Fundamentals of Low Cost Titanium Hydride Powder Metallurgy

Transcription

1 Fundamentals of Low Cost Titanium Hydride Powder Metallurgy Orest Ivasishin 1, Vladimir Moxson 2 1 Institute for Metal Physics, 36 Vernadsky str., Kiev 03142, Ukraine 2 ADMA Products Inc., 2035 Midway Drive, Twinsburg, Ohio 44087, USA

2 Introduction Ti TiH 2 +alloying powders Compaction Sintering Alloys, articles possessing desirable properties Titanium Hydride (TiH 2 )PM became an established technology ( US Patents , ; Eurasian patent ; Ukraine patents 70366, 65654, 92714) TiH 2 PM provides an economic and technical advantages as compared to conventional Ti metal PM Key feature of the TiH 2 PM is use of titanium hydride powder instead of titanium powder as the starting material

3 Advantage of the TiH 2 PM is not reduced to a simple substitution of the starting material; use of the TiH 2 powder leads to several specific features in the compaction and sintering stages that eventually result in properties equivalent to or better than those produced through conventional IM processing TiH 2 PM is based on a scientific background (fundamentals) which are generalized in this presentation

4 Characteristics of Titanium Hydride Characteristics Ti TiH 2 Lattice HCP/BCC FCC/FCT Young modulus 115 GPa 40 GPa Strength level MPa 150 MPa Ductility 15-45% (tensile) 0% Density 4.51 g/cm g/cm 3

5 TiH 2 Powder Particles Conventional, irregular shaped Spherical shaped

6 Compaction Tap density Work hardening (deformation) Particle rearrangement and fragmentation

7 Compaction 1 1 Tap density Work hardening (deformation) Particle rearrangement and fragmentation

8 Compaction 1 1 Tap density Work hardening (deformation) Particle rearrangement and fragmentation TiH 2 particles fragment during compaction resulting in refining the particles and formation of fresh surfaces After compaction Before compaction

9 Compaction: Green Density Density, % Ti H 2 Ti Pressure, MPa Rearrangement and fragmentation of the particles provides reliable connections between them thus making density and strength of the green compacts sufficient for handling

10 Compaction TiH 2 Pores are finer and better distributed in the green compact; contrary to Ti metal powder their size does not depend on powder size Interaction between TiH 2 particles and die wall is negligible so that ejection pressure is several times lower as compared to Ti metal powder Ti

11 Compaction: Processing Window Density Ti TiH 2 Pressure Strength Ti TiH 2 Pore size Powder size Powder Size

12 Compaction of Spherical Particles Compaction Not sufficient bonding of particles

13 Sintering: Heating of Compacts L/L 0 0,00-0,05-0,10 TiH 2 Low T TiH Temperature, C High T Intensity of hydrogen emission

14 Dehydrogenation: Compact Integrity Theor. shrinkage: 5,7 7,2% depending on hydrogen content 3 4% L/L 0 0,00-0,02-0,04-0,06-0,08 Shrinkage 6.8% Dehydrogenation Exper. 3.5%H, P=640 MPa, shrinkage: 6.8% (expected 6.7%) -0, Temperature, o C Before and after dehydrogenation

15 Surface Contamination (XPS) Surface Impurities

16 Surface contamination (XPS) H 2 O Ti/TiH 2 TiO 2 Oxygen: surface TiO 2 scale and absorbed H 2 O

17 Dehydrogenation: Cleaning Action (O) Mass spectrometry results H 2 O Intensity of H 2 O emission H 2 H 2 O(TiH 2 ) H 2 O(Ti) Intensity of H 2 emission TiO 2 O Ti Ti/ TiH 2 TiO 2 TiO 2 TiH 2 H 2 O Temperature, o C

18 Dehydrogenation: Cleaning Action (O) TiO 2 +4H = Ti+2H 2 O G(T)= T+3.46T lgt+ +2RT(ln 2ln 2

19 Dehydrogenation: Cleaning Action (O) TiO 2, O-Ti-OH, TiCl 3 subox. TiO 2p 2 3/ XPS Ti2p TiH 2p 1/2 2 20C air Intensity (arb. units) Ti-O-S 500C vac 15 min 500C vac 30 min Reduction of TiO 2 surface scales upon dehydrogenation: TiO 2 TiO Ti 500C va 60 min Binding Energy (ev)

20 Dehydrogenation: Cleaning Action (Cl) MgCl 2H Mg 2HCl 2 HCl emission indicates on cleaning of particle surface from chlorine by hydrogen

21 Dehydrogenation: Cleaning Action (C ) C 2 H 2 emission indicates on cleaning of particle surface from carbon by hydrogen

22 How to Decrease the Contamination? Transformation of the impurities into volatile products Allow them to get out from the articles open porosity is necessary Competitive process, e.g. dissolution of impurities in titanium should be avoided Gases Emission Dissolution of impurities Open porosity Closed porosity T, о С 22

23 Oxygen Content Typical change of oxygen content at different processing stages TiH 2 powder 0.16 Green compact 0.26 Sintered product 0.11 final oxygen content is generally lower than in green compacts

24 Sintering: Activation due to Hydrogen L/L o 0,00-0,02-0,04-0,06-0,08 Dehydrogenation 800 о С TiH 2-0, Temperature, о С 840 о С Ti Density, g/cm 3 4,2 4,0 3,8 3,6 3,4 3,2 3,0 Ti Densification TiH 2 Dehydrogenation Temperature, o C 1) Atomic hydrogen has a cleaning effect on the surface of the particles, thus increasing the chemical activity of the surface 2) Dehydrogenation and respective phase transformations lead to high concentration of vacancies and dislocations which activate sintering through increasing diffusion rate

25 Sintered Density Density, % Ti H 2 Ti Pressure, MPa Advantage of the TiH 2 PM is obvious. However, full densification is still difficult, especially at low pressures

26 Microstructure Evolution (CP Ti) Decrease in size and volume fraction of pores Optimized processing Pore healing Pore coalescence Grain growth Increase in size of pores, volume fraction of pores is constant Sintering temperature and time Because of low beta transus and fast grain growth full densification of the CP Ti is diifficult and needs some special measures

27 Sintering Activation due to Oxygen Density., g/cm 3 Impurities, % 4,46 (98.9%) O: 0.19 N: ,48 (99.3%) O: 0.24 N: ,49 (99.5%) O: 0.27 N: Density, g/cc 4,50 4,48 4,46 4,44 4,42 4,40 High oxygen content 99% Low oxygen content 98% Tensile stress (MPa) Tensile strain (%) 4,38 4, Pressure, MPa

28 TiH 2 BEPM Processing of Alloys Chemical homogenization, type of alloying powders Densification, structural integrity of green compacts due to TiH 2 / MA boundaries

29 Microstructure Evolution (Alloys) Pore healing Homogenization Optimized structure Pore coalescence Grain growth Synthesis temperature and time

30 Homogenization

31 Homogenization Alloying particles: elemental powders of and stabilizing elements EP Ti EP Ti Slow homogenization due to stable α+β condition: stabilizers do not penetrate into areas enriched with stabilizers

32 Homogenization Alloying particles: master alloy (contains and stabilizers) Ti MA Ti Ti Ti MA Route 1: stabilizers are trapped inside shell Route 2: both and stabilizers penetrate into titanium matrix

33 Densification Ti 0,00-0, MA Heating Exposure L/L o -0,04-0,06-0,08 Ti-1023 Porosity Dehydrogenation -0,10 Ti-6Al-4V CP-Ti Ti Temperature, o C h h h h Temperature, o C Time CP-Ti

34 Ti 6Al 4V Relative density,% % TiH 2 Alpha alloys and low alloyed alpha + beta alloys, e.g Ti TiH 2 Ti Compaction pressure, MPa Ti 64 are easily processed due to high beta transus, relatively low MA volume fraction (low fraction of TiH 2 / MA boundaries) Typical tensile properties achieved with TiH 2 powder YS, MPa UTS, MPa Elong., % RA, %

35 Beta Alloys Beta alloys, e.g. Ti 1023, Ti 5553 are difficult to process because of low betatransus, therefore fast grain growth, relatively high MA volume fraction (high fraction of TiH 2 / MA boundaries) Alloy Sinter. T Condition Density,% Grain size, m YS,MPa UTS, MPa El., % RA, % Ti Ti о С BEPM 97, ,1 16, о С BEPM 97, ,3 14, o C BEPM 96, ,0 13, o C BEPM 97, ,0 19,5

36 STA Beta Alloys Alloy Sintering Condition Density, Grain size, YS,MPa UTS, MPa El., % RA, % Temp. % m 5553 Ti- Ti о С STA 97, ,6 4, о С STA 97, ,76 1, o C STA 97, ,5 4,2 1023

37 Beta Alloys Alloy Sinter. T Condition Density,% Grain size, m YS,MPa UTS, MPa El., % RA, % 1350 о С STA 97, ,76 1, о С STA 97, ,6 4, o C STA 98, ,8 10,3 BMS Specification High strength condition Ti- Ti o C STA 97, ,5 4, o C STA 98, ,2 11,0 AMS 4984 Specification. High strength condition

38 Interplay of Processing Parameters Solid lines: noticeable influence, Dashed lines: influence appears only at some specific values of the parameters or minor influence Parameters Base powder size Alloying powder size Hydrogen content Compaction pressure Temperature Material characteristics Impurity content Density (residual porosity) Chemical and microstruct. homogeneity Grain size Mechanical properties

39 Conclusions TiH 2 approach exhibits advantages in both compaction and sintering stages compared to conventional Ti metal PM, providing wide opportunity to control characteristics of BEPM processed CP Ti and Ti alloys TiH 2 PM is based on solid fundamentals which defines general pattern of its practical application which allows to design the processing in a first approximation As in most advanced processes final result strongly depends on many processing parameters; for any particular alloy and/or part optimization of processing parameters is necessary