Material modelling for simulation of heat treatment

Detta är en avhandling från Luleå : Luleå tekniska universitet

Sammanfattning: Heat treatment of materials is a fundamental metallurgical process. Materials are subjected to heat treatment to relieve internal stresses, reduce brittleness and to improve machinability. The properties of materials can also be altered such as hardness, strength, toughness, and wear resistance to suit particular applications. Nevertheless, heat treatment can generate unwanted stresses and deformations, a fact that has to be taken into consideration when designing or changing the sequence of manufacturing for a given component. One way to decrease cost and reduce time in product development can be to use simulation tools that can reliably predict the final properties and shape of a component caused by the used manufacturing process. A decrease in cost and better knowledge of final properties already in product development can give the company a better market position and competitiveness. The objective of the work presented in this thesis is to develop efficient and reliable methods and models for simulation of heat treatment using the Finite Elements Method. The result of the simulation must be sufficiently accurate and completed within an acceptable time when the manufacturing simulation is to be used in product development. The models would enable us to predict residual stresses, distortion, final shape, and amount material phases after a heat treatment process. The formulation of constitutive equations for elasticity, plasticity, and creep is discussed, along with three unified models, bringing together plasticity/viscoplasticity, and creep into one model. There are many materials models to choose among but material parameters are usually lacking An approach where the same numerical algorithm for the implementation of a constitutive model in the finite element code is also used for material parameter identification is presented. A parameter fitting using a viscoplastic model with nonlinear isotropic hardening is performed. Combined welding and heat treatment simulation is performed on a geometrical complex shaped aerospace component. Efforts to accurately determine the boundary conditions have been made. During the cooling sequence most of the heat transfer is carried out by convective heat transfer. Therefore, a method is developed and a CFD analysis is carried out to obtain this convective heat transfer Moreover, comparisons how the use of different material models affects the response from combined welding and heat treatment on an aerospace component is made. The material models include different effects; rate-independent and viscoplastic models including microstructure calculation and transformation induced plasticity. Different creep models and different ways to apply them has also been investigated.