98 98

Volume 46

Issue 2

December 2010

Pages 98-107

International Scientific Journal

published monthly by the

World Academy of Materials

and Manufacturing Engineering

Archives of Materials Science and Engineering

© Copyright by International OCSCO World Press. All rights reserved. 2010

Influence of chemical composition variation and heat treatment on microstructure and mechanical properties of 6xxx alloys

G. Mrówka-Nowotnik*Department of Materials Science, Rzeszow University of Technology, ul. W. Pola 2, 35-959 Rzeszów, Poland* Corresponding author: E-mail address: mrowka@prz.edu.pl

Received 10.09.2010; published in revised form 01.12.2010

ABSTRACT

Purpose: The main task of this work was to study the effect a the precipitation hardening on the microstructure and mechanical properties of 6061, 6063 and 6082 aluminium alloys.Design/methodology/approach: In this paper differential scanning calorimetry (DSC) and hardness measurements have been utilized to study the effect of a precipitation hardening on the mechanical properties in 6xxx aluminium alloys. The mechanical (Rm and Rp0.2) and plastic (A, Z) properties of the examined alloys were evaluated by uniaxial tensile test at room temperature. The microstructure was observed using optical microscope - Nikon 300, scanning electron microscope HITACHI S-3400 (SEM) in a conventional back-scattered electron mode.Findings: The results show that the microstructure and mechanical properties changes during artificial aging due a the precipitation strengthening process. Therefore, the parameters (time and aging temperature) of precipitation strengthening process that may lead to the most favourable mechanical properties of 6061, 6063 and 6082 alloys were determined.Practical implications: This paper is the part of previous author’s investigations which results in modification of the heat treatment parameters that may lead to the most favorable mechanical properties of 6xxx alloys.Originality/value: The paper has provided essential data about influence chemical composition and aging parameters on the microstructure and mechanical properties of 6061, 6063 and 6082 alloys.Keywords: Metallic alloys; Microstructure; Mechanical properties; Heat tretmeantReference to this paper should be given in the following way: G. Mrówka-Nowotnik, Influence of chemical composition variation and heat treatment on microstructure and mechanical properties of 6xxx alloys, Archives of Materials Science and Engineering 46/2 (2010) 98-107.

MATERIALS

1. Introduction The 6xxx-group contains magnesium and silicon as major addition elements. These multiphase alloys belong to the group of commercial aluminum alloys, in which relative volume, chemical composition and morphology of structural constituents exert significant influence on their useful properties [1-22]. In the

technical 6xxx aluminium alloys contents of Si and Mg are in the range of 0.5-1.2 wt%, usually with a Si/Mg ratio larger than one. Besides the intentional additions, transition metals such as Fe and Mn are always present. If Si content in Al alloys exceed the amount that is necessary to form Mg2Si phase, the remaining Si is present in other phases, such as Al-Fe-Si and Al-Fe-Si-Mn particles [2,4,6-13].

1. Introduction

The aluminum alloys of 6xxx group have been studied extensively because of their technological importance and exceptional increase in strength obtained by precipitation hardening. The 6xxx aluminium alloys are mostly used as extruded products, as well as for construction and automotive application. The ease with which these alloys can be shaped, their low density, their very good corrosion and surface properties and good weldability are factors that together with a low price these make them commercially very attractive. The precipitation of metastable precursors of the equilibrium

(Mg2Si) phase occurs in one or more sequences which are quite complex. The precipitation sequence for 6xxx alloys, which is generally accepted in the literature [11-13], is: SSSS atomic clusters GP zones '' ' (stable) Some authors [8] consider the GP zones as GP1 zones while the '' is called a GP2 zone. The most effective hardening phase for this types of materials is ''. The medium strength AlMgSi aluminium alloys are commonly processed by extrusion. Their extradubility depends to a large extent on chemical composition, casting condition and heat treatment parameters (eg. homogenization treatment) which determine the microstructure of the billet before extrusion.

2. Material and experimental

The investigation has been carried out on the commercial 6061, 6063 and 6082 aluminum alloys. Chemical composition of the alloys is indicated in Table 1. Table 1. Chemical composition of the investigated alloys, %wt Alloy Si Mg Mn Cu Fe Zn Ni Cr Ti

6061 0.78 1.07 0.15 0.35 0.16 0.042 0.007 0.35 0.029

6063 0.55 0.55 0.07 0.026 0.18 0.02 0.005 - 0.018

6082 1.0 0.76 0.56 0.022 0.16 0.013 0.004 - 0.023

Thermal processing of the investigated alloys included a

homogenization treatment and T6 heat treatment (artificially ageing after solution treatment). The temperature of homogenization treatment of 6xxx alloys were determined on the basis of literature data and calorimetric investigations. The samples in as-cast state were preheated in an induction furnace to temperature 575°C held for 72 hours and subsequently cooled to room temperature. Additionally all alloys were heated in a resistance furnace for 12 hours at 565°C and then quenched into a water. Subsequently the specimens were subjected to artificial aging at temperature 175oC up to 98 h. In order to determine an influence of time on the kinetics of ageing the Brinell hardness was measured.

DSC samples of the supersaturated 6061, 6063 and 6082 alloys were investigated in SETARAM Setsys thermal analyzer fitted with a scanning differential calorimeter module. The heat effects associated with precipitation of GP zones and intermediate metastable and stable strengthening phase (Mg2Si) were obtained by subtracting a super purity Al baseline run.

After artificial aging, a set of specimens were prepared for tensile testing to study the effect of T6 heat treatments on mechanical properties of the examined alloys. The specimens were strained by tensile deformation on Instron TTF-1115 servohydraulic universal tester at a constant rates at room temperature in according to standard PN-EN 10002-1:2004 [23]. Tensile properties (tensile and yield strength; elongation) were evaluated using round test specimens of 8 mm diameter and 65 mm gauge length (according to ASTM E602-78T [24] standard).

A metallographic investigations were performed on the samples at as-cast state after homogenization treatment and extrusion forging process. The microstructure of the alloys was observed using optical microscope - Nikon 300 on polished sections etched in Keller solution containing 0.5 % HF in 50ml H2O. The surfaces of fracture of the damaged samples were prepared to microscopic examination by scanning electron microscopy (SEM).

3. Results and discussion The microstructure of the 6061 alloy in as-cast state and after

homogenization is given in Fig. 1 - as an example of as-cast state of the investigated alloys. a)

b)

Fig. 1. Microstructure of examined 6061 alloy: a) as-cast state, b) after homogenization at 575oC/72h

99READING DIRECT: www.archivesmse.org

1. Introduction The 6xxx-group contains magnesium and silicon as major addition elements. These multiphase alloys belong to the group of commercial aluminum alloys, in which relative volume, chemical composition and morphology of structural constituents exert significant influence on their useful properties [1-22]. In the

technical 6xxx aluminium alloys contents of Si and Mg are in the range of 0.5-1.2 wt%, usually with a Si/Mg ratio larger than one. Besides the intentional additions, transition metals such as Fe and Mn are always present. If Si content in Al alloys exceed the amount that is necessary to form Mg2Si phase, the remaining Si is present in other phases, such as Al-Fe-Si and Al-Fe-Si-Mn particles [2,4,6-13].

The aluminum alloys of 6xxx group have been studied extensively because of their technological importance and exceptional increase in strength obtained by precipitation hardening. The 6xxx aluminium alloys are mostly used as extruded products, as well as for construction and automotive application. The ease with which these alloys can be shaped, their low density, their very good corrosion and surface properties and good weldability are factors that together with a low price these make them commercially very attractive. The precipitation of metastable precursors of the equilibrium

(Mg2Si) phase occurs in one or more sequences which are quite complex. The precipitation sequence for 6xxx alloys, which is generally accepted in the literature [11-13], is: SSSS atomic clusters GP zones '' ' (stable) Some authors [8] consider the GP zones as GP1 zones while the '' is called a GP2 zone. The most effective hardening phase for this types of materials is ''. The medium strength AlMgSi aluminium alloys are commonly processed by extrusion. Their extradubility depends to a large extent on chemical composition, casting condition and heat treatment parameters (eg. homogenization treatment) which determine the microstructure of the billet before extrusion.

2. Material and experimental

The investigation has been carried out on the commercial 6061, 6063 and 6082 aluminum alloys. Chemical composition of the alloys is indicated in Table 1. Table 1. Chemical composition of the investigated alloys, %wt Alloy Si Mg Mn Cu Fe Zn Ni Cr Ti

6061 0.78 1.07 0.15 0.35 0.16 0.042 0.007 0.35 0.029

6063 0.55 0.55 0.07 0.026 0.18 0.02 0.005 - 0.018

6082 1.0 0.76 0.56 0.022 0.16 0.013 0.004 - 0.023

Thermal processing of the investigated alloys included a

homogenization treatment and T6 heat treatment (artificially ageing after solution treatment). The temperature of homogenization treatment of 6xxx alloys were determined on the basis of literature data and calorimetric investigations. The samples in as-cast state were preheated in an induction furnace to temperature 575°C held for 72 hours and subsequently cooled to room temperature. Additionally all alloys were heated in a resistance furnace for 12 hours at 565°C and then quenched into a water. Subsequently the specimens were subjected to artificial aging at temperature 175oC up to 98 h. In order to determine an influence of time on the kinetics of ageing the Brinell hardness was measured.

DSC samples of the supersaturated 6061, 6063 and 6082 alloys were investigated in SETARAM Setsys thermal analyzer fitted with a scanning differential calorimeter module. The heat effects associated with precipitation of GP zones and intermediate metastable and stable strengthening phase (Mg2Si) were obtained by subtracting a super purity Al baseline run.

After artificial aging, a set of specimens were prepared for tensile testing to study the effect of T6 heat treatments on mechanical properties of the examined alloys. The specimens were strained by tensile deformation on Instron TTF-1115 servohydraulic universal tester at a constant rates at room temperature in according to standard PN-EN 10002-1:2004 [23]. Tensile properties (tensile and yield strength; elongation) were evaluated using round test specimens of 8 mm diameter and 65 mm gauge length (according to ASTM E602-78T [24] standard).

A metallographic investigations were performed on the samples at as-cast state after homogenization treatment and extrusion forging process. The microstructure of the alloys was observed using optical microscope - Nikon 300 on polished sections etched in Keller solution containing 0.5 % HF in 50ml H2O. The surfaces of fracture of the damaged samples were prepared to microscopic examination by scanning electron microscopy (SEM).

3. Results and discussion The microstructure of the 6061 alloy in as-cast state and after

homogenization is given in Fig. 1 - as an example of as-cast state of the investigated alloys. a)

b)

Fig. 1. Microstructure of examined 6061 alloy: a) as-cast state, b) after homogenization at 575oC/72h

2. Material and experimental

3. Results and discussion

100 100

G. Mrówka-Nowotnik

Archives of Materials Science and Engineering

a)

b)

c)

Fig. 2. Mictostructure of the examined alloys after hot extrusion: a) 6063 alloy, b) 6082 alloy, c) 6061 alloy

The revealed particles of the intermetallic phases were formed

during casting of the alloy. A typical as-cast microstructure of 6xxx series aluminium alloys consisted of a mixture of Al3Fe,

AlFeSi and -AlFeMnSi intermetallic phases distributed at cell boundaries, accompanied sometimes with coarse Mg2Si (Fig. 1a). During homogenization of the alloy at temperature 575 C, the transformation -AlFeSi phase in more spheroidal -Al(FeMn)Si

phase may occur. It is supposed that the very fine dispersed precipitates shown in Figure 1b are particles of -Mg2Si phase.

During hot working of ingots, particles of intermetallic phases are arranged in positions parallel to direction of plastic deformation (along plastic flow direction of processed material) which allows for the formation of the band structure (Fig. 2). As a result, the reduction of size of larger particles may takes place. The accumulation of lattice defects in the material during hot extrusion forging process exerts a considerable influence on a structure formation. As a result, the strain hardening of the alloy takes place and, in consequence, an increase in mechanical properties occurred.

The DSC curves of the investigated 6xxx alloys obtained during calorimetric study of samples quenched immediately after solution heat treatment and heated with scaning rate of 15K/min revealed a total of 9 anthalpic effects. Seven of them were exothermic (Fig. 3).

100 200 300 400 500 600-220

-200

-180

-160

-140

-120

Endo

Exo

Hea

t flo

w, m

W

Temperature, °C

6063 6082 6061

1

2 3

4

5

6

7

8

9

Fig. 3. DSC trace of as-quenched samples of the examined 6xxx aluminium alloys taken at a scan rate of 15oC/min

Exothermic peaks 1 observed at the DSC curves are belived to be responsible for the extensive clustering activity in this group of alloys. The exothermic peak 2 and 3 at approximetly 165°C was linked to the formation of GP zones. GP zones dissolution occurred at ~ 187°C. The major exothermic peak 4 with a maximum at 242°C is clearly linked with the precipitation of the principal hardening phase ”. ” precipitation is promoted by precursor GP- zones [13]. The precipitation continued with the transformation of ” to the ’ phase producing neighbouring exothermic peak 5 with maximum at 331°C. In 6061 aluminum alloy besides of -Mg2Si phase precipitation of -Al2Cu and Q(Al5Cu2Mg8Si6) phases can be observed. The exothermic peak 6 with a maximum at 363°C is linked to the precipitation of the hardening phases ’ or Q’ The next two exothermic peaks 7 (428°C) and endothermic peak 8 (528°C) are associated with the precipitation and dissolution of the equilibrium -Mg2Si phase respectively. On the DSC curve small endothermic peak with maximum at about 585°C was observed. On the basis of literature date in the alloy

with a high Fe content (0.16%) eutectic melting of Al + -Mg2Si+ -Al5FeSi L+ ’-Al8Fe2Si or Al + -Mg2Si L occurred [8]. It can thus be concluded that the response to DSC heating of the present alloy independently of deformation agreed reasonably well with the precipitation sequence reported for AlMgSi alloys: Supersaturated solid solution SSSS atomic clusters GP zones

'' ' (stable). As can be observed in Table 2, the main difference in the

precipitation patterns of the examined alloys is the temperature of the peaks. This discrepancies of the order few degree of celsius can be explained in terms of chemical compositions of the alloys, similar explanation can be held for the sharpness of the peaks.

Table 2. Temperature of the peaks obtained during heating at rate of 15oC/min for the 6xxx alloys immediately after solutionizing and quenching

Alloy Temperature of the peaks, oC 1 2 3 4 5 6 7 8 9

6061 78.2 161.2 1762.1 267.7 304.0 363.0 500.4 530.2 577.3

6063 83.6 161.4 196.9 283.0 310.0 - 484.0 525.6 581.8

6082 83.2 166.0 198.1 274.4 310.4 - 508.1 528.0 585.6

In order to determine the influence of precipitation hardening conditions on the aging kinetic of the examined alloys, hardness measurements at specified time intervals were performed (see Table 3). Table 3. Changes in the HB hardness of investigated alloys in relation of time of aging at 175°C

Time of ageing, h Alloy

6061 6063 6082 0 55.4 35.8 55.3 0.5 70.9 36.9 64 1 98.3 39 84.3 1.5 106.3 52.6 101 2 115.4 68.8 112.8 3 116.6 73.1 113.2 4 118.5 81.2 118.3 5 120 87.7 119 6 121.5 91 122.6 8 125.3 88.8 121.7 10 127.2 89.7 123 12 123.3 92.5 120 15 122.9 94.2 120 20 123 87.5 116.7 24 125.8 89 115 30 124 91 119.7 35 126.6 91.3 122.1 48 128.7 88.6 115.5 60 125.6 86.4 113.4 72 124.4 83 105.1 98 122.2 81.4 100.9

Based upon the recent researches [1-3] one may conclude that (Mg2Si) precipitates play crucial role in hardening of these

alloys. According to [11] et al [6-9] the volume fraction of GP zones and (Mg2Si) precipitates increase with alloy’s components content. So far it has been shown that the mechanical properties were highly influenced by the precipitates of a hardening (Mg2Si) phase. Precipitates of this phase do not only affect mechanical properties. The strength is also influenced by the intermetallic phases formed during solidification of the alloys. This is evidently related to the reactions that occur during solidification of the 6xxx type alloys between some alloying elements (Fe, Mn and Cr) and main 6xxx alloys components – silicon and magnesium. Their formation have significant effect on decreasing volume fraction of hardening precipitates of (Mg2Si) phase. Therefore decreasing in the effectiveness of hardening via precipitates of this phase is observed. Treakner [25] suggested to utilize the equations by which either volume fraction of hardening (Mg2Si) phase and excessive content of Si can be calculated using a chemical composition of an alloy:

Si in precipitates of Al(FeMn)Si type zf(Si) = 0.25 (%Fe + %Mn) (1) Si in (Mg2Si) formed in reaction with Mg zm(Si) = 0.578 (%Mg) (2)

(Mg2Si) phase content w(Mg2Si ) = (Mg) + zm(Si) (3) remaining content of Si nf(Si) = (%Si) - zf(Si) (4) excessive content of Si n(Si) = nf(Si) - zm(Si) (5) Table 4. Calculated values of volume fraction of (Mg2Si) and of the excessive silicon content in the investigated alloys (equations 1-5) Alloy zf(Si) nf(Si) zm(Si) w(Mg2Si ) n(Si)

6061 0.077 0.703 0.618 1.688 0.085

6063 0.062 0.487 0.318 0.868 0.169

6082 0.177 0.823 0.439 1.199 0.384 Apart of quantitative determination of volume fraction of

(Mg2Si) in 6061, 6063 and 6082 alloys, additional information about excessive content of the main element of these alloys – Si and Mg that take part (in the presence of Fe and Mn) in formation of intermetallic phases were determined (Table 4). Previous works [2,26-28] and literature data [10-14] reveals that the presence of (Mg2Si) phase and precipitates of intermetallic phases containing Si, Fe and Mn play significant role in mechanical properties increasing. Fig. 4 shows the effect of artificial aging time on hardness of 6061, 6063 and 6082 alloys. It can clearly be seen that the hardness of examined alloys depends on the chemical composition and aging time. The plots for all alloys indicate that in all cases, the hardness initially increase rapidly with the increase in aging time reaching the peak value, after which hardness decrease. All of the investigated alloys reached maximum hardness after aging at 175°C – 128.7HB alloy 6061 for 48 hours, alloy 6063 –94.2HB for 15 hours and 6082 – 123.0 HB for 10 hours) (see Table 3 and Fig. 4).

101

Influence of chemical composition variation and heat treatment on microstructure and mechanical properties of 6xxx alloys

Volume 46 Issue 2 December 2010

a)

b)

c)

Fig. 2. Mictostructure of the examined alloys after hot extrusion: a) 6063 alloy, b) 6082 alloy, c) 6061 alloy

The revealed particles of the intermetallic phases were formed

during casting of the alloy. A typical as-cast microstructure of 6xxx series aluminium alloys consisted of a mixture of Al3Fe,

AlFeSi and -AlFeMnSi intermetallic phases distributed at cell boundaries, accompanied sometimes with coarse Mg2Si (Fig. 1a). During homogenization of the alloy at temperature 575 C, the transformation -AlFeSi phase in more spheroidal -Al(FeMn)Si

phase may occur. It is supposed that the very fine dispersed precipitates shown in Figure 1b are particles of -Mg2Si phase.

During hot working of ingots, particles of intermetallic phases are arranged in positions parallel to direction of plastic deformation (along plastic flow direction of processed material) which allows for the formation of the band structure (Fig. 2). As a result, the reduction of size of larger particles may takes place. The accumulation of lattice defects in the material during hot extrusion forging process exerts a considerable influence on a structure formation. As a result, the strain hardening of the alloy takes place and, in consequence, an increase in mechanical properties occurred.

The DSC curves of the investigated 6xxx alloys obtained during calorimetric study of samples quenched immediately after solution heat treatment and heated with scaning rate of 15K/min revealed a total of 9 anthalpic effects. Seven of them were exothermic (Fig. 3).

100 200 300 400 500 600-220

-200

-180

-160

-140

-120

Endo

Exo

Hea

t flo

w, m

W

Temperature, °C

6063 6082 6061

1

2 3

4

5

6

7

8

9

Fig. 3. DSC trace of as-quenched samples of the examined 6xxx aluminium alloys taken at a scan rate of 15oC/min

Exothermic peaks 1 observed at the DSC curves are belived to be responsible for the extensive clustering activity in this group of alloys. The exothermic peak 2 and 3 at approximetly 165°C was linked to the formation of GP zones. GP zones dissolution occurred at ~ 187°C. The major exothermic peak 4 with a maximum at 242°C is clearly linked with the precipitation of the principal hardening phase ”. ” precipitation is promoted by precursor GP- zones [13]. The precipitation continued with the transformation of ” to the ’ phase producing neighbouring exothermic peak 5 with maximum at 331°C. In 6061 aluminum alloy besides of -Mg2Si phase precipitation of -Al2Cu and Q(Al5Cu2Mg8Si6) phases can be observed. The exothermic peak 6 with a maximum at 363°C is linked to the precipitation of the hardening phases ’ or Q’ The next two exothermic peaks 7 (428°C) and endothermic peak 8 (528°C) are associated with the precipitation and dissolution of the equilibrium -Mg2Si phase respectively. On the DSC curve small endothermic peak with maximum at about 585°C was observed. On the basis of literature date in the alloy

with a high Fe content (0.16%) eutectic melting of Al + -Mg2Si+ -Al5FeSi L+ ’-Al8Fe2Si or Al + -Mg2Si L occurred [8]. It can thus be concluded that the response to DSC heating of the present alloy independently of deformation agreed reasonably well with the precipitation sequence reported for AlMgSi alloys: Supersaturated solid solution SSSS atomic clusters GP zones

'' ' (stable). As can be observed in Table 2, the main difference in the

precipitation patterns of the examined alloys is the temperature of the peaks. This discrepancies of the order few degree of celsius can be explained in terms of chemical compositions of the alloys, similar explanation can be held for the sharpness of the peaks.

Table 2. Temperature of the peaks obtained during heating at rate of 15oC/min for the 6xxx alloys immediately after solutionizing and quenching

Alloy Temperature of the peaks, oC 1 2 3 4 5 6 7 8 9

6061 78.2 161.2 1762.1 267.7 304.0 363.0 500.4 530.2 577.3

6063 83.6 161.4 196.9 283.0 310.0 - 484.0 525.6 581.8

6082 83.2 166.0 198.1 274.4 310.4 - 508.1 528.0 585.6

In order to determine the influence of precipitation hardening conditions on the aging kinetic of the examined alloys, hardness measurements at specified time intervals were performed (see Table 3). Table 3. Changes in the HB hardness of investigated alloys in relation of time of aging at 175°C

Time of ageing, h Alloy

6061 6063 6082 0 55.4 35.8 55.3 0.5 70.9 36.9 64 1 98.3 39 84.3 1.5 106.3 52.6 101 2 115.4 68.8 112.8 3 116.6 73.1 113.2 4 118.5 81.2 118.3 5 120 87.7 119 6 121.5 91 122.6 8 125.3 88.8 121.7 10 127.2 89.7 123 12 123.3 92.5 120 15 122.9 94.2 120 20 123 87.5 116.7 24 125.8 89 115 30 124 91 119.7 35 126.6 91.3 122.1 48 128.7 88.6 115.5 60 125.6 86.4 113.4 72 124.4 83 105.1 98 122.2 81.4 100.9

Based upon the recent researches [1-3] one may conclude that (Mg2Si) precipitates play crucial role in hardening of these

alloys. According to [11] et al [6-9] the volume fraction of GP zones and (Mg2Si) precipitates increase with alloy’s components content. So far it has been shown that the mechanical properties were highly influenced by the precipitates of a hardening (Mg2Si) phase. Precipitates of this phase do not only affect mechanical properties. The strength is also influenced by the intermetallic phases formed during solidification of the alloys. This is evidently related to the reactions that occur during solidification of the 6xxx type alloys between some alloying elements (Fe, Mn and Cr) and main 6xxx alloys components – silicon and magnesium. Their formation have significant effect on decreasing volume fraction of hardening precipitates of (Mg2Si) phase. Therefore decreasing in the effectiveness of hardening via precipitates of this phase is observed. Treakner [25] suggested to utilize the equations by which either volume fraction of hardening (Mg2Si) phase and excessive content of Si can be calculated using a chemical composition of an alloy:

Si in precipitates of Al(FeMn)Si type zf(Si) = 0.25 (%Fe + %Mn) (1) Si in (Mg2Si) formed in reaction with Mg zm(Si) = 0.578 (%Mg) (2)

(Mg2Si) phase content w(Mg2Si ) = (Mg) + zm(Si) (3) remaining content of Si nf(Si) = (%Si) - zf(Si) (4) excessive content of Si n(Si) = nf(Si) - zm(Si) (5) Table 4. Calculated values of volume fraction of (Mg2Si) and of the excessive silicon content in the investigated alloys (equations 1-5) Alloy zf(Si) nf(Si) zm(Si) w(Mg2Si ) n(Si)

6061 0.077 0.703 0.618 1.688 0.085

6063 0.062 0.487 0.318 0.868 0.169

6082 0.177 0.823 0.439 1.199 0.384 Apart of quantitative determination of volume fraction of

(Mg2Si) in 6061, 6063 and 6082 alloys, additional information about excessive content of the main element of these alloys – Si and Mg that take part (in the presence of Fe and Mn) in formation of intermetallic phases were determined (Table 4). Previous works [2,26-28] and literature data [10-14] reveals that the presence of (Mg2Si) phase and precipitates of intermetallic phases containing Si, Fe and Mn play significant role in mechanical properties increasing. Fig. 4 shows the effect of artificial aging time on hardness of 6061, 6063 and 6082 alloys. It can clearly be seen that the hardness of examined alloys depends on the chemical composition and aging time. The plots for all alloys indicate that in all cases, the hardness initially increase rapidly with the increase in aging time reaching the peak value, after which hardness decrease. All of the investigated alloys reached maximum hardness after aging at 175°C – 128.7HB alloy 6061 for 48 hours, alloy 6063 –94.2HB for 15 hours and 6082 – 123.0 HB for 10 hours) (see Table 3 and Fig. 4).

102 102

G. Mrówka-Nowotnik

Archives of Materials Science and Engineering

1 10 10030

40

50

60

70

80

90

100

110

120

130

Hard

ness

HB5

/250

log t, h

6061 6063 6082

Fig. 4. Variation of hardness with time of aging of investigated alloy at temperature of 175°C It is well known that silicon and magnesium content has a positive effect on mechanical properties after aging thus it is worth to determine the effect of Mg+Si concentration on the behavior of 6xxx alloys during aging (Fig. 5).

1,0 1,1 1,2 1,3 1,4 1,5 1,6 1,7 1,8 1,990

95

100

105

110

115

120

125

130

y = 45,116x + 44,467 R2 = 0,998

6082

6061

6063

Max

HB5

/250

Mg+Si, % Fig. 5. Influence of different Mg + Si levels on the maximum hardness of the examined alloys The results in (Fig. 5) demonstrate that progress of silicon and magnesium content improves the maximum hardness in examined alloys. In 6082 aluminium alloy, with the highest concentration of silicon and magnesium the highest hardness in T6 state was observed. The tensile (Rm and Rp0.2) and plastic properties (A, Z) of the aged alloy 6061, 6063 and 6082 are also strongly depend on the chemical composition and an aging time at 175°C. The results of the static tensile tests were summarized in Table 5. The increment alloys strength similarly to the observed increment in hardness can be treated as the effects of initial formation of GP zones followed by precipitation of metastable particles of ” and ’ phases.

Table 5. Mechanical properties of the examined alloys after T6 treatment

Alloy Aging time,h

Mechanical properties R0.2, MPa

Rm, MPa

A5, %

Z, %

6061

2 163.2 270.5 33.5 44.6 5 206.7 295.6 29.7 43.0 10 274.2 320.0 19.7 40.9 24 326.5 353.9 15.2 42.1 50 331.26 349.0 14.8 43.7 96 329.35 348.8 14.4 45.6

6063

2 128.5 202.3 25.1 75.4 5 257.0 275.6 16.1 51.8 10 271.3 281.6 14.1 45.6 30 269.2 289.9 12.1 40.4 50 267.5 290.4 13.9 35.5 96 229.5 252.3 13.3 48.4

6082

2 165.0 246.2 29.2 46.8 6 276.7 306.0 16.8 45.6 10 299.9 338.1 15.9 40 30 314.9 333.7 15.4 40.3 50 309.6 330.5 14.8 43.4 96 282.0 298.3 13.0 44.8

On the basic of the region of uniform plastic deformation from the stress-strain curves of and a simple power – curve relation:

= K n (6) the values of n (is the strain – hardening exponent) and K (is the strength coefficient) were determined. Additionaly on the basic of the stress-strain curves Young modulus E, and m – “hardening coefficient” from equation 7 was calculeted:

12.02.0

2.0

RR

RRRm mm (7)

were: R0.2 – yield strength, Rm – tensile strength (Tab. 6, Figs. 6-8). Table 6. The values of: E – Young modulus, K – strength coefficient, n – strain – hardening exponent and m – “hardening coefficient”

Alloy Aging time, h E/r K n m

6061

2 68 538/1.0000 297.78 0.0957 0.67357 5 71 053/0.9999 337.60 0.08030 0.43028 10 70 470/0.9999 394.18 0.0601 0.18429 24 71 155/0.9999 418.12 0.0367 0.08147 50 71 566/0.9999 394.46 0.0281 0.04910 96 71 999/0.9999 406.93 0.03420 0.05907

6063

2 63 256/0.9999 228.82 0.09592 0.56516 6 67 235/0.9999 303.74 0.0347 0.11250 10 66 322/0.9999 326.5 0.0300 0.0984 30 65 707/0.9999 324.12 0.0313 0.07534 50 68 935/0.9999 320.63 0.0312 0.08544 96 69 735/0.9999 299.29 0.0444 0.09321

6082

2 84 672/0.9999 290.27 0.0957 0.45930 6 69 683/0.9999 362.66 0.0402 0.10494 10 70 568/0.9999 378.5 0.0398 0.0765 30 71 287/0.9999 394.82 0.0336 0.05766 50 66 871/1.0000 391.92 0.0368 0.07073 96 70 756/0.9999 348.66 0.0294 0.04954

a)

0 10 20 30 40 50 60 70 80 90 100

150

180

210

240

270

300

330

360

390

420

R 0.2,

R m, K

, MPa

t, h

R0.2 Rm K

b)

0 10 20 30 40 50 60 70 80 90 100

10

15

20

25

30

35

40

45

50

A 5, Z,

%

t, h

A5 Z

c)

0 10 20 30 40 50 60 70 80 90 1000,0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

n, m

t, h

n m

Fig. 6. Effect of time aging at 175°C a) on yield R0.2, tensile Rm strength and strength coefficient K, b) elongation A5 and compression Z, c) n the strain – hardening exponent and m – “hardening coefficient” of 6061 alloy

a)

0 10 20 30 40 50 60 70 80 90 100

120

150

180

210

240

270

300

330

360

R 0.2,

R m, K

, MPa

t, h

R0.2 Rm K

b)

0 10 20 30 40 50 60 70 80 90 100

10

20

30

40

50

60

70

80

A 5, Z,

%

t, h

A5 Z

c)

0 10 20 30 40 50 60 70 80 90 1000,0

0,1

0,2

0,3

0,4

0,5

0,6

n, m

t, h

n m

Fig. 7. Effect of time aging at 175°C a) on yield R0.2, tensile Rm strength and strength coefficient K, b) elongation A5 and compression Z, c) n the strain – hardening exponent and m – “hardening coefficient”of 6063 alloy

103

Influence of chemical composition variation and heat treatment on microstructure and mechanical properties of 6xxx alloys

Volume 46 Issue 2 December 2010

1 10 10030

40

50

60

70

80

90

100

110

120

130

Hard

ness

HB5

/250

log t, h

6061 6063 6082

Fig. 4. Variation of hardness with time of aging of investigated alloy at temperature of 175°C It is well known that silicon and magnesium content has a positive effect on mechanical properties after aging thus it is worth to determine the effect of Mg+Si concentration on the behavior of 6xxx alloys during aging (Fig. 5).

1,0 1,1 1,2 1,3 1,4 1,5 1,6 1,7 1,8 1,990

95

100

105

110

115

120

125

130

y = 45,116x + 44,467 R2 = 0,998

6082

6061

6063

Max

HB5

/250

Mg+Si, % Fig. 5. Influence of different Mg + Si levels on the maximum hardness of the examined alloys The results in (Fig. 5) demonstrate that progress of silicon and magnesium content improves the maximum hardness in examined alloys. In 6082 aluminium alloy, with the highest concentration of silicon and magnesium the highest hardness in T6 state was observed. The tensile (Rm and Rp0.2) and plastic properties (A, Z) of the aged alloy 6061, 6063 and 6082 are also strongly depend on the chemical composition and an aging time at 175°C. The results of the static tensile tests were summarized in Table 5. The increment alloys strength similarly to the observed increment in hardness can be treated as the effects of initial formation of GP zones followed by precipitation of metastable particles of ” and ’ phases.

Table 5. Mechanical properties of the examined alloys after T6 treatment

Alloy Aging time,h

Mechanical properties R0.2, MPa

Rm, MPa

A5, %

Z, %

6061

2 163.2 270.5 33.5 44.6 5 206.7 295.6 29.7 43.0 10 274.2 320.0 19.7 40.9 24 326.5 353.9 15.2 42.1 50 331.26 349.0 14.8 43.7 96 329.35 348.8 14.4 45.6

6063

2 128.5 202.3 25.1 75.4 5 257.0 275.6 16.1 51.8 10 271.3 281.6 14.1 45.6 30 269.2 289.9 12.1 40.4 50 267.5 290.4 13.9 35.5 96 229.5 252.3 13.3 48.4

6082

2 165.0 246.2 29.2 46.8 6 276.7 306.0 16.8 45.6 10 299.9 338.1 15.9 40 30 314.9 333.7 15.4 40.3 50 309.6 330.5 14.8 43.4 96 282.0 298.3 13.0 44.8

On the basic of the region of uniform plastic deformation from the stress-strain curves of and a simple power – curve relation:

= K n (6) the values of n (is the strain – hardening exponent) and K (is the strength coefficient) were determined. Additionaly on the basic of the stress-strain curves Young modulus E, and m – “hardening coefficient” from equation 7 was calculeted:

12.02.0

2.0

RR

RRRm mm (7)

were: R0.2 – yield strength, Rm – tensile strength (Tab. 6, Figs. 6-8). Table 6. The values of: E – Young modulus, K – strength coefficient, n – strain – hardening exponent and m – “hardening coefficient”

Alloy Aging time, h E/r K n m

6061

2 68 538/1.0000 297.78 0.0957 0.67357 5 71 053/0.9999 337.60 0.08030 0.43028 10 70 470/0.9999 394.18 0.0601 0.18429 24 71 155/0.9999 418.12 0.0367 0.08147 50 71 566/0.9999 394.46 0.0281 0.04910 96 71 999/0.9999 406.93 0.03420 0.05907

6063

2 63 256/0.9999 228.82 0.09592 0.56516 6 67 235/0.9999 303.74 0.0347 0.11250 10 66 322/0.9999 326.5 0.0300 0.0984 30 65 707/0.9999 324.12 0.0313 0.07534 50 68 935/0.9999 320.63 0.0312 0.08544 96 69 735/0.9999 299.29 0.0444 0.09321

6082

2 84 672/0.9999 290.27 0.0957 0.45930 6 69 683/0.9999 362.66 0.0402 0.10494 10 70 568/0.9999 378.5 0.0398 0.0765 30 71 287/0.9999 394.82 0.0336 0.05766 50 66 871/1.0000 391.92 0.0368 0.07073 96 70 756/0.9999 348.66 0.0294 0.04954

a)

0 10 20 30 40 50 60 70 80 90 100

150

180

210

240

270

300

330

360

390

420

R 0.2,

R m, K

, MPa

t, h

R0.2 Rm K

b)

0 10 20 30 40 50 60 70 80 90 100

10

15

20

25

30

35

40

45

50

A 5, Z,

%

t, h

A5 Z

c)

0 10 20 30 40 50 60 70 80 90 1000,0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

n, m

t, h

n m

Fig. 6. Effect of time aging at 175°C a) on yield R0.2, tensile Rm strength and strength coefficient K, b) elongation A5 and compression Z, c) n the strain – hardening exponent and m – “hardening coefficient” of 6061 alloy

a)

0 10 20 30 40 50 60 70 80 90 100

120

150

180

210

240

270

300

330

360

R 0.2,

R m, K

, MPa

t, h

R0.2 Rm K

b)

0 10 20 30 40 50 60 70 80 90 100

10

20

30

40

50

60

70

80

A 5, Z,

%

t, h

A5 Z

c)

0 10 20 30 40 50 60 70 80 90 1000,0

0,1

0,2

0,3

0,4

0,5

0,6

n, m

t, h

n m

Fig. 7. Effect of time aging at 175°C a) on yield R0.2, tensile Rm strength and strength coefficient K, b) elongation A5 and compression Z, c) n the strain – hardening exponent and m – “hardening coefficient”of 6063 alloy

104 104

G. Mrówka-Nowotnik

Archives of Materials Science and Engineering

a)

0 10 20 30 40 50 60 70 80 90 100

150

180

210

240

270

300

330

360

390

R0.

2, R m

, K, M

Pa

t, h

R0.2 Rm K

b)

0 10 20 30 40 50 60 70 80 90 100

10

15

20

25

30

35

40

45

50

A 5, Z,

%

t, h

A5 Z

c)

0 10 20 30 40 50 60 70 80 90 1000,0

0,1

0,2

0,3

0,4

0,5

n, m

t, h

n m

Fig. 8. Effect of time aging at 175°C a) on yield R0.2, tensile Rm strength and strength coefficient K, b) elongation A5 and compression Z, c) n the strain – hardening exponent and m – “hardening coefficient” of 6082 alloy

The yield strength Rp0.2 (Fig. 9) and the tensile strength Rm (Fig. 10) increases with time of aging, however a significant increase in mechanical properties was achieved during aging for the first 20 hours. Further heating causes a steady increase of yield strength Rp0.2 , tensile strength Rm and strength coefficient K. After achieving the maximum values of R0.2, Rm and K continuously decreases with the time of aging were observed. The highest yield strength Rp0.2 (Fig. 9) and tensile strength Rm (Fig. 10) was achieved in 6061 alloy (Tables 5, 6).

0 20 40 60 80 100120

140

160

180

200

220

240

260

280

300

320

340

R0.

2, M

Pa

t, h

6061 6063 6082

Fig. 9. Effect of aging time on yield strength Rp0.2 of the alloys aged at 175°C

0 20 40 60 80 100180

210

240

270

300

330

360

R m, M

Pa

t, h

6061 6063 6082

Fig. 10. Effect of aging time on tensile strength Rm of the alloys aged at 175°C The highest tensile strength Rm was recorded for 6061 aluminium alloy with the maximum concentration of Mg2Si (Table 4, Fig. 11). One can see that, tensile strength Rm of 6061 alloy is higher by about 60 MPa compared to the 6063 alloy with the minimum concentration of Mg2Si (Tables 4, 5 and Figs. 6, 7, 10). It is worth to mention that 6061 possess the best plastic properties after aging at 175°C (Figs. 12, 13). As we can see on Fig. 12 the highest elongation A5 was observed in 6061 alloy, and the worst in 6063 alloy.

0,8 0,9 1,0 1,1 1,2 1,3 1,4 1,5 1,6 1,7 1,8

270

280

290

300

310

320

330

340

350

360

6061

6082

6063

max

R0.

2, m

ax R

m

Mg2Si, %

R0.2 Rm

Fig. 11. Influence of different Mg2Si levels on maximum on yield strength Rp0.2 and tensile strength Rm of the investigated alloys aged at 175°C

0 20 40 60 80 100

10

15

20

25

30

35

Elon

gatio

n A 5, %

t, h

6061 6063 6082

Fig. 12. Effect of aging time on elongation A5 of the investigated alloys aged at 175°C

0 20 40 60 80 10035

40

45

50

55

60

65

70

75

80

Com

pres

sion

Z, %

t, h

6061 6063 6082

Fig. 13. Effect of aging time on compression Z of the investigated alloys aged at 175°C

a)

b)

c)

Fig.14. Tensile fracture surface of the investigated alloys: a) 6061, b) 6063, c) 6082 after solid solution treatment and artificial aging at 175°C

105

Influence of chemical composition variation and heat treatment on microstructure and mechanical properties of 6xxx alloys

Volume 46 Issue 2 December 2010

a)

0 10 20 30 40 50 60 70 80 90 100

150

180

210

240

270

300

330

360

390

R0.

2, R m

, K, M

Pa

t, h

R0.2 Rm K

b)

0 10 20 30 40 50 60 70 80 90 100

10

15

20

25

30

35

40

45

50

A 5, Z,

%

t, h

A5 Z

c)

0 10 20 30 40 50 60 70 80 90 1000,0

0,1

0,2

0,3

0,4

0,5

n, m

t, h

n m

Fig. 8. Effect of time aging at 175°C a) on yield R0.2, tensile Rm strength and strength coefficient K, b) elongation A5 and compression Z, c) n the strain – hardening exponent and m – “hardening coefficient” of 6082 alloy

The yield strength Rp0.2 (Fig. 9) and the tensile strength Rm (Fig. 10) increases with time of aging, however a significant increase in mechanical properties was achieved during aging for the first 20 hours. Further heating causes a steady increase of yield strength Rp0.2 , tensile strength Rm and strength coefficient K. After achieving the maximum values of R0.2, Rm and K continuously decreases with the time of aging were observed. The highest yield strength Rp0.2 (Fig. 9) and tensile strength Rm (Fig. 10) was achieved in 6061 alloy (Tables 5, 6).

0 20 40 60 80 100120

140

160

180

200

220

240

260

280

300

320

340

R0.

2, M

Pa

t, h

6061 6063 6082

Fig. 9. Effect of aging time on yield strength Rp0.2 of the alloys aged at 175°C

0 20 40 60 80 100180

210

240

270

300

330

360

R m, M

Pa

t, h

6061 6063 6082

Fig. 10. Effect of aging time on tensile strength Rm of the alloys aged at 175°C The highest tensile strength Rm was recorded for 6061 aluminium alloy with the maximum concentration of Mg2Si (Table 4, Fig. 11). One can see that, tensile strength Rm of 6061 alloy is higher by about 60 MPa compared to the 6063 alloy with the minimum concentration of Mg2Si (Tables 4, 5 and Figs. 6, 7, 10). It is worth to mention that 6061 possess the best plastic properties after aging at 175°C (Figs. 12, 13). As we can see on Fig. 12 the highest elongation A5 was observed in 6061 alloy, and the worst in 6063 alloy.

0,8 0,9 1,0 1,1 1,2 1,3 1,4 1,5 1,6 1,7 1,8

270

280

290

300

310

320

330

340

350

360

6061

6082

6063

max

R0.

2, m

ax R

m

Mg2Si, %

R0.2 Rm

Fig. 11. Influence of different Mg2Si levels on maximum on yield strength Rp0.2 and tensile strength Rm of the investigated alloys aged at 175°C

0 20 40 60 80 100

10

15

20

25

30

35

Elon

gatio

n A 5, %

t, h

6061 6063 6082

Fig. 12. Effect of aging time on elongation A5 of the investigated alloys aged at 175°C

0 20 40 60 80 10035

40

45

50

55

60

65

70

75

80

Com

pres

sion

Z, %

t, h

6061 6063 6082

Fig. 13. Effect of aging time on compression Z of the investigated alloys aged at 175°C

a)

b)

c)

Fig.14. Tensile fracture surface of the investigated alloys: a) 6061, b) 6063, c) 6082 after solid solution treatment and artificial aging at 175°C

106 106

G. Mrówka-Nowotnik

Archives of Materials Science and Engineering

Figure 14 shows the fracture surface of the specimen of investigated 6xxx aluminum alloy in the peak aged condition after static tensile test. SEM observation of fracture processes in the sample with the highest tensile stress, confirmed that fracture initiates within void clusters as a result of a sequence of void nucleation, void growth, and void coalescence. Figure 5 presents the classical ductile profile with existence of dimples with different sizes which can be related to the presence of the two population of voids. Inside the dimples the presence of different particles are visible. Large dimples around hard intermetallic (Al8Fe2Si) and (Al5FeSi) precipitates and also smaller ones around dispersive hardening -Mg2Si Al2Cu and -Al(FeMn)Si precipitates were formed.

4. Conclusion It was found that the mechanical properties of the 6xxx series

aluminium alloys in the T6 tempers are strongly depend on the chemical composition and an aging time at 175°C. The degree of strengthening depends on the extent of ” precipitation which increases with increasing Mg and Si content in the chemical composition of the alloys.

The highest mechanical properties connected with a good plastic properties was achieved for 6061 alloy with the highest concentration of alloying elements Mg, Si, Cu, Mn and Fe. In 6061 aluminium alloy besides of -Mg2Si strengthening phase the precipitation of -Al2Cu and Q(Al5Cu2Mg8Si6) phases can be present.

Observation fracture surface (SEM) specimens after static tensile test showed that cracking of the examined alloys begin by nucleation and growth of voids. The sites of heterogenic nucleation of voids are the precipitates of intermetallic phases. Subsequent decohesion process initially proceeded at the interface between matrix and particle.

Acknowledgements This work was carried out with the financial support of the

Ministry of Science and Information Society Technologies under grant No. N507 3828 33.

References [1] S. Karabay, M. Zeren, M. Yilmaz, Investigation of extrusion

ratio effect on mechanical behaviour of extruded alloy AA-6101 from the billets homogenised-rapid quenched and as-cast conditions, Journal of Materials Processing Technology 160 (2004) 138-147.

[2] G. Mrówka-Nowotnik, J. Sieniawski, Influence of heat treatment on the microstructure and mechanical properties of 6005 and 6082 aluminium alloys, Proc. Int. Conf. "Achievements in Mechanical & Materials Engineering", Gliwice – Wis a, 2005, 447-450.

[3] M. Warmuzek, J. Sieniawski, A. Gazda, G. Mrówka, Analysis of phase formation in AlFeMnSi alloy with variable content of Fe and Mn transition elements, Materials Engineering 137 (2003) 821-824.

[4] M. Wierzbi ska, J. Sieniawski, Effect of morphology of eutectic silicon crystals on mechanical properties and clevage fractuce toughnessof AlSi5Cu1 alloy, Journal of Achievements in Materials and Manufacturing Engineering 14 (2006) 217-220.

[5] S. Zajac, B. Bengtsson , Ch. Jönsson, Influence of cooling after homogenization and reheating to extrusion on extrudability and final properties of AA 6063 and AA 6082 alloys, Materials Science Forum 396-402 (2002) 675-680.

[6] M. Warmuzek, G. Mrówka-Nowotnik, J. Sieniawski, Influence of the heat treatment on the precipitation of the intermetallic phases in commercial AlMn1FeSi alloy, Journal of Materials Processing Technology 157-158 (2004) 624-632.

[7] R.A. Siddiqui, H.A. Abdullah, K.R. Al.-Belushi, Influence of aging parameters on the mechanical properties of 6063 aluminium alloy, Journal of Materials Processing Technology 102 (2000) 234-240.

[8] N.C.W. Kuijpers, W.H. Kool, P.T.G. Koenis, K.E. Nilsen, I. Todd, S. van der Zwaag, Assessment of diffrent techniques for quantification of -Al(FeMn)Si and -AlFeSi intermetallics in AA 6xxx alloys, Materials Characterization 49 (2003) 409-420.

[9] G. Sha, K. O’Reilly, B. Cantor, J. Worth, R. Hamerton, Growth related phase selection in a 6xxx series wrought Al alloy, Materials Science and Engineering A304-306 (2001) 612-616.

[10] A.K. Gupta, D.J. Lloyd, S.A. Court, Precipitation hardening in Al-Mg-Si alloys with and without excess Si, Materials Science and Engineering A316 (2001) 11-17.

[11] G.A. Edwards, K. Stiller, G.L. Dunlop, M.J. Couper, The precipitation sequence in Al-Mg-Si alloys, Acta Materialia 46/11(1998) 3893-3904.

[12] W.F. Miao, D.E. Laughlin, Precipitation hardening in aluminum alloy 6022, Scripta Materialia 40/7 (1999) 873-878.

[13] G. Biroli, G. Caglioti, L. Martini, G. Riontino, Precipitation kinetics of AA4032 and AA6082 a comparison based on DSC and TEM, Scripta Materialia 39/2 (1998) 197-203.

[14] Z. Li, A.M. Samuel, C. Rayindran, S. Valtierra, H.W. Doty, Parameters controlling the performance of AA319-type alloys: Part II. Impact properties and fractography, Materials Science and Engineering A367 (2004) 111-122.

[15] G. Mrówka - Nowotnik, J. Sieniawski, A. Nowotnik, Tensile properties and fracture toughness of heat treated 6082 alloy, Journal of Achievements in Materials and Manufacturing Engineering 17 (2006) 105-108.

[16] M. Wierzbi ska, J. Sieniawski, Effect of morfology of eutectuc silicon crystals on mechanical properties and cleavage fracture toughness of AlSi5Cu1 alloy, Journal of Achievements in Materials and Manufacturing Engineering 14 (2006) 31-36.

[17] L.A. Dobrza ski, R. Maniara, J.H. Soko owski, The effect of cast Al-Si-Cu alloy solidification rate on alloy thermal characteristic, Journal of Achievements in Materials and Manufacturing Engineering 17 (2006) 217-220.

References

Acknowledgements

4. Conclusions

[18] L.A. Dobrza ski, W. Borek, R. Maniara, Influence of the crystallization condition on Al-Si-Cu casting alloys structure, Journal of Achievements in Materials and Manufacturing Engineering 18 (2006) 211-214.

[19] M. Kciuk, The structure, mechanical properties and corrosion resistance of aluminium AlMg1Si1 alloy, Journal of Achievements in Materials and Manufacturing Engineering 16 (2006) 51-56

[20] M. Czechowski, Fatigue life of friction stir welded Al-Mg alloys, Proceeding of the 13th International Scientific Conference on Achievements in Mechanical and Materials Engineering" AMME'2005, Gliwice – Wis a, 2005, 83-86.

[21] L.A. Dobrza ski, R. Maniara, M. Krupi ski, J.H. Sokolowski, Microstructure and mechanical properties of AC AlSi9CuX alloys, Journal of Achievements in Materials and Manufacturing Engineering 24/2 (2007) 51-54.

[22] M. Kciuk, S. Tkaczyk, Structure, mechanical properties and corrosion resistance of AlMg5 and AlMg1Si1 alloys, Journal of Achievements in Materials and Manufacturing Engineering 21/1 (2007) 51-54.

[23] PN-EN 10002-1+AC1. Metals. Standard method of tensile test at room temperature.

[24] ASTM E602-78T. Tentative method for Sharp-notch tension testing with cylindral specimens.

[25] F.O. Traekner, Factors Affecting the Physical Characteristics of Aluminium Magnesium Silicon Alloy Extrusions. Proc. of 2nd Int. Aluminium Technology Seminar, Atlanta 1 (1977) 339-347.

[26] G. Mrówka-Nowotnik, J. Sieniawski, Influence of heat treatment on the micrustructure and mechanical properties of 6005 and 6082 aluminium alloys, Journal of Materials Processing Technology 162-163 (2005) 367-372.

[27] G. Mrówka-Nowotnik, J. Sieniawski M. Wierzbi ska, Analysis of intermetallic particles in AlSi1MgMn aluminium alloys, Journal of Achievements in Materials and Manufacturing Engineering 20 (2007) 155-158.

[28] G. Mrówka-Nowotnik, J. Sieniawski, A. Nowotnik, Tensile properties and fracture toughness of heat treated 6082 alloy, Journal of Achievements in Materials and Manufacturing Engineering 17 (2006) 105-108.

107READING DIRECT: www.archivesmse.org

Figure 14 shows the fracture surface of the specimen of investigated 6xxx aluminum alloy in the peak aged condition after static tensile test. SEM observation of fracture processes in the sample with the highest tensile stress, confirmed that fracture initiates within void clusters as a result of a sequence of void nucleation, void growth, and void coalescence. Figure 5 presents the classical ductile profile with existence of dimples with different sizes which can be related to the presence of the two population of voids. Inside the dimples the presence of different particles are visible. Large dimples around hard intermetallic (Al8Fe2Si) and (Al5FeSi) precipitates and also smaller ones around dispersive hardening -Mg2Si Al2Cu and -Al(FeMn)Si precipitates were formed.

4. Conclusion It was found that the mechanical properties of the 6xxx series

aluminium alloys in the T6 tempers are strongly depend on the chemical composition and an aging time at 175°C. The degree of strengthening depends on the extent of ” precipitation which increases with increasing Mg and Si content in the chemical composition of the alloys.

The highest mechanical properties connected with a good plastic properties was achieved for 6061 alloy with the highest concentration of alloying elements Mg, Si, Cu, Mn and Fe. In 6061 aluminium alloy besides of -Mg2Si strengthening phase the precipitation of -Al2Cu and Q(Al5Cu2Mg8Si6) phases can be present.

Observation fracture surface (SEM) specimens after static tensile test showed that cracking of the examined alloys begin by nucleation and growth of voids. The sites of heterogenic nucleation of voids are the precipitates of intermetallic phases. Subsequent decohesion process initially proceeded at the interface between matrix and particle.

Acknowledgements This work was carried out with the financial support of the

Ministry of Science and Information Society Technologies under grant No. N507 3828 33.

References [1] S. Karabay, M. Zeren, M. Yilmaz, Investigation of extrusion

ratio effect on mechanical behaviour of extruded alloy AA-6101 from the billets homogenised-rapid quenched and as-cast conditions, Journal of Materials Processing Technology 160 (2004) 138-147.

[2] G. Mrówka-Nowotnik, J. Sieniawski, Influence of heat treatment on the microstructure and mechanical properties of 6005 and 6082 aluminium alloys, Proc. Int. Conf. "Achievements in Mechanical & Materials Engineering", Gliwice – Wis a, 2005, 447-450.

[3] M. Warmuzek, J. Sieniawski, A. Gazda, G. Mrówka, Analysis of phase formation in AlFeMnSi alloy with variable content of Fe and Mn transition elements, Materials Engineering 137 (2003) 821-824.

[4] M. Wierzbi ska, J. Sieniawski, Effect of morphology of eutectic silicon crystals on mechanical properties and clevage fractuce toughnessof AlSi5Cu1 alloy, Journal of Achievements in Materials and Manufacturing Engineering 14 (2006) 217-220.

[5] S. Zajac, B. Bengtsson , Ch. Jönsson, Influence of cooling after homogenization and reheating to extrusion on extrudability and final properties of AA 6063 and AA 6082 alloys, Materials Science Forum 396-402 (2002) 675-680.

[6] M. Warmuzek, G. Mrówka-Nowotnik, J. Sieniawski, Influence of the heat treatment on the precipitation of the intermetallic phases in commercial AlMn1FeSi alloy, Journal of Materials Processing Technology 157-158 (2004) 624-632.

[7] R.A. Siddiqui, H.A. Abdullah, K.R. Al.-Belushi, Influence of aging parameters on the mechanical properties of 6063 aluminium alloy, Journal of Materials Processing Technology 102 (2000) 234-240.

[8] N.C.W. Kuijpers, W.H. Kool, P.T.G. Koenis, K.E. Nilsen, I. Todd, S. van der Zwaag, Assessment of diffrent techniques for quantification of -Al(FeMn)Si and -AlFeSi intermetallics in AA 6xxx alloys, Materials Characterization 49 (2003) 409-420.

[9] G. Sha, K. O’Reilly, B. Cantor, J. Worth, R. Hamerton, Growth related phase selection in a 6xxx series wrought Al alloy, Materials Science and Engineering A304-306 (2001) 612-616.

[10] A.K. Gupta, D.J. Lloyd, S.A. Court, Precipitation hardening in Al-Mg-Si alloys with and without excess Si, Materials Science and Engineering A316 (2001) 11-17.

[11] G.A. Edwards, K. Stiller, G.L. Dunlop, M.J. Couper, The precipitation sequence in Al-Mg-Si alloys, Acta Materialia 46/11(1998) 3893-3904.

[12] W.F. Miao, D.E. Laughlin, Precipitation hardening in aluminum alloy 6022, Scripta Materialia 40/7 (1999) 873-878.

[13] G. Biroli, G. Caglioti, L. Martini, G. Riontino, Precipitation kinetics of AA4032 and AA6082 a comparison based on DSC and TEM, Scripta Materialia 39/2 (1998) 197-203.

[14] Z. Li, A.M. Samuel, C. Rayindran, S. Valtierra, H.W. Doty, Parameters controlling the performance of AA319-type alloys: Part II. Impact properties and fractography, Materials Science and Engineering A367 (2004) 111-122.

[15] G. Mrówka - Nowotnik, J. Sieniawski, A. Nowotnik, Tensile properties and fracture toughness of heat treated 6082 alloy, Journal of Achievements in Materials and Manufacturing Engineering 17 (2006) 105-108.

[16] M. Wierzbi ska, J. Sieniawski, Effect of morfology of eutectuc silicon crystals on mechanical properties and cleavage fracture toughness of AlSi5Cu1 alloy, Journal of Achievements in Materials and Manufacturing Engineering 14 (2006) 31-36.

[17] L.A. Dobrza ski, R. Maniara, J.H. Soko owski, The effect of cast Al-Si-Cu alloy solidification rate on alloy thermal characteristic, Journal of Achievements in Materials and Manufacturing Engineering 17 (2006) 217-220.

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[24] ASTM E602-78T. Tentative method for Sharp-notch tension testing with cylindral specimens.

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