Aluminum and Its Alloys 2.59Mechanical properties are a function of the alloy and temper as well as, in some cases, product form.. TABLE 2.17 Typical Mechanical Properties of Wrought All
Trang 1Thermal conductivity
at 77°F
Electrical conductivity at 68°F, percent of International Annealed Copper Standard
Electrical resistivity
T74 T6 T61, T651 T76, T651 T7351
1080 870 960 1020 1130
39 31 35 40 42
124 98 116 132 139
26 33 30 26 25 8017
H12, H22 H212 H221 H24
1600 1600
59 61 61 61
193 200 201 201
18 17 17 17
1 The following typical properties are not guaranteed, since in most cases they are averages for various sizes, product forms, and methods of manufacture and may
not be exactly representative of any particular product or size These data are intended only as a basis for comparing alloys and tempers and should not be specified
as engineering requirements or used for design purposes.
2 Coefficiet to be multiplied by 10 –6 Example: 12.2 × 10 –6 = 0.0000122.
3 Melting ranges shown apply to wrought products of 1/4 inch thickness or greater.
4 Based on typical composition of the indicated alloys.
5 English units = but-in./ft 2 hr°F
6 Eutectic melting can be completely eliminated by homogenization.
7 Eutectic melting is not eliminated by homogenization.
8 Homogenization may raise eutectic melting temperature 20–40°F but usually does not eliminate eutectic melting.
9 Although not formerly registered, the literature and some specifications have used T736 as the designation for this temper.
Trang 2Aluminum and Its Alloys 2.59
Mechanical properties are a function of the alloy and temper as well
as, in some cases, product form For example, 6061-T6 extrusions have
a minimum tensile ultimate strength of 38 ksi (260 MPa), while T6 sheet and plate have a minimum tensile ultimate strength of 42 ksi(290 MPa)
6061-2.4.1 Minimum and Typical Mechanical Properties
There are several bases for mechanical properties A typical property
is an average property; if you test enough samples, the average of the
test results will equal the typical property A minimum property is
de-fined by the aluminum industry as the value that 99% of samples willequal or exceed with a probability of 95% (The U.S military calls suchminimum values “A” values and also defines “B” values as those forwhich 90% of samples will equal or exceed with a probability of 95%, aslightly less stringent criterion that yields higher values) Typical me-chanical properties are given in Table 2.17 Some minimum mechani-cal properties are given in ASTM and other specifications; more aregiven in Table 2.18 for wrought alloys and Table 2.19 for cast alloys.Minimum mechanical properties are called “guaranteed” when prod-uct specifications require them to be met, and they are called “ex-pected” when they are not required by product specifications
Structural design of aluminum components is usually based on imum strengths The rules for such design are given in the Aluminum
min-Association’s Specification for Aluminum Structures, part of the
Alu-minum Design Manual Safety factors given there, varying from 1.65
to 2.64 by type of structure, type of failure (yielding or fracture), andtype of component (member or connection), are applied to the mini-mum strengths to determine the safe capacity of a component Typicalstrengths should be used to determine the capacity of fabricationequipment (for example, the force required to shear a piece) or thestrength of parts designed to fail at a given force to preclude failure of
an entire structure (Pressure relieving panels are an example of this,called frangible design) Maximum ultimate strengths are specified forsome aluminum products (usually in softer tempers), but these mate-rials are usually intended to be cold worked into final use products,changing their strength
Trang 3TABLE 2.17 Typical Mechanical Properties of Wrought Alloys 1,2
Alloy and
temper
Strength, ksi Elongation, % in 2 in.
Brinnell number
500 kg load
10 mm ball
Ultimate shearing strength, ksi
Endurance3limit, ksi
Modulus 4 of elasticity, ksi × 10 3 Ultimate Yield
1/16 in.
thick specimen
1/2 in dia.
specimen 1060-O
4 11 13 15 18
43 16 12 8 6
.
19 23 26 30 35
7 8 9 10 11
3 4 5 6.5 6.5
10.0 10.0 10.0 10.0 10.0 1100-O
5 15 17 20 22
35 12 9 6 5
45 25 20 17 15
23 28 32 38 44
9 10 11 12 13
5 6 7 9 9
10.0 10.0 10.0 10.0 10.0 1350-O
4 12 14 16 24
.
.5 .6
.
8 9 10 11 15
7
10.0 10.0 10.0 10.0 10.0 2011-T3
2011-T8
55 59
43 45
.
15 12
95 100
32 35
18 18
10.2 10.2
Trang 42014-T4, T451
2014-T6, T651
27 62 70
12 42 60
.
18 20 13
45 105 135
18 38 42
13 20 18
10.6 10.6 10.6 Alclad 2014-O
Alclad 2014-T3
Alclad 2014-T4, T451
Alclad 2014-T6, T651
25 63 61 68
10 40 37 60
21 20 22 10
.
.
18 37 37 41
.
10.5 10.5 10.5 10.5 2017-O
2017-T4, T451
26 62
10 40
.
22 22
47 105
18 38
13 18
10.5 10.5
11 50 47 57
20 18 20 13
22 19
47 120 120 130
18 41 41 42
13 20 20 18
10.6 10.6 10.6 10.6 Alclad 2025-O
11 45 42 63 60 66
20 18 19 11 6 6
.
.
18 40 40 41 40 42
.
10.6 10.6 10.6 10.6 10.6 10.6
Trang 5TABLE 2.17 Typical Mechanical Properties of Wrought Alloys 1,2 (Continued)
Alloy and
temper
Strength, ksi Elongation, % in 2 in.
Brinnell number
500 kg load
10 mm ball
Ultimate shearing strength, ksi
Endurance3limit, ksi
Modulus 4 of elasticity, ksi × 10 3 Ultimate Yield
1/16 in.
thick specimen
11 27 36 46 42 51 57
18 20 17 11 10 10 10
.
.
.
15 15 15
10.6 10.6 10.6 10.6 10.6 10.6 10.6
6 18 21 25 27
30 10 8 5 4
40 20 16 14 10
28 35 40 47 55
11 12 14 15 16
7 8 9 10 10
10.0 10.0 10.0 10.0 10.0 Alclad 3003-O
6 18 21 25 27
30 10 8 5 4
40 20 16 14 10
.
11 12 14 15 16
.
10.0 10.0 10.0 10.0 10.0
Trang 610 25 29 33 36
20 10 9 5 5
25 17 12 9 6
45 52 63 70 77
16 17 18 20 21
14 15 15 16 16
10.0 10.0 10.0 10.0 10.0
10 25 29 33 36
20 10 9 5 5
25 17 12 9 6
.
16 17 18 20 21
.
10.0 10.0 10.0 10.0 10.0
8 19 22 25 28 23
24 7 5 4 3 8
.
.
12 14 15 16 17 15
.
10.0 10.0 10.0 10.0 10.0 10.0
6 19 22 25 28 17 20 24 27
25 10 6 5 4 11 8 6 5
.
28 36 41 46 51
11 14 14 15 16 14 14 15 16
.
10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0
Trang 7TABLE 2.17 Typical Mechanical Properties of Wrought Alloys 1,2 (Continued)
Alloy and
temper
Strength, ksi Elongation, % in 2 in.
Brinnell number
500 kg load
10 mm ball
Ultimate shearing strength, ksi
Endurance3limit, ksi
Modulus 4 of elasticity, ksi × 10 3 Ultimate Yield
1/16 in.
thick specimen
1/2 in dia.
specimen 5050-O
8 21 24 26 29
24 9 8 7 6
.
36 46 53 58 63
15 17 18 19 20
12 13 13 14 14
10.0 10.0 10.0 10.0 10.0 5052-O
13 28 31 35 37
25 12 10 8 7
30 18 14 10 8
47 60 68 73 77
18 20 21 23 24
16 17 18 19 20
10.2 10.2 10.2 10.2 10.2 5056-O
5056-H18
5056-H38
42 63 60
2 59 50
.
35 10 15
65 105 100
26 34 32
20 22 22
10.3 10.3 10.3 5083-O
5083-H321, H116
42 46
21 33
.
22 16
.
25
23
10.3 10.3 5086-O
5086-H32, H116
5086-H34
5086-H112
38 42 47 39
17 30 37 19
22 12 10 14
.
.
23 27
.
10.3 10.3 10.3 10.3
Trang 817 30 33 36 39 17
27 15 13 12 10 25
.
58 67 73 78 80 63
22 22 24 26 28
17 18 19 20 21 17
10.2 10.2 10.2 10.2 10.2 10.2
5252-H25
5252-H38, H28
34 41
25 35
11 5
.
68 75
21 23
.
10.0 10.0
17 30 33 36 39 17
27 15 13 12 10 25
.
58 67 73 78 80 63
22 22 24 26 28
17 18 19 20 21 17
10.2 10.2 10.2 10.2 10.2 10.2
17 30 35 26 18
22 10 10 14 18
.
62 73 81 70 62
23 24 26 23 23
.
10.2 10.2 10.2 10.2 10.2
5456-H
5456-H25
5456-H321, H116
45 45 51
23 24 37
.
24 22 16
90
30
.
10.3 10.3 10.3
5457-O
5457-H25
5457-H38, H28
19 26 30
7 23 27
22 12 6
.
32 48 55
12 16 18
.
10.0 10.0 10.0
Trang 9TABLE 2.17 Typical Mechanical Properties of Wrought Alloys 1,2 (Continued)
Alloy and
temper
Strength, ksi Elongation, % in 2 in.
Brinnell number
500 kg load
10 mm ball
Ultimate shearing strength, ksi
Endurance3limit, ksi
Modulus 4 of elasticity, ksi × 10 3 Ultimate Yield
1/16 in.
thick specimen
1/2 in dia.
specimen 5652-HO
13 28 31 35 37
25 12 10 8 7
30 18 14 10 8
47 60 68 73 77
18 20 21 23 24
16 17 18 19 20
10.2 10.2 10.2 10.2 10.2 5657-H25
5657-H38, H28
23 28
20 24
12 7
.
40 50
12 15
.
10.0 10.0 6061-O
6061-T4, T451
6061-T6, T651
18 35 45
8 21 40
25 22 12
30 25 17
30 65 95
12 24 30
9 14 14
10.0 10.0 10.0 Alclad 6061-O
Alclad 6061-T4, T451
Alclad 6061-T6, T651
17 33 42
7 19 37
25 22 12
.
.
11 22 27
.
10.0 10.0 10.0 6063-O
7 13 13 21 31
20 22 12 12
.
25 42 60 73
10 14 17 22
8 9 10 10
10.0 10.0 10.0 10.0 10.0
Trang 106063-T831
6063-T832
37 30 42
35 27 39
9 10 12
.
82 70 95
22 18 27
.
10.0 10.0 10.0 6066-O
6066-T4, T451
6066-T6, T651
22 52 57
12 30 52
.
18 18 12
43 90 120
14 29 34
16
10.0 10.0 10.0
6101-H111
6101-T6
14 32
11 28
.
159
.
71
20
.
10.0 10.0
6351-T4
6351-T6
36 45
22 41
20 14
.
95
29
13
10.0 10.0 6463-T1
6463-T5
6463-T6
22 27 35
13 21 31
20 12 12
.
42 60 74
14 17 22
10 10 10
10.0 10.0 10.0 7049-T73
7049-T7352
75 75
65 63
.
12 11
135 135
44 43
.
10.4 10.4 7050-T73510, T73511
7050-T745110
7050-T7651
72 76 80
63 68 71
.
12 11 11
.
44 47
.
10.4 10.4 10.4 7075-O
7075-T6, T651
33 83
15 73
17 11
16 11
60 150
22 48
23
10.4 10.4 Alclad 7075-O
Alclad 7075-T6, T651
32 76
14 67
17 11
.
.
22 46
.
10.4 10.4
Trang 11TABLE 2.17 Typical Mechanical Properties of Wrought Alloys 1,2 (Continued)
Alloy and
temper
Strength, ksi Elongation, % in 2 in.
Brinnell number
500 kg load
10 mm ball
Ultimate shearing strength, ksi
Endurance3limit, ksi
Modulus4 of elasticity, ksi × 10 3 Ultimate Yield
1/16 in.
thick specimen
15 78 73
15 10
16 11 11
.
.
.
10.4 10.4 10.4 Alclad 7178-O
Alclad 7178-T6, T651
32 81
14 71
16 10
.
.
.
.
10.4 10.4
Trang 1271 74 61 65 67
11 12
13 13 12
.
.
.
10.2 10.4 10.4 10.2 10.4 Alclad 7475-T61
Alclad 7475-T761
75 71
66 61
11 12
.
.
.
.
10.2 10.2
1
The typical properties listed in this table are not guaranteed, since in most cases they are averages for various sizes, product forms, and
methods of manufacture and may not be exactly representative of any particular product or size These data are intended only as a basis
for comparing alloys and tempers and should not be used for design purposes.
2
The indicated typical mechanical properties for all except 0 temper material are higher than the specified minimum properties For 0
temper products typical ultimate and yield values are slightly lower than specified (maximum) values.
Trang 142.71
Trang 182.75
Trang 21TABLE 2.19 Mechanical Property Limits for Commonly Used Aluminum Sand Casting Alloys 1
Minimum properties Tensile strength
Ultimate Yield (0.% offset)
— 25.0 32.0
414 310 131 159 207 159 200 221 165 200 221 248 200 159 172 214 172 234
— 172 221
50.0 28.0 12.0
—
—
—
— 20.0 13.0 13.0 20.0 28.0 16.0 13.0
— 20.0 14.0 21.0
— 18.0 20.0
345 193 83
—
—
—
— 138 90 90 138 193 110 90
— 138 97 145
— 124 138
3.0 6.0 1.5
— 3.0 1.5
— 1.5 1.0 1.0
—
— 2.0
110–140
— 40–70 65–95 100–130 55–85 70–100 90–120 60–90 45–75 60–90 80–110 55–85 55–85 65–95 65–95 45–75 65–95
— 50–80 70–105
Trang 22—
— 17.0 17.0 17.0 22.0 42.0 35.0 30.0 33.0 37.0 32.0 34.0 32.0 52.0 32.0 36.0 36.0 42.0 48.0 16.0 17.0 24.0
241 207 248 131 159 207 214 172 234
—
—
— 117 117 117 152 209 241 207 228 255 221 234 221 290 221 248 248 290 331 110 117 165
— 22.0 25.0
— 16.0 2020 29.0 18.0 24.0
—
—
— 7.0 6.0 10.0 9.0 22.0 18.0 17.0 22.0 30.0 20.0 25.0 22.0 38.0 27.0 30.0 27.0 35.0 45.0
—
— 18.0
— 152 172
— 110 138 200 124 165
—
—
— 49 41 69 62 152 124 117 152 207 138 172 152 262 186 207 186 241 310
—
— 124
—
— 2.5 2.0
— 3.0
— 3.0 3.5
—
—
— 3.0 3.0
— 6.0 12.0 9.0 5.0 2.0 1.0 2.0 4.0 3.0 1.5 3.0 1.5 1.5 5.0 2.0 5.0 3.0
—
70–100 60–95 75–105 40–70 45–75 55–90 60–90 45–75 70–105
—
—
— 25–55 25–55 35–65 35–65 60–90 60–90 50–80 70–100 65–95 60–90 60–90 60–90 85–115 70–100 70–100
— 75–105 105–35 30–60 30–60 45–75
Trang 23TABLE 2.19 Mechanical Property Limits for Commonly Used Aluminum Permanent Mold Casting Alloys 1 (Continued)
Minimum properties Tensile strength
Ultimate Yield (0.% offset)
T65
T571
T61
T6 F F T6 T5 F T5 T6 T7 T551
T65
T61
T62
48.0 33.0 35.0 33.0 30.0 40.0 34.0 40.0 35.0 24.0 28.0 34.0 31.0 28.0 30.0 35.0 31.0 31.0 40.0 48.0 52.0
331 228 241 228 207 276 234 276 241 165 193 234 214 193 207 241 214 214 276 331 359
29.0 15.0 22.0 16.0
200 103 152 110
8.0 4.5 2.0 3.0
—
—
—
— 2.0
— 1.5 2.0
— 60–90 75–105 65–95 100–130 125–155 90–120 95–125 75–105 55–85 70–100 75–105 90–120 65–100 70–105 85–115 75–105 90–120 110–140
—
—
Trang 24186 255 290 248 234 276 145 172 228 172 172 255 310 310 310 324 145 145 138 152 241 255 310 193 221 124 117 124 186
—
—
—
— 27.0 30.0
—
— 22.0
—
— 26.0
— 36.0 34.0 38.0 7.0 6.0
— 12.0 18.0 17.0 35.0 18.0 22.0
—
— 152
—
— 179
— 248 234 262 49 41
— 83 124 117 241 124 152
—
—
— 3.0 3.0
— 3.0 3.0 3.0 5.0 3.0 3.0 4.0 3.0 2.0 2.5 20.0 2.5 8.0 10.0 3.0 7.0 4.0 8.0 3.0 8.0 3.0
60–90 75–105 90–120 70–100 65–95 75–105 40–70 55–85 65–95 60–90 60–90 70–100 75–105 85–115 75–105 85–115 30–60 30–60
— 45–75 60–90 55–85 80–110 55–85 60–90 30–60 30–60
— 55–85
Trang 251 Values represent properties obtained from separately cast test bars and are derived from ASTM B-26, Standard Specification for
Aluminum-Alloy Sand Castings; Federal Specification QQ-A-601e, Aluminum Aluminum-Alloy Sand Castings; and Military Specification MIL-A-21180c, Aluminum
Alloy Castings, High Strength Unless otherwise specified, the average tensile strength, average yield strength, and average elongation values
of specimens cut from castings shall be not less than 75 percent of the tensile and yield strength values and not less than 25 percent of the
elongation values given above The customer should keep in mind that (1) some foundries may offer additional tempers for the above alloys,
and (2) foundries are constantly improving casting techniques and, as a result, some may offer minimum properties in excess of the above.
2 Hardness values are given for information only; not required for acceptance.
3 F indicates “as cast” condition; refer to AA-CS-M11 for recommended times and temperatures of heat treatment for other tempers to achieve
properties specified.
4 Mechanical properties for these alloys depend on the casting process For further information, consult the individual foundries.
5 The T4 temper of Alloy 520.0 is unstable; significant room temperature aging occurs within life expectancy of most castings Elongation may
decrease by as much as 80 percent.
Notes for Table 2.19
Trang 26Aluminum and Its Alloys 2.83
2.4.2 Strengths
While the stress-strain curve of aluminum is approximately linear inthe elastic region, aluminum alloys do not exhibit a pronounced yieldpoint like mild carbon steels Therefore, an arbitrary definition for theyield strength has been adopted by the aluminum industry: a line par-allel to a tangent to the stress-strain curve at its initial point is drawn,passing through the 0.2% strain intercept on the x (strain) axis Thestress where this line intersects the stress-strain curve is defined as
the yield stress The shape of the stress-strain curve for H, O, T1, T2,
T3, and T4 tempers has a less pronounced knee at yield when pared to the shape of the curve for the T5, T6, T7, T8, and T9 tempers.(This causes the inelastic buckling strengths of these two groups oftempers to differ, since inelastic buckling strength is a function of theshape of the stress-strain curve after yield.)
com-Ultimate strength is the maximum stress the material can sustain.
All stresses given in aluminum product specifications are engineeringstresses; that is, they are calculated by dividing the force by the origi-nal cross sectional area of the specimen rather than the actual crosssectional area under stress The actual area is less than the originalarea, since necking occurs after yielding; thus the engineering stress
is slightly less than the actual stress
When strengths are not available, relationships between the known strength and known properties may be used The tensile ulti-
un-mate strength (F tu) is almost always known, and the tensile yield
strength (F ty) is usually known, so other properties are related to these:
F cy = 0.9 F ty for cold-worked tempers
F cy = F ty for heat-treatable alloys and annealed tempers
The strength of aluminum alloys is a function of temperature Mostalloys have a plateau of strength between roughly –150°F (–100°C)and 200°F (100°C), with higher strengths below this range, and lowerstrengths above it Ultimate strength increases 30 to 50% below thisrange, while the yield strength increase at low temperatures is not so
Trang 272.84 Chapter 2
dramatic, being on the order of 10% Both ultimate and yieldstrengths drop rapidly above 200°F, dropping to nearly zero at 750°F(400°C) Some alloys (such as 2219) retain useful (albeit lower)strengths as high as 600°F (300°C) Figure 2.2 shows the effect of tem-perature on strength for various alloys
Heating tempered alloys also has an effect on strength Heating for
a long enough period of time reduces the condition of the material tothe annealed state, which is the weakest temper for the material Thehigher the temperature, the briefer the period of time required pro-duce annealing The length of time of high temperature exposurecausing no more than a 5% reduction in strength is given in Table 2.20for 6061-T6 Since welding introduces heat to the parts being welded,welding reduces their strength This effect is discussed in Section2.8.1 below, and minimum reduced strengths for various alloys aregiven there
Under a constant stress, the deformation of an aluminum part may
increase over time, behavior known as creep Creep effects increase as
the temperature increases At room temperature, very little creep curs unless stresses are near the tensile strength Creep is usually not
oc-a foc-actor unless stresses oc-are sustoc-ained oc-at temperoc-atures over oc-about200°F (95°C)
2.4.3 Modulus of Elasticity, Modulus of Rigidity, and Poisson’s Ratio
The modulus of elasticity (E) (also called Young’s modulus) is the slope
of the stress-strain curve in its initial, elastic region prior to yield The
TABLE 2.20 Maximum Time at Elevated Temperatures, 6061-T6
Trang 28Figure 2.2 Typical tensile strengths of some aluminum alloys at various temperatures.
Trang 292.86 Chapter 2
modulus is significant, because it is a measure of a material’s stiffness,
or resistance to elastic deformation, and its buckling strength Themodulus varies slightly by alloy, since it is a function of the alloying el-ements It can be estimated by averaging the moduli of the alloying el-ements according to their proportion in the alloy, although magnesiumand lithium tend to have a disproportionate effect An approximatevalue of 10,000 ksi (69,000 MPa) is sometimes used, but moduli rangefrom 10,000 ksi for pure aluminum (1xxx series), manganese (3xxx se-ries), and magnesium-silicon alloys (6xxx series) to 10,800 ksi (75,000MPa) for the aluminum-copper alloys and 11,200 ksi (77,200 MPa) for
8090, an aluminum-lithium alloy Moduli of elasticity for various loys are given in Table 2.18 This compares to 29,000 ksi (200,000MPa) for steel alloys (about three times that of aluminum) and to6,500 ksi (45,000 MPa) for magnesium
al-For aluminum, the tensile modulus is about 2% less than the pressive modulus An average of tensile and compressive moduli isused to calculate bending deflections; the compressive modulus is used
com-to calculate buckling strength
Aluminum’s modulus of elasticity is a function of temperature, creasing about 10% around –300°F (–200°C) and decreasing about30% at 600°F (300°C)
in-At strains beyond yield, the slope of the stress-strain curve is called
the tangent modulus and is a function of stress, decreasing as the
stress increases Values for the tangent modulus or the
Ramberg-Os-good parameter n define the shape of the stress-strain curve in this elastic region and are given in the U.S Military Handbook on Metallic
in-Materials and Elements for Aerospace Structures (MIL HDBK 5) for
many aluminum alloys The Ramberg-Osgood equation is
(2.1)
where ε = strain
σ = stress
F y = yield strength
The modulus of rigidity (G) is the ratio of shear stress in a torsion
test to shear strain in the elastic range The modulus of rigidity is also
called the shear modulus An average value for aluminum alloys is
3,800 ksi (26,000 MPa)
Poisson’s ratio (ν) is the negative of the ratio of transverse strainthat accompanies longitudinal strain caused by axial load in the elas-tic range Poisson’s ratio is approximately 0.33 for aluminum alloys,similar to the ratio for steel While the ratio varies slightly by alloy
Trang 30Aluminum and Its Alloys 2.87
and decreases slightly as temperature decreases, such variations areinsignificant for most applications Poisson’s ratio can be used to re-
late the modulus of rigidity (G) and the modulus of elasticity (E)
through the formula
(2.2)
2.4.4 Fracture Toughness and Elongation
Fracture toughness is a measure of a material’s resistance to the
ex-tension of a crack Aluminum has a face-centered cubic crystal ture and so does not exhibit a transition temperature (like steel) belowwhich the material suffers a significant loss in fracture toughness.Furthermore, alloys of the 1xxx, 3xxx, 4xxx, 5xxx, and 6xxx series are
struc-so tough that their fracture toughness cannot be readily measured bythe methods commonly used for less tough materials and is rarely ofconcern Alloys of the 2xxx and 7xxx series are less tough, and whenthey are used in fracture critical applications such as aircraft, theirfracture toughness is of interest to the designer
The plane strain fracture toughness (K Ic) for some products of the2xxx and 7xxx alloys can be measured by ASTM B645 For those prod-ucts whose fracture toughness cannot be measured by this method(such as sheet, which is too thin for applying B645), nonplane strain
fracture toughness (K c) may be measured by ASTM B646 Fracturetoughness limits established by the Aluminum Association are given
in Table 2.21 Fracture toughness is a function of the orientation of thespecimen and the notch relative to the part, and so toughness is iden-tified by two letters: L for the length direction, T for the width (longtransverse) direction, and S for the thickness (short transverse) direc-tion The first letter denotes the specimen direction perpendicular tothe crack, and the second letter the direction of the notch
Ductility, the ability of a material to absorb plastic strain before
frac-ture, is related to elongation Elongation is the percentage increase in
the distance between two gage marks of a specimen tensile tested tofracture All other things being equal, the greater the elongation, thegreater the ductility The elongation of aluminum alloys tends to beless than mild carbon steels; while A36 steel has a minimum elonga-tion of 20%, the comparable aluminum alloy, 6061-T6, has a minimumelongation requirement of 8 or 10%, depending on the product form Analloy that is not ductile may fracture at a lower tensile stress than itsminimum ultimate tensile stress because it is unable to deform plasti-cally at local stress concentrations Instead, brittle fracture occurs at astress riser, leading to premature failure of the part
2 1( +ν) -
=
Trang 312.88 Chapter 2
The elongation of annealed tempers is greater than that of strainhardened or heat treated tempers, while the strength of annealed tem-pers is less Therefore, annealed material is more workable and able
to undergo more severe forming operations without cracking
Elongation values are affected by the thickness of the specimen, ing higher for thicker specimens For example, typical elongation val-ues for 1100-O material are 35% for a 1/16 in thick specimen, and 45%for a 1/2 in diameter specimen For this reason, it is important tospecify the type of specimen used to obtain the elongation value Elon-
be-TABLE 2.21 Fracture Toughness Limits
29 27 26 25 24
25 24 26 22 22
— 21 21 21 21 7050-T76512 1.000–2.000
2.001–3.000
36 34
24 23
— 20
7475-T7351 1.250–2.499
2.500–4.000
40 40
33 33
— 25
75 60
—
— 7475-T761 0.040–0.125
0.126–0.249
—
—
87 80
—
—
1 When tested per ASTM Test Method E399 and ASTM Practice B645.
2 Thickness for Klc specimens in the T-L and L-T test orientations: up through 2 in.
(ordered, nominal thickness), use full thickness; over 2 through 4 in., use 2-in specimen
thiccentered at T/2; over 4 in., use 2-in specimen thickness centered at T/4 Test location
for Klc specimensn the S-L test orientation: locate crack at T/2.
3 T74 type tempers, although not previously registered, have appeared in the literature
and in some specifications as T736 type tempers.
4 When tested per ASTM Practice B646 and ASTM Practice E561.
K lc,ksi in min.
Trang 32Aluminum and Its Alloys 2.89
gation is also very much a function of temperature, being lowest atroom temperature and increasing at both lower and higher tempera-tures
2.4.5 Hardness
The hardness of aluminum alloys can be measured by several ods, including Webster hardness (ASTM B647), Barcol hardness(ASTM B648), Newage hardness (ASTM B724), and Rockwell hard-ness (ASTM E18) The Brinnell hardness (ASTM E10)for a 500 kgload on a 10 mm ball is used most often and is given in Tables 2.17 and2.19 Hardness measurements are sometimes used for quality assur-ance purposes on temper The Brinnell hardness number (BHN) is ap-proximately related to minimum ultimate tensile strength: BHN =
meth-0.556 F tu; this relationship can be useful to help identify material orestimate its strength based on a simple hardness test The relation-ship between hardness and strength is not as dependable for alumi-num as for steel, however, so this equation is only approximate
2.4.6 Fatigue Strength
Tensile strengths established for metals are based on a single tion of load at a rate slow enough to be considered static The repeatedapplication of loads causing tensile stress in a part may result in frac-ture at a stress less than the static tensile strength This behavior is
applica-called fatigue While the fatigue strength of aluminum alloys varies by
alloy and temper, it does not vary as much as the static strength ure 2.3) For this reason, designers often consider fatigue strength to
(Fig-be independent of alloy and temper, especially when the num(Fig-ber ofload cycles is high
The fatigue strengths of the various aluminum alloys can be pared based on the endurance limits given in Table 2.17 These endur-ance limits are the stress range required to fail an R R Moorespecimen in 500 million cycles of completely reversed stress Endur-ance limits are not useful for designing components, however, becausethe conditions of the test by which endurance limits are establishedare rarely duplicated in actual applications Also, endurance limit testspecimens are small compared to actual components, and fatiguestrength is a function of size, being lower for larger components This
com-is because fatigue failure initiates at local dcom-iscontinuities such asscratches or weld inclusions and the probability that a discontinuitywill be present is greater the larger the component
Fatigue strength is strongly influenced by the number of cycles ofload and the geometry of the part Geometries such as connections
Trang 332.90 Chapter 2
that result in stress concentrations due to abrupt transitions such assharp corners or holes have lower fatigue strengths than plain metalwithout such details Therefore, for design purposes, applications arecategorized by the severity of the detail, from A (being least severe,such as base metal in plain components) to F (being most severe, such
as fillet weld metal) Design strengths in fatigue can be found in Table2.22 by substituting parameters given there into the equation
(2.3)
where S rd = allowable stress range, which is the algebraic difference
between the minimum and maximum stress (tension is positive, compression is negative)
C f = constant from Table 2.22
N = number of cycles of load
m = constant from Table 2.22
This equation is set so that there is a 95% probability that 97.7% ofcomponents subjected to fatigue will be strong enough to withstandthe stress range given by the equation
This equation shows that fatigue strength decreases rapidly as thenumber of load cycles increases For loads of constant amplitude, how-
Figure 2.3 Fatigue strengths of MIG welded butt joints.
S rd C f
N 1 m⁄ -
=
Trang 34Aluminum and Its Alloys 2.91
ever, it is believed that the fatigue strength of aluminum alloys doesnot decrease once the number of cycles reaches approximately 5 mil-
lion The fatigue strength predicted by the above equation for N = 5 million is called the constant amplitude fatigue limit (CAFL, or simply
fatigue limit) and is given in Table 2.22 Loads may also have variableamplitudes, such as the loads on a beam in a bridge carrying trafficcomposed of cars and trucks of various weights For variable ampli-tude loads, no lower bound on the fatigue strength is believed to exist,but some design codes use one-half of the constant amplitude fatiguelimit as the fatigue limit for variable amplitude loading Fatiguestrengths of aluminum alloys are 30 to 40% of those of steel undersimilar circumstances of loading and severity of the detail
Fatigue is also affected by environmental conditions The fatiguestrength of aluminum in corrosive environments such as salt spray can
be considerably less than the fatigue strength in laboratory air Thismay be because corrosion sites such as pits act as points of initiationfor cracks, much like flaws such as dents or scratches The more corro-sion resistant alloys of the 5xxx and 6xxx series suffer less reduction infatigue strength in corrosive environments than the less corrosion re-sistant alloys such as those of the 2xxx and 7xxx series On the otherhand, fatigue strengths are higher at cryogenic temperature than atroom temperature There isn’t enough data on these effects to establishdesign rules, so designers must test specific applications to determinethe magnitude of environmental factors on fatigue strength
The fatigue strength of castings is less than that of wrought ucts, and no fatigue design strengths are available for castings While
prod-TABLE 2.22 Fatigue Strengths of Aluminum Alloys
Category
C f
Fatigue limit
B 130 4.84 5.4 Members with groove welds parallel to the
direction of stress
transition radius 24 in ≥ R ≥ 6 in (610 mm
≥ R ≥ 150 mm)
transition radius 6 in ≥ R ≥ 2 in (150 mm ≥
R ≥ 50 mm)
Trang 352.92 Chapter 2
castings are less notch sensitive than wrought products, they, likewrought products, should be designed with as few stress concentra-tions as possible to improve fatigue life
2.5 Corrosion Resistance
As mentioned above, aluminum is resistant to corrosion from manyagents The hard aluminum oxide skin that forms on the surface inthe presence of oxygen discourages further oxidation of the metal.Thus aluminum is often used without any protective coating For someapplications, the metal may be protected with a coating An example isanodizing, a process that accelerates the formation of the protectiveoxide layer, as discussed in Section 2.9.4.1 below
2.5.1 General Corrosion Resistance
Most, but not all, aluminum alloys are less corrosion resistant thanpure aluminum General corrosion resistance of aluminum alloys isusually an inverse function of the amount of copper used in the alloy.Thus the 2xxx series alloys are the least corrosion resistant alloys,since copper is their primary alloying element and all have apprecia-ble (around 4%) levels of copper (The only alloy that the Aluminum
Association Specification for Aluminum Structures requires to be
painted for atmospheric exposure applications is 2014-T6) Some 7xxxseries alloys contain about 2% copper in combination with magnesiumand zinc to develop strength Such 7xxx series alloys (such as 7049,
7050, 7075, 7175, 7178, and 7475) are the strongest but least corrosionresistant of their series Low copper aluminum-zinc alloys, such as
7005, are also available, and have become more popular recently per does have a beneficial effect in 7xxx series alloys’ resistance tostress corrosion cracking (discussed further in Section 2.5.5 below),however, by allowing them to be precipitated at higher temperatureswithout loss in strength in the T73 temper, which has good strengthand good stress corrosion cracking resistance Among the 6xxx seriesalloys, higher copper content (1% in 6066) generally decreases corro-sion resistance, but most 6xxx series alloys contain little copper.Some other alloying elements also decrease corrosion resistance.Lead (added to 2011 and 6262 for machining characteristics), nickel(added to 2018, 2218, and 2618 for elevated temperature service), andtin (used in 8xx castings) all tend to decrease the corrosion resistance,but not enough to matter in most applications
Cop-Many of the 5xxx series alloys have general corrosion resistance asgood as commercially pure aluminum and are more resistant to saltwater, and so are useful in marine applications
Trang 36Aluminum and Its Alloys 2.93
Heat treatment parameters, such as the rate of quenching and thetemperature and time of artificial aging, can also affect general corro-sion resistance The relative general corrosion resistance of the vari-ous alloys is given in Tables 2.12 and 2.13
2.5.2 Galvanic Corrosion
Galvanic corrosion occurs when two conductors with different electricpotentials are electrically connected by an electrolyte, which is pro-vided by the moisture present in most applications The conductorthat is more anodic is corroded, and the conductor that is more ca-thodic is protected from corrosion In a saltwater solution, the order ofmost anodic to most cathodic is
While this list applies only when the electrolyte is a saline solution, it
is useful, because such a solution is similar to marine environments.The list shows that aluminum tends to be corroded in the presence ofsteel but protected when in electrical contact with zinc An exception
to the series is austenitic stainless steel, which, despite its place at theend of the list, tends to be polarized and therefore doesn’t corrode alu-minum The electric potential of different aluminum alloys variesslightly, so galvanic corrosion can also occur when different alloys ofaluminum are in contact, such as where 5xxx series filler wires areused to weld 6xxx series base metal The electric potentials of manycommon alloys of aluminum and other metals are listed in Table 2.23for a salt water electrolyte The electric potential of non-heat-treatablealloys is not a function of the temper, but the potential for heat-treat-able alloys varies slightly by temper and even for a given temper de-pending on the quenching rate
Trang 372.94 Chapter 2
The corrosion of aluminum in contact with more cathodic metals can
be minimized by maximizing the ratio of exposed surface area of thealuminum to that of the other metal This minimizes the current den-sity and thereby slows the rate of corrosion Coating the cathodicmetal is an effective way of doing this; coating the aluminum only isnot and can actually have the opposite effect Because of this princi-ple, the use of steel fasteners in an aluminum part usually will notcause significant corrosion of the aluminum, because the total surfacearea of the part is typically much greater than that of the fasteners,the part being much larger Aluminum fasteners in a steel component,
on the other hand, will tend to corrode rapidly
Some products can be provided with a thin coating of pure num or corrosion resistant aluminum alloy (such as 7072); the result-
alumi-ing product is called alclad This claddalumi-ing is metallurgically bonded to
one or both sides of sheet, plate, 3003 tube, or 5056 wire, and may be1.5 to 10% of the overall thickness The cladding alloy is chosen so it isanodic to the core alloy and so protects it from corrosion Any corrosionthat occurs proceeds only to the cladding-core interface and thenspreads laterally, making cladding very effective in protecting thinmaterials Because the coating generally has a lower strength thanthe base metal, alclad alloys have slightly lower strengths than non-
TABLE 2.23 Relative Electrical Potential of Metals in Saltwater 1
Trang 38Aluminum and Its Alloys 2.95
alclad alloys of the same thickness Alloys used in clad products aregiven in Table 2.24
Aluminum alloys can also be cathodically protected with zinc ormagnesium sacrificial anodes or with impressed currents Buried alu-minum pipelines are typically protected with anodes
2.5.3 Pitting Corrosion
Pitting corrosion is localized, resulting in small, randomly locatedpits roughly hemispherical in shape, and it most commonly occurs inthe presence of chloride ions The pits tend to be covered with corro-sion product, and so the rate of growth of the pit depth tends to di-minish over time Pit depth has been observed to be approximately afunction of the cube root of time Therefore, doubling the thickness of
a part increases the time to perforation eightfold If the part ness is sufficient and appearance is not an issue, pitting corrosion can
thick-be tolerated
2.5.4 Deposition Corrosion
The ions of heavy metals such as copper, mercury, lead, nickel, cobalt,and tin can cause severe localized corrosion of aluminum, especially in
acidic solutions where they are most soluble This action is called
dep-osition corrosion, because the metal reduced from these ions plates
onto the aluminum, setting up galvanic cells, usually resulting in ting A copper concentration of 0.02 to 0.05 ppm in neutral or acidic so-lutions is the approximate threshold above which pitting may occur.The exact threshold is a function of the particular aluminum alloy, the
pit-pH of the solution, the concentration of other ions (especially ate, chloride, and calcium), and whether the pits that develop are open
bicarbon-or occluded Significantly higher copper concentrations may be able, depending on these factors and the duration of exposure, sincepitting penetration rates decrease over time
accept-Mercury is among the most corrosive heavy metals to aluminum.Where aluminum’s oxide coat is damaged by chemical or mechanicalattack, mercury can corrode aluminum rapidly, especially in the pres-ence of moisture For this reason, a 0.005 ppm limit on the mercurycontent of water is suggested This compares to the U.S Environmen-tal Protection Agency maximum contaminant level goal for mercury indrinking water of 0.002 ppm
2.5.5 Stress Corrosion Cracking
Stress corrosion cracking (SCC) is localized directional crackingcaused by a combination of tensile stress and corrosive environment
Trang 392.96
Trang 402.97