Effect of scheme of powder feeding into arc on its losses and efficiency of plasma-powder surfacing process
A.I. SOM
" The Paton Welding Journal". - 2015. - №5-6. - pp. 22-25.
Area of investigation was powder losses in plasma-powder surfacing using plasmatrons with internal and
external schemes of its feeding into arc. Dependence of powder losses on its fraction, geometry of deposited
bead and surfacing efficiency was shown. It is determined that losses of powder from steel 10Kh18N10T
are 4—5 times lower at internal scheme of feeding, due to its more effective heating in the arc than at
external one and do not exceed 1—2 % under favorable conditions. Practical recommendations on efficient
application of plasmatrons with different systems of powder feeding are given. 4 Ref., 1 Table, 6 Figures.
Efficiency of process of plasma-powder surfacing
(PPS) mainly depends on heating efficiency and
melting of filler powder in arc. The better powder
heating in the arc is, the less its losses and the higher
surfacing productivity are under the same process
parameters. Besides, base metal penetration is lower
and formation of deposited bead is better.
It is well-known fact [1] that efficiency of
powder heating in PPS depends to significant
extent on scheme and parameters of its feeding
in arc. They determine flaying path and velocity
of particles as well as time of their staying in the
arc. Details on process of powder heating and
melting in the arc are given in works [2, 3].
Practical recommendations of these investigations
made a basis for development of current
plasmatrons [4].
Figure 1. Internal (a) and external (b) scheme of powder feeding in arc: 1 – electrode; 2—4 – plasma-shaping, protective
and focusing nozzle, respectively; 5 – part
Figure 1. Internal (a) and external (b) scheme of powder feeding in arc: 1 – electrode; 2—4 – plasma-shaping, protective
and focusing nozzle, respectively; 5 – part
Today internal and external schemes of powder
feeding in arc [3, 4] are the most widely used
for PPS. In the first case (Figure 1, a), powder
is fed in the arc inside the plasmatron in form of
uniformly distributed on circumference flow of
particles through cone slot formed by plasma and
focusing nozzles. In the second one (Figure 1,
b), it takes place outside the plasmatron through
one or several holes at the end of plasma-shaping
nozzle. Focusing nozzle is absent in this case.
Aim of the present work is providing quantitative
evaluation to these schemes from point of
view of powder losses, efficiency and safety of
surfacing process and, based on mentioned above,
make a proposal on practical recommendations
of their reasonable application.
Powder losses were investigated in surfacing
of beads of various width and thickness with
different efficiency over flat samples from steel
20. Bead width was varied in 5—30 mm range,variation of thickness made 1—5 mm and efficiency
10—50 g/min. Stainless steel powder of
10Kh18N10T type (analog AISI 321) of two fractions
63—100 and 100—200 μm were used as filler
material. As indicated in work [3], the dependencies
of movement and heating of this type of
powder in the arc are typical for majority of
powders used for PPS. Besides, it is relatively
refractory and has short pool in surfacing that is
very important in powder loss investigation.
The investigations were carried out on Plasma-
Master Ltd. equipment using PP-6-01 plasmatron
[4], which has internal system of powder feeding,
and experimental plasmatron with external powder
feeding system, similar to applied in plasmatrons
of leading world manufacturers Castolin, Deloro
Stellite, Commersald etc.
Figure 2. Plumes of powder outflow from PP-6-01 plasmatron (a) and experimental plasmatron (b)
Figure 2. Plumes of powder outflow from PP-6-01 plasmatron (a) and experimental plasmatron (b)
Powder in PP-6-01 plasmatron firstly enters
a special distribution chamber through inlet connection,
where it is uniformly distributed around
the circumference, and then being fed in the arc
via a system of slots, evenly located on conical
surface of plasma-shaping nozzle. The slots guide
the powder particles directly in the central most
heated part of the arc. Focus point of powder
particles is at 3 mm distance from the end of
focusing nozzle. Diameters of plasma-shaping
and focusing nozzles in these experiments equaled
5 and 8 mm, respectively.
Filler powder in the experimental plasmatron
is preliminary divided on two similar flows and
being fed in the arc directly through two holes
of 1.4 mm diameter made in diametric opposition
to each other at the end of plasma-shaping nozzle.
Angel of powder feeding in arc relatively to vertical
axis makes 35°, the same as in PP-6-01 plasmatron.
Focus point of powder particles is at
8 mm distance from the end of plasma-shaping
nozzle, diameter of which equaled 5 mm in experiments.
Plasmatron in surfacing was oriented
in such a way that powder feeding takes place
in plane normal to axis of deposited bead. In
both cases plasmatron distance to surface being
deposited was 10 mm.
Figure 3. Dependence of powder loss on width of deposited bead at external (1) and internal (2) schemes of its feeding
in arc: a – 63—100 μm powder fraction; b – 100—200 μm
Figure 3. Dependence of powder loss on width of deposited bead at external (1) and internal (2) schemes of its feeding
in arc: a – 63—100 μm powder fraction; b – 100—200 μm
Figure 2 shows plumes of powder flowing from
PP-6-01 plasmatron (Figure 2, a) and experimental
plasmatron (Figure 2, b) at 50 g/min
feeding. Consumption of transporting gas in both
cases was minimum for elimination of powder
blockage in the channels of plasmatrons and made
2 l/min. At that, initial velocity of particle flight
at the moment of their entrance in the arc equaled
1.0—1.5 m/s for PP-6-01 plasmatron and 2—
3 m/s for experimental one. Such variations are
caused by different resistance of channels for
powder feeding in these systems.
Powder loss was determined as difference between
general consumption of powder supplied by
feeder and mass of metal deposited for the same
time. Mass of deposited metal was determined by
means of weighting of the sample before and after
surfacing. Surfacing distance made 200 mm.
Weighing of the samples was carried out using
laboratory scales of up to 0.05 g accuracy.
Figure 4. Dependence of powder losses on height of deposited bead at external (1) and internal (2) schemes of its feeding
in arc: a – 63—100 μm powder fraction; b – 100—200 μm
Figure 4. Dependence of powder losses on height of deposited bead at external (1) and internal (2) schemes of its feeding
in arc: a – 63—100 μm powder fraction; b – 100—200 μm
Experiments showed that the largest powder
losses were observed with external scheme of its
feeding in the arc. Particularly, it becomes apparent
in deposition of narrow and thin beads
using coarse-grain powder. They achieve 20 %
(Figures 3, b and 4, b) in such cases. Increase of
width and thickness of the beads provides for
significant reduction of powder loss due to more
favorable conditions for powder entering into the
weld pool. Using of finer powder promotes more
significant decrease of losses, however, they are
still large enough and make 8—10 % (Figures 3,
a and 4, a).
Such significant powder losses at external
feeding scheme are firts of all caused by unfavorable
conditions of its heating in the arc. Time
of powder staying and heating in the arc is limited
due to low position of point for powder feeding
and large initial particle velocity. Moreover, local
jets of cold transporting gas deform arc column
and reduce plasma temperature in heating
zone. Calculation data of work [3] show that
only particles finer than 50 μm can be heated to
melting temperature of 10Kh18N10T steel at such
feeding scheme. Larger particles, which did not
come into weld pool, flew past and got lost.
Figure 5. Dependence of powder losses on value of feeding at different schemes of its feeding in arc: 1 – external; 2 – internal (63—200 μm powder fraction, bead width of 20 mm, height of 3 mm)
Figure 5. Dependence of powder losses on value of feeding at different schemes of its feeding in arc: 1 – external; 2 – internal (63—200 μm powder fraction, bead width of 20 mm, height of 3 mm)
Significantly better results are observed at internal
powder feeding. Regardless the fact that
dependencies are in general close, the level of
powder losses here is considerably lower. Also,
the difference between fine and coarse fractions
is not large. Under the most unfavorable conditions
powder losses do not exceed 5 % versus
20 % at external feeding. Powder losses make
only 1—2 % (see Figures 3, a and 4, a) in surfacing
of wide and full beads.
It was noted during the experiment that powder
losses depend not only on geometry of deposited
bead, but on productivity of process performance
as well. Increase of powder feeding promotes
for also increase of loss, in particular, at
external feeding (Figure 5).
Figure 6. View and macrosections of beads at internal (a) and external (b) schemes of powder feeding in arc
Figure 6. View and macrosections of beads at internal (a) and external (b) schemes of powder feeding in arc (Open image in full size: )
It can be explained by the fact that increase
of powder feeding provides for rise of particle
density in the arc and possibility of their elastic
collision between themselves at the moment of
entering in the arc is increased, as a result of which
they fly off from the arc and get lost. Probability
of particle collision is significantly more at external
feeding due to the fact that powder is fed in the
arc by more concentrated flows towards each other.
The important indices of surfacing efficiency in
addition to powder loss are also formation of layer, penetration of base metal and specific consumption
of energy. Here, internal scheme of powder
feeding also has obvious advantages, in particular
at fine layer surfacing.
Figure 6 shows view and macrosections of
cross section of 20 mm width and 1 mm height
deposited beads. It can be seen that formation of
bead deposited at internal powder feeding is significantly
better, and base metal penetration is
obviously lower than at external one. It is
achieved due to better heating of powder in the
arc and larger factor of its use. In this case, energy
distribution of plasma arc is so that its larger
part is consumed for powder heating, not for
part, i.e. heat input in the part is significantly
lower. Flow of transporting gas uniformly distributed
around the circumference provides for
additional stabilizing effect on arc and also promotes
improvement of bead formation. The result
of common effect of these factors shows that surfacing
with internal powder feeding is more energetically
profitable than at external one. Surfacing
current necessary in this case is 10—15 %
lower at the same efficiency.
The Table shows for example the optimum
values of arc current in surfacing of different
geometry beads using both schemes of powder
feeding in arc.
Optimum values of current at deposition of different geometry beads
Geometry of beads, mm
Surfacing speed, mm/min
Powder feeding rate, g/min
Powder fraction, μm
Surfacing current, A
Internal feeding
External feeding
b = 10
h = 3
140
26
63—100
170
190
100—200
180
205
b = 20
h = 3
70
34
63—100
175
200
100—200
180
210
b = 30
h = 3
50
38
63—100
180
220
100—200
190
230
Carried investigations showed that internal
scheme of powder feeding provides for significantly
better indices of surfacing than external
one. However, appearance of drops of liquid metal
at the output of focusing nozzle was observed
at long-term surfacing of powder fraction of less
than 100 μm with large productivity. It resulted
in deterioration of process stability and degradation
of deposited bead formation. Reduction of
productivity and, respectively, surfacing current
significantly decreases droplets appearance. They
were completely absent at less than 150 A current.
No droplets were also observed in use of
coarse fraction powder of more than 100 μm at
current up to 300 A. External scheme of powder
feeding in arc did not show such an effect in all
investigated range of currents (50—300 A). It
should be taken into account in practice, particularly,
in surfacing of fusible self-fluxing nickelbased
powders.
Conclusions
References
Conclusions
Powder losses are significantly less at internal scheme of its feeding in arc than at external one, and under favorable conditions do not exceed 2 %.
Internal scheme of powder feeding in the arc in general more efficient in comparison with external one. It provides for smaller powder losses, lower penetration of base metal and better formation of deposited bead at decreased energy consumption.
Increase of surfacing efficiency using of plasmatrons with external powder feeding requires application of powders with finer fraction of less than 100 μm, and more than 100 μm for plasmatrons with internal feeding.
In order to reduce losses of powder in surfacing by plasmatron with external powder feeding the process should be carried at small productivity of less than 30 g/min.
References
1. Gladky, P.V., Pavlenko, A.V., Zelnichenko, A.T.
(1989) Mathematical modeling of powder heating in
arc during plasma surfacing. Avtomatich. Svarka, 11,
17—21, 54.
Pavlenko, A.V., Gladky, P.V. (1989) Peculiarities of
filler powder heating in arc during surfacing. Ibid.,
11, 33—37.
