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Metal Casting Technologies : June 2008
TECHNICAL FEATURE Fluxing and degassing of aluminium F By Dr.-Ing. Wolfgang Kaettlitz - Foseco International Ltd. UK ABSTRACT luxing and degassing are the two most traditional and widely applied metal treatment operations we find in more or less extensive form in all aluminium foundries. Both operations have undergone significant changes during the last two decades – as a result of quality, cost and environmental aggravations. This article outlines the recent developments in fluxing and degassing technology and describes state of the art practices. In addition this article refers to latest developments in mechanised and automised fluxing and degassing processes and indicates how the progress in online hydrogen measurement technology will in future lead to a permanent hydrogen level monitoring and fully controlled melt degassing processes. INTRODUCTION It is well known that molten aluminium alloys have two inherent characteristics: the tendency to absorb hydrogen gas and the ability to readily oxidise. When aluminium alloy melts react with the atmosphere or moisture, they form amorphous continuous alumina (Al2 O3 O ? Al2 O3 ) films on the surface of the bath, according to the following reaction: 2 Al + 3 H2 + 3 H2 The simultaneously generated hydrogen gas is instantly picked up by the melt and dissolved. The alumina films are an intrinsic part of the melting process; they protect the metal underneath the film from further oxidation. However, in actual foundry operations, the surface of the molten bath always has some movement due to one or more of the following melting practices: charging, skimming, transferring and ladling. Any of these practices causes the thin alumina films to break which is leading to further re-oxidation and hydrogen pick up. As oxide formation and hydrogen pick up is leading to the most widespread casting defects, already in the early days of aluminium casting production practices have been established to remove both, oxide inclusions and dissolved hydrogen gas. The purpose of this article is to present an overview of recent progress in fluxing and degassing technology. In addition the latest developments in mechanisation and automation of metal treatment operations are considered. Finally this article will provide an outlook on fully controlled melt degassing processes by use of online hydrogen measurement technology. 40 www.metals.rala.com.au FLUXING Flux types Following general flux types are used in aluminium foundries: ¦ Melting flux This kind of flux is specifically used for melting of dirty foundry returns and machining chips. Melting fluxes are designed to be liquid at operating metal temperatures and having a double function, first to separate the non-metallic dirt from the liquid metal and secondly to form a liquid layer on the surface of the melt protecting it against further oxidation and hydrogen gas pick up. ¦ Drossing flux The purpose of drossing fluxes is to reduce the rich metallic aluminium content of the dross. Therefore drossing fluxes are able to react exothermically, generating heat and improving flux wettability. The work of drossing fluxes in separating metallic aluminium from the dross is considered to be due to both the surface tension effects supported by heat generation and the dissolution of aluminium oxides. ¦ Cleaning flux Melt cleaning fluxes are designed to remove aluminium oxide from the melt. Since a melt cleaning flux is non-exothermic and less reactive than a drossing flux it will yield less dry dross than a drossing flux. The work of metal cleaning fluxes is considered to be only due to the surface tension effects. ¦ Furnace wall cleaning flux Wall cleaning fluxes are specifically designed for the softening and removal of excessive aluminium oxide build-up that occurs on furnace walls, especially along the melt line. This type of flux helps keep crucible and furnace walls above and below the melt line free of oxide build-up. Flux formulations Historically most flux compositions contained high levels of fluoridic salts as fluorides are the most effective wetting and oxide dissolving agents. Due to environmental concerns about fluoride emissions the flux manufacturers developed effective flux compositions with minimised fluoride contents, and in the recent years even fluoride free products have been made available for specific applications. It has to be mentioned that fluoride free fluxes cannot be used effectively as a wall cleaning flux.