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Fluxes - Is There a Panoramic View? Fluxes - Is There A Panoramic View? K.C. Mills and D. See Department of Materials, Imperial College of Science, Technology and Medicine, London SW7 2BP, England (Received January 16, 2001.) ABSTRACT (ii) in Materials, "a substance used in soldering, welding and brazing to remove oxides from the Fluxes used in the following processes have been surfaces to be joined," and examined to determine what common features they (iii) in Metallurgy, "in liquid metal processing, a non- might have: soldering, brazing, welding, electroslag metallic material used to protect the metal and remelting (ESR), ingot and continuous casting, and the remove impurities." production of aluminium master alloys. A quick examination of the functions of fluxes used The fluxes have been inspected in terms of (i) the in a specific process (such as welding) will quickly functions that they have to perform in the process, (ii) indicate that properties such as 'promoting arc stability' the properties of the flux which are important to carry are not covered by the above definitions. out those functions, and (iii) typical compositions. It Consequently, in this review we have examined the was concluded that there were some common features following types of fluxes: for the fluxes; that they should (i) melt at least 100°C - soldering and brazing, below the metal, (ii) form a protective layer on the - welding, metal, (iii) absorb inclusions from the metal and, (iv) - electroslag refining, usually contain halides. On the other hand, frequently, - formation of master aluminium alloys containing key features were specific to the type of flux and its TiB , and application; for instance, powder consumption rates are 2 - mould and ingot fluxes for the casting of steel. essential to the performance of mould fluxes, as are factors affecting arc stability, to the performance of We have examined the above fluxes in terms of: welding fluxes. (i) the functions that the flux must perform in each application, (i) the physical properties which are important in carrying out these functions, and 1. INTRODUCTION (ii) typical compositions of the various fluxes used. The objective of this paper is to examine fluxes used 2 SOLDERING AND BRAZING FLUXES in different industries with a view to identifying any 2.1. Soldering features which (i) are common to all fluxes and (ii) are individual to specific applications. In soldering and brazing the surfaces of the metals We start with dictionary definitions of a flux /1 /: being joined remain solid throughout the entire (i) in Chemistry, "a substance promoting the operation. The filler metal from the solder makes the melting of another substance to which it is joint by intimate attachment to the base metal surfaces. added," The filler metal must 'wet' the workpiece surface and Vol. 20. Nos. 3-4, 2001 Fluxes - Is There A Panoramic View? must flow easily over the surfaces. These characteristics fluxes used must be capable of operating within will be promoted by low surface tensions and low temperature ranges of 600 - 800° C and 750 - 1150°C contact angle (solder on base metal) for the solder or respectively. The flux must be liquid and should not braze. The fluxes are added to facilitate this process and decompose at the operating temperature. must carry out the following tasks 121: Fluxes used in hard soldering contain low melting (i) remove any oxide films (usually Cu20 or SnO) compounds such as B203 NaF, KF, KHF2 borofluorides, present on the workpiece surfaces and protect borates, etc. these surfaces from any further oxidation, (ii) any remaining left-over fiux or reaction products 2.3. Brazing must be easily displaced from the metal, and (iii) promote the heat transfer from heat source to the Brazing is carried out on a variety of metals: Al, Mg, joint. Ti steels and Cu/Al alloys using a variety of filler The efficiency of the flux is controlled by its ability metals, each of which requires an appropriate flux. A to promote melting and is associated closely with the variety of compounds are used in these fluxes (e.g. LiCl, reactivity of the flux. However, if the flux is reactive, it LiF, NaF, and KF. KHF2, ZnCl2, KBF4, B203 etc). can cause corrosion of the joint in time. This is a major Brazing is very similar to hard soldering and the issue, especially in the case of soldering electronic requirements of the flux are identical to those for hard components 111. In some processes the fluxes are soldering. removed by washing after soldering. Fluxes for soft soldering tend to be of two types: 2.4. Discussion of Joining Fluxes (i) those soluble in organic fluids, and (ii) those soluble in water. The temperatures used for soft and hard soldering In some cases activators are added to the flux to and for brazing vary but the role of the flux is identical enhance the reactivity of the fluxes. Activated fluxes in all these applications. First, and most importantly, the nearly always contain halides. flux must promote the wetting of the filler metal and to do this, it must (i) remove the oxide films on the base (i) Fluxes soluble in organic liquids metal surface to permit wetting and (ii) have a low These are mostly based on colophony (rosin) which is melting point to provide a liquid layer to protect the a natural product derived from pine sap. The principle base metal from further oxidation. The presence of component of colophony is abietic acid. Colophony halides in fluxes is interesting. The main function of the has a melting point (mp) of 172°C and is soluble in halides is the chemical attack of the oxide films on the acetone and alcohol. It tends to have low reactivity surface of the base metal. However, there is evidence to and activators containing halides are usually added. suggest that Group VII elements (I, Br, CI, F) are very (ii) Fluxes soluble in water surface-active and thus the presence of F or CI at the These fluxes are frequently based on ZnCl2 (mp283°C) metal/solder (or braze) interface could reduce the or NH4CI or on mixtures of the two components. surface tension of the solder (or braze) significantly and There is little fluxing action in the absence of encourage spreading of the solder. moisture. 2.2. Hard Soldering 3 WELDING FLUXES The principles of this process are identical to those in soft soldering but the temperatures involved are In welding the metal pieces being joined are melted higher. In hard soldering the filler metals tend to be (cf soldering,brazing). Fluxes are used to protect the silver or copper to which elements such as zinc, tin etc. metal surface from oxidation. Failure to protect the are added to reduce the melting point. Consequently, the metal can lead to a weld with gas pores and poor surface 156 K.C.Mills and D. See High Temperature Materials and Processes appearance /3,4,5/. Fluxes are used in four welding rapid change in polarity in AC welding and thus fluxes processes: with significant F content can only be used in DC (i) Manual - Metal Arc /MMA/ shown in Figure la, welding (where F ions migrate to the anode). The ions (ii) Submerged -Arc Welding /SAW/ shown in produced in the arc also affect the droplet size. The Figure lb, basic fluxes tend to produce larger metal droplets from (iii) Flux-cored wire welding shown in Figure lc, and core wire whereas more acidic fluxes (which give less (iv) Electroslag - welding shown in Figure 1 d. oxygen protection) tend to produce 'spray transfer' 131. This may also be related to the surface tension of the Some of the requirements are common to all the core wire under the welding conditions. processes, but other tasks are specific to an individual Several types of fluxes are available for MMA welding process. Functions of welding fluxes include welding and the selection is dependent upon the type of /3,4,5/: weld required, for instance, practical (i.e. non- (i) to melt at a temperature below (e.g. 100°C) the horizontal) welding, is best performed with spray metal melting point of the base metal, transfer. The following fluxes are available: - (ii) to protect the weld from the atmosphere; fluxes (i) Acid (high Si02, FeO, MnO contents) - high Ο contain (a) carbonates and fluorides which and Η levels in welds leading to poor decompose to give C02 or F shielding and (b) in mechanised properties some cases contain metals (e.g. Al) which react (ii) Rutile - (Ti02 /30-50%/, CaO, Mn0,Si02) with oxygen to reduce the partial pressure of (iii) Basic - (30-40% CaF2, 25-40% CaO, 10-15% oxygen, S;02) low Ο and Η contents in weld metals and (iii) to form a slag crust which protects the weld from give large droplets, are difficult to use in AC subsequent oxidation during cooling, welding. (iv) to remove impurities from both the surface (e.g. rust) and the metal (e.g. inclusions) by absorbing them into the molten slag, (v) to encourage arc initiation and stability; sodium 3.2. Submerged Arc Welding (SAW) and potassium ions are beneficial for this task; In this process /3,4,5/ an arc is struck between a this is very important to the welding process, and consumable electrode and the workpiece which is (vi) to increase metal deposition rates of the metal covered with a layer of flux (Figure lb). A cavern is electrodes (in practice this is achieved by formed in the metal (Figure lb) and its shape is additions of ferrous alloy powders) and to maintained as the weld travels along the workpiece.
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