Aluminum

Aluminum and aluminum alloys are the most important nonferrous metals. Aluminum's density, ρ, is 2.7, which is about 1/3 of the density of iron. Its Young's modulus is 70 GPa (10 × 106 psi) which is also about 1/3 that of iron. The unique properties of aluminum that account for most of its usage are:

1. Good corrosion and oxidation resistance.

2. Good electrical and thermal conductivities.

3. Low density.

4. High reflectivity.

5. High ductility and reasonably high strength.

The uses of aluminum include foil, die castings, beverage cans, cooking and food processing, boats and canoes, and aircraft and automobile parts including sheet, engine blocks, and wheels. Aluminum's high reflectivity accounts for its use as foil for insulation and as reflective coatings on glass. Aluminum is used for power transmission lines and some wiring because of its high electrical conductivity. On an equal weight of cross section and equal cost bases, it is a better conductor than copper. Its high thermal conductivity is advantageous in its applications for radiators, air-cooled engines, and cooking utensils. The low density is important for lawn furniture, hand-held tools, and in cars, trucks, and aircraft. Aluminum's good strength and ductility is important in all structural uses where wrought products are used. Its chemical reactivity is important principally in its use in photoflash bulbs and the thermite reaction (Al + Fe2O3 → Fe + Al2O3). Its corrosion and oxidation resistance are important in packaging (foil, cans), architectural applications, and watercraft.

Composite materials have been used throughout history to achieve combinations of properties that could not be achieved with individual materials. Concrete is a composite of cement, sand, and gravel. Poured concrete is usually reinforced with steel rods. Other examples of composites include steel-belted tires; asphalt blended with gravel for roads; plywood with alternating directions of fibers, carbon, or glass fiber-reinforced polyester; or epoxy used for furniture, boats, and sporting goods. Composite materials offer combinations of properties otherwise unavailable. The reinforcing material may be in the form of particles, fibers, or sheet laminates.

Fiber-reinforced Composites

Fiber composites may also be classified according to the nature of the matrix and the fiber. Examples of a number of possibilities are listed in Table 17.1.

Different geometric arrangements of the fibers are possible. In two-dimensional products, the fibers may be unidirectionally aligned at 90° to one another in a woven fabric or randomly oriented (Figure 17.1). The fibers may be very long or chopped into short segments for easy fabrication. In thick sections it is possible to randomly orient short fibers in three dimensions. Fiber reinforcement is used to impart stiffness (increased modulus) or strength to the matrix. Fiber reinforcement also increases toughness.

Corrosion of metals can be classified as either corrosion in aqueous solutions or as direct oxidation at high temperatures. Both are electrochemical in nature.

Aqueous Corrosion

An aqueous corrosion cell consists of an anode where metal ions go into solution and electrons are produced, a cathode where electrons are consumed, an aqueous solution between the anode and cathode, and an external electrical connection between the anode and cathode (Figure 23.1). The anode reaction can be written M → Mn+ + ne−, where M stands for a metal. There are several possible cathode reactions:

1. Mn+ + ne− → M (This can occur only if there is a high concentration of Mn+ ions in solution.)

2. 2H+ + 2e− → H2 (This can occur only in an acid solution.)

3. O2 + 2H2O + 4e− → 4(OH)− (This is the most common cathode reaction. Note that it requires dissolved O2.)

4. O2+ 4H+ + 4e− → 2H2O (This occurs in acidic solutions and there must be O2 in an acid solution.)

The anode and cathode reactions must occur at the same rate. The corrosion rate is often limited by the cathode reaction.

The material of the anode is more active (less noble) than the material of the cathode. The electromotive series (Table 23.1) lists the relative activity of common metals in one-molar solutions of their own salts. The most noble (least reactive) metals appear at the top and most reactive at the bottom.

Ceramics

Ceramics are compounds consisting of metal and non-metal ions bonded either covalently or ionically. Most ceramics are crystalline. They tend to have high melting points and be very hard and brittle. Their tensile strengths are limited by brittle fracture but their compressive strengths are high. Because they retain high hardnesses at elevated temperatures, they are useful as refractories such as furnace linings. Oxidation at high temperature is not a problem with refractory oxides as it is with refractory metals. Magnesia, alumina, and silica are used for furnace linings. Ceramics are also used as tools for high-speed machining of metals. The high hardness of ceramics at room temperature leads to their use as abrasives as either loose powder or bonded into grinding tools. The low ductility of ceramics limits the structural use of ceramics mainly to applications in which the loading is primarily compressive. Iron-containing ceramic are used as magnets.

The bonding strength depends on the valences of the metal and non-metal. Compounds with higher valences (e.g., SiC, Si3N4) tend to be more strongly bonded than those of lower valences (e.g., NaCl, MgO) so they have higher melting points and higher hardnesses. Appendix 9 covers the geometric principles governing the crystal structures adopted by various ceramic compounds.

Cooling

Polymerization often releases a large amount of heat. Unless the heat is removed the reactants will become too hot.

Example Problem 12–1:

1. a. How much energy is released when one mole of phenol reacts with one mole of formaldehyde?

2. b. If the process were adiabatic (no heat released to the surroundings), how much would the temperature rise? The heat capacity of phenol formaldehyde is 1.193kJ/kg°C.

Solution:

1. a. Using the data in Appendix 7, the energy to break a C-O bond and an N-H bond is 360 and 430kJ/mole, and the energy released in forming a C-N bond and an O-H bond is 305 and 500kJ/mole. The net energy release is 805 −360 = 15kJ/mole.

2. b. The molecular weight of phenol formaldehyde is 90 g/mole (2O = 32 + 2N = 28 + 2C = 24 + 6H = 6) and the molecular weight of water is 18: ΔT = 15kJ/[(0.090kg) (1.193kJ/kg°C)+ 0.018kg)(4.186kJ/kg°C)] = 82°C. This would require cooling.

Injection Molding

Injection molding is similar to die casting of metals. A molten thermoplastic is injected into a metal mold at high pressure, and the molded part is ejected after it cools sufficiently. Injection molding is used to make a wide variety of parts, from small components to entire auto body panels. The most commonly used thermoplastics are polystyrene, ABS, nylon, polypropylene, polyethylene, and PVC. Although the properties of finished products benefit from a high molecular weight, very high molecular weights are not desirable for injection molding because viscosity increases with molecular weight.

Stages of Annealing

Annealing is the heating of a metal to soften it after it has been cold worked. Most of the energy expended in cold work is released as heat during the deformation; however, a small percentage is stored by lattice imperfections. There are three stages of annealing. In order of increasing time and temperature, they are:

1. Recovery, in which most of the electrical conductivity is restored. There is often a small drop in hardness; overall grain shape and orientation remain unchanged.

2. Recrystallization, which is the replacement of cold-worked grains by new ones with new orientations, a new grain size, and a new shape. Recrystallization causes the major hardness decrease.

3. Grain growth, which is the growth of recrystallized grains at the expense of other recrystallized grains.

Recovery

The energy release during recovery results from decreased numbers of point defects and rearrangement of dislocations. Most of the increase of electrical resisitivity during cold work is attributable to vacancies. These largely disappear during recovery so the electrical resistivity drops (Figure 6.1) before any major hardness changes occur. During recovery, residual stresses are relieved by creep, and this decreases the energy stored as elastic strains. Recovery causes no changes in microstructure that are observable under a light microscope.

Recrystallization

Recrystallization is the formation of new grains in cold-worked material. The new grains must first nucleate and then grow. Figure 6.2 shows the progress of recrystallization at 310°C of aluminum that had been cold worked 5%.

Casting

The production of most useful metallic objects involves casting, whether in final form or as ingots that are later shaped as solids by rolling, extruding, or forging. Cast metal components include engine blocks and suspension parts for railcars, trucks, and autos; valves, pumps, faucets, pipes, and fitting equipment for drilling oil wells; surgical equipment and prosthetic devices; and components for household and electronic devices. Injection molding, which is a form of casting, is used to produce many polymer objects.

A number of considerations are important in casting. These include liquid-to-solid shrinkage that requires a reservoir or riser of liquid to prevent void formation; thermal shrinkage of the solid that must be accounted for in designing molds; thermal gradients that can cause warping and residual stresses; segregation of components in solution and gas evolution during freezing; and surface appearance. Technological advances in the past few decades have improved the quality and decreased the cost of castings. Computer analysis has allowed prediction and control of the flow of molten metal in the mold, the temperature profiles, and the position of the solid-liquid interface during solidification. The result is the possibility of elimination of internal voids. The use of styrofoam patterns has increased productivity by simplifying mold making.

Macrostructure of Castings

Typically the outside skin of a casting is composed of fine grains of random orientation. As freezing progresses inward, grains that are more favorably oriented for growth crowd out less favorably oriented grains and form columnar crystals.

Ferromagnetism

All materials have some interaction with magnetic fields. However, the interaction is strong only in ferromagnetic materials. In this chapter, the term magnetic will mean ferromagnetic. Iron, nickel, and cobalt are the only elements that are magnetic at room temperature, although manganese may act magnetically in some alloys. Several rare earth elements are magnetic at temperatures below room temperature and are useful in certain magnetic alloys. Magnetic materials may be broadly classified as either soft or hard. In soft magnetic materials, the direction of magnetization is easily reversed. These are used in transformers, motors, generators, solenoids, relays, speakers, and electromagnets for separating scrap. Hard magnetic materials are those in which it is difficult to change the direction of magnetization. Uses of permanent magnets include compasses, starter motors, antilock brakes, motors, microphones, speakers, disc drives, and frictionless bearings.

Domains

A magnetic material consists of magnetic domains in which the directions of unbalanced electron spins of individual atoms are aligned with each other. Magnetization is a result of these electron spins. In a material that appears to be not magnetized, the fields of the domains are arranged so that their fields cancel as illustrated in Figure 22.1. The magnetization within the domains is in a crystallographic direction of easy magnetization.

When an external magnetic field is applied, the domains in closest alignment with that field grow as illustrated in Figure 22.2. This causes the magnetization to increase as shown in Figure 22.3.

Structure

Wood is composed of hollow cells that transport sap in the living tree. Annual growth rings are evident in the cross section of a tree trunk as shown in Figure A1.1. They are easily seen because faster growth in the spring results in larger cells (Figure A1.2). Softwoods come from conifer (e.g., pine, spruce, and fir) and hardwoods from broadleaf trees (e.g., oak, maple, hickory, poplar, and willow). Most hardwoods are denser and harder than softwoods, but there are exceptions. Douglas fir is harder and denser than willow and poplar.

Sapwood is the portion of the trunk through which sap is conducted. Heartwood is dead sapwood and is usually darker in color. The number of growth rings in the sapwood is between 20 and 40 for most hardwoods.

Bone-dry wood consists mainly of three compounds: cellulose (40 to 50%), hemicellulose (15 to 25%) in the cell walls, and lignin (15 to 30%), which holds the cells together. In a living tree, wood usually contains about 30% moisture. The shrinkage on drying varies with direction. Table A1.1 lists the shrinkage of several woods on drying to 6% moisture.

Because the shrinkage is greater in the tangential direction than in the radial direction, there is a strong tendency for a log to form radial cracks when drying, as shown in Figure A1.3. This tendency to split is aggravated because the outside dries before the inside.

Many products are made by pressing and sintering powders. Most ceramics are consolidated by sintering. These include clay products as well as refractory oxides. These ceramics cannot be fabricated by melting and freezing. Sintering is also used to produce parts of metals that are difficult to melt. Examples include carbide tools and tungsten for lamp filaments. Mixed powders are sintered to make composites that are not otherwise possible such as friction materials for brakes and clutches. Porous parts for filters or oil-less bearings are made by incomplete sintering. Teflon cannot be melted without decomposing so it is also processed as a powder. Pharmaceutical pills are made from powder. Powder processing is a simple and cheap way of fabricating large numbers of parts.

Powder Compaction

Figure 15.1 illustrates schematically how a part is pressed from powder. The process is highly automated with many parts being pressed per second.

There are limitations on the shape of the die. The shape of the die must be prismatic so the compact can be ejected. The ratio of height to diameter must not be too great. Otherwise friction on the sidewalls of the die will not allow sufficient compaction pressure in the center, as illustrated in Figure 15.2. The loss of compacted density is greater as the ratio of height to diameter increases.

Sintering

Sintering pressed powders at elevated temperature bonds the small powder particles together without melting them. The driving force for sintering is the reduction in surface area and the associated energy.