Robert Pederson

Study of alpha-case depth in Ti-6Al-2Sn-4Zr-2Mo and Ti-6Al-4V

At temperatures exceeding 480°C titanium alloys generally oxidises and forms a hard and brittle layer enriched with oxygen, which is called alpha case. This layer has negative effects on several mechanical properties and lowers the tensile ductility and the fatigue resistance. Therefore any alpha-case formed on titanium alloys during various manufacturing processes, such as heat treatment procedures, must be removed before the final part is mounted in an engine. In addition, long time exposure at elevated temperatures during operation of an engine could possibly also lead to formation of alpha-case on actual parts, therefore knowledge and understanding of the alpha-case formation and its effect on mechanical properties is important. Factors that contribute for growth of alpha-case are: presence of oxygen, exposure time, temperature and pressure. In the present study, isothermal oxidation experiments in air were performed on forged Ti-6Al-2Sn-4Zr-2Mo at 500°C and 593°C up to 500 hours. Similar studies were also performed on Ti-6Al-4V plate at 593°C and 700°C. Alpha-case depth for both alloys was quantified using metallography techniques and compared.

Evaluation of the bulk and alpha-case layer properties in Ti-6Al-4V at micro- and nano-metric length scale

In the present study the hardness of individual alpha ( )-Ti grains in Ti-6Al-4V was measured by nanoindentation using Berkovich tip indenter. Additionally, alpha-case layer was induced by performing isothermal heat treatment at 700 °C in air for 500 hours. The average hardness of the -Ti grains found in the bulk material and in the alpha-case layer were 6.7 ± 0.7 GPa and 9.4 ± 1.4 GPa, respectively. The high hardness of the -Ti grains in the alpha-case layer is due to solid solution strengthening caused by interstitial oxygen diffusion. The thickness of the developed alpha-case layer was estimated metallographically and compared with that measured from a hardness profile performed along the layer. Moreover, electron back-scattered diffraction was used to determine the local crystallographic orientation, the texture of the alloy microstructure, as well as phase fraction changes, where the nanoindentation measurements were performed.

Chemical Milling of Cast Ti-6Al-4V and Ti-6Al-2Sn-4Zr-2Mo Alloys in Hydrofluoric-Nitric Acid Solutions

The behavior of cast Ti-6Al-4V and Ti-6Al-2Sn-4Zr-2Mo during chemical milling in hydrofluoric-nitric (HF-HNO3) acid solutions with 1:3 and 1:11 molar ratios was investigated using electrochemical and atomic force microscopy (AFM) techniques. Faster corrosion rate in 1:3 solutions was measured for Ti-6Al-4V than for Ti-6Al-2Sn-4Zr-2Mo, whereas in 1:11 solution Ti-6Al-2Sn-4Zr-2Mo exhibited higher corrosion rate. Scanning Kelvin probe force microscopy measurements revealed difference in the Volta potential between the α-laths and the β-layers in the Widmansttaten microstructure indicating operation of microgalvanic cells between the microconstituents when in contact with HF-HNO3 solution. The AFM topography measurements demonstrated faster corrosion of the α-laths compared to the β-layers, in both alloys. In 1:3 solutions, higher α/β height difference was measured in Ti-6Al-4V, whereas in 1:11 solution, the difference was higher in Ti-6Al-2Sn-4Zr-2Mo. The results revealed that the chemical milling behavior o...

Microstructure characterisation of Ti-6Al-4V from different additive manufacturing processes

The focus of this work has been microstructure characterisation of Ti-6Al-4V manufactured by five different additive manufacturing (AM) processes. The microstructure features being characterised ar ...

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

Addition of small amounts of boron to cast Ti-6Al-4V alloy has shown to render a finer microstructure and improved mechanical properties. For such an improved alloy to be widely applicable for large aerospace structural components, successful welding of such castings is essential. In the present work, the microstructure and porosity of laser welds in a standard grade cast Ti-6Al-4V alloy as well as two modified alloy versions with different boron concentrations have been investigated. Prior-β grain reconstruction revealed the prior-β grain structure in the weld zones. In fusion zones of the welds, boron was found to refine the grain size significantly and rendered narrow elongated grains. TiB particles in the prior-β grain boundaries in the cast base material restricted grain growth in the heat-affected zone. The TiB particles that existed in the as cast alloys decreased in size in the fusion zones of welds. The hardness in the weld zones was higher than in the base material and boron did not have a significant effect on hardness of the weld zones. The fusion zones were smaller in the boron-modified alloys as compared with Ti-6Al-4V without boron. Computed tomography X-ray investigations of the laser welds showed that pores in the FZ of the boron modified alloys were confined to the lower part of the welds, suggesting that boron addition influences melt pool flow.

Failure Investigation of an Airline Engine Failure During Take Off

An Iran Airlines CF6-8002A2F engine had a turbine failure during takeoff from the Arlanda airport in Sweden, in January 2010. Failure investigation of the engine was performed by Lufthansa Technik AG (LHT). The Swedish Accident Investigation Board (SHK) requested a second opinion of the available High Pressure Turbine (HPT) Diffuser Aft Seal hardware, received from LHT, and because of GKN Aerospace Swedens extensive experience as Original Equipment Manufacturer (OEM) of Swedish fighter engines this second failure analysis work was performed by GKN. The failure investigation of the Diffuser Aft Seal pieces and fragments received from LHT revealed no fractures or fatigue cracks which are believed to be evidences of the primary cause to the failure. However, a secondary fatigue crack was found in an aft tooth fragment from the Diffuser Aft Seal. This tooth fragment has also a machining step on the forward surface, between a repair weld and the original tooth, with a geometry which gives a stress concentration factor of about 2.5 for radial and bending stresses in this area. This machining step may have contributed to initiate a fatigue crack in the seal tooth.

Influence of hydrogen environment on fatigue crack growth in forged Ti-6Al-4V: fractographic analysis

In this study, the influence of hydrogen environment (15 MPa) on the fatigue crack growth in forged Ti-6A1-4V at room temperature is investigated. It is observed that at 21 25 MPa√m, there exists ...