Positive improvements have been achieved in two already highly successful friction processes under development at TWI - with the simple addition of a shielding gas. Both friction seam welding (Fig. 1) and friction hydro pillar processing (Fig. 2) have recently come under the spotlight in a project tackled jointly by TWI and Air Products PLC.
During friction seam welding frictional heat is generated between the workpieces to be joined and a rotating consumable bar. It is this bar which presses into the joint line and travels along it depositing material into a previously machined joint preparation. The process exposes plasticised material to the atmosphere through the small joint gap and at the leading edge of the rotating consumable. To date this has made the technique vulnerable to atmospheric contamination (Fig. 3).
Fig.3 - Friction Seam Welding 316L stainless steel weld deposit onto 316L stainless steel substrate
However, by enveloping the plasticised material in a cloud of shielding gas, using a sliding shoe arrangement installed around the welding region, it has been possible to improve the finished weld quality.
Similar improvement have come to light using shielding gases to protect plasticised material exposed during the friction hydro pillar process.
In this process, which is well suited for repair work, fabrication and materials reprocessing, a rotating consumable is pressed hard into the base of a clearance hole in the parent material.
As the consumable softens, plasticised material is hydrostatically squeezed to the side as the rotational plane between consumable and deposit rises. This fills the hole in the parent material and makes a perfect repair.
So far, comparisons between welds made with and without shielding gases have proved firstly, that active gas shielding can minimise atmospheric contamination of the friction seam welding deposit surface, and secondly, that the use of active gas shielding has extended the range of suitable process conditions that can be used in creating high quality FHPP welds.
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