Numerical simulation of temperature distribution and heat transfer during solidification of titanium alloy ingots in vacuum arc remelting process

In order to get a better understanding of the vacuum consumable arc remelting(VAR) processes and thus to optimize them,a 3D finite element model was developed for the temperature fields and heat transfer of titanium alloy ingots during VAR process.The results show that the temperature fields obtained by the simulation are well validated through the experiment results.The temperature distribution is different during the whole VAR process and the steady-state molten pool forms at 329 s for d100 mm × 180 mm ingots.At the initial stage of remelting,the heat dissipation of crucible bottom plays an important role in the whole heat dissipation system.At the middle of remelting,the crucible wall becomes a major heat dissipation way.The effect of cooling velocity on the solidification structure of ingots was investigated based on the temperature fields and the results can well explain the macrostructure of titanium alloy ingots.

Effects of different helium cooling conditions on the structures of GH4169 alloy vacuum arc remelting ingots

The effects of different helium cooling conditions on the molten pool depth,dendrite structure,and microsegregation of GH4169 alloy ?508 mm vacuum arc remelting( VAR) ingots were studied using an optical microscope and an electron probe. The results show that under different helium cooling conditions,the growth of columnar crystals in the VAR ingot is the same with a certain angle running from the edge to the center and the dendrites at the edges are relatively small whereas the dendrites near the center are large. As the helium cooling increased,the molten pool depth decreased from 137 mm to 120 mm. Observations of the microstructure showed that as the helium cooling increased,the secondary dendrite arm spacing( SDAS) decreased at the center and R/2 region. Also,the Laves phase content markedly decreased. Under the same helium cooling conditions,the SDAS and Laves phase content at the center were higher than that at the R/2 region. Thus,more intense helium cooling effectively reduced segregation in VAR ingots and improved the metallurgical quality.

Mechanism of local solidification time variations with melt rate during vacuum arc remelting process of 8Cr4Mo4V high-strength steel

A 2D axisymmetric numerical model was established to investigate the variations of molten pool with different melt rates during the vacuum arc remelting of 8Cr4Mo4V high-strength steel,and the ingot growth was simulated by dynamic mesh techniques.The results show that as the ingot grows,the molten pool profile changes from shallow and flat to V-shaped,and both the molten pool depth and the mushy width increase.Meanwhile,the variation of both the molten pool shape and the mushy width melt rate is clarified by the thermal equilibrium analysis.As melt rate increases,both the molten pool depth and the mushy width increase.It is caused by the increment in sensible heat stored in the ingot due to the limitation of the cooling capacity of the mold.The nonlinear increment in sensible heat leads to a nonlinear increase in the mushy width.In addition,as melt rate increases,the local solidification time(LST)of ingot decreases obviously at first and then increases.When melt rate is controlled in a suitable range,LST is the lowest and the secondary dendrite arm spacing of the ingot is the smallest,which can effectively improve the compactness degree of 8Cr4Mo4V high-strength steel.

Effect of feeding parameters on ingot segregation and shrinkage pore in vacuum arc remelting

The feeding parameters in the final stage of vacuum arc remelting process obviously affect the solute segregation and shrinkage pore depth.Coupled with the electromagnetic field,fluid flow,and solute transport,a numerical model was built to investigate the effect of feeding parameters on the ingot solidification phenomena.The Nb segregation and shrinkage pore depth in the solidified ingot were measured.The results show that the liquid moves along the solidification front and the vertex flow is formed in the liquid pool,which promotes solute transport.In the solidified ingot,the Nb segregation in the lower part is negative,while that in the upper part is positive.With the differential electrode applied,the positive segregation is slightly reduced but the segregation distribution remains unchanged.As the feeding current decreases,the positive segregation zone moves toward the ingot top surface,due to the final solidification position moving upward.With the feeding time extended,the positive segregation moves to the top surface and the shrinkage pore depth is reduced.As the feeding time is set at 12 min,the shrinkage pore depth can be reduced to 21 mm.

Numerical Simulation of Fluid Flow Caused by Buoyancy Forces During Vacuum Arc Remelting Process

The metallurgical structure and composition of ingots which depend critically on the fluid motion within the molten pool during the vacuum arc remelting(VAR)process have important effect on the subsequent mechanical processes like forging,rolling and welding.In order to determine the fluid motion of molten pool,a 2D finite element model is established using ANSYS10.0 software,combined with the turbulent fluid flow and heat transfer.The fluid motion caused by thermo buoyancy forces is investigated at different VAR processes in the present study.The results indicate that the fluid flows symmetrically along the axis of the molten pool and clockwisely along the circle at the right pool’s profile.It is also shown that the maximum velocity increases with increasing melting rate and a direct proportional relationship exists.

Effect of Remelting Current on Molten Pool Profile of Titanium Alloy Ingot During Vacuum Arc Remelting Process

The performance of vacuum arc remelting (VAR) ingot depends largely on ingot structure and chemical uniformity,which are strongly influenced by molten pool profile that is influenced by VAR process.To better understand the effect of remelting current on molten pool profile of titanium alloy ingot during VAR process,a 3D finite element model is developed by the ANSYS software.The results show that there are three remelting stages during VAR process when the remelting current is 2.0 kA.The molten pool depth increases gradually from 30 to 320 s,then the change of molten pool depth is very small during the steady state stage from 320 to 386 s,and lastly the molten pool depth becomes shallow after 386 s.The melting rate and temperature of superheat increase with the remelting current increasing,which leads to the augment of molten pool volume.In the end,the total remelting time and steady state molten pool time decrease with the melting current from 1.6 to 2.8 kA.

Influence of Electromagnetic Stirring on the Macrosegregation in Arc Remelted Ti and Zr Alloy Ingots

A numerical model of the VAR process has been developed in close collaboration with titanium and zirconium producers.The model is based on the solution of the coupled heat,mass and momentum transport equations in the whole ingot(liquid pool,mushy zone and solidified part)using a finite volume method.It accounts for electromagnetic stirring, buoyancy flows and turbulence effects.Solidification mechanisms implemented in the model include a full coupling between energy and solute transport in the mushy zone.The numerical model has been applied to simulate the remelting of Ti6-4.The influence of the applied magnetic field on the fluid flow and segregation behaviour,for a number of different stirring practices,is presented and discussed.Also,the macrosegregation in Zy4 ingots is investigated.The comparison between the predicted segregation and the experimental results shows the importance of accounting for both the stirring and thermosolutal convection to forecast properly the segregation in remelted ingots.