Joint structure

A joint structure includes: a first same-type metal member; a second same-type metal member that can be mutually welded with the first same-type metal member; and a different-type member that has a penetrating portion, is interposed between the first same-type metal member and the second same-type metal member. In the plate thickness direction of an emission region in which a laser beam is emitted toward the penetrating portion, the plate thickness at the emission region of the first same-type metal member positioned on the side on which the laser beam is emitted is a predetermined thickness corresponding to a first gap. The first same-type metal member and the second same-type metal member are fused and bonded together via the penetrating portion, and the different-type member is compressed and fixed, such that the different-type member is fixed to the first same-type metal member and the second same-type metal member.

TECHNICAL FIELD

The present disclosure relates to a joint structure in which one-or-more different-type materials are fixed between same-type metal materials with application of laser light as a heat source.

BACKGROUND ART

Recent expansion of global transport of automobiles and other products has increased production volume of them. Under such a trend, there has been a growing demand for reduction of a total cost per product, particularly, for enhancing production efficiency by decreased production time.

At the same time, there has been a worldwide strong demand for constraint on carbon emissions to prevent global warming. To meet the demand, strenuous efforts on improvement of fuel efficiency are accelerating in transportation business including car industries. Decreasing weight of vehicles is a specific approach to improvement of fuel efficiency. Manufacturers are searching for using lightweight materials as possible.

As a welding method for transportation equipment including cars, a spot welding (i.e., a resistance welding) is widely employed. The spot welding has an upper electrode and a lower electrode as a welding gun for spot welding. Materials to be welded are tightly pressed between the upper electrode and the lower electrode and has application of current between the two electrodes. Therefore, the spot welding is not suitable for one-side welding and has limitations on shapes of material to be welded; the welding position of the material needs to be sandwiched between the welding gun for spot welding. Further, the welding gun needs a space on the upper and lower sides of the material to fit itself in and apply pressure to a welding position. Besides, due to heavy weight of the welding gun itself, the welding gun cannot move fast. After arriving the welding position, the welding gun needs pressure time for the material to be welded prior to welding. After welding, the welding gun needs to have cooling time for the welded material. That is, the spot welding needs lots of time other than the welding time.

As for weight saving of materials used for cars, manufacturers try to change a part of components from steel to light-metal material, such as aluminum. In such an effort, a technique and a structure suitable for connecting light-metal material with steel has been needed.

A spot welding using a rivet and a joining method using adhesives are conventionally used for connecting materials of different type. For example, Patent Literature 1 discloses a rivet shape, caulking, and a spot welding method capable of absorbing plastic flow that occurs in application of pressure on a different-type material between a rivet and a joint material similar to the material of the rivet and occurs in the different-type material caused by welding heat in spot welding. A different-type material often has a partial deformation in caulking and spot welding, and in other cases, such a different-type material sometimes has a depressed portion caused by a positional gap of the electrodes in spot welding. The structure disclosed in Patent Literature 1 addresses the problem above, maintaining joint strength so as not to have degradation.

CITATION LIST

Patent Literature

SUMMARY OF THE INVENTION

The joint structure of the present disclosure has a first material, a second material, which are same-type of metal and are weldable with each other, a different-type material that is difficult to weld to the first material and the second material. Having a penetrating part, the different-type material is sandwiched between the first material and the second material of same-type metal. In the plate thickness direction in which laser light is emitted toward an emission region in the penetrating part, the plate thickness at the emission region of the first material positioned on the side on which the laser light is emitted has a predetermined size so as to correspond to a first gap. The first gap is formed at the emission region in the plate thickness direction and exists before welding. The first material and the second material are fused and bonded together via the penetrating part, by which the different-type material is compressed and fixed. As a result, the different-type material is fixed to the first material and the second material of same-type metal.

DESCRIPTION OF EMBODIMENT

Background to the Present Disclosure

Prior to the description of exemplary embodiments, the background to the present disclosure will be briefly explained.

A conventional different-type joint material is described with reference toFIG. 8. If a part of different-type material200is deformed in caulking and spot welding, a space into which the deformed part moves has to be taken into account in designing a rivet. Further, if different-type material200has a depressed part due to, for example, a positional gap of the electrodes in spot welding, joint strength can be degraded. To deal with the problems, a rivet needs a complicated shape, such as chamfered edge30of an R (radius) shape and annular groove31. That is, rivet51has to have a precision work, which increases production cost. Further, since the spot welding (i.e., the resistance welding) takes long time for applying pressure and current, for cooling, and for moving before/after welding, which prolongs the total working time. Besides, joining materials100are welded such that the welding gun (not shown) holds joining materials100therebetween, which lowers the degree of flexibility in design of joining materials100.

In the resistance welding, molten metal is solidified into a weld nugget at a welding position. If a rivet disposed too close to an adjacent rivet, the welding current has a branch current, which fails sufficient nugget formation at an intended welding position. To obtain desired nugget formation with no branch current, a rivet has to be disposed with at least minimum joining pitch kept from an adjacent rivet. Due to the rivet arrangement with limitation in joining pitch, a conventional structure has often failed in increase in joint stiffness at an intended position.

The present disclosure provides a simple joint structure capable of joining different-type materials in laser welding, enhancing productivity.

First Exemplary Embodiment

The exemplary embodiment will be described with reference toFIG. 1throughFIG. 5B.

FIG. 1illustrates the joint structure in accordance with the first exemplary embodiment.FIG. 2AthroughFIG. 2Cillustrate a joint state in laser welding in accordance with the first exemplary embodiment.

Specifically,FIG. 2Aillustrates a joint state where first material1of circular shape is used as the upper plate.FIG. 2Billustrates a joint state where first material1of oval shape is used as the upper plate.FIG. 2Cillustrates a joint state where first material1of rectangular shape is used as the upper plate. Each of the cross sections taken along the line I-I inFIG. 2AthroughFIG. 2Ccorresponds to the cross section ofFIG. 1.

FIG. 1shows a joining state of first material1, second material2, and third material3. First material1and third material3are made of metal of same type, and second material2is made of a different material. Second material2is sandwiched between first material1and third material3. Each of first material1, second material2, and third material3is a plate material having a substantially uniform thickness.

As an example of the penetrating part of the structure in the present disclosure, through-hole4is formed in second material2in advance. Through-hole4forms a gap between first material1and third material3. In the state where second material2is sandwiched between first material1and third material3, no holes, projections, bumps are disposed in the gap formed by through-hole4. That is, the plate thickness of second material2substantially equals to first gap11between first material1and third material3in the plate thickness direction. Although the structure of the embodiment has through-hole4as an example of the penetrating part, it is not limited to; the penetrating part may be a penetrating groove.

The plate thickness direction mentioned above is the direction shown by the arrow inFIG. 1, which is perpendicular to the principal surface of first material1, second material2, and third material3in a state before welding.

The metal materials of same type mean the metals that are weldable with each other. It is not necessarily to be exactly the same; for example, a combination of ferrous metals or a combination of nonferrous metals may be employed, as long as they have a good affinity in welding. For example, the followings are the specific example of a combination of welding materials: as for ferrous-metal combinations of first material1and third material3, mild steel and mild steel; mild steel and stainless steel; stainless steel and stainless steel; mild steel and high-tensile steel; high-tensile steel and stainless steel; high-tensile steel and high-tensile steel, and as for nonferrous-metal combinations, aluminum and aluminum; aluminum and aluminum alloy; aluminum alloy and aluminum alloy.

Different-type second material2, which differs in material from first material1and third material3of same-type metal, is difficult to be welded to the same-type metal material. For example, when ferrous-metal material is chosen for first material1and third material3of same-type metal material, non-ferrous metal, such as copper and aluminum, is employed for different-type second material2. When metal material is chosen for first material1and third material3, resin material, such as CFRP (carbon fiber reinforced plastics) and PET (polyethylene terephthalate) is employed for different-type second material2.

As shown in a welding state ofFIG. 2A, laser light7is emitted from the upper side of first material1in the plate thickness direction. At that time, the emission effective range of laser light7(i.e., the weldable range) is the inside of through-hole4(seeFIG. 1) of second material2. Laser light7is circularly applied toward the inside of through-hole4, by which a weld bead is formed as weld part8shown inFIG. 1. When laser light7is incident to first material1, in the description, first material1and third material3correspond to a first material and second material, respectively, of same-type metal of the present disclosure.

Next, the joining state in and after welding is described with reference toFIG. 1. As described above, laser light7is circularly emitted from the upper side of first material1and third material3so as to be aimed to the inside of through-hole4as the incident region of laser light7. Weld part8is formed by applying laser light, and at that time, molten metal of first material1and third material3solidifies and shrinks at weld part8. At the same time, compression force6acts on different-type second material2sandwiched between first material1and third material3of same-type metal material. First gap11, as described earlier, is a space formed between first material1and third material3in the plate thickness direction along a plate thickness in the incident region of laser light7. The length of first gap11, which corresponds to the thickness of second material2, is compressed by compression force6.

When the length of first gap11(i.e., the plate thickness of second material2) is determined to be not less than 6% and not more than 38% of the plate thickness in the incident region of laser light7of first material1(as the upper plate with respect to the direction of laser emission) or of third material3(as the lower plate with respect to the direction of laser emission), the solidification and shrinkage of weld part8acts as compression force6on second material2. As a result, second material2is tightly compressed between first material1and third material3.

The determination of not less than 6% and not more than 38% the percentage of the size of first gap11corresponding to the plate thickness of second material2to the plate thickness of first material1(or third material3)—is obtained from experimental data.FIG. 3shows an example of the experimental data.FIG. 3is a graph of measurement data showing the relation of the plate thickness of first material1(as the upper plate with respect to the direction of laser emission) and the length of first gap11(i.e., the plate thickness of different-type second material2between first material1and third material3of same-type metal material).

FIG. 3is a graph of measurement data showing the relation of the plate thickness of first material1and the length of first gap11that is a space in the incident region of laser light7in the plate thickness direction and corresponds to the plate thickness of second material2.

In the measurement shown inFIG. 3, mild steel was employed for first material1and third material3, and PET as a resin material was employed for second material2. In the material combination above, first material1was disposed above third material3, and second material2was disposed between the same-type metal materials. Laser light7was circularly emitted from above the stacked materials toward the incident region (i.e., the inside of through-hole4of second material2). The laser output in the experiment was 3 kW.

For example, through-hole4as a penetrating part of second material2was determined to 12 mm. Laser light7was circularly incident in the plate thickness direction to the incident region of weld part8so that the incident region is smaller than through-hole4, keeping a distance from the outer edge of the diameter or the width of it. Specifically, the outer size of weld bead of weld part8was 8 mm.

For example, when first material1has a plate thickness of 0.8 mm, the effective range of the plate thickness of second material2(as first gap11that is a space between first material1and third material3in the plate thickness direction) for preferably compressed fixing (brought by solidification and shrinkage effect of molten metal at weld part8) is 0.1 mm to 0.3 mm. The plate thickness of second material2greater than 0.3 mm causes a partial deficiency in a weld line, such as holes (At least one opening holes formed in the weld due to insufficient filling of molten metal during welding), resulting in poor weld. The experiment shows that the effective plate thickness of second material2is 13%-38% of the plate thickness of first material1.

Further, when first material1has a plate thickness of 1.6 mm, the effective range of the plate thickness of second material2for preferably compressed fixing (brought by solidification and shrinkage effect of molten metal at weld part8) is 0.1 mm to 0.5 mm. The plate thickness of second material2greater than 0.5 mm causes a partial deficiency in a weld line, such as holes (At least one opening holes formed in the weld due to insufficient filling of molten metal during welding), resulting in poor weld. That is, the compressive fixing range by welding is determined by appropriate relation of plate thickness of different-type second material2to first material1of same-type metal material on the side of laser incident.

In other words, the experiment result shows that, when molten metal corresponding to 31%-38% of the maximum plate thickness of first material1falls into the inside of through-hole4, the joining work is successfully carried out without a defective part, such as holes. If the plate thickness of second material2becomes greater than 31%-38% of the maximum plate thickness of first material1, an amount of molten metal necessary for filling up the inside of through-hole4cannot be obtained, which causes a defective state, such as making holes.

In contrast, when the plate thickness of second material2(that corresponds to the length of first gap11as a space in the plate thickness direction) is smaller than 0.1 mm, first material1is too close to third material3, and the same-type metal materials fuse together. This causes lack of compression force6on different-type second material2.

As described above, insofar as an enough amount of molten metal in welding is obtained, i.e., insofar as having no holes, the greater first gap11(as a space in the plate thickness direction), the greater the solidification and shrinkage effect of molten metal at weld part8by incident of laser light7. The increase in solidification and shrinkage enhances compression force6(as the fixing force by first material1and third material3) that acts on second material2.

According to the embodiment, laser light7is emitted from the side on which first material1is disposed. When first gap11(as the plate thickness of second material2) is determined to 6%-38% of first material1or third material3, first gap11is filled with a sufficient amount of molten metal in welding; accordingly, the solidification and shrinkage effect of weld part8allows second material2between first material1and third material3to be fixed with compression force. The laser light may be emitted from the side of third material3, not from the side of first material1. In that case, the first material and the second material of same-type metal of the present disclosure correspond to third material3and first material1, respectively.

Besides, in the description, first material1and third material3are same-type metal materials and mild steel is employed for the above, but they are not limited to, as long as they are weldable with each other with sufficient joining strength. The followings are ferrous-metal combination examples of first material1and third material3of same-type metal material: mild steel and mild steel; stainless steel and stainless steel; high-tensile steel and high-tensile steel; mild steel and high-tensile steel; and high-tensile steel and stainless steel. As for a nonferrous-metal combination, the followings are employed: aluminum and aluminum; aluminum alloy and aluminum alloy; aluminum and aluminum alloy. They are weldable materials by laser.

In contrast, second material2is a material that is difficult to be welded due to poor absorptivity of laser light, such as copper and resin material, or a material having poor affinity in welding connection with first material1and third material3. For example, a specific combination of same-type materials1,3and second material2is mild steel (as first material1and third material3) and aluminum (second material2) and vice versa.

As described above, when the plate thickness of second material2sandwiched between first material1and third material3is determined to be not less than 6% and not more than 38% of the plate thickness of first material1(disposed on the laser emission side) and third material3, preferable compressive fixing in welding is obtained. In other words, when the plate thickness of first material1and third material3is determined to be not less than 250% and not more than 1600% of the first gap11(i.e., the plate thickness of second material2of different-type), preferable compressive fixing in welding is obtained. In this way, determining the plate thickness of joining materials to be in the effective range for compressive fixing allows the same-type metal materials to be fused together, providing the different-type material with compressive fixing.

Example

FIG. 4shows a joining example in which second material2and fourth material10both are different in material from one another are sandwiched between first material1and third material3of same-type material. In that case, the total plate thickness of second material2and fourth material10corresponds to first gap11. Prior to welding, the gap length of first gap11is appropriately determined.

For example, in the incident region in which laser light7is incident, when the gap length of first gap11is determined to be not less than 6% and not more than 38% of the plate thickness of first material1as the upper plate, the solidification and shrinkage effect of weld part8allows second material2and fourth material10between first material1and third material3to be fixed with compression force. Particularly, it will be effective joining when resin such as PET, or nonmetal material such as CFRP are employed for second material2and fourth material10as different materials from one another. Such a material tends to exhibit high transmittance for laser light7and is difficult to be directly welded with a material of different type.

FIG. 2Ashows an example in which first material1has a circular shape,FIG. 2BandFIG. 2Cshow examples in which first material1has other shapes. InFIG. 2B, the scanning trace of laser light7as a weld form (not shown), seen from above, has an oval shape. Width W2of second material2and third material3shown inFIG. 2Bis greater than width W1shownFIG. 2A. In the state ofFIG. 2B, the weld strength that weld part8needs in welding with emission of laser light7particularly acts in a certain direction, the major axis of oval first material1should be disposed along the direction, i.e., along width W2of second material2and third material3. Employing an oval shape for first material1eliminates the need for forming it into a large circular shape, reducing the area of first material1.

Further, as shown inFIG. 2C, employing a linear weld form allows first material1to have a rectangular shape, not a large circular shape, reducing the area of first material1. Further, forming the scanning trace of laser light7into one or more lines makes the scanning trace of welding simple. Compared to laser scanning with a circular or an oval shape carried out repeatedly for several times, it shortens welding time.

FIG. 5AandFIG. 5Bshow an example for enhancing welding strength and for making positioning easy.

InFIG. 5A, first material1has the function for positioning second material2in the joining process of third material3and second material2. Specifically, second material2of different type is sandwiched between first material1having a stepped part and third material3such that second material2has abutment against the stepped part of first material1. This makes positioning of second material2easy.

Besides, the joint structure ofFIG. 5Aenhances tensile strength of the joint section. To be specific, first material1is connected to third material3via second material2at weld part8; at the same time, first material1is directly connected with third material3, i.e., having no second material2therebetween at weld part8a. By virtue of the structure welded at two positions, when third material3undergoes tensile force from second material2or torsion from outside, the stress is shared by weld part8and weld part8a. The structure prevents weld part8—at which first material1is connected to third material3via second material2—from concentration of the stress. That is, the joint structure as a whole enhances joining strength between the same-type metal materials and the different-type material.

FIG. 5Bshows third material3having a folding-back structure. Such structured third material3doubles as first material1, forming a one-piece structure with no use of first material1as the upper plate. Second material2is inserted in third material3until having abutment against the folding-back section. The structure makes positioning of second material2easy.

InFIG. 5B, the upper-plate section of third material3with a folding-back structure corresponds to the first material of the present disclosure, and the lower-plate section of third material3with a folding-back structure corresponds to the second material of the present disclosure. That is, the first material and the second material may be a one-piece structure of same-type metal before welding.

As described above, the joint structure to be welded by laser light7of the embodiment has a first material, a second material, which are same-type metal and weldable with each other, and a different-type material disposed between the first material and the second material of same-type metal. The different-type material has through-hole4as an example of penetrating part. The different-type material is difficult to be welded to the first material and the second material of same-type metal.

The structure allows first material1and third material3of same-type metal to be fused and bonded together, providing different-type second material2with compressive fixing.

Second Exemplary Embodiment

Next, the structure of the second exemplary embodiment is described with reference toFIG. 1andFIG. 6.

The region on which laser light7is incident in the plate thickness direction has a predetermined distance from the edge of through-hole4of second material2, that is, the region has second gap5as the distance between the edge of through-hole4and the outer periphery of weld part8.

The outer size of weld part8corresponds to that of the region on which laser light7is incident.

With the incident of laser light7to the region, second material2receives welding heat input from weld part8. At that time, the positional relation of second material2and second gap5largely affects fusion of second material2.

With the incident of laser light7, weld part8of first material1and third material3of same-type metal receives welding heat input. When second gap5is properly positioned with respect to through-hole4, different-type second material2fuses by indirect influence of the welding heat input and flows into second gap5formed between the edge of through-hole4of second material2and the outer periphery of weld part8. That is, in the laser light incident of welding, different-type second material2is indirectly heated and fused by the welding heat input transferred from first material1. The fused second material2flows to the outer edge of weld part8—at which first material1and third material3(corresponding to the first material and the second material, respectively, of same-type metal) are fused and bonded together—and is tightly fixed to the outer edge of weld part8. As described above, the structure offers tightly-bonded surface connection of first material1, third material3, and second material2, in addition to compressive fixing by solidification and shrinkage effect of weld part8.

When second gap5has a short distance from the edge of through-hole4, welding heat of weld part8fed from laser light directly or indirectly influences through-hole4of second material2.

Receiving the influence of welding heat input, fused second material2flows into a space of weld part8of first material1and third material3. If second material2is a material with a low boiling point such as resin, vaporized resin material can spout from the space, resulting in poor weld. In contrast, when second gap5has a long distance from the edge of through-hole4, second material2has no fusion. Such a structure obtains only compressive fixing by solidification and shrinkage effect.

The relation of second gap5and melting condition of second material2by the heat effect of first material1and third material3depends on the material of second material2.

FIG. 6shows an example of experiment data.FIG. 6shows measurement result showing the relation of the material of second material2and second gap5.

FIG. 6is a graph of measurement data showing the relation of second gap5and the material of different-type second material2of the embodiment of the present disclosure.

The graph shows measurement data of the following materials employed for second material2: PET (polyethylene terephthalate) and CFRP (carbon fiber reinforced plastics) as resin material; A5000-series aluminum alloy as nonferrous metal.

In the measurement, a mild-steel material with a plate thickness of 1.6 mm was employed for first material1and third material3. As for different-type second material2, one of the aforementioned three materials with a plate thickness of 0.4 mm was sandwiched between first material1and third material3. The diameter of through-hole4of second material2was 12 mm. In welding, laser emission with laser output of 3 kW was applied to the structure so as to have a weld form of a circular shape. First gap11, which equals to the plate thickness of second material2, was 0.4 mm.

The description below is on second material2formed of PET as a resin material. When second gap5is not less than 0.8 mm, i.e., second gap5is not less than 200% of first gap11, compressive fixing of second material2is obtained. Even under the welding heat input effect, a space of weld part8of first material1and third material3has no flow-in of melted second material2and has no poor weld.

However, second gap5determined to be smaller than 0.8 mm, i.e., determined to be smaller than 200% of first gap11often invited poor weld. That is, when weld part8at which laser light7is incident has outer periphery close to the edge of through-hole4, second material2fuses by receiving the influence of welding heat input and flows into a space of weld part8of first material1and third material3. With the welding heat, melted and vaporized second material2can spout from the space, resulting in poor weld.

The description below is on the case in which a CFRP material (which is a fiber-reinforced resin in resin material) was employed for second material2. When second gap5is not less than 0.6 mm, i.e., second gap5is not less than 150% of first gap11, compressive fixing of second material2is obtained. Even under the welding heat input effect, a space of weld part8of first material1and third material3has no flow-in of melted second material2and has no poor weld.

However, second gap5determined to be smaller than 0.6 mm, i.e., determined to be smaller than 150% of first gap11can invite poor weld. That is, when weld part8at which laser light7is incident has outer periphery close to the edge of through-hole4, second material2fuses by receiving the influence of welding heat input and flows into a space of weld part8of first material1and third material3. With the welding heat, melted and vaporized second material2can spout from the space, resulting in poor weld. As shown by the measurement result, when a resin material is employed for different-type second material2sandwiched between the same-type metal materials, the properties of the resin material, such as a boiling point and a melting point, largely affect the allowable size of second gap5in welding.

Next, the description below is on the case in which A5000-series aluminum alloy material (as an example of nonferrous metal) was employed for second material2. When second gap5is not less than 0.5 mm, i.e., second gap5is not less than 125% of first gap11, compressive fixing of second material2is obtained. Even under the welding heat input effect, a space of weld part8of first material1and third material3has no flow-in of melted second material2and has no poor weld.

However, when second gap5is smaller than 0.5 mm, i.e., smaller than 125% of first gap11, second material2fuses by receiving the influence of welding heat input and flows into a space of weld part8of first material1and third material3, resulting in poor weld. The measurement data shows that poor weld depends on materials property.

The experiment data described above shows an example when the aforementioned different materials were employed for second material2. The allowable size of second gap5obtained in the case of the PET material tends to be similar to other resin materials. Similarly, the allowable size of second gap5obtained in the case of the CFRP material and in the case of the A5000-series aluminum alloy tends to be similar to other fiber-reinforce resins and other nonferrous metal, respectively. That is, when second gap5is smaller than the allowable size; specifically, when it is smaller than 0.5 mm (i.e., smaller than 125% of first gap11), poor weld may be the result. This is also almost true for other nonferrous metal.

Third Exemplary Embodiment

Next, the structure of the third exemplary embodiment is described with reference toFIG. 7. A description overlapped with that mentioned above will be omitted. The structure of the exemplary embodiment differs from that of the first exemplary embodiment in that spacer9is disposed in through-hole4. When second material2(disposed between first material1and third material3) has a large plate thickness, spacer9is disposed in the inside of through-hole4of second material2. Spacer9is a material of a metal similar to first material1and third material3and therefore weldable to them. Having spacer9in through-hole4contributes to decrease in size of first gap11that is the space formed in the plate thickness direction of first material1and third material3.

The plate thickness of spacer9is determined such that the size of first gap11(when spacer9is disposed between first material1and third material3in through-hole4) corresponds to 6%-38% of the plate thickness of first material1as the upper plate on which laser light7is incident. Spacer9is not necessarily additionally-disposed component; it may be formed between first material1and third material3by using welding materials, such as a filler and a consumable electrode.

In this case, too, like in the first and the second embodiments, the size of first gap11is determined to be in a predetermined range (seeFIG. 3) suitable for the plate thickness of first material1and third material3of same-type material, and keyhole laser welding is carried out so as to penetrate third material3as the lower plate. In this way, the solidification and shrinkage effect of weld part8enhances compression force6, providing second material2with compressive fixing.

If the first gap11becomes greater than 31%-38% of the maximum plate thickness of first material1, an amount of molten metal necessary for filling up first gap11cannot be obtained, which causes a defective state, such as making holes.

In contrast, when first gap11(as a space in the plate thickness direction) is smaller than 0.1 mm, first material1is too close to third material3, and first material1and third material3of same-type metal fuse together. This causes lack of compression force6on different-type second material2.

In the description of the embodiment, laser light7is emitted from the side of first material1as the upper plate with respect to the direction of laser emission, but it is not limited to; the laser light may be emitted from the side of third material3as the lower plate. In that case, third material3and first material1correspond to first material1and second material2, respectively, of same-type metal of the present disclosure.

In welding, a rivet is used for connecting a different-type material. When a part of the different-type material is deformed in caulking and spot welding, a space into which the deformed part moves has to be taken into account in designing rivets. Further, when the different-type material has a depressed part due to, for example, a positional gap of the electrodes in spot welding, joint strength can be degraded. To deal with the problems, a conventional rivet has needed a complicated shape, such as a chamfered edge of an R shape and an annular groove.

A rivet therefore has to be formed into a complicated shape with precision. That is, a rivet has to have a precision work, which increases production cost. Further, since the spot welding takes long time for applying pressure and current, for cooling, and for moving before/after welding, which decreases productivity. Besides, joining materials are welded such that a welding gun holds the joining materials therebetween, which lowers the degree of flexibility in design of the joining materials. In the resistance welding, molten metal is solidified into a weld nugget at a welding position. If a rivet disposed too close to an adjacent rivet, the welding current has a branch current, which fails sufficient nugget formation at an intended welding position. To obtain desired nugget formation with no branch current, a rivet has to be disposed with at least minimum joining pitch kept from an adjacent rivet. Due to the rivet arrangement with limitation in joining pitch, a conventional structure fails in increase in joint stiffness at an intended position. The structure of the present disclosure addresses the problems above.

As described above, the joint structure of the present disclosure has a first material, a second material, which are same-type metal materials and are weldable with each other, and a different-type material that is difficult to be welded to the first material and the second material. Having through-hole4as a penetrating part, the different-type material is to be sandwiched between the first material and the second material of same-type metal. In the plate thickness direction of an incident region in which laser light7is incident toward the penetrating part, the plate thickness at the incident region of the first material positioned on the side on which laser light7is incident has a predetermined size so as to be suitable for first gap11. First gap11is formed at the incident region in the plate thickness direction and exists before welding.

When laser light is incident to the incident region, the first material and the second material of same-type metal are fused and bonded together via through-hole4, by which the different-type material is compressively fixed to the first material and the second material of same-type metal.

Employing the joint structure eliminates a component that has to be formed with a high degree of precision due to its complicated structure. Besides, the joint structure is processed by laser welding, not spot welding. The process by laser welding allows the working time including welding to decrease to about 25% of the process by spot welding, significantly improving productivity. Further, the structure increases joint stiffness at an intended position, enhancing the degree of flexibility in design of joining materials.

First gap11may have a size corresponding to the length of through-hole4in the plate thickness direction in the incident region.

The length of first gap11in the incident region may be determined to be not less than 6% and not more than 38% of the thickness of the first material in the incident region.

The incident region in which laser light7is incident in the plate thickness direction may be formed small, having second gap5at a predetermined position with respect to through-hole4.

Different-type material2may flow onto the outer edge of weld part8at which the first material and the second material of same-type metal are fused and bonded together.

When a resin material is employed for the different-type material, the length of second gap5may be determined to be not less than 200% of that of first gap11.

When a fiber-reinforced resin material is employed for the different-type material, the length of second gap5may be determined to be not less than 150% of that of first gap11.

When nonferrous metal is employed for the different-type material, the length of second gap5may be determined to be not less than 125% of that of first gap11.

Spacer9formed of a material that is weldable to first material and second material of same-type metal may be disposed in through-hole4of the different-type material.

INDUSTRIAL APPLICABILITY

The joint structure is suitable for connecting different-type materials. Having a simple structure, the joint structure significantly decreases production take time and increases stiffness at a position that needs it. Further, the structure allows joining material to have increased flexibility in design. As described above, the structure has high industrial applicability as a joint structure for laser welding.

REFERENCE MARKS IN THE DRAWINGS