JP4690895B2 - Laser peening treatment method of metal object and metal object manufactured by laser peening treatment method - Google Patents

Description

  The present invention relates to a technique for improving the fatigue strength of a metal object by performing laser peening on the surface of the metal object.

  Generally, it is known that tensile residual stress is likely to occur in a welded part of a welded structure or a plastically deformed part of a press-molded part, which causes fatigue cracks when repeatedly loaded. As a method for improving fatigue strength, which suppresses the generation and propagation of fatigue cracks in metal parts and metal structures (collectively referred to as metal objects or structural members) subjected to repeated loads, compressive stress is applied to the portions subjected to repeated loads. There is a method of leaving As processing for applying such compressive stress, methods such as shot peening and ultrasonic peening are known.

  In addition, laser peening, which has been developed in recent years, utilizes the expansion reaction force of plasma generated by irradiation of a pulsed laser beam on the surface of a metal object through water or the like that absorbs less laser beam. This is a technique for applying residual compressive stress in the vicinity of the surface. For example, Patent Document 1 discloses the method.

  Laser peening has the following advantages over other peening methods such as shot peening and ultrasonic peening. First, since the irradiation spot of the laser beam can be controlled with high accuracy, it is suitable for selectively applying a compressive stress to a minute region such as a peripheral portion of a hole having a diameter of about 1 mm. In addition, there is an advantage that the change in the surface shape accompanying processing is small. In shot peening and ultrasonic peening, the processing surface is roughened by the processing, so that it is necessary to perform post-processing such as re-polishing the peening processing surface for application to machine parts that require surface shape accuracy. There was a problem. In laser peening, since the change in the surface shape accompanying processing is small, such a post-process can often be omitted.

In the laser peening process, several methods have been proposed for the irradiation method for applying a compressive stress to the processed surface. For example, Patent Document 2 discloses that a compressive stress can be uniformly left over the entire surface to be processed by processing so that irradiation spots of a pulse laser beam overlap each other.
Japanese Patent No. 3373638 Japanese Patent No. 3461948

  When applying laser peening to metal objects (hereinafter also referred to as workpieces), a large compressive stress is applied in the direction of the maximum principal stress (hereinafter referred to as the maximum principal stress direction) of the stress generated by repeated loads. The effect of improving fatigue strength increases as it is applied.

  The method of superimposing the irradiation spot of the laser beam as disclosed in Patent Document 2 so as not to create an unirradiated region applies a substantially uniform compressive stress in any direction to the entire processing surface. The irradiation method is suitable for the purpose of preventing stress corrosion cracking of the structure inside the reactor. However, for example, when a repeated load in one direction or a limited direction is applied to a metal object such as a machine part, applying a strong compressive stress in the maximum principal stress direction is effective in improving the fatigue strength.

  The first object of the present invention is to provide a method of applying a large compressive stress in a specific direction of a surface in a laser peening treatment method applied to a metal object. The fatigue strength of the metal object is increased by the compressive stress. This is the second purpose.

In order to achieve the above object, the present inventors have intensively studied the scanning method of the irradiation beam spot of the pulse laser beam on the surface of the workpiece in the laser peening process, and as a result, the residual compressive stress in the specific direction of the surface of the workpiece. As a result, the present invention has been achieved in which the fatigue strength of the workpiece can be increased. That is, the present invention is as follows.
(1) The laser peening treatment method of the present invention scans the surface with a beam spot obtained by condensing and irradiating a surface of a workpiece with a pulsed laser beam, and generates residual compressive stress on the surface, A laser peening treatment method for applying residual pulse stress in a specific direction within the surface of the surface by superimposing the pulsed laser beam on the surface, wherein the work material is a structural member and a load stress is applied when used. The specific direction is the direction of the maximum principal stress on the surface of the workpiece.
(2) The laser peening treatment method of the present invention is characterized in that a material layer that absorbs the pulsed laser beam is formed on the surface of the workpiece, and the pulsed laser beam is superimposed and irradiated (1). It is a laser peening processing method as described in above.
(3) In the laser peening processing method of the present invention, the beam spot is scanned in a direction orthogonal to the direction of the maximum principal stress on the surface of the workpiece by the superimposed irradiation of the pulsed laser beam. This is performed a plurality of times while shifting the position in the direction of the maximum principal stress, and the average number of times of superimposition that is the average value of the number of times of irradiation of the pulse laser beam at the same point on the surface is 4 times or more Is determined based on the strength of the workpiece, the irradiation condition of the pulsed laser beam, and a desired compressive stress value, in the laser peening processing method according to (1).
(4) The laser peening treatment method of the present invention is the laser peening treatment method according to (3), wherein the pulse laser beam is absorbed on the surface of the workpiece before the pulse laser beam is superimposed and irradiated. An average number of times of superimposition, which is an average value of the number of times of irradiation of the pulse laser beam at the same point on the surface, and the average number of times of superimposition is determined based on the strength of the workpiece, the pulse laser It is determined based on a beam irradiation condition and a desired compressive stress value.
(5) In the laser peening method according to the present invention, the workpiece has a hole on the surface and stress is applied, and the direction of the maximum principal stress around the hole is the circumferential direction of the hole. The laser peening processing method according to one of (1) to (4), wherein
(6) In the laser peening method according to the present invention, the workpiece has a groove on its surface and stress is applied, and the direction of the maximum principal stress at the bottom of the groove is the direction along the groove. The laser peening method according to one of (1) to (4), wherein the laser peening method is a direction that forms a right angle.
(7) Moreover, the metal object of this invention is a metal object manufactured with the laser peening processing method as described in any one of (1)-(6).

  In the method of the present invention, in the laser peening process for a metal object, the compressive stress in the maximum principal stress direction can be selectively increased by appropriately selecting the position, order, and density of the irradiation spot of the laser beam. Therefore, it is possible to stably obtain a metal object having a long fatigue life.

  In order to clarify the gist of the present invention, the following description will be given with reference to the accompanying drawings.

FIG. 1 shows an example of an apparatus for performing a laser peening process on a metal object (workpiece). The workpiece 7 is immersed in a water tank 5 containing water having high laser beam permeability. A pulse laser beam 2 having a pulse width of about 10 ns emitted from the laser beam oscillator 1 is collected by a condenser lens 3 and irradiated onto the surface of the workpiece 7 through a window 4 attached to the water tank 5. The peak power density on the workpiece surface of the pulse laser beam 2 is 1 to 100 TW / m ² . This pulsed laser beam irradiation generates plasma from the irradiation spot. However, since the expansion of the plasma is suppressed by the inertia of water in contact with the workpiece surface, the plasma pressure increases, and the reaction force causes the laser beam irradiation. It becomes possible to give plastic deformation near the surface of the spot. As a result, compressive stress can be applied to the surface of the workpiece. The treatment does not necessarily have to be performed in water, and a method such as spraying a jet of water on the processing surface, forming a water film on the surface of the workpiece, and using a medium that transmits laser light, such as an acrylic plate, etc. Even if it is used, since the effect of increasing the pressure by suppressing the expansion of the plasma can be obtained, it is possible to introduce a compressive stress to the workpiece. Further, in an apparatus in which the laser beam has a relatively long distance to penetrate through water as shown in FIG. 1, the wavelength of the laser beam is preferably a visible wavelength having good underwater permeability, but it is about 1 mm or less on the surface of the workpiece. When processing is performed by forming a water film, a laser beam having a near-infrared wavelength in the 1 μm band can also be used. The shape of the beam spot is often a circle or an ellipse, but may be a rectangle or the like. Hereinafter, a circular beam spot will be described as an example.

  Further, as shown in FIG. 2, processing may be performed by forming a material layer 6 that absorbs the laser beam 2 on the surface of the workpiece 7. As this absorbent material, plastic tape, black paint, or the like is used. By forming this absorbing material layer 6, it is possible to prevent the compressive stress near the surface layer of the spot portion from being reduced by melting the surface layer of the irradiated spot portion due to heat input from the laser beam and plasma. Therefore, there is an advantage that compressive stress can be applied with a smaller number of spots.

FIG. 3 shows a pulse laser beam irradiation method for applying a compressive stress to a planar workpiece using the method of the present invention. The maximum principal stress direction is defined as the Y direction. As shown in the figure, pulsed laser beam superimposition is performed by shifting the X direction, which is the direction orthogonal to the maximum principal stress direction of the workpiece surface, to the beam spot scanning direction and shifting the beam spot scanning area in the Y direction. While doing. The beam spots are scanned so that adjacent beam spots in the same scanning region overlap each other. In addition, processing is performed so that adjacent scanning regions overlap each other. The formation of the scanning region is continuously performed as “L ₁ → L ₂ → L ₃ →...” In FIG. Here, the irradiation order of the beam spots does not necessarily have to be the one-stroke drawing order as shown in FIG. 3. For example, “M5 → N5” → “N6 → M6” shown in FIG. You may scan in the process order of M5-> N5 "->"M6-> N6 ". When the area of the area S ₁ is superimposed and irradiated by N times of irradiation with the pulse laser beam with the area of the beam spot as S ₀ , the average value of the number of irradiation times of the pulse laser beam with respect to the same point is S ₀ × N / S ₁ . Define and call it the average number of superpositions. In the method of the present invention, the process is performed under the condition that the average number of superpositions is increased, thereby causing anisotropy in the residual compressive stress on the surface of the work material and selectively strengthening the compressive stress in the Y direction. As a result, a compressive stress equivalent to the tensile strength of the workpiece can be applied in the Y direction, and the fatigue strength can be greatly increased. Below, the mechanism of selective strengthening in the Y direction will be described.

The conditions to increase the average superposition times, stresses FIG midpoint P is surrendered at the time, i.e. 3 in L ₄ is formed scanned region from directly above is formed, X direction, Y direction The stress is saturated. At this time, there is no difference in stress in the X direction and the Y direction. Thereafter, a plurality of rows of scanning regions indicated by L ₅ , L ₆ , L ₇ ... In FIG. As will be described below, these scanning regions have an effect of selectively strengthening the compressive stress in the Y direction at the point P.

FIG. 5 shows the distribution of residual stress generated in the vertical direction (Y direction) when only one scanning region is formed in the X direction, as shown in FIG. 4, using an X-ray residual stress measuring device. It is a thing. The test piece was made using 440 MPa class carbon steel. The surface of the test piece was treated by providing a 200 μm thick plastic tape as an absorbing material layer that absorbs the laser beam. The processing was performed with the apparatus shown in FIG. 2, and the second harmonic (wavelength 532 nm) of an Nd: YAG laser having good underwater permeability was used as the laser beam. The time width of the pulse laser beam was 10 ns. The laser beam was condensed with a convex lens having a focal length of 100 mm. As shown in FIG. 4, the spot shape on the test piece was circular, the spot diameter was 0.4 mm, and the distance between the centers of adjacent beam spots was 0.1 mm. Here, the spot diameter was defined as the diameter at which 86% of the power of the entire beam enters (hereinafter, the same definition is used). Peak power density was 50TW / ^{m 2.} In the measurement results shown in FIG. 5, the minus sign means that the residual stress is a compressive stress. The larger the absolute value, the higher the compressive stress and the more effective the fatigue strength. The compressive stress also affects the outside of the beam spot scanning region (Y> 0.2 mm), and the magnitude of the compressive stress in the Y direction is larger than the compressive stress in the X direction. .

Based on this result, a plurality of rows of scanning regions indicated by L ₅ , L ₆ , L ₇ ... In FIG. 3 have a Y point rather than a compressive stress in the X direction with respect to the point P outside these scanning regions. It turns out that it has the effect | action which strengthens the compressive stress. Since the stress at the point P reaches yield when the L ₄ is formed and is already saturated, the compression stress in the X direction is reduced by the formation of these scanning regions, but at the cost of the maximum principal stress direction It is possible to further increase the compressive stress in the Y direction. Below, the result of having confirmed this effect by experiment is described.

In the experiment, the compressive stress applied by irradiating the beam spot in the single stroke order shown in FIG. 3 was examined while changing the average number of times of superimposition. Processing was performed so that the distance between adjacent beam spots in the same scanning region was equal to the distance between the center lines of adjacent scanning regions (for example, L ₁ and L _{2 in} FIG. 3). The test piece was made using 440 MPa class carbon steel. The apparatus shown in FIG. 1 or FIG. 2 was used for the treatment, and a test piece immersed in a water bath was irradiated with a laser beam. FIG. 2 corresponds to a case where an absorption material layer that absorbs a laser beam is provided and processed, and FIG. 1 corresponds to a case where a material layer that absorbs a laser beam is not formed on the surface of a test piece. The laser beam used was a second harmonic (wavelength 532 nm) of an Nd: YAG laser having good underwater permeability. The time width of the pulse laser beam was 10 ns. The laser beam was condensed with a convex lens having a focal length of 100 mm. The spot shape on the test piece was circular and the spot diameter was 0.4 mm. Peak power density was 50TW / ^{m 2.} The residual stress of the test piece was measured using an X-ray residual stress measuring device.

  6 and 7 show the dependence of the residual stress on the surface on the average number of times of superposition. FIG. 6 shows the result of processing with a 200 μm-thick plastic tape as the absorbing material layer that absorbs the laser beam, and FIG. 7 shows the result of processing without forming the absorbing material layer. In either case, the compressive stress increases as the average number of superpositions increases. If the average number of superpositions is 2 times or more when there is an absorbent material layer, and 4 times or more when there is no absorbent material layer, the Y direction is increased. It can be seen that the compressive stress can be selectively increased. Incidentally, the compressive stress in the Y direction is −498 MPa under the condition where the average number of superpositions is 14 without forming the absorbent material layer and is a high value of 113% of the uniaxial tensile strength (440 MPa) of the steel material.

  By the way, the compressive stress in the Y direction becomes the largest when the scanning direction is parallel to the X direction, but the scanning direction does not necessarily coincide with the X direction.

  The treatment performed without forming the absorbent material layer can omit the step of forming the absorbent material, which is advantageous from the viewpoint of productivity. However, as described above, this method has a problem that near the surface layer of the irradiated spot portion is melted by heat input from the laser beam and plasma, and the compressive stress near the surface layer of the spot portion is likely to be reduced. However, as can be seen from FIG. 7, if the average number of superpositions is set to 4 or more, the compressive stress in the Y direction near the outermost layer can be sufficiently strengthened. The compression of the melted part is the effect of applying compressive stress by irradiating a lot of laser pulses to the peripheral part after the melted part has been generated, as well as the average number of superpositions, and the removal process of the melted part This occurs because the compression effect due to the occurrence of the problem increases. However, these effects require that a laser beam is irradiated at the end of a series of processing because it is necessary to irradiate the peripheral part with a large number of laser pulses after the melted part is generated. As a result, this effect is reduced. Therefore, when processing is performed in a single stroke order as shown in FIG. 3, since the applied compressive stress is reduced for the last several scanning regions, the effect of improving fatigue strength is reduced. Therefore, it is effective to include the region where the treatment as shown in FIG. 3 is performed up to the region where the maximum principal stress on the surface of the metal object is small.

  In the treatment performed by forming the absorbent material layer, the outermost layer does not melt as described above. Therefore, by setting the average number of times of superposition twice or more, sufficient compressive stress can be applied from the outermost layer in the Y direction. Under conditions where the average number of superpositions is large, it is desirable to use a metal foil or the like as the laser beam absorbing material so that the absorbing material layer does not evaporate during processing.

  For workpieces such as steel, we examined in detail the amount of compressive stress applied to workpieces with different uniaxial tensile strengths by changing the pulse laser beam irradiation conditions and the average number of superpositions. As a result, the effect of strengthening the compressive stress in the Y direction increases as the average number of superpositions increases regardless of the presence or absence of the absorbing material layer, but this effect is saturated. The average number of times of superimposition necessary to saturate this effect is determined depending on the uniaxial tensile strength of the workpiece and the irradiation condition of the pulse laser beam. This average number of superpositions increases as the uniaxial tensile strength of the workpiece increases, and decreases as the peak power density of the pulse laser beam increases. When the strength of the workpiece is high and the peak power density of the pulse laser beam cannot be sufficiently obtained, the required average number of superpositions may exceed 100. In such a case, it may be undesirable from the viewpoint of cost to increase the average number of times of superimposition until the effect is saturated. Therefore, in such a case, the compressive stress required in the direction of the maximum principal stress is estimated from the desired fatigue strength determined by the intended use conditions, and is near the lower limit of the average number of times of superimposition necessary for applying the compressive stress. You may process on condition of this.

  Next, a method for applying compressive stress to a workpiece having holes will be described. Here, the process with respect to the to-be-processed material in which fatigue strength becomes a problem by the stress applied to the circumferential direction of a hole is shown. FIG. 8 shows the irradiation method of the present invention. As shown in the figure, the scanning direction is always in a direction perpendicular to the circumferential direction, and the entire periphery of the hole 8 is overlaid. As described above, if the process is performed under the condition that the average number of superpositions is increased, the circumferential compressive stress can be selectively increased over the entire circumference of the hole 8. When the region where fatigue strength is a problem is limited to a part of the circumference, it is not necessary to irradiate the pulse laser beam over the entire circumference of the hole, and the region including the part where the fatigue strength is a problem is not necessary. Only superimposing irradiation is necessary. For example, when the tangential direction of the circumference of the hole 8 is the maximum principal stress direction at two points on both ends of the hole 8, and it is necessary to strengthen this direction, it is effective to irradiate as shown in FIG. is there. As shown in the figure, the laser beam spot is scanned in a direction (X direction in FIG. 9) perpendicular to the maximum principal stress direction (Y direction in FIG. 9) at points A and B where fatigue strength is a problem. To do. By such a process, the compressive stress in the maximum principal stress direction at the points A and B can be selectively increased.

Next, a method for applying compressive stress to a workpiece having notches such as grooves will be described. Here, as shown in FIG. 10, at the bottom of the groove having a semicircular shape with a radius R, the direction (Y direction) perpendicular to the direction along the groove is the maximum principal stress direction, so that the fatigue strength is a problem. Consider the workpiece. As shown in the plan view of FIG. 10, the pulsed laser beam is superimposedly irradiated on the groove with the direction along the groove set in the scanning direction. To enhance the Y-direction compressive stress at the bottom of the groove selectively, after forming the scanning area indicated by L ₂ in the vicinity of the groove bottom center in FIG. 10, in right region of the center line of the groove bottom forming the scan region as shown by L _3. When irradiating a portion away from the center line of the groove bottom as in L _3, the treatment surface is inclined with respect to the Y direction. Therefore, when a laser beam is incident from the l ₁ direction in the cross-sectional view, The peak power density is lower than when irradiating on the groove bottom center line (L ₂ ). Here, when the predetermined peak power density (1 to 100 TW / m ² ) is not reached, it is desirable to tilt the laser incident direction from the Z direction as indicated by l ₂ . In addition, although the example of a process to the workpiece which has a groove | channel with a semicircle shape was shown here, the said method is applicable with respect to cross-sectional shapes with arbitrary curvatures, such as an ellipse shape, for example.

Hereinafter, as an example of the present invention, a result of performing a fatigue test after subjecting a test piece, which is a workpiece, to laser peening treatment under various pulse laser beam irradiation conditions will be described. The laser peening process was performed using the apparatus shown in FIG. 1 or FIG. FIG. 1 corresponds to the case of processing without forming a material layer that absorbs a laser beam on the surface of the test piece, and FIG. 2 corresponds to the case of processing by forming an absorbing material layer. The test piece was immersed in a water bath, and the surface of the test piece was irradiated with a pulse laser beam through a window attached to the water bath. The laser beam used was a second harmonic (wavelength 532 nm) of an Nd: YAG laser having good underwater permeability. The time width of the pulse laser beam was 10 ns. The laser beam was condensed with a convex lens having a focal length of 100 mm. The spot shape on the test piece was circular, and the spot diameter was 0.3 mm. Peak power density was 50TW / ^{m 2.} In the laser processing in the first to third embodiments described below, the beam spots are formed in the single stroke order shown in FIG. 3, but the interval between adjacent beam spots in the same scanning region and the adjacent scanning region are changed. Processing was performed so that the distances between the center lines were equal.

(First embodiment)
As a first embodiment of the present invention, the results of applying a plane bending fatigue test by applying a compressive stress to a planar workpiece are shown. The shape of the test piece created using 440 MPa class carbon steel is shown in FIG. The load acting direction was the longitudinal direction (Y direction in FIG. 11), and the load condition was double swing. As shown in the figure, the pulse laser beam was irradiated in a one-stroke writing order in which the direction perpendicular to the load direction of the test piece (the X direction in FIG. 11) was the scanning direction. The treatment was performed over a width of 20 mm in the load direction, and was performed on both sides. Table 1 shows the fatigue test results for various processing conditions. Table 1 also shows the result of measuring the residual stress on the surface of point P in FIG. 11 which is the center of the processing region, using an X-ray residual stress measuring apparatus. As the fatigue strength, a stress range in which the fracture life was 2 million times was used. Condition 1 is a comparative example, and is a result for a test piece not subjected to laser treatment. Conditions 2 to 5 are cases where laser treatment was performed without providing a material layer that absorbs the laser beam on the surface of the test piece. Condition 2 is a comparative example with an average number of superpositions of 1.7, but the residual stresses in the X direction and Y direction were both tensile, and fatigue strength was not improved. In condition 3 (comparative example) in which the average number of superpositions was 3.5, the fatigue strength improvement effect was small because the compressive stress in the Y direction was small, but in condition 4 (in this case, the average number of superpositions was 6.9) In Invention Example, the fatigue strength was improved by about 25%. Furthermore, in condition 5 (example of the present invention) where the average number of times of superposition is 14 times, a compressive stress (−451 MPa) of the same magnitude as 440 MPa, which is the tensile strength of the steel material, can be applied in the Y direction. A 40% improvement in fatigue strength was obtained. Thus, it was found that the compressive stress in the Y direction, which is the load direction, was selectively increased as the average number of superpositions was increased, and the fatigue strength was improved. Under conditions 6 to 9, a 200 μm-thick plastic tape was provided as the absorbing material layer for absorbing the laser beam on the surface of the test piece. Also in this case, it was found that the compressive stress in the Y direction can be selectively increased and the fatigue strength can be improved as the average number of superpositions is increased. Under condition 8 (invention example) where the average number of times of superposition was 3.5, an improvement in fatigue strength of about 25% was obtained. Conditions 6 and 7 are comparative examples in which the average number of times of superposition was less than 2, but there was almost no difference in residual stress in the X direction and Y direction, and the obtained improvement in fatigue strength was 15% or less.

(Second embodiment)
As a second embodiment of the present invention, the results of a fatigue test performed to verify the effect of improving fatigue strength around a hole on a metal object having a hole are shown. As shown in FIG. 12, an Ono type rotating bending fatigue test was conducted using a test piece having a through hole with a diameter of 1 mm in the center of a constricted portion with a diameter of 6 mm. The test piece was made using 440 MPa class carbon steel. Stress concentration occurs in the vicinity of the through hole of the test piece, the stress is maximum at points A and B in the figure, and the maximum principal stress direction is the Y direction. In order to increase the compressive stress in this direction, as shown in FIG. 12, superimposed irradiation was performed in a single stroke order with the X direction as the scanning direction. This irradiation was performed on a portion excluding a through hole in a 4 mm × 4 mm rectangular region (shaded portion in FIG. 12) centering on the center of the hole. Table 2 shows the fatigue test results. The table also shows the result of measuring the residual stress in the Y direction at point A. Condition 10 is a comparative example, and is a result for a test piece not subjected to laser treatment. Conditions 11 and 12 are when the material layer that absorbs the laser beam is not formed on the surface of the test piece, and Conditions 13 and 14 are when the plastic tape having a thickness of 200 μm is provided as the absorbing material layer. In any case, the fatigue strength of the example of the present invention processed under the condition that the average number of superpositions was larger than that of the comparative example was found to be larger. Under conditions 12 and 14 which are examples of the present invention, an improvement in fatigue strength exceeding 60% was obtained with respect to the comparative example.

(Third embodiment)
As a third embodiment of the present invention, the results of a fatigue test performed to verify the fatigue strength improvement effect at the bottom of the groove on a metal object having a groove are shown. As shown in FIG. 13, an Ono type rotating bending fatigue test was performed using a test piece having a groove of R = 0.5 mm over the entire circumference of the center portion of the constricted portion having a diameter of 6 mm. The test piece was made using 440 MPa class carbon steel. The maximum principal stress is generated in the Y direction, and in order to increase the compressive stress in this direction, the overlapping irradiation was performed spirally around the groove portion while the circumferential direction of the test piece was set as the scanning direction. The treatment was performed on the entire grooved portion (shaded area in FIG. 13). Table 3 shows the fatigue test results. The table also shows the result of measuring the residual stress in the Y direction at the groove bottom. Condition 15 is a comparative example, and is a result for a test piece not subjected to laser treatment. As shown in Condition 17, when the material layer that absorbs the laser beam was not formed on the surface of the test piece and the treatment was performed with an average number of superpositions of 14 times, an effect of improving fatigue strength exceeding 50% was obtained. Condition 16 is a comparative example in which the average number of superpositions was 1.7, but almost no improvement in fatigue strength was obtained.

It is a top view which shows a laser beam irradiation apparatus. It is a top view which shows a laser beam irradiation apparatus. It is a top view which shows the laser beam irradiation method of this invention. It is a top view which shows the laser beam irradiation method. It is a graph which shows the compressive-stress distribution around the spot scanning area | region of a laser beam. It is a graph which shows the stress of the sample which carried out the laser peening process. It is a graph which shows the stress of the sample which carried out the laser peening process. It is a top view which shows the laser beam irradiation method to a hole periphery. It is a top view which shows the laser beam irradiation method to a hole periphery. It is a figure which shows the laser beam irradiation method to a groove part. It is a top view which shows the test piece used in the Example of this invention. It is a top view which shows the test piece used in the Example of this invention. It is a top view which shows the test piece used in the Example of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Laser beam oscillator 2 ... Laser beam 3 ... Condensing lens 4 ... Optical window 5 ... Water tank 6 ... Material layer 7 which absorbs a laser beam ... Work material 8 ... Hole

Claims (7)

Scanning the surface with a beam spot obtained by focusing and irradiating the surface of the workpiece with a pulsed laser beam, generating residual compressive stress on the surface, and superimposing the pulsed laser beam on the surface , A laser peening treatment method for applying a residual compressive stress in a specific direction within the surface,
A laser peening treatment method, wherein a load stress is applied when the workpiece is a structural member, and the specific direction is a direction of a maximum principal stress on a surface of the workpiece.   2. The laser peening method according to claim 1, wherein a material layer that absorbs the pulse laser beam is formed on a surface of the workpiece, and the pulse laser beam is superimposed and irradiated. The superimposed irradiation of the pulse laser beam scans the beam spot in a direction orthogonal to the direction of the maximum principal stress on the surface of the workpiece, and the scanning is performed a plurality of times while shifting the position in the direction of the maximum principal stress. The average number of times of superimposition, which is an average value of the number of times of irradiation of the pulse laser beam at the same point on the surface, is 4 times or more, and the average number of times of superimposition depends on the strength of the workpiece, the pulse laser beam 2. The laser peening processing method according to claim 1, wherein the laser peening processing method is determined based on irradiation conditions and a desired compressive stress value. In the laser peening processing method according to claim 3,
Furthermore, before performing the superimposed irradiation of the pulse laser beam, a material layer that absorbs the pulse laser beam is formed on the surface of the workpiece, and an average value of the number of irradiation times of the pulse laser beam at the same point on the surface And the average number of times of superposition is determined based on the strength of the workpiece, the irradiation condition of the pulse laser beam, and a desired compressive stress value. Method.   2. The workpiece has a hole on its surface and is stressed during use, and the direction of the maximum principal stress around the hole is the circumferential direction of the hole. The laser peening processing method according to one of items 1 to 4.   The workpiece has a groove on its surface and is stressed during use, and the direction of the maximum principal stress at the bottom of the groove is a direction perpendicular to the direction along the groove. The laser peening processing method according to claim 1. The metal object manufactured with the laser peening processing method of any one of Claims 1-6.

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