JP2020153001A - Manufacturing method of aluminum alloy member - Google Patents

Abstract

To provide a manufacturing method of an aluminum alloy member in which characteristics is locally modified.SOLUTION: The present invention relates to the manufacturing method of an aluminum alloy member having a modified portion different in the characteristics from the surrounding portion in a base material made of an aluminum alloy. Specifically, a first electrode (1) made of a first metal and a second electrode (2) made of a second metal are energized therebetween while the first electrode (1) and the second electrode (2) are brought into pressurized contact with a local portion of a base material (W). Thus, a modification step of forming a modified portion in a joule-heated local portion is performed. According to the manufacturing method of the present invention, the first metal constituting the first electrode and the second metal constituting the second electrode are set to be larger in the electric conductivity than the aluminum alloy constituting the base material. Thus, the local portion is rapidly heated efficiently, and can be heated to the temperature needed to change the characteristics (metallurgical texture) in a very short time. Thus, an aluminum alloy member having a modified portion where only the local characteristics is modified can be efficiently obtained.SELECTED DRAWING: Figure 1

Description

The present invention relates to a method for manufacturing a locally modified aluminum alloy member and the like.

The required characteristics (mechanical properties) often differ depending on the part. For example, even a high-strength member may be required to have high ductility locally. To give a specific example, the part to be joined using a self-piercing rivet (Self Piercing Rivet / simply referred to as "SPR") that does not require drilling work (that is, self-piercing type) is highly ductile in order to prevent cracking and the like. It is hoped that there will be. A description related to such high ductility is found in the following patent documents.

Japanese Unexamined Patent Publication No. 2010-90459 JP-A-2018-94621 Japanese Unexamined Patent Publication No. 2004-124151 Japanese Unexamined Patent Publication No. 11-256227

Patent Document 1 proposes that a die-cast member made of an Al—Si alloy having an adjusted composition is heat-treated (solution heat treatment, aging treatment) to improve ductility, thereby suppressing cracking that occurs when the SPR is struck. are doing.

However, in Patent Document 1, it has sufficient characteristics without heat treatment except for the joint portion. Nevertheless, the entire casting is heat-treated only to prevent cracking due to local deformation when the SPR is struck. Further, in order to prevent blister (expansion of high-pressure gas nest) that occurs during heat treatment, a special high-vacuum die casting is performed. Therefore, the method as in Patent Document 1 causes a decrease in the physical strength of the entire die casting member, an occurrence of thermal strain, an increase in manufacturing cost, and the like.

Patent Document 2 proposes a method for manufacturing an aluminum alloy member in which only a joint portion where SPR is studded or the like is selectively made highly ductile. Specifically, the volume fraction (volume fraction) of primary crystal Al at the joint portion is increased by local pressurization during die casting, and local high ductility is realized. Since the wall thickness and the decrease in hot water temperature differ depending on the part, the pressure control and the mold structure during the casting process tend to be complicated.

Patent Document 3 aims at improving the press formability of a cold-rolled sheet, and proposes that a heated metal block is brought into contact with a portion having a large plastic deformation. As a result, the vicinity of the surface of the aluminum alloy is softened. However, since the amount of heat dissipated from the metal block and the amount of heat diffused to the aluminum alloy side are large and the temperature of the metal block drops significantly, it is not possible to sufficiently heat only the local part to the inside by that method.

Patent Document 4 proposes that the valve-to-valve portion of a cylinder head made of an aluminum alloy is remelted by energization heating and then rapidly cooled and solidified to reinforce it. The heating of the valve-to-valve portion is due to the self-resistance heat generation of the high-resistance carbon electrode and the resistance heat generation at the contact interface. Therefore, Patent Document 4 does not utilize Joule heat generation of the aluminum alloy itself constituting the valve-to-valve portion.

The present invention has been made in view of such circumstances, and an object of the present invention is to provide a method for manufacturing an aluminum alloy member whose characteristics are locally changed by a method different from the conventional method.

As a result of diligent research to solve this problem, the present inventor came up with the idea of modifying a local part of a base material made of an aluminum alloy by generating Joule heat, and actually realized this. By developing this result, the present invention described below has been completed.

<< Manufacturing method of aluminum alloy parts >>
(1) The present invention is a method for manufacturing an aluminum alloy member having a modified portion whose characteristics are different from those of the peripheral portion in a base material made of an aluminum alloy, which comprises a first electrode made of a first metal and a second metal. By energizing between the first electrode and the second electrode while pressing the second electrode against the local part of the base material, the local part is Joule-heated to form the modified part in the local part. The first metal and the second metal are a method for producing an aluminum alloy member having a higher electrical conductivity than the aluminum alloy.

(2) In the case of the production method of the present invention, the metal constituting each electrode has a higher electrical conductivity than the aluminum alloy (simply referred to as "Al alloy") constituting the base material to be heated. Therefore, when energized, the local part of the base material preferentially generates Joule heat over the electrode. As a result, the local area is efficiently and rapidly heated, and the temperature is raised to the temperature required for the change in characteristics (metal structure) in an extremely short time. In this way, according to the manufacturing method of the present invention, an aluminum alloy member having a modified portion in which only local characteristics are changed while avoiding deterioration of overall characteristics (strength, etc.) and occurrence of thermal strain (simply "Al"). "Alloy member") can be obtained efficiently.

(3) Furthermore, by relatively changing the form (shape, size, etc.) and material (electrical conductivity, thermal conductivity, etc.) between both electrodes, it is possible to control the characteristic level and characteristic distribution in the modified part. It becomes. For example, it is possible to adjust the hardness of the modified portion over the entire thickness direction and the hardness distribution in the thickness direction.

《Aluminum alloy member》
The present invention can also be grasped as an Al alloy member obtained by the above-mentioned manufacturing method. An example of this is an Al alloy member having a modified portion having a hardness distribution in which the position where the hardness is the minimum or the minimum in the thickness direction is biased toward the surface side from the center of the thickness. Further, an example thereof is a composite member in which another member is mechanically joined (for example, SPR joining) to a modified portion (high ductility portion) having high ductility. When the modified portion having high ductility is used as the joint portion, cracks and the like at the joint portion can be suppressed while maintaining the overall strength and the like.

<< Manufacturing method of metal parts >>
The present invention can be further grasped as a method for manufacturing a metal member based on the above-mentioned contents. That is, the present invention is a method for manufacturing a metal member having a modified portion in a metal base material having a characteristic different from that of a peripheral portion, and by energizing the electrode while pressurizing a local portion of the base material with the electrode. It can also be grasped as a method for manufacturing a metal member having a reforming step of heating the local part with Joule to form the reformed part in the local part, or a metal member obtained by the manufacturing method. The metal member is not limited to an Al alloy member (casting, plastically worked product, machined product, etc.), but may be an Mg alloy member, a Ti alloy member, an Fe-based (cast iron, steel) member, or the like.

<< Other >>
(1) The "base material" referred to in the present specification may be in any form as long as it can be energized from the pressure-welded electrodes. The "peripheral part" is a part that is in the peripheral area of the modified part and is not affected by energization and heating. Since the peripheral portion has the original characteristics of the base material, it may be paraphrased as the base portion. The local portion may be in any form (shape, size, etc.) as long as the electrode is a part of the base material to be pressure-contacted. The "modification part" may be substantially the same as the local part, or may be a part of the local part.

"Characteristics" can be selected according to the required specifications, but are mainly mechanical properties (ductility, toughness, strength, hardness, etc.). The characteristics should reflect changes (modification) in the metallographic structure. However, even if the change in the metal structure is not clearly observed before and after the reforming step, it is sufficient if the local characteristics change before and after the reforming step.

The "first", "second" and the like referred to in the present specification are names for convenience of explanation. However, when there is a surface side (treatment side) that is intended to be modified, the surface side may be appropriately set as the first electrode side.

(2) Unless otherwise specified, "x to y" in the present specification includes a lower limit value x and an upper limit value y. A range such as "ab" may be newly established with any numerical value included in the various numerical values or numerical ranges described in the present specification as a new lower limit value or upper limit value. Further, "x to ymsec" in the present specification means xmsec to ymsec. The same applies to other unit systems (A / mm ² , W / m · K,% IACS, μm, etc.).

It is explanatory drawing which shows typically the mode that the base material is energized and heated by the electrode pair. It is a schematic diagram which shows an example of a pressurization pattern and an energization pattern. It is a graph which shows the relationship between the depth from the surface, and the hardness of the modified part formed by using electrodes having different contact areas. It is a graph which shows the relationship between the depth from the surface and the hardness of the modified part formed by using electrodes having different electric conductivitys. It is a photograph which shows an example of the metal structure before and after the reforming process. It is a scatter diagram which shows the relationship between the electric conductivity and the thermal conductivity of a metal material.

One or more components arbitrarily selected from the present specification may be added to the components of the present invention described above. The contents described in the present specification appropriately apply not only to the production method of the present invention but also to Al alloy members and the like. Whether or not which embodiment is the best depends on the target, required performance, and the like.

"electrode"
(1) Form The electrode is composed of a pair of electrodes of a first electrode and a second electrode that are pressure-contacted with a base material (local part) to be modified. Each electrode may have the same or different form and material (electrical conductivity, thermal conductivity, etc.). Further, the electrode may also take various forms depending on the form of the base material. For example, each electrode does not necessarily have to be a pair of opposing electrodes arranged coaxially if energization is possible. Further, the contact surface (referred to as the tip surface) with the base material may be a flat surface or a curved surface, and one may be a flat surface and the other may be a curved surface.

The electrode may be a dedicated product, but if a general-purpose product such as a spot welding electrode is used, the electrode cost can be reduced. Many basic shapes of electrodes are specified in JIS C9304 (1999). For example, there are a flat type (F type), a radius type (R type), a dome type (D type), a dome radius type (DR type), a truncated cone type (CF type), a truncated cone radius type (CR type), and the like. If the local part of the base material is flat, an F-shaped electrode having a flat tip surface may be used. Spot welding electrodes are usually cylindrical or cylindrical with a hollow interior. An electrode having a recessed recess on the tip surface side may be used. The contact surface with the local part is, for example, substantially circular, substantially spherical, or substantially annular, depending on the pressing force. Of course, the electrode may be a prismatic shape or the like, and at this time, the contact surface has a substantially polygonal shape or a substantially pyramidal pyramidal shape.

The electrode may be detachable from the shank (cap tip type) or integrated with the shank (integrated type). By using a cap tip type electrode (also referred to as “electrode tip”), the cost of the reforming process can be reduced.

(2) Contact area The contact area between the electrodes and the base material may be the same or different between the pair of electrodes. By intentionally changing the contact area, the current density distribution in the local area can be changed when energized. As a result, the temperature distribution in the modified portion to be Joule-heated and thus the metal structure distribution also changes, and it is possible to form a modified portion having a desired characteristic distribution (for example, hardness distribution).

For example, the first contact area between the first electrode and the base material is made smaller than the second contact area between the second electrode and the base material. In this case, the current density in the local area when energized becomes large on the first electrode side, and a high temperature region is more likely to be formed on the first electrode side than on the second electrode side. As a result, for example, it is possible to form a modified portion in which the position where the hardness is the minimum or the minimum is closer to the first electrode side.

The contact area ratio (S1 / S2), which is the ratio of the first contact area (S1) to the second contact area (S2), is preferably 0.2 or more, 0.25 or more, and further 0.35 or more. If the contact area ratio is too small, local overheating, melting, etc. may occur due to current concentration. If you dare to say, the contact area ratio should be 0.7 or less and even 0.5 or less. When the contact area ratio is excessive (close to 1), there is no big difference between when the first contact area and the second contact area are substantially the same (when the contact area ratio is 1).

Incidentally, the "contact area" referred to in the present specification is an area where the tip surface of the electrode and the surface of the base material (local part) are considered to be in actual contact. If both sides in contact are flat surfaces, the area of the tip surface of the electrode may be the contact area. When at least one of the tip surface of the electrode and the surface of the base material is a curved surface, the horizontally projected area of the dent appearing at the contact portion between the electrode and the base material is substituted as the contact area.

(3) Electrical Conductivity The electrical conductivity of the metal (also referred to as “electrode material”) constituting the electrode may be the same or different between the pair of electrodes. By intentionally changing the electrical conductivity, the current density distribution in the local area can be changed. As a result, the temperature distribution in the modified portion to be Joule-heated and thus the metal structure distribution also changes, and it is possible to form a modified portion having a desired characteristic distribution (for example, hardness distribution).

For example, the electric conductivity of the first metal is made smaller than the electric conductivity of the second metal. In this case, the current density in the local area when energized becomes large on the first electrode side, and a high temperature region is more likely to be formed on the first electrode side than on the second electrode side. As a result, for example, it is possible to form a modified portion in which the position where the hardness is the minimum or the minimum is closer to the first electrode side.

The electric conductivity ratio (σ1 / σ2), which is the ratio of the electric conductivity (σ1) of the first metal to the electric conductivity (σ2) of the second metal, is 0.2 or more, 0.35 or more, and further 0.45 or more. It is good to do. If the electrical conduction ratio is too small, the amount of heat generated by the first electrode itself increases, which is inefficient. If you dare to say it, the electrical conduction ratio should be 0.8 or less and even 0.6 or less. When the electric conduction ratio is excessive (close to 1), there is no big difference from when the electric conduction ratio is 1.

Incidentally, the electric conductivity (conductivity) is the reciprocal of the resistivity (specific resistance), and "S / m" is used in the International System of Units (SI). However, in this specification, as appropriate, in conformity with Japan Powder Metallurgy Industry Standard JPMA-5, internationally adopted have been annealed standard soft copper (IACS: international annealed copper standard) of the specific resistance (1.7241 × 10 ^-2 The electrical conductivity is indexed by "% IACS", which is a ratio to the electrical conductivity (100% IACS) corresponding to μΩm).

The reason why the local heat generation distribution changes depending on the electrical conductivity of the electrode material is considered as follows. As shown in FIG. 6, in the metal material, the electric conductivity and the thermal conductivity have a substantially proportional relationship. Therefore, the electrode material having a small electric conductivity also has a small thermal conductivity, and conversely, the electrode material having a large electric conductivity also has a large thermal conductivity. For example, the first metal having a small electric conductivity has a small thermal conductivity, and the amount of heat radiated from the first electrode is also small. As a result, a high temperature region is more likely to be formed on the first electrode side than on the second electrode side. The electric conductivity and thermal conductivity shown in FIG. 6 are plotted based on the data extracted from the aluminum material database of the Japan Aluminum Association.

The electrode material is preferably a metal suitable for energizing a large current and applying a pressing force. For example, in addition to pure copper (oxygen-free copper, tough pitch copper, phosphorus deoxidized copper, etc.), chromium copper, zirconium copper, chromium / zirconium copper, alumina-dispersed copper, beryllium copper, etc. are used as electrode materials. Such an electrode material is selected in accordance with, for example, JIS Z3234 (2 types) or Group A (Class 2) of RWMA (Resistance Welder Manufacturer's Association). A typical example of the electrode material is chromium copper (Cr: 0.5 to 1.4% by mass, Cu: balance) having excellent electrical conductivity and strength. The electrical conductivity of such a copper-based electrode material is, for example, 55-95% IACS, 60-90% IACS, and even 65-85% IACS.

Other electrode materials include tungsten (W), molybdenum (Mo), copper tungsten (Cu-W), silver tungsten (Ag-W), heavy alloy (W-Ni-Cu-based material), W-TiN-based material, etc. is there. Since these electrode materials have a relatively small electric conductivity, they become the first metal (electrode material) when the electric conductivity ratio is less than 1 (σ1 / σ2 <1). The electrical conductivity of such an electrode material is, for example, 5 to 55% IACS, 10 to 50% IACS, and even 15 to 45% IACS.

(4) Others When the first contact area and the second contact area are different, the first metal and the second metal may have the same or different electric conductivity. Similarly, when the electric conductivity of the first metal and the electric conductivity of the second metal are different, the first contact area and the second contact area may be the same or different. The contact area and electrical conductivity of each electrode may be freely combined according to the desired local characteristic distribution.

By the way, when the electric conductivity or the contact area between the pair of electrodes is substantially the same, the temperature tends to be high in the central region (local central portion) between the electrodes. Joule heat itself is generated along the shortest path (energization path) between the electrodes, but the influence of cooling from the electrodes and heat dissipation from the local surface is small in the central region. The term "substantially the same" as used herein means that one is 90% to 110% of the other.

Since each electrode material has a higher electrical conductivity than the base material (Al alloy), the self-heating of each electrode is smaller than the self-heating of the base material when energized. However, the electrode can also be heated to a correspondingly high temperature due to self-heating or heat transfer from the base material. Therefore, in order to secure the strength of the electrode itself, the local cooling property of the electrode, and the like, the electrode may be forcibly cooled (water-cooled or the like).

《Reforming process》
The reforming step is performed by energizing each electrode that is pressure-welded (contacted while being pressurized) to the local part of the base material. Here, pressurization of the electrodes on the base material and energization between the electrodes will be described.

(1) Pressurization The pressing force (pressure contact force) from the electrode to the local area can be adjusted according to the local surface condition, the composition and type of the Al alloy, and the like. The pressing force is, for example, 5 to 70 N / mm ² , 15 to 50 N / mm ² or 25 to 45 N / mm ² . If the pressing force is too small, the contact resistance increases. Further, when the base material is a casting, blister is likely to occur locally. When the pressing force becomes excessive, deformation of the base material (depression, etc.) and deformation of the electrodes (wear, buckling, etc.) are likely to occur.

(2) Energization The energization from the electrode to the local part can also be adjusted according to the local form, the composition and type of the Al alloy, the desired characteristics, and the like. The current densities are, for example, 50-600 A / mm ² , 100-500 A / mm ² or 200-450 A / mm ² . The energizing time is, for example, 100 to 4000 msec, 400 to 3000 msec, or 500 to 2000 msec. If the current density or energization time is too small, heating will be insufficient. If the current density or energization time becomes excessive, the deformation will increase and the liquid phase will appear.

The reforming step should be performed within a range that does not cause a liquid phase locally. That is, it is preferable to energize in a range where the local temperature is lower than the liquidus temperature. When a liquid phase is generated locally, the structure becomes coarse and it becomes difficult to control the characteristics locally. The energization may be performed in a range where the local surface temperature is, for example, 350 to 650 ° C, 400 to 600 ° C, and further 450 to 550 ° C.

The temperature below the liquidus line temperature may be lower than the temperature at which 0.2% by mass of the liquid phase of the Al alloy appears. The "temperature at which the liquid phase of the Al alloy appears in an amount of 0.2% by mass" was obtained by performing a solidification calculation using the integrated thermodynamic calculation software "Thermo-Calc" for each Al alloy (composition). It is obtained from the relationship between the liquid phase ratio and the temperature.

The "local surface temperature" referred to in the present specification is a measurement of the surface temperature in the portion where the electrodes are in contact (electrode contact portion) or in the immediate vicinity thereof with a thermocouple installed in the vicinity thereof (for example, a K thermocouple). Warm and seek. The maximum temperature should be within the above range.

(3) Treatment pattern There are various forms in which the pressing force changes (referred to as "pressurization pattern") and the current density changes (referred to as "energization pattern") from the start to the end of the reforming process. possible. For example, there is a square pattern or a trapezoidal pattern (referred to as "basic pattern") in which the pressing force and the current density are maintained at a constant value except immediately after the start or just before the end.

An irregular pattern in which the pressing force and / or the current density changes in the middle may be adopted. The anomalous pattern may be a cycle pattern in which the current density or the like changes at regular intervals when one local area is heated (that is, during one reforming step), and this is performed in a plurality of cycles. Anomalous patterns usually only need to change the current density, which is easy to control.

《Modified part》
The modified portion may be one in which the characteristics of the local part (base material) before modification have changed. Its characteristics and degree are adjusted as appropriate. The characteristics are mechanical properties such as ductility, toughness, elongation, and strength, and hardness is a typical index thereof.

The modified portion may be, for example, a ductile portion having a higher ductility than the peripheral portion. The ductile portion has, for example, a smaller surface hardness and an average hardness in the thickness direction than the peripheral portion. Further, the modified portion may have a hardness distribution in which the position where the hardness is the minimum or the minimum in the thickness direction is biased toward the surface side from the center of the thickness. Such a ductile portion is preferable as a portion (joining portion) for joining another member to the Al alloy member by a mechanical method such as SPR or a bolt.

The hardness referred to in the present specification is measured by using a micro Vickers hardness tester (for example, MVK-E manufactured by Akashi Seisakusho Co., Ltd.) for a cross section including a target portion. The measurement is performed using an observation piece polished to a mirror surface, with a load of 100 g and a load time of 15 seconds. The Vickers hardness measured in this way is indicated by "HV0.1". The hardness is measured from a position deeper than 200 μm from the surface. The hardness of the modified portion is measured along the normal direction extending from the vicinity of the center of the outermost surface of the local portion where the tip surface of the electrode is in contact.

Unless otherwise specified, the hardness distribution is indicated by Vickers hardness measured at intervals of 200 μm pitch. The average hardness is the arithmetic (additional) average value of each hardness measured at intervals of 200 μm pitch from the surface to the center of thickness (center of wall thickness) extending in the normal direction. The surface hardness shall be the hardness measured at a depth of 200 μm from the surface.

The hardness of the reference peripheral portion (base: non-heated base material portion) shall be the hardness of the region not affected by local heating. Since the width of the heat-affected zone is at most about 1 to 2 mm, a region (position) separated from the outer peripheral edge (or inner peripheral edge) of the electrode contact portion by 2 mm or more to the outside (or inside) may be set as the peripheral portion.

When the local area is made highly ductile, the surface hardness of the modified portion is preferably 90% or less, 85% or less, and further 80% or less with respect to the surface hardness of the peripheral portion. Further, the minimum or minimum hardness of the modified portion is preferably 85% or less, 75% or less, and further 65% or less with respect to the surface hardness of the modified portion.

<< Al alloy member >>
(1) Base material The base material may be a casting made of an Al alloy or a wrought material. In any case, when the local part of the base material is Joule-heated, the characteristics (mainly mechanical properties) are improved by changing the metal structure and relaxing / releasing various strains introduced during manufacturing. ..

If the substrate is a cast, local ductility or toughness can be improved by the reforming step, depending on the Al alloy composition. The reason for this can be considered as follows. When the temperature of the casting is raised to a predetermined temperature or higher, the precipitated phase (Si, Mg ₂ Si, CuAl _2, etc.) in the metal structure (base) is re-solidified into α-Al. In addition, angular crystals (eutectic phase, etc.) crystallized during casting also change to a rounded shape (that is, spheroidized). Further, the internal strain accumulated during casting is also relaxed or eliminated. These things act additively or synergistically to soften (high ductility, high toughness).

In the reforming step, only the local temperature is rapidly raised to a desired temperature within a very short time, and then the local temperature is rapidly cooled through the electrode after the energization is completed. Therefore, the range in which the characteristic change occurs is limited to the contact portion of the electrode (tip surface) or the region extremely close to the contact portion (that is, the local region). As a result, it is possible to avoid a decrease in the physical strength of the Al alloy casting as a whole and the occurrence of thermal strain while softening the local part. Further, since the temperature rises while the local area is pressurized, even if the internal gas introduced at the time of casting is locally present, the generation of blister due to the internal gas is suppressed.

The casting may be die-cast, low-pressure cast, or gravity-cast. Further, the product is not limited to the mold-cast one, and may be a sand-cast one. The Al alloy for casting is, for example, an Al—Si based alloy, an Al—Si—Cu based alloy, an Al—Mg—Si based alloy, or the like. The Al—Si alloy is, for example, AC2B, AC4C, AC8A (JIS) or the like. The Al—Si—Cu based alloy is, for example, ADC10, ADC12 (JIS) and the like. The Al-Mg-Si based alloy is, for example, Magsimal-59 developed in Europe.

The casting may be made of a non-heat-treated alloy or a heat-treated alloy (Al—Cu alloy or the like). The cast made of the heat-treated alloy may be reformed after the desired heat treatment (this point also applies to the wrought material described later). As an example of an alloy for casting, for example, Si: 6 to 12%, optionally Mg: 0.1 to 1%, Mn: 0.2 to 1.2%, Fe: 0.05 to 1%, Cu : 1 to 5%, trace elements (total of Sr, Na, Sb, etc.): 0.003 to 0.05%, and there is an Al alloy in which the balance is Al and impurities.

The wrought material may be plastically processed such as press molding or forging, or may be machined. The wrought material may be an Al alloy of any type. Pure Al (JIS 1000 series) may be used, but an Al alloy system (JIS 2000 series to 7000 series) containing an alloy component as a base material for the reforming step is preferable. The wrought material is, for example, an Al alloy for wrought such as an Al—Cu based alloy (JIS 2000 series), an Al—Mg based alloy (JIS 5000 series), and an Al—Mg—Si based alloy (JIS 6000 series). It is good.

Even if the base material is a wrought material, if the Al alloy composition and the conditions of the reforming process (current density, energization time, etc.) are adjusted, ductility due to solid solution (solution hardening) and precipitation (aging) of solute elements A modified portion having improved toughness or strength can be formed. When a modification process is performed on a locally formed wrought material (base material) that has been plastically worked (especially cold plastic working), the modified part with improved ductility and toughness due to processing strain and release of residual stress, etc. can get.

Incidentally, as shown in FIG. 6, the wrought material (alloy for wrought) has a higher electric conductivity than the cast material (Al alloy for casting). Therefore, it can be said that the wrought material is less likely to generate heat by itself (it is less likely to be Joule heated) than the cast material. However, as described above, if the contact area and the electric conductivity of one of the electrodes are adjusted to increase the current density, the desired modification can be performed even in a local area made of a wrought material (base material). By the way, it is considered that the reason why the electric conductivity of the wrought material is high is that the metal structure is fine and homogeneous unlike the cast structure (see FIG. 5).

(2) Form and application There are various forms and applications of Al alloy members. The shape, size, number, etc. of the modified portion may be appropriately adjusted according to the form and application. The surface shape of each modified portion may be circular, square, square, or the like, and usually depends substantially on the shape of the tip surface of the electrode. Its size is, for example, about 10 to ¹ cm ^{2 and} further to about 5 to ² cm ² . When there are a plurality of modified portions, each modified portion may be arranged discretely or may be arranged continuously by partially superimposing them between adjacent portions.

The modified portion is, for example, a joint portion (including a planned joint portion) to which other members are mechanically joined. By using the softened or highly ductile modified portion for the joint portion, it is possible to suppress cracks during and after joining while maintaining the strength of the entire member.

The mechanical joining referred to in the present specification is performed by, for example, screws (bolts, nuts, etc.) or rivets. The rivet may be one that is crimped after being inserted into a preformed hole, or one that is self-perforated (SPR: Self Piercing Rivet). Cracking is prevented when the studs of the SPR with locally large plastic deformation are made in the softened or highly ductile modified portion.

In the case of SPR joining, the size of the modified portion may be, for example, 2 to 3 times the size of the head of the SPR. Specifically, for example, the diameter or one side of the modified portion may be a circular shape or a square shape of 15 to 25 mm. Such a modified portion is adjusted by the shape and size of the tip surface of the electrode.

The other member (member to be joined) to be mechanically joined may be the same material as the Al alloy member, but is usually a different material such as metal or resin. In the case of SPR joining, the member to be joined is preferably in the shape of a (thin) plate through which rivets can penetrate.

Various samples (Al alloy members) were produced, and their hardness distribution and metallographic structure were evaluated. The present invention will be described in more detail with reference to such specific examples.

<< Production of sample >>
(1) Base material As a base material (base material), a plate-shaped casting (100 mm × 30 mm × 3 mm) made of an Al alloy was prepared. This casting was made by die casting. The die casting was performed by injecting the prepared molten metal into the cavity of the mold made of tool steel (JIS SKD61). The casting conditions were hot water temperature: 630 ° C, mold temperature: 200 ° C, casting pressure: 40 MPa, and injection speed: 2 m / s.

The molten metal was prepared by dissolving the raw materials (metals, compounds) weighed to the following composition in the air. In the case of an Al alloy having the following composition, the temperature at which 0.2% by mass of the liquid phase appears is 572 ° C. The alloy composition is the mass ratio to the whole (molten metal) (100% by mass).
Alloy composition: Al-9% Si-0.21% Mg-0.38% Mn-0.12% Fe

(2) Pressurization / energization (reforming process)
As shown in FIG. 1, the central portion (local portion) of the above-mentioned base material w was sandwiched between a pair of electrodes 1 (first electrode) and electrode 2 (second electrode) to pressurize and energize. For each electrode, an F-shaped electrode tip having a flat tip surface was used. The size (outer diameter) or material of each electrode was variously changed as described later.

Pressurization and energization were performed by a spot welder (manufactured by ART-HIKARI Co., Ltd.). At that time, the surface temperature in the vicinity of the outer peripheral edge of the contact region between the electrode and the base material was appropriately measured by a K thermocouple.

The pressurization and energization patterns are shown in FIG. 2 (solid line portion). First, the tip surface of the electrode is brought into contact with the central surface of the base material, and a pressing force is applied from the electrode to the base material (section I). Next, in the pressurized state, energization is started from the electrode to the base material, and a constant current value is supplied for a predetermined time (section II). After the energization is completed, the pressing force from the electrode to the base material is removed to separate the electrode from the base material (section III). As shown by the broken line in FIG. 2, an irregular pattern in which the current value is changed during energization can also be adopted.

The energization time referred to in this embodiment is the time from the start of energization to the end of energization (the total time of section II in FIG. 2). The time when the current value is constant is about 90% of the energization time. Further, the pressing force and the current density referred to in this embodiment are values obtained by dividing the load and the current value applied from the electrode to the base material by the contact area between the electrode and the base material.

(3) Contact area Various chrome copper electrode tips (manufactured by SMK Corporation / 80% IACS) having different outer diameters (nominal diameters) on the tip side were prepared. The pair of electrodes selected from these was used as the electrode 1 and the electrode 2, and the above-mentioned pressurization and energization were performed. Table 1 also shows the current density, contact area ratio, and energization time of each electrode. The pressing force (load) between the electrodes was constant at 4 kN. Further, the area of the tip surfaces 1a and 2a of each electrode was defined as the contact area, and the area ratio thereof was defined as the contact area ratio. The current density shown in Table 1 was obtained by dividing the current value (constant control) at the time of energization by the contact area between the electrode and the base material.

Incidentally, the outer diameters of the electrodes 1 and 2 used in each sample are as follows.
Sample 11: φ10 mm (both electrodes have the same diameter), Sample 12: φ10 mm and φ15 mm,
Sample 13: φ8 mm and φ15 mm, Sample C1: φ6 mm and φ15 mm

(4) Electrical conductivity Various electrode chips made of different materials were prepared. The pair of electrodes selected from these was used as the electrode 1 and the electrode 2, and the above-mentioned pressurization and energization were performed. Table 2 also shows the ratio of the material (first metal, second metal) of each electrode to the electric conductivity (electrical conductivity ratio) and the energization time.

The outer diameter of each electrode: φ12 mm, the pressing force between the electrodes: 35.4 N / mm ² (load: 4 kN, both were constant. The current density shown in Table 2 is also the current value (constant) when energized. Control) was determined by dividing by the contact area between the electrode and the base material.

By the way, the electric conductivity of the electrode material used in each sample was as follows.
Cr copper: 80% IACS (Cr copper electrode manufactured by SMC Co., Ltd.)
W: 31% IACS (W electrode manufactured by Nippon Tungsten Co., Ltd.)
Graphite: 4% IACS (graphite electrode manufactured by Tokyo Carbon Industry Co., Ltd.)

《Measurement / Observation》
(1) Hardness The Vickers hardness (HV0.1) at each position was measured at each position using an observation piece produced from the test material of each sample by the method described above.

The hardness of each observation piece was measured at a position of about 200 μm from the outermost surface (reference: 0 μm). The hardness of the locally heated part (modified part) was measured from the surface to the inside near the center where the electrodes were in contact. In addition, the hardness of the observation piece (base material) of the sample C0 that was not energized and heated was measured in the same manner.

The measurement results for Samples 11 to 13 and Sample C1 in which the contact area ratio between the pair of electrodes was variously changed are shown in FIG. Further, FIG. 4 shows the measurement results of the samples 21 and 22 and the sample C2 in which the electric conduction ratio was variously changed between the pair of electrodes.

(2) Metal structure The local metal structure (sample 11) heated by energization and the metal structure (sample C0) of the unheated base material (base / peripheral part) are shown in comparison with FIG. For each metal structure, the observation pieces after mirror polishing were observed with an optical microscope (× 1000 times).

<< Evaluation >>
(1) Hardness As is clear from FIGS. 3 and 4, the local (modified portion) heated by energization is overall with respect to the base material (base / peripheral portion) before the treatment (sample C0). The hardness was reduced and it was softened (high ductility).

In addition, by changing the contact area ratio and the electrical conduction ratio, the position where the hardness is the minimum or the minimum is shifted from the center of the thickness to the surface side. Furthermore, it was confirmed that as the contact area ratio or the electrical conduction ratio was reduced, the position where the hardness was the minimum or the minimum became closer to the surface side, and the hardness at that position also tended to decrease.

(2) Metal structure As is clear from FIG. 5, it was found that the eutectic Si and the like were rounded in the local metal structure heated by energization with respect to the metal structure before the treatment. It is considered that such a change in the metal structure appears as the above-mentioned change in hardness.
(3) Others When the contact area ratio was made too small as in sample C1, the base material locally melted in the vicinity of the electrode 1 having a small contact area.

When the electric conductivity ratio was made too small as in sample C2, the electrode 1 itself having a small electric conductivity was overheated and became hot, and the heat transfer from the electrode 1 softened the vicinity of the local surface. However, since the Joule heat generation of the base material itself was small, the central side (inside) of the local part did not soften very much.

From the above, the local characteristics (for example, hardness) can be changed by the reforming process, and the change form of the characteristics (for example, hardness distribution) can be controlled by adjusting the contact area ratio and the electric conduction ratio. It was confirmed that it could be done.

Claims (14)

A method for manufacturing an aluminum alloy member having a modified portion in a base material made of an aluminum alloy having different characteristics from the peripheral portion.
The first electrode made of the first metal and the second electrode made of the second metal are brought into close contact with the local part of the base material, and the local part is heated by Joule by energizing between the first electrode and the second electrode. A reforming step for forming the reforming portion is provided in the local area.
The first metal and the second metal are a method for producing an aluminum alloy member having a higher electric conductivity than the aluminum alloy. The method for manufacturing an aluminum alloy member according to claim 1, wherein the reforming step does not generate a liquid phase locally. The method for manufacturing an aluminum alloy member according to claim 1 or 2, wherein the first contact area between the first electrode and the base material is smaller than the second contact area between the second electrode and the base material. The method for manufacturing an aluminum alloy member according to claim 3, wherein the contact area ratio (S1 / S2), which is the ratio of the first contact area (S1) to the second contact area (S2), is 0.2 or more. The method for producing an aluminum alloy member according to any one of claims 1 to 4, wherein the first metal and the second metal have substantially the same electrical conductivity. The method for manufacturing an aluminum alloy member according to any one of claims 1 to 4, wherein the electric conductivity of the first metal is smaller than the electric conductivity of the second metal. The aluminum alloy according to claim 6, wherein the electric conductivity ratio (σ1 / σ2), which is the ratio of the electric conductivity (σ1) of the first metal to the electric conductivity (σ2) of the second metal, is 0.2 or more. Method of manufacturing parts. The method for manufacturing an aluminum alloy member according to claim 1, 2, 6 or 7, wherein the first electrode and the second electrode have substantially the same contact area with the base material. The method for manufacturing an aluminum alloy member according to any one of claims 1 to 8, wherein the modified portion has higher ductility than the peripheral portion. The method for manufacturing an aluminum alloy member according to any one of claims 1 to 9, wherein the modified portion has at least a hardness smaller than that of the peripheral portion. The aluminum alloy member according to any one of claims 1 to 10, wherein the modified portion has a hardness distribution in which the position where the hardness is the minimum or the minimum in the thickness direction is biased toward the surface side from the center of the thickness. Production method. The method for manufacturing an aluminum alloy member according to any one of claims 1 to 11, wherein the modified portion is a joint portion to which another member can be mechanically bonded. The method for manufacturing an aluminum alloy member according to any one of claims 1 to 12, wherein the base material is a casting. The method for producing an aluminum alloy member according to any one of claims 1 to 12, wherein the base material is a plastic-processed wrought material.