Welding Process : A Joining Process

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Introduction To Welding:

  • Welding is a fabrication process to produce permanent joints between two similar and dissimilar materials, with or without the application of heat and pressure.
  • For example – Arc welding and gas welding : Only heat.
  • Function welding and resistance welding : Both heat and pressure
  • Explosion welding : Only pressure

Fundamentals of Arc Welding

1.Principle of Arc Welding :

To generate the arc, at first a contact is made between electrode and workpiece due to which partial melting and evaporation occurs at the interface and the two ends are severely heated due to which the electrons and ions at both the ends become electrically unstable.

As soon the contact is disengaged and small gap is maintained between the electrode and workpiece than at first the positively unstable ions emit from positive side and move towards the negative side to gain the stability.

Due to this ionic emission the gap between electrode and workpiece is ionized and through this ionized medium the unstable is travel towards positive side within the envelope of positive ions moving in opposite direction.

Those electrons and ions which are at the interface collide with each other and their kinetic energies convert into intense ultraviolet radiation and the arc heat.

Rest of the electrons and ions bombard at the positive and negative sides respectively where their kinetic energies convert into heat energy.

Since the kinetic energy of electron is more than that of ions therefore, larger amount of heat (about 2/3rd.) is developed at the positive side where the electrons bombard and relatively lesser amount of heat ( 1/3rd.) is developed at negative side where the ions bombard.

With direct current source if the workpiece is made positive and the electrode negative then it is called as straight positive and if the electrode is made positive and workpiece negative then it is called as reverse polarity.

This phenomena of arc generation is called as thermionic emission arc length can be sustained depends totally upon the arc voltage i.e voltage of power source must be a function of arc length and it must vary linearly with the arc length. In most of the welding process the arc voltage is limited upto 80V for the welders safety.

The Required Arc Current Depends on the Following Factors :

  • Type of material
  • Type of joint
  • Type of electrode
  • Diameter of electrode
  • Type of coating over the electrode
  • Thickness of the weldment
  • Position of the weld

The Required are Current Varies Linearly with the Electrode Diameter :

I=Kd

where, k = constant

  • The standard are length should be equal to the diameter of electrode while the standard width of the weld bead is three times the diameter of electrode.

Types of Electrodes :

  • Non Consumable Electrode
  • It is made up of high melting point materials such as tungsten and graphite which is only used to generate the arc and to keep the arc stable. Since the non-consumable electrodes do not melt therefore the arc is more stable and easier to be maintained. But the deposition rate with NC electrode will be nill and hence it will not provide any filler material.
  • Consumable Electrodes.

These electrodes melt during welding and hence provide filler metal and are made up of same material being welded until alloying is required.

These are of 2 types-

  • Bare electrode:

These electrodes are either in form of wire or Rod. Wire type electrodes are in form of a spool of a wire and are used in semi-automatic and automatic arc welding.

During welding the electrode holder has following two movements.

•The electrode holder needs to be fed towards the pool, with the same rate with which it is being consumed.

•The movement of electrode holder along the length of weldment.

If both of above movements are controlled manually then it is called as manual arc welding but if the first movement is controlled automatically and second manually then it is called as semi-automatic arc welding and if both of the above movements are controlled automatically then it is called as automatic arc welding.

In case of manual are welding it is impossible to keep the arc length constant. Therefore the arc length will keep changing, to keep the power input constant into the weld region the current being with drawn into the arc will also have to change. Therefore, in manual arc welding we use a constant Current transformer which has a drooping volt ampere characteristic because its secondary winding is made movable.

It can be observed that for a significant change in voltage there is in significant change in current to keep the роwer constant and therefore it is named as constant current transformer.

  • In semi-automatic and automatic arc welding since feeding is done automatically therefore the arc length and hence the arc voltage remains constant and hence a constant voltage transformer is used which has a linear volt ampere characteristic.

The VI line can be shifted up and down and its slope can be change by changing the OCV and sec to have different power characteristic

For Stable Arc:

  • In constant voltage transformer

V Arc =V Transformer

  • In constant current transformer.

L Arc = L Transformer

The linear volt-ampere characteristic constant voltage transformer in semi-automatic and automatic arc welding is given as follows:

V = OC V – [OCV/SSC]

Where,

OCV: Open Circuit Voltage: maximum voltage in circuit :

SCC: Short Circuit Current: Maximum current in circuit

Ohm’s law is invalid

Note: For a constant power (P-VI) voltage is inversely proportional to current and as we know from voltage arc length characteristic that arc length is directly proportional to arc -length. Therefore, arc currant will be inversely proportional to arc length.

Red type consumable electrode which arc bare are used in manual arc welding.

Coated Electrode:

  • Coated electrodes have a coated over the electrode wire which has following seating characteristics.
  • It provide protective shielding.
  • It stabilizes the arc It provide protective slag coating to accumulate impurities, prevent oxidation, slows down the cooling rate and hence produce strong and uniform green weldments.
  • It minimizes the effects of arc blow.
  • It reduces a spatter losses.
  • It may add elements.
  • It may add additional filler material.

Ingredients of Electrode Coating

Slag forming ingredients:

  • Asbestos, mica, Silica, Fluorspar, titanium dioxide, Aluminiumoxide, Calcium carbonate, magnesium carbonate

Arc Stabilizing Ingredients/Ionizing Ingredients :

Potassium Silicate, Sodium Silicate, Calcium Oxide, Sodium Oxide, Feldspar, Rutile (TiO₂ + ZnO₂).

Deoxidizing Ingredient:

Cellulose, starch, Dex trine, Saw dust, Dolomite, Aluminum.

Binding ingredient:

Sodium silicate and potassium silicate.

In AC arc welding the electrodes with potassium silicate binder is preferred because ionization potential of potassium is less then that of sodium.

TIM

Note: Ionization potential is the potent energy required to emit an e a neutral atom.

Arc Blow :

Are blow occurs due to unbalanced magnetic forces. While welding. the centre f near the workpiece the no. of magnetic line of forcer. sider of on the two the are will be approximately same and hence the are will not deflect but during welding near the corners or edges. the concentration of magnetic lines of forces will be maximum at one side. Therefore, the are will deflect and this phenomenon is called as are blow

  • The are blew and its effect will be severe if the рошем Source is DC and electrode is bare type and the material is. highly magnetic such as Nickel. alloy…

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