welding

Introduction to Welding

Dr. H. K. KhairaProfessor in MSME

MANIT, Bhopal

Welding

• Welding is the process of joining together pieces of metal or metallic parts by bringing them into intimate proximity and heating the place of content to a state of fusion or plasticity

Welding

• A concentrated heat source melts the material in the weld area; the molten area then solidifies to join the pieces together

• Sometimes a filler material is added to the molten pool to strengthen the weld

following are the key features of welding:• The welding structures are normally lighter than

riveted or bolted structures.• The welding joints provide maximum efficiency,

which is not possible in other type of joints.• The addition and alterations can be easily made in

the existing structure.• A welded joint has a great strength.• The welding provides very rigid joints.• The process of welding takes less time than other

type of joints.

largely used in the following fields of engineering:

• Manufacturing of machine tools, auto parts, cycle parts, etc.

• Fabrication of farm machinery & equipment.• Fabrication of buildings, bridges & ships.• Construction of boilers, furnaces, railways, cars,

aeroplanes, rockets and missiles.• Manufacturing of television sets, refrigerators,

kitchen cabinets, etc.

Types of Welding

• Fusion Welding• Pressure Welding

Fusion Welding

Fusion Welding

It is defined as melting together and joining metals by means of heat. It uses heat to melt the base metals and may add a filler metal.The thermal energy required for these operations is usually supplied by chemical or electrical means.Filler metals may or may not be used.

Welding Metallurgy

The base metal(s) and filler metal mix together during melting, forming an alloy when they solidify

The solidification of the metals can be considered as casting a small amount of metal in a metal mold

Fusion Welding

• All fusion welding process have three requirements.– Heat– Shielding– Filler metal

• The method used to meet these three requirements is the primary difference between welding processes.

Types of Fusion Welding(i). Arc welding • Carbon arc• Metal arc• Metal inert gas• Tungsten inert gas• Plasma arc• Submerged arc• Electro-slag(ii). Gas Welding• Oxy-acetylene• Air-acetylene• Oxy-hydrogen

(iv). Thermit Welding(vi)Newer Welding

– Electron-beam– Laser

Types of Fusion Welding

(i). Gas Welding (iv). Thermit Welding1. Oxy-acetylene (v)Newer Welding

2. Air-acetylene 1. Electron-beam3. Oxy-hydrogen 2. Laser

(ii). Arc welding 1. Carbon arc2. Metal arc3. Metal inert gas4. Tungsten inert gas5. Plasma arc6. Submerged arc7. Electro-slag

Pressure Welding

When pressure is used to join two metal parts with or without heat, it is known as pressure welding.

Types of Pressure Welding Processes

(i). Resistance Welding1. Butt2. Spot3. Seam4. Projection5. Percussion

(ii). Solid State Welding1. Friction2. Ultrasonic3. Diffusion4. Explosive

Factors Affecting Welding

• Heat source• Weld Metal Protection• Heat affected zone• Weldability

Heat Sources

Heat Sources in Welding

• 1. Combustion of fuel gas• 2. Electric arc• 3. Electrical resistance• 4. Friction• 5. Chemical reaction• 6. Other sources

Power density of heat sources

• As the power density of the heat source increases, the heat input to the workpiece that is required for welding decreases.

• Power density we get– Deeper weld penetration– Higher welding speeds– Better weld quality– Less damage to the workpiece

Variation of heat input to the workpiece withpower density of the heat source.

Variation of weld strength with unit thickness ofworkpiece heat input per unit length of weld.

Heat input Weld strength

Power density of heat sources

• Heat input Weld strength

Weld Metal Protection

Weld Metal Protection

• During fusion welding, the molten metal in the weld “puddle” is susceptible to oxidation

• Must protect weld puddle from the atmosphere

Weld Metal Protection

• Methods– Weld Fluxes– Inert Gases– Vacuum

Weld Fluxes

• Typical fluxes– SiO2, TiO2, FeO, MgO, Al2O3

– Produces a gaseous shield to prevent contamination

– Act as scavengers to reduce oxides– Add alloying elements to the weld– Influence shape of weld bead during solidification

Inert Gases

• Argon, helium, nitrogen, and carbon dioxideform a protective envelope around the weld area

• Used in– MIG– TIG– Shielded Metal Arc

Vacuum

• Produce high-quality welds• Used in electron beam welding• Nuclear/special metal applications– Zr, Hf, Ti

• Reduces impurities by a factor of 20 versus other methods

• Expensive and time-consuming

Heat affected zone

Heat affected zone

The surrounding area of base metal that did not melt, but was heated enough to affect its grain structure is known as heat affected zone

Heat affected zone

Heat affected zone

Heat affected zone

Weldability

Weldability

• Weldability is the ease of a material or a combination of materials to be welded under fabrication conditions into a specific, suitably designed structure, and to perform satisfactorily in the intended service

Weldability

• Metallurgical Capacity– Parent metal will join with the weld metal without

formation of deleterious constituents or alloys• Mechanical Soundness– Joint will be free from discontinuities, gas porosity,

shrinkage, slag, or cracks• Serviceability– Weld is able to perform under varying conditions of service

(e.g., extreme temperatures, corrosive environments, fatigue, high pressures, etc.)

Summary

Fusion welding melts the material then allows it to solidify and join it together

Solid-state welding uses pressure, and sometimes heat, to allow the metal to bond together without melting

Welding allows the production of parts that would be difficult or impossible to form as one piece