Resistance Welding Guidance and Tips for Good Welds

April 24, 2020


welding helmetResistance welding in its simplest form: As two sheets of mild steel are pressed under a controlled force by copper-alloy electrodes, a controlled and timed electric current passes through the two sheets. As the highest circuit resistance occurs at the sheets’ point of contact, this is where the weld current generates heat and causes a weld nugget to grow. After the current stops, with the sheets held under pressure, the molten material from the two pieces flow together and solidify to form a weld. The term resistance welding owes to the fact that the electrical property of resistance of the metal being welded causes heat generation as current flows through it.

Current, Time and Force—The Three Keys

Important factors in proper formation of the molten area between the pieces of metal include the magnitude of current, the length of time that it flows and the force squeezing the parts together.

The optimum values of these parameters vary with the types of metal workpieces and their thicknesses. For commonly used 1⁄16-in.-thick low-carbon steel, typical current is 10,000 amps for 0.25 sec., with 600 lb. of electrode force. Resistance welding schedules are available through the American Welding Society-Resistance Welder Manufacturers Alliance and most welding-machine manufacturers.

Obtaining such high electrical current requires the use of a purpose-built resistance welding transformer, which transforms high factory voltage to low (safer) welding voltage. As it transforms voltage, it simultaneously transforms low factory current to high welding currents.

As for process weld currents and weld times, a resistance welding control is responsible for turning the current on and off and forms a critical part of the system. A relay or hand-operated switch might be considered, but is unsuitable given the relatively slow speed of such operation. In the preceding example, the current must be on for only 0.25 sec., difficult to accomplish in such a short amount of time, let alone to perform consistently. Internally in the weld control, the power device used to switch current on is known as the silicon controlled rectifier (SCR). It operates by virtue of the fact that a small electrical signal applied to the device allows it to turn on in a small fraction of a second and conduct a large amount of current. Removing the electrical signal allows the device to turn off again. Fast turn-on and turn-off are possible due to no mechanical moving parts. SCRs operate on solid-state semi-conductor principles similar to those that govern transistors. Popular technology currently in use, highly efficient (mid-frequency DC (MFDC) inverters, use balanced three phase power and improve the process with many advantages in control of current, millisecond time resolution and the ability to produce high power systems.

The third critical factor in resistance welding, the force squeezing the metal parts together, or electrode force, is necessary to ensure good electrical contact between the parts being welded, and to hold the parts steady until the molten metal forming the welded joint has time to solidify. Depending on the size and type of welding machine, various methods of developing the electrode force are used, but the most common employs compressed air in a cylinder-and-piston arrangement. The cylinder attaches rigidly to the welding-machine frame, with the movable piston connected to the upper electrode. Compressed air introduced into the cylinder develops a force on the piston which, in turn, pushes the electrode down against the sheetmetal being welded. The amount of force applied depends on the area of the piston and the pressure of the compressed air. In the preceding example requiring 600 lb. of force, a 5-in.-dia. piston would require air pressure of 30 psi.

The welding control brings all of this together, coordinating the application of welding current with the mechanical motion of the welding machine. More specifically, it tells the electrodes when to close and open, and tells the welding current when to start and stop.

As the welding control provides control of both machine motion and welding current, it must produce two control signals: one to turn an electrically operated air valve on and off (for machine control ), and one to turn the SCRs on and off (for current control).

Tips for Success

  • Insufficient squeeze time can result in metal expulsion, burned electrodes, bad welds, marked work, and damaged ignitron tubes or SCRs.
  • Excessive weld time will shorten electrode life, cause excessive indentation, and cause internal cracks that can result in weld failures.
  • Insufficient weld time will result in low weld strength, assuming that all other factors are normal.
  • Insufficient hold time can result in surface expulsion, electrodes sticking, internal cracks in the weld nugget and, sometimes, cracks in parent metal. Follow the tables for minimum hold time.
  • Insufficient weld pressure can result in expulsion of metal, electrode damage (sticking), short electrode life, internal cracks in the weld nugget and, sometimes, excessive indentation.
  • Excessive weld pressure can result in low or variable weld strength, excessive weld-current requirements, mushrooming of electrodes, and excessive indentation.

With all other settings correct, adjust weld current to meet weld-quality standards.

  • An insufficient electrode-contact face will result in too small of a weld, excessive electrode mushrooming and excessive indentation. 
  • An excessive electrode-contact area will result in a weak weld (or too large of a weld, assuming that current is set accordingly) and internal cracks.
  • Misaligned or mismatched electrodes will result in expulsion, a displaced weld nugget and accelerated electrode wear.
  • Insufficient cooling will result in mushroomed electrodes and short life, cracks on the surface, and excessive indentation. Importantly, the water must flow in through the water quill and back out the outside of the quill. Also, the water quill must be bottomed gently against the inside of the electrode cavity each time that an electrode is replaced.
  • Dirt sticking on the material surface will shorten electrode life, and mark and burn the work surface.
  • Excessive electrode approach speed will accelerate electrode wear and damage equipment. During projection welding, it can damage the projection, resulting in poor weld quality.
  • Don’t perform a weld over the same spot twice to try to cover up for a bad weld. To do so effectively, the work must cool and then be hit with a much higher current. The inability to obtain a good weld in one hit indicates an incorrect machine setup or use of an insufficiently sized machine for the required weld. MF

Information provided by Entron Controls, LLC, Greer, SC; 864/416-0190; www.entroncontrols.com/resources/resistance-welding-knowledge.html.

Industry-Related Terms: Electrodes, Form, Forming, Nugget, Point, Projection Welding, Surface
View Glossary of Metalforming Terms

 

See also: Entron Controls LLC

Technologies: Welding and Joining

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