1.1 addition(s). Most of these heat-treatable alloys however

1.1       
HEAT TREATment of aluminum alloy

Heat
treatment in its broadest sense, refers to any of the heating and cooling methods
that are taken for the purpose of changing mechanical properties and the
metallurgical structure, or the residual stress state of a metal products38.

Some
of the marketable heat-treatable aluminum alloys are, with few exceptions,
based on ternary or quaternary systems with regard to the solutes that are involved
in establishing strength by precipitation. The Commercial alloys that their hardness
and strength can be significantly increased by the heat treatment include 2xxx,
6xxx, and 7xxx series, wrought alloys except the 7072 and 2xx. And some of
these have only copper, or have copper and silicon, as their primary
strengthening alloy addition(s). Most of these heat-treatable alloys however
may contain combinations of magnesium with one or more of the elements copper, zinc,
and silicon. Characteristically, even litle amounts of magnesium I concert that
have these elements accelerate and accentuate precipitation hardening, while
alloys in the 6xxx series have magnesium and silicon approximately in the
proportions required for the formulation of magnesium silicide (MgESi).
Although not as strong compared to most 2xxx and 7xxx alloys, 6xxx series
alloys have good machinability, formability, weldability, and corrosion
resistance, with medium strength39.

Heat
treatment to increase strength of aluminum alloys is a process that have three
steps;


Solution heat treatment: dissolution of the soluble phases


Quenching: development of the supersaturation


Age hardening: precipitation of solute atoms at room temperature (natural aging)
or either elevated temperature (precipitation heat treatment or artificial
aging)

1.1.1       
Aluminum heat
treatment

Aluminum
heat treating requires stringent controls. These controls are put in place to
avoid melting the aluminum alloy during solution heat treatment, and also to
ensure that a safe and durable product is manufactured and created40. Temperature uniformity requirements
inside furnaces are tight (±5ºF)in order to prevent eutectic melting, and also
insure uniform properties throughout the workload.

The
ideal properties of aluminum are achieved by alloying additions and heat
treatments. This promotes the creation of small hard precipitates that
interfere with the motion of dislocations and improve its mechanical
properties. 7075 aluminum alloy is one of the most commonly used aluminum alloy
for structural applications due to its attractive comprehensive properties such
as high strength, low density, toughness ductility and resistance to fatigue.
It has been completely utilized in aircraft structural parts and also in other
highly stressed structural applications.

Aluminum
alloy 7075 Chemical composition.

Element

%wt.

Zn

5.6

Mg

2.5

Cu

1.6

Al

Balance

 

1.1.2       
Defects that Occur
During Heat Treatment

During
the production of a part, defects may occur. These defects can come from
operations before heat treatment, such as midline porosity, inclusions which
are formed during casting of the ingot. More defects can form during
homogenization of the ingot, such as segregation, the formation of hard
intermetallic and second phase particles41. Most these defects associated with
heat treatment of aluminum can occur either during solution heat treatment, or
during quenching. Solution heat treating defects include incipient melting, oxidation
and under-heating. Defects which occur during quenching are typically
distortion or inadequate properties which is caused by a slow quench, resulting
in precipitation during quenching and inadequate supersaturation.

1.1.3       
Oxidation.

 If part is exposed to temperature for too
long, high temperature oxidation could become a problem41. This term high temperature
oxidation is really a misnomer. The culprit is actually moisture in the air
during the process of solution heat treatment. This moisture that is a source
of hydrogen, which diffuses into base metal. Voids form at the inclusions or
other discontinuities. The hydrogen gas accumulates, and then forms a surface
blister on the part. In general, 7XXX alloys is one of the most susceptible
(particularly 7050), then followed by 2XXX alloys. Extrusions are mostly prone
to blistering then followed by forgings.

Elimination
of moisture minimizes the problem of the surface blistering. This is
accomplished by the sequencing of door over quench tanks, and thoroughly drying
and then cleaning furnace loads prior to the solution heat treatment. It is
also important to sure that the load racks used for the solution heat treatment
are also dry. However, it is not always possible to eliminate the high humidity
in air to prevent surface blistering. Often the ambient relative humidity is very
high, so that the other measures may have to be taken42.

Use
of ammonium fluoroborate is typically used to prevent blistering on the 7XXX
extrusions. An amount equivalent to 5 g per m3 of workload space is usually
used in prevention of surface blistering. This is applied as a powder in the
shallow pan hanging from furnace load rack. This material is very corrosive and
it requires operators to wear the appropriate personal protective safety equipment’s.
The material is corrosive at temperature, it is highly recommended that the
inside panels in the furnace be manufactured using stainless steel. This reduces
corrosion and maintenance.

Anodizing
of parts prior to the solution heat treatment is an alternative to ammonium
fluoroborate. This is generally practical for the larger extrusions and
forgings, where cost of anodizing is small compared to cost of the part38.

Distortion during Quenching. Of all possible “defects” occurring
during heat treatment of aluminum, distortion during quenching is the very most
common. It is probably responsible for most of the non-value-added work
(straightening) and costs associated with the aluminum heat-treating. This distortion
during quenching is caused by differential thermal strains developed during
quenching, and differential cooling 17. These thermal strains could be
developed surface-to-surface or center-to-surface. Differential cooling can be
caused by large quench rates, so that center is cooled much slower than the
surface (non-Newtonian cooling) or by non-uniform heat transfer across surface
of the part.

1.1.4       
Stress Relief

Immediately
after the part is quenched, most aluminum alloys are nearly ductile as they are
in  annealed condition. Consequently, it
is often advantageous to stress relieve the parts by working the metal
immediately after quenching process. Numerous attempts have been made to
develop a thermal treatment which will remove, or appreciably reduce these
quenching stresses. The normal precipitation heat-treating temperatures are
generally too low in providing appreciable stress relief. Exposure to some
higher temperatures (which stresses are relieved more effectively) results in some
lower properties. However, such treatments are sometimes utilized when even the
moderate reduction of the residual stress levels is important enough so that
some sacrifices in mechanical properties can be accepted43.

1.1.5       
Mechanical Stress
Relief.

Deformation
consists of stretching (plate, extrusions, and bar) or compressing (forgings) product
sufficiently to achieve a small but a controlled amount (1 to 3%) of plastic
deformation. If  benefits of mechanical
stress relieving are in need, the user should refrain from the reheat treating44.

Effect
of the Precipitation Heat Treating on Residual Stress. The stresses that
developed during quenching from solution heat treatment are reduced during the
subsequent precipitation heat treatment. Degree of relaxation of stresses is
highly dependent upon the time and temperature of  precipitation treatment and alloy composition.
In general, the precipitation treatments that is used to obtain the T6 tempers
provide only modest reduction in stresses, ranging from around 10 to 35%. To
achieve a substantial lowering of a quenching stresses by a thermal stress
relaxation, higher-temperature treatments of T7 type are required. These
treatments are used when the lower strengths resulting from the averaging are
acceptable.

1.1.6       
Other thermal
stress-relief treatments,

They
are known as subzero treatment and cold stabilization, involve cycling of the
parts above and below room temperature. Temperatures chosen are those that can
be readily obtained with boiling water and mixtures of a dry ice and
alcohol–namely, 100 and -73 °C 1212 and -100 °F)–and the number of  the cycles ranges from one to five. The
maximum reduction in residual stresses that can be effected by some these
techniques is about 25%. The maximum effect that can be obtained only if the
subzero step is performed first, and immediately after the quenching from
solution- treating temperature while yield strength is low. No benefit that is
gained from more than one cycle. A 25% reduction in residual stresses is
sometimes sufficient to permit fabrication of parts that can not be made
without this reduction45. However, incase a general reduction
is needed, as much as 83% relief of residual stress is possible by increasing
the severity of uphill quench–that is, more closely approximating the reverse
of cooling rate differential during the original quench46.

1.1.7       
Dimensional Changes
during

Heat
Treatment In addition to completely reversible changes in dimension which are
simple functions of temperature change and caused by thermal expansion and
contraction, dimensional changes of a more permanent character are also
encountered during heat treatment. These changes are of different types, some are
of mechanical origin and others are caused by changes in metallurgical
structure. Changes of the mechanical origin include those arising from stresses
developed by the gravitational or other applied forces, from thermally induced
stresses or from the relaxation of the residual stresses. Dimensional changes
also accompany the recrystallization, solution, and the precipitation of
alloying elements.