2 corrosion. Besides of T73, another process named retrogression and re-aging or RRA
is another heat treatment process which is well known to enhance the corrosion resistance of aluminum alloy.
This project is done to study the effect of various heat treatment processes on the microstructure and stress corrosion cracking of aluminum alloy 7075. Heat treatment
is applied on the material specimens to enhance their corrosion resistance against the stress corrosion cracking. Two different types of heat treatment processes, namely
T73 tempering and RRA are applied on the aluminum alloy 7075, with T6 tempering as the reference. Direct tensile stress corrosion cracking test is done on the specimens,
where later tensile test is done to determine the residual stress of the material after it undergone DTSCC. Besides, the tensile strength of the materials after DTSCC is
then compared to the tensile strength of the materials which are not corroded under DTSCC, at which the difference of strength, indicates the occurrence of SCC within
the materials. This cause of such properties is explained by relating with the microstructure appearance of specimens.
1.2 Problem Statement
In modern day transportations, as well as aerospace applications, the demand for lighter materials that possess good mechanical properties have become the direction
for researches and developments, with the hope to find the best possible material for the structural components that suits the all requirements. Besides the aspect of light
weight and mechanical properties, one other important concern in the structural components for these applications is their susceptibility to corrosion, especially when
the materials are alloys.
The problem of corrosion is not a fresh issue in metals and alloys, and many solutions had been proposed to overcome corrosion problem. Among all corrosion
failures, stress corrosion cracking can be said as one of the important failure cause. Its occurrences are mostly undetectable or not apparent at the first place. Stress
3 corrosions cracks are mostly microcracks, where the cracks normally occur at the
grain level. This phenomenon happens in all alloys, depending on the environment and stress level, at which the effect and consequence can be catastrophic compared to
mechanical cracking. The corrosion takes place on the surfaces of the material, forming surface discontinuities that eventually becomes the stress raiser or notch to
crack propagation at microstructural level, but the mechanism of crack propagation in such phenomenon is not merely caused by the atomic dislocation due to the stress,
but it is also caused by the chemical attack on the crack tip, causing inter-granular cracks along the grain boundaries of the material. As the cracking happens within the
grain boundaries, it is invisible to naked eyes. These inter-granular cracks will then become a mechanical crack when the crack growth achieves a certain crack size that
is quite visible to naked eyes. SCC can happen in also all type of environment, but the corrosion rate maybe faster is the material is subjected to its susceptible
environment.
7000 series Al alloys have been widely used as structural materials in aeronautical and transportation purposes due to their attractive comprehensive properties, such as
low density, high strength, ductility, toughness and resistance to fatigue. Perhaps due to its lower price and cost of manufacturing, 7000 series AA gains higher popularity
as compared to a better but more expensive material known to be titanium alloys. 7075 Al alloy is one type of the 7000 series AA. Not only it possesses the same
principle alloying element as other 7000 series AA, i.e. zinc,
it is also sensitive to localized corrosion, such as inter-granular corrosion, exfoliation corrosion and stress
corrosion cracking. Li et al., 2007
For about 40 years as since aluminium alloys begin to gain their wide acceptance in the modern day applications, researches and developments in improving the
mechanical properties and the resistance to corrosion had been done vigorously. In the studies done by Li et al. 2007 and Reda et al. 2008, the stress corrosion
resistance, in the 7000 series aluminium alloys, including aluminium alloy7075 can be modified by heat-treatment. In the heat treatment done for such purposes, the most
common heat treatment methods for aluminium alloy 7075 would be over-aging, as in T7 tempering. Although the aluminium alloy 7075 with T6 tempered, possesses
high strength, however their localized corrosion resistance is poor. As enhancement
4 upon this problem in T6, over-aging treatments such as T73, T76 and T74 have been
developed. However, the strength of these over-aging treated aluminium alloy 7075- T7 is relatively poorer than T6. Retrogression and re-aging, RRA is then developed
as it produces a balance of properties for both strength and corrosion resistance for the 7000 series Al alloys. Reda et. al., 2008 However, the RRA treatment cannot be
used for large-section Al alloys due to its very short retrogression time.
In this investigation, by applying the three methods of T6, T73 and RRA tempering, the microstructure of the heat treated aluminium alloy 7075 is altered. T73
tempering and RRA are done to produce the microstructure that would resist the stress corrosion cracking, while the T6 tempering is done to produce materials with
properties referable by both T73 and RRA. It is not sufficient to compare only in stress corrosion resistance of both heat treatment processes to T6, as in the same time,
other mechanical properties are taken into account as well. In this study, it is crucial to identify the effect on the tensile strength and hardness of the material, besides of
looking in the performance of the material to resist stress corrosion cracking. Then, both heat treatment processes is compared to identify which one will actually be the
best process of giving the material good stress corrosion cracking resistance and mechanical properties. The identification of the best process is done by looking at the
formation of microstructure of both heat treatment processes compared to T6, as well as the reduction rate of the tensile strength when subject to stress corrosion. The
reduction rate of the tensile strength will eventually provide the indication of the occurrence of stress corrosion cracking. By identifying these measuring keys, it is
possible to answer the problem of which is the best process that would provide good stress corrosion cracking resistance, but in the same time having good mechanical
properties.
5
1.2 Objectives and Aims
