Three-layer steel plate spot welding process method

Abstract

The invention relates to the technical field of resistance spot welding, in particular to a three-layer steel plate spot welding process method which comprises the steps that three layers of stacked steel plates are placed between an upper electrode and a lower electrode; applying a pressure P of 3-6 kN to the electrode; a pulse spot welding process is adopted for welding, first pulse energy Q1, second pulse energy Q2, third pulse energy Q3 and fourth pulse energy Q4 are sequentially applied, and the pulse interval time is 25-35 ms; wherein, Q1lt; q2lt; Q2lt; q3lt; q4, and Q4 is greater than or equal to 1.5 Q3; the planar electrode cap and the spherical electrode cap are combined to form a pyramid-shaped contact condition, so that the resistance heat distribution of a welding area shows the gradient characteristic that the IF steel side is high and the hot forming steel side is low. The method has the advantages that gradual and controllable growth of nuggets is achieved, the defects of splashing, incomplete fusion or overburning and the like possibly generated by a single-pulse or simple double-pulse process are avoided, the finally obtained welding spot nuggets are full in size, interface bonding is firm, the process repeatability is good, and the strict automobile body welding quality standard can be met.

Description

Technical Field

[0001] This invention relates to the field of resistance spot welding technology, and in particular to a spot welding process for three-layer steel plates. Background Technology

[0002] To achieve lightweighting of automobile bodies, the widespread use of high-strength steel sheets in the manufacture of automobile body-in-white has become the mainstream technical solution. Currently, the application of hot-formed steel in body structures is increasing, with 1500MPa strength grade steel being the most common, while 1800MPa and 2000MPa grade hot-formed steel are also gradually being promoted and applied. In actual body assembly processes, it is often necessary to weld three different types of steel sheets simultaneously: hot-formed steel, ultra-high-strength steel, and outer sheet (usually IF steel). Due to significant differences in the material and microstructure of these three types of steel sheets, their resistivity also varies greatly. During resistance spot welding, steel sheets with higher resistivity and greater strength are more likely to form a weld nugget, while steel sheets with lower resistivity and lower strength are less likely to form an effective weld nugget. Therefore, when welding three dissimilar steel sheets together, the outer sheet (IF steel) often struggles to form a reliable connection with the hot-formed steel and ultra-high-strength steel, which has become a key technical bottleneck restricting the further promotion and application of hot-formed steel.

[0003] Existing patented technologies have attempted to solve the challenge of welding multi-layer dissimilar steel plates. For example, Chinese patent CN102458751B discloses a "resistance welding method and apparatus for resistance welding method," which increases the welding current on the thin plate side by adding a loop current, thereby promoting the formation of a weld nugget on the thin plate side. However, this solution requires the design of a complex loop structure and modification of existing welding machines, resulting in high implementation costs and limited adaptability.

[0004] Another Chinese patent, CN109773321B, discloses a "method for spot welding multilayer plates with different strength and thickness." This method adds a pre-forming and pre-forming maintenance process between the pre-pressing stage and the welding stage, essentially belonging to a preheating + welding dual-pulse welding process. Although this process improves the welding conditions of multilayer plates to some extent, it does not fundamentally change the problem of "high-resistance steel plates being easy to weld and low-resistance steel plates being difficult to fuse" caused by resistance differences. The welding effect for three layers of dissimilar steel plates is still not ideal.

[0005] Therefore, there is an urgent need for a simple, cost-controllable process that can effectively achieve reliable spot welding of three layers of steel plates (especially including hot-formed steel, ultra-high-strength steel and IF steel) to promote the further application of high-strength lightweight materials in automobile bodies. Summary of the Invention

[0006] The object of the present invention is to provide a three-layer steel sheet spot welding process method. By constructing a welding process model with a "pyramid" - shaped heat distribution, it solves the key technical problem that under the condition of superposed hot - formed steel and ultra - high - strength steel, IF steel cannot achieve reliable welding due to its low resistivity and difficulty in forming a fusion nucleus. Without relying on complex equipment modification, through the optimized matching of the electrode structure and welding timing sequence, it realizes the balanced growth of the fusion nucleus between dissimilar materials, thereby obtaining a welding solution with stable welding quality, strong process adaptability, low implementation cost and easy promotion.

[0007] To achieve the above object, the present invention is realized through the following technical solutions: A three - layer steel sheet spot welding process method. The three - layer plates are stacked in descending order of strength, namely hot - formed steel, ultra - high - strength steel, and IF steel in sequence. The strength of the hot - formed steel is ≥1000 MPa, the strength of the ultra - high - strength steel is ≥980 MPa, and the strength of the IF steel is ≥270 MPa. An asymmetric electrode cap combination is adopted. One electrode is a flat electrode cap, which contacts the steel sheet on the hot - formed steel side; the other electrode is a spherical electrode cap, which contacts the steel sheet on the IF steel side. The spot welding process includes: Place the stacked three - layer plates between the upper and lower electrodes. Apply a pressure P of 3 - 6 kN on the electrodes. Adopt a pulsed spot welding process for welding. The current frequency of the welding equipment is 1800 Hz and the current accuracy is 1 A, which is beneficial to forming a stable welding heat input. The spot welding process is carried out in 4 stages, successively applying the first pulse energy Q1, the second pulse energy Q2, the third pulse energy Q3, and the fourth pulse energy Q4. The pulse interval time is 25 - 35 ms. Among them, Q1 < Q2 < Q3 < Q4, and Q4 ≥ 1.5Q3. The combination of the flat electrode cap and the spherical electrode cap forms a "pyramid" - shaped contact condition, making the resistance heat distribution in the welding area show a gradient characteristic of high on the IF steel side and low on the hot - formed steel side.

[0008] The end - face plane diameter of the flat electrode cap is 8 - 10 mm; the spherical diameter of the spherical electrode cap is r = 10 - 16 mm.

[0009] The flat electrode cap and the spherical electrode cap are coaxially arranged and their end - faces are parallel.

[0010] The thicknesses of both the hot - formed steel and the ultra - high - strength steel are 1.0 - 2.5 mm, and the thickness of the IF steel is 0.6 mm - 1.0 mm.

[0011] The electrode pressure segmented control parameters are: When the thickness of the hot - formed steel ≤ 1.5 mm, the electrode pressure P = 3.5 - 4.5 kN; When the thickness of the hot - formed steel > 1.5 mm, the electrode pressure P = 4.0 - 5.5 kN.

[0012] The pulse energy of the pulse spot welding is set as: ①; ②; ③; In Formulas ① - ③: represents the end - face diameter of the flat electrode cap, with the unit of mm; represents the diameter of the spherical electrode cap, with the unit of mm; represents the material coefficient, and the value range is 0.8 - 1.2; represents the increment coefficient, and the value range is 0.1 - 0.3.

[0013] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. By adopting the asymmetric electrode combination of "flat electrode cap + spherical electrode cap", combined with the stacking order of the three - layer plates from high strength to low strength, a matching structure of "pyramid" - shaped contact conditions and "inverted pyramid" - shaped resistance distribution is constructed; this design makes the current density and resistance heat significantly concentrated on the IF steel side and moderately dispersed on the hot - formed steel side, thus effectively compensating for the problem of unbalanced fusion - nucleus formation tendency caused by the difference in material resistivity, and realizing the synchronous and reliable fusion of the three - layer dissimilar steel plates; 2. Adopting the multi - pulse spot - welding process, through the progressive energy - input strategy of Q1 < Q2 < Q3 < Q4 and Q4≥1.5Q3, the gradual and controllable growth of the fusion nucleus is realized, avoiding defects such as splash, incomplete fusion or over - burning that may occur in the single - pulse or simple double - pulse process. Finally, the obtained fusion - nucleus size of the solder joint is full, the interface bonding is firm, and the process has good repeatability, which can meet the strict welding quality standards of the automobile body; 3. The core of the present invention lies in the innovative combination of the electrode - cap shape and the optimization of the welding timing. There is no need to perform complex circuit modification or add additional auxiliary devices to the existing standard spot - welding machine. Both the flat and spherical electrode caps are conventional electrode types, which are easy to process and obtain. Therefore, this process is easy to be popularized and applied on the existing production line, with low transformation cost and strong adaptability; 4. By optimizing and defining the key parameters of the electrode - cap size (flat diameter 8 - 10 mm, spherical radius 10 - 16 mm), electrode pressure (3 - 6 kN), and pulse - energy sequence, a relatively wide and reliable process window is provided, so that in actual production, even if the material thickness fluctuates within a certain range or there are minor deviations in the equipment, the process can still remain stable, reducing the excessive dependence on the operator's experience, and improving the production rhythm and the qualified - product rate; 5. The point contact mode between the spherical electrode cap and IF steel, combined with an appropriate spherical radius and electrode pressure, can ensure sufficient current density while avoiding excessively deep electrode indentations or severe plate warping on the soft steel surface. The surface quality of the welded workpiece meets the appearance requirements, reducing subsequent finishing processes. Attached Figure Description

[0014] Figure 1 This is a schematic diagram of the spot welding process for three layers of steel plates.

[0015] Figure 2 This is a schematic diagram of a spherical electrode cap.

[0016] Figure 3 This is a schematic diagram of a planar electrode cap.

[0017] In the figure: 1. Planar electrode cap; 2. Hot-formed steel; 3. Ultra-high strength steel; 4. IF steel; 5. Spherical electrode cap. Detailed Implementation

[0018] The present invention will now be described in detail with reference to the accompanying drawings, but it should be noted that the implementation of the present invention is not limited to the following embodiments.

[0019] The following embodiments are implemented based on the technical solution of the present invention, providing detailed implementation methods and specific operation processes. However, the scope of protection of the present invention is not limited to the following embodiments. Unless otherwise specified, the methods used in the following embodiments are conventional methods.

[0020] Example: A spot welding process for three-layer steel plates is implemented using the following steps and parameters: S1. Welding materials and stacking order, see Figures 1-3 : S11 and the three layers of steel plates are hot-formed steel, ultra-high-strength steel, and IF steel, respectively, with strengths of: Hot-formed steel strength ≥ 1000 MPa; Ultra-high strength steel with a strength ≥980MPa; IF steel strength ≥ 270 MPa.

[0021] S12. The order of steel plate stacking from high to low strength is as follows: hot-formed steel (bottom layer), ultra-high strength steel (middle layer), IF steel (top layer).

[0022] S13, steel plate thickness is as follows: Hot-formed steel: 1.0~2.5mm; Ultra-high strength steel: 1.0~2.5mm; IF steel: 0.6~1.0mm.

[0023] S2, Electrode Cap Configuration: S21. Adopt an asymmetric electrode cap combination. The upper electrode is a flat electrode cap with a flat end face and a flat diameter of 8 mm to 10 mm. The lower electrode is a spherical electrode cap with a spherical radius of 10 mm to 16 mm.

[0024] S22. The flat electrode cap contacts the hot-formed steel side, and the spherical electrode cap contacts the IF steel side to form a "pyramid" contact condition.

[0025] S3. Spot welding process steps: S31. Stack the three-layer steel plates in sequence and place them between the upper and lower electrodes. S32. Apply an electrode pressure P with a value range of 3 to 6 kN to make the steel plates fit tightly together. S33. Use a multi-pulse spot welding process for welding. A total of four pulses are applied, and the pulse interval time is 30 ms. The energy of each pulse satisfies: Among them, Q1 < Q2 < Q3 < Q4, and Q4 ≥ 1.5Q3. The pulse energy of the pulse spot welding is set as: ①; ②; ③; In formulas ① - ③: represents the diameter of the end face of the flat electrode cap, with the unit of mm; represents the diameter of the spherical electrode cap, with the unit of mm; represents the material coefficient, with a value range of 1.0; represents the increasing coefficient, with a value range of 0.2.

[0026] S4. Spot welding process principle: By adopting an asymmetric combination of a flat and a spherical electrode cap and combining with the "inverted pyramid" resistance distribution characteristics of the steel plates, a "pyramid" contact condition is constructed, making the current density and resistance heat significantly higher on the IF steel side than on the hot-formed steel side, thereby effectively promoting the formation of the fusion nucleus on the IF steel side and achieving reliable welding of the three-layer dissimilar steel plates.

[0027] S5. Parameter priority description: When the diameter of the flat electrode cap is less than 8 mm, the hot-formed steel side overheats and the IF steel side is prone to poor welding. When the diameter of the flat electrode cap is greater than 10 mm, the heat on the hot-formed steel side is dispersed and the fusion nucleus is poor.

[0028] When the spherical radius of the spherical electrode cap is less than 10 mm, deep pits and deformation are likely to occur on the IF steel side; when the spherical radius of the spherical electrode cap is greater than 16 mm, the current density decreases and it is difficult to nucleate on the IF steel side.

[0029] When the electrode pressure is less than 3 kN, the fit is not tight, affecting the stability of the fusion nucleus; when the electrode pressure is higher than 6 kN, the indentation is too deep and the equipment requirements are high.

[0030] The spot welding process must be used in combination with the above electrode cap configuration. Using the spot welding process or symmetric electrode caps alone cannot achieve the same technical effect.

[0031] The following will specifically describe the present invention in combination with specific embodiments, and the advantages and various effects of the present invention will be presented more clearly therefrom.

[0032] Table 1 Welded steel plates and electrode caps.

[0033] Table 1 shows the thickness of the welding materials and the electrode cap size parameters used in 7 specific embodiments and 3 comparative examples of the present invention. The thicknesses of the hot-formed steel, ultra-high-strength steel, and IF steel are listed in the table, as well as the end face diameter of the flat electrode cap and the diameter of the spherical electrode cap used. All the embodiments adopt the asymmetric electrode combination of the present invention, while the comparative examples (Comparative Examples 1-3) only use flat electrode caps (with a diameter of 6 mm) and do not use spherical electrode caps, which are used to compare and illustrate the necessity of the electrode configuration of the present invention.

[0034] Table 2 Welding process regime.

[0035] Table 2 lists the welding process parameters corresponding to the embodiments and comparative examples, including four pulse energies (Q1, Q2, Q3, Q4) and the electrode pressure P. The embodiments adopt progressive multi-pulse energy input (Q1 < Q2 < Q3 < Q4), and Q4 ≥ 1.5Q3, while the comparative examples only use a single pulse energy (Q1) for welding. This table shows that the present invention can achieve the formation of more stable and larger-sized fusion nuclei through multi-pulse energy control.

[0036] Table 3 Welded joint inspection results.

[0037] Table 3 provides the welded joint inspection results, including the fusion nucleus diameters on the IF steel side and the hot-formed steel side and the final evaluation results. The fusion nucleus sizes of the welded joints in all the embodiments are uniform and the strength is qualified, while the fusion nucleus diameter of the IF steel in the comparative examples is significantly smaller and the welding quality is unqualified, further verifying the significant advantages of the present invention in achieving effective welding of three-layer dissimilar steels.

[0038] This invention employs an asymmetric electrode combination of a planar electrode cap and a spherical electrode cap, combined with a stacking order of three layers of plates in descending strength, to construct a matching structure of a pyramid-shaped contact condition and an inverted pyramid-shaped resistance distribution. This design significantly concentrates current density and resistive heat on the IF steel side and moderately disperses them on the hot-formed steel side, effectively compensating for the imbalance in weld nugget formation tendency caused by differences in material resistivity, and achieving synchronous and reliable fusion of three dissimilar steel plates. A multi-pulse spot welding process is used, through Q1...

Claims

1. A method for spot welding three layers of steel plates, characterized in that, The three-layer steel plates are stacked in order of decreasing strength, namely hot-formed steel, ultra-high-strength steel, and IF steel. The strength of the hot-formed steel is ≥1000 MPa, the strength of the ultra-high-strength steel is ≥980 MPa, and the strength of the IF steel is ≥270 MPa. An asymmetric electrode cap combination is used, with one electrode being a flat electrode cap that contacts the steel plate on the hot-formed steel side, and the other electrode being a spherical electrode cap that contacts the steel plate on the IF steel side. The spot welding process includes: Placing the stacked three-layer steel plates between the upper and lower electrodes; Applying a pressure P of 3 - 6 kN to the electrodes; Using the pulsed spot welding process for welding, successively applying the first pulse energy Q1, the second pulse energy Q2, the third pulse energy Q3, and the fourth pulse energy Q4, with a pulse interval time of 25 - 35 ms. Among them, Q1 < Q2 < Q3 < Q4, and Q4 ≥ 1.5Q3; The combination of the flat electrode cap and the spherical electrode cap forms a "pyramid" - shaped contact condition, making the resistance heat distribution in the welding area show a gradient characteristic of being high on the IF steel side and low on the hot-formed steel side.

2. The method for spot welding three-layer steel plates according to claim 1, characterized in that, The end face plane diameter of the described flat electrode cap is 8 - 10 mm; the spherical diameter r of the described spherical electrode cap is 10 - 16 mm.

3. The method for spot welding three-layer steel plates according to claim 2, characterized in that, The flat electrode cap and the spherical electrode cap are coaxially arranged and their end faces are parallel.

4. The method for spot welding three-layer steel plates according to claim 1, characterized in that, The thicknesses of both the hot-formed steel and the ultra-high-strength steel are 1.0 - 2.5 mm, and the thickness of the IF steel is 0.6 mm - 1.0 mm.

5. The method for spot welding three-layer steel plates according to claim 4, characterized in that, The electrode pressure segmented control parameters are: When the thickness of the hot-formed steel ≤ 1.5 mm, the electrode pressure P = 3.5 - 4.5 kN; When the thickness of the hot-formed steel > 1.5 mm, the electrode pressure P = 4.0 - 5.5 kN.

6. The method for spot welding three-layer steel plates according to claim 1, characterized in that, The pulse energy of the pulsed spot welding is set as: ①； ②； ③； In formulas ① - ③: This indicates the diameter of the end face of the planar electrode cap, in mm. This indicates the diameter of the spherical electrode cap, in mm. This represents the material coefficient, with a value ranging from 0.8 to 1.

2. This represents the increment factor, with a value range of 0.1 to 0.3.

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