AC and DC Welding

7/29/2019 AC and DC Welding

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ARC WELDING PRINCIPLE

Arc welding is one of several fusion processes for joining metals. By applying intense heat,

metal at the joint between two parts is melted and caused to intermix - directly, or more

commonly, with an intermediate molten filler metal. Upon cooling and solidification, a

metallurgical bond is created. Since the joining is an intermixture of metals, the final weldmentpotentially has the same strength properties as the metal of the parts. This is in sharp contrast to

non-fusion processes of joining (i.e. soldering, brazing etc.) in which the mechanical and

physical properties of the base materials cannot be duplicated at the joint.

Fig. 1 The basic arc-welding circuit

In arc welding, the intense heat needed to melt metal is produced by an electric arc. The arc is

formed between the actual work and an electrode (stick or wire) that is manually or mechanicallyguided along the joint. The electrode can either be a rod with the purpose of simply carrying the

current between the tip and the work. Or, it may be a specially prepared rod or wire that not only

conducts the current but also melts and supplies filler metal to the joint. Most welding in the

manufacture of steel products uses the second type of electrode.

Basic Welding CircuitThe basic arc-welding circuit is illustrated in Fig. 1. An AC or DC power source, fitted with

whatever controls may be needed, is connected by a work cable to the work piece and by a "hot"

cable to an electrode holder of some type, which makes an electrical contact with the welding

electrode.

An arc is created across the gap when the energized circuit and the electrode tip touches the workpiece and is withdrawn, yet still with in close contact.

The arc produces a temperature of about 6500F at the tip. This heat melts both the base metaland the electrode, producing a pool of molten metal sometimes called a "crater." The crater

solidifies behind the electrode as it is moved along the joint. The result is a fusion bond.


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Arc Shielding

However, joining metals requires more than moving an electrode along a joint. Metals at hightemperatures tend to react chemically with elements in the air - oxygen and nitrogen. When

metal in the molten pool comes into contact with air, oxides and nitrides form which destroy the

strength and toughness of the weld joint. Therefore, many arc-welding processes provide some

means of covering the arc and the molten pool with a protective shield of gas, vapor, or slag.This is called arc shielding. This shielding prevents or minimizes contact of the molten metal

with air. Shielding also may improve the weld. An example is a granular flux, which actuallyadds deoxidizers to the weld.

Fig. 2 This shows how the coating on a coated (stick) electrode

provides a gaseous shield around the arc and a slag covering on

the hot weld deposit.

Figure 2 illustrates the shielding of the welding arc and molten pool with a Stick electrode. Theextruded covering on the filler metal rod, provides a shielding gas at the point of contact while

the slag protects the fresh weld from the air.

The arc itself is a very complex phenomenon. In-depth understanding of the physics of the arc is

of little value to the welder, but some knowledge of its general characteristics can be useful.


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Nature of the ArcAn arc is an electric current flowing between two electrodes through an ionized column of gas. Anegatively charged cathode and a positively charged anode create the intense heat of the welding

arc. Negative and positive ions are bounced off of each other in the plasma column at an

accelerated rate.

In welding, the arc not only provides the heat needed to melt the electrode and the base metal,

but under certain conditions must also supply the means to transport the molten metal from thetip of the electrode to the work. Several mechanisms for metal transfer exist. Two (of many)examples include:

1. Surface Tension Transfer - a drop of molten metal touches the molten metal pool and isdrawn into it by surface tension

2. Spray Arc - the drop is ejected from the molten metal at the electrode tip by an electricpinch propelling it to the molten pool (great for overhead welding)

If an electrode is consumable, the tip melts under the heat of the arc and molten droplets are

detached and transported to the work through the arc column. Any arc welding system in which

the electrode is melted off to become part of the weld is described as metal-arc. In carbon ortungsten (TIG) welding there are no molten droplets to be forced across the gap and onto the

work. Filler metal is melted into the joint from a separate rod or wire.

More of the heat developed by the arc is transferred to the weld pool with consumable

electrodes. This produces higher thermal efficiencies and narrower heat-affected zones.

Since there must be an ionized path to conduct electricity across a gap, the mere switching on ofthe welding current with an electrically cold electrode posed over it will not start the arc. The arc

must be ignited. This is caused by either supplying an initial voltage high enough to cause a

discharge or by touching the electrode to the work and then withdrawing it as the contact area

becomes heated.

Arc welding may be done with direct current (DC) with the electrode either positive or negative

or alternating current (AC). The choice of current and polarity depends on the process, the typeof electrode, the arc atmosphere, and the metal being welded.


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What is the Difference between AC and DC Welding

AC continuously changes polarity, since directional of flow is reversed 120 times per second for

common 60 cycle electricity.

AC Current ideal for: Down hand Heavy plate Fast fill Aluminum TIG Welding with Hi frequency

Carbon arc torch

This welder has two types of AC current

1. Smooth Arcfor general-purpose work2. Force Arcfor carbon arc, TIG attachments and new fabrication

DC straight Polarity

With DC straight polarity (DCSP), the electrode is negative and the current flows in the opposite

directionelectrode to work.

DC Straight Polarity ideal for:

Hard facing Single Carbon Brazing Build-up Heavy Deposits Stainless Steel TIG Welding Cutting Tap. Cuts like 400 Amps.

DC Reverse Polarity

With DC reverse polarity (DCRP) The electrode is positive and the current flows from the work

to the electrode.

DC Reverse Polarity Ideal for: Overhead welding Vertical welding Cast iron welding Heavy aluminum Rivet welding Sheet Metal

Low Hydrogen Welding Arc Bronze Rod


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Differences between transformer and rectifier, two devices which are extensively used in

numerous electronic and electrical applications.

1. Need: Rectifier circuits are used to convert AC signals into their respective DC formats.

Transformers, on the other hand, are used to scale the voltage up or down as per the convenience

of concerned electronic gadget.

2. Construction Basics: Rectifiers are a combination of one to two pairs of diodes depending on

their application. They can be made in form of circuits by placing diodes in opposite polarity.

Besides circuits, rectifiers are also available as semiconductor diodes.

Transformers are constructed from copper coils which are wound around laminated cores.

Unlike rectifiers, no circuit construction for transformers is popular and they are available as

single component moldings.

3. Types: Rectifiers are popular in three types as per their application. They are:

a.Half wave rectifier.

b.Full wave rectifier.

c.Bridge rectifier.


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Half wave rectifier circuits

Half wave rectifiers only take half part of the input AC signal which is usually the positive part.

This rectifier is made only by a single diode. When the signal is positive, the diode is able to pass

it through and when signal is negative, diode doesnt conduct it. Hence, a half DC wave is

generated.

Full wave rectifier

Full wave rectifier can be referred to as two half wave rectifiers placed in opposite polarity, i.e.,

two diodes are connected to each other through opposite ends. Through this pattern, a full DC

wave is generated. The output wave is has ripples which are filtered using a capacitor.


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Bridge Rectifier

Bridge rectifiers are made using 2 to 3 pair of diodes in such a pattern that two diodes share

polarity. Bridge rectifiers are usually found in 3 phase circuits while the former two are preferred

for single phase ones.

Transformers:- are also available into several types depending on their application and circuit

characteristics. On the basis of voltage scaling, transformers can be divided into: Step up

transformer and step down transformer. Step up transformers are those which are used to

increase the voltage in the output while step down transformer are used for the opposite.

Besides these two types, transformers are further divided on basis of their application and

electrical properties. A few types of transformers on this basis are:

a. Power Transformers: These transformers are used for high power applications (usually more

than 500kVA).

b. Audio Transformers: These transformers are used in various audio applications such as guitar

amplifiers.

c. RF Transformers: These transformers work in the radio frequency range.


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3. Working Principle:A rectifierderives its working principle from a diodes. When diode is subjected to a voltage

which is positive beyond the threshold value, it begins to conduct. On the contrary, when a diode

is subjected to voltages which are negative in phase, it stops conducting. One or combination of

multiple diodes forms out a rectifier which uses this principle for signal conversion.

Transformer

A transformer is based on the principle of electromagnetism and mutual induction. The primary

coil, which is connected to the source, gets the input current. Any changes in the current

transmitted are linked to the secondary through flux changing and proportional to the current

change voltage is generated. The primary and secondary may be wound on a common core but

are separated to each other in terms of wire connections.

5. Applications:-Rectifiers find their application in more power supply concerning areas such as

AC to DC conversion during power transmission etc.


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Rectifiers have multiple types of technologies that are application specific and have different

respective power handling capacities. Some popular rectifier technologies are electromechanical,

plasma type, electrolytic, vacuum tube etc.

Transformers are used in almost electronic gadgets: in the adapters, in internal circuitry, in

amplifiers, in communication circuits etc.

Comparison of AC and DC Welding

When using a DC power source, the question of whether to use electrode negative or positive

polarity arises. Some electrodes operate on both DC straight and reverse polarity and others on

DC negative or DC positive polarity only.

Direct current flows in one direction in an electrical circuit and the direction of current flow and

the composition of the electrode coating will have a definite effect on the welding arc and weld

bead.

Figure 3 shows the connections and effects of straight and reverse polarity.

Electrode negative (-) produces welds with shallow penetration; however, the electrode melt-off

rate is high. The weld bead is rather wide and shallow as shown at "A" in Figure 3. Electrode

positive (+) produces welds with deep penetration and a narrower weld bead as shown at "B" in

Figure 3.

Straight Polarity Reverse Polarity

DC concentrates the majority of the heat at either the work piece or the tip of the electrode,

depending on polarity. AC does not have this benefit/detriment.

DC arcs don't like to break, AC or pulsed DC ones do.

Different methods of welding (TIG/MIG/Stick) require different application of voltage and

polarity to produce the desired best result.

With MIG welding mild steel, flux-cored filler requires the opposite polarity from gas shielded

welding to do a good job.


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when the electrode is given positive potential and the work piece is given negative potential,

the weld formed is shallow and wide, this method is called 'Direct Current Reverse Polarity'

(DCRP) welding procedure.

When the machine is set on straight polarity, the electrons flow from the electrode to the plate,

concentrating most of the heat on the work. With reverse polarity, the flow of electrons is from

the plate to the electrode, thus causing a greater concentration of heat at the electrode. Because of

this intense heat, the electrode tends to melt off; therefore, direct current reverse polarity (DCRP)

requires a larger diameter electrode than direct current straight polarity (DCSP). Notice that

DCSP produces a narrow, deep weld. Since the heat is concentrated on the work, the welding

process is more rapid and there is less distortion of the base metal. Overall, straight polarity is

preferred over reverse polarity because you can achieve better welds. DCRP forms a wide and

shallow weld and is rarely used in the GTAW process. The exception to this is when it is used to

weld sections of aluminum or magnesium.

DCRP has excellent cleaning power that results from the action of positive charged gas ions.

When these gas ions strike the metal, they pierce the oxide film and form a path for the welding

current to follow. This same cleaning action occurs in the reverse polarity half of an alternating

current welding cycle.


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Welding with DC Straight and Reverse Polarity

a. The electri cal arc welding circui tis the same as any electrical circuit. In the simplest electrical

circuits, there are three factors: current, or the flow of electricity; pressure, or the force required

causing the current to flow; and resistance, or the force required to regulate the flow of current.

(1) Current is a rate of flow and is measured by the amount of electricity that flows

through a wire in one second. The term ampere denotes the amount of current per second

that flows in a circuit. The letter I is used to designate current amperes.

(2) Pressure is the force that causes a current to flow. The measure of electrical pressure

is the volt. The voltage between two points in an electrical circuit is called the difference

in potential. This force or potential is called electromotive force or EMF. The difference

of potential or voltage causes current to flow in an electrical circuit. The letter E is used

to designate voltage or EMF.

(3) Resistance is the restriction to current flow in an electrical circuit. Every component in the

circuit, including the conductor, has some resistance to current flow. Current flows easier

through some conductors than others; that is, the resistance of some conductors is less thanothers. Resistance depends on the material, the cross-sectional area, and the temperature of the

conductor. The unit of electrical resistance is the ohm. It is designated by the letter R.

b. Welding electri cal circui ts. A simple electrical circuit is shown byfigure 10-12. This circuit

includes two meters for electrical measurement: a voltmeter, and an ammeter. It also shows a

symbol for a battery. The longer line of the symbol represents the positive terminal. Outside of a

device that sets up the EMF, such as a generator or a battery, the current flows from the negative

(-) to the positive (+). The arrow shows the direction of current flow. The ammeter is a low

resistance meter shown by the round circle and arrow adjacent to the letter I. The pressure or

voltage across the battery can be measured by a voltmeter. The voltmeter is a high resistance

meter shown by the round circle and arrow adjacent to the letter E. The resistance in the circuit is

shown by a zigzag symbol. The resistance of a resistor can be measured by an ohmmeter. An

ohmmeter must never be used to measure resistance in a circuit when current is flowing.
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c. Arc Welding Circuit. A few changes to the circuit shown byfigure 10-12, above, can be made

to represent an arc welding circuit. Replace the battery with a welding generator, since they are

both a source of EMF (or voltage), and replace the resistor with a welding arc which is also a

resistance to current flow. The arc welding circuit is shown by figure 10-13. The current will

flow from the negative terminal through the resistance of the arc to the positive terminal.

d. Reverse and Straight Polarity. In the early days of arc welding, when welding was done

with bare metal electrodes on steel, it was normal to connect the positive side of the generator to

the work and the negative side to the electrode. This provided 65 to 75 percent of the heat to the

work side of the circuit to increase penetration. When welding with the electrode negative, the

polarity of the welding current was termed straight. When conditions such as welding cast iron or

nonferrous metals made it advisable to minimize the heat in the base metal, the work was made

negative and the electrode positive, and the welding current polarity was said to be reverse. In

order to change the polarity of the welding current, it was necessary to remove the cables from

the machine terminals and replace them in the reverse position. The early coated electrodes for

welding steel gave best results with the electrode positive or reverse polarity; however, bare

electrodes were still used. It was necessary to change polarity frequently when using both bare

and covered electrodes. Welding machines were equipped with switches that changed thepolarity of the terminals and with dual reading meters. The welder could quickly change the

polarity of the welding current. In marking welding machines and polarity switches, these old

terms were used and indicated the polarity as straight when the electrode was negative, and

reverse when the electrode was positive. Thus, electrode negative (DCEN) is the same as straight

polarity (dcsp), and electrode positive (DCEP) is the same as reverse polarity (dcrp).

e. The ammeter used in a welding circuit is a millivoltmeter calibrated in amperes connected

across a high current shunt in the welding circuit. The shunt is a calibrated, very low resistance

conductor. The voltmeter shown in figure 10-12will measure the welding machine output and

the voltage across the arc, which are essentially the same. Before the arc is struck or if the arc isbroken, the voltmeter will read the voltage across the machine with no current flowing in the

circuit. This is known as the open circuit voltage, and is higher than the arc voltage or voltage

across the machine when current is flowing.

f. Another unit in an electrical circuit is the unit of power. The rate of producing or using energy

is called power, and is measured in watts. Power in circuit is the product of the current in
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amperes multiplied by the pressure in volts. Power is measured by a watt meter, which is a

combination of an ammeter and a voltmeter.

g. In addition to power, it is necessary to know the amount of work involved. Electrical work or

energy is the product of power multiplied by time, and is expressed as watt seconds, joules, or

kilowatt hours.

Resistance Welding

History of welding technologyIn ancient times, metal welding was done in the form of forge welding (metals heated up tomelting point are pressed together) and brazing (weld using alloy of low melting point). With the

advent of electricity, welding technology advanced remarkably; namely, resistance welding, arc

welding and gas welding were invented in the end of 19th century. Thereafter, various welding

technologies such as ultrasonic, friction, electron beam, plasma, laser welding have been

invented.Though we have very little chance to experience the welding technology, it is applied broadly in

a variety of industries and contributed their growth.

Principle of resistance weldingResistance welding is conducted as follows: Apply force and current through electrodes

contacted metal parts to be welded; and resistance heat is generated at the interface of metal partsand makes a nugget, resulting in melt joint. Though a large current flows, there is no danger of

an electric shock because only low voltage is impressed.

Features of resistance weldingFollowings feature in resistance welding:

No flux such as solder is necessary, so welded parts can be easily recycled. Spatter andultraviolet ray are most unlikely to be generated; consequently, clean and neat worksite is

realized. Easy operation as only pressing buttons facilitates process automation and does not

require trained skills unlike arc welding and gas welding.

As this welding is performed efficiently in a short period of time, it is suited for a high-volumes production of low-cost products.

Since welding is done in short time duration, it gives less heat-affected area onworkpieces, resulting inbeautiful appearance with less indentation.


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Electric facility is required in some cases due to use of large current. Optimum weldingparameters must be figured out before actual welding since those parameters depend on

material and thickness of parts to be welded. welding condition setting must be prepared.

Visual inspection is difficult because welded portion cannot be checked from the outside.

alescence of materials by heating them to the brazing temperature and by using a filler metal

(solder) having a liquidus above 840F (450C), and below the solidus of the base metals. For amore in-depth explanation, seeHow Brazing Works

Soldering - Soldering has the same definition as brazing except for the fact that thefiller metalpastesused has a liquidus below 840F (450C) and below the solidus of the base metals.

Welding - In welding, fusion takes place with melting of both the base metal and usually a filler

metal. See ourBrazing vs. Welding articlefor more in-depth answers.

What does a brazed joint provide?

Brazing provides:

Strong joints Lower temp/lower cost Maintains integrity of base metals Easily joins dissimilar metals Good joint appearance Skill easily acquired/automated

How does brazing work?

Brazing creates a metallurgical bond between the filler metal and the surfaces being joined. Heatis applied to the base metals and the filler metal is brought into contact with the heated parts.When the filler metal melts, it is drawn through the joint by capillary action.

What types of filler metal forms are there?

Filler metalscome in wire, strip, powder, or paste form. Availability in these forms depends on

the alloy that is chosen.Preformscan be made from strip and wire.

What is oxidization?

When metals are exposed to oxygen, oxides form from oxygen atoms that attach to the metal.The oxides that form prevent the molten alloy from metallurgically joining to the metal.

What is Flux?

Flux is a chemical compound that is applied and shields the joint surface from air and prevents

oxide formation. Although flux will dissolve and absorb oxides, the metals that are being joined

should be properly cleaned prior to brazing.
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What are the available flux forms?

Flux is available from Lucas-Milhaupt inpaste, slurry, liquid, andpowder formdepending on the

type of flux. Paste, slurry, and liquid fluxes are all water based whiledispensable fluxesare

petroleum based. Flux can also be delivered to the joint through the torch that is being used.

How can flux be cleaned off of joint after brazing?

The easiest way to clean flux off the brazed joint is to quench and soak the assembly in hotwater. HCl (up to 25%) can be added to the water for stubborn flux residue. Special cleaners may

also be purchased if needed.

What is the shelf life of flux?

If stored in the original unopened container, Lucas-Milhaupt flux is under warranty for twelve

months from the date of manufacture. This does not mean that after twelve months the flux is no

longer useful.

How can flux be reconstituted or thinned?

Water can be used to thin water-based flux. Usually distilled or de-ionized water is used for this

purpose.

What is brazing paste?

Brazing pasteis a material consisting of metal alloy in a powder form mixed with a binder.When it is required,brazing fluxis added to the brazing paste to provide protection from

oxidization.

In what applications is using paste feasible?

Paste can be used for torch, induction, and furnace applications. In these applications flux is

usually added to paste formulation. When using paste in a vacuum or atmosphere furnace flux

does not need to be added to the paste.

What is the shelf life of brazing paste?

Brazing paste that is in its original unopened container is warranted for 90 days. Similarly to the

flux, this warranty does not mean that after the 90 days, the paste will not function. Functionalitycan only be determined by melting the paste to see if flows or not. If the alloy flows then it is still

functional, conversely if the alloy balls up then it is not functional.

What is the proper clearance for brazing?

In general, a joint clearance ranging from 0.002"-0.005," will produce sound high strength joints

when flux brazing. When atmosphere and vacuum brazing, joint clearance should be 0.000"-0.002." Care does need to be taken when brazing metals with different coefficients of thermal

expansion. The amount the materials expand needs to be factored in when determining joint

clearance.
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What types of joint configurations can be used when brazing?

There are several different types of joints that can be used. The most common of these are butt

joints, lap joints, and butt lap joints. Pictures and calculations for determining the length of a lap

joint can be seen by mouse clicking here to go to the "Principles of Joint Design" article."

At what temperature should the furnace be at when furnace brazing?

Typically the furnace will be heated to a temperature 50F-100F above the liquidus of the fillermetal being used.

What is the strength of a brazed joint?

The strength of a brazed joint depends on several different factors. These being:

the base metals being joined joint clearance filler metal used

Joint strength varies with use of different base metals and filler metals.

Joint strength also depends on the gap between the two metals being joined. When the gap isincreased the joint strength decreases. Often times, under the correct conditions, the braze jointstrength will be equal to or greater than the strength of the base metals. For more in-depth

information on the types of brazed joints, see our article onbrazing joint design.

What is Handy One?

Handy One is a flux cored productthat simplifies the brazing process. Handy One is a brazing

alloy in strip form rolled around a measured amount of powdered flux. As the part is heated the

flux is released providing protection from oxidization. It is available in multiple filler metals andflux combinations to join virtually all common metals.

What alloy is recommended for brazing copper to copper?

In most cases, when brazing copper to copper in air conditioning and refrigeration service, one of

our Sil-Fos alloyswould be recommended. The phosphorous in the alloys allow the Sil Fosgroup to be self fluxing when brazing copper to copper which eliminates the use of a separate

flux.

Can stainless steel be brazed in vacuum below a temperature of 1700 F?

Stainless cannot be vacuumed brazed in at a temperature below 1700 F because of thepossibility of chrome-oxide formation which will prohibit the flow of the filler metal alloy on the

stainless steel. A brazing temperature of at least 1750 F is usually recommended in vacuum for

stainless steel. If the stainless steel is nickel plated it can be brazed at temperatures lower than1700 F.
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What braze alloy would be appropriate for brazing 300 series steel that will be subjected tocontact with water?

When brazing stainless steels that will be exposed to water, an alloy should be used that contains

nickel. Examples of these would be Easy Flo 3, Braze 505, Braze 630 and Braze 403. The nickel

in these alloys helps prohibit interface corrosion within the steel. To learn more about interfacecorrosion please readTechnical Bulletin No. T-9or contact Lucas-Milhaupt Technical Service.

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