Study on the mechanism of force calculations in flow forming a review

International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN
0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 3, April (2013), © IAEME
194
STUDY ON THE MECHANISM OF FORCE CALCULATIONS IN
FLOW FORMING: A REVIEW
1
G Venkateshwarlu, 2
K Ramesh kumar, 3
T. A. Janardhan Reddy, 4
G. Gopi,
1
Assistant professor, Department of Mechanical Engineering, University college of Engineering,
Osmania University, Hyderabad, Andhra Pradesh, India. –500007.
2&4
Scientist-‘G’ and Scientist-‘D’, Defence research development and laboratory,
Hyderabad, Andhrapradesh, India –500012
3
Professor and Head, Department of Mechanical Engineering, CVR college of Engineering,
Hyderabad, Andhrapradesh, India,–500013;
ABSTRACT
In metal working industry, flowforming processes is widely used. Flow forming is a metal
forming process in which a shorter and thinner ring, called preform, is elongated to a thin and long tube
over a rotating mandrel by means of three rollers. In this method, a hollow axisymmetric preform is
affixed to a mandrel. When the both are made to rotate, compression forces can be applied on the
outside diameter of the preform by hydraulically-driven CNC-controlled rollers. By a premeasured
quantity of wall thickness reduction, in one or more passes, the material is compressed above its yield
strength, plastically deformed and made to flow. The needed geometry of the workpiece is gained and
equipped when the outer diameter and the wall of the preform are reduced and the available material
volume is forced to flow longitudinally over the mandrel. Typically, the preform can be flowformed up
to six times its starting length before a need for reannealing of the metal is required. There are certain
parameters like mandrel speed, roller feed, roller geometry etc. which directly affect the dimensional
accuracy of flow formed component. In connection to these parameters the work piece metallurgical
properties like hardness variation and grain size variation will have effect on the dimensional accuracy
of flow formed components. The inside surface quality of the finished workpiece almost resembles
with the outside surface quality of the mandrel. Flow forming is used to produce rocket nose cones,
rocket motor cases, gas turbine components and dish antennas in the aerospace industry. It can also be
used to produce power train components and wheels in the automobile industry, gas bottles and
containers for storage applications. The analysis of flow forming has been undertaken by several
researchers. Most of the work is on the soft materials like lead, aluminum, low carbon steel, copper etc.
A very few authors have tried with the forming process of hard-to-work materials. Concise definitions
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ISSN 0976 - 6480 (Print)
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195
of the research done by other researchers on soft as well as hard-to-work materials is observed and
comprehended here. Advances in process simulation have been spurred essentially by the development
of general-purpose, finite-element-method (FEM) packges such as DEFORM, ABAQUS, and MSC.
Mar are generally applied for the study of elastic-plastic structural problems. The main advantages of
these methods are that they reduce the computational time, and make possible some degree of
optimization of the process parameters. This paper presents a study on the various mechanisms of flow
forming process and the potential applications of it in defense, automotive and aerospace industries.
Index Terms: CNC Flow forming, degree of optimization, Elasto-plastic FEM, Mechanics,
Power spinning, Preform, staggered flow forming.
NOMENCLATURE
= initial thickness (mm)
tf= final thickness (mm)
f= feed (mm/min)
R= roll radius (mm)
kc= strength coefficient of material
n= strain hardening coefficient
ft= tangential force (N)
fa= axial force (N)
fr= radial force (N)
ε= strain(three times of ln(ti/tf))
σm1 = Effective and mean effective stress
Φ = Reduction angle
CS= over roll depth (mm)
ρR=roller nose radius(mm)
α = half – apex angle of mandrel (deg)
= effective stress corresponding to an effective strain cot
= outer cone radius
= average value of deformation zone angle
dε= infinitesimal effective strain
At, Ar, Aa = projected areas of the contact surface between roller and cone in the tangential, radial, and axial
directions, respectively.
N = speed of rotation of the mandrel, rpm
σ = effective stress,
σm = mean effective stress.
dε = total effective strain
α = one half the cone angle, deg
θo, θo = angle for deformation zone and its average value, respectively, radian
θo’,θo’ = angle functionally related to θo and θo respectively, radian
1. INTRODUCTION
Flow forming is a relatively new technique which is ideally suited to take the place of the current
forging and machining processes in components like the mortar tube. The flow forming operation
begins with a considerably short and thick-walled hollow cylinder preform.The preform is fit to a
mandrel with the same diameter as the product. Circumferential rollers rotate axially along the preform,
thus cold working the billet, making it thinner and longer, until it has a near net shape to that of the
finished mortar tube. Flow forming produces a highly uniform, round and smooth, surface finish. The
amount of time spent on machining is additionally reduced, which effectively reduces fluid
replacement costs after machining. For most applications three rollers are used.
2. CLASSIFICATION OF FLOW FORMING

196
The classification of Flow forming process can be done based on two criteria.
(i) According to fixing necessities of preform shapes.
(ii) According to length of position of rolls during the forming process.
2.1. According to fixing necessities of preform shapes
Figure.1.Forward Flow forming Figure.2.Backward Flow forming
The First type is forward flow forming (Fig.1) and it is used to form preforms which have a
shape with one side is partially closed or fully closed. In forward flow forming, a tailstock is used to fix
the preform to the mandrel. The elongation of the workpiece during forward flow forming is at the same
direction with the relative axial movement of the rollers. Second type of flow forming is backward flow
forming (Fig.2) and it is used to form preforms with a constant hole inside. In backward flow forming,
a toothed ring is used to fix the preform to the mandrel and it is also used for reloading of the finished
workpiece. The elongation of the workpiece during backward flow forming is at the opposite direction
to the relative axial movement of the rollers. For precision long flow forming operations, typically three
rollers placed with 120° design is used. These rollers have precalculated radial and axial offsets
between each other to achieve necessary forming conditions shown in (Figs.3,4). The analysis of tube
making has been adopted by several researchers.
Figure.3. the flow form process Figure.4. Offsets of 3-rollers
2.2. According to the length of position of rolls during the forming process

197
2.2.1. Staggered-Roll Flow Forming Process
In the staggered-roll process, three rolls are staggered axially and radially. Each roll has a
specific geometry and job function during the forming process. The three rolls, which are positioned at
120o
from one another moving equally. They are driven by hydraulic force and rotate at the same speed
as the mandrel, so that, when they initially contact the preform they are moving at the same speed so as
not to score or gall the preform on initial contact. After the flow forming action is initiated, the rollers
are disengaged so that, they spin only by the friction of the rotation of the preform on the mandrel and
are then no longer driven by the hydraulic force. The staggered-roll process is used primarily for cold
forming, tubular products with thin walls open ends, and closed or semi-closed cylinders on one end.
The staggered-roll configuration allows for greater contact surface with the material than that of the
in-line process, which keeps the material from belling in front of the rolls and allows for a larger
reduction in cross sectional area in one pass. As noted, each roll has a specific geometry and job
function. The first roller is predominately responsible for making sure the “wave condition” on the
preform doesn’t fold over and flake. The second roller is predominately responsible for forming the
large wall reduction. Usually this angle is steeper than the first because, it is not concerned with the
“wave phenomena.” This facilitates and maximizes the amount of wall reduction that can be taken in
one pass. The last roller does not take as much of a reduction as the second roller, and it is responsible
for the finish or burnishing of the part. The sharper the leading angle, the better the surface finish will
be on the flow-formed part.
2.2.2. In-line Flow Forming Process
Unlike the staggered-roll method, flow forming with in-line rolls is characterized by either three
or four rolls that are in-line both axially and radially (Fig.6). The rollers also are not independently
driven as in the staggered roll process, rather than the rollers begin to rotate as they engage the spinning
workpiece.The rollers engage the workpiece at the specified angle and feed rate so as not to create
galling or other dimensional or surface imperfections. Some of the other distinctive characteristics of
the in-line process are described later in this section. Some of the distinctive characteristics of the
in-line process are
Both cold and hot flow-forming processes can be readily used depending on workpiece ductility
and the amount of area reduction taken at each pass.
Integral-end fittings or flanges can be rolled on both ends of the tube maintaining taper angles and
linear dimensions consistently during the run without adjustment to the tooling.
Springback, straightness, and dimensional distortion are controlled by roll configuration, feed rate,
and rate of rotation.
The roll arrangement permits self-centering of the component between the rollers, with the
provision that the work is divided equally between the rollers and the forces are perfectly balanced.
3. Flow Forming Process Details
In flow forming, as shown schematically in Fig.7, the blank is fitted into the rotating mandrel
and the rollers approach the blank in the axial direction and plasticize the metal under the contact point.
In this way, the wall thickness is reduced as material is encouraged to flow mainly in the axial direction,
increasing the length of the work piece. The flow of metal directly beneath the roller consists of two
components, axial and circumferential.

198
Figure.7. Principal of flow forming (deformation zone and forces)
If the length of circumferential contact is much longer than the axial contact length, then the
axial plastic flow will dominate the circumferential one. In this case, reduction in thickness will
resemble that of plane strain extrusion and a sound product will be produced. On the other hand, if
the opposite is true, then circumferential flow will dominate leading to high constraint of flow in
the axial direction. This situation will normally give rise to bulges in front of the rollers causing
defects. As the workpiece volume is constant, with negligible tangential flow, the final component
length can be calculated
As L1 = L0 S0 (di + S0) /S1 (di +S1) ……………. (1)
Where
L1 is the work piece length,
L0 is the blank length,
S0 is the starting wall thickness
S1 is the final wall thickness,
di is the internal diameter.
4. MAIN ADVANTAGES OF FLOW FORMING
Flow forming is a chipless, seamless and cold manufacturing processes. Improved material
properties such as yield strength, fatigue life, etc. Manufacturing capability of very accurate long
hollow parts. Preventing secondary operations such as turning, grinding, etc..
Highly Precise, seamless construction to net shapes
Improved mechanical properties
Tubular, conical & contoured geometry
Uniform axially-directional, and stable grain micro structure
Better surface finish
Very high diameter-to-length ratio
Repeatable accuracy part-to-part & lot-to-lot
5. TYPICAL APPLICATIONS OF FLOW FORMING
So=starting wall
thickness
S1=Finished wall
thickness
L0=Starting Length
di=Inside diameter
FR=Radial force
FA=Axial force
FT=Tangential force
δ =Trailing angle
γ =Leading angle
r = nose radius

199
Fast and economical production rates inclined to other methods various applications are listed
Tubular- type components i.e., Missile casings, flight and launch motor housings, Rockets and
cartridge case.
Rocket nose cones, rocket motor cases, gas turbine components and dish antennas in the
aero-space Industry.
Power trained components and wheels in the Automobile Industry, and gas bottles and
containers for storage applications.
The manufacturing of thin walled tubes and closed and cylinders for the chemical, nuclear,
food, pharmaceutical, cryogenic, beverage, filtration and printing Industries.
Mass production of small containment vessels for drum packages.
Various typical applications are shown in fig.8. Furthermore, flow forming produces a highly uniform,
round and smooth, surface finish. Flow forming method also allows achieving high dimensional
accuracies and required to mechanical properties.
Figure.8. Typical flow forming applications
6. LIMITATIONS OF FLOW FORMING
Following are the limitations of flowforming
The flow forming process is limited to forming radially symmetrical hallow parts such as
cones and cylinders.
Flow forming reduces the ductility and elongation of the material and spring back may be
problem with some results.
Not competitive with deep drawing for long production runs accepting even when counter are
complexes.
Tooling cost for flow forming are lesser than for drawing, but advantage is lost in long runs
because of longer operation time per piece.
7. CONCLUSIONS

200
According to the analytical models stated above, most of the researchers have worked on the soft
materials like lead, aluminum, low carbon steel, copper etc. A very few authors have tried with the
forming process of hard-to-work materials. A brief description of the work done by other researchers on
soft as well as hard-to-work materials was studied. The studies revealed that more power was consumed
while working on the hard materials. While working on soft materials observed that the increase in
percentage of reduction, increase in power consumption of radial force and axial force. As the diameter
of the roller increases there was increase in power consumption, axial force and the radial force. This is
due to the fact that as the diameter of the roller increases, more volume of the material comes into
contact due to which more power and forces are required.
Most of the analytical models developed the tangential force by few authors. This is because
the tangential force consumes most of the power in the Spinning, and it is thus significantly important
for the design of spinning machines. From the calculations the tool force based on the assumption that
the deformation mode in spinning is a combination of bending and shearing. Moreover, by assuming
uniform roller contact pressure,
Through comparing the finite element simulation results with the Thamasett algorithm
calculation results of spinning forces, the finite element simulation results differ slightly from the
Thamasett algorithm calculation results of the spinning force measurement experimental results.
As review, a result of the flow formed component will have considerably higher mechanical properties
than the ones of the starting material. Typically, the preform material is plastically deformed with wall
reductions causing a substantial refinement of the grain structure and a total realignment of the grain’s
microstructure in Flow forming method also allows to achieve high dimensional accuracies and
required mechanical properties.
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