Background

LLNL has been involved in the investigation of friction stir welding as a means of joining high-temperature radiation-resistant oxide dispersion strengthened (ODS) steels and other alloys for fusion energy applications since 2008. LLNL pioneered the use of photonically enhanced welds, through the introduction of laser peening technology for the closure of welds on spent nuclear fuel containers for Yucca Mountain in the mid-to-late 1990s through the early 2000's, LLNL has continued to investigate the use of photonically assisted joining processes.

In conventional friction stir welding (FSW) and friction stir processing (FSP), a rotating cylindrical tool is first plunged into the metal to be processed or welded, and then advanced horizontally along the surface of the metal. In the case of FSW, the rotating tool is advanced along a butt joint between two pieces of metal, the rotating motion of the work piece literally stirs metal from both sides of the joint together thereby producing a solid state joint between the two. In the case of FSP, the stirring causes the refinement of grains in the material, eliminates porosity and voids in castings thereby producing a material with superior material properties. In both processes, friction heat is generated between the tool and welded materials which softens the material enabling flow but without reaching the melting point.

Description

LLNL's invention uses energy efficient diode arrays for softening metals and alloys to enable friction stir process and friction stir welding. The use of intense light from compact, light-weight, and energy-efficient diode arrays to preheat the material being processed to the softening point eliminates defects associated with insufficient weld temperature such as tunnel voids.

The diode light heating enhancement (DLHE) of FSW also enables residual weld stress in the stir zone (SZ), the thermo-mechanically affected zone (TAZ) and the heat affected zone (HAZ) is controlled and minimized. The efficient and precisely controlled heating provided to FSW and FSP by DLHE can ultimately enable engineers to better control the microstructure that evolves in the weld.

The use of DLHE softens the material that the FSP or FSW tool must traverse thereby reducing load and wear which will substantially extend tool life and lower processing cost. Softening also enables welds to be traversed more quickly by the tool thereby increasing the processing rate which will also lower processing costs.

Advantages

The benefits of LLNL's invention which leverages diode technology developed at the Laboratory include:

  • pre-heating and softening of the material being processed greatly reduces the load on the FSP and FSW tool as well as the wear rate and can therefore extend tool life and reduce processing costs;
  • pre-heating and softening of the material also enables the tool to traverse the material more rapidly thereby increasing the rate of processing which also contributes to reduced processing costs;
  • reducing the number of defects associated with non-uniform and insufficient heating by the stir tool such as the formation of tunnel-type voids by controlling the heating of the weld zone;
  • the exit hole remaining after retraction of the stir tool and the defects related to this extraction are well known problems with FSP and FSW that can be mitigated with additional heating of exit point with diode arrays;
  • diode heating will also enable the application of FSP and FSW to a broader range of materials including refractory metals and alloys, cermets, ceramics polymers and plastics through the application of controlled localized softening;
  • the light source can be used for the thermal post processing to anneal the SZ, TMZ, and HAX thereby relating residual tensile stresses and reducing the changes of stress corrosion cracking and corrosion fatigue in these regions.
Potential Applications

Aerospace, nuclear and ship building companies may find LLNL's technology useful for improving and increasing the processing rate for the conventional friction stir welding, welding of new materials, and thermal post processing of welds.

Development Status

LLNL has filed for patent protection for this invention.

Reference Number
25612
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