Numerical modeling of the momentum, heat and mass transfer in the Vacuum Arc Remelting process

Vacu­um arc remel­ting (VAR) is a refi­ning pro­cess for the pro­duc­tion of high-puri­ty metal­lic mate­ri­als. It uses a very pure elec­tro­de made of mate­ri­al of its own type, which is often mel­ted in a vacu­um induc­tion fur­nace and refi­ned in the elec­tros­lag remel­ting process.

The VAR sys­tem essen­ti­al­ly con­sists of a water-coo­led cop­per mould and a power sup­p­ly to which the elec­tro­de to be remel­ted is atta­ched. A vol­ta­ge is appli­ed so that elec­tri­cal dischar­ges take place bet­ween the elec­tro­de and the block. The ener­gy released is used to melt the under­si­de of the elec­tro­de. After mass trans­fer in the form of metal dro­p­lets, a metal pool forms in the mould, which soli­di­fies con­ti­nuous­ly towards the edge.

The refi­ning effect is based on the high vacu­um sur­roun­ding the elec­tro­de. In com­bi­na­ti­on with the small metal dro­p­lets and the lar­ge dif­fu­si­on coef­fi­ci­ents at high tem­pe­ra­tures, dis­sol­ved gases can be extra­c­ted from the metal. A fur­ther goal of the pro­cess is the pro­duc­tion of a con­trol­led direc­tion­al soli­di­fi­ca­ti­on struc­tu­re with low segregation.

The pro­cess is very time-con­sum­ing and expen­si­ve, which is why it is only used for high-qua­li­ty steels, tita­ni­um, zir­co­ni­um and nickel super­al­loys. The mate­ri­als pro­du­ced in this way are used in tool making, the che­mi­cal indus­try as well as in the aero­space indus­try. The extre­me requi­re­ments in the­se appli­ca­ti­ons requi­re a high degree of pro­cess know­ledge and control.

In coope­ra­ti­on with VDM Metals GmbH, the VAR pro­cess is nume­ri­cal­ly inves­ti­ga­ted at the IOB. The simu­la­ti­on of flow, heat balan­ce and soli­di­fi­ca­ti­on is able to simu­la­te the shape of the metal pool, which is a decisi­ve qua­li­ty cha­rac­te­ristic of a known pro­cess. Thus, the resul­ting pool pro­fi­le can be cal­cu­la­ted with the model even with slight­ly chan­ged pro­cess parameters.

The vali­da­ti­on of the model is based on metall­ur­gi­cal micro­graphs of samples from the real pro­cess. The micros­truc­tures were made eva­luable accor­ding to the “macro­sco­pic micros­truc­tu­re image” method. Using a MATLAB pro­gram, the shape of the metal pool is recon­s­truc­ted from the growth direc­tions of the den­dri­tes, which can be com­pared with the simulation.