Derrill Holmes looks at the mechanical characteristics of the zirconium-to-titanium fusion weld.

Zirconium can be fusion welded to a number of other reactive and refractory metals including niobium, hafnium, vanadium, tantalum and titanium.

Fusion welding of zirconium to titanium can result in a weld exhibiting varying final properties which are unpredictable and generally non-reproducible. In the case of zirconium to titanium welds the corrosion resistance is generally less that that of either of the parent metals.

In addition, the mechanical properties of the weld are highly variable.

This article presents information on the mechanical characteristics of the zirconium-to-titanium fusion weld. This particular weld combination is not generally suitable for fabrication of equipment for use in the chemical processing industry (CPI).

Improved corrosion resistance

The zirconium to titanium fusion weld appears to be an attractive weld for several reasons. From the standpoint of possible cost savings with respect to titanium and availability, and perceived improved corrosion resistance, from the zirconium, this weld does indeed appear to be beneficial.

This article presents mechanical property data which indicates that fusion welding of zirconium to titanium by commonly used tungsten inert gas (TIG) method is not generally suitable for fabrication of equipment for use in the chemical processing industry (CPI).  

Fig. 1. shows the variation in chemical composition possible in a zirconium to titanium autogeneous weld. The light and dark colouration of the weld indicates variable weld pool chemistry.

Colour changes

An un-melted section of pure titanium is shown in the centre of the weld zone. These photomicrographs also show a range of chemistry, as indicated by colouration changes in this zirconium-titanium weld.

Each of these areas has different corrosion characteristics and mechanical properties.

This inherent variability in the zirconium- titanium weld composition is the basis for the unpredictable nature of this weld combination when standard welding methods are employed.

Here we are concerned with the zirconium-titanium fusion weld in the CPI equipment. For example, heat treatment in air at temperatures above stress relief can cause catastrophic oxidation.

If these welds are to be subjected to higher temperature (eg >570ºC) applications the variable oxidation rate should be carefully considered. The subject of catastrophic oxidation of Zr-Ti alloys is well covered by Nakayama1.  

Mechanical properties

Hardness data presented in Table 1 was taken from an average of three data points for each alloy condition. These alloys were produced as button melts and rolled to 0.060inches (1.5mm) in thickness. The Zr50Ti alloy was obtained from a commercial production lot.

The results of hardness testing for the titanium and zirconium parent materials are as expected, with the hardness being lower for material in the annealed condition (732ºC for 1 hour).

The zirconium-titanium alloys, however, show increased hardness values when the materials are in the annealed conditions. The zirconium-titanium alloys exhibit considerably higher hardness values than either of the parent metals.

Other authors obtained results from alloys containing 50 per cent zirconium indicating that the hardness was approximately 2.5 times higher when compared with pure titanium and pure zirconium. Kobayashi gives data for a range of zirconium–titanium compositions from 20–80 per cent zirconium2. Hardness data for the Zr-Ti weld area and heat affected zone (HAZ) is given in Table 2.

The increase in hardness after welding Zr 702 is typically less than 35 Vickers hardness units. This increase in hardness is due to the increase in oxygen content of the weld.

Oxygen contamination is possible even under the most meticulous welding practices. At least some of this oxygen uptake is from the oxide already formed on the surface of the metal being welded as well as from minor contamination in the welding gases.

The mechanical property data was based on sets of three tensile specimens from each alloy composition. The Zr-Ti alloys were fabricated from button melts into plate approximately 0.060 inches (1.5 mm) in thickness. The 50:50 wt per cent alloy composition was a commercially prepared alloy.

The data presented in Table 3 highlights the variability of the mechanical properties of the Zr-Ti welds.

In the case of ultimate tensile strength and yield strength, the values are considerably higher than the actual values for Zr 702 and the typical values for grade 2 titanium.

These property values are in line with other published data which indicates that the tensile strength of alloys containing 25–75 per cent zirconium increased about 2.3–3.0 times when compared with pure titanium and pure zirconium tensile strength2.

Summary

While it is possible to use standard fusion welding processes to weld titanium to zirconium, and while the resulting weld appears to be adequate visually, but should be used with caution because of the variable mechanical properties.

The purpose here was to highlight the metallurgical concerns of zirconium to titanium welds using traditional methods. Although some applications using these welds are being used successfully, careful consideration should be given in evaluating mechanical characteristics of this weld combination. 

Derrill Holmes is with ATI Wah Chang, based in Albany, Oregon, USA. www.wahchang.com