JP7152932B2 - Method for manufacturing aluminum alloy member - Google Patents

Description

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

A plurality of members with different shapes, materials, functions, etc. are often joined together in order to cope with the complication, enlargement, multi-functionality, weight reduction, etc. of members. Joining is performed by adhesion, welding, fastening, bonding, or the like according to required specifications. Among them, mechanical bonding is frequently used because it is relatively easy to achieve strong bonding. For example, members that need to be attached and detached are fastened with screws (bolts, nuts, etc.), and members that do not need to be attached and detached are joined with rivets.

By using rivets, dissimilar materials that cannot be welded can be easily joined together. Recently, attention has also been focused on joining using a self-piercing rivet (simply referred to as "SPR") that does not require a piercing operation (that is, self-piercing type) and enables strong bonding.

For example, the following patent document describes the joining of a steel plate or the like by riveting SPR to an aluminum alloy die-cast casting (member).

JP 2010-90459 A JP 2018-94621 A Japanese Patent Application Laid-Open No. 2004-124151

In Patent Document 1, a die-cast member (casting) made of an Al—Si alloy with an adjusted chemical composition is subjected to heat treatment (solution treatment, aging treatment) to increase ductility, thereby suppressing cracks that occur during riveting of SPR. are proposing to

However, in Patent Document 1, heat treatment of the entire casting is performed only in order to prevent cracking due to local deformation when the SPR is hammered, despite having sufficient properties without heat treatment except for the joint. Is going. In addition, in order to prevent blisters (expansion of high-pressure gas voids) that occur during heat treatment, special high-vacuum die casting is performed. Therefore, in the method of Patent Document 1, the overall strength of the die-cast member is lowered, thermal distortion occurs, and the manufacturing cost increases.

Patent Literature 2 proposes a method for manufacturing an aluminum alloy member in which only the joints where SPR is riveted are selectively made highly ductile. Specifically, by locally pressurizing during die casting, the volume ratio (primary crystal ratio) of the primary crystal Al in the joint is increased, and the high ductility is realized.

However, in the method of Patent Document 2, since the wall thickness and hot water temperature drop differ depending on the part, it is not easy to control the pressure during the casting process, and the mold structure is complicated.

Patent document 3 proposes contacting a heated metal block to partially soften the aluminum alloy. However, Patent Document 3 is aimed at improving the press formability of cold-rolled sheets rather than improving the bondability of castings, and it is proposed to preliminarily soften a portion that undergoes large plastic deformation (for example, an annular flange portion). I am just suggesting it. In addition, the method disclosed in Patent Document 3 is not suitable for heating a narrow local area such as a joint to a desired temperature because the amount of wasteful heat radiation from the metal block is large.

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 by partially softening an aluminum alloy casting by a method different from the conventional method. intended to

As a result of intensive research to solve this problem, the present inventor succeeded in effectively softening the localized area by bringing an electrode into contact with the localized area of the aluminum alloy casting and Joule heating only the localized area. Developing this result led to the completion of the present invention described below.

<<Manufacturing method of aluminum alloy member>>
(1) The present invention includes a local heating step of Joule-heating a local portion of an aluminum alloy casting by applying an electric current to the electrode while applying pressure to the electrode, thereby softening the local portion of the casting. It is a method of manufacturing an aluminum alloy member provided with.

(2) According to the production method of the present invention, castings (simply referred to as "castings") of aluminum alloys (also referred to as "Al alloys") can be produced while avoiding overall reduction in strength, thermal strain, etc. 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. By using the softened portion having higher ductility than the surroundings, for example, it is possible to join an Al alloy member and another member with stable quality while suppressing cracks and the like.

By the way, the reason why the manufacturing method of the present invention can effectively soften only a local portion (high ductility) is considered as follows. First, when the temperature of the local portion is raised to a predetermined temperature or higher, the precipitation phases (Si, Mg ₂ Si, CuAl ₂ , etc.) in the metal structure (base) that constitutes the local portion are dissolved again in α-Al. be done. In addition, angular crystals (eutectic phase, etc.) crystallized during casting also change to a rounded shape (that is, spheroidized). In addition, local internal strain accumulated during casting is also relieved or eliminated. These things act additively or synergistically to soften the local area.

Next, according to Joule heating, in which the Al alloy is self-heated, a local area can be rapidly heated to a desired temperature within an extremely short period of time. Then, after the energization is completed, the local area can be cooled through the electrodes brought into contact with the casting. Thus, during and after the local heating process according to the present invention, heating of the surroundings of the local area due to heat conduction is suppressed. For this reason, the softening range is limited to the contact portion of the electrode (tip surface) or its extremely neighboring region (that is, local area). As a result, while softening the local portion, the decrease in the actual strength of the Al alloy member (Al alloy casting) as a whole, the occurrence of thermal strain, etc. can be avoided.

In addition, in the local heating step of the present invention, the local part is heated while being pressurized. Therefore, even if the internal gas introduced during casting is present locally, the occurrence of blisters caused by it during the local heating process is suppressed.

《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, as one form using the softened portion, the present invention can also be grasped as a composite member in which an Al alloy member and another member are joined.

<<Manufacturing method of metal member>>
Based on the above-described content, the present invention can be further expanded to include a method for manufacturing a metal member. That is, the present invention provides a method for manufacturing a metal member provided with a softened portion, which includes a local heating step of Joule-heating the local portion by energizing the electrode while pressurizing the local portion with an electrode, Alternatively, it can be grasped as a metal member obtained by the manufacturing method thereof. Metal members are Al alloy members, Mg alloy members, Ti alloy members, Fe-based (cast iron, steel) members, etc., which are not limited to castings.

"others"
Unless otherwise specified, "x to y" as used herein includes the lower limit value x and the upper limit value y. A new range such as “a to b” can be established as a new lower or upper limit of any numerical value included in the various numerical values or numerical ranges described herein.

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 electricity supply pattern. It is a graph which shows the heating temperature of a casting, and the relationship of hardness. 4 is a graph showing the hardness distribution of an electrode contact portion and its surroundings. 4 is a graph showing hardness distribution in the depth direction at the center of the casting. 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 a 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 at the time of SPR riveting. It is a photograph showing the presence or absence of penetration cracks during SPR riveting.

In addition to the components of the present invention described above, one or more components arbitrarily selected from this specification may be added. The content described in this specification appropriately applies not only to the production method of the present invention, but also to Al alloy members, composite members using the same, and the like. Which embodiment is the best depends on the target, required performance, and the like.

<<Local heating process>>
The local heating process is performed by applying current to the local portion from the electrode while applying pressure to the local portion of the casting.

(1) Electrode The shape of the electrode does not matter as long as only the desired local area (and its vicinity) can be Joule-heated efficiently. The electrodes usually consist of a pair that clamps the area. However, the form of the electrodes is not necessarily limited to a pair facing each other as long as local pressure and local energization are possible.

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

The electrode may be detachable from the shank (cap chip type) or may be integrated with the shank (integrated type). Using a cap tip type electrode (also referred to as an "electrode tip") can reduce the cost of the local heating process.

A large current is applied and a pressure is applied by the electrodes. Therefore, the electrodes are preferably made of a material having excellent electrical conductivity (electrical conductivity), strength, and the like. 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, and the like are used. Such an electrode material may be selected according to 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% by mass, Cu: balance), which is excellent in electrical conductivity and strength.

Electrodes made of materials with high electrical conductivity (low resistivity) generally have excellent thermal conductivity. Therefore, the Joule-heated local area is rapidly cooled by the electrode serving as a chill after the energization is completed. Thus, the Joule heating effect remains localized or only in its extreme periphery.

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 performance of the electrode, it is preferable that the inside of the electrode is forcibly cooled (water-cooled, etc.). As a result, the local heating process can be stably and continuously performed even during mass production.

(2) Pressing The local pressing force by the electrode may be adjusted according to the local shape (thickness, etc.), Al alloy composition, casting method, heating temperature (softening degree), and the like. The applied pressure is, for example, 10-70 N/mm ² , 15-50 N/mm ² or 20-40 N/mm ² . If the applied pressure is too small, blisters tend to occur locally. If the applied pressure is excessively large, deformation (cavity, etc.) of the casting and deformation (wear, buckling, etc.) of the electrode tend to occur. As long as blisters do not occur, the lower limit of the applied pressure is not critical, but the applied pressure may be 5 N/mm ² or more in order to distinguish between the pressurized state and the mere contact state.

(3) Energization It is preferable to adjust the local energization conditions from the electrode according to the form of the local area, the composition of the Al alloy, the heating temperature (softening degree), and the like. The current density is for example 50-400 A/mm ² , 100-350 A/mm ² or 150-300 A/mm ² . The energization time is, for example, 100-4000 msec, 500-3000 msec, or 1000-2000 msec. If the current density or energization time is too small, the heating will be insufficient and the degree of softening will be small. If the current density or the energization time becomes excessive, the local area will be excessively softened or deformed.

The energization conditions may be controlled so that the local temperature is within a desired range. The local temperature may be, for example, the temperature at which 0.2% by mass of the Al alloy liquid phase appears or less. If the local temperature rises to a temperature at which the Al alloy liquid phase ratio exceeds 0.2% by mass, coarsening of α-Al and the eutectic phase proceeds, and local ductility may rather decrease. The "temperature at which 0.2% by mass of the liquid phase of the Al alloy appears" is obtained by 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 fraction and temperature.

Also, the local temperature may be, for example, 350° C. or higher, 400° C. or higher, or even 450° C. or higher. If the local temperature is too low, the improvement in softening and ductility in the local area is also small. Conversely, the local temperature may be, for example, below 600°C or even below 550°C.

The term "local temperature" as used in this specification refers to the surface temperature of the portion in contact with the electrode (simply referred to as the "electrode contact portion") or its extreme vicinity. 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 forms in which the applied pressure changes (referred to as "pressurization pattern") and changes in current density (referred to as "energization pattern") from the start to the end of the local heating process. could be. For example, a rectangular pattern or a trapezoidal pattern (referred to as “basic pattern”) may be employed in which the pressure and current density are kept constant except immediately after the start or just before the end. Also, an irregular pattern in which the pressure or current density changes in the middle may be adopted. When heating one local area (that is, during one local heating process), a cycle pattern may be employed in which the applied pressure or current density is changed in a constant cycle and this is repeated multiple times. It should be noted that both the applied pressure and the current density may be variously changed during one local heating step, but it is usually sufficient to change the current density, which is easy to control.

"casting"
As long as the casting is locally softened by the above-described local heating process, the Al alloy composition, casting method, metallographic structure, form, etc. are not limited.

(1) Alloy composition The Al alloy preferably contains 6 to 12%, 7 to 11%, and 8 to 10.5% Si, for example, when the entire Al alloy is 100%. If the amount of Si is too small, the castability deteriorates and the amount of shrinkage increases. Too much Si can degrade the mechanical properties (especially elongation) of the casting. In addition, unless otherwise specified, the alloy composition referred to in this specification is indicated by a mass ratio based on 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, 0.1-1%, 0.2-0.5% Mg. If the amount of Mg is too small, the effect cannot be sufficiently obtained, and if the amount of Mg is too much, crystallization of Mg ₂ Si and the like increases, and ductility and toughness may decrease.

Cu strengthens the Al matrix. The Al alloy may contain, for example, 1-5%, 2-4% Cu. If Cu is too little, the effect cannot be sufficiently obtained, and if Cu is too much, ductility and toughness may decrease.

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

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

Sr, Na or Sb refines the eutectic Si and improves the mechanical properties (especially ductility or toughness) of castings (mainly non-bonded portions). A trace amount of these elements is sufficient. These elements may be contained, for example, in a total amount of 0.003-0.05%, 0.01-0.03%.

(2) Casting Method/Metal Structure Castings are obtained by gravity casting, low-pressure casting, die casting, or the like. In the field of automobiles and the like, castings obtained by die casting (referred to as "die castings"), which are excellent in mass productivity, dimensional accuracy, etc., are frequently used. Since die casting has many thin-walled portions, it is preferable to soften the areas where cracks are likely to occur (for example, joints, etc.).

The above-mentioned 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 representative. In addition, Al alloys containing Si and Mg (Al--Si--Mg alloys) are used in die castings used for automobile body members in recent years.

Such die castings are mainly composed of α-Al (primary crystal) and eutectic structure, and often form a eutectic network structure in which eutectic crystals surround α-Al. The eutectic structure mainly consists of α-Al+Si (this is referred to as “eutectic Si”) for Al—Si alloys, and α—Al+Si+Mg ₂ Si for Al—Si—Mg alloys. In any case, the solute elements (Si, Mg, Cu, etc.) in the alloy composition may exist as a solid solution or a precipitated phase in α-Al.

(3) Form The form of the casting does not matter as long as it can be locally heated by the electrodes. However, it is preferable that the local portion has a shape that allows current to flow while being sandwiched by a pair of electrodes, so that the above-described local heating process can be performed efficiently.

《Softening part》
(1) Hardness The hardness and ductility (toughness) of the softened portion of the casting are adjusted by changing the conditions of the local heating process according to the required specifications. The hardness of the softened portion is, for example, 90% or less, 85% or less, 80% or less of 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. As a result, the ductility and crack resistance of the local portion (softened portion) are improved. I dare say that the hardness of the softened portion should be 60% or more, 65% or more, or even 70% or more of the hardness of the base portion. Thereby, the strength of the softened portion is also ensured.

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.) of a casting 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 thus measured is indicated by "HV0.1".

Unless otherwise specified, the hardness of a specific part (position) is the arithmetic (additive) average value of each hardness measured at 200 μm pitch intervals from the casting surface to the center of the thickness extending in the normal direction. (referred to as “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 with which the electrode tip surface was in contact).

Hardness distribution is represented by Vickers hardness measured at a pitch of 200 μm unless otherwise specified. The hardness distribution along the cast surface is based on the Vickers hardness at a depth of 200 μm from the outermost surface unless otherwise specified.

The reference hardness of the base portion (non-heated portion/substrate) is the hardness of the region not affected by localized heating. When 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 a locally heated Al alloy member, for example, the average hardness in a region (position) spaced 2 mm or more outward (or inward) from the outer peripheral edge (or inner peripheral edge) of the electrode contact portion may be the hardness of the base. When the hardness is measured at a plurality of points, the maximum value of the average hardness obtained 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 castings varies depending on the alloy composition, casting method (conditions), heat treatment, and the like. However, Al alloy castings usually contain at least Si, and have primary Al (α-Al) and eutectic Si (Si crystallized in the eutectic structure).

The eutectic Si (particles) in the softened portion of such castings tends to be more rounded (more spherical) than the eutectic Si in the base, which is not affected by localized heating. The degree of spheroidization (three-dimensional) of eutectic Si is indexed, for example, by the degree of circularity (two-dimensional) obtained by analyzing an image obtained by observing a specimen 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 than the circularity of the eutectic Si in the base portion, which is not affected by Joule heating.

This circularity is obtained as follows. First, a cross-section of a casting including a target portion is cut out and polished to a mirror finish to produce an observation piece. The observation piece may be chemically etched. 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: Tissue images (data) photographed at 1000 times are analyzed using an image processing device (for example, LUZEX manufactured by Nireco Corporation). Thereby, the degree of circularity is calculated for the eutectic Si extracted (traced) from the observation piece. The lower limit of eutectic Si (maximum particle length) extracted by image analysis is 1 μm.

Incidentally, the degree of circularity can be obtained from 4π·S/L ² , where S is the area occupied by the particle and L is the circumference of the particle. The circularity of a perfect circle (perfect sphere) is 1, and the more distorted the particle, the smaller the circularity. Unless otherwise specified, the circularity of the softened portion or the base as used herein is the arithmetic average of the circularity of each Si particle (eutectic Si) in the field of view observed with an optical microscope (×1000). and

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

Unless otherwise specified, the circularity of the base is the specific field of view of the observation piece cut out from the area spaced 2 mm or more outward (inward) from the outer peripheral edge (inner peripheral edge) of the contact area with the tip surface of the electrode (from the outermost casting surface Depth: 200 to 1000 μm × width: 500 μm) is calculated by image analysis of the tissue.

(3) Form/use A single casting may have a single softened portion (local portion) or a plurality of softened portions. Various shapes and sizes of the softened portion are possible. A softened portion with a complicated shape or a softened portion with a large area may be formed by performing energization once or several times using a dedicated electrode having a suitable shape, or by performing energization multiple times using a general-purpose electrode. may be

If one softened portion (continuously softened region) becomes excessively large, the original mechanical properties (strength, etc.) of the casting will be affected. Therefore, the size of one softened portion may be, for example, 9 cm ² or less, 7 cm ² or less, or 5 cm ² or less. Specifically, the softened portion may be, for example, a substantially circular shape with a diameter of 30 mm or less, a φ25 mm or less, or a φ20 mm or less, or a substantially rectangular shape with a 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 whose ductility and crack resistance are to be improved. As a representative example, the softened portion is a joint portion (including a portion to be joined) where another member is mechanically joined. Of course, the softened portion may be provided in addition to the joint portion.

It should be noted that the mechanical connection referred to in this specification is achieved by, for example, screws (bolts, nuts, etc.) or rivets. The rivets may be inserted into pre-formed holes and then crimped, or they may be self-piercing (SPR: Self Piercing Rivet). Cracking is preferably prevented when SPR riveting with large local plastic deformation is applied to the softened portion.

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

The other members mechanically joined to the casting referred to in this specification may be fasteners (screws, rivets, etc.) themselves, or other members to be joined. The member to be joined is a member different in material, shape, etc. from Al alloy castings. Composite members such as FRP and the like. The "X-based member" includes a pure metal member, an X-alloy member, and an X-based composite member. The joint portion of the member to be joined to be SPR-joined is preferably in the form of a (thin) plate through which a rivet can pass.

"casting"
A casting is obtained through a pouring step of pouring molten Al alloy into a mold and a cooling step of cooling and solidifying the molten Al alloy in the mold. In particular, the casting is preferably obtained by die casting in which molten metal is poured into a mold cavity while being pressurized. The cooling process of die casting is performed at a cooling rate of 50 to 500° C./s, for example, after the metal pouring process is completed or when the cooling process is started.

Various samples (Al alloy members) were produced, and their hardness, metal structure, crack resistance during joining, etc. were evaluated. The present invention will be described in more detail while citing such specific examples.

《Production of samples》
(1) Casting (base material)
A plate-shaped casting (100 mm×30 mm×3 mm) made of Al alloy was produced 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 a mold cavity 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 raw materials (metals and compounds) weighed to the following composition in the atmosphere. In the case of 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 of the (molten metal) to the whole (100 mass %).
Alloy composition: Al-9%Si-0.21%Mg-0.38%Mn-0.12%Fe

(2) Local Heating Process As shown in FIG. 1, the central portion (local portion) of the substrate was sandwiched between a pair of electrodes (chips), and pressure and current were applied as shown in Table 1. A chromium-copper D-type electrode tip (end face: φ8 mm/manufactured by SMK Corporation) was used. A spot welder (manufactured by ART-HIKARI Co., Ltd.) was used to locally pressurize and energize the electrode. At that time, the surface temperature near the outer edge of the contact area between the electrode and the substrate was measured with a K thermocouple.

Pressurization and energization were performed in a pattern as shown in FIG. That is, first, the front end surface of the electrode is brought into contact with the surface of the central portion of the substrate and pressed. As a result, a pressure is applied from the electrode to the substrate (section I). Next, while maintaining the pressurized state, electricity is started to be supplied from the electrode to the substrate, and a constant current value is supplied and held for a predetermined time (section II). It is also possible to employ an energization pattern in which the current value is changed during the energization, as indicated by the dashed line in FIG. After the energization is completed, the electrode is separated from the substrate by removing the pressure applied from the electrode to the substrate (section III).

The pressure and current density shown in Table 1 are values obtained by dividing the load and current value applied from the electrode to the substrate by the contact area between the electrode and the substrate. Load and current values are spot welder settings. The contact area (φ8 mm tip surface side) of this example was all 50.24 mm ²

The energization time is the time from the start of energization to the end of energization (the entire time of section II in FIG. 2). Note that the time during which the current value is constant is about 90% of the energization time. The heating temperatures shown in Table 1 are the maximum values (maximum temperatures) of the surface temperatures measured by the thermocouples described above.

Note that sample C1 shown in Table 1 is an as-cast base material that is not locally heated. In sample C2, the electrodes were brought into contact with the base material, and current was applied in a state in which no substantial pressure was applied to the base material.

《Measurement/Observation》
(1) Hardness Vickers hardness (HV 0.1) at each part was measured using an observation piece produced 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 electrode was in contact is shown in Table 1 as the hardness of the softened portion. Table 1 also shows the ratio of the hardness of the softened portion to the average hardness of the base portion (hardness of sample C1) not affected by the local heating as the degree of softening. Further, FIG. 3 summarizes the relationship between hardness and heating temperature (surface temperature) for each sample shown in Table 1. As shown in FIG. FIG. 3 also shows the hardness of sample C1 (substrate) that is not locally heated.

FIG. 4 shows the hardness distribution at a depth of 200 μm from the outermost surface of the casting using an observation piece manufactured from the test material of sample 12. As shown in FIG. The distance along the casting surface was based on the outer edge of the contact area (electrode contact portion) between the electrode and the casting (0 μm).

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

(2) Metal structure By the method described above, the metal structure of each sample was observed, and the circularity of eutectic Si was calculated. The metallographic structure was observed at the center of each casting (the center of the electrode contact portion for locally heated samples) at a depth of 200 to 1000 μm from the outermost surface. Table 1 summarizes the circularity obtained in this way. The ratio of the circularity of each sample to the circularity of the base not affected by local heating (the circularity of sample C1) is also shown in Table 1 as the circularity ratio.

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

《Bonding/cracking resistance》
A galvanized steel plate (thickness 1.6 mm) was superimposed on the casting of each sample and joined by SPR. The SPR used was made of boron steel, head (disc-shaped portion): φ8 mm, leg portion (cylindrical portion): φ5.3 mm×length 5.5 mm×thickness 1 mm.

Riveting 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 locally heated sample was riveted while the central axes of the softened portion (electrode contact portion) and the SPR were approximately aligned.

The presence or absence of cracks in the casting due to riveting was examined as follows. As shown in FIGS. 8A and 8B (both collectively referred to as "FIG. 8"), a dyeing liquid for dyeing flaw detection is applied to the non-bonded surface side (rear side) of the riveted casting. The joint is cut along the axial direction of the SPR. Remove the SPR and apply an anti-dye solution (developer) to the joint surface side of the casting. The presence or absence of permeation of the staining solution on the joint surface side was observed to determine the presence or absence of penetrating cracks. Table 1 summarizes the presence or absence of cracks at the time of bonding for each sample judged in this manner. In addition, FIG. 8 also shows an example of a case where there is a penetrating crack and an example where there is no penetrating crack.

"evaluation"
(1) Hardness As is clear from Table 1 and FIG. 3, when local heating is performed as in this example, softening occurs depending on the heating temperature. In particular, it has been found that when the temperature is 350° C. or higher, or 400° C. or higher, the softening progresses rapidly. It was also found that a localized portion (softened portion) heated to 450° C. or higher softens to 85% or less of that before heating (base portion).

As is clear from FIG. 4, when the electrode is locally heated, the electrode contact portion is stably softened, while the effect on the surroundings is negligible. For example, it was found that at a position about 1 mm away from the outer peripheral edge of the electrode contact portion, softening hardly occurs and the hardness is the same as that of the base material.

As is clear from FIG. 5, first, the hardness of the base material that was not locally heated decreased almost monotonically from the surface side toward the center. However, it was found that local heating using an electrode softened the hardness to a substantially uniform thickness over the depth direction (casting thickness direction).

(2) Structure As is clear from Table 1 and FIG. 6, local heating causes the eutectic Si to be rounded, increasing the degree of circularity.

As is clear from Table 1 and FIG. 7, it was found that blisters were generated inside unless pressure was applied during local heating. Conversely, it was confirmed that applying pressure can prevent blisters from occurring.

(3) Crack resistance As is clear from Table 1 and Fig. 8, cracks can be prevented by using a locally heated softened portion even when SPR riveting with large plastic deformation is performed on castings. I found out.

As described above, it was confirmed that the local heating process of the present invention softens only the local parts of the casting (electrode contact parts and their immediate vicinity) without generating blisters. It has also been confirmed that by using the localized portion (softened portion), for example, it is possible to perform SPR joining to Al alloy castings without causing cracks.

Claims (10)

A local heating step of joule-heating a local part of an aluminum alloy casting by energizing the electrode while pressurizing the local part of the aluminum alloy casting,
A method for producing an aluminum alloy member, wherein the softened portion softened by the change in local metallographic structure serves as a joining portion to be mechanically joined to another member in a cold state . 2. The method of manufacturing an aluminum alloy member according to claim 1, wherein the local heating step is performed by setting the temperature of the local area to a temperature at which 0.2 mass % of the liquid phase of the aluminum alloy appears or less. 3. The method of manufacturing an aluminum alloy member according to claim 1, wherein the local heating step is performed by setting the temperature of the local area to 350[deg.] C. or higher. The method for manufacturing an aluminum alloy member according to any one of claims 1 to 3, wherein the local heating step sets the hardness of the softened portion to 90% or less of the hardness of the base portion that is not affected by the Joule heating. . The method for manufacturing an aluminum alloy member according to any one of claims 1 to 4, wherein the casting is a die casting. The method for manufacturing an aluminum alloy member according to any one of claims 1 to 5, wherein the aluminum alloy contains 6 to 12% by mass of Si with respect to 100% by mass of the whole. 7. The method for manufacturing an aluminum alloy member according to claim 6, wherein the aluminum alloy further contains 0.1 to 1% by mass of Mg. 8. The local heating step according to claim 6, wherein the circularity of the eutectic Si in the softened portion is 1.2 times or more the circularity of the eutectic Si in the base portion which is not affected by the Joule heating. A method for manufacturing an aluminum alloy member of The method for manufacturing an aluminum alloy member according to any one of claims 1 to 8, wherein the electrode is internally water-cooled. The method for manufacturing an aluminum alloy member according to any one of claims 1 to 9, wherein the other member is a screw or a rivet .

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