Resistance welding was once considered only for joining sheet metals. No more.
Welded thin-wall tubes form a variety of sturdy, lightweight structures. But joining the parts has its challenges. Conventional laser, MIG, and TIG welders can burn holes in tubes before making proper welds unless a process is precisely controlled. Round parts tend to be difficult to fixture and often need rotating as they are welded. And they require careful fitting-up beforehand, a costly and time-consuming job.
But a technique called Annular Deformation Resistance Welding (ADRW) makes uniform-strength, leaktight welds in tubes faster than possible with traditional methods. Resistance (spot) welding has been used for years to rapidly join sheet metals such as automobile body panels. Here, a pair of blunt-ended electrodes apply electric current and mechanical force to a joint. Current passing through electrical resistance at the part interface locally heats the area to form a weld, without filler material.
ADRW uses the same power supplies as spot welders but replaces the blunt-ended electrodes with annular-shaped electrodes made specially for the job. The electrodes also apply electric current and mechanical force, the amounts of which depend on part size and geometry. Joining two 2.5-in.-diameter, 0.060-in. wall steel tubes takes about 100 to 120 kA of weld current and 4,000 to 5,000 lb of force, for example.
The annular electrodes let parts slide axially during the welding process to ensure a solid-state bond -- melting and solidifying (fusing) of material at joints is not necessary as with conventional spot welding. This is an important distinction because solid-state welding can join dissimilar metals, a big advantage of ADRW.
Welding cycle time is independent of tube diameter because an entire weld surface is done at once. For example, joining two 5-in.-diameter tubes with an automatic MIG setup may take 30 sec for welding and about 10 sec to load and unload. The ADRW process in this case also takes 10 sec for loading and unloading but welding consumes less than one second.
Design for ADRW
Key to quality welds is proper joint design. Consider the joining of two tubes, one with a slightly larger diameter than the other, inserted lengthwise. A flange formed on one end of the larger-diameter tube mates with a fold on the smaller-diameter tube. Folds are made by an upsetting process and locate slightly in from a tube end.
The process works for tubes up to 0.25-in. wall thickness and is limited by the ability to form flanges and folds. However, thick-thin tube welding is one of ADRW's greatest strengths. Thick tubes only require a flat face to which a folded, thinner tube welds.
Flange and fold dimensions need not be precise because low-current pulses heat parts so they conform to one another before current ramps to welding levels. Deforming parts at the weld interface also makes the process less sensitive to weld current. ADRW can vary weld current nearly 35% and still produce quality welds where spot welding has a much narrower 15% range between and high and low limits.
ADRW can join metal tubes to solids, sheet-metal stampings, and to other tubes of different diameters and cross sections and at various angles. Tubes can also be welded to flat or channel stampings and contoured sheets or to solid parts such as nuts. The system scales to fit tube diameters from 0.25 to 6 in., and larger diameters may be possible. ADRW works on a variety of metals including mild steels, 300 and 400 Series stainless steels, and nickel-based alloys.
Whatever the part geometry and material, parts must be free of nonconductive coatings and die lubricants that do not melt or dissolve in the base metals. Otherwise weld integrity may suffer.
Delphi Technologies Inc., (248) 813-8061, www.delphi.com
Tube construction reduces automobile weight, complexity
Fabricated-tube structures can replace heavy stampings and related (expensive) tooling for a variety of vehicle structural components. The use of thin-wall tubular steel helps cut both vehicle weight and parts count, without compromising structural integrity. Rectangular and round tubes of different metal compositions and wall thicknesses are readily available in long sections. NC or custom tube-bending machines and forming operations produce the finished shapes.
|TORSION TESTS OF TUBE-SHEET ASSEMBLIES|
|Extrusion||Angle at onset |
of linear region (°)
|Angle at end of |
linear region (°)
|5-mm gage length|
|50-mm gage length (same setup for fatigue testing)|
|CR = Cold rolled 1008 steel||Tension and torsion tests conducted at Delphi show welds made by ADRW are stronger than the base metal itself, the test criteria. Welds have also passed high-temperature corrosion fatigue tests, though results are proprietary.|
|HS = HSLA steel|
|Tubes for all tests are 9.5mm OD, 1.5-mm wall thickness|
|TORSION TESTS OF TUBE-SHEET ASSEMBLIES|
|107||CR||1.5||yes||8,393||6.9||tube broke at weld,little yield|
|CR = Cold rolled 1008 steel, HS = HSLA steel, Tubes for all tests are 9.5mm OD, 1.5-mm wall thickness|
|TENSILE TESTS OF NUT-TUBE ASSEMBLIES|
|Part ID||Yield torque (N-m)||Max torque (N-m)||Failure mode|
Cold rolled 1008 steel tube welded to sulfurized machinable steel nuts.
Maximum torque spec 15.5 N-m
A 90 N-m electric torque gun at 150 rpm to 5 N-m was quickstepped to 100 rpm until failure. Torques were measured with a 100 lb-ft external torque/angle transducer.