Edited by Jessica Shapiro
Engineers can gain design flexibility and save assembly costs by adding friction welding to their design toolkits.
Friction welding was first patented in the late 19th century and has been a staple of the auto industry since the early 1960s. However, engineers in many industries don’t realize they could be using it to join dissimilar metals quickly, cleanly, and accurately.
Friction welding differs from conventional welding in one key way: The parts being joined never melt. Instead frictional heat softens the interface between parts while they are still in a solid state. Forging force thrusts the parts together to create a single, homogenous piece.
And who says you can’t weld aluminum to copper or steel, or eliminate bolted joints, screwed interfaces, and additional flanges? Friction welding can do all of these things. It can also save money while creatively solving design challenges, especially when engineers keep the technique in mind from the start of the design process.
Friction welding takes several forms, but it never melts the parent material of either part being joined. Linear, friction-stir, and rotational welding all have the same basic principle: Frictional heat creates a plastic zone between two workpieces where they can be forged together under external force.
Linear friction welding (LFW) pushes the two workpieces together and rubs them parallel to one another. Friction-stir welding (FSW) also works on side-by-side workpieces, but a nonconsumable spinning pin tool inserted into the workpiece heats the interface material, extrudes it, and forges it in its wake. The resulting weld is formed with no loss of length or thickness.
Rotational friction welding (RFW) — including direct-drive and inertia welding — joins a spinning workpiece to a stationary one by rotational friction and externally applied force.
In a direct-drive setup, an electric motor keeps the spinning piece at a constant rpm. Welding machines move the workpieces together and apply an axial force. Direct drive angularly orients parts to within ±0.5° and permits more precise control over heat input into the parts.
Inertia friction welding uses energy stored in a flywheel rather than supplied by an electric motor, to keep one workpiece spinning.
One variant of these rotational processes is radial friction welding, in which machines apply force perpendicular to the axis of rotation. Radial welding can join tubular sections or attach collars to cylinder ODs.
In all types of RFW, the welding machine keeps the workpiece spinning a predetermined time, until the flywheel energy is exhausted, or until it reaches a preset amount of upset, i.e., the material displaced during the weld cycle, measured linearly. This loss of linear length is also called flash.
Even after rotation stops, the machine maintains or increases the welding (or forge) force a preset time to form a joint with the same strength as a conventionally forged joint.
All these techniques keep the base materials below their melting or liquidus temperatures. Lower temperatures and shorter times result in much smaller heat-affected zones (HAZ) on the interface after friction welding than after conventional welding.
Minimizing HAZ means there may be no need to heat treat parts before or after welding to relieve internal stresses. And there’s less worry about future corrosion and cracking in the HAZ.
Whether the process is RFW, FSW, or LFW, the relative motion that generates heat activates the forging process at the atomic level. The heat and force elastically displace the lattice planes of the surfaces being joined. This displacement creates the driving force for atoms to rebond to their new neighbors, atoms from the joining part.
Bonds can form between atoms in dissimilar materials. Aluminum can bond to stainless steel or copper, nickel alloys can bond to steel, and silver can bond to copper. The strength of the joint typically approaches that of the weaker of the two parent materials.
Full-strength welds require proper boundary-layer bonding, so there can be no contamination in the interface plane. Luckily, friction welding functions by displacing the original interface materials, so the parts being joined are self-cleaning and self-preparing.
Consequently, any forgeable material can also be friction welded. This includes a wide range of formed conditions such as wrought, broached or splined, forged, investment or die cast, powder-metal formed, and stamped.
All friction welds are by nature fully penetrating. In addition, because the complete interface is welded simultaneously, there are virtually no offset thermally induced stresses.
And because friction welding is a machine-controlled process, it is consistent and repeatable.
Cost savings and benefits
Many people assume friction welding is an expensive, customized, high-volume process. In fact, the process works for production lots of five or 500,000 parts, including fully automated high-volume production.
Most engineers who switch to friction welding find it takes less than half the time and one-third the power of conventional welding. Skipping pre or postweld heat treatments can also save time and money. And there are fewer expendable materials like fluxes and shielding gases to stock up on.
And because the process doesn’t emit heavy electromagnetic fields (EMF), engineers can avoid the cost of protecting production electrical lines, machine controls, and nearby electronic equipment from this interference.
Friction welding has the biggest impact on part cost and function when engineers integrate it into the design process. Parts that couldn’t be made by conventional welding may be possible with friction-welding techniques.
For example, delicate parts are usually added to assemblies after welding, but friction welding produces lower, more-localized heat, so it can join components later in the assembly process. For instance, it can tightly seal the ends of a tube after the tube has been filled.
Friction welding’s lower temperatures can join parts after they undergo final machining. There may be no need for postjoining stress relief that can distort dimensions.
Designs that need precise axial and angular positioning can use rotational friction welding as well. Some friction-welding machines — direct-drive machines in particular — can handle tolerances of 0.015 in. by controlling the amount of material upset with the weld. Angular orientation can be held to within 1°. And the technique is axially oriented, so it can do several concentric joints in a single step.
Because friction welding can join dissimilar metals, designers may not have to live with the trade-offs dictated by conventional joining techniques. It lets them join a stronger metal with one that has better conductivity or use a less-expensive material for the bulk of the design and place a metal that wears, resists corrosion, conducts heat or electricity, or bears stress better just where it’s needed.