Crystallization, microstructure and mechanical properties of silumins ...

ARCHIVES of FOUNDRY ENGINEERING Published quarterly as the organ of the Foundry Commission of the Polish Academy of Sciences

ISSN (1897-3310) Volume 10 Issue 1/2010 123 – 136

23/1

Crystallization, microstructure and mechanical properties of silumins with micro-additions of Cr, Mo, W and V S. Pietrowski*, T. Szymczak Department of Materials Technologies and Production Systems, Technical University of Łódź, Stefanowskiego 1/15 Street, 90-924 Łódź, Poland * Corresponding author: E-mail address: [email protected] Received 13.07.2009; accepted in revised form 22.07.2009

Abstract In this paper results of the crystallization, microstructure and mechanical properties studies of hypo-, hyper- and eutectic silumins with addition of: Cr, Mo, W and V in amount of about 0,05% are presented. The influence of Sb, Sr and P together with Ti + B on the silumins crystallization process has been given. Results of: the microstructure, Rm, Rp0,2, A5 and HB testing of silumins after precipitation hardening and heat treatment in temperature of 560°C/3min and water chilling are presented. Keywords: Innovative foundry technologies and materials, Micro-additions: Cr, Mo, W, V; Silumins, TDA control, Heat treatment; Microstructure, Mechanical properties

1. Introduction

2. Work methodology

Silumins with Cr, Mo, W and V are characterized by high mechanical properties due to complex phases precipitation, where a.m. elements are the main component. The type of crystallize phases containing: Cr, Mo, W and V are presented in work [1÷7]. Up to now the influence of above mentioned elements in amount of up to 0,1% in silumins was studied. The aim of this work was to test the Cr, Mo, W and V influence in amount of about 0,05% to the crystallization, the microstructure and mechanical properties of hypo-, hyper- and eutectic silumins modified by Sb, Sr and P together with Ti + B as-cast and after heat treatment.

Investigations were conducted on hypo-, hyper- and eutectic alloy silumins. Their chemical composition is presented in Table 1. To melt multicomponent silumins basic alloys AlSi7, AlSi11 and AlSi17 were used. The chemical composition of tested silumin was supplemented by technically pure metals: Cu, Ni, Si, Mg, Mo, W and alloys: AlCr15 and AlV10. Silumins were melted in the laboratory induction furnace with the graphite crucible. After melting and overheating multicomponent silumins were modified. Two versions of hypo- and eutectic silumins modification were used by addition to liquid metal properly: 0,30%Sb + 0,10%Ti + 0,02%B and 0,30%Sr + 0,10%Ti + 0,02%B. To hypereutectic silumins modification 0,45%P + 0,20%Sb + 0,12%TiB and 0,45%P + 0,30%Sb + 0,12%TiB were used. Modifiers were added as AlSr10, AlTi5B and CuP8 alloys and technically pure antimony. Mass fraction was selected to

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obtain the chemical composition of tested silumins presented in Tab. 1. After modification the liquid metal was refined by nitrogen during 15min. The control of tested silumins crystallization with use of TDA method was made too. Moreover chill castings of bars 15mm. were made. Specimens to metallographic, strength and hardness tests were made from them. Research were conducted both as-cast and after different heat treatment. On tested silumins precipitation hardening were carried out. Solution heat treatment was realized in condition as following: 520°C, 8h, water cooling whereas ageing: 160°C, 8h, free air

cooling. Additionally on tested silumins high-temperature and brief heat treatment was carried out. It consist in putting specimens into furnace to 560°C for 3min. and water chilling. Hardness of tested silumins was examined with use of Briviskop hardness testing machine on Brinell scale for 62,5/2,5/30 conditions. Load factor K = 10. Rm, Rp0,2 and A5 were examined with use of “Instron” universal testing machine in the ambient temperature. Metallographic testing were made with using „Eduko” optical microscope, magn. ×100 and ×400. Specimens were etched using 2% HF.

Table 1. The chemical composition of tested silumins Melt number

Si 7,00 6,97 12,00 11,96 17,31 17,47

No. 1 No. 2 No. 3 No. 4 No. 5 No. 6

Mg 0,49 0,49 0,51 0,33 0,52 0,50

Cu 4,02 3,99 4,04 4,04 5,13 5,19

Ni 3,95 4,01 4,01 4,04 4,03 3,97

Chemical composition, % Mo W V Fe 0,06 0,05 0,06 0,37 0,05 0,05 0,04 0,38 0,05 0,05 0,05 0,42 0,06 0,06 0,05 0,36 0,06 0,05 0,06 0,49 0,05 0,05 0,06 0,51

Cr 0,05 0,05 0,06 0,05 0,06 0,06

Point Pk A B D E M F G H J K

3. Results TDA curves and silumin microstructure from melt No. 1 are presented in Figure 1 (a, b). Silumin crystallization starts in temperature of tPK = 594°C from α(Al) phase precipitations. In tB = 569°C α + β + AlSiCrMoWVTiCuFe eutectic crystallization starts. FGH thermal effect is caused by Mg2Si phase crystallization and HJK – Al2Cu phase. Silumin crystallization finishes in tK = 485°C. a) 800

1

Pk A

B

DEM

FG HJ K

Sb 0,30 0,30 0,22 τ, s 61 75 131 182 192 205 343 361 390 401 419

Sr 0,30 0,30 0,30 t, °C 594 583 569 554 554 555 527 520 506 499 485

Ti 0,11 0,09 0,10 0,10 0,11 0,10

B 0,02 0,02 0,02 0,02 0,02 0,02

P 0,45 0,45

dt/dτ, °C/s -1,55 0,33 -0,62 0,00 0,13 0,00 -0,50 -0,20 -0,75 -0,47 -1,00

b)

700

0

-1

500

-2

t, oC

600

t = f( )

400

dt/d , oC/s

dt/d = f'( )

-3

300

-4 0

100

200

300

,s

400

500

600

Fig. 1 (a, b). TDA curves (a) and the microstructure (b) of silumin from melt No. 1 modified by Sb + Ti + B In Figure 2 (a, b) TDA curves (a) and the microstructure (b) of silumin from melt No. 2 are presented. Silumin crystallization

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ARCHIVES of FOUNDRY ENGINEERING Volume 10, Issue 1/2010, 123-136

starts in temperature of tPK = 606°C from α(Al) phase precipitations. In temperature of tB÷tC` (580÷550°C) α + AlSiCrMoWVTiCuFe binary eutectic crystallizes and afterwards phases like previous. Comparison of melts No. 1 and 2 shows, that silumin modification by Sr + Ti + B causes the increase of α + β + AlSiCrMoWVTiCuFe eutectic, Mg2Si and Al2Cu phases crystallization temperature. Finish of crystallization process take place at the same temperature. Antimony is not the classic silumins modifier, because do not change the lamellar Si structure to fibrous. It only decreases a distance among Si plates in the eutectic, whereas Sr causes the fibrous Si structure, what is shown in Fig. 1 (b) and 2 (b). Silumin modification by Sr caused α + AlSiCrMoWVTiCuFe binary eutectic crystallization, that there is not in silumin with Sb. Probably the previous crystallization of this eutectic, compared with silumin with Sb, caused decrease of α + β + AlSiCrMoWVTiCuFe ternary eutectic crystallization temperature.

b)

Fig. 2 (a, b). TDA curves (a) and the microstructure (b) of silumin from melt No. 2 modified by Sr + Ti + B

a) 800

1

Pk A

B C

C' DE M

FG HJ K

The microstructure of as-cast silumins from melt No. 1 and 2 is presented properly in Figure 3 and 4 (a, b).

700

0

a) -1

t, oC

600

500

-2

dt/d , oC/s

dt/d = f'( )

t = f( ) 400

-3

300

-4 0

100

200

300

400

500

600

,s

Point Pk A B C C’ D E M F G H J K

τ, s 96 122 162 197 254 283 301 322 382 397 420 429 471

t, °C 606 587 580 570 550 543 544 545 534 530 521 516 485

dt/dτ, °C/s -1,13 0,08 -0,34 -0,18 -0,43 0,00 0,11 -0,00 -0,40 -0,22 -0,60 -0,42 -1,06

b)

Fig. 3 (a, b). Silumin microstructure from melt No. 1. Phases: α, Mg2Si, Al2Cu, α + β + AlSiCrMoWVTiCuFe eutectic


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a)

a)

b)

b)

Fig. 4 (a, b). Silumin microstructure from melt No. 2. Phases: α, Mg2Si, Al2Cu, α + AlSiCrMoWVTiCuFe and α + β + AlSiCrMoWVTiCuFe eutectic

Fig. 5 (a, b). Silumin microstructure from melt No. 1 after precipitation hardening a)

From presented data results, that modification by Sr + Ti + B ensures a greater size reduction of α phase and α + β eutectic cells than Sb + Ti + B addition. It causes a size reduction of AlSiCrMoWVTiCuFe phase. There is the same effect after solutioning and ageing as it is shown in Figure 5 and 6 (a, b). Comparison of microstructures after solutioning and ageing from melt No. 1 and 2 shows, that a greater Si size reduction and its coagulation is obtained in silumin modified by Sr + Ti + B. In Figure 7 and 8 (a, b) the silumins microstructure after hightemperature and brief heat treatment are properly presented. It did not cause essential microstructure changes.

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b)

a)

Fig. 6 (a, b). Silumin microstructure from melt No. 2 after precipitation hardening

b)

a)

Fig. 8 (a, b). Silumin microstructure from melt No. 2 after precipitation hardening and high-temperature and brief heat treatment b)

Fig. 7 (a, b). Silumin microstructure from melt No. 1 after precipitation hardening and high-temperature and brief heat treatment

TDA curves and the microstructure of silumins from melt No. 3 and 4 are properly presented in Figure 9 and 10 (a, b). Silumin had got about 12,00% Si. There is 12,60% Si in eutectic point. From Fig. 9a results, that Sb addition displaced the eutectic point left to lesser Si concentration. Thereupon, on the derivative curve there is PKP’KT thermal effect from primary silicon crystallization, that is visible in the silumin microstructure (Fig. 9b). TDEMF thermal effect is caused by α + β + AlSiCrMoWVTiCuFe eutectic crystallization and FGH and HJK effects properly come from crystallization of Mg2Si and Al2Cu phases. Silumin modification by Sr + Ti + B causes crystallization of silumin from melt No. 4 as eutectic (Fig. 10a). PKDEMH thermal effect comes from α + β + AlSiCrMoWVTiCuFe ternary eutectic crystallization and HJK – from Al2Cu phase. There was not a thermal effect from Mg2Si phase, despite the fact, that there are its sparse, single precipitations in the microstructure (Fig. 10b).


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a)

a) 800

1

P'k TDE M

800

1

F GH K J

DE M Pk

HK J

Pk 700

0

700

0

600

-1

600

-1

500

-2

500

-2

t = f( )

t = f( )

400

-3

400

-3

300

-4

300

-4

0

100

200

300

400

500

600

0

100

200

,s

Point Pk P’k T D E M F G H J K

τ, s 65 77 84 106 110 142 353 376 388 396 418

dt/d , oC/s

t, oC

dt/d , oC/s

dt/d = f'( )

t, oC

dt/d = f'( )

300

400

500

600

,s

t, °C 594 579 574 558 559 562 522 511 506 503 485

dt/dτ, °C/s -2,00 -0,46 -0,87 0,00 0,30 0,00 -0,56 -0,34 -0,50 -0,29 -1,10

Point Pk D E M H J K

τ, s 76 89 94 136 376 384 396

t, °C 562 550 551 554 507 501 491

dt/dτ, °C/s -1,46 0,00 0,36 0,00 -0,88 -0,59 -1,10

b)

b)

Fig. 10 (a, b). TDA curves (a) and the microstructure (b) of silumin from melt No. 4 modified by Sr + Ti + B

Fig. 9 (a, b). TDA curves (a) and the microstructure (b) of silumin from melt No. 3 modified by Sb + Ti + B

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Comparison of eutectic crystallization temperature both silumins shows, that in silumin modified by Sr + Ti + B eutectic crystallize lower by 8°C temperature. There was the same the recalescence of eutectic crystallization temperature in both silumins and amount to 4°C. In Figure 11 and 12 (a, b) as-cast silumin microstructure from melt 3 and 4 are properly presented.


Comparison of microstructures shows a greater size reduction of eutectic cells by Sr than by Sb.

b)

a)

Fig. 12 (a, b). Silumin microstructure from melt No. 4 modified by Sr + Ti + B. Phases: Al2Cu, α + β + AlSiCrMoWVTiCuFe eutectic b) a)

Fig. 11 (a, b). Silumin microstructure from melt No. 3 modified by Sb + Ti + B. Phases: α, β, Mg2Si, Al2Cu, α + β + AlSiCrMoWVTiCuFe eutectic

b)

a)

Fig. 13 (a, b). Silumin microstructure modified by Sb + Ti + B after precipitation hardening. Phases: α, β, Mg2Si, Al2Cu, α + β + AlSiCrMoWVTiCuFe eutectic


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After solutioning and ageing there is a greater silicon coagulation in silumin modified by Sr than by Sb, as it is shown in Figure 13 and 14 (a, b).

a)

a)

b)

b)

Fig. 15 (a, b). Silumin microstructure modified by Sb + Ti + B after precipitation hardening and high-temperature and brief heat treatment Fig. 14 (a, b). Silumin microstructure modified by Sr + Ti + B after precipitation hardening. Phases: α, Al2Cu, α + β + AlSiCrMoWVTiCuFe eutectic

a)

After high-temperature and brief heat treatment there is the most intensive silicon coalescence and its coagulation in silumins modified by Sr as it is shown in Figure 15 and 16 (a, b).

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b)

a) 800

1

Pk T P'k

N O DM UE

FG H K J

700

0

-1

500

-2

t, oC

600

dt/d , oC/s

dt/d = f'( )

t = f( )

Fig. 16 (a, b). Silumin microstructure modified by Sr + Ti + B after precipitation hardening and high-temperature and brief heat treatment

400

-3

300

-4 0

100

200

300

400

500

600

700

,s

TDA curves and the microstructure of hypereutectic silumin from melt No. 5 and 6 are properly presented in Figure 17 and 18 (a, b). TDA curves of both silumins are very similar. In both silumins PKP’KT effect is caused by AlSiCrMoWVTiCuFe phase crystallization. NOU effect comes from pre-eutectic silicon crystallization. Crystallization heat of α + β + AlSiCrMoWVTiCuFe causes UDEMF effect. FGH and HJK effects are properly caused by Mg2Si and Al2Cu phases crystallization. It is characteristic, that for both silumins eutectic crystallization temperature is the same. Antimony decreases a distance among Si plates in eutectic. Strontium did not cause Si modification in eutectic. Its structure is still lamellar (Fig. 18b). Comparison with silumin modified by Sr shows, that in chill castings Sb causes a size reduction of primary and eutectic Si, as it is shown in Figure 19 and 20 (a, b).

Point Pk P’k T N O U D E M F G H J K

τ, s 27 39 52 204 231 248 263 268 280 446 464 483 495 522

t, °C 711 692 679 564 556 552 548 548 550 523 517 510 504 480

dt/dτ, °C/s -2,34 -0,69 -1,11 -0,58 0,03 -0,44 0,00 0,19 0,00 -0,40 -0,27 -0,53 -0,38 -1,10

b)

Fig. 17 (a, b). TDA curves (a) and the microstructure (b) of silumin from melt No. 5 modified by P + Sb + Ti + B


131

a)

a) 800

1

P'k T

NO DM UE

FG H K J

700

0

-1

500

-2

t, oC

600

dt/d , oC/s

dt/d = f'( )

t = f( ) 400

-3

300

-4

b) 0

100

200

300

400

500

600

700

,s

Point P’k T N O U D E M F G H J K

τ, s 27 32 159 177 187 200 204 223 409 425 454 463 485

t, °C 684 676 556 552 550 548 549 550 524 519 508 504 487

dt/dτ, °C/s -1,38 -1,73 -0,57 0,08 -0,28 0,00 0,12 0,00 -0,39 -0,27 -0,51 -0,29 -1,08

b)

Fig. 19 (a, b). Hypereutectic silumin microstructure modified by P + Sb + Ti + B. Phases: α, β, Mg2Si, Al2Cu, α + β + AlSiCrMoWVTiCuFe eutectic a)

Fig. 18 (a, b). TDA curves (a) and the microstructure (b) of silumin from melt No. 6 modified by P + Sr + Ti + B

132


b)

a)

Fig. 20 (a, b). Hypereutectic silumin microstructure modified by P + Sr + Ti + B. Phases: α, β, Mg2Si, Al2Cu, α + β + AlSiCrMoWVTiCuFe eutectic

b)

a)

Fig. 22 (a, b). Hypereutectic silumin microstructure modified by P + Sr + Ti + B after precipitation hardening

b)

After solutioning and ageing there is a partial coagulation of primary and eutectic silicon edges, stronger in silumins with Sb. It is exemplary shown in Figure 21 and 22 (a, b). High-temperature and brief heat treatment caused a size reduction of primary silicon and a coagulation of eutectic silicon in silumin modified by P + Sb + Ti + B, as it is shown in Figure 23 (a, b). In silumin from melt No. 6 modified by P + Sr + Ti + B the high-temperature and brief heat treatment did not change of primary silicon size. It caused partial coagulation and coalescence of eutectic silicon, as it is shown in Figure 24 (a, b).

Fig. 21 (a, b). Hypereutectic silumin microstructure modified by P + Sb + Ti + B after precipitation hardening


133

a)

a)

b)

b)

Fig. 23 (a, b). Silumin microstructure from melt No. 5 after hightemperature and brief heat treatment

Fig. 24 (a, b). Silumin microstructure from melt No. 6 after hightemperature and brief heat treatment Mechanical properties of tested silumins are presented in Table 2. Results from it, that the hardness of as-cast silumins is always greater than after next heat treatments. The highest hardness as-cast (197HB) had hypereutectic silumin with antimony (melt No. 5) and the lowest (115HB) hypoeutectic silumin (melt No. 1). The hardness after particular heat treatments changes similarly.

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Table 2. Mechanical properties of tested silumins No.

Melt number

1 1 2 2 3 3 4 4 5 5 6 6

Heat treatment as-cast solutioning ageing high-temp. as-cast solutioning ageing high-temp. as-cast solutioning ageing high-temp. as-cast solutioning ageing high-temp. as-cast solutioning ageing high-temp. as-cast solutioning ageing high-temp.

Rm, MPa 430 440 460 490 450 465 480 510 470 500 515 530 480 510 520 540 460 470 480 500 430 450 460 470

Rm tensile strength of tested silumins is the lowest for as-cast silumins and the highest after the high-temperature, brief heat treatment; similarly Rp0,2 and A5. The highest mechanical properties Rm = 540MPa, Rp0,2 = 470MPa and A5 = 6,5% were obtained in eutectic silumin modified by Sr + Ti + B. To recapitulate presented results it is necessary to state, that the chemical composition and the modifiers type have the

Mechanical properties Rp0,2, A5, MPa % 360 2,5 370 3,0 390 4,0 420 5,5 370 3,0 380 4,0 410 5,0 440 6,5 390 3,5 410 4,0 430 5,0 460 5,5 400 4,0 410 5,0 440 6,0 470 6,5 370 2,5 370 3,5 400 4,0 420 5,0 340 2,0 340 2,5 370 3,0 390 4,0

HB 115 107 111 86 122 108 111 96 140 115 121 100 134 107 109 106 197 150 152 141 171 145 147 126

significant influence on the crystallization process, microstructure after heat treatment and mechanical properties. Cr, Mo, W and V micro-additions cause silumins strengthening both as-cast and after heat treatment.


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4. Conclusions

References

The results have indicated the following: Cr, Mo, W and V presence in hypo-, hyper- and eutectic silumins depending on the modifier type and its grade causes pre-eutectic or eutectic crystallization of AlSiCrMoWVTiCuFe phase, AlSiCrMoWVTiCuFe phase causes substantial increase of silumins mechanical properties, antimony addition to silumin displaces the eutectic point to the lower silicon concentration, P + Sb + Ti + B simultaneous addition to hypereutectic silumins causes primary silicon crystals size reduction stronger than modification by phosphorus, comparison with as-cast silumins shows, that the precipitation hardening of silumins causes a coagulation of eutectic silicon as well as hardness and tensile strength decrease, the high-temperature, brief heat treatment of silumins decrease their HB hardness, but increase Rm, Rp0,2 and A5.

Acknowledgments

[1] S. Pietrowski, Silumins. PŁ Publishing House, Łódź, 2001 (in Polish). [2] S. Pietrowski, Multicomponent silumin. Patent No. 179730, 2001 (in Polish). [3] S. Pietrowski, Complex silumins, Journal of Achievements in Materials and Manufacturing Engineering, Vol. 24 Issue 1, 2007. [4] S. Pietrowski, Hypereutectic and eutectic silumins with addition of Cr, Mo, W, Co. Materials Engineering, No. 6, 2003 (in Polish). [5] S. Pietrowski, B. Pisarek, Silumins with high mechanical properties [w:] Sobczak J. (editor) Innovations in Foundry Engineering part I, Foundry Research Institute, Cracow, ISBN 978-83-88770-26-5, p. 69-72, 2006 (in Polish). [6] S. Pietrowski, Silumins with high mechanical properties [w:] Sobczak J. (editor) Innovations in Foundry Engineering part II, Foundry Research Institute, Cracow, ISBN 978-8388770-35-7, p. 71-97, 2008 (in Polish). [7] S. Pietrowski, T. Szymczak, Silumins alloy crystalization, Archives of Foundry Engineering, Vol. 9, Issue 3, 2009, 143158.

This study is done as a part of a development project No. 0 R00 0052 05.

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