PHYSICAL METALLURGY OF ALUMINIUM

Transcription

1 PHYSICAL METALLURGY OF ALUMINIUM Dr. C.S.Sivaramakrishnan Dy. Directr Natinal Metallurgical Labratry Jamshedpur Despite the fact that the prductin is highly energy intensive. aluminium and its allys are the mst widely used engineering materials next nly t irn and steel because f their functinal and ecnmical cmpetitiveness. The wrld ver, effrts are n t imprve the prperties f aluminium/allys with a view t replacing cstly and heavy weight materials in many applicatins, autmbile, space and aerspace fr example, fr energy saving and ecnmic benefits. The prperties f aluminium allys heavily depend n structural features and can be enhanced multifld by ally chemistry, heat treatment and therm-mechanical prcessing. This has been pssible with the increased insight int the physical metallurgy f aluminium allys. The basic advantages and applicatins areas f aluminium are the fllwing : 1. Prperties Light weight (1/3 density f steel) Resistant t weather High reflectivity Aluminium allys can equal the strength f steel High elasticity; prperties d nt deterirate at lw temperature Readily wrkable: can be easily thinned t 1/100 mm Cnducts electricity & heat cmparable with cpper Excellent funding prperties 2. Applicatin Areas Areas Due t Transprtatin- Light-weight, resistance t crrsin Architecture - Decrative aspects Package - Resistance t crrsin, decrative aspects Electrical industry - Gd electrical & heat cnductivity Husehld - Gd cnductivity, resistance t Crrsin Chemical & Fd industry - Resistance t crrsin Ally Classificatin Aluminium and its allys are bradly classified as cast and wrught under the 4 digit classificatin as per ASTM (American Sciety fr Testing Materials) specificatins. Aluminium allys can again be sub-classified int Heat treatable and Nn-heat treatable allys. The cast allys are used as cast cmpnents with minr

2 machining r heat-treatment. The wrught allys, n the ther hand, are used after the cast ingts are mechanically wrked, heat treated r therm-mechanically prcessed. Aluminium Allys Ally N. Al 99.00% Min. r greater lxxx Cpper 2x.xx Mn (with Mg) Silicn 4xxx Mg 5xxx Mg & Si 6xxx Zn, Mg 7xxx Other elements 8xxx Unused series 9xxx 1st digit is indicates the ally grup 2nd & 3rd indicate aluminium purity digit after decimal place indicate the prduct frm either casting r ingt Majr Ally System : Al-Cu allys : A1-8Cu, A1-10%Cu - als cntains Zn/Si besides Cu - Heat treatable Al-Cu ally Al-Mg Allys : Excellent crrsin resistance, gd machinability. - (Al-4Mg) ; attractive appearance - Als cntain Zn - die cast - Al-7Mg - sand cast - A1-10 Mg Al-Si allys : High crrsin resistance, gd weldability & lw specific gravity - Al-5.3% Si Als cntains Mg t strengthen by Mg2Si Al-Si-Mg, Al-Si, Al-Si-Mg-Cu Al-hyper : Cntain Ni. Cu.Mg, V - Engine blcks eutectic Si : Outstanding fluidity and machinability Al-Zn Al-Zn - Bearing and bushes Minr allying additins : Si - Nt gd fr tensile prperties Fe - Nt gd ductility Be - Used in Al-Mg allys Cr,Mn - imprves ductility Ni - imprves tensile strength at high temperature Ti, B,Zr - Grain refinement. P - Mdifies hyper eutectic Al- Si allys Bi,Pb,Sn - imprves machinability (Chip breakers) H- 2

3 Wrught Allts Al 99.00% min. r greater Cu Mn Si Mg Mg & Si Zn Other elements Unused series Ally N. lxxx 2xxx 3xxx 4xxx 5xxx 6xxx 7xxx 8xxx 9xxx 1st digit identifies the ally grup 2nd digit indicates impurities 3rd & 4th digit indicates minimum aluminium cntent Wrught Allys - Al-Mg, Al-Cu, Al-Cu-Mg, Al-Mg-Si, Al-Zn-Mg, Al-Zn- Mg-Cu. EFFECTS OF ALLOYING ELEMENTS & IMPURITIES ON PROPERTIES Density - Mg, Li & Si decrease density f aluminium - Cr, Cu, Fe, Mn, Ni, Ti and Zn increase density f Al - Si increase density till its slid slubility (S.S) limit (since lattice decreases). Abve S.S. the density decreases. * Thermal expansin - Mg/Zn increases thermal expansin, thers decrease. * Electrical Cnductivity - All elements reduce electrical cnductivity * Surface tensin- Bi Ca, Li, Mg, Pb, Sb and Sn reduce surface tensin. - Ag, Cu, Fe, Ge, Mn, Si, Zn have n effect. * viscsity - Cu, Fe, Ti increase viscsity - Zn has n effect n viscsity - Mg, Li reduces viscsity METALLOGRAPHY OF ALUMINIUM ALLOYS Etching t Reveal Structure Cast structure cnsists f grains. Etching causes disslutin f atm layers f metal frm individual grains prducing steps. The individual grains reflect light t different degrees. This enables the grains and phases t be distinguished (Fig.2a & 2b). Cmmnly used etching reagents fr aluminium/allys are Keller's reagent, hydrfluric acid and sdium hydrxide. Micr and Macr structures can be revealed by suitable etchants. CAST STRUCTURE Nucleatin & Grwth When the melt reaches the freezing pint, nuclei frm which increase in size and arrange themselves in certain clse packed

4 pattern. The latent heat released by the nuclei is cnducted away by the surrunding matrix. The grwth f the nuclei stps when the neighburing crystal meet each ther at grain bundaries and frm grains. Inside each grain the aluminium atms ( atms in ne grains) arrange in a lattice (Fig.3 & 4). Different grains have different rientatin. Grain bundaries are weak areas with regard t chemical crrsin. ln pure aluminium and allys undercling ccurs befre nucleatin if the heat. is remved faster mid n external surfaces are present. In cmmercial allys, Ti, Ii, Inc, etc. arc present. in line distributin n which nucleatin f aluminium takes place and the undercling will be minimum. Ingt Structures (Fig.5) Chill Zne Chill zne - Narrw regin f fine equiaxed crystals Clumnar zne - Parallel t heat flw directin Equiaxed zne f relatively carse equiaxed crystals. The 1st crystallizatin takes place near the cld muld wall cpiusly frming fine grained zne equiaxed in nature. Clumnar Zne Grains frm Chill Zne grw parallel t heat flw directin releasing heat t muld and t the centre, frming lng clumnar grains. Equiaxed Central Zne The central liquid slidifies with fresh nucleatin r nuclei derived frm chill zne due t cnvectin t frm carse equiaxed grains. Dendritic Grwth If ne lks int the inside f a grain a tree like structure can be seen which are dendrite arms. Crss sectin f a grain wuld reveal cells which are sectins f dendrite arms. Dendritic grwth ccurs due t the fact that heat is taken away frm crners and edges f cubic nuclei f aluminium. EQUILIBRIUM DIAGRAM (Fig.6, 7a & 7b) There are graphic representatin f temperature Vs cmpsitin indicating the phases, slidus/liquidus temperatures etc. e.g. Al-Cu, Al-Si. These diagrams are btained thrugh thermal analysis primarily in liquid t slid transfrmatin. Fr slid - slid transfrmatin varius ther methds like dilatmetry, metallgraphy, resistivity etc. arc emplyed t get. the equilibrium diagrams. The H-4

5 slidificatin prcess can be fllwed with the help f au equilibrium diagram in a hmgeneus ally system. The varius phases are designated as a. 0. y. 8. etc. Lever Rule Lever Rule will give the fractin f a particular phase. Eutectic:A liquid transfrms t tw slids at. a fixed temperature. Al 12.5%Si, A1-33%Cu. Refer Fig.7a. A-C-beginning f crystallizatin. Grwth f aluminium rich primary crystal (a S.S.) C-D-Simultaneus grwth f tw crystal types frming eutectic netwrk arund primary aluminium(a) E-D--Pure eutectic crystallizatin D- End f crystallizatin. Mdificatin Prcess fr refining the micr-structure thrugh additin f a third element befre casting an ally e.g.. Na fr hyp & eutectic Al-Si allys and P fr hyper eutectic 0 fir Al-S Allys. By mdificatin the mechanical prperties -ire significantly enhai iccd. GRAIN REFINEMENT Refining the grain size by hetergeneus r hmgeneus nucleatin. Fine grain size is desired fr excellent. rm temperature mechanical prperties. TYPES OF EQUILIBRIUM DIAGRAMS Simple Eutectics Be 11B Si 1VB Zn IIB Ga IIIB Ge IVB Sn 1V13 Fig 1113 Partly Miscible (Mntectie system,n Knwn Intermetallics are frmed with these elements) Cd 1113 In TA IVA Pb IBB Bi VB Na la K IA Rb IA CS IA Lanthanides, actinides are miscible in liquid state and frm cmplex binary systems with intermetallics. I-1-5

6 - Slid slutin with peritectics at Al end f phase diagram Ti IVA V VA Cr VIA Zr IVA Cb VA M VIA 11f IVA Tn VA W VIA All ther metallic elements are cmpletely miscible in liquid state f aluminium Si is cmpletely miscible in liquid Al B has nly 0.02% slubility in liquid Al C slightly sluble P & As insluble S is appreciably sluble SE & Te cmpletely miscible. Except 1-12 cmmn gases are nte sluble in aluminium. N element is cmpletely miscible in aluminium in slid state Zn has the greatest S.S. (66.4 at%) Ag, Mg, Li have greater than 10 at % slid slubility. HARDENING PROCESSES IN ALUMINIUM/ALUMINIUM ALLOYS Wrk hardening Ally hardening - Slid Slutin hardening (used in nn heat treatable allys) - Precipitatin hardening (used in heat treatable allys) Hardening phase Heat treatable Al-Mg-Si (6xxx) Mg2Si) Al-Cu (2xxx) - CuAl2 Al-Zn-Mg (7xxx) - MgZn2 Nn-heat treatable - Al-Mg (5xxx_) Al (lxxx) Al-Mn (3xxx) Precipitatin Hardening (Fig.8) * Slutin Treatment (abve slubility curve) * Quenching (water) * Ageing - (NA,AA); NA - Natural Ageing AA - Artificial Ageing Precipitatin Hardening (Fig. 8) * Slutin Treatment. (abve slubility curve) H- 6

7 * Quenching (Water) * Ageing - (NA-AA) NA - Natural Ageing AA - Artificial Ageing Precipitatin Stages (Fig.t) Zne frmatin due t atmic vibratin and clustering f unlike atms (Al <--> Cu, Mg <--> Zn, Mg <-->Si). Strengthening due t hindrance t dislcatin mvement. BIBLIOGRAPHY 1. Altenphl; Aluminium viewed frm within Aluminium Verlag, Dusseldrf, D.A.Granger, Slidificatin structures f aluminium ally ingts, Vl.9, ASM Handbk, 1985, M.C. Flemings, Slidificatin Prcessing, McGraw Ilill, Aluminium - Prperties and Physical Metallurgy, Ed. Jhn E.Hatch, ASM J. Plmear, Light Allys - Metallurgy f light metals, 2nd Editin, 1981, Edward Arnld. 6. Micrstructural cntrl in aluminium Allys : Defrmatin. recvery and recrystallizatin. Ed. E.11cnry Chia and II. J. McQueen, Met.Sc., Key t Aluminium Allys, Cmpiled by W.I lufnagel, Pub. Aluminium Zentrale,

8 Al-Cu-Mg AI Mg si - Al-Zrt-My --1 Age hat Bening allys T Al -Zit Mg 6-11 VA0 4* Al-Si Al-Si-Cu. Casting allys Al War k - _I 1 liar d( tiny allys Fig,. altanitun (Intl( it frmrdinte shiffs bright fleflctln f incident light by etched grains. The bserver sees frm abve "steps" In the crystal structure revealed by etching In the (darn f hne valln, ench r,lpn In the crystal sralm-n reflects mte ft lss light Int the nye f Ihn bserver. I tin crystal n the fr tell rllects the light directly back at the surce and therefre seems dark. Whereas. the crystals n the far right reflects the light directly In the eye f the bserver and appears bright. The schematic sectin, perpendicular t the etched surface, shws fur crystals with their cryslallgtraphic planes parallel t the page. Angle f Incident light: 45" (M. Schenck. Fig.2a richert aluminum surface (99 99% Al). Electrn micrgraph. One can see that the etchant remved layers f atms, ne alter the ther, preferentially frm crystallgraphic: planes parallel t Ihn laces l lh elementary cube x H-8

9 heat \ 0' i1; eali scbiii # c II i)? ---\ 660 cr,rtidyil ig IIIL 11e(11 01' li isii 1 Clinglslidilicatin) and heating (melting) curves fr 99.99% aluminum. I wilitin ) nlitichtia (1 rig.2b in Ille 'trll : tvclm 1111)1 wn/ 1' the atms Aluminum ()tms in 11)e :,slid stilly: Mrns l'il\cil in lhe lattice - ( 0- (% 11011aVIOr f Flf01111 during crystallizatin f niumaium (I. n., during slidificatin f nn aluminum melt). Fig N,, b ' --- ' --( _1.. I.13 d... n 'r) i 0, (?, I?. 0 '. _ 1 d p 0....,_ 0,.,,0 \ t._ 0, (it r _., (.F)(- ri..-.y,-i 1 c!.._1- "Slidificalin frnl ", that is,11)e y lie t,ecil (Ind slid taumir HMI 1-1-9

10 Structure f a hypeutectic - ally (Hanemann-Schrader). An aluminum-10% cpper ally, eutectic arund larger aluminum-rich primary crystals (a-slid slutin). 550x, 1' ṙ "..)*:1*-1 /1,-- 1.3i t / 4.:- I 4i> (.4( \lb,.1-0" 1,1-0./...., ( L, s c.; ;A:. j\,.iis. :(, I,, ;' t '''. )I'.,... 3 ; ' Viii, I) ; "'"v :::., Vilj ;'14 3..:ki,'-7":( ' i 4"'s,,, ) )L-:*-) 1,.,f,,, Aluminum ally with 8% silicn. Hypeutectic ally with aluminum-rich primary crystals. 85x. Aluminum ally with 20% silicn. Hypereutectic ally with primary silicn crystals. Alusuisse micrgraph. See Figure 38 fr eutectic ally structure. 120x. Structure f sandcast aluminum-silicn allys. F i g 4 H-

11 nw,l,wrihmnillrmorlacmmw16,11 1,1114alitmlittmlblq.1,0Wcw:lillrllitIOnquIrdialw) T hr,ptilli nlmchmthwilywrimv,11,,d 1hrim,0 71,1,1111.;COmitilmcirAilhOpttitrO,mthl;willithlmlm 7nns, wncl;tily In Thu flpi/of Inclrri HUI 13)It1111t1:1( Wahl.; dirt N itr if 011,1:tiOn O1i1t(P;it010 Iltci Val crl!will. Fig. 5.. Slidificatin f a hmgeneus ally under equilibrium cnditins. Liquidus Temp. Slidus Temp. 53 liquid slid (hmgeneus sfruclui.e) liquid 4 ra Slidificatin f a hmgeneus ally under nnequilibrium cnditins. iquidus Temp. Q, Therel. I Slidus 1 < _ K3 k-. Temp. Acludl 1--- cb Slidus Su Slidificatin f a hetergeneus ally under equilibrium cnditins. The 13 crystals are slid slutin crystals, rich in the allying metal.. Observing crystallizatin with th aid f an equilibrium diagram. At ( :1111 pt).', 711.i111 he/crvneus.,;lit.y14.1) e Athwi 'ruin/qui it 7 A,

12 Liquidus Tempe ratu Slidus TeMpeTatun Time Fig.7a. Schematic slidificatin curve 0 mushy Temperature Fig.7b I. t t. r.1. t t t I Mass % Cpper. Relatin between equilibrium diagram and structure fr the aluminium-cpper system. Representatin f 4% cpper cntent.

13 H-13 5 Slutin treatment 450 Quenching $.4 w Artificial ageing Natural ageing Tx:\AALL (13 Fig.8. Schematic precipitatin hardening f A1-4Cu-2Mg (2024) ally C 0 0 C: O 0c 0000C O 000C Fig.9a 000C 6 C. Atmic arrangement: : (a) Pure Metal (b) Substitutinal Slid Slutin (Cu/Mg) (c) Interstitial Slid Slutin (H2 ) Tw freign crystals (hetergeneities). Fig.9b :,.. \.*Y,;21,-,.-1' v. N':,;,7 c,, ' `.",: s' 1.. V Grain undary, ' 1.. l'between tw Al grains, Atmic arrangement in a hetergeneus structure ) Al atm 0 freign atm.