Introduction Alloy



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Introduction

Alloy

An alloy is a combination, either in solution or compound, of two or more elements, which has a combination of at least one metal, and where the resultant material has metallic properties. An alloy with two components is called a binary alloy; one with three is a ternary alloy; one with four is a quaternary alloy. The result is a metallic substance with properties different from those of its components.

Alloys are usually designed to have properties that are more desirable than those of their components. For instance, steel is stronger than iron, one of its main elements, and brass is more durable than copper, but more attractive than zinc.

Unlike pure metals, many alloys do not have a single melting point. Instead, they have a melting range in which the material is a mixture of solid and liquid phases. The temperature at which melting begins is called the solidus, and that at which melting is complete is called the liquidus. Special alloys can be designed with a single melting point, however, and these are called eutectic mixtures.

Sometimes an alloy is just named for the base metal, as 14 karat (58%) gold is an alloy of gold with other elements. The same holds for silver used in jewellery, and aluminium used structurally.
Steel
Of all the different metals and materials that we use in our trade, steel is by far the most important. When steel was developed, it revolutionized the American iron industry.  With it came skyscrapers, stronger and longer bridges, and railroad tracks that did not collapse. Steel is manufactured from pig iron by decreasing the amount of carbon and other impurities and adding specific amounts of alloying elements. Do not confuse steel with the two general classes of iron: cast iron (greater than 2% carbon) and pure iron (less than 0.15% carbon). In steel manufacturing, controlled amounts of alloying elements are added during the molten stage to produce the desired composition. The composition of steel is determined by its application and the specifications. Carbon steel is a term applied to a broad range of steel that falls between the commercially pure ingot iron and the cast irons. This range of carbon steel may be classified into four groups:
Low-Carbon Steel . . . . . . . . 0.05% to 0.30% carbon

Medium-Carbon Steel . . . . . 0.30% to 0.45% carbon

High-Carbon Steel . . . . . . . . 0.45% to 0.75% carbon

High-Carbon Steel . . . . . …..0.75% to 1.70% carbon


LOW-CARBON STEEL Steel in this classification is tough and ductile, easily machined, formed, and welded. It does not respond to any form of heat treating, except case hardening.
MEDIUM-CARBON STEEL —  These  steels  are strong and hard but cannot be welded or worked as easily as the low-carbon steels. They are used for crane, hooks, axles, shafts, setscrews, and so on.
HIGH-CARBON   STEEL/VERY   HIGH-CAR- BON STEEL Steel in these classes respond well to heat treatment and can be welded. When welding, special electrodes must be used along with preheating and stress-relieving procedures to prevent cracks in the weld areas. These steels are used for dies, cutting tools, mill tools, railroad car wheels, chisels, knives, and so on.
LOW-ALLOY,   HIGH-STRENGTH,   TEM- PERED  STRUCTURAL  STEEL —  A special low- carbon  steel,  containing  specific  small  amounts  of alloying elements, that is quenched and tempered to get a yield strength of greater than 50,000 psi and tensile strengths of 70,000 to 120,000 psi. Structural members made  from  these  high-strength  steels  may  have  smaller cross-sectional areas than common structural steels and  still  have  equal  or  greater  strength.  Additionally, these steels are normally more corrosion- and abrasion- resistant. High-strength steels are covered by ASTM specifications. NOTE: This type of steel is much tougher than low-carbon steels. Shearing machines for this type of steel must have twice the capacity than that required for low-carbon  steels.
STAINLESS STEELThis type of steel is classified by the American Iron and Steel Institute (AISI) into  two  general  series  named  the  200-300  series  and 400  series.  Each series includes several types of steel with different characteristics. The 200-300 series of stainless steel is known as AUSTENITIC. This type of steel is very tough and ductile in the as-welded condition; therefore, it is ideal for welding and requires no annealing under normal atmospheric conditions.  The most well-known types of steel in this series are the 302 and 304. They are commonly called 18-8 because they are composed of 18% chromium and 8% nickel.  The chromium nickel steels are the most widely used and are normally nonmagnetic. The 400 series of steel is subdivided according to their crystalline structure into two general groups. One group is known as FERRITIC CHROMIUM and the other group as MARTENSITIC CHROMIUM.
Bronze and Brass

Copper, brass and bronze alloys are non-ferrous metals with excellent electrical and thermal conductivity as well as good corrosion resistance, ductility and strength. Pure copper (Cu) is an unalloyed metallic element. Low alloy copper contains very small amounts of alloying elements such as aluminum and titanium.

Brass, bronze, leaded brass, nickel silver, copper nickel, aluminum bronze, tin bronze and silicon bronze are examples of copper alloys. Many copper, brass and bronze alloys are hardened or strengthened with cold working processes, solution treating, precipitation hardening, or spinodal decomposition.
Applications for copper, brass and bronze alloys include architectural and building materials; automotive parts; consumer, recreational and household products; tubes, pipes, and fittings; food processing; energy and gas transport; and welding and thermal processing equipment. Pure copper and high copper alloys are used widely in electrical power, current carrying, electronics and telecommunications applications.  They are also used in heat sinks, radiators, chillers, chill rolls, crucibles, and other heat transfer or cooling applications.

Bronze is, usually, a mixture of copper and tin, but sometimes also other metals. Brass is a mixture of copper and zinc. The various materials are recognisable from their colour: pure copper is red, brass is a golden yellow and bronze is brown. Bronze with a high percentage of tin can be greyish-brown to grey. However, it can sometimes be difficult to determine the exact material, whether it is copper, bronze or brass.

There are two basic types of copper, brass and bronze alloys: cast alloys and wrought alloys. Cast alloys are melted and then cast continuously, centrifugally, or statically into a mold. Wrought alloys are deformed mechanically during manufacturing in rolling, extrusion, or forging processes. Copper, brass and bronze alloys can also be fabricated through the pressing and sintering of copper powders.

Small amounts of alloying elements are often added to metals to improve certain characteristics of the metal.  Alloying can increase or reduce the strength, hardness, electrical and thermal conductivity, corrosion resistance, or change the color of a metal.  The addition of a substance to improve one property may have unintended effects on other properties. 



  • Strength

Solid solution strengthening of copper is a common strengthening method.  Small amounts of an alloying element added to molten copper will completely dissolve and form a homogeneous microstructure (a single phase).  At some point, additional amounts of the alloying element will not dissolve; the exact amount is dependent on the solid solubility of the particular element in copper.  When that solid solubility limit is exceeded, two distinct microstructures form with different compositions and hardnesses. 

Copper by itself is relatively soft compared with common structural metals.  An alloy with tin added to copper is known as bronze; the resulting alloy is stronger and harder than either of the pure metals.  The same is true when zinc is added to copper to form alloys known as brass.  Tin is more effective in strengthening copper than zinc, but is also more expensive and has a greater detrimental effect on the electrical and thermal conductivities than zinc.  Aluminum (forming alloys known as aluminum bronzes), Manganese, Nickel, and Silicon can also be added to strengthen copper.



  • Electrical and Thermal Conductivity

Pure copper is a very good conductor of both electricity and heat. The best way to increase the electrical and thermal conductivity of copper is to decrease the impurity levels.  The existence of impurities and all common alloying elements, except for silver, will decrease the electrical and thermal conductivity of copper.  As the amount of the second element increases, the electrical conductivity of the alloy decreases.  Cadmium has the smallest effect on resulting alloy's electrical conductivity, followed by increasing effects from zinc, tin, nickel, aluminum, manganese, silicon, then phosphorus.  Although different mechanisms are involved in thermal conductivity, the addition of increasing amounts of elements or impurities also produces a drop in thermal conductivity.  Zinc has very minor effect on the thermal conductivity of copper, followed by increasing effects from nickel, tin, manganese, silicon, and serious effects from phosphorus.  Phosphorus is often used to deoxidize copper, which can increase the hardness and strength, but severely affect the conductivity.  Silicon can be used instead of phosphorus to deoxidize copper when conductivity is important.

  • Color

Pure copper has a reddish gold color which quickly oxides to a dull green.  Since copper often contains natural impurities or is alloyed with more than one element, it is difficult to state the specific effect each alloying element has on the resulting alloy's color.  Electrolytic tough pitch copper contains silver and often trace amount of iron and sulfur and has a soft pink color.  Gilding copper is a reddish brown color and contains zinc, iron, and lead.  Brass is often used as an ornamental metal, since it has an appearance very similar to that of gold and is much less expensive.  Brasses contain varying amounts of zinc, iron, and lead and can vary from reddish to greenish to brownish gold.  Nickel silver, which contains nickel, zinc, iron, lead, and manganese, can have a grayish-white to silver appearance

Titanium Alloys
Since the introduction of titanium and titanium alloys in the early 1950s, these materials have in a relatively short time become backbone materials for the aerospace, energy, and chemical industries. The addition of alloying elements improves the mechanical properties (strength, hardness) of titanium while decreasing its thermal and electrical conductivities.
Titanium is a white metal, and has the best strength to weight ratio among the metals. Titanium is very reactive, and because of this it is often used for alloying and deoxidizing other metals. Titanium is a more powerful deoxidizer of steel than silicon or manganese. Titanium is 40% lighter than steel and 60% heavier than aluminum. This combination of high strength and low weight makes titanium a very useful structural metal. Titanium also features excellent corrosion resistance, which stems from a thin oxide surface film which protects it from atmospheric and ocean conditions as well as a wide variety of chemicals.
Although unalloyed titanium is not very useful for structural applications, titanium alloys are highly praised for their use in high-temperature and biomedical applications. Titanium is difficult to machine or weld, but has significant advantages over traditional metals. For medical implants, titanium is considered one of the most biocompatible materials available, especially where direct contact to tissue or bone is required.
Because of its high strength to weight ratio, titanium is used in a variety of applications, including products where weight is of importance such as aircraft, sporting equipment, etc. Because of its excellent corrosion resistance, titanium is also used for chemical processing, desalination, power generation equipment, valve and pump parts, marine hardware, and prosthetic devices. The combination of high strength-to-weight ratio, excellent mechanical properties, and corrosion resistance makes titanium the best material choice for many critical applications.
Another important characteristic of titanium- base materials is the reversible transformation of the crystal structure from alpha (α, hexagonal close-packed) structure to beta (β, body-centered cubic) structure when the temperatures exceed certain level. This allotropic behavior, which depends on the type and amount of alloy contents, allows complex variations in microstructure and more diverse strengthening opportunities than those of other nonferrous alloys such as copper or aluminum.

Titanium has the following advantages:



  • Good strength

  • Resistance to erosion and erosion-corrosion

  • Very thin, conductive oxide surface film

  • Hard, smooth surface that limits adhesion of foreign materials

  • Surface promotes drop-wise condensation

Titanium alloy compositions of various titanium alloys.

Because the allotropic behavior of titanium allows diverse changes in microstructures by variations in thermomechanical processing, a broad range of properties and applications can be served with a minimum number of grades. This is especially true of the alloys with a two-phase, α+β, crystal structure.

The most widely used titanium alloy is the Ti-6Al-4V alpha-beta alloy. This alloy is well understood and is also very tolerant on variations in fabrication operations, despite its relatively poor room-temperature shaping and forming characteristics compared to steel and aluminium. Alloy Ti-6Al-4V, which has limited section size hardenability, is most commonly used in the annealed condition.

Welding has the greatest potential for affecting material properties. In all types of welds, contamination by interstitial impurities such as oxygen and nitrogen must be minimized to maintain useful ductility in the weldment. Alloy composition, welding procedure, and subsequent heat treatment are highly important in determining the final properties of welded joints.



Some general principles can be summarized as follows:

  • Welding generally increases strength and hardness

  • Welding generally decreases tensile and bend ductility

  • Welds in more beta-rich alpha-beta alloys such as Ti-6Al-6V-2Sn have a high likelihood of fracturing with little or no plastic straining.

Titanium and titanium alloys are heat treated for the following purposes:

  • To reduce residual stresses developed during fabrication

  • To produce an optimal combination of ductility, machinability, and dimensional and structural stability (annealing)

  • To increase strength (solution treating and aging)

  • To optimise special properties such as fracture toughness, fatigue strength, and high-temperature creep strength.


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