Sindo Kou

Fluid flow and weld penetration in stationary arc welds

Weld pool fluid flow can affect the penetration of the resultant weld significantly. In this work, the computer simulation of weld pool fluid flow and its effect on weld penetration was carried out. Steady-state, 2-dimensional heat and fluid flow in stationary arc welds were computed, with three driving forces for fluid flow being considered: the buoyancy force, the electromagnetic force, and the surface tension gradient at the weld pool surface. The computer model developed agreed well with available analytical solutions and was consistent with weld convection phenomena experimentally observed by previous investigators and the authors. The relative importance of the influence of the three driving forces on fluid flow and weld penetration was evaluated, and the role of surface active agents was discussed. The effects of the thermal expansion coefficient of the liquid metal, the current density distribution in the workpiece, and the surface tension temperature coefficient of the liquid metal on weld pool fluid flow were demonstrated. Meanwhile, a new approach to free boundary problems involving simultaneous heat and fluid flow was developed, and the effort of computation was reduced significantly.

Three-dimensional convection in laser melted pools

Computer simulation was carried out to describe three-dimensional convection in laser melted pools,i.e., for the case where the workpiece is moving with respect to the laser beam. Two different types of driving forces for flow were considered in the model,i.e., the buoyancy force and the surface tension gradient at the pool surface. Laser surface melting of 6063 aluminum alloy was carried out using a continuous-wave CO2 laser, and the power delivered to the workpiece was measured calorimetrically. The calculated and observed fusion boundaries were compared and very good agreement was obtained. Finally, the effect of the surface tension temperature coefficient δγ/δT on the convection pattern and penetration of laser melted pools was demonstrated with the model.

A Fundamental Study of Laser Transformation Hardening

A theoretical and experimental study of heat flow and solid-state phase transformations during the laser surface hardening of 1018 steel was conducted. In the theoretical part of the study, a three-dimensional heat flow model was developed using the finite difference method. The surface heat loss, the temperature dependence of the surface absorptivity, and the temperature dependence of thermal properties were considered. This heat flow model was verified with the analytical solution of Jaeger and was used to provide general heat flow information, based on the assumptions of no surface heat loss, constant surface absorptivity, and constant thermal properties. The validity of each of these three assumptions was evaluated with the help of this heat flow model. In the experimental part of the study, on the other hand, a continuous-wave CO2 laser of 15 kW capacity was used in conjunction with a beam integrator to surface harden 1018 steel plates. The beam power and the travel speed of the workpiece were varied, and the onset of surface melting was determined. The configurations of the heat-affected zone observed were compared with those calculated using the heat flow model. The microstructure of the heat-affected zone was explained with the help of the calculated peak temperature, heating, and cooling rates.

Heat transfer, fluid flow and interface shapes in floating-zone crystal growth

Abstract Computer simulation was carried out to study heat transfer and fluid flow in the melt zone in floating-zone crystal growth. A high Prandtl-number material, i.e., NaNO 3 , and a low Prandtl-number material, i.e., Si, were considered. The unknown shapes of the melt/gas, melt/crystal and melt/feed interfaces were calculated for each of the following three cases: (1) conduction, (2) natural convection and (3) thermocapillary and natural convections. The effects of the growth rate, gravity and feed/crystal diameter ratio were demonstrated. It was observed that in both NaNO 3 and Si thermocapillary convection dominates over natural convection, at least for the conditions examined in the present study. This thermocapillary convection tends to reduce the stability of the melt zone, increase the convexity of the melt/crystal interface and reduce the maximum surface temperature. The effect of thermocapillary convection is significantly more pronounced in NaNO 3 than in Si, even though thermocapillary convection in Si is far stronger. The dynamic effect of convection on the shape of the melt/gas interface is very small.

Computer Simulation of Convection in Moving Arc Weld Pools

Computer simulation for three-dimensional convection in moving arc weld pools was described, with three distinct driving forces for flow considered — the electromagnetic force, the buoyancy force, and the surface tension gradient on the pool surface. Formulation of the electromagnetic force in the weld pool was presented. The calculated and experimentally observed fusion boundaries were compared. The arc efficiency and spatial distributions of the current density and power density used in the calculations were based on experimentally measured results, in order to verify the model. The effects of the electromagnetic and surface tension forces were discussed.

The effect of quenching on the solidification structure and transformation behavior of stainless steel welds

Weld solidification structure of three different types of stainless steel,i.e., 310 austenitic, 309 and 304 semiaustenitic, and 430 ferritic, was investigated. Welds of each material were made without any quenching, with water quenching, and with liquid-tin quenching during welding. The weld micro-structure obtained was explained with the help of the pseudobinary phase diagrams for Fe-Cr-Ni and Fe-Cr-C systems. It was found that, due to the postsolidification 5 → γ phase transformation in 309 and 304 stainless steels and the rapid homogenization of microsegregation in 430 stainless steel, their weld solidification structure could not be observed unless quenched from the solidification range with liquid tin. Moreover, the formation of acicular austenite, and hence, martensite, at the grain boundaries of 430 stainless steel welds was eliminated completely when quenched with liquid tin. The weld solidification structure of 310 stainless steel, on the other hand, was essentially unaffected by quenching. Based upon the observations made, the weld microstructure of these stainless steels was summarized. The effect of cooling rate on the formation of primary austenite in 309 stainless steel welds was discussed. Finally, a simple method for determining the relationship between the secondary dendrite arm spacing and the solidification time, based on welding speeds and weld pool configurations, was suggested.

Grain structure and solidification cracking in oscillated arc welds of 5052 aluminum alloy

The effect of arc oscillation on grain structure and solidification cracking in GTA welds of 5052 aluminum alloy was investigated using a four-pole magnetic arc oscillator and a modified fish-bone crack test. Two different mechanisms of crack reduction were identified: one in the low frequency range of arc oscillation and the other in the high frequency range. The former was the alteration of the orientation of columnar grains, while the latter was grain refining. Neither mechanism was operative in the intermediate frequency range and solidification cracking was severe, especially when the amplitude of arc oscillation was small. Alteration of grain orientation was obtained in welds made with transverse and circular arc oscillations, but not longitudinal arc oscillation. Grain refining, on the other hand, was achieved in welds made with all three types of arc oscillation patterns. The differences between the response of alloy 5052 to arc oscillation and that of alloy 2014 observed previously were discussed.

Alternating grain orientation and weld solidification cracking

A new mechanism for reducing weld solidification cracking was proposed, based on the concept of the crack path and resistance to crack propagation, and its effectiveness was verified in magnetically oscillated GTA welds of a rather crack susceptible material 2014 aluminum alloy. This mechanism,i.e., alternating grain orientation, was most pronounced in welds made with transverse arc oscillation of low frequency and high amplitude, and solidification cracking was dramatically reduced in these welds. The effect of the arc oscillation pattern, amplitude, and frequency on the formation of alternating columnar grains and the reduction of solidification cracking in GTA welds of 2014 aluminum alloy was examined and explained. The present study demonstrated for the first time that columnar grains can, in fact, be very effective in reducing solidification cracking, provided that they are oriented favorably.

Welding parameters and the grain structure of weld metal — A thermodynamic consideration

The grain structure of the weld metal can significantly affect its resistance to solidification cracking during welding and its mechanical properties after welding. An experimental study was conducted to investigate the effect of two basic welding parameters,i.e., the heat input and the welding speed, on the grain structure of aluminum-alloy welds. Gas-tungsten arc welding was performed under various heat inputs and welding speeds, with thermal measurements in the weld pool being carried out during welding and the amounts and nuclei of equiaxed grains in the resultant welds being examined using optical and electron microscopy. The experimentally measuredG/R ratios and the clearly revealed heterogeneous nuclei together demonstrated the thermodynamic effect of the heat input and welding speed on the weld metal grain structure.

Effects of rotation on heat transfer, fluid flow and interfaces in normal gravity floating-zone crystal growth

Abstract Computer simulation was conducted to study heat transfer and fluid flow in normal gravity floating-zone crystal growth of NaNO 3 with: (1) fast feed/crystal counterrotation and (2) fast single rotation of the feed or crystal. Natural convection, thermocapillary convection and forced convection were considered. The shapes of the melt/gas, melt/crystal and melt/feed interfaces were calculated and compared with those observed during crystal growth. The effects of key operating parameters were discussed, including the mode and speed of rotation, growth rate, heat input and growth direction.