Frictional spot joining method and frictional spot joining apparatus

In the third step of joining of the metal members, the pressing force of the rotational tool under rotation is maintained at the third pressing force that is smaller than the second pressing force until a specified period of time elapses after the tip of the pin portion of the rotational tool has reached the specified point that is at a distance from the joining face. Accordingly, the proper plastic flow of the metal member can be generated and thereby the joining strength with a sufficient period of time of agitation can be ensured, by maintaining a high temperature enough to soften the metal member and by preventing the rotational tool from coming into the metal member too deeply or from penetrating the metal member.

BACKGROUND OF THE INVENTION

The present invention relates to a frictional jointing, and in particular to a frictional joining technology in which different kinds of metal members that lap over are joined.

Members made of aluminum or aluminum alloy (hereinafter, referred to as “aluminum” simply) have been recently used as a body panel and the like of many automotive vehicles to reduce a vehicle weight. Accordingly, the joining of different kinds of metal members, such as aluminum and iron, or aluminum and steel, has been increasing. The frictional joining is also known as such a joining method. In this frictional joining, there is provided a work that is comprised of a first metal member, for example, that is made of aluminum alloy and a second metal member, for example, that is made of steel and has a higher melting point than the first metal member, which lap over. Then, a rotational tool is pushed against this work from a side of the first metal member, the first metal member is softened and made in a plastic flow state by a frictional heat generated via the rotational and pressing operation of the rotational tool, and so both metal members are joined in a solid state (joining in the solid state without melting) under a specified temperature that is lower than the melting point of the metal members.

Japanese Patent Application Laid-Open No. 2003-245782 discloses a certain spot joining technology. Herein, the rotational tool under rotation having its pin portion and shoulder portion is pushed against the work comprised of plural metal members at the high rotational speed and with the large pressing force at the initial stage in which only the pin portion contacts the work, so the generation of the frictional heat can be promoted. Meanwhile, the rotational tool under rotation is pushed at the low rotational speed and with the small pressing force at the terminal stage in which both the pin portion and the shoulder portion contact the work, so the proper agitation of the softened portion of the work can be promoted.

Also, the applicant has applied for a patent relating to the improved spot joining method and apparatus of the metal members in which the rotational tool is pushed with the stepwise increased pressing force to ensure the proper positioning of rotation by the pin portion of the rotational tool (U.S. patent application Ser. No. 11/000,063).

Meanwhile, in the case where different kinds of metal members, such as aluminum and steel, are joined, it is preferable in order to ensure the high joining strength that new uncovered surfaces of the metal members are exposed by pushing out the zinc plating layer existing on the joining boundary face from the joining portion of the members or by destroying the oxidation film. In order to do so, the metal member into which the rotational tool comes needs to be softened sufficiently and the plastic flow is generated. Here, in order to properly generate the plastic flow, it is necessary to promptly increase the heat generated at the joining portion to a temperature that can soften the metal member, and then to maintain the increased temperature for a while so that the continuous plastic flow of the metal member can be ensured. Herein, the invention of the above-described patent application is appropriate in order to promptly increase the temperature of the joining portion to the temperature to soften the metal member, because the considerably high pressing pressure is required. However, there are following concerns.

Namely, a certain period of time of agitation by the rotational tool under a relatively high pressing force at the terminal stage with both the pin portion and the shoulder portion of the rotational tool pushed into the metal member to generate the proper plastic flow at the metal member would cause an improper situation in which the rotational tool comes into the upper aluminum plate too deeply, so that the thickness of part of the aluminum existing between the tip of the rotational tool and the lower steel plate (remaining thickness) becomes too thin. Eventually, there would occur a situation in which the aluminum plate has been torn off. As a result, an aluminum loss would happen at the joining portion, and thus there would occur problems of galvanic corrosion or joining-strength decrease due to a difference in electric potential between the aluminum and the steel at this aluminum-loss portion. Also, the torn-off aluminum is attached to the rotational tool, which would prevent the rotational tool from properly operating for joining at subsequent joining portions. Further, the rotational tool penetrates the upper aluminum plate, and reaches the joining boundary face, hitting against the lower steel plate. As a result, the tip of the pin portion of the rotational tool would be worn improperly.

SUMMARY OF THE INVENTION

The present invention has been devised in view of the above-described problems of the joining of different kinds of metal members that lap over. An object of the present invention is to generate the proper plastic flow of the metal member and thereby to ensure the joining strength with a sufficient period of time of agitation, by maintaining a high temperature enough to soften the metal member and by preventing the rotational tool from coming into the metal member too deeply or from penetrating the metal member.

According to the present invention, there is provided a frictional spot joining method of metal members, in which a work comprising a first metal member and a second metal member that lap over is provided, a melting point of the second metal member being higher than that of the first metal member, and a rotational tool is pushed against the work from a side of the first metal member and the first metal member is softened and made in a plastic flow state by a frictional heat generated through a rotational and pressing operation of the rotational tool, the frictional spot joining method comprising a step of providing the rotational tool that comprises a shoulder portion to face the work and a pin portion that is located on a rotational axis of the rotational tool and projects toward the work from the shoulder portion, a first step of pushing the rotational tool under rotation with a first pressing force until the first metal member is softened by the rotational tool whose pin portion or both pin portion and shoulder portion come into the first metal member, a second step of pushing the rotational tool under rotation with a second pressing force that is greater than the first pressing force until a tip of the pin portion of the rotational tool reaches a specified point that is at a distance from a joining face of the first and second metal members, and a third step of maintaining the pressing force of the rotational tool under rotation at a third pressing force that is smaller than the second pressing force until a specified period of time elapses after the tip of the pin portion has reached the specified point that is at a distance from the joining face.

Thereby, the first metal member starts to be softened by the frictional heat in the first step, and in the second step, the temperature of the joining portion is promptly increased to the high temperature enough to soften the first metal member by the frictional heat and the plastic flow of the first metal member starts. Then, in the third step, since the pressing force is maintained at the low pressing force, the rotational tool can be prevented from coming into the metal member too deeply and the rotational tool can be held in the specified location. Accordingly, the high temperature enough to soften the first metal member can be maintained and thereby the proper plastic flow of the first metal member can be ensured, preventing the torn-off situation of the first metal member or the penetration of the rotational tool through the metal member. Thus, the proper plastic flow of the metal member can be maintained for a sufficiently long period of time in the third step, thereby ensuring the joining strength.

According to an embodiment of the present invention, the first pressing force is 1.47 kN or more and 3.43 kN or less, the second pressing force is 1.74 kN or more and 5.88 kN or less, and the third pressing force is 0.49 kN or more and 1.47 kN or less. Thereby, since the pressing forces in the respective steps are specifically defined, the effect described above can be surely obtained.

According to another embodiment of the present invention, the rotational tool is rotated at a specified rotational speed within a middle rotational speed through a high rotational speed in the first step, at the middle rotational speed in the second step, and at a specified rotational speed within a low rotational speed through the high rotational speed in the third step. Thereby, the first metal member can surely start to be softened by the frictional heat in the first step, the proper agitation of the softened portion of the metal member can be obtained in the second step, and the proper plastic flow can be maintained for a long period of time regardless of the rotational speed in the third step.

Herein, it is preferable that the rotational tool is rotated at a specified speed that is more than 2000 rpm and 3500 rpm or less in the first step, at a specified speed that is more than 2000 rpm and 3000 rpm or less in the second step, and at a specified speed that is 1500 rpm or more and 3500 rpm or less in the third step.

According to another embodiment of the present invention, the rotational tool comprises a ring-shaped concave that is formed at the shoulder portion around the pin portion. Thereby, since the rotational tool with the ring-shaped concave formed at the shoulder portion around the pin portion is used, the first metal member in the plastic flow state is prevented from flowing out from the portion right below the rotational tool and thereby the pressing force of the rotational tool is concentrated upon the portion right below the rotational tool. As a result, the plastic flow of the first metal member can be promoted.

Herein, it is preferable that the ring-shaped concave formed at the shoulder portion is a recess with a cone shape that has a center thereof aligning with the rotational axis of the rotational tool.

According to another embodiment of the present invention, the first metal member is made of aluminum alloy, the second metal member is made of steel, and both metal members are joined in a solid state at a joining boundary face thereof. Thereby, since the plastic flow is generated in the first metal member with the lower melting point, the energy necessary to join the first and second metal members can be kept smaller and the joining time can be kept shorter.

According to the present invention, there is further provided a frictional spot joining apparatus of metal members, in which a work comprising a first metal member and a second metal member that lap over is provided, a melting point of the second metal member being higher than that of the first metal member, and a rotational tool is pushed against the work from a side of the first metal member and the first metal member is softened and made in a plastic flow state by a frictional heat generated through a rotational and pressing operation of the rotational tool, wherein the rotational tool comprises a shoulder portion to face the work and a pin portion that is located on a rotational axis of the rotational tool and projects toward the work from the shoulder portion, and the frictional spot joining apparatus comprises a rotating device to rotate said rotational tool, a moving device to move and push the rotational tool relative to the work, and a pressing force control device to operate the rotating device and the moving device so that the rotational tool under rotation is pushed with a first pressing force until the first metal member is softened by the rotational tool whose pin portion or both pin portion and shoulder portion come into the first metal member, the rotational tool under rotation is pushed with a second pressing force that is greater than the first pressing force until a tip of the pin portion of the rotational tool reaches a specified point that is at a distance from a joining face of the first and second metal members, and the pressing force of the rotational tool under rotation is maintained at a third pressing force that is smaller than the second pressing force until a specified period of time elapses after the tip of the pin portion has reached the specified point that is at a distance from the joining face.

The above-described invention provides an apparatus that can obtain substantially the same effects as those by the above-described method.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, a preferred embodiment will be described referring to the accompanying drawings.FIG. 1is a schematic elevation view of a frictional spot joining apparatus1according to the present embodiment. The frictional spot joining apparatus1is used for joining of aluminum members or a aluminum member and a steel member of, for example, a body of an automotive vehicle and the like. This apparatus comprises a joining gun10and a robot40having the joining gun10at its hand as a major component. A six-axis multiple-articulated type of robot which has been used widely may be preferably used as the robot40.

As illustrated inFIGS. 2 and 3, the joining gun10comprises an attaching box11attached to the robot40, a L-shaped arm12extending downward from the bottom of the attaching box11, a body case13fixed to the side of the attaching box11above the arm12, a pressing motor14, and a rotating motor15. At the lower end of the body case13is provided a rotational tool16constituting part of a joining tool18. Meanwhile, a receiving member17constituting the other part of:the joining tool18is fixed to the tip of the arm12and located right below the rotational tool16, facing the rotational tool16.

As further illustrated inFIG. 4, in the body case13are provided a screw shaft (going-up-and-down shaft)24and a spline shaft (rotational shaft)25, which extend vertically in parallel and rotate, respectively. The shafts24,25extend upward, penetrating an upper cover member21, and their upper ends are located inside an upper cover22, where driven pulleys26,27are attached to them. Meanwhile, as illustrated inFIG. 5, the upper cover member21projects from the upper of the body case13toward the side of the body case13(seeFIG. 3), and to this projecting portion are fixed the pressing motor14and the rotating motor15. Herein, output shafts14a,15aof the motors14,15extend upward, penetrating the upper cover member21, and their upper ends are located inside the upper cover22, where drive pulleys14b,15bare attached to them. Belts for transmitting a drive power28,29are provided between the drive pulleys14b,15band the driven pulleys26,27, respectively. The screw shaft24is driven by the pressing motor14so as to rotate in a direction of a, b, while the spline shaft25is driven by the rotating motor15so as to rotate in a direction of c.

Returning toFIG. 4, a moving block31engages with a thread portion24aof the screw shaft24, and a rotational cylinder35is connected to a spline portion25aof the spline shaft25via spline connection. The rotational cylinder35is rotatably supported in a moving cylinder33that is integrally coupled to the moving block31via a coupling member32. A cylindrical lower projection13ais formed at the bottom of the body case13. At the lower end of the lower projection13ais provided a lower cover23, and the lower end of the moving cylinder33and the rotational cylinder35project downward beyond the lower cover23. Herein, the inside rotational cylinder35projects downward further below the outside moving cylinder33, and an attaching member36is fixed to the tip of the rotational cylinder35. The rotational tool16is attached to the attaching member36detachably (to be replaceable). Herein, between the lower cover23and the moving cylinder33are provided an expandable bellows34so as to prevent an outer peripheral face of the moving cylinder33from becoming dirty.

In the structure described above, when the screw shaft24is driven by the rotation of the pressing motor14so as to rotate in the direction of a inFIG. 5, the moving member30(including the moving block31, the coupling member32and the moving cylinder33) goes downward via its engagement with thread portion24a, so the rotational cylinder35in the moving cylinder33and the rotational tool16go down together. Meanwhile, when the screw shaft24is driven by the rotation of the pressing motor14so as to rotate in the direction of b inFIG. 5, the moving member30goes upward via its engagement with thread portion24a, so the rotational cylinder35in the moving cylinder33and the rotational tool16go up together. Also, when the spline shaft25is driven by the rotation of the rotating motor15so as to rotate in the direction of c inFIG. 5, the rotational cylinder35rotates in the same direction of c via the spline connection with the spline portion25aregardless of the movement of the moving member30described above, so the rotational tool16coupled to the rotational cylinder35rotates in the same direction of c together. Herein, the rotational axis of the rotational tool16is shown by reference character X inFIGS. 1 through 3.

Herein, a servomotor, which is easy to control and detect the rotational angle, is preferable as the pressing motor14. This servomotor or another type of induction motor that is easy to control the rotational speed is preferable as the rotating motor15.

FIG. 6illustrates the tip of the rotational tool16. The rotational tool16is especially designed so as to be suitable to the different kinds of metal members (aluminum and steel, for example), in which the lower end face (circular shape) of the cylindrical body16ais constituted as a shoulder portion16bto press the metal member. Herein, the shoulder portion16bis formed to be not flat, but slant with a specified angle (φ), so that it comprises a recess with a cone shape that has its center aligning with the rotational axis X (one example of a ring-shaped concave in the claim: this may be also formed of a recess with a curved slant surface toward the rotational axis X). A cylindrical pin portion16cis formed at the center of the shoulder portion16b, projecting from the lower end, i.e., the peripheral edge, of the shoulder portion16bwith a specified length (h). Specifically, the diameter of the shoulder portion16bis 10 mm, the diameter of the pin portion16cis 2 mm, the slant angle (φ) of the shoulder portion16bis 5°-7°, and the projecting length (h) of the pin portion16cis 0.35 mm or 0.3 mm, for example.

As illustrated inFIG. 1, the robot40is coupled to a control box50via a harness51. The joining gun10is coupled to the control box50via harnesses52,54,55and a junction box53. The operations of the pressing motor14and the rotating motor15are controlled by a control unit, not illustrated, in the control box50.

And, as illustrated inFIG. 7, in the present embodiment, a first metal member W1(aluminum plate, for example) with a relatively low melting point is located as the upper plate, and a second metal member W2(steel plate, for example) with a relatively high melting point is located as the lower plate. These plates W1, W2are placed so as to lap over, and constitute a work, which is fixed with a proper fixing means, not illustrated. Next, the robot40moves the joining gun10toward the work, and stops the gun's moving. Herein, the rotational tool16is positioned above the work, and the receiving member17is located below the work. At first, the joining gun10is moved upward, and the receiving member17contacts the lower face of the second metal member W2. Then, the rotational toll16under rotation is lowered toward the work, and then pressed against the first metal member W1. Thereby, the first metal member W1is softened by the frictional heat that is generated by the rotational and pressing operations of the rotational tool16to generate the plastic flow. Thus, the both metal members W1, W2are joined in the solid state.

In this joining, as described above, the first metal member W1is softened by the frictional heat generated by the rotational and pressing operations of the rotational tool16to generate the plastic flow, and thereby the first metal member W1with the relatively low melting point and the second metal member W2with the relatively high melting point are joined. Accordingly, the plastic flow is generated at the first metal member W1with the relatively low melting point, thus the energy necessary to join these metal members W1, W2can be kept smaller and the joining period of time can be kept shorter.

After the joining at one portion P has been done, the rotational tool.16is moved up and then the joining gun10is lowered. Subsequently, the joining gun10is moved laterally by a specified distance. Then, again the joining gun10is moved upward, and the rotational tool16is moved downward for the joining as described above. Thus, the frictional joining of the metal members W1, W2is executed at plural portions P . . . P.

More specifically, in the first step pressing illustrated inFIG. 8(“the first step” in the claims) in which only the lowered tip of the pin portion16cof the rotational tool16contacts the first metal member W1(corresponding to the period of time t1-t2inFIG. 12that will be described below), the frictional heat H is generated at the joining portion and diffused outward. The first metal member W1and the zinc plating layer Z on the surface of the second metal member W2, which has been coated to protect it from oxidation, start to be softened by the above-described frictional heat H at the joining portion.

Herein, the rotational tool16is pressed against the first metal member W1with the first pressing force that is relatively small, so the proper positioning of rotation by the pin portion16ccan be obtained. Both the tip of the pin portion16cand a peripheral edge portion of the shoulder portion16bmay be contacted against the first metal member W1to generate the frictional heat H in the above-described first step.

Subsequently, in the second step pressing illustrated inFIG. 9(“the second step” in the claims) in which the rotational tool16is further lowered and the tip of the shoulder portion16bcomes into the first metal member W1(corresponding to the period of time t2-t3inFIG. 12), the frictional heat H is generated more because of the rotation and pressing of the wide shoulder portion16bin addition to the pin portion16c. Accordingly, the first metal member W1is further softened enough to generate the plastic flow (A). Herein, since the shoulder portion16bof the rotational tool16comprises the recess with the cone shape that has its center aligning with the rotational axis X, the first metal member W1in the plastic flow state is prevented properly from flowing out from the portion right below the rotational tool16. As a result, the pressing force of the rotational tool16is concentrated upon the portion right below the rotational tool16, thus the plastic flow of the first metal member W1can be promoted.

At this stage, the rotational tool16is pushed against the first metal member W1with the second pressing force that is larger than the above-described first pressing force. Thus, after the first metal member W1has been softened to some degree by the frictional heat generated by the first pressing force, the rotational tool16is further pushed with the higher second pressing force. As a result, the plastic flow can be surely generated at the first metal member W1. And, the softened zinc plating layer Z is pushed out from the joining portion P by the pressing of the rotational tool16and the plastic flow of the first metal member W1, so a new uncovered surface of the second metal member W2is exposed at the joining boundary face of the metal members W1, W2(the upper surface of the second metal member W2). And, the oxidation film, not illustrated, that has been formed at the surface of the first metal member W1by oxygen in the air is destroyed at the joining portion P, so a new uncovered surface of the first metal member W1is exposed at the joining boundary face of the metal members W1, W2(the lower surface of the first metal member W1).

Next, in the third step pressing illustrated inFIG. 10(“the third step” in the claims) in which the rotational tool16is further lowered and the shoulder portion16bcomes into the first metal member W1deeply (corresponding to the period of time t3-t4inFIG. 12), the metal material pushed out by the rotational tool16rises as projections B. And, the zinc plating layer Z is further pushed out from the joining portion P, and the oxidation film is further destroyed. Thus, the new uncovered surfaces of the first and second metal members W1, W2are exposed widely (to an extent X in the figure). As a result, the joining strength of frictional spot joining (joining in the solid state) of the metal members W1, W2can be improved.

Herein, the long term pressing with the second pressing force makes the first metal member W1be softened too much would cause an improper situation in which the rotational tool16comes into the first metal member W1too deeply, so that the thickness of part of the first metal member W1existing between the rotational tool16and the second metal member W2becomes too thin. Eventually, there would occur a situation in which the first metal member W1is torn off. As a result, there would occur a problem that the contacting area between the first and second metal members W1, W2reduces and thereby the joining-strength decreases. Also, the pin portion16cof the rotational tool16would penetrate the upper first metal member W1and hit against the lower second metal member W2. As a result, a hole would be formed by the pin portion16cat the first metal member W1, which may cause a problem of galvanic corrosion due to difference in electric potential between the aluminum and steel at this portion.

Accordingly, the third pressing force that is smaller than the second pressing force is used in the third step to prevent the rotational tool16from coming into the first metal member W1too deeply. Thus, the plastic flow is generated at the first metal member W1while the temperature of the first metal member W1is maintained at the temperature in the second step pressing. Thereby, the pin portion16cis prevented from hitting against the second metal member W2, so the problems such as the torn-off situation and the galvanic corrosion are avoided.

Herein, a metal compound based on the metal of the first metal member W1and the metal of the zinc plating layer Z is formed at a portion of the zinc plating layer Z (Y).

As illustrated in FIG11, when the rotational tool16has been moved up after the joining, spot marks by the shoulder portion16band the pin portion16cremain on the surface of the work at the joining portion P, which is enclosed by the projections B.FIG. 12shows an exemplified change of the rotational speed and pressing force of the rotational tool16in the first, second and third steps.

As illustrated inFIG. 13, the temperature change was measured for the work comprising the first metal member W1as the upper plate and the second metal member W2as the lower plate, at measuring points of the center and the end portions of the joining portion P, with two thermocouples T1, T2that are inserted from the second metal member W2and placed at portions 0.5 mm away from the zinc plating layer Z at the above-described measuring points, for two cases, in which the third step pressing was done and the third step pressing was not done. Herein, a 6000-based aluminum alloy (with copper, 1.4 mm thick) was used as the upper plate W1, and a zinc plating steel plate (1.0 mm thick) was used as the lower plate W2. Joining conditions are as follows: the pressing force of 1.47 kN, the rotational speed of 3500 rpm, the pressing period of time of 1.0 sec for the first step pressing (for pressing in the first step); the pressing force of 4.90 kN, the rotational speed of 1500 rpm, the pressing period of time of 1.54 sec for the second step pressing (for pressing in the second step); and the pressing force of 0.98 kN, the rotational speed of 2500 rpm, the pressing period of time of 2.4 sec for the third step pressing (for pressing in the third step). The results are shown in graphs ofFIGS. 14 and 15.

As illustrated inFIG. 14, it was found that in the case where the third step pressing was not done, the temperature at the center and the end portions of the joining portion P in the second step pressing increased to a temperature that is high enough to soften the first metal member W1to generate the plastic flow, and then decreased after the second step pressing.

Meanwhile, as illustrated inFIG. 15, it was found that in the case where the third step pressing was done, the temperature at the joining portion P in the second step pressing increased to a temperature that is high enough to soften the first metal member W1to generate the plastic flow, and then was maintained for a certain period of time and subsequently decreased.

As a result, conducting the third step pressing after the second step pressing can provide the long period of time of joining of the metal members W1, W2with the proper plastic flow of the first metal member W1, and avoid the torn-off situation by preventing the rotational tool16from coming into too deeply and penetrating the first metal member W1, thereby ensuring the joining strength.

Next, a joining-strength testing was conducted for the work made by this frictional joining method, in which the different kinds of metal members of the first metal member (aluminum plate) W1and the second metal member (steel plate) W2are joined, with the third step pressing period of times of 0.0 sec, 0.5 sec, and 1.5 sec. First, a cross-draw testing illustrated inFIG. 16was used for the joining-strength testing. Herein, the first and second metal members W1, W2are overlapped with a cross shape and cramped, and then the frictional joining was applied to the joining portion P at the center of a cross portion from the first metal member W1. Subsequently, the first metal member W1is drawn in an upward direction M1and the second metal member W2is drawn in a downward direction M2. Thus, the draw force (cross tension strength) was measured the metal members W1, W2are separated by drawing them that way. Conditions for joining are shown inFIG. 17, and measurement results of the cross tension strength are shown inFIG. 18. The results ofFIG. 18shows the average measurement values of the cross tension strength that were obtained through three times of measurements in the above-described cross-draw testing, in which the 6000-based aluminum alloy plate (1.4 mm thick) was used for the first metal member W1and the zinc plating steel plate (1.0 mm thick) was used for the second metal member W2.

The measurement results of the cross tension strength inFIG. 18shows the cross tension strength in the case where there was the longer pressing period of time in the third step was greater than that in the case where there was the shorter pressing period of time in the third step, but the necessary joining strength was maintained in any cases. Also, the standard deviation of cross tension strength σ, which is shown along the axis of ordinate of the graph, shows that the longer the pressing period of time in the third step was, the smaller the value σ was, which means that the deviation of cross tension strength, namely the joining strength, became smaller, so the constant quality level was maintained.

Since the third step pressing can maintain the heating of the upper plate W1for the specified period of time after the second step pressing, the joining with the sufficient plastic flow at the upper plate W1can be done. Also, the joining strength can be prevented from decreasing due to the attachment of the upper plate W1to the rotational tool16(herein, the longer pressing period of time in the third step would be better).

Next, the joining-strength testing was conducted for the work made by this frictional joining method, in which the different kinds of metal members of the first metal member (aluminum plate) W1and the second metal member (steel plate) W2are joined, with the third step pressing period of times of 0.0 sec, 0.5 sec, and 1.5 sec.

In this joining-strength testing, the draw-shear testing as illustrated inFIG. 19was conducted. Herein, the first and second metal members W1, W2are overlapped partially and cramped, and then the frictional joining was applied to the joining portion P from the first metal member W1. Subsequently, the first metal member W1was drawn in a direction of arrow N1and the second metal member W2was drawn in a direction of arrow N2. Thus, the draw force (shearing force) was measured when the metal members W1, W2are separated by drawing them that way. The measurement results are shown inFIG. 20. The results ofFIG. 20shows the average measurement values of the shear strength that were obtained through three times of measurements in the above-described draw-shear testing, in which, like the above-described cross-draw testing, the 6000-based aluminum alloy plate (1.4 mm thick) was used for the first metal member W1and the zinc plating steel plate (1.0 mm thick) was used for the second metal member W2. The joining conditions shown inFIG. 17were used like the above-described cross-draw testing.

According to the measurement results of the shear-strength, as shown inFIG. 20, the shear-strength in the case where the pressing period of time in the third step was 0.5 sec or 1.5 sec was greater than that in the case where the pressing period of time in the third step was 0 sec, namely in the case where no pressing was conducted in the third step, so the joining strength was maintained.

Since the third step pressing with the pressing period of time of 0.5 sec or 1.5 sec can maintain the heating of the upper plate W1for the specified period of time after the second step pressing, the joining with the sufficient plastic flow at the upper plate W1can be done. Thus, the joining strength can be prevented from decreasing due to the attachment of the upper plate W1to the rotational tool16.

Next, for four samples of the work comprised of the first metal member (aluminum alloy) W1and the second metal member (steel plate) W2, the shear strength, i.e., the joining strength, of the first and second metal members W1, W2was measured by the above-described draw-shear testing, in which the pressing force, the rotational speed and the pressing period of time of the rotational tool16in the first, second and third steps were changed within respective specified ranges. The joining conditions of the draw-shear testing are shown inFIG. 21, and the measurement results of the shear strength are shown inFIG. 22. Herein, the following samples were used: a 6000-based aluminum alloy A (with copper, 0.7 mm thick) of the upper plate W1and a zinc plating steel plate (1.2 mm thick) of the lower plate W2; a 6000-based aluminum alloy A (with copper, 0.8 mm thick) of the upper plate W1and a zinc plating steel plate (1.2 mm thick) of the lower plate W2; a 6000-based aluminum alloy B (without copper, 1.4 mm thick) of the upper plate W1and a zinc plating steel plate (1.4 mm thick) of the lower plate W2; a 6000-based aluminum alloy A (with copper, 1.4 mm thick) of the upper plate W1and a zinc plating steel plate (1.0 mm thick) of the lower plate W2. For these four samples were conducted the shear-strength tests1through25.

As apparent from the measurement results inFIG. 22, in the cases with NG (not good) indicated in measurement result columns where the pressing force in the second step was too large regardless of the relatively small thickness of the upper plate W1(tests4,8and9), the tip of the pin portion16ccontacted the lower plate W2, penetrating through the upper plate W1, because the upper plate W1had been softened too much, so the torn-off situation of the aluminum of the upper plate W1attached was caused. Also, in the case where the pressing force in the first step was too small (test10), the softening by the tip of the pin portion16cwas not so enough that the rotational tool16could not be pressed properly against the upper plate W1in the subsequent step and thus the shear strength did not become sufficient.

Meanwhile, judging from the cases where the measurement results were OK (good), namely, the shear strength was beyond the standard strength (from the B-grade data of JIS), the preferable ranges of the pressing forces were such that the pressing force in the first step was 1.47 kN or more and 3.43 kN or less, the pressing force in the second step was 1.74 kN or more and 5.88 kN or less, and the pressing force in the third step was 0.49 kN or more and 1.47 kN or less. Also, the preferable ranges of the joining period of time were such that the joining period of time in the first step was 1.0 sec-2.5 sec, the joining period of time in the second step was 1.0 sec-1.5 sec, and the joining period of time in the third step was 0.5 sec-2.5 sec.

FIG. 23is a graph showing relationships between the pressing force in the third step and the shear strength. It is apparent from this graph that the third step pressing force of 0.49 kN or more and 1.47 kN or less showed the proper shear strength, i.e., the proper joining strength.

Herein, in the case where the pressing force in the third step was less than 0.49 kN, the pressing force was so small that the sufficient plastic flow at the upper plate W1was not generated by the frictional heat and thereby the shear strength was not sufficient. Accordingly, in the case where the pressing force in the third step was more than 1.47 kN, the possibility that the tip of the pin portion16cof the upper plate W1contacts the lower plate W2because the upper plate W1had been softened too much would increase, so that the possibility that the torn-off situation of the aluminum attached to the rotational tool16would occur increases.

According to the results described above, the setting of the pressing force in the third step within the proper range described above can surely prevent the shear strength, i.e., the joining strength, from reducing improperly due to the torn-off situation of the upper plate W1.

Next, the draw-shear testing (tests51,52and53) for the 6000-based aluminum alloy (1.4 mm thick) of the upper plate W1and the zinc plating steel plate (1.0 mm thick) of the lower plate W2was conducted under the joining conditions that the pressing forces and pressing period of time for the pressing in the first through third steps were set at constant values respectively, while the rotational speeds in the first through third steps were changed respectively within the rage of 1500 rpm-3500 rpm.FIG. 24shows the joining conditions.

FIG. 25is a graph of measurement results of the draw-shear testing (test51) of joining in which the rotational speeds in the first step were changed within the specified range,FIG. 26is a graph of measurement results of the draw-shear testing (test52) of joining in which the rotational speeds in the second step were changed within the specified range, andFIG. 27is a graph of measurement results of the draw-shear testing (test53) of joining in which the rotational speeds in the third step were changed within the specified range.

It is apparent from the results ofFIG. 25(test51) that the rotational speed of more than 2000 rpm and 3500 rpm or less (speed range enclosed by an oval in the figure) in the first step pressing showed the proper draw-shear strength. The reason for this seems that the upper plate W1was softened to a certain degree by the relative movement of the tip of the pin portion16cand then the plastic flow at the upper plate W1was surely generated by the pressing in the second and third steps, so that the proper shear strength could be obtained. When the rotational speed was 2000 rpm or less, however, the upper plate W1was not softened sufficiently by the rotational movement of the tip of the pin portion16cand thereby the rotational tool16was not pressed enough against the upper plate W1by the pressing in the second and third steps, so that the proper shear strength could not be obtained.

It is apparent from the results ofFIG. 26(test52) that the rotational speed of more than 2000 rpm and 3000 rpm or less (speed range enclosed by an oval in the figure) in the second step pressing showed the proper draw-shear strength. When the rotational speed was 2000 rpm or less, the frictional force between the upper plate W1and the rotational tool16increased greatly and thereby the rotational torque was increased, so that the energy consumption of the joining apparatus becomes large. When the rotational speed was more than 3000 rpm and 3500 rpm or less, the frictional force between the upper plate W1and the rotational tool16was relatively low, and thereby the upper plate W1was not be softened properly by the frictional heat and the plastic flow was not generated sufficiently, so that the proper shear strength could not be obtained.

It is apparent from the results ofFIG. 27(test53) that the rotational speed of 1500 rpm or more and 3500 rpm or less in the third step pressing showed the proper draw-shear strength. When the rotational speed was less than 1500 rpm, the frictional force between the upper plate W1and the rotational tool16increased greatly and thereby the rotational torque was increased, so that the energy consumption of the joining apparatus becomes large.

Accordingly, it is apparent that the preferable ranges of the rotational speed were such that the rotational speed for the first step pressing was more than 2000 rpm and 3500 rpm or less, the rotational speed for the second step pressing was more than 2000 rpm and 3000 rpm or less, and the rotational speed for the third step pressing was 1500 rpm or more and 3500 rpm or less.

Next, it was tested how the rotational speed, the pressing force and the kind of material of the first metal member W1in the third step pressing affect the temperature of the first and second metal members W1, W2. The same measurement method illustrated inFIG. 13was used. Namely, there was provided the work W comprised of the first metal member W1as the upper plate and the second metal member W2as the lower plate that lap over, and two thermocouples were inserted from the second metal member W2and placed at the center of the joining portion P of the second metal member W2to measure the changing of the temperature. The joining conditions are as follows: the pressing force of 1.47 kN, the rotational speed of 3500 rpm, the pressing period of time of 1.0 sec for the first step pressing; and the pressing force of 3.92 kN, the rotational speed of 2500 rpm, the pressing period of time of 1.54 sec for the second step pressing, where the joining conditions of the first and second step pressings were not changed. Meanwhile, the joining conditions for the third step pressing were the pressing force of 0.98 kN, the rotational speed of 2500 rpm, and the pressing period of time of 2.4 sec, where the joining conditions of the third step pressing were changed at need. Herein, the 6000-based aluminum alloy (1.4 mm thick) of the upper plate W1and the zinc plating steel plate (1.0 mm thick) of the lower plate W2were used to test the influence by the rotational speed and the pressing force in the third step.

FIGS. 28,29and30are graphs respectively showing relationships between the rotational speed, the pressing force, the kind of material of the upper plate W1and the temperature of the first and second metal members W1, W2at the pressing in the third step.

It was found, as apparent fromFIG. 28, that there was a tendency that the greater the rotational speed of the pressing in the third step was, the higher the temperature of the first and second metal members W1, W2(temperature at the joining portion P) was. It was also found that the temperature did not increase as the penetration of the rotational tool16occurred (at3500rpm), and that the temperature did not decrease as the proper plastic flow at the first metal member W1was prevented (at 1500 rpm).

It was found, as apparent fromFIG. 29, that there was a tendency that the greater the pressing force of the pressing in the third step was, the higher the temperature of the first and second metal members W1, W2(temperature at the joining portion P) was. It was also found that the temperature did not increase as the penetration of the rotational tool16occurred (at 1.47 kN), and that the temperature did not decrease as the proper plastic flow at the first metal member W1was prevented (at 0.49 kN). Also, it was found fromFIG. 30that there was no big difference in the temperature of the first and second metal members W1, W2(temperature at the joining portion P) between the case where a 6000-based aluminum alloy C (without copper, 1.4 mm thick) of the upper plate W1was used and the case where a 6000-based aluminum alloy D (with copper, 1.4 mm thick) of the upper plate W1was used.

The above-described embodiment just discloses the preferred embodiment, but any other modifications and improvements can be applied within the scope of a spirit of the present invention. For example, although the single pressing force was applied to the rotational tool in each step in the above-described embodiment, plural pressing forces may be applied stepwise or the pressing force may be changed within the preferable range.