Hot workability and microstructure control in Monel®400 (Ni–30Cu) alloy: An approach using processing map, constitutive equation and deformation modeling

Monel®400 alloy is widely used in marine and chemical industries due to its moderate strength and high resistance to corrosive environments. In the present work, hot workability of Monel®400 alloy in as-cast condition was studied using the processing maps approach. Hot-compression tests were performed in a Gleeble thermo-mechanical simulator in the temperature, T, range of 900–1200 °C and strain rate, $\dot{\epsilon }$, of 10⁻² – 10⁺¹ s⁻¹. The data obtained from the tests was used to construct processing map at a true strain, ε, of 0.5 utilizing the modified dynamic material model (DMM) proposed by Murty and Rao. Processing map identified three stable domains for possible hot working occurring in the following T and $\dot{\epsilon }$ range - Domain-1: T = 900–1200 °C and $\dot{\epsilon }$ = 10⁻²-10^−1.75 s⁻¹; Domain-2: T = 1150–1200 °C and $\dot{\epsilon }$ = 10^−1.75- 10⁻¹ s⁻¹; and Domain-3: T = 1100–1200 °C and $\dot{\epsilon }$ = 10⁺⁰ - 10⁺¹ s⁻¹. A broad flow instability regime in the form of flow localization was also identified at all temperatures and intermediate strain rates ($\dot{\epsilon }$ = 10^−1.5 - 10⁺⁰ s⁻¹). Complete recrystallized microstructure with discontinuous dynamic recrystallization (DDRX) as the dominating deformation mechanism was observed in Domain-3 (occurring at higher $\dot{\epsilon }$). In contrast, microstructural analysis of the deformed specimens in Domain-1 and Domain-2 occurring at lower $\dot{\epsilon }$ showed limited DRX at the serrated grain boundaries with dynamic recovery (DRV) and continuous DRX (CDRX) as the operating deformation mechanisms. With the aid of the processing map along with evolved microstructures of the deformed specimens, Domain-3 was identified as the optimum "safe" regime for thermo-mechanical processing. Results obtained from the processing map are then successfully validated through industrial trial forgings performed in the T range 950–1200 °C. The forging process was also simulated in identical process parameters using finite element method (FEM) software DEFORM®-3D to study the flow behavior and effective stress and strain distribution in the forged billets.