Parameters determining Solidification Structure and

EMC2013

Parameters determining Solidification Structure and Process Efficiency of ESR Superalloys S. Radwitz, J. Morscheiser, B. Friedrich

IME Process Metallurgy and Metal Recycling, RWTH Aachen University Prof. Dr.-Ing. Dr. h.c. Bernd Friedrich

Introduction General information about superalloys:  Developed in the middle of the 20th century, nomenclature due to superior high-temperature (HT) properties compared to steel:  Excellent HT-creep resistance  High mechanical HT-strength  Good HT-oxidation and -corrosion resistance  Common alloying elements: Ni, Co, Fe, Cr, Mo, Nb, W, Ta, Ti, Al Applications: Automotive

Off-shore Chemical industry

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Aviation & Space

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Motivation General tendency in turbine manufacturing  Increasing the degree of efficiency by

raising the maximum operating temperature  further development of the chemical composition  optimisation of the manufacturing processes

 Observance of strict safety requirements 

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

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© Courtesy of Rolls Royce

warranty of an accurate chemical composition minimisation of potential material and solidification defects

The ESR Process Functions: Removal of non-metallic inclusions (NMIs)

Avoidance of cavities and shrinkage holes

Controlled and “partially directed“ solidification

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Metall refining due to physical and chemical slag interactions interaction

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Aim of the Performed Work Interdendritic segregation Freckle formation

Material group prone to segregation and solidification defects!

Tree Rings

Solidification White Spots

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investigation of solidification processes during ESR

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process parameters

alloy dependence of various parameters slag skin

physical slag properties

Solidification during ESR Increase of the heat flow

Increase of the cooling rate (R)

Reduced local solidification times T − Ts T − Ts LST = L = L G∙v R

Decrease of the primary (1) and secondary (2) dendrite arm spacing 2 = C ∙

TL − Ts R

1 3

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reduced segregation Recycling

Concept of the Experiments investigated influencing factors process parameters 

power supply  height of the slag bath

phys. slag properties 

liquidus temperature (overheating)  viscosity (fluid flow behavior)  solidification path

selection of slag systems: 

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constant composition of mostly CaF2, CaO and Al2O3  addition of TiO2, MgO  especially suitable for the remelting of superalloys

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notably requirement: 

all physical properties are related to ionic slag structure  variation of a specific property with only the smallest change in the other attributes  CaF2, Al2O3, CaO

Trial

Average power input / kW

E1

110

E2

80

E3

Amount of slag Height of bulk slag / kg / mm

utilised 4 slags and their 90 viscosity at 1600 °C 90

140

4

85

E4

110

2.5

52

E5

110

8

174

ss te

chn ik

4

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Experimental Overview

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Trial

Composition

Tliquid / °C

Viscosity / P

E6

40/38/22

1230

0.3-0.45

E7

50/40/10 1780 utilised slag compositions in0.2-0.3

E8

their ternary phase 65/15/20 1480 diagram0.1-0.4

E9

50/30/20

1350

0.2-0.4

E10

45/37/18

1600

0.3-0.35

E11

80/10/10

1450

0.1-0.3

E12

15/48/37

1450

0.55-1.0

Combined (P)ESR Unit at IME Technical specifications:  Pmax: ~ 400 kW  transformer with two taps Umax/Imax:80V/5kA & 66V/6kA  AC power supply (50 Hz)  industrial process control unit  maximum pressure (PESR): 50 bar  Inertisation with protective gas (ESR)

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Mould geometries:  PESR: 160/180 Ø x 890 mm  ESR: 85/95 Ø x 800 mm, 145/160 Ø x 800 mm, 145/160 Ø x 960 mm, 170/215 Ø x 1 510 mm Recycling

Sampling from the Remelted Ingots

R C

M

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P

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Labelling

Analysed parameter

P

Depth of the metal pool

M

Solidification defects

R

SDAS, SEM-EDX

C

Chemical composition

Investigation of the Slag Skin Sampling of the slag skin 

uniform thickness over the ingot height  differences between the various trials

No significant influence of process parameters on slag skin thickness! slag skin thickness / mm

Clear influence of phys. properties on slag skin thickness!

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Influencing Factors on Slag Skin Formation Process parameters 

no detectable impact of power supply and slag bath height Phys. slag properties



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low liquidus temperature/ intense slag superheating supports formation of thin slag skins  high viscosities presumably lead to thick slag skins  lenght of solidification path affects slag skin thickness

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Results from Melting Paramters & Macrostructure rel. electrode position / mm

Power supply & amount of slag ↔ melt rate

Increase of the melt rate with:

E3, 140 kW, 4 kg E4, 110 kW, 2,5 kg E1, 110 kW, 4 kg E5, 110 kW, 8 kg E2, 80 kW, 4 kg

360 320 280 240 200 160 120 80 40 0 -40



rising power supply → increased slag temperature



decreasing slag bath height → reduced heat losses against the mould

Correlation melt rate ↔ pool depth 45 E3

Pool depth rises with increasing melt rate: 

E1-E5: linear relationship (no significant change in slag skin thickness)

pool depth / mm

process time / hh:mm:ss 40

E4

E1

35

30 E5 25 E2

ss te

chn ik

E6-E12: no identifiable correlation (varying heat extraction due to different slag skin thicknesses) Pro ze

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

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20 0,8

0,9

1

1,1

1,2

1,3

melt rate / kg/min

1,4

1,5

1,6

Microstructural Investigation Typical development of the SDAS over the ingot radius

Increase of the SDAS from the ingot surface to the center!

SDAS / µm

45 40 35

Why?

30



25



increase of the metal layer formation of the contraction gap

→ reduced heat flow

20 0

20 40 60 Distance from the ingot surface / mm

E8, T(liquid) = 1480 °C

E9, T(liquid) = 1350 °C

80

E10, T(liquid) = 1600 °C

Dependence slag skin thickness ↔ SDAS (ingot center) E1

45

E2

No distinct relationship: 

SDAS increases with rising slag skin thickness for a part of the performed trials



Additional influence of melt rate on dendrite arm spacing and the local solidification time

SDAS / µm

42 E3

39

E6

36

E9

33

E8

E5 E4

E11 E12

E10

30 27 0,3

0,5

0,7

0,9

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average slag skin thickness / mm Recycling

1,1

Microstructural Investigation 2 Correlation melt rate ↔ LST 30 E2

LST / s

25

E1

E5

20

E4

E8

E11

E3

15 E7 10

E9

E6 E12

E10 5 0 0,7

0,9

1,1

1,3

1,5

melt rate / kg/min

Explanation: 

increase of the solidification front velocity



slight rise of the thermal gradient

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→ decrease of SDAS / LST

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1,7

Microstructural Investigation 3 Distribution of elements in dendritic and interdendritic areas

content of the element / %

60

30

Cr

k>1

Fe

k>1

Ni

k1

Nb

k