Welding

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welding processes

Joining Processes: Welding, Brazing, Soldering

1. Brazing and Soldering: Melting of filler rod only

• Brazing: higher temperature, ~brass filler, strong

• Soldering: lower temp, ~tin-lead filler, weak

2. Welding: Melting of filler rod and base metals

3. Both: Join inexpensive parts to form complex product

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Welding is a process of metal joining by applying heat or Heat and pressure

WELDING

Classification of welding processesClassification of welding processes

welding

Forge or Pressure Welding

( Under pressure without additional filler metal )

Fusion or non pressure welding

( With additional filler metal )

Forge or Pressure welding :- The work piece are heated to plastic state & then work pieces are joined together by applying pressure on them. In this case no filler material is used.

Fusion or non- pressure welding :- Here edge of work piece to be joined & filler material

both are heated to a temp. above the melting point of the metal & then allow to solidify

Forge or pressure welding

Welding heat created by

Furnace Electric current friction

Forge or furnace welding Resistance welding friction welding

Spot seam projection flash butt

Fusion or non pressure welding

Welding heat created by

Gas Electric Arc Chemical reaction

Oxy-acetylene welding Thermit welding

Carbon arc metal arc atomic hydrogen submerged arc plasma arc electro slag TIG MIG

Other processes of welding are :

Solid State welding : The solid state welding depends upon the theory

that if two perfectly clean metallic surfaces are placed in an intimate contact ,

the cohesive force between atoms of the two surfaces is sufficient to hold them together.

The various solid state welding process are:

Friction Welding

Ultrasonic Welding

Diffusion Welding

Explosive Welding

Modern Welding Processes: Electron Beam Welding

Laser Beam Welding

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Types of Joints

©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

Welding positions defined here for groove welds: (a) flat, (b) horizontal, (c) vertical, and (d) overhead

Arc Welding Positions

Welding ProcessesElectric Arc Welding -- Polarity

SMAW - DC Polarity

Straight Polarity

Shallow penetration Deeper weld penetration

(thin metal)

Reverse Polarity

(+)(+)

(–)(–)

(–)(–)

(+)(+)

AC - Gives pulsing arc

- used for welding thick sections

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Shielded Metal Arc Welding (SMAW): “Stick welding”

• Older, simple technology

• The electrode is also the filler rod

• Only for steel

• Strong welds if done properly (but often not)

• Very high heat input: good for thick parts, bad for grain growth and distortion

Welding ProcessesSMAW – Shielded Metal Arc Welding

• Slag keeps oxygen off weld bead during cooling

• Consumable electrode

• Flux produces protective gas around weld pool

• Flux coated rod

Power = VI 10 kW

Power... Current I (50 - 300 amps)Voltage V (15 - 45 volts)

• General purpose welding—widely used

• Thicknesses 1/8” – 3/4”

• Portable

1. Protects the weld from oxidizing with atmosphere by producing a shield of gas (Inert Gas Co2) around the arc and weld pool

2. Provide the slag which floats at the top of molten metal so as to protect the weld from rapid cooling and to protect weld from atmosphere. The slag is then brushed off after weld gets cooled

3. Some elements for arc stabilization (Different for AC and DC)

4. Some alloying elements to improve strength and other properties

5. Coating normally insulator thus permit electrode to use in narrow groove for welding

6. Adding some material to make slag viscous or fluid.

7. Normal Dia. 3.2,4,5,6,8 and 9 Length : 350 or 450mm

Functions of Coated Electrodes( flux coated )

In conventional arc welding, the fluxes are used to shield atmosphere around the molten metal and arc.

In inert gas welding, separate inert gas such as argon, helium, carbon dioxide are used for surrounding the electric arc and molten metal from the metal .

Thus inert gas serves as a shielding gas which displaces the oxygen and nitrogen from the air surrounding the arc and molten metal. Thus it provides protective shield around arc and molten metal. Thus it eliminates formation of metal oxides and nitrides which lower weld metal ductility and toughness.

Inert gas welding is of following types

1. Gas tungsten arc welding (GTAW) or Tungsten inert gas ( TIG )

2. Gas metal arc welding ( GMAW ) or Metal inert gas ( MIG )

INERT GAS WELDING

Metal Inert Gas Welding (MIG, GMAW)

Metal Inert Gas Welding (Gas Metal Arc Welding) is the arc welding process, in which the weld is shielded by an external gas (argon, helium, CO2, argon + oxygene

or other gas mixtures).

Consumable electrode wire, having chemical composition simiilar to that of the parent material, is continuously fed from a spool to the arc zone. The arc heats and melts both the work pieces edges and the electrode wire. The fused electrode material is supplied to the surfaces of the work pieces, fills the weld pool and forms joint.

Due to automatic feeding of the filling wire (electrode) the process is referred to as a semi-automatic. The operator controls only the torch positioning and speed.

Similarities between TIG and MIG

1. No fluxes are used to shield the atmosphere around molten metal

2. Instead of that , both are using separate inert gas ( Argon, helium, Carbon dioxide ) for shielding purpose

Differences between TIG and MIG

INERT GAS WELDING

TIG1. Arc is stuck between a

non consumable tungsten electrode and work piece to be welded.

2. Filler material is added from a hand held filler rod or wire of the same composition as the work piece

MIG1. Arc is struck between

consumable electrode( which serves as a filler material ) and work piece to be welded.

2. The electrode / filler is a wire fed from a reel continuously to the welding zone. Consumable electrode is having the same composition as work piece material

Welding ProcessesGMAW – Gas Metal Arc Welding (MIG)

• DC reverse polarity - hottest arc

• MIG - Metal Inert Gas

• Consumable wire electrode

• AC - unstable arc

Groover, M., Fundamentals of Modern Manufacturing,, p. 734, 1996

Gas Metal Arc Welding (GMAW) Torch

• Shielding provided by gas

• Double productivity of SMAW

• Easily automated

- torch

- electric power source

- shielding gas source

- wire spool with wire drive control

GMAW ( MIG ) :- Main equipment

Equipment required for the GMAW

Gas Metal Arc Welding (GMAW): “MIG” (Metal-Inert-Gas)• ~Complex mechanism but simple to perform and easy to automate

• The electrode is also the filler rod, fed continuously from a spool. It melts in the arc.

• For steel or aluminum

• Low skill level can achieve good weld

• Medium heat input: distortion and grain growth are significant

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The heat is produced by an electric arc between the continuously fed metal electrode and the base metal. Both the base metal and the filler are melt. The weld area is protected by inert shield gases.

Weldable metals:

-steel carbon

- steel low-allow

- steel stainless

- aluminum

- copper and its allows

- nickel and its allows

- magnesium

- reactive metal (titanium, zirconium, tantalum)

The GMAW ( MIG ) process

Characteristics of the weld joint by GMAW

GMAW Process ParametersGMAW Process Parameters

Steel Material .035” wire Short-Arc Steel Material .035” wire Short-Arc ModeMode

ThicknessThickness Gas Gas 75%AR-75%AR-25%CO25%CO22

AmpsAmps Wire Wire

SpeedSpeedVoltsVolts

1/8”1/8” 18-1918-19 140-150140-150 280-300280-300 23-2423-24

3/16”3/16” 18-1918-19 160-170160-170 320-340320-340 24-2524-25

1/4”1/4” 21-2221-22 180-190180-190 360-380360-380 24-2524-25

5/16”5/16” 21-2221-22 200-210200-210 400-420400-420 25-2625-26

3/8”3/8” 23-2423-24 220-250220-250 420-520420-520 26-2726-27

Advantages of Metal Inert Gas Welding (MIG, GMAW):

•Continuous weld may be produced (no interruptions);•High level of operators skill is not required;•Slag removal is not required (no slag);• High welding speed and can be easily automated

Disadvantages of Metal Inert Gas Welding (MIG, GMAW):

•Expensive and non-portable equipment is required;•Outdoor application are limited because of effect of wind, dispersing the shielding gas.

Tungsten Inert Gas Arc Welding (TIG, GTAW)

Tungsten Inert Gas Arc Welding (Gas Tungsten Arc Welding) is the welding process, in which heat is generated by an electric arc struck between a tungsten non-consumable electrode and the work piece.

The weld pool is shielded by an inert gas (argon, helium, nitrogen) protecting the molten metal from atmospheric contamination.The heat produced by the arc melts the work pieces edges and joins them. Filler rod may be used, if required.

Tungsten Inert Gas Arc Welding produces a high quality weld of most of metals. Flux is not used in the process.

Gas Tungsten Arc Welding

Welding ProcessesGTAW – Gas Tungsten Arc Welding (TIG)

• Non-consumable electrode

• a.k.a. TIG - Tungsten Inert Gas

• Shield gas usually argon

• Used for thin sections of Al, Mg, Ti.

• With or without filler metal

Power 8-20 kW

Current I (200 A DC) (500 A AC)

• Most expensive, highest quality

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Gas Tungsten Arc Welding (GTAW): “TIG” (Tungsten-Inert-Gas)

• The electrode is tungsten (not consumed)

• The filler rod is separate and fed manually

• High skill level required to achieve good weld

• Difficult to automate

• Low heat input and small weld bead: distortion and grain growth are minimized

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Gas Tungsten Arc Welding (GTAW): “TIG” (Tungsten-Inert-Gas)

• Typical good quality TIG welds

Advantages of Tungsten Inert Gas Arc Welding (TIG, GTAW):

•Weld composition is close to that of the parent metal;•High quality weld structure •Slag removal is not required (no slag);•Thermal distortions of work pieces are minimal due to concentration of heat in small zone. •Flux is not used; therefore, finished welds do not require cleaning of corrosive residue.

Disadvantages of Tungsten Inert Gas Arc Welding (TIG, GTAW):

•Low welding rate;•Relatively expensive;•Requires high level of operators skill

Welding ProcessesSAW – Submerged Arc Welding

• 300 – 2000 amps (440 V)

• Consumable wire electrodeGas Metal Arc Welding (GMAW) Torch

• Shielding provided by flux granules

• Automated process (limited to flats)

• Low UV radiation & fumes

• Flux acts as thermal insulator

• High speed & quality (4 – 10x SMAW)

• Suitable for thick plates http://www.twi.co.uk

Plasma Arc Welding

Plasma Arc Welding (PAW)Plasma Arc Welding is utilizing heat generated by a constricted arc (plasma jet ) struck between a tungsten non-consumable electrode and either the work piece (transferred arc process) or water cooled constricting nozzle (non-transferred arc process).

Plasma is a high temperature ionised gas which is a mixture of positive ions, electrons and neutral gas molecules.

The gas is forced past an electric arc thtough a constricted opening at the end of water cooled nozzle.Due to this gas gets heated and becomes ionised which is a plasma. As the arc is constricted proportion of ionised gas increases and plasma jet is created. This results in a more concentrated heat source at a higher temperature that greatly increases the heat transfer efficiency, allowing for faster travel speeds.

This plasma jet will take a narrow, columnar shape that make it ideal for weldingThis process uses two inert gases , one forms the plasma and second shield the arc weld metal. Filler rod may or may not be supplied.

Transferred arc process produces plasma jet of high energy density and may be used for high speed welding and cutting of Ceramics, steels, Aluminum alloys, Copper alloys, Titanium alloys, Nickel alloys. Arc is struck between tungsten non-consumable electrode

Non-transferred arc process produces plasma of relatively low energy density. It is used for welding of various metals and for plasma spraying (coating).Arc is struck between non consumable electrode and water cooled constricted nozzle. Since the work piece in non-transferred plasma arc welding is not a part of electric circuit, the plasma arc torch may move from one work piece to other

Plasma Arc Welding (PAW)

Advantages of Plasma Arc Welding (PAW): Ensures greater arc stability

•Requires less operator skill due to good tolerance of arc to misalignments;•High welding rate;•High penetrating capability (keyhole effect);

Disadvantages of Plasma Arc Welding (PAW): •Expensive equipment;

High distortions and wide welds as a result of high heat input.

Soldering & Brazing Metal Joining Processes

Soldering & Brazing

• Filler metal distributed by capillary action

• Only filler metal is melted, not base metal

• Strength of joint typically

– Can join dissimilar metals

– Less heat - can join thinner sections (relative to welding)

– stronger than filler metal itself

– weaker than base metal

– Excessive heat during service can weaken joint

• Pros & Cons

• Lower temperatures than welding

– gap at joint important (0.001 – 0.010”)

• Metallurgical bond formed between filler & base metals

Soldering

Solder = Filler metal

Metal Joining Processes

Soldering

Applications:

• Printed Circuit Board (PCB) manufacture

• Pipe joining (copper pipe)

• Jewelry manufacture

Easy to solder: copper, silver, gold

Difficult to solder: aluminum, stainless steels

(can pre-plate difficult to solder metals to aid process)

• Alloys of Tin (silver, bismuth, lead)

• Melt point typically below 840 F

Flux used to clean joint & prevent oxidation

• Typically non-load bearing

Tinning = pre-coating with thin layer of solder

• separate or in core of wire (rosin-core)

PCB Soldering

• Soldering Iron & Solder Wire


Manual PCB Soldering

• Heating lead & placing solder

• Trim excess lead

• Heat for 2-3 sec. & place wire opposite iron

PTH - Pin-Through-Hole connectors

PCB Reflow Soldering Metal Joining Processes

Automated Reflow Soldering SMT = Surface Mount Technology

Printed solder paste on a printed circuit board (PCB)

• Solder Paste serves the following functions:– supply solder material to the soldering spot, – hold the components in place prior to soldering, – clean the solder lands and component leads – prevent further oxidation of the solder lands.

• Solder/Flux paste mixture applied to PCB using screen print or similar transfer method

• PCB assembly then heated in “Reflow” oven to melt solder and secure connection

Brazing• Steel base metal + Brass filler rod is common

• Lower temp than welding: retains heat treatment (if present), minimizes grain growth.

• Strong but slow (careful preparation, cleanup)

• Furnace brazing is easily automated

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Brazing

Use of low melt point filler metal to fill thin gap between mating surfaces to be joined utilizing capillary action


Brazing

Applications:

• Pipe/Tubing joining (HVAC)

• Filler metals include Al, Mg & Cu alloys (melt point typically above 840 F)

• Automotive - joining tubes

• Electrical equipment - joining wires

• Jewelry Making

• Flux also used

• Types of brazing classified by heating method:

– Torch, Furnace, Resistance

• Joint can possess significant strength

Brazing

Use of low melt point filler metal to fill thin gap between mating surfaces to be joined utilizing capillary action


Brazing

Applications:

• Pipe/Tubing joining (HVAC)

• Filler metals include Al, Mg & Cu alloys (melt point typically above 840 F)

• Automotive - joining tubes

• Electrical equipment - joining wires

• Jewelry Making

• Flux also used

• Types of brazing classified by heating method:

– Torch, Furnace, Resistance

• Joint can possess significant strength

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Resistance Spot Welding (RSW): “Spot Welding”

• No filler rod: electrical current is passed through metal under pressure

• Low skill level required

• Easy to automate

• Low heat input and no weld bead: distortion and grain growth are minimized

WELDING DEFECTS


Welding Defects

Cracks Cavities Solid inclusions Imperfect shape or unacceptable

contour Incomplete fusion Miscellaneous defects

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Distortion from Welding Processes

• Non-uniform shrinkage of weld bead

• Difficult to maintain alignments

• Solution: Rigid fixtures, pre-compensate for warping, loose tolerances

Weld bead profile: Convex or Concave?

• Solidification of molten bead leads to shrinkage

• Shrinkage of a concave bead leads to tension on surface tends to crack

• Shrinkage of a convex bead leads to compression on surface does not crack

• Generally, slightly convex beads are preferred.

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A weld bead in which fusion has not occurred throughout entire cross section of joint

Several forms of incomplete fusion are shown below

Incomplete Fusion


(a) Desired profile for single V-groove weld joint, (b) undercut - portion of base metal melted away, (c) underfill - depression in weld below adjacent base metal surface, and (d) overlap - weld metal spills beyond joint onto part surface but no fusion occurs

Weld Profile in AW

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Welding Defects

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Welding Flaws:

Incomplete Penetration

(not enough heat input)

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Engineering

Welding