Rotary Friction Welding is a solid state welding process in which two similar or dissimilar materials are made to rub against each other to produce sufficient heat at the interface and a subsequent upset force fuses the two materials together. The impurities at the interface are removed as flash and the bond formed will be stronger than either of the two materials.

The driving physical phenomena of Friction Welding is heat generation, heat conduction, plastic deformation, abrasion of the friction surfaces and material diffusion

It can be used to join a wide variety of Part Geometries like

Tube to Tube	Tube to Disk	Tube to Bar
Bar to Bar	Bar to Plate	Tube to Plate

How it works

Joining metals can be defined as the achievement of a metallic bond across the interface. In an ideal metallic bond, the valence electrons are not bound to any particular atom, but move freely throughout the metal. The reason for this behavior is that the valence electrons in a metal lie much further out from the core of the atom than do those in non-metals. Moving along wider orbits the valence electrons pass to regions remote from their parent atoms and are exposed to the attraction of neighboring nuclei. Consequently the valence electrons in metals are never permanently associated with any particular atom, but flow freely in a random arrangement as a kind of free “Electron Gas”. Thus the structure of a metal can be regarded as an assemblage of positive ions each consisting of the core of an atom (nucleus plus non-valence electrons) immersed in a “gas” or cloud of free electrons. The attraction between the positive ions and the electron gas give the metal its structure and coherence. This attraction forms the metallic bond.

In the welding of two pieces of the same material, if the two surfaces to be bonded are freed from oxide layers and all other contaminants then brought in intimate contact with each other so that there is mobility between the electrons of both surfaces and the attractive and repulsive forces between the atoms are established, then the two surfaces will weld and the bond between them will be similar to that between two grains of the same material. If the two pieces are of different materials, the ability to establish a metallic bond between their uncontaminated surfaces is still possible, but the strength of the bond will depend on the materials concerned.

The process

STAGE 1 – Preparation
There are two basic requirements, that have to be satisfied before friction welding can take place between metals. They are:

i) Produce two absolutely clean and uncontaminated surfaces;
ii) Bring this clean and uncontaminated surface in intimate contact with each other so that a metallic bond can be produced across the interface.

The surface of an actual piece of metal is far from the ideal or virgin surface.

It shows both waviness and roughness. Waviness is a macroscopic attribute of an actual metal surface and roughness is its microscopic quality. Surface contaminants are of three types. They are:
a) Organic films
b) Absorbed gases
c) Chemical compounds of the base metal, generally oxides.

To start the process, one component is placed in a clamp which is stationary and the other component is loaded into a rotating spindle. The rotating spindle then moves forward so that the two components touch each other. The spindle is then brought up to a predetermined angular velocity and subsequently a pre-defined axial force is applied.

STAGE 2 – Heating

The heat generated due to rubbing increases the temperature of the components and brings down the yield strength. Once the surfaces in contact reaches a plastic state, it yields and flows outwards to form the flash. This process removes the oxidised layer on both the materials. They are no longer in contact with the atmosphere and hence no new oxide layer can form. The nascent surfaces come into contact with each other The facing surfaces will typically have minute crevices and this step helps even out the surfaces into a perfect flat area.

Once the desired burn-off is achieved, the process then moves to the next stage- the Forging Stage.

STAGE 3 – Forging

In this step, the rotational motion is stopped and a forge force is applied for a predetermined time. This force results in the fusion of the two surfaces into a permanent bond.
The interatomic forces of attraction comes into play with the atomic movements of atoms and valences taking place to complete the weld.

Process Parameters and graph

The quality of Weld as a result of Friction Welding is determined by the following parameters

• Spindle Speed
• Axial Force
• Displacement
• Torque

The below graph shows the Weld parameters across the three phases:

Possible Weldable combinations

Rotary Friction Welding provides a unique opportunity to join dissimilar material types which are otherwise difficult to be welded together. The below chart details out what can/cannot be forged together using this process:-

Source: https://lib.ugent.be/fulltxt/RUG01/001/805/267/RUG01-001805267_2012_0001_AC.pdf

Advantages

There are quite some advantages of Rotary Friction Welding, as compared to other processes. Significant among them are

• Very short cycle time (few seconds)
• Ideal for mass production
• Savings in costly material if bi-metallic component is used (Eg. Drills- HSS/MCS)
• Low heat -affected zone and hence semi finished components can be welded
• No edge preparation, filler material, shielding gas, spattering, fumes and radiation
• Excellent welding: Joint as strong or stronger than parent material
• Metals as diverse as Cu to Al, Cu or Al to steel, Titanium to stainless steel etc. can be welded
• No inclusions such as oxide layer.
• Pre-treatment and post-treatment required only for few specific non-ferrous metals and some alloy steels.
• Highly productive with low cycle time.
• Dissimilar metals can be welded.
• Process is ecologically clean.
• No smoke, slag or consumables such as gas, flux or filler material.
• Components of shape such as hexagonal, square, elliptical etc. can be welded.
• Low energy consumption compared to other welding processes.
• Weld monitoring provides a 100% in process quality check.