This is a presentation of the two most basic types of friction welding.   We will examine how the energy is initially established for each method, and will also probe into the differences in the energy available for both of these friction welding methods.

Particular attention will be paid to the variability of the friction welding parameters, and to look closely at how these affect the overall welding process.  Expanding on this subject, we will see how these processes can be controlled to accommodate the friction welding of various metals and material shapes.  The pages that follow will provide relevant content that can potentially expand the interested manufacturer's knowledge of friction welding, thereby enabling him or her to intelligently evaluate the application of this process according to their own production needs.

When we speak of friction welding, we are actually talking about any of several techniques of joining metals.  All of these techniques employ the heat due to friction and pressure to accomplish the fusion of materials.  The more common, of course, are direct drive and inertia or stored energy friction welding.  The less common friction welding processes might include orbital oscillation, and linear or angular oscillation.  These technologies are demonstrating tremendous potential as they continue to emerge as viable processes.   In virtually all these welding techniques, heat is created by friction as the components are held against each other under pressure.  The significant differences in these processes lie in the relative motion of the components to each other, and in the sequence of applied pressure.

The main thrust of the efforts to develop the use of friction welding in manufacturing has been largely with the "stored energy" and "continuous drive" systems.  As the use of friction welding matures, it is becoming more apparent that the less familiar variations of friction welding are now beginning to gain more attention.

The United States has historically trailed other industrialized nations with respect to their willingness to accept friction welding.  The nations that have willingly accepted friction welding processes have realized the large savings in materials and cost these techniques can offer.  Interested manufacturers everywhere can help themselves tremendously by making every effort to acquire a basic knowledge of both continuous drive and the stored energy systems.

The basic operation for both of these systems consists of rotating one component in contact with a second component that is held stationary.  By applying axial pressure during rotation, heat is generated.  This process of pressure and heat creates a bond at the interface of the two mating parts.  It is significant to mention that the metal at the interface does not melt, but rather becomes plastic, We therefore find that the term "solid-state" welding is sometimes applied to this process.

One last point to consider is the fact that countless millions of welds have been made using both of these processes.  The selection of one process over the other ought not, therefore, to be taken into consideration in our discussion here.

In continuous drive friction welding, welding heat is obtained at the joint by rotating one part against the other at a constant or varied RPM, with an axial force applied to the mating components.  Energy is provided to this joint from a continuously running prime mover - usually an electric motor, directly connected to the machine spindle.  This energy source is infinite with respect to time, and is supplied to the interface until the proper total heat is obtained.  When this point is reached, the rotating member is stopped and a forging load is applied to the parts to be joined.

It's important to note that the speed is held constant for a selected time and/or distance, as pressure is varied.   The displacement of material at the interface is a function of heat input and pressure.  Rate of heat input is a function of rotational speed and axial pressure.   By combining specific rotational speeds and specific pressures, variations in the rate of metal displacement and volume of metal displacement can be obtained.

In order to better understand these relationships, 1et's put some order of magnitude on these parameters.  Speed is the peripheral velocity at the outside diameter of the smallest diameter part.  Speed is usually established between 250 to 500 SMF.  Good welds can be made at a slower or faster speed.  However, this range fits well into standard machine spindle and bearing design limits.

If we consider common steels, the following parameters would be applicable.  Friction pressure is totally variable.  It is usually lower during the initial stage of the process, then increased.  Pressure during the initial stage is usually 2 to 3 tons per sq. in. of cross sectional area.  The second friction pressure is increased to some magnitude between 1 to 6 tons per sq. in. of cross sectional area.

Forge pressure is applied following cessation of rotation, and is approximately 10 tons per sq. in. of cross sectional area.  Pressure is an important parameter and is selected after considering the metallurgy of the materials being joined, the shape of the parts, and to optimize weld quality.

As in the continuous drive system, the stored energy system also utilizes components one part is stationary and the other rotates with the machine spindle.  Energy for welding is supplied by the kinetic energy stored in a rotating system or mass.  The energy available in the continuous drive system is, for the sake of the discussion, infinite.  The energy available in the stored energy system is finite and selected to meet the requirements of the weld joint.  Due to considerations of machine design and total energy required, the rotative speed of the running component is considerably higher than in the continuous drive system.   Pressure is generally constant and the displacement curve takes on a different shape than that noted for the continuous drive system.  The majority of the total displacement comes at the end of the weld cycle.

The key to successful welds with this system is in the proper selection of the total energy required for a specific metal or metal combination and amount of cross sectional area at the interface.

A significant difference between these two systems of friction welding is the, control over energy input into the weld joint.  The Stored Energy system offers the potential to obtain higher input rates, the continuous drive system offers control over time of energy input as well as rate of input.  This becomes significant when greater total heat is required to slow the rate of cooling.

As mentioned earlier, both of these processes are known as "solid-phase" welding, with no melting of the materials being joined.

With the above serving as an introduction, let's develop the subject further.  The material that shall be presented subsequently is divided into three sections:

I. A discussion of welding processes leading to an investigation of solid-phase welding.

II. A metallographic comparison of fusion and friction welding, and a general discussion.

III. Friction welding variables, and their effect on the weld structure.

Fusion welding is undoubtedly one or the most common and important industrial joining techniques used today.  T1G (GTA), MIG (GMA) , stick electrode, electron beam, laser, plasma- arc, and others are all in common use.  In the case of electric-arc fusion welding, the process consists of high energy input resulting in melting of the filler and base material - and ultimate fusion.

Brazing and Soldering

This type of joining process, like fusion welding, is also in wide use and, therefore, of great importance.  Unlike fusion welding, however, there is no melting of the base material; i.e., workpiece.  Only the filler (braze alloy) which is sandwiched between the members to be joined is melted.  There are "braze" alloys available, however, that do cause some incipient melting of the base material due to diffusion of elements which causes a depression in the melting point.

Solid-Phase Welding

As the name implies, there is no melting of the material to be joined in solid-phase welding.  All bonding occurs strictly in the solid state condition.   Examples of solid-phase welding processes are as follows:

    1. Forge welding
    2. Some types of electric resistance welding
    3. Explosive welding
    4. Ultrasonic welding
    5. Friction and Inertia Welding

A satisfactory solid-phase weld requires that metallic bonds be established between the atoms at the surfaces of the metals to be joined.  In order to achieve these bonds, the surface must be brought together within atomic distances.

The cohesion and strength of a metal depends on the attractive force of constituent atoms.  Iron, for example, exists as a body centered cubic (BCC) lattice at low temperatures.  When in the equilibrium position, i.e., at rest, the respective atoms are separated by an atomic distance.

An applied load will cause a deformation in the atomic lattice resulting in an-internal stress.  If the load is sufficiently great, a displacement (elastic) will occur followed by atomic slip (plastic) along planes at weakness resulting in permanent deformation and increased strength in the localized slip areas.   Subsequent slip, therefore, must occur somewhere else in the lattice.   Eventually, a point reached where further slip cannot occur resulting in a fracture and the generation of two new surfaces and associated with these surfaces is free surface energy.

From a thermodynamic point of view, the energy of a system can be minimized by achieving bonding between surfaces which leads to a more stable state. Hence, the formation of bonds is favored.

A magnified view of a surface will indicate that it is composed of asperities, and that actual contact with a mating surface occurs on only a very small percentage of the actual cross-section.

In order to create a solid-phase bond between the mating pieces, the surfaces must be deformed sufficiently such that the surface atoms can establish bonds and contaminants (oxides, oil, etc.) that will inhibit bonding must be removed by some means.   Therefore two conditions must be satisfied prior to bonding:

    1. Surface cleanliness
    2. Surface deformation

A minimum amount at surface deformation is required before any strength is established in a weld.

In friction welding, if the criteria mentioned; i.e., cleanliness and deformation, are satisfied, then the result is creation of we1ds of the highest integrity.

Fusion Welding

A common setup for fusion welding might include two heavily cold worked bars of a material exhibiting a single phase, with a V-joint and land machined for weld penetration purposes.

A magnified view of the finished fusion weld would show changes in weld macro and microstructure resulting from thermal cycling during welding.  The center area of the welded segment, which becomes molten, solidifies leaving columnar grains which grow opposite to the direction of heat transfer.  Adjacent to this molten zone, there is an area of large equiaxed grains which recrystallizes from the cold worked grains and then grow in size due to the high temperature.  A zone of smaller equiaxed grains will be evident, sandwiched between the larger equiaxed and unchanged grains.  If the zone did not have sufficient time or temperature for the grains to grow; recrystallization might occur.  Overall, there is a severe change in microstructure as the weld is traversed.

Friction Welding

A common setup would be that of two cold worked unwelded bars, but with no end preparation.

A magnified view of the finished friction weld would show the weld macro and microstructure, and would show no evidence of melting.  The changes from the original structure would include a heat affected zone (HAZ) containing recrystallized, equiaxed grains and some heavily worked grains at the weld interface.  There is a considerable difference, then, between the fusion and friction welds.  Yet, some structural changes due to thermal cycling are common to both.

The preceding discussion was simplified since it only dealt with changing the actual size and shape of the individual grains and not with potential transformations.  When welding medium or high carbon steels, for example, it would normally be expected that in addition to changes in grain size and shape the high cooling rates common to all welding processes would create the transformation products often indicated by the representative TTT curve.

Preheating is a common practice in controlling cooling rates and we do have some capability to preheat in friction welding.  In the event undesirable transformation products form, post weld heat treatment will most likely be required.

The variables in the friction welding process can essentially be divided into two initial groups.  These are machine and non-machine.   Non-machine variables will include the material type to be welded and the part configuration and size.  These variables like in any other welding process will determine the selection of welding parameters.  The machine variables include friction and forge pressures, speed of rotation and axial shortening.  The rotational speed and pressure selected control the ultimate quality of the weld.

The rotational speed and pressure effect both the width and shape of the heat-affected zone.  Higher pressures tend to compress the HAZ, especially at the center, heavily work the interfacial material and cause a notch effect at the flash junction.  Higher speeds tend to increase the width of the HAZ and also the grain size.  Subsequent use of high pressure forging after spindle stop is used to work the structure and refine the grain size.