Several advanced alloy systems are susceptible to weld solidification cracking. One example is nickel-based superalloys, which are commonly used in critical applications such as aerospace engines and nuclear power plants. Weld solidification cracking is often expensive to repair and, if not repaired, can lead to catastrophic failure. This study, presented in three papers, presents an approach for simulating weld solidification cracking applicable to large-scale components. The results from finite element simulation of welding are post-processed and combined with models of metallurgy, as well as the behavior of the liquid film between the grain boundaries, in order to estimate the risk of crack initiation. The first paper in this study describes the crack criterion for crack initiation in a grain boundary liquid film. The second paper describes the model for computing the pressure and the thickness of the grain boundary liquid film, which are required to evaluate the crack criterion in paper 1. The third and final paper describes the application of the model to Varestraint tests of alloy 718. The derived model can fairly well predict crack locations, crack orientations, and crack widths for the Varestraint tests. The importance of liquid permeability and strain localization for the predicted crack susceptibility in Varestraint tests is shown.
Several advanced alloy systems are susceptible to weld solidification cracking. One example is nickel-based superalloys, which are commonly used in critical applications such as aerospace engines and nuclear power plants. Weld solidification cracking is often expensive to repair, and if not repaired, can lead to catastrophic failure. This study, presented in three papers, presents an approach for simulating weld solidification cracking applicable to large-scale components. The results from finite element simulation of welding are post-processed and combined with models of metallurgy, as well as the behavior of the liquid film between the grain boundaries, in order to estimate the risk of crack initiation. The first paper in this study describes the crack criterion for crack initiation in a grain boundary liquid film. The second paper describes the model for computing the pressure and the thickness of the grain boundary liquid film, which are required to evaluate the crack criterion in paper 1. The third and final paper describes the application of the model to Varestraint tests of Alloy 718. The derived model can fairly well predict crack locations, crack orientations, and crack widths for the Varestraint tests. The importance of liquid permeability and strain localization for the predicted crack susceptibility in Varestraint tests is shown.
Several advanced alloy systems are susceptible to weld solidification cracking. One example is nickel-based superalloys, which are commonly used in critical applications such as aerospace engines and nuclear power plants. Weld solidification cracking is often expensive to repair, and if not repaired, can lead to catastrophic failure. This study, presented in three papers, presents an approach for simulating weld solidification cracking applicable to large-scale components. The results from finite element simulation of welding are post-processed and combined with models of metallurgy, as well as the behavior of the liquid film between the grain boundaries, in order to estimate the risk of crack initiation. The first paper in this study describes the crack criterion for crack initiation in a grain boundary liquid film. The second paper describes the model required to compute the pressure and thickness of the liquid film required in the crack criterion. The third and final paper describes the application of the model to Varestraint tests of alloy 718. The derived model can fairly well predict crack locations, crack orientations, and crack widths for the Varestraint tests. The importance of liquid permeability and strain localization for the predicted crack susceptibility in Varestraint tests is shown.
Several nickel-based superalloys are susceptible to weld solidification cracking. Numerical simulation can be a powerful tool for optimizing the welding process such that solidification cracking can be avoided. In order to simulate the cracking, a crack model inspired by the RDG model is proposed. The model is based on a crack criterion that estimates the likelihood for a preexisting pore in a grain boundary liquid film to form a crack. The criterion depends on the thickness and the liquid pressure in the grain boundary liquid film, as well as the surface tension of the pore. The thickness of the liquid film is computed from the macroscopic mechanical strain field of an FE model with a double ellipsoidal heat source. A temperature-dependent length scale is used to partition the macroscopic strain to the liquid film. The liquid pressure in the film is evaluated using a combination of Poiseuille parallel plate flow and Darcy’s law for porous flows. The Poiseuille flow is used for the part of the grain boundary liquid film that extends into the region with liquid fraction less than 0.1, while Darcy’s law is used for the rest of the liquid film that extends into the regions with liquid fraction greater than 0.1. The proposed model was calibrated and evaluated in Varestraint tests of Alloy 718. Crack location, width, and orientation were all accurately predicted by the model.
Large tensile strains acting on the solidifying weld metal can cause the formation of eutectic bands along grain boundaries. These eutectic bands can lead to severe liquation in the partially melted zone of a subsequent overlapping weld. This can increase the risk of heat-affected zone liquation cracking. In this paper, we present a solidification model for modeling eutectic bands. The model is based on solute convection in grain boundary liquid films induced by tensile strains. The proposed model was used to study the influence of strain rate on the thickness of eutectic bands in Alloy 718. It was found that when the magnitude of the strain rate is 10 times larger than that of the solidification rate, the calculated eutectic band thickness is about 200 to 500% larger (depending on the solidification rate) as compared to when the strain rate is zero. In the paper, we also discuss how eutectic bands may form from hot cracks.
Welding simulations of the nickel-based superalloy Alloy 718 have been performed, combined with two fundamentally different hot crack indicators. The purpose of the indicators was to evaluate the risk of hot crack development in the weld. The result of the simulations has been compared with experiments. Advantages and limitations of the two hot crack indicators are discussed. Regions with a high value of the indicators in the simulations agree well with regions with hot cracks in the experiments.
This thesis consists of six papers on finite element simulations of the hot rolling of plates. Different friction and material models have been evaluated. The work includes both numerical simulation and experimental verification. It is shown that the constitutive model for the plate material is more important than the model for the friction between the rolled material and the rolls. It was also found that an explicit finite element method is more effective and easier to use than an implicit code. The models presented are one step towards a general and more complete computational model of flat rolling. They are not complete as they depend on a separate estimation of the roll bending. Otherwise, all three-dimensional aspects of the rolling process are included. The material models used work quite well for the simulation of the hot rolling operation. High accuracy is obtained for global quantities like rolling force and torque. A better material model would improve the prediction of the stress distribution in the rolled material. It should be able to describe the effect of rapidly changing strain rates and temperatures during deformation as well as recrystallization and thermal strains.
Weight reduction of mechanical components is becoming increasingly important as a way to provide more environment friendly production and operation of different equipment. This is true in almost any manufacturing industry, but is especially important to the aerospace industry. Casting has often been replaced by hot and cold metal working operations and welding, usually including an additional heat treatment. This gives components better material properties and provides components with less weight and cost but with increased strength and efficiency. This may even be true for rotating Ni- based superalloy components, and is enabled by welding methods. However, weld cracking of precipitation hardening Ni-based superalloys is a serious problem, both in manufacturing and overhaul since it endangers component life if cracks are allowed to propagate.
Cracks can appear in a weld and in it's surroundings. The triggering mechanisms depend on its location and when it is nucleated. Generally saying, weld cracking in precipitation hardening Ni-based superalloys consists of two different types of cracking, hot cracking and warm cracking which may be further divided into heat affected zone (HAZ) liquation cracking, solidification cracking and strain age cracking, respectively.
Finite element simulations of welding and heat treatment processes started in the seventies for small laboratory set-up cases and have today matured, and are now used on large-scale structures like aerospace components. But FE-based crack criteria that can predict the risk of cracking due to welding or heat treatments are rare. In a recent study both hot cracking and warm cracking have been investigated in Ni-based superalloys, and two FE-based indicators showing the risk of hot and warm cracks have been proposed. The objective of the investigation presented in this paper is to compare results from FE-simulations with experimental results from weldability tests, like the Varestraint test and the high temperature mechanical Gleeble test.
LKAB has tried new iron ore wagons for the 30 tonnes axle load. They got problems with cracking and material removal from the rim of the wheels during the tests. Martensite, which is more prone to cracking than other microstructures, was found at these locations. The initial material microstructure is supposed to contain no martensite. The purpose of this investigation is to find whether the thermal cycle due to braking, possibly with assistance of the mechanical load, can cause martensite formation
Rolling is simulated by a three-dimensional finite-element model with elastoplastic constitutive equations. The use of an explicit finite-element formulation, instead of the more commonly used implicit formulation, has reduced the required computing time. The larger of the models used in one step towards a general and complete computational model of rolling. Results from experiments and from two and three-dimensional calculations are compared.
A method for the evaluation of friction models is described. A wedge is rolled to uniform thickness, a range of reductions being investigated thereby in one experiment. Finite-element simulations are performed in order to estimate the friction parameters that can be used in the simulation of hot rolling. The influence of the material parameters and the friction parameters on the calculated results are investigated and the latter are compared with experimental results. It is shown that it is possible to separate the influence of the material parameters and the friction parameters, thus enabling the friction parameters to be evaluated from a minimum number of experiments.
A precipitate evolution model based on classical nucleation, growth and coarsening theory is adapted and solved using the multi-class approach for the superalloy IN718. The model accounts for dissolution of precipitates and is implemented in a finite element program. The model is used to simulate precipitate evolution in the fused zone and the adjacent heat affected zone for a welding simulation. The calculated size distribution of precipitates is used to predict Vickers hardness. The simulation model is compared with nanoindentation experiments. The agreement between simulated and measured hardness is good.
Coupled thermo-mechanical analysis of hot rolling is performed. The efficiency and accuracy when using an adaptive remeshing technique is compared with using a uniform, fine mesh. The advantages and limitations of the different techniques are discussed
The computational efficiency of the explicity finite element code DYNA2D and the implicit code NIKE2D are compared in the case of simulation of rolling. It is found that the explicit code is preferable. The advantages of the explicit formulation will be even more pronounced in three-dimensional simulations
Computing the evolution of thermal stresses accurately requires appropriate constitutive relations. This includes both the thermal and mechanical aspects, as temperature is the driver to thermal stresses. The paradigm of Integrated Computational Materials Engineering (ICME) aims at being able to quantitatively relate process-structure-property of a material. The article describes physics based models, denoted bridging elements, which are one step towards the vision of ICME. They couple material structure with heat capacity, heat conductivity, thermal and transformation strains and elastic properties for hypo-eutectoid steels. The models can account for the chemical composition of the steel and its processing, i.e. thermomechanical history, giving the evolution of the microstructure and the corresponding properties.
Simulation of some, or all, steps in a manufacturing chain may be important for certain applications in order to determine the final achieved properties of the component. The paper discusses the additional challenges in this context
Roll forming is cost-effective compared to other sheet metal forming processes for uniform profiles. The process has during the last 10 years developed into forming of profiles with varying cross sections and is thereby becoming more flexible. The motion of the rolls can now be controlled with respect to many axes enabling a large variation in the profiles along the formed sheet, the so-called 3D roll forming or flexible roll forming technology. The roll forming process has also advantages compared to conventional forming for high-strength materials. Furthermore, computer tools supporting the design of the process have also been developed during the last 10 years. This is quite important when designing the forming of complex profiles. The chapter describes the roll forming process, particularly from the designer’s perspective. It gives the basic understanding of the process and how it is designed. Furthermore, modern computer design and simulation tools are discussed.
Roll forming is cost-effective compared to other sheet metal forming processes for uniform profiles. The process has during the last 10 years developed into forming of profiles with varying cross sections and is thereby becoming more flexible. The motion of the rolls can now be controlled with respect to many axes enabling a large variation in the profiles along the formed sheet, the so-called 3D roll forming or flexible roll forming technology. The roll forming process has also advantages compared to conventional forming for high-strength materials. Furthermore, computer tools supporting the design of the process have also been developed during the last 10 years. This is quite important when designing the forming of complex profiles. The chapter describes the roll forming process, particularly from the designer’s perspective. It gives the basic understanding of the process and how it is designed. Furthermore, modern computer design and simulation tools are discussed
Superplastic-like forming is a newly improved sheet forming process that combines the mechanical pre-forming (also called hot drawing) with gas-driven blow forming (gas forming). Non-superplastic grade aluminium alloy 5083 (AA5083) was successfully formed using this process. In this paper, a physical-based material model with dislocation density and vacancy concentration as intrinsic foundations was employed. The model describes the overall flow stress evolution of AA5083 from ambient temperature up to 550 °C and strain rates from 10−4 up to 10−1 s−1. Experimental data in the form of stress–strain curves were used for the calibration of the model. The calibrated material model was implemented into simulation to model the macroscopic forming process. Hereby, finite element modelling (FEM) was used to estimate the optimum strain-rate forming path, and experiments were used to validate the model. In addition, the strain-rate controlled forming was conducted for the purpose of maintaining the gas forming with an average strain rate of 2 × 10−3 s−1. The predicted necking areas closely approximate the localized thinning observed in the part. Strain rate gradients as a result of geometric effects were considered to be the main reason accounting for thinning and plastic straining, which were demonstrated during hot drawing and gas forming by simulations.
The finite element method (FEM) is one of the most applicable mathematical analytic methods of rolling processes and is also an efficient method for analyzing coupled heat transfer. Thermal analysis of cold rolling process is not frequently used due to the widespread assumption of insignificant impact during rolling process. This research focuses on the development of coupled thermo-mechanical 2-D FE model analysis approach to study the thermal influence and varying coefficient of friction during the industrial cold rolling process of AA8015 aluminum alloy. Both deformable-rigid and deformable-deformable rigid contact algorithms were examined in the 2-D FE model. Findings revealed that temperature distribution in the roll bite rises steadily in a stepwise manner. The deformable-deformable contact algorithm is the best investigations of thermal behavior of the rolled metal and work rolls necessary for typical application in work roll design. The predicted roll separating force is validated with industrial measurements.
The cold rolling process is a strain-hardening mechanism, which is widely known for its strength improvement, excellent surface finish and dimensional tolerance. This work aims to model and simulate the first pass of the industrial cold rolling process of Aluminium 8015 alloy. Process parameters are obtained from practical industrial cold rolling of the aluminium alloy and are used in the development of 2-D and 3-D finite element models for the first pass. These were achieved using the MSC Marc-Mentat Software. Research results comprising of the roll force, contact frictional force, and shear stress were investigated. Findings reveal the deformation rate pattern and neutral points in the roll bite. The 3-D finite element model developed is effective in analyzing deformation of metals in the roll bite as compared to the 2-D model.