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Fundamentals of Joining Processes

Prepared By :

R.K.P.S Ranaweera BSc (Hons) MSc

Lecturer - Department of Mechanical Engineering

University of Moratuwa

(for educational purpose only)

ME3072 – MANUFACTURING ENGINEERING II

BSc Eng (Hons) in Mechanical Engineering

Semester - 4

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Outline

• Introduction to Welding

• Fusion-Welding Processes

• Solid-State Welding Processes

• Metallurgy of Welding

• Weld Quality

• Brazing & Soldering

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Joining Processes

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Classification of Joining Processes

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• Is a process by which two materials, usually metals are permanently joined together by coalescence, which is induced by a combination of temperature, pressure and metallurgical conditions.

• Is extensively used in fabrication as an alternative method for casting or forging and as a replacement for bolted and riveted joints. Also used as a repair medium to reunite metals.

• Types of Welding:�Fusion welding

�Solid-state (forge) welding

Introduction to Welding

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• Attention must be given to the cleanliness of the

metal surfaces prior to welding and to possible oxidation or contamination during welding process.

• Production of high quality weld requires:

�Source of satisfactory heat and/or pressure

�Means of protecting or cleaning the metal

�Caution to avoid harmful metallurgical effects

• Advantages of welding over other joints:

�Lighter in weight and has a great strength

�High corrosion resistance

�Fluid tight for tanks and vessels

�Can be altered easily (flexibility) and economically

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• Weldability has been defined as the capacity of

metal to be welded under the fabrication conditions imposed into a specific, suitably designed structure

& to perform satisfactorily in the intended service.

• The following metals have good weldability in the descending order: Iron, Carbon Steel, Cast Steel,

Cast Iron, Low Alloy Steels and Stainless Steels.

• Welding is extensively used in the following fields: automobile industry, aircraft machine frames, tanks,

structural work, machine repair work, ship building, pipe line fabrication ,thermal power plants and

refineries, fabrication of metal structures.

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• Steps in executing welding:

� Identification of welds, calculation of weld area by stress

analysis, preparation of drawings

�Selection of appropriate welding process

�Welding procedure – welding sequence, testing, etc

�Execution of welding with supervision & inspection

�Slag removal, weld dressing

�Stress relieving by proper treatment

�Testing, preferably by nondestructive methods

• Process of joining similar metals with the help of

filler rod of the same metal is called autogeneouswelding, and joining of metals using filler rod of is

called heterogeneous welding.

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• Types of welded joints:

�Lap joint

�Butt joint

�Corner joint

�Edge joint

�T-joint

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• Welding positions: Flat position, Horizontal position

Vertical position and overhead position.

• Welders have to protect themselves against spark, hot metal, ultraviolet, infrared and visible light rays, welding fumes, and other hazards.

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• Introduction�Is defined as the melting together & coalescing

of materials by means of heat, with or without

the application of pressure and with or without the use of filler metal.

�Thermal energy required for these operations is usually supplied by chemical (oxy-fuel gas, thermit) or electrical ( arc, resistance, electron beam, laser beam) means.

�Welds undergo important metallurgical & physical changes that will effect its performance.

Fusion-Welding Processes

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• Oxyfuel Gas Welding (OFW)

�Refers to a group of welding processes that use,

as their heat source, the flame produced by the combustion of fuel gas and oxygen.

� Types of Gas used:

� Oxyacetylene – high temperature

� Hydrogen – low temperature

� Methylacetylene propadiene – low temperature

�Heat is generated in accordance with a pair of

chemical reactions:

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Three basic types of oxyacetylene flames used in Oxyfuel-gas welding and cutting operations: (a) neutral flame; (b) oxidizing flame; (c) carburizing, or reducing, flame. The gas mixture in (a) is basically equal volumes of oxygen and acetylene.

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(a) General view of and (b) cross-section of a torch used in oxyacetylene welding. The acetylene valve is opened first; the gas is lit with a spark lighter or a pilot light; then the oxygen valve is opened and the flame adjusted.

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Basic equipment used in Oxyfuel-gas welding. To ensure correct connections, all threads on acetylene fittings are left-handed, whereas those for oxygen are right-handed. Oxygen regulators are usually painted green, acetylene regulators red.

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�Filler Metals

� Used to supply additional material to the weld zone

� Available as rod or wire made of metals compatible

with those to be welded

� Consumable filler rods may be bare, or they may be

coated with flux.

� Purposes of the flux:

- Retard oxidation of the surfaces of the part being welded, by generating gaseous shield around the weld zone

- Helps to dissolve and remove oxides and other substances from the workpiece and form a stronger joint

- Slag developed protects the molten puddle of metal against oxidation as it cools

- Provides means of adding various alloying elements into the

weld metal to enhance the properties of the joint

- Stabilizes the arc by providing certain chemicals

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�Pressure Gas Welding

Schematic illustration of the pressure-gas welding process.

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• Arc Welding: Consumable Electrode

�Heat is obtained from electrical energy.

�Arc is produced between the tip of the electrode

and the workpiece to be welded, by the use of an AC or a DC power supply.

�Arc produce temperatures about 30,000 0C

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• Shielded Metal Arc Welding (SMAW)

�About 50% of all industrial and maintenance

welding is currently performed by this process.

�Also known as stick welding.

�Electric arc is generated by touching the tip of a coated electrode against the workpiece and then withdrawing it quickly to a distance sufficient to

maintain the arc.

Schematic illustration of the shielded metal-arc welding

process.

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�Current used generally ranges between 50 A to

300 A & power requirements are generally 10kW

�Type of current:

� DC – straight & reverse polarity

� AC

Schematic illustration of the shielded metal-arc welding operations (also known as stick welding, because the electrode is in the shape of a stick).

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• Submerged Arc Welding (SAW)

�Weld arc is shielded by granular flux, consisting

of lime, manganese oxide, calcium fluoride, silica, and other compounds.

�It prevents spatter & sparks and suppresses the

intense ultraviolet radiation and fumes.

�Flux also act as a thermal insulator promoting

deeper penetration of heat into the workpiece.

�Consumable electrode is a coil of bare round wire 1.5 mm – 10 mm in diameter.

�Applications include thick plate welding for

shipbuilding and for pressure vessels.

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�Electrical current typically range between 300 A

to 3000 A & weld speed is high as 5 m/min.

Schematic illustration of the submerged-arc welding process and equipment. The unfused flux is recovered and reused.

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• Gas Metal Arc Welding (GMAW)

�Formerly called metal inert gas (MIG) welding.

�Weld area is shielded by an effective inert

atmosphere of argon, helium, carbon dioxide, or various other gas mixtures.

�In addition, deoxidizers are usually present in the electrode metal itself, prevent oxidation of

the molten weld puddle.

�Suitable for welding a variety of ferrous and nonferrous metals.

�Metal can be transferred by three methods:

spray, globular and short circuiting.

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Schematic illustration of the gas metal-arc welding process, formerly known as MIG (for metal inert gas) welding.

Basic equipment used in gas metal-arc welding

operations.

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• Flux-Cored Arc Welding (FCAW)

�Similar to GMAW, with the exception that the

electrode is tubular in shape & is filled with flux.

�Produce a more stable arc, improve weld contour , and produce better mechanical

properties of the weld metal.

�Electrodes are usually 0.5 mm – 4 mm in

diameter & the power required is about 20 kW.

�Used for welding of variety of joints, mainly on steels, stainless steels and nickel based alloys.

�Self-shielded cored electrodes are also available

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Schematic illustration of the flux-cored arc-welding process. This operation is similar to gas metal-arc welding.

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• Electrodes

�Is classified according to the strength of the

deposited weld metal, the current (AC or DC), & the type of coating.

�Identified by numbers or letters or by color code.

�Typical coated electrode numbers are 150 to 460 mm in length & 1.5 to 8 mm in diameter.

(Wire diameter must not vary more than 0.05 mm & Coatings must be concentric with wire)

�Electrodes are coated with claylike material that

include silicate binders & powder materials such as oxides, carbonates, fluorides, metal alloys,

and cellulose. 28

Designations for Mild Steel Coated Electrodes

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• Arc Welding: Non-consumable Electrode

�Unlike arc-welding processes, non-consumable

electrode processes typically use a tungsten electrode.

�Shielding gas is supplied from external source.

�Stable arc gap is maintained because the electrode is not consumed.

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• Gas Tungsten Arc Welding (GTAW)

�Also know as tungsten inert gas (TIG) welding.

�Filler metal is supplied from a wire & are similar

to the metals to be welded.

�Shielding gas is usually argon or helium.

�Is used for wide variety of metals & applications, particularly aluminium, magnesium, titanium & refractory metals.

�Power supply is either DC at 200 A or AC at 500

A and power requirements range from 8 kW to 20 kW.

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The gas tungsten-arc welding process, formerly known as TIG (for tungsten inert gas) welding.

Equipment for gas tungsten-arc welding operations.

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• Atomic Hydrogen Welding (AHW)

�Uses an arc in a shielding atmosphere of H2.

�Arc is between 2 tungsten or carbon electrodes.

�Hydrogen also cools the workpiece.

• Plasma Arc Welding (PAW)

�A concentrated plasma arc is produced and is

aimed at the weld area.

�Arc is stable and reaches temperatures as high as 33,000 0C.

�Plasma is ionized hot gas, composed of nearly

equal numbers of electron and ions

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�Plasma is initiated between tungsten electrode

and the orifice by a low current pilot arc.

�Shielding is supplied by means of an outer shielding rings and the uses of gases, such as argon, helium or mixtures.

�Two methods of plasma arc welding: transferred

arc method (a) or nontransferred arc method (b).

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• Thermit Welding (TW)

�Involves exothermic reactions between metal

oxides & metallic reducing agents and the heat produced in this reaction is used for welding.

�Common mixtures of materials used in welding

steel & cast iron are iron oxide, aluminium oxide, iron and aluminium.

�Mixtures may also contain other materials to impart special properties to the weld.

�Is suitable for welding & repairing large forgings

and castings.

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�Procedure - align the part to be joined � built a

mold �allow to flow superheated products

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• Electron Beam Welding (EBW)

�Heat is generated by high velocity narrow beam

electrons & the kinetic energy of the electrons is converted in to heat as they strike the workpiece

�Requires special equipment to focus the beam

on the workpiece in a vacuum.

• Laser Beam Welding (LBW)

�Utilizes a high power laser beam as the source of heat to produce a fusion weld.

�The beam has high energy density, therefore

deep penetrating capability.

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The relative sizes of the weld beads obtained by conventional (tungsten arc) and by electron-beam or laser-beam welding

�Comparison of Conventional and Electron- or

Laser-Beam Welding

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• Cutting

�A piece of metal can be separated in to two or

more pieces or into various contours by the use of heat source that melts and removes a narrow

zone in the workpiece.

�Oxyfuel Gas Cutting (OFC)

� Cutting occurs mainly by the oxidation of the steel

� Basic reaction with the steel are,

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(a)Flame cutting of steel plate with an oxyacetylene torch, and a cross-section of the torch nozzle. (b) Cross-section of a flame-cut plate showing drag lines.

�Arc Cutting

� Air carbon arc cutting

� Plasma arc cutting

� Lasers and electron beams40

• Introduction�Solid-phase welds are produced by bringing the

clean faces of components into intimate contact

to produce a metallic bond with or without application of heat, but application of pressure is

essential to induce plastic flow.

Solid-State Welding

Processes

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• Cold Welding (CW)

�Pressure is applied to the workpieces, through

either dies or rolls.

�Also known as roll bonding.

�Prior to welding, the interface is degreased, wire brushed, and wiped to remove oxide smudge.

�Can be used to join small workpieces made of soft, ductile metals.

Schematic illustration of the roll bonding, or cladding, process

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• Ultrasonic Welding (USW)

�Faying surfaces of the two components are

subjected to a static normal force and oscillating shearing (tangential) stress.

�Shearing stresses are applied by the tip of a

transducer and frequency of oscillation is generally in the range of 10 kHz to 75 kHz.

�Temperatures generated usually in the range of one-third to one-half of the melting point.

�Can be used with wide variety of metallic and

nonmetallic materials, including dissimilar metals and plastics.

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(a) (b)

(a)Components of an ultrasonic welding machine for lap welds. The lateral vibrations of the tool tip cause plastic deformation

and bonding at the interface of the workpieces. (b)Ultrasonic seam welding using a roller.

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• Friction Welding (FRW)

�Steps of operation

� On of the components remains stationary while the

other is placed in a chuck or collet and rotated at a

high constant speed.

� Two members to be joined are then brought into

contact under an axial force.

� Rotating member is then brought to a quick stop,

while the axial force is increased.

� Pressure at the interface and the resulting friction

produce sufficient heat for a strong joint to form.

�Types of FRW processes:Inertia friction welding, Linear friction welding

and Friction stir welding

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(a)

(b)

(a)Sequence of operations in the friction welding process

(b)Shape of fusion zone in friction welding, as a function of the force applied and the rotational speed.

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�Friction Stir Welding (RSW)

� Use of a third body to rub against the faying surfaces

� Probe at the tip heat and mix or stir the material

The principle of the friction stir welding process. Aluminum-alloy plates up to 75 mm (3 in.) thick have been welded by this process

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• Resistance Welding (RW)

�Heat required for welding is produced by means

of electrical resistance across two components to be joined.

�Actual temperature rise at the joint depends on

the specific heat and on the thermal conductivity of the metals to be joined.

�Magnitude of the current in resistance welding operations may be as high as 100,000 A,

although the voltage is typically only 0.5V – 10V.

�Similar or dissimilar metals can be joined.

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(a) Sequence in resistance spot welding. (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 (RSW)

� Tips of two opposing solid cylindrical electrodes touch

a lap joint of two sheet metals, and resistance heating

produces a spot weld

� Currents range from 3000 A to 40,000 A

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�Resistance Seam Welding (RSEW)

� Is a modification of spot welding wherein the

electrodes are replaced by rotating wheels or rollers.

� Using a continuous AC power supply, the electrically

conducting rollers produce a spot weld when ever the

current reaches a sufficient high level in the AC cycle.

� Can produce a joint that is liquid tight or gas tight.

� Roll spot welding is an extension of RSEW.

(a) (b)

Examples of Seam Welding(a) and (b) Seam-welded

cookware and muffler.

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(a) Seam-welding process in which rotating rolls act as electrodes. (b) Overlapping spots in a seam weld. (c) Roll spot welds. (d) Resistance-welded gasoline tank.

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�High-Frequency Butt Welding (HFRW)

� Similar to seam welding

� High frequency current (up to 450 kHz) is employed

� Types:

�High frequency resistance welding (fig. a)

�High frequency induction welding (fig. b)

Two methods of high-frequency butt welding of tubes

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�Resistance Projection Welding (RPW)

� Embossing one or more projections on one of the

surfaces to be welded to increase the electrical

resistance

Schematic illustration of resistance projection welding

Projection welding of nuts or threaded bosses and studs

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�Flash Welding (FW)

� Heat generated from the arc as the ends of the two

members begin to make contact and develop an

electrical resistance at the joint

� Form a flash at the joint

� Used to repair broken band-saw blades

Flash-welding process for end-to-end welding of solid rods or tubular parts

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�Stud Welding (SW)

� Similar to flash welding

� Part to be joined serves as one of the electrodes

while being joined to another component

� Disposable ceramic ring (ferrule) is placed around the

joint to concentrate the heat generated, prevent

oxidation and retain the metal in the weld zone

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

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• Explosion Welding (EXW)

�Pressure applied by detonating a layer of

explosives that has been placed over one of the components to be joined

�Cold pressure welding by plastic deformation (a) (b)

Schematic illustration of the explosion welding process: (a) constant interface clearance gap and (b) angular interface clearance gap

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(c) (d)

(c) and (d) Cross-sections of explosion-welded joints

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• Diffusion Bonding/Welding (DFW)

�Process in which the strength of the joint results

primarily from diffusion and secondarily from plastic deformation of the faying surfaces

�Ability to fabricate sheet-metal structures by

combining diffusion bonding with superplastic forming

� Eliminate use of mechanical fasteners

� High stiffness to weight ratio

� Good dimensional accuracy

� Low residual stresses

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• Standard Identification for Welds

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• Weld Joint

�Three distinct zones in a fusion-weld joint:

� Base metal

� Heat Affected Zone (HAZ)

� Weld metal

Metallurgy of Welding

Characteristics of a typical fusion weld zone in oxy-fuel gas and arc welding

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�Solidification of the Weld Metal

� Formation of columnar grains (dendritic)

� Grain structure and size depends on the specific

alloy, the specific welding process employed, and the

specific filler metal

Grain structure in (a) a deep weld (b) a shallow weld. Note that the grains in the solidified weld metal are perpendicular to the

surface of the base metal. In a good weld, the solidification line at the center in the deep weld shown in (a) has grain migration, which develops uniform strength in the weld bead

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�Heat Affected Zone (HAZ)

� Properties and microstructure of HAZ depends on:

�Rate of heat input and cooling

�Temperature to which the zone was raised

Schematic illustration of various regions in a fusion weld zone (and the corresponding phase diagram) for 0.30% carbon steel

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� Corrosion at HAZ

Intergranular corrosion of a 310-stainless-steel welded tube after exposure to a caustic solution. The weld line is at the center of

the photograph. Scanning electron micrograph at 20 X

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• Porosity

�Caused by,

� Gases released during melting of the weld area

� Chemical reactions during welding

� Contaminants

�In the shape of spheres or of elongated pockets.

• Slag Inclusions

�Are compounds such as oxides, fluxes, and electrode coating material trapped in the weld.

Weld Quality

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• Incomplete Fusion & Penetration

�Incomplete fusion or lack of fusion produces

poor weld beads.

�Incomplete penetration occurs when the depth of the welded joint is insufficient.

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�Penetration can be improved by,

� Increasing the heat input

� Reducing the travel speed during the welding

� Changing the joint design

� Ensuring that the surfaces to be joined fit properly

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• Weld Profile

Underfilling – when the weld is not filled with proper amount of weld metalUndercutting – melting away of the base metal & the consequent

generation of a groove.Overlap – surface discontinuity , usually caused by poor welding practiceand by the selection of improper materials.

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• Cracks

�Are classified as,

� Hot cracks – occur while the joint is still at elevated

temperatures

� Cold Cracks – develop after the weld metal has

solidified.

�Typical types of cracks are,

� Longitudinal

� Transverse

� Crater

� Underbead

� Toe cracks

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Types of cracks (in welded joints) caused by thermal stresses that develop during solidification and contraction of the weld bead and the surrounding structure. (a) Crater cracks. (b) Various types of cracks in butt and T joints.

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• Surface Damage

�Cause

� Metal may spatter during welding & be deposited as

small droplets on adjacent surfaces.

� Arc strikes at places other than the weld zone.

�Affect

� Poor surface appearance

� High surface roughness

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• Distortions

�Localized heating & cooling during welding

causes residual stresses in the workpiece.

Residual stresses developed during welding of a butt joint

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Distortion of parts after welding: (a) butt joints; (b) fillet welds. Distortion is caused by differential thermal expansion and contraction of different parts of the welded assembly.

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• Introduction

�Are processes that do not rely on fusion or

pressure at the interfaces; instead utilize filler material that requires some temperature rise in

the joint.

�Temperatures for soldering are lower than those for brazing, and the strength of a soldered joint is much lower.

�Can be used to join dissimilar metals of intricate

shapes and various thicknesses.

Brazing & Soldering

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• Brazing

�Basic Steps in Brazing

� Filler metal (low-melting-point nonferrous metal) is

placed at or between the faying surfaces to be joined,

and the temperature is raised enough to melt the filler

metal but not the workpieces.

� Molten metal is allowed to fill closely fitting space by

capillary action.

� Upon cooling and solidification of the filler metal, a

strong joint is obtained.

�Main types of brazing processes

� Ordinary Brazing

� Braze Welding – in which filler metal is deposited at

the joint with a technique similar to OFW.74

�Filler metal used for brazing melt above 4500C.

�Strength of the brazed joint depends on,

� Joint design

� Adhesion at the interfaces between the workpiece

and filler material

(a) Brazing and (b) braze welding operations.

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�Surfaces to be brazed should be chemically or

mechanically cleaned to ensure full capillary action

�Brazing Flux

� Prevent oxidation and to remove oxide film from

workpiece surfaces

� Use “wetting agents”, to improve both the wetting

characteristics of the molten filler metal and the

capillary action

� Made of borax, boric acid, borates, fluorides, &

chlorides

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�Brazing Methods

� Torch Brazing (TB) - Heat source is oxyfuel gas with a

carburizing flame

� Furnace Brazing (FB) – Brazing metal is preloaded in

appropriate configuration before placing it in a furnace

An example of furnace brazing: (a) before, (b) after. Note that the filler metal is a shaped wire

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� Induction Brazing (IB) – Source of heat is induction

heating by high frequency AC current, where the parts

with preloaded filler metal are placed near the

induction coils for rapid heating

� Resistance Brazing (RB) – Source of heat is the

electrical resistance of the components to be brazed

Schematic illustration of a

continuous induction-brazing setup, for

increased productivity

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� Dip Brazing (DB) – Dipping the assemblies to be

brazed into either a molten filler metal bath or a

molten salt bath, at a temperature just above the

melting point of the filler metal

� Infrared Brazing (IB)

� Diffusion Brazing (DFB)

� Etc.

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Joint designs commonly used in brazing operations. The clearance between the two parts being brazed is an important factor in joint

strength. If the clearance is too small, the molten braze metal will not fully penetrate the interface. If it is too large, there will be insufficient capillary action for the molten metal to fill the interface.

�Joint Design

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Examples of good and poor design for brazing

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• Soldering

�In soldering, the filler metal, called solder, melts

at a relatively low temperature and as in brazing solder fills the joint by capillary action.

�Heat sources are usually soldering irons, ovens,

or torches.

�Filler metal used for soldering melt below 4500C.

�Types of Solders and their ApplicationsTin-lead General purpose

Tin-zinc

Lead-silver

Cadmium-silver

Zinc-aluminum

Tin-silver

Tin-bismuth

Aluminum

Strength at higher than room temperature

Strength at high temperatures

Aluminum; corrosion resistance

Electronics

Electronics

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�Types of Soldering Techniques

� Torch Soldering (TS)

� Furnace Soldering (FS)

� Iron Soldering (INS)

� Induction Soldering (IS)

� Resistance Soldering (RS)

� Dip Soldering (DS)

� Infrared Soldering (IRS)

� Ultrasonic Soldering (US)

� Reflow Soldering (RS)

� Wave Soldering (WS)

�Types of Fluxes

� Inorganic acids or salts

� Resin based fluxes

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Joint designs commonly used for soldering.

Note that examples (e), (g), (i), and (j) are mechanically joined

prior to being soldered, for improved strength.

�Joint Design

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Reference Texts

• MANUFACTURING ENGINEERING & TECHNOLOGYSerope Kalpakjian, Steven R Schmid

Addison Wesley Longman (Singapore) Pte. Ltd.

Fourth Edition.

• MATERIALS & PROCESSES IN MANUFACTURINGE Paul Degarmo, JT Black, Ronald A Kohser.

Prentice-Hall India

Ninth Edition.