Holko Feature April 09:Layout 1 3/11/09 10:07 AM Page 53

BRAZING & SOLDERING TODAY Induction Soldering Gets Maglev Vehicle on Track

All design and operational requirements were met during the construction of more than 700 ft of track for an urban magnetic levitation vehicle

BY KENNETH H. HOLKO

unique approach to an urban mag- vantages of these vehicles include pollu- netic levitation vehicle (Refs. 1–3) tion-free operation, adaptability to exist- Awas developed by a multiorganiza- ing roadways, no vehicle/track friction, and tional team led by General Atomics. Pow- good acceleration and ride characteristics. erful NdFeB magnets arranged in a “Hal- bach” array were coupled with more than 700 ft of Litz wire ladder track to produce Tract Construction Details levitation and control. The conceptual ve- hicle, demonstration vehicle, and details A finished Litz bar track is shown in Magnets of track and magnet relationship are Fig. 2. The individual Litz track bars con- Track shown in Fig. 1. The Federal Transit Ad- sisted of ~144 insulated oxygen-free high- ministration sponsored the Urban Maglev conductivity (OFHC) copper wires en- Fig. 1 — Conceptual and demonstration program and the Litz ladder track was fab- cased in 1-in.-square Type 304/304L tubes. vehicles and track/magnet relationship. ricated for General Atomics, San Diego, The wires are originally installed in over- Calif. sized round tubes and then both are ex- The vehicle is propelled by a fixed lin- truded to the square shape. The wires ear synchronous motor reacting with per- have a spiral contour within the tubing manent propulsion magnets attached to that is important for proper magnetic per- the vehicle — Fig. 1. Two other sets of formance. It was also important to main- NdFeB magnets with poles aligned in a tain the integrity of insulation on each Litz specific pattern provide levitation. As wire due to the skin effect present at the these magnets pass over the fixed Litz lad- high operational frequencies. Shorting der track, opposing magnetic field loops from wire to wire had to be avoided. De- are generated. This sequential interaction tails of the Litz bar construction are seen or repulsion is responsible for levitation in Fig. 3. and path control. Successful operation of The OFHC copper extrusions had to the vehicle depends on copper wires inside be designed and purchased. This was a Fig. 2 — Litz bars soldered transverse to the the Litz ladder bars being metallurgically critical part of the project since the ends of pair of copper extrusions for the finished bonded to the copper extrusions shown in the Litz bars had to closely nest in the ex- track. Fig. 1. trusions so that the ends of the wires could The track also has a structural func- be soldered. Since 201 bars had to be sol- tion. As seen in Fig. 1, the Litz track is can- dered into both extrusions, some toler- tilevered off a fixed guideway foundation ance for out of straightness and twist had weldment. This means the track must be to be considered. At the same time, clear- able to support the weight of the vehicle ance of the stainless steel tube with the ex- through magnetic force levitation trusion “trough” could not be excessive or reaction. joint strength would suffer. One impor- Although this vehicle has a maximum tant extrusion detail was to provide un- speed of about 100 mph and normal oper- dercuts in the interior corners so interfer- ational speed of about 35 mph, magnetic ence with the stainless steel bars was Fig. 3 — Litz wire (top) removed from bar levitation vehicles in operation today have avoided. An extrusion and joint cross sec- and stainless steel bar enclosure (bottom). attained speeds in excess of 300 mph. Ad- tion are shown in Fig. 4.

KENNETH H. HOLKO, P.E., (kholko1@ san.rr.com) is with Holko Consulting, San Diego, Calif.

Based on a paper to be presented at the International Brazing & Soldering Conference, Orlando, Fla., April 26–29, 2009.

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AB

Fig. 4 — Litz bar, solder joint, and copper extrusion cross section. C D

Fig. 6 — A — Preheat; B — fluxing, solder; C — pot immersion; and D — final tinned ap- pearance.

Fig. 5 — Low (top) and higher (bottom) Fig. 7 — Untwisting the Litz bars. Fig. 8 — Fixture used to set up Litz bars and magnification cross-sectional views extrusions. through the Litz wire inside bar.

A cross section through the Litz wires sections fabricated had unique, nonlinear Development and bars is shown in Fig. 5 to illustrate the designs. These various track curvatures tight spacing and insulation coating. were necessary so the final test track could It was essential for the ends of each include a 165-ft radius turn. Total test tract After the copper extrusions were re- wire to be soldered to the copper extru- length was about 360 ft. Also, bar-to-bar ceived they were solvent cleaned and sions, so the magnetic loops necessary for spacing had to be held to 0.04 in. to ensure lightly etched to remove surface oxide. levitation could be developed. The low optimum vehicle performance. The Litz bars were also solvent cleaned. and repeatable interface resistance was Each joint in each track was tested by One-ft-long qualification samples were necessary for adequate lifting force to be applying high amperage and measuring soldered and provided to the customer for generated and to provide a smooth ride. resistance. Each joint was required to have electrical testing. Initial samples did not less than 40-μΩ electrical resistance. pass the resistance test. It was learned that Since measurement for contour and re- a pretinning step the customer suggested Track Requirements sistance was done at the customer’s facility, for the bar ends and the method of solder each completed track section had to be application were critical to successful sol- Track requirements were stringent in shipped on a custom strongback to avoid dering. This was learned through metal- that 17-ft track sections were required to damage. Since each track section weighed lurgical sectioning and examination. The be flat within 0.08 in. and within drawing in excess of 1500 lb, this also enabled safe problem was largely related to removal of contour to 0.04 in. About half of the 42 handling. the electrical insulation around each Litz

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Fig. 9 — Vellum used to fit tubes and extru- Fig. 12 — Coil providing heat for soldering. sions.

Fig. 13 — Dam used to control solder flow.

bar preparation for fitting later in the ex- trusions. About half the bars contained an unacceptable amount of twist and/or bend. Therefore, each bar had to be un- twisted or straightened in a hydraulic press. Care had to be taken not to over- Fig. 11 — Induction soldering equipment. twist or overstraighten the bars. This op- eration was done both before and during fitting into the extrusions. The untwisting Time in the solder pot was also critical as operation is shown in Fig. 7. Fig. 10 — Moving assembly into position for too much or too little solder could be After the bars were prepared they were induction soldering. drawn up between the wire ends. Figure 6 set up on an aluminum fixture with proper shows the preheat, flux, solder pot immer- spacing controlled by stainless steel shims. wire at the solder joint. sion steps, and final appearance after wip- As seen in Fig. 8, the aluminum fixture was So, the first challenge was to develop a ing off excess solder. built in two halves to clamp and hold the repeatable tinning technique for both Harris Stay Clean liquid flux (Fed. Litz bars. ends of the 8400 Litz bars that needed to Spec. OF506, Type 1, Form B) was used to After light clamping to hold position, the be soldered. The solder material the cus- facilitate wetting. The small holes near the copper extrusions were sequentially drawn tomer selected was 95 wt-% Sn–5 wt-% ends of the Litz tubes are vent tubes to into tight contact with bar ends through the Ag. This alloy was used for tinning and allow insulation vapor to escape. Reliable use of clamps as shown in Fig. 8. fabrication. wetting could not be achieved without During assembly and before soldering A necessary step was to preheat the these vents. It was also critical to remove could begin, the position of each tube and bars in an air furnace before fluxing and excess solder from the tube ends while the extrusion location and profile had to be tinning in a solder pot. The preheat and solder was still molten. A combination of checked against each unique drawing. Since solder pot temperatures were critical. (See snapping each tube and wiping with a wet a coordinate measuring machine was not the boxed item to review the actual pa- rag was used. Both tube ends had to be available, a number of full-size vellums were rameters and procedures for soldering the kept above the solder liquidus for this to created from the CAD files for the track track.) The function of these steps was to work. Excess solder on even one tube end segments. By laying the vellums on the as- burn off the insulation, but only from the would prevent proper fit, later, in the cop- sembled tubes and extrusions, position ends of the wires, and simultaneously wet per extrusion. could be corrected and fit into tolerance. the copper wires and stainless steel tubing. Another challenge remained in Litz Figure 9 shows the use of vellum.

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Induction Soldering Procedure

The final procedure developed and used on most of the 12. Starting with coil stationary and centered about 3 in. from maglev tracts was as follows. one end, squirt Harris Stay Clean liquid flux into joint, and begin 1. Check Litz bars for straightness and twist. Straighten bars heating at ~50% power setting. as necessary. 13. When melting temperature is reached, begin pushing the 1 2. Degrease Litz bars and copper extrusions with acetone. ⁄8-in.-diameter 95 wt-% Sn–5 wt-% Ag solder rod into joint. Note: Lightly etch copper extrusions in Citrinox at RT. Rinse in DI Best results were achieved with an operator on both sides of the water and dry. track. 3. Preheat Litz bars in air at 275°–300°F metal temperature for 14. When molten solder reaches dam, start coil travel away 30 min minimum. from dam while continuing to feed solder into joint. 1 4. Fill solder pot with 95 wt-% Sn–5 wt-% Ag solder and heat 15. Maintain ~ ⁄4-in. coil to extrusion gap, feed solder rod, add to 650°–700°F. flux as necessary for good wetting, and adjust power for control 5. Remove bars from preheat furnace and briefly immerse of melting and solidification in front and behind coil position. A ends in Harris Stay-Clean liquid flux. uniform solder fillet should form behind the coil position as the 6. Immediately tin ends of Litz bars by immersing in solder pot solder solidifies. to a depth of 1 in. for a minimum time of 3 min and a maximum 16. Turn off induction power and stop travel ~ 3 in. from end time of 6 min. of extrusion. Continue to add flux and feed solder until joint is 7. Remove from pot and use a combination of snapping bar filled and solidified. and wiping with a wet rag to remove excess solder. 17. Inspect entire length of extrusion, both sides, for joint fill 8. Inspect Litz wire ends for untinned areas and re-tin if and uniform fillet formation. Go back and repair questionable lo- necessary. cations while track is still in fixture. 9. Install bars in extrusions, apply fixtures and clamps. Lo- 18. Rotate track and solder other side in same manner. cated bars per applicable customer drawing using tape, precision 19. After both sides are finished and inspected, remove track. scale, and vellum. Install solder dams at both ends of extrusion. Check with appropriate vellum and correct/resolve any discrep- 10. Rotate track into position for soldering. ancies with customer representative. 11. Setup traveling Tocco 25-kW induction unit, custom two- 20. Use solvent to remove excess flux. Use sanding discs with 1 turn saddle coil, and track. Check coil spacing (~ ⁄4 in.) and travel fine grit to touch up areas with excess solder. along extrusion bar. Set travel speed for 3 in./min. 21. Strap to strongback and prepare shipping documents.

Induction Soldering power input to the joint, localization of the Tracks heating, and travel speed. If heating was too spread out over the extrusion, solder would run away from the joint and fill and Once the bars and extrusions were set solidification control would be lost. This and clamped into correct position, the was particularly a problem at the begin- same setup fixtures were used to hold and ning and end of each track where physical manipulate the tracks during induction “dams” had to be used as seen in Fig. 13. soldering. Rotating the fixtured assembly Several coil designs were tried and into position for soldering is shown in used for production. Best results were ob- Fig. 10. tained with the two-turn “saddle” coil de- Induction soldering was performed sign shown in Fig. 11. To obtain a tight coil with a 25-kW Tocco variable-frequency in- geometry for heat concentration and pro- duction unit, remote station, and custom vide for water cooling, ¼-in.-square water-cooled coil as seen in Fig. 11. OFHC copper tubing with a round hole As seen in Fig. 11, the power supply, re- was used. The coil was made from cut and mote station, and coil were mounted on an mitered segments silver brazed together adjustable stand and track so the coil to avoid bend problems. Insulation tape could travel under the extrusion/Litz bars was used to minimize coil shorting to the to provide heating. The coil in position extrusions. during soldering is shown in Fig. 12. When proper balance was achieved, in- Solder was manually introduced to the duction soldering could be done at 3 1 joint from ⁄8-in. straight lengths. The joint in./min without overdriving the coil or was fed simultaneously from both sides. power supply. The induction soldering op- Liquid flux was also added to the joint dur- eration went smoothly after the lessons ing soldering to aid wetting. Shims were described here were learned. Occasional left in place (top of Fig. 12) during solder- low fill areas or small fillets could be re- ing to hold the required bar-to-bar gap. paired by reheating with the induction coil Controlling the soldering operation Fig. 14 — Metallurgical section through in- and adding more solder. Cleanup of ex- was a careful balance between coil design, duction soldered joint, unetched. cess solder, as seen in Fig. 13, was eventu-

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Fig. 18 — World’s first cargo move using a Fig. 16 — Full-size vellum used to check fin- magnetically levitated vehicle in 2006. ished track dimensions. Thermal Processing, San Diego. Thanks are due to Charles Gee, then general man- ager, for his participation. Also, thanks to Chuck Ball of GCE Industries (now Don- casters) for preparing the vellums and Frauke Houge for the fine metallographic work shown here. Technical support from Dr. Sam Gurol and others at General Atomics is also appreciated.

References

1. Gurol, H., Baldi, R., and Bever, D. Fig. 15 — Metallurgical sections at wire to Fig. 17 — Finished track prepared for ship- 2004. Status of the General Atomics low solder (top) and stainless steel tube to sol- ment. speed urban maglev technology develop- der (bottom) interfaces, etched. ment program. 18th International Confer- ence on Magnetically Levitated Systems and fabrication experience as proven by the ally minimized and good cosmetic appear- Linear Drives. Lausanne, Switzerland. tracks meeting all design and operational 2. Gurol, H., and Baldi, R. 2002. ance was achieved as seen in Fig. 2. No re- requirements. The track was installed in jects for high resistance were reported for Overview of the General Atomics low 2004 and is still being operated today. In speed urban maglev technology develop- the 42 segments fabricated. fact, new interest in this system has devel- Metallography was used to determine ment program. 17th International Confer- oped for moving cargo containers at port ence on Magnetically Levitated Systems and interior quality by sectioning small sam- locations as seen in Fig. 18.♦ ples during the program. Examples of sec- Linear Drives. Lausanne, Switzerland. tions through the Litz bar to copper ex- Sept. 4–8. 3. Gurol, H., Baldi, R., Jeter, P., Kim, trusion joints are shown in Figs. 14 and 15. Acknowledgments Good fill, fillet, and joint formation I., and Borowy, B. 2006. General Atomics Maglev: Moving Towards Demonstration. were observed. Occasional oxide-type in- This work was conducted at Bodycote clusions can be seen that are probably a re- General Atomics, San Diego, Calif. sult of residual wire insulation. However, overall quality is good. The vellum approach previously de- scribed was used for final inspection as DO YOUR OWN TESTING seen in Fig. 16. On rare occasions, the track was reinstalled in the fixture, re- heated, and a dimension restored, such as Bend Testers - Bend Specimen Cutting bar-to-bar spacing. Fixtures - Coupons -Tensile Testers Track flatness was checked on the as- BT1B sembly table shown in Figs. 8 and 10, TT1 which had been ground flat prior to the be- Visit our website ginning of this program. Conventional for all sizes and height and feeler gauges were used. models available Preparation for shipment is shown in Fig. 17. Conclusions BSC-1PLT BT1C www.fischerengr.com ! (937)754-1750 This was a very successful program and For info go to www.aws.org/ad-index JUNE, 03 WELDING JOURNALWELDING JOURNAL 57