Properties of Copper Alloys, Nickel, Magnesium, Aluminum, and Annealing
Copper Alloys
Copper alloys: Great ductility without allowing thanks to that can be easily cold worked and matching not so easy. Corrosion resistance is outstanding in most normal environment. Mechanical properties are quite limited and they can improved with alloying materials. Most copper alloys cannot be hardened or strengthened by heat-treating procedures, cold working or solid solution to improve mechanical properties. Brass: copper + zinc. Alpha phase stable up to 35%. FCC crystal structure makes them soft, ductile and easy work. Increasing cu, we arrive to alpha&betta. Betta harder&stronger. Bronzes: copper with wowith tin, aluminum, silicon, nickel. corrosion resistant. Stronger than brass. If lubricated, interesting for wear resistance. Good elastic properties. Casted, hot worked or cold worked. By precipitation hardening: remarkable characteristics. Applications: bearing and sleeves. Cu + Sn: up to 10% Sn: alpha phase good for cold work. Over 10% Sn: very hard only workable by casting. Cu + Ni: total solubility, hot & cold wearing along whole % range. Including Zn, cold work and excellent corrosion. Coins. Cu + Si: 1-4% Si. Excellent corrosion. Cu + Al: 9% Al, alpha phase, cold work. Zn between 9-10%. Cu + Be: 0.4-2% Be, high tensile strength more expensive. Titanium: new materials. Good combination of properties. Medium density. Medium-high mech. Prop. Coefficient of thermal expansion lower than steel. High melting temperature. Common in earth. Expensive but not too much. Fatigue behavior worse in beta alloys. Regarding alloying elements: – alpha promoters: They have great solubility on the a phase and help to its stabilization. Most usual elements are Al, O2, N2 y C. – neutral: They have broad solubility both in phases a and b. Usual elements are Zr and Sn. Transformation temperature is not changed. Alpha alloys, excellent weldability, good high temperature behavior, not cold work. Neutral alloys better properties. Beta alloys can be welded and annealed. Suffer from crack and work worse in high temp
Nickel
NICKEL: used since ancient times (white copper) Extracted from minerals: oxidized, sulfurated, Cu-Ni. It is a heavy metal (density 8.9), & melting temp 1454, similar E to steel. Good mechanical properties & great chemical resistance to seawater & alkalis. Easily absorbs H2 & gets embrittled. In materials market, Nickel can be found as pure material, sintered Ni as dust with 75-95% Ni and the rest oxygen, and FeNi with 20-30 Ni and some Cr. Most common Ni alloys: Ni-Cu: monel, good corrosion resistance except for sulfur atmospheres. Ni-Cr: Ionel, good hot corrosion resistance. The more Cr better for oxidant agents. Ni improves the behavior against reducing agents, similar mech. prop. Ni-Cr-Fe: Incoloy, good for corrosion caused by H2 sulfurs, excellent for high temp, lower mech. prop. Ni-Cr-Co: Nimonic, excellent for hot corrosion and hot plastic strain, high mech. prop. Ni-Cr-Mo: Hasteloy, great thermal stability and low CTE. Corrosion resistance up to 1100.
Magnesium
Magnesium: low density (lowest of the metals), aerospace applications, magnesium extracted from MgO, crystalline structure Hc & soft, difficult cold work so usually hot work. Good corrosion resistant in normal environment but not in corrosion environments as the sea. Magnesium dast flammable. If zirconium added is finer and improves manufacturability. Not the most ductile but not brittle. Fatigue properties similar to steels. Aluminum: good electric conductor. Good thermal conductor. Easy to work. Low density & high corrosion resistance. Many alloys high density due to FCC, even at low temperatures. Low melting temp. mech strength improve by cold work and alloying materials, decrease corrosion resistance. Main alloying elements: Cu, Mg, Mn, Si and Zn. Cu: strength improvement, precipitation hardenable, easy to machine. Fe: improve strength & decrease crack, risk at higher temp. Mn: ductility improvement + Fe molding capacity. Mg: strength improvement, better corrosion resistance. Si: molding capacity impro, better corrosion resistance. Zn: decreases molding capacity, great strength improvement. To make alloys, elements used: principal: gives level of main strength as well as forming operations. Secondaries: less amount of materials to modify a main property easiness of transformation. Impurities. Aluminum alloys heat treated (series 2xxx) intermetallic precipitation, intergranular corrosion possible, best-known alloy 2024. Aluminum alloys not heat treated easy to mold
Annealing
ANNEALING: recrystallization of metals. Heat treatment in which a material is exposed to an elevated temp for an extended period and slowly cooled. To: relieve stress, increase stiffness, ductility and toughness, produce specific microstructure. 3 stages: heating to a T, holding T, cooling. Temp changes speed must be controlled to avoid large gradients that provoke crack failure. Annealing T is relatively low such that results from cold work are not affected. 1. Normalizing: Use to refine the grains and produce a more uniform and desirable size distribution. Fine-grained pearlitic steels are tougher than coarse-grained ones. It’s accomplished by heating at least over 55 above critical T. Continuous trans diagrams present the influence of the cooling rate. 2. Full anneal: in low medium and carbon steels that will be machined or will experience expensive plastic deformation during a dorming operation. 50 above A3 or A1 line. Then is furnace cooled (the heat treat furnace is turned off and both furnace and steel cool to room T) Microstructure with small grains & uniform grain structure results. 3. Spherodizing: to produce spheroidal cementite particles that give steels maximum ductility and smoothness for additional cold forming or machining processes. – by heating to T just below eutectoids, – over eutectoid and cooling very slowly – heating & cooling alternatively around +-50 eutectoid T.