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Thermoelectric power characterization of a 2024 aluminum alloy during solution treatment and aging
ELSEVIER
Thermoelectric Power Characterization of a 2024 Aluminum Alloy During Solution Treatment and Aging Daren Sun,*+ Xi-then Sun,* Derek 0. Northwood,” Jerry H. Sokolowski”
and
*Engineering Materials Group, Department of Mechanical and Materials Engineering, University of Windsor, Windsor, Ontario, N9B 3P4, Canada, and +Engineering Materials Department, Jilin University of Technology, Changchun, Peoples' Republic of China The solution treatment and aging of a 2024 aluminum alloy was studied using the thermoelectric power (TEP) measurement technique, and the results compared to those obtained by microhardness and optical microscopy. The TEP value changes with solution treatment temperature and duration and reaches a maximum value for solution treatment at 500°C. The changes in TEP during solution treatment are caused by changes in the solubility of the alloying elements in o-Al. In the artificial aging process, the TEP value decreases with increasing aging time, but exhibits different characteristics for different stages of aging. In the initial stage, the TEP value decreases slowly and shows a fluctuating behavior for aging in temperatures below 190°C. This fluctuation is caused by G.I? zone, G.P.B. zone, e”, 8’, S”, and S’ formation which make different contributions to the TEP value. The TEP values corresponding to maximum microhardness for different aging temperatures are the same for a given solution treatment temperature. After the peak age, the TEP values decrease very quickly because the solubility of the alloying elements in CY-Aldecreases with aging time. The micro structural changes caused by precipitation during aging which cannot be observed by the light optical microscope were successfully monitored by the TEP measurement technique.
system [2] (see Fig. 1) as an example. At temperatures below the solidus, the equilibrium state consists of two solid phases: (Y solid solution and an intermetallic-compound phase, 8 (AlzCu). The solid solubility of copper in the aluminum solid solution increases as the temperature increases, and at temperatures above the lower curve (solidus), copper is completely soluble in a-Al. However, at temperatures above the incipient melting temperature (the solidus line), the solubility of copper in aluminum decreases with increasing temperatures because of formation of a liquid phase which contains a higher copper content than in the solid. Therefore, the solution tempera-
INTRODUCTION The aluminum alloy 2024 is widely used in aircraft structures, rivet hardware, truck wheels, screw machine products, and other miscellaneous structural applications [ 11. It is a precipitation-hardening alloy which is subjected to a solution treatment, quenching, and an artificial aging treatment in order to obtain the optimum combination of mechanical properties. The solution treatment results in the dissolution of solid phases, and the temperature for this treatment must be carefully chosen. The solubility-temperature relationships can be illustrated by using the Al-Cu 83 MATERIALS CHARACTERIZATION 36:83-92 (1996) 0 Elsevier Science Inc., 1996 655 Avenue of the Americas, New York, NY 10010
1044-5803/96/$15.00 I’11 S 10445803(96)00002-2
D.Sun et al.
84
EXPERIMENT
DETAILS
MATERIALS
4
6
Copper. WI. % FIG. 1. Aluminum-rich end of the aluminum-copper equilibrium diagram [2].
The material used in this study was a 2024aluminum-alloy hot-rolled rod, with a diameter of 12mm. A detailed chemical analysis is presented in Table 1. The TEP specimens which were 70mm long, 3mm wide, and lmm thick were machined from the rod with the long dimension (70mm) being in the rolling direction, HEAT
ture should be between the solidus and the incipient melting temperature (solidus). In Al-Cu alloys, a succession of precipitates is developed upon aging a rapidly cooled supersaturated solid solution (SSS). These precipitates develop sequentially either with increasing temperature or with increasing time at temperature between room temperature and the solvus. The several stages are typically given as the following sequence [2]: SSS + G.P. zone -+ 0” + 6’ + 8 (AI,Cu) Most of the heat-treatable aluminum alloy systems exhibit multistage precipitation and undergo accompanying strength changes analogous to those of the Al-Cu system. The structural changes from the solid solution to the high-strength tempers involve precipitates that are too small to be resolved by the light microscope [2]. In studying the processes of solution treatment and aging, many measurement techniques have been used, including resistivity [3,4] and dilatometry [5,6]. In recent years, the experimental technique of thermoelectric power (TEP) measurement has been developed and has been used in a wide range of investigations [7]. It can sensitively monitor the variations in the alloying elements in solid solution in the alloy [S-lo] and detect changes induced by precipitation [lo-151. The objective of this work is to use the TEP technique to investigate the solution treatment and aging of a 2024 aluminum alloy.
TREATMENT
Effect of Solution Temperature. To investigate the effects of solution treatment temperature on the TEP values, temperatures ranging from 440°C to 560°C in 10” intervals were used in this investigation. The specimens were solution-treated for 30 minutes and then water-quenched.
Effectof Aging Process. To investigate the effects of thermal aging on the TEP values, samples were first solution treated at 480°C and 500°C for 30 minutes, water-quenched, and then aged at either 170”, 190”, 220”, or 250°C for times from 5 minutes to 100 h. To quantify the effects of the water quenching and the aging at ambient temperatures (or delay aging) on the TEP value, some samples were solution-treated at 500°C and aged at room temperature. The TEP values were measured at given times. OBSERVATION
OF MICROSTRUCTURE
After heat treatment, the metallographic samples were first ground down to 180 grit on a belt-grinder before grinding down to 4000 grit on Sic papers. They were finepolished on a wheel grinder using first a 3krn diamond paste followed by Struers
Table 1
Chemical Composition
of 2024
Aluminum Alloy (Weight Percent) Cu Mg
Mn
Fe
Si
Cr
Zn
4.4 1.5 CO.6 CO.5 CO.5 CO.1 CO.25
Ti
CO.15
TEP Characterization
of a 2024 Al Alloy
85
OP-S SiO2 (
POWER
AND
MEASUREMENTS
Figure 2 shows the principle of the TEP measurement system. Two copper blocks A and B were maintained at temperatures of T (15OC) and T + AT (25”C), respectively. The specimen was pressed into these Cu blocks to ensure a good thermal and electrical contact. A voltage, AV (kV) is generated across the specimen. The TEP values, AS, are calculated using equation [14] AS = AVIAT A more detailed description of the TEP measurement system can be found in Northwood et al. [15]. Microhardness was measured with a load of lOOg, a diamond pyramid indentor, and test time of 15 s. A minimum of six readings were taken randomly for each hardness determination to provide a statistical basis for the mean hardness values.
EXPERIMENTAL RESULTS DISCUSSION EFFECT OF SOLUTION TEMPERATURE AND
AND
TREATMENT DURATION
The variation in TEP values, AS (kV/K), and the microhardness as a function of solution treatment temperatures are shown in Fig. 3. Below 5OO”C,the TEP values and the
MA T ,
d, \
TCI
I
+“lCker*
_~.~~i
m,crahardnesa. H”
&TEP
FIG. 3. Effect of solution temperature on TEP value and microhardness of 2024 aluminum alloy.
microhardness increase with increasing solution treatment temperature, reaching a maximum value at about 5Oo”C, before decreasing with further increases in temperature. Figure 4 shows the relationship between the TEP and the duration of the solution treatment for the samples solution-treated at 500°C. It can readily be seen that initially the TEP values increase rapidly with time before reaching an approximately constant TEP value after 3 minutes of treatment. Recent studies have clearly shown that the TEP values measured at ambient temperature (20°C) are sensitive to the amount of alloying elements in solid solution [9,10, 161. As previously noted, for most aluminum alloys, the solubility of the alloying elements in solid solution changes with the solution treatment temperature and time. The Al-Cu phase diagram, shows that the solubility of copper in a-Al solid solution increases with temperature for solution
t BbCkB
TSAT
TC2
4
FIG. 2. Schematic diagram showing setup for the TEP measurements.
I
the experimental
0 4
3
10 Solution
FIG. 4. Effect of solution the TEP value.
20
15 treatment
25
30
time, t (minutes)
treatment
time at 500°C on
D. Sun et al.
86
FIG. 5. Microstructure at grain boundaries.
of 2024 aluminum
allov solution treated at 520°C for 30 minutes showing incipient meli :iIlg
treatment temperatures below the solidus. Upon incipient melting, that is above the solidus temperature, the solubility of copper in the alloy decreases with increasing temperature and the Cu content of the liquid increases. Between the incipient melting temperature and the solidus temperature, the solubility of copper in solid solution in alloy remains constant. In the 5.25% Cu alloy, these temperatures should be 548” and 536°C [2]. The 2024 aluminum alloy appears to behave in the same manner as the Al-Cu alloy. Below the solidus temperature, the solubility of the alloying elements in cx phase increases with increasing solution treatment temperature, and this is reflected in an increase in both the TEP values and the microhardness. These changes were not obvious from light optical metallographic examination. At approximately 500°C (between the solidus and the incipient melting temperature of approximately 502°C for the 2024 aluminum alloy [l]), the solubility of the alloying elements in the o-Al phases reaches a maximum; the TEP value and microhardness also reach their maximums. Above the incipient melting temperature,
the solubility of the alloying elements decreases with further increase in the solution treatment temperature, due to the formation of a liquid phase. Figure 5 shows a representative microstructure of a sample solution treated at 520°C. The liquid phase (Cu-rich) has formed at the grain boundaries, thus causing a decrease in the amount of alloy elements in solution in the a-Al phase. This decrease in alloying elements in solution led to a decrease in both the TEP value and the microhardness. There is an approximately linear relationship between the TEP values and the microhardness, shown in Fig. 6, although the relationship
FIG. 6. Relationship microhardness.
between
the TEP value and the
TEP Characterization
of a 2024 Al Alloy
is different for the specimens solutiontreated above and below the incipient melting temperature. This work suggests that the TEE’ measurements, which are easy and rapid to make, could be used effectively to determine the solution treatment time required in an actual production process for quality control. EFFECTS OF AGING The plots of the TEP values and microhardness versus aging time for aging temperatures 170”, 190”, 220”, 250°C are presented in Fig. 7(a-d) for a solution treatment temperature of 480°C and in Fig. 7(e-g) for a solution treatment temperature of 500°C. The following general observations can made from Fig. 7: (1) The microhardness shows a peak value at a certain aging time, but the TEP value monotonically decreases as the aging time increases. The TEP values, although they decrease with time, show differences in their rates of change with increasing aging times. Initially, the TEP value decreases slowly with time, and at low aging temperatures (<190”(Z), it shows a fluctuating nature. In this stage, the microhardness increases slowly. With increasing aging time, the microhardness begins to increase quickly (this time is different for different aging temperatures) and eventually reaches a peak value; it is interesting to note that the TEP reaches about the same value for the same solution treatment temperature, when the microhardness is at its peak value. For example, the TEP value of samples solution-treated at 500°C is about -4.65 I.LV/K and is -4.55 pV/K for samples solution treated at 480°C when peak microhardness is observed and does not depend on aging temperature. After the microhardness passes its peak value, the TEP value decreases quickly and then slowly but does not appear to have reached an equilibrium value. The decreases in the mean TEP value are larger than the scatter in measurements for an individual specimen. The microstructure as observed under the light optical micro-
87
scope in these processes shows no obvious changes, as seen in Fig. 8. (2) Comparable aging treatments, the TEP values, and hardness of the samples solution-treated at 500°C are higher than those of samples solution-treated at 480°C. (3) The maximum microhardness of the samples aged at high temperatures occurs more rapidly than in samples aged at lower temperatures. For example, the time corresponding to the aging peak for the samples aged at 250°C is about 2 h but is 2 days for samples aged at 170°C. The changes in the TEP values during aging can readily be explained by the existing precipitation theory. For this alloy, two successive transitions should exist, namely, G.P. -+0” -+ 0’ + 0 (Al&u) and G.P.B. + S” + S’ + S (CuMgAlz) [17-191. At the beginning of aging, the distribution of solute atoms changes from random to the disclike planar aggregates on particular crystallographic planes of the aluminum matrix, thus forming G.P. zones and G.P.B. zones (clusters). As the aging time increases, the Cl’. (G.P.B) zones + 0” (S”) phase, the 0” (S”) phase -+ 8’ (S’) phase, and the 0’ (S’) phase +I (S) phase transitions will take place. In these successive G.P. + 0” -+ 0’+ 8 and G.P.B. + S” + S’+ S transitions, the amount of alloying elements in solid solution will decrease and therefore the TEP value decreases. However, the interesting thing to note is that the TEP value does not simply decrease but can fluctuate in the early stages as can be clearly seen in the curves for low temperature aging and natural aging [Figs. 7(a) and 91. Pelletier et al. [ll] have shown that the types of precipitates formed initially during aging have a significant influence on the TEP value of Al-Cu alloys, that is, the contribution of the metastable platelike precipitates (for example, G.P. zones, 8” or 8’) to the TEP is either positive or negative. However, the incoherent 8 precipitates have no effect on the TEP. Therefore, the factors [the solubility of alloying elements in a-Al, the G. I’. (G.P.B.) zones, 0” (S”) and 8’ (S’)] will induce different contributions to TEP. For this reason, under certain conditions, the TEP value
D.Sun et al.
88
2
10
100
1000
Artificial aging time (min.) _ / ; -a-“ickerr hadnerr _TEP
1000*
I
10
100 Artificial
Value’
+Vickerr hardness +TEP i__~.. .__-
(4
1
10 Artificial ;Gkerr
lCmJ
value 1
(b)
100
1000
aging time, (min.1 hardness +TEP
value
(4
cc>
10
loo0
aging time, t (min.)
100
1ocm
1oclo
Artificial aging time, (min.) -.-“ickers A_-
hardness -TEP
value
1
(e)
1t”,ckers
hardness +TEP
(g)
value
1
0-d
FIG. 7. The TEP value and the hardness of 2024 aluminum alloy, solution-treated at 480°C and aged at (a) 17O”C, (b) 19O”C, (c) 22O”C, and (d) 250”; and solution-treated at 500°C and aged at (e) 17O”C, (f) 19O”C, (g) 22O”C, and (h) 250°C.
TEP Characterization
of a 2024 Al Alloy
89
20 pm
tb) FIG. 8. Microstructure of the sample solution-treated treated at 500°C and aged at 220°C for 3 h.
might not show a monotonic decrease with time. It has been shown that in the Al-Cu system the maximum temperature at which G.P. zones are stable is relatively low. If a quenched Al-Cu alloy containing G.P. zones is heated above 19o”C, the G.P. zones
and aged: (a) solution
treated at 500°C and (b) solution
redissolve in the cl-Al solid solution [20]. Stark et al. [19] in a review article on heattreatable aluminum alloys have pointed out that the G.P. zone forms during the initial stages of aging, and preferentially at the lower aging temperatures. Solution-
D.Sun etal.
Natural
aging time, t (minutes)
FIG. 9. The TEP value of 2024 aluminum rally aged, that is, at ambient temperature.
alloy natu-
treated and quenched aluminum alloys have high concentrations of trapped vacancies, and thus the vacancy diffusion mechanism becomes the predominant diffusional mode, which controls the rates of G.P. zone and metastable phases (G.P. zone, B’) formation [19, 211. At high aging temperatures, the aging processes take place so fast [the G.P. (G.P.B.) zones disappear quickly] that this fluctuating character can not be seen. As the aging time increases, the amount of the 0” (S”) and 8’ (S’) metastable phases increases, the hardness increases rapidly and eventually reaches its peak value, and the amount of alloying elements in solid solution in a-Al decreases continuously. As we have noted, the TEP value at peak microhardness is almost the same for all samples solution-treated at the same temperature. This shows that, according to the TEP characteristics of materials (TEP is sensitive to the amount of alloying elements in solid solution), the solubility of the alloying elements in the cx-Al is about the same at the aging peak, regardless of the aging temperature used. After the aging peak, the stable phases 8 and S begin to form and grow, at which overaging commences. The solubility of alloying elements in B-Al decreases rapidly, and the TEP curves show a continuous drop because of the solubility being the only factor to contribute to the TEP at this stage. The sample solution treated at 480°C may contain more undissolved phases than
sample solution treated at 500°C. Thus, during the aging treatment, any undissolved phases can act as nucleation sites for the precipitation of the metastable and equilibrium phases. This may explain why the sample solution treated at 500°C has a higher TEP value and microhardness than the sample solution treated at 480°C. This study has shown that the rate of decrease of the TEP value of samples aged at high temperatures is greater than that of samples aged at lower temperatures. This is expected and is consistent with previous studies [17, 201 which show that the aging rate is higher at higher temperatures. To summarize, the measured TEP value is related to the solubility of the alloying elements in solid solution in the c-w-Almatrix, and the formation and disappearance of metastable platelike precipitates (G.P. zones, G.P.B., 0’ ‘, 8’, S”, and S’). The microhardness is related both to the precipitation hardening and the solution hardening. In this study, both techniques are used to confirm the trends. However, these techniques are “indirect” and reflect not only precipitation but also other microstructural changes. The TEP technique is, however, attractive in that it is entirely nondestructive and one specimen can be used to follow the aging behavior at a given temperature by simply heating for a given time, making a TEP measurement at the desired “accumulated” time. The other advantage of the TEP measurement over microhardness is that microhardness measures only a small area of the sample and, if the microstructure is not homogeneous, there is considerable scatter in microhardness values. However, the TEP measurements are a volume “average” value (three-dimensional) of the properties of the sample.
CONCLUSIONS
The main conclusions as follows:
from this study are
1. The TEP values and the microhardness of 2024 aluminum alloy changed with solu-
TEP Characterization of a 2024 AI Alloy
q
L.
3.
4.
5.
tion treatment temperature. Below the solvus, the TEP values and the microhardness increase with increasing solution treatment temperature. Above the incipient melting temperature, the TEP values and the microhardness of the cr-Al matrix decrease with increasing solution treatment temperature. Between the solidus and incipient melting temperature, at a solution treatment temperature of about 5OO”C, the TEP values and the microhardness reach their maximum. The TEP values change with the duration of solution treatment. The TEE’ measurement technique can be used to determine the optimum solution treatment time. The TEP values monotonically decrease with aging time, but the microhardness has a peak value at a set aging time. At the beginning of the aging, the TEP When the hardness slowly decreases. reaches its peak value, the TEP value almost attains about the same value for all aging temperatures and then rapidly decreases as the microhardness decreases on further aging. For low-temperature aging treatments, at short aging times, the TEP values do not show a regular decrease but rather show fluctuating behavior due to the formation of G.P. zones, G.P.B. zones, 0”, S”, 0’, and S’ phases, with each of these phases having different contributions (positive or negative) to the TEP value. For the same aging time and temperature, the sample solutions treated at 500°C have higher TEP values and hardness than sample solutions treated at 480°C.
Financial assistance for this study was provided to Professors Northwood and Sokolowski by the Natural Sciences and Engineering Research Council of Canada (NSERCJ and by the Research Board of the University of Windsor.
References 1. W. H. Cubberly, Properties and selection: nonferrous alloys and pure metals, in Metals Handbook,
91 9th ed., American Society for Metals, Metals Park, OH, (1979). 2. W. H. Cubberly, Heat Treating, in Metals Handbook, 9th ed., American Society for Metals, Metals Park, OH, vol. 22, p. 674 (1981). 3. M. Corchia and F. Righini, Kinetics aspects of the transformation in zircaloy-2, I. Nucl. Mater. 97: 137-145 (1981). 4. M. R. Warren and J. A. Ytterhus, Precipitation reactions in Zr-2.0 at.% Cr-0.16 at.% Fe, Can. Metall. Q. 11:249-256 (1972). 5. G. F. Slattery, The effect of cooling on the B-o transformation in the Zr 2 at.% Cr 0.16 at.% Fe alloy, I. Less-Common Met. 1691-101 (1968). 6. G. F. Slattery, Some aspects of the B-o transformation in zirconium- 25 wt.% Nb alloy with variable cooling rate and soak temperature, I. Electrochem. Technol. 4:336-341 (1966). 7. G. W. Brindly, A thermoelectric study of the cold rolling and heat treatment of copper, in Report of a Conference on Strength of Solids, Physical Society, London, pp. 95-104 (1948). 8. J. M. Pelletier, J. Merlin, and R. Borrelly, Study of the development of Al-Zn solid solutions by measurements of thermoelectric power, Mater. Sci. Eng. 33:95-104 (1978). 9. R. Borrely, l’. Merle, and L. Adami, Study of the solubility of iron in zirconium by thermoelectric power measurements, I. Nucl. Muter. 170:147-156 (1990). 10. R. Borrely, J. Merlin, J. M. Pelletier, and G. Vigier, Application of thermoelectric power measurements to the metallurgical characterization of titanium alloy, 1. Less-Common Met. 69:49-61 (1980). 11. J. M. Pelletier, G. Vigier, J. Merlin, P. Merle, F. Fouquet, and R. Borrely, Precipitation effects on thermoelectric power in Al-Cu alloy, Acta Mefall. 32:1069-1078 (1984). 12. D. Benkirat, P. Merle, and R. Borrely, Effects of precipitation on the thermoelectric power of ironcarbon alloys, Acta Metall. 36:613-620 (1988). 13. R. Borrely, P. Merle, and D. Adenis, Seebeck effect of precipitation in 3004, Light Met. P. G. Campbell, ed., Minerals and Material Society, Warrendale, PA, pp. 703-712 (1989). in Metals and AI14. R. D. Barnard, Thermoelectricify lays, Taylor and Francis, London, (1972). 15. D. 0. Northwood, X-C. Sun, J. Hochreiter, J. H. Sokolowski, and D. E. Penrod, Thermoelectric power (TEL’) measurements as a materials characterization technique, in 23th International Conference on T!rermoeZectricity, B. Mathiprakasam, ed., Midwest Research Institute, Kansas City, MO, B4-2 (1994) 16. X-C. Sun and D. 0. Northwood, strain on TEP in a B-quenched I. Rare Met. 12:110-116 (1993). 17. J. M. Silcock,
The structural
Effect of microZr-Cr-Fe alloy,
aging characteristics
D. Sun et al.
92 of Al-Cu-Mg alloys with Cu: Mg weight ratio 711 and 2.2:1, I. Inst. Met. 89:203-210 (1960). 18. N. Sen and D. R. F. West, Some Factors influencing S precipitation in Al-Cu-Mg and Al-Mg-CuAg Alloys, J. Inst. Met. 9787-92 (1969). 19. E. A. Starke, Jr., A. K. Vasudevan, and R. D. Doherty, Aluminum Alloys: Contemporary Research and Applications, Academic Press, Boston, pp. 37 (1989).
20. P. G. Shewmon, Transformations McGraw-Hill, New York, p. 291(1969).
in
Metals,
21. A. Hatab and W. V. Youdelis, Effect of Be on the aging behaviour and precipitation kinetics of Al0.7%Mg-0.4%Si alloy, in Recent Developments in Light Metals: Proceedings of the International Symposium, M. Gilbert et al., eds., Metallurgical Society of the Canadian Institute of Mining, Metals, and Petroleum, pp. 269-279 (1994).
Thermoelectric Power Characterization of a 2024 Aluminum Alloy During Solution Treatment and Aging Daren Sun,*+ Xi-then Sun,* Derek 0. Northwood,” Jerry H. Sokolowski”
and
*Engineering Materials Group, Department of Mechanical and Materials Engineering, University of Windsor, Windsor, Ontario, N9B 3P4, Canada, and +Engineering Materials Department, Jilin University of Technology, Changchun, Peoples' Republic of China The solution treatment and aging of a 2024 aluminum alloy was studied using the thermoelectric power (TEP) measurement technique, and the results compared to those obtained by microhardness and optical microscopy. The TEP value changes with solution treatment temperature and duration and reaches a maximum value for solution treatment at 500°C. The changes in TEP during solution treatment are caused by changes in the solubility of the alloying elements in o-Al. In the artificial aging process, the TEP value decreases with increasing aging time, but exhibits different characteristics for different stages of aging. In the initial stage, the TEP value decreases slowly and shows a fluctuating behavior for aging in temperatures below 190°C. This fluctuation is caused by G.I? zone, G.P.B. zone, e”, 8’, S”, and S’ formation which make different contributions to the TEP value. The TEP values corresponding to maximum microhardness for different aging temperatures are the same for a given solution treatment temperature. After the peak age, the TEP values decrease very quickly because the solubility of the alloying elements in CY-Aldecreases with aging time. The micro structural changes caused by precipitation during aging which cannot be observed by the light optical microscope were successfully monitored by the TEP measurement technique.
system [2] (see Fig. 1) as an example. At temperatures below the solidus, the equilibrium state consists of two solid phases: (Y solid solution and an intermetallic-compound phase, 8 (AlzCu). The solid solubility of copper in the aluminum solid solution increases as the temperature increases, and at temperatures above the lower curve (solidus), copper is completely soluble in a-Al. However, at temperatures above the incipient melting temperature (the solidus line), the solubility of copper in aluminum decreases with increasing temperatures because of formation of a liquid phase which contains a higher copper content than in the solid. Therefore, the solution tempera-
INTRODUCTION The aluminum alloy 2024 is widely used in aircraft structures, rivet hardware, truck wheels, screw machine products, and other miscellaneous structural applications [ 11. It is a precipitation-hardening alloy which is subjected to a solution treatment, quenching, and an artificial aging treatment in order to obtain the optimum combination of mechanical properties. The solution treatment results in the dissolution of solid phases, and the temperature for this treatment must be carefully chosen. The solubility-temperature relationships can be illustrated by using the Al-Cu 83 MATERIALS CHARACTERIZATION 36:83-92 (1996) 0 Elsevier Science Inc., 1996 655 Avenue of the Americas, New York, NY 10010
1044-5803/96/$15.00 I’11 S 10445803(96)00002-2
D.Sun et al.
84
EXPERIMENT
DETAILS
MATERIALS
4
6
Copper. WI. % FIG. 1. Aluminum-rich end of the aluminum-copper equilibrium diagram [2].
The material used in this study was a 2024aluminum-alloy hot-rolled rod, with a diameter of 12mm. A detailed chemical analysis is presented in Table 1. The TEP specimens which were 70mm long, 3mm wide, and lmm thick were machined from the rod with the long dimension (70mm) being in the rolling direction, HEAT
ture should be between the solidus and the incipient melting temperature (solidus). In Al-Cu alloys, a succession of precipitates is developed upon aging a rapidly cooled supersaturated solid solution (SSS). These precipitates develop sequentially either with increasing temperature or with increasing time at temperature between room temperature and the solvus. The several stages are typically given as the following sequence [2]: SSS + G.P. zone -+ 0” + 6’ + 8 (AI,Cu) Most of the heat-treatable aluminum alloy systems exhibit multistage precipitation and undergo accompanying strength changes analogous to those of the Al-Cu system. The structural changes from the solid solution to the high-strength tempers involve precipitates that are too small to be resolved by the light microscope [2]. In studying the processes of solution treatment and aging, many measurement techniques have been used, including resistivity [3,4] and dilatometry [5,6]. In recent years, the experimental technique of thermoelectric power (TEP) measurement has been developed and has been used in a wide range of investigations [7]. It can sensitively monitor the variations in the alloying elements in solid solution in the alloy [S-lo] and detect changes induced by precipitation [lo-151. The objective of this work is to use the TEP technique to investigate the solution treatment and aging of a 2024 aluminum alloy.
TREATMENT
Effect of Solution Temperature. To investigate the effects of solution treatment temperature on the TEP values, temperatures ranging from 440°C to 560°C in 10” intervals were used in this investigation. The specimens were solution-treated for 30 minutes and then water-quenched.
Effectof Aging Process. To investigate the effects of thermal aging on the TEP values, samples were first solution treated at 480°C and 500°C for 30 minutes, water-quenched, and then aged at either 170”, 190”, 220”, or 250°C for times from 5 minutes to 100 h. To quantify the effects of the water quenching and the aging at ambient temperatures (or delay aging) on the TEP value, some samples were solution-treated at 500°C and aged at room temperature. The TEP values were measured at given times. OBSERVATION
OF MICROSTRUCTURE
After heat treatment, the metallographic samples were first ground down to 180 grit on a belt-grinder before grinding down to 4000 grit on Sic papers. They were finepolished on a wheel grinder using first a 3krn diamond paste followed by Struers
Table 1
Chemical Composition
of 2024
Aluminum Alloy (Weight Percent) Cu Mg
Mn
Fe
Si
Cr
Zn
4.4 1.5 CO.6 CO.5 CO.5 CO.1 CO.25
Ti
CO.15
TEP Characterization
of a 2024 Al Alloy
85
OP-S SiO2 (
POWER
AND
MEASUREMENTS
Figure 2 shows the principle of the TEP measurement system. Two copper blocks A and B were maintained at temperatures of T (15OC) and T + AT (25”C), respectively. The specimen was pressed into these Cu blocks to ensure a good thermal and electrical contact. A voltage, AV (kV) is generated across the specimen. The TEP values, AS, are calculated using equation [14] AS = AVIAT A more detailed description of the TEP measurement system can be found in Northwood et al. [15]. Microhardness was measured with a load of lOOg, a diamond pyramid indentor, and test time of 15 s. A minimum of six readings were taken randomly for each hardness determination to provide a statistical basis for the mean hardness values.
EXPERIMENTAL RESULTS DISCUSSION EFFECT OF SOLUTION TEMPERATURE AND
AND
TREATMENT DURATION
The variation in TEP values, AS (kV/K), and the microhardness as a function of solution treatment temperatures are shown in Fig. 3. Below 5OO”C,the TEP values and the
MA T ,
d, \
TCI
I
+“lCker*
_~.~~i
m,crahardnesa. H”
&TEP
FIG. 3. Effect of solution temperature on TEP value and microhardness of 2024 aluminum alloy.
microhardness increase with increasing solution treatment temperature, reaching a maximum value at about 5Oo”C, before decreasing with further increases in temperature. Figure 4 shows the relationship between the TEP and the duration of the solution treatment for the samples solution-treated at 500°C. It can readily be seen that initially the TEP values increase rapidly with time before reaching an approximately constant TEP value after 3 minutes of treatment. Recent studies have clearly shown that the TEP values measured at ambient temperature (20°C) are sensitive to the amount of alloying elements in solid solution [9,10, 161. As previously noted, for most aluminum alloys, the solubility of the alloying elements in solid solution changes with the solution treatment temperature and time. The Al-Cu phase diagram, shows that the solubility of copper in a-Al solid solution increases with temperature for solution
t BbCkB
TSAT
TC2
4
FIG. 2. Schematic diagram showing setup for the TEP measurements.
I
the experimental
0 4
3
10 Solution
FIG. 4. Effect of solution the TEP value.
20
15 treatment
25
30
time, t (minutes)
treatment
time at 500°C on
D. Sun et al.
86
FIG. 5. Microstructure at grain boundaries.
of 2024 aluminum
allov solution treated at 520°C for 30 minutes showing incipient meli :iIlg
treatment temperatures below the solidus. Upon incipient melting, that is above the solidus temperature, the solubility of copper in the alloy decreases with increasing temperature and the Cu content of the liquid increases. Between the incipient melting temperature and the solidus temperature, the solubility of copper in solid solution in alloy remains constant. In the 5.25% Cu alloy, these temperatures should be 548” and 536°C [2]. The 2024 aluminum alloy appears to behave in the same manner as the Al-Cu alloy. Below the solidus temperature, the solubility of the alloying elements in cx phase increases with increasing solution treatment temperature, and this is reflected in an increase in both the TEP values and the microhardness. These changes were not obvious from light optical metallographic examination. At approximately 500°C (between the solidus and the incipient melting temperature of approximately 502°C for the 2024 aluminum alloy [l]), the solubility of the alloying elements in the o-Al phases reaches a maximum; the TEP value and microhardness also reach their maximums. Above the incipient melting temperature,
the solubility of the alloying elements decreases with further increase in the solution treatment temperature, due to the formation of a liquid phase. Figure 5 shows a representative microstructure of a sample solution treated at 520°C. The liquid phase (Cu-rich) has formed at the grain boundaries, thus causing a decrease in the amount of alloy elements in solution in the a-Al phase. This decrease in alloying elements in solution led to a decrease in both the TEP value and the microhardness. There is an approximately linear relationship between the TEP values and the microhardness, shown in Fig. 6, although the relationship
FIG. 6. Relationship microhardness.
between
the TEP value and the
TEP Characterization
of a 2024 Al Alloy
is different for the specimens solutiontreated above and below the incipient melting temperature. This work suggests that the TEE’ measurements, which are easy and rapid to make, could be used effectively to determine the solution treatment time required in an actual production process for quality control. EFFECTS OF AGING The plots of the TEP values and microhardness versus aging time for aging temperatures 170”, 190”, 220”, 250°C are presented in Fig. 7(a-d) for a solution treatment temperature of 480°C and in Fig. 7(e-g) for a solution treatment temperature of 500°C. The following general observations can made from Fig. 7: (1) The microhardness shows a peak value at a certain aging time, but the TEP value monotonically decreases as the aging time increases. The TEP values, although they decrease with time, show differences in their rates of change with increasing aging times. Initially, the TEP value decreases slowly with time, and at low aging temperatures (<190”(Z), it shows a fluctuating nature. In this stage, the microhardness increases slowly. With increasing aging time, the microhardness begins to increase quickly (this time is different for different aging temperatures) and eventually reaches a peak value; it is interesting to note that the TEP reaches about the same value for the same solution treatment temperature, when the microhardness is at its peak value. For example, the TEP value of samples solution-treated at 500°C is about -4.65 I.LV/K and is -4.55 pV/K for samples solution treated at 480°C when peak microhardness is observed and does not depend on aging temperature. After the microhardness passes its peak value, the TEP value decreases quickly and then slowly but does not appear to have reached an equilibrium value. The decreases in the mean TEP value are larger than the scatter in measurements for an individual specimen. The microstructure as observed under the light optical micro-
87
scope in these processes shows no obvious changes, as seen in Fig. 8. (2) Comparable aging treatments, the TEP values, and hardness of the samples solution-treated at 500°C are higher than those of samples solution-treated at 480°C. (3) The maximum microhardness of the samples aged at high temperatures occurs more rapidly than in samples aged at lower temperatures. For example, the time corresponding to the aging peak for the samples aged at 250°C is about 2 h but is 2 days for samples aged at 170°C. The changes in the TEP values during aging can readily be explained by the existing precipitation theory. For this alloy, two successive transitions should exist, namely, G.P. -+0” -+ 0’ + 0 (Al&u) and G.P.B. + S” + S’ + S (CuMgAlz) [17-191. At the beginning of aging, the distribution of solute atoms changes from random to the disclike planar aggregates on particular crystallographic planes of the aluminum matrix, thus forming G.P. zones and G.P.B. zones (clusters). As the aging time increases, the Cl’. (G.P.B) zones + 0” (S”) phase, the 0” (S”) phase -+ 8’ (S’) phase, and the 0’ (S’) phase +I (S) phase transitions will take place. In these successive G.P. + 0” -+ 0’+ 8 and G.P.B. + S” + S’+ S transitions, the amount of alloying elements in solid solution will decrease and therefore the TEP value decreases. However, the interesting thing to note is that the TEP value does not simply decrease but can fluctuate in the early stages as can be clearly seen in the curves for low temperature aging and natural aging [Figs. 7(a) and 91. Pelletier et al. [ll] have shown that the types of precipitates formed initially during aging have a significant influence on the TEP value of Al-Cu alloys, that is, the contribution of the metastable platelike precipitates (for example, G.P. zones, 8” or 8’) to the TEP is either positive or negative. However, the incoherent 8 precipitates have no effect on the TEP. Therefore, the factors [the solubility of alloying elements in a-Al, the G. I’. (G.P.B.) zones, 0” (S”) and 8’ (S’)] will induce different contributions to TEP. For this reason, under certain conditions, the TEP value
D.Sun et al.
88
2
10
100
1000
Artificial aging time (min.) _ / ; -a-“ickerr hadnerr _TEP
1000*
I
10
100 Artificial
Value’
+Vickerr hardness +TEP i__~.. .__-
(4
1
10 Artificial ;Gkerr
lCmJ
value 1
(b)
100
1000
aging time, (min.1 hardness +TEP
value
(4
cc>
10
loo0
aging time, t (min.)
100
1ocm
1oclo
Artificial aging time, (min.) -.-“ickers A_-
hardness -TEP
value
1
(e)
1t”,ckers
hardness +TEP
(g)
value
1
0-d
FIG. 7. The TEP value and the hardness of 2024 aluminum alloy, solution-treated at 480°C and aged at (a) 17O”C, (b) 19O”C, (c) 22O”C, and (d) 250”; and solution-treated at 500°C and aged at (e) 17O”C, (f) 19O”C, (g) 22O”C, and (h) 250°C.
TEP Characterization
of a 2024 Al Alloy
89
20 pm
tb) FIG. 8. Microstructure of the sample solution-treated treated at 500°C and aged at 220°C for 3 h.
might not show a monotonic decrease with time. It has been shown that in the Al-Cu system the maximum temperature at which G.P. zones are stable is relatively low. If a quenched Al-Cu alloy containing G.P. zones is heated above 19o”C, the G.P. zones
and aged: (a) solution
treated at 500°C and (b) solution
redissolve in the cl-Al solid solution [20]. Stark et al. [19] in a review article on heattreatable aluminum alloys have pointed out that the G.P. zone forms during the initial stages of aging, and preferentially at the lower aging temperatures. Solution-
D.Sun etal.
Natural
aging time, t (minutes)
FIG. 9. The TEP value of 2024 aluminum rally aged, that is, at ambient temperature.
alloy natu-
treated and quenched aluminum alloys have high concentrations of trapped vacancies, and thus the vacancy diffusion mechanism becomes the predominant diffusional mode, which controls the rates of G.P. zone and metastable phases (G.P. zone, B’) formation [19, 211. At high aging temperatures, the aging processes take place so fast [the G.P. (G.P.B.) zones disappear quickly] that this fluctuating character can not be seen. As the aging time increases, the amount of the 0” (S”) and 8’ (S’) metastable phases increases, the hardness increases rapidly and eventually reaches its peak value, and the amount of alloying elements in solid solution in a-Al decreases continuously. As we have noted, the TEP value at peak microhardness is almost the same for all samples solution-treated at the same temperature. This shows that, according to the TEP characteristics of materials (TEP is sensitive to the amount of alloying elements in solid solution), the solubility of the alloying elements in the cx-Al is about the same at the aging peak, regardless of the aging temperature used. After the aging peak, the stable phases 8 and S begin to form and grow, at which overaging commences. The solubility of alloying elements in B-Al decreases rapidly, and the TEP curves show a continuous drop because of the solubility being the only factor to contribute to the TEP at this stage. The sample solution treated at 480°C may contain more undissolved phases than
sample solution treated at 500°C. Thus, during the aging treatment, any undissolved phases can act as nucleation sites for the precipitation of the metastable and equilibrium phases. This may explain why the sample solution treated at 500°C has a higher TEP value and microhardness than the sample solution treated at 480°C. This study has shown that the rate of decrease of the TEP value of samples aged at high temperatures is greater than that of samples aged at lower temperatures. This is expected and is consistent with previous studies [17, 201 which show that the aging rate is higher at higher temperatures. To summarize, the measured TEP value is related to the solubility of the alloying elements in solid solution in the c-w-Almatrix, and the formation and disappearance of metastable platelike precipitates (G.P. zones, G.P.B., 0’ ‘, 8’, S”, and S’). The microhardness is related both to the precipitation hardening and the solution hardening. In this study, both techniques are used to confirm the trends. However, these techniques are “indirect” and reflect not only precipitation but also other microstructural changes. The TEP technique is, however, attractive in that it is entirely nondestructive and one specimen can be used to follow the aging behavior at a given temperature by simply heating for a given time, making a TEP measurement at the desired “accumulated” time. The other advantage of the TEP measurement over microhardness is that microhardness measures only a small area of the sample and, if the microstructure is not homogeneous, there is considerable scatter in microhardness values. However, the TEP measurements are a volume “average” value (three-dimensional) of the properties of the sample.
CONCLUSIONS
The main conclusions as follows:
from this study are
1. The TEP values and the microhardness of 2024 aluminum alloy changed with solu-
TEP Characterization of a 2024 AI Alloy
q
L.
3.
4.
5.
tion treatment temperature. Below the solvus, the TEP values and the microhardness increase with increasing solution treatment temperature. Above the incipient melting temperature, the TEP values and the microhardness of the cr-Al matrix decrease with increasing solution treatment temperature. Between the solidus and incipient melting temperature, at a solution treatment temperature of about 5OO”C, the TEP values and the microhardness reach their maximum. The TEP values change with the duration of solution treatment. The TEE’ measurement technique can be used to determine the optimum solution treatment time. The TEP values monotonically decrease with aging time, but the microhardness has a peak value at a set aging time. At the beginning of the aging, the TEP When the hardness slowly decreases. reaches its peak value, the TEP value almost attains about the same value for all aging temperatures and then rapidly decreases as the microhardness decreases on further aging. For low-temperature aging treatments, at short aging times, the TEP values do not show a regular decrease but rather show fluctuating behavior due to the formation of G.P. zones, G.P.B. zones, 0”, S”, 0’, and S’ phases, with each of these phases having different contributions (positive or negative) to the TEP value. For the same aging time and temperature, the sample solutions treated at 500°C have higher TEP values and hardness than sample solutions treated at 480°C.
Financial assistance for this study was provided to Professors Northwood and Sokolowski by the Natural Sciences and Engineering Research Council of Canada (NSERCJ and by the Research Board of the University of Windsor.
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