Patent Application
20100000702
The present application claims the benefit of and priority from German Patent Application Serial No. DE 10 2008 031 777.2, filed Jul. 4, 2008, the entire disclosure of which is hereby incorporated by reference herein.
1. Field of the Invention
The present invention relates to an apparatus for determining the hot tearing susceptibility of metallic melts, comprising a casting mould, which has a first subvolume and a second subvolume, the second subvolume being formed as a tubular portion with a first end and a second end, the first end being connected to the first subvolume, comprising a measuring element, which extends into the second end of the tubular portion, and comprising a measuring device for recording a force on the measuring element and/or a change in position of the measuring element in the direction of extent of the tubular portion, the measuring device being coupled with the measuring element.
2. Discussion of the Prior Art
Hot tearing is a significant problem in the solidification of metallic melts. Hot tearing is the term used for intercrystalline or interdentritic discontinuities which occur during the solidifying or melting in a temperature range between solidus temperature and liquidus temperature. Hot cracks may be microscopically small or extend over several millimetres or even centimetres. In any event, they represent a defect for a workpiece that is intended to be created at least partially from a solidified metallic melt.
In particular with regard to strip casting or comparable continuous casting processes, knowledge of the hot tearing susceptibility of a melt is of great significance, since this has a direct influence on the strength of the cast product. This knowledge is also of significance for welds or the use of certain welding methods, since it has a direct influence on the durability of the weld.
In the past, there has been considerable activity in determining the hot tearing susceptibility of steels and aluminium alloys in particular. However, there has not so far been any success in sufficiently understanding or even simulating the phenomenon of hot tearing susceptibility. There is also no apparatus with which hot tearing susceptibility can be reliably determined.
When dealing with a solidification process of a metal melt in a casting mould, the typical approach so far has been to measure, and consider in the evaluation, not only the chemical composition but also melting parameters such as the melting bath temperature, the casting mould temperature, the variation in the temperature during the solidification and the forces occurring during the solidification in the casting mould as a result of shrinkage. A purely qualitative assessment may also be made by visual observation of the material in the region of the occurrence of a crack, its size and morphology.
A hot crack is qualitatively characterized with respect to its size. A distinction is generally drawn between cracks that can be seen with the naked eye, cracks that can be made visible with the aid of some means such as a magnifying glass and cracks that can only be seen under a microscope. The length of the crack is also often used as a criterion for hot tearing susceptibility. The known test methods have in common that, although they can assess the occurrence of cracks and their size, they do not allow conclusions to be drawn with respect to strains, and consequently with respect to resultant stresses. However, the knowledge of strains and stresses allows the susceptibility to cracks occurring in the future to be qualitatively determined or their further behaviour to be predicted.
Hot tearing susceptibility has so far been qualitatively determined by means of casting moulds in which, for example, a flow length is measured. The length of the solidified material up to which no crack occurs serves here as a characteristic number. This numerical value is specific to the geometry of the casting mould, i.e. the cross section of the casting channel and the chosen sprue system. Also specific to the respective test setup is the material from which the casting mould is produced. This may be, for example, either steel, if for example the casting mould is designed as a permanent mould, or sand. In the latter case, the grain size of the sand, the grain size distribution, the moisture content, the binder and the compaction and composition of the sand are also to be taken into consideration as further parameters. The melting bath temperature and the casting mould temperature are also determined as additional characteristic values.
A known casting mould for determining hot tearing susceptibility is based on annular geometries, in which the outside diameter is kept constant while the inside diameter can be varied. During the solidification, the material shrinks onto the core and, depending on the wall thickness, melting bath temperature, casting mould temperature and chemical composition, cracks can form. Problems with this type of casting mould occur in particular in the region of the sprue, since the conditions are often not reproducible there.
In S. Instone et al., “New apparatus for characterizing tensile strength development and hot cracking in the mushy zone”, International Journal of Cast Metals Research 12 (2000) 441-456, there is a description of an H-shaped casting mould with the sprue in the centre, in the case of which a tensile testing machine is also incorporated in the system. This is used to incorporate a force and attempt to draw conclusions with respect to the hot tearing susceptibility, while taking into consideration the crack occurring under a certain force and known temperature profiles.
Apart from that, casting moulds with a first receiving volume and a tubular measuring volume are also in use. In the case of these moulds, hotspots usually occur in the connecting region between the first receiving volume and the tubular measuring volume. A hotspot is an accumulation of melt during the solidification process that usually occurs in the central region of the volume of the casting mould that is furthest away from the cold wall of the casting mould. In the case of these casting moulds, the shrinkage behaviour of the melt during solidifying is measured over the length of the bar. Here too, parameters are the melting bath and casting mould temperature and the chemical composition of the melt. The hot tearing susceptibility is additionally influenced by the thermal content of hotspots.
An apparatus according to the precharacterizing clause of the claim is described in G. Cao, S. Kou, “Hot tearing of ternary Mg—Al—Ca alloy castings”, Metallurgical and Materials Transactions 37A (2006) 3647-3663. In it, Cao and Kou describe a direct measurement of the strains occurring during the solidification and the resultant forces with a bar-shaped subvolume of a casting mould. Y. Wang et al., “An understanding of the hot tearing mechanism in AZ91 magnesium alloy”, Materials Letters 53 (2002) 35-39, also describe a test setup with a bar-shaped casting mould geometry for determining hot tearing susceptibility.
However, with the apparatuses used, considerable frictional forces occur between the solidifying material and the inner wall of the casting mould, dependent on the temperatures of the solidifying material and of the casting mould, and ultimately on the shrinkage behaviour of both. Lubricants that are used under some circumstances bring with them further difficulties in the interpretation of the measured values. A meaningful interpretation is scarcely possible with frictional forces occurring in this way.
All the apparatuses known so far do allow a certain assessment of hot tearing susceptibility. However, because of the frictional forces occurring, none is suitable for the precise quantitative and qualitative determination of hot tearing susceptibility.
It is therefore the object of the present invention to provide an apparatus for determining the hot tearing susceptibility of metallic melts that allows an improved quantitative and qualitative determination of the hot tearing susceptibility of various metallic melts.
The present invention provides an apparatus for determining the hot tearing susceptibility of metallic melts, comprising a casting mould, which has a first subvolume and a second subvolume, the second subvolume being formed as a tubular portion with a first end and a second end, the first end being connected to the first subvolume, comprising a measuring element, which extends into the second end of the tubular portion, and comprising a measuring device for recording a force on the measuring element and/or a change in position of the measuring element in the direction of extent of the tubular portion, the measuring device being coupled with the measuring element, characterized in that the cross-sectional area of the tubular portion is reduced in the direction of the second end in each and every subportion of the tubular portion that may be selected substantially over the entire length.
Here, the term “casting mould” covers all receptacles that are suitable for a metallic melt to be contained or received in an inner volume or created therein.
Here, the term “volume” essentially means the inner volume that is available for receiving melts. The inner volume of the casting mould is made up here of at least two subvolumes.
The characterizing feature that the cross-sectional area of the tubular portion is reduced in the direction of the second end in each and every subportion of the tubular portion that may be selected substantially over the entire length is to be interpreted essentially as meaning that there is substantially no subportion over the entire length of the tubular portion in which the cross-sectional area is constant in the direction of extent of the tubular portion. This avoids falsification of the measurement by frictional forces between the solidifying material and the wall of the casting mould. For example, the tubular portion may have a circular cross section, with the diameter being reduced in every subportion towards the second end, and so the portion is frustoconical.
As compared with known measuring apparatuses, the apparatus according to the invention has the advantage in particular that the shrinking of the solidifying material in relation to the casting mould, which has an effect primarily in the direction of extent of the tubular portion, leads to the solidifying material immediately becoming detached from the wall of the casting mould, and consequently no frictional effects between the solidifying melt and the wall of the casting mould being measured.
Furthermore, the measurement is independent of the shrinkage behaviour of the casting mould itself, since the melt in the casting moulds generally shrinks more than the material of the casting mould itself.
The apparatus according to the invention has been developed primarily for determining the hot tearing susceptibility of magnesium melts. However, as a result of the test setup and the characteristic values determined, the apparatus can be used not only for magnesium alloys but also for aluminium, zinc and non-ferrous metals and for steels and other metallic melts.
The geometry according to the invention and the possibility of using a wide variety of casting mould materials mean that the apparatus essentially allows measurements on all materials during solidification. Consequently, a standardizable measuring cell for determining hot tearing susceptibility is available for the first time. Not only a qualitative assessment but at the same time also a quantitative assessment is therefore possible. This also allows for the first time the direct comparison of the hot tearing susceptibility of a wide variety of materials.
A great advantage of the invention is the simultaneous detection of cracks and the resultant changes in strain or stress in a test sample. Both quantitative and qualitative findings can be reached with respect to hot tearing susceptibility.
Apart from the chemical composition, melting parameters such as melting bath temperature, casting mould temperature, variation of the temperature during the solidification and the forces occurring during the solidification in the casting mould due to shrinkage are also measured with the apparatus and taken into consideration in the evaluation. In addition, a purely qualitative assessment of the material can be made at the same time with respect to the occurrence of a crack, its size, morphology and behaviour over time.
In order to minimize hydrostatic pressure influences or other influences on the measurement caused by gravitational force, it is advantageous if the tubular portion extends substantially in a horizontal direction.
Temperature measuring elements are preferably arranged along the direction of extent of the tubular portion. These may on the one hand serve for recording the temperatures for evaluation purposes. On the other hand, they may be used for regulating the temperature of the wall of the casting mould, and correspondingly the temperature of the solidifying melt. For this purpose, it is of advantage if at least one temperature measuring element is arranged in the first subvolume. This may, for example, be telescopically insertable into the inner volume of the casting mould, in order that the temperature at the hotspot in the connecting region between the first and second subvolumes can be measured.
It is advantageous for the temperature regulation and control if the first subvolume and/or the tubular portion is provided with a heating device and/or a cooling device. This allows the solidification process of the melt to be controlled in a specific manner. It is particularly advantageous in this respect if the temperature of the tubular portion can be controlled in such a way that the second end is colder than the first end, and consequently the melt solidifies first at the second end and thereby becomes connected to the measuring element in a tension-resistant manner. This allows the measurement of the tensile force that is caused by the shrinkage behaviour of the melt to be carried out accurately from the earliest possible stage of the solidification process.
A venting opening is preferably provided at the second end of the tubular portion, in order that the air in the tubular portion, which preferably extends substantially in a horizontal direction, can escape when the melt enters the portion. This venting opening may be, for example, a notch in the measuring element that extends on the upper side in the direction of extent of the measuring element.
The apparatus preferably has a sprue system located at least partially above the casting mould, the first subvolume of the casting mould being designed for receiving a metallic melt from the sprue system. In this case, the melt is created in the sprue system and, when the melt and the casting mould are at a suitable temperature, the melt is let out through a lower outlet into the first subvolume of the casting mould. Therefore, the first subvolume advantageously has an upper opening for receiving the melt from the sprue system located above it. As soon as the level of the melt has reached the connecting region between the first subvolume and the second subvolume of the casting mould, the melt is also distributed into the second subvolume.
In order that the measuring element can move in the axial direction without friction as far as possible, it is guided in relation to the casting mould, preferably in a sleeve, for example made of graphite or ceramic, with little frictional contact.
For the reading out and evaluation of the measurement results, the measuring device and any temperature measuring elements are preferably connected to a computer-controlled readout system and can be read out with this system.
This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the detailed description of the preferred embodiments. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter.
Various other aspects and advantages of the present invention will be apparent from the following detailed description of the preferred embodiments and the accompanying drawing figures.
Advantageous embodiments of the invention are explained in detail below on the basis of
The drawing figures do not limit the present invention to the specific embodiments disclosed and described herein. The drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the preferred embodiment.
The present invention is susceptible of embodiment in many different forms. While the drawings illustrate, and the specification describes, certain preferred embodiments of the invention, it is to be understood that such disclosure is by way of example only. There is no intent to limit the principles of the present invention to the particular disclosed embodiments.
Arranged underneath the outlet opening 13 of the melting furnace 9 is the casting mould 7, so that, when the plug 17 is raised, the melt 10 can flow through the outlet opening 13 into the casting mould 7. The casting mould 7 has two subvolumes, the first subvolume 27 being substantially a cylindrical receptacle with an opening on the upper side for receiving the melt 10 from the melting furnace 9. A second subvolume 27 extends in the form of a tubular portion 29 substantially horizontally away from the first subvolume 27. The tubular portion 29 has a first end 31, which is connected to a lower region of the first subvolume 27, and a second end 33, into which a measuring element 35 extends. When the casting mould 7 is filled from the melting furnace 9 on the upper side, the first subvolume 27 fills first, until a filling level at which the melt 10 flows into the tubular portion 29 is reached. For a measurement, the tubular portion 29 must be completely filled.
It can be seen still more clearly in
A large part of the outer surface area of the casting mould 7 is equipped with a heating or cooling system 37, so that the temperature of the casting mould 7 can be suitably controlled both in the first subvolume 27 and in the tubular portion 29. This allows the solidification behaviour of the melt 10 in the casting mould 7 to be controlled in a specific manner. For regulating the heating or cooling system 37 and for evaluation purposes, the casting mould 7 is also equipped with temperature measuring elements 39, 41, 43, 45, 47, which are respectively connected via a communication link to the computer-controlled readout system 25. A temperature measuring element 39 measures the temperature of the first subvolume 27 of the casting mould 7. Three temperature measuring elements 43, 45, 47 measure the temperature of the tubular portion 29 in various regions along the direction of extent of the tubular portion 29. A temperature measuring element 47 is in this case arranged at the second end 33 of the tubular portion 29. A temperature measuring element 41 is telescopically or endoscopically led through the first subvolume 27 into the connecting region between the first subvolume 27 and the tubular portion 29. This allows the temperature measuring element 41 to be used for measuring the temperature of the melt 10 at the hotspot.
At the second end 33 of the tubular portion 29 is the measuring element 35, which extends substantially in the form of a piston coaxially in the direction of extent of the tubular portion 29 and is guided in relation to the casting mould 7 in a sleeve 49 with little frictional contact, for example made of graphite or ceramic. The measuring element 35 is therefore mounted movably in the axial direction. For reading out the force and/or the position in the direction of extent of the tubular portion 29, the measuring element 35 is coupled with a measuring device 51. The measuring device 51 has in this case a universal joint 53 and a rod-shaped transmission element 55, the universal joint 53 connecting the measuring element 35 to the transmission element 55. The transmission element 55 transmits the force and/or the change in position in the axial direction to a force or position measuring instrument 57. The force or position measuring instrument 57 is likewise connected via a communication link to the computer-controlled readout system 25 for the reading out and evaluation of the measurement data.
The casting mould 7 and the force or position measuring instrument 57 have defined positions with respect to the platform 1, to which they are preferably fixedly connected. The casting mould 7 and/or the force or position measuring instrument 57 may also be connected to the platform 1 movably in the direction of extent of the tubular portion 29. This admittedly then necessitates a precise calibration of the axial positions. However, an additional tearing force can then be exerted on the melt 19, for example by means of a controlled stepping motor, in order to provoke a crack and/or to measure the load-bearing capacity of the material under certain stresses. Similarly, the sprue system 5 may be connected to the holding device 3 in a vertically adjustable manner, in order that during the initial casting the drop height of the melt 10 can be set and can be adapted appropriately to the material used. The overall apparatus may be designed, for example, as a benchtop experiment with a lateral extent of less than 1 metre. The tubular portion 29, tapering towards the second end 33, may have, for example, a length of approximately 20 cm and a diameter of between 12 and 8 mm.
With the apparatus shown, the method described below for determining the hot tearing susceptibility of a metallic melt is performed. First, a metallic material 10 of a known chemical composition that is to be tested, for example magnesium with an aluminium content of 3%, is filled into the melting furnace 9. The amount of material 10 must be adequate to fill the casting mould 7 adequately in liquid form to allow the tubular portion 29 to be completely filled. The melting furnace 9 is then heated to a temperature above the melting point of the material 10, for example to 715° C. At the same time, the casting mould 7 is preheated to a temperature of, for example, 350° C., in order to be able to control the later solidification behaviour of the melt 10 in the casting mould 7.
As soon as the temperature measuring elements 21, 39, 43, 45, 47 indicate the suitable temperature, the initial casting can commence. By means of the rod 19, the plug 17 closing the outlet opening 13 on the underside of the smelting furnace 9 is raised, so that the metallic melt 10 can flow through the outlet opening 13 into the first subvolume 27 of the casting mould 7 located under it. The melt 10 is thereby also distributed into the tubular portion 29, which is connected with its first end 31 to the first subvolume 27 of the casting mould 7. Since the temperature of the casting mould 7 lies below the melting point, the melt begins to solidify in the casting mould 7. This takes place at first at the wall of the casting mould 7, which is colder in relation to the melt, and after that proceeds inwards. As a result of the small diameter at the second end 33 of the tubular portion 29, the furthest distance from the first subvolume 27 and/or suitable temperature control by the heating or cooling system 37, the melt 10 solidifies first at the second end 33 of the tubular portion 29, into which the measuring element 35 extends. The solidification has the effect that a non-positive, and consequently tension-resistant, connection forms between the solidified melt 10 and the measuring element 35. During the solidifying, the metallic material 10 contracts. This manifests itself in particular in the direction of extent of the tubular portion 29. The casting mould 7 itself likewise contracts as it cools down, but many times less than the material 10, which passes through a phase transition from liquid to solid. Therefore, the measuring element 35 is drawn into the tubular portion 29 by means of the tension-resistant connection between the solidified melt 10 and the measuring element 35.
In the radial direction, the shrinkage of the solidifying material in the tubular portion 29 is scarcely noticeable. The tapering of the tubular portion 29 then prevents frictional effects between the wall of the casting mould 7 and the solidified material 10. This is so because the taper angle causes the material immediately to become detached from the wall, and not to pull itself along the wall, as a result of the notable shrinkage in the axial direction. As a result, the measurement of the tensile force on the measuring element 35 becomes many times more accurate than when conventional devices are used. It is then possible to use the measuring element 35 to measure the tensile force that the shrinkage of the material 10 brings about, or the change in length of the material 10.
During the solidification, the temperature at the hotspot, i.e. the connecting region between the first subvolume 27 and the tubular portion 29, is also recorded. In this region, the melt 10 remains liquid for the longest time, and the probability of the occurrence of a hot crack is greatest here. For the temperature measurement, a temperature measuring element 41 is telescopically or endoscopically led through the first subvolume 27 into the connecting region between the first subvolume 27 and the tubular portion 29 through a bore in the casting mould 27. The temperature measuring element 41 is heat-resistant and is provided with corresponding material, such as ceramic.
During the solidification, the readout system 25 controlled by the computer 23 reads both the temperatures measured with the temperature measuring elements 39, 41, 43, 45, 47 as a function of time and the tensile force on the measuring element 35, or the change in position of the latter, measured with the force or position measuring instrument 57.
The occurrence of a hot crack would show itself in the force curve as a step or a short fall. Consequently, the maximum tensile force that is measured in a test is dependent on whether one or more hot cracks have occurred and the form and extent they have. Consequently, the maximum force that is measured after a defined solidification interval is a quantitative measure of the size and/or number of hot cracks. For example, it has been found that, in the case of a magnesium melt with an aluminium content of 1% and a hotspot temperature of 300° C., a tensile force of less than 900 N indicates a large existing hot crack. Visual observations have confirmed this. A crack of just a moderate size here allows a tensile force of 900 N to 1100 N to be measured and the absence of a hot crack or the presence of only very small hot cracks allows tensile forces of more than 1100 N to be measured.
The preferred forms of the invention described above are to be used as illustration only, and should not be utilized in a limiting sense in interpreting the scope of the present invention. Obvious modifications to the exemplary embodiments, as hereinabove set forth, could be readily made by those skilled in the art without departing from the spirit of the present invention.
The inventors hereby state their intent to rely on the Doctrine of Equivalents to determine and access the reasonably fair scope of the present invention as pertains to any apparatus not materially departing from but outside the literal scope of the invention set forth in the following claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 102008031777.2 | Jul 2008 | DE | national |
