Method for resolving interfacial failure of resistance spot welded joint of 980 mpa grade galvanized complex phase steel sheets

Abstract

A method for resolving an interfacial failure of a resistance spot welded joint of 980 MPa grade galvanized complex phase steel sheets. The method comprises: on the basis of a sheet thickness of a welding specimen, determining the magnitude of a preheating current Ip and the magnitude of a preheating time tp; on the basis of the sheet thickness of the welding specimen, determining the number of welding pulses and a single-pulse welding time tw; on the basis of the sheet thickness of the welding specimen, determining a cooling interval time tc1 between a preheating pulse and a welding pulse, and an interval time tc2 between welding pulses; on the basis of the steel sheet thickness of the welding specimen, determining a welding pressure P; and on the basis of the sheet thickness of the welding specimen, determining a pressure hold time th. In the method, a pulse pattern which involves an ultra-high preheating current and a short preheating time is used, and a cooling interval time between welding pulses is relatively long, such that the joint performance and the weldability can be improved, thereby preventing interfacial failures.

Description

A method for solving the interface failure of resistance spot welded joints of 980MPa grade galvanized multiphase steel plates Technical Field

[0001] This invention relates to a method for solving the interface failure of resistance spot welded joints of 980MPa grade galvanized multiphase steel plates. Background Technology

[0002] In recent years, with increasingly stringent requirements from the automotive industry regarding lightweighting, safety, energy conservation, and environmental protection, the proportion of gigapascal-grade high-strength steel used in automotive bodies has continued to increase. Among these, galvanized multiphase steel CP980+Z, due to its high yield strength ratio and good mechanical properties, is widely used in important automotive structural parts. However, during resistance spot welding, galvanized multiphase steel CP980+Z frequently exhibits interface failure in the shear tensile test at the lower limit of the welding window, leading to reduced mechanical properties of the joint. Fatigue testing and energy absorption of the weld joint also fail to meet standards, which is unacceptable to OEMs and urgently requires improvement.

[0003] In the prior art, patent application number 2021105938672, entitled "A Resistance Spot Welding Method and Welding Apparatus for Galvanized Steel Sheets," involves adding a thin stainless steel sheet between the galvanized steel sheet base material and the electrode. Because the electrode and the galvanized steel sheet base material are separated by the stainless steel sheet and do not directly contact each other, zinc atoms cannot adhere to the electrode during welding to form an alloy, thus maintaining the electrode's original electrical and thermal conductivity. While this invention can improve the problem of electrode adhesion during zinc layer melting in galvanized steel sheets to some extent, thereby improving joint quality, the addition of stainless steel between the electrode and the galvanized steel sheet base material significantly reduces the cooling rate, resulting in a high soft phase content and lower strength in the weld nugget formation, making it unsuitable for ultra-high strength steels. Summary of the Invention

[0004] The purpose of this invention is to provide a method for solving the interface failure of resistance spot welding joints of 980MPa grade galvanized multiphase steel plates. The welding process using ultra-high current pulse form with short duration and long cooling time not only changes the failure mode of the joint and thus improves the mechanical properties, but is also applicable to resistance spot welding of ultra-high strength galvanized plates.

[0005] To achieve the above objectives, the present invention provides the following technical solution:

[0006] A method for solving the interface failure of resistance spot welded joints in 980MPa grade galvanized multiphase steel plates includes:

[0007] S1. Cut the galvanized multiphase steel template into square welding samples, remove the oil stains on the surface of the welding samples, and pre-press for 300-400ms.

[0008] S2. Determine the preheating current I based on the thickness of the welded sample.p With preheating time t p Size;

[0009] S3. Determine the number of welding pulses and the single-pulse welding time based on the plate thickness of the welding sample;

[0010] S4. Determine the cooling interval time tc1 between the preheating pulse and the welding pulse, and the welding pulse interval time tc2 based on the plate thickness of the welding sample.

[0011] S5. Determine the welding pressure P based on the thickness of the steel plate of the welding sample;

[0012] S6. Determine the holding time th based on the plate thickness of the welding sample.

[0013] In S2, the preheating current Ip ranges from 20kA to 25kA, and the preheating time is 10ms to tp.

[0014] In S3, when the thickness of the welded sample is 0.8mm ≤ plate thickness < 1.2mm, dual-pulse welding is used, with a single-pulse welding time of 140–160ms; when the thickness of the welded sample is 1.2mm ≤ plate thickness < 1.7mm, triple-pulse welding is used, with a single-pulse welding time of 120–140ms; when the thickness of the welded sample is 1.7mm ≤ plate thickness ≤ 2.0mm, quadruple-pulse welding is used, with a single-pulse welding time of 130–150ms.

[0015] In S4, when the thickness of the welded sample is less than 1.5 mm and the plate thickness is 0.8 mm, the welding pulse cooling interval tc1 is 15-25 ms and the welding pulse interval tc2 is 450-550 ms. When the thickness of the welded sample is less than 2.0 mm and the plate thickness is 1.5 mm, the welding pulse cooling interval tc1 is 35-45 ms and the welding pulse cooling interval tc2 is 650-750 ms.

[0016] In S5, when 0.8mm ≤ plate thickness of the welded specimen < 1.2mm, the welding pressure P = 2.5~3.0kN; when 1.2mm ≤ plate thickness of the welded specimen < 1.5mm, the welding pressure P = 3.0~3.5kN; when 1.5mm ≤ plate thickness of the welded specimen < 1.7mm, the welding pressure P = 3.5~4.0kN; when 1.7mm ≤ plate thickness of the welded specimen ≤ 2.0mm, the welding pressure P = 4.5~5.0kN.

[0017] In S6, when the thickness of the welded sample is 0.8mm ≤ 1.5mm, the holding time th = 150~200ms; when the thickness of the welded sample is 1.5mm ≤ 2.0mm, the holding time th = 300~350ms.

[0018] Then, a medium-frequency DC spot welding machine is used to perform resistance spot welding according to the parameters determined above and the sequence of pre-pressure, preheating, welding, and pressure holding.

[0019] Compared with the prior art, the beneficial effects of the present invention are:

[0020] 1. A pulsed pattern with ultra-high preheating current and short preheating time is used to disperse the zinc layer on the surface of galvanized multiphase steel, significantly reducing the zinc content in the weld nugget. The interval between multiple pulses is determined based on the thickness of the welded sample. The cooling interval between welding pulses is relatively long, which is to allow sufficient time for the weld nugget structure to stabilize after the previous welding pulse ends. The next pulse is applied when the weld point temperature is close to room temperature, further increasing the size of the weld nugget and completely fusing the two steel plates together. This improves the joint performance and weldability, resulting in a purer weld nugget structure, improved joint quality, and prevents interface failure.

[0021] 2. Extending the holding time can reduce shrinkage cavities inside the weld nugget and improve joint performance;

[0022] 3. The purpose of using multi-pulse welding is to gradually increase the diameter of the weld nugget, and it can also avoid spatter caused by long-term continuous welding, thus widening the welding process window and improving joint performance.

[0023] 4. This welding process can accelerate the wider application of 980MPa grade galvanized multiphase steel sheets in the automotive manufacturing field. Attached Figure Description

[0024] Figure 1 is a welding timing diagram. Detailed Implementation

[0025] 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.

[0026] 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.

[0027]

Example 1

[0028] The welding material is 0.8mm thick galvanized multiphase steel CP980+Z, and a medium-frequency DC spot welding machine is used, as shown in Figure 1. Details are as follows:

[0029] (1) Cut the 0.8mm thick CP980+Z template into 150mm×50mm welding samples, wipe the oil stains on the surface with alcohol, and the pre-press time is 300ms.

[0030] (2) Determine the preheating current Ip = 20kA and the preheating time tp = 10ms based on the thickness of the welded sample.

[0031] (3) Use dual-pulse welding and determine the single-pulse welding time tw = 150ms based on the plate thickness of the welding sample;

[0032] (4) Based on the thickness of the welded sample, determine the cooling interval time tc1 between the preheating pulse and the welding pulse as 20ms and the welding pulse interval time tc2 as 500ms.

[0033] (5) Select a welding pressure p = 2.5kN based on the thickness of the welded sample.

[0034] (6) Holding time th = 150ms.

[0035] Based on the above welding parameters, the resistance spot welding process window for 0.8mm thick galvanized multiphase steel CP980+Z is 2.2kA, which is much greater than the standard requirement of more than 1kA; the shear tensile properties of the joint are greater than 12kN, the cross tensile properties are greater than 4.1kN, the failure mode is pull-out of the weld nugget, and there is no spatter on the surface.

[0036] The traditional welding process was employed as follows: welding pressure of 2.6 kN, single-pulse welding, welding time of 230 ms, and holding time of 40 ms. The welding window was 1.1 kA (just meeting the standard requirement of greater than 1 kA). The shear tensile strength of the joint was 10.5 kN, and the cross tensile strength was 3.5 kN. A small number of interface failures occurred in the lower limit shear and cross tensile welding tests. Therefore, this welding method represents a significant improvement over traditional welding processes.

[0037]

Example 2

[0038] The welding material is 1.2mm thick galvanized multiphase steel CP980+Z, and a medium-frequency DC spot welding machine is used, as shown in Figure 1. Details are as follows:

[0039] (1) Cut the 1.2mm thick CP980+Z template into 150mm×50mm welding samples, wipe the oil stains on the surface with alcohol, and the pre-pressing time is 300ms.

[0040] (2) The preheating current Ip = 22kA and the preheating time tp = 15ms are determined based on the thickness of the welded sample.

[0041] (3) Using three-pulse welding, the single-pulse welding time tw = 130ms is determined according to the plate thickness of the welding sample;

[0042] (4) Based on the thickness of the welded sample, determine the cooling interval time tc1 between the preheating pulse and the welding pulse as 20ms and the welding pulse interval time tc2 as 500ms.

[0043] (5) Select a welding pressure p = 3.2 kN based on the thickness of the welded sample.

[0044] (6) Pressure holding time th = 180ms.

[0045] Based on the above welding parameters, the resistance spot welding process window for 1.2mm thick CP980+Z is 2.4kA, which is much greater than the standard requirement of more than 1kA; the shear tensile strength of the joint is greater than 20.3kN, the cross tensile strength of the joint is greater than 8.5kN, the failure mode is pull-out of the weld nugget, and there is no spatter on the surface.

[0046] The traditional welding process was employed as follows: welding pressure of 2.6 kN, single-pulse welding, welding time of 270 ms, and holding time of 40 ms. The welding window was 0.9 kA (not meeting the standard requirement of greater than 1 kA). The shear tensile strength of the joint was 17.3 kN, and the cross tensile strength was 6.7 kN. Interface failure occurred in the lower limit shear and cross tensile tests. Therefore, this welding method represents a significant improvement over traditional welding processes.

[0047]

Example 3

[0048] The welding material is 2.0mm thick CP980+Z, and a medium-frequency DC spot welding machine is used, as shown in Figure 1. The details are as follows:

[0049] (1) Cut the 2.0mm thick CP980+Z template into 150mm×50mm welding samples, wipe the oil stains on the surface with alcohol, and the pre-pressing time is 300ms.

[0050] (2) Determine the preheating current Ip = 25kA and the preheating time tp = 20ms based on the thickness of the welded sample.

[0051] (3) Four-pulse welding was used, and the single-pulse welding time tw = 140ms was determined according to the plate thickness of the welding sample;

[0052] (4) Based on the thickness of the welded sample, determine the cooling interval time tc1 between the preheating pulse and the welding pulse as 40ms and the welding pulse interval time tc2 as 700ms.

[0053] (5) Select a welding pressure p = 5.0 kN based on the thickness of the welded sample.

[0054] (6) Holding time th = 350ms.

[0055] Based on the above welding parameters, the resistance spot welding process window for 2.0mm thick CP980+Z is 1.7kA, which is much greater than the standard requirement of more than 1kA; the shear tensile strength of the joint is greater than 38.2kN, the cross tensile strength of the joint is greater than 20.2kN, the failure mode is pull-out of the weld nugget, and there is no spatter on the surface.

[0056] The traditional welding process used is as follows: welding pressure of 4kN, four-pulse welding, single-pulse welding time of 120ms, cooling time of 40ms, and holding time of 100ms. The welding window is 1.0kA (just meeting the standard requirement of greater than 1kA). The shear tensile strength of the joint is 31.3kN, and the cross tensile strength is 16kN. Interface failure occurred in the lower limit of the welding shear test. Therefore, this welding method represents a significant improvement over traditional welding processes.

[0057] This invention utilizes a pulsed approach with ultra-high preheating current and short preheating time to disperse the zinc layer on the surface of galvanized multiphase steel, significantly reducing the zinc content in the weld nugget. The multi-pulse interval is determined based on the thickness of the welded sample, with a relatively long cooling interval between welding pulses. This allows sufficient time for the weld nugget structure to stabilize after each pulse. The next pulse is applied when the weld point temperature is close to room temperature, further increasing the weld nugget size and ensuring complete fusion of the two steel plates. This improves joint performance and weldability, resulting in a purer weld nugget structure, enhanced joint quality, and prevention of interface failure. Extending the holding time reduces shrinkage cavities within the weld nugget, further improving joint performance. The multi-pulse welding method gradually increases the weld nugget diameter and avoids spatter caused by prolonged welding, widening the welding process window and improving joint performance. This welding process can accelerate the wider application of 980MPa grade galvanized multiphase steel sheets in the automotive manufacturing industry.

Claims

1. A method for solving the interface failure of a 980 MPa grade galvanized dual-phase steel sheet resistance spot welded joint, characterized in that, include: S1. Cut the galvanized multiphase steel template into square welding samples, remove the oil stains on the surface of the welding samples, and pre-press for 300-400ms. S2. Determine the magnitude of the preheating current Ip and the preheating time tp based on the thickness of the welded sample. S3. Determine the number of welding pulses and the single-pulse welding time based on the plate thickness of the welding sample; S4. Determine the cooling interval time tc1 between the preheating pulse and the welding pulse, and the welding pulse interval time tc2 based on the plate thickness of the welding sample. S5. Determine the welding pressure P based on the thickness of the steel plate of the welding sample; S6. Determine the holding time th based on the plate thickness of the welding sample.

2. The method according to claim 1, characterized in that, In S2, the preheating current Ip ranges from 20kA to 25kA, and the preheating time is 10ms to tp.

3. The method according to claim 1, characterized in that, In S3, when the thickness of the welded sample is 0.8mm ≤ plate thickness < 1.2mm, dual-pulse welding is used, with a single-pulse welding time of 140–160ms; when the thickness of the welded sample is 1.2mm ≤ plate thickness < 1.7mm, triple-pulse welding is used, with a single-pulse welding time of 120–140ms; when the thickness of the welded sample is 1.7mm ≤ plate thickness ≤ 2.0mm, quadruple-pulse welding is used, with a single-pulse welding time of 130–150ms.

4. The method according to claim 1, characterized in that, In S4, when the thickness of the welded sample is less than 1.5 mm and the plate thickness is 0.8 mm, the welding pulse cooling interval tc1 is 15-25 ms and the welding pulse interval tc2 is 450-550 ms. When the thickness of the welded sample is less than 2.0 mm and the plate thickness is 1.5 mm, the welding pulse cooling interval tc1 is 35-45 ms and the welding pulse cooling interval tc2 is 650-750 ms.

5. The method according to claim 1, characterized in that, In S5, when 0.8mm ≤ plate thickness of the welded specimen < 1.2mm, the welding pressure P = 2.5~3.0kN; when 1.2mm ≤ plate thickness of the welded specimen < 1.5mm, the welding pressure P = 3.0~3.5kN; when 1.5mm ≤ plate thickness of the welded specimen < 1.7mm, the welding pressure P = 3.5~4.0kN; when 1.7mm ≤ plate thickness of the welded specimen ≤ 2.0mm, the welding pressure P = 4.5~5.0kN.

6. The method according to claim 1, characterized in that, In S6, when the thickness of the welded sample is 0.8mm ≤ 1.5mm, the holding time th = 150~200ms; when the thickness of the welded sample is 1.5mm ≤ 2.0mm, the holding time th = 300~350ms.

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