A friction stir welding method

Abstract

The present application relates to a kind of friction stir welding methods, including stir head, the stir head includes shaft shoulder and stir needle, the stir needle is installed in the lower end of the shaft shoulder, the above-mentioned friction stir welding method, by which the shaft shoulder diameter is 5.5mm and the stir needle length is 1.4mm stir head is designed, and welding is carried out using the rotational speed of 1500rpm and the welding speed of 200mm / s, can improve welding quality, production efficiency and material utilization.

Description

Technical Field

[0001] This invention relates to the field of friction welding technology, and in particular to a friction stir welding method. Background Technology

[0002] The phone frame is a protective shell-like structure for the phone, consisting of only a frame-like component. The phone frame needs to be machined using CNC (Computer Numerical Control) machine tools. However, existing CNC machining methods are time-consuming, inefficient, and difficult to process intricate structures. To address these issues, a combination of CNC technology and die-casting technology has been developed, using friction stir welding.

[0003] Friction stir welding refers to the process of using the heat generated by the friction between a high-speed rotating welding tool and the workpiece to locally melt the materials being welded. As the welding tool moves forward along the welding interface, the plasticized material flows from the front to the rear of the welding tool under the rotational friction force, forming a dense solid-phase weld under the pressure of the welding tool. However, because the thickness of the mobile phone frame is relatively thin, using a conventional stirring head can easily cause deformation of the mobile phone frame, affecting the weld pass rate and increasing manufacturing costs. Summary of the Invention

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

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

[0006] A friction stir welding method includes a stirring head, the stirring head including a shoulder and a stirring pin, the stirring pin being mounted on the lower end of the shoulder;

[0007] The method for designing the shoulder radius includes the following steps:

[0008] A1. During the stirring and welding process, the stirring head is perpendicular to the workpiece for stable welding, thereby forming a plastic flow zone and a solid zone on the workpiece. Taking the dividing line between the plastic flow zone and the solid zone as an isotherm, and approximating this isotherm as a parabola, a rectangular coordinate system is established with the lowest point of the parabola as the origin. Then, the equation of the parabola satisfies the following relationship:

[0009] z=α(x 2 +y 2 Formula 1

[0010] A2. Based on the total heat Q generated when the stirring head is vertically and stably welded to the workpiece, the total heat Q includes heat generated by shoulder friction Q1, heat generated by stirring pin friction Q2, and heat generated by the material on the side of the stirring pin due to plastic deformation Q3, and Q3 << Q1, Q3 << Q2, therefore Q = Q1 + Q2, and Q, Q1, and Q2 satisfy the following relationship:

[0011]

[0012]

[0013]

[0014] Where η is the heat input efficiency, μ is the friction coefficient, ω is the rotational speed, τ is the pressure between the shoulder of the stirring head and the contact surface of the workpiece, and r is the shoulder diameter. da The diameter of the stirring needle;

[0015] A3. On the isotherm, the heat generated by friction of the stirring head is equal to the heat dissipation of the material's approximate critical heat dissipation surface, thus:

[0016] Q in =Q dis Formula 5

[0017] Among them, Q in Q is the thermal input power. dis This refers to heat dissipation power;

[0018] A4. The heat generated by the friction of the shaft shoulder, Q1, accounts for approximately 85% of the total heat generated by the friction of the stirring head, Q. Therefore,

[0019] Q in =Q×1.17 Equation 6

[0020] Q dis =λ / l·ΔT·A Equation 7

[0021] Where λ is the thermal conductivity of the material, l is the thickness of the interface, ΔT is the temperature difference across the interface, and A is the area of ​​the parabolic surface, we can obtain:

[0022]

[0023] Furthermore, according to the equation of the parabola, when x 2 +y 2 =r 2 We have z = h, where h is the workpiece thickness, therefore we have

[0024] a = h / r 2 Formula 9

[0025] Combining Equations 1-9, the shoulder radius r and the workpiece thickness h satisfy the following relationship:

[0026]

[0027] Where 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 at the workpiece contact surface, and ω is the rotational speed.

[0028] As a preferred technical solution, the method for designing the length of the stirring needle includes the following:

[0029] The length of the stirring needle is:

[0030] H pin =H w -(H plunge +H f Formula 11

[0031] Among them, H pin H is the length of the stirring needle. w H represents the thickness of the workpiece. plunge H represents the depth of the stirring head's downward pressure. f This is the distance between the bottom of the stirring needle and the bottom surface of the workpiece.

[0032] As a preferred technical solution, the angle between the stirring head and the direction perpendicular to the workpiece surface is 1-5°.

[0033] As a preferred technical solution, the stirring head is made of W360 material.

[0034] As a preferred technical solution, it includes the following steps:

[0035] B1. The stirring needle is inserted into the plate to be welded at a speed of 30 mm / min;

[0036] B2. Insertion dwell time: 8-10 seconds;

[0037] B3. After welding is completed, the stirring needle is pulled back at a speed of 100 mm / min to retract the welded plate.

[0038] B4. Retraction dwell time: 2 seconds.

[0039] The beneficial effects of the present invention are as follows: the above-mentioned friction stir welding method, by designing a stirring head with a shoulder diameter of 5.5 mm and a stirring pin length of 1.4 mm, and using a rotation speed of 1500 rpm and a welding speed of 200 mm / s for welding, can improve welding quality, production efficiency and material utilization. Attached Figure Description

[0040] Figure 1 This is a schematic diagram of the structure of the mobile phone mid-plate outer frame involved in the present invention;

[0041] Figure 2 This is a schematic diagram of the structure of the inner frame of the mobile phone's middle plate, as per the present invention.

[0042] Figure 3 This is a schematic diagram of the structure of the stirring head and the workpiece to be welded during welding, as per the present invention. Figure 1 ;

[0043] Figure 4 This is a graph showing the shoulder radius and weld depth involved in the present invention.

[0044] Figure 5 This is a schematic diagram of the structure of the stirring head and the workpiece to be welded during welding, as per the present invention. Figure 2 ;

[0045] Figure 6 This is a line graph showing the shoulder diameter, rotational speed, welding speed, and tensile strength of the shaft involved in this invention.

[0046] Figure 7 This is a schematic diagram of the tilt angle structure between the stirring head and the workpiece to be welded, as involved in the present invention; Detailed Implementation

[0047] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0048] A friction stir welding method includes a stirring head 100, which includes a shoulder 101 and a stirring pin 102. The stirring pin 102 is mounted on the lower end of the shoulder 101. In this embodiment, the outer frame of a mobile phone mid-plate made of AL6061 material (such as...) Figure 1 (as shown) and the inner frame of the mobile phone's mid-plate made of ADC12 material (as shown) Figure 2 As shown, the components are joined together using a stirring head 100 via friction stir welding. The required weld depth is 2mm and the weld length is 500mm. To improve weld quality, production efficiency, and material utilization, the dimensions of the shoulder and stirring pin are designed separately. The specific design method is as follows:

[0049] like Figure 3 As shown, the design method for the radius of shoulder 101 includes the following steps:

[0050] A1. During the stirring welding process, the stirring head is perpendicular to the workpiece for stable welding, forming a plastic flow zone and a solid zone on the workpiece. Taking the dividing line between the plastic flow zone and the solid zone as an isotherm, and approximating this isotherm as a parabola, a rectangular coordinate system is established with the lowest point of the parabola as the origin. Then, the equation of the parabola satisfies the following relationship:

[0051] z=α(x 2+y 2 Formula 1

[0052] A2. Based on the total heat Q generated when the stirring head is vertically and stably welded to the workpiece, the total heat Q includes heat generated by shoulder friction Q1, heat generated by stirring pin friction Q2, and heat generated by the material on the side of the stirring pin due to plastic deformation Q3, and Q3 << Q1, Q3 << Q2, therefore Q = Q1 + Q2, and Q, Q1, and Q2 satisfy the following relationship:

[0053]

[0054]

[0055]

[0056] Where η is the heat input efficiency, μ is the friction coefficient, ω is the rotational speed, τ is the pressure between the shoulder of the stirring head and the contact surface of the workpiece, and r is the shoulder diameter. da The diameter of the stirring needle;

[0057] A3. On the isotherm, the heat generated by friction of the stirring head is equal to the heat dissipation of the material's approximate critical heat dissipation surface, thus:

[0058] Q in =Q dis Formula 5

[0059] Among them, Q in Q is the thermal input power. dis This refers to heat dissipation power;

[0060] A4. The heat generated by the friction of the shaft shoulder, Q1, accounts for approximately 85% of the total heat generated by the friction of the stirring head, Q. Therefore,

[0061] Q in =Q×1.17 Equation 6

[0062] Q dis =λ / l·ΔT·A Equation 7

[0063] Where λ is the thermal conductivity of the material, l is the thickness of the interface, ΔT is the temperature difference across the interface, and A is the area of ​​the parabolic surface, we can obtain:

[0064]

[0065] Furthermore, according to the equation of the parabola, when x 2 +y 2 =r 2 We have z = h, where h is the workpiece thickness, therefore we have

[0066] a = h / r 2 Formula 9

[0067] Combining Equations 1-9, the shoulder radius r and the workpiece thickness h satisfy the following relationship:

[0068]

[0069] Wherein, λ is the thermal conductivity of the material. In the stable welding stage of friction stir welding, the temperature of the material in the plastic flow region is usually 3 / 4 of the material's melting point. The thermal conductivity at the temperature of 3 / 4 of the material's melting point can be selected; l is the thickness of the interface, generally taken as 1-2 mm; η is the heat input efficiency, generally taken as 0.6-0.9; μ is the friction coefficient, which is actually related to the material temperature and corresponds to different values ​​in different welding stages. However, for the sake of simplifying calculation and analysis, the friction coefficient is regarded as a constant value, usually taken as 0.3-0.6; τ is the pressure on the workpiece contact surface. The parameter τ can be determined by multiplying the yield strength of the workpiece material by a coefficient <1; ω is the rotational speed.

[0070] Based on the physical properties of aluminum alloys, and related experience and simulations, the values ​​of relevant parameters can be obtained, as shown in Table 1:

[0071] parameter η μ n τ / MPa λ l / mm numerical values 0.7 0.5 1500 150 200 1

[0072] The values ​​of the relevant parameters in Equation 10 of Table 1

[0073] After determining the relevant parameters, the relationship between the shoulder radius r and the weld depth h can be plotted as follows: Figure 4 The linear relationship in.

[0074] As shown in the figure, when the workpiece thickness is 2mm, the shoulder radius is approximately 2.5mm. Therefore, in the experiment, shoulders with diameters of 5mm, 5.5mm, and 6mm were selected as experimental subjects to explore suitable shoulder selection. Since the thermal conductivity of various aluminum alloy grades is very similar, this figure should also be applicable to other aluminum alloy grades.

[0075] After determining the shoulder dimensions and welding thickness, select the tool speed within a suitable range to conform to formula (10). Calculate the data using the formula, select left and right within the range, conduct experiments, determine welding speed and other parameters, and obtain optimal production parameters.

[0076] like Figure 5 As shown, the design method for the length of the stirring needle includes the following:

[0077] The length of the stirring needle is:

[0078] H pin =H w -(H plunge +H f Formula 11

[0079] Among them, H pinH is the length of the stirring needle; w H represents the thickness of the workpiece, which is 2mm. plunge The depth of the mixing head's downward pressure is typically 0.1-1mm, and the thicker the plate, the greater the downward pressure. H f This is the distance between the bottom of the stirring needle and the bottom surface of the workpiece, which is usually 0.1-1mm, and here we take 0.5mm.

[0080] Based on the workpiece dimensions, industry experience, and simulation, the values ​​of relevant parameters can be obtained, such as...

[0081] As shown in Table 2:

[0082] parameter <![CDATA[H w ]]> <![CDATA[H plunge ]]> <![CDATA[H f ]]> Value / mm 2 0.1 0.5

[0083] The values ​​of the relevant parameters in Equation 11 of Table 2

[0084] According to the formula (11) and parameters above, the length of the stirring needle is 1.4 mm.

[0085] Based on the welding parameters obtained from Formulas 10 and 11, the following parameters were selected as experimental subjects within a suitable range, as shown in Table 3:

[0086]

[0087] Table 3 Experimental welding parameters

[0088] Cross-group experiments were conducted on each data point, and the tensile strength of the welded product was tested. The strength coefficient was calculated by comparing it with the tensile strength of ADC12 in the mobile phone's middle plate outer frame (230 MPa). The weld was then visually evaluated, and the results of 27 sets of experiments were obtained, as shown in Table 4.

[0089]

[0090] Table 4 Welding Experiment Results

[0091] Based on Table 4, plot a line graph of shoulder diameter, rotational speed, welding speed, and tensile strength, as shown below. Figure 6 As shown.

[0092] From Table 4 above and Figure 6Under conditions of a 5.5mm shoulder diameter, a rotation speed of 1500rpm, and a welding speed of 200mm / s, the tensile strength reaches its maximum value. Controlling two variables, from the perspective of the single variable, the strength fluctuation caused by changing the rotation speed is mainly due to the fact that the rotation speed directly affects the frictional heat of the weld joint and the molten state of the aluminum alloy. Excessively high rotation speeds may lead to material overheating and weld defects, while excessively low rotation speeds will affect welding speed and weld quality. Too high a welding speed will result in insufficient heating of the weldment, leading to insufficient weld temperature and defects; while too slow a welding speed will increase heat input, causing deformation and other quality problems.

[0093] In summary, the shoulder diameter is generally about 3 times the weld thickness. Therefore, 5-6 mm was selected as the shoulder diameter for the experiment. Based on multiple cross-experiments, 5.5 mm was finally selected as the shoulder diameter.

[0094] The tilt angle (i.e., the angle between the stirring head 100° and the direction perpendicular to the workpiece surface) in friction stir welding is usually determined based on the characteristics and thickness of the materials being welded. For thin plates (1-6 mm thick), a tilt angle of 1-3° is generally used; for medium-thick plates (thickness greater than 6 mm), a tilt angle of 4-5° is generally used. In welding experiments, ADC12 and AL6061 aluminum alloy materials were used for welding. Considering both the welding pressure and the workpiece structure, the optimal tilt angle was determined to be 3°. This tilt angle parameter was selected based on pre-experiment preparation and adjustments during the welding process, so that the rear edge of the stirring head shoulder could apply a certain welding upsetting force to the weld during welding, improving the weld strength and uniformity, thereby ensuring the formation of plasticizing eddies and metal flow during the welding process.

[0095] The stirring head 100 is made of W360, which requires a much higher hardness than welding materials to reduce tool wear caused by welding. W360 has high hardness, high toughness and thermal stability, which can well support the tool material.

[0096] Based on the above parameters, DOE experimental analysis was conducted. Finally, experimental conditions of 5.5mm shoulder diameter, 1500rpm rotation speed, and 200mm / s welding speed were selected, resulting in a good tensile strength of 158MPa and a strength coefficient of 68.70%. The strength coefficient was calculated by comparing it with the tensile strength of 230MPa of the ADC12 outer frame of the mobile phone mid-plate.

[0097] The welding method using the above-mentioned stirring head includes the following steps:

[0098] B1. The stirring pin 102 is inserted into the plate to be welded at a speed of 30 mm / min to obtain a better welding effect. At the moment when the stirring pin contacts the plate to be welded, the axial force will increase sharply. If the insertion speed is too fast, it will cause great damage to the main shaft of the equipment before the plate to be welded has fully reached the thermoplastic state. If the insertion speed is too slow, it will cause overheating and affect the welding quality.

[0099] B2. Insertion dwell time of 8-10 seconds can effectively preheat the material and avoid heat damage. If the dwell time is too short, the plate being welded has not yet fully reached the thermoplastic state, and the weld temperature field has not reached the equilibrium state before welding begins, which will result in tunnel-shaped holes in the weld. If the dwell time is too long, the material being welded will be overheated and prone to component segregation, which will result in slag on the weld surface. At the same time, "S"-shaped black lines are also prone to appear inside the weld, affecting the weld quality.

[0100] B3. After welding, the stirring pin is pulled back at a speed of 100 mm / min to ensure good welding results. The pulling speed needs to be appropriate to pull the stirring head out of the molten pool. Too fast may introduce porosity, while too slow will increase the cooling time after welding.

[0101] B4. A 2-second retraction dwell time can achieve the best welding strength. Setting the retraction dwell time can effectively reduce the flow of liquid metal during the retraction process and ensure the uniformity of the weld.

[0102] The embodiments described above are merely preferred examples of the present invention and are not intended to limit the scope of the present invention. Therefore, all equivalent changes or modifications made to the structure, features and principles described in the claims of the present invention should be included within the scope of the present invention patent application.

Claims

1. A friction stir welding method, characterized in that, It includes a stirring head, which includes a shoulder and a stirring pin, with the stirring pin mounted on the lower end of the shoulder; The method for designing the shoulder radius includes the following steps: A1. During the stirring and welding process, the stirring head is perpendicular to the workpiece for stable welding, thereby forming a plastic flow zone and a solid zone on the workpiece. Taking the dividing line between the plastic flow zone and the solid zone as an isotherm, and approximating this isotherm as a parabola, a rectangular coordinate system is established with the lowest point of the parabola as the origin. Then, the equation of the parabola satisfies the following relationship: Formula 1 A2. The total heat Q generated when the stirring head is vertically and stably welded to the workpiece includes heat generated by shoulder friction Q1, heat generated by stirring pin friction Q2, and heat generated by the plastic deformation of the material on the side of the stirring pin Q3, and Q3... Q1, Q3 Q2, therefore Q = Q1 + Q2, and Q, Q1, and Q2 satisfy the following relationship: Formula 2 Formula 3 Formula 4 Where η is the heat input efficiency, μ is the coefficient of friction, ω is the rotational speed, τ is the pressure between the shoulder of the stirring head and the contact surface of the workpiece, and r is the shoulder diameter. The diameter of the stirring needle; A3. On the isotherm, the heat generated by friction of the stirring head is equal to the heat dissipation of the material's approximate critical heat dissipation surface, thus: Formula 5 in, For heat input power, This refers to heat dissipation power; A4. The heat generated by the friction of the shaft shoulder, Q1, accounts for 85% of the total heat generated by the friction of the stirring head, therefore... Formula 6 Formula 7 Where λ is the thermal conductivity of the material, l is the thickness of the interface, ΔT is the temperature difference across the interface, and A is the area of ​​the parabolic surface, we can obtain: Formula 8 Furthermore, according to the equation of a parabola, when We have z = h, where h is the workpiece thickness, therefore we have Formula 9 Combining Equations 1-9, the shoulder radius r and the workpiece thickness h satisfy the following relationship: Formula 10 Where 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 at the workpiece contact surface, and ω is the rotational speed.

2. The friction stir welding method according to claim 1, characterized in that, The method for designing the length of the stirring pin includes the following: The length of the stirring needle is: Formula 11 in, The length of the stirring needle. For the thickness of the workpiece, This refers to the depth of the stirring head's downward pressure. This is the distance between the bottom 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 angle between the stirring head and the direction perpendicular to the workpiece surface is 1-5°.

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

5. The friction stir welding method according to claim 4, characterized in that, It includes the following steps: B1. The stirring needle is inserted into the plate to be welded at a speed of 30 mm / min; B2. Insertion dwell time: 8-10 seconds; B3. After welding is completed, the stirring pin is pulled out of the welded plate from the molten pool at a speed of 100 mm / min; B4. Retraction dwell time: 2 seconds.

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