material joining isat 430 module 8. spring 2001isat 430 dr. ken lewis 2 joining technologies joining...

Material Joining

ISAT 430

Module 8

Spring 2001 ISAT 430 Dr. Ken Lewis 2Module 8

Joining Technologies Joining is a many splendored thing.

Welding Arc or melting Resistance or other

Soldering & brazing Mechanical fastening (bolts & nuts). Seaming and crimping Adhesive bonding

All are important for different reasons.


The Joined Vehicle


Why Join? The product may be impossible to make as a single piece. (See

previous slide) It may be more economical to make in pieces

A cooking pot (pot + handle) Complicated products may be needed to taken apart for repair

Sewing machines, cars, hair dryers,…….. Parts may have different functional purposes

Brake systems.

Rotors pads


Why Join2?

Making and assembling in one spot may be impossible The space shuttle Bridges Pianos …


Welding Fusion welding

Use heat to melt the base metals In many operations a filler metal is added to the

molten pool Adds bulk and strength to the joint.

Solid state welding Pressure or heat and pressure cause the coalescence

If heat is used, the temperature is below the melting point of the joined metals

No filler metal is used.


Weld joints


Fusion Welding Oxyfuel gas welding

Uses a fuel gas and oxygen to produce the heat. Arc welding

Heating is accomplished by an electric arc Resistance welding

Heating is accomplished by the passage of an electric current

Others Electron beam and laser welding


Flux One hears the word “FLUX” (Latin == ‘to flow’)

bandied about. All purpose word describing a material which

Protects the materials from unwanted oxidation Helps dissolve and remove oxides Provides a gaseous shield around the weld point

Oxy Acetylene Welding


Oxyfuel welding Most common process uses acetylene, C2H2 as the

fuel. Heat is generated by two chemical reactions:

C2H2 + O2 2CO + H2 + Heat

CO + H2 + O2 CO2 + H2O

Temperatures in these flames can reach 3,300°C (6,000°F)


Oxyacetylene Welding The flame is directed by a

welding torch. A filler metal is sometimes

added Usually coated with a

flux Heat efficiency is low ~0.1

– 0.3 Flame spreading, losses

to the air, etc.


Oxy acetylene flames Neutral flame

Gas:Fuel = 1:1 Useful

Oxidizing flame Excess oxygen Harmful to steel Useful in copper and

copper based alloys Thin film of slag forms

over the molten metal for protection.


Oxy acetylene flames Reducing flame

Excess fuel. Temperature is lower Used for brazing,

soldering. Other fuels,

Hydrogen, propane, butane, natural gas

Not useful, because the heat is low and the flames are reducing.


Fuel Temperatures and Heats.Temperature Heat of Combustion

Fuel °F °C Btu/ft3 MJ/m3

Acetylene (C2H2) 5589 3087 1470 54.8

MAPP1 (C3H4) 5301 2927 5460 91.7

Hydrogen (H2) 4820 2660 325 12.1

Propylene (C3H6) 5250 2900 2400 89.4

Propane (C3H8) 4579 2526 2498 93.1

Natural Gas 4600 2538 1000 37.3

1) Methylacetylene propadiene


Oxy-acetylene welding summary Relatively cheap Well suited to low volume and repair jobs. Rarely used on stock thicker than ¼ inch Rarely mechanized

Thus relies on the skill of the welder for quality.

Arc Welding


Shielded Welding -- overview


Arc Welding A fusion process wherein the coalescence of the

metals is achieved from the heat of an electric arc formed between an electrode and the work.

An electric arc is a discharge of electric current across a gap I a circuit.

It is sustained by the presence of a thermally ionized column of gas (called a plasma).

Temperatures up to 30,000°C (54,000°F) a generated


Arc Welding The clamp completes the

circuit A filler metal may be used.

Two kinds of welding electrode Consumable Non -- consumable


Consumable Electrode Welding Shielded Metal Arc Welding (SMAW)

Consumable electrode coated with chemicals that provide flux and shielding

Sometimes called ‘stick’ welding The filler metal (here the consumable electrode) is

usually very close in composition to the metal being welded.

The coating contains Cellulose, carbonates, a silicate binder Sometime other metals to alloy the joint


Shielded Metal Arc Welding


Consumable Electrode Welding Shielded Metal Arc Welding (SMAW)

The stick is about 12 – 18 inches long Applications

Construction, pipelines, shipbuilding, fabrication job shops

Used for Steels, stainless steels, cast irons

Not used for aluminum and its alloys, or copper and its alloys (energy density is too high)


Consumable Electrode Welding2

Gas Metal Arc Welding Electrode is a consumable bare metal wire

Usually continuous and fed from a spool The work area is protected by flooding it with a gas Wire diameters range from 1/32 to ¼ inch in

diameter


Gas Metal Arc Welding


Gas Metal Arc Welding Shielding Gases

Inert – Ar (Argon), He (Helium) Used for aluminum alloys and stainless steels

Active – CO2

Used for low and medium carbon steels

The combination of bare electrode wire and shielding gas eliminates slag in the work area Negates need for subsequent cleanup


Gas Metal Arc Welding Originally called “MIG” welding (for metal inert gas) Used widely in factory fabrication

Better metal usage (no stubs) Sticks or filler

High deposition rates No slag


Consumable Electrode Welding2

Flux – cored Arc Welding Developed in the 1950’ss Electrode is a continuous consumable tubing Interior of the electrode contains

Flux De-oxidizers Alloy elements


Flux – Cored Arc Welding


Flux – Cored Arc Welding Two versions

Self shielded Core contains flux and ingredients that generate a

shielding gas Gas shielded

Gas is applied externally

Used primarily for welding steels and stainless steels Noted for producing high quality smooth joints


Non-consumable Electrodes Gas Tungsten Arc Welding Known as “TIG” (tungsten inert gas) welding The electrode is W (tungsten)

Tm = 6170°F (3410°C)

Actually it is slowly consumed Shielding gases include Ar, He or a mixture


Gas Tungsten Arc Welding


Gas Tungsten Arc Welding (TIG) Used for a wide variety of metals

can also be used to weld dissimilar metals (but not very well)

Most commonly used for aluminum and stainless steel

For steel Slower and more costly than consumable welding Except for thin sections or where very high quality is

needed

Resistance Welding


Resistance Welding This is a group of fusion welding processes that use

heat and pressure to make the coalescence.

The heat comes from electrical resistance to current flow at the site of the weld.


Resistance Welding In order to obtain a strong

bond in the weld nugget pressure is applied until the current is turned off.

Strength depends on the initial surface condition Smoothness Cleanliness Presence of uniform thin

oxides is not critical


Resistance Welding Currents range from 3,000

to 40,000A Heat generated is given by:

2Q I Rt

Where: I = current (amperes) R = resistance (ohms) T = time of current

(seconds) Q = heat in Joules


Resistance Welding The reason that the current

is so high is because the R is usually so low ~~ 0.0001 ohm

2Q I Rt

Where: I = current (amperes) R = resistance (ohms) T = time of current

(seconds) Q = heat in Joules


A resistance spot welding operation is performed on two pieces of 0.0625 in thick sheet steel using 12,000 A for 0.23 seconds. The electrodes are 0.25 in. in diameter. Resistance is assumed to be 0.0001 ohm and the resulting weld nugget is 025 in. dia. And 0.1 in. thick. The melting energy for this metal is 155 BTU/in3.

What portion of the heat generated was used to form the weld and what portion was merely siphoned off and dissipated into the surrounding metal?

From the equation above: 2Q I Rt

3312 sec 3.141055 sec

BTUQ W BTU

W

2(12,000) (0.0001)(0.23) 3312 secQ W




The volume of the weld nugget is:2

4V d L

33

0.00491 0.761BTU

Q in BTUin

Assuming a disc

2 2 3(0.25) (0.1) 0.004914 4

V d L in

The heat necessary to melt this disc is




The wasted heat is just

3.14 0.761100 76%

3.14waste

BTU BTUQ

BTU


Resistance Spot Welding1. Parts inserted

2. Electrodes close and force is applied

3. Weld time. Current switched on, force maintained

4. Current off, force maintained. Note: current sometimes left on at a reduced level for stress relief (micro anneal

5. Electrodes opened – repeat.


Explosion Welding First, some elimination of a preconceived notion

When something explodes, it does not just go KABLOOIE There are different rates of explosion.

Deflagration The reaction front proceeds at various rates, but at less than the

speed of sound in the deflagrating material. Black powder

Explosion The reaction front proceeds at speeds greater than the speed of

sound in the exploding material Immense pressure build up.


Explosion Welding2

It just seems like it goes KABLOOIE All are fast compared to normal burning

A slow explosion has a detonation rate of ~2,000 m/s This is a heave

A fast explosion will have detonation rates approaching 8,000 m/s. Provides brisance (a slap)


Explosive welding Contact pressures are extremely

high The explosive may be flexible

plastic sheet. Detonation speeds are usually low

being 2400 – 3600 m/s The impact ejects the interface

effectively removing any oxides or surface impurities.

The impact mechanically interlocks the two surfaces


Explosion Welding


Explosive Welding Pieces of up to 20ft x 7 ft have been joined Very useful in cladding work

Cladding tantalum (Ta) to ordinary steel gives the inertness of Ta with the economy of steel.

Requires well trained personnel.

Brazing and Soldering


Brazing A process which a filler metal is placed at or between

the faying surfaces, the temperature is raised high enough to melt the filler metal but not the base metal. The molten metal fills the spaces by capillary

attraction. Two types

Ordinary brazing (above) Braze welding (similar to oxy-welding)

Faying surfaces = the surfaces to be joined.


Filler Metals for BrazingBase Metal Filler Metal Brazing Temperature

°C

Aluminum & its alloys Aluminum – silicon 570 – 620

Magnesium alloys Magnesium – aluminum

580 – 625

Copper & its alloys Copper – phosphorus 700 – 925

Ferrous and Non-ferrous (except Al,

Mg)

Ag & Cu alloys, copper phosphorus

620 – 1150

Fe, Ni, Co base alloys Gold 900 – 1100

Stainless steels, Ni, Co alloys

Nickel - Silver 925 – 1200


Brazing Torch Brazing

Oxy-fuel torch with a carburizing flame First heat the joint then add the filler metal

Requires skilled labor

Manual costs $300.00

Automated costs $50,000


Furnace Brazing Pre-cleaned and preloaded

with brazing metal The filler metal is a

shaped wire Vacuum or neutral

atmosphere furnaces are used for active metals

Note the completeness of the joint.


Brazing Capabilities Typical joints

Dissimilar metals can be assembled with good joint strength.

Shear strength can reach 120 ksi (800 MPa) using alloys containing silver.

Concerns Clearance too small,

metal will not penetrate Clearance to big,

insufficient capillary attraction.

Soldering


Soldering Solders (Latin solidare make solid) melt at a

temperature that his the melting point of the eutectic

Traditionally have been tin / lead

Others now are used. Pb is toxic and environmentally unfriendly


SoldersTin – Lead General purpose

Tin – Zinc Aluminum

Lead – Silver Strength at higher than room temperature

Cadmium – Silver Strength at high temperatures

Zinc – Aluminum Aluminum, corrosion resistance

Tin – Silver Electronics

Tin – Bismuth Electronics


Soldering Used extensively in the electronics industry Soldering temperatures are low Not used in load bearing members Butt joints rarely made If strength is needed, the joint may be mechanically

interlocked


Solder joints Typical joints Note that the starred

examples are mechanically joined first.

Copper and silver are easy

Fe, Al hard to solder because of their tough oxide films.

Adhesive Joints


Adhesive Bonding Joining process whereby a filler material is used to

hold two closely spaced parts together by surface attachment

Filler material (adhesive) Usually non-metal Usually a polymer

Curing Process (usually chemical) whereby the adhesive

physical properties are changed from a liquid to a solid.


Adhesive bonding Used in antiquity

Egyptians used gum Romans used pine tar to caulk their ships Glues were made from cheese, animal hooves, stag

horns for construction Plywood

Invented in 1905 Wood bonded with a phenol – formaldehyde resin


Adhesive systems


General Properties of some adhesives Acrylic

Thermoplastic, quick setting, tough bond at r.t.

Tennis racquets, metal parts

Epoxy Thermoset, strongest

engineering adhesive Metal, ceramic, rigid

plastic parts

Cyanoacrylate Thermoplastic, touch “Crazy Glue”

Hot Melt Thermoplastic, quick

setting, easy to apply Bonds most anything – Packaging, book

binding, metal can joints


General Properties of some adhesives Phenolic

Thermoset, strong, brittle Brake lining, clutch

pads, honeycomb structures

Silicone Thermoset, slow curing,

flexible, rubber like Gaskets sealants

Water base Animal Vegetable Rubbers

Inexpensive, non-toxic Wood, paper, fabric Leather, dry seal

envelopes


Joint Design Usually not as strong as welding or brazing joints Design principles

Maximize joint contact area Joints are strongest in shear and or tension so joints

should be designed to accommodate this Joints are weakest in cleavage or peel. Avoid these

stresses


Joint Peel

Brittle Tough


Buttjoints

Tjoints

Cornerjoints

Acceptable Joints


Adhesive bonding Advantages It provides a surface bond It distributes the load at the interface

Eliminates high stress areas caused by bolts, etc. Very thin and fragile components can be bonded without

significant increase in weight. Usually a near ambient temperature bond so distortion is

eliminated. Some adhesives are flexible after bonding Sealing as well as bonding can be achieved Joint design is often simplified


Adhesive bonding Disadvantages Joints are not as strong Adhesive must be compatible with materials being

joined Service temperatures are limited Cleanliness and surface preparation prior to adhesive

application are important Curing times can impose a limit on production rates Inspection of the bonded joint is limited.

material joining isat 430 module 8. spring 2001isat 430 dr. ken lewis 2 joining technologies joining...

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