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Extending Tool Life and the Hybrid Process

 

Extending Tool Life:

High Frequency Induction + Friction Stir Welding

 

 

The Problem

 Most processes existing for friction stir welding steel or high performance steel wear the tool out very quickly. The tool used for FSW is quite costly. Friction stir welding works great for welding aluminum, a relatively soft, friction stir friendly material, but high performance steels are much harder than aluminum. FSW would be an ideal process for joining high performance steels however, because of the hardness and strength of High Performance steel and the high energy heat energy required for plasticization, conventional FSW tools melt and wear out very quickly when used for joining that material.
 

Background

 Friction Stir Welding (FSW) was introduced in 1991 by The Welding Institute as a modified method of friction welding. The novelty was in not needing to rotate the parts. It presented a revolutionary method of joining dissimilar and previously un-weldable materials through solid-state bonding. The advantage of this process is that no melting is involved during welding, which eliminates the problems associated with solidification.

In conventional fusion welding, the work pieces are joined together by the application of heat, melting the materials together at the junction of the joint often with the addition of a filler material. The disadvantage of conventional fusion welding is that the melting of the parent material often brings about microstructure problems associated with re-solidification. Many materials lose strength due to the undesirable microstructure produced by melting and re-solidification. One way FSW differs from conventional fusion welding is that there is no melting, and thus, no microstructure re-solidification issues.

The way FSW works is a tool consisting of a pin and a shoulder is pressed into the junction between the parts to be joined. The tool spins as it moves along the seam of the weld. The tool is sunk into the parent material the depth of the pin; the pin stirs the material together while the shoulder stirs and helps to form and shape the surface of the weld.

Presently, the most common use of FSW is for welding aluminum. Aluminum alloys are vulnerable to re-solidification issues, so friction stir welding is an ideal process to use. Some alloys of aluminum used in aircraft were joinable only by rivets (which produce stress points) until the FSW process came about. The Eclipse 500 aircraft is one such application in which FSW replaces rivets. FSW is widely used in a growing number of aluminum applications from car parts to ship decks.

What We’re Doing 

 LeTourneau is doing several things to increase the FSW tool life while welding high performance steel. First we are using Tungsten Rhenium. This alloy is very hard, has very high wear resistance, and a very high melting point. The tool itself will be much more durable. Secondly, the process we are working on incorporates a cooling system for the FSW tool to reduce tool wear. Thirdly, to reduce wear, we are adding a high frequency induction (HFI) heat source at the beginning of the weld leading the tool to provide some of the energy used to plasticize the steel. This way, we are not wearing out our tool just heating the material to its plasticization temperature. The HFI heating cuts down on the amount of frictional heating required from the tool so the tool only has to stir the plasticized material together. To make the most of the HFI, we have incorporated a gap control system on the FSW machine preceding the weld.

 

 

Funding

FHWA R&D: $220,000 / 2 years

 

 

Partners

 Federal Highway Administration (FHWA)
 
Thermatool     
 
NASA
 
Carderock Division, Naval Surface Warfare Center

 

For another look at Friction Stir Welding and LeTourneau’s work on it, please see our FSW Senior Design webpage.  

 

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