topic 7 joining process welding brazing soldering fastening 160214

JOINING PROCESSES

An all-inclusive term covering processes

such as:

Welding

Brazing

Soldering

Adhesive bonding

Mechanical fastening

Overview on Joining Processes

Why Joining Processes

Some (even simple) products are too large to be made

by individual processes ( 3-D Hollow structural member, 5 m

diameter)

Easier, more economical to manufacture & join

individual components (Cooking pot with handle)

Products to be disassembled for maintenance (Appliances; engines)

Varying functionality of product (Carbide inserts in tool

steels; Brake shoes)

Transportation + assembly is less costly (Shelving units;

Machinery)

Product Example

Figure 8.1 Various parts in a typical automobile that are assembled by the joining processes.

Product Example (Cont.)

Figure 8.2: Examples of parts utilizing joining processes. (a) A tubular part fabricated by joining individual components. This product cannot be manufactured in one piece by any of the methods described in the previous chapters if it consists of thin-walled, large-diameter, tubular-shaped long arms. (b) A drill bit with a carbide cutting insert brazed to a steel shank—an example of a part in which two materials need to be joined for performance reasons. (c) Spot welding of automobile bodies.

WELDING PROCESSES

BS 499 part 1 Welding terms

A union between pieces of metal at faces rendered plastic or liquid by heat,pressure or both.

Overview on Joining Processes

Figure 8.3: Joining Method (AWS A3.0:2001)

AWS A3-0 Standard Welding Terms and Definitions.pdf

Fusion Welding

Fusion Welding

Any welding process that uses fusion

of the base metal to make the weld

(AWS A3.0: 2001)

Fusion Welding – Arc Welding

Arc Welding

A fusion welding process in which coalescence of the

metals is achieved by the heat from an electric arc

between an electrode and the work

Electric energy from the arc produces temperatures ~

10,000 F (5500 C), hot enough to melt any metal

Most AW processes add filler metal to increase volume

and strength of weld joint

An electric arc is a discharge of electric current across a

gap in a circuit

It is sustained by an ionized column of gas (plasma)

through which the current flows

To initiate the arc in AW, electrode is brought into

contact with work and then quickly separated from it by a

short distance

A pool of molten metal is formed near electrode

tip, and as electrode is moved along joint,

molten weld pool solidifies in its wake

Figure 31.1 Basic configuration of an arc welding process.

Fusion Welding – Arc Welding (Cont.)

Two Basic Types of AW Electrodes

Consumable – consumed during welding process

Source of filler metal in arc welding

Nonconsumable – not consumed during welding process

Filler metal must be added separately


Arc Shielding

At high temperatures in AW, metals are chemically

reactive to oxygen, nitrogen, and hydrogen in air

Mechanical properties of joint can be seriously

degraded by these reactions

To protect operation, arc must be shielded from

surrounding air in AW processes

Arc shielding is accomplished by:

Shielding gases, e.g., argon, helium, CO2

Flux


Power Source in Arc Welding

Direct current (DC) vs. Alternating current (AC)

AC machines less expensive to purchase and

operate, but generally restricted to ferrous

metals

DC equipment can be used on all metals and

is generally noted for better arc control


Resistance Welding (RW) A group of fusion welding processes that use a

combination of heat and pressure to accomplish

coalescence

Heat generated by electrical resistance to current flow at

junction to be welded

Principal RW process is resistance spot welding (RSW)

Fusion Welding – Resistance Welding

(Cont.)

Figure 31.12 Resistance

welding, showing the

components in spot

welding, the main

process in the RW

group.

Fusion Welding – Resistance Welding (Cont.)


(Cont.)

Components in Resistance Spot Welding

Parts to be welded (usually sheet metal)

Two opposing electrodes

Means of applying pressure to squeeze parts between

electrodes

Power supply from which a controlled current can be

applied for a specified time duration


(Cont.)

Advantages:

No filler metal required

High production rates possible

Lends itself to mechanization and automation

Lower operator skill level than for arc welding

Good repeatability and reliability

Disadvantages:

High initial equipment cost

Limited to lap joints for most RW processes

Skilled operators are required

Bigger job thickness cannot be welded

Fusion Welding – Resistance

Welding (Cont.)

Applications of resistance welding

Joining sheets, bars and tubes.

Making tubes and metal furniture.

Welding aircraft and automobile parts.

Making cutting tools.

Making fuel tanks of cars, tractors etc.

Making wire fabrics, grids, grills, mesh weld, containers etc.

Fusion Welding - Resistance Spot

Welding (RSW)

Resistance Spot Welding

Process in which fusion of faying surfaces of a lap joint is

achieved at one location by opposing electrodes

Used to join sheet metal parts using a series of spot welds

Widely used in mass production of automobiles,

appliances, metal furniture, and other products made of

sheet metal

Typical car body has ~ 10,000 spot welds

Annual production of automobiles in the world is

measured in tens of millions of units

Figure 31.13 (a) Spot welding cycle, (b) plot of squeezing force & current in cycle (1) parts inserted between electrodes, (2) electrodes close, force applied, (3) current on, (4) current off, (5) electrodes opened.

Fusion Welding - Resistance Spot

Welding (RSW) (Cont.)

Fusion Welding - Resistance Seam

Welding (RSEW)

Resistance Seam Welding (RSEW)

Uses rotating wheel electrodes to produce a

series of overlapping spot welds along lap joint

Can produce air-tight joints

Applications:

Gasoline tanks

Automobile mufflers

Various other sheet metal containers

Figure 31.15 Resistance seam welding (RSEW).

Fusion Welding - Resistance Seam

Welding (RSEW)

Fusion Welding - Oxyfuel Gas Welding

(OFW)

Oxyfuel Gas Welding

General term for welding operations that burn various fuels

mixed with oxygen

OFW employs several types of gases, which is the primary

distinction among the members of this group

Oxyfuel gas is also used in flame cutting torches to cut and

separate metal plates and other parts

Fusion Welding - Oxyfuel Gas Welding

(OFW)

Alternatives Fuel Gases for OFW

Acetylene

Gasoline

Hydrogen

MPS and MAPP gas

Propylene and Fuel Gas

Butane, propane and butane/propane mixes

Fusion Welding - Oxy-acetylene gas welding

(OAW)

Oxy-acetylene gas welding (OAW)

Oxy-acetylene gas welding is a group OFW process that

used acetylene gas as a fuel gas

Most popular fuel among OFW group because it is capable of

higher temperatures than any other - up to 3480C

(6300F)

Fusion welding performed by a high temperature flame from

combustion of acetylene and oxygen

Flame is directed by a welding torch

Filler metal is sometimes added

Composition must be similar to base metal

Filler rod often coated with flux to clean surfaces and prevent

oxidation

Figure 31.21 A typical oxyacetylene welding operation (OAW).

Fusion Welding - Oxy-acetylene

gas welding (OAW)

Fusion Welding - Oxy-acetylene gas

welding (OAW)

Chemical reaction during burning

Two stage chemical reaction of acetylene and

oxygen:

First stage reaction:

C2H2 + O2 2CO + H2 + heat

Second stage reaction:

2CO + H2 + 1.5O2 2CO2 + H2O + heat

Oxyacetylene Torch

Maximum temperature reached at tip of inner cone,

while outer envelope spreads out and shields work

surfaces from atmosphere

Figure 31.22 The neutral flame from an oxyacetylene torch

indicating temperatures achieved.

Fusion Welding - Oxy-acetylene

gas welding (OAW)

Fusion Welding - Other Processes

FW processes that cannot be classified as arc,

resistance, or oxyfuel welding

Use unique technologies to develop heat for

melting

Applications are typically unique

Processes include:

Electron beam welding

Laser beam welding

Electroslag welding

Thermit welding

Fusion Welding - Thermit Welding (TW)

Thermit Welding (TW)

FW process in which heat for coalescence is produced

by superheated molten metal from the chemical reaction

of thermite

Thermite = mixture of Al and Fe3O4 fine powders that

produce an exothermic reaction when ignited

Also used for incendiary bombs

Filler metal obtained from liquid metal

Process used for joining, but has more in common with

casting than welding

Figure 31.25 Thermit welding: (1) Thermit ignited; (2) crucible

tapped, superheated metal flows into mold; (3) metal solidifies to

produce weld joint.

Fusion Welding - Thermit

Welding (TW)

Fusion Welding - Thermit Welding (TW)

Applications

Joining of railroad rails

Repair of cracks in large steel castings and forgings

Weld surface is often smooth enough that no finishing is

required

Solid State Welding (SSW)

Solid State Welding (SSW)

Coalescence of part surfaces is achieved by:

Pressure alone, or

Heat and pressure

If both heat and pressure are used, heat is not enough to melt

work surfaces

For some SSW processes, time is also a factor

No filler metal is added

Each SSW process has its own way of creating a bond at the

faying surfaces

Essential factors for a successful solid state weld are that the

two faying surfaces must be:

Very clean

In very close physical contact with each other to permit atomic bonding

Solid State Welding (Cont)

SSW Advantages over FW Processes

If no melting, then no heat affected zone, so metal

around joint retains original properties

Many SSW processes produce welded joints that bond

the entire contact interface between two parts rather

than at distinct spots or seams

Some SSW processes can be used to bond dissimilar

metals, without concerns about relative melting points,

thermal expansions, and other problems that arise in FW

Solid State Welding (Cont.)

Processes under SSW group

Forge welding

Cold welding

Roll welding

Hot pressure welding

Diffusion welding

Explosion welding

Friction welding

Ultrasonic welding

Solid State Welding - Forge Welding

Forge Welding

SSW process in which

components to be joined are

heated to hot working temperature

range and then forged together by

hammering or similar means

Historic significance in

development of manufacturing

technology

Process dates from about

1000 B.C., when blacksmiths

learned to weld two pieces of

metal

Solid State Welding - Cold Welding

(CW)

Cold Welding (CW)

SSW process done by applying high

pressure between clean contacting

surfaces at room temperature

Cleaning usually done by degreasing

and wire brushing immediately before

joining

No heat is applied, but deformation

raises work temperature

At least one of the metals, preferably

both, must be very ductile

Soft aluminum and copper suited

to CW

Applications: making electrical

connections

Dies

WorkpieceWorkpiece

Before welding

After welding

Solid State Welding - Roll Welding

(ROW)

SSW process in which pressure sufficient to cause

coalescence is applied by means of rolls, either with or

without external heat

Variation of either forge welding or cold welding, depending

on whether heating of workparts is done prior to process

If no external heat, called cold roll welding

If heat is supplied, hot roll welding

Solid State Welding - Roll Welding

(ROW)

Applications

Cladding stainless steel to mild or low alloy steel for

corrosion resistance

Bimetallic strips for measuring temperature

"Sandwich" coins for U.S mint

Solid State Welding - Diffusion Welding

(DFW)

Diffusion Welding

SSW process that uses heat

and pressure, usually in a

controlled atmosphere, with

sufficient time for diffusion and

coalescence to occur

Temperatures 0.5 Tm

Plastic deformation at surfaces

is minimal

Primary coalescence

mechanism is solid state

diffusion

Limitation: time required for

diffusion can range from

seconds to hours

Work pieces

Schematic representation ofdiffusion welding using

electrical resistance for heating

A

B

Force

Solid State Welding - Diffusion Welding

(DFW)

DFW Applications Joining of high-strength and refractory metals in

aerospace and nuclear industries

Can be used to join either similar and dissimilar metals

For joining dissimilar metals, a filler layer of different

metal is often sandwiched between base metals to

promote diffusion

Solid State Welding - Explosion

Welding (EXW)

Explosion Welding (EXW)

SSW process in which rapid coalescence of two metallic

surfaces is caused by the energy of a detonated

explosive

No filler metal used

No external heat applied

No diffusion occurs - time is too short

Bonding is metallurgical, combined with mechanical

interlocking that results from a rippled or wavy interface

between the metals

Commonly used to bond two dissimilar metals, in

particular to clad one metal on top of a base metal over

large areas

Figure 31.27 Explosive welding (EXW): (1) setup in the

parallel configuration, and (2) during detonation of the

explosive charge.

Solid State Welding -

Explosion Welding (EXW)

Solid State Welding - Friction Welding

(FRW)

SSW process in which coalescence is achieved

by frictional heat combined with pressure

When properly carried out, no melting occurs at

faying surfaces

No filler metal, flux, or shielding gases normally

used

Process yields a narrow HAZ

Can be used to join dissimilar metals

Widely used commercial process, amenable to

automation and mass production

Figure 31.28 Friction welding (FRW): (1) rotating part, no contact; (2)

parts brought into contact to generate friction heat; (3) rotation

stopped and axial pressure applied; and (4) weld created.

Solid State Welding -

Friction Welding (FRW)


(FRW)

Two Types of Friction Welding

1. Continuous-drive friction welding

One part is driven at constant rpm against

stationary part to cause friction heat at

interface

At proper temperature, rotation is stopped

and parts are forced together

2. Inertia friction welding

Rotating part is connected to flywheel,

which is brought up to required speed

Flywheel is disengaged from drive, and

parts are forced together


(FRW)

Applications:

Shafts and tubular parts

Industries: automotive, aircraft, farm equipment,

petroleum and natural gas

Limitations:

At least one of the parts must be rotational

Flash must usually be removed

Upsetting reduces the part lengths (which must be

taken into consideration in product design)

Solid State Welding - Ultrasonic

Welding (USW)

Two components are held together, oscillatory

shear stresses of ultrasonic frequency are applied

to interface to cause coalescence

Oscillatory motion breaks down any surface films

to allow intimate contact and strong metallurgical

bonding between surfaces

Although heating of surfaces occurs,

temperatures are well below Tm

No filler metals, fluxes, or shielding gases

Generally limited to lap joints on soft materials

such as aluminum and copper

Figure 31.29 Ultrasonic welding (USW): (a) general setup for

a lap joint; and (b) close-up of weld area.

Ultrasonic Welding

Solid State Welding - Ultrasonic

Welding

Applications Wire terminations and splicing in electrical and

electronics industry

Eliminates need for soldering

Assembly of aluminum sheet metal panels

Welding of tubes to sheets in solar panels

Assembly of small parts in automotive industry

Weld Quality

Concerned with obtaining an acceptable

weld joint that is strong and absent of

defects, and the methods of inspecting and

testing the joint to assure its quality

Topics:

Residual stresses and distortion

Welding defects

Inspection and testing methods

Weld Quality

Residual Stresses and Distortion Rapid heating and cooling in localized regions during

FW result in thermal expansion and contraction that

cause residual stresses

These stresses, in turn, cause distortion and warpage

Situation in welding is complicated because:

Heating is very localized

Melting of base metals in these regions

Location of heating and melting is in motion (at

least in AW)

Weld Quality

Techniques to Minimize Warpage Welding fixtures to physically restrain parts

Heat sinks to rapidly remove heat

Tack welding at multiple points along joint to create a

rigid structure prior to seam welding

Selection of welding conditions (speed, amount of filler

metal used, etc.) to reduce warpage

Preheating base parts

Stress relief heat treatment of welded assembly

Proper design of weldment

Weld Quality - Welding

Defect/Imperfection

Imperfection is any deviation from the ideal weld.

Defect is an unacceptable imperfection

A perfect weld joint, when subjected to an external force,

provide a distribution of stress throughout its volume which is

not significantly greater than parent metal.


Defect/Imperfection

Cracks Fracture-type interruptions either in weld or in base metal

adjacent to weld

Serious defect because it is a discontinuity in the metal that

significantly reduces strength

Caused by embrittlement or low ductility of weld and/or base

metal combined with high restraint during contraction

In general, this defect must be repaired


Defect/Imperfection

Cavities

1. Porosity - small voids in weld metal formed by gases entrapped

during solidification. Caused by inclusion of atmospheric

gases, sulfur in weld metal, or surface contaminants

2. Shrinkage voids - cavities formed by shrinkage during

solidification. Cause by terminated arc at the end of a weld run

1 2


Defect/Imperfection

Solid inclusions - nonmetallic material entrapped in

weld metal

Most common form is slag inclusions generated during AW

processes that use flux

Instead of floating to top of weld pool, globules of slag become

encased during solidification

Metallic oxides that form during welding of certain metals such

as aluminum, which normally has a surface coating of Al2O3

Incomplete Fusion

Also known as lack of fusion, it is simply a weld bead in which

fusion has not occurred throughout entire cross section of joint


Defect/Imperfection


Defect/Imperfection

o Lack of Smoothly Blended Surfaces


Defect/Imperfection

o Miscellaneous defect

Misalignment Arc strikes

Spatter Burn Through

Inspection and Testing Methods

Visual inspection

Nondestructive evaluation

Destructive testing

Inspection and Testing Methods –

Visual Inspection

Visual Inspection

Most widely used welding inspection method

Human inspector visually examines for:

Conformance to dimensions

Warpage

Cracks, cavities, incomplete fusion, and

other surface defects

Limitations:

Only surface defects are detectable

Welding inspector must also determine if

additional tests are warranted


Nondestructive Evaluation (NDE) Tests

Nondestructive Evaluation (NDE) Tests

Ultrasonic testing - high frequency sound waves

directed through specimen - cracks, inclusions are

detected by loss in sound transmission

Radiographic testing - x-rays or gamma radiation

provide photograph of internal flaws

Dye-penetrant and fluorescent-penetrant tests -

methods for detecting small cracks and cavities that

are open at surface

Magnetic particle testing – iron filings sprinkled on

surface reveal subsurface defects by distorting

magnetic field in part


Destructive Testing

Destructive Testing

Tests in which weld is destroyed either during testing or

to prepare test specimen

Mechanical tests - purpose is similar to conventional

testing methods such as tensile tests, shear tests, etc

Metallurgical tests - preparation of metallurgical

specimens (e.g., photomicrographs) of weldment to

examine metallic structure, defects, extent and condition

of heat affected zone, and similar phenomena

Weldability Factors - Welding Process

Welding Process

Some metals or metal combinations can be readily

welded by one process but are difficult to weld by

others

Example: stainless steel readily welded by most AW

and RW processes, but difficult to weld by OFW

Weldability Factors – Base Metal

Base Metal

Some metals melt too easily; e.g., aluminum

Metals with high thermal conductivity transfer heat

away from weld, which causes problems; e.g., copper

High thermal expansion and contraction in metal

causes distortion problems

Dissimilar metals pose problems in welding when

their physical and/or mechanical properties are

substantially different

Weldability Factors - Other Factors

Filler metal

Must be compatible with base metal(s)

In general, elements mixed in liquid state that

form a solid solution upon solidification will

not cause a problem

Surface conditions

Moisture can result in porosity in fusion zone

Oxides and other films on metal surfaces can

prevent adequate contact and fusion

Design Considerations in Welding

Design for welding - product should be designed

from the start as a welded assembly, and not as a

casting or forging or other formed shape

Minimum parts - welded assemblies should

consist of fewest number of parts possible

Example: usually more cost efficient to perform

simple bending operations on a part than to

weld an assembly from flat plates and sheets

Arc Welding Design Guidelines

Good fit-up of parts - to maintain dimensional

control and minimize distortion

Machining is sometimes required to achieve

satisfactory fit-up

Assembly must allow access for welding gun to

reach welding area

Design of assembly should allow flat welding to

be performed as much as possible, since this

is fastest and most convenient welding position

Figure 31.35 Welding positions (defined here for groove welds): (a) flat, (b) horizontal, (c) vertical, and (d) overhead.

Flat welding is best position

Overhead welding is most difficult

Arc Welding Positions

BRAZING, SOLDERING, AND

ADHESIVE BONDING

1. Brazing

2. Soldering

3. Adhesive Bonding

Overview of Brazing and Soldering

Both use filler metals to permanently join metal

parts, but there is no melting of base metals

When to use brazing or soldering instead of

fusion welding:

Metals have poor weldability

Dissimilar metals are to be joined

Intense heat of welding may damage

components being joined

Geometry of joint not suitable for welding

High strength is not required

Overview of Adhesive Bonding

Uses forces of attachment between a filler

material and two closely-spaced surfaces to bond

the parts

Filler material in adhesive bonding is not

metallic

Joining process can be carried out at room

temperature or only modestly above

Brazing

Joining process in which a filler metal is melted

and distributed by capillary action between faying

surfaces of metal parts being joined

No melting of base metals occurs

Only the filler melts

Filler metal Tm greater than 450C (840F) but

less than Tm of base metal(s) to be joined

Strength of Brazed Joint

If joint is properly designed and brazing operation

is properly performed, solidified joint will be

stronger than filler metal out of which it was

formed

Why?

Small part clearances used in brazing

Metallurgical bonding that occurs between

base and filler metals

Geometric constrictions imposed on joint by

base parts

Brazing Compared to Welding

Any metals can be joined, including dissimilar

metals

Can be performed quickly and consistently,

permitting high production rates

Multiple joints can be brazed simultaneously

Less heat and power required than FW

Problems with HAZ in base metal are reduced

Joint areas that are inaccessible by many welding

processes can be brazed; capillary action draws

molten filler metal into joint

Disadvantages and Limitations of

Brazing

Joint strength is generally less than a welded joint

Joint strength is likely to be less than the base

metals

High service temperatures may weaken a brazed

joint

Color of brazing metal may not match color of

base metal parts, a possible aesthetic

disadvantage

Brazing Applications

Automotive (e.g., joining tubes and pipes)

Electrical equipment (e.g., joining wires and

cables)

Cutting tools (e.g., brazing cemented carbide

inserts to shanks)

Jewelry

Chemical process industry

Plumbing and heating contractors join metal pipes

and tubes by brazing

Repair and maintenance work

Brazed Joints

Butt and lap joints common

Geometry of butt joints is usually adapted for

brazing

Lap joints are more widely used, since they

provide larger interface area between parts

Filler metal in a brazed lap joint is bonded to base

parts throughout entire interface area, rather than

only at edges

Figure 32.1 (a) Conventional butt joint, and adaptations of the butt joint for brazing: (b) scarf joint, (c) stepped butt joint, (d) increased cross-section of the part at the joint.

Butt Joints for Brazing

Figure 32.2 (a) Conventional lap joint, and adaptations of the lap joint for brazing: (b) cylindrical parts, (c) sandwiched parts, and (d) use of sleeve to convert butt joint into lap joint.

Lap Joints for Brazing

Some Filler Metals for Brazing

Base metal(s) Filler metal(s)

Aluminum Aluminum and silicon

Nickel-copper alloy Copper

Copper Copper and phosphorous

Steel, cast iron Copper and zinc

Stainless steel Gold and silver

Desirable Brazing Metal Characteristics

Melting temperature of filler metal is compatible

with base metal

Low surface tension in liquid phase for good

wettability

High fluidity for penetration into interface

Capable of being brazed into a joint of adequate

strength for application

Avoid chemical and physical interactions with

base metal (e.g., galvanic reaction)

Figure 32.4 Several techniques for applying filler metal in brazing:

(a) torch and filler rod. Sequence: (1) before, and (2) after.

Applying Filler Metal

Figure 32.4 Several techniques for applying filler metal in brazing:

(b) ring of filler metal at entrance of gap. Sequence: (1) before,

and (2) after.

Applying Filler Metal

Brazing Fluxes

Similar purpose as in welding; they dissolve,

combine with, and otherwise inhibit formation of

oxides and other unwanted byproducts in brazing

process

Characteristics of a good flux include:

Low melting temperature

Low viscosity so it can be displaced by filler

metal

Facilitates wetting

Protects joint until solidification of filler metal

Heating Methods in Brazing

Torch Brazing - torch directs flame against work in

vicinity of joint

Furnace Brazing - furnace supplies heat

Induction Brazing – heating by electrical

resistance to high-frequency current in work

Resistance Brazing - heating by electrical

resistance in parts

Dip Brazing - molten salt or molten metal bath

Infrared Brazing - uses high-intensity infrared

lamp

Soldering

Joining process in which a filler metal with Tm less than or equal to 450C (840F) is melted and distributed by capillary action between faying surfaces of metal parts being joined

No melting of base metals, but filler metal wets and combines with base metal to form metallurgical bond

Soldering similar to brazing, and many of the same heating methods are used

Filler metal called solder

Most closely associated with electrical and electronics assembly (wire soldering)

Soldering Advantages / Disadvantages

Advantages:

Lower energy than brazing or fusion welding

Variety of heating methods available

Good electrical and thermal conductivity in joint

Easy repair and rework

Disadvantages:

Low joint strength unless reinforced by

mechanically means

Possible weakening or melting of joint in elevated

temperature service

Filler metal / Solder

Usually alloys of tin (Sn) and lead (Pb). Both metals have low Tm

Lead is poisonous and its percentage is minimized in most solders

Tin is chemically active at soldering temperatures and promotes wetting action for successful joining

In soldering copper, copper and tin form intermetallic compounds that strengthen bond

Silver and antimony also used in soldering alloys

Figure 32.8 Techniques for securing the joint by mechanical means prior to soldering in electrical connections: (a) crimped lead wire on PC board; (b) plated through-hole on PC board to maximize solder contact surface; (c) hooked wire on flat terminal; and (d) twisted wires.

Mechanical Means to Secure Joint

Functions of Soldering Fluxes

Be molten at soldering temperatures

Remove oxide films and tarnish from base part

surfaces

Prevent oxidation during heating

Promote wetting of faying surfaces

Be readily displaced by molten solder during

process

Leave residue that is non-corrosive and

nonconductive

Soldering Methods

Many soldering methods same as for brazing,

except less heat and lower temperatures are

required

Additional methods:

Hand soldering – manually operated soldering gun

Wave soldering – soldering of multiple lead

wires in printed circuit cards

Reflow soldering –used for surface mount

components on printed circuit cards

Figure 32.9 Wave soldering, in which molten solder is delivered up through a narrow slot onto the underside of a printed circuit board to connect the component lead wires.

Wave Soldering

Adhesive Bonding

Joining process in which a filler material is used to

hold two (or more) closely-spaced parts together

by surface attachment

Used in a wide range of bonding and sealing

applications for joining similar and dissimilar

materials such as metals, plastics, ceramics,

wood, paper, and cardboard

Considered a growth area because of

opportunities for increased applications

Adhesive Bonding - Terminology

Adhesive = filler material, nonmetallic, usually a

polymer

Adherends = parts being joined

Structural adhesives – of greatest interest in

engineering, capable of forming strong,

permanent joints between strong, rigid adherends

Curing in Adhesive Bonding

Process by which physical properties of the

adhesive are changed from liquid to solid, usually

by chemical reaction, to accomplish surface

attachment of parts

Curing often aided by heat and/or a catalyst

If heat used, temperatures are relatively low

Curing takes time - a disadvantage in production

Pressure sometimes applied between parts to

activate bonding process

Adhesive Bonding - Joint Strength

Depends on strength of:

Adhesive

Attachment between adhesive and adherends

Attachment mechanisms:

Chemical bonding – adhesive and adherend form primary bond on curing

Physical interactions - secondary bonding forces between surface atoms

Mechanical interlocking - roughness of adherend causes adhesive to become entangled in surface asperities

Adhesive Bonding - Joint Design

Adhesive joints are not as strong as welded,

brazed, or soldered joints

Joint contact area should be maximized

Adhesive joints are strongest in shear and tension

Joints should be designed so applied stresses

are of these types

Adhesive bonded joints are weakest in cleavageor peeling

Joints should be designed to avoid these types

of stresses

Figure 32.10 Types of stresses that must be

considered in adhesive bonded joints: (a) tension,

(b) shear, (c) cleavage, and (d) peeling.

Types of Stresses in Adhesive Bonding

Figure 32.11 Some joint designs for adhesive bonding: (a) through

(d) butt joints; (e) through (f) T-joints; (b) and (g) through (j)

corner joints.

Joint Designs in Adhesive Bonding

Adhesive Types

Natural adhesives - derived from natural sources,

including gums, starch, dextrin, soya flour,

collagen

Low-stress applications: cardboard cartons,

furniture, bookbinding, plywood

Inorganic - based principally on sodium silicate

and magnesium oxychloride

Low cost, low strength

Synthetic adhesives - various thermoplastic and

thermosetting polymers

Synthetic Adhesives

Most important category in manufacturing

Synthetic adhesives cured by various

mechanisms:

Mixing catalyst or reactive ingredient with

polymer prior to applying

Heating to initiate chemical reaction

Radiation curing, such as UV light

Curing by evaporation of water

Application as films or pressure-sensitive

coatings on surface of adherend

Applications of Adhesives

Automotive, aircraft, building products,

shipbuilding

Packaging industries

Footwear

Furniture

Bookbinding

Electrical and electronics

Surface Preparation

For adhesive bonding to succeed, part

surfaces must be extremely clean

Bond strength depends on degree of

adhesion between adhesive and adherend,

and this depends on cleanliness of surface

For metals, solvent wiping often used for

cleaning, and abrading surface by

sandblasting improves adhesion

For nonmetallic parts, surfaces are

sometimes mechanically abraded or

chemically etched to increase roughness

Application Methods

Manual brushing and rolling

Silk screening

Flowing, using manually operated dispensers

Spraying

Automatic applicators

Roll coating

Adhesive is dispensed

by a manually

controlled dispenser to

bond parts during

assembly (photo

courtesy of EFD Inc.).

Advantages of Adhesive Bonding

Applicable to a wide variety of materials

Bonding occurs over entire surface area of joint

Low temperature curing avoids damage to parts

being joined

Sealing as well as bonding

Joint design is often simplified, e.g., two flat

surfaces can be joined without providing special

part features such as screw holes

Limitations of Adhesive Bonding

Joints generally not as strong as other joining

methods

Adhesive must be compatible with materials being

joined

Service temperatures are limited

Cleanliness and surface preparation prior to

application of adhesive are important

Curing times can limit production rates

Inspection of bonded joint is difficult

©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing3/e

Mechanical Assembly Technology

1. Threaded Fasteners

2. Rivets and Eyelets

3. Assembly Methods Based on Interference Fits

4. Other Mechanical Fastening Methods

5. Molding Inserts and Integral Fasteners

6. Design for Assembly


Mechanical Assembly Defined

Use of various fastening methods to mechanically attach two or more parts together

In most cases, discrete hardware components, called fasteners, are added to the parts during assembly

In other cases, fastening involves shaping or reshaping of a component, and no separate fasteners are required


Products of Mechanical Assembly

Many consumer products are assembled largely by mechanical fastening methods

Examples: automobiles, large and small appliances, telephones

Many capital goods products are assembled using mechanical fastening methods

Examples: commercial airplanes, trucks, railway locomotives and cars, machine tools


Two Major Types of Mechanical

Assembly

1. Methods that allow for disassembly

Example: threaded fasteners

2. Methods that create a permanent joint

Example: rivets

Why Use Mechanical Assembly?

Low cost

Ease of manufacturing

Easy in creating design

Ease of assembly – can be accomplished with relatively ease by unskilled workers

Minimum of special tooling required

In a relatively short time

Ease of disassembly – at least for the methods that permit disassembly

Some disassembly is required for most


Threaded Fasteners

Discrete hardware components that have external or internal threads for assembly of parts

Most important category of mechanical assembly

In nearly all cases, threaded fasteners permit disassembly

Common threaded fastener types are screws, bolts, and nuts


Screws, Bolts, and Nuts

Screw - externally threaded fastener generally assembled into a blind threaded hole

Bolt - externally threaded fastener inserted into through holes and "screwed" into a nut on the opposite side

Nut - internally threaded fastener having standard threads that match those on bolts of the same diameter, pitch, and thread form

©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

Figure 33.1 Typical assemblies when screws and boltsare used.

Screws, Bolts, and Nuts


Some Facts About Screws and

Bolts

Screws and bolts come in a variety of sizes, threads, and shapes

Much standardization in threaded fasteners, which promotes interchangeability

U.S. is converting to metric, further reducing variations

Differences between threaded fasteners affect tooling

Example: different screw head styles and sizes require different screwdriver designs


Figure 33.2 Various head styles available on screws and bolts.

Head Styles on Screws and Bolts


Types of Screws

Greater variety than bolts, since functions vary more

Examples:

Machine screws - generic type, generally designed for assembly into tapped holes

Cap screws - same geometry as machine screws but made of higher strength metals and to closer tolerances


Hardened and designed for assembly functions such as fastening collars, gears, and pulleys to shafts

Figure 33.3 (a) Assembly of collar to shaft using a setscrew;

(b) various setscrew geometries (head types and points).

Setscrews


Designed to form or cut threads in a pre-existing hole into which it is being turned

Also called a tapping screw

Figure 33.4 Self-tapping

screws: thread-forming,

and thread-cutting.

Self-Tapping Screws


Screw Thread Inserts

Internally threaded plugs or wire coils designed to be inserted into an unthreaded hole and accept an externally threaded fastener

Assembled into weaker materials to provide strong threads

Upon assembly of screw into insert, insert barrel expands into hole to secure the assembly


Figure 33.6 Screw thread inserts: (a) before insertion, and (b) after insertion into hole and screw is turned into insert.

Screw Thread Inserts


Washer

Hardware component often used with threaded fasteners to ensure tightness of the mechanical joint

Simplest form = flat thin ring of sheet metal

Functions:

Distribute stresses

Provide support for large clearance holes

Protect part surfaces and seal the joint

Increase spring tension

Resist inadvertent unfastening


Figure 33.8 Types of washers: (a) plain (flat) washers; (b) spring

washers, used to dampen vibration or compensate for wear; and

(c) lock washer designed to resist loosening of the bolt or screw.

Washer Types


Bolt Strength

Two measures:

Tensile strength, which has the traditional definition

Proof strength - roughly equivalent to yield strength

Maximum tensile stress without permanent deformation


Figure 33.9 Typical stresses acting on a bolted joint.

Stresses in a Bolted Joint


Over-tightening in Bolted Joints

Potential problem in assembly, causing stresses that exceed strength of fastener or nut

Failure can occur in one of the following ways:

1. Stripping of external threads

2. Stripping of internal threads

3. Bolt fails due to excessive tensile stresses on cross-sectional area

Tensile failure of cross section is most common problem


Methods to Apply Required Torque

1. Operator feel - not very accurate, but adequate for most assemblies

2. Torque wrench – indicates amount of torque during tightening

3. Stall-motor - motorized wrench is set to stall when required torque is reached

4. Torque-turn tightening - fastener is initially tightened to a low torque level and then rotated a specified additional amount


Rivets

Unthreaded, headed pin used to join two or more parts by passing pin through holes in parts and forming a second head in the pin on the opposite side

Widely used fasteners for achieving a permanent mechanically fastened joint

Clearance hole into which rivet is inserted must be close to the diameter of the rivet


Figure 33.10 Five basic rivet types, also shown in assembled

configuration: (a) solid, (b) tubular, (c) semi-tubular, (d) bifurcated,

and (e) compression.

Types of Rivets


Applications and Advantages of

Rivets

Used primarily for lap joints

A primary fastening method in aircraft and aerospace industries

Advantages:

High production rates

Simplicity

Dependability

Low cost


Tooling and Methods for Rivets

1. Impact - pneumatic hammer delivers a succession of blows to upset rivet

2. Steady compression - riveting tool applies a continuous squeezing pressure to upset rivet

3. Combination of impact and compression


Interference Fits

Assembly methods based on mechanical interference between two mating parts being joined

The interference, either during assembly or after joining, holds the parts together

Interference fit methods include:

Press fitting

Shrink and expansion fits

Snap fits

Retaining rings


Snap FitsJoining of two parts in which

mating elements possess a temporary interference during assembly, but once assembled they interlock

During assembly, one or both parts elastically deform to accommodate temporary interference

Usually designed for slight interference after assembly

Originally conceived as a method ideally suited for industrial robots

Eureka! – it’s easier for humans too


Figure 33.13 Snap fit assembly, showing cross-sections of two

mating parts: (1) before assembly, and (2) parts snapped together.

Snap Fit Assembly


Retaining Ring

Fastener that snaps into a circumferential groove on a shaft or tube to form a shoulder

Used to locate or restrict movement of parts on a shaft

Figure 33.14 Retaining ring assembled into a groove


Design for Assembly (DFA)

Keys to successful DFA:

1. Design product with as few parts as possible

2. Design remaining parts so they are easy to assemble

Assembly cost is determined largely in product design, when the number of components in the product and how they are assembled is decided

Once these decisions are made, little can be done in manufacturing to reduce assembly costs


DFA Guidelines

Use modularity in product design

Each subassembly should have a maximum of 12 or so parts

Design the subassembly around a base part to which other components are added

Reduce the need for multiple components to be handled at once


More DFA Guidelines

Limit the required directions of access

Adding all components vertically from above is the ideal

Use high quality components

Poor quality parts jams feeding and assembly mechanisms

Minimize threaded fasteners

Use snap fit assembly

topic 7 joining process welding brazing soldering fastening 160214

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