CTU in Prague

Faculty of Mechanical Engineering

Ing. Petr Vondrouš, PhD., IWE

Theory of Joining and Cutting

1st semester 2015/2016

TJC - Background

Subject name: Theory of Joining and Cutting

Abreviation: TJC

Department of Manufacturing Technology

Total hours: 52,

Lecture 2 hrs/week

Practice 2 hrs/week

Finished: assessment + exam

Language: English

Teachers:

Ing. Vondrous: petr.vondrous@fs.cvut.cz

Ing. Kovanda, Ing. Kolarik, Ing. Rohan, Ing. Kramar

During practices others

+ engineers from companies during visits

TJC - Background

Contents:

Course covers technologies of welding, brazing and thermal cutting - SMAW, GMAW, GTAW, SAW, oxyacetylene cutting, plasma cutting..

Welding equipment and typical application of technologies in the industry is done.

Covered is also topic of material weldability and assessment of joint quality by destructive and non-destructive methods.

Assessment: 1. presence at the practices – max. 2 absences

2. submission and presentation of project report

Exam: Final evaluation of the subject is based on project and oral examination.

TJC – Expected knowledge

for students with finished Bc. degree

good level of language and technical knowledge is expected

Expected: good level of English

knowledge of materials, drawings, technology on the level of Bc. degree

do study at home if you lack some knowledge

Books available at CTU library and also at National Technical Library.

Internet – Focus on scientific and company information.

Eg.: www.scirus.com, scholar.google.com

Responsibilities of welding engineer

1. What does he do?

2. What is his responsibility?

3. What he must know?

4. Who is he subjected to do?

5. Who is he supervising?

6. Who does he cooperate with?

KNOWLEDGE – process, machines, materials,

drawings, norms

COOPERATION – welders, clients, boss

LEGAL RESPONSABILITY

TJC – Overall content 1

Order /

Day

Contents - lecture Practice

1 Organization, Contents, Welding industry,

technologies, mat., weld joints

Project introduction,

safety, welding catalogue

2 SMAW – arc, technology, use, limits, flux SMAW-labs

3 GMAW, GTAW – technology, welding torch,

use, SAW

GMAW, GTAW-labs

4 Laser welding, EBW, RSW Labs

5 Material weldability CET, preheat temperature

6 Excursion Exc. –Meiller-Kipper

7 Types of weld joints, joint preparation Load capacity of weld

8 Thermal cutting Oxy-fuel, laser cutting

9 Quality control, DT, NDT DT, NDT

10 Welding quality – welders qualification Welders qualification

11 Welding process qualification Process qualification

12 Excursion Excursion-Metrostav

13 Project discussion Assessment, project

closure

TJC –Contents of 1st lecture

1.Joining, welding process

2.Some specifics of welding industry

3.Technologies overview

4.Materials used

5.Weld joint design

Manufacturing process

Basic group of technologies

change of material`s shape

material removal

material addition

Where and why is welding important technology?

1. casting2. forming, forging

3. machining

4.assembly5. welding6. additive manufacturing

Joining processes

Aim: to join parts together to form assemblies which provide useful mechanical functions

E.g.: sewing with thread, zips, buttons, bricks and mortar, wood joints(dove-tailed joint)

Evolution of joining: sewing, weaving, dove-tail joint, metal rivets, srewed connections, welding

Permanent

Welding

Brazing

Adhesive

Rivets

Press Fit joints

Nails

Removable Fasteners

Bolts

Screws

Pins

Circlips

Keys

Welding processes

Welding – technology creating pernament connection between parts by creating atomic bonds between welded pieces accompanied by diffusion

Conditions of weld creation can be fullfiled by:

a)heating the pieces to melting point and fusing them – FUSION

b)applying pressure to the pieces in cold or hot state – PRESSURE

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, microstructure change, distortions in the weld. Harmful

effects: light, ultra violate radiation, fumes, high temperature.

Applications of welding: Buildings and bridges structures; Automotive,

ship and aircraft constructions; Pipe lines; Tanks and vessels; machinery

Fusion welding processes

Pressure welding processes

14

Importance: To Manufacturing

• Welding plays a key role in the

manufacturing challenge

• Welding is a key contributor to:

• Heavy Manufacturing

• Light Manufacturing

• Construction

• Transportation

• Electronic/Medical

• Maintenance & Repair

• Energy

15

• Welding is viewed as “dark & dirty”

• The critical role of welding is not fully appreciated in all corporate

and government offices.

• High schools phasing out welding programs and discouraging

careers despite annual earnings potential of $30,000 - $60,000.

• 1/2 of US industries report difficulties locating qualified welders.

• The US infrastructure is aging.

• Shortage “barrier” resulting in fewer competitive welding

techniques, less productivity and outsourcing to foreign markets.

• Natural disaster prone areas have immediate need for welders to

quickly rebuild homes and cities.

Status: Welding Industry

– Not the case anymore.

16

Status: Career Paths• Welder

Extensive and

growing job variety

• Engineers

• Researchers

• Business Owner

• Teachers

• Salesperson

• Machine Operator

• Underwater Welder

• Brazer

• Robotic Arc Welding

Technician/Operator

• Radiographic

Interpreter

• Inspector

• Maintenance and

Repair Tech.

17

Status: Opportunities

18

Status: Where Welders Work

• Military

• Aircraft and Aerospace

Industry

• Building Construction

• Automotive Industry

• Bridge/Highway

Construction

• Shipbuilding Industry

• Universities and Schools

• Safety Products

• Boiler Industry

• Medical Industry

• Mining Industry

• Robotics and Computer

Engineering

• Farming Equipment

Manufacturers

• Job Shops

• Specialty Gases

• Consumer Electronics

Civil engineering - Structures

Bridges

Buildings

Highways

Used technologies?Used materials?

Energy generation, pipelines

Used technologies?Used materials?

Transportation

Used technologies?Used materials?

Car

Used technologies?Used materials?

Is result of welding acceptable?

What is weldability?weldability is technological property that expresses suitability of material to create acceptable weld joint fulfilling all requirements by specific welding method and conditions

– 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.)

Welding technology

Welded material Structure design

All 3 have influence on the welding result.

Brief History of Welding

Late 19th Century

– Scientists/engineers apply advances in electricity to heat and/or join

metals (Le Chatelier, Joule, etc.)

Early 20th Century

– Prior to WWI welding was not trusted as a method to join two metals

due to crack issues

1930’s and 40’s

– Industrial welding gains acceptance and is used extensively in the

war effort to build tanks, aircraft, ships, etc.

Modern Welding

SSSR - Paton

USA -

France -

UK – TWI

Czechoslovakia

NOWADAYS SO MANY METHODS

Types of Welding

Fusion Welding Pressure Welding

Homogeneous Heterogeneous

Brazing SolderingGas Welding

Electroslag

High Energy Beam

Electric Arc

MIG

TIG

Shielded Metal Arc – “Stick”

Friction Welding

Other schemes, more detailed may exist as well-but basically divided by ENERGY SOURCE.

Fusion welding X Pressure welding

Fusion welding - Material is over the melting, fusion

temperature, upon cooling joint is created.

Base metal is melted, filler metal may be added

Heat is supplied by various means

– Oxyacetylene gas

– Electric Arc

– Plasma Arc

– Laser

Pressure welding – external force is needed, in molted or solid

state

– Diffusion welding

– Resistance welding

– Explosion

– Solid state welding

Joint Design

BUTT

JOINT

STRAP JOINT

LAP JOINT

FILLET JOINT

CORNER JOINT

Welding Positions

FLAT

HORIZONTAL

VERTICAL

OVERHEAD

INCREASING DIFFICULTY

Generalized Welding Symbol

FAR SIDE DETAILS

ARROW SIDE DETAILS

Field weld symbol

Weld all-around for

pipes, etc.

L1-L2

L1-L2

D = Weld Depth (usually equal to plate thickness)

L1 = Weld Length

L2 = Distance between centers for stitched welds

The Field Weld Symbol is a guide for installation. Shipyards

normally do not use it, except in modular construction.

Electrode

Material

D

D

Weld Geometry

Example Welding Symbol

5 5

5

5

One-sided welds are max 80% efficient

Two sided are 100% efficient

Geometry symbol for Y-groove5 150

Welded materials

Most widely used engineering materials:

Ferrous materials

Steels

Cast irons

Non ferrous

Al, Ti, Mg, Ni alloys

Where are these materials used?

What properties of the material and about welded structure we must know

before commencing the welding?

Material properties and weldability

What properties influence weldability?

Properties Examples

Mechanical properties Strength Young modulus Ductility

Physical properties Density Thermal expansion Heat conductivity

Chemical properties Reactivity Corrosion Flammability

Manufacturing properties Castability Weldability Formability

Cost, availability Prize Terms Place

Fusion welding

• These processes are associated with molten metal-

atmospheric reactions

– Oxides and nitrides are formed –oxide layer,

– Change of chemical composition

– Discontinuities such as porosity – emental cheese, leak proof ?

– Poor weld metal properties

• Need of shielding – during heating and cooling

– Vacuum

– Gas shielding – inert, active gasses

– Gas forming components

– Liquid layer – molten slag

Heat source energy density

• Heat source density influences speed of welding,

heat input per unit length, fase transformations and their

dynamics.

with increasse of energy density HAZ area, distortions

become smaller and welding speed increaces.

Oxyacetylene Welding

• Flame formed by burning a mix of acetylene (C2H2) and oxygen

• Fusion of metal is achieved by passing the inner cone of the flame over the metal

• Oxyacetylene can also be used for cutting metals

Inner Cone: 3300°C Combustion Envelope 2000°C

1200°CTORCH TIP

VIDEO - OAW

Arc welding processes

• Welding processes that employ an electric arc are the most

prevalent in industry

– Shielded Metal Arc Welding

– Gas Metal Arc Welding

– Flux Cored Arc Welding

– Submerged Arc Welding

– Gas Tungsten Arc Welding

SMAW

• Easy, equipment inexpensive, portable

• Filler metal and shielding of the weld puddle are provided by the

covered electrode

• Less sensitive to drafts, dirty parts, poor fit-up

• Can be used on carbon steels, low alloy steels, stainless steels, cast

irons, copper, nickel, aluminum

SMAW - MMA - drawbacks

• slow, low deposition rates, low productivity

• manual – operator dependent

• heat input is very high, small energy density

• Inadequate weld pool shielding for reactive metals – Ti, Zr, Ta

GMAW – MIG/MAG

There are four methods in which the wire is transferred to the molten pool:

•short circuiting

•globular

•pulsed spray

•spray transfer

AWS - Welding Handbook

GMAW – MIG/MAG – Metal transfer modes

According to setting of the welding parameters, there are 3 modes of molten

wire transfer to the molten pool:

•short circuiting – small welding parameters,

till 150 A, 20-200 Hz, typical for MMA

•globular – 180 A , Ar–2% O2 shielding

•spray transfer (for special invertor welders pulsing of current can be done -

pulsed spray transfer)

Above a critical current level, high

electromagnetic force (Lawrence force) -

small discrete metal drops, figure shows

spray transfer during GMAW of steel at 320A

and with Ar–2% O2 shielding.

Shielding in GMAW

Shielding gas can affect

– Weld bead shape

– Arc heat, stability, and starting

– Surface tension

– Drop size

– Puddle flow

– Spatter

Important gas characteristics:

ionization potenrial – arc start, stability, arc heat – heat input

heat transfer ratio – heat input

influence on surface tenstion – drop size, puddle flow, spatter

Ar Ar-He He CO2

GMAW characteristics

Advantages

•High deposition rates (higher than SMAW)

•Continuous electrode

•No slag creation

•Easily automated, versitality

•Deeper penetration (compared to SMAW)

Drawbacks

•Gun size

•Many process variables

Use in automotive:

•continuos, sealing welds - exhaust systems

•robotic welding of steel and Al

•arc spot welding can be variant to RSW

FCAW Flux cored arc welding

Uses tubular wire with flux inside.

Advantages

•High deposition rates (higher than SMAW)

•Continuous electrode

•No slag creation

•Easily automated

Drawbacks

•Gun size

•Many process variables

FCAW characteristics

Advantages

•Less cleaning, good self shielding protection

•High deposition rates (higher than SMAW)

•Continuous electrode

•Easily automated

•Deeper penetration

Drawbacks

•Slag creation

•Fumes

•Price

•spatter

Very reduced use in automotive:

•only small series production

•stainless welding, mufflers

SAW submerged arc welding

- use of granular flux which is fed into the joint from a flux hopper

- arc is struck between the wire and the workpiece beneath the flux

- arc and weld pool are shielded by the resulting envelope of molten flux

SAW submerged arc welding

Advantages

•continuous wire electrode – high diameter (max 5-6

mm)

•high current (1000 A), high deposition rates – for

thick materials

•no arc flash or glare

•Minimal smoke and fumes

•flux and wire added separately - extra dimension of

control

•easily automated

•joints can be prepared with narrow grooves

•used to weld carbon steels, low alloy steels,

stainless steels, chromium-molybdenum steels,

nickel base alloys

•twin wire, multiple torch …

Drawbacks - slag creation, much heat input, welding

process not visible

GTAW – TIG

- gas tungsten arc welding - arc is established between the tip of a tungsten

electrode and the workpiece to melt the base material (possibly also filler)

-tungsten electrode is considered to be non-consumable

-inert shielding gas protects the molten weld pool

GTAW – change of polarity

- non consumable electrode and invertor welder makes possible change of

polarity, stable arc, pulsating

Superior quality welds, generally free from spatter, porosity, or other defects

-Precise control of arc and fusion characteristics

-Weld almost all metals

-Used with or without filler wire

-Easily automated

-Used in all positions, intricate geometries weldable

GTAW - use

for thin sheets – very precise

sealing joints

precision welding

atomic energy, aircraft – turbine blades

repair of dies, forming tools

no spatter,

small distortion, high precision

especially for highly alloyed materials - SS, HSS, AHSS

especially for light alloys - Al, Mg

heat exchangers, exhausts, pressure wessels

PAW – Plasma arc welding

energy level of the arc plasma is constricted by flow of gas to increase energy

density in controlled manner

use of special gas nozzle around a tungsten electrode operating on DCEN

two variants - transferred arc process

- non-transferred arc process

PAW

-plasma gas – clean argon

-shielding gas – argon, helium, mixtures

Narrower welds can be made with PAW than with GTAW because of the

constricted arc. PAW can also be used on Titanium and other reactive metals.

High energy density – keyholing effect

Advantages

Greater Energy Concentration

Improved arc stability over other processes

Higher heat content, Higher travel speeds

Greater penetration capabilities

Finer sections can be welded with low current PAW than GTAW

Disadvantages

Operator skill required is slightly greater than for GTAW

Equipment more expensive

Orifice cleanliness, replacement

PAW - use

Gives better control than GTAW.

for thin sheets – very precise

sealing joints

precision welding

atomic energy, aircraft – turbine blades

repair of dies, forming tools

no spatter,

small distortion, high precision

especially for very thin materials – heat exchangers (Al)

fast – 5,5 m/min

high alloy materials, SS,

tailored welded blanks

very deep penetration

High energy beam welding

High Energy Density Beams – increase of energy density

possibility of keyhole welding

Plasma Arc welding

Electron beam welding

Laser beam welding

These processes focus the energy onto

a small area

EBW – electron beam welding

Deepest single pass weld penetration – max 300 mm

Fast travel speeds

Low heat input welds produce low distortion

Question:

Why EBW can produce the

deepest welds? What is the

biggest drawback of this

method?

LBW – Laser Beam Welding

principles of LBW will be explained in detail in specialized presentation

Heat

treatment Welding Cutting

Focal spot positionQuestion:

How can focal spot

position influence

the process?

Pressure welding

Resistance welding – spot

The resistance of metal to the localized flow of current produces heat

Process variables

Current

Time

Force

Spot and seam welding

used extensively for sheet metal joining

automotive, appliances …

High speed, < 0.1 seconds in automotive spot welds

Excellent for sheet metal < 5 mm

No filler metal

contact resistance between sheet metals is much higher than the bulk

resistance of the copper electrodes or of the sheet metal itself and the

highest resistive heating occurs between the two pieces of sheet metal

Resistance welding – seam

the welding electrodes are motor-driven wheels

power source is controled to produce ovelapped, water-tight

weld-gas tank

Roll spot weld Overlapping seam weld Continuous seam weld

Resistance welding

BIW has upto 5000 spot welds

process limitations:

voluminous equipment – non portable

only lap joints

high increase of current demand with thickness

dificcult NDT testing – nugget non visible

nugget acts like stress concentrator- low tensile

and fatique strength

Adaptive quality control for RSW

With increase of speed of electronics

devices, online NDT testing of weld

nuggets have become available.From Bosch Rexroth

Stud welding – arch stud welding

-stud is initially in contact with the plate

-use of ceramic sleeve (ferrule provides shielding and support)

-solenoid lifts the stud off the plate and arc is established an arc

-after an appropriate amount of arcing – heat input, the stud is driven into the

workpiece

Stud welding - capacitator discharge

-variation of the stud welding

-energy stored in a capacitor is discharged,

initiating an arc and melting

-stud is then forced into the workpiece

Stud welding - use

-very versitile

-fast

-economical

-automation industry - fixation of screws

-fastening, assembly of metal sheets (steel, Al, Mg), plastics …

-construction industry

-electronics, home eppliances

Pressure welding – solid state welding

Processes that produce a weld through the application of pressure at a

temperature below the melting temperature of the base material; no filler metal

is used.

Friction welding, friction stir welding

Diffusion welding

Ultrasonic welding

Explosion welding

By joining materials in the solid state, many of the difficulties of the fusion

processes are avoided.

dissimilar metals joining-thermal expansion mismatch between the two is

decreased in solid state processes because of the lower temperature

Friction welding - FRW

The workpieces are brought into contact and rotated very rapidly to produce

heat. Usually one piece is rotated against a stationary piece to produce the

heat at the junction.

Used for round bars, tubes, joining of bars to sheet …

fast, easy automation

easily joins Cu, Al, Cu+Al, Cu+steel, plastics

Drawbacks: clamping forces, rotational symmetry, forgeable

material is neccessary, S in steel is not good

Ultrasonic welding - USW

The parts to be joined are held together under pressure and are subjected to

ultrasonic vibrations at right angles to the contact area.

Ultrasonic welding processes occur when vertical oscillations at frequencies of 10

to 50 kHz are transmitted through polymers and dissipated in a bond line. Heat is

generated through a combination of friction and hysteresis.

Ultrasonic welding - USW

• Cost-effective process

• Large batch-sizes are possible – cycle time about 300 ms

• Very short cycle time

• clean

• precise

• Typical for thermoplastics, Al sheet, Cu wires

• Electrical connections, electronics, circuits – wires, chips, memories

• multiple ply joints – batteries

• hermetic sealing of containers at low temperature - lighter

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.)

Weld Metal Protection

During welding, the molten and heated metal is susceptible to

oxidation – need to protect weld from the atmosphere

•How?

– Weld Fluxes

– Inert Gases

– Vacuum

SMAW, SAW - Fluxes can be SiO2, TiO2, FeO, MgO, Al2O3

– Produces a gaseous shield to prevent contamination

– Act as scavengers to reduce oxides

– Add alloying elements to the weld

– Influence shape of weld bead during solidification

GMAW, GTAW – gases as Argon, helium, nitrogen, and carbon dioxide form a protective envelope around the weld area

Data source

This presentation used data from:

-Ladislav Kolařík, Ústav strojírenské technologie,

CTU in Prague

-AWS, American Welding Society

-other sources – list upon request