JP2015182135A - Welding method for projection bolt - Google Patents

Abstract

PROBLEM TO BE SOLVED: To properly ensure a molten state on a thin steel plate side when a projection bolt is welded to a thin steel plate by electric resistance welding.SOLUTION: A projection bolt 1, which is formed of a shaft part 2, a circular expanded diameter part 3, and a welding projection 4 consisting of an initial molten part 4A having a tapered part 6 and a main molten part 4B, is welded to a steel plate component 8 made of a thin steel plate by electric resistance welding. The space width C of a space 19 between the expanded diameter part 3 and the steel plate component 8 is reduced by electrode pressurization. At the same time of the reduction, air inside the space 19 is rapidly expanded by heat of melting to be rapidly discharged outside the space 19. Outside cool air is sucked into the space 19 by pressure drop in the space 19 generated by the discharge. A space width C1 after the completion of the welding is made approximately equal to the height dimension of the main molten part 4B. Consequently, the melting amount of the steel plate component 8 is properly ensured.

Description

  The present invention relates to a welding method for welding a projection bolt composed of a shaft portion, a diameter-enlarged portion formed integrally with the shaft portion, and a welding protrusion disposed in the center of the diameter-extended portion to a thin steel plate. It is related.

  Japanese Patent No. 4032313 (Patent Document 1) discloses a projection constituted by a shaft portion, a diameter-enlarged portion formed integrally with the shaft portion, and a welding protrusion disposed at the center of the diameter-extended portion. It is described that a bolt is welded to a steel plate part by electric resistance welding.

  The projection bolt disclosed in Patent Literature 1 has the shape shown in FIG. The projection bolt 20 is made of iron, and includes a shaft portion 21 in which a male screw is formed, a circular enlarged diameter portion 22 formed integrally with the shaft portion 21 and having a diameter larger than the diameter of the shaft portion 21, It is comprised by the circular welding protrusion 23 arrange | positioned in the center of the enlarged diameter part on the opposite side to the axial part 21. As shown in FIG. The welding protrusion 23 is a circular raised shape portion having a smaller diameter than the enlarged diameter portion 22, and includes a tapered portion 24 having a small inclination angle and a top portion 25 having a sharp central portion on the distal end surface side. . And the end surface of the enlarged diameter part 22 in parts other than the welding protrusion 23 is made into the taper surface 26 where the outer peripheral side became low.

Japanese Patent No. 4032313

  The invention described in Patent Document 1 (hereinafter referred to as the prior invention) has been put to practical use by Yoshitaka Aoyama and Shoji Aoyama, the inventors of the invention according to the present patent application. The inventors have succeeded in practical use of the prior invention by welding projection bolts to steel plate parts of an automobile body. That is, the welding protrusion 23 is welded to the steel plate part 27 at the center of the enlarged diameter portion 22, and the tapered surface 26 is in close contact with the surface of the steel plate part 27. The weld quality of a predetermined melted state and weld strength is ensured by such welding at the center and adhesion between the other parts, that is, “center welding / overall contact”.

  By the way, in the field of automobile bodies, for example, as an important measure for reducing the weight of a vehicle body, reduction of the plate thickness is promoted by improving the strength of a steel plate such as a high-strength steel plate. Special technical considerations are also required for electrical resistance welding.

  The situation is shown in FIG. A receiving hole 29 is formed in the movable electrode 28 that moves forward and backward, and the projection bolt 20 is held by the movable electrode 28 by inserting the shaft portion 21 therein. On the other hand, a steel plate part 27 made of a high-strength steel plate is placed on the fixed electrode 30, and the welding projection 23 is pressed onto the steel plate part 27 by the advancement of the movable electrode 28, so that a welding current is applied. Thereby, the welding projection 23 and the steel plate part 27 are in a molten state, and the bolt 20 is welded to the steel plate part 27 as shown in the figure.

  The welding state shown in FIG. 7B is an abnormal mode. The melted portion 32 painted black is formed across the entire thickness of the steel plate part 27. That is, as viewed in the thickness direction of the steel plate part 27, the entire plate thickness is once melted and then solidified. Such an excessive melting phenomenon is likely to occur when the plate thickness is reduced to 0.65 mm or 0.7 mm, and a large amount of heat on the projection bolt 20 side with respect to a thin plate thickness having a small heat capacity. It is thought that this occurred due to the influence. That is, the molten volume on the projection bolt 20 side is excessive with respect to the thin plate, and it is a thin plate even if the welding conditions such as the applied pressure, energization time and current value, and the volume of the molten metal are accurately controlled. Excessive melting occurs throughout the thickness.

  Furthermore, since the taper surface 26 is in close contact with the surface of the steel plate part 27, it is considered that the heat dissipation from the melting part 32 becomes insufficient, and the above-described excessive melting occurs.

  Usually, when bolts and nuts are welded to steel plate parts by electric resistance welding, the melting range of the steel plate viewed in the thickness direction of the steel plate parts is limited to half or two thirds of the plate thickness, and the required welding is performed. Strength is secured. That is, the base material which is a non-melting part is left behind. The weld strength can be ensured in this way because the melting range is the region as described above. This is considered to be because the bonding strength between the melted portion and the non-melted portion becomes a sufficient value due to widening.

  However, if the melting is performed over the entire thickness as shown in FIG. 7B, there is a problem that the welding strength between the bolt 20 and the steel plate part 27 cannot be sufficiently secured.

  Considering this problem, it is as follows. Since the melted part (Nugget) 32 is solidified by rapid cooling after completion of energization, it becomes a martensite structure, has extremely high hardness, and is brittle. In addition, in the region in the vicinity of the melted portion 32, the tissue change portion appears as a satin portion in the figure. Such a satin place is generally known as a heat affected zone (HAZ). This portion is indicated by reference numeral 33 and is not as brittle as the melted portion 32, but is more brittle than the base material portion.

  Therefore, when a bending force in the tilt direction is repeatedly applied to the bolt 20 on the fixed steel plate part 27, stress concentrates on the boundary portion between the melted portion 32 and the texture changing portion 33 having high hardness and brittleness. Cracks due to fatigue occur at this boundary. Alternatively, cracks occur in the texture change portion 33. In addition, since the plate thickness is thin, the area of the boundary portion between the melting portion 32 and the non-melting portion is reduced, and further, the boundary portion faces the plate thickness direction, so that the boundary portion is cracked. It is considered that the welding strength is not improved.

  The present invention is provided in order to solve the above problems, sucking external cold air into the gap formed between the enlarged diameter portion and the steel plate part, and promoting the cooling of the welded local portion, It aims at providing the welding method of the projection bolt which prevents the excessive fusion | melting of steel plate components.

  In the following description, the projection bolt may be simply expressed as a bolt.

According to the first aspect of the present invention, a shaft portion in which a male screw is formed, a circular enlarged diameter portion formed integrally with the shaft portion and having a diameter larger than the diameter of the shaft portion, and an outer peripheral side of the end surface are lowered. It was formed by a circular welding protrusion which is composed of an initial melting portion having a taper portion with a small inclination angle and a main melting portion connected to the initial melting portion and arranged at the center of the enlarged diameter portion on the opposite side to the shaft portion. The projection bolt is welded to the steel plate part by electric resistance welding in a state where the welding protrusion is pressed between the pair of electrodes to the steel plate part.
When the initial melted portion is pressed against the steel plate part, a welding current is applied and the initial melted portion starts melting, thereby reducing the gap width of the gap formed between the enlarged diameter portion and the steel plate part. Simultaneously with this reduction, the air in the gap is rapidly expanded by the heat of melting and rapidly discharged out of the gap, and due to the pressure drop in the gap caused by this discharge, external cold air is sucked into the gap. ,
The welding bolt welding method according to claim 1, wherein after the welding is completed, the gap width is substantially the same as a height dimension of the main melted portion.

  Since the welding protrusion is formed of an initial melting portion having a tapered portion with a small inclination angle whose outer peripheral side is lowered on the end surface and a main melting portion continuous with the initial melting portion, the melting portion of the initial melting portion is simultaneously melted. Progressively the surface of the steel plate part starts to melt. Along with the melting of the initial melting portion, the gap width of the gap formed between the enlarged diameter portion and the steel plate part is reduced. Simultaneously with the reduction of the gap width, the melting of a part of the main melting part proceeds following the melting of the initial melting part. However, since welding of the welding protrusions to the steel plate parts is substantially achieved by melting the entire initial melting portion, the melting of the main melting portion is very small or only the initial melting portion. It is said to be melted. Therefore, even if the main melted portion is melted, the gap width is substantially the same as the height of the main melted portion after the completion of welding. The gap formed between the enlarged diameter portion and the steel plate part is an annular space having a predetermined length in the diameter direction, in other words, a donut-shaped space.

  Since the applied pressure of the electrode is, for example, 2900 N, when the melting is started, the stationary electrode advances at an electrode advance distance corresponding to the amount of fusion at a high speed. Along with this, the expanded diameter part also advances at a high speed and the gap width is reduced, so even if one side of the gap is open to the outside air, the air in the gap is instantaneously compressed, and this compression At the same time, the air rapidly expands due to the heat of fusion. That is, when the compressed air is subjected to rapid heating, rapid expansion with a further increased expansion speed is formed. The high-pressure air obtained by this compression and rapid heating causes the air in the gap to be ejected out of the gap at a high speed. Since rapid ejection is performed in this manner, at the same time, the inside of the air gap becomes lean and the pressure in the air gap suddenly drops, and due to this pressure drop, cool air is sucked into the air gap.

  As described above, when the air in the gap rapidly jets out of the gap, the jet is in a direction along the surface of the steel plate part. Because of such a jet, the outside air is sucked into the gap from a direction away from the surface of the steel plate part. The reason why the jet of air is in the direction along the surface of the steel plate part is thought to be due to a phenomenon such as the Coanda effect.

  Such inhalation of cold air is performed at a stage of a liquid phase where the melted portion cannot be solidified, and therefore, excessive melting is suppressed. And since cold air will be in the state which contacted the lower surface of the enlarged diameter part heated by the said fusion | melting, the steel plate component surface, the main fusion part outer peripheral surface, a fusion | melting part exposure surface, etc., these heating parts are cooled favorably. Then, the amount of melting in the main melted part following the melting in the initial melted part is extremely small or not melted as described above, so that the height of the main melted part before melting remains almost unchanged. For this reason, after the welding is completed, the gap width is substantially the same as the height dimension of the main melted portion. Since the gap width is such a space width, heat radiation from the lower surface of the enlarged diameter portion, the surface of the steel plate part, the outer peripheral surface of the main melted portion, the exposed surface of the melted portion, etc. continues favorably.

  Since the heat of fusion is taken away by the above-described cold air suction, it is possible to prevent the melting from proceeding excessively and to avoid the excessive melting of the steel sheet component having a thin plate thickness. That is, the melted portion and the structure changing portion in the vicinity thereof are not formed over the entire plate thickness, and the unmelted base material portion is left between the structure changing portion in the vicinity of the melting portion and the steel plate surface. Therefore, this base material part fulfills the function of maintaining the strength as a steel plate part, and the weld joint strength of the bolt can be sufficiently secured. In addition, since the boundary area between the structure change part and the base material part can be secured over a wide region, the bonding strength of this boundary area part can be kept high, and even if an external force in the bending direction acts on the bolt, it can be easily cracked, etc. Will not occur.

  In addition, when the sucked cold air is heated again with residual heat such as the lower surface of the enlarged diameter portion, the steel plate component surface, the outer peripheral surface of the main melting portion, the exposed surface of the melting portion, the intake air expands and is discharged out of the gap, It is considered that cold air is sucked again due to the pressure drop accompanying this.

  After the welding is completed, the gap width is substantially the same as the height dimension of the main melted part. If the gap width is too narrow, the fluidity of the coating liquid in the gap cannot be obtained due to the viscosity of the coating liquid, so that the air stagnating in the gap is not exhausted and is contained in the coating liquid. There is a problem that rust is generated due to the enclosed air. Or when the length of a space | gap becomes excessive, a coating liquid does not enter into the back of a space | gap, and the same problem generate | occur | produces.

  However, as in the present invention, since the gap width after welding is almost the same as the height dimension of the main melted part, air bubbles are enclosed by the coating liquid between the enlarged diameter part and the steel plate part. A void with no size is left, and the problem of rust generation is solved. That is, since the gap width can be secured as a sufficient space, the coating liquid is actively introduced into the gap. By such a flow, the air in the gap is discharged, and the coating liquid adheres to the lower surface of the enlarged diameter part, the surface of the steel plate part, the outer peripheral surface of the main melting part, the exposed surface of the molten part, etc. This eliminates the problem of rust generation as described above.

  The above-mentioned coating is usually an undercoating by electrodeposition coating, and a steel plate part or an automobile body, etc., to which a bolt is welded in an aqueous paint bath having a relatively low concentration (normally 5 to 10% solids) is used as an electrode. On the other hand, a direct current is applied between both electrodes using the bathtub as a counter electrode, and the current is passed through to uniformly deposit a film having a thickness of 20 to 30 μm on the bolt or the steel plate part. The viscosity of the water-based paint is set to be very low because the solid content is the above value, and it flows without entraining air bubbles in the gaps and cavities of the product. And inflow into the gap between the steel plate parts.

  The invention according to claim 2 is the projection bolt welding method according to claim 1, wherein the ratio of the gap length viewed in the diameter direction of the enlarged diameter portion to the gap width is 2 to 8.

  As described above, the ratio of the gap length viewed in the diameter direction of the enlarged diameter portion to the gap width is set to 2 to 8, so that the outer peripheral surface of the main melting portion is located at the deepest part of the gap. Then, an elongated space is formed from there toward the outer peripheral side. Therefore, the rapid air ejection as described above is formed by synergy between the reduction of the gap width and the rapid thermal expansion of the air.

  A third aspect of the present invention is the projection bolt welding method according to the first or second aspect, wherein the gap width before the welding is 0.8 to 1.6 mm.

  Since the gap width is 0.8 to 1.6 mm, the gap width reduction and the rapid air expansion are formed simultaneously, and strong air injection is obtained. Such a phenomenon is considered to be related to the volume of the air gap, the viscosity of the air, the size of the air gap width, and the like. It is considered that these elements are well related because the gap width is set to 0.8 to 1.6 mm.

  Further, the gap width after completion of welding is substantially the same as the height of the main melted part. Therefore, when the coating liquid flows into the gap while discharging air and the above-described electrodeposition coating is performed, a very good coating film is uniformly deposited over the entire inner surface of the gap.

  A fourth aspect of the present invention is the projection bolt welding method according to any one of the first to third aspects, wherein the plate thickness of the steel plate component is 0.6 mm to 1.0 mm.

  As described above, even if the plate thickness is 0.6 mm to 1.0 mm, the above-described cooling action does not result in an excessive melting depth, and good welding strength can be obtained.

It is the side view and top view which show each part shape of a volt | bolt. It is sectional drawing which shows the state in which a volt | bolt is welded. It is sectional drawing which shows the welding process at the time of welding. It is sectional drawing which shows the ejection and suction | inhalation of air. It is a diagram which shows the ratio of the width | variety of width | variety and length, and the relationship between T1 and T2. It is sectional drawing which shows the state after a tension test. It is a figure which shows the welding state of the conventional volt | bolt.

  Next, an embodiment for carrying out the projection bolt welding method of the present invention will be described.

  1 to 6 show a first embodiment of the present invention.

  First, the dimensions and shape of the projection bolt will be described.

  The shape of the iron projection bolt 1 is shown in FIGS. 1 (A) and 1 (B). The bolt 1 includes a shaft portion 2 on which a male screw is formed, a circular enlarged diameter portion 3 formed integrally with the shaft portion 2 and having a diameter larger than the diameter of the shaft portion 2, and the shaft portion 2. Is formed by a circular welding protrusion 4 arranged in the center of the enlarged diameter portion on the opposite side. Reference numeral 5 denotes a male screw formed on the outer peripheral surface of the shaft portion 2 and has a valley portion and a mountain portion.

  As shown in FIGS. 1 and 4, the welding protrusion 4 includes an initial melting part 4 </ b> A and a main melting part 4 </ b> B. The initial fusion part 4A is a flat conical part formed by providing a taper part 6 with a small taper inclination angle at the outer peripheral side of the end face of the welding projection 4 to be lowered. A sharp top portion 7 is formed at the center of the initial melting portion 4A. The main melting part 4B is a truncated cone shaped part formed in a state of being connected to the initial melting part 4A. Since the bolt 1 is subjected to mold molding, roll processing, and the like, when observed in an enlarged manner, the top portion 7 actually has a slightly rounded shape instead of a sharply pointed shape.

  In FIG. 1C, the dimensions and inclination angles of each part are described for easy understanding of the dimensional state of the embodiment. As shown in this figure, the diameter (crest diameter) of the shaft portion 2 is 5.5 mm, the length of the shaft portion 2 is 24.5 mm, and the diameter and thickness of the enlarged diameter portion 3 are 20.5 mm and 1.2 mm, respectively. It is. Further, the diameter of the end face (tapered portion 6) of the welding projection 4 is 10.5 mm, the height (thickness) of the initial melting portion 4A is 0.35 mm, and the height (thickness) of the main melting portion 4B is 1. The inclination angle θ of the tapered portion 6 is 4.5 mm and 0 mm.

  Next, the welding state of the bolt 1 will be described.

  FIG. 2 is a cross-sectional view showing a state in which the bolt 1 is welded to the steel plate part 8. The movable electrode 9 is advanced and retracted by an air cylinder or an advancing / retracting output type electric motor (not shown). A receiving hole 10 is opened in the longitudinal direction of the movable electrode 9 at the center of the end face, and a permanent magnet 11 is attached to the inner part. The steel plate component 8 is placed on a fixed electrode 12 arranged coaxially with the movable electrode 9. The plate thickness of the steel plate component 8 is 0.65 mm.

  The shaft portion 2 is inserted into the receiving hole 10 of the movable electrode 9 by an operator or a supply rod, the shaft portion 2 is attracted by the permanent magnet 11, and the bolt 1 is held by the movable electrode 9. At this time, the back surface of the enlarged diameter portion 3 is in close contact with the end surface 13 of the movable electrode 9. FIG. 2 shows a state in which the movable electrode 9 holding the bolt 1 has advanced and the welding projection 4 is being pressed against the steel plate component 8. The top portion 7 and the tapered portion 6 in the vicinity thereof are recessed into the surface of the steel plate component 8 by this pressurization, although not shown. That is, the tip end portion of the taper portion 6 of the initial melting portion 4A slightly bites into the surface of the steel plate component 8, and the contact area between the welding projection 4 and the steel plate component 8 is increased. In this state, a welding current is applied and welding to the steel plate part 8 is performed.

  In the pressurized state shown in FIG. 2 and FIG. 4A, a gap 19 is formed between the lower surface of the enlarged diameter portion 3 and the surface of the steel plate component 8. The gap width when the gap 19 is viewed in the axial direction of the shaft portion 2 is the gap width C. The length of the gap 19 when viewed in the diameter direction of the enlarged diameter portion 3 is the gap length D. In this embodiment, as apparent from the dimensions shown in FIG. 1C, the gap width C is 1.35 mm and the gap length D is 5.0 mm. The ratio H of the gap length D to the gap width C is 3.7.

  Next, pressurizing energization conditions will be described.

  The pressing force by the movable electrode 9, that is, the pressing force of the welding projection 4 on the steel plate part 8, is 2900N, the welding current is 12000A, and the energization time is 9 cycles. This energization time of 9 cycles is the time from the start of energization to the start of melting of the initial melting part 4A after the elapse of a predetermined time and the subsequent end of melting of a part of the main melting part 4B. One cycle is 1/60 second.

  Good welding is possible under the above-mentioned conditions, but the setting range of each condition is preferably 2000 to 3300 N, welding current of 10,000 to 15000 A, and energization time of 5 to 12 cycles.

  Next, the welding process will be described.

  FIG. 3 shows the welding process. Although this figure is a cross-sectional view, the hatching description of the cross-sectional portion is omitted for easy viewing. FIG. 3A is an initial stage of energization in which a welding current is applied in the pressing state of FIG. 2, and the vicinity of the top portion 7 and the corresponding steel plate part 8 are slightly melted. This melting point is indicated by reference numeral 14.

  When the energization is further continued, the melted portion 14 becomes a radial and substantially flat melting range in the diameter direction and expands in a circular shape due to the inclination angle of the tapered portion 6. This enlarged transient state is shown in FIG.

  Thereafter, by continuing the energization with pressure, the melting of the main melting portion 4B is started simultaneously following the melting of the entire initial melting portion 4A. However, as described above, since the melting amount of the main melting portion 4B is very small, the height of the main melting portion 4B after the completion of welding hardly decreases. Due to the melting of the initial melting portion 4A, the entire surface area of the steel plate part 8 corresponding to the circular range of the initial melting portion 4A is melted as shown in FIGS. When the aforementioned energization time of 9 cycles elapses, the molten state shown in FIG.

  As is apparent from FIGS. 3C and 3D, the main melting portion 4B is melted very little or substantially so that only the initial melting portion 4A is melted. Welding conditions such as applied pressure, current value, and energization time are defined.

  As shown in FIGS. 1 (C) and 4 (A), the shape of the initial melting part 4A before melting is a flat conical shape, but the conical shape disappears at the initial stage of melting, and the main melting The melting of a part of the portion 4B is slightly added to form a flat melting region integrated with the melting portion of the surface portion of the steel plate part 8.

  FIG. 3D is a partial enlarged cross-sectional view showing the structure state after the completion of welding, and a blacked-out portion is the melted portion 14, which is the aforementioned nugget. And the part which appears in the vicinity of the fusion | melting location 14 in the layer form is the structure | tissue change part 15, and is the above-mentioned heat affected zone. The tissue change portion 15 is shown with a satin finish in the drawing.

  Reference numeral 16 denotes a non-melting portion, which is formed of a structure changing portion 15 and a base material 17 of a steel plate not subjected to thermal influence, and its thickness is indicated by T1. The thickness of the base material 17 alone is indicated by T2.

  Next, air ejection and inhalation will be described.

  Since the pressing force of the movable electrode 9 is 2900 N as described above, when the melting is started, the stationary movable electrode 9 advances at an electrode advance distance corresponding to the amount of fusion at a high speed. Along with this, the enlarged diameter portion 3 also advances at a high speed and the gap width C is reduced, so that the air in the gap 19 is instantaneously compressed even if one side of the gap 19 is open to the outside air. At the same time as this compression, the air rapidly expands due to the heat of fusion. That is, when the compressed air is subjected to rapid heating, rapid expansion with a further increased expansion speed is formed. The high-pressure air obtained by this compression and rapid heating causes the air in the gap 19 to be ejected outside the gap 19 at a high speed, as indicated by the arrow 20 shown in FIG. Since rapid ejection is performed in this manner, at the same time, the space 19 becomes lean and the pressure in the space 19 rapidly decreases, and this pressure drop causes cold air as shown by the arrow 21 in the figure. Outside air is sucked into the gap 19. Such inhalation of cold air is performed from the stage of the liquid phase where the melting part 14 cannot be solidified.

  As described above, when the air in the gap 19 rapidly jets out of the gap 19, the jet flows in the direction along the surface of the steel plate part 8 as indicated by the arrow 20. Since it is such a jet, the outside air is sucked into the gap from the direction separated from the surface of the steel plate part 8, that is, from above in FIG. This inhalation state is indicated by an arrow line 21. The reason why the air jet flows in the direction along the surface of the steel plate part 8 is considered to be due to a phenomenon such as the Coanda effect. Further, as the jet flow moves away from the gap 19, the velocity and pressure decrease and expands. For this reason, the expansion region 22 shown by the chain line in FIG. 4 (B) is formed.

  Next, specific values of T1 and T2 will be described.

  FIG. 5 is a diagram showing the relationship between the ratio H of the gap length D to the gap width C and T1, T2. In the ratio H3.7, as described above, T1 is 0.32 mm and T2 is 0.27 mm. This value is appropriate for the amount of penetration of the steel plate part 8 in the thickness direction and the thickness of the base material 17.

  At the same time as securing the values of T1 and T2, the gap C1 has a size such that bubbles are not sealed by the coating liquid between the vicinity of the outer periphery of the enlarged diameter portion 3 and the surface of the steel plate part 8 after completion of welding. Is kept. The gap C1 is a space width after welding, and is 0.95 mm which is substantially the same as the height dimension of the main melted part 4B. Further, the size of the gap C1 can be set to 1.0 mm by changing only the initial melting portion 4A by changing the welding conditions such as the applied pressure, the current value, and the energizing time.

  Next, the lower limit value and the upper limit value of each value will be described.

  The ratio H of the enlarged diameter portion D to the gap width C before welding is desirably 2-8.

  If the ratio H is less than 2, the gap length D becomes too small, so even if the gap width C is reduced or the air is rapidly expanded, the jet power cannot be sufficiently secured. Accordingly, the intake of cold air is insufficient and cooling becomes insufficient, and the penetration depth of the steel plate part 8 becomes excessive, that is, excessive melting occurs, resulting in insufficient welding strength.

  On the other hand, when the ratio H exceeds 8, the gap length D is excessive and the gap width C1 is too small, so that coating liquid such as electrodeposition coating can easily contain air in the gap 19 and rust is generated. The problem remains. Since the gap length D is excessive and the gap width C1 is too small, the intake air slowly reaches the back of the gap 19 and becomes insufficiently cooled and turns to the excessive melting side. The so-called melting heat becomes muddy. Therefore, it will go in the direction of insufficient welding strength. In addition, since the gap length D is excessive and the gap width C1 is too small, heat radiation from the lower surface of the enlarged diameter portion, the steel plate component surface, the outer peripheral surface of the main fusion portion, the exposed portion of the fusion portion, and the like becomes insufficient.

  It is desirable that the gap width C before welding is 0.8 to 1.6 mm.

  If the gap width C before welding is less than 0.8 mm, the volume of the gap 19 before welding is too small, so that sufficient air expansion cannot be ensured. Therefore, a strong jet cannot be obtained. If the gap width C exceeds 1.6 mm, air expansion is obtained, but the volume of the gap 19 before welding is too large, so that the amount of expansion is insufficient, and a strong jet pressure cannot be obtained.

  As is clear from FIG. 5, when the ratio H is 3.7, T1 is 0.32 mm and T2 is 0.27 mm, and when the ratio H is 6, T1 is 0.51 mm and T2 is 0. .46 mm. Correlation lines T1 and T2 in FIG. 5 are formed by connecting the values of T1 and T2 obtained by sequentially changing the ratio H in this way.

  When T1 = 0.32 mm and T2 = 0.27 mm in the ratio H = 3.7 are changed within the above-mentioned range by changing the welding conditions such as the applied pressure, the current value, and the energizing time, the ratio H remains 3.7. Can be converted to T1 = 0.38 mm and T2 = 0.34 mm.

  Next, the results of the evaluation test will be described.

  As described above, when the bolt 1 having a ratio H of the gap length D to the gap width C of 1 is welded to the steel plate part 8 under a pressurizing force of 2900 N, a welding current value of 12000 A, and an energization time of 9 cycles, T1 was 0.10 mm, and T2 was 0.04 mm. With such values, it was confirmed that sufficient welding strength could not be obtained. On the other hand, when the ratio H was 3.7 or 6.0, the values of T1 and T2 as described above were obtained.

  As described above, it is recognized that H = 1 is excessive melting, and H = 3.7 and H = 6.0 are proper melting.

  In addition, under the above-described welding conditions, bolts with H = 3.7 were welded to prepare 10 weld samples, and each T1 and T2 were measured. T1 = 0.30 to 0.35 mm T2 = 0.25 to 0.29 mm. Furthermore, bolts with H = 6.0 were welded to prepare 10 weld samples, and T1 and T2 were measured. T1 = 0.49 to 0.55 mm, T2 = 0.41 to 0 .49 mm. In any sample, it was confirmed that T1 and T2 were within the range of appropriate values.

  As shown in FIG. 6, the steel plate component 8 is fixed with a jig (not shown), and the bolt 1 is axially fixed to the welded steel plate component having such a ratio H of 3.7 or 6.0. As a result of the pulling test, it is recognized that the base material 17 and the melted portion 14 are broken from the steel plate part 8 in a sheared state to form a circular through hole 8B. This rupture occurs in the range of 3000 to 3500 N, and for example, it is determined that the welding strength is sufficient in a case where a wiring clamp fixing bolt is welded to a dash panel of an automobile.

  In addition, as a result of the repeated bending test in which the shaft portion 2 is tilted, cracks are generated from the boundary portion between the melted portion 14 and the structure change portion 15, the structure change portion 15 itself, or the boundary portion between the structure change portion 15 and the base material. I did not. It is determined that the welding strength is sufficient when the bolt 1 having such a size is welded to the steel plate part 8 having a very thin thickness of 0.65 mm.

  In the first embodiment, the shape of the initial melting portion 4A is a conical shape having the taper portion 6 and the apex portion 7, but may be a spherical shape instead. In the case of a spherical shape, a portion corresponding to the top portion 7 is pressed against the steel plate part 8, and melting is started from this pressed portion. The other welding processes are the same as those of the conical shape.

  The operational effects of the first embodiment described above are as follows.

  Since the welding protrusion 4 is formed of an initial melting portion 4A having a tapered portion 6 with a small inclination angle whose outer peripheral side is lowered on the end surface, and a main melting portion 4B connected to the initial melting portion 4A, Simultaneously with the melting of the volume portion, the surface of the steel plate part 8 starts to melt. With the melting of the initial melting portion 4A, the gap width C of the gap 19 formed between the enlarged diameter portion 3 and the steel plate part 8 is reduced. Simultaneously with the reduction of the gap width C, the melting of a part of the main melting part 4B proceeds following the melting of the initial melting part 4A. However, since the welding of the welding protrusion 4 to the steel plate part 8 is substantially achieved by the melting of the entire area of the initial melting portion 4A, the melting of the main melting portion 4B is extremely small, or Only the initial melting part 4A is melted. Therefore, even if the main melted part 4B is melted, the gap width C1 is substantially the same as the height dimension of the main melted part 4B after the completion of welding. The gap 19 formed between the enlarged diameter portion 3 and the steel plate part 8 is an annular space having a predetermined length in the diameter direction, in other words, a donut-shaped space.

  Since the applied pressure of the movable electrode 9 is 2900 N, for example, when the melting is started, the movable electrode 9 that has been stationary advances at an electrode advance distance corresponding to the amount of fusion at a high speed. Along with this, the enlarged diameter portion 3 also advances at a high speed and the gap width C is reduced, so that the air in the gap 19 is instantaneously compressed even if one side of the gap 19 is open to the outside air. At the same time as this compression, the air rapidly expands due to the heat of fusion. That is, when the compressed air is subjected to rapid heating, rapid expansion with a further increased expansion speed is formed. The high-pressure air obtained by this compression and rapid heating causes the air in the gap 19 to be ejected out of the gap 19 at a high speed. Since rapid ejection is performed in this manner, at the same time, the inside of the gap 19 becomes lean and the pressure in the gap 19 rapidly decreases. Due to this pressure drop, cool air is sucked into the gap 19.

  As described above, when the air in the gap 19 rapidly jets out of the gap 19, the jet flows in the direction along the surface of the steel plate part 8, that is, the direction indicated by the arrow 20. Since it is such a jet, the outside air is sucked into the gap 19 from the direction separated from the surface of the steel plate part 8, that is, the direction indicated by the arrow line 21. The reason why the air jet flows in the direction along the surface of the steel plate part 8 is considered to be due to a phenomenon such as the Coanda effect.

  Such cold air is sucked in a liquid phase where the melted portion 14 cannot be solidified, and therefore, excessive melting is suppressed. And since cold air will be in the state which contacted the lower surface of the enlarged diameter part 3 heated by the said fusion | melting, the surface of the steel plate component 8, the outer peripheral surface of the main fusion | melting part 4B, the exposed surface of the fusion | melting part 14, etc., these heating The part is cooled well. And since the amount of melting of the main melting part 4B subsequent to the melting of the initial melting part 4A is very small as described above or not melted, the height of the main melting part 4B before melting is left almost unchanged. . For this reason, after the welding is completed, the gap width C1 is substantially the same as the height dimension of the main melting portion 4B. Since the gap width C1 is such a space width, the heat radiation from the lower surface of the enlarged diameter portion, the steel plate component surface, the outer peripheral surface of the main melting portion, the exposed surface of the melting portion, etc. continues favorably.

  Since the heat of fusion is deprived by the cold air suction described above, excessive progress of melting is suppressed, and excessive melting of the thin steel plate part 8 having a thickness of 0.65 mm can be avoided. That is, the molten portion 14 and the structure changing portion 15 in the vicinity thereof are not formed over the entire plate thickness, and the base material portion 17 that is not melted between the structure changing portion 15 in the vicinity of the melting portion 14 and the steel plate surface is formed. Remained. Therefore, the base material portion 17 fulfills the function of maintaining the strength as the steel plate part 8, and the weld joint strength of the bolt 1 can be sufficiently secured. In addition, since the boundary area between the structure change portion 15 and the base material portion 17 can be secured over a wide region, the bonding strength of this boundary area portion can be kept high, and even if an external force in the bending direction acts on the bolt 1. No cracks occur.

  When the sucked cold air is heated again with residual heat such as the lower surface of the enlarged diameter portion, the steel plate part surface, the outer peripheral surface of the main melting portion, the exposed surface of the melting portion, etc., the intake air expands and is discharged out of the gap 19. It is considered that cold air is sucked again due to the pressure drop accompanying this.

  After the welding is completed, the gap width C1 is substantially the same as the height dimension of the main melted part. If the gap width C1 is too narrow, the fluidity of the coating liquid in the gap 19 cannot be obtained due to the viscosity of the coating liquid, so that the air stagnating in the gap 19 is not exhausted and is contained in the coating liquid. Become. There is a problem that rust is generated due to the enclosed air. Or when the length D of the space | gap 19 becomes excess, a coating liquid does not enter into the back of a space | gap, and the same problem generate | occur | produces.

  However, as in the present invention, since the gap width C1 after completion of welding is substantially the same as the height dimension of the main melted part 4B, bubbles are generated between the enlarged diameter part 3 and the steel plate part 8 by the paint liquid. A gap 19 having a size that is not sealed is left, and the problem of rust generation is solved. That is, since the gap width C1 can be secured as a sufficient space, the coating liquid is actively introduced into the gap 19. By such a flow, the air in the gap 19 is discharged, and the coating liquid adheres to the lower surface of the enlarged diameter part, the surface of the steel plate part, the outer peripheral surface of the main melting part, the exposed surface of the melting part, etc. This eliminates the problem of rust generation as described above.

  The above coating is usually an undercoating by electrodeposition coating, and a steel plate part 8 or a car body of an automobile, etc., to which bolts 1 are welded in a relatively low concentration (usually 5 to 10% solids) aqueous paint bath. The film is immersed as an electrode, and on the other hand, a direct current is applied between both electrodes with the bath as a counter electrode, and a film having a thickness of 20 to 30 μm is uniformly deposited on the bolt 1 and the steel plate part 8 through the current. The viscosity of the water-based paint is set to be very low because the solid content is the above value, and it flows without entraining air bubbles in the gaps and cavities of the product. 3 and the steel plate part 8 are allowed to flow into the gap 19 without stagnation.

  The ratio H of the gap length D seen in the diameter direction of the enlarged diameter portion 3 with respect to the gap width C is 2-8.

  As described above, the ratio H of the gap length D seen in the diameter direction of the enlarged diameter portion 3 with respect to the gap width C is set to 2 to 8, so that the main melted portion is located at the deepest part of the gap 19. The outer peripheral surface of 4B is located, and an elongate space is formed from there to the outer peripheral side. Therefore, the rapid air ejection as described above is formed in synergy with the reduction of the gap width C and the rapid thermal expansion of the air.

  The gap width C before welding is 0.8 to 1.6 mm.

  Since the gap width C before welding is 0.8 to 1.6 mm, the reduction of the gap width C and the rapid air expansion are formed simultaneously, and strong air injection is obtained. Such a phenomenon is considered to be related to the volume of the gap 19, the viscosity of the air, the size of the gap width C, and the like. It is considered that these elements are well related because the gap width C is set to 0.8 to 1.6 mm.

  Furthermore, the gap width C1 after completion of welding is substantially the same as the height of the main melted part 4B. Therefore, when the coating liquid flows into the gap 19 while discharging air and the above-described electrodeposition coating is performed, a very good coating film is uniformly deposited over the entire inner surface of the gap.

  The plate thickness of the steel plate component 8 is 0.6 mm to 1.0 mm.

  As described above, even if the plate thickness is 0.6 mm to 1.0 mm, the above-described cooling action does not result in an excessive melting depth, and good welding strength can be obtained.

  In Example 2, only the initial melting part 4A is melted, and the gap width C1 after welding is set to the same value as the height of the main melting part 4B. In order to melt only the initial melting portion 4A, the above-described welding conditions, that is, the applied pressure, the current value, the energization time, and the like are selected. Other configurations are the same as those of the first embodiment. In such a configuration, it is appropriate that the initial melting part is called, for example, “melting part” and the main melting part is called “base part” in terms of function.

  With such a configuration, the gap width C1 after completion of welding can be made the same as the height of the main melted portion 4B, and the gap width C1 can be ensured uniformly as an appropriate value. Other functions and effects are the same as those of the first embodiment.

  As described above, according to the method of the present invention, external cold air is sucked into the gap formed between the enlarged diameter portion and the steel plate part, and the cooling of the welded local part is promoted, so that the steel plate part is excessively melted. This is a method for preventing projection bolt welding. Therefore, it is possible to perform electric resistance welding with a bolt in a good state on a thin plate, and it can be used in a wide range of industrial fields such as a car body welding process for automobiles and a sheet metal welding process for household appliances.

DESCRIPTION OF SYMBOLS 1 Projection bolt 2 Shaft part 3 Expanded part 4 Welding protrusion 4A Initial fusion part 4B Main fusion part 6 Taper part 7 Top part 8 Steel plate part 8B Through hole 14 Melting part, melting part 15 Structure change part, Heat-affected part 16 Non-melting part 17 Base material, base material portion 19 Cavity C Cavity width C1 before welding Cavity width D after completion of welding D Cavity length H Ratio of Cavity length D to Cavity width C T1 Thickness dimension T2 of base material, base material Material part thickness dimension

According to the first aspect of the present invention, a shaft portion in which a male screw is formed, a circular enlarged diameter portion formed integrally with the shaft portion and having a diameter larger than the diameter of the shaft portion, and an outer peripheral side of the end surface are lowered. A projection formed by a circular welding protrusion, which is composed of an initial melted portion having a taper portion with an inclination angle and a main melted portion connected to the initial melted portion and arranged at the center of the enlarged diameter portion on the opposite side to the shaft portion. The bolt is welded to the steel plate part by electric resistance welding in a state where the welding protrusion is pressed between the pair of electrodes to the steel plate part.
When the initial melted portion is pressed against the steel plate part, a welding current is applied and the initial melted portion starts melting, thereby reducing the gap width of the gap formed between the enlarged diameter portion and the steel plate part. Simultaneously with this reduction, the air in the gap is rapidly expanded by the heat of melting and rapidly discharged out of the gap, and due to the pressure drop in the gap caused by this discharge, external cold air is sucked into the gap. ,
The ratio of the gap length seen in the diameter direction of the enlarged diameter portion to the gap width is 2 to 8,
The welding bolt welding method according to claim 1, wherein after the completion of welding, the gap width is the same as a height dimension of the main melted portion.

The ratio of the gap length viewed in the diameter direction of the enlarged diameter portion to the gap width is 2 to 8.

A second aspect of the present invention is the projection bolt welding method according to the first aspect, wherein the gap width before the welding is 0.8 to 1.6 mm.

A third aspect of the present invention is the projection bolt welding method according to the first or second aspect, wherein the thickness of the steel sheet component is 0.6 mm to 1.0 mm.

Claims (4)

A shaft portion in which a male screw is formed, a circular enlarged portion formed integrally with the shaft portion and having a diameter larger than the diameter of the shaft portion, and a taper portion having a small inclination angle with a lower outer peripheral side on the end face A projection bolt formed by a circular welding protrusion, which is composed of an initial melting portion and a main melting portion continuous to the initial melting portion and is disposed at the center of the enlarged diameter portion opposite to the shaft portion, is connected between a pair of electrodes. In the state where the welding protrusion is pressed on the steel plate part, the steel plate part is welded by electric resistance welding,
When the initial melted portion is pressed against the steel plate part, a welding current is applied and the initial melted portion starts melting, thereby reducing the gap width of the gap formed between the enlarged diameter portion and the steel plate part. Simultaneously with this reduction, the air in the gap is rapidly expanded by the heat of melting and rapidly discharged out of the gap, and due to the pressure drop in the gap caused by this discharge, external cold air is sucked into the gap. ,
The welding bolt welding method according to claim 1, wherein the gap width is substantially the same as a height dimension of the main melted portion after welding is completed.   The projection bolt welding method according to claim 1, wherein a ratio of a gap length viewed in a diameter direction of the enlarged diameter portion to the gap width is 2 to 8. 8.   The method for welding projection bolts according to claim 1 or 2, wherein the gap width before welding is 0.8 to 1.6 mm.   The method for welding projection bolts according to any one of claims 1 to 3, wherein a thickness of the steel sheet component is 0.6 mm to 1.0 mm.

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