Precipitation Hardening of Aluminum Alloys

Precipitation Hardening of Aluminum Alloys J.N.Astoveza Department of Mining, Metallurgical and Materials Engineering University of the Philippines, Diliman [email protected] jenastov [email protected] m

Abstract Precipitation hardening entails formation of fine-grained second phase particles which are uniformly dispersed along the lattice thereby serving as dislocation points with a general effect of strengthening a material. In this experiment, aluminum alloys specimens were subjected to different aging conditions (natural aging, artificial aging at elevated temperature and artificial aging at low temperature) after initially solutionizing them at 540 ᵒ C in the muffle furnace. Hardness reading reported in the following increasing order of samples: asreceived, naturally-aged, artificially-aged, and kept-in-freezer. In this same order, second phase precipitation precipitation and defined grain boundaries boundaries became more abundant abundant when the microstructures of the samples were taken after an interval of aging time.

1. Introduction Precipitation Hardening (or Age Hardening) provides one of the most widely used mechanism mechanism for the strengthening of metal alloys. The mechanism involves development of extremely small uniformly dispersed second phase precipitates responsible for an overall increase in strength of the aluminum alloy as these precipitates act as barriers to dislocation movement in the treated lattice. Accidentally discovered by Wilm during the years 1903-1911, the process of age hardening aluminum has outwitted cast iron in commercial applications following the establishment of its trade name Duralumin [6].

Three basic stages are involved in precipitation hardening namely: (1) Solutionizing, where the alloy is heated above solvus temperature until a homogenous solid solution (α) is produced upon dissolution of θ phase; (2) Quenching, where rapid cooling is operated such that a supersaturated solution of αSS is formed while diffusion of the original phase is prevented; and (3) Aging, where αSS is heated below the solvus temperature thereby inducing dispersion of fine precipitates which embodies the primary goal of precipitation hardening in producing fine precipitates that serve as impediments for dislocation movement yielding an overall increase in strength of the aluminum alloy [6]. Phase changes involved in the aforementioned stages may be effectively visualized with the following illustration (see Figure 1).

Figure 1. The aluminum rich end of the Al-Cu phase diagram illustrating stages of precipitation hardening [6]

In commercial applications, precipitation hardening treatments precede cold forming which modifies the final shape of the materials by inducing plastic deformation. Methods including rolling, extruding, forging, drawing or a combination of these are performed to produce what is known as wrought products. These products have modified grain size and shape on their final configuration. During deformation, deformation, the second-phase second-phase microconstituents present in irregular forms in the ingot are fragmented into more equiaxed particles, which tend to align in the direction of greatest extension [1] thereby giving the wrought materials directionality during the shaping process.

End-Quench Hardenab Hardenability ility Tes of an AISI AISI 1020 Steel. Page 1 of 7 J.N.Astoveza. Jominy End-Quench

2. Methodology Four aluminum strip specimens (15mm x 15mm) were initially prepared by stamping identification marks on their corners. The marks corresponded to: (1) as-received, (2) naturally-aged, (3) artificiallyaged, and (4) kept-in-freezer conditions. Three were loaded in the muffle furnace and heated for one hour at 540ᵒC excluding the one which which was taken as the “as“as-received” specimen. The operating temperature, 540ᵒC, was sufficiently above the solvus temperature (refer to Figure 1) thereby facilitating formation of a single homogenous phase representing solutionizing stage of the process. After heating, all three specimens were quenched in water until adequately cooled. Rapid cooling has to be operated to form a supersaturated solid solution of αss which contains excess of alloying elements and is not an equilibrium structure. The atoms do not have time to diffuse to potential nucleation sites and thus θ precipitates do not form [6]. Two specimens (“as(“asreceived” and “naturally“naturally-aged”) were then mounted to a PVC pipe fixture loaded with polymer resin in order to facilitate ease and functionality prior to polishing (see Figures 2 & 3 below).

simulated in order to create a comparison of their respective precipitation hardening mechanism. The etchant used for the specimens was Keller’s Keller’s etchant which is a standard etchant for aluminum samples. Using inappropriate etchant otherwise would not expose the microstructure samples when observed under the microscope.

2.1 Hardness Testing Since the available indenter of the Rockwell Hardness Tester (see Figure 4) in the laboratory was only appropriate for steel samples, hardness readings were taken instead using the Micro Vickers Hardness Tester (see Figure 5) which is also known as the Diamond Pyramid Hardness Test.

Figure 4. Rockwell Hardness Tester

Figures 2 & 3. Specimens were mounted in polymer resin to extend functionality for sample preparation.

Hardness reading and microstructure photos were obtained with the first two samples while the “artificially“artificially-aged” specimen was loaded again in the muffle furnace at heated at 190ᵒC for an hour. On the other hand, the “kept-in“kept-in-freezer” freezer” specimen was loaded in the freezer for fourteen days. Hardness readings and microstructure photos were then taken for both of the latter specimens preceded by mounting to the polymer resin and polishing. Three more hardness readings and microstructure photos were obtained for the “naturally“naturally-aged” specimen which was allowed to age at room temperature. With this procedure, different aging conditions were

Figure 5. Micro Vickers Hardess Tester

Micro Vickers Hardness tester measures the ability of materials to resist plastic deformation upon standard indention. The average length of diagonals of the diamond-pyramid impression (refer to Figure 6 ) is to be measured while taking note of the kilogram-force value of the indenter used. Hardness number is calculated by dividing the load applied over the area of indention which expression can be simplified to Equation 1 knowing that all ᵒ pyramidal diamond Vickers ranges use a 136 ᵒ indenter that forms square indent [7].


Figure 6. Diamond indentation using Micro Vickers Hardness Tester

Equation 1. Vickers Hardness Number adopted from [1] where: L = load in grams-for grams-force ce d = average length of diagonals in μm

In this experiment, 100p load was used for all hardness readings.

3. Results and Discussion 3.1 Experimental Results A table containing Hardness Readings, expressed in HV, is presented in Ta ble 1 (refer to Appendices). The specimen which attained the highest hardness reading was the one kept in freezer for fourteen days measuring 43.89 HV. On the other hand, the specimen with the lowest reading hardness reading was, as expected, the as-received specimen having the hardness of 20.55HV. The hardness reading obtained for the artificially-aged specimen was 41.31HV which was just slightly lower than the specimen which was kept in freezer but almost twice as hard as the as-received specimen. These differences in hardness readings indicated how significant the second-phase precipitation was in altering the hardness of the aluminum samples. Moreover, for the naturally-aged specimen a graph of hardness versus logarithm of aging time was Appendices) with the presented in Figure 6 (refer to Appendices corresponding data point values from Table 2 (refer to Appendices). It was observed that hardness increased as aging time proceeds until the seventh day of aging time. However, there was a significant decrease in hardness upon measurement on the ninth day.

The corresponding photos of microstructures are Appendices. No presented in Figures 7 to 13 in the Appendices clear grain boundary on a homogenous phase was observed for the as-received specimen (see Figure 7 ). ). It was expected that this initial condition would yield equi-axed grains. The microstructure for the naturally-aged specimen meanwhile indicated an increasingly evident development of grain boundaries and second-phase precipitates as aging time proceeds peaking on Day 7 and dropping on Day 9 (see Figures 8 to 11 ). At this point, secondphase precipitates started to form along grain boundaries taking globular forms as the supersaturated solution hinders the formation of the initially existing homogenous phase. Both the microstructures of artificially-aged and kept-in-freezer specimens exhibited high degree of second-phase precipitation (see Figures 12 and 13) from which high hardness readings were expected at a glance. The following discussions would cover elaboration on the governing theories possibly accounting for the trend observed in the experiment.

3.2 Aging Conditions In this experiment, three aging conditions following solutionizing and water-quenching were observed namely: natural aging, artificial aging at elevated temperature, and artificial low temperature aging. For natural aging, the specimen is kept at room temperature as it ages. As the aging time proceeds, grain boundaries continue to develop along with second-phase precipitates consisting mostly of alloying elements. Both of which properties result in an increased hardness since fine grain boundaries and second phase precipitates act as dislocation points as previously discussed in the introduction. Artificial Aging, on the other hand, involves aging at a temperature other than room temperature. This is also referred to as duplex aging. As discussed on a journal on precipitation hardening of AA7020 aluminum alloy [4]: “The main drawback of precipitation hardening of 7020 aluminium alloy is the formation of a precipitate free zone. To eliminate the formation of a PFZ, 7020 alloy is subjected to a duplex or triplex ageing treatment (Palmear 1981). Duplex ageing process aims at forming a large number of


precipitate nuclei at the low temperature stage, when the solid solution is appreciably supersaturated and diffusion process is low. These are then allowed to grow to optimum size at a fast rate by ageing at a second, but higher, temperature. Duplex ageing provid provides es a higher density and greater uniformity of distribution of precipitate than is possible in one-step ageing at an elevated temperature (Wolverton 2001) … Duplex ageing leads to marginally superior resistance to stress corrosion cracking as compared to that of singlestep ageing.”

Moreover, the kinetics of this process was clearly illustrated on Hardness versus Aging Time Graph of various aluminum alloy specimens aged at different temperature (see Figure 14).

Figure 14. Kinetics of Second-Phase Precipitation Aging

It can be inferred from the graph that operating duplex aging at higher temperatures correspond to shorter aging times required before the maximum hardness is obtained. Note that this trend is similar to the one obtained from this experiment (refer to Figure 6 ) where after reaching a certain aging time, hardness yield start to drop as the solution homogenize. Over-aging therefore has to be prevented if the goal is to achieve the maximum hardness. The reason behind the observed drop in hardness after a certain aging time was further elaborated in ASM Metals Handbook [1] given by the following excerpt: “With further heating at temperatures that cause strengthening or at higher temperatures, the precipitate particles grow, grow, but even more

importantly convert to the equilibrium phases, which generally are not coherent. These changes soften the material and, carried further, produce the softest or annealed condition. ”

It must be understood that coherence is measured by the degree of disregistry between atomic spacings [1] on the lattice of precipitate and that this parameter determines the mechanism of dislocation movement for the resulting phase after precipitation hardening.

3.3

Possible Sources of Error

It was easily observable that the specimens  were insufficiently etched as the grains detected from the some photomicrographs were unclear. Choosing a more appropriate etchant, or perhaps reviewing the proper method of etching for the Keller’s etchant, should be facilitated. Employing Vickers Hardness Test involves  optically measuring the indent size which is highly subjective and can possibly lead to human error. Accurate hardness readings require for the  samples to be indented inden ted along a flat surface. surface. After solutionizing however, the three specimens undergone significant deformation. This reason made it barely possible to obtain a perfectly flat surface after the specimens were mounted to the polymer resin.

4. Conclusions Precipitation hardening operated with different aging conditions resulted to varying hardness yields for the aluminum sheet samples in this experiment. Highest hardness reading reported in the following increasing order of samples: as-received, naturallyaged, artificially-aged, and kept-in-freezer. Moreover, in the latter order of increasing hardness, second-phase precipitation and defined grain boundaries became more abundant. The mechanism of strengthening metal alloys by precipitation hardening entails precipitation of finegrained second phase particles which are uniformly dispersed serving as dislocation points in the lattice with a general effect of decreasing possibility of deformation upon application of force. When a shorter aging time to achieve maximum hardness is desired, aging conditions after solutionizing may be altered. For duplex heating, operating operating at higher temperature corresponds to a shorter aging time needed to attain maximum hardness. For aging at low temperature on the other hand, an even higher hardness reading may be achieved as homogenization occurs at a faster rate thereby


resulting to finer precipitates. In this condition though, the level of strength may bring about higher susceptibility to cracking and other undesirable mechanical properties. In the end, appropriate heat treatment is selected based on mechanical property specifications required for the intended function of the material especially for the case of aluminum alloys which are vastly in-demand for high specific strengths applications in automobile and aerospace industries [5].

5. References [1] ASM International Handbook Committee. ASM Metals Handbook Handbook Desk Desk Edition. 1998. eBook. [2]

Marceau R.K.W. “Solute Diffusion Characteristics of a rapid Hardening Al-Cu-Mg Alloy During the Early Stage of Age Hardening.” The Minerals, Metals & Materials Society and ASM DOI: DOI:10.1007/s116-0100252-0: n.pag. Web. 26 August 2011.

[3] Esmaeili S. S. “On the Precipitation-Hardening Precipitation-Hardening Behavior of the Al-Mg-Si-Cu Al-Mg-Si- Cu Alloy AA6111.” Metallurgical Metallurgical and Materials Transactions Transactions A Volume 34A, March 2003 – 751:n.pag. Web. 26 August 2011. [4] Kumar S. “Precipitation Hardening and Hydrogen Embrittlement of Aluminum Alloy AA7020” Indian Academy of Sciences. Bull. Mater. Sci., Vol. 34, No. 2 (2011) pp. 309 – 309 – 319. 319. Web. 26 August 2011.

[5]

Li C. “Microstructure and Mechanical Properties of Multicomponent Aluminum Alloy Journal of Materials by Rapid Solidification.” Journal Engineering and Performance Performance 18(1) – 79 (2009): n. pag. Web. 26 August 2011.

Precipitation Hardening of [6] A l u m i n u m A l l o y . W e b . 2 6 A u g 2011..

[7] U d o m p h o l , T . . " L a b o r a t o r y 2 : N o t e b o o k Hardness Testing." M a n u a l s Guide . Mechanical Metallurgy Laboratory , n.d. Web. 26 Sept 2011.


6. Appendices 6.1 Tables and Graphs Table 1. Hardness Readings of Aluminum Specimens CALCULATED HARDNESS in HV 5-Aug

Specimen

9-Aug

10-Aug

12-Aug

15-Aug

17-Aug

As-Received

20.55

—

—

—

—

—

Naturally-Aged

26.92

30.48

32.11

29.71

—

—

Artificially-Aged

—

41.31

—

—

—

—

Kept-In-Freezer

—

—

—

—

—

43.89

Table 2. Data for Naturally-Aged Specimen Graph Naturally-Aged Hardness Reading

Aging Time (in Days)

Log (Aging Time)

26.92

2

0.30103

30.48

6

0.77815125

32.11

7

0.84509804

29.71

9

0.95424251

Figure 6. Graph of Hardness versus Logarithm of Aging Time for the Naturally-Aged Specimen

Hardness vs. Log(Aging Time) Naturally-Aged Specimen 33 32 31 30 29 28 27 26 0. 3

0. 4

0. 5

0. 6

0. 7

0. 8

0. 9

1


6.2

Photomicrographs of Aluminum Specimens (1000x magnification)

Figure7. Figure7. As-Received Specimen

Figure9. Figure9. Naturally-Aged Specimen (Day 6)




Figure12. Figure12. Artificially-Aged Specimen

Figure13. Kept-In-Freezer Specimen


Precipitation Hardening of Aluminum Alloys

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