Friction stir welding joint

Abstract

Provided is a friction stir welding joint which has excellent welding efficiency and which uses a ferrite-based steel plate as a material to be welded. The ratio of the average dislocation density of a bonding part to the average dislocation density of a base material is appropriately controlled.

Description

Technical Field

[0001] This invention relates to a friction stirring joint. Background Technology

[0002] In friction stir bonding, such as two-sided friction stir bonding, the materials to be joined are joined as follows: A pair of rotating tools are positioned on the surface and back sides of the materials to be joined, which have at least two metal plates. The rotating tools are then pressed against the surface and back sides of the materials to be joined, respectively, and moved in the joining direction while rotating. This softens the metal plates using frictional heat from the rotating tools and the materials to be joined, while simultaneously stirring the softened areas. Plastic flow then occurs in the region that becomes the joint area of ​​the materials to be joined, thus joining the materials. Hereinafter, the area in the buttressing or overlapping portions of the materials to be joined that is not yet joined will be referred to as the "unjoined portion," and the joined and integrated area will be referred to as the "joined portion."

[0003] As a technology related to such friction stir joining, for example, Patent Document 1 discloses: "A friction stir joining method, characterized in that, when two steel plates are butted or overlapped and friction stir joined, the rotational speed R of the tool rotating in the friction stir joining is set to more than 5 times / minute and less than 5000 times / minute, the front edge temperature T of the steel plate in contact with the front edge of the tool in the direction of travel is set to 25°C or more, the shoulder diameter D of the tool is set to 8 to 40 mm, and the joining speed V accompanying the tool is set to 0.1 to 5 m / minute, and the friction stir joining is performed."

[0004] Existing technical documents

[0005] Patent documents

[0006] Patent Document 1: Japanese Patent Application Publication No. 2019-166569 Summary of the Invention

[0007] However, in conventional friction stir bonding methods such as Patent Document 1, sufficient joint efficiency cannot be achieved when using a steel plate with a ferrite-based metal structure (hereinafter also referred to as a ferrite-based steel plate) as the bonding material. Therefore, it is desirable to improve this situation.

[0008] This invention was developed in view of the above-mentioned situation, and its object is to provide a friction stir joint that uses a ferrite-dominant steel plate as the joined material (in other words, a base material having a ferrite-dominant metal structure) and has excellent joint efficiency. It should be noted that in this specification, the numerical ranges indicated by “~” refer to the range including the values ​​before and after “~” as the lower and upper limits, respectively.

[0009] Therefore, the inventors conducted repeated and in-depth research to achieve the aforementioned objectives. First, they investigated why conventional friction stir bonding methods failed to achieve excellent joint efficiency in friction stir bonded joints using ferrite-based steel plates as the bonded materials. Specifically, the inventors used various materials as bonded materials and performed friction stir bonding under various conditions, analyzing the resulting friction stir bonded joints based on tensile tests and electron beam backscatter diffraction (EBSD) measurements.

[0010] As a result, the inventors obtained the following insight: Due to the microscopic dislocations introduced into the metal structure of the joint during friction stir bonding, the ductility of the joint of the friction stir bond is reduced at a microscopic level. This reduction in microscopic ductility makes the joint more prone to fracture, in other words, leading to a decrease in joint efficiency.

[0011] Regarding the above reasons, the inventors believe the following: In a ferrite-dominated metal microstructure, the hardness distribution typically does not change significantly. However, when regions with low microscopic ductility exist within the ferrite-dominated metal microstructure, fracture occurs due to insufficient ductility in these regions. Furthermore, this reduction in microscopic ductility is largely due to the increase in microdislocations in the joints introduced by heat treatment, processing, etc. Additionally, macroscopically, the mismatch between the joints and the base material also has a certain impact.

[0012] Furthermore, based on the above insights and the fact that the ferrite grains at the joint of the friction stir joint are fine, the inventors believed that the amount of microdislocations introduced into the joint might be related to the average properties of the matrix. Based on this idea, the inventors conducted further repeated studies to quantify the amount of microdislocations introduced into the joint. As a result, the inventors found that when the average dislocation density at the joint is significantly higher than the average dislocation density of the base material, the ductility of the joint of the friction stir joint decreases, leading to a reduction in joint efficiency.

[0013] Furthermore, based on the above insights, the inventors conducted further and repeated research and obtained the following insights: that is, by appropriately controlling the ratio of the average dislocation density of the joint to the average dislocation density of the base material, even when a ferrite-based steel plate is used as the joined material, a friction stir joint with excellent joint efficiency can be obtained.

[0014] This invention was completed based on the above insights and further research.

[0015] That is, the main structure of the present invention is as follows.

[0016] 1. A friction stirring joint, comprising two or more base materials and a joint portion between the base materials,

[0017] The ferrite area ratio of the aforementioned base material and the ferrite area ratio of the aforementioned joint are both 80% or more.

[0018] The relationship is given by equation (1).

[0019] Dw / Dm≤10.0 (1)

[0020] in,

[0021] Dw: Average dislocation density at the junction ( / m) 2 )

[0022] Dm: Average dislocation density of the parent material ( / m) 2 ).

[0023] 2. The friction stir joint according to 1 above, wherein the composition of the base material is, by mass %, C: less than 0.1%, Si: 2.0 to 8.0%, Al: less than 2.0% and Mn: less than 1.0%.

[0024] 3. The friction stirring joint according to 1 or 2 above, wherein the average dislocation density of the joint is 1.0 × 10⁻⁶. 16 / m 2 the following.

[0025] 4. The friction stirring joint according to any one of 1 to 3 above is a two-sided friction stirring joint.

[0026] According to the present invention, a friction stir joint with excellent joint efficiency can be obtained, using a ferrite-based steel plate as the bonding material. Furthermore, the friction stir joint of the present invention is also advantageous in terms of manufacturability. Attached Figure Description

[0027] Figure 1 This is an example of a KAM diagram of the base material and joint obtained through EBSD measurement. Detailed Implementation

[0028] The present invention will be described based on the following embodiments. First, a friction stirring joint according to one embodiment of the present invention will be described.

[0029] As described above, a friction stirring joint according to one embodiment of the present invention has two or more base materials and joints between the base materials (joints between the base materials).

[0030] [Material]

[0031] The base material has a ferrite-dominated metallic structure, specifically, a ferrite area ratio of 80% or more. It should be noted that each base material is composed of individual steel plates that are the materials to be joined.

[0032] The ferrite area fraction of the base metal is 80% or more, preferably 90% or more. The ferrite area fraction of the base metal can be 100%. Furthermore, the area fraction of the remaining microstructure other than ferrite in the base metal is 20% or less, preferably 10% or less, more preferably 5% or less. Examples of the remaining microstructure other than ferrite include bainite, martensite, sulfides, nitrides, carbides, etc. The area fraction of the remaining microstructure in the base metal can be 0%.

[0033] The area ratio of ferrite in the base material was determined as follows: A test piece was collected from the base material. Next, the observation surface of the test piece was ground, and then etched with 3 vol.% nitric acid ethanol (a solution of nitric acid and ethanol) to expose the microstructure. Then, it was photographed using an optical microscope at a magnification of 40 to 500x. The total area of ​​the observation region was preferably 50 μm square (50 μm × 50 μm) or larger. Next, based on the obtained microstructure image, the area of ​​ferrite was calculated using Adobe Photoshop from Adobe Systems. Then, the area of ​​ferrite calculated for each field of view was divided by the total area of ​​the observation region and multiplied by 100; the resulting value was taken as the area ratio of ferrite. It should be noted that the area ratio of ferrite can also be determined by EBSD measurement.

[0034] Furthermore, the composition of the base material can be exemplified by a composition, by mass%, of C: 0.1% or less, Si: 2.0 to 8.0%, Al: 2.0% or less, and Mn: 1.0% or less, with the remainder being Fe and unavoidable impurities. Unavoidable impurities are, for example, P: 0.2% or less, S: 0.01% or less, and N: 0.01% or less. In the above composition, at least one selected by mass% from Cr: 1% or less, Ni: 1% or less, Cu: 1% or less, Sn: 0.2% or less, Sb: 0.2% or less, Ca: 0.01% or less, REM: 0.05% or less, and Mg: 0.01% or less. It should be noted that elements other than Si and Fe can be 0% by mass. The Si content is more preferably 5.0% by mass or less. The C content is more preferably 0.001% by mass or more. The Mn content and Al content are each more preferably 0.01% by mass or more. Therefore, the thickness of a base material is preferably 0.2 to 3.2 mm.

[0035] It should be noted that the number of base materials joined at a single joint can be two or more. For example, the number of base materials joined at a single joint can be two, or three to five. In addition, for the base materials joined at a single joint, as long as they are ferritic steel plates, the steel grade and thickness can be the same or different from each other.

[0036] [Joint]

[0037] The joint is a region that undergoes thermal working based on frictional heat and plastic flow between a rotating tool and the joined materials, resulting in a recrystallized structure. The joint also has a ferrite-dominated metallic structure, specifically, a metallic structure with a ferrite area ratio of 80% or more.

[0038] The area ratio of ferrite at the joint is 80% or more, preferably 90% or more. The area ratio of ferrite at the joint can be 100%. Furthermore, the area ratio of the remaining microstructure other than ferrite at the joint is 20% or less, preferably 10% or less, more preferably 5% or less. Examples of the remaining microstructure other than ferrite include martensite, sulfides, nitrides, carbides, etc. The area ratio of the remaining microstructure at the joint can be 0%.

[0039] The area ratio of ferrite at the joint is determined, for example, as follows: The friction stir joint is cut along the thickness (vertical) direction with the joint cross-section (the surface of the friction stir joint including the vertical direction of the joint and the thickness direction, where the vertical direction of the joint is perpendicular to both the joint direction and the thickness direction) as the cutting surface. A test piece is then cut with the joint cross-section of the joint as the observation surface. Next, the observation surface of the test piece is ground, and then etched with 3 vol.% nitric acid ethanol to expose the microstructure. Next, it is photographed using an optical microscope at a magnification of 40 to 500x. The total area of ​​the observation area is preferably 50 μm square (50 μm × 50 μm) or larger. Next, based on the obtained microstructure image, the area of ​​ferrite is calculated using Adobe Photoshop from Adobe Systems. Then, the area of ​​ferrite calculated for each field of view is divided by the total area of ​​the observation area and multiplied by 100; the resulting value is taken as the area ratio of ferrite. It should be noted that the area ratio of ferrite can also be determined by methods such as EBSD measurement.

[0040] Alternatively, the base material and the joint can be separated, for example, as follows: The friction stir joint is cut along the thickness (vertical) direction with the joint cross-section as the cut surface. Then, the cut surface is ground and etched with 3 vol.% nitric acid ethanol. Next, the cut surface is observed using an optical microscope, and the base material and the joint are separated according to the degree of etching, etc.

[0041] Moreover, in a friction stir joint according to one embodiment of the present invention, it is important to properly control the relative relationship between the average dislocation density of the joint and the average dislocation density of the base material. Specifically, the ratio of the average dislocation density of the joint to the average dislocation density of the base material satisfies the following relationship (1).

[0042] Dw / Dm≤10.0 (1)

[0043] in,

[0044] Dw: Average dislocation density at the junction ( / m) 2 )

[0045] Dm: Average dislocation density of the parent material ( / m) 2 ).

[0046] It should be noted that, when the average dislocation density varies from the base material, Dm is the minimum average dislocation density among the average dislocation densities of the base materials joined by the joint.

[0047] As described above, the micro-dislocations introduced into the metal structure of the joint during friction stir bonding can be quantified by the average dislocation density. By controlling the relative relationship between the average dislocation density of the joint and the average dislocation density of the base material to satisfy the relationship in equation (1) above, excellent joint efficiency can be obtained. Therefore, Dw / Dm is set to ≤ 10.0. Preferably, Dw / Dm ≤ 5.0, more preferably Dw / Dm ≤ 3.0. It should be noted that the lower limit of Dw / Dm is not particularly limited, for example, it can be 1.0 ≤ Dw / Dm.

[0048] Furthermore, Dw is preferably 1.0 × 10⁻⁶. 16 / m 2 Below. Additionally, Dw is preferably 1.0 × 10⁻⁶. 13 / m 2 above.

[0049] Here, the average dislocation density is calculated, for example, by EBSD determination, using the following formula.

[0050] D = 2KAMave ÷ (u × |b|)

[0051] In the formula,

[0052] D: Average transfer density ( / m³) 2 )

[0053] KAMave: The average value of KAM

[0054] u: Measurement interval (m)

[0055] b: Burgers vector (m).

[0056] For example, the friction stir joint is cut along the thickness (vertical) direction with the joint cross-section as the cutting surface, and a test piece is cut with the joint cross-section of the base material and the joint as the observation surface. Next, EBSD measurements are performed to obtain local crystal orientation data for the base material and the joint. The measurement interval (step size) is preferably 0.1–20 μm. Furthermore, the measurement area is preferably 50 μm square (50 μm × 50 μm) or larger. Then, based on the analysis results of the local crystal orientation data, the KAM value (the average of the orientation differences between the measurement point and all its adjacent measurement points) is calculated for each measurement point, and their average values, KAMave, are calculated for both the base material and the joint. Then, the average dislocation density of the base material and the joint is calculated. For reference, Figure 1 This is an example of a friction stir joint obtained by measuring the base material and the joint using EBSD, where a ferritic steel sheet is used as the bonding material and friction stir is performed according to conventional methods.

[0057] Friction stir joints can be either single-sided or double-sided. A single-sided friction stir joint is obtained by joining materials using a single-sided friction stir process with a rotating tool positioned on one side of the materials. That is, a single-sided friction stir joint has weld beads only on one joint surface in the thickness direction. Conversely, a double-sided friction stir joint is obtained by joining materials using a double-sided friction stir process with rotating tools positioned on both the front and back sides of the materials. That is, a double-sided friction stir joint has weld beads on both joint surfaces in the thickness direction.

[0058] In addition, the friction stirring joint according to one embodiment of the present invention can be a butt joint or an overlapping joint.

[0059] Furthermore, according to one embodiment of the present invention, the friction stirring joint can further bond other base materials to the aforementioned base material via other joints. The types of other joints are not particularly limited.

[0060] In addition, the friction stirring joint according to one embodiment of the present invention can be manufactured as follows, for example.

[0061] First, as the materials to be joined, two or more steel plates with a ferrite-based main body are prepared; specifically, steel plates with a ferrite area ratio of 80% or more are prepared. Here, the materials to be joined form the base material after joining, and their metal structure, composition, thickness, etc., are the same as those of the base material. Therefore, the explanation is omitted here.

[0062] Next, the materials to be joined are bonded by friction stirring. There are no particular limitations on the bonding conditions; conventional methods can be used.

[0063] Examples of joining methods include butt joint and overlap joint. Butt joint refers to joining materials where the end faces of the materials to be joined are facing each other, and a rotating tool is pressed against the butt joint containing the end faces (butt surfaces) of the materials to be joined while rotating, and then the rotating tool is moved in the joining direction in this state to join the materials. Overlap joint refers to joining materials where at least a portion of the ends of the materials to be joined overlap, and a rotating tool is pressed against the overlap while rotating, and then the rotating tool is moved in the joining direction in this state to join the materials.

[0064] Next, in order to satisfy the relationship of the above equation (1), the friction stirring joint obtained as described above is subjected to post-heating under the following conditions.

[0065] Average heating rate (hereinafter also referred to as heating rate) in the temperature range of 100–700℃: 30–300℃ / second

[0066] Maximum temperature reached: 710~1200℃

[0067] Holding time at the highest reached temperature (hereinafter also referred to as holding time): 0.1 to 10 seconds

[0068] Average cooling rate (hereinafter also referred to as cooling rate) in the temperature range of 700–100°C: 30–300°C / second

[0069] It should be noted that the temperatures mentioned here are based on the surface temperature of the joint of the friction stir joint. Furthermore, it is preferable to control the heating rate, holding time, and cooling rate within the aforementioned ranges on both sides of the joint of the friction stir joint.

[0070] Furthermore, there are no particular limitations on the post-heating method; examples include high-frequency induction heating, laser irradiation, and gas heating. Alternatively, furnace heating can also be used.

[0071] There are no special restrictions on conditions other than those mentioned above; the conventional methods can be followed.

[0072] Example

[0073] Two steel plates, having the preferred composition described above (C: 0.1% or less, Si: 2.0–8.0% (by mass%), Al: 2.0% or less, and Mn: 1.0% or less, with a portion further containing at least one selected from Cr: 1% or less, Ni: 0.5% or less, Cu: 0.5% or less, Sn: 0.2% or less, Sb: 0.2% or less, Ca: 0.01% or less, REM: 0.05% or less, and Mg: 0.01% or less, with the remainder being Fe and unavoidable impurities) and a thickness of 2.0 mm, are butt-jointed and subjected to double-sided friction stir bonding under the conditions shown in Table 1 to obtain a friction stir bonded joint. Next, the friction stir bonded joint is post-heated under the conditions shown in Table 1. It should be noted that for conditions not explicitly described, the above-described or conventional methods are followed. Furthermore, in any case, the bonding conditions and post-heating conditions are substantially the same on both sides; therefore, only one of them is representatively described in Table 1.

[0074] For the friction stir joint thus obtained, the ferrite area ratio and Dw / Dm of the base material and the joint are calculated according to the above-described method. The results are recorded in Table 1. It should be noted that in Table 1, when the ferrite area ratios of the base materials joined in a joint are different, the value of the smaller ferrite area ratio is recorded representatively.

[0075] In addition, the joint efficiency of the obtained friction stir joints was evaluated. In the evaluation of joint efficiency, a joint efficiency (= fracture strength of the joint / fracture strength of the base material × 100) exceeding 90% was rated as "excellent," and below 90% was rated as "poor." The results are recorded in Table 1.

[0076] Here, the fracture strength of the joint is determined as follows. Specifically, from the obtained friction-stirred joint, a test piece of the same shape as test piece No. 1 as specified in JIS Z 3121 (2013) is collected, with the joint direction and thickness direction perpendicular to the long side direction of the test piece and the joint located at the center of the parallel portion. Then, a tensile test according to JIS Z 3121 (2013) is performed using the collected test piece to determine the maximum test force (N). The determined maximum test force (N) is then divided by the cross-sectional area (mm²) of the parallel portion of the test piece. 2 The value obtained is used as the fracture strength of the joint.

[0077] Alternatively, the fracture strength of the base material can also be referred to as the tensile strength (TS) of the base material (the joined materials). That is, the fracture strength of the base material can be determined by a tensile test according to JIS Z 2241 (2022). For example, a JIS 5 test piece is collected from the base material or a material identical to the base material. Then, using the collected test piece, a tensile test is performed at a crosshead speed of 10 mm / min to determine the tensile strength (TS). It should be noted that when the fracture strength varies depending on the base material, the highest fracture strength among the fracture strengths of each base material is used in the evaluation of joint efficiency.

[0078]

[0079] As shown in Table 1, in the inventive examples, a friction stir joint with excellent joint efficiency was obtained, using a ferrite-based steel plate as the bonding material. On the other hand, in the comparative examples, sufficient joint efficiency could not be obtained.

[0080] In addition, in friction stir joints and overlapping joints of various ferrite-based steel plates (with the above-mentioned preferred composition and thickness of 0.2 to 3.2 mm) which are separately manufactured as joining materials (i.e., the area ratio of ferrite in the base material and the area ratio of ferrite in the joint are both 80% or more), when Dw / Dm satisfies the above formula (1), excellent joint efficiency is obtained in the same way as above.

Claims

1. A friction stirring joint having two or more base materials and a joint portion between the base materials, The ferrite area ratio of the base material and the ferrite area ratio of the joint are both above 80%. Furthermore, the following relationship (1) is satisfied: Dw / Dm≤10.0 (1) in, Dw: Average dislocation density at the junction, in units of / m 2 Dm: Average dislocation density of the parent material, in units of / m 2 .

2. The friction stirring joint according to claim 1, wherein, The composition of the base material, by mass%, is: C: less than 0.1%, Si: 2.0 to 8.0%, Al: less than 2.0%, and Mn: less than 1.0%.

3. The friction stirring joint according to claim 1 or 2, wherein, The average dislocation density of the joint is 1.0 × 10⁻⁶. 16 / m 2 the following.

4. The friction stirring joint according to claim 1 or 2, wherein it is a two-sided friction stirring joint.

5. The friction stirring joint according to claim 3 is a two-sided friction stirring joint.

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