19. Welding, Soldering, and Brazing

• Welding is the process of joining two or more objects together. In general this is done by melting the adjacent surfaces, or by melting a third material that acts as a ‘glue’

• We can categorize welding by processes,

 

19.1 Arc Welding

• Basically, an electric arc is used to heat base metals and a consumable filler rod.

• This is the most common form of welding and is used in about half of all applications.

• A power supply is used to create a high potential between an electrode (guided by the welder) and a metal work piece. When moved close enough electrodes break down the air and start to flow. The local current of the flow is so high that it heats metals up to 30000C or 54000F.

 

• Material is added during this welding process.This material can come from a consumable electrode, or from a rod of material that is fed separately.

• The electrodes/rods are often coated. This coating serves a number of functions,

it protects the welder from contact

it deoxidizes and provides a gas shield

• Problems that arise in this form of welding is contamination of the metal with elements in the atmosphere (O, H, N, etc.). There can also be problems with surfaces that are not clean. Solutions to this include,

Gas shields: an inert gas is blown into the weld zone to drive away other atmospheric gases.

Flux: a material that is added to clean the surface, this may also give off a gas to drive away unwanted gases.

• Common types of processes include,

SMAW (Shielded Metal Arc Welding)/Stick Welding: A consumable electrode with a coating that will act as a flux to clean the metal, and to create a gas shield.

 

MIG (Metal Inert Gas): A consumable electrode in a gas shield. In addition to simple materials, this can handle aluminum, magnesium, titanium, stainless steel, copper, etc. This torch is normally water or air cooled.

 

TIG (Tungsten Inert Gas): A nonconsumable tungsten electrode is used with a filler rods and a gas shield. This can handle aluminum, titanium, stainless steel, copper, etc. This torch is normally water or air cooled.

 

SAW (Submerged Arc Welding): A normal wire is used as a consumable electrode, and the flux is applied generously around the weld. The weld occurs within the flux, and is protected from the air.

 

• Process variables include,

electrode current 50-300A is common

voltage

polarity

arc length

speed

materials

flux

workpiece thickness

19.2 Gas Welding

• Basically, filler and base materials are heated to the point of melting by a burning a gas.

• Two common types are,

oxygen-acetylene

mapp gas

• These are suited to a few applications, but they produce by-products that can contaminate the final weld.

• Typically the flame is adjusted to give a clean burn, and this is applied to the point of the weld.

 

• A welding rod will be fed in separately to melt and join the weld line.

• Flux can be used to clean the welds.

• Process variables include,

gas and oxygen flow rates

distance from surface

speed

material types

surface preparation of materials

19.3 Soldering and Brazing

• Basically, soldering and brazing involve melting a filler material that will flow into a narrow gap and solidify. It is distinct because the base materials should not be melted.

• The main difference is,

Soldering is done at a lower temperature, either with a propane torch, or an electric heater. It is intended for bonds with less required strength, such as electrical and plumbing applications.

Brazing is done at higher temperatures with oxyacetylene or mapp gas torches. These bonds tend to be higher and can be used for mechanical strength.

• General process considerations include,

Suitable for gaps from 0.001” to 0.01”

Surfaces must be sanded and cleaned before these processes are used.

Flux is often used to deoxidize a surface so that the filler will adhere better. Typical fluxes include,

Brazing flux: fused borax or alcohol and borax paste

Soldering flux: inorganic salts (zinc ammonium chloride), muriatic acid, resin based

Some fluxes are corrosive and should be removed after use.

• Materials include,

Solder is often an alloy combination of two of tin, lead, silver, zinc, antimony or bismuth.

Brazing metals are typically alloys such as,

brazing brass (60% Cu, 40%Zn)

manganese bronze

nickel silver

copper silicon

silver alloys (with/without phosphorous)

copper phosphorous

19.4 TITANIUM WELDING

• Titanium as a metal

above 885°C the material undergoes beta phase transition to body centered cubic arrangements

melts at 1800°C

resistance to corrosion

high affinity for carbon

soft and ductile when annealed

• Above 260°C titanium absorbs oxygen, nitrogen, and hydrogen. This causes when welding, because in excess they make titanium brittle.

• Titanium welding requires,

a very clean environment with no contaminants or other materials.

no drafts

the correct welding equipment

• To eliminate unwanted gases and moisture from being absorbed, a gas shield is used on both sides of the weld.

• The weld must be shielded until the temperature drops below 427°C.

• Gas tungsten arc welding,

gas is used to cover the tip of the torch, electrode and workpiece.

• The torch is,

a split copper collect holding a tungsten electrode. A nut tightens the collet and holds the electrode. The collet also serves to conduct current to the electrode.

tubes delivers gas to the torch, and it is channeled to the electrode in such a way as to ensure uniform coverage.

• Gas cups are,

Ceramic, metals or high temperature glass is used to direct the gas about the electrode. The size typically effects the gas consumption.

• An optional trailing shield focuses gas on the now welded joint, to allow proper cooling time.

 

• The electrode stickout (or electrode extension) is the distance that the electrode protrudes out the end of the collet. A larger stickout is proportional to the energy delivered, and the size of the gascap, and it allows better visibility of the work.

• A gas lens can be used to focus/balance the flow of gases, it can be used without a gas cup, or with one to improve gas coverage.

• Gas backups are placed on the back of the weld seam, purging is used when the back of the weld is enclosed (e.g., tubes).

• Typical welding parameters,

 

• Joints can be prepared by machining. If torch cutting has been used, the edges must be ground to remove the by-products of the cutting torch (typically > 1/16”). After grinding, burrs should be filed off.

• Surface cleaning should include,

1. degassing

2. brushing with stainless steel

3. sandblast off heavy scale

• Welding can also be done is a sealed chamber flooded with an inert gas. The chamber can have gas evacuated, and then reflooded, or gas flow will eventually exchange air for gas.

19.4.1 Problems

[an error occurred while processing this directive]

Problem 19.1 TRUE / FALSE: Oxygen is used to enhance oxidation when welding titanium.

19.5 Plastic Welding

• Well suited to joining of thermoplastics.

• Types of plastics used in welding are,

 

• Plastics to be joined should be compatible. A common method is based on trial and error testing.

• To determine plastic types burning small samples with a low flame gives the following observations,

 

• Welding of thermoplastics involves heating, contact, cooling and bonding.

• Joints may be weakened by incomplete fusion, oxidation or thermal degradation of the plastic.

• Melting may be done by,

gas or electric gun

heated tool

induction heating

friction

spinning

• Sheet welding,

heat and pressure are applied at an overlapped joint between thin sheets.

rollers join the sheets (one roller is often motor driven and heated while the other just applies pressure).

typical variables are,

roller temperatures

feed rate

pressure

underheating leads to a loose seam.

overheating leads to a hole formation.

parametric setting is very sensitive.

welds can be done on tables with a hand roller and a heat gun.

advantages

simple tools

disadvantages

hard to set parameters

preparation of welded sheets.

 

• if welding plastics to repair cracks, drill holes at the ends to stop crack propagation.

• Hot air/gas welding,

Used successfully with molded parts in,

PVC
polyvinylidene chloride
polyethylene
acrylic
polychlorotriflourethylene

Operation steps,

1. pieces positioned but a gap of 1/16” left

2. a suitable welding (often same material) rod is pushed into the gap

3. a hot blast often 400-600°F is directed at the tip of the welding rod and surfaces to be welded. * if a torch to focus distance of 1-2” is used, a drop in temperature of 200°F will occur, the resulting temperature should be the melting temperature of the plastic.

The final strength ranges from 50% maximum for high density materials, to near 100% for low densities.

The heating guns are similar to common hair dryers with heaters and fans, and vents to control air flow rates.

The heat calls for safety measures.

Nozzles: a variety of nozzles and tools are available.

Advantages,

simple tools

Disadvantages,

welding angles hard to set

• Tack welding,

parts are put in position.

the gun temperature is allowed to heat up (a tack welding nozzle is used).

the gun is put at an angle of 30-40°F to the weld and held in place until melting begins.

the gun is slowly drawn along the seam.

since the tack weld is weak (used for positioning) subsequent welding is required.

• General welding,

operation

1. the gun is held 90° to the weld and a rod is inserted.

2. once the rod starts to melt, the gun is turned to a 45° angle and moved steadily along the weld.

3. The gun is moved in an elliptical path over the weld with an amplitude of about 1”.

4. The rod is forced into the groove with a pressure of about 3-6 lbs. This pressure prevents air from entering the weld. An angle of 45° to 90° is used for the rod.

5. When ending a weld, the heat is turned off, and after cooling the rod is twisted off, or for continuous welds there should be an overlap of 1/2”.

6. If required a weld can be restarted by cutting the previous weld at an angle, and starting from that point.

• Speed Welding

the rod and gas are fed side by side.

the rod is heated in the gun, and is “wiped” out as it leaves the gun.

when starting pressure is applied to the rod and a sharpened tip is forced into the work.

as the rod starts to melt, the gun is lowered to 45° and drawn along. The welding rod is pulled in itself.

moving the tip too fast will result in beading and too slow will result in charring.

the weld is stopped by standing the gun at 90° to the surface and pulling the gun off. The rod is then cut off.

• Tractor Welding (Machine Welding),

a hot air gun and rollers are driven over a surface by motors.

a tape can be dispensed that will join the sheets or the two sheets can be overlapped.

advantages,

fully automated

easy to set parameters

disadvantages,

special equipment required

• Ultra Sonic Welding,

basically a high frequency vibration is directed through a plastic joint. The vibration causes friction, and then heat, often causing a solid bond in less than a second.

frequencies above 20 KHz.

the distance the vibration travels has a great deal to do with determining the classification.

very well suited to rigid thermo plastic parts.

good designs make direct application of the vibrations possible.

 

a smaller contact area increases the energy concentration. As a result V-notches, tongues, pins, and other special joints are commonly used.

 

if remote sealing is necessary, thicker walls should be incorporated into the part design

epoxy molds can be used to reinforce weaker parts when doing this operation.

advantages,

fast

clean

no extra materials needed

disadvantages,

tool design required

simple design rules not always available

• Linear Vibration Welding,

similar to Ultrasonic Welding, except that frequencies are about one hundred Hz and amplitude are mm.

this is best used with high coefficient of friction, low viscosity plastics.

• Spin/Friction Welding,

two parts are spun and the contact area builds up heat through friction and pressure. The pressure forces a good bond between parts and drives out bobbles.

flashing may occur with this method.

advantages,

produces a good weld

air does not enter during welding

inexpensive machines, such as drill presses may be used

disadvantages,

circular weld joints are required

• Testing Plastic Welds,

a hand held gun can be used to generate arcs. The sparks are generated with voltages up to 55 KV at 200 KHz.

operation,

1. The gun is calibrated to spark at distances just over the weld thickness (to a ground plate).

2. the ground plate is placed behind the weld.

3. as the probe is moved over the weld, sparks will jump when a gap in the weld moves between the probe and the ground plate.

19.6 Explosive Welding

• The basic mechanism is based on molecular bonding, as a result of high velocity impact. The high velocities are promoted by carefully detonated explosives.

 

• The process is done at room temperature in air, water or vacuum.

• Surface contaminants tend to be blown off the surface.

• Typical impact pressures are millions of psi.

• The process can be done in vacuum to reduce sound and blast.

• Well suited to metals that are prone to brittle joints when heat welded, such as,

aluminum on steel

titanium on steel

• The process does not work well for,

brittle metals with <5% tensile elongation

Charpy V-notch value < 10 ft.lb.

• If two materials can be brought close enough together, they will bond at a molecular level.

• This normally does not happen because surface contaminants (ie oxides, nitrides and absorbed gases) prevent a close approach of surfaces.

• Normal welding overcomes this problem by melting materials so that they mix in liquid phases.

• Important factors are,

critical velocity

critical angle

 

• When parameters match up, the surfaces form a liquid jet starting at the point they impact, and is directed away from the welded seam.

• The plates have an air gap between, and plastic, liquid or granular explosives are placed on the plate. The backer plate is rested on an anvil (e.g., sand could be used for lighter backers, or concrete/steel for stronger backers).

• The cladding plate can be supported with tack welded supports at the edges, or the metal inserts.

• Two plate shapes with straight constant interface or angled interface clearance.

• High velocity explosives require smaller gaps between plates, and buffers such as rubber and plexiglas. Angled interfaces are only used for high velocity explosives.

• Typically the detonation velocity should not exceed 120% of the sonic velocity in the metal.

• Typical explosive forms are,

plastic flexible sheet

cord

pressed shapes

cast shapes

powder/granular

• There is a maximum velocity for welding, above this the thermal effects weaken the joint.

• To efficiently use explosives the plate separation is 1/2 to 1 times the cladding plate thickness.

• High velocity explosives, 15-25,000 ft/sec. (4572-7620 m/s),

TNT

RDX

PETN

Composition B

Composition C4

Detasheet

Primacord

• Medium velocity explosives, 5-15,000 ft/sec. (1524-4572 m/s),

Ammonium nitrate

Ammonium perchlorate

Amatol

Nitroguonidine

Dynamites

diluted PETN

• 3 bond types,

straight, direct metal-to-metal: best type of bonding but difficult to obtain when collision velocity less than critical velocity.

wavy: the interface is strong and the interface has waves.

straight, but with a continuous layer: a weaker bond that results when the collision velocity is too high and the alloy bonds are strong.

• Advantages,

can bond many dissimilar, normally unweldable metals

the lack of heating preserves metal treatment

the process is compact, portable, and easy to contain

inexpensive

welds can be done from in2 to hundreds of ft2

no need for surface preparation

the backer plate has no size limits

• Disadvantages,

the metals must have high enough impact resistance, and ductility.

the geometries welded must be simple: flat, cylindrical, conical.

the cladding plate cannot be too large.

noise and blast can require worker protection, vacuum chambers, buried in sand/water.

• Typical applications,

spot welding

reinforcing aerospace materials with dissimilar metal ribs

seam and lap welds.

tubular transition joints

flat plates

19.6.1 Problems

[an error occurred while processing this directive]

Problem 19.2 TRUE / FALSE: Ductile metals can be welded with explosives.

Problem 19.3 What is the purpose of flux in welding?

Problem 19.4 List 20 parts you have seen that are welded. Indicate which welding process is the most appropriate for each.

Problem 19.5 What types of processes would be best suited for joining the following items? Indicate why.

a) two 12” dia. plastic pipes.

b) two 12” dia. steel pipes.

c) the sides of a plastic bag for potato chips.

d) two aluminum plates along one edge.

e) an aluminum and steel plate into a laminated plate.

f) steel muffler pipes.

Problem 19.6 What are the primary differences between welding soldering and gluing?

 

[an error occurred while processing this directive]