METAL JOINING PROCESSES

METAL JOINING PROCESSManufacturing processes, in which two or more parts are combined toform an assembly, are referred to as manufacturing processes forjoining and assembling. These processes can be divided into two majorclasses:

Mechanical assembly, which involves the use of various fasteningmethods to mechanically attach two (or more) parts and/orsubassemblies together. This group includes processes for permanent(riveting, press or shrink fitting) or non-permanent (assembly withthreaded fasteners) assembly.

Joining processes, in which two (or more) parts and/or subassembliesare joined together to form a permanent assembly. Examples arewelding, adhesive bonding, brazing and soldering.

Manufacturing processes, in which two or more parts are combined toform an assembly, are referred to as manufacturing processes forjoining and assembling. These processes can be divided into two majorclasses:

Mechanical assembly, which involves the use of various fasteningmethods to mechanically attach two (or more) parts and/orsubassemblies together. This group includes processes for permanent(riveting, press or shrink fitting) or non-permanent (assembly withthreaded fasteners) assembly.

Joining processes, in which two (or more) parts and/or subassembliesare joined together to form a permanent assembly. Examples arewelding, adhesive bonding, brazing and soldering.

MECHANICAL ASSEMBLYPROCESSES FOR NON-PERMANENT ASSEMBLY Threaded fasteners are components that have external or internal threads for

assembly of parts. The common threaded fastener types are screws, bolts,studs and nuts.

Bolt is an externally threaded fastener that is inserted through holes in theparts and screwed into a nut on the opposite side.

Screw is an externally threaded fastener that is generally assembled into ablind threaded hole and no nut is required.

Stud is an externally threaded fastener, but without the usual head possessedby a bolt. Studs can also be used to assemble two parts using a nut. They areavailable with threads on one end or both;

Nut is an internally threaded fastener having standard threads.

PROCESSES FOR NON-PERMANENT ASSEMBLY Threaded fasteners are components that have external or internal threads for

assembly of parts. The common threaded fastener types are screws, bolts,studs and nuts.

Bolt is an externally threaded fastener that is inserted through holes in theparts and screwed into a nut on the opposite side.

Screw is an externally threaded fastener that is generally assembled into ablind threaded hole and no nut is required.

Stud is an externally threaded fastener, but without the usual head possessedby a bolt. Studs can also be used to assemble two parts using a nut. They areavailable with threads on one end or both;

Nut is an internally threaded fastener having standard threads.

BOLT AND NUT

THREADED FASTENERSThreaded fasteners come in a variety of sizes, threads, and shapes.Numerous head styles are also available on bolts and screws, some ofwhich are shown in the figure. The geometries of these heads, as well asthe variety of sizes available, require different hand tools for the operator.

Flat Fillister Truss Hexagon Phillips Hex (internal) Square (internal)

Threaded fasteners come in a variety of sizes, threads, and shapes.Numerous head styles are also available on bolts and screws, some ofwhich are shown in the figure. The geometries of these heads, as well asthe variety of sizes available, require different hand tools for the operator.

Flat Fillister Truss Hexagon Phillips Hex (internal) Square (internal)

PROCESSES FOR PERMANENT ASSEMBLYRIVETING A rivet is an unthreaded, headed pin used to join two (or more) parts

by passing the pin through holes in the parts and then forming(upsetting) a second head in the pin on the opposite side.

The deforming operation can be performed hot or cold and byhammering or steady pressing. Once the rivet has been deformed, itcannot be removed except by breaking one of the heads.

RIVETING A rivet is an unthreaded, headed pin used to join two (or more) parts

by passing the pin through holes in the parts and then forming(upsetting) a second head in the pin on the opposite side.

The deforming operation can be performed hot or cold and byhammering or steady pressing. Once the rivet has been deformed, itcannot be removed except by breaking one of the heads.

PRESS AND SHRINK FITSA press fit assembly is one in which the two components have aninterference fit between them. The typical case is when a pin of adiameter Dp is pressed into a hole of a slightly smaller diameter Dc.

To assemble by shrink fitting, the external part is heated and enlarged bythermal Expansion. Then, the internal part either remains at roomtemperature or is cooled to contract its size. The parts are then assembledand brought back to room temperature so that the external part shrinksand, if previously cooled, the internal part expands to form a stronginterference fit.

A press fit assembly is one in which the two components have aninterference fit between them. The typical case is when a pin of adiameter Dp is pressed into a hole of a slightly smaller diameter Dc.

To assemble by shrink fitting, the external part is heated and enlarged bythermal Expansion. Then, the internal part either remains at roomtemperature or is cooled to contract its size. The parts are then assembledand brought back to room temperature so that the external part shrinksand, if previously cooled, the internal part expands to form a stronginterference fit.

WELDING Welding is a material joining process for permanent combining of two

(or more) parts that involves melting and subsequent solidification ofthe material from two parts thus forming a strong joint betweenthem. The assemblage of parts is called a weldment.

There are two groups of welding processes according to the state ofthe base material during the welding process: Liquid-state welding (fusion welding) Solid-state welding.

Welding is a material joining process for permanent combining of two(or more) parts that involves melting and subsequent solidification ofthe material from two parts thus forming a strong joint betweenthem. The assemblage of parts is called a weldment.

There are two groups of welding processes according to the state ofthe base material during the welding process: Liquid-state welding (fusion welding) Solid-state welding.

WELDING TYPES Fusion welding is by far the more important category. In fusion

welding, the base material is heated to melt. The most importantprocesses in this group fall in the following categories: Oxy-fuel gas welding: the metal surfaces to be joined are melted

progressively by heat from a gas flame (oxyacetylene), with orwithout filler metal, and are caused to flow together and solidifywithout the application of pressure to the parts being joined.

Arc welding: heating and melting of the material is accomplished byan electric arc created between an electrode and the base material.Arc welding processes may be manual, semi-automatic, or fullyautomated.

Resistance welding: the source of heat is the electrical resistance onthe interface between two parts held together under pressure.

Fusion welding is by far the more important category. In fusionwelding, the base material is heated to melt. The most importantprocesses in this group fall in the following categories: Oxy-fuel gas welding: the metal surfaces to be joined are melted

progressively by heat from a gas flame (oxyacetylene), with orwithout filler metal, and are caused to flow together and solidifywithout the application of pressure to the parts being joined.

Arc welding: heating and melting of the material is accomplished byan electric arc created between an electrode and the base material.Arc welding processes may be manual, semi-automatic, or fullyautomated.

Resistance welding: the source of heat is the electrical resistance onthe interface between two parts held together under pressure.

WELDING TYPES (contd..) Solid state welding: Two parts are joined together under pressure or

a combination of pressure and heat. If heat is applied, the contacttemperature is below the melting point of the base metal. Two weldingprocesses are the most popular from this group:

Diffusion welding: parts coalesce by solid-state diffusion. Friction welding: coalescence is achieved by the heat of friction

between two parts.

Solid state welding: Two parts are joined together under pressure ora combination of pressure and heat. If heat is applied, the contacttemperature is below the melting point of the base metal. Two weldingprocesses are the most popular from this group:

Diffusion welding: parts coalesce by solid-state diffusion. Friction welding: coalescence is achieved by the heat of friction

between two parts.

FUSION WELDING PROCESSOXYFUEL GAS WELDING Oxyfuel gas welding is the term used to describe the group of fusion operations

that burn various fuels mixed with oxygen to perform welding or cutting andseparate metal plates and other parts. The most important oxyfuel gas weldingprocess is oxyacetylene welding.

Oxyacetylene welding (OAW) is a fusion welding process performed by a high-temperature flame from combustion of acetylene and oxygen. The flame isdirected by a welding torch and a filler metal in the form of rod is added if theprocess is applied to weld. Composition of the filler must be similar to that ofthe base metal.

OXYFUEL GAS WELDING Oxyfuel gas welding is the term used to describe the group of fusion operations

that burn various fuels mixed with oxygen to perform welding or cutting andseparate metal plates and other parts. The most important oxyfuel gas weldingprocess is oxyacetylene welding.

Oxyacetylene welding (OAW) is a fusion welding process performed by a high-temperature flame from combustion of acetylene and oxygen. The flame isdirected by a welding torch and a filler metal in the form of rod is added if theprocess is applied to weld. Composition of the filler must be similar to that ofthe base metal.

Oxyacetylene welding uses equipment that is relatively inexpensiveand portable.It is therefore an economical, versatile process that is well suited tolow-quantity production and repair jobs.It is rarely used on the welding of sheet and plate stock thicker than 6mm because of the advantages of arc welding in such applications.Although OAW can be mechanized, it is usually performed manuallyand is hence dependent on the skill of the welder to produce a high-quality weld joint.

Oxyacetylene welding uses equipment that is relatively inexpensiveand portable.It is therefore an economical, versatile process that is well suited tolow-quantity production and repair jobs.It is rarely used on the welding of sheet and plate stock thicker than 6mm because of the advantages of arc welding in such applications.Although OAW can be mechanized, it is usually performed manuallyand is hence dependent on the skill of the welder to produce a high-quality weld joint.

TYPES OF FLAMES Reducing or Carburizing: More acetylene in mixture than Oxygen.

(Welding Al., Cast Irons, Alloy & High Carbon steels)

Neutral: Equal amounts of acetylene & oxygen (weld low carbonsteel, mild steels).

Oxidizing: More oxygen than acetylene in mixture. (WeldingBrass, Bronze & Cutting)

Reducing or Carburizing: More acetylene in mixture than Oxygen.(Welding Al., Cast Irons, Alloy & High Carbon steels)

Neutral: Equal amounts of acetylene & oxygen (weld low carbonsteel, mild steels).

Oxidizing: More oxygen than acetylene in mixture. (WeldingBrass, Bronze & Cutting)

ARC WELDING PROCESS Arc welding (AW) is a fusion welding process in which

coalescence of the metals is achieved by the heat from anelectric arc between an electrode and the work.

ARC WELDING PROCESS (Contd..) An electric arc is a discharge of electric current across a gap in a circuit.

To initiate the arc, the electrode is brought into contact with the workand then quickly separated from it by a short distance.

The electric energy from the arc produces temps. of 5000oC or higher,sufficiently hot to melt any metal. A pool of molten metal, consisting ofbase metal(s) and filler metal (if used) is formed near the tip of theelectrode.

In most arc welding processes, filler metal is added during the operationto increase the volume and strength of the weld joint. As the electrode ismoved along the joint, the molten weld pool solidifies in its wake region.

Electrodes in AW process are classified as- consumable, which melts continuously in the process of arc weldingthus providing the required filler material, and

- non-consumable, which resist melting by the arc. The filler materialmust be supplied separately.

An electric arc is a discharge of electric current across a gap in a circuit.To initiate the arc, the electrode is brought into contact with the workand then quickly separated from it by a short distance.

The electric energy from the arc produces temps. of 5000oC or higher,sufficiently hot to melt any metal. A pool of molten metal, consisting ofbase metal(s) and filler metal (if used) is formed near the tip of theelectrode.

In most arc welding processes, filler metal is added during the operationto increase the volume and strength of the weld joint. As the electrode ismoved along the joint, the molten weld pool solidifies in its wake region.

Electrodes in AW process are classified as- consumable, which melts continuously in the process of arc weldingthus providing the required filler material, and

- non-consumable, which resist melting by the arc. The filler materialmust be supplied separately.

GAS METAL ARC WELDINGGas Metal Arc Welding (GMAW) is an arc welding process in whichthe electrode is a consumable bare metal wire and shielding isaccomplished by flooding the arc with a gas. The bare wire is fedcontinuously and automatically from a spool through the weldinggun.

GAS METAL ARC WELDING (Contd..)Wire diameters ranging from 1 to 6 mm are used, the size depending onthe thickness of the parts being joined.Gases used for shielding include inert gases such as argon and heliumand active gases such as carbon dioxide.Selection of gases depends mainly on the metal being welded.Inert gases are used for welding aluminum alloys and stainless steel andin this case the process is often referred to as MIG/MAG welding (metal-inert gas/metal-argon welding).

Wire diameters ranging from 1 to 6 mm are used, the size depending onthe thickness of the parts being joined.Gases used for shielding include inert gases such as argon and heliumand active gases such as carbon dioxide.Selection of gases depends mainly on the metal being welded.Inert gases are used for welding aluminum alloys and stainless steel andin this case the process is often referred to as MIG/MAG welding (metal-inert gas/metal-argon welding).

ARC WELDING WITH NON-CONSUMABLE ELECTRODESGAS TUNGSTEN ARC WELDING (GTAW)This is an arc welding process that uses a non-consumable tungsten electrodeand an inert gas for arc shielding. Shielding gases typically used are argon,helium or a mixture of these gases. The GTAW process can be implementedwith or without a filler metal. The figure shows the latter case.When thin sheets are welded to close tolerances, filler metal is usually notadded. When a filler metal is used, it is added to the weld pool from aseparate rod or wire. The term TIG welding (tungsten inert gas welding) isoften applied to this process.

GAS TUNGSTEN ARC WELDING (GTAW)This is an arc welding process that uses a non-consumable tungsten electrodeand an inert gas for arc shielding. Shielding gases typically used are argon,helium or a mixture of these gases. The GTAW process can be implementedwith or without a filler metal. The figure shows the latter case.When thin sheets are welded to close tolerances, filler metal is usually notadded. When a filler metal is used, it is added to the weld pool from aseparate rod or wire. The term TIG welding (tungsten inert gas welding) isoften applied to this process.

RESISTANCE WELDING

- Resistance welding (RW) is a group of fusion welding processesthat utilizes a combination of heat and pressure to accomplishcoalescence.

- The heat required is generated by electrical resistance to currentflow at the interface of two parts to be welded.

- The resistance welding processes of most commercial importanceare spot and seam welding.

- Resistance welding (RW) is a group of fusion welding processesthat utilizes a combination of heat and pressure to accomplishcoalescence.

- The heat required is generated by electrical resistance to currentflow at the interface of two parts to be welded.

- The resistance welding processes of most commercial importanceare spot and seam welding.

RESISTANCE SPOT WELDINGResistance spot welding (RSW) is a resistance welding process inwhich fusion of the base metal is achieved at one location byopposing electrodes.

RESISTANCE SEAM WELDING (RSEW) The electrodes are two rotating wheels. The spacing between the weld nuggets

in resistance seam welding depends on the motion of the electrode wheelsrelative to the application of the weld current.

In the usual method of operation, called continuous motion welding, the wheelis rotated continuously at a constant velocity, and current is turned on at timingintervals consistent with the desired spacing between spot welds along theseam so that overlapping weld spots are produced.

But if the frequency of current switching isreduced sufficiently, there will be spacingbetween the weld spots, and this methodis termed roll spot welding. In another type,the welding current remains on at a constantlevel so that a truly continuous welding seamis produced.

The electrodes are two rotating wheels. The spacing between the weld nuggetsin resistance seam welding depends on the motion of the electrode wheelsrelative to the application of the weld current.

In the usual method of operation, called continuous motion welding, the wheelis rotated continuously at a constant velocity, and current is turned on at timingintervals consistent with the desired spacing between spot welds along theseam so that overlapping weld spots are produced.

But if the frequency of current switching isreduced sufficiently, there will be spacingbetween the weld spots, and this methodis termed roll spot welding. In another type,the welding current remains on at a constantlevel so that a truly continuous welding seamis produced.

Types of seam welding:conventional seam welding, roll spot welding, continuousresistance seam welding

SOLID-STATE WELDINGFORGE WELDINGForge welding is a welding process in which the components to bejoined are heated to hot working temperatures and then forgedtogether by hammer or other means.

Considerable skill is required by the craftsmen who practices this toachieve a good weld.

The process is of historic significance in the development ofmanufacturing technology; however, it is of minor commercialimportance today.

FORGE WELDINGForge welding is a welding process in which the components to bejoined are heated to hot working temperatures and then forgedtogether by hammer or other means.

Considerable skill is required by the craftsmen who practices this toachieve a good weld.

The process is of historic significance in the development ofmanufacturing technology; however, it is of minor commercialimportance today.

COLD ROLL WELDING Cold roll welding is a solid-state welding process accomplished by

applying high pressure by means of rolls between clean contactingsurfaces at room temperature.

Metals to be welded must be very ductile and free of work hardening.Contact surfaces must be exceptionally clean. Metals such as softaluminium, copper, gold and silver can be readily cold-welded. For smallparts, the forces may be applied by simple hand operated tools. Forheavier work, powered presses are required to exert the necessary force.

Cold roll welding is a solid-state welding process accomplished byapplying high pressure by means of rolls between clean contactingsurfaces at room temperature.

Metals to be welded must be very ductile and free of work hardening.Contact surfaces must be exceptionally clean. Metals such as softaluminium, copper, gold and silver can be readily cold-welded. For smallparts, the forces may be applied by simple hand operated tools. Forheavier work, powered presses are required to exert the necessary force.

DIFFUSION WELDING Diffusion Welding is a solid-state welding process that results

from the application of heat and pressure, usually in acontrolled atmosphere, with sufficient time allowed for solid-state diffusion and coalescence to occur.

Temperatures are well below the melting points of the metals,and plastic deformation at the surfaces is only minimal.

Diffusion Welding is a solid-state welding process that resultsfrom the application of heat and pressure, usually in acontrolled atmosphere, with sufficient time allowed for solid-state diffusion and coalescence to occur.

Temperatures are well below the melting points of the metals,and plastic deformation at the surfaces is only minimal.

FRICTION WELDING Friction welding is a solid-state welding process in which coalescence is

achieved by frictional heat combined with pressure. The heat is generated by the friction between the two surfaces, usually

by rotation of one part relative to the other. The parts are then driven toward each other with sufficient force to form

a metallurgical bond. The sequence is shown in the figure for a welding of two cylindrical parts. The axial compression force upsets the parts, and the material displaced

produces a flash. The flash must be subsequently trimmed to provide asmooth surface in the weld region. No filler metal, flux, or shielding gasesare required.

Friction welding is a solid-state welding process in which coalescence isachieved by frictional heat combined with pressure.

The heat is generated by the friction between the two surfaces, usuallyby rotation of one part relative to the other.

The parts are then driven toward each other with sufficient force to forma metallurgical bond.

The sequence is shown in the figure for a welding of two cylindrical parts. The axial compression force upsets the parts, and the material displaced

produces a flash. The flash must be subsequently trimmed to provide asmooth surface in the weld region. No filler metal, flux, or shielding gasesare required.

FRICTION WELDING (Contd..)

BRAZING and SOLDERINGBrazing and soldering are very similar methods of joining metals andother materials by applying heat and a filler metal without melting thebase materials. When heat is applied, the filler metal melts, wets the base materials,

and subsequently flows by capillary action into the gap between thebase metals or materials and join them by creating a metallurgical bondbetween them at the molecular level.

Brazing differs from soldering in that the filler metal is of greaterstrength with a higher melting point. Also, soldering occurs attemperatures less than 450C, and brazing at temperatures over 450C.

Soldering is used in electronics industry extensively.

BRAZING and SOLDERINGBrazing and soldering are very similar methods of joining metals andother materials by applying heat and a filler metal without melting thebase materials. When heat is applied, the filler metal melts, wets the base materials,

and subsequently flows by capillary action into the gap between thebase metals or materials and join them by creating a metallurgical bondbetween them at the molecular level.

Brazing differs from soldering in that the filler metal is of greaterstrength with a higher melting point. Also, soldering occurs attemperatures less than 450C, and brazing at temperatures over 450C.

Soldering is used in electronics industry extensively.