Joined body and method for manufacturing same

Abstract

This joined body comprises: an overlapping portion in which a first member and a second member overlap; and a stir welded portion which is provided in the overlapping portion and in which the first member and the second member are joined by friction stir spot welding. At least one of the first member and the second member is an aluminum casting. The stir welded portion includes a modified layer derived from the aluminum casting, the modified layer being formed by friction stirring of the aluminum casting. The modified layer comprises an upright wall portion facing an end portion of the overlapping boundary portion between the first member and the second member.

Description

Joined body and method for manufacturing the same

[0001] The present disclosure relates to a joined body formed by joining a plurality of members by friction stir spot welding, and a method for manufacturing the joined body.

[0002] As a joined body in which two or more members are overlapped, a joined body using friction stir spot welding is known. For friction stir spot welding, a single-acting or double-acting tool is used. The double-acting tool includes a pin and a shoulder having a hollow portion for accommodating the pin. For example, in the shoulder-first process, the shoulder is projected and press-fitted into the overlapping portion of the joining target members, while the pin is retracted to accommodate the material overflowing due to the press-fitting (see, for example, Patent Document 1).

[0003] An aluminum casting may be selected as a member to be joined by friction stir spot welding. Patent Document 2 discloses a dissimilar metal joined body formed by joining an iron member and an aluminum die-cast material by friction stir spot welding using a single-acting tool.

[0004] When at least one of the members to be joined by friction stir spot welding is made of an aluminum casting, interfacial fracture in which the interface of the overlapped members breaks tends to occur. For this reason, it has sometimes been difficult to obtain the required strength for the joined body.

[0005] Japanese Unexamined Patent Application Publication No. 2015-186869 Japanese Unexamined Patent Application Publication No. 2019-73080

[0006] An object of the present disclosure is to improve the strength of a joined body and optimize the fracture mode when forming a joined body including a member made of an aluminum casting by friction stir spot welding.

[0007] A joint according to one aspect of the present disclosure comprises an overlapping portion where a first member and a second member overlap, and a stir joint portion provided in the overlapping portion for joining the first member and the second member by friction stir point joining. At least one of the first member and the second member is an aluminum casting, and the stir joint portion includes a modified layer derived from the aluminum casting, formed by friction stirring of the aluminum casting. The modified layer has vertical wall portions facing the overlapping boundary portion between the first member and the second member.

[0008] A method for manufacturing a joint according to another aspect of the present disclosure includes: overlapping a first member and a second member, at least one of which is an aluminum casting, to form an overlapping portion; and pressing a double-acting friction stir point joining tool into the overlapping portion and performing friction stirring to form a stir joint that joins the first member and the second member by friction stir point joining. Forming the stir joint includes forming a modified layer derived from the aluminum casting by friction stirring of the aluminum casting, and forming a vertical wall portion in the modified layer that faces the overlapping boundary between the first member and the second member.

[0009] According to this disclosure, when a joint including an aluminum cast member is formed by friction stir point welding, the strength of the joint can be improved and the fracture mode can be optimized.

[0010] Figure 1 is a schematic cross-sectional view showing a joint according to the present disclosure. Figure 2 is a cross-sectional view showing the fracture mode of the joint in Figure 1. Figure 3 is a cross-sectional photograph with a close-up of the main part showing the results of a deloading test performed on a joint according to one embodiment of the present disclosure. Figure 4 is a schematic cross-sectional view for the joint in Figure 3 to explain the mechanism from interface expansion to base material fracture. Figure 5 is a schematic cross-sectional view showing the fracture mode when a shear force is applied to the joint. Figure 6 is a cross-sectional photograph with a close-up of the main part showing the results of a deloading test performed on a joint of a comparative example. Figure 7 is a schematic configuration diagram of a friction stir point welding apparatus used to manufacture the joint of this embodiment. Figure 8 is a cross-sectional view showing the friction stir welding process in the shoulder-first process. Figure 9 is a cross-sectional view showing an example of forming a vertical wall in the modified layer at the stir-welded portion of the joint. Figure 10 is a cross-sectional view showing another example of forming a vertical wall in the modified layer at the stir-welded portion of the joint. Figure 11 is a graph showing the relationship between the fracture mode of the joint and the tool press-fit depth. Figure 12 is a cross-sectional photograph of the joint to illustrate the preferred protrusion angle of the vertical wall portion of the modified layer. Figure 13 is a cross-sectional photograph of the joint showing an example of measuring the protrusion angle of the vertical wall portion according to the embodiment. Figure 14 is a cross-sectional photograph of the joint showing an example of measuring the protrusion angle of the vertical wall portion according to the comparative example. Figure 15 is a schematic cross-sectional view showing a stirred joint having a vertical wall portion with an arc-shaped bulge. Figure 16 is a graph showing the relationship between the vertical wall portion protrusion angle and the amount of vertical wall portion protrusion. Figure 17 is a graph showing the relationship between the vertical wall portion protrusion angle and the tool indentation amount. Figure 18 is a graph showing the relationship between the tool indentation amount, the amount of vertical wall portion protrusion, and the vertical wall portion protrusion angle when the shoulder rotation speed is changed in the shoulder press-fitting process in the shoulder pre-process for manufacturing the joint. Figure 19 is a graph showing the relationship between the tool indentation amount, the amount of vertical wall portion protrusion, and the vertical wall portion protrusion angle when the pin rising speed is changed in the shoulder press-fitting process. Figure 20 is a graph showing the relationship between the tool indentation amount, the vertical wall protrusion amount, and the vertical wall protrusion angle when the pin descent speed is changed during the backfilling process. Figure 21 is a graph showing the relationship between the tool indentation amount, the vertical wall protrusion amount, and the vertical wall protrusion angle when the pin indentation pressure is changed during the backfilling process.

[0011] Embodiments of the present disclosure will be described in detail below with reference to the drawings. The joint according to the present disclosure is manufactured by overlapping two or more members, at least one of which is an aluminum casting, and friction stir point joining at the overlapping portion. The overlapping members are structural members such as plates, frames, exterior materials, or columnar members. The manufactured joint becomes a component of a structure such as an aircraft, railway vehicle, automobile, or motorcycle.

[0012] [Structure of the Joint] Figure 1 is a schematic cross-sectional view of the joint 3 according to the present disclosure. Figure 1 is marked with vertical direction indicators. These direction indicators are for the convenience of explanation and do not limit the actual vertical direction of the joint 3. The joint 3 includes a first member 31, a second member 32, and a stir joint 4. The first member 31 and the second member 32 form an overlapping portion 30 where they overlap vertically. The overlapping portion 30 is formed by overlapping a part or all of the first member 31 and a part or all of the second member 32 vertically.

[0013] In the overlapping portion 30, the first member 31 is positioned on the upper side and the second member 32 is positioned on the lower side. The first member 31 is the side into which the friction stir point joining tool 1, described later, is first pressed, and the second member 32 is the side into which the tool 1 is last pressed. The area where the first member 31 and the second member 32 are in contact is the overlapping boundary portion 33. In the boundary portion 33, the lower surface of the first member 31 and the upper surface of the second member 32 face each other.

[0014] The agitated joint 4 is provided in the overlapping portion 30 and joins the first member 31 and the second member 32 by friction agitated point joining. The agitated joint 4 is formed by press-fitting and agitation with the friction agitated point joining tool 1 and has a cylindrical shape. In the example of Figure 1, the agitated joint 4 is formed to penetrate the first member 31 in the vertical direction and reach the upper portion of the second member 32. The agitated joint 4 includes an upper plate agitated layer 41 and a modified layer 42 as a lower plate agitated layer. The upper plate agitated layer 41 is a layer derived from the first member 31, formed by friction agitation of the first member 31. The modified layer 42 is a layer derived from the second member 32, formed by friction agitation of the second member 32.

[0015] The upper plate stirring layer 41 is a cylindrical portion having a diameter approximately equal to the outer diameter of the tool 1 that is pressed in. The modification layer 42 is connected to the lower end of the upper plate stirring layer 41. The modification layer 42 is formed in a region that extends from near the lower surface of the first member 31, across the boundary 33, to near the upper surface of the second member 32. In other words, the boundary 33 disappears due to the modification layer 42, and the modification layer 42 is formed so as to straddle the imaginary line of the disappeared boundary 33 above and below. It can also be said that the overlapping surface of the boundary 33 extending in the horizontal direction is divided by the modification layer 42.

[0016] The modified layer 42 includes a vertical wall portion 43 and a bottom portion 44. The vertical wall portion 43 is the side surface of the modified layer 42, and the bottom portion 44 is the bottom surface of the modified layer 42. The modified layer 42 includes a bulging portion that bulges radially outward from the other part of the agitated joint 4, i.e., the upper plate agitated layer 41. The surface of the bulging portion is the vertical wall portion 43. In Figure 1, a vertical wall portion 43 having the shape of an arc bulging radially outward is illustrated. The vertical wall portion 43 faces the overlapping boundary portion 33 of the first member 31 and the second member 32. In other words, the end of the boundary portion 33, which is separated by the formation of the modified layer 42, abuts against the vertical wall portion 43. As a result, the vertical wall portion 43 is in contact with the first member 31 and the second member 32 and the end of the boundary portion 33. The bottom portion 44 is a generally flat surface and is in contact with the second member 32. The actual bottom surface 44 is often a curved surface that is convex downwards.

[0017] At least one of the first member 31 and the second member 32 is made of an aluminum casting. In this embodiment, an example is shown where the first member 31 is a wrought metal and the second member 32 is an aluminum casting. An example of a wrought metal is an aluminum sheet. An aluminum casting is a casting or die-cast product made of an alloy of aluminum (Al) and silicon (Si). In this embodiment, the upper plate stirring layer 41 is a layer formed by friction stirring of a wrought material such as an aluminum sheet. The modified layer 42 is a layer derived from an aluminum casting, formed by friction stirring of the aluminum casting.

[0018] As the aluminum casting, Al-7%Si alloy and Al-12%Si alloy can be used. The aluminum casting may also be an alloy containing other additives. For example, Al-Si-Mg alloy, Al-Si-Cu alloy, Al-Si-Cu-Mg alloy, Al-Si-Mn-Mg alloy, etc., which contain magnesium or copper additives, may be used as the second member 32. As the first member 31, a material other than aluminum may be used, for example, a wrought magnesium alloy may be used.

[0019] Both the first member 31 and the second member 32 may be made of cast aluminum. Alternatively, the first member 31 may be made of cast aluminum and the second member 32 may be made of a wrought metal. Furthermore, other members may be interposed at the boundary 33. That is, the first member 31 and the second member 32 may not be in direct contact, and another sheet material, adhesive, or sealing material may be interposed between them as a third member. In addition, a coating layer or plating layer applied to the surface of the first member 31 or the second member 32 may be interposed at the boundary 33.

[0020] [Fracture Modes of the Joint] Figure 2 is a cross-sectional view showing the fracture modes of the joint 3 shown in Figure 1. In the joint 3 of this embodiment, when a horizontal shear force intersecting the vertical direction is applied, interface expansion BB and base material fracture MB occur. Interface expansion BB is a phenomenon in which a crack occurs in the horizontal direction at the overlapping boundary 33 between the first member 31 and the second member 32, due to the widening of the gap between the interfaces of the two members that are in contact with each other, or due to slippage. Base material fracture MB is a fracture that occurs in the base material of the second member 32, i.e., the non-agitated portion. The crack caused by interface expansion BB at the boundary 33 abuts against the vertical wall portion 43 of the modified layer 42. As a result, the extension of the crack along the direction of extension of the overlapping boundary 33 between the first member 31 and the second member 32, which in this embodiment is the horizontal direction, stops. As a result, the crack propagates downward from the vertical wall portion 43 along the base material, which is the non-agitated portion of the second member 32.

[0021] Figure 3 is a cross-sectional photograph with a close-up of the main part showing the results of an unloading test performed on the joint 3. An unloading test is a test in which the application of tensile force is interrupted during the process of applying tensile force to the joint 3 by pulling the first member 31 and the second member 32 in opposite directions in order to apply a shear force, and the progress of crack propagation is observed. The two photographs on the left side of Figure 3 are a cross-sectional photograph of the joint 3 before the initial crack initiation and a close-up of the main part, while the two photographs on the right side are a cross-sectional photograph of the joint 3 after the initial crack initiation and a close-up of the main part. In Figure 3, the area near the lower end of the upper plate stirring layer 41 and the modified layer 42 of the stirring joint 4, and the base material portions of the first member 31 and the second member 32 near the modified layer 42 are shown. Note that the boundaries of the stirring joint 4, the first member 31 and the second member 32 are unclear in the cross-sectional photograph, so contour lines indicating the boundaries have been added to the two photographs on the left side.

[0022] Before the initial crack initiation, the first member 31 and the second member 32 are in close contact at the overlapping boundary 33. They are also in close contact at the boundary between the stirred joint 4 and the first member 31 and the second member 32. The end of the boundary 33 faces the vicinity of the most protruding part of the vertical wall portion 43 of the modified layer 42.

[0023] After the initial crack initiation, interfacial expansion BB occurs at the boundary portion 33. However, the horizontal crack caused by interfacial expansion BB stops at the vertical wall portion 43. In other words, the crack does not propagate from the vertical wall portion 43 along the bottom portion 44 of the modified layer 42. From the vertical wall portion 43, a crack caused by base material fracture MB in the second member 32 extends downward in a zigzag pattern. That is, the presence of the vertical wall portion 43, which abuts against the end of the boundary portion 33, guides the crack propagation direction toward a direction that traverses the second member 32 longitudinally, resulting in a fracture mode in which the crack caused by interfacial expansion BB changes to a crack caused by base material fracture MB.

[0024] Figure 4 is a schematic cross-sectional view illustrating the mechanism of crack formation from interfacial expansion cracks (BB) to base material fracture cracks (MB) in the joint 3 shown in Figure 3. Figure 4 shows the internal structure of the second member 32, which is made of aluminum casting. The aluminum casting, which is an alloy of aluminum and silicon, contains a relatively coarse α-Al phase 34 and a eutectic phase. The α-Al phase 34 is a phase in which aluminum crystallizes exclusively. The eutectic phase is a phase in which silicon and aluminum crystallize, and has a high silicon concentration. In this embodiment, the eutectic phase is called the Si-enriched phase 35. Schematically, the α-Al phase 34 crystallizes in a dendritic manner, and the Si-enriched phase 35 crystallizes like joints filling the spaces between the branches. The Si-enriched phase 35 is comparable to the α-Al phase 34 in microscopic strength, but is more brittle than the α-Al phase 34. Therefore, when a crack enters the Si-enriched phase 35, the crack propagates rapidly. In other words, the Si-enriched phase 35, which is in a continuously connected state, is prone to crack propagation, and overall, the Si-enriched phase 35 has low strength.

[0025] In the modified layer 42, which is the portion of the aluminum casting that has been frictionally stirred, the α-Al phase 34 and the Si-enriched phase 35 are mixed by friction stirring, and the coarse α-Al phase 34 and Si-enriched phase 35 are no longer visible. In other words, the continuous Si-enriched phase 35, which is a brittle portion, is finely dispersed by friction stirring. As a result, crack propagation along the Si-enriched phase 35 becomes difficult in the modified layer 42, and as a result, the strength is improved. According to the inventors' tests, it was confirmed that the modified layer 42 has the same Vickers hardness as the second member 32 of the base material. Therefore, the modified layer 42 has the same strength as the second member 32 and is a layer that does not have mechanical weak points such as the Si-enriched phase 35. Because it has the same strength as the aluminum casting and its microstructure has been changed, it is called the modified layer 42 derived from the aluminum casting. The vertical wall portion 43, which is the side surface of the modified layer 42 having the mechanical properties described above, has a barrier function that stops the propagation of cracks in the interface expansion BB.

[0026] When a crack due to interfacial expansion BB begins to occur at the boundary 33 between the first member 31 and the second member 32, the crack abuts against the vertical wall portion 43 of the modified layer 42. Since the vertical wall portion 43 is a surface without brittle parts, the crack stops extending along the direction of extension of the boundary portion 33. On the other hand, the base material portion of the second member 32, which is made of aluminum casting that has not been friction stirred, contains a brittle Si-enriched phase 35. Therefore, the crack propagates from the vertical wall portion 43 along the Si-enriched phase 35 of the base material portion. The crack that propagates along the Si-enriched phase 35 becomes a base material fracture MB. In this way, by installing the vertical wall portion 43, it is possible to control the direction of crack propagation and make the fracture mode of the joint 3 a base material fracture MB rather than an interfacial fracture. Note that the base material fracture MB is not caused by deterioration of the second member 32 due to stress applied during the formation of the stirring joint portion 4. Rather, because the modified layer 42 becomes stronger due to the fine dispersion of the Si-enriched phase 35, the cracks in the interface expansion BB escape towards the second member 32, resulting in the base material fracture MB of the second member 32.

[0027] Based on the above fracture modes, the ratio T1:T2, which is the thickness T1 of the first member 31 made of a wrought material such as an aluminum sheet and the thickness T2 of the second member 32 made of an aluminum casting, is preferably selected from the range of 1:0.3 to 5. The preferred range is T1:T2 = 1:0.5 to 4, and the more preferred range is T1:T2 = 1:0.5 to 2. If the second member 32 is too thick compared to the first member 31, cracks due to interfacial expansion BB will have difficulty escaping to the second member 32 side, making it difficult for base material fracture MB to occur in the second member 32, meaning that the joint 3 will be damaged by interfacial fracture. If the second member 32 is too thin compared to the first member 31, base material fracture MB will easily occur in the second member 32, reducing the fracture strength of the joint 3. Furthermore, the greater the depth of the stirred joint 4, that is, the greater the depth to which the bottom 44 penetrates from the boundary 33 into the second member 32, the easier it is for cracks caused by interfacial expansion BB to escape downward or upward. In other words, interfacial fracture along the bottom 44 of the stirred joint 4 becomes less likely to occur. The preferred depth of the stirred joint 4, in other words, the length from the boundary 33 to the bottom 44, is 0.66 mm or more, more preferably 1 mm or more, and particularly preferably 1.4 mm or more.

[0028] Figure 5 is a schematic cross-sectional view showing the fracture mode when a shear force is applied to the joint 3. A shear force is applied to the joint 3, in which a stir joint 4 is formed at the overlapping portion 30 of a first member 31 made of a wrought material and a second member 32 made of an aluminum casting. The shear force is applied by tensile forces pulling the first member 31 and the second member 32 in opposite directions. The stir joint 4 penetrates the first member 31 and reaches the upper part of the second member 32.

[0029] Interfacial fracture is a fracture in which the first member 31 and the second member 32 separate at the boundary portion 33. The upper layer 401 of the stir joint 4 remains in the first member 31, and the lower layer 402 of the stir joint 4 remains in the second member 32. In other words, in interfacial fracture, the stir joint 4 also fractures along the direction of extension of the boundary portion 33. Lower plug fracture is a fracture in which the stir joint 4 and a portion 321 of the second member 32, which is a part of the second member 32 and directly below the stir joint 4, become completely separated from the second member 32. Base material fracture is a fracture that occurs in the non-stirred portion of the second member 32, as described above. In the case of aluminum castings, it is caused by a crack that propagates exclusively through the Si-enriched phase 35. The stir joint 4 and the portion 322 of the second member 32 remain in the first member 31.

[0030] If the joint 3 is damaged by interfacial fracture, undesirable problems arise from a quality control standpoint. In interfacial fracture, the first member 31 and the second member 32 separate at the boundary 33. The stir joint 4 also separates roughly along the line of the boundary 33. In this case, it becomes difficult to verify whether the first member 31 and the second member 32 were originally joined by the stir joint 4. Therefore, it becomes difficult to perform quality control that depends on the strength of the base material of the first member 31 and the second member 32 themselves. On the other hand, in the case of lower plug fracture or base material fracture, the shape of the stir joint 4 itself is maintained, and partial pieces 321 and 322 remain on the side of the first member 31. Therefore, there is an advantage in that it can be confirmed that the first member 31 and the second member 32 were joined by the stir joint 4 before the joint 3 was damaged.

[0031] As described above, the formation of the vertical wall portion 43 can induce base material fracture MB or lower plug fracture, thereby suppressing fracture of the joint 3 due to interfacial fracture. On the other hand, even in a stirred joint 4 equipped with a modified layer 42, if there is no portion that performs the function of a vertical wall portion 43, fracture due to interfacial fracture is more likely to occur.

[0032] Figure 6 is a cross-sectional photograph with a close-up of the main part showing the results of an unloading test performed on the comparative example joint 300. The photograph on the left of Figure 6 is a cross-sectional photograph of the joint 300 before the initial crack initiation, and the two photographs on the right are cross-sectional photographs and close-ups of the main part after the initial crack initiation. Similar to Figure 3, contour lines indicating the boundaries of the stirred joint 4, the first member 31, and the second member 32 have been added to the cross-sectional photograph before the initial crack initiation. The stirred joint 4 includes an upper plate stirred layer 41 and a modified layer 42, and the modified layer 42 has an end portion 430 facing the boundary portion 33. The end portion 430 is not a wall that blocks the extension direction of the boundary portion 33 like the vertical wall portion 43, but has a shape that is sharpened toward the boundary portion 33.

[0033] Before the initial crack initiation, the first member 31 and the second member 32 are in close contact at the overlapping boundary 33. They are also in close contact at the boundary between the stir-bonded joint 4 and the first member 31 and the second member 32. After the initial crack initiation, interfacial expansion BB occurs at the boundary 33. Furthermore, the crack caused by interfacial expansion BB propagates along the bottom 44 of the modified layer 42, leading to the separation of the first member 31 and the second member 32.

[0034] In other words, the end portion 430 facing the boundary portion 33 of the modified layer 42 does not have the function of a vertical wall that stops the propagation of cracks caused by interfacial expansion BB at the boundary portion 33. This is because the end portion 430 has a shape that is nearly parallel to the extension direction of the boundary portion 33, rather than a wall shape that is nearly perpendicular to the extension direction of the boundary portion 33. With an end portion 430 of this shape, cracks from interfacial expansion BB at the boundary portion 33 will pass through the end portion 430 and propagate along the bottom portion 44 of the modified layer 42, causing the joint 300 to break due to interfacial fracture. In contrast, the vertical wall portion 43 provided in the stirred joint portion 4 of this embodiment has a wall shape that is nearly perpendicular to the extension direction of the boundary portion 33. Therefore, the vertical wall portion 43 can stop the propagation of cracks caused by interfacial expansion BB at the boundary portion 33.

[0035] [Method for Manufacturing a Joined Body] The method for manufacturing the joined body 3 of the present disclosure includes the following steps 1 and 2. Step 1: A first member 31 and a second member 32, at least one of which is an aluminum casting, are stacked to form an overlapping portion 30. In this embodiment, a first member 31 made of a wrought material and a second member 32 made of an aluminum casting are stacked to form an overlapping portion 30. Step 2: A friction stir point joining tool, which includes a pin and a shoulder having a hollow portion into which the pin is inserted, is pressed into the overlapping portion 30 and friction stirring is performed to form a stir joint 4 that joins the first member 31 and the second member 32 by friction stir point joining. In step 2 above, a modified layer 42 derived from the aluminum casting is formed by friction stirring of the aluminum casting, which is the base material of the second member 32. In addition, a vertical wall portion 43 is formed in the modified layer 42 that faces the overlapping boundary portion 33 of the first member 31 and the second member 32.

[0036] <Configuration of the Friction Stir Point Welding Apparatus> Figure 7 is a schematic diagram showing the configuration of the friction stir point welding apparatus M used in the manufacture of the joined body 3. The friction stir point welding apparatus M includes a tool 1 for friction stir point welding, a tool drive unit 2 that rotates and moves the tool 1 up and down, and a controller 20 that controls the operation of each part of the friction stir point welding apparatus M. Note that the "up" and "down" direction indicators in Figure 7 are for explanatory purposes only and are not intended to limit the actual direction of use of the tool 1.

[0037] Tool 1 is supported by various tool fixing parts. For example, the tool fixing part is the tip of an articulated robot. A backup 15 is positioned opposite the lower end surface of tool 1. At least two members to be joined are positioned between tool 1 and backup 15. In this embodiment, an overlapping portion 30, in which a part of the first member 31 and a part of the second member 32 overlap in the vertical direction, is positioned between tool 1 and backup 15.

[0038] Tool 1 includes a pin 11, a shoulder 12, a clamp 13, and a spring 14. The pin 11 is a cylindrical member. The pin 11 is positioned so that its axis extends in the vertical direction. The pin 11 is rotatable around its axis as the rotation axis R, and can move up and down along the rotation axis R. When using Tool 1, the rotation axis R and the point contact position W at the overlapping portion 30 are aligned.

[0039] The shoulder 12 is a cylindrical member having a first hollow portion 12H inside into which the pin 11 is inserted. The axis of the shoulder 12 is coaxial with the axis of the pin 11, i.e., the rotation axis R. The shoulder 12 is rotatable about the rotation axis R and can move up and down along the rotation axis R.

[0040] The tool 1 of this embodiment is a double-acting tool in which the pin 11 and the shoulder 12 move independently in the axial direction. That is, the shoulder 12 and the pin 11 inserted into the hollow portion can both rotate around the axis of rotation R and move relative to each other in the direction of rotation axis R. Specifically, the pin 11 and the shoulder 12 can not only move up and down simultaneously along rotation axis R, but can also move independently, with one moving down and the other moving up.

[0041] The clamp 13 is a cylindrical member formed with a second hollow portion 13H inside through which the shoulder 12 is inserted. The axis of the clamp 13 is also coaxial with the rotation axis R. The clamp 13 does not rotate around its axis, but moves up and down, i.e., forward and backward, along the rotation axis R. The clamp 13 surrounds the shoulder 12. The clamp 13 plays a role in preventing the outflow of friction-agitated material when the pin 11 or shoulder 12 performs friction agitation. In other words, the enclosure of the clamp 13 prevents the friction-agitated material from scattering, and the friction-agitated point joint can be finished smoothly.

[0042] The spring 14 is attached to the upper end of the clamp 13 and biases the clamp 13 downward toward the overlapping portion 30. The clamp 13 is attached to the tool fixing portion via the spring 14. The backup 15 has a flat surface that contacts the lower surface of the overlapping portion 30 to be joined. The backup 15 is a backing member that supports the overlapping portion 30 when the pin 11 or shoulder 12 is pressed into the overlapping portion 30. The clamp 13, biased by the spring 14, presses the overlapping portion 30 against the backup 15.

[0043] The tool drive unit 2 includes a rotary drive unit 21, a pin drive unit 22, a shoulder drive unit 23, and a clamp drive unit 24. The rotary drive unit 21 includes a motor and drive gears, etc., and rotates the pin 11 and shoulder 12 around the rotation axis R. The pin drive unit 22 is a mechanism that moves the pin 11 forward and backward along the rotation axis R. The pin drive unit 22 drives the pin 11 to press it into the overlapping portion 30 and to retract it from the overlapping portion 30. The shoulder drive unit 23 is a mechanism that moves the shoulder 12 forward and backward along the rotation axis R. The shoulder drive unit 23 causes the shoulder 12 to press into the overlapping portion 30 and to retract it. The clamp drive unit 24 is a mechanism that moves the clamp 13 forward and backward along the rotation axis R. The clamp drive unit 24 moves the clamp 13 toward the overlapping portion 30 and presses the overlapping portion 30 against the backup 15. At this time, the biasing force of spring 14 comes into play.

[0044] The controller 20 consists of a microcomputer or the like, and controls the operation of the tool driving unit 2 by executing a predetermined control program. Specifically, the controller 20 controls the rotational driving unit 21 to cause the pin 11 and the shoulder 12 to perform the required rotational operations. Further, the controller 20 controls the pin driving unit 22, the shoulder driving unit 23, and the clamp driving unit 24 to cause the pin 11, the shoulder 12, and the clamp 13 to perform the required forward and backward movement operations.

[0045] <Usage method of the tool> Next, the usage method of the tool 1 exemplified in the present embodiment will be described. As the usage method of the friction stir spot welding apparatus M, roughly speaking, there are a pin-first process in which the pin 11 of the tool 1 is first press-fitted into the overlapping portion 30, and a shoulder-first process in which the shoulder 12 is first press-fitted into the overlapping portion 30. In the present embodiment, in the above-described step 2, the shoulder-first process is applied to form the stir welded joint 4 and manufacture the joined body 3. Hereinafter, the shoulder-first process will be described.

[0046] FIG. 8 is a diagram showing the processes P11 to P14 of the friction stir spot welding method by the shoulder-first process executed in the above-described step 2. The processes P11 to P14 schematically show the situation of friction stir spot welding the overlapping portion 30 between the first member 31 and the second member 32. Before the execution of the processes P11 to P14, the above-described step 1 is executed. In step 1, the first member 31 is disposed on the side where the tool 1 is first press-fitted, and the second member 32 is disposed on the side where the tool 1 is last press-fitted to form the overlapping portion 30.

[0047] Process P11 is the preheating process of the overlapping portion 30. The controller 20 rotates the pin 11 and the shoulder 12 around the axis at a predetermined rotational speed with the lower end of the tool 1 abutted against the surface of the first member 31. Process P12 is the press-fitting process of the shoulder 12. The controller 20 lowers the shoulder 12 to press it into the overlapping portion 30 while retracting the pin 11 upward. By this operation, the material in the press-fitting region of the shoulder 12 is stirred. Further, the overflow material OF overflowing from the overlapping portion 30 due to the press-fitting is discharged into the space of the first hollow portion 12H of the shoulder 12 generated by the retraction of the pin 11 as indicated by the arrow a1. Thus, during friction stir welding, the pin 11 is relatively moved upward with respect to the shoulder 12.

[0048] Process P13 is the process of filling back the overflow material OF. The controller 20 raises and retracts the shoulder 12 while lowering the pin 11. Due to the lowering of the pin 11, as indicated by the arrow a2, the overflow material OF discharged into the first hollow portion 12H is filled back into the press-fitting region of the shoulder 12. Process P14 is the finishing process. The controller 20 rotates both the pin 11 and the lower end surface of the shoulder 12 in a state where they are returned to the height position of the surface of the first member 31 to smooth the spot joining portion. Through the above processes, the stir joining portion 4 is formed.

[0049] <Example of forming the vertical wall portion> An example of forming the modified layer 42 and the vertical wall portion 43 in the stir joining portion 4 by applying the shoulder-first process described above will be described with reference to FIGS. 9 and 10. FIG. 9 is a cross-sectional view showing an example of forming the vertical wall portion 43 in the modified layer 42 by press-fitting the shoulder 12 only into the first member 31 which is the upper plate of the overlapping portion 30. FIG. 10 is an example of forming the vertical wall portion 43 in the modified layer 42 by press-fitting the shoulder 12 through the first member 31 of the upper plate and further into the second member 32 of the lower plate. Both examples in FIGS. 9 and 10 illustrate the press-fitting process of process P12 and the filling-back process of process P13 described based on FIG. 8.

[0050] In the example shown in Figure 9, during process P12, the shoulder 12 is press-fitted to the vicinity of the lower end of the first member 31, that is, just before the boundary portion 33, while the pin 11 is raised. This press-fitting causes the first fluid portion 41A, derived from the wrought material and formed by frictional stirring of the first member 31, to be housed inside the shoulder 12. The first fluid portion 41A later becomes the upper plate stirring layer 41 of the stirring joint 4. Near the upper end of the second member 32, a second fluid portion 42A derived from the aluminum casting is formed by frictional stirring of the second member 32 by the shoulder 12. The second fluid portion 42A later becomes the modified layer 42. At the stage of process P12, no portion corresponding to the vertical wall portion 43 has been formed in the second fluid portion 42A.

[0051] In process P13, the shoulder 12 is raised while the pin 11 is lowered. The first fluid portion 41A within the shoulder 12 is backfilled into the space created as the shoulder 12 rises. The pressing force from the first fluid portion 41A to the second fluid portion 42A during this backfilling causes the second fluid portion 42A to expand downward and laterally. As a result, a protruding portion 43A is formed that bulges radially outward from the outer diameter of the shoulder 12. The protruding portion 43A is formed on the portion facing the boundary portion 33. The protruding portion 43A becomes the vertical wall portion 43 after the process is completed.

[0052] In the example shown in Figure 10, during process P12, the pin 11 is raised while the shoulder 12 penetrates the first member 31 and is press-fitted to a position immediately after crossing the boundary portion 33, that is, to the upper portion of the second member 32. This press-fitting causes the first fluid portion 41A, derived from the wrought material and formed by the frictional stirring of the first member 31, to be housed inside the shoulder 12. Near the upper end of the second member 32, a second fluid portion 42A, derived from the aluminum casting, is formed by the frictional stirring of the second member 32 by the shoulder 12. The pressing force that presses the shoulder 12 into the second member 32 causes the second fluid portion 42A to expand downward and laterally. As a result, a protruding portion 43A is formed that bulges radially outward from the outer diameter of the shoulder 12, which will become the vertical wall portion 43 after the process is completed. The protruding portion 43A is formed in the portion facing the boundary portion 33.

[0053] In process P13, the pin 11 is lowered while the shoulder 12 is raised. The first fluid portion 41A within the shoulder 12 is filled into the space created as the shoulder 12 rises. At this time, the second fluid portion 42A is subjected to pressure and expands further downward and laterally. As a result, the protruding portion 43A, which becomes the vertical wall portion 43, grows larger.

[0054] Figure 11 is a graph showing the test results investigating the relationship between the fracture mode of the joint 3 and the press-fit depth of the tool 1. The horizontal axis of the graph shows the press-fit depth (mm) of the tool 1 into the overlapping portion 30. Figure 11 shows an example where the thickness of the first member 31 is 2.0 mm and the thickness of the second member 32 is 2.5 mm. The vertical axis of the graph shows the shear fracture strength (kN) of the joint 3. The fracture modes are represented by the symbols of the plots for the three modes shown in Figure 5. White circles indicate interface fracture, black circles indicate bottom plug fracture, and semi-black circles indicate base material fracture.

[0055] In the example shown in Figure 11, when the tool insertion depth is 2.0 mm, the tip of tool 1 reaches the boundary portion 33. The shear fracture strength is approximately 6.0 kN, regardless of the tool insertion depth. The fracture mode when the tool insertion depth is 1.6 mm is an undesirable interfacial fracture. The fracture mode when the tool insertion depth is 1.9 mm is a lower plug fracture. The fracture mode when the tool insertion depth is 2.0 mm or more is a base material fracture.

[0056] From the test results, it was found that if the tool 1 is pressed into the overlapping portion 30 at a tool press-in depth of 1.9 mm or more, the fracture mode will be either lower plug fracture or base material fracture. This means that by pressing in at a threshold of Th or more, a modified layer 42 having a vertical wall portion 43 that can stop cracks from interfacial fracture is formed in the stir joint portion 4. Note that a tool press-in depth of 1.9 mm corresponds to the press-in example in Figure 9. A tool press-in depth of 2.0 mm or more corresponds to the press-in example in Figure 10. It was confirmed that if the tool press-in depth is 2.0 mm or more, that is, a press-in depth that reaches the second member 32, the fracture mode will be base material fracture.

[0057] [Details of the Modified Layer] A preferred embodiment of the modified layer 42 of the agitated joint 4 will be explained. The vertical wall portion 43 of the modified layer 42 is preferably a surface that bulges radially outward from the upper plate agitated layer 41 and intersects the boundary portion 33 at an angle close to right with respect to the direction of extension. In other words, if the boundary portion 33 is a surface that extends horizontally, the vertical wall portion 43 is preferably a surface that is close to vertical, a mountain-shaped protruding surface, or a convex curved protruding surface. The shape of the vertical wall portion 43 formed by friction agitation is irregular, and it is difficult to describe its shape uniquely. Therefore, the shape of the vertical wall portion 43 will be described approximately.

[0058] Figure 12 is a cross-sectional photograph of the joint 3 to illustrate the preferred projection angle of the vertical wall portion 43 of the modified layer 42. Figure 12 shows an example in which the vertical wall portion 43 is approximately considered as a mountain-shaped projection surface. The upper and lower figures of Figure 12 are the same cross-sectional photograph. In the upper figure of Figure 12, contour lines have been added to clarify the boundary between the upper plate stirred layer 41 and modified layer 42 of the stirred joint 4 and the first member 31 and the second member 32, similar to Figure 3. In the lower figure of Figure 12, an approximate line of the vertical wall portion 43 has been added. The joint 3 shown in Figure 12 is in the state after the occurrence of interface expansion BB and base material fracture MB.

[0059] Figure 12 shows the position P1 of the side surface 41S of the upper plate stirring layer 41 and the position P2 of the apex that protrudes most radially from the vertical wall portion 43. The vertical wall portion 43 protrudes radially outward by a protrusion amount PR from the side surface 41S of the upper plate stirring layer 41. In the lower part of Figure 12, a apex V, a first line L1, a second line L2, and a third line L3 are indicated to approximate the shape of the vertical wall portion 43. The apex V is the position of the vertical wall portion 43 that protrudes most radially outward and is in contact with the end of the boundary portion 33. The first line L1 is an approximation line along the surface of the vertical wall portion 43 extending from the apex V toward the first member 31. The second line L2 is an approximation line along the surface of the vertical wall portion 43 extending from the apex V toward the second member 32. The third line L3 is an approximation line along the boundary portion 33, that is, an approximation line along the direction of extension of the boundary portion 33. Figure 12 shows an example where the third line L3 passes through vertex V, but the third line L3 and vertex V may be shifted vertically.

[0060] The angle formed by the first straight line L1 and the second straight line L2 at vertex V is defined as the protrusion angle FA. It is desirable that the protrusion angle FA be in the range of 52 degrees or more and 180 degrees or less. By setting the protrusion angle FA within the above range, the vertical wall portion 43 becomes a surface that intersects the third straight line L3, which is the extension direction of the boundary portion 33, at a relatively large angle. Therefore, the vertical wall portion 43 can be made more effective at stopping cracks in the interface expansion BB. In particular, if the protrusion angle FA is close to 180 degrees, the vertical wall portion 43 becomes a surface that is nearly perpendicular to the third straight line L3, making it easier to stop crack propagation. On the other hand, if the protrusion angle FA is less than 52 degrees, the crack tends to pass through the vertical wall portion 43 and propagate along the bottom 44 of the modified layer 42. In this case, it becomes difficult to guide the crack propagation to the Si-enriched phase 35 of the second member 32.

[0061] Figure 13 is a cross-sectional photograph of the joint 3 showing an example of measuring the protrusion angle FA of the vertical wall portion 43 of this embodiment illustrated in Figure 12. For the vertical wall portion 43 of this embodiment, the protrusion angle FA measured by setting the first straight line L1 and the second straight line L2 as approximation lines was approximately 114.2 degrees. The protrusion amount PR of the vertical wall portion 43 was 0.144 mm. It was confirmed that forming such a vertical wall portion 43 can induce base material fracture MB.

[0062] Figure 14 is a cross-sectional photograph of the joint 300 showing an example of measuring the protrusion angle FA of the end portion 430 corresponding to the vertical wall portion in the comparative example shown in Figure 6. The protrusion angle FA measured with the end portion 430 using the approximation lines, the first straight line L1 and the second straight line L2, was approximately 42.9 degrees. It was confirmed that when such a vertical wall portion is formed, it is not possible to induce base material fracture MB, and cracks in the interface expansion BB propagate horizontally, causing the joint 300 to break due to interface fracture.

[0063] Figure 15 is a schematic cross-sectional view showing a stir joint 4 having a vertical wall portion 43 with an arc-shaped bulge. In Figure 12, an example is shown in which the vertical wall portion 43 is approximately considered as a mountain-shaped protruding surface. In contrast, in the example of Figure 15, the vertical wall portion 43 is approximately considered as an arc-shaped surface that bulges radially outward. The arc-shaped vertical wall portion 43 bulges radially outward by a protrusion amount PR. The position that protrudes the most radially outward is the vertex V. The end of the boundary portion 33 faces the vertex V of the arc or its vicinity. In the example of Figure 15, the performance of the vertical wall portion 43 can be evaluated by the radius of curvature of the arc, etc., instead of the protrusion angle FA.

[0064] Figure 16 is a graph showing the relationship between the protrusion angle FA of the vertical wall portion 43 and the protrusion amount PR of the vertical wall portion 43. The horizontal axis of the graph is the protrusion amount PR (mm) of the vertical wall portion 43, and the vertical axis is the protrusion angle FA (degrees) of the vertical wall portion 43. In the test example in Figure 16, as in the example in Figure 11, multiple samples were used in which the plate thickness of the first member 31 was 2.0 mm and the plate thickness of the second member 32 was 2.5 mm. The position where the protrusion amount PR = 0 mm corresponds to the side surface 41S of the upper plate stirring layer 41. Here, white plots indicate interface fracture, black plots indicate lower plug fracture, and gray plots indicate base material fracture. Note that the triangular plots with C / C appended are test examples for the overlapping portion 30 in which both the first member 31 and the second member 32 are made of aluminum casting. The square plots with C / S indicated represent test examples for overlapping sections 30 where the first member 31 is an aluminum casting and the second member 32 is a wrought material. The other circular plots represent test examples for overlapping sections 30 where the first member 31 is a wrought material and the second member 32 is an aluminum casting, as described in the above embodiment. The aluminum casting used above is an Al-7%Si alloy. The diamond-shaped plots with S / D indicated represent test examples for overlapping sections 30 where the first member 31 is a wrought material with a plate thickness of 2.0 mm and the second member 32 is an aluminum die-cast material using an Al-12%Si alloy with a plate thickness of 2.0 mm.

[0065] The graph in Figure 16 shows that the protrusion angle FA tends to increase as the protrusion amount PR increases. Furthermore, it can be seen that interfacial fracture occurs when the protrusion angle FA is less than 52 degrees, plug fracture or base material fracture occurs in the range of 52 to 115 degrees, and only base material fracture occurs at 115 degrees or more. In other words, it has been confirmed that the protrusion angle FA is correlated with the fracture mode of the joint 3. If the fracture mode of the joint 3 is to be base material fracture, then it is sufficient to form a vertical wall portion 43 with a protrusion angle FA of 115 degrees or more.

[0066] [Relationship between process conditions and the shape of the vertical wall portion] Next, we will illustrate an example of a test concerning the relationship between the process conditions when forming the joint 3 by the shoulder-first process and the shape of the vertical wall portion 43. Figure 17 is a graph showing the relationship between the protrusion angle FA (degrees) of the vertical wall portion 43 and the amount of tool 1 pressed into the overlapping portion 30 (mm). The sample used is the same as the sample used to obtain the graph in Figure 16. The plotting conditions are also the same as those for the graph in Figure 16. The position where the tool pressing amount = 0 mm corresponds to the position of the boundary portion 33.

[0067] The graph in Figure 17 shows that the deeper the tool 1 is pressed into the overlapping portion 30, the larger the protruding angle FA of the vertical wall portion 43 becomes. In other words, by pushing the tool 1 beyond the boundary portion 33 to near the upper end of the second member 32, a vertical wall portion 43 that is more perpendicular to the boundary portion 33 can be formed. The fracture mode of the joint 3 is the same as in the example in Figure 16. The larger the protruding angle FA, the more likely it is that the fracture mode will be base material fracture.

[0068] Figure 18 is a graph showing the relationship between the amount of tool 1 pressed in (mm), the amount of vertical wall protrusion (mm), and the vertical wall protrusion angle (degrees) when the rotational speed of the shoulder 12 is changed during the press-fitting process of the shoulder 12 in process P12 of the shoulder-first process. In Figure 18 and the graphs in Figures 19 to 21 shown later, a negative number on the horizontal axis of the graph for the amount of tool 1 pressed in indicates that the tool press-fitting depth is shallower than the boundary 33 by that number, and a positive number indicates that the tool press-fitting depth is deeper than the boundary 33 by that number.

[0069] The rotation speeds of the shoulder 12 used in the test to obtain the graph in Figure 18 were 1000 rpm, 1500 rpm, and 2000 rpm. By pressing the shoulder 12 into the boundary portion 33 of the overlap portion 30 to a depth of -0.1 mm or +0.1 mm at these rotation speeds, it was possible to form a vertical wall portion 43 that met the required conditions. In other words, at the rotation speeds of the shoulder 12 used in the test, it was possible to form a stirring joint 4 having a vertical wall portion 43 that protrudes radially outward from the side surface 41S of the upper plate stirring layer 41 and has a protrusion angle FA of 55 degrees or more.

[0070] Figure 19 is a graph showing the relationship between the tool insertion depth (mm), the vertical wall protrusion depth (mm), and the vertical wall protrusion angle (degrees) when the upward speed of the pin 11 is changed during the shoulder press-fitting process. The upward speeds of the pin 11 used in the test were 10 mm / s and 20 mm / s. At either upward speed of the pin 11, a vertical wall 43 that met the required conditions could be formed by press-fitting the shoulder 12 to a depth of -0.1 mm or +0.1 mm at the boundary 33 of the overlapping portion 30. It was confirmed that when the tool insertion depth = -0.1 mm, setting the upward speed of the pin 11 to 20 mm / s was advantageous for forming a vertical wall 43 with a larger protrusion depth. It was also confirmed that increasing the tool insertion depth and increasing the upward speed of the pin 11 tended to increase the protrusion angle FA.

[0071] Figure 20 is a graph showing the relationship between the amount of tool 1 pressed in (mm), the amount of vertical wall protrusion (mm), and the vertical wall protrusion angle (degrees) when the descent speed of the pin 11 is changed during the backfilling process of process P13. The descent speeds of the pin 11 used in the test were 10 mm / s and 20 mm / s. At either pin 11 descent speed, a vertical wall 43 satisfying the required conditions could be formed by press-fitting the shoulder 12 to a depth of -0.1 mm or +0.1 mm at the boundary 33 of the overlapping portion 30. From the graph in Figure 20, it was confirmed that the protrusion angle FA tends to increase as the descent speed of the pin 11 increases. Note that a vertical wall 43 satisfying the required conditions could not be formed at a press-fitting depth of -0.4 mm.

[0072] Figure 21 is a graph showing the relationship between the amount of tool 1 pressed in (mm), the amount of vertical wall protrusion (mm), and the vertical wall protrusion angle (degrees) when the pressing pressure of the pin 11 is changed during the backfilling process of process P13. The total pressing pressure of tool 1 used in the test was 9 kN and 11 kN. The total pressing pressure is the pressure applied to the three components: the pin 11, the shoulder 12, and the clamp 13, during the backfilling process. With any of the total pressures of the pin 11, a vertical wall 43 that met the required conditions could be formed by pressing the shoulder 12 to a depth of -0.1 mm or +0.1 mm at the boundary 33 of the overlapping portion 30. However, a vertical wall 43 that met the required conditions could not be formed at a pressing depth of -0.4 mm.

[0073] [Summary of this disclosure] The specific embodiments described above include disclosures having the following configurations.

[0074] A joint according to a first aspect of the present disclosure comprises an overlapping portion where a first member and a second member overlap, and a stir joint portion provided in the overlapping portion for joining the first member and the second member by friction stir point joining, wherein at least one of the first member and the second member is an aluminum casting, and the stir joint portion includes a modified layer derived from the aluminum casting formed by friction stir of the aluminum casting, and the modified layer has vertical wall portions facing the ends of the overlapping boundary portion between the first member and the second member.

[0075] Aluminum castings consist of an alloy of aluminum (Al) and silicon (Si), and contain an α-Al phase and a eutectic phase. The α-Al phase is a phase in which aluminum crystallizes exclusively. The eutectic phase is a Si-enriched phase in which silicon and aluminum crystallize, with a high silicon concentration. According to the first embodiment, the friction-churned portion of the aluminum casting is a modified layer in which the α-Al and Si-enriched phases are mixed, and the Si-enriched phase, which is a brittle portion, disappears. When a crack begins to occur at the overlapping portion causing interfacial expansion between the first and second members, the crack abuts against the vertical wall of the modified layer. As a result, the crack stops extending along the direction of extension of the overlapping boundary between the first and second members. On the other hand, the base material of an aluminum casting that has not been friction-churned contains a Si-enriched phase. As a result, the crack propagates from the vertical wall along the Si-enriched phase of the base material. In other words, the vertical wall guides the crack propagation direction towards the Si-enriched phase of the base material, changing the crack from one caused by interfacial expansion to one caused by base material fracture. Eventually, base material fracture occurs, but the load required to reach fracture is greater than that required for interfacial fracture. Therefore, a joint with superior strength can be provided.

[0076] The joint according to the second embodiment is a joint according to the first embodiment, wherein the vertical wall portion has a vertex at the position where it is in contact with the end of the boundary portion, a first straight line along the plane extending from the vertex toward the first member, and a second straight line along the plane extending from the vertex toward the second member, and the angle between the first straight line and the second straight line at the vertex is in the range of 52 degrees or more and 180 degrees or less.

[0077] According to the second embodiment, the vertical wall portion intersects the boundary between the first member and the second member at a relatively large angle with respect to the extending direction of the boundary. Therefore, the vertical wall portion can be made more effective at stopping cracks that expand at the interface. In particular, if the angle is close to 180 degrees, the vertical wall portion becomes a surface that is nearly perpendicular to the extending direction of the boundary, making it easier to stop crack propagation. On the other hand, if the angle is less than 55 degrees, the crack tends to propagate along the vertical wall portion, making it difficult to guide the crack propagation to the Si-enriched phase of the base material.

[0078] The joint according to the third embodiment is the joint according to the first or second embodiment, wherein the modified layer includes a bulging portion that bulges radially outward from the other part of the stirred joint, and the surface of the bulging portion is the vertical wall portion.

[0079] Experiments conducted by the present inventors confirmed that both the inclusion of a bulging portion of the modified layer that extends radially outward, and the large amount of this bulging, contribute to improving the fracture strength of the joint and optimizing the fracture mode. It is believed that the greater the amount of radial outward bulging of the modified layer, the more easily a vertical wall portion is formed that can stop cracks from interfacial expansion. According to the third embodiment, a joint with even greater strength can be provided.

[0080] The joint according to the fourth embodiment is a joint according to the third embodiment in which the surface of the bulging portion has the shape of an arc that bulges approximately radially outward in a cross-section along the overlapping direction of the overlapping portion, and the boundary portion is opposite to the apex of the arc or its vicinity.

[0081] According to the fourth embodiment, by forming a bulging portion having the shape of a circular arc that expands radially outward, a vertical wall portion can be formed that is more effective at stopping cracks due to interface expansion.

[0082] The joint according to the fifth embodiment is a joint according to any of the first to fourth embodiments, wherein the friction stir point joint is formed by press-fitting a double-acting friction stir point jointing tool into the overlapping portion, the first member is positioned on the side into which the tool is first pressed, and the second member is positioned on the side into which the tool is last pressed, the first member being a wrought material and the second member being an aluminum casting.

[0083] According to the fifth embodiment, the joint between the wrought material and the aluminum casting can be formed as a joint that has high strength and an intended fracture mode.

[0084] In the joint according to the sixth embodiment, the thickness ratio of the wrought material to the aluminum casting is selected from the range of 1:0.3 to 5 in the joint according to the fifth embodiment.

[0085] According to the sixth aspect, by setting the thickness ratio within the above range, a joint can be provided in which the wrought material does not easily break.

[0086] The joint according to the seventh embodiment is a joint according to any of the first to sixth embodiments, wherein the vertical wall portion is a surface that converts the crack of interface expansion that occurred at the overlapping boundary portion into a crack of base material fracture of the aluminum casting.

[0087] According to the seventh embodiment, the vertical wall portion can change the fracture mode of the joint to a base material fracture with higher fracture strength.

[0088] A method for manufacturing a joint according to the eighth aspect includes: overlapping a first member and a second member, at least one of which is an aluminum casting, to form an overlapping portion; and pressing a friction stir point joining tool, which includes a pin and a shoulder having a hollow portion into which the pin is inserted, into the overlapping portion and performing friction stirring to form a stirred joint that joins the first member and the second member by friction stir point joining, wherein forming the stirred joint includes forming a modified layer derived from the aluminum casting by friction stirring of the aluminum casting, and forming a vertical wall portion in the modified layer that faces the overlapping boundary between the first member and the second member.

[0089] According to the eighth aspect, a modified layer containing a mixture of α-Al and a Si-enriched phase can be formed in an aluminum casting by friction stirring by press-fitting a friction stir point joining tool including a pin and a shoulder. The modified layer has vertical walls facing the edge of the overlapping boundary between the first member and the second member. The vertical walls can stop the extension of a crack that causes interfacial fracture between the first member and the second member at the overlapping portion.

[0090] The method for manufacturing a joint according to the ninth embodiment is, in the manufacturing method according to the eighth embodiment, to form the overlapping portion, the first member made of a wrought material is placed on the side to which the tool is first pressed in, and the second member made of an aluminum casting is placed on the side to which the tool is last pressed in, and to form the stirred joint portion, the stirred joint portion is formed by a shoulder-first process in which the shoulder of the tool is pressed into the overlapping portion ahead of the pin, and the modified layer is formed by pressing the tip of the shoulder into the first member up to a position immediately before the overlapping boundary between the first member and the second member, or into the second member up to a position immediately after crossing the overlapping boundary, thereby forming the modified layer which includes a bulging portion that bulges radially outward from the other parts of the stirred joint.

[0091] According to the ninth aspect, a modified layer including a bulging portion can be formed by the pressure in the backfilling step of the friction-stirred material in the shoulder-first process, or by the pressure in the pin-pressing step.

[0092] 1 Tool (friction stir point bonding tool) 11 Pin 12 Shoulder 3 Joint 30 Overlap 31 First member 32 Second member 33 Boundary 34 α-Al phase 35 Si-enriched phase 4 Stirred joint 41 Upper plate stir layer 42 Modified layer 43 Vertical wall BB Interface fracture MB Base material fracture L1, L2 First straight line, second straight line

Claims

1. A joint comprising: an overlapping portion where a first member and a second member overlap; a stir joint portion provided in the overlapping portion for joining the first member and the second member by friction stir point joining, wherein at least one of the first member and the second member is an aluminum casting; the stir joint portion includes a modified layer derived from the aluminum casting formed by friction stirring of the aluminum casting; and the modified layer has vertical wall portions facing the ends of the overlapping boundary portion between the first member and the second member.

2. The joint according to claim 1, wherein the vertical wall portion has a vertex at the position where it contacts the end of the boundary portion, a first straight line along the plane extending from the vertex toward the first member, and a second straight line along the plane extending from the vertex toward the second member, and the angle between the first straight line and the second straight line at the vertex is in the range of 52 degrees or more and 180 degrees or less.

3. The joint according to claim 1, wherein the modified layer includes a bulging portion that bulges radially outward from the other portion of the stirred joint, and the surface of the bulging portion is the vertical wall portion.

4. The joint according to claim 3, wherein the surface of the bulging portion has the shape of an arc that bulges approximately radially outward in a cross-section along the overlapping direction of the overlapping portion, and the boundary portion is opposite to the apex of the arc or in its vicinity.

5. A joint according to any one of claims 1 to 4, wherein the friction stir point joint is formed by press-fitting a friction stir point joining tool, which includes a pin and a shoulder having a hollow portion into which the pin is inserted, into the overlapping portion, the first member being positioned on the side into which the tool is first press-fitted, and the second member being positioned on the side into which the tool is last press-fitted, the first member being a wrought material and the second member being an aluminum casting.

6. The joint according to claim 5, wherein the thickness ratio of the wrought material to the aluminum casting is selected from the range of 1:0.3 to 5.

7. A joint according to any one of claims 1 to 4, wherein the vertical wall portion is a surface that converts a crack of interface expansion generated at the overlapping boundary portion into a crack of base material fracture of the aluminum casting.

8. A method for manufacturing a joined body, comprising: overlapping a first member and a second member, at least one of which is an aluminum casting, to form an overlapping portion; pressing a friction stir point joining tool, which includes a pin and a shoulder having a hollow portion into which the pin is inserted, into the overlapping portion and performing friction stirring to form a stirred joint that joins the first member and the second member by friction stir point joining; wherein forming the stirred joint comprises forming a modified layer derived from the aluminum casting by friction stirring of the aluminum casting; and forming a vertical wall portion in the modified layer that faces the overlapping boundary between the first member and the second member.

9. A method for manufacturing a joint according to claim 8, wherein forming the overlap portion is done by arranging the first member, made of a wrought material, on the side into which the tool is first pressed, and the second member, made of an aluminum casting, on the side into which the tool is last pressed; forming the agitated joint portion is done by a shoulder-first process, in which the shoulder of the tool is pressed into the overlap portion ahead of the pin; and the modified layer is formed by pressing the tip of the shoulder into the first member up to a position immediately before the overlap boundary between the first member and the second member, or into the second member up to a position immediately after the overlap boundary, thereby forming a bulging portion that bulges radially outward from the other parts of the agitated joint.

Images