JOINING PROCESSES An all-inclusive term covering processes such as: Welding Brazing Soldering Adhesive bonding Mechanical fastening
Aug 12, 2015
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)
Fusion Welding
Fusion Welding
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
Fusion Welding – Arc Welding (Cont.)
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
Fusion Welding – Arc Welding (Cont.)
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
Fusion Welding – Arc Welding (Cont.)
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.)
Fusion Welding – Resistance Welding
(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
Fusion Welding – Resistance Welding
(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)
Solid State Welding - Friction Welding
(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
Solid State Welding - Friction Welding
(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.
Weld Quality - Welding
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
Weld Quality - Welding
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
Weld Quality - Welding
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
Weld Quality - Welding
Defect/Imperfection
Weld Quality - Welding
Defect/Imperfection
o Lack of Smoothly Blended Surfaces
Weld Quality - Welding
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
Inspection and Testing Methods –
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
Inspection and Testing Methods –
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
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing3/e
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
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing3/e
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
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing3/e
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
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing3/e
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
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing3/e
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
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing3/e
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
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
Figure 33.2 Various head styles available on screws and bolts.
Head Styles on Screws and Bolts
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing3/e
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
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
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
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
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
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing3/e
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
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing3/e
Figure 33.6 Screw thread inserts: (a) before insertion, and (b) after insertion into hole and screw is turned into insert.
Screw Thread Inserts
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing3/e
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
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing3/e
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
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing3/e
Bolt Strength
Two measures:
Tensile strength, which has the traditional definition
Proof strength - roughly equivalent to yield strength
Maximum tensile stress without permanent deformation
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing3/e
Figure 33.9 Typical stresses acting on a bolted joint.
Stresses in a Bolted Joint
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing3/e
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
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing3/e
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
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing3/e
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
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing3/e
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
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing3/e
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
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing3/e
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
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing3/e
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
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing3/e
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
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing3/e
Figure 33.13 Snap fit assembly, showing cross-sections of two
mating parts: (1) before assembly, and (2) parts snapped together.
Snap Fit Assembly
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing3/e
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
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing3/e
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
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing3/e
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
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing3/e
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