Semi-Solid Microstructure Control of Wrought Al-Mg-Si Based Alloys

with Fe and Mn Additions in Deformation-Semi-Solid-Forming Process*1

Chakkrist Phongphisutthinan*2, Hiroyasu Tezuka and Tatsuo Sato

Department of Metallurgy and Ceramics Science, Tokyo Institute of Technology, Tokyo 152-8552, Japan

The effects of Fe, Mn and Fe/Mn-combined additions on the refinement of the spheroidized semi-solid �-Al grains in the wroughtaluminum 6000 based alloy was investigated. The semi-solid microstructure control of Al-1.2%Mg-1.3%Si based alloys with Fe (1.0 mass%),Mn (0.7 mass%) and Fe/Mn-combined (1.0/0.7 mass%) additions was studied by the Deformation Semi-Solid Forming (D-SSF) process. Thisprocess consists of high deformation before semi-solid heating in order to introduce high density of dislocations and fragmentation ofintermetallic compounds. Various second phase particles strongly affect the resultant �-Al grain size. The deformation process of the Fe-addedalloy with a low rolling ratio produces fine �-Al grains with an average size of 76 mm, which is apparently smaller than that of the Fe-free alloywith an average �-Al grain size over 100mm. The Mn addition required a high deformation ratio to achieve an average �-Al grain of 85mm. Thecombination of Fe and Mn additions produces the finest grain size of 64mm. The morphology of the Fe-intermetallic compound in the semi-solidforming process can be controlled by the cooling rate and Mn addition. The D-SSF process is a promising process to modify the harmful Feimpurity into the useful intermetallic compounds. [doi:10.2320/matertrans.L-MZ201119]

(Received October 1, 2010; Accepted January 28, 2011; Published April 20, 2011)

Keywords: semi-solid, thixoforming, deformation semi-solid forming process, iron impurity, 6082 aluminum alloy

1. Introduction

A semi-solid metal forming process is a manufacturingprocess of alloys in the semi-solid state, which consists ofglobular solid grains in the liquid phase at the temperaturesbetween solidus and liquidus. The unique characteristics ofthe semi-solid forming process enable the opportunity toproduce high quality products with high mechanical proper-ties and good formability compared with the solid forming orliquid casting process. However, this process requires narrowtolerances in many steps from the feedstock production toforming process.

The wrought Al-Mg-Si 6000 series alloys have manyadvantages such as good formability, good corrosion resist-ance, high mechanical properties and good precipitationhardening behaviors compared with conventional cast alu-minum alloys. The studies of wrought aluminum alloys in thesemi-solid forming process are still relatively limitedcompared with cast aluminum alloys because of the highsensitivity of the liquid fraction to the semi-solid temperaturein wrought aluminum alloys.1–4) In spite of that complication,the semi-solid forming process of wrought aluminum alloysis attractive to produce high quality products with goodformability and superior mechanical properties.5,6) Theprevious research work7,8) indicates that the additions ofMn and Cr in the Al-Mg-Si alloys are effective to producefine Al grains in the thixoforming process.

Nowadays, a recycling process of aluminum alloysbecomes very important from the view points of the priceand resources. The recycled aluminum scraps contain manykinds of trace elements. Fe is a major impurity in recycledaluminum scraps and extremely degrades properties ofaluminum alloys in the manufacturing process and final

products. The Mn addition to the Al-Si-Fe alloys is reportedto be effective to modify the morphologies of Fe-intermetal-lic compounds into the more favorable Chinese-scriptAlFeMnSi morphology.9,10) Furthermore, Mn-containingdispersoids are precipitated after homogenization and con-tribute to produce refined �-Al grains during heating andholding at the semi-solid temperature in the 6000 seriesalloy.5)

In this study, the semi-solid forming behaviors of thewrought Al-Mg-Si based alloys with Fe and Mn additionswere investigated by the Deformation-Semi-Solid Forming(D-SSF) process. The D-SSF process consists of plasticdeformation by cold-rolling to introduce high density ofdislocations, which are effective to refine the new equiaxedgrains by recrystallization during reheating to the semi-solidtemperature.2,7,8,11–13) At the semi-solid temperature, lowmelting points intermetallic compounds and �-Al grainboundaries start to melt and the solid grains becomesurrounded by the liquid phase. The small solid grains canimprove the formability of the semi-solid state and conse-quently enable to produce high quality near net shapeproducts. The fine solid grains with the size less than 100 mmand the adequate liquid fraction are required to take thebenefit of the semi-solid forming process.2–4) The plasticdeformation directly contribute to crush large size interme-tallic compounds such as the long plate-like �-AlFeSicompound commonly formed with impurity Fe into finefragmented dispersoids. The modifications of Fe-intermetal-lic compounds by the D-SSF process and alloy compositionmodification are demonstrated in this study.

2. Experimental

The chemical compositions of the Al-Mg-Si based alloysin this study are shown in Table 1. The high contents of basedMg and Si are in the standard compositions of the 6082 alloy.Fe, Mn and Fe/Mn combined additions were used to

*1The Paper Contains Partial Overlap with the ICAA12 Proceedings by

USB under the Permission of the Editorial Committee.*2Corresponding author, E-mail: chakkrist.p.aa@m.titech.ac.jp

Materials Transactions, Vol. 52, No. 5 (2011) pp. 834 to 841Special Issue on Aluminium Alloys 2010#2011 The Japan Institute of Light Metals

investigate the effect of various intermetallic compounds onthe spheroidization of the �-Al grains at the semi-solid stateand the resultant intermetallic compounds in the 6000 seriesalloy. Figure 1 illustrates the experimental procedure of theDeformation Semi-Solid Forming (D-SSF) process. Thealloy melt under an Ar protective atmosphere was cast intoingots using a cast iron mould. Then the ingots werehomogenized at 530�C for 86.4 ks in a salt bath. After thatthey were cut into 10–15 mm thickness and deformed bycold-rolling between 40–70% reduction ratios at roomtemperature. Finally, the specimens with the dimension of5� 8� 8 mm3 were heated to the semi-solid temperaturewith a horizontal infrared imaging furnace, with the heatingrate of 90�C/min to 575�C and subsequently 30�C/min to thesemi-solid temperatures as shown in Fig. 2 at 634 and 637�Cfor 600 and 300 s, respectively. The semi-solid specimenswere subsequently cooled with different cooling rates (waterquenching, air cooling and cooling at 30�C/min) to the roomtemperature for microstructure observation. The specimenswere polished and etched by the dilute Tucker’s reagent andcharacterized with an optical microscope for microstructuralobservation and measurement of sizes of the intermetallicparticles and �-Al grain. The characteristics of the semi-solidmicrostructure were evaluated by the image analysis with theScion Image software. The average grain size of �-Al (D) andshape factor (F) are defined as

D ¼ffiffiffiffiffiffiffiffiffiffiffi4A=�

pð1Þ

F ¼ P2=4�A ð2Þwhere A is the grain area and P is the grain perimeter. Thesolidus and liquidus temperatures were determined by adifferential scanning calorimetry (DSC). The 40.0 mg sam-ples were measured in a stainless steel pan at the heating rateof 5�C/min. The reference cell was an empty stainless steelpan. The liquid fraction was calculated by the area of anendothermic peak of the liquid phase. The in-situ observationof microstructure during semi-solid heating was conductedby a laser scanning microscope. The specimen with a 5 mmdiameter was heated in a sapphire pan under the Ar gasatmosphere with the heating rate of 50�C/min.

3. Result and Discussion

3.1 Liquid fraction evaluation by DSC and microstruc-ture

The measured DSC curves for the alloys are shown inFig. 3. The solidus, liquidus temperatures and the semi-solidtemperature ranges determined from the DSC results aresummarized in Table 2. The endothermic peaks indicated byarrows at around 580�C in these alloys correspond to themelting of Mg2Si. The liquid fraction was calculated as afunction of temperature from the area of the endothermicpeak of the liquid phase under the normalized line. Thecalculated liquid fractions of four alloys at 637�C aresummarized in Fig. 4. The Fe-free alloy has the lowestliquid fraction of 42% and the highest liquidus temperaturedue to its least amount of alloying elements. The Mn-addedand Fe/Mn-added alloys have liquid fraction of 46% and theFe-added alloy has the highest liquid fraction of 63%.

The liquid fraction of the water quenched semi-solidmicrostructures are summarized in Table 3. The maximumliquid fractions are 19%, 23%, 26% and 37% in the Fe-free,

Table 1 Chemical compositions of Al-Mg-Si based alloys (mass%).

Alloy Mg Si Fe Mn Mg2Si Excess Si Al

Fe-free 1.13 1.29 0.02 <0:01 1.53 0.89 Bal.

Fe-added 1.15 1.28 1.03 <0:01 1.56 0.87 Bal.

Mn-added 1.15 1.28 0.03 0.72 1.56 0.87 Bal.

Fe/Mn-added 1.15 1.28 1.03 0.73 1.56 0.87 Bal.

Semi-solid state

Homogenization530°°C, 24h

Casting

Tem

per

atu

re (

°C)

Time

Deformation

Liquidus

Solidus

R.T.

Fig. 1 Deformation-Semi-Solid Forming (D-SSF) Process.

0

100

200

300

400

500

600

700

0 120 240 360 480 600 720 840 960 1080 1200

Tem

per

atu

re, T

/°°C

Time, t /s

liquidus

90 °C/min

30 °C/minsolidus

Fig. 2 Semi-solid heating profile by horizontal infrared image furnace.

-90

-80

-70

-60

-50

-40

-30

-20

-10

0

550 570560 580 590 610600 620 630 650640 660

Hea

t fl

ow

(m

W)

end

oth

erm

ic e

xoth

erm

ic

Temperature, T /°°C

Fe-free alloy

Fe-added alloy

Mn-added alloy

Fe/Mn-added alloy

Fig. 3 Differential scanning calorimetry (DSC) results of the Fe-free alloy,

Fe-added alloy, Mn-added alloy and Fe/Mn added alloy.

Table 2 Solidus, liquids, semi-solid temperature range and liquid fraction

at 637�C of Al-Mg-Si based alloys measured by DSC.

Solidus

(�C)

Liquidus

(�C)

Semi-solid

temperature

range (�C)

Liquid fraction

at 637�C

(%)

Fe-free 561 658 97 42

Fe-added 575 650 75 63

Mn-added 571 654 83 46

Fe/Mn-added 579 651 72 46

Semi-Solid Microstructure Control of Wrought Al-Mg-Si Based Alloys with Fe and Mn Additions 835

Mn-added, Fe-added and Fe/Mn-added alloys, respectively.The DSC results clearly show higher liquid fraction values.Atkinson et al.14) observed a lower liquid fraction inthe delay-quenched semi-solid specimen than the rapid-quenched semi-solid specimen. Due to the low amount ofalloying element of the wrought Al-Mg-Si alloy, unlike incast aluminum alloy, some delay during quenching of thesemi-solid heated specimen from the horizontal imageinfrared furnace into water results in apparent decreasedliquid fraction by microstructural observation compared withthe heat absorption measurement by the DSC test. In order toget the accurate liquid fraction at the semi-solid temperatureby microstructure observation, an adequate rapid quenchingtechnique is required to avoid the growth of the primary solid�-Al toward the liquid phase which can result in theunderestimated liquid fraction. Moreover, the considerationand evaluation of the intermetallic particles as the liquidphase can be confusing, because of the thermal stability ofintermetallic compounds in semi-solid state. For example,some intermetallic compounds such as Fe-containing particleare thermally stable during heating to the semi-solid temper-ature. Figure 5 shows the remaining Fe-intermetallic com-pounds after semi-solid heating to 637�C with isothermalholding time of 10 s. These compounds are usually solid

phase and distributing in the liquid phase. However, the Fe-intermetallic compounds become completely melt after longholding time for 300 s.

3.2 Deformation process by cold-rollingIn the thermo-mechanical route of thixoforming process,2)

the deformation process is required to introduce strain energyand high density of dislocations in order to stimulate therecrystallization and resultant spheroidization in the semi-solid state. The deformation process was performed bycold-rolling at room temperature. The maximum thicknessreduction ratios in each alloy (60% in Fe-free, 40% in Fe-added, 70% in Mn-added and 60% in Fe/Mn added alloys)were conducted before the specimens broken. The wroughtAl-Mg-Si 6000 series alloys are well known as their excellentformability for the production of aluminum sheet andextrusion products. However, the presence of high Fe contentof 1 mass% in the Fe-added alloy results in the formationof coarse Fe-intermetallic compounds and consequentlydegrades the formability. Good Fe-added specimens withsome initiated cracks at the edge were obtained with 40%cold-rolling which was considered as maximum in thisexperiment. With further increased rolling ratio to 60%,the specimen becomes broken with severe cracks. TheMn-addition can improve the formability of the high Fe-containing Al-Mg-Si alloy with maximum 60% cold-rollingratio. Generally, Mn is intentionally added to wroughtaluminum alloys in order to improve the formability.Furthermore, Mn can substitute Fe in the AlFeSi compoundsduring crystallization and modify the harmful coarse Fe-containing compounds into more favorable Chinese-scriptAlFeMnSi-containing compound.

3.3 Effect of Fe and Mn additions on semi-solid micro-structures

The effects of various intermetallic compounds in the fouralloys on the average grain size, shape factor and liquidfraction were evaluated based on microstructure observationafter the semi-solid heating and are summarized in Table 3and Fig. 6.3.3.1 Semi-solid microstructures of the Fe-free alloy

The as-cast microstructure consists of Mg2Si in theeutectic regions in Figs. 7(a) and (b). After homogenization,

0

10

20

30

40

50

60

70

80

90

100

560 570 580 590 600 610 620 630 640 650 660

Liq

uid

fra

ctio

n (

%)

Temperature, T /°°C

Fe-free alloy

Fe-added alloy

Mn-added alloy

Fe/Mn-added alloy

Fig. 4 Calculated liquid fraction from DSC results of the Fe-free alloy, Fe-

added alloy, Mn-added alloy and Fe/Mn-added alloy.

Table 3 Average �-Al grain size of the Fe-free, Fe-added, Mn-added and

Fe/Mn added alloys.

AlloyCold-rolling

(%)

Average �-Al

grain size

(�m)

Shape factorLiquid fraction

(%)

Fe-free 40 122.8 1.54 16

60 109.9 1.65 19

Fe-added 40 76.0 1.55 26

Mn-added 40 98.4 1.47 17

60 84.6 1.38 23

Fe/Mn-added 40 70.6 1.37 34

60 63.8 1.36 37

Fe-interetallic

compound

Fig. 5 Remaining solid Fe-compounds after semi-solid heating at 637�C

for 10 s.

836 C. Phongphisutthinan, H. Tezuka and T. Sato

Mg2Si partially dissolves into the �-Al matrix and partiallyremains as spheroidized Mg2Si with small amounts ofimpurities in the eutectic regions as shown in Figs. 7(c)and (d). Consequently, there are less intermetallic com-pounds remaining during cold-rolling and semi-solid heatingcompared with the other alloys. The specimen becamedeformed by 40% and 60% cold-rolling as shown inFigs. 7(e) and (f), the spheroidized Mg2Si particles areslightly deformed whereas the elongated Mg2Si becomesslightly fragmented in Fig. 7(f). Mg2Si particles are mainlylocated at the original grain boundaries, similarly as in the as-cast specimen.

The melting of Mg2Si and grain boundaries of therecrystallized grains was investigated by a laser scanningmicroscope as shown in Figs. 8(a)–(c). The Mg2Si particlesremain as the solid particles at 560�C (Fig. 8(a)) which isbelow the solidus temperature. Mg2Si starts to melt into theliquid phase at the eutectic temperature around 575�C and

becomes a liquid pool at 590�C with the melting ofrecrystallized grain boundaries (Fig. 8(b)). No other solidsecond phase particles should be remaining beyond thesolidus temperature in the Fe-free alloy. With increasedtemperature at 620�C in Fig. 8(c), the liquid regionscontinuously expand around the liquid pool of Mg2Si andrecrystallized grain boundary as the melting of the solid-liquid interface and dissolution of solute atoms in the Almatrix. The properties such as surface wetting of the liquidphase to separate and decelerate the coalescence of the solid�-Al grains can be varied by the alloying elements and arestill unclear in the semi-solid forming process.

40

50

60

70

80

90

100

110

120

130

30 40 50 60 70 80

Ave

rag

e g

rain

siz

e, D

/μμm

Cold rolling (%)

Fe-free alloy

Fe-added alloy

Mn-added alloy

Fe/Mn added alloy

Fig. 6 Average grain sizes of �-Al phase as a fuction of cold-rolling ratio

in Al-Mg-Si based alloys after semi-solid heating at 637�C for 300 s.

(a)

(c)

(e)

Mg

Mg2Si

2Si

Mg2Si

(b)

impurities(d)

(f)

Fig. 7 Microstructures of the Fe-free alloy: (a),(b) as-cast, (c),(d) homo-

genized and (e),(f) 60% cold-rolled.

(a)

(b)

(c)

Melting Mg2Si

Melting recrystallized grain boundary

Melting recrystallized grain boundary

Melting Mg2Si

Mg2Si

Fig. 8 In-situ observation during semi-solid heating of the Fe-free alloy at

(a) 560�C, (b) 590�C and (c) 620�C.

Semi-Solid Microstructure Control of Wrought Al-Mg-Si Based Alloys with Fe and Mn Additions 837

After semi-solid heating at 637�C for 300 s, the averagegrain size of �-Al is 123 mm at 40% cold-rolling in Fig. 9(a)and slightly decreases to 110 mm at 60% cold-rolling inFig. 9(b). There are large amounts of the entrapped liquidphase inside solid grain as shown in Fig. 9(c) as a result ofgrain coarsening around a liquid region and eventuallyenclosed liquid pool by a solid region.3.3.2 Semi-solid microstructures of the Fe-added alloy

The as-cast microstructure in Figs. 10(a) and (b) consistsof the Chinese-script �-AlFeSi, plate-like �-AlFeSi andMg2Si compounds in the eutectic regions. During homoge-nization, Mg2Si is partially dissolved into the �-Al matrix,while Fe-containing compounds do not dissolve into theAl-matrix as shown in Figs. 10(c) and (d). The shape of�-AlFeSi becomes spheroidzed whereas the long �-AlFeSimorphologies remain unchanged after homogenization. Thedeformed microstructures are clearly shown in Figs. 10(e)and (f). The long �-AlFeSi compounds are finely fragmentedafter 40% cold-rolling. The average length of the long plate-like �-AlFeSi particles is refined from 6.9 mm in as-castspecimen to 1.6 mm after 40% cold-rolling.

After semi-solid heating at 637�C for 300 s, the average �-Al grain size is 76 mm as shown in Fig. 11(a), which is much

smaller than that in the case of the Fe-free alloy in Fig. 9(a).The fragmented Fe-compounds associated with high densityof dislocations are useful to produce fine recrystallized �-Algrains and prevent grain coarsening during heating to thesemi-solid temperature. During heating to the semi-solidstate, Fe-containing compounds are stable as the solidparticle in the experimental rapid heating condition unlikethe complete melting of Mg2Si in the Fe-free alloy. Figure 5shows the remaining Fe-compounds after semi-solid heatingat 637�C for 10 s. The finely fragmented Fe-compounds atlow temperature become coarsen during reheating to thesemi-solid state and remain as a thermally stable particle for ashort period in this condition. These remaining Fe-com-pounds effectively prevent the coarsening of the �-Al grain.Nevertheless, Fe-compounds completely melt into the liquidphase after a long holding time for 300 s in Fig. 11(b). There

(a)

αα-Al

(c)

entrapped liquid phase

(b)

α-Al

Fig. 9 Microstructures of the Fe-free after semi-solid heating at 637�C for

300 s with (a) 40%, (b) 60% cold-rolling and (c) typical entrapped liquid

phase inside solid grain and grain boundary of Mg2Si.

(a)

(c)

(e)

(d)

(b)

αα-AlFeSi

(f)

α-AlFeSi

β-AlFeSi

fragmented β

β-AlFeSi

-AlFeSi

Mg2Si

Mg2Si

Fig. 10 Microstructures of the Fe-added alloy: (a),(b) as-cast, (c),(d)

homogenized and (e),(f) 40% cold-rolled.

(a) αα-Al

(b)

Fig. 11 Microstructures of the Fe-added alloy after semi-solid heating at 637�C for 300 s with 40% cold-rolling (a) fine spheroidized �-Al

grain and (b) water quenched liquid region.

838 C. Phongphisutthinan, H. Tezuka and T. Sato

is only few amount of entrapped liquid phase inside the solid�-Al grains in the Fe-added alloy.

Hence, the fragmented Fe-intermetallic compounds areuseful to refine the �-Al grains by this process. However, thedeformation ratio in this alloy is limited because of lowformability due to the high amounts of coarse Fe-interme-tallic compounds.

The formation of Fe-intermetallic compounds in the finalmicrostructure can strongly affect the mechanical properties.One major parameter to control the morphologies of Fe-intermetallic compounds is the cooling rate from the liquidstate.15) Fe-intermetallic compounds can dissolve into theliquid phase at high semi-solid temperature and thencrystallize again during cooling. After semi-solid heating at634�C for 600 s the specimens were cooled down at differentcooling rates. The effects of cooling rates on the crystallizedFe-containing compounds in the Fe-added alloy are shown inFigs. 12(a)–(c). At the high cooling rate by water quenching,fine intermetallic compounds crystallize in the liquid region

as shown in Fig. 13(a). With decreasing cooling rate by aircooling, high amounts of Chinese-script �-AlFeSi com-pounds are observed in Fig. 12(b). Large amounts of longplate-like �-AlFeSi compounds crystallize with decreasingcooling rate at 30�C/min as shown in Fig. 12(c). Therefore,the control of the cooling rate in the Fe-added alloy aftersemi-solid heating strongly affects the formation of harmful�-AlFeSi compounds. In order to prevent the dissolution offine fragmented Fe-intermetallic compounds into liquidphase, lower semi-solid temperatures should be consideredin this Fe-added alloy.3.3.3 Semi-solid microstructures of the Mn-added alloy

The as-cast microstructures in Figs. 13(a) and (b) consistof small amounts of �-Al15(Fe,Mn)3Si2 compounds alonggrain boundaries. After homogenization, fine spherical androd-like Mn-containing dispersoids are precipitated neargrain boundaries as shown in Figs. 13(c) and (d). Thesedispersoids are useful to refine the semi-solid grainsize.8,11) Precipitate free zones (PFZ) are found at the grainboundaries. Figures 13(e) and (f) show deformed micro-structures of 70% cold-rolled specimen. The dendritricmicrostructures with Mn-containing dispersoids and Mg2Sibecome heavily deformed along the rolling direction.

After semi-solid heating at 637�C for 300 s, the average �-Al grain size is 98 mm at 40% cold-rolling as shown inFig. 14(a). Higher deformation of 70% cold-rolling isrequired to achieve fine �-Al grains with the average grainsize of 85 mm (Fig. 14(b)). Small amounts of the entrappedliquid phase are present in the Mn-added alloy. The waterquenched liquid phase is shown in Fig. 14(c). The Mndispersoids were reported as a thermally stable particleduring heating to the semi-solid state,11) which can decelerate

(a)

(b)

(c)

Chinese-script

fine intermetllic

αα-AlFeSi

-AlFeSiβ

compound

Fig. 12 Effect of cooling rate on the crystallization of Fe-intermetallic

compounds in the Fe-added alloy after by (a) water quenching, (b) air

cooling and (c) 30�C/min cooling rate.

(a)

(c)

(e)

(b)

dspersoids

(d)

(f)

rod-like

spherical disersoids

αα-Al15(Fe,Mn)3Si2

Mg2Si

Fig. 13 Microstructures of the Mn-added alloy: (a),(b) as-cast, (c),(d)

homogenized and (e),(f) 70% cold-rolled.

Semi-Solid Microstructure Control of Wrought Al-Mg-Si Based Alloys with Fe and Mn Additions 839

the grain coarsening in the semi-solid state. As a result, morerefined semi-solid grain size was achieved in the Mn-addedalloy compared with the Fe-free alloy.3.3.4 Semi-solid microstructure of the Fe/Mn-added

alloyThe combination of Fe and Mn additions was considered

as the modification of Fe-intermetallic compounds andprecipitation of Mn dispersoids. Fe-intermetallic compoundscan be modified to more favorable Chinese-script �-Al15(Fe,Mn)3Si2 phases by the Mn addition to the Al-Si-Fealloy as shown in Figs. 15(a) and (b). After homogenizationas shown in Figs. 15(c) and (d), fine spherical Mn-containingdispersoids are precipitated in the �-Al grains with precip-itate free zones (PFZ) at the grain boundaries similarly asin the Mn-added alloy. The deformed microstructures ofFe/Mn-added alloy consist of large amounts of fine Mndispersoids as well as the coarser fragmented particles of theFe/Mn-containing compound as shown in Figs. 15(e) and (f).This combination enhances the refinement of �-Al grainsduring heating to the semi-solid temperature.

After semi-solid heating at 637�C for 300 s, the averagegrain size of �-Al is 71 mm by 40% cold-rolling and decreasesto 64 mm by 60% cold-rolling as shown in Figs. 16(a) and (b),respectively. The Mn-containing dispersoids incorporatingwith fragmented AlFeMnSi compounds are extremely effec-tive to refine the �-Al grains.

The Mn addition to the Al-Mg-Si-Fe alloy affects not onlythe formation of the Chinese-script �-Al15(Fe,Mn)3Si2 phasein the as-cast microstructure but also the crystallizationof the AlFeMnSi compound after semi-solid heating.Figures 17(a)–(c) show the polyhedral �-Al15(Fe,Mn)3Si2phase which crystallize independently of cooling rates bywater quenching, air cooling and 30�C/min cooling rate,respectively. The Mn addition is very effective to modify themorphology of the Fe-intermetallic compound into the finepolyhedral compound instead of the Chinese-script shape orplate-like shape Fe-containing compounds.16)

In summary, the typical dendritric microstructures arefound in the as-cast specimens as shown in Figs. 7(a), 10(a),

13(a) and 15(a). After semi-solid heating, the spheroidized �-Al grains are produced as shown in Figs. 9(a) and (b), 11(a),14(a) and (b) and 16(a) and (b). The Fe-free alloy with theleast amount of intermetallic compounds shows the largestaverage grain size. The Mn-added alloy with Mn-containingdispersoids produced by homogenization shows finer averagegrain size. The fragmented Fe-compounds are effective torefine the average grain size in Fe-added alloy. Fe as anundesired impurity is a great alternative compared with themore expensive element such as Mn to produce fine semi-solid microstructure by the D-SSF process. The Fe/Mn addedalloy shows the smallest average grain size, because itconsists of large amounts of second phase particles from Mn-containing dispersoids and fragmented Fe/Mn-containingcompounds. The effect of various intermetallic compounds assecond phase particles strongly influences the recystallizationand spheroidization of the �-Al grain in the semi-solid state.The type, shape, size and distribution of second phaseparticles are well known to affect the recrystallizationbehavior. In the partially reheated semi-solid forming processsuch as the D-SSF process, the recystallization behaviorsbefore melting temperature and subsequently in partiallyremelting state are still not well studied. The thermal stabilityand transformation of intermetallic compounds in the semi-solid state must be taken into account.

4. Conclusions

The semi-solid microstructure control of wrought Al-Mg-Si based alloys with Fe and Mn additions is useful by theDeformation Semi-Solid Forming (D-SSF) process.

(1) The fragmentation of the harmful Fe-intermetalliccompound into finely fragmented particle can effectively

(a)

αα-Al

(c)

(b)

α-Al

Fig. 14 Microstructures of the Mn-added alloy after semi-solid heating at

637�C for 300 s with (a) 40% cold-rolling, (b) 70% cold-rolling and (c)

typical liquid phase in the Mn-added alloy.

(a)

(c)

(e)

(b)

(d)

(f)

Chinese-scipt -Al15(Fe, Mn)

Chinese-script αα

α

-Al15(FeMn)

spherical disersoids

Fragmented Chinese-script α-Al

3Si2

3Si2

sperical disersoids

15(Fe, Mn)3Si2

Fig. 15 Microstructures of the Fe/Mn-combinded added alloy: (a),(b) as-

cast, (c),(d) homogenized and (e),(f) 60% cold-rolled.

840 C. Phongphisutthinan, H. Tezuka and T. Sato

refine the spheroidized �-Al grains with only 40% cold-rolling. High cooling rate is preferable to refine the AlFeSicompounds after semi-solid heating at high temperature.

(2) The Mn-added alloy requires high deformation at 70%cold-rolling to produce fine �-Al grains.

(3) The Fe/Mn combined addition produce the finestspheroidized �-Al grains. The fine polyhedral morphology of�-Al15(Fe,Mn)3Si2 phase is obtained even at the differentcooling rate, indicating the preferable modification of the Fe-intermetallic compound.

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(a)

αα-Al

(b)

α-Al

Fig. 16 Microstructures of the Fe/Mn-combinded added alloy after semi-solid heating at 637�C for 300 s with (a) 40% and (b) 60% cold-

rolling.

(c)

(b)

(a)

polyhedral αα-Al15(Fe, Mn)

polyhedral α-Al15(Fe, Mn)

3Si2

3Si2

polyhedral α-Al15(Fe, Mn)3Si2

Fig. 17 Effect of cooling rate on the crystallization of AlFeMnSi

intermetallic compounds in the Fe/Mn-added alloy by (a) water quench-

ing, (b) air cooling and (c) 30�C/min cooling rate.

Semi-Solid Microstructure Control of Wrought Al-Mg-Si Based Alloys with Fe and Mn Additions 841