with the process was considerably smaller and of lower intensity than that produced during bead on plate laser beam welding (LBW). Flux addition during LBW produced thin and rough weld bead associated with humping. The development of such a weld bead is cause by reversal in the direction of Marangoni flow by oxygen induced inversion of surface tension gradient, widely fluctuating plasma plume and presence of oxides on the weld pool surface preventing free flow of the melt. Active flux laser weldments exhibited lower ductility than that of bead on plate laser weldments.

A study on laser drilling of thin steel sheet in air and underwater

In laser drilling of a thin stainless steel sheet in air with Nd:YAG laser pulses of 0.5–1 ms durations it was observed that the 0.5 ms duration laser pulse was more effective in drilling a through-hole than the relatively longer laser pulses with proportionately more energy. Further, laser drilling could be readily done when the sheet was placed at the focal point of the lens and below it but not above the focal point. On the other hand, the underwater laser drilling could be done when the sheet was placed above the focal point. An attempt has been made to explain these experimental observations considering various processes involved in laser drilling in air and underwater. While the recoil pressure of the vapor and plasma played an important role in laser drilling in air; the radial gradient of recoil pressure of evaporation, the Marangoni force induced by the surface tension gradient in melt pool and the cavitation effect of bubble collapse were believed to be responsible for the material removal in un...

Novel laser surface treatment approach to suppress sensitisation in modified type 316(N) stainless steel weld metal

Abstract Welded components are subjected to solution annealing heat treatment for achieving full stress relief and restoration of mechanical properties and corrosion resistance. During such heat treatments, optimum cooling rate has to be selected because very slow cooling rate will result in sensitisation and susceptibility to intergranular corrosion whereas fast cooling will result in reintroduction of residual stress. For 316 LN stainless steel which is welded using modified E316-15 electrodes (0·045–0·055%C), critical cooling rate above which there is no risk of sensitisation is 75 K h−1. This paper presents a novel laser surface treatment which suppresses sensitisation in weld metal, even at a slower cooling rate of 65 K h−1. Experiments involving laser surface melting were carried out with 150 W average power pulsed Nd:YAG laser and 10 kW CO2 laser, in both continuous wave and pulse modulated (100 Hz) modes. Best results were obtained when surface melting was performed with high frequency pulse modulated CO2 laser beam. The processed weld metal remained unsensitised after solution annealing followed by slower rate of cooling at 65 K h−1. Numerical simulation study was performed with ANSYS 7·0 software to understand the physical reason behind the difference in sensitisation behaviour of CO2 laser melted specimens under continuous wave and high frequency pulse modulated conditions and the predictions were validated using results of electron backscattered diffraction studies. Weld metal specimens treated with high frequency pulse modulated CO2 laser clearly showed evolution of fine grains near the fusion boundary region which enhanced sensitisation resistance.

TiC reinforced composite layer formation on Al–Si alloy by laser processing

Abstract Al–Si alloy was laser surface treated with various compositions of TiC and Ni under different laser processing parameters to produce a TiC reinforced composite layer on it. The study shows that TiC composite layer is formed when the composition is 75TiC–25Ni (wt-%) and laser power 2·5 kW and scan speed 0·2 m min−1. The layer exhibited an average microhardness of ∼750 HV, which is free from pores as well as cracks and contained TiC and Al–Si phases distributed in the Ni–Al matrix. Mechanical strength of layer is studied by the hardness test.