Welding ( Assembly/ Joining Process)

By: Mr. Sunil Kumar OjhaAssistant ProfessorMechanical Engineering DepartmentJRE Group of Institutions Greater Noida

Joining elements together, which shapes a final product. Assembly process can be made by human workers (uneducated but skilled) or by specialized machines and robots.

Example: Cars, computers, engines, cellophane etc.

What is assembly?

Aspect 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

Introduction

1. Welding 2. Soldering and Brazing 3. Mechanical Fastening 4. Adhesive Bonding

But our presentation will focus on welding

Joining Processes

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Mechanical methods◦ Screwed fasteners, rivets,

Adhesive bonding Brazing and Soldering

◦ Base metal does not fuse. ◦ Molten filler drawn into close-fit joints by capillary action (surface tension forces).◦ Brazing filler melts >450 C, solder <450 C

Welding

Overview of joining methods

Introduction to welding

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A joint produced by heat or pressure or both So there is continuity of material.

Filler (if used) has a melting temperature close to the base material

So Weld

Introduction to welding

welding is the process of joining in which heat is used to join similar or dis-similar metals with or without the application of pressure and filler metal.

WELDING◦ Welding is a materials joining process which produces

coalescence of materials by heating them to suitable temperatures with or without the application of pressure or by the application of pressure alone, and with or without the use of filler material.

◦ Welding is used for making permanent joints. ◦ It is used in the manufacture of automobile bodies, aircraft

frames, railway wagons, machine frames, structural works, tanks, furniture, boilers, general repair work and ship building.

Applications of welding Buildings and bridges structures; Automotive, ship and aircraft constructions; Pipe lines; Tanks and vessels; Railroads; Machinery elements

Advantages of welding Strong and tight joining; Cost effectiveness; Simplicity of welded structures design; Welding processes may be mechanized and

automated

Disadvantages of welding Internal stresses, distortions and changes of

micro-structure in the weld region; Harmful effects: light, ultra violate radiation, fumes,

high temperature.

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Solid phase welding◦ Carried out below the melting point without filler

additions◦ Pressure often used◦ Union is often by plastic flow

Fusion welding or Liquid Phase Welding◦ Welding in the liquid state with no pressure◦ Union is by molten metal bridging

Welding Processes

Introduction to 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

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 combustion 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+ O---->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)

Fusion Welding Process

Fusion Welding Process

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 adjusted.

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

Fusion Welding Process

2) PRESSURE GAS WELDING

- Involved with two components starts by heating the interface.- Once when the interface begins to melt, the torch is withdrawn.- 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 energy – 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 30,00 degrees Celsius.

Fusion Welding Process

Arc welding

Advantages◦ Most efficient way to join

metals◦ Lowest-cost joining

method◦ Affords lighter weight

through better utilization of materials

◦ Joins all commercial metals

◦ Provides design flexibility

Limitations Manually applied,

therefore high labor cost. Need high energy causing

danger Not convenient for

disassembly. Defects are hard to detect

at joints.

a.) 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 oxidation of the weld zone.

- DC is used and the polarity is important.

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

Arc Welding Processes

- 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

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

Non Consumable Electrode

- 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 electrodes to improve the electron emission characteristics.

Non Consumable Electrode

- 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.

Non Consumable Electrode

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 concentration – permitting deeper and narrower welds.

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

Non Consumable Electrode

- 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 currently 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.

Consumable Electrode

- The heat generated, melts a portion of the electrode tip, its coating, and the base metal in the intermediate 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 provides a shielding gas to protect it from oxygen in the environment.

Consumable Electrode

- The equipment consists of a power supply, cables and electrode 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.

Consumable Electrode

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.

Consumable Electrode

- 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 position.

Consumable Electrode

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

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

- Welding speeds: as high as 5 m/min.

Consumable Electrode

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 mixtures.

- The temperatures generated are relatively low.

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

Consumable Electrode

-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.

Consumable Electrode

Gas Metal-Arc Welding (MIG 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 electrode 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.

Consumable Electrode

- 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 conventional process.

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

Consumable Electrode

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.

Consumable Electrode

- 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, ferrous metals, copper.

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

Consumable Electrode

Advantages of LBW over EBW:

Laser beams can be shaped, manipulated, and focused optically 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 tendency for incomplete fusion, spatter, porosity, and less distortion.

Example of laser Welding: laser welding of razor blades

Consumable Electrode

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Introduction to weldingThermit welding

THERMIT WELDING (TW)

Thermit Pow-der

Molten Crucible

RAILS

Mould

Ig-niter

Molten metal flows down

developed in 1893

Flame heating gate

Thermit powder:

Fe2O3 + 2Al = 2Fe + Al2O3

THERMIT WELDING (TW)

Solid-State Welding Processes

• Forge Welding• Cold Welding• Roll Welding• Resistance Welding • Hot pressure Welding• Diffusion Welding• Explosion Welding• Friction Welding• Ultrasonic Welding

SOLID STATE WELDING PROCESSES

Cold Welding

Pressure is applied to the workpieces through dies or rolls

Preferably both work pieces should be ductile

The work pieces should cleaned thoroughly

Can not join dissimilar metals

Fig: The roll bonding or cladding process

Ultrasonic Welding

Surfaces of the two components are subjected to a static forces and oscillating shearing force

Produces a strong, solid-state bond

Versatile and reliable for joining metals

Fig: a) Components of an ultrasonic welding machine for lap welds.The lateral vibration of the tool tip cause plastic deformation and bonding at the in-terface of the work piece b)Ultrasonic some welding using a roller c)An ultrasonically welded part

Friction Welding

Developed in the 1940’s

Parts are circular in shape

Can be used to join a wide variety of materials

Fig: Sequence of operation in the friction welding process 1)Left-hand component is rotated at high speed. 2) Right-hand component is brought into contact under an axial force 3)Axial force is increased;the flash begins to form 4) Left-hand component stops rotating;weld is completed.The flash can subsequently be removed by ma-chining or grinding

Process can be fully automated

Can weld solid steel bars up to 250mm in outside diameter

Fig:Shape of friction zone in friction welding,as a function of the force applied and the rotational speed

Friction Welding

Resistance Welding

Developed in the early 1900’s

A process in which the heat required for welding is produced by means of electrical resistance across the two components

RW does not requiring the following:◦ Consumable electrodes◦ Shield gases◦ Flux

Resistance Spot Welding

RSW uses the tips of two opposing solid cylindrical electrodes

Pressure is applied to the lap joint until the current is turned off in order to obtain a strong weld

Fig: (a) Sequence in the resistance spot welding

Surfaces should be clean

Accurate control of and timing of electric current and of pressure are essential in resistance welding

Fig: b)Cross-section of a spot weld,showing the weld nugget and the indentation of the electrode on the sheet surfaces.This is one of the most commonly used process in sheet-metal fabrication and in automotive-body assembly

Resistance Spot Welding

Resistance Seam Welding

RSEM is modification of spot welding wherein the electrodes are replaced by rotating wheels or rollers

The electrically conducting rollers produce a spot weld

RSEM can produce a continuous seam & joint that is liquid and gas tight

Fig : (a) Seam-Welding Process in which rotating rolls act as electrode (b) Overlapping spots in a seam weld. (c) Roll spot weld (d) Resistance-welded gasoline

tank

Resistance Projection Welding

RPW is developed by introducing high electrical resistance at a joint by embossing one or more projections on the surface to be welded

Weld nuggets are similar to spot welding

Fig: a) Resistance projection Welding b)A welded bracket c) & d) Projection welding of nuts r threaded hosses and stack

The electrodes exert pressure to compress the projections

Nuts and bolts can be welded to sheet and plate by this process

Metal baskets, oven grills, and shopping carts can be made by RPW

Resistance Projection Welding

Flash Welding

Heat is generated from the arc as the ends as the two members contacts

An axial force is applied at a controlled rate Weld is formed in plastic deformation

Fig : (a)Flash-welding process for end-to –end welding of solid rods or tubular parts(b) & (c) Typical parts made by flash welding (d)Design Guidelines for flash welding

Stud Welding

Small part or a threaded rod or hanger serves as a electrode Also called as Stud arc welding Prevent oxidation to concentrate the heat generation Portable stud-welding is also available

Fig:The sequence of operation in stud welding,which is used for welding bars threaded rods and various fasteners onto metal plates

Percussion welding (PEW) is a type of resistance welding that blends dissimilar metals together. Percussion welding creates a high temperature arc that is formed from a short quick electrical discharge. Immediately following the electrical discharge, pressure is applied which forges the materials together. This type of joining brings the materials together in a percussive manner.

Percussion welding is similar to flash welding and upset welding but is generally considered to be more complex. It is considered to be more complex because it uses an electric discharge at the joint, followed by pressure being applied to join the materials together. Percussion welding is used to join dissimilar metals together, or used when flash is not required at the joint. This type of welding is limited to the materials having the same cross sectional areas and geometries. Percussion welding is used on materials that have small cross sectional areas.

Advantages of using percussion welding types include a shallow heat affected zone, and the time cycle involved is very short. Typical times can be found to be less than 16 milliseconds.

Percussion welding (PEW)

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)

Weldability

- Capacity of a metal or combination of metals to bewelded into a suitably designed structure, and for the result-ing 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 in welded joint

Weldability of a Metal Metallurgical Capacity

◦ Parent metal will join with the weld metal without formation of deleterious constituents or alloys

Mechanical Soundness◦ Joint will be free from discontinuities, gas

porosity, shrinkage, slag, or cracks Serviceability

◦ Weld is able to perform under varying conditions or service (e.g., extreme temperatures, corrosive environments, fatigue, high pressures, etc.)

Fusion Weld Zone

Figure Characteristics of a typical fusion weld zone in oxyfuel gas and arc weld-ing.

Metallurgy Of Welding

The metal used for the joining purpose is called solder. Solder are two types :1. Hard Solder 2. Soft Solder

Hard solder is an alloy of copper and zinc where is soft solder is an alloy of tin and lead.

SolderingSoldering is a process of joining two metals by using another low tempera-ture metal alloy ( Below 427 de-gree centigrade) .

Application soldering is widely used for sheet metal work and in radio, television work, electronic circuit work for joining of wires.

DisadvantageJoints formed are weak.

Process :The surface to be joined are cleaned are cleaned and are placed on each other. A flux is employed to prevent oxidation. Zinc chloride is commonly used for this purpose. The soldering iron is heated either electrically or by some ex-ternal heat . Then the hot end is dipped into the flux and solder is pressed against the sur-face to be joined. A joint is formed by melting the solder.

Brazing The joining of two metal pieces by means of heat and a

special filler (Spelter) having a melting point above 427 degree Centigrade but lower than the melting point of the parts to be joined. In brazing the ends of parent metal are not melted but an alloy, having low melting point is used. Temperature is raised to the fusion point of this alloy which when melted runs between the edges due to capillary action and produces thinning effect resulting in brazed joint. Through brazing dissimilar materials can be easily joined. The filler rod is used for joining are of Bronze material ( 60-40% brass main constituent with suitable amount of deoxidizer like silicon and tin etc used).

Application: joining of tubes, radiators, pipes and pipes fitting tool tips electrical items etc.

Advantages: useful for joining of dissimilar material, thin sections easily joined, good surface finish obtained, High production, Less skill, less cost.

Disadvantage: low strength, not applicable for hardened steel and aluminum alloys.

Inclusion: Entrapment of Slag, Scale, dirt, rust in weld zone etc

Cracks: Discontinuity of weld Metal

Distortion: Change in the indented shape and size of component or structure due to uneven contraction (shrinkage).

Poor Penetration: failure of weld molten metal to reach the bottom of joint.

Porosity: presence of small pores, voids, gases in the weld metal.

Spatter: Deposition of electrode metal particle adjutant to base metal

Undercut: Groove formed in the parent metal at the toe of a weld pass.

Overlapping: molten metal flows over the surface of the base metal.

Inadequate Fusion: sometime the deposited weld metal by electrode does not fuse fully with the base metal due to presence of oxides, dirt, slag or other foreign material.

Welding Defects

Welding DefectsDefects Reasons

1. Low Penetration

2. Cracks

3. Inclusions

4. Poor Fusion

5. Blow Holes (Porosity)

6. Wrapping

7. Scattering of weld

1.Incorrect current, fast speed of welding

2. selection of wrong electrode, metal contain too much carbon

3. dirty base metal, higher sulfur content, improper removal of slag

4. wrong current setting, wrong clearance between work piece and electrode, fast speed of welding.

5. Wrong arc length, impurities in in base metal, old electrode.

6. uneven heating, overheating, thin cross-section of metals

7. high current, long arc, faulty electrodes.