JP7586993B1 - Method for manufacturing metal joint - Google Patents

Abstract

The present invention provides a new manufacturing method for obtaining a metal joined body in which a high tensile steel plate, a hot stamped steel plate, or the like is firmly joined to an aluminum substrate.
[Solution] The present invention is a manufacturing method for a metal joined body, which includes a current passing step of passing a current between a steel sheet and an aluminum base that are sandwiched between electrodes. The steel sheet has a thickness of 1 to 3.5 mm at the joined portion. The current passing step is performed by passing a current of 3 to 9.5 kA for 300 to 1200 ms. The current passing step may be composed of a first current passing step of passing a current of 3 to 8 kA for 200 to 400 ms and a second current passing step of passing a current of 5 to 9 kA for 300 to 700 ms. By the manufacturing method of the present invention, a metal joined body in which a steel sheet and an aluminum base are joined via an intermetallic compound layer can be obtained.
[Selected figure] Figure 3

Description

The present invention relates to a method for manufacturing a metal joint consisting of a steel plate and an aluminum substrate.

In order to respond to weight reduction, high functionality, structural changes, etc., components in which dissimilar metals are joined (called "metal joints") are used. For example, roofs and outer panels in automobiles are made by joining lightweight aluminum alloy plates that make up the exterior surface to steel plates that form the skeleton or frame.

To date, dissimilar metals have mainly been joined by riveting (self-piercing rivet joining, etc.) or crimping. Such mechanical joining methods require auxiliary materials (rivets, etc.) and specialized equipment, which increases the weight and cost of the joined body. In addition, protrusions that form at mechanically joined parts can cause interference with surrounding parts and reduce workability.

Recently, there have been many proposals to use resistance welding (e.g., spot welding) to weld dissimilar metals (e.g., steel plate and aluminum alloy plate). For example, there are related descriptions in the following patent documents.

Patent Publication No. 2022-63070 Patent Publication 2023-66438

In both patent documents, a current of about 13 kA is passed to join the steel plate and the aluminum alloy plate ([0057] in Patent Document 1/[0076] in Patent Document 2). In addition, the steel plate joined to the aluminum alloy plate is a thin low-carbon steel plate (270 MPa class/plate thickness: 0.8 mm) ([0051] in Patent Document 1/[0054] in Patent Document 2).

The above current value is sufficiently smaller than the large current value (over 15 kA) that would directly melt the aluminum alloy plate itself using resistance heating alone, as in the past. However, it has been newly discovered that when, for example, thick high-tensile steel or hot stamped steel is spot welded to an Al alloy plate using the current values described in both patent documents, spatter (surface spatter, in which molten beads fly out from between the electrode and the joined material, and internal spatter, in which molten beads fly out from between the joined materials) can occur.

The present invention was made in consideration of these circumstances, and aims to provide a new manufacturing method for obtaining a metal joint between a steel plate and an aluminum substrate.

As a result of intensive research into solving this problem, the inventor came up with the idea of melting the area near the joining surface of the aluminum base by heat transfer from a resistance-heated steel plate, thereby joining the two, and successfully realizing this idea. By expanding on this result, the inventor has completed the present invention, which is described below.

<<Method for manufacturing a metal bonded body>>
The present invention is a manufacturing method including a current passing step of passing a current to a joint portion of a steel plate and an aluminum base held between electrodes, the joint portion of the steel plate having a thickness of 1 to 3.5 mm, and the current passing step is performed by passing a current of 3 to 9.5 kA for 300 to 1200 ms, to obtain a metal joined body in which the steel plate and the aluminum base are joined via an intermetallic compound layer.

According to the manufacturing method of the present invention, when resistance welding a thick steel plate with high electrical resistance to an aluminum substrate, it is possible to obtain a metal joint that is excellent in strength and reliability while suppressing the generation of spatter. The reason or mechanism for the manifestation of such excellent effects is currently thought to be as follows.

The amount of heat (Q) generated by the passage of electricity through the materials to be joined is roughly determined by their electrical resistance (R), the current value (I), and the time (t) through which electricity is passed (Q = ^I2 x R x t). The electrical resistance (R) is determined by the material (components, structure, etc.) and thickness of the materials to be joined. In particular, steel plate has a much higher electrical resistance than an aluminum substrate of roughly the same thickness, and the higher the strength, the higher the electrical resistance can become.

When a current is applied to a thick steel plate with high electrical resistance at the conventional high current level, the steel plate quickly overheats and spatters easily occur. In the present invention, a current is applied to such a steel plate for a long period of time at a relatively low current value, so spatters are less likely to occur.

Even if the materials to be joined do not melt when electricity is applied, the area near the joining surface of the aluminum base that comes into contact with the steel plate can melt due to the heat conducted from the steel plate (especially near the center) which has been resistively heated and has become hot. This initiation of melting causes a melting reaction (for example, a solid-liquid interdiffusion reaction: SLID) near the joining surface, producing an intermetallic compound layer between the steel plate and the aluminum base, allowing the two to be firmly joined (welded).

Metal Joint
The present invention can also be understood as a metal joined body. For example, the present invention may be a metal joined body in which a steel plate having a thickness of 1 to 3.5 mm is resistance-welded to an aluminum substrate. An example of the manufacturing method thereof is as described above.

The bonding layer of the materials to be joined (steel plate and aluminum substrate) does not necessarily have to be an intermetallic compound (layer) in its entirety. A part of the materials to be joined (e.g., iron substrate or aluminum substrate) may remain or be mixed in with it. Furthermore, the bonding layer may contain components of the coating layer that was on the part to be joined (e.g., Zn, O, etc.).

"others"
(1) Unless otherwise specified, the thickness and electrical resistance of the "joined portion" in this specification are specified in the state before joining (before the current application process). When it is not possible to directly measure the joined portion (for example, after joining), it is advisable to estimate the characteristics of the joined portion by measuring the area where the state before joining is almost maintained (the outer periphery of the joint or heat-affected zone (HAZ)).

(2) Unless otherwise specified, "x to y" in this specification includes a lower limit of x and an upper limit of y. Any numerical value included in the various numerical values or numerical ranges described in this specification may be used as a new lower limit or upper limit to create a new range such as "a to b." Additionally, unless otherwise specified, "x to ykA" in this specification means xkA to ykA. The same applies to other unit systems (ms, etc.).

FIG. 1 is a schematic diagram illustrating an overview of spot welding. 4 is an example of a time chart relating to the spot welding. 1 is a micrograph showing a fracture surface of a joint (sample 1).

One or more components selected from this specification may be added to the components of the present invention described above. The contents described in this specification may also be applied to the manufacturing method as well as to the product (metal bonded body, etc.) as appropriate. Which embodiment is best depends on the target, required performance, etc.

Steel Plate
The steel plate according to the present invention may have the following characteristics.

(1) Thickness The thickness of the steel plate (plate thickness) is, for example, 1 to 3.5 mm, 1.4 to 3.2 mm, 1.6 to 3 mm, or 1.8 to 2.6 mm. The plate thickness in this specification refers to the thickness of the parts to be joined before being used for joining.

If the plate thickness is too thick, the amount of heat generated when electricity is passed through the plate will also be too large, making it easier for spatter to occur. If the plate thickness is too small, the amount of heat generated will decrease and the amount of heat dissipated will increase, making it impossible to sufficiently heat the area near the surface of the aluminum substrate to be joined.

(2) Tensile strength/composition The steel sheet is, for example, a high tensile steel sheet (so-called high tensile strength steel sheet) or a hot stamp steel sheet. The tensile strength of the high tensile steel sheet is, for example, 590 MPa or more, 780 MPa or more, 980 MPa or more, or 1180 MPa or more. The tensile strength of the hot stamp steel sheet after quench forming is, for example, 780 MPa or more, 980 MPa or more, 1180 MPa or more, 1380 MPa or more, or 1580 MPa or more.

Such steel sheets contain a fair amount of alloy elements such as Ni, Si, or Mn in addition to C, and the composition is usually adjusted taking into consideration formability as well as strength. In addition, the steel sheets have a metal structure according to the strengthening mechanism (solid solution strengthening, precipitation strengthening, quenching, etc.).

(3) Coating layer The steel sheet may have a coating layer (or a surface treatment layer) at least on the aluminum substrate side of the joined portion. Of course, the coating layer may be provided on the entire surface of one or both sides of the steel sheet. The thickness and electrical resistance of the steel sheet (before joining) referred to in this specification include the coating layer. The thickness of the coating layer is usually 1 to 30 μm.

The coating layer is, for example, a zinc plating layer, a zinc oxide layer, a Zn-Fe alloy layer, etc. Examples of steel sheets having such a coating layer include galvanized steel sheets such as hot stamp steel sheets having a coating layer containing Zn (such as a Zn-Fe alloy layer), hot-dip galvanized steel sheets, electrogalvanized steel sheets, and alloyed hot-dip galvanized steel sheets. The alloyed hot-dip galvanized steel sheets may be aluminum-zinc alloy-plated steel sheets (so-called Galvalume steel sheets (registered trademark)).

Incidentally, the more the coating layer is alloyed or oxidized, the greater the electrical resistance and the more likely it is to generate heat. When a relatively low current is applied for a long period of time, as in the present invention, spattering can be suppressed even with such a coating layer. Conversely, such a coating layer can promote a melting reaction or the formation of an intermetallic compound layer near the joined surfaces.

<Aluminum base>
The aluminum substrate may be a wrought material or a cast material. Wrought materials include, for example, rolled materials, (hot) extruded materials, forged materials, etc. Cast materials include, for example, die cast materials, metal mold cast materials, sand cast materials, etc. The aluminum substrate may be of any form (shape, size), thickness of the joined portion, electrical resistivity, strength, composition, coating treatment, etc.

The aluminum substrate is preferably made of an aluminum alloy containing Mg and Si. Such an aluminum alloy is easily melted at the interface with the steel plate. Examples of wrought materials include the A4000 series, A5000 series, and A6000 series (JIS). Examples of cast materials include the ADC10 series, ADC12 series, and AC4 series (JIS). The aluminum alloy may contain other alloying elements (Cu, Mn, Zn, Fe, Ti, etc.).

The aluminum substrate is not limited to a plate material. Resistance welding is easier if at least the portion to be joined of the aluminum substrate is plate-shaped. The thickness of the portion to be joined is not important, but is, for example, 1.4 to 4 mm, 1.8 to 3.6 mm, or 2.2 to 3.2 mm.

Metal Joint
The metal joined body is not limited to one in which a single steel plate having the above-mentioned characteristics is joined to an aluminum substrate. The metal joined body may have another substrate (e.g., a steel plate) on the back side of the above-mentioned steel plate (the opposite side of the aluminum substrate). The other substrate may have any shape, thickness, electrical resistivity, strength, composition, coating treatment, etc. The other substrate may be a steel plate (hot-rolled steel plate, cold-rolled steel plate, high-strength steel plate, hot-stamped steel plate, etc.) or a metal body (plate) different from the steel plate.

The metal joint may be a joint in which multiple steel plates and an aluminum base are joined by the method of the present invention. There is no requirement for the multiple steel plates to be joined together. If an aluminum base is joined by the method of the present invention to a joint between previously joined steel plates, the joint (thickness) between the steel plates can be considered as the "joined portion (thickness) of the steel plate" as used in the present invention.

<Production Method>
The metal joint is formed by passing an electric current between the pressure-welded steel plate and aluminum substrate (electric current passing step).

(1) Current Value The current value applied to the steel sheet to be joined is, for example, 3 to 9.5 kA, 4 to 9 kA, 4.5 to 8 kA, or 5 to 7.5 kA. Incidentally, the current density calculated by dividing the current value by the tip area of the electrode on the steel sheet side may be, for example, 26.5 to 84.0 A/ ^mm2 or 44.2 to 66.3 A/ ^mm2 .

(2) Current Flow Time The current flow time (total time per joint) is, for example, 300 to 1200 ms, 400 to 1000 mm, or 500 to 900 ms. If the current flow time is too short, intermetallic compounds that provide the joint strength are not sufficiently generated on the joint surface of the aluminum base. If the current flow time is too long, adhesion occurs between the electrode and the aluminum base, reducing productivity and reducing joint quality.

(3) Current Flow Pattern The current flow process may be configured with a plurality of processes. For example, a first current flow process in which the current value at the start of current flow is relatively small is followed by a second current flow process in which the current value is relatively large, which makes it possible to form a sufficient joint (nugget) while effectively suppressing spattering.

The current value of the first current flow process is, for example, 3 to 8 kA, 3.5 to 7.5 kA, or 4 to 7 kA. The current flow time of the first current flow process is, for example, 200 to 400 ms or 250 to 350 ms.

The current value of the second current flow process is, for example, 5 to 9 kA or 6 to 8 kA. The current flow time of the second current flow process is, for example, 300 to 700 ms or 400 to 600 ms.

The current value of the first current flow process may be an upslope current flow that gradually increases the current. The upslope may be linear or curved. It is preferable that the current values at the start and end of the first current flow process are within the above-mentioned range, and that the current value at the end of the first current flow process is approximately equal to the current value at the start of the second current flow process. Such upslope current flow shortens the first current flow process and suppresses spattering, enabling efficient joining. The first current flow process may also serve as a pre-current flow process that familiarizes the contact state between the steel sheet and the aluminum substrate.

(4) Cooling step The parts to be joined after the current passing step may be naturally cooled or may be cooled through the electrodes (cooling step). The latter cooling step may be performed by cutting off the current (non-current passing), by passing a small current value, or by monotonically decreasing the current value (downslope current passing). In the case of joining hot stamped steel sheets, the parts to be joined after the current passing step may be rapidly cooled.

"electrode"
Electric current is usually applied through electrodes pressed against the steel sheet and aluminum substrate. The electrodes are selected according to the characteristics (material, shape, etc.) of the steel sheet and aluminum substrate to be joined. An example of an electrode for resistance spot welding will be described below.

(1) Form The electrode may be detachable from the shank (cap-tip type) or integral with the shank (integral type). Usually, a cap-tip type electrode (also called a "tip") is used to reduce costs.

The electrode (tip) has, for example, a bottomed, generally cylindrical tip portion and a generally cylindrical body portion continuing from the tip portion. The outer surface (pressure contact surface) of the tip portion may be convex or may be recessed. The size of the electrode does not matter. The outer diameter of the body portion (nominal diameter/D2) is, for example, φ10-20 mm or φ12-18 mm. The tip diameter (D1) is, for example, φ6-14 mm or φ8-12 mm.

It is recommended that a coolant (cooling liquid/cooling water) be introduced into the inner tube at the inside of the tip of the electrode. When the coolant is forcibly circulated, the temperature rise of the electrode can be suppressed and the plate material can be cooled stably through the electrode.

The basic shape of the tip of an electrode (especially a convex electrode) is specified in JIS C9304 (1999). Examples include flat (F-type), radius (R-type), dome (D-type), dome radius (DR-type), truncated cone (CF-type), truncated cone radius (CR-type), etc. For spot welding of steel plate and aluminum substrate, it is recommended to use electrodes of, for example, DR-type or R-type, from the viewpoint of versatility.

(2) Material The electrode (at least the tip) is preferably made of a material having excellent thermal conductivity, electrical conductivity, strength, etc. For example, an electrode made of a copper alloy having an electrical conductivity of 75 to 95% IACS, or even 80 to 90% IACS, is used. Examples of copper alloys include chromium copper, zirconium copper, chromium-zirconium copper, alumina-dispersed copper, and beryllium copper.

The electrode in contact with the steel plate and the electrode in contact with the aluminum substrate may be the same or different in form (shape, size (diameter)) and material.

(3) Pressing force The electrodes clamp the steel sheet and the aluminum substrate with, for example, 2 to 7 kN, or even 3 to 6 kN. If the pressing force is too small, spatter (dust) is likely to occur. If the pressing force is too large, deep dents (depressions) are likely to occur due to the electrodes, and deformation and wear of the electrodes are likely to occur. The pressing force is usually kept approximately constant, but may be changed during the process.

"others"
(1) The metal joint (particularly the joint) may be heat-treated (annealed, tempered, etc.) after welding. The heat treatment can adjust the structure of the joint (nugget) and its vicinity (heat-affected zone), remove residual stress, etc. The heating temperature and heating time may be adjusted depending on the composition and thickness of the steel sheets.

(2) Metal bonded bodies can be used to make various components and structures. For example, at least a portion (metal bonded body) of a vehicle frame, panel, housing, storage compartment, etc. can be obtained by the manufacturing method of the present invention.

A metal joint was produced by resistance spot welding a steel plate and an aluminum substrate, and the joint strength, fracture surface, etc. were evaluated. The present invention will be explained in more detail using such a specific example.

An overview of spot welding is shown in Figure 1. A sheet assembly consisting of a steel sheet and an Al alloy sheet stacked together was used as the material to be welded. Spot welding is performed by passing electricity through a pair of identical electrodes that are pressed against each surface of the sheet assembly (the top surface of the Al alloy sheet and the bottom surface of the steel sheet). In this example, for ease of explanation, unless otherwise specified, the directions of the arrows shown in Figure 1 are taken to be the up-down direction or the left-right direction.

<Sample Preparation>
(1) Welding material The steel plate used was a hot stamped material (thickness: 2.2 mm) made of hot-formed boron steel with a zinc-plated layer. Both sides of the plate were covered with an Fe-Zn alloy layer (average thickness: 15 μm).

The aluminum alloy plates used were die-cast material (ADC10/JIS) and hot-extruded material (A6005C/JIS). The plate thickness for both was 3.0 mm. The melting point (solidus temperature) of the die-cast material was 565°C. The melting point (solidus temperature) of the hot-extruded material was 590°C.

Each plate was cut into strips (30 mm x 100 mm) and used for spot welding as is without surface polishing.

(2) Electrode A commercially available tip (manufactured by OBARA Corporation) of DR type (JIS C9304) was used as the electrode. Forced circulating cooling water (flow rate: 2.7 L/min) was supplied to the inside (inner cylindrical part) of the tip to forcibly cool the tip. The electrode was made of chromium copper (Cr: 1 mass%, Cu: balance), and its electrical conductivity was 80% IACS.

As shown in the enlarged view of Figure 1, the electrode size is as follows: tip diameter (nominal diameter D2): φ16 mm, thickness of the tip base: 12 mm, radius of curvature (R) of tip shoulder: 8 mm, radius of curvature (R1) of tip surface: 40 mm, and tip diameter (D1) is 12 mm.

(3) Welding Spot welding was performed using a servo pressure spot welding machine (PA235KVAMF manufactured by ARO). An example of the current pattern is shown in Figure 2. The pressure (F) applied to the plate assembly by the electrodes was constant at 4 kN. Current was applied by controlling the DC current as follows.

The first current flow process was performed with a first current value of 4 to 7 kA and a first current flow time of 300 ms. The first current value was monotonically increased at a constant rate of change over time from 4 kA at the start of current flow to 7 kA at the end of current flow (linear upslope current flow).

Following the first current flow process, a second current flow process was carried out with a second current value of 7 kA (constant) and a second current flow time of 500 ms.

After the second current flow process, the current was cut off, and after 200 ms had elapsed, the electrodes were removed from the workpieces. In this way, a joint of hot stamped material and die cast material (sample 1) and a joint of hot stamped material and hot extruded Al alloy material (A6005C) (sample 2) were obtained.

"measurement"
Each sample was subjected to a tensile shear test (JIS Z 3136) to measure the load at break. This measurement was repeated three times to determine the tensile shear strength (average value). For example, sample 1 was 3894 N (-3σ: 277 N). When the tensile shear strength was divided by the area of the broken portion, a sufficient bonding strength of over 80 to 100 MPa was ensured for all samples.

"observation"
Fig. 3 shows a photograph of the bonding surface (fracture surface on the steel plate side) of sample 1 after the tensile test, observed with an inverted metallurgical microscope (GX53 manufactured by Olympus Corporation). An intermetallic compound layer was present on the bonding surface (outer circular area). Remnants of the broken die-cast material were also observed near the center of the bonding surface (inner circular area). As can be seen from Fig. 3, the steel plate and the Al alloy plate were bonded with a sufficiently large intermetallic compound layer (nugget), and it was also confirmed that the bonding strength was equal to or greater than the strength of the bonded material (Al alloy plate).

From the above, it was confirmed that the manufacturing method of the present invention can produce a dissimilar metal joint that is firmly bonded to an aluminum substrate, even when the steel plate has a high electrical resistivity and is thick.

Claims (13)

The method includes a current application step of applying current to a joint portion of a steel plate and an aluminum substrate sandwiched between electrodes,
The steel plate to be joined has a thickness of 1.4 to 3.5 mm,
The current application step is carried out by applying a current of 3 to 9.5 kA for 300 to 1200 ms,
The method for producing a metal joined body is provided in which the steel plate and the aluminum substrate are joined via an intermetallic compound layer. The method for manufacturing a metal joint according to claim 1, wherein the steel plate is a high-tensile steel plate having a tensile strength of 590 MPa or more. The method for manufacturing a metal joint body according to claim 1 or 2, wherein the steel sheet is a hot stamp steel sheet, a hot-dip galvanized steel sheet, or an alloyed hot-dip galvanized steel sheet. The method for manufacturing a metal joint body according to claim 3, wherein the portion to be joined of the hot stamped steel sheet has a coating layer containing Zn at least on the aluminum base side. The method for manufacturing a metal joint according to claim 1, wherein the portion to be joined of the aluminum substrate has a thickness of 1.4 to 4 mm. The method for manufacturing a metal joint body according to claim 1 or 5, wherein the aluminum substrate is made of a wrought material or a cast material. The method for producing a metal bonded body according to claim 1 or 5, wherein the aluminum substrate contains Mg and/or Si. The current application step includes a first current application step of applying current at 3 to 8 kA for 200 to 400 ms;
A second current application process of applying current at 5 to 9 kA for 300 to 700 ms;
The method for producing a metal joined body according to claim 1 , comprising: The method for manufacturing a metal joint body according to claim 8, wherein the first current application process is performed while gradually increasing the current. The method for manufacturing a metal joint body according to claim 1, wherein the electrode applies pressure to the jointed parts at 2 to 7 kN. The method for manufacturing a metal joint body according to claim 1, wherein the parts to be joined are resistance spot welded. The joined portion of the steel plate has a thickness of 1.6 to 3.5 mm,
The current application step is performed by applying a current of 3 to 8 kA for 400 to 1200 ms.
The method for producing the metal joined body according to claim 1 . The method for producing a metal joined body according to claim 8, wherein the second current application step is performed at 5 to 8 kA for 400 to 700 ms.

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