Hot Workability Behavior of Kovar: An Approach Using Constitutive Equations and Processing Maps
摘要
Kovar (Fe-29Ni-17Co) is a single-phase FCC iron-nickel-cobalt alloy widely utilized in electronic sensor packaging due to its low thermal expansion and compatibility with borosilicate glass for hermetic sealing. This study aims to evaluate the hot workability of Kovar and establish optimized processing parameters by investigating the effects of deformation temperature (T), strain rate (έ), and strain (ε) on flow behavior and microstructural evolution. Isothermal hot compression tests were conducted in a Gleeble thermomechanical simulator within the temperature range of 900–1100 °C at 50 °C intervals and strain rates ranging from 10−3 to 10 s−1, up to a true strain of 0.65. The strain-compensated Arrhenius model was employed to derive constitutive equations, yielding an activation energy for hot deformation of 338 kJ/mol. The onset of dynamic recrystallization (DRX) was analyzed using work-hardening rate (θ) versus flow stress (σ) curves to determine the critical stress associated with DRX initiation. Microstructural analysis revealed that the initial wrought austenitic structure gradually coarsened at lower strain rates and higher deformation temperatures. A processing map was developed at a true strain of 0.65 by overlaying regions of instability over the power dissipation efficiency map. The optimal hot working window was identified in the temperature range of 900–1050 °C and strain rates between 0.001-0.1 s−1, with a peak power dissipation efficiency of ~ 37% at 1050 °C and 0.001 s−1. These findings offer valuable insights into the deformation mechanisms and optimal processing conditions for Kovar alloy, enabling improved control over the microstructure and mechanical properties during hot working in several industrial applications.