JP2020063486A - Method for manufacturing aluminum alloy member - Google Patents

Abstract

To provide a method for manufacturing an aluminum alloy member which can suppress local crack generated in joining.SOLUTION: A method for manufacturing an aluminum alloy member includes a local heating step of Joule heating a local part of a cast of an aluminum alloy by performing energization from an electrode to a cast while pressurizing the local part with the electrode. Thereby, a softened part obtained by softening only the local part can be formed into the cast. A hardness of the softened part is, for instance, 90% or less with respect to a hardness of a base part not affected by Joule heating. In the case of die casting, circularity of eutectic Si in the softened part is 1.2 or more times as large as circularity of eutectic Si in the base part. When the softened part is used, for instance, even when a steel plate is joined to a cast by a self-piercing rivet (SPR), occurrence of cracking can be prevented.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a method for manufacturing an aluminum alloy member made of a cast aluminum alloy, and the like.

  A plurality of members having different shapes, materials, functions, etc. are often joined together in order to cope with complication, increase in size, multifunction, weight reduction, etc. of the members. The joining is performed by adhesion, welding, fastening, joining, etc. according to required specifications and the like. Among them, mechanical joining is often used, which can make strong joining relatively easily. For example, members that need to be attached and detached are fastened with screws (bolts, nuts, etc.), and members that need not be attached and detached are joined with rivets.

  If rivets are used, it is possible to easily join even dissimilar materials that cannot be welded. Recently, attention has also been paid to joining using a self-piercing rivet (simply referred to as “SPR”) that does not require a punching operation (that is, a self-piercing type) and can perform a strong coupling.

  For example, the following patent document describes a description relating to the bonding of a steel plate or the like by driving an SPR into a die cast casting (member) of an aluminum alloy.

JP, 2010-90459, A JP, 2008-94621, A JP, 2004-124151, A

  Patent Document 1 suppresses cracks generated during SPR driving by performing heat treatment (solution treatment, aging treatment) on a die-cast member (casting) made of an Al—Si alloy whose component composition is adjusted to enhance ductility. I suggest you do.

  However, in Patent Document 1, the heat treatment of the entire casting is performed only in order to prevent cracks due to local deformation when the SPR is driven, despite that the heat treatment is performed on the joints only, even though the heat treatment is performed on the joints. Is going. Moreover, in order to prevent blisters (expansion of high-pressure gas bubbles) that occur during the heat treatment, special high vacuum die casting is performed. Therefore, in the method of Patent Document 1, the physical strength of the entire die casting member is reduced, thermal distortion occurs, and the manufacturing cost increases.

  Patent Document 2 proposes a method of manufacturing an aluminum alloy member in which only the joint portion where SPR is driven into a tack or the like is selectively made highly ductile. Specifically, by locally applying pressure during die casting, the volume ratio (primary crystal ratio) of primary crystal Al in the joint is increased, and its ductility is increased.

  However, in the method of Patent Document 2, since the wall thickness and the decrease in the molten metal temperature differ depending on the part, it is not easy to control the pressure during the casting process and the die structure becomes complicated.

  Patent Document 3 proposes to bring a heated metal block into contact with the aluminum alloy to partially soften the aluminum alloy. However, Patent Document 3 is intended to improve the press formability of the cold rolled sheet, not to improve the bondability of the casting, and to soften a portion that undergoes large plastic deformation (for example, an annular flange portion) in advance. It's just a suggestion. Further, the method as disclosed in Patent Document 3 has a large amount of wasteful heat radiation from the metal block and is not suitable for heating a narrow local portion such as a joint portion to a desired temperature.

  The present invention has been made in view of such circumstances, and provides a manufacturing method and the like that can efficiently obtain an aluminum alloy member in which a local portion of an aluminum alloy casting is softened by a method different from the conventional method. The purpose is to do.

  As a result of earnest studies to solve this problem, the present inventor succeeded in efficiently softening the local part of the aluminum alloy casting by bringing the electrode into contact with the local part and Joule heating only the local part. By developing this result, the present invention described below has been completed.

<< Method of manufacturing aluminum alloy member >>
(1) The present invention comprises a local heating step of Joule heating the local portion of an aluminum alloy casting by energizing the electrode while pressing the local portion with an electrode, and a softening portion obtained by softening the local portion of the casting. Is a method for manufacturing an aluminum alloy member.

(2) According to the manufacturing method of the present invention, casting of an aluminum alloy (also referred to as “Al alloy”) (simply referred to as “casting”) is avoided while avoiding strength reduction, thermal strain, and the like that may occur as a whole. It is possible to efficiently obtain an aluminum alloy member (also referred to as “Al alloy member”) in which only a local portion is softened. If a softened portion having a higher ductility than the surroundings is used, for example, it is possible to join the Al alloy member and another member with stable quality while suppressing cracking and the like.

By the way, the reason why the local part can be efficiently softened (high ductility) by the manufacturing method of the present invention is considered as follows. First, when the temperature of the local part is raised to a predetermined temperature or higher, the precipitation phase (Si, Mg ₂ Si, CuAl ₂ etc.) in the metal structure (base) forming the local part is redissolved in α-Al. To be done. In addition, angular crystals that crystallized during casting (eutectic phase, etc.) also change into a rounded shape (that is, spheroidize). Further, the local internal strain accumulated during casting is relaxed or eliminated. These act additively or synergistically to soften the local area.

  Next, according to Joule heating that causes the Al alloy to self-heat, the local temperature can be rapidly raised to a desired temperature within an extremely short time. Then, after the completion of energization, the local portion can be cooled through the electrode that is in contact with the casting. In this way, heating of the periphery of the local portion due to heat conduction is suppressed during and after the local heating step according to the present invention. For this reason, the range of softening is limited to the contact portion of the electrode (tip surface) or the extremely close region (that is, the local portion). As a result, the local strength of the Al alloy member (Al alloy casting) as a whole is prevented from deteriorating and the occurrence of thermal strain is avoided while softening the local portion.

  In the local heating step of the present invention, the local area is heated while being pressurized. Therefore, even if the internal gas introduced at the time of casting is in the local area, it is possible to suppress the occurrence of blisters in the local heating step.

<< Aluminum alloy member or composite member >>
The present invention can also be grasped as an Al alloy member obtained by the manufacturing method described above. Furthermore, the present invention can be grasped as a composite member in which an Al alloy member and another member are joined, as one form utilizing the softened portion.

<< Metal member manufacturing method >>
The present invention can be further expanded to a method of manufacturing a metal member based on the above-mentioned contents. That is, the present invention comprises a local heating step of Joule heating the local portion by energizing the electrode while pressing the local portion with an electrode, and a method for manufacturing a metal member provided with a softened portion that softens the local portion, Alternatively, it can be grasped as a metal member obtained by the manufacturing method. The metal members are not limited to castings, but are Al alloy members, Mg alloy members, Ti alloy members, Fe-based (cast iron, steel) members, and the like.

《Others》
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 “a to b” may be newly established by setting any numerical value included in various numerical values or numerical ranges described in the present specification as a new lower limit value or upper limit value.

It is explanatory drawing which shows the mode of a local heating process typically. It is a schematic diagram which shows a pressurization pattern and an energization pattern. It is a graph which shows the heating temperature of a casting, and the relationship of hardness. It is a graph which shows the hardness distribution of an electrode contact part and its periphery. It is a graph which shows the hardness distribution of the depth direction in the casting center. It is a photograph showing the relationship between the presence or absence of local heating and the metal structure. It is a photograph which shows the influence on the metal structure by the presence or absence of pressurization at the time of local heating. It is a photograph showing how to evaluate the crack resistance during SPR driving. It is a photograph showing the presence or absence of through cracks at the time of SPR driving.

  One or more constituent elements arbitrarily selected from the specification may be added to the constituent elements of the present invention described above. The contents described in the present specification appropriately apply not only to the manufacturing method of the present invention, but also to Al alloy members, composite members using the same, and the like. Which of the embodiments is the best depends on the target, the required performance and the like.

<< Local heating process >>
The local heating step is performed by energizing the local area of the casting while pressing the local area of the casting with the electrode.

(1) Electrode The form of the electrode does not matter as long as it can efficiently Joule heat only a desired local portion (and the vicinity thereof). The electrodes are usually composed of a pair of members that sandwich the local pressure. However, the shape of the electrodes is not necessarily limited to the pair of electrodes facing each other as long as the local pressure can be applied and the local current can be supplied.

  The form, material, etc. of the electrode are appropriately selected according to the application and required specifications of the softening part. For the electrodes, general-purpose products such as spot welding electrodes may be used in addition to the dedicated products. The basic shape of the electrode is, for example, a flat shape (F shape), a radius shape (R shape), a dome shape (D shape), a dome radius shape (DR shape), as defined in JIS C9304 (1999). ), A truncated cone shape (CF type), and a truncated cone radius type (CR type). In the case of forming a flat softened portion, it is preferable to use an F-shaped electrode having a flat electrode tip surface in contact with the local portion. The spot welding electrode is usually cylindrical or columnar. In this case, the contact surface between the local portion and the electrode has a substantially circular shape or a substantially spherical shape. However, the electrodes used in the present invention may be prismatic or the like. In this case, the contact surface has a substantially polygonal shape or a substantially pyramidal surface cone shape.

  The electrode may be detachable from the shank (cap tip type) or integrated with the shank (integral type). When a cap tip type electrode (also referred to as “electrode tip”) is used, the cost of the local heating step can be reduced.

  A large current is applied and a pressure is applied by the electrodes. Therefore, the electrode is preferably made of a material having excellent conductivity (electrical conductivity) and strength. For example, in addition to pure copper (oxygen-free copper, tough pitch copper, phosphorous deoxidized copper, etc.), chromium copper, zirconium copper, chromium / zirconium copper, alumina dispersed copper, beryllium copper, etc. are used. Such an electrode material may be selected in accordance with JIS Z3234 (2 types) or Group A (Class 2) of RWMA (Resistance Welder Manufacturer's Association). A typical example is chromium copper (Cr: 0.5 to 1.4 mass%, Cu: balance), which has excellent electrical conductivity and strength.

  An electrode made of a material having high electrical conductivity (low resistivity) usually has excellent thermal conductivity. Therefore, in the locally heated Joule, the electrodes are cooled and rapidly cooled after the completion of energization. As a result, the effect of Joule heating is limited to the local area or its peripheral area.

  The electrode itself also becomes hot due to heat transfer from the casting side and self-heating during energization. In order to secure the strength of the electrode itself and the local cooling property of the electrode, it is preferable that the inside of the electrode is forcibly cooled (water-cooled or the like). As a result, the local heating process can be stably and continuously performed even during mass production.

(2) Pressurization The local pressure applied by the electrode may be adjusted according to the local form (thickness, etc.), the composition of the Al alloy, the casting method, the heating temperature (softening degree), etc. The applied pressure is, for example, 10 to 70 N / mm ² , 15 to 50 N / mm ² or 20 to 40 N / mm ² . If the applied pressure is too small, blisters are likely to occur locally. If the applied pressure becomes excessive, deformation of the casting (depression, etc.) and deformation of the electrode (wear, buckling, etc.) are likely to occur. It should be noted that the lower limit of the pressing force does not matter as long as the blister does not occur, but the pressing force may be 5 N / mm ² or more in order to distinguish the pressed state from the simple contact state.

(3) Energization The energization condition from the electrode to the local part may be adjusted according to the morphology of the local part, the composition of the Al alloy, the heating temperature (softening degree) and the like. The current density is, for example, 50 to 400 A / mm ² , 100 to 350 A / mm ² or 150 to 300 A / mm ² . The energization time is, for example, 100 to 4000 msec, 500 to 3000 msec or 1000 to 2000 msec. If the current density or the energization time is too short, the heating becomes insufficient and the softening degree becomes small. If the current density or the energization time becomes too long, the local parts will be excessively softened or deformed.

  The energization condition may be controlled so that the temperature of the local part falls within a desired range. The local temperature may be set to, for example, a temperature at which the liquid phase of the Al alloy appears at 0.2 mass% or less. When the local temperature rises to a temperature at which the liquid phase ratio of the Al alloy exceeds 0.2% by mass, α-Al and the eutectic phase are coarsened, and the local ductility may be deteriorated. The "temperature at which 0.2% by mass of liquid phase of Al alloy appears" was obtained by performing solidification calculation using the integrated thermodynamic calculation software "Thermo-Calc" for each Al alloy (composition). It can be obtained from the relationship between the obtained liquid phase ratio and temperature.

  The local temperature may be, for example, 350 ° C or higher, 400 ° C or higher, and further 450 ° C or higher. If the local temperature is too low, the softness and ductility in the local area are not improved so much. Conversely, the local temperature may be, for example, 600 ° C. or lower, or even 550 ° C. or lower.

  The "local temperature" referred to in the present specification is a surface temperature at a portion in contact with an electrode (simply referred to as "electrode contact portion") or in the vicinity thereof. The surface temperature is the maximum temperature obtained by measuring the temperature with a thermocouple (for example, K thermocouple) grounded on the surface of the electrode contact portion or its vicinity.

(4) Treatment pattern There are various modes in which the applied pressure changes (referred to as “pressurization pattern”) and the current density changes (referred to as “energization pattern”) from the start to the end of the local heating step. possible. For example, a square pattern or a trapezoidal pattern (referred to as “basic pattern”) in which the pressing force and the current density are maintained at constant values may be adopted immediately after the start or just before the end. An irregular pattern in which the pressing force and the current density change on the way may be adopted. When heating one local area (that is, during one local heating step), the pressing force or the current density may change in a constant cycle, and a cycle pattern may be adopted in which it is performed for a plurality of cycles. Note that both the pressing force and the current density may be variously changed during one local heating step, but it is usually sufficient to change the current density that is easy to control.

"casting"
The casting may be of any Al alloy composition, casting method, metallographic structure, morphology, etc., as long as it is locally softened by the above-described local heating step.

(1) Alloy Composition The Al alloy may contain 6% to 12%, 7% to 11%, and 8% to 10.5% of the entire Al alloy, for example. If the amount of Si is too small, the castability decreases and the shrinkage amount increases. If the amount of Si is excessive, the mechanical properties (especially elongation) of the casting may deteriorate. In addition, unless otherwise specified, the alloy composition in the present specification is represented by a mass ratio with 100% by mass (simply referred to as “%”) of the entire casting including the softened portion.

The Al alloy may contain modifying elements such as Mg, Cu, Fe, Mn, Sr, Na, and Sb in addition to Si. Mg strengthens the Al matrix. The Al alloy may contain, for example, 0.1 to 1% and 0.2 to 0.5% of Mg. If the amount of Mg is too small, the effect cannot be sufficiently obtained, and if the amount of Mg is too large, crystallization of Mg ₂ Si and the like increases, and ductility and toughness may decrease.

  Cu strengthens the Al base. The Al alloy may contain, for example, 1 to 5% and 2 to 4% Cu. If the amount of Cu is too small, the effect cannot be obtained sufficiently, and if the amount of Cu is too large, the ductility and toughness may decrease.

  Fe improves the seizure resistance of the mold. The Al alloy may include, for example, 0.05 to 1% and 0.1 to 0.4% Fe. If Fe is too small, the effect cannot be obtained sufficiently, and if Fe is too large, the ductility may decrease.

  Mn improves seizure resistance with respect to the mold. The Al alloy may contain Mn in an amount of 0.2 to 1.2% and 0.3 to 0.8%, for example. If Mn is too small, the effect cannot be obtained sufficiently, and if Mn is too large, the ductility may decrease.

  Sr, Na or Sb refines the eutectic Si to improve the mechanical properties (especially ductility or toughness) of the casting (mainly the non-bonded portion). Trace amounts of these elements are sufficient. These elements may be contained in a total amount of 0.003 to 0.05% and 0.01 to 0.03%, for example.

(2) Casting method / metal structure The casting is obtained by gravity casting, low pressure casting, die casting, or the like. In the automobile field and the like, die-cast castings (referred to as "die-cast castings") having excellent mass productivity and dimensional accuracy are often used. Since the die cast casting has many thin portions, it is advisable to set a region where cracks are likely to occur (for example, a joint portion) as the softened portion.

  The above-described Al alloy containing Si (Al-Si alloy) or Al alloy containing Si and Cu (Al-Si-Cu alloy) is used for the die casting. For example, Al alloys such as JIS ADC10 and ADC12 are typical. In addition, Al alloys containing Si and Mg (Al-Si-Mg based alloys) are used for die castings used for automobile body members in recent years.

Such die-cast castings are mainly composed of α-Al (primary crystal) and a eutectic structure, and often form a eutectic network structure in which eutectics are continuous so as to surround α-Al. Eutectic structure mainly, Al-Si-based alloy if alpha-Al + Si (this is called "eutectic Si."), Made of Al-Si-Mg based alloy, if _{α-Al + Si + Mg 2} Si. In any case, solute elements (Si, Mg, Cu, etc.) in the alloy composition may exist in α-Al as a solid solution or a precipitation phase.

(3) Morphology The shape of the casting does not matter as long as local heating by the electrode is possible. However, it is preferable that the local portion has a shape capable of conducting electricity while being sandwiched by a pair of electrodes, because the above-described local heating step can be efficiently performed.

<Softening part>
(1) Hardness The hardness and ductility (toughness) of the softened part of the casting are adjusted by changing the conditions of the local heating step according to the required specifications. The hardness of the softened portion is, for example, 90% or less, 85% or less, 80% or less with respect to the hardness of the base portion (or the base material / base material made of the casting itself before local heating) that is not affected by Joule heating. Is 75% or less. Thereby, the ductility and crack resistance in the local part (softened part) can be improved. Suffice it to say, the hardness of the softened portion is preferably 60% or more, 65% or more, and further 70% or more with respect to the hardness of the base portion. This ensures the strength of the softened portion.

  The hardness referred to in this specification is measured by using a micro Vickers hardness tester (for example, MVK-E manufactured by Akashi Seisakusho Co., Ltd.) on a cross section of a casting including a target site. 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 thus measured is indicated by "HV0.1".

  Unless otherwise specified, the hardness of a specific portion (position) is the arithmetic (additive) average value of the hardnesses measured at 200 μm pitch intervals from the casting surface to the center of the wall thickness extending in the normal direction. (It is called "average hardness"). Unless otherwise specified, the hardness of the softened portion is the average hardness measured and calculated in the vicinity of the center of the electrode contact portion (the center of the casting surface where the tip surface of the electrode was in contact).

  The hardness distribution is represented by Vickers hardness measured at 200 μm pitch intervals unless otherwise specified. The hardness distribution in the direction along the casting surface is based on the Vickers hardness at the depth of 200 μm from the outermost surface unless otherwise specified.

  The hardness of the reference base (non-heated portion / base material) is the hardness of a region that is not affected by local heating. When the local heating is performed according to the present invention, the width of the heat-affected zone is at most about 1 to 2 mm. Even in the locally heated Al alloy member, for example, the average hardness in a region (position) separated by 2 mm or more from the outer peripheral edge (or inner peripheral edge) of the electrode contact portion to the outer side (or inner side) may be the hardness of the base portion. When measuring the hardness at a plurality of points, the maximum value of the average hardness found at each point is adopted as the hardness of the softened portion or the hardness of the base portion.

(2) Structure The metal structure of the casting varies depending on the alloy composition, casting method (conditions), heat treatment and the like. However, an Al alloy casting usually contains at least Si, and has primary crystal Al (α-Al) and eutectic Si (Si crystallized in the eutectic structure).

  The eutectic Si (particles) in the softened portion of such a casting is more likely to have a rounded shape (more spherical shape) than the eutectic Si in the base portion that is not affected by local heating. The degree of spheroidization (three-dimensional) of eutectic Si is indexed by, for example, the circularity (two-dimensional) obtained by analyzing an image obtained by observing an observation piece with a microscope.

  The circularity of the eutectic Si in the softened portion can be 1.2 times or more, 1.3 times or more, or 1.4 times or more that of the eutectic Si in the base portion that is not affected by Joule heating.

  This circularity is obtained as follows. First, a casting cross section including a target portion is cut out, and an observation piece polished to a mirror surface is manufactured. The observation piece may be subjected to chemical etching. For example, a 0.25% HF aqueous solution can be used for 10 seconds, or a 0.2% NaOH aqueous solution can be used for 30 seconds. If the cooling rate of the casting is slow, 400 times may be selected.

  The observation piece thus obtained is observed with an optical microscope. Magnification: A tissue image (data) taken at 1000 times is analyzed using an image processing device (for example, LUZEX manufactured by Nireco Co., Ltd.). Thereby, the circularity of the eutectic Si extracted (traced) from the observation piece is calculated. The lower limit of eutectic Si (maximum particle length) extracted by image analysis is 1 μm.

By the way, the circularity is obtained from 4π · S / L ^{2 where} the occupied area of particles is S and the circumferential length of particles is L. The circularity of a perfect circle (true sphere) is 1, and the more distorted particles have the smaller circularity. Unless otherwise specified, the circularity in the softened part or the base in the present specification is a value obtained by arithmetically averaging the circularity of each Si particle (eutectic Si) in the visual field observed with an optical microscope (× 1000 times). And

  Unless otherwise specified, the circularity of the softened portion is the specific field of view of the observation piece cut out from the vicinity of the center of the region where the tip surface of the electrode was in contact (depth from the outermost surface of the casting: 200 to 1000 μm × width: 500 μm) The tissue is calculated by image analysis.

  Unless otherwise specified, the circularity of the base part is a specific field of view (from the outermost surface of the casting) cut out from an area separated by 2 mm or more from the outer edge (inner edge) of the contact area with the tip surface of the electrode to the outside (inner edge). (Depth of 200 to 1000 μm × width: 500 μm) is calculated by image analysis.

(3) Morphology / Use The number of softening parts (local parts) may be one or more in one casting. There may be various shapes and sizes of the softened portion. The softened portion having a complicated shape or the softened portion having a large area may be formed by energizing using a dedicated electrode having an appropriate shape once or several times, or by energizing using a general-purpose electrode multiple times. May be done.

If one softened portion (continuously softened region) becomes excessive, the original mechanical properties (strength, etc.) of the casting are affected. Therefore, the size of one softened portion may be, for example, 9 cm ² or less, 7 cm ² or less, and further 5 cm ² or less. Specifically, the softened portion may have, for example, a substantially circular shape with a diameter of 30 mm or less, a diameter of 25 mm or less, and a diameter of 20 mm or less, or a substantially rectangular shape with one side of 30 mm or less, 25 mm or less, or 20 mm or less.

  The softened portion is preferably provided in a part of the casting for which ductility, crack resistance, etc. are desired to be improved. As a typical example, a joint portion (including a joint portion to be joined) to which other members are mechanically joined is a softening portion. Of course, a softening part may be provided in addition to the joining part.

  The mechanical joining referred to in this specification is made 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-piercing (SPR: Self Piercing Rivet). It is preferable that the SPR tack with local large plastic deformation is applied to the softened portion because cracking is prevented.

  In the case of SPR joining, the size of the softened portion may be, for example, 2 to 3 times the size of the head of the SPR. Specifically, the softened portion may have a circular shape or a rectangular shape having a diameter or one side of 15 to 25 mm, for example. Such a softened portion can be easily formed by adjusting the ground area (heating area) of the electrode.

  The other member that is mechanically joined to the casting referred to in this specification may be the fastener (screw, rivet, etc.) itself, or any other member to be joined. The members to be joined are members different in material, shape, etc. from the Al alloy casting, and include, for example, metal members such as Fe-based members (particularly steel plates), Al-based members, Mg-based members, Ti-based members, resin members, and It is a composite member such as FRP. The "X-based member" includes a pure metal member, an X alloy member, and an X-based composite member. It should be noted that the joining portion of the joined members to be joined by SPR is preferably a (thin) plate shape through which a rivet can pass.

"casting"
The casting is obtained by a pouring step of pouring a molten aluminum alloy into a mold and a cooling step of cooling the molten metal in the mold to solidify it. Particularly, it is preferable that the casting is obtained by die casting in which the molten metal is poured into the cavity of the mold while being pressurized. The cooling process of die casting is performed, for example, at a cooling rate of 50 to 500 ° C./s after completion of the pouring process or at the start of the cooling process.

  Various samples (Al alloy members) were manufactured, and their hardness, metallographic structure, crack resistance during joining, etc. were evaluated. The present invention will be described in more detail with reference to such specific examples.

<< Manufacture of sample >>
(1) Casting (base material)
A plate-shaped casting (100 mm × 30 mm × 3 mm) made of an Al alloy was manufactured by die casting. This casting was used as a base material (base material). 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, injection speed: 2 m / s.

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

(2) Local heating step As shown in FIG. 1, the central portion (local portion) of the base material was sandwiched between a pair of electrodes (chips), and pressure and current application as shown in Table 1 were performed. For the electrode tip, a chrome copper D type (tip surface: φ8 mm / SMK Co., Ltd.) was used. Pressurization and energization to local areas by the electrodes 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 substrate was measured by a K thermocouple.

  Pressurization and energization were performed in a pattern as shown in FIG. That is, first, the tip end surface of the electrode is brought into contact with the surface of the central portion of the base material and pinched. As a result, a pressure 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, a constant current value is supplied, and the current is maintained for a predetermined time (section II). Note that, as shown by the broken line in FIG. 2, it is possible to adopt an energization pattern in which the current value is changed during energization. After the completion of energization, the pressure applied from the electrode to the base material is removed to separate the electrode from the base material (section III).

The pressing force and current density shown in Table 1 are values obtained by dividing the load and current value applied from the electrode to the base material by the contact area between the electrode and the base material. The load and current values are set values for the spot welder. The contact area (on the side of the tip surface of φ8 mm) in this example was 50.24 mm ² .

  The energization time 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. The heating temperature shown in Table 1 is the maximum value (maximum temperature) of the surface temperature measured by the thermocouple described above.

  The sample C1 shown in Table 1 is an as-cast base material that is not subjected to local heating. The sample C2 was energized only by bringing the electrode into contact with the base material without applying a substantial pressing force to the base material.

<< Measurement and observation >>
(1) Hardness Vickers hardness (HV0.1) in each part was measured by using the observation piece manufactured from the test material of each sample by the method described above. For the locally heated samples, the average hardness near the center where the electrodes were in contact is shown in Table 1 as the hardness of the softened portion. In addition, the ratio of the hardness of the softened portion to the average hardness of the base portion (hardness of sample C1) not affected by local heating is also shown in Table 1 as the softening degree. Furthermore, the relationship between hardness and heating temperature (surface temperature) for each sample shown in Table 1 is shown collectively in FIG. FIG. 3 also shows the hardness of the sample C1 (base material) that was not locally heated.

  FIG. 4 shows the hardness distribution at a position of a depth of 200 μm from the outermost surface of the casting, using the observation piece manufactured from the sample material of Sample 12. The distance along the casting surface was based on the outer periphery of the contact area (electrode contact portion) of the electrode and the casting (0 μm).

  Further, the hardness distribution in the depth direction (thickness direction of the casting) at the center (local center) of the electrode contact portion is also shown in FIG. 5 using the observation piece manufactured from the sample material of Sample 12. FIG. 5 also shows the hardness distribution at the same position for the sample C1 which was not locally heated.

(2) Metallographic Structure By the method described above, the metallic structure of each sample was observed and the circularity of eutectic Si was calculated. The observation of the metal structure was performed at the center of each casting (the center of the locally heated sample was at the electrode contact portion) and the depth from the outermost surface was in the range of 200 to 1000 μm. The circularity thus obtained is summarized in Table 1. The ratio of the circularity of each sample to the circularity of the base portion (circularity of sample C1) not affected by local heating is also shown in Table 1 as the circularity ratio.

  The observation piece after mirror-polishing cut out from the test material of each sample was observed with an optical microscope (magnification: 200 times) to confirm the presence or absence of blisters (gas holes) in the thickness direction of the center of the casting. The results are summarized in Table 1.

<Joinability / Crack resistance>
A galvanized steel plate (thickness: 1.6 mm) was overlaid on the casting of each sample and joined by SPR. For SPR, boron steel, head (disc-shaped portion): φ8 mm, leg portion (cylindrical portion): φ5.3 mm × length 5.5 mm × wall thickness 1 mm was used.

  Driving was performed from the steel plate side using a POP JOISPND hydraulic system with a load of 56 kN and a speed of 100 mm / s. The tacking on the locally heated sample was performed while the central axes of the softened part (electrode contact part) and the SPR were substantially aligned.

  The presence or absence of cracks in the casting due to the rivets was examined as follows. As shown in FIGS. 8A and 8B (both of which are collectively referred to as “FIG. 8”), a dyeing solution for dye flaw detection is applied to the non-bonding surface side (rear surface side) of the cast product after the rivet driving. The joint is cut along the axial direction of SPR. The SPR is removed, and the anti-staining solution (developing solution) is applied to the joint surface side of the casting. The presence or absence of penetration of the dyeing solution on the joint surface side was observed, and the presence or absence of penetration cracking was determined. Table 1 collectively shows the presence or absence of cracks at the time of joining of each sample thus judged. In addition, examples of cases where there is a through crack and cases where there is no through crack are also shown in FIG.

<< Evaluation >>
(1) Hardness As is clear from Table 1 and FIG. 3, it can be seen that when local heating is performed as in this example, the material softens depending on the heating temperature. In particular, it has been found that when the temperature is 350 ° C. or higher, or even 400 ° C. or higher, the softening rapidly progresses. It was also found that the local part (softened part) heated to 450 ° C. or higher is softened to 85% or less of that before heating (base part).

  As is apparent from FIG. 4, when the electrode is locally heated, the electrode contact portion is stably softened, while the influence on the surroundings is extremely small. For example, it was also found that at a position separated by about 1 mm from the outer peripheral edge of the electrode contact portion, almost no softening occurred and the hardness was similar to that of the base material.

  As is clear from FIG. 5, first, the hardness of the base material that was not locally heated was monotonically decreased from the surface side toward the center. However, it has been found that when the electrode is locally heated, it softens to a substantially uniform hardness in the depth direction (the casting thickness direction).

(2) Structure As is clear from Table 1 and FIG. 6, it was found that the eutectic Si is rounded and the circularity thereof is increased by the local heating.

  As is clear from Table 1 and FIG. 7, it was found that blisters are generated in the interior unless pressure is applied during local heating. Conversely, it has been confirmed that pressurization can prevent the formation of blisters.

(3) Cracking resistance As is clear from Table 1 and FIG. 8, even when SPR studs with large plastic deformation are applied to the casting, the occurrence of cracking can be prevented by using the locally heated softened portion. I understood.

  As described above, it was confirmed that when the local heating step of the present invention was performed, only the local parts of the casting (the electrode contact part and its immediate vicinity) could be softened without generating blisters. It was also confirmed that, by utilizing the local portion (softened portion), for example, SPR bonding can be performed on an Al alloy casting without causing cracks.

Claims (10)

By applying current to the electrode while pressurizing the local part of the cast aluminum alloy with the electrode, a local heating step of Joule heating the local part is provided,
A method for manufacturing an aluminum alloy member, wherein the casting is provided with a softened portion obtained by softening the local portion.   The method for producing an aluminum alloy member according to claim 1, wherein the local heating step is performed at a temperature of the local portion equal to or lower than a temperature at which a liquid phase of the aluminum alloy appears at 0.2% by mass.   The method for producing an aluminum alloy member according to claim 1, wherein the local heating step is performed at a temperature of the local part of 350 ° C. or higher.   The said local heating process makes the hardness of the said softening part 90% or less with respect to the hardness of the base part which is not influenced by the said Joule heating, The manufacturing method of the aluminum alloy member in any one of Claims 1-3. .   The method for manufacturing an aluminum alloy member according to claim 1, wherein the casting is a die casting.   The said aluminum alloy makes the whole 100 mass%, Si is 6-12 mass%, The manufacturing method of the aluminum alloy member in any one of Claims 1-5.   The method for manufacturing an aluminum alloy member according to claim 6, wherein the aluminum alloy further contains 0.1 to 1 mass% of Mg.   The said local heating process makes the circularity of the eutectic Si in the said softening part 1.2 times or more with respect to the circularity of the eutectic Si in the base part which is not influenced by the said Joule heating. Manufacturing method of aluminum alloy member.   The method for manufacturing an aluminum alloy member according to claim 1, wherein the electrode is water-cooled inside.   The method for manufacturing an aluminum alloy member according to claim 1, wherein the softened portion is a joined portion to which other members are mechanically joined.