10 mma welding

8/13/2019 10 MMA Welding

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Welding Inspector

MMA WeldingSection 10


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MMA - Principle of operation


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MMA welding

Main features:Shielding provided by decomposition of flux coveringElectrode consumable

Manual process

Welder controls: Arc length Angle of electrodeSpeed of travel

Amperage settings


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Power source

Holding oven

Inverter powersource

Electrode holder

Power cablesWelding visor

filter glass

Return lead

Electrodes

Electrodeoven

Control panel(amps, volts)

Manual Metal Arc Basic Equipment


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Transformer: Changes mains supply voltage to a voltage suitable for welding.

Has no moving parts and is often termed static plant.Rectifier: Changes a.c. to d.c., can be mechanically or statically achieved.Generator: Produces welding current. The generator consists of an armature

rotating in a magnetic field, the armature must be rotated at aconstant speed either by a motor unit or, in the absence ofelectrical power, by an internal combustion engine.

Inverter: An inverter changes d.c. to a.c. at a higher frequency.

MMA Welding Plant


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Voltage The arc voltage in the MMA process is measured as close to

the arc as possible. It is variable with a change in arc length

O.C.V. The open circuit voltage is the voltage required to initiate, or

re-ignite the electrical arc and will change with the type ofelectrode being used e.g 70-90 volts

Current The current used will be determined by the choice of

electrode, electrode diameter and material type and

thickness. Current has the most effect on penetration.Polarity Polarity is generally determined by operation and electrode

type e.g DC +ve, DC ve or AC

MMA Welding Variables


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20 8040 60 130 140120100 180160 200

10

60

50

40

30

20

80

70

90

100

Normal Operating

Voltage Range

Large voltage variation, e.g. +10v (due to changes in arclength)Small amperage changeresulting in virtually constantcurrent e.g. + 5A.

V o

l t a g e

Amperage

O.C.V. Striking voltage (typical) for arcinitiation

Constant Current Power Source(Drooping Characteristic)


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MMA welding parametersTravel speed

Travelspeed Too highToo low

wide weld bead contour lack of penetration burn-through

lack of root fusion incomplete rootpenetration undercut poor bead profile,

difficult slag removal


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MMA welding parametersType of current:

voltage drop in welding cables is lower with ACinductive looses can appear with AC if cables are coiledcheaper power source for ACno problems with arc blow with ACDC provides a more stable and easy to strike arc,especially with low current, better positional weld, thinsheet applicationswelding with a short arc length (low arc voltage) is easierwith DC, better mechanical propertiesDC provides a smoother metal transfer, less spatter


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MMA welding parametersWelding current

approx. 35 A/mm of diameter governed by thickness, type of joint and welding

position

Weldingcurrent Too highToo low

poor starting slag inclusions weld bead contour toohigh lack offusion/penetration

spatter excess

penetration undercut burn-through


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MMA welding parameters Arc length = arc voltage

Arcvoltage Too highToo low

arc can be extinguished

stubbing

spatter

porosity excesspenetration undercut burn-through

Polarity: DCEP generally gives deeper penetration


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MMA - Troubleshooting

MMA quality (left to right)current, arc length and travel speed normal;

current too low;current too high;

arc length too short;arc length too long;

travel speed too slow;

travel speed too high


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MMA electrode holder

Collet or twist type Tongs type withspring-loaded jaws


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The three main electrode covering types used in MMA welding

Cellulosic - deep penetration/fusion

Rutile - general purpose

Basic - low hydrogen

(Covered in more detail in Section 14)

MMA Welding Consumables

MMA Covered Electrodes


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Most welding defects in MMA are caused by a lack of welder

skill (not an easily controlled process), the incorrect settingsof the equipment, or the incorrect use, and treatment ofelectrodes

Typical Welding Defects:

Slag inclusions Arc strikes

Porosity

Undercut

Shape defects (overlap, excessive root penetration, etc.)

MMA welding typical defects


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Advantages: Field or shop use Range of consumables All positions Portable Simple equipment

Disadvantages: High welder skill required High levels of fume Hydrogen control (flux) Stop/start problems Comparatively uneconomic when compared with some

other processes i.e MAG, SAW and FCAW

Manual Metal Arc Welding (MMA)

10 mma welding

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