Light Metals And Their Alloys

Light Metals And Their Alloys Prepared By: 07301,07302,07303, 07304,07305,07307,07309,07117

LIGHT METALS AND THEIR ALLOYS SCOPE vLight alloys have become of great importance in engineering for construction of transportation equipment. Many of these light weight alloys have sufficiently high strength to warrant their use for structural purposes , and as a result of their use , the total weight of transportation equipment has been considerably decreased . Probably the greatest application of light metals is in the construction of aircraft. vAt present , the metals that serves as the base of the principal light alloys are aluminum and magnesium .Titanium and its alloys are included in this group since they have density much lower than that of steel.

ZINC § Centuries before zinc was discovered in the metallic form, its ores were used for making brass and zinc compounds were used for healing wounds and sore eyes. Brass was produced by the Romans in the time of Augustus (20 B.C. - 14 A.D.). § 75ppm (.oo7%) of earth’s crust. § 24th most abundant element on Earth. § 4th most common metal in use with annual production of around 10 million tons. § Found in association with base metals. § Large deposits in Australia, Canada and the United States. § At the current rate of consumption, these reserves are estimated to be depleted sometime between 2027 and 2055. § Naturally occurring zinc is composed of 5 stable isotopes, Zn64, Zn-66, Zn-67, Zn-68 and Zn-70 with Zn-64 being the most abundant (48.63% natural abundance).

ZINC(Zn) General Properties

Processing o 95% of the zinc is mined from Zinc Blende ore(ZnS) deposits mixed with sulfides of the Copper, Iron and Lead. o Zinc metal is produced using extractive metallurgy. 1. Froth flotation(zinc of about 50% is reached by this process). Main impurities after froth flotation are sulphur (32%), iron (13%), and SiO2 (5%). 2. Roasting converts the zinc sulphideconcentrate produced during processing to zinc oxide: 2 ZnS + 3 O2 → 2 ZnO + 2 SO2 3. After that zinc oxide Is treated with carbon or carbon monoxide at 950 °C (1,740 °F) into the metal, which is distilled as zinc vapor. The zinc vapor is collected in a condenser. 2 ZnO + C → 2 Zn + CO2

4.

Zinc oxide is converted into Zinc sulphate by treating it with Sulphuric acid. ZnO + H2SO4 → ZnSO4 + H2O

5. After this process electrolysis is done with Lead as anode and Aluminium as cathode. Zn is collected on Aluminium .

Alloys •

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Nickel Silvers : Alloys of Cu(50-70), Zn(5-40), Ni(5-30). Good corrosion resistance, ductile, high Young’s modulus, good luster etc. Used for screws, rivets, costume jewelry, surgical equipments etc. Al-Zn Alloys : Good corrosion resistance, good machineability, high strength etc. Used for aircraft str. Parts, turret housings, radio equipments, die casting. Brasses : Two types of brasses:α-Brass(5-36%Zn) 1. Yellow α Brass(20-36%): Good strength, corrosion resistance, high ductility Used for drastic cold working operations. 2. Red Brasses(5-20% Zn): High corrosion resistance,

α+β Brass(36-48%Zn) : Middle phase of both types. Naval Brass(60cu,39.75Zn,.75Sn): Used for propeller shafts, piston rods etc. Forging Brass(60cu,38Zn,2Pb): Used for hot forging, plumbing parts etc.

Applications Ø The most important application of zinc and the largest use of the metal is as an anti-corrosion agent. Ø Zinc used as material for the negative terminus or anode in batteries. Ø Zinc is used as a major alloying element. Ø Brass is used for utensils. Ø Brass is used for sculptures. Ø Zinc alloys are used in die casting. Ø Zinc is used for sheet metal. Ø Cadmium zinc telluride (CZT) is a semi conductive alloy that can be divided into an array of small sensing devices. Ø Zinc oxide is widely used as a white pigment in

MAGNESIUM ØCommercially pure magnesium has a specific gravity of 1.74 and is 99.8 percent pure. ØIn the annealed condition the wrought metal has a tensile strength of about 27000 lb/in.2 and elongation of 15% in 2 in. ØIt can be cold-rolled to a strength of about 37000 lb/in.2 but is cold-formed with difficulty. ØThe pure metal is used largely in Magnesium base alloys as a de-oxidiser and alloying agent in non-ferrous metals, in vacuumtube and dry-rectifier manufacture, and in pyrotechnics. ØThe desirable low specific gravity of Magnesium is offset by two disadvantages: namely, lack of stiffness and ease of oxidation. ØThe modulus elasticity of Magnesium is only 6.5×106 lb/in2 compared with 10.3×106 and 29.5×106 lb/in2 for Aluminium and steel respectively.

COATING OF Mg ALLOYS Ø Magnesium and its alloys has poor resistance to corrosion, particularly in salt water and salt atmosphere thus requiring a protective surface coating. Ø A protective coating may be applied to Magnesium and its alloys by dipping the material into a dichromate bath of by electrolytic anodizing. Ø Electrolytic anodizing treatment has the advantage of abrasion resistance and improved resistance to salts spray and provide an excellent base for paint. Ø The coating is applied in an aqueous acidic electrolyte containing phosphate, fluoride and chromate ions.

ALLOYING ELEMENTS IN Mg Ø The alloying elements commonly added to Mg are Al, Zn,Mn and for special purposes Sn,Zr,Ce,Th and Be. Ø Cu, Fe and Ni are considered impurities and must be kept at a minimum to provide the best corrosion resistance in the alloy. Ø Al in amounts ranging from 3-10% is the principle alloying element in most Mg alloys. It increases the strength, hardness and cast-ability of Mg.In excess of about 10%, the alloy becomes brittle. Ø . Zn is used together with Al in Mg based alloys in amounts upto about 3% to increase salt water corrosion resistance and to offset the harmful effects of Fe and Cu impurities. It also improves casting properties. An excessive amount of Zn produces porosity and brittleness because of the formation of the compound MgZn2

ØManganese has a very limited solubility in magnesium , particularly in the presence of Al. It is used In magnesiumaluminum and magnesium –aluminum-zinc alloys in quantities less than about 0.5percent to improve corrosion resistance and weld ability without effecting the strength properties. ØSilicon is not soluble in magnesium ,but it forms the compound Mg 2Si. Which increases the hardness of the alloy. It is usually held below 0.30% to avoid extreme brittleness. ØSn is soluble in Mg up to about 15% at 1200F(649 C), decreasing rapidly insolubility to room temperature with the precipitation of the beta phase(Mg2Sn). A Mg-Al-Mn alloy containing 5% Sn has improved hammer-forging properties.

CLASSIFICATION OF Mg ALLOYS

ON THE BASIS OF THE RECOMMENDATIONS OF THE AMERICAN SOCIETY FOR TESTING MATERIALS (ASTM DESIGNATION B275-59), THE Mg ALLOYS ARE BROADLY CLASSIFIED AS: ØCASTING Mg ALLOYS

DESIGNATIONS

For example: • • • •

As an example of this designation system, consider magnesium alloy AZ81A-T4. The first part of the designation, AZ, signifies that aluminium and zinc are the two principal alloying elements. The second part of the designation, 81, gives the rounded-off percentages of aluminium and zinc (8 and 1, respectively). The third part, A, indicates that it is the fifth alloy standardized with 8% Al and 1% Zn as the principal alloying additions. The fourth part, T4, denotes that the alloy is solution heat-treated.

Mg CASTING ALLOYS vTHE FIRST GROUP OF ALLOYS CONTAINS Al AND Zn AS THE ALLOYING ELEMENTS. vPOSSESS GOOD CASTING CHARACTERISTICS. vDEVELOP HIGH STRENGTH. vHAVE STABLE PROPERTIES AT TEMPERATURES AS HIGH AS ABOUT

Mg WROUGHT ALLOYS vMAY BE ROLLED AS SHEET AND PLATE, EXTRUDED AS BARS, SHAPES, OR TUBINGS , AND FORGED BY PRESS OR HAMMER FORGING. vREADILY HOT WORKED AND FORMED AT TEMPERATURES OF 400-700F. vENDURANCE CURVES SIMILAR TO THAT OF Al ALLOYS.

NICKEL • Nickel is the one of the most important metals used in the engineering. Pure nickel finds considerable application where resistance to corrosion is required under certain conditions. It finds extensive use in the chemical industry and in the production of caustic soda. • It serves as an excellent coatings for the electroplating of chromium. Nickel clad steel is used for the construction of heavy tanks and kettles. • “A” nickel is commercially pure nickel which is obtainable in the wrought form and contains 99.4 % nickel with the remainder particularly cobalt.

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“D” nickel> An alloy of the nickel containing 4.5% manganese and is used where improved resistance to the attack by sulfur compounds at temperature below about 1000F(540°C) is desired.

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“E ” nickel > It is similar to the D nickel and contain 2% manganese and is used for the similar purposes.

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“L” nickel > It contain low carbon content and is used for the applications where the large plastic deformations involved in the forming. This material is not as the work harden as the others.

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“Z” nickel> alloy of the nickel contain 4.5 % aluminum. This material is subjected to precipitation hardening and have good combination of the high strength and resistance to corrosion. It

NICKEL BASE ALLOYS • The alloys that contains a substantial portion of the nickel are of the great commercial importance. • There are low expansion alloys contain 36% nickel ,the glass sealing alloys with 30-50% nickel ,the alloys with low temperature coefficient of modulus of elasticity, and the magnetic alloys containing up to about 65% nickel. •

The alloys containing up to 50% nickel are classified as nickel based alloys.

MONEL • MONEL containing 67% Nickel 30% copper and containing small amount of iron and manganese. The Monels are particularly used in the applications that required resistance to acids alkalis, brines water and food products. The Monelfamily of alloys consist of five composition : Monel , R-Monel , K- Monel , H-Monel , S-Monel . R-Monel > It possesses same general characteristics as Monel , but it is a free machining alloy intended for processing in the automatic screw machine. The improved machining qualities are derived from the addition of 0.025 to 0.060 % sulfur. K-Monel > It is a precipitation hardenable Monel which contains 3% aluminum. Its strength approaches to that of heat treated alloy steel and having similar corrosion

• S-Monel > It is used primarily in casting and containing about 4% silicon. This alloy is responsive to precipitation hardening. A hardness of about 350 Brinell make it suitable for use when resistance to galling and erosion is important as in valve seats and where sliding contact is involved under corrosive conditions. • H-Monel> It is similar to the s Monel but contain 2.753.25% silicon. This alloy cannot be treated to as high a strength and hardness as the s-Monel , but for many application it is adequate.

• Inconel > It consist of 80% nickel,14% chromium and remainder is iron. Its specific gravity is 8.55 and melting point is 1395°C . It can be cast, rolled and cold drawn. It is used for making springs which have to withstand very high temperature and are exposed to corrosive action. • Nichrome > It consist of 65% nickel,15% chromium and 20% iron. It is used for making electric resistance wires for electric furnaces and heating elements. • Nimonic > It consist of 80% nickel and 20% chromium. It is widely used in the gas turbine engine.

COPPER -NICKEL ALLOYS • The addition of nickel to the copper has marked effect on the color of the alloy .The alloy become whiter in appearance until it 2% nickel it is practically white. • An alloy contain 45% nickel with remainder copper has very high resistivity and an extremely low temperature coefficient of resistivity which make its use in certain types of resistors. Used extensively in the thermocouples . this alloy is commonly called “constantan”.

CUPRONICKEL • It is a term which is not necessarily applied to any specific alloy, although it has used principally in connection with the alloy containing 15,20,30% nickel with remainder copper. • Others have been employed containing 2.5,5,10& 25% nickel. 30% cupronickel alloy has been used with considerable success for condenser tubes and tubing in conducting salt water and other corroding substances. • The alloy containing lower nickel content such as 20% cupronickel alloy are used for turbine blades and for parts requiring resistance to corrosion and erosion. These alloys are formed by hot forging and cold working operations.

NICKEL SILVER • The alloys referred to as nickel silver, their importance principally to their color and corrosion-resistance characteristics. The composition of nickel silver varies widely, but it is usually customary to maintain the copper content between 60 to 65 percent .These alloys can be obtained in cast, rolled, and extruded forms. Nickel silver is used principally as the base for silver-plated ware, the advantage being that in the event of wear of the silver plate, the color of the base substance does not differ greatly from that of the silver plate.

HIGH TEMPRATURE ALLOYS • Those alloys which can withstand temperature in excess of 2012F are called high temperature alloy. They are used in components of nuclear plants, jet and rocket engines. • Incoloy > It is a nickel base alloy. It consist of 42% nickel, 13% chromium,6% molybdenum 2.4% titanium 0.4% carbon and remaining is iron. • Hastelloy > It consist of 45% nickel, 22% chromium ,9% molybdenum, 1.5% cobalt, 0.5% tungsten 0.15% carbon and remaining is iron. • Vitallium> it consist of 62% cobalt,28% chromium,5.5% molybdenum, 2.5% nickel, 1.7%iron and 0.28% carbon.

NOMINAL COMPOSITION OF NICKEL BASED ALLOYS

OBTAINING ALUMINA(BAUXITE) v Mine Bauxite from Earth v Crush and spray with water. v Kiln-dry v Mix with soda ash and crushed lime. v Process in digester v Reduce under pressure v Settling tank where additional impurities are removed. v Precipitator v Thickener

ALUMINIUM HISTORY v Most Abundant Metal in Earth’s Crust v 1825 discovered v 1852 $545 per pound v 1888 Economical Processing

PROPERTIES OF ALUMINIUM v Commercially pure aluminum with a specific gravity of 2.71 is 99% pure ,the remainder consisting principally of iron and silicon . v In the annealed state , it has a tensile strength of 13,000 lb/in^2 .with an elongation of about 40% in 2 in. v Its resistance to action of atmosphere and to several chemicals gives it further advantages , although commercially pure aluminum is not usually resistant to strong alkalies or to some weak alkaline solutions. v Its corrosion resistance under oxidizing conditions depends upon the natural development of an aluminum oxide surface. v On weight basis ,the electrical conductivity of aluminum is about 200% that of copper; on a volume basis ,it is about 61% that of copper. v The aluminum alloys are used most extensively for structural purposes

ALLOYING ELEMENTS IN ALUMINIUM v The alloying elements commonly used in commercial aluminium alloys include copper, silicon, magnesium ,mangnese ,and occasionally zinc, nickel, and chromium. v The overall efect of these alloys additions is to raise the tensile strength,yeild strength, and hardness with corresponding reduction of percentage elongation. v Alloying elements are added extensively to aluminum casting to improve casting qualities as well as mechanical properties.. • • •

v Copper has been the principal alloying element in aluminum for many years. It is employed in amounts up to 4% in wrought alloys and up to about 8% in casting.Its effect is to decrease shrinkage and to provide a basis for age hardening in many aluminium alloys. vSilicon is probably second to copper in its importance as an alloying element ,principally in casting alloys.It is used in amounts ranging from about 1% to 14%. Silicon improves casting qualities, such as fluidity ,in addition to providing corrosion resistance,low thermal expansion,and high thermal conductivity. vMagnesium is alloyed with aluminium in amounts ranging from 1% to 10%.Such alloys are lighter than aluminium, posses good mechanical properties,and are easily machined. vZinc is added in amounts up to 10%, to improve mechanical properties. vManganese and chromium are added in small amounts to increase both strength and corrosion resistance of aluminium alloys.

CLASSIFICATION OF ALUMINIUM ALLOYS

v The aluminium alloys are broadly classified as: •

(1)casting alloys

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(2)wrought alloys

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CAST ALLOY DESIGNATION SYSTEM v No classification system for cast aluminium alloys has international acceptance. That of the Aluminium Association of the United States (AAUS) is the most widely used.

v The cast alloy designation system is based on a 3 digit-plus decimal designation xxx.x (i.e. 356.0).  The first digit (Xxx.x) indicates the principal alloying element, which has been added to the aluminum alloy

The second and third digits are significant in 1xx.x series, in which they provide the minimum aluminium percentage above 99% .For other series second and third digits (xXX.x) are arbitrary numbers given to identify a specific alloy in the series. The number following the decimal point indicates whether the alloy is a casting (.0) or an ingot (.1 or .2).  A capital letter prefix indicates a modification to a specific alloy. Example: Alloy - A356.0 the capital A (Axxx.x) indicates a modification of alloy 356.0. The number 3 (A3xx.x) indicates that it is of the silicon plus copper and/or magnesium series.  The 56 (Ax56.0) identifies the alloy within the 3xx.x series, and the .0 (Axxx.0) indicates that it is a final shape casting and not an ingot. ●

ALUMINIUM WROUGHT ALLOYS v The AAA (Aluminium Association of America) classification for wrought aluminium alloys has been adopted by the IADS (International Alloy Development System).The classification is based on a four –digit system. v We shall first consider the 4-digit wrought aluminum alloy identification system. v The first digit (Xxxx) indicates the principal alloying element, which has been added to the aluminum alloy and is often used to describe the aluminum alloy series, i.e., 1000 series, 2000 series, 3000 series, up to 8000 series.

The second single digit (xXxx), if different from 0, indicates a modification of the specific alloy, and). The third and fourth digits are significant in the 1xxx series but not in the others. In the 1xxx series the last two digits provides the minimum aluminium percentage above 99% ; thus 1145 has a minimum purity of 99.45%; 1200 has a minimum purity of 99.00%. In all other series, the third and fourth digits are simply serial numbers; thus 5082 and 5083 are two distinct aluminium-magnesium alloys. The second digit has a curious function: it indicates a close relationship: thus 5352 is closely related to 5052 and 5252; and 7075 and 7475 differ only slightly in composition. ●

In addition to four digits, specific letters are added to denote the thermal/mechanical treatment given. ●

O

annealed

H H1 H2 H3 H4 HX2 HX4

strain hardened strain hardened strain hardened strain hardened strain hardened quarter-hard half hard

only and recovery annealed and stabilized and painted

ALUMINUM CAST ALLOYS vThe nominal compositions and typical

v NOMINAL COMPOSITION OF SOME ALUMINIUM SAND-CASTING ALLOYS.

PROPERTIES OF CAST ALLOYS v Aluminium-magnesium alloy such as 214 and 220 are corrosion resistant even in marine atmosphere. v Aluminium-silicon alloy such as 43 and 356 are noted for excellent casting characteristics. v Aluminium-copper-silicon alloy,108,319 combine certain properties of aluminium copper and aluminium-silicon alloys to produce good casting characterstics ,weldibility,moderate strength and pressure tightness.

Aluminium wrought alloys v The nominal composition and typical mechanical properties of some standard wrought aluminium alloys are given in table:

PROPERTIES OF SOME IMPORTANT WROUGHT ALUMINIUM ALLOYS v Alloys such as 1100,3003,3004,4043,5052,5056,7072 are relatively low in cost,corrosion resistant and are welded easily. v Alloys 2014,2018,2025,4032,and 6151 are forging alloys,2014 is particularly used in aircraft-propeller forging. v Alloys 6053,6063 are extrusion alloys. The ability of 6063 to extrude compliclated shapes makes it useful for architecture work

L EA

What are the origins of the word Lead ? The name originates from the Greek word protos meaning 'first' and the Symbol Origin 'Pb' from the Latin word plumbum meaning 'lead'. Plumbism is the medical term for lead poisoning The Properties of the Element Lead Name of Element : Lead Symbol of Element : Pb Atomic Number of Lead : 82 Atomic Mass: 207.2 amu Melting Point: 327.5 °C - 600.65 °K Boiling Point: 1740.0 °C - 2013.15 °K Number of Protons/Electrons in Lead : 82 Number of Neutrons in Lead : 125

Lead is usually found in ore with zinc, silver and (most abundantly) copper, and is extracted together with these metals. The main lead mineral is galena (PbS), which contains 86.6% lead. Other common varieties are cerussite (PbCO3) and anglesite (PbSO4). The chief ore of lead is galena. In order to extract lead from galena, the ore is first concentrated by froth floatation The concentrated ore is roasted in air to convert it into lead oxide (PbO) and lead sulphate (PbSO4). Some galena is also left unchanged. If the air supply is now reduced; the unreacted PbS reacts with PbO and PbSO4 to In another method, the mixed sulphides (PbS and ZnS) are roasted to obtain oxides. The mixed oxides are reduced to their respective metals with coke by

The properties of lead that make it useful in a wide variety of applications are density, malleability, lubricity, flexibility, electrical conductivity, and coefficient of thermal expansion, all of which are quite high; and elastic modulus, elastic limit, strength, hardness, and melting Lead also has good resistance to corrosion under a wide variety of conditions. Lead is easily alloyed with many other metals and casts with little difficulty. The high density of lead (11.35 g/cm3, at room temperature) makes it very effective in shielding against x-rays and gamma radiation. The combination of high density, high limpness (low stiffness), and high damping capacity makes lead an excellent material for deadening

The low tensile strength and low creep strength of lead must always be considered when designing lead components. The principal limitation on the use of lead as a structural material is not its low tensile strength but its susceptibility to creep. Lead continuously deforms at low stresses and this deformation ultimately results in failure at stresses far below the ultimate tensile strength. The low strength of lead does not necessarily

Alloying with other metals, notably calcium or antimony, is a common method of strengthening lead for many applications. In general, consideration should always be given to supporting lead structures by lead-covered steel straps. When lead is used as a lining in a structure made of a stronger material, the lining can be supported by bonding it to the structure.

Compositions and Grades Bellow is listed the Unified Numbering System (UNS) designations for various pure lead grades and lead-base alloys. vPure leads L50000 - L50099 vLead - silver alloys L50100 - L50199 vLead - arsenic alloys L50300 - L50399 vLead - barium alloys L50500 - L50599 vLead - calcium alloys L50700 - L50899 vLead - cadmium alloys L50900 - L50999 vLead - copper alloys L51100 - L51199 vLead - indium alloys L51500 - L51599 vLead - lithium alloys L51700 - L51799 vLead - antimony alloys L52500 - L53799 vLead - tin alloys L54000 - L55099 vLead - strontium alloys L55200 - L55299

Grades are 1)pure lead (also called corroding lead) 2)common lead (both containing 99.94% min lead), 3)chemical lead and acid-copper lead (both containing 99.90% min lead) 4)Lead of higher specified purity (99.99%) is also available in commercial quantities. Specifications other than ASTM B 29 for grades of pig lead include federal specification QQ-L-171, German standard DIN 1719, British Corroding Lead. Most lead produced is pure (or corroding) lead (99.94% min Pb). Corroding lead which exhibits the outstanding corrosion resistance typical of lead and its alloys. Corroding lead is used in making pigments, lead oxides, and a wide variety of other lead chemicals. Chemical Lead. Refined lead with a residual copper content of 0.04 to 0.08% and a residual silver content of 0.002 to 0.02% is particularly desirable in the chemical industries and thus is called chemical lead. Copper-bearing lead provides corrosion protection comparable to that of chemical lead in most applications that require high corrosion resistance. Common lead, which contains higher

Lead-Base Alloys vBecause lead is very soft and ductile, it is normally used Commercially as lead alloys. Antimony, tin, arsenic, and calcium are the most common alloying elements. vAntimony generally is used to give greater hardness and strength, as in storage battery grids, sheet, pipe, and castings. Antimony contents of lead-antimony alloys can range from 0.5 to 25%, but they are usually 2 to 5%. vLead-calcium alloys have replaced lead-antimony alloys in a number of applications, in particular, storage battery grids and casting applications. These alloys contain 0.03 to 0.15% Ca. More recently, aluminum has been added to calcium-lead and calciumtin-lead alloys as a stabilizer for calcium. vAdding tin to lead or lead alloys increases hardness and strength, but lead-tin alloys are more commonly used for their good melting, casting, and wetting properties, as in type metals and solders. Tin gives the alloy the ability to wet and bond with metals such as steel and copper; unalloyed lead has poor wetting characteristics. vTin combined with lead and bismuth or cadmium forms the principal ingredient of many low-melting alloys.

A solder is a fusible metal alloy with a melting point or melting range of 90 to 450 °C (200 to 840 °F), used in a process called soldering where it is melted to join metallic surfaces. It is especially useful in electronics and plumbing. Alloys that melt between 180 and 190 °C are the most commonly used. Tin/lead solders, also called soft solders, are commercially available with tin concentrations between 5% and 70% by weight. The greater the tin concentration, the greater the solder’s tensile and shear strengths. At the retail level, the two most common alloys are 60/40 Sn/Pb which melts at 370 °F or 188 °C and 63/37 Sn/Pb used principally in electrical work. vIt has the lowest melting point (183 °C In plumbing, a higher proportion of the leadtin/lead was used, commonly or 361.4 °F) of all alloys; and 50/50. This had the advantage of making the alloy solidify more slowly, so that it could be wiped over the joint to ensure watertightness. Terne is an alloy coating of lead and tin used to cover steel, in the ratio of 20% tin and 80% lead. Terne is used to coat sheet steel to inhibit corrosion. It is the one of the cheapest alloys suitable for this, and the tin content is kept at a minimum while still adhering to a hot-dipped iron sheet, to minimize the cost.

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