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New Tool for Joining Ti-6Al-4V by Friction Stir Welding

Development and assessment of a new single-use tool for joining Ti-6Al-4V by friction stir welding

TWI Core Research Project 1140/2020

 

Overview

Friction stir welding (FSW) is now a well-established process for joining light alloys. The relatively low temperature (~500°C) required to soften and plasticise aluminium alloys allows the use of steel based FSW tools, a material which is readily available, low-cost and has a good welding life.

However, tool materials and designs for welding higher temperature alloys such as titanium present a significant challenge, because not only must they be able to withstand high welding forces and torque levels, they must also operate at high temperatures of typically 1000-1200°C. Refractory metals, such as tungsten and molybdenum, are the current preferred materials, but there is only very limited use reported by industry. These have historically provided a FSW tool with properties capable of making welds, however, the cost of the material, producing the tool features and the weld life put a very large burden on the commercial viability of FSW in all but very high value applications. Therefore, a lower cost tooling system could make FSW more financially accessible to a wide range of industries.

Objective

To develop and assess a low-cost, single-use, tooling system based on silicon nitride for welding high strength titanium alloys, particularly focusing on Ti-6Al-4V.

Approach

Following discussions with ceramic suppliers, it was identified that the significant cost elements in ceramic FSW tools were the bulk volume of ceramic and the finish grinding of the tool profile post firing.  To reduce bulk volume, a ceramic insert mounted in a reusable metallic holder was considered the best option.  To eliminate the final grinding operation, the insert would be machined in the green state and then fired.  The insert geometry adopted incorporated a tapered body for accurate location and flats for providing a drive feature.  The insert had a 3mm probe length.  A tool holder was designed to support and transmit the torque to the insert, as shown in Figure 1.

A weld procedure was developed for welding Ti-6Al-4V; ceramic inserts from two different suppliers were assessed making bead on plate (BoP) welds. The assessment consisted of visually inspecting the inserts then measuring their geometry using an Alicona Infinite Focus, non-contact profilometer. A two metre long BoP weld was then made, sectioned, and assessed visually by macrograph inspection and transverse tensile testing. Finally, the insert geometry was again inspected using an Alicona Infinite Focus non-contact profilometer. The pre- and post geometry inspection enabled an assessment of the amount of insert material lost during welding.

Figure 1. Model of insert and tooling system
Figure 1. Model of insert and tooling system

Results

The insert-based tooling system performed well during welding trials, and the insert holder proved reliable, reusable and provided a good accurate location for rotation. The insert wear observed from the first supplier was consistent; importantly, there was very little material lost from the probe tip, which will maintain the weld stir depth to minimise the chance of producing a weld with lack of penetration. The calculated volume loss of material from pre- and post weld scanning was 71.39 and 72.62mm³ for the two inserts assessed, representing 0.11% by volume of the stirred material volume of the 2m long weld.

The inserts supplied by the second supplier were similar to those of the first supplier, however, the wear was much less severe.  The calculated volume loss of material from the two inserts assessed was 45.48 and 43.95mm³ respectively; an average 38% lower than the first supplier and represents 0.07% by volume of the stirred material.  It should be noted that the welding parameters used to assess the inserts from the two suppliers were slightly different, which may have contributed to the difference in the calculated volume loss.  The most significant wear was again seen in the area where the insert initially contacts the plate in the plunge phase, a little off the probe tip and shoulder areas, with less wear from the curved surface of the probe.  Distribution of ceramic particles throughout the weld was not assessed.  The strength of the welds produced was slightly overmatched to the parent plate and the welds were free from voids.

 

Conclusion

In developing a tooling system capable of making 3mm deep, 2m long bead-on-plate (BoP) welds, in Ti-6Al-4V using single-use ceramic inserts, TWI has demonstrated the feasibility of using a single-use ceramic insert tooling system for welding high strength titanium alloys. With the inserts costing less than €50, this lower cost tooling system could make FSW more financially accessible to industries other than aerospace, where the product would benefit from the high quality welds produced by friction stir, but could not justify the cost of the current FSW tooling systems.

This project, therefore, represents an important step forward in high temperature FSW with results showing feasibility and practicality of approach. Follow on work will enable further investigation of the loss of material in the weld, leading to a new solution that can be adopted by industry.

 

This project was funded by TWI’s Core Research Programme.

Figure 2. A macrograph of the BOP weld taken from the end of the wear assessment plate
Figure 2. A macrograph of the BOP weld taken from the end of the wear assessment plate
Avatar Jonathan Martin Section Manager - Friction & Forge Processes

Jonathan, who is a metallurgist by training and a Fellow of The Welding Institute, joined TWI in 2005 and has more than 20 years’ of experience in friction stir welding (FSW) activities. Over the years, he has gained experience in all aspects of FSW including participating in FSW standard working groups, developing FSW training courses and delivering engineering solutions across the world. Since 2008, he has run the International Symposium on Friction Stir Welding and has published/presented over 40 scientific research reports and papers.

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