Foseco Non-Ferrous Foundryman’s Handbook Part 13 ppsx

Foseco products for the melting and treatment of copper and its alloys ALBRAL Fluxes for treatment of alloys containing Al, they dissolve and remove alumina.. HC copper Crucible Electric

Nearest equivalent in

old BS 1400 or

BS4577

BS EN or ISO symbol for castings (1)

BS EN material designation number for castings (2)

BS EN relevant casting processes and designations (3) GM

Diecasting

GS Sand

GZ Centrifugal

GP Pressure-die

GC Continuous

Cooper and Copper-chromium (High conductivity coppers)

Copper–zinc (Brasses)

CuZn37Pb2Ni1AlFe–C CC753S ✓

Copper–tin (Gunmetals and Phosphor-bronzes)

PB4 G–CuSn10PbP

Copper–tin–lead (Gunmetals and Leaded bronzes)

Copper–aluminium (Aluminium bronzes)

Copper–manganese–aluminium

Copper–nickel (cupro-nickels)

(1) Symbol finishes with B for material in ingot form.

(2) Number begins CB for material in ingot form.

(3) GM – permanent mould casting GS – sand casting GZ – centrifugal GP – pressure diecasting GC – continuous casting.

Method of casting affects properties significantly.

Note: Ingots are not specified for high conductivity coppers.

Melting copper and copper-based alloys

The melting of copper and copper-based alloys presents special problems Molten copper dissolves both oxygen and hydrogen and on solidification, the oxygen and hydrogen can combine to form water vapour which causes porosity in the casting, Figs 16.1–16.4 Without the presence of oxygen, hydrogen alone may also cause gas porosity Alloys containing aluminium form oxide skins which can cause problems in castings In other alloys, traces

of aluminium can cause defects and residual aluminium must be removed Special melting and metal treatment techniques have been developed to deal with these effects These include fluxing, degassing and deoxidation treatments Foseco supplies products for each of these treatments

Foseco products for the melting and treatment of copper and its alloys

ALBRAL Fluxes for treatment of alloys containing Al, they dissolve

and remove alumina

CUPREX Oxidising fluxes for preventing hydrogen pick-up during

melting, Table 16.3

Figure 16.1 Solubility of hydrogen in copper (From Neff, D.V (1989) Hydrogen

and oxygen in copper, AFS Trans., 97, 439–450.)

Figure 16.2 Effect of alloying elements on hydrogen solubility in copper melts (From Neff, D.V loc cit.)

Figure 16.3 Copper–copper oxide phase diagram (From Neff, D.V loc cit.)

CUPRIT Neutral or reducing fluxes, they protect alloys from

oxidation and reduce zinc loss, Table 16.4

DEOXIDISING

TUBES For deoxidising copper and its alloys, Table 16.6

ELIMINAL Flux for removing aluminium from melts

MDU Mobile Degassing Unit for the removal of hydrogen LOGAS 50 Briquettes for the removal of hydrogen, Table 16.5 PLUMBRAL Covering and scavenging flux for treating high lead

alloys

RECUPEX Fluxes for melting copper alloy swarf, skimmings and

scrap

RECUPEX 250 Reducing, protective flux for use when metal is held

molten for a long time, e.g during continuous casting SLAX 20 Slag coagulant

Figure 16.4 Equilibrium between hydrogen and oxygen in copper melts (From Neff, D.V.)

Table 16.3 CUPREX oxidising fluxes and their applications

Product Form Application

rate (%)

CUPREX 100 Powder 0.5–1 Tin/lead bronzes (<10% Pb) and

copper–nickel alloys

Fluid

CUPREX 160 Powder 1–2 Commercial copper, bronzes, gunmetal,

Ni–brass alloys melted in crucible or reverberatory furnaces

Plastic, dry

Table 16.4 CUPRIT reducing fluxes and their applications

type

Recommended procedure

Brass

Brazing metals

Gilding metals

Crucible 1 Use 1% addition rate of CUPRIT

Place briquettes in the bottom of the hot crucible and charge metal

on top Leave the slag intact until the crucible is withdrawn from the furnace

81 49

Add 1% CUPRIT at an early stage

in melting Leave slag intact until the crucible is withdrawn

Small reverberatory

1 Place briquettes at the bottom of the hot furnace and add the charge Use 1% CUPRIT

Electric 49 Add 0.5% CUPRIT in two stages,

add the major portion to the heel and the remainder for final drossing-off Skim before pouring

HC copper Crucible

Electric

81 Add 1% CUPRIT at an early stage

in melting Leave the slag intact until the metal is tapped or the crucible withdrawn

Brass

Brazing metals

Gilding metals

Comm copper

Electric low freq

induction

81 0.75% CUPRIT is needed Add

0.6% together with charge, stir in the balance before drossing-off More flux may be needed if the charge consists of swarf

Brass

Brazing metals

Gilding metals

Al–bronze

Si–bronze

Mn–bronze

All types

of reverb

furnace

81 Add 0.5% at the beginning of

melting During melting add more

to maintain a flux cover 1% total may suffice depending on the surface of molten metal exposed

Table 16.5 LOGAS 50 degassing tablets

CUPREX oxidising fluxes and their applications

CUPREX formulations evolve oxygen to produce oxidising conditions and a scavenging gas to remove most of the dissolved hydrogen, thus preventing the steam reaction which causes porosity in castings CUPREX also forms a flux cover to prevent the pick-up of more hydrogen from the furnace atmosphere and remove non-metallic material, Table 16.3

CUPRIT neutral or reducing fluxes

The CUPRIT range is produced in briquette and powder form:

Briquettes CUPRIT 1

Powder CUPRIT 49, 81, 103

CUPRIT briquettes are weighed while the powders are available in pre-weighed packets or in bulk The main functions of CUPRIT are:

To form a protective blanket over the metal during melting to prevent contamination of the melt from the furnace atmosphere and to protect alloying elements, especially zinc, from oxidation, thereby suppressing zinc fume and the formation of showers of zinc oxide in the air

Table 16.6 Grades of DEOXIDISING TUBES and their use

Alloy DEOX.

TUBE

Weight of melt (kg)

Commercial

copper

DS 2 × DS3 3 × DS4 6 × DS3 6 × DS4 3 × DS6 6 × DS6

HCC (high

conduct.)

DS &

CB

1 × DS1 +

1 × CB3

1 × DS2 +

2 × CB3

1 × DS3 +

3 × CB3

2 × DS4 +

1 × CB6

2 × DS4 +

2 × CB6

1 × DS6 +

4 × CB6 Brass DS 1 × DS1 1 × DS2 1 × DS3 1 × DS4 2 × DS4 1 × DS6 Bronze &

gunmetal

DS 1 × DS2 1 × DS3 1 × DS4 1 × DS5 3 × DS4 4 × DS5

Al-bronze &

Mn–bronze

Nickel–silver

castings

E &

DS

1 × E3 +

1 × DS2 2 1 × E3 +× DS4 3 2 × E3 +× DS3 4 × E3 +2 × DS6 8 × E3 +1 × DS6 16 × E3 +2 × DS6 Nickel–silver

for hot & cold

working

NS 1 × NS4 2 × NS4 3 × NS4 1 × NS6 2 × NS6 4 × NS6

Ni–bronze

Cu–Ni alloys

MG 2 × MG5 3 × MG5 2 × MG6 3 × MG6 6 × MG6 12MG6

To dissolve impurities from the melt.

To form an inert atmosphere for the melting of high conductivity copper (CUPRIT 81 flux)

To provide a mould and launder cover for the direct-chill casting of brass and copper (CUPRIT 103 flux)

Rotary degassing of copper and its alloys

The Mobile Degassing Unit (Fig 6.2) is effective for removing hydrogen from copper melts and should be used in the way described for aluminium alloys in Chapter 6

LOGAS degassing units

LOGAS degassing units comprise a mixture of chemicals which, on contact with molten metal, decompose to release a steady stream of non-reactive gas LOGAS is carefully dried and packed in foil, so the gas bubbles contain very little hydrogen and are able to flush out hydrogen from the melt

Deoxidants for copper and its alloys

The ideal deoxidant should function as follows:

1 It should combine with all the oxygen present to form a fluid slag

2 Deoxidation products should not be entrained in the solidified casting

3 Residual deoxidant should not adversely affect the physical properties of the alloy and should prevent further oxidation during pouring

Phosphorus satisfies most of these requirements, but a residual content of 0.025% is necessary to ensure adequate deoxidation This can seriously affect the conductivity of pure copper and causes embrittlement of high nickel bearing alloys

Alternative deoxidants are:

MAGNESIUM: Very effective and it eliminates the harmful effects of sulphur, but the oxide formed tends to remain entrapped in the metal

at grain boundaries, causing embrittlement

MANGANESE: An excellent deoxidant, present in DEOXIDISING TUBES E Manganese imparts some grain refinement

CALCIUM: A good deoxidant, although metal fluidity is slightly reduced

SILICON: Deoxidises well but the oxide formed may affect the surface appearance and pressure tightness of the casting

BORON: A satisfactory deoxidant having some grain-refining action Excess can cause embrittlement

DEOXIDISING TUBES L are also available for commercial and h.c copper, Ni–bronze, Cu–Ni alloys and Al–bronze They contain lithium and remove hydrogen as well as deoxidise

Copper-based alloy castings are usually made from charges using pre-alloyed ingot together with foundry returns (runners, risers and scrap castings) Such internal scrap must be carefully segregated to avoid mixing

of metal of different specifications With successive remelting there will be a tendency to lose volatile elements, particularly zinc, and to pick up contaminants such as iron The level of residual phosphorus may vary, depending on the deoxidation practice used, and it must be carefully monitored

The alloys are frequently melted in gas-fired furnaces, usually crucible furnaces Medium frequency induction fumaces are also used with silica or alumina linings Clay–graphite or silicon carbide crucibles can also be used, the electrical conductivity of the crucible allowing it to absorb induction power, yielding higher crucible temperatures and reduced stirring in the melt

The melting and treatment of each of the main alloy types will be dealt with in turn

Melting and treatment of high conductivity copper

The quality of high conductivity copper is measured by its electrical conductivity Pure copper in the annealed condition has a specific electrical resistance of 1.72 microhms per cubic cm at 20°C This is said to have 100% electrical conductivity IACS (International Annealed Copper Standard units) Cast copper can have a conductivity of 90% IACS and has both electrical and thermal applications since high electrical conductivity also implies high thermal conductivity Many of the impurities likely to be present in copper lower its electrical conductivity seriously, Table 16.7 Cu–C (HCC1) copper is used for water-cooled tuyeres and electrode clamps, it must have 86% IACS minimum so must be of high purity with only small additions of Cr or Ag to extend the freezing range and make casting 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 and for ease of machining

Where greater hardness and strength are required, copper–chromium castings CC1-TF may be used This alloy requires to be heat treated (1 hour

at 900°C, followed by quenching to room temperature and reheating to 500°C for 1–5 hrs) to realise its full properties

High purity copper is particularly prone to gas porosity problems due both to hydrogen and the hydrogen/oxygen reaction which occurs if any

oxygen is present in the molten metal Steps must be taken, during melting,

to exclude both hydrogen and oxygen from the melt The principles involved are:

Melt quickly, using the lowest temperature possible, under a reducing cover flux

Purge with an inert gas to remove hydrogen

Add deoxidants to remove residual oxygen, ensuring that residual deoxidant does not reduce the conductivity

Melting

The charge materials must be carefully selected to avoid impurities which can reduce the conductivity Before charging, the copper must be clean and degreased to avoid any hydrogen-containing contaminants Clean and dry crucibles, lids, plungers and slag stoppers must be used The crucible should

be preheated before charging to minimise the time that the copper is solid and unprotected by flux Melt down under a reducing cover of CUPRIT 81; the flux should be placed in the bottom of the crucible prior to charging 1 kg

of CUPRIT 81 is needed per 100 kg of metal

Table 16.7 The effect of impurities and alloying elements on the

electrical conductivity of pure copper

Hydrogen is removed from the melt by bubbling an inert gas through the melt This can be done using argon or nitrogen using the Mobile Degassing Unit (see Chapter 6) or less effectively by injecting gas through a graphite tube immersed deep into the melt 50–70 litres of gas are needed for each

100 kg of copper

An alternative way to degas is to plunge LOGAS 50 briquettes into the melt LOGAS is a granular material, strongly bonded and formed into a weighed unit with high surface area/volume ratio to ensure maximum contact area with the liquid metal On contact with the metal, LOGAS 50 decomposes releasing a steady stream of non-reactive gas which flushes out the hydrogen LOGAS 50 units are packed in foil, they are of annular shape having a central hole into which a refractory-coated steel plunger can be inserted, Table 16.5

Treatment takes from 3 to 10 minutes depending on the size of the melt Some loss of temperature occurs during treatment, so the initial treatment temperature must be chosen accordingly The minimum temperature practicable should be used

Deoxidation

A number of deoxidants are available for copper (Table 16.6) They combine with the dissolved oxygen in the metal forming stable oxides which float out

of the melt Phosphorus is the most widely used deoxidant for copper and its alloys because of its effectiveness and low cost It must be used sparingly with high conductivity copper since any residual phosphorus left in solution seriously lowers the conductivity of the copper (Table 16.7)

The recommended practice is to use phosphorus to remove most of the dissolved oxygen and to complete the deoxidation with a calcium boride or lithium-based deoxidant

The precise quantity of deoxidant needed depends on the melting practice used Simple tests can be made in the foundry to observe the solidification characteristics of the melt Open-topped cylindrical test moulds having impressions about 75 mm high by 50 mm diameter are needed They can be formed in a cold-setting resin or silicate sand mixture When the melt is ready for deoxidation, a sample of the copper should be ladled into one of the moulds and allowed to solidify If the head rises appreciably as shown

in Fig 16.5a very gassy metal is indicated DEOXIDISING TUBES DS containing phosphorus must be plunged and further test castings made At the point when the quantity of phosphorus added results in a shallow sink

in the head, as in Fig 16.5 b, it can be assumed that the residual phosphorus content of the melt is nil and a small amount, about 0.008% of oxygen, remains

Deoxidation is now completed by plunging DEOXIDISING TUBES CB or

L, adding sufficient to produce a test casting having a head with a deep sink

as in Fig 16.5 c The melt is now in a condition to produce castings free from porosity The approximate additions needed are shown below:

Weight of melt

DEOXIDISING

TUBES

DS & 1  DS1 1  DS2 1  DS3 2  DS4 2  DS4 1  DS6

CB 1  CB3 2  CB3 3  CB3 1  CB6 2  CB6 4  CB6

DEOXIDISING TUBES L, containing lithium, can be used as the final deoxidant in place of DEOXIDISING TUBES CB An application rate of 0.018–0.02% of product should be used In addition to being an excellent deoxidant, lithium also removes traces of hydrogen This is found to reduce the incidence of cracks in complex cast shapes

Casting conditions

HC 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 must not be too strong Resin bonded sand is suitable and the resin percentage must be as low as possible, the minimum necessary for handling the mould and cores Gating should be designed to minimise turbulence on pouring, in order to avoid the possibility of oxygen pick-up, Figs 16.6 16.7 (see Chapter 7) Feeding of the castings follows the practice used for steel castings (see Chapter 17)

Figure 16.5 The appearance of test castings: (a) Gassy metal (b) Partially deoxidised (c) Fully deoxidised.