H. ZHANG et al.: EFFECT OF THICKNESS REDUCTIONS ON MICROSTRUCTURE AND NANO-MECHANICAL ...
63–69
EFFECT OF THICKNESS REDUCTIONS ON MICROSTRUCTURE
AND NANO-MECHANICAL PROPERTIES OF AZ31 MAGNESIUM
ALLOY IN FLOW-FORMING
VPLIV ZMANJ[ANJA DEBELINE NA MIKROSTRUKTURO IN
NANOMEHANSKE LASTNOSTI MAGNEZIJEVE ZLITINE VRSTE
AZ31 MED PLASTI^NIM PREOBLIKOVANJEM
Han Zhang
1
, Tingting Zhang
2,3
, Jie Zhang
1
, Zhihang Yan
1
, Wenxian Wang
1,*
1
College of Materials Science and Engineering, Taiyuan University of Technology, Taiyuan, China
2
College of Mechanical and Vehicle Engineering, Taiyuan University of Technology, Taiyuan, China
3
Engineering Research Center of Advanced Metal Composites Forming Technology and Equipment, Ministry of Education, Taiyuan, China
Prejem rokopisa – received: 2022-10-14; sprejem za objavo – accepted for publication: 2022-12-22
doi:10.17222/mit.2022.659
The effect of reducing the thinning rate when flow forming AZ31B magnesium alloy barrels on the microstructure, crystal ori-
entation, micro-, nano- and macro-mechanical properties was investigated. The results demonstrate that when the magnesium al-
loy undergoes different degrees of plastic deformation after flow forming, the grain size and dislocation density inside the crys-
tal change greatly, and the evolution pattern of the organization has a significant effect on the mechanical properties of the
materials. The stress-strain curve of the cylindrical parts was obtained by nano-indentation and hardness. This is important to
improve the mechanical properties of the magnesium alloy.
Keywords: flow forming, microstructure, nanomechanical properties, hardness
V ~lanku je predstavljena raziskava vpliva zmanj{anja hitrosti tanj{anja materiala med plasti~nim preoblikovanjem valj~kov iz
magnezijeve zlitine vrsteAZ31B na njeno mikrostrukturo, orientacijo kristalov ter njene makro-, mikro- in nanomehanske
lastnosti. Rezultati raziskav so pokazali, da se zaradi razli~nih stopnej deformacije med izbranim protokolom plasti~nega
preoblikovanja mo~no spremeni velikost kristalnih zrn in gostota dislokacijizbrane zlitine, kar mo~no vpliva tudi na njene
mehanske lastnosti. Krivulje napetost-deformacija cilindri~nih delov so izdelali s pomo~jo nano indentacije (vtiskovanja nano
prizme) in meritev trdote. Rezultati te raziskave so pomembni s stali{~a izbolj{anja mehanskih lastnosti magnezijevih zlitin.
Klju~ne besede:plasti~no preoblikovanje, mikrostruktura, nano-mehanske lastnosti, trdota
1 INTRODUCTION
As lightweight materials, magnesium (Mg) and its al-
loys are relevant in many areas of the automotive, aero-
space and weapons industries.
1–4
Mg alloys usually
exhibit poor ductility and plasticity due to their hexago-
nal-close-packed (hcp) lattice.
5–7
Mg alloys are conven-
tionally formed by casting,
8
rolling,
9
forging
10
and extru-
sion.
11
Although progress has been made in these areas,
these processes are prone to defects such as cracking and
flaking, and these factors have severely restricted the ap-
plication and development of thin-walled magnesium al-
loy tubes.
12,13
Flow forming is an effective plastic deformation pro-
cess to fabricate high-precision, thin-walled cylindrical
components by employing an incremental rotary point
deformation technique.
14–16
As a chipless metal-forming
process, flow forming can reduce the wall thickness of a
pre-form or tube without changing the inner diameter,
and is an efficient and environmentally friendly
metal-forming process. In recent years, flow forming has
been widely utilized in the production of modern indus-
trial products, such as flanged components and seamless
tubes, for high precision and low processing cost.
17,18
It is
of great significance to utilize the advantage of flow
forming in the fabrication of thin-walled metal tubes to
produce high-precision magnesium alloy tubes.
Park et al.
19
determined the magnitude of radial, ax-
ial, and tangential forces on the barrel during the
flow-forming stage by analyzing the processing of the
spun barrel and combining the flow functions. Chang et
al.
20
focused on the effect of different forming passes on
the organization and properties of aluminum alloy spun
tubes. Katherine et al.
21
constructed a model by finite-el-
ement analysis software and verified the feasibility of the
model with experiments. The influence of different
flow-forming process parameters on the material form-
ing properties was investigated. However, the research on
the direct relationship between the microstructure evolu-
tion and the properties in the plastic deformation process
of flow forming cylindrical parts mainly stays at the
qualitative level through microstructure analysis, which
does not give quantitative results.
Materiali in tehnologije / Materials and technology 57 (2023) 1, 63–69 63
UDK 669.721.5:539.536 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 57(1)63(2023)
*Corresponding author's e-mail:
wangwenxian@tyut.edu.cn (Wenxian Wang)

In this work, the microstructure evolution and
nanomechanical behavior of magnesium alloy sin-
gle-layer barrels under different thinning rates during
flow forming were analyzed, and the load-displacement
curve obtained by indentation was transformed into a
stress-strain curve, which corresponded to the tensile ex-
periment. It is found that controlling the thinning rate of
the flow-forming process can effectively improve the
mechanical properties of the magnesium alloy.
2 EXPERIMENTAL PART
An extruded AZ31B alloy was used to investigate the
flow-forming process. The nominal composition of the
AZ31B alloy is 0.04 w/% Cu, 1.0 w/% Zn, 0.8 w/% Mn,
2.5 w/% Al, 0.08 w/% Ca and balance Mg. Before flow
forming, the billets were preheated to 623 K in an elec-
trical furnace and the mandrel was preheated to the same
temperature with electric heating pipes. The flow-form-
ing process is shown in Figure 1. The spindle revolu-
tions were 150 min
–1
, the feed ratio was 1 mm/revolution
and the thickness reduction was (0, 30 and 60) %, re-
spectively. Following the flow forming, the specimens
were cut parallel to the RD-AD plane (longitudinal sec-
tion).
To obtain reliable nano-indentation data, the nano-in-
dentation tester (Nano Indenter G200) micro- and
nano-mechanical tester with standard XP system from
Agilent was used to test the micro- and nano-mechanical
behavior of different micro-zones of the Al alloy cylin-
der. For each specimen, indentation tests were performed
with maximum loads of (20, 30, 40 and 50) mN, where
the loading duration was 15 s. At least three indentation
tests were performed at different maximum loads, while
maintaining sufficient intervals between each indenta-
tion. The micro- and nano-mechanical properties of the
characterized materials were further analyzed by
nano-indentation experiments. A detailed microstructural
analysis was performed by electron back-scattered dif-
fraction (EBSD) on a TESCAN MIRA3 scanning elec-
tron microscope. The testing plane of the EBSD was
RD*AD, and the measurement was taken at 20 kV with a
15-mm working distance, a tilt angle of 70°, and a scan-
ning step of 1.5 μm. The tensile properties of the mate-
rial along the AD were measured with a universal testing
machine (SDS-100) at a rate of 1 mm/min, the measured
length and width of the tensile specimens were 20 mm
and 5 mm, respectively.
3 RESULTS AND DISCUSSION
Figure 2 shows the grain sizes of the AZ31B Mg al-
loys with (0, 30 and 60) % thinning, respectively. With
an increase of the thinning rate, the average grain size of
AZ31B Mg alloys changes from 10.040 μm to 2.423 μm,
which is caused by large plastic deformation, and a large
number of low-angle boundaries appear at the same time.
During thermal deformation, it is easy to obtain a
strong texture, which has a great influence on the me-
chanical properties of magnesium alloys.
22
Figure 3
shows the pole figures of blanks at thickness reductions
of (0, 30 and 60) %, respectively. Through the evolution
of the microstructure, it can be seen that the grains of the
AZ31B Mg alloy in the initial state have a strong
{0001} basal texture. When the thinning rate is 30 %, a
large number of {-12-10 } and {01-10 } textures ap-
H. ZHANG et al.: EFFECT OF THICKNESS REDUCTIONS ON MICROSTRUCTURE AND NANO-MECHANICAL ...
64 Materiali in tehnologije / Materials and technology 57 (2023) 1, 63–69
Figure 1: Schematic diagram of the whole experimental procedures

pear, and a large number of{-12-10} tensile twins are
produced. The results reveal that{-12-10} tensile twins
are dominant in the early deformation process. Twins are
activated in most grains, but there are still some grains
without twins. When the thinning rate continues to in-
crease to 60 %, the texture type is consistent with that
when the thinning rate is 30 %, and the number of
{01-10 } stretching twins increases significantly, indi-
cating that when the deformation is large, the{01-10}
stretching twins are gradually activated due to the pres-
ence of external forces.
From Figure 4a it can be observed that most of the
grains of the base material on one side of the magnesium
alloy are concentrated around the orientation factor of
0.4–0.5. The closer the Schmidt factor value in the figure
is to 0.5, the color tends to be more red. The orientation
factor is called soft orientation, which means that it is
easier to trigger the opening of the slip system under the
H. ZHANG et al.: EFFECT OF THICKNESS REDUCTIONS ON MICROSTRUCTURE AND NANO-MECHANICAL ...
Materiali in tehnologije / Materials and technology 57 (2023) 1, 63–69 65
Figure 2: Microstructure evolution of flow-forming AZ31B Mg alloys

action of external forces to occur the macroscopic plastic
deformation. As the thinning rate increases to 30 %, the
value of the Schmidt factor on one side of the magne-
sium alloy decreases significantly, and the proportion of
the SF value less than 0.3 is about 95.7 %, and most of
them are blue-green in Figure 4b, so it is difficult for the
crystal to slip in the subsequent deformation. When the
thinning rate increases to 60 %, the values of the orienta-
tion factor are more evenly distributed, but most of them
are still concentrated below 0.3, as shown in Figure 4c.
As the thinning rate increases from 30 % to 60 %, the
crystals, which are not susceptible to slip, are deformed
significantly, so the material changes its mechanism at
larger deformations.
Dynamic recrystallization (DRX) has a significant
role in the large strain thermal deformation of magne-
sium alloys.
23,24
Therefore, it is necessary to further in-
vestigate the behavior of DRX in hot-flow-forming pro-
cesses. As observed in Figure 5, the base material of the
barrel is mainly composed of about 93 % recrystallized
grains and a small number of twins, indicating that dy-
namic recrystallization of the base material mainly oc-
curs during the extrusion process due to large deforma-
tion. In addition, a small number of sub-crystals are
distributed at the grain boundaries or twins of the
recrystallized grains. With the increase of the thinning
rate to 30 %, the barrel-shaped parts under the action of
external forces undergo large plastic deformation, the in-
terior of the magnesium alloy mainly consists of about
73 % of deformed grains and a small part of sub-crystals,
where the twin crystals are mainly distributed in the de-
formed grains. In this flow-forming process, the magne-
sium alloy side by a large number of recrystallized grains
in the base material after a large plastic deformation into
deformed grains. As can be seen from Figure 5b, the
magnesium alloy is more difficult to slip at the grain
boundaries, the deformation is not large, but a large de-
formation occurs inside the grain, so the deformation of
the whole grain is not uniform. As the thinning rate con-
tinues to increase to 60 %, the interior of the magnesium
H. ZHANG et al.: EFFECT OF THICKNESS REDUCTIONS ON MICROSTRUCTURE AND NANO-MECHANICAL ...
66 Materiali in tehnologije / Materials and technology 57 (2023) 1, 63–69
Figure 3: Pole figures of flow-forming AZ31B Mg at reduction rates:
a)0%,b)30%andc)60%
Figure 4: Schmid factor distribution on AZ31BMg under different thickness reductions after flow forming: a) and d) 0 %, b) and e) 30 %,
c) and f) 60 %

alloy side is mainly composed of about 71 % of de-
formed grains and about 26 % of recrystallized grains,
where the number of twins is significantly reduced com-
pared to the thinning rate of 30 %, and the grains un-
dergo significant refinement mainly due to the action of
twins driving dynamic recrystallization. Figure 5c illus-
trates that the sub-crystalline grains are basically trans-
formed into recrystallized grains during the flow-form-
ing process, which is due to the combined effect of the
dominant twin drive within the sub-crystal and the dislo-
cation plugging at the sub-crystal boundaries under large
strain conditions. During the thermal flow forming pro-
cess, the sub-crystal boundaries tend to migrate into
large-angle grain boundaries due to the large dislocation
difference around the sub-crystal boundaries, which are
used as the nuclei for recrystallization and growth.
From the distribution of dislocations in Figure 6,it
can be noted that the local orientation difference in the
parent material is small, indicating that the dislocation
density of the parent material on the side of magnesium
alloy is relatively small. With the increase of the thinning
rate, twin deformation occurs inside the magnesium al-
loy during the spinning process, then the two parts of de-
formed and undeformed crystals, i.e., twin crystals, will
be constrained by the matrix and produce larger strain.
Therefore, when the thinning rate reaches 60 %, the in-
ternal local orientation difference is relatively large. And
the average KAM of the base material is calculated to be
4.003°, and when the thinning rate reaches 30 %, the size
of the dislocation density inside the magnesium alloy in-
creases significantly to 17.495°, and with the increase of
the thinning rate the dislocation density inside the mate-
rial remains almost constant at about 17.547°.
Figure 7a to 7c show the load-displacement curves
of AZ31B Mg alloys with different thinning rates (0, 30
and 60) % at different maximum loads (20, 30, 40 and
H. ZHANG et al.: EFFECT OF THICKNESS REDUCTIONS ON MICROSTRUCTURE AND NANO-MECHANICAL ...
Materiali in tehnologije / Materials and technology 57 (2023) 1, 63–69 67
Figure 5: The distribution of recrystallized, deformed, and sub-structured grains of Mg under different thickness reductions after flow forming:
a)0%;b)30%;c)60%
Figure 6: Different thickness reductions of flow forming: a) and d) 0%, b) and e) 30 %, c) and f) 60 %

50) mN. Overall, the loading phase curves of the load-
displacement curves at different thinning rates almost
overlap, which indicates that the indentation behavior of
the flow-forming barrels is not sensitive to the loading
rate. With the increase of the thinning rate, the maximum
displacement depth gradually decreases, and its displace-
ment depth decreases more obviously with the increase
of the maximum indentation load. The stress-strain
curves of the AZ31B magnesium alloy are presented in
Figure 7d to 7f. The elastic modulus of the bar-
rel-shaped parts increased slightly from (50.3 ± 0.6) GPa
to (51.9 ± 0.4) GPa with the increase of the thinning rate
after flow forming, which is mainly due to the twinning
deformation inside the grains during the flow-forming
process, and the number and position of the twins have a
greater influence on the elastic modulus obtained from
the test. In addition, with the increase of the thinning
rate, the yield strength  y
of AZ31B magnesium alloy
has a significant increase, and the indentation yield
strength is (212.0 ± 0.5) MPa, after the thinning rate of
30 % and 60 % increased to (227.5 ± 0.8) MPa and
(248.2 ± 0.5) MPa, respectively, and the elastic modulus
increases from Figure 7g, Figure 7h and Figure 7i con-
tains the graphs of the tensile tests, which provide strong
evidence to verify the accuracy of the stress-strain data
calculated from the nanoindentation.
4 CONCLUSIONS
The change of thinning rate during flow forming af-
fects the relationship between the microstructure evolu-
tion and the mechanical properties of a magnesium alloy.
The following main conclusions were drawn:
1) There is a significant grain refinement after hot
flow forming. As the thickness decreases, the grain size
decreases and the average value of hardness increases
significantly.
2) This behavior is related to the microstructural evo-
lution, which is strongly influenced by the depression
H. ZHANG et al.: EFFECT OF THICKNESS REDUCTIONS ON MICROSTRUCTURE AND NANO-MECHANICAL ...
68 Materiali in tehnologije / Materials and technology 57 (2023) 1, 63–69
Figure 7: Typical load-displacement curves, stress-strain curves and tensile curves of Mg under different thickness reductions after flow forming:
a), d) and g) 0 %; b), e ) and h) 30 %; c), f) and i) 60 %

rate, due to the occurrence of grain-boundary sliding, dy-
namic recrystallization and grain growth.
3) High micro- and nano-mechanical properties are
obtained when hot flow forming is performed at a tem-
perature of 623 K, a spindle rotation of 150 min
–1
, a feed
ratio of 0.1 mm/revolution and a thickness reduction of
60 %. The modulus of elasticity and hardness reached
(51.9 ± 0.4) GPa and (248.2 ± 0.5) MPa.
Acknowledgment
This work was supported by National Natural Sci-
ence Foundation of China (Grant Nos. 52075360 and
51805359).
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Materiali in tehnologije / Materials and technology 57 (2023) 1, 63–69 69