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Rare Metals2019年第10期

Deep drawing of 6A16 aluminum alloy for automobile body with various blank-holder forces

Zhao-Yang Liu Bai-Qing Xiong Xi-Wu Li Li-Zhen Yan Zhi-Hui Li Yong-An Zhang Hong-Wei Liu

State Key Laboratory of Nonferrous Metals and Processes,General Research Institute for Nonferrous Metals

作者简介：*Bai-Qing Xiong e-mail:xiongbq@grinm.com;

收稿日期：12 November 2017

基金：financially supported by the National Key Research and Development Program of China (No. 2016YFB0300805);

Deep drawing of 6A16 aluminum alloy for automobile body with various blank-holder forces

Zhao-Yang Liu Bai-Qing Xiong Xi-Wu Li Li-Zhen Yan Zhi-Hui Li Yong-An Zhang Hong-Wei Liu

State Key Laboratory of Nonferrous Metals and Processes,General Research Institute for Nonferrous Metals

Abstract：

Based on the ABAQUS/explicit finite element method,the deep drawing of 6A16 alloy pre-aged and then storaged at room temperature for 1 week with various blank-holder forces(10,14,18 kN) was studied.The distribution and variation of stress and strain in deformation zones were investigated to drive the forming property and process of the alloy.Besides,the simulation result was verified combined with the deep drawing experiments.The results show that the stress and strain of the deformation zone have an incremental trend with the blank-holder force increasing while the deformation degree and grain size within a certain deformation zone have an obvious increase and an enlargement,respectively.After the deep drawing,the hardness of products also increases with the enhancement of blank-holder force.The blank-holder force of 18 kN is certified as the preferential one by the analysis of microstructure and simulation results.

Keyword：

ABAQUS; 6A16 aluminum alloy; Deep drawing test; Blank-holder force;

Received： 12 November 2017

1 Introduction

Owing to the less formability of aluminum alloys than that of steels,insufficient formability is a major problem in the manufacture of complex parts,particularly in the automotive industry.With the energy,environment and security issues become increasingly serious,and lightweight car design has become the mainstream of the development of modern automobile industry trends.Aluminum sheet is becoming mounting widespread as an automotive body panel material [ 1, 2, 3] .The heat-treatable aluminum alloys of the 6xxx series are widely used as an outer panel material,due to their ability of precipitation hardening during the paint-bake cycle,resulting in dent resistance improving [ 4, 5, 6] .Increasing the formability of these alloys would allow for designing panel parts with more complex geometry as a single piece,thereby avoiding the costs associated with any subsequent joining operations.The improvement of the deep drawing ability of aluminum alloy sheet is the forefront of the current international academic circles [ 7, 8, 9] .Blank-holder force (BHF) is not only the most easily controlled factor but also critical in the deep drawing procedure [ 10, 11] .6A16 aluminum alloy [ 12] studied and developed by our group has good formability and fast aging response,which has good capacity to be used as an outer panel material.The deep drawing process of 6A16 aluminum is unresearched.

In this work,the effect of BHF on sheet forming performance of 6A16 aluminum alloys was studied by finite element numerical simulation technology.

2 Experimental

According to the national standard GB/T 15825.3-2008,the deep drawing test of the cylindrical sample is shown in Fig.1.It includes punch,die,blank holder and blank.The dimensions are as follows:the radius of the punch fillet is5 mm,the radius of the die fillet is 10 mm,the diameter of the die hole is 50 mm [ 13] ,the clearance between the punch and the die is 1.82 mm,the thickness of the blank is1 mm,the friction factor is 0.1,and the thickness of the blank is 1 mm [ 14] and the diameter of the blank is100 mm [ 15] .The common sheet forming test machine(BCS-30D) was used as the experimental equipment,which was researched and developed by Beihang University independently.

Fig.1 Schematic of deep drawing test tooling (mm)

The material used in this test was 6A16 aluminum alloy,and the composition is listed in Table 1.The sheets were solution treated at 560℃for 0.5 h,followed by wind cooling [ 16] .Then,the as-quenched samples were immediately pre-aged at 140℃for 5 min and then naturally aged for 1 week (PA140,5 min+NA1W) [ 17, 18] .The mechanical properties of the 6A16 aluminum alloy sheets under aforementioned heat treatments were obtained by tensile test in different directions (Table 2).In Table 2,R_mis tensile strength,R_p0.2 is yield strength of the national standard,A is elongation,A₅₀ is elongation at a gauge length of 50 mm,N_10%＿20%is tensile strain hardening index at a deformation of 10%-20%and R_10%is Lankford value at deformation of 10%.The cups were paint baked at170℃for 0.5 h (PB170,30 min) after deep drawing test.

It is known that the deep drawability of alloys is greatly influenced by the r value which is defined as the ratio of the transverse strain to the normal strain [ 17] .It has been experimentally proven that there is a positive correlation between r value and the limit drawing ratio value.The average r andΔr (planar anisotropy index) values are expressed by:

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Table 1 Alloy composition of 6A16 aluminum alloy (wt%)

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Table 2 Room-temperature properties of 6A16 aluminum alloy-T4p

where r₀,r₄₅ andr₉₀ are the values in 0°,45°and 90°directions with respect to the rolling direction.According to the formula,r is about 0.66 andΔr is about 0.165.Generally speaking,high average r and lowΔr values are beneficial to deep drawing,which mainly depends on crystallographic texture [ 19] .AsΔr value is positive,four earings will be expected to appear in perpendicular to the rolling direction and along the rolling direction of the final deformed cup [ 20] .

The maximum BHF at the time of drawing the cup can be calculated as follows [ 21] :

where F_max is the maximum BHF,R_m is the tensile strength,D is the diameter of the blank,p is unit BHF,z is the reciprocal of the drawing coefficient,rd is radius of the die fillet and t is the thickness of the blank.According to the formula,F_max is about 20 kN.So the appropriate range of BHF was determined (10,14 and 18 kN).

3 Results and discussion

3.1 Simulation

The numerical analyses performed in this study were carried out using ABAQUS/explicit.Different strain paths were generated by varying the width of the modeled sheet.The following is a detailed description of the numerical modeling approach and the criterion used for determining the forming limit for each strain path [ 22, 23] .A solid model of the deep drawing testing tool set was modeled.The tooling set punch,die and blank-holder,were considered to be made of a rigid material and represented by rigid shells.The tooling was assumed to be made of steel.The tested sheet was modeled as an elastoplastic material and was meshed using C3D8R elements (an 8-node linear brick,reduced integration,hourglass control),as shown in Fig.2 [ 24] .

The contact between the sheet and the tooling was modeled using the master-slave contact approach with surface-to-surface contact option (one-way surface-to-surface contact).The sheet surfaces are considered to be the slave surfaces while the non-deforming rigid surfaces of the tooling (punch,blank holder and die) are considered to be master surfaces.The coefficients of friction used in the study were determined by an experimental load-displacement curve which is matched to the experimentally determined curve.A friction coefficient of 0.08 was used in the contact model between all three pairs of interacting surfaces,i.e.,blank-punch,blank-die and blank-blank holder [ 25] .

The strain and stress changes are shown in Figs.3 and 4,showing that circumferential stresses and max strains are different by the three different BHF.The horizontal axis represents the process of the simulation.There are about a thousand incremental steps in this process.Within the progress of the simulation,either stress or strain shows an upward trend obviously.The maximum relative error of the circumferential stress is 8.2%,and that of the equivalent strain is greater,which is 40.1%.The circumferential stress and the equivalent strain are essential to feed back the simulation of BHF when the energy method was used.And the change of negative strain is bigger than that of the positive strain.Particularly,as BHF increases,the fluctuation of stress curve becomes relatively smaller,and the overall trend increases.As BHF increases,the maximum strain value decreases in the first half of the stroke and increases in the second half.At the same time,as BHF increases,the curve becomes smoother.This shows that a larger BHF can ensure a more stable forming process.

Fig.3 Circumferential of stress by different BHFs

Fig.4 Equivalent strain by different BHFs

3.2 Earing and thickness

As shown in Fig.5,6A16 aluminum alloy can be deformed well in the experimental parameters and conditions.It can be intuitively found with the increase in BHF,the height of the cups has a small increase.There are four obvious earings in perpendicular to the rolling direction and along the rolling direction of every cup.The earing height data for every position of the cups under different BHFs are plotted in Fig.6.The specific average earing height and the earing rate were measured and calculated,as shown in Table 3.

Fig.2 Assembly diagram a and mesh of simulation b

Fig.5 Drawn cups by different BHFs

Figure 6 and Table 3 show that with the increase in the BHF,both the sidewall height and earing rate have significant increase.It is concluded that under experimental condition,the cup under bigger BHF has a faster radial growth.

The thickness of different positions of the cup is plotted in Fig.7.It can be seen that with the increase in BHF,the maximum thinning rate increases obviously but the maximum thickness increases slowly.It means that when BHF is smaller,the resistance at flange is smaller.So,the flow rate of the material is fast,resulting in incomplete material drawing and partial thickness.The deep drawing process makes the 100-mm-diameter original blanks become the cups whose final diameter is 50 mm.In the flange part,the blank has a large amount of shrinkage thickening [ 26] .The increment of BHF increases the frictional resistance of the blank toward the die fillet.It becomes difficult to fill the portion of the cylindrical wall by making the blank flow into the die,so that the maximum reduction ratio of the material increases.Meanwhile,the shrinkage thickening of the blank part of the flange is the same,but the increment of the BHF suppresses the radial flow [ 27] .As a result,the thickening rate also increases.As the BHF becomes larger,the resistance at the flange increases,leading to the fact that the thickness of straight wall becomes thinner and the material gets more deformation.Owing to the large deformation of the top and fillet,it can be speculated that the two parts of the grain are going to change more obviously.

Fig.6 Sidewall height of cups after deep drawing

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Table 3 Earing data at different BHF

Fig.7 Thickness in different positions of drawn cups by different BHFs

3.3 Microstructure

Significant recrystallization occurs during the deformation process.Variation in BHF is shown to have much qualitative effect on the recrystallized grain structure.Grain structure of top part of cups under different BHFs is shown in Fig.8.It shows that the smaller the BHF is,the more the recrystallized grain occurs.The details of the variation of grain size are shown in Table 4.With the increment in BHF,the grains in the top are significantly reduced,and the degree of recrystallization is higher.Owing to material shrinkage thickening,there is a growing trend of grain size in the top [ 28] .With the increase in BHF,there is more deformation in the top.It turns out the deformation storage capacity is higher,and the driving force of recrystallization is larger.Finally,the speed of recrystallization is faster [ 29] .The average grain size is 27.5μm,which is smaller than that of the test plate,resulting in grain refinement.This owes to that the grain boundary between adjacent grains is the main nucleation point of recrystallization.When the deformation is larger,there is more nucleation point.As the amount of deformation reaches a certain degree,this nucleation rate is greater than its growth rate,so it can be gotten the microstructure of smaller grain size than before.In the case of drawn cups by 10 and 14 kN BHF,the deformation is not big enough,so the deformation storage energy is low,the recrystallization driving force is small,and the recrystallization phenomenon is not obvious.At this point,the dominant trend of the grain is in growth [ 30] .

Fig.8 EBSD maps of top part of cups by a 10 kN,b 14 kN,c 18 kN BHF and d T4P

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Table 4 Details of grain under different BHFs

The grains are clearly elongated in the direction of deformation,as shown in Fig.9.But the variety of grains by different BHFs is little.The grain misorientation is different by different BHFs.The proportion of recrystallization increases as BHF increases.

3.4 Hardness

The curve of hardness is shown in Fig.10.In the process of deep drawing,a large number of dislocations and twins are produced in the deformed structure,resulting in deformation strengthening and reduced plasticity,especially in the areas with large deformation,and the strengthening effect is more remarkable.The top of the cups has a larger deformation.So,the top of its maximum thickening of the place has the highest strength.The top of cup has the highest hardness compared with other zones due to its largest deformation.As BHF increases,the thickening rate of the top of the cups increases.At the same time,the deformation becomes greater,so its relative hardness also increases.

3.5 Comparison of experiment and simulation

The comparison of the support and reaction force of the punch in the experiment and simulation with different BHFs is shown in Fig.11,as compared in the experimental process.It is found that the support and reaction force within the progress of the experiment increase first and then reduce.The maximum reaction force of experiment occurs at about 20 mm (half of the experimental process)in the stroke,while 17 mm (42%of the process) in simulation.Through the comparison of different blank-holder forces,it is found that with the increase in BHF,the maximum supporting force is increasing.The maximum reaction forces with a BHF of 10 kN in experimental and simulated processes are 29.1 and 30 kN,30.3 and 31.9 kN with a BHF of 14 kN,and 33.1 and 32.8 kN with a BHF of18 kN.The magnitude of the reaction force reflects the difficulty of the deformation of the material during the experiment.Generally,the greater reaction force can lead to greater stress and strain during the deep drawing test,and also increase the degree of deformation.This is consistent with the previous data.It can be concluded that ABAQUS/explicit simulation of the 6A16 aluminum alloys for deep drawing deformation has a relative reduction in the experiment.

Fig.9 EBSD maps of fillet part of cups by a,c 10 kN and b,d 18 kN BHF

Fig.10 Hardness in different positions at different BHFs

4 Conclusion

The effect of BHF on 6A16 aluminum alloy sheet forming performance was investigated.The major conclusions are summarized as follows.

With the increase in BHF,the stress,strain and deformation degree in deformation zone increase,while the grain size in deformation zone decreases.The integral hardness of the deformed cup increases with BHF increasing.The top part of the cups has the biggest hardness.ABAQUS/explicit simulation of the 6A16 aluminum alloys for the deep drawing deformation has a relative reduction in the experiment.The combination of simulation results and microstructure analysis of the 6A16aluminum alloy plate after drawing forming shows that the appropriate BHF is 18 kN.