Introduction
One of the directions of the progressive development of rail transport is the introduction of new metal materials for the manufacture of various structures. The main advantages include small specific gravity, strength, corrosion resistance, ease of recycling, etc.
Aluminium alloys are constantly improved, as well as their mechanical and physical properties, with the simultaneous development of innovative methods of their processing.
When making the structures of intricate shapes, welding is accompanied by certain difficulties. The aluminium alloys ability to weld is complicated by the formation of refractory foils, high thermal conductivity and castability, the tendency to form hot cracks, cavities and others.
Almost all of these disadvantages can be eliminated or significantly reduced when using welding without metal melting. Such methods are types of welding by pressure (Fig. 1), with the localization of heat dissipation in near-surface layers of the edges caused by transformation of mechanical energy (frictional forces) into the heat one.
Fig.
1. Friction welding process schemes:
a
– traditional; b
–
vibro friction; c
–
of sheet workpieces
using
a blade; P
– pressing force
Similarly, during the friction stir welding (FSW) [10], the heat source is the friction forces that arise between the surfaces of the welded edges and the working part of the special tool [5, 12–17].
The FSW process parameters include:
– Rotation frequency of the welding tool around its own axis;
– Speed of welding (speed of movement of the welding tool along the welding edges);
– Angle of the tool relative to the welded surface normal;
– Force of pressing the tool to the edges;
– Geometrical dimensions and shape of the working part of the welding tool.
The above characteristics have a different effect on the welding process and the quality of the weld joint [7, 13]. At the same time, the high sensitivity of FSW process to changes in process parameters makes it difficult to support stable welding conditions. For example, the rotation frequency of the tool and the speed of its movement along the edges allow for a wide range of changes in conditions of metal heating and connection. Another matter when it refers to geometric dimensions and tool shape. The intricate form of the elements of the tool significantly complicates its use when changing the range of products under the conditions of friction stir welding (Fig.2, а).
Each of the tool parts provides not only the transformation of mechanical energy into heat, but also its specific distribution along the intersection of the weld seam [17]. The action of the shoulder brings up to 90% of the total energy [12], the remainder is due to the pin. The above correlation indicates the main impact of the shoulder on the stability of FSW process.
In some cases, the working surfaces of the tool may have grooves, flutes, spirals [11], etc., which allows a high level of destruction of the oxide foil of aluminium alloy surfaces and increase in degree of diffusion between the weld metal, which in turn improves the mechanical properties of the joint [1, 12, 14, 15].
The working tool pin is intended for the distribution of thermal energy over the thickness of the welded joint, the crushing of the oxide foils of the ends of the welding surfaces and their activation, ensuring diffusion during welding.
The shape of the working tool pin can vary considerably depending on its diameter, the tool inclination angle during welding, the thickness of the base metal and its chemical composition, which in turn will affect the quality of the welded joint. Similarly to the surface, the shoulder pin may have a groove that helps control the flow of plasticized metal and direct it to the sealing of certain segments of the seam, which reduces the likelihood of the lack of root fusion.
Compared to the technologies of partial or full melting, the use of friction stir welding can significantly reduce the degree of overheating of the metal in the zone of thermal impact and, accordingly, the resulting deformation of the welded joint and the product as a whole [1, 2, 7].
Purpose
Improvement of the procedure for determining the optimum radius of the shoulder of a special tool depending on the variations of base metal thickness.
Material
and methodology
of
the research
The FSW process (Fig. 2, a) was carried out on a specially designed laboratory equipment using a variety of welding modes [2, 6, 12, 17].
The welding tool (fig. 2, b) was made of P9 grade high-speed steel, which retains its mechanical properties to the melting temperatures of aluminium-based alloys. The tool was used after a special heat treatment hardening.
For the purpose of high-quality metal filling, the axis of the working tool is located at an angle of 1...3° relative to the normal to the metal surface. Normal effort was chosen under the conditions of immersing the tool into the billet to a depth of up to 10% of the weld edge thicknessThe shoulder had a diameter of 12 mm and 1.72 mm long cone-shaped pin with a diameter of 3.4 mm.
a
b
Fig.
2. FSW process of two workpieces (a)
and
working
tool (b):
1A,
1B
– workpieces; 2
–
stick; 3
– working tool; 4
– pin;
5
– shoulder; 6
– source of rotation (engine);
7
– direction of welding
The speed of the tool rotation was changed in the range of 800...1600 min-1, with the force of pressing on plates not higher than 1.4 kN. The speed of tool movement along the seam was maintained constant – 50 mm/min.
Materials for researches were 1.85 mm thick plates of AMG3 alloy with the content of alloying elements within the brand composition.
The temperature and pressure from the tool in the welding zone were controlled in a specially designed stand mounted on a vertical cutting bench. The force of working tool pressing to the edges was measured by a dynamometer of type DS-0.1.
The metal heating temperature was measured by thermocouples of chromel-aluminium type with an arrangement at different distances from the axis of the weld seam.
Findings
In different ratios, the working tool rotation speed and its pressing to the connecting edges changed the degree of metal heating.
Fig.
3 shows the research results of the influence of the FSW process
parameters on the metal heating temperature under the shoulder of
the working tool.
Fig. 3 – Experimental curves of dependence
The
analysis of
the type
of the
ratio
Estimation
of the initial recrystallization temperature (TR)
for single-phase metallic materials
(TR
= 0.4·TM,
where
TM
is the metal melting point) showed a sufficiently qualitative
coincidence with the values of 76...78°С.
A
detailed analysis of the shape of the curves
On
this
basis,
the
moment
of
formation
of
the
horizontal
section
should
be
considered
as
a
limit
on
the
minimum
temperature
of
the
FSW
process,
and
a
sharp
decrease
The
shape of the curves and the qualitative change points in the ratio
Sufficiently
complex character of the simultaneous influence of several
technological parameters of the FSW process on plastic properties
demanded to estimate the required energy (Q)
for heating the metal to a temperature of 0.7TM.
The value Q
was determined by the ratio of type [9], taking into account the
experimental conditions:
where
The
calculation of Q
was carried out according to experimental data (Fig. 3) for the
alloy plates under steady state conditions:
Fig.
4 – Q dependence
on the
simultaneous influence
The
given dependence (Fig. 4) shows a rather complex simultaneous impact
on Q from
The
analysis
of
the
results
shows
that
in
the
case
of
constant
pressing
force,
the
double
change
of
The
result
of
Fig.
5 Change of Q
depending on
The
combined analysis of the Q
calculated
values (Figs 4, 5) indicates the need to determine the optimal
energy for friction stir welding and possible dependence on the
geometric dimensions of the working tool. Taking into account the
contribution of the shoulder to the energy balance of the FSW, an
attempt was made to estimate the optimal diameter of the work tool
shoulder for different thicknesses of the connecting edges. To this
end, the relationship between the thermal balance of the friction
welding process and the main technological parameters [3] was used:
where
Given
that the relation (2) includes unchanged values, the expression
should be simplified:
where
Then
the expression (2) will look like:
From
the relation (4), Rt
is equal to:
The
calculation results for
Rt
according to (5) using the experimental data [8] and the random
values of Q
for the studied change intervals
Fig. 6 – Diagram
of selecting the working
The
analysis of the results (Fig. 6) shows the existence of a
correlation between the energy Q
in the welding zone and the size of the working tool shoulder radius
Rt.
At higher speeds of the tool rotation (Fig. 6, curves 3 and 4), the
same values of Q are achieved with decreasing
Rt.
In
the process of welding, holding the tool at an angle of 1...3˚
relative to the normal to the blank
provides the necessary conditions for filling of the weld bath with
the metal. The expected unevenness of temperature distribution along
the plane of the contact spot (shoulder) occurs only in the initial
stages until reaching the optimal welding conditions. After this,
completely homogeneous conditions for the heat energy release
process are achieved.
When
designing a working tool, it should be borne in mind that excessive
increase in the shoulder diameter will increase the load on the
equipment and undesirably increase the width of the weld, as well as
using high frequency rotation of the tool can lead to overheating of
the weld connection, which negatively affects its mechanical
properties.
In
addition to the influence of these factors, the choice of welding
mode depends on the chemical and phase composition of the alloy, the
morphology and location in the matrix of the particles of the second
phase. Of particular importance is the thickness of the metal edges.
In
order to take into account the influence of the welded metal
thickness and its thermal properties on the achievement of optimal
conditions, it is proposed to use the similarity criterion [4]:
where
By
relation
(6), for
a
particular
alloy,
under
unchanged
conditions
The
value of the energy Q
for any thickness of the base metal is estimated by the following
expression:
The relation (7) taking into
account (8)
where
Taking
into account (1), the same level of Q
can be achieved by different combinations of
Fig. 7 –
Dependence of the required amount
The
given dependence allows us to determine the optimal conditions for
friction stir welding for plates of different thicknesses. For
example, for δ
= 3mm the required amount of heat energy should be of about 70 kJ.
Further, for this energy value, for a particular equipment with a
specified power, we determine the optimal rotational speed of the
tool and the corresponding radius of the shoulder.
Determination
of the value
Rt,
subject to the simultaneous consideration of the tool pressing
force, the rotation speed and the metal thickness, is proposed by
the ratio:
The
calculation results for (10) are given for the thicknesses 1<δ<10
mm in Fig. 8
The
constructed diagram for (10) corresponds to the conditions of
constant pressing force (0.275
kN), although the optimal Q
level for welding (FSW) is determined by the combination of
The
verification of the dependence (Fig. 8) was carried out
experimentally. So, under the constant pressure conditions (0.275
kN) for welding the 3
mm thick edges
with ω
= 1250 min-1,
the tool
radius should be 9...10 mm, and for the frequency of 1600 min-1
– approximately 8...9 mm.
Fig. 8 – Diagram
of determining the optimal working tool shoulder radius Rt
to achieve the established welding process at:
Under
conditions when the justified necessity of finding the optimal
welding mode is based on changing the working tool pressing force,
the calculation of
Rt
is carried out according to another diagram, for a specific value of
Thus,
according to the estimates, it is possible to improve the process of
finding the optimum welding conditions for the friction stir
welding.
Originality
and practical value
The
research determined the effect of the working tool rotation speed
and the magnitude of its pressure on the welded edges on the
temperature in the weld zone. The increase of
Conclusions
It
has been established that it is possible to obtain welded joints at
a temperature of about 0.7
Tm
during the friction stir welding.
At
different
ratios
of
the
working
tool
rotation
speed
and
pressing
to
the
connecting
edges,
the
conditions
for
achieving
the
effect
of
constant
softening
of
the
alloy
have
been
determined.
The
main technological parameters of the working tool have been
calculated and their influence on the generation of thermal energy
in the welding zone has been determined.
The
thermal analysis of the welding process resulted in development of
the procedure for determining the technological parameters of the
working tool depending on the weld metal thickness.
of the alloy
edge heating temperature under
the shoulder on the speed of
rotation
and the pressing force
of the working tool
(Vw = const):
1
– ω = 800 rpm; 2 –
ω = 1250 rpm; 3 –
ω = 1600 rpm
resulted in determination of the
optimal
temperature of
the heating
of the
metal edges.
First, a qualitatively different form of
dependence corresponded to a change in the structural
transformations in the metal edges. Regardless
of the
rotation speed
of the
tool and
its pressing
force, the
minimum
temperature was
in the
relatively
narrow range
of 70...85°С
(Fig. 3, points A, B, C). Taking into account possible uncontrolled
deviations with the support of stable conditions of the welding
process, to simplify the analysis of the thermal balance, the
average temperature value of 76...78°C was adopted, which
corresponded to the moment of change in the ratio type.
(Fig. 3),
taking into account the existing qualitative changes in structural
transformations under hot compression, suggests that from the moment
of formation of the horizontal section (A – D; B – E; C –
F) to a sharp reduction of the pressing force (point D, E, F), the
processes of internal alloy reorganization are due to the
development of dynamic polygonization and recrystallization.
–
as the
maximum
acceptable
value.
indicate the dependence of conditions for the achievement of the
metal superplastic state on the FSW process parameters. Indeed, if
the initial plasticization temperature remains practically
independent of the ratio of
and
,
then the
achievement of the superplastic state (P
decrease area) is more or less determined by the value
.
, (1)
– tool rotation frequency;
– pressing
force of the working tool,
–
temperature
(
),
–
power characteristic,
– exponent.
=
13.3
(800 min-1),
P = 1 kN;
= 20.8
(1250 min 1),
P = 0.59 kN
and
=
26.7
(1600 min-1),
P = 0.275 kN.
The temperature in the edge zone was 0.7
TM.
The result of the Q
estimate depending on
for points J,
H, G is shown in Fig. 4
of
and
(points J, H,
G) under
conditions
of
temperature reach
of 0.7 TM
and
.
On this basis, it is a definite interest to evaluate the impact of
any one characteristic. The result of the calculation of Q
depending
on
with
(0.275 kN) is
shown in Fig. 5.
leads to Q
change at the range of 10...12%, which corresponds to the average
value of
(Fig.
4, point G).
impact
on
,
in
comparison
with
the
pressing
force, has
a
definite
confirmation.
The fact is
that the change interval of
is 2 times
against the change of
more than
3.6
times. Thus, the experiment plan showed a different effect of the
FSW technological parameters on the processes of heating the
connecting edges.
with P=0.275kN, (2)
– energy
characteristic;
– friction
coefficient;
– specific
normal tool
pressure;
– angular
speed of
rotation; Rt
– radius of
the tool
shoulder.
, (3)
– constant coefficient.
, (4)
, (5)
and
are shown in
Fig. 6.
tool shoulder
radius Rt for
P=0.275 kN,
depending on
:
1
–
800;
2
–
1250;
3
– 1600; 4
– 2000
min-1
● –
Experimental data Q
from Fig. 4, (6)
– characteristic, similar to Q
from (2);
– thermal
conductivity;
– thickness of
the metal edges; Т
– temperature
in the welding zone, K.
and
Т
(optimal
heating
temperature
of
edges
in
FSW),
the
value
varies
in
a
very
small
range
of
values,
which
is
confirmed
by
the data
[4] and
experimentally
in
the work.
Given that
the thickness of the welded metal can vary in a wide range, the
calculations of
Rt
are limited in thicknesses from 1 to 10 mm, which are the most
common in the industry.
, (7)
, (8)
is constant, takes the form:
, (9)
and
.
Then,
for
(9), Q
is
in
fact
the
mean
for
the
range
of
changes
of
and
,
that
has
a
definite
confirmation.
So, for the
thickness δ = 1.85mm, the received energy value by the ratio (9)
and the average value Q
under the
three modes (40 kJ) have a qualitative coincidence (Fig. 7).
of energy Q by (9) for
the established process
of welding on the metal thickness
δ
, (10)
and
.
P
= 0.275 kN,
Vsim =
const:
1
– 800; 2
– 1250; 3
– 1600;
4
– 2000; 5
– 2500; 6
–
3000
min-1.
allows
reducing the pressure of the working tool during welding. The
conditions for the achievement of the superconducting state of the
metal in the welding zone and the optimal temperature range have
been determined.