Precipitation hardening strengthens an aluminium alloy in three steps: heat it to about 500 °C so the alloying elements dissolve, quench it in water to trap them in solution, then age it at 120-190 °C for 4-24 hours so fine particles form and block slip, lifting strength from about 120 MPa to over 500 MPa. Temperature control decides the result, because the solution temperature sits close to where the alloy starts to melt and too long or too hot an ageing softens it again. Welding or overheating undoes the effect.
Part of Heat Treatment
Precipitation hardening, or age hardening, is a heat treatment in which a supersaturated solid solution forms fine second-phase particles that make the alloy much stronger.
The Strength Comes From Particles Too Small To See
At high temperature an alloy of aluminium with copper, zinc or magnesium and silicon can hold more of the alloying element dissolved in the aluminium than it can at room temperature. Cooling slowly lets the extra come out as coarse particles that do little. Quenching rapidly freezes it in place, leaving a supersaturated solid solution that is soft. Ageing at moderate heat lets the atoms move a short distance and gather into tiny particles. Those particles stop planes of atoms from slipping, as described in Grain Boundaries for boundaries, and the metal gets far stronger. The relevant boundary on a phase diagram is the solvus, the line below which the element no longer dissolves.
Quenched steel and aged aluminium look alike on paper but differ. In steel, quenching makes the hard structure and Tempering softens it. In aluminium, quenching makes a soft state and ageing hardens it. The article on steel Heat Treatment and this one are opposites in that respect.
Method: The Three Steps
The Advanced Materials article states them:
- Solution treatment. Heat the alloy to 500 °C and hold it, which dissolves the alloying elements into solid solution.
- Quench. Plunge it into water at once. The speed traps the elements in a supersaturated solution. A slow transfer lets particles form coarsely and the treatment fails.
- Age. Hold at 120-190 °C for 4-24 hours, where fine precipitate particles form and strength rises.
The Advanced Materials article’s result is a rise from 120 MPa for the pure metal to 500 MPa or more for a 7075-T6 equivalent.
Reading the ageing numbers: the longer-time end suits the cooler temperature, and the shorter-time end the hotter. Peak strength comes at a particular time for each temperature. Ageing past it makes the particles coarsen and the alloy softens, which is called overageing. So find the peak by testing: age several identical pieces for different times and compare hardness.
Temperature Is The Whole Problem
The solution temperature is high enough to soften the metal and, for some alloys, close enough to the temperature where grain boundaries begin to melt that being a few tens of degrees too hot can ruin the part. Aluminium melts at 660 °C with no colour change, as the Advanced Materials article warns. The corpus’s colour method in Temperature Reading gives control to within 25-50 °C, and the “dark red” band covers 500-600 °C, so colour alone cannot tell 500 °C from a damaging 550 °C. Use a thermocouple or pyrometer if you possibly can, as the Advanced Materials article says. For the lower ageing range, melting points in the same article give fixed calibration points: tin melts at 232 °C and lead at 327 °C, both above the 190 °C ageing limit, and an oil bath thermometer works within the 120-190 °C range.
Test By Hardness, Not By Colour
Aluminium does not change colour with temper. Check the result by hardness: the Advanced Materials article’s Brinell test uses a 500 kg load on soft metals, as described in Hardness Testing, and compare against a piece of the same alloy that you left untreated. A tensile coupon from the same batch, per Tensile Testing, is better.
What Else This Applies To
Nickel alloys for hot parts are hardened by the same mechanism, with ageing at much higher temperatures. Mixed scrap aluminium usually does not respond predictably, as Aluminum Alloys explains, so use this treatment on known alloys.
Hot quench, steam and oil
A part at 500 °C dropped into water makes a burst of steam, so wear a face shield and stand clear of the splash, and keep the quench tank large and the part dry of trapped water. An oil bath for ageing must stay well below its flash point and never be left unattended. Never use a nitrate salt bath for aluminium without expert guidance, since molten nitrate with aluminium or organic material is explosive.
Common Mistakes
- Slow quench — the strengthening elements precipitate coarsely.
- Welding the aged part — the metal beside the weld overages and softens.
- Guessing 500 °C — the margin to damage is small.
- Ageing too long — overageing loses strength.
What’s Next
- Learn next: Hardness Testing — checking that ageing worked
- Combine with: Aluminum Alloys — which alloys respond