Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C1/00—Making non-ferrous alloys
  • C22C1/12—Making non-ferrous alloys by processing in a semi-solid state, e.g. holding the alloy in the solid-liquid phase
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C21/00—Alloys based on aluminium

Definitions

• the invention relates to a method for producing a material formed from a metal alloy according to the preamble of claim 1.
• the shaping of metal alloys in the semi-solid state using thixogies, thixo forging or thixopresses is becoming increasingly important as an alternative to the classic manufacturing methods for molded parts by means of casting, forging and pressing. So today it is possible, starting from a material in the semi-solid / semi-solid state - hereinafter referred to as semi-solid state - to manufacture cast or forged components with high quality standards.
• the semi-solid shape offers great economic potential, particularly for the manufacture of heavy-duty lightweight metal parts with complex geometries.
• the shaping of aluminum or magnesium alloys in the semi-solid state is a hybrid process that combines high design freedom and production speed of die casting processes with the quality advantages of forging processes.
• thixotropic behavior of the material, whereby thixotropy is understood to mean a special theological behavior in which a mechanical load due to shear stresses leads to a considerable decrease in viscosity. It should be noted that the viscosity changes by several orders of magnitude under load. In the unloaded state of a thixotropic metal alloy, its viscosity is approximately 10 6 to 10 9 Pas. which corresponds to the properties of a solid, whereas under a shear stress the viscosity drops to values of 1 Pas, which corresponds to a viscosity between that of honey (10 Pas) and olive oil (10 -1 Pas).
• the metal alloy is brought to an initial temperature above liquidus and then a grain refining agent is added to the melt formed in this way.
• the metal alloy is then cooled to an arbitrary temperature below the solid and the material thus produced is stored in the solid state for an essentially arbitrary time.
• the material is brought to the semi-solid state by heating to a holding temperature between solidus and liquidus and is held for a holding time of less than 15 minutes.
• the semi-solid material must be shaped within the holding time of less than 15 minutes.
• a disadvantage of the known method is that the materials that can be produced with it are not suitable for use in conventional molding systems due to the holding time limited to less than 15 minutes. Accordingly, processing by means of thixo casting, thixo forging or thixopressing of the materials produced using the known method requires special production facilities which ensure that the shaping is carried out within the processing window which is limited to less than 15 minutes.
• Another disadvantage of the known method arises from the fact that the material first has to be cooled from the molten state to the solid state and only then can it be brought into the semi-solid state for subsequent shaping. This intermittent setting is particularly undesirable for an automated manufacturing and molding process.
• the object of the invention is to improve a method of the generic type in order in particular to avoid the disadvantages mentioned.
• the metal alloy is brought to an initial temperature above liquidus and then an additional material is added which is capable of being transferred to reduce the interfacial energy between solid and liquid phase of the metal alloy mixed with the additional material in the semi-solid state.
• the proportion of the additional material should be selected so that in the semi-solid material with a solid phase proportion of 25% to 85%, the grain size and the degree of skeletonization remain essentially constant during a holding time of more than 15 minutes in order to maintain the ability of a suspension to be formed.
• the semi-solid material is phlegmatized, which permits production which is more economical from an ecological and ecological point of view. Sc the extension of the process window leads to a reduction in rejects, which always occurs with the previously known methods if the thixotropic properties of the material are lost as a result of an excessively long holding time. Outside which it is possible when applying the inventive method thanks to the recoverable desensitization to make the transfer of the Werkstoff ⁇ s in the semi-solid state for subsequent molding directly from the melt, ie an interim Wararr ⁇ nsewage of the material is not e- 'orderlich.
• the process flow can be largely homogenized in already existing production facilities thanks to the reduced structural sensitivity. If storage of the material is required. it can be cooled to a storage temperature below Solidus and only brought into the semi-solid state immediately before shaping, without the advantageous desensitization being lost.
• the following shaping can be used to produce components which have a good combination of strength and toughness and, moreover, are heat-treatable, weldable, pressure-tight and relatively inexpensive.
• the method can be used for the most varied types of metal alloys.
• the metal alloy contains aluminum as the main constituent, and barium is used as the additional material, the weight fraction of the barium according to claim 3 being 0.1% to 0.8% of the material.
• a dispersoid-forming element is added to the metal alloy in order to promote the formation of grains of small grain size.
• iron or chromium or titanium or zircon is expediently used as the dispersoid-forming element, the proportion by weight of the disoersoid-forming element being between 0.1% and 1% of the material.
• Figure 1 Average grain size D and form factor F for a state of the art
• aluminum alloy X Technically manufactured aluminum alloy (EN AW-6082, hereinafter: "aluminum alloy X") with a constant liquid phase content of 35% as a function of the isothermal hold time;
• FIG. 2 contiguity and contiguity volume of the aluminum alloy X produced according to the prior art with a constant liquid content of 35% as a function of the isothermal holding time;
• FIG. 3 contiguity and contiguity volume of the aluminum alloy X produced according to the state of the art as a function of the liquid phase component after a constant isothermal hold time of 5 minutes;
• FIG. 4 force-displacement curves of the aluminum alloy X produced according to the prior art as a function of the liquid phase fraction after an isothermal hold time of 5 minutes;
• FIG. 5 force-displacement curves of the aluminum alloy X produced according to the state of the art as a function of the isothermal holding time with a constant liquid phase fraction of 35%;
• FIG. 6 shows the volume of contiguity of a barium-containing aluminum alloy (X + Ba) produced according to the invention compared to the aluminum alloy X produced according to the prior art as a function of the isothermal holding time with a constant liquid phase component of
• FIG. 7 force-displacement curves of the barium-containing ones produced according to the invention
• thixotropy is understood to mean a special rheological behavior in which mechanical stress due to shear stress leads to a considerable decrease in viscosity.
• a thixotropic behavior can be expected for materials in the semi-solid state, ie at a temperature between the Solidus line and the Liquidus line, if the semi-solid solid material can be converted into a low-viscosity solid-liquid suspension under shear loading. This formability of a suspension requires a special structure in the semi-solid state, in which the solid components are not dendritic, but globulitic.
• the structure formation can be described by four structural parameters, namely by the solid phase fraction f s , the form factor of the solid phase F, the grain size of the solid phase D and the degree of skeletonization, the latter being expressed by the measurement quantity C s referred to as contiguity or preferably by the contiguity volume f s C s becomes.
• the liquid phase component f can also be specified, with the quantities f L and f s adding up to 1 under the permissible neglect of gaseous phase components.
• the solid phase component should be approximately 40% to 60%.
• the morphology and the connectivity of the solid phase are the process-determining structural parameters. A quantitative description of the structure morphology can be made with the help of the form factor F and the grain size D.
• the form factor F v / is defined
• grain size D Although there is no generally applicable upper limit value for grain size D in the prior art, experience shows that when shaping thin components, a grain size of about a twentieth of the wall thickness of the component should not be exceeded. For a wall thickness of 3 mm, a further criterion to be observed is a maximum grain size of approximately 150 ⁇ m.
• a commercially available thix alloy from Tap AlMgSi (hereinafter referred to as "aluminum alloy X") with a composition similar to the alloy with the designation EN AW-6082 according to the European standard EN 573-3, namely with a chemical composition of 1.1% by weight silicon, 0.85% /% magnesium, 0.61% manganese, 0.09% iron, 0.08% titanium, ⁇ 0.01% chromium, ⁇ 0.01 % Copper, ⁇ 0.01% nickel, ⁇ 0.01% lead and ⁇ 0.01% /% zinc was heated in an infrared oven to a desired temperature in the solidus-liquidus interval at 100 ° C / min heated, homogenized isothermally and then quenched.
• aluminum alloy X thix alloy from Tap AlMgSi
• thermocouple attached in the center of gravity of the sample (15 mm x 15 mm x 15 mm) ensures exact temperature determination (+/- 0.1 ° C) and heating control. Before each test, the accuracy of the thermocouple was checked in a calibration oven.
• the measurements were limited to the microstructural structural developments at 5 selected temperatures in the semi-solid range (613 ° C, 625 ° C, 633 ° C, 63 ⁇ c C and 638 ° C, corresponding to a liquid phase content of 10%, 20%. 30% , 35% and 40%) and with isothermal holding times of 1, 5, 10, 20 and 30 minutes. Subsequent metallographic examinations of the quenched samples showed the change in the structure during reheating depending on the test parameters.
• the parameters form factor F, grain size D and contiguity C s or contiguity volume f 3 C s enable the structural changes to be determined on the basis of the size, shape and spatial relationship of the solid alpha phase in the liquid matrix.
• FIG. 1 shows the change in form factor F and grain size D (in micrometers) as a function of the isothermal holding time t (in minutes) in the semi-solid state at a constant temperature of 636 ° C., corresponding to a liquid phase fraction f L of 35%.
• t in minutes
• f L liquid phase fraction
• S ss is the grain boundary surface between the solid phase, ie the surface between the continuous grains and not separated by melt, while S SL is the phase interface between solid phase and melt.
• the contiguity thus corresponds to the proportion that the interface to the same phase takes up in the entire interface of the solid phase.
• C s 0, the grains are isolated and completely surrounded by melt, while with increasing C s the grains have grown together more and accordingly the skeleton formation is more pronounced. Very low values of C s are undesirable because the semi-solid material then has no dimensional stability.
• C s ⁇ 1 the solid phase is fully agglomerated and cannot be converted into a suspension by applying shear stresses.
• FIG. 2 shows v / i ⁇ derum using the example of the aluminum alloy X the change in the contiguity C s and the contiguity volume f s C s as a function of the holding time t (in minutes) in the semi-solid state at a constant temperature of 63 ⁇ ° C, corresponding to a Liquid phase fraction f of 35%.
• FIG. 3 shows for the same material X the change in the contiguity C s and the contiguity volume f s C s after an isothermal hold time of 5 minutes as a function of the liquid phase fraction f L , v / o. Note that for f 1 -> 1 corresponding to C s - ⁇ 0 applies.
• the volume of contiguity f s C s increases with increasing holding time t and decreases with increasing liquid phase fraction f L , whereby, as expected, the skeleton formation increases with increasing holding time t.
• the properties necessary for successful shaping can, however, only be expected in a certain range of values for the contiguity volume f s C s .
• the evaluation of the rheological properties described below allows the appropriate interval for the volume of contiguity f s C s to be determined .
• FIG. 4 shows typical force-displacement curves of the aluminum alloy X after an isothermal holding time t of 5 minutes at various values of the liquid phase fraction f L , v / o the force K in kilonewtons and c ⁇ r path l in millimeters - g ⁇ g ⁇ b ⁇ n is.
• the force-displacement diagram With 20% of the liquid phase fraction f L bs, the force-displacement diagram has the characteristic shape for elastic-plastic behavior.
• the forming forces are very low, and one is thus in the thixotropic range to be attempted for the method.
• FIG. 5 shows the force : path curves after various isothermal holding times t (in minutes) for the same thixolegi ⁇ rung with a liquid phase fraction of 35% (corresponding to a temperature of 636 ° C.), the force K in kilonewtons and the path t is given in millimeters. While a thixotropic behavior can still be seen after a holding time t of 5 minutes, a longer holding time leads to a loss of the thixotropic properties
• FIG. 4 A comparison of FIG. 4 with FIG. 3 shows that the cemass of the figure ⁇ with a liquid phase nantil f L of 40% and 50% observed thixotropic behavior transferred to FIG. 3 with a decrease in the volume of contiguity f s C s to values below 0 3 goes hand in hand.
• FIG. 5 A comparison of FIG. 5 with FIG. 2, according to which the loss of thixotropic properties occurring after a holding time t of more than 5 minutes occurs according to FIG. 2 with an increase in the contiguity volume f s C s to values of over 0.3.
• the above-mentioned additional materials Z which are effective are the elements barium, which is particularly preferred, and antimony, strontium or bismuth. It must be pointed out that for some of these elements, in particular for silicon, it is known that their addition to an aluminum alloy brings about a positive refinement, for example by forming the aluminum-silicon eutectic. The quantitative proportions of these elements used for the refinement, however, are in the range of a few ppm and are in any case far too low to effect a desensitization of the thixotropic properties.
• the quantitative proportions of the additional material Z to be used in the method according to the invention are significantly higher than the quantitative proportions of finishing agent usually used for the modification of a eutectic.
• the effect achieved with the method according to the invention is based on the fact that, by reducing the interfacial energy between the solid phase and the liquid oasis of the semi-solid material, a driving force for the undesirable changes in the structure, in particular the grain coarsening and the increasingly include skeletonization, is reduced.
• the quantity of the additional material is to be selected so that the grain size D and the degree of skeletonization remain essentially constant during a holding time t of at least 15 minutes. This is illustrated in the exemplary embodiment below.
• aluminum alloy X-Ba The material thus formed (hereinafter referred to as "aluminum alloy X-Ba") with a chemical composition of 0.2% by weight barium, 0.8% by weight silicon, 0.41% by weight magnesium, 0.28% by weight manganese, 0.2% by weight % Iron, 0.01% by weight titanium, 0.19% by weight chromium, 0.35% by weight copper, ⁇ 0.01% by weight nickel, ⁇ 0.01% by weight lead and ⁇ 0.01% by weight zinc According to the characterization procedure described under number 2, heated in an infrared oven to a predetermined temperature in the solidus-liquidus interval at 100 ° C./min and then homogenized isothermally.
• FIG. 6 shows the course of the contiguity volume f s C s as a function of the isothermal holding time t (in minutes) with a constant liquid phase fraction f L of 35% on the one hand for the material produced by the process according to the invention, ie the aluminum alloy X + Ba, and on the other hand for the corresponding barium-free alloy X according to the state of the art.
• the change in structure was significantly reduced by using the method according to the invention.
• the critical value Y 0.3 for the contiguity volume f s C s was not reached even after a long holding time t of 30 minutes.
• inventive theory set out above using the example of an aluminum alloy can be applied in an analogous manner to other metal alloys X, for example to magnesium alloys but also to steels and heavy metal alloys. It is within the range of skill in the art to first determine in preliminary tests which values of grain size D and the degree of skeletonization or the volume of contiguity f s C s are to be observed in order to maintain the formability of a suspension in the semi-solid state and, moreover, a suitable one Additional material Z with interface energy-lowering properties to be selected.
• the aluminum alloys described in the previous exemplary embodiment with a composition similar to the alloy with the designation EN AW-6082 according to the European standard EN 573-3 contain, among other things, an admixture of iron, some of which act as a dispersoid-forming element, ie in the semi-solid state promotes the formation of grains of small grain size D. If other metal alloys X are used, a suitable dispersoid-forming element E must be admixed in addition to said additional material Z if necessary.

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• 2000
  • 2000-07-19 WO PCT/CH2000/000391 patent/WO2001009401A1/fr not_active Ceased
  • 2000-07-19 EP EP00941865A patent/EP1230409B1/fr not_active Expired - Lifetime
  • 2000-07-19 AT AT00941865T patent/ATE258233T1/de not_active IP Right Cessation
  • 2000-07-19 AU AU56699/00A patent/AU5669900A/en not_active Abandoned
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• 2001
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