Electron Beam Welding of Magnesium AZ91 D Plates

Transcription

1 Elctron Bam Wling of Magnsium AZ91 D Plats An invstigation into th impact of high cooling rats on microstructur an mchanical proprtis hlps trmin optimum wling conitions BY A. MUNITZ, C. COTLER, H. SHAHAM AND G. KOHN ABSTRACT. Magnsium AZ91D plats wr wl by an lctron bam (EB). Th intgrity of th wl spcimns was invstigat by X-ray raiography an ultrasonic tchniqus. Optical an scanning lctron microscopy wr us to invstigat th microstructural changs that occurr uring wling. Mchanical proprtis wr trmin by stanar tnsion tsts. It was foun that Mg AZ91D cast alloy can b EB wl with a ngligibl amount of iscontinuitis an with a minimal hat-affct zon (HAZ). Th rapi mlting an subsqunt rsoliification uring wling inucs microstructural rfinmnt, which incrass both strngth an uctility of th alloy. It was also obsrv th microstructur is finr with lss hat flux. Introuction Magnsium an magnsium alloys ar us in a wi varity of structural an nonstructural applications u to som uniqu proprtis such as thir low nsity an high spcific strngth an lastic moulus (Rfs. 1,2). As such, thy ar consir as avanc matrials for coping with nrgy consrvation an nvironmntal pollution rgulations. Thus, magnsium alloys ar us for parts in th automotiv an arospac inustris whr lightwight mtals ar n to minimiz wight or ruc th intrnal forcs at high acclrations. Howvr, th application of Mg alloys is limit by thir poor corrosion rsistanc an brittlnss. Mg-AZ91D, which contains approximatly 9 wt-% AI an 1 wt-% Zn, is wily us bcaus of its rlativly improv corrosion rsistanc. Nvrth- A. MUNITZ, C. COTLER an H. SHAHAM ar with Nuclar Rsarch Cntr-Ngv, Br Shva, Isral G. KOHN is with Rotm Inustris, Rotm Inustrial Park, Br Shva, Isral. lss, joining of magnsium parts, which may b crucial for ths applications, is limit for this alloy (Rf. 3). Thr ar svral wling tchniqus us for joining Mg alloys. Ths mthos inclu, among othrs, arc wling, friction stir wling (FSW) (Rf. 4) an high bam powr wling (lasr an lctron bam). Usually, aluminum contnt of up to 10 wt-% ais wlability by causing rfinmnt of th grain structur, whil a zinc contnt of mor than 1% incrass hot shortnss, which may caus wl cracking (Rf. 5). Gnrally, th suitability of a Mg alloy for EB wling is similar as in arc wling. Howvr, thr is a substantial iffrnc btwn gas tungstn arc wling (GTAW) an EB wling. Usually, GTAW involvs a high hat input, which causs a broa fusion zon an a vry larg hat-affct zon (HAZ). In contrast, EB an lasr bam wls, u to thir concntrat nrgy, crat only narrow fusion zons an hav larg pntration pths with minimal HAZ (Rfs. 6-8). In ths procsss, high cooling rats ar impos uring soliification (Rfs ). Ths high cooling rats chang th soliification path as KEY WORDS Elctron Bam Wling EBW Magnsium Alloys Mg Hat-Affct Zon HAZ Microstructur Cooling Rats compar to procsss involving slowr cooling rats. Sinc iffusion tims an iffusion istancs ar shortr, iffrnt phass may nuclat. A finr grain siz is usually associat with fastr cooling rats. Ths changs may ruc th problms involv in arc wling an bring about an improvmnt of th mchanical proprtis of th wl parts. Th prsnt work was aim at optimizing conitions for lctron bam wling of Mg AZ91D plats an obtaining a bttr unrstaning of th impact of high cooling rats on th microstructur. Exprimntal Magnsium AZ91D ingots of th composition list in Tabl 1 wr cast an soliifi in a prmannt mol, thn machin into plats of ithr 2 or 5 mm in thicknss. Aras fr of soliification pors wr chosn for wling using X-ray raiography an ultrasonic pulscho tchniqus. Aftr wling, th plats wr minimally machin on both sis. Th wl plats wr thn chck with both tchniqus. Raiography was prform with a mini-focus X-ray sourc using a 50-kV voltag an an 8-mA currnt. A fin-grain film was plac unr th plats at a istanc of 120 cm from th X-ray tub. Exposur uration was 18 s. Ultrasonic inspction was prform using watr immrsion. A singl 5-MHz transucr was oprat in a puls-cho mo. Th transucr was inclin at 22 g off th normal to th plat surfac so that shar wavs inclin at 50 g to th normal wr prouc insi th plat. Th plats wr linscann, with a rsolution of 0.2 mm. Each scanning lin was 0.5 mm apart from th othr. Th snsitivity of th systm was calibrat by comparing th ultrasonic imags to that of a machin 2O2-s I JULY 2000

2 A 1-2,I'' 42 I I tnsil spcimn thicknss 01o Typ. l" 10 ~-~ "-""~ I }..... L Fig Optical micrograph illustrating a cross-sctional viw that rvals th EB pntration an th rgion from which th tnsil spcimns wr manufactur. B Fig A -- Schmatic rawing of th tnsion tst spcimn; B -- schmatic rawing illustrating two (out of thr) tnsil spcimn typs us for analysis. cylinrical hol 1 mm in iamtr an 0.2 mm p that coul b tct clarly. In orr to fin propr lctron bam wling paramtrs, ba-on-plat wls wr prouc with th following wling paramtrs: vlocity, 10 mm/s; voltag, 6 kv; currnt, 1.5 or 5.5 ma focus on th surfac. Th wling conitions wr slct to yil full pntration using a minimum powr. Wling currnt of 5.5 ma was us for wling of 5-mm plats, an a wling currnt of 1.5 ma was us for th 2-mm-thick plats. To invstigat th impact of high cooling rats on th microstructur, Mg plat surfacs wr EB mlt using th following conitions: vlocity, 500 mm/s; voltag, 6 kv; currnt, 2-7 ma focus on th surfac. For microstructural xamination, th wl plats wr sction an mount in Baklit. Th spcimns wr thn polish using stanar mtallographic procurs up to a Y~-micron iamon past. Th spcimns wr thn tch in a solution compris of 5 ml actic aci + 5 g picric aci + 10 ml watr an 100 ml thyl alcohol for 20 to 80 s until th microstructur was rval. Th microstructural an compo- sitional analyss wr carri out using optical an scanning lctron microscopy (SEM). Th lattr was quipp with an nrgy isprsiv spctroscopy (EDS) attachmnt. For th EDS analysis, th spcimns wr tch vry lightly, just sufficint to rval th microstructur yt kp th surfac roughnss to a minimum. Th raw ata was corrct with a stanar ZAF computr program (Rf. 12). To avoi th graing influnc of porosity foun in th cast ingots on th tnsil strngth, only fct-fr spcimns wr us. Tnsil spcimns with 15 mm long an 4 mm wi gaug sctions wr machin out from th Mg plats, as illustrat in Fig. 1A. This typ of spcimn prparation allows on to machin from ach wling sampl thr tims as many spcimns than th numbr of sub-siz tnsil ASTM A 370 spcimns (60 mm long an 6 mm wi gaug sction with total lngth of 135 mm). Comparison of th rsults obtain using ths iffrnt typs of spcimns shows agrmnt within 5%. Thr typs of spcimns wr machin: 1) bas mtal in th as-cast conition; 2) spcimns containing th EB wl in thir cntr, as illustrat in Fig. 1B an mark with a subscript T; an 3) spcimns machin from th EB fusion zon, as illustrat in Fig. 2 an mark with a subscript L. Most spcimns wr 2 mm thick, xcpt for th spcimns manufactur from EB surfac-mlt aras, whos thicknsss rang btwn 1 an 1.4 mm, as scrib latr. To fin th mchanical proprtis of th B-mlt zon, th surfac of th Mg plats was covr with EB passs unr th sam conitions as in wling to a total with of 6-10 mm. Thr was an ovrlap of 20-50% btwn th passs. A cross sction of th mlt zon is shown in Fig. 2. Th thicknss of th tnsil spcimn was chosn so almost th ntir volum of th gaug sction was EB mlt, as shown by th ash lins in Fig. 2. To achiv this goal, th uppr surfac was machin to form a smooth surfac, thn th plat was machin from th othr si to th final thicknss accoring to th actual pntration pth foun by th cross-sctional mtallography -- Fig. 2. Each ata point rprsnts an avrag of at last 2-3 spcimns, which wr tnsion to fractur in a hyraulic tnsion machin at a cross-ha vlocity of 2 x 10.2 mm/s. Fractur surfacs wr xamin with a scanning lctron microscop. To improv rsolution by curbing oxiation, th fractur surfacs wr gol plat. For comparison, tnsil spcimns wr also machin from gas tungstn arc an friction stir wl plats (for tails on th lattr tchniqu, s Rf. 4). Sinc th wl was wir than 10 mm in both tchniqus, no problms wr ncountr in machining th L-typ spcimns from th wl itslf by th sam procur. Rsults an Discussion X-Ray Raiography an Ultrasonic Obsrvation Ultrasonic xamination of th Mg plats prior to wling rval shrinkag porosity of th orr of 20 pm, whil X-ray raiography fail to tct ths fcts (for mor tails s Rf. 13). It was foun th ara nar th chill, up to 30 mm from th bottom chill, is fct fr. All plats wr, thrfor, wl using spcimns prpar from this si WELDING RESEARCH SUPPLEMENT J 203-S

3 Fig Ovrall macrostructur of ba-on-plat wls on a 5-mm-thick Mg AZ91D plat. 13- iscontinuous prcipitation \ I~-MglTlAl,Zn)12 ~-Mg fusion zon: HA7 i j a b C Fig. 4 --Sconary lctron imags of a top viw of an EB wl at a currnt of 5 ma. A -- Top surfac ovrall viw; B -- as-cast microstructur; C an D -- th microstructur of th hat-affct zon far from an nar th fusion zon (circls c an mark in A), rspctivly; E -- fusion zon microstructur (circl mark in A). Tabl 1 -- Composition of a Typical AZ91D Ingot Us in th Prsnt Work Elmnt AI Mn Zn Si Cu Ni F B Mg Wt-% Bal. of th ingot. For both wling tchniqus, whn aras fr of shrinkag porosity wr us, th wl plats wr also fr of fcts such as cracks, vois or shrinkag porosity. Vois prsnt in th bas mtal almost always affct wl quality. Microstructural Obsrvation Ba-on-plat wls with varying wling paramtrs wr prform on a Mg alloy spcimn in orr to fin propr wling paramtrs. Th macrostructur of ths wls is shown in Fig. 3. In- crasing th currnt incras both th with of th fusion zons an that of th HAZ -- Tabl 2. Sconary lctron imags (SEI) illustrating th microstructur of a typical ba-on-plat wl an its vicinity ar shown in Fig. 4. A top ovrall viw is shown in Fig. 4A. Mg AZ91D contains 0.24 an 0.7 wt-% of Mn an Zn, rspctivly. Ths aitions o not chang th major phass of th bas mtal or of th wl. Thrfor, th Mg- AI binary phas iagram (Rf. 14) prsnt in Fig. 5 will b us to iscuss th iffrnt phass obsrv in th microstructur. Accoring to this phas iagram, bfor wling th bas mtal consist of AI12Mg17 (1~ phas, spac group 143m, a-mn prototyp) mb in btwn a-mg nrits -- Fig. 4B. Enrgy isprsiv spctroscopy (EDS) inicats th a-mg phas contains up to 11.3 an 1.2 at.-% of AI an Zn, rspctivly, whil th avrag a-mg contains only 5.0 an 0.5 at.-% of AI an Zn, rspctivly. This bhavior is typical of th coring ffct pricat by Schil's quation (Rf. 15). Th g phas contains about 30 an 2.7 at.-% AI an Zn, rspctivly. During cooling in th prmannt mol, a iscontinuous prcipitation (Rf. 16) occurr, as mark in Fig. 4B. Th [~ phas prcipitats in th soli stat from th suprsaturat a-mg lattic an appars having a "prlit" morphology. Gnrally this prcipitation procss bgins at grain bounaris an procs into th suprsaturat a-mg phas. Upon approaching th HAZ, on coul obsrv that, u to th hating ffct, th g iscontinuous prcipitats bgin to issolv in th Mg matrix -- Fig. 4C. Closr to th fusion zon, th Ig phas also bgins to issolv -- Fig. 4D. A nritic microstructur, yt with a rfin scal, was obsrv insi th fusion zon -- Fig. 4E. Sconary lctron imags (SEI) illustrating th microstructur of an EB wl prform at maximum vlocity with a bam currnt of 3 ma ar shown in Fig. 6. In Fig. 6A, a gnral viw of th spcimn is shown, monstrating th pntration of th moltn pool into th substrat. Th substrat microstructur is shown in mor tail in Fig. 4A an was scrib in tail in th prvious sction. Figur 6B shows th wl intrfac btwn th lctron bam fusion zon an th substrat at th maximum pntration pth an protrusion of th unmlt AI12Mg17 intrnritic phas into th moltn pool. About lam from th wl intrfac, AI-rich bans (as 204-s I JULY 2000

4 Tabl 2 -- With of th Fusion an Hat- Affct Zons as a Function of EB Currnt for Ba-on-Plat Wls on 5-mm-thick Tabl 3 -- Sconary Arm Spacing as a Function of Elctron Bam Currnt Mg AZ91D Plats Bam Currnt DAS Cooling Rats (ma) ([am) (K/s) EB Moltn HAZ Currnt Zon With With , (ma) (mm) (mm) ~ x 10, x 10, , x 10-' x 10~ x 10: Wight Prcnt Magnsium ( OO i oc ' 6'50oC I bas EB fusion HAZ mtal zon! / s c~ o@$ s Tabl 4 -- Sconary Arm Spacing as a Function of Local Soliification Tim for Mg AZ91 D (Aftr Bambrgr, t al., Rf.19) DAS ([am) Local soliification tim (s) ~:~~ ool/ II AI Atomic Prcnt Magnsium Mg s~4 200 llnl fll Z~ b Fig Th binary Mg-AI phas iagram (Rf. 14). compar to th avrag composition) wr obsrv in th fusion zon -- Fig. 6D. Th original AI~2Mg ~ 7 phas mlt but i not hav sufficint tim to unrgo long-rang mass transport u to th short local soliification tim. Th svr convction xisting in th moltn pool might caus long-rang mass transport crating ths compositional bans. Howvr, u to th vry short soliification tim in th zon nar th wl intrfac, th mlt rsoliifi bfor longrang iffusion coul occur. At a istanc of [am from th wl intrfac, a homognous mlt was crat u to th rlativly longr soliification tim, which allows for convction an iffusion to homogniz th mlt. Th microstructur of th cntr of th fusion zon is sn in Fig. 6E to 6F at two iffrnt magnifications. Du to th high cooling rats obtain uring soliification, microstructural rfinmnt was obsrv. Anothr outcom from th short mlting an soliification tims of th fusion zon is a vry limit imnsion of th hat-affct zon (HAZ). As can b sn in Fig. 6C, only a 30- to 50-pm-wi HAZ 12 Fig Sconary lctron imags illustrating an lctron-bam-mlt surfac of Mg AZ91D plat. A -- Ovrall wl cross sction viw; B -- microstructur of th wl intrfac at maximum pntration; C-- hat-affct zon nar th wl intrfac (circl c mark in A); D -- microstructur at maximum pntration (circl mark in B); E an F-- microstructur at fus ara cntr (circl mark in A). has bn crat. Sinc th mlting point of th AI 12Mgl 7 phas is about 450 C, as compar to about 600 C for th mlting point of th bas alloy, th AI12Mg17 phas in th HAZ might mlt uring wling an crat moltn pockts. Ths f moltn pockts may issolv som of th surrouning Mg matrix. Thrfor, upon rsoliification of ths moltn pockts, a broar hollow AI12Mg17 phas is crat, as sn on th right-han si of th photographs in Figs. 5D an 6C. WELDING RESEARCH SUPPLEMENT I 205-s

5 %, 100, lc "%. It b o C3 ~ lain! c.,,,,,j,,,,,..,i,,,,..a,,,,,,,. t.*.,,,,...i cooling rats Fig DAS as a function of cooling rat aftr Bambrgr, t al. (Rf. 19). Th ash lin is an xtrapolation of th curv to high cooling rats f EBJ. Fig Sconary lctron imags illustrating th microstructur rsulting from various wling currnts in ba-on-plat xprimnts. A -- 3 ma; B -- 5 ma; C ma; D ma; E ma; F ma. ~ ' Sconary lctron imags illustrating th microstructur of EB ba-onplat rsoliifi zons for various bam currnts ar shown in Fig. 7. As th bam currnt incras, largr sconary nrit arm spacing (DAS) was obsrv. A summary of th DAS as a function of th bam currnt is givn in Tabl 3. In gnral, th finnss of microstructur uring nritic soliification can b irctly rlat to th local soliification tim, tf, or to th avrag cooling rat uring soliification, avg (Rf. 17) = avg -a = b tf n (1) avg = AT/tf (2) whr AT is th soliification tmpratur rang, is th sconary arm spacing an a, b an n ar constants. Sinc th mlting tmpratur of Mg AZ91D is approximatly 600 C an th mlting tmpratur of th trnary utctic (Rf. 18) is about 430 C, w obtain AT = 170 C. Bambrgr, t al. (Rf. 19), has publish th ata givn in Tabl 4 for th DAS as a function of th local soliification tim. Using th ata of Tabl 4 an assuming 170 C as th soliification 100 rang, on may plot th DAS as a function of cooling rat, as shown in Fig. 8. If w assum on can xtrapolat th curv to 0" th limit of larg 0 cooling rats, on may also obtain th cooling rats uring EB wling (th cooling rats obtain in this procur ar also givn in Tabl 2). Cooling rats as high as 106 K/s coul thrfor xist uring EB wling. Mchanical Proprtis A summary of th mchanical proprtis for th various EB-mlt spcimns as compar to thos of th as-cast spcimns is givn in Tabl 5, an typical strss/strain curvs ar prsnt in Fig. 9. For comparison, in Fig. 9 an Tabl 5, w also inclu ata takn from friction stir an gas tungstn arc wl plats "ng-t, I i I I I i I, I ELONGATION [%] Fig Strss/strain curvs of various Mg-AZ9 I D spcimns. (for mor tails s Rfs. 4 an 13). As sn from Tabl 5 an Fig. 9, th yil strss, th ultimat tnsil strngth an th uniform longation of spcimns prpar from EB wling incras markly, as compar to thos of th ascast spcimns. Thr is, howvr, a mark iffrnc btwn th proprtis of th GTA-wl an thos of th EB-wl spcimns. As scrib in th work of Munitz, t al. (Rf. 13), th microstructur of th GTA wl matrial is similar to th microstructur of th bas mtal, but with slightly smallr in- 206-s I JULY 2000

6 trnritic arm spacing. Thrfor, th strss/strain curv of th GTA-wl matrial is similar to that of th bas mtal, with almost th sam yil strngth. Howvr, bcaus of th finr microstructur in th wl, it continu to work harn with th sam slop as that of th bas mtal, hnc th highr aut s an largr uniform longation. On th othr han, th microstructur of th EB surfac mlt is much finr, hnc th aitional incras in th yil strss an uniform longation, as compar to thos of th GTA-wl spcimns. Anothr iffrnc btwn th GTAwl an EB-wl spcimns is th much smallr HAZ in th lattr. This in turn markly changs th HAZ proprtis. Dgraation of th HAZ mchanical proprtis in spcimns that inclu th wl in th gaug lngth (not with T, Fig. 1 ) was obsrv (Tabl 5 an Fig. 9). All spcimns that ha th HAZ in thir gaug fail, whil xhibiting fractur in th HAZ itslf. W attribut this fact to th occurrnc of continuous I~-phas rgions along grain bounaris (Rf. 13). In contrast, th HAZ in EB wling is vry small, an th tnsil spcimns nvr Tabl 5 -- Summary of Mchanical Proprtis of As-Cast, Friction-Stir Wl, Gas Tungstn Arc Wl an EB Surfac Mlt Mg-AZ91 D Strss Elongation Sampl Yil UTS Uniform Total (Mpa) (Mpa) (%) (%) Litratur (Rf. 3) F tmpr As-cast without por with pors GTAW T spcimn L spcimn Friction stir wling-fsw EB surfac T spcimn mlt L spcimn fail in th HAZ, as illustrat in th top viw of tnsil spcimn microstructur aftr tnsion, which is shown in Fig 10. Sconary lctron imags illustrating th fractur morphology of as-cast spcimns as compar to thos of EB surfac-mlt spcimns ar prsnt in Fig. 11. Th fractur surfac of th ascast spcimns is mostly intrgranular, charactristic of a brittl fractur (Fig. 11A an B). Minor aras in th fractur surfac xhibit transgranular impl ruptur, charactristic of a uctil frac- tur -- Fig. 1 lb. W attribut ths rgions to th utctic rgions xisting in th as-cast matrial -- Fig. 4B. In contrast, th fractur surfac of EB-mlt spcimns is much finr. At low magnification, th fractur looks brittl -- Fig. 11C. Howvr, at largr magnifications, a impl microstructur is rval. This fin microstructur xplains th improvmnt in mchanical proprtis of th EB-surfac-mlt spcimns. It shoul b not in th fractur surfacs of th EB-mlt spcimns, som El a b c Fig Optical micrograph illustrating a top viw of a tnsion spcimn showing fractur outsi HAZ. Fig Sconary lctron imags illustrating th fractur microstructur. A an B -- Th as-cast; C-E -- lctron-bam-mlt surfacs. WELDING RESEARCH SUPPLEMENT I 207-S

7 rsiual original as-cast fractur aras occur. Sinc ths aras ar rlativly brittl, thy avrsly affct th mchanical proprtis of th spcimns. It was ifficult to trmin th fraction of volum that has not bn mlt by th EB. It was, howvr, rasonabl to assum th two EB surfac-mlt spcimns contain iffrnt amounts of rsiual as-cast microstructur. This may xplain th iffrncs in th mchanical proprtis btwn th two surfac-mlt spcimns. Summary Mg AZ91D plats of 2-5 mm thick wr succssfully lctron bam wl unr vacuum with no visibl fcts in ithr th wl or hat-affct zons. Du to th high cooling rats involv in EB wling, a minimal hat-affct zon with rfin microstructur, compar to that of th as-cast matrial, was obtain. This in turn incras th ultimat tnsil strngth, as wll as th uniform longation. Cooling rats up to 106 K/s wr stimat to xist uring EB surfac mlting of Mg AZ91 D. Acknowlgmnts Th authors wish to thank Z. Burshtin for his critical raing of th mansucript an G. Angl for his tchnical assistanc. Rfrncs 1. Hanbook of Trnary Alloy Phas Diagrams, Es. P. Villars, A. Princ an H. Okamoto p ASM Intrnational, 2. L, S., L, S. H., an Kim, D. H Effct of Y, Sr, an N aitions on th microstructur an microstructur mchanism of squz-cast AZ91 -X magnsium alloys. Mt. an Matr. Trans. A 29A: ASM Hanbook, Vol. 2, S. Housh, B. Mikucki an A. Stvnson Proprtis an slction: nonfrrous alloys an spcialpurpos matrials, p ASM Intrnational, Matrials Park, Ohio, 4. Kohn, G., Antonsson, S., an Munitz, A Friction stir wling of magnsium alloys. Procing of th 12th TM5 Annual Mting an Exhibition, San Digo, Calif., an also prsnt at th National Wling Confrnc, Isral. 5. Mtals Hanbook, 9th Eition, Vol. 6, E. F. Nipps, Coorinator Wling, brazing, an solring, p ASM Intrnational, 6. Wishit, A., Galun, R., an Morik, B. L CO 2 lasr bam wling of magnsium-bas alloys. Wling Journal 30: 149-s to 154-s. 7. Drauglats, U., Bouaifi, B., an Brozk, J Simulation of lasr bam wling of magnsium alloys. Proc. Th First Israli Intrnational Conf. on Magnsium Scinc an Tchnology, Es. E. Aghion an D. Elizr, pp , Da Sa, Isral. 8. Drauglats, U., Bouaifi, B., Stinborn, S., Schmi-Ftzr R., an Pisch A Proc. Th First Israli Intrnational Conf. on Magnsium Scinc an Tchnology, Es. E. Aghion an D. Elizr, pp , Da Sa, Isral. 9. Munitz, A., an Abbaschian, R Microstructur of Cu-Co alloys soliifi at various suprcooling. Mt. an Matr. Trans. A 27: Munitz, A., Elr, S., an Abbaschian, R Soliification of Unrcool Cu-10 wt-% Co alloys. Mt. Trans. A 23A: Munitz, A., an Abbaschian, G. J Soli stat transformation in F-Ni suprcool alloys. ]. of Matrials Scinc 26: R, S. J Elctron Microprob Analysis. Cambrig Univrsity Prss, Cambrig, U.K., pp Munitz, A., Cotlr, C., Strn, A., an Kohn, G. Mchanical proprtis an microstructur of gas tungstn arc wl magnsium AZ91 D plats. Submitt for publication in J. of Matrials Sci. 14. Binary Alloy Phas Diagrams, Vol. 1, Es. T. B. Massalski, J. L. Murray, L. H. Bnntt an H. Bakr, pp ASM Intrnational, 15. Kurz, W., an Fishr, D. J Funamntals of Soliification, p Trans Tch Publications. Switzrlan. 16. Portr, D. A., an Eastrling, K. E Phas Transformations in Mtals an AIIoys, p Van Nostran Rinhol, Grat Britain. 17. Mhrabian, R Rapi soliification. Intrnational Mtals Rviws 27(4): Mtals Hanbook, Vol. 2, 9th Proprtis an slction: nonfrrous alloys an pur mtals, pp ASM Intrnational, 19. Avishi, A., an Bambgr, M., Rlation btwn cooling rats an microstructurs in gravity-i-cast AZ91 D isks. Mtallurgical an Matrials Transactions B 30B: slJULY2000