PhD thesis Extended abstract

Transcription

1 TECHNICAL UNIVERSITY OF CLUJ NAPOCA FACULTY OF MATERIALS SCIENCE AND ENGINEERING Dipl. Eng.Adriana-Voica NEAG PhD thesis Extended abstract RESEARCHES REGARDING THE OBTAINING AND THE BEHAVIOUR TO DEFORMATION OF THIXOTROPIC ALUMINIUM ALLOYS Scientific Superviser: Prof. Dr. Eng. Traian CANTA Thesis jury : PRESIDENT: MEMBERS: Prof. dr. eng. Valer MICLE Deputy Dean, Faculty of Material Science and Engineering, Technical University of Cluj-Napoca Prof.dr.eng. Traian Canta - PhD Supervisor, Technical University of Cluj-Napoca Prof.dr.eng. Virgil GEAMAN - Reviewer,"Transilvania" University of Brasov Prof.dr.eng. Vasile Soporan Reviewer, Technical University of Cluj- Napoca Prof. dr. eng. Regis BIGOT - Reviewer, Ecole Nationale Supérieure d Arts et Métiers, Metz, Franta; CLUJ-NAPOCA 2008

2 TABLE OF CONTENTS INTRODUCTION... 7 CHAPTER I: STATE OF ART OF THE RESERCHES REGARDING THE OBTAINING AND THE BEHAVIOUR TO DEFORMATION OF THE THIXOTROPICAL ALLOYS 1.1 Theoretical considerations Semi-solid state The obtaining of the semi-solid state a) Methods based on partial solidification b) Methods based on partial remelting Thermodynamical equilibrium solid-liquid Microstructural evolution during solidification Methods for obtaining non-dendritical microstructure Methods for obtaining the non-dendritical microstructure from liquid material Methods for obtaining the non-dendritical microstructure from solid material Physical interpretations of non-dendritical solidification Advantages and disadvantages of semi-solid methods Behaviour of semisolid alloys Comparison between behaviour of dendritical and non-dendritical structure alloys The influence of the elaboration parameters on the microstructure a)cooling rate b) Shear rate c) Stirring staging on the semi-solid state Quantitative analysis Rheological methods of characterization a) Couette viscosimeter (rheometers) b) Compression through parallel plates c) Indentation d) Direct extrusion e) Backward extrusion Conclusions CHAPTER II: MATERIALS, EQUIPMENTS AND EXPERIMENTAL METHODS 2.1 General considerations Experimental conditions and equipments for obtaining the materials Experimental conditions for obtaining the Al-Si alloy Experimental conditions for obtaining the Al-Zn alloy Methods for determination the characteristic temperatures a) Thermal analyses b) Differential scanning calorimetry c) Analytical method Equipments for partial remelting of the billets Reheating equipment in ventilated furnace with electrical resistance (LGPMM)... 61

3 2.3.2 Reheating equipment in furnace with electrical resistance (IPM) Induction reheating equipment (LGPMM ) a) Billet position in induction device b) Temperature and reheating cycle control Experimental conditions and equipments used in semi-solid deformation Hydraulic press (Heckert) Hydraulic press (LLOYD) Hydraulic press (Loire from ENSAM Metz) Description of the technology used in IPM laboratory Description of the technology used in LGPMM laboratory Backward extrusion a) Die design used in IPM laboratory b) Die design used in LGPMM laboratory c) Thermal transfer estimation d) Backward extrusion stages e) Cup backward extrusion used for study the deformation behavior Conclusions CHAPTER III: EXPERIMENTAL TESTS AND RESULTS FOR CARACTERISATION OF THE DEFORMATION BEHAVIOUR IN THE SEMI-SOLID STATE OF ALUMINIUM ALLOY 3.1 General considerations Primary structure Microstructure evaluation of AlSi7Mg03 (A356) alloy Microstructure evaluation of AlZn5,5Mg2,5Cu1,6 (7075) alloy Microstructure evolution after partial reheating Partial reheating in electrical resistance furnace (LGPMM) a) Microstructure evolution and results analysis b) Quantitative analysis Partial reheating in electrical resistance furnace (IPM) Induction partial reheating (LGPMM) a) Microstructure evolution and results analysis Experimental results obtained at semi-solid deformation Behaviour characterization of AlSi7Mg03 alloy - Force evolution a) Influence of the microstructure on deformation force b) Pieces obtained c) Microstructures obtained after semi-solid deformation d) Mechanical characteristics of the piece after deformation in semi-solid state e) Problems encounted f) Technological modifications / improvements Behaviour characterization of AlZn5,5Mg2,5Cu1,6 alloy - Force evolution a) Billet temperature influence on extrusion force b) Press punch rate influence on extrusion force c) Pieces obtained by tixoextrusion of 7075 alloy d) The microstructure of tixoextrusion samples e) Problems encounted at tixoextrusion f) Improvements Conclusions

4 CHAPTER IV: MATHEMATICAL MODELLING AND COMPUTER SIMULATION OF THE THIXOFORGING BEHAVIOR 4.1 Theoretical consideration Mathematical models presents in literature Application of the modelling to study the behaviour on backward extrusion Theoretical consideration on the used models Modelling with a solid model a) Input data used for finite element analysis Micro-modelling or micro-macro modelling a) Principle of micro-macro modelling b) The limits of micro-macro model c) Constitutive equations Contributions to modelling of the backward extrusion process Norton-Hoff model a) Identification of the rheological parameters b) Study the influence of friction and thermal exchange conditions Simulation with a Norton-Hoff model a) Study of the material flow in a die Micro-macro model a) Microstructure description definition of the representative elementary volume Validation through simulating of the micro-macro mathematical model a) Parametrical analysis b) Comparison of the theoretical model with the experimental results Conclusions CHAPTER V: FINAL CONCLUSIONS AND PERSONAL CONTRIBUTIONS 5.1 Finale conclusions Further directions of development Personal contributions Dissemination of the studies and experimental results REFERENCE APPENDIX Short presentation of the thesis The importance of the subject of this work Researches regarding the obtaining and the behaviour to deformation of thixotropic aluminium alloys which represents a novelty on the national level - consist in the importance of knowing the mechanism involved in obtaining the materials having the required characteristics for semisolid state deformation, and also of their behaviour during semisolid deformation. The opportunity of choosing this theme it s confirmed on the international scale by the interest showed by the experienced team in this domain from the ENSAM, Metz (Regis Bigot, Veronique Favier), who gave me the possibility to study thoroughly this issue and ongoing the researches regarding the micro-modeling of the deformation process in the semisolid state.

5 Semi-solid material forming (SSM) is one of the near-net-shape forming processes which manufacture the final part by using the material at the temperature between liquidus and solidus. The key of the process is to obtain semi-solid slurry, free of dendrites, with: solid being present as non-agglomerated, fine and spherical particles, with minimum entrapped liquid in the solid, in order to obtain the necessary thixotropic flow behaviour. The semi-solid forming combines the advantages of conventional casting and forging. Its benefits from the material capacity to behave like a solid at rest, that eases its transportation, and like a liquid under deformation, that eases its flow. Other major advantages include near-net-shape production of complex geometry parts, weight savings in components with less porosity than conventionally. In general, that components exhibits better mechanical properties than conventional casting or forging. The semi-solid processing has a significant impact in a number of industries including aerospace, automotive and electronic components. The main objectives of the present doctoral thesis are the following: Establishing a technology to obtain aluminum alloys with thixotropical characteristics and developing some laboratory equipments to achieve this; Characterization tests of behaviour of this alloys to reheating in the semi-solid state; Studying the influence of reheating temperature and the extrusion rate on the semisolid deformation; Implementation of the experimental conditions into a specialized software code (FORGE 2005) Mathematical modeling of the deformation in semi-solid state using a solid model; Validation of the micro-macro model (developed by ENSAM) considering the experimental conditions of tixodeformation by backward extrusion; Studying the model parameters and realizing the necessary adaptations and optimizations in the software code to simulate the deformation process Simulation of the semi-solid deformation process. The doctoral thesis is structured on 5 chapters, developed on 183 pages, includes 146 figures, 12 tables and 82 relations, from which 10 tables and 105 figures are in the original part. The references list contains 133 titles, 9 of these are signed by the author and are based on the doctoral subject that were published in the volumes of national or international scientific manifestations. Chapter I realized an introduction in the thesis subject. Here is explained the importance and the actuality of the theme and are established the objectives of the thesis. It is presented the evolution of the researches regarding the semi-solid behaviour of various materials, highlighting the importance of this issue. It s presented some groups of specialists on semi-solid processing. Are emphasized the most important aspects of the materials behaviour in semi-solid state. Are presented some details about semi-solid state and is introduced a classification of semi-solid technologies with theirs definitions. Some experimental results, obtained by different researchers, explain through comparison, the behavior of dendritical alloys respect to non-dendritical alloys. In Chapter II are presented a cast aluminium alloy A356 ingot and an extruded aluminium alloy 7075-T651 bar, with 40 mm diameter. The experimental procedure for obtaining the A356 alloy is illustrated. There are also emphasized the experimental methods used to determine the characteristic temperatures of the alloys utilized in this work. Are described the equipments and the installations used for rheological behaviour characterization of the two aluminium alloys, on semi-solid deformation and the reheating installation used. The two dies and the principle of the cup backward extrusion are showed. Chapter III underscores in the first part, the primary microstructure of the two alloys considered and presents the evolution of their microstructure, when they are remelted into electrical resistance furnace respective electrical induction furnace. The quantitative analysis

6 results are presented. The temperature control during reheating and establishing the optimum heating cycle are essential in semisolid processes. The homogeneous microstructure with spherical grains is obtained in transversal and longitudinal section on the slug. The microstructure reveals a light primary solid phase and a dark eutectic phase, corresponding to the solid phase and to the liquid phase which is supposed to be present in the semi-solid state. Are also exposed the experimental technologies applied for the semisolid deformation of the two alloys. Throughout backward extrusion experimental tests it was possible to study the evolution of the deformation force depending of the punch stroke during deformation, showing the influence of some parameters, such as: temperature, strain rate and microstructure. We emphasize the presence of a solid fraction limit. Below this limit, the surface quality of the components decreases probably because of a too strong die/material relation. Were considered as well some samples obtained through thixoforging procedures and were underlined the problems which can occur during filling the die cavity. The information gained due to backward extrusion experiments were applied into modeling and simulations studies. Chapter IV is focused on mathematical modeling of the semisolid materials behaviour. A first behaviour modeling study of semisolid alloy was meant to verify the ability of a classical Norton-Hoff viscoplastic law to describe a relatively complex semisolid behaviour. We have demonstrated that the Norton-Hoff law appeared insufficient to describe some characteristics of semi-solid behaviour such as: the influence of strain rate, the temperature or the microstructure. We have considered that the theoretical morphological pattern proposed by micromechanical modeling is realized. We have implemented the experimental condition in FORGE2005 and we used the micromechanical modeling accounting for the key mechanical role of the solid bonds and free liquid to describe the semisolid behaviour. The aim of this model is to represent physical deformation phenomena and especially to take into account the agglomeration and disagglomeration processes of solid skeleton occurring during the deformation, which are mainly responsible for the semi-solid behaviour. Next, we analyzed the evolution of degree of agglomeration during the flow regarding with the strain rate and temperature fields. Micromodeling permits the validation of the micro-macro model in the conditions of backward extrusion deformation. The initial and boundary condition imposed for the simulations are as closed as possible to those recorded for the experimental tests. The obtained results using both models are presented in comparison to experimental results. Also, in both cases were presented the model equation parameters which were established by experimental tests. Chapter V contains final remarks result about characterization tests of rheological behaviour of the aluminium alloys studied. There are made some proposals for the further researches in the field and a brief summary of the most significant contributions of the thesis. Several of this personal contributions are the followings: A356 aluminum alloy elaboration with dendritical structure and realizing some laboratory equipments; It was compared and analyzed two technologies of dendritical structure modification to achieve a non-dendritical structure; It was studied the influence of isothermal maintaining period and of the maintaining temperature, on the microstructure evolution; There were established the optimal parameters to obtained the globular microstructure; It was establish an experimental technology of thixoextrusion which can be adapted to an industrial level as well; A ten times reduction of extrusion force was obtained to the deformation of a non-dendritical structure relatively to the dendritical one, for the A356 alloy; The samples obtained are healthiness without obvious micro or macro-porosities;

7 A mathematical model based on Norton-Hoff law it was elaborated and the constitutive equation parameters are presented; It was demonstrated that the model based on Norton-Hoff law, underestimate the level of necessary deformation force. This is not an adequate mathematical model for the rheological experimental tests presented. It clearly outcomes the necessity of a law implementation specific to the tixo materials behaviour. An original mathematical model micro-macro, still in development by the ENSAM team, was verified to backward extrusion process, being validate by the experimental results; Based on experimental results and using the inverse method, there was identified the parameters of micro-macro model in the conditions of 7075 alloy backward extruded; The original results obtained with the micro-macro model, has allow a connection between the experimental results and the simulation one for a cup backward extrusion. A significant number of simulation in Forge2005 was realized using the two models; The experimental curve of the force variation on the extrusion process was compared with the curve obtained through simulation. Analyzing the errors between the experimental curve and the modeled one, it was carried out a series of studies finalized with some proposals in order to reduces this errors; The results of realized modeling tests are the perfect illustration of the actual difficulty in modeling the semi-solid materials forming. The appendix that is presented in the final of thesis shows the results obtained during the experimental processes of establishing the reheating cycle and of the analyze of the model parameters. The research work in this field was extended in the projects entitled: AEROSPACE, period : ; Coordinator: S.C. ICPT TEHNOMAG CUG S.A., partners: Technical University of Cluj Napoca; Modelisation des effets thermiques en thixoforgeage, period : , bilateral contract - PAI-Brâncuşi with ENSAM, Metz, France director for the Romanian part.