METHOD FOR INSPECTING DISSIMILAR METAL BONDED JOINT AND METHOD FOR MANUFACTURING DISSIMILAR METAL BONDED JOINT

Abstract

To provide a dissimilar metal-joint inspection method which enables amount of the occurrence of blow holes to be easily determined, thereby enabling the occurrence of welding defect to be suppressed, and consequently enables a high-strength dissimilar-metal joint to be manufactured.SOLUTION: The dissimilar metal-joint inspection method includes: a step of forming a low-temperature sprayed coating 1 on a surface of an aluminum alloy material 2; a step of superimposing a steel material 3 on the side of the low-temperature sprayed coating 1; a step of applying laser beam from the steel material 3 side to thereby melt a part of the steel material 3 and a part of the low-temperature sprayed coating 1 together to form a weld metal 4 to join the steel material 3 and the aluminum alloy material 2; and a step of obtaining an excess weld metal height ratio by an expression A: (H / (Ts+Tf))×100 on the basis of a height H of the excess of weld metal 4 protruding from a surface of the steel material 3 and, a thickness Ts of steel 3 in the region forming weld metal 4, and a thickness Tf of the low-temperature sprayed coating 1, and when the excess weld metal height ratio exceeds a predetermined value, the method determines that a welding defect has occurred.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to a method for inspecting a dissimilar metal joint, a method for manufacturing a dissimilar metal joint, and a dissimilar metal joint. [Background technology]

[0002] In recent years, in the field of automobiles, high tensile strength steel (HTSS) has been used in the body frame and other components in order to reduce the weight of vehicles and improve collision safety in order to reduce CO2 emissions.

[0003] In addition, in order to further reduce the weight of vehicle bodies, there is an increasing demand for dissimilar metal joining materials in which non-ferrous metals such as lightweight aluminum or aluminum alloy materials are joined to steel materials. Generally, methods for joining dissimilar metals include joining methods using nails or screws, and joining methods using SPR (Self-Pierce Riveting) or FDS (Flow Drilling Screw; registered trademark).

[0004] However, methods using nails, screws or rivets have the problem that the nails and screws are relatively expensive or a process is required to create the rivets, which increases the manufacturing cost of the fastener and makes the resulting fastener heavy by the weight of the nails, screws and rivets.

[0005] On the other hand, when aluminum or aluminum alloy material and steel material are directly welded by a general method, a brittle intermetallic compound is formed at the joint interface, making it impossible to obtain a satisfactory strength. Therefore, there is an increasing demand for a welding method that can obtain a high-strength joint material in the joining of aluminum or aluminum alloy material and steel material.

[0006] Therefore, Patent Document 1 discloses a method for manufacturing a dissimilar material joint structure, in which a low-temperature sprayed coating is formed on at least a part of the surface of an aluminum alloy material by low-temperature spraying at least one metal powder selected from pure iron, carbon steel, nickel, nickel alloy, cobalt, and cobalt alloy, and the aluminum alloy material and the steel material are superimposed so that the low-temperature sprayed coating faces the steel material, and then laser welding is performed from the steel material side. According to the manufacturing method described in Patent Document 1, the low-temperature sprayed coating and the aluminum alloy material are mechanically and firmly joined by the anchor effect, and the low-temperature sprayed coating and the steel material are joined by laser welding. Therefore, the aluminum alloy material and the steel material can be joined with high joining strength.

[0007] However, when a dissimilar material joint structure is manufactured by the manufacturing method described in Patent Document 1, blowholes are likely to occur during welding due to gas contained in the low-temperature sprayed coating, which can lead to weld defects. If such weld defects occur, the joint strength decreases, so there is an increasing demand for a method for manufacturing a high-strength joint that can easily detect the presence or absence of blowholes. In addition, various methods have been proposed for detecting the occurrence of weld defects such as blowholes when joining materials to be welded that do not use low-temperature sprayed coatings.

[0008] For example, Patent Document 2 discloses an ultrasonic detection method in which a laser is irradiated onto a pair of members constituting a lap joint, and defects in the welded members obtained by laser welding are detected by using ultrasonic waves generated by the laser irradiation.

[0009] Patent Document 3 also proposes a method for determining defects in laser welding, in which a weld made by laser welding is photographed by an imaging means, texture analysis is performed on the photographed image, the value resulting from the texture analysis is compared with a predetermined threshold value, and if the value resulting from the texture analysis exceeds the threshold value, it is determined that a blowhole has occurred. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] JP 2020-011276 A [Patent Document 2] JP 2019-164037 A [Patent Document 3] JP 2003-205381 A Summary of the Invention [Problem to be solved by the invention]

[0011] However, the method of detecting defects by ultrasonic inspection described in Patent Document 2 takes a long time for inspection, and it is difficult to carry out the inspection within a production line.

[0012] Furthermore, the defect determination method described in Patent Document 3, when a blowhole occurs, identifies the uneven shape such as a crater formed on the surface of the welded part as a striped pattern based on brightness, and determines whether or not a blowhole occurs. However, when dissimilar metal materials are joined by laser welding using a low-temperature spray coating, even if a blowhole occurs, an uneven shape such as a crater does not necessarily occur on the surface of the welded part. Therefore, when a dissimilar material joint structure is manufactured using the manufacturing method described in Patent Document 1, the determination method described in Patent Document 3 cannot be used to determine the occurrence of a blowhole.

[0013] The present invention has been made in consideration of such problems, and has an object to provide a method for inspecting a dissimilar metal bonded joint and a method for manufacturing a dissimilar metal bonded joint that can easily determine the amount of blowholes generated, thereby suppressing the occurrence of welding defects and enabling the production of a high-strength dissimilar metal bonded joint, and a dissimilar metal bonded joint manufactured by this manufacturing method. [Means for solving the problem]

[0014] The above object of the present invention can be achieved by the following configuration (1) relating to a method for inspecting a dissimilar metal joint.

[0015] (1) forming a low-temperature sprayed coating by low-temperature spraying at least one metal powder selected from pure iron, carbon steel, ferritic stainless steel, austenitic stainless steel, nickel, nickel alloy, cobalt, and cobalt alloy on at least a part of the surface of an aluminum or aluminum alloy material; a step of overlapping the aluminum or aluminum alloy material and the steel material so that the low-temperature thermal spray coating and the steel material face each other; a step of melting a part of the steel material and a part of the low-temperature thermal spray coating by irradiating a laser from the steel material side to form a weld metal and join the steel material and the aluminum or aluminum alloy material; measuring a height H of the weld metal protruding from the surface of the steel material, calculating the sum of a thickness Ts of the steel material in the region where the weld metal is formed and a thickness Tf of the low-temperature spray coating, and determining a reinforcement height ratio (%) by the formula A: (H / (Ts+Tf))×100; A method for inspecting dissimilar metal joints, comprising determining that a welding defect has occurred when the reinforcement height rate (%) exceeds a predetermined value.

[0016] Further, preferred embodiments of the present invention relating to the inspection method for a dissimilar metal joint relate to the following (2) and (3).

[0017] (2) The inspection method for a dissimilar metal joint according to (1), characterized in that when the reinforcement height ratio is 15(%) or less, the blowhole rate is a predetermined value or less and the joint strength is judged to be good.

[0018] (3) The inspection method for a dissimilar metal joint according to (1), characterized in that when the reinforcement height ratio is 12(%) or less, the blowhole rate is a predetermined value or less and the joint strength is judged to be excellent.

[0019] The above object of the present invention can be achieved by the following configuration (4) relating to a method for producing a dissimilar metal joint.

[0020] (4) forming a low-temperature sprayed coating by low-temperature spraying at least one metal powder selected from pure iron, carbon steel, ferritic stainless steel, austenitic stainless steel, nickel, nickel alloy, cobalt, and cobalt alloy on at least a part of the surface of the aluminum or aluminum alloy material; a step of overlapping the aluminum or aluminum alloy material and the steel material so that the low-temperature thermal spray coating and the steel material face each other; a step of melting a part of the low-temperature spray coating and a part of the steel material by irradiating a laser from the steel material side to form a weld metal and join the steel material and the aluminum or aluminum alloy material; measuring a height H of the weld metal protruding from the surface of the steel material, calculating the sum of a thickness Ts of the steel material in the region where the weld metal is formed and a thickness Tf of the low-temperature spray coating, and determining a reinforcement height ratio (%) by the formula A: (H / (Ts+Tf))×100; If the above-mentioned weld height ratio exceeds a predetermined value, it is determined that a welding defect has occurred, The method includes an additional irradiation step of further irradiating the weld metal with a laser, and an additional determination step of determining whether or not the reinforcement height ratio after the additional irradiation step is equal to or less than a predetermined value, A method for manufacturing a dissimilar metal joint, comprising repeating the additional irradiation step and the additional determination step until the excess fill height ratio becomes equal to or less than a predetermined value.

[0021] A preferred embodiment of the present invention relating to a method for producing a dissimilar metal joint relates to the following (5).

[0022] (5) The method for manufacturing a dissimilar metal joint according to (4), characterized in that the laser output in the additional irradiation step is smaller than the laser output in the step of joining the steel material and the aluminum or aluminum alloy material.

[0023] The above object of the present invention can be achieved by the following configuration (6) relating to a method for producing a dissimilar metal joint.

[0024] (6) determining welding conditions through a preliminary welding test; A step of forming a low-temperature sprayed coating by low-temperature spraying at least one metal powder selected from pure iron, carbon steel, ferritic stainless steel, austenitic stainless steel, nickel, nickel alloy, cobalt, and cobalt alloy on at least a part of the surface of an aluminum or aluminum alloy material; a step of overlapping the aluminum or aluminum alloy material and the steel material so that the low-temperature thermal spray coating and the steel material face each other; and a step of irradiating a laser from the steel material side using the welding conditions determined by the preliminary welding test to melt a part of the low-temperature spray coating and a part of the steel material to form a weld metal, thereby joining the steel material and the aluminum or aluminum alloy material. The above preliminary welding test is A step of forming a test low-temperature sprayed coating by low-temperature spraying at least one metal powder selected from pure iron, carbon steel, nickel, nickel alloy, cobalt, and cobalt alloy on at least a part of the surface of a test aluminum or aluminum alloy material; a step of overlapping the test aluminum or aluminum alloy material and the test steel material so that the test low-temperature spray coating and the test steel material face each other; a step of melting a part of the test low-temperature spray coating and a part of the test steel by irradiating a laser from the test steel side under test welding conditions, forming a test weld metal, and joining the test steel and the test aluminum or aluminum alloy material; measuring a height H of the reinforcement of the test weld metal protruding from the surface of the test steel material, and calculating the sum of a thickness Ts of the steel material in the region where the test weld metal is formed and a thickness Tf of the test low-temperature sprayed coating, and determining a reinforcement height ratio (%) by the formula A: (H / (Ts+Tf))×100; The step of determining the welding conditions includes: a step of determining that a welding defect has occurred when the reinforcement height ratio exceeds a predetermined value, changing the test welding conditions and repeatedly carrying out the preliminary welding test, and selecting welding conditions under which the reinforcement height ratio is equal to or less than the predetermined value.

[0025] The above object of the present invention can be achieved by the following configuration (7) relating to a dissimilar metal joint.

[0026] (7) Aluminum or aluminum alloy material; a low-temperature thermal spray coating provided on at least a portion of the surface of the aluminum or aluminum alloy material and containing at least one metal selected from pure iron, carbon steel, ferritic stainless steel, austenitic stainless steel, nickel, a nickel alloy, cobalt, and a cobalt alloy; A steel material arranged so as to be in contact with the low-temperature thermal spray coating; A dissimilar metal joint having a weld metal formed so as to reach the low-temperature sprayed coating from a surface of the steel material opposite to a surface in contact with the low-temperature sprayed coating, and joining the low-temperature sprayed coating and the steel material, The weld metal has a reinforcement protruding from a surface of the steel material, A dissimilar metal joint, characterized in that, when the height of the reinforcement is H (mm), the thickness of the steel material in the region having the weld metal is Ts (mm), and the thickness of the low-temperature sprayed coating is Tf (mm), the reinforcement height ratio calculated by formula A: (H / (Ts+Tf))×100 is 15(%) or less. Effect of the Invention

[0027] According to the present invention, it is possible to provide a method for inspecting a dissimilar metal joint and a method for manufacturing a dissimilar metal joint, which can easily determine the amount of blowholes generated, thereby suppressing the occurrence of welding defects and enabling the manufacture of a high-strength dissimilar metal joint. Also, according to the present invention, it is possible to obtain a dissimilar metal joint having few blowholes in the weld metal and high strength. [Brief description of the drawings]

[0028] [Figure 1] FIG. 1 is a cross-sectional view showing a part of an inspection method for a dissimilar metal joint according to an embodiment of the present invention, illustrating a process of joining a steel material and an aluminum alloy material. [Diagram 2] FIG. 2 is a drawing-substitute photograph showing a part of the inspection method for a dissimilar metal joint according to the embodiment of the present invention, and showing an enlarged example of the obtained weld metal. [Diagram 3] FIG. 3 is a photograph, substituting a drawing, illustrating a part of the inspection method for a dissimilar metal joint according to an embodiment of the present invention and showing an enlarged view of another example of the obtained weld metal. [Figure 4] FIG. 4 is a graph showing the relationship between the reinforcement height ratio and the blowhole rate, with the vertical axis representing the reinforcement height ratio and the horizontal axis representing the blowhole rate. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0029] The present inventors have conducted extensive research into a method for easily determining the occurrence of blowholes when manufacturing a dissimilar metal joint between aluminum or aluminum alloy material and steel material, thereby manufacturing a high-strength dissimilar metal joint. As a result, they have found that when aluminum or aluminum alloy material and steel material are laser welded using a low-temperature sprayed coating, if blowholes occur in the weld metal, the height of the weld metal reinforcement changes depending on the amount of blowholes. In addition, the present inventors have found that the blowhole rate can be predicted by determining the value of the reinforcement height relative to the sum of the thicknesses of the steel material and the low-temperature sprayed coating. Therefore, by setting the reinforcement height so that it is equal to or less than a predetermined blowhole rate, a high-strength dissimilar metal joint with few blowholes can be manufactured. The present invention was made based on these findings.

[0030] Hereinafter, the embodiment of the inspection method for dissimilar metal joints, the manufacturing method for dissimilar metal joints, and the dissimilar metal joints according to the present invention will be described in detail with reference to the drawings. Note that the present invention is not limited to the embodiment described below, and can be modified as desired without departing from the gist of the present invention. In addition, in the embodiments shown below, aluminum or aluminum alloy material may be simply referred to as aluminum alloy material.

[0031] [Inspection method for dissimilar metal joints] Fig. 1 shows a part of an inspection method for a dissimilar metal joint according to an embodiment of the present invention, and is a cross-sectional view showing a process of joining a steel material and an aluminum alloy material. Fig. 2 shows a part of an inspection method for a dissimilar metal joint according to an embodiment of the present invention, and is a photograph substituting a drawing showing an enlarged example of an obtained weld metal. Fig. 3 shows a part of an inspection method for a dissimilar metal joint according to an embodiment of the present invention, and is a photograph substituting a drawing showing an enlarged example of another obtained weld metal.

[0032] <Process for forming low-temperature spray coating> As shown in Fig. 1, a low-temperature sprayed coating 1 is formed by low-temperature spraying a metal powder such as pure iron on at least a part of the surface of an aluminum alloy material 2. The low-temperature spraying method is also called cold spraying, and is a method in which gas and metal powder are sprayed onto an object at a high speed faster than the speed of sound. This method can be carried out by appropriately selecting the type of gas used, pressure, temperature, particle size of the metal powder, etc.

[0033] <Process of overlapping aluminum alloy material and steel material> Thereafter, a steel material 3 is laid on the obtained low-temperature sprayed coating 1 .

[0034] <Process for joining aluminum alloy material and steel material> Thereafter, the laser L is moved along a desired weld line direction while irradiating the steel material 3 side, i.e., the surface of the steel material 3 opposite to the surface in contact with the low-temperature sprayed coating 1. As shown in Fig. 2, by irradiating the laser L, a part of the steel material 3 is melted in the plate thickness direction, and at least a part of the low-temperature sprayed coating 1 is melted. As a result, a weld metal 4 is formed that penetrates the steel material 3 in the plate thickness direction and reaches a part of the low-temperature sprayed coating in the thickness direction from the surface of the low-temperature sprayed coating 1 on the steel material 3 side, and the aluminum alloy material 2 and the steel material 3 are joined.

[0035] Here, the reason why laser welding is applied as a process for joining an aluminum alloy material and a steel material is that laser welding has a low heat input and a small thermal effect compared to other welding methods such as arc welding. When arc welding is used, the heat generated during welding easily reaches the aluminum alloy material 2, and there is a risk that the aluminum alloy material 2 will melt as well, resulting in a decrease in joining strength. However, when laser welding is used, the thermal effect on the aluminum alloy material 2 can be minimized and the melting of the aluminum alloy material 2 can be suppressed.

[0036] As described above, when the steel material 3 and the low-temperature sprayed coating 1 are melted by irradiating the laser L, the low-temperature sprayed coating 1 contains gas, which easily causes blowholes 6 to occur in the weld metal 4. When blowholes 6 occur in the weld metal 4, the weld metal 4 protrudes from the surface of the steel material 3, and a reinforcement 5 is formed.

[0037] On the other hand, as shown in Fig. 3, when no blowholes are formed in the weld metal 4, the height of the reinforcement 5 protruding from the surface of the steel material 3 is low. Therefore, the occurrence of blowholes can be judged based on the height of the reinforcement 5, etc.

[0038] <Process for determining reinforcement height ratio> Specifically, in this embodiment, the height H of the reinforcement 5 is measured, and the sum of the thickness Ts of the steel material 3 and the thickness Tf of the low-temperature thermal sprayed coating 1 in the joining region between the aluminum alloy material 2 and the steel material 3, i.e., in the region where the weld metal 4 is formed, is calculated, and the reinforcement height ratio (%) is calculated by the following formula A. Formula A: (H / (Ts+Tf))×100

[0039] The height H of the reinforcement 5 may be measured by photography with a camera or by a laser, and the measurement method is not particularly limited. Since the surface of the weld metal 4 is likely to have a concave shape such as a crater at the weld start and weld end, it is preferable to select a stationary portion other than the weld start and weld end as the position for measuring the height H of the reinforcement 5.

[0040] The thickness Tf of the low-temperature sprayed coating 1 can be measured before the joining process. For example, the thickness of the aluminum alloy material is measured at the location where the low-temperature sprayed coating 1 is to be formed, and the thickness of the member at the corresponding location after the low-temperature sprayed coating 1 is formed is measured. Then, the thickness Tf of the low-temperature sprayed coating 1 can be calculated by subtracting the thickness of the aluminum alloy material before the coating is formed from the thickness of the member after the coating is formed.

[0041] The height H of the reinforcement 5 changes depending on the amount of blowholes 6 generated in the weld metal 4, and particularly when the amount of blowholes 6 generated is large, the value of the height H of the reinforcement 5 relative to the sum of the thickness Ts of the steel material and the thickness Tf of the low-temperature sprayed coating (reinforcement height ratio) becomes large. Therefore, when the reinforcement height ratio calculated by the above formula A exceeds a predetermined value, it can be determined that there are many blowholes 6 in the weld metal 4 and that poor welding has occurred.

[0042] In this embodiment, the blowhole rate can be predicted simply by measuring the height H of the reinforcement 5, the thickness Ts of the steel material, and the thickness Tf of the low-temperature sprayed coating, and calculating the reinforcement height rate using the above formula A, so it is possible to easily inspect whether a desired joint has been obtained in a production line. Note that in this specification, the blowhole rate is a value obtained by taking a cross-sectional macrophotograph of the weld metal 4, calculating the overall area Sw of the weld metal 4 including the blowholes, and the area Sb of the blowholes, and then using the following formula B. Formula B: (Sb / Sw) x 100

[0043] Incidentally, it has been reported that in arc welding, when the blowholes reach 20% or more, the joint strength drops sharply (Ishii, Influence of bubbles on strength in steel welds, Journal of the Japan Welding Society, Vol. 41 (1972) No. 4, pp. 368-378). The present inventors have experimentally confirmed that the blowholes are 20% or less when the reinforcement height ratio is 15% or less. Therefore, in order to make the blowhole ratio 20% or less, it is preferable to make the reinforcement height ratio 15% or less. In other words, when the reinforcement height ratio is 15% or less, it can be determined that the blowhole ratio is 20% or less and the joint strength is good. Also, for example, as shown in FIG. 3, it is preferable that the reinforcement height ratio is 12% or less, and in this case, it can be determined that almost no blowholes occur and the joint strength is superior.

[0044] [Manufacturing method for dissimilar metal joints] (First embodiment) In the present invention, a high-strength dissimilar metal joint with few welding defects can be manufactured by utilizing the above-mentioned inspection method for a dissimilar metal joint. Hereinafter, a manufacturing method for a dissimilar metal joint according to a first embodiment of the present invention will be described with reference to Figs. 1 to 3.

[0045] First, as shown in Figures 1 and 2, the steps of forming a low-temperature spray coating, overlapping an aluminum alloy material with the steel material, joining the aluminum alloy material with the steel material, and determining the weld height ratio are carried out in the same manner as in the inspection method for dissimilar metal joints. The method and position for measuring the height H of the weld 5 are as described above. If the weld height ratio exceeds a predetermined value, for example, 15%, it is determined that a welding defect has occurred, and the next additional irradiation step is carried out.

[0046] <Additional irradiation process> When it is determined based on the above-mentioned reinforcement height ratio that a dissimilar metal joint having the desired quality has not been obtained, the laser L is further irradiated along the weld metal 4. If the output of the laser L in this additional irradiation step is made larger than the output of the laser L in the step of joining the aluminum alloy material and the steel material, the low-temperature sprayed coating 1 may be melted again, and further blowholes 6 may be generated. Therefore, it is preferable to make the output of the laser L in the additional irradiation step smaller than the output of the laser L in the step of forming the weld metal. By this additional irradiation step, the blowholes 6 in the weld metal 4 can be discharged.

[0047] <Additional judgment process> Thereafter, the reinforcement height ratio after the additional irradiation process is determined in the same manner as in the process for determining the reinforcement height ratio, and it is confirmed whether it is 15% or less. If the reinforcement height ratio is 15% or less, it is determined that the blowholes have been reduced. On the other hand, if the reinforcement height ratio exceeds 15%, the additional irradiation process and the additional evaluation process are repeated until the reinforcement height ratio is 15% or less. By this method, a high-strength dissimilar metal joint can be manufactured.

[0048] The above-mentioned additional irradiation process and additional judgment process may be repeated two or more times. However, as the number of repetitions increases, the time required to manufacture one dissimilar metal joint increases significantly, so it is preferable to repeat the additional irradiation process and additional judgment process fewer times.

[0049] In this embodiment, by determining the relationship between the required strength and the blowhole rate in advance, the reinforcement height rate at which it is determined that a welding defect has occurred can be freely set. Specifically, when a sufficiently high strength is required, it is preferable that the blowhole rate is as low as possible, so the reinforcement height rate judgment standard can be set low, for example, to 12% or less. On the other hand, when the target strength is one that allows blowholes to a certain extent, the reinforcement height rate judgment standard can be set high.

[0050] Second embodiment In the second embodiment, a step of determining welding conditions by a preliminary welding test is carried out before the step of forming a low-temperature sprayed coating in the first embodiment, the step of overlapping an aluminum alloy material and the steel material, and the step of joining the aluminum alloy material and the steel material. Hereinafter, the preliminary welding test and the step of determining the welding conditions will be described. Note that the test aluminum alloy material and test steel material used in the preliminary welding test, and the test low-temperature sprayed coating and test weld metal formed in the preliminary welding test are similar to those actually produced, and therefore will be described with reference to Figs. 1 to 3.

[0051] [Preliminary welding test] <Process for forming test low-temperature sprayed coating> First, as shown in FIG. 1, a test aluminum alloy material, a test steel material, and a metal powder equivalent to those used in an actually manufactured dissimilar metal joint are prepared, and a test low-temperature sprayed coating (low-temperature sprayed coating 1) is formed by low-temperature spraying a metal powder such as pure iron onto the test aluminum alloy material (aluminum alloy material 2) in the same manner as in the actual manufacturing method.

[0052] <Step of overlapping the test aluminum alloy material and the test steel material> Next, the test aluminum alloy material and the test steel material (steel material 3) are overlapped so that the test low-temperature sprayed coating and the test steel material face each other.

[0053] <Step of joining test aluminum alloy material and test steel material> Thereafter, a laser L is irradiated from the test steel material side under arbitrarily set test welding conditions while moving the laser L along the desired weld line direction to melt a portion of the test low-temperature sprayed coating and a portion of the test steel material, thereby forming a test weld metal (weld metal 4) and joining the test aluminum alloy material and the test steel material.

[0054] <Process for determining reinforcement height ratio> The height H of the test weld metal protruding from the surface of the test steel is measured, and the sum of the thickness Ts of the steel in the area where the test weld metal is to be formed and the thickness Tf of the low-temperature spray coating is calculated, and the reinforcement height ratio (%) is calculated using formula A: (H / (Ts+Tf))×100.

[0055] [Determination of welding conditions] <Process for determining welding conditions> When the reinforcement height ratio exceeds a predetermined value, it is determined that a welding defect has occurred, and the test welding conditions are changed and the above preliminary welding test is repeated. As a method for changing the test welding conditions, it is preferable to control the amount of laser heat input, and specifically, it is preferable to change the laser output, welding speed, and beam focus position. Then, the test welding conditions when the reinforcement height ratio becomes equal to or less than the predetermined value are selected, and the actual welding process, i.e., the process of forming a low-temperature spray coating, the process of overlapping an aluminum alloy material and the steel material, and the process of joining the aluminum alloy material and the steel material, are performed.

[0056] According to the manufacturing method of the dissimilar metal joint of this embodiment, a metal powder such as pure iron is sprayed on the surface of the aluminum alloy material 2 at high speed to form the low-temperature spray coating 1, so that fine irregularities are formed on the surface of the aluminum alloy material 2. The low-temperature spray coating 1 made of pure iron and the aluminum alloy material 2 are mechanically fastened by the anchor effect, so that they are firmly bonded. In addition, since the low-temperature spray coating 1 and the steel material 3 are firmly joined by general laser welding, no intermetallic compound is generated, and the dissimilar metals of the steel material 3 and the aluminum alloy material 2 can be indirectly joined with high joining strength. In addition, according to the manufacturing method of this embodiment, additional laser irradiation or selection of conditions is performed until the reinforcement height rate becomes equal to or less than a predetermined value. Therefore, the blowhole rate in the weld metal 4 is reduced, and a dissimilar metal joint with high strength can be manufactured.

[0057] [Dissimilar metal joints] The present invention also relates to a dissimilar metal joint manufactured by the above-mentioned manufacturing method for a dissimilar metal joint. Referring to Fig. 3, a dissimilar metal joint according to an embodiment of the present invention will be described.

[0058] A dissimilar metal joint 10 according to this embodiment includes an aluminum alloy material 2, a low-temperature sprayed coating 1 provided on at least a portion of the surface of the aluminum alloy material 2, a steel material 3 arranged so as to be in contact with the low-temperature sprayed coating 1, and a weld metal 4 joining the low-temperature sprayed coating 1 and the steel material 3. The weld metal 4 is formed so as to reach the low-temperature sprayed coating 1 from the surface of the steel material 3 opposite to the surface in contact with the low-temperature sprayed coating 1, and has a weld metal 5 protruding from the surface of the steel material 3.

[0059] In addition, assuming that the height of the reinforcement 5 is H (mm), the thickness of the steel material 3 in the region having the weld metal 4 is Ts (mm), and the thickness of the low-temperature sprayed coating 1 is Tf (mm), the reinforcement height ratio calculated by the formula A: (H / (Ts+Tf))×100 is 15(%) or less.

[0060] In the dissimilar joint according to the present embodiment configured as described above, the low-temperature sprayed coating 1 is formed by spraying a metal powder such as pure iron onto the surface of the aluminum alloy material 2 at high speed, so that the low-temperature sprayed coating 1 and the aluminum alloy material 2 are mechanically fastened and firmly bonded by an anchor effect. In addition, the low-temperature sprayed coating 1 and the steel material 3 are firmly joined by the weld metal 4, so that the dissimilar metals of the steel material 3 and the aluminum alloy material 2 are indirectly joined with high joining strength. In addition, according to the present embodiment, the reinforcement height ratio is equal to or less than a predetermined value. Therefore, the blowhole ratio in the weld metal 4 is reduced, and the joint strength can be improved.

[0061] Next, the low-temperature thermal spray coating 1, the metal powder that is the material thereof, the aluminum alloy material 2, the steel material 3, and the welding conditions during laser irradiation in the inspection method for dissimilar metal joints, the manufacturing method for dissimilar metal joints, and the dissimilar metal joints according to this embodiment will be described in detail below.

[0062] <Type of metal powder: at least one selected from pure iron, carbon steel, ferritic stainless steel, austenitic stainless steel, nickel, nickel alloy, cobalt, and cobalt alloy> In order to join the low-temperature sprayed coating 1 and the steel material 3 by laser welding, it is important to select, as the material for the low-temperature sprayed coating 1, a metallic material that can be welded to the steel material 3 with a desired joining strength and that has good characteristics for the weld metal 4. Therefore, as the metallic powder used to form the low-temperature sprayed coating 1, a powder containing at least one metal selected from pure iron, carbon steel, ferritic stainless steel, austenitic stainless steel, nickel, nickel alloy, cobalt, and cobalt alloy is used.

[0063] In this specification, pure iron refers to iron that is easily available for industrial use and has a purity of 99.9% by mass or more. Carbon steel refers to a steel material that is mainly composed of iron and carbon and contains trace amounts of silicon, manganese, and impurities such as phosphorus, sulfur, and copper. Nickel alloys that are commonly called Inconel alloys, Incoloy alloys, and Hastelloy alloys, which are mainly composed of Ni with appropriate amounts of Mo, Fe, Co, Cr, Mn, etc., can be used.

[0064] <Metal powder average particle size and shape> There are no particular limitations on the average particle size of the metal powder that is the material for the low-temperature thermal spray coating 1, but when the gas pressure during low-temperature thermal spraying is a low-pressure condition of 1 MPa or less, it is preferably, for example, 20 μm or less, and more preferably 10 μm or less. On the other hand, when the gas pressure is set to a high pressure condition of 1 MPa to 5 MPa, the thickness is preferably, for example, 50 μm or less, and more preferably 30 μm or less. The particle shape of the metal powder is not particularly limited, but is preferably spherical from the viewpoint of flowability.

[0065] <Type of working gas> There is no particular limitation on the gas used when forming the low-temperature sprayed coating 1, but air, nitrogen, helium, or a mixture of these gases is generally used. On the other hand, if the low-temperature sprayed coating is oxidized, it may adversely affect the laser weldability, so it is preferable to use nitrogen or helium as the gas type.

[0066] <Working gas temperature> If the temperature of the gas used when forming the low-temperature thermal spray coating 1 is high, there is a risk of the powder clogging the thermal spray nozzle. Therefore, it is preferable that the temperature of the working gas is lower than the melting point of the metal powder used when forming the low-temperature thermal spray coating 1.

[0067] <Thickness of low-temperature sprayed coating> If the thickness of the low-temperature sprayed coating 1 is less than 0.3 mm, it is difficult to obtain sufficient strength, especially in the peeling direction, after welding. Therefore, by making the thickness of the low-temperature sprayed coating 1 0.3 mm or more, it is possible to obtain a joint with excellent strength. Therefore, the thickness of the low-temperature sprayed coating is preferably 0.3 mm or more, and more preferably 0.6 mm or more. On the other hand, if the thickness of the low-temperature sprayed coating 1 exceeds 3 mm, the deposition time becomes long and the manufacturing cost may increase. Therefore, the thickness of the low-temperature sprayed coating 1 is preferably 3 mm or less, and more preferably 2 mm or less.

[0068] <Aluminum or aluminum alloy material> Although there is no particular limitation on the aluminum or aluminum alloy material, when applied to members used in automobiles, etc., it is preferable to use aluminum alloy materials of 2000 series, 5000 series, 6000 series, 7000 series, etc., from the viewpoint of strength. In this embodiment, since laser welding that allows welding by one-sided construction from the steel material 3 side is used, even extruded materials with closed cross sections that are often used in the field of automobiles, etc. can be used without any problem.

[0069] <Steel> The steel material 3 is not particularly limited as long as it is a member made of a metal generally called iron and steel. However, in recent years, high-tensile steel materials (high-tensile materials) and the like have been widely used as steel plates for the body frames of automobiles in order to reduce the weight of the vehicle body and improve collision safety. It is difficult to apply the mechanical joining method, which is widely used as a dissimilar material joining method for joining steel materials to aluminum or aluminum alloy materials, to steel plates having a tensile strength of 590 MPa or more. Therefore, the present invention is particularly effective for high-tensile steel plates having a tensile strength of 590 MPa or more.

[0070] <Welding conditions during laser irradiation> In this embodiment, in order to further minimize the thermal influence on the aluminum alloy material 2, it is preferable to select appropriate welding conditions so as to melt only the low-temperature sprayed coating 1 and the steel material 3. In this embodiment, it is preferable to adjust the amount of laser heat input as a condition for controlling the reinforcement height ratio to a desired value. Specifically, the reinforcement height ratio can be controlled by variously changing the laser output, welding speed, and beam focusing diameter on the workpiece surface. In addition, in the manufacture of dissimilar metal joints using low-temperature sprayed coatings, it is generally preferable to set the power density (laser output / beam area on the workpiece surface) to 1.0×10 5 ~1.0×10 7 (W / cm 2 ), and the input energy (laser output / (beam focal diameter on the work surface × welding speed)) is 1.0 × 10 3 ~1.0×10 5 (J / cm 2 It is preferable to select the range of . EXAMPLES

[0071] Hereinafter, examples of the inspection method for a dissimilar metal joint according to this embodiment will be specifically described. First, a metal powder made of pure iron or nickel was sprayed at low temperature on the surface of an aluminum alloy plate to form a low-temperature sprayed coating. Next, the aluminum alloy plate and the steel plate were placed one on top of the other so that the obtained low-temperature sprayed coating and the steel plate faced each other. Then, a laser was irradiated from the steel plate side, i.e., the surface of the steel plate opposite to the surface in contact with the metal coating, to melt a part of the steel plate and a part of the low-temperature sprayed coating, form a weld metal, and join the steel plate and the aluminum alloy plate to produce a dissimilar material joint structure. The laser irradiation was controlled by a galvanometer scanner to perform linear welding.

[0072] The types of test materials, the conditions for forming the low-temperature sprayed coating, and the conditions for laser welding are shown below.

[0073] [Test material] Aluminum alloy plate: AA7204 (tensile strength 350MPa class), plate thickness 3mm Steel plate: High-tensile steel plate (tensile strength 980-1500MPa)

[0074] [Conditions for forming low-temperature thermal spray coating] Gas type: Nitrogen Gas pressure: 5MPa Gas temperature: 1000℃ Metal powder: iron powder, nickel powder Average particle size of metal powder: 20~50μm

[0075] [Laser welding conditions] Heat source: Fiber laser Speed: 4m / min Beam focusing diameter on work surface: 0.40~0.61mm Shielding gas: None

[0076] Thereafter, a cross-sectional macrophotograph was taken of the obtained dissimilar metal welded structure, and the height H of the reinforcement was measured from the cross-sectional macrophotograph, and the reinforcement height ratio was calculated from the thickness Ts of the steel plate and the thickness Tf of the low-temperature sprayed coating by the following formula A. In addition, the area Sw of the entire weld metal and the area Sb of the blowholes were measured from the cross-sectional macrophotograph, and the blowhole ratio was calculated by the following formula B.

[0077] Formula A: (H / (Ts+Tf))×100 Formula B: (Sb / Sw) x 100

[0078] The various conditions, measurement results and calculation results are shown in Table 1 below. In the blowhole rate shown in Table 1 below, 0(%) indicates that there were no blowholes at all.

[0079] [Table 1]

[0080] FIG. 4 is a graph showing the relationship between the reinforcement height ratio and the blowhole ratio, with the reinforcement height ratio on the vertical axis and the blowhole ratio on the horizontal axis. As shown in Table 1 and FIG. 4, it was shown that the blowhole ratio increases as the reinforcement height ratio increases, regardless of the type and thickness of the low-temperature spray coating, the thickness of the steel material, and the welding conditions. Also, as shown in Table 1, the test materials No. 1 to 15, which have a reinforcement height ratio of, for example, 15.0% or less, have a blowhole ratio of 20% or less, and it was determined that a high-strength dissimilar metal joint was manufactured. In particular, the test materials No. 1 to 13, which have a reinforcement height ratio of, for example, 12.0% or less, have a blowhole ratio of 1.5% or less, and blowholes are significantly reduced, and it was determined that a dissimilar metal joint with even higher strength was manufactured.

[0081] According to Table 1, a certain degree of correlation is observed between the reinforcement height H and the blowhole rate. However, for example, the reinforcement height H of test material No. 21 is 0.44 (mm), while that of test material No. 15 is 0.49 (mm). Test material No. 15 has a higher reinforcement height H, but the reinforcement height ratio is 15.0% or less and the blowhole rate is 20% or less, resulting in a dissimilar metal joint with higher strength than test material No. 21. From these results, it is not possible to judge the blowhole rate simply by comparing the reinforcement height H, and it has been shown that the present invention, which judges the blowhole rate from the reinforcement height ratio, is an extremely effective means. [Explanation of symbols]

[0082] 1. Low-temperature spray coating 2. Aluminum alloy material 3 Steel material 4. Weld metal 5 extra serving 6 Blowhole 10 Dissimilar metal joints

Claims

1. A step of forming a low-temperature sprayed coating by low-temperature spraying at least one metal powder selected from pure iron, carbon steel, ferritic stainless steel, austenitic stainless steel, nickel, nickel alloy, cobalt, and cobalt alloy on at least a part of the surface of an aluminum or aluminum alloy material; a step of overlapping the aluminum or aluminum alloy material and the steel material so that the low-temperature thermal spray coating and the steel material face each other; a step of melting a part of the steel material and a part of the low-temperature thermal spray coating by irradiating a laser from the steel material side to form a weld metal and join the steel material and the aluminum or aluminum alloy material; measuring a height H of the weld metal protruding from a surface of the steel material, calculating a sum of a thickness Ts of the steel material in a region where the weld metal is formed and a thickness Tf of the low-temperature thermal spray coating, and determining a reinforcement height ratio (%) by formula A: (H / (Ts+Tf))×100, A method for inspecting a dissimilar metal joint, comprising determining that a welding defect has occurred when the reinforcement height rate (%) exceeds a predetermined value.

2. 2. The method for inspecting a dissimilar metal joint according to claim 1, characterized in that, when the reinforcement height ratio is 15(%) or less, it is determined that the blowhole rate is a predetermined value or less and the joint strength is good.

3. 3. The method for inspecting a dissimilar metal joint according to claim 2, characterized in that, when the reinforcement height ratio is 12(%) or less, it is determined that the blowhole rate is a predetermined value or less and the joint strength is excellent.

4. A step of forming a low-temperature sprayed coating by low-temperature spraying at least one metal powder selected from pure iron, carbon steel, ferritic stainless steel, austenitic stainless steel, nickel, nickel alloy, cobalt, and cobalt alloy on at least a part of the surface of an aluminum or aluminum alloy material; a step of overlapping the aluminum or aluminum alloy material and the steel material so that the low-temperature thermal spray coating and the steel material face each other; a step of melting a part of the low-temperature spray coating and a part of the steel material by irradiating a laser from the steel material side to form a weld metal and join the steel material and the aluminum or aluminum alloy material; measuring a height H of the weld metal protruding from a surface of the steel material, calculating a sum of a thickness Ts of the steel material in a region where the weld metal is formed and a thickness Tf of the low-temperature thermal spray coating, and determining a reinforcement height ratio (%) by formula A: (H / (Ts+Tf))×100, When the reinforcement height ratio exceeds a predetermined value, it is determined that a welding defect has occurred, The method includes an additional irradiation step of further irradiating the weld metal with a laser, and an additional determination step of determining whether or not the reinforcement height ratio after the additional irradiation step is equal to or less than a predetermined value, A method for manufacturing a dissimilar metal joint, comprising repeating the additional irradiation step and the additional determination step until the excess fill height ratio becomes equal to or less than a predetermined value.

5. 5. The method for manufacturing a dissimilar metal joint according to claim 4, wherein a laser output in the additional irradiation step is smaller than a laser output in the step of joining the steel material and the aluminum or aluminum alloy material.

6. determining welding conditions through a preliminary welding test; A step of forming a low-temperature sprayed coating by low-temperature spraying at least one metal powder selected from pure iron, carbon steel, ferritic stainless steel, austenitic stainless steel, nickel, nickel alloy, cobalt, and cobalt alloy on at least a part of the surface of an aluminum or aluminum alloy material; a step of overlapping the aluminum or aluminum alloy material and the steel material so that the low-temperature thermal spray coating and the steel material face each other; and a step of irradiating a laser from the steel material side using the welding conditions determined by the preliminary welding test to melt a part of the low-temperature spray coating and a part of the steel material to form a weld metal, thereby joining the steel material and the aluminum or aluminum alloy material. The preliminary welding test is A step of forming a test low-temperature sprayed coating by low-temperature spraying at least one metal powder selected from pure iron, carbon steel, nickel, nickel alloy, cobalt, and cobalt alloy on at least a part of the surface of a test aluminum or aluminum alloy material; a step of overlapping the test aluminum or aluminum alloy material and the test steel material so that the test low-temperature spray coating and the test steel material face each other; a step of melting a part of the test low-temperature spray coating and a part of the test steel by irradiating a laser from the test steel side under test welding conditions, forming a test weld metal, and joining the test steel and the test aluminum or aluminum alloy material; measuring a height H of the reinforcement of the test weld metal protruding from the surface of the test steel material, calculating the sum of a thickness Ts of the steel material in the region where the test weld metal is formed and a thickness Tf of the test low-temperature sprayed coating, and determining a reinforcement height ratio (%) by the formula A: (H / (Ts+Tf))×100; The step of determining welding conditions includes: a step of determining that a welding defect has occurred when the reinforcement height ratio exceeds a predetermined value, changing the test welding conditions and repeatedly conducting the preliminary welding test, and selecting welding conditions under which the reinforcement height ratio is equal to or less than the predetermined value.

Images