Grain Refinement of Al-Si Alloys by Nb-B Inoculation. Part 1: Concept Development and Effect on Binary Alloys. Part 2: Application to Commercial

Grain Refinement of Al-Si Alloys by Nb-B Inoculation. Part 1: Concept Development and Effect on Binary Alloys. Part 2: Application to Commercial Alloys 1

Grain refinement of Al-Si alloys by Nb-B inoculation M. Nowak L. Bolzoni N. Hari Babu Brunel Centre for Advanced Solidification Technology Brunel University London, UK

Outline Grain refinement in Al alloys with Al-5Ti-B Concept development Application to Al-Si cast alloys Al-Nb-B master alloy Comparative study between Al-Nb-B and Al-5Ti-B master alloys

INTRODUCTION ALUMINIUM ALLOYS PROPERTIES LOW DENSITY, 2.7 g/cc GOOD MECHANICAL PROPERTIES HIGH CORROSION RESISTANCE HIGH THERMAL CONDUCTIVITY LOW ELECTRICAL RESISTIVITY ALLOYS WROUGHT CAST (Al-Si) GRAIN REFINEMENT IMPROVEMENT FLUIDITY/CASTABILITY MACHINABILITY CHEMICAL HOMOGENEITY MECHANICAL PROPERTIES SURFACE QUALITY REDUCED SHRINKAGE POROSITY

Nucleation / growth rate Factors determining grain size in as cast microstructure Nucleation & Growth Heterogeneous nucleation Homogeneous nucleation Growth Kinetics Temperature Atmosphere/Pressure Growth restriction Fragmentation Cooling rate T L Under cooling (DT) DT = T L -T g

EFFICIENT HETEROGENEOUS NUCLEATION SITES BASE MATERIAL 1. High melting Temp 2. Low lattice mismatch (atom position matching) 3. Chemical stability (should not react with alloying elements) A B A N. Hari Babu et al., Nature Materials 2005;4:476

GRAIN REFINERS IN ALUMINIUM INDUSTRY GRAIN REFINEMENT: Al-Ti-B Al Nuclei Al-Ti-C TiB 2 & Al 3 Ti Al 3 Ti Layer Orientation Relationships TiB 2 Particle {111}Al//{112}Al 3 Ti//{001}TiB 2 <110>Al//<201>Al 3 Ti <110>Al 3 Ti//<110>TiB 2 HREM image of Al/Al 3 Ti/TiB 2 interface MODIFICATION: Sr modification of the Si morphology P to nucleate the primary Si particles Source: B. J McKay

Influence of Al-Ti-B grain refiner for Al-Si alloys Not effective Wrought alloys Casting alloys Ti-Si phase formation M. Johnsson, Influence of Si And Fe on the Grain-Refinement of Aluminum, Zeitschrift für Metallkunde, 85 (1994), 781-785 Mats Johnsson Patent, 2000

Ti reaction with Si in Al-Si alloys T. E. Quested et al, Mater. Sci. Technol. 22, 1126 (2006) Ti is consumed by the formation of TiSi 2 and TiSi

Analogy between Al-Ti & Al-Nb phase diagrams Al 3 Ti Al 3 Nb

LATTICE MISMATCH Al 3 Nb 0.384 nm Al Al (face centred cubic)

Low Lattice Mismatch Coherent Interface Lattice mismatch ~0.9%

Nb chemical stability with Si Nb Si binary phase diagram Nb Ti Si ternary system J.L. Muray and A. J. Alister, Bulletin of Alloy Phase Diagrams 1984;5:74 J.C. Zhao et al., Materials Science and Engineering A 2004;372:21 Nb silicides form at higher temperature than Ti silicides thus preventing poisoning

Addition of Nb metal powder to liquid Al 20 mm 20 mm 20 mm Al-Nb 660 C Al-Nb 680 C Al-Nb 700 C Al-Nb 720 C 20 mm Al matrix 20 mm Nb particles 20 mm 200 μm 750-800 C Unreacted Nb metallic particulates

Nb 2 Al Nb 3 Al Poor dissolution of Nb in liquid Al Nb 2 Al Nb NbAl 3 Nb 3 Al 20 μm Requires high temperature for larger Nb particles and high concentrations

Addition of Nb fine metal powder to liquid Al Nb- Superconductivity 9K To verify the Nb dissolution, magnetic moment vs temperature measured <45 mm

EFFECT OF Nb on CP Al CP-Aluminium 700ºC Al with Nb-B

COMPARISON OF Al-Ti-B AND Nb-B ON CP Al

COMPARISON OF Al-Ti-B AND Nb-B TO HYPOEUTECTIC BINARY Al-Si Alloys Al-1Si Al-2Si Al-4Si Al-5Si Al-6Si Al-8Si Al-10Si Reference 0.1wt.% Al-5Ti-1B 0.1wt.% Nb & B (powders)

COMPARISON OF Al-Ti-B AND Nb-B TO HYPOEUTECTIC BINARY Al-Si Alloys 700ºC Practical alloys composition

Al-Si alloys for automotive applications Engine & transmission Components Crankcases Cylinder heads Intake manifolds Housings manual/automatic transmissions Housings power transfer units Chassis Components Subframes Knuckles Steering housings Source: VW VW Structural Components Body structures Instrument panels Door frames VW HL Wheels VW

Undercooling for Al-10Si alloy 0.3 o C/s ΔT Ref = 2.1ºC

Reference Grain size up to 1 cm ΔT Ref = 2.1ºC

Undercooling in the presence of NbB 2 /Al 3 Nb 0.3 o C/s ΔT Nb-B = 1.3ºC

GRAIN STRUCTURE Reference Grain size up to 1 cm Nb-B Grain size: 2-3 mm ΔT Ref = 2.1ºC ΔT Nb-B = 1.3ºC

dt/dt dt/dt Temerature [ o C ] 600 599 598 597 596 595 594 593 592 591 590 589 588 587 586 585 584 583 582 581 580 T nucl =588 0 C Cooling curves for Al-11Si (LM6) alloy a) b) LM6 dt/dt T rec ΔT=2.5 C T min =584.5 0 C 380 400 420 440 460 480 500 520 540 LM6 Time [ s ] T g =587 0 C 1 0-1 -2-3 Temeprature [ o C ] 595 594 593 592 591 590 589 588 587 586 T nucl =588.8 0 C ΔT=0.7 C T min =586.8 0 C LM6+NGR dt/dt 585-0.6 400 420 440 460 480 500 Time [ s ] LM6 with Grain refiner addition Nb-B T g =587.5 0 C 0.2 0.1 0.0-0.1-0.2-0.3-0.4-0.5 20 mm

HYPEREUTECTIC BINARY Al-Si ALLOYS Al-14Si 700ºC α-al dendrite α-al dendrite 200 μm 200 μm

HYPEREUTECTIC BINARY Al-Si ALLOYS - EUTECTIC Al-16Si 20 μm 20 μm 700ºC Al-18Si 20 μm 20 μm Al-27Si 20 μm 20 μm

HYPEREUTECTIC BINARY Al-Si ALLOYS PRIMARY SI Al-16Si 200 μm 200 μm Al-18Si 200 μm 200 μm Al-27Si 200 μm 200 μm

Application of Nb-B grain refiner to Al-Si commercial alloys

Commercial alloys tested with Nb-B GB USA Si Mg Mn Cu Ni Zn Fe LM6 A413 10.0-13.0 0.1max 0.5max 0.1max 0.1max 0.1max 0.6max LM13 336 10.0-13.0 0.2-0.4 0.5max 0.7-1.5 1.5max 0.1max 1max LM24 A380 7.5-9.5 3 max 0.5max 3.0-4.0 0.5 3 1.3max LM25 A356 6.5-7.5 0.2-0.6 0.3 0.2 0.1 0.1 0.5 9.99 0.005 0.005 0.0017 0.0044 0.005 0.09 10.98 0.268 0.21 2.134 0.068 0.778 0.83 6.06 0.275 0.265 2.725 0.0257 0.305 0.356 11.9 0.8 0.005 3.7 2 0.003 0.12

Nb-B Grain Refiner for Al-Si cast alloys Highly effective for Al-Si alloys & Mg alloys Fine & uniform grain structure Grain size is less sensitive to cooling rate Highly effective in sand casting cooling conditions Reduced porosity & macro defects Fine eutectic structure & intermetallics Improved ductility & strength Tolerant to Fe contamination Recycling of Al-Si scrap

α-al grain size [µm] COMMERCIAL HYPOEUTECTIC Al-Si ALLOYS Reference Al-5Ti-1B Nb-B LM25 500 μm 500 μm 500 μm 650ºC LM24 REFERENCE Nb-B Al-5Ti-1B 900 800 500 μm 500 μm 700 500 μm LM6 600 500 LM6 400 LM24 300 200 LM25 500 μm 500 μm 100 500 μm 0 600 610 620 630 640 650 660 670 680 690 Pouring temperature [ C]

Al-11Si (LM6) no addition Porosity ~1200mm ~200mm Al-11Si (LM6) with Nb-B ~160mm ~200mm 10 mm

Al-Nb-B Grain Refiner for Al-Si cast alloys Highly effective for Al-Si alloys & Mg alloys Fine & uniform grain structure Grain size is less sensitive to cooling rate Highly effective in sand casting cooling conditions Reduced porosity & macro defects Fine eutectic structure & intermetallics Improved ductility & strength Tolerant to Fe contamination Recycling of Al-Si scrap

EFFECT OF COOLING RATE 20 mm 20 mm 700ºC

α-al grain size, d [µm] α-al grain size, d [µm] EFFECT OF COOLING RATE 4500 4000 3500 Reference 10000 Nb-B inoculation d = 2072 (dt/dt) -0.34 3000 2500 2000 1500 1000 1000 d = 514 (dt/dt) -0.14 100 1000 10 0.1 Cooling rate, dt/dt [ C/s] 500 0 0 20 40 60 80 100 Cooling rate, dt/dt [ C/s]

ΔNv / Nv ref [%] Number of grains, Nv [m -3 ] EFFECT OF COOLING RATE 1000 900 800 700 600 500 400 300 200 100 0 Nv(ref) 3.50E+10 Nv(Nb-B inoculation) 3.00E+10 ΔNv ΔNv / Nv(ref) 2.50E+10 2.00E+10 1.50E+10 1.00E+10 5.00E+09 0.00E+00 0 20 40 60 80 100 Cooling rate, dt/dt [ C/s]

REFERENCE Al-Nb-B ADDITION Al-9Si-2Cu-0.7Mg-0.15Fe

A354 Alloy Condn. %Cu %Mg %Si %Fe %Mn %Ni %Zn %Pb %Sn %Ti %Sr Al-9Si-2Cu A354 CAST 1.60-2.0 0.50-0.60 8.6-9.44 0.154 0.05-0.10 0.054 0.10 0.014 0.054 0.10-0.154 0.02-0.030

Reduced Macro-porosity with Nb-B Al-11Si (LM6) with Al-5Ti-B 1 mm with Nb-B 1000µm 1 mm 1000µm 1 mm

Porosity 200000 Porosity Al-7Si alloy 150000 Porosity [ mm 2 ] 100000 50000 Al-7Si+Al-5Ti-B ~ /cm 2 Tp1 test 700 O C Al-7Si + Nb-B 0 Aluminium Al-Ti-B Novel grain refiner Reduced porosity in Nb-B grain refiner added castings

Al-7Si alloy Without With Nb-B addition Fine grain structure Reduced porosity

Fine eutectic structure Al-11Si (LM6) with Nb-B

Eutectic Si [µm] Finer Eutectic Si - wider range of cooling rates LM6 (Reference) LM6 + Nb-B 20 μm HPDC 20 μm Cooling rate [ C/s]

Al-10Si With Nb-B

Reference (Al-13Si) with Nb-B

UTS [ MPa ] Improved strength & ductility 240 Al-11Si alloy Machined from cast bars 230 220 Improved Crash performance Fatigue performance 210 200 190 180 LM6 LM6+Nb-B Casting temp: 800 170 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5 9.0 Elongation [ % ] Gravity mould Melting temp: 800 o C o C yield 82.4 Mpa 102 Mpa

Al-13Si piston alloy Al-Nb-B

Control of Fe-intermetallics in Al scrap A354 A354 + 1%Fe + 0.1%Nb-B 1% Fe Grain refiner to control Al-Fe-Si intermetallics Al 5 FeSi Al-Fe-Si large needle structure detrimental to mechanical properties

UTS (MPa) Recovery of properties in Fe-rich aluminium scrap 200 190 180 Properties recovery through refinement of intermetallics Virgin alloy 170 160 150 140 Virgin alloy with 1 wt% Fe impurity Reduced properties due to larger, needle structured intermetallics 0 0.5 1 1.5 2 Elongation, 4D (%)

Grain size, d [µm] FADING STUDY 680ºC 1200 1000 800 600 d = 1.12 (t) + 340 R² = 0.95 400 200 0 0 30 60 90 120 150 180 210 240 270 Contact time, t [min] LM6

Sedimentation height, h [cm] REQUIRED TIME TO SEDIMENT/FADE 20 18 16 14 12 10 8 6 4 2 0 NbB2 Al3Nb AlB2 20 µm 15 µm 10 µm Φ or ρ 0 10 20 30 40 50 Contact time, t [h]

Remelting Al-9Si-1.5Cu-0.6Mg-0.15Fe / RE-MELT No addition Al-Nb-B Addition 1 st Re-Melt ~ 3-4 mm 2 nd Re-Melt 3 rd Re-Melt 4 th Re-Melt

MASTER ALLOY DEVELOPMENT

Al-Nb-B Master Alloy 1. Nb metallic powder + KBF4 2. Addition of Nb metallic powder to diluted Al-B master alloy

MASTER ALLOY (METHOD 1) NbB 2 Al 3 Nb Al B Nb B Al Nb Al Al = 68.17 at.% Nb = 26.43 at.% O = 5.40 at.% Nb Al B Nb B

Al-2Nb-2B (METHOD 2) Al 3 Nb AlB 2-12 NbB 2

Al-4Nb-1B ON LM25 Reference 680ºC Nb-B (M.A.) Al-Nb-B (powders)

Al-4Nb-1B ON LM6 680ºC

Al-2Nb-1B ON Al-10Si

EFFECT OF Al-2Nb-2B ON UNDERCOOING FOR COMMERCIAL Al-Si ALLOYS

EFFECT OF Al-2Nb-2B ON COMMERCIAL Al-Si ALLOYS

RESULTS EFFECT OF Al-2Nb-2B ON COMMERCIAL Al-Si ALLOYS PISTON ALLOY (HYPER-EUTECTIC: 13 Wt.% Si) 740 C

EFFECT OF Al-2Nb-2B ON COMMERCIAL Al-Si ALLOYS 740 o C

EFFECT OF Al-2Nb-2B ON COMMERCIAL Al-Si ALLOYS 1600 Al-7Si (Ref) Al-7Si (Al-2Nb-2B) 1400 Al-8.5Si (Ref) Al-8.5Si (Al-2Nb-2B) α-al grain size [µm] 1200 1000 800 600 400 200 Al-10.5Si (Ref) Al-10.5Si (Al-2Nb-2B) 0 0 20 40 60 80 100 Cooling rate, dt/dt [ C/s]

Columnar grain structure Al-10 Si alloy - Direct Chill Cast Billets Fine (<0.5mm) equiaxed grains columnar Equiaxed Reference with Al-Nb-B Chill zone

Al-10Si DC billets Zone 1: chilled zone Zone 2: columnar crystals Zone 3: long columnar crystals Zone 4: equiaxed crystals Reference Nb-based compounds as heterogeneous nuclei Zone 1: chilled zone Inoculated Zone 2: very fine equiaxed crystals Zone 3: fine equiaxed crystals

DC-SIMULATOR

Comparative study between Al-Nb-B and Al-5Ti-B master alloys

Comparison between Ti-B and Nb-B Al-10Si Al-2Nb-B master alloy addition Al-5Ti-B master alloy addition ~ 4-5 mm ~ 300 µm 0.1% Nb 0.1%Ti

GRAIN REFINEMENT EFFICIENCY COMPARISON WHEEL ALLOYS (HYPO-EUTECTIC: 7 Wt.% Si) 740 C REFERENCE 0.1% Nb (AL-2Nb-2B) 0.2% MA (Al-5Ti-1B) 0.1% Ti (Al-5Ti-1B)

Summary Nb-B addition to Al-Si melt refines the grain structure of casting End-user benefits: Improved strength & ductility Lighter/thinner structures Homogeneous properties (thick & thin sections) Complex structures Tolerant to Fe contamination Closed loop recycling of scrap containing higher Fe Reduced shrinkage porosity - Improved soundness Component rejection ratio can be minimised