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Development of Cu-Cr alloy

Objectives

y To achieve solubility limit around 1 to 1.29% Cr by using various melting techniques

y Evaluate the three methods of melting practice for their performancey To study the characteristics and performance of Cu-Cr composite (Cr more than 10%)

Theoretical background

y The chromium content ranges from 0.5 to 1.2%.y Copper-chromium alloys can be made in all fabricated forms but are mostly available as rod,

bar or forgings.

y In the molten state the added chromium, like many other refractory metals, oxidizes readily,increasing the viscosity of the liquid and causing possible inclusions in the casting, but the

alloy can be readily cast by foundries with the required expertise.

y The Cu-Cr alloys are age hardenable which in this case means that a change in propertiesoccurs at elevated temp due to precipitation of chromium out of the solution.

y The strength of fully aged Cu-Cr is nearly twice that of pure Cu and its conductivity remainshigh at 85% IACS or 85% that of pure Cu.

y These high strength alloys retain their strength at elevated temperatures. The corrosionresistance of chromium copper alloys is better than that of pure copper because chromium

improves the chemical properties of the protective oxide film.

y The age hardening reaction occurs because the solid solubility of chromium in copperdecreases as the temperature decreases.

y The structure of slow cooled chromium copper is a two phase mixture of chromium andalpha copper. Superior mechanical properties are achieved by fast-cooling the chromium

copper alloys from the annealing temperature, so the chromium remains in a

supersaturated solid solution with the copper.y Followed by an aging treatment where the chromium precipitates from the solid solution

forming a very fine dispersion of precipitates in the matrix.

y The microstructure of a quenched or quickly cooled chromium copper alloy appears similarto that of the unalloyed copper.

y A fast cool prevents the chromium from precipitating out of the solid solution, so theresulting cast structure consists of a single phase alpha copper structure.

y The first material to solidify is pure copper, followed by a eutectic mixture of alpha Cu andchromium.

y The alpha and chromium eutectic material forms a lamellar structure in the interdendriticregions.

y The microstructure of the wrought alloy consists of equiaxed, twinned grains of alphacopper solid solution.

y Typically the alloy is cooled rapidly so the chromium remains in alpha copper solid solution.y The tempering treatment allows the chromium to precipitate out of solution forming a

dispersion of chromium precipitates throughout the matrix.

y The chromium precipitates, or hardening precipitates, can be very fine and may not bevisible at low magnification.

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Application:

Copper-chromium alloys are commonly used in rod form for spot welding electrodes, as bar

for high strength conductors and as forgings for seam welding wheels and aircraft brake

discs. As castings they find applications as electrode holders and electrical termination

equipment where the shape required is more complex than can be economically machined.

Effect of composition variation

y This alloy is normally known as Chromium-Copper alloy. It is high Cu alloyy And amount of Cr varies from 05-1.2 % (wt%)y In this range it is homogeneous solid solution, above this range there is no homogenous

structure. it makes banded structure

y As the Cr content increases there is increase in strength and decrease in conductivity.y The strength increase nearly twice when there is decrease in conductivity up to 80% IACS

Thermal treatment

y Cu-Cr alloy is age hardenable alloyy In this there is steps like

Solutionizing treatment followed by rapid quenching - then ageing is carried out

y SolutionizingIn this treatment, heating is carried out above 10000C to make homogeneous alloy andsoaking is done at this temp. for about 1-2 hr. and it is followed by rapid quenching to create

pores..

y AgeingIn this treatment, heating is carried out at 450-510 OC to precipitation hardening afterthis treatment there is considerable change in strength. And it is done for nearly 0.5-1 hr.

Melting practice

During manufacturing of this alloy there is problem of oxidation of Cr during heating/melting

To form Cr2O3. to avoid this it is covered with graphite powder. Graphite powder is

sprinkled on the surface to cover it

In this normally heating is carried out at 1350 -15000C.

Here there is different methods to develop this alloy. It will be explained in experimenta

setup

Plan of work

y Total 5 heats are taken for development of Cu-Cr alloyy 3 heats carried out in such manner to achieve max. 1.29% Cr recoveryy And other 2 heats with high Cr (more than 10%)

For 3 heatso Additions are kept same in 3 different melting techniqueso And recovery of Cr in every process is measureso And here in every heat min material should be such that at least 5 tensile test bar

can be produced

o And from this 5 test bars

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2 test bar is taken for further heat treatment (solutionizing and ageing) Other 2 test bar is taken directly for testing And other 1 bar is kept in spare This thing will be carried out to describe the effect of heat treatment

For 2 heatsy These heats are carried out for higher Cr content (more than 10%)y and the further testing is similar to above

Experimental set-up1. Material selection

During every heat the base material (Cu and Cr) will be kept same in manner of composition,

volume. And its main aim to keep same is to judge the Cr recovery during every process.

The Cu is in Form of 99% pure ingot.

And Cr is 99% pure and in form of small granules.

Charge calculation

For D=25 mm

H = 160 mm

As per the equation for the volume; volume V=78500 mm3

And for the other mould filling

Volume = 252000 mm3

Total volume=330500 mm3

= 330.5 cm3

Total mass M= volume *density

=330.5 * 8.94

=2954.67 gm

So weight of 5 bars = 14773.35 gm = 14.8kg

Melting techniques

1. Crucible meltingF

irst of all take a graphite crucible.And copper ingots are placed in crucible and then melting is carried out.

When Cu is completely melted further heating up to 1350oC is carried out.

And then small granules of Cr is added into the crucible to prevent sticking

Heating is carried out up to Cr is completely dissolved into the melt.

And the full melt is covered with coal.

And then the liquid metal is poured into the mould.

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2. Slag crucible meltingTheory : it was observed that aluminium added to the melt floated on top of the slag until ithad melted. After melting, the liquid aluminium does not join the metal pool below. This

may because the density of liquid Al is less than that of liquid slag (2.6-2.8 gm/cm3) and

significantly less than that of liq. Cu. hence the reaction shown below occurs in the bulk

phase of slag and the Cr generated in the slag phase joints the metal below thereby to Cu-Cr

alloy formation

Cr2O3 + 2Al = 2 Cr + Al2O3

In this process 600 gm of slag comprising of 20% CaF2 -30% CaO 30% SiO2 20%NaF is

prepared

I

n this composition of slag Cr2O3 90 gm mixed.Then the slag mixture was preheated in a furnace at 800oC for 2h.

The preheated slag was charged into the crucible after charging of Cu scrap.

The slag was rammed after charging and formed a sintered cake on top of Cu charge(4 kg).

The melt was held until Cu as well as slag had melted.

Aluminium pieces weighing in total 32gm and measuring 5*5*5 (mm3) were added to the

slag for in-situ reduction of Cr oxide present in the slag.

The melt was for 10 min.

During the process graphite powder was sprinkled after every charge.

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3. A

ddition of siliconFirst of all take graphite crucible

In this first melting a portion of Cr and then making additions of Cu and Si

The temp. of the melt should be kept high as much as 1500oC and melting may be done in

induction furnace.

The above described order of additions has many advantages in producing sound and

homogeneous alloy.

Composition within the part of the diagram marked zone of miscibility form a single

homogeneous liquid phase alloy.

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Thermal treatment

This alloy is age hardenable so post heat treatment will improve its strength and other

mechanical properties

This alloy is generally first solutionized at nearly 1000OC for nearly 1-2 hrs and ageing is

carried out at nearly 450-510 OC for nearly 0.5-1 hr

This solutionizing and ageing treatment is carried out in furnaceAfter this post heat treatment there is drastic change in mechanical properties like hardness,

strength and electrical conductivity.

Characterization and fractography

During this process the various test are necessary to check the quality of the material.

1) EDXRFThis test is carried out for measuring the composition of alloy at a single point.

Through this test we can get the amount of Cr% present in the alloy

2) Tensile testThis test is carried out for the measuring the strength of the alloy. Normally the strength

of Cu-Cr alloy is twice of that of pure Cu. And the strength is imp requirement to

development of this alloy.

3) Hardness testDuring ageing process there is precipitation of Cr take place. So hardness of this alloy is

more than the pure Cu. its theoretical data of hardness is near to 90HB.which is greater

than the pure Cu

4) Electrical conductivityHere there will be decrease in the electrical conductivity. But our aim is to get electrical

conductivity more than 75% IACS

5) Wear resistanceWear testing is carried out in machine. its wear resistance is greater than that of pure

Cu(Theo).

6) MicrographyMicrostructure will be seen in SEM or TEM

7) FractographyFractography will be done on specimen failed during the tensile testing