CN114247999A

CN114247999A - Laser spot welding method for high-strength steel lamination

Info

Publication number: CN114247999A
Application number: CN202011008038.5A
Authority: CN
Inventors: 杨上陆; 陶武
Original assignee: Shanghai Institute of Optics and Fine Mechanics of CAS
Current assignee: Shanghai Institute of Optics and Fine Mechanics of CAS
Priority date: 2020-09-23
Filing date: 2020-09-23
Publication date: 2022-03-29

Abstract

A high-strength steel lamination laser spot welding method comprises the following steps: superposing two or more steel workpieces to form a laminated combination, wherein the steel workpiece matrix is Q & P steel and the thickness is less than 3.0 mm; arranging air blowing openings around the position to be welded, wherein the air blowing openings are flat, the air outlet direction of the air blowing openings is approximately parallel to the welding surface, the vertical distance between the air blowing openings and the welding surface is 0-100 mm, and a compressed air source is used, and the air flow is 15-50L/min; enabling a laser beam to act on the upper surface of the laminated combination along a preset scanning path, setting welding parameters to enable the laser beam not to penetrate through the lower surface of the laminated combination, realizing melting and connection of a bottom workpiece through heat conduction, and finally forming a punctiform weld joint with the following characteristics: the area of the melted region of the lower surface of the laminate combination does not exceed 70% of the area of the melted region of the upper surface of the laminate combination. The invention can improve the utilization rate of laser energy, ensure the consistency of welding quality and improve the performance of a joint.

Description

Laser spot welding method for high-strength steel lamination

Technical Field

The invention belongs to the field of high-strength steel laser welding, and particularly relates to a high-strength steel laminated laser spot welding method.

Background

In the automobile manufacturing industry, the application of higher-strength steel in the automobile body manufacturing is promoted by the requirements of reducing the weight of the automobile, realizing energy conservation and emission reduction, improving the collision safety of the automobile and the like.

In automotive construction, multiple steel sheets are typically stacked to form a stack assembly, and welding of different stack steel sheet assemblies is required to achieve the manufacture of a vehicle body or component. The existing production method uses a resistance spot welding technology, needs higher welding current, has lower production efficiency, and can not solve the quality problem of high-strength steel welding. The laser welding technology is a method for applying laser beams on the surface of a welding material and melting the material by utilizing laser energy to realize welding, and has the advantages of single-side welding, good accessibility, no need of contact, high welding efficiency, small welding deformation, easiness in realizing automatic control and the like.

Generally, when welding a laminated steel sheet assembly with a laser beam, it is necessary to provide sufficient laser energy, such as a higher laser power, to achieve complete penetration of the laminated assembly and to ensure the performance of the welded joint. However, high-strength steel, especially second and third generation high-strength automobile steel, is sensitive to heat action due to high content of alloy elements in the steel substrate or through a relatively complicated and finely controlled heat treatment process; in the laser welding process, the original structure state of the material is damaged, and the higher alloy element content causes the problems of weld joint element segregation and the like, so that the performance of a welded joint is poor, and the application of a higher-strength steel plate and the popularization of a laser welding technology are limited.

Taking the third generation high strength steel QP steel as an example, the matrix structure of the material is mainly lath martensite and retained austenite, wherein the retained austenite generates phase transformation induced plasticity in the deformation process of the material, and the plasticity of the steel is greatly improved. In the laser welding process, the content of residual austenite in a welding seam is reduced rapidly due to the fact that materials are completely melted and then cooled and solidified, meanwhile, the welding seam and a heat affected zone have large thermal stress due to temperature gradient and tissue phase change, and finally the performance of a welding joint is poor, the strength and the plasticity are in low levels, and the conditions of production and application are difficult to meet.

It is noted that in the case of ultra-high strength steels, such as QP1180 (tensile strength of about 1200 MPa), high quality weld joints cannot be obtained by resistance spot welding (a double-sided welding method that uses contact resistance between laminated materials to produce heat at a large welding current) that is commonly used for automotive steel welding. Therefore, an effective welding method is needed to solve the above welding problems and promote the application of high strength steel such as QP steel and laser welding technology.

Disclosure of Invention

The method disclosed by the invention aims to solve the problems and realize the laser welding and application of the high-strength steel lamination combination.

The technical scheme adopted by the invention is as follows:

a high-strength steel lamination laser spot welding method comprises the following steps:

superposing two or more steel workpieces to form a laminated combination, wherein at least one steel workpiece is made of high-strength steel, and the tensile strength of the high-strength steel is not lower than 600 MPa;

arranging air blowing openings around the position to be welded of the laminated combination, wherein the air outlet direction of the air blowing openings is parallel to the welding surface, and the air flow is 10-50L/min;

enabling a laser beam to act on the upper surface of the laminated combination along a preset scanning path, setting welding parameters to enable the laser beam not to penetrate through the lower surface of the laminated combination, realizing melting and connection of the bottom workpiece through heat conduction of the upper workpiece, and enabling a finally formed punctiform weld joint to have the following characteristics: the area of the melted region of the lower surface of the laminate combination does not exceed 70% of the area of the melted region of the upper surface of the laminate combination.

The laser beam scanning path has a cyclic repeat feature such that the laser beam scans in a trailing path in a weld or weld puddle formed by a leading path scan.

The scanning path is an equidistant spiral line, the number of the windings of the spiral line is N, N is more than or equal to 8, and the adjacent distance L of the spiral line is smaller than the diameter of a laser beam focusing light spot.

The product of the number N of the spiral winding and the adjacent distance L (mm) of the spiral is 2.5-5.0.

The laser beam starts scanning from a spiral coil with a larger diameter and gradually develops towards the center, and the scanning speed is more than or equal to 8 m/min; the quotient of the product value of the laser power W (watt) and the laser beam scanning time t (second) and the value of the whole plate thickness H (mm) of the lamination combination welding position is 600-1000.

The laser beam is applied by scanning the weld head with a galvanometer.

The outlet of the air blowing opening is flat, and the air source is compressed air.

The vertical distance between the position of the air blowing opening and the welding surface is 0-90 mm.

The high-strength steel workpiece contains an austenite structure, the volume fraction of the austenite structure is 5% -40%, and the thickness of the steel workpiece is less than 3.0 mm.

The high-strength steel workpiece is Q & P (quenching-partitioning) steel.

The invention has the advantages and positive effects that:

1) by controlling the area of the melting region of the lower-layer workpiece of the laminated combination, the damage to the structure state of raw materials can be greatly reduced, and the residual stress level of a welding seam is reduced, so that the mechanical property of a welding joint is greatly improved.

2) For materials (such as QP1180 steel) which cannot be welded in high quality by a conventional welding method (such as resistance spot welding) and a conventional welding process (such as laser penetration welding), the method provided by the invention can realize high-quality welding, promote the application of high-strength steel, especially ultrahigh-strength steel materials in automobile production, and provide assistance for energy conservation and emission reduction; meanwhile, the method is beneficial to popularization and application of a laser welding technology, improves the production efficiency and reduces the energy consumption.

3) The method of the invention has the advantages of repeated scanning path, which is helpful for forming a stable and expanded molten pool and reducing the generation of welding spatter.

4) By the method, the laser beam develops from a position with larger outer diameter to the center, and the laser beam is matched with higher scanning speed (more than or equal to 8m/min), so that the utilization rate of laser energy can be improved, the requirement of laser power is reduced, and the energy loss is reduced.

5) The auxiliary blowing method is beneficial to reducing the absorption of the plasma to the laser beam and improving the stability of the welding quality.

6) The method has better effect of improving the performance of the high-strength steel laser welding head containing the residual austenite structure.

Drawings

In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly described below, and it is obvious that the drawings in the following description are only some embodiments of the present invention, and it is obvious for those skilled in the art that other alternative embodiments can be obtained according to the drawings without creative efforts.

FIG. 1 is a schematic view of laser spot welding of a laminated combination of an upper workpiece and a lower workpiece.

FIG. 2 is a schematic cross-sectional view of the lower surface of the upper and lower workpiece laminate assembly without laser spot welding.

FIG. 3 is a photograph of the lower surface of the actual workpiece without melting the combined lower surfaces of the two workpiece plies.

FIG. 4 is a cross-sectional photograph of the actual workpiece showing the combined bottom surfaces of the two-layer workpiece stack without melting.

FIG. 5 is a schematic cross-sectional view of the upper and lower layers of the workpiece stacked combination, in which the area of the lower surface melting region reaches 85% of the area of the upper surface melting region.

FIG. 6 is a photograph of a cross-section of an actual workpiece showing a 1.4mm QP980 two-layer workpiece stack having a combined lower surface melted to 90% melted area of the upper surface.

FIG. 7 is a comparison of tensile test curve data for a 1.4mm QP980 steel plate with two layers combined welded with the lower surface unmelted (inventive method) and the lower surface melted area reaching 90% of the upper surface melted area (conventional method).

FIG. 8 is a schematic cross-sectional view of the combined lower surface melt area of the upper and lower workpiece laminations being about 40% of the upper surface melt area.

FIG. 9 is a photograph of the lower surface of an actual workpiece with a 1.2mm QP980 two-layer workpiece stack having a combined lower surface melted to 40% melted area on the upper surface.

FIG. 10 is a cross-sectional photograph of a workpiece in which the combined lower surface of a two-layer workpiece stack of 1.2mm QP980 was melted to 40% of the melted area of the upper surface.

FIG. 11 is a comparison of tensile test curve data of a 1.0mm QP980 steel plate two-layer combination welded with a lower surface melting area of 55% of the upper surface melting area (inventive method) and a lower surface melting area of 95% of the upper surface melting area (conventional method).

FIG. 12 is a photograph of the fusion of the lower surface of a weld joint generated by the vertical distance between an air blowing port and the welding surface of a material in a two-layer combined welding process of a 1.0mm QP980 steel plate and 10 mm.

FIG. 13 is a photograph of the fusion of the lower surface of a weld joint generated by the vertical distance of 90mm between an air blowing port and the welding surface of a material in a two-layer combined welding process of a 1.0mm QP980 steel plate.

FIG. 14 is a photograph of the non-molten lower surface of a weld produced by a two-layer combination welding process of a 1.0mm QP980 steel plate without blowing.

FIG. 15 is a photograph of the unmelted lower surface of a weld produced in a two-layer combined welding process of a 1.0mm QP980 steel plate with a vertical distance between an air blowing port and the welding surface of the material being 140 mm.

Reference numeral, 1-laser beam, 21-laser beam scan path, 22-unmelted circular spot cross-sectional melted region of combined lower surface of two-layer stack, 23-melted region of combined lower surface of two-layer workpiece stack up to 85% cross-sectional melted region of area of upper surface melted region, 24-melted region of combined lower surface of two-layer workpiece stack up to about 40% cross-sectional melted region of area of upper surface melted region, 3-upper workpiece, 31-upper workpiece upper surface or first surface, 4-lower workpiece, 41-lower workpiece lower surface or second surface.

Detailed Description

In the drawings, the method of the invention is schematically depicted in different forms of existence, and the cross section of the welding spot obtained by the method of the invention and the traditional method is compared with the performance of a tension-shear test. The drawings are only for purposes of illustrating possible examples of the invention and are not intended to limit the scope of the invention. For better understanding of the above technical solutions, the following detailed descriptions are provided with reference to the accompanying drawings and specific embodiments.

Example 1

Referring to fig. 1, which is a schematic diagram of a laser spot welding process of the method of the present invention, a laser beam 1 is applied to a first surface 31 of a laminated assembly of an upper steel workpiece 3 and a lower steel workpiece 4 along a scanning path 21 to form a circular spot-shaped weld; in the process, the compressed air generates transverse airflow right above the welding seam through the air blowing port 5, and dust generated by laser welding is blown away, so that the laser beam is ensured to be absorbed by the welding seam.

In the embodiment, the welding lamination is composed of two layers of 1.4mm QP1180, the scanning path 21 is a spiral line with 10 turns and 0.292mm spacing, the laser power is 2500W, and the scanning speed of the laser beam is 10.0m/min, so that the weld joint 22 shown in fig. 2 is obtained, i.e., the lower surface 41 of the lower layer workpiece 4 is not melted. The welding spot is shown on the back side in fig. 3 and the cross section in fig. 4, and it can be seen that the material between the upper and lower workpieces is melted to form a good connection, and the lower part of the lower workpiece is not melted.

In contrast, conventional welding methods using different laser scanning paths and welding parameters will form a weld 23 as shown in FIG. 5, where the lower surface melt area reaches 85% of the upper surface melt area. In the comparative example, in the case where the other parameters of the above example were the same, the laser power was increased to 3500W, and the cross section of the obtained weld was as shown in FIG. 6, in which the lower surface melting area of the weld reached 90% of the upper surface melting area of the weld.

FIG. 7 shows the solder joint properties obtained in the above examples compared with those obtained in the comparative examples. The red curve is a pull-shear curve of the welding point obtained by the welding method, and the pull-shear displacement and the pull-shear load of the red curve are far higher than those of the blue pull-shear curve of the welding point obtained by the traditional welding method. Through statistics and calculation of specific values of all curves, the welding method can improve the tensile-shear load of the welding spot, namely the strength of the welding spot by about 25%, improve the tensile-shear displacement corresponding to the maximum load, namely the plasticity of the welding spot by about 80%, and the higher the plasticity of the welding spot, the greater the energy absorbed by the welding spot in the damage process, the more beneficial to the collision safety of vehicles.

Example 2

As shown in fig. 8, the weld 24 is obtained by adjusting the welding parameters such that the lower surface melt area is about 40% of the upper surface melt area. By adopting the welding process and parameter setting of the embodiment 1, the laminated material combination is two layers of QP980 plates with the thickness of 1.2mm, the laser power is set to be 2300W, the back surface of the welding spot after welding is finished is shown in figure 9, the cross section of the welding spot is shown in figure 10, and the melting area of the lower surface is 40% of the melting area of the upper surface.

Example 3

By adopting the welding process and parameter setting of the embodiment 1, the laminated material combination is a QP980 two-layer board with the thickness of 1.0mm, the laser power is set to 2000W, the area of the melting region on the lower surface of the obtained welding spot is 55% of the area of the melting region on the upper surface, and the corresponding tensile curve of the welding spot joint is shown in the red invention method in the figure 11; the laser power was 3000W, the area of the melted region on the lower surface of the resulting weld spot was 95% of the area of the melted region on the upper surface, and the corresponding weld joint tensile curve is shown in the blue conventional method in fig. 11. Therefore, the joint performance of the welding spot obtained by the method is improved, and the strength and the plasticity of the welding spot obtained by the method are obviously improved similar to the QP1180 with the thickness of 1.4mm in the embodiment 1.

Example 4

In the above embodiment, the blowing port is flat, and the gas is compressed air. Specifically, the laminated material combination is a QP980 two-layer plate with the thickness of 1.0mm, the laser power is set to 3000W, and the compressed air flow is set to 30L/min; when the vertical distance between the air blowing port and the welding surface of the material is 10mm, the lower surface of the laminated combination penetrates through, and the lower surface of the formed welding point is shown in figure 12.

Example 5

Similar to example 4, except that the vertical distance of the air blowing port from the welding surface of the material was set to 90mm, the lower surface of the formed weld spot was penetrated as shown in FIG. 13.

Comparative example 1

Similar to example 4, except that the gas flow rate of the gas blowing port was set to 0, that is, no gas was supplied, and the lower surface of the formed weld was not melted, as shown in fig. 14, it was demonstrated that the gas blowing method provided by the present invention can effectively suppress the influence of the laser welding fumes on the weld penetration.

Comparative example 2

Similar to example 4, except that the vertical distance between the blowing port and the welding surface of the material was set to 140mm, the lower surface of the formed weld was not melted, as shown in fig. 15, indicating that the location of the blowing port is a key factor in determining the blowing effect.

In the above embodiments, the two-layer plate lamination combination is taken as an example, and it is noted that the method of the present invention can be applied to lamination combinations of three or more layers. In the embodiment, compressed air is used as a blowing source, which is beneficial to reducing the actual use and production cost, and the use of other gases such as argon, nitrogen, carbon dioxide and the like is not limited, so that the same effect can be obtained.

The above description is only an illustrative embodiment of the present invention, but the scope of the present invention is not limited thereto, and any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope of the present invention should be covered by the present invention. Therefore, the protection scope of the present invention shall be subject to the protection scope of the appended claims.

Claims

1. A high-strength steel lamination laser spot welding method comprises the following steps:

2. The high strength steel laminate laser spot welding method of claim 1, wherein the laser beam scanning path has a cyclically repeating characteristic such that the laser beam is scanned in a trailing path in a weld or weld puddle formed by a leading path scan.

3. The high-strength steel laminated laser spot welding method according to claim 1 or 2, wherein the scanning path is an equidistant spiral, the number of windings of the spiral is N, N is more than or equal to 8, and the adjacent distance L of the spiral is smaller than the diameter of a laser beam focusing spot.

4. The laser spot welding method for high strength steel lamination according to claim 3, wherein the product of the number N of turns of the helix and the adjacent pitch L (mm) of the helix ranges from 2.5 to 5.0.

5. The laser spot welding method for high-strength steel lamination according to claim 3, wherein the laser beam is scanned from the spiral coil with a larger diameter and gradually progresses toward the center, and the scanning speed is more than or equal to 8 m/min; the quotient of the product value of the laser power W (watt) and the laser beam scanning time t (second) and the value of the whole plate thickness H (mm) of the lamination combination welding position is 600-1000.

6. The high strength steel laminate laser spot welding method according to claim 1 or 2, wherein the laser beam is performed by scanning a welding head with a galvanometer.

7. The high-strength steel lamination laser spot welding method according to claim 1, wherein a gas blowing port is arranged around a position to be welded of the lamination combination, the gas outlet direction of the gas blowing port is parallel to the welding surface, and the gas flow is 10-50L/min; the outlet of the air blowing opening is flat, and the air source is compressed air.

8. The laser spot welding method for high strength steel lamination according to claim 1 or 7, wherein the vertical distance between the blow port position and the welding surface is between 0 and 90 mm.

9. The high strength steel laminate laser spot welding method of claim 1, wherein the high strength steel workpiece has a retained austenite structure with a volume fraction of 5% to 40%, and the thickness of the steel workpiece is less than 3.0 mm.

10. The high strength steel laminate laser spot welding method of claim 8, wherein the high strength steel workpiece is Q & P (quench-and-dispense) steel.