SPOT WELDING METHOD FOR MULTI-LAYERS AND SPOT WELDING APPARATUS USING THE SAME

Abstract

A welding method and a welding apparatus using the same control heat emission amounts of interfaces of a welding subject to be similar by adjusting cross-sectional areas of two welding tips arranged on both outer facing surfaces of a panel type welding subject, superimposed in multi-layers, to simultaneously form a nugget diameter.

Claims

1. A welding method capable of multi-layer welding, the method comprising: heat emission amounts of interfaces of welding subjects are controlled to be similar by adjusting cross-sectional areas of two welding tips arranged on both outer facing surfaces of a panel type welding subject superimposed in multi-layers to simultaneously form a nugget diameter.

2. The method of claim 1, wherein a ratio (x/y) of the cross-sectional areas of two welding tips satisfies the following equation, which is a function of a thickness and resistivity of the welding subject, and wherein the ratio (x/y) of the cross-sectional areas of the welding tips has an allowable value of ±10%: $\frac{x}{y}=f\left({\rho }_i,t_i\right),$ wherein ρ represents resistivity of each layer of the welding subject, t represents a thickness of each layer of the welding subject, and i represents the number of welding subjects is a natural number of 2 or more.

3. The method of claim 2, wherein the ratio (x/y) of the cross-sectional areas of the welding tips has an allowable value of ±20%.

4. The method of claim 2, wherein a pitch, which is an interval between welding spots, is a short pitch in the range of 10 to 30 mm.

5. The method of claim 2, wherein the number of welding subjects is 3 or more.

6. A welding apparatus wherein the welding method of claim 1 is applied.

7. A welding apparatus wherein the welding method of claim 2 is applied.

8. A welding apparatus wherein the welding method of claim 3 is applied.

9. A welding apparatus wherein the welding method of claim 4 is applied.

10. A welding apparatus wherein the welding method of claim 5 is applied.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] FIG. 1 is a conceptual diagram for describing a shunt effect.

[0020] FIG. 2 is a graph of a welding machine in the related art when two-layer short pitch (10 mm pitch is applied) welding is applied and illustrates that the welding machine compensates calories loss by a shunt effect.

[0021] FIG. 3 is a graph of the welding machine in the related art when three-layer short pitch (10 mm pitch is applied) welding is applied and illustrates that strength deteriorates and nugget diameter decreases by the shunt effect.

[0022] FIGS. 4 and 5 illustrate compensations of shunt effects when the welding machine in the related art is applied to two-layer welding and three-layer welding.

[0023] FIG. 6 is the conceptual diagram of the three-layer welding.

[0024] FIG. 7 is proportional graph shown in a resistivity-strength (K grade).

[0025] FIG. 8 is a graph of resistivity scattering according to a material strength.

[0026] FIG. 9 is a photo diagram of various welding tips.

[0027] FIGS. 10A-10C are graphs showing a tensile strength and a nugget diameter size according to a welding tip ratio in Example 1.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0028] Hereinafter, a welding method capable of multi-layer welding through adjustment of a cross-sectional area of a welding tip and a configuration of a welding apparatus using the same according to the present disclosure are described in detail with reference to drawings.

[0029] However, the drawings are provided as an example for allowing those having ordinary skill in the art to sufficiently appreciate the spirit of the present disclosure. Accordingly, the present disclosure is not limited to drawings presented below but may be embodied in other aspects.

[0030] In addition, unless otherwise defined, the terms used in the description of the present disclosure have the same meaning as commonly understood by those having ordinary skill in the art to which the present disclosure belongs. In the following description and the accompanying drawings, a detailed description of known functions and configurations that may unnecessarily obscure the gist of the present invention have been omitted.

[0031] The present disclosure is basically contrived to provide a welding method capable of spot welding of multi layers, in particular, multiple steel plates for a vehicle and a welding apparatus using the same.

[0032] During Existing Insurance Institute for Highway Safety (IIHS) Small Overlap and Side Crash Test, there is a problem in that collision capability deficiency, i.e., reduced or poor crashworthiness, occurs due to an increase in vehicle body deformation caused by real side portion welding point interface breakage (occurring when nugget diameter is insufficient) and button breakage. As a result, there are many cases in which retest is performed by adding a welding point and structure adhesive. Further, since high cost is incurred for advance confirmation and test cost for a retest vehicle, an improvement measure therefor is required.

[0033] In general, in order to improve a collision capability, a lot of welding points should be secured by reducing a welding pitch interval, but it is difficult to increase the number of welding points because of a shunt effect.

[0034] FIG. 1 is a conceptual diagram for describing a shunt effect. As illustrated in FIG. 1, when a shunt effect occurs in which welding current flows at a place which is not related to a welding portion, a heat emission amount used for welding is reduced due to a decrease in current actually transmitted to the welding portion, and as a result, a welding strength deteriorates.

[0035] As a resolution for this problem, current and calories lost by the shunt effect may be supplemented by purchasing and using various welding machines. But in this case, only two-layer welding is generally possible. As a result, a fundamental solution may not be provided by a known welding machine in that most general collision members are welding of 3 layers or more.

[0036] Therefore, in the related art, a welding machine that adopts a scheme of supplementing the current and the calories according to a resistance pattern by the shunt effect is used.

[0037] Specifically, the welding machine in the related art is applicable to two-layer welding during short pitch (30 mm) welding. However, it is impossible to apply the welding machine in the related art during welding of 3 layers or more by a difference in contact resistance between interfaces. Such a problem becomes more significant in a short pitch of, in particular, less than 30 mm, and more specifically, a 10 mm level.

[0038] Calorie loss compensation and limitations of the welding machines in the related art may be confirmed through the graphs of FIGS. 2 and 3.

[0039] FIG. 2 is a graph of a welding machine in the related art when two-layer short pitch (10 mm pitch is applied) welding is applied. FIG. 2 illustrates that the welding machine compensates calorie loss by a shunt effect.

[0040] Further, FIG. 3 is a graph of the welding machine in the related art when three-layer short pitch (10 mm pitch is applied) welding is applied FIG. 3 illustrates that strength deteriorates and nugget diameter decreases because of the shunt effect.

[0041] Similarly, in respect to a reason why current compensation is not achieved in the three-layer welding in the welding machine in the related art, when a time difference of formation of a nugget diameter occurs due to an interface resistance difference, current compensation is not equally achieved and unequal compensation is achieved according to a contact surface. As a result, it is determined that it is impossible to apply the three-layer welding.

[0042] FIGS. 4 and 5 illustrate compensation of the shunt effect when the welding machine in the related art is applied to two-layer welding and three-layer welding.

[0043] In other words, as illustrated in FIG. 4, a welding condition may be set so that the intelligent welding machine compensates the current loss by the shunt effect when the welding machine in the related art is applied to the two-layer welding. As illustrated in FIG. 5, the time difference occurs in forming the nugget diameter when the welding machine in the related art is applied to the three-layer welding. In particular, more compensation current is consumed in super high tension steel and, as a result, a larger unequal compensation is generated.

[0044] In the present disclosure, in order to resolve such a problem, a method is devised in which the nugget diameter is simultaneously formed by controlling heat emission amounts of the interfaces to be similar during multi-layer welding. The method does so through adjustment of cross-sectional areas of two welding tips arranged on both outer facing surfaces of a panel type welding subject to enable equal calorie compensation between the contact surfaces.

[0045] Here, a heat emission amount during welding is in proportion to resistance of a steel plate as shown in Equation 1 below.

[00002]$\begin{array}{cc}P\left(\mathrm{Calories}\right)=\mathrm{VI}=I^{2}R.Math.R\propto \frac{\rho }{A}\{\begin{array}{c}\rho :\mathrm{Resistivity}\\ A:\mathrm{Cross}-\mathrm{sectional}\mathrm{area}\end{array}& \left(\mathrm{Equation}1\right)\end{array}$

[0046] Further, referring to the conceptual diagram of the three-layer welding in FIG. 6, resistivity between interfaces according to a cross-sectional area of the welding tip during the three-layer welding may be calculated as follows.

[0047] Referring to FIG. 6, a diameter of cross-sectional area A is

[00003]$\frac{x\left(t_2+t_3\right)+{\mathrm{yt}}_1}{t_1+t_2+t_3}$

and a diameter of cross-sectional area B is

[00004]$\frac{{\mathrm{xt}}_3+y\left(t_1+t_2\right)}{t_1+t_2+t_3},$

so resistivity of cross-sectional area A and cross-sectional area B is as follows.

[0048] Resistively in cross-sectional area A: average of resistively of materials a and b

[00005]$\frac{{\rho }_1+{\rho }_2}{2}$

[0049] Resistively in cross-sectional area B: average of resistively of materials a and b

[00006]$\frac{{\rho }_2+{\rho }_3}{2}$

[0050] Equation 2 below should be satisfied so that heat emission amounts of cross-sectional area A and cross-sectional area B are equal to each other.

[00007]$\begin{array}{cc}\left(\mathrm{Equation}2\right)& \\ R\propto \frac{\rho .Math.}{A.Math.}\frac{\frac{{\rho }_1+{\rho }_2}{2}}{{\left(\frac{x\left(t_2+t_3\right)+{\mathrm{yt}}_1}{t_1+t_2+t_3}\right)}^2}=\frac{\frac{{\rho }_2+{\rho }_3}{2}}{{\left(\frac{{\mathrm{xt}}_3+y\left(t_1+t_2\right)}{t_1+t_2+t_3}\right)}^2}& \left(1\right)\end{array}$

[0051] Here, when both sides of Equation 2 above are organized,

(ρ.sub.1+ρ.sub.2)(xt.sub.3+y(t.sub.1+t.sub.2)).sup.2−(ρ.sub.2−ρ.sub.3)(x(t.sub.2+t.sub.3)+yt.sub.1).sup.2=0

x.sup.2(t.sub.3.sup.2(ρ.sub.1+ρ.sub.2)−(ρ.sub.2+ρ.sub.3)(t.sub.2+t.sub.3).sup.2)−t.sub.1.sup.2y.sup.2(ρ.sub.2+ρ.sub.3)−y.sup.2(ρ.sub.1+ρ.sub.2)(t.sub.1+t.sub.2).sup.2+x(2t.sub.3y(ρ.sub.1+ρ.sub.2)(t.sub.1+t.sub.2)−2t.sub.1y(ρ.sub.2+ρ.sub.3)(t.sub.2+t.sub.3))=0

, and in a relational equation of cross-sectional areas of two welding tips arranged on both facing outer surfaces of the panel type welding subject superimposed in multi-layers, a ratio (x/y) of the cross-sectional areas of two welding tips is organized as Equation 3. Equation 3 is a function of a thickness and resistivity of the welding subject.

[00008]$\begin{array}{cc}\left(\mathrm{Equation}3\right)& \\ \begin{array}{cc}\propto & \{-2t,\left({\rho }_1+{\rho }_2\right)+\left(t_1+t_2\right)+2t_{\text{?}}\left({\rho }_2+{\rho }_{\text{?}}\right)\left(t_{\text{?}}+t_{\text{?}}\right)\\ \text{?}& \surd \begin{array}{c}{\left(2t_{\text{?}}\left({\rho }_1+{\rho }_2\right)\left(t_1+t_2\right)-2t_1\left({\rho }_2+{\rho }_{\text{?}}\right)\left(t_2+t_{\text{?}}\right)\right)}^2-4(t_{\text{?}}^2\left({\rho }_{\text{?}}+{\rho }_2\right)-\\ \left({\rho }_{\text{?}}+{\rho }_{\text{?}}\right){\left(t_2+t_3\right)}^2)\left(-{\text{?}}_{\text{?}}^{\text{?}}\left({\rho }_2+{\rho }_3\right)-\left({\rho }_2+{\rho }_3\right){\left(t_2+t_2\right)}^2\right)\}\end{array}\\ \text{?}& \left\{2\left(t_{\text{?}}^2\left({\rho }_1+{\rho }_2\right)-\left({\rho }_2+{\rho }_{\text{?}}\right){\left(t_{\text{?}}+t_{\text{?}}\right)}^2\right)\right\}\end{array}& \left(2\right)\end{array}$$\begin{array}{cc}.Math.\frac{x}{y}=f\left(p_i,t_i\right)& i=1,2,3\end{array}$$\text{?}\text{indicates text missing or illegible when filed}$

[0052] In Equation 3, p represents resistivity of each layer of the welding subject and t represents a thickness of each layer of the welding subject. Since the present disclosure relates to the panel type welding subject superimposed in multi-layers, i as the number of welding subjects is a natural number of 2 or more.

[0053] Furthermore, resistivity of the panel type welding subject, in particular, a steel plate, is in proportion to a carbon equivalent. Thus, the resistivity has characteristics shown in a resistivity-strength (K grade) proportional graph shown in FIG. 7.

[0054] As in the graph of FIG. 7, resistivity rapidly increases from an 80K grade strength steel plate. A low heat emission group is soft steel to 60 K grade steel plate, and a high heat emission group is a steel plate of an 80 K grade or more (including hot stamping). As a result, it may be that the heat emission amount used is in proportion to the resistivity and the resistivity is in proportion to a material carbon equivalent.

[0055] Meanwhile, in FIG. 7, resistivity ρ=10.7+0.61exp(C.sub.eq/0.12).

[0056] Therefore, a ratio of sizes of two welding tips is determined by Equation 3 described above, so the ratio of the sizes of the welding tips may be represented as a function of a thickness and resistivity of a steel plate and the resistivity of the steel plate increases according to the strength.

[0057] In this case, an average value of the resistivity of the steel plate universally used is shown in the above graph.

[0058] Furthermore, by considering scattering of the resistivity of the steel plate, the ratio of Equation 3 has an allowable value of ±20%, or an allowable value of ±10% as in Equation 4 below.

[00009]$\begin{array}{cc}\begin{array}{cc}0.9\times f\left({\rho }_i,t_i\right)\le \frac{x}{y}\le 1.1\times f\left({\rho }_i,t_i\right)& i=1,2,3\end{array}& \left(\mathrm{Equation}4\right)\end{array}$

[0059] In general, the shunt effect occurs when an internal between welding points is smaller than 30 mm, and as a result, the shunt effect becomes more severe as a pitch interval becomes smaller. A limitation of short pitch welding due to sizes of the welding tips and the nugget diameter is approximately 10 mm.

[0060] According to the present disclosure, in respect to the panel type welding subject superimposed in multi-layers, in particular, three layers or more steel plates, it is possible to secure an equal welding quality (tensile strength and nugget diameter size) which is irrespective of the shunt effect when the steel plate is applied to short pitch welding of an interval of 10 to 30 mm.

[0061] Further, referring to the graph of resistivity scattering according to a material strength of FIG. 8, it can be seen that, as the strength of the panel type welding subject, i.e., the steel plate increases, the resistivity scattering increases.

[0062] FIG. 9 is a photo diagram of various welding tips in the related art. The present disclosure provides a welding method capable of multi-layer welding through adjustment of a cross-sectional area of the welding tip. In particular, heat emission amounts of interfaces of the welding subject are controlled to be similar by adjusting cross-sectional areas of two welding tips arranged on both outer facing surfaces of the panel type welding subject superimposed in multi-layers to simultaneously form the nugget diameter.

[0063] Further, in the welding method of the present disclosure, the ratio (x/y) of the cross-sectional areas of two welding tips satisfies Equation 3 above, which is the function of the thickness and the resistivity of the welding subject. Also, the ratio (x/y) of the cross-sectional areas of the welding tips may have an allowable value of ±20%.

[0064] Furthermore, in the welding method of the present disclosure, the ratio (x/y) of the cross-sectional areas of the welding tips may have an allowable value of ±10%.

[0065] Moreover, in the welding method of the present disclosure, a pitch, which is an interval between welding spots, may be a short pitch in the range of 10 to 30 mm (10 or more and less than 30 mm).

[0066] Further, the number of welding subjects is 2 or more, and may be 2 to 5. The number of welding subjects may be 3 or more and in one example may be 3 to 5.

[0067] Further, the welding apparatus according to the present disclosure is achieved by applying the welding method.

[0068] The present disclosure may be better appreciated through the following example.

Example 1: 28 K (8 mm)-150K (12 mm)-100 K (10 mm) Three-Layer Welding, Pitch Interval of Welding Spots of 10 mm

[0069] In this case, in Example 1, an appropriate range of the ratio (x/y) of the cross-sectional areas of the welding tips is

[00010]$0.6\le \frac{x}{y}\le 0.73.$

[0070] FIGS. 10A-10C are graphs showing a tensile strength and a nugget diameter size according to a welding tip ratio in Example 1. Referring to the graph of FIG. 10A, when the ratio of the welding tip is in an appropriate welding tip ratio range, an equal welding quality within a deviation of 5% could be secured.

[0071] However, referring to the graph of FIG. 10B, when the ratio of the welding tip is smaller than the appropriate welding tip ratio, the tensile strength deteriorates, and the size of the nugget diameter decreases.

[0072] Further, referring to the graph of FIG. 10C, when the ratio of the welding tip is larger than the appropriate welding tip ratio, the tensile strength deteriorates, and the size of the nugget diameter decreases.

[0073] In Example 1 above, as shown in FIG. 10A, a reason why, when the ratio (x/y) of the cross-sectional areas of the welding tips is smaller or larger than the appropriate ratio range, the strength deteriorates and the nugget diameter size decreases is that, as described with reference to FIG. 5, a purpose of an asymmetric welding method and a welding apparatus using the same according to the present disclosure is to equally compensate calories between contact surfaces A and B by controlling heat emission amounts of interfaces to be similar during multi-layer welding through adjustment of the size of the welding tip. Accordingly, when the range of the ratio (x/y) of the cross-sectional areas of the welding tips is large, a sufficient heat emission amount may not be generated on contact surface A. Also, when the ratio range is small, more heat emission amount in contact surface A is still generated than that in contact surface B. As a result, the heat emission amounts of the contact surfaces are unequal.

[0074] As such, when an intelligent welding method in which multi-layer welding is possible through adjustment of the cross-sectional areas of the welding tips. In other words, when the asymmetric welding tip is used, in the case of three-layer welding and 10 mm-short pitch welding, it can be seen that the tensile strength and the nugget diameter size have equal values within 5%.

[0075] In particular, when quality scattering of the steel plate is considered and the asymmetric welding tip based on Equation 3 above is used, it can be seen that an equal and robust welding quality may be secured even in most severe 10 mm short-pitch welding.

Example 2: Range of Ratio (x/y) of Cross-Sectional Areas of the Welding Tips According to a Thickness Ratio and Strength of Three-Layer Superimposed Panel Type Welding Subject in which the Number of Welding Subjects is Three (a, B, and C), and in Particular, a Steel Plate

[0076] In Example 2 above, the appropriate range of the ratio (x/y) of the cross-sectional areas of the welding tip, in which the equal welding quality within the deviation of 5%, may be secured according to a thickness ratio and strength of three-layer superimposed panel type welding subject. In Example 2, the number of welding subjects is three (A, B, and C), and in particular, the welding subject is a steel plate measured and shown as Tables 1-4.

TABLE-US-00001 TABLE 2  A, B, and C material thickness ratio t.sub.1:t.sub.2:t.sub.3 = 8:10:10 Strength Welding tip ratio A B C [00011]$\left(\frac{x}{y}\right)$ 28K 150K 100K [00012]$0.61\le \frac{x}{y}\le 0.74$ 60K 150K 100K [00013]$0.73\le \frac{x}{y}\le 0.9$ 28K 150K 120K [00014]$0.56\le \frac{x}{y}\le 0.69$ 60K 150K 120K [00015]$0.69\le \frac{x}{y}\le 0.84$ indicates data missing or illegible when filed

TABLE-US-00002 TABLE 3  A, B, and C material thickness ratio t.sub.1:t.sub.2:t.sub.3 = 8:12:10 Strength Welding tip ratio A B C [00016]$\left(\frac{x}{y}\right)$ 28K 150K 100K [00017]$0.6\le \frac{x}{y}\le 0.73$ 60K 150K 100K [00018]$0.72\le \frac{x}{y}\le 0.89$ 28K 150K 120K [00019]$0.56\le \frac{x}{y}\le 0.68$ 60K 150K 120K [00020]$0.68\le \frac{x}{y}\le 0.83$ indicates data missing or illegible when filed

[0077] A welding tip ratio of 28 K (8 mm)-150 K (12 mm)-100 K (10 mm) in Table 3 is a range of Example 1 described above.

TABLE-US-00003 TABLE 4  A, B, and C material thickness ratio t.sub.1:t.sub.2:t.sub.3 = 7:14:10 Strength Welding tip ratio A B C [00021]$\left(\frac{x}{y}\right)$ 28K 150K 100K [00022]$0.62\le \frac{x}{y}\le 0.76$ 60K 150K 100K [00023]$0.76\le \frac{x}{y}\le 0.92$ 28K 150K 120K [00024]$0.58\le \frac{x}{y}\le 0.71$ 60K 150K 120K [00025]$0.71\le \frac{x}{y}\le 0.87$ indicates data missing or illegible when filed