Thixoforming : Semi-solid Metal Processing

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4.1 Production of Raw Material for Thixoforming Processesj107 advantages over other heating processes due to its inherent controllability and its frequency-dependent, relatively high penetration depth. During reheating, the eutectic phase melts first. The liquid portion rises steeply with minimal heating at temperatures above 560 C. From about 570 C, the entire eutectic phase is molten. At temperatures above 570 C, the aluminium dendrites begin to melt. The liquid portion rises only slowly in this temperature range. The working temperature for reheating is precisely within the transformation range between fully molten eutectics and commencement of dendrite melting. In addition to its low temperature sensitivity to the phase relationship, A356 has the characteristic that, in its partially liquid state, 50% primary phase and 50% eutectic exist. Due to this composition, there are material-specific advantages in terms of processing. Materials for which it is not possible to adjust the solid–liquid mixture so accurately require considerably higher processing costs [7]. In the slug on demand or rheocasting processes, the production of a suitable thixoformed raw material is done straight from the fluid melt with direct forming of the partially liquid metal [1]. Various new rheocasting processes are briefly explained in the following (see Figure 4.1, right-hand column). In new rheocasting (NRC), the melt is cast in a controlled manner into a crucible and thus cooled to the desired temperature between the solidus and liquidus states. This creates a fine-grained, globulitic solid phase. After a short homogenization phase, the partially solidified material is fed directly into the forging or casting process. In twin-screw rheomoulding, the melt is fed into the machine via a casting gate. There are two interacting screw feeders, with a special profile to achieve high shear rates, to convey the material axially into the machine and where it is simultaneous cooled to the partially liquid state. The high shear rate creates a fine-grained, globulitic grain structure. The partially liquid metal is dosed and pressed out of an opening into the filling chamber for forming in a modified die-casting machine. To date this process has been used only for modelling alloys of the types PbSn15 and MgZn30. One difficulty with this process is the choice of a suitable material for the screw feeders, in order to avoid adherence of the metal and to minimize wear [1]. The Vexocast process was developed by the company Ritter in Germany [1]. The metallic suspension is produced directly from the melt in a separate treatment chamber which is in combination with a holding furnace and with the casting chamber. The melt is inductively agitated in the treatment chamber and in the freezing range a small quantity of metal powder is added to cool it. One essential feature of the process is its short time range of only about 15 s for production of the metallic suspension. The channelling process represents a further application method of the slug on demand process. The two essential constituents of the trial setup for the channelling process are the casting channel and the forming basin. The channel has to provide for rapid cooling of the melt, down to a temperature below the liquidus temperature. This creates a very large number of active nuclei suspended in the melt. The melt flows from there into a ceramic forming basin installed directly underneath the channel. On cooling to the partially liquid range, the high nucleus density created results in the formation of a globulitic primary phase.
108j 4 Design of Al and Al–Li Alloys for Thixoforming The thixomoulding process entails injection die casting of partially liquid light metal alloys, predominantly Mg alloys [9, 10]. The original material, in the form of granulate or turnings, is transferred by powerful shear movements in a heated steel screw feeder into a liquid metallic suspension. Subsequently, the screw feeder performs a pushing motion to inject the material into the actual forming tool [11]. The proportion of solid material can be up to 65% [12]. 4.2 Chemical Grain Refinement of Commercial Thixoalloys If a metal alloy has to be processed in a partially liquid state, it must conform to many requirements. The following criteria are among the most important: . In the semi-solid range, the alloy constitutes a globulitic solid phase with grain sizes of
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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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Figure 5.7 The density of X210CrW12
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5.3 Calculations for the Bearing St
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Figure 5.11 Composition profiles of
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Figure 5.14 The density of 100Cr6.
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area which is available for heat tr
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Part Two Modelling the Flow Behavio
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170j 6 Modelling the Flow Behaviour
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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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