​
MULTI AXIAL FORGING
MCL 131, 2nd Semester 2019-20 Report
Anoushka Gupta
2018ME10590
Group 2
1. Introduction
Severe Plastic Deformation (SPD) is a top-down
technique to manufacture nanomaterials in bulk
without
expensive
powder
metallurgy
and
consolidation. Unlike conventional deformation
methods, the initial dimension and final dimension of
the sample are the same after the cycle. Multiple
rotations/reinsertions for forging and so as it becomes
possible to produce large strain deformation as we
apply strain to the same sample (no geometrical
change) of polycrystalline material (with grain
boundaries). The fine-grain refinement to enhance
properties like strength and ductility is controlled by:
a) Grain size which is controlled by Hall Petch
relationship, thus we can deform and obtain
microstructure of annealed and tailored
b) Crystallographic texture with higher elongation or
ductility or formidability or higher strength,
c) Dislocation density if high, microstructure has
strength but less ductility, or if recovered and didn’t
allow recrystallization, we can improve ductility.
d) Grain boundary character contribution to hardness.
Fig 1. Top-down Approach
SPD has various applications in industries like aerospace, biomedical, and automotive,
which need lightweight parts having high strength and reliability on microstructure, for
environmental harmony. Some of the SPD processes include material flow without axial
symmetry, e.g., Multiaxial Forging (MAF) and Cyclic Closed Die Forging. This report discusses
the details of MAF.
1
Multiaxial Forging (MAF) is a technique in which the sample is compressed
sequentially along three orthogonal axes by 90° rotation after each pressing (or forging), with
the help of existing equipment like a forging press, the process is easily modified. This
technique was developed to uniaxially compress the sample repeatedly on three orthogonal
axes. However, this repeated uniaxial compression would require the sample to be ground after
each pass to produce flat faces to compress it again. This process of grinding can be avoided if
a channel die is used, either as open die or confined. The channel die provides a plane strain
compression to the sample, which produces a higher effective strain than uniaxial compression
for the same reduction in height if the channel die is open in one direction, as was in Ref. [1] the
amount of strain per pass must be controlled to the desired amount for this process to be
repeatable. The open channel die has a disadvantage that one pair of faces bulges at the end of
a cycle leading to nonuniform deformation and early cracking. An alternate method is the use of
a confined channel die [2-5]. Here the sample is pressed till it fills the die after which it is
removed, rotated, and repressed. The confined channel die ensures that after each press, the
sample takes the shape of the die, and the face that would have bulged now becomes flat,
providing more uniform deformation.[6]
2. Literature Review​Somjeet
Biswas et al. experimented on magnesium alloy Mg-3Al-0.4
Mn
Multi axial forging was processed on magnesium alloy Mg-3Al-0.4 Mn with a strain of 0.3
imparted by uniaxial compression and 0.5 by plane strain compression to obtain a total VonMises strain of 0.8 in a single pass. The specimen could be extracted, rotated, and inserted in
the die for several passes. The first MAF cycle was carried out at 200 C and the second cycle at
150 C, as shown in Fig. 2. The billet so obtained was free from any external defects and gained
a sub-micron size of 0.9 micrometers from the initial average grain size of 25 micrometers. After
two cycles of operation, it was found that strength and ductility are influenced by fine grain size
and weak texture. The DRX recrystallization mechanism was indicated. [7]
Fig. 2. Schematic representation of the experiment parameters. Real
material before MAF is shown on top and after (bottom) is shown on
the right with the same nominal dimension. [7]
Research on grain refinement to submicron of Mg-2Zn-2Gd alloy when processed under
MAF concluded with observations made after the completion of the process as (i) Alloy
achieved UFG to submicron grain size (ii) High Tensile strength was obtained and suggested
improved mechanical properties which were suitable for automotive applications [8].
The grain size and texture modifications of AZ31 Mg alloy during MAF were studied to
attribute the enhancement of strength. T​he MDFed alloy showed excellent higher strength and
moderate ductility at room temperature, even at the grain size below one μm. [9]
A study investigated the mechanical properties and microstructure of Al 2014 alloy
subjected to multiaxial forging to a cumulative strain of 1.2, 1.8, and 2.4. They found that the
cumulative strain of 2.4 shows the formation of ultrafine grain sizes in the range of 100- 450 nm
with high-angle grain boundaries after four passes and also improvement of tensile strength and
hardness. The improvement in tensile properties of MDFed alloy was attributed to dislocation
strengthening and grain boundary strengthening effect. [10]
From literature on the grain refinement of low carbon steel through multidirectional
forging, it is found that the initial coarse grains of average 38 µm size fragmented into very fine
ferrite with grain sizes of about 1.2 µm. After MDF, the strength properties were improved
significantly, although uniform elongation and elongation decreased with increasing strain. [11]
The work on the superplastic behavior of Al7075 alloy by Multiaxial-forging observed the
decrease in tensile strength and better maximum elongation for the multiple forged aluminum
samples compared to the base samples. [12]
In literature, it is stated that the multiple directional forging is the simplest method to
achieve larger strains with the minimum change from its original shape and allows the
processing of bulk products. Due to observed increase in hardness from 54 HV to 91 HV after
three cycles of MDF. [13]
Multi-axial forging of IF and HSLA steel and achieved a total strain εc ≤ 20 with an
average grain size of 0.3 μm. An observed increase in strength and hardness was explained by
beneficial structure changes of the investigated material. The Vickers microhardness increased
to ~420 to 450 HV0.1 at the center of the sample after deformation at room temperature as well
as after 20–40 passes at 500–600 °C from its initial value of about 280 HV0.1 for undeformed
material. A further strain increased up to ​ε ~ 28 at 600 °C after 67 passes led to a precipitous
hardness decrease at the center of the sample, most likely, owing to the development of
dynamic recrystallization. [14]
Optimal Parameters in MAF
The MAF procedure is a metal flowing process working in shear method and
characterized by many parameters such as the strain which is imposed in each stain step[15].
The induced strain is evaluated by Eq. (1) (the logarithmic or true strain), where ℎ𝑓 is final height
and ℎ𝑖 is initial height.
𝜀 = 𝑙𝑛 ( ℎ𝑓 / ℎ𝑖 )
(Eq. 1)
Various parameters can be varied in the MAF process with each one referring to a
mechanical property and even in emission techniques[16].
Heat Treatment: ​One of the most important parameters is the forging temperature since
it has a significant effect on forming an ultrafine grain structure. The choice of the appropriate
temperature-strain rate regimes of deformation leads to the desired grain refinement. It is
reported that operating grain refinement mechanism during SPD is a function of strain rate and
temperature [17]. Therefore, it can be expressed by a single parameter called Zener–Hollomon
parameter (Z = 𝜀 exp (Q/RT), which takes into account the effects of both strain rate and
temperature on the deformation behavior. It is reported that a high Z value (lower temperature
and/or higher strain rate) promotes grain refinement through grain subdivision and dynamic
recovery whereas deformation at low Z value promotes refinement through grain subdivision
and dynamic recrystallization [17]
The operation takes over the temperature interval of 0.1–0.5 Tm, where Tm is the
absolute melting temperature, and it is useful for producing large-sized billets with
nanocrystalline structures. [18] It requires lower working temperatures than those commonly
used in conventional manufacturing of semi-finished products. The results from various
experiments exposed that there was an increase in the strength due to the grain refinement that
takes place with increasing temperature. In the experiment by Sojeet Biswas, The processing
was carried out with decreasing temperature in subsequent cycles.
Speed of pressing: Pressing by MAF is typically done using high capability tensile
machines that run normally with high speeds. Usually, the processing speeds will be in the
range of 1–20 mm/s. [19].
Load: ​MAF process is being conducted by universal testing machine (UTM) with the
help of multi axial forging die-set which consists of a female die, plunger and stud. To press the
material through the die chamber, the constant load is applied throughout the pressing process
and the press capacity maintained for this work is 200KN in order to avoid jamming or crack
induced in the workpiece and also inside the die [20].
3. Experiment
The MAF experiment demonstrated through
three non-parallel surfaces of billet are taken, as
shown in black, green, and red colors in Fig 3. ​The
operation takes over high Z value (temperature 500
C and strain rate 10 s^-1) on the AISI 1016 Steel
[21] with initial grains of average 17 μm size,
changing loading direction by 90 degrees to apply
uniaxial compression of e = 0.4 to the longest side
at each strain step, for initial shape after one cycle.
Graphite powder was used for lubrication
during forging. At large strains, the use of graphite
lubrication
causes
relatively
homogeneous
deformation.
initial
afterthe
X, review
Y, Z direction.
[1]
Because
of this,
is with multiples
of 3,6 or
9.​ion
4. Results & Analysis:
Fig 3. Schematic representation of Multiaxial Forging
4.1 Microstructure Evolution during MAF
Many times, the physical properties and, in
particular, the mechanical behavior of a material
depends on the microstructure. Grain size and shape
constitute microstructure. The coarse grains are
elongated in one direction with large amounts of Low
Angle Boundaries (LAB) in the grain interior at low
strain (Fig. 4a), and some deformation bands could
be seen. With increasing cumulative strain, the grain
size decreases as we see some fine and some
coarse grains with substructure. The fragmentation of
the grain is according to the strain gradient, while
leads to refinement, and the grain orientation
distribution becomes heterogeneous and random at
last (Fig. 4d). The structure evolved homogeneously
and gradually to an equiaxed one. No observation of
discontinuous dynamic recrystallization (DDRX).
Fig 4. Microstructural Evolution of Fe-​32%Ni alloy
with the cumulative strain of (a) 0.5, (b) 1.5, (c)
4.5, (d) 10.5 at 773 k and strain rate of 10−2 s−1)
4.2 Evolution mechanism during MAF
The multiaxial forging of the Fe-32% Ni Alloy
(Fig 5) with the elongated and some deformation
bands consists of high dislocation density & LAB
(Low Angle Boundaries). The process of MAF is
usually associated with dynamic recrystallization
(DRX) in single-phase metal alloys​. T
​ he MAFed
triggered dislocations are pinned and accumulated
near the low and high angle grain boundaries,
leading to the formation of dynamically recrystallized
UFG grain with subdivision of grain into subgrains
and with subgrain rotation the High Angle Grain
Boundaries (HAB) increase. This recrystallization
mechanism is shown schematically in figure 6 ​as
deformation bands are introduced, usually at 35-40
degrees, to the forging direction. Then changing the
loading direction leads to another deformation band
created intersecting the initial one. The rotation is like
dislocation movement or kink (slip process and offset
created in the first deformation band), again the new
band cuts all previous bands.
It is, therefore, that the deformed band which
crosses each-other subdivides the grain into several
subgrains, and these subgrains gradually rotate to
become grains with their boundaries being
transformed to high angle boundaries.
.
Fig. 5. Subdivided Fe–32%Ni alloy grains
Fig. 6. The evolution mechanism of continuous dynamic recrystallization during
multiaxial forging (a) initial grain, (b) after the first pass, (c) after the second pass with
deformation direction rotated 90°, (d) subgrains angled in subsequent passes.
4.3 G​rain boundary character distribution
The polycrystalline materials have some
atomic mismatch within the region where two grains
meet; this area, called a grain boundary. Initially,
the grains are well recrystallized and equiaxed as
the received annealed plain carbon steel had a high
95 fraction of High Angle Boundaries, but as seen
in Figure 7, it soon decreases to 30 fractions.
Because of multiaxial forging, grains subdivide, so
they increase in the fraction of LAB and decrease
HAB through subdivision or CDRX methods of
grain rotation, like conversion to HAB through the
continuous recovery of dislocation into the LAB.
Different materials have different mechanisms.LAB
friction is increasing to 70 percent after the third
pass. The initial phase is the development of
substructure, which is LAB within the grain. After
5th pass, again, HAB friction increases from 30 to
60 and further deformation to 70. This subgrain
boundary converts to HAB. After the 9th pass, the
UFG material and all are HABs.
Fig. 7 Variation of LAB and HAB with No. of
strain steps.
Grain boundary characterization indicates
grain refinement through grain subdivision and
recovery in steps three, six, and nine according to
the experimental steps to complete each cycle.
4.4 Distribution of grains during MAF
Various researchers [22-24] have observed
a decrease in strength with a reduction in average
grain size, due to the difference in texture. In
Figures 8 and 9, Multiaxial Forging was on as-cast
AZ61 Mg [25] into an extruded rod. Mg has HCP,
so deformation is difficult. We can achieve a strain
of 4 after 6th pass or so through the continuous
change of deformation. Initially, the inhomogeneous
microstructure was observed, but by the 3rd pass
(whole cycle,) it decreased and 5th or 6th. It’s
homogenous if considering the longer cycle,
multiple deformation directions. It is not in ARB, as
in it, the deformation direction was not changed. It
affects mechanical properties because, by 6th
pass, the hardness strongly increased 58 HV to 74
HV (Fig 9) as a function of equivalent strain.
Fig 8. Plot of grain size as a function of equivalent
strain.
Fig. 9 Plot of volume fraction of hardness vs.
equivalent strain
4.5 Softening by MAF
Softening is proposed to be through the
interplay between geometrically necessary
dislocations and grain size. ​Softening during
deformation (measure flow stress during
deformation) Al alloy deformed at room
temperature. In ref. [26], the first cycle was
100MPa next to 120/130, and continuously flow
stress is increasing, after the sixth pass, there is
a decrease in flow stress. Attributed change to:
a) The grain boundary is increasing.
Fine-grained materials will have a higher
density of grain boundaries per unit volume,
with decreasing grain size the effective obstacles ​Fig. 10. Stress as a function of the fraction of no.
to dislocation motion. Hall Petch relationship:
​of grain boundaries contributing to hardening
𝜎 = 𝜎0 + (𝑘 / √d)
​The average grain size is defined by d, and 𝜎 is the
yield strength. The value of 𝑘 is dependent on the
numbers of slip systems. It is higher for metals with
hexagonal close-packed structure (HCP) than
metals with body-centered cubic (BCC) or
face-centered cubic (FCC) structures. [16]
b) Decreasing dislocation density (contribute to
work hardening or strain hardening). When
both contributions increase but afterward no
contribution from grain size, therefore,
softening happens or maybe the effect of
microstructure
properties/change
in
mechanism like sliding.
Fig. 11. Flow stress during pressing vs.
equivalent strain. Each segment is for one
pass.
4.6​ H
​ ardening by MAF
Another mechanical property that
may be important to consider is hardness,
which is a measure of a material’s
resistance to localized plastic deformation.
Cherukuri and Srinivasan [27] conducted
multi-axial compressions/forgings (MAC/F)
at room temperature to get severe plastic
deformation (SPD) of AA6061 alloy. They
measured the microhardness across the
cross-section of each and every pass of
the MAC/F process and found that the
hardness distribution is not uniform during
early compressions/forgings and becomes
uniform with subsequent forgings
Double-n behavior is observed
during tensile testing of some multi axially
forged steels. The hardness values of
multiaxial forged steel increased by more
than 100% after eighteen warm multiaxial
forging strain steps. In another research,
Increase in the hardness of nearly 130%
was observed in the sample with
maximum forging passes from 127 HV for
0 pass material to 285HV for nine forged
pass materials.
Two different methods MAF and
ECAP (Equal Channel Angular Pressing)
showed similar trends in hardness;
therefore, as MAF can produce larger
specimen size compared with ECAP along
with less costly equipment compared to
ECAP.
​Fig. 12 Variation of hardness with strain
5. Conclusion
Based on the available literature, it is concluded that
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Various strengthening mechanisms are involved during the multiaxial forging process,
namely dislocation strengthening, grain boundary strengthening and accumulation of
high dislocation density, etc.
In all the studies, researchers proved that multiaxial forging is an effective process to
refine the grain size (up to nano level) of the materials, thereby increasing the properties
of the materials like tensile strength, flow stress, yield strength, and hardness.
MAF is one of the most appropriate methods for obtaining a fine grain size and a weak
texture in a material, leading to desired mechanical properties.
Enough laboratory results demonstrate the general feasibility of this approach. It is
recognized that these materials have a high innovation potential, as the results of
enhanced mechanical properties can be used in the aerospace industry as they need
light-weight micro-parts with reliable high strength.
MAF is also highly attractive primarily because of its ability to provide near isotropic
properties, therefore the improvement in ductility, tensile, and fatigue strength.
MAF has been used for Al and Ti alloys. Still, the research shows that it can be
implemented on the Mg alloy, which is generally hard to deform due to HCP lattice, low
ductility, and anisotropic structure but has tremendous potential.
Although MAF was first introduced in the 1990s to produce bulk billets, there have been
many modifications to give optimal properties of materials. If the suitable parameters like
temperature, or adding process like cold rolling showed high strain rate sensitivity and
elongation to failure in the range of 300–350C, which indicates good formability.
The research is done on Flowing stress, Hardness, and Uniformity of the grain
distribution during MAF. It shows that MAF can be implemented to make cost-effective
equipment requiring high tensile strength, biomedical devices, and automobile parts for
lightweight yet sturdy and robust travel gear.
The most significant benefit of the MAF process is that it can produce a large amount of
strain without the geometrical change in material and fracture.
This literature review was on the beneficial structural changes happening during the
MAF process.
This review would help improve thermo-mechanical processes to decide what amount of
strain to produce to obtain deformation, texture evolution, and grain refinement.
The contribution of the strain to the hardness or mechanical properties like elongation to
failure and ductility are discussed with respect to microstructure evolution and the
mechanism behind it.
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Figures:
Fig. 1 ​http://nanotech73.blogspot.com/p/preparation.html
Fig. 2​ IIT Kharagpur, India GIAN (​https://www.youtube.com/watch?v=TR2r15UakB4​)
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1016 steel, Materials & Design, Volume 31, Issue 8, 2010, Pages 3816-3824, ISSN 0261- 3069,
https://doi.org/10.1016/j.matdes.2010.03.030
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