fe alsi

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1/8Please cite this article in press as: T.-S. Shih, S.-H. Tu, Mater. Sci. Eng. A (2006), doi: 10.1016/j.msea.2006.11.017

ARTICLE IN PRESS+Model

MSA-21833; No.of Pages8

Materials Science and Engineering A xxx (2006) xxxxxx

Interaction of steel with pure Al, Al7Si and A356 alloys

Teng-Shih Shih , Shu-Hao Tu

Department of Mechanical Engineering, National Central University Chung-Li, Taiwan 32001 , Taiwan, ROC

Received 19 April 2006; received in revised form 6 November 2006; accepted 6 November 2006

Abstract

Carbon steel bars were dipped in aluminum alloy melts for different periods of time. A reactive ironaluminum intermetallic layer grew at the

interface between the melt and the steel substrate. This reactive layer was mainly composed of intermetallic FeAl (Fe 2Al5) and its thickness was

influenced by dipping time (360 min), surface roughness (0.74m versus 0.07m) and carbon content (0.20.4 wt%) of the steel bars and the

elements (mainly Si and Mg) added in the aluminum alloys. An oxide film, which existed at the interface between steel bar and melt, hampered theAlFe reaction. In comparison with a pure Al melt, the thickness of the reactive layer of the Al7Si alloy melt decreased after a very short dipping

time (3 min), increased following an increase in the dipping time (10 min) then decreased after a long dipping time (>20 min). The thickness of the

reactive layer of a 1040 steel bar dipped in an A356 alloy (Al7Si0.4Mg) melt for a given dipping time at 973 K, was the narrowest among all

melts studied; including pure Al, Al7Si, Al1Mg and Al7Si0.4Mg alloys.

2006 Elsevier B.V. All rights reserved.

Keywords: Oxide film; Reactive layer; Intermetallics

1. Introduction

In the casting process aluminum alloys are poured or injected

into the mold cavity to make the parts. The die set is preheated toreduce the temperature drop that occurs during the filling of the

melt. Themolten metal must be overheated (>Tm; melting point)

to enhancing its fluidity during filling. Therefore, metal dies that

come into contact with the molten aluminum are subjected to a

high temperature for a period of time. The die material often

contains 0.20.45 wt% carbon associated with some amount of

the alloys, such as Mo, Cr, Mn, etc., to improve the heat and/or

wear resistance. When a molten aluminum alloy comes into con-

tact with a die, a reactive layer develops between the interface of

the aluminum alloy and die substrate. Once die soldering occurs

at the interface, it is difficult to strip the cast parts from the

die cavity, resulting in both increased labor cost for removing

the casting from the die cavity and deterioration of the surface

quality of the casting.

In an equilibrious state, Fe can react with Al to form Fe 3Al,

FeAl, FeAl2, Fe2Al5 and FeAl3 phases [1]. However, not all

of these intermetallic compounds occur during the dipping test;

only the phases of Fe2Al5 and FeAl3 have been confirmed [2].

Corresponding author. Tel.: +886 3 4267317; fax: +886 3 4254501.

E-mail address: [email protected](T.-S. Shih).

Sundqvist and Hogmark found that three intermetallic phases of

FeAl, FeAl2, Fe2Al5 and FeAl3 formed on the surface of steel

during dipping tests [3]. Shankar and Apelian observed a reac-

tion at the interface between molten aluminum alloy and toolsteel. They found that phases of-(Al, Fe, Si), FeAl3, Fe2Al5formed, in that order, toward the tool steel matrix [4]. Wang et al.

described a crystalline Fe2Al5 orthorhombic structure that con-

tained 30% voids along the C-axis which made the Al diffuse

much more rapidly into the diffusion front. Consequently one

can see that the, Fe2Al5 phase favors a rapid influx of aluminum

atoms to the growth front of the crystal, resulting in the occur-

rence of a serrated interface, or tongue morphology between

the reactive layer and steel substrate [2]. Kobayashi and Yakou

found that Fe2Al5 mainly formed when the diffusion temper-

ature was less than 1073 K, but it might also occur when the

immersion temperature is as high as 1173 K for 300 s. Increas-

ing the diffusion temperature to 1273 K causes Fe3Al, FeAl and

Fe2Al5 phases to grow, in that order, in conjunction with steel

substrate [5].

Sundqvist and Hogmark also found that adding silicon to an

aluminum alloy hampered the growth of the Fe2Al5 phase in

the hot dip aluminizing process. When the surface of the H-13

tool steel specimen was protected by an oxide layer the initia-

tion and growth of the intermetallic phases was limited to some

area where the oxide layer wasdeficient [3]. The effect of silicon

hampering the reaction rate of the intermetallic phases is definite

0921-5093/$ see front matter 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.msea.2006.11.017
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MSA-21833; No. of Pages8

2 T.-S. Shih, S.-H. Tu / Materials Science and Engineering A xxx (2006) xxxxxx

but the reasons why silicon retards the growth of reacted layer

are controversial. Nicholls explains that silicon changes the dif-

fusion conditions of the Fe2Al5 phase, resulting in slower solid

state growth [6]. Komatsu et al. found that silicon accelerated

the velocity of iron enrichment in initially iron-free aluminum

melts [7]. Eggeler et al. used low carbon steel (0.18 wt% C)

and Al + 2 wt% Si alloys to study the influence of silicon on

the growth of the intermetallic layer during hot dip aluminizing.

They found that silicon acts on the solid state side and reduces

thickness of the layer after hot dip aluminizing [8]. Shankar and

Apelian found that silicon precipitates within the Fe2Al5 phase

and at the interfacial boundary between Fe2Al5 and the ternary

(Fe, Al, Si) phase layer [4].

Few researchers have focused on the initial transient interac-

tions occurring at the interface between the steel substrate and

molten aluminum alloys. Thus, in this study we measured the

reactive layers thickness and discuss the progressive develop-

ment of the interface between the steel substrate and the reactive

layerbased on microstructural observation and description of the

phenomenon. The discussion covers both the effects of oxidelayer decomposition and its interaction with silicon and magne-

sium during growth of the reactive phase (Fe2Al5).

2. Experimental procedure

In the hot dipping test, 1020 and 1040 carbon steel samples

16 mm in diameter and 50 mm in length were used; they were

0.20.4 wt% C, 0.450.9 wt% Mn. Before dipping, the steel bars

were polished by abrasive papers to remove any surface oxide.

The bars surface roughness remained at about 0.74 m (Ra)

to assess the effects of the carbon contents of different melts

on the reactive layer. A fine surface roughness of 0.07 m was

adopted for a comparison of the effects of the surface roughness

on the forming of the reactive layer. The alloys were prepared

from pure commercial-grade aluminum (99.86 wt%), Al2 wt%

Mg, Al50 wt% Si and A356 alloy (Al7wt% Si0.4 wt% Mg).

The different alloy melts are listed in Table 1. The pure Al and

aluminum alloys were melted in a muffle furnace and held at

a temperature 973 K. The steel bars were preheated at 523 K

for 10 min then dipped in the melts for different dipping time

periods: 360 min.

After dipping for a giving time, the steel bars were pulled out

from the melt, quenched in water then removed for sectioning

and sample preparation. The samples were then observed by anoptical microscope, and a scanning electron micrograph (SEM)

equipped with an EDS analyzer. After dipping, the surface of

steel bar was coated with a layer of aluminum or aluminum

alloy. The steel bar might become partially bonded with the

coated aluminum or aluminum alloys if the dipping time was

not long enough to generate a reactive layer (AlFe Intermetallic

phases). Once the bars surface was partially bonded, the coated

aluminum could be detached from the steel bar during sample

preparation and no reactive layer thickness would be recorded.

The thickness of the reactive layer of the sectioned sample was

measured for at least three observations to get a range of reactive

layer thicknesses.

3. Results and discussion

After the dipping test, the steel bars were sectioned and

removed for microscopic observation. The thickness of the reac-

tive layers between the aluminum alloy melt and the steel was

measured; see Table 2.

3.1. Interaction of 1040 carbon steel and pure aluminum

3.1.1. Effect of dipping time

Fig. 1ad shows the progressive development of the 1040

steel substrate, reactive layer and the interface between the steel

and the pure Al, for dipping times of 10, 20, 40 and 60 min,

respectively. When the dipping time was short, the thickness of

the reactive layer thickness varied significantly and the interface

was bumpy. Some areas displayed a thicker reactive layer, but in

some locations it was very narrow, as shown in Fig. 1a. Increas-

ing the dipping time caused the interface to become more ragged

and show a tongue morphology; see Fig. 1b. Further increasing

thedipping time causedthe raggedinterface to becomesmoother

but pores were generatedin thereactive layer as shown in Fig.1c.

The interface again becomes bumpy when the dipping time isextended to 60 min; see Fig. 1d. The morphology of the inter-

face between the reactive layer and the steel substrate varies

depending on the dipping time (or reaction rate) and is affected

by carbon diffusion in the substrate, aluminum diffusion in the

reactive layer and their interaction.

3.1.2. Effect of surface roughness

Highly polished 1040 steel bars (Ra= 0.07m) are first

dipped in pure Al melt for different time periods, and the

reactive layer thicknesses measured and listed in Table 2. A

comparison of the thickness of the reactive layers on 1040

steel bars with different surface roughnesses showed that thehighly polished samples had a wider layer of thickness when

the dipping time was less than 10 min; see Table 2 for a

comparison.

Table 1

Materials used in this study

Alloy Composition (wt%)

Al Cr Cu Fe Mg Mn Si Ti Zn

Al7Si Bal. 0.098 7.67 0.007

Al1Mg Bal. 0.10 0.95 0.09

A356.2 Bal. 0.10 0.12 0.30.45 0.05 6.77.5 0.20 0.05
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T.-S. Shih, S.-H. Tu / Materials Science and Engineering A xxx (2006) xxxxxx 3

Table 2

Measured reactive layer thickness (m) for 1020 and 1040 steel bars dipped in different Al alloy melts, including pure Al, Al7Si, Al1Mg and A356 alloys; dipping

times ranged from 3 to 60 min

Time (min) Parameters

1020 Pure Al abrasive

P80 (Ra: 0.74)


P80 (Ra: 0.74)


P1 000 (Ra: 0.07)

1040 Al7Si

abrasive P80

(Ra: 0.74)

1040 Al1Mg

abrasive P80

(Ra: 0.74)

1040 A356

abrasive P80

(Ra: 0.74)

3 NA 1622a 3037 34 3446a,b NA

10 82136 4550 4570 4776 4862 3040

20 91200 114136 77148 6694c 7195 3550

40 200318 227236 186210 8096c 119152 4560c

60 291336 264282 214257 90136c 210229 90100

Ra indicates the measured surface roughness of the polished steel bar (unit: m).a Partial bond.b Spalled intermetallic compounds from layer.c Film between the interfaces steel/intermetallic compounds.

Fig. 2 shows the interfacial and reactive layers of a highly

polished 1040 steel bar dipped in pure Al melt for 3 min. When

the steel bar was dipped in the aluminum melt, air trapped in the

surface notches would react with aluminum to form alumina.

The fine surface of the steel bar possesses fine notches, allow-

ing more fine oxide particles at the interface than for the rough

surface sample. These particles are potential sites for carbon

diffusion and lead to a higher extent of carbon depletion zones

ahead of the melt. A carbon depletion zone is defined as an area

of ferrite with few tiny spherical carbide particles. Consequently

a thicker reactive layer is produced on the fine steel bar than on

the rough steel bar when the dipping time is short, i.e., 3 min.

Sundqvist and Hogmark describes that the initiation and growth

of the intermetallic phases noting that they are affected by the

Fig. 1. Progressive development of reactive layer andinterface between 1040 steel andpure Al after different dipping time periods: (a) 10min; (b) 20min; (c) 40min;

(d) 60 min; the surface roughness of 1040 steel bar is Ra = 0.74m.

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Fig. 2. SEM micrograph showing the interface and reactive layer of 1040 steel

bar (Ra = 0.07m) dipped in pure Al melt for 3 min at 973 K; EDS analyses at

points 1 to 7 are also included.

oxide and limited to a few locations [3]. Both fine notches, which

offer more contacts for AlFe reaction, and a higher extent of

carbon depletion zones are beneficial for producing a thicker

reactive layer.

Wang et al. described that during aluminizing, Al reacts with

cementite. Carbon reduced from the cementite combines with

theiron to form FeAl intermetallics [2]. Fig.2 shows thatlamel-

lar cementite becomes spherical at a holding temperature 973 K.

The carbide particles are reduced in size by decomposition and

carbon diffusion. A comparison of the carbon concentrations

detected at different positions, from points 1 to 7 shows that

the carbon in the reactive layer is more soluble than in the fer-rite substrate. The area from point 1 to point 3 is covered by a

carbon depletion zone although the carbide particles at point 3

are slightly coarser than at point 2 or point 1. The lower twin-

tongue tip shows a small piece of extruded reactive phase into

steel substrate (or carbon depletion zone), as shown at A. EDS

analysis confirms that the reactive layer is indeed mainly com-

posed of mainly Fe2Al5 as detected by Kobayashi and Yakou

[5]. Burkhardt et al. describe the three-dimensional framework

of Fe and Al1 atoms in the Fe2Al5 structure and outline the

interatomic distance between the atoms building the framework.

The channels that form in the Fe2Al5 structure are shaped like

pentagonal antiprisms [9]. Carbon gradually decomposes from

Fig. 3. SEM micrograph along with line scanning of Al, O, Mn, Fe elements

across the valley of the reactive layer from point A to point B for a 1040

steel bar (Ra = 0.07m) dipped in the pure Al melt for 3 min at 973K.

spherical carbides to diffuse into the reactive layer via the chan-

nel. This can be partly confirmed by the tested carbon contents

from point 4 to point 7.

Fig. 3 shows the line scanning of Al, C, Mn and Fe from point

A to point B for the highly polished 1040 sample dipped in

pure Al melt for 20 min. The aluminum and iron profiles show

a gradual transition near the interface where transition phases,

such as FeAl may develop. Maritra and Gupta used a diffusion

couple experiment with pure Fe and a high purity AlSi eutectic

alloy. They found that Fe2Al5 was the only reactive phase at

a temperature of 873 K but that FeAl and Fe2Al5 phases were

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observed at 1073 K [10]. If reactive phase AlFe did occur, it

should be minor and lie between the Fe2Al5 phase and the ferrite

substrate as depicted by the Al and Fe profiles in Fig. 3.

A cavity (or pore) on the valley of the reactive layer is shown

in Fig. 2. After the steel bar comes into contact with the melt,

oxide particles form and/or the oxide film fractures, which pro-

vides potential sites for carbon diffusion and generates a carbon

depletion zone in the nearby substrate. Aluminum reacts with

Fe to form the reactive phase, Fe2Al5. The reactive phase pro-

trudes into the steel substrate.The preferred orientation develops

into the so-called tonguestructure. Oxide particles (with carbon)

may be trapped in the reactive layer or pushed into the valley

of the reactive layer. The reactive phase grows preferentially in

a direction perpendicular to the steel substrate but also normal

to the side-wall, depending on the carbon depletion zone devel-

oped ahead of the interface. The reactive layer grows side-walls

which seal as a gate when the two walls make contact. A piece of

carbon depletion zone is therefore trapped in the reactive layer,

as shown in area above point A in Fig. 2. Carbon persistently dif-

fuses into the reactive layer, decreasing the extent of the carbondepletion zone, to finally becomean entrapped pore, as indicated

by the arrow in Fig. 2.

3.1.3. Effect of carbon content

Decreasing the carbon content of the steel bar increases the

thickness of the reactive interfacial layer both for short and long

dipping times. A comparison of 1020 with 1040 steel is shown

in Table 2. The 1020 steel bar possesses both lower carbon con-

tent and a higher fraction of ferrite than does the 1040 steel bar.

Therefore, when the 1020 steel bar was dipped in the pure Al

melt, the reactive layer increased its thickness, because more fer-

rite was available at the interface to react with the Al melt. Thereactive layer would advance more quickly into the steel sub-

strate when there was more ferrite located ahead of the interface

front. In addition,the reactive phase shows favorable growthinto

the steel substrate via the ferrite grain boundaries. The tongue

morphology therefore becomes much more apparent comparing

with the 1040 steel bar dipped in the pure Al melt.

3.1.4. Reactions that occurred at the steel substrate ahead

of the interface

Fig. 4a shows the tongue morphologies of the reactive layer

after 1020 steel was dipped in the pure Al melt for 20 min.

The Fe, C and Al concentrations in the area ahead of the tip

of the tongue were measured by EPMA; see Fig. 4b. The car-bon content is relatively higher at the grain boundaries (both at

A and B) than within the grain. The aluminum concentra-

tion profile displays a peak at the grain boundary A ahead of

the tip of the reactive phase. The interdiffusion coefficients of

the intermetallics, FeAl3 and Fe2Al5, at 1073 K, are 9.2 106

and 1.84 104 cm2 s1, respectively [11]. The diffusion coef-

ficient of carbon in ferrite at 973 K is about 1.8 106 cm2 s1

[12]. Aluminum diffuses through the intermetallics toward

the ferrite grains but along the ferrite boundaries ahead of

the tip of the reactive phase. Aluminum reacts with carbon

(4Al+3C Al4C3; Gf= 163.16 kJ/mol at 1000K) to form

aluminum carbide at the grain boundaries, as at point A of

Fig. 4. SEM micrograph along with line scanning of C, Fe and Al elements

indicating the protruding reactive phases, entrapped aluminum carbide+ carbon

and side-wall growth of reactive phases for a 1020 steel bar dipped in pure Al

for 20min.

Fig. 4b. The carbon in the ferrite diffuses to the interface andthe grain boundaries; see A and B. Ferrite reacts with the

aluminums to form Fe2Al5 via the following reactions:

Fe + Al FeAl (11.09 kJ/mol),

FeAl + Al FeAl2 (16.99 kJ/mol),

FeAl2 + Al Fe2Al5 (19.64 kJ/mol).

The reactive product Fe2Al5 protrudes into the ferrite sub-

strate as shown in Fig. 2 below mark A. Aluminum carbide

(or carbon) would move along the interface to approach the val-

ley, to finally become a particle entrapped in the reactive layer.

Fig. 4 shows that the reactive phase protrudes into ferrite sub-

strate associated withaluminum carbide (or with carbon) trappedin the reactive layer. EDS analyses at the interface between the

steel substrate and the reactive phase indeed confirms the exis-

tence of carbon. Side-wall growth also occurs in the reactive

layer as indicated in Fig. 4.

3.2. Interaction of a 1040 steel bar with an Al7Si alloy

Table 2 shows that when a 1040 steel bar is dipped into a

Al7Si melt, after a short dipping time of 3 min the reactive

layer is 310m, which is far less than for the sample dipped

in the pure Al melt which is 1622m in thickness. However,

increasing the dipping time to 10 min in the Al7Si alloy melt

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only causes the thickness of the reactive layer to be slightly

greater than that dipped in the pure Al melt, i.e., 4776 m

versus 4550m. Extending the dipping time to longer than

20 min causes the reactive layer of the pure Al7Si alloy to be

significantly narrower than in pure Al. For a dipping time of

60 min, the thickness of the former is 90136m and the latter

is 264282m.

As explained previously, oxygen and/or an oxide film exists

in the surface notches of the steel bar when dipped in the alu-

minum melt. Theoxide film at the interfacebetween the steel bar

and aluminum melt tends to hamper the AlFe reaction and to

decrease the thickness of the reactive layer for a given dipping

time. The addition of silicon to the aluminum melt decreases

the surface tension of the melt slightly. When silicon comes in

contact with oxygen in the melt, the following reaction occurs

[13]:

3{Si} + 3(O2) 3[SiO2] + 4{Al} 3{Si} + 2[Al2O3],

(1)

where {}, () and [] represent the liquid, gas and solid states,respectively. Air trapped in the surface notches drives the above

reaction leading to the decomposition of alumina at the interface

between the steel bar and the Al7Si alloy melt.

Fig. 5 shows the morphology of the reactive layer that formed

between the 1040 steel bar and Al7Si alloy after 3 min of dip-

ping time. The reactive layer varies its thickness from 3 m

to about 10m. The white spot in the layer contains carbon

and indicates the possible existence particles of Al4C3, since no

oxygen was detected. Optical observation shows that the inter-

face between the reactive layer and the steel substrate becomes

smoother after dipping time is increased.When the 1040 steel bar was dipped in the Al7Si alloy melt,

oxide formed at interface. Carbon diffusing from the ferrite sub-

strate into grain boundaries produced a carbon depletion zone

for AlFe reaction. Aluminum carbide might also form at the

interface. The reactive Fe2Al5 layer is thin due to a mass deficit

of the Al content (93 wt% compared with pure Al) in the melt

after a short 3 min dipping time. Increasing the dipping time

to 10 min of the sample dipped in an Al7Si alloy melt leads

to a slightly greater extent of the reactive layer, probably due

to effect of silicon. The oxide is decomposed into fine particles

which are pushed into valleys in the reactive layer or at the inter-

face between the reactive layer and the steel substrate, in turn

offeringmore sites for carbondiffusion. A carbondepletion zonecan thus be readily formed ahead of the interface increasing the

driving force needed to form Fe2Al5.

Maritra and Gupta had studied an intermetallic compound

in a FeAlSi ternary system to find the solubility of Si in

FeAl3, Fe2Al5 and Fe3Al: it is in the range of 17 at% [10].

The solubility of silicon in the reactive layer, Fe2Al5, is partly

confirmed by the EDS test results shown in Fig. 5. The capa-

bility of the reactive layer for aluminum diffusion as well as

carbon diffusion is therefore greatly reduced. The driving force

needed to form the reactive phase is greatly decreased. Table 2

shows that when the dipping time is longer than 20 min the

extent of the reactive layer is greatly reduced; comparing the

Fig. 5. SEM micrograph showing the morphology of the reactive layer associ-

atedwith EDS analysesof white spots (aluminumcarbide) trapped in thereactive

layer for a 1040 steel bar dipped in Al7Si alloy melt for 20 min.

results of the 1040 sample dipped in pure Al and in the Al7Si

melt.

Fig. 6a shows the morphologies of reactive phase after a

20 min dipping in the Al7Si melt Fig. 6bd show the EDS

analyses at points B, C and D. The film-like structure

ahead of the interface contains mostly aluminum carbide, some

silicon or silicon carbide, and carbon. The SiC formation energy

is about 65.25 kJ/mol at 1000 K, which is lower than for forming

Al4C3, but higher than for forming Fe2Al5. The two white spots

detected in Fig. 6c and d are entrapped oxide.

3.3. Interaction of a 1040 steel bar with an Al1Mg alloy

For short dipping times there is a bigvariation in thethickness

of the reactive layer due to partial bonding and even peeling off

of the coated layer during sample preparation. The thickness of

the reactive layer is close to that of the sample dipped in the pure

Al melt when the dipping time is 10 min; see Table 2. When Mg

encounters oxygen and alumina, the following reaction occurs

[13]:

(2)

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Fig. 6. (a) SEM micrograph showing the morphology of the interfacial and reactive phase; EDS analyses (b) at interface between steel substrate and reactive

phase showing the existence of aluminum carbide and carbon; (c) oxide particle C; (d) oxide particle D for a 1040 steel bar dipped in Al7Si alloy melt for

20 min.

Shih and Liu also found that when the thermally formed alu-

mina was in contact with the Al0.5Mg alloy melt it resulted

in a thin film of MgAl2O4 forming at the interface between the

oxide and the melt [14]. When the 1040 steel bar came in con-

tact with the Al1Mg alloy melt, a film of spinel (MgAl2O4)

formed at the surface of the steel bar. If the dipping time was

short, the mass content of Mg was not enough to cover the

whole surface of the steel bar. The reactive phase, Fe 2Al5, wasgenerated on part of the surface and part incorporated spinel.

The measured data therefore show a big variation along with

partial bonding (even peeling off). Increasing the dipping time

caused the spinel to fracture and be trapped in the reactive layer.

The tongue morphology shown is much more apparent than

for the sample dipped in the Al7Si melt but is similar to that

obtained after dipping in the pure Al melt. It is detected that

Mg piled up in the area between the reactive phase and the melt

ahead of the interface (1.22 wt%, average of three tests). There

is no ternary AlFeMg intermetallic phase is in equilibrium

with the aluminum-rich solid solution [15]. When the reaction

layer increases in extent, Mg builds up ahead of the interface

between the reactive phase and the melt to form Mg2Al3. This

binary intermetallic phase hampers the diffusion of aluminum

atoms from the melt into the reacted phase and slows down the

AlFe reaction reducing the thickness of the reactive layer; see

Table 2.

3.4. Interaction of the 1040 steel bar with the A356 alloy

Thethickness of thereactive layer of the1040 steel bardipped

in the A356 alloy melt is the minimum among all the alloys stud-

ied; see Table 2. EDS analysis confirmed that there was silicon

in the reactive layer but no magnesium. Silicon has a smaller

atomic size than magnesium and tends to diffuse into the reac-

tive layer, greatly decreasing thecapability of Al atoms to diffuse

from the melt to react with the steel substrate. Magnesium accu-

mulates at the interface between the reactive layer and the melt

forming the binary intermetallic phase. The synergistic effects

of silicon and magnesium in the A356 melt significantly ham-

pered the growth of the reactive phase Fe2Al5 resulting in the

minimum layer thickness.

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4. Summary and conclusions

The interface between the reactive phase (Fe2Al5) and the

1040 steel substrate grew from a tongue structure to a smooth

structure to finally become ragged following the increasing dip-

ping time in the pure Al melt. The tongue structure was much

more apparent for the 1020 steel bar than for the 1040 steel bar.

When the dipping temperature was 973 K, lamellar cementite

in the steel substrate became spherical and gradually decom-

posed. Carbon diffused to the ferrite grain boundaries and to the

interface between the steel substrate and the reactive phase, to

react with the inward diffusion of Al to form Al4C3. The carbide

particles associate with the oxide particles would be trapped in

Fig. 7. Schematic illustrationsshowing the progressive developmentof: (a) con-

tact between melt and steel substrate; (b) diffusion of carbon into ferrite grain

boundaries and interface; (c) formation of carbon depletion zone in substrate

and aluminum carbide at interface; (d) the reacted phase protruding into fer-

rite substrate along with Al diffusion inward to steel substrate; (e) formation of

carbon depletion zone ahead of the tip of reactive phase; (f) morphologies of

protruding reacted phase associated with side-wall growth and entrapment of

oxide particles, aluminum carbide plus carbon for carbon steel dipped in pure

Al.

the reactive layer and it would grow into the carbon depletion

zone in the steel substrate. Fig. 7af schematically illustrate the

development of oxides at the interface, the carbon diffusion, the

formation of Al4C3, the entrapment of oxides and Al4C3 + C,

leading to the advancing and side-wall growth of the reactive

phase.

Dipping in the Al7Si alloy melt caused the silicon to react

with the alumina oxide. The oxide fragments (fine particles)

located at the interface provided potent sides for carbon dif-

fusion, increasing the driving force for the formation of the

reactive phase, if the dipping time was short. Silicon diffused

into the reactive layer greatly reducing the Al and carbon diffu-

sion capability and reducing the AlFe reaction rate, following

the increase in dipping time.

Dipping in the Al1Mg alloy melt caused spinel to form at

the interface between the steel substrate and the reactive phase.

Increasing the dipping time led to the fracturing of the spinel

and the fine pieces of oxide particles stimulated carbon diffu-

sion, increasing the AlFe reaction rate, for a short dipping time.

Mg would gradually build up at the interface between the reac-tive layer and the melt, leading to the formation of a binary

MgAl intermetallic phase. The AlFe reaction rate therefore

decreased if the dipping time was long, decreasing the reacted

layer thickness.

Dipping in the A356 alloy melt caused the synergistic effects

of: (a) Si being diffused into the matrix of reactive phase, which

reduced the Al and carbon diffusion capability, and (b) Mg

piled up at interface between the reactive phase and the melt.

These effects greatly hampered the growth of the reactive phase

resulting in the minimum layer thickness among all the alloys

studied.

References

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[11] K. Bouche, F. Barbier, A. Coulet, Mater. Sci. Eng. A-Struct. Mater. Prop.

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[12] J. Campbell, Casting, second ed., 2003, p. 13.

[13] L. Yang, D. Zhu, C. Xu, J. Zhang, Metall. Trans., A, Phys. Metall. Mater.

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[14] T.S. Shih, Z.B. Liu, Mater. Trans. 46 (8) (2005) 18681876.

[15] T. Lyman (Ed.), Metals Handbook, eighth ed., 1976, p. 392.

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