effect of technologies processing on material properties of selected aluminium alloys
TRANSCRIPT
Effect of Technologies Processing on Material Properties of Selected Aluminium Alloys
Petra Lacková1, a, Daniela Žabecká1, b, Ondrej Milkovič1, c, Milan Škrobian2, d and Matúš Bajcura3, e
1Technical University in Košice, Faculty of Metallurgy, Department of Materials Science, Park Komenského 11, 042 00 Košice, Slovak Republic
2Sapa Profiles a.s., Na Vastičke 7, 965 01 Žiar nad Hronom, Slovak Republic 3Constellium Extrusions Decin s.r.o., Ustecka 37, 405 35 Decin, Czech Republic
[email protected], [email protected], [email protected], [email protected], [email protected]
Keywords: aluminum alloys, aluminum structure, mechanical properties, fatigue properties, fatigue
fracture, Wӧhler curve
Abstract. The paper deals with an analysis of properties of industrially fabricated aluminum alloys,
specifically EN AW 6082 (AlSi1MgMn), EN AW 6061 (AlMg1SiCu) and EN AW 7075
(AlZn5.5MgCu). The alloys were processed by two different treatments T3 (solution annealing,
cold forming and natural aging) and T4 (solution annealing and natural aging). Mechanical
properties, structure as well as symmetrically torsional fatigue of these alloys were evaluated. The
paper was aimed to study the influence of processing technology of three aluminum alloys on their
properties in the conditions of tensile stress and cyclic load in torsion.
Introduction
Aluminum alloys belong to the materials which are mostly used in aircraft and automotive industry
with a requirement for fatigue resistance of the material [1]. Fatigue damage makes 75% of all
damages of the construction materials [4]. In this paper we focused on study of the aluminum alloys
of type 6XXX that is mainly alloyed by Mg, Si and of type 7XXX alloyed by Zn, Mg and the
matrix of composite material was aluminium alloy EN AW 6061, and as reinforcements were used
SiC particles. Due to the alloying the selected aluminum alloys have a good formability, mechanical
properties, weldability and corrosion resistance but on the other side they have a low fatigue
resistance [2, 3]. The factors which influence fatigue resistance of the material are type and
frequency of load, treatment, shape, and surface quality or other stress concentrators of material and
influence of the environment where the material is located [5]. Most of the published articles [6-8]
dealt with fatigue properties during bending and tensile load. Therefore, torsion fatigue properties
of the alloys are analyzed in this article.
Materials and experimental
Three aluminum based materials were analyzed: the composite EN AW 6061-T4 and alloys
EN AW 6082-T4, EN AW 7075-T3; with chemical composition shown in Table 1. Aluminum
alloys were industrially produced in a form of drawn rods and heat treated by T3 (solution
annealing, cold forming and natural aging) and T4 (solution annealing and natural aging)
procedures [9].
Table 1 Chemical composition of aluminum alloys [wt. %]
Material Al Si Mg Zn Mn Cu Fe
EN AW 6061 95.35 0.40 0.80 0.25 0.15 0.15 0.70
EN AW 6082 97.46 0.96 0.68 - 0.62 - 0.22
EN AW 7075 88.32 - 2.67 5.64 - 1.76 0.27
Materials Science Forum Vol. 782 (2014) pp 394-397Online available since 2014/Apr/09 at www.scientific.net© (2014) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/MSF.782.394
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Mechanical properties (yield strength - YS, ultimate tensile strength - UTS, elongation - A5
and contraction - Z) were measured by uni-axial tensile test carried out by ZWICK 1387 machine
according to STN EN ISO 6892-41 [10]. Young modulus was measured by using the extensometer
WN2 52497. Hardness was estimated in a cross-section by the Vickers test. Results of mechanical
properties evaluated as average value from six measurements are summarized in Table 2.
Table 2 Mechanical properties of aluminium alloys
Material YS
[MPa]
UTS
[MPa]
A5
[%]
Z
[%]
E
[GPa]
HV 10
[N.mm-2
]
HV 30
[N.mm-2
] YS/UTS
EN AW
6061 205 274 9 9 110 88 80 0.74
EN AW
6082 185 297 20 57 70 83 80 0.62
EN AW
7075 581 636 7 8 74 165 139 0.91
It is possible to estimate that the mechanical properties are the most dependent to type of treatment
of materials (T3 and T4). The highest YS and UTS was measured for EN AW 7075, but there was
applied cold drawn after solution annealing which led to the highest strain hardening. On the
contrary the alloys treated by T4 processing show lower strain hardening but the significant
differences in plasticity are found out in aluminum composite.
The samples for observation by light microscopy were prepared by metallographic
procedure. Samples etching were carried out by Kroll solution (92ml distilled water, 6ml HNO3,
and 2ml HF). Microstructures are documented in Fig. 1 by an optical microscope OLYMPUS
VANOX-T.
Fig. 1 Microstructures of the composite EN AW 6061 (a) and alloys EN AW 6082 (b),
EN AW 7075 (c)
Thin foils for transmission electron microscopy (TEM – Jeol JEM 200 FX operated at 200 kV)
observation were prepared by double jet electropolishing in TenuPol – 5 at voltage 15V. Solution of
33% HNO3 and 67% CH3OH was used as an electrolyte. Structures have been observed by TEM in
the bright field images which are shown in the Fig. 2.
Fig. 2 Microstructures (EN AW 6061 (a), EN AW 6082 (b)
and EN AW 7075 (c)) observed by TEM
The observed dislocation density and grain size of materials confirmed the assumption that the mechanical properties are highly dependent on the type of materials treatment (T3 and T4). The highest dislocation density and the lowest grain size were observed in EN AW 7075 alloy which was treated by T3 processing. Regardless of different chemical composition of materials, the TEM observation clearly correlated to the measured values YS/UTS (Table 2).
a b c
a b c
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Specimens for fatigue tests were prepared by turning machine in the form of rods with sizes
35.8 x 7.95 x 6.00 mm. Torsional fatigue was carried out by HENCK PWO 0298 machine.
Numbers of cycles to final rupture of materials are summarized in Table 3. The failure values are
displayed in semi-logarithmic plot of Wöhler curve in Fig. 3 which indicates non-existence of the
endurance limit typical for the aluminum alloys.
Table 3 Results of the fatigue in torsion
Material τ [MPa] Nc Material τ
[MPa] Nc Material
τ
[MPa] Nc
EN AW
6061
239 2700
EN AW
6082
215 19500
EN AW
7075
169 242300
172 662700 185 29100 124 457300
166 618400 175 55600 113 897400
110 1017300 130 337400 82 5736800
On the basis of a slope of diagonal branch Wohler curve can be noted the highest crack
initialization resistance of EN AW 6082 alloy. The lowest resistance was detected on the composite
EN AW 6061, which is related with ductility reduction of alloy. In Fig. 4 is shown nature of
samples breach during cycle loading in torsion for selected types of aluminium alloys. This breach
was occurred in the application of stress: 172 MPa for the EN AW 6061 composite, 175 MPa for
the EN AW 6082 alloy, 124 MPa for the EN AW 7075 alloy. Fracture surface documented in
Fig. 4, is perpendicular to the load direction on the EN AW 6082 alloy and EN AW 7075 alloy, too.
Surface of fracture on the EN AW 6061 composite took place in the direction of load axial.
Fig. 4 Fatigue fracture of the composite EN AW 6061 (a) and alloys EN AW 6082 (b),
EN AW 7075 (c)
Fraction areas of the observed alloys were assessed by SEM Jeol JSM–7000F. Fraction surfaces, documented by SEM, are shown in Fig. 5. Fracture fatigue surface can be divided to three characteristic areas: place of initialization, stable crack growth, and final rupture. Initialization of the fracture started from the surface of specimen and the crack growth continues in perpendicular direction of the specimen axis. The morphology of fracture surface was observed as characterized dimples with local presence of striation. Fine grained morphology was observed in the composite EN AW 6061.
Fig. 3 Wöhler curve of aluminium alloys
a b c
396 Metallography XV
Fig. 5 Fracture surfaces of the composite EN AW 6061 (a) and alloys EN AW 6082 (b),
EN AW 7075 (c)
Summary
Based on the results of the experimental part, it is possible to note the following:
- Treatment procedures and chemical composition of the three industrially fabricated
aluminum alloys caused to strong differences of strength properties (i.e. YS in the range
from 185 to 581 MPa) and plasticity (from 7% to 20 %).
- The highest crack initialization resistance was found out in the alloy EN AW 6082 and the
lowest in the composite EN AW 6061.
- Fracture surface is perpendicular of the load on the EN AW 6082 alloy and EN AW 7075
alloy, too. Surface of fracture on the EN AW 6061 composite took place in the direction of
load axial.
- The flat fracture with dimples and local presence of striations was observed on the EN AW
6082 and EN AW 7075 alloys. The composite EN AW 6061 had highly fractal fracture with
fine grained morphology.
- The grain size and dislocation density of aluminum alloys were observed by TEM and the
observation proved that the mechanical properties (especially YS/UTS) of alloys are the
most dependent on the treatment.
References
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a b c
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