The welding has the aim of ensuring the continuity of the matter to be assembled. In the case of metals, this continuity is carried out on the scale of the atomic building. Apart from the ideal case where the forces inter atomic and the diffusion slowly ensure the welding of the metal parts entirely put in following contact of surfaces perfectly compatible and free from any pollution, it is necessary to utilize an energy of activation to quickly carry out required continuity.

The operation can be applied to metals like with the Thermoplastique S (see plastic Soudage). Welding makes it possible to obtain a metal continuity of the assembly (continuity of the nature of assembled materials: metallic material, plastic, etc.). So mechanical techniques of assembly (Riveting, bolting, fastening…) or by adhesion (joining) do not answer the definition of welding.

Processes of welding of metal parts

Brazing with the blow lamp

Used with the Lead: This metal having a rather low melting point, the successive metal contributions are each time cooled by passage of a wet rag. This old method requires much tact, least overheating causing the collapse of the object to be welded.

Oxyacetylene welding or welding

Thermal energy is generated by the combustion of the mixture Oxygène - Acétylène. The filler is in general brought in the form of rod. The blowtorch presented on the photograph runs with the gasoline and not on acetylene. I have several of them.

Thermic welding

Used for the repair of massive parts such as the rails of railroad, is a method of welding chemical: the joint to be realized is imprisoned in a form, which one fills of a pulverulent mixture containing aluminum and iron oxide. The parts to be welded are heated with the red and the mixture is then ignited: the reduction of iron oxide by aluminum causes fusion and produced alumina is expelled upwards by decantation.

See also: Aluminothermy

Electric welding by resistance

(or by points, or with the serrated roller, or PSE, or PSR)

Welding is carried out by the combination of a strong amperage and a specific pressure. This process does not require an external contribution. The amperage heats the matter until fusion. The pressure maintains the contact between the electrode and the assembly. To weld, a grip plates the assembly with ends, or copper electrodes, conducting matter good of electricity and heat, which makes it possible less to heat the zone of contact with this grip and to avoid of it the fusion, which is limited to the zone of contact between the two sheets to weld. This technique is thus dependant on the resistivity (electrical resistance) of the matters, the total thickness of the assembly and the diameter of the electrodes. This process is mainly used in the sheet steel assembly low thickness (<6mm). This technique profits from a very important know-how and an incomparable productivity (in the applicability). For example, an automobile case is assembled with more than 80% by welded points.

Arc welding with coated electrodes

(MY: Manual Metal Arc , or SMAW: Shielded Metal Arc Welding )

The temperature of welding is generated by the electric arc between two electrodes which constitute the part to weld and the filler rod where the molten metal is protected by a slag.

See also: Arc welding

Welding the arc with nonfusible electrodes

(Welding TIG: Tungsten Inert Gas , GTAW Gas Tungten Arc Welding according to the American standards or proceeded 141 according to the ISO 4063)

An electric arc is established between the end of an infusible tungsten electrode and the part to be welded, under the protection of an inert gas (Argon, Hélium or mixes argon-helium…). The filler is added if necessary in the form of a rod or of a strip iron placed in the electric arc. This process can be even automated to be robotized in the frequent case of the orbital Soudage TIG.

Welding the arc with wire consumable electrodes or semi-automatic welding

(Welding MIG-MAG: Metal Inert Gas-Metal Activates Gas, GMAW Gas Metal Arc Welding according to the American standards or proceeded 131 (MIG) or 135 (MAG) according to the ISO 4063)

An electric arc is established between the end of a consumable electrode and the part to be assembled, under the protection of a gas mixture whose nature depends on the type of welding carried out. The electrode, supply automatically continuously since a reel, is appeared as a massive or filled wire.

Laser welding

Energy is brought in the form of a beam Laser. The laser sources can be of type CO or YAG or LED.

Welding plasma

(PAW: Plasma Arc Welding )

Regarded as an evolution of welding TIG, it is characterized some by the fact that the arc is mechanically (mechanical constriction) or pneumatically constrained (pneumatic constriction), thus generating a density of higher energy. The arc can spout out between the conduit and the electrode (untransfered arc) or between the part and the electrode (transferred arc) to even be semi-transferred.

Welding by electron beam

(EBW: Electron Beam Welding )

Use the kinetic energy of the electron S projected in an enclosure vacuum and focused on the part to weld to create a molten zone.

See also: Welding by electron beam

Welding friction

This type of welding is obtained by the heating of two parts in a hurry and moving one compared to the other. The relative movement involves a heating of the interface until local plasticization of material, then bonding diffusion atomic.

One distinguishes two families from welding friction:

  • the linear welding (LFW, Linear Welding Friction), obtained by a movement to go/linear return.

  • the orbital welding, obtained by relative rotation of the two parts.

This last type is composed of two families:

  • welding with controlled friction, for which the couple of the engine of drive is transmitted directly to the part in rotation.

  • welding with inertial friction, which uses a wheel of inertia to provide the couple of friction.

Welding friction malaxation or thixotropic welding

(FSW, Friction Stir Welding)

This type of welding is primarily used today for aluminum alloys because he asks very great efforts to be implemented. In the same way the tools used for the welding of nuances other than aluminum (steels) must be very hard and very resistant.

Welding the arc under flow

Welding by composition of processes

Known as hybrid, for example Laser plus TIG.

Electrogaz welding

Approaches the foundry.

Bonding diffusion

Consist in making use of the phenomenon of diffusion of the atoms to create a connection.

Explosive welding

This technique discovered fortuitously in 1957 during tests of explosive forming, is primarily employed to assemble different metals of nature, for example of aluminum on steel. Generally they are pre-welded sections which allow these assemblies (ex: aluminum superstructures on a boat with steel hull with an aim of lowering the center of gravity) Metals to be assembled are superimposed according to a certain angle and covered with a uniform layer of explosive, the fast Combustion (Détonation) of this one causes a fusion in corner which is propagated on all surface mixing intimate 2 metals. This type of welding is not thereafter prone to the galvanic corrosion.

Brazing with tin

Brazing with the tin is carried out at low temperature (200 °C - 250 °C), an alloy of tin and of Plomb molten and is used to join metal surfaces, in particular in the field of the electronic and the plumbing. It is of brazing and not about welding, because only the filler is amalgamated.

See also: Brazing

Problems of welding

Welding introduces two big families of problems:

  • metallurgical

  • mechanical

Metallurgy of welding

The operation of welding induced from its energy contribution and sometimes by the metal contribution of the metallurgical modifications on the level of the welded joint. These modifications will affect the microstructures of the molten zone and the thermically affected zones.

In fact, the welded joint is subjected to various problems resulting from these modifications:

  • hydrogen embrittlement: phenomenon of cold cracking

  • appearance of hot cracking (liquation) in the course of solidification: shrinkage pipes, intergranular cracking
  • held with corrosion different from the base metal: primarily due to the phenomena of segregation

These problems as well relate to the molten zone (which passed in the liquid state during the operation of welding) that the thermically affected zone. Zone affected thermically called ZAT is the metallurgical seat of modification of the base metal which can induce brittlenesses, falls of mechanical resistance, lacks of ductility… These modifications depend on welded material, the process used, the followed procedure…

Examples:

  • In steels C-mn slightly allied the ZAT is the seat of an increase in the mechanical properties (Re, Rm) and of fall of ductility

  • In thermomechanical steels with very high yield stress Re > 690 MPa, one can find in certain parts of the ZAT a phenomenon of softening which erases the effects of the thermomechanical rolling and which decreases the yield stress and the limit with the rupture.
  • an alloy of aluminum of series 5000 welded end to end always presents a fall of mechanical properties in ZAT.
  • an austenitic stainless steel of the type 304 L welded often presents a reduction in its behavior to corrosion on the level of the welding.
  • the Titanium alloys are very sensitive to the phenomena of oxidation during the operation of welding, which can make fall in a drastic way the mechanical properties of the welded joint.
  • …

Mechanical resistance of a welded joint

The metallurgical modifications impact the mechanical resistance of the welded joint. Also is necessary it to be ensured to obtain a sufficient mechanical behavior and to take account of the weldings in the calculation and the dimensioning of the parts. The operation of welding generates moreover creation of residual stresses due to the withdrawal created by the operation of welding on the parts. The behavior with the tiredness of the welded joints is fundamental problems in the design of the welded apparatuses. The geometrical defects of the weld beads play a great part in the behavior with the tiredness of the welded joints.

Defects of welding

Brittleness induced by the segregation

Welding consisting in locally heating metal, it acts of a thermal Traitement local. There are thus a local modification of the microstructure and metallurgical state of the zone of metal affected by heating (ZAT: thermically affected zone). Indeed, the cycle of temperature inherent in welding destroys age hardening and lowers the mechanical resistance to the circumference of the joint of welding.

The heating activates a certain number of mechanisms, whose in particular diffusion of the Atome S. It thus occurs a phenomenon called “segregation”: metal not being pure, the foreign atoms (impurities, elements of Alloy) migrate towards the grain boundary .

This can involve an embrittlement of the grain boundaries, and thus facilitate the intergranular Rupture fragile.

To avoid this problem, one carries out sometimes a Recuit of the part (heating of all the part in order to homogenize the unit).

Corrosion with the weld bead

The welding is the juxtaposition of two different metals. One can thus have a galvanic phenomenon of corrosion. This defect can arise the heterogeneous Soudage in the case of of a badly conceived assembly, subject to the presence of an electrolyte. Moreover, one can also see appearing a phenomenon of interfacial corrosion as that can be met during the segregation of to the grain boundaries in the bases Nickel or during the segregation of the Carbone to the grain boundaries in the stainless steels.

Porosities

They are hollow spherical defects which can be or not emerging. They are caused by the drafts, the lack of gas, the obstruction of the tube, a bad angle of welding, water or impuretées in the welding seam…

Blisters

This term indicates a group of emerging porosities or not emerging. When they are aligned, one speaks about vermicular blisters. If they are emerging, one speaks then about punctures.

Inclusions

They indicate a foreign compound with the welding and can contain Tungstène (Case of the Soudage TIG) or slag (Welding with the coated electrode or rod) or of oxides.

Shrinkage pipes

Following a shrinking of metal during its cooling, formed empty space appears visually on the surface of the cord.

Splits of solidification

Even defect which shrinkage pipes except that the defect is nonapparent.

Excess of penetration

Overflowing metal on the side towards cord.

Joining or lack of penetration

The base metal is nonmolten, which decreases the cross section of the welding. One distinguishes the cold forging where the interface between the base metal and the welding is empty (by a control with x-rays appears a dark spot) and the hot forging, where the interface is this time filled by molten oxides (this variety is undetectable with x-rays).

Cracks

One distinguishes:

  • the cracking cold caused by important residual mechanical constraints, a presence of Hydrogène dissolves and a fragile phase,

  • the hot cracking (or more exactly liquation) created by the segregation in the joint of an eutectic for example and,
  • With regard to the chrome steels or of the stainless steels the chromium cr23c6 carbide formation which precipitates on the level of the grain boundaries and causes the corrosion of joint welded by intergranular cracking (chromium is pumped by carbon and thus does not ensure any more its role of corrosion resistance in the vicinity of carbide).

Bites

Defect where the base metal is dug on part of the cord.

Gutters

Gutters is a bite of big size proportionally to the size of the base metal due to a too hot summer days of the filler compared to the thickness or the density of the metal which receives. (see bad parameter of the machine to be welded)

Ferrous pollution

Ferrous pollution is a corrosion of the stainless steels caused by the destruction of the layer of passivation and activated by the presence of iron. It generally results from the use of metal tools (brushes, shear, etc), after machining, formatted or is the consequence of molten metal projections at the time of operations of welding.

Geometrical defects

These defects can be misalignments between the parts, a cord too curvature…

Related techniques with welding

See too

  • Soldering iron
  • assembled Composant surfaces
  • plastic Soudage of it
  • Arc welding
  • Soudage TIG
  • Soudage ATIG
  • MIG Soudure
  • Soudage MIG-MAG

External bonds

  • File welding OTUA
  • Institute of welding
  • orbital welding
  • To succeed of beautiful weldings
  • Soudeur.com: The French-speaking site gate of the welder, the welding and welding
  • * Welding, article of the 1789-1815.com site, History differently
  • Otua
  • SDMS, white boiler making: control processes of welding (Nuclear - Vacuum, Ultra-vide, Cryogénie - Spacial - Industry)

Simple: Welding

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