Ht Al Age Hardening New

8/6/2019 Ht Al Age Hardening New

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SUBMITTED BY:GROUP- Y2

ROLL NO: 1850-1865

B.TECH (AE- 1)

SIXTH SEMESTER

SUBMITTED TO:GP. CAPT.

PRAVIN KHANNA


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The term heat treating for aluminum alloys is frequently restricted to

the specific operations employed to increase strength and hardness of

wrought and cast alloys.

These usually are referred to as the heat-treatable alloys to

distinguish them from those alloys in which no significant

strengthening can be achieved by heating and cooling.

Heat treatment to increase strength of aluminum alloys is a three-step

process:

Solution heat treatment: dissolution of soluble phases

Quenching: development of Supersaturation

Age hardening: precipitation of solute atoms either at room

temperature (natural aging) or elevated temperature (artificial aging

or precipitation heat treatment).

INTRODUCTIONINTRODUCTION


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After solution treatment and quenching, hardening is achieved either at

room temperature (natural aging) or with a precipitation heat

treatment (artificial aging).

In some alloys, sufficient precipitation occurs in a few days at room

temperature to yield stable products with properties that are adequate for

many applications.

These alloys sometimes are precipitation heat treated to provide increased

strength and hardness in wrought or cast products. Other alloys with slow

precipitations reactions at room temperature are always precipitation heat

treated before being used.

In some alloys, notably those of the 2xxx series, cold working or fresh

quenching of the material greatly increases its response to later

precipitation heat treatment.


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During this hardening and strengthening operation, precipitation of the

soluble constituents from the supersaturated solid solution takes place.

As precipitation progresses, the strength of the material increases, often by

a series of peaks, until a maximum is reached.

Further aging (over aging) causes the strength to steadily decline until a

somewhat stable condition is obtained.

The submicroscopic particles that are precipitated provide the keys or

locks within the grain structure and between the grains to resist internalslippage and distortion when a load of any type is applied. In this manner,

the strength and hardness of the alloy are increased.


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NaturalAging

The more highly alloyed members of the 6xxx wrought series, the

copper-containing alloys of the 7xxx group, and all of the 2xxx alloys

are almost always solution heat treated and quenched.

For some of these alloys, particularly the 2xxx alloys, the precipitation

hardening that results from natural aging alone produces useful tempersthat are characterized by

For the alloys that are used in these tempers, the relatively high

Supersaturation of atoms and vacancies retained by rapid quenching

causes rapid formation of GP zones, and strength increases rapidly,attaining nearly maximum stable values in four or five days.


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Precipitation heat treatments

They are generally low-temperature, long-term processes. Temperatures

range from 115115 toto 190190CC; times vary from 55 toto 4848 hourshours.

Production of material in T5- through T7-type tempers necessitates

precipitation heat treating at elevated temperatures (artificial aging).

Larger particles of precipitate result from longer times and higher

temperatures but must be fewer with greater distances between them.

The objective is to select the cycle that produces optimum precipitate size

and distribution pattern. Unfortunately, the cycle required to maximize one

property is usually different from that required to maximize others.

Consequently, the cycles used represent compromises that provide the bestbest

combinationscombinations ofof propertiesproperties..


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Precipitation hardening

. The process used to obtain the desired increase in strength is

therefore known as aging, or precipitation hardening.

The strengthening of the heat-treatable alloys by aging is not due merely

to the presence of a precipitate.

As a rule, the artificially aged alloys are slightly over aged to increase

their resistance to corrosion.

This is especially true with that are

susceptible to when inadequately aged.


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The 2xxx seriesaluminiumalloys are alloyed with copper from 1.9-6.8%

and often contain additions of manganese, magnesium and zinc.

They are used for applications such as:

Forgings, extrusions and liquefied gas storage tanks in civil transport and

supersonic aircraft.

These alloys have low crack growth rates and are therefore used on thelower wings and body skin. The alloys used are 2224, 2324 and 2524 and

are clad with 99.34% pure aluminium for increased corrosion resistance.

7xxx Seriesaluminiumalloys

Of all the aluminium alloys the Al-Zn-Mg system offers the greatestpotential for precipitation hardening. Stress corrosion cracking resistance

decreases with increasing Zinc to Magnesium ratio.

The stress corrosion cracking problems have been the biggest restriction

upon the use of these alloys but they have still been used in lightweight

military and civil aircraft.

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Ht Al Age Hardening New