The invention relates to an apparatus and a method for surface processing a metallic structure. Particularly but not exclusively, the invention relates to an apparatus and a method for surface processing a tubular metallic structure.
Tubular metallic structures are widely used in various industries including manufacturing and constructions for carrying loads or providing supports. Efforts have been made to improve the strength of these tubular metallic structures to enhance safety and stability. The improvement in the strength of the tubular metal structures also assists in replacing bulky and heavy metallic tubes with smaller and lighter tubes, and thus reducing the overall size and weight of the resulting products or structures.
Surface treating or processing is a convenient method for improving strength a structure, and particularly, a metallic structure. In 1999, the process of Surface Mechanical Attrition Treatment (SMAT) is first proposed by K. Lu and J. Lu, and since then the process has attracted increasing interests in the field. SMAT is an efficient method to create a layer of nano-crystallized structure on the surface of metals. Balls having a smooth, spherical surface generally made of stainless steel, tungsten-carbide and ceramics, etc., are placed in a working chamber along with a metallic sample to be surface-treated. The balls are then made to vibrate to resonance by a vibration generator, and that the sample is then subjected to collision by a large number of fast moving balls over a short period of time. Each collision creates an impact which induces plastic deformation with a high strain rate to the metal surface of the sample. As a consequence, the repeated multi-directional impacts at high strain rate onto the sample surface result in numerous plastic deformations and grain refinements, which progressively down to the nanometer regime over the entire sample surface and provides a significant enhancement on the strength of the surface being treated.
SMAT has been proved successful in enhancing strength of a planar surface or an outer surface of a tubular structure. However, treatment on an inner surface of a tubular structure remains a critical problem which significantly affects the efficacy of the treatment and thus the strength of the resulting structure. Accordingly, there has been a continual need for an effective and simple method in processing an inner surface of a tubular structure.
In accordance with a first aspect of the present invention, there is provided a method for processing a surface, comprising the steps of supporting a structure having an inner surface on a platform, disposing at least one ball adjacent to the inner surface, positioning a reflecting member adjacent to the inner surface, wherein the at least one ball is adapted to vibrate by a vibrating means and to collide with the inner surface and the reflecting member thereby creating an impact to the inner surface.
In an embodiment of the first aspect, at least part of the reflecting member is arranged within the structure.
In an embodiment of the first aspect, the vibrating means is positioned below the platform.
In an embodiment of the first aspect, the vibrating means is positioned at least partially within the structure.
In an embodiment of the first aspect, the structure is positioned such that a central axis thereof is substantially perpendicular to the platform.
In an embodiment of the first aspect, the central axis of the structure is arranged in parallel to a longitudinal axis of the reflecting member.
In an embodiment of the first aspect, the structure and the reflecting member are coaxially arranged.
In an embodiment of the first aspect, the structure, the reflecting member and the vibrating means are coaxially arranged.
In an embodiment of the first aspect, the structure is of tubular shape.
In an embodiment of the first aspect, the reflecting member comprises a circular side wall circumferentially abuts the inner surface of the structure.
In an embodiment of the first aspect, the reflecting member comprises at least one inclined wall extended downwardly and tapered inwardly from the circular side wall, the at least one inclined wall is adapted to collide with the ball.
In an embodiment of the first aspect, at least part of the reflecting member is of a shape of a frustum.
In an embodiment of the first aspect, the structure is made of metal or metal alloy.
In accordance with a second aspect of the present invention, there is provided an apparatus for processing a surface, comprising a platform arranged to support a structure having an inner surface, at least one ball disposed adjacent to the inner surface, and a reflecting member having at least one reflecting surface, wherein the at least one ball is adapted to vibrate by a vibrating means and to collide with the inner surface and the reflecting surface thereby creating an impact to the inner surface.
In an embodiment of the second aspect, at least part of the reflecting member is arranged within the structure.
In an embodiment of the second aspect, the vibrating means comprises a vibrating horn.
In an embodiment of the second aspect, the vibrating means is positioned below the platform surface.
In an embodiment of the second aspect, the vibrating means is positioned at least partially within the structure.
In an embodiment of the second aspect, the structure is positioned such that a central axis thereof is substantially perpendicular to the platform.
In an embodiment of the second aspect, the central axis of the structure is arranged in parallel to a longitudinal axis of the reflecting member.
In an embodiment of the second aspect, the structure and the reflecting member are coaxially arranged.
In an embodiment of the second aspect, the structure, the reflecting member and the vibrating means are coaxially arranged.
In an embodiment of the second aspect, the structure is of tubular shape.
In an embodiment of the second aspect, the reflecting member comprises a circular side wall circumferentially abuts the inner surface of the structure.
In an embodiment of the second aspect, the reflecting member comprises at least one inclined wall extended downwardly and tapered inwardly from the circular side wall, the at least one inclined wall is adapted to collide with the at least one ball.
In an embodiment of the second aspect, the reflecting member comprises a base portion extended downwardly from the at least one inclined wall and is positioned adjacent to the platform.
In an embodiment of the second aspect, at least part of the reflecting member is of a shape of a frustum.
In an embodiment of the second aspect, the tubular structure is made of metal or metal alloy
In an embodiment of the second aspect, the platform is supported by a supporting arrangement.
In an embodiment of the second aspect, the structure is fixedly positioned on the platform via a fixing means.
Further aspects of the invention will become apparent from the following description of the drawings, which are given by way of example only to illustrate the invention.
The present invention relates to an apparatus for processing a surface. The apparatus comprising a platform arranged to support a structure having an inner surface; vibrating means at least one ball disposed adjacent to the inner surface; and a reflecting member having at least one reflecting surface; wherein the at least one ball is adapted to vibrate by a vibrating means and to collide with the inner surface and the reflecting surface thereby creating an impact to the inner surface.
The present invention also relates to a method for processing a surface. The method comprising the steps of supporting a structure having an inner surface on a platform vibrating means; disposing at least one ball adjacent to the inner surface; positioning a reflecting member adjacent to the inner surface; wherein the at least one ball is adapted to vibrate by a vibrating means and to collide with the inner surface and the reflecting member thereby creating an impact to the inner surface.
Specifically, the present invention involves the use of the Surface Mechanical Attrition Treatment (SMAT) for processing a surface of a metallic structure, particularly but not exclusively, an inner surface of a metallic tubular structure. In addition, the term “metallic” may include metals, metal alloys or a mixture thereof. Nevertheless, a person skilled in the art would appreciate that the apparatus and the method of the present invention are also applicable in processing inner surfaces of structures having different shapes or geometric configurations, or structures being made of other materials, as long as the skilled person may consider appropriate in doing so.
The balls 18 are made of rigid materials such as stainless steel, tungsten carbide or ceramic etc. are introduced into the hollow center of the tubular structure 12. The balls 18 can be of a diameter of about 1 mm to about 3 mm, and will be set in motion inside the tubular structure 12 when the vibrating horn 16 is actuated to produce vibration to the balls 18. The number of balls 18 being used is mainly dependent on the geometric dimension of the apparatus including the tubular structure, the reflector and also the dimension of the ball itself.
A reflector 20 is positioned inside or partially inside the tubular structure 12. In this particular embodiment as shown in
This circular side wall 22a seals the hollow center of the tubular structure 12 from the external and thus encases the balls 18 within a tube cavity defined by the inner surface 12a of the tubular structure 12, the inclined side wall 24a of the middle portion 24, the cylindrical side wall 26a of the base portion 26, and the surface of the platform 14. Upon actuation of the vibrating horn 16, the balls 18 which are encased within the tube cavity will be set to vibration in a random motion. At a specific vibration frequency movement of the balls 18 will come to resonance, and will collide continuously with the inner surface 12a of the tubular structure 12. The collisions will also be reflected at the surfaces of the inclined side wall 24a and the cylindrical side wall 26a, which enhance the colliding effects to provide more vigorous impacts at the inner surface 12a. Each impact by the balls 18 induces plastic deformation with a high strain rate at the spot of collision of the inner surface 12a. Consequentially, the repeated multidirectional collisions result in significant mechanical impacts in the form of plastic deformations and grain refinements on the inner surface 12a of the tubular structure 12, and the impacts will progress down into the submicron regime over the inner surface 12a to create nano-crystallized structures at the inner surface 12a which improve the strength of the tubular structure 12.
Specifically, the central axis 12c of the tubular structure 12 coincides with the longitudinal axis 20c of the reflector 20, or both the longitudinal axes of the reflector 20 and the vibration horn 16 so as to provide maximized impacts to the tubular structure 12. Alternatively, the central axis 12c can be arranged in parallel to the longitudinal axis 20c of the reflector 20, or in parallel to both the longitudinal axes of the reflector 20 and the vibration horn 16 so as to provide impacts to the tubular structure 12.
Although a number of preferred designs of the reflector 20 have been described, a person skilled in the art will appreciate that the design of the reflector of the present invention should not be limited to the specific embodiments. Instead, the skilled person will understand that variations to the design will be applicable to the present invention as long as the reflector provides a reflecting surface to reflect the balls onto the inner surface to be treated of the tubular structure.
In an embodiment of treating a tubular structure with the apparatus of the present invention, a tubular structure 12 with an inner diameter of about 70 mm with a wall thickness of about 3 mm has been used. The tubular structure 12 is subjected to impact by the balls 18 for about 15 min under a vibration frequency of about 20 KHz. Each of the balls 18 is of a diameter of about 3 mm. Again, variation to the configurations of the tubular structure and the ball, and treatment conditions is applicable to the present invention as long as it is considered appropriate to the skilled person in the art.
The improvement of strength of the tubular structure as treated by the apparatus of the present invention having a reflector as shown in
The micro-hardness test is performed by using the Vickers hardness test method which consists of indenting the test material with a diamond indenter in the form of a right pyramid having a square base and an angle of 136 degree between the opposite faces, and the indenter is subjected to a load of 1 to 100 kgf. The full load is normally applied for 10 to 15 seconds. The two diagonals of the indentation left in the surface of the material after removal of the load are measured using a microscope and their average is calculated. The area of the sloping surface of the indentation is also calculated. The Vickers hardness is the quotient obtained by dividing the kgf load by the square mm area of indentation.
In this experiment, the sample tube was mounted in cross section in a conductive epoxy to conduct the hardness measurements. Surface of the sample being tested was first polished using successively fine grit size abrasives media prior to the measurements to eliminate surface damage. The measurements took place at different locations of the treated surface along the height of the sample, from top to bottom, and a testing force of 100 mN was applied for a duration of 10 s. Hardness of the cross-section of the SMAT treated sample surface was measured with an interval of 20 μm in the first 200 μm range. At least three measurements were taken at each distance and the average of these measurements was shown in each data point of
As shown in
The crystalline structure of the SMAT treated surface of a tube sample is further measured by X-ray diffraction (XRD) analysis with the results being demonstrated in
In general, XRD measures the intensity of an X-ray beam reflected from a small area. The atomic-level spacing within the crystal lattice of the specimen can then be determined based on the intensity results. XRD reveals different phases with identical compositions with finer details of the crystal structure such as the state of atomic “order” which accounts for their different properties. In addition, strain analysis and determination of the degree of crystallization can also be assessed.
The XRD analysis was carried out on the treated surface of a sample tube having an inner diameter of about 71 mm, a thickness of about 3 mm, and a height of about 25 mm. The SMAT was conducted by using the apparatus of the present invention having a reflector as shown in
To conduct the XRD analysis, the sample tube was first glued onto a silica base and the XRD patterns were measured using a (θ−2 θ) Philips diffractometer with Cu Kα radiation. The acquisition conditions were Δ (2 θ)=0.04°, Δt/step (2 θ)=1 s.
The XRD results of an untreated sample tube, a sample tube treated by a reflector as shown in
The results as revealed in the above experiments shown that the present invention is beneficial in improving the strength of the inner surface of a tubular metallic structure of the material, and that the tubular structure can then carry more loads when compared with an untreated structure, especially when both the inner and outer surfaces of the tubular structure are surface-treated. In addition, the present invention introduced a SMAT apparatus and surface treatment method which is efficient, simple and cost effective. Furthermore, the apparatus and method of the present invention is versatile, which can be easily set up and conducted in lab scale environment, and also capable of scaling up to meet various industrial applications.
It should be understood that the above only illustrates and describes examples whereby the present invention may be carried out, and that modifications and/or alterations may be made thereto without departing from the spirit of the invention.
It should also be understood that certain features of the invention, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the invention which are, for brevity, described in the context of a single embodiment, may also be provided or separately or in any suitable subcombination.
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