Method for preparing high-toughness magnesium alloy thin-wall tubule for intravascular stent through torsion die-free drawing

Abstract

The invention discloses a method for preparing a high-toughness magnesium alloy thin-wall tubule for an intravascular stent through torsional die-free drawing. According to the method, in the die-free drawing process, a clamp of a pipe is rotated, twin crystal generation is induced in a die-free drawing and torsion composite rheological mode, grain structure refinement of the magnesium alloy microtubule is achieved, and then the high-toughness magnesium alloy thin-wall tubule for the intravascular stent is obtained. The preparation method comprises the following steps: 1) preparing a magnesium alloy thin-wall small-diameter microtube by utilizing local heating die-free drawing; and (2) rotating the micro-tube chuck in the process of the step (1), inducing twin crystal formation in a stretching and rotating composite rheological manner, refining crystal grains and improving the toughness of the micro-tube. The preparation method is simple in process and low in machining cost, the magnesium alloy thin-wall thin-diameter microtube is high in size precision and wide in size range and has high obdurability, the outer diameter of the magnesium alloy thin-wall thin-diameter microtube ranges from phi 0.9 mm to phi 4 mm, the wall thickness ranges from 0.1 mm to 0.2 mm, the length of a single magnesium alloy thin-wall thin-diameter microtube is larger than or equal to 1000 mm, the ultimate tensile strength ranges from 300 MPa to 450 MPa, and the ductility ranges from 10% to 30%.

Description

Technical Field

[0001] The present invention discloses an innovative method for producing thin-walled, thin-diameter microtubes of high-strength and tough magnesium alloy for vascular stents. The core of the method is to introduce shear-assisted deformation during dieless drawing to induce twinning and refine the grain size, thereby significantly improving the strength and toughness of the microtubes. Background Art

[0002] With the development of medical care, interventional stent implantation has shown broad application prospects in the treatment of various diseases. Traditional non-degradable stents exist for a long time after implantation, leading to the formation of various complications after surgery. Using degradable stents instead of non-degradable stents can effectively avoid the occurrence of postoperative complications and realize the vision of intervention without implantation.

[0003] Among many degradable materials, magnesium alloy has good biocompatibility and excellent biodegradability, and the degradation products are beneficial to inhibit inflammatory response, showing good application prospects in degradable vascular stents. However, magnesium alloys are not strong enough, which limits the processing of magnesium alloy thin-walled and fine-diameter microtubes, resulting in poor radial support capacity of the stent after implantation. Therefore, improving the strength and toughness of magnesium alloy thin-walled and fine-diameter microtubes is the primary issue for the current market application of degradable magnesium alloy vascular stents.

[0004] The reason why magnesium alloys have poor strength and toughness is that during the plastic deformation process at room temperature, it is difficult to activate the non-basal slip system and it is easy to form {102} twins to reduce the yield strength. In order to obtain high-strength and high-toughness comprehensive mechanical properties, large plastic deformation is generally used to prepare ultrafine grained structures of magnesium alloys. According to the Hall-Petch formula, the finer the grains of metal polycrystalline materials, the higher the strength of the material. Its essence is to increase the volume fraction of grain boundaries in the material and increase the resistance to dislocation slip, which is called "fine grain strengthening."

[0005] However, magnesium alloys have low stacking fault energy, which leads to difficulty in sliding and climbing, slow dynamic recovery and high grain boundary diffusion. The degree of grain refinement that can be achieved through large plastic deformation technology is limited by its stable state, usually to the micron level. At the same time, the large plastic deformation process is complex and the size of the prepared sample is small, which cannot meet the requirements of large-scale production. This situation further limits the practical application of ultrafine-grained magnesium alloy thin-walled tubes.

[0006] Studies have shown that the twin structure produces geometrically required dislocations by coordinating the interface strain gradient, causing additional strain hardening, which can greatly improve the strength and toughness of magnesium alloys. In addition, the twin boundary is a low-energy interface with better thermal stability, so it is expected to form a nano-twin structure during the thermomechanical process and further improve the strength and toughness of magnesium alloys. In response to the difficulties in the strength and toughness of magnesium alloy thin-walled, fine-diameter microtubes, the present invention aims to introduce shear-assisted deformation in the processing of magnesium alloy thin-walled, fine-diameter microtubes by torsional dieless drawing, promote the formation of multiple twins in the magnesium alloy thin-walled microtubes, and then refine the grains, so as to achieve the goal of improving the strength and toughness of thin-walled microtubes for vascular stents. Summary of the invention

[0007] The purpose of the present invention is to provide a method suitable for processing high-strength and toughness magnesium alloy thin-walled and fine-diameter microtubes for vascular stents. The main method includes adopting shear-assisted dieless drawing technology to prepare high-strength and toughness magnesium alloy thin-walled and fine-diameter microtubes for vascular stents, thereby overcoming the shortcomings of insufficient strength and toughness of thin-walled and fine-diameter microtubes prepared by existing large plastic deformation technology, thereby promoting the clinical application of degradable magnesium alloy thin-walled and fine-diameter stents.

[0008] To achieve the above purpose, the scheme of the present invention is: to adopt local heating dieless drawing to efficiently prepare thin-diameter microtubes, to apply shear-assisted deformation to the magnesium alloy thin tubes by twisting the chuck, to induce the formation of multiple twins to improve the strength and toughness of magnesium alloy thin-walled thin-diameter microtubes.

[0009] The above-mentioned local heating dieless drawing includes a dieless drawing machine system, an online heating device and a cooling system. The shear-assisted deformation is mainly achieved by applying a rotational load to the fixture of the dieless drawing system, applying different degrees of shear strain to the thin-diameter microtubes of different sizes, thereby obtaining the corresponding high-strength and toughness thin-diameter thin-walled microtubes.

[0010] The shear-assisted dieless drawing system mentioned in the present invention is independently created, and is characterized in that the dieless drawing machine speed control system and the clamp torsion system are both precisely controlled by servo motors, and the drawing speed can be precisely controlled within the range of 0.1mm / min-5mm / min, and different shear strains can be precisely controlled within the range of 0.10rpm-10rpm.

[0011] In order to promote the formation of multiple twins, the present invention uses local heating to induce multiple twins in the magnesium alloy. The dislocation-like structure is activated, thereby inducing the formation of twins according to the high symmetry of the lattice. In addition, the introduction of the local heating system can also improve the stable rheological ability of the magnesium alloy, thereby improving the processing efficiency. The characteristic is that the heating form is online heating, the heating method includes thermocouple heating or electromagnetic induction heating, the heating power is 2 to 10 kW, and the mold temperature can be accurately controlled within the range of 100 to 600 ° C.

[0012] To achieve the purpose of the present invention, the method for preparing high-strength and tough magnesium alloy thin-wall microtubes by shear-assisted dieless drawing mainly includes the following steps:

[0013] 1) Preparation of magnesium alloy thin-walled and small-diameter microtubes using a local heating dieless drawing system;

[0014] 2) During the dieless drawing process described in step 1), a rotational load is applied to the chuck to introduce shear-assisted deformation to induce the formation of multiple twins, so that the prepared thin-walled, small-diameter microtubes have high strength and toughness.

[0015] Furthermore, the tube blank used for dieless drawing in the above step 1) is obtained by extrusion, and the wall thickness ranges from 0.2 mm to 0.5 mm.

[0016] Furthermore, the high-strength and toughness magnesium alloy thin-walled micro-tubes prepared in the above steps have an outer diameter of Φ0.9-Φ4 mm, a wall thickness of 0.1-0.2 mm, and a single length of not less than 1000 mm.

[0017] Furthermore, the ultimate tensile strength of the high-strength and tough magnesium alloy thin-walled microtube prepared in the above steps is 200-450MPa, and the elongation is 10-30%.

[0018] The beneficial effects of the present invention are: the formation of twins is greatly promoted through shear-assisted deformation, and the formation of twins improves the toughness of magnesium alloy thin-wall microtubes mainly through the following aspects: 1) The twin structure produces geometrically necessary dislocations by coordinating the interface strain gradient, causing additional strain hardening. 2) As the thickness of the twin layer decreases, the external stress required for dislocations to pass through the twin boundary increases, and at the nanoscale, the interaction between twins and dislocations will become the main mechanism for material strengthening.

[0019] Furthermore, compared with fine grain strengthening, twin strengthening can effectively improve the toughness of magnesium alloy microtubes, mainly because: 1) The thickness and strength of the twinned lamellae also follow the Hall-Petch relationship, and twin strengthening will not significantly reduce plasticity, which can achieve the purpose of "strong plasticity coordination". 2) The twin boundary is a low-energy interface with better thermal stability than the grain boundary, so it is expected to form a nano-twin structure during the thermomechanical process. In addition, magnesium alloys have low stacking fault energy, and full dislocations are easily decomposed into partial dislocations and stacking faults are easily formed, resulting in the easy formation of twin structures during the plastic deformation of magnesium alloys.

[0020] The use of shear-assisted dieless drawing to prepare high-strength and tough magnesium alloy thin-walled microtubes effectively breaks through the limitations of traditional processes such as long mandrel cold drawing, and can efficiently obtain microtubes of different sizes. Furthermore, the introduction of shear-assisted deformation promotes the formation of multiple twins, which can obtain high-strength and tough magnesium alloy microtubes.

[0021] The present invention provides an innovative, efficient and economical preparation method specifically for manufacturing magnesium alloy thin-walled, fine-diameter microtubes required for high-strength and toughness vascular stents. This method cleverly combines advanced process technology with material science principles, which not only greatly simplifies the cumbersome steps in the traditional preparation process, making the production process smoother and more efficient, but also significantly reduces processing costs, making large-scale production possible. More importantly, through unique process control, this method successfully improves the strength and toughness of magnesium alloy thin-walled, fine-diameter microtubes, so that while meeting the strict material requirements of vascular stents, it also has more outstanding mechanical properties. This groundbreaking progress has undoubtedly opened up a new path for the research and development and application of vascular stent materials, and is expected to lead the innovation and development of the medical device field. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 Schematic diagram of torsional dieless drawing according to the present invention;

[0023] Figure 2 Schematic diagram of the dieless drawing machine of the present invention;

[0024] Figure 3 The dieless drawing machine of the present invention;

[0025] Figure 4 Example 1 of the present invention prepares magnesium alloy thin-wall microtubes for vascular stents at different torsion speeds;

[0026] Figure 5 Example 1 of the present invention: magnesium alloy thin-wall microtubes for vascular stents prepared by torsion-dieless drawing;

[0027] Figure 6 The microstructure of magnesium alloy thin-wall microtubes for vascular stents prepared at different torsion speeds in Example 1 of the present invention;

[0028] Figure 7 Multiple twins prepared in Example 1 of the present invention;

[0029] Figure 8 The magnesium alloy ultrafine tube for the lacrimal duct stent prepared in Example 2 of the present invention;

[0030] Fig. 9 Engineering stress-strain curve of the magnesium alloy ultrafine tube for the lacrimal duct stent prepared in Example 2 of the present invention. DETAILED DESCRIPTION

[0031] In order to enable those skilled in the art to better understand the specific embodiments of the present invention, the following is a detailed description with reference to the accompanying drawings. The description in this section is only exemplary and explanatory and should not have any limiting effect on the scope of protection of the present invention.

[0032] like Figure 1 As shown, the specific structure of the present invention is: a method for preparing high-strength and tough magnesium alloy thin-walled tubes for vascular stents by torsional dieless drawing, wherein the torsional dieless drawing includes a dieless drawing machine system, an online heating system, a cooling system and a torsional device.

[0033] Preferably, the moldless drawing deformation mentioned in the present invention is controlled by the forward speed of the two clamps, which is characterized in that the drawing speed can be accurately controlled within the range of 0.1mm / min-5mm / min.

[0034] Preferably, the torsion system of the dieless drawing machine is precisely controlled by a servo motor, and can precisely control different shear strain amounts within the range of 0.10rpm-10rpm.

[0035] Preferably, the local heating system is heated in an online manner by thermocouple heating or electromagnetic induction heating, with a heating power of 2 to 10 kW, and can accurately control the temperature of the magnesium alloy thin-walled tube within a range of 100 to 600°C.

[0036] When used specifically, it is divided into the following embodiments:

[0037] Example 1

[0038] A method for preparing a vascular stent ZN20 alloy thin-walled tube by torsion dieless drawing comprises the following steps:

[0039] Step 1, selecting a ZN20 alloy tube blank prepared by hot extrusion, the extrusion temperature is 360°C, the extrusion ratio is 105, the tube blank wall thickness is 0.3mm, and the length is 500mm;

[0040] Step 2: Assemble the tube blank described in step 1 on a dieless drawing machine, and clamp the two ends of the tube blank respectively with two clamps.

[0041] Step 3, set the torsional dieless drawing parameters, the torsional angular velocity of fixture 1 is selected to be 0.5rpm, 1rpm, 2rpm, 4rpm, and 8rpm, and the torsional angular velocity of fixture 2 is 0rpm; the forward speed of fixture 1 is 0mm / s, the forward speed of fixture 2 is 1mm / s, and the single-pass cross-sectional reduction is 50%.

[0042] Step 4: The online heating system uses electromagnetic induction coil heating, sets the local heating temperature to 400°C, starts the cooling device, and starts the torsional dieless drawing process. The prepared magnesium alloy thin-walled tube is as follows: Figure 4 , Figure 5 shown.

[0043] Step 5: Microstructure observation was performed at 0 rpm, 0.5 rpm, 1 rpm, 2 rpm, 4 rpm, and 8 rpm. It can be seen that with the increase of the torsion speed, the number of twins in the magnesium alloy thin-walled tube increased significantly, and the sample achieved twin grain refinement, such as Figure 6 As shown, multiple twins are observed inside the grains, e.g. Figure 7 shown.

[0044] Example 2

[0045] A method for preparing a ZE21B alloy ultrafine tube for a tear duct stent by torsion dieless drawing comprises the following steps:

[0046] Step 1: Select a ZE21B alloy tube blank prepared by hot extrusion, the extrusion temperature is 400°C, the extrusion ratio is 150, the tube blank has a wall thickness of 0.25 mm, a length of 400 mm, and an outer diameter of Φ1.5 mm.

[0047] Step 2: Assemble the tube blank described in step 1 on the dieless drawing machine, clamp the two ends of the tube blank with two clamps respectively, set the torsion dieless drawing parameters, select the torsion angular velocity of clamp 1 to be 1rpm, and the torsion angular velocity of clamp 2 to be 0rpm; the forward speed of clamp 1 is 0mm / s, and the forward speed of clamp 2 is 0.5mm / s.

[0048] Step 3: The online heating system uses electromagnetic induction coil heating, sets the local heating temperature to 400°C, starts the cooling device, and starts the torsional dieless drawing process. The prepared magnesium alloy thin-walled tube is as follows: Figure 8 shown.

[0049] Step 4: The magnesium alloy ultrafine tube prepared in step 3 and the ultrafine tube prepared by dieless drawing without twisting are made into standard tensile specimens, and a tensile test is performed at room temperature at a tensile speed of 0.5 mm / min. The tensile test results are as follows: Fig. 9 It can be seen that the ZE21B alloy ultrafine tube prepared by the method of the present invention has good strength and toughness. Compared with the ultrafine tube without torsion-assisted dieless drawing, the yield strength, tensile strength and elongation after fracture of the magnesium alloy ultrafine tube prepared by the method are significantly improved, achieving simultaneous improvement of strength and toughness.

[0050] This article uses specific examples to illustrate the principles and implementation methods of the present invention. The above examples are only used to help understand the method and core ideas of the present invention. It should be pointed out that due to the limitations of textual expression and the objective existence of infinite specific structures, ordinary technicians in this technical field can make several improvements, modifications or changes without departing from the principles of the present invention, and can also combine the above technical features in an appropriate manner; these improvements, modifications, changes or combinations, or the direct application of the inventive concept and technical solution to other occasions without improvement, should be regarded as the protection scope of the present invention.

Claims

1. A method for preparing a magnesium alloy thin-walled, thin-diameter microtube for a high-strength and tough vascular stent by torsional dieless drawing, characterized in that In the process of local heating dieless drawing, by providing torsion-stretching conforming to rheological motion to magnesium alloy thin-walled microtubes, multiple twins are induced to form inside the grains, and the strength and toughness are improved by refining the microstructure through multiple twins. The dieless drawing includes a dieless drawing machine system, an online heating device and a cooling system. The torsion-assisted deformation is mainly achieved by applying a rotational load to the fixture of the dieless drawing system, and applying different degrees of shear strain according to different sizes of thin-diameter microtubes, thereby obtaining corresponding high-strength and tough thin-diameter thin-walled microtubes. The method for processing high-strength and toughness magnesium alloy thin-walled and small-diameter micro-tubes comprises the following steps: 1) Preparation of magnesium alloy thin-walled and small-diameter microtubes using a local heating dieless drawing system; 2) During the dieless drawing process described in step 1), a rotational load is applied to the chuck to introduce shear-assisted deformation to induce the formation of multiple twins, so that the prepared thin-walled, small-diameter microtubes have high strength and toughness.

2. The dieless drawing system according to claim 1, characterized in that The speed control system of the dieless drawing machine is precisely controlled by a servo motor, and can precisely control the drawing speed within the range of 0.1 mm / s-25 mm / s.

3. The local heating dieless drawing system according to claim 1, characterized in that The heating form is online heating, and the heating methods include thermocouple heating or electromagnetic induction heating. The heating power is 2 to 10 kW, and the mold temperature can be accurately controlled within the range of 100 to 600°C.

4. The shear-assisted dieless drawing system according to claim 1, characterized in that A rotational load is applied to the chuck, and the rotational load is precisely controlled by a servo motor, and different shear strain amounts can be precisely controlled within a range of 0.10rpm-10rpm.

5. The method according to claim 1, characterized in that The outer diameter of the prepared magnesium alloy thin-walled micro-tube is Φ0.9-Φ4mm, the wall thickness is 0.1-0.2mm, and the length of a single tube is not less than 1000mm.

6. The method according to claim 1 or 2, characterized in that The ultimate tensile strength of the prepared magnesium alloy thin-walled and small-diameter microtube is 200-450MPa, and the elongation is 10-30%.

7. The method according to claim 1, characterized in that The method has simple process and low processing cost, and the prepared magnesium alloy thin-walled and fine-diameter microtubes have high dimensional accuracy and a wide size range.

8. The high-strength and tough magnesium alloy thin-walled and small-diameter microtubes prepared by the method according to claim 1 are particularly suitable for use as degradable magnesium alloy vascular stent materials in various size ranges.

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