structural optimization of titanium alloys by electric upset forging

1. STRUCTURAL OPTIMIZATION OF TITANIUM ALLOYS BY ELECTRIC UPSET FORGING Lembit Kommel Tallinn University of Technology Department of Materials Engineering

3. Introduction The material thermophysics properties are for example the heat capacity, coefficients of thermal expansion and heat conductivity, heat convection, heat distribution and others... For part of bar heating and for thermal balance calculation needed the required capacity of heat addition, which depends on coefficient of electrical resistance of metal and density of high strength heat-proof titanium based alloys for the turbo-jet; steam turbines for fossil and nuclear power plants of structural parts such as blades are actual use. Components such as blades are highly loaded under the pressure of combined stresses from centrifugal stress during rotor rotation, bending stress and torsional stress from a compressed air or gas flow under service temperatures from -60 up to 650°C.

4. Process The electric upset forging (EUF) process combines with rapid heating and forming functions in one operation, speeding both perform and finished forging production. During heating process with conduction electric current the free electrons and hydrogen atoms moving in metal with increasing of temperature and in that time takes place the regulation of metal electron structure. The regulation of electron structure in ones turns to increase the internuclear interaction of metal.

5. Aims of investigation The aims of this structural optimization of heat-proof Ti-alloys are structure forming mechanism investigation at (a) electric conduction current density or velocity of rapid heating and at (b) deformation stress or deformation starting temperature during upsetting of bar in condition of severe plastic deformation.

6. Materials The Ti-alloys in this study (in wt. %) were: VT3-1 – a??-titanium alloy TiAl6Mo2Cr2Fe0.5Si VT18U - pseudo a-titanium alloy TiAl7.6 Zr11MoNbSnSi VT25U - a??-titanium alloy TiAl5.8W1.1Mo7.4Zr6.5Sn3.8Si0.5

7. Testing methods The natural cylindrical bars with diameter (d) from 15 to 50 mm were upsetted with testing of parameters on electric upsetting installation “Hasenclever” HG-125/560. Bars were heated by electric current density (i) from 10 to 26 A/m2 by heating velocity (??) from 12 to 250°C/s Severe deformed by deformation stress (?s) from 30 to 500 MPa. By these parameters use the temperature of deformation starting was in interval of 720-1300°C and was increased during deformation in the central part of heated and deformed bar. For texture and microstructure investigation the specimens were cut off from bars in three parts: initial, heated and deformed. These specimens (fabricated in three projections) were polished and etched for microstructure study. The texture and microstructure forming were studied with optical (Nikon CX) and scanning electron (Gemini LEO Supra-35) microscopes. For mechanical and physical properties measure the microindentation method with universal hardness tester Zwick Z2.5/TS1S was use. The universal hardness was measured by load of 100 N, creep and relaxation by load of 35 N during 5 minutes. The operational properties of compressor blades were tested on electrodynamics’ vibration stand by frequency f1,0 = 1010-1080 Hz of self-fluctuation. During each test step (up to 6) of fatigue strength testing the 2?107 cycles were made.

8. Mass heat capacity The thermophysics properties were determined by electric conduction heating method with heating velocity of 50°C/s to temperature 1100-1200°C. The coefficient of mass heat capacity (cp) was non-linear increased over two times. From temperatures at 600°C the heat capacity rapidly increase and also, the heat addition want to increase for thermal balance of EUF process calculation. Ti-alloys have maximal mass heat capacity in region of phases transformation temperature.

9. Coefficient of electrical resistance The coefficient of electric resistance has maximal value at temperatures 400-600°C and to take decrease to minimal value by temperatures at 800 °C for (a??)-titanium alloys and at 930°C for pseudo-a titanium alloy. The minimal value of electric conduction coefficient for all Ti-alloys was measured in region of phase’s transition. The coarse-grained ß-structure Ti-alloy has a lower coefficient of electrical resistance.

10. Coefficient of temperature conductivity Up to 700°C the coefficient of temperature conductivity was increased approximately linearly, then decrease by phase’s transitions and after phases transitions for ß-structure rapidly increases up to maximal value.

11. Physical properties of Ti-alloys As was shown these physical properties of Ti-alloys have non-linear character by temperature increase. Near region of phase transition the physical properties change significantly. For calculation of optimal EUF process parameters the numerical values of physical properties don’t use.

12. Microstructural investigation Microstructure of pseudo-a Ti-alloy is shown in initial state.

13. Microstructure formed During EUF Microtexture of pseudo-a Ti-alloy formed by EUF is shown.

14. Microstructure forming mechanism by minimal electric current density The initial coarse-grained microstructure was transformed by minimal electric current density of i = 13 A/m2. By this the heating velocity was only 12°C/s by minimal deformation rate 0.2 – 0.6 ? 10-3 m/s and maximal temperature 970°C by optimal short time deformation stress ?s = 140 MPa. The structure has view of deformed laminates of a- and ?-phases. Material was deformed by sliding on grain boundaries in condition of high velocity superplasticity.

15. Microstructure forming mechanism by maximal electric current density By electric current density increase up to 25 A/mm2 the heating velocity was increased up to 250 °C/s. From internal stresses in the subgrains the microstructure in view of fine laminates was formed. By this the high heating velocity influences on incubation time which was increased and as result the temperature of polymorphous phases transition was increased too, from 980° up to 1130°C. The strength characteristics were increased by mean plasticity.

16. Microstructure forming mechanism by minimal deformation stress The influence of deformation stress (by optimal electric current density i = 17 A/mm2) on microstructure forming is illustrated. The subgrains size was increased significantly by laminates thickening and was identical to cast Ti-alloy microstructure. This material was heated by velocity of ?t = 102°C/s and severe deformed by stress of ?s = 50 MPa at maximal temperature interval td = 1170-1280°C and has by high strength a low ductility properties. Maximal temperature in heated part of bar was increased up to temperature of 1170°C and as result the coarse laminates microstructure was formed. The material with this microstructure for blades manufacturing don’t use.

17. Microstructure forming mechanism by maximal deformation stress Microstructure of Ti-alloy after EUF by maximal deformation stress is shown. By deformation stress increase up to 300-500 MPa the deformation mechanism was changed. Large subgrains were crushed at rapid heating and during severe deformation the ultrafine microstructure with mean grain size of 600 nm was formed. This microstructure was formed by deformation stress at 500 MPa and deformation temperature af 800-850°C. This Ti-alloy with ultra-fine grained microstructure has by relative high strength high-cycles fatigue stress and good plastic properties. The compressor blades with ultra-fine grained microstructure have high life extension [1]. It was increased up to 2-3 times.

18. Influence of electric current density on heating velocity, EUF process duration and temperature of deformation starting for Ti-alloy VT18U For each curves the electric current density is shown.

19. Influence of deformation stress on temperature of deformation starting and the maximal temperature in the upsetted bar from VT18U Depending on deformation stress the mechanism of microstructure forming change: By minimal stress (50-125 MPa) the temperature and temperature interval increase up to maximal and large grains of ß-phase can be formed. By mean stress (130-140 MPa) the deformation took place at phase transition temperatures without phase transitions. By maximal stress (150-450 MPa) the temperature of deformation decrease and grain size decrease also at shear stress by SPD.

20. Effect of EUF on Ultimate Strength of titanium alloy VT25U

21. Effect of Processing on Fracture Toughness of Pseudo-a Ti-alloy Depending on Direction of Loading to Slip Lines

22. Effect of Processing on Elongation and Reduction of Area Depending on Direction of Loading to Slip Lines

23. Effetc of Synchronous Operation of the Deformation Stress and Electric Current Density on Temperature of Deformation Starting of the Ti-alloy VT3-1

24. Effect of electric current density and deformation stress on heating speed and structure forming mechanisms of titanium alloy VT3-1

25. Ti-alloys optimal parameters for EUF installation Hasenclever XG-125/560 Optimal values of electric current density (i, 102 x MA/m2 – curve 1) and deformation stress (??, MPa – curve 2) on heating velocity (??, °C/s) on the surface (curve 3) and in the central part of bar (curve 4) depending on bar cross-section area (S = 10-4 x m2) or for bar diameter from 16 to 50 mm. (Area, 10-4 x m2, S 10 = D 35.7 mm)

26. Optimal Hydraulic Pressure in Upsetting Cylinder for Hasenclever HG-125/560 During Processing of Ti-alloys VT25U and VT3-1 Depending on Bar Diameter and Degree of Deformation

27. Conclusions Grateful to the physical properties of Ti-alloys, their low coefficients of heat capacity, low densities, and high electrical resistance the electric upset forging is very effective method for this metallic materials manufacturing. During rapid electric conduction heating the defects of structure, such as cracks, porous and chemical inclusions can be (in result of solid-to-solid diffusion) welded, large subgrains with ?-phase coarse laminates crushed, and new dislocations formed. Grateful to the rapid electric conduction heating the temperature of phases transitions increase with heating velocity increase. Deformation stress influences on temperature, and also on texture and microstructure forming mechanism. The different optimized microstructures, formed during EUF process with optimal parameters, can be receiving the needed mechanical and in-services properties of Ti-alloys.