copper alloys Flashcards
main feature of copper
Very high thermal conductivity and electrical conductivity too
Extraction of Cu from ores
- Copper ores are found in nature under two distinct forms, as sulfides or oxides (native metallic copper can also be found in small amount)
- to obtain metalic cu we refine ores by a pyrometallurgy route which consists in:
1) smelting (enriched ores are reacted at high temperature with silica SiO2 to produce a «matte» rich in Cu2S and a slag)
2) The conversion of the (molten) matte (by blowing with O2 to preferentially oxidize sulfides, leading to «blister copper»)
3) fire refine blister Cu (by air blowing to preferentially oxidize more reactive metal impurities to slag)
4) deoxidation of residual CuO/Cu2O (by adding a phosporous to remove oxigen or by electrolitic refining this second option is expensive but we achieve high purity copper)
Factors affecting conductivity/resistivity of Cu
- dissolved elements
- temperature
- lattice defects:
main Cu alloys classification
wrought and cast alloys
main type of cu alloys seen in class
- pure copper grades
- Cu-Zn alloys (brass)
- Cu-Sn, Cu-Al, Cu-Ni alloys
- precipitation-strengthening Cu alloys
pure copper grades
they are classified according to achievable conductivity, namely on deoxidation process and purity:
-
Oxygen free copper:
the highest conductivity is achieved by limiting all solute elements.
(C10100 oxygen-free Cu is electrolitycally refined for electronic applications.
C10200 contains small Ag additions to refine grain size and control recrystallizaiton) -
Electrolytic tough pitch (ETP) copper:
still electrolytically refined but with small residuals of oxygen (300-400 ppm) used to bind solute elements as oxide particles (less harmful) -
Phosphorous-deoxidized copper:
P is a strong deoxidizer but depletes conductivity. The residual P content up to 0,04% reduces conductivity by 25%, not used as electrical conductor
Cu-Zn alloys (brass)
they are characterized by:
- Extended solid solution for alfa-brass alloys (FCC), with higher strength and workhardening ability.
(Typical alloy is C26000 (Cu-30Zn) known as
«cartridge brass») - For Zn>36% the beta-phase field is involved. beta-brass alloys feature a
disordered BCC crystal structure at high temperature which becomes ordered and stronger below about 460°C - beta-brass alloys can partially transform into a combination of alfa+beta grains at room temperature
- A typical beta-brass alloy is C36000 (Cu-36Zn-3Pb) which contains lead for
improved machinability. - Pb is insoluble in Cu and forms a dispersion of globules (former liquid
droplets) in the matrix which helps chip breakage and lubrication
Cu-Sn, Cu-Al, Cu-Ni alloys
these alloy groups take advantage of the solid solute strengthening given by Sn, Al and Ni, respectively
- A basic Cu-Sn alloy is C51000 (Cu-5Sn) with small amount of P, commonly called “phosphor bronze”
- A basic Cu-Al alloy is C60800 (Cu-5Al) for cold forming or C61000 (Cu-8Al) for hot forming
- A basic Cu-Ni alloy is C71300 (Cu-25Ni) mostly suitable for coinage
precipitation-strengthening Cu alloys
the steps are:
- high temperature
–> solid solution
- quenching
–> supersaturated solid solution
- aging
–> precipitation hardening
characteristics:
- Cu-Be / Cu-Cr-Zr / Cu-Fe-Cr alloys combine good electrical/thermal conductivity with significantly high strength for specific applications
- A classic Cu-Be alloy is C17200 (Cu-2Be) which can be solution treated at 800°C, water quenched and aged at 300°C to produce a dispersion of beta’ particles
thermal treatments of Cu alloys
- precipitation (dispersion) strengthening
- for alloys based on solid solution the main tt is annealing
- annealing to recover excess of workhardening
how do we improve mechanical properties of Cu
high conductivity is targeted by pure copper, strength and mechanical performance are mainly achieved by;
- alloying and workhardening in most Cu alloys
- precipitation strengthening in specific alloys
colour in Cu alloys
- For architecture, jewels, coins, materials colour plays an important role
- we can change colour of Cu alloys depending on alloying elements
✓ 10, 20, 50 cent : Cu-5Al-5Zn-1Sn (nordic gold)
✓ 1, 2 € (yellow part): Cu-20Zn-5Ni
✓ 1, 2 € (silver part): Cu-25Ni
corrosion resistance of Cu
- Copper behaves as a noble metal and is almost immune from galvanic corrosion.
- In most environments thanks to the naturally protective oxide film that forms on metal’s surface, passivation is very effective and corrosion resistance is achieved
- The oxide film has a reddish-brown aspect which darkens with time
- greenish colour on old roofings is produced by copper carbonate
- In brass alloys de-zincification can be a matter of concern (could happen that only the zinc rich phase (beta) react with environment and we loose zinc atoms
leading to a porous material) - Stress corrosion cracking occurs when an alloy is subjected to the combined affect of an applied load (or residual stress) and the chemical exposure to critical reagents like ammonia or ammonium compounds
- copper is also an excellent antibacterial element to treat wounds and deseases
copper key features
- High electrical and thermal conductivity (pure copper)
- Good machinability (free machining Cu alloys)
- Excellent corrosion resistance (Especially specific alloys)
- Good castability (Main alloys we have seen are casting alloys)
- Good formability by hot and cold processes
- Ability to be soldered, brazed, welded
- Fully recyclable, importance of scrap remelting (Good to recycle because is quite expensive)
copper applications
- heat exchangers
- pipes, tubins
- electrical wires
- electronic components
- construction and building
- electrical conductors
