Patent Application
20200061689
The invention relates to a method for machining a workpiece made of a metallic material, in which the workpiece is machined by ECAP.
Methods for machining workpieces are known which are summarized under the name “severe plastic deformation” (SPD). This involves a method in which a workpiece made of metal is subjected to severe plastic deformation in order to produce an ultrafine-grained (UFG) structure. The structure may attain an average grain size of less than 1 μm, and often has large orientation angles at the grain boundaries. The SPD methods include, among others, equal channel angular pressing (ECAP), accumulative roll bonding (ARB), high pressure torsion (HPT), repetitive corrugation and straightening (RCS), cyclic extrusion compression (CEC), torsion extrusion, severe torsion straining (STS), cyclic closed-die forging (CCDF), and super short multi-pass rolling (SSMR).
ECAP is a method in which a workpiece is pressed through at least two intermerging channels, wherein the channels have an identical cross section, and the transition between the channels is angled at an arbitrary angle, preferably between 80° and 140°. The plastic deformation of the material of the workpiece when pressed through the transition between the channels may result in a marked refinement of the material structure, and thus, improved material properties. Machining of a workpiece by ECAP is described in U.S. Pat. Nos. 5,513,512 A, 6,399,215 B1, and EP 2 366 808 A2, for example, wherein the machined workpieces are made, at least partially, of (pure) titanium and/or are used for manufacturing medical implants.
Medical implants are often made of pure titanium, which has very good biocompatibility. However, the relatively low mechanical strength of this material may be disadvantageous in such implants. Although the mechanical strength may be increased significantly by the use of titanium alloys, this generally occurs at the expense of biocompatibility, and thus possibly of the dwell time of titanium alloy implants in a human or animal body.
The devices used in the above-cited publications for carrying out ECAP are limited with respect to the dimensions and in particular with respect to the length of the workpieces to be machined. This is attributed in particular to the limited stroke of the stamp with which the workpieces are pressed through the channels of the particular tools. For eliminating this disadvantage, according to U.S. Pat. No. 7,152,448 B2 a suitable device is proposed, in which the workpiece has a first, partially circular channel that merges at an angle into a second, straight channel, wherein the first channel on the radially inner side is delimited by a rotatingly drivable, disk-shaped propulsion element. By rotation of the propulsion element, the workpiece is moved through the first channel by frictional engagement and pressed into the second channel in a continuous process. The aim is to allow machining of workpieces, having any length in principle, by ECAP in a continuous process.
In the publication “Severe Plastic Deformation by Equal Channel Angular Swaging” in Material Science Forum, Vols. 667-669, pages 103-107 (ISSN: 1662-9752) by Bruder et al., a method referred to as ECAS is described which combines the principles of ECAP and hammering (rotary swaging); however, the rotation of the workpiece relative to the tools, which is characteristic of conventional hammering, is not carried out. In addition, in the ECAS method the modified hammering always takes place concurrently with the ECAP. Diameter reduction for the workpiece is not achieved in the ECAS method.
US 2007/0256764 A1 discloses machining of a workpiece by a method referred to as ECAE, comparable to ECAP, in which the workpiece or the ECAE tool is set in vibration during the machining. This generation of vibration does not result either in deformation of the workpiece, or in particular diameter reduction. In addition, there are no forming strokes by forming tools during the generation of vibration.
In the publication by Pachla et al., “Effect of severe plastic deformation realized by hydrostatic extrusion and rotary swaging on the properties of CP Ti grade 2” in Journal of Material Processing Technology 221 (2015), pages 255-268, a method is described which combines hydrostatic extrusion with post-processing of a workpiece by hammering. The hammering is carried out to improve the surface quality of the workpiece.
Based on this prior art, the object of the invention is to provide a method by means of which in particular mechanical material properties of a workpiece that is machined by ECAP may be further improved.
This object is achieved by a method according to Patent claim 1. Advantageous embodiments of the method according to the invention and advantageous uses of a workpiece produced according to the invention are the subject matter of the further patent claims, and/or result from the following description of the invention.
According to the invention, a method for machining a workpiece that is made of a, or at least one, metallic material or contains such, wherein the workpiece is machined by ECAP, is characterized in that after the machining by ECAP, the workpiece is post-processed by hammering, thus achieving a (permanent) diameter reduction due to plastic deformation of the material of the workpiece. It has surprisingly been found that mechanical material properties of the material of a workpiece previously machined by ECAP, in particular its tensile strength and/or hardness, may thus be significantly improved or increased. This finding was surprising in particular due to the fact that other methods for post-processing forming, for example rolling, of a workpiece previously machined by ECAP do not always result in these improvements; on the contrary, improvements in the mechanical material properties previously achieved by ECAP are diminished, depending on the degree of deformation. Even when there is an improvement, usually only the strength is increased whereas the ductility is severely reduced, and when overloaded, the material becomes brittle or fractures with very little deformation. Such materials are not suitable as construction materials.
As stated above, machining by ECAP has basically been characterized in that a workpiece is pressed through at least two, or even three or more, intermerging channels, wherein the channels preferably have an identical cross section and the transition between the channels is angled. The plastic deformation of the material of the workpiece when passing through the transition between the channels may result in significant refinement of the material structure, and thus, improved material properties. In particular, the tensile strength and/or the hardness of the material may thus be greatly increased compared to the starting state (prior to the machining by ECAP).
A particularly notable improvement in the material properties may be achieved by carrying out ECAP multiple times, in particular twice. This may also take place in a machining pass when the tool used for ECAP has at least three channels, of which two in each case merge into one another at an angle, as shown by way of example in
It may preferably be provided that the forming of the workpiece takes place by hammering in multiple passes, each forming operation having a degree of deformation of 0.05 to 2, preferably 0.1 to 0.5, and particularly preferably 0.15 to 0.3.
Furthermore, it may be provided that the forming of the workpiece by hammering takes place at a temperature (of the workpiece) of between 10° C. and 600° C., preferably between 12° C. and 380° C., and particularly preferably between 14° C. and 250° C. Forming of the workpiece by hammering at a temperature of the workpiece of 100° C. and/or 350° C. may also be advantageous.
Hammering, also known as swaging or rotary swaging (for machining of workpieces having a circular cross section), is characterized in that two or more tools (3) that are situated on the circumferential side of the workpiece (1) exert radial forming strokes directed toward a workpiece center while the workpiece (1) rotates about the workpiece center relative to the tools (3), as schematically shown in
According to the invention, hammering may be provided in embodiments in which the workpiece (1) undergoes hot forming, warm forming, or cold forming. Various exterior and interior geometries of the workpiece (1) may be shaped by hammering. This may be achieved by using mandrels and by different relative movements of the tools (3) relative to the workpiece (1).
One advantage of hammering is that only intermittent forming takes place at a location (tool engagement) with each tool stroke. As a result, fewer overall stresses result in the component than with extrusion, for example, in which the entire cross section of the workpiece is pressed through a bottleneck. Because of this advantage with hammering, the material may be subjected to greater deformation without the expectation of crack formation or excessive embrittlement.
One preferred use of a method according to the invention is the manufacture of a resorbable or nonresorbable medical implant, for example a dental implant or tooth implant. Alternatively, such a medical implant may be designed in the form of a screw, a plate, a nail, a wire, a foil, or a scaffold, in particular a stent. The improvement in the mechanical material properties, achievable with a method according to the invention, may have beneficial effects in particular for implants.
Since the primary objective of post-processing of the workpiece by hammering within the scope of a method according to the invention is an improvement in the mechanical material properties, it may be provided to use a method according to the invention for producing a blank or a semi-finished product, i.e., a workpiece that is provided for further processing.
In particular, the objective of post-processing of the workpiece by hammering within the scope of a method according to the invention may be to achieve a diameter reduction for the workpiece. On account of the limited length of a workpiece in ECAP due to the stamp stroke of the ECPA device used, the length of blanks or workpieces that can be produced is generally limited, often to less than 500 mm, in particular less than 300 mm. As a result, further machining, for example for manufacturing screws, nails, plates, scaffolds, or stents, on appropriate devices such as lathes, long lathes, laser cutting devices, etc., is usually not cost-effective. In many cases, these devices have automatic conveying units, for example automated spindle bores, through which long rods or tubes are to be continuously supplied to the machining process.
Thus, a method according to the invention allows the manufacture of workpieces by hammering, for example blanks or semi-finished products such as rods, tubes, etc., whose lengths after the hammering may be ≥500 mm, preferably ≥1000 mm, and particularly preferably ≥2000 mm, and which may thus be further processed in a cost-effective manner.
The workpiece, i.e., the semi-finished product or the blank, may then undergo further processing, for example machining, to manufacture a component having a defined final contour, such as a resorbable or nonresorbable medical implant, for example a dental implant or tooth implant. Alternatively, such a medical implant may be designed in the form of a screw, a plate, a nail, a wire, a foil or a scaffold, in particular a stent.
On the other hand, in principle hammering also allows shaping of a machined workpiece, characterized by a great freedom of shape and very good dimensional stability (achievable tolerances of <0.03 mm, for example). Accordingly, it may also be provided to use a method according to the invention for manufacturing a component in such a way that predefined final contours of the component are produced by the hammering. Further post-processing may thus be dispensed with, so that the manufacturing costs for such a component may be kept low.
In one possible embodiment according to the invention, the hammering is carried out by means of a so-called double rotation unit (see
This procedure may be advantageously provided in particular for large-scale production (for example, manufacture of at least 1000 identical components), while production of blanks in small-scale production (manufacture of fewer than 1000 identical components) may be advantageous, since the additional costs incurred for post-processing machining of the blanks, for example, may be lower than the costs of multiple rotary swaging tools with which correspondingly different shapes for the components may be achieved. However, production of blanks may also be advantageously provided for large-scale production.
In one preferred embodiment of a method according to the invention, it may be provided that the metallic material includes titanium (pure titanium (Ti) or a titanium alloy) and/or magnesium (pure magnesium (Mg) or a magnesium alloy), in particular resorbable magnesium. The improvement in the mechanical material properties of the material, achievable by the post-processing according to the invention by hammering, has been achieved at least with pure titanium and with titanium alloys, in particular a preferred titanium alloy which in addition to titanium also includes (at least or solely) zirconium (Zr), preferably a mass fraction of approximately 10% to approximately 20%, in particular approximately 12% to approximately 14%, and in particular approximately 13%.
Titanium and magnesium are also suitable, but in particular titanium, due to its good biocompatibility, is a particularly advantageous material for medical implants that may preferably be manufactured using a method according to the invention.
In principle, the method according to the invention may advantageously be suited for machining workpieces made of metallic materials, particularly preferably using light metals (magnesium (Mg) or aluminum (Al), for example) or the alloys thereof. The method according to the invention is thus also suited, for example, for producing semi-finished products or blanks from resorbable magnesium or magnesium alloys, from which implants may likewise be manufactured, for example by subsequent turning, laser cutting, or other methods.
It may also preferably be provided that the temperature of the workpiece during the machining by ECAP is at least 200° C., at least 350° C., at least 450° C., or at least or approximately 500° C. For a workpiece made of magnesium or a magnesium alloy, in particular a temperature of between 200° C. and 350° C. may be advantageous. In contrast, for a workpiece made of pure titanium or a titanium alloy, a temperature of at least 450° C. and in particular 500° C. may be advantageous. If the temperature of such a workpiece made of titanium is less than 450° C. and in particular less than 500° C. during the machining by ECAP, this may result in pronounced crack formation in the workpiece due to the machining by ECAP.
In another preferred embodiment of a method according to the invention, it may be provided that the machining by ECAP includes at least four, six, or eight machining passes. A machining pass is understood to mean a pressing of the workpiece through an angled transition between two channels. In this regard, it may be provided that the workpiece is pressed by a tool having more than two channels, wherein two adjacent channels in each case form an angled transition, and at least two, preferably all, transitions are oriented differently (see
Within the scope of a method according to the invention, it may also be provided that the workpiece is additionally pressure-formed (in particular extruded) before and/or after the machining by ECAP. Further improvement in the mechanical material properties and/or a reduction in dimensions of the workpiece may be achieved in this way. Such additional pressure forming may be provided in particular before the workpiece is post-processed by hammering.
It may also preferably be provided that the workpiece is additionally heat treated. Further improvement and/or a targeted setting of material properties of the material may likewise be achieved in this way. The heat treatment may be provided before or after the machining by ECAP, and before or after the post-processing by hammering, and before or after optionally provided additional pressure forming. The temperature selected for the heat treatment may be a function of the selected material, and for titanium or a titanium alloy, for example, may be between 480° C. and 780° C., while for magnesium or a magnesium alloy the temperature may be between 120° C. and 580° C. Since the number and duration of such heat treatment steps may vary, cooling in air or by contact with some other medium, for example water, oil, or a gas such as argon, is possible.
The indefinite articles “a” and “an,” in particular in the claims and in the description which provides a general explanation of the claims, are understood as such, and not as numerals. Accordingly, specific components are to be understood in such a way that they may be present at least once, and may be present multiple times.
The improvements regarding certain characteristic values of the material of a machined workpiece that are achievable by use of a method according to the invention are explained below with reference to comparative tests.
An alloy composed of titanium and zirconium in a mass fraction of 13% (Ti-13% Zr) was used as material for the workpieces to be machined within the scope of the comparative tests. In the starting state, the workpieces, made of solid material, had a circular cross section with diameters of 10 mm or 16 mm. Two of these workpieces in the starting state (referred to below as “starting workpieces”) were provided in each case as comparative samples.
For the machining of the workpieces by ECAP, an ECAP tool was used having straight-running circular channels with a circular cross section, the channels having a cross-sectional diameter of 12 mm that was constant over the longitudinal extension, with the channels merging into one another at a (forming) angle of 120°. The workpieces were turned by 90° in each case between individual machining passes during the machining by ECAP.
Due to the 12-mm cross-sectional diameter of the channels of the ECAP tool, those workpieces having a diameter of 16 mm in the starting state were turned to provide a diameter of down to 12 mm prior to the machining by ECAP (referred to below as “solid material workpieces”). In contrast, those workpieces having a diameter of 10 mm in the starting state were enclosed by a tubular sleeve made of pure titanium and having an outer diameter of 12 mm (referred to below as “sleeve workpieces”).
Most of the sleeve workpieces were machined by ECAP at a temperature of 500° C. in two, four, or six machining passes. After four machining passes, cracks formed in the material of individual workpieces, in particular in the material of the tubular sleeve. When these sleeve workpieces were subjected to further machining passes, ends of the sleeve workpieces routinely broke off. One sleeve workpiece was machined by ECAP on a trial basis in four machining passes at a temperature of 450° C. However, even more pronounced crack formation occurred as a result.
A first series of eight solid material workpieces was machined by ECAP in four machining passes at a temperature of 500° C. After the machining by ECAP, these solid material workpieces had much better surface quality than the correspondingly machined sleeve workpieces. Seven of these solid material workpieces machined by ECAP were provided for post-processing by either hammering or rolling, while one of the solid material workpieces was provided as a comparative sample.
Also for one of the solid material workpieces, an attempt was made to reduce the forming temperature to 450° C. Four machining passes were carried out with this solid material workpiece. Here as well, however, severe crack formation and breaking off of the ends occurred.
For this reason, solid material workpieces of a second series were likewise machined by ECAP at a forming temperature of 500° C., but in this case with more machining passes than in the first series. It was shown that more than six machining passes would not be productive, since even with six machining passes, small pieces sometimes break off at the ends of the solid material workpieces, and there is only minor improvement in the mechanical properties. For a solid material workpiece in this second series, the machining by ECAP had to be terminated after five machining passes due to extreme crack formation.
Lastly, five solid material workpieces in the second series were machined by ECAP and provided for further use. Four of them were provided for post-processing, either by hammering or by rolling, while one solid material workpiece was once again used as a comparative sample.
In the post-processing by hammering, the diameter was reduced in multiple steps at room temperature (approximately 21° C.):
In the post-processing by rolling, at room temperature (approximately 21° C.) in a first forming step the diameter was reduced from 12 mm to 8 mm (ϑ=0.81), and in a second forming step was reduced from 8 mm to 6 mm (ϑ=0.58).
The post-processed workpieces and the workpieces provided as comparative samples were subsequently subjected to either one or more hardness tests or a tensile test.
The Vickers hardness (HV) was determined in the hardness test. For this purpose, the workpieces that were provided for the hardness test and embedded and polished for this purpose were each measured with a hardness tester (DuraScan 80 from EMCO-TEST Prüfmaschinen GmbH) with a force of 10 kp (=HV10) according to EN ISO 6507-1. For each workpiece, an average was determined from at least five individual tests (indentations).
The workpieces provided for the tensile tests, which were machined solely by ECAP, were reduced to a diameter of 6 mm by turning, the same as for the starting workpieces. For these workpieces, in addition a parallel measuring length of 30 mm (B6×30 specimens according to DIN 50125) was provided. The workpieces provided for the tensile tests, which were additionally rolled or hammered after the machining by ECAP, already had a reduction to a diameter of only 4 mm due to this post-processing. A parallel measuring length of 20 mm (B4×20 specimens) was provided for these workpieces. The initial strain rate was 3×10−4s−1 for all tensile tests.
The results of the hardness tests are summarized in the following table:
These results show that the hardness of the material increases as the number of ECAP machining passes increases, with the increase being greatest for the first machining passes. For the solid material workpieces, the increase in hardness is more pronounced than for the sleeve workpieces (+33% after four machining passes, compared to +38% after six machining passes).
The following conclusions may be drawn for the workpieces that were additionally rolled or hammered after the machining by ECAP: In the present case, rolling reduces the hardness, while hammering results in a further increase in the hardness. In both cases, however, the hardness distribution is not homogeneous. The workpieces that were additionally rolled are softer in the middle of the cross section, while the workpieces that were additionally hammered are harder in the middle than at the edge (see the table below). Such inhomogeneity was not observed in the workpieces machined solely by ECAP.
To make the hardness distribution apparent, for selected workpieces a location-dependent measurement was additionally carried out with a lower load (HV1). These measurements show slight inhomogeneity for a starting workpiece (16 mm) with slightly increased hardness at the edge, a largely homogeneous hardness distribution of a workpiece machined solely by ECAP (4×ECAP at 500° C.), and in comparison a reduction in hardness in the center of a workpiece post-processed by rolling, and once again an increase in hardness in the center of a workpiece post-processed by hammering in comparison to the workpiece machined solely by ECAP.
The results of the tensile tests are summarized in the following table. The elongation characteristic values are plastic elongations (i.e., without the elastic elongation), and all stresses are engineering stresses (technical stresses).
A comparison of these results shows that trends that are recognizable for the hardness tests also apply to the results of the tensile tests. The material of the workpieces becomes markedly stronger from machining by ECAP (for example, +48% in the tensile strength after four machining passes). Hammering further increases the strength (+30% tensile strength compared to machining solely by ECAP), while rolling decreases the strength. However, the stronger the material of the workpieces, the lower the respective ductility. The elongation at break of workpieces machined solely by ECAP is greater than 10%, while it decreases to approximately 5% due to subsequent hammering. Subsequent rolling results in severe embrittlement.
In summary, the following conclusions may be drawn: The strength and hardness of the starting material (Ti-13% Zr) was increased considerably by machining by ECAP, while maintaining ductility greater than 10%. Upon further machining of the workpieces previously machined by ECAP, it was shown that rolling did not provide good results, since both the strength and the ductility were greatly reduced. The decrease in ductility was expected, since this is a known effect with cold forming, in particular with rolling, and is referred to as strain hardening or cold embrittlement. The reduction in strength due to the rolling was unexpected, and is likely due to excessive stress caused by local increases in strain during deformation of the material, resulting in internal disruption and microcrack formation, for example. In contrast, hammering further increased the strength (yield strength and tensile strength greater than 1300 MPa), and resulted in only a tolerable reduction in the ductility.
One application example of a method according to the invention is the production of a semi-finished product, which is subsequently provided on a long lathe for manufacturing dental implants. For this purpose, a workpiece made of Ti13Zr was machined at 500° C. in four machining passes by ECAP and subsequently machined by hammering, wherein the hammering resulted in a reduction in the diameter of the cylindrical workpiece from 20 mm to 5 mm. The length of the workpiece increased from 150 mm to 2400 mm. The machining according to the invention resulted in an improvement of the tensile strength, from 750 MPa originally to 1300 MPa, and a change in the elongation at break from 25% (as cast and rolled) to 5%.
A further application example of a method according to the invention is the production of a semi-finished product, which is subsequently provided on a long lathe for manufacturing resorbable pins made of magnesium. For this purpose, a workpiece made of ZX00 (magnesium +<1% zinc +<1% calcium) was machined at 250° C. in four machining passes by ECAP and subsequently machined by hammering, wherein the hammering resulted in a reduction in the diameter of the cylindrical workpiece from 20 mm to 4 mm. The length of the workpiece increased from 150 mm to 3750 mm. The machining according to the invention resulted in an improvement of the tensile strength, from 220 MPa originally to 400 MPa, and a change in the elongation at break from 17% (as cast and rolled) to 8%.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2016 122 575.4 | Nov 2016 | DE | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2017/079270 | 11/15/2017 | WO | 00 |
