Flash-welding

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What is Flash-welding?

It is a mature economical process best adapted for butt welding mass production.

It is described as pertaining to resistance welding and forge welding, although sparks (uncontrolled arcs) are produced during its application.

This process consists in direct electrical heating of the ends of the pieces to be welded and then forging them together.

While heating, a thermal distribution characterized by a steep temperature gradient is established along the axial length of the pieces, much steeper than that developed in pure resistance welding.

This enables Flash-welding to process a much greater variety of materials and shapes than can be welded by resistance welding.

Flash welding needs clamping the extremities of metallic shapes to be joined in powerful current carrying fixtures capable of squeezing together, without deforming, those ends at the correct time in the cycle.

In this Site, a page on a similar but different process that should be considered for evaluating and comparing benefits and limitations of each one in any definite specific case can be seen by clicking on Friction Welding.

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Flash-welding: How does it work?

In Flash welding the abutting surfaces are first brought end to end with light contact so that the low voltage potential applied generates the passage of a certain current.

Minute protrusions of the surfaces short the circuit, while sparks (flash) bridge the gap and heat the metal.

This brief period of violent flashing is called burn-off and serves the function of squaring off the surfaces.

Flashing is a most effective heating procedure, rapidly producing surface high heat capable of melting drops of base material locally, where occurring.

Preheating, if necessary, is achieved by withdrawing the moving fixture and repeating the contact in rapid succession, possibly under automatic control, so that premature incomplete flash welding is avoided.

Increasing current produces resistive heating while a steady push brings the ends together.

An automatic trigger permits application of full force as soon as the heat reaches the right forge welding temperature, without substantial melting of material.

First all surface impurities and occasional molten drops are squeezed out of the joint and then in continuation an upsetting action takes place in the plastic zone, with macroscopic deformation and butt welding.

Bonding by Flash Welding occurs during the upsetting action, and some more metal is extruded from the weld zone to dispose of slag and other inclusions.

After removing the weld upset, no visual evidence of Flash-welding should remain but it can be found by metallographic techniques.

If porosity is detected near the outer surface on an etched section of the flash welded joint, it may indicate incomplete bonding because of either insufficient upsetting force or too low temperature and plasticity during upsetting.

Flash Welding Equipment is built for manual, semiautomatic or automatic operation. The force source can be manual, mechanical (through a cam), pneumatic or hydraulic.

Which Materials are joined by Flash Welding?

Flash Welding can be used for joining many ferrous and nonferrous alloys and combinations of dissimilar metals.

Practically Flash-welding can be applied to any metal that can be forged.

In addition to low-carbon steels, Flash-welding metals on a production basis includes low-alloy steels, tool steels, stainless steels, heat resisting alloys, aluminum alloys, magnesium alloys, nickel alloys, and copper alloys.

Cast iron (which cannot be forged) is reported as never having been successfully flash welded in production.

High quality joints can be obtained consistently by Flash welding, especially with automatic equipment that includes feedback controls in real time.

This reliability permits to qualify Flash welding for highly demanding applications as found in aerospace for solid and tubular sections.

Typical Applications:

Parts of aircraft landing gears and many rings of different materials are currently manufactured by Flash-welding (rolled and welded).

Many applications of Flash welding are found in automotive production. Continuous loops of band saws are formed by Flash-welding in a special attachment to the saw itself.

Sheet metal coils are welded together end to end by Flash-welding to provide the continuous supply of certain manufacturing processes.

Window frames and general frames are manufactured by Flash-welding for different industries. With very large equipment, the longitudinal seam of thick pipes can be manufactured by Flash-welding.

Flash welding as a process is quite similar to Upset Welding, the main difference being the measure of Flash being produced.

Advantages:

  • Economical in operation and in the use of metal.
  • Suitable for mass production.
  • Unskilled workforce needed.
  • Strong welds obtained.
  • Good fatigue properties available.
  • No special symmetry requirements (different from for Friction welding).

Limitations:

  • Removal of flash required (same as for Friction welding).
  • Process may affect or remove locally any strain hardening (cold work) properties.
  • Heat treatment after Flash-welding may be needed to develop full properties.
  • Costly maintenance of equipment due to flashing.
  • Fire hazards present.
  • Electric power and upsetting force in available equipment limit the weldable size.
  • Heat unbalance from different materials may cause upsetting problems.
  • Shunt effect for closed rings rolled and flash welded may need attention.
  • High accuracy alignment is required.

As a reference specification the following can be used:
MIL-W-6873
WELDING; FLASH, CARBON AND ALLOY STEEL
Note: The above document is Inactive for new design since 1997.

Useful notes applicable also to Flash-welding can be found in
The Professional Advisor on Resistance Welding
AWS code PARW

and in
ANSI/AWS C1.1M/C1.1:2000 (R2012)
Recommended Practices for Resistance Welding
American Welding Society, 01-Jan-2000
105 pages

An Article on Underwater Flash Butt Welding of Pipes was published (2) in Issue 69 of Practical Welding Letter for May 2009.
Click on PWL#069 to see it.

A special Issue of Practical Welding Letter, the Mid June Bulletin No.74, full of Resources on Resistance Welding was published on June 14, 2012.
Click on PWL#106B to see it.

An Article on Advancements in LFW was published (7) in Issue 150 of Practical Welding Letter for February 2016.
Click on PWL#150.

Watch the following Video of a
Flash Butt Welding Machine.


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Flash-welding is a versatile mass production process capable of high quality, high production butt welding in different materials. Could it be a solution for your welding problems? Read here...