Titanium Microstructure and Properties

To better understand how the microstructure of titanium alloys correlate to the physical properties of the material, numerous articles and papers will be reviewed which focus on identifying this correlation.

The first study to be review is titled: Experimental and numerical investigation on the tensile properties of a titanium alloy disc with dual microstructure [66]. This study explores in depth the effect of the microstructure of a Titanium alloy has on the mechanical properties of said alloy. It was found in this study that titanium alloys with a fully transformed β microstructure as shown in Figure 2.14a provides better creep and fracture toughness while an equiaxed microstructure (Figure 2.14b), consisting of both globular α and transformed β structures provides better fatigue resistance and ductility. An optimum combination of static and dynamic properties can be obtained by a microstructure comprising of 10-15% globular α in a transformed β matrix, as shown in Figure 2.14c.

25

Figure 2.14: Structure - Property Correlations in Titanium Alloys [66].

The next study under review also investigated how the phase structure effects the mechanical properties of the titanium alloy. This study is titled: Influence of α/β interface phase on the tensile properties of laser cladding deposited Ti–6Al–4V titanium alloy [67]. In this study Ti6Al4V is laser cladded using two different laser cladding deposition systems, one being a CO2 laser and the other a diode laser. Laser power and scanning speed was kept constant between the two systems at 2000W and 600mm/min. It was found through transmission electron microscopy (TEM) that the diode laser did produce the α/β interface phase (Figure 15) while the CO2 laser did not. The α/β interface phase acts as an inhibitor to dislocation motion which yielded increased mechanical strength. Tensile testing of the samples confirmed that the samples with the α/β interface phase had higher yield strength than the samples without the α/β interface. It was concluded that this work suggests an effective method of improving both the strength and plasticity of laser cladding deposited Ti6AlV based on the α/β interface phase [67].

Figure 2.15: TEM Images a) CO2 Laser without α/β interface phase; b) Diode laser with α/β interface phase [67].

26 The effect that laser power and scanning speed, and therefore the heat input, has on the achieved microstructure of the molten zone and the heat affected zone during laser welding was investigated in: Experimental and simulation study on the microstructure of TA15 titanium alloy laser beam welded joints [68]. In this study the laser power was varied between 2200, 2500 and 2800W while the welding speed was varied between 1.5, 2.0 and 2.5m/min. It was observe that for a lower heat input (low power or fast scanning speed) that short fine acicular martensite appears among the long parallel acicular martensite forming the basket weave structure. The relatively high heat input caused the distribution of the martensite to appear more scattered without obvious directionality. In summary the structure of welded seam was found to be columnar crystal structures while the heat affected zone consisted of isometric crystal structures [68].

The study titled: Microstructure characterization of Ti−6Al−4V titanium laser weld and its deformation [69], also focused on investigating the microstructure of laser welded Ti6Al4V.

This study kept the laser power, flow rate and defocussing amount constant while varying the scanning speed between 1.4, 1.6 and 2.0m/min. The compression tests were conducted and it was found that the 1.4m/min and the 2.0m/min achieved a similar ultimate strength of 1.121GPa and 1.191GPa, respectively. The stress-strain curve produced showed similar elastic modulus for all 3 samples. In conclusion the 2.0m/min scanning speed did achieve the highest ultimate strength. It was also found that the slip deformation in titanium laser welds can be activated by external forces (Figure 2.16). When applying an external force the minimum width of the slip was 600nm, without an external force and only activated by the welding residual stress the slip minimum thickness was 75nm.

Figure 2.16: SEM Images Slip Deformation a) with external force; b) without external force [69].

The next two studies reviewed focused on titanium composites and bimetallic joints, these studies were considered for review for the purpose of broadening the literature and to ensure a more holistic view and understanding is developed for how titanium alloys react to these types of applications. The first study is titled: Laser melting of titanium-diamond composites:

27 Microstructure and mechanical behavior study [70]. This study aimed to improve the mechanical properties of the titanium by adding 5, 10 and 15% diamond powder to commercially pure titanium powder and laser melting the powder using the laser engineered net shaping process. It was found that the addition of the diamond powder dramatically improved the mechanical properties of the titanium compounds as the percentage diamond powder increased the hardness increased to 984HV at 15% diamond powder with the Young’s modulus increasing to 629GPa. Such high mechanical properties for laser melted composites, make them ideal for wear resistant and high temperature applications [70]. The second study titled: Microstructure and mechanical properties of titanium/steel bimetallic joints [71], investigates Ti/Fe bimetallic plates joined with Cu-V filler. The main challenge with joining iron and titanium is that Fe-Ti intermetallics that form during the joining is brittle and is susceptible to cracking under load. However it was shown in this study that though the addition of the Cu-V as a filler material more Cu-Ti intermetallics were formed instead of Fe-Ti, which are more ductile and increases the overall resistance to cracking in these joints. The presence of the copper also assisted with the arresting of cracks that propagated through the Fe0.4V0.6 phase as illustrated in Figure 2.17.

Figure 2.17: Joint Microstructure – Crack arrest [71].