effective strains caused by different process conditions of...

Indian Journal of Engineering & Materials Sciences Vol. 22, December 2015, pp. 661-668

Effective strains caused by different process conditions of extrusion-shear and its

influences on the microstructures of AZ31 magnesium alloy

H-J Hua,b*, H Wanga, Y-Y Lia, M B Yanga & Zhongwen Oub aCollege of Material Science and Engineering, Chongqing University of Technology, Chongqing, 400050, China,

bPLA Chongqing Logistics Engineering College, 401311,China

Received 7 November 2014; accepted 18 June 2015

To improve strengths and microstructures evolution for magnesium alloy, extrusion-shear (ES) has been widely investigated because of its potentiality to produce ultra-fine grained microstructures of magnesium alloys. It is crucial to explore the effects of process parameters especially extrusion speeds and extrusion ratios on the deformation behaviors of magnesium alloy during ES process. Three-dimensional (3D) geometric models of ES processes with different extrusion speeds and extrusion ratios have been applied. Different extrusion speeds have been regarded as the initial conditions used

in DEFORMTM-3D software, and different extrusion ratios have been taken. The microstructures evolvements have been analyzed by the simulation and experimental results. The strains in the extruded rods have been predicted during ES process. ES process with different extrusion speeds and extrusion ratios have been applied to fabricate AZ31 magnesium alloy rod with preheated temperature 400°C. The strains of ES process slightly increase with the rise of extrusion speeds. Average grain sizes prepared by lower extrusion speeds are bigger than those prepared by higher speeds with the same die structures. The strains slightly decrease with the rise of extrusion ratios. The average sizes of microstructures for magnesium alloy prepared by ES extrusion decrease with the rise of extrusion ratios.

Keywords: Magnesium alloys, Microstructures, Grains refinement, Extrusion speed, Extrusion ratio

Magnesium alloy is the lightest of the structural metals. The ratio of strength to weight for the

magnesium alloys is comparable with that of the

strong alloys of aluminium or with the alloy steels, which is used when great strength is not necessary,

but where a thick, light form is desired1,2

. In recent

years bulk microstructure materials processed by methods of severe plastic deformation (SPD) such as

equal channel angular extrusion (ECAE) have

attracted the growing interest of specialists in

materials science3. ECAE usually includes more t

han 2 steps, and the material endures intricate

diversification of forming environments and may be

oxidized. Our research team engaged in the researches of the ES process, and the process include initial

forward extrusion and shearing process subsequently

as early as 20083,4

. The technology has been granted patents for Chinese inventions. Optimum extrusion

speeds and die structures for ES process could

improve satisfactory mechanical properties of

magnesium alloy. Effects of strain distributions on deformation characteristics can play a significant role

in the formation of ultra-fined grains (UFG) for

magnesium alloy, and which could affect the dislocation density and evolution of dislocation cells

and their subgrain structures5.

Direct extrusion is an important way to improve

workability and strength of magnesium alloys by

refining the grains size6. K. Matsuyama et al.

7 used a

new processing procedure to extrude a cast Mg-9% Al

alloy involving the sequential application of extrusion

and equal-channel angular pressing. Experiments showed that the Mg-9% Al alloy has an initial grain

size of ~50 µm after casting but this was reduced

to ~12 µm after extrusion and it was further reduced to ~0.7 µm when the extruded alloy was subjected to

ECAP for 2 passes at 473 K.

A fine-grained material is harder and stronger

because it has a greater total grain boundary area to

impede dislocation motion8. Bulk nanostructure

materials processed by methods of severe plastic

deformation (SPD) such as equal channel angular

extrusion (ECAE) have attracted the growing interest of specialists in materials science

9. The technique

could refine the microstructures of alloys, and

improve their strength according to the Hall-Petch relationship. ECAE is unique because significant cold

—————— *Corresponding author (E-mail: [email protected])

INDIAN J. ENG. MATER. SCI., DECEMBER 2015

662

work can be accomplished without reduction in the

cross-sectional area of the deformed work-piece10

.

In conventional extrusion effective strain is introduced by reduction in the cross-sectional

area. ECAE produces significant deformation strain

without reducing the cross-sectional area and is accomplished

11-14. Researches of Valiev et al.

15

combined the conventional extrusion and the

asymmetric extrusion to produce AZ31alloy sheets, the results showed that ASE approach can cause the

tilted weak basal texture, and improve mechanical

properties. Orlov et al.8 demonstrate the feasibility

of severe plastic deformation (SPD) techniques which combines conventional extrusion and equal

channel angular pressing in a single process. The

processed material exhibited an excellent balance of

strength and tensile ductility.

The defects of products and microstructures can be

controlled by extensive experimentation and try-out.

However which may spend time, manpower and

money. In addition if the rods require a specified microstructure, extrusion speeds and die structures

have to be controlled during ES process. Information

about these process variables is necessary to be optimized. But it is very difficult to obtain these data

from experiments. Finite element method (FEM) can

be used to reduce the amount of try-out necessary

to produce acceptable products16

.

In reference17

the stress location and wear depth of

the tooling components have been calculated by using

finite element models. The comparison is realized by

finite element simulation of the extrusion processes by using the code DEFORM F2.The researches

results show that the maximum friction load

contribution due to the container wall is much higher in the case of solid extrusion than in cup extrusion

18.

Numerical algorithms have been introduced to

optimize the bearing lengths that produce uniform velocity at the die exit, and this is based on a

finite element model to solve material flow during

extrusion. The solution approach involves iteratively computing velocity, temperature, and strain fields

during extrusion and updating the bearing lengths

until balanced flow is achieved 19

.

The main objective of this study is to investigate

the influences of extrusion speeds and extrusion ratios upon strain distributions and microstructures

evolutions during ES process. These results of the

thermo-mechanical reactions around formation zones are important. The strains evolutions during

ES process have been gained through computer

simulation based on the three-dimensional (3D) finite

element method. In the present research, an attempt has been made to simulate the ES process which

includes initial direct extrusion and subsequent

two shears. The geometrical and numerical models

based on simulation theories have been built to predict the evolutions of the strains distributions

by DEFORM-3D software. The microstructures of

AZ31 magnesium alloy sampled from ES formed rods have been observed. It concerns the characterization

of the thermo-mechanical response of wrought

magnesium alloy during ES process in term of strain rates affected by different ram speeds and extrusion

ratios. Simulations have been conducted to investigate

the effects of ram speeds and extrusion ratios on the distributions of effective strains. The simulated

results of AZ31 alloy have been validated by

actual experiments including ES experiments and microstructures observations.

Simulation Conditions DEFORM

TM-3D is a finite element method (FEM)

based process simulation system designed to analyze

various forming and heat treatment processes used by metal forming and related industries developed

by Scientific Forming Technologies Corporation

(2007). DEFORM-3D is capable of modeling complex three dimensional material flow patterns.

By simulating manufacturing processes on a

computer, this advanced tool allows designers

and engineers to reduce the need for costly shop floor trials and redesign of tooling and processes,

improve tool and die design to reduce production and

material costs, shorten lead time in bringing a new product to market, unlike general purpose FEM codes,

DEFORM is tailored for deformation modeling.

A user friendly graphical user interface provides

easy data preparation and analysis so engineers can focus on forming, not on learning a cumbersome

computer system. A key component of this is a fully

automatic, optimized remeshing system tailored for large deformation problems.

The simulation parameters are listed in Table 1.

The geometric models of STL format files were imported to the finite element program DEFORM 3D

v.6.1 software package to establish the finite element

meshes. Figure 1 is the FEM model of the ES process

including direct extrusion and two continuous simple shears. Two channels with an equal cross-section

intersect at an oblique angle Φ called die channel

HU et al: EFFECTIVE STRAINS CAUSED BY DIFFERENT PROCESS CONDITIONS OF EXTRUSION-SHEAR

663

angle, which is 135°. The material for magnesium

alloy AZ31 is considered as a plastic body during

the ES deformation process. Punch and die are assumed to be rigidity. The punch is moved in

X-axial direction. The geometries of the billets and

tooling are selected based on an actual extrusion condition. The computational conditions used are

shown in Table 1.

Extrusion speeds varies directly with metal temperature and pressure developed within the

container. Temperature and pressure are influenced

by extrusion speeds. Lower extrusion temperatures will usually produce shapes with better quality

surfaces and more accurate dimensions and

fine-grains. During the extrusion-shear process, the extrusion-shear speeds were selected as of

10 mm/s and 20 mm/s.

Table1 – Simulation parameters

Preheat temperature for billet,°C 400

Preheat temperature for ES die, °C 380

Billet length, mm 250

Billet diameter, mm 80

Container insider diameter, mm 82

Container outside diameter, mm 90

Channel angle 135°

Die bearing length, mm 5

extrusion ratio 12,28

Ram speed, mm/s 10,20

friction factor of the container–billet interface 0.4

Friction factor between the billet and die 0.4

Heat transfer coefficient between tooling and

billet, N/°C s mm2

11

Heat transfer coefficient between tooling/billet and air, N/°C s mm2

0.02

The extrusion ratio is defined as the starting

cross-sectional area divided by the cross-sectional

area of the final extrusion. When the extrusion ratio of a section is low, portions of the shape involving

the largest mass of metal will have little mechanical

work performed on it. This is particularly true on approximately the first ten feet of extruded

metal. Its metallurgical structure will approach the

as-cast (coarse grain) condition. This structure is mechanically weak and shapes with an extrusion

ratio of less than 10:1 may not be guaranteed as to

mechanical properties. One of the main advantages of the extrusion process is that this ratio can be

very large while still producing quality parts.

Extrusion ratios are 12 and 28 in this study. The flow stress-strain data of the AZ31 alloy

shown in Fig. 5b are determined through hot

compression tests. To take the effect of deformation heating during hot compression at strain rates on

the actual specimen temperature into account, a set

of flow stress-strain curves include the experimental

data over a temperature range of 250-400°C and a strain rate range of 0.01-10 s

−1. The flow model

of stress is implemented in the commercial FE code

DEFORMTM

-3D using the user subroutine, in which the yield stress is dependent on the strain, strain

rate and temperature. Contact boundary conditions

are applied to nodes of billet, and specify contact between those nodes and the surface of ram. The

friction at the work-piece and tool interfaces is

considered to be of shear-type.

Experimental Procedure ES process experiments have been carried out to

verify the results obtained from computer simulation

in laboratory. In order to validate the results of finite

Fig. 1 – (a) 3D Finite element model of ES process (1-ES die, 2-billet, 3-punch, 4-upsetting zone, 5- first shear zone, 6- second shear zone) and (b) curves of stress-strain with different strain rates and preheated temperature 400°C


664

Fig. 2 – (a) Schematic diagram of an ES die and (b) structure of ES die

element analysis, ES dies have been designed and manufactured to perform the actual ES processes,

cone-shaped ES dies with channel angle 135º and

different extrusion ratios have been designed and manufactured (seen from (a) die structure chart and

(b) ES die in Fig. 2.

Before extrusion the billets are machined to

a diameter of 59 mm. Real extrusion experiments have been carried out by employing a 500 ton

horizontal extrusion press with a resistance

heated container and a heater. The die material, die dimensions, billet dimensions and extrusion

conditions are all the same as those used in

numerical simulation as described above. The billets

have been heated in an external furnace up to 400°C. Ram speed was 10 mm/s and 20 mm/s

respectively during experimental verification.

The chemical composition of the as-received AZ31 billet used in this study (in mass percent) is

Mg3.02% Al1.01% Zn0.30%Mn. All the observation

samples have been taken from the center of rod. Microstructures in the as-received, and ES

extruded materials are examined following standard

metallographic procedures. The polished surface

is etched using either a solution of 1 vol%HNO3, 24 vol%C2H6O2, and 75 vol% H2O, or of 10 mL

acetic acid, 4.2 g picric acid, 10 mL H2O, or 70 mL

ethanol. Microstructure observations have been carried out using PME OLYMPUS TOKYO-type

optical microscope (OM). The average grain size

was measured by line-intercept method.

Results and Discussion

Effects of extrusion speeds on strains of magnesium alloy

during ES process

In order to gain the deformation characteristics

of ES process with different extrusion speeds, the

predicted strain provides quantitative insight into the deformation behaviors of magnesium alloy billets

during ES process. The effective strain distributions

illustrated in Figs 3a and 3b when billet temperatures are 400°C and extrusion speeds 10 mm/s and

20 mm/s respectively which provided the important

information regarding the strains distribution. The strain distributions for different extrusion

speeds are significantly different. The maximum

strains slightly increase with the rise of extrusion speeds. The deformation of extrusion is nonuniform,

and the maximum strain (10.555) in Fig. 3a locates

at channel corner, and which is bigger than that of Fig. 3b. The shear deformation can be obtained from

the intersection of two channels. It indicates that

the largest strain exists in the shear region, and the simple shear occurs. The distribution patterns are

very similar in spite of the fact that the actual values

are somewhat different. The extrusion speeds have significant effects on the strain distribution of the

rods. The shear deformation can be obtained at the

intersection of the two channels. It indicates that the largest strains still exist in the channel region,

where the simple shear occurs. It is obvious that the

shear strains caused by ES die increase with rise of ram speeds.


665

Fig. 3 – Strain distributions at extrusion stroke 49 mm with extrusion ratio 28 and different extrusion speeds (a) 10 mm/s and (b) 20 mm/s

Effects of extrusion speeds on grains refinements of

magnesium alloy

The AZ31 is worked under high pressure during

ES process. Optical microstructures for ES extrusions for different extrusion speed 10 mm/s and 20 mm/s

are shown in Figs 4a and 4b, respectively. There are

even equiaxed grains with an average grain size of 5 µm in ES hot-extruded rods as shown in Fig. 4a,

which indicate that intensive and full dynamic

recrystallization has taken place during ES process. After the hot ES extrusion, grains are refined and

more homogenous. But in Fig. 4b there are many

coarser grains and the sizes are more than 50 µm,

and the size of fine grains is around 5 µm so the part dynamic recrystallizations happen, and the

distribution of grains is nonhomogeneous. There are

almost no dynamic recrystallizations of fined grains exiting in the billet. Average grain size decreases with

extrusion speed rise. Average grain sizes prepared by

lower extrusion speeds are bigger than those prepared by higher speeds with the same die structures.

The curves of maximum strain-time which

locate at the channels during the ES process

at temperature 400°C have been shown in Fig. 5. It can be found the variation laws of the strain-time

shown in Fig. 5a. The strains in curve of strain-time

increase slowly, and the maximum value is

approximinately equal to 14 in Fig. 5a. The stress-time curve is a wave curve, and then continues

to go up to another meridian, till the strain value is

about 14, subsequently the stress decrease with the extrusion time. In stress-strain curve for Fig. 5b,

dynamic recrystallization can be recognized by

distinct peaks in the curves of flow stress, due to

the softening effect of recrystallization. Magnesium alloys display well-defined peaks when tested under

hot working conditions.

By comparing Fig. 5 (a) with Fig. 5 (b), it is clear that the maximum strain increase with extrusion

speeds. The total effective strain ε in the pressed

work-piece after extrusion-shear has been given with geometry ascertain methods, the principle of

extrusion-shear process is to introduce compressive

and accumulated shear strain into the samples. The character of extrusion-shear process is that the

sample is subjected to twice shear deformation.

The accumulative strain of extrusion-shear can be

expressed as Eq. (1)20

which include two parts: accumulative strain of direct extrusion and two

continuous ECAP steps.

2cot( ) csc( )2 2 2 2ln 2*

3

φ ψ φ ψψ

ε λ

+ + +

= + …(1)

Where ε is the accumulative strain, λ the extrusion

ratio the inner corner angle, Ψ the outer corner angle.

Obviously, the effective strains in the rods decrease

with the increase of the die channel angle Φ and the

die outer corner angle Ψ. However, the die channel angle Φ has more influence on the effective strain in

the rod than the die outer corner angle Ψ. The strain

rate of extrusion-shear can be expressed as Eq. (2).

.22

2 '

2

2cot( ) csc( )2 2 2 2

6

v

t R

φ ψ φ ψψ

εε

ψ

+ + +

= =

…(2)

Where v2 is extrusion speed, R2 is radius of rod made of magnesium alloy.


666

Fig. 4 – Microstructures of AZ31 Mg processed by ES at the temperature 400°C with extrusion ratio 28 and different extrusion speed (a) 10 mm/s and (b) 20 mm/s

Fig. 5 – The curves of strain-time for extrusion speed (a) 10 mm/s and (b) 20 mm/s with extrusion temperature 400°C

From the Eqs (1) and (2) it is found that the strain and strain rate distributions for different ram speeds are

very significantly different, and very closely related

with the parameters of ES die structures. The response

of magnesium alloy to be extruded can be influenced by the speeds of deformation. The temperature

developed in extrusion increases with increasing

extrusion speed. Temperature rise is due to the fact that

the strain rate is directly proportional to the ram speed, and the magnitude of the generated heat is proportional

to the strain rate. But the slower the extrusion speed is

the more time will be available for the generated heat to flow. Quantity of thermal rating caused by

extrusion-shear would influence the surface quality of

extruded rod. The volume thermal rating which includes plastic deformation of work-piece as well as

the contact surface friction may be expressed as20

.

.

1p

V

Q dVV

σ ε− −

= ∫ … (3)

Where σ−

stands for the equivalent stress,

.

pε−

the equivalent plastic rate of strain, v is the unit volume.

The metal extrusion process produces the heat

mainly concentrates in first and the second deformation range.

Effects of extrusion ratios on grain refinements of magnesium

alloy

It is obvious that the shear strain caused by ES die

with extrusion ratio 28 is bigger than which caused by ES die with extrusion ratio 12 at extrusion time

5.5 s in Fig. 6. The strain distributions for different

extrusion ratios of ES die are significantly different and not even. The maximum strains slightly increase


667

with the rise of extrusion ratios. The deformation of

the initial extrusion is nonuniform, and the highest

strain (10.98) in Fig. 6a. The maximum shear deformation can be obtained at the intersection of

the two channels. Distributions of the strains are

lamellar with distinct deformation gradients in the deformation zone. The deformation of this position

is close to the simple shear deformation.

Fig. 6 – The distributions of the strains with different extrusion ratios (a) 12 and (b) 28

Optical microstructures for ES process with different extrusion ratios 12 and 28, and extrusion

speed 20 mm/s are shown in Figs 7a and 7b, respectively. There are no even equiaxed grains in ES hot-extruded rods as shown in Fig. 7. After the hot ES process, grains are refined and more homogenous. But in Fig.7a there are many original grains and the sizes are more than 100 µm, and the size of fine grains is around 10 µm so the part dynamic recrystallization happen and the distribution of grains is nonhomogeneous. The microstructures show that the average sizes of microstructures for magnesium alloy prepared by ES extrusion decrease with the rise of extrusion ratios.

The relationship between the average recrystallization grain size (d) and the Zener-Hollomon parameter (Z) during dynamic recrystallization is given by Eq. (4)21.

3 1/3

0

( ) 10ndZ

d

− −= × … (4)

The temperature corrected strain rate Z is given by

Eq. (5).

.

exp( )Q

ZRT

ε= … (5)

Where έ is strain rate, Q is the activation energy for the deformation, T is the temperature and R is the

gas constant.

It can be seen from the Eqs (1)-(5) the strain of

deformation rods has a significant impact on the dynamic recrystallization of fined grains if the

process parameters of ES extrusion are the same.

The average sizes of magnesium increase with the rise of strains, and increase with the rise of extrusion

Fig. 7 – Microstructures of AZ31 Mg processed by ES with extrusion speed 20 mm/s and different extrusion ratios (a) 12 and (b) 28


668

speeds. The sizes and volume fraction of dynamic

recrystallization of fined grains are inversely

proportional to the extrusion speeds and extrusion ratios. It is obvious that average sizes of grains for

higher extrusion speeds and extrusion ratios are finer

than those prepared by ES extrusion with lower

extrusion speeds and extrusion ratios.

Conclusions

In accordance to finite element analysis and experimental results of different extrusion speeds and

extrusion ratios for ES extrusion have been

researched. 3D FEM simulation of as-cast AZ31 magnesium alloy billets subjected to ES process with

different extrusion speeds and extrusion ratios have

been carried out successfully. The strains in the

extruded rods have been predicted during ES process. ES process with different extrusion speeds and

extrusion ratios have been applied to fabricate AZ31

magnesium alloy rod with preheated temperature 400°C. The strains of ES process slightly increase

with the rise of extrusion speeds. Average grain sizes

prepared by lower extrusion speeds are bigger than

those prepared by higher speeds with the same die structures. The strains slightly decrease with

the rise of extrusion ratios. The average sizes of

microstructures for magnesium alloy prepared by ES extrusion decrease with the rise of extrusion ratios.

Acknowledgements

This work was supported by the foundation of

the post doctorate in Chongqing City and Project Number Xm201327, and China Postdoctoral Science

Foundation funded project (2015T81087 and

2014M552575), and Chongquing Natural Science

Foundation Project of cstc2014jcyjA50004 and cstc2015jcyjB0292, and Scientific and Technological

Research Program of Chongquing Municipal

Education Commission KJ1500939.

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