Arc welding Arc theory. CTU in Prague Faculty of Mechanical Engineering

Transcription

1 1st semester 2015/2016 Arc welding Arc theory CTU in Prague Faculty of Mechanical Engineering Ing. Petr Vondrouš, PhD., IWE

2 Electric arc An arc is an electric current flowing between two electrodes through an ionized column of gas. A negatively charged cathode and a positively charged anode create the intense heat of the welding arc. Negative and positive ions are bounced off of each other in the plasma column at an accelerated rate. Needs energy to ionize and to sustain plasma.

3 Characteristics Electric arc is connecting places with the lowest voltage plasma high speed of movement (100 m/s) Energy is released at the arc edges and is practically used. High temperature Intesive glow Diameter 1-10 mm At current over 0,3 A At Higher pressures Current A, voltage V. Electrones emitted from cathode passes through arc collumn and are removed by anode.

4 Power, shape of electric arc can be varied setting different welding paremeters. By this wide variety of welding methods can exist. SMAW GMAW GTAW PAW

5 Theory of electric arc Description is done on GTAW arc, because the arc is stricken btween tungsten non melting electrode and BM without changes in length, width, and is stable, with no change in U, I. Parts of arc: Cathode area Arc collumn Anode area

6 Electric arc On the heated cathode, where the emission energy is the lowest, the electrones are emitted. Emission of electrones causes decrease of potential, voltage (10-16 V). Thermically emitted electrones in electrical field increase speed, kinetic energy high enough to ionize gas atoms. Electrically conductive high temperature collumn (PLASMA), where U, I are stable. Thermodynamic processes of Disociation, ionization, excitation and recombination takes place. Anode is current in device, accepts electrones. Energy is transformed into heat and el.-mag. radiation. Recombination of electrones and iones, energy release higher temperature than at the cathode. Decrease of voltage 4-8 V. ČVUT, Fakulta strojní, Ústav strojírenské technologie, skupina svařování

7 Arc properties Arc is low temperature plasma, its properties depend on Electrode material Welded material Gas Geometry Parameters I, U

8 Disociation, Ionization At high temperatures molecules disintegrate to separate atoms energy needed is called DISSOCIATION energy. E.g. 0 2, H 2, N 2, CO 2 H H + 4,48 ev H + H Atoms recieving enough energy (ionization energy) release electron, so that free electron and positive ion are created. E.g. H + energy H + + e - easy to ionize elements easy to start the arc stability Lot of energy to ionize elements release more energy at the anode hotter arc.

9 Disociation, ionization energies Low ionization energy is good for arc stability Cs, K, Na additions

10 ČVUT, Fakulta strojní, Ústav strojírenské technologie, skupina svařování

11 Excitation, Recombination In the plasma, energy transfer is done by excitation and relaxation, i.e. emission. Due to much energy levels, wide spectra of energy is emitted UV, visible, IR radiation. At the outer edges of the arc recombination takes place energy example H + + H + H 2 + 4,48eV

12 Plazma Mixture of neutral and charged particles. Overall electrically negative. Plazma is: - Conductive - High thermal, electrical conductivity - Influenced by electric and magnetic field Gas x Plasma main difference is electrical conductivity

13 Arc Direct polarity, DC -, DCEN In direct polarity, DC+, DCEP

14 Thermal bilancy Arc has temperature between C. Created heat Q is heating up cathode, anode and environment. Q = Q a + Q c + Q e Depending on type of electrode the heating of electrode can be positive (SMAW, GMAW) o negative (GTAW). For every method there is different efficiency: k = η = (Q a + Q c ) / Q Efficiency factor: norm EN Heat input into WM: q = k (U.I) / v [J/mm] Method SAW SMAW, GMAW GTA, PAW Efficiency k 1,0 0,8 0,6

15 Material transfer Very important is molten metal transfer into the weld puddle. It depeds on: - method - material - electrode size - Current - voltage - Gas Heat of arc melts end of electrode, so that drop of molten metal is created. Forces present: Gravitation Surface tension molten metal Electromagnetic forces Lorenz forces PINCH EFEFCT radial el.-mag. force

16 Forces 1- electrode, 2 molten area, 3 molten filler wire, 4 cathode area, 5 arc column, 6 anode area, 7- radial pressure, a gravity, b radial el.-mag. force pressure (pinch efekt), c surface tension, d electro-static forces, e evaporation, f surface tension, g plasma, gas flow

17 Acting forces change material transfer: Short circuiting vs Spray transfer Surface tension (Fs) Evaporation force (Fp) Gravitation force (Fg) el-mag. force (Fm) hydrodynamic force (Fh) Lower I, U Lower I, U

18 Starting the arc Environment is the highest at the beginning, more energy must be supplied. Arc starting voltage U = V Electrode strike (SMAW, SAW, MIG/MAG) High frequency ionization (GTAW) Starting current (GTAW) Standard arc voltage V, current A.

19 Electric arc parameters Voltage influences arc length Current power, arc diameter Material, gas influence After starting the arc, temperature increases, so voltage can decrease. Arc resistence in non linear, it is influence by 1) Catode - non linear resistor 2) anode non linear resistor 3) arc column linear resistor, U=R.Ilinear U = U c + R.I + U a

20 Static arc characteristic Arc static characteristic expresses influence of voltage on current for constant arc length, for specific method and environment... It is often simplified to line practically used range of current. U 2 = ,04 I 2 Valid for SMAW upto 600 A

21 NORMALIZES arc characteristics GTAW (WIG, TIG) : U 2 = ,04 I 2 (V) upto 600 A, over 600 A U 2 = const. = 34 V GMAW (MIG, MAG) : U 2 = ,05 I 2 (V) upto 600 A, over 600 A U 2 = const. = 44 V SAW: U 2 = ,04 I 2 (V) U 2 = ,05 I 2 (V) upto 600 A, over 600 A U 2 = const. = 44 V

22 Welding source characteristic Arc characteristics Source setting changes the curve slope 1-4