Introduction SAW Seam Frict

8/12/2019 Introduction SAW Seam Frict

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Introduction

a) Submerged Arc Welding (SAW)

SAW is one of commons arc welding used in industry. It is a welding

process in which the arc is concealed by a blanket of granular fusible flux

consisting of lime, silica, manganese oxide, calcium fluoride, and other

compounds . Heat for submerged arc welding is generated by an arc between a

bare, solid-metal (or cored) consumable-wire or strip electrode and the work

piece. The arc is maintained in a cavity of molten flux or slag, which refines the

weld metal and protects it from atmospheric contamination. The flux becomes

conductive, and provides a current path between the electrode and the work.

This thick layer of flux completely covers the molten metal thus preventing

spatter. Alloy ingredients in the flux may be present to enhance the mechanical

properties and crack resistance of the weld deposit. A layer of granular flux, just

deep enough to prevent flash through, is deposited in front of the arc. Electrical

current, which produces the arc, is supplied to the electrode through the contact

tube. The current can be dc with electrode positive (reverse polarity, or DCEP),

dc with electrode negative (straight polarity, or DCEN), or alternating current

(ac). After welding is completed and the weld metal has solidified, the unfused

flux and slag are removed. Submerged arc welding is adaptable to both

semiautomatic and fully automatic operation, although the latter, because of its

inherent advantages, is more popular. In semiautomatic welding, the welder

controls the travel speed, direction, and placement of the weld. A semiautomatic

welding gun is designed to transport the flux and wire to the operator, who

welds by dragging the gun along the weld joint. Semiautomatic electrode

diameters are usually <2.4 mm (<3/32 in.) to provide sufficient flexibility and

feedability in the gun assembly. Manually guiding the gun over the joint

requires skill because the joint is obscured from view by the flux layer. This

process is normally limited to the flat or horizontal-fillet welding positions

(although horizontal groove position welds have been done with a special

arrangement to support the flux). Deposition rates approaching 100 lb/h

(45 kg/h) have been reported — this compares to ~10 lb/h (5 kg/h) (max)for shielded metal arc welding. Although currents ranging from 300 to 2000 A



are commonly utilized ,[1] currents of up to 5000 A have also been used

(multiple arcs). In automatic submerged arc welding, travel speed and direction

are controlled mechanically. Flux may be automatically deposited in front of the

arc, while the unfused flux may be picked by a vacuum recovery system behind

the arc. This process welding is widely used in heavy plate fabrication

including:

• Structural – bridge, building, and structure manufacturing

• Pipe – both longitudinal seams and circumferential welds

• Vessels and tanks for pressure storage use

• Heavy machine components

• Railcar manufacturing • Heavy construction/mining/crane

Welding process

The welding action can be initiated by introducing a piece of high resistance

conducting material like steel wool or carbon between the electrode and the

work piece. Once the welding action has been initiated the intense heat

produced by the flow of current in the high resistance path melts a path of theflux around the electrode forming a conducting pool. The molten filler displaces

the liquid flux and fuses with the molten base metal forming the weld. The

molten flux coating over the molten metal pool forms a blanket that eliminates

spatter losses and protects the welded joint from oxidation. As welding

proceeds, the molten weld metal and the liquid flux cool and solidify under a

layer of unused flux. The molten flux on solidification forms a brittle slag layer

which can be easily removed. Flux is an insulator but once it melts due to heat

of the arc, it becomes highly conductive and hence the current flow is

maintained between the electrode and the job through the molten flux. The

upper portion of the flux, in contact with atmosphere, which is visible remains

solid granular i.e., unchanged and can be reused. The lower, melted flux

becomes slag, which is waste material and must be removed after welding. The

electrode at a predetermined speed is continuously fed to the joint to be welded.

In semi-automatic welding sets the welding head is moved manually along the

joint whereas automatic welding a separate drive moves either the welding head

http://en.wikipedia.org/wiki/Submerged_arc_welding#cite_note-manufacturing-1






over the stationary job or the job moves/rotates under the stationary welding

head. The arc length is kept constant by using the principle of a self-adjusting

arc i.e., if due to certain reasons arc length decreases, arc voltage will increase,

arc current and therefore burn-off rate will increase thereby causing the arc to

lengthen. Backing plate of steel or copper may be used to control penetration

and to support large amounts of molten metal associated with the process.

Advantages of submerged arc welding

1) The arc is under a blanket of flux, which virtually eliminates arc flash,

spatter, and fume, making the process attractive from an

environmental standpoint.

2) High current densities increase penetration and decrease the need for

edge preparation.

3) High deposition rates and travel speeds are possible.

4) Cost per unit length of joint is relatively low.

5) The flux acts as a scavenger and deoxidizer to remove contaminants

such as oxygen, nitrogen, and sulfur from the molten weld pool. This

helps to produce sound welds with excellent mechanical properties.

6) Low-hydrogen weld deposits can be produced.



7) The shielding provided by the flux is substantial and is not sensitive to

wind as in SMAW and GMAW.

8) Minimal welder training is required, and relatively unskilled welders

can be employed.

9) The slag can be collected, reground, and sized for mixing back into

new flux as prescribed by manufacturers and qualified procedures.

Limitations of submerged arc welding

1) The initial cost of wire feeder, power supply, controls, and flux-

handling equipment is high.

2) The weld joint needs to be placed in the flat or horizontal position to

keep the flux positioned in the joint.

3) The slag must be removed before subsequent passes can be deposited.

4) Because of the high heat input, submerged arc welding is most

commonly used to join steels more than 6.4 mm (114 in.) thick.

Deposition rate — It is essential to recall that Deposition Rate is directly

proportional to the speed at which a particular wire diameter emerges from a

welding gun during welding. Deposition rate has nothing to do with how fast

the gun is neither traveling nor the voltage setting on the machine. Deposition

rate is simply a measure of how many pounds of wire come from the welding

gun in a certain amount of time, typically measured in lb./hr. If wire-feed speed

increases, deposition rate increases. We also understand that if we maintain the

wire-feed speed and change to a larger diameter wire, deposition rate will

increase as well. Armed with this understanding, calculating deposition rate

ends up being a very powerful exercise that gives you a number that can be used

to calculate key welding parameters.



There is formula to calculate the deposition rate of the SAW

Deposition rate=

x 60 minutes x

The welding parameters in submerged arc welding - arcvoltage, welding current, travel speed, stick-out, torch angle,wire diameter, wire feed speed and polarity - all influencethe shape and quality of the weld, and productivity.It is important to be aware of their individual andcombined influence. In this chapter we do not discusswelding defects that result from incorrect parameters – mostly set too low or too high. These are discussed in thetrouble shooting section of this catalog. The table reviewsthe effects when individual welding parameters are increased,while all other parameters remain unchanged.Decreasing them will have opposite effects.



Reference

1)http://www.mechanicalengineeringblog.com/tag/submerged-arc-

welding-introduction/

2) Author(s): F.C. Campbell, Editor, From: Joining: Understandingthe Basics (ASM International)

Published: Nov 2011Pages: 1-22

SPOT/SEAM

Resistance Spot Welding is one of the oldest of the electric weldingprocesses in use by industry today. The weld is made by a combinationof heat, pressure, and time. As the name implies, it is the resistance ofthe material to be welded to current flow that causes a localized heatingin the part. The pressure is exerted by the tongs and tips.The time ishow long current flows in the joint, which is determined by the materialthickness and type, amount of the current, and cross-sectional area ofthe welding tips and contact surfaces

Spot welding is a technique generally used to bond metals shaped into sheets no thickerthan 3 millimeters. Unlike other welding techniques, spot welding can create precisebonds without generating excessive heating that can affect the properties of the rest ofthe sheet. This is achieved by delivering a large amount of energy in a short time in orderto create controlled and reliable welds.

Typical spot welding machines make use of two copper alloy electrodes that arepositioned over the area where the bond is to be made. The two sheets of metal that arewelded are clamped by the two electrodes while a large electric current is run throughthem. The technique is also known as resistance spot welding, because the amount ofheat delivered on the spot is directly related to the resistance between the electrodes,the amplitude of the current and the duration of the applied electric current.

As a consequence, different metals with various thicknesses require different currentamplitudes, types of electrodes and time intervals. For example, if the machine is notproperly adjusted it could end up delivering too little or too much energy to the sheetsbeing bonded. In the first case, the amount of energy would simply be insufficient to meltthe metals and bond them, whereas if too much energy is inputted the sheets would meltexcessively, creating a whole in them rather than a weld.

The energy delivered during the bonding of two sheets must be

available instantaneously. In the case of high power demands, the

power supply is usually equipped with an energy storage unit,

otherwise this constructive element is completely useless.

The electric current required for such applications is produced with

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the help of a step down transformer (with the electrodes forming the secondary circuit of

the device), which lowers the voltage and increases the current (the voltage between the

two electrodes rarely exceeds 1.5 volts, except for when there is no galvanic connection

between the two, when the voltage increases to 5-10 volts, while the electric current can

reach values up to 100,000 amps).

devices that hold and clamp the workpiece and apply the welding forceSpecifications for resistance welding equipment have been standardized

by the Resistance Welder Manufacturers Association, and specificationsfor controls are issued by the National Electric ManufacturersAssociation.Single-spot welds are usually made by direct welding. Three arrangementsused for making this type of weld are shown in Fig. 3.3. In all threearrangements, one transformer secondary circuit makes one spot weld.The simplest and most common arrangement is two workpieces sandwiched

between opposing upper and lower electrodes (Fig. 3.3a). A conductive plate or mandrel having a large containing surface can be used asthe lower electrode (Fig. 3.3b); this reduces marking on the lower workpieceand conducts heat away from the weld more rapidly and may benecessary because of the shape of the workpiece. A conductive plate ormandrel beneath the lower workpiece can also be used in conjunction witha second upper electrode (Fig. 3.3~).

Spot welding is the most widely used joining technique for the assemblyof sheet metal products such as automotive body assemblies, appliances,furniture, and building products. Many assemblies of two or moresheet-metal stampings that do not require gas-tight or liquid-tight jointscan be more economically joined by high-speed spot welding than bymechanical methods. Containers are frequently spot welded. The attachmentof braces, brackets, pads, or clips to formed sheet-metal parts suchas cases, covers, bases, or trays is another common application of spotwelding.Major advantages of spot welding include high operating speeds andsuitability for automation or robots and inclusion in high-production assemblylines together with other fabricating operations. With automaticcontrol of current, timing, and electrode force, sound spot welds can be

produced consistently at high production rates and low unit labor costs

using semiskilled operators.



Seam welding

Resistance seam welding is a process in which heat caused by resistanceto the flow of electric current in the work metal is combined with

pressure to produce a welded seam. This seam, consisting of a series ofoverlapping spot welds, is normally gas-tight or liquid-tight. Two rotating,circular electrodes (electrode wheels), or one circular and one bar-typeelectrode, are used for transmitting the current to the work metal. Whentwo electrode wheels are used, one or both wheels are driven either bymeans of a gear-driven shaft or by a knurl or fi-iction drive that contactsthe peripheral surface of the electrode wheel. The series of spot welds ismade without retracting the electrode wheels or releasing the electrodeforce between spots, although the electrode wheels may advance eithercontinuously or intermittently.

Usually, two copper electrodes are simultaneously used to clamp the metal sheetstogether and to pass current through the sheets. When the current is passed

through the electrodes to the sheets, heat is generated due to the higher electrical

resistance where the surfaces contact each other. As the electrical resistance of

the material causes a heat buildup in the work pieces between the copper

electrodes, the rising temperature causes a rising resistance, and results in a

molten pool contained most of the time between the electrodes. As the heat

dissipates throughout the workpiece in less than a second (resistance welding time

is generally programmed as a quantity of AC cycles or milliseconds) the molten or

plastic state grows to meet the welding tips. When the current is stopped thecopper tips cool the spot weld, causing the metal to solidify under pressure. The

water cooled copper electrodes remove the surface heat quickly, accelerating the

solidification of the metal, since copper is an excellent conductor . Resistance spot

welding typically employs electrical power in the form of direct current, alternating

current, medium frequency half-wave direct current, or high-frequency half wave

direct current.

If excessive heat is applied or applied too quickly, or if the force between the base

materials is too low, or the coating is too thick or too conductive, then the molten

area may extend to the exterior of the work pieces, escaping the containment force

of the electrodes (often up to 30,000 psi). This burst of molten metal is called

expulsion, and when this occurs the metal will be thinner and have less strength

than a weld with no expulsion. The common method of checking a weld's quality is

a peel test. An alternative test is the restrained tensile test, which is much more

difficult to perform, and requires calibrated equipment. Because both tests are

destructive in nature (resulting in the loss of salable material), non-destructive

methods such as ultrasound evaluation are in various states of early adoption by

many OEMs.

http://en.wikipedia.org/wiki/Copper


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http://en.wikipedia.org/wiki/Thermal_conductor








Advantages of seam welding, compared with resistance spot welding, projection welding, and laser welding, are:Gas-tight or liquid-tight joints can be produced (not possible with spotwelding or projection welding).Seam width may be less than the diameter of spot welds, because the electrode contourcan be continuously dressed and is therefore of astable shape.High-speed welding (especially on thin stock) is possible.Tooling cost is generally favorable per inch of flange welded.Coated steels are generally more weldable using seam welding thanspot welding, because coating residue can be continuously removedfrom the electrode wheels if special provisions are made.Coated steels are generally more weldable using seam welding thanlaser welding, because coating volatility is minimized by the intense

pressure field in the weld zoneResistance seam welding is not particularly fit-up sensitive comparedwith laser welding. The hardness of resistance seam welds made with

air cooling is less than that of laser welds (120 HV vs. 250 HV, respectively,for drawing-quality bare steel, and 170 HV vs. 300 HV, respectively,for organic-coated drawing-quality steel).



Reference

Safety and Health, Fact Sheet No. 21 – 2/99, 1995 American Welding Society

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Friction welding

Friction welding is a process in which the heat for welding is produced by direct conversion of mechanical energy to thermal energy at the interfaceof the workpieces without the application of electrical energy or heatfrom other sources. Friction welds are made by holding a nonrotatingworkpiece in contact with a rotating workpiece under constant or graduallyincreasing pressure until the interface reaches welding temperatureand then stopping rotation to complete the weld (Fig. 6.16). The rotationalspeed, axial pressure, and welding time are the principal variables that arecontrolled in order to provide the necessary combination of heat and pressureto form the weldment. These parameters are adjusted so that the interfaceis heated into the plastic temperature range where welding can take

place. Once the interface is heated, axial pressure is used to bring the weldinterfaces into intimate contact. During this last stage of the welding process,



atomic difision occurs while the interfaces are in contact, allowinga metallurgical bond to form between the two materials. If incipient meltingdoes occur, there is no evidence in the finished weld because the metalis worked during the welding stage.There are two principal friction welding methods: direct-drive weldingand inertia-drive welding. Direct-drive friction welding, sometimes calledconventional friction welding, uses a motor running at constant speed toinput energy to the weld. Inertia-drive friction welding, sometimes calledflywheel friction welding, uses the energy stored in a flywheel to inputenergy to the weld. The major difference between the direct-drive andinertia-drive methods is the speed during the friction stage: in inertia-drivewelding the speed continuously decreases during the friction stage,whereas in direct-drive welding the speed remains constant. End preparationof workpieces, other than that necessary to ensure reasonably goodaxial alignment and to produce the required length tolerance for a specific set ofwelding conditions, is not critical. Frictional wear removes irregularities from the jointsurfaces and leaves clean, smooth surfaces that are

then heated to welding temperature.

In friction welding, the joint face of at least one of the workpieces must be essentially round. The rotating workpiece should be somewhat concentricin shape because it revolves at a relatively high speed. Workpiecesthat are not round, such as hexagon-shape workpieces, have been frictionwelded successfully, but the resulting weld upset is rough, asymmetrical,and difficult to remove without damaging the welded assembly. For specialapplications, welding machines have been modified so that the spindlestops at the same place each time, thus making it possible for workpiecesto be oriented to each other.Many ferrous and nonferrous alloys can be friction welded. Friction

welding also can be used to join metals of widely differing thermal andmechanical properties. Often combinations that can be friction weldedcannot be joined by other welding processes because of the formation of

brittle phases that would make such joints unserviceable. The submeltingtemperatures and short weld times of friction welding allow many combinationsof work metals to be joined.

The combination of fast joining times (on the order of a few seconds), and direct

heat input at the weld interface, yields relatively small heat-affected zones. Friction

welding techniques are generally melt-free, which avoids grain growth inengineered materials, such as high-strength heat-treated steels. Another



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Introduction SAW Seam Frict