1.1 hardening, while alloys in the 6xxx

1.1       
HEAT TREATING of aluminum alloy

Heat treatment in its
broadest sense, refers to any of the heating and cooling methods that are
performed for the purpose of changing the mechanical properties and the
metallurgical structure, or the residual stress state of a metal product38.

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The marketable
heat-treatable aluminum alloys are, with few exceptions, based on ternary or
quaternary systems with regard to the solutes involved in establishing strength
by precipitation. Commercial alloys that hardness and strength can be
significantly increased by heat treatment include 2xxx, 6xxx, and 7xxx series,
wrought alloys except 7072 and 2xx. 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.