Modelling and Numerical Simulation of Ductile Damage in Bulk Metal Forming

A complete set of fully coupled constitutive equations accounting for both combined isotropic and kinematic hardening as well as the ductile damage under anisothermal conditions at finite (visco)plastic strain is developed and implemented into the general purpose Finite Element code for metal forming simulation. First, the fully coupled anisotropic constitutive equations in the framework of Continuum Damage Mechanics are presented. Attention is paid to the strong coupling between the main thermomechanical fields as thermal effects, elasto‐viscoplasticity, mixed hardening, ductile isotropic damage and contact with friction. The associated numerical aspects concerning both the local integration of the coupled constitutive equations as well as the (global) equilibrium integration schemes are presented. The local integration is outlined thanks to the Newton iterative scheme applied to a reduced system of two differential equations. For the global resolution of the equilibrium problem, the classical dynamic explicit (DE) scheme with an adaptive time step control is used. A fully adaptive 2D methodology with mesh and loading sequences adaptation based on some appropriate error estimates is used. For 3D simulations only a constant appropriately refined 3D mesh is used. Various 2D and 3D examples are given in order to show the capability of the methodology to predict the ductile damage initiation and growth during metal forming processes.     

FE–Analysis of Simultaneous Hot/Cold Forging

A new, highly innovative manufacturing method is under development in research and industry, where the workpiece undergoes simultaneously a hot and a cold forging process followed by press hardening directly in the forging dies. Such a high complex forging technology is simulated by means of the finite element method, where the inductive electrical heating is modelled with substitute sources for the heat production besides the forming and the cooling process with thermal deformation and eigenstress evolution. The measured data for the temperature field and the final geometry of the forging test are compared to the results of the simulation, showing a very good agreement.     

Hot Workability of as‐Cast High Manganese‐High Carbon Steels

In order to produce new high Mn‐high C austenitic steels (R_m>700 MPa), different tests and methods were used to determine a suitable window of process parameters. In‐situ melting hot tensile tests and hot compression tests were carried out to investigate the hot ductility, fracture characteristics and flow behaviour during continuous casting and hot deformation of 3 steels with Mn and C contents between 9‐23% and 0.6‐0.9%, respectively. The results show that these steels are susceptible to interdendritic fracture at high temperatures. Decreasing Mn content improves the reduction of area at high temperatures to 60% or more. Hot deformation loads for processing the investigated steels are not higher in comparison to the stainless steel 1.4301.     

Strain‐Rate Sensitivity in Hot Forming of Steels – Influence of Microstructure,Temperature and Chemical Composition

A methodology to determine the strain‐rate sensitivity index was developed, based on rolling of a set of samples with the same draught but different speed at defined temperatures. It was shown that initial grain size has nearly negligible influence on the investigated variable, in contrast to phase composition whose influence is very considerable. Combined influence of strain rate and temperature on deformation resistance of various types of steel was studied. For a selected group of steels a universal equation was set up, which described, with a good accuracy, impact of reciprocal temperature and chemical composition (expressed simply by nickel equivalent) on strain‐rate sensitivity in hot state.     

Experimental Testing and Computer Simulations of Ductile Fracture in Tool Steels

Ductility was determined in experimental four‐point bending tests of smooth specimens of tool steel. The tool steels had different contents of carbides and carbide sizes and with a hardness of approximately 60HRc. Two of the materials tested were produced powder metallurgically, one was spray formed and one was conventionally uphill ingot cast. Carbide size distribution analysis was performed on planar polished sections of each material. Correlation between carbide microstructure and ductility performance was obtained. The fracture mechanisms were investigated with fractography. A 3D FE‐model was used to simulate the four‐point bending tests and thereby analyse the matrix flow curve. Also the strain at failure was analysed for each material when simulations were performed based on experimental data. SEM‐images of the materials carbide microstructure were used to create 2D FE‐models. The models simulated crack initiation and propagation by removing elements in the steel matrix as the plastic strain reached a critical level. With three variants, simulations of crack initiation and propagation at carbides were investigated. That was carbides with no cohesion to matrix, carbides fixed to the matrix and carbides with internal cracks. Comparison of strains at failure for the 2D and the 3D FE‐models showed good correlation.     

Processing Maps for Use in Hot Working of Ductile Iron

  The hot deformation characteristics of ductile iron are studied in the temperature range of 973 to 1273K and strain rate range of 0001 to 1 s-1 by using hot compression tests. Processing maps for hot working are developed on the basis of the variations of efficiency of power dissipation with temperature and strain rate. The results reveal that the flow stress of ductile iron is sensitive to strain rate. In the processing map under strain of 07, a domain is centered at 1273 K and 1 s-1, and the maximum efficiency is more than 36%. According to the maps, the zone with the temperature range of 1173 to 1273 K and strain rate range of 01 to 1 s-1 may be considered as the optimum region for hot working.     

变压器用热轧波纹板冷弯成形过程分析

 利用实验室模拟及有限元模型两种方法模拟波纹板冷弯压靠成形过程,结果表明,该材料所能承受的最大拉应变远大于最危险点所承受的拉应变,残余压应力位置处的显微硬度高于残余拉应力位置处。变形过程中的冷弯压靠危险点未发现冷弯裂纹,热轧酸洗板能够满足变压器用波纹板冷成形要求,变压器波纹板以热代冷方案是可行的,也符合节能降耗的要求。     

Semi‐Solid Processing of Metal Alloys

Semi‐solid metal casting is an innovative technology for the production of near‐net‐shape parts with demanding mechanical properties. The paper describes different processing routes and materials for semi‐solid‐metal casting (SSM), which have been investigated and also partially developed at the Foundry‐Institute of Aachen University. The standard thixocasting process for aluminium, highly reactive magnesium alloys and steel alloys with high melting points was investigated under variation of a wide range of process parameters. Specially adapted pre‐material production and reheating methods were developed for different materials and their application and future potential is pointed out. The thixocasting experiments were executed on a modified high pressure die‐casting machine with a specially designed “step‐die” providing wall thicknesses from 0.5 to 25 mm. The mechanical properties were tested in dependence of the wall thickness and the metal velocity. The results of these examination show high tensile strength values in combination with very good elongations. The rheocasting process is a new SSM‐forming method with liquid melt as feed‐stock and a high recycling potential. The research results of RCP‐technology (Rheo‐Container‐Process) invented at the Foundry‐Institute and of the Cooling‐Channel‐Process for aluminium and magnesium alloys are promising and are presented in this paper. Studies on semi‐solid processing of magnesium alloys and mixtures of them were conducted by Thixomolding^TM. To establish the most adequate process parameters, the temperature and the mixture relations were varied. Using a mould for tensile test specimens, the mechanical properties and the microstructure evolution could be evaluated. The chemical composition of the different phases was determined using SEM and EDX technologies. Evaluations of the flowing properties were conducted using a spiral mould with a total length of 2m and a cross section of 20mm x 1.5mm.     

Numerical Modelling of Semi‐Solid Flow under Processing Conditions

During the industrial process of semi‐solid forming (or thixoforming) of alloy slurries, typically the operation of die filling takes around 0.1s. During this time period the alloy slug is transformed from a solid‐like structure capable of maintaining its shape, into a liquid‐like slurry able to fill a complex die cavity: this involves a decrease in viscosity of some 6 orders of magnitude. Many attempts to measure thixotropic breakdown experimentally in alloy slurries have relied on the use of concentric cylindrical viscometers in which viscosity changes have been followed after shear rate changes over times above 1s to in excess of 1000 s, which have little relevance to actual processing conditions and therefore to modelling of flow in industrial practice. The present paper is an attempt to abstract thixotropic breakdown rates from rapid compression tests between parallel plates moving together at velocities of around 1m/s, similar to industrial conditions. From this analysis, a model of slurry flow has been developed in which rapid thixotropic breakdown of the slurry occurs at high shear rates.     

Multi‐scale Finite Element Cellular Automata Simulation of Multi‐step Cold Forging Operations

The application of new materials to produce forged connecting parts is presented in this work. Particular attention is put on modern bainitic steels due to their increased ductile and strength properties, which influence the behaviour of final products under further exploitation conditions. Bainitic steels do not require a series of thermo‐mechanical operations to obtain these elevated properties, which is one of the advantages of this material. Experimental analysis and numerical simulations of steel behaviour during multi‐step cold forging operations are described in this paper. Since it is one of the possible fracture initiation mechanisms, strain localization development during cold forging is investigated in detail. Conventional constitutive models used in finite element programs have limitations in modeling stochastic and discontinuous phenomena that are responsible for strain localization. The cellular automata model is used as constitutive law in this work to overcome these difficulties and investigate material flow during multi‐stage cold forging operations. Connection of the cellular automata (CA) and finite element (FE) methods creates a so‐called multi‐scale CAFE model. The main aspects of the model are described briefly in this paper. The experimental part of this work supports the numerical investigation. Comparison of the parameters measured and predicted by the CAFE model is presented and discussed as well.     

Ductile Fracture Prediction of 316LN Stainless Steel in Hot Deformation Process

A ductile fracture criterion of 316LN stainless steel, combined with the plastic deformation capacity of ma- terial and the stress state dependent damages, was proposed to predict ductile fracture during hot deformation. To the end, tensile tests at high temperatures were first performed to investigate the fracture behavior of 316LN stain- less steel. The experimental results show the variation of the critical fracture strain as a function of temperature and strain rate. Second, the criterion was calibrated by using the upsetting tests and the corresponding numerical simula- tions. Finally, the proposed fracture criterion was validated by the designed tests and the corresponding finite ele- ment （FE） simulation. The results show that the criterion can successfully predict the onset of ductile fracture at ele- vated temperatures.     

Semi‐Solid Rheoforging of Steel

To produce steel components with complex shapes excessive machining is necessary frequently since high pressure die casting of steel is not industrially applied. Forming steel in the semi‐solid state can in principle produce new components and geometries which cannot be realised by conventional closed die forging. Semi‐solid forging of steel combines the possibility of producing geometries not conventionally forgeable in one forming operation and of adding further functions during the same operation. In previous investigations on thixoforming of steels, the semi‐solid steel was generated by reheating precursor material billets. An alternative approach for generating semi‐solid steel from the liquid state with subsequent forging operation is presented in this paper for the first time. The steel grades X210CrW12 cold work tool steel and 100Cr6 bearing steel are molten and driven into a globular semi‐solid state using a cooling slope and a cup. By cooling the steel into the semi‐solid range instead of heating it, the required process temperatures are lower than in the process route via heating. Therefore, the load on the dies in a semi‐solid forging operation is decreased. Suggestions for the respective layout of the process are made for both steel grades. Future potentials and challenges to be solved are discussed, showing advantages especially in the field of high melting point alloys such as steels. This technique enables to produce pre‐shaped semi‐solid billets to optimise the materials flow and the homogeneity of the mechanical properties.     

热风炉管网系统耐材内衬的破损及热态修复研究

热风炉管道耐材内衬在使用过程中特别是在一代炉役的中后期,会产生不同程度的破损,时常发生渣化、变形、破裂、掉砖、甚至垮塌等现象,造成热风管网产生窜风漏气、局部过热、发红变形等危险隐患,甚至发生烧穿爆裂等恶性事故。针对上述问题,介绍了利用高炉短休风时间,对热风炉管道耐材内衬进行热态下维护与修复,延长管道内耐材的使用寿命。     

A Novel Process Control Model for Extracting Vanadium Semi‐Steel in Basic Oxygen Furnace Based on Steelmaking Mechanism

A novel process control model for basic oxygen furnace (BOF) steelmaking is proposed based on metallurgy mechanism, with lower contents of carbon, manganese, silicon, and lower temperature in semi‐steel after extracting vanadium. According to mass balance and heat balance, a static control model is built up, with slagging model, temperature model, and oxygen supply model. When actual amount of oxygen supply reaches 85% of theoretical value calculated by static model, quasi‐dynamic control model is activated to predict carbon and temperature in later period of steelmaking. A steelmaking process control system for semi‐steel smelting on 120 tonne BOF is developed in a steelmaking plant in China. Based on test of three steel grades, this model acts a guide for BOF steelmaking with semi‐steel, not only providing tactic, but also pointing out a feasible method for BOF process control without sub‐lance or off‐gas analyzer.     

Bake Hardening of Hot Rolled Multiphase Steels under Biaxial Pre‐strained Conditions

Multiphase steels show a strong bake hardening effect being of importance for shaping of car body structural parts. The raised yield strength is exploited for improved crash resistance. Especially the automotive industry has a growing interest in using this effect. Normally the bake hardening effect is examined in tensile tests whereas under industrial conditions shaping of structural parts shows a wide spread of stress strain behaviour, from uniaxial conditions over plain strain to biaxial ones. So it is obvious that the bake hardening behaviour of a material cannot be described with results of the uniaxial tension test only. To give a first answer to this question, the dependence of the bake hardening effect on different biaxial prestrains was investigated for several hot rolled multiphase steels using various baking temperatures and holding times whereas the bake hardening effect under uniaxial prestrain had already been examined in [5]. Considering the choices to generate biaxial strain, a Marciniak forming tool with a diameter of 250 mm mounted on a 2500 kN hydraulic press was chosen. For control of plastic deformation and adjustment the non‐contact measuring system ARGUS, was used. To reduce the quantity of experiments “Design of Experiments” and statistical methods were applied for a martensitic steel, a dual phase steel, a complex phase steel, a ferrite‐bainite steel, and a retained austenite steel known as TRIP, all in hot rolled condition. As a result, a formula for yield stress, tensile strength and residual deformability was developed. Furthermore, a method was found to predict easily whether a steel under investigation is qualified for additional experiments in regard to bake hardening or more exactly its response to different baking temperatures and holding times.     

Evolution of Globular Microstructure in New Rheocasting and Super Rheocasting Semi‐Solid Slurries

Based on the solidification theory for metal alloys, a simple recipe for the controlled processing of globular microstructures without external stirring is presented: Firstly, small solidification nuclei must be distributed homogeneously throughout a melt. In New Rheocasting (NRC) these nuclei are formed by forced homogeneous nucleation due to partial quenching of the melt, while in Super Rheocasting (SRC) the nuclei are “second phase particles” in specially designed alloys, which are grown in a controlled fashion in a certain temperature range. Potential alloy compositions for SRC are provided. Secondly, given these melts with small particles in them, globular growth can be assured by utilizing the Gibbs‐Thomson “self healing effect” and slow further cooling to allow diffusion in the melt and to suppress constitutional supercooling. This simple recipe is applicable to various ferrous and non‐ferrous alloys. If an SRC alloy is cooled more rapidly than necessary for globular growth of the primary phase, but is held sufficiently long in the SRC range for dispersoid formation, these dispersoids can act as potent grain refiners and possibly enhance elevated temperature properties. A combination of both processes by using SRC alloys in the NRC equipment may lead to pressure tight castings with low porosity and finer grain structure than can be achieved with NRC on its own, and consequently, better mechanical properties.