TECHNICAL FIELD

The present invention relates to a spot weldment obtained by spot welding of two or more stacked sheet materials and relevant techniques.

BACKGROUND ART

When joining two or more materials together, welding is often used to ensure the high joint strength. For example, sheet materials that constitute a vehicle body or the like of an automobile are joined by spot welding that allows a plurality of sites (spots) to be welded in a spot-like form.

The spot welding is a type of resistance welding performed such that both outer surfaces of workpieces to be joined (materials to be joined, such as sheet materials) are pressed (applied with pressing force) between a pair of electrodes arranged opposite to each other and, in this state, the workpieces to be joined are supplied with a large current via the electrodes for a short time. When performing the spot welding, the workpieces to be joined as such are melted and solidified to form a nugget inside the workpieces. The workpieces are thus joined to each other via the nugget. Many proposals for the spot welding have been made, and relevant descriptions are found, for example, in the patent documents listed below.

PRIOR ART DOCUMENTS

Patent Documents

[Patent Document 1] JP6136249B[Patent Document 2] JP10-244379A[Patent Document 3] Published Utility Model Application No. 4-83486

SUMMARY OF INVENTION

Technical Problem

(1) Patent Document 1 proposes a concave electrode (an electrode with a recessed central area of the pressure-contact surface to a workpiece to be joined) that can form a sufficiently large nugget while suppressing the occurrence of expulsion (a phenomenon in which the expanded molten metal spatters around the spot). Use of the concave electrode allows the molten metal, which is generated inside the workpieces during the spot welding, to bulge out to the recessed side of the electrode. This can suppress the occurrence of expulsion, and the desired joint strength can be stably ensured.

However, as will be understood, the nugget formed in the above process may take a form in which the bulging molten metal is directly solidified. When the concave electrode is used, therefore, protrusions may appear on the outer surface of the welded sheet materials. Such protrusions may cause deterioration of the appearance of a weldment, poor workability in the subsequent steps, etc.

(2) Patent Document 2 proposes an electrode for resistance spot welding of aluminum-based workpieces to be joined. Specifically, this document proposes an electrode that is made of a material (pure copper such as tough pitch copper or oxygen-free copper) with a high conductivity (97% IACS or more) and little resistance heat generation and that is provided inside with a small cooling hole. This electrode has high cooling efficiency, and the temperature rise of the pressure-contact surface in contact with a workpiece to be joined can be suppressed.

However, the conductivity of pure copper is high, but the strength is low. For example, the strength at 200° C. or lower is only about ½ as compared with copper alloys (such as chromium copper). If an electrode made of such a material is pressed with a large pressing force against a workpiece to be joined, buckling tends to occur at the outer circumferential side wall part around the small cooling hole, and the tip portion may readily be deformed and worn as the number of welding points increases. In order to stably form a desired nugget, it is necessary to frequently perform exchange of electrodes, reform (dressing) of the tip portion, etc. This may lead to the reduction in productivity and the increase in production cost due to the increase in tact time.

(3) Patent Document 3 (FIG. 10) discloses a bottomed cylindrical electrode for spot welding made of a copper alloy (chromium copper or alumina-dispersed copper). Copper alloys have higher strength (higher hardness) than pure copper, but the conductivity is low. Moreover, the electrode is thinner to some extent at the bottom part and cylindrical part as a whole. Accordingly, due to the combination of the material and the shape, the electrode of Patent Document 3 (FIG. 10) may readily be self-heated and softened when a large current is applied. This softening may also readily cause buckling when such an electrode is pressed with a large pressing force against a workpiece to be joined. Even if buckling does not occur, the electrode may readily be deformed as the number of welding points increases, and the frequency of exchange and reform of electrodes can increase as described above.

Furthermore, due to the temperature rise caused by the self-heating of the electrode, the temperature of the contact interface with a workpiece to be joined also rises, and a compound may readily be generated between the electrode and the workpiece to be joined. Generation of such a compound may lead to welding troubles such as pickup and welding adhesion. When the workpiece to be joined is an Al alloy material or a (high-tensile) steel sheet plated with zinc, Al—Si, or the like, a Cu-based compound may readily be generated due to the reaction in a molten state between Al or Zn on the side of the workpiece to be joined and Cu on the electrode side.

(4) In the automotive field, high-tensile steel sheets, aluminum alloy sheets, and other similar sheets are widely used for the purpose of weight reduction and the like, and spot welding thereof is also increasing. For the spot welding of high-tensile steel sheets, aluminum alloy sheets, and other similar sheets, a larger amount of heat (amount of heat (Q)=current value (I)2×resistance value (R)×time (t)) than that for the spot welding of soft steel sheets is required. This is because the low resistance of aluminum alloy requires a large current. Such an increase in the amount of heat (amount of energization) may cause the temperature of electrodes to increase and the generated amount of compounds to increase.

Moreover, in general, when high-tensile steel sheets, aluminum alloy sheets, or other similar sheets having poor press formability (springback is large, for example) are stacked after the press forming, a gap larger than that in the conventional technique may readily be generated in the parts to be welded. If the spot welding is performed under the presence of a gap, spattering (expulsion) occurs, which may not only deteriorate the working environment but also make it difficult to form a stable welding nugget. In order to perform spot welding without the gap, it is necessary to apply a large pressing force to the electrodes so that the workpieces to be joined are pressed strongly between the electrodes. Thus, the amount of energization (amount of heat generation) and the pressing force applied to the electrodes tend to increase more than ever. On the other hand, there is also a demand for reducing the frequency of exchange and reform of the electrodes, etc.

Conventionally, when spot welding is performed using general convex electrodes that are not concave electrodes, cleaning (such as dressing or reform) and exchange of electrodes (tips) have to be frequently performed to stably form a nugget having a certain size or more to ensure the joint strength of a weldment. Such a tendency is remarkable when a compound or alloy may readily be formed at the contact interfaces between the electrodes and sheet materials.

The present invention has been made in view of such circumstances and objects of the present invention include providing a spot weldment that has good appearance without prominent protrusions and the like on the surface and that allows for reduction of the production cost, etc.

Solution to Problem

As a result of intensive studies to achieve the above objects, the present inventors have newly found that a spot weldment can be efficiently obtained without deterioration of the appearance by forming a nugget into a desired shape between sheet materials to be joined. Developing this achievement, the present inventors have accomplished the present invention as will be described hereinafter.

«Spot Weldment»

(1) The present invention provides a spot weldment joined by a nugget formed inside stacked sheet materials. The nugget is formed through bringing a pair of electrodes arranged opposite to each other into pressure contact with the stacked sheet materials from outside and energizing the stacked sheet materials from the electrodes. The nugget has a nugget diameter (D) satisfying D≥4√t (t: thickness of sheet material) and a nugget flattening level (D/H) of 3.5 to 8. The nugget flattening level (D/H) is a ratio of the nugget diameter (D) to a nugget thickness (H). In this spot weldment, both outer surface parts of the sheet materials are free from protrusions formed due to bulging of molten metal.

(2) The spot weldment (simply referred to as a “weldment”) of the present invention has a sufficient joint strength because the nugget having a certain size (nugget diameter) or more is formed between the sheet materials. Moreover, the weldment of the present invention has good appearance without prominent protrusions and the like on the outer surface after the spot welding.

In the weldment of the present invention, the sheet materials are joined by the flattened nugget having a relatively smaller nugget thickness than that of conventional ones. When such a flattened nugget is formed, the temperature of the contact interfaces between the electrodes and the sheet materials tends to decrease. This can suppress the migration (diffusion, penetration) of the component elements (e.g., Cu), which constitute the electrodes (tips), to the sheet material side and the generation or the like of a compound or alloy in the vicinity of the interfaces.

As a result, it is possible to suppress the deformation/wear of electrodes and reduce the frequency of cleaning (such as dressing or reform) and exchange of electrodes, etc. Moreover, the change in the component composition is reduced in the vicinity of the outer surface of the spot-welded sheets (weldment) and in the nugget, and the weldment having originally expected characteristics (such as strength and corrosion resistance) can readily be obtained. Thus, according to the present invention, a weldment excellent in the appearance and characteristics can be obtained at low cost.

«Spot Welding Method, etc.»

The present invention can also be perceived as a spot welding method (also simply referred to as a “welding method”) with which the above-described weldment can be obtained. For example, the present invention can be perceived as a spot welding method using an electrode (tip) having a specific shape with which a flattened nugget is readily formed.

(1) The present invention may also provide, for example, a spot welding method performed using an electrode comprising a main body having a tip portion and a base portion. The tip portion is formed in a bottomed and substantially cylindrical shape. The base portion is formed in a substantially cylindrical shape and merges into the tip portion. The main body of the electrode comprises a copper alloy. The tip portion has a tip bottom part and a tip barrel part. The tip bottom part has a pressure-contact surface that is not recessed with respect to a workpiece to be pressed. The tip barrel part is formed in a substantially cylindrical shape and merges into the tip bottom part. The electrode has a tip inner diameter ratio (f/B) of 0.4 to 0.6. The tip inner diameter ratio (f/B) is a ratio of an inner diameter (f) of the tip barrel part to an outer diameter (B) of the base portion. The electrode has a tip bottom thickness ratio (L/B) of 0.15 to 0.5. The tip bottom thickness ratio (L/B) is a ratio of a thickness (L) of the tip bottom part to the outer diameter (B) of the base portion. The above-described spot weldment is obtained by this method.

(2) The electrode according to the present invention is made of a copper alloy and has a specific shape. Such material and form act synergistically to allow the electrode to be less susceptible to deformation such as buckling even when a large pressing force is applied. Furthermore, the electrode tends to suppress the temperature rise at the contact interface between the electrode and a workpiece to be joined, owing mainly to the form of the electrode. This can suppress the migration (diffusion, penetration) of the component elements (e.g., Cu), which constitute the electrode (tip), to the sheet material side and the generation or the like of a compound or alloy in the vicinity of the interface. In this way, when the electrode of the present invention is used for spot welding, the weldment can readily be obtained which is joined by a flattened nugget having a relatively smaller nugget thickness than that in the conventional ones.

Thus, when using the electrode according to the present invention in which the material and the form are matched, the frequency of cleaning (such as dressing or reform) and exchange of electrodes, etc., can be reduced while suppressing the deformation and wear of the electrode, and the formation of a stable nugget and the production of a weldment can be achieved. Moreover, use of the electrode may reduce the component composition change in the vicinity of the outer surface of the spot-welded sheets (weldment) and in the nugget, and the weldment having originally expected characteristics (such as strength and corrosion resistance) can readily be obtained. Thus, according to the welding method of the present invention, a weldment excellent in the appearance and characteristics can be stably or efficiently obtained.

«Others»

Unless otherwise stated, a numerical range “x to y” as referred to in the present description includes the lower limit x and the upper limit y. Any numerical value included in various numerical values or numerical ranges described in the present description may be selected or extracted as a new lower or upper limit, and any numerical range such as “a to b” can thereby be newly provided using such a new lower or upper limit.

