GB/Z 28283-2012Guide for simulation and modeling of thermal processing (English PDF)

1 Scope

This guiding technical document specifies the meaning of simulation and modelling technology for hot working processes, the principles by which its scales are divided, its main functions and its main content; puts forward the common requirements as to accuracy and speed, physical simulation and precise testing, and integration; and gives an explanation of the main technical content, the main steps and the key factors of the simulation and modelling of casting, forging and welding processes.

This guiding technical document applies to the research, development, application and assessment of simulation and modelling technology for the hot working processes of materials.

2 Overview of the simulation and modelling of hot working processes

Simulation and modelling technology for hot working processes is the general name for the technology which, by means such as modelling and simulation and experimental testing, simulates the working process of a material in a virtual environment for hot working processes such as the casting, forging, welding and heat treatment of metallic materials and the injection moulding of non-metallic materials, displays the evolution of the shape, the dimensions, the internal structure and the defects of the material during the working process, predicts the quality of its structure and properties, and so achieves optimized process design.

The main technologies it involves are: a) numerical simulation of the hot working process - by establishing a mathematical and physical model able to describe a particular hot working process accurately and by the simplified solution of the mathematical equations, the process is displayed dynamically and its result predicted; b) physical simulation of the hot working process - a necessary method of verification, in which, following the principle of similarity, specimens are made of the same or a similar material and tested under similar conditions, so as to obtain the relevant laws, data and criteria of the process and to check and verify the results of the numerical simulation;

c) expert systems - the relevant expert knowledge is expressed, in a definite logical structure, in a form the computer can use, so as to optimize the process parameters and the design; an expert system generally comprises three core parts, the knowledge base, the logical reasoning and judgement system and the man-machine interface. Because the factors affecting hot working are very complex, it is also an effective complement to numerical simulation; d) the basic theory of the hot working process and the analysis of defect formation - this is the technical foundation for establishing the mathematical and physical model of the process accurately and obtaining a scientific criterion for defects.

3 Division of scales in the simulation and modelling of hot working processes

Simulation is divided into three different scales: macroscopic (mm to m level), microscopic (micrometre to mm level) and atomic (nm to micrometre level). The macroscopic scale (mm to m level) generally has as its object the prediction of the macroscopic scale of the outline, such as shape and dimensions. The microscopic scale (micrometre to mm level) generally has as its object the prediction of structure, constitution and properties. The atomic scale (nm to micrometre level) generally has as its object the prediction of structure, constitution and properties.

4 Main functions of the numerical simulation of hot working processes

4.1 Dynamic simulation of the process: the implementation process of the various processes and the evolution of the shape, the outline, the dimensions, the defects and the structure of the material are displayed graphically.

5 Main content of the numerical simulation of hot working processes

5.2.2.4 Flow field - pressure field and velocity field. The flow field (pressure field, velocity field), built on the foundation of fluid mechanics, is an important model for simulating the mould filling process of a casting, and is used to predict defects of the casting such as cold shuts, entrapped gas, slag inclusions and sand wash, and to optimize the gating system.

5.2.3 Simulation of the microstructure and of defects. 5.2.3.1 Overview: this is the simulation of the evolution process and the result of the microstructure of the material during the hot working process - dendritic growth, eutectic growth, the transformation of equiaxed grains such as granular crystals, grain size, phase transformation - and of defects on the microscopic scale, such as mixed grains, segregation and hydrogen-induced cracking. The models describing the evolution of the microstructure and of defects are mainly stochastic statistical models, the phase field method, the phase transformation field and models for predicting particular defects.

5.2.3.2 Stochastic statistical models: there are the Monte Carlo method and the cellular automaton method, used mainly to simulate the formation and growth of the grain structure during the liquid-to-solid transformation. 5.2.3.3 The phase field method: through differential equations it reflects the combined action of diffusion, the ordering potential and the thermodynamic driving force during the liquid-to-solid transformation, and can simulate realistically the solidification process of a liquid metal and the formation and growth of the structure.

5.2.3.4 The phase transformation field: this is the mathematical and physical model that simulates the transformation of the structure during the heat treatment of a metal. The mutual relations and effects between the phase transformation and temperature (the latent heat of transformation) and stress (stress-induced transformation, and the occurrence of transformation stress and transformation plasticity) have to be considered together. 5.2.3.5 Models for predicting particular defects: the dynamic recrystallization model for the evolution of grain size during thermoplastic working (predicting mixed grains in a large forging), the model of the local hydrogen concentration build-up and diffusion during welding (predicting hydrogen-induced cracks), and others.

5.2.4 Coupled calculation of several physical fields. The various physical fields and methods are coupled locally or systemically. First comes the coupling of the various physical fields at the macroscopic simulation level, in which the temperature field is the basis on which the other physical fields are built; the common couplings are temperature field with stress/strain field and temperature field with flow field. Next, the continuum equations describing the macroscopic phenomena of the hot working process (temperature field, stress/strain field, velocity field) are coupled with the models describing the evolution of the microstructure, for instance temperature field with phase transformation field, stress/strain field with phase transformation field, temperature field with statistical model, temperature field with phase field, and temperature field with the stress/strain field microscopic defect prediction model - a coupling among several macroscopic and microscopic models.

5.3 Post-processing of the numerical simulation. The task of post-processing is to output the results of the numerical simulation as graphics, images and three-dimensional process animations having engineering meaning and used to guide process analysis, that is, dynamic visualization. It contains the basic effective models for the tooling design needed in manufacture.

6 Accuracy and speed of the numerical simulation of hot working processes

The accuracy and the speed of the numerical simulation are important factors affecting the numerical simulation; in selecting and applying the numerical simulation of a hot working process it is necessary to consider whether the accuracy and the speed can meet the requirements.

Annex A (informative) General rules of simulation and modelling technology for casting processes

A.1 Main content of simulation and modelling technology for casting processes. Simulation and modelling technology for casting processes comprises mainly the numerical simulation of the solidification process of the casting, the numerical simulation of the mould filling process of the casting, the numerical simulation of the thermal stress of the casting and the numerical simulation of the microstructure of the casting.

A.1.1 Numerical simulation of the solidification process. The numerical simulation of the solidification process of a casting predicts, by calculating the temperature gradient of the temperature field and the solid-fraction solidification time, and by a series of criteria, the position and size of the shrinkage cavities and porosity produced in the casting during solidification and the time at which they are produced. Through such prediction the casting process scheme drawn up can be modified and then verified by numerical simulation. At the same time, the method of solidification numerical simulation can determine the relation between the ingate and the casting when a sound casting is to be obtained.

A.1.2 Numerical simulation of the mould filling process. The numerical simulation of the mould filling process of a casting is obtained by calculating the fluid flow during the filling of the mould with liquid metal. The numerical simulation of the filling process can analyse the flow of the liquid metal in the gating system and inside the mould under given process conditions, including the distribution of flow rate, the distribution of flow velocity and the resulting temperature field of the casting. Such a simulation can provide correct initial conditions for the subsequent solidification analysis.

A.1.3 Numerical simulation of the thermal stress. The numerical simulation of the thermal stress of a casting predicts the hot-tearing-sensitive zones and the hot tears by calculating the thermal stress field during the solidification of the casting and the residual thermal stress during cooling. The formation of stress in the casting affects not only the final quality and the performance in use of the casting (cracks, distortion) but also the quality of the process design (the accurate specification of the shrinkage allowance). Simulating the stress distribution of the casting under a given process helps to lower the possibility of stress arising in the casting to a minimum by modifying the process, and so to guarantee the quality of the casting.

A.1.4 Numerical simulation of the microstructure. The numerical simulation of the microstructure of a casting calculates the nucleation and growth during the solidification process of the casting, and the microstructure of the casting after solidification and the properties it may possess. In the simulation of the microstructure, a mathematical model unifying macroscopic heat transfer and mass transfer with microscopic nucleation and growth is used to describe the dynamic process of the formation of the microstructure, including spontaneous or non-spontaneous nucleation, grain growth, dendritic growth and the distribution of second-phase particles.

A.2 Main steps of simulation and modelling technology for casting processes. The main steps in applying simulation and modelling technology for casting processes may be divided into the following three stages. A.2.1 Pre-processing: a solid model is built according to the structure and shape of the actual object and is automatically divided into polyhedral elements. Generally speaking, for a casting of simple shape, a fairly accurate result can be obtained by numerical simulation using a two-dimensional approximation; for a casting of complex structure, three-dimensional simulation calculation is needed to meet the accuracy requirement.