WELDING FOR ALL SEASONS 
 
MMA, TIG, MIG/MAG welding, plasma cutting, robotics and automation - technologies made possible by Selco generators and plants for a myriad professional applications: welding for major building projects, for the car, naval and aerospace industries, for metal structures, automatic systems, oil pipelines, rail transport and much more.
 
 
 
 Genesis_352_ro
Manual Metal Arc welding (MMA-SMAW)
a
Shielded Metal Arc Welding (SMAW), is the widespread welding technology, mostly because it needs simple, easy-to-use and versatile equipments. Moreover, thanks to its portability, it is the most suitable process to be used outdoor and in building sites.
  
Preparing the edges
To obtain good welding joints it is advisable to work on clean parts, free from oxidations, rust or other contaminating agents.
 
Choosing the electrode
The diameter of the electrode to be used depends on the thickness of the material, the
position, the type of joint and the type of preparation of the piece to be welded.
Electrodes of large diameter obviously require very high currents with consequent high heat
supply during the welding.

 
Type of coating
Rutile
Acid
Basic
Property
Easy to use
High melting speed
High quality of joint
Use
All position
Flat
All position
 
Choosing the welding current
The range of welding current related to the type of electrode used is specified by the manufacturer usually on the electrode packaging.
 
Striking and mantaining the arc
The electric arc is produced by scratching the electrode tip on the workpiece connected to the earth cable and, once the arc has been struck, by rapidly withdrawing the electrode to the normal welding distance.
Generally, to improve the arc striking behaviour a higher initial current is given in order to heat suddenly the tip of the electrode and so aid the arc establishing(Hot Start).
Once the arc has been struck, the central part of the electrode starts melting forming tiny globules which are transferred into the molten weld pool on the workpiece surface through the arc stream.
The external coating of the electrode is being consumed and this supplies the shielding gas for the weld pool, ensuring the good quality of the weld.
To prevent the molten material globules cause the extinguishing of the arc by short-circuiting and sticking the electrode to the weld pool, due to their proximity, a temporary increase of the welding current is given in order to melt the forming short-circuit (Arc Force).
If the electrode sticks to the workpiece, the short circuit current should be reduced to the minimum (antisticking).
 
Carrying out the welding
The welding position varies depending on the number of runs; the electrode movement is normally carried out with oscillations and stops at the sides of the bead, in such a way as to avoid an excessive accumulation of filler metal at the centre.
 
Removing the slag
Welding using covered electrodes requires the removal of the slag after each run.
The slag is removed by a small hammer or is brushed away if friable.
  

 
 
 TIG
TIG WELDING (GTAW)
a
The TIG (Tungsten lnert Gas) welding process is based on the presence of an electric arc struck between a non-consumable electrode (pure or alloyed tungsten with an approximate melting temperature of 3370°C) and the work-piece; an inert gas (argon) atmosphere protects the weld pool.
To avoid dangerous inclusions of tungsten in the joint, the electrode must never come in contact with the workpiece; for this reason the welding power source is usually equipped with an arc striking device that generates a high frequency, high voltage discharge between the tip of the electrode and the workpiece. Thus, thanks to the electric spark, ionizing the gas atmosphere, the welding arc is struck without any contact between electrode and workpiece.
Another type of start is also possible, with reduced tungsten inclusions: the lift start, which does not require high frequency, but only an initial short-circuit at low current between the electrode and the workpiece; when the electrode is lifted, the arc is established and the current increases until reaching the set welding value.
To improve the quality of the filling at the end of the welding bead it is important to control carefully the down slope of the current and it is necessary that the gas still flows in the welding pool for some seconds after the arc is extinguished.
Under many operating conditions, it is useful to be able to use two preset welding currents and to be able to switch easily from one to the other (BILEVEL).

 

Welding polarity
D.C.S.P. - Direct Current Straight Polarity
This is the most used polarity and ensures limited wear of the electrode (1), since 70% of the heat is concentrated in the anode (piece).
Narrow and deep weld pools are obtained, with high travel speeds and low heat supply.
Most materials, except for aluminium (and its alloys) and magnesium, are welded with this polarity.
 
D.C.R.P. - Direct Current Reverse Polarity
The reverse polarity is used for welding alloys covered with a layer of refractory oxide with higher melting temperature compared with metals.
High currents cannot be used, since they would cause excessive wear on the electrode.
 
D.C.S.P.-Pulsed - Direct Current Straight Polarity Pulsed
The use of pulsed direct current allows better control, in particular operating conditions, of the welding pool width and depth.
The welding pool is formed by the peak pulses (Ip), while the basic current (Ib) keeps the arc ignited.
This operating mode helps to weld thinner metal sheets with less deformations, a better form factor and consequently a lower danger of hot cracks and gas penetration.
Increasing the frequency (MF) the arc becomes narrower, more concentrated, more stable and the quality of welding on thin sheets is further increased.
 
Steel and copper TIG welding
The TIG procedure is very effective for welding both carbon and alloyed steel, for first runs on
pipes and for welding where good appearance is important.
Straight polarity is required (D.C.S.P.).

Preparing the edges_Careful cleaning and preparation of the edges are required.
 
Choosing and preparing the electrode_You are advised to use thorium tungsten electrodes (2% thorium-red coloured) or alternatively cerium or lanthanum electrodes with the folowing diameters:
Ø electrode (mm)
1.0
1.6
2.4
current range (A)
15÷75
60÷150
130÷240
The electrode must be sharpened.
 α (°)
30
60÷90
90÷120
current range (A)
0÷30
30÷120
120÷250 
 
Filler metal
The filler rods must have mechanical characteristics comparable to those of the parent metal.
Do not use strips obtained from the parent metal, since they may contain working impurities that can negatively affect the quality of the welds.
 
Shielding gas

Tipically, pure argon (99.99%) is used.

 
 
 
 
  

 
 
 MIG
MIG/MAG welding (GMAW)
a
A Mig/Mag system consists of a direct current power source, wire feeder, wire spool, torch and gas.
The current is transferred to the arc through the fusible electrode (wire connected to positive pole); in this procedure the melted metal is transferred onto the workpiece through the arc stream. The automatic feeding of the continuous filler material electrode (wire) is necessary to refill the wire that has melted during welding.

 

Methods
In MIG welding, two main metal transfer mechanisms are present and they can be classified according to the means by which metal is transferred from the electrode to the workpiece. The first one, defined “SHORT-ARC”, produces a small, fast-solidifying weld pool where metal is transferred from the electrode to the workpiece only for a short period when the electrode is in contact with the weld pool. In this timeframe, the electrode comes into direct contact with the weld pool generating a short circuit that melts the wire which is therefore interrupted. The arc then turn on again and the cycle is repeated.
Another mechanism for metal transfer is called the “SPRAY-ARC” method, where the metal transfer occurs in the form of very small drops that are formed and detached from the tip of the wire and transferred to the weld pool through the arc stream.
The Pulsed MIG process is a suitably managed and controlled Spray Arc MIG transfer process in which the energy is transferred to the arc in pulses, i.e. the welding current is suitably formed to provide a high energy pulse instantaneously. This gives rise to the separation and controlled transfer of a single droplet of filler material. The current is then held to a maintenance value capable of sustaining the welding arc, but without the transfer of material. In these conditions the welding pool is able to cool, thus allowing regular, controlled deposition of the molten metal particle on the base material. Pulsed MIG provides a welding speed that is far higher than the classic Spray Arc MIG process due to a faster, more efficient deposition process. Moreover, there is a substantial reduction in the amount of welding spatter and fumes and in the remachining times, resulting in fact in smaller deformations and an optimum degree of finish.
 
Welding parameters
The visibility of the arc reduces the need for the user to strictly observe the adjustment tables as he can directly monitor the weld pool.
• The voltage directly affects the appearance of the bead, but the dimensions of the weld bead can be varied according to requirements by manually moving the torch to obtain variable deposits with constant voltage.
• The wire feeding speed is proportional to the welding current.
Both in the Short-Arc/Spray-Arc MIG and Pulsed MIG processes, depending on the generator used, simple, rapid synergic settings of the welding parameters are available, automatically defined on the basis of the operating conditions (material, thickness, gas, wire, speed), which are dynamically controlled and kept in balance throughout the welding process by microprocessor control. This enables excellent welding results to be obtained in terms of quantity, quality and aesthetics in all conditions and in all applications.
 
Gases
MIG-MAG welding is defined mainly by the type of gas used: inert for MIG welding (Metal Inert Gas), active for MAG welding (Metal Active Gas).
Carbon dioxide (CO2)
Using CO2 as a shielding gas, high penetrations and low operating cost are obtained with high feeding speed and good mechanical properties. On the other hand, the use of this gas creates considerable problems with the final chemical composition of the joints as there is a loss of easily oxidisable elements with simultaneous enrichment of carbon in the weld pool.
Welding with pure CO2 also creates other types of problems such as excessive spatter and the formation of carbon monoxide porosity.
Argon
This inert gas is used pure in the welding of light alloys whereas, in chrome-nickel stainless steel welding, it is preferable using argon with the addition of oxygen and CO2 in a percentage of 2% as this contributes to the stability of the arc and improves the form of the bead.
Helium
This gas is used as an alternative to argon and permits greater penetration (on thick material) and faster wire feeding.
Argon-Helium mixture
Provides a more stable arc than pure helium, and greater penetration and travel speed than argon.
Argon-CO2 and Argon-CO2-Oxygen mixture
These mixtures are used in the welding of ferrous materials especially in SHORT-ARC operating mode as they improve the specific heat contribution. They can also be used in SPRAY-ARC. Normally the mixture contains a percentage of CO2 ranging from 8% to 20% and O2 around 5%.
 

 
 

 

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