AT9900070 Processing and characterization of aluminium alloys or composites exhibiting low-temperature or high-rate superplasticity Jacob C. Huang Institut of Materials Science and Engineering, National Sun Yat-Sen University, Kaohsiung, Taiwan, R.O.C. Wide applications of superplastic forming still face several problems, one is the high temperature that promotes grain growth, another is the low forming rate that makes economically inefficient. The current study is intended to develop a series of fabrication and thermomechanical processing, so as to result in materials possessing either low temperature superplasicity (LTSP) or high rate superplasticity (HRSP). The former has been achieved in the cast Al alloys, while the latter was accomplished in powder-metallurgy aluminium matrix composites. The aluminium alloys, after special thermomechanical processes, exhibited LTSP from 300 to 450 °C with elongations varying from 300 to 700%. The LTSP sheets after 700% elongation at 350 °C still possessed fine grains 3.7 pm size and narrow surface solutedepletion zones 11 ^m in whith, resulting in a post-SP T6 strength of 500 MPa, significantly higher than that of the HTSP superplasticity alloys tested at 525 °C or above. Meanwhile, it was found that LTSP materials may be transferred into HTSP materials simply by adding a preloading at 300-400 °C for a small amount of work. As for the endeavor in making HRSP materials, 2024Al/SiC, 6061Al/SiC and Al/Al3Ti systems processed by powder metallurgy or mechanical alloying methods are under investigation. The average sizes of the reinforeing SiC or A13Ti particles, as well as the grain size are all around 1 pra. The aluminium composites have exhibited HRSP at 525-620 °C and 10'2-10' s'1, with elongations varying from 150 to 350%. This ultimate goal is to produce an alloy or composite exhibiting low temperature and high strain rate superplasticity (LT&HRSP). 285 INTRODUCTION Processing and Characterization of 1. Superplasticity: Aluminum Alloys or Composites Exhibiting Superplasticity is referred to the capability of certain polycrystalline materials to undergo extensive Low-Temperature or High-Rate Superplasticity elongation, often preventing from continuous necking, prior to failure when tested at appropriate temperature and strain rates. ON Jacob C. Huang I B 1 1 i ii i 0 10 a 20 30 Institute of Materials Sci. and Eng. FIG. I. A uiperplaKic tlan|jiion of 4.850% ii demonstrated in a Pth-tl wi% Sn illoy"" by ihr iwo top umples and S.300% K demonsirated by the two bottom umpks in a commercial aluminum National Sun Yat-Sen University, bronze."" The cuircnt vrarld record is about 9.000% in the commercial aluminum bioiut"" Kaohsiung, Taiwan, R.O.C. Records of superplasticity HISTORICAL DATA ON SUPERPLASTICITY 1912 Bengough, a + p Brass, approx. 200% elong. Materials classification Material Elongation 1920 Resenheim ct al., Zn-AI-Cu in near-eutectic composition Metals Zn-Al 8800% 1928 Jenkfns, Cd-Zn and Sn-Pb alloys, approx. 400% clong. 1934 Pearson, Pb-Sn and Bi-Sn eutvctic alloys, 1950% elong. Metal matrix composites 6061A1/SJC 1400% 1945 Sviderskaya, Zn-Al alloys, observed large elongations 'sverkhplastichnost' (ultra-high plasticity) was coined and Intermetallic compounds Super a2 Ti3Al 1500% led the term superplasticity became public oo Supercooled amorphous mater. La55Al25Ni2o 15000% 1962 Underwood, review paper, led to interest in Western world Structure ceramics Y-TZP 800% 1970 Commercial applications, non-structural High Tc ceramics Y-Ba-Cu-O 110% 1974 Ti SPF applications, B-l and Space Shuttle 1981 Guiness Book of Word Records, 4850% eong. in Sn-38Pb Ceramic matrix composites AI2O3/Y-TZ 625% (Ahmed and Langdon, USC) High rate superplastic mater. MA-A1 1250% 1985 Higashi, -8000% clong. in Cu-Sn 1994 Langdon, -8800% clong. in Zn-Al Types of Superplasticity Micrograin superplasticity - Fully (or statically) 6: a recrystallized - Continuously (or dynamically) INITIAL recrystallized - Requirement: a. Fine and stable grain size INTEnMEOIATE e- o. (typically <10 jxm) b. Temperature > 0.4Tm c. Controlled strain rate oo oo d. Low flow stress riNAL e. Low tendency for cavitation f. High angle grain boundaries IE = o. csr g. grain size distribution h. grain aspect ratio Transformation superplasticity PIG 2 A schciiKHic illitslratian ol Me grain swilcliing meclianisin. involving grain boubdary sliding, proiwscd by Ashby and VCTMI. 6 6/ Internal strain-induced superplasticity formed Superform Metala Ltd and British Aluminium Company inow British Alcun Aluminium pic) begun to produce ttaa worlds first commercial superpluutio aluminum ulloy, Suprul 100. (Registered Com position i Al, 6Z titf. Cu, 0.4% Zr; Designation: AA2OO4 produeod per AMS42O8). In 1974 Superform Metals Ltd introduced unique forming techniques and equipment (Reforenco 1) which enublod deep male formed parts to be produced. This mulu forming equipment is shown in Figure 2. I m r— ja —4 •o > r- i/\ -I SO I SHEE 22 O O-o 2 T Ln TO m X -H ER ECU E ' rn m Figure 2 - Superform Male Forming Equipment to oo Detuiled discussions of itu operation liaa boen covered alaejhero (Reference 2). The complete male forming procaaa ia illuatratad by tha progressive aeries of formings shown in Figure 3. SLytt. Utte: 3 - The Complete Male Forming Cycle 303 <)0*/i cost savings and unproved dimen- sional accuracy complied with welded assemblies were achieved by the French Company. MbSSlTH HISI'ANO BUGAUI. by changing to SUPHAl four painted SUPRAl covws piotcct liydiaulic pipe work on die A300 Airbus undercarriage Dy seieclmg SUPRAL. M I AVIATION .icmcvcd greater consislency and inter- changeability on (lie lainngs loi Hie Alpha Jet Store Carrier More complex one piece shapes reduced hand limshiny ana avoided Hie Uisloiuon which could occur when welding drop hammered or lubber pressed parts. Increased Me. simpler assembly and icduced costs were bunelils gained by VICKERS MEDICAL when switching Irom Figure 6 - Male Formed Ejjiotor Seat Component Uaing Supral 100 glass librc lo SUPRAL iimei bodies on infant incubator uniis. to ':. /-•••. .*»;IB Short lead limes and case ol assembly o wcie bcneMs yarned wlien WESRANO MEIICOPTERS designed a new con- figuration engine intake lot an ciponmenlal programme. The 35 piece riv :HLCJ and welded construction w,r t ,,uncO limn lust 11 pressings wind l.nlhlully lepro- iluced ihe master An aliraciive design strong enough to withstand harsh industrial environments. ea. ^. eenmg against electrical noise and valuable heal sink properties were important bcnclils gained by SALTER INDUSTRIAL MEASUREMENTS on a weigh display uni| Figure 7 - Female Formed Pair of H6Ucopter.poor Panela Using Sugral^ipo SUPRAL panels loim most ol trie external body wuk ol the ASTON MARTIN I AfiONDA giving distinctive lines and a reduction in hand finishing compaied wHh lubber die pressed panels. 305 /. tSchG IN T E HME T AlI IC QLADE (t it unium- a luminum) ;' ^ Blades are produced by superplastic forging out of the TiAl interneta 11 ic allay'. Mechanical properties: 600 INITIAL: yield point, Mr'a ASPERrrY CONTACT mST STAGE: 2.5X relative elongation ASPERIIY CON IAL1 DEFORMATION AND INTERFACIAL 320 BOUNDARY FORMATION , fatigue limit Application: Blades are used in aircraft engines. The wight of gas- turbine engines with TiAl blades used in compressors decreases by 10-15%. SECOND STAGE: . > ! THIRD STAGE: GRAIN BOUNDARY MIGRATION VOLUME DIFFUSION AND AND PORE ELIMINATION PORE ELIMINATION -43- 4 I I 'yh i§ < I I -: —:•. ;;••:»' 3 V V IT \ m 1 1 I \ I \ ' 1 § I 292 Subject Covered * Superplastic (SP) Materials Development: (A) Aircraft-used and commercial aluminum alloys: (B) Advanced materials, including aluminum matrix composites* intermetallic compounds: Thermomechanical treatments (TMTs) Modified powder metallurgy Recrystallization routes High-T and low-T; low-rate and high-rate SP * SP Tensile Tests: Constant crosshead speed and constant strain rate With or without back pressure Strain rate change * Superplasticity Mechanism Analyses: * Post-SP Characterization: Microstructure characterization Grain boundary sliding contribution Mechanical property evaluation * Superplastic Forming (SPF): Forming system setup SPF practices Figure 2 SPF/DB of titanium heat cxcli;inger duct (Kef. 2), Comparison between theories and experiments Post-SPF property evaluation * Computer Simulation: * Joining for SP Thin Sheets: Diffusion bonding Electron-beam and laser-beam welding TEM micrographs of the precipitate and grain structure seen in the 7475 alloys: (a) large r\ precipates in the as-rolled samples (b)-(d) small and stable grains and the Cr- rich particles. The pinning effect can be clearly seen in (c) and (d). 294 7475 Route 1 (Reheat time- 15 min) ; ." Noi-deformed 51O°C ' 650% 525°C 730% Tca£>ile elongations of 7475 route 1 for initial strain rate 2x10-4 s-1 at rtifferen* temperatures 7475 Route 3 (Reheat time ~5 min) Not-deformed 2x1 Cr s"1 1500% Tensile elongations of 7475 route 3 at different initial mulates 510°C 295 The common working temperature for superplastic Al-Li alloys is 21 era 510-530 °C (the high-temperature superplasticity, HTSP) 00 d I * Surface Li- and/or Mg-depletion zone to 100-200 | * Severe orain growth from the S- a initial 5 |im to final 20 |xm * Appreciable cavrjation^ associated to P with concurreTit grain growth ON * Degraded post-form RT mechanical properties re re 5? 3 -* To develop S 3. 00 a superplastic Al-Li sheet with d optimum working temperature of o 350-450 °C rr I (the low-temperature superplasticity, LTSP) 00 -' — O :• & A 5 u o I g £ I 5 A «5 B =. M 0 •a 5 < oft S? IIO •z a I S8 u I -- msmm Mm ir o X 297 5? -8 S. (vdVi) sswis SIKU3) ds (Ul 1 JH ' f \ .o 298 Superplasticity in Metal Matrix Composites For conventional superplastic alloys e=10~4 ~ 10"3 ForMMCs ^lO"1 ~ 101 s Nieh et al. (1984)[1] first demonstrated Nieh et al.