Friction pressure welding method

Abstract

The present invention provides a friction welding method capable of reducing the welding temperature and a friction welding method capable of obtaining a welded portion free of defects regardless the type of material. A frictional welding method in which one member is brought into contact with the other member and slides while a load is applied substantially perpendicularly to the interface to be welded, the frictional welding method comprising: a first step in which frictional welding is carried out by setting a pressure calculated from the area and the load of the interface to be welded to be equal to or higher than the yield stress and the tensile strength of one member and/or the other member at a desired welding temperature; and a second step in which frictional welding is carried out by lowering the load, wherein the first step and the second step are continuously carried out.

Claims

1. A friction welding method, in which one member is brought into contact with other member and slides in a state that a load is applied perpendicularly to an interface to be welded, comprising: a first step of carrying out friction welding by setting a pressure (P.sub.1) calculated from an area of the interface to be welded and the load to be equal to or higher than a yield stress of the one member and/or the other member and equal to or lower than a tensile strength of the one member and/or the other member at a desired welding temperature, and a second step of carrying out friction welding by lowering the load, wherein the first step and the second step are carried out continuously.

2. The friction welding method in accordance with claim 1, wherein, in the second step, a true pressure (P.sub.2) is calculated by subtracting a softening area of the interface to be welded caused by an increase in temperature from the area, and the load is reduced so that the pressure (P.sub.1) and the true pressure (P.sub.2) are the same value.

3. The friction welding method in accordance with claim 2, wherein a softening region is set to 10 to 50% of the area.

4. The friction welding method in accordance with claim 1, wherein the pressure (P.sub.1) is set to the yield stress of the one member and/or the other member at the desired welding temperature.

5. The friction welding method in accordance with claim 1, wherein one member and/or the other member is made of an iron-based metal.

6. The friction welding method in accordance with claim 1, wherein the welding temperatures is set to be below the A.sub.1 temperature of the ferrous metals.

7. The friction welding method in accordance with claim 5, wherein the iron-based metal is a high-speed tool steel.

8. A welded structure having a welded portion of two metal materials, wherein at least one of the metal materials is a high-speed tool steel, prior austenite crystal grains of the high-speed tool steel at the welding interface of the welded portion are equiaxed grains, regions having crystal grain boundaries caused by the prior austenite crystal grains are distributed at constant intervals around the welding interface, a hardness within 5 mm of the welding interface is less than 500 HV, and all regions of the welding interface are metallurgically welded, wherein the prior austenite crystal grains are equiaxed grains, the austenite grains are recrystallized during the welding process.

9. A welded structure having a welded portion of two metal materials, wherein at least one of the metal materials is a high-speed tool steel, the welding interface of the welded portion mainly consists of recrystallized grains, regions having the recrystallized grains are distributed at constant intervals around the welding interface, a hardness within 5 mm of the welding interface is less than 500 HV, and all regions of the welding interface are metallurgically welded.

10. The welded structure in accordance with claim 8, wherein the high-speed tool steels are JIS-SKH51.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic view showing the welding process of the friction welding method of the present invention.

(2) FIG. 2 is a graph showing the deformation stress (yield stress) of carbon steel at each temperature.

(3) FIG. 3 is a graph showing the tensile strength of various metals at various temperatures.

(4) FIG. 4 is a schematic view showing a welding portion in the welded structure of the present invention.

(5) FIG. 5 is a photograph of the structure of the material to be welded.

(6) FIG. 6 is a graph showing changes in welding temperature during friction welding in Example 1.

(7) FIG. 7 is a macrophotograph of a longitudinal section including a welding center of the joint obtained in Example 1.

(8) FIG. 8 is an organizational photograph of the center of the welding interface in FIG. 7.

(9) FIG. 9 is a photograph of the structure of the side surface of the welding interface in FIG. 7.

(10) FIG. 10 is a hardness distribution of the joint portion obtained in Example 1.

(11) FIG. 11 is a graph showing the tensile strength of the joints obtained in Example 1 and Comparative Example 2.

(12) FIG. 12 is a graph showing changes in welding pressure and welding temperature during friction welding in Example 2.

(13) FIG. 13 is a graph showing a change in welding temperature during friction welding in Comparative Example 1.

(14) FIG. 14 is a macrophotograph of the joint obtained in Comparative Example 1 in a longitudinal section including the center of the joint.

(15) FIG. 15 is a fracture surface of the joint obtained in Comparative Example 1.

(16) FIG. 16 is a graph showing the change in welding temperature during friction welding in Comparative Example 2.

(17) FIG. 17 is a macrophotograph of the joint obtained in Comparative Example 2 in a longitudinal section including the center of the joint.

(18) FIG. 18 is an organizational photograph of the center of the welding interface in FIG. 17.

(19) FIG. 19 is a photograph of the structure of the side surface of the welding interface in FIG. 17.

DETAILED DESCRIPTION OF THE INVENTION

(20) Hereinafter, the friction welding method of the present invention and a typical embodiment of a joint structure obtained thereby will be described in detail with reference to the drawings, but the present invention is not limited thereto. In the following description, the same or corresponding components are denoted by the same reference numerals, and a repetitive description may be omitted. In addition, since the drawings are for conceptually explaining the present invention, the dimensions and ratios of the components shown in the drawings may differ from actual ones.

(21) FIG. 1 is a schematic view showing a welding process of the friction welding according to the present invention. The friction welding method of the present invention is a friction welding method in which one member 2 is brought into contact with the other member 4 and rotationally slid while a load is applied substantially perpendicularly to the interface 6 to be welded, wherein the friction welding method of the present invention includes a first step in which the friction welding is carried out by setting the area of the interface 6 to be welded and the pressure (P.sub.1) calculated from the load to be equal to or higher than the yield stress and the tensile strength of the member 2 and/or the other member 4 at a desired welding temperature, and a second step in which the load is lowered to perform the friction welding continuously. Hereinafter, each step will be described in detail.

(22) (1-1) First Step

(23) The first step is a step in which one member 2 is brought into contact with the other member 4 and is rotationally slid while a load is applied substantially perpendicularly to the interface 6 to be welded, and in this step, friction welding is carried out by setting the area of the interface 6 to be welded and the pressure (P.sub.1) calculated from the load to be equal to or higher than the yield stress and lower than the tensile strength of the one member 2 and/or the other member 4 at desired welding temperatures.

(24) Here, only one of the member 2 and the other member 4 may be rotated, or both may be rotated. Alternatively, one member 2 may be rotated before being brought into contact with the other member 4, or may be rotated after forming the interface 6 to be welded.

(25) The material of the one member 2 and the other member 4 is not particularly limited as long as the effect of the present invention is not impaired, and the material may have a metallic phase which can be welded by friction welding, but it is preferable that the material is an iron-based metal, titanium, or a titanium alloy, and it is more preferable that the material is a high-speed tool steel. Since the iron-based metal, titanium, or a titanium alloy has mechanical properties that can withstand the welding process of friction welding, deformation or the like at an unnecessary place during the welding process can be prevented by using these metals as the material to be welded. In addition, the friction welding is a solid-phase welding, and it is possible to suppress the deterioration of the mechanical properties of the welding portion, which is remarkably observed in general fusion welding. Further, even in the case of high-speed steel or a titanium alloy having a large plastic deformation resistance and a low thermal conductivity, by uniformizing the temperature distribution of the interface to be welded in the second step, it is possible to form a good welded portion in which no unwelded portion exists.

(26) The shape and size of the one member 2 and the other member 4 are not particularly limited as long as the effect of the present invention is not impaired, and the shape and size of the member to be welded can be set to the shape and size of the member to be welded by friction welding known in the art.

(27) Under a situation in which pressures (P.sub.1) are applied almost vertically to the welding surface 6, one member 2 and the other member 4, by rolling and moving on the same trajectory, burrs 8 are discharged from the welding surface 6. Here, in the friction welding, the welding temperature can be controlled by setting the pressure (P.sub.1) at the time of the friction welding to be equal to or higher than the yield stress of one member 2 and/or the other member 4 and equal to or lower than the tensile strength at a desired welding temperature. Here, the discharge of the flash 8 from the welded interface 6 is started by setting the pressure (P.sub.1) to be equal to or higher than the yield stress of the welded material, and the discharge of the burr 8 is accelerated by increasing the pressure (P.sub.1) up to the tensile strength. Like the yield stress, since the tensile strength at a (P.sub.1) temperature is also substantially constant depending on the material to be welded, the welding temperature corresponding to the set pressure P can be realized.

(28) As a specific example, the deformation stress (yield stress) of carbon steel at each temperature is shown in FIG. 2, and the tensile strength of various metals at each temperature is shown in FIG. 3. FIG. 2 is a graph published in “Iron and Steel, No. 11, 67 (1981), p. 140”, and FIG. 3 is a graph published in “Iron and Steel, No. 6, 72 (1986), p. 55”. As shown in these Fig.s, the tensile strength and yield stress at a particular temperature are approximately constant for different materials.

(29) That is, when the (P.sub.1) P at the time of welding is set high, the material to be welded having higher yield strength and tensile strength can be discharged as a flash, and the welding temperature can be lowered. Also, as shown in FIGS. 2 and 3, since the tensile strength and the yield stress at a specific temperature are substantially constant depending on the material, the welding temperature can be controlled very accurately.

(30) In order to control the welding (P.sub.1) more accurately, it is preferable to set the pressure P to the yield stress of one member and/or the other member at the desired welding temperature. In the frictional welding, the discharge of the flash 8 is started at the moment when the pressure (P.sub.1) reaches the yield stress, and the welding temperature can be more accurately defined as compared with the case where the pressure (P.sub.1) is set to a higher value (with the tensile strength as an upper limit).

(31) In other words, the (P.sub.1) rise caused by the frictional heat lowers the yield stress of the welded material, and the discharge of the flash is started at the instant when the yield stress becomes lower than the pressure P. Here, the temperature increasing speed is increased by increasing the rotation speed at which the material to be welded slides, but the maximum reaching temperature (welding temperature) is not changed.

(32) In friction welding, welding parameters other than the pressure (P.sub.1) (rotational speeds, welding times, allowances, and the like of the members to be welded) need to be set, but these values are not limited as long as the effects of the present invention are not impaired and may be appropriately set depending on the materials, shapes, sizes, and the like of the members to be welded.

(33) When the one member 2 and/or the other member 4 is made of an iron-based metal, it is preferable to set the welding temperature to a temperature equal to or lower than the A.sub.1 temperature of the iron-based metal used as the material to be welded. In iron-based metals, brittle martensite is formed by phase transformation, and there are cases in which welding is difficult and in which a welding portion is embrittled. On the other hand, by setting the welding temperature to the A.sub.1 temperature or lower by the friction welding method of the present invention, phase transformation does not occur, and therefore, the friable martensite can be completely suppressed from being formed. The A.sub.1 point (° C.) of the iron-based material can be known, for example, from “A1=750 8-26.6 C+17.6Si-11.6Mn-22.9Cu-23Ni+24.1Cr+22.5Mo-39 7V-5.7Ti+232.4Nb-169.4Al-894. 7B” (C, Si, etc. are substituted by weight %).

(34) When one member 2 and/or the other member 4 is made of titanium or a titanium alloy, it is preferable that the welding temperature be equal to or lower than the β transus temperature of titanium or a titanium alloy. By setting the welding temperature to be equal to or lower than the β transus temperature of titanium or a titanium alloy, the structure of the welded portion can be made fine equiaxed grains, and a welded portion having both high strength and toughness can be formed.

(35) (1-2) Second Step

(36) The second step is a step that is continuous with the first step, and the welding temperature in the entire area of the interface 6 to be welded, particularly in the center portion of the interface 6 to be welded, can be made uniform in the second step, and the formation of an unwelded portion can be effectively suppressed. In the friction welding, the peripheral speed increases on the outer peripheral side of the member to be welded, and the amount of friction heat generation increases as compared with the center portion. As a result, softening progresses in the vicinity of the outer periphery of the interface to be welded, and it becomes difficult to support the applied load. That is, the area of the interface 6 to be welded supporting the load is reduced, and the pressure actually applied to the center portion of the interface 6 to be welded is higher than the set value (P.sub.1). As described above, since the increase of the applied pressure lowers the welding temperature, in particular, when the desired welding temperature is low, when the deformation resistance of the material to be welded (2, 4) is large, and when the heat conduction at the interface to be welded 6 does not proceed quickly, an unwelded portion is formed due to the welding temperature of the center portion of the welding interface 6 being too low.

(37) On the other hand, by reducing the load applied to the interface 6 to be welded in the second step, the welding temperature can be increased. By raising the temperature of the central portion of the interface 6 to be welded by the second step to such an extent that welding is possible, it is possible to obtain a good friction-welded joint in which no unwelded portion exists. In general frictional welding, the applied pressure is increased in the final step of welding, but in the friction welding method of the present invention, it is possible to realize uniformity of the welding temperature at the interface to be welded by passing through completely different (reversed) steps.

(38) In the friction welding process, in the second step, it is preferable to calculate the true pressure (P.sub.2) by subtracting the softened region of the interface 6 to be welded due to an increase in temperature from the area, and reduce the load so that the pressure (P.sub.1) and the true pressure (P.sub.2) become substantially the same value.

(39) The softened region means a region in which plastic deformation is caused by the pressure (P.sub.1) in the temperature distribution of the interface 6 to be welded at the time of shifting from the first step to the second step. The softened region can be determined by observing the state of the interface 6 to be welded after the first step, but when the determination of the softened region by observing the interface 6 to be welded is omitted, the softened region is preferably 10 to 50% of the area of the interface 6 to be welded, more preferably 15 to 20%. By calculating the true pressure (P.sub.2) by setting the softened region to 10 to 50% of the area of the interface 6 to be welded, it is possible to effectively suppress the formation of an unwelded portion in the center portion of the interface 6 to be welded.

(40) The timing of shifting from the first step to the second step may be appropriately determined according to the material, shape, size, and the like of the material to be welded, but the defect suppressing effect can be sufficiently exhibited only by executing the second step for about several seconds.

(41) (B) Welded Structure

(42) FIG. 4 is a schematic view showing a welding portion in the welded structure of the present invention. The welded portion 10 is formed by welding the material to be welded 2 and the material to be welded 4, and the material to be welded 2 and/or the material to be welded 4 is high-speed tool steel. The welded structure of the present invention can be suitably produced by the friction welding method of the present invention, and FIG. 4 shows a welded portion welded by the friction welding method of the present invention.

(43) The welded portion 10 is not formed with a significant heat affected zone HAZ, and is an extremely reliable welded structure having a high joint efficiency. The welding interface 12 is mainly formed of recrystallized grains, and the structure in the vicinity of the welding interface 12 becomes microcrystallized grains of fine equiaxes, so that the welding portion 10 has high mechanical properties such as strength, toughness, reliability, and the like.

(44) Here, the recrystallization grains are formed by a decrease in recrystallization temperature due to plastic deformation of the material to be welded 2 and/or the material to be welded 4, and are one of the major features of the friction welding method of the present invention. On the other hand, in the conventional frictional welding method, the welding temperature is increased, so that a transformation structure including martensite is formed in the vicinity of the welding interface 12 of the tool steel.

(45) Further, in the welded structure of the present invention, the formation of martensite is suppressed, so that the hardness in the vicinity of the welding interface 12 is less than 500 HV. In addition, an unwelded portion does not exist at the welding interface 12, and an extremely good welded portion 10 is formed.

(46) Although the friction welding method of the present invention and the representative embodiment of the welded structure obtained thereby have been described above, the present invention is not limited to these methods, and various design modifications are possible, and all of these design modifications are included in the technical scope of the present invention.

EXAMPLE

Example 1

(47) High-speed tool steels with diameters of 10 mm and lengths of 90 mm: JIS-SKH51 (0.89% C-0.27% Si-0. 28% Mn-0. 020% P-0.001% S-3.90% Cr-6. 10% W-5.05% Mo-1. 84% V-Bal.Fe) round rods were used as the materials to be welded, and friction welding of the round rods was carried out using a friction welding machine manufactured by Nitto Seiki Co., Ltd. As shown in FIG. 5, the structure of the material to be welded is a tempered martensite and a spherical carbide.

(48) The friction welding conditions were as follows: the first step was carried out at a rotational speed of 100 rpm, a welding pressure of 360 MPa, and a side margin of 2 mm; and the second step was carried out at a rotational speed of 100 rpm, a welding pressure of 300 MPa, and a side margin of 1 mm. The friction welding was carried out by frictional length control, and the second step was carried out continuously at the time point when the margin reached 2 mm from the first step.

(49) The change in welding temperature during friction welding is shown in FIG. 6. Thermal imaging cameras (CPA-T640, manufactured by CINO) were used to measure the temperature of the sides of the welding interface to be welded. As shown in FIG. 6, the maximum attained temperature during friction welding is 790° C., which indicates that the maximum attained temperature is less than or equal to the A.sub.1 temperature of the material to be welded.

(50) FIG. 7 shows a macrophotograph of a longitudinal section including the center of the welded portion of the obtained joint. It can be confirmed that an unwelded portion is not formed at the center of the welded portion, and a good welded portion without defects is obtained. Although a region having white contrast exists at the outer periphery of the welding interface, the region is metallurgically welded and is not a defect.

(51) Photographs of the structure of the center and the side surface of the welding interface in FIG. 7 are shown in FIGS. 8 and 9, respectively. Both of these structures are composed of microstructurally fine ferrites and spherical carbides, and it is understood that the welding temperatures are suppressed below the A.sub.1 temperature in the whole area of the interface to be welded.

(52) The hardness distribution of the welded portion is shown in FIG. 10. The hardness measurement was carried out in a direction perpendicular to the center of the welded portion with respect to the cross section of the joint shown in FIG. 7. The hardness in the vicinity of the welding interface is slightly increased by the refinement of the structure, but remains at about 350 HV because the formation of martensite is suppressed.

(53) The tensile strength of the obtained joint is shown in FIG. 11. Since a good welded portion without defects is formed, the joint has a tensile strength substantially equal to that of the base material, and the joint efficiency is about 100%.

Example 2

(54) Friction welding was carried out in the same manner as in Example 1 except that the welding pressure in the second step was 180 MPa. FIG. 12 shows changes in the welding pressure and the welding temperature at the time of the friction welding. It can be seen that the welding temperature rises along with the transition from the first step (welding pressure 360 MPa) to the second step (welding pressure 180 MPa), and the welding temperature changes depending on the welding pressure (the welding temperature rises when the welding pressure is lowered).

Comparative Example 1

(55) Friction welding was carried out in the same manner as in Example 1 except that the margin of the first step was 3 mm and the second step was not carried out. The change in welding temperature during friction welding is shown in FIG. 13. It is understood that the maximum reaching temperature is 690° C., and the welding temperature is lower than or equal to the A.sub.1 temperature of the material to be welded.

(56) FIG. 14 shows a macrophotograph of a longitudinal section including the center of the welded portion of the obtained joint. An enlarged photograph of the center of the welded portion is also shown, but it can be confirmed that an unwelded portion exists at the center of the welded portion. It can be confirmed that an unwelded portion exists at the center of the welded portion even in the fractured surface of the welded portion shown in FIG. 15.

Comparative Example 2

(57) Friction welding was carried out in the same manner as in Example 1 except that the first step was carried out at a rotational speed of 200 rpm, a welding pressure of 240 MPa, and a margin of 2 mm, and the second step was not carried out. The change in welding temperature during friction welding is shown in FIG. 16. It is understood that the maximum reaching temperature is 1083° C., and the welding temperature is equal to or higher than the A, temperature of the material to be welded. In addition, from the result, it can be confirmed that the welding temperature is increased by decreasing the welding pressure.

(58) FIG. 17 shows a macrophotograph of a longitudinal section including the center of the welded portion of the obtained joint. Although the welding temperature is high and the formation of an unwelded portion is not observed even at the center of the welding portion, cracks are generated in the outer peripheral portion. The cracks are due to embrittlement due to the formation of martensite.

(59) Photographs of the structure of the center and the side surface of the welding interface in FIG. 17 are shown in FIGS. 18 and 19, respectively. Both of these structures are composed of martensite and spherical carbides, and it is understood that the welding temperatures are higher than the A.sub.1 temperature in the whole area of the interface to be welded.

(60) The hardness distribution of the welded portion is shown in FIG. 10. The hardness measurement was carried out in a direction perpendicular to the center of the welded portion with respect to the cross section of the joint shown in FIG. 17. The formation of martensite indicates that the hardness in the vicinity of the welding interface reaches 850 HV.

(61) The tensile strength of the obtained joint is shown in FIG. 11. The tensile strength is extremely low due to the embrittlement caused by martensite formation and is less than 100 MPa.

EXPLANATION OF NUMERALS

(62) 2,4 . . . materials to be welded, 6 . . . welding interface 8 . . . flash. 10 . . . welded portion, 12 . . . welding interface.