Super-elastic flexible structure, assembly tooling and method of making a super-elastic flexible structure

Abstract

This invention provides a hyperelastic flexible structure, forming tooling, and a method for fabricating the hyperelastic flexible structure. The hyperelastic flexible structure includes a first helical wire, a second helical wire, and welded mass microspheres. The length direction of the first helical wire is orthogonal to the length direction of the second helical wire. The welded mass microspheres are positioned at the intersection of the first and second helical wires to connect them. This invention connects the orthogonally arranged first and second helical wires with welded mass microspheres to form a hyperelastic flexible structure. Compared to existing hyperelastic structural materials, it possesses excellent multifunctional mechanical properties such as large deformation recovery elastic deformation and zero Poisson's ratio. It can be used as a skeletal reinforcement for novel multiphase composite hyperelastic structural materials or as a standalone component.

Description

Technical Field

[0001] This invention relates to the field of aerospace manufacturing technology, and more specifically, to a hyperelastic flexible structure, forming tooling, and a method for preparing the hyperelastic flexible structure. Background Technology

[0002] Based on the requirements and configuration characteristics of the variants, the variant structure can be decomposed into components such as flexible skin, deformable support frame, and actuation / drive devices. Existing approaches to achieving flexible skin mainly fall into three categories: ① flexible deformation based on the elastic deformation of the material itself; ② flexible deformation based on the deformation of the mechanism; and ③ flexible deformation based on the deformation of the structure. Among these, the basic principle of flexible deformation based on structural deformation is to use a specially shaped structure to amplify the small strain deformation of the parent material to obtain greater in-plane deformation. This approach has advantages such as strong structural designability and is currently a hot topic in flexible skin structure research.

[0003] In achieving flexible deformation based on structure, the main metal structural forms used include corrugated structures and honeycomb structures. A schematic diagram of corrugated structure A is shown below. Figure 1 As shown, each corrugation expands or contracts under the force along its direction, and the cumulative deformation of multiple corrugations results in a deformation much greater than that of a flat plate. Honeycomb structures can achieve large in-plane deformations through the bending deformation of the honeycomb walls. To eliminate the Poisson effect during the deformation process of the honeycomb core, Olympio et al. designed a honeycomb with zero Poisson's ratio, such as... Figure 2 As shown, Figure 2 The first honeycomb structure B shown in (a) is a mixed structure with positive and negative Poisson's ratios. Figure 2 The second honeycomb structure C shown in (b) is an "accordion" structure.

[0004] Currently, the main problems encountered in the use of hyperelastic metal structures for flexible skins are as follows:

[0005] (1) The existing structure has significant anisotropy, resulting in good deformation capacity in only one direction, while the deformation capacity perpendicular to that direction is insufficient. For example, the corrugated structure deforms well along the corrugation direction, but cannot deform perpendicular to the corrugation direction.

[0006] (2) When the existing structural deformation is insufficient or large deformation occurs, the change in the thickness dimension is large. For example, when a honeycomb structure deforms, the deformation is usually small due to the current structural geometry, generally less than 50%. For corrugated structures, the corrugation thickness decreases during deformation, and large deformation leads to a significant change in the thickness dimension of the structure, affecting the overall geometric shape of the structure.

[0007] (3) Due to the airtightness requirements, it is necessary to fill the superelastic metal structure with elastomers such as rubber. In the existing corrugated or honeycomb structures, the adjacent void filling areas are separated by corrugated walls or honeycomb core walls, and the adjacent void filling areas are not connected, resulting in discontinuous rubber filling in the structure. As a result, the load transfer of the rubber filling material is discontinuous during deformation, and the overall structural deformation is not coordinated. Summary of the Invention

[0008] (a) Technical problems to be solved

[0009] The technical problem to be solved by this invention is that current flexible skin superelastic metal structures have shortcomings such as anisotropy, insufficient structural deformation, large changes in thickness dimension during large deformation, and discontinuity after filling with elastic material.

[0010] (II) Technical Solution

[0011] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0012] In a first aspect, the present invention provides a superelastic flexible structure, comprising a first helical wire, a second helical wire, and a welded mass microsphere, wherein the length direction of the first helical wire is orthogonal to the length direction of the second helical wire; the welded mass microsphere is disposed at the intersection of the first helical wire and the second helical wire, so that the first helical wire and the second helical wire are connected.

[0013] Secondly, the present invention provides an assembly fixture for assembling the superelastic flexible structure, including a stretching component, a laying component, and a welding component; the stretching component is used to stretch metal wires to form a first spiral wire and a second spiral wire respectively; the laying component is used to pre-tighten and fix the first spiral wire and the second spiral wire, such that the first spiral wire and the second spiral wire are orthogonally arranged; the welding component is used to weld the intersection of the first spiral wire and the second spiral wire to form the welded quality microsphere.

[0014] In one embodiment, the stretching assembly includes a rotary winding device and a fixed-pitch stretching device. The rotary winding device is used to wind a metal wire to form a metal spiral coil with a first pitch. The fixed-pitch stretching device is used to stretch the metal spiral coil with the first pitch to form a first spiral wire / second spiral wire with a second pitch. The first pitch is smaller than the second pitch.

[0015] In one embodiment, the rotary winding device includes a tapered mandrel, a pressure roller, and a roller; the tapered mandrel is rotatable about its axis; the pressure roller is used to feed the metal wire so that the metal wire can rotate with the tapered mandrel and be wound around the tapered mandrel; the roller is located on the side of the tapered mandrel away from the pressure roller, and the opposite sides of the metal wire abut against the tapered mandrel and the pressure roller, respectively.

[0016] In one embodiment, the fixed-pitch stretching device includes a fixed platform and a plurality of fixed-pitch stretching pulley groups; the plurality of fixed-pitch stretching pulley groups are arranged at intervals along the length direction of the metal spiral coil, each fixed-pitch stretching pulley group includes two opposing first pulleys, with a first channel formed between the two first pulleys for the metal spiral coil to pass through, each first pulley is rotatably connected to the fixed platform, and the first pulley is provided with an involute rack in the circumferential direction, the rack being able to stretch the metal spiral coil with a first pitch to form a first spiral wire / second spiral wire with a second pitch when the rack rotates.

[0017] In one embodiment, the fixed pitch tensioning device further includes a springback compensation pulley group, which is located behind a plurality of fixed pitch tensioning pulley groups. The springback compensation pulley group is used to compensate for the reduction in pitch of the metal helical coil due to springback deformation.

[0018] In one embodiment, the rebound compensation pulley assembly includes two opposing second pulleys, with a second channel formed between the two second pulleys for the metal helical coil to pass through. Each second pulley is rotatably connected to the fixed platform, and the second pulley is provided with an involute rack in the circumferential direction.

[0019] In one embodiment, the laying assembly includes a clamp, a first pre-tightening member, and a second pre-tightening member. The clamp is provided with a first groove and a second groove arranged in an orthogonal array. The first pre-tightening member is used to apply pre-tightening force to both ends of a first spiral wire, and the second pre-tightening member is used to apply pre-tightening force to both ends of a second spiral wire. The first spiral wire is clamped in the first groove, and the second spiral wire is clamped in the second groove.

[0020] In one embodiment, the welding assembly includes a top cover plate, a brazing filler metal, and a brazing device; the top cover plate covers the laying assembly and has a plurality of square grooves formed on the top cover plate corresponding one-to-one with the positions of the cross nodes; the brazing filler metal is placed in the square grooves; the brazing device is used to weld the brazing filler metal to form weld quality microspheres.

[0021] Thirdly, the present invention provides a method for preparing a hyperelastic flexible structure, wherein the hyperelastic flexible structure is prepared using any of the assembly tooling described in the above-mentioned technical solutions, and the preparation method includes the following steps:

[0022] Step 1: Use a stretching assembly to stretch the metal wire to form the first spiral wire and the second spiral wire respectively;

[0023] Step 2: Pre-tighten and fix the first spiral wire and the second spiral wire respectively, so that the first spiral wire and the second spiral wire are arranged orthogonally on the laying assembly;

[0024] Step 3: Weld the intersection of the first spiral wire and the second spiral wire using a welding assembly to form welded quality microspheres.

[0025] (III) Beneficial Effects

[0026] The above-described technical solution of the present invention has at least the following advantages:

[0027] 1. This invention connects orthogonally arranged first and second helical wires with welded mass microspheres to form a superelastic flexible structure. Compared with existing superelastic structural materials, it has excellent multifunctional mechanical properties such as large deformation recovery elastic deformation and zero Poisson's ratio. It can be used as a skeleton reinforcement of a new type of multiphase composite superelastic structural material, or it can be used as a component on its own.

[0028] 2. This invention prepares a metal spiral coil by winding a metal wire using a rotary winding device; it then uses a fixed pitch stretching device to gradually stretch the metal spiral coil to produce a first spiral wire and a second spiral wire; finally, it uses a laying assembly to orthogonally lay and assemble the first and second spiral wires; and finally, it uses integrated preparation methods such as resistance spot welding and vacuum brazing to prepare a superelastic structural material. Attached Figure Description

[0029] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0030] Figure 1 This is a schematic diagram of a common corrugated structure in existing technology.

[0031] Figure 2 This is a schematic diagram of a common honeycomb structure in existing technology.

[0032] Figure 3 This is a schematic diagram of the superelastic flexible structure provided in an embodiment of the present invention.

[0033] Figure 4 This is a schematic diagram of the structure of the rotary winding device provided in an embodiment of the present invention.

[0034] Figure 5 This is a schematic diagram of the fixed pitch tensioning device provided in an embodiment of the present invention.

[0035] Figure 6 This is a schematic diagram of the structure of the first pulley provided in an embodiment of the present invention.

[0036] Figure 7 This is a schematic diagram of the laying component provided in an embodiment of the present invention.

[0037] Figure 8 This is a schematic diagram of the welding assembly provided in an embodiment of the present invention.

[0038] Figure 9 This is a cyclic loading force-displacement curve obtained from experiments on the hyperelastic flexible structure provided in the embodiments of the present invention.

[0039] The labels for the attached figures are as follows:

[0040] 100. Ultra-elastic flexible structure; 10. First spiral wire; 20. Second spiral wire; 30. Welded quality microspheres; 110. Metal wire; 200. Assembly tooling; 1. Tensioning assembly; 2. Laying assembly; 3. Welding assembly; 11. Rotary winding device; 12. Fixed pitch tensioning device; 111. Conical mandrel; 112. Pressure roller; 113. Roller; 121. Fixed platform; 122. Fixed pitch tensioning pulley block; 123. Springback compensation pulley block; 1221. First pulley; 1222. First channel; 1223. Rack; 1231. Second pulley; 1232. Second channel; 21. Clamp; 211. First groove; 212. Second groove; 31. Top cover plate; 32. Brazing filler metal; 311. Square groove. Detailed Implementation

[0041] To make the technical problems to be solved, the technical solutions, and the beneficial effects of the present invention clearer, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the present invention and are not intended to limit the present invention.

[0042] It should be noted that when a component is referred to as "fixed to" or "set on" another component, it can be located directly on or indirectly on the other component. When a component is referred to as "connected to" another component, it can be directly or indirectly connected to the other component.

[0043] It should be understood that the terms "length", "width", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, and are only for the convenience of describing the present invention, and do not indicate that the device or element must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of the present invention.

[0044] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating relative importance or the number of technical features. In the description of this invention, "a plurality of" means two or more, unless otherwise explicitly specified. The specific implementation of this invention will be described in more detail below with reference to specific embodiments:

[0045] like Figure 3 , Figure 4 , Figure 5 , Figure 6 , Figure 7 as well as Figure 8 As shown, this embodiment of the invention provides a superelastic flexible structure 100, including a first helical wire 10, a second helical wire 20, and welded mass microspheres 30. The length direction of the first helical wire 10 is orthogonal to the length direction of the second helical wire 20. The welded mass microspheres 30 are disposed at the intersection of the first helical wire 10 and the second helical wire 20 to connect them. The first helical wire 10 and the second helical wire 20 can be regarded as a series of microsprings connected in series and parallel. The elastic deformation capacity in both directions can be customized by adjusting the specifications such as the diameter and pitch of the helical wires of the first helical wire 10 and the second helical wire 20. The welded mass microspheres 30 play a reliable skeletal connection role and can also be used as a phase for controlling the mechanical properties of the superelastic structure, such as a dynamic resonant oscillator.

[0046] This invention also provides an assembly fixture 200 for assembling a superelastic flexible structure 100, including a stretching component 1, a laying component 2, and a welding component 3; the stretching component 1 is used to stretch metal wires 110 to form a first spiral wire 10 and a second spiral wire 20 respectively; the laying component 2 is used to pre-tighten and fix the first spiral wire 10 and the second spiral wire 20, and to make the first spiral wire 10 and the second spiral wire 20 orthogonally arranged; the welding component 3 is used to weld the intersection of the first spiral wire 10 and the second spiral wire 20 to form welded quality microspheres 30.

[0047] In one embodiment, the stretching assembly 1 includes a rotary winding device 11 and a fixed-pitch stretching device 12. The rotary winding device 11 is used to wind a metal wire 110 to form a metal spiral coil with a first pitch. The fixed-pitch stretching device 12 is used to stretch the metal spiral coil with the first pitch to form a first spiral wire 10 / second spiral wire 20 with a second pitch. The first pitch is smaller than the second pitch.

[0048] In one embodiment, the rotary winding device 11 includes a tapered mandrel 111, a pressure roller 112, and a roller 113; the tapered mandrel 111 is rotatable about its axis; the pressure roller 112 is used to feed the metal wire 110 so that the metal wire 110 can rotate with the tapered mandrel 111 and be wound on the tapered mandrel 111; the roller 113 is located on the side of the tapered mandrel 111 away from the pressure roller 112, and the opposite sides of the metal wire 110 abut against the tapered mandrel 111 and the pressure roller 112 respectively.

[0049] In one embodiment, the fixed-pitch stretching device 12 includes a fixed platform 121 and a plurality of fixed-pitch stretching pulley groups 122. The plurality of fixed-pitch stretching pulley groups 122 are arranged at intervals along the length direction of the metal helical coil. Each fixed-pitch stretching pulley group 122 includes two opposing first pulleys 1221, with a first channel 1222 formed between the two first pulleys 1221 for the metal helical coil to pass through. Each first pulley 1221 is rotatably connected to the fixed platform 121, and the first pulley 1221 is circumferentially provided with an involute rack 1223. When the rack 1223 rotates, it can stretch the metal helical coil with a first pitch to form a first helical wire 10 / second helical wire 20 with a second pitch. In this embodiment, the number of fixed-pitch stretching pulley groups 122 is preferably three, and the pitch stretching amplitude of the three fixed-pitch stretching pulley groups 122 can be 50%, 80%, and 100% of the designed target pitch (second pitch), respectively.

[0050] In one embodiment, the fixed-pitch stretching device 12 further includes a springback compensation pulley group 123. The springback compensation pulley group 123 is located behind the plurality of fixed-pitch stretching pulley groups 122 and is used to compensate for the reduction in pitch caused by springback deformation of the metal helical coil. During the fixed-pitch stretching process, the metal helical coil exhibits springback deformation; therefore, the springback compensation pulley group 123 is added after the fixed-pitch stretching pulley group 122. The rack spacing of this springback compensation pulley group 123, based on the designed pitch, considers the springback compensation factor between the cross-section and the pitch, and can compensate for the reduction in pitch caused by springback deformation.

[0051] In one embodiment, the springback compensation pulley assembly 123 includes two opposing second pulleys 1231, with a second channel 1232 formed between the two second pulleys 1231 for the passage of a metal spiral coil. Each second pulley 1231 is rotatably connected to the fixed platform 121, and the second pulley 1231 is provided with an involute rack in the circumferential direction.

[0052] In one embodiment, the laying assembly 2 includes a clamp 21, a first pre-tightening member (not shown), and a second pre-tightening member (not shown). The clamp 21 has a first groove 211 and a second groove 212 arranged in an orthogonal array. The first pre-tightening member applies pre-tightening force to both ends of the first spiral wire 10, and the second pre-tightening member applies pre-tightening force to both ends of the second spiral wire 20 to prevent bending and deformation during the spiral wire laying and assembly process. The first spiral wire 10 is clamped in the first groove 211, and the second spiral wire 20 is clamped in the second groove 212. The spacing and number of series and parallel connections of the first spiral wire 10 and the second spiral wire 20 arranged in an orthogonal array determine the longitudinal and transverse stiffness moduli of the hyperelastic cell structure.

[0053] In one embodiment, the welding assembly 3 includes an upper cover plate 31, a brazing filler metal 32, and a brazing device (not shown); the upper cover plate 31 covers the laying assembly 2, and a plurality of square grooves 311 corresponding one-to-one with the positions of the cross nodes are formed on the upper cover plate 31; the brazing filler metal 32 is placed in the square grooves 311; the brazing device is used to weld the brazing filler metal 32 to form weld quality microspheres 30.

[0054] This invention also provides a method for preparing a hyperelastic flexible structure, wherein the hyperelastic flexible structure 100 is prepared using any of the assembly fixtures 200 described in the above embodiments, and the preparation method includes the following steps:

[0055] Step 1: Use the stretching assembly 1 to stretch the metal wire to form the first spiral wire 10 and the second spiral wire 20 respectively;

[0056] Step 2: Pre-tighten and fix the first spiral wire 10 and the second spiral wire 20 respectively, so that the first spiral wire 10 and the second spiral wire 20 are orthogonally arranged on the laying component 2;

[0057] Step 3: Weld the intersection of the first spiral wire 10 and the second spiral wire 20 using the welding assembly 3 to form a welded quality microsphere 30.

[0058] The following is a specific embodiment provided by the present invention:

[0059] The metal wire 110 is made of TC4 alloy straight wire with a diameter of 0.3 mm. The metal wire 110 is placed against the outer circumference of a tapered mandrel 111 with a diameter of 3 mm. A roller 113 is then placed against the metal wire 110. Sufficient friction is generated between the roller 113 and the tapered mandrel 111, causing the metal wire 110 to undergo plastic deformation, forming a metal spiral coil with a median diameter of 3 mm. This 3 mm median diameter spiral coil is placed in a fixed-pitch stretching device 12. Through three sets of fixed-pitch stretching devices 12, the spiral coil is gradually stretched to 50%, 80%, and 100% of the preset pitch (second pitch). Finally, after being controlled by a set of springback compensation pulleys 123, a first spiral wire with a pitch of 4 mm is obtained. The above steps are repeated to prepare a second spiral wire with a pitch of 4 mm.

[0060] The first and second spiral wires with a pitch of 3mm are placed into the laying assembly 2. The first pre-tightening member is used to apply pre-tightening force to both ends of the first spiral wire 10, and the second pre-tightening member is used to apply pre-tightening force to both ends of the second spiral wire 20. The first spiral wire 10 is then secured in the first groove 211, with a spacing of 10mm between two adjacent first spiral wires 10. The second spiral wire 20 is then secured in the second groove 212, with a spacing of 10mm between two adjacent second spiral wires 20.

[0061] Ultrasonic cleaning removes oil and impurities from the surfaces of the materials to be welded and the assembly fixtures to prevent them from affecting the strength of the welded joint. Resistance spot welding is used at the intersection of the first spiral wire 10 and the second spiral wire 20, with diagonal welding performed one by one to fix the first spiral wire 10 and the second spiral wire 20. The resistance spot welding pulse time is 200 milliseconds, and the welding current is 500A. Vacuum brazing is performed in a vacuum brazing furnace at a temperature of 900℃ and a holding time of 60 minutes, with a vacuum degree of less than 2×10⁻⁶. -3 Pa; The sample was removed after the furnace temperature dropped to room temperature. During this period, the vacuum system needed to continue operating to prevent oxidation of the hyperelastic flexible structure 100. The hyperelastic flexible structure 100 was finally fabricated, and tests were conducted on it to obtain… Figure 9 The force-displacement curve shown is a cyclic loading force-displacement graph.

[0062] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. An assembly tool for assembling a super-elastic flexible structure, characterized in that, The super-elastic flexible structure comprises: a first helical wire; a second helical wire, a length direction of the first helical wire being orthogonal to a length direction of the second helical wire; a welding quality microsphere arranged at a cross node position of the first helical wire and the second helical wire to connect the first helical wire and the second helical wire; an assembly tooling comprising a stretching assembly, a laying assembly and a welding assembly; the stretching assembly is used to stretch a metal wire to form the first helical wire and the second helical wire respectively, the stretching assembly comprises a rotating winding device and a constant-pitch stretching device, the rotating winding device is used to wind the metal wire to form a metal helical wire turn with a first pitch, and the constant-pitch stretching device is used to stretch the metal helical wire turn with the first pitch to form the first helical wire / second helical wire with a second pitch, wherein the first pitch is smaller than the second pitch; the laying assembly is used to pre-tighten and fix the first helical wire and the second helical wire, and to arrange the first helical wire and the second helical wire orthogonally; the welding assembly is used to weld the cross node position of the first helical wire and the second helical wire to form the welding quality microsphere.

2. The assembly fixture of claim 1, wherein The rotating winding device comprises: a tapered mandrel capable of rotating about an axis thereof; a compression roller used to convey the metal wire so that the metal wire can rotate with the tapered mandrel and wind on the tapered mandrel; a roller wheel arranged on a side of the tapered mandrel away from the compression roller, and opposite sides of the metal wire abut against the tapered mandrel and the compression roller respectively.

3. The assembly tool of claim 1, wherein The constant-pitch stretching device comprises: a fixed platform; a plurality of constant-pitch stretching pulley blocks, the plurality of constant-pitch stretching pulley blocks are arranged at intervals along a length direction of the metal helical wire turn, each constant-pitch stretching pulley block comprises two oppositely arranged first pulleys, a first channel for the metal helical wire turn to pass through is formed between the two first pulleys, each first pulley is rotationally connected to the fixed platform, and a involute gear rack is arranged in a circumferential direction of the first pulley, and the involute gear rack can stretch the metal helical wire turn with the first pitch to form the first helical wire / second helical wire with the second pitch when the involute gear rack rotates.

4. The assembly tool of claim 3, wherein The constant-pitch stretching device further comprises a springback compensation pulley block arranged behind the plurality of constant-pitch stretching pulley blocks, and the springback compensation pulley block is used to compensate for a reduced pitch of the metal helical wire turn due to springback deformation.

5. The assembly tool of claim 4, wherein, The springback compensation pulley block comprises two oppositely arranged second pulleys, a second channel for the metal helical wire turn to pass through is formed between the two second pulleys, each second pulley is rotationally connected to the fixed platform, and an involute gear rack is arranged in a circumferential direction of the second pulley.

6. The assembly tool of claim 1, wherein The laying assembly comprises a clamp, a first pre-tightening member and a second pre-tightening member, the clamp is provided with first grooves and second grooves arranged in an orthogonal array, the first pre-tightening member is used to apply a pre-tightening force to two ends of the first helical wire, the second pre-tightening member is used to apply a pre-tightening force to two ends of the second helical wire, the first helical wire is clamped in the first grooves, and the second helical wire is clamped in the second grooves.

7. The assembly tool of claim 1, wherein The welding assembly comprises: An upper cover plate is arranged on the laying assembly, and a plurality of square grooves corresponding to the intersection node positions are formed on the upper cover plate; A filler is arranged in the square grooves; A soldering device is used to weld the filler to form the welded quality microspheres.

8. A method of making a super-elastic flexible structure, characterized by, The super-elastic flexible structure is prepared by using the assembly tooling according to any one of claims 1-7, and the preparation method comprises the following steps: Step one, the metal wires are respectively stretched to form the first spiral wire and the second spiral wire by using the stretching assembly; Step two, the first spiral wire and the second spiral wire are respectively pre-tightened and fixed, and the first spiral wire and the second spiral wire are arranged orthogonally on the laying assembly; Step three, the intersection node positions of the first spiral wire and the second spiral wire are welded by the welding assembly to form the welded quality microspheres.

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