PSI - Issue 43

John Campbell / Procedia Structural Integrity 43 (2023) 234–239 Author name / Structural Integrity Procedia 00 (2022) 000 – 000

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5. Eliminating the Pre-Existing Crack Population When first melted, metals tend to be contaminated with the oxide bifilm population they have inherited from previous melting and casting events. However, by simply floating or sinking of oxides the melt can be cleaned significantly. Filtering can also help if carried out appropriately. These strategies are sometimes not straightforward, but possible. They are described elsewhere (Campbell 2018, 2022). Having achieved an acceptably clean liquid metal, the problem arises now is how to cast it without re-introducing the bifilm defects. Vacuum casting is of no use because the vacuum seems never sufficiently good; the liquid metal is usually highly efficient in ‘getting’ the oxygen present in the vacuum. Thus, special precautions are required if the metal is to be poured, to avoid the normal masses of re-oxidation of the alloy, to produce the usual dense population of pre-existing cracks. Contact pouring is one major technique, in which the ladle is connected directly with the entrance to the mould (Campbell 2015, 2018). Additional refinements are possible to avoid even the residual air which contact pouring itself cannot avoid (Campbell 2018). By far the best solution to avoid the problem of the contamination of the liquid metal by air during pouring, is to eliminate pouring. Counter-gravity systems are used for many metals, mainly by pressurisation of the furnace or ladle, to force the melt up a riser tube and into the mould. Careful transfer of the metal in this way, keeping to below the known critical velocity of 0.5 m/s (Campbell 2015) ensures that the transfer can be made with perfection, generating zero bifilm defects. Aluminium is easy to transfer in this way; pumps are available. However, even steels can take advantage of this technology as witnessed by the Griffin process for steel railroad wheels which has been in use for over 50 years. 6. Conclusions 1. Current casting techniques widely use pouring techniques which are highly damaging to liquid metals, creating a population of bifilms, the pre-existing population of cracks in metals, which can explain many of our widely experienced failure mechanisms. 2. Improved gravity pouring processes are a significant help to address the problem, as is chemical alloying to ensure the elimination of a solid oxide on the liquid metal, and its replacement with a liquid oxide. 3. The ultimate solution to avoid bifilms is to avoid surface turbulence by casting metals counter-gravity.

References Campbell J. 2020 The Origin of Fracture – The Mechanisms of Metallurgical Failure. Elsevier.

Campbell J, 2015 The Complete Casting Handbook, Elsevier. Reed R C; 2006 Superalloys. Cambridge University Press. Tiryakioglu M; 2014, ‘ On the relationship between elongation and fatigue life in A206-T71 aluminum castings. ’ Materials Science & Engineering A601, 116 – 122. Campbell J; 2022 (1) , ‘VAR and ESR Steels – A Critical Appraisal’. Liquid Metals Processing and Casting (LMPC) Conference 2022 Philadelphia, USA. Campbell J. 2022 (2). ‘ Enhanced Steels by Ingot Casting - A Personal View ’ . Ingot Casting Rolling Forging (ICRF) Conference, Pittsburgh, Pa, USA. Campbell J. 2018 The Mini Casting Handbook. Published by Aspect Design Limited, Malvern, UK.