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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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