HEAT TREATING ALUMINIUM

HEAT TREATING ALUMINIUM

To understand the heat treatment and other ‘property improvement’ techniques used with aluminium and its alloys is necessary to understand what goes on inside the metal when it is first cast as either a pure metal or with the addition of alloys.

The aluminium molecule forms in the shape of a square with an atom on each corner and one in the center of each face of the square. The arrangement is called a face centered cubic cell.

As the metal chills from the molten state, these cells form at random in the metal and the cells join together building up to form the grain, A similar building of cells can be observed on the surface of freezing water. Unlike water however shrinkage rather than expansion occurs.

UNIT CELLS BUILD UP AS TEMPERATURE DROPS

Continuing to build up the grains eventually interfere with adjacent grains giving an uneven grain shape. The rate of cooling affects the shape, configuration and size of the grains. Generally the axis of the grains is at random and therefore cannot be compared with the unidirectional grain in timber.

GRAIN BUILD UP CONTINUED AS TEMPERATURE DROPS UNTIL RESTRICTED BY FORMATION OF ADJACENT GRAINS

FINAL GRAIN STRUCTURE: AS SEEN UNDER MICROSCOPE.

When the metal is subjected to a load the grain structure will be deformed. If the load exceeds the elastic limit the deformation will be permanent.

The deformation begins when the unit cells within the grain begin to slide over one another with the application of the load and takes place along the planes between the individual cells. The ease of slip determines the hardness of the alloy.

I will discuss heat treatment using copper as the major alloying element as the example.

Copper forms the same type of face centered cubic cells. When added as an alloy to aluminium, some of the copper atoms displace aluminium atoms within the cell and make the slip more difficult and hence the metal harder.

When alloys are added and the metal is first cast, two types of material are formed, Intermetallic Compounds (chemical compounds) and Solid Solutions (mixtures). The Intermetallic compounds tend to lie in relatively large particles. In this condition the metal is not very strong.

Before Solution Heat Treatment

CuAL₂Lies in large particles along the grain boundaries.

Aluminium can absorb small amounts of copper in solution at room temperatures. With an increase in temperature more copper is absorbed in solution.

There is a similarity when one dissolves sugar or copper sulfate crystals in water. More crystals can be dissolved in hot water than in cold water. If as many crystals as possible are dissolved in hot water and the water allowed to cool then the crystals will precipitate out of the solution.

In solution heat-treating we are carrying out a similar process. The larger proportion of copper is absorbed in the aluminium at elevated temperatures.

Quenching in cold water then rapidly cools the hot metal. This holds the saturated solution of copper in the alloy.

After Solution Heat Treatment

Cu AL₂is now dispersed in very fine particles uniformly throughout the grain.

This condition however is only temporary as the copper still precipitates out of solution the process is known as age hardening because as the copper precipitates out in small particles, evenly, and these form in the slip planes making the metal harder and stronger. This process takes about 4 days.

In the mean time the metal is in a soft, workable condition for about 20 minutes. It reaches 90% to 98% of its maximum hardness in 4 hours. This age hardening process may be halted if the newly treated metal is kept at temperatures below the freezing point of water.

Solution heat-treating puts the copper in the alloy in solid solution with the aluminium. It is then soft. After solution heat-treating, if the metal is allowed to reach temperatures above freezing, the metal age hardens to its strongest and hardest condition.

The age hardening process can be achieved artificially by Precipitation Heat-Treating (Artificial Ageing). The artificial ageing accomplished by precipitation heat-treating does a better and more even job than the natural method and with some alloys; the full hardness cannot be achieved without precipitation heat treatment.

The temperatures and soaking times shown on the tables are critical. If not then the solution and or precipitated alloy will contain pieces of free copper, which will cause intergranular corrosion.

Alloy	Annealing Treatment			Solution Heat Treatment (1)		Precipitation Heat Treatment
Alloy	Metal Temperature	Approx. Time of Heating Hours	Temper Designation	Metal Temperature F	Temper Designation	Metal Temperature F	Approx. Time of Heating Hours	Temper Designation
1100	650		-0
2017	775	2-3	-0	940	-T4
2024	775	2-3	-0	920	-T4	375	7-9	-T86
2117	775	2-3	-0	940	-T4
3003	775		-0
5052	650		-0
6061	775	2-3	-0	970	-T4	350	6-10	-T6
7075	775	2-3	-0	870	-W	250	24-28	-T6
(1) The time of heating varies with the product, the type of furnace and the size of load. For sheet metal heat treated in a bath of molten salt, the time may range from 10 minutes for thin material to 60 minutes for thick. Time of several hours may be required in air furnaces because the metal comes to temperature less rapidly. The material should be quenched from the solution heat-treating temperature as rapidly as possible and with a minimum delay after removal from the furnace. Quenching in cold water is preferred although less drastic chilling (hot or boiling water, air blast) is sometimes employed for bulky sections, such as forgings, to minimize quenching stresses.

Quenching is also critical. Parts should be immersed directly from the furnace, edge first. Cylindrical parts must be quenched end first. Loss of heat between removal from the furnace and quenching will result in uneven treatment and lead to intergranular corrosion.

There are two types of furnaces used for aluminium heat treatment. The air furnace, where hot air is the heating medium is the most satisfactory method for treating larger sections and the most usual production furnace in use.

Air furnaces are usually bottom loaded and located above the quench tank. Quench tanks should have a water spray on the surface to minimize the splash when parts are plunged into them from the furnace.

Salt baths are more easily controlled but because of their inherent dangers are generally reserved for small parts, rivets etc.

The Salt Bath

Salt baths contain a mixture of sodium nitrate and potassium nitrate. These salts melt at about 220 degrees C and in the molten condition they look like a pool of cool water. Hot salt can inflict nasty burns so don’t let appearances fool you, wear goggles, gloves, long sleeved overalls and leather aprons when using salt baths.

Jobs should be thoroughly dry before suspending in the salt bath. Water and salt definitely do not mix. Parts should be suspended so that they do not touch the sides or bottom of the bath.

If heating the bath from room temperature the surface of the salt should be broken. The bath should be cleaned regularly.

Magnesium rich alloys should not be treated in a salt bath.

In case of fire, evacuate the building and fight the fire from an external location, Water, in such cases cannot be used.

Non-heat treatable alloys may be annealed (softened) to enable extensive working. All alloys harden when worked as the working changes the shape of the grains.