8/12/2019 A Metallographic Study Study of Porosity and Fracture Behavior in Relation to the Tensile Properties in 319.2 End

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A Meta l lographic S tudyRelat ion to the Tensi le

of Porosi ty and Fracture Behavior inProper ties in 31 9 2 End C hi l l Ca st ing s

A.M. SAMU EL and F.H. SAMUEL

A metallographic stud y o f the poro sity and fracture beha vior in nnidirectionally solidified end chillcastings o f 319.2 aluminum alloy (A1-6.2 pct Si-3.8 pct Cu-0.5 pct Fe-0.14 pc t Mn-0.06 p ct Mg -0.073 pc t Ti) was carried out using optical micro scopy and scanning electron micro scop y (SEM ) to

determine their relationship with the tensile properties. The parameters varied in the production ofthese cast ings w ere the hydrogen (-0 .1 and - 0 . 37 mL/100 g A1) , modif ier (0 and 300 ppm Sr),and grain refiner (0 and 0.02 wt pct Ti) concentrations, as well as the solidification time, w hichincreased with increasing distance from the end chill bottom o f the casting, giving dendrite armspacings (DA Ss) ranging from --15 to - 95 /z m . Image analysis and energy dispers ive X-ray (EDX)analy sis we re emp loy ed for quantification of porosity/microstructural constituents a nd fracture surfaceanaly sis (phase identification), respe ctively. The results sho we d that the local solidification tim ev i z .DA S) significantly influences the duc til ity at low hyd rogen levels; at higher levels, ho wev er, hydro-gen has a more pronounced effect (porosity related) on the drop in ductil i ty. Porosity is mainlyobse rved in the form of elongated pores along the grain boundaries, with S r increasing the porosityvolum e percent and grain refining increasing the probability for pore branching. The beneficial effec tof Sr modification, how ever, im proves the alloy ductil ity. Fracture of the Si, /3-A15FeSi, a-AI15(Fe,Mn)3Si2, and A12Cu pha ses tak es plac e within the p hase particles rather than at the particle/A1matrix interface. Sensitivity of tensile properties to DA S allows for the use of the latter as an indicatorof the expected properties of the alloy.

I. I N T R O D U C T I O N

M A N Y variables affect the mechanical proper ties inaluminum alloy castings: the alloy composition; the metalsoundn ess in terms o f gas and shrinkage poro sity and in-clusions; the solidification rate; the metallurgical character-istics including g rain size, dendrite arm spacing (DA S), andthe shape, size, and distribution o f constituents; and theapp lied heat treatment. Th e microstructure is frequ ently

mod ified and grain refined to optimize the properties. W iththe aid o f special chilling techniques, solidification ratesthroughout the casting can be varied and correlated to theDAS/cell size and properties.

Among popular ly used aluminum cast ing a l loys , A356and 319 are two o f the com mercially important alloys usedin automotive applications. Comparing the two, while A356has been widely studied over the last 40 years, relativelyfew er investigations have been ca rried out on 319 alloy.The present work forms par t of an ongoing research pro-gram aimed at investigating the effect of various parame-ters, i . e . , solidification conditions, ad ditives, an d heattreatment, on the p erforman ce o f this alloy. The results pre-sented in this article highlight th e relations b etw een the so-lidification p arameters, po ros ity content, m icrostructure,tensile properties, a nd fracture behavior. In particular, themicrostructural aspec ts o f the different constituents o b-tained on solidification are elaborated upon. The tensileproperties, hav ing be en discu ssed in detail elsewhere, ]1~ areonly tabulated in the present article from the point o f view

A . M . S A M U E L , R e s e a r c h A s s o c i at e , an d F. H . S A M U E L , P r o f e s so r,a re wi th the Depar tment o f Appl ied Sc iences , Univers i ty o f Quebec a tChicout imi , Chicout imi , PQ, Can ada G 7H 2B 1 .

Manuscr ip t subm i t ted Nov em ber 30 , 1994 .

o f connecting the microstructural obse rvation s with sproperties. Wherever relevant, the results have been dcussed with respect to A356, for a com parison between two alloys.

I I E X P E R I M E N T A L P R O C E D U R E

The 319.2 aluminum alloy used in this study was supl ied by Alcan Ingot Al loys Canada, Guelph Wor(Guelph, ON, Canada) in the form of 12.5-kg ingots. Tchemical composi t ion of the as-received al loy is show nTable I. The ingots w ere melted in a sil icon carbide crucio f 7 kg capacity, usin g an electrical resistance furnace. Tmelting temperature was held at 735 ~ + 5 ~ At thtemperature, the mo lten metal was either d egassed usihigh-purity argon (melt hydrogen content

8/12/2019 A Metallographic Study Study of Porosity and Fracture Behavior in Relation to the Tensile Properties in 319.2 End

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Table I. Ch em icalCom position of the 319.2 AHoy Used in the Present W ork*

Element Si Cu Fe Mn Mg Ti Sr Zn Ni

ICP ana lysis 6.23 3.77 0.46 0.14 0.06 0.073 0.0003 0.08 0.008(wt pct)

*Average compositionobtained rom ICP analysisof 48 m elts.

C r ~

9 . . . . . . '

9 ,~. .f '~. . .. . : : ~

Gra in s i ze DAS

o I T .....~ . . . .. . . . . , . .

200 300 500 1,000Local solidification ime (s)

( a )

4 5 -

4 0

35 -

~ 2 O

2 O i 8 4

1 -

I0

0

1 0 0

7O

SO

3O

2O

A ~E