Welding

Question 1

What processes does the term joining refer to?

Answer 1

Welding, brazing, soldering, and adhesive bonding

Question 2

What is welding?

Answer 2

Welding is a materials joining process in which two or more parts are coalesced at their contacting surfaces by a suitable application of heat and/or pressure.

Question 3

What is a weldment?

Answer 3

The assemblage of parts that are joined by welding

Question 4

What are the main advantages of welding?

Answer 4

• Welding provides a permanent joint. The welded parts become a single entity.
• The welded joint can be stronger than the parent materials if a filler metal is used that has strength properties superior to those of the parents, and if proper welding techniques are used.
• Welding is usually the most economical way to join components in terms of material usage and fabrication costs. The alternative mechanical assembly is usually heavier than a corresponding weldment.
• Welding is not restricted to the factory environment. It can be accomplished “in the field”.

Question 5

What are the limitations / disadvantages of welding?

Answer 5

• Most welding operations are performed manually and are expensive in terms of labor cost. Many welding operations are considered “skilled trades”, and the labor to perform these operations may be scarce.
• Most welding processes are inherently dangerous because they involve the use of high energy.
• Since welding accomplishes a permanent bond between the components, it does not allow for convenient disassembly.
• The welded joint can suffer from certain quality defects that are difficult to detect. The defects can reduce the strength of the joint.

Question 6

What is the name for the surfaces that are to be joined through welding?

Answer 6

Faying surfaces

Question 7

What are the two main groups of welding process?

Answer 7

Fusion Welding

Solid-State Welding

Question 8

Give an overview of fusion welding

Answer 8

Fusion-welding processes use heat to melt the base metals. In many fusion welding operations, a filler metal is added to the molten pool to facilitate the process and provide bulk and strength to the welded joint.

Question 9

What is a fusion-welding operation with no filler called?

Answer 9

An autogenous weld

Question 10

What are the three main types of fusion welding operations?

Answer 10

Arc Welding
Resistance Welding
Oxyfuel Gas Welding

Question 11

Give an overview of Arc Welding (AW)

Answer 11

Arc welding refers to a group of welding processes in which heating of the metals is accomplished by an electric arc.

Question 12

Give an overview of Resistance Welding (RW)

Answer 12

Resistance welding achieves coalescence using heat from electrical resistance to the flow of a current passing between the faying surfaces of two parts held together under pressure.

Question 13

Give an overview of Oxyfuel Gas Welding (OGW)

Answer 13

These joining processes use an oxyfuel gas, such as a mixture of oxygen and acetylene, to produce a hot flame for melting the base metal and filler metal

Question 14

What is solid-state welding?

Answer 14

Solid-state welding refers to joining processes in which coalescence results from application of pressure alone or a combination of heat and pressure. If heat is used, the temperature in the process is below the melting point of the metals being welded. No filler metal is utilized.

Question 15

Give an overview of Diffusion Welding (DFW)

Answer 15

Two surfaces are held together under pressure at an elevated temperature and the parts coalesce by solid-state diffusion.

Question 16

Give an overview of Friction Welding (FRW)

Answer 16

Coalescence is achieved by the heat of friction between two surfaces.

Question 17

Give an overview of Ultrasonic Welding (USW)

Answer 17

Moderate pressure is applied between the two parts and an oscillating motion at ultrasonic frequencies is used in a direction parallel to the contacting surfaces. The combination of normal and vibratory forces results in shear stresses that remove surface films and achieve atomic bonding of the surfaces.

Question 18

What are the five basic types of joint

Answer 18

Butt Joint (Same plane)
Corner Joint (Joined at a right angle)
Lap Joint (Two overlapping parts)
Tee Joint (T shape)
Edge Joint (Joined at parallel edges)

Question 19

What are the typical zones of a fusion weld joint in which filler metal has been used?

Answer 19

1) Fusion Zone
2) Weld Interface
3) Heat-Affected Zone
4) Unaffected Base Metal Zone

Question 20

What are the characteristics of the fusion zone?

Answer 20

The fusion zone consists of a mixture of filler metal and base metal that have completely melted. This zone is characterized by a high degree of homogeneity among the component metals that have been melted during welding. The mixing of these components is motivated largely by convection in the molten weld pool.

Question 21

What is a significant difference between solidification during welding compared to during casting.

Answer 21

Epitaxial grain growth occurs in welding

Question 22

What is epitaxial grain growth?

Answer 22

In welding, by contrast, the nucleation stage of solidification is avoided by the mechanism of epitaxial grain growth, in which atoms from the molten pool solidify on pre-existing lattice sites of the adjacent solid base metal. Consequently, the grain structure in the fusion zone near the heat-affected zone tends to mimic the crystallographic orientation of the surrounding heat-affected zone.

Question 23

Describe the Weld Interface?

Answer 23

A narrow boundary that separates the fusion zone from the heat-affected zone. The interface consists of a thin band of base metal that was melted or partially melted (localized melting within the grains)during the welding process but then immediately solidified before any mixing with the metal in the fusion zone. Its chemical composition is therefore identical to that of the base metal.

Question 24

Describe the Heat-affected Zone (HAZ)

Answer 24

The metal in this zone has experienced temperatures that are below its melting point, yet high enough to cause microstructural changes in the solid metal.

Question 25

What is an electric arc?

Answer 25

An electric arc is a discharge of electric current across a gap in a circuit. It is sustained by the presence of a thermally ionized column of gas (called a plasma) through which current flows.

Question 26

Give an overview of consumable electrodes used in AW

Answer 26

Consumable electrodes provide the source of the filler metal in arc welding. These electrodes are available in two principal forms: rods (also called sticks) and wire. The problem with consumable welding rods is that they must be changed periodically, reducing arc time of the welder. Consumable weld wire has the advantage that it can be continuously fed into the weld pool from spools containing long lengths of wire, thus avoiding the frequent interruptions that occur when using welding sticks.

Question 27

Give an overview of Non-consumable electrodes

Answer 27

They are made of tungsten (or carbon, rarely), which resists melting by the arc. Despite its name, a non-consumable electrode is gradually depleted during the welding process (vaporization is the principal mechanism), analogous to the gradual wearing of a cutting tool in a machining operation. For AW processes that utilize non-consumable electrodes, any filler metal used in the operation must be supplied by means of a separate wire that is fed into the weld pool.

Question 28

What is the purpose of arc shielding?

Answer 28

At the high temperatures in arc welding, the metals being joined are chemically reactive to oxygen, nitrogen, and hydrogen in the air. The mechanical properties of the weld joint can be seriously degraded by the sereactions. Arc shielding prevents this.

Question 29

What are common shielding gases?

Answer 29

Common shielding gases include argon and helium, both of which are inert.

Question 30

What is a flux?

Answer 30

Afluxis a substance used to prevent the formation of oxides and other unwanted contaminants, or to dissolve them and facilitate removal.

Question 31

How does flux work?

Answer 31

During welding, the flux melts and becomes a liquid slag, covering the operation and protecting the molten weld metal. The slag hardens upon cooling and must be removed later by chipping or brushing.

Question 32

What are three main functions of flux.

Answer 32

(1) provide a protective atmosphere for welding, (2) stabilize the arc, and (3) reduce spattering.

Question 33

What are delivery methods of flux application?

Answer 33

(1) pouring granular flux onto the welding operation
(2) using a stick electrode coated with flux material in which the coating melts during welding to cover the operation
(3) using tubular electrodes in which flux is contained in the core and released as the electrode is consumed