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DOC NO. 5Q&A-002-SA1

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QUALIFICATION & CERTIFICATION BOARD Date of Issue: 23 May 2011ManagementSystem Manual IIW International Welding Specialist (IWS)

Sample Questions and AnswersSA1 WELDING PROCESS AND EQUIPMENT

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ABN 69 003 696 526

QUESTION 1

1.1 Briefly describe the main arc initiation mechanisms for GTAW welding and give advantages anddisadvantages of each. 12 marks

Scratch Starting MethodAs with MMAW the electrode is scratched on the work surface to start the arc, and then movedaway to the required arc length. Lowest cost equipment. Risk of tungsten contamination on work piece, and/or damage to electrode. Welder must master the technique to avoid electrode sticking to the work.

High Frequency (HF) Ignition

A high voltage, high frequency current provides an ionized path for the welding current to followwhen the electrode is brought within about 5mm of work piece. Less skill required than with scratch starting, and less risk of contamination. Enables stable AC welding with sinusoidal wave form. Requires a HF unit (built in or added to power source). HF may adversely affect other electrical and electronic equipment.

Contact or Lift Arc IgnitionThe electrode is brought into contact with the work, and then lifted off and the arc ignites. This isdone with electronic control that detects the short circuit situation and limits the current flow to verylow level. As soon as the electrode is lifted the arc is started. No HF even with AC (square wave form). Low skill, low risk of contamination. Highest cost, only available on some modern power sources.

1.2 With the aid of graphs, discuss the power source characteristics and applicable welding processesfor:a) Constant current drooping characteristicb) Constant voltage characteristic 8 marks

Drooping:The output voltage decreases as the output current increases;Low (restricted) short-circuit current.Suitable for the following processes

MMAWTIG


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A Large change in arc length (Voltage) causes a small change in amperage (melting rate).

Current

I1 I2

V2

V1

Voltage

A constant current machine will vary its output voltage to maintain a steady current while a constantvoltage machine will fluctuate its output current to maintain a set voltage.

Constant voltage:The output voltage remains substantially constant under output current alterations or a small changein arc volts results in a relatively large change in welding current.Suitable for constant feed electrode

GMAW (MIG/MAG)

FCAW (Flux cored arc)

SAW (Sub-arc)Small change in arc length (Voltage) causes big change in amperage.

Current

Vo

ltage

V1V2

I1 I2

V


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QUESTION 2

2.1 Briefly explain and describe the four main metal transfer mechanisms for GMAW welding with theaid of sketches. 15 marks

Dip (short arc, short circui ting) transfer:

Low voltage and low current (14-20V and ~60-200A on steel). Metal deposited during rapidly occurring shortcircuits (20-200/sec). Low heat input and narrow heat spread; weld metal 'freezes' rapidly. Spatter can occurif the short circuiting is violent. Uses: thin sections, positional welding, root pass to bridge a root gap.

Globular transfer:

Typically occurs at ~20-26V and >180A (on steel).Metal transfers in large globules at relatively low current density, sometimes during short circuits.The droplet size is characteristically greater diameter than the electrode.Uses: welding steel with CO2 shielding gas.

Spray transfer:High settings ~27V 180A (steel).Droplets


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Pulsed transferControlled spray transfer at the selected wire feed rate.

Spray transfer at current levels that would normally produce dip or globular transfer.

Positional welding in spray, improved weldability of thinner sections

Extends the current range in which spray transfer will occur.One form of improves the performance of dip (short arc) transfer. Synergic" power sources use electroniccontrol to manage the complex parameters.Advantages include;

Electronic control and feedback give a very stable arc with minimal spatter, and low risk of wire burnback or stubbing.

Simplified selection of parameters with power source containing programs for specific materials,wire diameters and shielding gases.

Larger diameter electrode wire diameters can be used (cheaper and easier to feed).

2.1 List an two advantages and two disadvantages of using argon based mixed shielding gas comparedto pure C02 shielding gas in MAG welding? 4 marks

Advantages:

Does not require a heating unit for the regulator

Less spatter

More stable arc

Disadvantages:

More expensive

Shallower and narrower penetration profile, thus greater risk of lack of fusion

Less tolerant of dirty steel

2.2 Discuss the differences between the three flame settings used in oxy-acetylene welding andbrazing, explain the features that distinguish them from each other and list an application where it beused. Use sketches if necessary. 16 marks

Neutral flame 1:1 gas ratio approx.

High temperature (3200C), concentrated zone at the end of the inner cone. adequate heat content minimum chemical reaction with parent and filler metals. Use for fusion weld steels, cast iron, stainless steel, copper, aluminium and for brazing.


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2.3 List two reasons why GMAW wires may be copper coated? Give one drawback of the coppercoating. 3 marks

Reason

It improves the current pickup between contact tip and the electrode. Helps prevent rusting of the wire when it is exposed to the atmosphere.

Drawback

Copper inclusions

2.4 List an advantage and a disadvantage of using pure C02 shielding compared to in argon basedshielding MAG welding? 2 marks

Advantages:

Cheaper

Deeper and wider penetration profile, thus lesser risk of lack of fusion

Disadvantages:

Spatter

Heating unit required in the regulator

Unstable arc

QUESTION 3

3.1 Explain the effect of current and polarity of the GTAW process in terms of heat distribution,penetration, electrode capacity, and cleaning action with the aid of a neat sketch. 10 marks

CurrentPolarity

DCEN DCEP AC(Balanced)

Oxide CleaningAction

No Yes Yes onceevery cycle

Heat balance 1/3 electrode2/3 work

2/3 electrode1/3 work

50/50

Penetration Deep, narrow Wide, shallow MediumElectrodecapacity

Excellent Poor Medium


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3.2 Explain the principles of the SAW process with the aid of a neat sketch. 10 marks

Process Principles

In the submerged arc welding (SAW) process the arc is established between a continuously fed wireelectrode and the work. The arc is "submerged" under a layer of granular flux.

Arc Character istics

Once the arc is established the flux melts and forms a liquid bubble around the arc, periodically expanding,

bursting and then reforming. Metal transfer is irregular, but confined to the flux bubble so no spatter isformed. A cavity at the front of the weldpool creates a strong rearward flow so that the weldpool issignificantly elongated.

Arc

Weldpool

Weld

Solidified

Flux recyclingsystem

Wire

Power

sourceGranular flux

Work

Wire

drive

Hopper

slag

Granular flux Solidified slag

Molten slag

Weldpool

Arc cavity

Electrode

Solidified weld metal


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QUESTION 4

4.1 Explain, with the aid of a sketch, the principles of the MMAW Process. 10 marks

Process Principles

Electrode Core wire

Flux coating

Arc

Slag coatedmetal droplet

Weld pool

Solidified slag

Weld deposit

Gas shielding PowerSource

The tip of the electrode is brought into contact with the work and then drawn a short distance away to forman arc. The heat of the arc (


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4.4 Identify a surface characteristic imparted by surfacing or spraying. 1 mark

Wear resistance

Heat resistance

Oxidation resistance

Corrosion resistance

Electrical resistance

Electrical conductance

Restoration of size

4.5 Explain, with the aid of a sketch, the principles of the FCAW Process. 10 marks

Flux-cored arc welding (FCAW) is a semi-automatic or automatic arc welding process. FCAW requires acontinuously-fed consumable tubular electrodecontaining a flux and a constant voltage or, less commonly, aconstantelectric currentwelding power supply. An externally supplied shielding gas is sometimes used, butoften the flux itself is relied upon to generate the necessary protection from the atmosphere. The process iswidely used in construction because of its high welding speed and portability.

FCAW EquipmentThe FCAW Process

Welding

Power

Wire feed

Shieldinggas

Weld

Shieldinggas nozzle

Contacttip

Arc

Gasshieldin

QUESTION 5

5.1 Define the following terms:

i) Duty Cycle (Power Source). 2 marks

The percentage of time, for an arbitrary test period, during which power supply can be operated atits rated output without overloading.

ii) Deposition Efficiency of a Welding Process 2 marks

Deposition efficiency in arc welding is the ratio of the weight of the weld metal deposited to the nett

weight of the filler material used.
http://en.wikipedia.org/wiki/Arc_weldinghttp://en.wikipedia.org/wiki/Electrodehttp://en.wikipedia.org/wiki/Flux_(metallurgy)http://en.wikipedia.org/wiki/Voltagehttp://en.wikipedia.org/wiki/Electric_currenthttp://en.wikipedia.org/wiki/Welding_power_supplyhttp://en.wikipedia.org/wiki/Shielding_gashttp://en.wikipedia.org/wiki/Shielding_gashttp://en.wikipedia.org/wiki/Welding_power_supplyhttp://en.wikipedia.org/wiki/Electric_currenthttp://en.wikipedia.org/wiki/Voltagehttp://en.wikipedia.org/wiki/Flux_(metallurgy)http://en.wikipedia.org/wiki/Electrodehttp://en.wikipedia.org/wiki/Arc_welding


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5.2 State the effects of the following on weld quality

i) Arc length that is too long using GMAW 2 marks

Increase spatter levels Increase the chance of undercut. Produce an erratic arc.

Increase the chance of contamination from the surrounding air.

Increase the voltage.

5.3 Explain the process principles of Friction Welding with the aid of a sketch. 4 marks

Friction welding is a solid-state joining process that produces a weld under compressiveforce between the contacting surfaces of work pieces moving or rotating relative to oneanother. Heat is produced and material is plastically displaced from the faying surfaces.

5.4 What is the fundamental difference between brazing and soldering. 2 marks

Brazing - the filler metal melting temperature is higherthan 450CSoldering - the filler metal melting temperature is lowerthan 450C

5.5 Explain the principles of Explosive Welding. 3 marks

Explosive welding is a solid state welding process (that is, no fusion or liquid state) that uses aprogressive explosive detonation to force two metals to impact together at high speed and pressureto form a metallurgical bond.

The explosive force brings the two metals progressively together at the collision front while expellinga jet of metal that forms just ahead. The jet cleans the surfaces from oxides and contaminants.

5.6 Explain with the aid of a sketch, the hot plate welding process used for weldable plastics. 5 marks

The technique involves heating the ends of the parts to be joined against an electrically heatedplaten until they are sufficiently molten. The heater plate is then removed and the parts pressedtogether. A cooling cycle follows, allowing the weld to develop strength. The platens are normallystainless steel or aluminium coated with PTFE to prevent the molten polymer sticking to them.Holding fixtures are required to hold the parts in alignment for welding. To make the weld, the joining

surfaces are brought into contact with the heated platen and held with sufficient pressure to allowthe surfaces to melt. Once the surfaces are melted, the holding pressure is released, the platenremoved and the two surfaces pushed together to form the joint. Pressure is maintained until themolten material has solidified.


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5.7 Explain with the aid of a sketch, the principle of carbon arc air gouging. 5 marks

Travel.

Electrode at 35-70

to work surface.

Air jet under electrode

Electrode clamp with air holes.

DCEP or AC

Copper coated electrode.

Stickout ~150mm max.

Molten material

Torch

An arc is struck between a consumable carbon-graphite electrode and this produces a molten pool on theworkpiece. The molten metal is then blown away by jets of compressed air, the forward movement of theelectrode producing a cut or groove in the metal.


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QUESTION 6

The following questions have multiple-choice answers and are worth 2 marks each.Only one answer is correct. Tick only one. If more than one box is ti cked, no marks will be awarded.

6.1 The development of the GMAW/MAG welding processes has been mainly due to?

a. Improvement of productivity and ease of mechanisation.

b. Applicability to welding of stainless steels.c. Use of inert gases.d. Improvement of mechanical strength of weld.

6.2 Which welding process you recommend for Cr-Mo steel pipes, 20 mm diameter and 2 mm wallthickness:

a. Manual Metal Arc Welding (MMAW)b. Gas Tungsten Arc Welding (GTAW)c. Plasma Keyhole Weldingd. Submerged Arc Welding (SAW)

6.3 Which of the following welding processes is only suitable for automation?

a. Manual Metal Arc Welding (MMAW)b. Submerged Arc Welding (SAW)c. Electron beam welding (EBW)d. Gas Tungsten Arc Welding (GTAW)

6.4 In arc welding processes, penetration is mostly affected by?

a. Preheatb. Voltagec. Arc lengthd. Current

6.5 The welding power source must:

a. Decrease the voltage of the power supply and increase the current.b. Maintain the parameters of the power supply.c. Increase the efficiency of the primary consumption.

d. Decrease the power supply current.

6.6 Which of the following power source types produces direct current?

a. A rectifierb. A diesel generatorc. An inverterd. All of the above

6.7 What is the difference between MIG and MAG Welding?

a. The base materialb. The type of shielding gas usedc. The wire electroded. The power source


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6.8 When welding austenitic stainless steels, backing gas is often used to prevent oxidation of thesurface of the material thereby reducing the corrosion resistance. Which of the following types is notused as backing gas for stainless steel?

a. Argonb. Argon and CO2 mixturec. Heliumd. Nitrogen and hydrogen mixture, eg. 90/10

6.9 What determines the selection of a wire feed roller for GMAW/MAG?

a. The type (chemical composition) of the wireb. Wire feed rate

c. The pre-bending of the wired. Diameter of the roller

6.10 Which MMAW electrodes produce the deepest penetration in the base metal?

a. Acid coated electrodesb. Rutile coated electrodesc. Basic coated electrodesd. Cellulose coated electrodes

6.11 In MMAW with covered electrodes when the arc length increases,

a. Current increases.b. Penetration increases.c. Arc voltage increases.d. Spattering decreases.

6.12 In submerged arc welding:

a. Only alternating current is usedb. Only direct current is used.c. The polarity has no importance.d. Alternating and/or direct current can be used.

6.13 Which of the following statements are not correct for the requirements of Laser Beam Welding,LBW?

a. Very accurate fit-up is necessaryb. Filler metal is always usedc. Mirrors or optic fibres are used to focus the laser beamd. The process calls requires automation

6.14 Which joining methods can be used for surfacing:

a. TIG-Hotwireb. Acetylene/Oxygen processc. Thermal sprayingd. Answers a), b) and c) are correct.

6.15 What are the four main process parameters of the resistance spot welding process?

a. Material, thickness, current type and pressureb. Current, pressure, time, electrode type and sizec. Time, pressure, material type, electrode size and typed. Polarity, pressure, material type, coating type

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