Well here it is Allan! I hope you can make sense of it for your students...
I concur as well that it is a poorly worded question, and please allow me to elaborate further if I may... The question is asking this:
"Which of the following shielding gases gives the highest arc temperature? They had the following choices: argon, helium, argon-helium mix, none of these."
First off, this question immediately raises other questions in my mind, and the first one that comes to mind is: Well, which welding process are they referring to, because I hope that this question doesn't simply imply that it pertains to all of the arc welding processes that use a shielding gas to protect the molten pool from contamination of the surrounding air in the atmosphere as well as to aid in transferring electrons from the power source across the air gap in order to establish as well as maintain & sustain the arc, and providing a certain type of heat transfer, arc temperature, removal of certain surface oxides, etc.
Having said that, it sure does make things a bit more complicated than when we first looked at the question initially don't you think?
Okay! So, let’s look at it from the point of view as written in The Welding Handbook Volume One - "Welding Technology" 8th Edition published by the AWS. As we read on page 43 in the "Physics of welding" chapter under the heading of: "Arc Characteristics" Starting from "Definitions" It states:
For all practical purposes, a welding arc can be considered a gaseous conductor which changes electrical energy into heat. The arc is the heat source for many welding processes because it produces a high intensity of heat and is easy to control through electrical means. Arcs are sources of radiation as well as heat sources. When used in welding processes, an arc may help remove surface oxides in addition to supplying heat. The arc also influences the mode of transfer of metal from the electrode to the work.
A welding arc is a particular group of electrical discharges that are formed and sustained by the development of a gaseous conduction medium. The current carriers for the gaseous medium are produced by thermal means and field emission. Many kinds of welding arcs have been conceived, each with a unique application in the field of metal joining, In some cases, the welding arc is a steady state device. More frequently, it is intermittent, subject to interruptions by electrical short circuiting, or continuously unsteady (This word is written as “nonsteady”), being influenced by an alternating directional flow of current or by a turbulent flow of the conducting gas medium.
THE PLASMA
The arc current is carried by a plasma, the ionized state of gas composed of nearly equal numbers of electrons and ions. The electrons, which support most of the current conduction, flow out of a negative terminal (Cathode) and move toward a positive terminal (Anode). Mixed with the plasma are other states of matter, including molten metals, slags, vapors, neutral and excited gaseous atoms, and molecules. The establishment of the neutral plasma state by thermal means, that is, by collision processes, requires the attainment of equilibrium temperatures according to the ionization potential of the material from which the plasma is produced. The formation of the plasma is governed by an extended concept of the Ideal Gas Law and The Law of Mass Action… Note: I will insert the equation @ a later time.
The particle densities of three kinds of particles can be determined by assuming the plasma is electrically neutral and that the ions have a single positive charge. Then the number of electrons is equal to the number of ions.
The expression of thermal equilibrium of the heated gas in an arc means that all kinetics and reactions of the particles in a microvolume may be represented by the same temperature. Thermal equilibrium in welding arcs is closely approached, but may be considered only approximate because of the influence of dominant processes of energy transport, including radiation, heat conduction, convection, and diffusion. The heated gas of the arc attains a maximum temperature of between 5000 and 50,000 K, depending on the kind of gas and the intensity of the current carried by the plasma.
The degree of ionization is between 1 and 100 percent; complete ionization is based on all particles being at a temperature corresponding to the first ionization potential. The attainment of a very close approximation to thermal equilibrium is more questionable in the region near to the arc terminals, where current conducting electrons are accelerated so suddenly by a high electric field that the required number of collisions does not occur. It is in the arc terminal regions that an explanation of current conduction based wholly on thermal ionization is insufficient and must be augmented by the theory of field emission or some other concept.
TEMPERATURE
Measured values of welding arc temperatures normally fall between 5000 and 30,000K, depending on the nature of the plasma and the current conducted by it. As a result of a high concentration of easily ionized materials such as sodium and potassium that are incorporated in the coating of covered electrodes (SMAW), the maximum temperature of a shielded metal arc is about 6000K.
In pure inert gas arcs, the axial temperature may approach 30,000 k. Some special arcs of extreme power loading may attain an axial temperature of 50,000 K. In most cases, the temperature of the arc is determined by measuring the spectral radiation emitted. The temperature attainable in arcs is limited by heat leakage rather than by a theoretical limit. The energy losses (Due to heat conduction, diffusion, convection, and radiation) characteristic of an arc plasma of specific composition and mass flow are in balance with the electrical power input. The energy losses from arcs vary in a complex way according to the magnitude of the temperatures and the influences of thermal conduction, convection, and radiation.
Figure 2.4 Thermal Conductivity of Some Representative Gases as a Function of Temperature shows in the graph that with argon, as the temperature rises in increments of 103 K will go up to 30,000K and have a thermal conductivity, ERG . CM-1 . SEC-1 . K-1 of approximately 5. With Helium at the same temperature of 30,000 K, the thermal conductivity goes all the way up on the chart to a little over 13 and just short of 14. Quite a significant difference I might add! The data shown for hydrogen and nitrogen indicated peaks due to the effect of thermal dissociation and association of the molecular and atomic forms (H2 &2H, and N2&2N) respectively.
RADIATION
The amount and character of radiation emitted by arcs depend upon the atomic mass and chemical structure of the gas, the temperature, and the pressure. Spectral analysis of arc radiation may show banks lines and continua. The analysis of radiation from organic type covered electrodes shows molecular bands revealing the existence of vibrational and rotational states, as well as line and continuum emissions from excited and ionized states. The inert gas arcs radiate predominantly by atomic excitation and ionization. As the energy input to the arcs increases, higher states of ionization occur, giving radiation of higher levels.
Radiation loss of energy may be over 20% of the total input in the case of argon welding arcs, while in other welding gases the radiation loss is not more than 10%. Intense radiation in the ultraviolet, visible, and infrared wavelengths is emitted by all exposed welding arcs. Ultraviolet radiation from argon shielded arcs is particularly strong because of mass effects and because little or no self-absorption occurs within the plasma volume.
ELECTRICAL FEATURES
A welding arc is an impedance to the flow of current, as are all normal conductors of electricity. The specific impedance is inversely proportional to the density of the charge carriers, and their mobility, with the total impedance depending on the radial and axial distribution of carrier density. The plasma column impedance is a function of temperature, but generally not in the regions of the arc near its terminals. The electrical power dissipated in each of the three spaces or regions of the arc is the product of the current flowing and the potential across the region. The current and potential across each region are expressed according to:
P = I(Ei + Ec + Ep)
Where:
P = power, W
Ec = anode voltage, V
Ec = cathode voltage, V
Ep = Plasma voltage, V
I = current, A
The regions are referred to as the cathode fall space, the plasma column fall space, and the anode fall space. However, there are intermediate region taken up in expanding or contracting the cross section of the gaseous conductor to accommodate each main region. As a consequence, the welding arcs assume bell or coned shapes, including the configuration of the arc terminals, gravitational and magnetic forces ,and interactions between plasma and ambient pressures,. The area over which the current flows into the arc terminals (anode and cthode spots) has a strong effect on the arc configuration and on the flow of heat energy into these terminals.
The current density at the workpiece terminal is of utmost importance to the size and shape of the fusion zone, and to the depth of fusion in a welded joint. The total potential of an arc falls with increasing current and rises again with a further increase in current. The decrease in total arc potential with increasing current can be attributed to a growth of thermal ionization and thermally induced electron emission at the arc cathode. The total potential of arcs generally increases as the spacing between the arc terminals increase. Because the arc column is continually losing charge carriers by radial migration to the cool boundary of the arc, lengthening the arc exposes more of the arc column to the cool boundary, imposing a greater requirement on the charge carrier maintenance. To accommodate this loss of energy and maintain stability, the applied voltage must be increased.
Much of the foregoing concerned the plasma column which is best understood. Although the mechanisms effective at the arc terminals have even more importance in welding arcs, they are less understood. The arc terminal materials must, in most cases, provide the means for achieving a continuity of conduction across the plasma column.
It is essential that the cathode material provide electrons by emission of sufficient density to carry the current. In the GTAW process, the tungsten electrode is chosen because it readily emits electrons when only a portion of the electrode tip is molten. Other cathode materials that are melted and transferred through the arc must also provide sufficient density of electrons to carry the arc current. In the case of consumable electrodes, additives in the form of coatings may be used to insure stable spatter-free transfer.
We’ll skip INFLUENCES OF MAGNETIC FIELDS IN THE ARC, and ARC BLOW found on pages 47thru 49 and start again on page 50 with;
METAL TRANSFER
Consumable electrode arc welding processes are used extensively because filler metal is deposited more efficiently and at higher rates than is possible with other welding processes. To be most effective, the filler metal needs to be transferred from the electrode with small losses due to spatter. Furthermore, uncontrolled short circuits between the electrode and the work should be avoided; otherwise, the welder or welding operator will have difficulty controlling the process.
In the case of the GMAW process, arc instability caused by erratic transfer can generate pressure fluctuations that draw air into the vicinity of the arc. So much care should be taken in order to properly “dial in” the optimal parameters when setting up this process regardless of the mode of metal transfer as well as maintaining as close to the optimal ESO (Electrode Stick Out) , CTWD (Contact Tip to Work Distance), and work & travel angles, and travel speed also.
The different types of transfer have been studied with motion pictures and by analysis of the short circuit oscillograms. Transfer through the arc stream of covered electrodes can be characterized as globular (massive drops) or as a showery spray (Large number of small drops). These modes are rarely found alone. More generally, material is transferred in some combination of both.
Transfer with the GMAW process varies greatly when used with argn shielding. When the current is above the transition level, the transfer mechanism can be best described as an axial spray, and short circuits are nonexistent. However, when helium or an active gas such as carbon dioxide is used for shielding, the transfer is globular, and some short circuiting is unavoidable. The GMAW short circuiting arc process has been adapted to use only short circuits for the transfer of metal to the pool.
The physics of metal transfer in arc welding is not well understood. The arcs are too small, and their temperatures too high for easy study, and metal transfers at high rates. Because of the difficulty involved in establishing the mechanisms that regulate the process, a great number of mechanisms have been suggested. These forces have been considered:
1) Pressure generated by the evolution of gas at the electrode tip.
2) The electrostatic attraction between electrodes.
3) Gravity.
4) The “pinch effect” caused by electromagnetic forces on the tip of the electrode.
5) Explosive evaporation of the necked filament between the drop and electrode due to the very high density of the conducting current.
6) Electromagnetic action produced by a divergence of current in the plasma around the drop.
7) Friction effects of the plasma jet.
In all probability, a combination of these forces functions to detach the liquid drop from the end of the electrode.
EFFECT OF POLARITY ON METAL TRANSFER IN ARGON
At low welding currents in argon, liquid metal from the electrode is transferred in the form of drops having a diameter greater than that of the electrode. With electrode positive, the drop size is roughly inversely proportional to the current, and the drops are released at the rate of a few per second. With a sufficiently long arc to minimize short circuits, drop transfer is reasonably stable and associated with a relative absence of spatter.
Above critical current level, however the characteristics of this transfer change to the axial spray mode. In axial spray transfer, the tip of the electrode becomes pointed, and minute drops are transferred at a rate of hundreds per second. The current at which this occurs is called the transition current. Often, as is in the case of steel, this change is very abrupt.
The axial spray transfer is unique not only because of its good stability but also because of the absence of spatter. Furthermore, the drops are transferred in line with the electrode rather than along the shortest path between the electrode and the workpiece. The metal, therefore, can be directed where needed when making vertical or overhead welds.
The key to spray transfer is in the pinch effect which automatically squeezes the drops off the electrode. This occurs as a result of the electromagnetic effects (Lorentz force) of the current. The transition current is dependent on a number of variables, including the electrode composition, diameter, electrode extension, and the shielding gas composition. A great difference in transition current is found with various metal systems.
The transition current is approximately proportional to the diameter as shown in Figure 2.15 which at this time I will not include in here because I want to stay focused on the question as much as possible without going into too much detail on something else.
Transition current is not dependent on current density, but is mildly dependent on the electrode extension. An increase in the extension allows a slight decrease in the current at which spray transfer develops. [In practical welding operations, electrode extension is usually 13 to 25 mm (1/2 to 1 inch) depending on the diameter size used].
I n GMAW of steels, the spray transfer arc mode is most often used with argon-based shielding gas. Small additions of oxygen to the shielding gas lower the transition current slightly while CO2 additions raise it. The transition current defines the lower limit of useful current for spray transfer.
At high welding current densities, a rotary arc mode takes place. With appropriate mixtures of shielding gases, wire feeding controls, and welding guns that perform well at high wire feed speeds, rotating transfer mode GMAW can be used to deposit 7kg/h (16lbs/h) or more steel weld metal. The useful upper current limit is the value where the rotational arc becomes unstable with loss of puddle control and high amounts of spatter.
The welding current at which axial spray disappears and rotational spray begins is proportional to the electrode diameter and varies inversely with electrode extension. Spray transfer can also be achieved at average current levels below the transition current using pulsed welding current. One or more drops of filler metal are transferred at a frequency of the current pulses. This technique increases the useful operating range of a given electrode size.
Solid state power sources that simplify the set up for pulsed power welding have increased the applications of pulsed spray welding. The relatively low average current levels permit out-of position welding of steel at relatively high deposition rates. It is also utilized for aluminum welding with larger diameter electrodes and lower average currents and wire feed rates.
ELECTRODE NEGATIVE
The GMAW process is normally used with direct current electrode positive power. When the electrode is negative, the arc becomes unstable, and spatter is excessive. The drop size is large, and arc forces propel the drops away from the workpiece. This action appears to result from a low rate of electron emission from the negative electrode. If the thermionic properties of the electrode are enhanced by light coatings of alkali metal compounds, metal transfer is significantly improved. Although the use of emissive coatings allows spray transfer in GMAW with DCEN, commercial filler metals are not generally available with such coatings.
Now I know that some want me to probably focus on the effects of other gases besides argon as they relate to the GTAW process, but that would only limit this discussion to only one process, and as one can already notice that the effects of argon and other gases such as helium and CO2 are equally important to study in depth as well as they relate to the GMAW process with its different modes of metal transfer also.
EFFECT OF OTHER GASES ON METAL TRANSFER
Although helium is inert, it is unlike argon for shielding a welding arc because it does not usually produce an axial spray arc. Instead, the transfer is globular at all current levels and with both polarities. Helium shielded arcs are useful, nevertheless, because they provide deep penetration. Spray transfer is produced in helium by mixing relatively small quantities of argon with it. Using dilute mixtures, the deep penetration is not adversely changed. Although 20 percent argon in helium is sufficient to achieve these results, the normal commercial mixtures contain 25% argon as an insurance factor. Argon-helium mixtures are used for welding nonferrous metals such as Aluminum and Copper. Generally, the thicker the material to be joined, the higher the percentage of helium in the shielding gas is used.
Active gases such as carbon dioxide and nitrogen are much like helium in their effects on the arc. Spray transfer cannot be achieved without treatment of the wire surface, In addition, greater instabilities in the arc and chemical reactions between the gas and superheated metal drops cause considerable spatter. The difficulty with spatter can be minimized by welding with the buried arc technique. This technique is common when Carbon Dioxide is used to shield Copper, and when Nitrogen is mixed with argon to shield Aluminum alloys.To offset the harsh globular transfer and spatter associated with CO2 shielding, Argon may be added to stabilize the arc and improve metal transfer characteristics. Short circuiting transfer is optimized using mixtures of 20 to 25% CO2 in Argon. Higher percentages of CO2 are used for joining thick steel plate.
Small amounts of Oxygen (2to5%) or carbon dioxide (5 to 10%) are added to Argon to stabilize the arc, alter the spray transition current, and improve wetting and bead shape. These mixtures are commonly used for welding steel.
At this time, we will skip the rest of page 52 in the book and instead, advance to page 58 where we will continue with:
PHYSICAL PROPERTIES OF METALS AND SHIELDING GASES
The physical properties of the metals or alloys being joined influence the efficiency and the applicability of the various joining processes. The nature and properties of the shielding gases and of the contaminants from the atmosphere may have pronounced effect on the resulting weld. The shielding gases may be generated either by the decomposition of fluxing materials or by direct introduction into the arc stream and the area surrounding the arc plasma. Both thermal conductivity and thermal expansion have a direct effect on distortion of the weldment. Base metal electrical resistivity and thermal conductivity have a pronounced effect on the application of both resistance and arc welding to the various metals, In the case of resistance welding, base metal resistivity, thermal conductivity, and specific heat influence the power requirements.
In the case of arc welding, arc starting and arc stability are greatly influenced by the ionization potentials of the metal and flux vapors as well as by the various electronic transitions that occur in the shielding gases under the extreme temperature conditions that exist in the arc. The thermionic work function of the electrode material, and to a lesser extent, that of the materials being welded, have direct bearing upon the efficiency of the energy transferred by the welding arc. Electrical resistivity also plays an important role in these processes as a result of resistance heating of the electrode between the contact tube or the electrode holder and the work. Resistance heating of the electrode may be an important contribution to the total energy input to the weld zone.
Weld bead shape is dependent to varying degrees upon the interfacial energy between the surrounding atmosphere and the molten metal. The surrounding atmosphere may consist either of a gas or a liquid flux. Elements in the surrounding medium may control the shape of the bead.
Another important material property that should be considered when determining the relative weldability of alloys is the rate of oxidation of the base metal. This rate is important in determining the degree of shielding required. A corollary to this is the relative stability of oxides that may be present. The specific heat and density of the shielding gases affect the heat of the arc and the shielding coverage.
So, if we look at table 2.10 titled:
Physical properties of shielding gases, we can see the differences between N2, Ar, He, H2, and CO2 respectively. However for the purpose of focusing on the question which only asks which of the two shielding gases, Ar, or He are “Hotter”? I will just list the physical properties of these 2 elements instead of all 5.
We will start with Argon Helium
Molecular weight = 39.948 4.0026
Normal boiling point @K = 87.280 4.224
@C = 185.88 - 268.94
@F = 302.57 - 452.07
Density @ 21.1 C (70 F),
1 atm:
Note: I atm = One atmosphere.
kg/m3 = - 1.656 0.1667
lb/ft3 = - 0.1034 0.01041
Specific Volume
@ 21.1 C (70 F), 1 atm:
M3/kg = 0.6039 5.999
Ft3/lb = 9.671 96.06
Specific Gravity @ 12.1 C = 1.380 0.1389
Specific Heat Constant
Pressure @ 12.1 C, 1atm:
J/kg K = 521.3 5192
Btu/lb F = 0.1246 1.241
Specific Heat constant
Volume @21.1 C, 1 atm:
J/kg K = 312.1 3861
Btu/lb F = 0.0746 0.7448
In conclusion, one cannot simply answer the question properly without having other very important variables necessary in order to choose the right answer… In other words, the question is incomplete!
Respectfully,
Henry
Topic Welding Industry / Technical Discussions / Argon...Helium...which is "hotter", yes this is a trick ?
Dear Allan,
