The setting of keyhole temperature in the laser welding

Hi,
i am trying to simulate the weld pool procedure of laser welding, and i have some questions:

if latent heat is ignored( namely, heat capacity is 800J/kg*K) and the temperature of keyhole is set as Tkeyhole=3000K, the coupling calculation of laminar flow and heat transfer is converged.

However, the heat capacity is set as consider the change of temperature, (0.03*T/Tsol+0.08)*MeltHeaCap*(T<=Tsol)+((0.89*T+0.11*Tmelt-Tsol)/(Tmelt-Tsol))*MeltHeaCap*(T>Tsol)*(T<=Tmelt)+((-0.88*T+Tliq-0.12*Tmelt)/(Tliq-Tmelt))*MeltHeaCap*(T>Tmelt)*(T<Tliq)+((0.02*T+0.12*Tevap-0.14*Tliq)/(Tevap-Tliq))*MeltHeaCap*(T>=Tliq)*(T<=TevapT) +MeltLatHea*exp(-((T-Tmelt)/DeltaT)^2)/(sqrt(pi)*DeltaT)(HERE CONSIDERING LATENT HEAT )

And this moment, if Tkeyhole is 500K, the simulation is Ok. But if the Tkeyhole is 3000K, the calculation is not converged( 4 iterations number displayed), and the error: Undefined value found, Detail: Undefined value found in the equation residual vector, there are 35 degrees of freedom giving NaN/Inf in the vector for the variable comp1.T, at coordinates: (-0.00344648,0,-0.0150501), (-0.00318545,0,-0.0171758),……

Could you help me to solve this problem?
I would be extremely grateful.
Regards,
RONG