Phase transformations and phase diagrams

In contrast to pure metals, which solidify at a constant temperature - freezing point, alloys solidify over a range of temperature, depending on the alloy components and their concentrations.

In course of solidification and subsequent cooling of solid alloy processes of phase transformations take place. The phases compositions and their quantities change with the temperature.

Phase diagrams are used for quantitative description of the phase transformation and changes.

Phase diagram of an alloy system is a graphical presentation of the relationships between the phases compositions and their relative amounts at any given temperature and under equilibrium conditions.

Despite the fact, that in real metallurgical processes, especially in the processes, occurring in solid state, the equilibrium conditions are not reached, phase diagram is a very useful instrument of analysis and quantitative evaluations of the alloy behavior.

Phase diagram of an alloy system consisting of two components is called binary phase diagram.

There are three main types of binary phase diagrams :

• Complete solid and liquid solution diagram,
• Eutectic diagram (including Eutectic diagram with partial solubility of the components in solid state and Eutectic diagram with intermetallic compound)
• Peritectic diagram.

These three diagrams and combinations of them describe behavior of most of binary alloys.

Complete solid and liquid solution diagram

The typical diagram of this type is illustrated by the figure below.

The diagram has two curves – liquidus (equilibrium conditions of liquid phase with first solid crystals – primary crystals) and solidus (equilibrium conditions of last liquid with nearly complete solid).

Consider solidification of an alloy with concentration C. When the alloy temperature is higher than T_L , single liquid phase exists (point M on the diagram).

When the temperature reaches the value T_L (point M₁ on the liquidus curve) solidification starts. According to solidus curve (point N₁ )the first solid crystals have different composition – C₁.

Further cooling of the alloy causes changing of the liquid phase composition according to the liquidus curve and when the alloy temperature reaches a certain intermediate value T (position M_T ), liquid phase of composition C_y and solid phase of composition C_x are in equilibrium.

Relative amounts of the two phases are determined by their compositions and may be calculated by the “lever rule” :

W_S / W_L = M_TY / M_TX

or

W_S / W_L = (C_Y-C) / (C-C_X)

Where:

W_S – weight of the solid phase;

W_L – weight of the liquid phase;

M_TY and M_TX – length of the corresponding lines in the diagram.

Solidification ends at the temperature T_S and the last remainders of liquid phase have the composition C₂ (according to the point F₂ on the liquidus curve).

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Eutectic diagram

Eutectic phase diagram describes behavior of the alloys, two components of which are completely soluble in liquid state and entirely insoluble in solid state.

This diagram has two liquidus curves, starting from the freezing points of the two metals and intersecting in a minimum point – eutectic point.

Consider solidification of an alloy with concentration C. When the alloy temperature is higher than T_L , single liquid phase exists (point M on the diagram).

When the temperature reaches the value T_L (point M₁ on the liquidus curve) solidification starts. The primary crystals, forming in this case are the crystals of the metal “A”.

Further cooling of the alloy causes enrichment of the liquid phase with the metal “B” according to the liquidus curve and when the alloy temperature reaches a certain intermediate value T (position M_T ), liquid phase of composition C_y and solid phase, consisting of “A” crystals, are in equilibrium.

At the temperature equal to T_E (eutectic temperature) formation of the primary crystals stops and the remainding liquid phase , having composition C_E (eutectic composition), transforms to an intimate mixture of small “A” and “B” solid crystals. This is the eutectic phase transformation.

Relative amounts of the primary crystals and the eutectic mixture may be calculated by the “lever rule” :

W_P / W_E = (C_E-C) / C

Where:

W_P – weight of the primary crystals;

W_E – weight of the eutectic mixture.

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Eutectic diagram with partial solubility of the components in solid state

This kind of phase diagram is a “hybrid” ofthe diagram with complete solid and liquid solution and the eutectic diagram (the metals are completely soluble in liquid state and entirely insoluble in solid state).

Consider solidification of an alloy with concentration C. When the alloy temperature is higher than T_L, single liquid phase exists (point M on the diagram).

When the temperature reaches the value T_L (point M₁ on the liquidus curve) solidification starts. According to solidus curve the first solid crystals (primary crystals) of the α-phase have composition C₁.

Further cooling of the alloy causes changing of the liquid phase composition according to the liquidus curve and when the alloy temperature reaches a certain intermediate value T (position M_T ), liquid phase of composition C_y and solid α-phase of composition C_x are in equilibrium.

At the temperature equal to T_E (eutectic temperature) formation of the primary crystals stops and the remainding liquid phase , having composition C_E (eutectic composition), transforms to a finely devided mixture of small solid crystals of α-phase and β-phase (eutectic phase transformation).

At this temperature all α-phase crystals have composition C_α and all crystals of β-phase have composition C_β.

Relative amounts of the α-phase primary crystals and the eutectic mixture may be calculated by the “lever rule” :

W_P / W_E = M₂E / M₂N

or

W_P / W_E = (C_E-C) / (C- C_α)

Where:

W_P – weight of the α-phase primary crystals;

W_E – weight of the eutectic mixture;

Just below the eutectic temperature T_E the alloy consists of two solid phase: α-phase and β-phase, relative amounts of whichis determined by the “lever rule” :

W_α / W_β = M₂F / M₂N

or

W_α / W_β = (C_β-C) / (C- C_α)

Where:

W_α – weight of the α-phase;

W_β – weight of theβ-phase;

During further cooling solid solution phases (α-phase and β-phase) change their compositions according to the solvus curves NN₃ and FF₃ .

Solvus curve determins formation of solid solution phase from another solid solution phase – similar to liquidus curve.

At the temperature T₃ α-phase crystals have composition C_α and all crystals of β-phase have composition C_β.

Hypo-eutectic alloys

If an alloy composition C is lower, than eutectic composition C_E , solidification of the alloy starts from formation of the primary crystals of α-phase according to the left branch of the liquidus curve. These alloys are called hypo-eutectic.

Hyper-eutectic alloys

If an alloy composition C is higher, than eutectic composition C_E , solidification of the alloy starts from formation of the primary crystals of β-phase according to the right branch of the liquidus curve. These alloys are called hyper-eutectic.

Eutectoid phase transformation is analogous to the eutectic transformation, however it occurs with a solid solution phase, breaking up into a mixture of two finely divided phases of different compositions.

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Eutectic diagram with intermetallic compound

Intermetallic compound (valence compound) is a phase, having chemical composition equal to a fixed simple ratio, like CuZn,Cu₃Sn, Mg₂Pb, etc.

Sometimes intermetallic compounds exist over a range of composition, differing from the valence law. Intermetallic compounds of this sort are called electron compounds or intermediate solutions.

An example of a phase diagram with intermetallic compound AB₂ is shown in the figure below.

This diagram may be considered as a combination two different diagrams: A- AB₂ and AB₂-B.

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Peritectic diagram

Sometimes a solid solution phase, which has already been formed, andthe residual liquid phase react and form another solid solution phase or intermetallic compound, having a composition between the compositions of the liquid and the first solid. This is peritectic transformation (peritectic reaction).

An example of a phase diagram with peritectic transformation is shown in the figure:

Consider solidification of an alloy with concentration C. When the alloy temperature is higher than T_L , single liquid phase exists (point M on the diagram).

When the temperature reaches the value T_L (point M₁ on the liquidus curve) solidification starts. According to solidus curve (point N₁ )the first solid crystals (primary crystals) of the α-phase have composition C₁.

Further cooling of the alloy causes changing of the liquid phase composition according to the liquidus curve and when the alloy temperature reaches a certain intermediate value T (position M_T ), liquid phase of composition C_y and solid α-phase of composition C_x are in equilibrium.

At the temperature equal to T_P (peritectic temperature) formation of the α-phase crystals stops and the remainding liquid phase , having composition C_L reacts with α-phase crystals , forming β-phase of composition C_P (peritectic phase transformation).

At this temperature remaining α-phase crystals have composition C_α and all crystals of β-phase have composition C_P (peritectic composition).

Relative amounts of the α-phase crystals and the liquid phase just above the peritectic transformation may be calculated by the “lever rule” :

W_α / W_L = (C_L-C) / (C- C_α)

Where:

W_α – weight of the α-phase crystals;

W_L – weight of the liquid phase;

Just below the peritectic temperature T_P the alloy consists of two solid phase: α-phase and β-phase, relative amounts of whichis determined by the “lever rule” :

W_α / W_β = (C_P-C) / (C- C_α)

Where:

W_α – weight of the α-phase;

W_β – weight of theβ-phase;

During further cooling solid solution phases (α-phase andβ-phase) change their compositions according to the corresponding solvus curves.

At the temperature T₃ α-phase crystals have composition C_α and all crystals of β-phase have composition C_β.

If the alloy composition is exactly equal to peritectic composition C_P , α-phase and liquid phase are consumed comletely in the peritectic reaction.

Alloys with composition C lower than C_P , some quantity of α-phase remains after the peritectic reaction (it may be calculated by the “lever rule”).

If the alloy composition C is higher than C_P , some liquid phase remains after the peritectic reaction. This remaining liquid transforms to β-phase during the further cooling.

Electrode potentials (corrosion)

Dr. Dmitri Kopeliovich

Electrode potential is a fundamental conception of electrochemistry and corrosion theory. It helps to predict the direction and intensity of an electrochemical process (eg. corrosion) and allows to control it.

• Oxidation and reduction
• Galvanic cell
• Standard electrode potential
• Electrochemical series
• Nernst equation

Oxidation and reduction

There are two meanings of the term oxidation.

The common (narrow) meaning of oxidation - chemical reaction (combining) between Oxygen and any other element resulting in formation of oxide.

For example calcium and oxygen combine forming calcium oxide:

Ca + 1/2O₂ = CaO

In chemistry the term oxidation has a wider meaning:

Oxidation - is a chemical process, in which an element loses one or more of its electrons giving it/them up to another element.

The element receiving the electrons is not necessarily oxygen.
For example the reaction between calcium and chlorine is considered oxidation:

Ca + Cl₂ = CaCl₂

In this reaction an atom of calcium oxidizes when it gives up two its electrons to two atoms of chlorine.

Both reactions may be presented in ionic form as follows:

Ca = Ca²⁺ + 2e^-

Reduction is a chemical process opposite to oxidation, in which an element gains one or more electrons given by another element.

For example reduction of copper from copper oxide:

CuO + H₂ = Cu + H₂O

In this reaction positive ion of copper receives two electrons from two atoms of Hydrogen.
Reduction of copper in ionic form may be presented as follows:

Cu²⁺ + 2e^- = Cu

Hydrogen losing its electron oxidizes in this reaction.

Oxidation of an element is always accompanied by reduction of another element therefore the reaction is often called oxidation-reduction or redox.

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Galvanic cell

If a piece of a metal M is dipped into a solution capable to dissolve it the metal begins to oxidize giving the electrons e^- to the metal specimen and forming positive ions M⁺ transferring to the electrolyte solution.

As a result a potential difference Δφ between the metal piece and electrolyte forms.

The absolute value of the potential difference can not be measured since the measurement would mean inserting another electrode into the electrolyte and formation another potential difference between them.

For relative measurements of potentials of various metals in various solutions galvanic cell is used.
Typical galvanic cell consists of two metallic electrodes (eg. Zn and Cu) immersed into different solutions (ZnSO₄ and CuSO₄). The solutions are electrically connected by a salt bridge (a piece of filter paper soaked in a salt solution).
The metallic electrodes are connected to a high resistance voltmeter.
The galvanic cell is schematically described by the cell notation:

Zn(s)|ZnSO₄(aq,m₁)||Cu(s)| CuSO₄(aq,m₂)

(s) indicates solid;
(aq, m₁), (aq, m₂) indicate aqueous solution of concentration m₁ and m₂
A single vertical line | indicates a boundary between e metallic electrode and the electrolyte;
The double vertical line || indicates the salt bridge between the two electrolytes.

It is accepted that the negative electrode is written in the left and the positive electrode - in right part of the diagram.



In this galvanic cell the zinc electrode is anode. It oxidizes and gives up the electrons of the atoms dissolving in the electrolyte in form of positive ions:

Zn = Zn²⁺ + 2e^-

The copper electrode is cathode. The dissolved copper ions gain electrons from the metallic electrode and converts to solid metal depositing on the cathode surface:

Cu²⁺ + 2e^- = Cu

If the value of the electric current is negligible (high resistance voltmeter is used) the measured potential difference between the electrodes is equal to the electomotive force (EMF) of the galvanic cell.

The measurement of electromotive force are usually made under standard conditions:

• Temperature: 77ºF (25ºC, 298K);
• Pressure of gases: 1 atm;
• Electrolyte solution activity (concentration): 1 M (1 mol/dm³).

Electromotive force measured under standard conditions has a notation E⁰.
EMF of a galvanic cell is a resulting sum of potential differences of the anode and cathode:

E⁰ = E_cathode - E_anode

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Standard electrode potential

Standard electrode potential is the electromotive force measured in the Galvanic cell consisting of the half-cell with the electrode under standard conditions and the half-cell with the standard hydrogen electrode.



Standard hydrogen electrode (SHE) consists of a platinum foil coated by platinum black (platinum powder), which is dipped into an acidic solution with concentration of hydrogen ions (H⁺) 1M and which is in contact with gaseous hydrogen bubbling around the electrode.
Platinum does not take part in the reaction but it serves as a catalyst for oxidation-reduction reaction of hydrogen.
Standard electrode potentials are measured relatively to the standard hydrogen electrode, potential of which is defined as 0 volt:

E⁰ = E⁰_R - E⁰(H/H₂) = E⁰_R

When a standard electrode potential is measured the electrode is connected to the positive terminal of the voltmeter and the standard hydrogen electrode is connected to the negative terminal of the voltmeter.

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Electrochemical series

Standard electrode potentials of metals are arranged in electrochemical (galvanic) series.

The greater the negativeness of a standard electrode potential the greater the tendency of the element to oxidize (dissolve).
Noble (non-reactive) metals having positive value of the standard electrode potential are located in the bottom part of the table.

Electrode	E0
Li+/Li	-3.045
Rb+/Rb	-2.925
K+/K	-2.925
Cs+/Cs	-2.923
Ba2+/Ba	-2.905
Ca2+/Ca	-2.866
Na+/Na	-2.714
Mg+/Mg	-2.37
Al3+/Al	-1.66
Ti2+/Ti	-1.630
Zr4+/Zr	-1.539
Mn2+/Mn	-1.179
V2+/V	-1.175
Cr2+/Cr	-0.913
Zn2+/Zn	-0.763
Cr3+/Cr	-0.744
Fe2+/Fe	-0.44
Cd2+/Cd	-0.403
Co2+/Co	-0.277
Ni2+/Ni	-0.250
Sn2+/Sn	-0.136
Pb2+/Pb	-0.126
Fe3+/Fe	-0.037
H+/H2	0.000
Cu2+/Cu	+0.337
Cu+/Cu	+0.521
Ag+/Ag	+0.799
Hg2+/Hg	+0.851
Pd2+/Pd	+0.987
Pt2+/Pt	+1.188
Au3+/Au	+1.50
Au+/Au	+1.692

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Nernst equation

If an electrode is not under standard conditions the electrode potential may be calculated according to the Nernst equation:

E = E⁰ - (RT/nF)*lnC_ion

Where:
E⁰ - Standard electrode potential, V;
R - gas constant R=8.3143 J/(mol*K);
T - temperature, K;
n - number of electrons transferred;
F - Faraday constant F=96500 C/mol (C-coulombs);
C_ion - molar activity (concentration) of ions.

At the temperature 298K the Nernst equation may be presented as follows:

E = E⁰ - (0.059/n)*lnC_ion

Nernst equation is used for constructing Pourbaix diagrams (E / PH diagrams), which help to determine the direction of electrochemical processes with metals in water solutions.

Welding precipitation hardening stainless steels

Welding austenitic stainless steels

Austenitic stainless steels include series 200 and 300 (examples: 201, 202, 216, 302 ,304, 310, 316, 321 ,347).

Due to their austenitic structure the steels have low thermal conductivity (half of that of ferritic steels) and therefore lower heat input is required for welding.
coefficient of thermal expansion of austenitic stainless steels is relatively high resulting in larger thermal distortions and internal stresses of the welded parts, which increase suceptibility of the weld to hot cracks. The presence of small amount of ferrite (about 5%) decreases the risk of hot cracks due to the ability of ferrite to dissolve low melting impurities.
Austenitic stainless steels are also characterized by lower wettability and higher viscosity of the liquid metal in weld pool, which may cause welding defects.
Commonly the compositions of filler materials for welding austenitic stainless steels match the steels compositions. The un-stabilized steels 201, 202,301, 302, 304, 305 are welded by the filler material of type 308 (21%Cr, 10%Ni).
308 alloy contains more chromium and nickel than 304, which results in:

• suppression of martensite formation ;
• controllable amount of ferrite limiting suceptibility of the weld to hot cracks.

SENSITIZATION
One of the possible welding defects of austenitic stainless steels is sensitization.
At the temperatures 900-1400ºF (482-760ºC) chromium carbides form along the austenite grains. This causes depletion of chromium from the grains resulting in decreasing the corrosion protective passive film.
This effect is called sensitization.
Sensitization is depressed in low carbon steels (0.03%) designated with suffix L (304L, 316L).
Formation of chromium carbides is also avoided in stabilized austenitic stainless steels (321, 347) containing carbide forming elements like titanium, niobium, tantalum, zirconium. Stabilization heat treatment of such steels results in preferred formation of carbides of the stabilizing elements instead of chromium carbides.

Welding ferritic stainless steels

Ferritic stainless steels include part of the steels from the series 400 (examples: 405, 409, 430, 442, 446).

The main welding problem of ferritic stainless steel is growth of ferrite grains caused by heating. Coarse grain structure results in low toughness of the weld material.
In order to prevent the grain coarsening the preheat is limited to 150-450ºF (65-230ºC). Low carbon ferritic steels (405, 409) are welded without preheating. Heat input should also be limited particularly when thick parts are welded (thicker, than 0.25”/6mm). Weld toughness may be improved if an Austenitic stainless steels filler material is used.

Commonly filler materials containing equal or excessive amount of chromium are used for welding ferritic stainless steels.
Some parts may be welded without filler material.

Welding martensitic stainless steels

Ferritic stainless steels include series 500 and part of the steels from the series 400 (examples: 403,410, 414, 416, 420, 422, 431, 440, 501, 502, 503, 504).

Weldability of martensitic stainless steels is low because of their sensitivity to cold cracks formation:

• Martensitic structure of the steels is determined by high carbon content, which reduces the steel ductility and therfore increase the sucsceptibility to cracks.
  
• Martensitic transformation causes changes of grains volume due to to the changes of the crystall lattice. These changes produce internall stresses, which increase the risk of cracks.
  
• Martensitic stailess steels easily pick-up and dissolve Hydrogen from the atmosphere and other sources, which may cause hydrogen embrittlement.
  

In order to prevent cracks formation the welded parts should be preheated to 400-570ºF (204-300ºC). Measures should be taken in order to diminish hydrogen pick-up during the welding process (dry flux, shielding gas). Post-weld heat treatment at 1000-1200ºF (540-650ºC) of high carbon (> 0.2%) martensitic steels is required for improvement of the steel toughness.

Commonly filler materials, in which contents of chromium and carbon match the composition of the welded parts, are used for welding martensitic stainless steels.
Weld toughness may be improved if an Austenitic stainless steels filler material (308, 309) is used.

Welding austenitic-ferritic (Duplex) stainless steels

austenitic-ferritic (Duplex) stainless steels (examples: 2205) have a mixed austenitic-ferritic structure and commonly contain 0.1-0.3% of Nitrogen.

Weldability of Duplex stainless steels is good.
In order to prevent nitrogen loss during welding shielding gas containing nitrogen is used.
Proper filler materials and controlled heat input help to obtain the required balance between the austenitic and ferritic phases.

Precipitation hardening stainless steels are categorized into three groups: martensitic (17-4PH, 15-5PH), semiaustenitic (17-7PH, PH 15-7Mo) and austenitic (17-10P, A286).

Commonly filler materials, composition of which close to the composition of the welded parts, are used for welding precipitation hardening stainless steels .

Weldability of austenitic precipitation hardening stainless steels is poor because of their susceptibility to hot cracks. Limited heat input and welding of parts in solution treated condition are required for diminishing risk of cracks. Nickel alloys (nickel-chromium-iron) are used as filler materials for welding austenitic precipitation hardening stainless steels.

Tungsten Inert Gas Arc Welding (TIG, GTAW)

Dr. Dmitri Kopeliovich

Tungsten Inert Gas Arc Welding (Gas Tungsten Arc Welding) is a welding process, in which heat is generated by an electric arc struck between a tungsten non-consumable electrode and the work piece.

The weld pool is shielded by an inert gas (Argon, helium, Nitrogen) protecting the molten metal from atmospheric contamination.

The heat produced by the arc melts the work pieces edges and joins them. Filler rod may be used, if required.

Tungsten Inert Gas Arc Welding produces a high quality weld of most of metals. Flux is not used in the process.


Advantages of Tungsten Inert Gas Arc Welding (TIG, GTAW):

• Weld composition is close to that of the parent metal;
• High quality weld structure
• Slag removal is not required (no slag);
• Thermal distortions of work pieces are minimal due to concentration of heat in small zone.
  

Disadvantages of Tungsten Inert Gas Arc Welding (TIG, GTAW):

• Low welding rate;
• Relatively expensive;
• Requres high level of operators skill.

Shielded Metal Arc Welding (SMAW)

Dr. Dmitri Kopeliovich

Shielded metal arc welding (Stick welding, Manual metal arc welding) uses a metallic consumable electrode of a proper composition for generating arc between itself and the parent work piece. The molten electrode metal fills the weld gap and joins the work pieces.

This is the most popular welding process capable to produce a great variety of welds.

The electrodes are coated with a shielding flux of a suitable composition. The flux melts together with the electrode metallic core, forming a gas and a slag, shielding the arc and the weld pool. The flux cleans the metal surface, supplies some alloying elements to the weld, protects the molten metal from oxidation and stabilizes the arc.
The slag is removed after Solidification.


Advantages of Shielded Metal Arc Welding (SMAW):

• Simple, portable and inexpensive equipment;
• Wide variety of metals, welding positions and electrodes are applicable;
• Suitable for outdoor applications.
  

Disadvantages of Shielded Metal Arc Welding (SMAW):

• The process is discontinuous due to limited length of the electrodes;
• Weld may contain slag inclusions;
• Fumes make difficult the process control.

Principles of arc welding

Dr. Dmitri Kopeliovich

Arc welding is a welding process, in which heat is generated by an electric arc struck between an electrode and the work piece.

Electric arc is luminous electrical discharge between two electrodes through ionized gas.

Any arc welding method is based on an electric circuit consisting of the following parts:

• Power supply (AC or DC);
• Welding electrode;
• Work piece;
• Welding leads (electric cables) connecting the electrode and work piece to the power supply.
  

Electric arc between the electrode and work piece closes the electric circuit. The arc temperature may reach 10000°F (5500°C), which is sufficient for fusion the work piece edges and joining them.

When a long join is required the arc is moved along the joint line. The front edge of the weld pool melts the welded surfaces when the rear edge of the weld pool solidifies forming the joint.

Types of weld joints are shown in the figure:

When a filler metal is required for better bonding, filling rod (wire) is used either as outside material fed to the arc region or as consumable welding electrode, which melts and fills the weld pool. Chemical compositions of filler metal is similar to that of work piece.

Molten metal in the weld pool is chemically active and it reacts with the surrounding atmosphere. As a result weld may be contaminated by oxide and nitride inclusions deteriorating its mechanical properties. Neutral shielding gases (argon, helium) and/or shielding fluxes are used for protection of the weld pool from atmospheric contamination. Shields are supplied to the weld zone in form of a flux coating of the electrode or in other forms.

Metal Inert Gas Welding (MIG, GMAW)

Dr. Dmitri Kopeliovich

Metal Inert Gas Welding (Gas Metal Arc Welding) is a arc welding process, in which the weld is shielded by an external gas (Argon, helium, CO₂, argon + Oxygen or other gas mixtures).

Consumable electrode wire, having chemical composition similar to that of the parent material, is continuously fed from a spool to the arc zone. The arc heats and melts both the work pieces edges and the electrode wire. The fused electrode material is supplied to the surfaces of the work pieces, fills the weld pool and forms joint.

Due to automatic feeding of the filling wire (electrode) the process is referred to as a semi-automatic. The operator controls only the torch positioning and speed.


Advantages of Metal Inert Gas Welding (MIG, GMAW):

• Continuous weld may be produced (no interruptions);
• High level of operators skill is not required;
• Slag removal is not required (no slag);
  

Disadvantages of Metal Inert Gas Welding (MIG, GMAW):

• Expensive and non-portable equipment is required;
• Outdoor application are limited because of effect of wind, dispersing the shielding gas