A method for evaluating the plastic flow of laser-shocked material surface

Abstract

The present application discloses a method for evaluating the plastic flow of the surface layer of a laser shock material, including: based on the parameter values ​​of the laser, numerically simulating the surface layer of the material with a single spot laser shock hardening; based on the simulation, obtaining the displacement deformation parameters under the laser shock hardening parameters, and the displacement data at different depth path nodes in the direction of the center line of the impact area; plotting the displacement data to obtain the depth variation trend of the displacement curve of each part of the material; fitting the displacement curve of each part respectively to obtain the deformation volume value of each part of the material. The present application can extract and analyze the volume plastic flow inside the laser shock hardening material, thereby realizing the analysis of the plastic flow inside the laser shock hardening material, and providing a research method for laser shock hardening combined with volume plastic flow and stress reconstruction, morphology improvement, microevolution, etc.

Description

Technical Field

[0001] The present application belongs to the field of laser processing and manufacturing technology, and specifically relates to a method for evaluating the plastic flow of the surface layer of a laser impact material. Background Art

[0002] Laser shock peening is an advanced surface modification technology. It mainly uses short pulse, high peak power laser to irradiate metal target to induce plasma shock wave, which acts on the surface of the material as a direct force source, and then does not transmit into the material. When the peak pressure of the shock wave exceeds the dynamic yield strength of the material, the metal material undergoes plastic deformation and generates a certain residual compressive stress. Its strain rate can reach 10 6 / s or above, the strain affects the layer depth, which can play a role in regulating the surface quality of the material and improving the surface performance of the material.

[0003] Plastic deformation is the direct basis for laser shock strengthening to reconstruct surface stress, improve surface morphology, and harden surface materials. Its origin is directly related to the attenuation of laser shock wave pressure, dynamic yield strength of materials, plastic strain rate, internal dislocations and microcracks of materials, etc. It is a very complex nonlinear dynamic process. In view of the plastic deformation process of laser shock strengthening, the laser shock strengthening is studied by simulation control test. The depth and diameter of the surface pits obtained by numerical simulation can be compared with the measured data of the test. The depth, size and depth of the residual stress in the laser shock strengthening process and the depth of surface plastic deformation are linearly related to the laser pulse pressure. In terms of surface profile and microscopic defects, a large number of crystal substructure defects such as dislocations, dislocation walls, and dislocation entanglements appear in the surface tissue after laser shock strengthening, which evolve into subgrain boundaries with the continuous action of strain. Combined with the correspondence between the plastic flow of laser shock strengthening and the morphological profile, surface stress changes, and microscopic feature evolution, the volume flow trend and driving law in the process of laser shock plastic deformation can be deeply analyzed. However, the existing experiments cannot extract and analyze the volume plastic flow inside the laser shock strengthened material. Summary of the invention

[0004] The present application provides a method for evaluating the plastic flow of the surface layer of a laser impact material to solve the above-mentioned technical problems.

[0005] In order to solve the above technical problems, a technical solution adopted in this application is: a method for evaluating the plastic flow of the surface layer of a laser impact material, comprising the following steps:

[0006] Based on the laser parameter values, a single-spot laser shock strengthening numerical simulation is performed on the surface of the material;

[0007] Based on simulation, the displacement deformation parameters under laser shock peening parameters and the displacement data of the center line direction of the impact area at different depth path nodes are obtained;

[0008] Plot the displacement data to obtain the depth variation trend of the displacement curve of each part of the material;

[0009] The displacement curves of each part are fitted separately to obtain the deformation volume value of each part of the material.

[0010] Furthermore, the laser parameter values ​​include laser energy, spot diameter and pulse width.

[0011] Furthermore, based on the parameter values ​​of the laser, a method for numerically simulating single-spot laser shock strengthening of the surface of the material includes:

[0012] The laser power density, spot diameter and pulse width are input in sequence to determine the laser equivalent peak pressure;

[0013] According to the sample size, a three-dimensional model of the material sample is established, where the model expression is:

[0014]

[0015] Where ε represents the equivalent plastic strain, is the dimensionless plastic strain rate; A is the initial yield stress; B is the strain hardening modulus; n is the hardening exponent; C is the strain rate hardening parameter;

[0016] The conditions of the model are set and the laser shock peening simulation is carried out.

[0017] Furthermore, based on the simulation, a method for obtaining displacement deformation parameters under laser shock peening parameters and displacement data of the center line direction of the impact area at different depth path nodes includes:

[0018] Based on the laser shock peening simulation, the laser shock displacement cloud map is obtained;

[0019] Based on the laser shock displacement cloud map, the displacement data of the center line direction of the impact area at different depth path nodes are obtained.

[0020] Furthermore, condition settings include meshing, material section property definition, model assembly, analysis step settings, loads, and constraints.

[0021] Further, the method of plotting the displacement data to obtain the depth variation trend of the displacement curve of each part of the material includes:

[0022] Based on the displacement curves of various parts of the material changing with depth, the cross-sectional changes of plastic flow are obtained;

[0023] Among them, the plastic flow changes on the material surface include surface pits and surface protrusions.

[0024] Furthermore, based on the laser shock displacement cloud map, a method for obtaining displacement data of the center line direction of the impact area at different depth path nodes includes:

[0025] Based on the displacement curves of various parts of the material and the displacement curve of the largest surface pit, the depth of the maximum convex displacement inside the material is obtained and the displacement data is extracted.

[0026] Furthermore, the method of fitting the displacement curves of each part to obtain the deformation volume value of each part of the material includes:

[0027] Divide each deformation area according to the displacement curve;

[0028] The corresponding rotation volume is calculated for the fitting curve of each deformation area to obtain the plastic flow volume of each protrusion or the plastic flow volume of the pit.

[0029] The beneficial effect of the present application is that the present application can extract and analyze the volume plastic flow conditions inside the laser shock strengthened material, thereby realizing the analysis of the internal plastic flow conditions of the laser shock strengthened material, and providing a research method for laser shock strengthening combined with volume plastic flow and stress reconstruction, morphology improvement, microevolution, etc. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 It is a schematic flow chart of an embodiment of a method for evaluating the plastic flow of a surface layer of a laser impact material of the present application;

[0031] Figure 2 The surface profile curves of different laser power densities of this application;

[0032] Figure 3 The residual stress distribution corresponding to different laser power densities of this application;

[0033] Figure 4 The centerline position of the laser shock simulation of the first embodiment of the method for evaluating the plastic flow of the surface layer of the laser shock material of the present application;

[0034] Figure 5 Deformation displacement curves at different depths of an embodiment of a method for evaluating the plastic flow of a surface layer of a laser impact material of the present application;

[0035] Figure 6 A schematic diagram of a surface displacement fitting curve of an embodiment of a method for evaluating the plastic flow of a surface layer of a laser impact material according to the present application;

[0036] Figure 7 A schematic diagram of an internal displacement fitting curve of an embodiment of a method for evaluating the surface plastic flow of a laser shocked material according to the present application. DETAILED DESCRIPTION

[0037] The technical solutions in the embodiments of the present application will be clearly and completely described below in conjunction with the drawings in the embodiments of the present application.

[0038] See also Figure 1 , Figure 1 1 is a flow chart of an embodiment of a method for evaluating the plastic flow of a surface layer of a laser-impacted material according to the present invention. The method comprises the following steps:

[0039] Step S1. Based on the parameter values ​​of the laser, a single-spot laser shock strengthening numerical simulation is performed on the surface of the material.

[0040] Specifically, the parameter values ​​of the laser shock are confirmed, including the laser energy, spot diameter and pulse width, and the laser equivalent peak pressure is confirmed.

[0041] The specific method of step S1 includes:

[0042] Step S11. The laser power density, spot diameter and pulse width are input in sequence to determine the laser equivalent peak pressure.

[0043] Specifically, a numerical simulation of single-spot laser shock strengthening was carried out in ABAQUS.

[0044] Step S12. Establish a three-dimensional model of the material sample according to the sample size, wherein the model expression is:

[0045]

[0046] Where ε represents the equivalent plastic strain, is the dimensionless plastic strain rate; A is the initial yield stress; B is the strain hardening modulus; n is the hardening exponent; and C is the strain rate hardening parameter.

[0047] Step S13: Setting conditions for the model and performing laser shock peening simulation.

[0048] Specifically, the model is meshed, material cross-section properties are defined, the model is assembled, analysis steps are set, loads and constraints are set, and other conditions are set to perform laser shock strengthening simulation.

[0049] Step S2. Based on the simulation, the displacement deformation parameters under the laser shock strengthening parameters and the displacement data of the center line direction of the impact area at different depth path nodes are obtained.

[0050] Specifically, the specific method of step S2 includes:

[0051] Step S21. Based on the laser shock peening simulation, a laser shock displacement cloud map is obtained.

[0052] Step S22. Based on the laser shock displacement cloud map, the displacement data of the center line direction of the impact area at different depth path nodes are obtained.

[0053] Specifically, according to the completed laser shock peening simulation, a laser shock displacement cloud map is obtained, and displacement data at different depth path nodes in the direction of the center line of the impact area are extracted.

[0054] Step S3: Plot the displacement data to obtain the depth variation trend of the displacement curve of each part of the material.

[0055] Specifically, the displacement data extracted at different depths were plotted using Origin software to obtain the changes in plastic flow under the influence of laser shock in each part. The changing trend of the displacement curve with depth was confirmed, and each plastic flow area was divided.

[0056] The plastic changes on the material surface are divided into two parts: surface pits and surface protrusions. The relative changes of the internal displacement curves at various depths are superimposed, and the maximum protrusion displacement inside the material is taken as the result of the internal protrusion deformation superposition.

[0057] Extract the displacement curve of each plastic flow part. The maximum pit displacement curve on the surface determines the depth of the maximum convex displacement inside the material and extracts the displacement data.

[0058] Step S4. Fit the displacement curves of each part respectively to obtain the deformation volume value of each part of the material.

[0059] Specifically, the displacement curves of each part are fitted respectively using Matlab software, and the displacement curve fitting equation is substituted into the corresponding rotation volume calculation formula to calculate the deformation volume value of each part.

[0060] Example 1

[0061] The following is combined with Figure 2-5 The specific implementation methods of this application are described in detail with specific implementation cases.

[0062] The numerical simulation of the single-spot laser shock processing includes the following steps:

[0063] 1.1 Establish geometric model and define material properties: geometric dimensions are 30mm*30mm*5mm, material density is 4500kg / m 3 , Poisson's ratio 0.342, elastic modulus 110GPa. Johnson-Cook simulation is used to describe the constitutive relationship of TC4 titanium alloy. The expression of this model is: where ε represents the equivalent plastic strain, is the dimensionless plastic strain rate; A is the initial yield stress; B is the strain hardening modulus; n is the hardening exponent; C is the strain rate hardening parameter. In this paper, the above parameters are taken as A = 950.228MPa, B = 603.382MPa, n = 0.1992, C = 0.0198;

[0064] 1.2 Define material interface properties and assemble the model.

[0065] 1.3 Set the explicit analysis step: According to the laser time history curve, the peak value is reached at 20ns, and the -6 s, its kinetic energy and plastic response no longer change. In this paper, the analysis step is set to 4×10 -6 s.

[0066] 1.4 Meshing: A mesh size of 0.08 mm × 0.08 mm × 0.08 mm was used in the laser shock strengthening area and the surrounding parts, and 0.15 mm × 0.15 mm × 0.08 mm was used in other parts. The C3D8R mesh type was used to mesh the model with 1,237,700 meshes. Full constraints were set at the bottom of the model during the analysis.

[0067] 1.5 Applied load: laser rate density is 3.02GW / cm 2 According to the Fabbro formula, the amplitude pressure is 4000MPa. A circular spot with a spot diameter of 2.5mm is used. The expression of the model is: where σ m is the peak pressure of the shock wave; I0 is the laser power density; α is the coefficient of the internal energy converted into thermal energy, usually α = 0.2 ~ 0.5; Z is the reduced acoustic impedance, its value is determined by the shock wave acoustic impedance between the target material and the water confinement layer, and the calculation formula is: Where Z1 and Z2 are the impedances of the material and the constrained layer, respectively, and z1 = 2.74 × 10 6 g / (cm 2 ·s), z2=1.5×10 6 g / (cm 2 ·s).

[0068] 1.6 Submit analysis work and post-processing: Complete finite element analysis and obtain numerical simulation results of laser shock peening, including stress, strain, displacement, etc.

[0069] 2. Use Origin to collect and process numerical simulation data, draw the extracted displacement data at different depths, and confirm the changing trend of the displacement curve with the change of depth.

[0070] 3. Divide each plastic flow area, extract the maximum surface pit displacement curve on the surface of the material, and divide it into two parts, surface pits and surface protrusions (around the light spot). Extract the internal maximum protrusion displacement curve as the cross-sectional curve of the internal protrusion deformation.

[0071] 4. Use Matlab software to fit the displacement curves of each part separately, and divide the surface pits and internal convex deformation fitting into Parabola fitting curves: y = a + b*x + c*x 2 , ExpMod2 fitting curve: y = exp(ab*x) in two parts.

[0072] 5. Substitute the fitting equation into the rotation volume formula to obtain the plastic deformation volume of each part. The rotation volume calculation formula is: Specifically, in this embodiment, the plastic deformation volume of each part corresponding to each power density is as shown in the following Table 1:

[0073] Table 1 Calculation table of plastic deformation volume of each part at different power densities

[0074]

[0075] The above description is only an embodiment of the present application and does not limit the patent scope of the present application. Any equivalent structure or equivalent process transformation made using the contents of the present application specification and drawings, or directly or indirectly used in other related technical fields, are also included in the patent protection scope of the present application.

Claims

1. A method for evaluating the plastic flow of the surface layer of a laser-shocked material, characterized in that: The following steps are involved: Step S1. Based on the parameter values ​​of the laser, a single-spot laser shock processing numerical simulation is performed on the surface of the material; Step S2. Based on the simulation, obtain the displacement deformation parameters under the laser shock peening parameters, and the displacement data of the center line direction of the impact area at different depth path nodes; Step S3. Plotting the displacement data to obtain the depth variation trend of the displacement curve of each part of the material; Step S4. Use Matlab software to fit the displacement curves of each part separately, and divide the surface pits and internal convex deformation fitting into Parabola fitting curves: y = a + b*x + c*x 2 And ExpMod2 fitting curve: y = exp (ab * x) two parts, the fitting equations are substituted into the rotation volume formula to obtain the deformation volume value of each part of the material, the rotation volume calculation formula is: The step S2 specifically includes: Step S21. Based on the laser shock peening simulation, a laser shock displacement cloud map is obtained; Step S22. Based on the laser shock displacement cloud map, the displacement data of the center line direction of the impact area at different depth path nodes are obtained; Wherein, the step S22 specifically includes obtaining the depth of the maximum convex displacement inside the material and extracting displacement data based on the displacement curves of each part of the material and the displacement curve of the largest surface pit; The step S3 specifically includes: Based on the displacement curves of various parts of the material changing with depth, obtaining the cross-sectional changes of the plastic flow, wherein the plastic flow changes on the surface of the material include surface pits and surface protrusions; The step S4 specifically includes: Dividing each deformation area according to the displacement curve; The corresponding rotation volume is calculated for the fitting curves of each deformation area to obtain the plastic flow volume of each part of the protrusion or the plastic flow volume of the pit.

2. The method according to claim 1, characterized in that The parameter values ​​of the laser include laser energy, spot diameter and pulse width.

3. The method according to claim 2, characterized in that The method for numerically simulating single-spot laser shock strengthening of the surface layer of a material based on the laser parameter value includes: The laser power density, the spot diameter and the pulse width are input in sequence to determine the laser equivalent peak pressure; According to the sample size, a three-dimensional model of the sample of the material is established, wherein the model expression is: Where ε represents the equivalent plastic strain, is the dimensionless plastic strain rate; A is the initial yield stress; B is the strain hardening modulus; n is the hardening exponent; C is the strain rate hardening parameter; Conditions are set for the model to perform laser shock peening simulation.

4. The method according to claim 3, characterized in that The condition setting includes mesh division, material section property definition, model assembly, analysis step setting, load and constraint conditions.

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