The Metallurgy of Commercially Pure Titanium Alloys Welded with a Pulsed Laser Beam

Alexander Buddery (2011). The Metallurgy of Commercially Pure Titanium Alloys Welded with a Pulsed Laser Beam PhD Thesis, School of Mechanical and Mining Engineering, The University of Queensland.

       
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Author Alexander Buddery
Thesis Title The Metallurgy of Commercially Pure Titanium Alloys Welded with a Pulsed Laser Beam
School, Centre or Institute School of Mechanical and Mining Engineering
Institution The University of Queensland
Publication date 2011-04
Thesis type PhD Thesis
Supervisor Matt Dargusch
John Drennan
Patrick Kelly
Total pages 143
Total colour pages 3
Total black and white pages 140
Subjects 09 Engineering
Abstract/Summary This thesis examines the effect of welding on the microstructure and properties of commercially pure titanium welded with a pulsed laser beam. Among other applications, pulsed laser welding is widely used in the construction of dental implants from pre-fabricated wrought components and also for the repair of both wrought and cast implants. In practice these devices experience an increased number of failures due to fracture of the laser welded titanium framework compared to traditional gold prostheses. The numerous studies which have focused on improving the properties of such welds, however, tend to be more clinically focused with only limited investigations into understanding the fundamental weld metallurgy. In the present work this issue is addressed with detailed investigation into the effect of alloying elements, external contamination from welding and changing process parameters. In addition, micros tensile testing and wave-dispersive spectroscopy techniques are developed to try and improve the characterisation of pulsed laser welds. Modern wave dispersive spectroscopy techniques were successfully employed to measure bulk oxygen content in titanium on a small area 100x100m with good agreement with the bulk ASTM standard technique down to oxygen contents of 500ppm. The technique was still not localised enough however to be applied to the intended application of laser welds. An all-weld longitudinal micro tensile test was successfully developed with the ability to accurately evaluate the elongation of the welds. It was identified however that the significance of such tensile test results could not be appreciated without further investigation into the weld metallurgy to develop a greater understanding of the cause of the measured changes in properties. Investigations into the weld metallurgy revealed that alloy composition and external contamination have the most significant effect on the weld microstructure and hardness of the weld with the effect of process parameters being more subtle. The welds consist of a fusion zone and a HAZ region which has undergone significant grain growth and morphology change due to heating above the beta transus. At low alloy contents the fusion zone and HAZ undergo a massive transformation on cooling which transits to a martensite transformation with increased alloy content. Iron was found to have the largest effect on weld microstructure, being a strong suppressor of the massive phase and promoting the martensitic transformation. Oxygen and nitrogen do not affect the nature of the allotropic beta to alpha phase transformation as much as iron but have a more pronounced effect on hardness due to solid solution strengthening. The transition from massive to martensitic transformation occurs at lower alloy contents in the fusion zone compared to the HAZ due to the higher cooling rate it experiences. A wide range of process parameters were trialled with little affect on the nature of the allotropic phase transformation. The main effect of process parameters was to coarsen the microstructure due to changes in the cooling rate linked mainly to changing energy per pulse. The hardness of high solute content alloys was more sensitive to changes in process parameters than low solute content alloys due to the effect solutes have on increasing the gradient of the Hall-Petch relation. For higher pulse energy welds an additional HAZ was identified associated with slow early stage recrystallization in the alpha phase field. This region requires a large number of dislocation to form however so is only present for welds on deformed substrates.
Keyword Titanium
Laser welding
microstrucutre
Contamination
Additional Notes the pages numbered 2, 72 and 87 are colour

 
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Created: Mon, 05 Sep 2011, 19:01:41 EST by Mr Alexander Buddery on behalf of Library - Information Access Service