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Physics of the Welding Arc in Magnetic Fields

Peck, William Francis
http://rave.ohiolink.edu/etdc/view?acc_num=osu1392733160

Abstract Details

1966, Master of Science, Ohio State University, Welding Engineering.
Still and high-speed photography have been employed to study the response of gas-tungsten arcs and gas-metal arcs to transverse magnetic fields of up to 200 gauss. Carbon steel, stainless steel and aluminum were studied using straight- and reverse-polarity arcs. A 30-minute color movie has been produced which demonstrates the effects of a constant transverse magnetic field on the gas-metal-arc. Photographic techniques were developed for 35-mm. color slides to yield realistic color and reveal metal transfer within the arc column while retaining definition of the plasma components. An 85-B color correction filter and 1.0 neutral density filter were used on an Exakta camera with a 135 mm. telephoto lens. Exposures at 1/1000 sec. on Kodachrome II daylight film were decreased 3 f-stops from those indicated by light-meter measurements. Color movies at 4000 fps using the same f-stop and filter adjustments showed metal transfer without the use of backlighting. Study of the effects of magnetic fields on welding arcs requires an understanding of what each color and component of the plasma re¬presents. This understanding is developed by considering four aspects of the arc plasma: (1) composition, (2) temperature, (3) color and (4) streaming. It is first shown that ionization of metal vapor is re¬sponsible for electrical conduction in metal vapor arcs. Saha's equation is then applied to calculate arc temperatures based on current density and ionization potential of the arc. A relationship is established between plasma temperature and color radiated to relate composition and temperature to the visual appearance of the arc. Finally, plasma streaming due to self-magnetic compression is related to the structure and dynamic behavior of the arc. The ferrous gas-metal-arc has four components. The blue- white core of ionized and excited metal vapor and excited argon atoms has a reported temperature of 6,000° K and high velocity due to the constriction at the electrode. The core originates from the end of the electrode and decreases in current density towards the base plate. A red-orange inner sheath of excited iron-oxide vapors originates from the electrode above the core. A blue outer sheath is composed of lower-temperature excited metal and argon atoms. Blue metal vapor and red iron-oxide vapor appear as tail flames at the base plate due to dispersion of the electrode plasma stream. Fleming's left-hand rule is useful in determining arc deflec¬tion but must be interpreted to consider the high velocity of plasma streams in the conducting core of the arc. Arc deflection is evidence of a skewed velocity distribution of the plasma stream from the elec¬trode. Distortion of a deflected arc occurs as the magnetic pumping action is unbalanced by the applied field producing an arc with high directional stability on one side and a lower velocity streaming on the other. Deflection of the main plasma stream from the electrode away from the base plate is accompanied by development of a plasma stream from the base plate. Streaming from the base plate occurs because the normal suppressing action of the electrode plasma stream is re¬duced and because the active spot on the base plate is constricted which results from a decrease in heat transferred to the active spot from the electrode stream. Plasma streams from the base plate do conduct current as evidenced by their deflection in magnetic fields. Anode and cathode streams are deflected in the same direction. Strong applied fields separate the cores of anode and cathode streams permitting study of streaming, plasma structure and metal transfer. Electrode streams may be deflected so far as to become parallel to each other. Electron conduction then occurs through the region between the visible plasma cores without emission of visible radiation due to low current density. The inertia of these plasma streams causes continuation of their motion even after they stop conduction but visible radiation disappears soon after the loss of Joule heating. Two forms of plasma instabilities exist that often lead to arc extinction but do not always occur as the arc extinguishes. Horizontal rotation of the electrode plasma stream occurs through about 90° and is accompanied by separation of the stream from the base plate. Vertical spinning of the electrode plasma stream may progress through several revolutions causing a pinwheel appearance. Spinning is accompanied by migration of the active spot on the base plate away from the electrode. Spinning and rotation do not usually occur simul¬taneously but either can lead to arc extinction in less than 1/1000 second. When spinning begins, forces are exerted on the electrode tip of sufficient strength to bend the end of a 1/16" diameter electrode into a U shape through a radius of less than 1/4" in a 150-gauss field as shown in high-speed films. Arc extinction in strong applied fields occurs as the result of excessive disturbance of the direction and symmetry of the velocity distribution of the plasma stream from the electrode. Development of a plasma stream from the base plate further deflects the stream from the electrode. Extinction occurs when the plasma streams from the two electrodes are separated so widely that current density be¬tween streams is so low that the plasma cools to the point where it will no longer conduct the arc current.
C. E. Jackson (Advisor)
160 p.
Industrial Engineering

Recommended Citations

Citations

  • Peck, W. F. (1966). Physics of the Welding Arc in Magnetic Fields [Master's thesis, Ohio State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=osu1392733160

    APA Style (7th edition)

  • Peck, William. Physics of the Welding Arc in Magnetic Fields. 1966. Ohio State University, Master's thesis. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=osu1392733160.

    MLA Style (8th edition)

  • Peck, William. "Physics of the Welding Arc in Magnetic Fields." Master's thesis, Ohio State University, 1966. http://rave.ohiolink.edu/etdc/view?acc_num=osu1392733160

    Chicago Manual of Style (17th edition)

osu1392733160
183
© 1966, all rights reserved.
This open access ETD is published by The Ohio State University and OhioLINK.