Английский для сварщиков. Гричин С.В. - 46 стр.

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46
In fusion welding a heat source generates sufficient heat to create and
maintain a molten pool of metal of the required size. The heat may be
supplied by electricity or by a gas flame. Electric resistance welding can be
considered fusion welding because some molten metal is formed.
Solid-phase processes produce welds without melting the base
material and without the addition of a filler metal. Pressure is always
employed, and generally some heat is provided. Frictional heat is developed
in ultrasonic and friction joining, and furnace heating is usually employed
in diffusion bonding.
The electric arc used in welding is a high-current, low-voltage
discharge generally in the range 10–2,000 amperes at 10–50 volts. An arc
column is complex but, broadly speaking, consists of a cathode that emits
electrons, a gas plasma for current conduction, and an anode region that
becomes comparatively hotter than the cathode due to electron bombardment.
Therefore, the electrode, if consumable, is made positive and, if
nonconsumable, is made negative. A direct current (dc) arc is usually used,
but alternating current (ac) arcs can be employed.
Total energy input in all welding processes exceeds that which is
required to produce a joint, because not all the heat generated can be
effectively utilized. Efficiencies vary from 60 to 90 percent, depending on the
process; some special processes deviate widely from this figure. Heat is lost
by conduction through the base metal and by radiation to the surroundings.
Most metals, when heated, react with the atmosphere or other nearby
metals. These reactions can be extremely detrimental to the properties of a
welded joint. Most metals, for example, rapidly oxidize when molten. A
layer of oxide can prevent proper bonding of the metal. Molten-metal
droplets coated with oxide become entrapped in the weld and make the joint
brittle. Some valuable materials added for specific properties react so quickly
on exposure to the air that the metal deposited does not have the same
composition as it had initially. These problems have led to the use of fluxes
and inert atmospheres.
In fusion welding the flux has a protective role in facilitating a
controlled reaction of the metal and then preventing oxidation by forming a
blanket over the molten material. Fluxes can be active and help in the process
or inactive and simply protect the surfaces during joining.
Inert atmospheres play a protective role similar to that of fluxes. In
gas-shielded metal-arc and gas-shielded tungsten-arc welding an inert gas —
usually argon—flows from an annulus surrounding the torch in a continuous
stream, displacing the air from around the arc. The gas does not chemically
react with the metal but simply protects it from contact with the oxygen in the
air.

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