STUDY OF SOLIDIFICATION DEFECTS OF ALUMINUM ALLOY

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1 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

2 Presentation Outline Introduction Entrainment of double oxide film Entrainment defects Evolution of bifilm Furling Unfurling Unfolding of bifilm 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

3 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

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

5 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.

6 Tangled bifilm and magnification of a portion of the bifilms Entrainment Defects

7 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

8 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 bifilmfrom the speed V, t = (3π 2 /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

9 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. TopLeft: Clusters of multiple pores. Top Right: At longer solidification times pore density decrease with increasing hydrogen content.

10 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. 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.

11 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.

12 Degassing Elimination of solidification defects Schematic diagram describing the various degassing steps

13 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, Aror 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.

14 Inclusion removal Schematic illustration of filter medium

15 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 containharmfulelementssuchascl,forp. 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. Inclusionscanbetrappedonthefrontorentryfaceofthefilter,andbuildupfilter cake layers.

16 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 ( MPa), hot isostatic pressing (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. Soitisimportanttotrytoreducetheformation ofoxideatallstagesofmeltpreparationand handling.

17 Elimination of entrained film Natural or automatic deactivation of film is not occurred in general practices. Soitisimportanttotrytoreducetheformation ofoxideatallstagesofmeltpreparationand handling. Theonlywaytosucceedinreducing theoxideincastingistousefilterintherunning 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.

18 Assessment of effects Fig (a) 0.50 s 3.00 s 4.00 s 6.20 s Fig(b) s 4.00s 6.75s Experimental Technique

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

20 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 entrainment by the implementation of improved casting technology The reduction in the density of bifilms in Al alloys can increase the mechanical properties.

21 References J. Campbell: Castings 2nd edn, ; 2003, Oxford, Butterworth-Heinemann. J. Campbell: Casting practice ; 2004, Oxford, Butterworth Heinemann. J.M. Boleauand 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, ircin 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 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,Schoolof Metallurgy and Materials, University of Birmingham, June 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.

22 References S. Fox and J. Campbell: Int. J Cast Met. Res., 2002, 14, Effect of the casting process variables on microporosityand 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 june 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, Qld4072, Australia, Materials Engineering, CSIRO Minerals, P.O. Box 883, Kenmore, Qld4069, Australia, Received in revised form 29 June 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. Wilcoxba IoffePhysico-Technical Institute of Russian Academy of Sciences, Politekhnicheskaya Street, 26, Saint- Petersburg , Russia International Center for Gravity Materials Science and Applications, Potsdam, NY , USA Received 19 March 2004; received in revised form 5 July 2004; accepted 19 July

Characteristics of dynamically formed oxide films on molten aluminium

Characteristics of dynamically formed oxide films on molten aluminium B. Nayebi and M. Divandari* The characteristics of oxide films formed within a short time of pouring were investigated using samples