Articles written in Sadhana
Volume 35 Issue 4 August 2010 pp 493-511
A pulse Nd: YAG laser with pulse duration 5–10 ns, beam radius at focal point 0·2–0·4 mm, wavelengths 1064 nm, 532 nm and 238 nm with linearly polarized radiation and Gaussian beam proﬁle, was impacted on a thin foil of titanium metal for generating plasma plume. Numerically, the above parameters were linked with average kinetic energy of the electrons and ions in the laser-induced plasma. In the present model, electrons having higher velocities are assumed to escape from plasma, that forms a negatively charged sheath around the plasma. It is seen from present computations that the forward directed nature of the laser evaporation process results from the anisotropic expansion velocities associated with different species. These velocities are mainly controlled by the initial dimension of the expanding plasma. An attempt was undertaken to estimate the length of the plume at different ambient gas pressures using an adiabatic expansion model. The rate of the plasma expansion for various Ar+ ion energies was derived from numerical calculations. A numerical deﬁnition of this plasma includes events like collisional/radiative, excitation/de-excitation and ionization/recombination processes involving multiples of energy levels with several ionization stages. Finally, based on a kinetic model, the plasma expansion rate across the laser beam axis was investigated.
Volume 35 Issue 5 October 2010 pp 609-618
The study of keyhole (KH) instability in deep penetration laser beam welding (LBW) is essential to understand welding process and appearance of weld seam defects. The main cause of keyhole collapse is the instability in KH dynamics during the LBW process. This is mainly due to the surface tension forces associated with the KH collapse and the stabilizing action of vapour pressure. A deep penetration high power CW CO2 laser was used to generate KH in mild steel (MS) in two different welding conditions i.e. ambient atmospheric welding (AAW) and under water welding (UWW). KH, formed in case of under water welding, was deeper and narrower than keyhole formed in ambient and atmospheric condition. The number and dimensions of irregular humps increased in case of ambient and under water condition due to larger and rapid keyhole collapse also studied. The thermocapillary convection is considered to explain KH instability, which in turn gives rise to irregular humps.
Volume 36 Issue 2 April 2011 pp 251-265
In the present study, the vorticity of melt motion in the keyhole and weld pool has been evaluated in case of high power CO2 laser beam welding. The circulation of vorticity is obtained as a function of Reynolds number for a given keyhole volume which is linked to Mach number variation. The shear stress and thermal ﬂuxes present in the turbulent pool are linked to diffusivity and Prandtl number variation. It was shown that below a critical value of Rayleigh number, the conduction mode of melt transfer signifying beam absorption becomes dominant. Above this value, convective heat transfer indicates melting and evaporation occurring in the weld pool during laser welding. The evaporative recoil pressure expels the liquid while surface tension and hydrostatic pressure help to retain the melt in the keyhole cavity in this high power laser beam welding. The understanding of several hydrodynamic phenomena occuring in the weld pool is valuable not only for understanding basic mechanistic aspects but also for process optimization involved in laser beam welding.
Volume 38 Issue 2 April 2013 pp 235-246
Fluid ﬂow mechanisms present in Keyhole (KH) during Laser Beam Welding (LBW) process inﬂuence the associated heat and mass transfer. In an attempt to describe these complexities for eventual optimization of LBW parameters, a dimensionless analysis using Mach (Ma), Raleigh (Ra), Reynolds (Re) and Marangoni (Mg) numbers have been carried out. This analysis describes hydrodynamics of melt and vapour phase appearing in the front and rear wall of KH. The non-dimensional hydrodynamic quantities describe the mechanism behind ﬂow pattern present in meltvapour in terms of ratio of convection–conduction heat transfer occurring within KH. The analysis shows that the higher Marangoni number indicates stronger Marangoni convection in the KH causing relatively higher capillary ﬂow in the melt pool. The laminar-turbulent ﬂow of melt-vapour in KH medium is described in terms of ratio of Reynolds and Mach numbers (Re/Ma). The pressure distribution in the KH accounts for the melt-vapour ejection rate. A relationship between depth and radius of KH has been obtained as a function of delivered laser power.