metals

Article Analysis of (Warm Stamping Process): A Study of the Variable Friction Coefficient on 6111 Aluminum Alloy

Shasha Dou 1,2,*, Xiaoping Wang 1,*, Jason Xia 2 and Lisa Wilson 3 1 College of Mechanical and Electrical Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China 2 School of Mechanical Engineering, Yancheng Institute of Technology, Yancheng 224051, China; [email protected] 3 The Wolfson Centre, University of Greenwich, Gillingham ME4 4TB, UK; [email protected] * Correspondence: [email protected] (S.D.); [email protected] (X.W.); Tel.: +86-158-5107-9040 (S.D.)

 Received: 27 July 2020; Accepted: 2 September 2020; Published: 4 September 2020 

Abstract: Aluminum alloy materials have been widely used in automobile, aerospace and other fields because of their low density, high specific strength and corrosion resistance. The process of the warm forming of aluminum alloy improves the of aluminum alloy sheets, reduces the deformation resistance and spring-back and improves the forming accuracy and quality of parts. For these reasons, it is frequently used. In this work, the effects of temperature, sliding speed and normal load on the friction coefficient of 6111 aluminum alloy were studied by using a CFT-I (Equipment Type) friction tester under boundary lubrication conditions. The surface morphology of the sample after the friction test was observed by optical microscopy. The results show that the surface quality of aluminum alloy is better at 200 ◦C, which was used as the temperature in the experiments. According to the test measurement results, the friction coefficient increases with the increase in temperature and decreases with the increase in sliding speed and normal load. Variable friction coefficient models of sliding speed and normal load were established. Using the optimal parameter combination as the simulation parameter, the established variable friction coefficient models were input into numerical simulation software, and two sets of comparative simulations were established. The thickness distribution of the sheet material obtained through the simulation was compared with the actual test measurement. Further verification was carried out through the amount of spring-back. The results show that the thickness distribution and spring-back of the sheet obtained by the variable friction coefficient model are closer to the actual measurements (the error of the spring-back angle decreased from more than 20% to less than 10%), which verifies the reliability and accuracy of the variable friction coefficient model.

Keywords: aluminum alloy; warm forming; friction coefficient; process parameters; numerical simulation

1. Introduction With the continuous development of lightweight technology, aluminum alloy materials are being more widely used in automotive, aerospace and other fields than ever before. This is because of the low density, high specific strength and corrosion resistance of aluminum alloy materials. However, when compared with steel materials, aluminum alloy demonstrates poor plasticity at room temperature, as well as difficulties for pressing a body panel with a complex shape. In addition, defects such as spring-back and fracture are prone to occur after the sheet metal stamping process. In many cases, it is difficult to guarantee the dimensional accuracy of parts, and the qualification rate of parts processed is comparatively low, which production costs [1].

Metals 2020, 10, 1189; doi:10.3390/met10091189 www.mdpi.com/journal/metals Metals 2020, 10, 1189 2 of 16

Warm metal forming generally refers to the forming process in which the material is heated to a temperature below the dynamic recovery or recrystallization temperature through the heat transfer of the furnace or . Under the warm forming condition, the elongation of the aluminum alloy sheet can reach levels between 20% and 25%, the strength of stamping parts increases, the spring-back decreases and the forming accuracy improves [2]. According to the temperature between the sheet metal and the die, warm forming can be divided into isothermal forming and non-isothermal forming. In the process of isothermal forming, the temperatures of the sheet metal and the die remain basically the same, which can improve the plasticity of the sheet metal and the uniform distribution of the wall thickness of parts. However, the warm forming process imposes higher demands on equipment, resulting in increased production cost and difficulty, and the anti-concave performance of parts can be poor. In the non-isothermal forming process, there is a specific temperature difference between the sheet metal and the die. During the stamping process, heat is transferred between the sheet metal and the die. The temperature of the sheet metal changes in real time, so it is widely used in the actual production process [3]. El Fakir et al. [4] put forward a new integrated processing technology that combines warm forming and heat treatment, called solution heat treatment forming cold die quenching, which can realize synchronous forming and quenching. This effectively solves the difficult problem of complex part processing while ensuring the plasticity of aluminum alloy. Finch and Wilson et al. [5,6] carried out experimental research on aluminum alloy after annealing and tempering in a cylinder and square box. The results showed that the properties of the aluminum alloy sheet were improved at 200 ◦C, and the shape of the formed parts had no effect on the sheet properties. Ayres and Wenner [7] studied the influence of forming temperature and punch speed on the forming limit of 5182 aluminum alloy through a punch bulging experiment. It was found that the forming limit of the sheet metal increased significantly with the increase in temperature and decrease in punch speed, and the results showed the strain distribution of the material tended to be within a reasonable range. D. Li et al. [8] studied the mechanical properties of 5182, 5754 and 6111 aluminum alloys at different temperatures and strain rates through a uniaxial tensile test. The results showed that the elongation of the sheet metal increased, corresponding to the increase in temperature and the decrease in strain rate, and the effect of temperature on the strain rate sensitivity coefficient was more pronounced. V.M. Simões [9] reported that the punch speed had a significant influence on the success of the sheet metal warm forming of aluminum alloys, and formability and spring-back remained stable or improved with the increase in punch speed. Lee et al. [10] also studied the influence of lubricant viscosity and surface roughness on the friction coefficient, established a friction model and calculated the relationship between the friction coefficient, lubricant viscosity and surface roughness by the least squares method. The Equation is as follows:

23.2 6 2 2 µ = 5.3 10− (ν 56.6) + 0.24(λ 0.76) 0.112 (1) 104.5 + ν0.98 − × − − − where ν is the lubricant viscosity and λ is the surface roughness. IM.Y.T. et al. [11] modified the classical Coulomb friction model by using the finite element method and an arctangent function and introduced the relative slip velocity. The modified model can maintain continuous friction stress on the boundary, and its shear friction model is as follows. ( !) 2 ur ur τ = mk arctan | | (2) π u ur 0 | | Y.Z. Zhao et al. [12] studied the relationship between load and the friction coefficient under boundary lubrication conditions and established a variable friction coefficient model based on different interface loads. 0.177 µ = 0.133(p/1.03)− (3) Metals 2020, 10, 1189 3 of 16

S.S. Dou [13] studied the influence of sliding speed and interface load on the friction coefficient under the boundary lubrication condition and established a comprehensive variable friction coefficient model with variable speed and variable load:

1.849P + 13.3 µ = (4) (P + 2.446)(0.9193P + 1.467 + ν) where P is the interface load and v is the sliding speed. Currently, there is no singular conclusion about the friction problem in the warm forming of aluminum alloy, as the stamping process for an aluminum alloy sheet is a highly nonlinear changing process, and the friction behavior between aluminum alloy and the die is exceedingly complex. It is important to study the friction characteristics of aluminum alloy warm forming to improve the accuracy of CAE (Computer Aided Engineering) simulation. The influence of the initial forming temperature, positive pressure (normal load) and sliding speed on the friction coefficient between 6111 aluminum alloy and an H13 die steel surface was investigated by using a CFT-I multifunctional material surface comprehensive performance tester (Zhongke Kaihua Technology Development Co., Ltd., Lanzhou, China) under boundary lubrication conditions. The surface morphology of 6111 aluminum alloy after a friction experiment with H13 die steel was observed and analyzed using a microscope. The influence law of multiple factors on the friction coefficient under different conditions was analyzed, leading to a friction model being established. In addition, the model of the variable friction coefficient was also established through experimentation, which is proved to improve simulation accuracy.

2. Materials and Methods

2.1. Materials In this test, 6111-T4 aluminum alloy with a thickness of 1 mm was used. Among the 6000-series alloys, 6111 aluminum alloy has the highest strength and has wide application prospects across the [14]. Its chemical composition is shown in Table1.

Table 1. Chemical composition of 6111 aluminum alloys (wt.%), data from [14].

Components Si Fe Cu Mg Zn Ti Cr Mn Al Mass fraction 0.75 0.40 0.50–0.90 0.70 0.10 0.10 0.10 0.15–0.45 96.5–97.2

H13 hot working die steel was used as the friction pair in the friction test, and its hardness was 55 HRC (Rockwell Hardness) after quenching and tempering. The test sample and fixture are shown in Figure1a, and the aluminum alloy sample after the friction test is shown in Figure1b. Abrasive was used to grind and polish the working surface of the die block, and the contact surface state of the actual stamping process was simulated. The contact surface of the aluminum alloy and the die was measured and observed through the use of an optical microscope, and its surface roughness Ra was recorded as 0.2–0.6 µm and 0.8–1.3 µm.

2.2. Lubrication Firstly, the contact surfaces between the H13 die steel and aluminum alloy were polished and then put into the beaker containing acetone and anhydrous ethanol for ultrasonic cleaning and dried with a blower. Through calculation, a certain amount of molybdenum-disulfide lubricant and boron-nitride (BN) high-temperature anti-oxidation agent were sprayed evenly on the surface of the sheet to form a certain thickness of oil film. The usage calculation is based on the ratio of the minimum oil film thickness to friction surface roughness λ—λ > 2 for fluid lubrication, λ < 1 for boundary lubrication and 1 < λ < 2 for mixed lubrication. Metals 2020, 10, 1189 4 of 16

Materials 2020, x, x FOR PEER REVIEW 4 of 17

(a) (b)

Figure 1. Diagram of the friction test sample. (a) Friction objects; (b) Aluminum plate samples. Figure 1. Diagram of the friction test sample. (a) Friction objects; (b) Aluminum plate samples.

2.3.2.2. Test Lubrication Equipment TheFirstly, experiment the contact was surfaces carried be outtween on athe CFT-I H13 multifunctionaldie steel and alum materialinum alloy surface were polished comprehensive and then put into the beaker containing acetone and anhydrous ethanol for ultrasonic cleaning and dried performance tester (shown in Figure2). In this device, the heating furnace is fixed on the reciprocating with a blower. Through calculation, a certain amount of molybdenum-disulfide lubricant and boron- motion test platform, which is heated by a resistance wire and controlled by a temperature controller, nitride (BN) high-temperature anti-oxidation agent were sprayed evenly on the surface of the sheet and the required temperature is output to the workbench. The sheet is fixed in the heating furnace to form a certain thickness of oil film. The usage calculation is based on the ratio of the minimum oil (Zhongkefilm thickness Kaihua Technologyto friction surface Development roughness Co., λ Ltd.,—λ > Lanzhou, 2 for fluid China) lubrication, through λ the< 1 fixture for boundary for heating, andlubrication the test is and carried 1 < λ out< 2 for after mixed the lubrication. sheet reaches the set temperature for 5 min. The reciprocating platform with the heating furnace and sheet sample moves along the x-axis direction, driven by an AC servo2.3. Test motor Equipment (Zhongke Kaihua Technology Development Co., Ltd., Lanzhou, China). The loading structureThe of experiment the H13 block was iscarried in the out upper on a partCFT-I and multifunctional in a relatively material static surface state. Whencomprehensive loading the sampleperformance or applying tester a (shown load, the in Figure H13 block 2). In this can devi movece, the up heating and down furnace along is fixed the on Z-axis the reciprocating with the lifting platform.motion After test platform, reaching which the set is heated temperature, by a resistance the plate–plate wire and controlled friction and by weara temperature test is conducted controller, after holdingand the for required 5 min. To temperature ensure that is the output temperature to the work ofbench. the sample The sheet corresponds is fixed in to the the heating real environment, furnace the temperature(Zhongke Kaihua controller Technology is divided Development into two Co., stages: Ltd., the Lanzhou, heating China) stage, through when the the heating fixture furnacefor temperatureheating, and rises the rapidly test is tocarried the systemout after temperature, the sheet reaches and the the insulation set temperature stage, for when 5 min. the The plate is insulated,reciprocating the z-axis platform lifts with and the the heating translation furnace table and loadingsheet sample the moldmoves sample along the drops x-axis in direction, the vertical direction.driven by After an AC the servo die motor comes (Zhongke into contact Kaihua with Technology the sheet Development metal, the normalCo., Ltd., load Lanzhou, is continuously China). The loading structure of the H13 block is in the upper part and in a relatively static state. When applied to the setting value. In the process of measurement, the loading structure of H13 steel remains loading the sample or applying a load, the H13 block can move up and down along the Z-axis with relatively static. The reciprocating platform with the heating furnace and aluminum alloy disk sample the lifting platform. After reaching the set temperature, the plate–plate friction and wear test is moves reciprocally along the x-axis direction, driven by the AC servo motor. conductedMaterials 2020 after, x, x FORholding PEER forREVIEW 5 min. To ensure that the temperature of the sample corresponds5 toof the17 real environment, the temperature controller is divided into two stages: the heating stage, when the heating furnace temperature rises rapidly to the system temperature, and the insulation stage, when the plate is insulated, the z-axis lifts and the translation table loading the mold sample drops in the vertical direction. After the die comes into contact with the sheet metal, the normal load is continuously applied to the setting value. In the process of measurement, the loading structure of H13 steel remains relatively static. The reciprocating platform with the heating furnace and aluminum alloy disk sample moves reciprocally along the x-axis direction, driven by the AC servo motor.

(a) (b)

Figure 2. CFT-I multifunctional material surface comprehensive performance tester. (a) Disk–pin Figure 2. CFT-I multifunctional material surface comprehensive performance tester. (a) Disk–pin friction test principle; (b) Temperature control table. friction test principle; (b) Temperature control table. During the friction process, the force sensor records the normal load and tangential friction force in real time, transmitting measured data to the computer. The real-time friction coefficient μ is calculated by Coulomb’s law Equation (5):

Fx μ = (5) F z

where Fx is the tangential friction force with the unit N, and Fz is the normal friction force with the unit N.

2.4. Experimental Arrangement The purpose of this friction experiment was to study the friction characteristics between H13 tool steel and 6111 aluminum alloy under different process parameters. The friction problem in the warm forming process for aluminum alloy, as referenced earlier, is very complex. The influence of the initial temperature, normal load and sliding speed on the friction characteristics of aluminum alloy are the main points of focus. The experimental parameters are shown in Table 2.

Table 2. Parameter settings of 6111 aluminum alloy for the friction test.

Sheet Temperature Load Fz Sliding Speed Vx Running Stroke L Contact (°C) (N) (mm/s) (mm) Conditions 25 10 20 5 100 20 30 5 Boundary 150 30 40 5 lubrication 200 40 50 5 250 50 60 5

3. Results

3.1. Effect of Sheet Temperature on Friction and Wear Behavior of Materials The friction coefficient can be used to measure the friction performance of materials during the forming process. The friction coefficient can reflect the friction force needed to overcome friction and slide up the deforming material. The larger the friction coefficient, the worse the friction performance of the material [15]. The sensor system monitors and records the positive pressure and horizontal friction in real time. The measured data are transmitted to the computer software synchronously, and

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During the friction process, the force sensor records the normal load and tangential friction force in real time, transmitting measured data to the computer. The real-time friction coefficient µ is calculated by Coulomb’s law Equation (5): F µ = x (5) Fz where Fx is the tangential friction force with the unit N, and Fz is the normal friction force with the unit N.

2.4. Experimental Arrangement The purpose of this friction experiment was to study the friction characteristics between H13 tool steel and 6111 aluminum alloy under different process parameters. The friction problem in the warm forming process for aluminum alloy, as referenced earlier, is very complex. The influence of the initial temperature, normal load and sliding speed on the friction characteristics of aluminum alloy are the main points of focus. The experimental parameters are shown in Table2.

Table 2. Parameter settings of 6111 aluminum alloy for the friction test.

Sheet Sliding Speed Vx Running Stroke L Contact Load Fz (N) Temperature (◦C) (mm/s) (mm) Conditions 25 10 20 5 100 20 30 5 Boundary 150 30 40 5 lubrication 200 40 50 5 250 50 60 5

3. Results

3.1. Effect of Sheet Temperature on Friction and Wear Behavior of Materials The friction coefficient can be used to measure the friction performance of materials during the forming process. The friction coefficient can reflect the friction force needed to overcome friction and slide up the deforming material. The larger the friction coefficient, the worse the friction performance of the material [15]. The sensor system monitors and records the positive pressure and horizontal friction in real time. The measured data are transmitted to the computer software synchronously, and the friction coefficient is calculated by Equation (5). Under the boundary lubrication condition and for a sliding speed of 30 mm/s and a normal load of 20 N, the variation curve of the interfacial friction coefficient over time for the 6111 aluminum alloy at different temperatures is shown in Figure3a. As seen in Figure3a, the friction coe fficient increases rapidly within the first few seconds and then decreases and finally slowly rises to a relatively stable fluctuation state. This is because more tangential force is needed to overcome the static friction force between the die block and the sheet metal at the beginning of the process, and the sliding friction force is smaller than the maximum static friction force. At the peak value of the friction coefficient, the maximum static friction occurs. After that, relative sliding occurs between the tool and the sheet metal, resulting in a slight decrease in the friction coefficient. It can be seen from the peak value of the curve that the maximum static friction needed to overcome increases with the increase in the temperature, and this is because the material softens with the increase in temperature, leading to decreases in the strength and hardness of the plate [16]. Under the conditions of the same sliding speed and loading load, the micro-convex bodies on the surface of the material are embedded in each other, which increases the contact area and makes the tangential friction force increase significantly. At the same time, given the increase in temperature, the viscosity of the lubricant on the surface of aluminum alloy decreases, and the ability of the lubricating oil film to resist load decreases. However, the oxide film on the surface of aluminum alloy remains relatively brittle and hard. After the oxide film is broken, the friction between friction Materials 2020, x, x FOR PEER REVIEW 6 of 17 the friction coefficient is calculated by Equation (5). Under the boundary lubrication condition and for a sliding speed of 30 mm/s and a normal load of 20 N, the variation curve of the interfacial friction coefficient over time for the 6111 aluminum alloy at different temperatures is shown in Figure 3a. As seen in Figure 3a, the friction coefficient increases rapidly within the first few seconds and then decreases and finally slowly rises to a relatively stable fluctuation state. This is because more tangential force is needed to overcome the static friction force between the die block and the sheet metal at the beginning of the process, and the sliding friction force is smaller than the maximum static friction force. At the peak value of the friction coefficient, the maximum static friction occurs. After that, relative sliding occurs between the tool and the sheet metal, resulting in a slight decrease in the friction coefficient. It can be seen from the peak value of the curve that the maximum static friction needed to overcome increases with the increase in the temperature, and this is because the material softens with the increase in temperature, leading to decreases in the strength and hardness of the plate [16]. Under the conditions of the same sliding speed and loading load, the micro-convex bodies on the surface of the material are embedded in each other, which increases the contact area and makes the tangential friction force increase significantly. At the same time, given the increase in temperature, the viscosity of the lubricant on the surface of aluminum alloy decreases, and the ability of the lubricating oil film to resist load decreases. However, the oxide film on the surface of aluminum alloy remains relatively brittle and hard. After the oxide film is broken, the friction between friction pairs is increased, and the friction coefficient increases, which is consistent with actual experience [17]. The friction coefficient curve can be divided into two stages: the rapid rise in the friction coefficient and

Metalsthe stable2020, 10 fluctuation, 1189 stage. 6 of 16 The actual production cycle of sheet metal stamping is fast. In order to make the test results better reflect the actual stamping process, the friction coefficient changes in the running-in stage are pairsconsistent is increased, with those and theof frictionthe actual coe ffisituation.cient increases, As seen which in Figure is consistent 3a, at the with beginning, actual experience the curve [17 is]. Thefluctuating. friction coeAfterfficient 5 s, the curve friction can coefficient be divided tends into twoto be stages: stable, the not rapid oscillating rise in violently. the friction Therefore, coefficient the andaverage the stable value fluctuationof the friction stage. test results from 5 to 15 s was taken as the effective friction coefficient, and Theeach actualgroup productionof experiments cycle was of repeated sheet metal three stamping times. The is fast.effective In order friction to coefficient make the testat different results bettertemperatures reflect the is shown actual stampingin Figure 3b. process, the friction coefficient changes in the running-in stage are consistentIt can withbe seen those from of the the figure actual that situation. the effective As seen friction in Figure coefficient3a, at will the increase beginning, gradually the curve in line is fluctuating.with the increaseAfter in 5 stemperature., the friction When coefficient the temperature tends to be increases stable, not from oscillating room temperature violently. Therefore, to 200 °C, thethe averagefriction valuecoefficient of the increases friction test rapidly results with from the 5 to increase 15 s was in taken temperature. as the effective At 200 friction °C, the coe ffifrictioncient, andcoefficient each group is about of experiments 0.155. Above was 200 repeated °C, the friction three times. coefficient The e increasesffective friction slowly, coe correspondingfficient at diff erentto the temperaturesincrease in temperature. is shown in Figure3b.

(a) (b)

Figure 3. The fluctuation curve of the friction coefficient with sliding stroke in boundary lubrication: Figure 3. The fluctuation curve of the friction coefficient with sliding stroke in boundary lubrication: (a) Variation curves of the friction coefficient with time under different temperatures; (b) (a) Variation curves of the friction coefficient with time under different temperatures; (b) Experimental frictionExperimental coefficients friction at dicoefficienfferent temperatures.ts at different temperatures.

It can be seen from the figure that the effective friction coefficient will increase gradually in line with the increase in temperature. When the temperature increases from room temperature to 200 ◦C, the friction coefficient increases rapidly with the increase in temperature. At 200 ◦C, the friction coefficient is about 0.155. Above 200 ◦C, the friction coefficient increases slowly, corresponding to the increase in temperature. For a stamping speed of 30 mm/s and a normal load of 20 N, the surface morphologies of the aluminum alloy at different temperatures were observed with a laser scanning microscope vk-x100 (KEYENCE, Osaka, Japan), as shown in Figure4. As shown in Figure4a, the surface is relatively stable with a small number of scratches and many fine abrasive particles, so abrasive wear and furrow wear occurred. As seen in Figure4b,c, the scratches on the surface became increasingly significant with the increase in test temperature, with scratch depth also increasing. The number of fine particles on the surface is seen to have decreased, indicating that the friction mechanism was mainly plow wear. As seen in Figure4e, there were many adhesion pits on the surface of the aluminum alloy, and the surface metal was peeled off and torn, resulting in serious adhesive wear. The original sheet, as shown in Figure4f, had fewer surface scratches on it. By analyzing the surface morphology of the 6111 aluminum alloy at different temperatures and considering the production cost and heating conditions in actual production, the warm forming temperature was determined to be 200 ◦C. Materials 2020, x, x FOR PEER REVIEW 7 of 17

For a stamping speed of 30 mm/s and a normal load of 20 N, the surface morphologies of the aluminum alloy at different temperatures were observed with a laser scanning microscope vk-x100 (KEYENCE, Osaka, Japan), as shown in Figure 4. As shown in Figure 4a, the surface is relatively stable with a small number of scratches and many fine abrasive particles, so abrasive wear and furrow wear occurred. As seen in Figure 4b,c, the scratches on the surface became increasingly significant with the increase in test temperature, with Metals 2020, 10, 1189 7 of 16 scratch depth also increasing. The number of fine particles on the surface is seen to have decreased, indicating that the friction mechanism was mainly plow wear.

(a) (b)

(c) (d)

(e) (f)

Figure 4. Surface morphology at different temperatures. (a) T = 25 °C; (b) T = 100 °C; (c) T = 150 °C; Figure(d) 4.T =Surface 200 °C; ( morphologye) T = 250 °C; ( atf) diOriginalfferent sheet. temperatures. (a)T = 25 ◦C; (b)T = 100 ◦C; (c)T = 150 ◦C; (d)T = 200 ◦C; (e)T = 250 ◦C; (f) Original sheet. As seen in Figure 4e, there were many adhesion pits on the surface of the aluminum alloy, and 3.2. Effect of Sliding Speed on Friction and Wear Behavior of Materials the surface metal was peeled off and torn, resulting in serious adhesive wear. The original sheet, as

shownUnder in boundary Figure 4f, had lubrication fewer surface conditions scratches and on for it. a temperature set at 200 ◦C and a load of 20 N, the change trend curve over time of the interface friction coefficient between the 6111 aluminum alloy and H13 tool steel at different sliding speeds is shown in Figure5. It can be seen from Figure5a that the friction coefficient curves under different sliding speeds have a common feature: during the first few seconds, the friction coefficient increases rapidly to a larger value and then decreases slightly, and it then enters a relatively stable fluctuation stage. This is because, with the increase in the sliding speed, the micro-convex bodies on the contact surface of the aluminum alloy sheet and the tool cannot be embedded in the pits in time, which reduces the actual contact area, reducing the friction coefficient of the sheet metal. On the other hand, the increase in the sliding speed causes the shear speed of the bonding points on the contact surface to decrease, and the number of shear bond points per unit time increases. Therefore, the friction coefficient between the 6111 aluminum alloy and the H13 steel surface decreases with an increase in sliding speed. Metals 2020, 10, 1189 8 of 16 Materials 2020, x, x FOR PEER REVIEW 9 of 17

(a) (b)

(c)

Figure 5. The fluctuation curve of the friction coefficient according to sliding speed: (a) Variation Figure 5. The fluctuation curve of the friction coefficient according to sliding speed: (a) Variation curves of the friction coefficient over time under different speeds; (b) Experimental friction curves of the friction coefficient over time under different speeds; (b) Experimental friction coefficients coefficients under different speeds; (c) Inverse function fitting curve. under different speeds; (c) Inverse function fitting curve. Table 3. Friction coefficient measurement and function model prediction. The experimental results indicate that the effective friction coefficients of the material contact interface are 0.162,Speed 0.128, (mm/s) 0.105, 0.09 and 0.079 15 when the25 sliding speeds35 are 20, 30,55 40, 50 and70 60 mm/s, respectively. Theμ1 (measured curve of thevalue) friction coefficient 0.194 change0.145 with velocity0.112 is shown0.082 in Figure 50.073b. It can be seen from Figureμ2 (calculated5b that when value) the speed is less 0.188 than 40 mm0.143/ s, the friction0.116 coeffi0.084cient decreases 0.07 rapidly Error rate（%） 3.09 1.38 3.57 2.44 4.11 with the increase in sliding speed; when the speed is greater than 40 mm/s, the rate of decrease of the friction coefficient with the increase in sliding speed gradually slows down. The change trend Under the conditions of a temperature of 200 °C and a load of 20 N, the microstructures of the of thealuminum curve conforms alloy surface to the with characteristics different sliding of the speeds inverse of 20, function, 30, 50 and and 60 the mm/s expression were observed, is shown as in Equationshown (6): in Figure 6. It can be seen in Figure 6a that there was a small number of fine abrasive particles a on the surface, and there was a large numberµ = of deep+ scratches,c as well as obvious spalling marks, (6) + mainly furrow wear and adhesive wear. Figureν 6b showsb that the adhesion pits and scratches on the wheresurfaceµ is the of frictionthe 6111 coe aluminumfficient; valloyis the became sliding shallo speed;wer, and and a, the b andnumber c are of constants. scratches was reduced. TheFigure inverse 6c reveals function that curvethe scratches of the friction on the coesurfacefficient were under increased different in number, sliding and speeds a few was scratches fitted with the Originappear software. to have been As seendeepened, in Figure with5 ac, few the adhesion values of pits the on function the surface. constants As seen of in a, Figure b and 6d, c are when 5.586, 15.52the and speed 0.005, increased respectively. to 60 mm/s, The fitting the surface degree was of rela thetively function stable, curve the scratches is 0.9998, were so the reduced curve and can not better reflectas the deep, variable and the relationship adhesion pits between were flattened. the sliding In speedconclusion, and frictionwith an coeincreasefficient. in sliding The fitting speed, function the modelactual was builtcontact and area is between shown in6111 Equation and H13 (7). decreases, thus reducing the friction coefficient.

5.586 µ = + 0.005 (7) ν + 15.52

In order to verify the effectiveness of the friction coefficient model, five groups of sliding speeds, with speeds of 15, 25, 35, 55 and 70 mm/s, were selected for test measurements. The five groups of sliding speed values were correspondingly substituted into Equation (7). The actual measurement Metals 2020, 10, 1189 9 of 16 and function prediction calculation results are shown in Table3. By contrast, the error between the experimental value of the friction coefficient µ1 and the prediction result of the function model µ2 was less than 5%. The model can effectively describe the relationship between the friction coefficient and the sliding speed in the actual stamping process, which verifies the effectiveness of the friction model.

Table 3. Friction coefficient measurement and function model prediction.

Speed (mm/s) 15 25 35 55 70

µ1 (measured value) 0.194 0.145 0.112 0.082 0.073 µ2 (calculated value) 0.188 0.143 0.116 0.084 0.07 Error rate (%) 3.09 1.38 3.57 2.44 4.11

Under the conditions of a temperature of 200 ◦C and a load of 20 N, the microstructures of the aluminum alloy surface with different sliding speeds of 20, 30, 50 and 60 mm/s were observed, as shown in Figure6. It can be seen in Figure6a that there was a small number of fine abrasive particles on the surface, and there was a large number of deep scratches, as well as obvious spalling marks, mainly furrow wear and adhesive wear. Figure6b shows that the adhesion pits and scratches on the surface of the 6111 aluminum alloy became shallower, and the number of scratches was reduced. Figure6c reveals that the scratches on the surface were increased in number, and a few scratches appear to have been deepened, with a few adhesion pits on the surface. As seen in Figure6d, when the speed increased to 60 mm/s, the surface was relatively stable, the scratches were reduced and not as deep, and the adhesion pits were flattened. In conclusion, with an increase in sliding speed, the actual contact area between 6111 and H13 decreases, thus reducing the friction coefficient. Materials 2020, x, x FOR PEER REVIEW 10 of 17

(a) (b)

(c) (d)

Figure 6. Surface morphology under different sliding speeds. (a) 20 mm/s; (b) 30 mm/s; (c) 50 mm/s; Figure 6. Surface morphology under different sliding speeds. (a) 20 mm/s; (b) 30 mm/s; (c) 50 mm/s; (d) 60 mm/s (d) 60 mm/s. 3.3. Effect of Normal Load on the Friction Behavior of Materials During the actual stamping process, normal loading refers to normal pressure on the pressing plate, which affects friction and wear by changing the contact area and deformation state [18]. Under the boundary lubrication condition and for an initial temperature of 200 °C and a sliding speed of 30 mm/s, the curve of the friction coefficient varying with time under different normal loads is shown in Figure 7. As shown in Figure 7a, the friction coefficient first rises sharply and fluctuates greatly, and then, it decreases gradually. As time goes on, the curve fluctuates gently and reaches a relatively stable fluctuation state, which is the friction coefficient decreasing with the increase in load. The fitting curve of the friction coefficient with load is shown in Figure 7c. It can be seen from the figure that the friction coefficient decreases with the increase in load, and the change trend of the curve is consistent with the tribological principle [19]. According to Y.Z. Zhao et al. [12], the new variable friction coefficient model was established, as shown in Equation (8):

F a−1 μ = μ（ ） +b (8) 0 F o

where F is the normal load, F0 is the reference load, μ0 is the measured friction coefficient and α is the model index (F0 > 0, 0.5 ≤ α ≤ 1). Under the boundary lubrication condition, the load corresponding to a friction coefficient close to 0.15 was selected as the reference load, that is, F0 = 15, μ0 = 0.138. The test results were fitted with the Origin software, and the fitting curve is shown in Figure 7c.

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3.3. Effect of Normal Load on the Friction Behavior of Materials During the actual stamping process, normal loading refers to normal pressure on the pressing plate, which affects friction and wear by changing the contact area and deformation state [18]. Under the boundary lubrication condition and for an initial temperature of 200 ◦C and a sliding speed of 30 mm/s, the curve of the friction coefficient varying with time under different normal loads is shown in Figure7. As shown in Figure7a, the friction coe fficient first rises sharply and fluctuates greatly, and then, it decreases gradually. As time goes on, the curve fluctuates gently and reaches a relatively stable fluctuation state, which is the friction coefficient decreasing with the increase in load. Materials 2020, x, x FOR PEER REVIEW 11 of 17

(a) (b)

(c)

FigureFigure 7. Friction 7. Friction coeffi coefficientcient curve curve under under di ffdifferenterent loads: loads: ( a(a)) VariationVariation curves curves of of the the friction friction coefficient coefficient over timeover under time diunderfferent different loads; loads; (b) Experimental (b) Experimental friction friction coeffi coefficientscients at di atff erentdifferent loads; loads; (c) Fitting(c) Fitting curve curve of the friction coefficient with load of the friction coefficient with load. From the fitting, the values of a and b are 0.793 and 0.01, respectively, and the fitting degree is The fitting curve of the friction coefficient with load is shown in Figure7c. It can be seen from the 0.9858, which shows that the model can better reflect the variable relationship between the load and figure that the friction coefficient decreases with the increase in load, and the change trend of the curve friction coefficient. The functional relationship between the friction coefficient and interface load is is consistentas follows. with the tribological principle [19]. According to Y.Z. Zhao et al. [12], the new variable friction coefficient model was established, −0.207 as shown in Equation (8): μ =0.138（F/15） +0.01 (9)

F a 1 Five groups of normal loads of 15,µ 25,= 45,µ0 55( and) − 65+ N bwere selected in order to verify the validity (8) Fo of the model. The values of the five groups of normal loads were substituted into Equation (9). The where actualF is the measurement normal load, andF0 functionis the reference prediction load, calculationµ0 is the results measured are shown friction in Table coe ffi4. cient and α is the model index (F > 0, 0.5 α 1). 0 ≤ ≤ Under the boundaryTable lubrication 4. Friction coefficient condition, measurement the load correspondingand function model to prediction. a friction coefficient close to 0.15 was selected as the reference load, that is, F = 15, µ = 0.138. The test results were fitted with the Load (N) 150 0 25 45 55 65 Origin software, and the fitting curve is shown in Figure7c. μ1 (Measured value) 0.138 0.125 0.115 0.108 0.104 From theμ fitting,2 (Calculated the values value) of a and b are 0.148 0.793 and0.134 0.01, respectively,0.120 and0.115 the fitting0.112 degree is 0.9858, which showsError that rate the (%) model can better 7.25 reflect the8.57 variable relationship4.26 6.90 between the7.50 load and As seen from the data in Table 4, the error rates between the measured value and the predicted result are less than 9%. Thus, the friction coefficient model is verified for the effectiveness in the stamping process. For a temperature of 200 °C and a sliding speed of 30 mm/s, the surface micro-morphology of the 6111 aluminum alloy under different normal loads is shown in Figure 8.

Metals 2020, 10, 1189 11 of 16 friction coefficient. The functional relationship between the friction coefficient and interface load is as follows. 0.207 µ = 0.138(F/15)− + 0.01 (9) Five groups of normal loads of 15, 25, 45, 55 and 65 N were selected in order to verify the validity of the model. The values of the five groups of normal loads were substituted into Equation (9). The actual measurement and function prediction calculation results are shown in Table4.

Table 4. Friction coefficient measurement and function model prediction.

Load (N) 15 25 45 55 65

µ1 (Measured value) 0.138 0.125 0.115 0.108 0.104 µ2 (Calculated value) 0.148 0.134 0.120 0.115 0.112 Error rate (%) 7.25 8.57 4.26 6.90 7.50

As seen from the data in Table4, the error rates between the measured value and the predicted result are less than 9%. Thus, the friction coefficient model is verified for the effectiveness in the stamping process. For a temperature of 200 ◦C and a sliding speed of 30 mm/s, the surface micro-morphology of the 6111 aluminum alloy under different normal loads is shown in Figure8. Materials 2020, x, x FOR PEER REVIEW 12 of 17

(a) (b)

(c) (d)

Figure 8. Surface morphology under different normal loads: (a) 10 N; (b) 20 N; (c) 40 N; (d) 50 N. Figure 8. Surface morphology under different normal loads: (a) 10 N; (b) 20 N; (c) 40 N; (d) 50 N. As seen in Figure 8a, there were slight scratches on the surface, which were caused by relative Assliding seen between in Figure the8a, convex there werepeak on slight the scratchesH13 steel onsurface the surface,and the concave which werevalley caused on the by6111 relative slidingaluminum between alloy the convex surface, peak resulting on the in furrow H13 steel wear. surface From andFigure the 8b, concave it can be valley seen that on thethe 6111scratches aluminum alloy surface,on the surface resulting decreased in furrow slightly, wear. and From there Figure were8 sob,me it cansmall be adhesion seen that pits; the scratchesthat is, the on friction the surface decreasedmechanism slightly, was and mainly there abrasive were some wear small and adhesive adhesion wear. pits; It thatcan be is, seen the frictionin Figure mechanism 8c that the small was mainly abrasiveadhesion wear pits and became adhesive shallow, wear. and It local can befurrows seen were in Figure deepened.8c that Figure the small8d shows adhesion that when pits the became shallow,normal and localload was furrows increased were to deepened. 60 N, the scratch Figure on8d the shows surface that increased when the and normal deepened, load and was slight increased peeling marks can be observed. to 60 N, the scratch on the surface increased and deepened, and slight peeling marks can be observed. 4. Discussion In order to test the effectiveness of the variable friction model in predicting the numerical simulation of sheet metal stamping, the friction model was inputted into the DYNAFORM 5.9 software (ETA CO, US) to simulate the thickness distribution within the U-bending part. For the spring-back analysis of hot stamping, “6*MAT_THERMAL_ISOTROPIC_TD_LC” was selected as the material models of the sheet and tooling, which are non-isothermal analysis models. Under the warm forming condition, the stamping speed is 2000 mm/s, the blank holder force is 30 KN and the friction coefficient is 0.12. The friction coefficient is set as the variable friction coefficient model and input with the table. The friction of the blank holder is controlled by the variable friction coefficient model for the load, and the friction between the sheet metal and the punch and die is controlled by a variable friction coefficient model for the speed. The simulation process parameters are shown in Table 5.

Metals 2020, 10, 1189 12 of 16

4. Discussion In order to test the effectiveness of the variable friction model in predicting the numerical simulation of sheet metal stamping, the friction model was inputted into the DYNAFORM 5.9 software (ETA CO, US) to simulate the thickness distribution within the U-bending part. For the spring-back analysis of hot stamping, “6*MAT_THERMAL_ISOTROPIC_TD_LC” was selected as the material models of the sheet and tooling, which are non-isothermal analysis models. Under the warm forming condition, the stamping speed is 2000 mm/s, the blank holder force is 30 KN and the friction coefficient is 0.12. The friction coefficient is set as the variable friction coefficient model and input with the table. The friction of the blank holder is controlled by the variable friction coefficient model for the load, and the friction between the sheet metal and the punch and die is controlled by a variable friction coefficient model for the speed. The simulation process parameters are shown in Table5. Materials 2020, x, x FOR PEER REVIEW 13 of 17 Table 5. The simulation process parameters. Table 5.The simulation process parameters. Sheet Holding Time Tooling Transfer Time Blank Holder Force Punch Pressure Punch Speed Temperature (◦C) (min) Temperature (◦C) (s) (KN)Blank (MPa) (mm/s) Sheet Holding Tooling Transfer Punch Punch 200 4 60 3 30Holder 3.0 20 Temperature Time Temperature Time Pressure Speed Force (°C) (min) (°C) (s) (MPa) (mm/s) (KN) The lubrication state of the material can be divided into fluid lubrication (µ 0.03), 200 4 60 3 30 3.0 20 ≤ mixed lubrication (0.03 < µ 0.1), boundary lubrication (0.1 < µ < 0.3) and dry friction (µ > 0.3). ≤ During the actualThe lubrication stamping state process, of the mostmaterial of ca then be sheet divided metal into interfacefluid lubrication friction (μ states≤ 0.03), aremixed boundary lubricationlubrication and mixed (0.03 lubrication.< μ ≤ 0.1), boundary Two groupslubrication of (0.1 constant < μ < 0.3) friction and dry coefrictionfficient (μ > 0.3). and During variable the friction actual stamping process, most of the sheet metal interface friction states are boundary lubrication and coefficient modified friction models with a friction coefficient of 0.08 (mixed lubrication) and an mixed lubrication. Two groups of constant friction coefficient and variable friction coefficient optimal combinationmodified friction friction models coe ffiwithcient a friction of 0.12 coeffici (boundaryent of lubrication)0.08 (mixed lubrication) were selected and foran optimal the simulation. The simulatedcombination thickness friction distributions coefficient of of0.12 the (boundary U-bend lubrication) under diff wereerent selected friction for coethe ffisimulation.cients are The shown in Figure9. simulated thickness distributions of the U-bend under different friction coefficients are shown in Figure 9.

(a) (b)

(c)

Figure 9. Thickness distributions under different friction coefficients of U-bending. (a) μ = 0.08; (b) μ Figure 9. Thickness distributions under different friction coefficients of U-bending. (a) µ = 0.08; = 0.12; (c) Variable friction coefficient model. (b) µ = 0.12; (c) Variable friction coefficient model. The warming stamping U-bend part test device was developed by the metal forming Laboratory of Jiangsu University, and it includes a temperature detection and control system, induction-heating furnace, U-shaped hot stamping die with water cooling, a hydraulic press, etc., as shown in Figure 10b. The hot stamping temperature control system collects the sheet temperature and die surface temperature warming by using an infrared thermometer, and it controls the temperature by water cooling. After being heated by the induction furnace, it is quickly sent to the hot die.

Metals 2020, 10, 1189 13 of 16

The warming stamping U-bend part test device was developed by the metal forming Laboratory of Jiangsu University, and it includes a temperature detection and control system, induction-heating furnace, U-shaped hot stamping die with water cooling, a hydraulic press, etc., as shown in Figure 10b. The hot stamping temperature control system collects the sheet temperature and die surface temperature warming by using an infrared thermometer, and it controls the temperature by water cooling. After being heated by the induction furnace, it is quickly sent to the hot die. MaterialsMaterials 2020 2020, x,, x,x xFOR FOR PEER PEER REVIEW REVIEW 14 14of of17 17

(a) (b) (a) (b) Figure 10. Friction coefficient curve under different loads and function fitting curve: (a) Warming Figure 10.10. Friction coefficient coefficient curve under didifferentfferent loads and functionfunction fittingfitting curve:curve: (a) Warming stamping mold; (b) Test device for hot stamping U-shaped parts. stamping mold; ((b)) TestTest devicedevice forfor hothot stampingstamping U-shapedU-shaped parts.parts. The working parts of the die are made of H13 steel and do not need to be heated, and there is a The working parts of the die are made of H13 steel and do not need to be heated, and there is coolingThe workingwater pipe parts for ofcooling, the die as are shown made in of Figure H13 steel 10a. and The do speed not needand load to be of heated, die movement and there are is a a cooling water pipe for cooling, as shown in Figure 10a. The speed and load of die movement are coolingcontrolled water by pipea hydraulic for cooling, press. as shown in Figure 10a. The speed and load of die movement are controlled by a hydraulic press. controlledThe byU-bend a hydraulic warming press. stamping parts are shown in Figure 11a. The thickness of the actual The U-bend warming stamping parts are shown in Figure 11a. The thickness of the actual stampingThe U-bend parts was warming measured stamping through parts the use are of shown an ultrasonic in Figure thickness 11a. The gauge, thickness and the of measuring the actual stamping parts was measured through the use of an ultrasonic thickness gauge, and the measuring stampingpoint was parts the wassymmetrical measured center through of the the sheet use widthof an ultrasonicdirection. Thethickness distribution gauge, curves and the of measuringthe plate point was the symmetrical center of the sheet width direction. The distribution curves of the plate pointthickness was the were symmetrical determined center by the of constant the sheet friction width coefficient, direction. and The the distribution modified friction curves model of the andplate thicknesstest are shown were determined in Figure 11b. by the constant friction coecoefficient,fficient, and the modifiedmodified frictionfriction model and test are shownshown inin FigureFigure 1111b.b.

(a) (a)

(b) (c)

Figure 11. Friction( bcoefficient) curve under different loads and function(c) fitting curve: (a) Warming stamping mold; (b) U-bend parts and thickness measurement; (c) Thickness distribution curve under Figuredifferent 11.11. conditions.Friction coefficient coe fficient curve under didifferentfferent loads and functionfunction fittingfitting curve:curve: (a) Warming stamping mold; (b) U-bend parts and thickness measurement; (c)) Thickness distribution curve under didifferentUnderfferent conditions.conditions.an optimal combination of process parameters, when the stamping speed is 20 mm/s and the blank holder force is 30 KN, a constant friction coefficient of 0.12 and the variable friction coefficientUnder anmodel optimal were combination selected to ofsimulate process the parameters, warm forming when theof 6111 stamping aluminum speed alloy, is 20 mmmm/sand/ sthe and thespring-back blankblank holder holder of forcethe force U-bend is 30is KN,30 was KN, a measured constant a constant friction (diagrams friction coeffi ofcient coefficientthe spring-back of 0.12 andof 0.12 theangle variableand and the measurement friction variable coe frictionffi arecient modelcoefficientshown were in modelFigure selected 12).wereto In selected simulateorder to toreduce the simulate warm the meas forming the urementwarm of 6111forming error aluminum of ofthe 6111 actual alloy,aluminum spring-back and the alloy, spring-backangle, and the the ofspring-backaverage the U-bend values of wasthe of U-bendfive measured actual was U-bend (diagrams measured parts of(diagrams were the spring-backtaken of as the the spring-back effective angle and spring-back measurementangle and angle. measurement are According shown are in shownto the in above Figure measurements 12). In order and to reduce calculation the measurement formula of the error variable of the friction actual coefficient spring-back model, angle, the the averagespring-back values angles of five Δθ actual1 and U-bendΔθ2 of the parts actual were stamping taken as parts the effectivewere 6.5° spring-back and −6.8°. The angle. spring-back According toangle the above reflects measurements the spring-back and deformation calculation degree formula of parts of theafter variable warm forming. friction In coefficient the post-processing model, the spring-backof DYNAFORM, angles the Δθ spring-back1 and Δθ2 of angle the actual under stamping the consta partsnt friction were coefficient 6.5° and −and6.8°. variable The spring-back friction anglecoefficient reflects model the spring-back were measured. deformation The meas degreeurement of parts results after are warmshown forming. in Table In5. the post-processing of DYNAFORM,As seen in theTable spring-back 6 the predicted angle values under ofthe the consta spring-backnt friction angles coefficient Δθ1 and and Δθ 2variable based on friction the coefficientconstant frictionmodel werecoefficient measured. model The were measurement 5.2° and 5.9°, results and the are errors shown between in Table the 5. predicted value of

As seen in Table 6 the predicted values of the spring-back angles Δθ1 and Δθ2 based on the constant friction coefficient model were 5.2° and 5.9°, and the errors between the predicted value of

Materials 2020, x, x FOR PEER REVIEW 15 of 17

the spring-back angle and the actual stamping measurement were 20% and 9.2%. The predicted values of the spring-back angles Δθ1 and Δθ2 were −5.4° and −6.2°, respectively, obtained by using the modified friction model, and the errors between them and the actual stamping measurement were 9.2% and 8.8%.

Metals 2020, 10, 1189 14 of 16 Table 6.Comparison of simulation and test results of the rebound angle.

Spring-Back/(°) Δθ1 Δθ2 Figure 12). In orderTrue to reduce value the measurement error of 6.5 the actual spring-back angle,−6.8 the average values of fiveConstant actual friction U-bend coefficient parts were taken as the 5.2 e(Error,ffective 20%) spring-back angle.−5.4 (Error, According 20.6%) to the above measurementsVariable friction and calculation coefficient formula of the 5.9 variable (Error friction, 9.2%) coefficient− model,6.2 (Error the, spring-back8.8%) angles ∆θ and ∆θ of the actual stamping parts were 6.5 and 6.8 . The spring-back angle reflects the 1 2 ◦ − ◦ spring-backTherefore, deformation the prediction degree ofof partsthe spring-back after warm angle forming. of sheet In themetal post-processing forming based of on DYNAFORM, the modified the spring-backfriction model angle is closer under to the the actual constant measurement friction coe resultsfficient and and can variable reflect frictionthe characteristics coefficient of model sheet weremetal measured. forming The more measurement realistically. results are shown in Table5.

(a) (b)

FigureFigure 12. Friction12. Friction coe coefficientfficient curve curve under under di ffdifferenterent loads loads and and function function fitting fitting curve: curve: ((aa)) Spring-backSpring-back angle;angle; (b) True(b) True spring-back spring-back measurement. measurement.

5.As Conclusions seen in Table 6 the predicted values of the spring-back angles ∆θ1 and ∆θ2 based on the constant friction coefficient model were 5.2 and 5.9 , and the errors between the predicted value The effects of temperature, sliding speed◦ and◦ normal load on friction and wear properties of theresulting spring-back from friction angle andbetween the actual 6111 aluminum stamping al measurementloy and H13 die were steel 20% were and studied 9.2%. by The using predicted a CFT- values of the spring-back angles ∆θ and ∆θ were 5.4 and 6.2 , respectively, obtained by using the I friction tester under different 1lubrication2 cond−itions.◦ The− surface◦ morphology of 6111 aluminum modifiedalloy under friction different model, andexperimental the errors conditions between them was andobserved the actual and stampinganalyzed measurementthrough an optical were 9.2%microscope, and 8.8%. and the influence mechanism of different experimental parameters affecting the friction coefficient was studied from a microscopic perspective. From the analysis of experimental data, Table 6. Comparison of simulation and test results of the rebound angle. variable friction coefficient models based on sliding speed and normal load were established using

the Origin software,Spring-Back and the validity/(◦) of the models∆ θwas1 verified. The conclusions∆θ2 are as follows. (1) Under differentTrue process value parameters, the fricti 6.5 on coefficient shows6.8 rapid fluctuation during − the running-inConstant stage and friction relati coevelyfficient stable levels 5.2 (Error,of fluctuation 20%) in the5.4 later (Error, stage. 20.6%) With the increase in − time, the frictionVariable coefficient friction first coeffi increasescient sharply 5.9 (Error, and 9.2%) then decreases6.2 to (Error, a relatively 8.8%) stable state. − (2) The friction coefficient increases with the increase in temperature, from room temperature to 200 °C, and the increasing trend of the friction coefficient becomes slower with the continuous Therefore, the prediction of the spring-back angle of sheet metal forming based on the modified increase in temperature; the influence of different temperatures on the hardness of 6111 aluminum friction model is closer to the actual measurement results and can reflect the characteristics of sheet alloy is not pronounced. According to the observation of surface micro-morphology, the number of metal forming more realistically. furrows on the 6111 aluminum alloy surface is limited, the surface is relatively smooth and the surface 5. Conclusionsmorphology is better at 200 °C. (3) When the temperature is 200 °C and the load is 20 N, the friction coefficient decreases with theThe increase effects ofin temperature, sliding speed. sliding According speed andto the normal analysis, load the on frictionvariable and friction wear coefficient properties model resulting of fromsliding friction speed between was 6111established, aluminum and alloythe errors and H13of the die model steel wereare all studied less than by using8%, which a CFT-I shows friction the testereffectiveness under different of the lubrication variable friction conditions. coefficient The model. surface morphology of 6111 aluminum alloy under different(4) experimental The results conditionsshow that the was friction observed coefficient and analyzed decreases through with the an increase optical in microscope, normal load and when the influencethe temperature mechanism is of 200 di ff°Cerent and experimental the sliding speed parameters is 30 mm/s. affecting When the the friction interface coe ffiloadcient is wasgreater studied than from a microscopic perspective. From the analysis of experimental data, variable friction coefficient models based on sliding speed and normal load were established using the Origin software, and the validity of the models was verified. The conclusions are as follows. (1) Under different process parameters, the friction coefficient shows rapid fluctuation during the running-in stage and relatively stable levels of fluctuation in the later stage. With the increase in time, the friction coefficient first increases sharply and then decreases to a relatively stable state. Metals 2020, 10, 1189 15 of 16

(2) The friction coefficient increases with the increase in temperature, from room temperature to 200 ◦C, and the increasing trend of the friction coefficient becomes slower with the continuous increase in temperature; the influence of different temperatures on the hardness of 6111 aluminum alloy is not pronounced. According to the observation of surface micro-morphology, the number of furrows on the 6111 aluminum alloy surface is limited, the surface is relatively smooth and the surface morphology is better at 200 ◦C. (3) When the temperature is 200 ◦C and the load is 20 N, the friction coefficient decreases with the increase in sliding speed. According to the analysis, the variable friction coefficient model of sliding speed was established, and the errors of the model are all less than 8%, which shows the effectiveness of the variable friction coefficient model. (4) The results show that the friction coefficient decreases with the increase in normal load when the temperature is 200 ◦C and the sliding speed is 30 mm/s. When the interface load is greater than 30 N, the decreasing trend gradually slows down. The morphology observations show that with the increase in the interface load, the scratches on the surface of 6111 aluminum alloy clearly increase and deepen, and surface exfoliation occurs. (5) With the optimal process parameters, a stamping simulation of typical U-bending parts was carried out by using finite element analysis software and the variable friction coefficient model established by the friction test. The thickness distribution simulation results under a mixed lubrication state and a boundary lubrication state were compared with the actual U-shaped stamping parts. Through the comparison of thickness distribution, it is observed that the simulation results for the thickness distribution based on the variable friction coefficient model are close to the actual measured values. Through the analysis of the spring-back angle, it is also observed that the spring-back measurement error of the variable friction coefficient model is smaller than that of the constant friction coefficient model. Therefore, the variable friction coefficient model improves simulation accuracy.

Author Contributions: Conceptualization, S.D.; methodology, S.D.; software, S.D.; advisors, X.W. and L.W.; formal analysis, J.X.; data curation, S.D.; writing—original draft preparation, S.D.; writing—review and editing, S.D.; funding acquisition, J.X. All authors have read and agreed to the published version of the manuscript. Funding: National Natural Science Foundation of China: 51505408. Conflicts of Interest: The authors declare no conflict of interest.

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