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METALS WELDING

Method of permanent joining of two metal bodies by establishing bonds between atoms of the two bodies under certain conditions of temperature and pressure.

Welding process = practically execution of a welds taking into account the thickness of the part, its position and the nature of the metal to weld.

Welding procedures used in modern technology allowed to produce different sized products - from the giant - to the miniature

Advantage

material economy (compared to riveting - overlapping tables); joint tightness is achieved; without holes for rivets increases the resistance assembly; may be obtained more flexible construction (to castings); less labor consuming; can get mixed construction consisting of several parts each of other material, obtained by different technological process; accessible and simple devices (to weld usual materials).

Disadvantages:

can not run large manufacturing series; often required heat treatment after welding; welding technological processes of high productivity requires expensive equipment.

The physical principle of welding

To create a connection between the objects it is necessary that the atoms arranged on the surface of one of the bodies to react with the atoms of the other body.

This condition can be achieved by two basic solutions: warming the surfaces to be joint in adjacent position through warming it will increases the plasticity of the metal and the amplitude of thermal oscillations of atoms (increases the number of holidays); Exerting a pressure between them.

By heating - increases the free energy of the atoms; - the inter-atomic bonds will be weaker. Carry out a molten metal bath its solidification will be the seam.

Through pressure - between the parts to be joined plastic deformation will cause the material flow along the surfaces in contact. If the pressure is high enough, it alone can achieve cold welding - producing structures with large grains worse mechanical properties.

Mechanism of binding forces between the parts to weld depends on the aggregate state: both liquids - the joint begin into the molten bath with the interaction of the molten materials and continuous with crystallization process;

both solid - the clamping forces of the different parts are obtained by putting the parts in adjacent position - the atoms near the surfaces in contact (distance between atoms of the two parts must be within the crystal lattice parameters) - oxide layer block the molecular cohesion.

The structure of welded joints

Welding = is the result of the welding operation;

Seam or weld joint = the area in which the inter-atomic cohesive forces act effectively. Seams obtained by melting have its own structure and chemical composition = dendritic structure.

Filler = metal or alloy - wires, rods or pellets, which participate at the formation of seam; its chemical composition has to be close to the base material.

Welding with filler material into the seam will be included base material too. The resulting bath shall enter into chemical reactions with elements in the environment: - O2 - gives rise to oxides; - H2 - encourages cracks; - N2 - form hard nitrides - reduce the plasticity of the seam);- with various alloying elements (Si, Mn, W, Cr).

The structure of welded joints MB1,2 - base material 1, 2 = elements of metals or alloy that are assembled;

CS - seam - dendrite structure which is typical for cast metal; ZIT1,2 - the heat-affected zone narrow zone between the choke andthe base metal = mixture of molten metal and the base metal superheat formed by diffusion of the constituents.Its depth depends on the thermal regime used. Depending onthe cooling rate there are obtained quenching structures, whichincrease the hardness of the steel.As the difference between the chemical composition of the weld metal and the base one is higher, as this area is more visible.

Depending on the type of energy used to heat the material: Welding heat source chemical - is achieved when the thermal energy is provided by chemical reactions:- Gas welding = Oxyacetylene Flames.

Electric welding - when heat energy is obtained by converting electricity through :- resistance - Spot and Seam welding;- electric arc - Metal-arc;- Submerged-arc;- Plasma-arc.

WELDING PROCESEES

1. Oxyacetylene Flames Used in Welding Is used to: welding carbon steel pieces with thickness less than 4mm; welding nonferrous alloys - bronze, brass; repair of cast iron components.

Source heat = controlled chemical reaction between a fuel gas (usually acetylene) mixed with oxygen.

As filler wire, chemically similar to the base material.

Three basic types of oxyacetylene flames used in oxy fuel - gas welding and cutting operations: (a) neutral flame; (b) oxidizing flame; (c) carburizing, or reducing, flame. The gas mixture in (a) is basically equal volumes of oxygen and acetylene.

General view;Cross-section of a torch used in oxyacetylene welding. The acetylene valve is opened first; the gas is lit with a spark lighter or a pilot light; then the oxygen valve is opened and the flame adjusted.Basic equipment used in oxyfuel-gas welding. To ensure correct connections, all threads on acetylene fittings are left-handed, whereas those for oxygen are right-handed. Oxygen regulators are usually painted green, acetylene regulators red.

Flame cutting of steel plate with an oxyacetylene torch, and a cross-section of the torch nozzle. Cross-section of a flame-cut plate showing drag lines.

2. Resistance spot weldingIt is used for thin sheets - up to 10mm - are used metal electrodes mounted in the grips of welding machine. High productivity - is achieved around 2000 points / minute.Used for welding flats or molded profile and the protective tapes of mechanisms. execution - with welding machines or devices - pliers, torches - portable - allowing welding in difficult positions: directly assembled parts; chassis vehicles.

By appropriate selection of the electrodes shape and of the arms that support them, they can achieve the most varied assembly work.

(a) Sequence in resistance spot welding. (b) Cross-section of a spot weld, showing the weld nugget and the indentation of the electrode on the sheet surfaces. This is one of the most commonly used process in sheetmetal fabrication and in automotive-body assembly.

(a) and (b) Spotwelded cookware and muffler. (c) An automated spotwelding machine with a programmable robot; the welding tip can move in three principal directions. Sheets as large as 2.2 m X 0.55 m can beaccommodated in this machine. Source: Courtesy of Taylor-Winfield Corporation.(a)(b)(c)Spot Welding Example

Robots equipped with spot-welding guns and operated by computer controls, in a mass-production line for automotive bodies. Source: Courtesy of Cincinnati Milacron, Inc.

Seamwelding process in which rotating rolls act as electrodes. Overlapping spots in a seam weld. Roll spot welds. Resistance-welded gasoline tank.3. Resistance Seam Welding

Applied in assembling fuel tanks, exhausts and bodywork, radiators from tin, vat washing machines, various types of carcasses.

Advantages of welding in line: higher mechanical strength of the joint ; welding line ensures tightness.

4. Metal-arc welding Arc = a stable electrical discharge between two electrodes at high current densities in gas. The points of contact between the electrode and part = areas to be heated to melting, due to high short-circuit current. Under the influence of a compression force, the number of contact points will increase continuously and crossing area electrode part a liquid metal bridge. When the temperature reach the metal boiling temperature, it occurs the breakage of metallic bridge and metal deck which is easily ionizable it will ensure the flow of current in the form of flashing. Arc welding - the arc is maintained between the electrode and part the electrode = the filler material

Schematic illustration of the shielded metal-arc welding process. About 50% of alllarge-scale industrial welding operations use this process.Schematic illustration of the shielded metal-arc welding operations (also known as stickwelding, because the electrode is in the shape of a stick).

Schematic illustration of the submerged-arc welding process andequipment. The unfused flux is recovered and reused. 5. Submerged-arc welding

welding quality is far superior to manual welding; electricity consumption is reduced; superior productivity.

Carry straight postweld, of relatively large length, or circular ones, horizontal, slightly inclined. MIG/MAG welding is a welding process by arc melting with fusible electrode. Protection of arc and metal bath uses a shielding gas. Depending on the nature of the gas used there are two different processes: MAG welding (metal - active - gas ) for an active gas; MIG (metal inert gas) - for an inert gas.MIG-MAG has a high degree of versatility being able to weld a wide range of materials: unalloyed steel with less carbon; low alloy steels or stainless steels; non ferrous metals and alloys - Cu, Al, Ni, Ti etc.

6. Plasma-arc welding

- plasma = mixture of electrons, ions and neutral particles - in constant motion - similar to the gaseous state of aggregation - open arc temperature may reach 6000 - 8000 0 CTwo types of plasma-arc welding processes: transferred, (b) non-transferred. Deep and narrow welds can be made by this process at high welding speeds.

Comparison of the size of weld beads in (a) electron-beam or laser-beam welding to that in (b) conventional (tungsten-arc) welding. Source: American Welding Society, Welding Handbook (8th ed.)

Metal bonding is achieved by mutual diffusion between the base metal and alloy for soldering.

The metal parts - in solid state.

Filler material - liquid (metal or non-metallic material - material 's composition differs from that sticks).

The heating of parts - to the melting temperature of the solder - which is lower than their melting temperature.

No thermal stresses occur in parts.

No deformations occur due to heating and cooling of parts .METAL SOLDERING

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