Friction stir welding method

Abstract

The invention relates to a friction stir welding method which comprises a stirring head, the stirring head comprises a shaft shoulder and a stirring needle, the stirring needle is installed at the lower end of the shaft shoulder, according to the friction stir welding method, the stirring head with the shaft shoulder diameter being 5.5 mm and the stirring needle length being 1.4 mm is designed through the method, welding is carried out at the rotating speed being 1500 rpm and the welding speed being 200 mm / s, and the welding speed is 200 mm / s. The welding quality, the production efficiency and the material utilization rate can be improved.

Description

Technical Field

[0001] The invention relates to the technical field of friction welding, in particular to a friction stir welding method. Background Art

[0002] The middle frame of a mobile phone is a shell-like thing that protects the mobile phone. It is just a frame-like thing. The middle frame of a mobile phone needs to be processed using a CNC, or computer numerical control, machine tool. However, the existing CNC processing method is time-consuming, inefficient, and difficult to process fine structures. To address this problem, a CNC process was developed that combines the die-casting process with a stir friction welding process.

[0003] Friction stir welding refers to the use of heat generated by the friction between the high-speed rotating welding tool and the workpiece to partially melt the welded material. When the welding tool moves forward along the welding interface, the plasticized material flows from the front of the welding tool to the back under the action of the rotating friction of the welding tool, and forms a dense solid phase weld under the extrusion of the welding tool. However, due to the thin thickness of the mobile phone middle frame, the use of conventional stirring heads is easy to cause deformation of the mobile phone middle frame, affecting the welding pass rate and causing an increase in manufacturing costs. Summary of the invention

[0004] The main purpose of the present invention is to provide a stir friction welding method to solve the above technical problems and improve welding quality, production efficiency and material utilization.

[0005] To achieve the above object, the present invention adopts the following technical solution:

[0006] A friction stir welding method includes a stirring head, wherein the stirring head includes a shaft shoulder and a stirring needle, and the stirring needle is installed at the lower end of the shaft shoulder;

[0007] The design method of the shoulder radius comprises the following steps:

[0008] A1. During the stirring welding process, the stirring head is vertically welded to the workpiece to form a plastic flow zone and a solid zone on the workpiece. The dividing line between the plastic flow zone and the solid zone is taken as the isotherm. The isotherm is approximately regarded as a parabola. A rectangular coordinate system is established with the lowest point of the parabola as the origin. The equation of the parabola satisfies the following relationship:

[0009] z=α(x 2 +y 2 ) Formula 1

[0011] A2. According to the total heat Q generated when the stirring head is vertically and stably welding the workpiece, the total heat Q includes the heat generated by the friction of the shaft shoulder Q1, the heat generated by the friction of the stirring needle Q2, and the heat generated by the plastic deformation of the material on the side of the stirring needle Q3, and Q3<<Q1, Q3<<Q2, so Q=Q1+Q2, and Q, Q1, Q2 satisfy the following relationship:

[0012]

[0013]

[0014]

[0015] Among them, η is the heat input efficiency, μ is the friction coefficient, ω is the rotation speed, τ is the pressure on the contact surface between the stirring head shoulder and the workpiece, r is the shoulder diameter, and r da is the stirring needle diameter;

[0016] A3. On the isotherm, the heat generated by the friction of the stirring head is equal to the heat dissipation of the approximate critical heat dissipation surface in the material, and it is obtained that:

[0017] Q in =Q dis Formula 5

[0019] Among them, Q in is the heat input power, Q dis is the heat dissipation power;

[0020] A4. The heat generated by the friction of the shaft shoulder Q1 accounts for about 85% of the total heat generated by the friction of the stirring head Q. Therefore,

[0021] Q in =Q×1.17 Formula 6

[0023] Q dis =λ / l·ΔT·A Formula 7

[0025] Among them, λ is the thermal conductivity of the material, l is the thickness of the interface, ΔT is the temperature difference on both sides of the interface, and A is the area of ​​the parabola. It can be obtained:

[0026]

[0027] In addition, according to the parabola equation, when x 2 +y 2 =r 2 , we have z = h, where h is the thickness of the workpiece, so we have

[0028] a=h / r 2 Formula 9

[0030] Combining equations 1-9, it can be concluded that the shoulder radius r and the workpiece thickness h satisfy the following relationship:

[0031]

[0032] Among them, λ is the thermal conductivity of the material, l is the thickness of the interface, η is the heat input efficiency, μ is the friction coefficient, τ is the pressure on the workpiece contact surface, and ω is the rotation speed.

[0033] As a preferred technical solution, the stirring needle length design method includes the following:

[0034] The stirring needle length is:

[0035] H pin =H w -(H plunge +H f ) Formula 11

[0037] Among them, H pin is the length of the stirring needle, H w is the thickness of the workpiece, H plunge H is the pressing depth of the mixing head, f It is the distance between the bottom end of the stirring needle and the bottom surface of the workpiece.

[0038] As a preferred technical solution, the inclination angle of the stirring head to the direction perpendicular to the workpiece surface is 1-5°.

[0039] As a preferred technical solution, the stirring head is made of W360.

[0040] As a preferred technical solution, the following steps are included:

[0041] B1. Insert the stirring needle into the plate to be welded at a speed of 30 mm / min;

[0042] B2, insertion dwell time 8-10s;

[0043] B3. After welding is completed, the stirring needle retracts the welded plate at a speed of 100mm / min;

[0044] B4. Withdrawal dwell time 2s.

[0045] The beneficial effect of the present invention is that the above-mentioned friction stir welding method, through which a stirring head with a shoulder diameter of 5.5 mm and a stirring needle length of 1.4 mm is designed, and welding is performed at a rotation speed of 1500 rpm and a welding speed of 200 mm / s, can improve welding quality, production efficiency and material utilization. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] Figure 1 It is a structural schematic diagram of the outer frame of the mobile phone middle plate involved in the present invention;

[0047] Figure 2 It is a structural schematic diagram of the inner frame of the mobile phone mid-plate involved in the present invention;

[0048] Figure 3 The structure of the stirring head and the welded workpiece during welding according to the present invention is shown in FIG. Figure 1 ;

[0049] Figure 4 A curve diagram of the shoulder radius and the welding depth involved in the present invention;

[0050] Figure 5 The structure of the stirring head and the welded workpiece during welding according to the present invention is shown in FIG. Figure 2 ;

[0051] Figure 6 A line graph of the shaft shoulder diameter, rotation speed, welding speed and tensile strength involved in the present invention;

[0052] Figure 7 It is a schematic diagram of the inclination angle structure of the stirring head and the workpiece to be welded according to the present invention; DETAILED DESCRIPTION

[0053] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0054] A friction stir welding method includes a stirring head 100, wherein the stirring head 100 includes a shoulder 101 and a stirring pin 102, and the stirring pin 102 is installed at the lower end of the shoulder 101. In this embodiment, a mobile phone middle plate outer frame (such as AL6061) is welded. Figure 1 As shown) and the mobile phone middle plate inner frame made of ADC12 (as shown Figure 2 As shown in the figure, the stirring head 100 is used for friction stir welding. The required welding depth is 2 mm and the welding length is 500 mm. In order to improve the welding quality, production efficiency and material utilization rate, the dimensions of the shaft shoulder and the stirring needle are designed respectively. The specific design method is as follows:

[0055] like Figure 3 As shown, the design method of the radius of the shoulder 101 includes the following steps:

[0056] A1. During the stirring welding process, the stirring head is vertically welded to the workpiece to form a plastic flow zone and a solid zone on the workpiece. The dividing line between the plastic flow zone and the solid zone is taken as the isotherm. The isotherm is approximately regarded as a parabola. A rectangular coordinate system is established with the lowest point of the parabola as the origin. The equation of the parabola satisfies the following relationship:

[0057] z=α(x 2+y 2 ) Formula 1

[0059] A2. According to the total heat Q generated when the stirring head is vertically and stably welding the workpiece, the total heat Q includes the heat generated by the friction of the shaft shoulder Q1, the heat generated by the friction of the stirring needle Q2, and the heat generated by the plastic deformation of the material on the side of the stirring needle Q3, and Q3<<Q1, Q3<<Q2, so Q=Q1+Q2, and Q, Q1, Q2 satisfy the following relationship:

[0060]

[0061]

[0062]

[0063] Among them, η is the heat input efficiency, μ is the friction coefficient, ω is the rotation speed, τ is the pressure on the contact surface between the stirring head shoulder and the workpiece, r is the shoulder diameter, and r da is the stirring needle diameter;

[0064] A3. On the isotherm, the heat generated by the friction of the stirring head is equal to the heat dissipation of the approximate critical heat dissipation surface in the material, and it is obtained that:

[0065] Q in =Q dis Formula 5

[0067] Among them, Q in is the heat input power, Q dis is the heat dissipation power;

[0068] A4. The heat generated by the friction of the shaft shoulder Q1 accounts for about 85% of the total heat generated by the friction of the stirring head Q. Therefore,

[0069] Q in =Q×1.17 Formula 6

[0071] Q dis =λ / l·ΔT·A Formula 7

[0073] Among them, λ is the thermal conductivity of the material, l is the thickness of the interface, ΔT is the temperature difference on both sides of the interface, and A is the area of ​​the parabola. It can be obtained:

[0074]

[0075] In addition, according to the parabola equation, when x 2 +y 2 =r 2 , we have z = h, where h is the thickness of the workpiece, so we have

[0076] a=h / r 2 Formula 9

[0078] Combining equations 1-9, it can be concluded that the shoulder radius r and the workpiece thickness h satisfy the following relationship:

[0079]

[0080] Among them, λ is the thermal conductivity of the material. In the stable welding stage of stir friction welding, the temperature of the material in the plastic flow area is usually 3 / 4 of the melting point of the material. The thermal conductivity at a temperature of 3 / 4 of the melting point of the material can be selected; l is the thickness of the interface, which is generally 1-2mm; η is the heat input efficiency, which is generally 0.6-0.9; μ is the friction coefficient, which is actually related to the material temperature. Different welding stages correspond to different values. However, in order to simplify calculation and analysis, the friction coefficient is regarded as a constant, usually 0.3-0.6; τ is the contact surface pressure of the workpiece. The parameter τ can be determined according to the yield strength of the workpiece material multiplied by a coefficient <1; ω is the rotation speed.

[0081] According to the physical properties of aluminum alloy, relevant experience, and simulation, the values ​​of relevant parameters can be obtained, as shown in Table 1:

[0082] parameter η μ n τ / MPa λ l / mm Numeric 0.7 0.5 1500 150 200 1

[0083] Table 1 Values ​​of relevant parameters in formula 10

[0084] After determining the relevant parameters, the relationship between the shoulder radius r and the welding depth h can be plotted as follows: Figure 4 The linear relationship in .

[0085] As can be seen from the figure, when the workpiece thickness is 2mm, the shoulder radius is about 2.5mm. Therefore, in the test, shoulders with diameters of 5mm, 5.5mm, and 6mm were selected as experimental objects to explore the appropriate shoulder selection. Since the thermal conductivity coefficients of various grades of aluminum alloys are very close, this figure should also be applicable to other grades of aluminum alloys.

[0086] After determining the size of the shaft shoulder and the welding thickness, select the tool speed within the appropriate range to make it conform to formula (10). According to the formula, the data is obtained, and the left and right selections are made within the range. Experiments are carried out to determine parameters such as welding speed and obtain the optimal production parameters.

[0087] like Figure 5 As shown, the stirring needle length design method includes the following:

[0088] The length of the stirring needle is:

[0089] H pin =H w -(H plunge +H f ) Formula 11

[0091] Among them, H pinis the length of the stirring needle; H w is the thickness of the workpiece, i.e. 2mm; H plunge H is the pressing depth of the stirring head, usually 0.1-1mm, and the thicker the plate, the greater the pressing amount; f It is the distance between the bottom end of the stirring needle and the bottom surface of the workpiece, usually 0.1-1mm, and 0.5mm is taken here.

[0092] According to the size of the workpiece, industry experience and simulation, the values ​​of relevant parameters can be obtained, such as

[0093] As shown in Table 2:

[0094] parameter <![CDATA[H w ]]> <![CDATA[H plunge ]]> <![CDATA[H f ]]> Value / mm 2 0.1 0.5

[0095] Table 2 Values ​​of relevant parameters in formula 11

[0096] According to the above formula (11) and parameters, the stirring needle length is 1.4 mm.

[0097] According to the welding parameters obtained by formula 10 and formula 11, the following parameters were selected as experimental research objects in a suitable range, as shown in Table 3:

[0098]

[0099] Table 3 Experimental welding parameters

[0100] A cross-group experiment was conducted on each data, and the tensile strength of the welded product was tested. The strength coefficient was calculated by comparing it with the tensile strength of 230MPa of the mobile phone mid-plate frame ADC12. The weld was evaluated for appearance, and 27 groups of experimental results were obtained, as shown in Table 4:

[0101]

[0102] Table 4 Welding test results

[0103] According to Table 4, a line graph of shoulder diameter, rotation speed, welding speed and tensile strength is drawn. Figure 6 shown.

[0104] From the above Table 4 and Figure 6, under the conditions of 5.5mm shoulder diameter, rotation speed of 1500rpm and welding speed of 200mm / s, the tensile strength can reach the maximum value. Controlling two variables, from the perspective of a single variable, the reason for the strength fluctuation caused by changing the rotation speed is mainly because the rotation speed directly affects the friction heat of the welding head and the molten state of the aluminum alloy. Too high a rotation speed may cause overheating of the material and weld defects, while too low a rotation speed will affect the welding speed and weld quality. Because the welding speed is too fast, the weldment will not be fully heated, the weld temperature is insufficient, and defects will occur; while too slow a welding speed will increase the heat input, causing deformation and other quality problems.

[0105] In summary, the shoulder diameter is generally about 3 times the welding thickness, so the experiment selected 5-6mm as the shoulder diameter for the experiment. Based on multiple groups of cross experiments, 5.5mm was finally selected as the shoulder diameter.

[0106] The inclination angle of friction stir welding (i.e. the inclination angle of the stirring head 100 and the direction perpendicular to the workpiece surface) is usually determined according to the characteristics and thickness of the material to be welded. For thin plates (thickness of 1-6mm), the inclination angle is generally 1-3°; for medium and thick plates (thickness greater than 6mm), the inclination angle is usually 4-5°. In the welding experiment, ADC12 and AL6061 aluminum alloy materials were used for welding. Taking into account the size of the welding pressure and the structure of the workpiece, the optimal inclination angle was determined to be 3°. The selection of this inclination parameter is based on the preparation before the experiment and the adjustment during the welding process, so that the rear edge of the shoulder of the stirring head can apply a certain welding forging force to the weld during welding, improve the strength and uniformity of the weld, and thus ensure the formation of plasticized eddy currents and the flow of metal during welding.

[0107] The material of the stirring head 100 is made of W360, which requires a hardness far greater than that of the welding material to reduce the wear of the cutter head caused by welding. W360 has high hardness, high toughness and thermal stability, and can well bear the tool material.

[0108] According to the above parameters, DOE experimental analysis was carried out, and finally the experimental conditions of 5.5mm shoulder diameter, rotation speed of 1500rpm, and welding speed of 200mm / s were selected, and a better tensile strength of 158MPa and a strength coefficient of 68.70% were obtained. The strength coefficient was calculated by comparing with the tensile strength of 230MPa of the mobile phone mid-plate frame ADC12.

[0109] The welding method using the above stirring head comprises the following steps:

[0110] B1. The stirring needle 102 is inserted into the welded plate at a speed of 30 mm / min to obtain a better welding effect. At the moment when the stirring needle contacts the welded plate, the axial force will increase sharply. If the insertion speed is too fast, the main shaft of the equipment will be greatly damaged when the welded plate has not yet fully reached the thermoplastic state. If the insertion speed is too slow, the temperature will be overheated and the welding quality will be affected.

[0111] B2. Insertion dwell time of 8-10s can effectively preheat the material and avoid thermal damage. If the dwell time is too short, the welded plate has not yet fully reached the thermoplastic state, and the weld temperature field has not reached the equilibrium state before welding begins, which will cause tunnel-shaped holes in the weld. If the dwell time is too long, the welded material will be overheated and components will be easily concentrated, resulting in slag on the weld surface. At the same time, "S"-shaped black lines are also likely to appear inside the weld, affecting the weld quality.

[0112] B3. After welding, the stirring needle is retracted at a speed of 100mm / min to ensure good welding results. The retraction speed needs to be appropriate to pull the stirring head out of the molten pool. Too fast may introduce pores, and too slow will increase the cooling time after welding.

[0113] B4. The withdrawal dwell time is 2s, which can achieve the best welding strength. The setting of the withdrawal dwell time can effectively reduce the flow of liquid metal during the withdrawal process and ensure the uniformity of the weld.

[0114] The above-described embodiments are only preferred examples of the present invention and are not intended to limit the scope of implementation of the present invention. Therefore, any equivalent changes or modifications made according to the structures, features and principles described in the scope of the patent application of the present invention should be included in the scope of the patent application of the present invention.

Claims

1. A friction stir welding method, characterized in that: It includes a stirring head, and the stirring head includes a shoulder and a stirring pin, and the stirring pin is installed at the lower end of the shoulder; The design method of the radius of the shoulder includes the following steps: A1. During the friction stir welding process of the above-mentioned stirring head, the stirring head vertically welds the workpiece stably to form a plastic flow zone and a solid state zone on the workpiece. Taking the dividing line between the plastic flow zone and the solid state zone as the isotherm, approximating this isotherm as a parabola, and establishing a rectangular coordinate system with the lowest point of the parabola as the origin, then the equation of the parabola satisfies the following relationship: z=α(x 2 +y 2 ) Equation 1 A2. According to the total heat Q generated when the stirring head vertically welds the workpiece stably, the total heat Q includes the heat Q1 generated by the friction of the shoulder, the heat Q2 generated by the friction of the stirring pin, and the heat Q3 generated by the plastic deformation of the material on the side of the stirring pin. And Q3 << Q1, Q3 << Q2, so Q = Q1 + Q2, and Q, Q1, Q2 satisfy the following relationship: Among them, η is the heat input efficiency, μ is the friction coefficient, ω is the rotation speed, τ is the pressure on the contact surface between the stirring head shoulder and the workpiece, r is the shoulder diameter, and r da is the stirring needle diameter; A3. On the isotherm, the heat generated by the friction of the stirring head is equal to the heat dissipated by the approximate critical heat dissipation surface in the material, and it is obtained that: Q in =Q dis Equation 5 Among them, Q in is the heat input power, Q dis is the heat dissipation power; A4. The heat Q1 generated by the friction of the shoulder accounts for about 85% of the total heat Q generated by the friction of the stirring head. Therefore, there is Q in =Q×1.17 Equation 6 Q dis =λ / l·ΔT·A Equation 7 Among them, λ is the thermal conductivity of the material, l is the thickness of the interface, ΔT is the temperature difference between both sides of the interface, A is the area of the paraboloid, and it can be obtained that: In addition, according to the parabola equation, when x 2 +y 2 =r 2 , z = h, where h is the thickness of the workpiece, so a=h / r 2 Equation 9 Combining Equations 1 - 9, it is obtained that the shoulder radius r and the workpiece thickness h satisfy the following relationship: Among them, λ is the thermal conductivity of the material, l is the thickness of the interface, η is the heat input efficiency, μ is the friction coefficient, τ is the pressure on the workpiece contact surface, and ω is the rotational speed.

2. The friction stir welding method according to claim 1, characterized in that: The design method of the length of the stirring pin includes the following: The length of the stirring pin is: H pin =H w -(H plunge +H f ) Equation 11 Among them, H pin is the length of the stirring needle, H w is the thickness of the workpiece, H plunge H is the pressing depth of the mixing head, f It is the distance between the bottom end of the stirring needle and the bottom surface of the workpiece.

3. The friction stir welding method according to claim 2, characterized in that: The inclination angle of the stirring head with respect to the direction perpendicular to the workpiece surface is 1 - 5°.

4. The friction stir welding method according to claim 3, characterized in that: The material of the stirring head is made of W360.

5. The friction stir welding method according to claim 4, characterized in that: It includes the following steps: B1. The stirring pin is inserted into the welded plate at a speed of 30 mm / min; B2. The insertion residence time is 8 - 10 s; B3. After welding is completed, the stirring pin is withdrawn from the welded plate at a speed of 100 mm / min; B4. The withdrawal residence time is 2 s.

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