Thermal Joining

Thermal Joining


Soft Soldering

1) Soldering Iron plugged in and turned on. Tip heats up

2) Tip of soldering iron is cleaned, fluxed and coated with solder (tinning)

3) Heated tip is placed on parts to be joined. More solder can be added if needed.

Soft soldering is typically used for soldering electronic parts.

Note: The method for soft soldering on here is now outdated, but the exam may still require it. Nowadays, solder contains flux in its core, so the process of applying flux to the iron/material is not required

Hard soldering equipment


○ Quick and easy process

○ Low cost for equipment

○ Easy to work on small joints, very simple process to do


○ Material cannot be exposed to high temperatures, may melt the solder

○ The joint must be close fitting and void free

Hard soldering

Hard soldering is a bit different to soft soldering:

○ Hard soldering does not use a soldering iron. It uses a torch which is powered by oxygen and acetylene

○ It is only used with brass or silver

○ In hard soldering, the joint is much stronger than soft soldering and so it has different applications

○ Higher temperatures are used in hard soldering

○ In hard soldering, the metal is heated with flux on it, then the solder melts around the joint. in soft soldering, the flux is placed on the iron itself

○ Hard soldering is often used with brass alloys as they are much harder than other materials

Hard soldering equipment

The picture above shows hard soldering equipment, oxygen and acetylene cylinder tanks


Provides a very strong joint


○ A cleaning agent is required to make sure that the surfaces are 100% clean and free of grease and dirt.

○ Limited range of materials

○ Requires much higher temperatures than soft soldering which are harder to reach, uses more energy and is less safe.

○ Not suitable for electronics

Fusion Welding


Oxyacetylene welding is a metal welding process that involves two gases: Oxygen and Acetylene.

Oxyacetylene welding


○ The equipment is much more portable than other welding methods (MIG/TIG)

○ The equipment is cheaper than most other types of welding rigs (MIG/TIG welding)

○ It's easy to learn.


○ Produces a much rougher finish, so further finishing may be required

○ Oxy/Acetylene welds have large heat affected zones, which means that the area around the weld is mechanically weaker than the rest of the material.


MIG - Metal Inert Gas Welding

MIG welding is a welding process used for metals where the electrode rod is consumed. The rollers (drive rolls) feed the electrode down the weld tool when the trigger is pressed. The electrode is usually electrically charged or heated. The shielding gas (argon) that comes out the copper nozzle protects the weld from oxygen, nitrogen and hydrogen contained in the air atmosphere. The reaction of these elements with the weld pool can create a variety of problems, including porosity (holes within the weld bead) and excessive spatter.

Oxyacetylene welding


○ Very high weld speeds compared to other types of welding

○ Low skill factor required, easier to do than other types of welding

○ Produces less slag than other types of welding

○ No fluxes required in most cases

○ MIG welding is versatile and can be used with a wide variety of metals and alloys

○ Portable

○ Doesn't require external filler rod


○ Higher initial setup cost

○ Limited positions: Can't do overhead or vertical welding due to weld pool

○ Atmosphere surrounding the welding process has to be stable (hence the shielding gasses), therefore this process is limited to draught free conditions


TIG - Tungsten Inert Gas Welding

TIG Welding is the same as MIG welding, however a separate metal filler rod is used and it is not consumed.

Oxyacetylene welding


○ Produces a much higher quality weld compared to other welding types (However see disadvantage 1)

○ Much more aesthetically pleasing finish than other weld types as no slag is created or flux needed

○ No sparks, spatter, smoke or fumes, so increased visibility

○ Can weld in all positions, unlike MIG


○ Slower than other welding techniques due to its high quality

○ More difficult to do than TIG, requires more skilled worker


Spot Welding

Spot welding is a method of welding used to join two pieces of sheet metal together with just heat and pressure. Two electrodes (usually copper) are heated via passing an electric current through them. The electrodes then press against each other with the metal in between them. This creates a weld nugget where the metals have been welded together.

Spot welding Spot welding


○ Automated process, so large amount of products can be produced and doesn't require a skilled worker

○ It is quick, just needs a few seconds to make the joint.

○ Doesn't require any external materials like weld filler rods etc


○ High initial cost

○ Can only be done with thin metal

○ Limited range of materials

Solid Phase Welding

Friction (spin welding)

Friction welding consists of rotating one component whilst the other remains fixed. They are pushed together and the friction joints the two together.

Friction welding


○ Simple and easy process

○ Very fast, takes a few seconds

○ Minimal surface preparation required

○ Automated process, large quantities can be produced, no skilled workers

○ Can be done with both plastics and metals

○ Can join dissimilar metals together


○ High initial cost

○ Limited weld positions, requires two flat surfaces



Ultrasonic welding uses high-frequency mechanical vibrations to join two materials being held under pressure. It is commonly used for plastics, and especially for joining dissimilar materials. The vibrations cause friction between the two plastics so that they heat up and merge together.

Ultrasonic welding


○ Very quick and easy to do in a workshop

○ Low cost equipment

○ Doesn't require skilled worker

○ Can weld both plastics and metals

○ Aesthetically pleasing as there is no waste, visible seam or anything else created by the weld


○ Limited to thin plastics or very thin metals

○ Very small affected area, so can only weld small parts

Hot Air Guns & Filler Rods



Used to join two pieces of metal together.

1) First, the pieces are cleaned (preferably with wire wool or an emery cloth) so grease and dirt are removed

2) Flux is brushed along the surface of the parts to be joined.

3) After this, a welding torch/heat gun/soldering iron is used to heat the joint until it is red in colour. Once it is red, the flame is kept on it, and a brazing rod (copper filler) is applied to the joint. The copper melts onto the joint, leaving a copper weld.

Can be done with one of the three: Heat gun, weld torch, soldering iron


○ Very low initial cost

○ Doesn't require very skilled worker

○ Brazing is easily adapted to mass production and it is easy to automate

○ Very cost effective process

○ Doesn't require expensive machines so can be done at home or in workshop


○ Not very strong weld as copper is used (copper is quite soft, weak and malleable)

○ Brazed joints can be easily damaged under high temperatures

○ Copper is prone to corrosion