Process-microstructure-mechanical property relationships for linear friction welded Ti-6Al-2Sn-4Zr-6Mo

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Webster, Toby (2020). Process-microstructure-mechanical property relationships for linear friction welded Ti-6Al-2Sn-4Zr-6Mo. University of Birmingham. Ph.D.

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Abstract

Titanium alloys are used in the aerospace industry for their high strength and low density, making them suitable for weight critical designs. The α+β heat-treatable alloy Ti-6Al-2Sn-4Zr-6Mo (Ti6246) has improved mechanical properties over the well characterised Ti-6Al-4V alloy. It is intended that Ti-6246 be used as a compressor disc material for future manufacturing technologies, such as Linear Friction Welding (LFW). LFW enables the solid-state production of a near-net shape component, offering savings in weight and size over tradition joining methods. Fundamental understanding of the relationship between welding parameters, microstructure and mechanical properties is required to assess the suitability of this alloy for LFW.
An investigation has been performed to determine the evolution of microstructure as a result of prior parent material morphology, Key Process Variables (KPV) of welding and weld geometry. The mechanical properties investigated include hardness, fatigue crack growth threshold, fracture toughness and tensile strength. The effect of test piece orientation on fatigue crack growth and fracture toughness is investigated, to determine the effect of weldline texture.
The microstructure produced as a result of changes to these variables has been characterised in both the As-Welded (AW) and Post Weld Heat Treated (PWHTed) conditions. Following welding, a dynamically recrystallised microstructure containing martensite (α”) is observed within the Central Weld Zone (CWZ) of microstructure variations tested in this study. Following a PWHT of 640°C for 2 hours, an increase in hardness is observed due to the reprecipitation of fine α laths within the CWZ. For a water quenched microstructure consisting of α” laths, two Heat Affected Zones have been observed, producing vastly different hardnesses.
Fatigue crack growth thresholds and fracture toughness have been investigated at room temperature using a four-point bend test configuration, on PWHTed specimens. Ti-6246 LFWs show no influence of test piece orientation, bonded width and the investigated KPVs, on the threshold and fracture toughness performance. From the minimal influence observed it is suggested that the CWZ may contain only low levels of α texture due to the LFW process. Ti-6246 LFWs show sensitivity to the two R ratios investigated, R = 0.5 and R = 0.1, lower thresholds were obtained for R = 0.5 due to a higher mean stress and K_max. The fracture toughness of the CWZ is much lower than the parent material due to the high hardness of the region. Tensile testing of different parent material microstructures has indicated superior Ultimate Tensile Strength (UTS) of the CWZ, compared to the parent material, due to necking and failure within the parent alloy.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):