Ti alloys

Question 1

Phase of Ti

Answer 1

Depending on temperature Ti could have two phases
- Alpha phase is HCP is under 888°C
- Beta phase is BCC is over 888°C is more ductile but is more prone to hydrogen absorption

Question 2

Properties of Ti

Answer 2

• Medium density
• High strength
• Medium E
• Low CTE
• Low Thermal cond
• High melting temp
• High static and fatigue strength, especially when Beta metastable produced strengthening precipitates

Question 3

Ti extraction and rifining

Answer 3

• Chlorination
• Purification
• oxidation and reduction
• From reduction we obtain titanium

Question 4

Effects of alloying elements (Al,Mo)

Answer 4

Elements are split on two side:
- Alpha stabilizer Al eq
- Beta stabilizer Mo eq

Question 5

Ti alloys depending on phase

Answer 5

• Commercially pure Ti
• Alpha or quasi-alpha Ti
• Alpha+Beta
• Metastable Beta alloys

Question 6

Thermal treatment

Answer 6

• For alpha and alpha+beta metastable–> sol. annealing+ art. aging
• Beta solutionising for beta homogenous structure
• rapid coooling in air or water to keep Beta metastable
• controlled decomposition of metastable phase by art aging to achive precipitation of alpha in strengthening particles.
• fast cooling promote the transformation of metastable beta into form of soft martensite called alpha’’.

Question 7

Corrosion resistance of Ti

Answer 7

• Excellent corrosion resistance due to chemically affinity to oxygen.
• Cathodic behaviour when in contact with most of other metals
• Corrosion sensitivity only in high reducing enviroments

Question 8

Effects of Beta stabilizer

Answer 8

• Higher density
• Higher response to thermal treatments
• Higher achivable stregth
• Improved formability

Question 9

Effects of Alpha stabilizer

Answer 9

• Increased Beta transus temperature
• Higher creep strength
• Improved weldability

Question 10

Ti alloys: CP titanium
Properties, thermal t., surface treat.

Answer 10

• Increasing of concentration of interstitials strength increased but ductility and corrosion resistance get worse.
• Only annealing is feasible
• Several grades of CP exist in order to stregth and corrosion resistance
• Forms of nitriding and carburizing are suitable in order to modify the surface layers.

Question 11

Issues of Ti with heat treatments

Answer 11

• The high reactivity with O2 suggests thermal treatment in inert atmosphere.
• O2 induces embrittlement, stabilize alpha phase. An alpha-case is created on surface after heating in air
• Beta phase also tends to pick up hydrogen, to avoid this a slightly oxidising enviroment is created

Question 12

Plastic def of Ti

Answer 12

• Hot plastic def is conveniently carried out in Beta or Apha+Beta phase.
• Beta is prone to grain growth, so Alpha+Beta phase is preferred.
• Oxide formed above 600°C is not passivating anymore the undelying metal.

Question 13

Welding of Ti

Answer 13

• Low thermal conductivity makes the welding operation easy to do.
• Care has to be paid to avoid interstitial pickup of O,H,N,C
• Welding edges have to be accurately prepared to avoid contamination by keeping low roughness
• Inter gasses are used nt only for weld zone but also for HAZ.

Question 14

Machinig of Ti

Answer 14

• Cooling
• Sticking of cutting tool, high wear
• Low efficiency, efficiency is further depleted adding Beta stab.
• For following het treatments is needed a accurate cleaning from the cooler liquid.
• Risk of chip burning.

Question 15

Application of Ti

Answer 15

• Turbines, submariner, space ships, military equipments
• Steam plants, boats, propellers, motorsport, exaust, biomedical sector.

Springs of Ti can store more energy than the steels ones.