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METALS WELDING



Method of permanent joining of two metal bodies by establishingbonds 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 themetal 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 othermaterial, 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 the 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:1. 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);

2. 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 where they are:

- both liquids: the joint begin into the molten bath with the interaction

of the molten material and continuous with crystallization process;

- both solid - the clamping forces of the different parts are obtained byputting 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.

Filler = metal or alloy - wires, rods or pellets, which participate at

the formation of seam; its chemical composition has to be close tothe 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 areassembled;

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 metalsuperheat – 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 weldmetal 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).

Electric welding - when heat energy is obtained by converting

electricity through resistance, electric arc, induction etc.

WELDING PROCESEES:

1. Oxyacetylene Flames Used in Welding

2. Metal-arc welding3. Submerged-arc welding

4. Resistance spot and seam welding



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 andacetylene.

1. Oxyacetylene Flames Used in Welding



(a) General view and (b)

cross-section of a torch

used in oxyacetylene

welding. The acetylenevalve 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.(c) Basic equipment used

in oxyfuel-gas welding. To

ensure correct

connections, all threads on

acetylene fittings are left-

handed, whereas those foroxygen are right-handed.

Oxygen

regulators are usually

painted green, acetylene

regulators red.



(a) Flame cutting of steel plate with an oxyacetylene torch, and a cross-

section of the torch nozzle.

(b) Cross-section of a flame-cut plate showing drag lines.



Schematic illustration of the

shielded metal-arc welding

process. About 50% of all

large-scale industrial welding

operations use this process.

Schematic illustration of the

shielded metal-arc welding

operations (also known as stick

welding, because the electrode is

in the shape of a stick).

2. Metal-arc welding



Schematic illustration of the submerged-arc welding process and

equipment. The unfused flux is recovered and reused.

3. Submerged-arc welding



4. 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 C

Two types of plasma-arc welding processes:

(a) 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.)



Laser welding of

razor blades.



(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 insheetmetal fabrication and in automotive-body assembly.

5. Resistance spot welding



(a) Seamwelding process in which rotating rolls act as electrodes.

(b) Overlapping spots in a seam weld.

(c) Roll spot welds.

(d) Resistance-welded gasoline tank.

Resistance

Seam

Welding



(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 be

accommodated 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.

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