STUDY OF THE EFFECT OF AGING CONDITION ON STRENGTH AND HARDNESS OF 6063-T5 ALLOY

Supervised by : Dr. G.I.P. De Silva

Presented by : E.M.A.N. Ekanayaka S.A.D.T. Dharmarathna

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INTRODUCTION Aluminium - The most abundant metal in the earth

crust • 8% by weight of the earth’s solid surface

Properties - Durability, light weight, good extrudability and surface finish

Pure metal and the alloy used as alternatives for other metals (ferrous and non-ferrous), ceramics and wood

Sri Lankan demand • Structural applications: Window and Door Frames,

Partitioning, L bars, U bars

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ALUMEX (PVT) LTD.

Project was industrially focused on “Alumex”

Product: Extruded Aluminium articles Raw material: 6063-T5 Aluminium alloy

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ISSUES

Production cannot meet the present increased demand

Relatively high cost of products

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PRODUCTION PROCESS

Casting

Homogenizing

Extrusion

Age Hardening

Surface Treatments

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REMEDY

Reduction of time in the Age Hardening process

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OBJECTIVES

To reduce the Age Hardening Time

To reduce the Energy Consumption

To upgrade the Mechanical Properties

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LITERATURE REVIEW

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6063-T5 Aluminium alloy

6063 Age Hardenable Aluminium alloy • Main alloying elements: Mg (0.2 ~ 0.6 wt%)

Si (0.45 ~ 0.9 wt%)

T5 - Cooled from an elevated temperature and artificially aged

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6063-T5 Aluminium alloy

Second Phase: Mg2Si

Solid solubility of Mg2Si decreases from 1.85 wt. % at the eutectic temperature of 595 °C

Al-Mg2Si quasi binary system forms

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Age hardening

Maximum hardness is achieved if the precipitates can resist cutting by dislocations, and are too close to permit by-passing of dislocations.

Strengthening a metal by introducing small particles of another phase which barriers dislocations motion

Cutting through:When the precipitates are too small

Bowing and By pass:When precipitates are too strong to be cut and inter-particle space become large

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The Age Hardening Process

SSSS

Solution treatment

Age hardening treatment

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Al-Mg2Si quasi binary system

GP zones – First form of precipitates (unstable) Needle Shaped with the long axis along [100] of the matrix

Sequence of precipitates in Al-Mg2Si

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Al-Mg2Si quasi binary systemSequence of precipitates in Al-Mg2Si

β΄ phase – Developed rod shape with Hexagonal crystal structure

Maximum hardness

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Al-Mg2Si quasi binary systemSequence of precipitates in Al-Mg2Si

β phase - Equilibrium phase with FCC crystal structure Alloy is over aged – Hardness decreases

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Al-Mg2Si quasi binary system

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Closely spaced fine precipitates • Resist dislocation Bowing and By pass

Strong large precipitates• Resist cutting by dislocations

This is called a Bimodal Precipitate Structure

Closely Spaced Fine Precipitates

Strong Large Precipitates

Increased Hardness+ =

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CONCEPT

Two Step Aging Process

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Homogeneous Nucleation of a Solute Cluster

r = radius of solute clusterΔG = free energy needed to form a spherical cluster of radius rGV = change in free energy per unit volume

σ = surface free energy per unit area

rc = critical radius of the cluster

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Gibbs-Thompson equation

r c x lnS = K

S = Amount of super saturation at a particular temperature

K = Temperature dependent constant ( K a 1/ T )

rc= Critical radius of a cluster at the relevant temperature

When T increases, rc increases

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At temp. T1 clusters nucleate and

grow - Size distribution: rmin – rmax

When temp. is raised from T1 to T2,

critical radius is raised from rc1 to rc2

If cluster radius r > rc2, the cluster will survive and continue to grow

If cluster radius r < rc2, the cluster will be unstable and will dissolve. But re-nucleation may occur.

This results a Bimodal Precipitate Structure with both closely spaced fine precipitates + strong large precipitates, which results better Mechanical Properties.

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Industrially Practiced Age Hardening ProcessSolution treatment

Age hardening treatment

Process was re-performed within the laboratoryResults were used as reference values

• Measured Hardness(HV) – 47.05• Total Time (Age Hardening) – 270 min

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Parameters Varied During the Process

1st step temperature - T1

Time to reach the 1st step temperature - t1

Soaking time in the 1st temperature - t2

2nd step temperature - T2

Time to reach the 2nd step temperature - t3

Soaking time in the 2nd temperature – t4

T2

T1

t5t4t3t2t1

Temperature (oC)

Time (min)

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LIMITATIONS

Furnace Limitation• The industrially acceptable range: 150oC to 250oC

Energy Consumption

Total Time Consumption• Below 270 min

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CONSTANTSTime to reach the 1st step temperature: t1

• 60 minutes

2nd step temperature: T2

• 225oC

Time to reach the 2nd step temperature: t3

• 30 minutes

T2

T1

t5t4t3t2t1

Temperature (oC)

Time (min)

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STAGE 1 - VARIABLES1st step temperature: T1

• Altered within150oC-200oC

Soaking time in the 1st temperature: t2

• Varied from 45 min- 90 min for each set of temperatures

Soaking time in the 2nd temperature: t4

• Varied Combinations-15 min and 30 min

T2

T1

t5t4t3t2t1

Temperature (oC)

Time (min)

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All Specimens were Solution Treated

• At 540oC for 3 hours

• To remove age hardening imposed • Dissolve all precipitates

Muffle Furnace

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A set of combinations among the above variables were developed

Heat treatments were performed using the Super C furnace for 2 samples per combination.

Super C Furnace

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Hardness was tested using Vickers Hardness tester

• 3 per sample 6 per combination• Average was recorded

Optimum suitable parameters determined using hardness obtained

Vickers Hardness Tester

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Aging Time and Temperature Combinations

1st step 2nd stepHardness

(HV)Temperature Time Temperature Time

150oC 60 min 225oC 15 min 37.85

150oC 90 min 225oC 15 min 38.25

175oC 45 min 225oC 15 min 39.10

175oC 60 min 225oC 15 min 41.68

175oC 75 min 225oC 15 min 47.58

175oC 90 min 225oC 15 min 45.93

200oC 60 min 225oC 15 min 35.05

200oC 90 min 225oC 15 min 37.87

t4 – maintained as 15 minT1 – varied from 150oC to 200oC

T2

T1

t5t4t3t2t1

Tem

pera

ture

(o C)

Time (min)

Hardness – Not Satisfactory

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t4 – maintained as 30 min

T1 – varied from 150oC to 200oC

T2

T1

t5t4t3t2t1

Tem

pera

ture

(o C)

Time (min)

1st step 2nd stepHardness

(HV)Temperature Time Temperature Time

150oC 60 min 225oC 30 min 41.47

150oC 90 min 225oC 30 min 41.25

175oC 45 min 225oC 30 min 40.92

175oC 60 min 225oC 30 min 51.68

175oC 75 min 225oC 30 min 52.05

175oC 90 min 225oC 30 min 43.78

200oC 60 min 225oC 30 min 36.42

200oC 90 min 225oC 30 min 40.62

Reference Hardness (HV) – 47.05

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DERIVATION1st Step Temperature (T1) : 175oC1st Step Soaking Time (t2) : 60 min

Rejections• 150oC – Low hardness in acceptable time duration

• 200oC – Higher energy consumption

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Current Status 1st step temperature : 175oC Time to reach the 1st step temperature : 60 min Soaking time in the 1st temperature : 60 min 2nd step temperature : 225oC Time to reach the 2nd step temperature : 30 min

225

175

30t4306060

Tem

pera

ture

(o C)

Time (min)

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2nd step soaking time “t4” was altered

• 0 ~ 60 min – 10 min intervals

Different sets of combinations were developed Samples prepared as standard tensile test

specimens

Tensile Test Sample

1.72mm(Gauge Length)

150 mm

14mm

66 mm

STAGE 2

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Heat Treatment - Super C• 2 specimens per combination

Hardness - Vickers Hardness Tester• 3 per sample 6 per combination• Average was recorded

Tensile Strength – Tensile Testing Machine

Tensometer

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Combinations and Results for varied “t4”

Sample noHeat Treatment

t4 (min) Hardness (HV)

Tensile Strength (N/mm2)1st Step 2nd Step

1 Reference _ 47.05 228.41

2 175oC - 60 min 225 oC - 0 min 0 45.08 170.27

3 175oC - 60 min 225 oC - 10 min 10 45.38 182.72

4 175oC - 60 min 225 oC - 20 min 20 46.83 199.34

5 175oC - 60 min 225 oC - 30 min 30 49.13 228.41

6 175oC - 60 min 225 oC - 40 min 40 51.10 240.86

7 175oC - 60 min 225 oC - 50 min 50 47.88 240.86

8 175oC - 60 min 225 oC - 60 min 60 45.33 232.56

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Graph of Hardness Vs “t4” value

0 10 20 30 40 50 6032.00

34.00

36.00

38.00

40.00

42.00

44.00

46.00

48.00

t4 value (min)

Har

dnes

s (H

V)

47.05

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Graph of Strength Vs “t4” value

0 10 20 30 40 50 60100.00

120.00

140.00

160.00

180.00

200.00

220.00

240.00

260.00

170.27

182.72

199.34

228.41

240.86 240.86

232.56

t4 value (min)

Tens

ile S

tren

gth

(N/m

m2)

228.41

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Theoretically• Absorbed heat energy (E) = mc

E= Heat energym = Mass of samplesc = Specific Heat Capacity = Temperature Difference

• Since m and c are constant • Energy Ratios = Ratio of areas under the graphs

Temperature (oC)

Time (min)

Energy Comparison

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EFFECTIVENESS – Varied “t4” Sample no

Heat Treatmentt4 (min) Total Time

(min)% Time Saving

% Energy Saving

1st Step 2nd Step

1 Reference_

270 0 0

2 175oC - 60 min 225oC- 0 min 0 180 33.33 45.59

3 175oC - 60 min 225oC- 10 min 10 190 29.63 39.96

4 175oC - 60 min 225oC- 20 min 20 200 25.93 34.33

5 175oC - 60 min 225oC- 30 min 30 210 22.22 28.71

6 175oC - 60 min 225oC- 40 min 40 220 18.52 23.08

7 175oC - 60 min 225oC- 50 min 50 230 14.81 17.45

8 175oC - 60 min 225oC- 60 min 60 240 11.11 11.82

Optimum was selected considering above results

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225

175

3040306060

Tem

pera

ture

(o C)

Time (min)

205

90

Tem

pera

ture

(o C)

Time (min)30150

Developed Process Process at Alumex

Hardness (HV) = 51.10

Tensile Strength (N/mm2) = 240.86

Total Time (min) = 220

Hardness (HV) = 47.05

Tensile Strength (N/mm2) = 228.41

Total Time (min) = 270

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Microstructure ObservationsMicrostructure

• Selected sample and reference• Viewed using Metallurgical microscope (X200)• Idea about grain size

Metallurgical Microscope

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After optimized Two Step Aging Treatment

Polishing Agent: Polycrystalline Diamond Powder Etchant: 5% HNO3 + 2% HF Solution

Microstructure Observation

After “Alumex” Practiced Aging Treatment

Polishing Agent: Polycrystalline Diamond Powder

Etchant: 5% HNO3 + 2% HF Solution

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225

175

3040306060

Tem

pera

ture

(o C)

Time (min)

PROGRESS

Property / parameter Practiced Process Developed Process

Hardness (HV) 47.05 51.10

Tensile Strength (N/mm2) 228.41 240.86

Total Time (min) 270 220

Time Saving (min) _ 50

% Time Saving _ 18.52

% Energy Saving _ 23.08

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CONCLUSION Considering Production Rate, Production Cost and

Enhanced Mechanical Properties the following Age Hardening Treatment is recommended.

225

175

3040306060

Tem

pera

ture

(o C)

Time (min)

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