NBSIR 74-579 V")

Metallographic Examination of

Cylinder Barrels Made From Aluminum

Alloy IA-356] Sand Castings

B. W. Christ

Mechanical Properties Section * ^

Metallurgy Division

Institute for Materials Research

National Bureau of Standards

Washington, D. C. 20234

September 1974

Failure Analysis Report

Prepared for Naval Ordnance Laboratory

Department of the Navy

Vhite Oak

Silver Spring, Md. 2D910

NBSIR 74-579

METALLOGRAPHiC EXAMINATION OF

CYLINDER BARRELS MADE FROM ALUMINUM

(A-356) SAND CASTINGS

B.W.Christ

Mechanical Properties Section

Metallurgy Division

Institute for Materials Research

National Bureau of Standards

Washington, D. C. 20234

September 1974

Failure Analysis Report

"This document has been prepared for the use of the Naval Ordnance Laboratory, Department of the Navy,

White Oak, Silver Spring, Md. Responsibility for its further use rests with that agency. NBS requests that if release

to the public is contemplated, such action be taken only

after consultation with the Office of Public Affairs at

the National Bureau of Standards."

Prepared for Naval Ordnance Laboratory

Department of the Navy

White Oak

Silver Spring, Md. 20910

U. S. DEPARTMENT OF COMMERCE, Frederick B. Dent, Secretary

NATIONAL BUREAU OF STANDARDS, Richard W. Roberts. Director

REFERENCE : Kr . Ray Wiley

Code 211 Magnetism & Metallurgy Division

Bldg. 24-20

Naval Ordnance Laboratory

White Oak, Silver Spring, Md . 20910

NOL Project Order No. N60921-75-PO 00074

JOB ORDER 551-03091-3SD

I. INTRODUCTION ;

A. Nature of the Investigation

At the request of the above reference, the Mechanical

Properties Section of the NBS Metallurgy Division undertook a

metallographic investigation of some specimens from aluminum alloy

sand castings used for cylinder barrels. It was reported to the

Mechanical Properties Section that the alloy was A-356. It was

also reported that some of these castings exhibited a tendency to

crack during machining or test firing, and that tensile specimens

taken from, some of them exhibited smaller elongations and ultimate

tensile strengths than those set forth in the specification. (See

Table I.) The objective of this investigation v;as to identify the

causes of the cracking and the anomalously-lov; mechanical properties. Initial analysis of these materials at NOL suggested that

cracking and anomalously-low m.echanical properties v/ere somehow

related to eutectic melting.

B. Chemical Composition, Heat Treatment and Microstructure

Aluminum alloy A-356 has the nominal composition, Al- 7%

Si- 0.3% Mg.l "A" designates a premium grade alloy with a maximum of 0.2 wt. pet. iron.-^ This casting alloy is a precipitation hardening material which is solution treated at 980

to 1010°F, cooled v/ith water at 150 to 212°F, then precipitation

hardened (or "aged") to a T6 condition by holding at 300 to 320°F

for 1 to 6 hours. 2a The precipitating phase which causes hardening is Mg2Si.l^ Iron concentration is kept low to minimize the

formation of the embrittling ternary phase, Fe2Si2Al9.1^

A prominent microstructural feature of the as-cast alloy is

the network of silicon particles which solidify at the binary

eutectic temperature of 1071°F.-'-'^ The binary eutectic is composed

of primary aluminum solid solution, showing up as the white matrix

phase in Figure la, and silicon solid solution (v/hich is almost

pure silicon) , showing up as the network of gray particles in

Figure la. A close-up of these eutectic silicon particles appears

in Figure lb. The "finger-print" smudge features appearing v/ithin

the eutectic silicon network of Figure 1 are due to a dispersion

of the hardening phase, which is thought to be Mg2Si.-'-^' ^'^

A close-up of one of these features, e.g.. Figure 4, has a

"pepper-like" appearance. The eutectic silicon network solidifies

when the casting solidifies, whereas the Mg2Si particles form in

the solid state during aging. For convenient reference below, a summary of the eutectics which may form in A-356 alloy appears in

Table II.

II. MATERIALS AND PROCEDURES:

Material supplied to NBS by NOL came from sand castings which

reportedly originated at either of tvro foundries, Ohio Aluminum

Industries or the Wellman Company. Some material, chiefly from

Ohio Aluminum Industries, exhibited cracks and anomalously-low

tensile properties. Seven as-mounted metallographic specimens of

the Ohio material \\ere supplied. (See Table III.) Also, two

fractured tensile bars were supplied, one from Ohio material and

one from Wellman material. Further, a small segment of an Ohio

casting containing a natural crack* near a drilled pin hole was

supplied. Finally, a number of photomicrographs resulting from

initial metallographic work at NOL v/ere supplied.

Three of the as-mounted metallographic specimens v/ere repolished and re-etched, and various photomicrographs were prepared.

Fracture surfaces of the broken tensile bars were photographed, and

one specimen for mounting and polishing was cut from a half of each

foundry's tensile bar. The natural crack in the small segment of

Ohio material .v/as photographed, then a small piece was carefully

cut from it for mounting and polishing so that details of the

natural crack could be observed at magnifications of 50 to 250X.

The ligament holding the cracked halves of the small segment

together v;as severed during this cutting operation, and the fracture

surfaces of the natural crack thus revealed were photographed in

the SEK. Finally, Knoop hardness measurements were made on the

metallographic specimen prepared from each foundry's tensile bar.

III. RESULTS AND DISCUSSION:

A. Metallographic Specimens Cut From Tensile Bars

The microstructure characteristic of Ohio material appears

in Figure 2a, and that characteristic of Wellman material appears

in Figure 2b. There is a noticeable difference in the porosity,

which shows up as the black regions in each photomicrograph. The

pore size appears larger, and the number of pores appears greater

in the Ohio material. The cause of this difference in the porosity

of the tv70 castings is not clear. The rounded dendrite arms of

primary aluminum solid solution projecting into the larger pores

of the Ohio material, and the location of these pores, suggest that

they are interdendritic shrinkage cavities. The location and shape

of the smaller pores in the Ohio material suggest that they may

be gas pores.

* "natural crack" is defined in Section III-B.

- 2 -

Comparison of Figures 2a and 2b indicates that the microstructure is coarser in the Ohio material than in the Wellman

material, i.e., on the average, the v/hite patches of primary

aluminum solid solution and the eutectic silicon particles are

coarser in the Ohio material than in the Wellman material. This

difference is possibly due to the difference in solidification

rates, for it is well known that a cast micros tructure becomes

finer as the solidification rate increases. However, influences

on the scale of the microstructure related to the m.aximum tem.perature reached during solution heat treatment (see Section III-D)

and to sodium additions (see Section III-E) should not be overlooked

Figures 3a and 3b compare eutectic silicon particles in Ohio

and Wellman material, respectively. Not only are the eutectic

silicon particles in the Ohio material larger than those in the

Wellman material, they are also plate-like, whereas those in the

Wellman material are rod-like. (This point is discussed further

in Section III-E.) Furthermore, comparison of Figures 3a and 3b

indicates that the eutectic silicon network is more continuous

in the Ohio material than in the Wellman material. This point

can be verified by placing an imaginary line through several

silicon particles in the interdendritic regions, then determining the number of intercepts with aluminum solid solution for

a fixed line length. On the average, there will be fev/er intercepts in the Ohio material than in the Wellman material. This

latter feature of microstructure, i.e., the continuity of the

eutectic silicon particles in the interdendritic region, is

important in the analysis of cracks and the loss of ductility.

Essentially, the brittle silicon particles represent an easy-tofracture phase in the microstructure, and the greater the

continuity of the eutectic silicon netvzork, the easier a crack

can propagate throughout the microstructure. This point will be

discussed further in Section III-B.

The "pepper-like" regions of Mg2Si appear in the Ohio

material as shown in Figure 4a and in the Wellman material as in

Figure 4b. Mg2Si is also evident in Figures 2a and 2b as the

dendrite shapes v/hich appear in the regions of primary aluminum

solid solution. The size and distribution of Mg2Si particles

seems to be quite similar in both the Ohio and Wellman materials.

This observation is consistent with the results of tensile and

hardness tests, which showed no significant differences in the

yield stresses and hardnesses of the Ohio and Wellman materials.

Yield stresses appear in Table I. Furthermore, average hardnesses,

as determined by placing a Knoop indenter in the primary aluminum

solid solution, were 110 and 106, respectively, for Ohio and

Wellman material. These hardness values convert to Rockwell-B

values of 53 and 49. A typical hardness impression appears at

the top-left in Figure 2a.

- 3 -

B. Relationship of Natural Crack Path to Microstructure

Natural cracks are defined as cracks which were found in

the castings. Natural cracks found in various specimens taken

from Ohio material almost always followed a path through the

silicon-rich interdendritic regions, as shown in Figures 5a and

5b. The crack in Figure 5a is near a specimen surface and runs

between two dark regions which may be sand particles trapped in

the casting. This is the only near surface crack of this type

which v/as found. The crack in Figure 5a appears at higher

magnification in Figure 6. The tendency for a crack to follov; a path through the silicon-rich interdendritic regions is evident

in the similarity betv/een the wandering nature of the crack path

on an as-machined surface, Figure 7, and that of the crack path

in Figure 5b.

Figure 8 shov;s examples of eutectic silicon particles

adhering to both surfaces of a natural crack. This feature was

observed along the entire crack length. The presence of silicon

particles along the crack edge suggests that the crack propagated

by fracturing any silicon particles V7hich it could reach. Indeed,

the secondary branches of the main crack appearing in Figure 8a

show the fracture of silicon particles. This fracture process is

vividly illustrated at higher magnification in Figure 8b. The

force v/hich propagated the crack probably came from residual

stresses in the casting, as well as from applied stresses accompanying machining operations and/or test firings.

It is evident from Figures 5 and 8 that the ease with v/hich

a crack can advance throughout the micros tructure increases as

the continuity of the netv/ork of eutectic silicon particles

increases, since the silicon particles are a brittle (and relatively

weak) phase. The difference in this feature of microstructure

between the Ohio and Wellman material, as discussed in Section

III-A, is probably the cause of the higher frequency of cracking

and the anom.alously-lov/ mechanical properties of the Ohio material

as reported by NOL. Probably the large interdendritic shrinkage

pores in the Ohio m^aterial , which were pointed out in Section III-A,

facilitate the nucleation of cracks in the interdendritic regions.

As mentioned in Section I-A, initial analysis of these materials at

NOL suggested that cracking and anomalously low mechanical properties

were somehov; related to eutectic melting. However, it is not obvious

that the features of microstructure in the Ohio material suspected

of causing cracking and anomalously-low mechanical properties,

namely, a nearly-continuous network of large eutectic silicon

platelets in the interdendritic regions and large interdendritic

pores, are a consequence of the overheating which causes eutectic

melting. (See Section III-D for a discussion of eutectic melting.)

- 4 -

c. Observations of Fracture Surfaces

1. Natural Crack Viev/ed in SEM

The fracture surfaces of the natural crack were viewed in

the SEM. The fracture surfaces exhibited numerous cleavage facets,

as is clear from Figures 9a, 9b, and 9c. Observations with the

stereographic viewer confirmed that most topographic features on

these fracture surfaces were cleavage facets. Very little

evidence of dimpled rupture was found.

Since Figure 8 shows silicon particles on both surfaces of

the natural crack, it may be inferred from Figures 8 and 9 that

the crack propagation mode was principally via cleavage of the

brittle and relatively weak silicon particles. The angular voids

which appear in Figures 9 probably resulted when silicon particles

were pulled out at some stage of the fracture process. The rounded

voids appearing in the top left quadrant of Figure 9b probably

resulted from the interdendritic shrinkage which occurred during

the initial solidification of the casting. The shadowy features

appearing in these voids are thought to be dendrite arms of the

primary aluminum solid solution.

2. Fracture Surfaces of Tensile Bars

Figure 10 shows the fracture surfaces of a tensile bar

machined from each foundry's castings. SEM results appear in

Figures ICa and 10b, and optical microscopy results appear in

Figures 10c and lOd. The polygonal regions in Figures 10a and

10b are interpreted as evidence of cleavage facets. The larger

scale of the microstructural features in the Ohio material as

compared to the Wellman material, a feature already noted in

Section III-A, is also evident from comparison of Figures 10a and

10b. The region of rounded features appearing in the bottom

center of Figure 10a is interpreted as an interdendritic shrinkage

pore, with the rounded features presumed to be the arms of

primary aluminum dendrites. The chief difference between the

optical micrographs. Figures 10c and lOd, is that a larger number

and larger size of bright facets appear on the tensile bar made

from the Ohio casting. These facets are probably the same

cleaved particles of silicon v;hich v/ere pointed out in the discussion of Figures 5, 8 and 9, in Sections III-B and III-C-1.

The presence of bright facets on the fracture surface of

the tensile bar made from a Wellman casting. Figure lOd, suggests

that cracks also propagate through silicon particles in this

material if the stress is high enough (ultimate tensile stress is

about 43,000 psi, as compared to about 35,500 psi for tensile bars

from the Ohio casting. Table I). However, to reiterate, the main

difference between the fracture surfaces in Figures 10a and 10b

is the number and size of bright facets, not just their presence.

This difference is easily seen on the fracture surface of tensile

bars v/ith the unaided eye, as well as with a lovz-magnif ication

,

hand-held magnifier.

- 5 -

D. Eutectic Melting

Eutectic melting is a phenomenon which sometimes occurs in

the heat treatment of precipitation hardening aluminum alloys

.

Available literature'^ / ^ describes eutectic melting principally

for v/rought aluminum alloys, but the phenomenon can also occur

during the heat treatmient of aluminum alloy castings. The

micros tructural features usually cited as evidence of eutectic

melting are rosettes and dark etching grain boundaries.^ Two

consequences of eutectic melting are reduced tensile strength

and reduced elongation, i.e., embrittlement . Changes in microstructural features and mechanical properties resulting from

eutectic melting are thought to be irreversible.

The type of rosette appearing in the middle of the primary

aluminum solid solution in Figure 11a is usually regarded as

resulting from solidification of the ternary eutectic, composed

of aluminum solid solution, silicon solid solution and jy[g2Si.

The expected three phases appear as white, light gray and dark

gray regions of the rosette. Features thought to be the same

ternary eutectic also appear in Figures 4a, lib, and 11c

although the rosette shape is not pronounced in Figure 11c.

The blade-like particles appearing in Figures 11 are thought

to be Fe2Si2Al9, a ternary phase of the Fe-Si-Al system. Blades

of Fe2Si2Al9 v;ere often observed to connect eutectic silicon

particles in the Ohio material, e.g., see Figure 11a, upper left

and upper right. Generally, there seemed to be more Fe2Si2Al9

blades in the Ohio material than in the Wellman material. The

reported iron analyses of castings from both foundries, < 0 . 05 wt.

pet., is well below the 0.2% max. appearing in the specifications

for A-356. Thus, embrittlement from iron bearing intermetallic

compounds is not likely.

Figures 11 were obtained from Ohio material, namely, the

metallographic specimen prepared from a segment of the fractured

tensile bar. Rosettes were found in specimens from both foundries,

but there was a larger number of rosettes in the Ohio material.

It is not clear v;hen the rosettes formed, i.e., during solidification of the casting or during the solution heat treatment.

Figure 12 shows a microstructural feature of Ohio material,

the triangular region projecting from a silicon particle, which is

interpreted as evidence of melting of the binary eutectic. This

eutectic is composed of the terminal aluminum solid solution and

the terminal silicon solid solution, and the eutectic temperature

is 1071°F.lt' Binary eutectic melting usually initiates at grain

boundaries, and indeed, this is the case in Figure 12. The

- 6 -

expected two phases, etching lightly and darkly, show up clearly

in the triangular region. The triangular shape, determined by

surface tension, suggests that the binary eutectic melted while

the surrounding material was solid. Furthermore, the smaller

size of the phases in the triangular region, as compared to the

size of the surrounding aluminum solid solution and silicon

particles, also suggests that the binary eutectic m.elted v/hile

the surrounding material v/as solid. For example, the surrounding

material could have acted as a heat sink to rapidly cool the

triangular region and thereby produce the smaller particle size.

It is possible that the melting occurred during a temperature

excursion during the solution heat treatment of the Ohio material.

Triangular regions such as that appearing in Figure 12 were

not detected in the Wellman material. The small size of the

triangular region, and the fine particle size of the phases in it,

makes detection of such regions extremely difficult. High

magnification, e.g., lOOOX, and extremely careful etching are

required to reveal such regions.

E. Morphology of Eutectic Silicon Particles in Al-Si Casting

Alloys

Eutectic silicon particles can be present in aluminumsilicon casting alloys as platelets or rods . 6 Solidification rates,

as well as sodium modification (i.e., addition of about .020%

sodium to the casting alloy) are known to influence the morphology

of eutectic silicon particles. ' -^-^ Thus, the difference betVv^een

the eutectic silicon particles in the Ohio and Wellman materials

namely, large, impinging platelets in the former and small, separated rods in the latter (see Section III-A) may be related to

solidification rate and sodium modification. The apparently greater

continuity of the network of eutectic silicon particles in the Ohio

material, as compared to Wellman material, was identified in

Section III-B as the difference in microstructures most likely to

explain the increased susceptibility of Ohio material to cracking.

An early report^*^ about sodium modification indicated that

there were problems "in obtaining gas free metal after introducing

sodium". Thus, it is possible that the difference in porosity

noted between the Ohio and Wellman material (see Section III-A)

may somehow be related to the introduction of sodium. The

possibility that pores might act as crack nucleation sites was

discussed in Section III-B.

IV. SUGGESTIONS FOR NON-DESTRUCTIVE EVALUATION : During the course of this metallographic investigation, a question arose as to which non-destructive evaluation methods

might be used on hardware in the field to evaluate its susceptibility to cracking. Results of this investigation suggest

the follov/ing possibilities:

- 7 -

1. Polish a metallographic surface of hardware in the field

and etch to reveal the nature of the eutectic silicon

particles in the interdendritic regions. II the particles

were large and platelike, and if the particles formed a nearly-continuous netvrork, it might be suspected that the

hardware v;ould be susceptible to cracking. This suggested

NDE method v;ould require some development, so that effects

on micros tructure due to different solidification rates

near and remote from surfaces could be recognized.

2. Rem.ove a sm.all sacrifical specimen from a piece of hardware,

fracture it and visually examine the fracture surface for

bright facets. A large number of large-size facets V70uld be

cause for suspecting that the hardv/are would be susceptible

to cracking. (See Section III-C-2.) Again, some development

v;ould be required so that effects on fracture surface facets

due to different solidification rates near and remote from

surfaces could be recognized.

3. Remove a sm>all sacrificial specimen from a piece of hardv:are

and carry out chem.ical analysis for sodium. A low sodium

concentration migh"c lead to the suspicion that the sodium,

addition intended to prcm.ote modification of the eutectic

silicon m.orphology (see Section III-E) was overlooked or

improperly handled. Admiittedly, some development would be

required so that effects on sodium dis uributions due to

differences in solidification rates near and remote from

surfaces could be recognized.

Whereas each of these suggested NDE methods provides some

inform.ation about the susceptibility of a piece of hardv;are to

cracking, it -iight be best if results from, all three suggested

methods (and perhaps results from other methods as well) v;ere

used as the basis for final decisions about the susceptibility

of a piece of hardware to cracking.

V. SUI'lI'IARY AND CONCLUSIONS : A metallographic investigation of some precipitation

hardened aluminum castings (A-356) used for cylinder barrels was

carried out. The material investigated cam.e from sand castings

supplied by tv/o foundries, Ohio Aluminum Industries and Wellm.an

Company. Som^e m.aterial, chiefly from Ohio Aluminum Industries,

exhibited cracks and anomalously-lov/ tensile properties. The

objective of this investigation was to identify the causes of

this behavior. Observations v;ere made with the light microscope

and the scanning electron microscope.

- 8 -

The natural crack path in Ohio material was closely related

to the micros tructure , i.e., natural cracks followed the network

of eutectic silicon particles in the interdendri tic regions.

Eutectic silicon particles were found on both sides of a natural

crack, and the fragmenting of silicon particles in the early

stages of crack propagation was evident. Numerous shrinkage pores

appeared in the interdendritic regions, and it v;as suggested that

these pores served as crack nucleation sites.

The microstructure of Ohio material was noticeably coarser

than the microstructure of Wellman material, i.e., the size of

primary aluminum dendrites was larger and the size of eutectic

silicon particles was larger. Moreover, eutectic silicon

particles in the Ohio material v/ere platelike, whereas those in

the Wellman material were rod-like. The network of eutectic

silicon particles was more continuous in the Ohio material than

in the Wellman material. Furthermore, the number and size of

pores attributed to interdendritic shrinkage was larger in the

Ohio material than in the Wellman laaterial. More evidence of

eutectic melting was found in the Ohio material than in the

Wellm.an material, but it was not obvious that the increased

susceptibility of Ohio material to cracking and its loss of

mechanical properties was a consequence of the overheating which

causes eutectic mtelting. There were no notable differences in

the hardening precipitate, Mg2Si, which appeared in the Ohio

and Wellman material.

The causes of the foregoing differences in microstructure

could not be positively identified. However, these differences

may arise from: differences in solidification rates, differences

in solution heat treating and aging conditions, and differences

in the use of sodium additions which m.odify the morphology of

eutectic silicon particles.

Three suggestions were made for non-destructive evaluation

methods v/hich might be useful for evaluating the susceptibility

to cracking of cylinder barrels in the field. These suggestions

were: 1) in situ metallographic examination of cylinder barrels,

2) fracture surface analysis of a small sacrifical specimen, and

3) chemical analysis of a small sacrifical specimen.

VI . ACKNOWLEDGEMENTS

Mr. C. H. Brady provided valuable skill and assistance with

the bulk of the metallographic work. Mr. T. R. Shives provided

assistance with the SEM and obtained the photos in Figure 9.

Mr. L. C. Smith assisted with manuscript preparation and obtained

the photos in Figures 5b and 8. Mr. D. B. Ballard obtained

Figures 7, 10a and 10b. Fruitful discussion with Dr. W. J.

Boettinger about the interpretations of some of the microstructures

is gratefully acknowledged.

- 9 -

BIBLIOGRAPHY

1, Metals Handbook, Eighth Edition. American Society for Metals,

Metals Park, Ohio.

(a) "forging and casting", vol. 5, 1970. Table I on p. 399.

(b) "metallography, structures and phase diagrams", vol. 8,

1973. A.luminum-Magnesium-Silicon Ternary Phase Diagram

on p . 3 96.

(c) Aluminum-Iron-Silicon Ternary Phase Diagram on p. 394

of Reference lb.

(d) "atlas of micros tructures of industrial alloys",

Figures 2120 to 2129 on pp. 257, 258.

2a. Specification: Mil H-6088E, "Heat Treatment of Aluminum

Alloys", 9 June, 1972.

2b. Specification: Mil A-2118 0C, "Aluminum Alloy Castings,

High Strength", 26 February, 1965.

3. "Optical Microscopy of Casting Alloys", described on p. 68

of Chapter 3 in Aluminum , Vol. 1, (Properties, Physical

Metallurgy and Phase Diagrams) . Edited by K. R. Van Horn.

American Society for Metals, Metals Park, Ohio. 1967.

4. "Eutectic m.elting and high temperature oxidation", described

on p. 353 (also p. 340) of Chapter 9 in Aluminum , Vol. Ill

(Fabrication and Finishing) . Edited by K . R. Van Horn.

American Society for Metals, Metals Park, Ohio, 19 67.

5. Metals Handbook, Eighth Edition (1964), Vol. 2, "heat

treating, cleaning and finishing". Figure 2 on p. 272.

6. Metals Handbook, Eighth Edition (1972), Vol. 7, "atlas of

microstructures of industrial alloys". Figures 2127 and

2128, p. 258.

7. "Premium-Quality Aluminum Castings", DMIC Report 211, Jan.,

1965. Battelle Memorial Institute, Columbus, Ohio 43201

8. M. C. Flemmings : "Solidification of Polyphase Alloys:

Castings and Ingots", pp. 181-182 in Chapter 6 of

Solidification Processing . McGraw-Hill, New York, 1974.

9. Metals Handbook, Eighth Edition (1972), Vol. 7, "atlas of

microstructures of industrial alloys". Figures 2125, 2126

and 2128 on p. 258.

- 10 -

ilO. G. N. Reinemann and L. E. Marsh: Metals Progress 76_ (1) , r 80-86 . (July, 1959) .

11. R. A. Zuech: Iron Age 183, 88 (Jan. 29, 1959).

12. T. A. V7elsko, D. B. Ballard and M. R. Meyerson: "Tagging of

Metal Objects", NBS Report No. 94 8 2 to U.S. Arms Control

and Disaririaiuent Agency, April, 1967 .

- 11 -

Table I

floom Temperature Tensile Properties of Specimens Taken

From Sand Cast and Heat Treated A-356 Cylinder Barrels^

u . z -5 ur r set u X uima te

•\r -J Q T J

X leiu Tensi le Hj JL \J L i <j CL U-LLJii l^ource of bpecimen strength Strength L-^^ i J- Uu.l.t:; Specimens L-Onaiuion" Location* * (psi) (psi) ^in ± mCii;

t O /-NT o o o cr o Zoo a A 33,300 35,700 1.0%

8285 a B 34 ,400 38,200 1.5

o o o c a Zoo a C 32 ,800 36,600 1.0

8040 b A 27 ,900 34,500 1.4

Ijhio 8040 uD

"DD z o ,300 3 z , / U U 1 . D

8040 b c 27,800 33 ,500 1.8

8107 c Jo , b U U

T Q /I n M J o , 4 U U U . /

I 8107 c B 35,500 39,200 0.6

8107 c c 31,100 37 ,200 0.4

I S/N 4032 d A 30,300 43,300 5.8%

//ellman 4032 d B 34 ,500 42,300 5.6 1—4032 d C 32,800 37 ,500 3,5

Military 28,000 38,000 5.0%

t*=pecif ication

or Class I

,

mm

^-356 Castings

(Ref. 2b)

IT

* I

|r4

min . elongation

in 2-inches

These data v;ere provided by the Naval Ordnance Laboratory.

Key: a - rough machined; no cracks, b - crack detected prior to

engine test. c - crack detected after engine test,

d - no cracks.

Key: A - radial. B - axial, at 90° to location A. C - region

between two cylinders. Tensile axis parallel to cylinder

axis

ii

Table II

Summary of Eutectics Which May Form In A-35 6 Alloys*

Type of

Eutectic

Ternary

Binary

Solid

Phases

Al^gg**, Mg2Al3, iMg2Si

(rosette)

Al. , Si, tss tss

(particles of eutectic

silicon in interdendritic

regions)

Eutectic Temperature

(°F)

1038

1071

(°C)

559

577

Quasi-Binary

"^''"tss' ^"^o^-^ 1103 595

Reference: Metals Handbook, Eighth Edition (1973), Vol. 8,

"metallography, structures and phase diagrams",

p. 396.

** The subscript, "tss", signifies terminal solid solution.
il

1
Table III

List of Metallographic Specimens From Sand Cast

and Heat Treated A-356 Cylinder Barrels

Source of

Specimens

Specimens Prepared

at NOL from Specified

Location on Cylinder

Barrels from Ohio

Foundry

Specimen

Number

8107*

8107

8107

8040*

8040

8285*

8285

Location

on Casting

1^

2b

3C

1

3

1

2

8107B

Specimens Prepared at NBS (Ohio)

from Fractured Tensile Bars 4032A

(Wellman)

* Re-polished and re-etched at NBS.

a Adjacent to drilled hole for pin.

b Adjacent to cylinder

c Near outer surface

Figure Legends

Solution-treated and aged microstructure of A-356 casting

alloy. (a) Regions of primary aluminum solid solution (white) , network of eutectic silicon particles in the interdendritic

regions (gray) , interdendritic shrinkage pores (black) and

clusters of Mg2Si particles (apparent smudges in primary

aluminum). (Ohio material: Specimen Number 8107-1.)

(b) Close-up of Mg2Si particles, and eutectic silicon platelets.

(Ohio material: Specimen Number 8040-1.)

Comparison of typical microstructures in solution-treated and

aged A-356 casting alloys. (a) Ohio material: (Specim.en

Number 8107B, cut from tensile bar.) (b) Wellman material:

(Specimen Number 4032A, cut from tensile bar.)

Comparison of eutectic silicon particles in solution-treated

and aged A-356 casting alloys. (a) Ohio material: (Specimen

Number 8 04 0B-1.) (b) Wellm.an material: (Specimen Number

4032A.

)

Comparison of Mg2Si particles in solution-treated and aged

A-356 casting alloys. (a) Ohio material: (Specimen Number

8107B, cut from tensile bar.) (b) Wellman material:

(Specimen Number 4032A, cut from tensile bar.)

Natural crack paths in Ohio material. (a) Near surface and

betv/een tv/o rounded black features thought to be embedded

particles of sand. (Specimen Number 8 04 0-1.) (b) Edge-on

vievz of crack near drilled hole. (Specimen cut from casting

near drilled hole.) See also Figure 6.

Close-up of near-surface crack appearing in Figure 5a.

(Ohio material: Specimen Number 8040-1.)

Typical crack path on as-machined surface. View of the

interior of a sectioned hole v/hich was drilled to accommodate

a pin. (Ohio material: Specimen cut from casting near

drilled hole.) Contrast due to topography, not etching.

Examples of how crack path follows eutectic silicon particles

in Ohio material. (a) Primary crack and associated secondary

cracks. (b) Close-up of secondary crack showing fracture of

silicon particles. (Specimen cut from casting near drilled

hole .

)

SEM views of the fracture surface of a natural crack in

Ohio material. (a) Large cleavage facet. (b) Interdendritic shrinkage pores, and cleavage facets. (c) Voids

due to pull-out of silicon particles, and cleavage facets.

(Specimen cut from casting near drilled hole.)

Fracture surfaces of tensile bars. (a) Cleavage facets and

an interdendritic shrinkage pore in Ohio material, as viewed

in SEM. (b) Cleavage facets in Wellman material, as viewed

in SEM. (c) Brightly reflecting cleavage facets in Ohio

material. (d) Faintly reflecting cleavage facets in Wellman

material. (Ohio material: Specimen Number 8107B. Wellman

material: Specimen No. 4032A.)

Rosettes interpreted as evidence of ternary eutectic phases

.

(a) and (b) Rosettes embedded in primary aluminum solid

solution. The needle-like and blade-like particles are

thought to be Fe2Si2Al9. (c) Non-rosette shaped regions

thought to contain ternary eutectic phases . (Ohio material

Specimen Number 8107B-1.)

Triangular regions interpreted as evidence of binary eutectic

melting. (Ohio material: Specimen Number 8040B-1.)

Figure 1. Solution-treated and aged microstructure of A-356 casting

alloy. (a) Regions of primary aluminum solid solution

(white) , network of eutectic silicon particles in the

interdendritic regions (gray) , interdendritic shrinkage

pores (black) and clusters of Mg2Si particles (apparent

smudges in primary aluminum) . (Ohio material: Specimen

Number 8107-1.) (b) Close-up of Mg2Si particles, and

eutectic silicon platelets. (Ohio material: Specimen

Number 8040-1.)

Figure 2, Comparison of typical microstructures in solutiontreated and aged A-356 casting alloys. (a) Ohio

material: (Specimen Number 8107B, cut from tensile

bar.) (b) Wellman material: (Specimen Number 4032A,

cut from tensile bar.)
I
Etchant: 1% HF

O

Oc7

Etchant: 1% HF

500X

(a)

C3o

0

-^^ O o

/n Q ^^'^ Q cc? 03 0 9 0

500X

(b)

Figure 3. Comparison of eutectic silicon particles in solutiontreated and aged A-356 casting alloys. (a) Ohio

material: (Specimen Number 8 040B-1.) (b) Wellman

material: (Specimen Number 4032A.)

Figure 4. Comparison of Mg2Si particles in solution-treated

and aged A-356 casting alloys. (a) Ohio material

(Specimen Number 8107B, cut from tensile bar.)

(b) Wellman material: (Specimen Number 4032A, cut

from tensile bar.)

Figure 5. Natural crack paths in Ohio material. (a) Near surface

and between two rounded black features thought to be

embedded particles of sand. (Specimen Number 8 040-1 .)SE£/lisafi'

(b) Edge-on view of crack near drilled hole. (Specimen

cut from casting near drilled hole.)

1^

As Polished

1

250X

Figure 6. Close-up of near-surface crack appearing in Figure

5a. (Ohio material: Specimen Number 8040-1.)

As Machined 75X

Figure 7. Typical crack path on as-machined surface. View of

the interior of a sectioned hole which was drilled

to accommodate a pin. (Ohio material: Specimen cut

from casting near drilled hole.) \Tf\^^''" <

As Polished 500X

(b)

Figure 8. Examples of how crack path follows eutectic silicon

particles in Ohio material. (a) Primary crack and

associated secondary cracks. (b) Close-up of

secondary crack showing fracture of silicon particles.

(Specimen cut from casting near drilled hole.)
V

I
As Fractured X17 0X

(b)

Figure 9. SEM views of the fracture surface of a natural

crack in Ohio material. (a) Large cleavage

facet. (b) Interdendritic shrinkage pores,

and cleavage facets. (Specimen cut from

casting near drilled hole.)

Figure 9c. Voids due to pull-out of silicon particles,

and cleavage facets. (Ohio material:

Specimen cut from casting near drilled hole.)
\
As-Fractured 112X

(b)

Figure 10. Fracture surfaces of tensile bars. (a) Cleavage

facets and an interdendritic shrinkage pore in

Ohio material, as viewed in SEM. (Specimen

Number 8107B.) (b) Cleavage facets in Wellman

material, as viewed in SEM. (Specimen Number

4032A.)

As -Fractured

(c)

ex

As-Fractured 6X

(d)

Figure 10. Fracture surfaces of tensile bars. (c) Brightly

reflecting cleavage facets in Ohio material.

(Specimen Number 8107B.) (d) Faintly reflecting

cleavage facets in Wellman material. (Specimen

Number 4032A.)

Etchant: 1% HF 500X

(b)

Figures 11a and lib. Rosettes interpreted as evidence of

ternary eutectic phases. Rosettes

embedded in primary aluminum solid

solution. The needle-like and bladelike particles are thought to be

Fe2Si2Al9. (Ohio material: Specimen

Number 8107B-1.)

I

Figure 11c. Non-rosette shaped regions thought to contain ternary eutectic phases. (Ohio material;

Specimen Number 8107B-1.)
1
Figure 12. Triangular regions interpreted as evidence

of binary eutectic melting. (Ohio

material: Specimen Number 804 0B-1.)

u 5 nr 1- 1 IF roMM.

BIBLIOGRAPHIC DATA

SHFFT

1. PUHLK A l lON OR RI-.PORT NO.

NBSIR 74-579

2, Ciov't Accession

No.

3. 1? (( ipient 's Accession No.

4. 1 1 1 1 1 ANP M IM 1 111,1 METALLOGRAPHIC EXAMINATION OF CYLINDER BARRELS MADE

FROM ALUMINUM ALLOY (A-356) SAND CASTINGS

5. 1 'uhl ication Date September 1974

6. l'< rfortning Organization Code

7. AniHOR(S)

B. W. Christ

8. I'rrforming Organ. Report No. NBSIR 74-579

9. PKKFORMINC. OIU, AN IZAl ION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No. 3120586

11. ( (ititract/Grant No.

12. '^pon'sorinp Organisation Name and Cojnplete Address (Street, City, State, ZIP)

Naval Ordnance Laboratory

White Oak, Silver Spring, Jyid. 20910

13. ' ype of Report & i-'eriod

< overed Pallure

Analysis Report

14. Sponsoring Agency Code

15. snPPI.l" MFNTARY NOTES

16. AilS l H Af 1 (A 200-word or less factual summary of most si^idcant information. If document jnchides a significant

biblinfir.iphv nr 1 itrralure survey, mention it here.)

A metallographic examination was carried out on cylinder barrels

made from aluminum alloy (A-356) sand castings. Sonie castings

exhibited a tendency to crack during machining and/or testing.

The examination revealed that cracks propagated through large

eutectic silicon platelets in the interdendritic regions.

Possible origins of the large eutectic silicon platelets were

discussed. Some suggestions were made about non-destructive

evaluation methods which would help to identify castings with a

potential for cracking.

7 I- ' \ (vfv 1.1 Iwrlvo vnlrivs; alpUnhrtical order; c/ipilnllxo only the first letter of thr first hoy word unless a proper

IH

M uni , i<iii .>(( il hv srtvicolotU^ ) A-^.SO aluminum; eutectic silicon platelets; interdendritic shrinkage;

non-dnH tructive evaluation, sand casting; yield and ultimate tensile

strenq ths

A\ All AMI! I I Y [ ' Unlimited

1 X ' "' ''tiiciiil Disirilnition. I''o Not Release to NTIS

' (Vil< ' 1 roin Sup. of Doc, U.S. Government Printing Office

W;. :|,in):'"n. \\( . 20402. SD Cat. No, CI 3

Hrrlrr i rom National Technical Information Service (NTIS)

Sprinfjficid, Virginia 22151

19. SF-CURI'l Y CLASS

(THIS RI'POR r)

UNCL ASSll' IF.D

21. NO. OF PAGES

32

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22. Price