Effect of Magnesium on 380 Alloy

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JOU RNA L OF MATERIALS SCIEN CE 30 1995) 2531-2540

Effec t o f magnes ium conten t on the age ingb e ha v iou r o f w a t e r-c h illed A I -S i -C u -M g -F e -M n 380 )alloy castings

F. H . S A M U E L , A . M . S A M U E L , H . L IUDe partem ent des Sciences AppliquOes, Un iversite du Quebec g Ch icoutim i, 555, Bo ulevardde I UniversitO, Ch icoutim i, Quebec, C anada G7H 2B1

A s tudy o f the e ffec t o f magne s ium concen t ra t ion on the age ing behav iour a s measured bythe hardness of 380 a l loy was conducted for three levels of magnesium, namely 0 .06 (basealloy), 0.33 and 0.5 w t , for wa ter-c hil led c asting s (den drite arm sp acing ,-~ 10-15 lam).Different ia l scanning calor imetry analys is of as-cas t samples was carr ied out to determinethe changes in the react ions of the phases obta ined dur ing a l loy sol id i f ica t ion, em ploy ingheat ing ra tes of 0 .1 and 1 .0~ -1, up to app rox ima tely 700~ Tw o heat t rea tm ents we re

appl ied to the as-cast a l loys: T5 com pris in g ageing a t 25 ( room tem perature ) , 155, 180, 200and 220~ fo r t imes u p t o 200 h , and T6 com pr i s ing so lu t ion hea t t r ea tmen t at 480~ o r515 ~ fo r 8 h , fo l lowed by quench ing in warm w ate r at 60 ~ fo l lowed by imm edia te a r t if ic i a lageing a t 155 or 180~ for varying t im es up to 100h. The resul ts show that the high erhardness values obta ined w i th T6 t reatm ent can be expla ined by the excess precipi ta t ion ofma gne sium -con ta ining phases in the as-sol id if ied a l loys . This precipi ta t ion could beel imina ted unde r the high cool ing-ra te con di t ions prevalen t in d ie-cas t ing operat ions so thatT5 t reatm ent m ay be used to replace T6 t reatm ent to prod uce the sam e hardness values , inaddi t ion, solut ion heat t rea tm ent in the low -tem pera ture range (480-515~ is adequate toproduce the requ i red changes in s i li con m orph o log y and d i s so lu t ion o f ma gnes ium in themat r ix . No s ign i fi can t d i ffe rence in ha rdness behav iour was obse rved w hen the m agnes iumconten t was inc reased beyond 0 .3 wt .

1 I n t r o d u c t i o n

O n e o f th e m o s t w i d e l y u se d a l u m i n i u m d i e c a s ti n ga l loys i n t he au tom ot ive ind us t ry i s t he 380- type a l loy,be long ing to t he A1-S i -Cu f ami ly. M an y f ac to r s a ff ec tthe m echan ica l p rope r t i e s o f t hese a l l oys, inc lud ingthe a l l oy ing e l emen t s and the i r conce n t r a t ions , andt h e a p p li e d h e a t t r e a t m e n t [ 1 - 3 ] . M a g n e s i u m , i n p a r -t icu l a r, a id s i n enhan c ing the s t r eng th o f t hese a l l oys ,bu t a t t he expense o f duc t il i ty. Wi th a h ighe r ma gne -

s ium co n ten t i n s t an da rd 380 a l loys ( i .e . above0 .1 wt ) , and wi th the ' app l i ca t ion o f a su i tab l e hea tt r ea tmen t , a l loys w i th h ighe r s t r eng th and ha rdnesscan be p roduced , hav ing be t t e r wea r r e s i s t ance[4 ] . The hea t t r e a tm en t can invo lve e i the r a l ow-t empera tu re age -ha rden ing (T5- type ) o r a so lu t iont r ea tm en t + age -ha rden ing (T6- type ) p roces s, t ha t a r epa r t i cu l a r ly su i t ab l e fo r a l l oys con ta in ing0 . 4 - 0 . 6 M g , t h e u n d e r l y i n g m e c h a n i s m b e i n g t h efo rma t ion o f MgzS i , a s t rong ha rden ing cons t i t uen t .How ever, i n t he ca se o f 380- type a l l oys , i nc reas ing them a g n e s i u m c o n t e n t m u s t b e c o n s i d e r e d i n c a r e fu l c o n -junc t ion w i th t he coppe r c on ten t o f t he a l l oy, a s cop -pe r i s a l so a s t rong ha rden ing cons t i t uen t [4 ] . Thein f luence o f t he a l l oy ing e l emen t s on the s t ruc tu ra ld e v e l o p m e n t a n d h e n c e o n t h e p r o p e r t i e s c a n b ee l u c id a t e d f r o m a n e x a m i n a t i o n o f t h e r e s u lt i ng

m i c r o s t r u c t u r e s [ 5 ] a n d b y t h e p o p u l a r m e d i u m o fthe rm a l ana lys i s , whe reby the so l id i fi ca t ion p roces s o fthe a l l oy can be m on i to r ed [6J .

The p re sen t s tudy was ca r r i ed ou t t o de t e rmine thee ffec t o f ma gnes ium con cen t r a t ion on the age ing be -hav iou r, a s me asu red by the ha rdn ess , o f 380 a l loy a tthree levels of m agn esium , 0 .06 (base a l loy) , 0 .33 a nd0 .5 wt , u s ing wa te r- ch i l l ed ca s t ings (dendr i t e a rmspac ing ~ 10 -15 gm) . The r e su l t s a re p re sen ted an d

discussed.

2 Experimental procedureThe base 380 a l loy was supp l i ed by Alcan In go t A l loysC a n a d a , G u e l p h , O n t a r i o , w i t h t h e c h e m i c a l c o m p o s i -t i on shown in Tab le I . The a l l oy was supp l i ed in t heu n m o d i f i e d f o r m . A b o u t 6 k g o f t h e m a t e r i a l w e r eme l t ed a t a t ime , and to each me l t , me asu re d quan t i t -i es o f m a g n e s i u m w e r e a d d e d i n t h e f o r m o fA1-50 wt M g ma s te r a l l oy to ob ta in fi na l ma gne -s ium levels of 0 .33 and 0 .5 wt . Each m el t wa s de-gas sed a t 740~ fo r 20 min by pas s ing d ry a rgonth rou gh a l ance . The hy drog en l eve l o f t he me l t a f t e rd e g a s s i n g w a s f o u n d t o b e a b o u t 0 . 1 0 m l / 1 0 0 g A 1 ,us ing an Al scanT hydrogen ana lyse r. No me l t t r ea t -m e n t s u c h a s m o d i f i c a t i o n o r g r a in r e f in e m e n t w a s

0022-2461 9 1995 Chapman & Hall 2531



T A B L E I Chemica l composi t ion of the as - rece ived 380 base a lloy

S i C u M g F e M n Ti Z n N i C r A 1

Specified conc. (wt ) 7.50- 9.50 3.00- 4.00 0.10 1.00 1.30 0.50 - 3.00 - Bal.

Ch em ical An alys is (wt ) 9.18 3.22 0.06 1.01 0.16 0.02 2.28 0.04 0.03 Bal.

110 ....25 mm

(a)

i s e r ( I

Watern l e t

r / ~ ~ 1 1 5 m m2 1 0 m m ~ ould ~ [

V A

Weter o otle t Wa te r ~ - ~ a s m ~L tank [ S t a n d /

= 310 mmb)

Figure 1 Schemat ic d iagram of the se t -up used for the water-ch i l ledcastings. (a) Top view, (b) section view.

used a s t hese tr ea tm en t s a r e n o t gene ra l ly requ i r ed ind ie ca s t ing ope ra t ions wh ere t he h igh coo l ing r a tei t se l f p rov ides t he neces sa ry s t ruc tu ra l r e f inemen t .

The degassed ma te r i a l was poured in to a g raph i t em o u l d o f 5 c m d i a m e t e r , m a i n t a i n e d a t 4 00 ~ ( t a k e nas t he r e f e rence ) , and a r ec t angu la r coppe r mou ldp laced in a t ank o f runn ing co ld wa te r ( a s chema t i cd i ag ra m o f t he app a ra tu s i s show n in F ig . 1). Th i sa r r angemen t pe rmi t t ed two d i f f e r en t coo l ing cond i -t i ons t o be ob ta ined .

Powder s fo r d i f f e r en t i a l s cann ing ca lo r ime t r i c( D S C ) m e a s u r e m e n t s w e r e p r e p a r e d f r o m t h e a s -

sol id i f ied mater ia ls ( refer red to he reaf ter as 380 (baseal loy) , 380 + 0 .33Mg, and 380 + 0 .5Mg, and corres-po nd ing to 380 a l loy conta in ing 0 .06, 0 .33 and0 .5 wt Mg , r e spec tive ly ) and exam ined in a D uP on t2100 app a ra tus u s ing d i f fe r en t hea t ing r a t e s (0 .1 and1 ~ s - a ) . DS C runs were in i ti a t ed a t room t emper-a t u r e a n d c o m p l e t e d a t a p p r o x i m a t e l y 7 0 0 ~ T h eo u t p u t w a s i n m W, a n d t h e n e t h e a t f l o w t o t h ere fe rence (h igh -pu r i ty anne a led a lum in ium) r e l a t ive t othe sample w as r eco rded a s a func t ion o f t empe ra tu re ,w i th t he base l i ne sub t r ac t ed f rom the da t a .

H e a t t r e a t m e n t s w e r e p e r f o r m e d i n a n a i r - f o r c e dove n ( _+ 1 ~ Sam ples were cut f rom the water-ch i l -l ed ca s t b locks no rm a l t o t he so l id i f i ca t ion d i r ec t ionand h ea t ed a t 0 .5 ~ s - 1 up to t he quench ing t emper-a tu re . Two so lu t ion t empera tu re s were used , 480 and515~ keep ing the so lu t ion t ime cons t an t a t 8 h ,

2532

fo l lowed by quench ing in ho t wa te r a t 60 ~ As -cas ta s we ll a s so lu t ion - t r ea t ed sam ples were na tu ra l ly andar t i f ic ia l ly aged a t 25 , 155, 180, 200 and 220 ~ forva ry ing t imes up to 200 h . The sa me oven was em -ployed for the ar t i f ic ia l ageing.

M ic ros t ruc tu ra l de t a il s o f p repa re d samples ( s amples ize 7 m m x 10 m m x 60 r am, p rope r ly po l i shed toob ta in a smoo th su r f ace ) were ana lysed us ing anO l y m p u s P M G 3 o p t ic a l m i c r o s co p e . H a r d n e s sm e a s u r e m e n t s w e r e m a d e o n b o t h s i d e s o f e a c hsample us ing a Br ine ll ha rdness t e s t e r (500 kg cha rge

and 10 mm d iam e te r s t ee l ba ll s) . An ave rage o f a t l e a s ts ix r ead ings were t aken fo r each r eco rded measu re -men t .

3 R e s u l t s a n d d is c u s s i o n3 1 S o l i d if i c a ti o n a n d r e m e l t in g

c h a r a c t e r i s t i c sFig . 2 shows the r ad iog ra ph o f a p l a t e s ample t ak enf rom a 380 _+ 0 .5M g a l loy cas t ing . The p l a t e con ta insa h o m o g e n e o u s a r e a t o w a r d s t h e c e n t r e o f th e c a s t i nga s in d i c a t e d b y t h e u n i f o r m g r e y m a t t a p p e a r a n c e . T h ec i r cu l a r fl ow pa t t e rn on the p r in t i s p robab ly causedby the tu rbu len t f l ow o f t he me ta l i n to t he co ld m ou ld( ~ 7 ~ dur in g cas t ing .

F rom pub l i shed r e su l t s I -6 -8 ] , t he expec ted se -quence o f reac t ions t ak ing p l ace fo r t he 380 base a l l oyat s low cool ing ra tes ( ~ 0 .2 ~ s -1) i s:

1 . deve lop me n t o f dendr i t i c ne two rk (~ -a lumi -n ium) : 578-577 ~

2. prec ip i ta t ion of [3-AlsFeSi i ron phase:572 564 ~

3 . ma in eu t ec t i c r eac t ion invo lv ing s il icon - an d~-Al ls (Fe , Mn)3Si2 i ron phase: 561-559 ~

4. pre c ip i ta t io n of Mg2Si : 559-491 ~

5 . p rec ip i t a t i on o f A lzCu : 491-487 ~6 . fo rm a t ion o f complex eu t ec t i c con ta in ing A12Cu

a n d A l s M g a S i 6 C u 2 : 4 8 5 - 4 7 9 ~The coo l ing cu rves ob ta ined fo r t he 380 ,

380 + 0 .33M g, and 380 + 0 .5M g a l loys a r e show n inF ig . 3a and b fo r g raph i t e a nd me ta l l i c mou lds , r e spec -t ive ly. The cu rves f ai l t o show the peak co r r e sp ond ingto the M gzS i phase . The r e su lt s show tha t i nc rease i nthe ma gnes ium con te n t up to 0 .3 w t does no t a ff ec tthe so l id if i ca t ion behav iou r. The eu t ec t ic t emp era tu rec h a n g e d b y a p p r o x i m a t e l y 1 ~ f o r e a c h 0 .1 w t M gadded . The coppe r phase t empera tu re (Reac t ion 5 ) i sobse rved to be sh i f t ed to a l ower t empera tu re . Thee ffec t o f m agne s ium c on ten t on the eu t ec t i c and pos t -eutec t ic reac t ions i s l i s ted in Table I I , whi le Table I I Ishow s the sol id i f ica t ion chara cter is t ics of the 380a l loys w i th d i f f e r en t magnes ium concen t r a t ions .



600

v

I3

580

560

540

520

500

480

46

440 t__0

( a )50

\\,

\\

\\

100 150 200 250 300 350 400 450

Ti m e ( s )

600

Figure Radi ograp h o f 380 + 0.5Mg alloy, arrow indicates thesolidif icat ion direct ion.

580

560

540v

520

o_E~ 500

480

F ig . 4 a - c a re t y pi ca l di ff er en ti al sc an ni ng 46o [ -ca lor imetry (DSC) p lo ts obta ined for the three 380

lloys (contain ing 0.06, 0.33 and 0.5 wt Mg), respec-t ively. Thes e al loys were solidified at 0.4 ~ s-1 and 440 2

rehea ted at 0.1 ~ s- 1. Tab le IV indicates the temp er- (b)a ture o f format ion of var ious prec ip i tates . Thedata reveal tha t bo th the Mg2Si an d A lzCu +

A15MgsSi6Cu2 reactions are shifted to lower temper-a tures when the magnes ium level i s increased to0 .33 wt , wi th an increase in the mel t ing temp era turerange . The la t te r i s caused by the depress ion in thestart-of-melt ing temperature, ls , and increase in theend-of-mel t ing tempera ture , l f . Fur ther increase inmag nes ium conten t, i .e . up to 0.5 wt , does not seemto have a significant influence on the melt ing charac- Cooling Mgteris tics of this allo y, rate (wt )

Fig . 5a-c are the DSC plo ts prod uced f rom the (ocs-1)same samples at a heat ing rate of 10 ~ s- 1. Th e mainobse rvatio ns to be no ted from these plots are: ~ 0.4 0.06

(a) al l reactions are shifted to higher temp eratur es, 0.330.50cont rary to the so l id i f ica t ion behaviour observed/

men tion ed above , and which could result from (b); ~ 10 o.o6(b) bo th A12Cu and Mg2Si reactio ns are join ed in o.33

0.50a cont inuous peak ( in some cases , the Mg2Si peak

\ \

\ \\\ \

\ .,, ,\ \

4 6 8 10 12 14 16

Ti m e ( s )

Figure 3 Coolin g curves obtained for the three al loys, (- -) 0.33,

( . ) 0.5 and (-- ) base 0.06 wt Mg, at two different coolingrates: (a) 0.4 ~ (b) 10~ -I .

TA BL E I I Effec t o f magnes ium conten t on the eu tect ic and pos t -eutect ic reactions [7]

Eutectic Post-eutect icreaction reactiontempera ture t empera ture

of) oc)

564.4 523, 498.7, 464563.6 492.6

561.7 488

560.0 496.8560.3 487.0556.5 484.0

2533



TABLE II I Solidification characteristics of the 380 alloys with different magnesium concentrations (wt )

Solidification parameters Cooling rate ~ 0.4 ~ s - 1 Cooling rate ~ 10~ 1

0.06Mg 0.33Mg 0.5Mg 0.06Mg 0.33Mg 0.5Mg

Start of solidification temperature (~Solidification temperature range (~Solidification time (s)End of solidification temperature (~

572.5 570.0 567.5 568.7 568.0 568.073.8 77.4 79.5 71.9 81.0 81.0

263.0 283.3 310.0 10.5 10.5 9.2498.7 492.6 488.0 496.8 487.0 487.0

T A B L E I V Remelting characteristics of the three alloys used, solidified at 0.4 ~ s

Mg Heating I AI2Cu b MgzSi Si~,t [3-iron lra Melting(wt ) rate (~ (~ (~ (~ (~ (~ range

( ~ s - 1) (~

0.06 0.1 508.2 c 511.8 c 527.23 c 568.84 576.5 588.2 80.40.33 490.5 500.8 516.87 566.45 576.5 590.8 100.30.50 492.9 500.45 515.18 564.47 576.0 591.0 98.1

0.06 1.0 520.0 526.56 537.0 577.53 N/A 608.2 88.20.33 502.3 515.94 528.57 577.81 N/A 612.9 110.60.50 502.3 516.81 527.6 575.95 N/A 611.76 109.6

Defined by the deviation of the curve from the base line.b With or without A15MgsSi6Cu2.r Identification no t certain.

canno t be dis t inguished clear ly or appears in the formof a hump); and

(c) an increase in the melting temperature range.The heat ing curves (0.1 ~ s-1) of samples solidified

at 10 ~ s - 1 reveal the pr esence of a single peak at lo w

-10

E

o

i-

-20

-30

-40

-50400(a)

55 .59 oCj g - 1

5 2 7 . 2 3 ~

566.84~

4;0 5;0 550 6;0 6;0 700

Temperature (~

- 1 5

-20 ~ 553.14

-25 500.80~ 7 J g-1

v 516.87~ >-30

3 5

-40-45 565.45~

2534

400(b)

4 5 0 5 ; 0 5 5 0 6 0 0 6 5 0 7 0 0

Temperature ~

temperatures , Fig. 6a-c . The temperat ure range(Table V) is very close to that obt aine d for MgzSi, forsamples solidified at 0.4 ~ s-1. Thus, inc reasing thesol idif icat ion rate may possibly resul t in e l iminat ingthe MgzSi reaction, with the excess magnesium beingprecipit ated as A15MgsSi6Cu2 (Reaction 6) instead, at

the end of solidification. This obs erva tion was con-f i rmed by microstru ctural exami nat ion, as wil l be dis-cussed in the next section.

The heat f low-te mpera ture plots for samples sol idi-fied at 10 ~ s- 1, an d usin g a heati ng rate of 1 ~ s- 1(Fig. 7a-c) are fou nd to be mo re or less the same asthose obt ain ed for samples solidified at 0.4 ~ s- 1, andheated at the same rate. However, there is a marginalincrease in the melt ing temp erat ure range ( ~ 10 ~Tabl e VI.

Figure DSC curves of samples solidified at 0.4~ -1 andreheated at 0.1 ~ 1: (a) 0.06, (b) 0.33, (c) 0.5 wt Mg.

v

o

QI

- 1 5

- 2 0

- 2 5

- 3 0

-35

-40-45

4OO(c)

551,48~500.45~ ~ _ . . ~ 9 3 . 7 J g 1

515 18~ ~

5 6 4 . 4 7 ~

4 5 0 5 0 0 5 6 0 6 0 0 6 5 0

Te m p e r a t u r e ( ~

7



o] 52 0 7 8 2~ 2 0 o o 95 ~

4 0

3060 . ~3 5

- 8 0 - 4 0575.95oc ~ 566.49~

-1 0 0 . . . . . . . . -4 5400 4 ; 0 5 ;0 5 ; 0 600 650 700 400 450 5 ; 0 5 5 0 6 ; 0 6 ; 0

E

O

1-

(a) Temperature (~ Temperature (~

- 2 0

- 4 0

- 6 0

- 8 0

557.73~ ~ ' ~

515.94 ~ /

577.81 ~

- 1 0 0400 450 500 550 600 650( b ) Temp erature ( ~ )

700

v

o

r

(a)

- 1 5

- 2 0

- 2 5

- 3 0

- 3 5

515.78 ~

- 2 0

-4Oo

v 560.11 ~-60 297 .4 Jg 1

8

577.53 ~

- 1 0 0400 450 500 550 600 650 700

(c) Temperature (~

Figure DS C curves of samples solidified at 0.4 ~ and re-heated at 1 ~ s- 1: (a) 0.06, (b) 0.33, (c) 0.5 wt Mg .

700

- 4 0

565.45 ~- 4 5

400 450 500 550 600 650

(b) Tem perature ( ~ )700

- 1 0

Ev

O

2 C

- 2 0

- 3 0

- 4 0

553.13 ~~ ~ 301.3 J g 1

5 1 4 . ~

564.22~

- 5 0400 450 500 550 6;0 650

(c ) Tempera tu re (~

700

Figure 6 DS C curves of samples solidified at 10~ -1 and re-hea ted at 0.1 ~ s- 1: (a) 0.06, (b) 0.33, (c) 0.5 wt M g.

3 2 M i c r o s t r u c t u r e

F i g. 8 a - c a r e t h e m i c r o s t r u c tu r e s o b t a i n e d f r o m s a m -ples corre spo ndin g to the three a l loys ( i.e . wi th 0 .06,0 .33 an d 0 .5 wt Mg, respect ive ly) , an d sol id i fied a t10~ s -1 . N o Mg2Si p rec ip it a t e s a r e obse rved , evena t t he h ighes t magn es ium c on ten t . How ever, f ine pa r-t i c l e s be l i eved to be Al sMgsS i6Cu2 a re s een be tweenthe A12Cu c rys t a l s (a r rowed). The am oun t o f t h i sphase i s found to i nc rease p rog res s ive ly wi th i nc reasei n m a g n e s i u m c o n t e n t . F o r c o m p a r i s o n , t he m i c r o -s t ruc ture o f 380 + 0 .5M g a l loy sol id i f ied a t 0 .4 ~ s - 1is sho wn in Fig . 8d , where Mg2S i prec ip i ta tes arec lear ly seen (ar rowed) .

F r o m Ta b l e I V, t h e s t a r t- o f -m e l t i n g t e m p e r a t u r e f o r380 + 0 .5Mg a l loy hea t ed a t0 . 1 ~ - 1 l i e s a round

492 ~ Thus , so lu t ion t r ea t ing th i s a l l oy a t l ow t em-pera tu res ( i .e . 480 ~ i s no t expecte d to cause s igni fi -can t chan ges in t he m ic ros t ruc tu re , F ig . 9a . So lu t iont rea tm ent , how ever, a t 515 for 8 h resul ts in par t ia lme l t i ng o f t he A leCu pha se w i thou t no t i ceab le cha -nges in the eutec t ic s i l icon morp ho log y, Fig . 9b . Incon t r a s t , so lu t ion t r ea t ing the wa te r- ch i l l ed samples o f380 and 380 + 0 .5M g a t 515 ~ fo r t he s ame pe r iod o ft ime is a s soc i a t ed wi th m arke d cha nges in t he s i l iconpa r t i c le m orph o logy , a s show n in F ig . 10a and b ,respect ive ly. Th e b lack spo ts appe ar ing in Fig . 10b area t t r i bu t ed to t he fu s ion o f t he A12Cu phase r e su l t ingf rom a lowe r ing o f t he s t a r t -o f -ma l t i ng t em pera tu redue to t he p re sence o f a h igh m agn es iu m leve l( ~> 0.3 w t ) in this alloy .

2 5 3 5



T A B L E V Rem elting characterist ics of the three alloys used, solidified at 10 ~ s-1

Mg He ating I~ (A12Cua, Mg 2Si) Sieur(wt ) rate (~ (~ (~

(~

~-iron If(~ (~

Mel t ingrange

o c )

0.06 0.1 520.0 525.90 566.490.33 503.5 516.78 565.450.50 505.9 514.51 564.22

0.06 1.0 516.5 523.88, 537.6b 578.34

0.33 503.5 513.17, 530.0b 576.390.50 505.9 512.50, 531.7b 576.27

576.5 591.8 71.8576.5 585.9 82.4574.1 585.9 80.0

N /A 621.2 104.7

N /A 625.9 122.4N /A 623.5 117.6

a W ith o r w ithout A15MgBSi6Cu2.b A hu mp instead o f a well-defined peak, ex tending ov er a tempe rature range.

TA B LE V I Comp a r i son be tween t he ha rdnes s va lue s r ep rot ed byDunn and Dicker t [4] and those obta ined in the present work

M g C o n d i t i o n Ha rdnes s (BHN)(wt )

[4] Present work

0.06 As -cast 81.9 84.04h/1 80 ~ (T5) 84.8 85.0

0.54 As -cast 89.7 101.9 :~4h/1 80 ~ (T5) 96.7 126.0

3 3 Ageing b e h a v i o u r

The role of magnesium content on the strength of 319alloy (6.25 Si, 3.72 Cu, 0.709 Fe, 0.33 Mg,0.3 Mn, sand mould cast, cooling rate < 1 ~ -1)in the as-cast and heat-treated condition (T5-artifi-cial ageing at 205 ~ for 8 h and quenching) has beenstudied by Das Gupta e t a l [9]. Their results showtha t increasing magnesium levels up to 0.6 hasa negligible effect on the hardness and tensile strengthof both the as-cast and heat-treated alloy. Further-more, they observed no significant microstructuralchanges with increasing magnesium content.

Dunn and Dickert [4] compared the effect of mag-nesium up to 0.55 on the mechanica l properties andhardness of A380 (3.25 Cu, 1.01 Fe, 9.15 Si,

0.15 Mg, 0.43 Mn and 1.81 Zn) and 383(2.69 Cu, 1.05 Fe, 10.5 Si, 0.035 Mg, 0.35Mn and 2.1 Zn) alloys. The alloys were examined atthree magnesium levels, i.e. 0.1, 0.35 and 0.55 wt .The presence of magnesium was seen to increase thetensile strength, yield strength and hardness at alltemperatures. Elongation was observed to be reducedby the presence of magnesium; however, the minimumvalue appeared to be acceptable provided the magne-sium content did not exceed 0.35 . According tothem, the optimum T5 treatment is 4h at 180~

The effect of varying magnesium content on themechanical properties of a 380 alumin ium die castingalloy as a function of ageing time at room temperatureas well as at 180~ was studied by Jonsson [10].He concluded that the tensile and yield strengthsincreased substantially with increasing magnesium

5 3 6

-20

-40

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(a)

561.62 ~ \

V 578 .34 oCI I

6

Tem perature (~-20

Ev

o

9

- r

-40

-60

-80

-100400 450 700

558.58

5 1 3 1 ;2 ~ 9 1 ~ I 1 / /

576.39 ~5 5 5 600 650

(b) Tem perature ( ~ )

-20

E

0

- r "

-40

-60

-80

-100400 o 7oo

555 48 ~ \

512.50 ~

576.27 ~

5 6 5;0 6oo o(c) Tem perature ( ~ )

Figure DS C curves of samples solidified at 10~ -1 and re-hea ted at 1 ~ s 1: (a) 0.06, (b) 0.33, (c) 0.5 wt M g.



Figure 8 Optical micrographs of the water-chilled alloys: (a) 0.06, (b) 0.33, (c) 0.5 wt Mg, (d) optical micrograph of 380 + 0.5 Mg a lloysolidified at 0.4 ~ s-1. Inse t shows the presence of A15MgsSi6Cu2 toge ther with A12Cu eutectic.

content when the al loys were subjected to art if icialageing. The hardness (measured by Brinell) increasedf rom 76 BH N to 96 BH N when 0 .57 w t Mg wasadd ed to the 380 alloy. Ageing for 8 h at 180 ~ in-creased this va lue to 112 BHN , a va lue h igher thantha t obta ined on ageing the cas t ing a t room temper-ature for 1 year, i.e. 107 BHN .

The w ork of Kle in [2] reveals tha t 380 d ie cas tings

do not reach the i r peak s t rength a t an ageing temper-ature of 140 ~ even after 60h. A high rate of increasein the value s of yield (YS) and tensile (UTS) s trength isnoted up to an ageing t ime of abo ut 8 h a t 160~ageing tempera ture . The va lues fa l l when the ageingt ime exceeds 24 h . Max imu m Brinel l hardness , YS andUTS are obta ined for an ageing t ime of 18h.

Singh et al [ -11] prop osed an empir ica l equat i on todeter min e the effect of (a) cop per percentag e, (b) mag-nesium percentage, (c) si l icon percentage, (d) solu-t ionizing temperature, (e) ageing temperature, and (f)ageing t ime on the hardness of A1 Si -C u- M g a l loys .Thei r resul t s indica te tha t both copper and magne-s ium enhance the s t rength prop er t ies of graphi te-cas tcas t and heat - t rea ted a l loys . Among var ious in terac-t ions , those be tween magnes ium and s i l icon , andmagnes ium, copper and s i l icon are most s igni f icant .

However, the obta ined equat ions are non- l inear innature , sugges t ing ad opt io n of a h igher order des ignfor the predic t ion of hardness . The auth ors recom-mend an op t imum t r ea tmen t combina t ion : so lu t i ont rea tmen t a t 530~ for 6h fo l lowed by ageing a t185~ for 4h , for max imu m s t rength proper t ies ofgraphite cast al loys.

Fig . l l a - c a re the p lo ts of hardness for the 380

alloys with the th ree levels of magn esium, aged im-mediately after solidification (T5 temper) at 25, 150,180, 200 and 220 ~ for t imes up to 200h. The mainobservat ions tha t can be made are as fo l lows.

1. Max im um a t t a inab le ha rden ing con t r i bu t ion dueto the addi t ion of 0 .33 wt Mg can be as h igh as52 . Fur th er increase in the magnes ium content , i .e .up to 0.5 wt , results in a marginal increas e in thealloy streng th ( ~ 6.5 ) .

2 . Peak-hardening i s obta ined when the a l loy i saged a t 155~ fo r 6 - 8h o r a t 180~ fo r 4h . P ro -longed ageing a t 180 ~ causes mater ia l sof tening dueto overageing . A s imi lar behaviour i s observed whenthe al lo y is aged at a high er temperature~ i .e. 200 ~

3. The harde ning effect produ ced a t 200 ~ i s com-parable to tha t exhib i ted by the a l loy upon ageing a t25 ~ indica t ing the comm encem ent of overageing .

537



Figure 9 Optica l micrographs of 380 + 0.5M g alloy (cooling rate 0.4 ~ s-1), solution treat.ed for 8 h at (a) 480 ~ and (b) 515 ~

Figure 10 Optical microg raphs of water-chilled samples solution treated for 8 h at 515 ~ (a) 0.06, (b) 0.5 wt Mg.

4 . Ageing a t 220~ resu l ts in mater ia l s t rengthmuch infer ior to tha t recorded immedia te ly a f te r cas t -ing.

Th e ma x i mu m ha rden ing e f fec t r epo r t ed by Dun nand Dicker t [ -4] when 0.57 wt Mg was added to 380

alloy in the T5 con dit i on (4 h at 180 ~ was less tha n10 (see Table VI) . The cool ing ra te used by them,however, was not given. Hence, i t is diff icult to assessthe cause for the d i ffe rence be tween the i r resu l t s andthose obta ined in the present case .

Fig. 12 displays the changes in the al loy hardnessa f t e r so lu t i o n t r e a tmen t a t 480~ i nw ar m wa t e r ( 60 ~ age ing a t 180~ fo rt imes up to 100 h (designa ted TT6). I t is evident tha tthere i s a remarkable improvement in the a l loys t r e ngth c om pa r ed t o t ha t show n in F ig . l l c . Whenthe a l loy was a ged 4h in both cases , the as -cas tha rd ne s s i nc r e a se d up t o 40 comp a red t o 16 fo rthe T5 t rea ted 380 + 0 .33Mg a l loy. Even for the basea l loy, age ing for 8h ra i sed the as -cas t hardness by30 , whereas the T5 t rea tmen t showed an increase ofa lmost 10 .

538

The work o f Ape l i anet al [12] shows tha t in ordert o ob t a in a max imum concen t r a t i on o f magnes iumand s i l icon par t ic les in so l id so lu t ion , the so lu t iontempera ture should be as c lose as poss ib le to theeutec t ic tempe ra ture . Some of the a l loy cons t i tuents

may form complex eu tec t ics , which mel t a t t emper-a tures much be low the equi l ibr ium eutec t ic temper-atures, i .e . A12Cu phase. T he di agra m of equi l ibriu msolubi l i ty of magn es ium in so l id a lum inium indica testha t about 0 .4 wt Mg can be p laced in so lu t ion a t480~ and increases to 0 .53 w t a t 515~ Thus ,the appl ied cool ing in the present work i s no t h ighenough to re ta in a l l magnes ium in so l id so lu t ion(Fig . 8). I t should be noted , how ever, tha t the cool ingra te used in a typ ica l d ie -cas t ing process is a t l eas t ano rde r o f magn i t ude g r ea t e r t han t ha t emp loyed he re .Therefore , i t i s expec ted tha t under indus t r ia l condi -t ions , the T5 tem per co uld wel l be used to rep lace theT6 temper (present work) , to achieve the same a l loystrength.

In the second case , the as -cas t samples were so lu-t i on hea t t r e a t ed fo r 8h a t 515 ~ quenched



( 6 0 ~ a g e d a t 1 5 5 a n d 1 8 0 ~ f o r t i m e su p t o 1 0 0 h ( d e s i g n a t e d T 6 ) . F i g . 1 3 a d e m o n s t r a t e s t h ev a r i a t i o n i n t h e a l l o y h a r d n e s s d u r i n g a g e i n g a t1 5 5 ~ T h e p e a k h a r d e n i n g f o r t h e b a s e a l l o y i sa t t a i n e d a f t er 2 0 h , w h e r e a s t h a t f o r 3 8 0 + 0 . 3 3 M go r 3 8 0 + 0 . 5 M g a l lo y s is a t t a i n e d i n t h e m u c h s h o r t e rt i m e o f ~ 8 h . A s e x p e c t e d , i n c r e a s in g t h e a g e i n gt e m p e r a t u r e t o 1 8 0 ~ s h if ts t h e p e a k h a r d e n i n gt o l o w e r t i m e s , i.e , 8 a n d 4 h , r e s p e c t iv e l y. C o m p a r i n g

F i g . 1 3 b w i t h F i g . 1 2 e m p h a s i z e s t h e i n f l u e n t i a lr o l e o f p l a c i n g m a g n e s i u m i n s o l id s o l u t i o n o n t h ea t t a i n a b l e h a r d n e s s l e v e l o f t h e a l lo y u p o n s u b s e q u e n ta g e i n g .

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F i g u r e 11 Va r i a t i o n i n a l l o y h a r d n e s s a s a f u n c t i o n o f a g e i n g t i m e

a t ( a ) 2 5 , ( b ) 1 5 5 , (c ) 1 8 0 , ( d ) 2 0 0 , a n d ( e ) 2 2 0 ~ f o r ( [Z ]) 0 . 0 6 w t % M g ,

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Figure (continued).

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Figure 2 Variation in a lloy hardness as a fun ction of ageing timeat 180 ~ for samples solution heat treated at 480 ~ for 8 h, for (El)0.06 wt Mg, (A) 0.33 wt Mg and (O) 0.50 wt Mg.

4 C o n c l u s i o n s

T h e e f f ec t o f m a g n e s i u m c o n c e n t r a t i o n o n t h e a g e i n gb e h a v i o u r a s m e a s u r e d b y t h e h a r d n e s s o f w a t e r - c h i l-l e d c a s t in g s o f 38 0 a l l o y c o n t a i n i n g 0 . 06 , 0 . 3 3 a n d0 .5 w t M g , r e s p e c t i v e l y, w a s s t u d i e d . F r o m a n a n a l -y s is o f t h e m i c r o s t r u c t u r a l , D S C a n d h a r d n e s s r e s u lt so b t a i n e d , t h e f o l l o w i n g c o n c l u s i o n s m a y b e d r a w n .

1. O f th e t w o h e a t t r e a t m e n t s a p p l i e d t o t h e a s - c a s ta l l o y s , t h e s i g n i f i c a n t ly h i g h e r h a r d n e s s v a l u e s o b -t a i n e d w i t h t h e T 6 t r e a t m e n t c a n b e e x p l a i n e d b y t h ee x c e ss p r e c i p i t a t i o n o f m a g n e s i u m - c o n t a i n i n g p h a s e s

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b) Ageing time h)

Figure 13 Va r i a t i o n i n h a r d n e s s o f a l l o y s s o l u t i o n h e a t tr e a t e d a t 5 1 5 ~ f o r 8 h a n d a g e d u p t o l O O h a t ( a) 1 55 a n d ( b) 1 8 0 ~ f o r (C ~)0 . 0 6 w t % M g , ( A ) 0 .3 3 w t % M g a n d ( 9 0 . 5 0 w t % M g .

i n t h e a s - so li d i f ie d a l l oys . Th i s exce s s p r ec ip i t a t i on canw e l l b e e l i m i n a t e d u n d e r t h e c o o l i n g r a t e c o n d i t i o n s

typ i c a l o f d i e - c a s t i ng ope ra t i ons , so t ha t a T 5 t r e a t -m e n t c a n b e e m p l o y e d in s t e a d o f t h e T 6 t r e a t m e n t t oa c h i e v e th e s a m e l ev e l o f i m p r o v e m e n t i n t h e a l l o ys t r e ng th .

2 . S o l u t i o n h e a t t r e a t m e n t i n t h e l o w - t e m p e r a t u r er a n g e o f 4 8 0 - 5 1 5 ~ i s a d e q u a t e t o p r o d u c e t h e r e-q u i r e d c h a n g e s i n s il i co n m o r p h o l o g y a n d d i s s o l u t i o no f m a g n e s i u m i n t h e a l u m i n i u m m a t r i x .

3 . N o s i g n i f ic a n t d if f e r ence i n ha rdnes s beh av io u r i so b s e r v e d w h e n t h e m a g n e s i u m c o n t e n t i s i n c r e a se db e y o n d 0 .3 w t .

c k n o w l e d g e m e n tF i n a n c i a l s u p p o r t r e c e i v e d f r o m t h e N a t u r a l S c ie n c esa n d E n g i n e e r i n g R e s e a r c h C o u n c i l o f C a n a d a is g ra t e -f u ll y a c k n o w l e d g e d .

R e f e r e n c e s1. R . W . B R U N E R , " M e t a l l u r g y o f D i e C a s t i n g A l l oy s " ( S D C E

Inc . , De t ro i t , M I , 1976) p . 25.

2 . F. K L E I N , i n " N A D C A D i e C a s t i n g C o n g r e s s a n d E x p o s i-t i o n " , D e t r o i t , P a p e r T 9 1 - 0 3 3 ( 1 9 9 1 ) p p . 5 9 - 6 5 .

3. E . G . M O R G A N ,F o u n d . Tr a d e J . 17 June (1982) 887.4 . R . P. D U N N a n d W. Y. D I C K E RT ,Die Cas t ing Eng . 19

(M arc h-A pr i l ) (1975) 12 .5 . E . K . H O L Z , S D C E T r a n s a c t i o n s (S o c ie t y o f D i e C a s t i n g

Eng inee r s , De t ro i t , M I , 1968) Pa per 112, 7 pp .6 . L . B A C K E R U D , G . C H A I a n d J . TA M M I N E N , "S o li d if i-

c a t i o n C h a r a c t e r i s ti c s o f A l u m i n u m A l l o y s" , Vo l. 2 , " F o u n d r yA llo ys ", A F S / S K A N A L U M I N I U M ( A F S / S K A N A L U M I N -I U M , D e s P l a i n e s , I L , 1 9 9 0 ) U S A .

7 . S . G O W R I a n d F. H . S A M U E L ,Meta l l . Mate r. Trans .2 5 A(1994) 437.

8 . A . M . S A M U E L a n d F. H . S A M U E L , J . M a t er . S ci. 3 0 1 9 9 5)in p ress .

9. R . D A S G U P T A , C . C . B R O W N a n d S. M A R E K ,A F S Tr a n s .97 (1989) 245.

1 0. W. J O N S S O N , S D C E Tr a n s a c t i o n s ( S o ci e ty o f D i e C a s t in gEngin ee r s , De t r io t , M I , 1964) Pa per 73 , 7 pp .

1 1. R . J . S I N G H , R . I . G A N G U L Y a n d B . K . D H I N D A W,Br.F o u n d r y m a n77(8) (1984) 436.

1 2. D . A P E L I A N , S . S H I V K U M A R a n d G . S I G W O RT H ,A F STr a n s . 97 (1987) 727.

R e c e i v e d 3 0 J u n ea n d a c c e p t e d 2 1 S e p t e m b e r 1 9 9 4

2 5 4 0

Documents

Effect of Magnesium on 380 Alloy