Welding lectures 11 13

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Casting Forming & WeldingCasting, Forming & Welding(ME31007)

Jinu Paul

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J u auDept. of Mechanical Engineering

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Welding Lecture – 11Welding ecture

Design of Weld joints

11 Oct 2012, Thursday 8.30 am‐9.30 am

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joints

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Design of Weld joints

(Refer class notes)

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Example No: 3

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Example No: 4

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Welding Lecture ‐ 12Welding ecture

Welding Processes‐

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12 October 2012, Friday 11.30 am ‐12.30 pm

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Other fusion welding processes

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Thermite mixture

Metallic fuel + Oxidiser Energy

Thermite Reaction

Metal oxide + Aluminum Metal + Aluminum oxide + Heat

• Bimolecular reactions and reaction rates are controlled by diffusion times between reactants. • Thermite mixtures of nano-sized reactants reduce the critical diffusion length thus increasing the overall reaction rate

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Thermite Reaction stages

(1/2)Fe3O4 + Al Fe + (1/2)FeAl2O4

2FeO + Al (3/2)Fe + (1/2)FeAl2O4

(1/2)FeAl2O4 + (1/3)Al (1/2)Fe + (2/3)Al2O3

Thermite types

Fuels Oxidisers

Aluminium,Magnesium,Titanium,Zinc,Silicon,Boron

Boron(III) oxide,Silicon(IV) oxide,Chromium(III) oxide,Manganese(IV) oxide,Iron(III) oxide,Iron(II,III) oxide,Copper(II) oxideCopper(II) oxide, Lead(II,III,IV) oxide,

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Thermite welding (TW)• Heat for coalescence is produced by

superheated molten metal from the chemical reaction of Thermitereaction of Thermite

• Example: 2Al + Fe2O3 2Fe + Al2O3 + heat• Filler metal is obtained from the liquid metal• More in common with casting than it does with

welding• Applications in joining of railroad rails and

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• Applications in joining of railroad rails and repair of cracks in large steel castings and forgings such as ingot moulds, large diameter shafts, frames for machinery, and ship rudders

Thermit welding (TW)

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Fe2O3 + Al 2Fe + Al2O3 + ~850kJ

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High-Energy-Density Beam Welding Processes

Electron beam and• Electron-beam and

• Laser-beam welding

• Focussed beam of electromagnetic energy– IR welding

Imaged arc welding

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– Imaged arc welding

– Microwave welding

Comparison of Conventional and E/Laser-Beam Welding

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Electron-beam welding (EBW)

• Uses kinetic energy of dense focused electrons

• Electrons emitted by cathode, accelerated by ring shaped anode, focused by electromagnetic field

• High energy density 10

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MW/mm2

• Heat focus on few micrometers

• Vacuum chamber

Electron speed VsAccelerating voltage

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E-Beam interaction with work piece

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Electron-beam penetration Vs operating pressure

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EBW or LBW of a butt joint

Butt joint i t

Melting occurs at the A key The keyhole

d it ltThe weld

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prior to welding

occurs at the point of impingement of the E-beam

hole forms

and its molten envelope penetrates workpiece

The weld forms upon solidification

Laser-beam welding (LBW)

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Laser-beam welding

• Coalescence is achieved by the energy of a highly concentrated, coherent light beam focused on the joint to be weldedjoint to be welded

• LBW is normally performed with shielding gases (e.g., helium, argon, nitrogen, and carbon dioxide) to prevent oxidation

• No vacuum chamber is required, no X-rays are emitted

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• Laser beams can be focused and directed by optical lenses and mirrors.

• LBW does not possess the capability for the deep welds and high depth-to-width ratios of EBW

Example‐1

A carbon dioxide laser with a power output of 1 kW operates in the continuous wave mode. (For CO2 laser, wavelength = 10 micron = 0.01 mm). Focal length f and diameter of the lens used is 100 mm and 8 mm respectively. The diameter of laser beam is 6 mm.

The laser-beam welding operation will join two pieces of steel plate together as shown in figure. The plates are 25 mm thick. The unit melting energy is 10 J/mm3 The heat transfer factor isThe unit melting energy is 10 J/mm3. The heat transfer factor is 0.70 and the melting factor is 0.55. Find the velocity of the laser beam movement if the beam penetrates the full thickness of the plates?

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Laser beam‐ Depth of penetration

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Focussed IR welding

• Infrared radiation from the sun or an artificial light source can be used • Radiation is focused into an intense, high-density spot directed onto the work

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Imaging arc welding

• High energy density due to focussing• Advantage is freedom from the electromotive Lorentz forces associated with conventional arc welding

Comparison of Electron‐Beam and Laser‐Beam Welding

EBW LBW

1 Deep penetration in all materials 1 Deep penetration in many materials but1. Deep penetration in all materials

2. Very narrow welds

3. High energy density/low linear

4. Best in vacuum, to permit electrons

5. Usually requires tight-fitting joints

6. Difficult to add filler for deep welds

1. Deep penetration in many materials, but not in metals that reflect laser light/or of specific wavelengths

2. Can be narrow (in keyhole mode)

3. Same

4. Can operate in air, inert gas, or vacuum

5. Same

6. Same

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7. Equipment is expensive

8. Very efficient electrically (99%)

9. Generates x-ray radiation

7. Same

8. Very inefficient electrically (- 12%)

9. No x-rays generated

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Welding Lecture ‐ 13Welding ecture 3

Solid state welding processes

17 October 2012 9.30 am ‐10.30 am

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processes

Solid state/Nonfusion welding

• Accomplish welding by bringing the atoms (or ions or molecules) to equilibrium spacing through plasticmolecules) to equilibrium spacing through plastic deformation application of pressure at temperatures below the melting point of the base material

• Without the addition of any filler

• Chemical bonds are formed and a weld is produced di l f h i i b i d

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as a direct result of the continuity obtained, always with the added assistance of solid‐state diffusion

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Solid state/Nonfusion welding

1. Pressure Welding By pressure and gross deformationdeformation

2. Friction welding By friction and microscopic deformation

3. Diffusion welding By diffusion, without or with some deformation

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4. Deposition welding Solid‐state deposition welding

Pressure WeldingCold welding

• Pressure is used at room temperature to produce l f t l ith b t ti l l ti

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coalescence of metals with substantial plastic deformation No heat

• The faying surfaces must be exceptionally clean• Cleaning is usually done by degreasing and wire brushing

immediately before joining

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Pressure Welding Cold welding

• At least one of the metals to be joined must be highly ductile and not exhibit extreme work hardeningductile and not exhibit extreme work hardening

• FCC metals and alloys are best suited for CW. Example‐ Al, Cu, and Pb

• To a lesser degree, Ni and soft alloys of these metals such as brasses, bronzes, babbitt metals (Sn, Cu, Sb, Pb), and pewter (Sn Cu Sb Bi)pewter (Sn, Cu, Sb, Bi)

• Precious metals, Au, Ag, Pd, and Pt, are also ideally suited to cold welding, as they are face‐centered cubic (soft) and are almost free of oxides

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Pressure Welding Cold welding

• Ideal for joining of dissimilar metals no intermixing of the base metals is required

• Allows inherent chemical incompatibilities that make fusion welding difficult to be overcome

• E.g. Cold welding of relatively pure Al to relatively pure Cu Electrical connections

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• Formation of brittle intermetallics (e.g., AI,Cu)either during postweld heat treatment or in service, (resistance heating in the electrical connector)

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Micro‐patterning of Organic Electronic Devices by Cold‐Welding

Calculated normal stress at the interface (yy) normalized to the applied pressure (P) as a function of distance (x) normalized to the stamp half-width (a)

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Micro‐patterning of Organic Electronic Devices by Cold‐Welding

• A prepatterned, metal‐coated stamp composed of a rigid material (Si) is pressed onto an unpatterned film consisting of the organic device layers coated with the same metal contact layer as that used to coat the stamp.coated with the same metal contact layer as that used to coat the stamp.

• Organic layer thickness ~ 100 nm, same thickness for the metal cathode

• When a sufficiently high pressure is applied, an intimate metallic junction is formed between the metal layers on the stamp and the film, leading to a cold‐welded bond (Fig 1, top).

• To induce selective lift‐off, additional pressure is applied to weaken the metal film at the edge of the stamp (Fig. 1 , middle).

• This additional pressure leads to substrate deformation, which is expected to enhance the local weakening of the metal film.

• When the stamp and film are separated, the metal cathode breaks sharply, forming a well‐defined patterned electrode (Fig. 1, bottom).

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Fabrication of OLEDs

(A) Optical micrograph of an array of 230-mm-diameter Mg-Ag alloy contacts patterned by cold-welding followed by cathode lift-off. (B) Scanning electron micrograph (SEM) of the edge of a 12-mm-wide stripe showing a clearly defined nearly featureless layer pattern.

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Cold welding of ultrathin gold nanowires

Singlecrystalline gold nanowires with diameters between 3 and 10 nm can be cold-welded together within seconds by mechanical contact alone

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Head‐to‐head welding of two Au‐ nano rodsa,b, One nanorod (right) is caused to approach another (left) until their front surfaces come into contactcontact. c–e, The welding process is completed within 1.5 s (c,d) followed by structure relaxation (d,e).f–i, After withdrawal of the STM probe (f–i), the as-welded nanowire is left in the free-standing state

(Triangles indicate the front edges of the two nanorods. Arrows indicate the withdrawing direction of the STM probe. Scale bars, 5 nm) 39

Pressure WeldingHot Pressure Welding

HEAT PRESSURE+

Vacuum or shielding

MACROSCOPIC DEFORMATION

COALESCENCEExamples: 1) Pressure gas welding 2) Forge welding

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Pressure Welding Forge welding (FOW)

• Earliest form of welding still used today by blacksmiths

• Produces the weld by heating work pieces to hot working temperatures and applying blows sufficient to cause deformation at the faying

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deformation at the faying surfaces

• Low‐carbon steels (most commonly forge‐welded metal), high‐carbon steel

Pressure Welding Forge welding

Schematic of typical joint designs for (a) manual and (b) automated forge

ldi

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welding

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Pressure WeldingRoll Welding

li d b ll f d h ld

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• Pressure applied by rollers Performed hot or cold• Applications cladding stainless steel to mild or low alloy steel

for corrosion resistance• Making bimetallic strips• Producing ‘‘sandwich’’ coins for the U.S. mint

Pressure WeldingExplosion welding

• Coalescence of two metallic surfaces is caused by the energy of a detonated explosive

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energy of a detonated explosive

• Commonly used to bond two dissimilar metals

• E.g. To clad one metal on top of a base metal over large areas

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Pressure WeldingExplosion welding: Applications

• Applications include production of corrosion‐i t t h t d ki i i tresistant sheet and making processing equipment

in the chemical and petroleum industries

• E.g. Commercially pure titanium clad to mild steel

• Often performed under water to enhance the shock wave to move and deform material

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shock wave to move and deform material

Compatible materials for Explosion welding

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2.1 Friction welding (FRW)

• Solid state welding Coalescence is achieved by frictional heat combined with pressurefrictional heat combined with pressure

• Friction is induced by mechanical rubbing between two surfaces usually by rotation of one part relative to the other raises the temperature at the joint interface to the hot working range Parts are driven toward each

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working range Parts are driven toward each other with sufficient force to form a metallurgical bond


Mechanical Rubbing FRICTION HEAT

MICROSCOPIC DEFORMATION

PRESSURE

+

COALESCENCENo melting occurs at the faying surfacesNo filler metal, flux, or shielding gases

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Drive parameter characteristics in FRW

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2.2 Friction stir welding (FSW),

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FSW Tool

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• A rotating tool is fed along the joint line between two work pieces pGenerates friction heat

• Mechanically stirring of the metal to form the weld seam

• The process derives its name from this stirring or mixing action

• FSW is distinguished from

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• FSW is distinguished from conventional FRW Friction heat is generated by a separate wear‐resistant tool rather than by the parts themselves


• The rotating tool is stepped, consisting of a cylindrical shoulderconsisting of a cylindrical shoulderand a smaller probe projecting beneath it

• The probe has a geometry designed to facilitate the mixing action

Th h ld i

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• The shoulder serves to constrain the plasticized metal flowing around the probe

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• During welding, the shoulder rubs against the top surfaces of the two parts, developing much of the friction heat

• While the probe generates additional heat by mechanically mixing the metal along the butt surfaces

• The heat produced by the combination of friction and mixing does not melt the metal but softens it to a highly plastic condition

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Heat generated in FSW

(Refer Class notes)

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• Typical applications butt joints on large aluminium parts• Other metals, include steel, copper, and titanium, as well as

l d ipolymers and composites• Advantages of FSW

– Good mechanical properties of the weld joint,– Avoidance of toxic fumes, warping, shielding issues, and other

problems associated with arc welding,– Little distortion or shrinkage– Good weld appearance

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• Disadvantages include – An exit hole is produced when the tool is withdrawn from the work,

and – Heavy‐duty clamping of the parts is required

Key benefits of friction stir weldingMetallurgical benefits Environmental

benefitsEnergy benefits

1. Solid phase process

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1. No shielding gas required

1. Improved materials use (e g joining2. Low distortion of work

piece

3. Good dimensional stability and repeatability

4. No loss of alloying elements

5 Excellent metallurgical

required

2. No surface cleaning required

3. Eliminate grinding wastes

4. Eliminate solvents required for degreasing

use (e.g., joining different thickness) allows reduction in weight

2. Only 2.5% of the energy needed for a laser weld

3. Decreased fuel 5. Excellent metallurgical properties in the joint area

6. Fine microstructure

7. Absence of cracking

8. Replace multiple parts joined by fasteners

degreasing

5. Consumable materials saving, such as rugs, wire or any other gases

consumption in light weight aircraft, automotive and ship applications

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2.3 Ultrasonic welding (USW)

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• Two components are held together under d t l i fmodest clamping force

• Oscillatory shear stresses of ultrasonic frequency are applied to the interface to cause coalescence

• Oscillatory motion between the two partsOscillatory motion between the two parts breaks down any surface films allows intimate contact and strong metallurgical bonding between the surfaces

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• Although heating of the contacting surfaces occurs due to interfacial rubbing and plastic deformation, the resulting temperatures are well below the melting pointtemperatures are well below the melting point

• No filler metals, fluxes, or shielding gases are required in USW.

• The oscillatory motion is transmitted to the upper work part by means of a sonotrode, which is coupled to an ultrasonic transducer. This device converts electrical powerultrasonic transducer. This device converts electrical power into high‐frequency vibratory motion. Typical frequencies used in USW are 15 to 75 kHz, with amplitudes of 0.018 to 0.13mm

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• Clamping pressures are well below those used in ld ldi d d i ifi t l ticold welding and produce no significant plastic

deformation between the surfaces.

• Welding times under these conditions are less than 1 sec.

• USW operations are generally limited to lap jointsUSW operations are generally limited to lap joints on soft materials such as aluminum and copper.

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3. Diffusion welding (DFW)

• Welding process results from the application of heat and pressure, usually in a controlled atmosphere, with p y psufficient time allowed for diffusion and coalescence to occur

• Temperatures are well below the melting points of the metals (about 0.5 Tm)

• Plastic deformation at the surfaces is minimal

• The primary mechanism of coalescence is solid state diffusion, which involves migration of atoms across the interface between contacting surfaces

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3. Diffusion welding (DFW)• Applications of DFW include the joining of high‐strength and refractory metals in the aerospace and nuclear industriesindustries.

• The process is used to join both similar and dissimilar metals, and in the latter case a filler layer of a different metal is often sandwiched between the two base metals to promote diffusion.

• The time for diffusion to occur between the faying• The time for diffusion to occur between the faying surfaces can be significant, requiring more than an hourin some applications

• Key parameters of the process‐ temperature, time, and pressure

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3. Diffusion welding (DFW)• Diffusion occurs by an Arrhenius relationship, that is, exponentially

with temperature:

• Where, – D is diffusion coefficient (of the diffusing species) at temperature T,

– Do is a constant of proportionality (dependent on the particular diffusing species and host),

– Q is the activation energy for diffusion to occur,

– k is Boltzmann’s constant, and

– T is the temperature on an absolute scaleT is the temperature on an absolute scale

• In general, diffusion welding begins to take place at a reasonable rate when the temperature exceeds half the absolute melting point of the base or host material(s), and, as a rule‐of‐thumb, the rate of diffusion doubles every time the temperature is raised approximately 30°C

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• Time is important because diffusion takes time to occur, since for atoms to jump from site to site takesoccur, since for atoms to jump from site to site takes time. Thus, the distance over which diffusion occurs depends on time:

• Where

– x is the diffusion distancex is the diffusion distance,

– D is the diffusion coefficient (as above),

– t is time, and C is a constant for the system.

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3. Diffusion welding (DFW)‐Features

1. Metals as well as ceramics can be joined directly to form a completely solid state weld

ll b f2. Filler can be used to permit increased micro deformation to provide more contact for bond formation and/or promote more rapid diffusion by providing a faster diffusing species

3. Dissimilar materials either by class or type, including metal‐to‐ceramic joints, can be joined directly or with the aid of a compatible filler or intermediate

4. Large areas can be bonded or welded, provided uniform intimate contact can be obtained and sustained

5. No heat‐affected zone as such, since the entire assembly in which the diffusion weld is being made is virtually always heated to the same temperature.

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