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Dissimilar friction welding of Titanium to Stainless Steel

 

TWI Industrial Member Report Summary 1040/2013

By Bertrand Flipo

Background

The joining of dissimilar materials is increasing in importance, as there is a drive to use hybrid structures to a greater extent in all industry sectors. Yet it presents specific challenges in terms of the selection of appropriate joining methods. Fusion welding of these dissimilar materials can sometimes be difficult due to intermetallic formation and thermal expansion co-efficient mismatches. Friction welding processes offer a credible joining method for dissimilar material combinations. The process involves relatively low temperatures (from 60 to 80% of the melting temperature) which reduce potential IMC's formation and residual stresses, shrinkage occurrence; those elements being the main issues when joining dissimilar metals.

Industrial Challenge

Intermetallic compounds (IMC) are generally considered detrimental to joint properties. They are usually very brittle and their thermo-mechanical properties are often different; their low thermal coefficient weakens the bond between the IMC and the alloy whereas their high electrical resistivity can lower a joint electrical conductivity. For some dissimilar material combinations the formation of intermetallic compounds is unavoidable during cooling from their liquid state

Key Findings

Joining of titanium directly to stainless steel by friction welding is not immediately suitable for industrial application as welds fail due to brittle fracture with joint tensile strengths below 85% of parent values. A study on single interlayers was carried out.

  • Ferro-titanium (FeTi, Fe2Ti) brittle intermetallics were found at the weld interface of direct Ti64 - 304L welds.
  • Niobium, Aluminium, Vanadium and Zirconium were considered as potential single interlayers.
  • Zirconium interlayers were the most successful, resulting in a maximum ultimate tensile strength of 483MPa was achieved.

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