JP3911134B2 - Spot welding method with excellent collision safety - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a spot welding method used for structural members for automobiles, and more particularly to a welding method that is optimal for preventing a spot welded portion of a member from breaking at the time of a collision.
[0002]
[Prior art]
In recent years, in the automobile industry, the development of a vehicle body structure that can reduce injury to passengers during a collision has become an urgent issue. Such a vehicle body structure excellent in collision safety can be realized by absorbing the impact energy at the time of collision by a structural member other than the passenger compartment to ensure the living space by minimizing the deformation of the passenger compartment. That is, it is important to absorb impact energy by the structural member. The main structural member that absorbs impact energy in automobile full-wrap collision and offset collision is the front side member. The front side member has a closed cross-section by spot welding after forming the member by press molding or the like. Usually, the impact energy is absorbed by buckling the front side member. In order to improve the absorption of impact energy, it is important to stabilize the buckling form and not to bend or break along the way.
[0003]
In order to stabilize the buckling of the above-mentioned members, if the spot welding interval is not optimized, fracture from the welding point occurs at the time of buckling. There is a problem that absorption decreases. In the past, in order to solve this problem, no. 9705JSAE SYMPOSIUM "New body structure forming technology" and so on, we prototyped a member with various spot welding intervals and conducted a buckling test to investigate the conditions for stable buckling without breaking at the weld point. . However, this method has a problem that trial and error such as trial manufacture and test for each automobile and each member is necessary, which requires manufacturing cost and takes time for designing. Japanese Patent Laid-Open No. 6-182561 proposes a structure for preventing separation of a welded portion where a load is applied to the floor panel, but it is a structure only for the floor panel. In order to find a spot welding method that prevents delamination and absorbs impact energy by stable buckling, trial and error by trial manufacture has been performed.
[0004]
[Problems to be solved by the invention]
It is an object of the present invention to provide a welding method that improves the absorption of impact energy by preventing the optimum spot welding interval of an impact energy absorbing member from being produced by prototyping the member and preventing breakage of the welded portion at the time of impact of the member. .
[0005]
[Means for Solving the Problems]
In order to solve the above-described problems, a spot welding method in which two metal plates according to the present invention are overlapped and welded is as follows.
(1) First, the thickness t [mm] of the metal plate, the ratio t / D [−] of the thickness of the plate thickness to the cross section of the member, the tensile strength TS of the base metal TS [MPa], and the spot welding interval P [mm] Is used as a parameter (variable), the finite element method analysis is performed by modeling the member after spot welding as a mesh model, and the external load f ₁ [kN] applied to the spot weld at the time of impact, which cannot be obtained by experiments, is calculated. Ask. It is preferable to obtain the external load f ₁ [kN] applied to the spot welded portion by multiple regression analysis as shown in Equation (1).
f ₁ = 0.00669t ^0.0242 (t / D) ^0.00103 TS ^0.593 P ^0.469 (1)
(2) Next, since the strength of the weld itself is not determined by a static test but is determined by a dynamic test, the thickness t [mm] of the metal plate, the tensile strength TS [MPa] of the base metal, and the welding With the nugget diameter r [mm] as a parameter, the peel strength f ₂ [kN] of the weld is determined by a special dynamic test called the onebar type high-speed tensile test. It is preferable to obtain the peel strength f ₂ [kN] at the time of impact of the spot welded part by multiple regression analysis as shown in Equation (2).
f ₂ = 0.0748t ^1.15 TS ^0.195 r ^1.12 (2)
(3) The spot welding interval is set so that the external load f ₁ applied to the spot welded portion is less than the peel strength f ₂ of the welded portion so that f ₁ <f ₂ from the formulas (1) and (2). Determine the welding conditions.
Further, the weld nugget diameter r [mm] is r = 5√t corresponding to the class A of JISZ3140 as the plate thickness t [mm], and r = 3 to 7 mm. It is a spot welding method.
[0006]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the present invention will be described in detail.
From the above formulas (1) and (2), if the base material tensile strength TS, plate thickness t, etc. are determined when designing the spot welding method for the impact energy absorbing member, Sometimes, the minimum necessary spot welding interval P for connecting the metal plates of the members for preventing breakage of the welded portion can be easily obtained. The influence of each parameter (variable) when obtaining the welding conditions from the equations (1) and (2) is as follows: plate thickness t = 1.1 mm, nugget diameter r = 5.2 mm, plate thickness / circumference ratio t / D = 0.02, the spot welding interval P and the base material tensile strength TS are as shown in FIG. The region where the vertical axis (welded part external load f ₁ ) − (weld part peel strength f ₂ ) is 0 or less is the optimum welding condition. become. Actually, it is difficult to predict the welding condition empirically because it varies even with the above variables, but the welding condition can be easily obtained by using this calculation formula.
[0007]
【Example】
Embodiments of the present invention shown in the drawings will be described below. FIG. 2 shows a hat-shaped cross-section member simulating a member member of an automobile. The cross-sectional shape is uniform in FIG. 2 and is 300 mm in the longitudinal direction (perpendicular to the plane of the drawing), and is composed of a hat-shaped member 1 and side plates 2. The thickness t of the steel plate of the member is 0.8 mm, the tensile strength TS is 300 MPa, the nugget diameter r = 4.5 mm, and the thickness / circumference ratio (t / D) is t / D = 0.018 from FIG. It becomes. From the determination of these variables, the external load f ₁ applied to the spot welded portion is obtained by the equation (1) by the welding interval as in the following equation.
f ₁ = 0.195P ^0.469
Further, according to the equation (2), the weld peel strength f _{2 is} 0.94 [kN].
[0008]
Therefore, the welded portion external load f ₁ for preventing fracture at the welded portion is determined only by the spot welding interval, and can be arranged as shown in FIG. That is, the spot welding interval can be determined to be less than 30 mm from the spot welding conditions where f ₁ <f ₂ . FIG. 4 is a comparison of the finite element method simulation results of the buckling configuration of the member in the 25 mm interval welding which is the optimum welding condition and the 75 mm interval welding which is the fracture of the weld based on the spot welding method according to the above-described example of the present invention. It is. As can be seen from the side view, the member with 25 mm interval welding does not cause fracture of the welded part and periodically buckles cleanly, while the member with 75 mm interval welding causes fracture of the welded part and the side plate peels off. It does not contribute to buckling deformation, and the buckling form is also irregular. The collision safety of the member is evaluated by the absorbed energy due to buckling deformation. FIG. 5 shows a comparison of the absorbed energy of the 50 mm interval welding in addition to the 25 mm and 75 mm interval welding shown in FIG. At any stage where the axial crushing length, which is the amount of buckling deformation, is 50, 100, or 150 mm, the absorbed energy characteristic of the member with the 25 mm interval spot welding condition according to the present invention is excellent, and is approximately compared with the member with 75 mm interval welding. The characteristic is improved by 8%.
[0009]
Needless to say, the spot welding interval determination method according to the present invention can be applied not only to the hat-shaped cross-section member shown in the embodiment but also to any polygonal cross-section and flanged member.
[0010]
【The invention's effect】
By finding the optimum spot welding conditions based on the present invention, trial and error with prototype members with different welding conditions can be omitted in order to ensure collision safety at the time of conventional member design. In addition to drastically reducing costs, design time can also be reduced. In addition, since the spot welding of the actual member can be performed at the minimum interval that satisfies the collision safety, the welding cost at the time of manufacturing can be reduced.
[Brief description of the drawings]
FIG. 1 is a diagram for explaining spot welding interval determination when viewed in terms of base material tensile strength and welding interval.
FIG. 2 is a cross-sectional view of a member model for performing welding condition determination.
FIG. 3 is a diagram illustrating welding interval determination conditions.
FIG. 4 is a diagram showing a change in buckling mode (at the time of 150 mm axial crush deformation) due to a difference in welding conditions.
FIG. 5 is a diagram showing an improvement in absorbed energy of a member under welding conditions.

Claims (3)

In a spot welding method in which two metal plates are stacked and welded,
(1) Parameters of the thickness t [mm] of the metal plate, the ratio t / D [−] of the plate thickness to the circumferential length of the member cross section, the base material tensile strength TS [MPa], and the spot welding interval P [mm] Then, the finite element method analysis is performed by modeling the member after spot welding, and the external load f ₁ [kN] applied to the spot weld is obtained.
(2) Peel strength f ₂ [kN] of the welded part by dynamic tests using the thickness t [mm] of the metal plate, the tensile strength TS [MPa] of the base metal, and the weld nugget diameter r [mm] as parameters. Seeking
(3) The spot welding interval is determined so that the external load f ₁ [kN] applied to the spot weld is less than the peel strength f ₂ [kN] of the weld.
A spot welding method excellent in collision safety characterized by this. 2. The collision safety according to claim 1, wherein the welding nugget diameter r [mm] is r = 5√t corresponding to the class A of JISZ3140 as the plate thickness t [mm], and r = 3 to 7 mm. excellent spot welding how to. Obtain the external load f ₁ [kN] applied to the spot weld by the following formula (1),
f ₁ = 0.00669t ^0.0242 (t / D) ^0.00103 TS ^0.593 P ^0.469 (1)
The spot welding method excellent in collision safety according to claim 1 or 2, wherein the peel strength f ₂ [kN] of the welded portion is obtained by the following equation (2).
f ₂ = 0.0748t ^1.15 TS ^0.195 r ^1.12 (2)

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