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Известия ТулГУ. Технические науки. 2015. Вып. 6. Ч. 2
УДК 621.791
INFLUENCE OF PULSED TIG WELDING PARAMETERS ON THE FORMATION OF SUBSURFACE CHANNELS IN METAL BODIES
Anna Tokar, Vladimir Ponomarov
The present work shows that welding has great potential not only in the area of joining materials, but also in other areas of manufacturing where machining or casting is usually used, for example, for making heating or cooling channels. This paper presents a channel making technique in metal bodies using Pulsed TIG welding. The idea was to enhance the defects called in the literature as "tunneling" (elongated cavities) and "convert" them in subsurface channels. Simple deposition on stainless steel plate tests were performed, varying the current pulse shape, pulse rate, arc length, electrode grinding angle, working angle and the welding speed seeking a greater robustness of the process. It was observed that the channel formation process was more fail-safe using a rectangular current pulse compared to a trapezoidal shape one. The pulse rate must be maintained between 1 and 3 Hz according to obtained results. The channels were more pronounced with sharpening angles around 60o.
Key words: Subsurface Channels, Elongated Cavities, Pulsed TIG Welding Process, Welding Parameters
1. Introduction
Many types of equipment for industry, transport or other area applications contain some parts subjected to excessive heating or cooling during operation and thus must be cooled or heated respectively to maintain their operational properties and an appropriate performance. The most commonly used approaches for this purpose are external fins (as in heat exchangers) or water jackets (as in combustion engines), which application is not always feasible as it may, for example, hinder the operation, as in the case of the matrices casting or mechanical forming. Subsurface channels offer a good alternative for these cases.
Among the methods for making subsurface channels (also called as internal channels) can be mentioned conventional drilling or the introduction of tubes made of high thermal conductivity material to parts manufactured by casting [1]. As some disadvantages of conventional drilling may be mentioned limitations regarding the use of high hardness materials (for example, those for molds and dies) and the impossibility to manufacture long channels and channels in curvilinear complex geometry parts, since the drills do not curve. In the case of method of tube incorporation [1], the tube must be cooled with air or water during the casting to avoid being fused over by the liquid metal (which would result in the loss of channels). In addition to be complicated, this method does not guarantee a proper contact of the tube surface with the workpiece material.
Another way to produce rectilinear and curvilinear subsurface channels in metal parts is a technique called Friction Stir Channeling - FSC [2]. This technique is based on the same principle that is used in Friction Stir Welding -
FSW, as described, for example, in the papers [3] and [4]. In this technique, a rotary tool (similar to a drill) penetrates the workpiece, plasticizes its material (by friction and plastic deformation), and then moves it away from the center of the plasticized region opening a channel in the bottom of that zone. The main disadvantage of this technique is that the metal being processed must be ductile enough and of a low hardness in order channels to be created with less effort and less tool wear, which is generally not the case for materials for molds and dies for forging and casting. In addition, this technique cannot be applied to surfaces with sharp profile changes (steps and corners, for example).
Alternatively, there are additive manufacturing techniques based on deposition of metal layers by sintering or fusion a metal powder using laser or electron beam [5, 6, 7, 8, 9 and 10]. These techniques allow to "print" (like a 3D printer) metal parts with various internal details, including subsurface channels. Disadvantages of these techniques are an enormous time required for production of details, limitations in terms of size of the parts produced (because they are usually produced in vacuum chambers or chambers filled with an inert gas), and the high cost of equipment used.
Thus, this paper presents a new manufacturing technique for subsurface channels, which is free from such limitations, and which allows a better operating flexibility and can be applied for a wide range of materials and workpiece geometries.
2. Materials and methods
The welding process used to manufacture subsurface channels was the Pulsed TIG with electrode negative polarity and with no metal deposition. The tungsten electrode was of the EWTh2% type of 4.0 mm diameter. The argon shielding gas was used with the 15 l/min flow rate. The welds were made on 304 stainless steel plates of 200 mm x 32 mm x 6,4 mm dimensions. The manipulation of the torch was performed using an X-Y coordinate table.
3. Results and discussion
3.1. Influence of the frequency of the rectangular shaped current pulse on bead geometry and on formation of channels
The main purpose of these tests was to find maximum and minimum values of pulse frequency (fp) for the current pulse rectangular format that determines the range with a greater reliability of the channel formation (table). The tests were made using the same average current (Im = 183,0 A) and the same amplitude of the pulse current (Ip = 350,0 A) and background current (Ib = 15,0 A), thereby maintaining the same difference between them (AI) of 335,0 A. The travel speed, the gap between the electrode tip and the plate and the electrode sharpening angle were kept constant at 150,0 mm/min, 3,5 mm and 60o respectively.
Regulated and monitored parameters for the tests on the influence ofpulse frequency on bead geometry and on formation of channels
Test Regulated parameters Monitored parameters (measured values)
fP (Hz) tP = h(s) IP (A) h (A) A1(A) Iraws (A) In (A) Inns (A) Im (A) u„, (V)
1 0,6 0,8 259,0 197,0 11,5
2 0,8 0,6 246,0 180,0 11,5
3 0,9 0,55 252,0 187,0 11,4
4 1,0 0,5 249,0 184,0 11,2
5 1,5 0,34 350,0 15,0 335,0 248,0 183,0 252,0 187,0 11,2
6 2,0 0,25 253,0 185,0 11,1
7 2,3 0,22 252,0 188,0 11,1
8 3,0 1,7 251,0 184,0 11,2
9 4.2 0,12 253,0 189,0 11,4
Note: tp, th> Ip arid Ib - pulse time, background time, pulse current and background current; AI = Ip - h; Im Um - mean current and voltage values.
The results show that the fused area (Aj), penetration (P), width (.L) and the channel formation reliability depend strongly on the pulse frequency. The increase in pulse rate causes a decrease of the penetration and the bead width and thus the fused area. The cavities were presented in all frequencies except in the highest one of 4,2 Hz but became more elongated (forming short or long channels) with the pulse frequency in the range from 0,9 to 2,5 Hz (fig. 1).
Fig. L Surface appearance (a) and longitudinal section (b) of a subsurface channel (see the test 3 of the table for the welding conditions)
These results might be explained by the next manner. Current pulses with durations within the range of 0,22 - 0,55 s causes an expulsion of a large mass of molten metal from the weld pool and its dislocation father from the
formed crater. Under these conditions the molten metal is less likely to return to its previous location (i.e., to flow down to the crater in full) before being solidified, creating a cavity. This mechanism of the cavity/channel formation is explained in more details in the next session (item 3.2). The parameter settings of these tests did not ensured a high reliability of the channel formation. That is why, before to make further tests, an hypothesis of subsurface channel formation on metal bodies by pulsed TIG welding arc action should be discussed.
3.2. The mechanism of subsurface channel formation on metal bodies by pulsed TIG welding arc action
It is supposed that subsurface channels, illustrated by fig. 2, are formed through the next mechanism.
Fig. 2. Illustration of the channel formation mechanism
An intensive melting of the base metal occurs during the high current pulse accompanied by an arc high pressure action, which expulses the molten metal from the front region of the weld pool to its rear region. As a result of this, there is a crater formed in the front region of the pool. At the beginning of the background time, the arc pressure is decreasing, causing an opposite displacement of the molten metal towards the crater region. On its way back the metal solidifies, and its lower layers being solidified before than upper ones (the solidification starts from the border with the solid metal). As a result, the metal of upper layers solidifies being hung on the rear wall of the crater forming a subsurface channel at its bottom.
3.3. Effect of the current pulse trapezoidal shape on bead geometry and on formation of channels
These tests were conducted with the aim to evaluate the effect of a trapezoidal shape of the current pulse on robustness of the channel formation. Four tests were made using the same welding conditions as mentioned in the item 3.1,
except that the pulse frequency was kept constant equal to 1,25 Hz. The pulse current rising and decreasing rates were varied. It was observed that, unfortunately, the use of a current pulse trapezoidal format did not result in an increase of the robustness of channel formation. There were no continuous cavities observed.
3.4. Influence of electrode sharpening angle on bead geometry and formation of channels
Erohin A.A. [11] showed that the electrode sharpening angle influences the occurrence of cavities in the case of continuous (not pulsed) current. To evaluate this relationship for the case of a pulsed current some test were conducted with varied electrode sharpening angle (y). As in the tests described above, there were made simple beads on plate under the welding conditions mentioned in item 3.1, except that the pulse frequency (fp) was kept constant of 2 Hz and the electrode sharpening angle (y) varied from 30° to 80°.
The results show that the acute electrode sharpening angles (30°, 40° and 60°) provide a triangular shaped profile of the melted zone with high fused area and penetration, while more obtuse angles (70° and 80°) result in a reduced fused area with a flat profile (wide and shallow). The registered average voltage values may help to explain these results. A lower voltage was recorded for tests with obtuse electrodes. This fact would suggest that, in this case, the arc was shorter, though all tests were conducted with the same distance between the electrode tip and the workpiece equal to 3,5 mm. It can have the next explanation. When the current is increased (during the pulse), the arc becomes wider for obtuse angle electrode tips with a limited increment in its length, whereas in the case of acute angles, the arc length grows more because of the requirement of having the same electrode - arc coupling area for the same current.
These results confirm the hypothesis of Erohin [11], which showed that the reduction of the electrode sharpening angle provoked a higher concentration of heat flow towards the workpiece, namely the current density, and thus an increase of the arc pressure (causing some gain of the penetration). It is worth mentioning that there are some contradictory results in the literature regarding the influence of the electrode sharpening angle on the penetration. For example, Key [12] has shown that the smaller is the electrode sharpening angle, the lower the penetration becomes.
The electrode sharpening angle, besides exerting an influence on bead geometric parameter, also influences the formation of subsurface channels. The longest channels (up to 120 - 130 mm) were observed with the grinding angles at 60°. The sharpening angles of 30° to 40° resulted in somewhat shorter channels (reaching at maximum 50 - 70 mm), while for the angles of 70° to 80°, there were no channels observed. According to Erohin, Bukarov and Ishenko [12] this phenomenon can be explained by an influence of the electrode tip shape on the dynamic pressure of the arc plasma flow. Smaller angles cause increased con-
centration of the arc pressure on the weld pool, moving the liquid metal upwards. Under obtuse electrode sharpening angles, the arc flow is distributed more evenly over the entire surface of the liquid metal. In this case, the arc does not deep into the weld pool too much, forming a shallower crater.
3.5. Influence of the torch travel angle on bead geometry and formation of channels
As in the tests described above, there were made simple beads on plate under the welding conditions mentioned in item 3.1, but in this case the parameter submitted to variations was the torch travel angle ((3), assuming drag or push angels, as shown on fig. 3. The gap between the electrode tip and the plate was measured as shown on fig. 3 and it was kept constant at 3,5 mm.
As shown in fig. 3 the penetration and the bead width were always higher when the right torch travel angle was used. The penetration was higher for drag angles as compared to push angles, but smaller than that for the right angle. It was noticed that the higher drag or push angle was, the lower penetration was observed, which can be explained by the heat reflection from the plate with higher inclination angle.
The torch travel push angle of 5° resulted in a wider and shallower fused area as compared with that of the 5° drag angle. This result was expected, since when the drag technique is used, the arc is more steady (less shaky) and the "arc - metal base" coupling favors to a high fusion efficiency. Whereas under push angle technique, the "arc - base metal" coupling is more shaky contributing to base metal preheating, and thus, to a better wettability by the molten metal, leading to a wider bead with low penetration.
Push angles
A== 15,7 mm L- 9,7mm P=2,l mm
Weldina direction
Dras anales
L„ = const
A = 15 J mm2 L— 8,7 mm P= 2.6 mm
ftp
4r= 21,8 mm2 fL= 'J. 8 mm P=4,l mm
Iy= 19,4 mm21 JL= 8,5 mm P=3.7 mm
\f= 15,5 mm2 i = 8,6 mm P=3,4mm
Fig. 3. Illustration of the influence of torch travel angle on bead geometry
and formation of channels
The torch travel angle influences, besides on the bead geometry, also on the formation of cavities/channels. The cavities/channels were more pronounced when the right angle was used (fig. 3). They were of smaller cross section and shorter when drag angles were used, and they almost did not appear under push angles welding technique. Moiseenko, V.P. et all [13] received somewhat similar results. They also observed that push angles did not favor the formation of cavities/channels. On the other hand, their tests showed that drag angels are more favorable to cavities/channels formation than the right angle (what, actually, disagrees with the results described above). This discrepancy in results may be related to different welding conditions such as the current type and value, welding travel, pulse frequency, electrode grinding angle, base metal properties, etc.
3.6. Influence of arc length on bead geometry and formation of channels
These tests were carried out maintaining the same welding conditions as in previous tests. The distance from the electrode tip to the plate (La) was varied from 2,0 to 12,5 mm. The welds were made on 304 stainless steel plates of 200 mm x 38 mm x 6,1 mm dimensions. It was expected that an increase in the arc length would result in a reduction in the penetration and increased width of the bead. What actually was not confirmed. The test results showed that when the arc length was changed from 2 to 6 mm, the bead geometric parameters remained almost unchanged. The penetration started decreasing and the bead width increasing only after the arc length reached 9 and 12,5 mm. It was found that shorter arcs (in the range of 2,0 to 4,0 mm) are prone to generate channels long enough and with a large channel cross-section.
3.7. Influence of travel speed on bead geometry and formation of channels
The tests performed so far gave understanding that one of the most important parameters, with regard to the formation of channels, is the travel speed. Therefore, this parameter was intentionally varied to find its most appropriate value to produce channels with large cross-section and as long as possible. The welds were performed with the same parameters mentioned in item 3.6, except the travel speed, which value was varied from 120 to 180 mm/min. The obtained results showed that travel speeds in the rage of 150 - 160 mm/min provided the best conditions for the formation of channels (they had a larger section and a higher length). The travel speed of 120 mm/min was not favorable for the channel formation (for these particular welding conditions). Travel speeds above 160 mm/min generated irregular weld beads with craters, undercuts and short length cavities.
Figure 4 shows some examples of beads with channels received in these tests. As it can be seen, this technique is quite promising for subsurface channel manufacturing.
Fig. 4. Some examples of beads with channels (304 stainless steely pulse current - 380 A, background current -15 A, pulse time - 300 ms, background time - 700 ms, pulse frequency - 1 Hz, travel speed -140 mm/min,
arc length - 3,5 mm)
4. Conclusions
The formation of subsurface channels in metallic bodies by the Pulsed TIG Welding arc action depends heavily on many welding parameters, namely:
- the pulse frequency must be maintained between 1 and 3 Hz (for the 304 stainless steel plates of approximately 6 mm thickness);
- the channels were more pronounced (reaching 12 - 13 cm length) with electrode sharpening angles around 60°. Cavities were not continuous for sharpening angles from 30° to 40° and they were not observed at all for more obtuse angles from 70° to 80°;
- channels were more pronounced when straight travel angle was used. Channels were shorter with narrow cross section when drag travel angels were used and there were almost no channels with push travel angles;
- shorter arcs (2 - 3 mm) are more favorable for the formation of subsurface channels;
- travel speeds which provided optimum conditions for the generation of channels (with a higher cross section and a longer length) were of 150 - 160 mm/min for the welding conditions used;
- the trapezoidal format of current pulses was not favorable for the robustness of the channel formation (formed cavities were not continuous);
- the technique for making subsurface channels in metal bodies with Pulsed TIG welding is quite promising. However, the robustness of the channel formation for this technique is not acceptable yet. The formation of the subsurface channel is very susceptible to variations of many welding parameters.
5. Acknowledgements
The authors would like to acknowledge the technical support offered by the LAPROSOLDA/UFU, the financial support received from the CNPq (a research grant) and from the FAPEMIG, which were very essential for the success of this work.
References
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Anna Tokar, master, annatokar@mecanica.ufu.br, Brazil, Uberlandia, Federal University of Uberlandia,
Vladimir Ponomarov, candidate of technical science, ponomare vamecanicajifu.hr, Brazil, Uberlandia, Federal University of Uberlandia
ВЛИЯНИЕ ПАРАМЕТРОВ ИМПУЛЬСНО-ДУГОВОЙ СВАРКИ НЕПЛАВЯЩИМСЯ ЭЛЕКТРОДОМ НА ФОРМИРОВАНИЕ ПОДПОВЕРХНОСТНЫХ КАНАЛОВ В
МЕТАЛЛИЧЕСКИХ ДЕТАЛЯХ
Анна Токарь, Владимир Пономарев
Настоящая работа показывает, что сварка может быть использована не только для соединения материалов, но и там, где обычно применяются способы механообработки или литья, например для выполнения каналов охлаждения/нагрева. Эта работа представляет способ выполнения каналов в металлических загатовках с использованием импульсно-дуговой сварки неплавящимся электродом. Основная идея заключается в трансформации дефектов сварного шва, типа удлиненных полостей, в подповерхностные каналы. Эксперименты были выполненны простой наплавкой на пластины из нержавеющей стали с изменением таких параметров, как форма импульса тока, частота пульсации, длина дуги, угол заточки электрода, угол наклона горелки и скорость сварки с целью повышения надежности формирования канала. Было установлено, что прямоугольный импульс тока обеспечивает лучшие результаты, чем трапецоидальный. Частота пульсации должна находиться в диапазоне 1 - 3 Гц. Угол заточки электрода должен быть близким к 60 o.
Ключевые слова: Подповерхностные каналы, удлиненные полости, импульсно-дуговая сварка неплавящимся электродомб параметры сварки.
Анна Токар, магистр, annatokar@mecanica.ufu.br, Бразилия, Уберландия, Федеральный Университет города Уберландия,
Владимир Пономарев, канд. техн. наук, ponomarev@mecanica.ufu.br, Бразилия, Уберландия, Федеральный Университет города Уберландия
