CHAPTER 3. METHODOLOGY. This chapter describes the background theory and different

Transcription

1 CHAPTER 3. METHODOLOGY This chapter describes the background theory and different methods of simulations, details on finite element analysis, different types of simulation used in the sand castings, the research methods, working flow and standard operation procedures of ProCAST were interpreted. First, the capabililities of soft ware s, types of simulation used in sand castings, secondly the research flow of present work was shown, and then a brief introduction of operation guide of ProCAST is developed SOFTWARE PACKAGES CAPABILITIES Most of commercially available simulation software packages, that support solidification and mould filling analysis, are developed based on either the Finite Element Method (FEM) or the Finite Difference Method (FDM) [82] Finite Element Method The finite element method for analysing structural parts has been since the 1950s. The method was primary developed for use in the aerospace and nuclear power industries. Here, the safety of the structures is critical: they involve large capital expenditure and the economic consequences of a failure are very severe, so the cost of the analysis is justified. Today the method is also extensively used in areas such as the automotive industry, where components are relatively cheap but are manufactured in large volumes. Furthermore, 30

2 any small reduction in the safe weight of a component such as a connecting rod can lead to additional benefits in areas such as vibration reduction and fuel economy [83]. The growth in the usage of finite element methods is due to the developments in computing technology, in recent years. Today there are a number of large software companies developing and marketing finite element and associated modelling software. As a result, there exist commercial finite element packages capable of solving the most sophisticated problems, in a wide variety of areas [84]. In spite of the abundance and power of commercial software, it is still very important to have an understanding of the principles of the technique, so that an appropriate simulation can be selected, correctly defined and interpreted. The complete mathematical discussion is not important to the sand casting engineer, it is important to realize the practical differences. FDM uses an orthogonal mesh to represent cavity and mould geometry. It is very hard to model a complex casting shape, especially a thin wall casting, with a reasonable number of elements. Even use of millions of elements may not improve the analysis result too much because the interface length on any out-of-plane section is still incorrect. This casting/mould interface is one of the most influential variables in sand mould process. FEM can use not only a non-orthogonal mesh, but also different types of mesh where needed. In FEM, it is easy to fit in zero-thickness interface elements, which model the cast metal/mould contact. Be-cause of this boundary 31

3 element there is a chance to divide the casting and mould surface temperatures. This is not the case with FDM. The geometric flexibility of FEM is essential for sand casting engineers to obtain accurate analysis results. In addition, stress analysis of parts can be performed with FEM with little or no modification of the mesh. Successful use of the same FEM mesh for casting solidification analysis and stress analysis has been reported. Thus, FEM is better suited to sand casting analysis [86]. Popularly recognized casting simulation programs presently available to foundry engineers are listed in Table:3.1 Table:3.1 Recognized casting simulation programs [75] CastCAE Software Program Company and Location CT-Castech Inc. Oy, Espoo, Finland Castflow, Castherm Walkington Engineering, Inc., Australia PAM-CAST/ProCAST MAGMASoft JSCast SOLIDCast ESI Group, Paris, France MAGMA GmbH, Aachen, Germany Komatsu Soft Ltd., Osaka, Japan Finite Solutions, Inc., Illinois, USA AutoCAST Advanced Reasoning Technologies P. Ltd., Mumbai 32

4 Simulation software s SOLIDCast uses the Finite Difference Method (FDM) of heat transfer calculation, combined with a unique tracking of volumetric changes in the metal, to predict the temperature and volume changes in a casting as it is poured, solidified and cooled. This combined thermal-volumetric approach has proven to be an extremely accurate method of predicting various casting problems, including micro- and macro-porosity, hot spots and other defects [85]. MAGMASOFT was created by MAGMA, a German company founded in This Finite Deference Method program may help the user stay away from gating and feeding problems, forecast casting quality, aids permanent mould design and reduce fettling costs. It contains many modules for calculating specific processes, such as high-pressure die casting, low-pressure die casting, lost foam, lowpressure die casting for wheel castings, etc., optimization modules, and modules that could predict thermo elastic/thermoplastic stresses, residual stresses and strain in castings and moulds, modules that can predict the solidification sequence of cast iron alloys using microscopic kinetic growth models, heat treatment modules, etc. 3.2 ProCAST OVERVIEW Recognized for years as the leading Finite Element solution for casting process simulation, ProCAST 2004 offers new graphical user interfaces and improved performances in speed, accuracy and 33

5 modelling capabilities. Is a modular software solution offering an extensive suite of modules and engineering tools to meet the most challenging requirements of the casting industry. In order to address a wide variety of casting processes and related issues, the software capabilities include automatic mesh generation, thermal analysis including radiation effects, flow analysis for mould filling, fully coupled thermal, flow and stress analysis, and advanced metallurgical options. It is leading finite element solution for Casting Process Simulation. Based on proven Finite Element technology, it provides a complete solution covering a wide range of casting processes and alloy systems [86] Introduction Is a three dimensional solidification and fluid flow package developed to perform numerical simulation of molten metal flow and solidification phenomena in various casting processes, primarily die casting (gravity, low pressure and high pressure die casting) and sand casting. It is particularly helpful for foundry applications to visualize and predict the casting results so as to provide guidelines for improving product as well as mould design in order to achieve the desired casting qualities. Prior to applying the simulation extensively to create sand casting and die casting models for the simulation of molten metal flow (mould filling) and solidification (crystallization in the process of cooling).the cast and mould design of the experiment is transformed into a 3D model and imported into ProCAST to conduct 34

6 the sand casting process simulation. In the present work simulation of mould filling solidification of casting of CO 2 sand alloy steel castings are carried out Research Flow The purpose of this present work is to simulate the mechanism of the solidification of alloy steel sand castings, and analyze the results to give some aspects of logical thoughts for experiments designation, and to optimize the casting parameters in order to achieve better properties of steel castings. The procedures were mainly divided into three stages. They were Simulation Preparation, Computer Aided Simulation, and Analysis. Each stage contained several steps. Researcher followed this operation flow to try and examine different influencing factors, such as molten metal temperature, mould material, inlet velocity, substrate pre heating temperature, and radiation. In the first stage, observation of fluid flow was most important because all the model construction, parameters designation, and questions description are based on observing substantial experiments. The purpose of this stage was to gather more data for simulation experiments, and all the material properties, mould properties, relationships between materials and surroundings are needed. In addition, the second stage was the simulation, and this stage was totally under computer operation, including model construction, input factors setting, problem solving, result obtainment. Finally, the final stage was to show the results of 35

7 simulation, to build a data base and to analyze, then to find out convincing conclusions that would improve the casting. Fig: 3.1(a)and Fig:3.1(b) is the flow charts showing the entire procedures conducting to this research. Fig:3.1 (a) Research Procedure[71]. Fig:3.1(b) Steps needed to- Make simulation[71] Solid modelling of casting in ProENGINEER Wildfire-2.0 The solid model of a cast product is the backbone for various software programs that help in improving the consistency and speed 36

8 of different tasks in casting development. First step is to convert a part drawing sheet shown in Fig:3.2 and 3.3 given by the customer in to a CAD model, generally first we will make a 3D solid model using CAD software and then make a orthographic 2D drawing regarding specific direction. After this, from the 3D solid model of the part, we will get directly the information required for the gating system design like, surface area,total volume which is required for calculating the modulus (V/A) of casting, and other properties like weight, and mass can be find from the Fig:3.4 and Fig: 3.5 showing a 3D model parts. After this we have to calculate the total allowances including shrinkage, machining allowance, and draft required whenever necessary. Decide the Parting surface and parting direction and then calculate the gating system design, runner riser dimensions and position. After this calculation is over, we are going to modify the 3D model part to get the required patterns, Fig:3.6& Fig:3.7. shows the 3D models of patterns, and thus we will get finished pattern from the cast part model. Now to model the Sand block and sand core whenever necessary. So first is to calculate the dimensions of sand block, and model it on the same pattern model on parting surface. And then by using various Boolean Functions, we are getting the required mould cavity (e.g. subtracting pattern from sand block model) Fig:3.8 Showing the 3D model of sand block. Now at this time all our modelling part is over, and we have to go for analysis of modeled part. 37

9 Fig: D Part Drawing of Straight Bar Fig:3.3.3D model of Straight Bar Casting with gating system 38

10 Fig:3.4. 2D Part Drawing of Flange Bar Fig:3.5. 3D model of Flange bar with gating system Fig:3.6 Straight bar pattern Fig:3.7 Flanged bar pattern Fig: 3.6 & Fig:3.7 3D part models of patterns 39

11 Fig:3.8. 3D model of sand block Finite element modelling of Casting Process The development of solidification process is difficult in nature and the Simulation of such process is required in industry before it is essentially undertaken. Finite element method is used to simulate the heat transfer process accompanying the solidification process. The metal and the mould along with the air gap formation is accounted in the heat transfer simulation. deformation of the casting is caused due to non-uniform shrinkage associated with the process. Residual stresses are induced in the final castings. Simulation of the shrinkage and the thermal stresses are also carried out using finite element methods. The material behavior is considered as visco plastic. Stress investigation of castings poses several difficulties not seen in more traditional problems in mechanics. The residual stress formation during castings is a consequence of various regions of a geometrically complicated casting cooling at different rates. Stress response is the result of coupled thermal, micro thermal and stress histories. Stress predictions are strongly influenced by the thermal and micro 40

12 structural histories. The accuracy of thermal and micro structural predictions is a primary factor in the accuracy of residual stress predictions. An overall planning of a comprehensive solidification modelling system is shown in Fig:3.9. This Figure depicts the various modules available in the current state-of-the-art solidification simulation of casting processes, the information available from each module and the interconnection between each module. The early models of cooling of casting were straightforward heat conduction analysis. However, the mechanics of fluid flow are important for both mould-filling effects and physics based models of inter-dendrite porosity formation. Fig:3.9 Typical architecture of a comprehensive casting modelling system [88] This wide solidification study given an account of several aspects of modelling of heat transfer, fluid flow and thermodynamics in castings. Solidification kinetics including phase selection, 41

13 nucleation and growth are now being investigated in several laboratories. The incorporation of these principles into the more traditional thermo-fluid models promises to enable quantitative micro structural predictions in the near future, and predictions of engineering properties such as tensile strength and elongation will be possible before long. These predictions will enable product-design engineers to evaluate the effects of non-uniform properties and defects on the life cycle performance of components. Finally, the coupling of mechanical analysis with thermal analysis enables the predictions of residual stresses and distortions in castings The ProCAST System Is a physics based computer program designed for the calculation of fluid flow, thermal and thermo mechanical phenomena encountered during the production of metal castings. A typical casting is produced by pouring molten liquid metal into a suitably prepared mould cavity containing the topology of the part to be manufactured. As a result of heat energy extraction through the mould walls, the liquid metal cools and solidifies producing a desired metal part. The soundness and overall quality of cast parts is strongly affected by the liquid metal poured in mould, flow during mould filling, and of the time dependent temperature fields during solidification. Macro and micro structural characteristics of the cast components are determined by the flow and thermal history of the casting and these in turn determine the mechanical and other physical properties of the 42

14 material. It is based on a finite element methodology that is coupled with a Volume of Fluid technique for the computation of mould filling. The present study was conducted using Version 2004 of the ProCAST System. The simulation module is the computational engine that carries out the necessary mathematical calculations and produces computed values of metal velocity, temperature, fraction solidified, shrinkage porosity and the like. Finally, the ViewCAST module allows detailed examination of the computed results Typical Steps for thermal analysis After completing the 3D modelling in Solid Modeler Application, save the part in STEP Format. The STEP file is imported in to GeoMESH software. Where the solid model gets meshed and analyzed for any possible meshing errors. After the surface mesh is generated save the part. Now import the mesh file from GeoMESH to software, it will show a different module in a window, select the MeshCAST module. MeshCAST generates a 3-D tetrahedral mesh using the Finite Element Method (FEM). A triangular surface mesh of the object is the prerequisite for MeshCAST "tet-mesh" generation. Based upon the IGES, PARASOLIDS, STEP, STL model, MeshCAST can generate the triangular surface mesh. Alternatively, MeshCAST can use the surface mesh from CAD or CAE package as input for tet-mesh generation. The following Fig:3.10 shows the steps involved in ProCAST. 43

15 Fig: 3.10 Typical ProCAST Steps for thermal analysis [87] There are six major steps in MeshCAST which are required in order to produce a high quality tetrahedral mesh. Input file that is *.unv file. Every MeshCAST session will begin with the designation. The surface mesh in the Meshing Environment as necessary. MeshCAST automatically checks the input file geometry and attempts to resolve flaws as it is loaded. Edit the surface mesh in the repair environment. In this step MeshCAST actually generates the 3-D tetrahedral mesh of the solid model. View the mesh and enhance its quality, as show in (Fig:3.11 volume meshes of a part). 44

16 Fig:3.11. Volume mesh model in MeshCAST Fig:3.12 Materials assignment in PreCAST: The first operation to perform is to assign material properties to the domains. The first domain should be selected and the desired material properties should be selected in the database. IS1030 steel is selected in this case as shown in Fig

17 Fig: 3.13 Interface menu in PreCAST In the interface menu shown in Fig:3.13, create the interfaces between the different Material domains and give the desired interface heat transfer coefficients. The type of interface should be specified. The desired interface heat transfer coefficient should be selected in the database and assigned to the corresponding interface. Fig:3.14 Boundary Conditions in PreCAST The cooling of the outside of the mould with the air, as well as the top surface of the casting should be defined in the "Boundary Conditions" list of options. The type of boundary condition should be selected. the "Heat" type should be selected in the list which is 46

18 appearing as shown in Fig Then, this "Heat" boundary condition appears in the data base. Fig:3.15 Gravity Sand Casting- Process Menu The gravity should be defined in the "Process menu". This will open the "Gravity" panel as shown in Fig Gravity in the appropriate direction regarding the part is set. Fig: Initial Conditions Menu The initial temperature of both material domains should be specified in the "Initial Conditions" menu. Each domain should be selected and the initial temperature should be entered in the field as shown in the Fig:

19 Fig: 3.17 Run Parameters Menu Finally, the calculations parameters should be specified in the "Run Parameters" list of options Fig: Fig:3.18 ProCAST solver in put window The calculation can now be launched as show in Fig On Windows, a Command window will open and the DataCAST and ProCAST are automatically launched. 48