“Microstructure and mechanical characterisation of titanium alloy linear friction welds”

¹ TWI Ltd, Cambridge, United Kingdom
² CAV Advanced Technologies Ltd, Consett, United Kingdom

Abstract

The increasing use of composite materials in new aircraft builds leads to a significant demand for titanium alloy structural parts. The increasing costs and popularity of this material, together with restrictions in supply and processing, are driving the aerospace industry to make increasingly efficient use of available material.

Linear Friction Welding (LFW) is a rapid, high integrity, solid-state forging process that has the potential to decrease the buy-to-fly ratio, production time and time to market of aerostructure components. Its dependability has already been proven for the production of key components in some of the latest generation aero engines.

The LFW process is yet to be used for aerostructures. This is primarily due to the LFW process not being widely known, but also due to a lack of performance data on aerostructures manufactured by LFW being available to the supply chain and design community.

To address this issue, a large series of titanium alloy weldments was produced and assessed via metallographic examinations and mechanical testing in both as-welded and post-weld heat-treated condition. The matrix of experiments was able to capture the LFW process window of this titanium alloy, and to measure the impact of the parametric conditions.

Metallographic examination revealed a high integrity weld free from contaminants and oxides at the weld interface; with a characteristic recrystallised Widmanstätten martensitic Beta weld centre zone microstructure, in as welded condition, and a finegrained equiaxed recrystallised to alpha-beta microstructure in post-weld heat-treated condition.

As-welded joints were tested under tensile and alternating fatigue conditions to provide an extended set of joint performance data. Joints demonstrated tensile performance equivalent to that of the parent material in all cases, with near-parent fatigue properties and improved (reduced) fatigue scatter in the post-weld heat-treated condition.