Applied Mechanics and Materials Vol. 770 (2015) pp 253-257
© (2015) Trans Tech Publications, Switzerland
doi:10.4028/www.scientific.net/AMM.770.253
Submitted: 2015-03-04
Accepted: 2015-03-18
Online: 2015-06-22
Study of Wind Velocity Impact Upon the Quality of Shielding and Upon
the Thermal Processes Under MAG Welding
D.A. Chinakhov 1,a, A.V. Vorobyev1, E.G. Grigorieva1, E.I. Mayorova1
1
Yurga Institute of Technology Tomsk Polytechnic University
652055, Russia, Kemerovo region, Yurga, Leningradskaya str. 26,
a
chinakhov@tpu.ru
Keywords: welding, shielding gas, thermal fields, simulation.
Abstract. In the given paper we consider the impact of wind velocity upon the active shielding gas
flow and changes of thermal processes in the MAG welding area. The authors completed numerical
simulation of consumable electrode welding under traditional and two-jet gas shielding. It was
established that application of two-jet gas shielding for MAG welding increases the hardness of the
shielding gas jet and reduces wind-related displacement of thermal fields in the welded item. This
ensures more qualitative shielding of the welding area under the windy conditions and uniform heat
distribution in the welded item which leads to more homogeneous structure of weld and HAZ metal
in comparison to the traditional (one-jet) gas shielding.
Introduction
At the present time a topical issue of field welding is protecting welding area against the weather
impact. The windy conditions significantly deteriorate the protective properties of shielding gas and
affect the processes in the welding area. MMA is widely applied under the field conditions but it is
characterized with low efficiency, so producers are trying to replace it with machine metal active
gas welding (MAG).
Reliable protection of the welding area is one of the basic conditions of producing a high-quality
weld joint. Protection should be provided until complete hardening of the weld pool. In most cases
the protection is achieved by supplying a jet of shielding gas to the welding spot. The outward part
of the shielding gas jet is inevitably mixed with the air and only its inward part consists of 100%
shielding gas (Fig. 1) [1-6].
mm
Fig. 1. Composition of carbon dioxide gas jet flowing from weld nozzle:
1 – 100 % СО2; 2 – СО2 + 10 % of air; 3 – СО2 + 60 % of air; 4 – СО2 + 80 % of air
Gas-shielded arc welding is based on the principle of forcing the air out of the welding area by a
flow of shielding gas. At the present time there are many widely used methods of gas-shielded arc
welding.
Gas-shielding is one of the most frequently used protection methods when welding. Through
varying active shielding gas flow rate and distance from the nozzle to the surface of the welded
metal we can control the rate of weld metal crystallization. When welding with active gas shielding
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Urgent Problems of Up-to-Date Mechanical Engineering
we can provide protection only for the fusion zone. The gas flow is laminar under local gas
shielding, but if the gas flow rate reaches its critical value the laminar flow imbalance is observed
leading to turbulent shielding gas flow (with strong whirling). As a result up to 50% of air are
mixed into the shielding gas flow at the distance of several millimeters (6-10 mm) [1-3, 6].
The form of the gas flow and efficiency of shielding depend upon the type of shielding gas, the type
of weld joint and welding rate, movement of surrounding air (wind, draught). When welding in CO2
it is easier to provide reliable shielding than when welding in inert gases, their mixtures with CO2.
When welding the inner side of the fillet welds and square welds the shielding is much better than
when welding the outer side of the fillet welds. To improve the quality of shielding in this case it is
recommended to apply temporary welding screens. Under the increased welding rates the efficiency
of shielding is deteriorated. The quality and the performance characteristics of weld joints produced
in the open air depend not only upon the appropriate choice of welding materials, welding variables,
qualification of welders and following the welding procedures but it is to a great extent determined
by the conditions under which the welding is completed. Welding processes completed in windy,
rainy, foggy, snowy or other weather can result in welding defects.
The aim of the research is simulation and studying of wind velocity impact upon the active
shielding gas flow and thermal distribution in the MAG welding area in the process of consumable
electrode welding under traditional and two-jet gas shielding.
Methodology
When welding under the windy conditions gas shielding is ensured only by hard shielding jets
which are not deflected and are not carried away by the wind, hardness of the jet depending upon
the flow rate of the shielding gas. Scientists of UTI TPU have developed a method of welding with
two-jet gas shielding [7, 8] which ensures hardness of the inside jet of the supplied gas (Fig. 2),
protects metal in the heat-affected zone, reduces turbulence in the near-weld area and prevents air
inflow into the welding area. Control of the gas dynamics of the inside jet of the shielding gas
allows effecting the molten metal of the droplet and that of the weld pool, leads to intensive stirring
of molten electrode and basic metal in the weld pool, changes the cooling rate and the time of weld
and HAZ metal stay in the high-temperature region. The outside annular jet ensures reliable
protection of the welding area from the atmospheric effects. According to the results of the
previously conducted research works [7-10] we can establish that the method of gas shielding and
the rate of gas flow have a considerable effect upon the quality of welding area protection and upon
the welding processes.
a)
b)
Fig. 2. Shielded arc welding: a) traditional one-jet gas shielding; b) two-jet gas shielding
Results and Discussion
The authors completed simulation of gas jets flow from the nozzle with traditional and two-jet
shielding without wind influence (Fig. 3). Calculation and simulation were completed in the
program SolidWorks FlowSimulation with the computing cluster “SKIF-polytech” (TPU
supercomputer, cluster.tpu.ru).
Applied Mechanics and Materials Vol. 770
255
The basic input data: gas СО2, gas flow rate 20 l/min, geometry of the welding torch for machine
welding with traditional (one-jet) and two-jet gas shielding, front surface of gas flow – plane,
distance from the nozzle edge to the plane (welded item) – 12 mm.
As a result of simulation of shielding gas flow from the welding nozzle under consumable electrode
welding with traditional and two-jet gas shielding (Fig. 3) we established that with application of
two-jet shielding the rate of gas flow from the welding nozzle increases, the gas jet becomes harder
and, thus, compresses the arc. It leads to increased effect of shielding gas jet upon the droplet of
electrode metal, increases the frequency of electrode metal droplets transfer into the weld pool and
intensity of metallurgical processes on the surface of the droplet [9, 10]. Significant change of
shielding gas dynamics causes change of processes taking place in the welding area.
a)
b)
Fig. 3. Results of simulation of shielding gas flow from the welding nozzle: a) traditional one-jet
gas shielding; b) two-jet gas shielding
The authors completed the study of thermal fields in the welding area for the chosen methods of gas
shielding without wind influence. They also completed simulation of how active shielding gas flow
rate from various welding nozzles – traditional (one-jet) and two-jet – effects the heat distribution in
MAG welding area (Fig. 4) [9, 10].
a)
b)
Fig. 4. The results of simulation of heat distribution in the welding area:
a) traditional one-jet gas shielding; b) two-jet gas shielding
The simulation results showed that under the two-jet shielding the thermal fields in the welding area
are squeezed, i.e. the arc is constricted. It leads to the increase of heat input into the item and into
the droplet of electrode metal, heat efficiency upgrading, reduction (under the given conditions) of
the heat-affected zone (HAZ) in comparison to traditional (one-jet) shielding. Reduction of HAZ
reduces the risk of weld joints weakening and improves their working properties [8, 11, 12].
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Urgent Problems of Up-to-Date Mechanical Engineering
To estimate the quality of gas shielding of the welding area we completed simulation of the wind
velocity impact (3 m/s and 10 m/s) upon gas flow and heat distribution in the welding area for onejet and two-jet gas shielding (Fig. 5 and 6).
a)
b)
Fig. 5. Results of simulation of the wind impact upon shielding gas flow under the wind velocity
of 3 m/s: a) traditional one-jet gas shielding; b) two-jet gas shielding
a)
b)
Fig. 6. Results of simulation of the wind impact upon shielding gas flow under the wind velocity
of 10 m/s: a) traditional one-jet gas shielding; b) two-jet gas shielding
The results of simulation showed that being exposed to the wind the shielding gas is carried away
from the welding area and heat is redistributed downwind. Gas shielding quality and heat
redistribution in the welding area under traditional one-jet shielding are changed under the wind
velocity of 3 m/s. Under the same wind conditions and two-jet gas shielding, there is practically no
carrying shielding gas downwind and thermal fields redistribution. When the wind velocity
increases up to 10 m/s traditional gas shielding is carried downwind and the thermal fields change
abruptly, i.e. we observe considerable runaway of heat from the welding area leading to asymmetry
and alteration of weld and HAZ metal properties [13-15]. In case of two-jet gas shielding we
observe shielding gas being carried away and displacement of thermal fields in the arc zone but the
thermal picture of the item remains practically unaltered. It allows ensuring symmetry and
homogeneity of structure and properties of the weld and HAZ metal when completing MAG
welding at the open sites.
Conclusion
According to the results of the study it was established that application of MAG welding with twojet gas shielding increases the shielding gas flow rate from the welding nozzle, i.e. arc constriction
occurs. It ensures more qualitative shielding of the welding area under windy conditions and
uniform heat distribution in the welded item resulting in more homogeneous weld and HAZ metal
structure in comparison to traditional (one-jet) gas shielding.
Applied Mechanics and Materials Vol. 770
257
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Urgent Problems of Up-to-Date Mechanical Engineering
10.4028/www.scientific.net/AMM.770
Study of Wind Velocity Impact upon the Quality of Shielding and upon the Thermal Processes under
MAG Welding
10.4028/www.scientific.net/AMM.770.253