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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

Fig. 2 — 100-ton mechanical LFW system. Courtesy of APCI.

Fig. 3 — As-welded aluminum-to-steel joint.

oscillation—much like a crankshaft and piston/rod assembly. A schematic of the oscillator function is shown in Fig. 1, in which the circle represents the variable stroke crank. Variable frequency is obtained simply by changing the motor speed that drives the crank. The phase change of a second rotating cam that changes the reference location of the driving crank provides the variable amplitude. Schematically, this increases or de- creases the size of the circle in Fig. 1. The oscillation method easily aligns parts at the end of the welding cycle by changing the variable amplitude at the end of the cycle to zero. Application of normal force and fixturing is similar to other linear friction machine designs, applying load perpendicular to the os- cillating interface. A 100-ton mechani- cal LFW system is shown in Fig. 2.

This mechanical oscillation de- sign greatly reduces machine cost and footprint to approximately one-third or less than the size of a comparable hydraulic LFW system. The new system also allows complexity to be added to the weld process, including multiple phases, extending low pressure fric- tional pre-heats of the surface, and changing the frequency and/or ampli- tude in the middle of oscillation. Addi- tionally, the mechanical system allows for amplitudes exceeding 6 mm and 70 Hz of oscillation, thus expanding the available parameters. CASE STUDY The automotive and aerospace industries are both seeking weight re- ductions via new materials with high strength-to-weight ratios and multi-ma- terial designs, commonly known as

lightweighting initiatives. Recent ad- vancements in LFW technology enable it to join aluminum alloys with over 90% efficiency [4] . As an extension of this work, EWI has examined joining aluminum to steel with a mechanical LFW system. Using this system, EWI joined 6061- T6 aluminum to 1018 steel with joint strengths matching that of the 6061-T6 base material. Joined pieces were 12.7 × 12.7-mm square faces with a 161-mm 2 cross-sectional area. Processing fre- quency and amplitudes used to join the pieces surpass traditional LFW capabili- ty, resulting in joints exceeding 280 MPa ultimate tensile strength. A photo of the as-welded joint is shown in Fig. 3 and a cross-section is shown in Fig. 4. Joining dissimilar metals, includ- ing this combination, typically results in the formation of brittle interme- tallic compounds with low strength,

Fig. 4 — Cross-section of aluminum-to-steel joint.

Fig. 5 — Aluminum-to-steel interface under high magnification using SEM.