Deformation-resistance welding simplifies tube joining.
Delphi Energy & Chassis Systems Corp., Kettering, Ohio, originally used gas metal-arc welding (GMAW) to attach UNS 1018 pinch bolts to ASTM A512 tubes. They are used in monotube shocks that provide vehicles with an ultrasmooth ride. Problem is, welding leaktight annular joints between the cast-iron pinch bolt and carbon-steel tube isn't necessarily an easy or economical proposition.
The key to the monotube shock's damping prowess is a magnetorheological (MR) fluid housed inside the tubing. The MR fluid from Lord Corp., Cary, N.C., changes its apparent viscosity when exposed to a magnetic field. This reversible change in viscosity is nearly instantaneous and is infinitely variable, letting mono-tube shocks quickly adjust suspension-dampening properties as terrain changes.
But welding tubes requires tight process controls so there's no chance of fluid leaks. Otherwise, GMAW can easily burn holes in the shock's thin-walled monotube. And it's often tricky for technicians to accurately position the parts before welding. Fixturing can also be problematic for cylinders and other round parts because they often rotate during welding.
To solve these woes, the Energy & Chassis group has pulled an ace from Delphi's corporate sleeve. A solid-state welding process developed by Anthony Ananthanarayanan, a Delphi technical-welding fellow and his engineering partner Desi Herbst, recently emerged from the lab and is being commercialized at SpaceForm Inc., a recent Delphi Technology Inc., spin-off. The Michigan-based startup is transitioning the Delphi Deformation Resistance Welding (DRW) process into applications well beyond the fuel-system components originally envisioned by its inventors.
In addition to the monotube shocks, other potential automotive applications include under-body, roof frame, and rocker-panel assemblies. Compared to stamped metal, tubular structures made with DRW result in lighter, more fuel-efficient cars that are stronger and safer. Race-car designers use similar construction to boost performance and protect drivers. SpaceForm researchers are also looking to use DRW for trailer hitches, suspension links, and exhaust systems. They have recently teamed with NASA to investigate DRW for use in extraterrestrial-manufacturing cells. The process might be used to build mobile suspension subframes in orbit or on the moon and Mars, if in-space power requirements are small.
THE DRW PROCESS
DRW uses conventional resist-ance-welding equipment. Resistance welders use suitably shaped electrodes to concentrate a localized mechanical force and current to join sheet-metal assemblies. One of the important functions of the electrodes in conventional resistance welding is to prevent sliding of parts along the weld interface during welding.
The process typically melts the metals being welded locally to create the weld joint. At point of contact, the electrodes heat a small, localized area to the melting point of the metal. This creates a "nugget" of welded metal after current shuts off. Conventional resist-ance-welded joints are of questionable quality if welded entirely in the solid state, says Delphi's Ananthanarayanan, and consequently cannot effectively join dissimilar material combinations such as cast iron to steel.
DRW, in contrast, uses electrodes that promote relative sliding of parts being welded along the weld interface, creating an effective solid-state or melted and solidified weld joint. This makes it ideal for joining tubes to any type of part, says Ananthanarayanan. Tube cross sections including rectangular, teardrop, hexagonal, square, and oval can be welded to solids, flat or channel sheet-metal stampings, and tubes of varying diameters and angles.
DRW applies mechanical force to the parts while the joint around the tube sees pulses of high current (up to 300 kA) that pass through the special and sometimes annular electrodes. The electrodes produce uniform, leak-tight welds around the entire tube circumference with no need to rotate the assembly, says Ananthanarayanan. "Joints have tested stronger than the parent metals and are strong enough to hold fluids and gases under pressure and heat."
DRW can join similar or dis-similar metals including mild steels, 300 and 400 Series stainless, nickel-based alloys, and cast-iron-to-carbon steel as in the monotube shock. Space-Form is also looking at additional steel-to-steel, aluminumto-steel, and other metal combinations for NASA.
One key feature is that DRW uses conventional midfrequency dc-resistance-welding systems with transformer packs that run at 1,000 to 1,200-Hz frequencies while welding tubes of large diameter that require large weld currents, says Ananthanarayanan. "This lets manufacturers use existing welding buses. The welder draws power from all three phases of the bus, and transformer combination packs optimize the ratio of secondary-weld current to primary-bus current. For welding tubes of smaller diameter, conventional equipment operating on line frequency is also satisfactory."
Prior to DRW, tubes must run through preforming or upsetting processes that put small folds and flanges near their ends. The folds compress against the tube when they are put into the welder and current is applied through them. As the current heats the fold to near melting, solid-state deformation at the interface creates the weld.
Part fit-up need not be precise and cycle times are shorter than conventional arc or laser welding techniques for manufacturing tubular components, says Ananthanarayanan. "There is also no need for filler materials, but the use of filler metals can be engineered when appropriate."