A method of optimizing the performance of a welded joint of a thermoformed part

Abstract

The application discloses a method for optimizing the performance of a welded joint of a hot-formed part, comprising the following steps: laser tempering a specified area of a hot-formed part to be spot-welded to form a laser-tempered soft area; splicing and fixing the hot-formed part, with the spot-welding area located at the center of the laser-tempered soft area after splicing; and starting spot welding to obtain a hot-formed assembly part with a gradient hardness distribution of a weld nugget area. The application heats and tempers the spot-welding area and the periphery in advance through a laser process to obtain a laser-tempered soft area, then performs a spot-welding process in the laser-tempered soft area, controls the laser-tempering softening process to adapt to the size and performance requirements of the spot, and controls the size of the weld nugget heat-affected zone through the range of the laser-tempered soft area. Compared with a non-tempered spot-welding process, the application provides a wider heat-affected zone and better plasticity and impact energy absorption effect under dynamic load. Compared with a post-spot-welding tempering process, the application does not lose the strength of the base body, has a small heat-affected zone, and has a small part deformation effect.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of spot welding, in particular to a method for optimizing the performance of a welded joint of a hot-formed part. BACKGROUND

[0002] In recent years, under the dual requirements of lightweight and high safety of automobiles, hot-formed steel has become the core material of lightweight structure and collision safety parts of new energy vehicles due to its excellent specific strength.

[0003] Hot-formed steel (such as 22MnB5) has a surface hardness of HV450-HV550 after hot stamping and quenching. There is a problem in steel-steel resistance spot welding: the heat-affected zone (HAZ) around the weld nugget is greater than 1mm wide, the martensite is not tempered, the stress is concentrated at the grain boundary, micro-cracks are induced, the fatigue strength of the joint under dynamic load is only 30%-50% of that of soft steel, and the HAZ of the weld nugget is prone to cracking during the collision process, thereby causing the part to fail prematurely.

[0004] To solve this problem, the existing solution is to temper the local area after resistance spot welding, and to perform secondary tempering on the weld nugget area by induction heating to reduce internal stress. However, the temperature control precision of induction heating for tempering is low, the strength loss of the base material after post-weld tempering is more than 15%, and there is a problem of large degree of deformation of the part. SUMMARY

[0005] The present application provides a method for optimizing the performance of a welded joint of a hot-formed part, which heats and tempers the spot welding area and the outer periphery in advance through laser technology to obtain a laser-tempered soft zone, and then performs steel-steel resistance spot welding process on the hot-formed part in the laser-tempered soft zone, so that the width of the weld nugget heat-affected zone is controlled by the size of the laser-tempered soft zone, and the problem of stress concentration in the weld nugget heat-affected zone can be solved.

[0006] To solve the above technical problems, the present application provides a method for optimizing the performance of a welded joint of a hot-formed part, comprising the following steps:

[0007] S1, laser tempering the designated area of the hot-formed part to be spot welded respectively to form a laser-tempered soft zone;

[0008] S2, splicing and fixing the hot-formed part after laser tempering on a resistance spot welding fixture, wherein the spot welding area is located at the center of the spliced laser-tempered soft zone;

[0009] S3, starting spot welding to obtain a hot-formed assembly part with a gradient hardness distribution around the weld nugget area.

[0010] The application heats and tempers the spot welding area and the periphery in advance through a laser process to obtain a laser tempering soft area, and then performs a steel-steel resistance spot welding process of a hot-formed part in the laser tempering soft area, controls the laser tempering softening process to adapt to the size and performance requirements of the welding spot, and controls the size of the welding nugget heat affected zone; compared with the untempered spot welding process, a wider welding nugget heat affected zone is provided, which has better plasticity and impact energy absorption effect under dynamic load, solves the problem of stress concentration in the welding nugget heat affected zone, and provides a hot-formed assembly part with better performance in the spot welding area; compared with the post-spot welding tempering process, the base body strength is not lost, the welding nugget heat affected zone is small, and the part deformation is small.

[0011] Further, in S1, the laser tempering temperature is 200℃-Ac1 (austenite transformation start) temperature.

[0012] Further, in S1, the laser tempering power is 0.3kW-8kW, the wavelength is 600-1200mm, the defocusing amount is -10mm~+20mm, and the scanning speed is 4-20mm / s.

[0013] Further, in S1, the material of the hot-formed part is 1000MPa, 1500MPa or 2000MPa hot-formed steel.

[0014] Further, for 1000MPa hot-formed steel, the hardness of the laser tempering soft area is 200-300HV; for 1500MPa hot-formed steel, the hardness of the laser tempering soft area is 220-430HV; for 2000MPa hot-formed steel, the hardness of the laser tempering soft area is 250-600HV.

[0015] Further, in S1, the laser tempering area temperature is monitored in real time by an infrared temperature sensor, and the laser power is adjusted according to the real-time monitored temperature to maintain the set tempering temperature.

[0016] Further, in S3, the spot welding parameters are: preheating current 2.5-5kA, welding current 5-15kA, electrode pressure 2-6kN, welding time 150-700ms, and pulse number 1-3 times.

[0017] Further, in S3, the width of the laser tempering soft area of the periphery of the welding nugget area is 2-10mm, and the width of the heat affected zone of the periphery of the laser tempering soft area is 4-10mm; the hardness of the welding nugget area is 300-700HV, the elongation of the laser tempering soft area of the periphery of the welding nugget area is 6%-20%, and the tensile strength is 700-1400MPa.

[0018] Further, the thickness of the hot-formed part is 0.8-3.0mm, and the overall deformation of the hot-formed part before and after welding is ≤1mm.

[0019] The second aspect of the present application provides the hot forming assembly part prepared by the welding method of the first aspect, wherein the hot forming assembly part has 40%-1500% increase in spot welding tensile shear displacement and 100%-500% increase in spot welding tensile shear energy absorption compared with a non-tempered spot welded part.

[0020] The beneficial effects of the present application are as follows:

[0021] The present application heats and tempers the spot welding area and the outer periphery in advance by a laser process to obtain a laser tempered soft zone, and then performs a spot welding process in the laser tempered soft zone, controls the laser tempering softening process to adapt to the size and performance requirements of the welding spot, and controls the size of the weld nugget heat affected zone through the range of the laser tempered soft zone.

[0022] The process of laser tempering + spot welding of the present application provides a wider heat affected zone compared with the non-tempered spot welding process, the heat affected zone has better plasticity and impact energy absorption effect under dynamic load, the spot welding tensile shear displacement increases by 40%-1500%, the spot welding tensile shear energy absorption increases by 100%-500%, solves the problem of stress concentration in the weld nugget heat affected zone, and provides a hot forming assembly part with better performance in the spot welding area; compared with the post-spot welding tempering process, the base body strength is not lost, the heat affected zone is small, and the part deformation is small. BRIEF DESCRIPTION OF DRAWINGS

[0023] In order to more clearly illustrate the technical solutions of the present application, the drawings needed in the embodiments will be briefly introduced as follows. Obviously, the drawings described below are only embodiments of the present application, and other drawings can also be obtained by those skilled in the art without creative labor.

[0024] Figure 1 is a schematic diagram of the spot welding process of the hot forming part of the present application;

[0025] Figure 2 is a schematic diagram of the relationship between the various zones in the hot forming assembly part of the present application;

[0026] Figure 3 is a hardness distribution diagram of the weld nugget zone of the hot forming assembly parts of Examples 1-3 and Comparative Example 1 of the present application;

[0027] Figure 4 is a tensile shear performance comparison diagram of the hot forming assembly parts of Example 3 and Comparative Example 1 of the present application;

[0028] Explanation of reference numerals in the drawings: 1, hot forming part, 11, weld nugget zone, 12, laser tempered soft zone, 13, heat affected zone, 14, base material zone, 2, resistance spot welding mechanism. DETAILED DESCRIPTION

[0029] The technical solutions of the present application will be described clearly and completely below in connection with the embodiments of the present application. Obviously, the described embodiments are only some of the embodiments of the present application, but not all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative efforts fall within the scope of the present application.

[0030] The present embodiment provides a method for optimizing the performance of a welded joint of a hot-formed part, comprising the following steps:

[0031] S1, laser tempering the designated area of the hot-formed part to be spot-welded respectively to form a laser-tempered soft area;

[0032] S2, splicing and fixing the hot-formed parts after laser tempering on a resistance spot welding fixture, wherein the area to be spot-welded is located at the center of the spliced laser-tempered soft area;

[0033] S3, starting spot welding, referring to Figure 1 , two hot-formed parts 1 are spot-welded under the action of a resistance spot welding mechanism 2 to obtain a hot-formed assembly part with a gradient hardness distribution around the nugget zone, wherein the hot-formed assembly part comprises, in order from the center to the periphery, a nugget zone 11, a laser-tempered soft zone 12, a heat-affected zone 13 and a base material zone 14, referring to Figure 2 .

[0034] The present application heats and tempers the spot-welding area and the periphery in advance through a laser process to obtain a laser-tempered soft area, and then performs a steel-steel resistance spot welding process on the hot-formed parts in the laser-tempered soft area, controls the laser tempering softening process to adapt to the size and performance requirements of the welding spot, and controls the size of the nugget heat-affected zone; compared with the un-tempered spot welding process, a wider nugget heat-affected zone is provided, which has better plasticity and impact energy absorption effect under dynamic load, solves the problem of stress concentration in the nugget heat-affected zone, and provides a hot-formed assembly part with better performance of the spot-welding area; compared with the post-spot-welding tempering process, the base strength is not lost, the nugget heat-affected zone is small, and the part deformation is small.

[0035] As a preferred embodiment, in S1, the temperature of laser tempering is 200℃-Ac1 temperature, the temperature of the laser-tempered area is monitored in real time by an infrared temperature sensor, and the laser power is adjusted according to the real-time monitored temperature to maintain the set tempering temperature; the power of laser tempering is 0.3kW-8kW, the wavelength is 600-1200mm, the defocusing amount is -10mm~+20mm, and the scanning speed is 4-20mm / s.

[0036] As a preferred embodiment, in S1, the material of the hot-formed part is a 1000 MPa grade, 1500 MPa grade or 2000 MPa grade hot-formed steel; for the 1000 MPa grade hot-formed steel, the hardness of the laser tempering soft zone is 200-300 HV; for the 1500 MPa grade hot-formed steel, the hardness of the laser tempering soft zone is 220-430 HV; for the 2000 MPa grade hot-formed steel, the hardness of the laser tempering soft zone is 250-600 HV.

[0037] As a preferred embodiment, in S3, the spot welding parameters are: preheating current 2.5-5 kA, welding current 5-15 kA, electrode pressure 2-6 kN, welding time 150-700 ms, and pulse number 1-3 times; the width (ring width) of the laser tempering soft zone at the periphery of the nugget zone is 2-10 mm, the width (ring width) of the heat-affected zone at the periphery of the laser tempering soft zone is 4-10 mm; the hardness of the nugget zone is 300-700 HV, the elongation of the laser tempering soft zone at the periphery of the nugget zone is 6%-20%, and the tensile strength is 700-1400 MPa.

[0038] As a preferred embodiment, the thickness of the hot-formed part is 0.8-3.0 mm, and the overall deformation of the hot-formed part before and after welding is ≤1 mm.

[0039] Another embodiment provides a hot-formed assembly part made by the welding method described in the above embodiments, wherein the hot-formed assembly part has a 40%-1500% increase in spot welding tensile shear displacement and a 100%-500% increase in spot welding tensile shear energy absorption compared to a non-tempered spot-welded part.

[0040] Embodiment 1

[0041] The present embodiment provides a method for optimizing the performance of a welded joint of a hot-formed part, comprising the following steps:

[0042] (1) Place a 1500 Mpa aluminum-silicon plated hot-formed part with a thickness of 1.8 mm on a laser processing table, and set the laser processing program, wherein the laser tempering temperature is 600°C, the scanning speed is 10 mm / s, the wavelength is 1080 mm, the defocusing amount is +15 mm, and the laser equipment program is started to perform laser tempering to obtain a laser tempering soft zone, the size of the laser tempering soft zone is 20 mm x 20 mm, and the part is taken out after the program is completed;

[0043] (2) splice and fix the laser-tempered hot-formed part on a resistance spot welding jig, wherein the spot welding area is located at the center of the spliced laser-tempered soft zone;

[0044] (3) start spot welding, wherein the preheating current is 4 kA, the welding current is 9.5 kA, the electrode pressure is 4.3 kN, the welding time is 500 ms, the pulse number is 2, and two hot forming assembly parts are spot welded under the action of the resistance spot welding mechanism to obtain a hot forming assembly part with a gradient hardness distribution in the nugget zone.

[0045] Example 2

[0046] The difference between this example and Example 1 is that the laser tempering temperature is 700°C, and other parameters and steps remain unchanged.

[0047] Example 3

[0048] The difference between this example and Example 1 is that the laser tempering temperature is 800°C, and other parameters and steps remain unchanged.

[0049] Comparative Example 1

[0050] The difference between this comparative example and Example 3 is that the laser tempering step of step (1) is omitted, and other parameters and steps remain unchanged.

[0051] The hardness of the nugget and the periphery of the hot forming assembly parts obtained in Examples 1-3 and Comparative Example 1 is detected, and the results are shown in Figure 3 It can be seen that the comparative example has only about 1 mm of the nugget heat affected zone, and the stress distribution is concentrated. Compared with Comparative Example 1, the laser tempering after spot welding of the example forms a gradient hardness distribution around the nugget zone, and the laser tempering provides a wider heat affected zone, solving the problem of stress concentration in the nugget heat affected zone.

[0052] The spot welding tensile shear performance of the hot forming assembly parts obtained in Example 3 and Comparative Example 1 is compared, and the results are shown in Figure 4 It can be seen that the hot forming assembly part obtained in Example 3 has better plasticity and impact energy absorption effect. In addition, through performance analysis test, it is known that the fatigue strength of the spot welded joint of the hot forming assembly part tempered in advance is increased by 30%-60% compared with the unsoftened spot welding, and the crack propagation rate under dynamic load is reduced by more than 50%.

[0053] In summary, the present application heats and tempers the spot welding area and the periphery in advance by laser process to obtain a laser tempering soft zone, and then performs spot welding process in the laser tempering soft zone, controls the laser tempering softening process to adapt to the size and performance requirements of the welding spot, and controls the size of the weld nugget heat affected zone through the laser tempering soft zone range. Compared with the non-tempering spot welding process, the laser tempering + spot welding process provides a wider heat affected zone, the spot welding tensile shear displacement increases by 40%-1500%, the spot welding tensile shear energy absorption increases by 100%-500%, the heat affected zone has better plasticity and impact energy absorption effect under dynamic load, solves the problem of stress concentration in the weld nugget heat affected zone, and provides a hot forming assembly part with better performance in the spot welding area; compared with the post-spot welding tempering process, the base body strength is not lost, the heat affected zone is small, and the part deformation is small.

[0054] The present application is described in detail above in combination with the specific embodiments and exemplary examples, but these descriptions cannot be understood as limiting the present application. Those skilled in the art understand that the technical solutions and embodiments of the present application can be variously replaced, modified or improved without departing from the spirit and scope of the present application, and these all fall within the scope of the present application. The protection scope of the present application is subject to the appended claims.

Claims

1. A method of optimizing the performance of a welded joint of a thermoformed part, characterized in that, The method comprises the following steps: S1, laser tempering is performed on a designated area of a hot-formed part to be spot-welded to form a laser-tempered soft area; S2, the hot-formed part after laser tempering is spliced and fixed on a resistance spot welding jig, wherein the spot-welding area is located at the center of the laser-tempered soft area after splicing; S3, spot welding is started to obtain a hot-formed assembly part with a gradient hardness distribution around the nugget zone.

2. The method of optimizing the weld joint performance of a thermoformed part of claim 1, wherein, In S1, the temperature of laser tempering is 200℃-Ac1.

3. The method of optimizing the weld joint performance of a thermoformed part of claim 1, wherein, In S1, the power of laser tempering is 0.3kW-8kW, the wavelength is 600-1200mm, the defocusing amount is-10mm~+20mm, and the scanning speed is 4-20mm / s.

4. The method of optimizing the weld joint performance of a thermoformed part of claim 1, wherein, In S1, the material of the hot-formed part is 1000MPa-grade, 1500MPa-grade or 2000MPa-grade hot-formed steel.

5. The method of optimizing the performance of a welded joint of a thermoformed part according to claim 4, wherein, For 1000MPa-grade hot-formed steel, the hardness of the laser-tempered soft area is 200-300HV; for 1500MPa-grade hot-formed steel, the hardness of the laser-tempered soft area is 220-430HV; and for 2000MPa-grade hot-formed steel, the hardness of the laser-tempered soft area is 250-600HV.

6. The method of optimizing the weld joint performance of a thermoformed part of claim 1, wherein, In S1, the temperature of the laser-tempered area is monitored in real time by an infrared temperature sensor.

7. The method of optimizing the weld joint performance of a thermoformed part of claim 1 wherein, In S3, the spot welding parameters are as follows: preheating current 2.5-5kA, welding current 5-15kA, electrode pressure 2-6kN, welding time 150-700ms, and pulse number 1-3 times.

8. The method of optimizing the weld joint performance of a thermoformed part of claim 1, wherein, In S3, the width of the laser-tempered soft area around the nugget zone is 2-10mm, the width of the heat-affected zone around the laser-tempered soft area is 4-10mm, the hardness of the nugget zone is 300-700HV, the elongation of the laser-tempered soft area around the nugget zone is 6%-20%, and the tensile strength is 700-1400MPa.

9. The method of optimizing the weld joint performance of a thermoformed part of claim 1, wherein, The thickness of the hot-formed part is 0.8-3.0mm, and the overall deformation of the hot-formed part before and after welding is ≤1mm.

10. A thermoformed assembly part made by the method of any one of claims 1-9, wherein, Compared with the spot-welding tensile shear displacement of a non-tempered spot-welded part, the spot-welding tensile shear displacement of the hot-formed assembly part is increased by 40%-1500%, and the spot-welding tensile shear energy absorption is increased by 100%-500%.

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