Melting copper–nickel alloys

Nickel increases the solubility of hydrogen in copper melts, so it is necessaryto melt under an oxidising cover of CUPREX, followed by degassing withthe Rotary Degassing Unit or LOGAS 50 to eliminate the hydrogen, thenfinally to deoxidise.

For a 100 kg melt, use 2 kg of CUPREX 1 blocks, degas with one LOGAS 50 briquette and deoxidise with DEOXIDISING TUBES MG (3 MG6 tubes for 100 kg). Note that Cu–Ni alloys may be embrittled by phosphorus, so DEOXIDISING TUBES DS should not be used.

Recommended pouring temperatures

Light castings, under 15 mm section 1400°CMedium castings, 15–40 mm section 1350°CHeavy castings, over 40 mm section 1280°C

Filtration of copper-based alloys

Copper-based alloys, particularly those containing aluminium such as theAl–bronzes and some of the brasses, benefit greatly from filtration in themould. SEDEX ceramic foam filters are recommended.

Melting and treatment of high conductivity copperThe quality of high conductivity copper is measured by its electricalconductivity. Pure copper in the annealed condition has a specific electricalresistance of 1.72 microhms per cubic cm at 20°C. This is said to have 100%electrical conductivity IACS (International Annealed Copper Standardunits). Cast copper can have a conductivity of 90% IACS and has bothelectrical and thermal applications since high electrical conductivity alsoimplies high thermal conductivity. Many of the impurities likely to bepresent in copper lower its electrical conductivity seriously, Table 16.7.Cu–C (HCC1) copper is used for water-cooled tuyeres and electrodeclamps, it must have 86% IACS minimum so must be of high purity withonly small additions of Cr or Ag to extend the freezing range and makecasting easier.For less onerous duties, copper having tin or zinc up to 2% may be used.A degree of conductivity is sacrificed to allow better casting properties andfor ease of machining.Where greater hardness and strength are required, copper–chromiumcastings CC1-TF may be used. This alloy requires to be heat treated (1 hourat 900°C, followed by quenching to room temperature and reheating to500°C for 1–5 hrs) to realise its full properties.High purity copper is particularly prone to gas porosity problems dueboth to hydrogen and the hydrogen/oxygen reaction which occurs if anyoxygen is present in the molten metal. Steps must be taken, during melting,to exclude both hydrogen and oxygen from the melt. The principlesinvolved are:Melt quickly, using the lowest temperature possible, under a reducingcover fluxPurge with an inert gas to remove hydrogenAdd deoxidants to remove residual oxygen, ensuring that residualdeoxidant does not reduce the conductivityMeltingThe charge materials must be carefully selected to avoid impurities whichcan reduce the conductivity. Before charging, the copper must be clean anddegreased to avoid any hydrogen-containing contaminants. Clean and drycrucibles, lids, plungers and slag stoppers must be used. The crucible shouldbe preheated before charging to minimise the time that the copper is solidand unprotected by flux. Melt down under a reducing cover of CUPRIT 8;the flux should be placed in the bottom of the crucible prior to charging. 1 kgof CUPRIT 8 is needed per 100 kg of metal.

The effect of impurities and alloying elements on theelectrical conductivity of pure copperImpurity % % IACSAluminium 0.1 85Antimony 0.1 90Arsenic 0.1 75Beryllium 0.1 85Cadmium 1.0 90Chromium 1.0 80Calcium 0.1 98Iron 0.1 70Magnesium 0.1 94Manganese 0.1 88Nickel 0.1 95Phosphorus 0.1 50Silicon 0.1 65Silver 1.0 97Tin 1.0 55Zinc 1.0 90DegassingHydrogen is removed from the melt by bubbling an inert gas through themelt. This can be done using argon or nitrogen using the Mobile DegassingUnit or less effectively by injecting gas through a graphitetube immersed deep into the melt. 50–70 litres of gas are needed for each100 kg of copper.An alternative way to degas is to plunge LOGAS 50 briquettes into themelt. LOGAS is a granular material, strongly bonded and formed into aweighed unit with high surface area/volume ratio to ensure maximumcontact area with the liquid metal. On contact with the metal, LOGAS 50decomposes releasing a steady stream of non-reactive gas which flushes outthe hydrogen. LOGAS 50 units are packed in foil, they are of annular shapehaving a central hole into which a refractory-coated steel plunger can beinserted,Treatment takes from 3 to 10 minutes depending on the size of the melt.Some loss of temperature occurs during treatment, so the initial treatmenttemperature must be chosen accordingly. The minimum temperaturepracticable should be used.

Deoxidation

A number of deoxidants are available for copper. They combinewith the dissolved oxygen in the metal forming stable oxides which float outof the melt. Phosphorus is the most widely used deoxidant for copper andits alloys because of its effectiveness and low cost. It must be used sparinglywith high conductivity copper since any residual phosphorus left in solutionseriously lowers the conductivity of the copper.The recommended practice is to use phosphorus to remove most of thedissolved oxygen and to complete the deoxidation with a calcium boride orlithium-based deoxidant.The precise quantity of deoxidant needed depends on the melting practiceused. Simple tests can be made in the foundry to observe the solidificationcharacteristics of the melt. Open-topped cylindrical test moulds havingimpressions about 75 mm high by 50 mm diameter are needed. They can beformed in a cold-setting resin or silicate sand mixture. When the melt isready for deoxidation, a sample of the copper should be ladled into one ofthe moulds and allowed to solidify. If the head rises appreciably , a very gassy metal is indicated. DEOXIDISING TUBES DScontaining phosphorus must be plunged and further test castings made. Atthe point when the quantity of phosphorus added results in a shallow sinkin the head, it can be assumed that the residual phosphoruscontent of the melt is nil and a small amount, about 0.008% of oxygen,remains.Deoxidation is now completed by plunging DEOXIDISING TUBES CB orL, adding sufficient to produce a test casting having a head with a deep sink. The melt is now in a condition to produce castings free fromporosity. The approximate additions needed are shown below:Weight of meltDEOXIDISING TUBES25 kg 50 kg 75 kg 100 kg 200 kg

400 kgDS1 & 1 DS1 1 DS2 1 DS3 2 DS4 2 DS4 1 DS6CB 1 CB3 2 CB3 3 CB3 1 CB6 2 CB6 4 CB6DEOXIDISING TUBES L, containing lithium, can be used as the finaldeoxidant in place of DEOXIDISING TUBES CB. An application rate of0.018–0.02% of product should be used. In addition to being an excellentdeoxidant, lithium also removes traces of hydrogen. This is found to reducethe incidence of cracks in complex cast shapes.Casting conditionsHC copper, being almost pure copper, has an extremely short freezing range.It is very weak at the point of solidification so that moulds and cores mustnot be too strong. Resin bonded sand is suitable and the resin percentagemust be as low as possible, the minimum necessary for handling the mouldand cores. Gating should be designed to minimise turbulence on pouring, inorder to avoid the possibility of oxygen pick-up,Recommended casting temperaturesLight castings < 15 mm section 1250°C

Medium castings 15–40 mm section 1200°CHeavy castings >40 mm section 1150°CMelting and treatment of high conductivity copper alloysCopper– Silver additions should be made in the form of Cu–Ag master alloy andintroduced into the melt after degassing but prior to deoxidation. The samedual deoxidation process used for pure copper is recommended.The simplest form of gating for small to medium sizecastings.1. Tapered sprue to reduce formation of air bubbles.2. Deep basin to receive first turbulent impact of metal.3. Ingate tapering out to reduce metal velocity. Noteposition of ingate at top of sprue basin.2. Progressively narrowing runner to keeprunner bar full; this reduces dross formation.Note runner bar extention to trap dross.

Copper–cadmiumDegassing and deoxidation by the dual treatment must be completed beforecadmium is added. The molten copper can be tapped directly onto purecadmium metal as the metal is transferred from the melting furnace to apouring ladle. The use of a Cu–Cd master alloy is preferable, since lowercadmium losses occur. Molten cadmium evolves toxic brown fumes so goodventilation is needed.Copper–chromiumCu–Cr master alloy should be added after degassing but before deoxidation.The chromium alloy should be thoroughly stirred in to ensure ahomogeneous solution. A chromium loss of 10–30% may be expecteddepending on the state of oxidation of the melt. Phosphorus additionsshould only be made if a test casting shows a rising head. Normally thechromium addition and a final deoxidation with calcium-boride or lithiumis sufficient. Any residual phosphorus left in the alloy will upset its responseto heat treatment.