Welding

Welding / Assembly Joining Process

What is assembly?

joining elements together, which shapes a final product. Assembly process can be made by hu-man workers (uneducated but skilled) or by spe-cialized machines and robots.

Example: Cars, computers, engines, celphone etc.

IntroductionAspect of manufacturing:

1.Impossible to manufacture as a single producte.g chairs, computer, etc.

2.More economical to manufacture as individual components, which are then assembled e.g bicycle

3.For maintenance or replacement purposes e.g. car accessories and engines.

4.Different materials due to different properties requirement e.g. cooking pots and pans.

5.Ease and less costly of transportatione.g. Bicycle

Joining Processes

1. Welding2. Soldering and Brazing3. Mechanical Fastening4. Adhesive Bonding

But our presentation will focus on welding

Welding

Fusion Welding Solid State

Welding

Fusion WeldingOxyfuel-

Gas Weld-ing

Pressure-Gas Weld-

ing

Arc WeldingElectron-

Beam Weld-ing

Laser-Beam Welding

Consumable Electrode

Non Con-sumable electrode

- Gas tungsten-arc welding- Plasma-arc welding

- Atomic Hydrogen Welding

- Shielded metal-arc welding-Submerged-arc welding-Gas Metal-arc welding-Flux cored-arc welding-Electrogas welding- Electroslag welding

Solid State WeldingRoll Bond-

ing/ Weld-ing

Cold Weld-ing

Ultrasonic Welding

Resistance Welding

Friction Welding

Explosion Welding

Diffusion Welding

-Inertia friction welding- Linear friction welding- Friction stir welding

-Resistance spot welding- Resistance seam welding- High-frequency resistance welding- Resistance projection welding- Flash welding- Stud welding- Percussion welding

Fusion Welding Process

1) OXYFUEL-GAS WELDING (OFW)

- OFW uses a fuel gas combined with oxygen to produce flame

- Function of the flame - act as a source of the heat to melt the metals at the joint.

- Common gas welding process uses acetylene (oxyacetylene gas welding - OAW).

-Application: structural sheet metal fabrication, automotive bodies, and various repair work.

Fusion Welding Process- OAW process utilizes the heat generated by the com-

bustion of acetylene gas (C2H2)in a mixture of oxygen.- These primary combustion process, occurs in the inner

core of the flame, involves the reaction of:

C2H2+ O2---------->2CO + H2+ Heat (1/3 total heat generated in the flame)

- The secondary combustion process involves further burning of hydrogen and carbon monoxide:

2CO + H2+1.5O2------------>2CO2+ H2O + Heat (2/3 of the total heat)


a. General view of oxy torchb. Cross-section of a torch

used in oxyacetylene welding. The acetelyne valve is opened first; the gas is lit with spark lighter or a pilot light; then the oxygen valve is opened and the flame ad-justed.

c. Basic equipment used in oxyfuel-gas welding. All acetylene fittings are left handed while oxygen are right handed. Oxygen regulators are usually painted green, acetelyne regulators red.

Flame types1. Neutral - ratio 1:1 , no excess oxygen2. Oxidizing - greater oxygen supply (excess oxygen), harmful for steel due to oxidizes. Only suit for nonferrous metal like copper & copper based alloys.3. Carburizing - insuffientof oxygen (excess acetytelene), low temperature, thus suitfor applications requiring low heat like brazing, soldering, flame hardening.

Filler metals

1. To supply additional metal to the weld zone during weld-ing.2. Filler rods or wire and may be coated by flux3. The purpose of flux is to retard oxidation of the welded surfaces.

Oxyacetelene Flames Use in Welding


2) PRESSURE GAS WELDING

- Involved with two components starts by heating the inter-face.- Once when the interface begins to melt, the torch is with-drawn.- A force is applied to press both components together and maintain until the interface solidifies.- The joined end with the occurrence of a flash.

Pressure Gas Welding

3) ARC-WELDING PROCESSES

- In arc welding, the heat is obtained from electrical en-ergy – by using AC or a DC power supply.

- The process involved can be either consumable or non-consumable electrode.

- An arc is produced between the tip of electrode and the work piece which need to be welded.

- The arc produces temperatures approximately 30,000 de-grees celsius.


Arc Welding Processesa.) NON CONSUMABLE ELECTRODE

- The electrode is a tungsten electrode type.

- Need externally supplied shielding gas because of the high temperature involved in order to prevent oxida-tion of the weld zone.

- DC is used and the polarity is important.

- For straight polarity which is also known as direct-cur-rent electrode negative (DCEN); the workpiece is positve (anode) , while the electrode is negative (cathode).

- It will produce welds that are narrow and deep.

- For reverse polarity which is also known as direct-current electrode positive (DECP); the workpiece is negative and electrode positive.

- In this process, weld penetration is less, and the weld zone is shallower and wider.

Arc Welding Processes

Non Consumable Electrode

i) GAS TUNGSTEN-WELDING (GTAW)

- Also known as TIG welding

- Suitable for thin metals.

- This process is expensive because of the cost of inert gas

- Provides welds with very high quality and surface finish

- Filler metal is supplied from a filler wire

- The shielding gas is usually argon or helium

- This filler metals are similar to the metal that need to be welded, and flux is not used.

- In this operation, tungsten electrode is not consumed, therefore a constant and stable arc gap is maintained at a constant current level.

- Power supply either 200A DC or 500A AC; depending on the metals to be welded.

- Generally, AC is suitable for aluminum and magnesium.

- Thorium or zirconium may be used in the tungsten elec-trodes to improve the electron emission characteristics.


- Contamination of the tungsten electrode by molten metal ca cause discontinuities in the weld.

- Therefore, contact between the electrode with the molten metal pool should be avoided.


Gas Tungsten-Arc Welding

ii) PLASMA-ARC WELDING (PAW)

- In this welding operation, a concentrated plasma arc is produced and directed towards the weld area.

- The arc is stable and the temperature can reaches up to 33,000 degrees celsius.

- PAW has less thermal distortion, and higher energy con-centration – permitting deeper and narrower welds.

- Plasma: it is an ionized hot gas composed of nearly equal number of electrons and ions.


- This plasma initiated between the tungsten electrode and the small orifice by a low current pilot arc.

- Operating current: usually below 100A.

- Filler metal is fed into the arc during welding process.

- There are two methods of plasma-arc welding:a) Transferred-arc method - Work piece being welded is part of the electrical

circuit. The arc transfers from the electrode to

the work piece.b) Nontransferred method - The arc occurs between the electrode and the

nozzle. The heat is carried to the workpiece by the plasma gas.

- Welding speeds from 120 to 1000 mm/min.

- Can be welded with part thickness less than 6mm.

Plasma-arc Welding Process

i) SHIELDING METAL-ARC WELDING

- Old method , simplest, held manually.

- Most of all industries and maintenance welding cur-rently performed with this process.

- The electric arc is generated by touching the tip of a coated electrode against the workpiece.

- Need to have a sufficient distance and movement to maintain the arc.


- The heat generated, melts a portion of the electrode tip, its coating, and the base metal in the intermmediate arc area.

- The molten metal consists of a mixture of the base metal (work piece), the electrode metal, and substance from the coating on the electrode; thus this mixture forms the weld when it solidifies.

- The electrode coating deoxidizes the weld area and pro-vides a shielding gas to protect it from oxygen in the envi-ronment.


- The equipment consists of a power supply, cables and elec-trode holder.

- Power supply: can be either DCor AC, ranges between 50 to 300A.

- For sheet metal welding, DC is preferred because of the steady arc produces.


Shielded Metal-Arc Welding

ii) SUBMERGED-ARC WELDING (SAW)

- The weld arc is shielded by a granular flux consisting of lime, silica, manganese oxide,calcium flouride.

- The flux is fed into the weld zone from a hopper by gravity flow through a nozzle.

- The thick layer of flux completely cover s the molten metal and it prevents from spatterand sparks.

- The flux also acts as a thermal insulator by promoting deep penetration of heat into theworkpiece.


- The consumable electrode is a coil of bare round wire 1.5 to 10 mm in diameter; andfed automatically through a tube which is called welding gun.

- Electric current: range between 300 to 2000 A.

- Power supply: single or three phase power point; rating up to 440V.

- Due to flux is a gravity fed type; therefore this welding process is limited largely towelds into flat or horizontal po-sition.


- Circular weld can be made on pipes or cylinders ²provided that they are rotatedduring welding process.

-Suitable for carbon and alloy steel and stainless steel sheet or plates.

- Welding speeds: as high as 5 m/min.


Submerged-Arc Welding

iii) GAS METAL-ARC WELDING

- Also known as metal inert-gas (MIG).

- The weld area is shielded by an effectively inert atmosphere of argon, helium, carbondioxide, or other various gas mix-tures.

- The temperatures generated are relatively low.

- Suitable only for thin sheets which is less than 6mm.


-The consumable bare wire is fed automatically through a nozzle into the weld arccontrolled by wire-feed drive motor.

-There are 3 types of GMAW process:a)Spray transfer.b)Globular transfer.c)Short circuiting.


Gas Metal-Arc Welding

a) SPRAY TRANSFER- Small size of molten metal droplets from the electrode are transferred to the weld area at a rate of several hundred droplets per second.- The transfer is spatter free and very stable.- Using high DC current and voltages with large diameter of electrodes.

- The electrodes are used with argon or an argon rich gas mixture act as a shielding gas.

Types of Gas-Metal Arc Process

b) GLOBULAR TRANSFER- Utilizes with carbon-dioxide-rich gases, and globules are propelled by the forces of the electric-arc transfer of a metal, resulting in considerable spatter.- High welding current are used - greater weld penetration and higher welding speed

c) SHORT CIRCUITING- The metal is transferred in individual droplets, as the elec-trode tip touches the molten weldmetal and short circuits.- Low currents and voltages are utilized.- Electrodes are made from small-diameter wire.- Power required: § 2 kW.

Types of Gas-Metal Arc Process

iv) ELECTRON BEAM WELDING

- Can be welded almost any metal; butt or lap welded and the thicknesses up to 150mm.

- The thickness of the workpiececan range from foil to plate.

- Generally, there is no involvement of shielding gas, flux, or filler metal.

- Distortion and shrinkage in the weld area is minimal.

- Heat is generated by high velocity narrow-beam electrons.

- Capacity of electron guns range up to 100 kW.


- The kinetic energy of the electrons is converted into heat as they strike the workpiece.

- Required special equipment to focus the beam on the workpiece, typically in vacuum.

- The higher the vacuum, the more the beam penetrates, and the greater is the depth-to width ratio, range between 10 and 30.

- Sizes of the welds are much smaller compared to conven-tional process.

- Parameters can be controlled accurately at welding speeds as high as 12 m/min; thiscan be done by using automation and servo motor.


v) LASER-BEAM WELDING

- Utilizes a high power laser beam as the source of heat.

- The beam can focused onto a very small area, and due to this it has high energy density and deep penetrating capability.

- This process is suitable for welding deep and narrow joints with depth-to-width ratios ranging from 4 to 10.

- The laser beam may be pulsed for a application such as the spot welding of thinmaterials with power level up to 100 kW.


- Minimum shrinkage and distortion, good strength and generally are ductile and free ofporosity.

- Can be automated to be used on a variety of materials with thicknesses up to 25mm.

- Typical metals and alloys welded: aluminum, titanium, fer-rous metals, copper.

- Welding speeds: range from 2.5 m/min to as high as 80 m/min for thin metals.


Advantages of LBW over EBW:

Laser beams can be shaped, manipulated, and focused op-tically by using fiber optics, therefore the process can be automated easily. The beams do not generate x-rays. The quality of the weld is better than in EBW with less ten-dency for incomplete fusion, spatter, porosity, and less dis-tortion.

Example of laser Welding: laser welding of razor blades


Solid State Welding Processes

Forgew

elding

Coldw

elding

Rollw

elding

Hot pressurew

elding

Diffusionw

elding

Explosionw

elding

Frictionw

elding

Ultrasonicw

elding

SOLID STATE WELDING PROCESSES

• Forge Welding• Cold Welding• Roll Welding

• Hot pressure Welding• Diffusion Welding• Explosion Welding

• Friction Welding• Ultrasonic Welding

Forge Welding

- Welding process inwhich 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 manufacturingTechnology

- Process dates from about 1000 B.C., When blacksmiths learned to weld two pieces of metal

- Of minor commercial importance today except for its variants

Roll Welding (ROW)

- SSW process in which pressure sufficient to cause coales-cence is applied by means of rolls, either with or without ex-ternal heat

- Variation of either forge welding or cold welding, depending on whether heating of work parts is done prior to process

- If no external heat, called cold roll welding

- If heat is supplied, hot roll welding

Roll Welding

Roll Welding Application

- Cladding stainless steel to mild or low alloy steel for corrosion resistance

-Bimetallic strips for measuring temperature

- “Sandwich" coins for U.S mint

Diffusion Welding (DFW)

- SSW process uses heat and pressure, usually in a controlled atmosphere, with sufficient time for diffusion and coalescence to occur

- Plastic deformation at surfaces is minimal

- Primary coalescence mechanism is solid state diffusion

- Limitation: time required for diffusion can range from seconds to hours

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

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

Explosive Welding

-Commonly used to bond two dissimilar metals, in particular to clad one metal on top of abase metal over large areas

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

Friction Welding

Application and Limitation of 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)

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 allowintimate 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

Ultrasonic Welding

USW 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

Weldability

- Capacity of a metal or combination of metals to bewelded into a suitably designed structure, and for the resulting weld joint(s) to possess the required metallurgical properties to perform satisfactorily in intended service

Good weldability characterized by:- Ease with which welding process is accomplished

-Absence of weld defects

Acceptable strength, ductility, and toughness inwelded joint

Documents

Welding