Welding arc

Introduction

In the summer, we often see lightning in the sky, which is a gas discharge phenomenon. The gaseous medium between the two electrodes, a strong and lasting arc discharge phenomenon is called. High temperature (up to a temperature of 6000 deg.] C) and the light arc discharge. Human understanding of this phenomenon, and is used in industrial production. It may be used to heat the arc cutting arc, carbon arc gouging and an arc steelmaking; arc energy illumination light (e.g., headlight) Screening of the film using an arc lamp or the like.

arc welding produces

an excited gas atoms, ionization and electron-emitting

neutral gas originally electrically non-conductive , in order to generate an arc current through the gas, it is necessary that the gas molecules (or atoms) become ionized electrons and positive ions. Further, in order to maintain the arc burning arc required cathode electron emission continuously, which must be continuously supplying electrical energy to the arc, to supplement the energy consumed. Gas ionization and electron emission arc is the most basic physical phenomena.

1. Excitation and ionization of gas atoms

obtained if the additional energy of gas atoms and electrons may jump from a lower energy level to a higher energy level to another, this when the atom is in the "excited" state. Atomic energy required for the jump "excited" energy state is called the excitation. Ionised gas atoms is bound electrons completely out of the nucleus, the formation of ions and free electrons. The energy required by the positive ions are formed is called an ionization energy atoms.

in the welding arc, the ionization caused by the energy source, has the following three forms:

(1) impact ionization. Refers to the electric field in the ionization occurs after accelerated charged particles (electrons, ions) and neutral (atoms) collision.

(2) thermal ionization. Means at high temperatures, having a high kinetic energy of the gas atoms (or molecules) collide with each other due to ionization.

(3) photoionization. Refers to gas atoms (or molecules) absorb ionization energy ray photons produced.

while generating gas atoms ionized particles with opposite charges will collide, positive ions and electrons to point in nature, i.e., generation and phenomena. When the composite velocity and the velocity is equal to the ionization, ionization will tend to be relatively stable balancing state. Generally, the greater the number of charged particles of the arc space, the more stable arc, while the charged particles and phenomena will reduce the number of charged particles, thereby reducing the stability of the arc.

2. Electron-emitting

on the cathode surface atoms or molecules, to accept external energy to release free electrons is called an electron emission phenomenon. Electron emission is to maintain a stable arc and arc burning a very important factor. According to their different sources of energy, it can be divided into thermal emission, photoemission, heavy particles emitted from the emitter colliding under strong electric field and the like.

(1) heat emission. After a solid or liquid surface of the object is heated, some of the kinetic energy of the electron has a work function of greater than escape into the space in the outer surface of the thermal emission phenomenon is referred to. Thermal emission plays an important role in the welding arc, it is enhanced as the temperature increases.

(2) photoemission. Solid or liquid material surface receiving light rays to release the energy of free electrons in a phenomenon known as photoemission. For various metal oxides and, only when the light ray wavelength less than the wavelength limit can emit electrons thereof, to produce photoemission.

(3) heavy particle impact emission. Large energy heavy particles (e.g., positive) hit the cathode, causing the escape of electrons, known as the heavy particle impact emission. Heavy particles the greater the energy, the more intense electron emission.

(4) emitted from the strong electric field effect. When the surface of a solid or liquid substance, although the temperature is not high, but the presence of a strong electric field and a large potential difference is formed near the surface of the cathode to have more electrons are emitted, which is called a self-emission under strong electric field , referred to as self-emissive. The stronger the electric field, the current density of emitted electrons is formed is greater. Since emission plays an important role in the welding arc, especially in a non-contact arc, which is more obvious.

In summary, the arc welding is a form of gas discharge, and maintaining the welding arc is formed in the electric field, heat, light, particle kinetic energy effect, the gas atoms are constantly excited, ionized and the results of electron emission. At the same time, there is produced a composite negative ions, positive ions and electrons. Obviously, the pilot arc welding power source mainly by heat, light and electric field and the kinetic energy generated therefrom, and this field is the no-load voltage supplied by the power generated by the arc.

ignition of the welding arc

weldment between the electrode and the voltage is, when they are in contact with each other, corresponding to the arc welding power source shorted . Since the point of contact resistance is large, a large short-circuit current, a large amount of Joule heat is generated, so that the molten metal, even, and vaporized, causing a strong electron emission and ionization of the gas. In this case, between the pull wire and the weldment and then a little distance, so that, due to the supply voltage, within this distance, form a strong electric field, but also to promote electron emission. Meanwhile, the ionized gas is accelerated, charged particles in the electric field, the directional movement of the poles. Welding power supply constant electric power, the new added continuously charged particles, to form a continuous arc burning.

physical basis of the welding arc

(a) and the electric field intensity distribution of arc

is the arc of a gas discharge phenomenon, it is a charged particle through both electrodes a conductive space between the process gas.

arc has three components: a cathode region, the anode region, the arc column region.

(b) generating an arc of charged particles

1, ionized gas

in the action of external energy, so that the neutral gas molecules or atoms separation into electrons and positive ions in a process known as gas ionization.

which is essentially neutral gas particles absorb sufficient energy, electrons from the shackles of the nucleus become a process of free electrons and positive ions.

ionizable species:

(1) away from the thermoelectric generating ionized

heated gas particles is known as thermal ionization effect. Its nature is intense thermal motion of the particles, ionization collision generated.

(2) field ionization

to accelerate charged particles in an electric field, and wherein the neutral particles generated inelastic collision ionization.

The degree of ionization:

ionization degree: the ratio of the unit volume of the total particle number of particles before bath ionized gas ionization degree of ionization is called. Ionization

(3) photoionization

neutral gas particles by the action of light radiation generated called photoionization.

2, the electron emission cathode

(1) electron emission: the free electrons in the cathode by the applied energy from a process known as electron-emitting cathode surface escaping. The size of its transmission capability is represented by the work function Aw.

(2) the cathode spot

bright surface of the cathode region is called the cathode spot.

having a cathode spot "cathode cleaning" ( "cathode broken") effect reason: since the work function lower than that of pure metal oxide, as an oxide cathode spot will move to the place, the oxide removal.

(3) type electron-emitting

1) heat emission

cathode surface portion of thermally induced electron kinetic energy electrons generated reaches or exceeds the work function of the emitter.

to heat thermal emission cathode as the main form of emission.

2) field-emission

cathode surface is subjected to electric power, when the electric field strength reaches a certain level the electron emitting surface of the electron emission cathode.

In the cold cathode field emission mainly in the form of emission.

Welding arc

3) light-emitting

as a cathode surface by the free electrons of the optical radiation energy to escape to a certain extent the metal surface emission.

4) emitting particle collision

to obtain the surface energy of the free electrons in the arc motion high speed collision of positive ions escape cathode cathode surface phenomenon.

(c) the disappearance of charged particles

1, the diffusion

migration of charged particles from a high density to a low density center portion surrounding the phenomenon.

2, the composite

positive and negative particles combine into the peripheral arc phenomenon neutral particles.

3, the formation of negative ions

neutral particles adsorbed portion of electrons to form negative ions procedure.

(d) harm the welding arc

arc welding is mainly ultraviolet, visible light and ultraviolet light composition, can cause damage to the skin, resulting in peeling, burning phenomenon, it is necessary to strengthen the radiation protection arc welding, the exposed portion and avoid skin contact with the welding arc.

conductive property

conductive properties (a) of the arc column region

arc column comprising a large number of electrons, positively charged ions and neutral particles the gaseous state particles aggregated together, outside this electrically neutral state is referred to the arc plasma.

minimum voltage Principle: arc column in stable combustion when there is a make their own minimal energy consumption characteristics, i.e. when constant current and arc ambient conditions, stable burning arc will automatically select a certain conductive cross-section, so that the arc energy consumption is minimized. When the arc length is also a constant value, i.e., the size of the electric field strength represents the size of the arc heat production, and therefore, the lowest intensity of the electric field energy consumption is at a minimum, i.e. the arc length of the fixed minimum voltage drop, which is the principle of the minimum voltage.

conductive properties (b) the cathode region

acts: receiving transmitted by the positive ion current arc column; to provide the desired arc conduction electron streams to the arc column region.

which emits mainly in the form:

1, the heat-emitting

2, field-emission

1, the anode spot

on the anode surface can be seen bright light emission region, referred to as the anode spot.

anode spots will automatically find the relatively low melting point to avoid a pure metal oxide surface, metal surface walk.

2, the conductive form of the anode region

(1) Field Ionization

(2) Thermal Ionization

welding arc process characteristics

(a) thermal characteristics of arc

1, the heat generating arc column

low current density, high temperature, energy mainly by particle collision, heat losses.

2, the cathode region of the heat generating

high current density, low temperature, energy is used to heat and heat radiation loss to the cathode of the cathode region, may also be used to heat or weld filler pieces.

3, the heat-anode region

high current density, low temperature, energy is mainly used for heating and loss of the anode can also be used to heat the filler material or weldment.

(ii) the mechanical properties of the arc

arc force and penetration affect droplet transfer, the bath was stirred, and weld splatter, so the arc force weldment a direct impact on the quality of the weld.

1, and the arc force acting

(1) electromagnetic pinch force

Cause: the attraction generated between arc current line.

due to the different diameters across the electrodes, the arc inverted tapered shape. Axial thrust arc the arc cross-section unevenly distributed, the axis of the arc column at the maximum, gradually decreases outward, the force is reflected in the pressure welding member is formed of the bath, called electromagnetic static pressure.

effects: concave molten bath; stirring of the molten bath to produce grain refinement; promoting removal of impurity gases and slag; promoting droplet transfer; constraints extension arc, the arc straight, energy concentration.

(2) the plasma jet force

axial static pressure solenoid driven high-temperature air near the electrodes (plasma stream) Length toward weldment, weld pool formation of an additional pressure, the pressure of the plasma stream is called a force (electromagnetic dynamic pressure).

effects: plasma stream may increase the force of the straight arc; promoting droplet transfer; increase penetration and weld pool formed agitation.

(3) spot force

when the spot is formed on the electrode, since the pressure at the spot by the reaction of the charged particles strike or metal evaporated while speckles, called spots or pressure dot force.

direction of the spot and the force is always opposite to the direction of droplet transfer, thus always hinder droplet transfer, spatter.

In general, the cathode spot force than the anode spot force.

main factors

2, arc force

(1) welding current and arc voltage

(2) wire diameter

( 3) the polarity of the electrodes

(4) a gaseous medium