Thixoforming : Semi-solid Metal Processing

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6.1 Empirical Analysis of the Flow Behaviourj171 Figure 6.3 Structural changes and corresponding viscosity changes as a function of shear rate. can reach such stability that it is able to withstand its own weight and can be handled like a solid. As soon as shear is applied, the network in the billet breaks up and the material starts to flow. These structural changes of the solid phase are reflected by the time-dependant flow properties. Quaak et al. [10] proposed a two-step model to describe the structural evolution during a shear rate jump and a shear rate drop. The influence of particle diameter on the viscosity is clearly visible on taking a closer look at the procedure to generate the metallic suspension in a rotational rheometer, subsequently called material preparation (Figure 6.4). First, the alloy is completely liquefied. The subsequent cooling down to the semi-solid state (section I) is followed by an isothermal shearing period of 3 h (sections II and III). During this time, Ostwald ripening (larger particles grow at the expense of smaller ones) takes place, leading to a continuous decay in viscosity. This phenomenon just occurs in a rotational rheometer where the experimental time is orders of magnitude higher than in capillary rheometers or compression tests. Ostwald ripening overlaps each Figure 6.4 Ostwald ripening during material preparation, exemplarily shown for tin–lead.
172j 6 Modelling the Flow Behaviour of Semi-solid Metal Alloys rheological measurement. Koke and Modigell [11, 12] introduced an approach to predict the influence of convective Ostwald ripening on the viscosity evolution during material preparation in a rotational rheometer. This approach is used here to eliminate particle growth from experimental results just to reflect rheological phenomena. 6.1.2 Mathematical Modelling of the Flow Behaviour The mathematical approach used to describe experimentally discovered flow phenomena depends on the solid fraction of the sample under investigation. Semi-solid material used for thixocasting with solid fraction between 40 and 60% is described using fluid mechanics whereas material used for thixoforging with higher solid fraction up to 90% requires approaches from solid-state physics [13]. 6.1.2.1 Approach from Fluid Mechanics It was mentioned earlier that in the semi-solid state the globular solid particles are dispersed in a liquid Newtonian matrix and consequently modelling approaches known from classical suspension rheology can be applied. In the equilibrium state, semi-solid alloys are shear-thinning and the most common descriptions are as follows: Ostwald de Waele: t ¼ kg_ n Yh ¼ kg_ n 1 Herschel–Bulkley: t ¼ t0 þ kg_ n Yh ¼ t0 Cross model: þ kg_ n 1 g_ t ¼ t¥ þ t0 t¥ 1 þ kg_ nYh ¼ h ¥ þ h0 h ¥ 1 þ kg_ n The first two models just differ in the yield stress t0, which is a property thoroughly discussed in the literature. Barnes and Walters [14] doubt the existence of a yield stress and claim it to be the stress which cannot be measured due to device-related limits. Contrary to this, Cheng [15] defines a static and a dynamic yield stress, both of which are influenced by the internal structure of the material. In Section 6.2, recent investigations on the low-melting tin–lead alloy using creep tests at very low stresses show the necessity to distinguish between isostructural, dynamic and static yield stress. In addition to the yield stress, the models contain the consistency factor k and the flow exponent n. The stress approaches infinity for very low shear rates and zero for very high shear rates. In the Cross model, it is assumed that thixotropic fluids effectively behave like a Newtonian fluid in the case of very low or very high shear rates. The viscosity tends towards h0 for shear rates ! 0 and towards h¥ for shear rates ! ¥. Atkinson [2] fitted several experimental data for tin–lead to the Cross model but stated that experimental values at the transition to the plateau regions are sparse or even missing. To account for thixotropic effects in metallic suspensions, the common theories can be divided into three groups [2, 16]:
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Thixoforming Semi-solid Metal Proce
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Further Reading Herlach, D. M. (ed.
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The Editors Prof. Dr. G. Hirt Insti
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VI Contents 1.3.5.2 Other Aluminium
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VIII Contents 4.6.1 Thixocasting 13
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X Contents 7.4.2 Isothermal Non-ste
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XII Contents 9.4.1.3 Simulation Res
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Preface Semi-solid forming of metal
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XVIII List of Contributors Andreas
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XX List of Contributors Alexander S
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2j 1 Semi-solid Forming of Aluminiu
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10j 1 Semi-solid Forming of Alumini
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Table 1.1 Overview of semi-solid pr
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Part One Material Fundamentals of M
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32j 2 Metallurgical Aspects of SSM
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44j 3 Material Aspects of Steel Thi
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Temperature (ºC) 1550 1500 1450 14
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100j 3 Material Aspects of Steel Th
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106j 4 Design of Al and Al-Li Alloy
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7 A Physical and Micromechanical Mo
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3. Macroscopic averages or homogeni
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displacement boundary conditions or
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and liquid are both assumed to be i
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are the strain rate concentration t
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Semi-solid viscosity (Pa s) into cl
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Load (kN) strain to rupture, leadin
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Load (kN) 7 6 5 4 3 2 1 0 0 7.5 Con
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adius R radius of the spherical inc
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Part Three Tool Technologies for Fo
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242j 8 Tool Technologies for Formin
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9 Rheocasting of Aluminium Alloys a
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Figure 9.2 Processes for the semi-s
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Two Italian companies gained the fi
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9.2 SSM Casting Processesj317 For t
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Figure 9.7 Oil pump cover for Honda
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Figure 9.9 Middle temperature cours
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Figure 9.12 Components of cartridge
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Figure 9.15 Process adapted multipa
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step-shot experiments. It is also s
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Figure 9.18 (a) Meander tool for fl
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Figure 9.19 (a, b) Wear appears in
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Table 9.2 Temperature determination
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and are plausible on the left. Furt
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Figure 9.27 Development of the micr
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some places can be observed as smal
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Figure 9.31 Simulation of the metal
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Figure 9.33 So-called Constant Temp
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Figure 9.35 Comparison of the micro
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9.4 Rheoroutej347 If rounding of th
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9.4.1.2 Parameters: Cooling to the
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Figure 9.36 Results of the 2D-MICRE
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homogeneous microstructure and no d
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grain fragment density [mm -1 ] 200
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can be considered for rheocasting o
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Figure 9.44 Microstructure of Zr/Sc
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Figure 9.45 MAGMAsoft simulation: c
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The cooling to the process temperat
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D grain diameter of a circle DGM De
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27 Doppelbauer, M. (2003) Wirtschaf
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10 Thixoforging and Rheoforging of
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1970s [7]. Also, the quality of the
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10.3 Heating and Forming Operations
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With the solution of the Equations
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10.3.1.2 Modelling of Inductive Hea
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material properties have to be cons
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10.3 Heating and Forming Operations
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Figure 10.9 Experimental results, d
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Figure 10.12 Segregation in compone
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10.3.4 Thixoforging of Steel In thi
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10.3.5 Thixojoining of Steel In the
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einforcing element. The possibiliti
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Figure 10.20 The robot controller a
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Figure 10.21 Experimental results w
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Figure 10.24 Design and working pri
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The investigations show that the rh
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Figure 10.31 Scheme of the heat tra
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Table 10.2 Definition of boundary c
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Figure 10.35 Experimental and simul
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References 1 Koesling, D., Tinius,
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steel billets into the semi-solid s
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412j 11 Thixoextrusion The followin
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414j 11 Thixoextrusion channel with
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416j 11 Thixoextrusion parameter co
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418j 11 Thixoextrusion Both concept
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420j 11 Thixoextrusion should be ju
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422j 11 Thixoextrusion Figure 11.8
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424j 11 Thixoextrusion Table 11.4 C
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426j 11 Thixoextrusion Temperature
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428j 11 Thixoextrusion impacts. Ext
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432j 11 Thixoextrusion shell format
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436j 11 Thixoextrusion Table 11.6 P
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440j 11 Thixoextrusion solidificati
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442j 11 Thixoextrusion GPa pressure
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444j Index arbitrary volume element
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446j Index equilibrium tests 141 eq
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448j Index - importance 273 ion flu
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450j Index oxygen-sensitive element
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452j Index - thixoforming 241 semi-
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454j Index - high pressure 131 - sc
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Thixoforming : Semi-solid Metal Processing

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