Department of Mechanical and Industrial Engineering

CASTING AND SOLIDIFICATION OF MATERIALS

ME-8109

Dr. C. Ravindran

STUDY OF SOLIDIFICATION DEFECTS OF ALUMINUM ALLOY

Prepared by: Vikas Minhas

Presentation Outline

� Introduction

� Entrainment of double oxide film

� Entrainment defects

� Evolution of bifilm

• Furling

• Unfurling

� Unfolding of bifilm

� Effect of hydrogen and dendrite spacing� Effect of hydrogen and dendrite spacing

� Problems due to submerged bifilm

� Effect on Mechanical Properties

� Elimination of solidification defects

• Degassing

• Inclusion removal

• Elimination of bifilm

� Conclusion

� References

Introduction

� Entrainment defects are caused by the folding action of the (oxidized) liquid

surface.

� Formation and Entrainment of bifilms,

1. On the surface of melt in furnace

2. During pouring

3. In mould

Entrainment of a double oxide film

� A breaking wave is formed by surface turbulence.

� The two un-wetted sides of the oxide films contact each other.oxide films contact each other.

� A double oxide film is submerged into the bulk liquid as a crack like defect.

Entrainment Defects

Surface turbulence causing the entrainment of

bifilms and associated bubbles.

� A - B:- Small entrained bubbles

form pores in bifilm.

� C and D:- Large bubbles are

buoyant ,creating trails prior to capture elsewhere

or eventual detrainment.

Entrainment Defects

Tangled bifilm and magnification of a portion of the bifilms

Evolution of bifilm

Furling

� Bifilm is pummelled and ravelled into

Compact convoluted form.

� If Liquid beneath the surface experience

bulk turbulance,Re>2000

Unfurling

� Filling of casting is complete, Re<2000

� Precipitation of gas through alumina film

� Interdendritic pore formation

� Defines the properties of cast material

Unfolding of bifilm

� Average resisting force = 3πηRV/2

� Unfurling effort by gas pressure = 2PRh(h/2)

2PRh(h/2) = 3πηRV(R/2)

� Time to unfurl the bifilm from � Time to unfurl the bifilm from

the speed V,

t = (3π2/2) (η/P)( R/h)2

Where

V=πR/t

RxR = Square fold area

P = gas pressure acting over the

small area hR Unfurling process by hydrogen penetration

Effect of Hydrogen Content and Dendrite Arm Spacing

� Lower Left:

Micro-inflation

� Lower Right:

At short solidification times Pore

density increases with increase in

hydrogen content.

� Top Left:

Clusters of multiple pores.

� Top Right:

At longer solidification times pore

density decrease with increasing

hydrogen content.

Problems due to Submerged Films

Machining Problem

� Oxides are much harder than the metal itself, causing dragging out during machining, leaving unsightly grooves.

� The cutting edge of tool is often chipped or blunted by encounters with such problems

Leak Tightness

� For thin-sectioned castings 5 mm and below, film defects can be extended from wall to wall across the mould cavity and so connect the casting cavity, surfaces with a leak path.with a leak path.

� Bubble defects are specially troublesome with respect to leak tightness, since they necessarily start at one casting surface and connect to the surface above.

Fluidity

� The fluidity of clean melt is always higher than that of dirty melt, and can be cast at a lower temperature.

� The cumulative benefits are valuable.

Effect on Mechanical Properties

UTS and Elongation

� Dendrite arm spacing increases

� Strength Decreases

� Elongation Decreases

Fracture strength

� Decreases

� Fatigue properties decreases

Corrosion

� Corrosion pits and corrosion

filaments occur principally on

entrained Casting defects.

Elimination of solidification defects

Degassing

Schematic diagram describing the various degassing steps

Degassing

� To remove hydrogen from the liquid aluminium, a degassing process is essential.

� Flux, purge gasses, vacuum degassing and ultrasonic vibration can be used as degassing methods. The most popular method is bubbling inert or reactive gases from near the bottom of the furnace.

� The purge gasses are usually, Ar or some mixture of these gases. During their passage through the melt, the bubbles, which are of low initial hydrogen content, absorb hydrogen from the melt and then escape at the surface.

� The melting temperature, size of the gas bubbles and gas composition are important factors for this degassing method. The solubility of hydrogen doubles when the liquid aluminium temperature increases by 60 °C.

� The size of the gas bubbles determines the area/volume ratio (A/V). For high degassing efficiency, the A/V should be maximised, and bubble smaller than 5 mm are the most effective.

Inclusion removal

Schematic illustration of filter medium

Inclusion removal

� Inclusions such as aluminium oxide and Mg come from ingots or reactions with

oxygen in the air or humidity during melting and casting.

� Sedimentation, flotation and filtration in the furnace are used by the foundry

industries to remove these inclusions.

� Once at the surface, they can be removed by skimming. However, these fluxes

contain harmful elements such as Cl, F or P.contain harmful elements such as Cl, F or P.

� The flux itself can be an inclusion in the casting if it remains in the melt after

treatment. After cleaning during the melting process, a filter within the mould can

help to remove inclusions such as fluxes, ceramic particles or oxide film during the

filling process.

� Inclusions can be trapped on the front or entry face of the filter, and build up filter

cake layers .

Elimination of entrained Film

� When submerged, the film continues to grow by consuming the stored gases

oxygen to form oxides,

nitrogen to form nitirdes,

other goes in solution

Eventually all the gases are consumed and the most damaging effects of the film causing leaks, nucleating bubbles, cavities, cracks will have been removed.

� This automatic deactivation of entrained film usually occurs in cases where the metal is subjected to pressure (squeeze casting (50-150 MPa), hot isostaticpressing (200 MPa), even some benefits are obtained in sand casting with moderate pressure of only 0.7 MPa).

Elimination of entrained film

Natural or automatic deactivation of film is

not occurred in general practices.

� So it is important to try to reduce the formation

of oxide at all stages of melt preparation and

handling.handling.

� The only way to succeed in reducing

the oxide in casting is to use filter in the running

system, after pouring, but before the metal enters

the mould, to ensure that the flow conditions into

the mould cavity do not reintroduce new oxide.

Assessment of effects

Fig (a) 0.50 s 3.00 s 4.00 s 6.20 s

Fig(b) 0.50 3.00s 4.00s 6.75s

Experimental Technique

Assessment of effects

2D-Section of Model During Filling (a) Unfiltered, (b) Filtered

Conclusion

� Many metals that currently exhibit brittle failure may only do so because of their

high content of bifilms

� Bifilms can be closed by the application of pressure applied in the liquid or solid

state, with benefits to mechanical properties

� Deactivation remains, however, an expensive second best to the avoidance of� Deactivation remains, however, an expensive second best to the avoidance of

entrainment by the implementation of improved casting technology

� The reduction in the density of bifilms in Al alloys can increase the mechanical

properties.

References

� J. Campbell: ‘Castings’ 2nd edn, 443–452; 2003, Oxford, Butterworth-Heinemann.

� J. Campbell: ‘Casting practice’; 2004, Oxford, Butterworth Heinemann.

� J.M. Boleau and J.E.Allison, The Effect of Porosity Size on the Fatigue Properties in a Cast 319 Aluminum Alloy, SAE International, 2001

� Visualisation of oxide film defects during solidification of aluminum alloys, s fox, and j Campbell, cast metal research group, irc in materials for high performance applications, university of Birmingham, uk june 21, 2000

� Assessment of the effect of grain refinement on the solidification characteristics of 319 aluminum alloy using thermal analysis by S.G. Shabestari, M. Malekan. Center 319 aluminum alloy using thermal analysis by S.G. Shabestari, M. Malekan. Center of Excellence for Advanced Materials and Processing (CEAMP), School of Materials and Metallurgical Engineering, Iran University of Science and Technology (IUST), Narmak, Tehran, Iran.

� Behaviours of Bifilms in A356 Alloy during Solidification: Developing Observation Techniques with 3-D Micro X-ray Tomography By Jun Mo Park , The University of Birmingham,School of Metallurgy and Materials, University of Birmingham, June 2009.

� Settling behaviour of different grain refiners in aluminum by Paul L. Schaffer, Arne K. Dahle. Division of Materials Engineering, the University of Queensland, Brisbane, Qld 4072, Australia. Received in revised form 30 June 2005.

References

� S. Fox and J. Campbell: Int. J Cast Met. Res., 2002, 14, 335–340.

� Effect of the casting process variables on microporosity and mechanical properties in an investment cast aluminum alloy by Y.M. Li and R.D. Li. Foundry division department of engineering Shenyang polytechnic university Shenyang. 16 june1999.

� Eutectic modification and microstructure development in Al–Si Alloys by A.K. Dahle, K. Nogita, S.D. McDonald, C. Dinnis, L. Luc, CRC for Cast Metals Manufacturing, The University of Queensland, Brisbane, Qld 4072, Australia, Materials Engineering, CSIRO Minerals, P.O. Box 883, Kenmore, Qld 4069, Australia, Received in revised form 29 June 2005.Received in revised form 29 June 2005.

� Structural and mechanical properties of Al–Si alloys obtained by fast cooling of a levitated melt by S.P. Nikanorova, M.P. Volkova, V.N. Gurina, Yu.A. Burenkova, L.I. Derkachenkoa, B.K. Kardasheva, L.L. Regelb, W.R. Wilcoxb a Ioffe Physico-Technical Institute of Russian Academy of Sciences, Politekhnicheskaya Street, 26, Saint-Petersburg 194021, Russia International Center for Gravity Materials Science and Applications, Potsdam, NY 13699-5814, USA Received 19 March 2004; received in revised form 5 July 2004; accepted 19 July 2004.

� www.light-alloy-wheels.com

� www.world-aluminium.com

� www.aluminiumlearning.com

� www.aluminium.matter.org.uk

� www.Azom.com

� www.en.wikipedia.org/wiki/aluminium