Title: Precipitation Hardening
Course: Materials of Engineering > Section: MET 2200

PRECIPITATION HARDENING OF ALUMINUM ALLOYS
MET 2200-001 Lab Rev 5

INTRODUCTION:

Certain aluminum alloys have limited room temperature solubility for certain hardening elements.  These alloys are represented by partial solid solubility phase equilibrium diagrams.  Two examples are AA6061 which has silicon as its primary hardening element and AA2024 which has copper as its primary hardening element.  Each of these alloys can be thermally hardened via precipitation hardening.  In their precipitation hardened state, they have much different mechanical and electrical properties as compared to commercially pure aluminum alloys.

OBJECTIVE:

It is the objective of this experiment to demonstrate the effects of precipitation hardening on AA6061 and AA2024 aluminum alloys.

PRODEDURE:

Due to restrictions of furnace capacity, material availability, and testing equipment; precipitation hardening will only be monitored during this experiment with Rockwell hardness which is representative of strength changes which will be occurring during the experiment.  Alloy coupons approximately 1.5 inches square will be used as the test samples in this experiment.  A sample of the as-received material will be used as the reference for each alloy.

The procedure is below:

- Mark the samples on their edge with a vibrating etch tool.
>li>Test the Rockwell hardness on only one flat face of each sample using the Rockwell B or F scale.
- Solution heat treat 20 samples of each alloy to dissolve the excess hardening elements.
- AA2024 will be heated to 910 – 930 degrees Fahrenheit for a minimum of one (1) hour followed be an immediate room temperature water quench. (Caution – the small furnace controller causes over-temperature conditions.)
- AA6061 will be heated to 960 – 1010 degrees Fahrenheit for a minimum of one (1) hour followed be an immediate room temperature water quench.
- Hardness tests will be performed on each solution-treated sample using the Rockwell B or the Rockwell F scale depending on the hardness range required for the sample hardness.  Rockwell B scale is to be used when the sample hardness is too high for the Rockwell F scale.
- The solution treated samples will be precipitation hardened using the following sequences and hardness tested using Rockwell B or Rockwell F scales as needed:
  1. Four (4) samples of each alloy will be precipitation hardened at room temperature for 48 hours (T4); placed into a freezer to restrict any further precipitation; and then be hardness tested at the second lab meeting.
  2. Four (4) samples of the AA2024 alloy will be precipitation hardened at 365 – 385 degrees Fahrenheit for 12-16 hours (T6); hardness tested; and then placed into a freezer to restrict any further precipitation.
Four (4) samples of the AA6061 alloy will be precipitation hardened at 345 – 365 degrees Fahrenheit for 8 hours (T6); hardness tested; and then placed into a freezer to restrict any further precipitation.
  3. Four (4) samples of the AA2024 alloy will be precipitation hardened at 365 – 385 degrees Fahrenheit for 144 or more hours and then be hardness tested to check for any overaging at the second lab session...
Four (4) samples of the AA6061 alloy will be precipitation hardened at 345 – 365 degrees Fahrenheit for 144 or more hours and then be hardness tested to check for any overaging at the second lab session.
  4. Four (4) samples of each alloy will be precipitation hardened at room temperature for 144 or more hours (T4) to determine hardness changes when held at room temperature for an extended period.
  5. Four (4) samples of each alloy will be then placed into a freezer for 144 or more hours to restrict precipitation and then be hardness tested.
DATA ANALYSIS:
Discuss how all variations of experimental equipment, material, procedure, and data collection may have changed the results of the experiment and include reasons for these observations.  The hardness data will need to be normalized to Brinell if any situations arise where the hardness values for any of the samples require the use of more than one (1) Rockwell hardness test range.   Reference comparison data is to be obtained from handbooks or from the on-line Aluminum Association website for the alloys and for the precipitation treatments used in this experiment.  Graphing temperature as the ordinate and time as the abscissa will provide a record of the thermal history for the sample lots.  Graphing hardness as the ordinate and using the same time scale as the thermal history graph will allow hardness trends to be directly compared with thermal history.
DISCUSSION:
The hardness data obtained for each of these two alloys will need to be discussed using the relationship to the thermal treatments and to the representative phase equilibrium diagrams for each alloy.  The discussion must include precipitation hardening mechanisms to explain why the hardness changed during as the experiment progressed from as-received coupons forward through the experiment.  A working knowledge of the precipitation hardening process is to be evident in your discussion.
CONCLUSION:
What conclusion(s) can be drawn from this experiment on these aluminum alloy materials?