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A D V A N C E D M A T E R I A L S & P R O C E S S E S | J A N U A R Y 2 0 1 6 2 6 L inear friction welding (LFW) is a solid-state process that uses friction and plastic deformation to gener- LINEAR FRICTIONWELDING UPDATE: LOWERCOSTS, BROADERAPPLICATIONS From joining railroad rails to producing strong aluminum-to-steel joints, recent advancements in linear friction welding are reducing equipment costs and expanding potential uses. Michael Eff, Jerry Gould, and Tim Stotler EWI, Columbus, Ohio

As the material is heating, it is extruded away from the joint and a new surface, called a nascent surface, is formed. By stopping oscillation and forging once the nascent surface is formed, a weld is made between the two pieces. A sche- matic of this process is shown in Fig. 1. Key variables for LFW include the axial load along with oscillation fre- quency, amplitude, and duration. LFW machines must achieve the desired relative velocities between two parts, apply axial loads, and precisely stop oscillation to align parts after weld- ing. In order to maintain high relative velocities under an axial load, shear loads during welding can become large. Therefore, designing an oscilla- tor that can withstand the shear loads opposing oscillation is one of the most critical—and costly—factors of an LFW machine. Current LFW systems are most- ly hydraulic actuation systems, which store energy under high fluid pressure that is first directed to one side of a drive cylinder and then to the other side to generate oscillation. High speed valves with large flow rates, many paral- lel circuits, and hydraulic accumulators are required for hydraulic control and must change flow direction in 1/60th of a second to achieve a 60-Hz oscilla- tion. Hydraulic servo valves operating at speeds up to four times faster than typical industrial servo valves provide amplitude control [1] .

ate heat. A metallurgical bond between two pieces of material is achieved via relative motion (i.e., friction) of mate- rials under applied force. Solid-state welding processes join without melting materials and are in high demand due to their superior weld quality, ability to join non-fusion-weldablematerials, and over- all lower peak temperatures than fusion welding processes. LFW is closely related to rotary fric- tion welding, which uses relative angu- lar motion under force to generate heat. However, LFW uses translational motion rather than rotational motion and is thus able to join noncircular cross-sections as well. Despite its advantages, industrial applications of LFWhave been limited to high value-added components such as jet engine components due to prohibi- tive equipment costs. Recently, new advancements in os- cillator technology have reduced equip- ment costs and expanded LFW’s com- mercial viability into applications ranging from producing aluminum-to-steel joints to the joining of railroad rails. OSCILLATOR TECHNOLOGY ADVANCEMENTS LFW achieves friction heating and plastic deformation at the interface be- tween the two components tobe joined.

Fig. 1 — Mechanical LFW schematic.

These machines require a signif- icant investment in both capital and floor space and are also complex to operate and maintain. Due to their size and complexity, hydraulic LFW machines have been relegated to pro- ducing only the highest value parts for the most demanding applications. A primary application for these sys- tems is welding blades to disks for jet engines [2] . One specialized equipment builder, APCI LLC, South Bend, Ind., recently developed a unique me- chanically based oscillator for LFW. Instead of the complex hydraulic sys- tems used to oscillate a part, a motor drive with a continuously variable stroke crank [3] performs this task. Mo- tor rotation drives the crank, which translates rotary motion into linear