Optimizing the Parameters in Heat Treatment for Achieving High Hardness and Efficient Bending

Mechanics, Materials Science & Engineering, September 2016

ISSN 2412-5954

Optimizing the Parameters in Heat Treatment for Achieving High Hardness and Efficient Bending of Thin BS 2014 Aluminium Alloy Sheets Abirami Priyadarshini B. 1

GTN Engineering India Ltd, Tamil Nadu, India DOI 10.13140/RG.2.2.10632.42242

Keywords: hardness measurement, aluminium alloys, bending, aging.

ABSTRACT. The present work targets in setting a standard heat treatment procedure for obtaining high hardness values of the order of 80 HRB in BS 2014 aluminium alloy sheets of 2mm thick commonly used in aerospace industries. A hardness range of 60HRB to 72HRB is possible in low thickness sheets as stated in the standard BS EN 485-2:2013.

Experiments were performed to achieve higher hardness values by controlling the heat treatment temperatures thereby understanding the ageing mechanism of the Al-Cu alloy to a wider extent. The validated process sequence in turn resulted in complications where bending of the sheets resulted in cracking. Further investigation was performed and it was found that the BS 2014 alloy has to be bent within two hours of solution annealing in order to have an efficient bending. The results showed that the natural ageing is so rapid in this alloy, which strengthens the material so quickly by the formation of CuAl2 precipitates, thereby, demanding the bending procedure to be performed before the growth of precipitates becomes dominant.

Introduction. The research and innovation at the aircraft industry focuses on reducing the weight of the aircraft for improving the efficiency, safety and performance. It also demands a positive step in environmental and economic factors thereby resulting in a favorable combination of high corrosion resistance, fatigue resistance, formability and strength coupled with low density[1]. Aluminium is one of the most important materials facing these challenges where it finds a wide variety of applications in the aerospace industry depending on their complexity and performance requirements.

With copper as the main alloying element, the 2xxx series of aluminium alloys are of significant interest possessing high strength to density ratio and thereby being used for structural applications in variety of fields such as the aviation and military sectors[2], [3].

Aluminium, being a sheet material, demands a predominant level of bending and forming. Among the 2xxx series, the BS 2024 is the most popular alloy used in the manufacturing of aircraft skins, cowls and structures[4]. Currently, the BS 2014 aluminium alloy is gathering attention due to its ability to achieve higher hardness and therefore it is used mainly for the interface beam assembly in aircraft structures and casings. These applications require a balance to be struck between the higher degree of hardness produced with the ability to bend and form the alloy. Work was performed in identifying the precipitates that are responsible for hardening the 2014 Al alloy where the due to the presence of copper was of significant importance[5]. A good combination of mechanical properties can be achieved by controlling the precipitation mechanism where the elements such as Magnesium and Silicon are also responsible in improving the hardness of this alloy[6].

In the present work, the BS 2014 alloy was targeted to produce an increased hardness by optimizing the heat treatment factors thereby having an efficient control over the ageing mechanism. BS EN 4852:2013 states a maximum hardness of 72 HRB that could be achieved in a 2014 alloy[7]. However, this alloy has been studied widely for its ageing process where the precipitates are solely responsible in hardening the alloy resulting from the copper addition[2], [5]. Sadeler et al. studied the effect of T4 (solution treated and naturally aged) and T6 (solution treated and artificially aged) tempers where they concluded that the T6 temper has positive effects on the mechanical properties of this alloy[8].

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Mechanics, Materials Science & Engineering, September 2016

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Various failures were reported from the aircraft industries where there were issues in the bending of this alloy even if the required hardness was achieved. This defect from the application side of industries demanded a more appropriate methodology in order to successfully bend these alloys considering the mechanism of hardening. Hence, work was also done in performing a successful 90o bend without the formation of cracks where an efficient heat treatment cycle was investigated and the process was optimized.

Experimental work. The BS 2014 aluminium alloy used in this investigation was procured at the T6 temper condition owing to its better properties compared to the T4 temper[8] and was subjected to a chemical analysis treatment which had the composition as shown in table 1. The alloy plates with the dimensions of 2mm x 65mm x 300mm were considered for the experiments. Also, the standard BS EN 485-2:2013 states a hardness value of 72 HBW for a thickness of 1.5-3 mm and hence a thickness of 2 mm was considered for performing a comparison in the achieved hardness.

Table 1. Chemical analysis of 2014 aluminium alloy.

Elements Specified values[9] (%) Observed values (%) Cu

3.8-5.0

4.051

Si

0.5-1.2

0.941

Fe

0.70 max

0.151

Mn

0.3-1.2

0.714

Mg

0.2-0.8

0.546

Cr

0.3 max

0.004

Zn

0.2 max

0.034

Ti

0.3 max

0.023

Al

Remainder

93.458

Initially seven sample plates were considered and prepared for undergoing the heat treatment trials.

The surface was cleaned to remove any foreign bodies, oxides and impurities if present. The 2014 aluminium sheets that satisfied the standard composition were cut into the required dimensions using laser-cutting process. The trials that were performed had the following sequence as depicted in table 2. It should be noted that the various trials performed had different process parameters where every trial sequence followed the standard heat treatment procedure for Aluminium alloys.

Various standards such as the AMS-H-6088 B, MIL-S-10699B, ASTM-B597-1992 and IS: 88601978 were considered in selecting the appropriate temperatures of heat treatment. These standards provided the code of practice for the heat treatment of aluminium alloys and the required conditions that are maintained throughout the process. These factors essentially include the salt composition, heat treatment baths and the procedure of heat treatment.

Furnace annealing was done at certain trials at a temperature of 410oC for two hours. The sample was then furnace cooled with a maximum cooling rate of 28oC per hour until the specimen reached 260oC which was then followed by air cooling. This is an important pre-step to solution annealing for the effective dissolution of precipitates in order to avoid cracking.

Solution annealing was done at 510oC for 35-40 minutes with water as the quenching medium. The main purpose of solution annealing was to achieve proper homogenization of the alloy to facilitate an efficient aging mechanism.

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Mechanics, Materials Science & Engineering, September 2016

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Table 2. Sequence of trials for the heat treatment process.

Trial number

Process sequence

Trial 1

Laser cutting

Bending

Trial 2

Laser cutting

Solution annealing

Trial 3

Laser cutting Solution annealing solution annealing Aging

Trial 4

Laser cutting Solution annealing annealing Aging

Trial 5

Laser cutting Furnace annealing within 6 hours of solution annealing

Solution annealing Aging

Bending

Trial 6

Laser cutting Furnace annealing within 4 hours of solution annealing

Solution annealing Aging

Bending

Trial 7

Laser cutting Furnace annealing within 2 hours of solution annealing

Solution annealing Aging

Bending

Aging

Bending

Bending within 12 hours of Bending within 6 hours of solution

The bending of the alloy sheets was performed using the Yawei bending machine with a capacity of 220 tons for various trials as mentioned in table 2 depending on the time after solution treatment. The sample under trial 7 (see table 3) that passed the bending test was approved and considered for studying the aging mechanism to achieve the required hardness.

As the heat treatment procedure for bending is now validated, eight other samples of 2014-T6 were subjected to the process of laser cutting, furnace annealing, solution annealing, straightening and bending within 2 hours of solution annealing according to trial 7. These samples were successfully bent and were subjected to the aging process with a temperature of 175oC. The soak time varied from 2 hours to 18 hours to see the variation in hardness produced depending on the temperature changes (see table 5). The hardness was measured using a standard Rockwell hardness tester at B scale.

Results and discussion Bending factors and parameters. The trials performed using different sequences of heat treatment yielded the following results after bending.

Table 3. Bending results of the trials performed Trial number

Process validation

Trial 1 -Trial 6

Failed due to the formation of cracks

Trial 7

A successful 90o bend performed without the formation of crack or irregularities.

The sample under the trial 1 methodology failed as expected, as there were no surface modifications performed. The bending of this sample resulted in the obvious formation of crack thereby leading to breakage. The sample under trial 2 had a complete heat treatment cycle following the theoretical reasoning where the solution annealing, quenching and ageing resulted in a significant formation of the precipitates. This sample also failed due to the formation of cracks, which is a result of the precipitation of CuAl2 along with various other insoluble compounds. The material has a tendency to MMSE Journal. Open Access www.mmse.xyz

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Mechanics, Materials Science & Engineering, September 2016

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naturally age at a rapid rate thereby resulting in the growth of precipitates leading to a significant improvement in hardness. However, this results in increasing the brittleness of the material leading to the formation of cracks.

A different methodology was followed where bending of the material was performed within a certain specific time after solution annealing to assess the rate of growth of the precipitates that affects bending. In order to achieve this, a time limit has to be deduced for performing the bending before the precipitates start to age. Trails 3 to 7 were bent within a specified time after solution annealing with the time available for bending reduced from 12 hours to 2 hours where a successful bend was performed.

Fig. 1. Failure of the sheets.

Time is a critical parameter where the bending was totally dependent on the rate of growth of precipitates which in turn increases the hardness and brittleness of the thin sheet. Thus, for an efficient bending of the Aluminium 2014 alloy, the thin sheet has to be bent within two hours of solution annealing so that the material can be formed before the rate of growth of precipitates reaches the critical limit. The failure of the sheets is as depicted in Fig. 1 when undergoing the process from trial 1 to trial 6.

Parameters for high hardness. It was also noted that the aerospace industries require a certain minimum hardness that has to satisfy the component working conditions. After performing a successful bend, the target was laid on achieving a high hardness value for the bent sheet and this was satisfied by setting the proper ageing time, restricting the over ageing process. In order to establish this target, the hardness values were recorded as shown in table 5.

Table 4. Hardness obtained.

TRAIL NOS 1 2 3 4 5 6 7 8

TEMP

SOAK TIME

OBTAINED HARDNESS

2.0 Hrs 4.0 Hrs 6.0 Hrs 8.0 Hrs 10.0 Hrs 12.0 Hrs 14.0 Hrs 18.0 Hrs

48 HRB 49 HRB 59 HRB 68 HRB 78 HRB 67 HRB 65 HRB 63 HRB

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Mechanics, Materials Science & Engineering, September 2016

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It was seen that the hardness value at 175oC for an ageing time of 10 hours yielded high hardness value of 78 HRB for the heat treatment parameters that was considered throughout this experiment.

Beyond this time, over ageing occurred where the hardness values dropped down. The sample surfaces were polished with the use of Silicon carbide paper and diamond paste and then etched by microscope after every ageing cycle so that the growth of the precipitates can be efficiently related to its hardness as shown in Fig. 3. It can be seen that the growth of precipitates is so rapid where they reach a maximum hardness at the time of ten hours. The precipitates might act as a stress raiser where the crack propagation starts to initiate. The microstructures confirmed the rapid growth of precipitates, which supports the experimental hardness values that are obtained.

Precipitate formation. The evolution of hardness at the performed trials is directly proportional to the Cu-Al precipitates that are formed. Various research in the past confirms the precipitates to be CuAl2 phase where during the process of quenching, Cu is contained as a super saturated solid solution in the Aluminium rich phase at room temperature[8]. During the aging phase, the combination of copper and aluminium results in the formation of fine crystals of CuAl2 in the solution.

The increase in hardness values are a result of the formation of these crystals owing to the solubility of copper in aluminium. From the microstructures obtained in Fig. 3, it is evident that the CuAl2 phase is present by the difference in contrast that is produced. The main elements of the microstructure are characterized as dark, insoluble precipitates composed of complex compounds such as Fe, Mn, Al, Si and also the presence of particles of CuAl2 phase which are the white areas in a matrix of solid solution[8]. It can also be seen that the condition of reduced hardness obtained after the aging time of 11 hours produced a state of over aging as shown in Fig. 2. Hence, for achieving the maximum hardness, the region showing a maximum peak was utilized thereby fixing the aging time to 10 hours at a temperature of 170oC. These parameters yielded hardness values that were higher than the hardness mentioned in the standard[7]. It has to be noted that aging was performed after the samples were bent so that the required shape of the component can be progressed to the desired level of hardness without failure.

80

Hardness (HRB)

75 70 65 60 55 50

45 40 0

2

4

6

8

10

12

14

16

Aging time (Hours)

Fig. 2. Hardness vs Aging time.

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Mechanics, Materials Science & Engineering, September 2016

a

b

c

d

ISSN 2412-5954

Fig. 3. Optical microscope images showing the growth of precipitates at (a) 2h (b) 8h (c) 10h (d) 14h taken at 200x.

Summary. The various samples of 2mm thin sheets of Aluminium 2014 alloy were optimized for heat treatment and bending parameters, where the following conclusions were drawn.

(1) The bending of aluminium 2014 alloy has to be performed within two hours of solution annealing, as the degree of natural aging in this alloy is significantly high. A successful bend can be performed within that time before the rapid growth of precipitates significantly increases the hardness resulting in cracking of the sheets during bending.

(2) The aging of the specimen after bending yields high hardness values than the standard hardness mentioned in BS EN 485-2:2013 when the process of aging is carried out for 10 hours at a temperature of 175oC. The furnace annealing proved to have a positive impact by being a successful pre-process to solution annealing. Proper control of temperature and environment resulted in a hardness of 78 HRB which is more than the value that was previously achieved.

The improved parameters for achieving high hardness value with successful bending will be highly desired by the aerospace industries where thin sheets of aluminium 2014 alloy plays a significant role. The scope of the future work lies in improving the aging conditions to perform a successful bend and achieve higher hardness values for thinner sheets of the order of 1mm that will significantly improve the efficiency of weight reduction in an aircraft. Work is also demanded in areas of fracture mechanics where the mode and mechanics of fracture in this alloy can be analyzed to a greater extent.

Acknowledgement. I take this opportunity to express my gratitude to Mr.K.B. Babu, CEO, GTN Engineering India Ltd for permitting me to undertake a project at his reputed industry. His constant support and guidance is highly appreciated. I also thank Mr. K. Vijayabaskar, Chief Operations officer, GTN Engineering India Ltd for his extended support throughout the course of this project.

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Mechanics, Materials Science & Engineering, September 2016

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In addition, I gratefully acknowledge the assistance and contribution of my guide, Mr. Gowtham, Operations manager, GTN Engineering India Ltd, for his cordial support, valuable information and guidance, which helped me in completing this task through various stages.

References Material Science Enineering. A, vol. 528, no. 22 23, pp. 7068 7076, 2011, doi: 10.1016/j.msea.2011.05.055 - Cu - Mg / Al - Zn Journal of Alloys and Compounds, vol. 684, pp. 195 200, 2016, doi: 10.1016/j.jallcom.2016.05.132 [3] I J Polmear, Light alloys: Metallurgy of the light metals. 1995. info/articles/aircraft-aluminum.php. [Accessed: 10-Aug-2016].

[5

Cu

Si

Mg

298, 2005. vol. 2344236, pp. 165 168, 1997.

[7] BS EN 485Mechanical properties.

Sheet , strip and plate

Aluminium Alloy with the Age-Har

277, 2014.

-

2014.

Cite the paper Abirami Priyadarshini B. (2016). Optimizing the Parameters in Heat Treatment for Achieving High Hardness and Efficient Bending of Thin BS 2014 Aluminium Alloy Sheets. Mechanics, Materials Science & Engineering Vol.6, doi:10.13140/RG.2.2.10632.42242

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