eas 107 lab2

Mohd Ashraf Mohd Ismail

Laboratory Experiment 2

Name : Mohammed Ashraf Bin Mohammed Ismail

Student No: N0806406

Contact No: 98225529

Date Submitted: 29th September 2008

Group:

Lab. : Heat Treatment of Aluminuim

Course Instructor: Mr Beh Hang Meng

Mr Nah Kiat Gee

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Table of Contents

ABSTRACT .................................................................................................................. 3

INTRODUCTION ......................................................................................................... 4

HEAT TREATMENT OF ALUMINIUM ALLOY...................................................... 6

OBJECTIVES................................................................................................................ 7

ORIGINAL COMPOSITION FOR SPECIMENT ALUMINIUM ALLOY ................ 8

EXPIREMENT PROCEDURE ..................................................................................... 9

EXPIREMENT RESULT............................................................................................ 11

HARDNESS GRAPH.......................................................................................... 11

TENSILE GRAPH............................................................................................... 12

MAX TENSILE ELONGATION GRAPH ......................................................... 12

DISCUSSION OF RESULT........................................................................................ 13

CONCLUSION............................................................................................................ 14

REFERENCE .............................................................................................................. 15

APPENDIX.................................................................................................................. 16

Introduction to Introduction to Engineering Material and Aeromaterial

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Abstract

The heat treatment behavior of aluminium 2xxx(Duralumin) is investigated. In this experiment, the effects Precipitation Hardening are being studied and how it affects the formation and decomposition of precipitates and other phases of aluminum. The experiment makes extensive use of the Al2Cu equilibrium phase diagram. The results of the heat treatment are evaluated using the Rockwell Hardness Test and Tensile Elongation test. The analysis of the microstructure of certain specimen is also being carried out. We also use an electron microscope to find out the alloy composition of the specimen.

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Introduction

Introduction

Pure aluminium is too soft for most structural applications and therefore is usually alloyed with several elements to improve its corrosion resistance, inhibit grain growth and of course to increase the strength. The optimum strengthening of aluminium is achieved by alloying and heat treatments that promote the formation of small, hard precipitates, which interfere with the motion of dislocations. Aluminium alloys that can be heat treated to form these precipitates are considered heat treatable alloys. Many heat treatable aluminum alloys are not weldable because welding would destroy the microstructure produced by careful heat treatment.

Whilst pure aluminium has low specific gravity, good corrosion resistance and excellent thermal and electrical conductivity it is too weak and ductile to be used on its own. Aluminium alloyed with copper and heat treated correctly could be made far stronger. The alloy of aluminium with 4% copper is called Duralumin. These alloys have typically low specific gravity (around 2.7) and high strength (450 MPa). They are limited by a maximum service temperature of about 660°C.It is usually naturally or artificially aged. Virtually all heat treatable aluminum alloys are strengthened by precipitation hardening. The aim of heat treatment is to produce a large number of fine precipitates in the aluminium grains. These interfere with the movement of dislocations when the metal yields. This has the effect of increasing the strength of the alloy. The heat treatment used to produce the precipitates involves a high temperature solution treatment, quenching and then ageing.


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Figure 1 - Aluminium Copper Phase Diagram

Figure 2 - More detailed Aluminium Copper Phase Diagram

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Heat Treatment of Aluminium Alloys

The heat treatment of aluminium alloys is different from conventional heat treatment applied to steel. It consist of (a) Solution Treatment, and (b) Quenching (C) Aging.

a) Solution Treatment - This heat treatment of aluminum method is usually done to increase the strength of an alloy. It involves raising the temperature of the alloy into the single phase region so that all of the precipitates dissolve.(See (1) from figure 2 (550˚C )). Solution heat-treating aims to puts the maximum possible concentration of copper in the alloy in solid solution with the aluminium. The alloy is then rapidly quenched to form a supersaturated solid solution and to trap excess vacancies and dislocation loops which can later act as nucleation sites for precipitation. In this condition, the alloy will have a fine grain structure soft and very ductile.

b) Quenching- After solution treatment, the next step is quenching. During quenching, the sample is rapidly cooled, usually in water at room temperature, which gives through to a new structure. The structure that is formed from the water quenching is a supersaturated solid solution that is not in equilibrium. This happens because the atoms do not have time to diffuse to potential nucleation sites. The rapid quenching also improves resistance to corrosion and stress-corrosion cracking, creating the highest strengths attainable and also the best combinations of strength and toughness in the alloys.

c) Aging - There are two types of aging, natural and artificial.

Natural aging can be accomplished by allowing the alloy to sit air cooled.

Artificial aging requires that the supersaturated solid solution be reheated at a temperature below the solvus temperature to start the aging, which allows atoms to diffuse only short distances. The purpose of aging is to produce the finely dispersed precipitates. The fine precipitates in the alloy impede the movement of the dislocations during the deformation by forcing the dislocations to either cut through the precipitates or go around them. Eventually if we hold the alloy for a sufficient time at the aging temperature, the equilibrium ( ) structure is produced. We would produce ultra fine uniform dispersed 2nd phase precipitates particle.

The first step in the process of aging is the formation of GP Zones(Guinier-Preston). GP Zones are solute atoms that have diffused into coherent clusters. Coherent clusters are clusters of the solute atoms that distort the crystal structure, but are still connected to the rest of the crystal structure. Incoherent(Overaged) clusters are clusters that have grown too big, and that do not connect to the rest of the matrix. Incoherent structures actually weaken the alloys because they do not impede dislocation movement the way coherent clusters do, instead they allow dislocations to slide by. As aging continues, more copper atoms diffuse to the precipitate and the GP I Zones thicken into thin disks called GP II Zones. If diffusion continues, the GP II Zones will


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grow into coherent equiaxed theta prime precipitates. Finally, incoherent stable theta precipitates form, as shown in figure 3, which means that the alloy has been overaged (Smith W., 181).

How long to let the material age and which temperature (Time and Temperature) for the heat treatment during aging is very important.

Figure 3. (a) A noncoherent precipitate has no relationship with the crystal structure if the surrounding matrix. (b)

A coherent precipitate forms so that there is a definite relationship between the precipitate’s and the matrix crystal

structure. (Askeland 1994)

Objectives

From the experiment we were able to :

I. Find out the Alloy composition of the Al-Cu Alloy.

II. How different types of teat treatment process of aluminum. (Solution

Treatment, Quenching and Age Hardening) affect the hardness and tensile

strength.

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Original Composition of Aluminum Alloy

Spectrum processing :

Peak possibly omitted : 0.263 keV

Processing option : All elements analysed (Normalized)

Table 1 - Elements Composition in Aluminium Alloy

Element App Intensity Weight% Weight% Atomic %

Conc. Corrn. Sigma

Al K 47.83 1.1378 94.37 0.33 97.63

Mn K 0.27 0.8405 0.72 0.11 0.37

Cu K 1.55 0.8516 4.09 0.26 1.80

Ag L 0.23 0.6247 0.82 0.19 0.21

Totals 100.00

Figure 4 - Aluminium Alloy Composition


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Experimental Procedure

We used 8 specimen for the experiment.

Step 1) We heat up the specimen as stated below

Specimen 1. (O) – Original State (Unchanged)

Specimen 2. (A) – Heat up at 165ºC for 10 hrs. Air-cooled.

Specimen 3. (B) – Heat up at 545 ºC for 45 min Quench in water, Reheat to 190ºC for 30 min. Air-Cooled.

Specimen 4. (C) - Heat up at 545 ºC for 45 min Quench in water, Reheat to 190ºC for 90 min. Air-Cooled.

Specimen 5. (D) - Heat up at 545 ºC for 45 min Quench in water, Reheat to 260ºC for 5 min. Air-Cooled.

Specimen 6. (€) - Heat up at 545 ºC for 45 min Quench in water, Reheat to 260ºC for 60 min. Air-Cooled.

Specimen 7. (∆) - Heat up at 545 ºC for 45 min Quench in water, Reheat to 190ºC for 150 min. Air-Cooled.

Specimen 8. (BB) - Heat up at 545 ºC for 45 min Quench immediately in water, Air-Cooled

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Step 2) After specimen is cooled to room temperature, we take the reading for the hardness test using the Rockwell hardness test 16mm ball (HRB)

Step 3) When the hardness test has been done, we proceed to do the tensile stress. After the specimen has been plastically being deformed we print out the tensile chart

Step4) After all the reading has been found and recorded, we tabulate the result in a table form. For clear comparison, we plotted a chart to compare the hardness and tensile elongation with all the specimen.

Note: Only for the original specimen, (O) we magnify the aluminum alloy to find out the alloy composition using an electron microscope.

Figure 5 - Furnace

Figure 3 - Electron Microscope Figure 4 - Tensile Test Machine

Figure 6 - Image Analyzer (Microstructure)


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Experiment Result

Hardness Test

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Tensile Strength

Maximum Tensile Elongation


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Discussion of Result

During aging, the alloy can either be underaged, overaged, or critically aged. When

item is being underaged or overaged, there always some loss in yield strength.

From the graph above, there are benefits using to using lower aging temperature.

1st the maximum strength increase as the aging temperature decrease. Specimen ‘A’ should have the best yield strength(heated at 165˚C for 10 hrs)

2nd The alloy maintains its maximum strength over a longer period of time.

3rd The properties are more uniform. Example Specimen ‘D’(260˚C) has the poorest hardness test result. By right it should have been ideally strengthen because it’s aged at 260˚C for 5 min (0.1h = 6 min)(see graph above). Most probably only the surface of the specimen reached the proper temperature and strengthens but the centre still remains cools and ages slightly or probably have yet to.

Specimen ‘∆’(190˚C) and Specimen ‘€’ (260˚C) Overage . This happens because the theta precipitates grow into incoherent grains of solute atoms that are not effective in stopping the movement of the dislocations.

Specimen ‘B’(190˚C), Specimen ‘C’(190˚C) - Critically Aged. They have the best hardness and tensile strength.

Natural Aging requires long time often several days to reach maximum strength. However the peak strength is higher and more stable than that obtained in artificial aging and no overaging occurs.

Despite our best efforts, we still found some discrepancy in our result that might due to : Not letting the specimen cool down enough before all the test are being carried out. Human Error – Improper set up of experiment and calculation error.

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Conclusion

Precipitation hardening is a heat-treatment process that increases strength and hardness in an alloy. There are three steps in the process of precipitation hardening: solution treatment, quenching, and aging. During these three steps the alloy is transformed to a homogeneous, one phase solution, rapidly quenched in a medium such as water, and then aged either naturally or artificially. Aluminum based alloys are commonly hardened and strengthened through precipitation hardening.

Different alloy groups have different strengths and structural applications. For example, the 6xxx group is used for general structural purposes while the 7xxx group is used for aircraft structures. Precipitation hardening strengthens materials by forming coherent clusters of precipitate that impede dislocation movement. Precipitation hardening is an economical way to increase the performance of common alloys and is widely used.


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Reference

1) http://www.avcorp.com/heat-treatment-of-aluminum.asp

2) http://www.key-to-metals.com/Article39.htm

3) http://www.metalbashatorium.com/heat_treating_aluminium.htm

4) http://pwatlas.mt.umist.ac.uk/internetmicroscope/micrographs/microstructures

/more-metals/al-alloys/age-hardened.html

5) http://www.hsc.csu.edu.au/engineering_studies/aero_eng/2580/aluminium_all

oys.html

6) http://en.wikipedia.org/wiki/Aluminium_alloy

7)

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Appendix I

eas 107 lab2

Documents

alloy of aluminium

conventional heat treatment

careful heat treatment

aim of heat treatment

heat treatments

heat treatable alloys

speciment aluminium

aluminium grains