Method for manufacturing magnesium alloy bone screw through ultrafast laser

Abstract

The invention discloses a method for manufacturing a magnesium alloy bone screw through ultrafast laser, and belongs to the technical field of laser precision machining. Based on layered laser milling, the dynamic compensation adjustment method for the laser scanning paths of the different milling layers is innovatively provided, a mathematical model is established by adjusting the inclination angles of the scanning paths of the different milling layers and parallelogram filling line segment distribution, cooperative control over the width of a laser ablation area and the depth of a thread tooth is achieved, and the machining precision is improved. High-precision machining of thread teeth in different shapes can be achieved, and controllable and uniform micron-sized texture features are generated on the surfaces of the thread teeth. And meanwhile, the machining precision and the process stability are remarkably improved, and the method is not only suitable for medical instruments such as magnesium alloy bone screws, but also can be popularized to precise manufacturing of continuous rotary body structures of other materials.

Description

Technical Field

[0001] The present invention belongs to the field of laser processing technology, and specifically relates to a method for high-precision three-dimensional laser manufacturing of medical equipment with rotating structures such as magnesium alloy bone screws. Background Art

[0002] Three-dimensional bodies of revolution are a key design feature in medical device manufacturing, and their machining accuracy directly impacts product performance and quality. The three-dimensional geometry of the threaded portion of medical devices, such as bone screws, places high demands on machining processes. Traditional machining methods, such as turning and casting, present challenges such as burrs, cracks, and stress concentration, and struggle to achieve high-precision machining of complex three-dimensional shapes. Magnesium alloy bone screws, in particular, have become a research hotspot in the field of orthopedic implant materials due to their excellent biocompatibility, mechanical properties, and biodegradability. However, their manufacturing process remains limited by the machining difficulties of bodies of revolution, such as the high-precision formation of complex thread profiles, the precise control and uniformity of surface microfeatures, and the optimization of overall mechanical properties. These technical challenges not only limit product processing efficiency and quality consistency but also place greater demands on their future clinical applications. Ultrafast laser processing technologies, such as picosecond lasers, leverage their ultrashort pulse characteristics to achieve low heat-affected zones, high-precision machining, and high-quality surfaces, making them a key area of ​​research in micro- and nano-manufacturing. However, existing laser technology still faces numerous challenges in processing continuously rotating three-dimensional geometric shapes. First, the forming efficiency of large-format three-dimensional spiral structures is low, making it difficult to meet the needs of large-scale production. Second, processing strategies, precision, and process stability need to be further improved. This is especially true for heat-sensitive materials such as magnesium alloys, and the processing technology for complex rotating structures such as bone screws still lacks a systematic optimization solution. These issues not only restrict processing efficiency and quality, but also limit the widespread application of ultrafast laser technology in medical device manufacturing. Summary of the Invention

[0003] To address these issues, the present invention provides a method for ultrafast laser manufacturing of magnesium alloy bone screws. This method utilizes ultrafast (picosecond) laser processing technology to achieve high-efficiency, high-precision, and high-quality manufacturing of magnesium alloy bone screws with a body of revolution, while also meeting the requirements of complex geometric structures. The core of this invention lies in a method for compensating for the accuracy of a deep layered laser scanning path used in picosecond laser processing. This method includes a coordinated control method for the laser ablation zone width (pitch) and thread depth, as well as a method for dynamic compensation adjustment of the laser scanning path.

[0004] The method for coordinated control of the width (pitch) of the laser ablation zone and the depth of the thread tooth described in the above method is characterized in that the picosecond laser milling removal area is analogized to an inverted asymmetric (unequal) trapezoid. In the picosecond laser milling process of processing the original magnesium alloy round bar blank into a bone screw, the processing is divided into multiple milling layers based on the single optimal removal depth (h) when the picosecond laser interacts with the magnesium alloy. The total number of layers is n, and the milling is not performed in one go. Different thread tooth depths have different widths of the laser ablation zone, resulting in different thread tooth thicknesses. The present invention is particularly suitable for the forming of asymmetric (unequal trapezoidal) thread tooth structures. The width of the laser ablation zone varies linearly with the thread depth. The specific mathematical model is defined as:

[0005]

[0006] Among them, ω k is the width of the laser ablation zone at the milling layer k, ω1 is the width of the laser ablation zone of the first layer, ω n is the width of the laser ablation area of ​​the nth layer, that is, the last layer of the milling layer, where 0≤k≤n, k is an integer, 0≤ω1≤1.75mm, 0≤ω n ≤1.75mm; the geometric shape of the thread teeth is based on ω1 and ω n The value of ω is used to adjust the geometry of the thread teeth. n =ω1, the thread tooth type is a rectangular thread in the transmission thread type. When ω n＜ When ω1, the thread tooth profile is a trapezoidal thread or a serrated thread among the dynamic thread types, and a common thread among the connecting thread types, including but not limited to an irregular trapezoid and a triangle; the mechanical properties of the thread teeth, including tensile strength and torsional strength, are optimized through the tooth profile.

[0007] The laser scanning path dynamic compensation adjustment method described in the above method is characterized in that the laser scanning processing pattern is set to a parallelogram, but the parallelogram has no outer contour; the interior of the parallelogram is filled with parallel line segments of equal size and spacing to serve as the laser scanning path, the length of a single laser scanning path is defined as l, 0<l≤100.0mm, and the laser is unidirectional scanning; each line segment (scanning path) can constitute the hypotenuse of a right triangle, the adjacent angle of the line segment is defined as α, and the α angle is equal to the acute angle inside the parallelogram; the adjacent side of the α angle is the base of the right triangle, that is, the width of a single laser scanning path, which is also the ablation width of a single path in the axial direction of the bone screw, and is also the thread tooth pitch of the screw; the total length of the entire laser scanning area (the length of the parallelogram), that is, the total length of the projection on the original round rod blank before screw processing is the total thread length, is defined as L, and the laser scanning path generation rule is described as follows:

[0008] On the basis of depth stratification, the thread tooth profile size is controlled by dynamically adjusting the parallelogram inclination angle α in each milling layer during processing. The relationship between the laser scanning path length l, the laser ablation width ω and the scanning path inclination angle α is set as: In order to achieve the ablation width ω k From wide to narrow, the angle α k It needs to be adjusted from small to large, and the mathematical model of the laser ablation zone width and the inclination angle of the laser scanning path is defined as:

[0009]

[0010] Among them, α (k) is the inclination angle of the laser scanning path at the milling layer k; the milling layer k of the laser is related to the inclination angle α k The established mathematical model is:

[0011]

[0012] Among them, α k is the inclination angle of the laser scanning path at depth z.

[0013] The milling layer laser path used in the method of the present invention is established by AutoCAD software and stored in the picosecond laser control system in dxf format; when performing picosecond laser processing, the original magnesium alloy round rod is clamped on a rotating motor, and during the processing, the magnesium alloy round rod rotates at a constant speed. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 Figure 1 shows the laser-formed magnesium alloy bone screw, where 1 is the original round bar blank, 2 is the laser-formed bone screw, 3 is the area removed by the laser in the original round bar blank, and 4 is the area not removed from the original round bar blank. Figure 2 Schematic diagram of the strategy for dynamic compensation adjustment of the laser scanning path, where 5 is the length of the entire laser scanning processing area (total thread length L), 6 is the triangular relationship constructed by a single laser scanning path, 7 is the length of a single laser scanning path, 8 is the width of the ablation zone of a single laser path, 9 is the scanning direction of the laser, 10 is the spacing of the laser scanning paths, 11 is the tilt angle α of the laser scanning path, and 12 is the overall height of the parallelogram of the laser scanning processing area; Figure 3 13 is a schematic diagram of laser-formed bone screw processing, wherein 13 is the overall processing of the laser-formed bone screw, 14 is an enlarged diagram of the layered milling of the laser-formed bone screw, and 15 is a diagram of the laser scanning path of different milling layers during laser processing; Figure 4 This is a photo of the magnesium alloy bone screw formed by laser milling. Figure 5 Schematic diagram of an embodiment. DETAILED DESCRIPTION

[0015] To make the objectives, technical solutions, and advantages of the embodiments of the present invention more clear, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts shall fall within the scope of protection of the present invention.

[0016] The mathematical model provided by the present invention is proposed under the condition that the processed substrate is at a fixed rotation speed. When processing a rotary structure with a round rod-like substrate, the difference between the theoretical design length and the actual forming length must be fully considered. Since the rotation of the substrate will cause the laser scanning path to extend along the spiral line, different rotation speeds will lead to changes in the actual forming length. To this end, all the process parameters used in the examples of the present invention are derived by introducing the geometric characteristics of the thread spiral line to derive the relationship between the laser scanning speed and the rotation speed and the standard thread pitch, thereby effectively solving the problem of the difference between the theoretical design length and the actual forming length, and ensuring that the processing accuracy meets the requirements. This method is particularly suitable for the precision manufacturing of magnesium alloy bone screws based on picosecond lasers, and provides a new theoretical basis and technical support for the processing of similar rotary structures; the specific implementation steps are:

[0017] Step 1: Determine the initial parameters. The bone screw size selected in this example is the HA3.5 type, which is specified in the "Medical Industry Standard of the People's Republic of China (YY 0018-2016)" for metal bone screws, including cortical bone HA-type bone screws in bone-bonding implants. The figure below shows the dimensional parameters of the HA3.5 type bone screw. The thread base radius is 1.20 mm, the tooth tip radius is 1.75 mm, the thread pitch is 1.25 mm, and the tooth depth is 550 μm. The specific parameters and dimensions are shown in the figure below.

[0018] In this example, based on data from actual experiments, under certain picosecond laser power and scanning speed, the optimal milling depth for a single layer of ZK60 magnesium alloy round bar with a diameter of 3.5 mm, with no molten residue and minimal roughness, is preferably 110 μm. That is, a total number of layers n = 5 of magnesium alloy round bar with a radius of 1.75 mm can achieve a tooth depth of 550 μm for HA3.5 type bone screws. The specific layering diagram is shown in the figure below. Figure 2 As shown; in this example, the thread length is set to L = 21.0 mm, that is, the long side length of the overall parallelogram in the processing path is 21.0 mm. Preferably, the height of the parallelogram is set to 1.0 mm.

[0019] Step 2: Based on the design parameters of the HA3.5 bone screw described above, according to the formula The width of the picosecond laser ablation zone was calculated. Since the magnesium alloy round rod substrate was always in a constant speed rotation state during processing, the theoretically calculated ablation zone width was smaller than the design value. At the same time, combined with the constant speed rotation of the substrate, the actual design value could be achieved. The specific calculated values ​​were: the ablation width of the first layer was 1.07 mm, the ablation width of the second layer was 0.99 mm, the ablation width of the third layer was 0.90 mm, the ablation width of the fourth layer was 0.82 mm, and the ablation width of the fifth layer was 0.74 mm.

[0020] Step 3: Based on the design parameters of the HA3.5 bone screw described above and the total number of layers obtained in the first step, calculate the inclination angle α of the picosecond laser scanning path in different milling layers. The specific scanning path diagram is shown in the figure below. Figure 3 As shown; according to the formula The calculated inclination angle α1 of the first layer is 21.87°, the inclination angle α2 of the second layer is 31.08°, the inclination angle α3 of the third layer is 38.48°, the inclination angle α4 of the fourth layer is 45.21°, and the inclination angle α5 of the fifth layer is 51.68°. k The width of the picosecond laser ablation gradually decreases as it increases layer by layer, meeting the needs of irregular thread shape.

[0021] The fourth step is to set the parameters of the picosecond laser and the fixture. In this example, the picosecond laser power P = 22W, the scanning speed V = 10.00mm / s, the scanning path spacing is 15μm, the laser pulse repetition frequency is 500kHz, and the speed of the magnesium alloy round bar when clamping is 55rpm. The specific milling example is as follows: Figure 4 shown.

Claims

1. A method for manufacturing magnesium alloy bone screws using ultrafast laser, characterized in that: include: The laser milling forming process of magnesium alloy bone screws with a body of revolution structure is divided into n layers, where n is a positive integer. A dynamic adjustment strategy for the laser scanning path of each milling layer is established to compensate for the laser scanning path of each milling layer. The dynamic adjustment strategy of establishing the laser scanning path of each milling layer and compensating the laser scanning path of each milling layer specifically includes: setting the scanning processing range of each layer of the laser to a parallelogram filled with multiple parallel line segments, the multiple parallel line segments are the scanning path of the laser, and the scanning direction of the laser is a single direction scanning; each of the multiple parallel line segments constitutes the hypotenuse of a right triangle, and in the right triangle, the acute angle below the line segment is defined as α, 0<α≤90°, that is, the inclination angle of the laser scanning path, which is also the internal angle of the parallelogram, the adjacent side of the angle α is the base of the right triangle, and the width is defined as ω, 0≤ω≤1.75mm, which is the width of a single laser scanning path, which is also the laser ablation width of a single path in the axial direction of the bone screw, and also the thread tooth spacing of the screw; the original round rod blank of the screw is in a rotating state during processing, so the scanning widths controlled by the multiple scanning paths are not affected by each other; According to the regular changes of the rotating structure during the laser ablation process, the width of the laser ablation zone ω is established. k The mathematical model of the relationship between the number of thread tooth cutting layers k, where 0≤k≤n, k is an integer, calculates the change of the laser ablation zone width in each layer with depth. The specific mathematical model is: Where ω1 is the width of the first laser ablation area, 0≤ω1≤1.75mm, ω n is the width of the laser ablation area of ​​the nth layer, 0≤ω n ≤1.75mm; The original round bar blank is placed horizontally and clamped on a rotating fixture. The laser is projected vertically for processing, and the laser focus is located on the upper generatrix of the original round bar blank. The width of the overall laser scanning path area, that is, the height of the parallelogram, ranges from 0 to 5.0 mm. The length of the overall laser scanning area, that is, the length of the parallelogram, is also the total length of the thread and the total length of the processing projection on the original round bar blank, defined as L, 0 < L ≤ 100.0 mm. The length of a single laser scanning path is defined as l, 0 < l ≤ 100.0 mm. At different milling layers, the inclination angle of the laser scanning path when the milling layer is k is defined as α (k) , 0<α (k) ≤90°; According to the trigonometric function relationship in the triangle, the laser scanning path length l, the laser scanning path inclination angle α (k) Control the width of the laser ablation zone ω k , that is, the thread tooth spacing of the screw. The thread tooth spacing and shape are precisely controlled through a mathematical model. The specific mathematical model is: According to the laser ablation width ω under the milling layer k of laser ablation k With the inclination angle α (k) The corresponding relationship is used to obtain the inclination angle α of the final laser scanning path. (k) Mathematical model with different milling layers: The length of l is inconsistent in different milling layers, but consistent in the same milling layer. (k) The angles are inconsistent in different milling layers, but consistent in the same milling layer, and the laser ablation width ω k The width is inconsistent in different milling layers, but consistent in the same milling layer, so the thread tooth geometric parameters, including thread tooth spacing and tooth shape, are inconsistent; ω1 and ω n There are two relationships. When ω n =ω1, the thread tooth type is a rectangular thread in the transmission thread type. When ω n＜ When ω1, the thread tooth profile is a trapezoidal thread, a serrated thread in the dynamic thread type, and a common thread in the connecting thread type, and the tooth profile is an unequal-segmented trapezoid, an isosceles triangle, or an equilateral triangle.

2. The method for manufacturing magnesium alloy bone screws using ultrafast laser according to claim 1, characterized in that: The interior of the parallelogram is filled with multiple parallel and equally spaced line segments, and the parallelogram has no external contour line; the spacing range of the filled line segments is less than 30μm, and must be less than or equal to the diameter of the laser spot used; the width of a single texture is controlled by the equal-size spacing of the above-mentioned line segment groups, that is, the spacing of the laser scanning path, and the texture width is in the range of 15 to 100μm.

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