Method for judging fracture form of ring-shaped nugget resistance spot welded joint under tensile-shear load

Abstract

The present application relates to a kind of ring-shaped nugget resistance spot welding joint fracture form judging method under tensile shear load, belong to resistance spot welding quality detection field.The welding joint of ring-shaped nugget resistance spot welding is ultrasonically tested, the inner diameter size of ring-shaped nugget, outer diameter size are extracted as characteristic value, and the fracture form of welding joint is judged based on ring-shaped nugget resistance spot welding joint fracture model.Firstly, the fracture form criterion of ring-shaped nugget under tensile shear load is proposed, and the nugget failure of resistance spot welding is fully considered as a complex phenomenon involving the interaction between nugget geometric factor, welding joint metallurgical performance and mechanical loading mode.The ring-shaped nugget model is idealized as the ring-shaped nugget with drum-shaped vertical section, and the numerical relationship between the maximum shear force of ring-shaped nugget horizontal interface and the maximum tension of ring-shaped nugget outer heat affected zone and base material (fracture position) is compared, so that the failure form of ring-shaped nugget is more accurately determined.

Description

Technical Field

[0001] This invention relates to the field of resistance spot welding quality inspection, and particularly to a method for predicting the fracture mode transformation of annular welded resistance spot joints under tensile and shear loads. Specifically, it relates to a method for judging the fracture mode of annular welded resistance spot joints under tensile and shear loads, which can distinguish between weld nugget fracture failure (IF) or pull-out fracture failure (PF) of annular welded resistance spot joints along the workpiece mating surface under tensile and shear loads. Background Technology

[0002] Resistance spot welding (RSW) is a key joining technology in the automotive industry, playing a crucial role in automobile manufacturing. Modifying the failure modes of RSW joints can improve structural load-bearing capacity and energy absorption. The failure modes of RSW can be mainly divided into two types: interfacial fracture failure (IF) and pull-out fracture failure (PF). In IF mode, the crack propagates along the interface between the two plates through the weld nugget, exhibiting brittle failure behavior. Once the weld nugget fails, the joint's load-bearing capacity is completely lost, adversely affecting the vehicle's crashworthiness. In PF mode, the weld nugget is pulled out from a single sheet of metal, and the crack propagates primarily around the nugget. This non-linear crack propagation path, compared to the linear path in IF mode, leads to greater plastic deformation and energy absorption. Therefore, the ductile failure behavior of PF mode is usually the preferred choice in the automotive industry.

[0003] Current methods for determining the failure behavior of resistance spot weld nuggets mainly focus on elliptical or drum-shaped nuggets, lacking suitable criteria for determining the failure behavior of annular nuggets. Furthermore, as shown in equation (13), the nugget failure behavior transformation criteria specified in the American Welding Society and Japanese and German welding standards only consider plate thickness as a single factor. However, the failure of actual spot welds is influenced by multiple factors, including the shape of the nuggets, microstructure, and defects. Therefore, this criterion cannot predict the failure behavior transformation of most steel spot welds. In recent years, scholars such as Pouranvari have further explored the transformation of nugget fracture modes by excluding the influence of the base metal and the yield strength of the nuggets, as shown in equation (14). However, the maximum shear force and maximum tensile force are obtained from the average stress, which does not conform to the sinusoidal distribution of tensile stress around the nuggets proposed by Stephens et al. Moreover, the idealized model of a cylindrical nuggets differs significantly from the actual nuggets, making it unsuitable for further discussion of the failure behavior characteristics of annular nuggets.

[0004]

[0005]

[0006] Where D CWhere is the critical melt nugget diameter, K is a constant of 4 or 5, T is the thickness of the base material, P is the porosity factor, f is a coefficient of 0.5, and H... PFL H represents the hardness at the pull-out fracture failure site. FZ The hardness at the interface fracture failure location is given by D, where D is the diameter of the melt nucleus and τ is the hardness. FZ Shear strength of the melt core, σ PFL Tensile strength at the pull-out fracture failure location. Summary of the Invention

[0007] The purpose of this invention is to provide a method for determining the fracture mode of annular weld nugget resistance spot welds under tensile-shear loads. This method fully considers that weld nugget failure in resistance spot welding is a complex phenomenon involving the interaction between weld nugget geometry, the metallurgical properties of the weld joint, and the mechanical loading method. It addresses the problems of current spot weld nugget failure mode transformation criteria having low universality, inaccuracies caused by significant differences between columnar weld nugget models and actual elliptical or drum-shaped models, the use of a single stress value around the weld nugget leading to excessive calculation errors, and the inability to predict annular weld nugget failure modes.

[0008] The above-mentioned objective of the present invention is achieved through the following technical solution:

[0009] A method for determining the fracture mode of a resistance spot welded joint with an annular fusion core under tensile and shear loads is proposed. The welded joint of the resistance spot welded joint with an annular fusion core is subjected to ultrasonic testing, and the inner and outer diameters of the annular fusion core are extracted as feature values. The fracture mode of the welded joint is determined based on the fracture model of the resistance spot welded joint with an annular fusion core.

[0010] The fracture model of the annular fusion nugget resistance spot welded joint is established as follows: based on the inner diameter, outer diameter, axial thickness on one side of the fusion nugget, Vickers hardness of the joint fusion nugget area, Vickers hardness of the heat-affected zone, and Vickers hardness of the base material, the ultimate shear force F that the horizontal interface of the joint fusion nugget can withstand is calculated. IF The maximum tensile force F that the heat-affected zone of the joint and the base material can withstand. PF And determine the fracture mode based on the following model:

[0011] When F PF >F IF When the interface breaks (IF),

[0012] When F PF ≤F IF At that time, pull-out fracture (PF)

[0013] The method for determining the Vickers hardness of the joint weld nugget area, the Vickers hardness of the heat-affected zone, and the Vickers hardness of the base material is as follows: For the experimental specimens under the selected materials, plate thickness combination, and process parameters, the annular weld nugget resistance spot welding technology is used to cut the obtained welded joint and prepare metallographic specimens. The Vickers hardness of the joint weld nugget area, the Vickers hardness of the heat-affected zone, and the Vickers hardness of the base material are measured. After multiple measurements of multiple specimens, the average value is taken and set as a constant.

[0014] The inner and outer diameters of the annular melt core are calculated as follows: An equivalent circle transformation is performed based on the ultrasonic C-scan detection results; N points are selected on the outer diameter circle of the melt core; any two points are chosen to form a perpendicular bisector, and the intersection of these perpendicular bisectors is the center of the circle. The average value of the resulting cluster of center points is used as the center coordinates. The average distance from the center coordinate point to the N boundary points is the equivalent outer diameter R of the annular melt core. 外 Pick M points on the inner diameter circle of the molten core, and take the average distance between the M points and the center of the circle. This average distance is the equivalent inner diameter R of the annular molten core. 内 ;

[0015] The axial thickness of the weld nugget on one side is measured as follows: Macroscopic images of cross-section metallographic specimens under selected material thickness combinations and process parameters are measured using Image-Pro Plus software to obtain the axial thickness of the weld nugget on one side under these conditions. The average value is then calculated after multiple measurements on multiple specimens. Similarly, welding is performed multiple times under different process conditions with the same material and thickness combination to obtain the average axial thickness of the weld nugget on one side under different process conditions. The axial thickness of the weld nugget on one side under a large number of different process parameter conditions, along with the equivalent inner and outer diameters of the annular weld nugget, are imported into MATLAB for cftool polynomial fitting to obtain a three-parameter function equation: Z(H)=f(R 内 ,R 外 Based on the ultrasound C-scan results, R... 内 ,R 外 Substituting the parameters into the three-parameter function equation, we can obtain the axial thickness of the melt core on one side under the current conditions.

[0016] The annular melt core is idealized as an annular melt core with a drum-shaped cross-section; the radius of curvature of the drum shape is the equivalent outer diameter R of the annular melt core after the equivalent circle transformation. 外 .

[0017] The method for determining the fracture mode of annular fusion nugget resistance spot welded joint under tensile and shear loads comprises the following specific steps:

[0018] (1) Calculate the ultimate shear force that the horizontal interface of the annular melt core can withstand after idealization;

[0019] The shear stress at the horizontal interface x of the melt nucleus is expressed as:

[0020]

[0021] The differential equation for the area at the horizontal interface x of the melt nucleus is:

[0022]

[0023] The differential equation for the shear force that the horizontal interface of the molten core can withstand is:

[0024] dF IF =τ x ·d s

[0025]

[0026]

[0027] In the formula τ x Let τ be the ultimate shear strength at the horizontal interface x of the melt nucleus. max R is the maximum tensile-shear stress that the melt core can withstand when interfacial tearing failure occurs. 内 R is the equivalent inner diameter of the annular melt core. 外 F is the equivalent outer diameter of the annular melt core. IF This is the ultimate shear force that the horizontal interface of the molten nucleus can withstand;

[0028] (2) Calculate the maximum tensile force that the heat-affected zone of the joint and the base material can withstand after the idealization of the annular weld nugget;

[0029] The tensile stress at point α on the horizontal interface side of the annular melt nugget is:

[0030] σ α =σ max1 cosα

[0031] The tensile stress at any point (α, β) outside the annular melt core is:

[0032] σ β =σ max1 cosα·cosβ (4)

[0033] The differential of the area at (α, β) is:

[0034] ds = R α ·dβ·dh (5)

[0035] Among them, R α =R 外 ·cosα

[0036] Therefore, the differential equation for the maximum tension at (α, β) is:

[0037] dF PF1 =σ β·ds·cosβ

[0038] Combining the above equations, we get:

[0039]

[0040] When a spot weld joint fractures during pull-out, the crack continues to propagate along the thickness of the base material after passing through the heat-affected zone outside the drum-shaped weld nugget; the tensile stress at α′ on the outer side of the horizontal interface of the axial thickness of the annular weld nugget after idealization is as follows:

[0041] σ α′ =σ max2

[0042] The tensile stress at any point (α′, β′) in the thickness direction of the outer base material of the annular weld nugget is:

[0043] σ β′ =σ max2 cosβ′ (7)

[0044] The idealized fracture area of ​​the annular melt nugget model on the parent material side is:

[0045]

[0046] Therefore, the differential equation for the maximum tension at (α′, β′) is:

[0047] dF PF2 =σ α′ ·S′·cosβ′dβ′

[0048] Combining the above equations, we get:

[0049]

[0050]

[0051] In the formula σ α σ represents the tensile stress at point α on the outer side of the horizontal interface of the melt nucleus. α′ σ is the tensile stress at α′ outside the horizontal interface of the axial thickness of the single-sided melt core, cosα is the cosine of the angle between the outer side α of the annular melt core and the horizontal interface of the melt core, and cosα′ is the cosine of the angle between the outer side α′ of the horizontal interface of the single-sided melt core and the horizontal interface of the melt core. β Let σ be the tensile stress at any point (α, β) outside the annular melt core. β′Let be the tensile stress at any point (α′, β′) in the thickness direction of the base plate outside the annular weld nugget; h be the vertical height of any point (α, β) outside the annular weld nugget from the horizontal interface of the weld nugget; cosβ be the cosine of the angle between any point (α, β) outside the annular weld nugget and point α; cosβ′ be the cosine of the angle between any point (α′, β′) in the thickness direction of the base plate outside the annular weld nugget and the point where the maximum tensile stress on the horizontal plane is reached; H be the axial thickness of the annular weld nugget on one side; T be the thickness of the base plate being welded; and σ be the tensile stress. max1 σ represents the ultimate tensile strength that the side of the annular melt nugget can withstand when pull-out failure occurs. max2 F represents the ultimate tensile strength in the thickness direction that the base material outside the annular weld nugget can withstand when pull-out fracture occurs. PF1 F is the maximum tensile force that the side of the annular melt core can withstand when pull-out fracture failure occurs. PF2 When pull-out fracture occurs, the maximum tensile force that the outer base material of the annular weld nugget can withstand in the thickness direction is S′, which is the fracture area on the base material side of the idealized model of the annular weld nugget, and R. 内 R is the equivalent inner diameter of the annular melt core. 外 F is the equivalent outer diameter of the annular melt core. PF This refers to the maximum tensile force that the heat-affected zone of the joint and the base material can withstand.

[0052] (3) Optimize the calculation parameters and obtain the relevant criteria for the transformation of the fracture mode of the annular spot weld nugget.

[0053] According to the Tresca failure criterion, the ultimate tensile strength of a material is twice its ultimate shear strength. Furthermore, there is a linear relationship of coefficient B between the ultimate tensile strength and the Vickers hardness V, namely:

[0054]

[0055]

[0056] In the formula σ HAZ σ represents the ultimate tensile strength of the heat-affected zone. FZ σ is the ultimate tensile strength of the fusion nucleus region. BM V represents the ultimate tensile strength of the base material. HAZ Vickers hardness of the heat-affected zone, V FZ V represents the Vickers hardness of the melt nucleus region. BM B1, B2, and B3 are the coefficients representing the Vickers hardness of the base material, and the coefficients representing the linear relationship between the ultimate tensile strength of the material and its Vickers hardness.

[0057] Therefore, the formulas obtained in (1) and (2) can be regarded as related functions of the equivalent inner and outer diameters of the annular melt core, the axial thickness of the melt core on one side, the heat-affected zone inside the annular melt core, the heat-affected zone outside the annular melt core, and the Vickers hardness value of the base material; substituting the above parameters into equations (3), (6), and (9) yields F.IF With F PF The value can be used to determine the failure mode of the annular melt core;

[0058]

[0059] The beneficial effects of this invention are as follows: It proposes for the first time a criterion for the fracture mode of annular weld nuggets under tensile-shear loads, fully considering that weld nugget failure in resistance spot welding is a complex phenomenon involving the interaction between weld nugget geometry, weld joint metallurgical properties, and mechanical loading methods. The influence of parameters such as the inner and outer diameters of the weld nugget, the axial thickness of the weld nugget on one side, the thickness of the base metal plate, and the Vickers hardness of the weld nugget zone, heat-affected zone, and base metal on the fracture mode of annular weld nuggets under tensile-shear loads was investigated. This invention is more accurate than methods that only consider plate thickness as a criterion for weld nugget failure mode discrimination. By idealizing the annular melt core model as an annular melt core with a drum-shaped vertical cross-section, the model assumption is more realistic and accurate than that of a cylindrical melt core. In the process of formula derivation, the linear relationship of shear stress distribution at the horizontal interface of the melt core and the sinusoidal distribution of tensile stress on the side of the melt core are taken into account. By comparing the numerical relationship between the maximum shear force at the horizontal interface of the annular melt core and the maximum tensile force in the heat-affected zone outside the annular melt core and the base material (fracture location), the failure mode of the annular melt core can be determined more accurately. In addition, this formula is also universally applicable to the determination of failure modes of non-annular melt cores, that is, conventional elliptical or drum-shaped melt cores. Attached Figure Description

[0060] The accompanying drawings, which are included to provide a further understanding of the invention and form part of this application, illustrate the invention and are used to explain it, but do not constitute an undue limitation of the invention.

[0061] Figure 1 This is a schematic diagram of an idealized model of the annular melt core with a drum-shaped cross-section according to the present invention;

[0062] Figure 2 This is a schematic diagram showing the horizontal interface dimensions of the annular melt core of the present invention and the shear stress distribution.

[0063] Figure 3 This is a schematic diagram showing the tensile stress distribution and the outer dimensions of the annular melt core of the present invention.

[0064] Figure 4 This is a schematic diagram of the tensile stress distribution at a point (α, β) on the outer side of the annular melt core of the present invention. Detailed Implementation

[0065] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0066] See Figures 1 to 4 As shown, the method for determining the fracture mode of annular welded joints under tensile-shear loads according to the present invention fills the gap in the criteria for predicting the fracture mode transformation of annular welded joints. This invention addresses the shortcomings of existing prediction criteria that do not adequately consider the complex influences of weld nugget geometry, weld joint metallurgical properties, and mechanical loading methods on the fracture mode transformation of the weld nugget. Furthermore, it solves the problem of calculation errors caused by the failure of the idealized weld nugget model to accurately reproduce the weld nugget shape and stress state, thereby more accurately determining the failure mode of the weld nugget. The method includes the following steps:

[0067] Step S1: Perform ultrasonic C-scan on the annular weld nugget resistance spot weld joint, and perform equivalent circle transformation on the obtained results to obtain the equivalent inner and outer diameter dimensions of the annular weld nugget;

[0068] Step S2: By measuring the dimensions of the macroscopic metallographic photographs of the cut sample, the average axial thickness of the melt nugget on one side under the process conditions is obtained (and a three-parameter equation between the axial thickness of the melt nugget on one side and the equivalent inner and outer diameters of the melt nugget is established under different process conditions through a large amount of different experimental data).

[0069] Step S3: Measure the Vickers hardness of the weld nugget zone, the Vickers hardness of the heat-affected zone, and the Vickers hardness of the base material of the cut sample joint. After taking the average value of multiple measurements on multiple samples, set it as a constant.

[0070] Step S4: Idealize the annular weld nugget model obtained by the annular weld nugget resistance spot welding technology into an annular weld nugget with a drum-shaped cross-section.

[0071] Step S5: Using the idealized annular melt core obtained in step S4 as the calculation reference, the ultimate shear force that the annular melt core can withstand at the horizontal interface after idealization is obtained.

[0072] Step S6: Using the idealized annular weld nugget obtained in step S4 as the calculation reference, the maximum tensile force that the heat-affected zone of the joint and the base material can withstand is obtained.

[0073] Step S7: Optimize the calculation parameters, compare the relationship between the maximum shear force and the maximum tensile force obtained in steps S5 and S6, and derive the criterion for the transformation of the fracture mode of the annular spot weld nugget.

[0074] In step S1, ultrasonic C-scan is performed on the annular weld nugget resistance spot welded joint, and the obtained results are transformed into an equivalent circle to obtain the equivalent inner and outer diameters of the annular weld nugget. This includes the following steps:

[0075] Step S101: Perform ultrasonic C-scan on the annular fusion nugget resistance spot weld joint. Analyze the obtained pixels using a cubic polynomial. To equivalently approximate the difference function The image is then processed using a 7×7 median filter and output. The expression is:

[0076]

[0077] Step S102: Perform an equivalent circle transformation on the output image, pick N points on the outer diameter circle of the molten core, and take any two points from the N points to draw a perpendicular bisector. The intersection of the perpendicular bisectors is the center of the circle. Take the average value of the obtained cluster of center points as the center coordinates. The average distance from the center coordinate point to the N boundary points is the equivalent outer diameter R of the annular molten core. 外 Pick M points on the inner diameter circle of the molten core, and take the average distance between the M points and the center of the circle. This average distance is the equivalent inner diameter R of the annular molten core. 内 .

[0078] In step S2, the average axial thickness of the melt nugget on one side under the process conditions is obtained by measuring the dimensions of the macroscopic metallographic photographs of the cut sample (and a three-parameter equation between the axial thickness of the melt nugget on one side and the equivalent inner and outer diameters of the melt nugget is established under different process conditions based on a large amount of different experimental data); this includes the following steps:

[0079] Step S201: The dimensions of the cross-section metallographic specimen under the selected material plate thickness combination and process parameters are measured by using Image-Pro Plus software to obtain the axial thickness of the melt core on one side under the selected material plate thickness combination and process parameters, and the average value is taken after multiple measurements of multiple specimens.

[0080] Step S202: Welding is performed multiple times under different process conditions with the same material and plate thickness combination to obtain the average axial thickness of the weld nugget on one side under different process conditions.

[0081] Step S203: Import the numerical values ​​of the axial thickness of the melt core on one side and the equivalent inner and outer diameters of the annular melt core under a large number of different process parameters into MATLAB for cftool polynomial fitting, and obtain the three-parameter function equation, Z(H)=f(R 内 ,R 外 Based on the ultrasound C-scan results, R... 内 ,R 外 Substituting the parameters into the three-parameter function equation, we can obtain the axial thickness of the melt core on one side under the current conditions.

[0082] In step S3, the Vickers hardness of the weld nugget zone, the Vickers hardness of the heat-affected zone, and the Vickers hardness of the base material are measured in the sectioned sample joint. After multiple measurements on multiple samples, the average value is set as a constant. This includes the following steps:

[0083] Step S301: For the experimental specimens under the selected materials, plate thickness combination and process parameters, the annular fusion core resistance spot welding technology is used to cut the obtained weld joint and prepare metallographic specimens.

[0084] Step S302: Measure the Vickers hardness of the joint weld nugget zone, the Vickers hardness of the heat-affected zone, and the Vickers hardness of the base material. After taking the average value of multiple measurements on multiple samples, set it as a constant.

[0085] In step S4, the annular weld nugget of the resistance spot weld is idealized as an annular model with a drum-shaped cross-section, which includes the following steps:

[0086] Step S401: Draw an annular melt core with a vertical cross-section in the shape of a drum.

[0087] Step S402, mark the equivalent inner diameter of the annular melt core as R. 内 The equivalent outer diameter of the annular melt core is designated as R. 外 The axial thickness of the annular melt core on one side is marked as H, any point outside the annular melt core is marked as (α, β), and the vertical height of any point (α, β) outside the annular melt core from the horizontal interface of the melt core is marked as h.

[0088] In step S5, the maximum shear force that the annular melt core model can withstand at the horizontal interface is calculated, which includes the following steps:

[0089] Step S501, calculate the shear stress at the horizontal interface x of the annular melt nugget:

[0090]

[0091] Step S502, calculate the differential area at the horizontal interface x of the annular melt core:

[0092]

[0093] Step S503, calculate the differential shear force that the horizontal interface of the annular melt core can withstand:

[0094] dF IF =τ x ·d s

[0095] Step S504, calculate the maximum shear force that the horizontal interface of the annular melt nugget can withstand:

[0096]

[0097] In the formula, τ x Let τ be the ultimate shear strength at the horizontal interface x of the melt nucleus. max F is the maximum tensile and shear stress that the melt core can withstand when interfacial tearing failure occurs. IF R is the ultimate shear force that the horizontal interface of the molten nucleus can withstand. 内 R is the equivalent inner diameter of the annular melt core. 外 It is the equivalent outer diameter of the annular melt core.

[0098] In step S6, the calculation of the maximum tensile force that the heat-affected zone of the joint and the base material can withstand includes the following steps:

[0099] Step S601, calculate the tensile stress at the side α of the horizontal interface of the annular melt core:

[0100] σ α =σ max1 cosα

[0101] In the formula σ α For the tensile stress on the outer side of the horizontal interface of the melt nucleus, σ max This represents the ultimate tensile strength that the side of the annular melt core can withstand when pull-out failure occurs.

[0102] Step S602, calculate the tensile stress at any point (α, β) on the outer side of the annular melt core:

[0103] σ β =σ max1 cosα·cosβ (4)

[0104] In the formula, σ β Let be the tensile stress at any point (α, β) outside the annular melt core.

[0105] Step S603, calculate the area differential at (α, β):

[0106] ds = R α ·dβ·dh (5)

[0107] Step S604, calculate the differential of the maximum tension at (α, β):

[0108] dF PF1 =σ β ·ds·cosβ

[0109] Step S605, Replace relevant parameters:

[0110]

[0111] Step S606: Calculate the maximum tensile force that the side of the annular melt core can withstand when pull-out fracture failure occurs.

[0112]

[0113] Step S607: Determine the maximum tensile force that the model can withstand at point α′ on the parent material side after the idealization of the annular melt nugget:

[0114] σ α′ =σ max2

[0115] Step S608: Calculate the tensile stress at any point (α′, β′) in the thickness direction of the base material outside the annular weld nugget:

[0116] σ β′ =σ max2 cosβ′ (7)

[0117] Step S609, calculate the fracture area of ​​the parent material side of the idealized annular melt nugget model:

[0118]

[0119] Step S610, calculate the differential equation for the maximum tension at (α′, β′):

[0120] dF PF2 =σ α′ ·S′·cosβ′dβ′

[0121] Step S611: Calculate the maximum tensile force that the parent material side plate of the idealized annular melt core model can withstand in the thickness direction:

[0122]

[0123] Step S612, determine the tensile force required for the spot weld joint to break during pull-out:

[0124]

[0125] In the formula σ α The tensile stress at point α on the outer side of the horizontal interface of the melt nucleus is σ. α′ σ is the tensile stress at α′ outside the horizontal interface of the axial thickness of the single-sided melt core, cosα is the cosine of the angle between the outer side α of the annular melt core and the horizontal interface of the melt core, and cosα′ is the cosine of the angle between the outer side α′ of the horizontal interface of the axial thickness of the single-sided melt core and the horizontal interface of the melt core. β Let σ be the tensile stress at any point (α, β) outside the annular melt core. β′Let be the tensile stress at any point (α′, β′) in the thickness direction of the base plate outside the annular weld nugget; h be the vertical height of any point (α, β) outside the annular weld nugget from the horizontal interface of the weld nugget; cosβ be the cosine of the angle between any point (α, β) outside the annular weld nugget and point α; cosβ′ be the cosine of the angle between any point (α′, β′) in the thickness direction of the base plate outside the annular weld nugget and the point where the maximum tensile stress on the horizontal plane is reached; H be the axial thickness of the annular weld nugget on one side; T be the thickness of the base plate being welded; and σ be the tensile stress. max1 σ represents the ultimate tensile strength that the side of the annular melt nugget can withstand when pull-out failure occurs. max2 F represents the ultimate tensile strength in the thickness direction that the base material outside the annular weld nugget can withstand when pull-out fracture occurs. PF1 F is the maximum tensile force that the side of the annular melt core can withstand when pull-out fracture failure occurs. PF2 When pull-out fracture occurs, the maximum tensile force that the outer base material of the annular weld nugget can withstand in the thickness direction is S′, which is the fracture area on the base material side of the idealized model of the annular weld nugget, and R. 内 R is the equivalent inner diameter of the annular melt core. 外 F is the equivalent outer diameter of the annular melt core. PF This refers to the maximum tensile force that the heat-affected zone of the joint and the base material can withstand.

[0126] In step S7, the calculation parameters are optimized, and the relationship between the maximum shear force and the maximum tensile force obtained in steps S5 and S6 is compared to derive the criterion for the fracture mode transformation of the annular welded joint. This includes the following steps:

[0127] Step S701: The ultimate tensile strength that the material can withstand is twice its ultimate shear strength, that is:

[0128]

[0129] Step S702: There is a linear relationship with coefficient B between the ultimate tensile strength of the material and its Vickers hardness V, that is:

[0130]

[0131] In the formula σ HAZ σ represents the ultimate tensile strength of the heat-affected zone. FZ σ is the ultimate tensile strength of the fusion nucleus region. BM V represents the ultimate tensile strength of the base material. HAZ Vickers hardness of the heat-affected zone, V FZ V represents the Vickers hardness of the melt nucleus region. BM B1, B2, and B3 are the coefficients representing the Vickers hardness of the base material, and the coefficients representing the linear relationship between the ultimate tensile strength of the material and its Vickers hardness.

[0132] Step S703: Substitute the above parameters into equations (3), (6), and (9) to obtain F.IF With F PF Value, used to determine the failure mode of the annular melt core:

[0133]

[0134] Where IF represents the interface fracture failure mode and PF represents the pull-out fracture failure mode.

[0135] Therefore, the formulas obtained in (3), (6), and (9) can be regarded as related functions of the inner and outer diameters of the annular melt core, the axial thickness of the melt core on one side, and the Vickers hardness values ​​of the heat-affected zone inside the annular melt core, the outer side of the annular melt core, and the base material (fracture location).

[0136] Example:

[0137] To address the low universality of current spot weld nugget failure mode transformation criteria, and the inaccuracy caused by the significant difference between columnar weld nugget models and actual elliptical or drum-shaped models, which fails to predict the failure mode of annular weld nuggets, this invention provides a method for determining the fracture mode of annular resistance spot welded joints under tensile-shear loads. This invention fully considers that the failure of resistance spot welded joints involves the complex interaction between the geometric factors of the weld nugget, the metallurgical properties of the welded joint, and the mechanical loading method, thus providing a more accurate prediction of the failure mode of resistance spot welded joints. The specific process steps are as follows:

[0138] 1) Before welding, the DP600 duplex steel plate is ground to remove the oxide film, then ultrasonically cleaned with acetone solution, and then dried.

[0139] 2) Replace the spot welding electrode cap with a core-insulated electrode cap, start the spot welding equipment, the cooling circulating water flow rate is about 6L / min, and adjust the welding machine press stroke.

[0140] 3) The steel plate thickness can be selected from 0.5mm to 2.5mm. The core insulation size of the core insulating electrode cap can be selected based on the thickness relationship between the two plates. The diameter of the core insulating electrode cap can be selected from 2.4mm to 4.8mm. Adjust the welding parameters of the press and welding machine: Select the welding current and welding time during the welding process to form a weld nugget without welding spatter. Fix the welding time to 12 cycles, the post-weld holding time to 6 cycles, and the electrode pressure to 6kN. Select the welding current as a variable parameter to control the size of the annular weld nugget morphology, and select the welding current range of 7-11kA. Complete the welding.

[0141] 4) Perform ultrasonic C-scan on the annular fusion nugget resistance spot weld joint. Approximate the difference function using a cubic polynomial to obtain the obtained pixels. The image is then processed by a 7×7 median filter and output. An equivalent circular transformation is then performed on the output image to obtain the equivalent inner and outer diameters (R) of the annular melt core. 内 R 外 ).

[0142] 5) Under tensile and shear loads, select plates of the same thickness and place them under the clamping device to prevent additional torque from being generated during equipment operation. The tensile and shear speed is set to 2 mm / min.

[0143] Furthermore, the two sets of samples were cut along the central axis of the annular melt core. The cross-section was then ground, polished, and etched. The obtained cross-section was placed under an optical microscope to measure the axial thickness of the melt core on one side. The hardness of 10 points inside the melt core was measured using a Vickers hardness tester, and the average value H was taken. FZ And the average hardness H at 10 points at the pull-out fracture failure site. PFL Let be a constant.

[0144] Furthermore, by importing the numerical values ​​of the unilateral axial thickness of the melt core and the equivalent inner and outer diameters of the annular melt core under a large number of different process parameters into MATLAB for cftool polynomial fitting, a three-parameter function equation was obtained: Z(H)=f(R 内 ,R 外 This method allows for the non-destructive acquisition of the axial thickness of the melt core on one side.

[0145] Furthermore, substitute the dimensional values ​​obtained from the above steps into the resulting formula:

[0146]

[0147]

[0148]

[0149]

[0150] Therefore:

[0151]

[0152] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the invention by those skilled in the art. Any modifications, equivalent substitutions, or improvements made to the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for determining the fracture mode of annular fusion nugget resistance spot welded joint under tensile and shear load, characterized in that: Ultrasonic testing was performed on the welded joint of the annular fusion nugget resistance spot welding. The inner and outer diameters of the annular fusion nugget were extracted as feature values, and the fracture mode of the welded joint was determined based on the fracture model of the annular fusion nugget resistance spot welding joint. The fracture model of the annular fusion nugget resistance spot welded joint is established as follows: based on the inner diameter, outer diameter, axial thickness on one side of the fusion nugget, Vickers hardness of the joint fusion nugget area, Vickers hardness of the heat-affected zone, and Vickers hardness of the base material, the ultimate shear force that the horizontal interface of the joint fusion nugget can withstand is calculated. The maximum tensile force that the heat-affected zone of the joint and the base material can withstand. And determine the fracture mode based on the following model: when At that time, the interface fractured ( ) when At that time, pull-out fracture ( ) The method for determining the Vickers hardness of the joint weld nugget area, the Vickers hardness of the heat-affected zone, and the Vickers hardness of the base material is as follows: For the experimental specimens under the selected materials, plate thickness combination, and process parameters, the annular weld nugget resistance spot welding technology is used to cut the obtained welded joint and prepare metallographic specimens. The Vickers hardness of the joint weld nugget area, the Vickers hardness of the heat-affected zone, and the Vickers hardness of the base material are measured. After multiple measurements of multiple specimens, the average value is taken and set as a constant. The inner and outer diameters of the annular melt core are calculated as follows: An equivalent circle transformation is performed based on the ultrasonic C-scan detection results; N points are selected on the outer diameter circle of the melt core; any two points are chosen to form a perpendicular bisector, and the intersection of these perpendicular bisectors is the center of the circle. The average value of the resulting cluster of center points is used as the center coordinates. The average distance from the center coordinate point to the N boundary points is the equivalent outer diameter of the annular melt core. Pick M points on the inner diameter circle of the molten core, and take the average distance between the M points and the center of the circle. This average distance is the equivalent inner diameter of the annular molten core. ; The axial thickness of the melt core on one side is measured by using Image-Pro Plus software to measure the size of the macroscopic photograph of the cross-section metallographic sample under the selected material plate thickness combination and process parameters, and the axial thickness of the melt core on one side under the selected material plate thickness combination and process parameters is obtained. The average value is taken after multiple measurements of multiple samples. Similarly, welding was performed multiple times under different process conditions with the same material and plate thickness combination. The average axial thickness of the weld nugget on one side under different process conditions was obtained. The axial thickness of the weld nugget on one side and the equivalent inner and outer diameters of the annular weld nugget under a large number of different process parameters were imported into MATLAB for polynomial fitting using cftool to obtain a three-parameter function equation. Based on the ultrasound C-scan results, Substituting the parameters into the three-parameter function equation, we can obtain the axial thickness of the melt core on one side under the current conditions.

2. The method for determining the fracture mode of annular fusion nugget resistance spot welded joint under tensile and shear load as described in claim 1, characterized in that: The annular melt core is idealized as an annular melt core with a drum-shaped cross-section; the radius of curvature of the drum shape is the equivalent outer diameter of the annular melt core after the equivalent circle transformation. .

3. The method for determining the fracture mode of annular fusion nugget resistance spot welded joint under tensile and shear load as described in claim 1, characterized in that: The specific steps are as follows: (1) Calculate the ultimate shear force that the horizontal interface of the annular melt core can withstand after idealization; The shear stress at the horizontal interface x of the melt nucleus is expressed as: （1）； The differential equation for the area at the horizontal interface x of the melt nucleus is: （2）； The differential equation for the shear force that the horizontal interface of the molten core can withstand is: ； （3）； In the formula Let x be the ultimate shear strength at the horizontal interface of the melt nucleus. This represents the maximum tensile and shear stress that the melt core can withstand when interfacial tearing failure occurs. The equivalent inner diameter of the annular melt core, The equivalent outer diameter of the annular melt nugget. This is the ultimate shear force that the horizontal interface of the molten nucleus can withstand; (2) Calculate the maximum tensile force that the heat-affected zone of the joint and the base material can withstand after the idealization of the annular weld nugget; (3) Optimize the calculation parameters and obtain the relevant criteria for the transformation of the fracture mode of the annular spot weld nugget.

4. The method for determining the fracture mode of annular fusion nugget resistance spot welded joint under tensile and shear load as described in claim 3, characterized in that: Step (2) involves calculating the maximum tensile force that the idealized heat-affected zone of the annular weld nugget and the base material can withstand. Specifically, this means: Horizontal interface side of the ring-shaped melt nugget The tensile stress at the point is: ； Any point outside the ring-shaped melt core The tensile stress at the point is: （4）； exist The differential of the area is: （5）； in, ; Therefore The differential equation for the maximum tension is: ； Combining the above equations, we get: （6）； When a spot welded joint fractures during pull-out, the crack, after passing through the heat-affected zone outside the drum-shaped weld nugget, will continue to propagate along the thickness direction of the base material; after idealizing the annular weld nugget, the outer side of the axial thickness horizontal interface of the single-sided weld nugget... Tensile stress at the point: ； Any point in the thickness direction of the outer base material of the annular melt core The tensile stress at the point is: （7）； The idealized fracture area of ​​the annular melt nugget model on the parent material side is: （8）； Therefore The differential equation for the maximum tension is: ； Combining the above equations, we get: （9）； （10）； In the formula The outer side of the horizontal interface of the molten nucleus Tensile stress, The outer side of the horizontal interface with axial thickness of the single-sided melt nugget Tensile stress, Outside the ring-shaped melt core The cosine value of the angle between the point and the horizontal interface of the molten core. The outer side of the horizontal interface at the axial thickness of the single-sided melt nugget. The cosine of the angle between the weld nugget and the horizontal interface. Any point outside the ring-shaped melt core Tensile stress, For any point in the thickness direction of the outer base material of the annular melt nugget Tensile stress, Any point outside the ring-shaped melt core Vertical height from the horizontal interface of the molten core Any point outside the ring-shaped melt core Place and The cosine of the angle between the two points, For any point in the thickness direction of the outer base material of the annular melt nugget The cosine of the angle between the point and the point where the tensile stress on the horizontal plane is at its maximum is given; H is the axial thickness of the annular weld nugget on one side; and T is the thickness of the base material plate being welded. This represents the ultimate tensile strength that the side surface of the annular melt nugget can withstand in the event of pull-out failure. This represents the ultimate tensile strength in the thickness direction that the base material outside the annular weld nugget can withstand when pull-out fracture occurs. This is the maximum tensile force that the side of the annular melt core can withstand when pull-out fracture failure occurs. The maximum tensile force that the base material outside the annular weld nugget can withstand in the thickness direction when pull-out fracture occurs. The idealized fracture area of ​​the annular melt nugget model is the fracture area on the parent material side. The equivalent inner diameter of the annular melt core, The equivalent outer diameter of the annular melt nugget. This refers to the maximum tensile force that the heat-affected zone of the joint and the base material can withstand.

5. The method for determining the fracture mode of annular fusion nugget resistance spot welded joint under tensile and shear load as described in claim 3, characterized in that: The optimized calculation parameters described in step (3) yield the relevant criterion for the transformation of the fracture mode of the circumferential spot weld nugget. Specifically, according to the Tresca failure criterion, the ultimate tensile strength that the material can withstand is twice the ultimate shear strength. In addition, the ultimate tensile strength of the material is related to the Vickers hardness. There are coefficients between them. The linear relationship, that is: ； （11）； In the formula The ultimate tensile strength of the heat-affected zone. The ultimate tensile strength of the fusion nucleus region. The ultimate tensile strength of the base material. Vickers hardness of the heat-affected zone. The Vickers hardness of the melt nucleus region. Vickers hardness of the base material This is the coefficient representing the linear relationship between the ultimate tensile strength and Vickers hardness of a material; Therefore, the formulas obtained in (1) and (2) can be regarded as relevant functions of the equivalent inner and outer diameters of the annular melt core, the axial thickness of the melt core on one side, the heat-affected zone inside the annular melt core, the heat-affected zone outside the annular melt core, and the Vickers hardness value of the base material; substituting the above parameters into equations (3), (6), and (9) yields the results. and The value can be used to determine the failure mode of the annular melt core; （12）。

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