Thixoforming : Semi-solid Metal Processing

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Figure 8.51 SEM images of alumina die surfaces after steel thixoextrusion experiments. (a) Die inlet; (b) Die land. 8.6 Bulk Ceramic Forming Toolsj299 ceramic material. Adhesion of steel on the die surfaces was observed sporadically and only locally, owing to the poor wetting of alumina by steel. This advantageous behaviour promotes the aforementioned high surface quality of formed parts. Microscopic analysis of the dies carried out using SEM and EDS substantiates these findings. Alumina die surfaces applied in steel thixoextrusion experiments, as depicted exemplarily in Figure 8.51, appear to be unaffected by tribochemical attack of the semi-solid steel slurry. This is demonstrated by the machining grooves originating from green machining of the die still being visible after application, thereby acting as indicators of tribological attack (Figure 8.51a). Although the accumulated contact time of alumina dies with semi-solid steel was short in small-scale forming series performed using self-heating ceramic dies, the findings are consistent with investigations on the chemical interaction of alumina ceramics with steel alloys in the temperature range of interest for steel thixoforming [5]. Thus, the potential of oxide ceramics in semi-solid metal forming with regard to improved tool life is clearly discernible. 8.6.4.4 Applicability of Self-heating Ceramic Dies for Steel Thixoforming The die concept of self-heating ceramic dies for the semi-solid processing of steel has been realized and tested by the construction of prototypes for thixocasting, thixoforging and thixoextrusion. The intended beneficial effects on the flow behaviour of the semi-solid slurries, reduction of process forces and geometric complexity of work pieces have been accomplished in small-scale forming series. Regarding the thermomechanically induced stresses, denoted short-term effects according to the general load profile depicted in Figure 8.1, self-heating alumina dies showed sufficient resistance in near-isothermal steel thixoextrusion experiments. A detailed analysis of the temperature-dependent mechanical strength of the alumina dies in steel thixoextrusion is given elsewhere [2]. Regardless of the excellent wear and corrosion resistance of thixoextrusion dies observed herein, a prediction on longterm performance of such tools is not feasible on the basis of current experiments, since contact times with the semi-solid slurry were too short.
300j 8 Tool Technologies for Forming of Semi-solid Metals The technical drawbacks of the existing prototypes are directly related to tool construction and may be addressed by a straight bottom-up tool design. This includes finite-element calculations on thermomechanical stresses arising during preheating and service. The critical aspects of this concept have been identified and should be addressed in future work. Owing to the severe impact on the process layout itself, the influence on process parameters and work piece quality has to be monitored carefully, as discussed in Chapter 11. Concerning general applicability, the vast perspective of this tool concept should be pointed out, in particular with a view to tapping the full potential of the advantages offered by forming metals in the semi-solid state, and even higher melting alloys than steel. 8.6.5 Perspectives for the Application of Bulk Ceramic Dies in Steel Thixoforming Two different ceramic die concepts for the semi-solid processing of steel are introduced,consideringtheprocessdemandsondiepropertiesandthespecificdemandsof distinct ceramic die materials on their operating conditions. The characteristic strengths and limitations of both die concepts are identified on the basis of adapted modeltests,theirperformanceinsmall-scaleformingseriesandtheworkpiecequality of thixoformed steel parts. Regarding the application of conventionally heated Si3N4 dies operating at temperatures 1000 C was validated on the example of steel thixoextrusion. It was shown that the notably increased die temperatures not only allowed the application of thermal shock-sensitive ceramics of high corrosion and wear resistance, but also significantly improved work piece quality in view of the geometric complexity of the formed parts and phase homogeneity in comparison with previous studies by other workers [87–89] on steel thixoextrusion. Further work has to be carried out on the appropriate design of such tools for target applications. The principal results of tool construction and process parameters may be transferred to innovative metal forming technologies, such as isothermal forging and superplastic forming of other highmelting alloys that are difficult to shape using conventional forming methods. Thus, two die concepts have been developed that provide useful means for successful semi-solid processing of steels. However, the narrow process window
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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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5 Thermochemical Simulation of Phas
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here it can be assumed that the dat
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Figure 5.2 Calculated temperature v
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Figure 5.4 Composition profiles of
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It is clear that C has by far the s
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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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Page 334 and 335: 9 Rheocasting of Aluminium Alloys a
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Page 340 and 341: 9.2 SSM Casting Processesj317 For t
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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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