Patent Application
20220161312
The present invention relates to a method for producing a shaped part comprising a bulk metallic glass and a shaped part produced according to the method and its use. Metallic glasses have been the subject of extensive research since they were discovered about 50 years ago at the California Institute of Technology. Over the years, it has been possible to continuously improve the processability and properties of this class of materials. While the first metallic glasses were still simple, binary alloys (made up of two components), the production of which required cooling rates in the range of 106 Kelvin per second (K/s), more recent, more complex alloys can already be transferred into the glassy state at significantly lower cooling rates in the range of a few K/s. This has a significant influence on process management and the components that can be implemented. The cooling rate at which the melt does no longer crystallize and solidifies in the glassy state is referred to as the critical cooling rate. It is a system-specific variable that is heavily dependent on the composition of the melt and also defines the maximum component thicknesses that can be achieved. If you consider that the thermal energy stored in the melt must be discharged through the system sufficiently fast, it becomes clear that only components with a small thickness can be manufactured from systems with high critical cooling rates. In the beginning, metallic glasses were therefore mostly produced using the melt spinning process. The melt is stripped onto a rotating copper wheel and solidifies like a glass in the form of thin strips or films with thicknesses in the range of a few hundredths to tenths of a millimeter. With the development of new, complex alloys having significantly lower critical cooling rates, other manufacturing processes can increasingly be used. Today's solid glass-forming metallic alloys can be converted into the glassy state by pouring a melt into cooled copper molds. The implementable component thicknesses are alloy-specific in the range from a few millimeters to centimeters. Such alloys are called bulk metallic glasses (BMG). A large number of such alloy systems are known today. The subdivision of bulk metallic glasses is usually based on their composition, wherein the alloy element with the highest weight percentage is referred to as the base element. The existing systems include, for example, precious metal-based alloys such as gold, platinum, and palladium-based bulk metallic glasses, early transition metal-based alloys such as titanium or zirconium-based bulk metallic glasses, late transition metal-based systems based on copper, nickel, or iron, but also systems based on rare earths, such as neodymium or terbium. Bulk metallic glasses typically have the following properties compared to classic crystalline metals:
There are various methods of manufacturing shaped parts from bulk metallic glass. The melt spinning process as described in U.S. Pat. No. 4,116,682 A is suitable for producing thin metal strips having a thickness of approximately 100 μm.
Due to the high cooling rates that can be achieved, alloys that have comparatively high critical cooling rates can also be processed amorphously in this way. For the production of cast parts with dimensions in the range of a few millimeters, special alloys have been developed that have lower critical cooling rates and still solidify amorphously, at casting thicknesses in the range of a few millimeters. Such alloys are described in US 20150307975 A1, for example.
The prior art has certain disadvantages. Although thin structures with a high degree of homogeneity can be produced using melt spinning, thicker shaped parts with a diameter of more than 150 μm are not easily accessible. Thicker shaped parts can be produced by melt casting than by melt spinning, but these shaped parts often have highly fluctuating mechanical properties.
Furthermore, shaped parts with a thickness of less than 500 μm are often difficult to manufacture using casting processes, since the viscosity of the melt increases rapidly during casting.
Shaped parts made from bulk metallic glass using casting processes often show a high degree of heterogeneity in terms of mechanical properties, such as flexural strength. This makes the use of shaped parts made of bulk metallic glass difficult or impossible in precision applications, for example in precision engineering.
Furthermore, shaped parts often have defects in the form of tiny gas inclusions, which have a negative impact on the mechanical properties.
It was an object of the present invention to provide a method which solves at least one of the aforementioned problems.
A preferred object of the present invention was to provide a method which allows producing shaped parts comprising a bulk metallic glass, wherein the shaped parts have defined and homogeneous mechanical properties.
It was an object of the present invention to provide a method which allows producing shaped parts comprising a bulk metallic glass and which have a reduced number of defects, such as air inclusions, compared to cast molded parts.
Furthermore, it was an object of the invention to provide an improved method which allows producing shaped parts comprising a bulk metallic glass in fewer work steps.
Another preferred object of the invention was to provide a method with which shaped parts can be produced from bulk metallic glasses with a diameter of 600 μm or less, particularly 400 μm or less, by means of the casting process.
A contribution to achieving at least one of the objects mentioned is made by the subject matter of the independent claims.
A first aspect of the invention relates to a method for producing a shaped part comprising a bulk metallic glass, characterized by the following steps:
Optionally, further steps can be carried out before, during or after the steps mentioned, as long as the specified sequence of steps a)-c) is adhered to.
The method according to the invention provides a production route for shaped parts having a bulk metallic glass. According to the invention, the shape of the shaped part is not restricted any further. In one possible embodiment, the shaped part can be selected from the group consisting of strips, cuboids, wires, rods, or sheets.
The method according to the invention is particularly suitable for the production of shaped parts, particularly sheets and strips with a thickness of 100-600 μm, particularly 200 μm-500 μm. Such shaped parts are typically too thick to be produced by melt spinning and too thin to be produced by injection molding.
Furthermore, with the process parameters remaining the same, multiple shaped parts with homogeneous mechanical properties can be obtained using the method according to the invention, based on the relative standard deviation over several components. The relative standard deviation of the strength in the case of multiple shaped parts produced according to the invention is preferably not greater than 10% and particularly not greater than 5%.
In one aspect, the invention relates to a method for producing a shaped part. The shaped part according to the invention comprises a bulk metallic glass or consists thereof. Bulk metallic glasses are alloys that have a metallic binding character and at the same time an amorphous, i.e. non-crystalline, phase. In the context of the invention, an alloy can be referred to as bulk metallic glass if the respective alloy can be brought into the glassy state in a body with dimensions of 1 mm×1 mm×1 mm at a suitable cooling rate.
The bulk metallic glasses can be based on different elements. In this context, “based” means that the element mentioned in each case represents the largest proportion in relation to the weight of the alloy. Typical components, which can preferably also form the basis of the alloy, can be selected from:
Preferred combinations of elements contained in bulk metallic glasses are selected from:
Other particularly preferred examples of alloys that form bulk metallic glasses are selected from the group consisting of Ni—Nb—Sn, Co—Fe—Ta—B, Ca—Mg—Ag—Cu, Co—Fe—B—Si Nb, Fe—Ga—(Cr, Mo) (P, C, B), Ti—Ni—Cu—Sn, Fe—Co—Ln—B, Co—(Al, Ga)—(P, B, Si), Fe—B—Si—Nb and Ni—(Nb, Ta)—Zr—Ti. In one embodiment of the invention, alloys based on copper and/or zirconium are preferred. Particularly, the bulk metallic glass can be a Zr—Cu—Al—Nb alloy. In addition to zirconium, this Zr—Cu—Al—Nb alloy preferably also contains 23.5-24.5% by weight copper, 3.5-4.0% by weight aluminum and 1.5-2.0% by weight niobium, the proportions by weight adding up to 100% by weight. The latter alloy is commercially available under the name AMZ4® from Heraeus Deutschland GmbH. In another particularly preferred embodiment, the bulk metallic glass can contain the elements zirconium, titanium, copper, nickel and aluminum. Particularly suitable bulk metallic glasses for the production of shaped parts have the composition Zr52.5Ti5Cu17.9Ni14.6Al10 and Zr59.3Cu28.8Al10.4Nb1.5, wherein the indices specify at-% of the respective elements in the alloy. Another preferred group of alloys can contain the elements Zr, Al, Ni, Cu and Pd, particularly Zr60Al10Ni10Cu15Pd5 (indices in at.-%). Another preferred group of alloys contains at least 85 wt-% Pt as well as Cu and phosphorus, wherein the alloy may also contain Co and/or nickel, for example Pt57.5Cu14.5Ni5P23 (indices in at.-%).
In step a) of the method according to the invention, a preform comprising a bulk metallic glass is provided. The preform preferably consists of a bulk metallic glass. In a preferred embodiment, the preform is made with a bulk metallic glass using a casting process, particularly injection molding or suction casting.
For example, the starting components of the solid glass-forming alloy can be melted in an arc under vacuum until a homogeneous alloy is created to produce this preform comprising a bulk metallic glass. The alloy obtained can be processed into a preform comprising a bulk metallic glass by suction or injection molding, for example. The casting process preferably takes place in an argon atmosphere. The preform comprising a bulk metallic glass is preferably a strip, a cuboid, a wire, a rod, or a sheet metal. The preform preferably has a solid diameter (that is to say without cavities or recesses) of at least 1 mm in the smallest dimension.
The critical casting thickness should not be exceeded, such that the preform comprising a bulk metallic glass solidifies amorphously during casting. The melts of optimized alloys have critical casting thicknesses of one millimeter or more, such that at a sufficient cooling rate, e.g. in a copper mold with optional water cooling, completely amorphous preforms comprising a bulk metallic glass can be obtained. A person skilled in the art knows how preforms comprising a bulk metallic glass can be produced by means of a casting processes.
The preform comprising a bulk metallic glass obtained preferably has a weight proportion of bulk metallic glass that is at least 95%, particularly at least 98%. Particularly preferably, the preform comprising a bulk metallic glass is completely amorphous, measured by means of XRD due to the absence of crystalline signals in the diffractogram.
In step b), the preform comprising a bulk metallic glass is repeatedly plastically deformed, wherein the temperature T1 is below the glass transition temperature of the bulk metallic glass. Particularly, the temperature T1 during the deformation is at least 15% below the glass transition temperature, measured in ° C. According to the invention, the plastic deformation takes place in multiple deformation steps, each with a deformation Δd (Δd=d before the deformation step−d after the deformation step). The diameter d of the preform is measured along the deformation direction in the respective deformation step. According to the invention, the plastic deformation in step b) takes place in such a way that the deformation Δd of the preform made of bulk metallic glass increases with an increasing number of steps. In an optional embodiment, the deformation Δd, after it has increased, can decrease again with an increasing number of steps For example, the deformation Δd can increase from 5 μm to 50 μm and then decrease again to 5 μm or 10 μm. By means of processes in which the deformation Δd first increases and then decreases again, particularly thin shaped parts with a diameter of less than 300 μm can be produced efficiently and homogeneously.
Deformation steps can be saved if the deformation Δd increases with increasing deformation steps, which can make the method according to the invention more cost-effective than conventional methods in which the deformation Δd is kept constant. The deformation Δd is preferably at least 1 μm, particularly at least 5 μm and very particularly preferably at least 10 μm. At most, the deformation Δd can be a maximum of 100 μm, particularly a maximum of 50 μm, per deformation step. The change in the deformation Δd with an increasing number of steps can preferably take place continuously or in step-by-step. In this context, continuous means that the deformation Δd changes with each further deformation step. Step-by-step means here that multiple deformation steps with the same deformation Δd are carried out before one or more deformation steps with the next larger deformation Δd take place. The increase in deformation Δd with increasing deformation steps can increase linearly, e.g. in steps of 5 μm (Δd 5 μm→Δd 10 μm→Δd 15 μm→ . . . ) or increase further with increasing deformation steps, e.g. Δd 5 μm→Δd 10 μm→Δd 20 μm→Δd 50 μm, . . . .
In the example of a preform made of bulk metallic glass in the form of a cast strip with a thickness of 3 mm, the deformation Δd can start at 5 μm in the first step and end with a deformation Δd of 50 μm in the last step of the deformation, with the final thickness of the strip is about 500 μm. In this example, the reduction in thickness can preferably take place in 50-150 steps.
The degree of deformation per deformation step is preferably in the range of 0.1-0.5% of the diameter d, wherein the degree of deformation is calculated by: (1−(d after the deformation step/d before the deformation step)). The summed degree of deformation Δdtotal of the preform over all deformation steps can preferably be up to 90%, the summed degree of deformation Δdtotal being calculated by: (Δdtotal=1−(d after deformation/d before deformation)).
Forming is preferably carried out by rolling. Optionally, two or more rollers can be used for forming. In another preferred embodiment, the forming, particularly rolling, takes place in such a way that the shaped part is deformed with a force which is in the range of serration in the stress-strain diagram. The serration in the stress-strain diagram is to be understood as a curve in which, with increasing strain, there is a sudden drop in stress, and this process is repeated multiple times before the specimen breaks.
In a preferred embodiment of the invention, the preform comprising a bulk metallic glass is either deformed in the same direction or in different directions in each deformation step. If the preform comprising a bulk metallic glass is deformed in different directions, the deformation directions of successive deformation steps can be parallel or perpendicular to one another. A particularly homogeneous material can be obtained by a vertical orientation of successive deformation steps.
The number of deformation steps, particularly rolling steps, is not restricted any further according to the invention. In a preferred embodiment, the shaped part is deformed in at least two deformation steps, particularly in at least ten deformation steps and specifically preferably in at least 30 deformation steps, and particularly in at least 50 deformation steps. The preform comprising a bulk metallic glass is particularly preferably deformed in a maximum of 300 deformation steps, particularly a maximum of 200 deformation steps and specifically preferably in a maximum of 150 or a maximum of 100 deformation steps.
Forming can optionally take place after step b). Forming can preferably be selected from bending, hammering, and deep drawing.
In step c) of the method according to the invention, the preform comprising a bulk metallic glass is heated to a temperature T2 above the glass transition temperature and below the crystallization temperature to obtain a shaped part comprising a bulk metallic glass. The shaped part preferably consists of a bulk metallic glass. In another preferred embodiment, the shaped part contains both amorphous bulk metallic glass and at least one crystalline phase. Such mixtures of amorphous and crystalline phases are also called bulk metallic glass composites (BMGC).
The heating in step c) is preferably carried out below the extrapolated initial crystallization temperature (according to DIN EN ISO 11357-3:2018-07). The shaped part produced by the method can have mechanical properties, particularly strengths, such as flexural strength, which are similar to those of the preform before the deformation. The preform comprising a bulk metallic glass is preferably heated in step c) such that the bending strength of the shaped part obtained corresponds to the initial value of the bending strength of the preform comprising a bulk metallic glass in step a). The flexural strength after step c) is preferably at most 15%, particularly at most 10% lower than the flexural strength of the cast preform. In the context of the invention, the flexural strength according to DIN EN ISO 7438:2016-07 can be determined in a bending test. In a preferred embodiment, the preform comprosing a bulk metallic glass is heated for a period of 0.1 to 3000 s, particularly 5 to 300 s. In a particularly preferred embodiment, the heating is terminated before the crystallization temperature in the TTT-diagram of the alloy is achieved for the respective heating rate.
The preform comprising a bulk metallic glass is preferably heated to a temperature T2 which fulfills the following condition: TG<T2<TG+(60/100)*(TX−TG), particularly the condition TG<T2<TG+(30/100)*(TX−TG). In this case, TG is the glass transition temperature and TX is the crystallization temperature. Under these conditions, particularly advantageous mechanical properties of the finished shaped part can be obtained.
In a preferred embodiment, the heating in step c) takes place while applying additional pressure to the shaped part. This can lead to particularly small piece-to-piece deviations in the mechanical properties of the resulting components. The pressure applied to the shaped part is preferably from 1 to 600 MPa, particularly from 5 to 300 MPa and very particularly preferably from 10 to 150 MPa.
In a particularly preferred embodiment of the invention, the heating in step c) takes place as thermoplastic forming (TPF). TPF can change the overall dimensions of the shaped part, for example the thickness of a sheet metal, or, alternatively, structures can be embossed into the shaped part, that is, the preform is locally deformed. Alternatively, the surface of the shaped part can be changed using TPF. Heating is preferably carried out in a heatable press.
Preferably, the preform comprising a bulk metallic glass can be pressurized or thermoplastically shaped between two plane-parallel surfaces, whereby flat shaped parts can be obtained. Flat means with a thickness variation in the range of at most 20% around the mean value, e.g. +/−200 μm with an average thickness of 1 mm or +/−30 μm with an average thickness of 150 μm.
In one possible embodiment of the invention, the component obtained after step c) can be used for technical applications directly and without further treatment.
For example, the method according to the invention can be used to produce a shaped part comprising a bulk metallic glass, wherein the shaped part has a diameter of at least 200 μm in at least one dimension. The shaped part comprising a bulk metallic glass can preferably have a flexural strength (in N/mm2) which at most is 15% below the flexural strength of the cast alloy. The shaped part preferably has a diameter of at least 200 μm in at least two dimensions. In one possible embodiment of the invention, the shaped part has a thickness not greater than 600 μm, particularly not greater than 400 μm. The bulk metallic glass of the shaped part which has these properties is preferably selected from Zr52.5Ti5Cu17.9Ni14.6Al10 and Zr59.30Cu28.8Al10.4Nb1.5, wherein the indices indicate at-% of the respective elements in the alloy.
Optionally, step c) can then be followed by one or more aftertreatment steps for the shaped part comprising a bulk metallic glass. The aftertreatment steps can be selected, for example, from laser cutting, water jet cutting, milling, drilling, grinding, polishing and sandblasting.
The process according to the invention can be used to produce shaped parts for a wide variety of applications. The use of the method is particularly advantageous wherever shaped parts are required that have high geometric precision and isotropic mechanical properties and can be produced with little piece-to-piece deviation.
The use of the method according to the invention for the production of precision mechanical components, such as springs, gears, etc., is particularly preferred. For example, such components can be used for the manufacture of watches.
DSC Measurement
The DSC measurements in the context of the invention are carried out in accordance with DIN EN ISO 11357-1:2017-02 and DIN EN ISO 11357-3:2018-07. The sample to be measured in the form of a thin disc or film (approx. 80-100 mg) is placed in the measuring device (NETZSCH DSC 404F1, NETZSCH GmbH, Germany). The heating rate is 20.0 K/min. Al2O3 is used as the crucible material. The heat flow is measured against an empty reference crucible, such that only the thermal behavior of the sample is measured.
The measurement method is carried out according to the following steps:
Glass Transition Temperature
In the context of the present invention, the glass transition temperature is measured according to ASTM E1365-03 as follows.
The sample to be examined is placed in a crucible in a DSC device (NETZSCH DSC 404F1, NETZSCH GmbH, Germany). The system is heated and cooled according to the following scheme, and the respective heat flow is measured in steps a) and c).
As a result of the experiment, the enthalpy is obtained as a function of the temperature for the sample. In step a), the amorphous sample is crystallized. In step c), the thermal behavior of the already completely crystallized sample is recorded.
In order to determine the glass transition temperature, the measurement from step c) is subtracted from the measurement from step a). The resulting curve includes an endothermic transition at a lower temperature and an exothermic signal at a higher temperature. The signal at a higher temperature corresponds to the crystallization process. The endothermic signal corresponds to the glass transition. In order to determine the glass transition temperature, a tangent line to the baseline is determined before the glass transition range (by linear fitting). A second tangent is determined at the turning point (corresponding to the peak value of the first derivative over time) of the glass transition range. The temperature value at the intersection of the two tangents indicates the glass transition temperature (Tf according to AST 1356-03).
Crystallization Temperature
The crystallization temperature was determined by means of DSC in accordance with the DIN EN ISO standard 11357-3:2018-07. This standard is designed for polymers, but can be used analogously for metallic glasses. In the context of the invention, the crystallization temperature corresponds to the peak crystallization temperature Tp,c as used in the standard mentioned herein. The heating rate was 20 K/min.
The alloy (Zr59.30Cu28.8Al10.4Nb1.5) was produced by melting the elements in a vacuum arc. A preform was made from the alloy produced by means of suction casting by pouring the homogeneous, liquid melt of the alloy into a copper casting mold. The copper mold was kept at room temperature. The casting obtained in the form of a tape had the dimensions 3×15×40 mm.
The cast part obtained in the form of a strip was rolled in a rolling mill at room temperature with increasing deformation steps to a thickness of 0.5 mm. The deformation steps started with a thickness reduction of 5 μm, and the rolling ended at a deformation Δd of 50 μm per deformation steps after 70 rolling processes.
The cold-rolled strip was then heated using a heated press below the crystallization temperature in the TTT diagram of the alloy for 60 seconds to achieve the desired flexural strength of approximately 2250 N/mm2 adjust.
According to the example described, 50 shaped parts were produced. For the components obtained, the stress-strain behavior was measured using a 3-point bending test (in accordance with DIN EN ISO 7438:2016-07). The results of the measurements are summarized in Table 1.
Table 1 summarizes the measured flexural strengths of the manufactured parts for different stages of manufacture and gives the standard deviation of the flexural strength over several parts for each processing step. It can be seen that shaped parts can be obtained using the method of the invention (3rd row), which parts have an average flexural strength of 2261 N/mm2 which is close to the initial value of the cast preform of 2447 N/mm2 while the homogeneity of the components (expressed by the lower standard deviation) has increased by a factor of 3.4 compared to the cast parts (row 1).
| Number | Date | Country | Kind |
|---|---|---|---|
| 19162224.0 | Mar 2019 | EP | regional |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2020/054719 | 2/24/2020 | WO | 00 |
