Friction stir welding apparatus for structural steel
10766099 ยท 2020-09-08
Assignee
Inventors
Cpc classification
B23K20/1225
PERFORMING OPERATIONS; TRANSPORTING
B23K20/1235
PERFORMING OPERATIONS; TRANSPORTING
B23K20/12
PERFORMING OPERATIONS; TRANSPORTING
B23K20/1245
PERFORMING OPERATIONS; TRANSPORTING
B23K37/003
PERFORMING OPERATIONS; TRANSPORTING
International classification
Abstract
A friction stir welding apparatus that includes a rotary tool and a heating device. The heating device is placed in front of the rotary tool to heat steel sheets that are used as a workpiece. The rotary tool is moved in the welding direction so that steel sheet is softened by frictional heat. Additionally, the surface temperature, the area, and the position of the heated region during the heating process are strictly controlled. The heating of the steel sheets by the welding apparatus provides sufficient strength and good welding workability by advantageously eliminating plastic flow defects generated due to insufficient heating of workpieces.
Claims
1. A friction stir welding apparatus for structural steel, the welding apparatus comprising: a rotary tool that includes a shoulder and a pin disposed on the shoulder, the pin sharing a rotation axis with the shoulder, at least the shoulder and the pin being made of a material that is harder than steel sheets that are used as workpieces, a front heating device that is provided in front of the rotary tool and positioned higher than a top surface of the rotary tool, and a rear heating device provided behind the rotary tool and positioned lower than the front heating device, wherein: the front heating device is adapted to: form a heated region on the steel sheets such that: (a) a center of the heated region is located on the advancing side; and (b) a distance from the center of the heated region to the welding centerline is from 0.5 times to 1 time a maximum radius of the pin of the rotary tool, the heated region having a temperature T.sub.S ( C.) that satisfies T.sub.S0.8T.sub.A1 (such that T.sub.A1 is described in Formula (1) below)
T.sub.A1( C.)=72310.7[% Mn]16.9[% Ni]+29.1[% Si]+16.9[% Cr]+290 [% As]+6.38[% W]Formula (1), where [% M] represents an amount (mass %) of element M in the steel sheets used as a workpiece, the rotary tool is adapted to penetrate into a non-welded part of the steel sheets and to move in a welding direction while the rotary tool rotates so that a portion of the steel sheets which has been preheated by the front heating device is: (i) softened by frictional heat between the rotary tool and the steel sheets and (ii) the softened portion of the steels sheet is stirred with the rotary tool to produce plastic flow, and the welding centerline is a straight line that passes through the rotation axis of the rotary tool and is parallel to the welding direction.
2. The friction stir welding apparatus for structural steel according to claim 1, wherein, where a maximum depth of a region that extends from the surface of the steel sheets in the heated region, in a thickness direction, and has a temperature T.sub.D ( C.) satisfying T.sub.D0.8T.sub.A1 (such that T.sub.A1 is described in the Formula (1)) is defined as a depth D of the heated region, the depth D of the heated region being equal to or larger than 30% of a total thickness t of the steel sheets.
3. The friction stir welding apparatus for structural steel according to claim 1, wherein the front heating device is a laser heating device.
4. The friction stir welding apparatus for structural steel according to claim 1, wherein the rear heating device is adapted to heat a welded part of the steel sheets.
5. The friction stir welding apparatus for structural steel according to claim 4, further comprising a cooling device that is provided behind the rotary tool and the rear heating device and that is adapted to cool the welded part of the steel sheets.
6. The friction stir welding apparatus for structural steel according to claim 1, further comprising a cooling device that is provided behind the rotary tool and that is adapted to cool a welded part of the steel sheets.
7. The friction stir welding apparatus for structural steel according to claim 6, wherein the rear heating device is provided behind the cooling device and is adapted to reheat the welded part of the steel sheets.
8. The friction stir welding apparatus for structural steel according to claim 2, wherein the front heating device is a laser heating device.
9. The friction stir welding apparatus for structural steel according to claim 2, wherein the rear heating device is adapted to heat a welded part of the steel sheets.
10. The friction stir welding apparatus for structural steel according to claim 3, wherein the rear heating device is adapted to heat a welded part of the steel sheets.
11. The friction stir welding apparatus for structural steel according to claim 8, wherein the rear heating device is adapted to heat a welded part of the steel sheets.
12. The friction stir welding apparatus for structural steel according to claim 9, further comprising a cooling device that is provided behind the rotary tool and the rear heating device and that is adapted to cool the welded part of the steel sheets.
13. The friction stir welding apparatus for structural steel according to claim 10, further comprising a cooling device that is provided behind the rotary tool and the rear heating device and that is adapted to cool the welded part of the steel sheets.
14. The friction stir welding apparatus for structural steel according to claim 11, further comprising a cooling device that is provided behind the rotary tool and the rear heating device and that is adapted to cool the welded part of the steel sheets.
15. The friction stir welding apparatus for structural steel according to claim 2, further comprising a cooling device that is provided behind the rotary tool and that is adapted to cool a welded part of the steel sheets.
16. The friction stir welding apparatus for structural steel according to claim 3, further comprising a cooling device that is provided behind the rotary tool and that is adapted to cool a welded part of the steel sheets.
17. The friction stir welding apparatus for structural steel according to claim 15, wherein the rear heating device is adapted to reheat the welded part of the steel sheets.
18. The friction stir welding apparatus for structural steel according to claim 16, wherein the rear heating device is adapted to reheat the welded part of the steel sheets.
19. The friction stir welding apparatus for structural steel according to claim 8, further comprising a cooling device that is provided behind the rotary tool and that is adapted to cool a welded part of the steel sheets.
20. The friction stir welding apparatus for structural steel according to claim 19, wherein the rear heating device is adapted to reheat the welded part of the steel sheets.
Description
BRIEF DESCRIPTION OF DRAWINGS
(1)
(2)
(3)
(4)
DESCRIPTION OF EMBODIMENTS
(5) Exemplary embodiments of the present disclosure are described in detail below.
(6) The present disclosure is directed to a friction stir welding apparatus for structural steel. As illustrated in
(7) In
(8) The advancing side denotes the side on which the direction of rotation of the tool is the same as the welding direction. The retreating side denotes the side on which the direction of rotation of the tool is opposite to the welding direction.
(9) In friction stir welding with the friction stir welding apparatus of the present disclosure, the preheating process for heating the steel sheet with the heating device in front of the rotary tool moving in the welding direction is important. The conditions of the preheating process will be described below with reference to
(10) In
(11) [Surface Temperature T.sub.s of Steel Sheet in Heated Region: T.sub.s0.8T.sub.A1]
(12) As illustrated in
T.sub.A1( C.)=72310.7[% Mn]16.9[% Ni]+29.1[% Si]+16.9[% Cr]+290[% As]+6.38[% W](1)
where [% M] represents an amount (mass %) of element M in the steel sheet used as a workpiece, with [% M] being 0 when the steel sheet is free of element M.
(13) To ensure the heated region in the thickness direction, the surface of the heated region may have a temperature gradient. In this case, the surface temperature of the steel sheet in the heated region is preferably 1.5T.sub.M C. or lower. To avoid damage to the rotary tool and changes in microstructure due to an excessive increase in the temperature of the welded part, the surface temperature of the steel sheet in the heated region is preferably lower than T.sub.M C. before the heated region contacts the rotary tool passing through the heated region.
(14) The temperature T.sub.M ( C.) is a melting point of the steel sheet used as a workpiece.
(15) [Minimum Distance between Rotary Tool and Heated Region on Surface of Steel Sheet: Equal to or Less Than Diameter of Shoulder of Rotary Tool]
(16) If the distance between the rotary tool and the heated region on the surface of the steel sheet is too large, the temperature of the heated region before welding decreases, which hinders obtaining a sufficient preheating effect. Thus, the minimum distance X between the heated region on the surface of the steel sheet and the rotary tool moving in the welding direction is set to a distance equal to or less than the diameter of the shoulder of the rotary tool.
(17) If the distance between the heated region and the rotary tool is too small, the rotary tool may be damaged by the heat from the heating device. Thus, the minimum distance between the heated region on the surface of the steel sheet and the rotary tool moving in the welding direction is preferably set to 0.1 times or more of the diameter of the shoulder of the rotary tool.
(18) The diameter of the shoulder of the rotary tool is about 8 to 60 mm.
(19) [Area of Heated Region on Surface of Steel Sheet: Equal to or Less than Area of Maximum Diameter Part of Pin of Rotary Tool]
(20) If the heated region 12 is too large, the microstructures of the heated region and the surrounding region change. In particular, in the case of high-tensile steel sheets reinforced by the martensite structure, heating at the ferrite-austenite transformation temperature or lower still tempers martensite to cause softening and thus significantly reduces the weld joint strength. For this, the area of the heated region on the surface of the steel sheet is set to an area equal to or less than the area of the maximum diameter part of the pin of the rotary tool.
(21) The maximum radius of the pin of the rotary tool corresponds to b in the projection view of the shape of the tool illustrated in
(22) If the area of the heated region is too small, a sufficient preheating effect is not obtained. Thus, the area of the heated region on the surface of the steel sheet is preferably 0.1 times or more of the area of the maximum diameter part in the pin of the rotary tool.
(23) The maximum diameter of the pin of the rotary tool is about 2 to 50 mm.
(24) [Area of Heated Region Located Between Welding Centerline and AS Line on Surface of Steel Sheet: 50% or More of Area of Heated Region on Surface of Steel Sheet]
(25) In friction stir welding of steel material, plastic flow starts on the advancing side, moves in the direction of rotation of the rotary tool, passes through the front side in the welding direction, through the retreating side, and through the rear side in the welding direction, and ends on the advancing side. Since the plastic flow starts on the advancing side as described above, the steel sheet used as a workpiece on the advancing side tends to undergo insufficient heating. If defects are generated due to insufficient plastic flow, most of the defects are generated on the advancing side.
(26) When 50% or more of the area of the heated region 12 on the surface of the steel sheet is located between the welding centerline 10 and the AS line 11 parallel to the welding centerline 10, and the surface of the steel sheet on the advancing side is preferentially heated, plastic flow can be promoted to reduce generation of defects and increase the welding speed. The proportion of the area of the heated region 12 located between the welding centerline 10 and the AS line 11 is preferably 60% or more, and more preferably 80% or more. The proportion may be 100%.
(27) The center of the heated region is located between the AS line and the straight line passing through the midpoint between the welding centerline and the AS line in order to preferentially heat the steel sheet on the advancing side. In other words, the center of the heated region is preferably located on the advancing side of the welding centerline, and the distance from the center of the heated region to the welding centerline is preferably from 0.5 times to 1 time the maximum radius of the pin of the rotary tool.
(28) [Temperature T.sub.D of Region in Thickness Direction of Heated Region: T.sub.D0.8T.sub.A1]
(29) As described above, the steel sheets to be welded with the friction stir welding apparatus of the present disclosure normally have a strength of about 30% of room-temperature strength at a temperature of about 80% of T.sub.A1, which is the transformation temperature of steel. As the temperature is higher than about 80% of T.sub.A1, the steel sheets have lower strength. Preferably, a region of the steel sheet in the thickness direction of the heated region is also softened in advance by increasing the temperature to 0.8T.sub.A1 C. or higher, and the softened portion of the steel sheet is stirred to promote plastic flow, which further reduces the load on the rotary tool and further increases the welding speed. Therefore, the temperature T.sub.D of the heated region in the thickness direction, which defines the depth D of the heated region described below, is set to 0.8T.sub.A1 C. or higher. The temperature T.sub.A1 ( C.) can be obtained from Formula (1) below.
T.sub.A1( C.)=72310.7[% Mn]16.9[% Ni]+29.1[% Si]+16.9[% Cr]+290[% As]+6.38[% W](1)
where [% M] represents an amount (mass %) of element M in the steel sheet used as a workpiece.
(30) To ensure the heated region in the thickness direction, the heated region may have a temperature gradient in the thickness direction. In this case, the temperature of the steel sheet in the heated region in the thickness direction is preferably 1.5T.sub.M C. or lower. To avoid damage to the rotary tool and changes in microstructure due to an excessive increase in the temperature of the welded part, the temperature of the steel sheet in the heated region in the thickness direction is preferably lower than T.sub.M C. before the heated region contacts the rotary tool passing through the heated region.
(31) The temperature T.sub.M ( C.) is a melting point of the steel sheet used as a workpiece.
(32) [Depth D of Heated Region: 30% or More of Total Thickness of Steel Sheet]
(33) The depth D of the heated region is defined as the maximum depth of a region that has a temperature T.sub.D of 0.8T.sub.A1 C. or higher in the heated region in the thickness direction and that extends from the surface of the steel sheet used as a workpiece. In this case, the depth D of the heated region is preferably 30% or more of the total thickness of the steel sheet. This is because plastic flow is further promoted by setting the depth D of this heated region to 30% or more of the total thickness of the steel sheet, which is advantageous in reducing the load on the rotary tool and increasing the welding speed. More preferably, the depth D of the heated region is 50% or more of the total thickness of the steel sheet.
(34) However, if the depth D of the heated region exceeds 90% of the total thickness of the steel sheet, excessive heating may cause changes in microstructure. Thus, the depth D of the heated region is preferably 90% or less of the total thickness of the steel sheet.
(35) The heating device used in the preheating process is not limited to a particular device, but a laser heating device is preferably used.
(36) This is because using a laser with high energy density as a heat source enables accurate control of the conditions of the preheating process and improves welding workability without impairing weld joint properties.
(37) The conditions other than the above-described conditions are not limited and, for example, the moving speed of the heating device used in the preheating process is set to substantially the same as the welding speed. When a laser heating device is used as the heating device, the laser output and the beam diameter are set appropriately according to the welding conditions.
(38) The preheating process in the friction stir welding apparatus of the present disclosure is described above. In the friction stir welding apparatus of the present disclosure, a cooling device is provided behind the rotary tool moving in the welding direction, and the cooling device can improve the weld joint strength.
(39) This is because the welded part is normally cooled naturally after completion of welding, and thus sufficient weld joint strength is not obtained when steel material being workpiece material has low hardenability. Fortunately, the cooling device behind the rotary tool moving in the welding direction can cool the welded part of the steel sheet, and appropriate control of the cooling rate allows the strength to be improved by quench hardening. A specific cooling device is preferably a cooling device using injection of inert gas. The cooling rate in this case is preferably, for example, 30 to 300 C./s from 800 to 500 C.
(40) If steel material being workpiece material has high hardenability, there is a problem in that the welded part may be excessively hardened to reduce the toughness of the weld joint. Fortunately, excessive hardening can be suppressed by placing the rear heating device that heats the rear part close to the rotary tool and appropriately controlling the cooling rate to slowly cool the steel sheets. A specific heating device is preferably a heating device using high-frequency induction heating or using a laser as a heat source. The slow cooling rate in this case is preferably, for example, 10 to 30 C./s from 800 to 500 C.
(41) A rear heating device may be provided behind the rotary tool moving in the welding direction and after the cooling device. The rear heating device may reheat the welded part of the steel sheets.
(42) If the welded part is quench-hardened by cooling with the cooling device and excessively hardened, the hardness can be reduced by tempering the welded part with the rear heating device to obtain good joint properties, namely, both high strength and toughness. The cooling rate in this case is preferably, for example, 30 to 300 C./s from 800 to 500 C. The reheating temperature is preferably, for example, from 550 to 650 C.
(43) A cooling device may be provided behind the rotary tool moving in the welding direction and after the rear heating device. The cooling device may cool the welded part of the steel sheets.
(44) In this case, a composite microstructure can be formed by, just after welding, reducing the cooling rate with the rear heating device and then increasing the cooling rate with the cooling device, which provides good joint properties, namely, both high strength and ductility. For example, the cooling rate in this case is preferably about 10 to 30 C./s from 800 to 600 C. and then 30 to 300 C./s from 600 to 400 C.
(45) The welding conditions other than the above-described conditions are set according to an ordinary method. A larger torque of the rotary tool means lower plastic flowability of the steel sheets and higher tendency to generate defects or the like.
(46) In the present disclosure, attempts are made to regulate the torque of the rotary tool by controlling the rotation speed of the rotary tool in the range of 100 to 1000 rpm and to increase the welding speed to 1000 mm/min or higher.
(47) As the type of steel targeted in the present disclosure, general structural steel or carbon steel, such as rolled steel for weld structure of JIS G 3106 or carbon steel for machine structure of JIS G 4051, can be preferably used. The present disclosure can also be advantageously applied to high-strength structural steel having tensile strength of 800 MPa or higher. In this case, the strength of the welded part is equal to or higher than 85% of the tensile strength of the steel sheet (base material) or equal to or higher than 90% of the tensile strength of the steel sheet (base material).
EXAMPLES
Example 1
(48) Friction stir welding was performed using a steel sheet having a thickness of 1.6 mm and having the chemical composition and the tensile strength shown in Table 1. The joint abutting faces formed a non-angled square groove and were subjected to one-side one-pass welding while the surface conditions were similar to those after milling. The welding conditions for friction stir welding are shown in Table 2. A rotary tool made of tungsten carbide (WC) and having the cross-sectional dimensions shown in
(49) TABLE-US-00001 TABLE 1 Chemical Composition (mass %) Tensile Strength 0.8 No. C Si Mn P S (MPa) T.sub.A1 ( C.) 1 0.3 0.21 0.69 0.012 0.003 1015 577 2 0.16 0.07 0.69 0.016 0.009 420 574
(50) TABLE-US-00002 TABLE 2 Inclined Angle Rotation Speed (rpm) of () of Tool Tool Welding Speed (mm/min) 3 400 400
(51) To determine the region heated by preheating using a laser as a heat source before welding, steel sheet 1 shown in Table 1 was irradiated with a laser beam under different irradiation conditions (laser moving speed, laser output, and beam diameter) shown in Table 3, and the surface temperature was measured by thermography. Microstructure observation using a Nital etching solution was carried out by observing the cross section of the laser-irradiated part.
(52) The region having temperatures equal to or higher than the transformation point (T.sub.A1 C.) was etched in the darkest gray, and its surrounding region where the temperature was lower than the transformation point (T.sub.A1 C.) but the structure with high hardness such as martensite in the base material was tempered was etched relatively in light gray. The region having temperatures equal to or higher than the transformation point (T.sub.A1 C.), the region tempered at temperatures lower than the transformation point (T.sub.A1 C.), and the base-material region can be distinguished from each other. On the basis of the knowledge about steel heat treatment, it is known that the region tempered at temperatures lower than the transformation point (T.sub.A1 C.) corresponds to the region having temperatures of 0.8T.sub.A1 C. or higher and lower than T.sub.A1 C. The depth D.sub.0 of the region having temperatures equal to or higher than the transformation point (T.sub.A1 C.) and the depth of the region having temperatures equal to or higher than 0.8T.sub.A1 C. (the depth D of the heated region) were determined by such microstructure observation using a Nital etching solution.
(53) The measurement results are shown in Table 4.
(54) TABLE-US-00003 TABLE 3 Irradiation Moving Speed Laser Output Beam Diameter Conditions (mm/min) (kW) (mm) A 400 1.4 3.0 B 400 1.4 1.8 C 400 0.7 4.0
(55) TABLE-US-00004 TABLE 4 Shape of Diameter (mm) of Depth D (mm) Irradiation Heated Heated Region on D.sub.0 of Heated Conditions Region Surface of Steel Sheet (mm) Region A circle 3.5 0.28 0.30 B circle 2.0 0.47 0.50 C circle 4.5 0.09 0.10
(56) As shown in Table 4, the measurement results of the surface temperature by thermography indicate that, under the irradiation conditions A, the region having temperatures equal to or higher than 0.8T.sub.A1 C. had a circular shape with a diameter of 3.5 mm. Since the maximum diameter of the pin of the rotary tool used here is 4.0 mm, the area of the heated region on the surface of the steel sheet is equal to or less than the area of the maximum diameter part of the pin of the rotary tool.
(57) Under the irradiation conditions B, the region having temperatures equal to or higher than 0.8T.sub.A1 C. had a circular shape with a diameter of 2.0 mm. Therefore, the area of the heated region on the surface of the steel sheet is equal to or less than the area of the maximum diameter part of the pin of the rotary tool similarly as described above.
(58) Under the irradiation conditions C, the region having temperatures equal to or higher than 0.8T.sub.A1 C. had a circular shape with a diameter of 4.5 mm. Since the maximum diameter of the pin of the rotary tool used here is 4.0 mm, the area of the heated region on the surface of the steel sheet is larger than the area of the maximum diameter part of the pin of the rotary tool.
(59) As shown in Table 4, observation of the cross section of the laser-irradiated part indicates that, under the irradiation conditions A, the depth D.sub.0 of the region having temperatures equal to or higher than T.sub.A1 C. and the depth of the region having temperatures equal to or higher than 0.8T.sub.A1 C. (the depth D of the heated region) were 0.28 mm and 0.30 mm, respectively. Since the thickness t of the steel sheet used as a workpiece is 1.6 mm, the depth D of the heated region, which is the depth of the region having temperatures equal to or higher than 0.8T.sub.A1 C., is about 18.8% of the thickness t of the steel sheet.
(60) Under the irradiation conditions B, the depth D.sub.0 of the region having temperatures equal to or higher than T.sub.Al C. and the depth of the region having temperatures equal to or higher than 0.8T.sub.A1 C. (the depth D of the heated region) were 0.47 mm and 0.50 mm, respectively. Since the thickness t of the steel sheet used as a workpiece is 1.6 mm, the depth D of the heated region is about 31.3% of the thickness t of the steel sheet.
(61) Under the irradiation conditions C, the depth D.sub.0 of the region having temperatures equal to or higher than T.sub.Al C. and the depth of the region having temperatures equal to or higher than 0.8T.sub.A1 C. (the depth D of the heated region) were 0.09 mm and 0.10 mm, respectively. Since the thickness t of the steel sheet used as a workpiece is 1.6 mm, the depth D of the heated region is about 6.3% of the thickness t of the steel sheet.
(62) Table 5 shows the conditions of the preheating process by laser irradiation before welding of the workpieces and the conditions of the process performed after welding. In the process performed after welding, cooling was performed by gas injection and heating (and reheating) was performed by induction heating.
(63) The sign - in the conditions of the preheating process and the conditions of the process after welding in Table 5 indicates that the preheating process or the process after welding such as cooling and/or heating was not carried out. The characters (AS) or (RS) regarding the distance from the welding centerline to the center of the heated region indicate whether the center of the heated region is located on the advancing side of the welding centerline or on the retreating side of the welding centerline.
(64) TABLE-US-00005 TABLE 5 Conditions of Preheating Process Minimum Distance (mm) Proportion (%) of Conditions of Process after Welding Distance X (mm) from Welding Area of Heated Cooling Reheating Sample Laser between Heated Centerline to Region Located between (Slow Cool- Temper- Steel Irradiation Region and Center of Welding Centerline Additional ing) Rate ature Sheet Conditions Rotary Tool Heated Region and AS Line Process ( C./s) ( C.) Example 1 1 A 1 1.0 (AS) 50 or more 70 Example 2 1 B 1 0.5 (AS) 50 or more 80 Example 3 1 B 1 1.0 (AS) 50 or more 100 Example 4 1 B 1 1.8 (AS) 50 or more 63 Example 5 1 B 3 1.8 (AS) 50 or more 63 Example 6 1 B 1 1.8 (AS) 50 or more 63 cooling .fwdarw. 250*.sup.1 600 reheating Example 7 1 B 3 1.8 (AS) 50 or more 63 cooling .fwdarw. 250*.sup.1 600 reheating Example 8 2 B 3 1.8 (AS) 50 or more 63 cooling only 100*.sup.1 Example 9 1 B 3 1.8 (AS) 50 or more 63 heating only 10*.sup.1 Example 10 1 B 3 1.8 (AS) 50 or more 63 heating .fwdarw. 10*.sup.2 .fwdarw. 100*.sup.3 cooling Comparative 1 Example 1 Comparative 1 B 13 1.8 (AS) 50 or more 63 Example 2 Comparative 1 B 1 2.3 (AS) less than 50 31 Example 3 Comparative 1 B 1 1.8 (AS) less than 50 0 Example 4 Comparative 1 C 1 2.3 (AS) less than 50 42 Example 5 *.sup.1cooling rate from 800 C. to 500 C. *.sup.2cooling rate from 800 C. to 600 C. *.sup.3cooling rate from 600 C. to 400 C.
(65) Table 6 shows the measurements of the torque of the rotary tool during welding and the tensile strength obtained by performing tensile testing on tensile test specimens that were taken from the obtained weld joint with the size of No. 1 specimen defined in JIS Z 3121.
(66) A larger torque of the rotary tool means lower plastic flowability and higher tendency to generate defects or the like.
(67) TABLE-US-00006 TABLE 6 Torque (N .Math. m) of Tensile Strength Rotary Tool (MPa) Example 1 75 918 Example 2 68 920 Example 3 60 916 Example 4 53 931 Example 5 72 911 Example 6 52 1010 Example 7 71 1012 Example 8 70 418 Example 9 72 950 Example 10 71 975 Comparative Example 1 90 919 Comparative Example 2 88 914 Comparative Example 3 82 905 Comparative Example 4 87 918 Comparative Example 5 85 921
(68) Table 6 shows that, in Examples 1 to 10, a high welding speed still resulted in a strength corresponding to 85% or higher of the tensile strength of the steel sheet used as a base material as well as a torque of the rotary tool of 75 N.Math.m or less, which indicates good plastic flowability. In particular, Examples 6 and 7 involving cooling and reheating after welding exhibited a strength corresponding to 99% or higher of the tensile strength of the base material.
(69) In Comparative Examples 1 to 5, the torque of the rotary tool was 80 N.Math.m or higher, which indicates poor plastic flowability.
Example 2
(70) As in Example 1, friction stir welding was performed using a steel sheet having a thickness of 1.6 mm and having the chemical composition and the tensile strength shown in Table 1. The joint abutting faces formed a non-angled square groove and were subjected to one-side one-pass welding while the surface conditions were similar to those after milling. The welding conditions for friction stir welding are shown in Table 7. As in Example 1, a rotary tool made of tungsten carbide (WC) and having the cross-sectional dimensions shown in
(71) TABLE-US-00007 TABLE 7 Inclined Angle Rotation Speed (rpm) of () of Tool Tool Welding Speed (mm/min) 3 600 1000
(72) To determine the region heated by preheating using a laser as a heat source before welding, steel sheet 1 shown in Table 1 was irradiated with a laser beam under different irradiation conditions (laser moving speed, laser output, and beam diameter) shown in Table 8, and the surface temperature was measured by thermography. Moreover, microstructure observation using a Nital etching solution was carried out by observing the cross section of the laser-irradiated part as in Example 1 to determine the depth D.sub.0 of the region having temperatures equal to or higher than the transformation point (T.sub.A1 C.) and the depth of the region having temperatures equal to or higher than 0.8T.sub.A1 C. (the depth D of the heated region).
(73) The measurement results are shown in Table 9.
(74) TABLE-US-00008 TABLE 8 Irradiation Moving Speed Laser Output Beam Diameter Conditions (mm/min) (kW) (mm) D 1000 3.5 3.0 E 1000 3.5 1.8 F 1000 1.8 4.0
(75) TABLE-US-00009 TABLE 9 Major Minor Axis (mm) Axis (mm) of Heated of Heated Depth D Shape of Region on Region on (mm) of Irradiation Heated Surface of Surface of D.sub.0 Heated Conditions Region Steel Sheet Steel Sheet (mm) Region D ellipse 3.8 3.2 0.30 0.32 E ellipse 2.2 1.8 0.51 0.54 F ellipse 4.9 4.1 0.10 0.11
(76) As shown in Table 9, the measurement results of the surface temperature by thermography indicate that, under the irradiation conditions D, the region having temperatures equal to or higher than 0.8T.sub.A1 C. had an elliptical shape with a major axis in the laser moving direction and a minor axis in the direction perpendicular to the laser moving direction, specifically, with a major axis of 3.8 mm and a minor axis of 3.2 mm. Since the maximum diameter of the pin of the rotary tool used here is 4.0 mm, the area of the heated region on the surface of the steel sheet is equal to or less than the area of the maximum diameter part of the pin of the rotary tool.
(77) Under the irradiation conditions E, the region having temperatures equal to or higher than 0.8T.sub.A1 C. had an elliptical shape with a major axis in the laser moving direction and a minor axis in the direction perpendicular to the laser moving direction, specifically, with a major axis of 2.2 mm and a minor axis of 1.8 mm. Therefore, the area of the heated region on the surface of the steel sheet is equal to or less than the area of the maximum diameter part of the pin of the rotary tool similarly as described above.
(78) Under the irradiation conditions F, the region having temperatures equal to or higher than 0.8T.sub.A1 C. had an elliptical shape with a major axis in the laser moving direction and a minor axis in the direction perpendicular to the laser moving direction, specifically, with a major axis of 4.9 mm and a minor axis of 4.1 mm. Since the maximum diameter of the pin of the rotary tool used here is 4.0 mm, the area of the heated region on the surface of the steel sheet is larger than the area of the maximum diameter part of the pin of the rotary tool.
(79) As shown in Table 9, observation of the cross section of the laser-irradiated part indicates that, under the irradiation conditions D, the depth D.sub.0 of the region having temperatures equal to or higher than T.sub.A1 C. and the depth of the region having temperatures equal to or higher than 0.8T.sub.A1 C. (the depth D of the heated region) were 0.30 mm and 0.32 mm, respectively. Since the thickness t of the steel sheet used as a workpiece is 1.6 mm, the depth D of the heated region, which is the depth of the region having temperatures equal to or higher than 0.8T.sub.A1 C., is about 20.0% of the thickness t of the steel sheet.
(80) Under the irradiation conditions E, the depth D.sub.0 of the region having temperatures equal to or higher than T.sub.Al C. and the depth of the region having temperatures equal to or higher than 0.8T.sub.A1 C. (the depth D of the heated region) were 0.51 mm and 0.54 mm, respectively. Since the thickness t of the steel sheet used as a workpiece is 1.6 mm, the depth D of the heated region is about 33.8% of the thickness t of the steel sheet.
(81) Under the irradiation conditions F, the depth D.sub.0 of the region having temperatures equal to or higher than T.sub.Al C. and the depth of the region having temperatures equal to or higher than 0.8T.sub.A1 C. (the depth D of the heated region) were 0.10 mm and 0.11 mm, respectively. Since the thickness t of the steel sheet used as a workpiece is 1.6 mm, the depth D of the heated region is about 6.9% of the thickness t of the steel sheet.
(82) Table 10 shows that the conditions of the preheating process by laser irradiation before welding of the workpieces and the conditions of the process performed after welding. In the process performed after welding, cooling was performed by gas injection and heating (and reheating) was performed by induction heating.
(83) The sign - in the conditions of the preheating process and the conditions of the process after welding in Table 10 indicates that the preheating process or the process after welding such as cooling and/or heating was not carried out. The characters (AS) or (RS) regarding the distance from the welding centerline to the center of the heated region indicate whether the center of the heated region is located on the advancing side of the welding centerline or on the retreating side of the welding centerline.
(84) TABLE-US-00010 TABLE 10 Conditions of Preheating Process Minimum Distance (mm) Proportion (%) of Conditions of Process after Welding Distance X (mm) from Welding Area of Heated Cooling Reheating Sample Laser between Heated Centerline to Region Located between (Slow Cool- Temper- Steel Irradiation Region and Center of Welding Centerline Additional ing) Rate ature Sheet Conditions Rotary Tool Heated Region and AS Line Process ( C./s) ( C.) Example 11 1 D 1 1.0 (AS) 50 or more 75 Example 12 1 E 1 0.5 (AS) 50 or more 85 Example 13 1 E 1 1.0 (AS) 50 or more 100 Example 14 1 E 1 1.8 (AS) 50 or more 65 Example 15 1 E 3 1.8 (AS) 50 or more 65 Example 16 1 E 1 1.8 (AS) 50 or more 65 cooling .fwdarw. 300*.sup.1 600 reheating Example 17 1 E 3 1.8 (AS) 50 or more 65 cooling .fwdarw. 300*.sup.1 600 reheating Example 18 2 E 3 1.8 (AS) 50 or more 65 cooling only 150*.sup.1 Example 19 1 E 3 1.8 (AS) 50 or more 65 heating only 30*.sup.1 Example 20 1 E 3 1.8 (AS) 50 or more 65 heating .fwdarw. 30*.sup.2 .fwdarw. 150*.sup.3 cooling Comparative 1 Example 6 Comparative 1 E 13 1.8 (AS) 50 or more 65 Example 7 Comparative 1 E 1 2.3 (AS) less than 50 28 Example 8 Comparative 1 E 1 1.8 (AS) less than 50 0 Example 9 Comparative 1 F 1 2.3 (AS) less than 50 41 Example 10 *.sup.1cooling rate from 800 C. to 500 C. *.sup.2cooling rate from 800 C. to 600 C. *.sup.3cooling rate from 600 C. to 400 C.
(85) Table 11 shows the measurements of the torque of the rotary tool during welding and the tensile strength obtained by performing tensile testing on tensile test specimens that were taken from the obtained weld joint with the size of No. 1 specimen defined in JIS Z 3121.
(86) A larger torque of the rotary tool means lower plastic flowability and higher tendency to generate defects or the like.
(87) TABLE-US-00011 TABLE 11 Torque (N .Math. m) of Tensile Strength Rotary Tool (MPa) Example 11 95 940 Example 12 94 952 Example 13 87 934 Example 14 85 946 Example 15 95 933 Example 16 87 1015 Example 17 96 1014 Example 18 91 418 Example 19 95 971 Example 20 95 990 Comparative Example 6 Comparative Example 7 Comparative Example 8 Comparative Example 9 Comparative Example 10
(88) Table 11 shows that, in Examples 11 to 20, a high welding speed of 1000 mm/min still allowed welding at a torque of the rotary tool of 100 N.Math.m or lower and resulted in a strength corresponding to 85% or higher of the tensile strength of the steel sheet used as a base material, which provided a satisfactory joint. In particular, Examples 16 and 17 involving cooling and reheating after welding exhibited a strength of 99% or higher of the tensile strength of the base material.
(89) In Comparative Example 6, welding was failed because the rotary tool was damaged during welding. In Comparative Examples 7 to 10, welding was failed because the non-welded part remained, so that a satisfactory joint was not obtained. Therefore, the torque of the rotary tool and the like were not measured in Comparative Examples 6 to 10.
REFERENCE SIGNS LIST
(90) 1 Rotary Tool
(91) 2 Rotation Axis
(92) 3 Steel Sheet
(93) 4 Welded Part
(94) 5 Heating Device
(95) 6 Cooling Device
(96) 7 Rear Heating Device
(97) 8 Shoulder of Rotary Tool
(98) 9 Pin of Rotary Tool
(99) 10 Welding Centerline
(100) 11 AS Line
(101) 12 Heated Region
(102) 13 Cooled Region
(103) 14 Reheated Region
(104) a Diameter of Shoulder of Rotary Tool
(105) b Maximum Diameter of Pin of Rotary Tool
(106) c Probe Length of Rotary Tool
(107) X Minimum Distance between Heated Region and Rotary Tool
(108) D Depth of Heated Region
(109) t Thickness of Steel Sheet
(110) Inclined Angle of Rotary Tool