JP7359245B2 - Rotating tools and joining methods - Google Patents

Description

The present invention relates to a rotary tool for friction stir and a welding method.

2. Description of the Related Art As a rotary tool used for friction stir welding, one that includes a shoulder portion and a stirring pin that hangs down from the shoulder portion is known. The rotary tool performs friction stir welding with the lower end surface of the shoulder portion pressed into a metal member. By pushing the shoulder portion into the metal member, it is possible to press down the plastic flow material and suppress the occurrence of burrs. However, if the height position of the bond changes, defects are likely to occur, and there are problems in that the groove becomes larger and more burrs occur.

On the other hand, there is a friction stir welding method in which two metal members are joined using a rotating tool equipped with a stirring pin, in which a rotated stirring pin is inserted into the abutting part of the metal members, and only the stirring pin contacts the metal member. A friction stir welding method is known that includes a main welding step in which friction stir welding is performed in a state where the welding is performed (Patent Document 1). According to the related art, a spiral groove is carved on the outer circumferential surface of the stirring pin, and friction stir welding is performed with only the stirring pin in contact with the workpiece and the base end exposed. Even if the height position of the weld changes, the occurrence of defects can be suppressed, and the load on the friction stirrer can also be reduced. However, since the plastic flow material cannot be pressed by the shoulder portion, there are problems in that the grooves on the surface of the metal member become larger and the bonding surface roughness becomes larger. Another problem is that a bulge (a portion where the surface of the metal member swells compared to before joining) is formed beside the groove.

On the other hand, Patent Document 2 describes a rotary tool that includes a shoulder portion and a stirring pin that hangs down from the shoulder portion. A tapered surface is formed on the shoulder portion and the outer circumferential surface of the stirring pin, respectively. A spiral groove in a plan view is formed on the tapered surface of the shoulder portion. The cross-sectional shape of the groove is semicircular. By providing a tapered surface, stable joining can be achieved even if the thickness of the metal members or the height position of joining changes. Furthermore, by entering the plastic flow material into the groove, the flow of the plastic flow material can be controlled to form a suitable plasticized region.

JP2013-39613A Patent No. 4210148

However, the prior art disclosed in Patent Document 2 has a problem in that the plastic flow material enters the grooves of the tapered surface, causing the grooves to malfunction. Further, when the plastic fluid material enters the groove, the plastic fluid material adheres to the groove and is friction stirred, so there is a problem in that the metal members to be welded and the deposits rub against each other, resulting in a decrease in bonding quality. Further, there are problems in that the surface of the metal member to be welded becomes rough and there are many burrs, and the grooves on the surface of the metal member also become large.

From this point of view, an object of the present invention is to provide a rotary tool and a joining method that can reduce the grooves on the surface of a metal member and reduce the roughness of the joining surface.

In order to solve such problems, the present invention provides a rotary tool for friction stirring comprising a proximal pin and a distal pin, wherein the taper angle of the proximal pin is equal to the taper angle of the distal pin. The taper angle of the proximal pin is 135 to 160°, and the outer peripheral surface of the proximal pin has a spiral shape when viewed from above and a stepped shape when viewed from the side. , a spiral step portion is formed, the angle between the bottom surface of the step portion and the side surface of the step is 85 to 120 degrees, and a spiral groove is carved on the outer peripheral surface of the tip side pin. The spiral groove is characterized in that it is composed of a spiral bottom surface and a spiral side surface, and the spiral angle formed by the spiral bottom surface and the spiral side surface is 45° to 90°.

The present invention also provides a welding method for friction stir welding a butt portion formed by abutting the end surfaces of a pair of metal members using a rotating tool, wherein the rotating tool connects a proximal pin and a distal pin. The taper angle of the proximal pin is larger than the taper angle of the distal pin, the taper angle of the proximal pin is 135 to 160°, and the outer circumference of the proximal pin is greater than the taper angle of the proximal pin. A spiral stepped portion is formed on the surface, which is spiral- shaped when viewed from above and stepped when viewed from the side, and the angle formed by the bottom surface of the stepped portion and the side surface of the stepped portion is 85 to 120°. A spiral groove is carved on the outer circumferential surface of the tip side pin, and the spiral groove is composed of a spiral bottom surface and a spiral side surface, and the spiral groove is composed of the spiral bottom surface and the spiral side surface. The helical angle is 45° to 90°, and while the outer peripheral surface of the proximal pin is in contact with the surface of the metal member, friction stirring is performed while pressing the plastic flow material with the bottom surface of the stepped portion. It is characterized by doing the following.

According to this joining method, the metal member can be held down by the outer circumferential surface of the proximal pin with a large taper angle, so the groove on the joining surface can be made smaller and the bulge formed on the side of the groove can be reduced. The protrusion can be eliminated or made smaller. Since the stepped portion is shallow and the exit is wide, the plastic flow material is difficult to adhere to the outer circumferential surface of the proximal pin even if the metal member is pressed by the proximal pin. Therefore, it is possible to reduce the bonding surface roughness and to suitably stabilize the bonding quality. Furthermore, by providing the tip end pin, it can be easily inserted to a deep position.

Further, it is preferable that the angle between the bottom surface of the step and the side surface of the step is an acute angle of 85° or more, or an obtuse angle of 120° or less. Further, it is preferable that the taper angle of the proximal pin is 135 to 160°. Further, it is preferable that the height of the stepped side surface of the stepped portion is 0.1 to 0.4 mm. Further, it is preferable that the height of the stepped side surface of the stepped portion is 0.1 to 0.25 mm . Further, a spiral groove is carved on the outer peripheral surface of the tip side pin, and the spiral groove is composed of a spiral bottom surface and a spiral side surface, and the spiral groove is composed of the spiral bottom surface and the spiral side surface. It is preferred that the helical angle is between 45° and 90°. Moreover, it is preferable that the step bottom surface is parallel to a horizontal surface. Further, it is preferable that the step bottom surface is inclined upward by 15 degrees or less or downward by -5 degrees or more with respect to a horizontal plane from the rotation axis of the tool toward the outer circumference. According to this joining method, the grooves on the surface of the metal member can be made smaller, and the roughness of the joining surface can be made smaller.

According to the bonding method according to the present invention, it is possible to reduce the grooves on the surface of the metal member and to reduce the roughness of the bonding surface.

FIG. 1 is a side view showing a rotary tool according to an embodiment of the present invention. FIG. 3 is an enlarged cross-sectional view of the rotary tool. 1 is a perspective view showing a joining method according to an embodiment of the present invention. 1 is a cross-sectional view showing a joining method according to an embodiment of the present invention. FIG. 2 is a conceptual diagram showing a conventional shoulderless rotary tool. FIG. 2 is a conceptual diagram showing a conventional rotary tool. It is a sectional view showing a first modification of a rotary tool. It is a sectional view showing a second modification of the rotary tool. It is a sectional view showing a third modification of a rotary tool. 1 is a table showing conditions of Example 1. 3 is a graph showing the results of Example 1. 3 is a table showing conditions of Example 2. FIG. 7 is a side view showing a rotary tool of a comparative example of Example 2. 3 is a graph showing the results of Example 2. 3 is a table showing conditions of Example 3. It is a graph showing the results of Example 3-1. 3 is a graph showing the results of Example 3-2. It is a graph showing the results of Example 3-3.

Embodiments of the present invention will be described with reference to the drawings as appropriate. As shown in FIG. 1, a rotary tool 1 is a tool used for friction stir welding. The rotary tool 1 is made of tool steel, for example. The rotary tool 1 mainly includes a base shaft portion 2, a proximal pin 3, and a distal pin 4. The base shaft portion 2 has a cylindrical shape and is a portion connected to the main shaft of the friction stirrer.

The proximal pin 3 is continuous with the base shaft portion 2 and tapers toward the distal end. The proximal pin 3 has a truncated cone shape. The taper angle A of the proximal pin 3 may be set as appropriate, and is, for example, 135 to 160°. When the taper angle A is less than 135° or more than 160°, the bonded surface roughness after friction stirring becomes large. The taper angle A is larger than the taper angle B of the distal end pin 4, which will be described later. As shown in FIG. 2, a stepped portion 10 is formed on the outer circumferential surface of the proximal pin 3 over the entire height thereof. The stepped portion 10 is spirally formed clockwise or counterclockwise. In other words, the stepped portion 10 has a spiral shape when viewed from above and has a step-like shape when viewed from the side. In this embodiment, in order to rotate the rotary tool clockwise, the stepped portion 10 is set counterclockwise from the base end side to the distal end side.

In addition, when rotating the rotary tool counterclockwise, it is preferable to set the stepped portion 10 clockwise from the base end side to the distal end side. As a result, the plastic fluid material is guided to the tip side by the stepped portion 10, so that it is possible to reduce metal overflowing to the outside of the metal members to be welded. The step portion 10 includes a step bottom surface 10a and a step side surface 10b. The distance X1 (horizontal distance) between the vertices 10c, 10c of the adjacent step portions 10 is appropriately set according to the step angle C and the height Y1 of the step side surface 10b, which will be described later.

The height Y1 of the stepped side surface 10b may be set as appropriate, and is set, for example, to 0.1 to 0.4 mm. When the height Y1 is less than 0.1 mm, the bonding surface roughness becomes large. On the other hand, if the height Y1 exceeds 0.4 mm, the bonding surface roughness tends to increase, and the number of effective step portions (the number of step portions 10 in contact with the metal members to be welded) also decreases.

The step angle C between the step bottom surface 10a and the step side surface 10b may be set as appropriate, and is set, for example, to 85 to 120 degrees. In this embodiment, the stepped bottom surface 10a is parallel to the horizontal surface. The step bottom surface 10a may be inclined within a range of -5° to 15° with respect to the horizontal plane from the rotation axis of the tool toward the outer circumference (minus indicates downward with respect to the horizontal plane, positive indicates downward with respect to the horizontal plane). above). The distance X1, the height Y1 of the step side surface 10b, the step angle C, and the angle of the step bottom surface 10a with respect to the horizontal plane are such that the plastic fluid material does not stay inside the step portion 10 and adhere to the outside when performing friction stirring. It is appropriately set so that the step bottom surface 10a can suppress the plastic flow material and reduce the joint surface roughness.

The distal end pin 4 is formed continuously from the proximal end pin 3. The tip end pin 4 has a truncated cone shape. The tip of the tip side pin 4 is a flat surface. The taper angle B of the distal pin 4 is smaller than the taper angle of the proximal pin 3. A spiral groove 11 is carved on the outer circumferential surface of the tip end pin 4. The spiral groove 11 may be clockwise or counterclockwise, but in this embodiment, in order to rotate the rotary tool 1 clockwise, it is carved counterclockwise from the base end toward the distal end.

In addition, when rotating the rotary tool counterclockwise, it is preferable to set the spiral groove 11 clockwise from the base end side to the distal end side. As a result, the plastic fluid material is guided to the tip side by the spiral groove 11, so that it is possible to reduce the amount of metal overflowing to the outside of the metal members to be welded. The spiral groove 11 includes a spiral bottom surface 11a and a spiral side surface 11b. The distance (horizontal distance) between the vertices 11c and 11c of adjacent spiral grooves 11 is defined as length X2. The height of the spiral side surface 11b is defined as a height Y2. The spiral angle D formed by the spiral bottom surface 11a and the spiral side surface 11b is, for example, 45 to 90 degrees. The spiral groove 11 has the role of increasing frictional heat by coming into contact with the metal member to be welded, and also has the role of guiding the plastic fluid material to the tip side.

Next, a joining method according to the present invention will be explained. In the joining method according to this embodiment, a butting process and a friction stirring process are performed. The butting process is a process of butting the respective end surfaces 20a, 20a of the metal members 20, 20, as shown in FIG. Each front surface and each back surface of the metal members 20, 20 are flush with each other.

The friction stir process is a process of friction stir welding the butt portion J1 using the rotary tool 1. In the friction stirring process, the rotary tool 1 rotated clockwise is inserted into the abutting portion J1, and is relatively moved so as to trace the abutting portion J1. A plasticized region W is formed on the movement locus of the rotary tool 1. As shown in FIG. 4, in the friction stir process, friction stir welding is performed while pressing the surfaces 20b, 20b of the metal members 20, 20 with the outer peripheral surface of the proximal pin 3 of the rotary tool 1. The insertion depth of the rotary tool 1 is set such that at least a portion of the proximal pin 3 comes into contact with the surface 20b of the metal member 20. In this embodiment, the insertion depth is set so that the center portion of the outer peripheral surface of the proximal pin 3 in the height direction comes into contact with the surface 20b of the metal member 20.

Here, as shown in FIG. 5, in the conventional shoulderless rotary tool 100, since the shoulder part cannot press the surface of the metal member 110 to be welded, the concave groove (the surface of the metal member to be welded and the surface of the plasticized region) There is a problem in that as the concave grooves (concave grooves) become larger, the bonding surface roughness increases. Another problem is that a bulge (a portion where the surface of the metal member to be welded swells compared to before joining) is formed beside the groove. On the other hand, when the taper angle β of the rotary tool 101 is made larger than the taper angle α of the shoulderless rotary tool 100, as in the rotary tool 101 shown in FIG. Since it can be pressed down, the groove becomes smaller and the bulge also becomes smaller. However, since the downward plastic flow becomes stronger, kissing bonds are more likely to be formed at the bottom of the plasticized region.

On the other hand, the rotary tool 1 of this embodiment includes a proximal pin 3 and a distal pin 4 having a smaller taper angle than the taper angle A of the proximal pin 3. This makes it easier to insert the rotary tool 1 into the metal members 20, 20. Further, since the taper angle B of the tip end pin 4 is small, the rotary tool 1 can be easily inserted to a deep position in the metal members 20, 20. Further, since the taper angle B of the tip end pin 4 is small, downward plastic flow can be suppressed compared to the rotary tool 101. Therefore, it is possible to prevent kissing bonds from being formed in the lower part of the plasticized region W. On the other hand, since the taper angle A of the proximal pin 3 is large, compared to conventional rotary tools, stable welding can be achieved even if the thickness of the metal members to be welded or the height position of the weld changes.

In addition, since the plastic flow material can be held down by the outer circumferential surface of the proximal pin 3, the groove formed on the joint surface can be made smaller, and the bulge formed on the side of the groove can be eliminated. It can be made smaller or smaller. Furthermore, since the step-like step portion 10 is shallow and has a wide outlet, the plastic flow material can easily escape to the outside of the step portion 10 while being held down by the step bottom surface 10a. Therefore, even if the plastic flow material is held down by the base pin 3, it is difficult for the plastic flow material to adhere to the outer circumferential surface of the base pin 3. Therefore, it is possible to reduce the bonding surface roughness and to suitably stabilize the bonding quality.

The design of the rotary tool 1 of the present invention can be changed as appropriate. FIG. 7 is a side view showing a first modification of the rotary tool of the present invention. As shown in FIG. 7, in the rotary tool 1A according to the first modification, the step angle C between the step bottom surface 10a and the step side surface 10b of the step portion 10 is 85 degrees. The step bottom surface 10a is parallel to the horizontal surface. In this way, the step bottom surface 10a is parallel to the horizontal plane, and the step angle C may be an acute angle within a range where the plastic fluid material stays in the step portion 10 during friction stirring and escapes to the outside without adhering.

FIG. 8 is a side view showing a second modification of the rotary tool of the present invention. As shown in FIG. 8, in the rotary tool 1B according to the second modification, the step angle C of the step portion 10 is 115°. The step bottom surface 10a is parallel to the horizontal surface. In this way, the step bottom surface 10a may be parallel to the horizontal plane, and the step angle C may be an obtuse angle within the range that functions as the step portion 10.

FIG. 9 is a side view showing a third modification of the rotary tool of the present invention. As shown in FIG. 9, the step bottom surface 10a is inclined upward by 10 degrees with respect to the horizontal plane from the rotation axis of the tool toward the outer circumference. The stepped side surface 10b is parallel to the vertical plane. In this manner, the step bottom surface 10a may be formed to be inclined upward from the horizontal plane toward the outer circumferential direction from the rotation axis of the tool within a range that can suppress the plastic flow material during friction stirring. The first to third modified examples of the rotary tool described above can also achieve the same effects as the present embodiment.

Next, examples of the present invention will be described. In Examples, three types of tests were conducted in Examples 1, 2, and 3, and the bonded surface roughness after the friction stirring process was measured.

[Example 1]
In Example 1, the rotary tool 1 is inserted into the surface of a single metal member (aluminum alloy: A5052-H34), and moved relative to the surface by a predetermined distance to measure the surface roughness along the plasticized region generated after friction stirring. Rz (μm) was measured with a surface roughness meter (main unit: Surfcom 1400D, controller: KA9801CF). The measurement conditions were as follows: measurement length: 5 mm, measurement speed: 0.6 mm/s, cutoff type: Gaussian, cutoff wavelength: λs = 0.8 mm according to JIS01. The width of the metal member was 100 mm, the length was 300 mm, and the plate thickness was 2 mm. The rotation speed of the rotary tool 1 was 5000 rpm, and the welding speed was 500 mm/min. The insertion depth of the rotary tool 1 (the distance from the tip of the rotary tool 1 to the surface of the metal member) was 1.8 mm. The step angle C of the step portion 10 was 90°. The taper angle B of the tip side pin 4 was set to 75°. The distance X2 of the spiral groove 11 (see FIG. 2) of the tip end pin 4 was 0.18 mm, and the height Y2 was 0.22 mm. The length of the tip side pin 4 was 1 mm, and the diameter of the tip was 2 mm (the above are the basic conditions).

As shown in FIG. 10, in Example 1, the taper angle A of the proximal pin 3 of the rotary tool 1 is 105°, 120°, 135°, 142.5°, 150°, 157.5°, and 165°. The correlation between the taper angle and the bond surface roughness was investigated by changing the The distance X1 is set to be approximately constant, and the height Y1 of the stepped side surface 10b of the stepped portion 10 at this time is as shown in FIG. That is, as the taper angle A increases, the height Y1 of the stepped side surface 10b decreases.

As shown in FIG. 11, it was found that when the taper angle A of the proximal pin 3 was 135° to 160°, the surface bonding roughness was reduced. It has been found that when the taper angle A is less than 135°, the joint surface roughness tends to increase. If the taper angle A is less than 135°, the shape is similar to that of a shoulderless tool, so the effect of suppressing the plastic flow material is lost, and it is thought that the joint surface roughness increases. On the other hand, when the taper angle A exceeds 160°, the height Y1 becomes smaller, and the step of the stepped portion 10 becomes smaller. In other words, it is considered that the function of the stepped portion 10 deteriorates and the bonding surface roughness increases.

[Example 2]
In Example 2, as shown in FIG. 12, the taper angle A is fixed at 150°, and the height Y1 of the stepped side surface 10b is set to 0.05 mm, 0.10 mm, 0.18 mm, 0.25 mm, 0.33 mm, The correlation between the height Y1 of the stepped side surface 10b and the bonding surface roughness was investigated by changing the height to 0.40 mm. The other conditions except for the taper angle A and the height Y1 are the same as the basic conditions of the first embodiment.

Further, in Example 2, as shown in FIG. 13, friction stirring was performed using a rotating tool 200 of a comparative example shown in Patent Document 2. The rotation tool 200 of the comparative example includes a proximal pin 203 and a distal pin 204. The taper angle of the proximal pin 203 is larger than the taper angle of the distal pin 204. A spiral groove 13 is formed on the outer peripheral surface of the proximal pin 203. The cross-sectional shape of the groove 13 is approximately semicircular. The radius of curvature of the groove 13 was set to 0.5 mm. The depth of the groove 13 was 0.3 mm, and the distance between adjacent grooves 13, 13 was 1.2 mm. A spiral groove 11 is formed on the outer circumferential surface of the tip end pin 204 .

As shown in FIG. 14, it was found that when the height Y1 of the stepped side surface 10b was 0.10 to 0.40 mm, the bonding surface roughness was reduced. In the rotary tool 200 of the comparative example, the joint surface roughness was 55 μm. It was found that when the height Y1 was 0.05 mm, the bonding surface roughness became significantly large. If the height Y1 is less than 0.10 mm, the state approaches a state where there is no step, so it is considered that the amount of plastic flow based on the step portion 10 decreases and the bonding surface roughness increases.

On the other hand, when the height Y1 exceeds 0.40 mm, the bonding surface roughness tends to increase. This is because when the height Y1 increases, the distance X1 of the step bottom surface 10a also inevitably increases. For example, when the taper angle A exceeds 150°, the distance X1 increases significantly. When the distance X1 of the step bottom surface 10a increases, the effective number of steps (the number of steps in contact with the metal parts to be welded) at the same insertion depth decreases, so the amount of plastic flow based on the step 10 decreases and the welding It is thought that the surface roughness increases. Although the pressing force of the rotary tool 1 against the metal members to be welded was changed to 2600N, 2800N, and 3000N, almost no difference in the pressing force was observed.

[Example 3]
In Example 3, as shown in FIG. 15, the step angle C was varied and the correlation between the step angle C and the bonding surface roughness was investigated. In Example 3, the taper angle A was 150°. The step angle C was changed to 60°, 75°, 85°, 90°, 105°, 120°, and 135°. In addition, Example 3-1 is a case in which the height Y1 of the stepped side surface 10b is 0.1 mm, Example 3-2 is a case in which the height Y1 is 0.18 mm, and Example 3-2 is a case in which the height Y1 is 0.25 mm. The case was designated as Example 3-3. Other conditions are the same as the basic conditions of Example 1.

As shown in FIGS. 16 to 18, it was found that the joint surface roughness was reduced when the step angle C was set to 85° to 120°. When the step angle C is less than 85°, the plastic flow material tends to accumulate inside the step portion 10, and the plastic flow material adheres to the inside of the step portion 10, so that the step portion 10 no longer functions. Furthermore, if the plastic flow material adheres to the stepped portion 10, there is a risk that the plastic flow material and the metal member to be welded will be damaged. On the other hand, if the step angle C exceeds 120°, the plastic flow material cannot be held down, so it is thought that the joint surface roughness increases. Further, among Examples 3-1, 3-2, and 3-3, the joint surface roughness of Example 3-3 was the smallest. In other words, it was found that the bonding surface roughness tends to decrease as the height Y1 of the stepped side surface 10b is at least in the range of 0.1 to 0.25 mm.

1 Rotary tool 2 Base portion 3 Base end pin 4 Distal end pin 10 Step portion 10a Step bottom surface 10b Step side surface 11 Spiral groove A Taper angle (of the base end pin)
B Taper angle C Step angle D Spiral groove angle J1 Butt part X1 Distance (of proximal pin)
X2 Distance Y1 Height (of the side of the step)
Y2 height

Claims (7)

A rotating tool for friction stirring comprising a proximal end pin and a distal end pin,
The taper angle of the proximal pin is larger than the taper angle of the distal pin,
The taper angle of the proximal pin is 135 to 160°,
A spiral stepped portion is formed on the outer circumferential surface of the proximal pin, which is spiral- shaped in plan view and step- shaped in side view;
The angle formed by the bottom surface of the step portion and the side surface of the step is 85 to 120 degrees,
A spiral groove is carved on the outer peripheral surface of the tip side pin,
The rotary tool is characterized in that the helical groove is composed of a helical bottom surface and a helical side surface, and a helical angle formed by the helical bottom surface and the helical side surface is 45° to 90°. 2. The rotary tool according to claim 1, wherein the angle between the bottom surface of the step and the side surface of the step is an acute angle of 85 degrees or more, or an obtuse angle of 120 degrees or less. The rotary tool according to claim 1 or 2, wherein the height of the stepped side surface of the stepped portion is 0.1 to 0.4 mm. The rotary tool according to claim 1 or 2, wherein the height of the stepped side surface of the stepped portion is 0.1 to 0.25 mm. The rotary tool according to any one of claims 1 to 4, wherein the step bottom surface is parallel to a horizontal plane. 2. The stepped bottom surface is inclined upward by 15 degrees or less, or downward by -5 degrees or more with respect to a horizontal plane from the rotation axis of the tool toward the outer circumference. The rotary tool according to any one of claims 1 to 4. A joining method in which a butt part formed by butting end surfaces of a pair of metal members is friction stir welded using a rotating tool,
The rotary tool is
Equipped with a proximal pin and a distal pin,
The taper angle of the proximal pin is larger than the taper angle of the distal pin,
The taper angle of the proximal pin is 135 to 160°,
A spiral stepped portion is formed on the outer circumferential surface of the proximal pin, which is spiral- shaped in plan view and step- shaped in side view;
The angle formed by the bottom surface of the step portion and the side surface of the step is 85 to 120 degrees,
A spiral groove is carved on the outer peripheral surface of the tip side pin,
The spiral groove is composed of a spiral bottom surface and a spiral side surface, and the spiral angle formed by the spiral bottom surface and the spiral side surface is 45° to 90°,
A joining method characterized in that friction stirring is performed while the outer circumferential surface of the proximal pin is brought into contact with the surface of the metal member and the plastic flow material is held down by the bottom surface of the step portion.

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