Mais conteúdo relacionado


Unit 1 Physics of Welding Arc Welding.pptx

  1. Physics of Welding Arc: The physics of welding deals with complex physical phenomenon associated with weld induced heat, electricity, magnetism, light etc. Chapter contains:- i. Welding arc; ii. voltage distribution along the arc; iii. thermionic and non-thermionic cathodes; iv. theories of cathode and anode mechanism; v. arc characteristics and its relationship with power source; vi. arc efficiency; vii. heat generation; viii.effect of type of shielding gas on arc; ix. isotherms of arcs.
  2. Introduction In fusion welding process, the material around the joints is melted in both parts to be joined. The most important factors governing a fusion welding process are: ✓ The characteristics of the heat source. ✓ The characteristics of the arc. ✓ The heat flow characteristics in the joints. ✓ The nature of deposition of filler material in the Fusion Zone (FZ), known as weld pool.
  3. General characteristics of heat sources Heat Source: A heat source, suitable for welding, should release the heat in sharply defined isolated zone. Moreover, the heat should be produced at high temperature and at a high rate.  In fusion welding process one of the most important source of heat is electric arc. P/S Jobs Electrode, cathode (-ve) Arc Anode (+ve)
  4. Welding arc Welding Arc: It has been defined as a sustained electrical discharge through an ionized gas. The discharge is initiated by an avalanche of electrons emitted from hot cathode (i.e. thermionic emission) and maintained by the thermal ionization of the hot gas. This electrical discharge through an ionized gas produces a good amount of heat energy. A welding arc is a high current (up to 2000 amp) and low voltage (10 to 50) discharge. Electric welding arc besides being a heat source, transfer material, create turbulence in weld pool.  First of all, let us see in details, how an electric arc is created and maintained between 2 opposing polarity.
  5. thermionic emission Initially, a good contact is made between the electrode and work. Thereafter, the electrode is withdrawn. As a result, the metallic bridges starts breaking, thus increasing current density per bridge. Finally, the current density rises to such a high value that the bridges start boiling. Under such conditions, the electrons come out of both the surfaces by a process known as thermionic emission Note: Obviously, the electrons ( having (-ve charge ) coming out of the anode (+ve terminal) are pulled back, whereas those coming out of the cathode (-ve terminal ) are also attracted towards the anode. e Job (+) Anode e Electrode, cathode (-ve)
  6. thermionic emission (cont.) According to the Richardson-Dushman equation the emitted electron current density, Ie (A/m2), is related to the absolute temperature T by the following equation: 𝐼𝑒 = 𝐶𝑇2 exp − Ø 𝑘𝑇 Where, C= Richardson Constant 𝐴 𝑚2𝑘2 T= Absolute temperature (K) Ø= thermionic work K= Boltzmann Constant (8.6173*10−5 eV/K) C= 4π. 𝑚. 𝑒. 𝑘2 ℎ3 m = mass of an electron = 9.10938*10−31 𝐾𝑔 e = charge of an electron = 1.602*10−19 coulomb h = Planks constant = 6.626*10−34 𝐽 − 𝑆𝑒𝑐 1eV = 1.602*10−19𝐽
  7. thermionic emission (cont.) Work function : The minimum amount of energy needed for an electron to leave a material surface is called the work function. For most metals, it is on the order of several electron-volts. Table: Thermionic work function (approx.) Metal type Ø (eV) Aluminium 4.1 Cu 4.4 Fe (iron) 4.4 Tungsten 4.5 Na (Sodium) 2.4 K (Potassium) 2.2 Nickel 5.0 One electron volt is equal to 1.602*10−19𝐽
  8. Ionization The inter-particle collisions, taking place in the gap between electrodes, give rise to a process called, 'thermal ionization’ Ionization Potentials of some commonly used gas: N 15.6 eV Ar 15.8 He 24.6 Job (+) Anode e Electrode, cathode (-ve) + e e e + Ionization e ion Voltage gradient (E); Volt/distance Eed Eed > Potential energy of atom Electron gain some force Fe = E*e (N) d Fe=E*e*d (N-m)
  9. Conduction of Current in the Arc Once arc started, the arc itself becomes a source of ions through a process of ionization. These ions are attracted by the cathode (-ve terminal) and the resulting collisions keep the cathode hot. The total current in the arc is carried by 2 sets of electrons • Primary electrons: It is emitted by cathode (-ve terminal). • Secondary electrons: It is produced as a result of the ionization the arc gap.
  10. Arc Structure ❑ The conditions in the arc column are quite different from the region where the arc comes in contact with electrode (i.e., cathode) and the workpiece (i.e., anode in the DCEN). ❑ in the immediate vicinity of the electrode or the job, the plasma can no longer maintain its high temperature because it comes in contact with comparatively much colder workpiece and electrode. ❑ High temperature gradients exist on both the ends of the arc column and naturally the arc gets divided into 3 distinct zones i.e.: i. The most concentrated source of heat is the cathode spot ii. Hottest region is the arc column iii. The largest quantity of heat is produced at the anode The cathode is negative, anode is positive, and the arc column is electrically neutral as it contains equal number of ions and electrons. Job Arc Electrode
  11. Arc Structure Job (+), Anode (+) Cathode spot (emit electron) Cathode (-) - + + + ++ + Arc column i. Cathode spot ii. Cathode space (10−2) mm iii. Arc column iv. Anode space (10−2 ) mm v. Anode spot Anode Spot
  12. Arc Structure cont. Cathode spot: This is relatively a very small area on the cathode surface, emitting the electrons. Cathode space: it is a gaseous region adjacent to the cathode and has a thickness of the order of 10-2 mm. This region has the +ve space charge, so the voltage drop is necessary as the electrons arc to be pulled from this region. Arc column: This is the visible portion of the arc consisting of the plasma (hot ionized gas) where the voltage drop is not sharp. Anode space: This is gaseous region (thickness 10-2 mm ) and adjacent to the anode surface when a sharp drop of voltage takes place.  This is because the electrons have to penetrate the anode surface after overcoming the repulsion of the thermionically emitted electrons from anode surface. Anode spot: This is the area on the anode surface where the electrons are absorbed. This is larger than cathode spot.
  13. Arc Structure cont.  Different potential drop regions of arc: Anode (+) + + + + + Cathode (-) L Arc length Voltage Voltage drop in Cathode drop region Voltage drop in anode drop region Arc column voltage drop Cathode drop region Anode drop region 10−2 𝑚𝑚 10−2𝑚𝑚
  14. Approximate temperature in electrode and plasma - ve Anode (+) Tungsten electrode, melting temp (6191 °F, 3422 °C) 4000 °F 4900 °F 5300 °F 11000 °F
  15. Potential Drop Characteristics The potential/voltage drop changes if the material or spacing between electrode changes. However, a change in spacing and the current essentially changes only the drop in arc column. It has been experimentally found that, for given spacing, the voltage reduces up to a current value of around 50 amp and increases thereafter, as shown in below: I, amp V, volt I, amp V, volt L1 , arc length 3 mm 2 mm L1, 1 mm 50 A
  16. Potential Drop Characteristics cont.  Up to around 50 amp of current, the shape of the are is almost cylindrical. This results in a higher conductivity and consequently lower resistance.  However, beyond 50 amp of current, the arc bulges out and the current path becomes more than the are gap which again increases the resistance of the arc.  Due to these opposite effects, i.e., higher temperature and longer current path, the voltage drop remains almost constant over a wide range of current values. • The electrode drops are also independent of the arc length.  Here, we can write voltage drop across the entire are as: V=A+Bl (As voltage drop line depends on arc length) where A is the electrode drop and Bl represents the column drop.
  17. Arc Power  The power of an arc varies with its length and there optimum length for which the arc power is maximum. optimum arc length (lopt) can be determined as follows: I, amp V, volt V 1 V 2 I2 I1 I, Arc length Power Iopt P = V*I; = V(l)*I(l) For opt, 𝑑𝑃 𝑑𝑙 = 0 , lopt = ? P = V*I ; (watt) V = A+Bl …….(1) V1 = A+Bl1 ; V2 = A+Bl2; I1 = f(l1); I2 = f(l2); P1 = V1*I1 ; P2 = V2*I2; Pmax = f(Iopt)
  18. Problem - 1 The voltage-length characteristics of a direct current arc is given by V= (20+40L) volts. Where ‘L’ is the arc length in cm. the power source characteristics is approximated by a straight line with an open circuit voltage = 80 V and short circuit current =1000 amp. Determine the optimum arc length and corresponding power. Sol: Given Data: V= (20+40L)…..(i), Vo = 80 V, Is = 1000 A, Lopt ?, Popt ? We Know P = V*I B Vo I, amp V, volt A D C Is E From similarity of two triangle ABC and ADE 𝐴𝐵 𝐴𝐷 = 𝐵𝐶 𝐷𝐸 = 𝑉𝑜 − 𝑉 𝑉𝑜 = 𝐼 𝐼𝑠 𝑉 = 𝑉𝑜 − 𝑉𝑜 𝐼 𝐼𝑠 𝑉 = 80 − 80 ∗ 𝐼 1000 ……(ii) (i) = (ii) 𝐼 = 60 − 40𝑙 ∗ 25 2 ………(iii) 𝑃 = 𝑉 ∗ 𝐼 𝑃 = 20 + 40𝑙 ∗ 60 − 40𝑙 ∗ 25 2 for opt. 𝑑𝑃 𝑑𝑙 = 0 𝑙𝑜𝑝𝑡 = 0.5𝑐𝑚 𝑃𝑜𝑝𝑡 = 20 𝑘𝑤𝑎𝑡𝑡
  19. Problem - 2 The voltage — length characteristics of a DC arc is given by V= (15+4l) volts. where ‘l’ is the arc length in mm which varied between 4 mm to 6 mm. Here the current varied between 500 A to 400 A. The power source characteristics is approximated by a straight line. Find the open circuit voltage and short circuit current. Also determine the optimum arc length and corresponding power. Given Data: V= (15+4l)….(i) At l = 4mm, I = 500 A At l = 6mm, I = 400 A Vos = ? Is = ? Lopt ? Popt ? From similarity of two triangle ABC and ADE 𝐴𝐵 𝐴𝐷 = 𝐵𝐶 𝐷𝐸 = 𝑉𝑜−𝑉 𝑉𝑜 = 𝐼 𝐼𝑠 ……..(ii) B Vos I, amp V, volt A D C Is E From eqn (i) For l= 4mm; V = 15+4l; V = 15+4*4 = 31V l= 6mm; V = 15+4l; V= 15+4*6 = 39V By eqn (2) 𝑉𝑜𝑠 − 31 𝑉𝑜𝑠 = 500 𝐼𝑠 … … . (iii) 𝑉𝑜𝑠 − 39 𝑉𝑜𝑠 = 400 𝐼𝑠 … … . (𝑖𝑣)
  20. Problem – 2 cont. The voltage — length characteristics of a DC arc is given by V= (15+4l) volts. where ‘l’ is the arc length in mm which varied between 4 mm to 6 mm. Here the current varied between 500 A to 400 A. The power source characteristics is approximated by a straight line. Find the open circuit voltage and short circuit current. Also determine the optimum arc length and corresponding power. Now, eqn (iii) / eqn (iv) 𝑉𝑜𝑠 − 31 𝑉𝑜𝑠 − 39 = 500 400 Vos = 71 V Now putting Vos in eqn (iii) We get, Is = 887.5 A For Lopt and Popt V= (15+4l)……(i) 𝑉𝑜𝑠 − 𝑉 𝑉𝑜𝑠 = 𝐼 𝐼𝑠 71 − 𝑉 71 = 𝐼 887.5 V = 71 − 71 ∗ 𝐼 887.5 …….(v) Eqn(i) = eqn(v) 𝐼 = 56 − 4𝑙 ∗ 887.5 71 𝑃 = 𝑉 ∗ 𝐼 𝑃 = 15 + 4𝑙 ∗ 56 − 4𝑙 ∗ 887.5 71 ……(vi) For lopt; 𝑑𝑃 𝑑𝑙 = 0, 𝑙𝑜𝑝𝑡 = 5.125𝑚𝑚 Putting ‘lopt’ in eqn (vi), 𝑃𝑜𝑝𝑡 = 15.753 𝑘𝑊
  21. Arc Initiation Arc initiation:  Arc is initiated by providing a conducting path between the electrode and job/workpiece.  Or Ionizing the gap between the two.  It can be initiated by following ways:  Tapping method  Scratching method  By steel wool  By a carbon rod  By a high frequency unit Work-piece Electrode
  22. Arc Initiation cont.  Tap Start - tap the rod against the base metal. By momentarily touching the electrode with job and taking it away.  Scratch start - scratch the electrode on the base metal like a match.  Example: SMAW Process Fig. Tapping method of arc starting Fig Scratch method of arc starting
  23. Arc Initiation cont.  By steel wool: In this case, the steel wool kept pressed between the electrode and the workpiece.  When the welding current is switched on, steel wool provides a conducting path for the arc to establish.  Ex.: SAW and automatic WC welding.
  24. Arc Initiation cont. By a carbon rod: Arc can be initiated with help of a carbon rod.  Suitable arc gap is kept between the electrode and workpiece, current is switched on, then the electrode and the job simultaneously are momentarily touched with a carbon rod.  Ex.: Automatic metal arc welding
  25. Arc Initiation cont. H. F. (high frequency) unit: In order to eliminate the chances of electrode contamination a H. F. ( high frequency ) unit is inserted in the circuit to initiate the arc.  High frequency unit is a device which supplies high voltage of the order of few kV along with high frequency of few kHz with low current.  When electrode tip is brought within 3 to 2 mm from the job/workpiece, a spark jumps across the air gap between the electrode and the job. Then this high voltage ionizes the medium between electrode and workpiece starting pilot arc which ultimately leads to the start of main arc.  Ex.: GTAW and PAW.  Note: Although high voltage may be fatal for the operator but when it is associated with high frequencies then current does not enter into the body but it causes only skin effect i.e. current passes through the skin of operator.
  26. Types of welding arc  The welding arcs may be categorized of the following types: i. Steady arc (generally in DC, electrical discharge between two electrodes) ii. Unsteady arc (this is due to electrical short circuiting metal transfer where the arc interrupted) iii. Continuously non-steady arc (this is due to AC current flow) iv. Pulsed arc (intermittent current pulses are superimposed on a regular arc to obtained spray metal transfer during pulse interval) t, sec I, amp Peak Current Background Current
  27. Types of welding arc cont.  Depending upon the geometry of the tungsten electrode (cathode) tip 2 different modes of welding arcs were observed by Olsen; • Cathode spot mode & • Normal mode ❑ Note: The normal mode is more stable and readily obtainable.  The cathode spot mode exhibits a constriction of the plasma at the cathode and is accompanied by a higher voltage for a given arc length. ✓ For same welding current and arc gap, the rise or peak temperature is more in cathode spot mode than the normal mode.
  28. Functions of current flow In an arc, current density, magnetic field strength and pressure all decrease from cathode drop region towards the arc column, because arc cross- section increases rapidly in the arc column. The current flow through the arc gives rise to self induced magnetic field which compresses the arc plasma resulting in appreciable axial and radial pressure gradient in the arc.  The radial pressure gradient constrict the arc (pinch effect) and raises the temperature of the arc discharge.  Whereas axial pressure gradient give rise to plasma streaming which transports material and heat from electrode to the work piece.  Plasma streams stabilise the arc and exert a pressure on the molten pool which helps to increase the penetration.  Note: Both these effects are proportional to square of arc current. Anode Cathode (-) Molten pool Arc Magnetic flux
  29. Arc stability  Arc is said to be stable if it is uniform and steady.  A stable arc will produce good weld bead & defect free nuggets.  The stability of a welding arc is governed by so many factors, as mentioned below:  Suitable matching of arc and power source characteristics, a little variation in arc length, i.e., arc voltage should not extinguish the arc.  Continuous and proper emission of electrons from electrode (say cathode) and thermal ionization in the arc column.  Arc length.  Electrode tip geometry in GTAW. Higher the included angle better the arc stability.  Presence of dampness, oil, grease etc. on the surface of workpiece increase arc instability.  Limited practice of the welder. Anode 𝛼 Included angle Tungsten electrode
  30. Arc stability cont . Polarity have significant effect on arc stability:  The welding processes in which electrode is expected to emit free electrons (i.e. SMAW, GTAW, PAW) required for easy arc initiation and their stability, selection of polarity affects the arc stability.  In consumable electrode welding i.e. in SMAW using covered electrode having low ionization potential elements provide better stable arc with DCEN than DCEP.  However, SMA welding with DCEP gives smoother metal transfer.  Similarly, in case of non-consumable electrode welding i.e. in GTAW welding, tungsten electrode is expected to emit electrons for providing stable arc and therefore DCEN is commonly used except in case of reactive metals e.g. Al, Mg, Ti. DCEN (-) ve Stable arc
  31. Arc Blow  The unwanted deflection or the wandering of a welding arc from its intended path is termed as arc blow or arc bow.  Arc blow is the result of magnetic disturbances which unbalance the symmetry of self-induced magnetic field around the electrode, arc and workpiece.  Arc blow becomes severe when welding is carried out in confined spaces and corners on a heavy metal plate, using DC power source.
  32. Arc Blow cont.  AC arcs are less susceptible to arc blow than DC arcs. • Because AC reverses direction which in turn, reverses the magnetic field build up, collapses and rebuilds as current reverses from +ve to -ve. • This phenomenon does not permit the magnetic field strength to build up to a value so as to cause arc blow.  On the other hand in DC welding, the magnetic field set up in workpiece (adjacent to the arc) continuously builds up and the arc blow occurs.
  33. Factors affecting arc blow  Magnetic fields produced in the workpiece adjacent to the welding arc, due to current flow through the arc.  With multiple welding heads, arc at one electrode may be affected by the magnetic field of the arc at the other electrode.  The magnetic field produced in the workpiece around the earth connection may tend to drive the arc away from the point from where this connection is made.  This magnetic field is also produced because of current flow from the earth connection to the workpiece.
  34. Types of arc blow  There are three kinds of arc blow depending upon the above factors: i. A forward blow at the starting end of a weld and backward blow at the finishing end of a weld. ii. A sideward deflection. iii. Arc rotate.
  35. Forward and Backward arc blow  Magnetic flux lines get crowded near the starting and finishing end of the workpiece because they find an easier path through the workpiece than that through the air.  The arc seeks the path of least resistance and deflects towards the weak flux side.  Which causes the forward arc blow at the starting end and backward arc blow at the finishing end of the weld bead in a workpiece.
  36. Sideward arc blow  It depends on workpiece earth clamp.  As the arc comes near the arc clamp, it deflects sidewardly in a direction away from the clamp, perhaps because of the magnetic flux enacted in the workpiece by the earth clamp (i.e., ground).  This magnetic flux is produced by the flow of the current from clamp to workpiece.  It is also noted that as the arc crosses the earth connection, it has a tendency to come to the original line of travel .
  37. Arc Rotation ❑ Here the arc deflection as well as rotation occur. ❑ Arc rotation indicates that under certain conditions of arc blow, perhaps arc experiences magnetic field lines parallel to the arc axis. Fig. Arc rotation and deflection
  38. Effect of arc blow  Increased arc blow results in an unstable arc  Poor weld bead appearance  Irregular and erratic weld deposition  Undercutting and lack of fusion  Spatter  Uneven and weak welded joints  Slag entrapment  Porosity
  39. Remedies arc blow  It can be minimized by keeping the following factors in view:  Changing the position of the earth clamp and welding away from the earth connection.  Storing workpiece away from the magnetic sources, such as welding power sources.  Employing ground connection more than one.  Using a short arc.  Lowering arc current.  Using smaller diameter electrodes.  Decreasing arc travel speed.  Superimposing a counteracting externally applied longitudinal magnetic field.
  40. Metal Transfer Introduction:  When an electric arc is struck between the job and the consumable electrode,  The arcing end of the electrode starts melting,  Takes approximately a spherical shape, hangs towards the job, and  Ultimately drop down on the same, either with a free flight through the arc or by short circuiting the job.  The size of the droplet and the drop transfer rate affects the weld bead geometry, weld metal microstructure and strength of welded joints etc.  Metal transfer can be studied by a high speed movie camera (3000-7000 frames/sec). Like GMAW (here metal transfer is well visible).  But where the metal is not visible like (like SMAW or SAW) X-rays have been used to study metal transfer.
  41. Metal Transfer
  42. Classification of Metal Transfer  There are 3 main types of metal transfer : a) Free flight transfer b) Contact transfer c) Slag protected transfer a) Free flight transfer: i. Globular ii. Spray and iii. Explosive b) Contact transfer: i. Bridging ii. Short-circuiting (i.e. in short-arc GMAW) c) Slag protected transfer: i. Flux wall guided ii. Other modes(SMA, cored wire, ESW)
  43. Free Flight Metal Transfer  Free flight transfer: In which metal drops get detached from the electrode, pass through the arc and fall on the job.  Categories of Free flight transfer (depends on approx. size of droplet): • Sub-threshold metal transfer • Globular metal transfer • Spray metal transfer • Jet metal transfer Fig. Types of metal transfer
  44. Globular Metal Transfer  Here the drop diameter is approximately twice the electrode wire diameter. Ex: SMAW, SAW.  It is observed at low arc current and with larger arc.  The no. of drop transferred per second is very less (1 to 10 drops/s).  The globules may pass freely through the welding arc or depending upon the size and gap of the arc they may short circuit the arc.  This transfer is associated with spatter loss and shallow penetration height.
  45. Spray Metal Transfer  Here the drop diameter is approximately equal to or less than the electrode core wire diameter.  The rate of drop transfer is much higher than the globular transfer.  Here is a continuous spray of drops.  It occurs at high arc currents and low arc lengths.  Spray mode of transfer produces stable arc, good weld bead, deep penetration, a strong joint and is recommended for thicker plates .  Jet type: in this case the electrode end becomes tapered and a jet of drops comes out from the electrode.
  46. Categories of Spray Transfer  There are mainly 3 different categories of spray metal transfer: i. Projected (Due to Intermediate-current in GMAW) ii. Streaming (Due to Medium-current in GMAW) iii. Rotating (Due to High-current in GMAW)
  47. Categories of Spray Transfer cont.  projected Spray Metal Transfer:  Electromagnetic force is the main governing force.  Projected spray is characterized by small droplets (close to the electrode diameter) transferring from the electrode tip to the weld pool at a rate of about hundreds per second, without short-circuiting the pool.  Very regular and no significant amounts of spatter arc observed.  Projected spray transfer can only be used in the flat position, because of the large volume of the molten metal in the weld pool.  A projected spray is obtained at high voltage (long arc) and intermediate-current i.e. just above transition current in GMAW.  Note: The transition current is dependent on a great number of parameters, such as filler material, shielding gas composition and electrode extension/ diameter.
  48. Categories of Spray Transfer cont.  Streaming Spray Metal Transfer:  Electromagnetic force is the main governing force.  With a further increase of the welding current, projected spray metal transfer transforms into "streaming spray" transfer.  Greater heat is produced in the electrode tip.  The anodic area increases due to higher current arriving the wire end. As a result, a wire volume above the arc-wire coupling is heated enough to become plastic, resulting in the "tapered" shape of the electrode end.  At the tip, very fine droplets arc formed and detached. As long as this tapered end does not touch the pool, there is no spatter.
  49. Categories of Spray Transfer cont.  Rotating Spray Metal Transfer:  Here also electromagnetic force is the main governing force.  This mode of metal transfer takes place by a further increase in the current level from that of streaming spray.  The wire electrode tapering effect is more pronounced with overheating, resulting in an extended metal filament.  Strong electromagnetic forces, caused by the excessively high welding current applied, move the column away from its straight line of flow.  The combination of asymmetric radial forces and azimuthal forces results in spiral motion of the column.  The droplets (extremely fine) are detached from the tip of the rotational filament in tangential direction, producing a lot of spatter.  This process is generally used to improve the sidewall penetration in the flat position and prevent the molten pool sagging during the horizontal welding.
  50. Short-Circuiting Transfer  Short-circuiting type of metal transfer:  Here the arcing end of the electrode starts melting  Develops to a spherical shape  Makes contact with molten pool in the base metal and get detach from the electrode.  When the hanging drop touches the base metal, the circuit is shorted and arc extinguishes.  Electrode pinch effect increases due to increase of current during shorting, neck formation quickens and ultimately the drops get detached from the electrode.  The moment the drop is detached from the electrode, the circuit again opens ana arc gets
  51. Short-Circuiting Transfer cont.  Normal short circuiting ranges from 20 to 200 per second.  The short circuiting frequency is mainly dependent on: electrode wire dia. and arc voltage, i.e., with increases of wire dia. and voltage decreases the short circuit frequency.  At CO2 atmosphere of steel welding the short circuiting voltage is about 20 V and maximum short circuiting frequency:  For 1.5 mm electrode dia. = 75/sec.  For 0.75 mm electrode dia. = 150/sec.
  52. t, sec I, amp Peak Current Background Current Pulsed Transfer Pulsed arc welding is a controlled method of spray transfer welding requiring a more sophisticated power source.  In spray transfer, metal transfer along arc generally occurs at constant current.  In short circuit transfer, the current generally irregular in nature.  But in this case, transfer of metal from the wire tip to molten pool occurs only at a period of pulse or peak current.  During the interval between pulses, a low 'background' current maintains the arc to keep the wire tip molten but no metal is. Note: Here we can control the deposition rate (by adjusting pulse rate i.e. by controlling the peak current and background current pulse).
  53. Pulsed Transfer (cont.) Fig. Sequence of events in pulsed metal transfer  Time vs. current during the sequence of events is as below:
  54. Forces affecting metal transfer In general various forces involved in the phenomenon are given below: • Surface tension • Viscosity of the liquid metal • High velocity gas jets • Force due to metal vapour • Gravity • Force due to impact of charge carriers • Lorentz forces.
  55. Forces affecting metal transfer cont.  Surface tension force: It is a retarding force which tries to keep the drop in its position.  The force of surface tension acting on the drop when it is just to detach is given by: where d = electrode diameter σ = surface tension (Force/distance) k = it is a function dependent on electrode diameter and capillarity constant of the material. Normally it varies from 0.6 to 1.0.  Note: The force of surface tension ranges from 400 to 800 dyne for electrode from 1.5 to 3 mm. At higher temperature the surface tension is lowered. 𝑭𝒔 = 𝝅𝒅𝝈𝒌 𝐹𝑠 = 2σπ𝑅𝑒2 4𝑅 ∗ 𝑘 𝑅𝑒 = 𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑑𝑒 𝑑𝑖𝑎. 𝑅 = 𝐷𝑟𝑜𝑝𝑙𝑒𝑡 𝑑𝑖𝑎.
  56. Forces affecting metal transfer cont.  Viscosity of liquid metal: It is retaining force.  The high velocity gas jets striking the job and getting back may retard the movement of metal drop tending to fall down in the molten pool.  Force due to metal vapour  Vapours generate due to molten metal evaporation from bottom of the drop and weld pool move in upward direction. Forces generated due to upward movement of metal vapours act against the molten metal drop hanging at the tip of the electrode.  So, this force also tends to hinder the detachment of metal droplet.  Gravity: It acts as a detaching force when welding in flat position and it is a retarding force when welding overhead.  The force of gravity (𝜌𝑣𝑔), which depends upon volume of globule.  It is almost negligible on small diameter droplets. τ = μ ∗ 𝑑𝑢 𝑑𝑦 𝐹𝑔 = 𝑚𝑔 = 𝜌𝑣𝑔
  57. Forces affecting metal transfer cont. ❑ Force due to impact of charge carriers  As the polarity charged particles (ions & electrons), move towards anode or cathode and eventually impact/collide with them.  Force generated due to impact of charged particles on to the molten metal drop hanging at the tip of electrode tends to hinder the droplet detachment. Force due to impact of charged particles where m = the mass of charge particles, v = the velocity and t = the time. 𝐹𝑐𝑝 = 𝑚 𝑑𝑣 𝑑𝑡
  58. Forces affecting metal transfer cont.  Lorentz force:  This force is the result of interaction of the arc current with its self- induced magnetic field.  This force exercises pinch effect on the globule, aids in the neck formation and drop detachment.  The self-induced magnetic field of the arc plasma results in plasma streaming which carries the detached drop to the workpiece.  The pinch force varies from 250 to 1000 dyne for 1.5 to 3 mm electrode dia.  Note: Surface tension and viscosity of the liquid metal help droplet to grow in size. Whereas electromagnetic forces constrict (i.e., neck) the molten end of the electrode due to that drop separates from electrode. 𝐹𝑙 = 𝐹𝑝 = μ ∗ 𝐼2 8π μ = magnetic permeability = 𝑁 𝑎𝑚𝑝2
  59. Polarity in Welding Process
  60. • Melting Rates • Effect of Parameters on Metal Transfer • Efficiency of Welding
  61. Melting Rates  Electrical resistance heating of the electrode by welding current affects the electrodes melting rate (M.R.) • Electrode melting rate (MR.) can be represented as: 𝑀. 𝑅. = 𝑎𝐼 + 𝑏𝐿𝐼2 where a = anode or cathode constant of proportionality for heating. b = constant of proportionality for electrical resistance heating and includes the electrode resistivity. L = electrode extension or stick out. I = welding current. Metals 𝒂 ( 𝒌𝒈 𝒉. 𝑨 ) 𝒃( 𝒌𝒈 𝒉. 𝑨. 𝒎𝒎 ) Aluminum (DCEP) 5.4*10−3 4.4*10−6 Mild Steel (DCEP) 8.6*10−3 2.5*10−5 Mild Steel (DCEN) 1.8*10−2 2.5*10−5 Table. Relative magnitude of heating coefficients of 1.6 mm diameter wire electrode
  62. Effect of Parameters on Metal Transfer  The drop transfer rate also depends on  Arc current  Arc length  Type of polarity  Electrode material  Electrode extension and  Shielding gas  Note: The drop transfer increases with DCEP (i.e. DCRP), with increase in arc current and electrode extension.
  63. Effect of voltage and Current on Metal transfer  Arc current increases drop transfer rate increases. Because increasing the current increases the electrode burn off rate. ❖ At a constant current, drop transfer rate decreases as the voltage increases, because the heat losses increase as at high voltage, the arc length increases. ❖ For longer arc heat loss is more. Fig. Effect of arc current and voltage on drop transfer Drop rates Current V1 V2 R1 R2 I V2 > V1 V = IR V α R α L/A
  64. Effect of Polarity on Metal transfer  Electrode Positive:  By this polarity stable arc and drop transfer can be obtained.  With DCEP by varying the current the drop transfer rate and type can be varied.  At low welding currents the size of the droplet in argon develops to a diameter more than the diameter of electrode (i.e. globular). On The droplet size is roughly Inversely proportional to the current and only a few droplets are release per second.  With long arc length, the droplets are transferred without short circuit. ✓ At high current the spray transfer occurs, here the tip of the electrode becomes pointed and the drops are transferred at a rate of about a hundred per second. The current at which this occurs is called transition current.
  65. Effect of Polarity on Metal transfer  Electrode Negative: GMAW arc becomes unstable and spattery when electrode negative is used. The drop size is big and due to arc forces the drops are propelled away from the workpiece as spatter. Spray transfer may be observed in argon shielded consumable electrode arc only. It appears that argon provides the unique plasma properties with the self-magnetic force to develop axial spray transfer through the arc.
  66. Effect of Other Gases on Metal transfer  Helium gas:  Helium, although inert gas, does not produce axial spray transfer.  The transfer is globular with both polarities at all current levels.  Helium arcs are useful, nevertheless, because they provide deep penetration.  Spray transfer can be obtained by mixing small quantities of Argon gas (about 20 %). Ionization Potential Ar = 15.8 eV He = 24.8 eV
  67. Effect of Other Gases on Metal transfer Carbon dioxide and Nitrogen:  Active gases like carbon dioxide and nitrogen do not produce spray transfer, spatter on the other hand is increased.  The amount of spatter, massiveness of the drops and instability of transfer generally are greater when the electrode is negative.  Spray transfer can be achieved by painting cesium and sodium on steel wire surface with carbon dioxide shield using direct current electrode negative polarity (i.e. DCEN).  Some applications of these gases as a shielding medium:  Carbon dioxide can be used to shield arcs in mild steel.  Nitrogen can be used mixed with argon to shield aluminium alloys.  Nitrogen is used to shield coroner.
  68. Welding Energy Input  The energy input per unit length, ‘H’ is computed as the ratio of total input power ‘P’ of the heat source to its welding travel velocity, v. 𝐻 = 𝑃/𝑣  If the source of heat is an electric arc then, 𝐻 = 𝐸𝐼 𝑣  Preciously speaking the net energy input would be 𝐻𝑛𝑒𝑡 = 𝜂𝐸𝐼 𝑣  where η = the heat transfer efficiency. H=P/v = watt/(m/s) = (J/s)/(m/s) = J/m
  69. Efficiency of Welding  In Welding there are two different types of efficiency i. Heat source efficiency ii. Melting or Heat transfer efficiency
  70. Heat Source Efficiency  In the case of arc welding, having a constant voltage E and a constant current I, the arc/ heat source efficiency can be expressed as; 𝜂 = 𝑄 ∗ 𝑡𝑤𝑒𝑙𝑑 𝑄𝑛𝑜𝑚𝑖𝑛𝑎𝑙 ∗ 𝑡𝑤𝑒𝑙𝑑 = 𝑄 ∗ 𝑡𝑤𝑒𝑙𝑑 𝐸𝐼 ∗ 𝑡𝑤𝑒𝑙𝑑 = 𝑄 𝐸𝐼 where, Q = is the rate of heat transfer from heat source to work piece Qnominal = is the nominal rate of heat of the power source tweld = is the welding time P/ S Jobs Arc
  71. Measurement of Heat source efficiency  Heat source efficiency can be measured using a calorimeter (by measuring the heat transfer from the heat source to the workpiece and then to the calorimeter).  The temperature rise in the cooling water (Tout - Tin) can be measured using thermocouples or thermistors. Heat transfer from the workpiece to the calorimeter is given by: Where; W is the mass flow rate of water C is the specific heat of water Tout is the outlet water temperature Tin is the inlet water temperature t is time 𝑄 ∗ 𝑡𝑤𝑒𝑙𝑑 = 0 ∞ 𝑊𝐶 𝑇𝑜𝑢𝑡 − 𝑇𝑖𝑛 𝑑𝑡 ≈ 𝑊𝐶 0 ∞ 𝑇𝑜𝑢𝑡 − 𝑇𝑖𝑛 𝑑𝑡  Note: The calorimeter can be a round cross section if the workpiece is a pipe or a rectangular cross section if the workpiece is a sheet.
  72. Heat Source Efficiency Fig. Heat source efficiency in various welding processes
  73. Melting Efficiency  Melting efficiency is the ability of the heat source to melt the base metal (as well as the filler metal).  The melting efficiency of the arc , ηm can be defined as follows:  Note: The quantity inside the parentheses represents the volume or material  And the denominator represents the heat transfer from the heat source to the workpiece.  Increase of ‘V’ results in increase in melting efficiency of the arc ηm 𝜂𝑚 = Abase. V. tweld . Hbase + Afiller. V. tweld . Hfiller/(η. E. I. tweld) Where; ‘V’ is the welding speed ‘Hbase’ is the energy required to raise a unit volume of base metal to the melting point and melt it. ‘Hfiller’ is the energy required to raise a unit volume of tiller metal to the melting point and melt it. ‘tweld’ is the welding time. ‘Abase’ is cross-sectional area base metal which is melted Arm. ‘Afiller’ is cross-sectional area filler metal. Afiller Abase Aweld = Afiller + Abase
  74. Melting Efficiency cont.  With the help of following equation for determining Afiller, Where, ‘Rfiller’ and ‘Vfiller’ arc the radius and feeding Speed of the filler metal, respectively Afiller. V. tweld = π𝑅2 𝑓𝑖𝑙𝑙𝑒𝑟. 𝑉𝑓𝑖𝑙𝑙𝑒𝑟. 𝑡𝑤𝑒𝑙𝑑 𝐴𝑓𝑖𝑙𝑙𝑒𝑟 = 𝜋𝑅2𝑓𝑖𝑙𝑙𝑒𝑟. 𝑉𝑓𝑖𝑙𝑙𝑒𝑟 𝑉 Afiller Abase
  75. Welding Parameters and Their Effects  Weld quality and weld deposition rate both are influenced by various weldirame rs and joint geometry. These parameters are the process variables as given below:  Welding current  Arc voltage  Welding speed  Electrode feed rate  Electrode extension (stick-out)  Electrode diameter  Joint geometry
  76. Welding Parameters and Their Effects cont.  Each of the above parameters affects, to varying extent, the following:  Deposition rate  Weld-bead shape  Depth of penetration  Cooling rate  Weld induced distortion.  So, a proper understanding of the effects of welding parameters is important to obtain a sound welded joint with adequate metal deposition rate and minimum distortion
  77. Welding Parameters and Their Effects cont.  Welding Current:  For a given electrode and polarity in DC welding, melting rate is directly proportional to the energy (current and voltage).  Part of this energy Q is used to melt the base metal (qb), part goes to melt electrode and flux (qf) rest is dissipated as conduction (qcp, + qcc), convection (qv,) and radiation (qr). Q = qb + qf + (qcp + qcc) + qv, + qr Here, Q = EI (Watt i.e. J/s) = 𝐼2. 𝑅𝑎 (J/s)  Where, Q = electrical power consumed  1= welding current  E = arc voltage  Ra= arc resistance
  78. Welding Parameters and Their Effects cont.  Welding Current:  it is most important variable affecting melting rate, the deposition rate, the depth of penetration and the amount of base metal melted.  if the current (for a given welding speed) is too high, it will result in:  Excessive penetration (thinner plates will melt through)  Excessive melting of electrode-excessive reinforcement  More heat input to plates being joined increased distortions  if the welding current is too low, it will result in:  inadequate penetration  Lack of fusion  Note: Current could be AC or DC. DC provides steady arc, smooth metal transfer, good wetting action and uniform weld bead size.
  79. Welding Parameters and Their Effects cont.  Arc voltage:  Arc voltage is the voltage between the job and the electrode during welding. For a given electrode it depends upon the arc length.  Open circuit voltage approximately varies between 50-100 V whereas arc-voltage are between 15 V to 40 V. When the arc is struck, the open circuit voltage drops to arc voltage and welding load comes on power supply.  The arc voltage depends on arc length and type of electrode.  As the length increases, arc resistance increases, resulting in higher voltage drop i.e., arc-voltage increases. Fig. Effect of welding voltage on weld bead shape, keeping all other parameters constant
  80. Welding Parameters and Their Effects cont.  Arc voltage:  Proper arc length is important in obtaining a sound joint.  Short arc: It may causes short circuits during metal transfer  Long arc: lacks direction and intensity, gives heavy spatter and formation of undercuts.  Weld-bead appearance depends on arc-voltage.  Increase in arc-voltage tends to: cause porosity, spatter, flatten the weld bead and increase weld width.  Reduction in arc-voltage leads to: narrower weld-bead and higher crown.  Trials are, therefore, made to obtain optimum arc voltage.
  81. Welding Parameters and Their Effects cont.  Welding speed: It generally conforms to a given combinations of welding current and arc voltage.  If welding speed is more than required:  Heat input to the joint decreases  Less filler metal is deposited than requires, less weld reinforcement height  Undercut, arc blow, porosity and uneven bead shape may result.  If welding speed is slow:  Filler metal deposition per length increases, more weld reinforcement  Heat input per unit length increases  Weld width increases and reinforcement height also increases more convexity  Penetration decreases beyond a certain decrease in speed.  A large weld pool, rough bead and possible slag inclusion.
  82. Welding Parameters and Their Effects cont.  Welding speed:  With all variables held constant, weld penetration depth attains a maximum at a certain intermediate speed.  At excessively low welding speeds the arc strikes a large molten pool, the penetrating force get cushioned by the molten pool.  With excessively high welding speeds, there is substantial drop in thermal energy per unit length of welded joint resulting in undercutting along the edges of the weld bead.  it is because of insufficient backflow of filler metal to fill the path melted by the arc.  Note: Welding speed is to be adjusted within limits to control weld size and depth of penetration.
  83. Welding Parameters and Their Effects cont.  Electrode feed speed:  Electrode feed rate determines the amount of metal deposited per unit length or per unit time.  In most welding machines the welding current adjusts itself with electrode feed speed to maintain proper arc length.  Electrode Extension:  Electrode extension, also known as length of stick out, is the distance between the end of the contact tube and the end of the electrode.  An increase in electrode extension results in an increase in electrical resistance.  This causes resistance heating of electrode extended length, resulting in additional heat generation and increase of electrode melting rate.  But the energy so consumed reduces the power delivered to the arc thus decreases bead width and penetration depth.
  84. Welding Parameters and Their Effects cont.  Electrode Extension:  To maintain proper bead geometry along with a desired penetration and higher melting rate (i.e., large electrode extension), the machines voltage setting must be increased to maintain proper arc length.  At current densities above 125 A/mm2, electrode extension becomes important.  An increase of up to 50% in deposition rate can be achieved by using long electrode extensions without increasing welding current.  This increase in deposition rate is accompanied with decrease with decrease in penetration.  Thus when deep penetration is desired long electrode extension is not desirable.  On the other hand, for thinner plates, to avoid the possibility of melting through, a longer electrode extension becomes beneficial.
  85. Welding Parameters and Their Effects cont.  Electrode Diameter:  It affects bead configuration, affecting penetration and deposition rate.  At any given current, a smaller diameter electrode will give higher current density causing a higher deposition rate compared to large diameter electrode.  A larger diameter electrode, however requires a higher minimum current to achieve the same metal transfer characteristics. Thus larger electrode will produce higher deposition rate at higher current.  In case of poor fit-up or thick plates welding larger electrode size is better to bridge large root openings than smaller ones. Fig: Effect of electrode size on Bead geometry keeping current voltage and speed constant Fig. Effect of electrode size on bead geometry keeping current voltage and speed constant
  86. Voltage distribution along the arc
  87. Thermionic and non-thermionic cathodes:  Emission of free electron  Free electrons and charged particles are needed between the electrode and work for initiating the arc and their maintenance. Ease of emitting electrons by a material assessed on the basis of two parameters work function and ionization potential.  Themo-ionic emission:  Increase in temperature of metal increases the kinetic energy of free electrons and as it goes beyond certain limit, electrons are ejected from the metal surface.  Field emission:  High potential difference (107 V/cm) between the work piece and electrode is established for the field emission purpose.  Secondary emission:  High velocity electrons moving from cathode to anode in the arc gap collide with other gaseous molecules
  88. Thermal emission Thermally induced flow of electron from a surface because the thermal energy given to the surface overcomes the work function of the material The mechanism of emission of electron due to heating of metal is called thermo- ionic emission Increase in temperature of metal(T) increases the kinetic energy(MV2) of free electrons and as it goes beyond certain limit, electrons are ejected from the metal surface The temperature at which thermo-ionic emission takes place, most of the metals melt. Hence, refractory materials like tungsten and carbon, having high melting point exhibit thermo-ionic electron emission tendency.
  89. Field Electron Emission: In this approach high strength electro-magnetic field that generated during thermal emission process, leads to free electrons pull out of the metal surface. High potential difference(107 V/cm) between the work piece and electrodes established (to overcome the work function) for the field electron emission purpose.
  90. Secondary Electron Emission High velocity electrons moving from cathode to anode in the arc gap collide with other gaseous molecules. This collision results in decomposition of gaseous molecules into atoms and charged particles (electrons and ions).
  91. Theories of cathode and anode mechanism: Zones in Arc Gap • On establishing the welding arc, drop in arc voltage is observed across the arc gap. • The rate of drop in arc voltage varies with distance from the electrode tip to the weld pool • Generally, five different zones are observed in the arc gap: • Cathode spot, • Cathode drop zone, • Plasma, • Anode drop zone and • Anode spot
  92. Cathode spot  This is a region of cathode where from electrons are emitted.  There are three types of cathode spots :, 2. pointed, and 3.normal.  There can be one or more than one cathode spots moving at high speed ranging from 5-10 m/sec.  Mobile cathode spot is usually produced at current density 100-1000 A/mm2. it is generally found during the welding of aluminum and magnesium.
  93. Cathode spot  Mobile cathode spot loosens the oxide layer on reactive metal like aluminum, Mg and stainless steel. Therefore, mobile cathode spot helps in cleaning action when reverse polarity is used i.e. work piece is cathode.  Pointed cathode spot is formed at a point only mostly in case of tungsten inert gas welding at about 100A/mm2. Pointed tungsten electrode forms the pointed cathode-spot.  Coated steel electrode with ball shaped tip of forms normal cathode spot.
  94. Cathode drop region:  This region is very close to the cathode and a very sharp drop of voltage takes place in this zone  Due to cooling effect of cathode voltage drop in this region directly affects the heat generation near the cathode which in turn governs melting rate of the electrode in case of the consumable arc welding process with straight polarity (electrode is cathode).
  95. Plasma  Plasma is the region between electrode and work where mostly flow of charged particles namely free electrons and positive ions takes place.  In this region, uniform voltage drop takes place. Heat generated in this region has minor effect on melting of the work piece and electrode.
  96. Anode drop region:  Like cathode drop region, anode drop region is also very close to the anode and a very sharp drop in voltage takes place in this region due to cooling effect of the anode voltage drop in this region.  Thus voltage drop in this region affects the heat generation near the anode & so melting of anode.  In case of direct current electrode negative (DCEN), voltage drop in this zone affects melting of the work piece.
  97. Anode spot: • Anode spot is the region of a anode where electrons get merged and their impact generates heat for melting. • However, no fixed anode spot is generally noticed on the anode like cathode spot.
  98. Arc characteristics and its relationship with power source: Fig Static arc characteristic
  99. Arc characteristics and its relationship with power source: Fig Constant current power source characteristic
  100. Arc characteristics and its relationship with power source: Fig Constant voltage power source characteristic
  101. Arc characteristics and its relationship with power source:
  102. Arc efficiency:  In the case of arc welding, having a constant voltage E and a constant current I, the arc/ heat source efficiency can be expressed as; 𝜂 = 𝑄 ∗ 𝑡𝑤𝑒𝑙𝑑 𝑄𝑛𝑜𝑚𝑖𝑛𝑎𝑙 ∗ 𝑡𝑤𝑒𝑙𝑑 = 𝑄 ∗ 𝑡𝑤𝑒𝑙𝑑 𝐸𝐼 ∗ 𝑡𝑤𝑒𝑙𝑑 = 𝑄 𝐸𝐼 where, Q = is the rate of heat transfer from heat source to work piece Qnominal = is the nominal rate of heat of the power source tweld = is the welding time
  103. Heat generation:
  104. Heat generation:
  105. Effect of type of shielding gas on arc:  1. Filler Metal Deposition Rate and Efficiency  2. Spatter Control and Post weld Cleaning  3. Bead Profile and Over welding  4. Bead Penetration, Potential for Burn-through  5. Out-of-position Weldability  6. Welding Fume Generation Rates  7. Weld Metal Mechanical Properties
  106. Isotherms of arcs: