Thixoforming : Semi-solid Metal Processing

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inherent to steel thixoforming and the necessity for evaluating process parameters and designing specific tools for each intended work piece geometry make die construction a challenging task. In this respect, the latter concept of high-temperature dies is advantageous in terms of process stability owing to the comparatively stable defined operation parameters. 8.7 Conclusions and Perspectives 8.7 Conclusions and Perspectivesj301 The work on PVD process technology focused on the improvement of tool life in semi-solid metal forming by developing an adapted deposition process for oxide coatings on hot working steel and TZM substrates. Two different candidates were identified as suitable coating materials, g-Al2O3 and t-ZrO2, using tailored model tests, owing to the high hardness of alumina in the relevant temperature range and less adhesion of the semi-solid steel forming material on zirconium oxide. The PVD coatings g-Al2O3 and t-ZrO2 were confirmed as suitable coating materials. In order to improve adhesion to the substrate, a TiAlN interlayer was provided to increase the coating performance. Regarding thin-film synthesis, the objectives of the study were to reduce the deposition temperature to values below the annealing temperatures of the substrate materials and to determine parameters for reliable and reproducible thin-film properties. This was achieved by a pulsed PVD process with tight and fast process control to prevent target poisoning. Coating experiments showed that substrates with surface features up to an aspect ratio of 1:2 can be coated successfully with a constant coating thickness. The applicability of PVD-coated hot working steel and TZM dies was demonstrated in small-scale steel thixoforming (thixoforging, - casting and -extrusion) series. The long-term stability of g-Al2O3 and t-ZrO2 coatings has to be evaluated in industrial forming series. Future work will focus on enhancement of the PVD process on the target for coating substrates of complex surface morphology. The aim of the PECVD process technology development was to ensure a reduction in the deposition temperatures for a-Al2O3 at substrate temperatures allowing deposition on steel dies, namely at 550 C. To achieve that goal, a bipolar DC PECVD method was developed. The constitution and mechanical properties of the films were found to be strongly dependent on the deposition conditions such as substrate temperature, gas mixture and peak voltage. The elastic properties of the films were strongly affected by the formation of the pores in the structure and were intimately linked to the normalized ion flux. Performance-related tests such as adhesion, high-temperature tribological and thermal shock evaluations were performed to evaluate the behaviour of coatings in a close to real environment. The results underline that alumina thin films deposited by PECVD are very promising candidates for die protection during semi-solid processing of steel. The excellent high-temperature properties inherent to ceramics make them candidates for die materials for high-temperature metal forming, for example,
302j 8 Tool Technologies for Forming of Semi-solid Metals semi-solid processing of steel, in a first approach. Although ceramic coatings on metal substrates are one possible strategy to benefit from these properties, this implies that tools need to be cooled during operation. Consequently, the process layout is determined by the thermal boundary conditions, limiting the applicability of bulk ceramics to those exhibiting the highest available thermal shock resistance and fracture toughness, that is, silicon nitride. The suitability of silicon nitride has been demonstrated, the operating limits identified and design guidelines derived. Conversely, this process layout leads to restrictions on work piece quality and geometric complexity of produced parts due to rapid solidification. Hence an alternative bulk ceramic tool concept based on the separation of forming and solidification has been introduced, increasing freedom of shape and allowing for the application of highly corrosion-resistant but thermal shock-sensitive ceramics. The viability of this tool concept has been demonstrated on the example of thixoextrusion. Forming results and work piece quality are highly encouraging, opening new perspectives for future metal forming processes. List of Symbols and Abbreviations a deposition rate [nm s 1 ] AIP arc ion plating Al2O3 aluminium oxide Al2TiO5 aluminium titanate b empirical constant [g cm 3 ] CrAlN chromium aluminium nitride CTE coefficient of thermal expansion CVD chemical vapour deposition DC direct current E elastic modulus of a porous body [GPa] E0 elastic modulus of a non-porous body [GPa] EDS energy-dispersive spectroscopy FC critical load [N] GPSN gas pressure sintered silicon nitride HPDC high-pressure die casting HSS high-speed-steel JCPDS Joint Committee on Powder Diffraction Standards m.f. middle frequency MSIP magnetron sputter ion plating NIF normalized ion flux OES optical emission spectroscopy P pore volume fraction [g cm 3 ] PC precursor content [%] PD power density [Wcm 2 ] PECVD plasma-enhanced chemical vapour deposition precursor ratio PR
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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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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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