JP2006224127A - Method for manufacturing steel/aluminum joined structure - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for manufacturing a steel/aluminum joined structure having a high joining strength by controlling a current supply pattern in spot welding. <P>SOLUTION: When an aluminum material is overlapped on a hot-dip aluminum plated steel sheet and is integrated with each other by spot welding, the setting value W of a welding current is determined within the range from 8 to 14 kA, and the following conditions are satisfied: Q<SB>1</SB>/Q<SB>2</SB>is within the range from 0.05 to 3.0, and Q<SB>1</SB>+Q<SB>2</SB>is within the range from 1 to 5 kA by second, where Q<SB>1</SB>is an accumulated current within an up-slope period (t<SB>0</SB>→t<SB>1</SB>) from the starting time of the current supply until the time at which the welding current has reached the setting value W, and Q<SB>2</SB>is an accumulated current within a constant current welding period (t<SB>1</SB>→t<SB>2</SB>). <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention is a steel / aluminum suitable for various structural materials, such as a vehicle member and a heat exchanger, which have both light weight and corrosion resistance of an aluminum material and excellent mechanical strength of a steel material, and are required to be light and have high strength. The present invention relates to a method for manufacturing a bonded structure.

  Aluminum materials such as aluminum and aluminum alloys are used in various fields by utilizing their light weight and excellent corrosion resistance. However, in applications where strength is important, the required strength is satisfied by increasing the thickness. However, the thickening impairs the lightness that is an advantage of the aluminum material and is not suitable as a structural member corresponding to a compact design. When a steel material with high mechanical strength is laminated with an aluminum material, the required strength can be obtained without the need for thickening.

  For the lamination of the aluminum material and the steel material, a mechanical coupling method such as bolts, nuts, rivets, and fitting has been adopted. However, an excellent joint is difficult to obtain by the mechanical coupling method, and the productivity is low. When the aluminum material / steel material can be welded together, a steel / aluminum joint structure having significantly higher productivity and better characteristics than the mechanical joining method can be obtained. However, when steel materials and aluminum materials are joined by a normal melt joining method, a large amount of very fragile intermetallic compounds are generated at the joining interface, and the joining strength is significantly reduced.

An intermetallic compound is produced by an interdiffusion reaction of atoms of steel and aluminum materials at the interface. A method (Patent Document 1) is known that suppresses the formation of intermetallic compounds by appropriately controlling the reaction temperature, time, and the like that control the diffusion reaction during friction welding. Ingenuity is required, and there is room for improvement in simplifying the joining process. As seen in the method of resistance welding a hot-dip aluminized steel sheet to an aluminum material (Patent Document 2), application of spot welding has also been studied.
JP 2003-33885 A JP-A-6-39558

  Since a hot-dip aluminum-plated steel sheet has a hot-dip aluminum plating layer as a surface layer, it tends to be considered that the hot-dip aluminum-plated steel sheet exhibits the same behavior as an aluminum material at the time of joining. However, since the joining interface is heated to a high temperature exceeding the melting point (660 ° C.) of Al during spot welding, the molten Al produced by the high temperature heating is changed from the Al—Fe—Si ternary alloy layer at the base steel / plating layer interface to Fe. , Si, etc. diffuse. The diffused Fe reprecipitates during the cooling process during welding, and Si having a large diffusion coefficient is dispersed throughout the nugget. As a result, when the bonded interface after cooling is observed, a nugget in which a fragile Al—Fe binary alloy layer is formed in the entire bonded interface is detected, and the bonding strength is extremely low.

By regulating the proportion of the intermetallic compound in the joint interface, the adverse effect of the Al—Fe binary alloy layer on the joint strength can be suppressed (Patent Document 3). In order to suppress the formation of intermetallic compounds, the heat generated during spot welding is biased toward the hot-dip aluminum-plated steel sheet with the hot-dip aluminum-plated steel sheet on the positive electrode side and the aluminum material on the negative electrode side. Inevitable.
Japanese Patent Laid-Open No. 2003-145278

As a result of investigating and examining the behavior of Fe and Si that diffuses and reprecipitates in Al melted by high-temperature heating during spot welding, the present inventors have found that Al and Al are appropriately controlled by controlling the Fe and Si concentrations of the hot-dip aluminized steel sheet. The inventors filed a patent application that founded that a steel / aluminum bonded structure with high bonding strength was produced while suppressing the adverse effect of the -Fe binary alloy layer (Patent Document 4). In the prior application, the Fe and Si concentrations are adjusted to the ranges of 3 to 12% by mass and 0.5 to 5% by mass, respectively, and the area ratio of the Al—Fe binary alloy layer occupying the bonding interface is lowered to reduce the bonding strength. It is improving.
Japanese Patent Application No. 2003-336641

We further investigated and investigated the relationship between the nugget formed at the joint interface of the steel / aluminum joint structure and the joint strength. As a result, it was found that the nugget diameter increases as the nugget diameter increases, but the nugget diameter effective for improving the joint strength can be controlled by the energization pattern during spot welding.
The present invention is based on knowledge about the relationship between the nugget diameter and the energization pattern, and adopts an energization pattern with a gentle upslope at the start of energization, thereby generating a nugget with an appropriate diameter that is not excessively deep. An object of the present invention is to provide a steel / aluminum bonded structure having high bonding strength.

The present invention superimposes an aluminum material and the molten aluminum-plated steel sheet, making the steel / aluminum joined structure was integrated by spot welding, the welding current from the energization start relative integrated current Q ₂ of the constant current welding period Welding current with an energization pattern in which the ratio Q ₁ / Q ₂ of the integrated current Q ₁ during the up slope period until the value reaches the set value W is 0.05 to 3.0 and the sum Q ₁ + Q ₂ is ₁ to 5 kA · sec. Is supplied to the material to be welded. Preferably, the set value W of the welding current is set to 8 to 14 kA, and the up slope period is set to 0.01 to 0.5 seconds.

  Hot-dip aluminum-plated steel sheets include steel sheets with plated layers of pure Al, Al-Si, etc., but any hot-dip aluminum-plated steel sheet has an N enriched layer of N: 3.0 atomic% or more at the base steel / plated layer interface. What formed is preferable. The N enriched layer can be formed by heat-treating the base steel with adjusted N and Al contents after aluminum plating. Aluminum or various aluminum alloys can be used as the mating material to be joined to the hot-dip aluminum-plated steel sheet, but Mg: 0.1 to 6.0 mass is used for precipitating Mg-Si intermetallic compounds that contribute to strength. %, Si: An aluminum alloy containing 3.0% by mass or less is preferable.

  In spot welding using an AC power supply, the welding current is supplied to the workpiece by an energization pattern (FIG. 1) that is increased to a set value through an up slope and returns to a steady state through a down slope. The material to be welded is pressurized from a predetermined time (squeeze time) before the start of energization to a predetermined time (holding time, off time) after the end of energization. The material to be welded is heated by Joule heat by energization, becomes a molten state, and is melt-joined with the counterpart material.

  Nuggets are formed at the melt-bonded portion with the counterpart material, but the nugget shape is affected by the energization pattern. Among them, the influence of the up slope is large, and the gentle up slope becomes a shallow and wide nugget, and the steep up slope becomes a deep and narrow nugget. The change of the nugget shape according to the slope of the upslope is considered to depend on the form of propagation of heat applied to the material to be welded immediately after the start of welding, and can be explained as follows.

  When the welding current is increased with a steep upslope, the propagation of heat input is limited to the portion where the welding electrode is in contact with the workpiece (welding point) and the very narrow range near the welding point. Therefore, the tendency for the material to be welded to increase in temperature in the plate thickness direction appears strongly, and a deep and narrow nugget is formed. On the other hand, when the welding current is increased with a gentle upslope, the heat input is propagated over a relatively wide range centering on the welding point, and a relatively shallow and wide nugget is formed.

  The nugget shape has a great influence on the physical properties of the joint interface formed by the dissimilar welding of the aluminum material / hot-dipped galvanized steel sheet, and thus the joint strength. With a steep up-slope welding current, melting proceeds to a deep portion along the plate thickness direction of the material to be welded, and diffusion of Fe and Si from the base steel / plated layer interface to the molten Al is promoted. The promotion of diffusion causes a shortage of Si necessary for the Al—Fe—Si ternary alloy layer effective for bonding strength, and increases the ratio of the fragile Al—Fe binary alloy layer to the bonding interface. Conversely, with a gentle upslope welding current, the diffusion of Fe and Si along the thickness direction (depth direction of the nugget) is suppressed, and the proportion of the Al—Fe—Si ternary alloy layer occupying the bonding interface is large. The Al—Fe binary alloy layer is also relatively small.

Next, the relationship between the nugget shape and the energization pattern was investigated, and the energization pattern in which the nugget shape effective for improving the bonding strength was formed was quantitatively investigated. As will be apparent from the examples described later, the integrated current during the up slope period (t ₀ → t ₁ ) from the energization start time t ₀ to the time t ₁ when the welding current reaches the set value W is Q ₁ , constant current welding. When the integrated current in the period (t ₁ → t ₂ ) is Q ₂ , the integrated current ratio Q ₁ / Q ₂ is set to 0.05 to 3.0, and the integrated current sum Q ₁ + Q ₂ is set to ₁ to 5 kA · sec. It is effective to supply a welding current to a material to be welded in an energization pattern in forming a nugget having a target shape (FIG. 2).
Further, the integrated current ratio Q ₁ / Q ₂ : 0.05 to 3.0 means that the welding current is raised from zero to the set value W over a long time as compared with the conventional spot welding. Since the up-slope period is lengthened, a sufficient joint strength can be obtained even if the welding current set value W is lowered, and there is no need to use a spot welding machine having a large capacity.

The hot dip galvanized steel sheet is manufactured by a continuous hot dip plating line, and a hot dip plated layer of Al, Al-Si or the like is provided on the surface of the original plate.
There are low carbon steel, medium carbon steel, low alloy steel, etc. as the plating base plate, and steel types to which Si, Mn, Cr, Ni, Al, etc. are added depending on the application are used. Among these, steel added with 0.002 to 0.020 mass% of N for suppressing the Al—Fe interdiffusion reaction is preferable. When N-added steel is used for the plating base plate, the Al content is restricted to 0.03 mass% or less in order to ensure an effective N amount for suppressing the diffusion of Al—Fe.

  Although the growth of the fragile Al—Fe binary alloy layer is delayed as the hot-dip aluminum plating layer becomes thicker, the workability of the hot-dip aluminum-plated steel sheet is lowered if it is too thick, so a film thickness of 5 to 50 μm is preferable. Further, in forming an Al—Fe—Si ternary alloy layer effective in bonding strength by suppressing the growth of the Al—Fe binary alloy layer at the base steel / plating layer interface, Fe, The Si content is preferably regulated in the range of Fe: 0.5 to 5% by mass and Si: 3 to 12% by mass, respectively. In addition to Fe and Si, elements such as Ti, Sr, B, Cr, Mn, and Zn can be included in the molten aluminum plating layer as necessary.

The aluminum material of the mating member does not impose any particular restrictions on the material, but most aluminum or aluminum alloy can be used as long as it is a wrought material. In applications where strength is required, it is preferable to use an aluminum alloy containing Mg: 0.1 to 6.0 mass% and Si: 3.0 mass% or less. Mg and Si in the alloy solidify and strengthen the matrix and precipitate as fine Mg ₂ Si by heat treatment such as aging treatment, thereby imparting strength to the aluminum alloy.

The welded materials collected from the hot-dip aluminized steel sheet and aluminum material are superposed on each other after degreasing and cleaning, and are sandwiched between the electrodes for spot welding. Next, electricity is applied in a state where the material to be welded is pressurized, and both are integrally joined. The welding current supplied to the material to be welded is generally increased or decreased according to the energization pattern shown in FIG. The energization pattern includes an up-slope period (t ₀ → t ₁ ) from the start of energization until reaching the set value W of the welding current, a constant current welding period (t ₁ → t ₂ ) in which the welding current of the set value W is supplied, It is divided into a down slope period (t ₂ → t ₃ ) in which the welding current is lowered from the set value W to zero.

The set value W of the welding current is preferably determined in the range of 8 to 14 kA depending on the thickness and material of the material to be welded. The welding current of 8 to 14 kA is slightly lower than the conventional welding current, and it is an advantage of the present invention that welding is possible at a low current. Setting value W: Under the condition of ₈ to 14 kA, the ratio Q ₁ / Q ₂ of the integrated current Q ₁ during the upslope period and the integrated current Q ₂ during the constant current welding period is 0.05 to 3.0, and the sum Q ₁ + Q2 is controlled within the range of ₁ to 5 kA · sec.
The integrated current Q ₁ is calculated as W × (t ₁ −t ₀ ) / 2 in the energization pattern in which the welding current increases linearly to the set value W during the up slope period (t ₀ → t ₁ ). The integrated current Q ₂ is calculated as W × (t ₂ −t ₁ ) because the welding current having the set value W is supplied over the period (t ₂ −t ₁ ).

When the integrated current ratio Q ₁ / Q ₂ is less than 0.05, good bonding strength cannot be obtained with a relatively low welding current setting value W of 12 kA or less. On the other hand, when the cumulative current ratio Q ₁ / Q ₂ exceeds 3.0, a large amount of Al—Fe binary alloy layer is formed at the joint interface, so that stable joint strength cannot be obtained and welding takes time. This causes an increase in manufacturing costs. With the integrated current sum Q ₁ + Q _{2 of} less than 1 kA · sec, heat input during welding is insufficient and good bonding strength cannot be obtained. However, when the accumulated current sum Q ₁ + Q ₂ exceeds 5 kA · sec, it occupies the bonding interface. The ratio of the Al—Fe binary alloy layer becomes 90% or more, and the fracture form after cross tension also exhibits interface fracture that makes the bonding strength unstable.

As the integrated current ratio Q ₁ / Q ₂ increases, the nugget formed at the bonding interface changes from a deep and narrow shape to a shallow and wide shape, and the bonding strength of the steel / aluminum bonding structure increases. Specifically, the effect of the upslope period on the nugget shape is observed at 0.01 seconds or more, and is particularly noticeable in a joint structure spot-welded with a low welding current of around 10 kA. Obtaining a high joint strength with a low welding current is advantageous in that a spot welder having a large capacity is not required.

The following hot-dip galvanized steel sheet, aluminum material is used as the material to be welded, and the upslope period (t ₀ → t ₁ ) and welding current set value W are the steel / aluminum joint structure joint strength, nugget shape, alloy at the joint interface The effect on strata was investigated.

[Hot-aluminized steel sheet]
N: Hot-dip galvanized steel sheet in which an Al-9.0 mass% Si plating layer is provided at a thickness of 20 μm on a plating original sheet of 0.01 mass% and plate thickness: 1.0 mm [aluminum material]
Thickness including Si: 1.2 mass%, Mg: 0.6 mass%: Aluminum alloy of 1.0 mm (JIS A6022)

A 50 mm × 150 mm test piece cut out from a hot-dip aluminum-plated steel sheet and aluminum material was degreased and washed, and then overlapped and sandwiched between electrodes of a spot welder, and a load of 3 kN was applied. The electrode uses a copper alloy tip having a tip diameter of 6 mm, a tip radius of 40 mm, and a shoulder radius of 8 mm, and a constant current welding period (t ₁ → t ₂ ) in which a welding current of a set value W is supplied is 0.10 seconds. The up-slope period (t ₀ → t ₁ ) during which the welding current rises to the set value W was set to three modes of 0.008 seconds, 0.017 seconds, and 0.167 seconds.

The spot welded joint structure was subjected to a cross tension test (JIS Z2317), and the joint strength along the peeling direction was measured. The measurement result (FIG. 3) shows that when the upslope period is 0.008 seconds and compared with the joint strength N _0.5 when the spot welding is performed, the upslope period is 0.017 seconds and 0.1167 seconds when the spot welding is performed. It shows that the strengths N ₁ and N ₁₀ are high. The bonding strength difference ΔN tended to increase as the welding current set value W decreased.

A cross-section of the bonded portion of the bonded structure spot-welded at a set value W of 10 kA was observed, and the nugget shape and the alloy layer generated at the bonded interface were investigated. In the joint section, a nugget melted in the thickness direction of the aluminum alloy plate at the hitting point is generated (FIG. 4). The thickness of the aluminum alloy of the RBI portion B _2, and the melt thickness as B _1, penetration thickness ratio the ratio of the melt thickness B ₁ with respect to the thickness B _2: calculated as B ₁ / B _2. Penetration thickness ratio: The nugget shape was evaluated from B ₁ / B ₂ and the nugget diameter L ₁ . Also, SEM • EDX (840A, manufactured by JEOL Ltd.) observes the alloy layer at the bonding interface to determine the Al—Fe—Si ternary alloy layer or Al—Fe binary alloy layer, and determines the nugget diameter from the determination result. As a ratio of the generation length of the Al—Fe binary alloy layer to the Al—Fe binary alloy layer, the occupation ratio of the Al—Fe binary alloy layer in the bonding interface was calculated.

As can be seen from the investigation results in Table 1, the longer the upslope period, the shallower and wider the nugget is formed, and the Al—Fe binary alloy layer occupying the bonding interface becomes smaller.
On the other hand, in the joint structure manufactured by spot welding with an up slope period of 0.008 seconds, the nugget has a deep and narrow shape, indicating that heat input has propagated exclusively in the plate thickness direction. The proportion of the Al—Fe binary alloy layer in the joint interface was also increased.

The results of investigating the influence of the upslope period on the nugget diameter, penetration thickness ratio, and Al-Fe binary alloy layer occupancy under the conditions where the welding current set value W is 10 kA and 14 kA are shown in FIGS. .
From the above results, it can be inferred that by increasing the upslope period, an appropriately shaped nugget is formed with less Al—Fe binary alloy layers at the bonding interface, and the bonding strength of the bonded structure is increased.

Next molten aluminum-plated steel sheet, an aluminum material to be welded material, the ratio of the integrated current Q ₂ of the integrated current Q ₁ and the constant current welding period up slope period _{(t 0 → t 1) (} t 1 → t 2) The effects of Q ₁ / Q ₂ and the sum Q ₁ + Q _{2 on} the joint strength, nugget shape, alloy layer at the joint interface, and fracture morphology after the cross tension test were investigated.

[Hot-aluminized steel sheet]
N: Hot-dip galvanized steel sheet in which an Al-9.5 mass% Si plating layer is provided at a thickness of 30 μm on a plating original sheet of 0.0076 mass% and plate thickness: 1.0 mm [aluminum material]
Thickness including Si: 1.2 mass%, Mg: 0.6 mass%: Aluminum alloy of 1.0 mm (JIS A6022)

A 50 mm × 150 mm test piece cut out from a hot-dip aluminum-plated steel sheet and aluminum material was degreased and washed, and then superimposed and sandwiched between electrodes of an AC spot welder, and a load of 3 kN was applied. . The same copper alloy tip as in Example 1 was used for the electrodes, and various up-slope periods (t ₀ → t ₁ ), constant current welding periods (t ₁ → t ₂ ), and welding current set values W as shown in Table 2. Changed and spot welded.

The joint strength of the spot welded joint structure is measured by cross tension test (JIS Z2317), and the nugget shape (nugget diameter, penetration thickness ratio) and Al-Fe binary occupying the joint interface are observed from the cross-section observation results The ratio of the alloy layer was determined.
As a result, the integrated current ratio Q ₁ / Q _2: 0.05~3.0, integrated current sum Q ₁ + Q _2: integrated current to Q ₁ up slope period as 1~5kA of-seconds condition is satisfied, when controlling integrated current Q ₂ of the constant current welding period, good joint strength, nugget shape, alloy layer forms a bonding interface that obtained was found (Table 3). The fracture form after the cross tensile test was also the base metal fracture, and it was confirmed that stable joint strength was obtained.

As described above, when spot welding is performed by overlapping hot-dip galvanized steel sheets and aluminum materials, the accumulated current Q ₁ during the up slope period (t ₀ → t ₁ ) in which the welding current rises to the set value W from the start of energization and By managing the integrated current Q ₂ during the constant current welding period (t ₁ → t ₂ ), it is possible to suppress the formation of a fragile Al—Fe binary alloy layer at the joint interface and to improve the joint strength. Forming a nugget. Therefore, the produced steel / aluminum joint structure has higher joint strength than the conventional spot welded joint structure, has the advantages of aluminum material and steel material, and is used as various structural members.

The figure which shows the energization pattern of the welding current supplied to a material to be welded in relation to a pressurization state A graph showing that the welding current changes from the start of energization to the end of energization A graph showing that the bonding strength changes according to the upslope period Explanatory drawing for calculating penetration thickness ratio Graph showing the effect of upslope period on nugget diameter Graph showing the effect of upslope period on penetration thickness ratio Graph showing the effect of the upslope period on the occupancy of the Al-Fe binary alloy layer

Claims (2)

Superposing the aluminum material and the molten aluminum-plated steel sheets, when integrated by spot welding, integration of up slope period from the energization start until the welding current reaches the set value W to integrated current Q ₂ of the constant current welding period A steel characterized in that a welding current is supplied to a material to be welded in an energization pattern with a current Q ₁ ratio Q ₁ / Q ₂ of 0.05 to 3.0 and a sum Q ₁ + Q ₂ of ₁ to 5 kA · sec. / Production method of aluminum joined structure.   The manufacturing method according to claim 1, wherein the set value W of the welding current is 8 to 14 kA and the up slope period is 0.01 to 0.5 seconds.

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