504
T. Balusamy et al.
Chen et al. [96] have evaluated the corrosion behavior and corrosion fatigue of
untreated AZ31B Mg alloy and those subjected to SMAT using 2 and 3 mm Ø
stainless steel balls at 20 kHz for 15 min in 3.5% NaCl. An increase in surface
roughness after SMAT and the defects induced during the treatment has caused a
deleterious influence on the corrosion resistance of the treated alloy. The formation
of more number of deeper corrosion pits on the treated alloy further substantiates it.
Nevertheless, the grain refinement, surface nanocrystallization and the compressive
residual stress induced during SMAT have led to a significant improvement in the
fatigue life of the treated alloy. This is validated by the increase in fatigue life with
an increase in diameter of the balls from 2 mm to 3 mm. At lower stress amplitude, a
decrease in fatigue life of the alloy is observed when the test environment is changed
from air to 3.5% NaCl. However, at higher stress amplitude, the improvement in
fatigue life is much pronounced in 3.5% NaCl environment.
The tribological behavior of untreated GW63K Mg alloy and those subjected to
SMAT at two different sliding speeds of 0.05 m/s and 0.2 m/s is studied by Liu
et al. [97]. A similar study is also performed by Xia et al. [98] using AZ31 Mg
alloy at 0.05 and 0.5 m/s. The wear mechanism of Mg alloys varies with the sliding
speed; abrasive wear at 0.05 m/s and oxidative wear at 0.2 and 0.5 m/s. According
to Liu et al. [97], the poor wear resistance of SMAT GW63K Mg alloy observed at
0.05 m/s is due to its low ductility and toughness. With an increase in sliding speed
to 0.2 m/s, the transformation of abrasive wear to oxidative wear occurs, which is
likely to increase the wear resistance. Nevertheless, filling the worn surface with
wear debris promotes delamination leading to poor wear resistance. In contrast, Xia
et al. [98] have observed an improvement in wear resistance of SMAT AZ31 Mg
alloy at 0.5 m/s due to its ability to form a compact oxide film which reduced the
coefficient of friction. Studies on cell adhesion and spreading on SMAT Mg alloys
are rather limited.
7 Concluding Remarks
SMAT has enabled surface nanocrystallization, promoted the formation of a graded
layer structure having nanosized grains at the top surface, submicron-sized grains at
the subsurface with coarse grains near the bulk. The microstructure exhibits considerable variation as a function of deformation depth from the top surface. SMAT induces
compressive residual stress. The change in deformation thickness, microstructure,
compressive residual stress induced during treatment and type of phases formed are
largely a function of the level of strain and strain rate. The repeated multidirectional
impingement of the balls results in the generation of numerous dimples and craters
on the surface, which increases the R a . The hardness and mechanical properties are
also increased after SMAT. Grain refinement, work hardening, strain-induced dislocations, and compressive residual stress are responsible for the increase in hardness
and mechanical properties. The maximum hardness is reached at few microns from
the top of the surface. The hardness exhibits a steady decrease with an increase in
T. Balusamy et al.
Chen et al. [96] have evaluated the corrosion behavior and corrosion fatigue of
untreated AZ31B Mg alloy and those subjected to SMAT using 2 and 3 mm Ø
stainless steel balls at 20 kHz for 15 min in 3.5% NaCl. An increase in surface
roughness after SMAT and the defects induced during the treatment has caused a
deleterious influence on the corrosion resistance of the treated alloy. The formation
of more number of deeper corrosion pits on the treated alloy further substantiates it.
Nevertheless, the grain refinement, surface nanocrystallization and the compressive
residual stress induced during SMAT have led to a significant improvement in the
fatigue life of the treated alloy. This is validated by the increase in fatigue life with
an increase in diameter of the balls from 2 mm to 3 mm. At lower stress amplitude, a
decrease in fatigue life of the alloy is observed when the test environment is changed
from air to 3.5% NaCl. However, at higher stress amplitude, the improvement in
fatigue life is much pronounced in 3.5% NaCl environment.
The tribological behavior of untreated GW63K Mg alloy and those subjected to
SMAT at two different sliding speeds of 0.05 m/s and 0.2 m/s is studied by Liu
et al. [97]. A similar study is also performed by Xia et al. [98] using AZ31 Mg
alloy at 0.05 and 0.5 m/s. The wear mechanism of Mg alloys varies with the sliding
speed; abrasive wear at 0.05 m/s and oxidative wear at 0.2 and 0.5 m/s. According
to Liu et al. [97], the poor wear resistance of SMAT GW63K Mg alloy observed at
0.05 m/s is due to its low ductility and toughness. With an increase in sliding speed
to 0.2 m/s, the transformation of abrasive wear to oxidative wear occurs, which is
likely to increase the wear resistance. Nevertheless, filling the worn surface with
wear debris promotes delamination leading to poor wear resistance. In contrast, Xia
et al. [98] have observed an improvement in wear resistance of SMAT AZ31 Mg
alloy at 0.5 m/s due to its ability to form a compact oxide film which reduced the
coefficient of friction. Studies on cell adhesion and spreading on SMAT Mg alloys
are rather limited.
7 Concluding Remarks
SMAT has enabled surface nanocrystallization, promoted the formation of a graded
layer structure having nanosized grains at the top surface, submicron-sized grains at
the subsurface with coarse grains near the bulk. The microstructure exhibits considerable variation as a function of deformation depth from the top surface. SMAT induces
compressive residual stress. The change in deformation thickness, microstructure,
compressive residual stress induced during treatment and type of phases formed are
largely a function of the level of strain and strain rate. The repeated multidirectional
impingement of the balls results in the generation of numerous dimples and craters
on the surface, which increases the R a . The hardness and mechanical properties are
also increased after SMAT. Grain refinement, work hardening, strain-induced dislocations, and compressive residual stress are responsible for the increase in hardness
and mechanical properties. The maximum hardness is reached at few microns from
the top of the surface. The hardness exhibits a steady decrease with an increase in