JP5215986B2 - Dissimilar material joint and dissimilar material joining method - Google Patents

Description

  The present invention relates to a dissimilar material joint having excellent joint strength and a dissimilar material joining method.

  In recent years, with respect to global environmental problems caused by exhaust gas and the like, improvement in fuel efficiency has been pursued by reducing the weight of the body of a transport aircraft such as an automobile. In addition, it has been pursued to improve safety at the time of automobile body collision without hindering the weight reduction as much as possible. For this reason, in particular, the use of aluminum alloy materials such as aluminum alloy plates that are lighter in weight and superior in energy absorption, instead of steel materials such as steel plates that have been used in the past, has increased for automobile body structures. It's getting on. The aluminum alloy plate referred to here is a general term for rolled plates such as a hot-rolled plate and a cold-rolled plate of aluminum alloy (including not only a rolled plate but also a tempered plate).

  For example, panels such as outer panels (outer plates) and inner panels (inner plates) of panel structures such as automobile hoods, fenders, doors, roofs, trunk lids, etc. are made of Al-Mg-Si AA to JIS6000 series. The use of aluminum alloy plates such as AA to JIS5000 (hereinafter simply referred to as 5000), such as Al-Mg based (hereinafter simply referred to as 6000), is being studied.

  These aluminum alloy plates are used by being joined to steel materials (steel members) such as steel plates and mold steels that are generally used in ordinary automobile bodies unless they are all aluminum automobile bodies. Therefore, when an aluminum alloy material is used for an automobile body (a member in which a steel material and an aluminum alloy material are combined), this also inevitably involves the joining of dissimilar materials of Fe-Al (bonding between dissimilar metal members of iron-aluminum). ) Is necessary.

  However, as well known as a problem when performing this Fe-Al dissimilar material joining by welding, as is well known, an intermetallic compound layer of Fe and Al that is highly brittle and extremely brittle at the joint interface (hereinafter also referred to simply as an interface). (Reaction layer) is generated. For this reason, even if they are joined to each other, even though they are joined to each other, due to the formation at the interface of this intermetallic compound layer, in the Fe-Al dissimilar material joint by welding, it becomes a dissimilar material joint or dissimilar material welded joint. In many cases, sufficient bonding strength cannot be obtained.

  Reflecting this, conventionally, these dissimilar material joined bodies (joints of dissimilar metal members) are joined not only by welding, but also by using bolts, rivets, etc., or using an adhesive, There are problems such as complexity of the joining operation and an increase in joining cost.

  Therefore, conventionally, it has been studied to use efficient spot welding, which is widely used for joining steel plates of a normal automobile body, as a welding method for joining different materials of Fe-Al.

  As such spot welding, for example, a method of inserting an aluminum-steel clad material between an aluminum material and a steel material in order to suppress the formation at the interface between the brittle Fe and Al intermetallic compound layers described above. Proposed. In addition, methods have been proposed in which a metal having a low melting point is plated or inserted on the steel material side. Furthermore, a method of sandwiching insulator particles between an aluminum material and a steel material, a method of providing unevenness in a member in advance, and the like have been proposed. Further, after removing the non-uniform oxide film of the aluminum material, it is heated in the atmosphere to form a uniform oxide film, and in a state where the contact resistance of the aluminum surface is increased, two layers of aluminum-steel are formed. A method of spot welding using a steel plate as an insert material has also been proposed.

  On the other hand, when elements that easily form oxides such as Si, Mn, and Al are added on the steel material side to increase the strength of the steel sheet, the surface of the base material contains oxides containing these Si, Mn, and Al. Is known to form. It is also known that these oxides containing Si, Mn, Al, etc. inhibit the adhesion between the surface coating such as galvanization and the steel sheet. On the other hand, if the thickness of the oxide layer containing Si, Mn, Al, etc. is in the range of 0.05 to 1 μm by pickling the steel plate, the adhesion between the surface coating such as galvanization and the steel plate It is also known that spot weldability between steel plates is improved (see Patent Document 1).

  However, in these prior arts, sufficient joining strength is provided to the welded joint of Fe-Al dissimilar materials, which is commonly used for joining the body of an ordinary automobile, and under the joining conditions by efficient spot welding. I can't get it. In other words, the spot welding conditions for obtaining the joint strength must be complicated, which is not realistic.

  On the other hand, in particular, a technique intended for spot welding of a dissimilar welded joint between a 6000 series aluminum alloy material, etc., which is widely used for automobile bodies, and a high-strength steel plate (high-tensile material) with a tensile strength of 450 MPa or more. Various proposals have also been made.

  For example, in Patent Documents 2 and 3, spot welding is performed in such a manner that two or more steel materials are stacked or steel materials with a plate thickness limited to 3 mm or less or steel materials are sandwiched between aluminum alloy materials. Proposed. Patent Document 4 proposes to improve the joint strength by defining the nugget area and the thickness of the interface reaction layer in the spot weld. In Patent Documents 5 and 6, it is proposed that the composition, thickness, area, and the like of each generated compound on the steel material side and the aluminum alloy material side at the welding interface are each finely defined to improve the joint strength. .

  Further, in Patent Document 7, in a high-strength steel plate having a specific composition, an existing oxide layer on the steel plate surface is removed once, and then an outer oxide layer newly formed and an inner oxide layer immediately below the steel fabric surface It is proposed to obtain a high joint strength of a dissimilar material joined body under appropriate spot welding conditions. This patent document 7 suppresses the diffusion of Fe and Al during spot welding by the outer oxide layer and the inner oxide layer, and causes excessive generation of an Al—Fe-based brittle intermetallic compound layer at the bonding interface. It is to suppress.

  Further, in Patent Document 8, a Cu alloy layer is disposed on the surface to be joined on the iron-based material side with a thickness of 0.1 to 1 μm or as a foil with a thickness of 0.01 to 1 mm. The layer is used as a brazing filler metal in spot welding to suppress excessive formation of an Al—Fe brittle intermetallic compound layer. And in order to guarantee the effect of this Cu alloy layer, when joining by spot welding, a fluoride-type flux is applied to the joining surface on the aluminum material side, and the oxide film on the aluminum material side is removed to join. It is essential to do.

JP 2002-294487 A JP 2007-144473 A JP 2007-283313 A JP 2006-167801 A JP 2006-289552 A JP 2007-260777 A JP 2006-336070 A JP 2004-351507 A

  These Patent Documents 2-7 are commonly intended for spot welding of a dissimilar welded joint between an aluminum alloy material and a high-strength steel plate, have few restrictions on application conditions, etc., have excellent versatility, and are brittle at the joint. The object is to improve the bonding strength by suppressing the formation of intermetallic compounds.

  However, even in these Patent Documents 2 to 8, regarding spot welding of a dissimilar welded joint between an aluminum alloy material and a high-strength steel plate, the joint strength is still less than about 2 kN and is not high enough, and there is room for improvement. . In these conventional technologies, although excessive generation of an Al—Fe-based brittle intermetallic compound layer at the joint interface of spot welding is suppressed in common, generation of a substantial amount is unavoidable. In other words, these conventional techniques do not eliminate the formation of Al—Fe-based brittle intermetallic compound layers.

  The present invention has been made in view of such problems, and its object is to eliminate the formation of an Al-Fe-based brittle intermetallic compound layer at the joint interface of spot welding, and to achieve high joint strength. Another object is to provide a dissimilar material joint and a dissimilar material joining method of aluminum alloy plate.

  In order to achieve the above object, the gist of the dissimilar joint joint of the present invention is a dissimilar joint joint in which an aluminum alloy plate and a steel plate that has been plated with Cu in advance are overlapped and joined by spot welding. The interfacial reaction layer formed by spot welding at the interface between the steel plate and the aluminum alloy plate is composed of a eutectic structure of an intermetallic compound phase of Al and Cu and a metal Al phase. As a proportion of the phase, the average area ratio per unit area of the cross section in the plate thickness direction of the interface reaction layer, the intermetallic compound phase of Al and Cu is 70 to 90%, and the metal Al phase is 10 to 10%. Suppose that it is 30%.

  Further, the gist of the dissimilar material joining method of the present invention for achieving the above object is a dissimilar material joining method in which a steel plate and an aluminum alloy plate are overlapped and joined by spot welding, and is preliminarily attached to a surface to be joined of the steel plate. After applying Cu plating with a thickness of 3 to 10 μm, the applied pressure is over 3 kN, 6 kN or less, the welding current is over 18 kA, 30 kA or less, and the welding time is 40 to 500 msec. Spot welding without using, and the interface reaction layer formed by the spot welding at the interface between the steel plate and the aluminum alloy plate is a eutectic structure of an intermetallic compound phase of Al and Cu and a metallic Al phase, As a ratio of each phase in the eutectic structure, the intermetallic compound phase of Al and Cu is 70 in terms of an average area ratio per unit area in the cross section in the thickness direction of the interface reaction layer. While 90% of the metal Al phase is to have 10 to 30%.

  Here, the interfacial reaction layer of the present invention is not only composed of the eutectic structure of the intermetallic compound phase of Al and Cu and the metallic Al phase, but also in the rest of the eutectic structure or in the co-crystal structure. In addition to the crystal structure, other intermetallic compound phases or metal phases may be included. However, the interfacial reaction layer or eutectic structure of the present invention, together with other intermetallic compound phases and metal phases, is based on the definition of the area ratio of the intermetallic compound phase of Al and Cu and the metallic Al phase. In particular, the structure regulates the harmful intermetallic compound of Fe and Al. That is, the interfacial reaction layer of the present invention contains a small amount or no amount of the harmful intermetallic compound of Fe and Al as the balance of the eutectic structure.

  In the above-mentioned Patent Document 8 in which the Cu alloy layer is disposed on the surface to be joined on the iron-based material side, the present inventor still cannot suppress the formation of an Al—Fe-based brittle intermetallic compound layer at the joint interface of spot welding. Investigated anew. As a result, there is a novel interfacial reaction layer structure (composition) that can suppress the formation of the Al—Fe-based brittle intermetallic compound layer and obtain a dissimilar joint with excellent joint strength. It was found that the spot welding conditions for obtaining the layer composition are separately from the conventional spot welding conditions.

  That is, when placing pure Cu plating on the surface to be joined on the iron-based material side, if welding under the special spot welding conditions, in the interfacial reaction layer between different materials formed by spot welding, it is completely different from normal, It has been found that a novel interfacial reaction layer structure defined in the present invention can be obtained. In such a novel interfacial reaction layer structure, the formation of the Al—Fe-based brittle intermetallic compound layer is suppressed, and a dissimilar joint joint excellent in bonding strength can be obtained. In other words, even if a Cu alloy layer is disposed on the surface to be joined on the iron-based material side or pure Cu plating is disposed, when welding is performed within the range of the spot welding conditions described in Patent Document 8, Thus, the structure of the interface reaction layer defined in the present invention is not achieved, and the formation of an Al—Fe-based brittle intermetallic compound layer at the joint interface of spot welding cannot be suppressed.

  According to the present invention, after pre-plating Cu on the surface to be joined on the steel plate side, a flux is used so as to satisfy special conditions in which the pressurizing force and welding current are increased and the welding time is shortened. The formed interfacial reaction layer has a composition composed of an intermetallic compound of Al and Cu and metal Al. Accordingly, the remainder of the interfacial reaction layer may be an intermetallic compound of Fe and Al, or the interfacial reaction layer may have no intermetallic compound of Fe and Al, and the bonding strength It is possible to obtain a dissimilar joint joint excellent in

It is a drawing substitute photograph which shows the interface structure of this invention dissimilar material joint. It is explanatory drawing which shows the composition in each site | part of the interface structure | tissue in FIG.

(Interface structure)
FIG. 1 shows a cross-sectional view in the thickness direction of a nugget of a lap joint by spot welding of a high-tensile steel plate and a high-strength 6000 series aluminum alloy plate in a reflected electron image of an SEM as in an example described later. The structure was observed and Example 1 of Table 4 (welding conditions are g in Table 3).

  In FIG. 1, a whitish dense portion where a flat portion on the upper side of the drawing and an indented dent (crater) are mixed is a region of the high-tensile steel plate. And the whitish rough part which branches downward in the shape of a root on the lower side of the figure is the region of the 6000 series aluminum alloy plate.

  On the other hand, in the region between these regions (in the middle portion of FIG. 1), a whitish ridge-like (peak-like) portion and a black elongated valley-like (groove-like) sandwiched between these whitish ridge-like portions. The portion is folded in layers (stratified) or striped (overlapped).

  The structure of the intermediate region is the structure of the interfacial reaction layer defined in the present invention, and the intermetallic compound phase of Al and Cu and the metal formed by the spot welding at the interface between the steel plate and the aluminum alloy plate. It is an interface reaction layer composed of a eutectic structure with an Al phase. That is, the interface reaction layer has two solid phases (intermetallic compound phase of Al and Cu and metallic Al phase) crystallized (solidified) in the course of cooling after melting by spot welding as shown in FIG. It is a so-called “eutectic” structure in terms of material that is mixed (mixed) regularly (in layers).

  Here, in the present invention, two solid phase “phases” in the eutectic structure, that is, an intermetallic compound phase of Al and Cu, and a metal Al phase are used by using the term “phase”. It is used separately from the “layer” of the Fe-based brittle intermetallic compound layer. This is because an Al—Fe-based brittle intermetallic compound layer is a “layer” that has a relatively large thickness and area as an interface reaction layer, and thus, as previously stated, a “layer”. This is because the term “phase” is technically different and expressed separately.

  In this eutectic structure in the present invention, the ratio of the respective phases is such that the intermetallic compound phase of Al and Cu is 70 to 90 in terms of the average area ratio per unit area of the cross section in the thickness direction of the interface reaction layer. %, And the metal Al phase is 10 to 30%. That is, the remainder of this eutectic structure contains a small amount of an intermetallic compound of Fe and Al, or does not include an intermetallic compound of Fe and Al as an amount that can be analyzed (quantitatively). It is characterized by a structure that suppresses the formation of an Al—Fe brittle intermetallic compound layer at the interface.

  FIG. 2 shows the atomic concentrations of O, Al, Fe, and Cu by EDX analysis in the respective portions of the interfacial reaction layer 4-1, 4-2, and 4-3 in FIG. The composition in atomic concentration%) is shown. In the interfacial reaction layer of FIG. 1, the whitish bowl-shaped portions to which the respective numbers are attached are common to the respective parts of 4-1, 4-2, 4-3 as shown in FIG. Cu occupies much and Fe is remarkably small. This fact confirms that the whitish bowl-shaped portion is composed of an intermetallic compound phase of Al and Cu and has a structure that suppresses the formation of an Al—Fe-based brittle intermetallic compound layer. In FIG. 2, the amount of Fe in the 4-1 part is larger than the amount of Fe in each of the parts 4-2 and 4-3. This is thought to be due to detection.

  Further, in the interfacial reaction layer folded in a layered manner, the black elongated valley-like portion sandwiched between the whitish ridge-like portions is also extremely low in Fe and also very low in Cu according to the EDX analysis. On the other hand, Al occupied the majority. This fact confirms that the black elongated valley-shaped portion is made of only metal Al and has a structure that suppresses the formation of an Al—Fe-based brittle intermetallic compound layer.

  From these facts, the interfacial reaction layer formed by spot welding at the interface between the steel plate and the aluminum alloy plate has a whitish saddle-like portion made of an intermetallic compound of Al and Cu, and has a whitish saddle-like portion. It can be seen that the sandwiched black elongated portion is made of only metal Al. It can be seen that the interface reaction layer formed by spot welding has a layered structure in which the intermetallic compound phase of Al and Cu and the metallic Al phase overlap (fold) each other.

  And in various examples in which the spot welding conditions are changed, the intermetallic compound phase of Al and Cu in the interfacial structure of FIG. 1 (white whitish portion) and the metal Al phase (white whitish portion) are sandwiched. The area ratio occupied by the black elongated portions) was measured. As a result, as an average area ratio per unit area of the cross section in the thickness direction of the interface structure of FIG. 1, the intermetallic compound phase of Al and Cu is 70 to 90%, and the metal Al phase is 10 to 30%. It has been found that the joint strength of the dissimilar material joint is improved when the composition is made of.

  That is, in the range of these prescribed average area ratios, the eutectic structure (interfacial reaction structure) of FIG. 1 is almost composed of the intermetallic compound phase of Al and Cu and the metal Al phase, The remainder of this average area ratio contains the intermetallic compound (layer) of Fe and Al in the eutectic structure, or the intermetallic compound of Fe and Al (in the interface reaction layer) in the eutectic structure ( Layer) is not included as an amount that can be analyzed (quantified).

  On the other hand, in the eutectic structure (interfacial reaction structure), when the intermetallic compound of Al and Cu is less than 70% and the metal Al is less than 10%, the remainder is Fe and Al. As a result, the ratio of brittle intermetallic compounds of Fe and Al increases, which is similar to the conventional interfacial reaction structure, and the joint strength of the joint decreases. In other words, when the eutectic structure is below the specified average area ratio, the formation of an Al—Fe brittle intermetallic compound layer at the joint interface of spot welding cannot be suppressed.

  On the other hand, the formation of an interfacial reaction structure in which the intermetallic compound of Al and Cu exceeds 90% or the metal Al exceeds 30% depends on the thickness of the Cu plating layer and spot welding conditions described later. It is difficult even if it is optimized, and there is no need to increase the intermetallic compound of Al and Cu or the ratio of the metal Al. Therefore, the composition of the interfacial reaction structure is in the above-described range.

(Cu plating of steel sheet)
In the present invention, Cu plating is performed in advance on the bonded surfaces of the steel plates before spot welding. By applying Cu plating to the surface to be joined of the steel plate in advance, the interfacial reaction layer formed in combination with spot welding under special conditions in which the pressurizing force and welding current are increased and the welding time is shortened is made of Al and Cu. A composition comprising an intermetallic compound and metal Al. That is, the remainder of the interfacial reaction layer can be an intermetallic compound of Fe and Al, or the interfacial reaction layer can be free of an intermetallic compound of Fe and Al. An excellent dissimilar joint joint can be obtained.

  Spot welding is energization heating. However, when a steel plate and an aluminum alloy plate are brought into contact with each other, a plurality of point contacts are made, so that it is generally difficult to uniformly heat the joint (interface portion). Here, by applying Cu plating to the surface (surface) to be joined on the steel plate side, the aluminum alloy plate and Cu plating are still in point contact, but because the Cu conductivity is good, the steel plate side generates heat uniformly. As a result, a more uniform interface reaction layer having the above composition can be obtained.

Cu plating thickness:
The average thickness of the Cu plating for this purpose is in the range of 3 to 10 μm. If the average thickness of the Cu plating is less than 3 μm, the amount of Cu that makes point contact with the aluminum alloy plate is too small, and the effect of the good conductivity of the Cu is not exhibited, and the steel plate side does not generate heat uniformly. As shown in FIG. 1, a thin, thin or layered interface reaction layer (interface layer after spot welding) cannot be obtained. That is, even in combination with spot welding under the above special conditions, the interface reaction layer cannot be made of a composition composed of an intermetallic compound of Al and Cu and metallic Al, and the composition of intermetallic compound of Fe and Al is increased. In addition, since only a portion of this intermetallic compound appears, it becomes a non-uniform layered interface reaction layer, resulting in insufficient bonding strength.

  On the other hand, when the average thickness of the Cu plating exceeds 10 μm, the thickness becomes thick like a foil, and on the contrary, the effect of the good conductivity of Cu cannot be obtained. That is, even in combination with the spot welding under the special conditions, the interface reaction layer cannot have a composition composed of an intermetallic compound of Al and Cu and metal Al, and the composition is uniform and thin as shown in FIG. A layered interfacial reaction layer cannot be obtained. That is, the intermetallic compound of Fe and Al increases, and furthermore, only the intermetallic compound appears, resulting in a non-uniform layered interface reaction layer, resulting in insufficient bonding strength.

Composition of Cu plating:
The Cu plating in the present invention is also a raw material for making the interface reaction layer a composition composed of an intermetallic compound of Al and Cu and metal Al. In order to exhibit the effect of the good conductivity of Cu, Cu plating The plating composition is preferably pure Cu (pure copper) plating. That is, when the Cu alloy containing a large amount of the third element as described in Patent Document 8 is used, the conductivity of the Cu alloy plating that is in point contact with the aluminum alloy plate is lowered, and the interface reaction occurs. It becomes difficult for the layer to have a composition composed of an intermetallic compound of Al and Cu and metal Al.

  In the spot welding described in Patent Document 8, as described, the Cu alloy itself coated on the surface to be joined of the iron-based material becomes a brazing material to form an interface reaction layer, and brittle Fe and Suppresses formation of an intermetallic compound layer with Al. For this reason, in Patent Document 8, the composition of the interface reaction layer and the joint strength of the joint are largely governed by the coated Cu alloy layer.

  Therefore, the surface to be joined of the iron-based material in Patent Document 8 is not pure Cu having low strength, but mainly Cu, and Si, Mn, Zn, Cr, Mg, Ni, Al, Sn. A Cu alloy having a high strength containing at least one element selected from the group consisting of Fe, Ti and Zr. For example, the total addition amount of Al and Fe is a large amount of 30% by mass or less. Moreover, the total addition amount to Cu alloy layers, such as Si, Mn, Zn, Cr, Mg, Sn, Ti, Zr, and Ni, is also a large amount of 30% by mass or less.

  However, in the Cu plating according to the present invention, as described above, pure Cu plating is preferable, all of these alloy elements are unnecessary, and the smaller the content of alloy elements, the more preferable, the total content of all these elements in Cu plating. The amount is 5% by mass or less (including 0%). Incidentally, the Cu plating in the present invention can be formed on the bonded surface of the steel sheet by a known plating method such as immersion or electroplating by ordinary methods or vacuum deposition.

  The Cu plating needs to be applied to the surface to be joined (one side or one side) of the steel sheet, which becomes the joint surface with the aluminum alloy plate, but it needs to be applied to the other side (both sides) Not necessarily. However, if it is difficult to plate only one side of the steel plate, such as by dipping the steel plate in a plating solution, it may be plated on both sides, and if necessary, the other side (both sides) Other plating or coating such as galvanization may be applied as appropriate.

(Spot welding conditions)
The spot welding condition in the present invention is also a combination with a Cu plating layer applied in advance to the joint surface of the steel sheet, so that the formed interface reaction layer has a composition composed of an intermetallic compound of Al and Cu and metal Al. is important. That is, the remainder of the interfacial reaction layer can be an intermetallic compound of Fe and Al, or the interfacial reaction layer can be free of an intermetallic compound of Fe and Al. It is important to obtain an excellent dissimilar joint joint.

  The spot welding conditions in the present invention are different from the spot welding conditions of the conventional method and Patent Document 8 and are special conditions in which the welding pressure and the welding current are increased and the welding time is shortened. Specifically, the applied pressure is over 3 kN, 6 kN or less, the welding current is over 18 kA, 30 kA or less, and the welding time is 40 to 500 msec. A general-purpose spot welding apparatus can be used for spot welding under such conditions.

  If the applied pressure is too low outside the above range, the contact point between the steel plate (Cu plating) and the aluminum alloy plate is few, so the interface reaction during spot welding becomes non-uniform, and the interface reaction layer is made of Al and Cu. Thus, a composition composed of an intermetallic compound and metal Al cannot be obtained, and an interface reaction layer having a uniform composition and a thin or layered structure as shown in FIG. 1 cannot be obtained. That is, the intermetallic compound of Fe and Al increases, and furthermore, only the intermetallic compound appears, resulting in a non-uniform layered interface reaction layer, resulting in insufficient bonding strength. On the other hand, when the applied pressure is too high, the melted portion scatters from the nugget during spot welding, resulting in insufficient bonding strength.

  Even if the welding current is too low, the welding time is too short, or conversely too long, the interface reaction during spot welding is insufficient and the nugget is sufficiently formed. The interface reaction layer is composed of the above-described intermetallic compound of Al and Cu and metal Al, and cannot have a composition that does not include the intermetallic compound of Fe and Al.

  On the other hand, in the case of high current or long-time spot welding where the welding current is increased outside the above range, or the welding time is increased outside the above range, the interfacial reaction proceeds too much, and the interfacial reaction layer formed on the contrary is formed. The composition does not include the intermetallic compound of Fe and Al, and the bonding strength is insufficient.

  Incidentally, in the said patent document 8, welding current is 8-18 kA, a pressurizing force is 1.5-3 kN, and welding time is 1 s or less as preferable spot welding conditions. When the applied pressure exceeds 3 kN, element diffusion is promoted, and a large amount of Cu—Al intermetallic compound is formed, so that a good joint cannot be obtained. Further, when the welding time exceeds 1 s, the growth of the Cu—Al-based intermetallic compound is promoted and a good joint cannot be obtained. Therefore, the usefulness of making the interfacial reaction layer formed as in the present invention a composition comprising an intermetallic compound of Al and Cu and metal Al is not recognized, and deviated from the present invention. Since the spot welding conditions are employed, the composition of the present invention is not obtained.

  Thus, in the said patent document 8, it recognizes that the production | generation and growth of a Cu-Al type | system | group intermetallic compound are encouraged by spot welding conditions. However, the usefulness of this Cu—Al-based intermetallic compound is not recognized at all, and on the contrary, it is actively trying to eliminate it as a harmful intermetallic compound.

  In the spot welding joining method in the present invention, the use of a flux as in Patent Document 8 is unnecessary, and it is harmful. According to the spot welding conditions in the present invention, it is possible to remove the oxide film from the bonded surface of the aluminum alloy plate without applying the fluoride-based flux to the bonded surface of the aluminum alloy plate. Further, it is not necessary to improve the wettability between the molten Cu from the Cu plating of the steel plate and the aluminum alloy plate surface. When the flux such as the fluoride is used, it becomes difficult to make the formed interface reaction layer in the spot welding condition according to the present invention different from the composition containing no intermetallic compound of Fe and Al in the present invention.

(Material steel plate)
About the raw steel plate used for the dissimilar joint joint of the present invention, a normal mild steel plate can be used in the present invention. However, it is preferable to use a high-strength steel plate (high tensile) having a tensile strength of 450 MPa or more in view of applications that require weight reduction and high strength such as automobile members.

  In addition, ordinary mild steel sheets tend to form a brittle intermetallic compound layer (reaction layer) of Fe and Al during spot welding as compared with high-strength steel sheets. For example, mild steel sheets are generally low-alloy steels, and the oxide film is almost iron oxide. Therefore, diffusion of Fe and Al is facilitated, and compared with high-strength steel sheets, a brittle metal between Fe and Al. A compound layer (reaction layer) is easily formed. In addition, mild steel sheet is less brittle than Fe steel and Al, because it contains less Si and Mn, which has the effect of suppressing the diffusion of Fe and Al as the base material, and it is easy to diffuse Fe and Al. It is easy to form an intermetallic compound layer of Fe and Al. Furthermore, mild steel sheets have lower steel strength than high-strength steel sheets, and depending on the plate thickness, the deformation of the steel sheet tends to increase due to the pressure applied by the electrode tips during spot welding. There is also a tendency that a brittle intermetallic compound layer of Fe and Al is easily formed without increasing the pressure (because there is a limit).

Component composition of steel sheet:
The component composition of the high-strength steel plate is preferably a steel plate composition containing a predetermined amount of Si, Mn, etc., for example, by mass%, C: 0.01 to 0.30%, Si: 0.1 to 3.00% , Mn: 0.1 to 3.00% each, and the balance is preferably composed of Fe and inevitable impurities. In addition to this, Al: 0.002 to 0.1% may be further contained. Further, in addition to or instead of Al, Nb: 0.005 to 0.10%, Ti: 0.005 to 0.10%, Zr: 0.005 to 0.10%, Cr : 0.05 to 3.00%, Mo: 0.01 to 3.00%, Cu: 0.01 to 3.00%, Ni: 0.01 to 3.00%, one or more May be contained. In addition, the unit (content of each element) of the chemical component in this invention is mass% altogether including an aluminum alloy.

Steel plate thickness:
The thickness t1 of the steel plates to be joined is preferably selected from the range of 0.5 to 5.0 mm according to the thickness t2 on the aluminum material side. If the plate thickness t1 of the steel plate is too thin at less than 0.5 mm, the strength and rigidity required for the automobile member and the like cannot be ensured. Moreover, when the thermal deformation of the steel material at the time of spot welding becomes large and this is remarkable, the material of the steel material falls off and a sound welded joint cannot be obtained. On the other hand, if the steel plate thickness t1 exceeds 5.0 mm, it becomes difficult to control the amount of heat input, spot welding becomes difficult, and the weight reduction, which is one of the advantages of dissimilar joint materials, is sacrificed.

(Aluminum alloy plate)
The material aluminum alloy plate used for the dissimilar joint joint of the present invention is not particularly limited in the type of the alloy, but, as in the case of the steel plate, from the application that requires weight reduction and high strength of automobile members and the like. Therefore, higher strength is desirable. In this regard, among aluminum alloys, it is optimal to use an AA (or JIS) 6000 series, 7000 series, or 5000 series aluminum alloy plate that has high strength and is widely used as this kind of structural material.

  The thickness t2 of the aluminum alloy plate to be used is preferably in the range of 0.5 to 4.0 mm. If the thickness t2 of the aluminum alloy plate is too thin at less than 0.5 mm, the strength as a structural material is insufficient, which is inappropriate. Moreover, when the thermal deformation of the aluminum material at the time of fusion welding becomes large and this is remarkable, the material of the aluminum material falls off and a sound welding joint cannot be obtained. On the other hand, when the thickness t2 of the aluminum material exceeds 4.0 mm and is too thick, the weight reduction which is one of the advantages of the dissimilar material joint is sacrificed. In addition, it becomes difficult to control the amount of heat input required to generate an interface reaction layer having an appropriate thickness.

  EXAMPLES Hereinafter, although an Example is given and this invention is demonstrated more concretely, this invention is not limitedly interpreted by this Example.

  A 980 MPa class high-tensile steel plate having a plate thickness of 1.2 mm and a 6000 series aluminum alloy plate having a plate thickness of 1.0 mm and a 0.2% proof stress of 250 MPa or more are spot-welded without using a flux, Dissimilar material joints were manufactured, and the structure and performance were investigated and evaluated.

  More specifically, a steel plate having the component composition shown in Table 1 and an aluminum alloy plate having the component composition shown in Table 2 were processed into a cross tensile test piece shape described in JIS A 3137 and overlapped. Spot welding was performed under each of the conditions a to n, and different materials were joined. Here, as evaluated from the peel strength shown in Table 4 described later, a to f and l to n shown in Table 3 are inappropriate in which any one of the applied pressure, the welding current, and the welding time is out of the appropriate range. Spot welding conditions, g to k, are appropriate spot welding conditions.

  Both the steel plate and the aluminum alloy plate were processed into the shape of the cross tensile test piece (50 mm width × 150 mm length), overlapped with each other, and the overlapped central portion was spot welded in the width direction. For spot welding, a single-layer rectifying resistance spot welder (capacity 90 KVA) is commonly used, and spot welding is performed at 10 points under the conditions shown in Table 3 for each of pressure, welding current, and welding time. Went. In this case, a dome-shaped electrode made of a Cu—Cr alloy was used in common, and the positive electrode was an aluminum material and the negative electrode was a steel material.

  The surface to be joined of the steel plate was preliminarily plated with 5 μm of pure Cu before being spot welded. In addition, the steel plate which has not given pure Cu plating to the to-be-joined surface was also tested for the comparison.

(Interfacial reaction layer thickness and formation range)
The thickness and the formation range of the interface reaction layer of each of the dissimilar joints thus produced were measured. These results are also shown in Table 4. The thickness of the interfacial reaction layer in the plate thickness direction is measured by cutting in the plate thickness direction at the center of each spot weld, embedding it in resin, polishing it, and performing SEM (0.5 mm intervals over the entire spot weld). Observation by reflected electron image). When the thickness of the interface reaction layer is 1 μm or more, it is measured with a field of view of 3000 times, and when it is less than 1 μm, it is measured with a field of view of 10,000 times. The value averaged at the spot welds was taken as the average thickness of the interface reaction layer. The results are shown in Table 4.

  Furthermore, during this SEM observation, the interface reaction layer is subjected to elemental analysis as shown in FIG. 2 by EDX to determine what each layered structure shown in FIG. 1 is, and in the interface reaction layer, The presence state of these intermetallic compounds of Al and Cu and metal Al, and the average area ratio per unit area of the cross section in the thickness direction were determined. That is, from the 3000 times SEM (reflection electron image) photograph, a region having a width of 1 μm is taken in the thickness direction of the interface reaction layer (vertical direction in FIG. 1) with respect to the boundary between the steel plate and the interface reaction layer, The areas of the intermetallic compound of Al and Cu and the metal Al in the interface reaction layer in this region were determined. Then, the respective areas were averaged by the observation results at the intervals, and the average area ratio (%) per unit area of the cross section in the plate thickness direction of the interface reaction layer was obtained. The results are shown in Table 4, and the rest of these are regarded as brittle intermetallic compounds of Fe and Al, and Table 4 also lists their average area ratios.

  Each of the manufactured joints was subjected to a cross tensile test with a tensile tester to determine the peel strength (maximum load). These results are also shown in Table 5. With reference to the spot weld joint strength between A6022 aluminum materials = 1.0 kN, the peel strength was evaluated as ◯ if it was 2.0 kN or more, and x if it was less than 2.0 kN.

  As is apparent from Table 4, the dissimilar material joint of the invention example has an appropriate pure Cu plating applied to the surface to be joined of the steel plate in advance, and is spot-welded under the appropriate conditions. As a result, the formed interfacial reaction layer had an average area ratio of 70 to 90% per unit area of the cross section in the thickness direction of the interfacial reaction layer, an intermetallic compound of Al and Cu, and the average area ratio of 10 It consists of a uniform layered structure with ~ 30% metal Al. The remainder of the interface reaction layer is an intermetallic compound of Fe and Al, or the interface reaction layer does not have an intermetallic compound of Fe and Al. As a result, the dissimilar material joint according to the invention example has excellent joint strength (peel strength).

  On the other hand, as apparent from Table 4, the dissimilar joints of the comparative examples have no pure Cu plating on the surfaces to be joined of the steel plates, or the spot welding conditions are not appropriate. As a result, the formed interface reaction layer is not a uniform layered structure of the intermetallic compound of Al and Cu and the metal Al, which has the predetermined area range. Further, the amount of the intermetallic compound of Fe and Al remaining in the interface reaction layer is relatively large. As a result, the bonded joint of the dissimilar material of the comparative example is significantly inferior in bonding strength (peeling strength) as compared with the inventive example.

  In other words, even if pure Cu plating is preliminarily arranged (applied) on the surface to be joined on the steel plate side, it is not a suitable spot welding condition indicated by g to k in Table 3; It is not an interfacial reaction layer composed of a eutectic structure. As a result, it can be seen that formation of an Al—Fe brittle intermetallic compound layer at the joint interface of spot welding cannot be suppressed, and the joint strength of the dissimilar material joint is lowered.

  Therefore, from these facts, the critical strength of the spot welding condition, which is pure Cu plating of the bonded surface of the steel sheet of the present invention, high pressurization force, high current, and short welding time, with respect to the joint strength of the dissimilar joint joint. The significance is supported.

  According to the present invention, there is provided a dissimilar joint joint and dissimilar material joining method between a steel plate and an aluminum alloy plate, which can eliminate the formation of an Al-Fe brittle intermetallic compound layer itself at the joint interface of spot welding and achieve high joint strength. Can be provided. Such a dissimilar material joint and dissimilar material joining method can be usefully applied as various structural members and welding methods thereof in the transportation field such as automobiles and railway vehicles, machine parts, building structures, and the like.

Claims (3)

  A dissimilar joint joint in which an aluminum alloy plate and a steel plate that has been subjected to Cu plating in advance are overlapped and joined by spot welding, and formed by spot welding at the interface between the steel plate and the aluminum alloy plate The interfacial reaction layer is composed of a eutectic structure of an intermetallic compound phase of Al and Cu and a metallic Al phase, and the ratio of each phase in the eutectic structure is as a unit area of the cross section in the plate thickness direction of the interfacial reaction layer. A dissimilar joint with excellent joint strength, characterized in that the intermetallic compound phase of Al and Cu is 70 to 90% and the metal Al phase is 10 to 30%.   The average thickness of the Cu plating layer applied in advance to the joint surface of the steel sheet is 3 to 10 μm, and the spot welding is performed with a pressure of over 3 kN, 6 kN or less, a welding current of over 18 kA, 30 kA or less, a welding time of 40 to 40 The dissimilar joint joint excellent in joining strength according to claim 1, which satisfies each condition of 500 msec and does not use a flux.   A dissimilar material joining method in which a steel plate and an aluminum alloy plate are overlapped and joined by spot welding, and the surface to be joined of the steel plate is subjected to Cu plating with a thickness of 3 to 10 μm in advance, and the applied pressure exceeds 3 kN. 6 kN or less, welding current: exceeding 18 kA, 30 kA or less, spot welding is performed without using flux so as to satisfy each condition of welding time 40 to 500 msec, and the spot is formed at the interface between the steel plate and the aluminum alloy plate. The interfacial reaction layer formed by welding is a eutectic structure of an intermetallic compound phase of Al and Cu and a metallic Al phase, and the ratio of each phase in the eutectic structure is a cross section in the thickness direction of the interfacial reaction layer. The average area ratio per unit area of the intermetallic compound phase of Al and Cu was 70 to 90%, and the metal Al phase was 10 to 30%. Wherein, excellent dissimilar materials bonded manner bonding strength.

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