physics of welding better version

8/9/2019 Physics of welding better version

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List and briefly explain the three factors that prohibit the joining of various engineering

metals.

Surface Roughness all materials will have a varying level of discontinuity at the surfaceconsisting of peaks and valleys (asperities) that will prevent full/direct contact between

adjoining materials. When a surface is prepared by smoothing the surface (machining, polishing,plastic deformation between adjoining materials) there will likely still be some variance between

the two surfaces. Surface Contamination Contaminant Layers such as absorbed gases,moisture and/or greases can cause different problems in welding like forming unwantedparticles/voids in solidifying weld metal or can be the reason for cracks appearing in the fusion

zone. Also, oxide layers that form at the metal surface can impede electrical flow by varying thelocation of anode/cathode spots in the arc during arc welding. Oxide layers can also react with

the molten pool in fusion welding. Energy of Adhesion Free energy at the material surfacecauses surface tension that must overcome interfacial free energy occurring between two

adjoining materials in order for joining to occur. This can be accomplished by supplying energy(kinetic, chemical, electrical potential, etc.). Strong interfacial bonding requires crystallographic

matching at the interface, which will help to lower the interfacial free energy.

Define the Lorentz force and explain why the plasma gas always travels from the electrode

to the workpiece regardless of the welding position.

The Lorentz force is the electromagnetic force associated with the current density vector carriedthroughout the arc and the induced magnetic flux associated with the flow of current, where

FLorentz =J x B. The arc flows towards the work piece, provided conditions are appropriate(electrical input, electrode type, work piece distance, etc.), because this is the path of least

resistance in order to complete the electric circuit.As the current density field within the arc diverges/converges to/from the small area at the

electrode tip to/from a larger area at the work piece (typically giving a conical formation) therewill a vertical component of the Lorentz force that is directed in the downward direction. This

occurs because the current density vectors/flowlines at the boundary of the arc are notperpendicular to the work piece, resulting in a net electromagnetic (Lorentz) force that is both

inward and downward. [Sin Do Kou, The Physics of Welding]

Describe the arc force (Note: Arc force is not the Lorentz force. But, Lorentz force will

influence the arc force).

The arc force is the combination of forces induced by the Lorentz/electromagnetic pressure andarc gas/vapour pressure. Where electromagnetic pressures are effected by current density flow

and magnitude (effected by factors such as electrode size/shape) and gas/vapour pressures areeffected by gas flow rates (and the distance the gas/vapour travels), arc gas/vapour densities

(where the difference between Argon and Helium gives differences as shown in the notes;Helium is a lighter than air gas and in overhead welding this can be beneficial to arc

flow). Gravitational forces can also play a role in the arc force.

Explain the self-stabilization nature of the C.V. power supply with a constant wire feed

rate.


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Using a constant voltage (CV) power supply with a corresponding constant wire feed rate, anequilibrium state will arise so that the stickout will return to this established point after there is a

variance in the arc length. With a decrease in arc length there will be a decrease in the resistanceof the system, this will bring an increase in current that will increase the melt off rate at the tip of

the consumable electrode, and with an increase in melt off rate the electrode tip will tend to

return back to the equilibrium operating point. The opposite will occur if the arc lengthincreases.

Define and differentiate weld joint and weld types.

A weld joint is how the work piece to be joined is constructed. Examples are a T-joints andButt-joints:

A weld type describes the weld that is being used at the given joint. Examples are a Fillet orGroove weld.

Will you have thermionic emission when welding aluminum with DCRP? Explain.

In DCEP welding of Aluminum it is not expected that there will be thermionic emission. Thereasons for this expectation are: (note that Thermionic emission exists when a material reaches

an elevated temperature and beings to emit electrons and possibly ions, this temperature is itsincandescence temperature)

Aluminum is not a refractory metal, which are more likely to produce thermionic emissions ofmagnitudes large enough to affect current density (this doesnt mean that thermionic emission

from the surface is impossible, but that a detectable quantity of emission is reduced).An Aluminum work piece will act as a powerful heat sink and will tend to distribute heat away

from the weld, where this will act to cool down the cathode area/spot. Thermionic emission isthe release of electrons/charged ions on a surface due to elevated surface temperature. With less

heat at the surface there is less chance for emission to occur. [the physics of welding]

In a welding arc, at what node will most of the heat be generated?

Generally the anode will generate the most heat. This is the location within the arc that has the

highest concentration of electrons, which in the column will generate the highest temperature. Pg126 The Physics of Welding

For a non-autogenous welding process (i.e., filler metal added), what is the most commonly

used polarity?

DCEP (DCRP).

Describe the basic components of an inverter arc welding power supply. What are the

advantages of using inverter technology in arc welding machines?

Rectifier changes AC to DC, High-speed switch (inverter) converts DC to high frequency AC,Transformer brings to a lower voltage and isolate the output from the utility line (transformer

is small due to the incoming high frequency AC power), solid state rectifier converts back to DC,


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filtered smooth DC characteristic.Advantages of using an inverter power supply are: smaller, more compact than other power

sources make it more accessible, especially during field work. Precise control of current allowsfor better control of varying welding processes and is more easily manipulated to give required

output characteristics (pulsing and waveform manipulation). [Modern Welding Technology]

The curves of group a have lower voltages than the curves in group b because the electrodeused in group a is Thoirated Tungsten, which operates below the melting point (ie, the

electrode tip will not melt and become spherical like with a pure Tungsten electrode, which tendsto cause the cathode spot to wander). The lower work function of thoriated material permits

equal current density of electron emission at lower temperatures. [The Physics of Welding,pg153] This reduction in the power usage means that less voltage is required to achieve the

same current.

(Anode/Cathode only molten In either case the conductivity in the arc rises due to higherconcentration of metal vapour that is being evaporated from the molten pool. This increased

concentration is a possible reason for increased conductivity in the arc, which will give highercurrents at a lower voltage. The anode generates slightly higher heat than the cathode and

therefore should also mean a slightly higher concentration of metal vapour in the arc.

Both Cathode and Anode Non-molten This situation occurs when heat sinking is greatest out of

both cathode and anode materials, giving less energy for metal evaporation to occur and hencelower conductivity (higher resistance) in the arc.

A general definition given from The Physics of Welding is, The arc power is equal to the heatlosses from the cathode, anode and arc column added together and that the less heat transmitted

to the surroundings will give a correspondingly lower arc voltage. So the case of the greater heatsinking (cathode and anode non-molten) suggests more heat is being transmitted to the

surrounding area, instead of through the arc. This statement also corresponds to part 4.a) in thatthe smaller diameter electrode of group a is also transmitting less heat to its surroundings and

this will also be a contributing factor as to its lower voltage.

physics of welding better version

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