10 mma welding
TRANSCRIPT
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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)