This document summarizes the process of precipitate hardening or age hardening in an Al-Cu alloy. It involves three steps: 1) solution heat treatment to dissolve soluble phases, 2) quenching to develop supersaturation, and 3) age hardening through precipitation either at room temperature (natural aging) or elevated temperatures (artificial aging). Samples of an Al-4%Cu alloy were solution heat treated and quenched, then aged at different temperatures from 150-230C to test the effect on hardness. Hardness increased with increasing aging temperature, demonstrating successful precipitation hardening.

1. 1
Abstract:
This report contains a brief introduction about an important heat treatment
technique for aluminum-copper alloy called “precipitate hardening”, objective of this
technique, different types of aging, how to perform this treatment and the results obtained.
Objective:
To study the effect of Age Hardening in Al-Cu alloy
Material and Equipment:
 Al-Cu alloy (4% CU)
 Heating furnace
 Quenching medium (water)
 Rockwell Hardness Testing machine
Introduction:
The term “heat treating” for aluminum alloys is frequently restricted to the
specific operations employed to increase their strength and hardness. 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. The process is called
“Precipitate hardening or age hardening”. 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).

2. 2
The mayor aluminum alloy systems with precipitation hardening include:
 Aluminum-copper systems with strengthening from CuAl2
 Aluminum-copper-magnesium systems (magnesium intensifies precipitation)
 Aluminum-magnesium-silicon systems with strengthening from Mg2Si
 Aluminum-zinc-magnesium systems with strengthening from MgZn2
Solution Heat Treating:
To take advantage of the precipitation hardening reaction, it is
necessary first to produce a solid solution. The process by which this is accomplished is called
solution heat treating and its objective is to take into solid solution the maximum practical
amounts of the soluble hardening elements in the alloy. The process consists of soaking the
alloy at a temperature sufficiently high and for a time long enough to achieve a nearly
homogeneous solid solution. The equilibrium solid solubility of copper in aluminum increases
as temperature increases--from about 0.20% at 250 °C (480 °F) to a maximum of 5.65% at the
eutectic melting temperature of 548 °C (1018 °F). The time requirement can vary from less
than a minute for thin sheet to as much as 20 h for large sand or plaster-mold castings.
Fig 1. Aluminum-Copper Binary Phase Diagram

3. 3
Quenching:
Quenching is in many ways the most critical step in the sequence of heat-
treating operations. The objective of quenching is to preserve the solid solution formed at the
solution heat-treating temperature, by rapidly cooling to some lower temperature, usually
near room temperature; to produce supersaturated solution at room temperature - the
optimum condition for precipitation hardening. Most frequently, parts are quenched by
immersion in cold water, or in continuous heat treating of sheet, plate, or extrusions in
primary fabricating mills, by progressive flooding or high-velocity spraying with cold water.
Age Hardening:
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.
Natural Aging:
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 tempers (T3 and T4 types)
that are characterized by high ratios of tensile to yield strength and high fracture toughness
and resistance to fatigue. 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 (solute rich microstructural domains) and strength increases rapidly, attaining
nearly maximum stable values in four or five days.

4. 4
Precipitation Heat Treatment:
Generally are low-temperature, long-term processes.
Temperatures range from 115 to 190°C; times vary from 5 to 48 h. Choice of time-
temperature cycles for precipitation heat treatment should receive careful consideration.
Larger particles of precipitate result from longer times and higher temperatures. Precipitation
heat treatment following solution heat treatment and quenching produces T6- and T7-type
tempers. Alloys in T6-type tempers generally have the highest strengths. Precipitation heat
treatment following solution heat treatment and quenching produces T6- and T7-type
tempers. Alloys in T6-type tempers generally have the highest strengths.
Procedure:
 First of all, given 6 sample of Al-Cu alloy were solution heat treated at 550°C for 1h
 After that, all the samples were quenched in water bath
 After quenching, 2 samples were aged at 150°C, other 2 samples were aged at 190°C
and remaining 2 samples were aged at 230°C for 24h
 After cooling, their hardness were measured
Observations and Calculations:
Specimen No. Solution Treating
Temp. (°C)
Solution Treating
Time (h)
Aging
Temp. (°C)
Aging
Time (h)
HRc
1 550 1 150 24 54
2 550 1 150 24 65
3 550 1 190 24 71
4 550 1 190 24 82
5 550 1 230 24 61
6 550 1 230 24 68

5. 5
Fig 1. Graph representing relation b/w aging temperature and hardness
References:
 ASM Handbook Volume 4
 “Introduction to Physical Metallurgy” by Sydney H. Avner
 “Heat Treatment Principles And Techniques” by T.V Rajan, C.P Sharma and Ashok
Sharma
0
10
20
30
40
50
60
70
80
90
0 50 100 150 200 250
HRc
Aging Temperature
Aging Temperature Vs Hardness