ALUMINUM

WELDING

Why al welding is important?? Low Melting temperature (660°C) of Al.

Aluminum Oxide Surface Coating:-Aluminum reacts with air and

produce thin film of aluminum oxide which has melting point

approximately 1920°C(almost three times pure AL).

AL boils at lower Temperature then its oxide melts. The oxide is also

heavier then AL and when melted tends to be trapped in the molten

aluminum.

When Aluminum Oxide become thicker it will absorbs moisture from air.

Moisture is source of H2 which cause porosity in AL welding. Hydrogen

may also come from oil, paint and Dirt in the weld area. Hydrogen will

enter in weld pool and is soluble in molten aluminum. With a rapid

cooling rate, free hydrogen is retained within the weld and cause porosity.

Porosity will decrease weld strength and ductility.

Due to High Thermal Conductivity(205 W/m.k approx.) it requires larger

heat input when welding is begun, since much heat is lost in heating the

surrounding the metal

Higher thermal expansion co-efficient .(23.1* )

Absence of color change as Temperature approaches the melting point.

Aluminum suffers a reduction in strength in the weld area [unlike steel].

When stressed, a welded Aluminum structure will incur local deformation

in the welded area first.

Some Al alloy exhibit “hot short” tendencies and are creak sensitive.

These means at the range of temperature where liquid is slushy it has not

quite enough strength to resists the shrinkage stress that are occurring

from cooling and transformation.

General techniques used for Aluminum welding

• Pulsed MIG welding.

• TIG welding.

• Variable-Polarity Plasma arc welding.

• Laser welding.

But from all above the most common welding method used for

AL welding for T-178 CNA tank is TIG welding.

GAS TUNGSTEN ARC WELDING or TIG

WELDING.

• GENERAL SETUP:-TIG welding is widely used for welding aluminum and it

produces welds of good appearance and quality. TIG welding requires three thing,

HEAT, SHELDING, and FILLER METAL. The heat is produced by electricity

passing through the tungsten electrode by creating an arc to the metal. The shielding

comes from a compressed bottle of gas that flows to the weld area to protect it from air.

The filler metal is just a wire that is dipped by hand into the arc and melted. The way

these three things come together is pretty simple. First the welder turns on the gas flow,

many times by a valve on the TIG torch itself.

• The gas begins to flow and starts protecting the weld area from the air. The torch is held

over the weld joint just far enough for the torch not to touch the metal. Then the welder

presses a foot pedal and the TIG torches tungsten electrode starts an arc. Once the arc is

started the two pieces of metal begin to melt by creating a puddle of metal. Once the

puddle is established the welder with the other hand starts filling the joint by manually

dipping a welding wire into the arc to fill the joint. Ultimately this process creates a

single piece of metal.

• ELECTRODE:-Tungsten is the best choice for a non-consumable electrode Due to

High melting temperature and Good electrical conductivity.As the tungsten electrode

becomes hot the arc between the electrode and the work stabilizes But a clean and

correctly ground tungsten is needed. . The tungsten needs to pure tungsten and tungsten

alloyed with thorium oxide (ThO2) or zirconium oxide (ZrO2), which is added to improve

the arc ignition and to increase the life of electrode. The tungsten also needs a ball shape

at the end of it to spread the heat properly

• Because of the intense heat some erosion of the electrode will occur, the electrode may get

consumed quickly if it is allowed to get oxidized, since tungsten oxide has a lower melting

temperature. The oxidation occurs when the electrode is allowed to cool in the atmosphere

after welding. Hence, the shielding gas flow should be maintained for some time after

extinguishing the arc so that the electrode gets sufficiently cooled in a protective

atmosphere rather than in the oxidizing normal atmosphere. These include In the last years

some alloy elements have been incorporated, such as cerium and lanthanum, which also

increase the life of the electrode and tend to decrease the risk of radiation that is produced

when electrodes of high thorium content are employed. Zirconium electrodes are preferred

for AC, because they present a higher melting point than pure tungsten or tungsten-

thorium. During the welding process, it is assumed that the electrode tip is hemispherical

type. This is a very important aspect, because the arc stability depends in a greater manner

of tip geometry.

Shielding Gas for welding:- Primarily two inert gases are used for shielding

purposes. They are argon, helium, or a mixture of the two. They must be of high purity

for welding applications. The purity is normally held to 99.995%(UHP). Each of these

gases have characteristics which must be understood in making a selection of a shielding

gas for a particular application. The primary function of the gas is to exclude the active

properties in the surrounding air from the weld area. The type of gas also has an influence

on the characteristics and behaviour of the arc and resultant weld. The chief factor

influencing the effectiveness of a shielding gas is the gas density. Argon with an atomic

weight of 40 is about one and one third times as heavy as air and ten times as heavy as

helium with an atomic weight of 4. Argon after leaving the torch nozzle tends to form a

blanket over the weld area whereas helium tends to rise rapidly from the arc area. In order

to obtain equivalent shielding, flow rates for helium are usually two to three times that of

argon. The same is true of helium-argon mixtures, especially those with high helium

content.

Argon shielding gas is supplied in BLUE cylinders. Argon may be obtained in the gaseous

state in cylinders or as a liquid in specially constructed cylinders or bulk tanks. The

ionization potential of argon is 15.7 volts. Ionization potential is the voltage necessary to

remove an electron from the gas atom making it an ion or charged atom. Argon has low

thermal conductivity which means it is not a good conductor of heat. This results in a

higher arc density. Arc density refers to the concentration of energy in the arc. With argon

this energy is confined to a narrow area or "pinpointed" in a small area. With a given arc

length, the arc voltage with argon will be less than with helium. Argon provides excellent

arc stability and cleaning action even at low amperages. The correct flow rate is an

adequate amount to shield the "molten weld pool area" and protect the tungsten electrode.

Any greater flow than this is a waste. The correct flow rate in cubic feet per hour (CFH) is

influenced by many variables that must be considered on each application.

Helium shielding gas is supplied in BROWN cylinders. The ionization potential of helium

is 24.5 volts. Helium has excellent thermal conductivity. The helium arc column will

expand under the heat of the arc reducing the arc density. The arc column is more flared

out than the arc column with argon. The more flared out the arc column the more work

surface area is being heated. The heat at the centre of the arc, therefore, can move more

readily downward towards the colder metal at the bottom of the work piece. This results in

a deeper penetrating arc. With an equivalent arc length helium will produce a higher arc

voltage than will argon. With helium shielding any slight variation of arc length can have

quite an effect on arc voltage and, consequently, total arc power. For this reason helium is

not as desirable as argon for manual operations.

Helium is, therefore, used on applications where the torch is machine guided or held.

Because helium is a light gas, flow rates are usually two or three times that of argon for

equivalent shielding. With the increased flow rate, total cost of shielding goes up sharply.

The cost must be weighed against increased penetration on thick material, and the

increased travel speed attainable.

Helium / argon mixtures are sometimes used for their higher heat characteristics. Gas

mixtures, usually 25% helium and 75% argon are sometimes used and can help to increase

travel speeds when AC - gas tungsten arc welding. Mixtures of more than 25% helium for

AC – gas tungsten arc welding are used, but not often, as they can tend to produce

instability, under certain circumstances, in the AC arc.

FILLER WIRE USED:- The compositions of filler rods should be chosen to

suit the parent metals being welded. Filler rods should be stored in clean dry conditions to

prevent deterioration. They should be free from rust, scale, oil grease and moisture, which

would contaminate welds, and be cleaned with stainless steel wool or smooth aluminum

oxide cloth immediately before use.

Material of Construction for all Concentrated Nitric Acid Storage Tank (T–123, T–178 &

T–179) at Aniline TDI Complex is ASTM SB209 Al 1060 TEMPER ‘O’. For welding of

that material we use AWS/SFA 5.10 ER 1100 (2.50*1000) TIG-400.[Al-99.0%,ZN-

0.10%,MN-0.05,Cu-0.05to0.20%].

where 5.10- stands for Aluminum alloys.

5.1-for carbon steel

5.4-for stainless steel

5.8-for alloy steel

AWS/SFA 5.10 ER 1100. ER 1100 is designed for welding alloys

1060,1100,1350.welding of al 99.8,al 99.7al 99.5.

POWER SOURCE FOR WELDING:- It is possible to use Direct

Current (DC) or Alternating Current (AC) to weld by GTAW.

Alternating Current, commonly used when welding aluminium manually or semi-

automatically, combines the two direct currents by making the electrode and base material

alternate between positive and negative charge. This causes the electron flow to switch

directions constantly, preventing the tungsten electrode from overheating while

maintaining the heat in the base material. During the electrode-positive portion of the

cycle the thin, refractory aluminium oxide layer is removed, thus providing as self-

cleaning effect. We can use direct polarity (electrode negative, DCEN) or reverse polarity

(positive electrode, DCEP).

DCEN:-In this case, the electrode is connected to the negative pole of the heat source and

the electrons are emitted from the electrode and they are accelerated as they travel through

the arc (plasma). This effect produces a high heat in the work piece and therefore gives a

good penetration and a relatively narrow weld shape.

DCEP:-On the other hand, when reverse polarity is used, the electrode is connected to the

positive pole of the heat source. Now the effect of the heating due to the bombardment of

the electrons is higher in the electrode than that of work piece, which results in a wide

weld bead and shallower than that generated by direct polarity. In this case, due to high

energy concentration in the electrode it is necessary to employ a thicker diameter and a

cooling system to eliminate the electrode tip melting possibility. The bombardment effect

by positive ions of the inert gas removes the oxide film and produces a cleaning effect on

the welding surface. Therefore, reverse polarity can be used to weld materials that are

resistant to oxides such as aluminium, if it is not required a high penetration.

When alternating current is used, is possible to obtain a good combination of oxides

elimination (cleanliness) and penetration,. This polarity is the most employed to weld

aluminium alloys.

TORCHES:- The water cooled torch is designed so that water is circulated through

the torch cooling it and the power cable. The power cable is contained inside a hose, and

the water returning from the torch flows around the power cable, thereby cooling it.

PRECUTION TO BE TAKEN WHILE ALUMINUM WELDING

CUTTING:- An ordinary steel angle grinding disc would become clogged when

cutting or grinding aluminum. Special grinding discs are available for aluminium. Stone

cutting and grinding discs work well. Alternatively aluminium is quite soft and can be cut

using a metal blade on a jig saw. Aluminum dust and shavings left over from cutting or

filing when combined with either steel or iron dust can produce an explosive compound

known as Thermite. Thermite burns at a very high temperature and can cause serious

injury. To avoid this hazard, only use grinding wheels that have been used to grind

aluminum or use a new grinding wheel. Also Aluminium being a soft metal steel or iron

dust will easily get embedded in it and will create problem during welding process.

REMOVING THE OXIDE AND FINISHING:-The oxide layer

needs to be removed from the aluminium immediately before welding. If it is not removed

small particles of un-melted oxide will be entrapped in the weld pool and will cause a

reduction in ductility, luck of fusion, and may cause weld cracking. A stainless steel wire

brush is sufficient to remove the oxide and leave a clean surface. It's best to use a new wire

brush that hasn't been used on steel, and to brush in a single direction so the oxide isn't

rubbed into the aluminum.

PREHEATING OF ALUMINUM:-Preheat is needed when there is a risk

that, if a welding operation is carried out „cold‟, an unsound weld could be produced.

Aluminium Alloys have a high thermal conductivity and preheat is used to provide

additional heat to the weld area in order to help ensure full fusion of the weld. Application

of preheat is also used to drive off any moisture in the surface oxide. Preheating may not

be necessary when welding thin sheet. The actual preheat temperature required for a

specific welding operation depends not only on the material or materials being welded, but

also on the combined thickness of the joint, the heat input from the welding process being

used, and the amount of restraint imposed upon the components. Aluminium alloys are

preheated to temperatures between 150°C and 350°C. Certain heat treatable aluminium

alloys (Al-Si-Mg) are sensitive to HAZ liquation cracking if overheated, and preheat must

be carefully controlled within this range. Try pre-heating the aluminium with a blowtorch

first. Remember that aluminium alloys have relatively low melting points and care must be

taken to avoid overheating, which can result in poor

USING A BRASS / SS HEAT SINK:-Brass / SS have a much higher

melting point than aluminium and it is also quite inert which makes it an ideal material to

use as a heat sink. Heat sinks are effective when clamped immediately behind the

aluminum to be welded. The heat sink takes a great deal of heat from the weld, so the

welding motion can be much slower and hence neater. The reverse of the weld is flat

where the molten aluminium contacted the heat sink. The weld is strong and there is very

little aluminum to file away.

COOLING AND POST WELD CLEANING: -Allow the joint to

cool slowly in ambient air. Apply SS wire brush to clean the weld joint. Don‟t use CS wire

brush.

WELD BEAD TECHNIQUE:-Stringer bead: - In stringer bead technique

the arc stays in the middle of the puddle, so the heat energy first goes to the molten weld

metal pool and then is transferred away by the base metal. This will result in the puddle

staying in molten state longer. Aluminium tends to distort quite a bit when a local hotspot

is caused by a welding torch. Welding torch has to be moved quickly otherwise a single

part of the aluminium will get overheated and burn through. Never stop moving the

welding gun along the weld, as this will cause a localized area of the aluminium stock to

heat up

Weaver bead: - In Weaver bead technique weld bead is formed by moving the electrode

along the joint in a weaving motion. Do the weaving very rapidly but with a distinct pause

on each side. The pause does not have to be visible, just enough to feel it in your hands.

Also do the pause with enough frequency that you are keeping most of the puddle molten.

Here the electrode will help spread the heat to the base metal more evenly. This will

reduce the time of centre of the bead to remain hot.

TIG WELDING OF ALUMINUM TIPS

When using AC current on aluminum, the material will become shiny when it is ready to

accept a filler rod. The material will not accept filler metal if it is not shiny. Watch the area

closely underneath your tungsten rod until you see it get shiny. Exception: DC welding

aluminum with helium shield pool does not cha nge color –it sinks.

When you add rod to an aluminum weld, the pool will rise. Take this into account when

you position your tungsten over your part or else your weld will rise up and contaminate

your tungsten electrode.

Rod to torch angle is critical. Heat will reflect off of your part and melt your rod if your

are not aware of it. Slide rod in under heat. Rod needs to be pushed into the pool not

drawn in by the heat of the arc or you will never achieve uniformity. This applies to any

process in which you have to manually feed in a rod with your hand. Rod and torch angle

should never be the same if it can be avoided.

Keep your arc length as short as possible. Arc length is the distance between the tungsten

and the workpiece. The more stable your hand is the closer you can get. Different arc

lengths can be used for different materials.Experiment. Less heat is inputted when your arc

is short, Your arc is concentrated in a smaller areas. Welding with higher amps and shorter

arc will input less heat into part than a long arc and less amperes. The hotter you weld the

faster you can go, within reason.

Tungsten profiles change arc characteristics.