Prepared by: A.L. Mwahid S. Jaafar
Kufa University
College of Engineering
Materials Eng. Department
Introduction
The ferrous alloys even stainless steels and cast irons use similar methods
for controlling microstructures and properties. However, the structures
and behavior of the different groups of nonferrous alloys have enormous
differences. Melting temperatures, for example, vary from near room
temperature for gallium to over 3000°C for tungsten.
Strengths vary from 5 MN.m-2 to over l500 MN.m-2. Aluminum, magnesium, and beryllium (the light metals) have very low densities, whereas
lead and tungsten have exceptionally high densities.
In many applications, weight is a critical factor. To relate the strength of
the material to its weight, a specific strength or strength-to-weight ratio,
is defined:
Specific strength = strength/density
Table 6.1 compares the specific strength of some light-strength
nonferrous alloys.
Table 6.1 Specific strength and cost of nonferrous alloys
Another factor in designing with nonferrous metals is their cost, winch
also varies considerably. Table 13.1 gives the appropriate price of metals
in 1992. One should note, however, that the price of the metal is only a
small portion of the cost of a part. Fabrication and finishing, not to
1
mention marketing and distribution, often contribute much more to die
overall cost of a part.
Aluminum and Its Alloys
Extraction of Aluminum
The only important ore of aluminum is bauxite, which contains aluminum
oxide (AI2O3). Unfortunately, this cannot be reduced to the metal by
heating it with coke (as in the case of iron ore), because aluminum atoms
are, so to speak, too firmly combined with oxygen atoms to be detached
by carbon. For this reason, an expensive electrolytic process must be used
to decompose the bauxite and release aluminum.
Crude pig iron can be purified (turned into steel) by blowing oxygen over
it, to burn out the impurities, but this would not be possible in the case of
aluminium, since the metal would burn away first, and leave us with the
impurities. Instead the crude bauxite ore is first purified by means of a
chemical process, and the pure aluminium oxide is then decomposed by
electrolysis.
When the electric current passes, Al ions being positively charged, are
attracted to the lining of the furnace, which constituents the negative
electrode(or cathode).
Figure 6.1 Production of aluminum in an electrolytic cell.
General Properties of Aluminum
Aluminum has a density of 2.70 g/cm3 or one-third the density of steel,
and a modulus of elasticity of 10x106 psi. Although aluminum alloys
2
have low tensile properties compared with those of steel, their specific
strength (or strength-to-weight ratio) is excellent. Aluminum is often used
when weight is an important factor, as in aircraft and automotive
applications..
Aluminum also responds readily to strengthening mechanisms. Table 6.2
compares the strength of pure annealed aluminum with that of alloys
strengthened by various techniques. The alloys may be 30 times stronger
than pure aluminum.
Aluminumʼs beneficial physical properties include high electrical and
thermal conductivity, nonmagnetic (paramagnetic) behavior, and
excellent resistance to oxidation and corrosion. Aluminium reacts with
oxygen, even at room temperature, to produce an extremely thin
aluminium oxide (Al2O3) layer that protects the underlying metal from
many corrosive environments.
Aluminium does not, however, display a high fatigue limit, so failure by
fatigue may eventually occur even at low stresses. Because of its low
melting temperature, aluminium does not perform well at elevated
temperatures. Finally, aluminium alloys have a low hardness, leading to
poor wear resistance.
Table 6.2 the effect of strengthening mechanisms in aluminum and its
alloys
Example: Strength-to-Weight Ratio in Design
A steel cable 0.5 in. in diameter has a yield strength of 70,000 psi. The
density of steel is about 7.87 g/cm3. Based on the data in Table 6.5,
determine (a) the maximum load that the steel cable can support, (b) the
diameter of a cold-worked aluminum-manganese alloy (3004-H 18)
required to support the same load as the steel, and (c) the weight per foot
of the steel cable versus the aluminum alloy cable.
3
SOLUTION
a. Load = F = σy  A = 70.000 (π/4) (0.5 in.)2 = 13,744 lb
b. The yield strength of the aluminum alloy is 36,000 psi. Thus:
A = (π/4)d2 = F/σy = 13,744/36,000 = 0.38 in.2
d = 0.697 in.
Density of steel = ρ = 7.87 g/cm3 = 0.284 lb/in.3
Density of
3
3
aluminum = ρ = 2.70 g/cm = 0.097 lb/in
c. Weight of steel = Alρ = (π/4)(0.5in)2(12)(0284)
= 0.669 lb/ft
Weight of aluminum = Alρ = (π/4)(0.697)2 (2) (12)
(0.097) = 0.444 lb/ft
Although the yield strength of the aluminum is lower than that of
the steel and the cable must be larger in diameter, the aluminum cable
weighs only about half as much as the steel cable. When comparing
materials, a proper factor-of-safety should also be included during design.
Example 2: Design/Materials Selection for a Cryogenic Tank
Design the material to be used to contain liquid hydrogen fuel for the
space shuttle.
SOLUTION:
Liquid hydrogen is stored below -253oC; therefore, our tank must have
good cryogenic properties.
Lightweight aluminum would appear to be a good choice. Aluminum
does not show a ductile to brittle transition. Because of its good ductility,
we expect aluminum to also have good fracture toughness, particularly
when the alloy is in the annealed condition.
One of the most common cryogenic aluminum alloys is 5083-O.
Aluminum-lithium alloys are also being considered for low-temperature
applications to take advantage of their even lower density.
Designation
Aluminium alloys can be divided into two major groups: wrought and
casting alloys, depending on their method ol fabrication. Wrought
alloys, which are shaped by plastic deformation (hot and/or cold
working), have compositions and microstructures significantly
different from casting alloys, reflecting the different requirements of
the manufacturing process.
4
Within each major group we can divide the alloys into two
subgroups: heat-treatable and non heat-treatable alloys.
Aluminium alloys are designated by the numbering system shown in
Table 6.3, The first number specifies the principle alloying elements,
and the remaining numbers refer to the specific composition of the
alloy. This IADS (International Alloy Designation System) numbering
system has been adopted by most countries.
The degree of strengthening is given by the temper designation T or
H, depending on whether the alloy is heat-treated or strain-hardened
(Table 6.4).
Other designations indicate whether the alloy is annealed (O),
solution-treated (W), or used in the as-fabricated condition (F). The
numbers following the T or H indicate the amount of strain
hardening, the exact type of heat treatment, or other special aspects
of the processing of the alloy. Typical alloys and their properties are
included in Table 6.5.
Table 6.3 Designations system for aluminum alloys
5
Table 6.4 Temper designations for aluminum alloys
Table 6.5 Properties of a typical aluminum alloys
6
Aluminum Heat Treatment
Heat treating is a critical step in the aluminum manufacturing process to
achieve required end-use properties. The heat treatment of aluminum
alloys requires precise control of the time-temperature profile, tight
temperature uniformity and compliance with industry-wide specifications
so as to achieve repeatable results and produce a high-quality, functional
product.
The most widely used specifications are AMS2770 (Heat Treatment of
Wrought Aluminum Alloy Parts) and AMS2771 (Heat Treatment of
Aluminum Alloy Castings)-SAE standards (Society of Automotive
Engineers), which detail heat-treatment processes such as aging,
annealing and solution heat treating in addition to parameters such as
times, temperatures and quenchants.
Wrought aluminum alloys (Table 6.3) can be divided into two categories:
non-heat treatable and heat treatable. Non-heat-treatable alloys, which
include the 1xxx, 3xxx, 4xxx and 5xxx series alloys, derive their strength
from solid solutioning and are further strengthened by strain hardening
or, in limited cases, aging.
Heat-treatable alloys include the 2xxx, 6xxx, 7xxx and 8xxx series alloys
and are strengthened by solution heat treatment followed by precipitation
hardening (aging).
Cast aluminum alloys (Table 6.3) cannot be work hardened, so they are
used in either the as-cast or heat-treated conditions. Common heat
treatments include homogenization, annealing, solution treatment, aging
and stress relief.
Heat Treatment Processes
In general, the principles and procedures for heat treating wrought and
cast alloys are similar. For cast alloys, however, soak times tend to be
longer if the casting is allowed to cool below a process-critical
temperature for the particular alloy. Solution soak times for castings can
be significantly reduced to durations similar to that for wrought alloys if
the castings are placed into the solution furnace while still hot (above the
7
process-critical temperature) immediately following mold filling and
solidification.
The reduction of stress in complex cast shapes is achieved in large part by
the control of quenching parameters such as agitation rate, quenchant
temperature, rate of entry and part orientation in the quench.
Aging
The goal of aging is to cause precipitation dispersion of the alloy solute to
occur. The degree of stable equilibrium achieved for a given grade is a
function of both time and temperature. In order to achieve this, the
microstructure must recover from an unstable or “metastable” condition
produced by solution treating and quenching or by cold working.
The effects of age hardening or precipitation hardening on mechanical
properties are greatly accelerated, and usually accentuated, by reheating
the quenched material to about (100˚C-200˚C). A characteristic feature of
elevated-temperature aging effects on tensile properties is that the
increase in yield strength is more pronounced than the increase in tensile
strength. Also ductility – as measured by percentage elongation – may
decrease. Thus an alloy in the T6 temper has higher strength but lower
ductility than the same alloy in the T4 temper.
In certain alloys, precipitation heat treating can occur without prior
solution heat treatment since some alloys are relatively insensitive to
cooling rate during quenching.
Thus they can be either air cooled or water quenched. In either condition,
these alloys will respond strongly to precipitation Heat treatment.
In most precipitation-hardenable systems, a complex sequence of timedependent and temperature-dependent changes is involved. The relative
rates at which solution and precipitation reactions occur with different
solutes depend upon the respective diffusion rates, in addition to
solubility and alloy contents.
8
Annealing
Annealing is used for both heat-treatable and non-heat-treatable alloys to
increase part ductility with a slight reduction in strength. There are
several types of annealing treatments dependent to a large extent on the
alloy type, initial and final microstructure and temper condition. In
annealing it is important to ensure that the proper temperature is reached
in all portions of the load.
The maximum annealing temperature needs to be carefully controlled.
During annealing, the rate of softening is strongly temperature dependent
– the time required can vary from a few hours at low temperature to a few
seconds at high temperature. Full annealing (temper designation “O”)
produces the softest, most ductile and most versatile condition. Other
forms of annealing include: stress-relief annealing, used to remove the
effects of strain hardening in cold-worked alloys; partial annealing (or
recovery annealing) done on non-heat-treatable wrought alloys to obtain
intermediate mechanical properties; and recrystallization characterized by
the gradual formation and appearance of a microscopically resolvable
grain structure.
Homogenization (Ingot Preheating Treatments)
The initial thermal operation applied to castings or ingots (prior to hot
working) is homogenization, which has one or more purposes depending
upon the alloy, product and fabricating process involved. One of the
principal objectives is improved workability since the microstructure of
most alloys in the as-cast condition is quite heterogeneous. This is true
for alloys that form solid solutions under equilibrium conditions and even
for relatively dilute alloys.
Preheating
Preheating of aluminum ingots prior to rolling, extruding, forming,
forging or melting (Fig. 4) is used to reduce energy consumption by
improved process efficiency, reducing cycle time and increasing safety.
9
Solution Heat Treatment
The purpose of solution heat treatment is the dissolution of the maximum
amount of soluble elements from the alloy into solid solution. The
process consists of heating and holding the alloy at a temperature
sufficiently high and for a long enough period of time to achieve a nearly
homogenous solid solution in which all phases have dissolved (Fig. 5).
Care must be taken to avoid overheating or underheating. In the case of
overheating, eutectic melting can occur with a corresponding degradation
of properties such as tensile strength, ductility and fracture toughness. If
underheated, solution treatment is incomplete and strength values lower
than normal can be expected. In certain cases extreme property loss can
occur. The solution soak times for castings can be reduced significantly
by placing the casting directly into the solution furnace immediately
following solidification. The casting is maintained at a temperature above
a process-critical temperature (PCT), and the alloy solute is still in
solution.
In general, a temperature variation of ±10˚F (±5.5˚C) from control set
point is allowable, but certain alloys require even tighter tolerances.
Tighter thermal variation (±5˚F) allows the set point to be controlled
closer to the eutectic, thus improving proportion and reducing required
soak time. The time at temperature is a function of the solubility of the
alloy solute and the temperature at which the aluminum casting or
wrought alloy is removed from the mold and placed into the solution
furnace. This time may vary from several minutes to many hours. The
time required to heat a load to the treatment temperature increases with
section thickness, air space around the casting for hot air to flow and the
loading arrangement.
Quenching
Rapid and uninterrupted quenching in water or poly (alkylene) glycol in
water is, in most instances, required to avoid precipitation detrimental to
mechanical properties and corrosion resistance. The solid solution formed
by solution heat treatment must be cooled rapidly enough to produce a
10
supersaturated solution at room temperature that provides the optimal
condition for subsequent age (precipitation) hardening. Quench types
include hot water immersion, ambient water immersion, water spray,
forced air, forced air with mist and poly (alkylene) glycol in water.
Quenching is, in many ways, the most critical step in the sequence of heat
treating. In immersion quenching, cooling rates can be reduced by
increasing the quenchant temperature. Conditions that increase the
stability of a vapor film around the part decrease the cooling rate. Four
factors that minimize distortion in the aluminum include:




Temperature of the quenchant
Agitation rate of the quenchant
Speed of entry of casting into the quenchant
Orientation of the aluminum part as it enters the quenchant
Stress Relief
Stress-relief annealing can be used to remove the effects of strain
hardening in cold-worked alloys. No appreciable holding time is required
after the parts have reached temperature. Stress-relief annealing of
castings provides maximum stability for service applications where
elevated temperatures are involved.
Tempering
Tempering can be performed on heat-treatable aluminum alloys to
provide the best combination of strength, ductility and toughness. These
may be classified by the following designations:





“F” - as fabricated
“H” - strain hardened
“O” - annealed
“T” - thermally treated
“W” - solution treated
The temper designation (Table 3) follows the alloy designation and
consists of letters. Subdivisions, where required, are indicated by one or
more digits following the letters.
11