Non-uniform constitutive parameter characterization method of thick plate friction stir welding seam

Abstract

The invention discloses a non-uniform constitutive parameter characterization method for a thick plate friction stir welding seam. The characterization method comprises the following steps: carrying out a metallographic observation test on the cross section of a welding seam, and dividing the whole welding seam entity into a plurality of microscopic characteristic areas according to the grain size and morphology observed by the metallographic observation test; carrying out microhardness testing on the cross section of the welding seam to obtain microhardness distribution of the welding seam in the horizontal and thickness directions; carrying out a uniaxial tensile test on the whole welding seam to obtain tensile force-displacement data of the whole welding seam; establishing a stress constraint equation of the whole welding seam according to a force balance principle aiming at all microscopic characteristic areas in the welding seam; and according to the microhardness distribution rule in each microcosmic characteristic area, solving a stress constraint equation of the welding seam area, and obtaining a stress expression and a non-uniform constitutive parameter of each microcosmic characteristic area. According to the characterization method, accurate characterization of the non-uniform mechanical property of the welding seam can be achieved.

Description

Technical Field

[0001] The invention relates to a method for characterizing the mechanical properties of a friction stir weld, and in particular to a method for characterizing the non-uniform constitutive parameters of a thick plate friction stir weld. Background Art

[0002] Friction stir welding technology is an emerging solid-phase connection technology that uses the friction between the stirring head and the workpiece to heat and plasticize the workpiece, thereby forming a tight weld joint. With the improvement of the load-bearing capacity of aerospace products, the thickness of the workpiece to be welded is also increasing. Compared with thin plates, the heat transfer of thick plates during the friction stir welding process is insufficient, and the performance non-uniformity of the weld along the thickness direction is more significant, which directly affects the overall stress distribution and deformation of the structure, bringing difficulties to the stress and deformation prediction of large-thickness welded structures. Therefore, it is necessary to accurately characterize the non-uniform mechanical properties of thick plate friction stir welds in order to improve the simulation prediction accuracy of large-thickness welded structures.

[0003] In the prior art, the Chinese invention patent (application number 2023105141819) "A method for characterizing constitutive parameters of welds in tailored plates at high temperatures based on virtual field method" discloses the precise characterization of the non-uniform constitutive parameters of the weld surface.

[0004] The current problem is:

[0005] The existing methods cannot obtain the constitutive parameter information inside the weld, especially the non-uniform constitutive parameters along the thickness direction of the weld. Therefore, they are not suitable for welds with thicker thickness. It is necessary to develop a constitutive parameter characterization method for thick friction stir welds. Summary of the invention

[0006] The purpose of the present invention is to provide a method for characterizing the non-uniform constitutive parameters of thick plate friction stir welds, which can achieve accurate characterization of the non-uniform mechanical properties of welds and solve the problem that the internal mechanical properties of thick plate friction stir welds are difficult to accurately characterize.

[0007] In order to achieve the above technical objectives, the present invention adopts the following technical solutions:

[0008] A method for characterizing non-uniform constitutive parameters of a thick plate friction stir weld, the method comprising:

[0009] Carry out metallographic observation test on the cross section of weld, and divide the whole weld entity into several microscopic characteristic areas according to the grain size and morphology observed in the metallographic observation test;

[0010] Carry out microhardness test on the cross section of the weld to obtain the microhardness distribution of the weld along the horizontal and thickness directions;

[0011] Carry out uniaxial tensile test on the entire weld to obtain the tensile force-displacement data of the entire weld;

[0012] For all microscopic characteristic areas in the weld, the stress constraint equation of the weld as a whole is established based on the force balance principle;

[0013] According to the microhardness distribution law in each microscopic characteristic area, the stress constraint equation of the weld area is solved to obtain the stress expression and non-uniform constitutive parameters of each microscopic characteristic area.

[0014] Furthermore, the number of divided microscopic characteristic zones is four, and the four microscopic characteristic zones are respectively a weld nugget zone, a thermo-mechanical affected zone, a heat affected zone and a base metal zone;

[0015] The weld core zone is the direct friction and stirring area of ​​the welding tool head, and the microstructure of the weld core zone is equiaxed crystals with fine size and uniform shape;

[0016] The thermomechanically affected zone has significant plastic deformation, and the grains therein are in a state of thermal growth and bending elongation;

[0017] The heat-affected zone mainly contains coarse recrystallized grains with larger sizes;

[0018] The parent material region mainly comprises horizontally elongated coarse grains.

[0019] Furthermore, the microhardness test is a Vickers hardness test, and the adopted test path includes a horizontal test path and a vertical test path, the horizontal test path is located at the center layer of the weld, and the vertical test path is located at the center axis of the weld.

[0020] Furthermore, the corresponding function expression is selected for characterization according to the microhardness variation data of each microscopic characteristic area;

[0021] The function expressions used include linear function, quadratic function, exponential function and logarithmic function.

[0022] Furthermore, the stretching direction of the uniaxial tensile test is parallel to the direction of the weld.

[0023] Furthermore, the resultant tensile force of all microscopic characteristic areas is equal to the total tensile force of the weld uniaxial tensile specimen. The total tensile force of the weld uniaxial tensile specimen is expressed as:

[0024] F M =∫∫σ N dA N +∫∫σ T dA T +∫∫σ H dA H +∫∫σB dA B ,

[0025] In the formula,

[0026] F M It represents the total tensile force of the weld uniaxial tensile specimen.

[0027] σ N The stress-strain expression for the weld nugget zone is:

[0028] σ T The stress-strain expression for the thermomechanically affected zone is,

[0029] σ H The stress-strain expression for the heat affected zone is,

[0030] σ B Represents the stress-strain expression of the parent material region,

[0031] A M represents the cross-sectional area of ​​the weld uniaxial tensile specimen,

[0032] A N represents the cross-sectional area of ​​the weld nugget zone,

[0033] A T represents the cross-sectional area of ​​the thermomechanically affected zone,

[0034] A H represents the cross-sectional area of ​​the heat affected zone,

[0035] A B Represents the cross-sectional area of ​​the parent material region.

[0036] Further, the stress constraint equation is solved according to the mapping relationship between microhardness and stress;

[0037] The mapping relationship between microhardness and stress includes a linear relationship and a quadratic relationship;

[0038] The non-uniform constitutive parameters include a strength coefficient and a hardening exponent.

[0039] Furthermore, the weld is a friction stir weld.

[0040] Furthermore, the thickness of the weld is greater than 15 mm.

[0041] Compared with the prior art, the characterization method of the present invention has the following beneficial effects:

[0042] 1) In the characterization method of the present invention, the performance changes of the weld in the horizontal and vertical directions are simultaneously introduced into the stress constraint equation, which solves the problem that the internal mechanical properties of the thick plate friction stir weld are difficult to accurately characterize, thereby improving the stress and deformation simulation calculation accuracy of the thick welded structure;

[0043] 2) The non-uniform constitutive parameters of the weld are solved by combining material performance testing with theoretical analysis. In this way, the experimental workload can be reduced as much as possible while ensuring the accuracy of the solution, thereby improving the characterization efficiency of the non-uniform constitutive parameters of the weld. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Figure 1 It is an overall flow chart of the method for characterizing the non-uniform constitutive parameters of the thick plate friction stir weld of the present invention;

[0045] Figure 2 It is a schematic diagram of the division of microscopic characteristic areas in the characterization method of the present invention;

[0046] Figure 3 It is a schematic diagram of the microhardness test path in the characterization method of the present invention;

[0047] Figure 4 is a microhardness distribution diagram along the horizontal direction in the characterization method of the present invention;

[0048] Figure 5 is a microhardness distribution diagram along the vertical direction in the characterization method of the present invention;

[0049] Figure 6 It is a dimension marking diagram of the weld uniaxial tensile specimen in the characterization method of the present invention;

[0050] Figure 7 It is a cloud diagram of the non-uniform constitutive parameter distribution of the weld in the characterization method of the present invention. DETAILED DESCRIPTION

[0051] The present invention will be further described below with specific embodiments:

[0052] This embodiment provides a method for characterizing the non-uniform constitutive parameters of a thick plate friction stir weld, which can achieve accurate characterization of the non-uniform mechanical properties of the weld.

[0053] It should be noted that the welds mentioned in this article are all friction stir welds, that is, welds formed by friction stir welding.

[0054] In this embodiment, the thickness of the weld is more than 15 mm, and there are no obvious welding defects inside the weld.

[0055] See also Figure 1The characterization method of this embodiment includes the following steps S1 to S5.

[0056] S1, a metallographic observation test is carried out on the weld cross section. According to the grain size and morphology observed in the metallographic observation test, the entire weld entity is divided into four solid areas as microscopic characteristic areas.

[0057] The four microscopic characteristic zones are called weld core zone, thermo-mechanical affected zone, heat affected zone and base material zone in order from near to far from the center axis of the weld.

[0058] The weld core zone is a direct friction and stirring area of ​​the welding tool head, and the microstructure of the weld core zone is equiaxed crystals with fine size and uniform shape.

[0059] The thermomechanically affected zone has significant plastic deformation, and the grains therein are in a state of thermal growth and bending elongation.

[0060] The heat affected zone mainly contains coarse recrystallized grains with larger sizes.

[0061] The parent material region mainly comprises horizontally elongated coarse grains.

[0062] S2, conduct microhardness test on the cross section of the weld to obtain the microhardness distribution of the weld along the horizontal and thickness directions, or in other words, understand the microhardness distribution law of the weld.

[0063] It should be noted that the microhardness test is a Vickers hardness test, and the test path used includes a horizontal test path and a vertical test path. The horizontal test path is located at the center layer of the weld, and the vertical test path is located at the center axis of the weld.

[0064] It should be noted that an appropriate function expression is used to characterize the distribution law of weld microhardness.

[0065] Specifically,

[0066] According to the microhardness variation data of each microscopic characteristic area, a suitable function expression is selected for characterization. The function expression used includes but is not limited to linear function, quadratic function, exponential function, logarithmic function, etc.

[0067] S3, a uniaxial tensile test is performed on the entire weld to obtain tensile force-displacement data of the weld uniaxial tensile specimen (or the weld as a whole).

[0068] The weld uniaxial tensile specimen contains the four microscopic characteristic zones mentioned above, namely, the weld nugget zone, the thermomechanical affected zone, the heat affected zone and the parent metal zone.

[0069] The tensile direction of the weld uniaxial tensile specimen is parallel to the direction of the weld, and the size of the weld uniaxial tensile specimen should meet the domestic or international standards for tensile testing of metal materials.

[0070] S4, for all microscopic characteristic areas in the weld, the stress constraint equation of the entire weld is established based on the force balance principle.

[0071] It should be noted that the resultant tensile force of all microscopic characteristic areas is equal to the total tensile force of the entire weld uniaxial tensile specimen. In this way, the total tensile force of the entire weld uniaxial tensile specimen can be expressed as:

[0072] F M =∫∫σ N dA N +∫∫σ T dA T +∫∫σ H dA H +∫∫σ B dA B

[0073] In the formula,

[0074] F M It represents the total tensile force of the weld uniaxial tensile specimen.

[0075] σ N The stress-strain expression for the weld nugget zone is:

[0076] σ T The stress-strain expression for the thermomechanically affected zone is,

[0077] σ H The stress-strain expression for the heat affected zone is,

[0078] σ B Represents the stress-strain expression of the parent material region,

[0079] A M represents the total cross-sectional area of ​​the weld uniaxial tensile specimen,

[0080] A N represents the cross-sectional area of ​​the weld nugget zone,

[0081] A T represents the cross-sectional area of ​​the thermomechanically affected zone,

[0082] A H represents the cross-sectional area of ​​the heat affected zone,

[0083] A B Represents the cross-sectional area of ​​the parent material region.

[0084] S5, according to the microhardness distribution law in each microscopic characteristic area, the stress constraint equation of the entire weld is solved according to the mapping relationship between microhardness and stress to obtain the stress expression and non-uniform constitutive parameters of each microscopic characteristic area.

[0085] It should be noted that the mapping relationship between microhardness and stress is an existing empirical relationship, which includes but is not limited to a linear relationship, a quadratic relationship, and the like.

[0086] In this embodiment, the mapping relationship between microhardness and stress is specifically:

[0087]

[0088] In the formula,

[0089] HV N Indicates the Vickers microhardness of the weld nugget zone,

[0090] HV T represents the Vickers microhardness of the thermomechanically affected zone,

[0091] HV H represents the Vickers microhardness of the heat affected zone,

[0092] HV B Indicates the Vickers microhardness of the base material area.

[0093] It should be noted that the non-uniform constitutive parameters include the strength coefficient K and the hardening exponent n. The strength coefficient and the hardening exponent are data sets related to the position coordinates and vary non-uniformly inside the weld.

[0094] It should be noted that the stress-strain relationship at different positions inside the weld is obtained based on the strength coefficient and hardening index. By writing the stress-strain data into the material parameter module of the commercial finite element software, accurate finite element simulation calculations of thick plate friction stir welding structures can be achieved, thereby improving the deformation prediction accuracy of large welded structures.

[0095] The characterization method of this embodiment has the advantages that:

[0096] 1) In the characterization method of this embodiment, the performance changes of the weld in the horizontal and vertical directions are simultaneously introduced into the stress constraint equation, which solves the problem that the internal mechanical properties of the thick plate friction stir weld are difficult to accurately characterize, thereby improving the stress and deformation simulation calculation accuracy of the thick welded structure;

[0097] 2) The non-uniform constitutive parameters of the weld are solved by combining material performance testing with theoretical analysis. In this way, the experimental workload can be reduced as much as possible while ensuring the accuracy of the solution, thereby improving the characterization efficiency of the non-uniform constitutive parameters of the weld.

[0098] It should be noted that, based on the basic concept of this embodiment, the characterization method of the present invention can be applied to different types of welded materials, welded workpiece geometries and welding process parameters.

[0099] A specific implementation case is provided below, in which the welding object is an aluminum alloy thick plate.

[0100] See also Figure 2 , (corresponding to step S1) metallographic observation is carried out on the cross section of the friction stir weld of the aluminum alloy thick plate, and the weld is divided into four microscopic characteristic zones according to the microscopic grain size and morphology, including weld nugget zone 1, thermomechanical affected zone 2, heat affected zone 3 and base material zone 4.

[0101] The microstructure of the weld nugget zone 1 is equiaxed grains with fine size and uniform shape; the micro grains of the thermomechanical affected zone 2 are in a state of thermal growth and bending elongation; the heat affected zone 3 mainly contains coarse recrystallized grains with larger size; the parent material zone 4 mainly contains coarse grains elongated horizontally.

[0102] like Figure 2 As shown in Figure 2, polynomial functions are used to fit the boundaries of different microscopic features, including β N-Tr , β T-Hr , β H-Br , β N-Ta , β T-Ha , β H-Ba Among them, the subscripts NT, TH and HB refer to the boundary between the weld nugget zone 1 and the thermomechanical affected zone 2, the boundary between the thermomechanical affected zone 2 and the heat affected zone 3, and the boundary between the heat affected zone 3 and the parent metal zone 4, respectively, and the subscripts a and r represent the advancing side and retreating side of the weld, respectively.

[0103] See also Figures 3 to 5 , (corresponding to step S2) Vickers microhardness tests are carried out along the horizontal and vertical directions of the weld respectively to obtain the microhardness distribution law of the thick plate friction stir weld along the horizontal and vertical directions.

[0104] like Figure 3 As shown, the horizontal test path 5 is located at the center layer of the thick plate friction stir weld, and the vertical test path 6 is located at the center axis of the thick plate friction stir weld.

[0105] According to Figure 4 and Figure 5 The microhardness distribution law shown in the figure is selected to fit the microhardness change data of each microscopic characteristic area. Among them, the microhardness HV N It can be expressed as a linear function of the vertical coordinate y and is independent of the horizontal coordinate x; the microhardness HV of the thermomechanical affected zone 2 TIt can be expressed as a linear function of the horizontal coordinate x and is related to the vertical coordinate y; the microhardness HV of the heat affected zone 3 H It can be expressed as a logarithmic function of the horizontal coordinate x and is related to the vertical coordinate y; the microhardness HV of the base material area 4 B is a constant and is independent of the horizontal coordinate x and the vertical coordinate y.

[0106] See also Figure 6 and Figure 7 , (corresponding to step S3) a uniaxial tensile test is carried out to obtain tensile force-displacement data of the uniaxial tensile specimen of the thick plate friction stir weld.

[0107] like Figure 6 and Figure 7 As shown in the figure, a uniaxial tensile specimen of a thick plate friction stir weld is prepared along the weld direction, which contains four microscopic characteristic areas, and the specimen size meets the "GB / T 228.1-2021 Metallic Material Tensile Test Method" for the tensile test of metallic materials. During the uniaxial tensile test, the force-displacement data of the specimen is recorded.

[0108] (Corresponding to step S4) According to the force balance principle, the stress constraint equations of different microscopic characteristic zones of the thick plate friction stir weld are established; the stress constraint equation is the force balance equation of the weld as a whole, including the stress terms of each microscopic characteristic zone, that is, the resultant force of each microscopic characteristic zone is equal to the total tensile force of the weld uniaxial tensile specimen:

[0109]

[0110] Among them, F M is the total tensile force of the uniaxial tensile specimen of the friction stir weld, σ N is the stress-strain relationship of the weld nugget zone 1, σ Ta With σ Tr are the stress-strain relationship of the advancing and retreating sides of the thermomechanical affected zone 2, σ Ha With σ Hr are the stress-strain relationship of the advancing and retreating sides of the heat affected zone 3, σ Ba With σ Br are the stress-strain relationships of the advancing and retreating sides of the parent metal zone 4, respectively, and t is the thickness of the weld uniaxial tensile specimen.

[0111] (Corresponding to step S5) Solve the stress constraint equation according to the mapping relationship between microhardness and stress to obtain the stress expression and non-uniform constitutive parameters of each microscopic characteristic area.

[0112] For aluminum alloys, the mapping relationship between Vickers microhardness and stress can be approximated as an equiproportional relationship:

[0113]

[0114] The Swift constitutive model is used to characterize the stress term. Combining the stress constraint equation and the mapping relationship between Vickers microhardness and stress, the strength coefficient K and hardening index n of different microscopic characteristic areas can be iteratively solved. The stress-strain data of each point in the weld area can be further obtained to improve the finite element simulation calculation accuracy of thick plate welding structures. The solution of the strength coefficient K is shown in Figure 1 shown.

[0115] The above are only preferred embodiments of the present invention and are not intended to limit the protection scope of the present invention. Therefore, any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for characterizing non-uniform constitutive parameters of thick plate friction stir welds, characterized in that: The characterization method comprises: Carry out metallographic observation test on the cross section of weld, and divide the whole weld entity into several microscopic characteristic areas according to the grain size and morphology observed in the metallographic observation test; Carry out microhardness test on the cross section of the weld to obtain the microhardness distribution of the weld along the horizontal and thickness directions; Carry out uniaxial tensile test on the entire weld to obtain the tensile force-displacement data of the entire weld; For all microscopic characteristic areas in the weld, the stress constraint equation of the weld as a whole is established based on the force balance principle; According to the microhardness distribution law in each microscopic characteristic area, the stress constraint equation of the weld area is solved to obtain the stress expression and non-uniform constitutive parameters of each microscopic characteristic area.

2. The method for characterizing non-uniform constitutive parameters of thick plate friction stir weld according to claim 1, characterized in that: There are four micro-feature zones divided, namely, weld nugget zone, thermo-mechanical affected zone, heat affected zone and parent metal zone; The weld core zone is the direct friction and stirring area of ​​the welding tool head, and the microstructure of the weld core zone is equiaxed crystals with fine size and uniform shape; The thermomechanically affected zone has significant plastic deformation, and the grains therein are in a state of thermal growth and bending elongation; The heat-affected zone mainly contains coarse recrystallized grains with larger sizes; The parent material region mainly comprises horizontally elongated coarse grains.

3. The method for characterizing non-uniform constitutive parameters of thick plate friction stir weld according to claim 1, characterized in that: The microhardness test is a Vickers hardness test, and the adopted test path includes a horizontal test path and a vertical test path. The horizontal test path is located at the center layer of the weld, and the vertical test path is located at the center axis of the weld.

4. The method for characterizing non-uniform constitutive parameters of thick plate friction stir weld according to claim 1, characterized in that: According to the microhardness variation data of each microscopic characteristic area, the corresponding function expression is selected for characterization; The function expressions used include linear function, quadratic function, exponential function and logarithmic function.

5. The method for characterizing non-uniform constitutive parameters of thick plate friction stir weld according to claim 1, characterized in that: The tensile direction of the uniaxial tensile test is parallel to the direction of the weld.

6. The method for characterizing non-uniform constitutive parameters of thick plate friction stir weld according to claim 1, characterized in that: The resultant tensile force of all microscopic characteristic areas is equal to the total tensile force of the weld uniaxial tensile specimen. The total tensile force of the weld uniaxial tensile specimen is expressed as: In the formula, F M It represents the total tensile force of the weld uniaxial tensile specimen. σ N The stress-strain expression for the weld nugget zone is: σ T The stress-strain expression for the thermomechanically affected zone is, σ H The stress-strain expression for the heat affected zone is, σ B Represents the stress-strain expression of the parent material region, A M represents the cross-sectional area of ​​the weld uniaxial tensile specimen, A N represents the cross-sectional area of ​​the weld nugget zone, A T represents the cross-sectional area of ​​the thermomechanically affected zone, A H represents the cross-sectional area of ​​the heat affected zone, A B Represents the cross-sectional area of ​​the parent material region.

7. The method for characterizing non-uniform constitutive parameters of thick plate friction stir weld according to claim 1, characterized in that: The stress constraint equation is solved according to the mapping relationship between microhardness and stress; The mapping relationship between microhardness and stress includes a linear relationship and a quadratic relationship; The non-uniform constitutive parameters include a strength coefficient and a hardening exponent.

8. The method for characterizing non-uniform constitutive parameters of thick plate friction stir weld according to claim 1, characterized in that: The weld is a friction stir weld.

9. The method for characterizing non-uniform constitutive parameters of thick plate friction stir weld according to claim 1, characterized in that: The thickness of the weld is greater than 15 mm.

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