WO2006114316A2

WO2006114316A2 - Method and device for determining the type of microstructure of a metal work piece during heating

Info

Publication number: WO2006114316A2
Application number: PCT/EP2006/003925
Authority: WO
Inventors: Daniele Marcello Borrelli; Alexander Brenner; Günther PRITSCHOW
Original assignee: Universität Stuttgart
Priority date: 2005-04-28
Filing date: 2006-04-27
Publication date: 2006-11-02
Also published as: WO2006114316A3; EP1877766A2; DE102005054316B3

Abstract

The invention relates to a method for determining the type of microstructure of a metal work piece (2) during heating, especially for a subsequent thixoforming step, whereby the work piece (2) is heated by inducing an electric current into the same. The current induced into the work piece (2) is pulsed by pulsing the applied voltage. The resistivity or the magnetic permeability of the work piece (2) is determined from the flow of the current in the induction coil. The resistivity or the magnetic permeability of the work piece (2) is determined by determining the time constant or the type of current drop in the induction coil (5) after a voltage pulse. The resistivity or the magnetic permeability of the work piece (2) are used to determine the microstructure of the work piece (2).

Description

Method and device for determining the structural state of a metallic workpiece during heating

The invention relates to a method for determining the structural state of a metallic workpiece during heating, in particular for a subsequent Thixoforming, according to the closer defined in the preamble of claim 1. Art Furthermore, the invention relates to a device for determining the microstructural state of a metallic workpiece during heating , in particular for a subsequent thixoforming, according to the preamble of claim 7.

As part of the efforts to increasingly use light metal materials in both automotive and aerospace technology, a new forming process has recently been created, referred to as thixoforming or thixoforging. Here, the material to be reshaped is in a semi-liquid or thixotropic state. Although the technological advantages of the products produced by this process, in particular with regard to the complicated manufacturable forms with very high strength values, not doubted, for mass production, the existing processes are not yet suitable, due among other things to the insufficient process reliability in the Heating of the components is due to deformation. One of the boundary conditions of thixoforming is namely that at least for certain materials at the time of forming a uniform material structure should form with fine-grained, globular solid phase skeleton in a defined proportion of a molten matrix in the entire blank. According to the generally known state of the art, the heating of the components is usually carried out in induction heating systems during thixoforming. Basically, although the energy-saving heating in a very short time enabling inductive heating leads to the fact that primarily the edge regions of the component are heated, whereas the heating of the core can only be done by the material-dependent heat diffusion. To achieve this, compensating diffusion phases with reduced supplied power are required, which leads to a not inconsiderable prolongation of the heating time.

Another fundamental problem in the heating of the components is that the proportion of solid and liquid structural components can not be measured directly. However, due to the above-described requirements on the microstructure during the transformation, it is necessary to know this ratio, since if the liquid fraction is exceeded, undesired microstructural changes occur and if the liquid fraction is not reached, the thixoforming processes can not be successfully carried out.

In the methods of the prior art, the desired thixotropic region is often determined empirically by means of iterative methods via heating experiments and a subsequent Schuffbildanalyse in the laboratory. By the thus obtained relationships between the temperature of the component and the transition region from solid to liquid state of the desired microstructure is reproduced in series parts. However, these are a very complex and extremely inaccurate procedure, since the micrograph analysis can only be used for blanks with exactly the same alloy composition. The relationship between the thixotropic region and device temperature is significantly altered by variations in the alloy composition, which can be as much as one or more percent within a batch of material due to the manufacturing process. A secure process control in series production can not be guaranteed in this way.

Among other things, the temperature measurement fails due to the fact that with some alloys an accuracy of less than one Kelvin has to be achieved in order to be able to draw conclusions about the structural condition. However, this accuracy can not be achieved with measurements in different surface layers with realistic effort.

From EP 0 728 304 Bl it is known to detect resistance changes in the blank by means of an additional sensor coil. However, such an additional sensor coil is sensitive to external disturbances or stray fields, which can easily lead to a falsification of the measured values. Furthermore, with an additional sensor coil, the homogeneity of the heating can not be determined, since only the front or lateral surface of the blank can be measured.

DE 199 21 827 A1 proposes the use of a microphone with connected sound analysis for determining the microstructure state. However, this procedure is also In view of the very narrow range in which thixoforging can be used, not effective.

The same applies to the proposal to determine the microstructure state based on the sole measurement of the effective current in the circle of the induction coil, as proposed in DE 199 48 843 Al.

J. Baur describes in "Werkstattstechnik online" a generic method and a corresponding device for determining the microstructure state of a metallic workpiece during heating, but even here, it is not possible to make a sufficiently accurate statement about the microstructure state.

It is therefore an object of the present invention to provide a method for determining the structural state of a metallic workpiece during heating, in particular for a subsequent Thixoforming, which is able to determine the current microstructure state during the heating of the workpiece very accurately and thus to allow heating of the workpiece to a certain ratio between solid and liquid phase.

According to the invention, this object is achieved by the features mentioned in claim 1.

The inventors have found that there is a direct relationship between the resistivity or the magnetic permeability of the workpiece and its microstructure, in particular in the region of the transition between the solid and liquid phases, which is determined by the invention. The method according to the invention is utilized in a manner with which the microstructure state of the workpiece can be determined very precisely. In this way, the heating process for the workpiece to be formed can be controlled very precisely to a certain required ratio between solid and liquid phase, whereby the problems of the prior art can be circumvented and significantly better processing results with a lower reject rate can be expected in the subsequent Thixoforming. In particular, neither a 100% test nor a destructive test of the workpieces is required.

Another advantage of the method according to the invention is that the heating process does not have to be substantially interrupted and no disturbances thereof are to be expected.

A particularly suitable method according to the invention for determining the specific resistance or the magnetic permeability of the workpiece is to determine one of these values by determining the time constant or the form of the drop in the current in the induction coil after a voltage pulse. Here, the existing relationship between the time constant and the resistivity or the magnetic permeability is utilized.

If, in an advantageous embodiment of the invention, the applied voltage is interrupted for a certain time, it is advantageously possible to record the decay process and thus the time constant to be determined longer and thus more accurately. In order to achieve a measurement of the heating of the workpiece, it can be provided in an advantageous development of the invention that the frequency of the voltage at the induction coil during the heating of the workpiece at least once, but advantageously modulated in short successive intervals, where is particularly advantageous if, in at least two different frequencies of the voltage across the induction coil in this context, the time constant of the drop of the current in the induction coil and the difference and change is determined. In this way it is possible to deduce the degree of homogeneity of the heating through the cross section of the workpiece. Such a procedure is not possible with systems which operate according to the resonant circuit principle, since they do not permit rapid successive frequency switching.

A device for determining the structural state of a metallic workpiece during heating, in particular for a subsequent thixoforming, can be taken from the features of claim 7.

The inventive use of the induction coil as a sensor coil, it is possible to adapt the strategy for heating the workpiece during operation to the actual state, whereby compared to known devices, the process duration can be reduced and the microstructure can be improved.

In this case, a voltage source with an impressed or imposed frequency is used, which not only causes significantly lower costs, but also a simple has a better construction and more reliable operation, thus allowing a more flexible use of the entire system.

By one or more additional ohmic or complex resistors in the circuit of the flyback diodes results in a maximum dependence of the time constant of the drop in the current in the induction coil of the change in the resistivity or the permeability of the workpiece, so that the inventive device is a very accurate determination of Microstructure state of the heated workpiece allows.

Further advantageous embodiments and modifications of the invention will become apparent from the remaining dependent claims. Hereinafter, an embodiment of the invention with reference to the drawing is shown in principle.

It shows:

1 shows a device for carrying out the method according to the invention;

Fig. 2 is an equivalent circuit diagram of the device of Fig. 1;

3 shows the course of the voltage and the current in the method according to the invention;

Fig. 4 shows the resistivity versus temperature for a particular aluminum alloy; 5 shows an exemplary circuit diagram of a voltage source of the device according to the invention; and

6 shows a possible construction of additional resistors of the voltage source from FIG. 5.

The device 1 shown in Fig. 1 is used for heating a metallic workpiece 2, which is then to be formed by means of a forging press, not shown, or a similar suitable device. In this case, the workpiece 2 is heated to a temperature at which a part of the workpiece 2 in a solid and the other part is in a liquid state and formed in this thixotropic state. This type of forming is called thixoforming or thixoforging. Here, the workpiece 2, the most diverse materials are used, such as aluminum or titanium alloys, but also various magnetic materials such as steel materials and the like.

The device 1 has a multi-frequency voltage source 3, which is provided with a controller 4. The workpiece 2 is located within an induction coil 5, which is connected to the voltage source 3 via a transformer 6. In the circuit of the induction coil 5 are an ammeter 7 and a voltmeter 8 for measuring the current or the voltage in this circuit. The respective course of the current and voltage signals is evaluated by a digital signal processing system, not shown, and the control information necessary for carrying out the heating described in more detail below is calculated. The voltage source 3 has a rectifier, a DC link for reactive current compensation and a single-phase inverter in bridge circuit as an output stage in a manner known per se, as explained in greater detail in FIGS. 5 and 6. In the present case, a voltage source is used with an impressed or imposed frequency, wherein the frequency is variable via a controller, not shown. The energy input into the workpiece 2 can be controlled both by frequency modulation and by pulse width modulation.

In the simplified equivalent circuit diagram of the device 1 according to FIG. 2, in which parasitic capacitances, inductances and resistances are not shown, the voltage source 3 is assigned a voltage source 16 and a resistor 9 and the transformer 6 a transformer 15 and resistors 10 and 13. The induction coil 5, an ohmic resistance 11 and inductors 12 and 14 are assigned.

With the described device 1, not only the workpiece 2 can be heated to the desired temperature by inducing an electric current therein, but it is also possible to use the induction coil 5 as a sensor coil and thus to detect the structural condition of the workpiece 2 upon heating thereof. In this case, the current induced in the workpiece 2 and generating an electromagnetic alternating field is shaped characteristically by pulses of the applied voltage in the induction coil 5 or coil voltage and the specific resistance of the workpiece 2 is determined by evaluating the current profile of the induction coil. Ie 5, for example, determined by determining the time constant of the drop of the current after one of the voltage pulses, ie in a decay. The phrase "after one of the voltage pulses" means a pulse pause which reduces the highest voltage, ie it is not absolutely necessary that there is no more voltage.This lowering or switching off of the voltage takes place only for a short time up to a few If the workpiece 2 is made of a ferromagnetic material, then in addition to the resistivity and the magnetic permeability or its change can be determined.

An exemplary profile of the voltage and the current at the induction coil 5 is shown in FIG. 3 on the basis of a transient analysis. In this case, the voltage is represented by the solid, the current by means of the dashed and the time constant of the current by means of the dotted line. The diagram shows that after a voltage pulse, ie in a pulse pause, the current drops relatively quickly. This drop or decrease of the current is recorded in the present exemplary case by the time constant τ in a measured value. From the change of this time constant τ, which can thus be determined with a standard method well-known from electrical engineering, can now on the change in the property of the workpiece 2, in the present case to the change in the resistivity or on the magnetic permeability of the same and thus be closed to the microstructure state. In this case, an equation corresponding to τ = L / R can be used to calculate the resistivity. In order to set the microstructural state desired for the subsequent forming, the change in the coil current and thus both the ohmic resistance and the inductance of the arrangement are monitored. From the change behavior, a conclusion can be drawn for the above-described relationship between resistance or inductance and temperature or structural state of the workpiece.

From the diagram of FIG. 4, in which the area of the solid phase of the workpiece 1 with A, the area in which solid and liquid phases are present next to each other, B and the liquid phase area C, it is clear that the resistivity changes very much even in the region of the transition between the solid and the liquid phase, so that even small changes in the temperature lead to a large change in the specific resistance, which is why the resistivity is very good for determining the structural state of the workpiece 2 can be used.

So that the change in the time constants of the current profile depends maximally on the change in the specific resistance or permeability of the workpiece 2, one or more ohmic or complex resistors can be inserted into the circuits of the freewheeling diodes of the bridge circuit of the single-phase inverter in the voltage source 3. These should be selected to a maximum, but only so large that the switching elements, such as transistors, which are currentless at this time, are not destroyed by the voltage dropping across them. In the simplified circuit diagram of the voltage source 3 according to FIG. 5, after a rectifier 17 provided with a mains connection, a DC intermediate circuit is connected to the inductors 18 and 19 and a capacitor 20. A plurality of semiconductor power switches 21, 22, 23 and 24 are arranged in a known manner and are oppositely electrically conductive in pairs. Respective freewheeling diodes 25, 26, 27 and 28 serve the continuous current profile when using an inductive load 31. For the maximum dependence of the time constant of the property changes of the workpiece during the heating process additional ohmic or complex resistors 29 and 30 are inserted into the circuit of the freewheeling diodes, as exemplified in Fig. 5. Fig. 6 shows the internal structure of these complex resistors 29 and 30 by a real resistor 32, an inductor 33 and a capacitor 34 in an exemplary manner.

In the method for determining the microstructure state of the workpiece 2 may further be provided that the frequency of the voltage at the induction coil 5 during the heating of the workpiece at least once, but advantageously at intervals repeatedly modulated. This results in a lower penetration of a deeper penetration of the current-enforced area. By such a change in frequency, the penetration depth of the stream is changed in the workpiece 2, whereby the microstructure state can be determined in different edge volume and closed on the degree of soak. With a subsequent heating control so a homogeneous heating of the workpiece can be made possible. If, at different frequencies of the voltage, the change and / or the speed of the If the change in the time constant of the drop in the current is determined, then the current microstructure state can be determined at different distances from the edge of the workpiece 2, ie the degree of homogeneous heating, which in turn can be used to heat those layers of the workpiece 2 in each case. which have not yet reached the desired material state or insert suitable diffusion times with a lower power supply and to achieve the target state exactly.

Claims

Patent claims

A method of determining the microstructure state of a metallic workpiece during heating, in particular for subsequent thixoforming, wherein the workpiece is heated by inducing an electric current therein, wherein the current induced in the workpiece is pulsed by pulsing the applied voltage, and wherein the resistivity or magnetic permeability of the workpiece is determined from the course of the current in the induction coil, since the resistivity or magnetic permeability of the workpiece (2) is determined by determining the time constant or the shape of the current drop in the induction coil Induction coil (5) is determined after a voltage pulse, and that from the resistivity or the magnetic permeability of the workpiece (2) of the structural state of the workpiece (2) is determined.

2. The method of claim 1, wherein the applied voltage is interrupted for a certain period of time.

3. The method according to claim 1 or 2, characterized in that the voltage is pulsed substantially rectangular.

4. The method according to claim 1, wherein the frequency of the voltage at the induction coil is modulated at least once during heating of the workpiece.

5. Method according to claim 4, wherein the frequency of the voltage at the induction coil is modulated a plurality of times during heating of the workpiece in a succession of intervals.

6. The method of claim 4 or 5, characterized in that at at least two different frequencies of the voltage at the induction coil (5) the time constant of the drop of the current in the induction coil (5) and their difference and change is determined.

7. A device for determining the structural state of a metallic workpiece during heating, in particular for a subsequent thixoforming, with an induction coil connected to a voltage source for inducing an electrical current into the workpiece, wherein in the circuit of the induction coil at least one current measuring device is present, and the voltage source has an inverter in bridge circuit, since you rchge ke nn records that in the circuit of the freewheeling diodes of the bridge circuit at least one ohmic or complex resistor is integrated, and that the induction coil (5) as a sensor coil for determining a profile of the workpiece (2) induced electrical current is formed.

Device according to claim 7, characterized in that the electrical resistance of the at least one ohmic or complex resistor is maximally so high that the voltage dropping thereon is the switching elements of the

Bridge circuit not destroyed.