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MEL207 Manufacturing Processes-I
Joining
Joining and Assembly Defined
Joining - welding, brazing, soldering, and adhesive bonding
These processes form a permanent joint between parts
Assembly - mechanical methods (usually) of fastening parts together
Some of these methods allow for easy disassembly, while others do not
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Welding
Joining process in which two parts are coalesced at their contacting surfaces by application of heat and/or pressure
Many welding processes are accomplished by heat alone
Others by a combination of heat and pressure
Still others by pressure alone with no external heat
In some welding processes a filler material is added to facilitate coalescence
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Provides a permanent joint
Usually the most economical way to join components in terms of material usage and fabrication costs
Not restricted to a factory environment
__________
Manual operated and expensive in terms of labor cost
Utilize high energy and are inherently dangerous
Welded joints do not allow for convenient disassembly
Welded joints -defects that are difficult to detect
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Advantages & Limitations
Welding Processes Fusion welding - melting the two parts, in some cases adding
filler metal to the joint
Arc welding-Electric Arc
Resistance spot welding-surfaces held under pressure
Oxyfuel gas welding-Acetylene
Solid state welding - heat and/or pressure without melting
Ultrasonic: by ultrasonic oscillation motion
Diffusion welding-held under pressure at elevated temperature
Friction welding-heat friction between two surfaces
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Heat Density
Power:
Power density -low, heat into work-melting never occurs
Too high, localized temp . vaporizes metal
Practical range of values in which welding is performed
Power transferred to work per unit surface area
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Comparison
Oxyfuel gas welding -large amounts of heat-heat density is relatively low (heat is spread over a large area)
Oxyacetylene gas-the hottest of the OFW fuels-burns at 3500C
Arc welding -high energy over a smaller area, 5500-6600C
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Fusion Welded Joint Fusion weld joint with added filler metal consists of:
Fusion zone, Weld interface, HAZ, unaffected metal
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HAZ
Metal experiences temperature below melting point-enough to cause micro structural changes
Chemical composition-as base metal, but region is heat treated so that its properties/structure have been altered- effect on mechanical properties in HAZ is usually negative, and it is here that welding failures often occur
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WELDING PROCESSES
Arc Welding
Resistance Welding
Oxyfuel Gas Welding
Other Fusion Welding Processes
Solid State Welding
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Arc Welding A pool of molten metal is formed near electrode tip
As electrode is moved along joint, molten weld pool solidifies
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electrical circuit of an arc welding process
Manual Arc Welding and Arc Time
Problems with manual welding:
Weld joint quality
Productivity
Arc Time = (time arc is on) divided by (hours worked)
Also called arc-on time
Typical values:
Manual welding arc time = 20%
Machine welding improves arc time to ~ 50%
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Types of AW Electrodes
Consumable consumed during welding process
Source of filler metal in arc welding
Nonconsumable not consumed during welding process
Any filler metal must be added separately
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Consumable Electrodes Forms of Consumable Electrodes
Welding rods (also called sticks) are 9 to 18 inches and 3/8 inch or less in diameter and must be changed periodically
Weld wire can be continuously fed from spools with long lengths of wire, avoiding frequent interruptions
Electrode is consumed by arc and added to weld joint as filler metal
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Nonconsumable Electrodes Made of tungsten which resists melting
Gradually depleted during welding (vaporization)
Any filler metal must be supplied by a separate wire
Power Source
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
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AW Processes-Consumable Electrodes Shielded Metal Arc Welding
Gas Metal Arc Welding
Flux-Cored Arc Welding
Electrogas Welding
Submerged Arc Welding
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Shielded Metal Arc Welding (SMAW) Uses a consumable electrode with a filler metal coating with
chemicals that provide flux and shielding- "stick welding"
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Composition of filler metal usually close to base metal
Coating is of powdered cellulose with oxides, carbonates, other ingredients, held together by a silicate binder
Stick -clamped in electrode holder with power source
Disadvantages of stick welding: Periodical changing, higher current may melt coating prematurely
Applications: steels, SS, CI, and nonferrous alloys
Not recommended for Al & alloys, Cu alloys, Ti
Gas Metal Arc Welding (GMAW) Uses a consumable bare metal wire as electrode and
shielding accomplished by flooding arc with a gas
Wire is fed continuously and automatically from a spool through the welding gun
Shielding gases include inert gases such as argon and helium for aluminum welding, and active gases such as CO2 for steel welding
Bare electrode wire plus shielding gases eliminate slag covering on weld bead - no need for manual grinding and cleaning of slag
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Gas metal arc welding (GMAW)
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GMAW Vs SMAW
Better arc time because of continuous wire electrode Sticks must be periodically changed in SMAW
Better use of electrode filler metal than SMAW The end of the stick cannot be used in SMAW
Higher deposition rates
Eliminates problem of slag removal
Can be readily automated
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Gas Tungsten Arc Welding (GTAW)
Tungsten electrode and an inert gas for arc shielding
Melting point of tungsten = 3410C (6170F)
Also called TIG welding (Tungsten Inert Gas welding) In Europe, called "WIG welding"
Used with or without a filler metal When used, filler metal is added to weld pool from separate rod or wire
Applications: aluminum and stainless steel most common
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Adv. & Disadv. of GTAW
Advantages:
High quality welds for suitable applications
No spatter because no filler metal through arc
Little or no post weld cleaning because no flux
Disadvantages:
Generally slower and more costly than consumable electrode AW processes
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Resistance Welding (RW)
A group of fusion welding processes that use a combination of heat and pressure:
Heat generated by electrical resistance to current flow at junction to be welded
Principal RW process = resistance spot welding (RSW)
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Advantages and Drawbacks of RW
Advantages: No filler metal is required
High production rates are possible
Lends itself to mechanization and automation
Operator skill level is lower than for arc welding
Good repeatability and reliability
Disadvantages: High initial equipment cost
Limited to lap joints for most RW processes
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Ultrasonic Welding (USW) SSW process in which two components are held together, and oscillatory
shear stresses of ultrasonic frequency applied:
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 Al & Cu
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USW Applications
Wire terminations and specializing in electrical and electronics industry (eliminates need for soldering)
Assembly of aluminum sheet metal panels
Welding of tubes to sheets in solar panels
Small parts assembly tasks in automotive industry
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Overview of Brazing and Soldering
Both use filler metals to join metal parts, there is no melting of base metal
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 does not lend itself to welding
High strength is not required
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Adv., Disadv. Limitations of Brazing Any metals can be joined, including dissimilar metals
Quick and consistent, permitting high production rates
Multiple joints can be brazed simultaneously
In general, less heat and power required than FW
Problems with HAZ in base metal near joint are reduced
Joint areas that are inaccessible by many welding processes can be brazed, since capillary action draws molten filler metal into joint
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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
Soldering
Joining in which a filler metal with Tm less than or equal to 450C (840F) is melted and distributed between surfaces of metal being joined
No melting of base metals, but filler metal wets and combines with base metal to form metallurgical bond
Details of 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)
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Adv. & Disadv. of Soldering Advantages:
Lower energy input than brazing or fusion welding
Variety of heating methods available
Good electrical and thermal conductivity in joint
Easy to repair and rework
Disadvantages: Low joint strength unless reinforced by mechanically means
Possible weakening or melting of joint in elevated temp.
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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
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Adhesive Types Natural adhesives - from natural sources-gums, starch, dextrin, soya flour
Low-stress applications: cartons, furniture, bookbinding; plywood
Inorganic based on sodium silicate and magnesium oxychloride
Low cost, low strength
Synthetic adhesives - various thermoplastic and thermosetting polymers
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
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Major Classes of Mechanical Assembly
1. Methods that allow for disassembly, ex: threaded fasteners
2. Methods that create a permanent joint, ex: Rivets
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
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Some Facts About Screws and Bolts Screws and bolts come in a variety of sizes, threads, and shapes
There is much standardization in threaded fasteners-interchangeability
Differences between threaded fasteners affect tooling
Example: different screw head styles and sizes require different driver designs
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Various head styles available on screws and bolts
Self-Tapping Screws Designed to form or cut threads in a pre-existing hole
Also called a tapping screw
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(a) thread-forming, and
(b) thread-cutting
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
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Bolt Strength Two measures:
Tensile strength, which has the traditional definition
Proof strength - roughly equivalent to yield strength Maximum tensile stress without permanent deformation
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Rivets Unthreaded, headed pin used to join two or more parts by passing pin
through holes in parts and forming a second head on the opposite side
Widely used for achieving a permanent mechanically fastened joint
Clearance hole into which rivet is inserted must be close to the diameter of the rivet
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5 basic rivet types:
(a) solid
(b) Tubular
(c) Semi tubular
(d) Bifurcated
(e) compression
Rivets Applications and Advantages
Used primarily for lap joints
Example: a primary fastening method in aircraft and aerospace industries
Advantages: High production rates
Simplicity
Dependability
Low cost
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