Method for improving welding spot quality of resistance spot welding of three-layer board

Abstract

The invention relates to a method for improving the welding spot quality of resistance spot welding of a three-layer plate, which adopts an upper electrode cap and a lower electrode cap which are different from each other, effectively improves the spot welding nugget deviation of the three-layer steel plate, and widens the spot welding process window phenomenon. The spot welding method comprises the following steps: 1, performing surface treatment on a plate, and assembling in a lap joint form; 2, selecting an electrode cap; and thirdly, welding is conducted, and resistance spot welding of the three-layer steel plate is completed. The method has the advantages that the surface quality is good in the welding process, and no shrinkage cavity is generated; 2, the nugget deviation is small; thirdly, the spot welding process window reaches more than 2kA, and the spot welding process window is improved; and fourthly, the process is simple, production efficiency is high, cost is low, spot-welded joints are good in quality, and production and application are easy.

Description

Technical Field

[0001] This patent application belongs to the technical field of steel plate welding, and more specifically, it relates to a method for improving the quality of resistance spot welding welds of three-layer plates, and is mainly used for resistance spot welding of three-layer steel plates. Background Art

[0002] Resistance spot welding of automobile panels is the main connection process in the automobile body-in-white due to its characteristics of concentrated heat, simple operation and high efficiency. The quality of the 5,000 to 6,000 welds on a car directly affects the performance of the car body.

[0003] Resistance spot welding is a method of welding by applying pressure to the assembled weldment through electrodes and using the resistance heat generated by the current passing through the contact surface of the joint and the adjacent area. It has been widely used in aerospace, electronics, automobiles, household appliances and other fields, especially in key parts such as the front longitudinal beam and B-pillar of the car body. The matching conditions of three-layer steel plates with different strength and thickness between low-carbon steel inner and outer panels and dual-phase steel reinforcements have increased significantly, and the application ratio has reached 33%.

[0004] Resistance spot welding of automobile plates is generally performed with upper and lower electrode caps. In normal circumstances, the upper and lower electrode caps are welded with traditional round flat head electrode caps. When operating on multi-layer steel plates with different strengths and thicknesses, it often causes the molten core to shift, resulting in shrinkage, severe welding spatter, narrow process window and severe electrode wear, which in turn affects the quality of welding.

[0005] Therefore, it is necessary to improve the offset problem of the molten core in the resistance spot welding process of the three-layer plate to widen the spot welding process window and improve the welding quality. Summary of the invention

[0006] The technical problem to be solved by the present invention is to provide a method for improving the quality of resistance spot welding welds of three-layer plates, so as to solve the problems of nugget deviation and narrow welding process window of three-layer plates mentioned in the background technology.

[0007] In order to solve the above problems, the technical solution adopted by the present invention is: A method for improving the quality of resistance spot welding of three-layer boards comprises the following steps: S1. The plate surface is treated and assembled in overlapping form to obtain clean welding parent material; S2, electrode cap selection, use upper and lower heterogeneous electrode caps; S3, welding, using variable electrode pressure to complete the resistance spot welding of three layers of steel plates.

[0008] Furthermore, in S2, a ball-head electrode cap is used on the thin plate side, and a round flat-head electrode cap is used on the thick plate side, and the end face diameter is 5 mm to 7 mm.

[0009] Furthermore, in S2, the electrode cap material is Cr-Zr-Cu.

[0010] Further, S3 is specifically, (1) Preloading: The steel plate is preloaded by electrode pressure P1, P1: 2.0~3.0kN, preloading time is t u =200~300ms; (2) Welding current 1: electrode pressure P1: 2.0~3.0kN, welding current I1=(0.5~0.6) *I2, welding time t w1 =100~200ms, cooling time is t c =20~40ms; (3) Welding current 2: electrode pressure P2: 3.5~4.5kN, welding current I2=I F -0.2, welding time t w2 =250~350ms; F is the spatter current, the minimum current that produces welding spatter when single pulse spot welding is used, the unit is kA, and the cooling time is t c1 =20~40ms; (4) Welding current 3: electrode pressure P2: 3.5~4.5kN, welding current I3=(0.7~0.8) *I2, welding time t w3 =100~200ms, cooling time is t c2 =20~40ms; (5) After the welding current stops, the electrode pressure P2 is used to maintain the pressure for a holding time of t H =200~300ms.

[0011] Furthermore, the first steel plate is IF steel (DC04, DC05 or DC06), the second steel plate is dual-phase steel (DP590, DP780 or DP980), and the third steel plate is hot stamped steel T1500HS or dual-phase steel (DP590, DP780 or DP980).

[0012] Furthermore, in S3, a medium frequency inverter resistance welding machine is used.

[0013] Furthermore, the process window for spot welding of steel plates is greater than 2kA; the nugget offset value is -1≈0, that is, the nugget offset value is close to 1.

[0014] Due to the adoption of the above technical solution, the beneficial effects achieved by the present invention are: By adopting the upper and lower electrode caps of the present invention and the upper and lower dissimilar electrode cap resistance spot welding method, the current density on the thin plate side of the three-layer steel plate is increased, so that the resistance heat around the contact surface is generated faster and more, thereby accelerating the heating rate of the base material on the thin plate side, so that the molten core on this side is formed in advance, and finally achieving the purpose of balancing the molten cores on both sides of the three-layer steel plate.

[0015] The method of asymmetric electric field input can effectively increase the size of the weld nugget on the thin plate side, thereby reducing the difference in the diameter of the upper and lower weld nuggets in the three-layer steel plate, improving the phenomenon of weld nugget offset in the three-layer steel plate, and improving the welding quality. Compared with the window under the traditional round flat head electrode cap, the left boundary moves to the left; and the right boundary also moves to the right, and the width of the entire window is widened. This result fully demonstrates that the use of heterogeneous electrode caps also has a good effect on improving the weldability of three-layer steel plates. In addition, the resistance spot welding method of the present invention has low production cost, does not require additional tooling and fixtures, has high welding efficiency and automation, and is conducive to large-scale three-layer steel plate welding. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 is a schematic diagram of the structure of the electrode cap in the present invention; Figure 2 It is a schematic diagram of the structure of the electrode cap in the present invention; Figure 3 It is the resistance spot welding process diagram of the present invention; Figure 4 is a comparison diagram of the spot welding process window of the three-layer plate of Example 1; Figure 5 is a comparison diagram of the nugget photo of Example 1; Figure 6 is a comparison diagram of the spot welding process window of the three-layer plate of Example 2; Figure 7 is a comparison diagram of the nugget photo of Example 2; Among them: 1. ball head electrode cap, 11. ball head, 2. round flat head electrode cap, 21. round flat head, 22. end face, 3. steel plate one, 4. steel plate two, 5. steel plate three. DETAILED DESCRIPTION

[0017] The present invention is further described in detail below with reference to the embodiments.

[0018] A method for improving the quality of resistance spot welding of three-layer boards, such as Figure 1 , Figure 2 , including the following steps: S1. The plate surface is treated and assembled in overlapping form to obtain clean welding parent material; S2. Selection of electrode caps. Use upper and lower heterogeneous electrode caps, covering the thin plate side, thick plate side, and end face 22. The electrode cap material is Cr-Zr-Cu. The thin plate side uses a ball head electrode cap 1, and the thick plate side uses a round flat head electrode cap 2. The end face 22 has a diameter of 5mm to 7mm, preferably 6mm. The electrode cap head shapes of the ball head electrode cap 1 and the round flat head electrode cap 2 are different. The ball head electrode cap 1 is an arc, which is a ball head 11, and the round flat head electrode cap 2 is a straight line, which is a round flat head 21. The end face 22 is the length of the straight line part of the round flat head 21. The thin plate side has the upper electrode cap on the thin plate, and the thick plate side has the lower electrode cap on the thick plate. It is a professional term.

[0019] S3, welding, using variable electrode pressure, complete the three-layer steel plate resistance spot welding. Figure 3 As shown, specifically: (1) Preloading: The steel plate is preloaded by electrode pressure P1, P1: 2.0~3.0kN, preloading time is t u =200~300ms; (2) Welding current 1: electrode pressure P1: 2.0~3.0kN, welding current I1=(0.5~0.6) *I2, welding time t w1 =100~200ms, cooling time is t c =20~40ms; (3) Welding current 2: electrode pressure P2: 3.5~4.5kN, welding current I2=I F -0.2, welding time t w2 =250~350ms; F is the spatter current, the minimum current that produces welding spatter when single pulse spot welding is used, the unit is kA, and the cooling time is t c1 =20~40ms; (4) Welding current 3: electrode pressure P2: 3.5~4.5kN, welding current I3=(0.7~0.8) *I2, welding time t w3 =100~200ms, cooling time is t c2 =20~40ms; (5) After the welding current stops, the electrode pressure P2 is used to maintain the pressure for a holding time of t H =200~300ms.

[0020] Steel plate 1 3 is IF steel (DC04, DC05 or DC06), steel plate 2 4 is dual-phase steel (DP590, DP780 or DP980), and steel plate 3 5 is hot stamping steel T1500HS or dual-phase steel (DP590, DP780 or DP980).

[0021] In S3, a medium frequency inverter resistance welding machine is used.

[0022] The process window for spot welding of steel plates is >2kA; the nugget offset value is -1≈0, that is, the nugget offset value is close to 1.

[0023] The following example explains this in detail. Example 1

[0024] A method for improving the quality of resistance spot welding of three-layer boards is specifically completed by the following steps: Steel plate 1 3 is a 0.7 mm DC06 steel plate, steel plate 2 4 is a 1.2 mm DP780 dual-phase steel, and steel plate 3 5 is a 1.2 mm T1500HS steel plate. A comparative test was conducted using upper and lower heterogeneous electrode caps and a traditional electrode cap.

[0025] 1. Plate surface treatment: Clean the oil, rust, iron oxide and other substances on the surface of the test plate to obtain a clean welding base material.

[0026] 2. Selection of electrode caps: The upper and lower heterogeneous electrode caps are made of Cr-Zr-Cu (chromium zirconium copper). The upper electrode cap uses a ball head electrode cap 1, and the lower electrode cap uses a round flat head electrode cap 2, with an end face 22 diameter of 6 mm. In the comparative test, both the upper and lower electrode caps use traditional round flat head electrode caps 22, with an end face diameter of 6 mm.

[0027] 3. Welding: (1) Preloading: The steel plate is preloaded by electrode pressure P1, P1: 2.0 kN, preloading time t u =200ms; (2) Welding current 1: electrode pressure P1: 2.0 kN, welding current I1 = 4.5 kA, welding time t w1 =100ms, t c =30ms; (3) Welding current 2: electrode pressure P2: 3.5 kN, welding current I2 = 7.8 kA, welding time t w2 =250ms, I F =8.0kA, t c1 =40ms; (4) Welding current 3: electrode pressure P2: 3.5 kN, welding current I3 = 6 kA, welding time t w3 =100ms, t c2 =20ms; (5) After the welding current stops, keep t H =200ms.

[0028] Through testing, it can be seen that in Example 1, the three-layer steel plate uses a traditional electrode cap spot welding process window of 1.2 kA, a nugget offset value of 1.43, and no shrinkage cavity is generated in the nugget. The method of the present invention obtains a spot welding process window of 2.2 kA, a nugget offset value of 1.22, and no shrinkage cavity is generated in the nugget. Figure 4 and Figure 5 shown. Example 2

[0029] A method for improving the quality of resistance spot welding of three-layer boards is specifically completed by the following steps: Steel plate 1 3 is a 0.7 mm DC06 steel plate, steel plate 2 4 is a 1.2 mm DP600 dual-phase steel, and steel plate 3 5 is a 2.0 mm DP600 dual-phase steel. A comparative test is performed using upper and lower heterogeneous electrode caps and a traditional electrode cap.

[0030] 1. Plate surface treatment: Clean the oil, rust, iron oxide and other substances on the surface of the test plate to obtain a clean welding base material.

[0031] 2. Selection of electrode caps: The electrode cap material is Cr-Zr-Cu, the upper electrode cap uses a ball head electrode cap 1, the lower electrode cap side uses a round flat head electrode cap 2, and the end face 22 diameter is 6mm; in the comparative test, the upper and lower electrode caps use traditional round flat head electrode caps 2, and the end face 22 diameter is 6mm.

[0032] 3. Welding: (1) Preloading: The steel plate is preloaded by electrode pressure P1, P1: 3.0 kN, preloading time t u =200ms; (2) Welding current 1: electrode pressure P1: 3.0 kN, welding current I1 = 5 kA, welding time t w1 =200ms, t c =30ms; (3) Welding current 2: electrode pressure P2: 4.5 kN, welding current I2 = 8.6 kA, welding time t w2 =300ms, I F =8.8kA, t c1 =40ms; (4) Welding current 3: electrode pressure P2: 4.5 kN, welding current I3 = 6.6 kA, welding time t w3 =200ms, t c2 =20ms; (5) After the welding current stops, keep t H =300ms.

[0033] Through testing, it can be seen that in Example 2, the three-layer steel plate uses the traditional electrode cap spot welding process window of 1.0 kA, the nugget offset value of 1.35, and a large shrinkage cavity is generated. The method of the present invention obtains a spot welding process window of 2.1 kA, a nugget offset value of 1.19, and no shrinkage cavity is generated in the nugget. Figure 6 and Figure 7 shown.

[0034] From the above two embodiments, it can be seen that the steel plate spot welding process window of the present method is greater than 2kA, and the nugget offset value approaches 1, which can effectively increase the size of the nugget on the thin plate side, thereby reducing the difference in the upper and lower nugget diameters in the three-layer steel plate, improving the nugget offset phenomenon in the three-layer steel plate, and improving the welding quality. Compared with the window under the traditional round flat-head electrode cap, the left boundary moves to the left; and the right boundary also moves to the right, and the width of the entire window is widened. This result fully demonstrates that the use of heterogeneous electrode caps also has a good effect on improving the weldability of three-layer steel plates. In addition, the resistance spot welding method of the present invention has low production cost, does not require additional tooling and fixtures, has high welding efficiency and automation, is conducive to large-scale three-layer steel plate welding, and has extremely high promotion and application value.

Claims

1. A method for improving the quality of resistance spot welding of three-layer boards, characterized in that The steps include: S1. The plate surface is treated and assembled in overlapping form to obtain clean welding parent material; S2, electrode cap selection, use upper and lower heterogeneous electrode caps; S3, welding, using variable electrode pressure to complete the resistance spot welding of three layers of steel plates.

2. A method for improving the quality of resistance spot welding of three-layer boards according to claim 1, characterized in that: In S2, a ball-head electrode cap is used on the thin plate side, and a round flat-head electrode cap is used on the thick plate side, and the end face diameter is 5mm to 7mm.

3. A method for improving the quality of resistance spot welding of three-layer boards according to claim 2, characterized in that: In S2, the electrode cap material is Cr-Zr-Cu.

4. A method for improving the quality of resistance spot welding of three-layer boards according to claim 2, characterized in that: S3 is specifically: (1) Preloading: The steel plate is preloaded by electrode pressure P1, P1: 2.0~3.0kN, preloading time is t u =200~300ms; (2) Welding current 1: electrode pressure P1: 2.0~3.0kN, welding current I1=(0.5~0.6) *I2, welding time t w1 =100~200ms, cooling time is t c =20~40ms; (3) Welding current 2: electrode pressure P2: 3.5~4.5kN, welding current I2=I F -0.2, welding time t w2 =250~350ms; F is the spatter current, the minimum current that produces welding spatter when single pulse spot welding is used, the unit is kA, and the cooling time is t c1 =20~40ms; (4) Welding current 3: electrode pressure P2: 3.5~4.5kN, welding current I3=(0.7~0.8) *I2, welding time t w3 =100~200ms, cooling time is t c2 =20~40ms; (5) After the welding current stops, the electrode pressure P2 is used to maintain the pressure for a holding time of t H =200~300ms.

5. A method for improving the quality of resistance spot welding of three-layer boards according to claim 1, characterized in that: The first steel plate is IF steel, the second steel plate is duplex steel, and the third steel plate is hot stamped steel T1500HS or duplex steel.

6. A method for improving the quality of resistance spot welding of three-layer boards according to any one of claims 1 to 5, characterized in that: In S3, a medium frequency inverter resistance welding machine is used.

7. A method for improving the quality of resistance spot welding of three-layer boards according to claim 6, characterized in that: The process window for spot welding of steel plates is >2kA, and the nugget offset value is -1≈0.

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