Welding of Ti-6Al-4V: Influence of welding process and alloy composition on microstructure and properties
Doktorsavhandling, 2018

Titanium alloys are widely used for components in the fan and compressor sections of aeroengines mainly because of their superior strength-to-weight ratio. Large static compressor components can be manufactured by welding together smaller subcomponents, which has potential to provide benefits such as higher buy-to-fly ratio and improved performance of the components. This is the background for why welding and the mechanical properties of welds have been investigated in this project. The aim of the work was to study what kind of microstructures and defects are formed in welding of Ti-6Al-4V with different welding processes and how these aspects affect the mechanical properties of the welds. Furthermore, the influence of chemical composition of the alloy on the formation of microstructures and defects was studied.

The welding processes compared were electron beam welding, laser beam welding, plasma arc welding and TIG welding. High energy beam welding processes rendered a finer weld microstructure in comparison to the coarser microstructure produced by arc welding processes. The finer weld microstructure was found to be beneficial for tensile ductility and low cycle fatigue performance. Porosity was observed in welds produced by all the processes. Large pores and pores located close to the specimen surface the most detrimental to the fatigue strength.  Fatigue life in the welds produced by arc weld processes was more sensitive to porosity than in the high energy beam welds. The finer microstructure has a higher resistance to micro crack initiation and growth which contributed to the better fatigue performance of welds produced by electron beam welding and laser beam welding.

The alloy composition had a significant influence on the microstructure of the welds and the formation of defects. A small boron addition induced significant grain refinement in weld in boron alloyed material. Narrow columnar prior-β grains were formed in the fusion zones of the boron alloyed welds. The α colonies and α plates were also refined, as compared to the standard Ti-6Al-4V welds. In the cast base material, the TiB particles were located along the prior-β grain boundaries restricting the grain growth in the heat affected zone. In the fusion zone of welds, TiB particles had decreased in size and formed networks of stripes along the interdendritic regions. EBSD combined with prior-β grain reconstruction was an effective method to reveal the prior-β grain structure in the different weld zones. A significant batch to batch variation in amount of porosity was observed in laser welding of Ti-6Al-4V. The most significant factors affecting formation of porosity were the material batch, pulse length and welding speed. The material batches that were most susceptible to formation of porosity had increased amount of carbon and oxygen. The formation of porosity could be minimized in all material batches by optimizing the welding parameters.

fatigue

microstructure

Ti-6Al-4V

porosity

welding

defects

Virtual Development Laboratory, Hörsalsvägen 7A
Opponent: Prof. Lars Pejryd, School of Science and Technology, Örebro University, Sweden.

Författare

Sakari Tolvanen

Yt- och mikrostrukturteknik

Fatigue strength dependence on microstructure and defects in Ti-6Al-4V welds

Proceedings of the 13th World Conference on Titanium,; (2016)p. 311-315

Paper i proceeding

TIG welding and laser welding of boron alloyed Ti-6Al-4V

Proceedings of the 10th International Conference on Trends in Welding Research,; (2016)p. 321-324

Paper i proceeding

Tolvanen, S., Pederson, R., Klement, U. Microstructure and mechanical properties of Ti-6Al-4V welds produced with different processes

Microstructure and Porosity of Laser Welds in Cast Ti-6Al-4V with Addition of Boron

Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science,; Vol. 49(2018)p. 1683-1691

Artikel i vetenskaplig tidskrift

Tolvanen, S., Germain, L., Pederson, R., Klement, U. Phase transformation mechanisms in boron alloyed Ti-6Al-4V weld studied using beta grain reconstruction

Tolvanen, S., Johansson, J., Pederson, R., Klement, U. Batch to batch variation in formation of porosity in laser welding of Ti-6Al-4V

Titanium alloys are widely used in aviation industry mainly because of their superior combination of high strength and low weight. Titanium alloys can be found on parts for landing gear, internal components of wings, and engine components. Large titanium alloy components used in aircraft engines have been traditionally manufactured out of large single piece casting that are machined into final shape. A significant amount of the initial casting is machined away in order to achieve the finished geometry of the part. This is neither environmentally friendly nor economically viable. An alternative method to manufacture large aeroengine components is to weld together smaller subcomponents. The benefit of this approach is that less scrap material is produced and the performance of the components can be improved. However, fusion welding involves localized melting which produces changes in geometry and microstructure, weld defects and residual stresses in the welded material. These factors are detrimental to the mechanical performance of welded structures. This is the background for why welding and the mechanical properties of welds have been investigated in this project. The aim of the work was to study what kind of microstructures and defects are formed in welding of Ti-6Al-4V with different welding processes and how these aspects affect the mechanical properties of the welds. Furthermore, the influence of chemical composition of the alloy on the formation of microstructures and defects was studied.

The welding processes compared were electron beam welding, laser beam welding, plasma arc welding and TIG welding. High energy beam welding processes produced a finer weld microstructure in comparison to the coarser microstructure produced by arc welding processes. The finer weld microstructure was found to be beneficial for tensile ductility and low cycle fatigue performance. Porosity was observed in welds produced by all the processes and this caused variation in the fatigue performance of the welds. Large pores and pores located close to the specimen surface the most detrimental to the fatigue strength.  Fatigue life in the welds produced by arc weld processes was more sensitive to porosity than in the high energy beam welds. The finer microstructure has a higher resistance to micro crack initiation and micro crack growth which contributed to the better fatigue performance of welds produced by laser beam welding and electron beam welding.

The chemical composition of the base material had a significant influence on the microstructure of the welds and the formation of defects. A small boron addition induced significant grain refinement in boron alloyed welds. Narrow columnar prior-β grains were formed in the fusion zones in the welds in boron alloyed materials. The α colonies and α plates were also refined, as compared to the standard Ti-6Al-4V welds. TiB particles located in the prior-β grain boundaries restricted the grain growth in the heat affected zone. A significant batch to batch variation in amount of porosity was observed in laser welding of Ti-6Al-4V. The material batches that were most susceptible to formation of porosity had increased amount of carbon and oxygen. The formation of porosity could be minimized in all material batches by optimizing the welding parameters.

Bildande av defekter vid svetsning och deras inverkan på mekaniska egenskaper hos Ti-6Al-4V och Ti-6Al-2Sn-4Zr-2Mo

VINNOVA, 2013-08-01 -- 2017-06-30.

Ämneskategorier

Bearbetnings-, yt- och fogningsteknik

Annan materialteknik

Metallurgi och metalliska material

Styrkeområden

Materialvetenskap

ISBN

978-91-7597-771-3

Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 4452

Utgivare

Chalmers tekniska högskola

Virtual Development Laboratory, Hörsalsvägen 7A

Opponent: Prof. Lars Pejryd, School of Science and Technology, Örebro University, Sweden.

Mer information

Senast uppdaterat

2018-08-13