A method for predicting the fracture mode of spot welding of peeled samples

Abstract

The present invention discloses a method for predicting the fracture mode of a stripped sample spot welding, comprising the following steps: Step 1: obtain a plurality of stripped samples by resistance spot welding, measure the diameter of the nugget of the stripped samples, and perform a tensile test on the stripped samples; Step 2: select stress triaxiality as a constraint parameter characterizing the stress state of the spot welding nugget, and calculate the stress triaxiality of the stripped sample; Step 3: fit a fracture mode prediction curve based on the measured nugget diameter and stress triaxiality as a fracture mode criterion. The present invention can be applied to the fracture mode prediction of the stripped samples, and can improve the accuracy and reliability of the fracture mode prediction.

Description

Technical Field

[0001] The present invention belongs to the technical field of welding fracture prediction, and specifically relates to a method for predicting the fracture mode of a peeled sample spot weld. Background Art

[0002] At present, technologies such as self-piercing riveting, stamping connection, hot-melt self-tapping, and laser brazing that can be used to connect aluminum and steel are limited by equipment costs and connection effects and cannot be used for large-scale production. Resistance spot welding, as the main connection technology in the automotive industry, is widely used by automakers for its advantages such as low cost, high efficiency, and high robustness. However, brittle intermetallic compounds are inevitably formed at the welding interface between aluminum alloys and steel plates, which brings challenges to the application of resistance spot welding technology in the connection of dissimilar materials between aluminum and steel. For aluminum and steel resistance spot welded joints that meet the actual application strength, when the weld is overloaded, different forms of fracture will occur, namely interface fracture and pull-out fracture.

[0003] At present, the critical nugget diameter concept method is widely used in academia and industry to predict the fracture mode of resistance spot welded joints under tensile load. However, the above method is only applicable to specimens with a single load, such as tensile shear, and is not applicable to specimens with complex loads). For example, the stress form of peeling specimens is very common in the frame structure, such as the connection structure between the door frame and the bottom plate. The critical nugget direct method is not applicable to the prediction of the fracture mode of peeling specimens. This is reflected in the fact that when the nugget diameter of the peeling specimen is smaller than the critical nugget diameter, the fracture mode is still not a pull-out fracture, but an interface fracture. However, in actual welding, it is difficult to weld a small nugget diameter while ensuring that the post-weld strength meets the use requirements. In addition, there is currently no fracture mode prediction method for such aluminum and steel peeling specimens. Summary of the invention

[0004] The purpose of the present invention is to provide a method for predicting the fracture mode of a peeled sample spot weld. The present invention can be applied to the fracture mode prediction of a peeled sample and can improve the accuracy and reliability of the fracture mode prediction.

[0005] The technical solution of the present invention is a method for predicting the fracture mode of a peeled sample spot weld, comprising the following steps:

[0006] Step 1: obtain several peeling samples by resistance spot welding, measure the nugget diameter of the peeling samples, and perform tensile tests on the peeling samples;

[0007] Step 2: Select stress triaxiality as the constraint parameter to characterize the stress state of the spot welding nugget, and calculate the stress triaxiality of the peeled sample;

[0008] Step 3: Based on the measured nugget diameter and stress triaxiality, a fracture mode prediction curve is fitted as a fracture mode criterion.

[0009] In the aforementioned method for predicting the fracture mode of the peeled sample spot welding, in step 1, resistance spot welding between different steel plate thicknesses and different steel plate materials is used to obtain a peeled sample with a small nugget diameter.

[0010] In the aforementioned method for predicting the spot welding fracture mode of the peeled sample, in step one, the welding pressure is 3500N-4000N, the pressurization time is 1000ms-1500ms, the preheating time is 20ms-40ms, the preheating current is 6kA-8kA, followed by cooling for 10ms-30ms, the root mean square value of the welding current is 9kA-11kA, the total welding time is 1500ms-1700ms, and the holding time after welding is 200ms-300ms.

[0011] The aforementioned method for predicting the spot welding fracture mode of the peeled sample has a welding pressure of 3900N, a pressurization time of 1500ms, a preheating time of 20ms, a preheating current of 7.5kA, followed by cooling for 10ms, a welding current root mean square value of 10.9kA, a total welding time of 1615ms, and a post-weld holding time of 250ms.

[0012] In the aforementioned method for predicting the fracture mode of the spot weld of the peeled sample, in step 1, the peeled sample is subjected to tensile testing and displacement loading, the tensile rate is 1-3 mm / min, and the fracture mode of the peeled sample after tensile fracture is statistically analyzed.

[0013] In the aforementioned method for predicting the fracture mode of spot welding of peeled specimens, the stress triaxiality calculation formula is as follows:

[0014]

[0015] Among them, σ m is the hydrostatic pressure, σ e is the VonMises equivalent stress, σ1, σ2, σ3 are the stress components in the XYZ directions respectively.

[0016] In the aforementioned method for predicting the fracture mode of spot welding of peeled specimens, the stress triaxiality is used as the abscissa, the nugget diameter is used as the ordinate, and the stress triaxiality and the nugget diameter of the peeled specimen are used as data points to obtain a curve graph.

[0017] The above-mentioned method for predicting the fracture mode of the spot welding of the peeled sample is to fit the formula for the fracture mode prediction curve as follows:

[0018] D=6.469S -1.041 ;

[0019] Where D is the diameter of the weld core and S is the stress triaxiality.

[0020] In the aforementioned method for predicting the fracture mode of spot welding of peeled samples, the fracture mode criterion is that when the data point is located at the upper right of the fracture mode prediction curve, it is a pull-out fracture mode, and when the data point is located at the lower left of the fracture mode prediction curve, it is an interface fracture.

[0021] Compared with the prior art, the present invention has the following beneficial effects:

[0022] 1. The present invention measures the diameter of the weld nugget and calculates the stress triaxiality, and combines it with finite element analysis. This method can more accurately predict the fracture mode of the welding sample, which helps to identify potential welding defects in advance in actual production and reduce uncertainty and scrap rate in the production process.

[0023] 2. The present invention calculates stress triaxiality and divides material properties, and deeply considers the stress distribution and material characteristics inside the welding sample, which helps to understand the fracture mechanism more comprehensively and provides a more solid data basis for the prediction of fracture mode.

[0024] 3. The present invention uses stress triaxiality and nugget diameter as data points to draw a curve graph. This method provides an intuitive fracture mode prediction curve, facilitates analysis and identification of characteristics of different fracture modes, and further improves the intuitiveness and interpretability of the prediction. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 The influence of stress triaxiality and nugget diameter on the tensile fracture mode of the peeled sample of the present invention;

[0026] Figure 2 The present invention provides a fracture mode prediction method based on stress triaxiality and nugget diameter and verification results;

[0027] Figure 3 It is a schematic diagram of the micro-region of the present invention;

[0028] Figure 4 , Figure 5 Schematic diagram of the micro shearing sample of the present invention. DETAILED DESCRIPTION

[0029] The present invention is further described below in conjunction with the accompanying drawings and embodiments, but they are not intended to limit the present invention.

[0030] Embodiment: A method for predicting a spot welding fracture mode of a peeled sample comprises the following steps:

[0031] Step 1: obtain several peeling samples by resistance spot welding, measure the nugget diameter of the peeling samples, and perform tensile tests on the peeling samples;

[0032] On the basis of ensuring the performance of the sample, the stripping samples with small nugget diameter were welded as much as possible. Different steel plate thicknesses and different steel plate materials were resistance spot welded to obtain stripping samples with various nugget diameters, such as aluminum and steel stripping samples with large nugget diameter (8.6mm) and small nugget diameter (6.7mm). The welding pressure was 3900N, the pressurization time was 1500ms, the preheating time was 20ms, the preheating current was 7.5kA, and then cooled for 10ms. The root mean square value of the welding current was 10.9kA, the total welding time was 1615ms, and the post-welding holding time was 250ms. The nugget diameter corresponding to each stripping sample was measured, and the stripping samples of each welding combination were subjected to tensile testing on the MTS mechanical testing system, with displacement loading and a tensile rate of 2mm / min. The fracture mode of the weld after tensile fracture was statistically analyzed and listed in Table 1.

[0033] Table 1 Nugget diameter and tensile fracture mode of peeled specimens with different welding combinations

[0034] Stripping sample welding plate assembly Nugget diameter(mm) Fracture mode 1.2mmAA6022-2.0mmLCS 8.6 Pull out the fracture 1.2mmAA6022-2.0mmLCS 8.0 Pull out the fracture 1.2mmAA6022-2.0mmLCS 6.7 Interface fracture 1.2mmAA6022-1.2mmLCS 8.2 Pull out the fracture 1.2mmAA6022-0.9mmLCS 7.9 Interface fracture 1.2mmAA6022-1.2mmHSLA 7.8 Pull out the fracture 1.2mmAA6022-2.0mmHSLA 7.1 Interface fracture

[0035] Step 2: Select stress triaxiality as the constraint parameter to characterize the stress state of the spot welding nugget, and calculate the stress triaxiality of the peeled sample;

[0036] The stress triaxiality is selected as the constraint parameter to characterize the stress state of the weld core. The stress triaxiality of each peeled sample in Table 1 is calculated, and the double-parameter relationship is obtained by combining the diameter of the weld core. The calculation formula of the stress triaxiality is as follows:

[0037]

[0038] Among them, σ m is the hydrostatic pressure, σ e is the Von Mises equivalent stress, σ1, σ2, and σ3 are stress components in the XYZ direction respectively. The above stress components can be calculated by the finite element software Abaqus. Affected by temperature, the structure of the peeled specimen after welding is divided into three micro-zones: nugget, heat-affected zone and parent material. The mechanical properties of the materials in the above zones are very different. Previous finite element methods did not consider the mechanical properties of the materials in the above micro-zones, resulting in inaccurate calculated stress results. During this calculation process, the partitioning function of the software is used during modeling to divide the welded structure into micro-zones, and divide the nugget, heat-affected zone and parent material into three zones (such as Figure 3 As shown in the figure), material properties and stress-strain data are assigned to the materials in the above regions, and each stress component is calculated and substituted into the formula to obtain the stress triaxiality.

[0039] The mechanical properties of the above-mentioned micro-area materials were measured using newly developed micro-shear specimens, such as Figure 5 shown. Figure 5Based on the original welding sample, a shear zone is processed in the heat affected zone and the nugget. The length of the shear zone is 4mm-6mm, the width is 1mm-1.5mm, and the thickness is 0.5mm-1mm. Tensile loading is performed on the MTS with a loading rate of 0.2mm / min. At the same time, the strain of the shear zone is measured with the help of digital image correlation technology to obtain stress-strain curve data, which is input into the micro-area material properties of the finite element model.

[0040] Step 3: Based on the measured nugget diameter and stress triaxiality, a fracture mode prediction curve is fitted as a fracture mode criterion.

[0041] The calculated stress triaxiality and core diameter distribution of each peeled sample are used as the horizontal and vertical coordinate parameters, and the stress triaxiality and core diameter of the peeled sample are used as data points to obtain Figure 1 In the curve graph shown, the square shape represents the pull-out fracture mode, and the circle represents the interface fracture mode. After fitting the data points, an obvious trend is found, that is, the data points of the pull-out fracture are located in the upper right of the fitted fracture mode prediction curve, and the data points of the interface fracture are located in the lower left of the fitted fracture mode prediction curve. Therefore, the middle is selected as the fracture mode criterion, which is represented by the thick solid line. The formula for the fracture mode prediction curve obtained by fitting is:

[0042] D=6.469S -1.041 ;

[0043] Where D is the diameter of the weld core and S is the stress triaxiality.

[0044] The present invention measures the diameter of the weld nugget and calculates the stress triaxiality, and combines it with finite element analysis that considers the micro-region performance of the material after welding. This method can more accurately predict the fracture mode of the welding sample, which helps to identify potential welding defects in advance in actual production and reduce uncertainty and scrap rates in the production process. The present invention calculates the stress triaxiality and divides the material properties, and deeply considers the stress distribution and material characteristics inside the welding sample, which helps to more comprehensively understand the fracture mechanism and provide a more solid data foundation for the prediction of the fracture mode. The present invention uses the stress triaxiality and the weld nugget diameter as data points to draw a curve graph. The method provides an intuitive fracture mode prediction curve, which is convenient for analyzing and identifying the characteristics of different fracture modes, and further improves the intuitiveness and interpretability of the prediction.

[0045] In order to verify the above method, a set of 1.2mm AA6022 aluminum alloy and 1.2mm low carbon steel plates were selected for resistance spot welding. The nugget diameter of the weld point is 8.2mm. The stress triaxiality calculated by the finite element method is 1.02, and the fracture mode is pull-out fracture. The nugget diameter and stress triaxiality of this combination are plotted on Figure 2It is found that the point is located at the upper right of the line corresponding to Formula 2, which should be a pull-out fracture, and is consistent with the test results.

[0046] In summary, the present invention can be applied to the prediction of fracture modes of peeled samples, and can improve the accuracy and reliability of fracture mode prediction.

Claims

1. A method for predicting the fracture mode of a peeled sample spot weld, characterized in that: The following steps are involved: Step 1: obtain multiple peeling samples by resistance spot welding, measure the nugget diameters of the peeling samples, and perform tensile tests on the peeling samples; Step 2: Select stress triaxiality as the constraint parameter to characterize the stress state of the spot welding nugget, and calculate the stress triaxiality of the peeled sample; Step 3: Based on the measured nugget diameter and stress triaxiality, a fracture mode prediction curve is fitted as a fracture mode criterion; The stress triaxiality calculation formula is as follows: Among them, σ m is the hydrostatic pressure, σ e is the VonMises equivalent stress, σ1, σ2, and σ3 are the stress components in the XYZ directions respectively; The stress triaxiality is used as the abscissa, the nugget diameter is used as the ordinate, and the stress triaxiality and the nugget diameter of the peeled sample are used as data points to obtain a curve graph; The formula for fitting the fracture mode prediction curve is: D=6.469σ* -1.041 ; Where D is the diameter of the weld core, σ * is the stress triaxiality; The fracture mode criterion is that when the data point is located at the upper right of the fracture mode prediction curve, it is a pull-out fracture mode, and when the data point is located at the lower left of the fracture mode prediction curve, it is an interface fracture.

2. The method for predicting the fracture mode of a peeled sample spot weld according to claim 1, characterized in that: In step 1, resistance spot welding is performed between different steel plate thicknesses and different steel plate materials to obtain a stripping sample with a small nugget diameter.

3. The method for predicting the fracture mode of a peeled sample spot weld according to claim 2, characterized in that: In step one, the welding pressure of the stripped sample obtained by resistance spot welding is 3500N-4000N, the pressurization time is 1000ms-1500ms, the preheating time is 20ms-40ms, the preheating current is 6kA-8kA, and then cooling is 10ms-30ms. The root mean square value of the welding current is 9kA-11kA, the total welding time is 1500ms-1700ms, and the holding time after welding is 200ms-300ms.

4. The method for predicting the fracture mode of a peeled sample spot weld according to claim 3, characterized in that: The welding pressure is 3900N, the pressurization time is 1500ms, the preheating time is 20ms, the preheating current is 7.5kA, followed by cooling for 10ms, the root mean square value of the welding current is 10.9kA, the total welding time is 1615ms, and the holding time after welding is 250ms.

5. The method for predicting the fracture mode of a peeled sample spot weld according to claim 2, characterized in that: In step 1, the peeled sample is subjected to tensile testing and displacement loading at a tensile rate of 1-3 mm / min, and the fracture mode of the peeled sample after tensile fracture is statistically analyzed.

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