Deep drawing of 6A16 aluminum alloy for automobile body with various blank-holder forces
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
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
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
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
Table 1 Alloy composition of 6A16 aluminum alloy (wt%)
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
The maximum BHF at the time of drawing the cup can be calculated as follows
where Fmax is the maximum BHF,Rm 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,Fmax 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
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
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
Fig.6 Sidewall height of cups after deep drawing
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
Fig.8 EBSD maps of top part of cups by a 10 kN,b 14 kN,c 18 kN BHF and d T4P
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.
Fig.11 Reaction forces of experiment and simulation by a 10 kN,b 14 kN and c 18 kN BHF
参考文献
[17] Davies G.Materials for Automobile Bodies.2nd ed.Oxford:Butterworth-Heinemann;2004.1.