Deep-drawing of magnesium alloy sheets at warm temperatures
Journal of Materials Processing Technology185(2007)
147–151
Deep-drawing of magnesium alloy sheets at warm temperatures S.H.Zhang a,?,K.Zhang a,Y.C.Xu a,Z.T.Wang a,Y.Xu a,Z.G.Wang b
a Institute of Metal Research,Chinese Academy of Sciences,72Wenhua Road,Shenyang110016,China
b Department of Mechanical Engineering,Gifu University,Japan
Abstract
In magnesium alloy sheet products have been attracting more and more attention in recent years because of their application potentials as coverings of portable electrical devices and automotive panels.However,magnesium alloy sheets are usually formed at temperatures between 250and400?C because of their poor plasticity at room temperature,which makes the tooling system quite complex and also the products cost expensive.
A proper forming temperature range was determined.The effects of blank holding forces on the workpiece quality were analyzed by warm deep drawing of cups from magnesium alloy sheets.Therefore,appropriate process parameters were selected to avoid forming defects effectively.In the paper,a rigid blank holder was used to adjust blank holding forces.A special liquid lubricant PTFE was used on the tool surfaces.The conditions of process defects as?ange wrinkling and ruptures were analyzed.Experiments were carried out to verify the computer simulation results.Efforts were made to optimize process parameters by analyzing the causes of defects in order to improve the Limit Drawing Ratio of magnesium alloy https://www.sodocs.net/doc/954144151.html,puter simulation with explicit?nite element method was used to optimize the process parameters before carrying out the actual experiments.It is found that rolled magnesium alloy sheets have good deep drawing formability at a forming temperature range of105–170?C with the limit drawing ratio up to2.6.It is also necessary to control the time of heating blanks.The formability will be reduced severely by excessive heating duration.
?2006Published by Elsevier B.V.
Keywords:Magnesium alloy;Warm deep drawing;Forming temperature;Liquid lubricant;Limit drawing ratio
1.Introduction
Magnesium alloys are increasingly becoming the ideal mate-rials for modern industrial products with the characteristics of light weight and recycling.Because of lower density,better col-lision safety property and electromagnetic interference shielding capability,magnesium alloys are available for producing some structural parts such as the coverings of mobile telephones,note-book computers and portable mini-disks(MD).In the past,the demand for this alloy as a structural material was not high because of its less availability commercially as well as lim-ited manufacturing methods.Recently,die casting of magnesium alloys has been the prevailing method for manufacturing parts in the automotive industry.However,this process is not ideal in producing thin-walled Mg structures because of excessive amount of waste materials and casting defects.So sheet metal forming processes(such as thermal deep-drawing process[1], isothermal gas forming[2])have been developed to manufacture ?Corresponding author.
E-mail address:shzangs@https://www.sodocs.net/doc/954144151.html,(S.H.Zhang).thin-walled parts with good mechanical property and surface quality to avoid the defects above.
Deep-drawing process is an important and popular process in assessment of formability of sheet metal.Magnesium possesses poor formability at room temperature because of its hexagonal closed packed structure.It is necessary to enhance the forming temperature in order to improve formability of magnesium alloys effectively[3].Usually,magnesium alloys sheets are formed at temperatures between250and400?C,which makes the tool-ing system quite complex and also the products cost expensive. Sheet metal forming processes of magnesium alloys,which need to be improved continually,are developed and industrialized in Japan,Germany,Taiwan and Singapore[4–6].
Finite element method(FEM)is a very effective method to simulate the forming processes with accurate prediction of the deformation behaviors.FEM can be used not only in the analysis but also in the design to estimate the optimum conditions of the forming processes.This can be done before carrying out the actual experiments for an economical and successful application of SPF to industrial components[5].
The research group has studied sheet metal forming processes of magnesium alloys in recent years[7–10].In this paper,warm
0924-0136/$–see front matter?2006Published by Elsevier B.V. doi:10.1016/j.jmatprotec.2006.03.150
148S.H.Zhang et al./Journal of Materials Processing Technology 185(2007)147–151
deep drawing experiments of rolled magnesium alloy sheets were utilized to ?nd the proper forming temperature range as low as possible and to discuss the effects of major factors on the workpiece quality.At the same time,a ?nite element analysis was performed to simulate technological defects of blanks.2.Experiments and numerical simulation
The warm deep drawing experiments of magnesium alloy sheets were carried out using a 100tonnes four-post multifunc-tional hydraulic press.A rigid blank holder was used to adjust blank holding forces to avoid wrinkling of blanks.External heating method and internal heating method may be conducted to raise the temperature of blanks.When the external heating method is adopted,the blank,which ?rst is heated to the form-ing temperature in an oven,is formed subsequently in the heated tool.When the internal heating method is adopted,the blank is heated and formed inside the tool which is preheated to a certain temperature.In this paper,the internal heating method was selected,and electric heating elements were uniformly dis-tributed in the die.The temperatures of tools and blanks were measured by using the portable digital temperature tester,which had the sensor of electric resistance Pt(100)with a measuring contactor sensitive to the temperature.
The warm deep drawing setup is shown in Fig.1.The main tool dimensions are as follows:punch diameter 65.6mm/66.6mm,punch shoulder radius 5mm,die hole diame-ter 68mm,and die shoulder radius 10mm.Meanwhile,the ?nite element method (FEM)was applied to predict the effects of pro-cess parameters such as the blank holding force on the forming quality of workpieces.The FEM model is presented in Fig.2.The blanks used in the experiments are rolled magnesium alloy AZ31sheets with an initial thickness of 0.4and 0.8mm.The rolled sheets were annealed at the temperature of 250?C with the holding time of 25min [11].And the microstructure and tensile fractograph of 0.8mm are shown in Figs.3and 4.The average grain size is about 5?
m.
Fig.1.Schematic of warm deep drawing:(1)blank holder;(2)punch;(3)blank;(4)die;(5)electric heating
elements.
Fig.2.FEM model:(I)blank holder;(II)punch;(III)blank;(IV)
die.
Fig.3.Micro structure of rolled magnesium alloy sheets after annealing.
Process conditions include the forming temperature,the sur-face temperature of the punch,the forming speed,the blank holding force and the lubricant.The forming temperatures were selected at a temperature range of 105–170?C.The surface
tem-
Fig.4.Tensile fractograph of rolled magnesium alloy sheets after annealing.
S.H.Zhang et al./Journal of Materials Processing Technology 185(2007)147–151149
peratures of the punch were at temperatures from 80to 100?C.The forming speeds varied from 0.7to 1mm/s.Liquid PTFE was used as the lubricant,which could be handled conveniently.Unlike the conventional lubricants,PTFE is easy to be cleaned and can supply the uniform lubricating layer.In addition,it is not readily volatile at the forming temperatures.3.Results and discussion 3.1.Thickness distribution
Thickness distribution of magnesium alloy workpiece was simulated by FEM before carrying out experiments,which was useful for predicting fracture.In Fig.5,the thickness of 0.8mm sheet is presented graphically at different region.The lowest point of curve shows the thickness of blank at punch-nose where is easiest to fracture.Generally,when thinning rate exceeds 25%,the workpiece is thought to fail to form.Some values greater than original thickness indicate the thickening on the ?ange.3.2.Effect of forming temperatures on the LDR
Limit drawing ratio (LDR)is an important index to assess deep drawing formability of sheets.Forming temperature and heating duration of blanks have apparent effects on LDR in warm deep drawing experiments of magnesium alloy sheets.The experiments were characteristic of being carried out at tempera-tures as low as possible.The forming temperatures were selected at a temperature range of 105–170?C.The LDR of workpieces could exceed 2.0at the forming temperature of 140?C and be up to 2.6at 170?C,which might completely meet the requests of coverings of portable electrical devices at the aspect of the deformation amount.
It was found that magnesium alloy sheets had good deep drawing formability at the determined temperature range.In Fig.6,LDR is presented graphically at different forming tem-
peratures.
Fig.5.Thickness of workpiece at different
region.
Fig.6.LDR at different forming temperatures.
Because of better thermal conductivity,magnesium alloy blanks should not be heated to the forming temperature lasting over 10min in the preheated tools.With longer heating duration,deep drawing formability of sheets could be reduced severely caused by interior coarse grains,and sheets were brittle to be cracked at the punch-nose.
3.3.Effect of blank holding forces on the LDR
Blank holding force (BHF)was one of the important process parameters for warm deep drawing of magnesium alloy sheets.When the blank holding forces were lower,the blank was dif-?cult to be formed because of wrinkling on the ?ange that was impeded by the clearance of tools and caused the workpiece to fracture.Higher blank holder forces could increase the tensile stress of sheets at the punch-nose,which eventually caused frac-tures when the bearing capacity of sheets was exceeded.The wrinkling and fracture defects caused by inappropriate blank holding forces were analyzed using ?nite element method.Sim-ulation results and experimental results showed good agreement,as shown in Table 1.In this paper,the method of a constant gap between the blank holder and the die surface was applied to adjust blank holding forces with which satisfactory cups could be formed.
3.4.Effects of tooling geometries on workpiece quality Proper die clearance is very important to obtain workpieces with better quality,especially for thin blanks.
Smaller clearance can lead to the fractures owing to being impeded,whereas bigger one is easy to wrinkle the blank.
For instance,for the blank with 0.4mm thickness,when the die clearance on each side is 0.7mm,good workpieces can be obtained,as shown in Fig.7.And when the clearance is 1.2mm,side wall causes wrinkles,as shown in Fig.8.
It was also found that a smaller punch radius reduced the formability of magnesium alloy cups,which was because that material was restrained from being stretched in different direc-tions simultaneously,resulting in a dramatic increase in major
150S.H.Zhang et al./Journal of Materials Processing Technology 185(2007)147–151
Table 1
Simulation results and experimental results of defects with inappropriate blank holding forces
Wrinkling defect
Fracture defect
Expermintal
result
Simulation
result
Fig.7.Workpieces with proper tool clearance (0.4mm sheet).
strain causing an early fracture [12].A proper punch with the radius of 5mm was applied during experiments.3.5.Other process parameters
Besides forming temperatures and blank holding forces,forming speeds and the lubricant were important process param-eters in warm deep drawing experiments of magnesium alloy sheets.Analyzing the effects of these factors on deep draw-ing formability,appropriate process parameters were selected to avoid forming defects in order to improve the LDR of work-
pieces.
Fig.8.Workpieces with excessive tool clearance (0.4mm sheet).
The forming speeds were at a range of 0.7–1mm/s,and the forming limit graph with two values of 0.7and 1mm/s is pre-sented in Fig.4,respectively.It was found that LDR decreased as the increase of forming speeds at the same temperature.Higher forming speeds could cause severe hardening on the ?ange and increase deformation resistance,which made bearing capacity of sheets near the die shoulder decrease and resulted in lower LDR.
In comparison,forming speed of 1mm/s was easier to cause anisotropic deformation behavior for workpieces than one of 0.7mm/s,which was resulted in because of shorter stress relax-ation duration.Fig.9displays workpieces at a forming speed of 1mm/s.
Table 2
Workpieces of different process parameters
Wrinkle
Rupture at die shoulder Rupture at punch-nose Rupture at side wall Heating time Moderate Moderate Excessive Moderate BHF
De?cient Moderate Moderate Excessive Forming speed
Moderate
Excessive
Moderate
Moderate
Workpiece
S.H.Zhang et al./Journal of Materials Processing Technology 185(2007)147–151
151
Fig.9.Workpieces at a forming speed of 1
mm/s.
Fig.10.Workpiece with LDR 2.6under proper process parameters.
For the lubricant,liquid PTFE was uniformly painted on the die and the blank holder,especially at the die shoulder,which could not only ensure the deep drawing to be carried out but also avoid the scratches on the surface of blanks.
Table 2shows the forming quality of cups under the condition of different process parameters,and the causes of defects are analyzed.
Using appropriate process parameters,magnesium alloy cups with LDR of 2.6could be obtained,as shown in Fig.10.4.Conclusions
A setup for warm deep drawing experiments of magnesium alloy sheets was developed.The proper lower forming tem-perature range was determined,and the effects of blank hold-ing forces on the workpiece quality were analyzed.Therefore,appropriate process parameters were selected to avoid forming defects effectively.The following conclusions are obtained:
1.In warm deep drawing experiments,rolled magnesium alloy sheets had good deep drawing formability at a forming tem-perature range of 105–170?C,and cup workpieces could be formed successfully.
2.The LDR of workpieces could exceed 2.0at the forming temperature of 140?C,which might meet the requests of coverings of portable electrical devices at the aspect of the deformation amount and be suitable for commercial appli-cation.The LDR can reach 2.6at the forming temperature of 170?C.And the cup of 0.4mm thickness had also been formed at this temperature.
3.Magnesium alloy blanks should not be heated to the form-ing temperature for a period of over 10min in the preheated tools as the formability will be reduced severely by excessive heating duration.
4.Efforts are made to optimize process parameters by analyzing the causes of defects in order to improve the limit drawing ratio of magnesium alloy workpieces.Acknowledgement
The authors would like to thank the Key Project of the National High Technology Research and Development Program of China (863Program)for providing ?nancial support for this work (Grant No.2003AA331120).References
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